▎ 摘　　要

Topological defects in graphene, such as dislocations and grain boundaries, can produce out-of-plane buckling defects, which can bulge either up or down and, therefore, introduce two different polarities (namely, "up" and "down"). These defects have strong effects on graphene properties. We perform very long-time-scale, temperature accelerated molecular dynamics simulations of these topological defects and observe that they are able to flip polarity through concerted motion of multiple carbon atoms without breaking any covalent bonds; the simulations reveal the polarity flipping transition pathway. Polarity flipping has activation barriers ranging from 0.80 to 1.55 eV - several times lower than those of breaking covalent bonds (e.g., Stone-Wales bond rotations) and implies that flipping can occur over seconds or years, depending on the defect structure. The simulations further show that when multiple defects are present, polarity flipping can propagate through the defects system leading to interesting and complex higher-order polarity changes. (C) 2012 Elsevier Ltd. All rights reserved.