Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session W1: Focus Session: Charge & Energy Transfer IV |
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Sponsoring Units: DCP Chair: Carlos Silva, University of Montreal Room: 103/105 |
Thursday, March 6, 2014 2:30PM - 2:42PM |
W1.00001: Carrier dynamics in colloidal indium arsenide quantum dots in the weak excitation limit Austin Spencer, William Peters, Vivek Tiwari, Byungmoon Cho, Nathan Neale, David Jonas The dynamics of photo-excited carriers in colloidal indium arsenide (InAs) quantum dots are characterized by degenerate pump--probe spectroscopy at 1.5 times the band gap. This material is of particular interest due to reports of efficient multiple exciton generation and its potential application in third-generation photovoltaic devices. Use of a sample renewal technique based on laser beam scanning enables long resampling times ($>$0.5 s) with minimal spatial overlap between successive laser shots thereby minimizing repetitive excitation. Pump--probe transients at a range of excitation probabilities are reported, from 2.6\%, where signal from biexcitons is small (1.9\%), to 36\%, where the biexcitons contribute 45\% of the signal. These transients are well described by a tri-exponential fit which includes time constants of approximately 1 ps, 16 ps, and 750 ps tentatively attributed to carrier cooling, multi-exciton recombination, and single exciton recombination respectively. By an excitation probability of 10\%, biexciton dynamics are detectable and continue to grow in magnitude as the excitation probability increases. The pump power dependence of the signal at 20 ps, which deviates from linearity at an excitation probability of 10\%, reflects biexciton recombination. [Preview Abstract] |
Thursday, March 6, 2014 2:42PM - 2:54PM |
W1.00002: Photoinduced Electron Transfer to Engineered Surface Traps in CdSe Nanocrystals Marco Califano, Haiming Zhu, Ye Yang, Kim Hyeon-Deuk, Nianhui Song, Youwei Wang, Wenqing Zhang, Oleg Prezhdo, Tianquan Lian Quantum confined nanomaterials, such as semiconductor nanocrystals (NCs), have emerged in the past decade as a new class of materials for solar energy conversion. An appropriate model for describing photoinduced charge transfer in these systems is, however, still lacking. Recently we observed that the rate of photoinduced electron transfer from CdSe NCs to molecular acceptors increased with decreasing NC size (and increasing driving force) exhibiting a lack of Marcus inverted regime behaviour over an apparent driving force range of ~ 0-1.3 V. Our atomistic semiempirical pseudopotential calculations show that an Auger assisted ET mechanism, in which the transfer of the electron is coupled to the excitation of the hole, can circumvent the unfavourable Frank-Condon overlap (that is a signature of inter- or intra- molecular electron transfer) in the Marcus inverted regime, reproducing our observed ET rates with remarkable accuracy. We conclude that electron transfer from quantum dots differs from electron transfer originating from both molecules and bulk semiconductors. It proceeds via a novel Auger-assisted pathway which we believe is available to most excitonic nanomaterials. This new finding will have a major impact on the design of next generation solar energy harvesting devices. [Preview Abstract] |
Thursday, March 6, 2014 2:54PM - 3:06PM |
W1.00003: Electron - acoustic phonon coupling in colloidal lead sulfide quantum dots Byungmoon Cho, Vivek Tiwari, Austin Spencer, Dmitry Baranov, Samuel Park, David Jonas Lead chalcogenide quantum dots (QDs) with bandgaps in the shortwave infrared are candidate materials for next generation photovoltaics exceeding the Shockley-Queisser limit. Despite ongoing controversy, multiple exciton generation (MEG) in QDs offers potential for improved photovoltaic efficiency. Hot carriers from high energy photoexcitation dissipate excess energy via coupled phonons; this is detrimental to MEG. The electron-phonon coupling (EPC) magnitude, partitioning among modes and dependence on the size/shape are poorly understood. We performed degenerate femtosecond pump-probe spectroscopy to investigate Auger recombination dynamics, a reverse process of MEG. We observe a quantum beat due to coherent acoustic phonons in femtosecond pump-probe signals from oleate capped colloidal lead sulfide QDs in toluene. A 3.4 ps period oscillation decays with 4.6 ps damping constant in 8 nm diameter dots; the amplitude increases linearly with pump energy and modulation is weaker than reported in smaller dots. An elastic continuum model for acoustic phonon frequency vs. dot diameter suggests a not yet understood quantitative discrepancy with prior work. These relaxation processes have important implications for QD photovoltaics. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:42PM |
