Variation of electronic structures of CeAl2 thin films with thickness studied by X-ray absorption near-edge structure spectroscopy

We report the X-ray absorption near-edge structures (XANES) of CeAl2 thin films of various thickness, 40-120 nm, at Al K-, Ce L3-, and Ce M4,5-edges. It is found that the threshold of near-edge absorption features at the Al K-edge shifts to the higher photon energy as film thickness decreases, implying that Al loses p-orbital charge and the valence of Ce increases slightly as revealed from the XANES features at Ce L3- and M4,5-edges. Above observations suggest that there is charge transfer from Al to Ce as the surface to bulk ratio is varied. This induces change in the electronic structures of CeAl2 thin films. The Ce 4f electrons are surface sensitive in nanoparticles compared to thin films. This work also shows that 4f electronic states of Ce ions are sensitive to the reduction of the coordination number induced by surface effects that would change their hybridization.