Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session A38: Focus Session: The Chemical Physics of Biological and Biologically-inspired Solar Energy Harvesting I |
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Sponsoring Units: DCP Chair: Greg Scholes, University of Toronto Room: 410 |
Monday, March 16, 2009 8:00AM - 8:36AM |
A38.00001: Investigation of Excitonic Coherence in LHCII by 2D Electronic Spectroscopy Invited Speaker: Photosynthesis has evolved with the ability to transfer energy through a matrix of light-harvesting pigment-protein complexes with almost no loss. The accomplishment of this near unity quantum efficiency is a feat that man has yet to understand or replicate. One proposed mechanism integral to this process requires long-lived coherent superpositions of the excitons, delocalized electronic excitations, in these systems. Two-dimensional Fourier transform electronic spectroscopy, already proven to be an ideal technique for investigating these coherences, has been employed to study Light Harvesting Complex II (LHCII), the most abundant light harvesting complex in higher plants. As in other photosynthetic systems previously studied, we observe long-lived coherence lasting beyond many of the excitons' lifetimes. Furthermore, unique coherence signatures allow the energies of the individual excitons to be located in an otherwise highly congested spectrum. This technique, by which 2D FT electronic spectroscopy can pinpoint excitonic spectral positions, and the resulting implications for LHCII will be discussed. [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 9:12AM |
A38.00002: Luminescent Solar Concentrators Employing Phycobilisomes Invited Speaker: At current manufacturing growth rates, it is expected to take at least 20 years to produce enough Si-based solar cells to make a significant impact on the world energy supply. Solar concentrators could alleviate manufacturing constraints by focusing light on small solar cells. Luminescent solar concentrators (LSCs) are especially promising because they do not need to track the sun to obtain high optical concentration factors. Light incident on an LSC is absorbed by dyes, re- emitted into a guided mode in the slab, and finally collected by a PV cell mounted at the edge of the slab. The maximum optical concentration of an LSC is theoretically limited by the wavelength shift between absorption and emission in the dye. In this presentation, we describe LSCs that mimic a four energy level laser design, maximizing the wavelength shift and minimizing re-absorption losses. We employ phycobilisomes - photosynthetic antenna complexes that concentrate excited states in red algae and cyanobacteria. The phycobilisomes are cast in a solid-state matrix that preserves their internal Förster energy-transfer pathways and large wavelength shift between absorption and emission. Casting is a simple fabrication technique that also eliminates any need for expensive high-index glass or plastic. By comparing the performance of intact and decoupled complexes, we establish that energy transfer within intact phycobilisomes reduces LSC self-absorption losses by approximately (48$\pm$5)\%. These results suggest that phycobilisomes are the model for a new generation of cast LSCs with improved efficiency at high optical concentrations. [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:48AM |
A38.00003: Coherent Multidimensional Spectroscopy of Photosynthetic Complexes: Manipulating Quantum Pathways by Optical Pulse Sequences Invited Speaker: The response of chromophore aggregates to sequences of femtosecond laser pulses is simulated using the nonlinear exciton equations. The nonlinear response can be interpreted in terms of the scattering of elementary excitations, quasiparticles, rather than as transitions among eigenstates. Applications are made to the Fenna-Matthews-Olson (FMO) and the PSI light harvesting complexes. Some fundamental symmetries of multidimensional optical signals are used to design techniques that can selectively resolve coherent quantum dynamics and incoherent energy dissipation. Simulations show damped oscillations of cross peaks corresponding to evolution of coherences, without interference from incoherent population relaxation. Energy transfer pathways are seen through the redistribution of crosspeak amplitudes. Resolution is enhanced by employing specific pulse polarization configurations to generate chirality-induced signals. New pulse sequences are designed to generate signals that are induced by correlations among elementary excitations. Specific phase-matching directions can target the correlated dynamics of double excitations. Cross peaks in 2D correlation plots are interpreted in terms of quasiparticle scattering and shown to reveal the double-exciton wavefunction, projected into products of single-excitons. Uncorrelated double-exciton states do not show up in the spectra due to quantum interference among pathways. The proposed techniques amplify cooperative dynamical features and reveal information on the robustness of quantum states to fluctuating environments. In collaboration with Darius Abramavicius and Dmitri Voronine, University of California, Irvine, CA 92697. [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:00AM |
A38.00004: Exploring Nanophotovoltaic Molecules using STM Chenggang Tao, Jibin Sun, Xiaowei Zhang, R. Yamachika, D. Wegner, Y. Bahri, G. Samsonidze, S. Louie, T. Tilly, R. Segalman, M. Crommie Composite molecular solar cells are a promising and exciting alternative to traditional silicon or gallium arsenide solar cells, but the power conversion efficiency remains low. In order to further increase this efficiency, a deeper understanding of the microscopic mechanisms at work in organic solar cells is needed. Using scanning tunneling microscopy and spectroscopy we have investigated nanophotovoltaic molecules that combine both donor and acceptor elements. Submolecular spectral resolution reveals the energy level alignment within these composite molecular structures. This information should be useful for understanding the energy conversion pathways within molecular solar cells, and for developing higher efficiency solar cell materials. [Preview Abstract] |
