Symposium Organizers
Matthew F. Doty University of Delaware
Sayantani Ghosh University of California, Merced
Armando Rastelli IFW Dresden e.V.
Marija Drndic University of Pennsylvania
H2: Synthesis and Characterization of Materials for Quantum Dot Coupling
Session Chairs
Monday PM, November 29, 2010
Room 300 (Hynes)
H1: Understanding and Engineering Charge Interactions in Colloidal Quantum Dots
Session Chairs
Monday PM, November 29, 2010
Room 300 (Hynes)
9:30 AM - **H1.1
Tunable Coupling of Electronic and Magnetic Degrees of Freedom in Colloidal Quantum Dots.
Scott Crooker 1
1 , National High Magnetic Field Lab, Los Alamos, New Mexico, United States
Show AbstractMagnetic doping of semiconductor nanostructures is actively pursued for applications in magnetic memory and spin-based electronics. A primary goal of these efforts is to control the interaction strength between carriers (electrons and holes) and the embedded magnetic atoms. In this respect, colloidal nanocrystal heterostructures (
e.g., core-shell designs) provide great flexibility through growth-controlled ‘engineering’ of electron and hole wavefunctions in individual nanocrystal quantum dots. Moreover, the truly 0-D nature of these nanocrystal quantum dots permits access to a regime of strong confinement – and therefore carrier-magnetic interactions – considerably beyond that which can be realized in quantum wells or in bulk materials.
This talk will describe widely tunable magnetic sp–d exchange interactions between electron–hole excitations (excitons) and paramagnetic spin-5/2 manganese ions using core/shell nanocrystals composed of Mn2+-doped ZnSe cores that are overcoated with shells of narrower-gap CdSe [1]. Magnetic circular dichroism studies reveal giant Zeeman spin splittings of the band-edge exciton that, surprisingly, are tunable in both magnitude and sign. Effective exciton g-factors are controllably tuned from -200 to +30 solely by increasing the CdSe shell thickness, demonstrating that strong quantum confinement and wavefunction engineering in heterostructured nanocrystals can be used to manipulate carrier– Mn2+ wavefunction overlaps and the sp–d exchange parameters themselves. In parallel, very recent magneto-photoluminescence studies suggest that strong quantum confinement also directly influences the emission properties of the embedded Mn2+ ions. In marked contrast with their bulk or 2-D counterparts, we observe a strong circular polarization of the characteristic yellow/red (4T1->6A1) emission from the Mn2+ ions in these nanocrystals. The polarization precisely tracks the strength of the sp-d exchange in these quantum dots as a function of temperature, field, and nanocrystal composition. [1] D. A. Bussian et al., Nature Materials 8, 35 (2009).
*In collaboration with R. Viswanatha, D. A. Bussian, M. Yin, M. Brynda, Al. L. Efros, and V. I. Klimov. Supported by the DOE Office of Basic Energy Sciences, and DOE Center for Integrated Nanotechnologies.
10:00 AM - H1.2
Circularly Polarized Photoluminescence from the Internal Mn Transition in Mn-doped Nanocrystals.
Ranjani Viswanatha 1 , Scott Crooker 2 , Jeffrey Pietryga 1 , Victor Klimov 1
1 Physical Chemistry and Applied Spectroscopy, Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 2 National High Magnetic Field Laboratory, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractMagnetically doped colloidal II-VI nanocrystals provide great flexibility for controlling electronic and magnetic interactions via growth-controlled “engineering” of wave functions. This class of materials is characterized by strong sp-d exchange coupling between carriers and embedded magnetic atoms, giving rise to enhanced Zeeman splittings of band-edge exciton [1,2] Another interesting aspect of Mn-doped II-VI semiconductors in general is the red (~2.1 eV) emission from internal Mn (T->A) transitions, whose origin has been debated (eg, [3]) and studied in both bulk materials and in quantum wells. However, the behavior of the Mn emission in colloidal nanocrystals has not been extensively studied. In this work, we discuss the effect of strong three dimensional quantum confinement on this internal Mn emission and show that it is qualitatively different from that of bulk or quantum well systems.We investigate the magneto-optical properties of colloidal Mn-doped ZnSe/CdSe core/shell nanocrystals using magnetic circular dichroism (MCD) and circularly polarized photoluminescence (MCPL). In particular we measure the emission properties of the Mn d-d transition as a function of magnetic field and temperature and compare it with similar measurements in bulk doped semiconductors. Surprisingly, magneto-PL studies reveal a strongly circularly polarized emission from internal Mn transitions at ~2.15 eV with applied magnetic fields, which follows the same field- and temperature-dependent (Brillouin-like) magnetization of the Mn spins. This behavior is qualitatively different than in the bulk systems. The mechanism of this behavior of the local Mn spins that is solely dictated by the strength of the quantum confinement will be discussed.[1] D. A. Bussian, S. A. Crooker, M. Yin, M. Brynda, A. L. Efros and V. I. Klimov, Nature Materials 8, 35 (2009).[2] V. A. Vlaskin, R. Beaulac, and D. R Gamelin, Nano lett., 9, 4376 (2009).[3] A. V. Chernenko, P. S. Dorozhkin, V. D. Kulakovskii, A. S. Brichkin, S. V. Ivanov and A. A. Toropov, Phys. Rev. B, 72, 45302 (2005).
10:15 AM - **H1.3
Multiexciton Generation at the Nanoscale.
Eran Rabani 1
1 , Tel Aviv University, Tel Aviv Israel
Show AbstractCarrier multiplication is a process where several charge carriers are generated upon the absorption of a single photon in semiconductors. This process is of great technological ramifications for solar cells and other light harvesting technologies. For example, it is expected that when more charge carriers created shortly after the photon is absorbed, the larger fraction of the photon energy can successfully be converted into electricity, thus increasing the device efficiency. In this talk we will discuss the process of multiexction generation leading to carrier multiplication in semiconducting nanocrystals and carbon nanotubes. A general theoretical framework will be presented and the limits of indirect absorption and impact ionization will be derived. The role of composition material, size, geometry and energy will be discussed.
10:45 AM - H1.4
The Role of Composition and Surface Treatment in Multiple Exciton Generation Efficiency of PbSe Quantum Dots.
Aaron Midgett 1 2 , Hugh Hillhouse 3 1 , Barbara Hughes 1 2 , Arthur Nozik 1 2 , Matthew Beard 1
1 , NREL, Golden, Colorado, United States, 2 , University of Colorado, Boulder, Colorado, United States, 3 Department of Chemical Engineering , University of Washington, Seatle, Washington, United States
Show AbstractRecent reports have suggested that due to the repetitive nature of pulsed fs-laser experiments for measuring the photon-to-exciton quantum yields (QYs) that result from multiple exciton generation (MEG), an alternative relaxation pathway may produce artificially high results. We present transient-absorption (TA) data for several lead chalcogenide quantum dots (QDs) at a variety of pump photon energies. We examine the dynamics of a charge separated state for PbSe QDs and obtain photon-to-exciton QYs for all samples studied under flowed or stirred conditions. We find differences in the observed TA dynamics between flowing and static conditions that depend upon photon fluence, pump photon energy, and quality of the QD surfaces. The results are modeled with a spatially resolved population balance of generation, recombination, convective replacement, and accumulation of long-lived excited QDs. By comparing the simulations and experiments, the steady-state population of the long-lived QD excited states and their kinetics are determined for different experimental conditions. We also find that by treating the surface of QDs, we can increase QYs and decrease the charging probability by a factor of 3-4. The fact that the MEG efficiency was modified with a surface treatment (although modestly) shows that materials can be designed to increase QYs and eventually make better use of the available energy in the solar spectrum.
11:30 AM - H1.5
Carrier Multiplication Studies of Lead-salt Nanorods with a Superconducting Nanowire Single-photon Detector.
Richard Sandberg 1 , M. Qazilbash 1 , Bishnu Khanal 1 , Richard Schaller 2 , Jeffrey Pietryga 1 , Martin Stevens 3 , Burm Baek 3 , Saw Woo Nam 3 , Victor Klimov 1
1 Center for Advanced Solar Photophysics, Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 2 Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois, United States, 3 , National Institute of Standards and Technology, Boulder, Colorado, United States
Show AbstractColloidal lead-salt nanomaterials have generated great interest as potential materials to boost photovoltaic efficiencies through carrier multiplication (CM) [1,2]. The strong quantum confinement of these zero-dimensional semiconducting nanomaterials leads to strong charge-coupling. While the exact physical mechanism behind CM is debated in the literature, it is generally believed that CM is enhanced in these quantum dots via the strong charge coupling of a hot carrier produced by an above band-gap photon with a valence band carrier. Lead-salt nanorods have been suggested as an attractive alternative to their spherical nanocrystal counterparts as they provide a natural pathway for charge transport. Furthermore, it has been suggested that they may show higher CM efficiency due to proposed symmetry breaking and a lifting of the ground state degeneracy. Here we report the first measured carrier multiplication efficiencies and time resolved photoluminescence (TRPL) studies of colloidal PbSe nanorods prepared by two methods: sequential cation exchange and a direct synthesis method. The efficiencies of ensemble PbSe nanorods in solution were measured by time-resolved photoluminescence upconversion (UPL) and by time-correlated single-photon counting (TCSPC) with a superconducting nanowire single photon detector (SNSPD) [4]. The PbSe nanorods show bright photoluminescence with high quantum yield, but only show modest improvements in carrier multiplication yields when compared to spherical PbSe nanocrystals. Additionally, the early-time PL intensity scaling follows the same free carrier model as spherical nanocrystals [2,3]. Finally, we will discuss how the use of an SNSPD enabled CM measurements from TRPL at shorter pump wavelengths and with much lower fluences than possible with traditional UPL. This new tool will find application in numerous nanomaterial studies both on ensemble and single nanocrystal measurements.[1] R. D. Schaller, V. I. Klimov, Physical Review Letters 92, 186601 (2004).[2] J. A. McGuire, M. Sykora, J. Joo, J. M. Pietryga, V. I. Klimov, Nano Letters 10, 2049 (2010).[3] J. A. McGuire, J. Joo, J. M. Pietryga, R. D. Schaller, V. I. Klimov, Accounts of Chemical Research 41, 1810 (2008).[4] E. A. Dauler, A. J. Kerman, B. S. Robinson, J. K. W. Yang, B. Voronov, G. Goltsman, S. A. Hamilton and K. K. Berggren, J. Mod. Optics 56, 364 (2009).
11:45 AM - H1.6
Size Dependence of Multiple Exciton Generation Rate in CdSe and PbSe Quantum Dots.
Zhibin Lin 1 , Alberto Franceschetti 2 , Mark Lusk 1
1 Physics, Colorado School of Mines, Golden, Colorado, United States, 2 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractIt is theoretically possible for a photon of sufficiently high energy to generate two or more lower energy excitons in a process known as multiple exciton generation (MEG). Provided that this process occurs faster than competing carrier relaxation processes, more than one pair of charge carriers might be collected per photon absorbed. An issue is whether or not quantum confinement associated with quantum dots enhances MEG. If so, this raises the prospect of designing nanostructured devices that exploit MEG in order to capture high energy photons with greater efficiency than is currently possible. It would also follow that smaller dots should be preferred because of the greater degree of confinement. We consider this issue by calculating the multiplication rates of hot carriers in CdSe and PbSe quantum dots using an atomistic pseudopotential approach and first order perturbation theory. Both excited holes and electrons are considered, and electron-hole Coulomb interactions are accounted for. For CdSe quantum dots, we find that holes have much higher multiplication rates than electrons with the same excess energy, due to the larger density of final states (positive trions). When electron-hole pairs are generated by photon absorption, however, the net carrier multiplication rate is dominated by photogenerated electrons, because they have on average much higher excess energy. We also find, contrary to earlier studies, that the effective Coulomb coupling governing carrier multiplication is energy dependent. We show that smaller dots result in a decrease in the carrier multiplication rate for a given absolute photon energy. However, if the photon energy is scaled by the volume dependent optical gap, then smaller dots exhibit an enhancement in carrier multiplication for a given relative energy. These results are compared and contrasted with those associated with PbSe quantum dots.
12:00 PM - **H1.7
Engineered Exciton-exciton Interactions in Core-shell Semiconductor Nanocrystals.
Victor Klimov 1
1 , Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractUsing semiconductor nanocrystals (NCs) one can produce extremely strong spatial confinement of electronic wave functions not accessible with other types of nanostructures. One consequence of this effect is a significant enhancement in carrier-carrier interactions that lead to a number of novel physical phenomena including ultrafast mutiexciton decay due to Auger recombination. Significant recent interest in multiexciton phenomena in NCs has been stimulated by studies of NC lasing [1] as well as multiexciton generation by single photons via carrier multiplication [2]. The focus of this presentation is on Auger recombination discussed in the context of NC lasing. Because of relaxation of momentum conservation, the efficiency of Auger recombination in NCs is greatly enhanced compared to bulk semiconductors [3-5]. One consequence of fast Auger decay is very short lifetimes of optical gain in NCs that greatly complicate the applications of these nanostructures in practical lasing technologies [6]. Recently, we have developed several approaches for resolving the problem of ultrafast Auger decay. In one approach, we utilize core/shell hetero-NCs engineered in such a way as to spatially separate electrons and holes between the core and the shell. The resulting imbalance between negative and positive charges produces a strong local electric field, which allows us to demonstrate optical amplification in the single-exciton regime when Auger recombination is inactive [1]. In another approach, we use “giant” NCs that comprise a small CdSe core overcoated with a thick shell of wider-gap CdS. These structures show greatly suppressed Auger recombination, which allows us to realize broadband optical gain (extends over 500 meV) due to emission from multiexcitons of high orders (up to 10 and above) [7]. To elucidate the mechanism underlying suppression of Auger recombination in these NCs, we conduct detailed structural and spectroscopic studies of a series of samples with progressively increasing shell thickness. We observe a sharp transition to Auger-recombination-free behavior for 5-to-7 monolayer-thick shells. Simultaneously, we detect the development of an intense phonon mode, which is characteristic of a CdSeS alloy. These results suggest that the likely reason for suppressed Auger decay in these nanostructures is the “smoothing out” of the confinement potential due to formation of a graded interfacial CdSeS layer between the CdSe core and the CdS shell, as was recently proposed in ref. [8]. [1]V. I. Klimov et al., Nature 447, 441 (2007).[2]R. D. Schaller, and V. I. Klimov, Phys. Rev. Lett. 92, 186601 (2004).[3]V. I. Klimov et al., Science 287, 1011 (2000).[4]J. M. Pietryga et al., Phys. Rev. Lett. 101, 217401 (2008).[5]I. Robel et al., Phys. Rev. Lett. 102, 177404 (2009).[6]V. I. Klimov et al., Science 290, 314 (2000).[7]F. Garcia-Santamara et al., Nano Lett. 9, 3482 (2009).[8]G. E. Cragg, and A. L. Efros, Nano Lett. 10, 313 (2010).
