Plasmon resonance shifts in oxide-coated silver nanoparticles

Abstract

Silver-silica nanocomposites have been synthesized by an electrodeposition technique. Silver oxide layers have been grown on the silver particles by an oxidation treatment of the nanocomposites in the temperature range 478-653 K. Optical absorption spectra of films of these nanocomposites dispersed in polystyrene were studied in the wavelength range 250-800 nm. Two absorption peaks were observed. The one around 370 nm is shown to be due to the uncoated silver nanoparticles. The other in the range 550-700 nm arises due to the presence of silver-core-silver-oxide nanoshells in these materials. The data were analyzed by the relevant theory of optical scattering from ultrafine composite particles which took into account the size-dependent dielectric permittivity of silver metal cores. The analysis indicates that nanoshells of thicknesses in the range 1.0-10.6 nm are grown on the metal core depending on the oxidation temperature. The metal cores having diameters equal to or less than 3 nm show an electrical conductivity which is less than Mott's minimum metallic conductivity. The analysis of the first absorption peak also leads to the conclusion that the silver metal particles causing this absorption show a metal-insulator Mott transition. This implies that the metal particles have diameters less than 3 nm. The growth rate of the nanoshell indicates an activation energy of 0.29 eV, which is consistent with that for oxygen diffusion in a silicate glass.