Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session E29: First-principles Modeling of Excited-State Phenomena in Materials IV: Nanoscale SystemsFocus
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Sponsoring Units: DCOMP DMP DCP DCMP Chair: Noa Marom, Carnegie Mellon Univ Room: LACC 406A |
Tuesday, March 6, 2018 8:00AM - 8:36AM |
E29.00001: Electronic and Optical Excitations in Confined Nanostructures Invited Speaker: Serdar Ogut Electronic and optical excitations in confined nanostructures have been in the center of an intense research effort for the last two decades. Achieving a detailed understanding of how light interacts with matter at the nanoscale and how it can be manipulated to tune material properties is a challenging endeavor that necessitates a reliably predictive modeling and simulation effort to aid and interpret experiments. To this end, computational work during the last two decades on time-dependent density-functional-theory (TDDFT) and Green’s function-based many-body perturbation theory methods, such as the GW approximation and the Bethe-Salpeter equation (BSE), have provided reliable methodologies to examine electronic and optical excited states from first principles within similar frameworks as ground state properties. This talk will focus on our recent applications of real-space TDDFT and GW-BSE methods to a variety of confined nanostructures. They include our studies on (i) the nature of electronic and optical excitations in bulk-truncated TiO2 nanocrystals, (ii) the effects of self-consistency and vertex corrections in the GW-BSE formalism in predicting excitation spectra of a set of aromatic molecules, and (iii) comparison of predictions from density-functional, many-body perturbation, and quantum chemistry techniques for photoelectron spectra of small copper oxide and related transition metal oxide clusters. |
Tuesday, March 6, 2018 8:36AM - 8:48AM |
E29.00002: First-principles molecular dynamics simulations of ligand-passivated cadmium selenide quantum dots Siyoung Kim, Marton Voeroes, Wooje Cho, Francois Gygi, Dmitri Talapin, Giulia Galli Semiconductor quantum dots (QD) are heterogeneous ligand-passivated nanostructures used in photovoltaic cells and light emitting devices. Predictions of the optoelectronic properties of QDs are challenging as most synthesis methods yield broad size distributions; in addition, in most cases only indirect and partial information is available from experiments on the structure of the QD/ligand interface. Recently, small ligand terminated cadmium selenide clusters, with a known number of core atoms, were synthesized [1], providing ideal test-beds to cross-validate experiments and theory. Using the experimentally determined QD core structures, we conducted first-principles molecular dynamics simulations at finite temperatures to uncover the effects of the passivating ligand layer on the structural and optoelectronic properties of the clusters. |
Tuesday, March 6, 2018 8:48AM - 9:00AM |
E29.00003: First-Principles Study of Hot Electron Dynamics in Silicon Quantum Dots Jian Cheng Wong, Lesheng Li, Yosuke Kanai In our earlier study on effects of surface passivation on a small silicon quantum dot, a unique relaxation behavior of hot electron was found when the surface was terminated with fluorine atoms. In this work, we examine effects of the quantum dot size as well as the effect of decoherence on the hot electron relaxation process. Hot electron dynamics in silicon quantum dots is investigated using first-principles electron dynamics simulations in the framework of the fewest switches surface hopping method. The study shows that the interesting surface passivation effect on the hot electron relaxation is already diminished when the dot size is increased to a few nanometers as typically synthesized in experiments. Our study also shows that decoherence plays an important role in determining the relaxation rate. We will also discuss effects of passivating the quantum dot surface with organic molecules. |
Tuesday, March 6, 2018 9:00AM - 9:12AM |
E29.00004: Simulation of Laser-Induced Rectification in a Nano-scale Diode Model Xiaojia Xu, Daniel Kidd, Kazuyuki Watanabe, Kalman Varga Time-dependent density functional theory is utilized to simulate a periodic jellium model system, representing a nano-scale vacuum-tube diode, subject to a continuous laser. Such devices are suggested as a means of achieving petahertz operational speeds due to improved transport velocities. Local geometric sharpness causes enhanced field emission and results in laser-induced rectification. The rate of electron transfer between jellium clusters is meausured for various field intensities, and separation distances. |
