Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session W41: Focus Session: Electronic Structure and Applications to Energy Conversion I |
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Sponsoring Units: DCP Chair: Sergei Tretiak, Los Alamos National Laboratory Room: A115/117 |
Thursday, March 24, 2011 11:15AM - 11:51AM |
W41.00001: Tuning the electronic structure of II-VI semiconductors and nanostructures for energy applications Invited Speaker: Using first-principles calculations within density functional theory (DFT), we study the impacts of quantum confinement, strain, and surface ligand passivation on the electronic structure of typical II-VI wurtzite semiconductors and nanostructures. In CdSe/CdTe core/shell nanowires, large anisotropic strains develop due to the large lattice mismatch. These strains result in significant reductions in band gap in the CdSe core with increasing CdTe shell thickness, by amounts comparable to that expected from reduced quantum confinement [1]. The response of band gaps of wurtzite compounds to anisotropic strain is further shown to be large and highly non-linear, and system-dependent [2]. In addition, we also explore the effects of chemisorbed ligand on the electronic structure of CdSe surfaces. Substantial shifts in band edge energies are predicted due to the induced dipole at the CdSe-ligand interface and the intrinsic dipole of the ligand [3]. Our studies suggest well-defined routes to control both the band gaps and band edge energies of nanomaterials for light-harvesting applications. \\[4pt] [1] S. Yang, D. Prendergast, and J. B. Neaton, Nano Lett. 10, 3156 (2010).\\[0pt] [2] S. Yang, D. Prendergast, and J. B. Neaton, Appl. Phys. Lett., in press (2011).\\[0pt] [3] S. Yang, D. Prendergast, and J. B. Neaton, in preparation (2011). [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:03PM |
W41.00002: TR-2PPE Studies of Ultrafast Charge Separation at Organic Photovoltaic Interfaces S.W. Robey, G.J. Dutton, W. Jin, J.E. Reutt-Robey Dissociation of excitons in organic photovoltaic (OPV) devices occurs exclusively at interfaces between donor and acceptor molecular materials. To help understand critical charge separation processes, we have performed time-resolved two-photon photoemission (TR-2PPE) studies of sub-picosecond exciton dynamics at well-characterized organic donor-acceptor interfaces. Interfaces between phthalocyanines and C$_{60}$ were engineered using organic MBE and characterized using STM, STS, and UPS. Ultrafast TR-2PPE measurements were performed on CuPc$\backslash $C$_{60}$ structures by pumping the lowest optical $\pi \to \pi $* transitions (Q-band) to generate CuPc singlet (S$_{1})$ excitons and probing this population with a time-delayed UV pulse. For thick films, CuPc S$_{1}$ decay is dominated by vibrational relaxation (several 100's femtoseconds) and singlet-to-triplet conversion ($\sim $ 1 picosecond). Directly at the interface, however, charge transfer to C$_{60}$ dominates decay of S$_{1}$ exciton, ($\sim $ 100 femtoseconds) . We also find evidence for important recombination routes from the charge separated state back to lower-lying CuPc T$_{1}$ triplet excitons. To test the impact of intersystem crossing to triplet levels, we have performed analogous investigations for H$_{2}$Pc$\backslash $C$_{60}$ interfaces. Results for this interface will be compared and contrasted with the CuPc$\backslash $C$_{60}$ case. [Preview Abstract] |
Thursday, March 24, 2011 12:03PM - 12:15PM |
W41.00003: Near- and Far-Field Effects on Excited States at Organic Semiconductor and Metal Interfaces Oliver Monti, Mary Steele, Nahid Ilyas, Leah Kelly We present an investigation of the evolution of excited states at the interface of the dipolar organic semiconductor vanadyl naphthalocyanine on highly oriented pyrolytic graphite. Using two-photon photoemission we observe several excited states at sub-monolayer to few-monolayer coverages. Excited states of this organic semiconductor are progressively stabilized with coverage, an effect that is somewhat mirrored in the image state manifold as well. These findings can be understood in the context of a simple electrostatic model that considers how molecular levels and vacuum level are influenced differentially by the the strong electrostatic fields present at the interface with dipolar molecules: While the vacuum level rises continuously with coverage, the molecular states are significantly depolarized as a function of electric fields in the near-field regime. This indicates that the interfacial excited state electronic structure is strongly sensitive to long-range intermolecular interactions mediated by the surface, with direct implications for energy level alignment and charge transfer dynamics at the interface. Interfacial electrostatic fields may therefore be used to manipulate in a concrete fashion interfacial charge transfer processes such as photoinduced interfacial electron transfer. [Preview Abstract] |
Thursday, March 24, 2011 12:15PM - 12:51PM |
