Polyhedral Crystal Films of Covalent Organic Frameworks
Xiansong Shi1, Qixing Liu1, Haipei Shao2
1Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117585, Singapore.
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Porous crystal films offer great potential for tackling energy and environmental challenges. However, despite over a decade of intensive research, covalent organic framework (COF) films exhibiting polyhedral textures reflective of their crystalline nature remain exceedingly rare. Here we present a scalable and adaptable biphasic strategy to synthesize pyrene (Py)-COF polyhedral crystal films with exceptional crystalline order under ambient conditions. This method enables precise control over polycrystal formation in solution and crystal growth on substrates, yielding Py-COF polyhedral films with tunable nanometer-scale thicknesses on a 4 in. wafer scale. Among them, a Py-1P film achieves a record-high Brunauer-Emmett-Teller surface area reported for COF films with comparable pore sizes alongside remarkable chemical stability. Real-space electron microscopy reveals previously unreported details of the growth of COF crystal films. These polyhedral crystal films show anisotropic thermal responses along the [001] lattice direction, with dynamically adaptive thermal expansion coefficients of (1.64-2.15) × 10-4 K-1. Through sensor testing, we correlate film crystallinity with device performance and longevity. This research expands the boundaries of COF films, revealing tremendous possibilities to advance film science and its applications.
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