First-principles study of substrate-mediated interactions on a compressed Ag(111) surface

We used density-functional theory (DFT) to resolve interactions between Ag atoms on a compressed Ag(111) surface from the first to the 53rd neighbor distance, a separation range of over 2nm. We find that these interactions are primarily pairwise and electronic in origin. A spatially resolved plot of the pair interactions reveals that they form concentric rings that alternate between attraction and repulsion. Calculations of the band structure of the strained Ag(111) surface reveal that there is a surface state at the Γ̄ point just below the Fermi level, similar to what is observed for unstrained Ag(111). The band structure of the strained Ag(111) surface is consistent with the magnitude and spatial dependence of the oscillatory interactions that we find in the DFT calculations. A comparison between a theory developed to describe adsorbate pair interactions mediated by surface-state electrons and our DFT results indicates that the theory describes the DFT results in an average way at distances beyond the third neighbor-although the DFT results contain angular variations in the interactions not predicted by the theory. At short adsorbate separations, we find that interactions at the first-neighbor and second-neighbor distances are strongly attractive and indicative of direct chemical bonds between the adatoms. The third-neighbor interaction is repulsive and is possibly mediated by bulk electrons. The results of these studies may be useful in the design of metal thin heteroepitaxial films.