Polymeric Templates Enhancing the Quality of Two-dimensional Covalent Organic Framework Films Prepared on Water Surface

Abstract

Air-water interfacial polymerization is an emerging route to crystalline 2D covalent organic framework (COF) thin films, which can be highly influenced by surfactants used to guide crystal growth. Here we screen six surfactant-like soft templates under identical Schiff-base conditions. Poly(acrylic acid) (PAA) and perfluoropolyether (PFPE)-OH deliver highly continuous polycrystalline films, whereas the other four anionic small-molecule surfactants produce fragmented islands or films with limited long-range order. Multi-scale characterization of 2D COF films resolves a tetragonal lattice with face-on orientation and atomically resolved square pores. Raman spectroscopy identifies a crystallinity-sensitive peak at 1562 cm⁻¹, assigned by density-functional-based tight-binding (DFTB) to a Raman-active, IR-silent free-base porphyrin skeletal mode. This peak is weak or absent in small-molecule-templated films and provides a bench-scale crystallinity marker specific to porphyrin-node 2D COFs that retain local centrosymmetry. Atomic force microscopy (AFM) deep indentation on suspended Si₃N₄ membranes (1 µm radius) probes intra-domain mechanics, and the measured moduli (309–613 MPa across surfactants) are consistent with the film-quality hierarchy. The results suggest that a polymeric surfactant backbone, which creates a continuous interfacial template, plays a major role in 2D COF film continuity; intra-domain stiffness is largely independent of macroscopic film continuity. The mechanism, by which polymeric templates outperform small-molecule surfactants remains to be fully elucidated.

Keywords

2D covalent organic framework / Air-water interface / Surfactant template / Raman spectroscopy / AFM nanoindentation

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