Thermal Stability and Phonon Thermal Transport in Spherical Silicon Nanoclusters

Silicon nanoclusters/nanoparticles have attracted increasing attention in innovative technology due to their unique properties, which differ from those of bulk materials. Structural and thermodynamic properties of nanoclusters are fundamentally important for the performance and stability of nanocluster-based devices. Unlike the homogeneous melting of bulk silicon, the melting of crystalline silicon nanospheres proceeds over a finite temperature range due to surface effects, which shows the heterogeneous melting of nanoclusters. The melting temperature of silicon nanoclusters is lower than that of bulk silicon and rises with the increase of the cluster size. Structure changes upon heating indicates that the melting of silicon nanospheres is progressively developed from the surface and into the core. The structure of the spherical silicon nanocluster can change gradually from the bulk diamond structure to a non-diamond structure with the decrease in the cluster size. Hydrogenated silicon nanoclusters are thermally stable if the hydrogen coverage is more than 50%. The thermal conductivity of the spherical silicon nanoclusters shows a size-dependent effect arising from the remarkable phonon-boundary scattering and can be about three orders of magnitude lower than that of bulk silicon. The thermal conductivity of crystalline nanospheres also decreases as the temperature increases from 50 to 1000 K because of the stronger phonon-phonon scattering at higher temperatures.