Characterising ultrathin ceria films at the nanoscale: Combining spectroscopy and microscopy

CeO2−x(1 1 1) ultrathin films consisting of small, discrete islands decorating a Pt(1 1 1) substrate have been studied using a combination of Scanning Tunnelling Microscopy, Low-Energy Electron Microscopy, and Low-Energy Electron Diffraction. Significantly, the chemical nature of the ceria film has also been probed using X-ray Absorption Spectroscopy (XAS) combined with X-ray PhotoEmission Electron Microscopy (XPEEM) in the same ultrahigh vacuum system. XAS spectra over the Ce M5 absorption edge demonstrated that the ceria islands contained ∼50% Ce4+and ∼50% Ce3+, leading to an overall stoichiometry of CeO1.75, which was uniform across the film. The unique advantage of this experimental setup is the application of multiple techniques on the same sample: high-resolution STM to monitor the morphology, XPEEM to probe the stoichiometry, and LEEM to act as a bridge between the two.