Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session J31: Focus Session: Beyond Graphene: Synthesis, Defects, Structure, and Properties III |
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Sponsoring Units: DMP Chair: Arend van der Zande, Columbia University Room: 607 |
Tuesday, March 4, 2014 2:30PM - 2:42PM |
J31.00001: Effect of monolayer substrates on the electronic structure of single-layer MoS$_{2}$ Alfredo Ramirez Torres, Duy Le, Talat S. Rahman We have performed first-principles calculations based on density functional theory (DFT) to study structural and electronic properties of a single layer of MoS$_{2}$ deposited on single-layer substrates of hexagonal boron nitride (BN), graphene and silicene. All have a honeycomb structure; hence the formation of heterostructures is expected. Since the lattice mismatch between MoS$_{2}$ and these substrates is large, we have considered different periodicities among layers to reduce as far as possible the incommensurability between lattices. Our results show that BN barely affects the electronic structure of isolated single-layer MoS$_{2}$; the DFT gap remains $\sim$1.8 eV. Graphene and silicene severely modify the electronic structure introducing additional states within the optical gap. Adsorption on graphene turns the system into a zero band gap semiconductor bringing the conduction bands of MoS$_{2}$ down to the Fermi level of graphene. Adsorption on silicene shifts both MoS$_{2}$ bands, valence and conduction, towards the silicene Fermi level, in addition to inducing a gap of 55 meV in the silicene itself. We present analysis of possible charge transfer in these systems and discuss the relevance of these hetero structures for practical applications. [Preview Abstract] |
Tuesday, March 4, 2014 2:42PM - 2:54PM |
J31.00002: Phonon Properties in 2D Transition Metal Dichalcogenide Crystals: Symmetry and Dimensionality Matter Yanyuan Zhao, Xin Luo, Hai Li, Paulo Araujo, Hua Zhang, Su Ying Quek, Mildred Dresselhaus, Jun Zhang, Qihua Xiong 2D transition-metal dichalcogenide (TMD) crystals possess lower symmetry compared to their bulk counterparts and thus have different phonon modes and behaviors. While in general the Raman active modes in 3D are also Raman active in 2D, the reverse is not always true due to the reduced symmetry in 2D. Here, using both Raman spectroscopy and first principles calculations, we uncover the ultra-low frequency (5 $\sim$ 55 cm$^{-1})$ interlayer breathing and shear modes in few-layer MoS$_{2}$, MoSe$_{2}$, WS$_{2}$ and WSe$_{2}$, prototypical layered TMDs. The interlayer breathing modes correspond to an optically inactive mode in the bulk, and thus only exist in the 2D case. Remarkably, the frequencies of these modes can be perfectly described using a simple linear chain model. Besides, two new Raman peaks located at 176 cm$^{-1}$ and 310 cm$^{-1}$ were observed ONLY in few-layer WSe$_{2}$, as a result of the lower symmetry in 2D. Our results shed light on a general understanding of the Raman/IR activities of the phonon modes in layered TMD materials and their evolution behaviors from 3D to the 2D. [Preview Abstract] |
Tuesday, March 4, 2014 2:54PM - 3:06PM |
J31.00003: Outstanding mechanical properties of monolayer MoS$_2$ and its application in elastic energy storage Qing Peng, Suvranu De The structural and mechanical properties of graphene-like honeycomb monolayer structures of MoS$_2$(g-MoS$_2$) under various large strains are investigated using density functional theory (DFT). g-MoS$_2$ is mechanically stable and can sustain extra large strains: the ultimate strains are 0.24, 0.37, and 0.26 for armchair, zigzag, and biaxial deformation, respectively. The in-plane stiffness is as high as 120 N/m (184 GPa equivalently). The third, fourth, and fifth order elastic constants are indispensable for accurate modeling of the mechanical