Large freestanding 2D covalent organic framework nanofilms exhibiting high strength and stiffness

Two-dimensional covalent organic frameworks (2D COFs) represent an ideal platform to develop novel technological applications. The integration of 2D COFs into thin-film device architectures requires a deep knowledge of their mechanical performance, especially as large-area ultrathin films. Here, we report the synthesis, transfer, and mechanical characterization of large-area freestanding 2D COF films with nanometer thicknesses. Imine-linked COF nanofilms are prepared by condensation reaction at air–water interface, which provides freestanding, uniform centimeter-scale 2D COF films with controlled thickness. The developed procedure enables the direct transfer of the synthetized large-area COF nanofilm onto patterned substrates for mechanical characterization. Tensile tests are performed on freestanding 2D COF films with 85 nm thickness and with a testing area as large as 0.3 mm2. The measured strength of the COF nanofilms is 188 ± 57 MPa, while the Young's modulus is 37 ± 15 GPa. Our findings not only demonstrate the high stiffness and strength of COF nanofilms over a large-area, which make them suitable for applications where high mechanical performance is required, but also pave the way for a fundamental understanding of the relationship between the structures and macroscopic mechanical properties of 2D COFs. Thus, the method that we developed herein will enable a broad exploration of the properties of large-area 2D COFs that will guide their engineering design toward the development of novel COF-based devices.