Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session R33: Advanced SpectroscopyFocus
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Sponsoring Units: DMP Chair: Edbert Sie, Massachusetts Institute of Technology Room: 296 |
Thursday, March 16, 2017 8:00AM - 8:12AM |
R33.00001: First-principles study of polarized Raman spectra of few-layer IV-VI compounds (GeS, GeSe, SnS, and SnSe) Jia-An Yan, Lucas Webster, David Houston, Danna Doratotaj IV-VI compounds such as GeS, GeSe, SnS, and SnSe are layered compounds with weak interlayer interactions and have attracted recent attention due to their interesting thermoelectric and optoelectronic properties. In this talk, we will present our recent comparative study of the polarized Raman spectra of few-layer MX (with M$=$Ge and Sn, and X$=$S and Se) based on density-functional theory. Possible effects of strain and phase transitions on the Raman response will be compared with available experimental data. [Preview Abstract] |
Thursday, March 16, 2017 8:12AM - 8:24AM |
R33.00002: In-situ Raman and PL spectroscopy of phosphorene under high-pressure Meysam Akhtar, Sahar Pishgar, Gamini Sumanesekra, Jacek Jasinski Few-layer black phosphorus (phosphorene), a novel two-dimensional (2D) material is gaining attention, particularly for electronic applications, because of high carrier mobility (\textasciitilde 10$^{3}$ cm$^{2}$V$^{-1}$s$^{-1})$ and a direct, layer number-dependent bandgap, changing from 0.3 eV for bulk to \textasciitilde 2 eV for a monolayer BP. Several recent theoretical studies have indicated that strain engineering can be a viable strategy to additionally tune the electronic structure of phosphorene. Reversible direct-indirect bandgap and semiconductor-metal transitions have been predicted under compression strain for monolayer as well as few-layer phosphorene. Here, we conducted a systematic experimental study of these phenomena, by \textit{in situ} high-pressure Raman and PL spectroscopy. Few-layer black phosphorus (phosphorene) samples, with varying sizes and number of layers, was prepared by liquid exfoliation, a diamond anvil cell (DAC) was used to create high-pressure conditions (up to \textasciitilde 15 GPa), and \textit{in situ}$,$ optical spectra were measured using a micro-Raman/PL system. The experiment accompanied with theoretical calculations of vibrational modes to a better understanding of high-pressure effects on optical properties and band structure of this material system. [Preview Abstract] |
Thursday, March 16, 2017 8:24AM - 8:36AM |
R33.00003: Ab initio calculation of resonant Raman intensities of transition metal dichalcogenides Henrique Miranda, Sven Reichardt, Alejandro Molina-Sanchez, Ludger Wirtz Raman spectroscopy is used to characterize optical and vibrational properties of materials. Its computational simulation is important for the interpretation of experimental results. Two approaches are the bond polarizability model and density functional perturbation theory. However, both are known to not capture resonance effects. These resonances and quantum interference effects are important to correctly reproduce the intensities as a function of laser energy as, e.g., reported for the case of multi-layer MoTe$_{\mathrm{2}}^{\mathrm{1}}$. We present two fully \textit{ab initio} approaches that overcome this limitation. In the first, we calculate finite difference derivatives of the dielectric susceptibility with the phonon displacements$^{\mathrm{2}}$. In the second we calculate electron-light and electron-phonon matrix elements from density functional theory and use them to evaluate expressions for the Raman intensity derived from time-dependent perturbation theory. These expressions are implemented in a computer code that performs the calculations as a post-processing step. We compare both methods and study the case of triple-layer MoTe$_{\mathrm{2}}$. $^{\mathrm{1}}$ G. Froehlicher et al. Nano Lett. \textbf{15,} 6481 (2015) $^{\mathrm{2\thinspace }}$Y. Gillet et al. Phys. Rev. B \textbf{88,} 094305 (2013) [Preview Abstract] |
Thursday, March 16, 2017 8:36AM - 8:48AM |
