Anomalous twin boundaries in 2D materials

The high mechanical strength and excellent flexibility of 2D materials such as graphene are some of their most important properties [1]. Good flexibility is key for exploiting 2D materials in many emerging technologies, such as wearable electronics, bioelectronics, protective coatings and composites [1] and recently bending has been suggested as a route to tune electronic transport behaviour [2]. For virtually all crystalline materials macroscopic deformation is accommodated by the movement of dislocations and through the formation of twinning defects [3]; it is the geometry of the resulting microstructure that largely determines the mechanical and electronic properties. Despite this, the atomic microstructure of 2D materials after mechanical deformation has not been widely investigated: only by understanding these deformed microstructures can the resulting properties be accurately predicted and controlled. In this paper we describe the different structural features that can form as a result of bending in van der Waals (vdW) crystals of 2D materials. We show that twin boundaries, an important class of crystal defect, are delocalised by several nm and not atomically sharp as has been assumed for over half a century [4]. In addition, we demonstrate that different classes of microstructure are present in the deformed material and can be predicted from just the atomic structure, bend angle, and flake thickness. We anticipate that this new knowledge of the deformation structure for 2D materials will provide foundations for tailoring transport behaviour[2], mechanical properties, liquid-phase [5,6] and scotch-tape exfoliation [7,8], and crystal growth.