Monday, March 16, 2009 10:00AM - 10:12AM |
A38.00005: New highly polar semiconductor ferroelectrics for solar energy conversion devices Andrew M. Rappe, Ilya Grinberg, Joseph W. Bennett Solar energy is a promising long-term solution for future energy requirements; however, current solar energy conversion devices are plagued by low efficiency. The use of ferroelectric ABO$_3$ perovskite oxides is one approach for boosting conversion efficiency. Ferroelectric oxides possess spontaneous polarization and have been shown to produce a bulk photovoltaic effect, in which charged carriers, specifically electrons and holes, separate to prevent recombination. Once separated, the high-energy electrons are available for electrical work or for the catalytic splitting of water into hydrogen and oxygen. Currently, most solid oxide ferroelectrics have a band gap of at least 3~eV, absorbing primarily in the ultra-violet (UV) region. Since UV light comprises only 8$\%$ of the solar spectrum, new materials with a decreased band gap and large polarization would be highly desirable. We use first-principles density functional theory (DFT) calculations to investigate the ground state structures of PbTiO$_{3}$ solid solutions containing Ni, Pd and Pt. We predict that these proposed materials will display a decreased band gap when compared to PbTiO$_{3}$, while maintaining or enhancing polarization. They are promising candidates for use as semi-conducting ferroelectric substrates for solar conversion devices. [Preview Abstract] |
Monday, March 16, 2009 10:12AM - 10:24AM |
A38.00006: Femtosecond electronic relaxation in cylindrical molecular aggregates Andrew Moran, Jordan Womick, Stephen Miller Natural light harvesting systems have evolved with correlated pigment fluctuations whose fine-tuning promotes efficient energy transfer and photosynthesis. We investigate similar correlations in a double-walled cylindrical molecular aggregate with a diameter of 10 nm. A variety of nonlinear laser spectroscopies are utilized in this work. Excitons localized on different regions of the cylindrical structure are found to undergo correlated energy level fluctuations by analysis of photon echo line shapes. Particular electronic relaxation channels are resolved with a specialized coherent Raman spectroscopy. The importance of correlated pigment fluctuations for excitonic energy transfer is discussed. [Preview Abstract] |
Monday, March 16, 2009 10:24AM - 10:36AM |
A38.00007: F\"orster coupling in realistic nanoparticle circuits Patrick Rebentrost, Mike Stopa, Alan Aspuru-Guzik Semiconductor nanoparticles could potentially be used to build artificial light-harvesting systems and electrically controlled excitonic circuits. We derive a new method for computing the F\"orster coupling between nanoparticles in an arbitrary electrostatic environment. We calculate the self-consistent electronic structure of an exciton, including the electron-hole attraction, in a nanoparticle within the two band effective mass approximation. Self-interaction of the electron and the hole are removed and the eigenstate is approximated as a product state of the electron and the hole wavefunctions. The environment is incorporated via boundary conditions on Poisson's equation and arbitrary dielectric background. The transfer rate of the exciton via F\"orster coupling to a neighboring nanoparticle is computed, without making a dipole approximation, from the results of the self-consistent calculation. Departure from the usual $1/R^3$ dependence are calculated, as well as specific cases where gates or additional nanoparticles are present. [Preview Abstract] |
Monday, March 16, 2009 10:36AM - 10:48AM |
A38.00008: Non-Markovian Environmental Contributions to the Efficiency of Energy Transfer Cesar Rodriguez-Rosario, Patrick Rebentrost, Alan Aspuru-Guzik Non-Markovian environmental effects have been experimentally observed in the Fenna-Matthews-Olson photosynthetic complex, but their role is not understood. We study the dynamical contribution of the environment to the efficiency of energy transfer by considering a non-Markovian environment and its interplay with the system Hamiltonian. We focus on the role of memory effects of different orders in time, and their competition that affect the energy transfer by defining the efficiency of the non-Markovian process. This efficiency measure has applications to the study of the quantum transport efficiency and engineering of light-harvesting devices. [Preview Abstract] |
Monday, March 16, 2009 10:48AM - 11:00AM |
A38.00009: Spatially correlated fluctuations and coherence dynamics in photosynthesis Z. G. Yu, M. A. Berding, Haobin Wang Recent multicolor photon-echo experiments revealed a long-lasting quantum coherence between excitations on donor and acceptor in photosynthetic systems. Identifying the origin of the quantum coherence is essential to fully understand photosynthesis. Here we present a generic model in which a strong intermolecular steric restoring force in densely packed pigment-protein complexes results in a spatial correlation in conformational (static) variations of chromophores, which in turn induces an effective coupling between high-frequency (dynamic) fluctuations in donor and acceptor. The spatially correlated static and dynamic fluctuations provide a favorable environment to maintain quantum coherence, which can consistently explain the photon-echo measurements [1]. [1] Z. G. Yu, M. A. Berding, and Haobin Wang, Phys. Rev. E {\bf 78}, 050902 (Rapid Communications) (2008). [Preview Abstract] |
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