  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.
