Thermal transport in ultrathin Si nanowires: a first principles study

TL;DR

This study uses first-principles calculations to analyze phonon properties and thermal conductivity in ultrathin silicon nanowires, revealing size-dependent phonon behavior and limitations of boundary scattering models.

Contribution

It provides detailed first-principles insights into phonon dynamics and thermal transport in Si nanowires, highlighting size effects and comparing with classical boundary models.

Findings

Thermal conductivity ranges from 15 to 102 W/mK, increasing with nanowire diameter.

Phonon scattering rates are highest in the smallest nanowires and decrease with size.

Casimir boundary scattering underestimates thermal conductivity in these nanowires.

Abstract

Phonon properties of small Si nanowires in [110] direction have been analyzed using density functional perturbation theory. Several samples with varying diameters ranging from 0.38 to 1.5 nm have been calculated. It is found that the frequency of optical phonons at the zone center tend to decrease with increasing size of the nanowire. Investigation of the phonon scattering rates has revealed very high values in the smallest sample which decrease with increasing nanowire size. A remarkable change in scattering rates is shown for increasing diameter from 0.53 and 0.78 nm to 0.86 nm. The higher phonon scattering could be attributed to an alignment of phonon modes at a specific frequency. Results of the thermal conductivity are lower with respect to bulk Si and are found between 15 and 102 W/mK. A trend of increasing thermal conductivity with increasing diameter can be observed. This effect is attributed to several changes in the phonon dispersion which are not necessarily correlated to the wire size. These explicit results have been compared to the thermal conductivity when boundary effect is approximated with Casimir scattering. The Casimir method substantially underestimates the results for explicit nanowires.