Symposium Organizers
Eric A. Stach Purdue University
Curtis R. Taylor Virginia Commonwealth University
Zhiming M. Wang University of Arkansas
Qi-Kun Xue Tsinghua University
M1: Synthesis & Characterization of Quantum Dots - Chemical
Session Chairs
Alexander Govorov
Eric Stach
Monday PM, November 27, 2006
Room 208 (Hynes)
9:00 AM - **M1.1
Engineering Nanocrystal Solids for Solution-Processed Field-Effect Transistors, Floating Gate Memories, Solar Cells and Thermoelectric Devices
Dmitri Talapin 1 2 , Marcus Scheele 1 , Elena Shevchenko 1 , David Mitzi 2 , Christopher Murray 2
1 The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 2 IBM Research Division, T. J. Watson Research Center, Yorktown Heights, New York, United States
Show Abstract9:30 AM - M1.2
CdSe-core Quantum Dots with Novel Structural and Electronic Properties
Rodion Belosludov 1 , Hiroshi Mizuseki 2 , Atsuo Kasuya 3 , Michael Philpott 2 , Yoshiyuki Kawazoe 2
1 ARCMG, Institute for Materials Research, Tohoku University, Sendai, Miyagi, Japan, 2 Institute for Materials Research, Tohoku University, Sendai, Miyagi, Japan, 3 Center for Interdisciplinary Research, Tohoku University, Sendai, Miyagi, Japan
Show AbstractDue to the bright fluorescence, narrow emission, broad UV excitation and high photostability the CdSe-core quantum dots (QDs) have been shown to be useful as an alternative to conventional organic fluorophores for use in biological imaging. Several groups have demonstrated the use of such QDs for fluorescence imaging in vivo [1]. The currently used CdSe-core QDs have are crystalline in structure and there is a potential cyclotoxicity that is correlated with the liberation of free Cd2+ ions due to deterioration of the CdSe lattice. One solution of this problem is a design and synthesis of new nanocomposites those are more stable than crystalline QDs. Recently our group used first-principles calculations to predict the existence of (CdSe)n (n=33 and 34) nanoparticles which were also isolated from a synthesis by mass spectrometry [2]. These nanoparticles do not adopt the crystal form of CdSe, resembling more closely the carbon fullerenes, are remarkably stable. In order to increase the biocompatible efficiency the proposed CdSe nanoparticles the following shell-core QDs (M1X1/M2X2)n (M1=Zn, M2=Cd, and X1,2=S, Se) have been investigated. Ab initio ultrasoft pseudopotential plane wave method with generalized gradient approximation for the exchange and correlation energy has been used for simulation of QD nanoclusters. As a basis for initial structure, the stable configuration of fullerene-like (CdSe)34 ((CdSe)28 as shell and (Cd6Se)6 as core) has been selected [2]. Due to bond difference between Zn-S, Zn-Se in shell and Cd-S, Cd-Se in core, the core part has been substituted from (M2X2)6 to (M2X2)4. The new shell-core particles (ZnX1)28/(CdX2)4 are found to be energetically stable and the outer cage structure based on ZnS and ZnSe are undistorted. These results indicate the possibility of formation stable shell-core nanoparticles with small concentration of Cd which can significantly reduce the toxic effect of fluorescence II-VI semiconductor QDs. We also tried to develop the basic strategy for building a series of clusters of greater size with one basic design feature evident in current systems namely core and shell(cage) as in the case of (CdSe)13 and (CdSe)34 proposed in [2]. We have found design rules for several cage systems that predict their chemical stochiometry and dimensions. These rules show how to construct several series of cages with octahedral symmetry Oh. These cages can reshape to tetrahedral symmetry the Td subgroup of Oh without any bond fission. With the help of these rules the schedule of tasks shifts away from random search to a more methodical approach. New tasks include a stronger focus on finding cores that can stabilize a given series of cages. [1] M. E. Akerman et al., Nat. Acad. Sci. USA 2002, 99, 12617.[2] A. Kasuya et al., Nature Materials 3, 99-102, 2004.
9:45 AM - M1.3
High Quantum Yield Blue-Emitting Nanocrystals for Efficient Quantum Dot Light Emitting Diodes
Jonathan Halpert 1 , Pollina Anikeeva 2 , Moungi Bawendi 1 , Vladimir Bulovic 2
1 Chemistry, MIT, Cambridge, Massachusetts, United States, 2 EECS, MIT, Cambridge, Massachusetts, United States
Show AbstractWe present bright and efficient electroluminescent (EL) blue light emitting diodes (LED) using blue-emitting colloidal quantum dots (QD) as the emissive layer. Devices are constructed with organic hole and electron transport materials that surround an electroluminescent layer of ZnCdS/ZnS core/shell QDs in a sandwich structure, as previously reported [Coe et al., Nature 2002]. Quantum dots are prepared using synthetic methods adapted from Knoll et al., J.Am.Chem.Soc. 2003 and overcoated in a two-pot synthesis using diethyl zinc and hexamethyldisilathiane (TMS2-S). The synthesized materials emit at peak wavelength of 440nm with a solution quantum yield of >50%. The QD-LEDs are color saturated blue emitters with minimal organic emission and CIE coordinates of (0.19, 0.11), capable of video brightness and 0.4% external quantum efficiency at 8V and 0.6 mA/cm2. These new devices significantly improve on the efficiency of previously reported blue QD LEDs and represent the final step in creating full-spectrum white light QD-LEDs as well as blue sub-pixels in red, green and blue (RGB) QD-LED displays.
10:00 AM - M1.4
One-pot Synthesis of Ag2S Quantum Dots and Their Thin Film Devices via Selective Degradation Process of Single Precursor Molecules
Hee Cheul Choi 1 , Qun Tang 1
1 Department of Chemistry, Pohang University of Science and Technology, Pohang Korea (the Republic of)
Show AbstractSelective formation of metallic Ag and semiconducting Ag2S nanocrystals has been achieved via a modified hot-injection process from a single-source precursor molecule, Ag(SCOPh) which can potentially generate both [Ag] and [AgS] fragments simultaneously. When the precursor molecules are injected into a preheated reaction system at 160 oC, spherical Ag2S nanocrystals are directly obtained even without a molecular activator, such as alkylamines. Mixtures of Ag and Ag2S or pure metallic Ag nanocrystals are obtained if the precursor molecules are injected at lower or room temperature. These results are attributed to the direct transfer of thermal energies to precursor molecules, which are enough to dissociate S-C as well as Ag-S bonds simultaneously. Such synthetic strategy has been further applied for the facile formation of Ag2S nanocrystal thin films on solid substrates, which are fabricated into three-electrode transistor type devices. As one of the potential applications, highly responsive optoelectric phenomena from Ag2S device have been demonstrated.References(1) Tang, Q.; Song, H. J.; Choi, H. C. Adv. Mater. 2006, Submitted.(2) Tang, Q.; Yoon, S. M.; Yang, H. J.; Lee, Y.; Song, H. J.; Byon, H. R.; Choi, H. C. Langmuir 2006, 22, 2802.
10:15 AM - M1.5
Synthesis of Large Uniform Arrays of III-oxide and III-Nitride Nanocrystals
Sreekar Bhaviripudi 1 , Jifa Qi 1 , Angela Belcher 1 2
1 Materials science and Engineering, MIT, Cambridge, Massachusetts, United States, 2 Biological Engineering Division, MIT, Cambridge, Massachusetts, United States
Show AbstractIII-nitride materials such as GaN and InN are primary candidates for various high performance opto-electronic devices including laser diodes and blue-LEDs. We report a novel approach for synthesizing arrays of III-nitride nanocrystals on various substrates. First, large uniform arrays of Ga2O3, In2O3 and (Ga-In) oxide nanoparticles with average diameters ranging between 2-5 nm have been synthesized using block copolymer micellar templates. Then, these arrays of nanoparticles were treated with ammonia in the temperature range of 350-700 °C and respective nitride nanocrystals were formed, as confirmed by TEM and XPS analysis. Interestingly, at higher temperatures (>800 °C) the Ga2O3 nanoparticles function as self-catalysts for the growth of GaN nanowires.
10:30 AM - M1.6
Multicomponent Nanoparticle Periodic Arrays.
Elena Shevchenko 1 , Dmitri Talapin 1 2 , A. Alivisatos 1 , Stephen O'Brien 3 , Christopher Murray 2
1 The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 2 IBM Reasearch Division, T. J. Watson Research Center, Yorktown Heights, New York, United States, 3 Department of Applied Physics & Applied Mathematics, Columbia University, New York, New York, United States
Show Abstract10:45 AM - M1.7
Coherently Embedded Quantum Dots by Heteroepitaxy of Immiscible Semiconductors
Wolfgang Heiss 1 , Heiko Groiss 1 , Erich Kaufmann 1 , Günter Hesser 1 , Michaela Böberl 1 , Gunther Springholz 1 , Friedrich Schäffler 1 , Roman Leitsmann 2 , Luis Ramos 2 , Friedhelm Bechstedt 2 , Kazuto Koike 3 , Hisashi Harada 3 , Mitsuaki Yano 3
1 Institut für Festkörperphysik, University Linz, Linz Austria, 2 Institut für Festkörpertheorie und -optik, Friedrich-Schiller-Universität , Jena Germany, 3 , Osaka Institute of Technology, Osaka Japan
Show AbstractAs yet, efficient fabrication of semiconductor quantum dots (QD) is based either on chemical synthesis of colloidal nanocrystals, or on strain-driven 3D growth of semiconductor heterostructures. Distinct disadvantages are the organic shell of the former, which impedes carrier injection, and the asymmetric shape and strong interdiffusion of the latter. Here we demonstrate an entirely different approach that yields almost perfect QDs in a single-crystalline, conducting matrix. It is based on low-temperature heteroepitaxial growth of two semiconductors with a large miscibility gap, which decompose into coherently embedded QDs upon annealing. As a model system we combine the narrow-gap IV-VI semiconductor PbTe with the wide-gap II-VI compound CdTe. They have almost identical cubic lattice constants, but the different bonding configurations let them crystallize as rocksalt, and zincblende lattices, respectively, with a shared Te fcc sub-lattice. After annealing, the PbTe layer disintegrates into highly symmetric, coherent and defect-free precipitates. In comparison to bulk PbTe these QDs show drastically enhanced photoluminescence in the mid infrared range, due to quantum confinement and enhanced overlap of the electron-hole-pair wave functions. High-resolution transmission electron microscopy (HRTEM) reveals [1] that the coherently embedded PbTe QDs assume the shape of small-rhomboedric-cubo-octahedrons, which contain {100}, {110} and {111}interfaces only. These interfaces are of new types, because the adjacent crystals do not only have different chemical properties, but also possess different atomic geometries and bonding arrangements. To gain further insight into the structural and electronic interface properties, we quantitatively compare first-principles total-energy calculations in the repeated-slab approximation with multi-slice simulations of the HRTEM images. The most drastic effect occurs at the electrostatically neutral (110) interface, where we find a lateral spatial offset between the two crystal halves due to rebonding across the interface. For the two polar (001) interfaces, significantly different lattice-plane spacings are observed, depending on whether the polar CdTe (001) face is cation- or anion-terminated. The agreement between the first-principles calculations and the HRTEM data is excellent for both interface classes. Self assembly of QDs driven by lattice-type mismatch is a promising approach toward coherently embedded and highly ideal QDs. In addition, these QDs provide a model system for the physical properties of coherent interfaces with bonding mismatch. [1] W. Heiss et al, Appl. Phys. Lett. 88, 192109 (2006)
11:30 AM - M1.8
Ligand Control of the Morphology and Properties of Colloidal CdSe Nanorods.
Wei Wang 1 2 , Sarbajit Banerjee 1 3 , Shengguo Jia 1 3 , Michael Steigerwald 1 2 , Irving Herman 1 3
1 Materials Research Science and Engineering Center, Columbia University, New York, New York, United States, 2 Department of Chemistry, Columbia University, New York, New York, United States, 3 Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York, United States
Show Abstract11:45 AM - M1.9
Physical Properties of Quantum-Confined Europium Sulfide Nanocrystals.
Marcela Redigolo 1 3 , Dmitry Koktysh 2 3 , Sandra Rosenthal 2 3 , James Dickerson 1 3
1 Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee, United States, 3 Vanderbilt Institute for Nanoscale Science and Engineering, Vanderbilt University, Nashville, Tennessee, United States, 2 Department of Chemistry, Vanderbilt University, Nashville, Tennessee, United States
Show AbstractSynthesis and characterization of europium sulfide (EuS) nanoscale particles are presented. Historically, research on europium sulfide has focused on bulk (dNCs > 100 nm), nanocrystalline (2.0 nm < dNCs < 100 nm), and quantum confined (dNCs < 2.0 nm) materials. The initial studies on EuS materials focused on bulk, that is, on macroscopic crystals, micropowders, and epitaxial layers. However, interest on nanoscale EuS has increased considerably during the last 5 years. The physical properties of the nanomaterials are dominated by Eu2+ ionic transitions, which are affected directly by the nanocrystal size, due to nearest neighbor couplings, surface anisotropies, and strain effects. We believe these phenomena markedly affect the observable physical characteristics of the nanomaterials. In this study, we observed a reversal in the magnetization hysteresis curves for the materials as a function of nanocrystal size. Furthermore, we explored the size-dependent phenomenon behavior of the net magnetic dipole moment and the remnant magnetization of the nanoparticles both as the nanocrystal diameter decreased, approaching the quantum confinement regime, and as the ambient temperature changed. Electron microscopy techniques (TEM, STEM, HRTEM, FE-TEM), selected area electron diffraction, and x-ray diffraction were employed to characterize the structure of these materials. Optical spectroscopic analyses complimented these characterizations.
12:00 PM - M1.10
Luminescent Colloidal Nanocrystals Modification of Functionalized Poly(methyl Methacrylate) Based Co-polymers: Novel Functional Materials for Nano Imprint Lithography.
Michela Tamborra 1 2 , Marinella Striccoli 1 , Maria Lucia Curri 1 , Angela Agostiano 1 2 , David Mecerreyes 3 , Juan Antonio Alduncin 3 , Juan Antonio Pomposo 3 , Nikolaos Kehagias 4 , Vincent Rebaud 4 , Clivia Sotomayor Torres 4
1 IPCF, CNR , Bari Italy, 2 Department of Chemistry, Università di Bari, Bari Italy, 3 , CIDETEC, San Sebastian Spain, 4 Tyndall National Institute, University College Cork, Lee Maltings, Cork Ireland
Show Abstract12:15 PM - M1.11
Solution NMR on InP and PbSe Colloidal Nanocrystals: in situ Ligand Identification and Thermodynamic Analysis of Ligand-nanocrystal Adsorption.
Iwan Moreels 1 , Jose Martins 2 , Zeger Hens 1
1 Inorganic and Physical Chemistry, Gent University, Gent Belgium, 2 Organic Chemistry, Gent University, Gent Belgium
Show AbstractColloidal semiconductor nanocrystals or quantum dots are an important building block in bottom-up nanotechnology. They consist of an inorganic, crystalline core surrounded by a monolayer of organic ligands. As these ligands can be modified or exchanged for others, they provide a convenient way to give the quantum dots functionality. However, no generally accepted experimental method exists for identifying the ligands and studying the kinetic and thermodynamic properties of ligand-nanocrystal adsorption. In this presentation, we demonstrate that solution NMR is an extremely useful tool to investigate the ligands of colloidal nanocrystals. We will present results on InP quantum dots with trioctylphospine oxide [1] and PbSe quantum dots with oleic acid ligands, combining more advanced solution NMR techniques like 1H-13C HSQC spectroscopy and pulsed field gradient diffusion NMR. This not only leads to an unequivocal identification of the resonances of the bound ligands. It enables us to determine the diffusion coefficient of the nanocrystals in solution and to verify capping exchange procedures as well. Moreover, by calibrating the surface area of the NMR resonances using a solute of known concentration, the density of ligands at the nanocrystal surface can be quantified. In the case of InP|TOPO, this approach is used to demonstrate the presence of a dynamic equilibrium between bound and free ligands. Analysis of the corresponding adsorption isotherm - determined using NMR - leads to an estimation of the free energy of adsorption and the free energy of ligand-ligand interaction at the nanocrystals surface [2]. [1] Z. Hens, I. Moreels, J.C. Martins, ChemPhysChem 6, 2578 (2005). [2] I. Moreels, J.C. Martins, Z. Hens, ChemPhysChem 7, 1028 (2006).
12:30 PM - M1.12
Amine-functionalized Boron Nitride Nanotubes as Templates for Quantum Dot Array
Takashi Ikuno 1 2 , Toby Sainsbury 1 2 , David Okawa 2 3 , Jean Frechet 1 3 , Alex Zettl 1 2
1 Materials Sciences Div., Lawrence Berkeley National Laboratory, Berkeley, California, United States, 2 Dept of Physics, UC Berkeley, Berkeley, California, United States, 3 Dept of Chemistry, UC Berkeley, Berkeley, California, United States
Show AbstractProgrammable self-assembly of nanoparticles is one of the most promising bottom-up approaches to the synthesis of nano-electronic and nano-optelectronic devices such as single electron transistors, molecular sensors, and plasmon waveguides. For this purpose, a template is needed which determines the desired device shape. Boron nitride nanotubes (BNNTs) are ideal materials for this purpose because (1) BNNTs have a quasi-one-dimensional structure similar to that of carbon nanotubes, (2) BNNTs are insulators, with a uniform wide bandgap (~5.5 eV) regardless of the radius, chirality or the number of walls of the tube, and (3) BNNTs have a high resistance to oxidation and are structurally stable and inert to most chemicals. Although BNNTs have attractive properties as templates, the surface of BNNTs has low chemical reactivity and low wettability. In particular, it is difficult to chemically alter the surface of pristine BNNTs under thermal equilibrium condition such as in solution based reaction media. To create active surfaces that are suitable for the templated assembly of quantum dot arrays, reactive treatments under non-equilibrium condition such as glow plasma treatment and functionalization with chemically active species, are desirable.In this study, amine functional groups have been generated at the surface of BNNTs using an ammonia glow plasma treatment. A conventional microwave plasma system has been adopted for this experiment. An ammonia plasma was generated by applying microwave power of 200 W to the gas molecules and a negative bias voltage of -100V to the BNNT mats. In order to investigate the presence of amine functional groups at the surface of the BNNTs, a short chain organic molecule terminated with bromine has been coupled with the amine groups via amide formation. The modified BNNTs were then characterized using Fourier transform infrared (FT-IR) spectroscopy and electron energy loss spectroscopy (EELS) in order to confirm the presence of the molecules and hence the chemical reactivity of amine-functionalized BNNTs. By virtue of existing amine functional group at the surface, we then have confirmed that colloidal gold nanoparticles were uniformly coated on BNNTs. The nanoparticle arrays could be utilized for quantum nanodevices.
12:45 PM - M1.13
Controlled Assembly of Nanoparticles onto Carbon Nanotubes
Junhong Chen 1 , Ganhua Lu 1 , Liying Zhu 1 , Carol Hirschmugl 2
1 Mechanical Engineering, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin, United States, 2 Physics, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin, United States
Show AbstractAssembly of nanoparticles along the external surface of carbon nanotubes (CNTs) is of both fundamental and technological interest. Potentially displaying the unique properties of both CNTs and nanoparticles, the nanoparticle-nanotube hybrid structure attracts a broad range of advanced applications including nanoelectronics, chemical sensors, biosensors, catalysis, solar cells, fuel cells, and hydrogen storage. Here we report on an efficient and material-independent dry route to assemble nanoparticles onto both single-walled CNTs (SWCNTs) and multiwalled CNTs (MWCNTs) with considerable control based on electrostatic force directed assembly. Nanoparticles produced through aerosol routes or aerosolization of colloidal nanoparticles are introduced into an electrode gap in an inert stagnation flow. One electrode is the grounded metal tubing that delivers the nanoaerosol and the other electrode is a CNT-coated copper grid that is electrically biased with a dc voltage. The electric field near the CNT is significantly enhanced and nanoparticles are attracted to the external surface of CNTs. Transmission electron microscopy (TEM), high resolution TEM (HRTEM), energy dispersive X-ray Spectroscopy (EDX), and synchrotron Fourier Transform Infrared Spectroscopy (FTIR) are used to characterize the interesting hybrid structures. Potential for large-scale coating has been observed based on the low-magnification TEM images. HRTEM images show that the nonagglomerated entity of nanoparticles and the crystallinity of both nanoparticles and CNTs are preserved during the assembly. In addition, there is an intrinsic nanoparticle size selection over the assembly, which results in a narrower nanoparticle size distribution on CNTs. Moreover, the packing density and the final nanoparticle size distribution on CNTs can be controlled. A comparison of the FTIR spectra of the CNT sample before and after the nanoparticle coating suggests that the nanoparticles are attached to CNTs through van der Waals force. The unique properties of CNTs are most likely preserved with the noncovalent coating. The new method enables in-situ coating of CNTs with nanoparticles and coating CNTs with nanoparticles of multiple materials. These novel multicomponent nanostructures will open up new opportunities for many applications.
M2: Applications of Quantum Dots as Biological Probes
Session Chairs
Monday PM, November 27, 2006
Room 208 (Hynes)
2:30 PM - M2.1
Composition-tunable Quantum Dots and Their Novel Applications.
Mingyong Han 1
1 Institute of Materials Research and Engineering, National University of Singpaore, Singapore Singapore
Show Abstract2:45 PM - M2.2
Synthesis and Microstructural Analysis of Benzylthiol-functionalized Au Nanocrystals.
Floriana Vitale 1 4 , Luciana Mirenghi 1 , Emanuela Piscopiello 1 , Maria Protopapa 1 , Leander Tapfer 1 , Cinzia Giannini 2 , Antonietta Guagliardi 2 , Antonio Cervellino 2 , Giovanni Pellegrini 3 , Enrico Trave 3 , Giovanni Mattei 3 , Paolo Mazzoldi 3 , Ilaria Fratoddi 4 , Maria Russo 4
1 UTS MAT, ENEA, Brindisi Italy, 4 Dip. di Chimica, Università di Roma "La Sapienza", Roma Italy, 2 IC, CNR, Bari Italy, 3 Dip. di Fisica, Università di Padova, Padova Italy
Show AbstractWe report on the colloidal synthesis and detailed microstructural and microanalytical analysis of dodecanethiol- and benzylthiol-stabilized Au nanocrystals (NC) of diameter between 2nm and 10nm. The stable functionalized Au NC were synthesized using two different colloidal routes: a two-phase liquid-liquid method and a one-phase method. In both methods the gold nanocrystals were prepared by the chemical reduction of a metal salt precursor (in liquid phase) in the presence of the capping agent, which isolates the metal cluster preventing the growth by formation of covalent links. The size-control of the Au NC was achieved by choosing the suitable AuCl4-/thiol molar ratio. The size, strain, shape and crystalline structure of the nanocrystals were determined by a full-pattern X-ray powder diffraction analysis and high-resolution electron microscopy. In particular, the X-ray powder diffraction reveal the coexistence of cuboctahedra, formed by Au fcc crystals, Au icosahedra and decahedra (also known as Multiple Twin Particles, MPTs) characterized by five-fold axes of symmetry.The chemical environment of the Au nanocrystals and their interaction with the thiols was investigated by X-ray photoelectron spectroscopy. The experimental Au 4f7/2 peak position of these systems was found intermediate between the value 83.80eV reported for the Au0 bulk and the value 84.9eV corresponding to the AuI state associated to the covalent Au-S chemical bond. The chemical shift of the Au 4f7/2 line was interpreted considering the interaction between the thiol chains and the Au nanocrystals, resulting in a different electronic distribution of the surface region and of the core of the Au-nanocrystal. The position of the Au 4f7/2 represents the sum of two contributions: the Au-S surface bonds and Au-Au core bonds. Other considerations on the S 2p peak exclude the presence of only Au-Au bonds and reveal a different chemical behaviour of samples synthesized by double-phase and single-phase methods. The surface plasmon resonance (SPR) peak appearing in the optical absorption spectra is related to the matrix refractive index and to the parameters characterizing the metallic clusters (dielectric constant of the metal, clusters size, shape and spatial distribution) and has been monitored for each sample. Moreover, in case of capped-nanoparticles, the ratio between the capping-shell volume and the metallic cluster volume as well as the possible changes induced by the capping agent on the core charge state strongly influence the main features characterizing the SPR resonance (peak height and spectral position). The Maxwell-Garnett effective medium theory has been used in order to interpret the surface plasmon absorption resonance and to evaluate the effects of the capping agent on the core charge state. PL spectroscopy measurements show the characteristic IR emission at λ=960nm of small sized Au nanocrystals (~3nm), while at larger size the PL emission is strongly reduced.
3:00 PM - M2.3
Modified Blinking Kinetics in Solid State Quantum Dot/Conjugated Organic Polymer Composite Nanostructures.
Nathan Hammer 1 , Kevin Early 1 , Kevin Sill 2 , Ravisubhash Tangirala 2 , Michael Odoi 1 , Todd Emrick 2 , Michael Barnes 1
1 Chemistry, University of Massachusetts, Amherst, Massachusetts, United States, 2 Polymer Science & Engineering, University of Massachusetts, Amherst, Massachusetts, United States
Show AbstractFluorescence intermittency, or "blinking" in quantum dot systems has been the subject of great interest since the first observation of this phenomenon nearly 10 years ago. The stability of quantum dot fluorescence emission is especially important in the context of photovoltaic, optoelectronic, and biological applications, where device performance, or the ability to track labeled particles, is affected adversely by fluorescence intermittency. Single-molecule spectroscopy combined with atomic force microscopy measurements reveal that CdSe quantum dots functionalized with oligo(phenylene vinylene), OPV, ligands exhibit modified optical properties such as suppression of blinking when compared to conventional TOPO covered or ZnS-capped CdSe quantum dots. The blinking suppression is shown to be highly sensitive to the degree of ligand coverage on the quantum dot surface and this effect is interpreted as resulting from charge transport from photoexcited OPV into vacant trap sites on the quantum dot surface. This direct surface derivatization of quantum dots with organic ligands also enables a "tunable" quantum dot surface that allows dispersion of quantum dots in a variety of polymer supported thin films without phase segregation. This facilitates straightforward inclusion of these new hybrid materials into solid state formats and suggests exciting new applications of composite quantum dot/organic systems in optoelectronic systems.
