Symposium Organizers
Prashant Nagpal Los Alamos National Laboratory
Matthew A. Pelton Argonne National Laboratory
Kurtis S. Leschkies Applied Materials Inc.
Hedi Mattoussi Florida State University
Patanjali Kambhampati McGill University
CC1: Optical, Electronic, and Magnetic Functionalities Using Novel Semiconductor Nanocrystal Synthesis.
Session Chairs
Monday PM, November 28, 2011
Room 207 (Hynes)
9:30 AM - **CC1.1
Silicon and Copper Selenide Nanocrystals for Biological Applications.
Colin Hessel 1 , Michael Rasch 1 , Brian Korgel 1
1 Department of Chemical Engineering, University of Texas at Austin, Austin, Texas, United States
Show AbstractSemiconductor and metal nanocrystals have been extensively studied for use in biological applications, for both disease detection and therapy. This research, however, has focused on a relatively narrow range of materials, primarily on gold and silver nanocrystals in the case of metals and Cd-based nanocrystals in the case of semiconductors. In recent years, the nanocrystal "tool kit" has been expanding, including light-emitting silicon nanocrystals and more recently, transition metal chalcogenides like copper selenide for plasmonic heating. Here, we present recent research on the synthesis and surface functionalization of silicon nanocrystals for biological applications, including their inclusion in liposomes. We will also present our efforts on the synthesis and use of copper-deficient copper selenide nanocrystals for photothermal heating. Ligand-stabilized Cu2-xSe nanocrystals were synthesized by a colloidal hot injection method and coated with amphiphilic polymer. The nanocrystals readily disperse in water and exhibit strong near infrared (NIR) optical absorption with a high molar extinction coefficient. The NIR absorption is due to a plasmonic resonance related to the high free carrier density in the nanocrystals due to copper vacancies. When excited with 800 nm light, the Cu2-xSe nanocrystals produce significant photothermal heating with a photothermal transduction efficiency of 22%, comparable to nanorods and nanoshells of gold (Au). In vitro photothermal heating of Cu2-xSe nanocrystals in the presence of human colorectal cancer cell (HCT-116) led to cell destruction, demonstrating the viability of Cu2-xSe nanocrystals for photothermal therapy applications.
10:00 AM - CC1.2
An All-Gas-Phase Approach for the Fabrication of Silicon Quantum Dot Light Emitting Devices.
Rebecca Anthony 1 , Kai-Yuan Cheng 2 , Zachary Holman 1 , Russel Holmes 2 , Uwe Kortshagen 1
1 Mechanical Engineering, University of Minnesota, Minneapolis, Minnesota, United States, 2 Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota, United States
Show AbstractQuantum dots offer unique advantages for the manufacture of light emitting devices including their size tunable optical luminescence, and compatibility with device fabrication on low-cost, flexible substrates that may open up routes to roll-to-roll manufacturing. To date, the fabrication of light emitting devices using quantum dot luminophores exclusively has relied on colloidal chemistry for the quantum dot synthesis, surface treatment, and deposition. In this presentation we demonstrate, to our knowledge for the first time, an all-gas-phase approach for the formation of light emitting devices from silicon quantum dots. In a single gas phase reactor, silicon quantum dots are synthesized, their surfaces functionalized with organic ligands, and deposited onto glass substrates carrying a transparent conductive oxide bottom contact. Production of silicon nanocrystals is achieved through plasma decomposition of the mono-silane precursor, leading to the formation of monodisperse silicon nanocrystals. Quantum dot surfaces are functionalized with organic monolayers by injecting the vapor of various alkenes into the afterglow of the synthesis plasma. Inertial impaction of the functionalized silicon nanocrystals is used to form dense nanocrystal films. Devices are completed by metal evaporation of a top contact. The approach presented here completely avoids the use of solvents and allows the formation of electroluminescent quantum dot devices with as little as three deposition steps for the top and bottom contact layers and the silicon quantum dot layer. Primary support for this work was received from the National Science Foundation (NSF) Award Number ECCS-0925624. Partial support was also received from the NSF MRSEC Program under Award Number DMR-0819885. R.J.H. would also like to acknowledge support from 3M Company through a Non-Tenured Faculty Grant.
10:15 AM - CC1.3
Pyrite Nanocrystals: Shape-Controlled Synthesis and Tunable Optical Properties via Reversible Self-Assembly.
Wei Li 1 , Markus Doeblinger 2 , Aleksandar Vaneski 3 , Andrey L. Rogach 3 , Frank Jaeckel 1 , Jochen Feldmann 1
1 , Photonics and Optoelectronics Group, Faculty of Physics and Center for Nanoscience, Ludwig-Maximilians-Universität München, Munich Germany, 2 , Department of Chemistry, Ludwig-Maximilians-Universität München, Munich Germany, 3 , Department of Physics and Materials Science and Centre for Functional Photonics, City University of Hong Kong, Hong Kong Hong Kong
Show AbstractNanocrystals from non-toxic, earth abundant materials have recently received great interest for their potential large-scale application in photovoltaics and photocatalysis. Here, we report for the first time on the shape-controlled and scalable synthesis of phase-pure pyrite (FeS2) nanocrystals employing a simple and low-cost thermal reaction.[1] Either dendritic nanocrystals (nanodendrites) or nanocubes are obtained by controlling the kinetics of the nanocrystal growth. Pyrite nanodendrites can be reversibly assembled and redispersed, which is accompanied by an increased absorption in the red/near-infrared spectral region due to interactions between the closed-packed nanocrystals. High-concentration nanodendrite dispersions are used to prepare pyrite thin films with strong broadband extinction in the visible and near-infrared. We also show first results on combinations of pyrite nanodendrites with other semicondonctor materials that may be relevant for photovoltaic applications.[1] W. Li, M. Döblinger, A. Vaneski, A. L. Rogach, F. Jäckel, J. Feldmann, submitted
10:30 AM - CC1.4
Multimodal I-III-VI2 Quantum Dots for In Vivo near Infrared Fluorescence and Magnetic Resonance Imaging.
Gary Sitbon 1 , Elsa Cassette 1 , Amaury Patissier 1 , Nicolas Lequeux 2 , Marion Helle 2 , Lina Bolotine 2 , Frédéric Marchal 2 , Benoit Dubertret 2 , Thomas Pons 1
1 , LPEM - ESPCI , Paris France, 2 , CRAN - Centre Alexis Vautrin, Nancy France
Show AbstractSemiconductor nanocrystals, or quantum dots (QDs) have the potential to significantly impact the performance of near infrared fluorescence imaging for biomedical research, diagnostics and optically assisted surgery. For example, QDs could be applied to detection of the sentinel lymph node, the status of which is a key prognostic factor for treatment of many types of cancer. Unfortunately, QDs emitting in the near infrared have been until recently composed of toxic compounds (Cd, Pb, Hg, Te, As…). The potential long term release of these toxic elements in the body has thus been a major obstacle to the QD clinical use, but would also represent an important hurdle for large scale optoelectronic applications.Here we present the synthesis and optical characterization of core and core-shell QDs based on CuInS2 and CuInSe2 that do not contain any toxic heavy metals. Their emission is tunable in the whole near infrared imaging window from 600 nm to 1100 nm. Growth of a ZnS shell around the CuInS2 and CuInSe2 cores increases the fluorescence quantum yield up to 60 % in organic solvents. Structural, composition and optical characterization of these QDs suggest that photoluminescence occurs from defect states inside the QD core. Core/shell QDs maintain their fluorescence properties after solubilisation in water and for long periods of time in vivo. We demonstrate their use for in vivo imaging and detection of regional lymph nodes in mice. Interestingly, CuInS2 -based QDs show a much reduced in vivo toxicity compared to CdTeSe/CdZnS QDs.In addition, we present the synthesis of novel multimodal I-III-VI2 QDs based on doping of near infrared CuInS2/ZnS QDs with paramagnetic Mn2+ ions. We characterize the structural and optical properties of these novel probes. The level of Mn doping is examined under varying synthesis conditions using elemental analysis and electron paramagnetic resonance. SQUID magnetic measurements show the super-paramagnetic behaviour of these nano-particles. These Mn-doped CuInS2/ZnS QDs constitute promising compact nano-probes for multimodal magnetic resonance imaging and near infrared fluorescence and could be useful for whole body pre-clinical MRI mapping and intra-operative optical tracking.
10:45 AM - CC1.5
Synthesis of Mn Doped CdS Nanoparticles in Zincblende Phase with Control of Their Composition and Size.
Masafumi Nakaya 1 , Itaru Tanaka 1 , Atsushi Muramatsu 1
1 Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai, Miyagi, Japan
Show AbstractMagnetic semiconductor, where the host cation is randomly substituted by transition metal ions, is the important material for application of spintronic devices. In order to integrate magnetic semiconductors into electronic devices, magnetic semiconductor materials will need to have nanosized structure. Transition metal ion doped cadmium sulfide nanoparticle is one of the most important magnetic semiconductor materials. However, the random dope of transition metal ions into CdS in zincblende phase and their control in composition become more difficult with decreasing in size to nano-level. Here we report the preparation of zinc blende (CdMn)S nanoparticles with various Mn concentration by mild condition and the effect of the particle size and Mn concentration on their magnetic properties. Mn doped CdS nanoparticles in zincblende phase was prepared by following procedure. Cd and Mn precursors, 1,3-dibutylthiourea, oleylamine and 1-dodecanthiol were added into di-n-octylether. The solution was heated up to 150 °C with vigorously stirring under nitrogen atmosphere and kept for 2 h. Then the solution was heated up to 230 °C and kept for 3~24 h. The solution was cooled down to room temperature and the resulting nanoparticles were precipitated by adding ethanol. The resulting nanoparticles were redispersed into hexane.Mn concentration of as-prepared (Cd1-xMnx)S nanoparticles can be controlled from x = 0 to 0.36 by change the initial feed ratio. The particle sizes were ca. 3 nm in narrow size distribution, independent from Mn content. Their crystal structures were defined as zinc blende type crystal structure without by-product present such as manganese sulfide or oxide. The magnetic properties of the nanoparticles showed diamagnetism at x = 0 (this results corresponds to bulk CdS property), paramagnetism at x = 0.1 and superparamagnetism at x ≥ 0.2. From these results, Mn ions might be randomly doped into CdS in zincblende phase and magnetic long-range order in the particle might be developed by doped Mn. In addition, synthesis of larger zinc blende (CdMn)S nanoparticles and the effect of size on magnetic and optical properties will be also presented.
11:00 AM - CC1:OEMF
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11:30 AM - **CC1.6
Hybrid Metal-Semiconductor Nanoparticles: From Photocatalysis to Doping.
Uri Banin 1
1 Institute of Chemistry & Center for Nanoscience and Nanotechnology, Hebrew University of Jerusalem, Jerusalem Israel
Show AbstractThe ability to selectively arrange nano-sized domains of metallic, semiconducting and magnetic materials into a single “hybrid” nanoparticle offers an intriguing route to engineer nanomaterials with multiple functionalities or the enhanced properties of one domain. In this talk we will present recent strategies used to create semiconductor-metal hybrid nanoparticles, highlight the emergent properties of these multi-component materials, and discuss their potential applicability for different technologies.One aspect of the multifunctionalilty of hybrid metal-semiconductor nanoprticles is related to light induced charge separation that was found to take place at the metal-semiconductor interface in such hybrids opening the path to their implementation in solar energy harvesting. Experiments examining the potential function of metal-semiconductor hybrid nanoparticles as photocatalysts will be reported. Another aspect related to metal-semiconductor combinations is that of impurity doping in such colloidal nanocrystals which presents an important challenge. From the synthesis side, the introduction of a few impurity atoms into a nanocrystal which contains only a few hundred atoms may lead to their expulsion to the surface or compromise the crystal structure. From a physical viewpoint, impurities inherently create a heavily doped nanocrystal under strong quantum confinement, and the electronic and optical properties in such circumstances are still unresolved. We developed a solution based method to dope semiconductor nanocrystals with metal impurities providing control of the band gap and Fermi energy. A combination of optical measurements, scanning tunnelling spectroscopy and theory revealed the emergence of a confined impurity band and band-tailing effects. Successful control of doping and its understanding provide n- and p-doped semiconductor nanocrystals which greatly enhance the potential application of such materials in solar cells, thin-film transistors, and optoelectronic devices prepared by facile bottom-up methods.
12:00 PM - CC1.7
Electronic-Impurity Doping of CdSe Colloidal Quantum Dots.
Ayaskanta Sahu 1 2 , Andrew Wills 3 , Moon Sung Kang 2 , Alexander Kompch 4 , Christian Notthoff 4 , Katherine Wentz 5 , Sophia Hayes 5 , Wayne Gladfelter 3 , Markus Winterer 4 , C. Daniel Frisbie 2 , David Norris 1
1 Optical Materials Engineering Lab, ETH Zurich, Zurich Switzerland, 2 Dept. of Chemical Engineering & Materials Science, University of Minnesota, Minneapolis, Minnesota, United States, 3 Dept. of Chemistry, University of Minnesota, Minneapolis, Minnesota, United States, 4 Nanoparticle Process Technology, University Duisburg-Essen, Duisburg Germany, 5 Dept. of Chemistry, Washington University, St. Louis, Missouri, United States
Show AbstractDespite the importance of doping for controlling the electronic conductivity of bulk semiconductors, very few examples exist where impurities have been incorporated into semiconductor nanocrystals (or colloidal quantum dots) to affect their electronic properties. In particular, little progress has been made on the electronic doping of the most heavily studied nanocrystal system, CdSe. Here, we will discuss an approach to lightly dope CdSe nanocrystals with a controllable amount of electronic impurities. The physical characterization of these particles then shows that the addition of even one impurity (on average) has a dramatic effect on the optical properties of the nanocrystals. Furthermore, studies of the electronic transport through films of these quantum dots reveal complex behavior in the Fermi level as a function of dopant concentration. Using techniques to characterize the crystal structure near the impurities, their location within the nanocrystal was determined. These results are consistent with the physical properties and together can explain the observed optical and electrical trends. However, the results also show that dopant behavior in nanocrystals is not as simple as one might expect. Thus, these experiments begin to reveal the properties of a new class of nanocrystal materials that may be important for future nanocrystal devices.
12:15 PM - CC1.8
Doping Position Dependent Energy Transfer Dynamics in Mn-Doped Core/Shell Semiconductor Nanocrystals.
Dong Hee Son 1
1 Chemistry, Texas A&M Univ, College Station, Texas, United States
Show AbstractWe investigated the rate of exciton-to-Mn energy transfer in Mn-doped CdS/ZnS nanocrystals as a function of Mn doping radius and doping density. The rate of exciton-to-Mn energy transfer in Mn-doped semiconductor nanocrystals, responsible for intense Mn phosphorescence, should depends strongly on the spatial overlap of donor (exciton) and acceptor (Mn ion). To study how the energy transfer rate varies with the radial location of Mn ions, we synthesized radial doping position-controlled CdS/ZnS nanocrystals of varying doping density. Pump-probe transient absorption signals at the bandedge and near IR region were measured to probe the recovery of exciton absorption bleach and relaxation of intraband electron absorption reflecting the dynamics of energy transfer and potential trapping of charge carriers. The data indicate that the dynamics of bleach recovery and decay of near IR absorption become faster and increasingly multiexponential as Mn ions are doped close to the center of the core for a given doping density. We discuss this observation in terms of increasing donor-acceptor wavefunction overlap and its heterogeneity with decreasing doping radius. The result from this study provides a useful information on structural optimization of the energy transfer yield in doped quantum dots, where the exciton energy can be stored in a long-lived Mn ligand field state with less nonradiative loss.
12:30 PM - CC1.9
Incorporation of ``Permanent" Optically Active Holes by Copper-Doping of Inverted Core/Shell Nanocrystals.
Sergio Brovelli 1 , Ranjani Viswanatha 1 , Anshu Pandey 1 , Scott Crooker 2 , Victor Klimov 1
1 , Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 2 , National High Magnetic Field Laboratory, Los Alamos, New Mexico, United States
Show AbstractColloidal semiconductor nanocrystals (NCs) are an emerging class of tunable materials with a large promise for applications in low-cost, solution-processable devices. At present, however, their applicability is greatly limited by the difficulty of controlled introduction of permanent electrically and/or optically active charges. The resolution of this problem would greatly benefit applications of NCs in areas such as photovoltaics and light-emitting diodes (LEDs) and could also enable new advanced device concepts such as “zero-threshold” optical gain. So far, most successful NC doping efforts have focused on magnetically active Mn ions. Several recent publications have also reported the incorporation of ions such as Cu and Ag into NCs. The physical functions of these impurities, however, are still not well understood, including their role as charge-donating species (electron vs. hole donor) and magnetic behaviors (diamagnetic vs. paramagnetic). Here, we introduce a new class of tunable colloidal nanostructures composed of Cu-doped ZnSe cores overcoated with shells of CdSe. Via spectroscopic and magneto-optical studies, we conclusively demonstrate that Cu impurities represent paramagnetic 2+ species, and hence, can serve as a source of optically-active permanent holes. This implies that activation of optical emission due to the Cu level requires injection of only an electron without a need for a valence-band hole. This peculiar electron-only emission mechanism is confirmed by experiments in which the titration of the NCs with hole-withdrawing molecules leads to enhancement of Cu-related photoluminescence (PL) while simultaneously suppressing the intrinsic, band-edge exciton PL. In addition to containing permanent optically active holes, these newly developed materials show unprecedented emission tunability from near infrared (1.2 eV) to the blue (3.1 eV) and reduced losses from re-absorption due to a large Stokes shift (up to 0.7 eV). These properties make them very attractive for applications in light-emission and lasing technologies and especially for the realization of novel device concepts such as “zero-threshold” optical gain and “electron-only” LEDs.
12:45 PM - CC1.10
Nucleation-Controlled Doping of CdSe:Mn2+ 2-D Quantum Well Nanoribbons.
Jung Ho Yu 1 , Xinyu Liu 2 , Kyoung Eun Kweon 3 , Jin Joo 1 , Taehee Kim 4 , Jae Sung Son 1 , Kyungkon Kim 4 , Gyeong S. Hwang 3 , Margaret Dobrowolska 2 , Jacek K. Furdyna 2 , Taeghwan Hyeon 1
1 School of Chemical and Biological Engineering, Seoul National University, Seoul Korea (the Republic of), 2 Physics, University of Notre Dame, Notre Dame, Indiana, United States, 3 Chemical Engineering, The University of Texas at Austin, Austin, Texas, United States, 4 , Korea Institute of Science and Technology, Seoul Korea (the Republic of)
Show AbstractDoping of semiconductor nanocrystals has attracted a great deal of attention for the expectation of tuning the electronic, optical and magnetic properties of semiconductor nanocrystals with the utilization of their quantum-confined energy. Such doping is, however, difficult to achieve in low-dimensional strongly quantum confined nanostructures by growth-controlled doping procedures. In this presentation, we will introduce a new doping strategy-“nucleation-controlled doping”. We demonstrate that the incorporation of manganese ions up to 10 % into 1.4 nm thick CdSe quantum nanoribbons can be readily achieved by the nucleation-controlled doping process. The cation-exchange reaction of (CdSe)13 clusters with Mn2+ ions governs the Mn2+ incorporation during the nucleation stage. This highly efficient Mn2+ doping of the CdSe quantum nanoribbons results in giant exciton Zeeman splitting with an effective g-factor of ~600, the largest value seen so far in diluted magnetic semiconductor nanocrystals. The nucleation-controlled doping strategy demonstrated here thus opens the possibility of doping various strongly quantum confined nanocrystals for diverse applications, including spintronic and photovoltaic applications. [J. H. Yu et al. Nat. Mater. 2010, 9, 47; J. S. Son et al. Adv. Mater. 2011, in press]
CC2: Interaction in Coupled Metal and Semiconductor Nanostructures I
Session Chairs
Monday PM, November 28, 2011
Room 207 (Hynes)
2:30 PM - **CC2.1
Plexcitonics: Metal-Hybrid Nanostructures Designed for Plasmon-Exciton Interactions.
Naomi Halas 1
1 , Rice University, Houston, Texas, United States
Show AbstractTo date, a wide variety of complex plasmonic nanostructures have been designed for specific plasmon-plasmon interactions, giving rise to new hybridized plasmonic media. This basic approach can be extended to hybrid metal-semiconductor nanostructures, whose properties arise from plasmon-exciton coupling. Such “plexcitonic” media can be constructed using nanoparticle-based layered spherical geometries and planar geometries. The coupling of localized plasmons with J-aggregates in layered plexcitonic nanoparticles provides a highly robust and practical system for studying the new modes formed by this interaction. Time-resolved studies of J-aggregate-Au nanoshell complexes when the nanoshell plasmon and J-aggregate exciton energies are degenerate probe the dynamical behavior of the coupled system. Transient absorption is observed, in dramatic contrast to the photoinduced transmission of the pristine J-aggregate. A transient Fano-shaped modulation within the Fano dip is also observable. A combined one-exciton and two-exciton state model coupled to the nanoshell plasmon describes the behavior of the J-aggregate-Au nanoshell complex. Several geometries designed for the study of plasmon-semiconductor quantum dot coupling are also investigated.
3:00 PM - CC2.2
Coupling of Silicon Nanocrystals and Plasmonic Nanostructures.
Tyler Roschuk 1 , Enrico Massa 1 , Iain Crowe 2 , Pengyuan Yang 3 , Andrew Knights 4 , Russell Gwilliam 3 , Matthew Halsall 2 , Stefan Maier 1
1 Physics, Imperial College London, London United Kingdom, 2 Electrical and Electronic Engineering, University of Manchester, Manchester United Kingdom, 3 Surrey Ion Beam Centre, Advanced Technology Institute, University of Surrey, Guildford United Kingdom, 4 Engineering Physics, McMaster University, Hamilton, Ontario, Canada
Show AbstractOver the past two decades, much emphasis has been placed on the potential for silicon nanocrystals (Si-ncs) to act as a Si-based light source for integrated photonic applications. In spite of large increases in the luminescence efficiency of Si through quantum confinement, light emission from Si-ncs exhibits lifetimes of 10s of microseconds. By placing such structures in close proximity to plasmonic structures it is possible to achieve both an enhancement of the interaction of the Si-ncs with the excitation source and a reduction in the luminescence lifetime [1,2]. Here we consider the coupling of Si-ncs formed in SiO2 through an ion implantation process into SiO2 or glass cover slips followed by thermal annealing. In order to have strong interactions between the plasmonic structures and the Si-ncs, Si ions were either implanted at low energy or the sample surfaces have been etched to place the Si-ncs at a depth ~10-15 nm from the surface. FDTD simulations have then been used for the design of rectangular Au nanoantennas with gap sizes from 20-50 nm having a strong overlap in their plasmonic resonance and the measured Si-nc emissions. The simulations predict up to a 25X enhancement of the average E-field in the Si-nc layer in the vicinity of the antennas. Au-nanoantennas were then fabricated on the sample surface using electron beam lithography process. Fluorescence and lifetime measurements were performed on the samples using either a 375 or 405 nm laser diode as the excitation source in a confocal microscopy setup. A significant increase in the emission intensity is observed in the region of the nanoantennas. Furthermore, preliminary results also demonstrate a reduction in the fluorescence lifetime. In addition to Si-ncs, another method of achieving luminescence in Si-based materials is through rare earth doping of silicon oxide (both with and without Si-ncs). Here, we briefly consider the use of the above mentioned nanoantenna structures to enhance the fluorescence of rare earth ions (Eu3+ and Tb3+) implanted into SiO2. Finally, we discuss the potential methods of integration, applications, and suitability of both Si-nc and rare earth doped systems in various areas of photonics. [1] Giannini, V., et al. Small 6, 2498 (2010).[2]Biteen, J.S., et al., Journal of Physical Chemistry C 111, 13372 (2007).
3:15 PM - CC2.3
Quantum Dot/Plasmonic Nanoparticle Hybrid Systems with Quantum Yields That Vary with Excitation Wavelength.
Keiko Munechika 1 , Yeechi Chen 1 , Andreas Tillack 1 , Abhishek Kulkarni 1 , Ilan Jen-La Plante 3 , Andrea Munro 2 , David Ginger 1
1 Chemistry, University of Washington, Seattle, Washington, United States, 3 , Ben-Gurion University of the Negev, Beer Sheva Israel, 2 Chemistry , Pacific Lutheran University , Tacoma, Washington, United States
Show AbstractCoupled plasmonic/chromophore systems are of interest in applications ranging from fluorescent biosensors to solar photovoltaics and photoelectrochemical cells because near-field coupling to metal nanostructures can dramatically alter the optical performance of nearby materials. In this presentation, we describe CdSe quantum dots (QDs) near single silver nanoprisms that exhibit photoluminescence lifetimes and quantum yields that depend on the excitation wavelength, in apparent violation of the Kasha-Vavilov rule. These variations are observed only when the QDs are near the metal particles. We attribute the variation in QD lifetime with excitation wavelength to the wavelength-dependent coupling of higher-order plasmon modes to different spatial subpopulations of nearby QDs. At the QD emission wavelength, these subpopulations are coupled to far-field radiation with varying efficiency by the nanoprism dipolar resonance. These results offer an easily accessible new route to design metal-hybrid structures with tailored optical properties.
3:30 PM - CC2.4
Phosphine and Amine Complexes of Chloride Terminated CdSe Nanocrystals.
Nicholas Anderson 1 , Zachariah Norman 1 , Jonathan Owen 1
1 Chemistry, Columbia University, New York, New York, United States
Show AbstractThis talk will present our studies on the chlorination of alkylcarboxylate- and alkylphosphonate-terminated cadmium selenide nanocrystals with chlorotrimethylsilane in the presence of trialkylphosphine and amine ligands. Little change to the optical absorbance and photoluminescence are visible unless the chlorination is conducted with alkylphosphonate terminated nanocrystals, or samples contaminated with adventitious, acid, alcohols, or primary amines, in which case a blue shift of the absorption features and an increase in the photoluminescence quantum yield are observed. Control experiments show that acid formed from trimethylsilyl chloride and surface bound monhydrogen phosphonate ligands, adventitious carboxylic acid, alcohols or primary amines causes etching. Under these conditions, phosphine and amine ligands are converted to adsorbed phosphonium or ammonium chloride ligands. Thin films of chlorine-terminated nanocrystals were prepared by spin coating and thermal evaporation of the organic ligands, giving rise to small internanocrystal spacing. Preliminary investigations of the optical properties and electrical transport characteristics of these films will be presented.
3:45 PM - CC2.5
Strong Modification of Photoluminescence Decay Characteristics in Metal/Porous-Silicon System.
Toshihiro Nakamura 1 , Bishnu Tiwari 1 , Sadao Adachi 1
1 Graduate School of Engineering, Gunma University, Kiryu, Gunma, Japan
Show AbstractThe luminescence decay characteristics in various materials are known to be strongly modified by metal nanostructure. This is caused by the electromagnetic interactions via excitation of the surface plasmons in the metal. This type of modifications has been observed in a wide variety of materials, such as semiconductor nanocrystals and organic molecules. Nanocrystalline Si has been observed to exhibit strong photoluminescence (PL) in the visible to near-infrared range even at room temperature, and is a promising material for various optoelectronic device applications. There are, however, a few works on the PL decay characteristics of nanocrystalline Si modified by metal nanostructure. This is because the nanocrystalline Si usually shows a complex decay process due to the broad distribution of radiative and non-radiative decay rates caused by the phonon-assisted recombination processes and also fluctuation of Si nanoparticle size and morphology. In this work, we investigate the modification of photoluminescence decay characteristics due to metal nanoparticle deposition on porous Si, which is a typical nanocrystalline Si assembly. Thin porous Si layer of ∼60 nm thickness was formed by the conventional anodic etching. Subsequently, MgF2 spacer layer and Au or Ag nanoparticles were deposited. PL decay curves were measured for samples of various MgF2 and metal layer thicknesses. In the presence of metal film, the luminescence decay curves can be strongly modified, i.e., the decay lifetime becomes short and exhibits remarkable non-single exponential feature. The observed decay curves are well described by a log-normal distribution of decay rates. From these analyses, we determine the distribution of decay rate for each sample. It is found that the deposition of metal film leads to much wider decay rate distribution with its peak shifted toward longer decay rate side. Moreover, the decay rate distribution is strongly dependent both on the MgF2 spacer and metal thicknesses.
4:00 PM - CC2:ICMSN1
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4:30 PM - CC2.6
Impact of Metal Nanoparticles on Excitons in Organic Thin-Films.
Bjoern Niesen 1 2 , Barry Rand 1 , Pol Van Dorpe 1 2 , David Cheyns 1 , Eduard Fron 3 , Paul Heremans 1 2
1 , IMEC, Leuven Belgium, 2 Electrical Engineering (ESAT), Katholieke Universiteit Leuven, Leuven Belgium, 3 Chemistry, Katholieke Universiteit Leuven, Leuven Belgium
Show AbstractWhen metal nanoparticles (NPs) are embedded in a light absorbing organic medium, the localized surface plasmons excited in the NPs interact with excitons generated in the organic medium. These interactions include absorption enhancement due to an increase in the electric near-field as well as luminescence quenching or enhancement. This gives metal nanoparticles potential applications in photovoltaics, light emitting devices, and optical sensors. In order to gain a better understanding of the interactions in such an organic molecule-metal nanoparticle hybrid system, we combined steady-state absorption and photoluminescence (PL) measurements with transient PL measurements of Ag NPs embedded in a thin-film of a small-molecule organic semiconductor. In order to exclusively study near-field effects, the Ag NPs used for these experiments had a size of less than 15 nm, such that light scattering could be neglected. Thin-films with and without spacer layers between the NPs and the semiconductor were examined to study the distance-dependence of the interactions. We observed an increase in light absorption of the organic semiconductor copper phthalocyanine (CuPc) in the presence of Ag NPs with an enhancement factor of up to 1.4. By introducing a spacer layer, the range of this absorption enhancement was found to be around 5 nm. These experimental results are in good agreement with finite element simulations. We studied the effect of Ag NPs on the PL of several organic semiconductors with different natural quantum yield and emission wavelength. The PL intensity and decay-time of these organic semiconductors were strongly affected by the presence of the Ag NPs, with a distance-dependence that heavily depended on the spectral overlap between the NP surface plasmon resonance and the PL emission band, the natural quantum yield of the organic, and the refractive index of the embedding medium. By employing an analytical model, we could qualitatively describe this behaviour and attribute it to changes in the radiative and non-radiative decay rates. In fact, we found, both experimentally and analytically, situations where PL could be enhanced by 25% or reduced by 80%.
4:45 PM - CC2.7
Epitaxial CdX-Au (X=S, Se) Nanocrystal Heterostructures by Thermal Annealing: Thermal and Photostability Studies of Semiconductor-Metal Heterostructures.
Albert Figuerola 1 , Marijn van Huis 2 3 , Changming Fang 2 4 , Marco Zanella 1 , Alessandro Genovese 1 , Sergio Marras 1 , Andrea Falqui 1 , Henny Zandbergen 2 , Roberto Cingolani 1 , Liberato Manna 1 2
1 , Istituto Italiano di Tecnologia, Genoa Italy, 2 , Kavli Institute of Nanoscience; Delft University of Technology, Delft Netherlands, 3 , EMAT, University of Antwerp, Antwerp Belgium, 4 , Materials innovation institute (M2i), Delft Netherlands
Show AbstractSemiconductor inorganic nanocrystals are promising materials in a variety of applications, such as photovoltaics, photocatalysis and opto-electronics.1 The separation ability of the charge carriers generated in the semiconductor sections and the construction of suitable nanocrystal-electrode interfaces will affect strongly the effective performance of the ultimate device. The growth of metallic domains directly on the surface of semiconductor nanocrystals has been suggested as a possible strategy for improving these issues: rod and tetrapod-shaped semiconductor nanocrystals can be nowadays decorated with small Au domains, and several studies have been performed to investigate their charge separation capacity.2,3 However, all these studies are made with Au-CdX heterostructures (X = S, Se, Te) that show a random crystallographic orientation between the two domains which changes from particle to particle in the sample. The variability of the metal-semiconductor nanointerface from one unit to another can easily introduce a certain dispersion of the electronic structure among nanocrystals, seriously degrading the performance of the device. Hence, high-performing and reproducible nanoelectronic devices can only be thought of when using monodisperse semiconductor-metal building blocks with well-defined thermally and photostable interfaces. We will show our results concerning the thermal evolution of a collection of heterogeneous CdSe-Au and CdS-Au nanosystems prepared by wet-chemical approaches. They were all observed while annealed in-situ in a transmission electron microscope with atomic resolution. Heating under vacuum conditions induces Au migration along the semiconductor particle until a extremely high selectivity at their tips is achieved. More interestingly, the annealing also results in distinct and well-defined epitaxial CdSe(CdS)//Au interfaces, in contrast to interfaces that are formed during kinetically controlled wet chemical synthesis, as the only ones available so far. The high quality of these new interfaces should make the heterostructures more suitable for use in nanoscale electronic devices. We also studied the stability of such systems under exposure to electron irradiation: no significant changes were observed for the Se-derived hybrids. On the other hand, CdS-Au heterostructures did suffer a drastic morphological and chemical transformation under these conditions: both experimental and theoretical studies were conducted which allowed to suggest a mechanism and driving force responsible for the transformations. The poor photostability shown by CdS-Au heterostructures under electron irradiation could compromise their long-term use in solar cells, and allows to consider the selenium analogue as a longer-duration active material for the photovoltaic industry.(1) Talapin, D. V. et al. Chem. Rev. 2010, 110, 389-458.(2) Mokari, T. et al. Science 2004, 304, 1787-1790.(3) Sheldon, M. T. et al. Nano Lett. 2009, 9, 3676-3682.
5:00 PM - CC2.8
Pt-Decorated CdS Nanorods: Slow-down of Photoelectron Transfer under Hydrogen Generation Conditions.
Frank Jaeckel 1 , Maximilian Berr 1 , Aleksandar Vaneski 2 , Andrei Susha 2 , Jessica Rodriguez-Fernandez 1 , Christian Mauser 1 , Stefan Fischbach 1 , Markus Doeblinger 3 , Andrey Rogach 1 , Jochen Feldmann 1
1 Physics, Ludwig-Maximilians-University Munich, Munich Germany, 2 Physics and Materials Science, City University of Hong Kong, Hong Kong Hong Kong, 3 Chemistry, Ludwig-Maximilians-University Munich, Munich Germany
Show AbstractColloidal CdS nanorods decorated with sub-nanometer sized Pt clusters have been shown to generate hydrogen photocatalytically with quantum efficiencies of up to 3.9% in the presence of a sacrificial hole scavenger. [1] Using transient absorption spectroscopy the charge carrier dynamics of these hybrid metal-semiconductor systems are investigated under hydrogen generation conditions [2]. Surprisingly, we find a slow-down of the photoelectron transfer rate under hydrogen generation conditions, i.e, in the presence of the hole scavenger, compared to situations where no significant hydrogen is generated. This unexpected behavior is explained by different degrees of localization of the electron wave function on the nanorod in absence and presence of the hole on the nanostructure, respectively. We also explore new semiconductor and catalytic nanomaterial combinations for solar hydrogen generation.[1] M. Berr, A. Vaneski, A. S. Susha, J. Rodríguez-Fernández, M. Döblinger, F. Jäckel, A. L. Rogach, J Feldmann Appl. Phys. Lett. 97, 093108 (2010).[2] M.J. Berr, A. Vaneski, C. Mauser, S. Fischbach, A.S. Susha, A.L. Rogach, F. Jäckel, J. Feldmann, submitted.
Symposium Organizers
Prashant Nagpal Los Alamos National Laboratory
Matthew A. Pelton Argonne National Laboratory
Kurtis S. Leschkies Applied Materials Inc.
Hedi Mattoussi Florida State University
Patanjali Kambhampati McGill University
CC3: Surface Plasmon Polaritons in Metals and Semiconductors
Session Chairs
Tuesday AM, November 29, 2011
Room 207 (Hynes)
9:30 AM - **CC3.1
Tuning Molecular Properties via Hybrid Photon-Matter States.
Thomas Ebbesen 1
1 ISIS, University of Strasbourg, Strasbourg France
Show AbstractStrong coupling of light and matter can give rise to a multitude of exciting physical effects through the formation of hybrid states. Organic molecules have been increasingly used for the study of strong coupling since their large transition dipole moment permits the observation of Rabi splitting in the range of a few hundreds of meV at room temperature. Such large modifications in the energy levels have significant implications for molecular and material sciences. Our recent research on this topic will be presented.
10:00 AM - CC3.2
Excitons and Localized Surface Plasmons in Copper Chalcogenide Nanocrystals.
Ilka Kriegel 1 , Jessica Rodriguez-Fernandez 1 , Enrico Da Como 1 , Chengyang Jiang 2 , Richard Schaller 3 , Dimitri Talapin 2 , Jochen Feldmann 1
1 Photonics and Optoelectronics Group, Department of Physics and CeNS, Ludwig-Maximilians-University, Munich Germany, 2 Department of Chemistry, University of Chicago, Chicago, Illinois, United States, 3 Argonne National Laboratory, Center for Nanoscale Materials, Argonne, Illinois, United States
Show AbstractTransition metal chalcogenide nanocrystals (NCs) from cheap and earth abundant transition metals like copper have been focus of large attention in recent years for photovoltaic application.[1] Indeed, copper sulfide (Cu2-xS) and copper selenide (Cu2-xSe) NCs show interesting optical properties as they have a low bandgap extending in the near infrared (~1.2eV). The absorption spectrum of stoichiometric Cu2S and Cu2Se, produced under oxygen-free conditions, is characterized by a low intensity absorption onset due to an indirect transition and increases towards the blue because of the direct transition. In the presence of oxygen, their non-stoichiometric counterparts, Cu2-xS and Cu2-xSe, are formed. Those develop an intense band in the NIR, which has recently been assigned to free carrier absorption, i.e. plasmonic absorption.[2-3]In this contribution, we provide evidence of localized surface plasmons in Cu2-xS and Cu2-xSe NCs via dielectric sensitivity, supercrystal disassembly and controlled oxidation/reduction experiments. In particular, we demonstrate its formation and control upon oxidation, correlated to the phase transition from stoichiometric to non-stoichiometric. This gives the unique opportunity to tune the position of the plasmon band in the near infrared. Moreover, we investigated the behavior of the excitons in Cu2-xS NCs upon localized surface plasmon formation. The coexistence of excitons and a large density of free carriers results in a quenching of the exciton photoluminescence (PL). By means of transient absorption spectroscopy probing the excitonic resonances we show how the carrier lifetime is faster in the presence of free carriers suggesting an Auger mechanism as the origin of the quenched PL from excitons. Our results not only demonstrate that non-stoichiometric copper chalcogenide NCs simultaneously exhibit excitons and tunable localized surface plasmons in one material, but also that these materials are a unique platform to study at the nanoscale the interaction between excitons and free carriers.[1]C. Wadia, A. P. Alivisatos, and D. M. Kammen, Environmental Science & Technology 43, 2072 (2009).[2]Y. Zhao et al., Journal of the American Chemical Society 131, 4253 (2009).[3]I. Kriegel et al., Chem. Mat. 23, 1830 (2011).
10:15 AM - CC3.3
Graphene Coated Metallic Nanostructures: An Improved Surface-Enhanced Raman Scattering Substrate.
Qingzhen Hao 1 4 , Seth Morton 2 , Bei Wang 1 , Jeremy Bossard 3 , Douglas Werner 3 , Lasse Jensen 2 , Tony Jun Huang 4
1 Department of Physics, The Pennsylvania State University, University Park, Pennsylvania, United States, 4 Department of Engineering Science and Mechanics, The Pennsylvania State University, University Park, Pennsylvania, United States, 2 Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania, United States, 3 Department of Electrical Engineering, The Pennsylvania State University, University Park, Pennsylvania, United States
Show AbstractSurface-enhanced Raman Scattering (SERS) is currently the only method capable of simultaneously detecting a single molecule and providing its chemical fingerprint. A wealth of metallic nanostructures has been proposed to increase SERS sensitivity. Graphene has recently been shown to be SERS active. In the present study, we combine graphene with conventional metallic substrates to achieve higher sensitivity of SERS detection. SERS measurements are carried out on methylene blue (MB) molecules. We synthesize high quality single-layer graphene sheets by chemical vapor deposition (CVD) on copper foils and transfer them to gold nanostructures, i.e., nanoparticle or nanohole arrays. Bare graphene enhances the Raman signal of MB by a factor of ~16. The combined graphene nanostructure substrates show about threefold or ninefold enhancement in the Raman signal of MB compared with the bare nanohole or nanoparticle substrates, respectively. The difference in the enhancement factors between the nanohole and nanoparticle substrates is explained by the different morphologies of graphene on the two substrates. SERS enhancement of graphene is further investigated on mechanically exfoliated graphene. We found that SERS enhancement of graphene can be tuned by changing its Fermi level through doping. Both molecular doping and gate doping experiments show that hole-doped graphene yields a larger SERS enhancement in MB than electron-doped one. Density functional theory (DFT) simulations, using long-range corrected functional LC-omegaPBE0, are then used to study the interaction between MB molecules and a graphene cluster. Our observations show strong evidence that SERS enhancement of graphene is a chemical effect. SERS enhancement from metallic nanostructures, on the other hand, is mainly electromagnetic effect, relying on the plasmonic properties of the nanostructures. Full-wave electromagnetic simulations indicate that graphene does not alter the plasmonic properties of nanostructures significantly, and consequently there is little influence on the electromagnetic SERS enhancement. However, graphene offers additional chemical enhancement which could be combined with the conventional SERS enhancement of bare gold nanostructures to achieve higher detection sensitivity. We believe that the application of graphene to SERS offers a potential way to further improve the sensitivity of conventional metallic SERS substrates.
10:30 AM - CC3.4
GaAs Nanowire Based Plasmon Emitting Diode: An Integrated Subwavelength Light Source and Waveguide.
Pengyu Fan 1 , Carlo Colombo 2 , Kevin Huang 1 , Anna Fontcuberta i Morral 2 , Mark Brongersma 1
1 Materials Science and Engineering, Stanford University, Stanford, California, United States, 2 , École Polytechnique Fédérale de Lausanne, Lausanne Switzerland
Show AbstractWe have demonstrated electroluminescenc from a coreshell GaAs nanowire with radial p-n junction. The volume of light emission is only confined to a subwavelength volume near the electrical contacts of the nanowire, which makes it essentially a point source. We also show this point source naturally couples to surface plasmon when the wire is covered by metal and could be guided along subwavelength nanostrips. Due to the deep subwavelength nature of both the source and guide, we were able to realize truly point-to-ponit routing of light in plane even with 90 degree bending and Y-splitter. Such electrically pumped integrated light source and waveguide, which works at room temperature and power-efficient, could act as essential component of realizing on-chip nanophotonic circuit.
10:45 AM - CC3:SPPMS
BREAK
11:15 AM - **CC3.5
Fabricating and Probing Optical Metamaterials at the Nanoscale.
Albert Polman 1
1 Center for Nanophotonics, FOM Institute AMOLF, Amsterdam Netherlands
Show AbstractWe introduce a new optical metamaterial design composed of a three-dimensional architecture of strongly coupled plasmonic waveguides, in which light can be manipulated and controlled in unique ways. The basic building block in this structure is the plasmonic metal-insulator-metal waveguide, of which the dispersion can be tuned by varying the dielectric layer thickness. Plasmons are excited electrically through impact excitation of silicon quantum dots that are embedded in the plasmonic waveguide. Both planar and coaxial three-dimensional plasmonic waveguide architectures are presented and show an isotropic refractive index that is tunable between -10 and + 10. The figure of merit is as large as 10, higher than in any other metamaterial in the visible. We use focused ion beam milling of ultra-thin membranes to fabricate the structures. Optical refraction experiments on metamaterial microprisms show clear negative refraction at a wavelength of 365 nm. Using interferometry we directly demonstrate the negative phase advance in this new material, corresponding to a refractive index n=-1.0.With the light propagating and confined within these metallodielectric structures, a key question is how these modes can be addressed from the outside, so that their dispersion and local density of states, two key parameters describing propagation and confinement, can be measured. Here, we introduce a new technique, angle-resolved cathodoluminescence imaging spectroscopy, to determine these quantities. The metamaterial structures are excited by a 30 keV electron beam, focused on the sample surface to a spot size of 1-10 nm. The incident point charge, together with its image charge in the substrate, forms a broadband point dipole source that, according to Fermi’s Golden Rule, excites the metamaterials’ modes over a broad spectral range. The radiation from these modes is collected in the far field by a parabolic mirror and imaged onto a CCD detector. In this way, spatial maps (resolution 10 nm) of the plasmonic resonant mode intensities can be determined spectra from which the plasmon wavelength and hence dispersion can be directly derived. Information of the modal phase advance is acquired by collecting the angle-resolved radiation patterns.We apply this technique to coaxial Ag/SiO2/Ag waveguides fabricated using deposition and focused ion beam milling and directly demonstrate that these structures, at the cutoff frequency, show zero phase advance of light, corresponding to an effective refractive index n=0. At cutoff, the enhanced density of optical states is directly observed from a peak in the electron-beam induced emission spectrum. The cutoff frequency can be tuned throughout the entire visible-infrared spectral range (400-1800 nm) by varying the geometry. Applications of these waveguides in planar integrated circuit architectures will be discussed.
