First-Principles Calculations of the Quantum Size Effects on the Stability and Reactivity of Ultrathin Ru(0001) Films

We carry out first-principles calculations of Ru(0001) films up to 30 monolayers (MLs) to study the quantum size effect (QSE) of Ru films for two cases: the freestanding Ru films and Ru films on Pt(111) substrates. Our studies show that the properties of these films (surface energy, work-function, charge density decay length in a vacuum and chemical reactivity) exhibit pronounced oscillatory behavior as a function of the film thickness, with an oscillation period of about four MLs for both cases due to the relationship of the match between the Fermi wave vector and the film thickness. Due to the localization of d-electron of Ru films, these quantum oscillations almost disappear when the thickness of the film is more than ～20 ML for the free standing Ru films, while for the Ru films on Pt substrates the oscillations disappear quickly when the thickness of the film is beyond ～13 ML. Our results reveal that the stability and reactivity of the Ru films could be tailored through QSE and the Ru bilayer grown on Pt substrates observed in the experiment is also related to the effect.