Large Out-of-Plane Deformations of Two-Dimensional Covalent Organic Framework (COF) Sheets

Flexibility and deformability are important factors influencing the assembly and application of two-dimensional covalent organic frameworks (2D COFs). On one hand, the formation of stable, extended 2D sheets is a prerequisite for the fabrication of high-quality 2D crystals. On the other hand, characterizing these properties will eventually provide a path toward the inclusion of electron-vibration couplings when evaluating their electronic properties such as their band structures. Here, atomistic molecular dynamics simulations are used to investigate the mechanical properties of 2D COF sheets, taking the prototypical COF-5 as a representative example. Large out-of-plane deformations are found, about 400% higher than those encountered in graphene. In addition, structural defects lead to significantly larger twists and deformations, which underlines the challenges in fabricating stand-alone, large-size 2D COF sheets. Stacking, on the other hand, effectively reduces the out-of-plane deformations and suppresses the role of defects, two aspects beneficial to the lateral extension of 2D COF sheets.