Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session K30: Transition Metal Dichalcogenides: Processing and ApplicationsFocus
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Sponsoring Units: DMP Chair: Abhay Pasupathy, Columbia University Room: 293 |
Wednesday, March 15, 2017 8:00AM - 8:12AM |
K30.00001: In situ photothermal oxidation kinetics in MoS2 Rahul Rao, Ahmad Islam, Phillip Campbell, Eric Vogel, Benji Maruyama It is important to understand the thermal and chemical stability of mono- and few-layer MoS$_{\mathrm{2}}$ for their use in applications. Oxidative environments are of particular interest due to the potential for use of MoS$_{\mathrm{2}}$ in electronics, sensing and energy storage. Here we present an in situ study of the oxidation kinetics of few-layer MoS$_{\mathrm{2}}$ over a wide range of temperatures. In situ monitoring of the MoS$_{\mathrm{2}}$ Raman spectra under oxidation revealed a decrease in intensity of the peaks following sigmoidal decay kinetics and that was initiated at temperatures as low as 300 \textordmasculine C. Ex situ resonance Raman spectroscopy, scanning electron and atomic force microscopy analysis indicated breaking up and thinning of the MoS$_{\mathrm{2}}$ films down to mono- and bi-layer regions. The process likely originated at defect sites in the film, and based on the Raman peak frequencies, resulted in p-doped islands. From the temperature dependence of the data we extracted a reaction energy of \textasciitilde 0.54 eV, which can be attributed to oxidation of the MoS$_{\mathrm{2}}$ at defect sites. Finally, oxidation of films with varying defect densities revealed a clear dependence of oxidation rate and reaction energy on structural defects. [Preview Abstract] |
Wednesday, March 15, 2017 8:12AM - 8:24AM |
K30.00002: Defect formation on MoS$_{\mathrm{2}}$ via methanol to methoxy conversion Prescott Evans, Hae Kyung Jeong, Sumit Beniwal, Peter Dowben, Ludwig Bartels, Duy Ley, Talat Rahman Coverage dependent defect formation, via methanol adsorption on MoS$_{\mathrm{2\thinspace }}$and conversion into methoxy, was investigated utilizing scanning tunneling microscopy. Low Temperature adsorption of methanol on MoS$_{\mathrm{2}}$ at 110 K followed by annealing of the sample near 350 K conversion as well as adsorption of methanol on MoS$_{\mathrm{2}}$ at 350 K results in the formation of numerous point defects at the surface of the MoS$_{\mathrm{2}}$ substrate. Larger multi-point defects, nominally \textasciitilde 1 nm in size as well as line defects on the MoS$_{\mathrm{2}}$ sample surface become increasingly apparent with multiple cycles of methanol exposure and annealing. Preliminary luminescence studies of extensive methanol exposure to MoS$_{\mathrm{2}}$ supports defect formation in MoS$_{\mathrm{2\thinspace }}$monolayers, based on the significant quenching of luminescence. Temperature dependent luminescence of micro-particles of MoS$_{\mathrm{2}}$ with excess methanol again indicate compositional changes and defect formation of MoS$_{\mathrm{2}}$ via observed color change of material and sharp quenching of luminescence near the 350 K conversion temperature. The experimental results will be compared with density functional theory. [Preview Abstract] |
Wednesday, March 15, 2017 8:24AM - 8:36AM |
K30.00003: In-plane Negative Poisson’s Ratios in 1T-Type Crystalline Two-Dimensional Transition Metal Dichalcogenides Liping Yu, Qimin Yan, Adrienn Ruzsinszky Materials with a negative Poisson’s ratio, also known as auxetics, exhibit counterintuitive mechanical behavior -- becoming fatter when stretched and thinner when compressed. Such materials have enormous potential in many applications such as biomedicine and sensors but are exceedingly rare in nature. Despite that a variety of man-made auxetic materials have been discovered and fabricated, almost all of them are bulk materials with specially engineered porous structure with low density and stiffness. In this work, using first-principles calculations, we discover twelve single-layer two-dimensional transition metal dichalcogenides, MX$_2$ (M = Mo, W, Tc, Re; X = S, Se, Te), exhibiting intrinsic in-plane negative Poisson’s ratios in their 1T-type crystalline structure. The in-plane stiffness is predicted to be in the order of 10$^2$ GPa, at least three orders higher than most man-made auxetic materials. We attribute the occurrence of such auxetic behavior to the strong coupling between the chalcogen p orbitals and the intermetal t$_{2g}$-bonding orbitals within the basic triangular pyramid structure unit. [Preview Abstract] |
