Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session P30: Transition Metal Dichalcogenides: Optical PropertiesFocus
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Sponsoring Units: DMP Chair: Manish Chhowalla, Rutgers University Room: 293 |
Wednesday, March 15, 2017 2:30PM - 3:06PM |
P30.00001: Optical and magneto-optical properties of atomically thin transition metal dichalcogenides Invited Speaker: Jie Shan Recent advances in the development of atomically thin layers of van der Waals (vdW) bonded solids have opened up new possibilities for the exploration of two-dimensional (2D) physics as well as for materials for applications. Among them, semiconductor transition metal dichalcogenides have been shown to possess direct bandgaps in the near-infrared to the visible region in the monolayer limit, a property well suited for photonics and optoelectronics applications. This property is also widely tunable by doping, external fields and environmental effects, owing to the materials' atomic thickness. By stacking 2D vdW materials layer-by-layer, we exploit a vdW heterostructure device platform to improve the quality of atomically thin semiconductor transition metal dichalcogenides and to achieve independent control of the doping density and electric field in them. In this talk, we will discuss our recent studies on the doping-dependent optical properties and their dynamics, and the observation of a unique Landau level structure under a magnetic field in 2D WSe2 enabled by the heterostructure devices. [Preview Abstract] |
Wednesday, March 15, 2017 3:06PM - 3:18PM |
P30.00002: Optical properties of monolayer MoS$_{2}$ nanoribbons Guohua Wei, Erik J. Lenferink, Nathaniel P. Stern Confinement of carriers in semiconductors is a powerful mechanism for manipulating optical and electronic properties of materials. Although atomically-thin monolayer semiconductors such as transition metal dichalcogenides naturally confine carriers in the out-of-plane direction, achieving appreciable confinement effects in the in-plane dimensions is less well-studied because their optical processes are dominated by tightly bound excitons. In earlier work, we have shown that lateral confinement effects can be controlled in monolayer MoS$_2$ using high-resolution top-down nanopatterning\footnote{G. Wei, D. Czaplewski, E. Lenferink, T. Stanev, I. Jung, N. Stern. arXiv:1510.09135}. Here, we use similar techniques to create monolayer MoS$_2$ nanoribbons that exhibit size-tunable photoluminescence and anisotropic Raman scattering. Our process also allows characterization of transport properties of the nanoribbons. This approach demonstrates how dimensionality influences monolayer semiconductors, which could impact charge and valley dynamics relevant to nano-scale opto-electronic devices. [Preview Abstract] |
Wednesday, March 15, 2017 3:18PM - 3:30PM |
P30.00003: Probing the intrinsic optical quality of CVD grown MoS$_{\mathrm{2}}$ Amina Zafar, Haiyan Nan, Zainab Zafar, Yumeng You, Zhenhua Ni Optical emission efficiency of two-dimensional layered transition metal dichalcogenides (TMDs) is one of the most important parameters affecting their optoelectronic performance. The optimization of the growth parameters by chemical vapor deposition (CVD) to achieve optoelectronic-grade quality TMDs is, therefore, highly desirable. Here, we present a systematic photoluminescence (PL) spectroscopic approach to assess the intrinsic optical and crystalline quality of CVD grown MoS$_{\mathrm{2}}$ (CVD MoS$_{\mathrm{2}})$. We propose the use of the intensity ratio between the PL measured in air and vacuum as an effective way to monitor the intrinsic optical quality of CVD MoS$_{\mathrm{2}}$. Low-temperature PL measurements are also used to evaluate the structural defects in MoS$_{\mathrm{2}}$, via defect-associated bound exciton emission, which well correlates with the field-effect carrier mobility of MoS$_{\mathrm{2}}$ grown at different temperatures. This work therefore provides a sensitive, noninvasive method to characterize the optical properties of TMDs, allowing the tuning of the growth parameters for the development of optoelectronic devices. [Preview Abstract] |
Wednesday, March 15, 2017 3:30PM - 3:42PM |