properties under strains larger than 0.04, 0.07, and 0.13 respectively. The second order elastic constants, including in-plane stiffness, are predicted to monotonically increase with pressure while the Poisson ratio monotonically decreases with increasing pressure. With the prominent mechanical properties including large ultimate strains and in-plane stiffness, g-MoS$_2$ is a promising candidate of elastic energy storage for clean energy. It possesses a theoretical energy storage capacity as high as 8.8 MJ/L and 1.7 MJ/kg, or 476 Wh/kg, larger than a Li-ion battery and is environmentally friendly. [Preview Abstract] |
Tuesday, March 4, 2014 3:06PM - 3:42PM |
J31.00004: Watching Silica's Dance: Imaging the Structure and Dynamics of the Atomic (Re-) Arrangements in 2D Glass Invited Speaker: David Muller Even though glasses are almost ubiquitous---in our windows, on our iPhones, even on our faces---they are also mysterious. Because glasses are notoriously difficult to study, basic questions like: ``How are the atoms arranged? Where and how do glasses break?'' are still under contention. We use aberration corrected transmission electron microscopy (TEM) to image the atoms in a new two-dimensional phase of silica glass -- freestanding it becomes the world's thinnest pane of glass at only 3-atoms thick, and take a unique look into these questions. Using atom-by-atom imaging and spectroscopy, we are able to reconstruct the full structure and bonding of this 2D glass and identify it as a bi-tetrahedral layer of SiO$_{2}$ [1]. Our images also strikingly resemble Zachariasen's original cartoon models of glasses, drawn in 1932. As such, our work realizes an 80-year-old vision for easily understandable glassy systems and introduces promising methods to test theoretical predictions against experimental data. We image atoms in the disordered solid [1] and track their motions in response to local strain [2]. We directly obtain ring statistics and pair distribution functions that span short-, medium-, and long-range order, and test these against long-standing theoretical predictions of glass structure and dynamics. We use the electron beam to excite atomic rearrangements, producing surprisingly rich and beautiful videos of how a glass bends and breaks, as well as the exchange of atoms at a solid/liquid interface. Detailed analyses of these videos reveal a complex dance of elastic and plastic deformations, phase transitions, and their interplay. These examples illustrate the wide-ranging and fundamental materials physics that can now be studied at atomic-resolution via transmission electron microscopy of two-dimensional glasses. Work in collaboration with: S. Kurasch, U. Kaiser, R. Hovden, Q. Mao, J. Kotakoski, J. S. Alden, A. Shekhawat, A. A. Alemi, J. P. Sethna, P. L. McEuen, A.V. Krasheninnikov, A. Srivastava, V. Skakalova, J. C. Meyer, and J.H. Smet. \\[4pt] [1] P. Y. Huang, et al., \textit{Nano Lett.}, \textbf{12} 1081--1086 (2012).\\[0pt] [2] P. Y. Huang et. al, \textit{Science} \textbf{342}, 224-227 (2013) [Preview Abstract] |
Tuesday, March 4, 2014 3:42PM - 3:54PM |
J31.00005: The formation and pinning of folds in two-dimensional materials Yuanxi Wang, Vincent Crespi The isolation of two-dimensional materials such as graphene, hexagonal boron nitride and transition metal dichalcogenides from their bulk counterparts allows a new kind of crystal defect - a fold. Using density functional theory simulations, we characterize the geometry, energetics and formation mechanism of different kinds of folds in fluorinated graphene and modified molybdenum disulfide sheets with sulfur vacancies and selenium substitutions. Furthermore we demonstrate two methods of pinning a fold once it is formed: by the preferential adsorption of adatoms along the fold-line, or by forcing the registry of the folded sheets via selective p- or n-type doping. The latter can be applied to the design and self-assembly of two-dimensional sheets into more complex geometries. [Preview Abstract] |