W41.00004: Time-domain ab initio studies of excitation dynamics in semiconductor quantum dots Invited Speaker: Solar energy applications require understanding of dynamical response of novel materials on nanometer scale. Our state-of-the-art non-adiabatic molecular dynamics techniques, implemented within time-dependent density functional theory, allow us to model such response at the atomistic level and in real time. The talk will focus on single and multiple exciton generation, relaxation, annihilation and dephasing in semiconductor quantum dots.\\[4pt] References:\\[0pt] [1] O. V. Prezhdo, ``Multiple excitons and electron-phonon bottleneck in semiconductor quantum dots: Insights from ab initio studies'', \textit{Chem. Phys. Lett. -- Frontier Article}, \textbf{460}, 1 (2008) \\[0pt] [2] O. V. Prezhdo ``Photoinduced dynamics in semiconductor quantum-dots: insights from time-domain ab initio studies'', \textit{Acc. Chem. Res.}, \textbf{42}, 2005 (2009) \\[0pt] [3] A. B. Madrid, H.-D. Kim, O. V. Prezhdo, ``Phonon-induced dephasing of excitons in silicon quantum dots: multiple exciton generation, fission and luminescence'', \textit{ACS-Nano}, \textbf{3}, 2487 (2009) \\[0pt] [4] C. M. Isborn, O. V. Prezhdo, ``Quantum dot charging quenches multiple exciton generation: first-principles calculations on small PbSe clusters'', \textit{J. Phys. Chem. C,} \textbf{113}, 12617 (2009) \\[0pt] [5] S. V. Kilina, D. S. Kilin, O. V. Prezhdo, ``Breaking the phonon bottleneck in PbSe and CdSe quantum dots: time-domain density functional theory of charge carrier relaxation'', \textit{ACS-Nano}, \textbf{3}, 93 (2009). \\[0pt] [6] S. A. Fischer, A. B. Madrid, C. M. Isborn, O. V. Prezhdo, ``Multiple exciton generation in small Si clusters: A high-level, ab initio study'', \textit{J. Phys. Chem. Lett.,} \textbf{1}, 232 (2010). [Preview Abstract] |
Thursday, March 24, 2011 12:51PM - 1:03PM |
W41.00005: Hole Localization in Molecular Crystals From Hybrid Density Functional Theory Na Sai, Paul F. Barbara, Kevin Leung Charge trapping in organic solids and interfaces plays an important role in organic photovoltaic efficiencies. Experimental confirmation of intrinsic charge trapping at the atomic scale and the tools to directly probe the trap energy landscape, however, remain lacking. We use first principles computational methods to examine hole trapping in organic molecular crystals. We present a computational scheme based on the tuning of the fraction of exact exchange in hybrid density functional theory to eliminate the many-electron self-interaction error [1]. With small organic molecules, we show that this scheme gives accurate descriptions of ionization and dimer dissociation. We demonstrate that the excess hole in perfect molecular crystals can form self-trapped hole polarons. The predicted absolute ionization potentials of both localized and delocalized holes are consistent with experimental values.\\[4pt] [1] N. Sai, P. Barbara, and K. Leung (submitted). [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W41.00006: Linear Dichroism and Photoluminescence Microscopy Imaging of Grain Boundaries in Crystalline Metal-Free Phthalocyanine Thin Films Zhenwen Pan, Cody Lamarche, Ishviene Cour, Naveen Rawat, Lane Manning, Randall Headrick, Madalina Furis We employed a combination of linear dichroism and photoluminescence microscopy with spatial resolution of 5$\mu $m to study the excitonic properties of solution-processed metal-free phthalocyanine (H2Pc) crystalline thin films with millimeter-sized grains. We observe a highly-localized, sharp, monomer-like emission at the high angle grain boundaries, in contrast to samples with more uniform grain orientation where no such feature has been observed. The energy difference between the grain boundary luminescence and the HOMO-LUMO singlet exciton recombination of the crystalline H2Pc is measured to be 160meV. Our systematic survey of grain boundaries indicates this localized state is never present at low angle boundaries where the $\pi $-orbital overlap between adjacent grains is significant. It supports recent results which associated a decrease in carrier mobility with the presence of large angle boundaries in similar crystalline pentacene films. [Preview Abstract] |
Thursday, March 24, 2011 1:15PM - 1:51PM |
W41.00007: Theories and applications for characterizing electronic coupling factors Invited Speaker: The transport of charges and excitation energy are two processes of fundamental importance in many biological and material systems. One of the fundamental parameters in the transport rates is the electronic coupling, which is essentially an off-diagonal Hamiltonian matrix element between the initial and final diabatic states. We have developed ways to define the diabatic states and calculate the coupling factors, including those for electron transfer (ET) and excitation energy transfer (EET). The fundamental method development and applications will be discussed. For characterizing TEET, the Fragment Spin Difference (FSD) was developed and it can be to calculate the TEET coupling over a general class of systems. TEET in bacterial light-harvesting complex LH2 and the peridinin chlorophyll-a protein (PCP) of dinoflagellates were calculated and analyzed. Our results are in good agreement with experimental results and it offers limits to the photoprotection models. Therefore, with the FSD scheme, it is possible to quantify and analyze the electronic couplings in TEET processes in large systems, and to derive insights and limits of theoretical models. [Preview Abstract] |
Thursday, March 24, 2011 1:51PM - 2:03PM |
W41.00008: Ab initio calculation of optical spectra of solvated molecules: GW+BSE method for liquid environments Jeehye Lee, Tomas Arias Electronic excitations for solvated systems have drawn a great interest in the energy community because they provide a possibility to engineer photoexcitation processes. Time-dependent DFT (TDDFT) and the hybrid QM/MM approach successfully calculate the solvent shift in excitation energies (solvatochromic shift) for confined systems, but are well known to work best for small systems and Frenkel excitons. Here we present a new modification of the GW and Bethe-Salpeter equation (GW+BSE) methods which allows treatment of solvated systems beyond the TDDFT level by including the frequency-dependent polarizability of the solvent at the diagrammatic level. In this initial work, we present the solvatochromic and ionization potential shifts for a series of molecules in aqueous solution. [Preview Abstract] |
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