Structure-dependent electronic properties of nanocrystalline cerium oxide films

We investigate the electronic properties of nanocrystalline cerium oxide (CeOx) films, grown by various techniques, and we establish universal relations between them and the film structure, composition, and morphology. The nanocrystalline CeOx films mainly consist of CeO2 grains, while a considerable concentration of trivalent Ce3+ is distributed at the CeO2 grain boundaries forming amorphous Ce2O3. A small portion of Ce3+ is also located around O-vacancy sites. The optical properties of the CeOx films are considered, taking into account the reported band-structure calculations. The fundamental gap E-g of CeOx is due to the indirect O2p-->Ce4f electronic transition along the L high-symmetry lines of the Brillouin zone and it is correlated with the [Ce3+] content, explaining the redshift of E-g in nanostructured CeOx which is due to the Ce3+ at the grain boundaries and not due to the quantum-size effect itself. We also correlate the energy position of the O2p --> Ce4f electronic transition, which varies up to 160-meV wide, with the lattice constant of the CeO2 grains. We also show that the higher-order transitions are more sensitive to film composition. The refractive index, far below E-g, is explicitly correlated with the film density, independently of the Ce3+/Ce4+ and O concentrations, grain size, and lattice parameter. The density is also found to be the major factor affecting the absolute value of the epsilon(2) peak, which corresponds to the O2p-->Ce4f electronic transition.