X-ray absorption spectroscopy of vanadium dioxide thin films across the phase-transition boundary

X-ray absorption spectroscopy (XAS) and x-ray photoemission spectroscopy of the V L edge and O K edge were performed on VO2 thin films rf sputtered at various conditions. The spectra give evidence of the changes in the electronic structure depending on the film quality. XAS of the O K edge shows a decrease of the spacing between 3d(pi) and 3d(sigma) bands by 0.8 eV with concurrent broadening of both bands for the sample sputtered at lower substrate temperature and consequently having more polycrystalline and disordered character. 3d(sigma) band position appears to be more sensitive to the sample quality, indicating that the cation-ligand interaction is mostly affected likely due to the distortion of the local O coordination surrounding a V ion. The observed variation of the spectra in films of different morphologies may reflect the changes of the density of states responsible for the considerable variation of the metal-insulator transition (MIT) properties reported for VO2 thin films synthesized at different conditions. The study of the temperature dependence of the XAS spectra including repeated measurements across the MIT revealed both reversible and irreversible V L-edge and O K-edge changes. The thermal cycling of the VO2 films through the MIT shows irreversible shifts of the conduction bands toward lower photon energies apparently caused by the sample deterioration due to the lattice transformations at the MIT. The signature of a phase transition in a VO2 film at MIT temperature (T-MIT) is clearly seen in the XAS O K-edge spectra which show reversible switches of the 3d(pi) and 3d(sigma) bandwidths by approximately 20% depending on the sample being above or below T-MIT.