Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session F2: Focus Session: Beyond Graphene - Optics in 2D semiconductors II |
Hide Abstracts |
Sponsoring Units: DMP Chair: Sefaattin Tongay , Arizona State University Room: 001B |
Tuesday, March 3, 2015 8:00AM - 8:12AM |
F2.00001: Measurement of the optical dielectric function of transition metal dichalcogenide monolayers: MoS2, MoSe2, WS2 and WSe2 Albert Rigosi, Yilei Li, Alexey Chernikov, Xian Zhang, Heather Hill, Arend van der Zande, Daniel Chenet, En-min Shih, James Hone, Tony Heinz We report a determination of the complex in-plane dielectric function of monolayers of four transition-metal dichalcogenides: MoS$_2$, MoSe$_2$, WS$_2$, and WSe$_2$, for photon energies from 1.5 -- 3 eV. The results were obtained from reflection spectra using a Kramers-Kronig constrained variational analysis. From the dielectric functions, we obtain the absolute absorbance of the monolayers. We also provide a comparison of the dielectric function for the monolayers with the corresponding bulk materials. [Preview Abstract] |
Tuesday, March 3, 2015 8:12AM - 8:24AM |
F2.00002: Optical generation and detection of pure valley current in monolayer transition metal dichalcogenides Wenyu Shan, Jianhui Zhou, Di Xiao Recent years have seen a surge of interest in the manipulation of the valley index of Bloch electrons, largely driven by its potential applications as valleytronics. Transition metal dichalcogenides are new types of two-dimensional materials with spin-valley coupling, and show some promise as a realization of valleytronics. In this work, we propose a practical scheme to generate a pure valley current in monolayer transition metal dichalcogenides by one-photon absorption of linearly polarized light. We show that the pure valley current can be detected by either photoluminescence measurements or the ultrafast pump-probe technique. Our method, together with the previously demonstrated generation of valley polarization, opens up the exciting possibility of ultrafast optical-only manipulation of the valley index. The tilted field effect on the valley current in experiment is also discussed. [Preview Abstract] |
Tuesday, March 3, 2015 8:24AM - 8:36AM |
F2.00003: Photoluminescence and photocurrent measurement in monolayer MoTe$_{2}$ Ya-Qing Bie, Gabriele Grosso, Dmitri Efetov, Efren Navarro-Moratalla, Dirk Englund, Pablo Jarillo-Herrero 2D transition metal dichalcogenides (2D-TMD), such as MoS$_{2}$, WS$_{2}$, WSe$_{2}$, MoSe$_{2}$, have been verified with many remarkable physical properties including the indirect to direct band transition and valley dependent spin polarization. As one of the 2D-TMD family member, monolayer 2H-MoTe$_{2}$ is proved to be a direct bandgap semicoductor with strong spin orbital interaction and a significantly low bandgap $\sim$ 1.10eV. However, the effect of the enhanced coulomb interaction arising from reduced dielectric screening in monolayer MoTe$_{2}$ has yet to be experimentally demonstrated. Here we employ the near infrared (NIR) photoluminescence and photocurrent measurement to study the quasi-particle interactions at different carrier concentration. This study sheds light on manipulating excitons in MoTe$_{2}$ and designing highly efficient NIR optoelectronic devices. [Preview Abstract] |
Tuesday, March 3, 2015 8:36AM - 9:12AM |
F2.00004: Excitons and Valley Dynamics in MoS$_{2}$, MoSe$_{2}$ and WSe$_{2}$ monolayers Invited Speaker: Xavier Marie We have investigated the optical and valley properties for both neutral and charged excitons in transition metal dichalcogenide monolayers (ML): MoS$_{2}$, MoSe$_{2}$ and WSe$_{2}$. In WSe$_{2}$ MLs, we have combined linear and non-linear optical spectroscopy (one and two-photon PLE, Second Harmonic Generation spectroscopy) to uncover the excited states of the neutral exciton. The clear identification of s and p exciton excited states combined with first principle calculations allows us to determine an exciton binding energy of the order of 600 meV. The deviation of the excited exciton spectrum from the standard Rydberg series will be discussed. Moreover we show that exciton valley coherence