Synthesis and characterization of tungsten disulphide

Abstract

Tungsten disulphide (WS₂) is a 2-dimensional layered material with each layer consisting S-W-S in strong covalent bond whereas weak van der Waals' force between layers. Weak bonding between layers allows the preparation of monolayers by mechanical exfoliation, which is the method firstly used for preparing graphene. However it is challenging to prepare large area flakes by such a method. Different techniques have been developed for the growth of WS₂ and novel properties at few-layer and monolayer were gradually discovered in recent years. In this project, two methods have been applied for the growth of WS₂. Direct sulphurization was applied on sputtered W films. It was found that WS₂ nanotubes were formed even at moderate temperatures. The crucial factor was the thickness of the precursor films. Ultrathin W films (0.5 and 0.7 nm) formed WS₂ nanotubes while thicker films showed nanoparticle-like surfaces. This morphology change was found to be stress-related. By patterning the films into micrometer-wide tracks, intrinsic stress in the ultrathin films was released and the critical loading stress for buckling was increased. The manipulation of WS₂ morphologies could be achieved simply by controlling the precursor films thickness or by conventional patterning technique. In addition, doping of WS₂ was studied by sulphurization of co-sputtered Ni-W films. Different doping concentrations were realized simply by controlling sputtering rates of Ni and W, which was confirmed by X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray spectroscopy (EDX). Structural and chemical analyses, including Raman spectroscopy, X-ray diffraction (XRD) and transmission electron microscopy (TEM) were performed. Electrical properties of the samples were also studied by measuring the resistance-temperature behaviour, confirming the semiconducting nature of pure and doped WS₂. Photodetectors were constructed from such WS₂ films. Doping with 5.1% Ni-WS₂showed the best photodetection performance among pure and heavily Ni-doped samples. Chemical vapour deposition (CVD), which allows the growth of single crystalline flakes by controlling the parameters involved in the reaction, was applied for the preparation of WS₂. The flakes were transferred to SiO₂/Si substrates. Electrical properties of WS₂ flakes were studied by constructing back-gate transistor. Spin Hall effect was preliminarily investigated by constructing Pt/WS₂/YIG tri-layer samples.