Theoretical investigations of the anisotropic optical properties of distorted 1T ReS2 and ReSe2 monolayers, bilayers, and in the bulk limit

We present a theoretical study of electronic and optical properties of the layered ReX2 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 correctedGW 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 ReS2 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 ReSe2, 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.