Effects of Ni vacancies and crystallite size on the O 1s and Ni 2p x-ray absorption spectra of nanocrystalline NiO

We have studied the electronic structure of nanocrystalline NiO thin films, grown by radio-frequency magnetron sputtering under different experimental conditions, using x-ray absorption spectroscopy. The O 1s and Ni 2p spectra showed distinct changes as a function of O-2 content in the plasma, which were reproduced with cluster model calculations. These changes are attributed to the incrementing of the surface contribution due to a decrease of the crystallite size as the O-2 content in the plasma increases, and to the presence of induced nickel vacancies. Thus, the changes in the electronic structure can be related to the modification of structural and transport properties of these nanocrystalline films.