P30.00004: Enhanced Photoluminescence of Monolayer WS2 on Ag Films and Nanowire$-$WS2$-$Film Hybrids Fei Cheng, Alex D Johnson, Ping-Hsiang Su, Yutsung Tsai, Chih-Kang Shih Monolayer transition metal dichalcogenides is a promising material for integrated optoelectronic devices. Nevertheless, their small absorption length and moderate photoluminescence (PL) need to be compensated for effective utilization. We demonstrate here an enhanced light-matter interaction in monolayer WS$_{\mathrm{2}}$ by utilizing hybrid plasmonic nanostructures. Since a gain media in close proximity to metal may quench PL, we investigate systematically how the PL of WS$_{\mathrm{2}}$, as a function of temperature, depends on the spacer thickness of hybrids. Unlike typical gain media-plasmonic composites where an optimal thickness of spacer layer is \textasciitilde 5 nm or larger, we find that the maximum enhancement occurs at \textasciitilde 1 nm and the PL is increased by more than an order of magnitude on Ag films due to exciton-coupled surface plasmon polaritons (SPPs). We also explore a composite, Ag nanowire$-$WS$_{\mathrm{2}}-$Ag film, and observe not only additional enhancement of PL (by a factor of 3) by SPPs reflected from wire end but also improvement of epitaxial film over thermal one (by factor of 2), which is attributed to suppressed propagation loss of SPPs on epitaxial films. [Preview Abstract] |
Wednesday, March 15, 2017 3:42PM - 3:54PM |
P30.00005: Coupling and Stacking Order of ReS$_{\mathrm{\mathbf{2}}}$\textbf{~Atomic Layers Revealed by Ultralow Frequency Raman Spectroscopy} Chun Hung Lui, Jia-An Yan, Zongyou Yin, Zhipeng Ye, Gaihua Ye, Jason Cheng, Ju Li, Rui He We investigate the ultralow-frequency Raman response of atomically thin ReS$_{\mathrm{2}}$, a special type of two-dimensional (2D) semiconductors with unique distorted 1T structure. Bilayer and few-layer ReS$_{\mathrm{2}}$~exhibit rich Raman spectra at frequencies below 50 cm$^{\mathrm{-1}}$, where a panoply of interlayer shear and breathing modes are observed. The emergence of these interlayer phonon modes indicate that the ReS$_{\mathrm{2}}$~layers are coupled and orderly stacked. Whereas the interlayer breathing modes behave similarly to those in other 2D layered crystals, the shear modes exhibit distinctive behavior due to the in-plane lattice distortion. In particular, the two shear modes in bilayer ReS$_{\mathrm{2}}$ are nondegenerate and clearly resolved in the Raman spectrum, in contrast to the doubly degenerate shear modes in other 2D materials. By carrying out comprehensive first-principles calculations, we can account for the frequency and Raman intensity of the interlayer modes and determine the stacking order in bilayer ReS$_{\mathrm{2}}$. [Preview Abstract] |
Wednesday, March 15, 2017 3:54PM - 4:06PM |
P30.00006: Nanogap-enhanced Raman spectroscopy of monolayer MoS2 Xifan Wang, Pavlo Zolotavin, Charlotte Evens, Douglas Natelson Surface enhanced Raman spectroscopy (SERS) is a common method to obtain vibrational and chemical information of materials. However SERS relies on plasmon excitation in metallic nanostructures to provide large field enhancements under laser illumination. We have fabricated nanogaps located within Au nanowires linking extended electrodes that gives largest SERS enhancements when the incident light is polarized transverse to the nanowire. With this structure, we are able to combine both vibrational spectroscopy and electronic transport measurements, and study the interactions between applied bias voltage and Raman spectra, as demonstrated in C60 and other molecules.. We report extending this approach to examine MoS2, one of the most popular two-dimensional materials. Due to the energy difference between monolayer MoS2 band gap (1.68eV) and 785 nm excitation laser (1.58eV), Raman spectra of monolayer MoS2 are not easily observed by conventional Raman spectroscopy. We find nanogap enhancement of the Raman spectrum of monolayer MoS2, with a polarization dependence consistent with the plasmonic enhancement in the nanogap. We will present preliminary findings of the effects of source-drain and gate bias on such enhanced Raman spectra. [Preview Abstract] |
Wednesday, March 15, 2017 4:06PM - 4:18PM |
