Theoretical investigations of the anisotropic optical properties of distorted 1TReS2 and ReSe2 monolayers, bilayers, and in the bulk limit

Abstract

We present a theoretical study of electronic and optical properties of the layered $\mathrm{Re}{X}_{2}$ compounds ($X$ = S, Se) upon dimensional reduction. The effect on the band-gap character due to interlayer coupling is studied by means of the self-energy corrected $GW$ method for optimized and experimental sets of a structure's data. Induced changes on the optical properties as well as optical anisotropy are studied through optical spectra as obtained by solving the Bethe-Salpeter equation. At the ${G}_{0}{W}_{0}$ level of theory, when decreasing the thickness of the ${\mathrm{ReS}}_{2}$ sample from bulk to bilayer and to a freestanding monolayer, the band gap remains direct, despite a change of the band-gap nature, with values increasing from 1.6, 2.0, and 2.4 eV, respectively. For ${\mathrm{ReSe}}_{2}$, the fundamental band gap changes from direct for the bulk phase (1.38 eV) to indirect in the bilayer (1.73 eV) and becomes direct again for a single layer (2.05 eV). We discuss these results in terms of the renormalization of the band structure. We produce the polarization angular-dependent optical response to explore the optical anisotropy present in our results, as well as the fine structure of the lowest excitonic peaks present in the absorption spectra.