Thermal transport in H-terminated ultrathin [110] Si nanowires: a first principles study

Phonon properties of ultrathin Si nanowires in [110] direction have been calculated by density functional perturbation theory. Several samples with varying diameters ranging from 0.38 to 1.5 nm have been investigated. It is found that the frequencies of optical phonons at the zone center increase with decrease in size of the nanowire, giving rise to avoided-crossing with longitudinal acoustic phonons. This feature determines a corresponding increase in the scattering rates and flattening of the longitudinal acoustic mode. More specifically, a remarkable change in scattering rates is shown for decreasing diameter. Results of the thermal conductivity are much lower with respect to bulk Si and are found between 40 and 119 Wm^-1 K^-1 , also providing evidence of increasing thermal conductivity with increase in diameter. This effect is attributed to several changes in the phonon dispersions. Finally, it is shown that approximating the boundary scattering of phonons by Casimir scattering leads to a severe underestimation of the thermal conductivity in these systems.