R33.00004: Imaging strain distribution in monolayer transition metal dichalcogenides Hongchao Xie, Zefang Wang, Kin Fai Mak, Jie Shan Atomically thin transition metal dichalcogenides (TMDs) have demonstrated many remarkable optical and electronic properties that are of interest from the viewpoint of both fundamental studies and potential applications. Because of the atomic thickness, these materials can also withstand large mechanical deformation, presenting an effective handle for engineering of their physical properties. Control and measurement of strain in atomically thin TMDs is thus needed. In this work, we develop a device structure that can continuously tune the strain level in suspended TMD membranes by an electrostatic force and a hyperspectral imaging method that can map the spatial distribution of strain. As an example, results on strain engineering of the optical properties of monolayer WSe2 will be presented. [Preview Abstract] |
Thursday, March 16, 2017 8:48AM - 9:00AM |
R33.00005: Auger Recombination in monolayer WS2 Paul Cunningham, Kathleen McCreary, Berend Jonker Reduced dimensionality and strong Coulombic interactions are expected to enhance many-body interactions, like Auger recombination, in 2-D semiconductors. This may limit the performance of LEDs and Lasers based on monolayer transition metal dichalcogenides. We use ultrafast transient absorption spectroscopy to measure Auger recombination, e.g. exciton-exciton recombination, in CVD-grown monolayer WS2. We experimentally determine the Auger rate to be 0.089 cm\textasciicircum 2/s at room temperature, which is an order of magnitude larger than the bulk value. This nonradiative pathway dominates for exciton densities larger than 10\textasciicircum 11 cm\textasciicircum -2 and below the Mott density. Higher energy excitation above the A-exciton resonance produces a hot electron-hole gas that precedes exciton formation. Measurements in vacuum remove surface-bound oxygen that neutralizes n-type WS2 so that trions with a binding energy of 30 meV are observed, which decay on an ultrafast time scale. [Preview Abstract] |
Thursday, March 16, 2017 9:00AM - 9:12AM |
R33.00006: Broadband Femtosecond Transient Absorption Spectroscopy for CVD MoS$_{2}$ monolayer Shrouq Aleithan, Maksim Livshits, sudiksha Khadka, Jeffrey Rack, Martin Kordesch, Eric Stinaff Carrier dynamics in monolayer MoS2 have been investigated using broadband femtosecond transient absorption spectroscopy (FTAS). A tunable pump pulse was used while a probe pulse of white-light continuum over the spectral range of 350 nm - 800 nm revealed ground and excited state carrier dynamics. For MoS$_{2}$ we observe previously reported features related to ground state bleaching along with higher energy features that can be related to states identified as the C and D excitons, which have been reported to arise from band nesting. Interestingly, for pump wavelengths both resonant and non-resonant with the A and B excitons, we observe a broad ground state bleach around 2.9 eV, with decay components similar to A and B. Associating this bleach with the band nesting region between K and Gamma in the band structure indicates significant k-space delocalization and overlap among excitonic wave functions identified as A, B, C, and D. Comparison of time dynamics for all features in resonance and non-resonance excitation is consistent with this finding. The results on these dynamics may prove useful to a greater understanding of the electronic structure of this material. [Preview Abstract] |
Thursday, March 16, 2017 9:12AM - 9:48AM |
R33.00007: Many-Body Effects on the Electronic and Optical Properties of Quasi-2D Semiconductors Invited Speaker: Diana Qiu Quasi-two-dimensional (quasi-2D) semiconductors are the subject of intense research interest as platforms for both developing atomically thin devices and exploring novel physics. These are layered materials with covalent bonding in each layer and weak coupling between layers so that individual layers can be easily peeled off. Changes to confinement and screening in reduced dimensions can lead to drastic changes in quasiparticle (QP) and optical properties when compared with bulk materials. We use the GW and GW-BSE methods to explain and predict the QP and optical properties of quasi-2D semiconductors. We find that quasi-2D materials have diverse, strongly-bound excitons including some with unusual (massless) dispersion. Moreover, we explore the effects of the screening environment on the QP and excitonic properties and find that encapsulation and substrate engineering can tune the QP gap and exciton binding energies by an order of magnitude. This sensitivity allows us to treat screening and confinement as separate degrees of freedom, opening new pathways for engineering the properties of low-dimensional materials.