Epitaxial growth of CeO2(111) film on Ru(0001): Scanning tunneling microscopy (STM) and x-ray photoemission spectroscopy (XPS) study

Abstract

We formed an epitaxial film of CeO2(111) by sublimating Ce atoms on Ru(0001) surface kept at elevated temperature in an oxygen ambient. X-ray photoemission spectroscopy measurement revealed a decrease of Ce4+/Ce3+ ratio in a small temperature window of the growth temperature between 1070 and 1096 K, which corresponds to the reduction of the CeO2(111). Scanning tunneling microscope image showed that a film with a wide terrace and a sharp step edge was obtained when the film was grown at the temperatures close to the reduction temperature, and the terrace width observed on the sample grown at 1060 K was more than twice of that grown at 1040 K. On the surface grown above the reduction temperature, the surface with a wide terrace and a sharp step was confirmed, but small dots were also seen in the terrace part, which are considerably Ce atoms adsorbed at the oxygen vacancies on the reduced surface. This experiment demonstrated that it is required to use the substrate temperature close to the reduction temperature to obtain CeO2(111) with wide terrace width and sharp step edges.

A perfectly stoichiometric and flat CeO2(111) surface on a bulk-like ceria film

“…3(b) by fitting the spectrum with 4 Gaussian functions at 530.2 eV (peak 1), 531.4 eV (peak 2), 532.6 eV (peak 3) and 533.3 eV (peak 4). Peak 1 and 2 are assigned to oxygen bound to Ce 4+ and Ce 3+ , respectively2324 whereas peak 3 is assigned to hydroxyls and/or H 2 O molecules bound to Ce 3+  2526. From Peaks 1 and 2, we extract the relative content of Ce 3+ .…”

Structure, Morphology and Reducibility of Epitaxial Cerium Oxide Ultrathin Films and Nanostructures

“…This specific epitaxy is often more stable than the formation of polycrystalline films with (111) surface orientation and random in-plane orientation and it is typically stabilized by the post-growth thermal treatment. For example, the Cu(111) surface lattice parameter is almost exactly two-thirds smaller than the cerium oxide one and a 2:3 coincidence cell has been shown to be established [10], while on the Ru(0001) surface, a 5:7 coincidence cell is observed [18,22]. The formation of coincidence cells can be observed in STM images and in low energy electron diffraction (LEED) patterns.…”

“…Ce 3 d XPS spectra can be resolved by a well-established fitting procedure, obtaining the Ce 3+ and Ce 4+ concentration within the XPS probing depth [36,37]. Indeed, other XPS lines, like the Ce 4 d or even the O 1 s , also contain contributions from different oxidation states [22], though the fitting procedure contains more ambiguities. The use of synchrotron radiation allows us to tune the photon energy around the Ce 4 d → 4 f resonance, increasing the sensitivity to the different oxidation states and decreasing the probing depth [6,38].…”

“…A recent work on the Ru(001) substrate clearly shows that the best morphology in terms of the terrace width and the sharpness of the terrace step edges can be obtained by reactive deposition at temperatures close to the onset temperature for film reduction [22]. In ultrathin films on Pt(111), the post-growth thermal treatments in O 2 at high temperatures (700–1100 K) induce an agglomeration and a flattening of the cerium oxide features observed after growth at room temperature, leading to the formation of islands with definite shape and size, as shown in Figure 3.…”

“…More recent studies use the reactive growth of cerium in oxygen atmosphere, followed by a post-growth thermal treatment—also in oxygen atmosphere—to optimize the film stoichiometry, structure, and morphology [17,18,19]. Accurate studies of the influence of the growth parameters on the film properties allowed researchers to obtain systems with tunable stoichiometry and size of morphological surface features [6,20,21,22]. Typically, above approximately 1000–1100 K, depending on the substrate used, the oxide films showed to be thermally unstable and they reconverted into complex surface alloys [6,9,15].…”