Effect of Alloying on the Optical Properties of Ag-Au Nanoparticles

The optical response of Au-Ag bimetallic nanoalloys has been studied using pseudopotential time-dependent density-functional theory calculations. The structures included the magic-number icosahedral nanoparticles of 55 and 147 atoms, 37-atom pentagonal rods, and 20-atom tetrahedra. Our results show strong resonances for the pure Ag nanoparticles and strongly broadened spectra with many transitions for the pure gold structures, in qualitative agreement with available experiment and previous calculations. For bimetallic core-shell particles, the outer shell determines the overall character of the optical response; a single outer layer of Ag can produce an Ag-like resonance even in a gold-rich structure. The inclusion of a gold core within a silver shell leads to a distinct red-shift of the silver-like resonances as well as to some damping. The bimetallic nanoparticles are found to be very sensitive to the chemical configuration, the position of the atomic species in some cases outweighing the effect of changing composition. For randomly alloyed configurations of 147-atom nanoparticles, the spectra show a smooth transition between pure Ag and Au and are in good qualitative agreement with available experiment.