Coverage-dependent stability of RuSi 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 (RuSi). 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 RuSi. Overlayers of surface-adsorbed Si adatoms and RuSi 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 RuSi formation with respect to oxide formation, focusing on coverage-dependent energy differences. Our results show RuSi 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 RuSi and the subsequent oxidation of Ru and RuSi were also investigated experimentally using in situ XPS. This confirmed the DFT prediction, with negligible oxide formation on the RuSi 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.