Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session M52: Surface Studies of Transition Metal Dichalcogenides |
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Sponsoring Units: DCMP Chair: Daniel Dougherty, North Carolina State University Room: Mile High Ballroom 1E |
Wednesday, March 4, 2020 11:15AM - 11:27AM |
M52.00001: Surface Potential Mapping of MoS2 Monolayers on Au Nanostripe Arrays Soyeong Kwon, Yugyeong Je, Jungeun Song, Eunah Kim, Bora Kim, Sang Wook Lee, Dongwook Kim Understanding the physical properties of metal-semiconductor contacts is crucial for both electrical characterizations and device applications. In this work, MoS2 monolayers were transferred on Au nanostripe arrays. The surface potential distributions of the samples in dark and under light illumination were investigated using Kelvin probe force microscopy, which has a few tens of nm spatial resolution. To explain the surface potential contrast change in dark and light, we proposed the energy band diagrams considering the strain and electronic interaction. Optical micro-reflectivity spectra was also obtained and compared with numerical simulation results. The relationship between the surface potential maps and the optical characteristics of the samples will be discussed in this presentation. |
Wednesday, March 4, 2020 11:27AM - 11:39AM |
M52.00002: Studies of Band Alignment in WS2/MoS2/Al2O3 Heterostructures using Kelvin Probe Force Microscopy Bora Kim, Po-Cheng Tsai, Jayeong Kim, Eunah Kim, Soyeong Kwon, Seokhyun Yoon, Shih-Yen Lin, Dongwook Kim Two-dimensional (2D) crystals can be assembled into van der Waals (vdW) heterostructures by mechanical exfoliation or chemical vapor deposition. Charge transport and recombination processes are determined by the band alignment in the heterostructures. Therefore, investigation of the band alignment is a crucial step toward the applications of the 2D vdW heterostructures. In this work, we fabricated WS2/MoS2 heterostructures on single crystal Al2O3 wafers, using sulfurization of pre-deposited transition metal layers. The surface potential of the sample in dark and light was measured using Kelvin probe force microscopy, which allowed us to suggest the band diagrams of the vdW heterostructures. The measured Raman spectra of the heterostructures supported the proposed band alignment, considering the electronic interaction at the interface and the interlayer screening effects. |
Wednesday, March 4, 2020 11:39AM - 11:51AM |
M52.00003: Mottness versus unit-cell doubling as the driver of the insulating state in 1T-TaS2 Christopher Butler, Masaro Yoshida, Tetsuo Hanaguri, Yoshihiro Iwasa Recent debate about the nature of the insulating ground state in 1T-TaS2 has largely hinged around the stacking pattern of its quasi-2D charge ordered layers. The stacking determines whether the 3D unit-cell contains one or two ‘Star-of-David’ (SD) clusters, each of which hosts 13 unpaired Ta orbitals. For simple stacking with one SD per cell, the insulating state must be explained by Mott physics, but for stacking with a dimerized pair of SDs per cell, ab initio calculations predict a band insulator [1,2]. |
Wednesday, March 4, 2020 11:51AM - 12:03PM |
M52.00004: Confinement Heteroepitaxy: A Novel Route for Realizaing Exotic 2D Materials Alexander Vera, Wilson Yanez, Natalie Briggs, Timothy Bowen, Siavash Rajabpour, Nitin Samarth, JOSHUA ROBINSON Advancement of nascent electronic quantum frontiers (spintronics, photonics, and sensing) has been mediated through development of materials platforms with non-trivial band topologies, allowing distinct access to quantum information otherwise lost through decoherence. However, access to the more exotic quantum phenomena within these frontiers still pose a materials challenge due to synthesis difficulties or small windows of performance, which render these routes limited in scalability. We have demonstrated facile synthesis of air-stable, atomically thin, single crystal two-dimensional (2D) metals (gallium, indium, tin, lead, and silver) within a silicon carbide and epitaxial graphene interface through high-pressure intercalation (i.e. confinement heteroepitaxy, or CHet). The non-centrosymmetric nature, coupled with potentially strong spin orbit coupling and superconductivity, in these 2D metals, suggest this novel synthesis route as an avenue for manifesting exotic effects (such as topological superconductivity) due to non-trivial band topology. |
