Bulk and surface electronic structure of 1T-TiS2 and 1T-TiSe2

Ab initio band-structure calculations were performed for bulk, single slab, and thin films of TiX2 (X = S, Se) using the localized spherical wave method. According to these calculations, bulk TiS2 and TiSe2 are semimetallic. The calculations show that TiS2 thin films are semiconductors, but thin films of TiSe2 an semimetallic. The indirect gap for single slab TiS2 is about 1.0 eV, and the gap becomes smaller with increasing number of layers. When the number of layers increases to 11. the TiS2 thin films are semimetallic. All but the surface layers are found to be electrically neutral. The density of states as a function of the energy for the surface layer is different from that of the bulk. The Madelung energy of the Ti atoms on the surface is about 0.35 eV lower than that for the Ti atoms in the bulk. The calculations are compared with photoemission spectra, reported in the literature.