Structural and Dynamic Properties of Graphene Grain Boundaries

Graphene grain boundaries (GBs) have significant effects on the material properties of graphene sheets. However, the structures of graphene GBs are not yet clear. In this work, we carried out a series of hybrid molecular dynamics simulations to study graphene symmetric tilt GB structures and energetics over a wide range of GB misorientation angles. Our simulations revealed that the graphene GBs were comprised of regular/irregular array of GB dislocations, depending on the GB misorientation angles. The structures of the GB dislocations depends strongly on the GB misorientation angles, see the insets of Fig. a. The GB energy [IMAGE png] as the function of GB misorientation angle is displayed in Fig. a. From Fig. a we found that there exists two cusps in GB energies, which correspond to the GBs with the highest GB dislocation densities (see the insets in Fig. a). This project has been published in the journal CARBON (DOI:10.1016/j.carbon.2011.01.063).

The GB dislocations often bulges out and introduce out-of-plane bucklings. These defects can either bulge up or down, and therefore, introduce two distinct polarities - namely, [IMAGE png] and [IMAGE png] polarities. By carrying out a series of very-long-time-scale, temperature accelerated dynamics (TAD) simulations, we found that these defects can flip their polarities via concerted motion involving multiple carbon atoms without breaking any covalent bond, see Fig. b. These polarity flipping transitions have 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. Furthermore, our TAD simulations show that when multiple defects are present, polarity flipping can propagate through the defects system leading to interesting and complex higher-order polarity combination changes, see Fig. c. We also showed that the conformations of graphene nanoribbon are strongly correlated with graphene defect polarity combinations. Hence, our TAD simulations demonstrate that a static structural view of defects in graphene is insufficient for describing defect effects on graphene properties, and defect polarity combinations represents the exercise of introducing extra degrees of freedom of graphene besides defect types. This work has been published in CARBON (DOI:10.1016/j.carbon.2012.02.055).

Figure: (a) graphene GB energy [IMAGE png] as the function of GB misorientation angles (symbols), the insets show the corresponding GB dislocation structures. (b) [IMAGE png]/[IMAGE png] defect polarity flipping transition pathway from TAD simulations. (c) time sequences of the evolution of the [IMAGE png] GB containing four periods along the GB direction (upper panels), and the defect blister pattern at T = 1000 K. The red arrows highlight bulges with [IMAGE png] polarity.