12:30 PM - H1.8
New Paradigm for Controlling Exciton Dynamics via Engineered Electron-hole Exchange Interaction in Semiconductor Nanocrystals.
Sergio Brovelli 1 , Richard Schaller 1 3 , Scott Crooker 2 , Florencio Garcia-Santamaria 1 , Ranjani Viswanatha 1 , Yongfen Chen 1 , Jennifer Hollingsworth 1 , Han Htoon 1 3 , Victor Klimov 1 3
1 Chemistry Division, Physical Chemistry and Applied Spectroscopy, Los Alamos National Laboratory, Los Alamos , New Mexico, United States, 3 Center for Advanced Solar Photophysics, Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 2 National High Magnetic Field Laboratory, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractThe electron-hole exchange interaction (EI) in spherically shaped, quantum-confined semiconductor nanocrystals (NCs) of CdSe gives rise to an optically forbidden (“dark”) exciton ground state that is several meV below an optically allowed (“bright”) exciton. This dark-bright splitting has a profound effect on exciton dynamics, and specifically, leads to nearly two-order-of-magnitude increase in the exciton radiative lifetime as sample is cooled from room temperature to liquid helium temperature [1,2]. EI has also been shown to dramatically affect other important physical properties of nanostructures, such as exciton spin dynamics[3], coherent interdot coupling[4] and electron transfer kinetics to external quenchers[5]. Here we demonstrate a nanoengineering-based approach that provides extrinsic control over the exchange energy for nearly constant emission energy. Specifically, we show that the dark-bright energy splitting can be tuned over an order of magnitude by tuning the electron-hole spatial overlap in core-shell CdSe/CdS NCs with a variable shell width. Direct control of EI is demonstrated via temperature- and magnetic-field-dependent measurements of time-resolved photoluminescence (PL) and fluorescence line narrowing (FLN). Upon growing an appropriately thick CdS shell on a CdSe core, we produce a material that emits with a nearly constant rate, within a factor of 2.5, over a temperature range from 1.5 to 300 K and magnetic fields from 0 to 7 Tesla. Further, we show that the low-temperature exciton lifetime can be reduced by nearly an order of magnitude by increasing the CdS shell width. This observation seems to contradict to traditional expectations since in samples with a thicker shell electrons and holes experience stronger spatial separation, which should result in increased radiative recombination times.[6] However, this paradoxical result can be rationalized if one considers the effect of electron-hole spatial overlap on the EI energy, which further affects the occupation factors of dark and bright exciton states.References:[1] Nirmal, M. et al. Observation of the dark exciton in CdSe quantum dots. Phys. Rev. Lett. 75, 3728-3731, (1995).[2]Crooker, S. A. et al. Multiple temperature regimes of radiative decay in CdSe nanocrystal quantum dots: Intrinsic limits to the dark-exciton lifetime. Appl. Phys. Lett. 82, 2793-2795, (2003).[3] Astakhov, G. V. et al. Exciton Spin Decay Modified by Strong Electron-Hole Exchange Interaction. Phys. Rev. Lett. 99, 016601, (2007).[4] Fält, S. et al. Strong Electron-Hole Exchange in Coherently Coupled Quantum Dots. Phys. Rev. Lett. 100, 106401, (2008).[5] Nonoguchi, Y. et al. Low-Temperature Observation of Photoinduced Electron Transfer from CdTe Nanocrystals. The Journal of Physical Chemistry C 113, 11464-11468, (2009).[6] Garcia-Santamaria, F. et al. Suppressed Auger Recombination in "Giant" Nanocrystals Boosts Optical Gain Performance. Nano Lett. 9, 3482-3488, (2009).
12:45 PM - H1.9
Ultrafast Competition in Quantum Dots: Interior Charge-transfer vs Surface Auger Relaxation.
Edbert Sie 1 , Tzechien Sum 1 , Alfred Huan 1 , Hairuo Xu 2 , Wee-Shong Chin 2 , Prashanth Upadhya 3 , Rohit Prasankumar 3 , Antoinette Taylor 3
1 Physics and Applied Physics, Nanyang Technological University, Singapore Singapore, 2 Chemistry, National University of Singapore, Singapore Singapore, 3 , Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractAuger processes are commonly known in QDs to deplete 1Se-electrons via the surface-states. Here, we propose that deep-levels arising from interior defects can compete effectively with the Auger process for charge-transfer (CT) by the trapping of holes, leading to longer lifetime of the 1Se-state. In this respect, we have carefully prepared the deep-levels by forming interior defects, i.e. Zn vacancies (VZn), inside ZnS QDs through chemical synthesis. These interior defects also give rise to an emission band in the optical spectra that replaces the commonly assigned surface-states emission. Here, the increased rate of the trapping of holes and the prolonged 1Se-state lifetime are demonstrated in transient photoabsorption and photobleaching spectroscopy, respectively. Following this CT process, the interior defects (VZn) are found to exhibit Jahn-Teller symmetry breaking (Td to C3v) as well as charge-localization within 260 fs as posited by Zunger et. al. [PRL 103, 016404 (2009)]. Spectrally-dispersed transient photoluminescence studies by a streak camera suggests that a spin-selective CT process occurs here, which may have implications in the use of arrayed QDs for spintronics devices without using any magnetic elements. All these findings are especially valuable for achieving the criteria of selective doping in QDs (by rare-earths and/or transition-metals) in which the interplay between energy conservation, spin and excited-state lifetimes play a crucial role in the electronic and optical properties of doped QDs.
H2: Synthesis and Characterization of Materials for Quantum Dot Coupling
Session Chairs
Monday PM, November 29, 2010
Room 300 (Hynes)
2:45 PM - **H2.1
cQED in Quantum Dot Micropillar Cavities – Fundamental Research and Applications.
Stephan Reitzenstein 1 , Caroline Kistner 1 , Steffen Munch 1 , Christian Schneider 1 , Arash Rahimi-Iman 1 , Tobias Heindel 1 , Micha Strauss 1 , Sven Hofling 1 , Ilya Ponomarev 2 , Tom Reinecke 2 , Lukas Worschech 1 , Alfred Forchel 1
1 , University of Wuerzburg, Wuerzburg Germany, 2 , Naval Research Laboratory, Washington, District of Columbia, United States
Show AbstractFollowing the first demonstration of strongly coupled quantum dot (QD)-microcavity systems enormous effort has been devoted to this field of cavity quantum electrodynamics (cQED) [1,2]. For applications in non-classical light sources and for fundamental cQED studies it is crucial to control the light-matter coupling strength by external parameters. To date most experimental studies involving strong coupling in QD-microcavity systems have relied on temperature tuning while electric and magnetic fields are required to fully explore their potential in terms of switching speed, local tuning and in-situ control of the interaction strength. In this contribution, we will present recent progress in the field of cQED in QD-micropillar systems controlled by external parameters. It will be demonstrated that electrically contacted high-Q micropillars with a low density layer of InGaAs QDs in the active region allow for highly efficient emission of single photons and electro-optical resonance tuning in the strong coupling regime [3,4]. Besides, we will show that external magnetic fields can induce a transition from the strong to the weak coupling regime [5] and allow for a photon-photon coupling mediated by an excitonic transition. References[1] J. P. Reithmaier et al., Nature 432, 197-200 (2004)[2] T. Yoshie et al., Nature 432, 200 (2004)[3] T. Heindel et al., Appl. Phys. Lett. 96, 011107 (2010)[4] C. Kistner et al., Opt. Exp. 16, 15006 (2008)[5] S. Reitzenstein et al., Phys. Rev. Lett. 103, 127401 (2009)
3:15 PM - **H2.3
The Physics and Technology of Stressed Quantum Dots.
Oliver Schmidt 1
1 Institute for Integrative Nanosciences, IFW Dresden, Dresden Germany
Show AbstractWe transfer GaAs nanomembranes incorporating self-assembled quantum dots onto piezoelectric substrates [1]. In this way, we are able to apply huge biaxial as well as uniaxial stress, which causes the excitonic emission lines of the quantum dots to shift in energy up to 18 meV [2]. At the same time, binding energies are altered and biexitonic and neutral excitonic transitions can be tuned into perfect resonance [3], thus, theoretically allowing to generate entangled photons in a time reordering scheme [4]. The GaAs nanomembranes are doped to create charge tuneable quantum dot samples. These samples are contacted to operate single QD LEDs with huge wavelength tuning range and a simultaneous control over the QD’s charging state [2]. Uniaxial strain is able to engineer the wavefunction’s symmetry inside quantum dots so that the fine structure splitting can be tuned deterministically [5]. Our work opens new horizons to control and manipulate electronic properties at the single quantum dot level simply by applying mechanical stress.[1] T. Zander et al., Opt. Express 17, 22452 (2009)[2] R. Trotta et al., unpublished[3] F. Ding et al., Phys. Rev. Lett. 104, 067405 (2010)[4] J. E. Avron et al., Phys. Rev. Lett. 100, 120501 (2008)[5] J. Plumhof et al., unpublished
3:45 PM - H2.4
Multiple-stacking of Columnar InAs Quantum Dot Layer Using Modulated Tensile-strained InGaAsP Barriers.
Shigekazu Okumura 1 2 , Nami Yasuoka 1 2 , Kenichi Kawaguchi 3 , Yu Tanaka 1 2 , Mitsuru Ekawa 3
1 , PETRA, Tokyo Japan, 2 , Fujitsu Limited, Atsugi Japan, 3 , Fujitsu Laboratories Limited, Atsugi Japan
Show AbstractElectronically coupled InAs columnar quantum dots (CQDs) are composed of multi-stacked InAs QD layers and thin tensile-strained InGaAsP barrier layers on an InP substrate. They are promising candidates for the active layer of high-performance semiconductor optical amplifiers (SOA) in the 1.55-μm telecom region. This is because we can control the height and strain of CQDs by changing the number of stacking and the thickness of tensile-strained barrier layers, respectively [1]. To further increase the optical gain of an SOA, the CQDs layer needs to be multi-stacked. In this work, we investigated two types of CQDs structures with uniform tensile-strained barriers and modulated tensile-strained barriers, where the tensile-strain of the upper barriers was larger than that of the lower barriers. A 12-fold CQDs layer was grown by alternatively depositing InAs QD layers and InGaAsP barrier layers on an InP (001) substrate by MOVPE. The deposition amount of InAs was 1.2 ML except for the first layer which had 2.5 ML of InAs, and that of InGaAsP barriers was fixed at 2.5 ML. Five samples, A, B, C, D and E, were fabricated. Samples A and B were composed of uniform barriers with a tensile strain of 0.6% and 1.0%, respectively. Samples C, D, and E were composed of modulated tensile-strained barriers with the lower four barriers having a tensile strain fixed at 0.4% and the upper seven barriers having a tensile strain fixed at 1.0%, 1.2% and 1.4%, respectively. Triple-stacked CQDs layers applying these CQDs structural parameters with the insertion of a 30-nm-thick unstrained InGaAsP spacer layer were fabricated. The photoluminescence (PL) wavelength of samples A and B were 1.55 μm to 1.50 μm, respectively. The reason for this difference in PL wavelength is that the strain of CQDs approaches a hydrostatic condition by receiving compressive stress in the growth direction as the tensile strain of the barriers increases. Triple-stacked CQDs layers emitting at 1.50 μm (sample A) resulted in a good PL spectrum and a surface morphology, while the layers emitting at 1.55 μm (sample B) degraded due to a higher averaged strain, compressively 0.78%, of the CQDs layer. On the other hand, the PL wavelength of samples C, D and E were almost same as 1.55 μm although the tensile strain of the upper barriers was increased from 1.0% to 1.4%. This means that the upper barriers do not affect the PL wavelength but act to compensate the compressive strain of the CQDs layer. By applying sample E with the lowest averaged strain of compressively 0.5%, we succeeded in triple-stacking a 12-fold CQDs layer emitting at 1.55 μm without degrading the crystallinity. It was indicated that the use CQDs with modulated tensile-strained barriers is effective for multi-staking CQDs layers. Part of this work was performed under the management of PETRA supported by NEDO.[1] K. Kawaguchi et al., Appl. Phys. Lett. 93, 121908 (2008)
4:30 PM - **H2.5
Anomalous Hall Effect in Anisotropic Media.
