Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session K20: First-principles Modeling of Excited-state Phenomena in Materials VII: 2D Materials and SurfacesFocus
|
Hide Abstracts |
Sponsoring Units: DCOMP DMP Chair: Ismaila Dabo, Pennsylvania State University Room: BCEC 157A |
Wednesday, March 6, 2019 8:00AM - 8:36AM |
K20.00001: Ab initio time-dependent optical spectroscopy applied to spin and valley dynamics in monolayer transition metal dichalcogenides Invited Speaker: Alejandro Molina-Sanchez Monolayer transition metal dichalcogenides (TMDs) like MoS2 or WSe2 are promising semiconducting 2D materials. Not only they have a bandgap in the optical range and suitable transport properties, but also provide a stimulating arena to study fundamental physics. For instance, the valence and conduction bands are spin-split because of the strong spin-orbit interaction. Moreover, due to the lack of inversion symmetry, with circularly polarized light, one can create excitons at selected K valley. The resulting imbalance in the population of the K valleys is the basics of valleytronics. |
Wednesday, March 6, 2019 8:36AM - 8:48AM |
K20.00002: The origin of single photon emission in 2D WSe2 Yu Jie Zheng, Yifeng Chen, Yu Li Huang, Pranjal Kumar Gogoi, Ming-Yang Li, Lain-Jong Li, Paolo E Trevisanutto, Qixing Wang, Stephen J Pennycook, Andrew Thye Shen Wee, Su Ying Quek Several experimental groups have shown that defect structures in 2D WSe2 result in single photon emission (SPE). However, the origin of SPE is still unknown. We present a first principles study of the nature and optical properties of point defects in 2D WSe2, together with scanning tunneling microscopy (STM) and scanning transmission electron microscopy images. We predict that O2 can dissociate easily at Se vacancies, resulting in O-passivated Se vacancies (OSe) and O interstitials (Oins), which give STM images in good agreement with experiment. Our GW-Bethe-Salpeter-equation calculations show that Oins defects give exciton peaks ~50-100 meV below the free exciton peak, in good agreement with the localized excitons observed in independent SPE experiments. No other point defect (OSe, Se vacancies, W vacancies, and SeW antisite defects) gives excitons in the same energy range. We conclude that the Oins defect is a source for the SPE previously observed in 2D WSe2. |
Wednesday, March 6, 2019 8:48AM - 9:00AM |
K20.00003: Moiré Patterns of Excitons in Twisted Bilayer Transition-Metal Dichalcogenides Heterostructure Xiaobo Lu, Shiyuan Gao, Xiaoqin (Elaine) Li, Li Yang Twisted van der Waals heterostructures and the corresponding superlattices, moiré patterns, have been regarded as remarkable platforms to modulate many-electron interactions and optical excitations of two-dimensional structures. We employ first-principles many-body perturbation theory to study excitons and their moiré patterns in twisted bilayer MoSe2/WSe2 heterostructures. Because of significant type-II band offsets of heterostructures, direct interlayer excitons are always the lowest-energy excitons. We find that the energy variation of interlayer excitons is more significant in the R-type twisting structures than that in the H-type ones. Moreover, although the electron-hole binding energy is nearly spatially homogenous, the optical oscillator strength and radiative lifetime of interlayer excitons are very sensitive to the local stacking style, and they can vary in a few orders of magnitude in different regions of twisted bilayer structures. As a result, optical moiré patterns of interlayer excitons with high contrasts of brightness and radiative lifetime are expected. |
Wednesday, March 6, 2019 9:00AM - 9:12AM |
K20.00004: Dimensionality Dependence of Radiative Recombination in Black Phosphorus from First-principles Feng Wu, Dario Rocca, Yuan Ping Monolayer black phosphorus is a unique anisotropic 2D material with a sizable direct band gap and high mobility that has promising optoelectronic applications. However, the origin of how excitonic effects and excited state lifetime change with dimensionality has not been well understood. In this work we studied the electronic excitations and the radiative recombination in black phosphorus monolayer, nanoribbons and quantum dots by employing GW approximation (GW) and solving Bethe-Salpeter Equation (BSE). We demonstrate that 1)the exciton wavefunctions in 0D, 1D and 2D nanostructures are similar, which extend more along the armchair direction than zigzag, and reducing the size of nanostructures along the armchair direction significantly affects the exciton energy while that along the zigzag direction does not. 2)The radiative lifetime increases dramatically when the dimension shrinks from 2D to 1D to 0D, because the constraint of energy and momentum conversation makes that the radiative lifetime increases approximately 103 times for each dimension reduction. This study provides important insights on engineering excited state properties of nanostructure materials. |
Wednesday, March 6, 2019 9:12AM - 9:24AM |
