Study of the growth of NiO on highly oriented pyrolytic graphite by X-ray absorption spectroscopy

In this work, we present a X-ray absorption spectroscopy (XAS) study of the growth of NiO on highly oriented pyrolytic graphite (HOPG). NiO has been grown by reactive evaporation of metallic Ni in an oxygen atmosphere (2 × 10−5 Torr) at room temperature. We paid special attention to the study of the early stages of growth. Both, Ni 2p and O 1s core-level XAS spectra were measured. For large NiO coverages, the spectra resemble that of a NiO single crystal, thus indicating the formation of a stoichiometric NiO thin film on the HOPG substrate. The Ni 2p XAS spectra remain similar during the whole growth process, indicating that Ni atoms are present in the high spin Ni2+ form, as supported by multiplet calculations. In contrast, for low coverages the line-shape of the O 1s XAS spectra differ strongly from that of bulk NiO. Cluster calculations of the spectra in octahedral and pyramidal symmetries support the formation of nanometric planar NiO islands at the graphite steps as previously observed by atomic force microscopy (AFM) in this system.