Density functional investigation of structural and electronic properties of small bimetallic silver-gold clusters

摘要

Structural and electronic properties of bimetallic silver-gold clusters up to eight atoms are investigated by the density functional theory using Wu and Cohen generalized gradient approximation functional. By substitution of Ag and Au atoms, in the optimized lowest energy structures of pure gold and silver clusters, we determine the ground state conformations of the bimetallic silver-gold ones. We reveal that Ag atoms prefer internal positions whereas Au atoms prefer exposed ones favoring charge transfer from Ag to Au atoms. For each size and composition, binding energy, HOMO-LUMO gap, magnetic moment, vertical ionization potential, electron affinity and chemical hardness were calculated. On increasing the size of the cluster by varying number of Ag atoms with fixed number of Au ones, vertical ionization potential and electron affinity show obvious odd-even oscillations consistent with the pure Ag and Au clusters. Au atoms inclusion in the cluster increases the binding energy and vertical ionization potential, indicating higher stability as the number of Au atoms grows. The variation of chemical hardness with the composition in a cluster with the same size shows peaks when the number of Ag atoms is greater than or equal to Au ones, corresponding to transition from planar to tri-dimensional structures. For clusters with even number of atoms, the peaks indicate that the clusters with the same number of Ag and Au atoms are the most stable ones. Analyzing the density of states, we found that increasing the concentration of Ag atoms affects the energy separation between the HOMO and the low lying occupied states.