PNNL

Abstract

The surface structure of a-Cr2O3(0001) before and after exposure to activated oxygen from an ECR plasma source was investigated by x-ray photoelectron spectroscopy (XPS) and x-ray photoelectron diffraction (XPD). Epitaxial Cr2O3(0001) thin films were deposited on Al2O3(0001) substrates by oxygen-plasma-assisted molecular beam epitaxy (OPA-MBE). When cooled or annealed in vacuum, strong evidence for a Cr-Cr-O3- termination was obtained by comparing the Cr3+ XPD azimuthal scan to single scattering simulations. However, after plasma exposure, a high binding energy feature was observed in the Cr 2p XPS spectrum that possesses an ordered structure distinct from the underlying Cr3+ of Cr2O3, which remains Cr-Cr-O3-like. Investigation of this new surface structure with simulations of various candidate structures tentatively rules out CrO2-like configurations. The high binding energy feature likely arises from a higher oxidation state of Cr. One possibility is the oxidation of the surface layer of Cr to Cr6- with a double chromyl structure (O=Cr=O).