Soliton instability and fold formation in laterally compressed graphene

We investigate-through simulations and analytical calculations-the consequences of uniaxial lateral compression applied to the upper layer of multilayer graphene. The simulations of compressed graphene show that strains larger than 2.8% induce soliton-like deformations that further develop into large, mobile folds. Such folds were indeed experimentally observed in graphene and other solid lubricants two-dimensional (2D) materials. Interestingly, in the soliton-fold regime, the shear stress decreases with the strain s, initially as s(-2/3) and rapidly going to zero. Such instability is consistent with the recently observed negative dynamic compressibility of 2D materials. We also predict that the curvatures of the soliton-folds are given by r(c) = delta root beta/2 alpha, where 1 <= delta <= 2, and beta and alpha are respectively related to the layer bending modulus and to the interlayer binding energy of the material. This finding might allow experimental estimates of the beta/alpha a ratio of 2D materials from fold morphology.