Wednesday, March 4, 2020 12:03PM - 12:15PM |
M52.00005: Surface Engineering of Monolayer MoS2 for Atomic Layer Deposition of TiO2 Jaron Kropp, Can Ataca, Theodosia Gougousi We investigate methods to improve the growth of high-quality TiO2 films on MoS2 via atomic layer deposition (ALD). Untreated MoS2 surfaces are hydrophobic, and as a result, ALD results in discontinuous films that nucleate only on defect sites. Since pristine MoS2 monolayers have low defect density, argon ion bombardment is used to create sulfur vacancies. The vacancy concentration is monitored in-situ with x-ray photoelectron spectroscopy. We use thiols to passivate the vacancies and act as a seed layer for ALD. However, spontaneous oxidation of the defective MoS2 layer can occur, and ALD on argon sputtered MoS2 results in uniform TiO2 films with and without the thiol treatment. Without thiol treatment, dissociative adsorption of oxygen molecules on the sulfur vacancies may seed the ALD by leaving behind adsorbed atomic oxygen. With density functional theory, we investigate the electronic properties of defective, thiol- and oxygen-passivated sulfur vacancies in MoS2. Thiol- or oxygen-passivated MoS2 retains desirable electronic properties. Careful control of the sulfur vacancy concentration followed by functionalization can provide a means of tuning the electronic properties of MoS2 and providing seed sites for high quality ALD film growth. |
Wednesday, March 4, 2020 12:15PM - 12:27PM |
M52.00006: Photoluminescence and Surface Potential Characterizations of MoS2 Monolayers on Periodic Au Nanostructures Jungeun Song, Soyeong Kwon, Eunah Kim, Bora Kim, Dongwook Kim, Seongyeon Lee, Kiju Yee MoS2 monolayers were prepared on hexagonal arrays of Au nanotriangles (NTs) and nanoholes (NHs), fabricated by nanosphere lithography. Photoluminescence (PL) and optical reflectivity spectra of the samples on the Au NT and NH arrays were investigated. The surface plasmon excitation and Wood anomaly affected the reflectivity data, which could be clearly explained by numerical simulations. Also, light-induced surface potential change, determined by spatial distribution of the photo-generated charge carriers, was studied using Kelvin probe force microscopy. The comparison of the two kinds of samples on the NT and NH arrays could show us how the periodic metal nanostructures influenced the physical characteristics of the MoS2 layers. |
Wednesday, March 4, 2020 12:27PM - 12:39PM |
M52.00007: Scanning tunneling microscopy study of monolayer NbSe2 Mengke Liu, Jia Yu, Chih-Kang Shih Two-dimensional transition metal dichalcogenides (TMD) have received significant interest due to their unique structural and electrical properties. Within the family of TMD materials, NbSe2 has a 2H phase which is metallic in nature and interestingly exhibits a charge density wave and superconducting phase transition. Naturally occurring bulk NbSe2 is a 2H phase, however, recently a 1T phase of NbSe2 was grown by MBE. Intriguingly, this 1T phase exhibits an energy gap and insulating behavior at low temperatures. In this work, we study MBE grown monolayer NbSe2 using scanning tunneling microscopy and spectroscopy (STM/S). We report both 2H and 1T phases are present in the growth, and whose relative abundance suggests a growth temperature dependence. We further report comparative studies of the electronic properties of both 2H and 1T phases using STM/S. |
Wednesday, March 4, 2020 12:39PM - 12:51PM |
