Elastic films of two-dimensional single-crystal covalent organic frameworks
Naturally occurring and synthetic materials can be strong or tough, but rarely both; and when they are highly crystalline, they are normally bittle and fragile due to the existence of defects. This is especially true for two-dimensional (2D) crystals which can be even divided and disrupted by a line defecct such as grain boudnary. Now, an international team led by Zhikun Zheng of Sun Yat-sen University and Guangdong University of Technology, China, developed a sacrificial go-between guided interfacial synthesis method to introduce interwoven structure typically exist in amorphous linear polymers into macroscopic polycrystalline films of an emerging class of 2D crystals – 2D covalent organic frameworks to connect their single-crystal domains and enabled them with high strength, toughness and elasticity. The film could be easily synthesized on water surface in air. When it was fractured, its crack diffused shortly along grain boudary and most cracked area rebounded back to their initial position before fracture while crystals typically encountered serious crack propagation. The Young’s moduli and breaking strength of the 2D COFs can be as high as 73.4 ± 11.6 GPa, which is similar to that of alluminium alloy. The mechanical measurements were performed and analyzed by the group of Armin Gölzhäuser of Bielefeld University.
The groups of Ute Kaiser of Universität Ulm and Stefan C. B. Mannsfeld of Dresden University of Technology monitored the polymerization and crystallization of the 2D COFs using spherical-aerration-corrected high resolution transmission electron microscopy and grazing incidence wide-angle X-ray scattering. Kaiser and her collegures revealed the molecular structures and grain boundary structures of the films of the 2D COFs with near atomic resolution.
It is expected that 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.