Vas Kunets 1 , S. Prosandeev 1 , Yu Mazur 1 , Greg Salamo 1
1 Physics, University of Arkansas, Fayetteville, Arkansas, United States
Show AbstractOne-dimensional conductors have numerous potential applications but suffer from an instability in the electronic structure characterized by an opening of a band-gap at the Fermi energy. By using a molecular beam epitaxy, we succeeded to achieve stable one-dimensional conductivity by embedding one-dimensional conductors into a three-dimensional semiconductor. Evidence of this success is the observation of strong anisotropic lateral conductivity. This is further supported by the observation of the anomalous Hall effect, which confirms a strong coupling between the three-dimensional and one-dimensional subsystems. These finding provide an opportunity to take advantage of the higher performance of one-dimensional systems.
5:00 PM - H2.6
Placement Control of Nano Dots by Selective Heating.
Haeyeon Yang 1 , Casey Clegg 1
1 Physics, Utah State University, Logan, Utah, United States
Show AbstractSelf-assembly is one of the promising methods to fabricate optoelectronic devices for sustainability in energy and environment through higher energy conversion efficiencies because it is easy and economical. However, difficulties in control of the size and position of self-assembled nanostructures have been regarded as major obstacles in bringing their full potentials into reality. Many different methods were tried to control placements of nano dots, which includes patterning of substrate surfaces to use different diffusion kinetics depending surface orientation, oxide layer deposition and etching, and on high index surfaces. However, majority of device fabrication has been developed on the (001) orientation and the pattering involves complicated additional steps. Interferential irradiation is a relatively simple process to control the placement and can be applied to any substrates and coupled with various growth techniques. We report that the interferential irradiation of high power laser induces self-assembly of nano dots on epitaxial growth fronts. Spatial thermal modulations in nanoscale were created in-situ on the epitaxial surfaces in the Molecular Beam Epitaxy (MBE) growth chamber by applying interferential irradiations of high power laser pulses. The as-irradiated surfaces were examined using the attached ultra-high vacuum Scanning Tunneling Microscope (STM). The STM images indicate that self-assembled dots are formed along the lines separated by the interferential periodicity and larger dots were observed when the substrate is at a higher temperature. The results suggest that the position of nucleation and size of nano dots can be controlled by simple adjustment of interference parameters and substrate temperature respectively.
5:15 PM - H2.7
Diffuse X-ray Diffraction at (In,Ga)As Quantum Dot Molecules.
Michael Hanke 1 , Zhiming Wang 2 , Yuriy Mazur 2 , Peter Lytvyn 2 4 , Jihoon Lee 3 , Greg Salamo 2
1 , Paul-Drude-Institute, Berlin Germany, 2 Department of Physics, University of Arkansas, Fayetteville, Arkansas, United States, 4 V. Lashkaryov Institute of Semiconductor Physics, NAS of Ukraine, Kiev Ukraine, 3 Department of Electrical Engineering, Kwangwoon University, Seoul Korea (the Republic of)
Show AbstractA widely recognized development in the self-assembly of semiconductor nanostructures is the growth of quantum dot molecules (QDMs). These are local arrangements of a small number (in many cases 2 to 8) of individual quantum dots (QDs). Besides the most simple, kind of prototypical QDM containing just two QDs more sophisticated QDMs made of three, four or even six QDs are frequently discussed. All the different types (bi-QDMs, and multi-QDMs) can be considered as building blocks of different functionality for opto-electronic device applications, future quantum computational devices and quantum communication. In that context a detailed knowledge on elastic strain evolution (with respect to the particular shape) becomes a key issue for a better understanding of the QDM's evolution.In that context we discuss the evolution of surface quantum dot molecules (QDM) in the system (In,Ga)As/GaAs(001) in terms of shape and elastic strain evolution. QDMs are grown by a combined approach using droplet epitaxy for initial homoepitaxial GaAs mounds, which subsequently serve as nucleation spots for surrounding (In,Ga)As surface quantum dots. Atomic force micrographs trace a detailed pathway toward the final QDM containing up to six quantum dots with perfect inherent symmetry. Synchrotron-based grazing incidence diffraction together with grazing incidence small angle x-ray scattering reveal a relaxation behavior, which for all growth stages comprises a strained lattice along [-110] and partial elastic relaxation along [110]. Numerical finite element calculations on the three-dimensional strain profile support the experimental findings.
5:30 PM - H2.8
Coupling of InAs and As(Sb) Quantum Dots in Epitaxial GaAs Films.
Vladimir Chaldyshev 1 , Nikolay Bert 1 , Vladimir Nevedomsky 1 , Valerii Preobrazhenskii 2 , Mikhail Putyato 2 , Boris Semyagin 2
1 , Ioffe Institute, St.Petersburg Russian Federation, 2 , Institute of Semiconductor Physics, Novosibirsk Russian Federation
Show AbstractHybrid nanostructures based on quantum dots (QDs) of metals and semiconductors have attracted constantly increasing attention during the last few years. These objects may have unusual interesting properties due to interaction and hybridization of intrinsic excitations, namely excitons and plasmons. Formation of the hybrid nanostructures is a challenging task since the formation procedures are specific and often inconsistent for the QDs of different origin.In this presentation we first report about possibility of self-organization of semiconductor InAs QDs and semimetal As(Sb) QDs in a close vicinity to each other in GaAs matrix, utilizing a combined process of the molecular beam epitaxy (MBE).The samples were grown on GaAs substrates with (001) orientation and contained a single layer or stack of coupled InAs QDs. These QDs were self-organized by using the Stranski-Krastanow growth mode. The InAs QDs were overgrown by GaAs at low (200°C) temperature. The low-temperature grown layers contained delta-layers of In or Sb that served as precursors for self-organization of As or AsSb QDs, respectively, during post-growth annealing. The resulting structures were studied by transmission electron microscopy (TEM), atomic force microscopy, x-ray diffractometry and optical techniques.The TEM study reveals a single layer or double layer of vertically coupled InAs QDs, an array of As QDs in the low-temperature grown top layer and close pairs of InAs - As(Sb) QDs, i.e. hybrid "nanomolecules". The origin for the observed coupling seems to be the overlapping local mechanical fields of strained QDs. In case of AsSb QDs these fields are much stronger. That results in coupling of apparently all the InAs QDs to AsSb QDs. The ripening rate upon annealing for both As and AsSb QDs coupled to InAs QDs appeared to be higher than for As QDs in a plain GaAs matrix. It gives a tool for adjustment of the size of the As(Sb) QDs in the hybrid "nanomolecules" by accumulation an extra arsenic from reservoir of small As inclusions in the top layer. The microstructure of the hybrid couples of QDs was investigated along with an optical study to reveal specific features of such objects.
5:45 PM - H2.9
Synthesis and Assembly of PbSe Nanorod and PbSe@CdSe Two-dots-in-a-Rod Nanocrystals.
Marianna Casavola 1 , Karel Lambert 2 , Zeger Hens 2 , Daniel Vanmaekelbergh 1
1 Condensed Matter and Interfaces, Debye Institute for Nanomaterials Science, Utrecht University, Utrecht Netherlands, 2 Physics and Chemistry of Nanostructures, Ghent University , Ghent Belgium
Show AbstractNanotechnology aims at fabricating functional materials and devices, whose constituents and their spatial organization are engineered at the nanoscale. One way to achieve this goal is through the exploitation of colloidal nanocrystals, whose size- and shape- dependent properties can be easily controlled, as building blocks for novel devices.Recent achievements have been reported on the realization of heterostructured nanocrystal (HNCs), i.e. containing two different compounds in a single particle, often with topological control.1 These systems are expected to benefit from the individual properties of the single components, and additionally show novel properties arising from inter-particle quantum mechanical coupling. Here, we present the synthesis and characterization of novel types of HNCs based on asymmetrically-shaped PbSe NCs, which have been used as templates for the growth of PbSe-CdSe structures with controlled topological distribution of material sections.2 We have firstly developed a novel, catalyst-free synthetic strategy for the growth of rock-salt PbSe in well-defined nanorods (NRs), uniform in diameter and length, formed by oriented attachment.2-4 PbSe-CdSe HNC structures have been obtained from PbSe nanorods (NRs) by partial replacement of Pb with Cd cations (cation exchange) at low temperature.2,5 Interestingly, HRTEM and tomography studies reveal that the firstly formed PbSe@CdSe core@shell NCs evolve in unprecedented structures, named two-dots-in-a-rod, which comprise CdSe nanorods (NRs), each embedding two PbSe NCs.2 These materials may provide a novel platform for the study of electronic coupling between semiconductor NCs. In addition, the growth of PbSe-based NCs in anisotropic shapes could be particularly advantageous for providing semiconductor NCs with emission of polarized light in the near-IR. For this purpose, we have achieved both horizontal and vertical assemblies of the NRs by slow solvent evaporation of NR solution on functionalized substrates, systems which opto-electronic properties are currently under investigation.1.M. Casavola et al. Eur. J. Inor. Chem. 2008, (6), 837-854.2.M. Casavola et al. Unpublished results.3.K.-S. Cho et al. JACS 2005, 127, 7140.4.W.-k Koh et al JACS 2010, 132, 3909.5.J.M. Pietryga et al. JACS 2008, 130, 4879.
Symposium Organizers
Matthew F. Doty University of Delaware
Sayantani Ghosh University of California, Merced
Armando Rastelli IFW Dresden e.V.
Marija Drndic University of Pennsylvania
H3: Plasmon-mediated Interactions and Photovoltaic Applications
Session Chairs
Tuesday AM, November 30, 2010
Room 300 (Hynes)
9:30 AM - **H3.1
Exciton-plasmon Interactions and Energy Transfer in Nanostructures.
Alexander Govorov 1
1 , Ohio University, Athens, Ohio, United States
Show AbstractCoulomb and electromagnetic interactions between excitons and plasmons in nanocrystals cause several interesting effects: energy transfer between nanoparticles (NPs), plasmon enhancement, reduced exciton diffusion in nanowires (NWs), exciton energy shifts, Fano interference effect, and non-linear phenomena [1-3]. Using transport equations for excitons, we model exciton transfer in NWs and explain the origin of the blue shift of exciton emission observed during recent experiments with hybrid NW-NP assemblies [2]. We also look at optical responses of artificial light-harvesting complexes composed of chlorophylls, bacterial reaction centers, and NPs [3]. We show that, using superior optical properties of metal and semiconductor NPs, it is possible to strongly enhance the efficiency of light harvesting in such complexes [3]. An interaction between a discrete state of exciton and a continuum of plasmonic states can give rise to interference effects (Fano-like asymmetric resonances and anti-resonances). These interference effects greatly enhance visibility of relatively weak exciton signals and can be used for spectroscopy of single nanoparticle and molecules. In a nonlinear regime, the Fano effect becomes strongly amplified and results in interesting non-linear responses [4]. If an exciton-plasmon system includes chiral elements (chiral molecules or nanocrystals), the Fano-like interference effects strongly enhance the circular dichroism signals [5]. In conclusion, our theory explains current experimental results and also provides rationale for future experiments and applications. Potential applications of dynamic exciton-plasmon systems include sensors and light-harvesting. [1] 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). [2] J. Lee, P. Hernandez, J. Lee, A. Govorov, and N. Kotov, Nature Materials 6, 291 (2007). [3] A. O. Govorov and I. Carmeli, Nano Letters 7, 620 (2007). [4] M. Kroner, A. O. Govorov, S. Remi, B. Biedermann, S. Seidl, A. Badolato, P. M. Petroff, W. Zhang, R.Barbour, B. D. Gerardot, R. J. Warburton, and K. Karrai, Nature 451, 311 (2008). [5] A.O. Govorov, Z. Fan, P. Hernandez, J.M. Slocik, R.R. Naik, Nano Letters 10, 1374 (2010).
10:00 AM - H3.2
Moving Beyond Plasmonics for High-efficiency Light Emission from Semiconductor Nanostructures.
Antonio Llopis 1 , Jie Lin 1 , Sergio Pereira 2 , Tito Trinidade 2 , Ian Watson 3 , Zhiming Wang 4 , Greg Salamo 4 , Arkadii Krokhin 1 , Arup Neogi 1
1 , Univ. of North Texas, Denton, Texas, United States, 2 CICECO, University of Aveiro, Aveiro Portugal, 3 , University of Strathclyde, Strathclyde United Kingdom, 4 , University of Arkansas, Fayetteville, Arkansas, United States
Show AbstractPlasmonics has been identified as the next generation technology for nanoscale photonic devices such as light emitters, optical interconnects and detectors. It has been shown that coupling to surface plasmons can produce an enhancement in photoluminescence intensity as well as strongly confined light emission. Surface plasmon enhanced light emission or detection, however, is limited by strong dissipative effects as well as strict energy and momentum matching conditions for resonant interaction with the emitter or absorber. This combination of loss due to dissipation and the narrow window for resonant interaction with the plasmons limits the overall efficiency of devices and requires careful tailoring of the system.Instead of the electrodynamically-induced modification of the radiative recombination rate produced via plasmonics, we present an alternative mechanism for increasing the internal quantum efficiency of a light emitter based on carrier concentration modulation via electrostatic interaction with embedded metallic nanoparticles. The Coulomb attraction of carriers to the image charges induced within the metal nanoparticles results in a high concentration of electron-hole pairs around the nanoparticle. This in turn leads to a marked enhancement and spatial concentration of photoluminescence at low input power. This universal electrostatic effect is not dissipative and lacks the stringent frequency and energy matching requirements imposed on surface plasmon coupling. Here we demonstrate strongly enhanced light emission from various solid state emitters ranging from the UV to the near-IR based on non-resonant interaction of e-h pairs with metallic nanoparticles. In each of these systems the surface plasmon resonance of the metal nanoparticles is known to be below the emission energy of the emitter, thus excluding effects due to resonant enhancement. The nanomaterial systems demonstrating efficient light emission can be synthesized using various techniques. We present here enhanced emission from Au nanoparticles embedded in epitaxially (MOCVD) grown InGaN/GaN multiple quantum wells, Au nanoparticles embedded in ion-implanted Si/ SiO2 nanocrystals, and metallic Ga nanoparticles on the surface of an MBE grown AlGaAs/GaAs quantum well. We experimentally demonstrate a significant increase of emission intensity along with features that deviate from those associated with plasmonics including an increase in the radiative decay lifetime of the system and a power-dependent saturation in intensity and carrier concentration enhancement.
