Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session H19: Calculating Optical Properties from First PrinciplesInvited
|
Hide Abstracts |
Sponsoring Units: DCOMP Chair: Audrius Alkauskas, Center for Physical Sciences and Technology Vilnius, Lithuania Room: 278-279 |
Tuesday, March 14, 2017 2:30PM - 3:06PM |
H19.00001: Excited States and Optical Spectra Based on GW-BSE: Dimensionality and Screening Invited Speaker: Steven G. Louie In this talk, I discuss some recent developments and applications of first-principles GW plus Bethe Salpeter equation (GW-BSE) approach to the understanding and prediction of photo-excited states, optical responses, and related spectroscopic properties of materials, in particular atomically thin two-dimensional (2D) crystals. Owing to their reduced dimensionality, quasi-2D materials and their nanostructures can exhibit highly unusual behaviors. Symmetry, many-body interactions, doping, and substrate screening effects play a critical role in shaping qualitatively and quantitatively their excited-state properties. Accurate treatment of these effects, in particular many-electron interactions, poses new theoretical and computational challenges. I will present some new developments in addressing these challenges, and present studies on monolayer and few-layer transition metal dichalcogenides and metal monochalcogenides, as well as black phosphorus and other 2D crystals. Several highly interesting and unexpected phenomena are discovered: unusual excitonic level structures and optical selection rules; exchange-induced light-like (massless) exciton dispersion in 2D; tunable optical and plasmonic properties; and the dominant influence of substrate screening. [Preview Abstract] |
Tuesday, March 14, 2017 3:06PM - 3:42PM |
H19.00002: Extreme quantum confinement in nitrides for improved LED efficiency. Invited Speaker: Emmanouil Kioupakis Nitride materials are indispensable for LEDs in the ultraviolet and the short-wavelength part of the visible spectrum (2014 Nobel Prize in Physics). However, their efficiency is reduced at high power (efficiency droop) and longer wavelengths (green-gap problem). In this talk, I will discuss how first-principles calculations provide insights into the efficiency-limiting mechanisms in nitride LEDs. We identified the origin of the efficiency problems to be nonradiative Auger recombination and its interplay with the intrinsic polarization fields and alloy composition fluctuations of InGaN quantum wells. Our predictive calculations also suggest engineering solutions to improve the LED efficiency. I will discuss how extreme quantum confinement in atomically thin binary nitrides (GaN and InN) is a promising method to stabilize excitons at room temperature and realize efficient LEDs in the deep-ultraviolet and green part of the spectrum. I will also present our results for the design of BInGaN alloys that are lattice-matched to GaN for visible-light emission. This work was supported by the NSF CAREER (1254314) and DMREF (1534221) programs. Computational resources were provided by the DOE NERSC facility (DE-AC02-05CH11231) and by XSEDE, supported by NSF grant ACI-1053575. [Preview Abstract] |
Tuesday, March 14, 2017 3:42PM - 4:18PM |
H19.00003: Many-Body Perturbation Theory for Understanding Optical Excitations in Organic Molecules and Solids Invited Speaker: Sahar Sharifzadeh Organic semiconductors are promising as light-weight, flexible, and strongly absorbing materials for next-generation optoelectronics. The advancement of such technologies relies on understanding the fundamental excited-state properties of organic molecules and solids, motivating the development of accurate computational approaches for this purpose. Here, I will present first-principles many-body perturbation theory (MBPT) calculations aimed at understanding the spectroscopic properties of select organic molecules and crystalline semiconductors, and improving these properties for enhanced photovoltaic performance. We show that for both gas-phase molecules and condensed-phase crystals, MBPT within the GW/BSE approximation provides quantitative accuracy of transport gaps extracted from photoemission spectroscopy and conductance measurements, as well as with measured polarization-dependent optical absorption spectra. We discuss the implications of standard approximations within GW/BSE on accuracy of these results. Additionally, we demonstrate significant exciton binding energies and charge-transfer character in the crystalline systems, which can be controlled through solid-state morphology or change of conjugation length, suggesting a new strategy for the design of optoelectronic materials. [Preview Abstract] |
Tuesday, March 14, 2017 4:18PM - 4:54PM |
H19.00004: Finding order in disorder: Raman spectroscopy of amorphous silicon, from ab initio to multiscale modeling Invited Speaker: David Strubbe Amorphous silicon is an interesting material for photovoltaics, thermoelectrics, batteries, detectors, or thin-film transistors, whether as active material, a route to nano/microcrystalline silicon, or a passivation for crystalline silicon. Amorphous silicon is also a well-studied model system for amorphous materials in general, which present unique opportunities and challenges compared to crystalline materials. Raman spectroscopy is an important characterization tool for furthering our understanding and future applications. To enable its use for mapping local strain distributions, we calculated the Raman spectrum with ab initio density-functional perturbation theory under different strains and found peak shifts proportional to the trace of the strain. We showed this is the general form for isotropic amorphous vibrational modes, by symmetry analysis and explicit computation. These results were confirmed by accompanying experimental measurements. [1] Much larger structural models are needed for other interesting questions about a-Si:H, such as medium- and long-range order, nano/microcrystalline Si, amorphous/crystalline interfaces, or amorphous nanostructures. While quantum-mechanical calculations become infeasible, classical inter-atomic potentials can provide vibrations but need a model for Raman intensities. We retool the old idea of a bond-polarizability model by parametrization from our detailed database of ab initio Raman tensors, opening the way for large-scale Raman calculations on complex Si materials.\\ \\$[1]$ David A. Strubbe, Eric C. Johlin, Timothy R. Kirkpatrick, Tonio Buonassisi, and Jeffrey C. Grossman, "Stress effects on the Raman spectrum of an amorphous material: theory and experiment on a-Si:H," Phys. Rev. B 92, 241202(R) (2015). [Preview Abstract] |
Tuesday, March 14, 2017 4:54PM - 5:30PM |
H19.00005: Electronic structures and optical properties of colloidal quantum dots Invited Speaker: Lin-Wang Wang Colloidal quantum dots can be synthesized by relatively cheap wet chemistry approaches with large quantity. Despite of their simple synthetic methods, they can have high optical qualities, e.g., with close to 100{\%} photoluminescence (PL) quantum efficiency. Their optical properties can be tuned by changing the size, shape, hetero structure and composition of the quantum dots. The surface ligand passivation of the quantum dot also plays an important role in determining the electronic and optical properties of the quantum dots. In this talk, we will present the calculations of the electronic structures and optical properties of colloidal nanocrystals. Charge patching method is used to construct the Hamiltonian of a colloidal quantum dot, followed by the calculation of its electronic structure and optical properties. Limited configuration interaction formalism can be used to study the fine structures of the exciton or multiexciton behavior, as well as the Auger effects in such systems. For a heterostructure nanocrystal, when its size becomes large (e.g., 10 nm in size), the internal strain might play an important role. The exciton might be localized in particular region of a quantum dot. The optical gap and PL intensity can also be altered by external stress on the nanocrystal. The surface chemistry of the colloidal quantum dots, especially for Pb chalcogenide systems, will also be discussed. It will be shown that the electronic structure and optical properties of Pb chalcogenide systems are robust against defects and imperfection of surface passivation, explaining why these nanocrystals can have such high optical qualities. We will show how one can change the electronic and optical properties by using different surface ligand passivations. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700