# 3D strain in 2D materials: Experimental test in unsupported monolayer graphene under pressure

## TL;DR

This study measures the in-plane and out-of-plane stiffness of unsupported monolayer graphene under pressure using Raman spectroscopy, revealing similar in-plane properties to graphite and providing estimates for out-of-plane stiffness.

## Contribution

First experimental measurement of 3D strain in unsupported monolayer graphene under pressure, clarifying its mechanical properties without substrate effects.

## Key findings

- In-plane stiffness of graphene is similar to graphite (~5.4 cm$^{-1}$GPa$^{-1}$).
- Out-of-plane stiffness of graphene estimated at 1.4 GPa, comparable to graphite.
- Unsupported graphene exhibits comparable anharmonicity to graphite.

## Abstract

Previous Raman measurements on supported graphene under high pressure reported a very different shift rate of in-plane phonon frequency of graphene (16 cm$^{-1}$GPa$^{-1}$) from graphite (4.7 cm$^{-1}$GPa$^{-1}$), implying very different in-plane anharmonicity that graphene gets stiffer than graphite in-plane under the same pressure. It was suggested that it could be due to the adhesion of graphene to substrates. We have therefore performed high pressure Raman measurements on unsupported graphene and we find a similar in-plane stiffness and anharmonicity of graphene (5.4 cm$^{-1}$GPa$^{-1}$) to graphite. On the other hand, the out-of-plane stiffness of graphene is hard to define, due to the 2D nature of graphene. However, we estimate a similar out-of-plane stiffness of graphene (1.4$\pm$295 GPa) to that of graphite (38.7$\pm$7 GPa), by measuring its effect on the shift of the in-plane phonon frequency with pressure.