11:45 AM - CC3.6
Plasmonic Properties of Single Nanoantennas and Nanoantenna Dimers Investigated by Scanning Transmission Electron Microscopy.
Ina Alber 1 , Reinhard Neumann 1 , Wilfried Sigle 2 , Maria Eugenia Toimil-Molares 1
1 Materials Research, GSI Helmholtz Centre for Heavy Ion Research, Darmstadt Germany, 2 , Max-Planck-Institute for Intelligent Systems, Stuttgart Germany
Show AbstractMetallic nanoantennas are promising candidates for photoactive devices due to their ability to collect solar energy by excitation of localized surface plasmon resonances (LSPR) and confine this energy into subwavelength volumes in the nearfield of the antenna [1]. Recent investigations focus in particular on LSPR excitations in nanoantennas separated by small gaps of few nanometres (so-called nanoantenna dimers), because coupling of individual antennas leads to large energy densities confined exactly at the position of the gap [2,3]. Application of such single nanoantennas and dimers requires a fundamental understanding of their nearfield characteristics. Here, we will present high-resolution nearfield maps of single gold nanoantennas and nanoantenna dimers separated by gaps with sizes ranging from 30 to 8 nm minimum. The synthesis of the nanoantennas was performed by means of electrochemical deposition in etched ion-track membranes, providing control on wire dimensions and gap size [4,5].Combining scanning transmission electron microscopy and electron energy loss spectroscopy, we analyse the LSPRs of individual and dimer antennas with high spatial resolution (~ 4 nm), exciting both bright as well as dark plasmons. Our results reveal the splitting of LSPR into bonding and antibonding modes for at least three multipole orders for the nanoantenna dimers. The distance in energy between bonding and antibonding mode depending on gap size and multipole order as well as the dispersion relations for single nanowire and dimer will be discussed. [1] L. Tang et al., Nature Photonics 2, 226, (2008).[2] J. Aizpurua et al., Phys. Rev. B 71, 235420 (2005).[3] B. Willingham et al., Appl. Phys. B 93, 209 (2008).[4] F. Neubrech et al., Appl. Phys. Lett. 89, 253104 (2006). [5] S. Karim et al., Appl. Phys. A 84, 403 (2006).
12:00 PM - CC3.7
Optimal Plasmonic Metamaterials for Omnidirectional Broadband and Multispectral Enhancement.
Sencer Ayas 1 , Hasan Guner 1 , Burak Turker 1 , Oner Ekiz 1 , Faruk Dirisaglik 2 , Aykutlu Dana 1
1 , Institute of Material science and Nanotechnology, Ankara Turkey, 2 Electrical and Computer Engineering department, University of Connecticut, Storrs, Connecticut, United States
Show AbstractWe report the experimental realization of ultra thin, omnidirectional, broadband multispectral plasmonic metamaterial absorbers for solar cell and spectroscopic applications over the whole the visible spectrum. The spectral responses of such structures are tunable over a broad spectrum. The experimental results are verified with the RCWA and FDTD simulations. Recently, near unity MAs in the microwave [1], IR [2] and NIR [3] regime has been experimentally demonstrated. By tuning the duty-cycle of such structures, it is possible to obtain broadband and multispectral MAs. Efficient omni-directional absorption over a wide spectral band is very important for efficient enhancement in real-world applications. Using numerical simulations we have show that, wide angle and over 40nm absorption bandwidth can be obtained by using one dimensional MAs. We also verify broadband and multispectral MAs using two dimensional structures. In practice, the effect of metal layer surface roughness is found to be critically important in understanding the experimental data. The roughness of silver film on glass results in unexpected spectral response and by employing a germanium wetting layer to decrease the roughness of the silver film, ideal structures can be fabricated and characterized. The structures are defined with e-beam lithography and lift-off of a 50nm thick silver film. The periods of both one and two dimensional structures are around 250nm. The pitch is tuned from 100nm to 210nm. Spectral characterization is performed with a reflection mode experimental setup [4]. We observe only one mode for pitch lengths smaller than 100 nm and that this mode splits into multiple modes as the pitch length is increased. Such multiply split modes form a band-like absorption spectrum. We show using RCWA simulations that the absorbance is omnidirectional, with acceptance up to 80 degrees of angle of incidence. Such structures with extended angular acceptance are very suitable to be used with high numerical aperture objectives, as well as in solar energy conversion applications. Through optimization, we obtain doubly and multiply-resonant omnidirectional structures for simultaneous excitation and emission enhancement, where only weakly-coupled localized modes are used, as opposed to previous studies [5-6]. We employ the absorber structures as SERS substrates. Peak and average Raman enhancement factors are investigated. Tuning of the emission properties of fluorescent dyes and quantum dots is experimentally demonstrated by designing absorber structures that have resonances at the absorption and emission maxima of fluorescent layers.1.N. I. Landy et.al. ,Phys. Rev.Let., 100, 207402 (2008) 2.X. Liu et.al, Phys. Rev. Let. 104, 207403 (2010)3.J. Hao et.al.,App.Phys. Let., 96, 251104 _20104.Y. Chu et.al., Optics Let. ,Vol. 34, No. 3 / 20095.Y. Chu et. al., ACS nano VOL. 5,NO. 1,20116.M.G. Banaee et.al, ACS nano VOL. 4 NO. 5,2011
12:15 PM - CC3.8
Nanocrystalline Thermoelectrics Synthesized by Alkalide Reduction.
Jason Michel 1 , Michael Wagner 1
1 Chemistry, George Washington University, Washington, District of Columbia, United States
Show AbstractNanocrystalline thermoelectric materials have recently gained much attention due to their improved conversion efficiency, which arises from drastic reduction in thermal conductivity, as well as enhancement of the Seebeck coefficient. Thermal conductivity is reduced through phonon scattering by the vast number of grain boundaries, while Seebeck coefficient is improved by electronic quantum confinement. In the present study Bi2Te3 nanocrystals were formed with controllable average crystal size and a narrow size distribution. Varying the bismuth and tellurium halide precursor complexes resulted in differences in the average crystal size between 2.7 nm and 4.9 nm. The average crystallite size was further modified to between 3 nm and 40 nm by simple heating protocols. Potentially optimal crystal size regimes may be attained in this way. Other phases, such as Bi, Te, PbTe, PbSe, the Sb-and Se-substituted alloys of Bi2Te3 have also been synthesized in this manner. Cold-pressing the nanocrystalline powders formed disk-shaped pellets whose electrical conductivity, Seebeck coefficient, and thermal conductivity were measured independently using customized instrumentation.
CC4: Investigations of Novel Physical Phenomenon in Devices I
Session Chairs
Tuesday PM, November 29, 2011
Room 207 (Hynes)
2:30 PM - **CC4.1
Structural and Functional Design of Nanocrystal Solids.
Jong-Soo Lee 1 , Angshuman Nag 1 , Dae-Sung Chung 1 , Wenyong Liu 1 , Dmitri Talapin 1
1 Chemistry, University of Chicago, Chicago, Illinois, United States
Show AbstractColloidal nanocrystals can combine the advantages of crystalline inorganic semiconductors with the size-tunable electronic structure and inexpensive solution-based device fabrication. Single- and multicomponent nanocrystal assemblies, also known as superlattices, provide a powerful general platform for designing two- and three-dimensional solids with tailored electronic, magnetic, and optical properties. Unlike atomic and molecular crystals where atoms, lattice geometry, and interatomic distances are fixed entities, the nanocrystal arrays represent ensembles of “designer atoms” with potential for tuning their electronic structure and transport properties. However, the insulating nature of the surface ligands used for nanocrystal synthesis typically results in the poor electronic coupling between individual nanocrystals. To address this fundamental problem, we introduced electronically transparent inorganic surface ligands, including molecular metal chalcogenide complexes like Sn2S64-, In2Se42-, etc as well as metal-free inorganic ligands like S2-, Se2-, Te2-. These new approaches to surface termination of colloidal nanocrystals provide a set of advantages such as all-inorganic design and diverse compositional tunability for both nanocrystal and ligand constituents. By using optimized inorganic surface ligands we prepared various nanocrystal solids exhibiting band-like charge transport with electron mobility of >10 cm2/Vs, high photoconductivity and tunable doping level. We will demonstrate the power of this approach for thermoelectric and photovoltaic applications.
3:00 PM - CC4.2
Role of Quantized and Mid-Gap States in ``Dark'' Charge Transport and Photoconductivity in Semiconductor Nanocrystal Films.
Prashant Nagpal 1 2 , Victor Klimov 1 2
1 , Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 2 , Center for Advanced Solar Photophysics, Los Alamos, New Mexico, United States
Show AbstractColloidal semiconductor nanocrystals (NCs) have attracted significant interest for applications in solution-processable devices such as light-emitting diodes and solar cells. However, poor understanding of charge transport in NC assemblies, specifically the relation between electrical conductance in dark and under light illumination, hinders their technological applicability. Here, we simultaneously address the issues of “dark” transport and photoconductivity in films of PbS NCs by incorporating them into optical field-effect transistors (OFETs), in which the channel conductance is controlled by both gate voltage and incident radiation. Spectrally resolved photoresponses of OFETs reveal a weakly conductive mid-gap band (MGB) which is responsible for charge transport in dark. The mechanism for conductance, however, changes under illumination when it becomes dominated by band-edge quantized states. In this case, the MGB still plays an important role as its occupancy (tuned by the gate voltage) controls the dynamics of band-edge charges. Our study has broad implications for NC-based electronics and optoelectronics, and specifically, suggests that design guidelines for NC devices might be different depending on whether they are intended for operation in dark (diodes and transistors) or under illumination (photodetectors and solar cells).
3:15 PM - CC4.3
Quantum Dots, Defect States and Solar Cell Efficiency.
Frank Riehle 1 2 , Yunfei Zhou 1 2 , Michael Eck 1 2 , Michael Krueger 1 2
1 Freiburger Materials Research Centre, University of Freiburg, Freiburg Germany, 2 Department of Microsystems Engineering, University of Freiburg, Freiburg Germany
Show AbstractQuantum dots (QDs) are colloidal semiconducting nanocrystals which exhibit unique optical features such as size-tunable fluorescence and absorption spectra arising from to the quantum confinement. When QDs absorb light, electrons are promoted from the valence band to the conduction band leaving a bound electron-hole pair (exciton) behind. After the excitation electrons and holes can recombine by e.g. emitting light of lower energy, known as fluorescence. The fluorescence quantum yield is strongly influenced by the crystal quality, surface chemistry and the surrounding medium. Thus the reproducible synthesis of highly crystalline QDs with tailored surface properties is a prerequisite for their successful implementation into e.g. photovoltaic devices. CdSe QDs made from cadmium carboxylate and TOPSe in a coordinating TOPO-HDA solvent mixture show absolute quantum yields of more than 80% which is a good indication for their high crystalline structure. After a special surface treatment with hexadecanoic acid the CdSe QDs are ready to be used in polymer-hybrid solar cells. There the bound electron-hole pairs, which need to be dissociated into free charge carriers, are generated in the polymer (donor) after light absorption. The electrons are further transferred to the QDs (acceptor) from where they travel to the cathode by hopping events. The corresponding holes stay in the polymer and are transported to the anode. Both, polymer and QDs contribute to the resulting photocurrent. However, the electron-hole recombination mediated by crystal defects in the QDs competes with the charge transfer and thus reduces the solar cell efficiency. In this presentation the synthesis and surface modification of nearly defect-free CdSe QDs will be highlighted and the correlation between fluorescence quantum yield, surface structure and power efficiency will be discussed.
3:30 PM - CC4.4
Plasmonic Light Management for Colloidal Quantum Dot Photodetectors.
F. Pelayo de Arquer 1 , Fiona Beck 1 , Gerasimos Konstantatos 1
1 Solution Processed Nanophotonics, Institut de Ciènces Fotòniques, Barcelona, Catalunya, Spain
Show AbstractColloidal quantum dots (CQDs) have recently attracted significant attention as a candidate material for optoelectronic devices, and in particular solar cells and photodetectors[1]. These materials can be manufactured in the solution phase and spin-cast onto a variety of substrates, significantly reducing the cost of device fabrication. Additionally, the bandgap of CQD films can be tuned to target the near IR and SWIR, of interest for communications, night vision, and bio-imaging, simply by varying the diameter of the CQDs, due to the quantum confinement size effect. To date, PbS CQDs photoconductors have been shown to outperform their best epitaxial counterparts[2]. However, the elevated dark current of these devices limits their operation. The dark current can be minimised by reducing the device thickness, but this results in incomplete light absorption. To improve performances further, it is necessary to decouple the optical thickness from the electrical thickness by employing novel light-trapping schemes.Plasmonic scattering structures can couple incident light into trapped modes inside the thin photodetectors, increasing absorption. Discrete metal nanoparticles (NP) can be fabricated on a glass substrate before the CQD are spin-cast, by simple evaporation and annealing of thin metal films. AFM measurements confirm that uniform CQD films as thin as 50 nm can be successfully spin-cast over Ag NP with dimensions typically used for light trapping[3], with average heights of 30 nm. This technique allows plasmonic scattering structures to be incorporated into optoelectronic devices without significantly increasing the complexity or cost of device fabrication.Preliminary studies have shown that evaporated and annealed Ag NP films can increase the measured absorption of 100 nm thick PbS CQD films, by a factor of 4 or more over a broadband wavelength region around the exciton peak at λ=915 nm (from λ=750 to 1000 nm). Initial spectrally resolved photocurrent measurements of PbS CQD photoconductor devices sensitised with Ag NP films show broadband enhancements of over 100%. By comparing the photocurrent spectra with absorption measurements we can investigate the contribution of light trapping. Additionally, we study the effect of integrated Ag NP on the electrical properties of the device, by varying the thickness of PbS CQD films. By over-coating the Ag NP with ultra-thin LiF dielectric layers, we attempt to minimise recombination effects at the Ag/CQD PbS interface, without significantly affecting the efficacy of the NP in increasing absorption in the film.References[1]E. H. Sargent, Nat. Phot. 3, 325, 2009[2]G. Konstantatos, et al., Nat, Mat. 442, 180, 2006.[3]F. J. Beck, et al., J. App. Phys., 105 (11),114310, 2009.
3:45 PM - CC4.5
Colloidal Quantum Dot Mid-Infrared Emitters and Photodetectors.
Sean Keuleyan 1 , Emmanuel Lhuillier 1 , Philippe Guyot-Sionnest 1
1 JFI / Chemistry, The University of Chicago, Chicago , Illinois, United States
Show AbstractColloidal quantum dots are well known for their bright, robust fluorescence, easily tunable across the full visible spectrum by control over particle size. Narrow gap materials such as the lead chalcoge¬¬nides have extended this tunability into the near-IR, and with PbSe, fluorescence as far as 4.3 µm has been reached. These materials are limited in range though by their bulk bandgap and while zero bandgap materials provide a starting point for tunable electronic transitions through the full infrared range, colloidal synthesis of these materials is is not as developed as that of the cadmium and lead based materials. Colloidal mercury telluride has previously reached a band edge just beyond 3 µm, and we present a new method giving narrow size dispersions and spectral features across the range of 1 to 5 µm. Photoluminescence and electron-hole separation are important for application as emitters and for photodetection but these processes compete with other relaxation pathways. In the mid-IR these are especially important, where organic molecules have intense vibrational transitions which can quench electronic excitations. While current devices for mid-IR optoelectronics rely on expensive, complicated processing, colloidal quantum dots enable cheap and easy deposition but further study is needed to improve performance to compete with bulk materials. We have developed a new synthesis of HgTe quantum dots, giving narrow size distributions tunable to 5 µm, important for spectral selectivity and transport. We demonstrate fluorescence and photoconductive detection using thin films of HgTe quantum dots, covering the atmospheric transparency between 3 and 5 µm, in spite of the presence of organic capping ligands. Improvements will come with development of the material and understanding the processes behind transport and noise, which limits sensitivity in photodetection. Such a cheap and flexible material, optically active in this spectral region could dramatically change the IR imaging and atmospheric communications fields.
4:00 PM - CC4:INPPD
BREAK
4:30 PM - **CC4.6
Third Generation Photovoltaics: Multiple Exciton Generation in Colloidal Quantum Dots, Quantum Dot Arrays, Quantum Dot Solar Cells, and Effects of Solar Concentration.
M. Beard 1 , J. Luther 1 , O. Semonin 2 , J. Gao 2 , A. Midgett 1 , M. Hanna 2 , Barbara Hughes 1 , Arthur Nozik 1 2
1 , National Renewable Energy Laboratory, Golden, Colorado, United States, 2 , University of Colorado, Boulder, Boulder, Colorado, United States
Show AbstractOne potential, long-term approach to more efficient future generation solar cells is to utilize the unique properties of quantum dots (QDs) and unique molecular chromophores to control the relaxation pathways of excited states to produce enhanced conversion efficiency through efficient multiple electron-hole pair generation from single photons . We have observed efficient multiple exciton generation (MEG) in PbSe, PbS, PbTe, and Si. We have studied MEG in close-packed QD arrays where the QDs are electronically coupled in the films and thus exhibit good transport. External and interal photocurrent quantum yield with values > 100% have been measured. We have developed simple, all-inorganic QD solar cells that produce large short-circuit photocurrents and power conversion efficiencies in the 3-5% range via both nanocrystalline Schottky junctions and nanocrystalline p-n junctions. Various possible configurations for novel solar cells based on MEG in QDs that could produce high conversion efficiencies will be presented, along with progress in developing such new types of solar cells. Recent analyses of the effect of MEG combined with solar concentration on the conversion efficiency of solar cells will be discussed.
5:00 PM - CC4.7
Engineering the Energy Landscape in Colloidal Quantum Dot Films for Photovoltaic Device Efficiency.
David Zhitomirsky 1 , Illan Kramer 1 , Ratan Debnath 1 , Osman Bakr 2 , Edward Sargent 1
1 Electrical and Computer Engineering, University of Toronto, Toronto, Ontario, Canada, 2 Materials Science and Engineering, King Abdullah University of Science and Technology, Thuwal Saudi Arabia
Show AbstractColloidal quantum dot (CQD) photovoltaics exploit quantum size effect tuning to make more efficient use of the sun’s broad visible and infrared spectrum. Quantum size effect tuning is especially powerful in realizing tandem and, ultimately, multijunction cells based on a single class of solution-processed materials.The dependence of effective bandgap on nanoparticle size also presents a challenge for the engineering of high-efficiency devices. A rough energy landscape impedes charge carrier delocalization over long lengthscales, yet this is needed to maximize mobility. Even in a hopping picture of transport, small bandgap inclusions act as traps, resulting in the accumulation of photocarriers, in turn promoting their rapid recombination. Here, we report a combined modeling and experimental study that seeks to quantify the requirements on the smoothness of the energy landscape in CQD films, specifically with respect to achieving maximal photovoltaic device performance.Experimentally, we synthesized substantially monodispersed populations of CQDs each centered about a different excitonic energy. We fabricated CQD films using each monodisperse population and characterized the photoluminescence spectrum of each population. We then ‘doped’ the larger-bandgap (1.35 eV) population using various concentrations of the small bandgap (1.0 eV) population ranging from 0.01% - 5% doping of smaller bandgap dots into the larger gap matrix. We found that, for doping concentrations below 1%, the luminescence spectrum was dominated by the matrix. For concentrations above 1%, the luminescence spectrum became dominated by the dopant. The concentration range 0.01%-1% represents the transition region. From this result, we conclude that – for the effects of polydispersity on the open-circuit voltage in CQD photovoltaics to be minimal – the concentration of traps (including small bandgap CQDs) having energy depth of 0.3 eV must be less than 1%.To explain these results in detail, we employed numerical modeling of CQD devices made from various distributions of quantum dots. Here we varied the width of Gaussian-distributed dot populations and observed the impact on key parameters controlling performance. We found that a distribution linewidth of 50 meV or less ensures no dramatic loss in device performance compared to the ideal case. For broader distributions, recombination through low bandgap dots, preventing full quasi-Fermi level splitting, limits and degrades device open-circuit voltage dramatically. This work has motivated a new experimental effort aimed at removing the long, small bandgap tail in QDs both in solutions and solids. We have found that ultracentrifugation allows isolation of monodisperse cultures with the necessary linewidths, which, when incorporated into films, gave superior photovoltaic power conversion efficiency.
5:15 PM - CC4.8
Confocal Photocurrent Mapping of Nanowires Embedded in Nanocrystal Solids.
August Dorn 1 2 , David Strasfeld 2 , Daniel Harris 2 , Hee-Sun Han 2 , Moungi Bawendi 2
1 Institute of Applied Physics, University of Hamburg, Hamburg Germany, 2 Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractNanocrystal solids are promising materials for photo- and particle detection owing to their size tunable optical and electronic properties. However, low charge carrier mobilities in nanocrystal solids pose challenges to efficient exciton extraction, which is of critical importance for applications in optoelectronic devices and for particle detector systems. We will explore how the photocurrent extraction challenge in nanocrystal solids can be overcome by using a CdSe/CdS nanocrystal-CdSe nanowire hybrid photodetector device, which relies on non-radiative exciton energy transfer from the nanocrystals to the nanowires. By combining the wide electronic tuneability of nanocrystals with the excellent one-dimensional charge transport characteristics obtainable in nanowires, we are able to demonstrate a two to three orders of magnitude increase in photocurrent extraction efficiency from the nanocrystal solid. In addition, we correlate local device morphology with optoelectronic functionality by measuring the local photocurrent response in a scanning confocal microscope.
5:30 PM - CC4.9
Tuning Charge Separated States in Type II CdSe/CdTe Nanorod Heterostructures for Photovoltaics.
Hunter McDaniel 1 2 , Moonsub Shim 1
1 Materials Science and Engineering, Univeristy of Illinois at Urbana-Champaign, Urbana, Illinois, United States, 2 Center for Advanced Solar Photophysics, Los Alamos National Lab, Los Alamos, New Mexico, United States
Show AbstractType II nanorod heterostructures (NRHs) exhibit ultrafast exciton dissociation, long carrier lifetimes and broad red-shifted absorption compared with their single phase nanorod (NR) counterparts. Due to properties that are strongly dependent on size, shape and composition, and furthermore, due to the necessity for electronically accessing both components of the heterostructure (which may be achieved via enhancing anisotropy), the key step towards functional multi-component systems is the ability to control not only the size and shape but also spatial orientation of each component with respect to each other. In order to better understand growth mechanism leading to enhanced anisotropy and charge separation in NRHs synthesized from monodisperse NR seeds, we have examined various factors that contribute to and the effects of structural diversification in the type II CdSe/CdTe system. Highly Stokes-shifted emission which arises from charge separated state (CSS) recombination is used to quantify the energy of interfacial band gaps and can be enhanced or suppressed via controlled positioning of CdTe on CdSe seeds. Further, by tuning the composition of the second component to form CdSe/CdSexTe1-x alloyed NRHs, an additional avenue of heterointerfacial band structure engineering is opened up. A careful examination of these NRHs using atomic resolution STEM allows for a spatial mapping of the composition which is then correlated with electronic and optical features. Additionally, our investigation of a new photovoltaic concept incorporating type II NRHs as an extremely thin absorbing layer demonstrates that CSS absorption can contribute meaningfully to photocurrent and gives design criteria for NRH solar cells. Finally, ultrafast transient absorption gives insight into the kinetics of exciton dissociation and carrier trapping in type II NRHs that will be crucial for their application in solar cells.
5:45 PM - CC4.10
Improved Current Extraction from ZnO/PbS Quantum Dot Heterojunction Photovoltaics Using a MoO3 Interfacial Layer.
Patrick Brown 1 , Richard Lunt 2 , Ni Zhao 3 , Timothy Osedach 4 , Darcy Wanger 5 , Liang-Yi Chang 6 , Moungi Bawendi 5 , Vladimir Bulovic 7
1 Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Chemical Engineering & Materials Science, Michigan State University, East Lansing, Michigan, United States, 3 Electronic Engineering, Chinese University of Hong Kong, Hong Kong China, 4 Applied Physics, Harvard University, Cambridge, Massachusetts, United States, 5 Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 6 Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 7 Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractLead sulfide quantum dots (PbS QDs) have attracted growing attention for use in solar cells due to their highly tunable bandgap (0.5 to 2.0 eV), high elemental abundance, and amenability to ambient-atmosphere solution-phase deposition. ZnO/PbS and TiO2/PbS np-heterojunction photovoltaics have shown particular promise and have demonstrated recent gains in both efficiency and stability. However, the presence of a Schottky diode at the PbS/anode interface in opposition to the main oxide/PbS heterojunction diode has been shown to limit the performance of these devices. Here, we demonstrate that this Schottky barrier may be attenuated through the insertion of a thin film of molybdenum oxide (MoO3) between the PbS photoactive layer and the top anode contact. The high-work function MoO3 layer is found to pin the Fermi level of the top electrode, enabling the formation of an ohmic contact to PbS and allowing even low-work function metals to be used as the anode. The MoO3 thus effectively decouples the device performance from the properties of the anode, which enables the use of a transparent ITO anode to measure the external quantum efficiency under illumination from either the anode side or cathode side. A high open-circuit voltage (VOC = 0.59 ± 0.01 V) is obtained for a range of anode materials, and corresponding increases in short-circuit current and fill factor for MoO3-containing devices enable 1.5-fold, 2.3-fold, and 4.5-fold enhancements in photovoltaic device efficiency for gold, silver, and ITO anodes, respectively.
CC5: Poster Session I
Session Chairs
Wednesday AM, November 30, 2011
Exhibition Hall C (Hynes)
9:00 PM - CC5.1
Synthesis of Wurtzite-Type ZnS Nanoparticles by the Pulsed Electric Discharge.
Emil Omurzak 1 , Tsutomu Mashimo 1 , Saadat Sulaimankulova 2 , Shintaro Takebe 1 , Liliang Chen 1 , Zhypargul Abdullaeva 1 , Chihiro Iwamoto 1 , Yudai Oishi 1 , Hirotaka Ihara 1 , Hiroshi Okudera 3 , Akira Yoshiasa 1
1 , Kumamoto University , Kumamoto Japan, 2 , Institute of Chemistry and Chemical Technology, National Academy of Sciences, Bishkek Kyrgyzstan, 3 , Kanazawa University, Kanazawa Japan
Show AbstractSynthesis of wurtzite-type ZnS nanoparticles by an electric discharge submerged in molten sulfur is reported. By the pulsed plasma between two zinc electrodes of 5 mm diameter in molten sulfur, we have synthesized high-temperature phase (wurtzite-type) ZnS nanocrystals with an average size of about 20 nm. Refined lattice parameters of the synthesized wurtzite-type ZnS nanoparticles were found to be larger than those of the reported ZnS (JCPDS 36-1450). Synthesis of ZnMgS (solid solution of ZnS and MgS) was achieved by using ZnMg alloys as both cathode and anode electrodes. UV-Visible absorption spectroscopy analysis showed that the absorption peak of the as-prepared ZnS sample (319 nm) displays a blue-shift comparing to the bulk ZnS (335 nm) due to the quantum confinement. Photoluminescence spectra of the samples revealed peaks at 340, 397, 423, 455 and 471 nm, which were related to excitonic emission and stoichiometric defects.
9:00 PM - CC5.10
Light-Induced Selective Deposition of Metals on Gold-Tipped CdSe Seeded CdS Nanorods.
Jie Lian 1 , Xinheng Li 1 , Ming Lin 2 , Yin Thai Chan 1 2
1 Chemistry, National University of Singapore, Singapore Singapore, 2 , Institute of Materials Research and Engineering, Agency for Science, Technology and Research, Singapore Singapore
Show AbstractWe introduce a facile approach for the selective deposition of metals on Au-tipped CdSe-seeded CdS nanorods that exploits the transfer of electrons from CdS to the Au tips upon UV excitation. This light-induced deposition method was used for the deposition of Pd under mild conditions, which produced a Pd/Au alloyed tip while preserving the rest of the semiconductor nanoarchitecture. The highly site-selective deposition method was extended to the deposition of Fe, yielding monodispersed, structurally complex Au core/FexOy hollow shell-tipped semiconductor nanorods. These structurally well-defined rods were found to exhibit magnetic functionality. The synthetic strategies described in this work expand on the range of metals that can be deposited on heterostructured semiconductor nanorods, opening up new avenues for the hierarchical buildup of structural complexity and therefore multifunctionality in nanoparticles.References:Li, X;† Lian, J;† Lin, M; Chan, Y. J. Am. Chem. Soc. 2011, 133, 672-675. (†These authors contributed equally to this work.)
9:00 PM - CC5.11
Chemical and Structural Transitions between Nickel and Nickel Phosphide Nanoparticles Studied through X-Ray Absorption.
Liane Moreau 1 , Don-Hyung Ha 1 , Haitao Zhang 1 , Richard Robinson 1
1 Materials Science and Engineering, Cornell University, Ithaca, New York, United States
Show AbstractNickel phosphides are under increasing interest as a catalyst for hydrotreating in fuels. Ni2P has been reported to outperform commercially used catalysts in terms of resistance to poisoning. Several new studies have reported synthetic methods for nickel phosphide nanoparticles but little is known about their chemical and structural attributes. In this study we examine nickel and nickel phosphide nanoparticles through x-ray absorption spectroscopy (XAS), and report on the differences between the phases. XAS is a useful and innovative technique to study nanoscale systems where other characterization techniques fail due to resolution and sensitivity limits. Techniques such as x-ray diffraction (XRD), for example, are insufficient to analyze nanoparticle systems which lack long-range order, particularly in intermediate phases. In XAS the XANES region provides details about sample geometric and electronic structure, and the EXAFS region provides short-range order, on the subnanometer scale, making it particularly important for nanoscale and amorphous materials. Through the combination of these techniques, along with TEM and DFT calculations, a thorough characterization and analysis of nickel phosphide nanoparticle phases is provided. In this study of the Ni-P nanoparticle system we: (1) Compare nanoparticle pure and intermediate phases and gain additional insight into structural differences of particles on the nanoscale. This unique structural behavior can account for deviation of nanoparticle properties from those of bulk structure. EXAFS reveals that there is actually a significant amount of phosphorus in an amorphous sample which appears to be nickel from XRD. This suggests that Ni-P intermediate phases retain the long range order of a phosphorus-poor structure despite excess Ni-P bonds with short-range ordering. (2) Characterize and elucidate several nickel phosphide nanoparticle phases and investigate their potential as promising materials for energy applications. The XANES region of the XAS spectra is used to analyze the electronic structure of the nickel 3d and 4p orbital overlap with phosphorus 3p orbitals. Through this we analyze differences between phases and their implications for applications.(3) Compare the long-range structural characterization by XRD to the short-range order displayed by EXAFS in order to investigate the limitations of XRD in nanoparticle characterization and use a combination of both techniques to gain a complete understanding of nanoparticle short and long-range behavior. This enables us to resolve the nanoparticle phase transition properties and diffusion mechanisms, which can lead to optimization of nanoparticle synthesis as well as nanoparticle use in device technology.
9:00 PM - CC5.13
Freestanding, Stabilized Silicon Nanoparticles for Photovoltaic Applications.
Robert Bywalez 1 , Sonja Hartner 1 , Anoop Gupta 1 , Hartmut Wiggers 1 2
1 IVG, Institute for Combustion and Gasdynamics, University of Duisburg-Essen, Duisburg Germany, 2 CeNIDE, Center for Nanointegration Duisburg-Essen, University of Duisburg-Essen, Duisburg Germany
Show AbstractSilicon nanoparticles are envisioned for a broad range of applications ranging from electroluminescent devices over biomarkers to emitters for solar cells. One of the current major challenges is to effectively stabilize the silicon particles against oxidation and form stable dispersions in order to facilitate industrial production by applying efficient low cost processes.In this work a fast and efficient process to functionalize silicon nanoparticles with acrylic acid is introduced. Major advantages of using acrylic acid are related to its low cost and toxicity. The presence of terminal carboxylic groups in the functionalized nanoparticles makes them soluble in polar solvents and therefore facilitates their implementation for industrial purposes.Silicon nanoparticles were produced in a low pressure microwave plasma process by the decomposition of silane (SiH4). Variation of microwave power and reactor pressure enabled the tuning of the particle size between 3 and about 50 nm in diameter. The native oxide shell covering the particles was removed by hydrofluoric acid etch and the particles were subsequently grafted with acrylic acid by thermal alkylation in order to stabilize them for the synthesis of dispersions and to prevent the particles from re-oxidation. Special emphasis was put on elucidating the binding nature of the functionalization agent onto the nanoparticle surface. For this purpose FTIR and XPS measurements were performed proving the covalent attachment by the formation of a Si-C bond. Dispersion properties were investigated by dynamic light scattering showing no sign of degradation over months. These dispersions were utilized to print layers of the functionalized particles. Investigations of the electrical properties were conducted by impedance spectroscopy, revealing higher conductivity values, then the hitherto favored dodecene functionalization. The semiconducting layers were tested for their photoconductive properties, in order to assess their applicability for photovoltaic implementation.
9:00 PM - CC5.14
Large Scale Solution Phase Synthesis of Carbon Quantum Dots as Sensitizers for Solar Cells.
Peter Mirtchev 1 , Eric Henderson 4 , Navid Soheilnia 1 , Christopher Yip 5 2 3 , Geoffrey Ozin 1
1 Chemistry, University of Toronto, Toronto, Ontario, Canada, 4 , Opalux Inc., Toronto, Ontario, Canada, 5 Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, Ontario, Canada, 2 Biochemistry, University of Toronto, Toronto, Ontario, Canada, 3 Institute of Biomaterials and Biomedical Engineering, University of Toronto, Toronto, Ontario, Canada
Show AbstractCarbon quantum dots (CQD’s) are an intriguing recent member of the carbon nanomaterials family which includes graphene, fullerenes, and carbon nanotubes. CQD’s combine many of the favourable properties of traditional semiconductor quantum dots with facile, low-cost solution phase synthesis methods, and reduced toxicity. In this work, we have synthesized and thoroughly characterized aqueous CQD’s prepared via the dehydration of γ-butyrolactone using sulphuric acid. The prepared carbon dots exhibited characteristic tuneable blue-green photoluminescence without the need for a surface passivation agent. The surface was found to contain C=O, O-H and -SO3 functionalities by IR, NMR, and X-ray photoelectron spectroscopy. Due to the compatible surface functionality and absorption edge tail extending into the near infrared region of the spectrum, we have utilised these as low cost solution processable sensitizers for photovoltaic applications. Nanocrystalline TiO2 solar cells sensitized with CQD's were prepared exhibiting a maximum short circuit current density of 0.53 mA/cm2 and open circuit voltage of 380 mV with a fill factor of 64.4% for a power conversion efficiency of 0.13%.
9:00 PM - CC5.15
Semiconductor Quantum Dots for Solar Hydrogen Production.
Fen Qiu 1 , Zhiji Han 1 , Richard Eisenberg 1 , Todd Krauss 1
1 Department of Chemistry, University of Rochester, Rochester, New York, United States
Show AbstractThe abundance and availability of solar energy makes it an attractive primary energy source for hydrogen (H2) generation independent of fossil fuels. Photocatalytic water splitting in particular has attracted great interest recently. For the reductive side of water splitting, the photosensitizer absorbs a photon and transfers the electron to a catalyst, which can reduce aqueous protons to hydrogen. Colloidal semiconductor quantum dots (QDs) have several potential advantages over small molecule photosensitizers typically used with molecular catalysts for H2 production including broad absorption covering the solar spectrum, size tunable band-gap, the ability to store and deliver two electrons, and enhanced photostability. We will present studies of water-soluble CdSe QDs used as the photosensitizer to transfer electrons to a novel Ni catalyst, which could reduce protons to H2. CdSe QDs were made water-soluble via capping with cysteine in the presence of Zn(NO3)2 and NaOH. The fluorescence of the water-soluble QDs was found to be quenched in the presence of ascorbic acid, establishing this molecule as a potential sacrificial electron donor. Reversible oxidative quenching of QDs by methyl viologen (MV2+) indicated these particular QDs have potential as a photosentizer. Several combinations of proton reducing catalysts and operating conditions were tried in order to maximize H2 production efficiency. Good results were obtained by mixing QDs and triethanolamine (sacrificial electron donor) with a Ni pyridyl thiolate catalyst at pH 13 in a 1:1 H2O/Ethanol solution. Under these conditions, H2 production occurred with 100 turnovers with respect to catalyst and 1000 turnovers with respect to QDs. These preliminary results, while not superior to the best organic dye photosensitizers, show that QDs can be potentially used in multicomponent photocatalytic H2 production.
9:00 PM - CC5.16
From Sulfur-Amine Solutions to Metal Sulfide Nanocrystals: Peering into the Sulfur-Oleylamine Black Box.
Jordan Thomson 1 , Kaz Nagashima 2 , Peter Macdonald 1 2 , Geoffrey Ozin 1
1 Department of Chemistry, University of Toronto, Toronto, Ontario, Canada, 2 Department of Chemical and Physical Sciences, University of Toronto Missisauga, Missisauga, Ontario, Canada
Show AbstractWhile significant progress has been made in the synthesis of monodisperse semiconductor nanocrystals of technologically relevant compositions, very little is still understood about the mechanism of precursor reaction and the formation of reactive species. In this work, we present our findings on the formation of metal sulfide nanocrystals from sulfur-amine solutions, an increasingly popular but little studied reaction compared to phosphine-based syntheses. By Pulsed Field Gradient diffusion NMR along with the standard toolbox of 1D and 2D NMR, we determined that sulfur-amine solutions used as a sulfur precursor exist as alkylammonium polysulfides at low temperatures. Upon heating to temperatures used in nanocrystal synthesis, the polysulfide ions react with excess amine to generate H2S, which combines with the metal precursor to form metal sulfide. Four different reaction pathways were found, each of which produced H2S and the byproducts identified. Thioamides were identified as an intermediate and were shown to exhibit much more rapid kinetics than sulfur-alkylamine solutions at low temperatures in the synthesis of metal sulfide nanocrystals.
9:00 PM - CC5.17
Synthesis of Colloidal InP/ZnS Nanocrystals for a Photosensitizer.
Seungyong Lee 1 , Vanga Reddy 1 , Omar Manasreh 1
1 Electrical Engineering, University of Arkansas, Fayetteville, Arkansas, United States
Show Abstract Intrinsic nontoxicity made the direct band gap InP/ZnS core/shell to be one of the most promising semiconductor nanocrystals. InP nanocrystals have the advantage of tuning the optical absorption range in the desired solar spectrum region. Highly luminescent (due to the direct band gap) and monodisperse InP/ZnS nanocrystals were synthesized in a non-coordinating solvent under a thorough degassing process. The research goal here is to synthesize InP/ZnS with different nanocrystal size to generate a broader emission color range, high emission efficiency, and good chemical stability. By varying the molar concentration of indium to ligand with reaction time, different size InP/ZnS nanocrystals were grown. For the purpose of ensuring air stability, ZnS shell around the InP was grown. This ZnS shell improves on the chemical instability in terms of oxidation prevention and on the photoluminescence due to quantum confinement. The InP/ZnS nanocrystals where characterized using both optical and structural tools. Measurements of absorption and emission were performed on different InP/ZnS nanocrystals with different sizes. As expected, the measurements show a red-shift in the band gap as the size of the InP/ZnS nanocrystals is increased. Coupling InP/ZnS to TiO2 is in progress where the photovoltaic devices based on this coupling are expected to produce high solar energy conversion efficiency.
9:00 PM - CC5.18
Colloidal CuInS2 Based Nanocrystals/TiO2 Nanotubes Arrays Composite Solar Cells Fabrication and Testing.
Vanga Reddy 1 , William Wilson 1 , Rick Eyi 1 , Jiang Wu 1 , Scott Mangham 1 , Omar Manasreh 1 , John Dixion 2 , Andrew Wang 2
1 Electrical Engineering, University of Arkansas, Fayatteville, Arkansas, United States, 2 , Ocean Nanotech, Springdale, Arkansas, United States
Show Abstract In our laboratory, we have synthesized a series of cadmium free, and environmental friendly colloidal nano crystals such as CuInS2 (CIS) or CuInSe2 based materials with high quality and high purity and formulated nano inks as well as explored their optoelectronic properties and approached for photovoltaic device fabrication. As a matter of fact, these colloidal materials have high demand in the present photovoltaic market due to their proven ability as a absorbers in solid state solar cells or nano-structured three-dimensional (3D) solar cells. In addition, low cost and ease of chemical synthesis and tunability of crystal size and composition etc., made them as a promising building blocks for optoelectronic device fabrication such as solar cells or LED’s. Though there are several thinfilm deposition techniques are available, they are considered to be very expensive. Furthermore, sophisticated instrumentation is required to form thinfilms, where as a chemical flask based colloidal synthesis of nanocrystals are relatively much easier as compared to chemical or physical vapor deposition.Thus, we have diligently, approached colloidal nanocrystals for the first time to fabricated a 3 D photovoltaic devices by couplings nanocrystals of CIS with TiO2 nanotubular arrays (TNAs), which were grown by novel electrochemical anodization on the surface of titanium metal foil in a polar organic and fluoride contain electrolyte solution. The purpose of using TiO2 tubes is that they have higher surface area as compared regular TiO2 nano particlulate film, secondly we do not require additional back contact hence the TiO2 tubes are fabricated on the surface of Ti metal foil, which is conductive. Moreover, the higher light harvesting efficiency has been attributed to enhanced light scattering properties of TNA’s.The 3 D solar cells fabricated via formation of p-n junctions in a very simple and elegant way. Typically the CIS is considered as a p- type where as a TiO2 is considered as a n- type material. For example, the nano ink of 2-10 nm of CIS nanocrystals or ZnS capped chalcopyrite nanocrystals thin layer formed appropiately on to TiO2 tubes by spraying or simple deposition of colloidal ink on the surface of TiO2 nanotube arrays and dried formed a device. Device performance was tested under standard illumination conditions and current voltage (I-V) characteristics were measured to test the proof of concept. Some of the interesting findings will be presented on synthesis of colloidal crystals with varied composition such as CuInS2, CuInSe2, CuIn(Ga)Se2, stable colloidal nano inks formation as well as their performance will be evaluated in solar cells as an efficient absorber materials.
9:00 PM - CC5.19
Optical Bragg Reflection by the System of Self-Organized Metallic SbAs Nanoinclusions in GaAs and AlGaAs.
Vladimir Chaldyshev 1 , Pavel Lukin 1 , Marina Baidakova 1 , Nikolay Bert 1 , Vladimir Nevedomsky 1 , Maria Yagovkina 1 , Valerii Preobrazhenskii 2 , Mikhail Putyato 2 , Boris Semyagin 2
1 , Ioffe Institute, St. Petersburg Russian Federation, 2 , Institute of Semiconductor Physics, Novosibirsk Russian Federation
Show AbstractMetal-semiconductor metamaterials possess very interesting optical properties due to combination of the plasmon resonance in the metal part with the dielectric response in the semiconductor part of the structure. However, fabrication of such metamaterials is challenging since the metal and semiconductor production technologies are, with rear exclusions, inconsistent to each other. In this paper we employ a self-organization process to form one-dimensional lattices of two-dimensional sheets of metallic SbAs nanoinclusions in the epitaxial semiconducting GaAs and AlGaAs matrices. The periods of these lattices correspond to the resonant Bragg condition at 1.55 um for the case of GaAs matrix and at 0.75 um for the case of AlGaAs matrix. The diameter of the SbAs nanoinclusions was varied from 5 to 20 nm by modification of the parameters of the self-organization process. The microstructure of the metamaterials was documented with the transmission electron microscopy and high resolution x-ray diffractometry. Our optical study revealed a pronounced Bragg resonance in the optical reflection spectra from the SbAs/GaAs system. This optical Bragg resonance, however, is relatively weak since its frequency is far from the plasmon resonance in the SbAs nanoparticles. Wider band gap in the AlGaAs allow us to arrange the Bragg conditions much close to the resonant plasmon frequency for the SbAs nanoinclusions in AlGaAs. It results in a strong resonant optical reflectivity with the absolute minimum-to-maximum magnitude of about 60%. The resonant optical properties are analyzed in connection to the structural data on the SbAs/AlGaAs metamaterials.