10:15 AM - H3.3
A Plasmonic Nanoantenna Enhances the Optical Excitation Efficiency of Single Self-assembled GaAs Quantum Dots.
Markus Pfeiffer 1 2 , Klas Lindfors 1 2 , Paola Atkinson 3 , Armando Rastelli 3 , Oliver Schmidt 3 , Harald Giessen 2 , Markus Lippitz 1 2
1 , Max Planck Institute for Solid State Research, Stuttgart Germany, 2 , 4th Physics Institute and Research Center SCOPE, Stuttgart Germany, 3 , IFW, Dresden Germany
Show AbstractThe light emission properties of a single quantum system may be significantly modified by placing the emitter close to a nanostructure. This allows controlling, e.g., the excitation and emission rate as well as the emission pattern of the emitter. Plasmon resonant metal structures are a particularly interesting choice for the nanostructure as the electromagnetic field is significantly enhanced at the plasmon resonance wavelength. This offers exciting possibilities in both fundamental light-matter studies as well as in applications. We experimentally investigate the influence of plasmon resonant nanoantennas on the photoluminescence properties of individual semiconductor quantum dots (QDs). As nanoantennas we use spherical gold nanoparticles. The quantum dots are epitaxially grown AlGaAs/GaAs QDs which are buried a few nanometers beneath the semiconductor surface and photoluminesce at approximately 760 nm wavelength. The advantage of this system is that the optical properties of the QDs are very stable and the transition dipole moments have a fixed orientation. The thin barrier layer allows efficient coupling between the quantum dot and a nanoparticle positioned in a well controlled way above the emitter. We observe a reduced spectral linewidth and an increase in the photoluminescence intensity of up to a factor of 8 due to the nanoantenna. Based on time resolved measurements and spectral analysis of the photoluminescence we attribute the enhancement in luminescence to an increase in the excitation rate of the quantum dot. Our results demonstrate that the combination of plasmonic nanostructures and epitaxial quantum dots is fruitful for both sides, leading to stable quantum emitters for plasmonics and enhanced coupling of the quantum dots to the light field.
10:30 AM - H3.4
Synthesis of Metal-semiconductor Hybrid Nanostructures.
Bishnu Khanal 1 , Jeffrey Pietryga 1 , Victor Klimov 1
1 Chemistry, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractCoupling to the surface plasmons of nearby metal nanostructures can have significant effects on the excitation and relaxation processes of quantum dots (QDs). In the last few years, tremendous interest has grown in such metal-semiconductor interactions because of potential applications in lasing, solid-state lighting, photovoltaics and sensors. However, advantageous coupling depends on a number of factors, including spectral overlap and interparticle separation distance. Advances in this field have been slowed by the difficulties of chemical fabrication of metal-semiconductor hybrid nanostructures with the requisite control over such parameters. This presentation will describe a generalizable method for the chemical synthesis of metal/dielectric/QD core/shell/shell hybrid nanostructures with straightforward control over shape, size, separation distance and QD loading. In a particularly striking example, gold nanorods of nearly any desired aspect ratio, having strong longitudinal plasmon features ranging from the visible well into the infrared, are synthesized by a tip-selective growth and dissolution technique. They are then coated with 5-20 nm thick silica layers. Finally, visible or infrared emitting QDs are chemically attached to the surface of the silica-coated gold nanorods, so as to match QD PL peak and metal plasmon energies. The effect of the metal surface plasmon on QD PL will also be discussed in this presentation.
10:45 AM - H3.5
Development of In-situ Tuning Method of Quantum-dot Spacing and Optical Coupling between Gold Nano-clusters.
Mitsuru Inada 1 , Yoshihiro Yoshihara 1 , Hideya Kawasaki 3 , Yasuhiko Iwasaki 3 , Tadashi Saitoh 1 , Ikurou Umezu 2 , Akira Sugimura 2
1 Department of Pure and Applied Physics, Kansai University, Osaka Japan, 3 Department of Chemistry and Materials Engineering, Kansai University, Osaka Japan, 2 Department of Physics, Konan University, Kobe Japan
Show AbstractNanostructured materials have attracted much attention in recent years for their potential applications. In particular electric structure of nanomaterials, which consist of coupled quantum dots is of considerable interest both fundamentally and technologically, because such electric structure can be described as Hubbard model due to a competition between quantum size effect and inter-dot coupling as well as electron-electron interaction. However there are some technological problems to investigate the coupling effect, for instance, it is difficult to control the inter-dot spacing with sub-nanometer range. In this study, we report on a development of in situ precise tuning method of quantum-dot spacing and investigate fluorescence resonant energy transfer in Au nano-clusters.The tuning device consists of a DC motor and capacitance-positioning sensor. After Au nano-clusters, which include Au5, Au8 and Au13 clusters, were deposited on poly(dimethylsiloxane) film, the film was gently and continuously stretched by the DC motor. The stretching the film means an increasing the inter-dot spacing. The degree of stretching was monitored by the capacitance-positioning sensor. We investigated photoluminescence(PL) spectra by stretching the film. We found the blue shift of the PL peak with stretching the film. So the stretching the film means an increasing the inter-dot spacing, the peak shift can be explained by the emission from smaller nano-clusters. The quenching the emission from smaller nano-cluster can be explained by fluorescence resonant energy transfer from smaller nano-cluster to lager one, when the inter-dot distance is smaller.
11:30 AM - **H3.6
Coupling Between Semiconductor Quantum Dots and Plasmonic Cavities, Antennas and Waveguides.
Harry Atwater 1 2 , C. Hoffmann 1 2 , J. Fakonas 1 2 , D. O'Carroll 1 2
1 Kavli Nanoscience Institute, California Institute of Technology, Pasadena, California, United States, 2 T. J. Watson Laboratories of Applied Physics, California Institute of Technology, Pasadena, California, United States
Show AbstractSince 2000, intensive research on plasmonic light localization in metal-dielectric nanostructures has enabled a truly nanoscale photonics discipline. Widespread use of quantitative full field electromagnetic modeling and wide availability of powerful tools for nanostructure fabrication have accelerated progress in plasmonics. In the next ten years, we can foresee an ability to design a variety of coupled plasmonic/semiconductor excitonic nanoscale systems that exploit both weak and strong exciton/plasmon coupling. Weakly-coupled systems can enable control of the spontaneous emission rate and direction for semiconductor nanostructures such as GaAs, InGaN quantum dots and wires, as well as polymeric semiconductor dots, coupled to plasmonic cavities with modest quality factor but extremely small mode volume. Systems that can lead to dramatic (> 1000x) enhancement in spontaneous emission rate will be described. Further, one can imagine generating strongly-coupled exciton/plasmon systems that enable interesting phenomena such as correlated photon emission and plasmon entanglement, and prospects for these will be discussed.
12:00 PM - H3.7
PbS and PbSe Quantum Dot Schottky Junction and Heterojunction Solar Cells.
Joseph Luther 1 , Jianbo Gao 1 3 , Octavi Semonin 1 2 , Matthew Lloyd 1 , Matthew Beard 1 , Arthur Nozik 1 2
1 , NREL, Golden , Colorado, United States, 3 , University of Toledo, Toledo, Ohio, United States, 2 , University of Colorado, Boulder, Colorado, United States
Show AbstractMultiple exciton generation provides a promise to extend theefficiency of a single junction solar cell beyond the ShockleyQueisser limit of ~33% by using a high-energy photon to generatemultiple electrons. This effect is most commonly studied in colloidalnanocrystals or quantum dots of PbS or PbSe. Over the past 3 years,lead chalcogenide based solar cells have seen rapid progress,including the highest measured photocurrents under 1-sun conditionsfor nanostructured solar cells. The open circuit voltages of deviceshave surpassed the bulk bandgap indicating quantum confinement canhelp create a fascinating thin film with new collective properties.This talk will discuss coupling strategies for creating conductivequantum dot networks, alloying strategies to improve device metrics,as well as various device structures and contacts to collect electronsfrom the light absorbing quantum dot arrays. We will present data froman NREL certified device, which sets the benchmark for futureimprovements in this field. We will also discuss recent advances incontrolling the stability of the devices, oxygen-induced effects onthe electrical properties, and quantum dot size-dependent propertiesof the devices.
12:15 PM - H3.8
Single-nanocrystal Spectroscopy for Quantitative Studies of Emission Efficiencies of Multiexciton States.
Young-Shin Park 1 , Anton Malko 2 , Javier Vela 1 , Yongfen Chen 1 , Jennifer Hollingsworth 1 , Victor Klimov 1 , Han Htoon 1
1 Center for Integrated nanotechnologies, Los Alamos National Lab, Los Alamos, New Mexico, United States, 2 Physics Department, University of Texas at Dallas, Richardson, Texas, United States
Show AbstractSize-controlled emission colors and high intrinsic photoluminescence quantum yields (QYs) are among the reasons that make nanocrystal quantum dots (NQDs) attractive materials for applications such as solid state lighting and lasing. One problem in all of these applications, however, is significant quenching of emission from charged excitations (such as trions) and neutral multi-electron-hole-pair states (multiexcitons) due to non-radiative Auger recombination. In this process, the electron-hole recombination energy is not emitted as a photon, but rather dissipates in the form of a kinetic energy of the third carrier. Thus, the ability to harvest the emission from multiexcitonic and charged states could dramatically enhance the potential of NQDs for light-emitting applications, especially, in the case of lasing, where multiexcitons are required to produce optical gain.Recently, we have observed clear signature of multiexcitonic emission from a new class of NQDs, in which a 3-4 nm diameter CdSe core is over-coated with a thick (>12 monolayer) CdS shell (dubbed "giant" NQDs or g-NQDs due to their large size)[1]. Here, we present two independent quantitative measurements examining the photoluminescence (PL) QYs of multiexciton states from individual g-NQDs. We also discuss the application of these techniques to coupled g-NQD-metal-nanoparticle structures with the goal to elucidate the effect of plasmonic-field enhancement on multi-exciton PL quantum yield.In the first approach, we extract the ratio of the QYs of bi-exciton and single-exciton states (QYBX/QYX) from the variation of the spectrally integrated PL intensity as a function of pump power. This approach requires the knowledge of NQD absorption cross-sections and certain assumptions on scaling of radiative and non-radiative decay rates with exciton multiplicity. In the second method, the ratio QYBX/QYX is measured from the normalized amplitudes of the zero-time coincidence feature in the second-order intensity correlation function (g(2)) obtained from single g-NQDs under weak excitation. While the second approach allows for direct measurements of QYBX/QYX, pump-dependent measurements of PL intensity are simpler compared to photon-correlation studies and less time consuming. The results of these two independent experiments are in excellent mutual agreement. They indicate that while the near-perfect multiexciton QYs (e.g. QYBX/QYX > 0.8) can be achieved in g-NQDs with shell thickness >12 CdS monolayers, the QYBX/QYX ratio varies widely from one g-NQD to another. Finally, we present the application of these two approaches to g-NQDs coupled to metal nanoparticles. In these hybrid structures, the coupling strength is controlled by tuning the spectral overlap of Plasmon modes with NQD excitons and varying the semiconductor-metal separation with dielectric spacers.[1] H. Htoon et. al., Nano Lett. DOI:10.1021/nl1004652 (2010)
H4: Understanding Quantum Dot Interactions
Session Chairs
Tuesday PM, November 30, 2010
Room 300 (Hynes)
2:30 PM - **H4.1
Characteristic Properties of Holes in Vertically Coupled Quantum Dots.
Juan Climente 1 , Matthew Doty 2 , Marek Korkusinski 3 , Alex Greilich 4 , Michael Yakes 4 , Allan Bracker 4 , Michael Scheibner 4 , Pawel Hawrylak 3 , Dan Gammon 4
1 Dpt. Physical and Analytical Chemistry, Universitat Jaume I, Castellon, Castellon, Spain, 2 , University of Delaware, Newark, Delaware, United States, 3 , Institute for Microstuctural Sciences, Ottawa, Ontario, Canada, 4 , Naval Research Laboratory, Washington, District of Columbia, United States
Show AbstractTunneling of excitons between vertically coupled quantum dots enables the formation of molecular-like orbitals which are important in many quantum dot-based devices. The differences in size and composition of the quantum dots is overcome by a vertical electric field, which brings the two quantum dot levels into resonance and induces either electron or hole tunneling. While early works investigating coupled quantum dots used resonant electrons, recently it has become apparent that the use of resonant holes poses a number of advantadges, such as slower decoherence rates and improved suitability for local spin manipulation using electric fields.In this talk we shall see that holes in InAs double quantum dots exhibit a number of surprising properties which are not observed for electrons. First we show that increasing the interdot distance leads to the formation of molecular ground states whose orbital character is antisymmetric instead of symmetric.[1,2] This phenomenon, which has no precedent in natural molecules, has implications both for engineering of nanostructures with new optoelectronic devices and for controlling spin tunneling. The possibility to change from symmetric to antisymmetric ground state after growth, by means of longitudinal magnetic fields, is then discussed.[3] Next, we investigate the origin of anticrossings observed in photoluminescence spectra between hole states with opposite quantum dot localization and spin projection. The magnitude of these anticrossings, which are over ten times larger than those of electrons, implies important degree of spin mixing between states which are separate in distance.[4] All these features can be readily understood from the strong spin-orbit coupling of the valence band. Using a Luttinger spinor description of the hole states, rather than the usual heavy hole approximation, a natural explanation is found for the characteristic behavior of holes. [1] J.I. Climente et al., Phys. Rev. B 78, 115323 (2008).[2] M.F. Doty et al., Phys. Rev. Lett. 102, 047401 (2009). [3] J.I. Climente, Appl. Phys. Lett. 93, 223109 (2008). [4] M.F. Doty et al., Phys. Rev. B 81, 035308 (2010).
