Rhenium diselenide and rhenium disulphide are layered semiconductors that belong to the transition metal dichalcogenide (TMD) family. Like graphene and other TMDs, these materials can be exfoliated down to a few atomic layers. However, unlike other TMDs, the rhenium dichalcogenides are only stable in a triclinic structure that exhibits in-plane anisotropy. This anisotropy manifests itself in the vibrational, optical and electronic transport properties ofthese crystals.Ab initio calculations and experimental results are presented to describe the Raman spectra of the rhenium dichalcogenides. From Raman spectroscopy the anisotropy of these crystals can be observed. Flipping a flake (a C2 rotation about an axis in the layer plane) is not a symmetry of the system. Therefore, there are two non-equivalent vertical orientations. Raman spectroscopy can be used to identify whether a flake is facing "up" or "down". The latticedynamics of these crystals are described using a simple ball and spring model. It is shown that low mass impurities, such as sulphur, in ReSe2 can occupy four non-equivalent positions of the unit cell; there are four local vibrational modes corresponding to these four positions and Raman spectroscopy can be used to find them. An unusual experimental geometry (edge-on excitation) helps enhance these signals.The electronic band structures of bulk ReSe2 and ReS2 are explored using angle-resolved photoemission spectroscopy (ARPES). From the measurements and complementary DFT calculations it is shown that: (i) there is anisotropy in the electronic dispersions; (ii) the valence band maxima are not located along any of the high symmetry directions; and (iii) both of these crystals have indirect band gaps. The rhenium dichalcogenides were thought to act as electronically decoupled monolayers; it is demonstrated that this is not the case and that thereis signicant electronic coupling between the layers. Finally, ARPES results of a monolayer of ReSe2 are presented; again, anisotropy in the electronic band structure is observed.
|Date of Award||13 Jul 2018|
|Supervisor||Daniel Wolverson (Supervisor) & Simon Bending (Supervisor)|