Bulletin of the American Physical Society
2014 Annual Meeting of the Mid-Atlantic Section of the APS
Volume 59, Number 9
Friday–Sunday, October 3–5, 2014; University Park, Pennsylvania
Session C5: 2D Materials - Beyond Graphene |
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Chair: Jie Shan, Pennsylvania State University Room: Life Sciences Building 009 |
Saturday, October 4, 2014 10:30AM - 11:06AM |
C5.00001: Observation of the valley Hall effect in MoS2 transistors Invited Speaker: Kin Fai Mak Two-dimensional (2D) atomic layers of molybdenum disulfide (MoS2) have attracted much recent attention due to their unique electronic properties. In addition to charge and spin, electrons in MoS$_2$ monolayers possess a new valley degree of freedom (DOF) that has finite Berry curvatures. As a result, not only optical control of the valley DOF is allowed, but each valley is also predicted to exhibit an anomalous Hall effect whose sign depends on the valley index. In this talk, we will discuss our recent observation of this new valley Hall effect (VHE) in monolayer MoS$_2$ transistors. This is manifested experimentally as a finite anomalous Hall effect when circularly polarized light is used to preferentially excite electrons into a specific valley. We will describe the dependence of the anomalous Hall conductivity on photon helicity, photon energy, doping levels and crystal symmetry, and will compare these observations with theoretical predictions. Possibilities of using the valley DOF as an information carrier in next-generation electronics and optoelectronics will also be discussed. [Preview Abstract] |
Saturday, October 4, 2014 11:06AM - 11:18AM |
C5.00002: ABSTRACT WITHDRAWN |
Saturday, October 4, 2014 11:18AM - 11:30AM |
C5.00003: Electronic properties of rhenium doped tungsten disulfide monolayers Eduardo Cruz-Silva, Amber McCreary, Nestor Perea-Lopez, Ana Laura Elias, Humberto Terrones, Mauricio Terrones Layered transition metal dichalcogenides (TMDs) have attracted attention due to their electronic and optical properties. In particular, MoS$_2$ and WS$_2$ show an indirect to direct electronic band gap transition when reduced to a monolayer, and display strong photoluminescence. While there are proposed applications for MoS$_2$ and WS$_2$ as electronic and optoelectronic devices, control of their electronic properties needs to be reached before these applications can be scaled. Chemical doping has been recently shown to allow the modification of the electronic properties of MoS$_2$ monolayers by substitution of either transition metals or the chalcogen. Here we present an experimental and computational study of the electronic and optical properties of doped WS$_2$ monolayers. Re-doped WS$_2$ monolayers have been produced by chemical vapor deposition (CVD). Photoluminescence and Raman spectroscopy studies suggest that rhenium atoms have been successfully incorporated into WS$_2$ lattice. \textit{Ab initio} calculations indicate that substitution of W atoms by Re results in the formation of new states in the vicinity of the Fermi energy that allows tailoring of the electronic band gaps, which also results in different optical properties. [Preview Abstract] |
Saturday, October 4, 2014 11:30AM - 11:42AM |
C5.00004: Thermal Conductivity of Monolayer Molybdenum Disulfide Obtained from Temperature-Dependent Raman Spectroscopy J.R. Simpson, R. Yan, M. Watson, D.B. Romero, A. Briggs, X.G. Xing, A.R. Hight Walker Atomically-thin transition metal dichalcogenides (TMDs) offer potential for an alternative to graphene in advanced devices owing to their unique electronic and optical properties. We report the temperature-dependent Raman spectra of the monolayer TMD molybdenum disulfide (MoS$_{2})$. Mechanical exfoliation of bulk MoS$_{2}$ provides monolayer flakes, which are then transferred to either sapphire substrates (with and without HfO$_{2}$ overcoating) or suspended over holes in a Si/Si$_{3}$N$_{4}$ substrate. The temperature dependence of Raman spectra from (100 to 400) K reveals two strong phonon modes, the planar$ E_{2g}^{1}$ and out-of-plane $A_{1g}$, both of which soften linearly with increasing temperature as a result of anharmonic effects. We extract a linear temperature coefficient for both Raman-active modes. These data, when combined with the first-order coefficients from laser power-dependent measurements, enable extraction of the thermal conductivity. The resulting room-temperature thermal conductivity, $\kappa = $ 35 W m$^{-1}$ K$^{-1}$, agrees well with first-principles lattice dynamics simulations, however, this value is significantly lower than that of graphene. The impact of the dielectric and substrate environment on extraction of $\kappa $ will be discussed. Additionally, we will present preliminary Raman spectra of related TMDs, $e.g$., TaSe$_{2}$. [Preview Abstract] |
Saturday, October 4, 2014 11:42AM - 11:54AM |
