Vibrations and thermal transport in nanocrystalline silicon

We use a combination of vibrational-mode analysis and molecular-dynamics simulations to study the effect of grain size on the nature of thermal vibrations, their localization, and their ability to carry heat in nanocrystalline silicon. Vibrational-mode analysis demonstrates that the vibrations that carry most of the heat in small-grain (less than or similar to 3 nm) structurally heterogeneous nanocrystalline silicon are almost identical in nature to those in homogeneous amorphous silicon, where the majority of the vibrations are delocalized and unpolarized. Consequently, the principal thermal conductivity mechanism in such a nanocrystalline material is the same as in the amorphous material. With increasing grain size, the vibrational modes become progressively more like that of a crystalline material; this is reflected in a crossover in the mechanism of thermal transport to that of a crystalline material.