Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session B51: Focus Session: Beyond Graphene: Synthesis, Defects, Structure, and Properties II |
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Sponsoring Units: DMP Chair: Xiaodong Xu, University of Washington Room: Mile High Ballroom 1E |
Monday, March 3, 2014 11:15AM - 11:27AM |
B51.00001: Photoluminescence and reflectivity measurements of single-layer transition-metal dichalcogenides A.T. Hanbicki, M. Currie, D. Gunlycke, A.L. Friedman, G. Kioseoglou, B.T. Jonker Single layers of transition metal dichalcogenides, MX$_{2}$ (M = Mo, W and X = S, Se) have been the focus of intense research recently because they are direct gap semiconductors with inequivalent K-points making them prime candidates for valleytronics. We have performed various optical measurements including photoluminescence (PL) and differential reflectivity in an attempt to elucidate such issues as intervalley scattering, peak assignment and spin-orbit splitting. For all of the MX$_{2}$ materials, excitonic emission is observed with wavelengths in the visible regime with the main emission occurring at the A-exciton. Additionally, identification of the B-exciton yields spin-orbit energies ranging from 170 meV to 390 meV. We will also discuss intervalley scattering based on measurements of the change in optical polarization as a function of excitation energy. This work was supported by ONR directly, and by NRL and the NRL Nanoscience Institute. [Preview Abstract] |
Monday, March 3, 2014 11:27AM - 11:39AM |
B51.00002: Exciton Rydberg series in mono- and few-layer WS2 Alexey Chernikov, Timothy C. Berkelbach, Heather M. Hill, Albert Rigosi, Yilei Li, \"Ozgur B. Aslan, Mark S. Hybertsen, David R. Reichman, Tony F. Heinz Considered a long-awaited semiconducting analogue to graphene, the family of atomically thin transition metal dichalcogenides (TMDs) attracted intense interest in the scientific community due to their remarkable physical properties resulting from the reduced dimensionality. A fundamental manifestation of the two-dimensional nature is a strong increase in the Coulomb interaction. The resulting formation of tightly bound excitons plays a crucial role for a majority of optical and transport phenomena. In our work, we investigate the excitons in atomically thin TMDs by optical micro-spectroscopy and apply a microscopic, \textit{ab-initio} theoretical approach. We observe a full sequence of excited exciton states, i.e., the Rydberg series, in the monolayer WS$_{\mathrm{2}}$, identifying tightly bound excitons with energies exceeding 0.3 eV - almost an order of magnitude higher than in the corresponding, three-dimensional crystal. We also find significant deviations of the excitonic properties from the conventional hydrogenic physics - a direct evidence of a non-uniform dielectric environment. Finally, an excellent quantitative agreement is obtained between the experimental findings and the developed theoretical approach. [Preview Abstract] |
Monday, March 3, 2014 11:39AM - 11:51AM |
B51.00003: Excitons as massless and massive Dirac particles in monolayer transition metal dichalcogenides Hongyi Yu, Guibin Liu, Xiaodong Xu, Wang Yao In monolayer transition metal dichalcogenides, tightly bound excitons can form at +-K valleys, where optical generation of excitonic valley polarization and coherence can be realized through a polarization selection rule. Here, we show that the the electron-hole Coulomb exchange leads to the strong coupling between the valley pseudospin of bright exciton and its motion. In the light cone, the exciton dispersion exhibits a massless Dirac cone with chirality index I=2. Moderate tensile strain provides a powerful approach to tune the exciton dispersion. When the exciton binds an electron to form a negatively charged trion, the exchange interaction with the excess electron opens up a gap and the trion behaves as a massive Dirac particle. With the optical addressability at specifiable momentum and energy, excitons in monolayer transition metal dichalcogenides may provide unique opportunities to study Dirac particles. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:27PM |
B51.00004: Optoelectronic Control of Spin and Pseudospin in Layered WSe$_2$ Invited Speaker: Aaron Jones Coherent manipulation of spin-like quantum numbers facilitates the development of new quantum technologies. Layered transition metal dichalcogenides provide an ideal laboratory to exploit such dynamic control of spin, pseudospin, and their interplay. Here, we discuss two examples based on monolayer and bilayer WSe$_2$. Due to the inversion asymmetry in monolayer WSe$_2$, valley pseudospins, which index the degenerate extrema of the energy-momentum bands, possess circularly polarized optical selection rules. In addition to the generation of valley polarization through optical pumping of valley excitons, we demonstrate the creation of a coherent superposition between valley states in monolayer WSe$_2$ by linearly polarized excitation. On the other hand, bilayer WSe$_2$ provides an additional quantum degree of freedom, the layer pseudospin, which corresponds to layer polarization. In AB stacked bilayers, we find the real spin is locked to layer pseudospin for a given valley, which results in the suppression of spin relaxation and electrical control of spin Zeeman splitting without an applied magnetic field. Additionally, we obtain spectroscopic evidence of interlayer and intralayer trion species, an important step toward coherent optical control in van der Waals 2D heterostructures. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B51.00005: Observation of negative terahertz photoconductivity in monolayer MoS$_{2}$ under femtosecond laser excitation Chun Hung Lui, Alex J. Frenzel, Daniel V. Pilon, Yi-Hsien Lee, Jing Kong, Nuh Gedik We observed a pronounced transient decrease of terahertz conductivity in doped monolayer molybdenum disulfide (MoS$_{2})$ after pulsed laser excitation. This anomalous phenomenon arises from the strong many-body interactions in the system, where optically produced electron-hole pairs join the doped charges to form trions, bound states of two electrons and one hole, and substantially diminish the carrier conductivity by the resulting increase of effective mass. Our results reveal the ultrafast formation and decay of trions in monolayer MoS$_{2}$ and their influence on the conductivity of the material. [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B51.00006: Asymmetry in optical absorption spectra of 2D metal dichalcogenides Deepika Saini, Ramakrishna Podila, Apparao Rao Two dimensional (2D) inorganic materials have recently been shown to exhibit interesting optical phenomena that are strikingly different from their bulk counterparts. For example, exfoliated dichalcogenides exhibit an increased absorption and photoluminescence due to a change in the nature of its bandgap, i.e. a transition from an indirect to a direct bandgap. We find that the bandgap transitions in MoS$_{\mathrm{2}}$ and WS$_{\mathrm{2}}$ in the UV-VIS regime are invariably accompanied by an asymmetric Briet-Wigner-Fano lineshape. In this talk we will discuss the origin of Fano lineshape in terms of phonon scattering and changes in the electronic band structure. [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:03PM |
B51.00007: Effects of metal contact on 2D semiconductors Xiaoping Hong, Sufei Shi, Jonghwan Kim, Yinghui Sun, Feng Wang Thin layers of transition metal dichalcogenides (TMD) such as MoS2 have recently attracted intense interest in both fundamental research and electronic and optoelectronic applications. As atomically thin semiconductors the electronic and optical properties of single layer TMDs are greatly sensitive to their metal contacts. We use optical method to characterize the effects of metal contact on single layer MoS2, which provides valuable information about how to improve the performance of MoS2 based devices. Also it sheds light on possible manipulation of excitons in atomically thin TMDs through their metallic environment. [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B51.00008: Edge Nonlinear Optics on a MoS2 Atomic Monolayer Xiaobo Yin, Ziliang Ye, Daniel Chenet, Yu Ye, Kevin O'Brien, James Hone, Xiang Zhang The structural discontinuity at an interface yields significant electric dipolar contributions to the surface optical nonlinearity, making the highly surface- and molecular-specific second-harmonic spectroscopy an indispensable tool for noninvasive study of surface sciences. Not only does it measure dipolar width across interfaces, but it also probes real-time dynamics of surface, such as atomic reconstructions, charge transfer and molecular conformational transitions. Here we study experimentally the second-order nonlinear optics on the one-dimensional edges of hexagonal molybdenum disulfide (MoS2) atomic membranes. The broken inversion symmetry of the atomically thin monolayer shows strong second-harmonic generation (SHG), in stark contrast to the centrosymmetric bulk material which is immune to the second order nonlinear processes. The destructive interference and annihilation of nonlinear waves from neighboring atomic membranes not only reveals the few-atom-wide line defects that stitch different crystal grains together but also allows the rapid mapping of crystal grains and grain boundaries over large areas which typically requires a cumbersome diffraction-filtered dark-field transmission electron microscope (TEM). More interestingly, this unique optical imaging technique enables the nonlinear optical detection of the electronic edge state at the atomic edges of two-dimensional crystals where the translational symmetry is broken. The observed edge resonance of SHG clearly indicates the electronic structure variation at the atomic edges that have been long suspected to be the active sites for electrocatalytic hydrogen evolutions. [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B51.00009: Dynamics of valley polarized excitations in monolayer MoSe2 using transient spin-grating spectroscopy Fahad Mahmood, Edbert Sie, Yi-Hsien Lee, Jing Kong, Nuh Gedik We report on a transient spin-grating measurement on CVD-grown monolayer MoSe2. Two cross-polarized 1.5 eV short laser pulses interfere on the sample to generate a polarization grating. This selectively induces K and K' valley excitations, the populations of which vary sinusoidally across the