Wednesday, March 15, 2017 8:36AM - 8:48AM |
K30.00004: Electron-doping by hydrogen in transition-metal dichalcogenides Sehoon Oh, Seongil Im, Hyoung Joon Choi Using first-principles calculations, we investigate the atomic and electronic structures of 2H-phase transition-metal dichalcogenides (TMDC), 2H-MX$_{2}$, with and without defects, where M is Mo or W and X is S, Se or Te. We find that doping of atomic hydrogen on 2H-MX$_{2}$ induces electron doping in the conduction band. To understand the mechanism of this electron doping, we analyze the electronic structures with and without impurities. We also calculate the diffusion energy barrier to discuss the spatial stability of the doping. Based on these results, we suggest a possible way to fabricate elaborately-patterned circuits by modulating the carrier type of 2H-MoTe$_{2}$. We also discuss possible applications of this doping in designing nano-devices. This work was supported by NRF of Korea (Grant No. 2011-0018306) and KISTI supercomputing center (Project No. KSC-2016-C3-0052). [Preview Abstract] |
Wednesday, March 15, 2017 8:48AM - 9:00AM |
K30.00005: Effect of hydrogen adsorption on the structure of the basal plane of MoS$_2$ Duy Le, Zahra Hooshmand, Talat S. Rahman The ability to generate and tailor sulfur vacancies on the basal plane of transition metal dichalcogenide is of interest because it opens the way to utilize the material for catalytic applications. We will present our density functional theory based study of the effect of H$_2$ adsorption on the structure of the basal plane of single-layer MoS$_2$. We will show that the 1T’ phase of MoS$_2$ can be a catalyst for H$_2$ dissociation, that the dissociation barrier is about 1.15 eV, and that the resultant atomic H can extract S atoms from the basal plane to create vacancies after overcoming a barrier of 0.15 eV. In contrast, H$_2$ is found to have hardly any effect on the 2H phase of MoS$_2$. Moreover, in the lateral heterostructure composed of the 2H and 1T’ phases of single-layer MoS$_2$, the spillover of atomic hydrogen from the 1T’ to 2H phase is expected. Once the atomic hydrogen is on the 2H phase, it is mobile and also has the propensity to bind with sulfur atom to form vacancies. We will also discuss the effect of temperature and pressure on defect creation on the basal plane of MoS$_2$. [Preview Abstract] |
Wednesday, March 15, 2017 9:00AM - 9:12AM |
K30.00006: Zinc doping of large-area MoS2 films via chemical vapor deposition Enzhi Xu, Haoming Liu, Kyungwha Park, Zhen Li, Yaroslav Losovyj, Matthew Starr, Madilynn Werbianskyj, Herbert Fertig, Shixiong Zhang Atomically thin molybdenum disulfide (MoS2) has attracted significant attention because of its great potential for electronic and optoelectronic applications. Undoped MoS2 is n-type presumably due to the formation of native defects, and realizing p-type conduction has often turned out to be challenging. In this work, we report on the synthesis and characterizations of large-area Zn-doped MoS2 thin films in which the zinc dopant is demonstrated to be p-type. The films were grown by chemical vapor deposition and are monolayers or bilayers with a lateral dimension on the order of millimeters. The p-type nature of Zn dopants was evidenced by the suppression of n-type conduction and a downward shift of the Fermi level with doping. Density-functional-theory calculations were carried out to demonstrate the stability of the Zn dopants and to determine the impurity states. A p-type gate transfer characteristic was observed after the Zn-MoS2 film was thermally annealed in a sulfur atmosphere. [Preview Abstract] |
Wednesday, March 15, 2017 9:12AM - 9:48AM |
K30.00007: 2D dichalcogenide electronic materials and devices Invited Speaker: Andras Kis The discovery of graphene marked the start of research in 2D electronic materials which was expanded in new directions with MoS2 and other layered semiconducting materials. They have a wide range of promising potential applications, including those in digital electronics, optoelectronics and flexible devices. Combining 2D materials in heterostructures can increase their reach even further. In my talk, I will present our recent efforts in growing 2D semiconducting transition metal dichalcogenides (TMDCs) and heterostructures using a variety of techniques such as CVD and MBE, starting from epitaxial growth of MoS2 on sapphire with a high degree of control over lattice orientation. Next, I will show our work on atomically thin rhenium disulphide (ReS$_{\mathrm{2}})$ liquid-electrolyte gated transistors with atypical behaviour at high charge densities related to the peculiar band structure of this material. I will finish by presenting new results on spin/valley transport in semiconducting monolayer TMDC materials. [Preview Abstract] |
