Electronic structure ofReS2andReSe2from first-principles calculations, photoelectron spectroscopy, and electrolyte electroreflectance

The electronic structures of ${\mathrm{ReS}}_{2}$ and ${\mathrm{ReSe}}_{2}$ single crystals are investigated using a first-principles density-of-states (DOS) calculation, ultraviolet photoelectron spectroscopy (UPS), and electrolyte electroreflectance (EER). The total and partial DOS were calculated by the full-potential linearized-augmented-plane-wave method. From the calculations, the main contribution near the band edge of $\mathrm{Re}{X}_{2}$ $(X=\mathrm{S},\mathrm{S}\mathrm{e})$ is determined to be dominated by the nonbonding Re d orbitals. The valence-band DOS is experimentally verified by the UPS measurements. EER measurements were performed in the energy range of 1.3\char21{}6 eV. The EER spectra exhibit sharp derivativelike structures in the vicinity of the band-edge excitonic transitions as well as higher-lying interband transitions. Transition energies are determined accurately. From the experimental and the theoretically calculated results, probable energy-band structures of ${\mathrm{ReS}}_{2}$ and ${\mathrm{ReSe}}_{2}$ are constructed.