M52.00008: Thickness Dependent Electronic Structures of Pd Dichalcogenides: A First Principles Study Liang-Ying Feng, Rovi Angelo B. Villaos, Zhi-Quan Huang, Chia-Hsiu Hsu, Feng-Chuan Chuang Among the family of transition metal dichalcogenides (TMDs), Pd-based TMDs have been one of the less explored materials. In this study, using first-principles calculations, we investigate the electronic properties of PdX2 (X=S, Se, or Te) with respect to film thickness. With regards to the structural stability, the bulk and thin film structures of PdS2 and PdSe2 exhibit pyrite phase, while PdTe2 exhibits 1T phase as their most stable configurations. For the electronic properties, the most stable bulk configurations demonstrate semi-metallic features, while their corresponding monolayer structures for pyrite PdS2 and PdSe2 are insulating with band gaps of 1.399 eV and 1.548 eV, respectively, while 1T PdTe2 remained to be semi-metallic. For the band properties, we observe that all these materials manifest decreasing/closing of indirect band gap with increasing thickness. Moreover, all the stable monolayer band structures exhibit flat bands and diverging density of states near the Fermi level, indicating the presence of van Hove singularity. |
Wednesday, March 4, 2020 12:51PM - 1:03PM |
M52.00009: Electronic Growth Modes in Metal Dichalgogenide Interfacees Tim Kidd, Evan O'Leary, Andrew Stollenwerk The transition metal dichalcogenides (TMDCs) ara a family of materials of great interest for chemical tunability and novel electronic properties. The TMDC surface terminates at a van der Waals gap, making a stable, highly non-interacting growth surface. We have found that this lack of interaction can induce a variety of metals to spontaneously organize into films or nanostructures with strongly preferred heights upon the TMDC surface. The terraces of these features is atomically flat and highly oriented. Their structure can be predicted using only the electronic structure of the metal. This electronic growth differs from previously reported magic height systems in that high temperature annealing enhances growth properties, implying true equilibrium conditions. This process could be extended to a great number of highly 2D materials and open new avenues of research for self-assembled nanostructures with various electronic or spin-based confinement. |
Wednesday, March 4, 2020 1:03PM - 1:15PM |
M52.00010: Doping WS2 armchair nanoribbons with transition metals Yan-Hong Chen, Chi-Hsuan Lee, Shun-Jen Cheng, Chih-Kai Yang Armchair WS2 nanoribbons are semiconductors with band gaps close to 0.5 eV. When a W atom is replaced by a transition metal, impurity states can have tremendous effect on the overall electronic structures. By using first-principles calculations based on density functional theory, we investigated substitutional of Ti, V, Cr, Mn, Fe, Co for WS2 ribbons of different widths. We found that Fe-doped ribbons have two-channel conduction separately in the middle part of the ribbon and at the edges. Co-doped ribbons are turned in to spin filters, having 100% spin-polarized conductions. These results should be useful for nano-electronic circuit design. |
Wednesday, March 4, 2020 1:15PM - 1:27PM |
M52.00011: Induction of Phase Transition in Transition Metal Dichalcogenides on Metal Substrates Chi Sin Tang, Xinmao Yin, Di Wu, Weilong Kong, Qixing Wang, Ming Yang, Mark B. H. Breese, Wenjing Zhang, Andrivo Rusydi, Andrew Thye Shen Wee Two-dimensional transition metal dichalcogenides (2D-TMDs) possess unique structural phases including the semiconducting 1H and quasi-metallic 1T′ phases. They show intriguing optical and electronic properties. 1H to 1T′ phase transition processes are important to be utilitzed for novel device applications. We present a high-yield 1H to1T′ phase transition of monolayer-MoS2 on Cu and monolayer-WSe2 on Au via an annealing-based process. Both experimental and first-principles studies are conducted to study the phase transition mechanism. The 1T' phase yield is increased via interfacial hybridizations enhancement through interfacial binding energy, charge transfer, shorter interfacial spacing, and weaker bond strength. |
Wednesday, March 4, 2020 1:27PM - 1:39PM |