3:15 PM - M2.4
Silica Colloids Containing Multiple Controlled Embedded Quantum Dots - A Highly Biocompatible and Photoactivable System for Various Applications
Christina Graf 1 , Sofia Dembski 1 , Tim Krüger 2 , Andrea Ewald 3 , Eckart Rühl 4 1
1 Institut für Physikalische Chemie, Universität Würzburg, Würzburg Germany, 2 Theodor-Boveri-Institut für Biowissenschaften, Biozentrum, Universität Würzburg, Würzburg Germany, 3 Abteilung und Lehrstuhl für Funktionswerkstoffe der Medizin und der Zahnheilkunde, Universität Würzburg, Würzburg Germany, 4 Institut für Chemie und Biochemie, Physikalische und Theoretische Chemie, Freie Universität Berlin, Berlin Germany
Show AbstractQuantum dots (QD) are nowadays widely used as markers in life science applications. However, the long-term stability against oxidation processes leading to the leakage of toxic ions and to the reduction of the photoluminescence (PL) are still unsolved issues for in vivo applications. We present a novel system, so-called multicore colloids, providing an excellent stability against ion leakage and therefore low cytotoxicity as well as an adjustable and highly activable PL in various media.Multiple CdSe/ZnS QD are embedded in a controlled way into silica colloids by a recently developed method.1 Controlled adsorption of QD on monodisperse silica spheres is followed by their subsequent coverage by a silica layer. The lateral distance between the QD and the thickness of the outer protective silica shell can be adjusted with nanometer accuracy. The multicore particles have a radius of 20 – 1000 nm and they show a bright PL. The stability of the multicore colloids was studied under physiological conditions. The particles show an extremely low release of toxic Cd2+ ions, which is about 20 times lower than for other water-soluble CdSe/ZnS systems.2 The exposure to ultraviolet radiation (365 nm) has only a minor influence on the release of Cd2+ from the multicore colloids. Even in the case of particles with a thin (< 5 nm) silica shell the amount of released Cd2+ was about 1000 times lower than that for ZnS/CdSe QD capped with organic molecules or coated with polyacrylate/streptavidin under similar conditions.2 Studies with living cells confirm the high biocompatibility of the multicore colloids.The photoactivation of the multicore particles depends strongly on the outer environment. We observe an up to ten times enhancement of the PL in aqueous solutions. The colloids show a long-term photostability under continuous photoactivation with ultraviolet-radiation (365 nm). A slight blueshift of the PL is observed for colloids in water with a thin 6 nm silica shell, where the PL maximum remains unchanged in samples with thicker silica shells. Obviously, the thickness-dependent permeability of the silica shell controls the influence of the outer media on the QD. Further, the control of the interparticle distance inside the silica matrix permits the investigation of QD coupling effects during photoactivation. The multicore colloids retain their bright fluorescence also inside living cells. Moreover, they can be more than four times photoactivated by laser illumination (405 nm) in living cells and remain activated for many hours. This opens novel applications in life sciences, such as diffusion studies in selected parts of living cells. The outer silica shell is well suited for controlled modifications with biomolecules. Studies on the specific functionalization of multicore colloids that specifically bind on the nucleolus of living cells are currently in progress.(1) C. Graf et al., Langmuir 2006, 22, 5604(2) A. M. Derfus et al., Nano Lett. 2004, 4, 11
4:30 PM - **M2.5
Development of Photoluminescent Colloidal Semiconductor Nanocrystals with Detailed Characterization on Structure.
Kui Yu 1 , Christopher Ratcliffe 1 , Cristina Badarau 1 , Shanti Singh 1 , Badruz Zaman 1 , Maxime Vincent 1 , John Ripmeester 1
1 National Research Council Canada , Steacie Institute for Molecular Sciences, Ottawa, Ontario , Ontario, Canada
Show Abstract5:00 PM - M2.6
Developing Semiconductor Quantum Rods as Fluorescent Biological Probes.
Aihua Fu 1 , Weiwei Gu 2 3 , Benjamin Boussert 1 , Kristie Koski 1 , Daniele Gerion 1 , Liberato Manna 1 , Mark Le Gros 2 3 , Carolyn Larabell 2 3 , A. Paul Alivisatos 1 3
1 Chemistry, Univ. of California Berkeley, University of California Berkeley, California, United States, 2 , University of California, San Francisco, San Francisco, California, United States, 3 , Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show Abstract5:15 PM - M2.7
Synthesis and Application of High Quality Nanocrystals.
William Yu 1 , Vicki Colvin 1
1 Department of Chemistry, Rice University, Houston, Texas, United States
Show Abstract5:30 PM - M2.8
Ratiometric Nanocrystal Sensors based on Resonant Energy Transfer.
Rebecca Somers 1 , Preston Snee 2 , Andrew Greytak 1 , Moungi Bawendi 1 , Daniel Nocera 1
1 chemistry, MIT, Cambridge, Massachusetts, United States, 2 Chemistry, University of Illinois-Chicago, Chicago, Illinois, United States
Show AbstractA reversible and ratiometric nanocrystal (NC) pH sensor was synthesized by tethering CdSe/ZnS NCs (also known as quantum dots) to fluorescent sensing dyes. The construct combines the advantages offered by the NCs such as broad excitation profiles and high quantum yields, with the specific sensing ability of the dye. The sensor functions via modulation of Forster Resonant Energy Transfer (FRET) between donor NCs and acceptor dyes and is found to be naturally ratiometric. The presence of an isosbestic point between the two emission maxima allows the sensor to be self-calibrating. Compared to typical sensors that display a single intensity-based response to analytes, the internal reference is advantageous as it is not sensitive to fluctuations in excitation intensity or collection efficiency. The pH-sensing construct is the first example of a CdSe nanocrystal based sensor that is both reversible and ratiometric. The synthetic approach utilized in our constructs is general and can be exploited for sensing other analytes both reversibly and ratiometrically. Progress towards incorporating the NC sensors in biological studies will also be discussed.
5:45 PM - M2.9
Enhanced Luminescence Efficiency from Hydrogel Microbead Encapsulated Quantum Dots.
Arup Neogi 1 , Brett Garner 1 , Santaneel Ghosh 1 , Jianyou Li 1 , Tong Cai 1 , Zhibing Hu 1
1 Department of Physics, University of North Texas, Denton, Texas, United States
Show AbstractM3: Poster Session: Synthesis & Characterization
Session Chairs
Eric Stach
Curtis Taylor
Zhiming Wang
Qikun Xue
Tuesday AM, November 28, 2006
Exhibition Hall D (Hynes)
9:00 PM - M3.1
Measurement of the Temperature Distribution on Resistively Heated Nanowires using CdSe/ZnS Nanocrystals.
Peter Löw 1 2 , Nobuyuki Takama 2 , Beomjoon Kim 2 , Christian Bergaud 1
1 , LAAS - CNRS, Toulouse France, 2 LIMMS - CNRS/IIS, The University of Tokyo, Tokyo Japan
Show AbstractThermal imaging of microstructures can normally be performed using infrared (IR) microscopy, giving spatial resolutions of down to a few micrometers. However, when dealing with smaller structures, e.g. nanoscale structures, the diffraction of light sets a fundamental limit to IR measurements. To perform thermal characterization of nanoscale structures, we have investigated the potential of using CdSe/ZnS semiconductor nanocrystals as optical temperature probes by exploiting their temperature dependent fluorescence [1]. To achieve locally confined heating on a nanoscale, nanowires were used as resistive heating elements.Nickel nanowires with widths down to 200 nm were fabricated using E-beam lithography and subsequently passivated with a thin sputtered SiO2 layer. A highly concentrated CdSe/ZnS nanocrystal solution was deposited onto the sample surface. As the liquid evaporated, a dense and uniform film of nanocrystals covering the entire surface was obtained.The temperature dependence of the nanocrystal film was calibrated by externally heating the sample using a microscope heating stage. Fluorescence intensities of the CdSe/ZnS nanocrystals were captured at temperatures between 30°C and 60°C with intervals of 5°C. A calibration curve of the temperature dependence could thereby be obtained. All measurements were done in ambient air.Resistive heating was achieved by applying a DC current through a nanowire. The increase in temperature resulted in a decrease of the fluorescence intensity on top of the nanowire. Using the data obtained in the calibration process, the intensity change could be converted to temperature. In the case of a 40 μm long and 500 nm wide nanowire, a current of 2.5 mA, equaling a power of approximately 6 mW, resulted in temperatures between 55°C and 65°C along the nanowire. The central part of the nanowire was observed to be hotter than the ends. To further improve the accuracy of our technique, investigations aiming to reduce effects such as photobleaching, thermal curing [2] and blinking [3] of nanocrystals are underway.Our results on nanoscale thermal characterization and nanowire heating will be used for the development of a device for thermal manipulation and study of single molecules, particularly proteins.References:[1]G. W. Walker, V. C. Sundar, C. M. Rudzinski, A. W. Wun, M. G. Bawendi, and D. G. Nocera, “Quantum-dot optical temperature probes”, Appl. Phys. Letter, vol. 83, no. 17, pp. 3555-3557, 2003.[2]V. Biju, Y. Makita, A. Sonoda, H. Yokoyama, Y. Baba, and M. Ishikawa, ”Temperature-Sensitive Photoluminescence of CdSe Quantum Dot Clusters”, J. Phys. Chem. B, Vol. 109, No. 29, pp. 13899-13905, 2005.[3]S. Hohng, and T. Ha, “Near-Complete Suppression of Quantum Dot Blinking in Ambient Conditions”, J. of Am. Chem. Soc., Vol. 126, pp. 1324-1325, 2004.
9:00 PM - M3.10
Synthesis and Characterization of Size- and Shape-controlled Bismuth nanoparticles
Seon Kim 1 , Sung Yang 1
1 Chemistry, Kyung Hee Univ., Yongin Korea (the Republic of)
Show AbstractControlling the size and shape of semi-metal nanoparticles is technologically important because of the strong correlation between these parameters and semimetal-semiconductor transition and potentially applicable to optical, electronic devices. Bi has anisotropic Fermi surface, low carrier density and small carrier effective mass, hence holds strong possibility of application to magnetic sensor or thermoelectric devices. Bismuth nanoparticles and nanorods have been synthesized by reducing Bismuth chloride with superhydride in the presence of Oleic acid and Oleyl amine The crystal structures of Bi nanoparticles were characterized by X-ray powder diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). The Reaction temperature and time played important roles in determining the size and shape of the nanocrystals. The size could be controlled over a wide range, from 10 to 70 nm by varying the reaction time and precursor concentration.
9:00 PM - M3.11
Low-temperature Synthesis of CdSe Multipod Quantum Dots.
Fotios Papadimitrakopoulos 1 2 3 , Jonathan Doll 1 2 , Kushan Biswas 1 3
1 Nanomaterials Optoelectronic Laboratory, University of Connecticut, Storrs, Connecticut, United States, 2 Polymer Program, Institute of Materials Science, University of Connecticut, Storrs, Connecticut, United States, 3 Department of Chemistry, University of Connecticut, Storrs, Connecticut, United States
Show Abstract9:00 PM - M3.12
Monoatomic Chains Deposited onto Silicon Steps Studied by Density Functional and Scattering Theories.
Anna Mazzone 1
1 IMM, CNR , Bologna Italy
Show Abstract9:00 PM - M3.13
Electronic Structure Studies of CdSe Nanocrystals Using Synchrotron Radiation
Robert Meulenberg 1 , Jonathan Lee 1 , Louis Terminello 1 , Tony van Buuren 1
1 , Lawrence Livermore National Laboratory, Livermore, California, United States
Show Abstract9:00 PM - M3.14
Spatial correlations in III-V nanostructures arrays: effect of compositional modulation and buried stress fields
Jose Bortoleto 2 , Joao Zelcovit 1 , Jefferson Bettini 3 , Monica Cotta 1
2 GPM, Unesp, Sorocaba, São Paulo, Brazil, 1 DFA, UNICAMP, Campinas, São Paulo, Brazil, 3 LME, LNLS, Campinas, São Paulo, Brazil
Show Abstract9:00 PM - M3.2
Self-Assembly of Nanoparticles and Quantum Dots at the Surface of Functionalized Boron Nitride Nanotubes.
Toby Sainsbury 1 2 , Takashi Ikuno 1 2 , David Okawa 3 1 , Jean Frechet 3 2 , Alex Zettl 1 2
1 Department of Physics, University of California, Berkeley, Berkeley, California, United States, 2 Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 3 Department of Chemistry, University of California, Berkeley, Berkeley, California, United States
Show AbstractIn recent years considerable research effort has been directed towards the synthesis, characterization and functionalization of novel nanoscale materials such as quantum dots, nanoparticles, nanorods and nanotubes. In particular the surface functionalization of many of these materials has been used as a means by which their assembly may be controlled in solution and at substrates. The controlled assembly of such materials is seen as a fundamental step towards utilization of the novel intrinsic properties which many nanoscale materials possess. In this context, the immobilization of quantum dots and nanoparticles at substrates which result in the formation of arrays of the materials is of interest. Nanotubes are one such material that may act as a substrate which can template the assembly of nanoparticles and quantum dots from solution. Such a strategy is believed to have much promise whereby the intrinsic properties of the nanotubes and particles might be utilized for a range of electronic, catalytic, sensing and materials science applications.Nanotubes of carbon and boron nitride have generated considerable interest due to their remarkable intrinsic properties and their use in a range of technological applications. A key feature which has enabled many of the developments concerning carbon nanotubes (CNTs) has been the ability for the surface of the nanotubes to be chemically modified or functionalized. While there have been significant developments concerning CNTs, there has been comparatively little progress concerning boron nitride nanotubes (BNNTs) and their integration with nanoscale materials. In this context, the modification of BNNTs which results in the generation of high densities of functional groups at the surface of BNNTs is believed to have much potential for the utilization of these materials as nanoscale templates and for the integration with host materials to form composites.Boron nitride nanotubes (BNNTs) have been functionalized by the generation of amine groups at their surface following an ammonia plasma irradiation treatment. The amine-functionalized BNNTs were covalently modified by the coupling of a series of organic molecules terminated with functional groups. The modification of the BNNTs was characterized using FT-IR and EDS. The resulting functionalized BNNTs were used to template the assembly of nanoparticles and quantum dots in solution. This results in complete monolayer coverage of the particles at the surface of the BNNTs which was characterized by TEM. This approach constitutes a basis for the preparation of highly functionalized BNNTs and for their utilization as nanoscale templates for the assembly and integration with other nanoscale materials.
9:00 PM - M3.3
Novel Synthesis of Water Soluble PbSe Quantum Dots
Lioz Etgar 1 , Efrat Lifshitz 2 , Rina Tannenbaum 3 4
1 Nanoscience and Nanotechnology Program, Technion, Haifa Israel, 2 Department of Chemistry, Technion, Haifa Israel, 3 Department of Chemical Engineering, Technion, Haifa Israel, 4 School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractThe potential use of nanoparticles in biological applications such as drug delivery has generated much interest and triggered rapidly expanding directions of research. In particular, quantum dots (QD’s) are regarded as ideal candidates as sensors in processes involving molecular recognition of biological markers. The properties of QD’s result from quantum-size confinement, which occurs when metal and semiconductor nanoparticles are smaller than their exciton Bohr radii. Hence, they are sensitive to their size and to small changes in the chemical structure of their surface. In order to utilize these nanoparticles in biological applications, they have to be compatible for use in cell biology, immunoassay and biosensors. To prepare such conjugates from nanoparticles, the nanoparticles must be stabilized by ligands that possess terminal functional groups that are available and compatible for biochemical coupling reactions. We have, therefore, studied the possibility to synthesize QD’s such as PbSe that would be compatible with an aqueous environment.In order to render the PbSe nanoparticles water soluble without changing their properties, the stabilizing ligands used in the traditional synthesis of PbSe, oleic acid, were exchanged with 2-aminoethanethiol (AET) ligands that promoted solubilization of the QD’s attracting the surrounding water molecules and essentially allowing the particles to exist in the water environment. Moreover the absorbance spectra, transmission electron microscopy (TEM) images and Fourier transform infra-red (FTIR) spectra of the PbSe QD’s in both organic media and water indicate that the morphology, size, size distribution and chemical composition of the PbSe QD's remain unchanged during the transfer to an aqueous medium.In conclusion, the ability to synthesize water soluble PbSe QD’s with excellent properties and very good size distribution, will allow them to have substantial advantages for biological applications such as biosensors and drug delivery.
9:00 PM - M3.5
Kinetic Study on the Hydrothermal Synthesis of Organic-Hybridized Cerium Oxide Nanoparticles.
Seiichi Takami 1 2 , Satoshi Ohara 2 , Takashi Naka 2 , Tadafumi Adschiri 2 , Yutaka Wakayama 1 , Toyohiro Chikyow 1
1 Advanced Electronic Materials Center, National Institute for Materials Science, Tsukuba, Ibaraki, Japan, 2 Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai, Miyagi, Japan
Show AbstractWe report kinetic study on the hydrothermal synthesis of cerium oxide nanoparticles using a flow-type reactor. The synthesis was performed in the presence of organic molecules to hybridize them on the surface of the nanoparticles. The amount of reacted cerium ion was evaluated using inductively coupled plasma (ICP) method. Arrhenius plots assuming first-order reaction showed that the presence of organic molecules slowed the consumption of cerium ion, which was plausibly caused by the capping of the growing surface of the metal oxide nanoparticles; the size of synthesized nanoparticles in the presence of the surface modifiers was much smaller than that of the products without surface modifiers. Based on these kinetic studies, we can optimize the configuration of flow-type reactors and synthetic conditions for desired products. For example, we have successfully synthesized cerium oxide nanoparticles whose diameter was ~4 nm using the flow-type reactor. Their surface was capped by organic molecules that increased the affinity with water and prevented agglomeration between them.
9:00 PM - M3.6
Aqueous Synthesis of ZnS Quantum Dots with Visible Photoluminescence
Hui Li 1 , Wan Shih 1 , Wei-Heng Shih 1
1 Materials Science & Engineering, Drexel University, Philadelphia, Pennsylvania, United States
Show AbstractThrough an environmentally friendly aqueous method, non-heavy metal zinc sulfide (ZnS) quantum dots (QDs) with visible fluorescent emission were synthesized with size 2-5 nm. The trap-state fluorescence emission is bright with a quantum yield comparable to commercial core-shell QDs. The capping molecules, including carboxyl and amino functional groups which were ready for bioconjugation, helped stabilize QDs in aqueous suspension against aggregation and photobleaching. Higher amount and longer chain of capping molecules improved the stability of QDs. The particle size of ZnS QDs could be controlled by adjusting the ratio of capping molecules to the precursor. Furthermore, excess Zn concentration could also enhance the photoluminescence intensity.
9:00 PM - M3.7
Size-selective Absorption of Colloidal CdSe Nanoparticles by Mesoporous Molecular Sieves.
Alexey Lukashin 1 , Andrey Eliseev 1 , Irina Kolesnik 1 , Dmitry Petukhov 1
1 Dept. of Materials Science, Moscow State University, Moscow Russian Federation
Show Abstract9:00 PM - M3.8
Modeling and Simulation of Quantum Dot Synthesis in Microemulsions and Liquid Crystals.
Borislava Kostova 1 , Sreekumar Kuriyedath 1 , Joungmo Cho 1 , Triantafillos Mountziaris 1 , Ioannis Kevrekidis 2
1 Chemical Engineering, University of Massachusetts, Amherst, Massachusetts, United States, 2 Chemical Engineering, Princeton University, Princeton, New Jersey, United States
Show AbstractA lattice-Monte Carlo simulation technique has been developed that describes the formation of ZnSe nanocrystals (quantum dots) inside the spherical nanodomains formed by self-assembly of a ternary system containing an amphiphilic block copolymer, a polar continuous phase (formamide) and a non-polar dispersed phase (heptane) [1,2,3]. The stochastic model describes diffusion of diethylzinc molecules that are confined in the dispersed phase, nucleation of ZnSe by a spontaneous and irreversible reaction between dielthylzinc and hydrogen selenide diffusing through the interface, as well as diffusion and coalescence of ZnSe clusters leading to the formation of a single nanocrystal per nanodroplet. The stochastic simulation was calibrated by using a deterministic diffusion-reaction model describing diethylzinc depletion from a nanodroplet due to a fast interfacial reaction. The motion of molecules and clusters was programmed according to their diffusivity, which was estimated by using the Stokes-Einstein equation. The formation of stable "magic" clusters with close-caged structure can be tracked in the simulations and the predicted size variation of the final nanoparticle particle population can be recorded. A thermal analysis of cluster-cluster coalescence was performed using a macroscopic model describing: (1) Energy released due to surface area reduction. (2) Energy accumulation in the coalescing particles that can lead to melting. (3) Energy dissipation to the surrounding medium that can lead to evaporation of the heptane and formation of a thin insulating layer of vapor around the particle (in analogy to the Leidenfrost effect in boiling). The simulations reveal the possibility of local melting and recrystallization of nanoparticles, thus explaining the formation of single crystals in a medium that is at room temperature. A microfluidic system for quantum dot synthesis in microemulsions has been designed by coupling a model describing the two phases of the microemulsion as interpenetrating continua with the mesoscopic lattice-Monte Carlo model of quantum dot formation inside a single nanodroplet. The models were coupled through the interfacial flux of hydrogen selenide. The effects of operating conditions, such as inlet flow rate and supply of hydrogen selenide to the microemulsion, on the growth rate of clusters and nanocrystals have been studied using this multi-scale modeling approach.References1. G.N. Karanikolos, et al., Langmuir, 20(3), 550-553 (2004) 2. G.N. Karanikolos, et al., Nanotechnology, 16, 2372-2380 (2005)3. G.N. Karanikolos, et al., Nanotechnology, 17, 3121-3128(2006).
9:00 PM - M3.9
Modelling the Effect of Shape on the Phase Stability of Colloidal Nanoparticles.
Amanda Barnard 1 , Rakesh Yeredla 2 , Huifang Xu 2
1 Department of Materials, University of Oxford, Oxford United Kingdom, 2 Department of Geology & Geophysics and Materials Science Program, The University of Wisconsin, Madison, Wisconsin, United States
Show AbstractIn recent years it has been found that many colloidal nanoparticles exhibit a reversal of phase stability with respect to the bulk analogues. In addition to this, the same colloidal nanoparticles may form in a variety of different shapes, depending in part upon the size and phase. Examples include the tetragonal and monoclinic polymorphs of zirconium dioxide, and the anatase and rutile polymorphs of titanium dioxide. Although considerable attention has been given to finding methods for controlling the phase of these nanoparticles, the role of nanomorphology in affecting the size-dependent phase transition has been largely ignored. To address this issue, we have used a shape-dependant thermodynamic model to investigate the relationship between nanomorphology and phase stability. Our results provide the free energy of formation for each polymorph with a variety of shapes, and show how the transition size is intrinsically linked to prevalence of particular surface facets. From these results it becomes apparent that variations in the thermochemical results reported in the literature may also be partially attributed to variations in nanocrystal shapes.
Symposium Organizers
Eric A. Stach Purdue University
Curtis R. Taylor Virginia Commonwealth University
Zhiming M. Wang University of Arkansas
Qi-Kun Xue Tsinghua University
M4: Applications in Electronics and Optoelectronics
Session Chairs
Tuesday AM, November 28, 2006
Room 208 (Hynes)
9:00 AM - **M4.1
Colloidal Nanocrystals of Complex Shape: Synthesis, Properties, Applications.
Armand Alivisatos 1
1 Materials Sciences, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show Abstract9:30 AM - M4.2
Efficient White Light QD-LEDs based on Red, Green and Blue Emitting Colloidal Quantum Dots
Polina Anikeeva 1 3 , Jonathan Halpert 2 , Moungi Bawendi 2 , Vladimir Bulović 1
1 Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 3 Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractWe demonstrate, for the first time, efficient stable white light LEDs based on simultaneous electroluminescence from a monolayer of mixed red, green and blue emitting quantum dots (QDs) in a hybrid organic/inorganic structure. Our devices exhibit external quantum efficiencies of 0.36%, the highest value so far reported among white hybrid organic/QD LEDs, and CIE coordinates of (0.35, 0.41) at video brightness. Independent processing of organic charge transport layers and QD luminescent layer allows for tunability of CIE coordinates without changing the device structure. We demonstrate that the white QD-LED emission spectrum can be tuned by simply changing the ratio of QDs in the electroluminescent layer, enabling precise adjustments in the QD-LED color temperature with high color rendering index. We also use colloidal QDs synthesized via novel procedures allowing for a highly luminescent and reproducible materials for the device applications. The demonstrated devices suggest future white QD LED use in solid-state lighting and information display applications.
9:45 AM - M4.3
Resonant Cavity Quantum Dot LEDs.
Vanessa Wood 1 , Jonathan Tischler 1 , Polina Anikeeva 1 , Jonathan Halpert 2 , Moungi Bawendi 2 , Vladimir Bulović 1
1 Department of Electrical Engineering and Computer Science, MIT, Cambridge, Massachusetts, United States, 2 Department of Chemistry, MIT, Cambridge, Massachusetts, United States
Show AbstractWe present the first demonstration of directed emission and increased spectral purity from a quantum dot LED (QD-LED) in a resonant cavity. A resonant cavity QD-LED (RC QD-LED) is attractive for a variety of applications in fields such as optical communications, spectroscopy, metrology, environmental and industrial sensing, and biomedical instrumentation. RC LEDs are less expensive and more robust than lasers, which currently dominate the market in these areas.The fabrication of the device makes it easily tunable across a wide range of wavelengths. The high reflection (98% reflectivity) mirror of the RC QD-LED was deposited by RF sputtering of alternating layers of λ/4, TiO2 and Al2O3. A standard QD-LED structure was subsequently deposited to form the remainder of a λ-cavity. A λ/2, ITO electrode was sputtered on top of the DBR, followed by thermal evaporation of an organic hole transport layer. A monolayer of colloidally synthesized, CdSe QDs was then deposited via a microcontact printing at the optical antinode of the device. Organic hole-blocking and electron transport layers were thermally evaporated on top of the QDs and topped with a metal electrode, which doubles as the second cavity mirror. The RC QD-LED exhibits directed emission, with 85% of the emitted photons in the emission cone 40 degrees away from the normal. The typically narrow QD luminescence spectrum (of 40nm full width at half maximum) is further narrowed to 8nm in RC QD-LED electroluminescence spectrum. The absolute external quantum efficiency of the RC QD-LED is lower than the QD-LED without the cavity. However, if it is accounted for that only a small fraction of the cavity emission is collected in the normal direction, the two efficiencies are comparable. Furthermore, time resolved photoluminescence measurements on QD-LEDs and RC QD-LEDs show no change in the spontaneous emission lifetime for the resonant structure, indicating that spectral filtering occurs in the RC QD-LED.
