Surface metallic nature caused by an in-gap state of reduced NiO: a photoemission study

The electronic state of oxygen-reduced NiO(1-bar 0-bar 0) surface has been investigated using angle-resolved and resonant photoemission spectroscopy with synchrotron radiation. We focus on the behavior of an in-gap state induced by oxygen-defects, which is observed at ∼ 0.5-bar eV below the Fermi level (EF). The in-gap state shows slight energy dispersion; however, with a closer look around EF, it seems to create metallic Fermi cut-off both in hν-dependent normal-emission spectra and in off-normal emission ones. The rest of the peaks due to bulk bands are identical to those of stoichiometric bulk NiO; therefore, the observed feature of the in-gap state reveals the fact that the surface of reduced NiO is a metal while the vast underlying bulk is still an insulator