3:00 PM - H4.2
Transient Absorption for Characterization of Quantum Dot Intermediate Band Solar Cells.
Steve Smith 1 , Praveen Kollay 1 , Andrew Norman 2
1 , South Dakota School of Mines and Technology, Rapid City, South Dakota, United States, 2 , NREL, Golden, Colorado, United States
Show AbstractWe use transient absorption methods to characterize the sequential two-photon absorption in a quantum-dot super-lattice based intermediate band solar cell (QD-IBSC). Using collinear, orthogonally polarized beams generated from an Optical Parametric Oscillator (OPO) at varying time delay, tuned stepwise from 1050nm to 1250nm, we use the solar cell photocurrent as a direct measure of the transient absorption by measuring the differential photo-current as a function of time delay between two energetically degenerate, 100fs pulses. For comparison, we measure the pulse autocorrelation in the same geometry using a GaAsP photodiode, where all observed photocurrent is derived from instantaneous two-photon absorption. Our measurements show that at high intensity, the measurement is dominated by instantaneous two photon absorption, with a simultaneous sequential two-photon photocurrent which persists beyond the pulse overlap. Our measurements demonstrate the method can reveal carrier dynamics in a working QD-IBSC, and their dependence on energy. The method could potentially give details of the band structure formed in the QD-IBSC. Such knowledge may benefit device development and future designs of IBSCs based on QD superlattices or alternative intermediate band materials or device structures.
3:15 PM - **H4.3
Atomistic Calculations of Energy Levels and Interactions in Epitaxial Lateral Quantum-dot Molecules.
Gabriel Bester 1
1 , Max Planck Institute for Solid State Research, Stuttgart Germany
Show AbstractIn this contribution I will outline the framework, based on empirical pseudopotentials and configuration interaction [1], to obtain quantitative predictions of the excited state properties of semiconductor nanostructures using their experimental sizes, compositions and shapes. The methodology can be used to describe colloidal nanostructure of few hundred atoms all the way to epitaxial structures requiring millions of atoms. We will illustrate the capability of the method by a recent application to lateral quantum dot molecules [2]. We show that by the combination of the aforementioned theory, knowledge of the detailed morphology of the QDMs, and PL experiments, we reach a thorough understanding of the underlying processes involved in the optical experiment. We highlight the importance of electronic coupling in lateral dot molecules fostered by the presence of an In-poor basin connecting the dots from below. Beyond the static picture we find strong evidence, backed up by PL measurements, for a dynamical model ensuing from the lack of potential barrier felt by the electron in opposition to the decoupled holes. This model leads to a qualitatively different behavior for absorption and emission processes under non-resonant excitation. We present further predictions for different charged excitons states and analyze the regions where optically accessible anticrossings should be expected. We conclude that these type of structures represent an interesting type of nano-laboratory where the dynamics of carriers comes prominently into play.
[1] G. Bester, J. Phys.: Condens. Matter. 21 023202 (2009).
[2] J. Peng, C. Hermannstadter, M. Witzany, M. Heldmaier, L. Wang, S. Kiravittaya, A. Rastelli, O. G. Schmidt, P. Michler and G. Bester, Phys. Rev. B. 81 205315 (2010).
3:45 PM - H4.4
Coherence and Decoherence in Quantum Dot Exciton Dynamics: Driven Dots and Excitonic Energy Transfer.
Ahsan Nazir 1
1 Physics and Astronomy, University College London, London United Kingdom
Show AbstractIn this presentation, I shall explore two aspects of the interplay of quantum coherence and decoherence in quantum dot exciton dynamics. In the first part, I shall describe recent theoretical work demonstrating that the dephasing observed experimentally in single, coherently-driven quantum dots can be accurately described within a simple weak-coupling master equation formalism [1], identifying acoustic-phonons as the dominant source of excitonic decoherence.In the second part, I shall investigate excitonic energy transfer dynamics in a coupled quantum dot pair beyond the weak inter-dot or exciton-phonon coupling regimes. I identify a transition from coherent to incoherent exciton dynamics with increasing temperature, due to multiphonon effects not captured by a standard weak system-environment (i.e. exciton-phonon) coupling treatment. The crossover temperature has a marked dependence on the degree of spatial correlation between fluctuations experienced at the two quantum dot sites. For strong correlations, this leads to the possibility of coherence surviving into a high-temperature regime [2].[1] A. J. Ramsay et al., Phys. Rev. Lett. 104, 017402 (2010)[2] A. Nazir, Phys. Rev. Lett. 103, 146404 (2009)
4:30 PM - **H4.5
Optical Signatures of Kondo Effect in Self Assembled Quantum Dots.
Christian Latta 1 , Florian Haupt 1 , Parisa Fallahi 1 , Atac Imamoglu 1
1 Institute for quantum electronics, ETH, Zürich Switzerland
Show AbstractRecently, the observation of coupling of a quantum dot to a Fermi sea in photoluminescence measurements has been reported[1]. We optically investigate transitions in self-assembled quantum dots where either the ground or the excited state interacts strongly with a Fermi sea of electrons in differential transmission. This technique allows us to gain detailed information about the exact form of the absorption lineshape, thereby minimally disturbing the system. In the perturbative regime, the coupling to the Fermi sea at 4 K is reflected in the measurements in a two-fold way: A renormalized transition energy, and characteristic 1/δ power law tails to blue laser detunings in the absorption lineshapes. If the temperature is lowered below a parameter-dependent value TK, the system undergoes a crossover associated with the formation of a new stongly correlated many body state [2]. The non-perturbative coupling between the optically excited electron and the Fermi sea leads to an orthogonality catastrophe; the associated absorption lineshape exhibits a divergence that scales as 1/δβ≈0.5.Our experimental work is supported by a combination of numerical renormalization group and analytical results [3].[1] N. A. J. M. Kleemans et al., Nature Physics, DOI: 10.1038/NPHYS1673[2] J. Kondo, Progress of Theoretical Physics 32,37[3] H. Türeci et al., arXiv:0907.3854
5:00 PM - H4.6
Single Shot Optical Readout of a Quantum Dot Electron Spin via Resonance Fluorescence.
Nick Vamivakas 1 , Chao-yang Lu 1 , Clemens Matthiesen 1 , Yong Zhao 1 2 , Stefan Falt 3 , Antonio Badalato 4 , Mete Atature 1
1 , University of Cambridge, Boston United Kingdom, 2 , Ruprecht-Karls-Universität Heidelberg , Heidelberg Germany, 3 , Sol Voltaics AB, Lund Sweden, 4 , University of Rochester, Rochester, New York, United States
Show AbstractBy monitoring the intermittency of resonance fluorescence [1] from a quantum dot molecule (QDM) we perform optical single-shot measurements of a resident electron’s spin. The QDM consists of two vertically stacked Indium Arsenide quantum dots, separated by 13 nm, in Gallium Arsenide embedded in a Schottky diode heterostructure. The bottom quantum dot (QD) of the pair is 35 nm above an n+ layer of doped Gallium Arsenide, which serves as an electron reservoir, allowing for deterministic electron charging of the QDM. We operate in a gate voltage regime where the QD closest to the electron reservoir (host QD) is single electron charged while the other (readout) QD remains empty in the ground state. The ground states consist of a single electron in the host QD with spin projection down (up) and the excited trion states of the host QD are identical to a single QD. These transitions are used for optical preparation of the host QD spin. The QDM also supports distributed trion states consisting of an electron in the host QD and an exciton in the readout QD. This is the only charge configuration which exhibits electronic tunnel coupling between the host and readout QD. Under a finite magnetic field along the QDM growth direction spin-dependent interactions lift the degeneracy of the 12 molecular states of this configuration and each state becomes spectrally addressable. By monitoring the resonance fluorescence from the optical transition between the host QD electron spin down ground state (single charge configuration) and the host QD electron spin down/readout QD electron spin down/readout QD hole spin up excited trion state (three charge configuration) we can resolve flips in the orientation of the host QD spin through suppression of the resonance fluorescence signal. Two-color experiments confirm the conditionality of the measured resonance fluorescence on the host QD spin orientation. We find the observed spin dynamics are a result of the measurement laser driving other nearby QDM optical transitions with a finite spectral detuning from the readout transition energy.[1] A. N. Vamivakas, Y. Zhao, C.-Y. Lu, and M. Atatüre, “Spin-resolved quantum dot resonance fluorescence.”Nature Phys. 5, 198-202 (2009).
5:15 PM - H4.7
Quantum Coupling of Charged Exciton States in Lateral Quantum Dot Molecules.
Xinran Zhou 1 , J. Lee 2 , Zh. Wang 3 , G. Salamo 3 , M. Doty 1
1 Department of Materials Science and Engineering, University of Delaware, Newark, Delaware, United States, 2 Department of Electrical Engineering, Kwangwoon University , Seoul Korea (the Republic of), 3 , University of Arkansas, Little Rock, Arkansas, United States
Show AbstractControllable quantum coupling between separate quantum dots is one of the most challenging obstacles to the development of scalable devices that operate at the quantum limit. The development of such devices is of great interest because of the opportunity to implement novel information processing architectures like quantum computing. Coherent coupling has been demonstrated in vertically-stacked quantum dot molecules (QDMs), but this architecture cannot be scaled to large arrays with independently controlled coupling between arbitrary nearest neighbor pairs. Lateral QDMs can be scaled up and independently controlled, but this approach has been slow to develop because it is challenging to grow quantum dot pairs with small lateral inter-dot separations and because the degree of coupling is substantially weaker than in vertically-stacked QDMs. Even more challenging is the identification of the signatures of coupling when the dots are charged with electrons or holes that can serve as single charge or spin bits.We report spectroscopic evidence of quantum coupling in the charged exciton states of lateral quantum dot molecules. The lateral InAs QDMs are fabricated by molecular beam epitaxy using controlled shape evolution under a thin GaAs cap. The quantum coupling is controlled with a three-terminal device design that can apply electric fields along both the growth and molecular axes. Spectral maps as a function of bias along the growth direction identify the charging of the QDMs. Spectral maps as a function of bias along the molecular axis control the degree of quantum coupling in both neutral and charged exciton states. The observed spectra agree with the results of atomistic empirical pseudopotential calculations.
5:30 PM - H4.8
Coupling between Donor and Acceptor Charge Transfer States in Colloidal Nanocrystals.
Marcus Jones 1
1 Chemistry, UNC Charlotte, Charlotte, North Carolina, United States
Show AbstractParticle size and shape effects determine how the optoelectronic properties of nanoscale materials deviate from those of the bulk. Quantum confinement of photo-generated electrons and holes in colloidal semiconductor nanoparticles is well understood and results in a wide array of potentially important qualities that have been widely studied in recent years. In addition to these properties, application of nanocrystals in photovoltaics requires rapid and efficient generation of separated charges, which must be transported away from the chromophore to do useful work. These charge separation processes originate from highly delocalized exciton states and can be understood in terms of electron transfer reactions that can localize an electron or hole into a ligand orbital or a nanocrystal trap state.At the nanoscale, electronic coupling of exciton states with states in the surrounding environment is dramatically more influential on relaxation dynamics than in bulk materials. Charge transfer reaction in nanocrystals typically produce states with very low absorption cross sections, so we tend only to have a qualitative understanding of their role and relative impact on the underlying dynamics. Time resolved fluorescence is a technique that reflects both the radiative recombination rates from intrinsic exciton states and the non-radiative transitions rates to extrinsic surface or ligand states. Unfortunately, interpretation of fluorescence transients is not easy and typical multi- or stretched exponential decay models yield little specific photophysical information.I will describe the application of a stochastic model of exciton and charge-transfer dynamics to understand the fluorescence decay signals from a series of nanocrystal samples. Specifically I will discuss the modulation of trap states in UV-degraded materials, the trapping dynamics or core versus core-shell nanocrystals and the enhanced coupling of hot excitons with surface states. General features of charge transfer reactions are emerging from these analyses, which will be discussed in the context of future photovoltaic materials.
H5: Poster Session: Quantum Dot Coupling
Session Chairs
Wednesday AM, December 01, 2010
Exhibition Hall D (Hynes)
9:00 PM - H5.1
Room-temperature Long-wavelength (~1.45μm) Emission from Self-assembled InAs/GaAs Quantum Dots.
