Electronic structure of Co²⁺ ions in anatase Co:TiO₂ in relation to heterogeneity and structural defects

We have studied the electronic structure of Co ions in semiconducting anatase Co:TiO2(1.4 at.% Co) with x-ray-absorption spectroscopy. The density of structural defects is varied by (i) the oxygen vacancy concentration depending on the oxygen background pressure during growth, and (ii) the film thickness. TheCoL2,3-edge spectra, which are characteristic of Co2+ (3d7) ions, show particularly pronounced ligand-to-metal charge-transfer (CT) satellites for the thickest oxygen-poor films, which also exhibit the highest structural defect density. With charge-transfer multiplet calculations, the CT satellites are modeled by configuration interaction, originating from hybridization between Co 3d- and ligand states. This hybridization adds to the complex interplay between Co heterogeneity, defects, and electronic and magnetic properties in Co:TiO2.