One-dimensional (1D) materials such as various kinds of nanowires and nanotubes have attracted considerable attention due to their potential applications in electronic and energy conversion devices. The thermal transport phenomena in these nanowires and nanotubes could be significantly different from that in bulk material due to boundary scattering, phonon dispersion relation change, and quantum confinement. It is very important to understand the thermal transport phenomena in these materials so that we can apply them in the thermal design of microelectronic, photonic, and energy conversion devices. While intensive experimental efforts are being carried out to investigate the thermal transport in nanowires and nanotube, an accurate numerical prediction can help the understanding of phonon scattering mechanisms, which is of fundamental theoretical significance. A Monte Carlo simulation was developed and applied to investigate phonon transport in single crystalline Si nanowires. The Phonon-phonon Normal (N) and Umklapp (U) scattering processes were modeled with a genetic algorithm to satisfy both the energy and the momentum conservation. The scattering rates of N and U scattering processes were given from the first perturbation theory. Ballistic phonon transport was modeled with the code and the numerical results fit the theoretical prediction very well. The thermal conductivity of bulk Si was then simulated and good agreement was achieved with the experimental data. Si nanowire thermal conductivity was then studied and compared with some recent experimental results. In order to study the confinement effects on phonon transport in nanowires, two different phonon dispersions, one based on bulk Si and the other solved from the elastic wave theory for nanowires, were adopted in the simulation. The discrepancy from the simulations based on different phonon dispersions increases as the nanowire diameter decreases, which suggests that the confinement effect is significant when the nanowire diameter goes down to tens nanometer range. It was found that the U scattering probability engaged in Si nanowires was increased from that in bulk Si due to the decrease of the frequency gap between different modes and the reduced phonon group velocity. Simulation results suggest that the dispersion relation for nanowire solved from the elasticity theory should be used to evaluate nanowire thermal conductivity as the nanowire diameter reduced to tens nanometer.
Decoupling electron and phonon transport in single-nanowire hybrid materials for high-performance thermoelectrics
