Atomistic simulation of phonon heat transport across metallic nanogaps

TL;DR

This paper develops a combined atomistic simulation framework to study phonon-mediated heat transport across metallic nanogaps, revealing significant tunneling effects and the roles of anharmonicity and electrostatics.

Contribution

It introduces a novel simulation approach combining MD and NEGF to analyze heat transfer at nanometer gaps, providing new insights into phonon tunneling and near-field heat transport.

Findings

Phonon tunneling is a major heat transfer channel below 1nm gap.

Lattice anharmonicity accounts for 20-30% of phonon tunneling.

Electrostatic interactions are negligible at typical experimental biases.

Abstract

The understanding and modeling of the heat transport across nanometer and sub-nanometer gaps where the distinction between thermal radiation and conduction become blurred remains an open question. In this work, we present a three-dimensional atomistic simulation framework by combining the molecular dynamics (MD) and phonon non-equilibrium Green's function (NEGF) methods. The relaxed atomic configuration and interaction force constants of metallic nanogaps are generated from MD as inputs into harmonic phonon NEGF. Phonon tunneling across gold-gold and copper-copper nanogaps is quantified, and is shown to be a significant heat transport channel below gap size of 1nm. We demonstrate that lattice anharmonicity contributes to within 20-30% of phonon tunneling depending on gap size, whereas electrostatic interactions turn out to have negligible effect for the small bias voltage typically used in experimental measurements. This work provides detailed information of the heat current spectrum and interprets the recent experimental determination of thermal conductance across gold-gold nanogaps. Our study contributes to deeper insight into heat transport in the extremely near-field regime, as well as hints for the future experimental investigation.