Changes in electronic structure within NiSx (0.60 < x < 1.53) compound series

In this article, changes in electronic properties of NiS_x series of films with different sulfur content (x = S/Ni) are studied using X-ray (XPS) and ultra-violet (UPS) photoelectron spectroscopy techniques. NiS_x (0.60 < x < 1.53) films are synthesized on native oxide of Si(111) substrates via reactive deposition of Ni and S in an ultra-high vacuum chamber by varying the sulfur pressures and substrate temperatures. The binding energy (BE) of Ni2p_3/2 core level, the separation between the Ni2p_3/2 main peaks and satellite peaks, and the Ni3d valence band binding energies are all found to vary nearly linearly with the sulfur content in NiS_x series of compounds, suggesting a direct dependence of Ni electronic charge on the sulfur content. The shift of the Ni3d and S3p states (to higher and lower BE, respectively) with an increase in the S content provides experimental confirmation of the theoretical prediction previously reported (Wen et al., 2020) [1]. All NiS_x films showed density of states (DOS) at the Fermi level, indicating their metallic nature. The Ni2p_3/2 binding energy shift of NiS_x films are much smaller than that of oxide-based compounds, confirming the significantly more covalent nature of the sulfides. NiS_x film grown on Si (111) substrate showed nickel silicide interface formation. This study provides information on the growth and trends in the electronic properties of NiS_x relevant for their applications in catalysis, energy storage, and electronic devices.