Quantifying phonon particle and wave transport in nanostructures--The unexpectedly strong particle effect in silicon nanophononic metamaterial with cross junction

TL;DR

This study introduces a method combining Monte Carlo and atomic Green's function techniques to separately quantify phonon particle and wave effects in nanostructures, revealing a surprisingly strong particle contribution in silicon nanophononic metamaterials.

Contribution

It provides a novel approach to distinguish and quantify phonon particle and wave effects, demonstrating the significant particle effect in structures previously thought to be dominated by wave effects.

Findings

Particle effect contributes up to 39% to thermal conductivity reduction.

Particle effect remains strong even at very small cross-sectional areas.

Methodology enables detailed phonon transport analysis in nanostructures.

Abstract

Understanding phonon transport mechanisms in nanostructures is of great importance for delicately tailoring thermal properties. Combining phonon particle and wave effects through different strategies, previous studies have obtained ultra-low thermal conductivity in nanostructures. However, phonon particle and wave effects are coupled together, that is their individual contributions to phonon transport cannot be figured out. Here, we present how to quantify the particle and wave effects on phonon transport by combining Monte Carlo and atomic green function methods. We apply it to 1D silicon nanophononic metamaterial with cross-junctions, where it has been thought that the wave effect was the main modulator to block phonon transport and the particle effect was negligibly weak. Surprisingly, we find that the particle effect is quite significant as well and can contribute as much as 39% to the total thermal conductivity reduction. Moreover, the particle effect does not decrease much as the cross section area (CSA) of the structure decreases and still keeps quite strong even for CSA as small as 2.23 nm2. Further phonon transmission analysis by reducing the junction leg length also qualitatively demonstrates the strong particle effect. The results highlight the importance of mutually controlling particle and wave characteristics, and the methodologies for quantifying phonon particle and wave effect are important for phonon engineering by nanostructuring.