Anomalous Strength Characteristics of Tilt Grain Boundaries in Graphene

TL;DR

This study reveals that large-angle tilt grain boundaries in graphene, with higher defect densities, can be stronger than low-angle boundaries, challenging traditional fracture mechanics predictions and informing improved growth strategies.

Contribution

The paper demonstrates that large-angle tilt grain boundaries in graphene are stronger than low-angle boundaries, contradicting continuum fracture mechanics, and identifies the atomic bonds responsible for failure.

Findings

Large-angle boundaries with more defects are stronger.

Failure is due to strained bonds in 7-member rings.

Results challenge Griffith-type fracture predictions.

Abstract

Using molecular dynamics simulations and first principles calculations, we have studied the structure and mechanical strength of tilt grain boundaries in graphene sheets that arise during CVD growth of graphene on metal substrates. Surprisingly, we find that for tilt boundaries in the vicinity of both the zig-zag and arm-chair orientations, large angle boundaries with a higher density of 5-7 defect pairs are stronger than the low-angle boundaries which are comprised of fewer defects per unit length. Interestingly, the trends in our results cannot be explained by a continuum Griffith-type fracture mechanics criterion, which predicts the opposite trend due to that fact that it does not account for the critical bonds that are responsible for the failure mechanism. We have identified the highly-strained bonds in the 7-member rings that lead to the failure of the sheets, and we have found that large angle boundaries are able to better accommodate the strained 7-rings. Our results provide guidelines for designing growth methods to obtain grain boundary structures that can have strengths close to that of pristine graphene.