Trends and Properties of 13-Atom Ag-Au Nanoalloys I: Structure and Electronic Properties

We present a systematic study of the structures and the electronic and magnetic properties of 13-atom Ag-Au nanoalloys, using spin-polarized ab initio calculations based on density functional theory. To this end, we use all possible chemical configurations of four different initial symmetries as starting structures: icosahedra, decahedra, cuboctahedra, and the buckled biplanar (BBP) cluster. Mixing is energetically favored; there is no indication of segregation. We find a general tendency to minimize the number of Au-Au bonds. Many of the clusters undergo strong morphology changes. The resulting structures of lowest energy, independent of the starting geometry, are distorted biplanar clusters. The cuboctahedra are a rather stable local minimum against geometry changes following the introduction of the mixing. All the lowest-energy structures have a Kohn-Sham HOMO-LUMO gap of about 0.2 eV and a total spin of 1 mu(B). Higher total spin values are found for some of the icosahedra and decahedra, but they have an energy much higher than that of the lowest-energy structures of the respective compositions. The quasi-particle gap is about 3.7 eV across the composition range. It does not vary appreciably with the composition and structural details of the clusters.