surface. The decay of this valley polarization grating is studied through a diffracted probe beam. We find that the decay strongly depends on the initial population and exhibits a characteristic temperature dependence. These results provide important insights into the lifetime and mechanisms for inter-valley scattering as well as possible scattering to dark exciton states in monolayer transition metal dichalcogenides. [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B51.00010: Biexciton formation in monolayer MoS2 as observed by transient absorption spectroscopy Edbert J. Sie, Yi-Hsien Lee, Alex J. Frenzel, Jing Kong, Nuh Gedik We report on the observation of biexcitons and heterobiexcitons in monolayer MoS2 measured using optical pump and probe spectroscopy. The binding energies of these biexcitons were found to be as large as 35 meV and 60 meV, respectively. This renders the four-particle, or even higher-order, electronic correlations stable against thermal fluctuations at room temperature. These results could serve as a guide for first-principle calculations of high-order electronic correlations in 2D atomic crystals, and to facilitate further investigation toward device applications. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B51.00011: Probing the dark excitons in TMDC with two photon absorption spectroscopy Ziliang Ye, Kevin Obrien, Yu Ye, Mervin Zhao, Ying Wang, Jun Xiao, Hanyu Zhu, Xiaobo Yin, Yuan Wang, Xiang Zhang When the transition metal dichalcogenide (TMDC) is reduced to the monolayer, its electronic bandgap is shifted from indirect type to direct type, resulting in a great enhancement in the photoluminescence efficiency. On the other hand, excitons usually play a very important role in determining the materials' optical properties, especially in the low dimensional forms. However, the exciton contribution has yet been experimentally identified in TMDC. Here we use the two photon absorption spectroscopy technique to probe the TMDC dark exciton transition, which has the complementary selection rule to bright exciton transition. By comparing the dark-bright exciton separation with the theoretical model, we confirm the extraordinarily large binding energy of exciton in two-dimensional TMDC. The identification of exciton contribution as well as the electronic band gap size would help to design the TMDC electronics and optoelectronics in the future. [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B51.00012: Valley Carrier Dynamics in Monolayer Molybdenum Disulphide from Helicity Resolved Ultrafast Pump-probe Spectroscopy Dong Sun, Qinsheng Wang, Shaofeng Ge, Xiao Li, Jun Qiu, Yanxin Ji, Ji Feng We investigate the valley related carrier dynamics in monolayer MoS$_{\mathrm{2}}$ using helicity resolved non-degenerate ultrafast pump-probe spectroscopy at the vicinity of the high-symmetry K point under the temperature down to 78 K. Monolayer MoS$_{\mathrm{2}}$ shows remarkable helicity resolved transient reflection signals, in stark contrast to bilayer and bulk MoS$_{\mathrm{2}}$ due to the enhancement of many body effect at reduced dimensionality. The ultrafast time-resolved result shows that the valley polarization is preserved for only around 1 ps before scattering process makes it undistinguishable. We suggest that the dynamical degradation of valley polarization is attributable primarily to the exciton trapping by defect states in the exfoliated MoS$_{\mathrm{2}}$ samples. Our experiment and a tight-binding model analysis also show that the perfect valley CD selectivity is fairly robust against disorder at the K point, but quickly decays from the high-symmetry point in the momentum space in the presence of disorder. [Preview Abstract] |
Monday, March 3, 2014 2:03PM - 2:15PM |
B51.00013: Topologic connection between a (quasi) 2-D hexagonal and 3-D diamond or wurtzite structure Jianwei Wang, Yong Zhang The similarity in the geometrical arrangements of C atoms between the (111) plane of diamond and the basal plane of graphite has long been recognized.[1] Individual (111) monolayers of various IV and III-V compounds have been explored theoretically as candidates for 2-D materials beyond graphene.[2] We further point out the topologic connection between the 2-D hexagonal and 3-D diamond (zinc-blende) or wurtzite structure, namely a buckled 2-D hexagonal structure (e.g., silicene) can be viewed as a partially collapsed 3-D (111) or (0001) monolayer when the monolayer spacing is increased and in the meantime allowing the structure to relax into a (meta)stable configuration. Graphene is one of the special cases the monolayer collapses entirely. Using a density functional theory, we examine this topologic evolution for IV, III-V, and II-VI compounds and calculate the electronic structures for the quasi 2D structures so derived. When large atoms are involved, the weakened $\pi $-bond of the monolayer leads to chemical instability. Capping layers can be used to stabilize the material, which then forms an ultra-thin quantum well or superlattice.[3] [1] Li et al., JAP 73, 711(1993).[2] \c{S}ahin et al., PRB 80, 155453 (2009). [3] Esaki {\&} Tsu, IBM Res. Develop. 14, 61 (1970). [Preview Abstract] |
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