In Situ Surface-State Transformation on a Nickel Single Crystal for the Oxidation Process

Layer-by-layer oxidation of a Ni(111) single crystal was performed to investigate the interface electronic states at the buried NiO/Ni interface. The transformation of nickel from its metallic to oxidized states was studied using in situ X-ray photoelectron spectroscopy and synchrotron radiation X-ray absorption spectroscopy after oxidation at high temperatures in an oxygen atmosphere. Multiple electronic surface states were detected during the initial oxidation stage when the NiO thickness was below one monolayer. The oxidation process involved the coupled migration of Ni atoms from the interior to the surface and the concurrent penetration of oxygen atoms from the exterior to the interior. The presence of metallic nickel on the surface after oxidation indicates that the partial decomposition of NiO occurs at high temperatures under ultrahigh vacuum conditions.