Optical properties of (AuxAg1-x)(n) clusters embedded in alumina: Evolution with size and stoichiometry - art. no. 085407

Mixed (AuxAg1-x)(n) clusters of various relative compositions are produced in a laser vaporization source, and embedded in an alumina matrix. The size and concentration effects in the optical properties are investigated for d ifferent stoichiometries (from x=0 up to I in steps of 0.25), in the diamet er range 1.5-5 nm for which the size evolution is ruled by quantum effects. For a given size, we measure a steady evolution of the surface plasmon fre quency versus x. For a given relative composition, the blueshift (relative to the prediction of Mie's theory), damping, and broadening of the resonanc e all become more important as the size decreases and as the gold concentra tion increases. These spectra are compared with model calculations, based o n the time-dependent local-density-approximation formalism, including both the porosity of the matrix and an inner skin of reduced ionic-core polariza bility. Two models are introduced to describe the effective dielectric func tion of the ionic background in the bimetallic cluster. Agreement between e xperimental results and both models is discussed.