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Dislocations represent one of the most fascinating and fundamental concepts in materials science1-3. Most importantly, dislocations are the main carriers of plastic deformation in crystalline materials4-6. Furthermore, they can strongly affect the local electronic and optical properties of semiconductors and ionic crystals7,8. In materials with small dimensions, they experience extensive image forces, which attract them to the surface to release strain energy9. 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 carbide10 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 investigation by diffraction contrast analysis and atomistic simulations. Our investigation reveals two striking size effects. First, the absence of stacking-fault energy, a unique property of bilayer graphene, leads to a characteristic dislocation pattern that corresponds to an alternating AB [Lef-right arrow] AC change of the stacking order. Second, our experiments in combination with atomistic simulations reveal a pronounced buckling of the bilayer graphene membrane that results directly from accommodation of strain. In fact, the buckling changes the strain state of the bilayer graphene and is of key importance for its electronic properties11-14. Our findings will contribute to the understanding of dislocations and of their role in the structural, mechanical and electronic properties of bilayer and few-layer graphene.
In graphite, the bulk material most closely related to graphene, basal- plane dislocations have been well known since the early 1960s15. The dissociation of perfect basal-plane dislocations into pairs of Shockley partial dislocations (partials) bounding a stacking-fault ribbon in the basal plane was observed using conventional transmission electron micro- scopy15,16 (TEM). The separation of partials (or the width of the stacking- fault ribbon) is determined by the balance between the repulsive forces (reduction of total strain energy) and the attractive forces (minimization of stacking-fault energy). Similarly, the dissociation of perfect disloca- tions...