•A detachable all-carbon linked 3D COF film was produced•Substrate-catalyzed synthesis in continuous flow enables high control over COF growth•Heterojunction of semiconductor/COF realizes a high current rectification ratio of 104 For each year, the sophistication of technologies relying on organic molecules is increasing. Organic molecules and materials have gone from being mostly used as dyes and drugs to being integrated into electronics and molecular machines. Today, only the imagination together with some fundamental physical constraints limits what organic materials are able to do. The covalent organic framework (COF) is an upcoming material that shows a unique covalent bond supported crystallinity and high-mass diffusion, which are vital for transfer behaviors of particles and charges. However, the powdery nature that resulted from the common COF synthesis is limiting the exploration for thin film devices. Methods to fabricate this material in the form of films are therefore a necessary step for its utilization. The approach introduced here can produce 3D COF films of high quality, enabling the fabrication of high-performance thin film devices and showing the potential of 3D COFs in organic electronics. New synthetic strategies are ceaselessly being explored to fabricate covalent organic frameworks (COFs) as continuous materials with controlled morphology. However, making substrate detachable thin layers of 3D COFs, which would allow for film-based applications, has turned out to be particularly difficult. Here, we developed a substrate-catalyzed synthesis in a continuous flow method that successfully realized the fabrication of crystalline detachable all-carbon-linked 3D SBFdiyne-COF films. The key element in the method is a highly controlled interfacial reaction between the Cu substrate and a base flowing over the substrate. Benefiting from the detachability and high quality of the fabricated film, an organic semiconductor/COF heterojunction was constructed, showing high current rectification performance of 104 under forward/reverse bias. New synthetic strategies are ceaselessly being explored to fabricate covalent organic frameworks (COFs) as continuous materials with controlled morphology. However, making substrate detachable thin layers of 3D COFs, which would allow for film-based applications, has turned out to be particularly difficult. Here, we developed a substrate-catalyzed synthesis in a continuous flow method that successfully realized the fabrication of crystalline detachable all-carbon-linked 3D SBFdiyne-COF films. The key element in the method is a highly controlled interfacial reaction between the Cu substrate and a base flowing over the substrate. Benefiting from the detachability and high quality of the fabricated film, an organic semiconductor/COF heterojunction was constructed, showing high current rectification performance of 104 under forward/reverse bias. Covalent organic framework (COF) materials have drawn intense attention in recent years due to their covalent bond supported crystallinity resulting in precisely controlled atom and pore distribution.1Geng K. He T. Liu R. Dalapati S. Tan K.T. Li Z. Tao S. Gong Y. Jiang Q. Jiang D. Covalent organic frameworks: design, synthesis, and functions.Chem. Rev. 2020; 120: 8814-8933Crossref PubMed Scopus (997) Google Scholar, 2Li J. Jing X. Li Q. Li S. Gao X. Feng X. Wang B. Bulk COFs and COF nanosheets for electrochemical energy storage and conversion.Chem. Soc. Rev. 2020; 49: 3565-3604Crossref PubMed Google Scholar, 3Sun T. Xie J. Guo W. Li D.-S. Zhang Q. Covalent–organic frameworks: advanced organic electrode materials for rechargeable batteries.Adv. 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Recently, we reported a templated surface reaction in a continuous flow and showed successful fabrication of high-quality 3D COF films, which transform from an insulating state to a conductive state when doped.25Yang Y. Mallick S. Izquierdo-Ruiz F. Schäfer C. Xing X. Rahm M. Börjesson K. A highly conductive all-carbon linked 3D covalent organic framework film.Small. 2021; 17: 2103152Crossref PubMed Scopus (7) Google Scholar However, the fabricated film was chemically bonded to the substrate via a self-assembled monolayer (SAM), which limits its further application. Substrate-catalyzed reactions are a synthetic method where the substrate itself plays the role as catalyst resource, and the reaction is therefore kept close to the surface. The on-surface Glaser polycondensation is a typical substrate-catalyzed reaction, which is applied for the synthesis of acetylenic polymers.26Sun H. Dong C. Liu Q. Yuan Y. Zhang T. Zhang J. Hou Y. Zhang D. Feng X. 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Progress in research into 2D graphdiyne-based materials.Chem. Rev. 2018; 118: 7744-7803Crossref PubMed Scopus (541) Google Scholar, 31Sakamoto R. Fukui N. Maeda H. Matsuoka R. Toyoda R. Nishihara H. The accelerating world of Graphdiynes.Adv. Mater. 