Substrate orientation effect in covalent organic frameworks/2D materials heterostructure by high-resolution atomic force microscopy

Heterostructures based on covalent organic frameworks (COFs) and other two-dimensional (2D) materials attract considerable attention due to their extraordinary properties and tremendous application potential. Substrate effects play a crucial role in the integration of ultrathin COF films onto 2D materials through direct polymerization. In this study, highly ordered monolayer COFs were successfully constructed on the surfaces of highly oriented pyrolytic graphite (HOPG), hexagonal boron nitride (hBN), and molybdenum disulfide (MoS₂). High-resolution atomic force microscopy (HR-AFM) imaging clearly reveals the substrate orientation effect in COFs/2D materials heterostructure. Honeycomb networks formed via Schiff-base reaction and boronic acid condensation reaction can epitaxially grow in specific orientations relative to the underlying substrate lattices. This work provides direct evidence for substrate effects in the on-surface synthesis of COFs and paves the way for further investigation into the intrinsic electronic properties of monolayer COFs and the development of multifunctional hybrid devices.