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Structures and the Electronic Properties of Au19X Clusters (X = Li, Na, K, Rb, Cs, Cu, and Ag)

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posted on 14.01.2010, 00:00 by Tapan K. Ghanty, Arup Banerjee, Aparna Chakrabarti
We employ an ab initio scalar relativistic density functional theory based method to calculate the ground state structures and the electronic properties for Au19X clusters, X being the alkali metal atoms, Li, Na, K, Rb, and Cs as well as the coinage metal atoms, Ag and Cu. The tetrahedral Au20 clusters have been doped exohedrally with these atoms at three different types of unique sites where the dopant atom substitutes one gold atom from (i) the vertex, (ii) the surface, and (iii) the edge sites. In addition to the structures based on tetrahedral Au20, we also consider cage-like structures for Au19X clusters with the dopant atom located at an endohedral position. We first optimize the geometries of these clusters and then we carry out vibrational analysis on these optimized structures of the substituted Au20 clusters in order to check the stability of the final optimized structures. Further, using the optimized geometries of these doped clusters, we calculate the binding energy, interaction energy of the dopant atom with the Au19 cluster, vertical ionization potential, vertical electron affinity, and HOMO−LUMO gaps of these doped clusters. For these systems, we also carry out the charge population analysis. We compare these properties of the doped clusters with those of the pure Au20 cluster to characterize the stability and chemical inertness of the doped clusters. Few cage like endohedrally doped Au19X clusters (X = Li, Na, and Cu) are found to have binding energies comparable to those of the corresponding exohedrally doped clusters. For the larger atoms (X = K, Rb, Cs, and Ag), all of the endohedrally doped cage-like structures have been found to be less stable than the corresponding exohedral structures. Nevertheless, exohedrally doped Au19X clusters with X located at one of the surfaces of tetrahedral structure correspond to the most stable isomer for all the dopants. We observe that the Li and Cu doped gold clusters, where the dopant atom is located at one of the surface sites of the Au20 cluster are more stable than the pure Au20 cluster. This leads to the possibility of finding highly reactive anions of these doped clusters. Geometric as well as energetic considerations indicate that it may be possible to characterize these species experimentally using photoelectron spectroscopy.