Tuesday, March 4, 2014 3:54PM - 4:06PM |
J31.00006: Possible Structural Phase Transitions in Transition Metal Dichalcogenides Engin Durgun, Hasan Sahin, Francois Peeters Most of the the transition metal dichalcogenides (TMD) have graphene-like hexagonal crystal structure which are composed of metal atom layers (M) sandwiched between layers of chalcogen atoms (X) and these structures have MX$_2$ stoichiometry. Chalcogen layers can be stacked on top of each other in two different forms: H phase made of trigonal prismatic holes for metal atoms and T phase that consists staggered chalcogen layers forming octahedral holes for metals. Among the TMDs that have been reported to be stable, individual layers of MoS$_2$, MoSe$_2$, WS$_2$ and WSe$_2$ have 1H structure in their ground state while dichalcogens of Ti, V and Ta prefer the 1T phase. In our study we investigate the physical mechanisms underlying for the possible phase transitions in TMDs. Our calculations based on first-principles techniques reveal that in addition to H and T phases various distorted H and T phases can be also stabilized by point defects. These new phases have entirely different electronic properties. [Preview Abstract] |
Tuesday, March 4, 2014 4:06PM - 4:18PM |
J31.00007: SPE-LEEM Studies on the Surface and Electronic Structure of 2-D Transition Metal Dichalcogenides (Part II) Wencan Jin, Po-Chun Yeh, Nader Zaki, Datong Zhang, Jerzy Sadowski, Abdullah Al-Mahboob, Arend van de Zande, Daniel Chenet, Jerry Dadap, Irving Herman, Peter Sutter, James Hone, Richard Osgood In this work, we studied the surface and electronic structure of monolayer and few-layer exfoliated MoS$_{\mathrm{2}}$ and WSe$_{\mathrm{2}}$, as well as chemical-vapor-deposition (CVD) grown MoS$_{\mathrm{2}}$, using Spectroscopic Photoemission and Low Energy Electron Microscope (SPE-LEEM). LEEM measurements reveal that, unlike exfoliated MoS$_{\mathrm{2}}$, CVD-grown MoS$_{\mathrm{2}}$ exhibits grain-boundary alterations due to surface strain. However, LEEM and micro-probe low energy electron diffraction show that the quality of CVD-grown MoS$_{\mathrm{2}}$ is comparable to that of exfoliated MoS$_{\mathrm{2}}$. Micrometer-scale angle-resolved photoemission spectroscopy (ARPES) measurement on exfoliated MoS$_{\mathrm{2}}$ and WSe$_{\mathrm{2}}$ single-crystals provides direct evidence for the shifting of the valence band maximum from $\Gamma $ to ${\rm K}$, when the layer number is thinned down to one, as predicted by density functional theory. Our measurements of the k-space resolved electronic structure allow for further comparison with other theoretical predictions and with transport measurements. [Preview Abstract] |
Tuesday, March 4, 2014 4:18PM - 4:30PM |
J31.00008: Characterization of large area molybdenum disulphide by low energy electron microscopy K.L. Man, H. Yamaguchi, S. Najmaei, S. Lei, P.M. Ajayan, J. Lou, G. Gupta, A.D. Mohite, K.M. Dani Molybdenum disulphide (MoS$_{2}$) is a new 2D direct-bandgap semiconductor material which has recently attracted substantial interest due to its potential applications in electronics, optics and energy storage. One of the challenges that needed to be overcome is in the large scale synthesis of high quality single crystal MoS$_{2}$. Recently, it is shown that chemical vapor deposition (CVD) is a promising way of in the production of single layer MoS$_{2}$. Here we report our study using low energy electron microscopy (LEEM) of large area MoS$_{2}$ synthesized by CVD technique. The MoS$_{2}$ samples are grown on Si/SiO$_{2}$ substrates and then transferred onto n-doped Si substrates. In the LEEM images, we observe large triangular shaped MoS$_{2}$ flakes along with irregular shaped flakes. Using low energy electron diffraction (LEED) and dark field imaging technique, we identify the triangularly shaped flakes as MoS$_{2}$ single crystal while the irregular ones contain multiple domains orientations. These studies provide insight into the growth of large area single domain MoS$_{2}$ crystals using CVD technique and the transfer process onto different substrates for potential device applications. [Preview Abstract] |