W1.00004: Engineered semiconductor nanocrystals with enhanced carrier multiplication yields Invited Speaker: Victor Klimov Carrier multiplication (CM) is a process whereby absorption of a single photon results in multiple electron-hole pairs (excitons). This process could benefit a number of solar-energy conversion technologies, most notably photocatalysis and photovoltaics. This presentation overviews recent progress in understanding the CM process in semiconductor nanocrystals, motivated by an outstanding challenge in this field - the lack of capability to predict the CM performance of nanocrystals based on their known photophysical properties or documented parameters of parental bulk solids. Here, we present a possible solution to this problem by showing that, using biexciton Auger lifetimes and intraband relaxation rates inferred from ultrafast spectroscopic studies, we can rationalize relative changes in CM yields as a function of nanocrystal composition, size and shape. Further, guided by this model, we demonstrate a two-fold enhancement in multiexciton yields in PbSe nanorods vs. quantum dots attributed to enhanced Coulomb interactions. We also explore the control of competing intra-band cooling for increasing multiexciton production. Specifically, we design a new type of hetero-structured PbSe/CdSe quantum dots with reduced rates of intra-band relaxation and demonstrate a four-fold boost in the multiexciton yield. These studies provide useful guidelines for future efforts to achieve the ultimate, energy-conservation-defined CM efficiencies. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 3:54PM |
W1.00005: Characterization and 2D FT electronic spectra of colloidal PbSe nanocrystals Dmitry Baranov, Samuel Park, Byungmoon Cho, David Jonas In the last decade, colloidal lead selenide nanocrystals (PbSe NCs) have been actively studied by ultrafast laser spectroscopy techniques due to interesting electronic structure and rich carrier dynamics. Theoretical studies of small PbSe NCs by Zunger group predicted intervalley splitting of the electron and hole ground states which are degenerate in the bulk. Recent theoretical work by Goupalov group predicted that magnitude of the splitting depends on the structure of NC's core and presence of surface shell. These predictions and abundant reports of sample to sample variation in observed photophysics of PbSe NCs point towards the need for a better relation of sample's structure and composition to results of spectroscopic measurements. In this work, we investigate well-defined colloidal PbSe NCs by 2D FT electronic spectroscopy in the short-wave infrared region. Samples of oleate-capped PbSe colloidal NCs (3.3 nm average diameter, 1.09 eV band gap) have uniform shapes with a size distribution varying by less than half of bulk PbSe lattice constant. Studied samples of PbSe NCs were prepared following a protocol which has been reported to produce Pb-rich NCs, suitable for testing predictions about the intervalley splitting. [Preview Abstract] |
Thursday, March 6, 2014 3:54PM - 4:06PM |
W1.00006: Single Molecule Spectroscopy and Scanning Probe Microscopy to Investigate Excited State Energy Transport in Quantum Dot Higher Order Structures Alan Van Orden, Martin Gelfand, Duncan Ryan, Kevin Whitcomb Single molecule fluorescence spectroscopy and scanning probe microscopy have been used to investigate small isolated clusters of CdSe/ZnS nanocrystalline quantum dots dispersed on insulating, conducting, and semiconducting surfaces. The aggregated quantum dots exhibit excited state energy transfer and charge transport which affects the time dependent autocorrelation of the photoluminescence (PL) emission intensity, photon counting statistics, blinking statistics, and PL lifetime, as observed by single molecule fluorescence spectroscopy. The structural arrangement of the nanocrystals and the electron transfer between the quantum dots and substrate can be investigated using atomic force microscopy, transmission electron microscopy, and scanning tunneling microscopy. These combined experiments provide novel perspectives on energy and electron transport in quantum dot higher order structures and the effects of structural arrangements, substrates, and attached ligands. These insights will enhance the development of technological applications of quantum dots, including bioimaging, display technology, and alternative energy technology. [Preview Abstract] |
Thursday, March 6, 2014 4:06PM - 4:18PM |
