Microstructure of Au nanoclusters formed in and on SiO2

Au nanocrystals dispersed in or on an amorphous SiO2 substrate were formed by a self-organization procedure based on sputtering deposition and subsequent annealing. The kinetic mechanisms of the Au nanocrystals self-organization induced by the annealing processes were studied by transmission electron microscopy. A clear difference between the Au nanocrystals growth in and on SiO2 was found: in either cases the growth process was a ripening mechanism limited by diffusion but with a different growth rate. On the other hand, high-resolution transmission electron microscopy analyses allowed us to study the nanocrystals equilibrium shape and internal atomic structure evidencing a strong influence of the surrounding environment. Quantitative data on the cluster surface free energy were obtained via the inverse Wulff construction. An angular and radial symmetric surface free energy function for nanocrystals isotropically surrounded (Au nanocrystals in SiO2) was found, determining symmetric nanocrystals equilibrium shapes. An angular and radial asymmetrical surface free energy function for nanocrystals anisotropically surrounded (Au nanocrystals on SiO2) was found, determining asymmetrical nanocrystals equilibrium shapes. (C) 2008 Elsevier Ltd. All rights reserved.