10:00 AM - M4.4
Microcavity Light Emitting Devices Based on Colloidal Semiconductor Nanocrystal Quantum Dots
Jian Xu 1 , Ting Zhu 1 , Fan Zhang 1 , Michael Gerhold 2 , Andrew Wang 3
1 , Penn State University, State College, Pennsylvania, United States, 2 Electrical Engineering, North Carolina State University, Raleigh, North Carolina, United States, 3 , Ocean NanoTech, LLC, Fayetteville, Arkansas, United States
Show Abstract10:15 AM - M4.5
Soft-chemistry Route to P-I-N Heterostructured Quantum Dot Electroluminescence Device: All Solution-processed Polymer-Inorganic Hybrid QD-EL Device
Soon-Jae Kwon 1 , Kyung-Sang Cho 1 , Byoung-Lyong Choi 1 , Byung-Ki Kim 1
1 Display Device and Materials Lab, Samsung Advanced Institute of Technology, Suwon Korea (the Republic of)
Show AbstractWe have fabricated QD-EL devices with sequential layers of ITO/HTL(Polymer)/QD/ETL (inorganic)/Al by soft-chemical process, where cross-linking agent immobilizes the QDs upon solution-processed ETL construction. It is well known that vacuum-deposited small-molecules play a role of electron transport layer (ETL) in OLED or QD-EL devices, but such small molecule has a tendency to recrystallize under operating conditions. Furthermore, molecular materials such as Alq3 are easily damaged by the moisture and oxygen, making dark spots and device degradation. Furthermore, the dry method of depositing small molecule inevitably needs high-cost equipments, experienced skills, and encapsulation process against moisture and oxygen. In contrast, wet-chemical process guarantees low-cost and facile route for the ETL fabrication, if the “wet”-fabricated film acts as low electron-injection barrier. Here we have tried to fabricate P-I-N heterostructured QD-EL devices adopting all-solution-process, which have low turn-on voltage comparable to OLEDs. To produce the multilayered device structures, p-type polymer film was deposited on the ITO glass by successive process of coating and thermal curing, thereupon a few monolayers of QD was spin-coated. A cross-linkable ETL material was spin-coated onto a chemically-immobilized quantum dot layers and then thermally cured. As a cathodic electrode, relatively air-stable aluminium was deposited. The conduction band of the integrated inorganic moiety is located on the proximity of aluminum Fermi level (EF), which not only guarantees operating stability and lifetime of the electroluminescence (EL) device but lowers the turn-on voltage. Adsorption of the amines (ex.,-NH2) onto the inorganic ETL surface, due to non-bonding electrons associated with nitrogen groups in immobilizing QD layers, will result in a spatially intimate contact between the QD layer and the n-type inorganic moiety, which will allow orbital overlap at the interface and there by promote charge transfer. This polymer-inorganic hybrid EL device may provide a pathway to all-inorganic based QD-EL devices with enhanced efficiency and durability in the near future.
10:30 AM - M4.6
Plasmonic All-optical Modulation of Subwavelength Aperture Transmission via Saturable and Photo-induced Absorption of CdSe Quantum Dots on Corrugated Metallic Films.
Domenico Pacifici 1 , Henri Lezec 1 2 , Harry Atwater 1
1 Applied Physics, California Institute of Technology, Pasadena, California, United States, 2 , Centre National de la Recherche Scientifique, Paris Cedex 16 France
Show Abstract10:45 AM - M4.7
Synthesis and Characterization of ZnSe Quantum Dots Doped with Single Manganese Atoms
Andreas Hofmann 1 2 , Christina Graf 2 , Christine Boeglin 3 , Vladimir Korsunskiy 4 , Reinhard Neder 4 , Eckart Rühl 1 2
1 Institut für Chemie und Biochemie, Physikalische und Theoretische Chemie, Freie Universität Berlin, Berlin Germany, 2 Institut für Physikalische Chemie, Universität Würzburg, Würzburg Germany, 3 , Institute de Physique et Chimie des Materiaux de Strasbourg, Strasbourg France, 4 Institut für Mineralogie, Universität Würzburg, Würzburg Germany
Show AbstractDilute magnetic semiconductor nanoparticles represent a new class of quantum dots with unique optical, electronic, and magnetooptical properties. Initially, the choice of such structures was motivated by analogous bulk materials which are known as dilute magnetic semiconductors (DMS). Especially, manganese doped materials show interesting and promising characteristics based on Mn2+ as a paramagnetic center (S = 5/2) which substitutes a group II cation in the quantum dot lattice. ZnSe nanoparticles doped with manganese were prepared in a high temperature organic approach [1]. Their structure and composition were investigated by various methods. An ICP analysis (ICP: Inductive Coupled Plasma) of the particles show that the nanoparticles have a low Mn content of 0.1–0.3% in respect to Zn and that the Zn to Se ratio is 1.1. In photoluminescence measurements the characteristic 4T1 -> 6A1 Mn transition is observed, which is typical for Mn2+ inside a crystal lattice. Electron paramagnetic resonance (EPR) measurements also yield a hyperfine coupling constant of 61.8×10-4 cm-1 which is nearly identical which those of bulk ZnSe:Mn samples. We have investigated for the first time dilute magnetic nanoparticles by X-Ray Magnetic Circular Dichroism (XMCD) experiments in order to study of the local environment of the manganese ions in nanoparticles. The comparison of the present results with previous experimental and theoretical data on single manganese ions (Mn2+ d5)[2] shows that the ZnSe:Mn nanoparticles contain single, well separated manganese ions. Neither a Mn-Mn coupling nor traces of oxidation of the manganese ions were observed in the present results. The crystal structure of the ZnSe:Mn nanoparticles was studied by high resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD). These measurements reveal that the nanoparticles have a low polydispersity (15%), a slightly elongated shape (aspect ratio = 1.3), and crystallize in a typical zincblende structure. In addition, pair distribution function analysis (PDF) of the particles was carried out in order to determine the size and the interatomic distances between the Zn and Se sites. A new theoretical model was used to fit PDF data. Explicit structures of the nanoparticles were simulated and the PDF calculated from their respective actual atom positions. The parameters for the simulation included the lattice constants, an isotropic atomic displacement parameter for Zn and Se, a stacking fault probability, and the size and shape of the crystal. All parameters were refined to yield a good fit between the experimental and calculated PDF. This yields an excellent agreement between the size and shape data obtained from HRTEM, XRD, and PDF.[1] D. J. Norris et al. Nano Lett. 1, 3 (2001) [2] P. Gambardella et al. Phys. Rev. B 72, 045117 (2005)
M5: Electronic Structure (Theory & Some Experiments)
Session Chairs
Tuesday PM, November 28, 2006
Room 208 (Hynes)
11:30 AM - **M5.1
Defect Properties in Semiconductor Quantum Dots
Jingbo Li 1 , Su-Huai Wei 1 , Shu-Shen Li 2 , Jian-Bai Xia 2
1 , National Renewable Energy Laboratory, Golden, Colorado, United States, 2 , State Key Lab for Superlattice and Microstructure, Institute of Semiconductor, Chinese Academy of Science, Beijing China
Show AbstractSemiconductor nanostructures such as quantum dots are of great current interest for novel device applications. The application of semiconductors as novel electronic devices depends critically on its doping properties. Although defect properties have been extensively studied in the past for bulk semiconductors and various approaches have been proposed to overcome the doping limit in semiconductors [1], very few studies have been carried out to understand how the formation of nanostructure affect the defect properties in semiconductors. Using first–principles band-structure method, we have calculated the formation energies and transition energy levels of point defects in the prototype CdSe quantum dots (QDs) and investigated the limiting factors for n-type and p-type doping in the nanostructure. We found that (i) both defect formation energy and transition energy level increases when the QD size decreases. (ii) The DX center will be stable if the size of the QD is small enough [2], although it is unstable in the bulk material. (iii) The AX center in QD become even more unstable when the QD size decreases. We also show that the defect formation energy and transition energy levels are strongly affected by the presence of surface in the QDs. Our studies, therefore, show that defect properties in quantum dots could be significantly different from that in bulk semiconductors.[1]S.-H. Wei, Computational Materials Science 30, 337 (2004).[2]J. Li and S.-H. Wei, Phys. Rev. Lett. 94, 185501 (2005).
12:00 PM - M5.2
Virtual Synthesis and Optoelectronic Properties of Prismatic Artificial Molecules of In-N, Zn-O, Si-C and Zn-S: Comparative Studies.
Liudmila Pozhar 1 , Gail Brown 2 , William Mitchel 2
1 Chemistry, Western Kentucky University, Bowling Green, Kentucky, United States, 2 Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson AFB, Ohio, United States
Show AbstractCurrent “bottom-up” methods of experimental synthesis of arrays of small quantum dots (QDs, or artificial molecules) composed of semiconductor compound atoms realize nanoheterostructures (NHSs) based on units that are from several tenths to hundredths of atoms in size. Further increase in the density of elements of NHSs requires scaling down the size of the NHS units, thus minimizing the QD size to include only a few atoms (artificial molecules) whose electronic properties should be finely tuned to specific applications. While this task is beyond contemporary technologies, virtual (i.e., fundamental theory-based, computational) synthesis method can be used to provide theoretical predictions for physico-chemical properties of the artificial molecules, including those nucleated and stabilized in quantum confinement, surfaces and at interfaces, to guide experimental developments in the near future. Thus, Hartree-Fock (HF), restricted open shell HF, and multiconfiguration self-consistent field (CI/CAS/MCSCF) methods provide sophisticated theoretical tools to study structure, composition and electronic properties of small artificial molecules composed of semiconductor compound atoms. In the reported study, these tools are used to synthesize virtually several prismatic In-N, Zn-O, Si-C and Zn-S artificial molecules whose structure is derived from that of the symmetry elements of the respective wurtzite bulk lattices. Applications of spatial constraints to the atomic coordinates allow modeling the molecular synthesis in quantum confinement, to obtain pre-designed molecules with tunable electronic properties. Relaxation of these constraints, or vacuum optimization, leads to the corresponding molecules synthesized in “vacuum”. The developed of computational templates of the studied artificial molecule synthesized in confinement reflects the influence of the quantum confinement on the electronic level structure (ELS), type of bonding, the direct optical transition energy (OTE) and charge and spin density distributions (CDD and SDD, respectively) of the molecules. Comparing the structure and properties of these molecules to those of their vacuum counterparts, one is led to a conclusion that a small changes in atomic positions in otherwise structurally similar molecules may lead to a significant change in their electronic properties. This can be used to tune the properties of artificial molecules by changing the atomistic details of quantum confinement where the molecules are synthesized, and conditions of their synthesis.
12:15 PM - M5.3
High-Efficiency Carrier Multiplication in Semiconductor Nanocrystals: Recent Progress and Challenges.
Victor Klimov 1 , Richard Schaller 1 , Jeffrey Pietryga 1 , Milan Sykora 1
1 Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractThe efficient conversion of photon energy into electrical charges is a central goal of much research in physics, chemistry, and biology. A usual assumption is that absorption of a single photon by a material produces a single electron-hole pair (exciton), while the photon energy in excess of the energy gap is dissipated as heat. In 2004, we reported for the first time that nanocrystals (NCs) of PbSe could respond to absorption of a single photon by producing two or more excitons with the unity probability (Phys. Rev. Lett. 92, 186601, 2004). This presentation reviews our recent follow-up work that addresses such issues as the generality and the mechanism of this carrier multiplication phenomenon, the limits on photon-to-exciton conversion efficiencies, and implications of carrier multiplication in photovoltaics and photocatalysis. One remarkable feature of carrier multiplication in NCs is that it can produce multiple charges with quantum efficiencies that correspond to the ultimate limit dictated by energy conservation. For example, for photon energy of 7.8 energy gaps, a maximal possible number of photogenerated excitons based on energy conservation is 7, which is exactly the number measured in our experiments (Nano Lett. 6, 424, 2006). In this case, 90% of the photon energy is used to produce a useful effect (multiple charges) and only 10% is lost as heat. In the normal scenario (one exciton per photon), 90% of the photon energy would be dissipated as heat. Another unexpected feature of carrier multiplication is that it results in unusual distributions of carrier populations that cannot be described by Poissonian statistics. Specifically, by selecting certain photon energies, we obtain photoexcited NC ensembles with nearly pure single multiplicities (i.e., all excited NCs contain the same number of excitons) that can be tuned in the controlled way from 1 to 7 (Phys. Rev. Lett. 96, 097402, 2006).While the exact mechanism for carrier multiplication in NCs is still under debate, one factor, which certainly contributes to high efficiencies for this process, is a unique property of NCs to produce significant exciton-exciton interactions. We believe that because of these strong interactions a high-energy exciton initially excited by a photon only exists in its “virtual,” short-lived state, which rapidly decays to produce a more stable state that comprises two or more excitons (Nature Phys. 1, 189, 2005). Based on measured carrier-multiplication efficiencies, we project that a power-conversion limit of a single-junction solar cell can be increased up to ~42%, which is roughly a 40% improvement compared to the situation without multiple-exciton generation. Carrier multiplication can also significantly improve the performance of photocatalytic structures particularly in reactions that involve multiple reduction/oxidation steps such as water splitting. Other potential applications include nonlinear optics, lasing, and quantum information.
12:30 PM - M5.4
Multipolar Photonic Interactions between Quantum Dots of Different Sizes.
Hideaki Matsueda 1
1 Department of Information Science, Kochi University, Kochi Japan
Show Abstract Multipolar interactions between quantum dots (QDs) of different sizes involving real and virtual photons are compared, and suggested as the principle of intra-device, inter-device, and inter-chip interconnections and logical operations, with the device concepts for solid state nano-networks and a quantum computer. We have been interested in expanding the freedom of mankind to harness fast and seemingly weak correlations among QDs ensemble, manifesting them at macroscopic level [1, 2]. Especially, the inclusion of the resonance dynamic dipole-dipole interaction (RDDDI, where excitation energy sufficient only for some sites out of numbers of sites is supplied) has been our major concern both in theory and in experiment [1, 2]. In this paper, first a theoretical outlook of the possible coherent multipolar interactions involving virtual as well as real photons will be given, and then some interactions that may be useful for intra-device (short range), inter-device (intra-chip, medium range) and inter-chip (long range) interconnections and logic operations are selected. Using the time-energy uncertainty, each interaction energy will give the lifetime of the photon mediating the interaction, or the coherence time for the interaction. Then the coherence length or the range of the interaction is estimated, yielding the limiting distance for the possible interconnection by each multipolar interaction. Moreover, possible device concepts to implement these interconnections and the logical operations are given. [1] H. Matsueda, "Solid State Quantum Computation", Chap.10 of Coherence and Statistics of Photons and Atoms, ed. Jan Perina (John Wiley, New York, 2001) pp. 422-469. [2] H. Matsueda, K. Leosson, Z. Xu, J. M. Hvam, Y. Ducommun, A. Hartmann, and E. Kapon, "Dynamic Dipole-Dipole Interactions between Excitons in Quantum Dots of Different Sizes", IEEE Transactions on Nanotechnology, 3 (2), pp.318-327 (2004).
12:45 PM - M5.5
Excitons and Plasmons in Semiconductor-metal Nanoparticle Assemblies: Energy Transfer, Fano Resonances, and Nonlinear Effects.
Alexander Govorov 1 , Wei Zhang 1 , Garnett Bryant 2 , Jaebeom Lee 3 , Nicholas Kotov 3
1 Physics and Astronomy, Ohio University, Athens, Ohio, United States, 2 Atomic Physics Division, NIST, Gaithersburg, Maryland, United States, 3 Department of Chemical Engineering, University of Michigan , Ann Arbor, Michigan, United States
Show AbstractMotivated by recent experiments on bio-conjugated semiconductor-metal hybrid nanocrystal superstructures [1,2], we have developed a microscopic, density-matrix theory to describe systems composed of semiconductor quantum dots (QDs) and metal nanoparticles (NPs) in the presence of external electric fields. The interaction between an exciton in the QD and plasmons in the NPs leads to interesting optical properties: excitonic energy transfer between building blocks of a nano-assembly, electromagnetic enhancement, exciton energy shift, and interference and non-linear phenomena [3,4]. We explore both the linear regime (for weak external field) and the non-linear regime (for strong external field). The interference between the external field and the induced internal field results in strong enhancement of energy absorption (compared with the energy absorption of a QD in the absence of a metal NP) and also leads to an asymmetric peak and valley in the total energy absorption (Fano-like shape). We also consider Rayleigh scattering, which further reveals this type of behavior. Our theory is useful for understanding present experimental results and can give guidance for future experiments and applications.[1] J. Lee, A. O. Govorov, J. Dulka, and N. A. Kotov, Nano Letters, 4, 2323 (2004).[2] J. Lee, A. O. Govorov, and N. A. Kotov, Angewandte Chemie, 117, 7605 (2005).[3] A. O. Govorov, G. W. Bryant, W. Zhang, T. Skeini, J. Lee, N. A. Kotov, J. M. Slocik,and R. R. Naik, Nano Letters, 6, 984 (2006).[4] W. Zhang, A. O. Govorov, and G. W. Bryant, submitted to Phys. Rev. Lett.
M6: Optoelectronic Properties and Characterization
Session Chairs
Tuesday PM, November 28, 2006
Room 208 (Hynes)
2:30 PM - **M6.1
Intraband Spectra and Electron Relaxation for II-VI Core/shell Nanocrystals.
Philippe Guyot-Sionnest 1 , Anshu Pandey 1
1 Chemistry and Physics, University of Chicago, Chicago, Illinois, United States
Show Abstract3:00 PM - M6.2
Initial Excitonic State Selective Ultrafast Dynamics of Semiconductor Quantum Dots.
Patanjali Kambhampati 1 , Samuel Sewall 1 , Ryan Cooney 1 , Kevin Anderson 1 , Eva Dias 1
1 Chemistry, McGill University, Montreal, Quebec, Canada
Show AbstractAn understanding of carrier relaxation dynamics in semiconductor quantum dots is important both for fundamental physics as well as for their incorporation into devices such as gain media for lasers [1]. Here, we report on the first femtosecond experiments on high quality colloidal semiconductor quantum dots in which the initial excitonic state has been prescribed. These data show dramatic differences at all probe wavelengths based upon the initial state. These experiments produce the first direct measure of electron and hole relaxation times as well as the first observation, to our knowledge, of quantum state specific Stark effects where the field is internally generated by the excitonic state.These experiments show that the dynamics depend upon the excitation wavelength due to different initial excitonic states. These experiments directly measure electron relaxation times through state filling. The observed instrument response limited bleaching when pumping into a 1S electron state are in contrast to induction times when pumping into 1P and higher excited states. The difference between these transients provides the first direct measure of 1P to 1S electron relaxation times. From these data we unambiguously assign and electron relaxation time of 140 fs for these CdSe quantum dots. These data also show induced absorption features which arise from carrier induced Stark shifting rather than state filling. These data most dramatically illustrate the initial state specific dynamics. The induced absorption features arises from the carrier induced Stark redshift of the band edge absorption. The magnitude and the induction times strongly depend upon the initial state. These observations indicate that the Stark shifting is determined by the specific quantum state of the exciton. Pump wavelengths were chosen in which the electron is in the same 1S state whereas the hole is in either is 1S(3/2) or 2S(3/2) state. The difference in these transients directly reflect hole cooling. From these data we unambiguously assign the lowest hole relaxation timescale as 300 fs. Perhaps most surprising is that these Stark induced signals are still evolving past 1 ps. By this time, the hot electrons have fully cooled and are in their 1S state, regardless of initial excitonic state. That this state specificity persists on timescales longer than 1 ps suggests that it is the still relaxing hole envelope function which creates a state specific Stark effect. [1] V.I. Klimov, “Optical Nonlinearities and Ultrafast Carrier Dynamics in Semiconductor Nanocrystals”, J. Phys. Chem. B 104, 6112-6123 (2000).
3:15 PM - M6.3
Radiative Recombination of Tri-excitons in CdSe Quantum Dots.
Alberto Franceschetti 1 , Claudia Troparevsky 2
1 , National Renewable Energy Laboratory, Golden, Colorado, United States, 2 , Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show Abstract3:30 PM - M6.4
Optical Properties of Colloidal CdSe/ZnS core/shell Nanocrystals Embedded in a UV Curable Resin.
Abhi Joshi 1 , Edwin Davis 2 , K. Narsingi 1 , Omar Manasreh 1 , Brad Weaver 3
1 Electrical Engineering, University of Arkansas, Fayetteville, Arkansas, United States, 2 Optical Engineering, Norfolk State University, Norfolk, Virginia, United States, 3 Code 6818, Naval Research Lab, Washington, District of Columbia, United States
Show Abstract3:45 PM - M6.5
Nonradiative Recombination in Heavily Doped CdSe Quantum Dots for Intersubband Device Applications.
Shengkun Zhang 1 , Xuecong Zhou 1 , Aidong Shen 1 , Wubao Wang 1 , B b Das 1 , Hong Lu 1 , Maria Tamargo 1 , Robert Alfano 1
1 , City College of New York, New York, New York, United States
Show AbstractDoped CdSe quantum dots have important applications in fabricating intersubband infrared devices. The performance of these devices depends very much on the role of nonradiative recombinations in the CdSe QDs. In this research, nonradiative recombination in heavily doped n-type CdSe quantum dots was investigated by time and temperature dependent photoluminescence (PL) spectroscopy. Three multiple CdSe quantum dot samples were grown over ZnCdMgSe barrier layers on InP (001) substrate by molecular beam epitaxy. Samples A and B were intentionally doped by Cl with a concentration of 1x1018cm-3 while control sample C was undoped. The deposition thicknesses are 3 nm for samples A and C, and 4 nm for sample B. The sizes of the CdSe quantum dots in samples A and C were nearly same and smaller than that in sample B. From temperature dependence of the PL intensity, nonradiative recombinations in the three samples were characterized by different activation energies for doped and undoped quantum dots. Despite the different QD sizes of the doped samples A and B, they have almost the same activation energy of nonradiative recombination, which is about 50 meV. The undoped sample C has an activation energy of 89 meV that is much higher than the doped samples. Strong carrier screening in heavily doped samples may reduce the barrier height of those nonradiative recombination centers.Time resolved PL spectra show that the PL decay times of the doped samples are nearly same and are much longer than that of the undoped sample. In the heavily doped samples, the PL decay is dominated by the radiative recombination process, which is determined by the doping level. The high concentration of electrons also saturate the nonradiative recombination centers and make them less active. This saturation effect leads to significant enhancement of luminescence efficiency in the doped CdSe quantum dots.
4:00 PM - M6.6
Probing Spin Structure of Exciton States in Individual Colloidal CdSe Nanocrystals.
Han Htoon 1 , M. Furis 2 , S. Crooker 2 , V. Klimov 1
1 Chemistry Division , Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 2 , National High Magnetic Field Laboratory, Los Alamos, New Mexico, United States
Show AbstractStrong quantum confinement provided by semiconductor nanocrystal (NCs) has a pronounced effect on the spin structure of band-edge exciton states. Understanding and ultimately controlling this spin structure may allow additional tunability of spectral and dynamical responses of NCs. While single-NC photoluminescence (PL) studies are now routine, there are no reports of spin-resolved magneto-PL studies with single-NC sensitivity. To this end, we constructed a low temperature magneto-PL system (T > 4 K, B up to 5 Tesla) to simultaneously detect both orthogonal polarizations of the emission from individual CdSe NCs. This system allows us to spectrally resolve the left/right circularly-polarized PL (or horizontal/vertical linearly polarized PL) from single NCs without complications arising from blinking and spectral diffusion. Magneto-PL studies clearly reveal an energy splitting between left- and right-circularly polarized PL peaks and a strong degree of circular polarization in a subset (<10%) of NCs. We attribute this to the Zeeman splitting of spin-up and spin-down bright excitons in those NCs that have near-ideal cylindrical symmetry and wurzite c-axes aligned parallel to B. In addition, the B = 0 PL spectra reveal that some of the NCs (~10%) emit two spectrally distinct, linearly (and orthogonally) polarized PL peaks. These peaks arise from the intrinsic fine structure splitting of bright excitons which is due to the anisotropic exchange interaction in those NCs with imperfect cylindrical symmetry [1]. The observed anisotropic exchange splitting ranges from 1 to 2 meV, which is considerably larger than the corresponding splitting typically observed in epitaxial quantum dots. In magnetic fields of 5 T, these fine structure states show mixed linear and circular polarization due to interplay between anisotropic exchange and Zeeman splitting effects.[1] M. Furis, H. Htoon, M. A. Petruska, V. I. Klimov, T. Barrick and S. A. Crooker, Phys. Rev. B (Rapid Comm.) (2006)
4:30 PM - M6.7
Transition Energies and Extinction Coefficients in PbS Colloidal Quantum Dots.
Ludovico Cademartiri 1 , Erica Montanari 2 , Gianluca Calestani 2 5 , Andrea Migliori 3 , Antonietta Guagliardi 4 , Geoffrey Ozin 1
1 Department of Chemistry, University of Toronto, Toronto, Ontario, Canada, 2 Chemistry, University of Parma, Parma Italy, 5 , CNR-IMEM, Parma Italy, 3 , CNR-IMM, Bologna Italy, 4 , CNR-IC, Bari Italy
Show Abstract4:45 PM - M6.8
Water-based Synthesis of II-VI Quantum Dots using Microemulsions and Liquid Crystals as Templates.