Tuesday, March 6, 2018 9:12AM - 9:24AM |
E29.00005: First principles Theory for Non-linear Transport in Nano-structures Oscar Grånäs, Grigory Kolesov, Efthimios Kaxiras We apply a recent implementation of the real-time formulation of time-dependent density functional theory to investigate the transient behaviour of strong currents through graphene nano-ribbons. By mapping out the time-dependent local currents, density changes and potentials, we associate the microscopic behaviour with macroscopic observables. We attribute the non-linearity to deviations from the ground-state band-structure appearing in situations with high current densities. |
Tuesday, March 6, 2018 9:24AM - 9:36AM |
E29.00006: Photoinduced topological phase transition in black phosphorous and graphene Jiatao Sun, Hang Liu, Sheng Meng Recent advances in ultrafast spectroscopy open a route toward engineering new phase of solids with optical pumping. The nonequilibrium electronic states of solids driven by a strong AC electromagnetic field manifest many topological states different from equilibrium ones. Using first-principles calculations and Floquet theorem, we studied the dressed states of black phosphorous and graphene under periodic driven of laser. Intriguing photo-dressed electronic states including Floquet Dirac semimetals, Floquet topological insulators etc can be engineered in black phosphorous by changing the direction, intensity and frequency of incident laser. The coexistence of type-I and type-II Floquet Dirac fermion can be realized spontaneously, which can be monitored by simulating the pump-probe time and angular-resolved photoelectron spectroscopy. Our works demonstrate the Floquet engineering of ultrafast changing the topological properties of solids. 1. Hang Liu et al., arxiv: 1710.05546. |
Tuesday, March 6, 2018 9:36AM - 9:48AM |
E29.00007: Dependence of Hot Electron Transfer on Surface Coverage and Adsorbate Species at Semiconductor-Molecule Hybrid Interfaces Lesheng Li, Yosuke Kanai Developing a molecular-level understanding of how one could enhance hot electron transfer (HET) at semiconductor-molecule hybrid interfaces is central to advancing various future technologies. Using first-principles simulations, we investigate how surface coverage and adsorbate species influence HET at semiconductor-molecule interfaces. Counterintuitively, increasing surface coverage was found to suppress HET because nonadiabatic couplings (NACs) at the interface are noticeably altered by the increased delocalization of hot electron accepting states. Adsorbate species itself is an important factor in HET not simply because of energy levels, but because the transfer is quite sensitive to NACs. Our work shows that relatively minor variations of the NACs could lead to significant changes in the HET characteristics. Developing a “design principle” at a molecular level for enhancing HET remains a great challenge, and our work shows that controlling NACs must be part of such a design principle, in addition to the energy level alignment. |
Tuesday, March 6, 2018 9:48AM - 10:00AM |
E29.00008: Electronic and Optical Excitations in Transition-Metal Oxide Clusters Young-Moo Byun, Bin Shi, Meisam Rezaei, Serdar Ogut Transition-metal oxide systems are technologically and scientifically important, but it is challenging to correctly compute their excited-state properties from first principles due to strong electron correlations inherent in them. We apply various flavors of GW-BSE and TDDFT methods to early (Sc and Ti) and late (Ni, Cu, and Zn) transition-metal oxide clusters and compare our computational results with experimental data. Based on the comparison, we discuss the level of theory and approximations that allow us to predict the electronic and optical properties of transition-metal oxide systems accurately and efficiently. |
Tuesday, March 6, 2018 10:00AM - 10:12AM |
