Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session D23: Invited Session: Electron-Hole Interaction in Nanoparticles |
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Sponsoring Units: DCOMP Chair: Ari Chakraborthy, Syracuse University Room: 505-507 |
Monday, March 3, 2014 2:30PM - 3:06PM |
D23.00001: Photoexcitations in embedded semiconducting nanoparticles Invited Speaker: Giulia Galli We will discuss technical challenges involved in describing photo-excitation processes from first principles, in realistic materials [1], and we will present some results obtained using density functional and many body perturbation theory for semiconducting nanoparticles embedded in complex solid matrices, and for nanoparticles [2] with unusual core structures [3]. These are systems with promising properties for solar energy conversion. \\[4pt] [1] Yuan Ping, Dario Rocca, and Giulia Galli, \textit{Chem. Soc. Rev. }\textbf{42}, 2437 (2013).\\[0pt] [2] Stefan Wippermann, M\'{a}rton V\"{o}r\"{o}s, Dario Rocca, Adam Gali, Gergely Zimanyi and Giulia Galli \textit{Phys. Rev. Lett. }\textbf{110}, 046804 (2013).\\[0pt] [3] Stefan Wippermann, Marton Voros, Adam Gali, Francois Gygi,Gergely T. Zimanyi, and Giulia Galli 2013 (submitted for publication). [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:42PM |
D23.00002: Calculating recombination rates and biexciton binding/antibinding in core-shell dots and nano-rods Invited Speaker: John Shumway Predicting radiative lifetimes and photoluminescence (PL) emission energies from electron-hole recombination in nano structures is complicated by correlation. Quantum correlations---particularly the attraction between the recombining electron and hole---reduce the PL emission energy but also modify the wave functions, enhancing recombination rates. Interactions with spectator particles can also affect energies and lifetimes, though sometimes the sign of these changes is non-intuitive. Path-integral quantum Monte Carlo (PI-QMC) is a wave-function free computational quantum approach that can easily handle interactions between several electrons and holes in a nanostructure. We present an application to core-shell dots and nano-rods, where proper treatment of correlation is necessary to understand the binding/antibinding transition in the biexciton [1]. The imaginary-time paths provide further insights into the properties of the electron-hole states. We show how changing the topology of the paths can be used to calculate recombination rates and give insights into the recombination process. Fluctuations in the paths are used to calculate responses to electric and magnetic fields. These calculations are performed with the open source pi-qmc code available on GitHub and as a community resource on the nanoHUB.\\[4pt] [1] P. G. McDonald, E. J. Tyrrell, J. Shumway, J. M. Smith, and I. Galbraith, Phys. Rev. B 86, 125310, (2012). [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 4:18PM |
D23.00003: Exciton Scattering approach for conjugated macromolecules: from electronic spectra to electron-phonon coupling Invited Speaker: Sergei Tretiak The exciton scattering (ES) technique is a multiscale approach developed for efficient calculations of excited-state electronic structure and optical spectra in low-dimensional conjugated macromolecules. Within the ES method, the electronic excitations in the molecular structure are attributed to standing waves representing quantum quasi-particles (excitons), which reside on the graph. The exciton propagation on the linear segments is characterized by the exciton dispersion, whereas the exciton scattering on the branching centers is determined by the energy-dependent scattering matrices. Using these ES energetic parameters, the excitation energies are then found by solving a set of generalized ``particle in a box'' problems on the graph that represents the molecule. All parameters can be extracted from quantum-chemical computations of small molecular fragments and tabulated in the ES library for further applications. Subsequently, spectroscopic modeling for any macrostructure within considered molecular family could be performed with negligible numerical effort. The exciton scattering properties of molecular vertices can be further described by tight-binding or equivalently lattice models. The on-site energies and hopping constants are obtained from the exciton dispersion and scattering matrices. Such tight-binding model approach is particularly useful to describe the exciton-phonon coupling, energetic disorder and incoherent energy transfer in large branched conjugated molecules. Overall the ES applications accurately reproduce the optical spectra compared to the reference quantum chemistry results, and make possible to predict spectra of complex macromolecules, where conventional electronic structure calculations are unfeasible. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:54PM |
D23.00004: Excitons in time-dependent density-functional theory Invited Speaker: Carsten Ullrich Excitons are the dominant feature in the optical spectra of insulators and semiconductors close to the absorption edge. They are collective excitations of the many-body system, but can often be discussed in a simplified picture as bound electron-hole pairs. To describe excitons in bulk materials with time-dependent density-functional theory (TDDFT), exchange-correlation functionals with a proper long-range behavior are required. The first part of this talk will present a TDDFT approach for directly calculating singlet and triplet exciton binding energies, which is based on an adaptation of the Casida formalism for periodic solids. Several exchange-correlation kernels have been tested for a variety of semiconductors and large-gap insulators. The second part of this talk will discuss a method to visualize exciton dynamics in large organic molecules in real time, based on the time-dependent transition density matrix. The method is applied to study the optical properties of intramolecular charge-transfer excitons in photoexcited molecular donor-acceptor systems that are of interest in organic photovoltaics. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:30PM |
D23.00005: Quantum dots -- artificial atoms, large molecules, or small pieces of bulk? Insights from time-domain ab ignition studies Invited Speaker: Oleg Prezhdo Quantum dots (QD) are quasi-zero dimensional structures with a unique combination of solid-state and atom-like properties. Unlike bulk or atomic materials, QD properties can be modified continuously by changing QD shape and size. Often, the bulk and atomic viewpoints contradict each other. The atomic view suggests strong electron-hole and charge-phonon interactions, and slow energy relaxation due to mismatch between electronic energy gaps and phonon frequencies. The bulk view advocates that the kinetic energy of quantum confinement is greater than electron-hole interactions, that charge-phonon coupling is weak, and that the relaxation through quasi-continuous bands is rapid. QDs exhibit new physical phenomena. The phonon bottleneck to electron energy relaxation and generation of multiple excitons can improve efficiencies of photovoltaic devices. Our state-of-the-art non-adiabatic molecular dynamics techniques, implemented within time-dependent density-functional-theory, allow us to model QDs at the atomistic level and in time-domain, providing a unifying description of quantum dynamics on the nanoscale. [Preview Abstract] |
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