Qi (Grace) Qiu 1 , Georgios Karanikolos 2 , Paschalis Alexandridis 2 , Tracy Heckler 1 , Jun Wang 1 , Mesut Yasar 3 , Athos Petrou 3 , Triantafillos Mountziaris 1
1 Chemical Engineering, University of Massachusetts, Amherst, Massachusetts, United States, 2 Chemical and Biological Engineering, University at Buffalo - SUNY, Amherst, New York, United States, 3 Physics, University at Buffalo - SUNY, Amherst, New York, United States
Show AbstractA technique for simultaneous size and shape control of compound semiconductor nanostructures using microemulsions and lyotropic liquid crystals as templates has been developed[1-3]. The templates are formed by self-assembly of a poly (ethylene oxide)-poly (propylene oxide)-poly (ethylene oxide) (PEO-PPO-PEO) block copolymer in the presence of a polar and a non-polar solvent. PEO-PPO-PEO block copolymers, due to their amphiphilic character, exhibit very rich structural polymorphism when dissolved in selective solvents of different polarity, and can attain a number of microstructures such as spheres, cylinders, and lamellae. Luminescent ZnSe quantum dots[1] and single-crystalline nanorods[2] with average diameters below 10 nm were synthesized in oil-in-water microemulsions and lyotropic liquid crystals respectively, using the PEO-PPO-PEO/heptane/formamide self-assembled system and a reaction between diethylzinc dissolved in heptane with hydrogen selenide. In this work, the PEO-PPO-PEO/water/p-xylene system was used as a template for II-VI nanocrystal synthesis in the dispersed water phase. Quantum dots of ZnSe, CdSe, ZnMnSe, and CdMnSe were grown in the spherical domains of reverse (water-in-oil) microemulsions and cubic liquid crystals by reacting metal acetates with hydrogen selenide. Hollow nanospheres and hollow nanotubes of ZnSe were grown around the dispersed spherical and cylindrical oil nanodomains of the normal (oil-in-water) cubic and hexagonal liquid crystal phases, respectively. Free-standing quantum wells (nanoplates or nanolaminates) were grown in the lamellar liquid crystals. The nuclei of the nanoparticles are formed by a spontaneous and irreversible reaction between metal acetates and hydrogen selenide,that is either bubbled through the microemulsion or allowed to diffuse into the liquid crystalline templates. The nuclei grow by surface reactions with unreacted precursors and by cluster-cluster coalescence to yield nanocrystals that acquire the shape and morphology of the self-assembled liquid crystalline domains. The nanostructures were characterized by HR-TEM, XRD, EDX and optical spectroscopy. The shape of the nanocrystals can be controlled by selecting the structure of the template and the phase where the precursor is dissolved. The size of the nanocrystals can be controlled by the size of the nanodomains and the concentration of the precursors in them. [1] G.N. Karanikolos, et al., Langmuir, 20(3), 550 (2004)[2] G.N. Karanikolos, et al., Nanotechnology, 16(10), 2372 (2005)[3] G.N. Karanikolos, et al., Nanotechnology, 17, 3121 (2006)
5:00 PM - M6.9
Energy Transfer between Semiconductor Colloidal Quantum Dots and J-aggregates of Cyanine Dye
Qiang Zhang 1 , Tolga Atay 2 , Hayato Urabe 1 , Arto Nurmikko 1 2 , Jonathan Tishchler 3 , Scott Bradley 3 , Vladimir Bulovic 3
1 Engineering, Brown University, Providence, Rhode Island, United States, 2 Physics, Brown University, Providence, Rhode Island, United States, 3 Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractThe need for materials having better optical and optoelectronic properties to be used in applications has been driving researchers in materials science to develop novel compounds and structures. The possibility to grow tailored systems incorporating both inorganic and organic materials has attracted great attentions for reasons from both fundamental and application point of view. In the strong coupling regime, the hybridization of the Frenkel exciton in organic materials and the Wannier-Mott excitons in inorganic materials could lead to some exciting linear and nonlinear optical properties of the hybrid material; In the weak coupling regime, the efficient incoherent energy transfer between the organic and inorgainic materials could facilitate the development of new optoelectronic devices with improved performance, in application areas such as light emitting devices, photodetectors and sensors, etc. Semiconductor colloidal quantum dots with superior photostability and wide wavelength tunability and aggregate of molecular dyes with large absorption oscillator strength within the J-band are two good examples in each category.Here, we report the study of energy transfer between semiconductor quantum dots and J-aggregate of cyanine dyes, which are encapsulated in water-in-oil micelles. Recently, we demonstrated that TOPO-coated CdSe-ZnS colloidal quantum dot could be efficiently transported into water-in-oil micelles with NP-5 as the surfactant and cyclohexane as the solvent. With addition of controlled amount of water-soluble TDBC dye molecules at appropriate temperature, we found that J-aggregate of TDBC can readily form in the micelles as well, presumably in close physical proximity to the quantum dots as due to the small size of the micelles and the charge attraction. Transmission, photoluminescence (PL), photoluminescence excitation (PLE) spectroscopy and PL lifetime measurement have been performed to characterize this composite system. We have found evidences for Förster-type of energy transfer between quantum dots and J-aggregate, with the details and the direction of energy transfer depending on the spectral overlap between the absorption and emission spectrum of the two materials.
5:15 PM - M6.10
Ligand Effects on Quantum Dot Photoluminescence studied via Single Molecule Spectroscopy.
Andrea Munro 1 , David Ginger 1
1 Chemistry, University of Washington, Seattle, Washington, United States
Show AbstractColloidal nanocrystal quantum dots are being investigated as chromophores for a variety of applications. It is known that quantum dot photoluminescence is strongly dependent upon surface chemistry; therefore a more detailed understanding of surface effects on quantum dot photoluminescence is desirable, especially for applications where thick inorganic shells may be detrimental. To this end, we study the effects of thiol and amine concentration on CdSe, CdSe/CdS and CdSe/CdZnS/ZnS quantum dot photoluminescence in chloroform and toluene solutions and in thin polymer films at both the ensemble and single molecule level. We unify disparate reports that amines both increase and decrease photoluminescence by demonstrating that alkylamines cause both effects on CdSe luminescence depending on amine concentration. In contrast, we show that alkanethiols decrease the photoluminescence for all studied quantum dots, except those with ZnS shells. We fit the photoluminescence quenching as a function of free thiol concentration with a Langmuir adsorption isotherm and conclude that the binding constant for primary alkanethiols is significantly higher than has been previously reported. In addition we use the fits to determine the number of accessible thiol binding sites per quantum dot. As thiols quench the photoluminescence, the ensemble lifetime shows a decrease, as well a broadening of the lifetime distribution. We describe the PL quenching by alkanethiols in the context of both intrinsic and ligand-induced trap states. We test this hypothesis against observed changes in both single quantum dot blinking dynamics and single quantum dot fluorescence lifetimes.
5:30 PM - M6.11
Resonant Nonlinear Refraction at Telecom Wavelengths, a Spectroscopic Study on PbSe Quantum Dots.
Iwan Moreels 1 , Pacal Kockaert 2 , Jerome Loicq 4 , Dries Van Thourhout 3 , Zeger Hens 1
1 Inorganic and Physical Chemistry, Gent University, Gent Belgium, 2 Optics and Acoustics Department, Université Libre de Bruxelles, Brussels Belgium, 4 , Liège Space Centre, Liège Belgium, 3 Department of Information Technology, Gent University, Gent Belgium
Show Abstract5:45 PM - M6.12
Heterostructure Nanostructures Based on Cd Chalcogenides.
Sandeep Kumar 1 , Gregory Scholes 1
1 Chemistry, University Toronto, Toronto, Ontario, Canada
Show AbstractCadmium chalcogenide nanocrystals have been of interest as prototypical colloidal quantum dots and because of their potential applications. To date, most of the research on these nanocrystals has focused on either single component nanostructures, or alloy type of nanostructures. Further manipulation of the properties of these materials may be possible by means of a new configuration, where the constituents are combined on the nanoscale like a traditional composite material. In such a configuration the individual components retain their identity and contact each other via an identifiable interface. Such types of catalytically grown III-V and IV semiconductor nanowires with alternating compositions has already been reported for the development of single nanowire light emitting diodes and single electron transistors.In this report we will present new types of solution grown composite heterostructures based on Cd chalcogenides where, depending on the energy level alignment of individual components, both type I and type II heterostructures can be made. In addition to the composite formation, shape anisotropy was also introduced in the system and heterostructure nanorods were synthesized. These heterostructures showed evidence of charge transfer in type II and energy transfer in type I heterostructures. The charge transfer band of these heterostructures was investigated in details for its solvent dependency. The solvent shift of the charge transfer band from the nonpolar solvent toluene to polar solvents like dimethyl formamide was found to agree reasonably well with the calculated value of solvent shift using simple Onsager cavity model. The photophysics of these materials will be reported.
Symposium Organizers
Eric A. Stach Purdue University
Curtis R. Taylor Virginia Commonwealth University
Zhiming M. Wang University of Arkansas
Qi-Kun Xue Tsinghua University
M7: Assembly & Integration I
Session Chairs
Michael Hanke
Regina Ragan
Wednesday AM, November 29, 2006
Room 208 (Hynes)
9:00 AM - **M7.1
Self-Assembled Nanocrystals and Nanowires for Electronics and Solar Cells
Yi Cui 1 , Jia Zhu 1
1 , Stanford University, Stanford, California, United States
Show AbstractTo exploit the properties of nanocrystals and nanowires in electronics and solar energy conversion, it is necessary to organize them into designed networks or architectures. Here I present our recent progress on the self-organized nanostructures. In one example, colloidal nanocrystals are assembled by the surface tension force into desired location forming arbitrary patterns by design. It is possible to directly assemble them into circuit. In another example, nanowires form regular networks with controlled orientation during chemical synthesis, opening the opportunities in applications such as solar cells, which require effecient charge carrier collection.
9:30 AM - **M7.2
Spin Dynamics and Cavity Enhanced Spectroscopy in Semiconductor Quantum Dots.
David Steuerman 1 , Nathaniel Stern 1 , Jesse Berezovsky 1 , David Awschalom 1
1 Center for Spintronics and Quantum Computation, University of California, Santa Barbara, California, United States
Show Abstract10:00 AM - M7.3
Self-assembled Monolayer of Nanocrystals onto Three Dimensional Substrate for Light Emitting Device.
Seong Jae Choi 1 , Dong Kee Yi 1 , Jae-Young Choi 1 , Seon-Mi Yoon 1 , Hyeon Jin Shin 1 , In-Yong Song 2 , Jong-Bong Park 2
1 Display Emissive Material Project Team, Samsung Advanced Institute of Technology, Yongin-si, Gyeonggi-do, Korea (the Republic of), 2 Analytical Engineering Group, Samsung Advanced Institute of Technology, Yongin-si, Gyeonggi-do, Korea (the Republic of)
Show Abstract10:15 AM - M7.4
Quantum Dot Photolithography
Massimo Bertino 1
1 physics, university of missouri-Rolla, Rolla, Missouri, United States
Show AbstractWe present a general method that enables surface and three-dimensional photolithography of high quality quantum dots.In our method, the solvent of a sol-gel material such as a silica hydrogel is exchanged with a solution of metal chalcogenide precursors. The precursors are then photodissociated, and patterns are generated in the exposed regions. Several precursor combinations have been tailored, which allow photolithography of CdS and CdSe quantum dots with infrared, ultraviolet, and X-ray radiation. Patterns are produced either by masking, or by moving the samples in front of a focused photon source. Nanoparticle size is controlled by adding capping agents to the precursor solution. The quantum yield of the composites is increased to reach a value as high as 30% by photoactivation.
10:30 AM - M7.5
Spatially Controlled Periodic Arrays of Individual II-VI Semiconductor Nanocrystals For Single Photon Generation.
Qiang Zhang 1 , Hayato Urabe 1 , Shouheng Sun 2 , Arto Nurmikko 1
1 Engineering, Brown University, Providence, Rhode Island, United States, 2 Chemistry, Brown University, Providence, Rhode Island, United States
Show AbstractThe development of single photon emitters from semiconductor nanoparticles is a contemporary challenge, with applications to quantum cryptography and quantum computing. Number of reports have been published where single photon emission with antibunching statistics appear as expected in a system which approximates a two-level atom. Quantum dots prepared by both epitaxial crystal growth in III-V compounds and those synthesized via colloidal solution-based techniques in II-VI compounds have been used as demonstration testbeds for single photon on command, while the stability of excitons in II-VI semiconductor nanocrystals offers a ready pathway to room temperature emission. A generic problem with most of these approaches is the inability to control the spatial location of the quantum dots within a heterojunction or on a solid substrate, due to their random spatial distribution. As a consequence, the optical-pumping access to these particles and their embedment into microcavity structures have been quite cumbersome and inefficient. Here we demonstrate an experimental approach, which has enabled us to synthesize CdSe/ZnS nanocrystals individually encapsulated in spherical silica particles. Through self-assembling, these QD-silica composite particles can be placed into geometrically well-defined templates with submicrometer precision on large spatial scale, so as to unambiguously access and enable single photon emission from a specific nanocrystal source.Our synthesis approach begins with conventional CdSe/ZnS colloidally prepared nanocrystals whose surface chemistry is subsequently modified to enable a relatively thin silica shell growth in a water-in-oil microemulsion. An amine-based surface coupling reagent is chosen to provide good surface protection for the nanocrystals and assist silica growth. The appearance of free silica particles and silica particles containing multiple nanocrystals are suppressed by fine tuning of the nanocrystal, water and surfactant concentrations. After centrifugation and purification, a much thicker silica cladding can then be applied via a Stöber process, resulting in a final particles size of more than 100nm. The spatial organization of the QD-silica particles is accomplished by self-assembling these particles into an array of submicron wells in a thin poly(methyl methacrylate) (PMMA) film precisely defined through electron-beam lithography. Highly ordered individual nanocrystal array is produced via a capillary force approach, by taking advantage of the size and hydroxyl group terminated silica surface of the composite particles. Finally the optical properties of spatially controlled nanocrystals are studied and characterized by fluorescence microscopy, spectroscopy and single-photon correlation measurement at room temperature. Photo antibunching has been observed in the optical emission of individual composite particles, as due to the effectiveness of Auger recombination.Research Supported by DARPA and NSF
11:15 AM - **M7.7
Beyond Epitaxical Self-Assembled and Nanocrystal Quantum Dots: Composite Systems.
Anupam Madhukar 1
1 Nanostructure Materials and Devices Laboratory, University of Southern California, Los Angeles, California, United States
Show AbstractIntegration of epitaxical and colloidal semiconductor nanostructures into hybrid structures can potentially open unprecedented functionalities that combine the strengths of the epitaxical nanostructures in optoelectronics with the versatility of the nanocrystal quantum nanostructures and their application in solution environment [1, 2, 3]. In this talk I shall present some approaches to the synthesis of such hybrid structures and the attendant materials challenges which, when overcome, will open vast areas of applications ranging from energy conversion to detection of weak biochemical signals such as in early detection of biohazards and disease.This work is supported by DARPA/AFOSR under the DURINT01 program.[1] A. Madhukar, S. Lu, A. Konkar, Y. Zhang, M. Ho, S. M. Hughes, and A. P. Alivisatos, “Integrated semiconductor nanocrystal and epitaxical nanostructure systems: structural and optical behavior” Nano Lett. 5, 479 (2005).[2] A. Konkar, S. Lu, A. Madhukar, S. M. Hughes, A. P. Alivisatos, "Semiconductor nanocrystal quantum dots on single crystal semiconductor substrates: High resolution transmission electron microscopy", NanoLett. 5, 969 (2005).[3] S. Lu and A. Madhukar, “Optical response of InAs/ZnSe nanocrystal quantum dot submonolayers adsorbed on GaAs(001)” [To be published]
12:15 PM - M7.9
Nucleation of Self-Assembled SiGe Quantum Dot Molecules at Pre-Defined Sites Using Focused Ion Beam Patterning.
Jennifer Gray 1 , Robert Hull 1 , Jerrold Floro 2
1 Materials Science and Engineering, University of Virginia, Charlottesville, Virginia, United States, 2 , Sandia National Laboratories, Albuquerque, New Mexico, United States
Show AbstractThe ability to produce quantum dots at specific locations is important for applications such as quantum computing where quantum dots need to be arranged into logic structures. By combining substrate patterning with heteroepitaxial growth of SiGe on Si under kinetically limited conditions, we can self assemble four-fold island nanostructures, or “quantum dot molecules”, at pre-defined locations. Under growth conditions where ad-atom mobility is limited, strain-relieving islands form only at energetically favorable sites. These sites include the edges of pits that form in the Si buffer and SiGe film at locations where the substrate was initially patterned ex-situ using a focused ion beam (FIB). Since the pits are pyramidal in shape, an island forms at each of the four pit edges, resulting in four closely spaced islands locally arranged in a square. By adjusting the composition of the alloy, we can control the lateral length scale of these structures. This method therefore provides a way of controlling island size as well as placement. In addition, these structures are very uniform in size and shape when the initial FIB milling depths are less than 5nm. This technique is therefore an attractive method for producing complex nanoelectronic architectures, where quantum dot size, placement and uniformity are important parameters for potential applications. Results for a variety of FIB patterns and milling conditions will be demonstrated.This work was partially supported by the DOE Office of Basic Energy Sciences. Sandia is a multiprogram laboratory of the United States Department of Energy operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.
12:45 PM - M7.11
Simultaneous Electron Beam Induced Etching And Deposition For Formation Of High-Purity Contacts To Nanostructures.
Charlene Lobo 1 , Milos Toth 2 , Gavin Hartigan 2 , Ralph Knowles 2
1 Microstructural analysis unit, University of Technology Sydney, Sydney, New South Wales, Australia, 2 , FEI company, Newburyport, Massachusetts, United States
Show AbstractGas-mediated electron beam induced etching (EBIE) and deposition (EBID) permit nano-scale modification of surface material via chemical reactions involving electron-dissociated precursor molecules. EBID has been used to fabricate arrays of metallic quantum dots for electrical transport measurements, and is a promising technique for direct writing of electrical contacts to nanocrystals and nanotubes1-3. However, the purity and conductivity of metallic EBID nanostructures is typically limited by carbonaceous film growth fed by hydrocarbon components of metalorganic precursor molecules, and contaminants present on the sample surface. Such contamination can be reduced by simultaneous EBIE using a gaseous etch precursor such as H2O or O2 (Ref. 4).Here, we report a combined theoretical and experimental study of processes in which EBIE and EBID occur simultaneously. We show that switching between nanostructure deposition and etching can be induced simply by changing the electron flux (controlled by the beam diameter or beam current). The switching is observed when one of the processes has both a higher efficiency and a lower precursor partial pressure than the other. We present a theoretical model that describes the growth rate of deposits as a function of electron flux, based only on the molecular properties and partial pressures of the deposit and etch precursor gases. The model describes our experimental results, and enables us to eliminate other possible explanations for our observations (such as beam heating).Our results enable a new level of control over gas-mediated electron beam induced deposition and etching for nanostructure fabrication. The electron flux dependence of simultaneous EBID and EBIE plays a key role in reducing contamination buildup rates during deposition of high purity nanostructures in the presence of oxygen-containing gases, and can be used to enhance the spatial localization of etch profiles. 1. Fabrication and properties of dot array using electron beam induced deposition. M. Komuro & H. Hiroshima, Microel. Eng. 35, 273 (1997).2. Electron transport in an array of platinum quantum dots. Z. M. Liao, J. Xun & D.-P. Yu, Phys. Lett. A 345, 386 (2005).3. Electrodes for carbon nanotube devices by focused electron beam induceddeposition of gold. T. Brintlinger et al, J. Vac. Sci. Tech. B. 23, 3174 (2005).4. Solid gold nanostructures fabricated by electron beam deposition. K. Molhave et al, Nano Lett. 3, 1499 (2003).5. Electron flux controlled switching between electron beam induced etching and deposition, M. Toth, C. J. Lobo, G. Hartigan & R. Knowles, submitted to Nano Lett.
M8: Assembly & Integration II
Session Chairs
Wednesday PM, November 29, 2006
Room 208 (Hynes)
2:30 PM - M8.1
Partial Ordering in the Nucleation and Growth of Self–organized Ge/Si Nanostructures.
Fulvio Ratto 1 , Andrea Locatelli 2 , Stefano Fontana 2 , Sharmin Kharrazi 3 , Shriwas Ashtaputre 3 , Sulabha Kulkarni 3 , Stefan Heun 4 , Federico Rosei 1
1 , INRS - EMT, Varennes , Quebec, Canada, 2 , Sincrotrone Trieste S.C.p.A., Basovizza, TS, Italy, 3 , University of Pune, Pune India, 4 , TASC – INFM, Basovizza, TS, Italy
Show Abstract2:45 PM - M8.2
Novel Morphologies of InAs Quantum Dot Growth on GaAs Surfaces Containing Nanostructures Formed by Droplet Epitaxy.
Jihoon Lee 1 2 , Zh. M. Wang 1 , Baolai. L. Liang 1 2 , Kim. Sablon 1 2 , Neil Strom 2 , Gregory Salamo 1
1 Physics, University of Arkansas, Fayetteville, Arkansas, United States, 2 MicroEP (electronics and photonics), University of Arkansas, Fayetteville, Arkansas, United States
Show AbstractThe growth of InAs on GaAs (100) proceeds by the Stranski-Krastanov mode, where lattice strain drives 2-dimensional growth into 3-dimensional growth. The resulting InAs quantum dots are typically distributed randomly on the GaAs (100) surface due to the stochastic nature of self-assembly. Recently, great efforts have been made to laterally arrange such quantum dots into configurations with short or long range ordering. For example, InAs quantum dots can be aligned into laterally ordered arrays by patterning the GaAs surface photolithographically. In this presentation, we report a different approach to fabricating quantum-dot clusters by depositing InAs on GaAs surfaces containing GaAs nanostructures that have been formed by droplet homoepitaxy. With the absence of As flux, the deposition of Ga introduces Ga droplets on the GaAs (100) surface. Through subsequent exposure to an As flux, the Ga droplets can spontaneously form into different GaAs nanostructures like GaAs mounds and holed nanostructures. When InAs is then deposited onto these nanostructured GaAs (100) surfaces, different quantum structures can form. For example, InAs quantum-dot “bracelets” can form around GaAs mounds. Also, InAs quantum dots can develop in the middle of holed GaAs nanostructures before the typical InAs strain-driven transition takes place, and satellite InAs quantum dots can form around the holed GaAs nanostructures when additional InAs is deposited. These novel quantum dot configurations show potential for optoelectronic applications.
3:00 PM - M8.3
Patterning Ordered Metal Nanoparticle Arrays via Integration of Self-Assembly and Standard Manufacturing Technology
Regina Ragan 1
1 Chemical Engineering and Materials Science, University of California, Irvine, Irvine, California, United States
Show AbstractNoble metal nanostructures have demonstrated: the capacity for single molecule detection limits in plasmon resonance biosensors, chemical selectivity and higher performance in catalytic processes than their bulk counterparts; and the transport of electromagnetic energy along particle chains in optical circuits to name a few examples. One of the most significant challenges to technological developments that capitalize on unique properties of metal nanoparticles and nanowires is the fabrication of nanostructure arrays with monodisperse size, shape and spacing and high density using a low cost and high throughput technique since in all applications, structural properties affect performance. We will present two unique methods for producing ordered arrays of monodisperse metal nanocrystals. The first process involves a Si–compatible fabrication process for dense (~10^11/cm^2) parallel arrays of bimetallic nanostructures with monodisperse size and shape, over large area (> 1 mm^2), and having feature size and inter-particle spacing unattainable with state of the art electron beam lithography. Noble metal deposited on a self-assembled nanowire template via physical vapor deposition combined with reactive ion etching produced noble metal nanostructures with particle diameters of ~ 8nm with a narrow size distribution < +/-1.4 nm and inter-particle spacing of 1 nm. Thermodynamic and kinetic driving forces that lead to nanostructure formation will be discussed. The second process involves selective chemical attachment of metal nanoparticles on a self-assembled hexagonally ordered cylindrical PS-b-PMMA diblock copolymer template via a bifunctional molecular linker. Diblock copolymer films are chosen as a template for metal nanoparticle assembly since previous work has demonstrated the ability to fabricate ordered arrays over areas hundreds of microns in length as well as the ability to integrate diblock copolymer film deposition with polymer inline manufacturing methods. The experiment is mainly divided into three steps: (a) Au and Ag nanoparticle synthesis with monolayer protection groups; (b) modification of surface functional group of polymer templates; and (c) cross linking between Ag or Au nanoparticles functional groups and polymer surface group. Selective binding of ligand functionalized Ag nanoparticles onto PMMA versus PS was achieved. By varying the ratio of domain size of the cylindrical region to nanoparticle diameter, the number of metal nanoparticles that can chemically assemble into a cluster on a domain can be varied. We will report structural characterization of nanoparticle arrays as well as optical characterization of nanoparticle clusters. These two processes are chosen for the ability to integrate metal nanoparticle array with Si microelectronics and polymer based microsystems.
3:15 PM - M8.4
Three-dimensional Self-organization of InAs/InP(001) Quantum Dot Multilayers: Correlation Diagram Describing the Transition from Aligned to Antialigned Stuctures.
Annie Levesque 1 , Nikolay Shtinkov 1 , Patrick Desjardins 1 , Remo Masut 1
1 Génie Physique, École Polytechnique de Montréal, Montréal, Quebec, Canada
Show Abstract3:30 PM - M8.5
Linear Alignment of InGaAs Quantum Dots on Nominal GaAs(001) Surfaces.
Haeyeon Yang 1 , Dongjun Kim 1 , E. Everett 1
1 Physics, Utah State University, Logan, Utah, United States
Show Abstract3:45 PM - M8.6
Room Temperature Exciton Storage in Elongated Semiconductor Nanocrystals
Robert Kraus 1 , Pavlos Lagoudakis 1 , Andrey Rogach 1 , Dmitri Talapin 2 , Horst Weller 2 , John Lupton 1 , Jochen Feldmann 1
1 Photonics and Optoelectronics Group, Physics Department and CeNS, Ludwig-Maximilians-Universität München, Munich Germany, 2 Institute of Physical Chemistry, University of Hamburg, Hamburg Germany
Show AbstractSpherical CdSe nanocrystals capped by a CdS rod-like shell, referred to as nanorods, exhibit interesting spectral dynamics on the single particle level.[1,2] However, for the purpose of applications, the ensemble properties of nanorods are most interesting. We are especially interested in the behaviour of an ensemble of nanorods under the influence of an electric field, as this holds great relevance for future devices. We show here that by applying an electric field to an ensemble of nanorods in a vertical sample geometry fluorescence quenching coincides with a suppression of radiative rate without increasing ionization and non-radiative decay. After turning off the electric field, a significant fraction of quenched - and therefore stored - excitons recombines radiatively, even for field lengths up to 100 µs. Despite the reduction of the wavefunction overlap arising from the field induced separation of electron and hole the coulombic binding of the exciton prevents enhanced trapping of carriers on the highly reactive surface or in potential fluctuations in the nanocrystal lattice. As exciton storage selects the most polarizable particles, a significant quantum confined Stark shift of ~15 meV along with a correlated broadening of the spectrum is observed in the time-resolved emission of the ensemble at room temperature. This controlled modulation of both the emission wavelength and width together with the dramatic extension of the nanoparticle radiative lifetime opens up promising applications in optical modulators and optical memory cells.[1] J. Müller et al., Phys. Rev. Lett. 93, 167402 (2004)[2] J. Müller et al., NanoLett. 5, 2044 (2005)
M9: Theory of Self-assembly
Session Chairs
Wednesday PM, November 29, 2006
Room 208 (Hynes)
4:30 PM - **M9.1
Understanding the Growth of Quantum Dots.
