Synthesis and characterisation of large area monolayer tungsten disulphide

Two-dimensional (2D) transition metal dichalcogenides (TMDs), equipped with direct bandgaps in the visible range of electromagnetic spectrum have extended the promise of 2D materials (graphene) to optoelectronics. Towards facilitating the future industrialization of 2D TMDs, this project focused on developing chemical vapour deposition (CVD) techniques for the growth of large area monolayer tungsten disulphide (WS2), which was followed by characterisations of their structural and semiconducting properties. It was demonstrated that controlling the introduction time and the amount of sulphur (S) vapour relative to the tungsten trioxide (WO3) precursor during the CVD growth of WS2 was critical to achieving large crystal domains on the surface of silicon wafers with a 300 nm oxide layer. This improvement for CVD techniques enabled the formation of single crystalline WS2 monolayers with edges up to 370 μm which were visible to the naked eye. Synthetic 2D materials grown by CVD are typically polycrystalline, and determining grain size within domains and continuous films is crucial for determining their structure. The thesis showed that grain boundaries (GBs) in monolayer WS2, grown by CVD, could be preferentially oxidized by controlled heating in air. Under apposite conditions, the degradation of GBs led to their clear and rapid identification using a standard optical microscope. Subsequent studies showed that how these monolayer WS2 GBs influenced the electroluminescence (EL) behaviour in lateral source-drain devices under bias. Real time imaging of the WS2 EL detected arcing between the electrodes when probing across a GB, which then localized at the GB region as it eroded under high bias conditions. Analysis of the eroded GB region showed the formation of micro- and nanoribbons across the monolayer WS2 domains. These results provide important insights into future EL devices that utilize CVD grown monolayer TMDs when GBs are present in the active device region.