Nucleation of Silicon Nanoparticles in Amorphous Silicon Dioxide Matrices
In this contribution I summarize our research on the formation of nanoparticles embedded in a-SiO2-x matrices. In particular, I focus on the Si diffusion in SiO2-x matrices, and amorphous-to-crystalline phase transition in Si nanoparticles embedded in SiO2-x. I show that Si diffusion can be rationalized in terms of three mechanisms: O-driven (i.e. driven by the change of O-coordination), Si-driven, and bond-swapping (i.e. driven by the change of "local" stoichiometry). The relevance of each of these mechanisms depends on the silicon concentration in the sample and on the temperature. The O-driven mechanism dominates at low Si concentrations, and Si-driven and bond-swapping dominate at higher ones. At higher T the effect is mitigating the difference of contribution among the various mechanisms. Concerning amorphous-to-crystalline phase transition, I show that in small nanoparticles (radius <= 2 nm) the more stable phase at low Ts is the amorphous one, i.e. there is an inversion of the relative stability between the two phases with respect to bulk systems. In larger nanoparticles the bulk-like behavior is recovered. High temperatures favors the crystal phase in small nanoparticles. These results can explain the experimental observations that nanoparticles undergo to a phase transition in preparation protocol adopting "high" annealing temperatures (T >= 1150 degrees C).