can be achieved following resonant one or two photon excitation [1]. The neutral and charged exciton dynamics have been measured by time-resolved photoluminescence and pump-probe Kerr rotation dynamics [2,3]. The neutral exciton valley polarization decays within about 6 ps, as a result of the intervalley coupling due the strong electron-hole Coulomb exchange interaction in bright excitons. The temperature dependence is well explained by the developed theory, taking into account the long-range exchange interaction [4]. In contrast the valley polarization decay time for the charged exciton is much longer ($\sim$ 1ns) [5]. Finally we will compare the exciton dynamics in WSe$_{2}$ mono and bi-layers [6] \\[4pt] [1] G. Wang et al, arXiv:1404.0056 (2014)\\[0pt] [2] D. Lagarde et al, PRL 112, 047401 (2014)\\[0pt] [3] C.R. Zhu et al, PRB 90, 161302(R) (2014)\\[0pt] [4] M. Glazov et al, PRB 89, 201302(R) (2014)\\[0pt] [5] G. Wang et al, PRB 90, 075413 (2014)\\[0pt] [6] G. Wang et al, APL 105, 182105 (2014) [Preview Abstract] |
Tuesday, March 3, 2015 9:12AM - 9:24AM |
F2.00005: Imaging the grain boundaries in polycrystalline MoS$_{2}$ monolayer by non-invasive second harmonic generation Jinxin Cheng, Tao Jiang, Qingqing Ji, Yanfeng Zhang, Xingao Gong, Wei-tao Liu, Shiwei Wu Atomically thin transition metal dichalcogenide monolayers have showed intriguing physical properties for high performance quantum electronics. In order to utilize them in technological applications at industrial scale, mass production of this two dimensional materials via chemical vapor deposition (CVD) is demanded and urged. Despite the success of growing large-scale monolayer, limited grain size and emergence of grain boundary remain as the major hurdle being single crystalline sheets. To resolve this issue, it is necessary to image the grain and grain boundary, and further understand their formation with statistical significance. Here we used second harmonic generation (SHG) microscopy, a noninvasive coherent imaging technique, to image the grain and grain boundary in CVD grown monolayer molybdenum disulfide. The destructive interference between neighboring grains enabled us to pinpoint the location of grain boundary; the anisotropic polarization pattern permitted us to determine the type of grain boundary. Furthermore, this high-throughput characterization technique allows statistical analysis of hundreds of grain and grain boundary, unambiguously revealing that the CVD growth mechanism of monolayer MoS$_{2}$. [Preview Abstract] |
Tuesday, March 3, 2015 9:24AM - 9:36AM |
F2.00006: Tunable polaritons from plasmon-phonon coupling in hyperbolic media Siyuan Dai, Qiong Ma, Shou-En Zhu, Mengkun Liu, Trond Andersen, Zhe Fei, Michael Goldflam, Martin Wagner, Kenji Watanabe, Takashi Taniguchi, Mark Thiemens, Fritz Keilmann, G.C.A.M. Janssen, Pablo Jarillo-Herrero, Michael Fogler, D.N. Basov Using infrared nano-imaging and nano-spectroscopy, we report on the tunable hyperbolic response in heterostructures comprised of a monolayer graphene deposited on hexagonal boron nitride (G-hBN). Electrostatic gating of the top graphene layer allows for modification of the wavelength and intensity of hyperbolic phonon polaritons in bulk hBN. When compared with the pristine hBN polaritons, the graphene modified ones exhibit a longer wavelength in the Type II hyperbolic region and shorter wavelength in the Type I region. Because of this modification, we achieve a 90{\%} increase of the propagation length for Type II polaritons in hBN. The physical origin of the modification is attributed to plasmon-phonon coupling in the G-hBN heterostructure. Our work provides a comprehensive study of plasmon-phonon coupling in a hyperbolic medium with the exploration of graphene's potential for modification of collective modes in other materials. [Preview Abstract] |
Tuesday, March 3, 2015 9:36AM - 9:48AM |