C5.00005: Strain Engineering of 2D Transition Metal Dichalcogenides Amber McCreary, Matin Amani, Avinash Dongare, Terrance O'Regan, Mauricio Terrones, Raju Namburu, Madan Dubey, Rudresh Ghosh The potential of ultrathin molybdenum disulfide (MoS$_{2})$ nanostructures for applications in electronic and optoelectronic devices requires a fundamental understanding of their electronic structure as a function of strain. The strain dependence of the electronic properties of bilayer sheets of 2H-MoS$_{2}$ has already been studied using \textit{ab initio} simulations based on density functional theory (DFT).\footnote{L. Dong \textit{et al. } \textit{Appl. Phys. Lett}. \textbf{104}, 053107 (2014).} In this work, we use CVD grown bilayer MoS$_{2}$ triangles to verify the predicted results, both through optical and electrical measurements as a function of dynamic and static strains. By transferring the MoS$_{2}$ onto a flexible substrate and performing Raman characterization as a function of uniaxial strain, it was observed that while the monolayer MoS$_{2}$ triangles were able to withstand strains of up to 1.2{\%} before slippage, the bilayer triangles slipped at strains less than or equal to 0.5{\%}, suggesting that it is possible the strain is distributed differently in the two layers. With this in mind, we looked at the Raman as a function of strain for vertically grown triangles of MoS$_{2}$ consisting of 1, 2, 3, 4, and \textgreater 4 layers on a single triangle to study the distribution of strain in multilayered 2D materials. [Preview Abstract] |
Saturday, October 4, 2014 11:54AM - 12:06PM |
C5.00006: Facile Synthesis of MoS$_{2}$ and Mo$_{\mathrm{x}}$W$_{\mathrm{1-x}}$S$_{2}$ Triangular Monolayers Zhong Lin, Michael Thee, Ana Elias, Simin Feng, Chanjing Zhou, Kazunori Fujisawa, Nestor Perea-Lopez, Victor Carozo, Humberto Terrones, Mauricio Terrones Single- and few-layered transition metal dichalcogenides (TMDs) such as MoS$_{2}$ and WS$_{2}$ are emerging two dimensional materials exhibiting numerous and unusual physico-chemical properties that could be advantageous in the fabrication of unprecedented optoelectronic devices. Here we report a novel and alternative route to synthesize triangular monocrystals of MoS$_{2}$ and Mo$_{\mathrm{x}}$W$_{\mathrm{1-x}}$S$_{2}$ by annealing MoS$_{2}$ and MoS$_{2}$/WO$_{3}$ precursors, respectively, in the presence of sulfur vapor. In particular, the Mo$_{\mathrm{x}}$W$_{\mathrm{1-x}}$S$_{2}$ triangular monolayers show gradual concentration profiles of W and Mo whereby Mo concentrates in the islands' center and W is more abundant on the outskirts of the triangular monocrystals. These observations were confirmed by atomic force microscopy (AFM), and high-resolution transmission electron microscopy (HRTEM), as well as Raman and photoluminescence (PL) spectroscopy. The presence of tunable PL signals depending on the Mo$_{\mathrm{x}}$W$_{\mathrm{1-x}}$S$_{2}$ stoichiometries in 2D monocrystals opens up a wide range of applications in electronics and optoelectronics. [Preview Abstract] |
Saturday, October 4, 2014 12:06PM - 12:18PM |
C5.00007: Investigating the Work Function Evolution of WS$_{x}$Se$_{2-x}$ Alloys Jacob Shevrin, Junjie Wang, An Nguyen, Tom Mallouk, Jun Zhu Two-dimensional layered transition metal dichalcogenides (TMDs), such as WS$_{2}$ and WSe$_{2,}$ are an important class of materials because of their novel physical and electrical properties. The work function of the material can inform the choice of metal to use when making contacts and can also provide valuable information regarding the band alignment in heterostructures made of dissimilar materials. Here we present work function measurements of multi-layer WS$_{x}$Se$_{2-x}$ (x ranges from 0 to 2) sheets exfoliated from bulk alloys using Kelvin Probe Force Microscopy. Using a graphite work function W$_{G\, }=$ 4.5 eV as reference, we find the average work function of WS$_{x}$Se$_{2-x}$ to linearly interpolate between W$=$4.52 eV for WSe$_{2}$ to W$=$4.74 eV for WS$_{2}$ as x varies from 0 to 2. At every alloy composition, W varies from sheet to sheet in a range of approximately 0.15 eV. Our experimental results provide useful information to the design of transistors and heterostructures of these materials. [Preview Abstract] |
Saturday, October 4, 2014 12:18PM - 12:30PM |
C5.00008: Center for the Computational Design of Functional Layered Materials: A New Energy Frontier Research Center at Temple University Xiaoxing Xi, John P. Perdew, Maria Iavarone, Xifan Wu, Adrienn Ruzsinszky, Jianwei Sun With Temple as the lead institution, seven universities (including Drexel, Duke, North Carolina State, Pennsylvania, Princeton, and Rice) and one national lab (Brookhaven) have partnered to form a new DOE-supported Energy Frontier Research Center. New or modified materials with desired functionalities play an essential role in the development of clean-energy technologies such as solar cells, batteries, and the generation of hydrogen fuel by water-splitting. Computation of materials properties, based on first-principles theory and modeling, is a promising direction for materials design: quicker and cheaper than experiment, and slower but more reliable than intuition. We aim to design new or defect-modified functional layered materials by theory, modeling, and computation. Candidate materials of special interest will be grown and experimentally characterized in the Center. Detailed experimental and theoretical work will be carried out for catalysis on layered materials, e.g., water splitting. The work of the Center can have important benefits, including new, accurate, and widely useful methods of electronic structure theory, new insights into the materials-by-design problem, and new materials for clean-energy technologies. [Preview Abstract] |
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