Tuesday, March 4, 2014 4:30PM - 4:42PM |
J31.00009: SPE-LEEM Studies on the Surface and Electronic Structure of 2-D Transition Metal Dichalcogenides Po-Chun Yeh, Wencan Jin, Nader Zaki, Datong Zhang, Jerzy Sadowski, Abdullah Al-Mahboob, Arend van de Zande, Daniel Chenet, Jerry Dadap, Irving Herman, Petter Sutter, James Hone, Richard Osgood In this work, we studied the surface and electronic structure of monolayer and few-layer exfoliated MoS$_{\mathrm{2}}$ and WSe$_{\mathrm{2}}$, as well as chemical-vapor-deposition (CVD) grown MoS$_{\mathrm{2}}$, using Spectroscopic Photoemission and Low Energy Electron Microscope (SPE-LEEM). LEEM measurements reveal that, unlike exfoliated MoS$_{\mathrm{2}}$, CVD-grown MoS$_{\mathrm{2}}$ exhibits grain-boundary alterations due to surface strain. However, LEEM and micro-probe low energy electron diffraction show that the quality of CVD-grown MoS$_{\mathrm{2}}$ is comparable to that of exfoliated MoS$_{\mathrm{2}}$. Micrometer-scale angle-resolved photoemission spectroscopy (ARPES) measurement on exfoliated MoS$_{\mathrm{2}}$ and WSe$_{\mathrm{2}}$ single-crystals provides direct evidence for the shifting of the valence band maximum from $\Gamma $ to K, when the layer number is thinned down to one, as predicted by density functional theory. Our measurements of the k-space resolved electronic structure allow for further comparison with other theoretical predictions and with transport measurements. [Preview Abstract] |
Tuesday, March 4, 2014 4:42PM - 4:54PM |
J31.00010: ABSTRACT WITHDRAWN |
Tuesday, March 4, 2014 4:54PM - 5:30PM |
J31.00011: Inorganic nanotubes and fullerene-like nanoparticles at the crossroad between materials science and nanotechnology Invited Speaker: Reshef Tenne This presentation is aimed at underlying the principles, synthesis, characterization and applications of inorganic nanotubes (INT) and fullerne-like (IF) nanoparticles (NP) from 2-D layered compounds. While the high temperature synthesis and study of IF materials and INT from layered metal dichalcogenides, like WS$_{2}$ and MoS$_{2}$ remain a major challenge, progress with the synthesis of IF and INT structures from various other compounds has been realized, as well. Intercalation and doping of these nanostructures, which lends itself to interesting electronic properties, has been realized, too. Core-shell nanotubular structures, like PbI$_{2}$@WS$_{2}$ and SnS/SnS$_{2}$ and PbS/NbS$_{2}$ nanotubes from ``misfit'' compounds have been recently reported. Re doping of the IF and INT endow them with interesting electrical and other physio-chemical properties. Major progress has been achieved in elucidating the structure of INT and IF using advanced microscopy techniques, like aberration corrected TEM and electron tomography. Also recently, scaling up efforts in collaboration with ``NanoMaterials'' resulted in multikilogram production of (almost) pure multiwall WS$_{2}$ nanotubes phases. Extensive experimental and theoretical analysis of the mechanical properties of individual INT and more recently IF NP was performed casting light on their behavior in the macroscopic world. IF-MS$_{2}$ (M$=$W,Mo, etc) were shown to be superior solid lubricants in variety of forms, including an additive to various lubricating fluids/greases and for various self-lubricating coating. Full commercialization of products based on this technology is taking place now. [Preview Abstract] |
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