Jerry Tersoff 1
1 , IBM Watson Center, Yorktown Heights, New York, United States
Show AbstractSelf-assembled semiconductor quantum dots have potential applications in future nanoscale electronic devices. However, their growth exhibits many surprising and remarkable features, which can include lateral motion and complex patterns of alloy intermixing. These must be understood in order to grow well-controlled nanostructures. Examples will be shown, illustrating the use of both continuum simulations and simple theoretical arguments to understand observed features of growth.
5:00 PM - M9.2
Effects of Surface and Interface Stresses on Film Stress during the Heteroepitaxial Film Growth
Chun-Wei Pao 1 , David Srolovitz 1
1 Mechanical & Aerospace Engineering, Princeton University, Princeton, New Jersey, United States
Show AbstractThe stress state within a heteroepitaxial films plays a key role in determining both the opto/electronic properties and morphology evolution of thin films. The wafer curvature method is widely used as a means of measuring stresses in thin film. However, the effects of surface and interface stresses on wafer curvature are rarely considered in analyzing such data. We rederive the conventional Stoney equation for application to both islands and films on thin wafers and demonstrate that such effects can be profound for the case of islands and very thin films. We complement this theoretical analysis with a series of atomistic simulations of heteroeptiaxial film growth. The simulations demonstrate that, even under positive lattice mismatch, the measured stress-thickness product (proportional to the wafer curvature) can still be negative and therefore does not reflect the stress state within the films. The discrepancy is associated with neglect of surface and interface stresses in the analysis. A new theoretical analysis, including the effects of lattice mismatch, surface and interface stresses, continuous films and islands is presented and validated via comparison with atomistic simulation results.
5:15 PM - M9.3
Microstructural Evolution of Strained Heteroepitaxial Multilayers
Ramanarayan Hariharaputran 1 , Vivek Shenoy 1
1 Division of Engineering, Brown university, Providence, Rhode Island, United States
Show AbstractNanopatterns formed in the microstructures during strained heteroepitaxial multilayer growth is of current interest in electronic material research for their unique optical and electronic properties. Recent experimental studies on multilayer growth report two kinds of growth namely (a) alternate tensile and compressive misfit strained layers and (b) strained and spacer layers on a substrate. The patterns observed arise due to Asaro Tiller Grinfeld instability where competition between relaxation of the misfit stress in the individual layers with respect to the substrate and surface energy results in a characteristic wavelength. Earlier numerical studies were predominantly linear stability analyses of the strained layers to identify the stability diagram as a function of kinetic parameters by assuming no bulk diffusivity and with simplifications to calculate the elastic state of the system. Here, we present a new model using phase field approach to simulate the evolution of strained multilayer growth. In contrast to earlier studies, our model predicts microstructural evolution in the non linear regime with finite bulk diffusivity and with calculation of elastic state without assumptions. We observe three types of patterns in the microstructure namely, layered growth leading to a superlattice of alternate phases, correlated islands and lateral composition modulation of the two phases which match with existing experimental microstructures. We present our results in terms of kinetic parameters namely, deposition flux, surface and bulk diffusivities and material parameters like misfit strain and surface energies of the individual layers.
5:30 PM - M9.4
Order, Randomness and Fluctuations in Heteroepitaxial Quantum Dot Growth.
Lawrence Friedman 1
1 Engineering Science and Mechanics, Pennsylvania State University, University Park, Pennsylvania, United States
Show AbstractEpitaxial self-assembled quantum dots (SAQDs) represent an important step in the advancement of semiconductor fabrication at the nanoscale that will allow breakthroughs in electronics and optoelectronics. SAQDs are a result of Stranski-Krastanow growth whereby a growing planar film becomes unstable after an initial wetting layer is formed. In the case of SAQDs, this instability is driven by lattice-mismatch strain energy, but also tempered by surface energy and wetting energies. Typical systems include GexSi1-x deposited on a Si substrate and InxGa1-xAs/GaAs. In applications, order of SAQD arrays is a key factor. There are two types of order, spatial and size disorder. Spatial order is needed for nanoelectronic applications, while size order is required for both nanoelectronic and optoelectronic applications. Here, the role of crystal anisotropy, random initial conditions and thermal fluctuations in influencing SAQD order during early stages of SAQD formation is studied through a simple stochastic model of surface diffusion. Surface diffusion is analyzed through a linear and perturbatively nonlinear analysis. The role of crystal anisotropy in enhancing SAQD order is elucidated. Furthermore, it is also found that spatial and size order are related and characterized similarly in terms of correlation lengths. There are two relevant and predictable correlation lengths that grow as the square-root of time. One of them is present in both the isotropic and anisotropic cases, but the other one is due to crystal anisotropy. This second anisotropy related length plays a limiting role for SAQD order. Finally, it is also found that SAQD order is enhanced when the deposited film is allowed to evolve at heights near the critical wetting surface height that marks the onset of non-planar film growth.
5:45 PM - M9.5
Elastic Energy Relaxation in Buried Quantum Dots.
Vladimir Chaldyshev 1 , Anna Kolesnikova 2 , Alexei Romanov 1
1 , Ioffe Institute, St. Petersburg Russian Federation, 2 , Institute of Problems of Mechanical Engineering, St.Petersburg Russian Federation
Show AbstractElastic distortions play an important role in the material systems with buried quantum dots. These distortions often govern the dot self-organization and ordering processes, electronic and atomic structure, optical and electronic properties. Relaxation of the elastic energy is usually undesirable due to crucial impact on the material crystallinity and corresponding device performance. In this paper we theoretically analyze three models, which correspond to three different ways of elastic energy relaxation in buried quantum dots. The first model considers formation of a pair of prismatic dislocation loops. One of them lies on dot/matrix interface, whereas the other is a satellite and locates in the adjacent matrix. The second model also includes the satellite loop and differs from the first one by non-local reduction of the dot plastic distortion. The origin of the satellite loop is the materials conservation requirement. The third model considers the case when this requirement is violated and only the misfit dislocation loop is formed. We determine the critical radii of the dots and loops, as well as the dependence of the satellite loop size on the dot size. The model calculation are compared to the relevant experimental data, which are InGaAs dots in GaAs, SiGe dots in Si, AsSb clusters in GaAs, InN dots in GaN, PbCdTeSe nanoparticles in different media, ErSb dots in GaSb, and MnAs inclusions in GaAs.
M10: Poster Session: Applications: Electronics & Optoelectronics
Session Chairs
Eric Stach
Curtis Taylor
Zhiming Wang
Qikun Xue
Thursday AM, November 30, 2006
Exhibition Hall D (Hynes)
9:00 PM - M10.1
Control of Spontaneous Emission from Colloidal Quantum Dots in a Polymer Microcavity
Vinod Menon 1 , Nikesh Valappil 1 , Iosif Zeylikovich 2 , Taposh Gayen 2 , Bidyut Das 2 , Robert Alfano 2
1 Physics, Queens College of The City University of New York, Flushing, New York, United States, 2 Physics, City College of The City University of New York, New York, New York, United States
Show AbstractWe report on a six fold enhancement of spontaneous emission rate from colloidal CdSe/ZnS core/shell quantum dots embedded in a one dimensional polymer microcavity structure at room temperature. The polymer microcavity structures were fabricated using spin coating. Alternating layers of polymers of two different refractive indices were stacked to form the Distributed Bragg reflectors (DBRs). To achieve high reflectivity, the polymers for the DBR structure were chosen such that they have relatively high refractive index ratio. The high and low refractive index polymers chosen were poly-N(vinylcarbazole) (PVK), and poly acrylic acid (PAA), with refractive indices of 1.683 and 1.420 at 600 nm respectively. Thin films of quarter wavelength thickness of the two polymers were alternately spin coated on a glass substrate to make the DBR structure. Greater than 90% reflectivity was obtained using ten periods of the structure. A PVK cavity layer of λ thickness embedded with CdSe/ZnS core/shell quantum dots (Evident technologies) is sandwiched between two such DBRs to form the entire microcavity structure. The bottom and top DBRs comprise of ten and five periods respectively.The possibility to enhance the spontaneous emission rate (Purcell Effect) through the use of cavities is attractive for realizing high-efficiency light emitters. Here, we demonstrate significant reduction in spontaneous emission lifetime of the CdSe/ZnS quantum dots embedded in a polymer microcavity structure realized by spin coating. Room temperature time-resolved photoluminescence measurements were carried out on this structure. The second harmonic of a Ti-Sapphire laser (400 nm) with pulse duration of 150 fs was used as the excitation source and the photoluminescence was collected using a streak camera with 10 ps resolution. The average pump power was 20 kW/cm2. The photoluminescence decay time of the bare CdSe/ZnS core/shell quantum dots was found to be ~1 ns while for the quantum dots embedded in the microcavity it is observed to be ~150 ps.
9:00 PM - M10.2
Investigating Charge Carrier Mobilities in Nanocrystal-Polymer Hybrid Photovoltaic Devices
Fan Zhang 1 , Ting Zhu 1 , Jian Xu 1
1 Engineering Science and Mechanics, Penn State Univ., State College, Pennsylvania, United States
Show Abstract9:00 PM - M10.3
Novel Electron Transport Behavior in Au-CdSe Core-shell Quantum Dot.
Wei Lu 1 , Bing Wang 1 , Jianguo Hou 1
1 Hefei National Lab for Physical Science at the Microscale, University of Science & Technology of China, Hefei, Anhui, China
Show Abstract9:00 PM - M10.4
Shape-controlled Bismuth Sulfide Colloidal Nanostructures and their Nanocrystal Plasma Polymerization.
Ludovico Cademartiri 1 , Reihaneh Malakooti 1 , Yasemin Akçakir 1 , Srebri Petrov 1 , Andrea Migliori 2 , Geoffrey Ozin 1
1 Department of Chemistry, University of Toronto, Toronto, Ontario, Canada, 2 , CNR-IMM, Bologna Italy
Show AbstractBismuth chalcogenides are a increasingly interesting class of materials, especially at the nanoscale, because of their potential for thermopower generation and as contrast agents for CT. For example Bi2Te3 is still the state-of-the-art thermoelectric material and Bi2S3 nanocrystals have recently been found to be the most performing contrast agents for X-ray CT to date. The synthesis and characterization of Bi2S3 nanostructures with controlled and monodisperse shape is thus highly relevant and will be discussed in detail[1]. Depending on the reaction conditions the nanocrystal shape can be changed from spherical to rod-like to ultranarrow wires[2]. The reaction we employed is a nearly solvent-less “hot injection” route conducted in an heterogeneous system. Our success in producing monodisperse, shape controlled and colloidally stable nanocrystals further expands the possibilities of such routes[3]. We will thus expand on the relevance of heterogeneity and precursor ratio on the growth behaviour of the nanocrystals as well as highlighting what still needs to be learnt from these “unusual” reaction conditions. In order to further extend the applicability of these materials we will describe the successful water solubilization of such nanostructures by using biocompatible polymeric ligands.The nanocrystal plasma polymerization [4] of the abovementioned nanostructures was performed to yield flexible and inorganic nanocrystal films and their full chemical characterization will be presented, demonstrating the formation of an inorganic and flexible nanocomposite built from colloidal nanocrystals building blocks. In the case of nanorods the composites showed a long-range smectic order which will be demonstrated via SAXS and HRSEM analysis.[1] “Shape-Controlled Bi2S3 Nanocrystals and their Plasma Polymerization into Flexible Films”R. Malakooti, L. Cademartiri, Y. Akçakir, S. Petrov, A. Migliori, G. A. OzinAdvanced Materials – accepted[2] R. Malakooti, L. Cademartiri, S. Petrov, A. Migliori, G.A.Ozin – manuscript in preparation[3] Multigram Scale, Solventless and Diffusion-Controlled Route to Highly Monodisperse PbS NanocrystalsL. Cademartiri, J. Bertolotti, R. Sapienza, D. S. Wiersma, G. von Freymann, G. A. OzinJournal of Physical Chemistry B 2006, 110(2), 671-673[4] Nanocrystals as Precursors for Flexible Functional FilmsL. Cademartiri, G. von Freymann, A. C. Arsenault, J. Bertolotti, D. S. Wiersma, V. Kitaev, G. A. OzinSmall 2005, 1 (12), 1184-1187
M11: Poster Session: Optoelectronic Properties
Session Chairs
Eric Stach
Curtis Taylor
Zhiming Wang
Qikun Xue
Thursday AM, November 30, 2006
Exhibition Hall D (Hynes)
9:00 PM - M11.2
``Forbidden" Fluorescence from Lead Selenide Quantum Dots.
Jeffrey Peterson 1 , Todd Krauss 1
1 Dept of Chemistry, University of Rochester, Rochester, New York, United States
Show Abstract9:00 PM - M11.3
Evidence for Ultrafast Carrier Transport in Barrier Separated Quantum Dot / Quantum Shell Structures.
Patanjali Kambhampati 1 , Eva Dias 1 , Samuel Sewall 1 , Kevin Anderson 1
1 Chemistry, McGill University, Montreal, Quebec, Canada
Show AbstractEvidence for Ultrafast Carrier Transport in Barrier Separated Quantum Dot / Quantum Shell StructuresIn order to extend the functionality of colloidal semiconductor quantum dots, epitaxial growth of additional layers of semiconductors has been explored. These colloidal studies offer a parallel to the developments of quantum well superlattices and multiple quantum wells. A well developed example for colloidal quantum dots is the CdS/HgS quantum dot / quantum well system. The ultrafast carrier dynamics in this system have furthermore been studied using femtosecond pump-probe spectroscopy. In the CdS/HgS system, carriers can directly relax into the low gap HgS quantum well on an ultrafast timescale. Broadly, this class of materials offers the potential of bandgap engineering along the radial coordinate for colloidal quantum dots. Recently, an interesting variation has been developed in which the core and the outer shell is separated by a tunneling barrier [1]. In this CdSe/ZnS/CdSe system, the inner ZnS shell serves as a tunable tunneling barrier offering a unique material geometry with which to probe charge carrier dynamics in coupled quantum dot based materials. Here, we provide spectroscopic evidence that ultrafast carrier transport from the core to the outer shell proceeds with a sub-picosecond timescale. Fabrication of a core/barrier/shell semiconductor nanocrystal is described consisting of CdSe/ZnS/CdSe. Photoluminescence is observed both from the CdSe core and CdSe outer shell. The photoluminescence excitation spectrum for the core in this structure shows dramatic differences from the bare core. The core/shell structure shows a distinct drop in the quantum yield for emission from the core, upon resonance with the band edge of the outer shell. This observation suggests the presence of carrier transport mechanism that effectively competes with intraband relaxation of the core. Based upon recent measurements in our laboratory of < 300 fs intraband relaxation of the electron and the hole in CdSe cores [2], we assign the drop in the quantum yield for emission from the core to ultrafast carrier transport through the ZnS tunneling barrier. Femtosecond experiments are underway to directly measure the ultrafast transport process suggested by the steady state spectroscopic data. The rapid and efficient charge separation within these colloidal core/shell structures holds promise for optoelectronic applications. [1] D. Battaglia, B. Blackman, and X. Peng, J. Am. Chem. Soc., 127, 10889 (2005). [2] S.L. Sewall, R.R. Cooney, K.E.H. Anderson, E.A. Dias, and P. Kambhampati, Phys. Rev. Lett., Submitted (2006).
9:00 PM - M11.4
Synthesis, Characterization, and Spectroscopy of CdSe-TiO2 Nanocomposites
Jin Young Kim 1 , Sung Bum Choi 1 , Sung Hun Youn 1 , Hyun Suk Jung 2 , Kug Sun Hong 1
1 Materials Science and Engineering, Seoul National University, Seoul Korea (the Republic of), 2 Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractSemiconductor nanocrystals, particularly the CdSe nanocrystals have attracted great interests over the past years due to their opto-electronic properties which are remarkably different from bulk materials and their potential applications to photonic band-gap crystals, light-emitting diodes, lasers, biological imaging, and so on. To improve their optical and electronic properties, much effort has been devoted to the preparation of composite semiconductors by combining different semiconductors such as CdS, CdTe, and ZnS. However, there have been only very few attempts to prepare CdSe composites combined with oxide semiconductors. In this study, a new approach to synthesize CdSe composite nanocrystals combined with TiO2 was investigated. CdSe-TiO2 nanocomposites were synthesized via a modified aminolysis route using Ti-isopropoxide (TIP), oleic acid (OLA), and oleylamine (OA). CdSe nanocrystals as the seeds for the reaction were prepared by the precursor injection based on the 1-octadecene (ODE) without using the trioctylphosphine oxide (TOPO). Effects of the aminolysis temperatures and the amounts of OLA on morphology and phase evolution of the CdSe-TiO2 nanocomposites were systematically investigated. The successful formation of CdSe-TiO2 nanocomposites was confirmed by a high-resolution transmission electron microscope (HR-TEM), X-ray diffraction, and Raman spectroscopy. Optical characterizations also confirmed the CdSe-TiO2 nanocomposites. UV-Vis spectroscopy and spectrofluorometry for the CdSe-TiO2 nanocomposites exhibited lower-energy absorption and emission relative to the CdSe seeds, which could be ascribed to the excitation and recombination between CdSe and TiO2. The TiO2 phase played roles as both separation of photoelectrons and passivation of the CdSe seeds, which could support the feasibility as the photovoltaic applications.
M12: Poster Session: Electronic Structure
Session Chairs
Eric Stach
Curtis Taylor
Zhiming Wang
Qikun Xue
Thursday AM, November 30, 2006
Exhibition Hall D (Hynes)
9:00 PM - M12.1
3D-Imaging of Non-spherical Silicon Nanoparticles Embedded in Silicon Oxide by Plasmon Tomography
Aycan Yurtsever 1 , Matthew Weyland 1 , David Muller 1
1 Applied&Engineering Physics, Cornell University, Ithaca, New York, United States
Show Abstract9:00 PM - M12.2
Zinc-blende or Wurtzite: Phase tunable in ZnS Nanocrystals
Hao Zhu 1 , Yuwen Zhao 1 , John Q Xiao 1
1 Department of Physics and Astronomy, University of Delaware, Newark, Delaware, United States
Show Abstract9:00 PM - M12.3
Iron Nanoparticles Embedded in Silica Glass: A Computational Study.
Peter Kroll 1 , Jens Theuerkauf 1 , Thomas Wieland 1
1 Inorganic Chemistry, RWTH Aachen University, Aachen Germany
Show AbstractM13: Poster Session: Biological Applications & Sensing
Session Chairs
Eric Stach
Curtis Taylor
Zhiming Wang
Qikun Xue
Thursday AM, November 30, 2006
Exhibition Hall D (Hynes)
9:00 PM - M13.1
Preparation of Organic-inorganic Composite Containing Single Quantum Dot.
Kyoungja Woo 1 , Donghyun Koo 1
1 Nano-Materials Research Center, Korea Institute of Science and Technology, Seoul Korea (the Republic of)
Show Abstract9:00 PM - M13.2
Preparation and Characterization of Quantum Dot - Hyaluronic Acid Conjugates For Tissue Engineering Applications
Ji Seok Kim 1 , Sei Kwang Hahn 1 , Sung Jee Kim 2
1 Advanced Materials Sciences and Engineering, Pohang University of Science and Technology (POSTECH), Pohang, Kyungbuk, Korea (the Republic of), 2 Chemistry, Pohang University of Science and Technology (POSTECH), Pohang, Kyungbuk, Korea (the Republic of)
Show AbstractQuantum dot - hyaluronic acid (HA) conjugates were synthesized for real-time-imaging of HA degradation and the following biological phenomena. HA is a biodegradable, biocompatible, non-immunogenic, and non-inflammatory linear polysaccharide which has been widely used for arthritis treatment, ophthalmic surgery, drug delivery, and tissue engineering. HA has been recently reported to be degraded at its binding site of LYVE-1 on lymphatic vessels, which initiate angiogenesis for tumor metastasis. Interestingly, HA has a different conformational structure in water and in organic solvent, and the carboxyl group of HA is known to be the recognition site of CD44 and hyaluronidase. Based on these facts, HA was chemically modified in the mixed solvent of water and ethanol by grafting adipic acid dihydrazide (ADH) to the carboxyl group of HA, which resulted in controlled degradation of HA in the body. The resulting stealth HA was conjugated with quantum dots and additionally crosslinked to prepare HA hydrogels incorporating quantum dots. Quantum dots are advantageous for long time imaging since they do not photo-bleach and can be easily multiplexed. In this work, the preparation and characterization of quantum dot – HA conjugate and HA hydrogel incorporating quantum dots were carried out to illustrate the biological roles of HA in the body for future tissue engineering applications.
9:00 PM - M13.3
Phospholipid-Functionalized Core/Shell CdSe/ZnS Quantum Dots for Photoluminescence Enhancement.
Peng He 1
1 Chemistry, North Carolina State University, Raleigh, North Carolina, United States
Show Abstract9:00 PM - M13.5
Engineering InAsxP1-x/InP/ZnSe III-V Alloyed Core/Shell Quantum Dots for the Near Infrared.
Sangwook Kim 1 , John Zimmer 2 , John Frangioni 3 , Moungi Bawendi 2
1 Molecular science and Technology, Ajou University, Suwon Korea (the Republic of), 2 chemistry, MIT, Boston, Massachusetts, United States, 3 Hematology/Oncology,, Beth Israel Deaconess Medical Center,, Boston, Massachusetts, United States
Show AbstractQuantum dots with a core/shell/shell structure consisting of an alloyed core of InAs1-xPx, an intermediate shell of InP, and an outer shell of ZnSe were developed. The InAs1-xPx alloyed core has a graded internal composition with increasing arsenic content from the center to the edge of the dots. This compositional gradient results from two apparent effects: (1) the faster reaction kinetics of the phosphorus precursor compared to the arsenic precursor, and (2) a post-growth arsenic-phosphorus exchange reaction that increases the arsenic content. The cores have a zinc blend structure for all compositions and show tunable emission in the Near Infrared (NIR) region. A first shell of InP leads to a red-shift and an increase in quantum yield. The final shell of ZnSe serves to stabilize the dots for applications in aqueous environments, including for NIR biomedical fluorescence imaging. These NIR emitting core/shell/shell InAsxP1-x/InP/ZnSe were successfully used in a sentinel lymph node mapping experiment.
Symposium Organizers
Eric A. Stach Purdue University
Curtis R. Taylor Virginia Commonwealth University
Zhiming M. Wang University of Arkansas
Qi-Kun Xue Tsinghua University
M14: Theory of Self-assembly
Session Chairs
Thursday AM, November 30, 2006
Room 208 (Hynes)
9:00 AM - **M14.1
Self-organized Interfacial Growth in Diffusion-limited System.
Mu Wang 1
1 National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093, Jiangsu, China
Show AbstractIn diffusion-limited growth very often long-range ordering effect can be observed, which is characterized either by forming crystallite branches with strict zigzag features [1-3], or by consecutively rotating the crystallographic orientation with certain axis and leading to periodic faceting-roughening transitions on the surface of crystallite aggregate [4]; or by nucleating crystallites with specific facets on a single crystalline substrate, forming an agglomerate of nano-platelets with a very random appearance, yet in fact the whole agglomerate is single-crystalline [5]. We demonstrate that these unusual growth phenomena are related to surface tensions during the interfacial growth. We show that the first two effects are due to the imbalance of surface/interface tensions in the early stage of nucleation at the concave corner of crystal facet and the foreign substrate, and the subsequent successive rotation of crystallographic orientation in lateral growth. A theory based on thermodynamics is developed to describe this unusual growth behavior. The third effect can be interpreted by epitaxial nucleation followed by diffusion-limited growth. These experimental observations provide interesting examples how interfacial tensions play a role in self-organized growth. [1] Mu Wang, X.Y. Liu, C. Strom, et al., Phys. Rev. Lett. 80, 3089 (1998) [2] D. W. Li, Mu Wang, P. Liu, et.al., J. Phys. Chem. B107,96-101(2003)[3] X. Y. Liu, Mu Wang, D. W. Li, et al., J. Cryst. Growth 208, 687-695 (2000)[4] Mu Wang, D.-W. Li, D.-J. Shu, P. Bennema, et.al., Phys. Rev. Lett., 94, 125505 (2005)[5] Tao Liu, et al., to be published
9:30 AM - M14.2
Morphological Evolution of Ge Quantum Dots on Si(001) with Deposition in Terms of Competition Between Elastic and Plastic Relaxation Mechanisms.
Leo Miglio 1 , Francesco Montalenti 1
1 Dept. of Materials Science, University of Milano Bicocca, Milano Italy
Show Abstract9:45 AM - M14.3
Forces that Drive the Self-assembly of Metallic Dots on Semiconductor Substrates.
David Salac 1 , Wei Lu 1
1 Mechanical Engineering, University of Michigan, Ann Arbor, Michigan, United States
Show Abstract10:00 AM - M14.4
Thermodynamic Modeling of Island Size Distributions for InGaAs/GaAs Self Assembled Quantum Dots: A Quantitative Effort to Understand Ensemble Size Nonuniformity.
Jeff Cederberg 1 , Alexana Roshko 2 , Brit Hyland 2 , Michael Coltrin 1
1 Advanced Materials Sciences, Sandia National Laboratories, Albuquerque, New Mexico, United States, 2 Optoelectronics Division, National Institute of Standards and Technology, Boulder, Colorado, United States
Show Abstract10:15 AM - M14.5
Examination of Self-Assembly of Quantum Dot Superlattices Using the Phase-Field Model of a Heteroepitaxial Thin Film
Nirand Pisutha-Arnond 1 , Steven Wise 3 , John Lowengrub 3 , Peter Voorhees 2 , Katsuyo Thornton 1
1 Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States, 3 Mathematics, University of California, Irvine, Illinois, United States, 2 Materials Science and Engineering, Northwestern University, Evanston, Illinois, United States
Show AbstractA phase field model was employed to examine quantum-dot-superlattice formation during heteroepitaxial growth. It is well known that quantum dots form due to the stress-driven Asaro-Tiller-Grinfeld (ATG) instability. While the size of the dots is related to the misfit strain as found in the ATG theory, kinetic effects result in a large variation in dot size and spatial distribution. The quantum dot superlattices provide more uniform dot size and spatial distributions, as well as vertical alignment. Using two- and three-dimensional phase-field simulations, we examined the formation of quantum dot superlattices. We found that more regular dot sizes and spacing could be achieved in a quantum dot superlattice when the layers were allowed to relax sufficiently and the amount of deposited material was tuned to cover each of the layers in the relaxed configuration. The effect of surface roughness was also examined.
