First-Principles Investigation of Electronic Structures and Optical Properties of TiS2 (001) Surface

This study investigates the optical characteristics and electronic structure of the (001) surface of TiS2 using first-principles calculations. The electronic structure of the TiS2 (001) surface was determined to be analogous to that of its bulk counterpart. The band gap of the TiS2 (001) surface is 0.91 eV, and it is 0.81 eV for bulk TiS2. Nevertheless, there is a marked discrepancy in the density of states distribution close to the Fermi level, which could be ascribed to van der Waals forces between layers. When the Ti 3d and S 2p electrons transition, they are not influenced by van der Waals forces for the TiS2 (001) surface, while those in bulk are influenced by van der Waals forces. The results of optical properties show that the dielectric constant of the TiS2 (001) surface is less than that of bulk TiS2. The dielectric peak band edge, extinction coefficient band edge, and absorption band edge of the TiS2 (001) surface show a red shift. The reason for this phenomenon is that the polarization energy of the TiS2 (001) surface is smaller than that of bulk TiS2, and the displacement polarization of the surface decreases. Based on the electronic structure and optical properties results, the electrons of the TiS2 (001) surface are more active, easier to excite, and easier to absorb the photoelectron energy. It enhances the optical response and improves the optical utilization rate.