Reversible wrinkles of monolayer graphene on a polymer substrate: toward stretchable and flexible electronics.

The surface instability of monolayer graphene supported by a soft (polymer) substrate under equal-biaxial compression has been explored through large scale coarse-grained molecular simulations. Regardless of the interfacial adhesion strength between the graphene and the substrate, herringbone wrinkles have always been observed due to their lowest energy status, compared with the checkerboard, hexagonal, triangular and one dimensional sinusoidal modes. Moreover, the graphene-polymer substrate interaction energy has a negligible effect on the critical strain for the onset of these wrinkles. Yet, if the graphene is bonded to a rigid (non-deformable) substrate, the critical strain increases with increasing graphene-substrate interfacial strength. The surface wrinkles of graphene are delayed and suppressed by the strong bonding of graphene to the rigid substrate. Besides, only localized folds and crumples have been observed on the surface of graphene, when graphene-substrate interaction energy is strong enough. All these observations signal that the deformability (stiffness) of the substrate plays an essential role in determining the morphology of supported graphene under compression. In addition, when a flat graphene is attached on a highly pre-strained (50%) polymer substrate, wrinkles will be formed on its surface during the relaxation of pre-strain within the polymer substrate. The wrinkled graphene could be stretched up to 50% without fracture, accompanied by the diminishing of surface wrinkles. Therefore, it opens a new avenue to enhance the stretchability of graphene materials, and enables the future applications of graphene and other 2D materials in stretchable and flexible electronics.