P30.00007: Probing Anisotropy in Transition-Metal Dichalcogenides using Polarized Raman Spectroscopy J. Harding, J. R. Simpson, J.-A. Yan, A. McCreary, M. Terrones, D. Rhodes, L. Balicas, R. Ghosh, S. Banerjee, A. R. Hight Walker Highly-anisotropic, 2D, transition metal-dichalcogenides (TMD) have generated interest as a result of their polarization-dependent, Raman-active vibrational modes. Such polarization dependence offers a possible approach to practically characterize crystallographic axes, which are crucial for orientation-dependent, device applications. We systematically measure the polarized Raman spectra for the first-order Raman active modes in ReS$_2$, an anisotropic TMD. Mechanical exfoliation prepares few- and single-layer ReS$_2$ flakes on SiO$_2$/Si substrates. Control of sample orientation and incident/scattered polarization directions affords acquisition of Raman spectra as a function of the polarization angle. Additionally, we induce anisotropy in MoS$_2$, a normally isotropic TMD, through the application of strain. Monolayer MoS$_2$ is synthesized using CVD and transferred onto flexible PET substrates, to which mechanical strain is applied and polarized Raman spectra acquired.\footnote{A. McCreary \textit{et al.}, ACS Nano \textbf{10}, 3186 (2016).} We will discuss polarized Raman measurements with predictions from density function theory.$^1$ [Preview Abstract] |
Wednesday, March 15, 2017 4:18PM - 4:30PM |
P30.00008: Intrinsic phonon bands in high quality monolayer $T'$ molybdenum ditelluride Shao-Yu Chen, Carl Naylor, Thomas Goldstein, Charlie Johnson, Jun Yan Distorted octahedral ($T')$ transition metal dichalcogenide (TMDC) is a type of layered semimetal that has attracted significant recent attention because of its fascination physical, chemical and nontrivial topological properties.~ Unlike its hexagonal counterpart,~monolayer (1L) $T'$-TMDC is challenging to work with due to rapid sample degradation in air. In this talk, I will discuss well-protected 1L-$T'-$MoTe$_{\mathrm{2}}$~that exhibits sharp and robust intrinsic Raman bands, with intensities about one order of magnitude stronger than those from bulk~$T'$-MoTe$_{\mathrm{2}}$. The high quality samples enable us to reveal for the first time the set of all nine even-parity zone-center optical phonons. Crystal angle and light polarization resolved measurements further indicate that all the intrinsic Raman modes belong to either $z$-mode (vibrating along the zigzag Mo atomic chain) or $m$-modes (vibrating in the mirror plane). Moreover, with the knowledge of vibrational symmetry, we can effectively distinguish the intrinsic modes from Te-metalloid-like modes with energy around 122 and 141 cm$^{\mathrm{-1}}$~which are associated to the sample degradation.~ Our studies offer a powerful non-destructive method for assessing sample quality, providing the fingerprint as well as key insights in understanding the fundamental properties of 1L $T'$-TMDCs. [Preview Abstract] |
Wednesday, March 15, 2017 4:30PM - 4:42PM |
P30.00009: Raman Scattering of Atomic Layers of Mo$_(x)W_(1-x)Te_2$ Alloy Thomas Goldstein, Shao-Yu Chen, Ashwin Ramasubramaniam, Jun Yan The various compounds and phases of atomically thin transition metal chalcogenides (TMDCs) are of great interest for both optical/electrical applications and fundamental physics research. This talk addresses the growth and polarization resolved Raman analysis Mo$_(x)W_(1-x)Te_2$ alloy crystals. In chemical vapor transport (CVT) growth, differing reaction rates between the transport gas and the transition metals leads to crystals of varying composition forming as a function of growth time. For Mo$_(x)W_(1-x)Te_2$ this causes both H phase (low W) and T' phase (high W) crystals to form. Raman analysis of atomically thin H phase exfoliated crystals shows all zone center modes of H$-MoTe_2$ (http://www.nature.com/articles/srep28024), consistent with previous measurements. In addition we observe several new Raman bands. From an analysis of their energies, layer number dependence, and symmetries, two of these additional peaks may be associated with H$-WTe_2.$ [Preview Abstract] |
Wednesday, March 15, 2017 4:42PM - 4:54PM |
P30.00010: Spin-resolved photoemission study of epitaxially grown MoSe$_2$ and WSe$_2$ thin films Sung-Kwan Mo, Yi Zhang, Zahid Hussain, Zhi-Xun Shen, Mauro Fanciulli, Stefan Muff, J. Hugo Dil, Choongyu Hwang We report spin-resolved photoemission results from MoSe$_2$ and WSe$_2$ thin films grown by molecular beam epitaxy on bilayer graphene substrates. We found spin polarization only in single- and tri-layer samples, but not in bi-layer sample, as expected from the inversion symmetry breaking and strong spin-orbit coupling inherent in few-layer thick transition metal dichalcogenides. The spin polarization is mostly along the out-of-plane direction of the sample surface and it is strongly dependent on the light polarization as well as the measurement geometry. This reveals intricate coupling between spin and orbital degrees of freedom in this material class. *S.-K. Mo et al. J. Phys.: Condens. Matter 28, 454001 (2016). [Preview Abstract] |
