Density functional theory global optimization of chemical ordering in AgAu nanoalloys

The chemical ordering in AgAu nanoalloys is determined by global optimization searches at the DFT level. A simulation code based on the Basin Hopping algorithm is developed and applied to truncated octahedral nanoalloys of 38 atoms and to icosahedral nanoalloys of 55 atoms. The optimization results show a different behaviour of these clusters for what concerns the optimal chemical ordering on the Ag-rich side. In the truncated octahedron, up to two Au atoms are accommodated in the cluster core, while in the icosahedron, all atoms are placed on the surface. When increasing Au content, string-like Au patterns form on the cluster surface. The different behaviours of the icosahedron and of the truncated octahedron are rationalized in terms of the electronic properties of these nanoalloys, which demonstrate that a subtle interplay of charge transfer and stress effects is taking place.