Do Ag_n (up to n = 8) clusters retain their identity on graphite? Insights from first-principles calculations including dispersion interactions

Adsorption of pre-formed Ag_n clusters for n = 1 - 8 on a graphite substrate is studied within the density functional theory employing the vdW-DF2 functional to treat dispersion interactions. Top sites above surface layer carbon atoms turn out to be most favorable for a Ag adatom, in agreement with experimental observations. The same feature is observed for clusters of almost all sizes which have the lowest energies when the Ag atoms are positioned over top sites. Most gas phase isomers retain their structures over the substrate, though a couple of them undergo significant distortions. Energetics of the adsorption can be understood in terms of a competition between energy cost of disturbing Ag-Ag bonds in the cluster and energy gain from Ag-C interactions at the surface. Ag₃ turns out to be an exceptional candidate in this regard that undergoes significant structural distortion and has only two of the Ag atoms close to surface C atoms in its lowest energy structure.