Wednesday, March 15, 2017 4:54PM - 5:06PM |
P30.00011: Synthesis and integration of TMDs into nonlinear optical devices Ana Laura Elias, Corey Janisch, Alex Cocking, Zhong Lin, Ethan Khan, Nestor Perea-Lopez, Pulickel M. Ajayan, Mauricio Terrones, Humberto Terrones, Zhiwen Liu 2-Dimensional atomic layers constitute and emerging platform for the development of novel multifunctional ultra-thin and transparent materials for novel photonic devices. In particular, several routes have been expored to achive the isolation of single and few layered semiconducting transition metal dichalcogenides (TMDs). Powder pyrolysis is suitable approach for the synthesis of monocrystalline 2D transition metal dichalcogenides (TMDs). We have used this method to grow 2D TMDs into various forms, including pristine MoS2, WS2, as well as their alloys and heterostructures. The properties and growth dynamics of 2D TMDs are greatly affected by the choice of substrate. By transferring the 2D layers into arbitrary substrates, interesting applications of semiconducting TMDs in novel photonic applications can be envisaged. Integration of such devices constitutes a step forward in the exploitation of semiconducting TMDs’ unique extremely large nonlinearity. We have demonstrated the enhanced absorption and photoluminescence generation from MoS2 monolayers coupled with planar nanocavities. Second Harmonic Generation (SHG) in monolayer WS2 will also be discussed. [Preview Abstract] |
Wednesday, March 15, 2017 5:06PM - 5:18PM |
P30.00012: Electric Field-Dependent Photoluminescence in Multilayer Transition Metal Dichalcogenides T. K. Stanev, A. Henning, V. K. Sangwan, N. Speiser, N. P. Stern, L. J. Lauhon, M. C. Hersam, K. Wang, D. Valencia, J. Charles, T. C. Kubis Owing to interlayer coupling, transition metal dichalcogenides (TMDCs) such as MoS$_{\mathrm{2}}$ exhibit strong layer dependence of optical and electronic phenomena such as the band gap and trion and neutral exciton population dynamics. Here, we systematically measure the effect of layer number on the optical response of multilayer MoS$_{\mathrm{2}}$ in an external electric field, observing field and layer number dependent emission energy and photoluminescence intensity. These effects are studied in few (2-6) and bulk (11$+)$ layered structures at low temperatures. In MoS$_{\mathrm{2\thinspace }}$the observed layer dependence arises from several mechanisms, including interlayer charge transfer, band structure, Stark Effect, Fermi level changes, screening, and surface effects, so it can be challenging to isolate how these mechanisms impact the observables. Because it behaves like a stack of weakly interacting monolayers rather than multilayer or bulk, ReS$_{\mathrm{2}}$ provides a comparison to traditional TMDCs to help isolate the underlying physical mechanisms dictating the response of multilayers. [Preview Abstract] |
Wednesday, March 15, 2017 5:18PM - 5:30PM |
P30.00013: In-situ Epitaxial Growth of Lateral WS2/WS2xSe2(1-x)/WS2 Multijunctions with 100-fold Photoconductivity Yutsung Tsai, Zhaodong Chu, Cheng Fei, Alex Johnson, Di Wu, Keji Lai, Xiaoqin Li, Chih-Kang Ken Shih Conventional semiconductor Heterojunctions (HJ) have played a critical role in advanced electronic and photonic devices. Consequently, after the discovery of atomically thin transition metal dichalcogenides (TMDs), as 2D semiconductors, TMD-based HJs have quickly attracted a lot of attentions. TMD HJ can be formed either vertically (in this sense, similar to conventional HJ but with atomically thin individual layers) or laterally (in this case, to a lower dimension). CVD growth has been shown to be a powerful technique to create lateral HJ. However, to bring the technological potential to another level, multiple heterojunctions (MJ) such as quantum wells will need to be developed. Here, we report a successful in-situ 3-step epitaxial growth of lateral WS2/WS2xSe2(1-x)/WS2 MJ by following suitable growth conditions. Photoluminescence (PL) and Raman characterizations have verified our lateral MJ of the triangular core-ring-ring configuration and indicated the composition x to be 0.85. Unexpectedly, Microwave-impedance-microscopy measurements have extracted the photoconductivity in WS2xSe2(1-x) alloy domain to be 100-fold comparing to the photoconductivity in WS2 domain. [Preview Abstract] |
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