Thermal behavior of indium nanoclusters in ion-implanted silica

Fused silica substrates were implanted with 2x10(17) In(2+)/cm(2) ions at 320 keV. Indium crystalline nanoclusters with an average size of about 15-20 nm were found in the as-implanted samples. The thermal behavior of the nanoclusters was studied by performing heating-cooling cycles in vacuum and by using in-situ techniques based on glancing-incidence x-ray diffraction and transmission electron microscopy. The precipitates showed both superheating and supercooling. Moreover, no evidence of clusters growth or reorientation during the thermal cycle was found. A detailed study of the heating sequence showed that the melting temperature of the Indium precipitates depended on their size, i.e., the smallest particles melt first and at a temperature which is about 7 K below the bulk melting point, while the largest ones were superheated until about 13 K above it. Moreover, a remarkable stability of the In cluster well above their melting temperature (up to about 980 K) was evidenced by in-situ transmission electron microscopy analysis. From a thermodynamic point of view, the experimental results were explained by considering two effects acting on the clusters: the thermodynamic size effect and the pressure of the silica matrix.