9:00 PM - CC5.2
Electrical and Material Characteristics of N Doped GeBiTe for the High Speed PRAM Device.
Dong-Hyun Im 1 , Byoung-Jae Bae 1 , Gyu-Hwan Oh 1 , Sung-Lae Cho 1 , Doo-Hwan Park 1 , Kyung-Min Chung 1 , Se-Phyo Kim 1 , Dong-Hyun Kim 1 , Youngsu Chung 1 , Joon Kim 1 , Seok-Woo Nam 1 , Ho-Kyu Kang 1
1 Process Development Team, Semiconductor R&D Center, Samsung Electronics, Hwasung-City Korea (the Republic of)
Show AbstractPhase change random access memories (PRAM) is a promising candidate for high-density non-volatile memories due to its scalability. Because of fast and reversible phase transition between high-resistivity amorphous and low-resistivity crystalline phases of chalcogenides, PRAM has attracted considerable attention. Presently commercial developments of PRAMs are based on the use of Ge2Sb2Te5 materials, which have been utilized in the semiconductor and optical data storage applications. However the improved properties of phase change material such as low melting temperature, high crystallization temperature, and low electrical resistivity of the crystalline phase are required for hybrid mobile applications recently. Therefore we evaluated the GeBiTe material which shows high speed phase change transition and we especially focused on the effect of nitrogen doping on GeBiTe thin films. We observed the morphology of undoped and N-doped GeBiTe thin films on SiO2 substrate with increasing deposition temperature. The surface bond nature of N-doped GeBiTe films was examined by XPS and the composition of N-doped GeBiTe is measured by AES and XRF with deposition temperature and N2 gas flow rate. The XPS results show that the Ge composition increases with nitrogen content because of stable Ge-N bond formation in GeBiTe material. We also found that N-doped GeBiTe thin film remarkably increases the crystallization temperature up to 300 °C, although the crystallization temperature of an undoped GeBiTe thin film is at 240 °C from XRD and SEM measurements. In order to ensure the resistance to the heat and plasma damage during subsequent processes, the conventional PCM (Phase Change Material) device was fabricated to characterize the electrical behavior of N-doped GeBiTe. The resistivity ratio between SET and RESET state in the N-doped GeBiTe shows about 10 ~ 20 and the set speed of N-doped GeBiTe is 3 times as fast as Ge2Sb2Te5.
9:00 PM - CC5.20
The Nanoporous Metallisation of Insulating Substrates through Semiconductor Photocatalysis.
Michael Bromley 1 , Colin Boxall 1
1 Engineering Department, Lancaster University, Lancaster, Lancashire, United Kingdom
Show AbstractA significant property of semiconductor materials lies in the formation of dissociated and un-dissociated (excitonic) electron-hole pairs upon the absorption of ultra-band gap light energy. These photogenerated charge carriers are able to interact chemically with local species, most notably in the facilitation of redox reactions though photocatalysis. We report the novel use of semiconductor photocatalysis for the deposition of metal onto insulating surfaces and the in-process formation of nano-structured porosity within this metal.In the process of Photocatalytically Initiated Electroless Deposition (PIED) we have developed a one-step metal deposition process which utilises photocatalysis to directly metallise mesoporous and/or nanoparticulate TiO2 sensitised insulating substrate surfaces. PIED has been successfully utilised to produce electrically conductive layers of various metals including Ag and Pd on glass, quartz and polymer substrates. The process is spatially selective and offers several advantages over traditional, non-photocatalytic techniques such as enhanced controllability and purity of the deposit as well as reduced operational costs and environmental impact.With the addition of a self-assembled, hexagonally close-packed polystyrene microsphere template to the substrate prior to metal deposition, PIED can be used to fabricate a thin metal films with highly ordered porosity on the nano-scale. Metallisation occurs directly onto the TiO2 sensitised substrate through the interstitial spacing in the microsphere array, which is subsequently removed by dissolution, to produce a conductive, nanoporous metal film. The thickness of the deposited metal is readily controlled by deposition time allowing the selective metallisation of substrates with single or multi-layer porosity.The fabrication of nanoporous metal by this novel method adds a conductive and permeable metallic structure of high surface area to an otherwise electrically insulating surface. Such metallised insulating materials have potentially wide applications in membrane and separation technology, desalination, electrode/solid electrolyte composites for fuel cells, energy storage and sensors – especially surface enhanced resonance Raman spectroscopy (SERRS).
9:00 PM - CC5.21
Highly Efficient Regeneration of NADH via CdS, CdSe, CdTe Nanocrystals.
Dong Heon Nam 1 , Sahng Ha Lee 1 , Chan Beum Park 1
1 Materials Science and Engineering, KAIST, Daejeon Korea (the Republic of)
Show AbstractIn many biocatalytic synthesis reactions, cofactors such as NADH are critically required for the performance of enzymes catalyzing redox reactions. Due to the high cost of cofactors, a reliable supply is a key barrier to the promotion of economically feasible processes for the synthesis of fine chemicals through biocatalysis. In the past two decades, many researchers have focused on the in situ regeneration of oxidized cofactors, but have not given valuable results until now. Recently, a photochemical route of cofactor regeneration has come into the spotlight because of its potential use of abundant solar energy and clean process. However, extremely low efficiencies of cofactor regeneration have been observed due to insufficient photoefficiency of photosensitizers under visible light ranges. In this work, we report a new application of CdTe, CdSe and CdS nanocrystals to the photochemical regeneration of nicotinamide cofactors under visible light range. We found CdS, CdSe, and CdTe nanocrystals, have a high capability to drive in situ photochemical regeneration of NADH under visible light. In particular, CdTe nanocrystals exhibited a turnover frequency and a turnover number for NADH regeneration that were superior to those of other inorganic photosensitizers reported previously. We investigated the effect of nanocrystal size on NADH regeneration by varying the size of CdSe nanocrystals from 2.8 nm to 4.5 nm. According to our results, NADH yield was higher with smaller nanocrystals, which was attributed to their enhanced photoefficiency and increased number of surface active sites. Furthermore, we investigated the electron transfer pathway from nanocrystals to NAD+ through the analysis using photoluminescence and cyclic voltammetry. This work shows that nanocrystals have a potential to become an efficient light harvesting component that can photochemically boost enzymatic synthesis reactions critically requiring cofactors.Our Recent Publications Related to This Presentation:D. H. Nam, S. H. Lee, C. B. Park, Small, 2010, 6, 922-926.
9:00 PM - CC5.23
Compositional Distribution and Its Effect on the Electronic Properties of Semiconductor Ternary Quantum Dots.
Sumeet Pandey 1 , T. Mountziaris 1 , Dimitrios Maroudas 1
1 Department of Chemical Engineering, University of Massachusetts, Amherst, Massachusetts, United States
Show Abstract
The enhanced functionality of II-VI, IV-VI, and III-V semiconductor ternary quantum dots (TQDs) enables applications in photovoltaic devices, optoelectronics, and fluorescent biological labels. Design and efficient synthesis of such TQDs requires a comprehensive understanding of the underlying process-structure/composition-property-function relationships. To this end, we have integrated first-principles density functional theory (DFT) calculations, atomistic Monte Carlo (MC) simulations, species transport theory, colloidal synthesis, and materials characterization to address challenging issues of compositional distribution effects on TQD properties.
Atomic-scale assembly of hetero-nanostructure TQDs: We have analyzed surface segregation as a potential means for self-assembly of TQD heterostructures with diffuse interfaces. We have determined the equilibrium concentration distribution as a function of nanocrystal size and composition for TQD morphologies that include faceted equilibrium nanocrystal shapes for ZnSe1-xTex, InxGa1-xAs, and InxGa1-xP TQDs; such predictions are based on coupled compositional, structural, and volume relaxation of the nanocrystals according to MC and conjugate-gradient methods employing a DFT-parameterized description of interatomic interactions. We have also developed a phenomenological species transport theory for surface-segregation-induced ordering of constituent and dopant atoms in the dilute limit, which explains the computed equilibrium concentration profiles.
Interfacial Stability of ZnSe/ZnS Core/Shell TQDs: The nm-scale diffusion lengths in nanocrystals introduce an interesting interplay between the kinetic and thermodynamic stability of inorganic core/shell and inorganic/organic TQD/surfactant interfaces. We investigate the thermodynamic stability of such interfaces in TQDs based on DFT calculations combined with XPS and photoluminescence (PL) spectra of TQDs that we have synthesized and annealed. Our findings explain the possibility of compositional redistribution that may cause degradation over time of the core/shell TQD electronic properties, with far reaching implications for the use of such nanostructures in devices.
Compositional Effects on the Electronic Properties of TQDs: We report DFT calculations of the electronic structure of ZnSe1-xSx (type-I) and ZnSe1-xTex (type-II) TQDs and the impact of TQD composition and compositional distribution on the electron density distribution, electronic density of states, and band gap of the TQDs. The resulting relationships obtained for core/shell and alloyed ZnSe1-xSx nanocrystals provide an interpretation for the key features observed in the PL spectra.
9:00 PM - CC5.24
Understanding ``Giant” Nanocrystal Quantum Dots by Synthetic Manipulations and Spectroscopic Methods.
Ben Mangum 1 2 , Yagnaseni Ghosh 2 , Joanna Casson 1 , Han Htoon 1 2 , Jennifer Hollingsworth 2
1 Chemistry Division, Physical Chemistry and Spectroscopy(PCS), Los Alamos National Lab, Los Alamos, New Mexico, United States, 2 Materials Physics and Applications Division, Center for Integrated Nanotechnologies (CINT), Los Alamos National Lab, Los Alamos, New Mexico, United States
Show AbstractSemiconductor nanocrystal quantum dots (NQDs) are increasingly being considered as nearly ideal candidates for light-emission applications due to high quantum efficiencies and narrow-band and particle-size-tunable photoluminescence. However, they suffer from certain disadvantages, including chemical-environment-dependent photo-instability at the ensemble level and intermittency in fluorescence intensity, or “blinking”, at the single NQD level. Prior work in our research team showed for the first time that the growth of ultra-thick shells (number of shell monolayers, n, = 10-20) of the higher bandgap material, CdS, over CdSe NQD cores leads to remarkable photostability and significant suppression of blinking behavior,1,2 which is likely related to the concurrently observed suppression of nonradiative Auger recombination.3,4 The new photophysical behavior afforded by this structurally new class of NQD, the so-called “giant” NQD (g-NQD), promises significant advantages for their application in novel light emitting devices (LEDs).We now emphasize further improvements toward complete suppression of blinking and tunability of optical properties by manipulation of the particle volume and the nature of the core/shell interface, especially with respect to fine-tuning the interfacial composition. This new degree of control over g-NQD structural properties leads to control over emission wavelength, single and multi-excitonic decay lifetimes, and fluorescence intermittency (blinking) at single NQD levels. We demonstrate the unique optical properties of these novel CdSe/nCdS core-shell NQDs by ensemble and single quantum dot spectroscopies. Specifically, time-correlated single photon counting (TCSPC) techniques are used to examine both intensity and lifetime fluctuations as a function of time. Furthermore, by performing 2nd order photon correlation experiments at the single dot level5, bi-exciton quantum yields for these same samples were also determined as a function of the particle volume (itself a combined function of NQD core and shell volumes) and interfacial composition. We demonstrate that by appropriate manipulation of such key structural properties, it is possible to further suppress nonradiative Auger recombination processes and to achieve complete suppression of blinking.1.Chen, Y. et.al., J. Am. Chem. Soc., 2008, 130, 5026.2.Vela, J. et.al., J. Biophotonics, 2010, 3, 706.3.García-Santamaría, F. et al., Nano Lett., 2009, 9, 3482.4.Htoon, H. et al., Nano Lett., 2010, 10, 2401.5.Park, Y.S. et al., Phys. Rev. Lett., 2011, 106, 187401.
9:00 PM - CC5.25
Enhanced Optical Absorption from No-Phonon Transitions in Silicon Quantum Dots.
Benjamin Lee 1 , Jun-Wei Luo 2 , Ingrid Anderson 3 , Daniel Hiller 4 , Margit Zacharias 4 , Paul Stradins 1
1 National Center for Photovoltaics, National Renewable Energy Lab, Golden, Colorado, United States, 2 Basic Energy Sciences, National Renewable Energy Lab, Golden, Colorado, United States, 3 Department of Physics, Colorado School of Mines, Golden, Colorado, United States, 4 IMTEK, Faculty of Engineering, Albert-Ludwigs-University Freiburg, Freiburg Germany
Show AbstractWe report theoretical and experimental results showing enhanced above-gap optical absorption of silicon quantum dots (QDs), as compared to bulk Si. In particular, we measure an increase in the absorption strength of Si QDs, above that of an equivalent volume of bulk Si, for transition energies between ~2.5-3.2 eV. While it is well-established that quantum confinement relaxes the momentum conservation rule and can allow no-phonon bandedge transitions, it is usually expected that the absorption strength of QDs is similar to the bulk at higher energies. Interestingly, we find increased absorption for Si QDs at higher energies, from both experiment and theoretical calculations. The latter reveal the mechanism governing the absorption enhancement. Our observation of enhanced absorption in Si QDs may be interesting for applications, for example in photovoltaics.Experimentally, absorption was determined for Si QDs with sizes from 2-5 nm, made by two different methods: a) Si nanocrystals (NCs) grown in SiO2 matrix [1]; b) NCs grown in an argon-silane plasma, passivated and dispersed in a liquid [2]. NCs were synthesized with narrow size distribution. The absorbance of the samples was obtained using a UV-Vis spectrophotometer with an integrating sphere and by photothermal deflection spectroscopy. For the NCs in oxide, the amount of Si present in the NCs was determined by elastic recoil detection analysis. For the plasma-synthesized NCs, they were handled in an inert environment, their mass measured, and passivated with ligand-chemistry to prevent oxidation. With both types of NCs, we see near-gap absorption consistent with an indirect-like quantum-confined bandgap, but the higher-energy absorption (~2.5-3.2 eV) is enhanced above that of an equivalent volume of bulk crystalline Si.We attribute the enhanced absorption to mixing of Γ and X states for conduction electrons of the Si QDs [3]. The valence and conduction electronic states were calculated for a variety of QD sizes, using an atomistic screened pseudopotential method. The bulk Γ Bloch component appears in lower-energy states of the conduction band of the QDs, mixing with the X component corresponding to the conduction band minimum in bulk Si. For QDs with size <5 nm, there is appreciable mixing of these states, for electronic energies from just above the bandgap to ~3.2 eV, which corresponds to the direct-gap Γ point in bulk Si. The greatest state-mixing occurs at higher energies. This leads to an enhancement in the strength of no-phonon transitions, giving increased absorption, particularly at higher energies close to the bulk direct-gap.This work was supported by the U.S. Department of Energy under Contract No. DE-AC36-08GO28308.[1] M. Zacharias et al., Appl. Phys. Lett. 80, 661-663 (2002).[2] L. Mangolini et al., Nano Lett. 5, 655-659 (2005).[3] J.-W. Luo, P. Stradins, A. Zunger, Energy Environ. Sci., DOI:10.1039/C1EE01026C (2011).
9:00 PM - CC5.26
Understanding Charge Transfer Interactions in Quantum Dot-Dopamine Redox Complexes.
Xin Ji 1 , Goutam Palui 1 , Hyon Bin Na 1 , Hedi Mattoussi 1
1 Department of Chemistry and Biochemistry , Florida State University, Tallahassee, Florida, United States
Show AbstractDue to some of their unique optical and spectroscopic properties, luminescent quantum dots offer a promising platform for developing sensors based on energy transfer or charge transfer interactions. They have a large fraction of their atoms arrayed on their surfaces, and their photoemission is highly sensitive to interactions with proximal dyes, redox complexes and certain metal ions. Dopamine is a chemical neurotransmitter that is involved in a variety of brain activities. It has two intrinsic states, a reduced catechol and the oxidized quinone structure, and it exhibits strong pH-sensitive redox properties. In the current study we investigate the effects of arraying a controllable number of dopamine groups (around the nanocrystal surface) on the optical and spectroscopic properties of QDs. For this, amine-functionalized nanocrystals are directly conjugated to dopamine isothiocyanate, where the fraction of amine-functionalized liagnds and the separation distance between the QD and dopamine are varied. These QDs are rendered water-compatible using end-functionalized DHLA-PEG ligands where the length of PEG moieties and faction of the end functions are varied. This sample configuration allowed us to probe effects of density of redox active complexes, separation distance, as well as pH, all on a biologically inert QD platform.We measured pronounced PL quenching rates for all QD-dopamine assemblies. Furthermore, we found that several parameters affect the PL loss. First of all, the quenching efficiency at basic pH is substantially higher than that measured at more acidic pH. Second, this pH-dependent PL loss is closely correlated with the presence of oxygen in the medium. Third, the quenching efficiency strongly depends on the number of dopamine molecules per QD-conjugate. These findings are supported by time-resolved fluorescence measurements where condition-dependent reduction in the exciton lifetime is measured. We will detail the materials design, the QD and reactive dopamine preparation, QD-redox conjugation and data analysis. We will also discuss the mechanism of PL quenching within the framework of photoinduced charge transfer interactions.
9:00 PM - CC5.28
Synthesis of Gold Nanoparticles via Dimethylacetamide-Controlled Reduction of Gold-Catechol Complexes.
Jihyeon Yeom 1 , Haeshin Lee 1 , Yoon Sung Nam 2 3
1 Department of Chemistry, Korea Advanced Institute of Science and Technology, Daejeon Korea (the Republic of), 2 Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Daejeon Korea (the Republic of), 3 Department of Biological Sciences, Korea Advanced Institute of Science and Technology, Daejeon Korea (the Republic of)
Show AbstractSynthesis of colloidal Au nanoparticles (Au NPs) has been extensively studied to modulate their size, shape, and surface chemistry. These properties largely determine the optical and biological functions of Au NPs that are critically important for their applications to sensors, bio-imaging agents, catalysts, therapeutic agents, etc. Here we present a new route for the synthesis of Au NPs using catechol, a mussel-inspired adhesion molecule, as a reducing and capping agent for chloroauric acid. A mixed solvent of dimethylacetamide (DMA) and water is used as a reaction medium because DMA effectively suppresses the reduction of chloroauric acid presumably via the decreased dissociation and diffusivity of the ionic precursor, while it is very quickly reduced by catechol in water. Our results show that the size and morphology of Au NPs highly depend upon the DMA/water ratio and the functional groups of catechol. Nearly monodisperse, spherical Au NPs of sub-10 nm in diameter can be reproducibly prepared using either of pyrocatechol, 3,4-dihydroxyphenylacetic acid, or 4-tert-butylcatechol in an 85:15 mixture of DMA and water. Dextran-coated Au NPs are also prepared using catechol-grafted dextran oligomers that sterically stabilize the surface of Au NPs in aqueous milieu. This study suggests that the DMA-controlled reduction of gold-catechol complexes provides a useful option for the synthesis of functionalized Au NPs.
9:00 PM - CC5.29
Virus-Templated SWNT-TiO2 Core-Shell Nanotubes for Highly Efficient Electron Collection in Photovoltaic Devices.
Xiangnan Dang 1 2 , Hyunjung Yi 1 2 , Paula Hammond 2 3 , Angela Belcher 1 2 4
1 Department of Materaisl Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 The David. H. Koch Institute for Integrative Cancer Research, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 3 Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 4 Department of Biological Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractThe performance of photovoltaic devices could be improved by using rationally-designed nanocomposites with high electron mobility to efficiently collect photo-generated electrons. Single-walled carbon nanotubes (SWNT) exhibit very high electron mobility, but the incorporation of such nanotubes into nanocomposites to create efficient photovoltaic devices is challenging. Here we report the synthesis of SWNT-TiO2 nanocrystal core-shell nanocomposites using a genetically engineered M13 virus as a template. Using this method, SWNTs are stabilized without surfactants and surface modifications, and their electronic properties are preserved. In addition, close contact is achieved between SWNTs and TiO2 nanocrystals. With the developed biological template approach, we demonstrate that well-dispersed semiconducting SWNTs can improve the power conversion efficiency of DSSCs. Even small fractions of nanotubes (typically 0.1 wt%) improve the power conversion efficiency by increasing the electron collection efficiency. We also show that both the electronic type and degree of bundling of the nanotubes in the SWNT/TiO2 complex are critical factors in device performance. Semiconducting SWNTs improve the electron collection and metallic SWNTs decrease the electron collection in DSSCs. Moreover, debundled SWNTs affect the device performance more than bundled SWNTs. By optimizing the electronic type and aggregation state of SWNTs, we achieve a power conversion efficiency in the dye-sensitized solar cells of 10.6%.Because SWNTs have good thermal conductivity in addition to high electron mobility, this approach might improve the stability of large DSSC modules. Moreover, biological engineering of multiple genes of the virus can extend this approach to creation of more complex structures. Though the route to DSSC improvement lies in the development of dyes with absorption extending into the infrared and better redox couples for higher voltages, we believe that our approach will enable the utilization of SWNTs in many practical photovoltaic devices that require efficient electron diffusion and reduced electron recombination, for instance, quantum dot solar cells, organic solar cells, and photoelectrochemical cells.
9:00 PM - CC5.3
Tunable Multi-Photon Absorption Cross-Sections Using Seeded CdSe/CdS Nanorod Heterostructures.
Tze Chien Sum 1 , Guichuan Xing 1 , Sabyasachi Chakrabortty 2 , Song Wee Ngiam 1 , Kok Loong Chou 1 , Yin Thai Chan 2
1 Division of Physics and Applied Physics, Nanyang Technological University, Singapore Singapore, 2 Department of Chemistry, National University of Singapore, Singapore Singapore
Show AbstractOver the last two decades, multi-photon absorption (MPA) in colloidal semiconductor quantum dots (QDs) has been intensively investigated for potential applications in bio-imaging, upconversion lasing, three dimensional data storage and optical limiting. These applications leverage on the unique characteristics of QDs: size-dependent optoelectronic properties, large MPA cross-sections, relatively high quantum yields, good photostability and flexible surface chemistry. The dependence of the MPA cross-sections of QDs on the size is attributed to the available density of states. Hence, increasing the MPA cross-sections of QDs without significantly degrading its quantum yield or altering its emission wavelength can be highly desirable for example, in multi-photon fluorescence imaging where greater signal may be achieved using less average incident power, thereby minimizing sample damage. While the pronounced size-dependence of the emission of fluorescent QDs in the strong confinement regime presents a convenient way to achieve desired emission wavelengths by simply changing the dot size, however, it also simultaneously imposes severe restrictions on the ability to vary the absorption cross-section while maintaining the emission at a required wavelength. Thus from the stand point of wavelength-specific applications, increasing the MPA cross-section of a QD without significantly modifying its size-dependent emission is an important and yet non-trivial challenge to overcome. Herein we present a method that permits the independent tuning of the MPA cross-section and its corresponding luminescence properties using semiconductor core/enlarged-shell QDs. We demonstrate this with a representative CdSe/CdS nanodot/nanorod system. The elongated CdS shell functions as a photon-capturing “antenna”, which can greatly enhance the overall MPA cross-section of the QD. Photoexcitation of the CdS shell leads to ultrafast carrier transfer to the CdSe core where radiative recombination subsequently occurs. We show that further elongating the CdS shell in rods of a certain length result in substantial gains in the MPA cross-section without significantly red-shifting their emission. The emission peak, on the other hand, can be tuned by appropriately changing the core size. Importantly, these results suggest a strategy for enhancing the MPA whilst independently tuning the emissive wavelengths of semiconductor QDs in a way which is highly relevant to their applications in multi-photon bio-imaging and upconversion lasing. Our latest results on the charge transfer mechanisms, as well as the 3PA studies will also be presented at the conference. A unifying relation that provides a clear basis of comparison between the 3PA cross-sections of various II-VI semiconductor nano-materials is proposed so as to facilitate a more judicious choice of their use in 3PA applications.
9:00 PM - CC5.30
Synthesis of Well Defined CdSe Tetrapods Based on Continuous Precursor Injection and Surface Ligand Control.
Jaehoon Lim 1 , Wan Ki Bae 3 , Go Un Park 2 , Kookheon Char 1 , Seonghoon Lee 2
1 The National Creative Research Initiative Center for Intelligent Hybrids, School of Chemical Engineering, Seoul National University, Seoul Korea (the Republic of), 3 Center for Advanced Solar Photophysics, C-PCS, Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 2 School of Chemistry, Seoul National University, Seoul Korea (the Republic of)
Show AbstractCadmium selenide (CdSe) tetrapods have been regarded as one of ideal nanostructures for high-performance optoelectronic devices based on their optical or electronic quantum phenomena. However, the conventional approach to prepare such nanocrystals (i.e., hot-injection method with alkylphosphonic acid ligands) has shown some limitations to realize cost-efficient and high-performance optoelectronic devices based on CdSe tetrapods, mainly due to expensive ligands and complicated synthetic procedures for mass production. In order to overcome such a limitation, we have developed a “continuous precursor injection” approach using cheap alkylammonium halides as branching agents substituting alkylphosphonic acids. The “continuous precursor injection” implies the successive injection of precursor solution and branching agent using a syringe pump with a controlled injection rate. The CdSe tetrapods prepared with a new synthetic route exhibit exceptional shape selectivity above 90 % as well as finely tunable dimensions varying from 1 to above 20 in aspect ratio simply by controlling the reaction temperature and injection rate. The superior quality of tetrapods apparently originates from the minimal fluctuation of precursor concentration and temperature which would maximize the probability of the complete branching of wurtzite arms at four-equivalent {111} facets of zincblende seeds. This allowed us to examine the branching mechanism in detail, induced by alkylammonium halides introduced in the present study. With the systematic investigation on the relationship of basicity of anions contained in the branching agents with the shape evolution of CdSe nanocrystals as well as their binding states analyzed by X-ray photoelectron spectroscopy, we verify that the weak Lewis base adsorbed on {111} facets modulates the surface energy, causing the growth of wurtzite arms.
9:00 PM - CC5.31
Electric Field Distribution of Quasi-3D Plasmonic Nanostructures as a Function of Dielectric Medium, Substrate and Depth.
Jiajie Xu 1 , Pavel Kvasnicka 2 , Jiri Homola 2 , Qiuming Yu 1
1 Chemical Engineering, University of Washington, Seattle, Washington, United States, 2 Institute of Photonics and Electronics, Academy of Sciences of the Czech Republic, Prague Czechia
Show AbstractQuasi-3D plasmonic nanostructures are composed of a metallic thin film with nanoholes on top and metallic nanodiscs at the bottom of each well which are physically separated by the dielectric medium. Localized surface plasmons (LSPs) can be excited at both 2D nanoholes and 0D nanodots. By engineering different plasmonic elements in a single quasi-3D plasmonic nanostructure, additional freedoms can be achieved in tuning plasmonic properties. In addition, the upper surface of the gold nanodiscs at the bottom and the bottom surfaces of the nanoholes on the top create a Fabry-Pérot (FP) resonant nanocavity with strong field confinement. Therefore, upon electromagnetic irradiation, the complex interplay of the plasmonic resonances, FP resonances, diffraction, and interference results in a unique electric field distribution that can tailor the performance of optoelectronic devices when integrated with optoelectronic materials. In this presentation, we focused on the investigation of the effects of dielectric medium, substrate, and depth on the local electric field distribution of quasi-3D gold nanohole arrays by the integration of three-dimensional finite-difference time-domain (3D-FDTD) simulations and experimental surface-enhanced Raman scattering (SERS) measurements on nanofabricated quasi-3D gold nanohole arrays. The quasi-3D nanostructures in heterogeneous dielectric media, such as PMMA on silicon, pure ITO and glass, and ITO coated glass, as well as homogeneous dielectric medium, such as PDMS, PU, and silicon, were simulated using 3D-FDTD. The depth was varied from 100 to 1200 nm while the diameter and spacing were maintained 400 and 100 nm, respectively, for all arrays. It was found that the intensity of the maximum local electric fields oscillates with the depth and the stronger local electric fields occurring at the top or bottom gold layer strongly depend on the dielectric medium, substrate, and depth. To verify the trend of electric fields as a function of depth and dielectric medium predicted by 3D-FDTD simulations, SERS spectra of 4-mercaptopyridine adsorbed on the quasi-3D gold nanohole arrays fabricated on silicon and ITO coated glass substrates via EBL were taken. The enhancement factors (EFs) obtained from simulations and experiments were compared. The capabilities of tuning not only the intensity but also the location of maximum local electric fields by varying the depth, dielectric media, and substrates make quasi-3D plasmonic nanostrucrure arrays for enhancing the performance of nanostructured optoelectronic devices including photovoltaic and light emitting devices.
9:00 PM - CC5.32
Fluorescent Hybrid Metal-Semiconductor Nanostructures from the Selective Deposition of Metal Nanoparticles on Semiconductor Nanorods.
Sabyasachi Chakrabortty 1 , Guichuan Xing 2 , Tze Chien Sum 2 , Yinthai Chan 1
1 Chemistry, National University of Singapore, Singapore Singapore, 2 Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore Singapore
Show AbstractWe demonstrate that high selectivity over the growth of gold nanoparticles at distinct positions on CdSe seeded CdS nanoheterostructures can be readily achieved by changing the Au precursor concentration as the only parameter varied. This is facilitated by a hierarchical order of reactivities between the different facets at the tips and sides of the nanorod, with the facets at the end of the rod furthest from the CdSe seed being the most reactive for Au deposition. Importantly, we show that Au-CdSe seeded CdS nanorods with a matchstick-like configuration retained a significant amount of its original fluorescence for sufficiently long rod lengths, exhibiting quantum yields of up to 23%. Instead of an expected dipole-dipole energy transfer quenching mechanism between the radiative CdSe core and the Au nanoparticle, we found that the quenching efficiency was strongly modulated by both the size of the CdSe core as well as its distance from the Au nanoparticle, suggesting the participation of more complex non-radiative pathways. A proof-of-concept type application of these fluorescent metal-tipped semiconductor nanorods is given, exploiting the different chemical affinities of the metal and semiconductor moieties to facilitate directed assembly on a chemically patterned surface.
9:00 PM - CC5.33
Characterisation of Nanocrystal Core-Shell Structures in Sub-Nanometer Resolution by Anomalous Small Angle X-Ray Scattering.
Rainer Lechner 1 , Gerhard Fritz-Popovski 1 , Maksym Yarema 2 , Wolfgang Heiss 2 , Oskar Paris 1
1 Institute of Physics, Montanuniversitaet Leoben, Leoben Austria, 2 Institute for Semiconductor and Solid State Physics, Johannes Kepler Universitaet Linz, 4040 Linz Austria
Show AbstractSemiconducting NCs quantum dots are of great interest for numerous applications requiring bright and stable fluorophores [1], especially after stabilizing the NC-core with a hard protective shell [2, 3]. The IV-VI lead chalcogenides NCs have shown a great potential as fluorophores from the mid to the near infrared [3]. In contrast to an epitaxial shell growth on top of the core material [2, 4], we investigate in this study the core/shell formation driven by cation exchange. Cationic exchange of Pb by Cd is achieved by adding an excess of Cd-oleate to the NC suspension in toluene [3]. We use PbS cores with two different starting diameters of 9.6 nm and 5.2 nm [5]. After the addition of the Cd oleate solution, aliquots are removed at several reaction times, resulting in NCs solutions in toluene containing 1-2 wt% of NCs. The core/shell formation is indicated directly after growth by a strong enhancement of the photoluminescence intensity and is also directly visible in TEM images.In this work, we perform anomalous small angle x-ray scattering (ASAXS) experiments at the synchrotron radiation sources ESRF (Grenoble) and at BESSY II (HZB-Berlin). Tuning the x-ray energy just below the Pb-LIII-edge at 13.035 keV allows to record SAXS spectra at different energies, where the contribution of Pb to the total scattering signal is varied [6]. This allows for resolving the total electron density as well as the Pb-atom density inside the NCs independently by applying a core/shell spherical model to fit the data. The outer diameter of the large core/shell NCs shrinks from 9.8 nm to 8.6 nm after the first shell growth step. During the further growth the PbS core shrinks, the CdS shell thickness increases, while the outer diameter remains constant. The final PbS core diameter is determined to be 7 nm with a 0.8 nm pure CdS shell. In the intermediate not fully formed shells still a small amount of around 1 Pb per nm3 is detected. In the smaller core/shell NCs the outer diameter remains constant at 5.2 nm and the final shell thickness of 1.4 nm is formed fast with a core diameter of 2.4 nm. We relate these findings to an increased cationic exchange due to an increased surface to volume ratio for the smaller NCs. We demonstrate that with ASAXS sub-nanometer resolution of the core/shell dimensions with an elemental sensitivity of around one Pb-atoms per nm3 is achieved.[1] V.I. Klimov, M.G. Bawendi, MRS Bull. 26, 998-1004 (2001)[2] D.V. Talapin, I. Mekis, S. Götzinger, et al., J. Phys. Chem. B, 108, 18826–18831 (2004)[3] J.M. Pietryga, D.J Werder, D.J. Williams, et al., JACS 130, 4879-4885 (2008)[4] M. Yarema, S. Pichler, M. Sytnyk, R. Seyrkammer, R.T. Lechner, et al., ACS Nano 5, 3758-3765 (2011)[5] A.M. Hines, G.D. Scholes, Adv. Mater., 15, 1844-1849 (2003)[6] M.Sztucki, E. Di Cola and T. Narayanan, J. Appl. Cryst. 43, 1479–1487 (2010)
9:00 PM - CC5.34
Response of Luminescent Silver Nanodots as Probes.
Junhua Yu 1 , Sungmoon Choi 1 2 , Jin-Kyu Lee 2
1 Chemistry Education, Seoul National University, Seoul Korea (the Republic of), 2 Chemistry, Seoul National University, Seoul Korea (the Republic of)
Show AbstractVarious structures stabilize luminescent silver nanodots, yielding nanodots with low cytotoxicity and excellent brightness and photostability. These nanodots can be reproduced to a high concentration in a variety of biological media, suggesting possible biological applications. The photophysical responses of nanodots strongly depend on the environment they locate, which enables silver nanodots acting not only as biolabeling agents, but also as probes of microenvironments. The primary applications of such silver nanodots have shown promising results. We present here the photophysical responses of nanodots in cellular matrices and other microenvironments and their possible applications.
9:00 PM - CC5.36
Synthesis and Oxidation Behavior of Colloidal Copper Nanoparticles Obtained from Organometallic Precursors Chemistry.
Clement Barriere 6 , Kilian Piettre 1 2 4 , Virginie Latour 1 2 , Olivier Margeat 3 , Bruno Chaudret 5 2 , Pierre Fau 1 2
6 The Hamlyn Centre, Imperial College, London United Kingdom, 1 Laboratoire de Chimie de Coordination, CNRS, Toulouse France, 2 , PRES Université de Toulouse, Toulouse France, 4 , ST Microelectronics SAS , Tours France, 3 CINaM , Université de la Méditerranée, Marseille France, 5 LPCNO, INSA, Toulouse France
Show AbstractNanosized metal particles (NPs) synthesis is a very active research area since such objects display enhanced properties derived from both quantum confinement effects and high surface to volume ratio. With regards to their high chemical activity and air sensitivity, nanoparticles are preferably prepared with noble metals like gold, silver, platinum1 or more directly as oxide compounds. Because copper is prone to oxidation, the literature on copper nanoparticles (NPs) synthesis is less developed than other metals. Copper is nonetheless a versatile material which can be found in various applications as in advanced microelectronics for conductive lines, medicine as an antibacterial agent, or catalysis for chemistry or biochemistry reactions.2 In all cases, the control of the size and size dispersion of nanoparticles of copper remains a major problem and these nanoparticles when exposed to the action of oxygen from ambient air give rise at least to surface oxidation or to full oxidation. In this talk we will describe the formation of copper colloidal solutions obtained from low temperature and low pressure hydrogenolysis (3 bars) in organic solvent of various metal organic copper precursors. This mild chemistry method offers large choices of synthesis parameters (precursor chemistry, solvent, ligand type, temperature…) which participate to the control of the resulting NPs properties (size, capping agent nature …) and a good control of size dispersion has been achieved (20%). We have obtained stable copper NPs from copper amidinate, copper mesytile and copper isobutyrate precursors, with the help of low levels (fraction of molar eq.) of long chain alkylamine or carboxylic acid capping agents. The resulting nano objects are characterized by transmission electron microscopy and spectroscopic UV-Vis methods. An emphasis is given on the role of the precursor chemistry in order to explain the resulting size and size distribution of the copper nanoparticles. This procedure could easily be extended to more complex bimetallic systems (like CuZn or CuAg) as an example, in the aim of an improved air stability system through the protection of the copper NPs surface from oxidation. (1)Tao, A. R.; Habas, S.; Yang, P. D. Small 2008, 4, 310.(2)(a) Wang, Y. F.; Biradar, A. V.; Wang, G.; Sharma, K. K.; Duncan, C. T.; Rangan, S.; Asefa, T. Chem.-Eur. J. 2010, 16, 10735(b) Meng, H.; Chen, Z.; Xing, G. M.; Yuan, H.; Chen, C. Y.; Zhao, F.; Zhang, C. C.; Zhao, Y. L. Toxicology Letters 2007, 175, 102(c) Luisier, A.; Utke, I.; Bret, T.; Cicoira, F.; Hauert, R.; Rhee, S. W.; Doppelt, P.; Hoffmann, P. Journal of the Electrochemical Society 2004, 151, C535
9:00 PM - CC5.5
Synthesis, Characterization and Optical Properties of CdSe Nanosheets.
Rick Morasse 1 3 , Luibov Shapoval 2 , Masaru Kuno 3
1 Slatt Fellow for Energy Systems Research, University of Notre Dame, Notre Dame, Indiana, United States, 3 Department of Chemistry & Biochemistry, University of Notre Dame, Notre Dame, Indiana, United States, 2 Fulbright Visiting Student Researcher, Herzen State Pedagogical University, St. Petersburg Russian Federation
Show AbstractThe synthesis, characterization, and optical properties of high quality CdSe nanosheets are described. A solution-based approach is used to synthesize the nanosheets by mixing Cd and Se precursors with an organic fatty acid and noncoordinating solvent at low temperatures with a subsequent cadmium acetate (in solvent) injection in order to create a Cd rich environment and induce two dimensional (2D) growth. This leverages advances in the development of high quality colloidal quantum dots (QDs) with those of producing 1D nanowires in order to create scale-up synthetic procedures for 2D nanosheets. Resulting rectangular CdSe nanosheets are approximately 20nm in width and 50nm in length with a size distribution of approximately 25%. Intrasheet width variations are very small, although sheets exhibit both straight and curved edges along their widths. High resolution transmission electron microscopy (TEM) images reveal that the sheets are crystalline. Quantum confinement effects are observed in the UV-visible absorption spectra of the CdSe nanosheets, with first and second absorbances at 465nm and 435nm in addition to red shifted absorbances based upon the varying thickness of the nanosheets. Synthetic approaches used to vary the thickness of CdSe nanosheets are reported. Emission studies of sheets and ensembles were also performed. Measurement of the absorption cross sections of single nanosheets can be accomplished by single molecule microscopy using spatial modulation. This facile synthesis affords more opportunities for further investigations of the optical and electrical properties of 2D nanomaterials. The decoration of CdSe sheets with gold particles, for use in photochemical hydrogen generation, will be explored in future experiments. Overall, this investigation presents simple synthetic routes to the size control of 2D CdSe nanosheets which have potential uses in photovoltaics, optoelectronics, functional materials, and solar hydrogen generation.
9:00 PM - CC5.6
Secondary Phosphine Based Synthesis and Mechanism of II-VI Semiconductor Quantum Dots.
Helen Wei 1 , Christopher Evans 1 , Todd Krauss 1
1 Chemistry, University of Rochester, Rochester, New York, United States
Show AbstractIn conventional syntheses of many II-VI and IV-VI colloidal semiconductor quantum dots (QDs), tertiary phosphine chalcogenides are mixed with cadmium salts to form the QDs. However, this procedure suffers from known irreproducibilities and exhibits low conversion yield (< 2%). We recently showed that secondary phosphines, which are common impurities in commercially obtainable tertiary phosphines, were in fact the primary species responsible for QD formation at low temperatures. Tertiary phosphine chalcogenides serve as a source of the anion. Here we will present the synthesis of CdS QDs using pure secondary phosphine sulfide (DPP-S) and Cd-stearate in tetradecane. By using only secondary phosphine selenide precursors, which react with quantitative yield, we can control very accurately the size and stoichiometry of the QDs, including their surface composition. Unexpectedly, we found the specific surface termination of CdS QDs had a huge effect on their fluorescence properties. Band edge emission was completely quenched in S-terminated QDs, but completely recovered when the QDs were Cd-terminated. In fact, the fluorescence intensity from the CdS particle depended directly on the relative amount of Cd versus S on the surface. We also found that QDs synthesized with DPP-S in saturated and unsaturated solvents were dramatically different. Saturated solvents potentially inhibit hydrophosphination across the olefin, which converts DPP to TPP and thus may change the mechanism of QD formation. Altogether, these results, and in particular the use of highly reactive QD chemical precursors to synthesis QDs, opens up the exciting possibility to tune QD photophysical properties through controllable changes in QD surface composition.
9:00 PM - CC5.7
All-Inorganic Solution-Dispersible Silicon Nanocrystals-Controlling Dispersibility by Impurity Doping.
Minoru Fujii 1 , Masatoshi Fukuda 1 , Hiroshi Sugimoto 1 , Kenji Imakita 1 , Shinji Hayashi 1
1 Department of Electrical & Electronic Engineering, Graduate School of Engineering, Kobe University, Kobe Japan
Show AbstractSi nanocrystals dispersible in polar liquid without surface functionalization by organic molecules have been realized by simultaneously doping n- and p-type impurities. We show that the co-doped Si nanocrystals are stable in methanol for more than five months, while intrinsic Si nanocrystals prepared by the same procedure form large agglomerates. The different behavior of the intrinsic and co-doped Si nanocrystals in solution suggests that doped impurities exist on the surface of Si nanocrystals and the surface potential is large enough to prevent the agglomeration. The solution of co-doped Si nanocrystals exhibits broad photoluminescence with the maximum in the near infrared range (1.1-1.3 eV).
9:00 PM - CC5.8
Zinc Oxide Quantum Dot Activated Inorganic Electroluminescent Device.
Takahisa Omata 1 , Yuki Tani 2 , Satoshi Kibayashi 2 3 , Kazuyuki Takahashi 1 , Akira Miyanaga 1 , Yasuhiro Maeda 1 , Shinya Otsuka-Yao-Matsuo 1
1 Division of Material and Manufacturing Science, Graduate School of Engineering, Osaka University, Suita Japan, 2 R&D Center, HOYA Corporation, Akishima Japan, 3 , Samsung Yokohama Research Institute, Yokohama Japan
Show AbstractWe report fabrication of all-inorganic electroluminescent (EL) device, in which the solution synthesized ZnO QDs are used as a chromophore, and EL-emission at ~3.3 eV due to the recombination of the electron and hole that occupy quantum confined levels in the ZnO QDs. The device structure fabricated in the present study, Glass/ITO/TaOx/MgO/ZnO-QDs/MgO/TaOx/Au, was similar with the CdSe Q-EL developed by Kobayashi et al. The substrates used were commercially available amorphous-ITO-coated SiO2-glass. For the ZnO QD chromophore, we prepared colloidal ZnO QDs with ~5 nm diameter dispersed in a mixed solvent of toluene and chloroform by hydrolysis of zinc-di-n-butoxide and successive surface-capping ligand exchange to oleylamine. The ZnO QD layer was deposited by using a LIQUID apparatus that is one of the ion-beam deposition techniques. The LIQUID apparatus enable us to fabricate organics free and dense solid QD-films. The TaOx, MgO and Au layers were deposited by conventional rf-magnetron sputtering or evaporation technique. The device was operated by sinusoidal ac-voltage with a peak-to-peak voltage from 100 to 175 V. In cases of the applied ac-voltage higher than 140 V, a clear UV-emission at ~3.3 eV appeared in addition to the visible emission centered at ~2 eV. The EL profile for the 170 V drive was almost identical with the photoluminescence spectrum of the starting colloidal ZnO QDs. We successfully achieved EL emission in UV-region due to the recombination of electron and hole that occupy quantum confined levels in the ZnO QDs. We also examine the validity of the device structure for UV EL-emission and the role of MgO layer will be presented.
