Phonon Scattering at Kinks in Suspended Graphene

TL;DR

This paper explains large thermal time constants in suspended graphene by modeling phonon scattering at kinks caused by edge adhesion, revealing a geometrically induced thermal interface resistance within a single 2D material.

Contribution

The study introduces a model showing how edge kinks in graphene create a thermal resistance, aligning with experimental data and highlighting a new type of interface resistance in 2D materials.

Findings

Thermal time constants match experimental observations.

Edge kinks significantly limit phonon transmission.

A new geometric thermal resistance mechanism is proposed.

Abstract

Recent experiments have shown surprisingly large thermal time constants in suspended graphene ranging from 10 to 100 ns in drums with a diameter ranging from 2 to 7 microns. The large time constants and their scaling with diameter points towards a thermal resistance at the edge of the drum. However, an explanation of the microscopic origin of this resistance is lacking. Here, we show how phonon scattering at a kink in the graphene, e.g. formed by sidewall adhesion at the edge of the suspended membrane, can cause a large thermal time constant. This kink strongly limits the fraction of flexural phonons that cross the suspended graphene edge, which causes a thermal interface resistance at its boundary. Our model predicts thermal time constants that are of the same order of magnitude as experimental data, and shows a similar dependence on the circumference. Furthermore, the model predicts the relative in-plane and out-of-plane phonon contributions to graphene's thermal expansion force, in agreement with experiments. We thus show, that in contrast to conventional thermal (Kapitza) resistance which occurs between two different materials, in 2D materials another type of thermal interface resistance can be geometrically induced in a single material.