10:30 AM - M14.6
A Level-Set Method for Epitaxial Growth and Self-Organization of Quantum Dots
Christian Ratsch 1 , Young-Ju Lee 1 , Xiaobin Niu 1 , Russel Caflisch 1
1 Mathematics, UCLA, Los Angeles, California, United States
Show AbstractWe have developed an island dynamics model for heteroepitaxial growththat employs the level-set technique in combination with a fullyself-consistent elastic model. At every timestep in the simulation,we solve the elastic equations for the entire system. This ispossible within our approach because the numerical timestep canbe chosen much larger than in an atomistic simulation. At everylattice site strain then changes the potential energy surface, and thusthe microscopic parameters of the simulation. In particular, strainwill enhance the rate of detachment from island edges. We will presentresults that show that a strain induced enhancement of the rateof detachment leads to a more regular distribution of island sizes.The reason is that bigger islands are typically more strained thansmaller islands, and thus their growth is slowed down. We alsopresent results that show that a strain induced enhancement ofthe detachment rate moves the system from layer-by-layer growthto the formation of coherent islands.
10:45 AM - M14.7
Surface Diffusion and Scaling Behavior in the Nucleation and Growth of InAs Quantum Dots on GaAs(001).
Arciprete Fabrizio 1 , Ernesto Placidi 2 , Massimo Fanfoni 1 , Fulvia Patella 1 , Enrico Orsini 1 , Adalberto Balzarotti 1
1 Department of Physics, University of Rome "Tor Vergata", Rome Italy, 2 , CNR-INFM, Rome Italy
Show AbstractM15: Heteroepitaxy - Growth & Characterization I
Session Chairs
Thursday PM, November 30, 2006
Room 208 (Hynes)
11:30 AM - **M15.1
Engineering Nanostructures and their Behavior
Gregory Salamo 1
1 physics, University of Arkansas, Fayetteville, Arkansas, United States
Show AbstractRecent clever techniques for fabricating nanosize materials, one-atomic-layer-at-a-time, have simultaneously opened the door to new physics, chemistry, biology, and engineering. Nanosize materials simply do not behave as the bulk. Indeed, the rules that govern the growth and behavior of these tiny structures must be uncovered. In this talk we will discuss our recent efforts to engineer shape, size, density, and position of nanostructures and the interactions between them, and to develop a clear understanding of their optical and electrical behavior.
12:00 PM - M15.2
Shape Transitions and Intermixing of Self-assembled PbSe Quantum Dots Studied by Scanning Tunneling Microscopy.
Gunther Springholz 1 , Laurel Abtin 1 , Vaclav Holy 2
1 , University of Linz, Linz Austria, 2 , Charles University, Prague Czech Republic
Show AbstractSelf-assembled semiconductor quantum dots generally exhibit strong changes in composition and shape during overgrowth with a protective capping layer, which has pronounced effects on the electronic and optical properties of the dots. To determine the structure of buried quantum dots, we have developed a new technique based on the analysis of the surface lattice deformations measured by in situ scanning tunneling microscopy. For the investigated material system of PbSe quantum dots grown on PbTe (111) it is thus found that a very strong intermixing occurs when the dots are overgrowth with PbTe capping layers due to strong Se/Te ex-change reactions. This results in a rapid shrinking of the dots and to a transition to truncated pyramids after embedding in the matrix material. From a series of systematic overgrowth studies, it is found that this effect can be suppressed using a thin EuTe layer as barrier for surface exchange. In this way the sharp triangluar pyramidal structure of the as grown surface dots is completely preserved. The analysis of the STM surface profiles by elastic strain calculations reveals that these dots also exhibit a higher Se concentration, i.e., less material intermixing, as well as much sharper interfaces as compared to the solely PbTe capped dots. This has significant consequences for the modeling of the electronic properties of the dots.
12:15 PM - M15.3
Interdiffusion Processes in SiGe:Si(001) Islands.
Marina Leite 1 2 , Gilberto Medeiros-Ribeiro 1 3 , Theodore Kamins 3 , R. Stanley Williams 3
1 , LNLS, Campinas, Sao Paulo, Brazil, 2 , Instituto de Fisica Gleb Wataghin - UNICAMP, Campinas, Sao Paulo, Brazil, 3 , Hewlett-Packard Laboratories, Palo Alto, California, United States
Show AbstractThe mechanisms involving SiGe:Si(001) island formation have been intensively studied due to the interesting prospects for epitaxial self-assembly. In particular, the understanding of how alloying can arise in these structures - from intermixing during growth, bulkinterdiffusion or surface diffusion - is of primary importance in the knowledge of self-assembled island growth. In order to elucidate the contribution of each different mechanism during Ge:Si island formation, four samples were grown by Chemical Vapor Deposition with distinct experimental parameters. The growth temperature was kept at T=600 °C with growth rates of 6ML/min and 3ML/min (F, S). To separate the influence of surfacediffusion from bulk interdiffusion in island evolution, two samples grown at 600 °C and 3 ML/min were subsequently annealed for 10 minutes in PH3 and H2 environments (P, H). Selective etching experiments were performed using a H2O2:NH4OH solution, suitable for attacking SiGe alloys with high selectivityand low etch rates (approximately 0.1 Å/s). Atomic Force Microscopy (AFM) analysis was carried out before and after etching steps. The post-growth annealing experiments were performed with the aim of evaluating the effects of surface diffusion and bulk interdiffusion. For sample H, both processes contributed to the island compositionprofile, whereas for sample P, the PH3 environment hinders the Si surface diffusion, making bulk interdiffusion the primary process for alloying. All samples were grown to produce a surface with a predominant dome population. A few superdomes (SDs) were observed in samples S and H, and some pyramids were observed for all samples. Theetching results were quite different for all island shapes. The islands in sample P were attacked preferentially along the [100] direction. After 1 hour of etching, the domes exhibited a flower-shape morphology, with mounds along the [110] directions. This unique observation is a strong indication of a favored interdiffusion along this direction. As a result, one can infer that the material under the {311} facets was richer in Si. In contrast, the islands treated in a H2 environment showed significant Si interdiffusion from the sides. Distinct from all other cases, the domes were attacked asymmetrically. Also noteworthy from the AFM results obtained for sample D, the SDs were attackedfrom inside, leaving a Si rich ring in the outer region of the SDs. This observation confirms the simultaneous contribution of the two processes for sample H. The present data allowed separation of the three main alloying mechanisms, which permits a better understanding of the composition evolution. It was found that the PH3 effectively suppresses surface diffusion, whereas the annealing with H2 allowed both bulk and surface processes. These results indicate the complexity and importance of alloying in island formation and composition profile.
12:30 PM - M15.4
Characterization of Self Assembled Germanium Quantum Dots using STEM and CBED.
Trinity Biggerstaff 1 , Donovan Leonard 1 , Konrad Jarausch 2 , Vy Yam 3 , Daniel Bouchier 3 , Osama Aboelfotoh 1 , Gerd Duscher 1 4
1 Material Science and Engineering, North Carolina State University, Raleigh, North Carolina, United States, 2 Electron Microscopy Division, Hitachi High Technologies America, Inc., Pleasanton, California, United States, 3 Institute of Fundamental Electronics, University Paris-Sud, Paris France, 4 Condensed Matter Sciences Division, Oak Ridge National Lab, Oak Ridge, Tennessee, United States
Show Abstract12:45 PM - M15.5
Photoluminescence Properties of InAs/GaAs Heterostructure with Surface Quantum Dots Sitting on Multiple-layer Buried Quantum Dots.
Baolai Liang 1 , Zhiming Wang 1 , Yuriy Mazur 1 , Eric DeCuir Jr 2 , Omar Manasreh 2 , Gregory Salamo 1
1 Department of Physics , University of Arkansas, Fayetteville, Arkansas, United States, 2 Department of Electrical Engineering, University of Arkansas, Fayetteville, Arkansas, United States
Show Abstract Semiconductor quantum dots (QDs) hold the promise for new generation of electronic/photonic devices. In the case of surface quantum dots (SQDs) directly exposed to air without any capping layers on top of them, the optical and electronic behavior of such SQDs is very sensitive to any minimum fluctuation of the surface potential. These surface-sensitive nanostructures combining together with biological/chemical functional molecular composites are expected to play an important role in future sensor applications. As an example, the optical properties of the InAs SQDs could be extremely sensitive to variations in the surface environment and InAs SQDs may be a better candidate for sensing biological agents. But the problem is that the optical behavior of InAs SQDs is very poor due to surface states, as shown by their weak and broad photoluminescence (PL). We report on an approach that overcomes this limitation through a stacking heterostructure with several layers of InAs buried QDs (BQDs) underneath the InAs SQDs. The samples are grown on semi-insulated GaAs (100) by molecular beam epitaxy (MBE) with adjacent InAs QD layers separated by a 10nm GaAs spacer. The InAs QDs are vertically-aligned across all the QD layers due to the strain field transmitted. Because of the vertically-aligned structure and a thin spacer, there is strong carrier transfer from the BQDs to the SQDs, which consequently results that the PL emission from surface quantum dots is significantly improved. The optical properties are investigated through a series of samples with a different number of BQDs layers. PL spectra show two prominent peaks, one from surface quantum dots and one from buried quantum dots. As the layer number of BQDs increases, the PL peak from the SQDs becomes narrower and stronger. With 4-layers of BQDs underneath, the SQDs have a PL peak redshifted about 283meV with respect to the one of BQDs, which mainly due to the strain variation before and after the growth of the GaAs cap layer. Remarkably, the PL peak from the SQDs is more than 7 times stronger than that of the sample with only a single layer of SQDs. Meanwhile, the linewidth of the SQD peak is significantly narrower than that of the sample with only a single layer of SQDs. The improvement of the SQDs spectrum is attributed to the vertical coupling between the BQDs and SQDs. Temperature and excitation density depended PL measurement prove that there is strong carrier transfer from BQDs to SQDs. The carrier tunneling time is determined to be about 450ps by time-resolved PL investigation. So the performance of the SQDs obtains significant enhancement in our approach. The close spatial and optical correlation between surface and buried quantum dots enable this hybrid QD structure great potential for biological sensing.
M16: Heteroepitaxy - Growth & Characterization II
Session Chairs
Gunther Springholz
Zhiming Wang
Thursday PM, November 30, 2006
Room 208 (Hynes)
2:30 PM - **M16.1
The Atomic and Step Structure of III-V Semiconductors Wetting Layers.
Jennifer Lee 1 , Jessica Bickel 1 , Chris Pearson 2 , Joanna Mirecki-Millunchick 1
1 Department of Materials Science and Engineering, The University of Michigan, Ann Arbor, Michigan, United States, 2 Department of Computer Science, Engineering Science and Physics, The University of Michigan-Flint, Flint, Mississippi, United States
Show AbstractWe are using a combined Molecular Beam Epitaxy- Scanning Tunneling Microscope system to investigate the atomic structure and diffusion of the adatoms during the growth of strained alloy films in order to understand the step structure of the wetting layer of both tensile strained GaAs/InP(001) and compressively strained GaSb/GaAs(001) films. Models of strained film growth based on elastic strain energy predict unique morphologies for different strain: islands upon large terraces for compressive films and mesas separated by trenches for tensile films. Compressive GaSb/GaAs(001) films have the expected islanded morphology for thicknesses greater than 2.2 MLs. The surface reconstruction of the wetting layer, however, is a combination of β2(2x4), which is the reconstruction of the GaAs substrate, and an Sb-rich(4x3) reconstruction, which is typical of unstrained GaSb(001). Based upon these observations, we have developed a model regarding the initial growth of GaSb upon GaAs(001). A salient feature of this model is the exchange between the excess layer of Sb of the (4x3) reconstruction and Ga in order to maintain stoichiometry of the β2(2x4). In the case of Sb deposition in the absence of a Ga flux, the coverage of the β2(2x4) is lower, as expected from the model. Furthermore, the step edge density of these films is higher, suggesting that some of the necessary Ga may be obtained from disassociation at step edges. Tensile GaAs/InP(001) also have the expected mesa-trench morphology, except the terraces or mesas have significantly rougher steps and higher surface anisotropies. Image analysis of GaAs provides a quantitative assessment of step edge density versus growth rate and temperature for comparison with computational models. Conventional kinetic models predict that the step density decreases with increasing temperature or decreasing growth rate. Our data show that this decrease is mitigated by elastic interactions with step edges, resulting in an overall rougher morphology. Furthermore, films grown at high temperature have higher step edge density and pits extend into the substrate, indicating significant intermixing in this regime. The resulting morphology has been shown to be quite regular, inducing the formation of periodic lateral composition modulation.
3:00 PM - M16.2
Guided Self Assembly of InAs Quantum Dots on a Cleaved Facet: Experimental Results and Growth Kinetics.
Emanuele Uccelli 1 , Dieter Schuh 2 , Jochen Bauer 1 , Max Bichler 1 , Jonathan Finley 1 , Matthew Grayson 1 , Gerhard Abstreiter 1 , Anna Fontcuberta i Morral 1
1 Walter Schottky Institut, Technische Universität München, Garching Germany, 2 Institut für Angewandte und Experimentelle Physik II, Universität Regensburg, Regensburg Germany
Show AbstractSemiconductor quantum dots (QDs) have attracted in the past years significant interest worldwide because they are very promising active media for advanced device applications as well as they permit to investigate new quantum physics phenomena. For this purpose, the assembly of QDs in a deterministic way in pre-designed arrangements is highly desirable. Recently, we were able to fabricate long range ordered chains of InAs QDs by combining selective growth with self-assembly (1). InAs growth was realized on a (110) in situ cleaving surface consisting of AlAs nanostripes embedded in GaAs.Here, we present an extended phase diagram for the fabrication of QDs arrays, showing under which conditions preferential growth of QDs on the AlAs stripes occurs. For that, we investigated the QDs selective occurrence as a function of the AlAs stripe thickness, distance between the stripes, temperature, In rate and As4-vapor overpressure. Atomic Force Microscopy and Scanning Electron Microscopy measurements highlight that the nucleation of InAs QDs chain takes place only on the AlAs stripes. We found that the lateral dimensions of the QDs directly reflect the thickness of the underlying AlAs layer, while QDs density along AlAs stripe can be tuned by changing the distance between the stripes (GaAs layer thickness) and adjusting the As-supply. We also found a narrow optimum MBE growth conditions that inhibit larger triangular structures growth onto AlAs regions, and partially also onto GaAs regions: a very low In-rate (≤0.01nm/s) combined with a very high As4-vapor overpressure (4-7x10-5mbar) and a low substrate temperature range (430-480°C).A comprehensive model is presented, for the understanding of the mechanism of selective nucleation of the QDs on the AlAs stripes as a function of the growth conditions. The model gives insight for the mastering of the density and uniformity of the dots arrays. Finally, micro-photoluminescence spectroscopy measurements are presented. Spectra taken on ensemble of QDs showed a very small FWHM (~18-20meV), also indicating a small size distribution.Reference:(1) Bauer J. et al., Long-range ordered self-assembled InAs quantum dots epitaxially grown on (110) GaAs, Applied Physics Letters 85, 4750 (2004)
3:15 PM - M16.3
Selective Growth of Ordered Ge Quantum Dot Arrays on Si(111) by Self-Organized Adsorbate Induced Nanopattering.
Jens Falta 1 , Thomas Schmidt 1 , Jan Flege 1 , Subhashis Gangopadhyay 1 , Torben Clausen 1 , Andrea Locatelli 2 , Stephan Heun 3
1 Institute of Solid State Physics, University of Bremen, Bremen Germany, 2 , ELETTRA Synchrotron Light Source, Trieste Italy, 3 , TASC-INFM Laboratory, Trieste Italy
Show AbstractMonolayer deposition of adsorbates can drastically change the growth mode of heteroepitaxial systems. E. g., surfactants like Sb or Bi have been shown to suppress the formation of large 3D Ge islands and enable the growth of smooth Ge on Si(111)-7×7 [1-3]. In the present work, a novel approach is presented [4,5], using a partially Ga-covered Si(111) surface as template for the selective growth of aligned Ge nano-islands. In-situ low energy electron microscopy (LEEM) and x-ray photoemission electron microscopy (XPEEM) have been used to study the evolution of the surface. During submonolayer deposition of Ga at a substrate temeprature of 600° C, Ge:Si(111)-√3×√3 domains are formed, which exclusively nucleate at the Si(111)-7×7 domain boundaries and substrate step edges. As a consequence, a two-dimensional nanopattern of Ga-covered and Ga-free surface areas is formed. The size and the shape of this nanopattern can be tuned by the miscut angle and miscut orientation. These findings can be explained in terms of line and kink energies of the √3×√3 - 7×7 domain boundaries. Subsequent deposition of Ge on such a nanopattern leads to the selective growth of Ge nano-islands in the Ga-rich regions of the surface, which is attributed to the adsorbate induced modulation of the surface chemical potential. These heterogenous nucleation conditions result in well ordered arrays of small, homogeneously sized Ge dots which are aligned at the substrate step edges and initial 7×7 domain boundaries.References:[1] M. Horn-von Hoegen et al., Phys. Rev. Lett. 67, 1130 (1991)[2] G. Meyer, B. Voigtländer, and N.M. Amer, Surf. Sci. 274, 1932 (1994)[3] Th. Schmidt et al., Appl. Phys. Lett. 74, 1391 (1999)[4] Th. Schmidt et al., New J. Phys. 7, 193 (2005)[5] Th. Schmidt et al., submitted to Phys. Rev. Lett.
3:30 PM - M16.4
X-ray Diffraction Analysis of 2-D Nanostructures in AlAs/AlSb Superlattices.
Rebecca Forrest 1 , J. Throckmorton 1 , C. Canedy 2 , J. Meyer 2
1 Physics, University of Houston, Houston, Texas, United States, 2 , Naval Research Laboratory, Washington, District of Columbia, United States
Show AbstractX-ray diffraction has been used to identify self assembled lateral modulation in AlAs/AlSb strain balanced superlattices. This appears to be the first report of lateral modulation in this system. The superlattice structures consist of 122 periods of approximately 25 monolayers AlSb and 2 monolayers AlAs, grown by molecular beam epitaxy on GaSb. Both vertical superlattice and lateral modulation satellite peaks were observed. X-ray reciprocal space maps were made to determine the average morphology of the modulated structures. The vertical superlattice has a period of approximately 8 nm, while the lateral modulation has a period on the order of 100 nm in only one <110> direction, indicating the formation of two-dimensional nanostructures. The occurrence of lateral modulation in these AlAs/AsSb superlattices is consistent with recent work by Li, et. al. linking it not only to the strain in the layers, but also to the strain in the interfacial bonds. This may provide a means of controlling the occurrence of lateral modulation in superlattices.
3:45 PM - M16.5
Controlling Three-Dimensional Ordering in (In,Ga)As Quantum Dot Lattices by GaAs Surface Orientation.
Martin Schmidbauer 1 , Shahram Seydmohamadi 2 , Daniil Grigoriev 3 , Zhiming Wang 2 , Yuriy Mazur 2 , Peter Schaefer 3 , Michael Hanke 4 , Rolf Koehler 3 , Greg Salamo 2
1 , Institute for Crystal Growth, Berlin Germany, 2 Department of Physics, University of Arkansas, Fayetteville, Arkansas, United States, 3 Institute of Physics, Humboldt-University Berlin, Berlin Germany, 4 , Martin-Luther University Halle-Wittenberg, Halle/Saale Germany
Show AbstractIn the Stranski-Krastanow growth mode, while the growth conditions can be optimized to produce nanostructures of near identical size and shape, often only a random spatial distribution of the quantum dots (QDs) is observed for a single layer of QDs. However, for multiple layers a range of different results, from near perfect QD chains to three-dimensional (3D) lattices, has been reported and discussed. In this case, it has been suggested that the anisotropy in surface diffusion for (In,Ga)As QDs on GaAs (100), which is mainly caused by the (2x4) surface reconstruction with dimer rows running along [0-11], is responsible for the formation of QD chains along the [0-11]-direction. In particular, the surface diffusion length along [0-11] is larger than along [011], causing an asymmetric separation between neighboring dots and consequently leads to QD chain structures. In this paper, we report on experiments that use natural surface steps on high index substrates to further uncover the role of both surface diffusion and strain in producing 3D ordering of QDs. In particular we examine the formation and development of 3D lattices of (In,Ga)As QDs in a GaAs matrix. These lattices are created by vertically stacking QD layers while simultaneously introducing surface steps in each layer in order to vary and control the symmetry of the diffusion and strain pattern in each layer. Our findings show that by using different high index substrates we can in fact use surface steps to fine tune surface diffusion and strain in order to encourage 3D organized growth. For example, for multi-layered growth on the (100) surface of GaAs the 3D array of QDs is laterally aligned as a dot chain while vertically aligned directly along the [100]-direction. This situation is dramatically changed, however, when the substrate surface orientation is different from (100). The lateral ordering of the QD array transforms from a well aligned chain-like arrangement to a planar rhombic lattice when the surface orientation is systematically changed from (100) in the direction towards (111)B. We observe an interesting linear relationship between the in-plane rhombic angle and the surface step density. Meanwhile, these changes are coupled to systematic changes in the alignment along the growth direction. Theoretical investigations based on a linear elasticity theory show a systematic variation of the angle α of inclined vertical inheritance of the lateral QD positions when the surface orientation is changed from (100) towards (111)B. The calculated values for the inclination angle α are mainly determined by the elastic anisotropy of the GaAs matrix and are in excellent qualitative and quantitative agreement with experimental findings based on x-ray diffuse scattering.
4:30 PM - **M16.6
Polar and Non-polar GaN Quantum Dots.
Bruno Daudin 1
1 DRFMC/SP2M, CEA-Grenoble, Grenoble France
Show Abstract5:15 PM - M16.8
Temperature Dependence of Self-Assembled Quantum Dot Lateral, Density and Size Distributions.
Alexana Roshko 1 , Jeffrey Cederberg 2 , Brittany Hyland 1
1 , National Institute of Standards and Technology, Boulder, Colorado, United States, 2 , Sandia National Laboratory, Albuquerque, New Mexico, United States
Show AbstractControl of self-assembled quantum dot (SAQD) size and density is critical for optimizing device performance in many applications. Of particular importance is obtaining lateral uniformity of the dots across wafers. We have examined the influence of growth temperature on the lateral uniformity of In(Ga)As SAQD density and height across 5.1 and 7.6 cm (100) GaAs wafers. Samples were grown using three different techniques: pulsed molecular beam epitaxy (MBE), continuous MBE, and metal-organic chemical vapor deposition (MOCVD). The dot density and height distributions were measured as functions of position across the central third of the wafer area, using atomic force microscopy.For pulsed MBE there are relatively large variations in dot density and height across the wafers. The standard deviation of the dot density can vary up to 30 % of the average density and the standard deviation of the height can vary up to 10 % of the average. The lateral uniformity improves as the growth temperature decreases and the dot density increases. For continuous, low-growth rate MBE the dot density and height are significantly more uniform, with standard deviations being less than 5 % of their averages even with increasing growth temperature and dot densities down to 100 dots/μm2. For samples grown by MOCVD the standard deviations in dot density and height are less than 10 and 5 % of the averages respectively, even for densities as low as 10 dots/µm2. In spite of the considerable differences in dot uniformity for the pulsed and continuous MBE growth conditions, the temperature dependence of the average dot density for these samples is very similar. Assuming an Arrhenius relationship of the form ρSAQD = A exp (-Eact/kT), both the activation energies (-1.61 and -1.64 eV) and the pre-exponentials (10-8 and 9x10-9 dots/um2, for the pulsed and continuous samples respectively) are nearly identical. For the MOCVD sample the dependence is quite different. The activation energy is larger -3.63 eV and the pre-exponential is smaller 10-22 dots/um2. The difference in fit for the MBE and MOCVD dots is likely related to the substantial difference in composition for the samples investigated and the concomitant differences in strain. The MBE grown SAQDs are nominally 45 % In while the MOCVD SAQDs are nominally pure InAs. Tests are underway to establish whether the density of pure InAs dots grown by MBE have the same temperature dependence as the MOCVD grown dots. These results will be interpreted in terms of the relative importance of strain in the dot nucleation.
5:30 PM - M16.9
RHEED for In-situ Metrology of Stranski Krastanov InAs Qunatum Dots.
Andrea Feltrin 1 , Alex Freundlich 1
1 Center for Advanced Materials, University of Houston, Houston, Texas, United States
Show Abstract5:45 PM - M16.10
InAs Quantum Dots for Intersubband Devices
Aaron Andrews 1 , Matthias Schramboeck 1 , Tomas Roch 1 , Maximilian Austerer 1 , Werner Schrenk 1 , Gottfried Strasser 1
1 Center for Micro- and Nanostructures, TU Vienna, Wien, Wien, Austria
Show AbstractIntersubband devices, such as quantum cascade lasers (QCLs) and quantum well infrared photodetectors (QWIPs), exhibit strong photon emission and detection in the mid-infrared (MIR) (3-20 µm) and terahertz (THz) (65-260 µm) regions, which are of particular interest for chemical sensing. Traditional QCLs utilize the energy levels of a two-dimensional quantum well or superlattice injector and active region. A strong performance limitation of MIR devices is the low carrier lifetimes due to LO phonon emission. This parasitic channel can be strongly suppressed by using zero-dimensional quantum dot (QD) states in the active region. In a GaAs matrix the ground and excited states of an In-based QD are below the GaAs band edge. These dots have already been proven to significantly enhance the performance of interband lasers as well as MIR photodetectors. To use the discrete levels of QDs in intraband emitters, like QCLs, the electronic states must be above the conduction band edge to achieve efficient extraction of the electrons by the conventional two-dimensional minibands. To accomplish this, the QD matrix material must be the barrier, we utilize Al0.45Ga0.55As for MIR and Al0.15Ga0.85As for THz, and the dot size and composition must produce a ground state energy above the GaAs band edge. Previously we used InxAl1-xAs QDs, but the lower mismatch strain produced larger QDs and thus only a small increase in ground state energy. We present results for InAs QDs in AlxGa1-xAs barriers, where the QD ground states lie between the wetting layer energy and the GaAs band edge. This is only possible within a small range of QD sizes. We have determined that the InAs QD size and density are controlled by growth rate, monolayer thickness, growth temperature, in-situ annealing time, growth interruption, and sample rotation. Table 1 illustrates the large sensitivity of QD density on substrate temperature and AlxGa1-xAs composition for an identical InAs growth rate and monolayer thickness. The QD density can be varied over two orders of magnitude within a 20 °C range in growth temperature. QD density is increased significantly on AlxGa1-xAs surfaces with increasing Al concentration, while the QD size is reduced. The AlxGa1-xAs barrier thickness and growth temperature are adjusted to investigate the confinement of the carriers in the QD. Negative differential resistance with multiple peaks was observed in a resonant tunnelling diode (RTD) structure, where the quantum well was replaced by a single layer of InAs QDs with Al0.45Ga0.55As barriers.
