Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session L7: First-Principles Modeling of Excited State Phenomena V: Low-Dimensional SystemsFocus
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Sponsoring Units: DCOMP DMP DCP Chair: Johannes Lischner, Imperial College London Room: 266 |
Wednesday, March 15, 2017 11:15AM - 11:51AM |
L7.00001: Graphene revisited: From orbital mapping to its impact as a substrate Invited Speaker: Claudia Draxl Graphene, the material of the 21st century, is without doubt one of the best characterized solids. Despite the enormous amount of investigations and related publications, it still it offers a variety of exciting aspects to explore, in particular in view of its excitations. Combining density-functional theory with many-body perturbation theory, as implemented in the all-electron full-potential package \textbf{exciting }[1], provides a powerful framework for this purpose. (i) The first example concerns the question, whether we can ``see'' orbitals in an electron microscope. Indeed, transmission electron microscopy can be used for mapping atomic orbitals, as demonstrated recently by a first-principles approach [2]. For defected graphene, exhibiting either an isolated vacancy or a substitutional nitrogen atom different kinds of images are to be expected, depending on the orbital character. (ii) Graphene/BN heterostructures absorb light over a broad frequency range, from the near-infrared to the ultraviolet region, exhibiting novel features induced by the stacking [3]. Peculiar features of their excitations are inter-layer excitons that can be modulated upon layer patterning. By choosing the stacking arrangement, the electronic coupling between the individual components can be tuned to enhance light-matter interaction. (iii) As demonstrated for azobenzene monolayers, graphene as a substrate strongly impacts the photo-switching behavior of molecules [4]. Despite the weak hybridization, the photo-absorption of the molecules is remarkably modulated. While substrate polarization reduces the band-gap of the adsorbate, enhanced dielectric screening weakens the attractive interaction between electrons and holes. Furthermore, excitations corresponding to intermolecular electron-hole pairs, which are dark in the isolated monolayers, are activated by the presence of the substrate. (iv) Finally, we ask how first- and second-order Raman spectra of graphene [5] are affected by strain that may be induced by an underlying substrate. References: [1] A. Gulans, et al., J. Phys.: Condens. Matter 26, 363202 (2014). [2] L. Pardini, S. L\"{o}ffler, G. Biddau, R. Hambach, U. Kaiser, C. Draxl, and P. Schattschneider, Phys. Rev. Lett. 117, 036801 (2016). [3] W. Aggoune, C. Cocchi, D. Nabok, K.Rezouali, M. Belkhir, and C. Draxl, preprint. [4] Q. Fu, C. Cocchi, D. Nabok, A. Gulans, and C. Draxl, preprint. [5] A. Hertrich, C. Cocchi, P. Pavone, and C. Draxl, in preparation. [Preview Abstract] |
Wednesday, March 15, 2017 11:51AM - 12:03PM |
L7.00002: Dynamical Excitonic Effects in Doped Two-Dimensional Semiconductors Shiyuan Gao, Yufeng Liang, Catalin Spataru, Li Yang It is well-known that excitonic effects can dominate the optical properties of two-dimensional materials. These effects, however, can be substantially modified by doping free carriers. We investigate these doping effects by solving the first-principles Bethe-Salpeter Equation. Dynamical screening effects, included via the sum-rule preserving generalized plasmon-pole model, are found to be important in the doped system. Using monolayer $\mathrm{MoS_2}$ as an example, we find that upon moderate doping, the exciton binding energy can be tuned by a few hundred meVs, while the exciton peak position stays nearly constant due to a cancellation with the quasiparticle band gap renormalization. At higher doping densities, the exciton peak position increases linearly in energy and gradually merges into a Fermi-edge singularity. Our results are crucial for the quantitative interpretation of optical properties of two-dimensional materials and the further development of ab initio theories of studying charged excitations such as trions. [Preview Abstract] |
Wednesday, March 15, 2017 12:03PM - 12:15PM |
