The properties of Ce overlayers grown on a Ru(0001) single crystal have been studied during annealing, and for exposure to oxygen. We have been using X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and low energy electron diffraction (LEED).
The systems have been studied with two different Ce overlayer thicknesses: > 1 ML (5-7 Å), and < 1 ML (2-3 Å). The annealing study was carried out by annealing the sample stepwise from room temperature up to 1000 °C, while recording the photoemission spectra and LEED pictures after each step. In the oxidation study, the Ce-Ru system was annealed to 1000 °C before it was exposed to oxygen at 500 °C. The oxygen doses used was 1, 3, 8, 18, and 68 L.
The results obtained indicate that there is a very low degree of interface alloy formation and intermixing of the two metals. The system with Ce > 1 ML, shows a higher degree of alloy formation right after deposition compared to the sub-monolayer Ce system. Upon annealing, the Ce surface layer forms a more ordered structure on top of the Ru substrate. At 1000 °C a (3x3) structure containing some disorder is formed.
There are no major differences between the two overlayer thicknesses when the Ce-Ru systems are exposed to oxygen. In both cases the result is a formation of Ce oxides. The conversion of Ce2O3 to CeO2 increases with higher oxygen exposures. From the results obtained in this study there are no indications of oxidation of Ru. The oxygen doses are probably to low. While increasing the oxygen exposure, an oxide overlayer with a partly ordered (4x1) structure is formed.
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