Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session S33: Structural and Electronic PropertiesFocus
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Sponsoring Units: DMP Chair: Rui He, University of Northern Iowa Room: 296 |
Thursday, March 16, 2017 11:15AM - 11:27AM |
S33.00001: The Role of the Moire Pattern on the Electronic Band Structure of Mono- to Few Layer MoS2 D. Trainer, A. Putilov, T. Saari, T.-R. Chang, H.-T. Jeng, H. Lin, B. Wang, C. Lane, J. Nieminen, A. Bansil, X. X. Xi, M. Iavarone Using low temperature high resolution scanning tunneling microscopy and spectroscopy (STM/STS) we have investigated MoS2 grown by chemical vapor deposition (CVD) on a substrate of highly oriented pyrolytic graphite (HOPG). Atomic resolution images reveal clear Moir\'{e} patterns, which in general arise due to a lattice mismatch or rotational misalignment between weakly interacting MoS2 layers and between the first MoS2 layer and the substrate. The quasi-particle band-gap was analyzed as a function of the lattice rotation. Changes in the band structure were supported by density functional theory (DFT) calculations. [Preview Abstract] |
Thursday, March 16, 2017 11:27AM - 11:39AM |
S33.00002: Probing the Band Structure of Ultrathin MoTe$_{\mathrm{2}}$ via Strain Burak Aslan, Isha Datye, Hsueh-Hui Kuo, Michal Mleczko, Ian Fisher, Eric Pop, Tony Heinz Molybdenum ditelluride (MoTe$_{\mathrm{2}})$ is a semiconducting layered group VI transition metal dichalcogenide with an optical band gap of 1.1 and 0.9 eV in the monolayer and bulk, respectively. The bulk crystal possesses an indirect gap whereas the monolayer has a direct one. It is still under debate whether the direct-to-indirect gap crossover occurs at the monolayer or bilayer limit at room temperature, resulting from the fact that the two gaps are very close to one another in ultrathin crystals. We take advantage of this closeness by tuning the two gaps with in-plane tensile strain. In particular, we employ photoluminescence and absorption spectroscopy to probe the near-band-edge optical transitions and study their line-shapes to distinguish the direct and indirect gaps in few-layer MoTe$_{\mathrm{2}}$. We observe that the applied strain redshifts the direct and indirect gaps at different rates and strongly affects the spectral widths of the optical transitions. Our observations help us understand what contributes to the broadening of the A exciton peak in ultrathin MoTe$_{\mathrm{2}}$ and how the direct-to-indirect gap crossover occurs with decreasing thickness. [Preview Abstract] |
Thursday, March 16, 2017 11:39AM - 11:51AM |
S33.00003: Anisotropic electronic structure of ReS$_2$ D. Biswas, J. M. Riley, L. Bawden, O. J. Clark, L. Collins-Mcintyre, J. Feng, W. Meevasana, R. Yano, T. Sasagawa, A. Ganose, D. O. Scanlon, P. D. C. King The recent discovery that the optical properties of the group VIIB transition metal dichalcogenides (TMDCs), ReX$_2$ (X=Se,S) depend only weakly on material thickness has opened the possibility to achieve optical response from bulk ReX$_2$ which can only be realised by fabrication of single-layer samples in group VIB semiconducting TMDCs such as MoS$_2$ and WSe$_2$ [1]. While anisotropy in the optical and electronic properties of ReX$_2$ has been extensively studied, the electronic structure which underpins this remains almost completely unexplored experimentally to date. We present direct measurements of the electronic structure of ReS$_2$ from angle resolved photoelectron spectroscopy. Through this, we uncover an intriguing energy dependence of the underlying electronic structure anisotropy. Particularly we find that the states at the valence band top are rather three dimensional, with the fundamental band gap located away from the Brillouin zone centre. At higher binding energies, the electronic bands become quasi one-dimensional, reflecting the Re chains which form due to a pronounced structural distortion in ReS$_2$. [1] S. Tongay, {\it et. al.}, Nat. Commun. {\bf 5}, 3252 (2014). [Preview Abstract] |
Thursday, March 16, 2017 11:51AM - 12:03PM |