L7.00003: Enhanced quasiparticle band gap renormalization in doped two-dimensional semiconductors Li Yang, Shiyuan Gao, Yufeng Liang One of the most prominent features of two-dimensional (2D) materials is the enhanced many-body interactions because of quantum confinement and reduced electronic screening. Doping is widely observed in 2D materials by either inevitable defects or intended electrostatic and chemical processes. The doped free carriers introduce additional screening and plasmon excitations, which substantially modifies many-body interactions. This factor and enhanced van Hove singularities lead to a large, nonlinear quasiparticle band gap renormalization (BGR) in 2D semiconductors. Using the GW approximation, we developed an efficient scheme to calculate the BGR by separating the doping contributions to the quasiparticle self-energy from the intrinsic one. Based on our first-principles calculation, we report enhanced BGR (a few hundred meV) of monolayer transition metal dichalcogenides, black phosphorus, and hexagonal BN. Our result is crucial for interpreting experimental measurements and quantitatively understanding and engineering quasiparticle band gaps in 2D semiconductors. [Preview Abstract] |
Wednesday, March 15, 2017 12:15PM - 12:27PM |
L7.00004: Anomalous Size Dependence of Optical Properties in Black Phosphorus Quantum Dots xianghong niu, yunhai li, huabing shu, jinlan wang Understanding electron transitions in black phosphorus nanostructures plays a crucial role for applications in electronics and optoelectronics. In this work, by employing time-dependent density functional theory calculations, we systematically study the size-dependent electronic, optical absorption and emission properties of black phosphorus quantum dots (BPQDs). Both the electronic gap and the absorption gap follow an inversely proportional law to the diameter of BPQDs in conformity to the quantum confinement effect. In contrast, the emission gap exhibits anomalous size dependence in the range of 0.8-1.8 nm which is blue-shift with the increase of size. The anomaly, in fact, arises from the structure distortion induced by the excited state relaxation and it leads to huge Stokes shift in small BPQDs. [Preview Abstract] |
Wednesday, March 15, 2017 12:27PM - 12:39PM |
L7.00005: Optical properties of 2D monochalcogenides: single-layer GaSe and GaTe from first-principles calculations Gabriel Antonius, Steven G. Louie Two-dimensional (2D) metal monochalcogenides such as gallium selenide and gallium telluride show good electrical properties, and an exceptionally large photoresponsitivity, making it suitable for photodetectors and phototransistor devices. We compute the absorption spectrum of single-layer GaSe and GaTe from first-principles calculations, using the GW and BSE formalism. The hydrogenic series of bound excitons (derived from the lowest interband transition) found in these materials is dark in linear optical response, due to the different symmetry of the first valence and conduction band. The high-energy bright excitons assume a non-hydrogenic behavior located neither in the center nor the boundary of the Brillouin zone, where a high density of states allows for large absorbance. [Preview Abstract] |
Wednesday, March 15, 2017 12:39PM - 12:51PM |
L7.00006: Electronic and Optical Properties of Borophene, a Two-dimensional Transparent Metal. Lyudmyla Adamska, Sridhar Sadasivam, Pierre Darancet, Sahar Sharifzadeh Borophene is a recently synthesized metallic sheet that displays many similarities to graphene and has been predicted to be complimentary to graphene as a high density of states, optically transparent 2D conductor. The atomic arrangement of boron in the monolayer strongly depends on the growth substrate and significantly alters the optoelectronic properties. Here, we report a first-principles density functional theory and many-body perturbation theory study aimed at understanding the optoelectronic properties of two likely allotropes of monolayer boron that are consistent with experimental scanning tunneling microscopy images. We predict that despite both systems are metallic, the two allotropes have substantially different bandstructure and optical properties, with one structure being transparent up to 3 eV and the second weakly absorbing in the UV/Vis region. We demonstrate that this strong structure-dependence of optoelectronic properties is present with the application of strain. Lastly, we discuss the strength of electron-phonon and electron-hole interactions within these materials. Overall, we determine that precise control of the growth conditions in necessary for controlled optical properties. [Preview Abstract] |
Wednesday, March 15, 2017 12:51PM - 1:03PM |
