Size and composition dependence in the optical properties of mixed (transition metal/noble metal) embedded clusters

Abstract

Optical properties of mixed clusters Ni/Ag, Co/Ag, and Ni/Au of 2--5 nm in diameter, produced by laser vaporization and embedded in an alumina matrix, are investigated. The first part is devoted to $({\mathrm{Ni}}_{x}{\mathrm{Ag}}_{1\ensuremath{-}x}{)}_{n}$ clusters whose photoabsorption spectra reveal a surface plasmon resonance, damped, broadened, and blueshifted as compared to pure silver clusters. For a given mean size, a blueshift of the resonance band as well as a damping and broadening with increasing nickel proportion is observed, in good qualitative agreement with classical predictions, assuming a Ni-core/Ag-shell geometry. This core-shell structure is confirmed by low energy ion spectroscopy measurements showing that the cluster surface is essentially composed of silver. For a given composition, the size evolution of the plasmon band consists of a damping and broadening with decreasing size whereas no clear shift is noticed. Although the classical predictions, including the surface scattering limited mean free path contribution of the conduction electrons in the silver shell, account well for the size effects, inhomogeneous effects (size, shape, and local porosity) also contribute to the broadening and damping of the resonance in experiment. The second part concerns the optical properties of $({\mathrm{Co}}_{0.5}{\mathrm{Ag}}_{0.5}{)}_{n}$ and $({\mathrm{Ni}}_{0.5}{\mathrm{Au}}_{0.5}{)}_{n}$ clusters. The size evolution of the optical properties of $({\mathrm{Co}}_{0.5}{\mathrm{Ag}}_{0.5}{)}_{n}$ clusters is similar to that of $({\mathrm{Ni}}_{0.5}{\mathrm{Ag}}_{0.5}{)}_{n}$ clusters, with in addition, a weak blueshift of the resonance with decreasing size. As for the $({\mathrm{Ni}}_{0.5}{\mathrm{Au}}_{0.5}{)}_{n}$ clusters, their optical spectra do not display a marked resonance. The comparison with a core-shell model including a size dependent damping constant in the gold or silver shell gives a good understanding of these features for the two systems Co/Ag and Ni/Au.