Clasping folds enable superior elastic strength of graphene films

Two-dimensional graphene sheets show strong potential for use in fabrication of macroscopic films, as they have exceptional mechanical properties owing to their in-plane strength^1-6. Improvements in the stacking order of flat two-dimensional sheets have led to corresponding improvements in the strength and toughness of such films^7-11. In addition, modulation of the spatial conformation of individual units has enabled the development of benchmark materials with superior properties^12,13; however, this has remained elusive in the case of two-dimensional sheets. Here we report a strategy for preparation of graphene films with a combination of high strength (2.9 GPa), toughness (84.5 MJ m^-3) and elastic strain (5%). We attribute these performance gains to a fold-clasping lamellar structure achieved by modular assembly of surface-folded multilayer graphene oxide flakes. We identify a pull-to-push transition of force chain topology and propose a shear-lock stress transfer strategy in the fold-clasping lamellar graphene film that enables advances in mechanical properties. This work demonstrates the modular design of spatial conformation of two-dimensional flakes on a large scale and further improves the performance of macroscopic films for potential applications in lightweight structural components.