Ultralow thermal conductivity of single crystalline porous silicon nanowires

TL;DR

This study demonstrates that single crystalline porous silicon nanowires exhibit ultralow thermal conductivity due to reduced phonon group velocity, with potential applications in electronics and nano-electromechanical systems.

Contribution

It establishes a direct relationship between porosity and thermal conductivity in crystalline silicon nanowires using novel measurement techniques and molecular dynamics simulations.

Findings

Thermal conductivity as low as 0.33 W/m·K at 43% porosity

Reduction in phonon group velocity verified by Young's modulus measurements

Porous silicon nanowires have structural sizes less than 5 nm

Abstract

Porous materials provide a large surface to volume ratio, thereby providing a knob to alter fundamental properties in unprecedented ways. In thermal transport, porous nanomaterials can reduce thermal conductivity by not only enhancing phonon scattering from the boundaries of the pores and therefore decreasing the phonon mean free path, but also by reducing the phonon group velocity. Here we establish a structure-property relationship by measuring the porosity and thermal conductivity of individual electrolessly etched single crystalline silicon nanowires using a novel electron beam heating technique. Such porous silicon nanowires exhibit extremely low diffusive thermal conductivity (as low as 0.33 Wm-1K-1 at 300K for 43% porosity), even lower than that of amorphous silicon. The origin of such ultralow thermal conductivity is understood as a reduction in the phonon group velocity, experimentally verified by measuring the Young modulus, as well as the smallest structural size ever reported in crystalline Silicon (less than 5nm). Molecular dynamics simulations support the observation of a drastic reduction in thermal conductivity of silicon nanowires as a function of porosity. Such porous materials provide an intriguing platform to tune phonon transport, which can be useful in the design of functional materials towards electronics and nano-electromechanical systems.