Dislocations in bilayer graphene

Basal-plane dislocations, identified as fundamental defects in bilayer graphene by transmission electron microscopy and atomistic simulations, reveal striking size effects, most notably a pronounced buckling of the graphene membrane, which drastically alters the strain state and is of key importance for the material’s mechanical and electronic properties. Bilayer graphene buckles under strain of dislocations Dislocations in crystalline materials — line defects where atoms are dislodged from their position in the crystal — can carry plastic deformation and also modify the material's electronic properties. Benjamin Butz et al . report the experimental and theoretical study of line defects in epitaxial bilayer graphene — the thinnest imaginable crystalline material that can host such dislocations — and describe their properties in terms of classical dislocation theory. As a consequence of the novel two-dimensional organization of this material, a characteristic equidistant pattern of dislocations develops, associated with a pronounced buckling of the bilayer graphene membrane. The buckling dramatically alters the strain state of the bilayer graphene and potentially changes its electronic structure and mechanical properties too. Dislocations represent one of the most fascinating and fundamental concepts in materials science 1 , 2 , 3 . Most importantly, dislocations are the main carriers of plastic deformation in crystalline materials 4 , 5 , 6 . Furthermore, they can strongly affect the local electronic and optical properties of semiconductors and ionic crystals 7 , 8 . In materials with small dimensions, they experience extensive image forces, which attract them to the surface to release strain energy 9 . However, in layered crystals such as graphite, dislocation movement is mainly restricted to the basal plane. Thus, the dislocations cannot escape, enabling their confinement in crystals as thin as only two monolayers. To explore the nature of dislocations under such extreme boundary conditions, the material of choice is bilayer graphene, the thinnest possible quasi-two-dimensional crystal in which such linear defects can be confined. Homogeneous and robust graphene membranes derived from high-quality epitaxial graphene on silicon carbide 10 provide an ideal platform for their investigation. Here we report the direct observation of basal-plane dislocations in freestanding bilayer graphene using transmission electron microscopy and their detailed investigatio