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