Wednesday, March 15, 2017 9:48AM - 10:00AM |
K30.00008: Effect of single-layer MoS$_{\mathrm{2}}$ support on the geometry and electronic structure of transition metal nanoparticles Takat B. Rawal, Duy Le, Talat S. Rahman We present results of density functional theory based calculations of the geometry and electronic structure of small (29-atom) transition (TM) nanoparticles (NPs) supported on single-layer MoS$_{\mathrm{2}}$, with and without S-vacancy defects. Among the prototypes considered, the NPs bind more strongly on defect-laden rather than pristine MoS$_{\mathrm{2}}$ -- in the order Cu$_{\mathrm{29}}$\textgreater Ag$_{\mathrm{29}}$\textgreater Au$_{\mathrm{29}}$ for defect-laden and of Cu$_{\mathrm{29}}$\textgreater Au$_{\mathrm{29}}$\textgreater Ag$_{\mathrm{29}}$ for pristine. Interestingly, van der Waals interactions play a stronger role in the case of Au$_{\mathrm{29}}$ than in the other two. Strong interaction between the NPs and defect-laden MoS$_{\mathrm{2}}$ is also facilitated by the close contact of their ``boat-shape'' with v-like-shape of single-layer MoS$_{\mathrm{2}}$ formed when laden with S vacancies. We also find that the trend for the charge transfer from NPs to MoS$_{\mathrm{2}}$, regardless of its form (pristine or defect-laden)$_{\mathrm{,}}$ is$_{\mathrm{\thinspace }}$Cu$_{\mathrm{29}}$\textgreater Ag$_{\mathrm{29}}$\textgreater Au$_{\mathrm{29}}$ and that defect-laden MoS$_{\mathrm{2}}$ donates more charge than its pristine counterpart does. Among all NPs, the largest shift of d-band center toward the Fermi level occurs in the case of defect-laden-supported Au$_{\mathrm{29}}$. Our results suggest that defect-laden MoS$_{\mathrm{2}}$ is good support for anchoring and for tuning electronic properties of TM NPs, which have potential in catalytic applications. [Preview Abstract] |
Wednesday, March 15, 2017 10:00AM - 10:12AM |
K30.00009: hBN-encapsulated Group-VI TMDC van der Waals Heterostructures : Fabrication and Optical properties Mitsuhiro Okada, Yusuke Kureishi, Shohei Higuchi, Kenji Watanabe, Takashi Taniguchi, Hisanori Shinohara, Ryo Kitaura Two-dimensional (2D) semiconductors, including MoS$_{\mathrm{2}}$, WS$_{\mathrm{2}}$, MoSe$_{\mathrm{2}}$, etc., have provided a fascinating opportunity to explore optical properties in 2 dimensions. In particular, van der Waals (vdW) heterostructures composed of these 2D semiconductors, such as WS$_{\mathrm{2}}$/MoS$_{\mathrm{2}}$, offer a novel platform for optical physics, where strong inter-layer interaction drastically alters optical transitions. To explore the intrinsic properties of vdW heterostructures, high-quality samples are indispensable. Here, we report preparation and optical properties of high-quality vdW heterostructure (WS$_{\mathrm{2}}$/MoS$_{\mathrm{2}})$ which are fully encapsulated by hexagonal boron nitride (hBN). The hBN-encapsulated heterostructures were prepared through the polymer-assisted dry-transfer process, where control of the stacking angle was possible. Fabricated heterostructures show PL emissions not only from A-exciton peaks but also from interlayer excitons at 1.4-1.7 eV. The observed peaks from interlayer excitons can be decomposed into three peaks, which due to an ideal flatness of the hBN-encapsulated sample. [Preview Abstract] |
Wednesday, March 15, 2017 10:12AM - 10:24AM |
K30.00010: Exploration of vertical dipole in 2D layered polar crystal Jun Xiao, Hanyu Zhu, AngYu Lu, Yuan Wang, Lain-Jong Li, Xiang Zhang Conventional two-dimensional (2D) transition metal dichalcogenide (TMD) monolayers such as MoS$_{\mathrm{2}}$ have attracted huge interest for their unique valley and excitonic physics. Additional freedom in spin/valley manipulation and functionalization can be introduced by breaking out-of-plane mirror symmetry, but has yet been experimentally realized so far. Here we report first study on monolayer polar TMD. With advanced optical and mechanical characterization techniques, we demonstrated the presence of vertical dipole and related functionality in such synthetic polar crystal. Our work opens the way to achieve asymmetric TMD monolayer by design, providing an excellent platform for studying dipole-coupled physics and out-of-plane motion control at 2D limit. [Preview Abstract] |
