Nontrivial impact of interlayer coupling on thermal conductivity: opposing trends in in-plane and out-of-plane phonons

TL;DR

This paper investigates how interlayer coupling in 2D materials affects thermal conductivity, revealing opposing effects on in-plane and out-of-plane phonons and providing new insights into thermal management.

Contribution

It demonstrates that interlayer coupling can simultaneously reduce out-of-plane phonon conductivity and increase in-plane phonon conductivity in 2D materials, a nontrivial effect previously unexplored.

Findings

Out-of-plane phonon thermal conductivity decreases with increased coupling.

In-plane phonon thermal conductivity increases with increased coupling.

Phonon spectrum analysis links these effects to mode upshift and flattening.

Abstract

The study of heat transport in two-dimensional (2D) materials reveals novel behaviors due to quantum confinement effects, where in-plane and out-of-plane phonons play crucial roles. In 2D materials like graphene, it is widely recognized that the out-of-plane vibrational mode is the primary contributor to thermal conductivity owing to the mirror symmetry. Based on this perspective, the introduction of interlayer coupling, which breaks this symmetry, is expected to induce a significant reduction in thermal conductivity within 2D materials. Nevertheless, recent studies have presented unexpected findings, indicating that interlayer coupling can actually increase thermal conductivity of 2D materials. This controversial result suggests a nontrivial underlying mechanism governing the effects of interlayer coupling on thermal conductivity in 2D materials, necessitating further exploration. In our work, we investigate the modulation of thermal conductivity through interlayer coupling in a sandwich structure composed of hexagonal boron nitride (h-BN) and bilayer graphene (BG), specifically a h- BN/BG/h-BN system. Through molecular dynamics simulations, we find that the thermal conductivity from out-of-plane phonons can be significantly reduced, while that from in-plane phonons can be significantly increased, as the interlayer coupling strength increases. This results in a nontrivial, coupling-strength-dependent overall thermal conductivity. The phonon spectrum analysis conducted using our modified package reveals that the upshift and flattening of the out-of-plane (ZA and ZO) phonon modes are mainly responsible for these variations, and the extent of the upshift and flattening is proportional to the strength of interlayer coupling. This work offers new insights into manipulating the thermal conductivity of 2D materials.