Subhananda Chakrabarti 1 , Shreyas Shah 1
1 Center of Nanoelectronics, Department of Electrical Engineering , Indian Institute of Technology Bombay, Mumbai, Maharashtra, India
Show AbstractSelf-assembled InAs quantum dots (QDs) grown epitaxially on GaAs QDs typically emit in the 1.1μm-1.2μm range. To obtain QD-based communication lasers, efforts are being made to obtain emission in the telecommunication bands around 1.3μm and 1.55μm. Increasing the InAs monolayer coverage is a possible route to longer-wavelength emission. However, there are drawbacks of using this method with a single layer of QDs (SQD). The bilayer (BQD) structure can alleviate some of these problems. Since stacking multiple layers of such QDs within the active region of devices such as VCSELs is important in order to obtain high modal gain, it is of interest to study the effects of stacking, with a very thin barrier, on the structural and optical properties. In this work, we study structural and optical properties of strain-coupled, multiple layers of InAs QDs, separated by a very thin GaAs spacer layer, and observe that the multilayer (MQD) structure with a higher InAs monolayer coverage significantly extends the emission wavelength towards the 1.55μm window at room temperature.All samples were grown on a GaAs (100) wafer with an EPI MOD GEN II MBE system. The growth rate for all InAs layers was ~0.03ML/s. QDs in the SQD structure were grown at a substrate temperature of 520 deg.C. The QDs in the seed layer of the BQD and the MQD structures were grown at a substrate temperature of 500 deg.C, while the upper layers in each structure and the GaAs barrier layers were grown at 460 deg.C. PL spectra of the SQD, BQD and the MQD samples gave respective GS-peaks at 1.157μm, 1.229μm and 1.330μm with linewidths of 37.2nm, 20.3nm and 53.5nm at 8K. These are red-shifted to 1.258μm, 1.33μm and 1.45μm at 300K. The activation energies of the GS-peaks of these samples are calculated to be approximately 140meV, 122meV and 47meV respectively.The emission wavelength of the MQD sample is considerably red-shifted towards 1.55μm as compared to those of the SQD and BQD samples. We attribute this to the effects of electronic-coupling, and the lattice-unstrain caused by propagation of strain through the thin GaAs spacers in this sample. Strain-coupled QDs typically exhibit a lower linewidth, as in our BQD sample. However, stacking large dots up to 10 layers with very thin barrier nullifies this advantage, as indicated by the large FWHM for MQD. Power-dependent PL spectra of the MQD sample display two independent peaks at all excitation intensities, indicating a bimodal dot-size distribution. TEM images also show two distinct classes of stacks. The particularly low activation energy for the MQD sample is attributed to a higher defect density in the stacks caused by a high amount of strain. Despite such defects, distinct PL peaks are observed at room temperature, indicating the viability of this structure for practical applications.Acknowledgment : DST for financial support, the European Commission for providing partial funding under contract SES6-CT-2003-502620 (FULLSPECTRUM)
9:00 PM - H5.10
Hot Electron Transfer from Semiconductor Nanocrystals.
William Tisdale 1 , Kenrick Williams 2 3 , Brooke Timp 2 , David Norris 1 , Eray Aydil 1 , Xiaoyang Zhu 2 3
1 Chemical Engineering & Materials Science, University of Minnesota, Minneapolis, Minnesota, United States, 2 Chemistry, University of Minnesota, Minneapolis, Minnesota, United States, 3 Chemistry & Biochemistry, University of Texas at Austin, Austin, Texas, United States
Show AbstractIn conventional semiconductor solar cells, absorption of photons with energies greater than the semiconductor band gap generate “hot” charge carriers that quickly “cool” before all of their energy can be captured – a process that limits device efficiency. Semiconductor nanocrystals (or quantum dots) have been touted as promising materials for photovoltaics because discretization of their electronic energy levels can slow down this cooling process, which might enable the extraction of photogenerated charge carriers before their excess energy is converted to heat. In this presentation, I will demonstrate sub-50 fs electron transfer from hot energy levels of PbSe nanocrystals to delocalized conduction band sates of TiO2. In order to make these measurements, we developed the use of optical second harmonic generation for femtosecond time-resolved studies of interfacial charge separation. I will discuss the information we obtain from this technique as well as the effect of temperature, nanocrystal size, and surface chemistry. Additionally, I will show how ultrafast electron transfer excites coherent vibration of the first layer of TiO2 surface atoms, whose collective atomic motions can be followed in real time.
9:00 PM - H5.11
Toward the Realization of an Efficient Si-based Light Source Emitting at 1.55 um.
Joan Ramirez 1 , Olivier Jambois 1 , Yonder Berencen 1 , Daniel Navarro 1 , Sergi Hernandez 1 , Oleksiy Anopchenko 2 , Alessandro Marconi 2 , Nikola Prtljaga 2 , Nicola Daldosso 2 , Lorenzo Pavesi 2 , Jean-Philipe Colonna 3 , Jean-Marc Fedeli 3 , Blas Garrido 1
1 Electronics, University of Barcelona, Barcelona Spain, 2 Physics, University of Trento, Trento Italy, 3 , CEA LETI, Grenoble France
Show AbstractFor several years, silicon-based photonic devices have been widely considered in order to develop integrated circuits allowing to overcome the microelectronic bottlenecks. The challenge for silicon photonics is to manufacture low-cost information processing components by using standard and mature CMOS technology. Numerous photonic devices have already been developed in the last years for light propagation, modulation or detection on silicon substrates. The ultimate goal for the photonic and electronic convergence would be to monolithically integrate powerful Si-based light sources into the CMOS photonic integrated circuits. Some encouraging works have reported the possibility to obtain efficient Si-based light emitting diodes [1,2]. Even stimulated emission by optical pumping of an Er doped Si-based material has been reported [3], but some efforts have still to be pursued in order to increase the emitted power, and achieve the realization of an injection Si-based laser.In this work, we report a systematic study of the electroluminescence (EL) properties and mechanisms of Si-rich silicon oxides doped with Er ions, by correlating the study of transport mechanisms and electroluminescence spectra. Three different techniques have been used to fabricate the active layer: LPCVD, PECVD, and ion implantation. Different Si excess were introduced, and various annealing treatment were performed in order to form Si nanoclusters (Si-nc) from the SRSO layers that are known to have an enhanced radiative emission in the visible when they are crystalline (figure 1a), or act as efficient sensitizers for the luminescence of Er ions at 1.5 um when they are amorphous (figure 1b). A total of about 20 different wafers have been processed.The devices were excited in DC excitation. We find that the three techniques of deposition give different power efficiency that we attribute to the different matrix defect concentration induced by each technique. Two kinds of transport mechanisms could be observed in function of the annealing treatment. In one case, the transport is made by hopping from Si-nc to Si-nc and low EL power is obtained. In the other case, the injection of hot electrons is observed (see Fowler-Nordheim plot in inset of figure 2), leading to a higher EL power of some 0.1-1 uW at 1.5 um, that we attribute to direct impact excitation of the Er ions by the injected electrons. In order to achieve optical gain for the realization of a Si-based laser, the number of inverted Er ions has been estimated. In the best case, it has been possible to invert about 10% of the Er population (figure 2). Final results shows the electroluminescence can be improved just optimizing the way our nanocrystals are excited.
9:00 PM - H5.12
Effects of MBE Growth on the Optical Properties of AlGaN Quantum Wells.
Wen Feng 1 , Gautam Rajanna 1 , Boris Borisov 1 , Ayrton Bernussi 1 , Sergey Nikishin 1 , Mark Holtz 1
1 Nano Tech Center, Texas Tech University, Lubbock, Texas, United States
Show AbstractWe have carried out temperature dependent photoluminescence (PL) and optical transmission measurements on AlGaN MQWs grown by gas source molecular beam epitaxy (GSMBE) with ammonia on (0001) sapphire. The growth began with nitridation of the (0001) sapphire substrate for 30 min at 900 0C. Next 150 nm thick AlN buffer and a 70 nm thick Al0.55Ga0.45N buffer layer were grown. The AlN and Al0.55Ga0.45N buffers were grown at 860 0C and 820 0C, respectively. This thickness was sufficient to reach a two dimensional (2D) growth mode with 1x1 surface reconstruction, as confirmed by streaky reflection high energy electron diffraction (RHEED) patterns. The growth continued with a QW structure consisting of five pairs of Al0.45Ga0.55N wells and Al0.55Ga0.45N barriers. QW structures were grown at temperatures between 755oC and 840oC. The well and barrier widths are estimated to be ~2 nm and ~5 nm respectively. The growth times for the wells and barriers were maintained at 20 s and 50 s respectively. All the structures were completed with a 10 nm thick cap layer of AlN. We found that the transition from the 2D to 3D growth mode, induced by temperature of epitaxy, has a dramatic effect on the PL and absorption properties of QWs. Continuous wave PL experiments were performed in the temperature range 10-300 K using a UV laser emitting at 266 nm. We observed that the low temperature PL peak energies ranged from 4.35 eV to 4.48 eV when the growth temperature was varied from 755 0C and 810 0C. The highest PL emission efficiency was determined for the Al0.45Ga0.55N/Al0.55Ga0.45N samples grown at 795 0C. Some of the investigated samples exhibited strong PL peak blue-shift and red-shift as the excitation intensity is increased. The obtained results are attributed to distinct competing recombination mechanisms involving regions of the samples with 2D and/or 3D growth modes, the dimensions and distribution of QD-like structures in the well layer and differences in the thicknesses of the wetting layer. We found that under conditions, for which QDs do not readily form in the well regions, the absorption increases above ~ 4.8 eV at temperature 30 K due to the thick AlGaN buffer layer present in this sample. The absorption edge is consistent with AlN content of ~ 60. Below this band edge we observed a weaker shoulder at ~ 4.5 eV, which was attributed to the absorption by the MQWs. A comparison between absorption and PL emission of the MQWs revealed a Stokes shift as large as 200 meV. This suggests that the origin of the large Stokes shift observed in our MQW samples are related to the formation of QDs. This work was partly supported by the National Science Foundation (ECS-0609416) and U.S. Army CERDEC (W15P7T-07-D-P040) and J. F Maddox Foundation.
9:00 PM - H5.13
Optical Characterization of Trap State and Emission Dynamics in Lead Chalcogenide Quantum Dot Films.
Helen Chappell 1 2 , Jianbo Gao 2 , Joey Luther 2 , Arthur Nozik 2 3 , Justin Johnson 2
1 Department of Physics, University of Colorado, Boulder, Colorado, United States, 2 , NREL, Golden, Colorado, United States, 3 Department of Chemistry, University of Colorado, Boulder, Colorado, United States
Show AbstractAlthough reasonably efficient photonic and photovoltaic quantum dot devices have been demonstrated, further advances will likely require an understanding of the underlying photophysics and charge transport mechanisms. The characterization of below-gap trap states, in particular, is crucial to building this understanding.Towards this end, we conduct a series of temperature-dependent optical studies on PbS, PbSe, and PbTe quantum dots in both close-packed and electrically coupled films, as well as dispersed in solution.Steady-state photoluminescence measurements indicate the presence of multiple sources of radiative decay, including band-to-band recombination and emission involving a trap state within the gap. The energy of this red-shifted emission decreases in larger nanocrystals, but its decrease is roughly a factor of two less than that of the band gap’s decrease. This trend is suggestive of the recombination of a free carrier with a fixed-energy trap; the nearly-equal effective masses of the electron and hole in the PbX system produce such a dependence. In terms of dynamics, the trapping rate is slowed in smaller nanocrystals (those with larger band gap), as predicted by the energy gap law. Energy transfer between nanocrystals also plays a role, especially in films of close-packed nanocrystals, and is included in a comprehensive model of emission dynamics.Treatment of the films with ethanedithiol and similar chemical agents produces higher mobilities and altered optical properties; in these treated films, competition between charge transfer between nanocrystals and trapping at surfaces generally leads to weak emission that is strongly temperature-dependent. We further characterize the energy and density of trap states through steady-state photomodulation studies, which directly probe long-lived carriers with photoinduced absorption. We compare this optical data with mobility studies on films that mimic those used in working devices. A broad picture of carrier dynamics in quantum dot films from photoabsorption through the production of photocurrent is expected to result.
9:00 PM - H5.14
Advanced Structure Control of Interconnected NC Assemblies through Pulsed Laser Annealing.
William Baumgardner 1 2 , Joshua Choi 3 2 , Kaifu Bian 2 , Lena Kourkoutis 3 , Detlef Smilgies 5 , Michael Thompson 4 , Tobias Hanrath 2
1 Department of Chemical and Chemical Biology, Cornell University, Ithaca, New York, United States, 2 School of Chemical and Biomolecular Engineering, Cornell University, Ithaca, New York, United States, 3 School of Applied and Engineering Physics, Cornell University, Ithaca, New York, United States, 5 Cornell High Energy Synchrotron Source, Cornell University, Ithaca, New York, United States, 4 Department of Materials Science and Engineering, Cornell University, Ithaca, New York, United States
Show Abstract There is growing recognition in the nanocrystal (NC) research community that controlling the interconnection between NCs is just as important as control over individual NC properties themselves. A major challenge is therefore to balance the seemingly contradictory requirements of coupling nanostructures while preserving the effects of quantum confinement. NC superlattices formed by self-assembly from colloidal NC suspensions generally lack efficient inter-NC coupling due to the presence of insulating ligands bound to the NC surface. Chemical ligand exchange, plasma treatment, and thermal centering had previously been investigated to improve NC coupling. Laser annealing presents a promising, yet under explored, technique to process NC assemblies with advanced control over film morphology, NC coupling, and film crystallinity. NC superlattices with controlled translational and orientation order were prepared by drop casting PbS and PbSe NC suspensions under controlled solvent vapor conditions. We explored various approaches to tailor the composition of the interstitial volume of the NC assembly. Amorphous silicon films were sputter deposited on select NC assemblies. NC assemblies were processed with sub-microsecond pulsed laser annealing. We systematically explored laser pulse duration and intensity to tune kinetic aspects of matrix and/or NC phase melting and recrystallization. Processed films were characterized by scanning and transmission electron microscopy, tomography, wide- and small-angle X-ray scattering. We applied infrared spectroscopy to monitor the NC surface chemistry throughout various processing steps. Charge transport characteristics (conductivity and capacitance) of processed assemblies were investigated to establish structure-property relationships. Absorbance and photoluminescence spectroscopy were used to probe optical properties.
9:00 PM - H5.2
Hairloss in Nanocrystals: Controlling Nanocrystal Superlattice Symmetry and Shape-anisotropic Interactions through Variable Ligand Surface Coverage.
