Heat conductance is strongly anisotropic for pristine silicon nanowires

TL;DR

This study reveals that heat conductance in pristine silicon nanowires is highly anisotropic, with significant differences depending on wire orientation, and provides atomistic insights into the underlying phonon behavior.

Contribution

The paper demonstrates the anisotropic heat conductance in silicon nanowires and links it to bulk phonon band structure, using DFT and empirical potential models for different diameters.

Findings

Wires along <110> have 50-75% higher conductance than <100> and <111>

DFT and TEP results agree within 10% for smallest wires

Anisotropy can be qualitatively explained by bulk phonon band structure

Abstract

We compute atomistically the heat conductance for ultra-thin pristine silicon nanowires (SiNWs) with diameters ranging from 1 to 5 nm. The room temperature thermal conductance is found to be highly anisotropic: wires oriented along the <110> direction have 50-75% larger conductance than wires oriented along the <100> and <111> directions. We show that the anisotropies can be qualitatively understood and reproduced from the bulk phonon band structure. Ab initio density functional theory (DFT) is used to study the thinnest wires, but becomes computationally prohibitive for larger diameters, where we instead use the Tersoff empirical potential model (TEP). For the smallest wires, the thermal conductances obtained from DFT- and TEP calculations agree within 10%. The presented results could be relevant for future phonon-engineering of nanowire devices.