M52.00012: Tuning the Electronic Band Structure of Copper Selenide Cu2Se Thin Films Grown via Molecular Beam Epitaxy Ryan Trey Van Haren, Toyanath Joshi, David Lederman Cu2Se has long been known to be an excellent thermoelectric material whose electronic band structure that is tunable by introducing copper vacancies into the crystal structure. More recently, this compound has been predicted to host topologically-protected surface states. In this work, we will present our successful epitaxial growths of Cu2-xSe thin films via molecular beam epitaxy (MBE). Using reflection high energy electron diffraction (RHEED) and x-ray diffraction (XRD) measurements, we will show how we are able to quantify the copper concentration by analyzing the subtle shifts in our observed XRD spectra corresponding to small changes in the lattice spacing due to these copper vacancies and how these vacancies influence electronic transport in the film. In this manner we will demonstrate how we are able to tune the copper vacancies and electronic band structure by precise control of the crystal’s growth parameters. |
Wednesday, March 4, 2020 1:39PM - 1:51PM |
M52.00013: Fabrication of Copper Selenate Thin Films by Pulsed Laser Deposition David King, Jinke Tang, Uppalaiah Erugu, John Ackerman Cu2OSeO3 is a rare material with properties that are highly desirable for spintronic devices: it is an insulating ferrimagnet that also hosts magnetic skyrmions. Previous investigations of Cu2OSeO3 have emphasized single crystals. However, skyrmions are typically more stable in thin films than in bulk materials. Also, thin films are more suitable for device applications than single crystals. In this research, we present results of using pulsed laser deposition (PLD) to fabricate thin films of Cu2OSeO3, specifically accounting for the high disparity in vapor pressure between the reactants CuO and SeO2. Data showing the film's magnetic properties, crystallography, and electrical properties are presented. |
Wednesday, March 4, 2020 1:51PM - 2:03PM |
M52.00014: Dimensionality-Mediated Semimetal-Semiconductor Transition in Ultrathin PtTe2 Films Meng-Kai Lin, Rovi Angelo B. Villaos, Joseph Hlevyack, Peng Chen, Ro-Ya Liu, Chia-Hsiu Hsu, José Avila, Sung-Kwan Mo, Feng-chuan Chuang, Tai-Chang Chiang Platinum ditelluride (PtTe2), a type-II Dirac semimetal in the bulk form, is made of Te-Pt-Te tri-atomic layers (TLs) loosely bonded together by van der Waals interactions. It remains semimetallic in ultrathin films down to just two TLs, but a further reduction of the film thickness to just a single TL induces a Lifshitz electronic transition to a semiconductor with a sizable gap. This transition is evidenced by experimental mapping of the band structure by angle-resolved photoemission spectroscopy for films of various thicknesses prepared by molecular beam epitaxy on a bilayer-graphene-terminated SiC substrate. Layer-by-layer evolution of the band structure is well resolved, which facilitates absolute layer counting. The measured semiconducting band structure for the single TL is in excellent agreement with theoretical calculations. Our results demonstrate a novel electronic transition at the single-layer, or two-dimensional limit through film thickness control. |
Wednesday, March 4, 2020 2:03PM - 2:15PM |
M52.00015: Structural response of a slit-shaped graphene nanopore to adsorption: Observation by in situ neutron diffraction Joseph Schaeperkoetter, Haskell Taub, Helmut Kaiser, Zachary Buck, Matthew Connolly, Carlos Wexler We have investigated adsorption-induced deformation in graphene oxide framework materials (GOFs) using neutron diffraction at sample pressures up to 140 bar. GOFs, made by solvothermal reaction of graphite oxide (GO) and benzene-1,4- diboronic acid (DBA), are a suitable candidate for deformation studies due to their narrow (∼1 nm), monodispersed, slit-shaped pores whose width can be measured by diffraction techniques. We have observed, in situ, a monotonic expansion of the slit width with increasing pressure upon adsorption of xenon, methane, and hydrogen under supercritical conditions [1]. We find that the expansion of the three gases can be mapped onto a common curve based solely on their Lennard-Jones parameters, in a manner similar to a law of corresponding states. All scattering measurements were performed on the 2-axis powder diffractometer at the Missouri University Research Reactor (MURR). |
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