Synthesis of well-ordered COF monolayers: surface growth of nanocrystalline precursors versus direct on-surface polycondensation.

Two different straightforward synthetic approaches are presented to fabricate long-range-ordered monolayers of a covalent organic framework (COF) on an inert, catalytically inactive graphite surface. Boronic acid condensation (dehydration) is employed as the polymerization reaction. In the first approach, the monomer is prepolymerized by a mere thermal treatment into nanocrystalline precursor COFs. The precursors are then deposited by drop-casting onto a graphite substrate and characterized by scanning tunneling microscopy (STM). While in the precursors monomers are already covalently interlinked into the final COF structure, the resulting domain size is still rather small. We show that a thermal treatment under reversible reaction conditions facilitates on-surface ripening associated with a striking increase of the domain size. Although this first approach allows studying different stages of the polymerization, the direct polymerization, that is, without the necessity of preceding reaction steps, is desirable. We demonstrate that even for a comparatively small diboronic acid monomer a direct thermally activated polymerization into extended COF monolayers is achievable.