Ab initio electronic-structure calculations on the Au/Ag multilayer system and Au epitaxy on the Ag(110) surface

Ab initio electronic-structure calculations are performed for Au/Ag multilayers with small modulation wavelengths in the [100], [110], and [111] fcc modulation directions and for Ag [110] slabs covered with none, one, two, or three Au(110) layers. The total energies of the multilayers can be ordered according to the number of Au-Ag nearest-neighbor pairs in these systems, from which the bonding at the interface can be estimated. The multilayer and slab results taken together allow for a decomposition of the energetics of Au adlayer behavior on the Ag(110) surface. It is found that a bare Ag(110) surface is energetically more favorable than an Ag(110) surface covered by Au monolayers. This is in agreement with the difference in surface energy of Au and Ag surfaces, and it implies that the bonding energy gained at the Au-Ag interface is insufficient to overcome this difference. Therefore Au will tend to grow in islands on an Ag(110) surface rather than wetting it. Our results, based on Au-Ag bonding and the open geometry of the Ag(110) surface, support the bilayer-growth model that has recently been reported in the literature, but this support is only for the first two layers in the islands. Whereas the density-of-states (DOS) curves of the multilayers show hardly any variation for the different systems, the layer-resolved densities of states for the slabs show pronounced changes on going from the surface layers inward. For all slabs, with or without Au atop, an interface state can be observed, localized between the first and second surface layer. Furthermore, a shift in the d-state density to higher energies at the interface and to lower energies for the layer below the interface occurs as compared with the elemental DOS curves