9:00 PM - CC5.9
Lead Sulfide Quantum Dots Synthesis, Deposition, and Temperature Dependence Studies of the Stokes Shift.
Joanna Wang 1 , Bruno Ullrich 1 , Gail Brown 1
1 Materials and Manufacturing Directorate, Wright Patterson AFB, Dayton, Ohio, United States
Show Abstract Lead Sulfide (PbS) nanoparticles (NPs) have attracted considerable attention owing to the narrow direct band gap (0.41 eV) and large exciton Bohr radius, which provides an excellent system for studying quantum confinement effects. However, despite these extensive studies there are still opportunities to further understanding of the physics of the optical properties associated with the confined states. One such area is the understanding of the origin of the well known Stokes shift between the photoluminescence (PL) emission peak and the intrasubband absorption peak of the PbS quantum dots(QDs). The PbS NPs used in this study were synthesized in organic solvents with oleic acid as a capping agent based on a method published in the literature.ref After synthesis, the PbS NPs were washed with methanol, which removed excess oleic acid, octadecene, and other substances. The washed PbS NPs were then dispersed in toluene. The size of the PbS NPs was controlled by changing the oleic acid/octadecene ratios and also by changing the bis(trimethylsilyl)sulfide injection and growth temperatures. Several PbS NPs of different sizes were synthesized and investigated for uniform deposition and long range ordering. A specially fabricated device for creating very uniform deposition over a 0.5 inch diameter area on glass was used for the PbS solvent deposition. The photoluminescence (PL) and optical absorption (OA) properties of the PbS NP layer formed on glass using our solvent evaporation deposition technique were studied. Particular attention was paid to the thermal shift of the PL and OA spectra of the PbS QDs. As is well known, the room temperature PL emission peak is red-shifted with respect to the intrasubband absorption peak. This separation is called Stokes shift and is usually attributed to multi-phonon driven energy relaxations into the dark exciton state before luminescent transitions occur. Similarly, the PL and OA peaks at cryogenic temperatures are also separated in energy. The influence of the particle size (2.2–5.3 nm) and stoichiometry on the Stokes shift was studied previously for PbS QDs but not its temperature dependence ΔS(T). The investigation of ΔS(T), which is of considerable importance for further understanding of the PL transitions and the realization of technological applications including anti-Stokes cooling, is presented. The thermal dependence (5K-300K) of PL and optical absorbance of the PbS sample was measured by employing Fourier spectroscopy. We demonstrate ΔS(T) can be described asΔS(T)=S(0)-
/{exp(/kT)-1} where, ΔS(0) is the Stokes shift at temperatures approaching 0K, is interpreted as the average Fan parameter shared by the OA and PL processes, and is the average energy spend in the sample for the band gap alteration, and kT is the thermal energy. The critical temperature Tc at which S(T)=0 was found to be Tc=463 K (=39.9 meV), corresponding to the sum of prominent PbS phonon energies.
Symposium Organizers
Prashant Nagpal Los Alamos National Laboratory
Matthew A. Pelton Argonne National Laboratory
Kurtis S. Leschkies Applied Materials Inc.
Hedi Mattoussi Florida State University
Patanjali Kambhampati McGill University
CC6: Spectroscopic Studies in Semiconductor and Metal-Hybrids I
Session Chairs
Wednesday AM, November 30, 2011
Room 202 (Hynes)
9:30 AM - **CC6.1
Hot-Carrier Dynamics in Semiconductor Nanocrystals in Relation to Carrier Multiplication and Photoluminescence Blinking.
Victor Klimov 1
1 Chemistry Division, Los Alamos Natl Lab, Los Alamos, New Mexico, United States
Show AbstractDue to small, “sub-excitonic” dimensions, semiconductor nanocrystals can produce novel electronic interactions that involve charges residing in intrinsic quantized states as well as species located at nanocrystal surfaces. Strong coupling between quantum-confined carriers opens new energy relaxation and recombination channels associated with various types of Auger processes. For example, the interaction of a hot conduction-band electron with valence-band charges can lead to an interesting relaxation regime in which the kinetic energy of a hot carrier is not lost as heat but is used to produce additional electron-hole pairs, a process known as carrier multiplication. Alternatively, a hot electron can escape from the particle due to direct coupling to surface species leading to nanocrystal ionization. I will discuss the interplay between various channels for energy relaxation and charge recombination in the nanocrystals, with specific focus on three topics: (1) spectroscopic aspects of carrier multiplication and photoionization; (2) hot-electron transfer in relation to nanocrystal blinking; and (3) Auger decay engineering in core/shell nanostructures. I will also discuss the implications of these studies for applications of nanocrystals in solar-energy conversion and light emission.
CC7/K6: Joint Session
Session Chairs
Wednesday PM, November 30, 2011
Room 207 (Hynes)
9:30 AM - CC7.1/K6.1
ENZ-Enhanced Transmission through a Subwavelength Slit.
Sandeep Inampudi 1 , David Slocum 1 , David Adams 1 , Shiva Vangala 1 , Nicholas Kuhta 2 , William Goodhue 1 , Daniel Wasserman 1 , Viktor Podolskiy 1
1 Physics and Applied Physics, University Of Massachusetts Lowell, Lowell, Massachusetts, United States, 2 Department of Physics, Oregon State University, Corvallis, Oregon, United States
Show AbstractEfficient transmission of light through device structures with subwavelength openings has numerous practical applications in the fields of optical communication, subwavelength imaging and lithography. One approach to enhance the transmission efficiency through subwavelength structures relies on periodically modulated plasmonic systems on composite-based structures, which is often accompanied with substantial fabrication challenges. A promising alternative mechanism to achieve enhanced transmission through subwavelength regions involves materials with vanishingly small dielectric permittivity, also known as epsilon-near-zero (ENZ) materials. A single flat layer of ENZ-material is expected to provide efficient coupling between free-space radiation and sub-wavelength guiding structures. In this work we performed a comprehensive analysis of the role of ENZ materials in the enhancement of light transmission through a subwavelength slit. We have experimentally verified the enhanced transmission through the bulk ENZ material at optical frequencies and developed an analytical model capable of calculating the field distribution throughout the system. Our results show that the transmission enhancement is dominated by the plasma resonance of ENZ coupling layer. In the limit of extremely small material absorption, the overall transmission can be further enhanced by filling the inside of the slit with ENZ medium. In realistic plasmonic systems, however, transmission enhancement due to ENZ filling of the slit needs to be weighed against material absorption accompanying the plasma resonance.
9:45 AM - CC7.2/K6.2
Vertical and Lateral Gap Mode Plasmonic Cavities with Coupled Organic Gain Medium.
Shanying Cui 1 , Kasey Russell 1 , Tsung-li Liu 1 , Evelyn Hu 1
1 School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, United States
Show AbstractSurface plasmon polaritons concentrate optical energy to sub-wavelength length scales, leading to considerable interest in nanoscale metal devices. Metal-based cavities that concentrate electromagnetic fields in the vicinity of optical emitters are of particular interest for investigations into quantum electrodynamics. Although metal losses limit the maximum cavity quality (Q) factors in metal-based cavities, the much smaller mode volumes (V) in these structures provide a Q/V comparable to that of dielectric cavities. We have tested the limits of small mode volumes with two different plasmonic nanocavity geometries: a vertical and horizontal one. The vertical cavity consists of a silver nanowire lying parallel to a silver substrate. The lateral cavity is a narrow trench milled into a silver substrate. In this report, we explore the use of use organic emitters in both geometries. An important advantage of these metal cavities is the wide choice of optically active layers that may be incorporated into the device. Our group has recently demonstrated colloidal PbS nanocrystal coupled to the vertical cavity. Quantum dots, however, are typically several nanometers in diameter, and their incorporation within cavities requires care to achieve thin layers with uniform thicknesses that can effectively couple to the cavity modes. Fluorophores, a class of organic dyes, can be covalently bound as a thin, conformal monolayer to fit in small gaps. They are ubiquitous, versatile and easily functionalizable for further applications, such as single-molecule detection or biosensing. In order to have ultrathin gaps in the cavities with controlled modes and maximized electromagnetic fields, it is imperative to have ultrasmooth silver surfaces. Silver substrates with sub-nanometer roughness is achieved through a ‘template stripping’ method. A thin film of silver is deposited onto an atomically-smooth Si wafer and subsequently transferred to another substrate. In the lateral gap mode cavities, the smooth surfaces are a result of careful optimization of the focused ion beam milling used to form the trenches. In both the vertical and lateral cavity geometries, the coupled gain medium is a self-assembled monolayer of fluorophore dye in the gap, either between the nanowire and the substrate, or within the trench between two metal surfaces. A thin layer of dielectric is deposited through atomic layer deposition between the metal surfaces and the dye to prevent metal quenching of the dye emission. We have observed clear peaks on the dye emission spectrum, evidence of fluorescence coupled to the modes of the cavity, in both cavity geometries. The gap size and trench depth are essential variables to understanding the limits to our system. Organic dyes give us the capability to explore these limits by controlling the gap thicknesses with layer-by-layer growth or by binding it to molecules self-assembled on thickness controlled atomic layer deposition of dielectrics.
CC6: Spectroscopic Studies in Semiconductor and Metal-Hybrids I
Session Chairs
Wednesday PM, November 30, 2011
Room 202 (Hynes)
10:00 AM - CC6.2
The Excitonics of the Surface of Semiconductors Nanocrystals: Implications for Optical Gain, Multiple Exciton Generation, and Single Dot Blinking.
Patanjali Kambhampati 1
1 Chemistry, McGill University, Montreal, Quebec, Canada
Show AbstractThe surface of semiconductor nanocrystals is well known to be a key aspect of its function, yet it remains poorly understood. The surface of the nanocrystal is particularly important in that it is related to the creation of a photoproduct that obscures measurements of multiple exciton generation, optical gain, and single dot photoluminescence blinking. The production of surface trapped charges has seen recent interest in terms of hot carrier charging. Here, we show ultrafast pump/probe as well as simple continuous wave experiments that unravel the nature of the surface of the nanocrystal, as well as its importance in key processes such as optical gain and multiple exciton generation. These experiments show that surface can be described in terms of an activated electron transfer process to surface excitonic states. We connect this excitonic surface state to the core of the nanocrystal via real-time femtosecond measurements of the pathways by which hot excitons populate these surface excitonic states. These femtosecond measurements are consistent with the continuous wave experiments, which reveal a photo-action spectrum for hot exciton surface trapping. Once a surface trapped exciton is produced, we show its role in blocking optical gain, yielding incorrect measurements of Auger recombination processes, and creating false multiple exciton generation signals.
CC7/K6: Joint Session
Session Chairs
Wednesday PM, November 30, 2011
Room 207 (Hynes)
10:00 AM - CC7.3/K6.3
Demonstration of a SPASER Mechanism by the Coupling of Diamond and Plasmonic Au Nanoparticles.
Silvia Orlanducci 1 , Ilaria Cianchetta 1 , Emanuela Tamburri 1 , Valeria Guglielmotti 1 , Francesco Toschi 1 , Massimiliano Lucci 2 , Maria Letizia Terranova 1
1 Dip. di Scienze e Tecnologie Chimiche, Università di Roma Tor Vergata, Roma Italy, 2 Dip. di Fisica, Università di Roma Tor Vergata, Roma Italy
Show AbstractThe coupling of diamond color centers with surface plasmons (SP) and/or structures represents an interesting field of research in solid state physics, material science, and nanophotonics.The basic idea underlying the present research is the coupling of Si optical centers of various diamond systems with plasmonic Au nanoparticles in a SPASER (short for Surface Plasmon Amplification by Stimulated Emission of Radiation) configurations. SPASER, one of the most attractive and recent concept in nanophotonic, it is a synergic process occurring when a resonance between SP and the emission from an active medium (named gain) is achieved. The emission properties of the gain and the SP modes define the SPASER’s working frequency, which are completely unrelated to the used excitation field energy [1]. SPASER was theoretically introduced for the first time by Bergman & Stockman in 2003 [1], whereas first experimental demonstration was only found in 2009 with the work of Noginov’s group [2]. In the frame of Stockman’s theory, in our labs the issue of producing systems suitable for the spasing mechanism has been addressed by coupling diamond Si color centers with properly designed plasmonic Au nanoparticles. A high concentration of Si defects in the diamond lattice is achieved by inserting Si nanoparticles during the CVD growth of diamond [3], whereas size-controlled Au nanoparticles are produced using chemical routes. In our experiments the excitation of the diamond photoluminescence (PL) Si defects (emission at 738 nm) is provided by a laser line at 514 nm, whereas the Au surface plasmons have a maximum in the absorption spectrum at 740 nm perfectly coupled with the 738 nm PL band.The resonant energy transfer from the excited diamond defects to the gold plasmonic modes able to locally enhance the Si photoluminescence (PL), the threshold behavior of the PL enhancement vs pumping energy, the decrease of the FWHM of PL with increasing the pumping laser energy, represent as a whole the first experimental evidence of an efficient SPASER mechanism provided by a diamond-based system [4].The relevant spasing performance demonstrated by our diamond-Au samples, characterized by the stability and the chemical inertness of an all solid-state source, opens an exciting prospective for the development of novel nanophotonic devices .[1]Bergman, D. J. & Stockman M., Phys. Rev. Lett. 90, 027402, (2003)[2] Noginov, M. A., Nature, 460, 1110-1112, (2009)[3] Orlanducci, S. et al., Surface & Coatings Technology 201, 9389–9394, (2007)[4] Orlanducci, S., Cianchetta, I., Tamburri, E., Guglielmotti, V., Terranova, M.L., Nature Photonics, under consideration
CC6: Spectroscopic Studies in Semiconductor and Metal-Hybrids I
Session Chairs
Wednesday PM, November 30, 2011
Room 202 (Hynes)
10:15 AM - CC6.3
Thermalization Dynamics in Semiconductor Nanocrystals.
Daniel Hannah 2 , Nicholas Dunn 2 , Sandrine Ithurria 3 , Dmitri Talapin 3 1 , Lin Chen 1 2 , Matthew Pelton 1 , George Schatz 2 , Richard Schaller 1 2
2 Department of Chemistry, Northwestern University, Evanston, Illinois, United States, 3 Department of Chemistry, University of Chicago, Chicago, Illinois, United States, 1 Center for Nanoscale Materials, Argonne National Lab, Lemont, Illinois, United States
Show AbstractColloidal semiconductor nanocrystal (NCs) quantum dots of materials such as CdSe offer size-controlled bandgaps, intense absorption features, and substantial photoluminescence quantum yields. Such desirable light-absorbing and emitting properties, in addition to facile solution processing, make NCs attractive for solid-state lighting, photovoltaics, bio-labeling and optical amplification applications. Exciton relaxation in quantum-confined matter following photoexcitation has drawn substantial interest for both fundamental understanding and applied purposes. Intraband relaxation takes place on single-picosecond timescales while radiative recombination require tens of nanoseconds at room temperature. On the other hand, photoluminescence lifetimes becomes substantially longer with decreasing temperature owing to the material-specific exciton fine structure and, specifically, due to a lowest-energy optically passive exciton state. Here, we describe detailed studies of radiative recombination in CdSe NCs wherein we reveal signatures of NC thermalization. Specifically, measurements of spectrally and temporally-resolved photoluminescence as a function of temperature reveal signatures of phonon dissipation with clear NC size-dependence.
CC7/K6: Joint Session
Session Chairs
Wednesday PM, November 30, 2011
Room 207 (Hynes)
10:15 AM - CC7.4/K6.4
Correlated Optical Measurements and Plasmon Mapping of Metallic Nanostructures.
Beth Guiton 1 2 , Vighter Iberi 3 , Shuzhou Li 4 , Donovan Leonard 2 5 , Chad Parish 2 , Paul Kotula 6 , George Schatz 7 , Stephen Pennycook 2 8 , Jon Camden 3
1 Department of Chemistry, University of Kentucky, Lexington, Kentucky, United States, 2 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 3 Department of Chemistry, University of Tennessee, Knoxville, Tennessee, United States, 4 Division of Materials Science, Nanyang Technological University, Singapore Singapore, 5 Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee, United States, 6 Materials Characterization Department, Sandia National Laboratories, Albuquerque, New Mexico, United States, 7 Department of Chemistry, Northwestern University, Evanston, Illinois, United States, 8 Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee, United States
Show AbstractPlasmonics is a rapidly growing field with applications spanning communications, computing, photovoltaics, sensors and medical diagnosis. The continuing trend toward increasing complexity and reduced size makes imaging of the plasmonic modes extremely challenging. We will show how spatial maps of the localized surface plasmon (LSP) modes of high-aspect-ratio silver nanorods can be directly obtained using electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM), and correlated for the first time to optical data from the exact same particles. Multivariate statistical analysis (MVSA) enables the extraction of both dark and light modes, which show excellent agreement with classical electrodynamics calculations of the near-electric-field enhancements obtained from planewave excitation. EELS mapping is thus demonstrated to be an invaluable technique for elucidating complex and overlapping plasmon modes. As such this result should have a powerful impact on the growing nano-plasmonics and related energy research communities.
CC6: Spectroscopic Studies in Semiconductor and Metal-Hybrids I
Session Chairs
Wednesday PM, November 30, 2011
Room 202 (Hynes)
10:30 AM - CC6.4
Ultrafast Photoinduced Intraband Absorption in PbS, PbSe and PbSe/CdSe Core/Shell Nanocrystals for near-Infrared to Mid-Infrared All-Optical Signal Processing.
Bram De Geyter 1 2 3 , Pieter Geiregat 1 2 3 , Arjan Houtepen 4 , Dries Van Thourhout 1 3 , Laurens Siebbeles 4 , Zeger Hens 2 3
1 Photonics Research Group, INTEC, Ghent University IMEC, Gent Belgium, 2 Physics and Chemistry of Nanostructures, Inorganic and Physical Chemistry Departement, Ghent University, Gent Belgium, 3 Center for Nano- and Biophotonics (NB-Photonics), Ghent University, Gent Belgium, 4 Optoelectronic Materials, Faculty of Applied Sciences, Delft University of Technology, Delft Netherlands
Show AbstractColloidal nanocrystal quantum dots and dot-in-dots are highly tunable optical materials due to strong quantum confinement of the carriers. The quantum confinement not only raises the bandgap energy, it also relaxes intraband selection rules, making multicarrier Auger recombination ultrafast and optical dipole transitions between intraband states allowed. We present a transient absorption study which shows that photoinduced absorption below the bandgap in PbS, PbSe and PbSe/CdSe colloidal quantum dots can be attributed to intraband absorption. The power dependence agrees with a state-filling model of the bandgap excitonic states and the dynamics of the absorption decay indicate that initial state population decays through Auger recombination. The strength of the intraband transition is wavelength-independent up to (at least) 2000 nm and is about 10% of the bandgap transition strength. The broadband nature, the ultrafast dynamics and the high cross section make these material excellent candidates for all-optical high-speed signal processing on the silicon photonics platform in both the near and the mid-infrared wavelength range.
CC7/K6: Joint Session
Session Chairs
Wednesday PM, November 30, 2011
Room 207 (Hynes)
CC6: Spectroscopic Studies in Semiconductor and Metal-Hybrids I
Session Chairs
Wednesday PM, November 30, 2011
Room 202 (Hynes)
10:45 AM - CC6.5
Size and Composition Dependent Carrier Multiplication Studies on PbX QDs.
Jayson Stewart 1 , Aaron Midgett 2 3 , Lazaro Padilha 1 , Danielle Smith 2 , Jeffrey Pietryga 1 , Joseph Luther 2 , Matthew Beard 2 , Arthur Nozik 2 3 , Victor Klimov 1
1 , Los Alamos National Laboratory, Los Alamo, New Mexico, United States, 2 , National Renewable Energy Laboratory, Golden, Colorado, United States, 3 Chemistry, University of Colorado, Boulder, Colorado, United States
Show AbstractWe present the results of recent CASP efforts in which we compare Auger-recombination lifetimes and carrier multiplication (CM) yields for strongly confined PbSe, PbS and PbTe nanocrystal quantum dots (QDs). QDs made of PbX are attractive candidates for third-generation solar cells because of their narrow band gaps, large Bohr exciton radii, and good natural abundance. In this work, we study a large collection of samples using two different experimental techniques (photoluminescence up-conversion and transient absorption) spread over two different institutions. We discuss the similarities and differences in the PbX QDs and analyze these results in the context of known trends for phonon emission rates in bulk PbX materials as well as CM rates based on the measured Auger recombination lifetimes.
11:00 AM - CC6:SSSM
BREAK
CC7/K6: Joint Session
Session Chairs
Wednesday PM, November 30, 2011
Room 207 (Hynes)
11:00 AM - CC7.5/K6.5
Surface Plasmon and Photonic Mode Propagation in Gold Nanotubes with Wall Thickness on the Order of the Electron Mean Free Path.
Jesse Kohl 1 , Deirdre O'Carroll 2
1 Materials Science and Engineering, Rutgers University, Piscataway , New Jersey, United States, 2 2.Department of Chemistry and Chemical Biology and IAMDN, Rutgers, The State University of New Jersey, Piscataway, New Jersey, United States
Show AbstractLarge inner diameter (ID, 200 nm) gold nanotubes (AuNT’s) were synthesized to study the optical properties and surface plasmon polariton (SPP) mode propagation as function of wall thickness (WT), excitation angle and polarization direction. Fabrication was carried out using nanoporous alumina templates which yield ~30 mm2 arrays with ~108 AuNTs with WT tuned from 30 nm to > 140 nm. Dark-field spectroscopy of nanotube arrays and single nanowire/nanotube heterostructures show a red shift (> 100 eV) over this WT range. Finite-difference-time-domain simulations of AuNT’s of constant 200 nm ID and WT of 30, 60 and 140 nm exhibit red shifts of 63 and 128 eV under axial and normal excitation, while exhibiting SPP propagation lengths of 0.40, 1.15 and 1.30 µm under 660 nm excitation and 1.1, 2.2 and 2.0 µm under 770 nm excitation for the three WT’s, respectively. The AuNT’s also exhibit photonic mode propagation which increases with increasing WT and longer excitation wavelengths (2 µm under 660 nm excitation and 4.7 µm under 770 nm excitation for ID = 140 nm). Increasing SPP and photonic mode propagation length is attributed to the WT becoming greater than the bulk electron mean free path for gold (38 nm), resulting in sufficient electron density to confine and propagate higher energy modes. Thinner structures can only support lower energy modes excited by longer wavelengths which results in the apparent red shift. Electric-field intensity profiles show inter-wall coupling, analogous to insulator-metal-insulator waveguides only under normal excitation. Axial excitation results in SPP mode propagation on the outer surface but no inter-wall coupling. With suitable choice of WT, AuNT’s may be employed for anisotropic light confinement and large-area arrays of vertically aligned waveguides or optical cavities.
11:15 AM - CC7.6/K6.6
Hybrid Optoplasmonic Elements and Materials for High-Performance Optical Information Processing and Sensing.
Svetlana Boriskina 1 , Wonmi Ahn 1 , Yan Hong 1 , Bjoern Reinhard 1
1 Chemistry Department, Boston University, Boston, Massachusetts, United States
Show AbstractNoble-metal nanostructures that support localized surface plasmon (SP) resonances offer unsurpassed capabilities for sub-wavelength light focusing and high sensitivity to environmental changes, which explains their rapidly expanding role in nanoimaging and biomedical research. While spectral positioning of the localized SP resonances can be achieved by varying nanoparticles morphology, dissipative losses in metals hinder efficient control over SP resonances linewidths. These losses also limit the use of propagating SP polaritons as the long-distance signal carriers and challenge the construction of extended functional plasmonic nanocircuits. Finally, development of robust schemes for active nanoscale field modulation, frequency switching, and reversible energy transfer between photons, SPs and nanoscale emitters is stifled by fundamental restrictions on scaling existing tuning mechanisms to nanoscale dimensions.To address these challenges, we have recently proposed hybrid optoplasmonic structures that combine the capability of optical microcavities to insulate emitter-photon systems from decohering environmental effects with the superior nanofocusing properties of SP nanostructures [1]. Efficient photon trapping and re-cycling in photonic microcavities provides mechanisms of strong spectral selectivity in the proposed optoplasmonic elements and strong resonant modification of radiative rates of embedded emitters [1]. We will show that novel classes of optoplasmonic amplifiers and frequency (de)multiplexers can be realized through tailored coupling of optoplasmonic elements into discrete networks [1]. We will demonstrate the capability of optoplasmonic circuits to enable adaptive light switching on nanoscale and active control over cascaded photon-emitter interactions over both short (nanometers) and long (hundreds of microns) length scales [1,2]. Finally, we will highlight the opportunities of hybrid optoplasmonic structures as biosensing platforms that combine high sensitivity of plasmonic elements with high spectral resolution of high-Q microcavities and thus can feature improved detection limits as compared to individual photonic and plasmonic sensors [3]. While this paper focuses on theoretical concepts of optoplasmonic elements and networks, recent advances in nanofabrication technologies put the fabrication of these structures within reach, and we will briefly discuss the available fabrication strategies for practical realization of hybrid resonant optoplasmonic structures developed in our group [4] and elsewhere. [1] S.V. Boriskina & B.M. Reinhard, Proc. Natl. Acad. Sci. USA 108(8), 3147-3151 (2011).[2] S.V. Boriskina & B.M. Reinhard, Opt. Express, Focus Issue Collective Phenomena in Photonic, Plasmonic and Hybrid Structures (S.V. Boriskina et al Eds.) (2011).[3] M.A. Santiago-Cordoba, S.V. Boriskina, F. Vollmer & M.C. Demirel, Appl. Phys. Lett. Aug (2011).[4] W. Ahn, S.V. Boriskina, Y. Hong & B.M. Reinhard, submitted to Symposium CC
CC6: Spectroscopic Studies in Semiconductor and Metal-Hybrids I
Session Chairs
Wednesday PM, November 30, 2011
Room 202 (Hynes)
11:30 AM - CC6.6
Zero-Phonon Optical Transitions in Si Nanocrystals.
Ilya Sychugov 1 , Jan Valenta 2 , Kazutaka Mitsuishi 3 , Minoru Fujii 4 , Jan Linnros 5
1 ICYS, NIMS, Tsukuba, Ibaraki, Japan, 2 Chemical Physics & Optics, Charles University, Prague Czechia, 3 HVEM Station, NIMS, Tsukuba, Ibaraki, Japan, 4 Electrical and Electronic Engineering, Kobe University, Kobe, Hyogo, Japan, 5 Material Physics, Royal Institute of Technology, Stockholm, Kista, Sweden
Show AbstractOptical transitions in silicon nanocrystals with different surface passivation were probed at low temperature on a single particle level. A new type of a quasidirect recombination process is identified different from the quantum-confined exciton transition. The luminescence spectra have different emission energies but the contribution of a no-phonon transition is significantly higher than expected from the quantum confinement model. Its relative strength was found to be temperature dependent suggesting spatial localization of excitons as a possible origin.
CC7/K6: Joint Session
Session Chairs
Wednesday PM, November 30, 2011
Room 207 (Hynes)
11:30 AM - CC7.7/K6.7
Epsilon near Zero Material with Ag/TiO2 Composite.
Ganapathi Subramania 1 , Arthur Fischer 1
1 , Sandia National Laboratory, Albuquerque, New Mexico, United States
Show AbstractEpsilon near zero (ENZ) materials are metamaterials whose effective dielectric permittivity (epsilon) is close to zero for a wavelength range of interest. This results in unique electromagnetic properties such as unusually large effective wavelength inside the medium even at optical frequencies, important for nanoscale optical circuits[1]. Such materials can be achieved utilizing metal-dielectric multilayer composites which provide the negative and positive components of the dielectric permittivity respectively, needed to achieve an effective near zero value. The small values of epsilon near the ENZ wavelength also makes them attractive for enhancing non-linear behavior by utilizing the non-linearity of metals. Large changes in transmission with low power inputs have been predicted for such systems[2, 3] leading to interesting optical bistability behaviors[4]. Silver (Ag) is an attractive option as it has high non-linear coefficient (3.16 × 10−16 m2/V2) while having a high plasma frequency. We will describe a metal-dielectric multilayer composite consisting of Ag and titanium dioxide (TiO2) fabricated using electron beam evaporation to achieve an ENZ behavior near 630nm wavelength . The typical thicknesses of Ag and TiO2 are kept around 15-20nm and 45-55nm respectively. High dielectric constant of TiO2 enables keeping the layer thicknesses small in order to minimize non-local effects that can adversely affect effective medium behavior. We will discuss the optical transmission and effective dispersion response of the composite structure obtained through spectroscopic and interference measurements. We will also discuss the effective non-linearity of the Ag/ TiO2 composite as well as the role of metal losses.Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.References:[1]N. Engheta, Science 2007, 317, 1698.[2]A. Husakou, J. Herrmann, Phys. Rev. Lett. 2007, 99, 127402.[3]N. N. Lepeshkin, A. Schweinsberg, G. Piredda, R. S. Bennink, R. W. Boyd, Phys. Rev. Lett. 2004, 93, 123902.[4]A. Ciattoni, C. Rizza, E. Palange, Physical Review A 2011, 83, 043813.
CC6: Spectroscopic Studies in Semiconductor and Metal-Hybrids I
Session Chairs
Wednesday PM, November 30, 2011
Room 202 (Hynes)
11:45 AM - CC6.7
Engineering Enhanced Optical Properties of near-IR Upconverting Nanoparticles.
Daniel Gargas 1 , Alexis Ostrowski 1 , Emory Chan 1 , Delia Milliron 1 , Bruce Cohen 1 , P. James Schuck 1
1 Molecular Foundry, Lawrence Berkeley Laboratory, Berkeley, California, United States
Show AbstractDue to their unique properties in converting low energy light into higher energy electronic transitions, upconverting nano-materials have garnered considerable interest in bio-imaging, photovoltaic, and opto-electronic applications. In particular, lanthanide-doped upconverting nanoparticles (UCNPs) have demonstrated a host of functionalities due to their nanoscale dimensions and wide range in transition-metal doped compounds. Unlike quantum dots and other single emitters that exhibit luminescence blinking and photo-bleaching, UCNPs undergo upconverting energy processes regardless of material size and provide high photostability in both aqueous and ambient environments.[1] In addition, their mixed electric and magnetic dipole transitions make them ideal materials for study of plasmon-enhanced properties with metal nanostructures. However, despite recent progress in the optical properties of UCNPs, a fundamental understanding of the photo-physics remains unclear, particularly at nanoscale dimensions where surface properties can dominate energy transfer processes. More specifically, it is unclear how sensitive the photo-physical properties are to their environment, especially as UCNP diameters are reduced below 10nm.Here we report on the luminescence properties of Er3+, Yb3+-doped NaYF4 UCNPs with diameters ranging from 5 – 50 nm in both core and core-shell architectures. Optical characterization of the luminescence lifetime and spectral emission from both UCNP films and single particles reveal a strong dependence on UCNP size and surface functionalization, which allows quantification of particle-environment interactions and the enhancement of surface versus bulk properties. Furthermore, by exhibiting a large shift (anti-stokes) in absorption energy versus transition energy, UCNPs offer a model platform for investigating the interaction of energy transfer across metal-semiconductor nano-interfaces, whereby the intrinsic luminescence lifetimes (from μs to ms) are probed for Purcell enhancement and spontaneous emission rate modification.[1] – S. Wu, P.J. Schuck, et al Proc. Nat. Acad. Sci. 106, 10917, 2009
CC7/K6: Joint Session
Session Chairs
Wednesday PM, November 30, 2011
Room 207 (Hynes)
11:45 AM - CC7.8/K6.8
Third-Order Nonlinear Optical Properties of Uniaxial Metal-Dielectric Composites.
Joseph Geddes 1 2 , Erik Nelson 3 , Paul Braun 1 2
1 Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States, 2 Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States, 3 Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts, United States
Show AbstractWe show how uniaxial composites comprising alternating thin planar subwavelength layers of metal and dielectric materials can exhibit third-order optical nonlinearities greater than those of the already large intrinsic nonlinearities of the metal, when the appropriate volume fraction of metal is chosen. The enhancement occurs in the direction perpendicular to the plane of the layers. Furthermore, the absorption in these materials could be limited or even less than that of the bulk metal component in some cases. We report z-scan measurements of the nonlinear refractive index of several such composites.
CC6: Spectroscopic Studies in Semiconductor and Metal-Hybrids I
Session Chairs
Wednesday PM, November 30, 2011
Room 202 (Hynes)
12:00 PM - CC6.8
Giant Absorption Enhancement in Close Packed Monolayers of Colloidal Quantum Dots through Dipolar Coupling Effects.
Pieter Geiregat 1 , Yolanda Justo 1 , Zeger Hens 1
1 , Ghent University, Gent Belgium
Show AbstractQuantum dots (QDs) are widely studied for use in solar cells, light emitting diodes, photodetectors, etc.. In such a device context, they are generally deposited in thin layers. However, colloidal QD properties (e.g. absorption cross sections, exciton lifetime, ...) are typically evaluated in solution using effective medium approaches such as the Maxwell-Garnett model. These models assume that the QDs behave as uncoupled dipoles embedded in a matrix with given permittivity. Clearly, this assumption no longer holds for a close packed film of QDs where the dipoles can couple through electromagnetic multipolar interactions. Using properties measured in solution to evaluate the performance and physics of thin film devices is therefore incorrect, or at least a serious simplification. We propose to treat the coupling of QDs in thin films using the 'coupled dipole model' (CD model). This model was developed to understand the localised plasmonic response of coupled arrays of metallic nanoparticles. Its basic idea is that the internal field of a particle in close proximity to other dipoles will be a superposition of the external field and the induced dipolar fields of the neighboring particles. Here, we apply this theory to hexagonally close packed monolayers of QDs. To compare light absorption by particles in a close packed monolayer with that of particles dispersed in a dielectric, we define an 'enhancement factor' E, being the ratio between the absorption cross section in film to that in solution. We consider PbS and CdSe QDs, both passivated with oleic acid, as model systems since these show a bulk like dielectric function at energies well above the first exciton transition. In this way, we can calculate E in close packed monolayers as a function of particle size within the CD model. The only free parameter in this procedure is the dielectric constant of the film environment which we chose to be 3.5, resembling an environment of air, glass and other nanoparticles. To access the enhancement experimentally, we measure the absorption spectrum of single, close packed Langmuir-Blodgett monolayers of QDs on glass substrates using a standard UV-VIS-NIR spectrophotometer. To correct for increased reflection, we also measure the absolute reflectance of the monolayers. The latter amounts to a correction of maximum 10% to the initially measured absorption. The enhancement predicted by theory fits the experimental data very well: giant, particle-size dependent enhancements up to 9 are predicted and measured for PbS monolayers. Smaller enhancements, of the order 2-4 are found for CdSe, which reflects the reduced dielectric screening in these materials and the concomitant decrease of the influence a dipole has on its neighbors. We conclude that dipolar coupling can induce a strong, tunable enhancement of the absorption of QDs in monolayers, which is an extremely useful collective property of QD assemblies with applications in photovoltaics and photodetection.
CC7/K6: Joint Session
Session Chairs
Wednesday PM, November 30, 2011
Room 207 (Hynes)
12:00 PM - CC7.9/K6.9
Mode Matching Analysis for Negative Refraction in a Two Dimensional Plasmonic Metamaterial.
Sandeep Inampudi 1 , Igor I Smolyaninov 2 , Viktor Podolskiy 1
1 Physics and Applied Physics, University Of Massachusetts Lowell, Lowell, Massachusetts, United States, 2 Department of Electrical and Computer Engineering, University of Maryland, College Park, Maryland, United States
Show AbstractThe phenomenon of negative refraction, well understood in homogeneous magnetic or anisotropic systems remains a mystery in planar plasmonic metamaterials formed by periodic arrays of thin PMMA ridges deposited on metal substrate. Up to date there exists at least two different theoretical explanations for this phenomenon, one based on the energy-momentum directions of the SPP at the single metal-dielectric interface, and the other based on Dyakonov waves. However, neither of these explanations accounts for the complex three-dimensional geometry of the system and the non-trivial coupling between SPPs and free-space modes associated with this geometry. Here we present a theoretical analysis of this phenomenon of negative refraction of surface plasmons in planar plasmonic metamaterials. We use mode matching technique to analyze the dynamics of the plasmonic mode and its non-trivial coupling to the free space waves and to the other guided modes of the system, taking into account the extended 3D-geometry and the finite thickness of the PMMA ridges. The eigen states of the periodic system dominated by the surface waves are identified and their dispersion is analyzed via generalization of mode-matching formalism and Bloch-periodic approach.
CC6: Spectroscopic Studies in Semiconductor and Metal-Hybrids I
Session Chairs
Wednesday PM, November 30, 2011
Room 202 (Hynes)
12:15 PM - CC6.9
The Life and Fate of Optical Excitations in Quantum-Dot Solids.
Arjan Houtepen 1 , Elise Talgorn 1 , Yunan Gao 1 2 , Michiel Aerts 1 , Sandeep Sukumaran 1 , Lucas Kunneman 1 , Juleon Schins 1 , Tom Savenije 1 , Herre Van der Zant 2 , Laurens Siebbeles 1
1 Chemical Engineering, Delft University of Technology, Delft Netherlands, 2 Kavli Institute of Nanoscience, Delft University of Technology, Delft Netherlands
Show AbstractFilms of colloidal semiconductor nanocrystals, so-called Quantum-Dot solids, are heavily investigated as candidates for photodetectors and solar cells. The operation of such devices requires efficient charge-carrier photogeneration, high charge-carrier mobilities and long charge carrier lifetimes. For photovoltaic application Quantum Dots are of particular interest since it has been demonstrated in recent years that absorption of a single high energy photon can result in the generation of multiple excitons (multiple exciton generation, MEG). If these excitons can be converted into mobile charge carriers this process will lead to an increased photocurrent and, hence, an increased power conversion efficiency in solar cells.We have prepared QD solids composed of PbSe nanocrystals with very short interparticle distances that fulfill all these requirements. The processes of (multiple) carrier photogeneration, diffusion and decay have been studied by a combination of terahertz spectroscopy, transient absorption spectroscopy and temperature-dependent time-resolved microwave conductivity. Using these techniques we demonstrate that the charge carrier mobilities are as high as 3 cm2/Vs and that all photogenerated excitons dissociate into mobile charge carriers. The exciton dissociation rate is sufficiently high to allow even multiple excitons, created efficiently via MEG, to dissociate into mobile charge carriers. As a result, multiple carriers per absorbed photon, generated efficiently via MEG, are directly available, without the aid of an electric field or interfaces that provide an additional driving force for exciton dissociation.Our results demonstrate that Quantum-Dot solids are very promising candidates for simple and cheap optoelectronic devices.
CC7/K6: Joint Session
Session Chairs
Wednesday PM, November 30, 2011
Room 207 (Hynes)
12:15 PM - CC7.10/K6.10
Mode Engineering with Organic Microcavities Using Thin Metal Layers.
Susanne Hintschich 1 , Alexander Zakhidov 1 , Robert Brueckner 1 , Markas Sudzius 1 , Hartmut Froeb 1 , Vadim Lyssenko 1 , Karl Leo 1
1 , Institut für Angewandte Photophysik, Dresden Germany
Show AbstractMicrocavities are widely used systems for studying the non-linear interactions of light and matter. Recently polariton lasing [1] and photon condensation [2] were demonstrated at room temperature in organic microcavities. Such observations are made possible by the large oscillator strengths and exciton binding energies provided by organic semiconductors. In our work, we fabricate a high quality organic microcavity (OMC), where an active λ/2 layer of the organic host:guest system Alq(3):DCM (2 wt.%) is embedded between two dielectric mirrors (DBRs). The resonance wavelength of this structure is chosen slightly detuned from the peak of the wide emission spectrum of DCM. Between the active layer and the bottom DBR, we incorporate a 40 nm silver layer. This structure is then excited off-resonantly, with a tightly focused laser beam. Via far-field emission spectroscopy, we demonstrate the formation of hybrid Tamm plasmon-polariton states in the OMC. We are further able to pattern the metal layer using a lift-off process to create elliptical defects or photo-lithography to fabricate gratings. As a consequence, the resonances discretise due to optical confinement. In the case of the grating, linear dispersion branches of are observed near the bottom of the cavity mode by angle-resolved photoluminescence. The above, discrete Tamm states are observed to couple to the cavity resonance to form a macroscopically coherent state: Above an optical excitation threshold, we experimentally observe coherent emission at room temperature, comparable to those observed by Lai et al. [3] We estimate a coherence length spanning at least four periods of the grating. This is larger than the quasimode radius in a comparable unpatterned microcavity. Using numerical simulations, we are able to reproduce the experimental data for the E(k) dispersion of the cavity emission. This procedure provides fundamental insights into the physical mechanisms of phase coupling between cavity and Tamm polaritons. Thereby, we demonstrate how an absorptive metal pattern is employed for mode engineering in an organic microcavity at room temperature. [1] Kena-Cohen, S. and Forrest, S.R.; Room-temperature polariton lasing in an organic single-crystal microcavity. Nature Photonics, 4, 371-375 (2010). [2] Klaers, J., Schmitt, J., Vewinger, F. and Weitz, M.; Bose-Einstein condensation of photons in an optical microcavity. Nature, 468, 545-548, (2010). [3] Lai, C.W. et al.; Coherent zero-state and p-state in an exciton-polariton condensate array. Nature 450, 529-U8, (2007).
CC8: Interactions in Coupled Metal and Semiconductor Nanostructures II
Session Chairs
Wednesday PM, November 30, 2011
Room 202 (Hynes)
2:30 PM - **CC8.1
Hybrid Metal-Semiconductor Nanostructures for New Frontiers of Quantum Nano-Optics.
Min Ouyang 1 2
1 Department of Physics, University of Maryland - College Park, College Park, Maryland, United States, 2 Department of Materials Science and Engineering, University of Maryland - College Park, College Park, Maryland, United States
Show AbstractNanoscience & nanotechnology offer the promise of producing revolutionary advances in areas ranging from fundamental science to emerging technological applications. In this talk, a few recent advances on the topic of chemistry and physics of new types of hybrid nanostructures will be presented. I will start from materials standpoint and show how to achieve precise control of hybrid nanostructures with desired property and functionality based on bottom-up chemical synthetic paradigm. Enabled by these significant material advances, fundamental chemical and physical properties can be finely tailored within nanostructures, thus leading to novel technology concepts and applications. Examples centering on the emerging nanoscale light-matter interactions within well-defined hybrid nanostructures will be further discussed.
CC9/K7: Joint Session
Session Chairs
Wednesday PM, November 30, 2011
Room 207 (Hynes)
2:30 PM - CC9.1/K7.1
The Use of Gold Clusters to Enhance Nonlinear Optical Absorption.
Paul Day 1 2 , Kiet Nguyen 1 3 , Ruth Pachter 1
1 Materials & Manufacturing Directorate, Air Force Research Lab, Wright-Patterson AFB, Ohio, United States, 2 , General Dynamics Information Technology, Inc., Dayton, Ohio, United States, 3 , UES, Inc., Dayton, Ohio, United States
Show AbstractGold clusters have been studied for their potential as nonlinear optical materials, and indications are that they can compete successfully as two-photon absorbing (TPA) materials with organic compounds. In particular, a TPA cross-section of over 400000 GM has been measured for the thiolated gold cluster Au25(SR)18-1. Another possibility involves combining gold clusters with organic TPA chromophores to obtain TPA materials for specific requirements. In this work, time-dependent density functional theory (TDDFT) has been used to calculate the linear absorption and TPA spectra for a compound where the Au25(SR)18-1 cluster has been coordinated to the donor amino group in the “donor-π-acceptor” compound dimethylamino nitrostilbene (DANS). Challenges for modeling the hybrid material are in finding a level of theory that can successfully model both parts of the system, for example in using an exchange-correlation functional that is also accurate in the asympototic regions, shown to be necessary in modeling chromophores with significant charge transfer such as DANS, and in including relativistic effects, imperative for gold compounds. Other proposed gold clusters and conjugated organic chromophores have been combined in various ways, as will be discussed. The calculated results can aid in the design of new TPA materials.
2:45 PM - CC9.2/K7.2
Plasmonic Nanotubes Analogous to Graphene: Dynamic Control of Three-Dimensional Plasmonic Swiss Rolls.
