In addition to its unique electrical properties, graphene is prized due to its amazing strength-to-thickness ratio. However, as the current method for growing large area sheets involves chemical vapor deposition (CVD) onto metallic substrates, the size of graphene grains is determined by the size of the substrate domains. This introduces defects into the graphene sheet in the form of grain boundaries, which are expected to have a devastating effect on the material’s strength.
However, contrary to what may be expected from conventional wisdom, researchers from Brown University and the University of Texas have determined that domains might not be as big a problem as previously thought. In fact, while pristine, domain-free graphene is preferred, if defects are present, more is better than few [Grantab et al., (2010) Science 330, 946].
Graphene is composed of a sheet of carbon atoms, linked in a hexagon formation. When two misorientated grains come together a defect is introduced in the form of a pentagon and heptagon. If the mismatch between the two grains is small, there will be very few defects along the boundary, as the material in-between the defects is stable with a hexagonal arrangement. As the misorientation angle increases, the number of defects increases, until eventually the whole boundary wall is composed of pentagons and heptagons.
Shenoy and coworkers found that when a graphene sheet is stretched perpendicular to the grain boundary, it is actually strongest when the mismatch between grains is just large enough to produce a full boundary of defects. Such a revelation came as a great surprise, as it can not be explained on the basis of continuum mechanics. Instead the researchers had to employ an atomic-level study of the bond failure in order to understand the result.
Using density functional theory (DFT) calculations the researchers found that an existing strain exists within imperfect graphene, even before the materials is mechanically stretched. This strain is a result of the bond lengths at the heptagon-septagon sites. However, for larger mismatch angles this strain is reduced, as the bond length at the heptagon-septagon sites approaches the bond length present in pristine graphene.
Professor Vivek Shenoy, coauthor of the paper, talked to Materials Today about the possibility of manipulating the growth such that the optimum mismatch angle can be obtained. He told us that “At present we do not have a very good understanding of the way in which graphene islands nucleate during growth. This is a very active area of research and I would not be surprised if a clear picture of the nucleation process emerges in a year or so. Once that happens, one would be able to control grain orientations”. If this is the case we could soon be seeing graphene with designer defects.
Stewart Bland