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.

多晶材料的性能通常由缺陷决定，二维（2D）晶体甚至可以被线缺陷分割和破坏^1-3。相比之下，二维晶体通常需要加工成薄膜，不可避免地是多晶且含有大量晶界，因此脆且易碎，阻碍了在柔性电子、光电子和分离中的应用^1-4。此外，与玻璃、木材和塑料类似，它们也受到机械强度和韧性之间的权衡影响。^{5, 6}在这里，我们报告了一种生产高强度、坚韧和弹性薄膜的新兴二维晶体薄膜的方法 - 二维共价有机框架 (COF)，由单晶域组成，通过水表面上交织的晶界连接，使用脂肪族双胺作为牺牲媒介。展示了两种 2DCOF 薄膜，其杨氏模量和断裂强度分别为 56.7 ± 7.4 GPa 和 73.4 ± 11.6 GPa，以及 82.2 ± 9.1 N/m 和 29.5 ± 7.2 N/m。我们预计，牺牲引导合成方法和交织晶界将激发各种多晶材料的晶界工程，赋予它们新的性能，增强其当前的应用，并为新的应用铺平道路。