Jeremy Baumberg 1 , Fumin Huang 1 , Jatin Sinha 2 , Nick Gibbons 1 , Phil Bartlett 2
1 Cavendish Laboratory, University of Cambridge, Cambridge United Kingdom, 2 Department of Chemistry, University of Southampton, Southampton United Kingdom
Show AbstractThree dimensional plasmonic nanostructures and metamaterials exhibit unusual optical properties such as negative refraction, superlensing, and optical cloaking. While recent approaches for fabricating 3D plasmonic nanostructures include electron beam lithography, laser writing, or atomic layer deposition, all of these are compromised in terms of sub-wavelength feature size, scalability of manufacture and cost. Here we report a new class of 3D metamaterials which are the photonic analogue of graphene rolled up into nanotubes.Plasmonic swiss-rolls are fabricated through strain-induced self-rolling from two-dimensional metallic nanopore films. Rolls with lengths up to a several millimetres and core diameters of a few microns are produced displaying strong and striking colours. Their optical properties are readily tuneable across a broad spectral range by simply varying the hole size and thickness of the base nanopore films (an approach not available for the atomic-lattice of graphene). More interestingly they can be dynamically driven by light irradiation, rolling or unrolling with increasing or decreasing light intensity, providing a first demonstration of active tuning of 3D plasmonic nanostructures in the optical frequency range. Such 3D plasmonic metamaterials based on rolling up planar plasmonic structures offer a new general route to 3D construction,[1] and deliver a new set of tuneable plasmonic modes from the quantization around the cylinder axis.[2] The orientation of the roll-up vectors define the symmetry and properties of the plasmonic swiss rolls in an analagous fashion to the chiral indices of carbon nanotubes.The extra degree of freedom in modifying the base properties of the planar films provides a host of new functionalities for these meta-structurs. Various potential interesting applications are opened up, including nano-actuators, nano-motors, and nano-generators.References:[1] Scalable Cylindrical Metallodielectric Metamaterials, N. Gibbons, J.J. Baumberg, et al., Advanced Materials 21, 3933 (2009).[2] Fu Min Huang et al, submitted to Nature Nanotechnology (2011).
CC8: Interactions in Coupled Metal and Semiconductor Nanostructures II
Session Chairs
Wednesday PM, November 30, 2011
Room 202 (Hynes)
3:00 PM - CC8.2
Exciton-Plasmon Interaction, Chirality, and Circular Dichroism in Hybrid Nanostructures.
Alexander Govorov 1 , Zhiyuan Fan 1 , Tim Liedl 2 , Valerie Gerard 3 , Yurii Gun’ko 3
1 Department of Physics, Ohio University, Athens, Ohio, United States, 2 Fakultät für Physik, Ludwig-Maximilians-Universität, München Germany, 3 School of Chemistry, University of Dublin, Trinity College, Dublin Ireland
Show AbstractCoulomb and electromagnetic interactions between nanocrystals result in several prominent effects such as non-radiative Energy transfer, plasmon enhancement, exciton energy shifts, Fano effects [1-4], and new mechanisms of optical chirality [5-8]. In this study we focus on induced optical chirality and related circular dichroism (CD) of nanoscale assemblies [5-8]. Many molecules and biomolecules are chiral and exhibit remarkably strong optical chirality (circular dichroism) due to their amazingly uniform atomic composition in a large ensemble. It is challenging to realize artificial nanoscale systems with optical chirality since the atomic structure of artificial nanostructures may not be always controlled or even known. Nevertheless, the nanoscale systems with artificial optical chirality can be realized [5-8]. To accomplish this goal, we propose to use assemblies incorporating chiral molecules and metal nanocrystals. Two main mechanisms are responsible for the creation of new CD signals in nanocrystal superstructures. The first mechanism is the Coulomb interaction between a non-chiral plasmonic nanocrystal and a chiral molecule (e.g. a protein or DNA) [5,7,8]. The second mechanism comes from the plasmon-plasmon interaction in a nanocrystal complex with a chiral geometry. One example is a nanoparticle helix [6] that can be assembled with the help of biomolecules. Several recent experiments reported plasmon-induced and plasmon-enhanced circular dichroism effects in nanoscale assemblies incorporating both molecules and nanocrystals. To conclude, the results obtained in this study can be used to design a variety of hybrid nanostructures and nanomaterials with enhanced and tailored optical chirality in the visible spectral range. Potential applications of chiral exciton-plasmon systems include chiral sensing and new plasmonic devices. [1] A. O. Govorov, G. W. Bryant, W. Zhang, T. Skeini, J. Lee, N. A. Kotov, J. M. Slocik, R. R. Naik, Nano Letters 6, 984 (2006).[2] W. Zhang, A. O. Govorov, G. W. Bryant, Phys. Rev. Lett. 97, 146804 (2006). [3] M. Kroner, A. O. Govorov, S. Remi, B. Biedermann, S. Seidl, A. Badolato, P. M. Petroff, W. Zhang, R. Barbour, B. D. Gerardot, R. J. Warburton, K. Karrai, Nature 451, 311 (2008). [4] J. Lee, P. Hernandez, J. Lee, A. Govorov, N. Kotov, Nature Materials 6, 291 (2007).[5] A. O. Govorov, Z. Fan, P. Hernandez, J. M. Slocik, R. R. Naik, Nano Letters 10, 1374 (2010). [6] Z. Fan, A.O. Govorov, Nano Letters, 10, 2580– 2587 (2010). [7] J. M. Slocik, A. O. Govorov, R. R. Naik, Nano Lett., 11, 701 (2011). [8] V. A. Gérard, Y. K. Gun'ko, E. Defrancq, A. O. Govorov, Chem. Commun. DOI: 10.1039/C1CC11083G (2011).
CC9/K7: Joint Session
Session Chairs
Wednesday PM, November 30, 2011
Room 207 (Hynes)
3:00 PM - CC9.3/K7.3
Experimental Determination of the Confinement of a Plasmonic Nanocavity.
Kasey Russell 1 , Kitty Yeung 1 , Evelyn Hu 1
1 School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, United States
Show AbstractOptical cavities can tightly confine electromagnetic energy and couple it to optically-active material. Although in the visible and near-IR part of the spectrum this has conventionally been done in dielectric material systems, progress has recently been made in fabricating all-metal optical cavities with coupled emitters. These cavities are designed to have sufficiently small mode volume to enable cavity electrodynamical effects despite their metal-loss limited cavity quality. At such extreme levels of confinement, however, imperfections such as nm-scale roughness of the metal surfaces can lead to significant deviations from ideal behavior, making it imperative to have a method to experimentally measure the degree of optical confinement.Here we present direct measurements of the confinement of a metal-based optical cavity. Our method is based on the observation that the high field confinement of the cavity makes the cavity resonances sensitive probes of the local environment surrounding the cavity. By coating the cavity in a conformal dielectric layer of known thickness and dielectric constant, the frequency of the cavity resonances are shifted. The magnitude of this shift and the thickness of dielectric coating at which the shift saturates provide quantitative measurements of both the mode volume of the cavity (~10-5 μm3) and the exponential decay length of the evanescent fields escaping from the cavity (~10 nm). These measurements are in excellent agreement with finite-difference time-domain simulations.
CC8: Interactions in Coupled Metal and Semiconductor Nanostructures II
Session Chairs
Wednesday PM, November 30, 2011
Room 202 (Hynes)
3:15 PM - CC8.3
Plasmonic Manipulation on Photon Emission Statistics of Individual Core-Shell Nanocrystal Quantum Dots.
Young-Shin Park 1 2 , Yongfen Chen 1 2 , Yagnaseni Ghosh 2 , Ping Xu 1 , Nathan Mack 1 , Hsing-Lin Wang 1 , Jennifer Hollingsworth 2 , Victor Klimov 1 2 3 , Han Htoon 1 2
1 Chemistry Division, Los Alamos National Lab, Los Alamos, New Mexico, United States, 2 Center for Integrated Nanotechnologies, Los Alamos National Lab, Los Alamos, New Mexico, United States, 3 Center for Advanced Solar Photophysics, Los Alamos National Lab, Los Alamos, New Mexico, United States
Show AbstractPhoton emission statistics plays a key role in identifying the nature of nanoscale light sources such as quantum dots, diamond nanocrystals, dye molecules, and carbon nanotubes. Specifically, while the observation of sub-Poissonian statistics or anti-bunching behavior in the second-order photon correlation function (g(2)) provides an unambiguous proof on non-classical nature of their radiation, super-Poissonian or photon bunching from single quantum emitters signifies a strong correlation between a pair of photons emitted from two successive electronic transitions. An ability to manipulate this statistical property of quantum emitters could bring breakthroughs in their potential applications as single and/or entangled photon pair sources that are critically needed for quantum communication.Here, we present photon statistics studies of photoluminescence (PL) emission from individual core/shell CdSe/CdS nanocrystal quantum dots(NQDs) that are coupled with local plasmonic fields in the proximity of a rough silver surface. We demonstrate that the interaction with localized plasmons can modify the photon emission statistics of an individual core-shell NQD from photon antibunching to strong photon-bunching. This unique phenomenon was observed in the low excitation limit (average electron-hole pair,〈N〉<< 1) where the degree of photon-bunching is determined by the ratio between single exciton and bi-exciton quantum yields. Interestingly, we also observe that, as the pump power increases, the degree of photon-bunching (i.e. the height of zero-delay peak in g(2) measurement : g(2)(0)) is first reduced, featuring a dramatic transition from photon-bunching to incomplete photon anti-bunching (g(2)(0)<1) , and then gradually grows toward the Poissonian limit (i.e. g(2)(0) = 1) at very high pump power (〈N〉>> 1). Our detailed analysis allows us to attribute these interesting behaviors to plasmonic effects on the competition between radiative and non-radiative recombination channels, which ultimately lead to a condition where the quantum yield of the bi-exciton states becomes greater than that of the single exciton state. In addition to photon pair generation technology, this observation may have a profound impact on lasing applications, which require efficient emission of bi-exciton states. Furthermore, since this phenomenon can in principle be manifest in any quantum emitter that allows a relatively efficient radiative recombination of bi-exciton state, our work also presents plasmonic coupling as a new, novel route toward achieving manipulation on photon emission statistics of individual quantum emitters in general.
CC9/K7: Joint Session
Session Chairs
Wednesday PM, November 30, 2011
Room 207 (Hynes)
3:15 PM - CC9.4/K7.4
Angularly Independent Structural Color of Nanostructured Metal Surfaces.
Sylvanus Lee 1 2 , Alyssa Pasquale 2 , Gary Walsh 2 , Marco Romagnoli 3 , Luca Dal Negro 2
1 Mechanical Engineering, Boston University, Boston, Massachusetts, United States, 2 Electrical and Computer Engineering & Photonics Center, Boston University, Boston, Massachusetts, United States, 3 , MIT/Photonic Corp, Culver City, California, United States
Show AbstractThe color of metal surfaces is defined by the absorption and reflection properties of the materials. Although the color of metal surfaces can be drastically modified by engineering resonant phenomena in periodic nanostructures, the incorporation of aperiodic elements is needed in order to demonstrate angularly-insensitive structural colors. Unlike periodic grating arrays, it has been shown that two-dimensional (2D) deterministic aperiodic nanostructures feature broadband frequency responses with wide angular intensity distributions. In particular, the proposed plasmonic Pinwheel and hyper-uniform patterns featuring full rotational invariance in Fourier space are statistically homogeneous and isotropic structures. Therefore, designing nanostructures based on these aperiodic, though deterministic, patterns is ideally suited as a robust approach to generate structural color of artificial metal surfaces. We first experimentally measure dark-field scattering and reflection spectra of periodic and Pinwheel arrays using dark-field scattering spectroscopy and angle-resolved reflection spectroscopy under white light illumination. By measuring the peak wavelength of the dark-field scattering spectra in response to increasing objective magnification, which corresponds to an increase in the angle of light collection, we see that the scattering peaks of the pinwheel gold nanostructured metal surfaces are largely independent on the observation angle and feature a scattering peak at approximately 500 nm. Reflection spectra taken at angles spanning from 30 to 60 degrees demonstrate that periodic arrays red-shift with an increase in detection angle, while the Pinwheel patterns are insensitive to changes in the detection angle. Three-dimensional finite-difference time-domain (3D FDTD) simulations were performed to model the far-field scattering properties of single particles, periodic arrays and aperiodic Pinwheels on gold films. We calculate that gold nanoparticles can support far-field scattering in the green (500-550 nm) when they are on gold films. Simulations of periodic arrays and Pinwheel arrays with N=337 particles demonstrate that pinwheel arrays support isotropic far-field scattering properties, as well as resonances around 550 nm. Our results demonstrate for the first time that, without the incorporation of extrinsic materials and pigments, angularly independent structural color can be observed and detected in nanostructured metal surfaces by dark-field scattering spectroscopy and angle-resolved reflection spectroscopy in deterministic aperiodic Pinwheel arrays under white light illumination. Such nanostructured metal surfaces can be produced in large scale by nano-imprint technology. Hence the coloration mechanism and the insensitivity of observation angle of the proposed device can potentially advance plasmon enhanced optical filters, displays, decorative, sensing and security applications.
CC8: Interactions in Coupled Metal and Semiconductor Nanostructures II
Session Chairs
Wednesday PM, November 30, 2011
Room 202 (Hynes)
3:30 PM - CC8.4
Tailoring Hot-Excitonic Emission in Semiconductor Nanowires by Controlling Lifetimes with Nanocavity Plasmons.
Chang-Hee Cho 1 , Carlos Aspetti 1 , Michael Turk 2 , James Kikkawa 2 , Sung-Wook Nam 1 , Ritesh Agarwal 1
1 Materials Science & Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States, 2 Physics and Astronomy, University of Pennsylvania, Philadelphia, Pennsylvania, United States
Show AbstractControlling the radiative properties of light emitters with an engineered surface plasmon cavity has been of great importance for understanding the underlying physics and designing new nanoscale optoelectronic devices. In this work, we show the non-thermalized hot-excitonic emission in single CdS-SiO2-Ag core-shell plasmonic nanowires, which is absent for simple photonic CdS nanowires. By tuning the plasmonic cavity size to match the whispering gallery mode resonances, an almost complete transition from thermalized excitonic to hot-excitonic emission was achieved, which reflects exceptionally high radiative rate enhancement. Time-resolved measurements for the plasmonic nanowires showed the excited-state lifetime shortening by a factor of >103, resulting in sub-picosecond lifetimes. Numerical calculations also confirmed that the electromagnetic field enhancement by the whispering gallery plasmon nanocavity is as high as 40000 and the calculated Purcell factor reaches up to 3800, which is consistent with the measured lifetime. This observation indicates that the intrinsic emission properties of semiconductors can be engineered by means of integrating with nanocavity plasmons and is important for understanding and designing nanoscale emitters with novel properties.
CC9/K7: Joint Session
Session Chairs
Wednesday PM, November 30, 2011
Room 207 (Hynes)
CC8: Interactions in Coupled Metal and Semiconductor Nanostructures II
Session Chairs
Wednesday PM, November 30, 2011
Room 202 (Hynes)
CC9/K7: Joint Session
Session Chairs
Wednesday PM, November 30, 2011
Room 207 (Hynes)
4:00 PM - CC9.5/K7.5
High Reflectivity of Nanoporous GaN Distributed Bragg Reflector Made by Doping Dependent Electrochemical Etching.
Sang-Wan Ryu 1 , Joonmo Park 1 2 , Yoon-Han Lee 1 , Sang-Mook Kim 2 , Seung-Jae Lee 2 , Jong Hyeob Baek 2
1 Department of Physics, Chonnam National University, Gwangju Korea (the Republic of), 2 , Korea Photonics Technology Institute, Gwangju Korea (the Republic of)
Show AbstractA distributed Bragg reflector (DBR) is an essential structure for resonant-cavity light-emitting diodes and vertical-cavity surface emitting lasers. For nitride materials, it is very difficult to fabricate epitaxial DBR so novel structures such as air-gap DBR was investigated. In this work, we fabricated nanoporous GaN DBR using doping selective electrochemical etching. Ten pairs of n-GaN/undoped-GaN were grown first, then periodic circular patterns (diameter= 20 μm, separation= 40 μm) were formed by reactive ion etching. The nanoporous etching was conducted from the etched sidewall at controlled voltage. Only n-GaN was converted into nanoporous layer because of the selectivity of the etching. The nanoporous DBR showed better structural stability than the completely etched air-gap DBR. Reflection spectra were measured and compared with simulation.The fabricated nanoporous DBR was characterized by reflectance spectrum measurement. For the sample etched at 17.5 V, clear stopband was observed and the maximum reflectivity of DBR region was about 85%. The reflection spectrum matched well with the simulation and the refractive index of nanoporous GaN was estimated to be 1.8.
CC8: Interactions in Coupled Metal and Semiconductor Nanostructures II
Session Chairs
Wednesday PM, November 30, 2011
Room 202 (Hynes)
4:15 PM - **CC8.5
Quantum Plasmonics and Plexcitonics.
Peter Nordlander 1
1 Physics and Astronomy, Rice Univ, Houston, Texas, United States
Show AbstractA recently developed fully quantum mechanical approach for the description of plasmonic and excitonic nanoparticles and their interactions is presented.[1] Quantum effects can have a pronounced influence on the electric field enhancements near the nanoparticle surfaces and on the optical properties strongly coupled nanoparticles.[2] For closely spaced metallic nanoparticles, electron transfer and nonlocal screening can drastically reduce the electric field enhancements across the gap and result in a Charge Transfer Plasmon (CTP) where an oscillatory electric current is induced between the different particles.[2] The energy of the CTP is found to depend strongly on the electronic structure of the junction and the presence of molecules inside the gap.[3,4] For the coupled plasmonic-excitonic system where bonding and antibonding hybrid plexciton states are formed,[1] quantum effects can lead to a drastic enhancement of the excitation cross section of the excitons. [1] A. Manjavacas et al., Nano Lett. 11(2011)2318, N. Fofang et al., Nano Lett. 8(2008)3481[2] J. Zuloaga et al., Nano Lett. 9(2009)887, ACS Nano 4(2010)5269[3] O. Perez et al., Nano Lett. 10(2010)3090, L.Slaughter et al., ACS Nano 4(2010)4657[4] P. Song et al., J. Chem. Phys. 134(2011)074701
CC9/K7: Joint Session
Session Chairs
Wednesday PM, November 30, 2011
Room 207 (Hynes)
4:15 PM - CC9.6/K7.6
Loss Optimization in Dielectric/Metal Coated Flexible Terahertz Waveguides.
Pallavi Doradla 1 , Cecil Joseph 1 , Jayant Kumar 1 , Robert Giles 1
1 Physics and Applied Physics, University Of Massachusetts Lowell, Lowell, Massachusetts, United States
Show AbstractThe Terahertz (THz) frequency range, located midway between the microwave and infrared region, is a rapidly developing area for source and detection technologies that has shown a promise in biomedical imaging, remote sensing and security screening. For many applications like invivo medical imaging it is necessary to transport and guide THz radiation with minimal transmission loss. A flexible THz waveguide is an essential tool for these applications.Low loss, hollow, flexible waveguides have been designed and fabricated for the maximal transmission of Terahertz (THz) radiation. Polycarbonate (PC) was chosen to be the base material due to its lowest surface roughness. THz radiation can be guided by coating the inner surface of PC tubing with a high reflective metal like Silver (Ag) or Gold (Au). Attenuation characteristics of Terahertz radiation in Ag and Au coated waveguides with bore diameters 4.1mm, 3.2mm, 2 mm were studied at 215μm wavelength. Minimal Propagation loss of 2dB/m was obtained by coupling the lowest loss TE11 mode from an optically pumped terahertz laser into Ag coated waveguide. Transmission loss can be reduced substantially by creating corrugation in metal coated waveguides and by coupling linearly polarized Hybrid mode into it. Corrugation can be achieved in waveguides by the addition of a dielectric layer to the metal coating. Polystyrene (PS) was chosen to be the dielectric, due to its lowest extinction coefficient, which enhances the transmission through the waveguide. By coupling HE11 mode into PS/Ag coated waveguides a minimal propagation loss of less than 1dB/m was achieved. The results will be presented during MRS meeting.
CC8: Interactions in Coupled Metal and Semiconductor Nanostructures II
Session Chairs
Wednesday PM, November 30, 2011
Room 202 (Hynes)
4:45 PM - CC8.6
Coherent Coupling of Surface Plasmon Polaritons and Adsorbed Molecules.
Michael Stopa 1 , Alan Aspuru-Guzik 2 , Semion Saikin 2
1 Physics, Harvard University, Cambridge, Massachusetts, United States, 2 Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts, United States
Show AbstractWe theoretically consider the coherent coupling between surface plasmon polaritons (SPPs) at a metal-insulator interface with a layer of molecules adsorbed to the surface. Such coupling of molecules to SPPs is an important feature employed in various schemes for nanoscale sensing structures. In particular, we investigate the radiating properties of the molecules when they are taken to comprise the N-particle state known as a Dicke state, which is the appropriate description when all the molecules interact with equal strength with the plasmon (or with one specific plasmon mode). Such a Dicke state has an interesting radiation behavior where the maximum light emission occurs in the state where half of the molecules (treated as two-level systems) are in their excited states. This is the well-known phenomenon of Dicke superradiance. We calculate how, for non-equal coupling strengths (between the plasmons and the molecules), the initially coherent Dicke state degenerates into an incoherent superposition of single-particle states. It is important to note that this decohering process occurs without the need to posit any external coupling to a thermal heat bath or energy loss mechanism to the environment. In the process, the transition rate from excited to ground state evolves from the coherent limit to the fully incoherent limit. Finally, close to the incoherent limit we show how small coherences between neighboring groups of molecules can cause the radiative behavior to depart from the purely classical limit and we discuss cases where these departures could be experimentally observed.
5:00 PM - CC8.7
Semiconductor Heterostructures and Metal-Hybrid Nanocrystals for Photovoltaic Applications.
Arnaud Demortiere 1 , Chunxing She 1 , Matthew Pelton 1 , Richard Schaller 1 , Seth Darling 1 , Elena Shevchenko 1
1 Center for Nanoscale Materials, Argonne National Laboratory, Chicago, Illinois, United States
Show AbstractHeterostructure semiconductor nanocrystals (NCs) are promising materials for many applications such as light emitting devices and luminescent solar concentrators, due to their high photoluminescence quantum yield (PLQY) and good photo-stability. Core/shell heterostructures have been observed to exhibit PLQY up to 80% with large quasi-Stokes shifts, which minimize reabsorption of emitted light in materials containing NCs. Here, we report the elaboration of different high PLQY core/shell nanocrystals in order to explore the applicability of inorganic materials across a wide range of solar spectrum (500 to 1500 nm) for luminescent solar concentrator (LSC). In the way, we focus on CdSe/CdS, CdTe/CdSe and PbSe/PbS core/shell hetero-nanocrystals synthesized by colloidal chemical approach. We present the systematic studies of the effect of the reaction conditions on the growth kinetics, crystalline structures and optical properties in a larger range of size. STEM-HAADF, EXAFS and EELS analyses are used to characterize the structure and the interface of these semiconductor hetero-junctions. Furthermore, the evolution of PLQYs, PL lifetimes and bleaching relaxation times are studied as a function of the size of core and the thickness of the shell. Then, the homogeneity and the optical properties of slabs of NCs/polymer nanocomposites are studied to improve the efficiency of LSC devices. Finally, we showed the influence of the growth of metal nanoparticles, i.e. gold, platinum and cobalt, on their optical properties and charge-carrier mobilities.
CC10: Poster Session II
Session Chairs
Thursday AM, December 01, 2011
Exhibition Hall C (Hynes)
9:00 PM - CC10.1
Photoluminescence Enhancement in CdS Nanoparticles by Surface-Plasmon Resonance.
Dae-Ryong Jung 1 , Jongmin Kim 1 , Seunghoon Nam 1 , Changwoo Nahm 1 , Hongsik Choi 1 , Jae Ik Kim 1 , Junhee Lee 1 , Chohui Kim 1 , Byungwoo Park 1
1 WCU Hybrid Materials Program, Department of Materials Science and Engineering, Seoul National University, Seoul Korea (the Republic of)
Show AbstractTo examine the influence of metal nanoparticles on the photoluminescence of semiconductors, colloidal mixtures of CdS and Au nanoparticles were prepared with different CdS/Au fractions. Compared to the cadmium-sulfide nanocrystals (quantum efficiency = 7%), the CdS/Au mixtures showed enhanced luminescence properties (quantum efficiency = 14%). The existence of an optimum ratio of metal to semiconductor nanoparticles for the photoluminescence intensity indicates that interactions between the metal and semiconductor nanoparticles induced by surface-plasmon resonance occur constructively at appropriate distances. [1] D.-R. Jung, J. Kim, S. Nam, C. Nahm, H. Choi, J. I. Kim, J. Lee, C. Kim, and B. Park,
submitted to Appl. Phys. Lett. [2] K. Leong, Y. Chen, D. J. Masiello, M. T. Zin, M. Hnilova, H. Ma, C. Tamerler, M. Sarikaya, D. S. Ginger, and A. K.-Y. Jen,
Adv. Funct. Mater. 20, 2675 (2010). Corresponding Author: Byungwoo Park:
[email protected]9:00 PM - CC10.10
Superparafragilistic Plasmonic/Magnetic Bifunctional Nanoparticles.
Sheng Peng 1 , Changhui Lei 2 , Yang Ren 3 , Russell Cook 4 , Yugang Sun 1
1 Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois, United States, 2 Center for Microanalysis of Materials, Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States, 3 X-Ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois, United States, 4 Electron Microscopy Center, Materials Science Division, Argonne National Laboratory, Argonne, Illinois, United States
Show AbstractHybrid nanoparticles composed of multiple components usually exhibit multiple functionalities for applications that are difficult (or even impossible) to achieve from single-component nanoparticles. For example, nanoparticles consisting of both noble metals and iron oxides exhibit not only unique optical properties but also magnetic responses. Large-scale synthesis of such hybrid nanoparticles is still challenging in general.A facile amorphous-seed mediated strategy has been developed to synthesize bifunctional nanoparticles composed of silver (Ag) nanodomains and hollow iron oxide (Fe3O4) nanoshells.[1] These hybrid particles exhibit unique optical properties originating from strong surface plasmon resonance of the silver and strong superparamagnetic responses from the iron oxide. The key for this success relies on the precise formation of thin amorphous coatings on the seed nanoparticles that facilitate the formation of hybrid nanoparticles as well as maintain strong interfacial adhesion between the two components in each particle. Such multifunctional hybrid nanoparticles are expected to be very useful in surface-enhanced Raman scattering (SERS) for chemical and biological sensing, magnetic/optical dual-modal imaging, and drug delivery. [1] S. Peng, C. Lei, Y. Ren, R. E. Cook, Y. Sun, Angew. Chem. Int. Ed., 2011, 50, 3158-3163.
9:00 PM - CC10.11
The Structural Evolution and Diffusion during the Chemical Transformation from Cobalt to Cobalt Phosphide Nanoparticles.
Don-Hyung Ha 1 , Liane Moreau 1 , Clive Bealing 1 , Haitao Zhang 1 , Richard Hennig 1 , Richard Robinson 1
1 Materials Science and Engineering, Cornell University, Ithaca, New York, United States
Show AbstractNanoscale systems can display interesting and unique transformation kinetics that increase the structural complexity of the original material. Some of the most interesting nanoparticle morphologies and heterostructures in recent years have come from chemical transformations applied to nanoparticles, for example Kirkendall hollowing and partial cation exchange. Characterization of the transformation routes to form complex final structures is one of the major challenges in nanoscience. A more complete understanding of the transformation pathways would provide directions to improve synthesis techniques, leading to optimization of nanoparticles for use in applications. It also would provide insight into the control of nanoparticle chemical and physical properties. Here we report the structural evolution and the diffusion processes which occur during the phase transformation of nanoparticle ε-Co to Co2P to CoP, from a reaction with tri-n-octylphosphine (TOP). Extended X-ray absorption fine structure (EXAFS) investigations were used to elucidate the changes in the local structure of cobalt atoms which occur as the chemical transformation progresses. Results from EXAFS show both the Co2P and CoP phases contain excess Co. Results from EXAFS, transmission electron microscopy, X-ray diffraction, and density functional theory calculations reveal that the inward diffusion of phosphorus is more favorable at the beginning of the transformation from ε-Co to Co2P by forming an amorphous Co-P shell, while retaining a crystalline cobalt core. When the major phase of the sample turns to Co2P, the diffusion processes reverse and cobalt atom out-diffusion is favored, leaving a hollow void, characteristic of the nanoscale Kirkendall effect. For the transformation from Co2P to CoP, theory predicts an outward diffusion of cobalt while the anion lattice remains intact. In real samples, however, the Co-rich nanoparticles continue Kirkendall hollowing.
9:00 PM - CC10.12
Creation, Templating, and Electrical Characterization of Focused Ion Beam Produced InAs Nanospikes.
Kevin Grossklaus 1 , Jacob Jokisaari 1 , Xiaoqing Pan 1 , Joanna Millunchick 1
1 Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractWe report on a method for creating indium droplet capped InAs metal-semiconductor hybrid spike nanostructures by focused ion beam (FIB) irradiation. Under specific conditions, normal incidence FIB irradiation erodes InAs films to create high aspect ratio spikes, or “nanospikes,” with average heights of several hundred nanometers and maximum heights greater than 800 nm. The nanospike formation mechanism has been examined in situ by scanning electron microscopy (SEM) observation, and nanospike formation has been found to proceed via an ion-induced droplet masking process. Metallic In droplets form on the InAs surface due to preferential sputtering of As and shield the underlying material. As the surrounding material recedes due to FIB erosion, the nanospikes form under the persistent In droplet. By creating nanospikes using an InAs/InP heterostructure we have found that the location of the InAs/InP interface strongly influences nanospike size and location, as nanospikes will not form on areas where the InP substrate has been exposed. This effect may be exploited to template nanospikes into regular arrays by pre-patterning an InAs/InP heterostructure to locally control InAs film thickness and In droplet location. The structures of nanospikes created using both InAs and InAs/InP films have been examined by transmission electron microscopy (TEM), by scanning transmission electron microscopy (STEM), and by STEM energy dispersive spectrometry (EDS). The nanospikes have been observed to possess a metallic indium cap, a damaged outer layer, and a range of internal structures, with some spikes containing a single crystalline core and while others are heavily ion damaged and polycrystalline. The electrical properties of the different types of nanospikes produced in this study have been characterized using a combined in-situ TEM/nanoprobe technique, which allows for simultaneous TEM imaging and current-voltage measurements. The nanospikes have been found to be conductive and show non-Ohmic IV behavior. Furthermore, microstructural changes arising from the applied voltage are observed with commensurate changes in the IV characteristics. The electrical conductivity and ion disrupted structure of the nanospikes may make them useful for nanoscale thermoelectric applications.
9:00 PM - CC10.13
Development of Light-Soaking Free Nano-Crystal/Amorphous Silicon Thin Film by Neutral Beam Assisted CVD Process at Room Temperature.
Jin Nyoung Jang 1 , Dong hyeok Lee 1 , Hyun Wook So 1 , SukJae Yoo 2 , Bonju Lee 2 , MunPyo Hong 1
1 Display and Semiconductor Physics, Korea University, Chungnam Korea (the Republic of), 2 Plasma Application, National Fusion Research Institute, Deajeon, Chungnam, Korea (the Republic of)
Show AbstractIn order to apply various portable devices lightweight forms, such flexible solar panels, fabrication require low temperature processes and must be composed of high stability material. The structural state of absorption part is directly connected to the solar device performance and stability under light stress. Amorphous silicon (a-Si) exhibits the Staebler Wronski effect. A micro-crystalline Si base thin film solar cell is more stable but possesses lower absorption in comparison to an a-Si solar cell. Additionally, the micro-crystalline Si base thin film solar cell contains an incubation layer that causes high series resistance between the inter layers. In order to resolve light induced instability, nc-Si was used. Nc-Si is more stable than a-Si and exhibits higher absorption in comparison to micro-crystalline Si (μc-Si). The nano-crystalline or polymorphous-Si development for TFT and solar cells have been employed to compensate for disadvantage inherent to a-Si and μ-Si:H. We recently developed a new chemical vapor deposition (CVD) technology with a neutral particle beam (NPB) source which controls the energy of incident neutral particles in the range of 1~300eV in order to enhance the atomic activation and crystalline of thin films deposited at low temperatures. In this study, nano-crystalline Si thin films were deposited using the NBaCVD system under various process conditions.Neutral beam energy effectively increases crystal fraction (~80%) with 3~10nm grain size. In doped case, the doping efficiency also increased as reflector bias. At 330V of reflector bias, the activation energy was 0.001 eV. This means dopants are fully occupied as substitutional site even nano-size grian. Generally as grain size decrease, the dopant binding energy increases as ratio of 1 of diameter of grain size. As these results, NB energy sufficiently transports its energy to doping and crystallization.TEM image shows no incubation layer between nc-Si film of substrate and high crystallized nc-Si film. The nucleation which is occurred from first layer is growing, but it does not become micro-size grain such as micro-crystalline silicon growth. This uniform of nano-cyrstalline formation is caused by competition of nucleation and crystal growing which depend on neutral beam energy and kind. In light induced degradation (Staebler-Wronski effect) of photoconductivity, both conventional intrinsic and n-type doped amorphous silicon show general degradation of photoconductivity. A light-indued degradation of just a few % of magnitude is observed for NBaCVD processed nc-Si. Energetic hydrogen passivates and chemically anneals under the lying nc-Si grain and the amorphous region. In FTIR data, 2000, 2100 cm-1 represent the Si-H, Si-H2 stretching mode. As the reflector bias increased, the intensity of 2100 cm-1 also increased. This means the energetic hydrogen passivated the nano-grain boundary during the deposition because of its high diffusivity and chemical potential.
9:00 PM - CC10.14
Inorganic Surface Functionalization of Semiconducting Nanocrystals.
Haitao Zhang 1 , Bo Hu 1 , Richard Robinson 1
1 Materials Science and Engineering, Cornell University, Ithaca, New York, United States
Show AbstractInorganic colloidal nanocrystals (NCs) have attracted much attention in the past decade due to their unique size and shape dependent properties. Thin films of semiconductor NCs have emerged as promising new materials for electronic and optoelectronic devices. The presence of bulky hydrocarbon (C8-C18) surfactant ligands in most of colloidal NCs, however, creates highly insulating barriers which block the electronic communication between NCs, limiting the applications of assemblies of colloidal NCs. Thermal removal of surfactant ligands has proven difficult because NCs become thermally unstable at temperatures well below those required for the pyrolysis of surfactant ligands. Research into surface modification of NCs is thus mainly focused on replacing the long chain ligands with small molecules, such as amines, thiols, hydrazine, (BF4)NO, and metal chalcogenides. These surfactant ligand modifications have decreased the inter-particle distances and, in some cases, introduced conductive inorganic ligands, thus resulting in higher conductance of the NCs thin films.We report here the first use of a simple inorganic compound of (NH4)2S to remove the bulky organic surfactant ligands of semiconducting NCs. This surface modification process is unique in that 1) no inorganic surfactant ligands ((NH4)2S or (NH4)S-) are detected on the NCs after ligand removal and 2) the original organic surfactant ligands is efficiently eliminated in only a few seconds. The NCs are left terminated by bare inorganic atoms. A significant decrease on inter-particle spacing is detected by small angle X-ray diffraction. TEM studies show that the NCs are connected to each other after (NH4)2S treatment, but still maintain quantum confinement. Such "connected but confined" nanocrystal assemblies exhibit much stronger inter-particle coupling, and are promising new materials for fabricating electronic and optoelectronic devices.
9:00 PM - CC10.16
New Transparent Light-Emitting Diode Using by Indium Phosphide Colloidal Quantum Dots.
Jiwan Kim 1 , Yohan Kim 1 2 , Se Min Kim 1 2 , Min Suk Oh 1 , Tonino Greco 3 , Christian Ippen 3 , Armin Wedel 3 , Jungwon Kang 2 , Chul Jong Han 1
1 Flexible Display Research Center, Korea Electronics Technology Institute, Seongnam, Gyeonggi-do, Korea (the Republic of), 2 , Dankook University, Yongin, Gyeongg-do, Korea (the Republic of), 3 Functional Polymer System, Fraunhofer Institute for Applied Polymer Research, Potsdam Germany
Show AbstractRecently, quantum dots (QDs) have received wide attention due to their unique optical and electrical properties for various applications, ranging from displays to biomarkers. Especially, a size-tunable photoluminescence with a narrow bandwidth as well as high quantum yield make QDs as a promising material for future display applications. Since the first report on colloidal QDs based light-emitting diodes (QD-LEDs) in 1994, new colloidal QDs materials and synthesis processes have been developed by chemists to increase their stability and optical properties, and various device structures and efficient transport materials for QD-LEDs have been approached by engineers for maximizing electroluminescence efficiency. While the performance of QD-LEDs has been upgraded dramatically by leading groups, a future task is the substitution of Cd-based QDs by less toxic materials. In this report, we show unique transparent Cd-free QD-LEDs using InP colloidal quantum dots for the first time. Therefore, a multi-shell and one-pot approach has been chosen to realize stable, highly luminescent InP-QDs, with a luminescence FWHM below 50 nm. Based on the developed InP quantum dots, the device structure with proper hall/electron transport materials was optimized to maximize the electroluminescence for the display application. Different electrode materials were also tested for better efficiency and possible future other applications. The developed transparent InP QD-LEDs show applicable luminescence values (> 500 cd/m2) and transparencies at 550 nm (> 50%). These findings suggest that Cd-free QD-LEDs will be very promising for use in the future display applications. The developed technology for optimized device structures and transport materials can be also applicable in various fields.
9:00 PM - CC10.17
Synthesis of Cadmium Arsenide Quantum Dots.
Daniel Harris 1 , Moungi Bawendi 2
1 Department of Materials Science, MIT, Cambridge, Massachusetts, United States, 2 Chemistry, MIT, Cambridge, Massachusetts, United States
Show AbstractA method for the synthesis of Cd3As2 colloidal quantum dotsluminescent from 530 to 2000 nm is presented. Previous reports on quantum dots emitting in the infrared are primarily limited to the lead chalcogenides and indium arsenide and rely on discrete precursor injection(s) to control size. In contrast, we discuss a continuous injection method that allows precise tuning of the emission wavelength while simultaneously improving size distribution. This work expands the availability of high quality infrared emitters. Furthermore, the unusual inverted band structure of bulk cadmium arsenide makes high quality QDs made from this material of interest for fundamental study.
9:00 PM - CC10.18
Novel Fabrication Method for High Mobility Poly-Si Thin-Film Transistor with Oriented Crystalline.
Min-Sun Kim 1 , Hyun Min Cho 1 , Sung-Jei Hong 1 , Seung Ki Joo 2
1 1)Display components and materials research center, Korea Electronics Technology Institute, Seungnam, Gyunggi Korea (the Republic of), 2 2)School of Materials Science and Engineering, College of Engineering, Seoul National University, Seoul Korea (the Republic of)
Show AbstractIn these days, flat panel display devices are needed high quality moving image. So, the performance of display panel should be advanced more and more - such as brightness, resolution, image transport speed, etc. In fact, high performance of display panel are very much dependent on the carrier mobility of thin-films transistors (TFTs), many workers have focused on characteristics of TFTs. Especially, 3D display panels and large size display panels (over 50 inch) are needed faster response speed than conventional display panels because human-eyes fatigue. Therefore, much attention has focused on crystallization techniques and low temperature polycrystalline silicon (LTPS) TFTs, because of the several hundred times higher carrier mobility than that of amorphous Si (a-Si) TFTs and promising applications of LTPS TFTs in 3D flat panel displays. As a crystallization technique, excimer laser scanning (ELS), and metal-induced lateral crystallization (MILC) are representative methods. However, these crystallization techniques have some problems. ELS is the sole technology for LTPS crystallization method, it suffers from two major problems such as non-uniform crystal quality and high production cost. On the other hand, MILC has many advantages such as low temperature processes, smoother surface, higher degree of crystal uniformity, and etc. However, the carrier mobility of MILC poly-Si is lower than that of excimer laser crystallization. Generally, the MILC poly-Si grains have many different orientations, so it has many grain boundary defects. The presence of poly-Si grain boundary defects in the channel region of TFTs affects the electrical performance. Therefore, control of the orientation of Si crystal grains enables us to control the alignment of grain boundaries thus electrical performance can be improved because of reducing randomly oriented grain boundaries and grain boundary defects from device area. Previous research on the orientation of MILC poly-Si has been reported.In this work, a series of TFTs having crystal filtered (CF) poly-Si grains (here-in-after CF-TFTs) with various crystal filter widths were fabricated to study the relationship between the crystal filtering and the field-effect mobility. A conventional MILC-TFT (channel regions are crystallized by MILC, so TFTs have randomly oriented poly-Si grains; here-in-after MILC-TFTs) was also fabricated for comparison. In addition, we investigated the effect of catalytic metal for MILC in CF-TFT. This TFT structure and materials are able to realize novel TFT and Display device performance.
9:00 PM - CC10.19
Template-Assisted Electrochemical Synthesis of ZnTe Semiconductor Nanowires and Their Electrical Properties.
Dong-uk Kim 1 , Ki-moon Park 2 , Bongyoung Yoo 1 2
1 Department of Bionanotechnology, Hanyang University, Ansan Korea (the Republic of), 2 Department of Materials Engineering, Hanyang University, Ansan Korea (the Republic of)
Show AbstractRecently, one-dimensional(1-D) semiconductor nanostructures have received much attention because of their unique physical and chemical properties that could be exploited in nanoscale devices. Many attempts have been made to synthesis 1-D semiconductor nanostructure using various techniques, such as vapor-liquid-solid(VLS) growth, chemical vapor deposition(CVD), solvothermal method and template-assisted electrochemical deposition. Among these methods, the template-based technique is one of most effective methods for the synthesis of 1-D nanosturcrues, because it is a simple, cost-effective process and the size of nanostructures can be precisely controlled. ZnTe is one of important II-VI group semiconductor materials with a direct optical band gap of 2.26 eV at room temperature, offering potential applications in optoelectronic and energy conversion devices. The intrinsic ZnTe usually exhibits p-type conductivity, while most II-VI group semiconductor materials, such as ZnSe, ZnS and ZnO, show n-type conductivity. This property of ZnTe can be allowed to form p-n heteojunctions with other n-type semiconductors. In this study, high-ordered semiconductor ZnTe nanowires were prepared in anodic aluminium oxide(AAO) template by using electrochemical deposition. Morphological, compositional and structural properties of ZnTe nanowires were investigated by varying electrodeposition parameters. Electrical conductivity and transfer properties were also observed by connecting a single ZnTe nanowire in field-effect transistor(FET) configuration.
9:00 PM - CC10.2
Shape Mediated Selectivity of Metal Deposition on Anisotropic Semiconductor Nanostructures.
Nimai Mishra 1 , Yinthai Chan 1
1 Chemistry, National University of Singapore (NUS), Singapore Singapore
Show AbstractThe topological selectivity of metal deposition onto an anisotropic semiconductor nanostructure is pivotal to its applications in directed assembly, but systematic studies on the deposition process as a function of particle shape have been relatively limited. Herein we describe a systematic, surfactant-driven method to synthesize monodisperse CdSe seeded CdS nanoheterostructures of 4 representative geometries: (i) cylinder- and cone-like nanorods and (ii) tetrapods with cylinder- and cone-like arms in order to examine the effects of facet distribution on the location-specific deposition of Au under ambient conditions. While preferential growth at the vertices of the different semiconductor structures was expected due to an overwhelming presence of dangling bonds, we found that Au growth at the high index side facets of the cone-like structures was significantly suppressed. Further investigation revealed this to be due to a highly efficient electrochemical Ostwald ripening process in which Au clusters below a critical size dissolved and re-deposited onto the largest Au particle on the semiconductor structure. We exploited this phenomenon to fabricate hierarchically complex uniform CdSe seeded CdS tetrapods with cone-like arms, where a solitary large Au tip is found on one of the arms while the other three arms bear Ag2S tips.
9:00 PM - CC10.20
Electrolyte Gated Nanowire Field-Effect Transistors.