F2.00007: Observation of biexciton states in monolayer WSe$_{2}$ Yumeng You, Xiaoxiao Zhang, Timothy Berkelbach, Mark Hybertsen, David Reichman, Tony Heinz We report the identification of strongly-bound biexcitonic states in monolayer crystals of WSe$_{2}$. The presence of biexcitons was identified by the emergence of a new photoluminescence feature at the high exciton density. From the spectral shift of the biexciton emission, we infer a biexciton binding energy of about 50 meV. We also present results on the thermal stability and ultrafast dynamics of the biexciton states. In comparison with the behavior in conventional quantum-well structures, the biexciton binding energy in monolayer WSe$_{2}$ is enhanced by more than an order of magnitude. A variational calculation of the biexciton state reveals that the high binding energy arises not only from strong carrier confinement in two dimensions, but also from the reduced and nonlocal dielectric screening in this atomically thin material. [Preview Abstract] |
Tuesday, March 3, 2015 9:48AM - 10:00AM |
F2.00008: Spontaneous circular polarization of photoluminescence from WS$_{2}$ single layers Thomas Scrace, Yutsung Tsai, Biplob Barman, Peiyao Zhang, Athos Petrou, George Kioseoglou, Marek Korkusinski, Isil Ozfidan, Pawel Hawrylak We have carried out a magnetoluminescence study of WS$_{2}$ single layer crystals excited with linearly polarized light. The photoluminescence (PL) contains two features. The first is associated with the neutral exciton ($X^{0})$; the second feature is due to the recombination of negatively charged excitons ($X^{-})$ in the presence of a two-dimensional electron gas (2DEG). The $X^{-}-2DEG$ feature has a non-zero circular polarization up to 19{\%} at zero magnetic field even though the PL excitation light is linearly polarized. The circular polarization is effected by an external magnetic field applied perpendicular to the crystal plane at $2\% /Tesla$. The zero field circular polarization of the $X^{-}-2DEG$ photoluminescence feature is interpreted as due to the existence of a spontaneously valley polarized 2DEG. This is a new state possible in WS$_{2}$ due to valley and spin locking and a strong electron-electron interaction. [Preview Abstract] |
Tuesday, March 3, 2015 10:00AM - 10:12AM |
F2.00009: Intrinsic Exciton Linewidth in Monolayer Transition Metal Dichalcogenides Kai Hao, Galan Moody, Chandriker Dass, Chang-Hsiao Chen, Lain-Jong Li, Akshay Singh, Kha Tran, Genevieve Clark, Xiaodong Xu, Gunnar Berg\"auser, Ermin Malic, Andreas Knorr, Xiaoqin Li Excitons in monolayer transition metal dichalcogenides (TMDCs) exhibit exceptionally large binding energy, strong optical absorption, and spin valley coupling. These characteristics make TMDCs a promising system for optoelectronics and valleytronics. An important yet unknown property of excitons in TMDCs is the intrinsic homogeneous linewidth, which reflect radiative recombination and irreversible dissipative decay. Here, we use optical coherent two-dimensional spectroscopy to reveal the exciton homogeneous linewidth in monolayer CVD grown Tungsten Diselenide (WSe2). With excitation density and temperature dependent measurements, exciton-exciton interaction and exciton-phonon interactions are quantitatively evaluated. Extrapolating to zero density and temperature, we obtain a residual homogeneous linewidth of $\sim$ 1.5 meV, which places a lower bound of 0.2 ps on the exciton radiative lifetime. This result is consistent with microscopic calculations, which suggest that fast radiative decay of delocalized excitons arises from their large oscillator strength. [Preview Abstract] |
Tuesday, March 3, 2015 10:12AM - 10:24AM |
F2.00010: Absorption spectrum and ultrafast response of monolayer and bilayer transition-metal dichalcogenides Volodymyr Turkowski, Alfredo Ramirez-Torres, Talat S. Rahman We apply a combined time-dependent density functional theory and many-body theory approach to examine the absorption spectrum and nonequilibrium response of monolayer and bilayer MoS2, MoSe2, WS2 and WSe2 systems. In particular, we evaluate the possibility of existence of bound states - excitons and trions in the undoped systems. In a previous work we have already demonstrated [1] that the binding energies of these states in the monolayer systems are large which makes them available for room temperature applications. We analyze the possibility of ultrafast electron-hole