Atomistic Structure, Motion, and Thermal Conductivity of van der Waals Grain Boundaries in Two‐Dimensional Covalent Organic Frameworks

Two‐dimensional covalent organic frameworks (2D COFs) are typically viewed as ideal crystals with periodic structures, yet experimentally fabricated 2D COF films and powders consist of nanoscale grains with abundant van der Waals (vdW) grain boundaries. To elucidate the structural and functional impact of these ubiquitous interfaces, we conducted extensive molecular dynamics simulations on tens of vdW grain boundary configurations for representative 2D COFs. We found that these grain boundaries often exhibit significant in‐plane layer offsets, monomer insertions, and deformations, making them partially responsible for the interlayer displacement patterns commonly observed in X‐ray diffraction data. Free‐energy calculations reveal that out‐of‐plane sliding is the least energetically costly, while in‐plane motions at certain types of grain boundaries can require more energy than interlayer separation in perfect crystals. Additionally, pronounced interfacial thermal resistance was observed at grain boundaries (over ∼2–3 nm depth), with thermal conductivity reduced to 1%–20% of intrinsic values, depending on contact area and degree of monomer insertion. These findings underline the critical role played by grain boundary configurations in packing structure, motions, and thermal transport properties in 2D COFs, and emphasize the need to account for such interfacial effects in both fundamental studies and device applications.