Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session Y26: Electron-phonon Interactions in 2D Materials (Raman/ ARPES) |
Hide Abstracts |
Sponsoring Units: DCMP Chair: Jyoti Katoch, The Ohio State University Room: 325 |
Friday, March 18, 2016 11:15AM - 11:27AM |
Y26.00001: Raman characterization of few-layered 1T'-MoTe$_{\mathrm{2}}$ Irving Herman, Dennis Wang, Ali Dadgar, Sang-Wook Cheong, Abhay Pasupathy Transition metal dichalcogenides (TMDs) exhibit a wealth of physical phenomena that have been studied via electronic transport and optical characterization because of their potential device applications in the 2-D limit. In particular, theory has predicted that a certain subset of TMDs, specifically those in the structurally distorted octahedral (1T') phase, are large-gap quantum spin Hall (QSH) insulators. Here we characterize the thickness of one such TMD, 1T'-MoTe$_{\mathrm{2}}$, down to the monolayer limit using Raman spectroscopy and compare our results to atomic force microscopy (AFM) measurements. Our goal is to determine how thinning it down via micromechanical exfoliation changes the intensities and frequencies of its Raman modes, thus enabling one to track layer dependence in a definitive yet minimally invasive manner in much the same way used for graphene and other layered materials. [Preview Abstract] |
Friday, March 18, 2016 11:27AM - 11:39AM |
Y26.00002: ABSTRACT WITHDRAWN |
Friday, March 18, 2016 11:39AM - 11:51AM |
Y26.00003: Resonant-Raman Intensities of N-layer Transition Metal Dichalcogenides from First Principles Henrique Miranda, Guillaume Froehlicher, Ettienne Lorchat, François Fernique, Alejandro Molina-Sánchez, Stéphane Berciaud, Ludger Wirtz Transition metal dichalcogenides (TMDs) have interesting optical and electronic properties that make them good candidates for nano-engineering applications. Raman spectroscopy provides information about the vibrational modes and optical spectrum at the same time: when the laser energy is close to an electronic transition, the intensity is increased due to resonance. We investigate these effects combining different ab initio methods: we obtain ground-state and vibrational properties from density functional theory and the optical absorption spectrum using GW corrections and the Bethe-Salpeter equation to account for the excitonic effects which are known to play an important role in TMDs. Using a quasi-static finite differences approach [1], we calculate the dielectric susceptibility for different light polarizations and different phonon modes in order to determine the Raman tensor of TMDs, in particular of multi-layer and bulk MoTe$_{2}$. We explain recent experimental results for the splitting of high-frequency modes [2] and deviations from the non-resonant Raman model. We also give a brief outlook on possible improvements of the methodology. [1] Y. Gillet et. al., Phys. Rev. B 88, 094305 (2013). [2] G. Froehlicher et. al., Nano Lett. 15, 6481 (2015). [Preview Abstract] |
Friday, March 18, 2016 11:51AM - 12:03PM |
Y26.00004: Raman investigation of molybdenum disulfide with different polytypes Jae-Ung Lee, Kangwon Kim, Songhee Han, Gyeong Hee Ryu, Zonghoon Lee, Hyeonsik Cheong The Raman spectra of molybdenum disulfide (MoS$_{2})$ with different polytypes are investigated. Although 2H-MoS$_{2}$ is most common in nature, the 3R phase can exist due to a small difference in the formation energy. However, only a few studies are reported for the 3R phase, and most studies have focused on the 2H phase. We found the 2H, 3R and mixed phases of exfoliated few-layer MoS$_{2}$ from natural molybdenite crystals. The crystal structures of 2H- and 3R-MoS$_{2}$ are confirmed by the HR-TEM measurements. By using 3 different excitation energies, we compared the Raman spectra of different polytypes in detail. We show that the Raman spectroscopy can be used to identify not only the number of layers but also the polytypes of MoS$_{2}$. [Preview Abstract] |
Friday, March 18, 2016 12:03PM - 12:15PM |
Y26.00005: Observation of anomalous Stokes versus anti-Stokes ratio in MoTe2 atomic layers Thomas Goldstein, Shao-Yu Chen, Di Xiao, Ashwin Ramasubramaniam, Jun Yan We grow hexagonal molybdenum ditelluride (MoTe2), a prototypical transition metal dichalcogenide (TMDC) semiconductor, with chemical vapor transport methods and investigate its atomic layers with Stokes and anti-Stokes Raman scattering. We report observation of all six types of zone center optical phonons. Quite remarkably, the anti-Stokes Raman intensity of the low energy layer-breathing mode becomes more intense than the Stokes peak under certain experimental conditions, creating an illusion of 'negative temperature'. This effect is tunable, and can be switched from anti-Stokes enhancement to suppression by varying the excitation wavelength. We interpret this observation to be a result of resonance effects arising from the C excitons in the vicinity of the Brillouin zone center, which are robust even for multiple layers of MoTe2. The intense anti-Stokes Raman scattering provides a cooling channel for the crystal and opens up opportunities for laser cooling of atomically thin TMDC semiconductor devices. [Preview Abstract] |