S33.00004: Coulomb engineering of the bandgap in 2D semiconductors Archana Raja, Andrey Chaves, Jaeeun Yu, Ghidewon Arefe, Heather Hill, Albert Rigosi, Timothy Berkelbach, Philipp Nagler, Christian Sch\"{u}ller, Tobias Korn, Colin Nuckolls, James Hone, Louis Brus, Tony Heinz, David Reichman, Alexey Chernikov Here we demonstrate a novel approach for bandgap engineering in 2D semiconductors. It is based on the modification of the dielectric environment, rather than on any change in the material itself. The unique environmental sensitivity of the strong Coulomb interaction in the 2D limit makes Coulomb engineering of the bandgap particularly effective. It also preserves the favorable characteristics of the 2D layer. In our studies, we determine the bandgap of the semiconductor by measuring ground and excited exciton transitions by optical spectroscopy and extrapolating to the quasi-particle band edge. In this fashion, we have directly demonstrated tuning of the bandgap of monolayer WS$_2$ and WSe$_2$ by 100’s of meV through control of the external dielectric environment. We have identified lateral jumps in the bandgap by preparing external dielectric media with abrupt boundaries. Moreover, bandgap renormalization is maximized within a 1 nm thick capping dielectric, suggesting that lateral junctions with nanoscale spatial resolution can be prepared within a homogenous 2D material. [Preview Abstract] |
Thursday, March 16, 2017 12:03PM - 12:15PM |
S33.00005: In-plane, commensurate GaN/AlN junctions: single-layer composite structures, multiple quantum wells and quantum dots Engin Durgun, Abdullatif Onen, Deniz Kecik, Salim Ciraci In-plane composite structures constructed of the stripes or core/shells of single-layer GaN and AlN, which are joined commensurately display diversity of electronic properties, that can be tuned by the size of their constituents. In heterostructures, the dimensionality of electrons change from 2D to 1D upon their confinements in wide constituent stripes leading to the type-I band alignment and hence multiple quantum well structure in the direct space. The $\delta$-doping of one wide stripe by other narrow stripe results in local narrowing or widening of the band gap. The direct-indirect transition of the fundamental band gap of composite structures can be attained depending on the odd or even values of formula unit in the armchair edged heterojunction. In a patterned array of GaN/AlN core/shells, the dimensionality of the electronic states are reduced from 2D to 0D forming multiple quantum dots in large GaN-cores, while 2D electrons propagate in multiply connected AlN shell as if they are in a supercrystal. These predictions are obtained from first-principles calculations based on density functional theory on single-layer GaN and AlN compound semiconductors which were synthesized recently. [Preview Abstract] |
Thursday, March 16, 2017 12:15PM - 12:27PM |
S33.00006: Carrier Scattering in Decorated Transition Metal Dichalcogenide Monolayers Diana Meneses Gustin, Sergio Ulloa, Victor Lopez The modification of electronic properties of 2D crystals by adsorption of molecules, such as deposition of photosensitive azobenzene on MoS2 monolayers, has been recently achieved [1]. Azobenzene modifies the electronic structure, which may be described by local charge transfer and strain fields. These potentials produce carrier scattering processes that may be described in terms of scattering phase shifts and corresponding cross sections. A comparison of Schrodinger and massive Dirac formulations for such problem are found to provide interesting insights in two asymptotic regimes. At low energies, where both dispersions can be seen as parabolic bands, the process is dominated by low angular momentum channels which exhibit different wave number dependence, and yet result in nearly isotropic scattering amplitudes. On the other hand, the differential cross section at high energies has clear signatures of the different band dispersions, with anisotropic behavior and different energy dependence in the Dirac or Schrodinger problem. The understanding of the electronic dynamics in these systems hold the promise for successful design of structures with desired functionality, as we exemplify by presenting differential cross sections for different types of scattering potentials. [Preview Abstract] |
Thursday, March 16, 2017 12:27PM - 1:03PM |