\\ \\I would like to acknowledge collaborations with members of the Louie group. This work was supported by U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences Engineering Division, and by National Science Foundation. [Preview Abstract] |
Thursday, March 16, 2017 9:48AM - 10:00AM |
R33.00008: Ultrafast Extreme-UV ARPES Studies of Electron and Exciton Dynamics in the Transition-Metal Dichalcogenide MoSe$_{\mathrm{\mathbf{2}}}$ Jan H. Buss, Frederic Joucken, Yiming Xu, Julian Maklar, He Wang, Changhyun Ko, Sefaattin Tongay, Jinqiao Wu, Robert A. Kaindl Semiconducting transition-metal dichalcogenides exhibit intriguing physical properties, including a large spin-orbit splitting, strong Coulomb interactions, and optical access to the valley degree of freedom. Important insight into the fundamental microscopic interactions can be obtained via studies of the momentum-resolved non-equilibrium carrier dynamics. Here, we present time-resolved ARPES investigations of MoSe$_{\mathrm{2}}$ crystals using high-repetition-rate extreme-UV femtosecond pulses, enabling us to track the electron dynamics within the full Brillouin zone with high sensitivity. After resonantly driving excitons at the K-point, the transient ARPES signals reveal a rapid time evolution governed by inter-valley scattering to the conduction band minimum on a 70-fs time scale. We will discuss the momentum-space dynamics as well as distinct temporal and spectral features that provide evidence for the first observation of excitons via angle-resolved photoemission spectroscopy. [Preview Abstract] |
Thursday, March 16, 2017 10:00AM - 10:12AM |
R33.00009: Attosecond Carrier Dynamics in Quasi-2D SnS$_{\mathrm{2}}$ Oliver Monti, Calley Eads, Dmytro Bandak, Dennis Nordlund, Mahesh Neupane The electronic structure of SnS$_{\mathrm{2}}$, a van der Waals layered semiconductor with minimal spin-orbit splitting and an indirect bandgap in the visible, is highly anisotropic. Here, we investigate ultrafast carrier dynamics in SnS$_{\mathrm{2}}$ as a means to assess intra- and inter-layer coupling. Using resonant photoemission spectroscopy, we show that carrier dynamics in SnS$_{\mathrm{2}}$ are indeed highly anisotropic. Strong intralayer coupling leads to attosecond carrier dynamics within a layer, while interlayer coupling is much weaker and interlayer hopping occurs on much longer time-scales. These differences arise from the different orbitals contributing to the intra- and inter-layer coupling. Our study highlights with atomic detail the different time-scales involved in charge delocalization dynamics in layered materials and paves the way for tailoring layer-to-layer interactions. [Preview Abstract] |
Thursday, March 16, 2017 10:12AM - 10:24AM |
R33.00010: ARPES and XRD study of ReSe$_{\mathrm{2}}$ monolayer Byoung Ki Choi, Soren Ulstrup, Seo Hyoung Chang, Luca Moreschini, Chris Jozwiak, Aaron Bostwick, Eli Rotenberg, Young Jun Chang ReSe2, among many transition metal dichalcogenides materials, has the largest interlayer distance and show direct band gap with strong photoluminescence signal even for thick layers. Its electronic structures should be useful for understanding the unique optical properties. Although many theoretical calculations, experimental evidences are still limited for identifying the band structure of their ultrathin layers. Here, we grew ReSe2 monolayers on graphene/SiC substrates by using MBE. We could perform ARPES and grazing XRD measurements on the ReSe2 films. We could identify that the ReSe$_{\mathrm{2}}$ monolayer has top of valence band near the $\Gamma $ point. We discuss band structure hybridization between ReSe2 and graphene bands. From the XRD analysis, we precisely measured interlayer distance of ReSe2 monolayer and bilayer. [NRF-2014R1A1A1002868, NRF-2016K1A3A7A09005337] [Preview Abstract] |
