Atomic and electronic structure in van der Waals heterostructures

Two-dimensional materials (2DM) have strong in-plane bonding, but typically only weak van der Waals interactions with neighbouring material. It is this anisotropy which allows them to be isolated as layers, but it also dominates their behaviour in heterostructures. Unlike conventional three-dimensional heterostructures, where epitaxial growth is required to maintain high quality crystalline layers throughout, the van der Waals interactions allow highly crystalline layers to be stacked with atomic precision but without direct atomic registry, relaxing the need to lattice match or even symmetry match consecutive layers. This offers almost limitless possibilities for combining different 2DM in heterostructure stacks with designed functionalities, and has significant implications for the growth of 2DM and for thin film growth on 2DM. The weak interlayer interactions also introduce an additional degree of freedom – the twist angle between neighbouring layers. However, although the interlayer interactions are weak, they are not insignificant and play a role both in growth processes (van der Waals epitaxy) and in orbital hybridisation between layers, hence affecting both the atomic and electronic structure. I will present some of our recent work demonstrating the role of van der Waals epitaxy in the growth of and on 2DM, and in studying electronic structure in 2DM stacks. A particular focus will be on the use of angle resolved photoemission spectroscopy (ARPES) to directly measure band parameters and band alignments, demonstrating that spatially resolved ARPES can determine inter-layer hybridisation effects in micron-scale samples.

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