Irina Lokteva 1 , Saghar Khodabakhsh 2 , Stefan Thiemann 1 , Florentina Niebelschuetz 1 , Jana Zaumseil 1
1 Institute of Polymer Materials, Department of Materials Science and Engineering, University of Erlangen-Nuremberg, Erlangen Germany, 2 Cavendish Laboratory, University of Cambridge, Cambridge United Kingdom
Show AbstractOne-dimensional inorganic nanostructures have emerged as promising materials for the fabrication of prototypical optoelectronic devices. Semiconducting chalcogenide nanowires can be grown in solution from metal catalyst particles via a solution-liquid-solid (SLS) mechanism [1,2]. For that metal nanoparticles are introduced into a precursor solution resulting in nanowire growth. However, subsequent deposition of these nanowires onto electrodes while ensuring good electrical contact e.g. for field-effect transistors (FETs) is challenging. In this work, we present a method to grow n-type CdSe nanowires from bismuth catalyst nanoparticles that are formed directly on the source-drain electrodes via thin film break-up at high temperatures. The metal and catalyst layer are patterned by photolithography in one step. Thus nanowires grow only from the electrodes and form good electrical contacts via the bismuth particles, which improves overall device characteristics. Parameters such as thickness of the evaporated bismuth layer, preparation conditions of the Cd and Se precursors, injection and growth temperature, etc. were found to influence the final length and diameter of the CdSe nanowires. Additionally, the choice of the source-drain metal is crucial to obtain nanowire growth avoiding bismuth alloys at high temperatures. Nanowires produced by SLS with a diameter of about 10 nm reach lengths over 6 µm thus being able to bridge moderate FET channels. We used electrolyte gating with ionic liquids to study the charge transport properties of these CdSe nanowires after hydrazine treatment and found reasonably high electron mobilities (0.08 cm2V-1s-1) at gate voltages below 2 V. We further extended this technique to produce high mobility ambipolar PbSe nanowire FETs.[1] L. Ouyang, K. N. Maher, C. L. Yu, J. McCarty, and H. Park, J. Am. Chem. Soc. 2007, 129, 133-138[2] K. L. Hull, J. W. Grebinski, T. H. Kosel, and M. Kuno, Chem. Mater. 2005, 17, 4416-4425
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Atomistic Modeling on Grain Boundaries in Multicrystalline Silicon for Photovoltaic Cell.
Hiroshi Mizuseki 1 , Ambigapathy Suvitha 1 , Ryoji Sahara 1 , Yoshiyuki Kawazoe 1
1 , Institute for Materials Research, Tohoku Univ., Sendai, Miyagi, Japan
Show AbstractGrain boundaries (GB) form an important component of the microstructure of a multicrystalline material. Optimization of the grain-boundary structures of multicrystalline Si is a key issue to achieving high energy conversion efficiency, because these regions act as recombination centers for carriers in solar cell materials. Multicrystalline Si with artificially-controlled grain orientations has been proposed as a means of reducing the number of electrically-active grain boundaries that lead to undesirable carrier recombination[1,2]. In the present study, we applied the spherical model [2] as proposed by Lee and Choi. To make a spherical model including a grain boundary under a given misorientation, the right cluster is rotated about the [110] or the [112] orientation and the left cluster is rotated about the [110] or the [112] orientation. The [110] and [112] axes correspond to the preferred growth directions. These geometries correspond to a grain boundary in multi-crystalline silicon under dendrite growth conditions. In this case, the prepared grain boundary is located at the center of the spherical model. To perform structural relaxation, we apply a Monte Carlo (MC) method based on the Tersoff potential [4] for the silicon system. We then evaluate the potential energy of the spherical model including all 1000 Si atoms, allowing for relaxation at the surface and at the grain boundary. Moreover, we have carried out a density functional theory study on the Sigma 3(111) silicon grain boundary, and calculated the impurity effect of Ni, Fe, Cu and Cr atoms doping them near the grain boundary both at interstitial and substitutional sites. The segregation energy for the impurities follows the order Fe greater than Cu, Ni and Cr at the substitutional site and Cr greater than Cu, Fe and Ni, at the interstitial site. The calculated values were in positive, indicating segregation is not favored in the Sigma 3(111) grain boundary. When the metal impurity is placed at the substitutional site, new state in fundamental gap was observed in the density of states which leads to reduction of band gap that may have implication in the solar cell performance [6]. This work was partially supported by New Energy and Industrial Technology Development Organization (NEDO) of Japan.[1] N. Usami et al., Jpn. J. Appl. Phys., 45, 1734 (2006).[2] I. Takahashi et al., J. Appl. Phys., 109, 033504 (2011).[3] B.-J. Lee and S.-H. Choi, Modelling Simul. Mater. Sci. Eng., 12, 621 (2004).[4] J. Tersoff, Phys. Rev. B, 39, 5566 (1989).[5] H. Ogawa, Mater. Trans., 47, 2706 (2006).[6] A. Suvitha et al., Jpn. J. Appl. Phys., 49, 04DP02 (2010).
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Synthesis of the Highly Monodisperse Blue Emitting InP/ZnS Quantum Dots.
Kipil Lim 1 , Ho Seong Jang 1 , Kyoungja Woo 1
1 Nano-Materials Research Center, KIST, Seoul Korea (the Republic of)
Show AbstractSemiconductor nanoparticles, quantum dots (QDs), have been studied in the past decade due to their unique opto-electrical properties, the control of the band-gap energy according to their size. Recently the semiconductor materials with non-toxic elements attract many researchers for the application to the industry, such as light emitting diodes, bio imaging and photovoltaic devices. InP semiconductor QDs are one of non-toxic materials and attract many researcher due to its high luminescence and appropriate bandgap for visible light emission.In this study, we present the synthesis method of highly monodisperse blue emitting InP core /ZnS shell QDs, which was hard to achieve by conventional synthesis methods, and investigation of its properties. Smaller QDs are necessary in order to acquire InP QDs with higher bandgap. We dropped the temperature dramatically after mixing of the In and P precursor, and attained small InP QDs, whose band edge absorption is 425 nm and around 2.5 nm in diameter. Luminescent intensity and stability can be increased if InP cores are encapsulated by ZnS shell material. It is important that the size of InP cores remain unchanged during formation of the ZnS shell. By reducing shell synthesis temperature, we could successfully attain blue emitting InP core /ZnS shell QDs, whose band edge absorption was 425 nm and band edge emission was 475 nm. Also the size distribution of InP core /ZnS shell QDs were quite narrow that the full width at half maximum was below 40 nm.In summary, blue emitting InP core /ZnS shell QDs with band edge emission of 475 nm were synthesized. The luminescence, crystalline and element analysis were investigated in this study.
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Porous Silicon Nanocrystals in a Silica Aerogel Matrix.
Jamaree Amonkosolpan 1 , Daniel Wolverson 1 , Bernhard Goller 1 , Dmitry Kovalev 1 , Matthew Rollings 1 , Michael Grogan 2 , Tim Birks 1
1 Physics, University of Bath, Bath United Kingdom, 2 Electrical & Computer Engineering, Boston University, Boston, Massachusetts, United States
Show AbstractThe past decade has seen the rapid development of silicon (Si) nanostructures. Due to quantum confined effects, nano-Si can emit tunable photoluminescence (PL) over a wide range. The long radiative lifetime of excitons makes nano-Si able to store the energy of a photo-generated electronic excitation and transfer energy to oxygen molecules (O2), a process normally forbidden by spin selection rules in direct optical excitation. Silica aerogel is a nano-porous silica glass. It has large pores filled with air (over 90% of the total volume) and is structured on a scale of tens of nm. The aerogel has many useful physical properties; for instance, a low refractive index (≈ 1.05), low density, low thermal conductivity and high optical transparency. The incorporation of nanoparticles to form a doped aerogel introduces potential applications for sensing or for nonlinear photonic devices. We have made Si-doped aerogels by sol-gel chemistry. We start with a “wet gel” stage to obtain a desired gel structure, followed by a low temperature supercritical CO2 drying process. Different sizes of porous Si nanocrystals with a solid Si core and a porous Si shell having either oxide or hydride-terminated surfaces are introduced into the aqueous colloid to achieve Si-doped aerogels of varying concentrations. Photoluminescence (PL) was used to compare the undoped and Si-doped aerogels and so to investigate the existence of porous Si nanoparticles inside the aerogels. Even undoped aerogels show some PL, but introducing Si nanoparticles gives significant changes in the PL spectra in the energy region that corresponds to the PL band of the nanoparticles before incorporation. Raman spectra were also obtained from aerogels with different types of Si particles. The undoped aerogel shows a strong Raman peak around 620 cm-1 and small peaks near 483 and 850 cm-1. When the concentration of Si nanoparticles is increased, the intense 620 cm-1 peak is replaced by a 506-520 cm-1 peak, determined by the size and oxidation state of the Si nanoparticles. The intensity ratio of the nanoparticle and aerogel Raman peaks gives a measure of the nanoparticle concentration and furthermore the nanoparticle peak’s position indicates the structure of the nanoparticles after incorporation. The results have shown obviously that the sample, which contains the hydride surface, remains only the Si core part evident from the 520 cm-1 peak.It appears that the preparation process of the aerogel leads to the complete oxidation of porous silicon nanoparticles having a hydride-terminated surface; we were therefore unable to detect the energy transfer of between them and O2. However, by “refreshing” oxide-terminated porous Si aerogels in HF vapour, we aim to recover the energy transfer process.
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Au Rod-Titania Nanocomposite as the Efficient Photocatalytic Material.
Joohyun Lim 1 , Jin-Kyu Lee 1
1 Chemistry, Seoul National University, Seoul Korea (the Republic of)
Show AbstractTitania is not only used in a wide range of fields as white pigment or sunscreen but also have attracted a lot of attention for their possible application in various fields such as gas sensing, electronic device, solar cell, and photocatalyst. Among others, great photocatalytic property of titania has been actively researched for their advantages to remove organic pollutants or split water following the craze on green chemistry. But the partial use of sunlight limits the efficiency of titania. Au rod-titania nanocomposite was prepared to use visible-near IR region of sunlight. The composite was obtained from seed mediate growth of Au rod and mild sol-gel method of titania. Tuning the size of titania and the absorption region of Au rod was possible and characterized by SEM, TEM and UV-Vis. The formation of composite could be clearly monitored by STEM and EDX. The crystallinity of titania and composite was confirmed by XRD. Photcatalytic property of nanocomposite was tested by the degradation of organic dye, and photooxidation/reduction of simple organic compound. The composite showed the photocatalytic property under visible light as well as UV light. We monitored the charge separation between titania and Au rod by transient absorption spectroscopy under both visible and IR region. It would be the surface plasmon resonance of Au rod that made the composite has photocatalytic property even under visible light. The size effect of titania on the property of composite is under researching.
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Evaluating the Influence of Different Reaction Parameters on CdSe-CNTs Composites Interaction.
Cristina Palencia 1 , Leonor de la Cueva 1 , Fabiola Iacono 2 , Beatriz H. Juarez 1
1 , IMDEA Nanoscience Instituto Madrileño de Estudios Avanzados en Nanociencia, Madrid Spain, 2 Departamento de Física de la Materia Condensada, Universidad Autónoma de Madrid, Madrid Spain
Show AbstractSurrounding organic ligands covering semiconductor nanoparticles play a critical role controlling the size, shape, optical properties, and further interactions. The focus of our research is the interaction between semiconductor CdSe nanoparticles and carbon nanotubes surfaces or, in general, carbon sp2 obtained by the so-called hot injection method. The ligand environment has been analyzed by means of NMR (both 1H, and {1H} 31P), absorbance and FTIR-ATR (Attenuated Total Reflectance) spectroscopy. The results have been quantitatively compared with those obtained by X-Ray- photoelectron spectroscopy on CdSe nanoparticles attached to HOPG (Highly Oriented Pyrolytic Graphite) surfaces, showing a good agreement. The work shows the influence of chlorine mediating the interaction, and emphasizes the importance of the ligand characterization to control interactions at the nanoscale.
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Photoconductivity Characteristics of Silicon Quantum Dots Embedded in Silicon-Rich Silicon Nitride Films Prepared by Cat-CVD.
Kyoung-Min Lee 1 , Sin-Young Kang 2 , Ki-Su Keum 2 , Kil-Sun No 2 , Jung-Hoon Park 2 , Wan-Shick Hong 1 2
1 Nano Engineering, University of Seoul, Seoul Korea (the Republic of), 2 Nano Science and technology, University of Seoul, Seoul Korea (the Republic of)
Show AbstractSilicon rich silicon nitride (SRSN) films containing silicon quantum dots were deposited by catalytic chemical vapor deposition (Cat-CVD). The size and density of these silicon quantum dots inside the SRSN films could be easily controlled by mixing ratios of the source gas [1, 2]. In this work, we fabricated simple metal-insulator-semiconductor (MIS) and p-i-n structures with these SRSN films to measure electrical properties of these SRSN films. Photoconductivity characteristics were observed in current-voltage (I-V) curves of the silicon quantum dots embedded in the SRSN films. The photo-to-dark current ratios in the I-V characteristics changed by the size and density of the silicon quantum dots inside the SRSN films. Also, we studied influence of electrical properties of matrix SRSN films without silicon quantum dots on photoconductivity characteristics of silicon quantum dots embedded in the SRSN films. We attempted to control height and width of potential barriers for improvement of photoconductivity. The photo-to-dark current ratios could be increased by changing composition and thickness of the SRSN films without silicon quantum dots.[1] Kyoung-Min Lee, Tae-Hwan Kim and Wan-Shick Hong, Scripta Materialia, 59, (2008) pp. 1190-1192 [2] Kyoung-Min Lee, Tae-Hwan Kim and Wan-Shick Hong et al. Scripta Materialia, 60, (2009) pp. 703-705
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Plasmon-Exciton Interactions on Single Thermo-Responsive Platforms Demonstrated by Optical Tweezers.
Silvia Hormeno 1 2 , Neus Bastus 3 4 , Andreas Salcher 4 , Horst Weller 4 , Ricardo Arias-Gonzalez 1 2 , Beatriz Hernandez 1
1 , IMDEA Nanoscience, Madrid Spain, 2 Macromolecular Structure, Centro Nacional de Biotecnologia (CSIC), Madrid Spain, 3 , Institut Català de Nanotecnologia (ICN), Barcelona Spain, 4 , Institute of Physical Chemistry and Center for Applied Nanotechnology, Hamburg Germany
Show AbstractOptical tweezers have recently shown to be suitable to study nanoparticles for biophysical and biomedical purposes (1,2). We have studied the system composed by semiconductor fluorescent nanoparticles (quantum dots, QDs) and Au nanoparticles immobilized on individual thermo-responsive microspheres by optical trapping. In particular, we have characterized the optical and size behaviour of single bare pNIPAM (poly-(N-isopropylacrylamide)) microspheres, and microspheres covered either with Au nanoparticles, CdSe/CdS QDs or a combination of both. The optical design used is a counter-propagating dual-beam laser trap which operates as a force sensor independently of the size, shape or refractive index of the trapped object (3). In addition, we have integrated an automated hydrodynamic flow system that provides a precise method to inject low volumes of the microsphere suspension in the proximity of the optical trap and at a controlled rate. Au nanoparticles and QDs produced in organic media were ligand exchanged with amine-PEO ligands to make them biocompatible (4). The optical trap causes a photo-thermal heating effect of water in the focal region that triggers pNIPAM hydrogel volume phase transition. This effect is enhanced in the presence of Au nanoparticles, which act as an additional heat source. Final size is modulated by the nature and concentration of the nanoparticles on the microspheres.The IR continuous wave lasers used for trapping also excite QDs through a two-photon absorption mechanism, without need of any other excitation source (1). We have detected fluorescence emission by using the CCD camera and videomicroscopy. Regarding the optical properties, while we registered constant emission (minutes range) from the pNIPAM/QDs beads, emission and subsequent quenching due to energy transfer from the QDs to the metallic nanoparticles take place for pNIPAM/Au/QDs microspheres held in the optical trap. This effect is the consequence of plasmon-exciton interactions as QDs and Au nanoparticles get closer upon pNIPAM shrinkage.These systems combine the potential ability for drug delivery (due to the properties of pNIPAM systems), local thermal processes (due to the Au nanoparticles) and labelling (due to the QDs), representing very interesting platforms for the design of thermal sensors in biological studies. In addition, the use of thermo-responsive elements may also be an effective way to calibrate the temperature in optical trapping.1.Jauffred, L.; Oddershede, L. B. Nano Lett 2010, 10, (5), 1927-30.2.Bendix, P. M.; Reihani, S. N.; Oddershede, L. B. ACS Nano 2010, 4, (4), 2256-62.3.Smith, S. B.; Cui, Y.; Bustamante, C. Methods Enzymol 2003, 361, 134-62.4.Salcher, A.; Nikolic, M. S.; Casado, S.; Velez, M.; Weller, H.; Juarez, B. H. Journal of Materials Chemistry 2010 20, (7), 1367-1374.
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Capping-Free Ultrafine Au Nanoparticles on ZnO Exhibiting High Catalytic Activity for CO Oxidation at Room Temperature.
Paromita Kundu 1 , Nisha Singhania 1 , Giridhar Madras 2 , N. Ravishankar 1
1 Materials Research Centre, Indian Institute of Science, Bangalore India, 2 Department of Chemical Engineering, Indian Institute of Science, Bangalore India
Show AbstractSemiconductor-based metal hybrids of nanoscale dimension have emerged as versatile material for nanoelectronics, nanobiotechnology, sensor applications, catalysis ranging from CO oxidation to glucose oxidation, photodetectors, photovoltaics and photocatalysis. The performance of the hybrid in all the above cases depends significantly on the metal-semiconductor interface, metal particle dispersion on the semiconductor, size and shape of both the counterparts. Besides, for biological applications, dispersibility in aqueous phase, clean surface and low cytotoxicity of the material is highly desired. The common strategy of tethering metal nanoparticles to semiconductor base involves use of molecular linkers and surfactants to control the morphology of the hybrid. Hence the rigorous post treatment reduces the yield and utility of the hybrid and linkers prevent obtaining a clean interface that alters the efficiency of the material to a large extent. We demonstrate an ultrafast microwave assisted route for ZnO based metal hybrids synthesis in aqueous medium with precise control over the distribution and size of the metal nanoparticles in absence of any surfactant/capping agent. The resulting hybrid shows a uniform distribution of sub 2 nm Au nanoparticles on ZnO nanorods/nanowires. The loading of the metal nanoparticles can be tuned by changing the precursor concentration and reaction temperature. Direct heterogeneous nucleation of the metal ensures a clean interface which is expected to enhance the electron transport kinetics at the junction. Such electronic interaction at the semiconductor-metal interface can enhance the optoelectronic behavior, catalytic and sensing properties of the hybrid. We investigated the role of the ZnO-Au catalyst for CO oxidation and it shows excellent activity at room temperature. The effect of different morphology of ZnO support is also studied to understand the contribution of different facets towards the catalysis. The microstructural and morphological charaterization of the nanohybrid is carried out using advanced microscopic techniques like HAADF-STEM and HRTEM. XPS analysis is done to understand the role of the catalyst and its chemical composition. The catalyst is found to be extremely stable to higher temperatures as well, which indicates a strong semiconductor-metal interaction. The method of synthesis is general and can be extended for designing other noble metal hybrids based on semiconductors.
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Layer-by-Layer Assembly for Highly Efficient Quantum-Dot-Sensitized Solar Cells.
Sukyung Choi 1 , Ho Jin 1 , Sungjee Kim 1
1 Chemistry, Pohang University of Science and Technology, Pohang Korea (the Republic of)
Show AbstractQuantum dots (QDs) have received great interest for the application of alternative light harvesters in dye-sensitized solar cells since they have advantages over conventional organic chromophores such as high extinction coefficients, broad absorption ranges and tunability of the bandgap. For the QD-sensitized solar cells, a wide bandgap nanostructured electrode (TiO2 or ZnO) is desired to be well covered by QDs. It has been demonstrated that highly surface charged QDs of opposite signs can be alternatively deposited onto TiO2 as a dense and robust fashion. A multilayer of CdSe QDs was prepared onto the mesoporous surface of nanoparticulated-TiO2 film by a layer-by-layer (LBL) assembly using the electrostatic interaction between oppositely-charged QD surfaces. To maximize the absorption of incident light and the generation of excitons by CdSe QDs present within the fixed geometry of TiO2 film, experimental conditions of the QD deposition were optimized by controlling the ionic strength of the medium. A proper ionic strength was found to be very critical in providing a deep penetration of QDs into the mesopores, thus leading to a dense and uniform distribution throughout the whole TiO2 matrix. To enhance the photovoltaic properties of the QD-sensitized solar cells, a series of post-treatments have been performed with (1) CdCl2, (2) thermal annealing and (3) ZnS-coating, which significantly improved the effective photovoltaic properties. It is proposed that metal chalcogenide complexes (MCCs) can behave as a highly conductive continuous channels between QDs. A new series of quantum dot (QD) surface ligands are synthesized containing MCCs and highly charged functional groups. They can provide MCC-based QD surfaces decorated by negatively charged or positively charged functional groups. MCCs are designed to thermally decompose upon heat treatment and can be converted into metal chalcogenides, which is expected to result in the decrease of interparticle spacing and the enhancement of electronic coupling between QDs. The photovoltaic properties of the solar cells fabricated by the MCC-based QDs will be discussed.
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The Interaction between CdSe Nanoparticles and sp2 Carbon Surfaces Studied by Scanning Tunneling Microscopy and X–Ray Photoelectron Spectroscopy.
Fabiola Iacono 1 , Leonor de la Cueva 2 , Cristina Palencia 2 , Jonathan Rodriguez-Fernandez 1 , Jose Maria Gallego 1 , Beatriz H. Juarez 2 , Roberto Otero 1 2
1 , Universidad Autonoma de Madrid, Madrid Spain, 2 , imdea nanoscience, Madrid Spain
Show AbstractRecent experiments on the synthesis and photoconductive properties of hybrid systems consisting of carbon nanotubes (CNTs) and CdSe nanoparticles1 have raised the question of the interactions between such pyramidal NP’s and sp2 carbon surfaces. To learn about such interaction we have used Scanning Tunnelling Microscopy (STM) and X–Ray Photoemission Spectroscopy (XPS) to investigate the adsorption of CdSe nanoparticles on HOPG (Highly Oriented Pyrolytic Graphite) substrates. From STM experiments we observe that nanoparticles are self-organized in compact islands, and the images suggest preferential directions of growth on the HOPG surface. From XPS experiments we obtained information about the chemical composition, in particular about the organic ligands surrounding the NPs. We obtained the expected signal of phosphorous (coming from the octadecylphosphonic acid used as ligand) and, in addition, a signal of chlorine. The presence of this element could arise only from the injection of a small volume of 1,2-dichloroethane during the synthesis process. The results of our experiments show a direct correlation between the quantity of chlorine incorporated to the NPs and sticking coefficient of NPs on HOPG. This observation supports and quantifies the fact that, for the hybrid CNT–NP system, the presence of chlorine–containing solvents during the synthesis improves the quality of the hybrid system2. Further analysis of the XPS peaks points towards an incorporation of chlorine in the organic shell around the NPs that might mediate the interaction with the highly-polarizable sp2 carbon surface.1: B.H. Juárez, C.Klinke, A. Kornowski, , H. Weller, Nanoletters 2007, 7(12), 3564_35682: B.H. Juárez, Michaela Meyns, Alina Chanaewa, Yuxue Cai, Christina Klinke, Horst Weller, JACS 2008, 130, 15282_15284.
9:00 PM - CC10.31
Organic-Inorganic Hybrids for Optoelectronic Device.
Krisztina Szendrei 1 , Dorota Jarzab 1 , Wolfgang Heiss 2 , Maria Antonietta Loi 1
1 , University of Groningen, Groningen Netherlands, 2 , University of Linz, Linz Austria
Show AbstractColloidal inorganic nanocrystals have recently received increasing attention due to their unique photophysical properties resulting in promising applications in biological labeling, light-emitting diodes and photovoltaic devices. For most of these applications the key issue is to be able to control the surface chemistry of the NCs, since a ligand shell is needed to prevent aggregation. NCs are generally stabilised with electrical insulating ligands which prevent their application in efficient optoelectronic devices. I will demonstrate different strategies we have been using to extract carriers from PbS nanocrystals by means of interface with organic molecules [1] and how this can lead to efficient photodetector and [2] and solar cells[3]. [1] Dorota Jarzab, Krisztina Szendrei, Maksym Yarema, Stefan Pichler, Wolfgang Heiss, Maria A. Loi, Advanced Functional Materials, DOI:10.1002/adfm.201001999 (2011).[2] K. Szendrei, F. Cordella, M. V. Kovalenko, M. Böberl, G. Hesser, M. Yarema, D. M. Jarzab, O. V. Mikhnenko, A. Gocalinska, M. Saba, F. Quochi, A. Mura, G. Bongiovanni, P. W. M. Blom, W. Heiss, M. A. Loi, Adv. Mat. 21, 683 (2009).[3]K. Szendrei, W. Gomulya, M. Yarema, W. Heiss, and M. A. Loi, Appl. Phys. Lett. 97, 203501 (2010).
9:00 PM - CC10.32
Photon Antibunching Behavior of a Single CdSe/ZnS Quantum Dot-Metal Nanostructure System.
Sadahiro Masuo 1 , Hiroyuki Naiki 2 , Shinjiro Machida 3 , Akira Itaya 3
1 Department of Chemistry, Kwansei Gakuin Universtiy, Sanda Japan, 2 Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai Japan, 3 Department of Macromolecular Science and Engineering, Kyoto Institute of Technology, Kyoto Japan
Show Abstract Semiconductor quantum dots (QDs) have been studied extensively because of their potential for optoelectronic applications and biolabeling. One of the interesting optical properties of single QDs is their single-photon emission behavior, i.e., photon antibunching behavior at room temperature. However, single QDs show strong fluorescence blinking, which inhibits single-photon emission. It has been reported that this blinking behavior is strikingly suppressed by localized surface plasmon resonance (LSPR) in metal nanostructures. LSPR is also reported to give rise to an increase in the fluorescence intensity and a shortening of the lifetime, which is known as fluorescence enhancement. These effects are highly desirable for efficient single-photon sources. In recent years, a few reports about photon antibunching behavior of single QD-metal nanostructure systems have been published. However, a detail mechanism is still unclear. In this study, we investigated photon antibunching behavior of a single QD interacted with LSPR of metal nanostructure in details. By simultaneously measuring time traces of fluorescence intensity, lifetime, and photon correlations of single QDs, we have found a strong relationship between the degree of enhancement, the lifetime, and the probability of single-photon emission. CdSe/ZnS core/shell QDs were used. As the metal nanostructures, silver nanoparticles (AgNPs) were prepared by the conventional reduction method of silver nitrates. In order to enhance the fluorescence from the single QDs, a AgNPs/QD in PMMA/coverslip sample structure was prepared by drop-casting AgNPs-dispersed aqueous solution onto a PMMA thin film (thickness: 25 nm) in which isolated QDs were embedded. The fluorescence behavior of single QDs was measured using a Hanbury-Brown and Twiss type photon correlation set-up in combination with fs-pulsed laser excitation under a sample-scanning confocal microscope. As results of measurements for single QDs near AgNPs, we revealed that the probability of single-photon emission strongly depended on the fluorescence lifetime and intensity, i.e., the probability of single-photon emission decreased with a decrease in the fluorescence lifetime and with an increase in the fluorescence intensity. Based on the estimation of both radiative and non-radiative decay rates enhanced by LSPR, the following mechanism was proposed. The radiative decay rate enhanced by LSPR competes with the Auger recombination process, thus suppressing the Auger recombination. Therefore, the probability of single-photon emission decreases when the lifetime is shortened. Above results indicate that the control of multi-photon or single-photon emission, and the suppression of the Auger recombination are possible by the control of the QD-LSPR interaction. The present results yield new insights into fundamentals of QD-metal nanostructure interactions, and are also important to control the exciton dynamics by LSPR.
9:00 PM - CC10.33
Synthesis of Segmented Au/Ag/Au Nanostructures and Nanogaps.
Ina Alber 1 , Sven Mueller 1 , Oliver Picht 1 , Markus Rauber 2 , Reinhard Neumann 1 , Maria Eugenia Toimil-Molares 1
1 Materials Research, GSI Helmholtzcentre for Heavy Ion Research , Darmstadt Germany, 2 , Darmstadt University of Technology, Darmstadt Germany
Show AbstractBy interaction with electromagnetic radiation of appropriate wavelength, resonant surface charge density oscillations are excited in metallic nanowires (NW). These, so-called surface plasmons, are responsible for large electromagnetic field enhancements in the NW near-field that render these wires promising devices for, e.g., highly sensitive detection of molecules using surface enhanced infrared spectroscopy [1] and surface enhanced Raman spectroscopy [2, 3].Since most recently, efforts are being directed towards the fabrication and characterization of novel structures such as NW dimers separated by narrow gaps of controlled size. Experimental and theoretical results reveal that coupling of the localized surface plasmon resonances (LSPR) of two individual NWs lead to strongly increased field enhancements at the gap [4, 5]. However, the controllable and cost-efficient synthesis of these structures remains an experimental challenge. Here we present the fabrication and morphological characterization of narrow gaps with sizes ranging from 7 to 80 nm separating two gold NWs. The gaps are created in a two-step process. First, segmented Au/Ag/Au NWs are grown by sequential pulsed electrochemical deposition from a single electrolyte containing Au(CN)2- and Ag(CN)2- ions in the pores of etched ion-track membranes [6]. Secondly, the silver segments are dissolved by chemical etching using nitric acid, leading to the generation of gaps [7]. We will show how all important dimer parameters, i.e. length and diameter of the gold wires, the dimension of the gap, and the morphology of the remaining gold segments can be adjusted by adequate variation of the synthesis parameters, in particular pore diameter, electrolyte content, and pulse sequence.[1] F. Neubrech et al., Phys. Rev. Lett. 101, 157403(2008). [2] S. Nie et al., Science 275, 1102 (1997).[3] K. Kneipp et al., Phys. Rev. Lett. 78, 1667 (1997). [4] J. Aizpurua et al., Phys. Rev. B 71, 235420 (2005).[5] B. Willingham et al., Appl. Phys. B 93, 209 (2008).[6] C. Ji et al., J. Electrochem. Soc. 150 , C523 (2003).[7] N. Van Hoang et al., Nanotechnology 20 125607 (2009).
9:00 PM - CC10.34
The Effect of Adventitious Water and Post-Injection Temperature Control on the Shape and Reproducibility of Colloidal PbSe Nanocrystal Synthesis.
William Baumgardner 1 , Zewei Quan 2 , Tobias Hanrath 1
1 , Cornell University, Ithaca, New York, United States, 2 , Binghampton University, Binghampton, New York, United States
Show AbstractIntensive research efforts on colloidal nanocrystals (NCs) have led to tremendous progress in the controlled synthesis of NCs with a wide variety of size, shape, and composition. Small variations in the NC nucleation and growth processes significantly influence the morphology of the resulting product. Consequently, reliably reproducing colloidal NC synthesis protocols carried out in different laboratories remains a challenge. To elucidate possible causes of irreproducibility, we investigated aspects of colloidal NC synthesis that are not typically reported, including specific drying procedures, and the post-injection reaction temperature profile. Based on the example of a PbSe nanocube reaction protocol, we performed syntheses with varying amounts of adventitious water in the reaction solution, as well as the direct introduction of significant amounts of water. We found that reactions with as much water removal as possible lead to more single crystalline products, and caused the oriented attachment of nanoparticles into nanorods. Larger amounts of water caused cloverlike, multicrystalline NC aggregates. Using this procedure, the synthesis of well defined, monodisperse NCs required the inclusion of small amounts of adventitious water in the reaction vessel. The temporal evolution of the synthesis temperature is a key parameter in the hot-injection synthesis. The specific time-dependent temperature profile is critical to temporally separating the nucleation and growth stages of the synthesis. In the literature, the time required to ramp the temperature back up to set point is generally not reported in sufficient detail. In the case of PbSe NCs, we performed a set of reactions with identical “literature descriptions”, while systematically varying the post-injection temperature ramp rate to produce a wide variety of shapes, including pseudo-spherical, clover-like, and cubic. We conclusively show that while both the drying procedures and the post-injection temperature profile are both aspects of the synthesis procedure that often go unreported, they are critical aspects of NC shape control.
9:00 PM - CC10.35
Bismuth Tri-Iodide Nanoparticles Synthesized from Octadecene Suspension.
Ivana Aguiar 1 , Laura Fornaro 2
1 Facultad de Química, Compound Semiconductors Group, Universidad de la República, Montevideo Uruguay, 2 Centro Universitario de la Región Este, Compound Semiconductors Group, Universidad de la República, Rocha Uruguay
Show AbstractBismuth tri-iodide is a semiconductor layered compound of growing interest for direct and digital imaging. Each bismuth atom coordinates in an octahedron with six iodine atoms, with structural planes consisting of I-Bi-I layers, bonded between them by Van der Waals interactions. This layered structure determines that bismuth tri-iodide crystals can grow with platelet habit. Thereby, we consider of great importance to study if such structure can determine nanoparticles of this material as well.In light of this, the present work reports the synthesis of bismuth tri-iodide nanoparticles by the suspension method. Bi(NO3)3.5H2O, I2 and KI were used as source materials, and 1-octadecene was used as suspension agent. The best synthesis conditions were 4 hrs. at 80-110 °C, followed by 10 min. at 180-210 °C. The synthesized compounds were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area diffraction (SAD) and energy dispersive spectroscopy (EDS). Rounded and rod-like bismuth tri-iodide nanoparticles, 5-500 nm in size were obtained, depending on the synthesis conditions.Seeking for a first approach to nanoscale properties of bismuth tri-iodide, we also investigated the UV-Vis spectra of suspensions of its nanoparticles. We found that nanoparticles suspensions exhibited peak maxima shifts in their UV-Vis spectra, which may be of great interest for its applications in the area of radiation detectors.
9:00 PM - CC10.36
Fabrication of Hybrid Optoplasmonic Structures by Microspheres Self-Assembly on Lithographically-Patterned Substrates for High Efficiency Photonic-Plasmonic Mode Coupling.
Wonmi Ahn 1 , Svetlana Boriskina 1 , Yan Hong 1 , Bjoern Reinhard 1
1 Chemistry, Boston University, Boston, Massachusetts, United States
Show AbstractHybrid optoplasmonic structures that combine high-quality optical microcavity resonators and plasmonic nanostructures achieve much narrower linewidths of photonic modes than those of plasmonic resonances, which can improve sensing capabilities of a plasmonic sensor beyond its limits. In addition, photonic microresonators offer great opportunities for cascaded hot-spot intensity enhancement, long distance energy transfer, adaptive switching of spatial light distributions, and resonant manipulation of radiative rates of embedded emitters. Although theoretical researches have been spurred by the aforementioned advantages of hybrid optoplasmonic structures, development of robust fabrication strategies for practical realization of hybrid resonant optoplasmonic structures is still in infancy. Here we introduce a novel nanofabrication approach that enables a precise and controllable vertical and horizontal positioning of plasmonic elements relative to embedded microspheres by self-assembling microspheres on lithographically patterned, dry-etched surfaces. This approach ensures that the whispering gallery mode evanescent field and plasmonic mode interact synergistically at equatorial plane of the microspheres, which has been one of the greatest challenges of existing fabrication methods. We will also show high design flexibility of our template-assisted self-assembly approach for further performance optimization, and characterize its photonic-plasmonic mode coupling behavior through experimental spectroscopy, which will be supported by computational studies performed in our group [1,2] and elsewhere.[1] S.V.Boriskina & B.M.Reinhard, Proc. Natl. Acad. Sci. USA 108(8), 3147-3151 (2011).[2] S.V.Boriskina, W.Ahn, Y.Hong & B.M.Reinhard, submitted to Symposium K.
9:00 PM - CC10.37
High-Efficiency Silicon-Compatible Photodetectors Based on Ge Quantum Dots.
S. Cosentino 3 , Pei Liu 2 1 , Son Le 2 1 , S. Lee 1 , D. Paine 1 , A. Zaslavsky 1 , S. Mirabella 3 , M. Miritello 3 , I. Crupi 3 , A. Terrasi 3 , D. Pacifici 1
3 MATIS-IMM-CNR and Dipartimento di Fisica ed Astronomia, Università di Catania, Catania Italy, 2 Department of Physics, Brown University, Providence, Rhode Island, United States, 1 School of Engineering, Brown University, Providence, Rhode Island, United States
Show AbstractThese days, quantum confined structures have been intensively used for designing and tuning the electrical and optical properties of semiconductor devices. Particularly, Group IV semiconductor Quantum Dots (QDs) have been recently introduced in optoelectronic and photovoltaic applications. For example, Si QDs has recently been shown to increase the efficiency of photodetectors up to 200% in the visible range. However, compared with Si QDs, Ge QDs show even more advantages. SiO2-encapsulated Ge QDs are easily fabricated at low temperature and have higher absorption coefficients due to localized defect states at the Ge/SiO2 interface, making it an ideal candidate for several applications. In this work, we report on high-efficiency broad-band metal-insulator-semiconductor (MIS) photodetectors, with amorphous Ge (a-Ge) QDs embedded in a SiO2 matrix as the insulating layer. Our Ge-rich SiO2 film (230nm-thick) was fabricated through rf-magnetron co-sputtering deposition of a SiO2 and a Ge target onto a heavily doped n-type Si substrate maintained at 400°C. Densely packed a-Ge QDs in the as-grown SiO2 film were observed by high resolution transmission electron microscopy, showing an average QDs size of 3nm. The optical bandgap of these a-Ge QDs is measured to be 1.6eV. A fully transparent indium-zinc-oxide film (55nm-thick) was finally deposited on top of the a-Ge QD-doped SiO2 film. In order to understand the role of Ge QDs and to characterize the performance of the device, I-V curves were measured both under dark and illumination. Compared to the dark measurement, a large increase of photocurrents was observed under illumination, in reverse bias conditions. A similar SiO2 sample but without a-Ge QDs was also fabricated, showing no photogenerated currents under illumination. This indicates the key role of a-Ge QDs in this device. The internal quantum efficiency (IQE) experimentally obtained shows a broad-band response from 500nm-900nm with peak IQE values up to 700%. Spectral responsivities between 1–4 A/W were achieved in the same range, outperforming commercially available Si-based photodiodes and reference cells. We explain the high efficiency of our device as a result of the transport mechanism through photoexcited QDs. To further investigate this mechanism, we modeled the optical field distribution by means of finite-difference-time-domain calculations, revealing the light absorption behavior inside the device, indicating the broad-band absorption originates from a joint contribution of both the a-Ge QDs and the Si substrate. Moreover, we will also report a series of time-resolved experiments aimed at characterizing the typical time-response of these Ge QDs. Our results suggest that a-Ge QDs are promising candidates to enhance the performance of CMOS-compatible semiconductor devices suitable for integration with on-chip optical communication modules, with the advantage of easy fabrication due to the relatively low processing temperature.
9:00 PM - CC10.4
Increased Electronic Coupling in Silicon Nanocrystal Networks Doped with F4-TCNQ.
Alexandra Carvalho 1 , Jose Coutinho 1 , Sven Oberg 2 , Mark Rayson 2 , Patrick Briddon 3
1 , University of Aveiro, Aveiro Portugal, 2 Department of Mathematics, Luleå University of Technology, Luleå Sweden, 3 School of Electrical, Electronic and Computer Engineering, University of Newcastle upon Tyne, Newcastle United Kingdom
Show AbstractSilicon nanocrystal films are emerging as attractive functional units for silicon-based electronic materials. Easily integrated into bulk silicon substrates, they offer the means for band engineering, and relaxation of the momentum conservation restriction, both conditions necessary for the use of silicon materials for light emitting devices.We performed a modelling study of the oxidation of silicon nanoparticles by an organic agent (tetrafluoro-tetracyanoquinodimethane, F4-TCNQ) using first-principles calculations. We studied the charge distribution in the hybrid system as a function of the nanocrystal size. For nanocrystals with diameters between 2 nm and 3 nm, there is a formation of a hybrid semi-filled level shared by the nanocrystal and the F4-TCNQ molecule, and the nanocrystal is oxidised in the region close to the contact point. This type of oxidation treatment has the potential of enhancing p-type conduction both by mediating inter-nanocrystal hole transfer and by enhancing the doping. Additionally, electron transfer between them requires a small activation barrier (~0.8-1 eV). It is argued that the organic molecule will serve as a mediator for hole hopping between adjacent nanocrystals.
9:00 PM - CC10.5
Electronic and Optical Properties of Chlorinated Silicon Nanoparticles.
Alexandra Carvalho 1 , Sven Oberg 2 , Mark Rayson 2 , Patrick Briddon 3
1 I3N, University of Aveiro, Aveiro Portugal, 2 Department of Mathematics, Luleå University of Technology, Luleå Sweden, 3 School of Electrical, Electronic and Computer Engineering, University of Newcastle upon Tyne, Newcastle United Kingdom
Show AbstractFreestanding silicon nanocrystals (Si-NCs) grown using plasma deposition offer an exceptional potential for electronic structure engineering through the control of electronic confinement (size) effects and chemical manipulation of the surface[1]. The nanocrystals can be synthetised with a bare surface with H- or Cl- termination [2].We compare the electronic structure of chlorinated and hydrogenated silicon nanocrystals using a first principles model based on Density Functional Theory. Energy levels, ionisation energy, and the localisation of the near-gap levels for perfect, spherical nanocrystals are calculated as a function of the nanocrystal size. It is found that chlorinated Si-NCs have a smaller bandgap, and their electron affinity is higher by about 2~eV for nanocrystal diameters of 1-2 nm.Due to the higher electronegativity of chlorine, the localisation of the highest occupied electron state on the surface terminators is greater for Cl-covered Si-NCs. Additionally, surface dangling bonds are found to be less reactive. Hence, they are stable in the neutral charge state for a wider range of electron chemical potentials than H-terminated nanocrystals.
9:00 PM - CC10.6
Processing Dependence on the Thermoelectric Properties of Nanostructured Thermoelectric Materials.
Anuja Datta 1 , Kaya Wei 1 , Adrian Popescu 1 , Lilia Woods 1 , George Nolas 1
1 Department of Physics, University of South Florida, Tampa, Florida, United States
Show AbstractThe property requirements for thermoelectric (TE) materials are quantified by the dimensionless figure of merit, ZT = S2σT/κ, where S = Seebeck coefficient, σ = electrical conductivity, T = absolute temperature, and κ = total thermal conductivity (κ = κL + κe, the lattice and electronic contributions, respectively). Nanoscale effects, such as enhanced interfacial phonon scattering and charge carrier filtering, have so far been established as a means of improving ZT in TE materials. However limitations in applicability of this technology for nanostructured TE materials exists to a degree because of the difficulties in employing scalable and cost-effective synthesis processes for the preparation of nano-size materials, and also due to the lack of suitable processing techniques for preparing polycrystalline bulk TE materials with nano-scale domains (dimensional nanocomposites) for further TE applications. We have developed a two-step bottom-up strategy for preparing nanocomposites that involves composition and size controlled syntheses of TE materials as nanocrystals by facile solution based processes followed by densification of nanocrystals into bulk pellets by Spark Plasma Sintering (SPS) resulting in the uniform distribution of nano-scale domains in the bulk matrix. The carrier concentrations of the nanocomposites were modified by doping the nanocrystals prior to densification. In this work we discuss the synthesis and densification dependent TE properties in nanostructured state-of-art materials systems. The TE properties were experimentally measured and theoretically modeled. Experimental results and theoretical calculations revealed that the inclusion of electron/grain and phonon/grain interface scattering is crucial for the correct description and interpretation of the measured properties. These systems show improvement in ZT as compared to the bulk with similar carrier concentrations. *This work is supported by the U.S. Army Medical Research and Materiel Command under Grant No. W81XWH-07-1-0708 and the National Science Foundation under Grant Nos. CBET-0932526 and CMMI-0927637.
9:00 PM - CC10.7
Mechanistic Study of Monomer Formation in CdSe Quatum Dots Synthesis.
Raul Garcia Rodriguez 1 , Haitao Liu 1
1 Department of Chemistry, University of Pittsburgh, Pittsburgh , Pennsylvania, United States
Show AbstractWe report our findings on the reaction mechanism responsible for CdSe QD nucleation. Pure SePMe3 can react with cadmium carboxylates to yield CdSe QDs. The use of alcohols leads the formation of an intermediate alkoxyphosphonium species. The formation of this compound can be rationalized from the more reactive acyloxyphosphonium which results after the nucleophilic attack of a carboxylic ligand to the phosphorus atom of the selenide phosphine. A reaction mechanism is proposed based on our experimental observations and DFT studies
9:00 PM - CC10.8
Visible-Light Photocatalytic Activity of La2Ti2O7 Nanosheets Originated from Band Gap Narrowing.
