Browsing by Subject "Tungsten diselenide"
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Item Growth optimization of WSe₂ and its sulfurization to WS₂(2019-07-09) Liu, Chison Qishan; Banerjee, SanjayTungsten diselenide has gained much interest within recent years after it was reported to have both p-type and ambipolar transport properties. And because most other transition metal dichalcogenides exhibit n-type transport properties, tungsten diselenide would help to further realize CMOS technology if one could find a more reliable way to synthesize it in large areas with high quality crystallinity. In this report I will be detailing my work on successfully synthesizing WSe₂, its sulfurization into WS₂, and discussing what I’ve observed in both the crystal quality and growth mechanisms. My goal is to provide a better understanding of the growth process in hopes of moving forward with improving future growth recipesItem Magnetotransport studies of tungsten diselenide holes(2018-10-10) Movva, Hema Chandra Prakash; Tutuc, Emanuel, 1974-; Banerjee, Sanjay; Register, Leonard F; Yu, Edward T; Akinwande, Deji; Demkov, Alexander AThis dissertation describes the electronic transport properties of holes in tungsten diselenide (WSe₂), a prototypical transition metal dichalcogenide (TMD) material, probed using low-temperature magnetotransport measurements, and facilitated by a device structure with platinum (Pt) bottom-contacts and hexagonal boron nitride (h-BN) encapsulation. The discovery of graphene has stimulated an intense interest in exploration of materials with stable two-dimensional allotropes, TMDs being one of them. Of the myriad variety of TMDs, sulfides and selenides of molybdenum and tungsten have garnered great attention on account of their semiconducting nature. A major roadblock to investigation of TMDs' electronic transport properties has been the poor quality of electrical contacts. The top-gated device structure with Pt bottom-electrodes presented in this dissertation ensures Ohmic hole contacts to WSe₂ down to a temperature of 0.3 K and permits low-temperature magnetotransport measurements. Encapsulating WSe₂ in h-BN preserves its intrinsic quality, resulting in high hole mobilities at low-temperatures, and thereby enabling observation of Shubnikov-de Haas (SdH) oscillations and quantum Hall states (QHS) in perpendicular magnetic fields. Analysis of the SdH oscillations in monolayer and bilayer WSe₂ reveals two-fold degenerate Landau levels and a hole effective mass of 0.45m [subscript e]; m [subscript e] is the free electron mass. Bilayer data also show carrier localization in the two layers signifying weak interlayer coupling, and negative compressibility of holes in the bottom layer. The QHS data reveal interesting transitions between even and odd filling factors as the hole density is tuned, which can be explained by a Zeeman-to-cyclotron energy ratio that changes as a function of density due to strong electron-electron interactions. Tilted magnetic field measurements reveal that the holes reside in the K valleys, as evinced by their spins which are locked perpendicular to the WSe₂ plane. In trilayer WSe₂, holes are found to populate two subbands with different effective masses, 0.5m [subscript e] and 1.2m [subscript e], associated with the K and Γ valleys, respectively. At a fixed total hole density, the K and Γ occupations are tunable via an applied transverse electric field, an observation which can be explained in terms of an electric field dependent bandstructure.