Cobalt growth on two related close-packed noble metal surfaces

We report on scanning tunneling microscopy (STM) studies of submonolayer growth of cobalt on the close-packed (1 1 1) surfaces of Au and Ag. Both substrates belong to the category of noble metals, and they both exhibit a lattice misfit of ∼13% with respect to the (0 0 0 1) plane of Co. However, whereas the Au(1 1 1) surface reconstructs into the rather complex herringbone structure that disperses the cobalt into nanoclusters, the Ag(1 1 1) surface does not reconstruct in its clean state, and the surface dispersion of Co on this surface is therefore different. For Ag(1 1 1) at temperatures ranging from 160 to 200 K and for Au(1 1 1) at room temperature, the Co growth is three-dimensional starting with double layer islands followed by additional single layers. For both the Co/Au(1 1 1) and the Co/Ag(1 1 1) system, a Moiré pattern develops in the first bilayer of the Co islands, indicating an epitaxial but not commensurate growth. For Co islands with more than two layers, the subsequent layers are commensurate with the lower Co layers in the islands, but exhibit a decreasing corrugation of the Moiré pattern as observed in STM images. Despite a difference in the Moiré lattice constant and rotational angle, we show that the cobalt lattice constant is the same on both surfaces. We furthermore relate defect nucleation on the herringbone reconstruction on Au(1 1 1) to defect nucleation on steps on Ag(1 1 1).