S33.00007: Imaging the motion of electrons in 2D semiconductor heterostructures. Invited Speaker: Keshav Dani Technological progress since the late 20th century has centered on semiconductor devices, such as transistors, diodes, and solar cells. At the heart of these devices, is the internal motion of electrons through semiconductor materials due to applied electric fields or by the excitation of photocarriers. Imaging the motion of these electrons would provide unprecedented insight into this important phenomenon, but requires high spatial and temporal resolution. Current studies of electron dynamics in semiconductors are generally limited by the spatial resolution of optical probes, or by the temporal resolution of electronic probes. In this talk, we combine femtosecond pump-probe techniques with spectroscopic photoemission electron microscopy to image the motion of photoexcited electrons from high-energy to low-energy states in a 2D InSe/GaAs heterostructure exhibiting a type-II band alignment. At the instant of photoexcitation, energy-resolved photoelectron images reveal a highly non-equilibrium distribution of photocarriers in space and energy. Thereafter, in response to the out-of-equilibrium photocarriers, we observe the spatial redistribution of charges, thus forming internal electric fields, bending the semiconductor bands, and finally impeding further charge transfer. By assembling images taken at different time-delays, we make a movie lasting a few tens of picoseconds of the electron transfer process in the photoexcited type-II heterostructure -- a fundamental phenomenon in semiconductor devices like solar cells. Quantitative analysis and theoretical modeling of spatial variations in the video provide insight into future solar cells, electron dynamics in 2D materials, and other semiconductor devices. [Preview Abstract] |
Thursday, March 16, 2017 1:03PM - 1:15PM |
S33.00008: Tunable electron phonon interaction in MoS$_{2}$ Bastian Miller, Alexander W. Holleitner, Ursula Wurstbauer Transition metal dichalcogenides such as MoS$_{2}$ are of current interest for optoelectronic application, but also for studying fundamental aspects of light-matter interaction and excitonic properties in strictly two-dimensional semiconductors. We explore the impact of the charge carrier density on the electron phonon interaction strength by non-resonant and resonant Raman spectroscopy. We utilize MoS$_{2}$ field effect structures with polymer electrolyte gate facilitating a change of the 2D electron density by more than two orders of magnitude [1]. We report unusual behavior in polarization and charge carrier dependent resonant Raman spectra that point towards strong electron-phonon coupling in MoS$_{2}$ and the importance of excitonic phenomena.\newline [1] Miller et al., APL 106, 122103 (2015). [Preview Abstract] |
Thursday, March 16, 2017 1:15PM - 1:27PM |
S33.00009: Slater-Koster Tight-Binding parametrization of single and few-layer Black-Phosphorus from first-principles calculations Marcos Menezes, Rodrigo Capaz Black Phosphorus (BP) is a promising material for applications in electronics, especially due to the tuning of its band gap by increasing the number of layers. In single-layer BP, also called Phosphorene, the P atoms form two staggered chains bonded by $sp^3$ hybridization, while neighboring layers are bonded by Van-der-Waals interactions. In this work, we present a Tight-Binding (TB) parametrization of the electronic structure of single and few-layer BP, based on the Slater-Koster model within the two-center approximation. Our model includes all 3s and 3p orbitals, which makes this problem more complex than that of graphene, where only 2pz orbitals are needed for most purposes. The TB parameters are obtained from a least-squares fit of DFT calculations carried on the SIESTA code. We compare the results for different basis-sets used to expand the ab-initio wavefunctions and discuss their applicability. Our model can fit a larger number of bands than previously reported calculations based on Wannier functions. Moreover, our parameters have a clear physical interpretation based on chemical bonding. As such, we expect our results to be useful in a further understanding of multilayer BP and other 2D-materials characterized by strong $sp^3$ hybridization. [Preview Abstract] |
Thursday, March 16, 2017 1:27PM - 1:39PM |
S33.00010: Tuning the electronic properties of WS$_{\mathrm{2}}$ on hBN by adatom engineering Jyoti Katoch, Søren Ulstrup, Simon Moser, Roland J. Koch, Kathleen M. McCreary, Simranjeet Singh, Jinsong Xu, Berend T. Jonker, Roland Kawakami, Aaron Bostwick, Eli Rotenberg, Chris Jozwiak Among transition metal dichalogenides (TMDs), monolayer tungsten disulfide (WS$_{\mathrm{2}})$ is gaining interest due to its large band gap, relatively high charge carrier mobilites and high spin-orbit coupling. The possibility to tune the electronic, optical and spin-valley related properties by substrate and adatom (e.g. alkali and heavy metal doping) engineering are one of the most interesting yet unexplored areas of research in the field of TMDs. Recently, we investigated the electronic band structure of monolayer WS$_{\mathrm{2}}$ on h-BN by angle-resolved photoemission (ARPES). The 10$\mu $m sized spatial resolution of the $\mu $ARPES endstation at the newly commissioned MAESTRO facility of the Advanced Light Source allowed us to identify these flakes and to obtain high quality band structure and core level information in a full spectro-microscopic approach. As particularly interesting finding we will discuss the effect of alkali and heavy metal doping on the spin-orbit coupling in the valence band of WS$_{\mathrm{2}}$ on h-BN. [Preview Abstract] |
