ACS Nano (2014)

Graphene crystals usually don't come with neat, straight edges. Rather, a graphene edge is a mix of armchair, zigzag or reconstructed-zigzag fragments — the latter resulting from two carbon atoms forming a pentagon with three neighbouring atoms of a zigzag edge. Kuang He and colleagues have now observed a fourth edge type known as 'bearded edges': dangling carbon–carbon bonds protruding from a zigzag edge, which were previously predicted to be unstable.

The authors performed aberration-corrected transmission electron microscopy experiments on 'bulk' graphene and graphene nanoribbons. In both cases, all four edge types were observed. By analysing images recorded at successive time steps, He et al. were able to reconstruct the edge dynamics. They found that the electron beam induced structural transformations like bond formation and breaking — or atom loss. The dangling bonds forming the bearded edges were found to lie slightly out of the graphene, an observation corroborated by density-functional-theory calculations.

Edge types directly influence electronic properties, so a bearded graphene edge is expected to result in a peculiar band structure. Indeed, it is known that for photonic crystals with the same beard structure, a novel type of edge state arises, associated with a highly dispersed, nearly flat band.