Disorder and cluster formation during ion irradiation of Au nanoparticles inSiO2

Abstract

Au nanoparticles (NPs) have been formed by ion beam synthesis in $600\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ thin $\mathrm{Si}{\mathrm{O}}_{2}$. Subsequently the NPs were irradiated with $2.3\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$ Sn ions at liquid nitrogen temperature. Samples were analyzed using extended x-ray absorption fine structure (EXAFS) spectroscopy and small angle x-ray scattering (SAXS) as a function of Sn irradiation dose. Transmission electron microscopy shows that the NPs largely retain their spherical shape upon irradiation. However, we observe a reduction in average NP size and a concomitant significant narrowing of the size distribution with increasing irradiation dose as consistent with inverse Ostwald ripening. At lower irradiation doses, significant structural disorder is apparent with an effective bond length expansion as consistent with amorphous material. At higher irradiation doses, EXAFS measurements indicate dissolution of a significant fraction of Au from the NPs into the $\mathrm{Si}{\mathrm{O}}_{2}$ matrix (as monomers) and the formation of small Au clusters (dimers, trimers, etc.). We estimate the volume fraction of such monomers/clusters. Ion irradiation thus yields disordering then dissolution of Au NPs.