Effects of surface non-stoichiometry on the electronic structure of ultrathin NiO(0 0 1) film

Highlights: • Oxygen vacancy defects were created by Ar⁺ ion bombardment and high temperature UHV annealing treatment to NiO ultrathin films grown epitaxially on Ag(0 0 1) substrate. • Low energy electron diffraction was used to investigate into the surface structure. • Angle resolved photoemission experiments were performed to probe the defect states near the Fermi level and modifications in valence band electronic structures. • X-ray photoemission and its angular dependency probed the changes in the core levels as well as valence bands of the oxide films. - Abstract: Surface structure and electronic structure of ultra thin NiO flms grown on Ag(0 0 1) substrate have been investigated in detail by means of low energy electron diffraction and photoemission spectroscopic studies under systematic modifications of surface Ni to O stoichiometry. Mild Ar⁺ bombardments (300 eV) were applied to the oxide films to produce predominant surface oxygen vacancies. These vacancy defects, which are mostly localized to the near surface region of the film, produce 'defect states' within the forbidden energy gap which pin the Fermi level (E_F) away from valence band maxima. Angle-dependent X-ray photoemission spectroscopic studies have been undertaken to characterize the core level as well as the valence band electronic structures of the film which confirm the existence of metallic nickel clusters at the sputtered surface. The depletion of oxygen at the NiO surface produced by annealing in ultra high vacuum at sufficient temperature and its enhanced effect on the monolayer NiO film have been elucidated.