Optical properties of silver clusters formed by ion irradiation

Precipitation of silver clusters in silica is achieved by dierent methods: ion implantation, ion beam mixing of superimposed layers and ion irradiation of lms deposited by means of co-sputtering or sol-gel technique. Main features of the nanoparticles depending on the preparation method are investigated by TEM. The optical extinction resonance of these clusters is analysed in terms of sizes and interaction between the clusters on the basis of calculations. We found that resonances in sputtered and gel lms with low metal concentrations are well described by plasmon polaritons in isolated clusters and calculations based on Mie theory allow the study of their growth under irradiation. This theory is not appropriate to describe the optical response of silver clusters in silica implanted with Ag concentrations higher than 5 at.% or in ion beam mixed lms, because of the interaction between clusters. Using an eective medium model, it is demonstrated that the random dispersion of clusters in implantation lms causes fluctuations and, on average, an increase of the clusters polarization. On the contrary, the particular arrangement of the clusters with a bimodal size distribution in ion beam mixed lms induces a screening eect between the clusters of largest size.