E29.00009: Real-Time Density Functional Tight Binding: A New Computational Approach for Probing Plasmonic Properties of Large Material Systems Bryan Wong, Niranjan Ilawe, Maria Oviedo Light-harvesting systems and plasmonic materials continue to garner significant attention due to their importance in both energy conversion and detection technologies. However, several challenges exist in improving their performance: these electronic multifunctional systems are coupled, many-body systems with complex electronic interactions with their surrounding environments. All of these processes occur at different time and length scales, and span an immense multi-scale space (i.e., chemical interactions within their environment, all in tandem with external light and electric fields). To this end, we have utilized a new computational approach based on density functional tight binding (DFTB) theory to directly probe and calculate these complex systems. This implementation allows us to calculate the electronic and structural properties of large systems (~10,000 atoms), whereas conventional DFT approaches are typically limited to only hundreds of atoms. Using this approach, I will highlight our work on photo-initiated dynamics in large plasmonic nanoantennas. In particular, these time-domain, quantum-mechanical studies provide an approach to probe real-time electron dynamics mechanisms to understand and tailor these complex material systems for realistic applications. |
Tuesday, March 6, 2018 10:12AM - 10:24AM |
E29.00010: Electron Affinity of Liquid Water Alex Gaiduk, Tuan Anh Pham, Marco Govoni, Francesco Paesani, Giulia Galli Understanding redox reactions in aqueous environments requires a precise knowledge of the ionization potential and electron affinity of liquid water. The former has been measured, but the latter remains unknown. We predict the electron affinity (EA) of liquid water and of its surface from first principles, coupling path-integral molecular dynamics with ab initio potentials and many-body perturbation theory calculations. Our results for the surface agree with recent pump-probe spectroscopy measurements, while those for the bulk differ from several estimates adopted in the literature. We revisit these estimates and present an updated energy diagram for an electron in water. We show that the ionization potential of the bulk and surface are almost identical; instead their EAs differ substantially, with the conduction band edge of the surface much deeper in energy than that of the bulk. We also discuss the significant impact of nuclear quantum effects on the fundamental gap and band edges of the liquid. |
Tuesday, March 6, 2018 10:24AM - 10:36AM |
E29.00011: Laser-induced water decomposition near 2D sheets studied by TDDFT Yoshiyuki Miyamoto, Hong Zhang, Xinlu Cheng, Angel Rubio In this presentation, we will show the TDDFT-Ehrenfest dynamics for pulse-laser induced decomposition of water molecules near 2D sheets, graphene, hBN and gC3N4. The assumed full-width of half-maximum (FWHM) of the field intensity spans from 10 fs to 20 fs, and the assumed wavelengths are 800nm and 400nm. We have found reduction of threshold intensity of optical E-field for the decomposition when water molecules are located near the 2D sheet. This fact is understood by optical field enhancement near the 2D sheet rather than their catalytic effect. The case with FWHM=20 fs gives smaller threshold intensity of the laser field than the case of FWHM=10 fs, yet the fluence per pulse is smaller with shorter FWHM. We further performed the simulation of several water molecules above graphene sheet keeping inter-water distances very close to those in bulk water. However, the threshold intensity for the water decomposition was less sensitive to the presence of the water-water interactions. The details of the present results was published in Phys. Rev. B96, 115451 (2017) |
Tuesday, March 6, 2018 10:36AM - 10:48AM |
E29.00012: Calculation of Excited-State Properties by Analysis of Electronic Response to Initial-State, and
External-Field Perturbations Martin Mosquera, Lin Chen, Mark Ratner, George Schatz Fueled by advances in transient spectroscopy and high-performance computational science, theoretical methods for the computation of excited state dynamics are quickly progressing. In this talk we discuss an extension of standard linear-response TDDFT for the calculation of excited state-excited state transition multipoles, and ideas for the computation of pure excited-state properties. The techniques reported in this talk are based on the linear response analysis of the polarizability of a system of non-relativistic electrons that are subject to weak external fields, where, at the initial propagation time the ground-state is perturbed by a base excited state of interest. We present successful applications to ultrafast transient X-ray spectroscopy and exciton absorption in organic photovoltaics. |
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