Anomalous Strength Characteristics of Tilt Grain Boundaries in Graphene

Perfect Imperfections Graphene is composed of six-atom rings, but will include a number of five- and seven-atom rings as defects. Using simulations, Grantab et al. (p. 946) show that more defects do not necessarily lead to greater deterioration of mechanical properties. Mismatches caused by differences in the orientation of neighboring crystals are divided into low- and high-angle grain boundaries, and typically it is the lower-angle boundaries that are stronger. In graphene, by contrast, the larger-angle boundaries, which consist of higher-defect densities, are better able to accommodate the strain and prevent failure that originates in the breakup of the seven-member graphene rings. This suggests ways for synthesizing imperfect graphene sheets that will have mechanical properties that are close to those of perfect graphene. Simulations indicate that high-angle boundaries can better accommodate strain and prevent failure in graphene. Graphene in its pristine form is one of the strongest materials tested, but defects influence its strength. Using atomistic calculations, we find that, counter to standard reasoning, graphene sheets with large-angle tilt boundaries that have a high density of defects are as strong as the pristine material and, unexpectedly, are much stronger than those with low-angle boundaries having fewer defects. We show that this trend is not explained by continuum fracture models but can be understood by considering the critical bonds in the strained seven-membered carbon rings that lead to failure; the large-angle boundaries are stronger because they are able to better accommodate these strained rings. Our results provide guidelines for designing growth methods to obtain sheets with strengths close to that of pristine graphene.