Joshua Choi 1 2 , Clive Bealing 3 , Kaifu Bian 2 , Detlef Smilgies 4 , Richard Hennig 3 , Tobias Hanrath 2
1 Applied and Engineering Physics, Cornell Universiy, Ithaca, New York, United States, 2 Chemical and Biomolecular Engineering, Cornell University, Ithaca, New York, United States, 3 Materials Science and Engineering, Cornell Universiy, Ithaca, New York, United States, 4 , Cornell High Energy Synchrotron Source, Ithaca, New York, United States
Show AbstractThe prospect of combined control over individual nanocrystal (NC) and ensemble (i.e., NC superlattice) properties presents a fertile area of fundamental research with many exciting opportunities to create novel metamaterials with coherent electrical and optical properties relevant to a number of nanotechnology applications. As in the analogous atomic crystals, collective interactions among NCs in these artificial solids are governed by the NC energy levels, coupling between adjacent sites, and the symmetry and spacing of the superlattice. Compared to the immense progress made in synthetic control of size, shape and composition of individual NCs, the structural control over their ordered assemblies is less well developed. Better understanding, and ultimately control, over the molecular interactions governing the self-assembly processes is required to bridge the current gap in superlattice structure-property relationships. To a first approximation, interactions between colloidal NCs can be modeled as a soft sphere interaction potential which predicts the formation of superstructures with face centered cubic (fcc) symmetry. However, this basic model is often inadequate. Electrostatic interactions and NC shape effects present two prominent perturbations to the spherical interaction potential which may direct the formation of other superlattice symmetries. We show that differences in the coverage of ligands bound to specific NC facets can be exploited to tune the interaction potential towards the self-assembly of superlattices with either fcc or non-close packed, body centered cubic (bcc) symmetries. We analyzed the structure of PbS NC superlattice with synchrotron based grazing incident small-angle X-ray scattering and transmission electron microscopy. NCs with dense ligand coverage were found to form fcc assemblies, whereas the same NCs with reduced ligand coverage formed bcc assemblies with high degree of preferential orientation of individual NCs. The asymmetric ligand-ligand interactions were enhanced by tuning the ligand density at different nanocrystal facets via several different methods such as oxidation, filtering and anti-solvent washing - FTIR characterization shows that up to 60% surface ligands can be lost. Using computational methods, we show that different degree of ligand loss between {111}NC and {100}NC facets occur due to varying binding energies of surface lead oleate. Our results highlight ligand-ligand interaction as one of the crucial factors to consider in designing NC superlattices with desired symmetry and orientational order.
9:00 PM - H5.3
A Cadmium Free Quantum Dot for Visble Luminescence: New Reverse type –I ZnSe core / Shell / Shell Structure.
SungWoo Kim 1 , Sang-Wook Kim 1
1 Molecular Science and Technology, Ajou University, Suwon Korea (the Republic of)
Show AbstractQuanntum Dots (QDs) have been attracted much attention due to their unique optical properties originating from the quantum confinement effect and also studied extensively over the past two decades for their potential application such as light-emitting diodes lasers, and biological labels. Their opto-electronic properties can be engineered by controlled size, composition and band gap offsets of the core/shell QDs. Core-shell QDs have been classified into three categories, type-I, type-II and reverse type-I according to their band alignments. First, Type-I core/shells, such as CdSe/CdS and InP/ZnS, contain shell materials with a wider bandgap than the core, which can improve the quantum yield. Second, Type-II core/shells, such as ZnTe/CdSe, ZnSe/CdS, have staggered band alignment between the core and shell and show a wide tunable emission wavelength. Finally, reverse type-I structure, such as CdS/HgS, CdTe/InP and ZnSe/CdSe, has a wider band gap than that of shell , which suggests the possibility of an outward-shift of electrons and holes. This results in increased delocalization of the electrons and holes, leading to a red-shift in the absorption and emission.We report development of cadmium free reverse type-I QDs of ZnSe core/shell/shell.ZnSe core/shell/shell nanocrystals also showed improved optical properties, with photoluminescence quantum yield up to about 58% and an emission peak position tunable from 430 to 620 nm.
9:00 PM - H5.4
Large-scale Production of InP Quantum Dots.
Tae Hoon Kim 1 , Sang-Wook Kim 1
1 , Ajou University, Suwon Korea (the Republic of)
Show AbstractQuantum Dots (QDs) have optical and electronic properties from quantum confinement effect. As a result, they could be used in various applications such as transistors, solar cells, LEDs, and medical imaging. Although II-VI QDs have been studied from the beginning, toxicity problems from Cd ion could not have been solved so far. Therefore, cadmium-free QDs, such as InP and CuInS2, have been researched recently. However, the process of synthesizing III-V QDs is much more complicated than II-VI groups quantum dot, and also it is hard to achieve large scale synthesis. For the reason, we researched on the process to develop large scale synthesis of InP/ZnS, which is representative of nontoxic quantum dot, and we could succeed it with dropwise method. QD’s quality by the method is satisfied as comparing to that by the hot-injection method. The absorption and emission of InP/ZnS QDs by the large scale synthesis are measured. Emission spectrum range is shown in 490 nm through over 600nm. Until now, we could obtain 2 g of solid QDs in a batch.
9:00 PM - H5.6
Energy Level Shifts of Charged Excitons in Colloidal CdSe Quantum Dots Films.
Heng Liu 1 , Philippe Guyot-Sionnest 1
1 , James Franck Institute, Chicago, Illinois, United States
Show AbstractWe report a systematic study about the effect of extra charges on the 1Se-1Sh and 1Pe-1Ph transitions. Spectator charges are injected into monodisperse CdSe quantum dots by electrochemistry. The redshift of the 1Se-1Sh transition is ~10meV/electron, and redshift of the 1Pe-1Ph transition is ~30meV/electron, which are in a fairly good agreement of the theoretical calculation. The energy shifts do not have strong dependence on the size of quantum dots ensembles.
9:00 PM - H5.7
Coadsorption (CdSe)ZnS Quantum Dot and Au Nanoparticles on TiO2 Films with Enhanced Photoconversion Efficiency
Mi-Hee Jung 1 , Man Gu Kang* 1
1 Thin Film Solar Cell Technology Research Team, Advanced Solar Technology Research Department, Convergence Components & Materials Research Laboratory , Electronics and Telecommunications Research Institute (ETRI), Daejeon Korea (the Republic of)
Show AbstractSemiconductor quantum dots (QDs) sensitized solar cell offer significant advantages over dyes sensitized solar cells. QDs provided ability to match the solar spectrum better because their absorption spectrum can be tuned with particle size. In addition, it has been shown recently that the QDs can generate multiple exciton which could improve the efficiency of the device. However, despite the advantages of using QDs as a sensitizer, QD cells are less efficient than dye cells because of their insufficient long-term stability due to the surface damage/oxidation under illumination and the charge recombination between electrons injected in the conduction band of the QDs and the electrolyte.Herein, we report the coadsorption of core-shell(CdSe)ZnS dots and gold nanoparticles on TiO2 surface for quantum dot sensitized solar cells. Au nanoparticles bound to TiO2 via mercaptopropionic acid and then the core-shell(CdSe)ZnS dots are linked to the Au nanoparticle assembly. The resulting photocurrent is 5-fold higher than the photocurrent originating form a (CdSe)ZnS layer that lacks the Au nanopartilces. The enhanced photocurrent in the Au/(CdSe)ZnS nanoparticles array is attributed to effective charge separation of the electron-hole pair by the injection of conduction-band electrons from the CdSe to the Au nanopartilces. As a result of the improved performance, the overall energy conversion efficiency was increased by 60%, as compared to that for a reference cell without the Au nanoparticles at 100mW/cm2
9:00 PM - H5.9
Computational Insight for ``Capped" Nanocrystals Self-assembly.
Ananth Prakash 1 , Paulette Clancy 1
1 Chemical and Biomolecular Engineering, Cornell University, Ithaca, New York, United States
Show AbstractNanocrystalline solids have become the subject of intense study due to their unique optical properties and their capacity to form self-assembled superlattices. These properties make them suitable for use in a variety of applications such as solar cells, light emitting devices, transistors, etc. The self-assembly of these nanoparticles is governed by interactions at the molecular level, and hence, understanding the nature of these interactions could be instrumental in achieving precise control over the self-assembly process. The nanocrystals are "capped" by organic molecules which are believed to drive the self-assembly and stabilize the final superlattice, and which are responsible for nearly 99% of the interparticle forces. Despite the importance of the ligand-ligand interactions, there is very little fundamental understanding of these hybrid systems. In our work, we use atomically and molecularly explicit Molecular Dynamics simulations to understand the interactions between the ligands on the surface of the nanocrystals. Specifically, we observe the energetic interactions of experimentally-sized nanocrystals capped with ligands in various lattices including face-centered cubic (fcc), body-centered cubic (bcc), body-centered tetragonal (bct) and hexagonal close packed (hcp) structures. We also undertake the study of key controlling parameters of self-assembly including nanocrystal diameter, ligand grafting density, ligand length, nanocrystal orientation and size of nanocrystal facets etc. This study provides key insights into molecular scale information about ligand interactions which can be leveraged into more coarse-grained mesoscale simulations of nucleation and growth phenomena.
Symposium Organizers
Matthew F. Doty University of Delaware
Sayantani Ghosh University of California, Merced
Armando Rastelli IFW Dresden e.V.
Marija Drndic University of Pennsylvania
H6: Towards Bulk Quantum Dot Materials with Tuned Properties
Session Chairs
Wednesday AM, December 01, 2010
Room 300 (Hynes)
9:30 AM - **H6.1
Multiscale Modeling of Functional Materials with Self-assembled Nanoscale Components.
Petr Kral 1
1 Department of Chemistry M/C 111, University of Illinois at Chicago, Chicago, Illinois, United States
Show AbstractWe use multiscale modeling to describe stabilization and material parameters of superlattices and membranes with nanoscale components. First, we investigate multilayer superlattices formed of monodisperse semiconducting nanoparticles and nanorods with attached organic ligands and solvated in organic solvents. We describe stabilization of superlattices with different observed lattice types by modeling the averaged van der Waals and Coulombic coupling (nonlocal dipoles) of their components. We also describe stabilization of different observed types of superlattices formed of ligated metal nanocubes.Next, we use coarse-grained molecular dynamics simulations to describe clusters of nanoparticles, nanorods, nanodiscs, and graphene nanostructures self-assembled in the interior of the hydrated phospholipid membranes. These model hybrid materials might have novel bioelectronic applications.Finally, we apply coarse-grained and atomistic molecular dynamics simulations to describe freestanding membranes of ligated nanoparticles that were recently prepared. We show how their parameters depend on the particle size, composition, ligand type, and number of particle monolayers.
10:00 AM - H6.2
Mott and Efros-Shklovskii Variable Range Hopping in CdSe Quantum Dots Films.
Heng Liu 1 , Alexandre Pourret 1 , Philippe Guyot-Sionnest 1
1 Department of Physics, James Franck Institute, Chicago, Illinois, United States
Show AbstractThe model of variable range hopping conductivity predicts a crossover between Mott and Efros-Shklovskii as a function of temperature and density of states. This is observed using monodispersed CdSe colloidal quantum dot 3D solids where the density of states at the Fermi level is varied by electrochemistry. At low density of states, both below the lowest state (<0.4 e−/dot) and in the conductivity gap between the first and second state (2e−/dot), the temperature dependence of the conductivity shows the 1/4 exponent of Mott hopping. At other fillings up to 6e−/dot, the conductivity shows the 1/2 exponent of Efros-Shklovskii hopping. The non-Ohmic conductivity is also found to be explained quantitatively by the variable range hopping model.
10:15 AM - H6.3
Entropy-driven Formation of Binary Semiconductor-nanocrystal Superlattices.
Wiel Evers 1 , Bart de Nijs 1 , Laura Filion 1 , Sonja Castillo 1 , Marjolein Dijkstra 1 , Daniel Vanmaekelbergh 1
1 Debye Instituut for NanoMaterials Science , Utrecht University, Utrecht Netherlands
Show AbstractFormation of binary superlattices (BNSLs) from colloidal nanocrystals (NCs) are of huge interest due to their great potential as novel materials with unique optoelectronic properties. In such systems, different nanocrystals are in close contact in a well-ordered 3-D geometry. While a host of different crystal structures and material combinations have been reported, the driving forces behind BNSL formation are not fully understood. This information, if available, would provide avenues to novel classes of nanostructured materials.I will present a study of the driving forces for the formation of binary nanocrystal superlattices by comparing the formed structures with full free energy calculations. The nature (metallic or semiconducting) and the size-ratio of the two nanocrystals are varied systematically. We investigated BNSL formation from PbSe/CdSe and PbSe/Au NCs in a size ratio range between 0.47 and 0.86. In addition, crystallization induced by increased particle concentration is compared with that induced by increased attractive interactions. With semiconductor nanocrystals, self-organization at high temperature leads to superlattices (AlB2, NaZn13, MgZn2) in accordance with the phase diagrams for binary hard-sphere mixtures; hence entropy increase is the dominant driving force. On the other hand, a slight change of the conditions in the environment or the replacement of semiconductor by metallic nanocrystals leads to crystals structures that are energetically stabilized. It follows from this study that if nanocrystal crystallization is driven by entropy as is with semiconductor nanocrystals, the resulting structures are determined by the size-ratio of the nanocrystals, enabling the rational design of novel nanostructured materials. The BNSLs formation can even be extended to ternary superlattices composed of PbSe and CdSe NCs.References:1.Overgaag, K., Evers, W., de Nijs, B., Koole, R., Meeldijk, J. & Vanmaekelbergh, D. Binary superlattices of PbSe and CdSe nanocrystals. Journal of the American Chemical Society 130, 7833-7835, (2008).2.Friedrich, H., Gommes, C. J., Overgaag, K., Meeldijk, J. D., Evers, W. H., Nijs, B. d., Boneschanscher, M. P., de Jongh, P. E., Verkleij, A. J., de Jong, K. P., van Blaaderen, A. & Vanmaekelbergh, D. Quantitative structural analysis of binary nanocrystal superlattices by electron tomography. Nano Letters 9, 2719-2724, (2009).3.Evers, Wiel H., Friedrich, H., Filion, L., Dijkstra, M. & Vanmaekelbergh, D. Observation of a ternary nanocrystal superlattice and its structural characterization by electron tomography. Angewandte Chemie International Edition 48, 9655-9657, (2009).4.Evers, W. H., De Nijs, B., Filion, L., Castillo, S. & Vanmaekelbergh, D. Entropy-driven formation of binary semiconductor-nanocrystal superlattices. Nature Nanotech. (Submitted).