M17: Poster Session: Heteroepitaxy
Session Chairs
Eric Stach
Curtis Taylor
Zhiming Wang
Qikun Xue
Friday AM, December 01, 2006
Exhibition Hall D (Hynes)
9:00 PM - M17.1
First-principles Calculations of Energetic Properties of Vacancies, Mn-atomic Defects in CdTe.
Rerbal Benali 1 , Merad Ghouti 1 , Cibert Joel 2 , Aourag Hafid 1
1 , LEPM-URMER. A. Belkaid University, B.P. 119, Tlemcen Algeria, 2 , Laboratoire Louis Néel, CNRS, BP 166, 38042 Grenoble cedex 9 France
Show Abstract9:00 PM - M17.10
Purcell Effect for CdSe Quantum Dots Embedded in High-Q Monolithic II-VI Pillar Microcavities.
Henning Lohmeyer 1 , Kathrin Sebald 1 , Carsten Kruse 1 , Detlef Hommel 1 , Jürgen Gutowski 1
1 Institute of Solid State Physics, University of Bremen, Bremen Germany
Show AbstractA promising temperature stability of the emission of single epitaxial CdSe/ZnSSe quantum dots (QDs) has been demonstrated by the observation of suppressed multiphoton emission up to 200 K [1]. If QDs couple to discrete optical modes of a high-Q microcavity (MC) the Purcell effect can yield optimized photon collection important for the realization of an efficient and compact source of single photons for applications in quantum information processing and communication. We report on the enhanced spontaneous emission of CdSe/ZnSe QDs embedded in all-epitaxial ZnSe-based pillar MCs. The samples were grown by molecular beam epitaxy on GaAs substrates. A ZnSSe λ cavity contains a single sheet of CdSe QDs (density ≈ 5*1010 cm-2). The cavity is embedded in an 18-periods bottom and a 15-periods top distributed Bragg reflector made of ZnSSe as high-index and MgS/ZnCdSe short-period superlattices as low-index layers (cavity resonance set to 2.41 eV, quality factor Q = 2500). Pillar MCs with diameters between 500 nm and 2.5 µm have been fabricated from the epitaxial sample by use of focused-ion-beam milling [2]. The QD ensemble shows a nearly Gaussian photoluminescence (PL) emission band (FWHM ≈ 50 meV) centered at 2.5 eV at low temperatures if measured for a reference sample (RS) without cavity. PL spectra of pillars measured at elevated temperature exhibit distinct resonant modes in good agreement with theory (see [2] for quantum-well MCs). For the smaller pillars, a Q ≈ 1500 can be determined leading to calculated Purcell factors of up to 19. At 5 K, partly resolution limited emission lines are observed superimposed to the modes becoming of increasing dominance for decreasing pillar diameters. These emission lines of individual QDs can be traced easily up to 50 K. If thus tuned in resonance with the pillar modes they show a pronounced enhancement of the detected PL intensity. For time-resolved measurements, pulsed excitation by a Ti:sapphire laser at 2.71 eV was used. On the low-energy shoulder of the QD emission band in the RS, an average PL lifetime (LT) of 480 ps is obtained. In the pillar MCs, QD emission being off-resonant to the pillar modes but coupling to leaky modes exhibits LTs comparable to the RS data. In contrast to this, the LTs measured for QDs resonant to the fundamental pillar modes are reduced by a factor of up to 3.8 depending systematically on the pillar diameter. This enhancement can be well understood as an average effect due to the random spatial and spectral distribution of the probed QDs with respect to the optical modes. Thus, these findings are a clear fingerprint of the Purcell effect and show the efficient coupling of the QDs to the pillar modes which is demonstrated here for the first time for QDs in an all-epitaxial ZnSe-based cavity.[1] K. Sebald et al., Appl. Phys. Lett. 81, 2920 (2002). [2] H. Lohmeyer et al., Appl. Phys. Lett. 88, 51101 (2006).
9:00 PM - M17.11
Real-time Observation of Nucleation and Evolution of InAs Quantum Dots on GaAs(001) During MBE.
Itaru Kamiya 1 , Kohtaro Matsuura 1 , Tsuyoshi Higashinakagawa 1
1 , Toyota Technological Institute, Nagoya Japan
Show Abstract Self-assembled (SA) quantum dots (QDs) have been widely studied due to the facileness in their preparation. Unlike other types of QDs that require complicated fabrication processes, SA QDs are prepared merely by depositing materials that have different bandgaps and lattice constants to the with respect to the substrate by epitaxial crystal growth techniques. InAs QDs on GaAs(001) grown by MBE or MOCVD have been a typical example, and their optoelectronic properties have been extensively investigated. For device applications, it is essential that their size and spatial distribution are controlled. However, since SA QDs are formed through random processes, it is not easy to achieve size and distribution uniformity without prior processing the substrate prior to crystal growth. A number of studies have been performed to understand the fundamental mechanisms of SA QD formation that would provide us with information to achieve such goal. Here, we performed real-time observation of SA InAs QD growth on GaAs(001) by MBE. In contrast to most previous reports that employed growth interruption, by following the time transient of RHEED specular beam in detail, we obtained information about nucleation and evolution of the QDs, and have been able to distinguish processes that are dependent and independent of growth rate. In addition, the results reveal that surface migration of In/As atoms and their incorporation into QDs, with the aid of the wetting layer, can be observed. We will also provide a quantitative discussion on these processes.
9:00 PM - M17.12
Modeling of Silicon Nanocrystals Nucleation and Growth Deposited by LPCVD on SiO2 : From Molecule/surface Interactions to Reactor Scale Simulations.
Ilyes Menouar Zahi 1 2 3 , Brigitte Caussat 2 , Hugues Vergnes 2 , Alain Esteve 1 , Medhi Djafari Rouhani 1 , Pierre Mur 3 , Philippe Blaise 3 , Emmanuel Scheid 1
1 , LAAS/CNRS, Toulouse France, 2 , LGC/CNRS, Toulouse France, 3 , CEA-DRT-LETI, Grenoble France
Show AbstractThe microelectronic industry is in permanent evolution, due to the need of integration in many systems of the everyday life (PC, car, MP3, mobile, …). A typical example concerns non volatile memories for which the poly-silicon floating gate of the Flash memories could be replaced by a floating gate made up of silicon nanocrystals. The deposition of nanocrystals by LPCVD (Low Pressure Chemical Vapor Deposition) from silane SiH4 on SiO2 surfaces remains one of the most promising ways of synthesis. In particular, it is mandatory to reach an area density of 10 exp.12 dots/cm2 with a radius lower than 5 nm so as convenient and reliable Flash memories could be industrialized. To overcome these various problems, we have decided to model physical and chemical phenomena from the scale of the precursor molecules and surface bonds to that of the industrial reactor.Despite a huge experimental effort, fundamental understanding of the key mechanisms for nucleation and growth remains elusive.Experiments presented in the literature have shown that a surface pre-treatment forming silanol Si-OH groups to the detriment of epoxy Si-O-Si bonds increased the nucleation rate. Previous modelings of LPCVD reactors showed that the silane pyrolysis leads to the formation of unsaturated species which are poorly concentrated but extremely reactive with surfaces. In this paper, we propose a multiscale approach combining several models in hierarchical order to extend progressively the experimental requirements while keeping the predictive nature of the results by transferring parameters from a model to another. Specifically, we study the fundamental chemical reaction taking place at the initial stage of the deposition, i.e. nucleation and growth, via first principles calculations (density functional theory). Chemical pathways and associated activation barriers for SiH2/SiH4 reactions onto oxide surface species are detailed. Parameters are thus defined (kinetics of the reactions, sticking coefficients) for further chemistry modeling at the continuum level coupled with reactor scale modeling features (CFD code Fluent associated to homogeneous and heterogeneous kinetic laws).The link between the multi-scale simulation procedure and some experimental results will be explained, we will demonstrate how simulation can enrich and guide future experiments. In particular, we find that an improvement of the reproducibility and uniformity of the Si nanocrystals deposition is possible by highly diluting silane in a carrier gas such as hydrogen or nitrogen, since the deposition kinetics of silane decreases sharply. Therefore, the deposition time could be significantly increased. A positive consequence of this high dilution seems to be the contribution to deposition of unsaturated species such as silylene highly increases; this could favor the dot nucleation and consequently improve the dot density and size uniformity.
9:00 PM - M17.13
Novel, Oxide Glass-Based Complex Nanocomposite Materials: Fabrication and Structural Characterization
Kristina Lipinska-Kalita 1 , Patricia Kalita 2 , Carlo Segre 3 , Russell Hemley 4 , Thomas Hartmann 5
1 Geosciences, University of Nevada Las Vegas, Las Vegas, Nevada, United States, 2 Physics, University of Nevada Las Vegas, Las Vegas, Nevada, United States, 3 Physics, Illinois Institute of Technology, Chicago, Illinois, United States, 4 Geophysical Laboratory, Carnegie Institution of Washington, Washington DC, District of Columbia, United States, 5 , Harry Reid Center for Environmental Studies, Las Vegas, Nevada, United States
Show Abstract9:00 PM - M17.14
Multi-Color Long Wavelength Infrared Detectors Based On Intersuband Transitions In InGaAs and InAs Quantum Structures.
Omar Manasreh 1 , Brandon Passmore 1 , Vasyl Kunets 2 , Greg Salamo 2 , Jinqiao Xie 3 , Lin Zhou 4 , David Smith 4
1 Electrical Engineering, University of Arkansas, Fayetteville, Arkansas, United States, 2 Department of Physics,, University of Arkansas, Fayetteville, Arkansas, United States, 3 Department of Electrical Engineering, VCU, Richmond , Virginia, United States, 4 The Center for Solid State Science, Arizona State University, Tempe, Arizona, United States
Show AbstractIntersubband transitions in several InGaAs, InAs multiple quantum dots and multiple quantum wells structures grown by the molecular beam epitaxy (MBE) technique have been investigated for their use in single, two-color, and broad-band long wavelength infrared detectors. The materials were characterized using optical absorption, photoluminescence, electrochemical capacitance-voltage, cross-sectional TEM, and XRD techniques. Devices were fabricated from various wafers and their photoresponse was measured at 77K. Furthermore, the dark current was measured in the temperature range of 77-300K for all devices. Single-band infrared detectors fabricated from InAs/GaAs multiple quantum dots were found to possess a photoresponse around 7 micron. However, a broad-band photoresponse was measured in the spectral range of 3.5 – 12 micron for detectors fabricated from InAs quantum dots embedded in graded InGaAs quantum wells with GaAs barrier. Two-band voltage-tunable photodetectors were fabricated from two stacks of InGaAs/AlGaAs multiple quantum well grown by the MBE technique with a photoresponse spectrum peaking at 6.0 micron and 10.0 micron. The voltage tunability is demonstrated by measuring the photoresponse as a function of bias voltage. The 6.0 micron band was found to be dominant at lower bias voltages while the 10.0 micron band is dominant at higher voltages.
9:00 PM - M17.15
Synthesis and Electronic Application of Germanium Nanocrystals in Silicon Oxide Matrix.
Wee Choi 1 2 , Wai Chim 1 2
1 Electrical and Computer Engineering, National University of Singapore, Singapore Singapore, 2 Advanced Materials for Micro- and Nano- Systems Programme, Singapore-MIT Alliance, National University of Singapore, Singapore Singapore
Show AbstractThe increasing use of portable electronics and embedded systems has resulted in a need for low-power high-density non-volatile memories. The current floating-gate flash memory cells use a relatively thick tunnel oxide to prevent direct tunneling current leakage to ensure good data retention capability and to reduce the off-state power consumption of the memory array. However, such a thick tunnel oxide means that the write and erase pulse durations during programming of the flash memory are relatively long and these compromise the programming speed of the device. Alternative proposal to use nanocrystal for memory application has demonstrated that the device can be programmed at fast speeds (hundreds of nanoseconds) using low voltages for direct tunneling and storage of electrons in the nanocrystals. Also by using such nanocrystal charge storage sites that are isolated electrically, charge leakage through localized oxide defects was reduced. In this talk, we will examine some fundamental aspects in the synthesis of Ge nanocrystals in silicon oxide matrix. The talk will be divided into (i) Synthesis of Germanium nanocrystal in silicon oxide matrix and (ii) Germanium nanocrystal formation in thin silicon oxide matrix for memory application. In Part (i), we will discuss the influence of Ge concentration, oxide diffusion barrier and the annealing conditions on the nanocrystal formation in thick co-sputtered Ge plus silicon oxide films. The basic mechanism of the Ge nanocrystal formation will be discussed. In Part (ii), we present results of Ge nanocrystal formation in thin (5-10 nm) silicon oxide matrix. The results on the optimization of the growth of Ge nanocrystals for memory applications will be discussed. The memory device consists of a metal-insulator-semiconductor structure. The insulator region consists of a tunnel oxide layer, a thin sputtered oxide layer with Ge nanocrystals embedded and a sputtered oxide cap layer. The roles of the cap oxide, tunnel oxide, and Ge concentration on the performance of the memory devices will be discussed extensively.
9:00 PM - M17.16
Controlling Lateral Ordering of InGaAs Quantum Dots with Arsenic Background
Euclydes Marega 1 , Mohammad Hussein 1 , Ziad Abu Waar 1 , Gregory Salamo 1
1 Physics, University of Arkansas, Fayetteville, Arkansas, United States
Show AbstractSelf-assembled quantum dots (QDs) have been investigated and proposed as a promising candidate to a new generation of lasers, detectors and photonic crystals. However, most of these applications require uniformity in size, shape, and spatial distribution of QDs to achieve their advantages over other materials. Lateral ordering of QDs grown on flat surfaces, like GaAs (100), has been hindered due to the random arrangement of self-assembled QDs. While recent methods using lithography techniques have demonstrated successful lateral control of the QDs positions, these methods require processing steps in addition to QDs growth and might suffer from greater probability of defects associated with processing.Lateral and vertical ordering of InGaAs QDs on GaAs (001) surface has been recently reported utilizing vertical stacking of QDs and growth at relatively high temperature. In fact “chains” of ordered QDs has been fabricated using this approach that has been discussed in terms of selective nucleation on strain minima and enhanced adatom diffusion in the (0-11) direction.We present in this work our method of breaking the anisotropy of such structures by changing the growth environment. We have shown experimentally that using As2 molecules instead of As4 as a background flux is promising to achieve an isotropic ordering of stacked quantum dots layers over GaAs (001) as a result of enhanced adatom diffusion along (011) direction. Our results are consistent with reported experimental and theoretical studies on surface structure and diffusion mechanism over GaAs (001). As confirmed by low temperature Photoluminescence (PL) measurements, the optical properties of laterally ordered QDs has been improved by using As2 as a background flux.
9:00 PM - M17.17
LO-phonon Overheating in Quantum Dots: Low Electronic Densities.
Karel Kral 1
1 , Institute of Physics, Acad. Sci. Czech Republic, Prague 8 Czech Republic
Show Abstract9:00 PM - M17.18
InAs/GaAs VS InP/GaAs Quantum Dot Molecules and their Potentials for Photovoltaic Applications.
Wipakorn Jevasuwan 1 , Sirichai Ruangdet 1 , Somchai Ratanathammaphan 1 , Somsak Panyakeow 1
1 Electrical Engneering, Chulalongkorn University, Bangkok, Bangkok, Thailand
Show Abstract9:00 PM - M17.19
Optical and Transmission Electron Microscopy studies of colloidal Au nanocrystals: Towards stable nanocrystals with narrow size- and shape-distributions.
Bo Jun Kim 1 , Atul Konkar 1
1 Mork Family Department of Chemical Engineering and Materials Science, University of Southern California, Los Angeles, California, United States
Show Abstract9:00 PM - M17.2
The Emission Wavelength Tailoring of Self-assembled InAs/InP Quantum Dots Grown on GaInAsP and InP Buffers.
Satya Barik 1 , H. Tan 1 , C. Jagadish 1 , N. Vukmirović 2 , P. Harrison 2
1 Department of Electronic Materials Engineering, The Australian National University, Canberra, Australian Capital Territory, Australia, 2 School of Electronic and Electrical Engineering, The University of Leeds, Leeds United Kingdom
Show Abstract9:00 PM - M17.21
Fabrication and Electrical Characteristics of Field-effect Thin-film Transistors with p-channels of HgTe Nanocrystals.
Hyunsuk Kim 1 2 , Kyoungah Cho 1 2 , Dong-Won Kim 1 2 , Hye-Ryoung Lee 1 2 , Sangsig Kim 1 2
1 Electrical Engineering, Korea Univ., Seoul Korea (the Republic of), 2 Institute for Nano Science, Korea Univ., Seoul Korea (the Republic of)
Show Abstract9:00 PM - M17.22
Dependences of Ferromagnetic MnAs Nanocluster Formation by Metal-Organic Vapor Phase Epitaxy on Crystallographic Orientations of GaInAs/InP Layers
Shinjiro Hara 1 , Junichi Motohisa 1 , Takashi Fukui 1
1 Research Center for Integrated Quantum Electronics (RCIQE), and Graduate School of Information Science and Technology, Hokkaido University, Sapporo Japan
Show AbstractEpitaxial growth of nanostructures with the hetero-junctions between ferromagnetic and III-V compound semiconductor materials is promising for the realization of nano-spintronic devices using the characteristics of the spin-polarized carriers. MnAs ‘thin films’ on GaAs layers have been most intensively grown by low temperature molecular beam epitaxy. We have been investigating ferromagnetic MnAs nanoclusters (NCs) embedded in GaInAs (001) layers [1] and self-assembled on GaInAs (111) B surfaces [2] by metal-organic vapor phase epitaxy (MOVPE). We have found that a [001] direction (c-axis) of hexagonal MnAs NCs is parallel to a [-1-1-1] direction of GaInAs layers, and that the samples with the MnAs NCs show strong ferromagnetic coupling at room temperature for the applied magnetic fields in a direction parallel to a wafer plane of InP (111)B substrate [2]. In the current work, we report wafer orientation dependences of MnAs NC self-assembly on GaInAs/InP layers by MOVPE. For the MnAs growth, Bis-(methyl-cyclopentadienyl) manganese ((MeCp)2Mn) was chosen as a manganese organometallic precursor. MnAs NCs were typically grown under the conditions of the growth temperature (Tg) = 600 oC and the V/Mn ratio = 750. Here, a ‘V/Mn ratio’ is defined as a partial pressure ratio between a group V source material, p[AsH3], and a manganese precursor, p[(MeCp)2Mn], that is, V/Mn = p[AsH3]/p[(MeCp)2Mn]. We have found that hexagonal MnAs NCs with well-defined crystallographic facets are formed after the MnAs growth on GaInAs/InP (111) A and B layers, whereas rectangular NCs are formed on the (001) ones. Typical NCs measure around 250 nm in diameter and 20 nm in height on the GaInAs (111) B surfaces. The densities of the NCs were estimated to be 1.0 x 109 cm-2 and 1.9 x 108 cm-2 on the (111) A and B surfaces, respectively. The size and density of the MnAs NCs depend strongly on MOVPE growth conditions and crystallographic orientations of the wafers. On the (111) B surfaces, by increasing p[(MeCp)2Mn], the lateral size of the NCs is increased, whereas almost no change is observed in density and height. On the (111) A surfaces, on the other hand, the density of the NCs is drastically increased from 108 to 109 cm-2 by increasing p[(MeCp)2Mn]. On the basis of kinetics, the thermally activated surface diffusion of Mn ad-atoms can be enhanced with increasing a Tg and/or decreasing a V/Mn ratio. Therefore, these differences with respect to the NC formation are possibly attributed to the differences in the number of absorption sites for the Mn ad-atoms, that is, in the atomic arrangements (or surface reconstructions) between on the GaInAs (111) A and B surfaces. [1] S. Hara et al., J. Cryst. Growth 261, 330 (2004); Nanotechnology 16, 957 (2005); [2] S. Hara and T. Fukui, accepted for publication in Appl. Phys. Lett.
9:00 PM - M17.24
MOCVD Growth and Structural, Optical and Magnetic Characterization of GaN:Mn and GaN: Fe Nanostructures.
Shalini Gupta 1 , Hun Kang 1 , Matthew Kane 1 2 , William Fenwick 1 , Yongqiang Wang 1 , Ian Ferguson 1 2
1 Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States, 2 Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Show Abstract9:00 PM - M17.25
Self- Organized Single and Double Ring- Like Nanostructures.
Ziad Abu Waar 1 , Zhiming Wang 1 , Jihoon Lee 1 , Yuriy Mazur 1 , Gregory Salamo 1
1 Physics, University of Arkansas, Fayetteville, Arkansas, United States
Show Abstract9:00 PM - M17.26
Microscopical Study of Growth Mechanisms Involved in the Formation of Au Nanoislands on Si Substrates.
Emanuela Piscopiello 1 , Leander Tapfer 1 , Marco Vittori Antisari 2 , Paola Prete 3 , Nicola Lovergine 4 5
1 UTS MAT, ENEA-c.r. Brindisi, Brindisi Italy, 2 UTS Materiali e Nuove Tecnologie, ENEA-c.r. Casaccia, Rome Italy, 3 Instituto per la Microelettronica e Microsistemi, CNR-IMM, Unità di Lecce, Lecce Italy, 4 Dip. di Ingegneria dell’Innovazione, Univ. of Lecce, Lecce Italy, 5 CNISM, Univ. of Lecce, Lecce Italy
Show AbstractThis work reports on the structural characterization of Au nanoparticles directly prepared onto p-type (100)-oriented Si substrates by a physical methodology, consisting in the UHV evaporation of a thin (1-4 nm) Au film and its successive high (815°C) temperature annealing. The morphology, orientation, and crystalline structure of Au nanoparticles were characterized by scanning and high-resolution transmission electron microscopy (STEM, HRTEM) and X-ray diffraction (XRD), respectively.Samples have been prepared on two different substrates consisting of either clean Si or Si with a 500 nm thick thermally-deposited SiO2 layer, with the aim of understanding the reaction mechanisms between the two phases, which are important for the adhesion and/or epitaxial relationship between Au nanocrystals and substrate. Furthermore, Au nanoislands on Si surfaces play an important role in the so-called metal catalyst assisted vapour phase growth for the bottom-up fabrication of semiconductor nanowires.Experimental results show that the type of substrate strongly influences the process of Au nanoisland formation upon heat treatment, by affecting the interaction of deposited Au with the underlying substrate. In fact, for a clean Si substrate the Au strongly interacts with Si so that the Au nanoisland lattice shows a well defined crystallographic relationship with the substrate; in the case of SiO2 coated substrates, the chemical interaction between Au and Si is much reduced and spherical nanoislands with a random orientation are produced upon thermal treatment. Considerations concerning coherent and incoherent growth of Au on Si are also included.
9:00 PM - M17.3
Selective-area Growth of Self-assembled InAs-QDs by Metal Mask Method for Optical Integrated Circuit Applications.
Nobuhiko Ozaki 1 , Yoshiaki Takata 1 , Shunsuke Ohkouchi 2 , Yoshimasa Sugimoto 1 3 , Naoki Ikeda 1 3 , Yoshinori Watanabe 1 , Yoshinori Kitagawa 1 , Akio Mizutani 1 , Kiyoshi Asakawa 1
1 Center for TARA, Univ. of Tsukuba, Tsukuba, Ibaraki, Japan, 2 Fundamental and Environmental Research Laboratories, NEC Corporation, Tsukuba, Ibaraki, Japan, 3 Ultrafast Photonic Devices Laboratory, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki, Japan
Show Abstract9:00 PM - M17.4
Characteristics of Ten-layer 1.3-μm InAs/InGaAs Quantum Dot Lasers Fabricated using Pulsed Anodic Oxidation.
Chongyang Liu 1 , Soon Fatt Yoon 1 , Qi Cao 1
1 School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore Singapore
Show Abstract9:00 PM - M17.5
Fabrication of Phase-separated Films in Various Metal Silicide-silicon and Metal Germanide-germanium Systems.
Nobuhiro Yasui 1 , Ryoko Horie 1 , Yoshihiro Ohashi 1 , Tohru Den 1
1 Inorganic Material Research, Canon Research Center, Ohta-ku, Tokyo, Japan
Show AbstractIn recent years, there has been great interest in phase-separated materials, such as patterned structure of well-aligned rods embedded in a matrix. Since these materials have fine pitch structure over lithography limitation, they have been examined vigorously in film growth. For example, Fe-LaSrFeO4[1], BaTiO3-CoFe2O4[2], Al-Si[3], Al-Ge[4] system have already been developed by conventional physical vapor deposition methods. However, there are not so mach kinds of above-mentioned structural materials. And we have newly developed many phase-separated films in various metal silicide-silicon and metal germanide-germanium systems by a magnetron sputtering method. Metal materials were chosen from Ti,Cr,Fe,Co,Ni,Cu,Zr,Mo,Pd,Hf,Ta and W etc. These metal silicide-silicon systems(metal germanide-germanium systems) have been exhibited metal-silicide(metal-germanide) rods, perpendicular to the substrate and parallel to each other, in an a-Si matrix because they have an eutectic phase diagram between metal silicide and silicon(metal-germanide and germanium). We can control the rod’s diameter in the range of about 2nm to 20nm. And its diameter depends on every metal material as well as sputtering condition or composition of metal. Then we found out the correlation between the diameter and eutectic temperature in the same sputtering condition. In this presentation, we will show the various images observed by scanning electron microscopy and transparent electron microscopy and discuss what it is important factor for diameter size control in these phase-separated films.[1]L.Mohaddes-Ardacili, H.Zheng, S.B.Ogale, B.Hannoyer, W.Tian, J.Wang, S.E.Lofland, S.R.Shinde, T.Zhao, Y.Jia, L.Salamanca-Riba, D.G.Schlom, M.Wutting, R.Ramesh, Nature Materials,3,533(2004)[2]H.Zheng, J.Wang, S.E.Lofland, Z.Ma, L.Mohadded-Ardabili, T.Zhao, L.Salamanca-Riba, S.R.Shinde, S.B.Ogale, F.Bai, D.Viehland, Y.Jia, D.G.Schlom, M.Wutting, A.Roytburd, R.Ramesh, Science 303,661(2004)[3]K.Fukutani, K.Tanji, T.Motoi, T.Den, Adv.Mater.16,1456(2004)[4]K.Fukutani, Y.Ishida, K.Tanji, T.Den, MRS spring meeting abstract P13.16(2006)
9:00 PM - M17.7
Interfacial Structure and Consequent Systematic Crystal Tilt in Epitaxial InAs Films on <100> GaAs.
