Alloying Effects on the Optical Properties of Ag-Au Nanoclusters from TDDFT Calculations

The optical properties of alloyed Ag-Au 147-atom cuboctahedral nanoclusters are theoretically investigated as a function of composition and chemical ordering via a time-dependent density functional theory (TDDFT) approach. Compositions 37-63%, 46-54%, and 63-37%, in Ag-Au, and three types of chemical ordering, core shell, multishell and maximum mixing, are considered. Additionally, the optical spectra of pure Ag clusters with several structural motifs are also studied. It is found that (a) pure Ag dusters exhibit a neater dependence of the absorption peak on the shape of the cluster than Au clusters, (b) the absorption spectrum of alloyed clusters is not strongly affected by changes in chemical, ordering, possibly because of their limited size, and (c) the optical absorption peak smoothly shifts to higher energies, gets narrower, and substantially gains in intensity by increasing Ag concentration, in excellent agreement with available experimental data. An analysis of the character of the electronic transitions mostly contributing to the absorption peak allows us to rationalize the notable difference between Ag and Au in terms of optical properties and the effect of alloying.