Elastic films of single-crystal two-dimensional covalent organic frameworks – Nature

The properties of polycrystalline materials are often dominated by defects, and two-dimensional (2D) crystals can even be divided and disrupted by a line defect^1-3. In contrast, 2D crystals are often required to be processed into films, which are inevitably polycrystalline and contain numerous grain boundaries, and therefore are brittle and fragile, hindering application in flexible electronics, optoelectronics and separation^1-4. Moreover, similar to glass, wood, and plastics, they suffer from trade-off effects between mechanical strength and toughness.^{5, 6} Here, we report a method to produce highly strong, tough and elastic films of an emerging class of 2D crystals – 2D covalent organic frameworks (COFs) composed of single-crystal domains connected by interwoven grain boundary on water surface using an aliphatic bi-amine as a sacrificial go-between. Films of two 2DCOFs were demonstrated, which showed Young’s moduli and breaking strength of 56.7 ± 7.4 GPa and 73.4 ± 11.6 GPa, and 82.2 ± 9.1 N/m and 29.5 ± 7.2 N/m, respectively. We envisage the sacrificial go-between guided synthesis method and the interwoven grain boundary will inspire grain boundary enigineering of various polycrystalline materials, endowing them with new properties, enhancing their current applications and paving the way for new applications.