10:30 AM - H6.4
CdSe Nanoparticle/Carbon Nanotube Hybrid Materials: Synthesis and Optical Properties.
Austin Akey 1 , Chenguang Lu 1 , Irving Herman 1
1 Applied Physics and Applied Mathematics, Columbia University, New York, New York, United States
Show AbstractCarbon nanotubes present opportunities for constructing of nanomaterials with unique properties, for use in sensors and optoelectronic device applications. We report the synthesis of heterostructures composed of single-walled carbon nanotubes and chemically attached, monodisperse cadmium selenide nanoparticles (3.5 to 6 nm in diameter) that form after treatment with pyridine. The resulting hybrid material is stable and resists aggregation; TEM and SEM characterization shows the nanotubes to be densely covered in nanoparticles. The optical properties of the hybrid material differ significantly from those of the unbound nanoparticles and nanotubes, pointing to the existence of strong electronic/optical interactions between the two. In contrast to unbound nanoparticles, the Stokes shift in the nanoparticle photoluminescence decreases with particle size, both for hybrids formed using mixtures of semiconductor/metallic tubes and those using separated semiconductor tubes, and this will be explained in terms of coupling. This coupling has important implications for the use of quantum dots in energy harvesting devices and sensors. This work is primarily supported by the Nanoscale Science and Engineering Center at Columbia University, which is supported by NSF under Award Number CHE-0641523.
10:45 AM - H6.5
Colloidal Nanocrystals Capped with Metal Chalcogenide Complexes: Preparation, Properties and Self-assembly.
Maksym Kovalenko 1 , Dmitri Talapin 1
1 Department of Chemistry, University of Chicago, Chicago, Illinois, United States
Show AbstractChemically-synthesized nanocrystals are considered as potentially useful building blocks for the broad spectrum of applications in electronic and optoelectronic devices. Despite an impressive progress in the synthesis of colloidal nanocrystals, their surface chemistry, the interface structure and the interparticle connectivity need to be properly designed to achieve high degree of electronic coupling. To address these issues, we have recently developed a general approach for removing insulating organic capping from the nanocrystal surface by replacing it with electronically conductive inorganic ligands [1]. The combinations of common metals and chalcogens such as SnS44-, SnTe44-, In2Se42-, Sn2S64-, SbSe43- etc. were employed as surface capping ligands. We will discuss various aspects of the synthesis, assembly and device integration of these novel nanomaterials, covering (i) preparation of stable colloidal dispersions of inorganically-capped nanocrystals [2]; (ii) Solution-based processing and self-assembly of this new class of colloids into electronically coupled nanocrystal solids such as arrays of metallic and semiconductor nanocrystals; (iii) the role of metal chalcogenide capping ligands on the luminescent properties of semiconductor nanocrystals, and (iv) the rational choice of nanocrystal and their inorganic capping for solution-processed thermoelectric materials. The solid-state chemical reactions between nanocrystal cores and surrounding metal chalcogenide complexes were used to prepare highly-conductive biphase PbTe-Sb2Te3 nanocomposites and nanostructured ternary Bi2-xSbxTe3 [3].[1]M. V. Kovalenko, M. Scheele, D. V. Talapin. Science 2009, 324, 1417-1420[2]M. V. Kovalenko, M. I. Bodnarchuk, J. Zaumseil, J.-S. Lee, D. V. Talapin. J. Am. Chem. Soc. 2010, in print[3]M. V. Kovalenko, B. Spokoyny, J.-S. Lee, M. Scheele, A. Weber, S. Perera, D. Landry, D. V. Talapin. J. Am. Chem. Soc. 2010, 132, 6686-6695
11:30 AM - **H6.6
Hierarchical Self-assembly of Nanoparticles in Amphiphilic Block-copolymer Assemblies.
So-Jung Park 1 , Brenda Sanchez-Gaytan 1 , Robert Hickey 1 , Amanda Kamps 1
1 Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania, United States
Show AbstractFor the past couple of decades, there has been a considerable effort in combining nanoparticles and polymers in materials synthesis and device fabrication in order to take advantage of the unique physical properties of nanoparticles and the excellent processibility of polymers. An important issue in this effort is to control the arrangement of nanoparticles in polymer matrix because the dispersion of nanoparticles significantly impacts the electronic, transport, and mechanical properties of the composite materials. Here, we present that the cooperative self-assembly of nanoparticles and amphiphilic block-copolymers offers an effective way to control the organization of nanoparticles. We have demonstrated that unique hierarchical self-assembly structure such as radial assemblies of quantum dots and magnetic nanoparticles can be spontaneously formed by manipulating the interactions between nanoparticles and polymers. This talk will discuss what affects the co-assembly formation and how to use that knowledge to control the organization of nanoparticles.
12:00 PM - H6.7
Control over Nanocrystal Superlattice Symmetry via Shape-anisotropy Driven Ligand-solvent Interactions.
Kaifu Bian 1 , Joshua Choi 2 , Ananth Prakash 1 , Paulette Clancy 1 , Detlef Smilgies 3 , Tobias Hanrath 1
1 School of Chemical and Biomolecular Engineering, Cornell University, Ithaca, New York, United States, 2 School of Applied and Engineering Physics, Cornell University, Ithaca, New York, United States, 3 CHESS, Cornell University, Ithaca, New York, United States
Show AbstractDespite intense research efforts by research groups world-wide, the potential of self-assembled nanocrystal superlattices (NCSL) has not been realized due to an incomplete understanding of the fundamental molecular interactions governing the self-assembly process. Because NCSLs reside naturally at length-scales between atomic crystals and colloidal assemblies, synthetic control over the properties of constituent nanocrystal (NC) building blocks and their coupling in ordered assemblies is expected to yield a new class of materials with remarkable optical, electronic and vibrational characteristics. Progress towards the formation of suitable test structures and subsequent development of NCSL-based technologies has been held back by the limited control over superlattice spacing and symmetry. Here we show that NCSL symmetry can be controlled by manipulating molecular interactions between ligands bound to the NC surface and the surrounding solvent. Specifically, we demonstrate solvent vapor-mediated assembly of NCSL with unprecedented control of lattice symmetry and orientational coherence of NCs within the lattice. The assembly of various superlattice allotropes, including face-centered cubic (fcc), body-centered cubic (bcc), and body-centered tetragonal (bct) structures, is studied in real time using in-situ grazing incidence small-angle X-ray scattering (GISAXS) under controlled solvent vapor exposure. This approach provides quantitative insights into the molecular level physics that controls solvent-ligand interactions and assembly of NCSL. Computer simulations based on all-atom molecular dynamics techniques confirm several key insights gained from experiment. Initial structure-property relationships are presented in the comparison of optical spectra and ellipsometry of superlattices with controlled fcc and bcc symmetry
12:15 PM - H6.8
Geometric Packing, Electronic Coupling, and the Origins of Optical Redshifts in PbSe Quantum Dot Solids.
Abraham Wolcott 1 , Vincent Doyeux 2 , Cory Nelson 1 , Kin Lei 1 , Kenrick Williams 1 , Xiao Zhu 1
1 Chemistry and Biochemistry, UT Austin, Austin, Texas, United States, 2 UFR de Physique, Universite Joseph Fourier, Grenoble, Dauphiné, France
Show AbstractUnderstanding how semiconductor quantum dots (QD) assemble, couple and behave electronically in a QD solid is an open challenge with implications for both electronic and optoelectronic applications. We expand on this premise by manipulating the inter-QD distance in two-dimensional (2D) and three-dimensional (3D) arrays through the use of alkanedithiol linking molecules with various lengths. With decreasing length of linking molecules, optical absorption spectroscopy of the QD assembly shows a tunable redshift in the 1st and 2nd exciton peak positions, but with minimal broadening of the peak width (≤5%). High resolution scanning electron microscopy (HRSEM) reveals a transition in packing configuration from hexagonally close-packing with C18 (oleic acid), C8 and C6 alkanedithiols to a mixed hexagonal/cubic/disordered packing motif when C5→C2 or no linking molecules are used. A lack of broadening in the optical transitions is attributed to the dominance of dipole-dipole interactions, and is also responsible for the red-shift. This conclusion is confirmed in experiments with QD solids composed of mixed sizes. Optical broadening is only observed with an extended hydrazine treatment wherein the complete removal of capping molecules leads to the agglomeration and sintering of QDs. Our work demonstrates two significant regime changes in QD-QD interaction: (1) a transition from hexagonal packing with longer chain alkanedithiols to mixed hexagonal/cubic/disordered geometries, and (2) a transition in inter-QD coupling from a dipole-dipole dominated regime at inter-QD distances of 0.7-3.5 nm to electronic coupling at distances less than 0.5 nm.
12:30 PM - H6.9
Low Temperature Annealing of Lead Chalcogenide Quantum Dot Thin Films and Its Controllability of Inter-dot Coupling.
Seung Jae Baik 1 , Koeng Su Lim 1 , Su Yeon Lim 1 , Dong Geon Han 1 , Seung Hyup Yoo 1 , Sohee Jeong 2 , Kyungnam Kim 2 , Chang-Soo Han 2
1 Electrical Engineering, KAIST, Daejeon Korea (the Republic of), 2 , KIMM, Daejeon Korea (the Republic of)
Show AbstractLead chalcogenide quantum dot (QD) thin films have potential application in large area electronic devices employing solution processing. Transport property of QD thin film is mainly governed by inter-dot electronic coupling, and it is known that surface ligand exchange is a viable method to control inter-dot coupling. In this work, we provide further controllability of inter-dot electronic coupling through low temperature thermal annealing, which results in bandgap narrowing and conductivity enhancement of lead chalcogenide QD thin films. Electronic coupling energy increases about 7~8 meV, and conductivity increases by 10^6 after annealing at 170C. Electronic coupling energy and conductivity increases according to ’logistic’ relation with the inverse of annealing temperature. Physical modeling on the annealing effect of optical and electrical properties of quantum dot films will be presented with a reasoning based on inter-dot electronic coupling. Moreover, as a device application, Schottky solar cell will be presented to show that a low temperature anneal could enhance the power conversion efficiency of solar cells based on quantum dot solids.
12:45 PM - H6.10
Surface Passivation of Silicon Nanowires.
Yaping Dan 1 , Kwanyong Seo 1 , Kuniharu Takei 2 , Ali Javey 2 , Kenneth Crozier 1
1 School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, United States, 2 Department of Electrical Engineering and Computer Sciences, University of California, Berkeley, California, United States
Show AbstractDue to their high volume-to-surface ratio, recombination through surface states largely dominates the lifetime of charge carriers in nanowires. This limits the diffusion length to be approximately equal to the wire diameter.[1] When nanowires are used as photodiodes, therefore, only the electron-hole pairs excited near the electrodes are collected. Those far from the electrodes recombine before being collected. Nanowire photodetectors thus have much lower quantum efficiency than their traditional planar counterparts. It is known that the density of interface states at the Si-SiO2 interface for thermally-grown SiO2 is significantly reduced, compared to the case of an unmodified Si surface. This alleviates the problem of recombination through interface states. Here we investigate the hypothesis that silicon nanowires with a surface passivation layer should have longer carrier lifetimes (and diffusion lengths) and higher quantum efficiency. We synthesize p-type Si nanowires using gold catalysts in a vapor-liquid-solid process at 460 °C. The nanowires exhibit a surface layer that is approximately 10nm thick. The surface layer is believed to be SiO2, although this, and the mechanism by which it is formed, requires further investigation. Through electrical measurements, the acceptor concentration is estimated to be ~10^17 cm^-3. To make contacts to the wire, a pair of Pd/Ti electrodes is deposited after the surface layer in the contact region has been removed with buffered HF. The fabricated devices have Schottky barrier contacts. To determine the diffusion length of charge carriers, scanning photocurrent microscopy (SPCM) is performed using a near-field scanning optical microscope (NSOM). A laser beam (532nm) chopped at 1.8 kHz is sent through the tip of the contact-mode NSOM probe (aperture diameter ~ 90nm). The photocurrent is recorded as a function of the tip position using a lock-in amplifier for different bias conditions. The diffusion lengths, hence lifetimes, of minority charge carriers for wires with diameters ranging from 30 to 170nm are determined from the photocurrent maps.Experimental results indicate that the diffusion lengths, and hence minority carrier lifetimes, are significantly larger for the Si nanowires with the surface layer than bare Si nanowires. From experimental data, we estimate that the density of states for the Si nanowires with the surface layer is on the order of ~10^10 cm^-2. Our results suggest that with the surface passivated, the dominant limitation on the minority carrier lifetime becomes recombination through gold impurities originating from the catalyst employed during VLS growth.Reference[1] J. E. Allen, E. R. Hemesath, D. E. Perea, et al, Nature Nanotechnology 3 (2008) p. 168.