separation in bilayer systems through inter-layer hole transfer, and show that such a possibility exists, in agreement with experimental observations. For doped systems we consider the possibility of Mahan excitonic states in monolayers and show that the binding energy for these states is of the order of 10 meV. We perform a detailed analysis of the relaxation of doped monolayers excited by ultrafast laser pulse by taking into account electron-phonon scattering effects, and demonstrate that ultrafast (10-100fs) processes, including luminescence, may be relevant for these materials. \\[4pt] [1] A. Ramirez-Torres, V. Turkowski, and T.S. Rahman, Phys. Rev. B 90, 085419 (2014). [Preview Abstract] |
Tuesday, March 3, 2015 10:24AM - 10:36AM |
F2.00011: Second-Harmonic Generation in a Phase-Match Free Nonlinear 2D Crystal Mervin Zhao, Ziliang Ye, Yu Ye, Hanyu Zhu, Yuan Wang, Xiang Zhang The second harmonic generation (SHG) produced from two-dimensional atomic crystals have been utilized to great effect in studying the grain boundaries and electronic structure of such crystals. However, the SHG in many transition metal dichalcogenides (TMDCs) only occur in odd numbered layers due to their noncentrosymmetric nature, limiting the applicability of their SHG. Here, we probe the SHG from the bulk noncentrosymmetric molybdenum disulfide (MoS2). Whereas the commonly studied 2H crystal phase's antiparallel dipoles in adjacent layers give an oscillatory SH response, the parallel dipoles of each atomic layer in the 3R phase constructively interfere to amplify the second harmonic intensity. Due to this interference, we observed the phase-match free condition yielding a quadratic dependence between the intensity and layer number. Additionally, we probed the layer evolution of the A and B excitonic transitions in 3R-MoS2 using SHG spectroscopy. We find exciton splittings distinct from 2H-MoS2, resulting from the different interlayer interactions of the two polytypes. [Preview Abstract] |
Tuesday, March 3, 2015 10:36AM - 10:48AM |
F2.00012: Light matter interactions in 2D transitional metal dichalcogenides: excitonic emission and valley splitting Ting Yu Two-dimensional (2D) semiconductors, such as transitional-metal-dichalcogenide monolayers (TMD 1Ls), have aroused great interest because of the underlying fundamental physics (e.g. many body effects and wealth excitonic states) and the promising optoelectronic applications such as light-emitting diodes and solar cells. Here, we report excitonic emission and valley splitting of monolayer WS2 and MoS2 under electrical, optical and magnetic manipulation. Through electrical and optical injection of charge carriers, tunable excitonic emission has been realized due to interplay of various excitonic states, and basic binding energies of trions have been extracted. At low temperature, the Zeeman shifts of excitons and trions have been determined by polarization-dependent photoluminescence measurements under perpendicular magnetic fields, which reveal the breaking of valley degeneracy. Our studies provide the fundamental understanding on large excitonic and unique valleytronic effects in TMD 1Ls. Moreover, we also develop multiple strategies for managing the light emission, which opens up many possibilities for improving the performance and creating the multifunction of 2D TMD-based light emitting applications. [Preview Abstract] |
Tuesday, March 3, 2015 10:48AM - 11:00AM |
F2.00013: Optical Properties and Band Gap of Single- and Few-Layer MoTe2 Crystals Ozgur Burak Aslan, Claudia Ruppert, Tony Heinz Single- and few-layer crystals of exfoliated MoTe2 have been characterized spectroscopically by photoluminescence, Raman scattering, and optical absorption measurements. We find that MoTe2 in the monolayer limit displays strong photoluminescence. On the basis of complementary optical absorption results, we conclude that monolayer MoTe2 is a direct-gap semiconductor with an optical band gap of 1.10 eV. This new monolayer material extends the spectral range of atomically thin direct-gap materials from the visible to the near-infrared. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700