Metal Alloy Nanoclusters by Ion Implantation in Silica

In the last decade, metal nanoclusters (NCs) embedded in insulating matrices have received increasing interest due to their peculiar optical, magnetic and catalytic properties when the size becomes comparable to or less than the electronic mean free path. Glass-based composites are, in general, expected to play an important role as materials for various nanotechnology application, due to the low cost, ease of processing, high durability, resistance and high transparency, as well as the possibility of tailoring the behavior of the glass-based structures. Metallic NCs embedded in glass can increase the optical third-order susceptibility of the matrix by several orders of magnitude, making such systems interesting candidates to be used as optical switches. Among different possible synthesis processing, ion-beam-based techniques proved to be very suitable in synthesizing NCcontaining glasses. Moreover, the composition of the clusters can be varied easily by sequential ion implantation in the matrix of two different elements whose energy and dose can be tailored so as to maximize the overlap between the implanted species and to control their local relative concentration. Nevertheless, for achieving tunability of the NCs properties for actual devices, a careful control over alloy clusters synthesis and stability has to be achieved in order to clarify which are the parameters (i.e., implantation conditions, subsequent thermal or laser annealings, ion irradiation, etc.) that can promote separation (via oxidation, for instance) instead of alloying of the implanted species. This chapter deals with metal nanocluster composites synthesized by ion implantation and formed by clusters of binary transition metals alloy embedded in silicate glasses, in which the cluster concentration is below the percolation limit (dispersed clusters). Some case studies will be presented in which ion-beam-based techniques are exploited either for the synthesis or for the structural or compositional modification of composites. In particular, the analysis will be focused on the formation of metal alloy nanoclusters in SiO 2 matrix obtained by sequential ion implantation of Au-Cu, Au-Ag, Co-Ni, Cu-Ni and Co-Cu. The most relevant technological parameters for ion implantation (dose, energy) and for the subsequent treatments (thermal annealing, ion-irradiation with light ions or laser annealing) are investigated to control cluster composition, size and stability. As far as the technological properties of the studied systems are concerned, some results will be presented about the nonlinear optical properties of Au-based alloy NCs composites.