Optoelectronic properties of the two-dimensional van der Waals semiconductor indium selenide (InSe)

This thesis presents an investigation of the optoelectronic properties of the two-dimensional (2D) van der Waals (vdW) semiconductor indium selenide (InSe) and exploits these properties in InSe-graphene vdW heterostructure devices. These heterostructures are fabricated by mechanical exfoliation and dry transfer of InSe and graphene nanosheets. A novel method of device fabrication by needle transfer of graphene microsheets is also described and used for some devices. The optical properties of InSe nanosheets differ qualitatively from those reported for other 2D materials, such as transition metal dichalcogenides (TMDCs). In particular, this thesis reports on the controlled modulation of optical signals by exploiting the inherent optical anisotropy and mechanical flexibility of atomically thin 2D vdW InSe bent onto a periodic array of silicon (Si) nanopillars. A series of vertical and planar vdW heterostructures, including tunnelling transistors and photodetectors, are investigated. The optoelectronic transport characteristics of these devices exploit a favourable band alignment between InSe and graphene. Moreover, 2D energy subbands of InSe exhibit strong quantum confinement, offering a route to the modulation of electrical properties. Optical absorption studies on bulk InSe by variable angle spectroscopic ellipsometry (VASE) are presented, which indicates strong resonances in the ultra-violet (UV) range of the absorption spectrum. A fast, ultra-high photoresponsivity is demonstrated in a hybrid phototransistor based on an InSe/graphene heterostructure that exploits the light-induced charge transfer at the interface of InSe and graphene