K20.00005: Electronic screening of quasiparticle excitations by atomically thin substrates Keian Noori, Nicholas Cheng, Fengyuan Xuan, Su Ying Quek The weaker screening response within two-dimensional (2D) materials has manifested in novel physical phenomena, with strongly bound excitons being a well-known example. Here, we study the screening properties in reduced dimensions by employing first principles GW calculations, focusing on the electronic screening of point charge perturbations adjacent to a 2D material. We find that this is an excellent approximation for predicting the HOMO-LUMO gaps of benzene adsorbed on 2D materials, without expensive GW calculations for the full system. Interestingly, by comparing the screening response of ~15 2D and 3D substrates to point charge perturbations above the substrate, we find that both 2D and 3D substrates have a screening response that obeys the same approximately linear relation with their quasiparticle gaps. This is in contrast to the much weaker screening response of 2D materials to excitations within the material. Our results can be attributed to the fact that most of the induced charge responding to the perturbing potential is located within ~2-3 Å of the surface atomic plane in both 2D and 3D substrates. This work implies that 2D materials are effective atomically thin dielectrics, and uncovers new insights into the unusual physics of screening in reduced dimensions. |
Wednesday, March 6, 2019 9:24AM - 9:36AM |
K20.00006: Impact of defects on the opto-electronic properties of monolayer GeSe; a many-body perturbation theory perspective Kirk Lewis, Sahar Sharifzadeh An accurate and detailed knowledge of the influence of defects will be central to the design of promising new 2D materials. Due to their reduced dimensionality and screening, these materials are even more likely to be impacted by defects than their 3D counterparts. We employ first-principles many-body perturbation theory within the GW/BSE approximation to investigate the influence of point defects on the opto-electronic properties of a monolayer semiconductor composed of GeSe. We systematically study a series of charged vacancies, their trap state energies, and their impact on optical absorption. We determine that the excitonic properties of the material are significantly affected by the presence of defects, with implications for devices fabricated using this material system. |
Wednesday, March 6, 2019 9:36AM - 9:48AM |
K20.00007: Molecules at Conducting Surfaces from First-Principles: Dark States and Distance-Dependent Broadening Alina Umerbekova, Michele Pavanello Subsystem Density Functional Theory (DFT) can be taken to the time domain enabling first-principles simulations of electron dynamics of complex systems. Upon inspection, we observe all the relevant regimes proper of non-Markovian open quantum system dynamics: electronic energy transfer and screening. Contrary to interactions between molecular systems, when molecules interact with conducting surfaces the electron dynamics is strongly non-Markovian with dramatic repercussions to the molecule’s response to external perturbations. Metals and semiconductors typically have large polarizabilities, and even in a regime of low coupling their effect on molecular species is significant: line broadening, peak shift, and intensity borrowing are observed and explained in terms of inter-subsystem dynamical interactions and a many-body decomposition of the system’s response function in a way that transcends Fermi Golden Rule. We characterize the decay of an energy level's broadening with molecule-surface distance, and inspect bright as well as dark states. |
Wednesday, March 6, 2019 9:48AM - 10:00AM |
K20.00008: Multiple Effects of Inhomogeneous Strain Field on Carrier Distribution in Bending 2D Materials Jiuyu Sun, Jinlong Yang It is critical to investigate optoelectronic properties in bending 2D materials, especially for inhomogeneous strain field (iHSF), for application of flexible nanoelectronics. Using first-principles calculations, we find there are multiple effects of iHSF on charge distribution in g-C3N4 nanosheets and black phosphorus nanoribbons (BPNRs). On the one hand, in rippled g-C3N4, the valence band maximum (VBM) and conduction band minimum (CBM) of the whole structure are spatially separated, thus the photogenerated electrons and holes are driven to different regions, respectively. This indicates the rippled g-C3N4, with potential advantage of good efficiency of electron-hole separation, is a flexible and promising platform for metal-free photocatalytic water splitting. On the other hand, the iHSF forms multiple exciton funnels in BPNRs. We find the type of funnels (spatial distribution and motion of excitons) can be tuned by changing the intensity and sign (tensile or compressed) of the strain field, thickness and periodicity (zigzag or armchair) of BPNRs. By forming different funnels, the excitons are able to be accumulated in unstrained, tensile or compressed regions, which enriches the means of controlling excitons for photo-detecting or photo-emitting. |
Wednesday, March 6, 2019 10:00AM - 10:12AM |
K20.00009: Realizing an excitonic insulator by decoupling exciton binding energy from the minimum band gap Jiang Zeyu, Yuanchang Li, Duan Wenhui, Shengbai Zhang Realizing excitonic insulator state in real materials has long been an attractive subject of condensed matter physics, as motivated by both fundamental interest in many-body physics and the potential application in transport devices due to its bosonic nature. Direct-gap materials serve as promising candidates for excitonic insulators, where the difficulty to distinguish from a Peierls charge density wave is avoided. However, direct-gap materials still suffer from a divergence of polarizability when the band gap approaches zero, leading to a diminishing exciton binding energy. We propose that one can decouple the exciton binding energy from the band gap in materials where band-edge states have the same parity. As a concrete example, we show by first-principles calculations that two-dimensional GaAs and experimentally mechanically exfoliated single-layer TiS3 support prefectly to this principle, thus hold the possiblility for experimental realization of excitonic insulators. |
