The electronic and structural properties of graphene- and metal chalcogenide-based van der Waals heterostructures

In this thesis, we present the electronic and structural properties of graphene and metal chalcogenide-based van der Waals heterostructures. Graphene and post transition metal chalcogenides are 2 dimensional (2D) materials that have drawn a lot of attention over the last few years, leading them to be used in new electronic devices, such as field effect transistors.

First, we outline how defects in stacked graphene/hBN/graphene van der Waals heterostructures can induce localised states in the boron nitride barrier and can enable single electron tunnelling. Boron nitride is often used as an insulating material within graphene-based devices, and defects in the lattice can induce localised states. We investigate using density function theory the properties of ten defect states and determine their energies and their likelihood to give rise to tunnelling channels in graphene/hBN tunnel transistors.

Second, we study the effects that an external electric field has on these heterostructures and the localised states. Our density functional theory analysis reveals that for mono- and bi-layer hexagonal boron nitride barriers, the two graphene layers either side can be strongly coupled giving rise to a large band gap on the graphene layers. Moreover, we find that the band gap can be controlled by application of the electric field. We develop a model of the defect state based on the linear combination of atomic orbitals and use this model to understand the effect of the electric field. Our orbital analysis reveals that the electric field induces a large Stark effect shift of the energies of the state when the defects in the hexagonal boron nitride layer were coupled to the graphene layer. These results are consistent with recent experimental measurements. The density functional theory analysis of the electric field effect on the defects revealed that the coupling of the state with the graphene layers gave rise to strong redistribution of the charge when the defect is close to the graphene layers.

Finally, we study a new family of 2D materials the post transition metal chalcogenides. These materials have recently drawn a lot of attention, due to their potential applications in new electronic devices. Their smaller band gap makes them an alternative to boron nitride, in graphene-based transistors. We show our investigations of the structural properties of two similar phases of In₂Se₃. Our theoretical atomistic modelling, in combination with experimental measurements, revealed a large dependence of the structural properties on the stacking configuration of two similar phases of In₂Se₃. We also investigate stacks of graphene with InTe and In₂Se₃ to determine their properties and examine their potential in future electronic devices.