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 Eg of CeOₓ is due to the indirect O2→pCe4f electronic transition along the L high-symmetry lines of the Brillouin zone and it is correlated with the [Ce³⁺] content, explaining the redshift of Eg 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 O2→pCe4f 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 Eg, 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 ε₂ peak, which corresponds to the O2→pCe4f electronic transition.