Wednesday, March 6, 2019 10:12AM - 10:24AM |
K20.00010: Linear response and nonlocal dielectric function of freestanding and substrate-supported borophene from first-principles Anubhab Haldar, Sahar Sharifzadeh Two dimensional boron, or borophene, is a metallic monolayer material that has been recently synthesized on metallic substrates. First principles studies on the mechanical, optical, and electronic properties of borophene have predicted novel applications of borophene in flexible and power electronics. Additionally, borophene is a flexible, transparent, metallic material with highly nonlocal and anisotropic permittivity, and is a candidate as a plasmonic material. Here, we utilized first-principles density functional theory to conduct a systematic investigation of the linear response in isolated and metal-supported borophene, in order to quantitatively describe plasmon resonances in the visible region in the presence of the substrate. We determine that the dielectric properties of borophene are significantly affected by a silver substrate. This has implications for monolayer and thin film plasmonic and optoelectronic devices based on this platform. |
Wednesday, March 6, 2019 10:24AM - 10:36AM |
K20.00011: Charge dynamics in proton-irradiated aluminum sheets Alina Kononov, Aneesh Jonelagadda, Andre Schleife Materials capable of withstanding continual ion radiation are highly desirable in space and nuclear applications. A detailed understanding of the mechanisms leading to degradation of materials under ion bombardment would enable targeted development of radiation resistant materials. As an energetic charged particle penetrates a material's surface, it deposits energy and excites electrons, leading to secondary electron (SE) emission and localized charge within the material. Even when direct collisions with nuclei are rare, fs scale surface charge dynamics may cause Coulomb explosion, which would damage and erode the material surface. We use time-dependent density functional theory to characterize SE emission, surface charge dynamics, and atomic forces in few-layer aluminum sheets under proton irradiation. From first-principles, we compute exit-side and entrance-side SE yields, SE energy spectra, and time scales of charge equilibration within the material as the projectile velocity and material thickness are varied. We also estimate the momentum acquired by aluminum atoms near the impact point. These simulations provide unprecedented insight into the dynamical response of materials' surfaces to ion bombardment. |
Wednesday, March 6, 2019 10:36AM - 10:48AM |
K20.00012: Level Alignment in Large-Scale Hybrid Organic-Inorganic Systems from Hybrid Density Functional Theory Svenja Janke, Mariana Rossi, Sergey V. Levchenko, Matthias Scheffler, Manoj Kumar Jana, Chi Liu, David B Mitzi, Volker Blum Hybrid organic-inorganic systems allow to combine the properties of organic and inorganic substances at the nanoscale and hence open up a wide area for design of new materials with tunable properties. The positions of carrier levels and their alignment determine electronic properties of hybrid materials. A key challenge is that the systems in question tend to be large, due to alignment of components with inherently different lattice parameters or due to complex crystal structure packing. We here use hybrid density functional theory (FHI-aims all-electron code) for systems comprising over 1,000 atoms to reliably predict level alignments in two types of systems. For the paradigmatic interface system tetracene and pentacene at H/Si(111), we demonstrate the necessity of choosing large cells of up to 1,200 atoms that reflect the coincidence pattern and find type II heterojunction behavior with potential separation of charge between organic and inorganic component. For a layered double perovskite we demonstrate how level alignment between organic and inorganic compound is affected when the metal ion is exchanged, and rationalize experimentally observed photoluminescence in these systems. |
Wednesday, March 6, 2019 10:48AM - 11:00AM |
K20.00013: Ehrenfest molecular dynamics approach to a light-induced softening of aluminum slab based on time-dependent density functional theory Hiroki Katow, Yoshiyuki Miyamoto Modulation in lattice kinetics of simple metals under the irradiation of an ultra-short laser pulse is of great interest to both industrial applications and fundamental physics. In our study we performed Ehrenfest molecular dynamics (Ehrenfest MD) simulation of a nine-atomic-layer-thick aluminum slab based on time-dependent density functional approach. In our simulation the slab has (111) surface of aluminum fcc structure. The slab is put in vacuum, and is exposed to a femtosecond laser pulse. To analyze the Ehrenfest MD results, we employed a quasi one-dimensional model that bounds neighboring layers by a quadratic potential. Model parameters are fitted so that they reproduce Ehrenfest MD trajectory. We found a significant and non-uniform reduction of the force constants with the increase of laser intensity suggesting lattice softening by electronic excitation. In this presentation, we compare the results with the case of Born-Oppenheimer MD under finite electron temperature and discuss the correspondence between these two distinct types of theoretical methods. |
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