L7.00007: The Interaction of Atomic Hydrogen with Au under Optical Plasmonic Excitation Christopher Lane, Devika Sil, Ethan Glor, Kyle Gilroy, Safiya Sylla, B. Barbiellini, R.S. Markiewicz, Svetlana Neretina, Arun Bansil, Zahra Fakhraai, Eric Borguet The interaction of Au with hydrogen, especially the formation of Au-H bonds, is far from being understood due to the inert nature of bulk Au. Since the Au is non-reactive, thermodynamics alone cannot drive the dissociation of H$_2$ due the large activation energy of 4.51 eV, leaving the difficult task to find methods to facilitate gold hydride formation. However, when irradiated with low intensity visible photons, nanometer sized gold structures produce localized surface plasmons, which decay into hot electrons able to dissociate H$_2$ molecules at the surface. Experimentally we find as a result of the dissociation reaction a change in the optical properties of the Au nanostructures, manifest as a blue-shift in the dielectric function. We will present work, wherein we combine the results of density functional theory (DFT) and insitu-spectroscopic ellipsometry, to justify the proposed mechanism of the blue-shift based on gold hydride formation, where the coverage of the hydride formation is directly proportional to the blue shift. We will also provide unique insights on the interaction of Au nanoparticles with atomic hydrogen. Work supported in part by the US Department of Energy. [Preview Abstract] |
Wednesday, March 15, 2017 1:03PM - 1:15PM |
L7.00008: Electronic structure and optical properties of Ag$_{\mathrm{\mathbf{44}}}${(MNBA)}$_{\mathrm{\mathbf{2}}}${ nanoclusters: An }{{ab initio}}{ study } Zahra Hooshmand, Duy Le, Volodymyr Turkowski, Talat S Rahman In a recent study,Abdulhalim et al. have presented a new strategy for tailoring the optical properties of thiol-protected small Ag nanoclusters by protonation-deprotonation of the ligand shell[1].Specifically,the absorption spectrum of the 44-atom Ag nanoparticles with 30 MNBA (5 mercapto-2-nitrobenzoic acid) ligands, showed dependence on the pH environment.Here we present results of our density functional theory (DFT) and TDDFT based calculations of the effects of the MNBA ligand on the electronic and optical properties of Ag$_{\mathrm{44}}$.In particular,we find when 2 MNBA molecules are adsorbed on Ag$_{\mathrm{44}}$ there is a strong tendency of dimerization between adjacent molecules via H\textellipsis O-H groups.This dimerization is proposed to be the origin of the experimentally observed nanocluster's optical behavior at lower pH values. In the case of high pH environment, the deprotonation significantly modifies the binding of sulfur atoms to the silver cluster,and as a result the optical properties of the system.TDDFT results for the excitation energies of the system demonstrate that the optical absorption spectrum is defined by transitions that involve hybridized ligand-nanoparticle states.Obtained results may lead to a better understanding of the connection between the electronic structure and the optical response of noble-metal nanoparticle-ligand systems.[1] Abdulhalim,L.,et al., \textit{Inorg.Chem.},2016,55~(21), p 11522. [Preview Abstract] |
Wednesday, March 15, 2017 1:15PM - 1:27PM |
L7.00009: Carrier Multiplication in Chiral Single-Walled Carbon Nanotubes: DFT-Based Study. Deyan Mihaylov, Andrei Kryjevski, Svetlana Kilina, Dmitri Kilin It is understood that the conclusion about multiple exciton generation (MEG) efficiency in a nanoparticle can only be made by including competition between different relaxation channels, such as phonon-mediated carrier thermalization, exciton multiplication and recombination, Auger scattering, etc. Here, we study time evolution of photo-excited states using Boltzmann transport equation (BE) that includes phonon emission/absorption terms together with the exciton multiplication and recombination terms. BE coefficients are computed using finite-temperature many-body perturbation theory (MBPT) (sometimes called NEGF) combined with the DFT simulations. Exciton effects are included by solving the Bethe-Salpeter equation based on RPA-screened Coulomb interaction (with additional simplifying approximations). In particular, we calculate internal efficiency, the number of excitons generated from a single energetic photon. We find that efficient MEG in chiral single-wall carbon nanotubes (SWCNTs), such as (6,2), (10,5), (6,5), and in nm-sized amorphous H-passivated Si nano-wires is present within the solar spectrum range. In SWCNTs MEG strength depends on chirality. We find that MEG efficiency in SWCNTs with Cl atoms adsorbed to the surface is enhanced compared to the pristine case. [Preview Abstract] |