W1.00007: Breaking the symmetry of a PbSe quantum dot by a hot electron-hole pair Minh-Tuan Trinh, Xiaoyang Zhu Optical excitation of semiconductor nanocrystals or quantum dots (QDs) near the bandgap is now well-understood, but the same cannot be said about excitation significantly above the bandgap. Here, we report an observation of symmetry breaking imposed by a hot electron-hole pair from above-gap excitation in PbSe QDs using ultrafast pump-probe spectroscopy. The breaking of symmetry results in a modification of optical dipole selection rules, as well as the broadening and redshift of dipole-allowed transitions, during the picosecond lifetime of the hot carriers. The observations can be interpreted as a transient Stark effect resulting from the bulk-like behavior of the hot electron-hole pair. At a short time scale right after excitation, the hot electron and hole can be viewed as independent carriers that generate a net transient internal electric field which breaks the symmetry of the QD. By varying the excitation energy we show that the symmetry breaking effect increases with excitation energy and disappears at the bandgap excitation. Such a breaking of symmetry via transient Stark effect should be of general significance to the understanding of QD photophysics above the bandgap. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:54PM |
W1.00008: Multiexciton Generation in Nanocrystals and Nanorods Invited Speaker: Eran Rabani Multiexciton generation (MEG) is a process where several excitons are generated upon the absorption of a single photon in semiconductors. This process enjoys great technological ramifications for solar cells and other light harvesting technologies. For example, it is expected that the 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. Strict selection rules and competing processes in the bulk allows MEG at energies of five times the band gap. It was suggested that nanocrystals, where quantum confinement effects are important, may exhibit MEG at lower values of (typically 2 to 3 times the band gap). Indeed, MEG in nanocrystals has been reported recently for several systems, showing that the threshold was size and band-gap independent. However, more recent studies have questioned the high efficiency of MEG in nanocrystals. In this talk we will discuss the process of MEG in semiconducting nanocrystals (NCs) and nanorods (NRs). 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 on the MEG efficiencies will be explored using a stochastic approach to calculate MEG with a numerical effort that scales linearly with system size. [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W1.00009: ABSTRACT WITHDRAWN |
Thursday, March 6, 2014 5:06PM - 5:18PM |
W1.00010: Hierarchical Equation of Motion Investigation of Decoherence and Relaxation Dynamics in Nonequilibrium Transport through Interacting Quantum Dots Rainer Hartle, Guy Cohen, David R. Reichman, Andrew J. Millis A recently developed hierarchical quantum master equation approach [1,2] is used to investigate nonequilibrium electron transport through an interacting double quantum dot system in the regime where the inter-dot coupling is weaker than the coupling to the electrodes. The corresponding eigenstates provide tunneling paths that may interfere constructively or destructively, depending on the energy of the tunneling electrons [3]. Electron-electron interactions are shown to quench these interference effects in bias-voltage dependent ways, leading, in particular, to negative differential resistance, population inversion and an enhanced broadening of resonances in the respective transport characteristics [2]. Relaxation times are found to be very long, and to be correlated with very slow dynamics of the inter-dot coherences (off diagonal density matrix elements). The ability of the hierarchical quantum master equation approach to access very long time scales is crucial for the study of this physics. [1] JCP 128, 234703 (2008) [2] arXiv:1309.1170 (2013) [3] PRB 87, 085422 (2013) [Preview Abstract] |
Thursday, March 6, 2014 5:18PM - 5:30PM |
W1.00011: Studying the Photoexcitation Quenching Mechanism in Quantum Dot-Nitroxide Radical Hybrid Sytems Poulami Dutta, Jeffrey Sayen, Remi Beaulac The study of energy/electron transfer processes in inorganic/organic complexes is an active area of research, with applications in fields ranging from energy conversion to chemical catalysis. A really interesting variation of this is the use of colloidal semiconductor nanostructures as one of the donor and/or acceptor units; since this area has remained quite unexplored. In the present work, nitronyl- and imino-nitroxide organic free radicals have been synthesized and coupled to colloidal II-VI quantum dots (QDs). The nature of the donor-acceptor interactions in these QD-radical hybrids has been investigated using spectroscopic techniques such as ground state UV-Vis absorption, steady-state and time-resolved photoluminescence(PL) spectroscopy. In all cases, these radicals quench the Luminescence intensity from the photo-excited QDs through an efficient non-radiative process. The excited-state quenching rate constant is highly dependent on parameters like QD size and composition, the identity and concentration of the organic free radical, and the nature of the medium surrounding the QDs. The quenching rates can be further correlated to the surface-binding sites on the QDs and can also be tuned by modifying the nature of the QD surface by growing different QD hetero-structures. [Preview Abstract] |
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