Crystallizing Sub 10 nm Covalent Organic Framework Thin Films via Interfacial–Residual Concomitance

Abstract

Synthesis of covalent organic framework (COF) thin films on different supports with high crystallinity and porosity is crucial for their potential applications. We have designed a new synchronized methodology, residual crystallization (RC), to synthesize sub 10 nm COF thin films. These residual crystallized COF thin films showcase high surface area, crystallinity, and conductivity at room temperature. We have used interfacial crystallization (IC) as a rate-controlling tool for simultaneous residual crystallization. We have also diversified the methodology of residual crystallization by utilizing two different crystallization pathways: fiber-to-film (F–F) and sphere-to-film (S–F). In both cases, we could obtain continuous COF thin films with high crystallinity and porosity grown on various substrates (the highest surface area of a TpAzo COF thin film being 2093 m2 g–1). Precise control over the crystallization allows the synthesis of macroscopic defect-free sub 10 nm COF thin films with a minimum thickness of ∼1.8 nm. We have synthesized two COF thin films (TpAzo and TpDPP) using F–F and S–F pathways on different supports such as borosilicate glass, FTO, silicon, Cu, metal, and ITO. Also, we have investigated the mechanism of the growth of these thin films on various substrates with different wettability. Further, a hydrophilic support (glass) was used to grow the thin films in situ for four-probe system device fabrication. All residual crystallized COF thin films exhibit outstanding conductivity values. We could obtain a conductivity of 3.7 × 10–2 mS cm–1 for the TpAzo film synthesized by S–F residual crystallization.