Fanke Meng 1 , Nianqiang Wu 1
1 mechanical and aerospace engineering, west virginia university, Morgantown, West Virginia, United States
Show AbstractNitrogen-doped lanthanum titanate (La2Ti2O7) nanosheets (100-500nm) with single crystal structure have been prepared by hydrothermal processing and subsequent heat treatment in NH3 at 600 °C. The crystal structure of La2Ti2O7 nanosheets is not changed after nitrogen doping. Ultraviolet-Visible light (UV-Vis) absorption curves show that light absorption region of nitrogen-doped La2Ti2O7 is extended to the visible light regime (495 nm). This extension of absorption region is attributed to band gap narrowing, which arises from hybridization of N 2p and O 2p orbitals. Nitrogen-doped La2Ti2O7 has higher photocatalytic activity for decomposition of methyl orange (MO) compared with pristine La2Ti2O7 under both visible light and UV light irradiation.
9:00 PM - CC10.9
Mechanism of Photo-Luminescence from Au25 Nanoclusters.
Isamu Sakanaga 1 , Mitsuru Inada 2 , Tadashi Saitoh 2 , Hideya Kawasaki 3 , Yasuhiko Iwasaki 3 , Toshiki Yamada 4 , Ikurou Umezu 1 , Akira Sugimura 1
1 Physics, Konan University, Kobe, Hyogo, Japan, 2 Pure and Applied Physics, Kansai University, Suita, Osaka, Japan, 3 Chemical and Material Engineer, Kansai University, Suita, Osaka, Japan, 4 , NICT-KARC, Kobe, Hyogo, Japan
Show AbstractGold nanoclusters, which are composed of only sevral tens of atoms, attract much deal of interest, because they show strong luminescence with high quantum yields. Since this system includes both light-emitting and plasmon-existing regions within each dot, we may expect some new functions based on the integrated phenomena. The emission mechanism, however, remains unclear. One of the possible mechanisms is a plasmon-related model. Another is a model based on one-electron motion. In order to clarify the dominating physical process that causes the strong luminescence, we studied photo-luminescence (PL) properties of thiol-protected Au25 clusters, whose size corresponds to the border between the plasmon effect model region and the one-electron model region.We observed luminescence from 2.8 eV band in addition to that from 1.8 eV band. Decay of the time resolved PL measurement at higher energy band indicates that the decay can be fitted by stretched exponential function with the mean relaxation time of 2.2 ns. In contrast, the measurement of lower energy band luminescence decay shows double exponential function with much slower relaxation times. We compared these results with a one-electron energy band structure estimated using a simplified model that the 6sp electrons move freely inside the gold nanoparticle while the 5d-electrons are localized. As a result, it is found that the PL properties can be consistently explained by assuming that the lower energy band luminescence corresponds to the transition between 2P and 1F bands of 6sp states and the higher band emission to that between the 1F band of 6sp state and 5d state.Present results suggest that the light emission process of Au25 is primarily determined by the transition between 6sp states in which free electrons are involved.
Symposium Organizers
Prashant Nagpal Los Alamos National Laboratory
Matthew A. Pelton Argonne National Laboratory
Kurtis S. Leschkies Applied Materials Inc.
Hedi Mattoussi Florida State University
Patanjali Kambhampati McGill University
CC13: Poster Session III
Session Chairs
Thursday PM, December 01, 2011
Exhibition Hall C (Hynes)
CC11: Spectroscopic Studies in Semiconductor and Metal-Hybrids II
Session Chairs
Thursday PM, December 01, 2011
Room 207 (Hynes)
9:30 AM - **CC11.1
Multi-Exciton Generation by a Single Photon in Nanocrystals.
Alexander Efros 1
1 , Naval Research Laboratory, Washington DC, District of Columbia, United States
Show AbstractSolar light would be an important source of clean and renewable energy if the efficiency of inexpensive solar cells could be increased. Increased efficiency can be achieved through carriermultiplication: Photo-generated carriers, whose excess energy is greater than the energy gap, can create secondary electron-hole pairs via impact ionization of the filled band. Through this process, two (or more) electron-hole pairs are collected from each photon instead of just one. As first suggested by Nozik, impact ionization may effectively compete with cooling in nanocrystals (NCs), due to the enhanced rate of inverse Auger processes for carrier multiplication and the ``phonon bottleneck" suppression of carrier relaxation, leading to efficient MEG. Soon after this publication, Schaller and Klimov [1] observed ultra-efficient MEG by a single photon in PbSe NCs, using band-edge transient absorption measurements. Later, efficient MEG was observed by many groups using different techniques in NCs of many semiconductors: PbSe, PbS, Si, CdSe, InAs , and in carbon nanotubes. At the same time, some groups were not able to observe MEG in CdSe and InAs NCs and found that the efficiency of MEG measured in PbSe NCs was appreciably smaller than that reported earlier. The diverse experimental data on the MEG efficiency are now converging to more modest values for PbS and PbSe NCs, but MEG in NCs has been shown to be significantly more efficient than impact ionization in bulk semiconductors. In my talk I will discuss briefly the origin of this controversy and explain why MEG could be very efficient in semiconductor NCs. There are two models that explain the high efficiency of multiexciton generation (MEG) observed in nanocrystals (NCs): the coherent superposition model, based on the strong quasi-resonant coupling between exciton and multi-exciton states in a NC [2,3] and non-coherent models that are based on the important observation that the density of biexciton states is significantly larger than the density of exciton states at the same energy [4]. We unify both approaches and consider a single-photon excitation coherently coupled with many multi-exciton-states in a NC within a full quantum-state evolution approach [5]. Including phonon relaxation confirms that efficient MEG requires the exciton--biexciton coupling time to be faster than exciton relaxation time. [1] R. Schaller and V. Klimov, Phys. Rev. Lett. 92, 186601 (2004).[2] R. J. Ellingson, et. al, Nano Lett. 5, 865 (2005).[3] A. Shabaev, Al. L. Efros, and A. J. Nozik, Nano Lett. 6, 2856 (2006).[4] R. D. Schaller, V. M. Agranovitch, and V. I. Klimov, Nature Physics 1, 189 (2005). [5] W. Witzel, M. Shabaev, C. S. Hellberg, V. L. Jacobs, and Al. L. Efros, Phys. Rev. Lett. 105, 137401 (2010)
10:00 AM - CC11.2
Interfacial Alloying in CdSe/CdS Heteronanocrystals,a Raman Spectroscopy Analysis.
Norman Tschirner 2 , Holger Lange 2 , Andrei Schliwa 2 , Christian Thomsen 2 , Karel Lambert 1 , Zeger Hens 1
2 , Technische Universitaet Berlin, Berlin Germany, 1 , Ghent University, Gent Belgium
Show AbstractIn the field of colloidal semiconductor nanocrystals or quantum dots (QDs), coating a core QD with an inorganic shell has become a widely used technique to enhance the materials performance and to tune its opto-electronic properties. In this context, the atomic structure of the core/shell interface has recently come into focus since the elimination of blinking in single QD fluorescence and the reduction of the Auger recombination rate of multiple excitons has been linked to a graded transition between core and shell1. Although this concept has by now been demonstrated for a number of heteronanocrystals, such as1 (Cd,Zn)Se/ZnSe and2 CdSe/CdS, the question remains as to how alloying at these nanoscale heterointerfaces can be analyzed, demonstrated and, possibly, quantified. Recent literature based on phonon replica measured using fluorescence line narrowing on CdSe/CdS heteronanocrystals suggests that the phonon spectrum may contain a fingerprint of interfacial alloying3. In this contribution, we take this idea as a starting point to measure the phonon spectrum of CdSe/CdS with much better resolution using Raman spectroscopy. For this work, we start from zincblende CdSe cores and grow a CdS shell using a successive ion layer addition and reaction (SILAR) approach. This leads to CdSe/nCdS core/shell heterenanocrystals with an adjustable number n of CdS layers. Combining transmission electron microscopy (TEM) and x-ray diffraction, we show that the CdS shell grows coherently on the CdSe core. Based in the shifts of the fundamental CdSe and CdS LO bands in the Raman spectrum, this results in compressive strain in the core and tensile strain in the shell. Continuum mechanics based calculations confirm this picture, although the strain in both core and shell is less than predicted by theory. In addition, specific Raman modes beside the CdSe and CdS LO and their (mixed) overtones appear after shell growth. These modes cannot be understood in an abrupt heterointerface model. Based on the evolution of their frequency and intensity with increasing shell thickness, we propose a model of the CdSe/CdS dot-in-dot heteronanocrystals studied here that features a core-alloyed interface-shell structure. By identifying a distinct Raman fingerprint of an alloyed interface, this work provides the basis for an extended study of interfacial alloying in CdSe/CdS or similar core/shell systems. In particular, the combination of ab-initio density functional theory calculations of the vibrational properties of Cd(Se,S) alloys with (non-destructive) Raman measurements could allow a detailed understanding of these nanoscale interfaces, which is out of reach of other methods like, e.g., TEM. 1. Wang, X. Y. et al., Nature 2009, 459 (7247), 686-689.2. Htoon, H. et al., Nano Letters 2010, 10 (7), 2401-2407.3. Garcia-Santamaria, F. et al., Nano Letters 2011, 11 (2), 687-693.
10:15 AM - CC11.3
Real-Time Mapping of Multiexciton Dynamics in Semiconductor Nanocrystals: Towards Hole Control of Multiexcitonic Processes in Nanostructures.
Patanjali Kambhampati 1
1 Chemistry, McGill University, Montreal, Quebec, Canada
Show AbstractThe formation of multiexcitons in semiconductor nanocrystals is the main process driving the development of nanocrystal lasers, nanostructures for multiple exciton generation, and non-classical photon sources. The measure of multiexciton formation is its binding energy, which has been reported only for the ground state in nanocrystals. In contrast, the structure of excited multiexcitons in colloidal nanocrystals has remained elusive due to their ultrafast transient nature. Here, we report on the first observation of the factors which govern the structure of the simplest multiexciton - biexciton in CdSe semiconductor nanocrystals, revealing that the biexciton binding energy is dictated by the configuration of the hole. These experiments suggest controlling the hole wavefunction as a route towards tailoring the performance of devices which exploit multiexcitons in nanostructures. This idea is confirmed by correlating the development of optical gain to the transient biexciton binding energy.
10:30 AM - CC11.4
Charged-Excitons, Auger Recombination and Optical Gain in CdSe/CdS Nanocrystals.
Michele Saba 1 , Marco Marceddu 2 1 , Francesco Quochi 1 , Mauro Aresti 1 , Andrea Mura 1 , Giovanni Bongiovanni 1
1 Dipartimento di Fisica, Università di Cagliari, Monserrato Italy, 2 Centro Grandi Strumenti, Università di Cagliari, Monserrato Italy
Show AbstractConcerning optoelectronic applications for colloidal nanocrystals, non-radiative recombination phenomena have represented a significant roadblock towards successful exploitation of their optical properties. Whenever multiexciton or charged excitons are generated in NCs, Auger recombination occurs, a process in which, due to the Coulomb interaction, one exciton disappears by conferring its entire energy to another exciton in the NC. As a result of Auger recombination, the excited-state lifetime, typically of the order of 10 ns for single-exciton states, can be reduced to hundred of ps or less for multiexcitons or charged excitons, a detrimental effect if excitons are to be exploited for optical emission or photoconversion. Fast Auger recombination has been linked to a variety of unwanted optical phenomena in NCs: blinking of photoluminescence, ultrashort optical gain lifetime and narrow bandwidth for optical gain. The recent demonstration of core/shell NCs engineered for efficient multiexciton photoluminescence emission, even under cw excitation, have therefore represented a major advance. Measurements of long multiexciton decay times in such nanocrystals have been interpreted as evidence for suppression of Auger recombination. In the present work we set up ultrafast spectroscopy experiments to spectrally and temporally separate exciton, charged excitons, biexciton and multiexciton optical transitions in CdSe/CdS core/shell NCs and directly measure in the time domain their lifetimes. Our time-resolved photoluminescence and transient absorption measurements directly accessed the lifetime of such excited states, providing a clear picture of Auger phenomena in CdSe/CdS NCs. Our data demonstrated that long decay tails in nonlinear photoluminescence coexisted with fast Auger recombination of biexciton and multiexciton states. The several ns-long decay of nonlinear photoluminescence and optical gain could be attributed to charged exciton states. Excitation with variable repetition rate was employed to study the statistics and dynamics of photoionization and charge trapping leading to the appearance of charged-exciton states.
10:45 AM - CC11.5
First Exciton Fine Structure Splitting in Colloidal CdSe/CdS Dot in Plate Heterostructures.
Elsa Cassette 1 , Benoit Mahler 1 , Benoit Dubertret 1 , Thomas Pons 1
1 LPEM, ESPCI, Paris France
Show AbstractCdSe/CdS heterostructure has become a model system for semiconductor nanocrystals (NCs) since the weak lattice mismatch between these two materials allows an epitaxial growth, even for “giant shells”. These NCs have shown non blinking behavior and their optical properties are still being studied. Moreover, CdSe/CdS dot in rod NCs have been widely investigated for their linear polarization properties.Here, we present a novel class of NCs, with mixed dimensionality: a dot in plate heterostructure. This system was made by growing an anisotropic 2D CdS shell on initial 0D CdSe core. In the opposite of CdSe/CdS dot in rod NCs, the growth of CdS on the wurtzite c-axis is reduced compared to the (a, b) plane (final aspect ratio around 2, for different shell thickness). We show how to tune the SILAR (Successive Ion Layer Adsorption and Reaction) synthesis to obtain either spherical (3D) or flat (2D) shell, on spherical CdSe cores. The different NCs were characterized and compared by TEM and XRD measurements.We show that these new dot in plate structures present original electronic properties. Photoluminescence excitation spectra reveal that the first exciton (1S3/2) fine structure dramatically splits in two groups during the CdS growth (separation up to 60 meV). At the same time, the full width at half maximum of the emission spectrum is reduced down to 23 nm at room temperature (RT) and the Stokes shift decreases. In this heterostructure, the splitting cannot result directly from the anisotropic shape since the hole stays confined in the spherical core. Moreover, it cannot be explained by the exchange interaction which decreases, due to the delocalization of the electron in the entire NC. We thus attribute it to the effect of the anisotropic pressure induced by the shell. This apparent asymmetric crystal field splits the valence band in two, the heavy hole (hh) and the light hole (lh) bands.The first exciton (1S3/2-hh) corresponds to a single bright state doubly degenerated (±1L), which is responsible of the emission at RT. Ensemble and single NC polarization measurement confirms that transition dipole between this state and the ground state is two-dimensional in the (a, b) plane. In addition, the 1S3/2-lh state, which is higher in energy and composed of two close bright states, 0U and ±1U, presents mixed polarization properties (parallel and perpendicular to the c-axis respectively).Moreover, we surprisingly see that these anisotropic NCs tend to lie down flat on the substrate for relatively thick shells. This spontaneous alignment, together with their electronic properties, make this novel system interesting for fundamental physics studies and potential applications.
11:30 AM - CC11.6
Enhanced Suppression of Auger Processes in Core/Thick Shell CdSe/CdS Quantum Dots.
Clementine Javaux 1 , Benoit Mahler 1 , Alexander Efros 2 , Benoit Dubertret 1
1 LPEM, ESPCI, Paris France, 2 , Naval Research Laboratory, Washington, District of Columbia, United States
Show AbstractAuger processes are a non-radiative way for the exciton recombination in which the released energy is transferred to a third charge carrier instead of a photon. Because of their very high efficiency in confined structures (τA ≈ 10-100ps) compared to the radiative processes (τr ≈ 10ns), these non-radiative processes are an obstacle to applications such as lasing or light emitting diodes. In particular the blinking phenomenon has been attributed to Auger processes, and leads the quantum dots (QDs) to turn on and off randomly. In the last years, a huge effort has been done to reduce this effect. The teams of B.Dubertret, V.Klimov, T.Krauss and E.Lifshitz among others have been able to synthesize nanocrystals with strongly reduced Auger processes efficiency. Particularly, two kinds of core/thick shell CdSe/CdS QDs have shown suppressed Auger recombination. In the team of V.Klimov, a CdSeS alloy layer is formed at the core/shell interface. This leads to the smoothing of the confinement potential, suppressing Auger processes. In our team, the CdSe/CdS interface is sharp, and the reduction of Auger processes is therefore related to the shell thickness. With a shell thickness of 5nm, it has been shown that the Auger decay time of a trion (an exciton and a third charge carrier) reaches the order of magnitude of its radiative decay time. In order to go further in the reduction of Auger processes in these structures, we now synthesize CdSe/CdS nanocrystals with a shell thickness of 10nm. The study of these huge nanocrystals at the single dot level reveals great improvements: at room temperature, the Auger lifetime of a trion is increased up to hundreds of nanoseconds, becoming much longer than its radiative decay time. We thus obtain non-blinking core/shell emitters with very stable emission intensity. A theoretical model explaining the reduction of Auger processes with increasing shell thickness in structures with a sharp interface is proposed. These interesting results provide better understanding and control of non-radiative Auger processes in nanostructures, and suggest methods for their complete suppression in core/thick shell QDs.
11:45 AM - CC11.7
Colloidal Nanoplatelets with Two-Dimensional Electronic Structure and Ultrafast Fluorescence Lifetime.
Benoit Dubertret 1 , Sandrine Ithurria 1 , Mickael Tessier 1 , Benoit Mahler 1 , Alexander Efros 2
1 LPEM, ESPCI, CNRS, Paris France, 2 Center for Computational Material Science, Code 6390, Naval Research Laboratory, Washington DC, District of Columbia, United States
Show AbstractSize and shape control of colloidal semiconductor nanocrystals result in a diverse and complex assortment of three dimensional structures with unique optical and electrical properties. Sphere-, rod, and tetrapod- shaped nanocrystals have been synthesised, and their formation mechanisms are now well understood. However, the syntheses of strongly anisotropic nanocrystals with one dimension much smaller than the two others, such as nanoplatelets, are still largely underdeveloped. Some examples provide interesting methods to grow CdSe nanoribbons with 1,4nm thickness or PbSe ultrathinfilms with randomly varying thickness, but clear demonstration of perfect quasi 2D semiconductor systems is still missing. In this work, we demonstrate the formation of atomically flat quasi-two dimensional colloidal CdSe, CdS and CdTe nanoplatelets with well defined thicknesses - from 4 to 11 monolayers - using transmission electron microscopy and optical spectroscopy. These nanoplatelets present an extremely narrow emission spectra with full width half maximum < 40meV at room temperature, and the precise control of their thickness results in discrete shift of their absorption and emission spectra that can reach up to one eV. The nanoplatelets have the electronic properties of two dimensional quantum wells and their thickness-dependent absorption and emission spectra are described very well within an 8 band Pidgeon-Brown model that accounts for the degeneracy of the valence band as well as the non parabolicity of both the conduction and the valence bands. The radiative fluorescent lifetime measured in CdSe nanoplatelets decreases with temperature, reaching 1ns at 6K. This time is 3 orders of magnitude shorter than the typical lifetime in colloidal quantum dots. This makes the nanoplatelets the fastest colloidal fluorescent emitters and confirms that they are the colloidal equivalent of semiconductor quantum wells formed by molecular beam epitaxy.
12:00 PM - CC11.8
Observing the Onset of Superionicity in Copper (I) Sulfide Nanodisks Using Ultrafast X-Ray Probes.
Timothy Miller 1 , Joshua Wittenberg 1 , Aaron Lindenberg 1 2 3
1 Materials Science, Stanford University, Stanford, California, United States, 2 PULSE Institute, SLAC National Accelerator Laboratory, Menlo Park, California, United States, 3 SIMES Institute, SLAC National Accelerator Laboratory, Menlo Park, California, United States
Show AbstractNanoscale superionic materials have recently generated interest as part of a push for small, clean and reliable energy capture and storage materials. Superionics are crystalline compounds where, above a critical temperature, one ionic component of the crystal acquires ionic conductivity of order 1(Ωcm)-1, similar to conductivities measured in the liquid phase. At the nanoscale unique behavior emerges that enables control of the phase transition temperature and stabilization of the superionic phase at ambient conditions, creating the opportunity for new functionality in superionic nanocrystals. Building a complete picture of the transformation rate and mechanism will allow incorporation of these materials into ultrafast nanoionic switches and data and energy storage devices. Using ultrafast x-ray spectroscopic and diffraction techniques we directly observe the transformation kinetics of copper (I) sulfide nanodisks, capturing the structural and bonding changes associated with the onset of superionicity. The transformation is induced by femtosecond optical pulses through an ultrafast temperature jump to a superheated state. Time-resolved measurements of the x-ray absorption spectrum with femtosecond time resolution at the copper L3 edge reveal a spectral sharpening on the 10-20 picosecond timescale, corresponding to a change in the average electron density localized near the copper nucleus. These measurements provide the first glimpse into the bonding rearrangements at the interface between the ionic-metallic copper ions and the rigid sulfur lattice that occur at the onset of superionicity. Direct structural information was also gathered in the form of ultrafast x-ray diffraction. Large-amplitude changes on several diffraction peaks were observed upon driving the superionic transition, with an additional oscillatory response seen, providing precise information about the structural pathway of the superionic transformation at the nanoscale.
12:15 PM - CC11.9
Single CdSe Nanoplatelets Spectroscopy.
Mickael Tessier 1 , Benoit Dubertret 1
1 LPEM, ESPCI, Paris France
Show AbstractPhysical parameters of semiconductors nanocrystals (NCs) depend on their size, their shape, their surface and their composition. Control of these parameters is important to obtain more efficient NCs for opto-electronic applications. In 2008, our team synthesized the first colloidal NCs with perfect 1D confinement. These NCs have the shape of platelets. These nanoplatelets (NPLs) are atomically flat and have a thickness in the range of 1-2nm. Their lateral dimensions vary from 10 to 50nm. NPLs have visible fluorescence and their quantum yield in n-hexane solution can reach 50%. Such colloidal nano-objects have electronic properties similar to two dimensional quantum wells and consequently are potential candidates for various quantum devices such as laser-diode, light-emitting diodes or infrared photodetectors.We present single CdSe nanoplatelets spectroscopy at room and cryogenic temperature. We observe photoluminescence, lifetime intensity decay and fluorescence intensity at different temperatures. We also analyze the NPLs blinking statistics as well as their antibunching properties. This study shows that NPLs have optical properties which strongly differ from classical colloidal CdSe NCs. Control of the confinement at the atomic level leads to a perfect monodispersity of the optical properties. At low temperature, the NPLs radiative lifetime is 3 orders of magnitude shorter than typical lifetime in colloidal NCs. This makes the nanoplatelets the fastest colloidal fluorescent emitters and confirms that they are the colloidal equivalent of semiconductor quantum wells formed by molecular beam epitaxy.
CC12: Investigations of Novel Physical Phenomenon in Devices II
Session Chairs
Thursday PM, December 01, 2011
Room 207 (Hynes)
2:30 PM - **CC12.1
Limits and Benefits of Practical Nanostructured Solar Cells.
Vladimir Bulovic 1
1 , M.I.T., Cambridge, Massachusetts, United States
Show AbstractThe practical limits of the power conversion efficiencies of nanostructured solar cells, including quantum dot, molecular organic, polymeric, and dye-sensitized cells will be compared by surveying the best demonstrations of the last decade. Insights that led to the most recent technical advances will be highlighted through examples of colloidal quantum dot solar cell. Merits of the newly-proposed nanostructured solar cell approaches such as multi-exciton generation in colloidal quantum dot films and singlet-fission in molecular devices will be shown to be minimal in comparison to the demonstrated performance of the stacked nanostructured solar cells. Scale and speed of the solar technology deployment will be highlighted as the paramount technical challenges in which nanostructured solar cells can make a notable impact.
3:00 PM - CC12.2
Giant Nanocrystal Quantum Dots (g-NQDs) as Stable and Efficient Down-Conversion Phosphor Material.
Janardan Kundu 1 , Yagnaseni Ghosh 1 , Allison Dennis 1 , Han Htoon 2 , Jennifer Hollingsworth 1
1 Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 2 Physical Chemistry & Applied Spectroscopy Division, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractQuantum dot (QD) based light emitting diodes (LEDs) are currently in the forefront of solid state lighting (SSL) applications research. Maintaining high stability and efficiency for these devices over extensive operation times is a long standing technical challenge. Here, we report on giant nanocrystal quantum dots (g-NQDs) as a suitable candidate for efficient and stable color conversion phosphor material for down-conversion LEDs. g-NQDs are uniquely characterized by a high QY, large Stokes’ shift as well as unparalleled chemical robustness and photostability. We fabricated visible AC down-conversion devices with g-NQDs demonstrating high operational stability, high efficiency (without any significant self re-absorption losses at high g-NQD concentration), and steady down-conversion efficiencies. Such simple and robust device architecture utilizing red and green emitting g-NQDs as phosphors is a promising avenue for enhancing efficiency and stability of all-solid-state white LEDs.
3:15 PM - CC12.3
A New Class of High Figure of Merit Bulk Nanothermoelectrics by the Bottom-up Synthesis and Assembly of Sulfur-Doped Nanoplates.
Rutvik Mehta 1 , Yanliang Zhang 2 , Richard Siegel 1 , Theodorian Borca-Tasciuc 2 , Ganpati Ramanath 1
1 Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States, 2 Mechanical Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States
Show AbstractSculpting nanoscale building blocks with novel properties and retaining the nanostructuring-induced properties in larger scale assemblies are key to obtaining bulk nanomaterials with properties otherwise not attainable in non-nanostructured bulk materials. Here, we demonstrate a new class of both p- and n-type bulk nanomaterials with room-temperature thermoelectric figures of merit ZT as high as 1.1 from assemblies of pnictogen chalcogenide nanoplates obtained by a scalable (10 g/min) bottom-up approach. Even without optimization, our nanobulk pnictogen chalcogenides exhibit 25-250% higher ZT than their non-nanostructured bulk counterparts and state-of-the-art alloys. A unique aspect of our work is that our binary nanobulk thermoelectrics exhibit ZT~1 by a combination of doping and nanostructuring, without any alloying additions. We show that sub-atomic-percent sulfur doping from a nanoplate-sculpting and surface-passivating agent used in our synthesis, and mixing different pnictogen chalcogenide nanoplates, yield single-crystal-like charge carrier mobilities and control over the majority carrier type, leading to high electrical conductivities of 30 ≤ σ ≤ 250 kSm-1 and control over the Seebeck coefficients of -240 ≤ α ≤ 300 μVK-1. We show that α and σ can be further enhanced by heterostructuring the nanoplates to form metal-chalcogenide hybrids, and controlling the dopant level and site occupancy, respectively. We also obtain low lattice thermal conductivities of 0.2 < κL ≤ 0.6 Wm-1K-1 close to the lower limit of theoretical calculations invoking phonon scattering from nanograins and nanopores. Tuning the electron-crystal phonon-glass behavior by adapting our bottom-up approach for designing nanoscale building blocks should enable nanobulk thermoelectrics with further increases in ZT for transforming thermoelectric refrigeration and power harvesting technologies.
3:45 PM - CC12.5
Copper Sulfide Colloidal Nanocrystal Films – The Right Stuff for Economical Photovoltaics?
Adam Brewer 1 , Michael Arnold 1
1 Materials Science and Engineering, University of Wisconsin at Madison, Madison, Wisconsin, United States
Show AbstractCopper sulfide nanocrystals (NCs) have potential as the primary light harvesting material in an economical solar cell. Bulk Cu2S has an ideal band gap, >105 1/cm absorption coefficient and is made of naturally abundant elements, while NCs can be cheaply solution processed. To fully take advantage of this system we need to quantify the properties of Cu2S NCs. Alivisatos’ group demonstrated cost effective scalable Cu2S nanocrystal synthesis and its application in heterojunction photovoltaic devices with CdS nanocrystals, but opportunities remain for removing the toxic CdS electron acceptor, and to deepen and broaden our understanding of both Cu2S and colloidal nanocrystal systems in general. In our work we have verified the synthesis of 6 - 10nm Cu2S NCs first indexing the films to hexagonal Cu2S by XRD, with size calculated from the Debye-Scherrer equation. The crystal size and monodispersity were further verified by SEM and TEM. Thin film absorption and PL measurements verified a 105 1/cm absorption coefficient and a band gap of 1.3 eV. The band gap increase from bulk 1.2 eV is potentially due to either quantum confinement or non-stoichiometric Cu(2-x)S. Neither EDS nor XPS indicated any copper deficiency, but more testing is required as neither method is accurate to the 1% needed to be conclusive. We have spun these NCs into smooth films for electrical measurements, and have increased the NC film conductivity from 5x10-6 to greater than 2x10-2 S/cm by performing a ligand exchange on thin films of as synthesized NCs, exchanging long insulating dodecanethiol ligands by spinning on an excess of ethanedithiol. This method also enables layer by layer deposition of nanocrystals to build films of arbitrary thickness. XPS further checked the extent of ligand exchange, and showed clean films without oxidation, indicating at least moderate air stability. Many problems remain to be solved including characterization of the Cu2S film properties, such as doping and mobility. These factors are crucial in understanding the behavior of Cu2S NC solar cells using different electron accepting materials.
4:30 PM - CC12.6
Quantum Confinement and Phase Transition Effects on the Optical and Electronic Properties of Metastable Silver Selenide Nanocrystals.
Ayaskanta Sahu 1 2 , Moon Sung Kang 2 , Ankur Khare 2 , Donna Deng 2 , David Norris 1
1 Institut fur Verfahrenstechnik, Optical Materials Engineering Laboratory, ETH Zurich, Zurich, Zurich, Switzerland, 2 Department of Chemical Engineering and Materials Science, University of Minnesota, Twin Cities, Minneapolis, Minnesota, United States
Show AbstractIn the bulk, silver selenide (Ag2Se) is an interesting narrow band gap semiconductor possessing a wide array of intriguing properties, from superionic conductivity to giant magnetoresistance. In addition, it also undergoes a reversible first-order temperature-dependent phase transition which induces significant changes in its electronic and ionic properties. While most of these properties have been extensively examined in the bulk, very few studies have been conducted at the nanoscale. Decreasing the size provides an additional route to explore the unique properties of this material. We have recently developed a versatile synthesis that yields colloidal silver selenide nanocrystals in a metastable tetragonal phase not observed in the bulk. We observe a pronounced size dependent quantum confinement effect on the electronic absorption spectra of these semiconductor nanocrystals. We also proceed to study the size dependence of the phase-transition temperature in nanocrystalline Ag2Se. We utilize differential scanning calorimetry and in-situ X-ray diffraction analyses to observe the phase transition in these nanocrystal assemblies. Results indicate a significant deviation from the bulk phase-transition temperature of 135 °C when we reduce the nanocrystal size to a couple of nanometers, thus allowing us to tailor both the optical and phase transition properties by tuning the size of the crystal. Furthermore, we also investigate the temperature-dependence of the electronic properties of thin films of Ag2Se nanocrystals.
4:45 PM - CC12.7
Cu2ZnSnS4 Nanocrystals for Solar Cells.
Ankur Khare 1 , Boris Chernomordik 1 , Andrew Wills 1 , Lauren Ammerman 1 , David Norris 2 , Eray Aydil 1
1 Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota, United States, 2 Optical Materials Engineering Laboratory, ETH Zürich, Zürich Switzerland
Show AbstractCopper zinc tin sulfide (Cu2ZnSnS4 or CZTS) is emerging as an abundant nontoxic light absorbing material for low-cost thin film solar cells. In one approach, the absorber layer is formed by annealing thin films cast from colloidal dispersions of CZTS nanocrystals. We have developed a novel CZTS nanocrystal synthesis method based on rapid low-temperature (150-200 °C) decomposition of copper, zinc and tin dithiocarbamate complexes in presence of oleylamine. Oleylamine lowers the decomposition temperature of all three metal dithiocarbamates to a narrow range. Injecting oleylamine into a stoichiometric mixture of copper, zinc and tin dithiocarbamates results in immediate nucleation and growth of CZTS without the formation of binary metal sulfides. The formation of CZTS is confirmed by a suite of characterization methods including X-ray diffraction and Raman spectroscopy. Oleylamine concentration and synthesis temperature determine the nanocrystal size, which could be controlled between 2 and 7 nm. The optical absorption edge of nanocrystals with diameters less than 3 nm shifted to higher energies due to quantum confinement. This is the first report of quantum confinement in quaternary colloidal nanocrystals. Moreover, nanocrystals melt at temperatures much lower (~550 °C) than the melting temperature of bulk CZTS (931 °C). Melting and recrystallization of CZTS nanocrystals deposited on molybdenum-coated glass forms thin films with large grains suitable for solar cells.
5:15 PM - CC12.9
Detection of Quantum Dot Blinking by Dynamic Conductive Atomic Force Microscopy.
Klara Maturova 1 , Sanjini Nanayakkara 1 , Manuel Romero 1 , Nikos Kopidakis 1 , Jao van de Lagemaat 1
1 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractColloidal semiconductor quantum dots (QDs) are promising candidates for a variety of applications given their strong tunable fluorescence, stability and small luminescence bandwidth. However it is known that they exhibit sudden change of fluorescence intensity. Discontinuous quantum dot fluorescence, known as blinking or fluorescence intermittency, is observed despite the continuous laser excitation. The “off” periods are associated with trapping of excited charge carrier in a long-lived trap.Here we present results on quantum dots blinking observed by c-AFM in dynamic contact mode, where the tip oscillates above the surface and periodically briefly touches the surface. When the metallic tip approaches the sample, current can be injected into unoccupied states. On millisecond timescale we have found the “on” periods with high current injection and the “off” periods of no current. During the “on” state, electrons or holes are injected into the quantum dot depending on the polarity of the applied bias. On the other hand, during the “off” state, trapped charges prevent current injection by a Coulomb blockade effect. Detail examination of the “on” periods shows that each period consists of periodically recurring current injection peaks. The resonant frequency of the tip and high speed data acquisition enables us to see single current injection events as ~5us wide peaks which are 20us apart. We have examined colloidal CdSe and PbSe quantum dots covalently bound to a self-assembled monolayer of hexanedithiol. Next to the current measurements, we also correlate our results with I-V spectroscopy and detect quantum dot current blinking both in the dark and under illumination. Illumination appears to shorten the on/off period between blinking events.Together with c-AFM, the luminescence following the current injection is examined. The time-delay between current injection and when light emission occurs enables us to measure the rate of energy transport. Simultaneous topographical, current and luminescence measurement gives us control when and where is exciton created.
CC13: Poster Session III
Session Chairs
Friday AM, December 02, 2011
Exhibition Hall C (Hynes)
9:00 PM - CC13.1
Synthesis of One-Dimensional Semiconductor/Metal Heterostructures by Controlled Heterogeneous Nucleation.
Subhajit Kundu 1 , Paromita Kundu 1 , N. Ravishankar 1
1 Materials Research Centre, Indian Institute of Science, Bangalore India
Show AbstractOne-dimensional semiconductor/metal heterostructures are potential candidates for applications in photodiodes, photo-detectors, field-effect transistors and sensing. In all these cases, the heterojunction plays a crucial role in controlling the electron dynamics and thus a clean interface is desired. Though, there are methods for the synthesis of 1-D wires/rods of semiconductors and metal, integrating them together is non-trivial. We present a method for the attachment of anisotropic Au nanostructures to different semiconductor rods and wires by a simple wet chemical route. The semiconductor support materials are chosen to span a broad bandgap range from IR to visible in order to tune the optical properties of the hybrid. Heterogeneous nucleation of the fine Au nanoparticles on the substrate is the key step in obtaining site specific attachment of Au nanostructures viz. tetrapods and wires. Oriented attachment of Au nuclei along 111 facet results in the formation of such structures. Our method does not involve any template/molecular linker to anchor the metal counterpart, hence offer minimal resistance for electron transport at the junction. The morphological characterization has been carried out using microscopy and spectroscopy techniques to obtain complete mechanistic understanding of such hybrids.
9:00 PM - CC13.11
Embedded Nanoscale Metamaterial for Enhanced Optical Absorption in Ultrathin Silicon.
Fan Ye 1 , Michael Burns 1 , Stephen Shepard 1 , Michael Naughton 1
1 , Boston College, Chesnut Hill, Massachusetts, United States
Show AbstractA nanoscale metallic metamaterial embedded into an ultrathin silicon film is shown to substantially increase optical absorption. Computer simulations on 20 nm-thick metal patterns embedded in ~80 nm-thick amorphous silicon films find more than 100% increase in integrated absorption in the visible regime (400-800 nm), the majority of which occurs in the red/NIR. In experiments on samples with these same thicknesses, we find ~50% increase in wavelength-integrated absorption. These results may prove useful in thin film, and especially ultrathin film, photovoltaics. Computer simulations are done with varied embedding depth, film thicknesses, and metamaterial shapes and patterns in efforts to optimize absorption. Moreover, different metals are employed in both simulations and experiment to obtain information on whether and to what extent (if any) plasmonic effects contribute to the observed effects.
9:00 PM - CC13.12
Study of CdSe/Zns Quantum Dots for Thermal History Sensing Application.
Nitin Shukla 1 , Gang Chen 1 , Taofang Zeng 1 , Yucheng Lan 2 , Zhifeng Ren 2
1 , M. I. T. , Cambridge, Massachusetts, United States, 2 Physics Department, Boston College, Chesnut Hill, Massachusetts, United States
Show AbstractIn this work, we aim to develop quantum dot based thermal history sensors for recording temperature profile of extreme thermal events such as explosion, combustion, and geothermal events. In general, temperature profile of a thermal event is recorded in real time using conventional sensors such as thermocouples. However, this strategy may not work in extreme thermal environments due to the fact that sensor will be destroyed by the extreme nature of the event. We adopt a two step strategy in such here: i) we use temperature induced mass-diffusion/reaction process in core-shell quantum dots to record the temperature profile during the thermal event, and ii) use photoluminescence (PL) spectroscopy to measure this mass-diffusion subsequent to the thermal event. The temperature field can be extracted from the PL data by assuming a moving boundary mass-diffusion approach where core is shrinking.We used CdSe/ZnS core-shell type colloidal quantum dots of ~5-10 nm size with peak emission wavelength in the range of ~600-650 nm. It is very important to accurately determine time and temperature dependence on the PL energy shift. In the experiment, we accurately controlled the time and the temperature of the quantum dots by using well prepared thin wires as a heater and a temperature sensor. We observed that PL energy blue-shifts with temperature, a clear indication of mass-diffusion process. The time and temperature PL data show that the mass-diffusion process follows an Arrhenius type of kinetic process. Using PL data, we determined key parameters for the mass-diffusion process such as activation energy, diffusion coefficient and time exponent. We also conducted annealing experiments in vacuum to validate the mass-diffusion/reaction process. Finally, we used transmission electron microscopy (TEM) to quantitatively relate mass-diffusion with the PL energy shift.
9:00 PM - CC13.14
Seeded Growth of Shape-Controlled Wurtzite CdSe Nanocrystals: Cubes, Platelets, and Bullets.
Katherine Rice 1 , Mark Stoykovich 1 , Aaron Saunders 1
1 Chemical and Biological Engineering, University of Colorado, Boulder, Colorado, United States
Show AbstractPrevious investigations into the synthesis of wurtzite CdSe nanocrystals have given rise to well-developed methods for producing particles with anisotropic shapes such as, rods, tetrapods and wires; however, the synthesis of other anisotropic shapes has proved challenging. Here, we demonstrate the use of a seeded-growth approach to produce colloidal, shape controlled wurtzite CdSe nanoparticles with previously unobserved morphologies. The synthesis, which makes use of small CdSe nanocrystals as nucleation sites for subsequent growth, can be tuned to selectively yield colloidal CdSe cube- and hexagonal platelet-shaped nanocrystals, in addition to previously observed rod- and bullet-shaped particles. We characterize the structure and discuss possible growth mechanisms for these new shapes, and demonstrate a quantitative analysis technique for shape classification based on Fourier descriptors obtained from transmission electron micrographs.
9:00 PM - CC13.15
Postfocused Nanocrystal Diameter Tuning through Control of the Reaction Rate and the Solubility: Experiment vs. Realistic Modeling.
Bram De Geyter 1 2 3 , Sofie Abe 1 , Zeger Hens 1 3
1 Physics and Chemistry of Nanostructures, Inorganic and Physical Chemistry Departement, Ghent University, Gent Belgium, 2 Photonics Research Group, INTEC, Ghent University IMEC, Gent Belgium, 3 Center for Nano- and Biophotonics (NB-Photonics), Ghent University, Gent Belgium
Show AbstractWe show that adjusting the reaction rate and solubility through ligand engineering in a hot injection synthesis is a viable strategy to tune the diameter of colloidal nanocrystals at the end of the size distribution focusing, i.e., the post-focused diameter. The approach is introduced by synthesis simulations, which describe nucleation and growth of colloidal nanocrystals from a solute or monomer that is formed in-situ out of the injected precursors. We present a coupled set of continuous rate equations, including monomer generation, nucleation and growth in one model. Instead of dimensionless parameters, we use common dimensions of diameter, time and concentration to keep comparison with experiments straightforward. We explore a three dimensional parameter space by adjusting the reaction rate for monomer generation, the solubility and the temperature. These simulations indicate that the post-focused diameter is reached at almost full yield, and that it can be adjusted by the rate of monomer formation or the appropriate choice of ligand. We implement this size tuning strategy using a particular CdSe quantum dot synthesis that shows excellent agreement with the model synthesis. After demonstrating that the reaction rate depends in first order on the Cd and Se precursor concentration, the proposed strategy of size control is explored by varying the precursor concentration. This enables the synthesis of colloidal nanocrystals with a predefined size at almost full yield and sharp size distributions, which is highly relevant especially in the context of reaction upscaling and automation. Moreover, the results obtained challenge the traditional interpretation of the hot injection synthesis, in particular the link between hot injection, burst nucleation and sharp size distributions.
9:00 PM - CC13.16
Localized Defects of SiGe/Si Superlattice Structures for Sensor Application Using Ion Beam Bombardment.
Patrick Grayson 1 2 , Claudiu Muntele 1 2 , Cydale Smith 1 2
1 Department of Physics, Alabama A&M University, Normal, Alabama, United States, 2 Center for Irradiation of Materials, Alabama A&M University, Normal, Alabama, United States
Show AbstractWe fabricated and characterized SiGe/Si superlattice structures for sensor applications. The structures are 10 periodic nano-layers of SiGe/Si superlattice thin films, with the alternating layers between 5-10 nm thick. The objective of this study is to fabricate an ultra-sensitive oxygen-gas-sensor for civilian and military applications. The superlattices were bombarded by 5 MeV Si ions at three different fluences to fabricate localized and specific defects. Rutherford Backscattering Spectrometry (RBS), X-ray Photoelectron Spectroscopy, photoluminescence, and X-ray diffraction were used to determine the film thickness, stoichiometry, and structural quality before and after MeV bombardment.
9:00 PM - CC13.17
Manipulating the Properties of Metal Nanostructures: Sculptured Thin Films Coated by Atomic Layer Deposition.
Daniel Schmidt 1 , Natale Ianno 1 , Eva Schubert 1 , Mathias Schubert 1
1 Electrical Engineering, University of Nebraska-Lincoln, Lincoln, Nebraska, United States
Show AbstractThe fabrication of three-dimensional metal nanostructures with tailored geometry is one of the central challenges of nanotechnology because geometrical and material parameters are responsible for the optical, electrical, mechanical, chemical, or magnetic properties of such nanostructured thin films. Engineered artificial sculptured thin films (STFs) with designed anisotropies are potential candidates for applications in various fields such as optics, magneto-optics, as well as chemical and biological sensing and detection. However, in order to utilize metallic nanostructures for novel applications their size-, structure-, and material-driven physical properties have to be understood and quantified.We utilize glancing angle electron-beam deposition, which exploits physical atomic-scale shadowing and dynamically varying particle flux azimuth for fabrication of three-dimensional highly spatially coherent STFs with different morphologies. Subsequently, nanostructures are individually covered with a thin conformal coating (cladding) by means of atomic layer deposition (ALD). The ALD process is monitored with in-situ generalized ellipsometry.We will present the fabrication processes as well as structural and optical properties of highly anisotropic ALD coated metal STFs determined by generalized spectroscopic ellipsometry in the visible and near-infrared spectral region. The analysis of our multilayer slanted columnar thin films deposited at glancing angle revealed that such STFs possess biaxial (monoclinic) optical properties, and the optical response may be described by an effective medium dielectric homogenization approach. It will be discussed how the anisotropic Bruggeman effective medium approximation (AB-EMA) allows for determination of structural parameters as well as fractions of individual film constituents. Furthermore, the AB-EMA analysis reveals that the anisotropic dielectric properties of the metal core changes upon deposition of a dielectric cladding.