Thursday, March 16, 2017 10:24AM - 10:36AM |
R33.00011: Linear polarized photoluminescence properties of few-layer ReS$_{\mathrm{\mathbf{2}}}$\textbf{ and ReSe}$_{\mathrm{\mathbf{2}}}$ Zhengguang Lu, Nihar Pradhan, Daniel Rhodes, Shahriar Memaran, Komalavalli Thirunavukkuarasu, Zhigang Jiang, Luis Balicas, Dmitry Smirnov Here, we present layer-, temperature- and polarization-dependent photoluminescence (PL) measurements on ReS$_{\mathrm{2}}$ and ReSe$_{\mathrm{2}}$ thin crystals. The few-layer samples were prepared by mechanical exfoliation of bulk crystals on SiO$_{\mathrm{2}}$/Si substrates and characterized by AFM and low frequency Raman spectroscopy. At 300K, the PL spectra of both ReS$_{\mathrm{2}}$ and ReSe$_{\mathrm{2}}$ show a weak broad peak. A multicomponent PL structure becomes clearly resolved in the low temperature spectra featuring well-separated peaks, and being dominated by two linearly polarized excitonic peaks observed on all samples from bulk to bilayer. Both ReS$_{\mathrm{2}}$ and ReSe$_{\mathrm{2\thinspace }}$exhibit a very similar near-IR anisotropic PL response, although the polarization angle and the relative intensities of these PL peaks depend on the material and vary with the thickness of the layer. [Preview Abstract] |
Thursday, March 16, 2017 10:36AM - 10:48AM |
R33.00012: Probing the Anisotropic Light-Matter Interaction in Ultrathin ReS$_{\mathrm{2}}$ Daniel Chenet, Burak Aslan, Pinshane Huang, Chris Fan, Arend van der Zande, James Hone, Tony Heinz Rhenium disulfide (ReS$_{\mathrm{2}})$ is a semiconducting layered group VII transition metal dichalcogenide that exhibits a stable distorted 1T phase. We demonstrate that the reduced crystal symmetry, as compared to the molybdenum and tungsten dichalcogenides, leads to anisotropic optical properties that persist from the bulk down to the monolayer limit. We find that the direct optical gap blueshifts from 1.47 eV in the bulk to 1.61 eV in the monolayer limit. In the ultrathin limit, we observe polarization-dependent absorption and polarized emission from the band-edge optical transitions. We thus establish ultrathin ReS$_{\mathrm{2}}$ as a birefringent material with strongly polarized direct optical transitions that vary in energy and orientation with sample thickness. We also demonstrate the strong anisotropy in the Raman scattering response for linearly polarized excitation. Polarized Raman scattering is shown to permit a determination of the crystallographic orientation of ReS$_{\mathrm{2}}$ through comparison with direct structural analysis by scanning transmission electron microscopy (STEM). Analysis of the frequency difference of appropriate Raman modes is also shown to provide a means of precisely determining layer thickness up to four layers. [Preview Abstract] |
Thursday, March 16, 2017 10:48AM - 11:00AM |
R33.00013: Probing excitonic effects with valence-electron-energy-loss spectroscopy: application to hexagonal boron nitride MYRON KAPETANAKIS, Ritesh Sachan, Mark Oxley, Maureen Lagos, Philip Batson, William Weber, Matthew Chisholm, Sokrates Pantelides The interplay between surface and bulk phenomena gives rise to a variety of interesting features, especially for lower-dimensionality systems, and provide the opportunity for the realization of applications in, for instance, optoelectronics and photonics. Such features are triggered by excitonic states that are suppressed at the bulk counterparts of the material. Here we use a combination of monochromated, aberration-corrected scanning transmission electron microscopy (STEM) and density functional theory (DFT) calculations to study the effect of excitons on the valence-electron energy-loss (VEEL) spectra of the wide-band-gap hexagonal boron nitride (hBN). The experimental VEEL spectra are acquired using the state-of-art monochromated aberration corrected Nion UltraSTEM with 8 meV energy resolution. Theoretically, the excitonic effects on the VEEL spectra are understood by solving the Bethe-Salpeter equation (BSE). Within the combined theoretical scheme, we are able to study hBN systems with increasing number of sheets and demonstrate the transition from the pure 2D monolayer to the bulk hBN. [Preview Abstract] |
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