Wednesday, March 15, 2017 1:27PM - 1:39PM |
L7.00010: Excitonic states and defect physics of two-dimensional group-IV monochalcogenides. Lidia Gomes, Alexandra Carvalho, Paolo Trevisanutto, Aleksandr Rodin, Antonio Neto Layered group-IV monochalcogenides have become an important group of materials within the ever-growing family of two-dimensional crystals. Among the binary IV-VI compounds, SnS, SnSe, GeS, and GeSe form a subgroup with orthorhombic structure which has shown exciting particularities and has been considered of high potential for numerous application. We give a brief overview of some important properties of the 2D form of this group and focus on recent results addressing the excitonic properties and the impact of the introduction of point defects on their structures. Vacancies and oxygen defects are modeled using first principles calculations. Energetic and structural analysis of five different models for chemisorbed oxygen atoms, reveals a better resistance of these materials to oxidation if compared to their isostructural partner, phosphorene. We also discuss a parallel work where quasi-particle band structure and excitonic properties of GeS and GeSe monolayers are investigated through ab initio GW and Bethe-Salpeter equation calculations. Within the main results, we show that the optical spectra of both materials are dominated by excitonic effects, however, GeS presents a remarkably larger binding energy of 1 eV. [Preview Abstract] |
Wednesday, March 15, 2017 1:39PM - 1:51PM |
L7.00011: Encapsulation-induced Renormalization of the Electronic and Optical Bandgaps in Black Phosphorene Nanostructures Diana Y. Qiu, Felipe H. da Jornada, Steven G. Louie Few-layer black phosphorus has recently emerged as a promising 2D semiconductor, notable for its widely tunable bandgap, highly anisotropic properties, and theoretically-predicted large exciton binding energies. In order to avoid degradation under ambient conditions, it has become common practice to encapsulate black phosphorus devices, and it is generally assumed that this encapsulation does not significantly affect their electronic and optical properties. We have performed ab-initio GW and GW plus Bethe Salpeter equation (GW-BSE) calculations to determine the quasiparticle (QP) bandstructure and optical spectrum of one (1L)- through four-layer (4L) black phosphorus, with and without encapsulation between hexagonal boron nitride and sapphire, two of the most common capping and substrate materials. We find that due to its small intrinsic screening, black phosphorus is exceptionally sensitive to environmental screening. Encapsulation reduces the exciton binding energy in 1L by as much as 70\% and completely eliminates the presence of a bound exciton in 4L. The reduction in the exciton binding energies is offset by a similar renormalization of the QP bandgap so that the optical gap remains unchanged, though the qualitative features of the absorption spectrum change dramatically. [Preview Abstract] |
Wednesday, March 15, 2017 1:51PM - 2:03PM |
L7.00012: Many-body Effect, Carrier Mobility, and Device Performance of Hexagonal Arsenene and Antimonene Yangyang Wang, Meng Ye, Ruge Quhe, Pu Huang, Jing Lu Monolayer (ML) arsenene and antimonene, as new members of group V-enes, have attracted great interest. Experimentally, multilayer arsenene/antimonene nanoribbons have been fabricated on an InAs/InSb substrate. ML and multilayer antimonene have been isolated by mechanical exfoliation and liquid-phase exfoliation. More importantly, they are highly stable under ambient condition. Together with their wide band gaps predicted by the HSE theory, arsenene and antimonene are very attractive for nanoscale optoelectronic and electronic devices. We investigate the many-body effect and device performance of ML hexagonal arsenene and antimonene using ab initio GW, GW plus Bethe-Salpeter equation and nonequilibrium Green's function approach. The quasi-particle and optical band gaps are calculated in ML arsenene and antimonene for the first time. Low (21/66 cm2/V$\cdot $s for electron/hole) and moderate carrier mobilities (150/510 cm2/V$\cdot $s for electron/hole) are obtained, for arsenene and antimonene, respectively. Quantum transport simulation reveals that the performance limits of sub-10 nm ML arsenene and antimonene FETs can satisfy both low power and high performance requirements of the ITRS target in the next decade. [Preview Abstract] |
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