Wednesday, March 15, 2017 10:24AM - 10:36AM |
K30.00011: Effect of surface relaxation and inter-layer interaction on electronic properties of lattice-matched 2D/3D Heterostructures: A first-principles study Mahesh Neupane, Decarlos Taylor, Edward Byrd, Dmitry Ruzmetov, Terrance O'Regan, Tony Ivanov Transition metal dichalcogenide two-dimensional (2D) materials have shown great potential as next-generation materials for the post-Si era mainly due to their layer-dependent electronic and optical properties. Using these features of 2D materials, many exploratory devices using small-scale exfoliated samples have been designed and studied. Transfer-free large scale growth of 2D materials, however, remains elusive. However, recent large scale growth efforts focusing on defect-free growth of atomically thin 2D materials on 3D substrates such as SiO$_{\mathrm{2\thinspace }}$[1], SiC [2], and GaN [3, 4] have shown some early promise for technologically relevant 2D on 3D growth. Most of these experimental studies highlight the importance of bulk-like surface relaxation in 3D substrates and strong 2D/3D surface interactions during growth. Motivated by these recent experimental advancements, we performed a theoretical/computational study of 2D (MoS$_{\mathrm{2}})$/3D (GaN) heterostructure using a first-principle calculation. An attempt will be made to establish a structure-property relationships in the MoS$_{\mathrm{2}}$/GaN heterostructures by correlating the MoS$_{\mathrm{2}}$-GaN surface interaction and the electronic properties such as band gap and work-function.1. ACS Nano 2016, 10, 2819-2826; 2. Appl. Phys. Lett 105, 203404, 2014; 3. ACS Nano 2016, 10, 3580-3588; 4. ACS Appl. Mater. Interfaces 2016, 8, 20267$-$20273. [Preview Abstract] |
Wednesday, March 15, 2017 10:36AM - 10:48AM |
K30.00012: Raman study of atomically thin GaSe: photo induced oxidation and identification of polar vibrational modes Alaric Bergeron, John Ibrahim, Richard Leonelli, Sebastien Francoeur GaSe is a direct gap 2D material with strong non-linear optical coefficients and distinctive interband selection rules, making it interesting for optoelectronic and spintronic applications. In this presentation, we use Raman spectroscopy to address two important issues relating to GaSe. First, we investigate the rapid photo-oxidation of GaSe in ambient conditions and the emergence of several oxidation products: Ga$_2$Se$_3$, Ga$_2$O$_3$ and Se. Oxidation rates increase exponentially with illumination power and no safe optical power threshold exists for atomically thin GaSe. Since this process simultaneously requires oxygen, humidity, and optical excitation, removing one of these three effectively protects GaSe. Second, the assignment of polar phonons remains conflicted in the literature. Here, we measure and model the polarization-resolved Raman signal as a function of wave vector orientation. We find that two features in the 230-260 cm$^{-1}$ range exhibit unexpected selection rules and wave vector dependence. We develop a polar phonon model to explain this behavior and identify the crystalline vibrational modes. [Preview Abstract] |
Wednesday, March 15, 2017 10:48AM - 11:00AM |
K30.00013: In-Plane Raman Spectra and Electrical Anisotropies of Layered Tin Selenide Xiaolong Xu, Lun Dai, Yu Ye Tin selenide has recently attracted particular interest by showing an unexpectedly low thermal conductivity and high power factor, providing great potential for thermoelectric applications. However, tin selenide shows strong anisotropy of their respective power factor. A complete study of the optical and electrical anisotropies of SnSe nanoplates can lead to material/device designs with better performances. In this paper,we synthetize the single-crystal SnSe nanoplates (NPs) on mica substrates by chemical vapor deposition(CVD). We have systemically studied the in-plane anisotropy of Raman modes in SnSe nanoplate by angle dependent and polarized Raman spectroscopy. We also demonstrate that the angular dependence of the Raman response drastically depends on the incoming photon energy and the thickness of SnSe NP. We have also performed the angle-resolved conductance measurement of SnSe NP using 12 electrodes on the same flake spaced at an angle of 30°. Therefore, our work offers key insights into the light-matter interaction and electrical property in anisotropic SnSe layered material, thereby paving a coherent route for advancing the study of their anisotropic thermal and thermoelectric properties. [Preview Abstract] |
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