Friday, March 18, 2016 12:15PM - 12:27PM |
Y26.00006: ABSTRACT WITHDRAWN |
Friday, March 18, 2016 12:27PM - 12:39PM |
Y26.00007: Magnetism and Raman Spectroscopy of Pristine and Hydrogenated TaSe2 Monolayer tuned by Tensile and Pure Shear Strain Sugata Chowdhury, Jeffrey Simpson, T. L. Einstein, Angela R. Hight Walker 2D-materials with controllable optical, electronic and magnetic properties are desirable for novel nanodevices. Here we studied these properties for both pristine and hydrogenated TaSe2 (TaSe2-H) monolayer (ML) in the framework of DFT using the PAW method. We considered uniaxial and biaxial tensile strain, as well as shear strain along the basal planes in the range between 1\% and 16\%. Previous theoretical works (e.g. APL 107, 032402 (2015)) considered only symmetrical biaxial tensile. Pristine ML is ferromagnetic for uniaxial tensile strain along \^{x} or \^{y}. For tensile strain in \^{y}, the calculated magnetic moments of the Ta atoms are twice those for the same strain in \^{x}. Under pure shear strain (expansion along \^{y} and compression along \^{x}), a pristine ML is ferromagnetic, but becomes non-magnetic when the strain directions are interchanged. Due to carrier-mediated double-exchange, the pristine ML is ferromagnetic when the Se-Ta-Se bond angle is $<$ 82$^{\circ}$ and the ML thickness is $<$ 3.25{\AA}. We find that all Raman-active phonon modes show obvious red-shifting due to bond elongation and the E2 modes degeneracy is lifted as strain increases. For a TaSe2-H ML, the same trends were observed. Results show the ability to tune the properties of 2D-materials. [Preview Abstract] |
Friday, March 18, 2016 12:39PM - 12:51PM |
Y26.00008: Quantum plasmonic nano-imaging of few-layer MoS2. Dmitri Voronine Transition metal dichalcogenides such as MoS2 are promising 2D materials with many applications. Their diffraction-limited optical characterization using Raman spectroscopy provides important structure-functional information. In this work, nanoscale tip-enhanced Raman scattering (TERS) signals of few-layer MoS$_{\mathrm{2}}$ are presented and limits of signal enhancement are investigated by varying the tip-sample gap. Quantum plasmonic quenching of gold photoluminescence signals was observed for subnanometer gaps. Similar quantum plasmonic behavior was observed for more than 3 nm gaps between gold substrate and tip with few-layer MoS$_{\mathrm{2}}$ junctions. These results may be used for designing new generation quantum optoelectronic devices. [Preview Abstract] |
Friday, March 18, 2016 12:51PM - 1:03PM |
Y26.00009: Nanoscale control of phonon excitations in graphene Hyo Won Kim, Wonhee Ko, JiYeon Ku, Seunghwa Ryu, Sung Woo Hwang Phonons, which are collective excitations in a lattice of atoms or molecules, play a major role in determining various physical properties of condensed matter, such as thermal and electrical conductivities. In particular, phonons in graphene interact strongly with electrons; however, unlike in usual metals, these interactions between phonons and massless Dirac fermions appear to mirror the rather complicated physics of those between light and relativistic electrons. Therefore, a fundamental understanding of the underlying physics through systematic studies of phonon interactions and excitations in graphene is crucial for realizing graphene-based devices. In this study, we demonstrate that the local phonon properties of graphene can be controlled at the nanoscale by tuning the interaction strength between graphene and an underlying Pt substrate. Using scanning probe methods, we determine that the reduced interaction due to embedded Ar atoms facilitates electron-phonon excitations, further influencing phonon-assisted inelastic electron tunneling. [Preview Abstract] |
Friday, March 18, 2016 1:03PM - 1:15PM |
Y26.00010: Observation of angle resolved bands of C$_{60}$ by photoemission spectroscopy Claudia Ojeda-Aristizabal, Drew Latzke, Jonathan Delinger, Alex Zettl, Alessandra Lanzara The band structure of a C$_{60}$ thick film deposited in-situ on a crystalline surface is studied by angle resolved photoemission spectroscopy (ARPES). We observe the presence of a well-defined low energy diffraction pattern (LEED) and dispersive HOMO bands, suggesting a crystalline arrangement of the C$_{60}$ molecules. The momentum and photon energy dependence of these bands is presented. [Preview Abstract] |
Friday, March 18, 2016 1:15PM - 1:27PM |
