Electronic and optoelectronic devices based on 2-dimensional crystals

Two dimensional (2D) crystals including graphene and monolayer transition metal dichalcogenides (TMD) such as molybdenum disulphide (MoS2) and tungsten disulphide (WS2) have attracted widespread attention for both basic scientific interest and potential in device applications. In this thesis, we investigate the electronic and optoelectronic devices based on 2D crystals grown using chemical vapour deposition (CVD). In our study on graphene transistors, we show that surface charge transfer between graphene and vertically stacked WS2 causes local modulation to doping levels of graphene, and can be further stimulated by photo-irradiation. We demonstrate the formation of an isotype junction in graphene, where two lateral sections of a graphene ribbon contain different dopant concentrations. Ultrathin lateral photodetectors of 2 nm thickness are made by incorporating graphene as electrodes and WS2 as active semiconductor. We show that graphene is different to conventional metal electrodes due to the finite density of states from the Dirac cones of the valence and conduction bands, which enables modulation of photoresponsivity through electrostatic gating and light input control. We demonstrate photodetectors with laterally spaced graphene electrodes and vertically stacked WS2/MoS2 as active light-adsorbing component, achieving photoresponsivity as high as 2340 A W-1. We observe in hybrid WS2/MoS2 photodetector massive enhancements in photogain compared to homobilayer and monolayer devices of MoS2 or WS2 due to efficient charge transfer between WS2 and MoS2 and long carrier lifetimes. Our results show that high performance photodetectors using all 2D materials can be realized using graphene electrodes and TMD heterostructures with type II band alignment.