Coverage-dependent stability of Ru_xSi_y on Ru(0001): a comparative DFT and XPS study

This work investigates the interaction of silicon with ruthenium, extending from Si-defect centers in ruthenium bulk to the adsorption of Si on the Ru(0001) surface. Using density functional theory (DFT) we calculate the interaction energies of up to 2 monolayers (MLs) of Si with this surface, uncovering the initial formation of ruthenium silicide (Ru_xSi_y). Our results demonstrate that Si readily forms substitutional defects (Si_Ru) in bulk ruthenium. These defects are further stabilized on the Ru(0001) surface, resulting in a distinct propensity for forming Ru-Si_Ru mixed layers - which can thus be described by stoichiometry Ru_xSi_y. Overlayers of surface-adsorbed Si adatoms and Ru_xSi_y mixed layers are iso-energetic at 0.5 ML, with the latter becoming increasingly energetically favored at higher Si coverages. We further examine the influence of Ru_xSi_y formation with respect to oxide formation, focusing on coverage-dependent energy differences. Our results show Ru_xSi_y layers are energetically favored with respect to the forming oxide for silicon and oxygen coverages above 1.1 ML, respectively. In addition, the formation of Ru_xSi_y and the subsequent oxidation of Ru and Ru_xSi_y were also investigated experimentally using in situ XPS. This confirmed the DFT prediction, with negligible oxide formation on the Ru_xSi_y sample, whereas the unprotected Ru surface showed extensive RuO₂ formation under the same conditions. Our study not only enhances the understanding of Ru surface chemistry but also suggests a straightforward computational approach for screening the oxidation resistance of surface coatings.