The geometric structure of silver-doped silicon clusters.

Cationic silver-doped silicon clusters, Si(n)Ag(+) (n=6-15), are studied using infrared multiple photon dissociation in combination with density functional theory computations. Candidate structures are identified using a basin-hopping global optimizations method. Based on the comparison of experimental and calculated IR spectra for the identified low-energy isomers, structures are assigned. It is found that all investigated clusters have exohedral structures, that is, the Ag atom is located at the surface. This is a surprising result because many transition-metal dopant atoms have been shown to induce the formation of endohedral silicon clusters. The silicon framework of Si(n)Ag(+) (n=7-9) has a pentagonal bipyramidal building block, whereas the larger Si(n)Ag(+) (n=10-12, 14, 15) clusters have trigonal prism-based structures. On comparing the structures of Si(n)Ag(+) with those of Si(n)Cu(+) (for n=6-11) it is found that both Cu and Ag adsorb on a surface site of bare Si(n)(+) clusters. However, the Ag dopant atom takes a lower coordinated site and is more weakly bound to the Si(n)(+) framework than the Cu dopant atom.