First-principles study of the contractive reconstruction of gold and silver monolayers on gold, silver, and aluminum

We investigated the structures of Au and Ag monolayers on a number of metal surfaces. It is energetically favorable for both Au and Ag to transform from a square to hexagonal arrangement and to contract to a higher surface density, but Au gains substantially more energy than Ag. This is true both for a monolayer in isolation as well as on top of a jellium surface. We calculated the mismatch energy (energy loss when the top layer loses registry with the substrate) for Au and Ag, and found that Ag has a slightly higher mismatch energy. The results offer a strong indication that Au(100) can reconstruct but Ag will not. The reconstruction is further studied with a 2 dimensional Frenkel-Kontorowa model, with parameters extracted from our total energy calculations. It is energetically favorable for the top layer of Au(100), but not for Ag, to transform to a hexagonal-close-packed structure and contract.;Au(111) has a (22 x√3) surface unit cell, and there is strong experimental evidence that the reconstruction pattern is formed by an uniaxial contraction of the top layer of Au atoms, so that both the fcc (ABC stacking) and hcp (ABA stacking) surface sites are occupied by the surface Au atoms. We found that the fcc and hcp sites are almost equal in energy, the bridge site is slightly higher while the top site is very energetically unfavorable. The Au(111) surface reconstruction is simulated using a Frenkel-Kontorowa model. The lowest energy pattern is a unilaterally contracted hexagonal layer, occupying both the fcc and the hcp sites. The surface band structures for the top layers occupying various sites are also studied.;Ag grows on Al(111) with the Stranski-Krastanov mode. For coverage less than 1 monolayer, Ag grows pseudomorphically on Al. For larger coverage, Ag cluster formation takes place, and the Ag monolayer is contracted. We study the energetics of Ag on Al(111) and decide under what conditions it is more favorable for Ag to grow epitaxially or to form clusters. The contraction of the Ag monolayer is considered using a Frenkel-Kontorowa model. (Abstract shortened with permission of author.