Corrugation-dominated mechanical softening of defect-engineered graphene

TL;DR

This study investigates how defect engineering via ion irradiation affects the elastic properties of graphene, revealing that corrugations caused by vacancies significantly soften the material, contrasting with some previous findings.

Contribution

It provides experimental measurements of elastic softening in defect-engineered graphene and identifies corrugations from vacancies as the primary cause, supported by atomistic simulations.

Findings

Elastic modulus decreases from 286 to 158 N/m after defect introduction.

Corrugations caused by vacancies lead to significant softening.

Surface contamination removal can reverse the softening effect.

Abstract

We measure the two-dimensional elastic modulus $E^\text{2D}$ of atomically clean defect-engineered graphene with a known defect distribution and density in correlated ultra-high vacuum experiments. The vacancies are introduced via low-energy (< 200 eV) Ar ion irradiation and the atomic structure is obtained via semi-autonomous scanning transmission electron microscopy and image analysis. Based on atomic force microscopy nanoindentation measurements, a decrease of $E^\text{2D}$ from 286 to 158 N/m is observed when measuring the same graphene membrane before and after an ion irradiation-induced vacancy density of $1.0\times 10^{13}$ cm$^{-2}$. This decrease is significantly greater than what is predicted by most theoretical studies and in stark contrast to some measurements presented in the literature. With the assistance of atomistic simulations, we show that this softening is mostly due to corrugations caused by local strain at vacancies with two or more missing atoms, while the influence of single vacancies is negligible. We further demonstrate that the opposite effect can be measured when surface contamination is not removed before defect engineering