Optical properties and band structure of a layered Tl$_2$S crystal

The optical absorption spectra of a layered Tl$_2$S crystal have been studied at $T$ = 293 K. In has been found that the fundamental absorption edge has an exponential shape in all samples under study. The ab initio calculations of the energy band structure of Tl$_2$S have been performed in terms of the density functional theory for the first time. Based on the complementary analysis of the results of the optical measurements and calculation of the energy band structure of the material, the model of formation of the exponential fundamental absorption edge of the layered semiconductor Tl$_2$S has been proposed.