Growth and Local probing of Transition Metal Dichalcogenides

Abstract

This work focuses on synthesising and investigating functional properties of selected two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs), by combining material science approaches with a range of characterization techniques.
We uncovered a versatile Chemical Vapor Deposition (CVD) process capable of producing in one single stream several WS2 polymorphs: few-layer nanotubes; an out-of-plane, pure WS2, 2D nanomesh; and in-plane mono- and few-layer 2D domains. This entails a two-stage process in which, first, various morphologies of nanowires or nanorods of WO2.9/WO2.92 sub-oxides (belonging to the class of Magnéli phases) were formed, followed by their sulfurization to undergo reduction to the aforementioned WS2 polymorphs. This approach meant intentionally decoupling the 2WO3 + 7S → 2WS2 + 3SO2 reaction in two steps, and in this way demonstrate a long-time hypothesized mechanism via sub-oxide (WO3-x) intermediate as the path to 2D domain growth. 2D, in-plane WS2 domains grow via a “self-seeding and feeding” mechanism where short WO2.9/WO2.92 nanorods provide both the nucleation sites and the precursor feed-stock. Understanding the reaction path (here, in the W-O-S space) is an emerging approach towards controlling the nucleation, growth and morphology of 2D domains and films of transition metal dichalcogenides (TMDs). Finally, large-area (millimeter scale) WS2 films could be synthesized by a simple hydrogen-free chemical vapour deposition (CVD) method on SiO2/Si substrates.
Transport anisotropy in few-layer ReS2 was locally investigated with a multiple-probe scanning probe microscope (MP-SPM: (i) by directly measuring the angular dependence of the current, and (ii) by injecting current and measuring the resulting surface potential distribution using Kelvin probe microscopy. Current anisotropy with unprecedented angular (5º) resolution was obtained, revealing effects of finite size of ReS2, and potential strain or structural modifications in the material. The resulting electric field distribution under current injection is strongly influenced by the low conductance/mobility axis in ReS2, which tends to rotate the field lines towards it. Finite-element analysis simulations corroborate these findings, and reveal how various parameters of a probing configuration influence the measured anisotropy; thus, informing of the necessity of careful design of real devices involving such low-symmetry, anisotropic 2D materials.
Finally, a double gated structure involving a traditional back-gate and a scanning probe top gate (within a Scanning Tunneling Microscope) was realized on multi-layer WSe2, and preliminary measurements show a degree of control of the Fermi energy within the band-gap.

Date of Award	2019
Original language	English
Awarding Institution	University of Bath
Supervisor	Daniel Wolverson (Supervisor) & Adelina Ilie (Supervisor)