Further theoretical analyses (2D and 3D) of Ni1/4TiS2 probed by XPS/STM studies

The surface of Ni1/4TiS2 was investigated by X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). Electronic calculations were performed using the ab initio Hartree–Fock program crystal. The XPS results (chemical shift of core peaks) are satisfactorily accounted for through a Mulliken population analysis. Concerning the STM results, we imaged the top sulfur plane (0 0 1) and interpret the data on the basis of the partial electron density of a slab which consist of seven (0 0 1) Ni1/4TiS2 layers. It was found that the bright spots in experimental STM images correspond to sulfur atoms in a single metallic environment (Ti atoms). Beside higher electronic density, these atoms are highlighted because of their proximity to the tip compared with sulfur atoms in a double environment (Ti and Ni).