Co Substitution Effect on the Electronic Structure of NiS Studied by X-Ray Photoemission Spectroscopy

Abstract

X-ray photoemission spectroscopy (XPS) measurements of the valence band, Ni 2p core level, Co 2p core level and S 2s, 2p core levels were carried out on the metallic state of Ni_1−vCo_vS (0≤v≤0.06). The main peak of the valence band spectra shifts toward lower binding energy (E_B) with increasing v (dE_B⁄dv=−2.6±0.4 eV/mol). This behavior is different from those reported on vacancy introduction or on the Se substitution in Ni_1−xS or NiS_1−ySe_y (dE_B⁄dx≈0; dE_B⁄dy≈0). The Ni 2p_3⁄2 core level peak also shifts toward lower E_B (dE_B⁄dv=−1.8±0.6 eV/mol), while no change in the S 2s or 2p core level peak is observed. We propose the possibility that the hybridization between the Co d⁷L and the Ni d⁸L peaks near the E_F, where L denotes a ligand hole, leads to the modification of the electronic state of Ni sites in Ni_1−vCo_vS.