Optoelectronics of two-dimensional van der Waals heterostructures

Van der Waals heterostructures composed of various two-dimensional nanomaterials were assembled and their optoelectronic properties were investigated. Graphene, hexagonal boron nitride (hBN), molybdenum and tungsten disulphide (MoS2 and WS2) layers were stacked together and light emission, photovoltaic activity, and valley coherence in the mixed structures were studied. A stacked percolating network of semi-metallic reduced graphene oxide and insulating hBN is prepared via chemical and liquid exfoliation techniques. The optical, electrical and structural properties of the mixed hBN-rGO composite are investigated. It is determined that the transport mechanism is via hopping conduction through defect states in the hBN domains. Furthermore, AC electroluminescence was observed in the mixed structure which is attributed to the recombination of electrons and holes in the hBN crystal defects. WS2 is directly grown on graphene sheets via chemical vapor deposition. WS2 is selectively grown on monolayer graphene grains transferred onto silicon dioxide substrate by controlling the growth temperature. WS2 is grown on graphene films transferred onto n-type silicon substrate forming a photovoltaic cell. The deposition of a few layers of WS2 atop the graphene layer led to a six-fold improvement in the power conversion efficiency compared to a graphene/n-Si cell. This has been attributed to the van Hove singularity induced increased light absorption of the WS2 layer and the reconfiguration of the electronic band structure at the interface. A mixed 2D van der Waals structure of the semiconducting transition metal dichalcogenides, MoS2 and WS2, are grown via CVD and their optical, structural and electrical properties are investigated. A 2D variable range hopping charge transport mechanism was determined via temperature dependent conductivity measurements of the field effect transistors with the mixed MoS2/WS2 films as channel materials. The FET device also exhibited light effect with the photo-response decreasing with increasing temperature and gate voltage due to the increased occupation of energy states at higher temperatures and gate voltages. Valley coherence was exhibited by the mixed structure which was determined by the polarization of the photoluminescence peak observed when the scattered light is analyzed in a direction parallel and perpendicular to the linearly polarized incident light.