9:00 PM - CC13.18
Alkene-Functionalized Silicon Nanocrystals and Air-Stable Solution-Processable Hybrid Solar Cells.
Jihua Yang 1 , Rebecca Anthony 1 , Uwe Kortshagen 1
1 Department of Mechanical Engineering, University of Minnesota, Minneapolis, Minnesota, United States
Show AbstractFor new generation of optoelectronic devices, silicon nanocrystals (SiNCs) are distinguished in natural abundance and low toxicity. SiNCs can be synthesized by nonthermal plasma technique with large yields, good crystal quality, and size control. Surface modification of SiNCs with alkene ligands can passivate surface defects and make the SiNCs soluble in organic solvents. The hydrosilylated SiNCs have shown potential for biological labeling or inorganic-organic hybrid light emitting devices, but no progress has yet been reported for application in solar cells. The large steric barrier due to the alkyl chains can result in poor electronic coupling of neighboring SiNCs or between the SiNCs and polymer electron donors, limiting the viability of alkyl chain-capped SiNCs for hybrid solar cells. Our current efforts focus on functionalization of clusters rather than single particles of SiNCs for enhancement of their photovoltaic activity. Soluble clusters of SiNCs may benefit electronic coupling in the network of the SiNCs because there will be no ligand barriers within the clusters and the overall number of barriers among SiNCs or at SiNCs:polymer interface can be significantly reduced. As the first part of this work, the functionalized clusters of SiNCs are obtained by controlling the reaction concentration of 1-dodecene ligands for thermal hydrosilylation, as shown by Fourier transform infrared spectroscopy (FTIR), atomic force microscope (AFM) and photoluminescence. The clusters of SiNCs results in a reduced overall coverage of alkyl chains on surface, but maintain excellent solubility and good luminescence, as well as improved photovoltaic activity. Second, the functionalized clusters of SiNCs are obtained by a one-step plasma-assisted synthesis and in-flight hydrosilylation of SiNCs with short-chain 1-octene ligands, in a compact nonthermal plasma reactor. These clusters readily form a stable Si ink with excellent solubility and desirable photovoltaic activity. The hybrid solar cells based on the bulk heterojunction of the Si NC clusters and polymer poly(3-hexyl thiophene) show a permanent positive initial aging effect on performance and obtain a open circuit voltage of 1 V. Furthermore, the solar cells show excellent stability in air, retaining 60% of the initial highest efficiency after 30 days of air exposure.This work was supported by the Center for Advanced Solar Photophysics, an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences (BES).
9:00 PM - CC13.19
Plasmon Spectroscopy of Nonspherical Gold and Silver Nanoparticles: Orientational Interpolation and Applications to Colloidal Synthesis.
Raman Shah 1 , Stephen Gray 2 , Philippe Guyot-Sionnest 1
1 James Franck Institute, University of Chicago, Chicago, Illinois, United States, 2 Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois, United States
Show AbstractNoble metal nanoparticles in dielectric media exhibit localized surface plasmon resonances. In the case of a nonspherical particle, the optical absorption and scattering spectra due to such resonances depend on the orientation of the particle with respect to the incidentbeam of light. We present an approximate, general method called orientational interpolation (OI) to compute arbitrary-orientation and orientation-averaged spectra of such particles starting from several simulated spectra at fixed orientations. The method uses interpolation on the configuration manifold of the particle and exploits symmetry to make efficient use of simulation results. We show that OI gives results in excellent agreement with the exact solution for an ellipsoidal silver particle in the electrostatic limit. We then show that OI effectively models the plasmon spectra of nanoparticles that violate these assumptions, including large particles with quadrupole resonances and nonellipsoidal particles. We apply OI to two practical problems: assigning the ensemble UV-Vis spectrum of high-quality silver nanorods and purifying gold bipyramids.
9:00 PM - CC13.2
Evaluation of Colloidal CdSe Quantum Dots with Metal Chalcogenide Ligands for Optoelectronic Applications.
Yiqiang Zhang 1 , Rajeev Acharya 1 , Xian-An Cao 1
1 Department of Computer Science and Electrical Engineering, West Virginia University, Morgantown, West Virginia, United States
Show AbstractThe effects of ligand exchange on the optical, electrical and optoelectronic properties of colloidal CdSe core and CdSe/CdS/ZnS core/multishell quantum dots (QDs) solids were investigated. Exchanging the original organic ligands of colloidal CdSe with inorganic metal chalcogenide (SnS4) ligands resulted in carrier delocalization and enhanced inter-QD electronic coupling, as inferred from peak redshift and broadening of the absorption and photoluminescence (PL) spectra. The SnS4-capped QDs retained strong excitonic absorption, but suffered significant PL quenching. Mild thermal treatment below 350 °C transformed the SnS4 ligands into a more conductive phase which linked QDs into an assembly of strongly-coupled functional blocks, leading to complete PL quenching and further enhanced charge transport. Wavelength-dependent photocurrents in ITO/core QDs/metal structures have been measured. The spectral responses resembled the absorption spectra of the QDs, and the quantum yield of photocurrent generation was substantially increased after the ligand exchange. These findings suggest that colloidal QD solids with metal chalcogenide ligands may be better suited for solar energy conversion in solar cells than use in light-emitting devices as luminophores.
9:00 PM - CC13.20
Connecting Surface Ligand Passivation to Performance in Highly Efficient Silicon Nanocrystal-Organic Light-Emitting Devices.
Kai-Yuan Cheng 1 , Rebecca Anthony 2 , Ting Chen 1 , Uwe Kortshagen 2 , Russell Holmes 1
1 Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota, United States, 2 Department of Mechanical Engineering, University of Minnesota, Minneapolis, Minnesota, United States
Show AbstractIn this work we examine the connection between surface ligand coverage and electroluminescence efficiency in hybrid silicon nanocrystal-organic light-emitting devices (NC-OLEDs). We show that in an optimized device architecture, silicon nanocrystals (SiNCs) are capable of showing very high external quantum efficiencies exceeding 9%. The SiNCs are synthesized using a non-thermal plasma-based process, and are chemically passivated with ligands of 1-dodecene. Interestingly, while efficient electroluminescence can be realized from these systems, the role of the surface ligand coverage has not been quantitatively connected to device performance under electrical injection. Here, we examine the dependence of the electroluminescence efficiency and device drive voltage on the degree of surface ligand coverage. In quantifying the ligand coverage using thermogravimetric analysis, we are able to more clearly identify the optimum SiNC coverage conditions, balancing the need to reduce quenching at the SiNC surface with the need for effective charge injection. Overall, this work provides additional insight into the design of chemically passivated SiNCs for optoelectronic applications.
9:00 PM - CC13.21
Photoluminescence of Silicon Nanocrystals Embedded in Silicon Dioxide.
Arif Alagoz 1 2 , Ayse Seyhan 2 , Steinar Foss 3 , Terje Finstad 3 , Rasit Turan 2
1 Applied Science, University of Arkansas at Little Rock, Little Rock, Arkansas, United States, 2 Physics, Middle East Technical University, Ankara Turkey, 3 Physics, University of Oslo, Oslo Norway
Show AbstractSilicon has been the leader of microelectronics for decades with its excellent electronic properties and oxidized form silicon dioxide. On the other hand, indirect electronic band structure of bulk silicon does not allow silicon based light emitting devices. As oppose to bulk silicon, its nanocrystal form shows luminescence at visible and infrared region due to quantum confinement. In this work, we present the effects of deposition conditions and post annealing on formation silicon nanocrystals and its photoluminescence characteristics. Silicon nanocrystals were formed in silicon rich silicon dioxide layer fabricated by magnetron co-sputtering and high temperature furnace annealing. Effect of excess silicon concentration and annealing temperature on photoluminescence spectrum was investigated. In addition, post-annealing under hydrogen gas environment is carried out in order to understand the effect of defect passivation in silicon nanocrystal on photoluminescence.
9:00 PM - CC13.22
Reduction of Gold Penetration through Phenyl-Terminated Organic Monolayers on Si.
Azadeh Akhtari-Zavareh 1 , Karen Kavanagh 1 , Richard Popoff 2 , Hogan Yu 2
1 physics, Simon Fraser University, Burnaby, British Columbia, Canada, 2 Chemistry, Simon Fraser University, Burnaby, British Columbia, Canada
Show AbstractThermally-evaporated gold can penetrate organic monolayers attached to Si. The amount of penetration can be reduced by manipulating the monolayers including changing their length, the attachment chemistry, the end group, and/or coverage. By using ballistic electron emission microscopy (BEEM), we have previously shown that changing the length of an n-alkyl monolayer bonded to Si via covalent carbon bonds, in a Au/n-alkyl/Si junction does not change the BEEM transmission significantly. But thioacetete-terminated n-alkyl monolayers did significantly reduce the BEEM transmission current and increased the effective diode barrier height from current-voltage properties. This dependence was attributed to additional methylene groups interacting with Au atoms and inhibiting gold penetration. Changing the end group to larger phenyl molecules also did not, by itself, reduce the BEEM transmission through the interface noticeably. However, keeping the phenyl end group plus increasing the monolayer density using phenylacetylene rather than allyl benzene did make a significant difference. BEEM measurement showed a reduction of transmission current through Au/phenyl-terminated-alkyl/Si junctions by a factor of 10 indicating significantly less Au penetration and more insulating monolayers. This data is supported by current-voltage measurements using mercury (Hg) probe contacts that have shown significantly less leakage current for phenyl-terminated monolayers compared to plain n-alkyl monolayers. And high resolution XPS measurement after Au evaporation indicates that the molecules are still bonded to the substrate as evidenced by deconvolution of the C1s (C-Si) signal. The authors acknowledge funding support from NSERC.
9:00 PM - CC13.24
Submicron Spherical Zinc Oxide Particles as Optical Scatterers.
Naoto Koshizaki 1 , Hongqiang Wang 1 , Yoshie Ishikawa 2
1 , AIST, Tsukuba Japan, 2 , Kagawa University, Takamatsu Japan
Show AbstractFabrication of size-tailored submicron spherical particles has recently attracted significant research interest due to their unique physicochemical properties and emerging various applications. However, due to the intrinsic anisotropic crystal growth behavior, they always grow anisotropically with a high tendency to form non-spherical nanostructures. ZnO, with a wide bandgap of 3.37 eV and a large exciton binding energy of 60 meV, is one of the key semiconductors. Due to the intrinsic nature of polar hexagonal-phase ZnO, diverse well-defined 1D nanostructures have been synthesized and utilized in a variety of functional device applications. Comparatively, the creation of spherical crystal of ZnO, which are thought to be an attractive material for photonic crystals, solar cells and photocatalysts, has seldom been reported. Furthermore, even within those few existing reports, the resultant submicron ZnO spherical particles have usually been built up by secondary nanoparticles, and therefore the lack of close contact between nanostructures will inevitably influence or even reduce the performance of ZnO submicron spheres in various applications. Herein, by using a simple bottom-up laser processing, i.e. pulsed laser irradiation of colloidal nanoparticles of ZnO, we demonstrate the synthesis of ZnO submicron spheres constructed without any subunits. Particle size of ZnO submicron spherical particles changed from 195 nm to 320 nm by increasing the laser fluence from 67 to 133 mJ/pulse/cm2. The unique selective pulsed heating involved in the laser processing assures the formation of spherical crystal of ZnO. The UV-vis extinction spectra of obtained particles demonstrated a red shift from 375 to 440 nm with the increase in laser fluence. It has been reported that the red shift can be resulted from the size increase of the nanoparticles. However, this cannot explain the phenomena because the absorption edge of bulk ZnO is at around 375 nm (3.3 eV), and the resultant submicron spheres cannot be much more bulk. One reasonable explanation is that a resonant scattering most likely occurs when the size of particles is comparable with the wavelength of the incident light. We also investigated the room-temperature photoluminescence of the ZnO spherical particles with different size. The typical PL spectra of ZnO spheres consist of UV emission and visible emission. In addition, the intensity of the defect-related visible emission gradually decreases with the increasing of the laser fluence. This might be due to the gradual relaxation in ZnO crystals upon the increasing of the growth temperature.This finding could thus be of great importance for the expansion of ZnO submicron-sphere-based research. The interesting size-related extinction indicated the potential of ZnO submicron spheres in optics devices.
9:00 PM - CC13.25
Formation of Optical Barriers in Silicon Crystals for Optical Waveguide Applications through Ion Implantation Assisted Synthesis of Multilayer Structures of Silver Nanoparticles.
Perveen Akhter 1 , Nirag Kadakia 2 , William Spratt 2 , Mengbing Huang 2
1 Department of Physics, University at Albany-SUNY, Albany, New York, United States, 2 College of Nanoscale Science and Engineering, University at Albany-SUNY, Albany, New York, United States
Show AbstractThis work reports a novel method based on ion implantation, thin film deposition and thermal annealing to engineer the refractive index profile for fabrication of plasmonic waveguides in silicon crystals. Starting with Si substrate, a silicon dioxide layer of 200 nm was first grown by thermal annealing at 950 °C for 45 minutes in oxygen ambient. Depending on the number of optical barriers to be generated in Si, these samples received a single H implant or multiple H implants at different energies. For example, to create three optical barrier layers in Si, three H implants with a fixed dose of 3e16 /cm^2 for each, were conducted at three different energies at 30, 100 and 160 keV, respectively, corresponding to the projected ranges of 0.32, 0.87 and 1.4 μm, respectively. These energies were chose to avoid any overlap of the implantation regions tails. Following post-implantation annealing between 600 and 900 °C for 1 hour in Ar ambient, thin layers of Ag were deposited on the samples and post-deposition annealing at temperatures 700-900 °C was performed to diffuse Ag atoms in Si. Rutherford backscattering spectroscopy (RBS) indicated the accumulation of a large density of Ag atoms around the implanted H projected ranges. Transmission electron microscopy (TEM) also showed evidence for formation of Ag nanocrystals at those locations. Optical reflectivity of those resultant samples was found to oscillate with light wavelength, implying the presence of layer structures of varied index of refraction. This was further confirmed by spectroscopic ellipsometry (SE) measurements. Through modeling of SE data, the optical constants including the refractive index and the absorption coefficient of the samples were determined as a function of light wavelengths, yielding evidence for the formation of optical barriers at the depths close to the H projected ranges. The implications of these results, including possible plasmonic effects of Ag nanoparticles, on optical waveguiding in Si, will be discussed.
9:00 PM - CC13.26
RGB-Light-Emitting Si Quantum Dot: Fabrication by Pulsed Laser Ablation in Supercritical Fluid and Liquid Phases.
Ken-ichi Saitow 1 2 , Kazuyuki Nishio 2 , Takumi Kitasako 2
1 Natural Science Center for Basic R&D (N-BARD), Hiroshima Univ., Higashi-Hiroshima, Hiroshima, Japan, 2 Department of Chemistry, Hiroshima University, Higashi-Hiroshima Japan
Show AbstractWe performed the pulsed laser ablation of bulk silicon (Si) crystal in a supercritical fluid to obtain Si nanomaterials. According to electron microscope, photoluminescence, Raman, EPMA, the product has been confirmed as the Si quantum dot (QD) emitting in RGB (red, green, and blue) light. These colors were changed by the fluid pressures during the laser ablation. Here we conduct the pulsed laser ablation of silicon crystal in several organic liquids. It becomes easier to obtain the Si QD. That is, the QD size is ranging from 1 to 4 nm, according to DLS and TEM experiments. The particle was also dispersed in organic liquids. Note that the luminescence intensity of Si QD is changed significantly by the solvents during the laser ablation. That is, the intensity difference as a function of solvent is a factor of 50. We present this new finding from the data of PL spectra, dynamic light scattering, the electron microscope, vibrational spectra, and elemental analysis, and so on.
9:00 PM - CC13.27
Unconventional Hybrid Plasmonic Sensors for Optical Detection in Complex Media: From Actuatable Dimers to Photoswitchable Clusters.
Jennifer Chen 1 , Yunqi Yan 1 , Heather Durkee 1 , Beth Traxler 2 , David Ginger 1
1 Chemistry, University of Washington, Seattle, Washington, United States, 2 Microbiology, University of Washington, Seattle, Washington, United States
Show AbstractWe present sensing modalities based on the actuation of discrete nanoparticle dimers and photoswitching of multi-nanoparticle assemblies. First, detection by discrete dimers is demonstrated by the unconventional spectral blue shift in the hybridized plasmon mode when the target geometrically extends the dimer upon binding, as monitored by darkfield scattering spectroscopy. Significantly, and distinct from nearly all other LSPR sensors, this concept allows us to distinguish between target binding and non-specific adsorption by the direction (red or blue) of the wavelength shift. Second, detection by biomediated photoswitchable clusters is demonstrated by monitoring light-modulation dependence of the LSPR of the colloids. We explore the detection of DNA and an exemplary protein, and demonstrate the capability for sensing in complex media like serum. We envision that these sensing modalities could be applied to the detection of a number of species with appropriately designed biological linkers, and open the door to colorimetric remote standoff sensing platforms.
9:00 PM - CC13.28
Photoelectrochemical Properties of Nanodiamond-Conducting Polymer Hybrid Structures.
Manoj Ram 1 3 , Farah Alvi 2 , Humberto Gomez 2 , Lee Stefanakos 3 , Ashok Kumar 1 2
1 Nanotechnology Research and Education Center (NREC), University of South Florida, Tampa, Florida, United States, 3 Clean Energy Research Center (CERC) , University of South Florida, Tampa, Florida, United States, 2 Department of Mechanical Engineering, University of South Florida, Tampa, Florida, United States
Show AbstractThe photoelectrochemical properties of nanoparticles (TiO2, ZnO) blended with conducting polymer (hexyl polythiophene, poly(3-methylthiophene) (PMTh), ragioregular polyhexylthiophene (RRPHTh)) have been studied due to their high electron mobility, chemical and physical stability of inorganic nanoparticles, and the flexibility in fabrication of larger cells at low cost. The quantum yield in conjugated polymer blended with metal oxide as well as quantum dot still remains a challenge. Nano-hybrid films of ND-RRPHTh, ND-PHTh, and ND-PMTh were coated on indium tin oxide (ITO) coated glass plate, n-Si and gold coated glass surface and characterized using a combination of physical and electrochemical techniques. This paper reports the interesting photoelectrochemical properties of blend ND-RRPHTh, ND-PHTh, and ND-PMTh nano-hybrid films.A standard electrochemical cell with a three-electrode system was employed for the photo electrochemical photocurrent response for the RRPHTh blend films. The electrochemical studies were performed in electrolytes 0.1 M HCl, 0.1 M H2SO4, O.1 M LiClO4 in PC, 0.1 M camphor sulfonic acid, and 0.1 tertaethylammoniumtetrafluoroborate in acetonitrile. The electrochemical cell contained film coated on discussed substrates as working electrode, platinum as a counter electrode and Ag/AgCl as reference electrode for water based electrolyte, whereas silver wire electrode for non-aqueous was used as a reference electrode. The photo electrochemical properties of the blend films of ND-RRPHTh, ND-PHTh, and ND-PMTh films at different electrolytes were studied using the 60 to 100 watt lamp...It has been demonstrated that the incorporation of small quantities of nanodiamond enhances photoconductivity in ND-RRPHTh, ND-PHTh, and ND-PMTh polymers. The photo electrochemical properties of NDs-RRPHTh deposited on either n-type silicon or ITO coated glass plate in TBATFB electrolyte is around 8 to 10 times higher in current density, and energy conversion efficiencies as compared to other nano-hybrid films. In the present paper, we have investigated the photoelectrochemical properties aiming to assemble photovoltaic devices or photosensors based on nanodiamond-conjugated polymer blend films.
9:00 PM - CC13.29
Self-Organized Freestanding One-Dimensional Au Nanoparticle Arrays.
Yu Yuwen 1 , Wenchong Hu 1 , Seokho Yun 1 , Theresa Mayer 1 2 , Mahalingam Krishnamurthy 3 , Kurt Eyink 3
1 Department of Electrical Engineering, The Pennsylvania State University, University Park, Pennsylvania, United States, 2 Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania, United States, 3 Air Force Research Laboratory, Wright Patterson Air Force Base, Greene, Ohio, United States
Show AbstractOne-dimensional (1D) arrays of noble metal nanoparticles with controlled particle diameter and interparticle spacing can be optimized to have extremely narrow plasmon resonances that are of interest for applications ranging from biodetection to anticounterfeiting. These structures are typically fabricated on planar substrates by direct-write electron beam lithography, which is not a scalable manufacturing process. This talk will present a new method to synthesize self-organized 1D Au nanoparticle arrays encapsulated within freestanding SiO2 nanowires. The process begins by forming a template composed of a dense array of Si nanowires. The wire diameter is modulated by adjusting the etching and deposition cycles during deep reactive ion etching (DRIE) of a lithographically patterned Si substrate. The Si nanowire template is then coated with a conformal Au thin film, and the entire structure is thermally oxidized in dry O2. The Au film enhances the oxidation of Si to form SiO2-coated Au wires that have the same diameter variation as the starting Si wires. Additional thermal treatment causes the Au wires to break into a linear chain of Au nanoparticles, where the Au particle diameter and interparticle spacing is determined by the starting diameter of the Si wire, the modulation wavelength, and the Au film thickness. This method was used to fabricate 1D Au nanoparticle arrays with uniform particle diameters between 88 nm and 200 nm and interparticle spacing between 230 nm and 630 nm. Optical absorption measurements of the 1D particle arrays dispersed in solution show a peak at 550 nm, which corresponds to the plasmon resonance wavelength of Au. Individual 1D Au nanoparticle arrays are characterized using electron energy loss spectroscopy (EELS) and energy filtered transmission electron microscopy (EFTEM), which show that the plasmonic properties of the Au nanoparticles are uniform along the 1D array.
9:00 PM - CC13.3
Bifunctional Silica Sphere Encapsulating CdSe/CdS Quantum Dots and Maghemite Nanoparticles.
Wooyoung Park 1 , Ho Seong Jang 1 , Kyoungja Woo 1
1 , Korea Institute of Science and Technology, Seoul Korea (the Republic of)
Show AbstractFunctional nanocomposites consisting of silica and quantum dots (QDs) have been widely studied due to high stability and bio-compatibility of silica and unique optical properties of QDs. Also, magnetic silica microspheres have been spotlighted in various fields such as bio-separation, bio-imaging, and so on. Recently, bifunctional silica composites started to be actively studied, since silica nano/micro composites including both QDs and magnetic nanoparticles can provide bifunctionality at the same time.In this study, we synthesized superparamagnetic silica spheres with strong photoluminescence. These bifunctional silica spheres are composed of a silica core, a QD and maghemite nanoparticle (MNP) layer, and silica shell. Due to a CdSe/CdS QD layer located at equidistance from the center in the silica sphere, the silica composite showed enhanced PL intensity compared with CdSe/CdS QDs. Additionally, supplement of maghemite nanoparticles in the CdSe/CdS QD layer let the silica composite spheres show bifunctionality of luminescence and superparamagnetism. The saturation magnetization value of the silica sphere encapsulating CdSe/CdS and maghemite nanoparticles varied from 0.15 emu/g to 0.62 emu/g with increasing the ratio of MNPs to CdSe/CdS QDs.In summary, we will present various sized silica spheres encapsulating QDs and MNPs and discuss their luminescent and magnetic properties. Since these silica composite spheres provide advantages such as structural stability, enhanced PL intensity, and superparamagnetism, they can be a favorable candidate for several applications such as biological labeling, bio-separation, and so on.
9:00 PM - CC13.30
Routes to Optimizing the Photoluminescence and Electroluminescence Quantum Yields of Plasma Produced Silicon Nanocrystals.
Rebecca Anthony 1 , David Rowe 1 , Matthias Stein 2 , Jihua Yang 1 , Uwe Kortshagen 1
1 Mechanical Engineering, University of Minnesota, Minneapolis, Minnesota, United States, 2 , University of Duisburg-Essen, Duisburg Germany
Show AbstractThe pursuit of efficient and sustainable light emitting technologies has led to great interest in the optical characteristics of silicon nanocrystals (SiNCs). Our group recently demonstrated the potential of SiNCs for electroluminscence with devices that achieved 9% external quantum efficiency. In order to attain these record efficiencies for any nanocrystal light emitting device reported to date, the photoluminescence quantum yield of SiNCs needed to be optimized. In this presentation, we discuss the means that we have found to be necessary in order to achieve optimal photoluminescence quantum yields from SiNCs. The nonthermal plasma synthesis used in our approach has been well-studied for synthesis of luminescent SiNCs, yet we found that some crucial parameters were not yet understood and remained poorly controlled. Here we present our recent work on the effect of the plasma afterglow on the efficiency of photoluminescence from SiNCs produced in a nonthermal plasma reactor. We studied this issue by injecting different gases into the afterglow and examining the surfaces and the photoluminescence quantum yields of the SiNCs. We found that hydrogen injection is crucial to high-efficiency photoluminescence from SiNCs, likely due to the passivation of surface defects. This discovery will be instrumental in understanding the role of the SiNC surface in photoluminescence efficiency and in the design of reactors for synthesis of high-quality SiNCs. Primary support for this work was received from the National Science Foundation (NSF) Award Number ECCS-0925624. Partial support was also received from the NSF MRSEC Program under Award Number DMR-0819885.
9:00 PM - CC13.31
THz Dielectric Anisotropy of Metal Slanted Columnar Thin Films.
Tino Hofmann 1 , D. Schmidt 1 , A. Boosalis 1 , P. Kuehne 1 , R. Skomski 2 , C. Herzinger 3 , J. Woollam 3 , M. Schubert 1 , E. Schubert 1
1 Department of Electrical Engineering, University of Nebraska-Lincoln, Lincoln, Nebraska, United States, 2 Department of Physics and Astronomy, University of Nebraska-Lincoln, Lincoln, Nebraska, United States, 3 , J.A. Woollam Co. Inc., Lincoln, Nebraska, United States
Show AbstractSculptured thin films (STFs) are artificially made materials with three-dimensional, highly spatially coherent arrangements of nanostructures. Contemporary interest in materials for terahertz (THz) electronic, optoelectronic, and optical applications is redrawing attention to STFs that may enable designed optical properties for the THz frequency region.We report on the anisotropic optical dielectric functions of a metal (cobalt) slanted columnar thin film deposited by electron-beam glancing angle deposition for the THz frequency domain using generalized spectroscopic ellipsometry. A simple anisotropic Bruggeman effective medium dielectric function homogenization approach is successfully employed to describe the observed optical response. This approach describes isolated, electrically conductive columns which render the thin film biaxial (orthorhombic). We find that probed at large wavelengths in the THz frequency range, the Bruggeman depolarization factors render the shape of the polarization induced by the nanocolumns almost as a sphere, however, with subtle differences along and perpendicular to the columnar axis. These subtle differences are responsible for the anisotropic dielectric functions, which differ all drastically from the corresponding bulk (cobalt) dielectric properties. Furthermore, we will demonstrate that the anisotropic optical response of STFs changes drastically as the function of the dielectric properties of the ambient of the nanocolumns.The anisotropic Bruggeman effective medium approach predicts upon slight modifications of Drude, fraction and/or depolarization parameters that targeted optical properties of STFs in the THz range can be achieved by variation of slanting angle, lateral column density, and material.
9:00 PM - CC13.32
Charge Transport in Lead-Salt Nanocrystal Thin Films Coupled by Variable Length Linker Molecules.
Kevin Whitham 1 , Byoungnam Park 2 , Tobias Hanrath 2
1 Materials Science and Engineering, Cornell University, Ithaca, New York, United States, 2 Chemical and Biomolecular Engineering, Cornell University, Ithaca, New York, United States
Show AbstractSemiconductor nanocrystal quantum dots (NQDs) have emerged as versatile building blocks for a range of optoelectronic applications. Advances in the synthetic control over the particle size, shape, and composition have introduced new design principles for materials with optical and electronic properties by design. The vast majority of the proposed technologies depend critically on efficient electronic coupling, both between the quantum-confined NQDs themselves and to external microscopic contacts. Despite continued progress in understanding as well as controlling the basic charge transport processes, significant knowledge gaps with regards to the fundamental transport mechanism and the role of chemical and physical properties of the interface on electronic coupling remain. We investigated charge transport in PbSe NQD films treated with both conjugated and unconjugated dithiol molecular linkers including: ethanedithiol, benzene-dithiol, biphenyl-4,4’-dithiol, terphenyl-4,4’-dithiol, and 4,4’-dimercaptostilbene. We use the field-effect transistor as a model to measure and compare carrier mobility and trap density as a function of ligand length and conjugation. To a first approximation, the extent of electronic coupling is expected to decrease exponentially with increasing inter-NQD separation. Surprisingly, we found that transport in films treated with linkers of similar length, namely biphenyl-4,4’-dithiol and 4,4’-dimercaptostilbene show markedly different behavior. These results suggest that electronic coupling between NQDs is also influenced by the chemical nature of the cross linking molecule. Moreover, our results indicate a fundamental difference in the energy landscape due to the alkyl moiety of dimercaptostilbene, or due to defect related traps or ligand density.
9:00 PM - CC13.33
Growth of Three-Dimensional Complex Structure of Uniform ZnO Nanorods on Metal Grid.
Satyaprakash Sahoo 1 , J. Scott 1 2 , Akhilesh Arora 1 3 , Ram S. Katiyar 1
1 Physics, University of Puerto Rico, San Juan United States, 2 Department of Physics, Cavendish Laboratory, University of Cambridge , Cambridge United Kingdom, 3 Condensed Matter Physics Division , Indira Gandhi Centre for Atomic Research, Kalpakkam India
Show AbstractWe fabricate three-dimensional fractal structures of ZnO using vapor-liquid-solid (VLS) technique and simply choosing suitable metal grid as substrate. From the morphological studies using the FESEM and TEM it has been found that the fractal structures are formed by highly uniform hexagonal nanorods of ZnO which are connected by nano-point contacts. These nano-point contacts are stable enough to support a large three dimensional microstructure. Based on the structural characterization the possible growth mechanism of these fractal structures has been explained by analogy with that of metal fractals formed by electrochemical deposition. Raman scattering and room temperature photoluminescence studies confirm high crystallinity of ZnO nanorods. We believe that these three-dimensional morphologies may be an alternative to nanorods in nano-mechanical reinforcement.
9:00 PM - CC13.34
Synthesis and Thermoelectric Properties of Mg2(Si, Sn) Solid Solutions.
Fang Hu 1 , Kamyar Ravaji 1 , Choongho Yu 1
1 , Texas A&M University, College station, Texas, United States
Show AbstractMagnesium silicide solid solutions have great potentials for efficient thermoelectric materials in a medium to high temperature range due to their availability and high stability at high temperatures. The environmentally friendly n-type Mg2(Si, Sn) thermoelectric dual phase bulk materials were synthesized by direct melting under inert gas atmosphere protection and by current assisted hot press sintering in air. The aim is to get phonon scattering centers such as nanoparticles embedded on the phase boundaries and lamellar structures which can reduce the thermal conductivity to improve the value of the dimensionless parameter called “figure of merit” or ZT. The electrical conductivity, the Seebeck coefficient, and thermal conductivity were measured as functions of temperature from room temperature to 600 °C. The nano structure size, size distribution as well as crystallinity were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM) and x-ray diffraction (XRD). The chemical composition was examined by energy-dispersive x-ray spectroscopy (EDS). Furthermore, the influence of doping materials, such as antimony, copper, silver, and tungsten on thermoelectric properties of magnesium silicide has been compared as well.
9:00 PM - CC13.35
Photophysical Properties of Surface Modified PbSe Quantum Dots.
Barbara Hughes 2 1 , Justin Johnson 1 , Daniel Ruddy 1 , Arthur Nozik 1 2 , Matthew Beard 1
2 Department of Chemistry and Biochemistry, University of Colorado, Boulder, Boulder, Colorado, United States, 1 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractMany QD properties such as photoluminescence, carrier mobilities, andelectron- hole-pair multiplication, depend heavily on the QD surface. It is well accepted that as-made, spherical, PbSe QDs are cation-rich. With a stoichiometric core, this would allude to a completely Pb-terminated surface. It has been shown that this Pb-oleate surface termination leads to the production of non-radiative traps within minutes of oxygen exposure. In order to explore the origin of the secondary emission states in PbSe QDs as well as the creation of non-emitting traps, this study attempts to change the anion/cation ratio at the QD surface. In order to shift the anion/cation ratio in favor of an anion-rich QD, it is necessary to passivate the large Se anion at the particle surface. An alkyl selenide compound was prepared and used in both the preparation of PbSe QDs as well as in surface treatments for ligand exchange on oleate-capped QDs. This alkyl selenide, which binds to surface Pb's through a bond more covalent in character than traditional ligands, appears to slow oxidation and thereby the production of non-radiative surface trapping.
9:00 PM - CC13.4
Hybrid Humic Acid–Gold Nanoparticle Composite Materials for Sensitive Detection of Organic Dyes by Surface-Enhanced Raman Scattering.
Paola Corio 1 , Cintia Petroni 1 , Gustavo Andrade 2
1 , University of São Paulo, São Paulo Brazil, 2 Chemistry, Universidade Federal de Juiz de Fora, Juiz de Fora, MG, Brazil
Show AbstractOrganic dyes are an important class on environmental pollutants, and the nature of the interaction of such species with humic acid strongly determines their environmental behavior and fate. This work investigates different approaches for the synthesis of humic acid-gold nanoparticles (HA-AuNP), the performance of these particles as sensors for trace determination of organic dyes and the nature of their chemical interaction with humic acids by surface-enhanced Raman scattering (SERS). The Au nanoparticles were obtained by direct fabrication in aqueous media according to a modified procedure as proposed by Alvarez-Puebla et al.1.The synthesis were carried out in the presence of humic acid at various pH values, and in the presence and absence of sodium citrate. In this approach, humic acid serves different purposes. In the synthesis of the nanoparticles, they serve as reducing agent and for surface stabilization, preventing coalescence of the nanoparticles in aqueous media. Considering the use of the nanoparticles as SERS-active substrates, the humic acid serves as an extraction phase associated with the SERS platform, favoring the interaction of the organic dyes with the metallic nanoparticles. This approach can be used to enhance the sensitivity and selectivity of SERS technique and avoid interference from other species in solution. It is also important to mention that the humic acid gives rise to a very weak Raman spectrum, and therefore does not interfere significantly in the spectroscopic detection of the species of interest. The obtained HA-AuNPs were characterized by electronic spectroscopy, scanning electron microscopy and SERS, in order to establish a correlation between their morphology, surface plasmon, and their potential use as SERS platforms. The HA-AuNPs have been applied as SERS substrates for two textile dyes, Congo red (an anionic dye) and Nile Blue (a cationic dye). The recorded Raman spectra are, in these cases, assigned to the complex formed by the humic acid and each of the species of interest. The obtained results revealed a significant dependence on the morphological and electronic properties of the HA-AuNPs with the synthesis procedure, and also a very strong interaction between the nanoparticles and the Nile blue cationic dye. We conclude that HA-AuNP may provide a valuable approach for the spectroscopic characterization of environmentally relevant species and their chemical interaction with humic acids, through the use of surface enhanced Raman spectroscopy. 1. R. A. Alvarez-Puebla, D. S. Santos, R. F. Aroca, The Analyst, 2007, 132, 1210-1214.
9:00 PM - CC13.5
Highly Stable Multidentate Polyzwitterionic Quantum Dot Ligands for Long-Term Bioimaging.
Emerson Giovanelli 1 , Eleonora Muro 1 , Gary Sitbon 1 , Alexandra Fragola 1 , Thomas Pons 1 , Nicolas Lequeux 1 , Benoit Dubertret 1
1 , LPEM/ESPCI, Paris France
Show AbstractThe use of semiconductor quantum dots in live-cell imaging requires their complete solubility in water as well as an excellent compatibility with biological media. However, typical quantum dot syntheses provide fluorescent nanocrystals capped with hydrophobic ligands. To make them water-soluble, we turned to a cap exchange, consisting in the replacement of original ligands by hydrophilic ones, bearing a chemical function able to bind to the nanocrystal surface. But the design of these new ligands is also guided by further needs for biological applications of quantum dots, namely: small size; stability over a large pH range, at elevated salt concentrations and in a cellular medium; low non-specific adsorption; and possible functionalization afterwards. To match these criteria, we first developed a zwitterionic ligand connecting a dithiol (as linking function) to a sulfobetaine part (as aqueous solubility promotor). CdSe/CdS/ZnS core-shell quantum dots capped with this ligand exhibited all of the expected properties. Nevertheless, such a bidentate ligand showed insufficient stability at high nanoparticle dilutions.In order to improve ligand stability, we synthesized a hydrophilic polymer based on the radical random copolymerization of two methacrylamides: one containing the dithiol function, the other including the sulfobetaine group. Such a multiplication of attachment points and hydrophilicity-promoting groups in the same structure covering the quantum dot surface proved to be highly efficient. Monitoring the absorbance of diluted quantum dots capped either with our first zwitterionic ligand or the polymeric one demonstrated indeed the superiority of the latter in terms of colloidal stability. This exceptional behavior was verified regardless of the pH or salt concentrations, and DLS measurements showed a suitable compact size for the nanoparticles. Moreover, easy subsequent functionalization of the polymeric ligand could be achieved by copolymerizing the above mentioned monomers with another methacrylamide, ending with a reacting group. As an example, the resulting terpolymer was coupled with a fluorescein-type dye and capped onto quantum dots. This led us to determine the desorption rate constant of the terpolymer in the presence of a great amount of the bidentate ligand as a competitor. The measure confirmed the outclassing stability of the polymer, which resulted also in an increased intracellular stability of the corresponding quantum dots, making this multidentate polyzwitterion a remarkable ligand for long-term live-cell imaging experiments based on fluorescent nanocrystals.
9:00 PM - CC13.6
Structural Characterization of Silver NPs Modified Titanosilicate ETS-10.
Sezin Galioglu 1 , Burcu Akata 1 2
1 Micro and Nanotechnology, Middle East Technical University, Ankara Turkey, 2 Central Laboratory, Middle East Technical University, Ankara Turkey
Show AbstractETS-10, which is a synthetic microporous material (pore size 4.9 x 7.6 Å) consisting of uniquely arranged building units to form semiconducting -Ti-O-Ti-O-Ti- wires that run in the crystal in a and b directions, has been of interest in optoelectronic and photovoltaic applications due to its combined microporous structure with its semiconductor electronic behavior. Different kinds of nanoparticles, such as metal nanoparticles, quantum dots can be incorporated into ETS-10 in order to use them in photocatalysis, adsorption and photovoltaic applications [1-3].Silver NPs modified ETS-10 crystals were prepared and characterized in detail to investigate the interaction of silver nanoparticles with the synthesized titanosilicate ETS-10 crystals. For this purpose, as-prepared, silver ion-exchanged, and silver NPs modified ETS-10 (with three molar concentrations) have been characterized using, XRD, SEM, XPS, UV/VIS Spectroscopy and Raman Spectroscopy. The silver NPs modified ETS-10 was prepared by the ion-exchange of Ag+ ions with the extra framework Na+ and K+ ions in ETS-10 using different AgNO3 concentrations followed by the reduction of the Ag+ ions within the cavities of ETS-10 by using NaBH4 solution. The chemical reduction method seems to form silver nanoparticles ‘inside’ the ETS-10 material with respect to heat treatment methodology. XRD results showed that after all treatments (ion-exchange, reduction) ETS-10 samples remain crystalline, and ion-exchange occurred with ‘site selectivity’. UV-VIS studies indicated the formation of silver nanoparticles. Furthermore, the broadness of SPR band indicated different size of silver nanoparticles that formed. XPS studies confirmed the formation of silver nanoparticles within ETS-10 samples. BEs of Ti 2p3/2, Si 2p of the samples indicated that there was no structural collapse upon ion-exchange and reduction, as well. Raman studies showed that: As the silver ion-exchange concentration increased (for ion-exchanged and reduced samples), the intensity of the band decreased. Raman results for silver NPs modified ETS-10 indicated distortion of -Ti-O-Ti-O-Ti- chains. However, the band was still observable. Structural characterization showed that incorporation of metal nanoparticles within ETS-10 effects quality of -Ti-O-Ti-O-Ti chains. The first time study of full characterization of silver NPs modified ETS-10 was done. Although there was significant distortion of titania chain was observed upon reduction with chemical methods, the Raman band at 733-739 cm-1 was observed, which indicated that the chains regarded as a 1D quantum-confined form of titania were not fully destroyed. [1] M.J. Nash, D.J. Doren, Applied Catalysis B: Environmental 88 (2009) 232–239[2] I. Tiscornia, C. Tellez, J. Coronas, and J. Santamaria, J. Phys. Chem. C 2007, 111, 4702-4709[3] P. Atienzar, S. Valencia, A. Corma, and H. Garcia, ChemPhysChem 2007, 8,1115–1119
9:00 PM - CC13.8
Degradation of Photoluminescence in Si/SiGe Three-Dimensional Nanostructures.
Nikhil Modi 1 , Leonid Tsybeskov 1 , David Lockwood 2 , Xiao Wu 2 , Jean-Marc Baribeau 2
1 Electrical and Computer Engineering, New Jersey Institute of Technology, Newark, New Jersey, United States, 2 Institute of Microstructural Sciences, National Research Council, Ottawa, Ontario, Canada
Show AbstractWe find a reversible degradation of low temperature photoluminescence (PL) in Si/SiGe three-dimensional, cluster morphology nanostructures. Under continuous excitation, the PL intensity initially decreases slowly by ≤ 10%, and then decreases much more rapidly by ~ 80%. After the PL signal stabilizes, a complete recovery requires warming the sample to room temperature. We propose that charge accumulation and Auger auto ionization enhancement at the Si/SiGe heterointerface with presumably a type II energy band alignment are responsible for this phenomenon. More specifically, we suggest that SiGe cluster charging is accompanied by carrier diffusion to, and capture on, deep level traps associated with lattice mismatch induced dislocations within the Si substrate and/or surface defect states.
9:00 PM - CC13.9
Toward Nano-Device Applications of Metals and Metal Oxides Nano-Clusters.
Chi Won Ahn 1 , Il-Suk Kang 1
1 , NNFC at KAIST, Daejon Korea (the Republic of)
Show AbstractSize and shape control of active materials on nano-device is critical technology to enhance their performance. We review on the fabrications of nano-electronic devices using nano-clusters as active building blocks of nano-devices. Metal nanoclusters were synthesized by inert-gas condensation in a sputtering reactor. Techniques described include those based on percolation, aggregation, and oxidation of nano-clusters [1]. We then review the application of cluster techniques to designs of functional nanostructured devices [2-4].The high retention nonvolatile memory (nanofloating gate memory) with nanostructure composed of clusters of different sizes. And we fabricated a nanofloating gate memory based on ZnO-TFT for transparent and flexible nano-electronic devices.Plasmon-sensitive device is facilely fabricated by depositing copper clusters to the brink of percolation. The results prove that photon-induced surface plasmons contribute to electron transport between clusters. Therefore, the device can detect the surface plasmon resonance by simply monitoring the current.Graphene gas sensors (specifically for nitrogen dioxide) are fabricated using a process based on the oxidation of tin clusters. The reduced graphene oxide (rGO) without cluster showed p-type semiconducting behavior towards the gas, but there was little recovery. As the stannic oxide nanocluster deposition time increased to the percolation, the recovery increased. This is because the rGO forms Schottky junctions with n-typed stannic oxide clusters. However, after the percolation threshold, another pathway through stannic oxide clusters was formed between the contacts. Finally, both of the sensitivity and the recovery decreased.As a solar application, the graphene has multi-functions, such as a component of Schottky junction, hole transporting layer, and transparent electrode and nanoclusters have the light trapping not only by scattering but also by the excitation of localized surface plasmons.[1] I.-S. Kang et al., J. Nanosci. Nanotechnol. 10, 3671 (2010).[2] K. Y. Yang et al., Opt. Express 18, 16379 (2010).[3] I.-S. Kang et al., Nanotechnology 22, 254018 (2011).[4] I.-S. Kang et al., Appl. Phys. Lett. 98, 212102 (2011).