Moiré Patterns of Two-Dimensional Covalent Organic Framework Single Crystals Created by Superstacking

Two-dimensional (2D) covalent organic frameworks (COFs) are useful due to their molecular and crystalline features. However, the knowledge of the interlayer stacking of 2D COFs remains limited. Here, we report a 2D COF, COF-400, exhibiting an unprecedented ABCD stacking mode. Such a stacking mode was inherited in its reduction and oxidation derivatives, COF-400-Re and -Ox, respectively. Their structures were solved by single crystal X-ray/electron diffraction with a resolution up to 0.75 Å, as well as real-space imaging using transmission electron microscopy (TEM) with an interpretable resolution up to 0.95 Å. Furthermore, Moiré patterns were observed in the superstacking of exfoliated COF films with the twisted angle varying from 6.4 to 69.7°. Individual sets of 2D COF single crystals in the Moiré superstructures were clearly identified by TEM with a resolution of 1.6 Å. These findings not only enrich the knowledge on the stacking behaviors of 2D COFs but also offer extra means for the manipulation of 2D COFs, potentially useful for device fabrication.