Y26.00011: Optical properties of transition metal dichalcogenide monolayers Benedikt Scharf, Tobias Frank, Martin Gmitra, Jaroslav Fabian, Igor Zutic, Vasili Perebeinos In recent years, 2D materials, such as transition metal dichalcogenide (TMDCs) monolayers, have attracted a great deal of attention due to their excellent transport and optical properties. Using a tight-binding description and the Bethe-Salpeter equation, we theoretically investigate optical and excitonic properties of TMDC monolayers in different setups. Such 2D materials exhibit peculiar screening properties and excitons with large binding energies. This can also lead to important spin-based applications as the spin-orbit coupling in the valence band results in a splitting close to the K and K' points and yields spin-relaxation times orders of magnitude larger than in III-V semiconductors. [Preview Abstract] |
Friday, March 18, 2016 1:27PM - 1:39PM |
Y26.00012: Band Structure Studies of in situ Deposited C$_{60}$ and the Effects of Doping Drew Latzke, Claudia Ojeda-Aristizabal, Jonathan Denlinger, Alex Zettl, Alessandra Lanzara We present electronic band structure studies of the unique system of in situ deposited thin film C$_{60}$ on a bulk substrate through high-resolution angle-resolved photoemission spectroscopy (ARPES) measurements. We discuss the electronic band structure in relation to novel phenomena recently found in other low-dimensional samples. Finally, we investigate the doping dependence of the thin film C$_{60}$ band structure as we deposit dopants on the surface in situ. [Preview Abstract] |
Friday, March 18, 2016 1:39PM - 1:51PM |
Y26.00013: ARPES studies of van der Waals heterostructure Eryin Wang, Xiaobo Lu, Guorui Chen, Alexei V. Fedorov, Yuanbo Zhang, Guangyu Zhang, Shuyun Zhou Van der Waals heterostructures are a novel class of ``materials by design" which are formed by stacking different two-dimensional crystals together via van der Waals interaction. The periodic potential by the Moir$\acute{e}$ superlattice can be used as a control knob for tuning the electronic properties of two dimensional materials and can induce various novel quantum phenomena. Here we report direct electronic structure studies the of a model van der Waals heterostructure using angle-resolved photoemission spectroscopy (ARPES). [Preview Abstract] |
Friday, March 18, 2016 1:51PM - 2:03PM |
Y26.00014: Unified Description of the Optical Phonon Modes in $N$-Layer MoTe$_2$ Guillaume Froehlicher, Etienne Lorchat, Fran\c cois Fernique, Chaitanya Joshi, Alejandro Molina-S\'anchez, Ludger Wirtz, St\'ephane Berciaud $N$-layer transition metal dichalcogenides (denoted MX$_2$) provide a unique platform to investigate the evolution of the physical properties between the bulk (3D) and monolayer (quasi-2D) limits. Here, we present a unified analysis of the optical phonon modes in $N$-layer $2H$-MX$_2$ [1]. The $2H$-phase (or hexagonal phase) is the most common polytype for semiconducting MX$_2$ (such as MoS$_2$). Using Raman spectroscopy, we have measured the manifold of low-frequency (rigid layer), mid-frequency (involving intralayer displacement of the chalcogen atoms only), and high-frequency (involving intralayer displacements of all atoms) Raman-active modes in $N=1$ to $12$ layer $2H$-molybdenenum ditelluride (MoTe$_2$). For each monolayer mode, the $N$-dependent phonon frequencies give rise to fan diagrams that are quantitatively fit to a force constant model. This analysis allows us to deduce the frequencies of $all$ the bulk (including silent) optical phonon modes. [1] G. Froehlicher, E. Lorchat, F. Fernique, C. Joshi, A. Monlina-S\'anchez, L. Wirtz, and S. Berciaud, Unified Description of the Optical Phonon Modes in $N$-Layer MoTe$_2$, Nano Letters, \textbf{15} (10), pp 6481-6489 (2015) [Preview Abstract] |
Friday, March 18, 2016 2:03PM - 2:15PM |
Y26.00015: Mapping wavevector dependent electron-phonon coupling and nonequilibrium phonon dynamics in thin graphite with ultrafast electron diffuse scattering Jean-Philippe Boisvert, Robert P. Chatelain, Mark J. Stern, Mark Sutton, Bradley J. Siwick Radio-frequency compressed ultrafast electron diffraction has been used to probe the coherent and incoherent coupling of impulsive electronic excitation at 1.55 eV (800 nm) to optical and acoustic phonon modes directly from the perspective of the lattice degrees of freedom. Recent improvements in source brightness for ultrafast diffraction experiments are now allowing for the study of diffuse scattering signals. Here, we show that ultrafast electron diffuse scattering (UEDS) can yield time and momentum-resolved phonon population data. Beyond the possibility of directly probing the dynamics of thermalization, this can also be used to determine the wavevector dependent electron-phonon coupling strength in materials. This new information provides significant insights into the electron relaxation pathways of graphitic materials. In particular, we present the first direct measurement of the $K$-point phonon population dynamics after impulsive electronic excitation and maps of the phonon population in momentum-space. Finally, we propose a simple mechanism for the thermalization of graphitic materials after impulsive electronic excitation which unifies all ultrafast measurements of this process to date. [Preview Abstract] |
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