Xueyan Song 2 , Ganesan Suryanarayanan 3 , Anish Khandekar 4 , Thomas Kuech 4 3 , Susan Babcock 2 3
2 Materials Science and Engineering, Univeristy of Wisconsin, Madison, Wisconsin, United States, 3 Materials Science Program, Univeristy of Wisconsin, Madison, Wisconsin, United States, 4 Chemical and Biological Engineering, Univeristy of Wisconsin, Madison, Wisconsin, United States
Show AbstractThe use of semi-insulating GaAs as a substrate of choice for novel devices containing the low band gap semiconductors InAs, GaSb and AlSb drives interest in developing new substrates and growth processes that circumvent defects induced by the 7% mismatch between the film and substrate. InAs films grown on unmodified GaAs substrates under conditions that favor low nucleation and/or high growth rates show a systematic crystal tilt of the InAs grains of ~4° in six preferred directions relative to the GaAs substrate. Foregoing work has demonstrated that a systematic pattern of tilted domains forms in isolated islands during the island growth stage in this materials system, provided that the island lateral dimensions reach ~1 µm prior to coalescence. The tilt developed at this stage persists as the film coalesces and thickens.1 The purpose of this study was to explore the structural origins of the tilt. Transmission electron microscopy (TEM) diffraction and high resolution imaging techniques were used to measure the pattern of tilt a long the heterointerface in both plan view and cross section and correlated it with the local interfacial dislocation structure. These TEM investigations confirmed on the nanoscale and at the interface the orientation distribution that was deduced previously using Backscattered Electron Kikuchi pattern analysis on the island surfaces.1 The center of the island is shown to be crystallographically aligned with the GaAs substrate. The interfacial structure near the center of the island consists of paired 60° dislocations, the 60° dislocations paired so as to produce zero or near zero net out of plane burgers vector density near the center of the island. In contrast, at the outer edges of micron-sized islands, the InAs is tilted 2° from the GaAs in the direction that locally relaxes the compressive stresses in the film. That is, the [001] InAs tilts away from [001] GaAs toward the nearest outer edge of the island. The interfacial dislocation network near the edge of the island where tilted InAs exists is composed of 60o dislocations almost all of like burgers vector. As a result, the dislocation network that accommodates the mismatch also has a net out of plane burgers vector density that most likely accounts for the observed 2° tilt. These results support the model of Spenser and Tersoff for development of mosaic spread in epitaxial films.2
9:00 PM - M17.8
Modelling Size-selected Growth of Nanodots by Using Reaction Kinetic Approach
Ismo Koponen 1 , Kirsi Nevalainen 2 , Marko Rusanen 2
1 Department of Physical Sciences, University of Helsinki, Helsinki Finland, 2 Laboratory of Physics, Helsinki University of Technology, Espoo Finland
Show AbstractSimple and efficient production of large amounts of nanodots with sharply defined size distribution is essential for many technological applications. Self-assembly of growing nanostructures is a promising phenomenon for such an efficient production of nanodots [1-3]. In self assembly, the existence of free energy minimum is the essential feature of growth, although there are also other features [4-5]. Consequently, in this work we study the growth of nanodots under conditions, where the size dependent free energy of the islands has a minimum. The purpose is to find a region of growth, where the energy minimum causes the dot size distribution to sharpen, take nearly a Gaussian form [5] and the mean dot size to stay constant. Special attention is paid to the large dot size region, where enhanced adatom detachment from dots occurs and a metastable region of growth is attained.The nanodot growth is modelled by using reversible reaction kinetic model including only adatom attachment and detachment. The time development of dot size distribution is described by rate equations, where the rates are defined in accordance with the self-consistent theory [6]. In defining the rates, we have used the phenomenological energy function for size-dependent island free energy [7], and taken into account also the entropy contribution.The rate equations are solved using the particle coalescence method (PCM), which is an efficient and versatile numerical scheme for studying the growth near equilibrium. Without deposition the island distribution indeed finds a metastable state near the energy minimum and the distribution obtained is nearly a Gaussian, although there are systematic deviations. This is consistent with continuum-model results of Jesson et al. [5]. In growth process with deposition, the distribution finds a metastable state as well, but now with a pronounced power-law type singular distribution of small islands, in addition to the Gaussian part. The increase in temperature shifts the metastable region towards smaller sizes, in agreement with Monte Carlo simulations by Meixner et al. [5].The accuracy and efficiency of the calculations allows us to study in detail the effects of changing the energy parameters on distributions. The reaction kinetic model thus provides the same simplicity as the continuum approach but relaxes the most restricting approximations behind it and allows the modelling in more microscopic level, better comparable to Kinetic Monte Carlo schemes. [1] J.L. Gray et al., Phys. Rev. Lett. 92, 135504 (2004)[2] E. Ogando et al., Phys. Rev. B 65, 153410 (2004)[3] F. Rosei, J. Phys. : Cond. Matt. 16, S1373 (2004)[4] M. Meixner et al., Phys. Rev. Lett. 87, 236101 (2001)[5] D.E. Jesson et al., Phys. Rev. Lett. 92, 115503 (2004)[6] G.S. Bales and A. Zangwill, Phys. Rev. B 55, R1973 (1997)[7] Z. Gai et al., Phys. Rev. Lett. 89, 235502 (2002)
9:00 PM - M17.9
Isotropic Lateral Ordering of III-V Quantum Dots Over GaAs (001) By Self-Assembly.
Mohammad Hussein 1 , Euclydes Marega 1 2 , Gregory Salamo 1
1 Microelectronics-Photonics, University of Arkansas, Fayetteville, Arkansas, United States, 2 Physics, University of Sao Paulo, Sao Paulo Brazil
Show Abstract
Symposium Organizers
Eric A. Stach Purdue University
Curtis R. Taylor Virginia Commonwealth University
Zhiming M. Wang University of Arkansas
Qi-Kun Xue Tsinghua University
M18: Physical Synthesis of Quantum Dots
Session Chairs
Friday AM, December 01, 2006
Room 208 (Hynes)
9:00 AM - **M18.1
Self-assembly of Semiconductor Quantum Dots by Droplet Epitaxy
Nobuyuki Koguchi 1
1 Quantum Dot Research Center, NIMS, Japan, Tsukuba, Ibaraki, Japan
Show AbstractSelf assembling quantum dots (QDs) has been attracted much attention to form structures with dimensions on the order of a few tens of nm that are necessary for the realization of advanced quantum devices such as lasers or infrared detectors or quantum computer. Frequently, numerous fine islands structures grown on a substrate at the initial stage of thin film growth, caused by Stranski-Krastanow growth mode appeared only in the lattice-mismatched systems such as InGaAs/GaAs, has been used to realize these QDs structures. In 1990, we have proposed a novel self-organized growth method, termed Droplet Epitaxy, for the direct formation of QDs with homogeneous size[1]. Compared with the island formation based on the Stranski-Krastanow growth mode, the Droplet Epitaxy is applicable to the formation of quantum dots not only in lattice-mismatched but also in lattice-matched systems such as GaAs/AlGaAs. The process of the Droplet Epitaxy in MBE chamber consists of forming numerous III-column element droplets such as Ga or InGa with homogeneous size of around 10 nm on the substrate surface first by supplying their molecular beams, and then reacting the droplets with As molecular beam to produce GaAs or InGaAs epitaxial microcrystals. Another advantage of the Droplet Epitaxy is that we can fabricate QDs structures without wetting layer by cotrolling the stoichiometry of the substrate surface just before the deposition of III-column element droplets[2]. Also we can control the QDs structures self-organizingly as pyramidal shape[2], ring shape[2,3] or concentric double ring shape[4,5]. Here we review our recent progress on the Droplet Epitaxy. [1] N. Koguchi, S. Takahashi and T. Chikyow : Proc. Int. Conf. MBE, San Diego, 1990, VIB- 4, J. Cryst. Growth 111, 688 (1991).[2] K. Watanabe, N. Koguchi and Y. Gotoh, Jpn. J. Appl. Phys. 39 (2000) L79-L81.[3] T. Mano, N. Koguchi, J. Cryst. Growth 278, 108 (2005).[4] T. Mano, T. Kuroda, S. Sanguinetti, T. Ochiai, T. Tateno, J.S. Kim, T. Noda, M. Kawabe, K. Sakoda, G. Kido, and N. Koguchi, Nano Lett. 5, 425 (2005). [5]
9:30 AM - M18.2
Morphology and Self-assembling of SiGe/Si Islands Grown by Liquid Phase Epitaxy in the Near- and Far Non-equilibrium Growth Limits.
Michael Hanke 1 , Torsten Boeck 2 , Anne-Katrin Gerlitzke 2 , Frank Syrowatka 3 , Frank Heyroth 3
1 Department of Physics, Martin-Luther-University Halle-Wittenberg, Halle /Saale Germany, 2 , Institute of Crystal Growth, Berlin Germany, 3 , Center of Materials Science, Halle/Saale Germany
Show AbstractLiquid phase epitaxy (LPE) serves as a growth method which usually operates closer to thermodynamical equilibrium than any other growth technique. We first discuss a characteristic shape transition of SiGe/Si(001) Stranski-Krastanov islands and the evolution of lateral positional correlation in a near-equilibrium LPE experiment, and relate these findings to the three-dimensional strain distribution as revealed by diffuse x-ray scattering and finite element calculations. Extended island rows forming a chess-board like formation indicate a nucleation scenario governed by a long-range strain interaction of, at least, the second last island in a chain. The reduced symmetry of Si(113) substrates, on the other hand, results in a unidirectional ordering of the SiGe islands. An initial homoepitaxial layer grown from indium solution at 930 C yields a perfectly linear pattern which serves in the subsequent heteroepitxial growth from a bismuth solution at a considerably lower temperatures of 590 C as a natural template for island nucleation. Generally the applied lattice mismatch restricts the final dimensions to a certain minimal value. Thus, several attempts are directed to further shrinking of the island size. Applying energy dispersive x-ray microanalysis we could prove that in the far non-equilibrium limit of LPE, using an exceptional high cooling rate of 10K/min, the final island size does not depend on the lattice mismatch within a given concentration window. Moreover it shows a way to drop down the island size below the strain restricted value by more than a factor of two. Lateral positional correlation exclusively happens by next-to-island interactions forming square-like island configurations with subsequently smaller islands outwards. Eventually subsequent island stages indicate a morphological transition with decreasing facet angles from steep whisker-like objects towards island with flatter facets.
9:45 AM - M18.3
Evaporation Induced Self-assembly of Nanoparticles.
Zhiqun Lin 1 , Jun Xu 1
1 Materials Science and Engineering, Iowa State University, Ames, Iowa, United States
Show Abstract10:00 AM - M18.4
Synthesis of ZnS Nanocrystals by Spray Pyrolysis
Hongwang Zhang 1 , Ken-Tye Yong 1 , Mark Swihart 1
1 Chemical and Biological Engineering, The University at Buffalo (SUNY), Buffalo, New York, United States
Show AbstractZinc sulfide is a II-VI semiconductor with a direct band gap in the near-UV region, with potential applications ranging from bioimaging to optoelectronics. Here, we demonstrate the preparation of ZnS nanoparticles (NPs) by spray pyrolysis using inexpensive, commercially available zinc diethyldithiocarbamate as a single-source precursor. Solutions of this precursor in toluene are dispersed into fine droplets by an atomizer, and then carried into a tube furnace where they evaporate fully and ZnS NPs nucleate from the vapor phase. Particles can be collected on filters or directly into a solvent. The as-produced particles form stable dispersions in most organic solvents, including toluene, acetone, methanol, and ethanol. The NPs were characterized by transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and optical spectroscopies. They are about 3 – 20 nm in diameter and are crystalline, as indicated by the presence of lattice fringes in HRTEM, and by selective-area electron diffraction. Their XRD pattern shows a single peak, indexed to the (111) plane of the cubic zinc blende lattice. The UV absorption spectrum shows peaks in the range of 330-380 nm, and the NP’s exhibit bright blue photoluminescent emission under UV illumination. The presence of heterogeneous ZnS nanorods was also observed in ZnS NPs dispersions. These nanorods have corrugated surfaces and appear to have formed by “oriented attachment” of ZnS NPs. Small nanorods (10 nm by 50 nm) with smooth surfaces were also observed in some cases, and may have formed directly in the gas phase. The formation mechanisms remain a subject of ongoing investigation. This approach can potentially be extended to related materials for which dithiocarbamate precursors are available, including ZnSe, CdS, and PbS. Similarly, ZnO nanocrystals can be prepared by this method, using an aqueous zinc acetate solution as the precursor. Preliminary results on synthesis of these other materials will also be presented.
10:15 AM - M18.5
Spray Produced Coral-Shaped Assemblies of MnS Nanocrystal Clusters.
Lilac Amirav 1 , Efrat Lifshitz 1
1 Department of Chemistry and Solid State Institute , Technion, Haifa Israel
Show AbstractA novel spray-based technique enables the production of high quality, free, uncoated semiconductor nanocrystals. Their collection, following spray droplet desolvation during flight, could result in unusual structures. We report on spray-produced ordered clusters (~50nm diameter) of MnS nanocrystals with crystal size range of 1-2nm, and their assembly into micron-sized coral-shaped fractal aggregates. Ballistic cluster-particle aggregation, with the introduction of physical interaction between particles, is suggested as a model for the assemblies' growth.
M19: Applications in Electronic & Optoelectronics
Session Chairs
Friday PM, December 01, 2006
Room 208 (Hynes)
11:00 AM - **M19.1
Deterministic Control of Confined Electronic and Photonic States in Single Semiconductor Nanostructures.
Armando Rastelli 1 , Oliver Schmidt 1
1 , Max-Planck-Institut fuer Festkoerperforschung, Stuttgart Germany
Show AbstractThe fabrication of semiconductor nanocrystals by Stranski-Krastanow growth has proven to be a successful approach to obtain quantum dots (QDs) with excellent structural, optical and electronic properties. Because of the statistical nature of the self-assembly process, QDs in an ensemble are characterized by emission spectra differing from QD to QD. This is an intrinsic problem of self-assembled nanostructures, and needs to be tackled for any application requiring identical or well-defined nano-objects. Noteworthy, similar arguments apply to microstructures obtained by the top-down approach, where unavoidable uncertainties on the fabrication parameters result in properties differing from the nominal specifications. In the case of high-quality optical microcavities, even small fluctuations affect the energy of the confined modes. Since it is hard to envision fabrication methods able to yield structures with confined energy states known a priori within the typical width of a few tens of micro-eV, post-fabrication tuning must be used.Here we present a simple approach to tune, within a broad range and with resolution-limited accuracy, single confined states of individual III-V QDs and microdisk cavities. The method is based on the use of the focused laser of a standard micro-photoluminescence setup both as a probe and as a tool to in-situ modify a nano- or micro-structure. The laser beam locally heats the structure from cryogenic temperatures up to a few hundreds °C, thus activating processes such as bulk interdiffusion between QD material and the surrounding matrix and As desorption. The effect of such structural changes is quickly tested at low laser power. The desired energy is gradually reached by applying several heating/measuring steps.With this approach we bring the excitonic transitions of initially different QDs into resonance. Blue-shifts of more than 15 meV, comparable with typical inhomogeneous broadenings of QD ensembles, are successfully achieved.The emission lines of the annealed QDs remain resolution-limited during the process, suggesting that no appreciable deterioration of the structure is produced by the annealing. Since the tuning accuracy is only limited by the resolution of our experimental setup (70 micro-eV), we believe the method to be controllable down to the homogeneous broadening of the transitions (typically 2-20 micro-eV at low temperatures). With the same method we finely tune the optical modes of microdisk microcavities, which are observed to blue-shift after each annealing step. Also in this case relatively large shifts with resolution-limited accuracy can be obtained with no appreciable deterioration of the mode qualities.This approach may open the way to the long-sought eterministic control of electronic confined states in QDs and confined light states in semiconductor microcavities.
11:30 AM - M19.2
Progress and Perspectives for InGaN Quantum Dots and Monolithic Nitride Cavities.
Kathrin Sebald 1 , Henning Lohmeyer 1 , Jürgen Gutowski 1 , Tomohiro Yamaguchi 2 , Carsten Kruse 2 , Detlef Hommel 2
1 Semiconductor Optics, Institute of Solid State Physics, University of Bremen, Bremen Germany, 2 Semiconductor Epitaxy, Institute of Solid State Physics, University of Bremen, Bremen Germany
Show Abstract11:45 AM - M19.3
Temperature-dependence of Optical Transitions of One Dimensional InGaAs/GaAs Quantum Structures.
Zhixun Ma 1 2 , Todd Holden 1 , Zhiming Wang 3 , Samuel Mao 2 , Gregory Salamo 3
1 Physics Department, New York State Center for Untrafast Photonic Materials and Applications, Brooklyn College of CUNY, Brooklyn, New York, United States, 2 , Lawrence Berkekely Laboratory, Berkeley, California, United States, 3 Department of Physics, University of Arkansas, Berkeley, Arkansas, United States
Show Abstract12:00 PM - M19.4
Tunnel QW-QDs InGaAs-InAs High Gain Medium for All-Epitaxial VCSELs.
Vadim Tokranov 1 , Michael Yakimov 1 , Jobert Van Eisden 1 , Serge Oktyabrsky 1
1 College of Nanoscale Science and Engineering, SUNY at Albany, Albany, New York, United States
Show AbstractRelatively broad size distribution of self-assembled quantum dot (QD), limitations in carrier capture and thermalization rates are still limiting the maximum saturated gain of QD-based optoelectronic devices, such as in-plane laser diodes and vertical cavity surface emitting lasers (VCSELs). To overcome these problems, QD – coupled – to – quantum well (QW) structures with tunnel injection of carriers into QDs were developed. For VCSELs laser medium with high gain and fast carrier dynamics, we investigated structures of tunnel-coupled pairs consisting of InGaAs QWs grown on top of shape-engineered self-assembled InAs QDs (QW-on-QDs). Photoluminescence (PL), transmission electron microscopy and electroluminescence studies were used to characterize the properties of QW-on-QDs active medium. QW-on-QDs structures with 3 nm-tick GaAs/AlAs tunnel barrier were grown. Composition of InGaAs QWs was varied from 29 to 36% to adjust QW-to-QDs ground state (GS) energy separation, while keeping the thickness of QW constant. Room temperature optical properties of tunnel well-on-dots structures at low excitation were found to be sensitive to energy separation between GS energies of QDs and QW. The spectra also shows that QD-related luminescence tends to peak at discrete energy separations from the QW peak, multiples of ~ 35 meV, which is close to the LO phonon energy. Resonant tunneling multi-peak QD-related spectra with one and two LO-phonon emission were measured. Optimized GS energy separation between QW and QDs was found to be close to the energy of LO phonon. This structure demonstrated narrowing of QD PL line down to 21.6 meV at T=77K, likely indicating an efficient LO phonon-assisted tunneling of carriers from QW into QD ensemble states. Optimized triple-pair tunnel QW-on-QDs structures were evaluated in VCSELs. All-epitaxial VCSELs with triple-pair tunnel QW-on-QDs as active medium demonstrated continuous wave mode lasing. Tunnel QW-QDs VCSELs with doped mirrors exhibited 1.8 mA (Jth ~ 800 A/cm2) minimum threshold current at QD GS emission wavelength, 1135 nm, with 0.7 mW optical power and 12% light-current efficiency.
12:15 PM - M19.5
Metalorganic Vapor Phase Epitaxy of InGaAs/GaAs Quantum Dot Lasers for 1.3 µm Applications
Andre Strittmatter 1 , T. Germann 1 , K. Posilovic 1 , Th. Kettler 1 , U. Pohl 1 , D. Bimberg 1
1 Institut für Festkörperphysik, Technische Universität Berlin, Berlin Germany
Show AbstractGrowth details of of InGaAs/GaAs quantum dot (QD) lasers emitting at the 1.3 µm datacom window will be presented. The structures were grown by low-pressure MOCVD using alternative precursors for the group-V elements. In order to shift the QD emission towards 1.3 µm, the concepts of overgrowing Inx>0.5Ga1-xAs QDs with a strain-reducing Inx<0.2GaAs layer (SRL) and that of embedding InGaAs QDs in an InGaAs quantum well of lower In content were compared. The embedding method was found to lead to a progressive blue shift of the QD luminescence as the thickness of the InGaAs layer underneath the QDs is increased, in contrast to the SRL method. Applying overgrowth by a SRL, high performance QD lasers were already demonstrated at 1.25 µm [1]. These devices show very low threshold current densities of below 60 A/cm2 and internal efficiencies well exceeding 90%. Therefore, the SRL method was chosen for further studies to obtain 1.3 µm laser devices. First a reliable PL monitor using test structures with a QD region identical to that of the lasers was established. Under low and high excitation conditions, the PL intensities reflect the defect density and the dot areal density, respectively. By comparing high excitation PL spectra of test structures with lasing spectra of respective devices, we were able to define a lower limit of ground state PL intensity required for ground state lasing of laser diodes. These findings were used to develop stacked QD layers for the lasers.Composition of QDs and SRL, V/III ratio, growth interruption time, and spacer layer thick¬ness turned out to be the most critical parameters. Intense PL emission above 1.3 µm wavelength is obtained for xIn=0.75 (QDs) and xIn=0.12 (SRL). Highest PL efficiency and smooth surfaces of stacked QD layers are attained using low V/III ratios (<5) during growth of all layers, and spacer thicknesses above 40 nm.
12:30 PM - M19.6
Intermediate-band GaAs Quantum Dot Solar Cells Employing Energy Fences.
Guodan Wei 1 , Stephen Forrest 1
1 , University of Michigan, Ann Arbor, Michigan, United States
Show Abstract It has been suggested that incorporation of a high density of quantum dots (QDs) in a p-i-n solar cell is a means to obtain a significant increase of power conversion efficiency to >60% by overcoming the difficulty of increasing the solar cell photocurrent without degrading its voltage [1,2]. The narrow gap QDs with 3D closely packed layers can form an intermediate band that allows for the absorption of significant amounts of below-band gap radiation that would otherwise not be useful in a wide gap semiconductor bulk cell. Unfortunately, the very high efficiencies predicted for these cells has not yet been realized, due in part to non-ideal band structures that result in charge trapping followed by recombination of the photocarriers in the QDs. To eliminate this charge trapping effect in a conventional (GaAs) cell, a novel structure employing QDs buried in a high bandgap barrier layer is proposed and analyzed. The structure consists of a GaAs p-i-n structure that is the host to a high density of InAs QDs surrounded by AlxGa1-xAs barrier “fences”. The calculated average electron and holes transmission coefficient of these fenced dots (called a DFENCE heterostructure) quantitatively confirms that photocarriers from the GaAs junction diode can be effectively blocked from trapping in the QDs. In contrast to conventional InAs/GaAs QD structures, the calculated band structure of the DFENCE structure in the framework of spherical Bessel function suggests that an additional AlxGa1-xAs fence barrier lowers the ground state of InAs quantum dots. Hence, the incorporation of the fence does not affect the photocarrier generation and escape rate in the QDs regions. The maximum solar power conversion efficiency of the DFENCE structure employing 10-20 QD layers in the junction depletion region can be as high as 40%, compared with GaAs homojunction cells with efficiencies of approximately 25%. Reduction in fill factor resulting from the additional energy barriers, and optimal materials choices to achieve the highest possible performance will be discussed.1.Luque A., Marti A., Increasing the efficiency of ideal solar cells by photon induced transitions at intermediate levels, Phys. Rev. Lett.78 (26), 5014-5017(1997). 2.Marti A., Cuadra L., Luque A., Quasi-drift diffusion model for the quantum dot intermediate band solar cell, IEEE TRANSACTIONS ON ELECTRON DEVICES 49(9), 1632-1639 (2002).
12:45 PM - M19.7
Plasmon-Enhanced Luminescence from Silicon Quantum Dots Coupled to Nanostructured Metals.
Julie Biteen 1 , Hans Mertens 2 , Luke Sweatlock 1 , Nathan Lewis 1 , Albert Polman 2 , Harry Atwater 1
1 , California Institute of Technology, Pasadena, California, United States, 2 , AMOLF-FOM, Amsterdam Netherlands
Show AbstractWe report enhanced photoluminescence (PL) and electroluminescence (EL) from silicon quantum dot (QD) emitters interacting with the enhanced local field when in close proximity to noble metal nanostructures. This enhanced luminescence is attributed to a resonant increase in quantum efficiency, radiative decay rate, and absorbance cross section. Electron beam lithography is used to tune the plasmon resonance energy of arrays of Ag islands by varying the particle diameters and array pitches. Taking advantage of the broad emission spectrum of Si QDs, the wavelength of maximum PL enhancement is tuned from 600 to 900 nm, and this wavelength is found to coincide with the plasmon resonance wavelength, determined from confocal transmission measurements. Since no enhancement is observed when the plasmon energy is resonant with the excitation source at 532 nm, the enhancement effect is attributed to an enhanced radiative emission rate due to the locally enhanced electric field at the position of the excited nc-Si dipole.We have used finite-integration time-domain computations to perform full-field electromagnetic simulations to investigate enhanced emission from Si-QD-doped SiO2 coupled to arrays of silver nanoparticles. These studies show the shifting of the plasmon resonance energy with particle size and array pitch, demonstrating that the local field is enhanced up to 50 nm deep into the quartz substrate (and therefore felt by the embedded Si QD), and reproducing the trend of diminishing field intensity with increasing particle diameter. The calculated average enhanced local field intensity in the plane of the Si QDs should correspond to the average radiative rate enhancement, and indeed we find good numerical agreement between our computations and experiments. Finally, the plasmon-coupling experiments are extended to Si QDs in a field-effect light-emitting diode, and we observe enhanced EL from this system. This enhanced EL increases with increasing Ag nanoisland coverage, and the greatest enhancements (200%) are observed upon coupling to a continuous rough Ag film. In general, the radiative dipole enhancement effect in a coupled metal-semiconductor emitter system is a function of the metal-Si separation distance and the structure of the metal, and analytic modeling suggests enhancements of the radiative emission rate by more than 100x are possible. If this can be achieved, emission rates for Si QDs can be similar to those for direct bandgap semiconductors, implying a range of previously inaccessible applications for Si QD radiative emission such as bright and efficient Si LEDs.