Thursday, March 16, 2017 1:39PM - 1:51PM |
S33.00011: Origin of the Counterintuitive Dynamic Charge in the Transition-Metal Dichalcogenides Nicholas Pike, Benoit Van Troeye, Antoine Dewandre, Xavier Gonze, Matthieu Verstraete Our recent first-principles calculations of the electronic and vibrational properties of the hexagonal transition-metal dichalcogenides reveal that their Born effective charges display a counterintuitive sign when compared to most other materials or transition-metal dichalcogenides with trigonal symmetry. We determine the origin of this counterintuitive sign by calculating the electronic, vibrational, and optical properties of these systems. We show that the sign of the Born effective charge is directly related to the electric field response of the electronic density, and, in turn, to the bonding characteristics of the material.There is a filled anti-bonding molecular orbital at the Fermi level, which is localized on the transition-metal atom and corresponds to a form of solid state $\pi$ back-bonding in these material. We propose a method of determining if other materials display a similar counterintuitive sign, based on their bonding characteristics, and propose experiments which could measure the sign of the Born effective charge using different spectroscopies. [Preview Abstract] |
Thursday, March 16, 2017 1:51PM - 2:03PM |
S33.00012: A first-principles study on second-order ferroelectric phase transition in two-dimensional puckered group V materials. Sang-Hoon Lee, Seung-Hoon Jhi We study two-dimensional group V materials (P, As, Sb, and Bi) in puckered honeycomb structure using first-principles calculations. Two factors, the degree of puckering and buckling characterize not only the atomic structure but also the electronic structure and its topological phase. By analyzing the lone-pair character of constituent elements and the softening of the phonon mode, we clarify the origin of the buckling. We show that the phonon softening leads the second-order type structural phase transition from a flat to a buckled configuration. The inversion symmetry breaking associated with the structural transition induces the spontaneous polarization in these homogenous materials. Our calculations suggest that external strains or n-type doping are effective methods to control the degree of buckling. We find that the ferroelectric and non-trivial topological phase can coexist in puckered Bi when tensile strains are applied. [Preview Abstract] |
Thursday, March 16, 2017 2:03PM - 2:15PM |
S33.00013: Phase Transitions of Bulk Black Phosphorus and Few-Layer Phosphorene from First Principles Keian Noori, Su Ying Quek Bulk black phosphorus (BP) undergoes phase transitions under pressure, transitioning from the orthorhombic to the rhombohedral (A-7) phase at 40-80 kbar [1], and from the A-7 to the simple cubic phase at ca. 110 kbar [1], the latter being superconducting. Phosphorene, a monolayer of the orthorhombic phase of BP, demonstrates many attractive characteristics, including high carrier mobility, high optical and UV absorption, and anisotropic mechanical, electronic, optical, and transport properties [2]. The pressure-induced phase transitions of few-layer phosphorene, however, have not yet been thoroughly studied, and it remains unclear if and at what pressures it transitions to the corresponding rhombohedral and simple cubic phases. In this work we study the pressure-induced phase transitions of bulk BP and few-layer phosphorene from first-principles density functional theory calculations, exploring the complex effects of different exchange-correlation functionals and finite temperature. \par \noindent [1] J. C. Jamieson, Science 139, 1291 (1963); T. Kikegawa and H. Iwasaki, Acta. Cryst. B39, 158 (1983); H. Kawamura, et al. Solid State. Commun. 49, 879 (1983) \par \noindent [2] A. Carvalho, et al., Nat. Rev. Mater. 1, 16061 (2016) [Preview Abstract] |
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