Why covalent organic frameworks grow twisted on graphite

Van der Waals 2D materials made of graphene and Covalent Organic Frameworks (COFs) show a large potential for advanced applications, such as in superconductive and twistronic devices. High-quality pyrene diboronic acid (PDBA) COFs have been recently achieved on graphite, showcasing highly reproducible twisted stackings and moiré superlattices. A fundamental understanding of the processes ruling the formation of these unexpected materials is yet lacking. Hereby, we address this issue by studying growing COFs at different twist angles with an ad-hoc hybrid molecular mechanics force field with near-DFT accuracy. This reveals that as PDBA COFs grow, the number of available thermodynamic minima decreases dramatically, which may effectively lock the COF in the unique twisted stacking found experimentally. In addition, the mobility on the surface is found to show large alterations with size and twist angle. We discuss how both findings are expected to play a fundamental role in the synthesis of twisted PDBA COFs monolayers on graphitic surfaces.