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 different stoichiometries (from x=0 up to I in steps of 0.25), in the diameter 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 frequency versus x. For a given relative composition, the blueshift (relative to the prediction of Mie's theory), damping, and broadening of the resonance all become more important as the size decreases and as the gold concentration increases. These spectra are compared with model calculations, based on the time-dependent local-density-approximation formalism, including both the porosity of the matrix and an inner skin of reduced ionic-core polarizability. Two models are introduced to describe the effective dielectric function of the ionic background in the bimetallic cluster. Agreement between experimental results and both models is discussed.