2019; 31e1804211Google Scholar In this reaction, the copper source—for instance, a copper foil—serves as both the substrate for holding the polymeric product and the source of catalyst. In a solution containing an organic base, the copper atoms are extracted from the surface of the copper foil into the solution as CuI/CuII ion species forming a thin diffusion layer of catalytic-active species at the interface between the copper foil and solution.32Borrelli M. Querebillo C.J. Pastoetter D.L. Wang T. Milani A. Casari C. Khoa Ly H. He F. Hou Y. Neumann C. et al.Thiophene-based conjugated acetylenic polymers with dual active sites for efficient co-catalyst-free photoelectrochemical water reduction in alkaline medium.Angew. Chem. Int. Ed. 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All-carbon-linked continuous three-dimensional porous aromatic framework films with nanometer-precise controllable thickness.J. Am. Chem. Soc. 2020; 142: 6548-6553Crossref PubMed Scopus (17) Google Scholar Furthermore, when integrated with a quartz crystal microbalance (QCM), the reaction process can even be monitored on a surface in real time. Here, we combine substrate catalysis and continuous flow to realize the successful preparation of a detachable large area 3D COF film with high continuity and surface smoothness. The reaction is based on self-coupling of acetylenic monomers (SBFyne) in pyridine under catalysis of Cu ions provided by a copper surface. The continuous flow system provides a controllable thickness of the catalyst diffusion layer, stable reactant concentration, and reaction temperature, which are of vital importance for high-quality films. Finally, the film was detached from the surface and transferred onto a silicon wafer for construction of a semiconductor/COF heterojunction, which exhibits high current rectification under forward and reverse bias with on-off ratio as high as 104. As an outlook, heterojunctions with outstanding rectification are the cornerstone of many high-performance optoelectronic devices, for instance, the bipolar junction transistors (BJTs) and alternating current-direct current (AC-DC) rectifiers.37Martin A.S. Sambles J.R. A few-monolayer organic rectifier.Adv. Mater. 1993; 5: 580-582Crossref Scopus (10) Google Scholar, 38Nijhuis C.A. Reus W.F. Whitesides G.M. Molecular rectification in metal−SAM−metal oxide−metal junctions.J. Am. Chem. Soc. 2009; 131: 17814-17827Crossref PubMed Scopus (223) Google Scholar, 39Viola F.A. Brigante B. Colpani P. Dell’Erba G. Mattoli V. Natali D. Caironi M. A 13.56 MHz rectifier based on fully inkjet printed organic diodes.Adv. Mater. 2020; 32e2002329Crossref Scopus (16) Google Scholar, 40Su B.-W. Zhang X.-L. Yao B.-W. Guo H.-W. Li D.-K. Chen X.-D. Liu Z.-B. Tian J.-G. Laser writable multifunctional van der Waals heterostructures.Small. 2020; 16e2003593Crossref Scopus (6) Google Scholar When making a continuous 3D COF film with high surface smoothness and controlled thickness, accurate control of the reaction conditions is required. This to realize directional growth and to avoid random elongation in full 3D space. Here, we explored the method of substrate-catalyzed synthesis in continuous flow to fabricate a 3D COF film based on the SBFyne monomer (Figure 1A). The film exhibits high quality, featuring an all-carbon-linked three-dimensional network with conjugation bands throughout the entire structure. A QCM was applied to monitor the growth process. In the QCM, the vibrational frequency of the quartz crystal changes upon absorption/desorption events on the surface in a linear fashion with regards to adsorbed/desorbed mass. The reactant SBFyne monomers dissolved in pyridine and pure pyridine were separated in two input channels. In the first stage, only pyridine was injected into the reaction flow cell where a QCM chip with a copper surface acts as both a catalyst source and a sensor for monitoring the film growth dynamics. As shown in Figure 1B, the initial solvent rush causes a slightly positive shift of frequency corresponding to mass loss, indicating that Cu atoms were removed from the surface of the QCM chip by pyridine (see Figure S1). In this process, a catalyst diffusion layer containing ionic CuI/CuII species was formed at the substrate-solution interface (as confirmed by Figure S2). In the next stage, the SBFyne monomer solution was injected into the chamber and started a self-coupling reaction under catalysis of CuI/CuII species (Figure 1C) at an optimized temperature (Figure S3). The observed negative frequency shift shows the gradual SBFdiyne-COF film growth on the substrate due to the polycondensation of SBFyne. By tracking the frequency shift, the film growth rate and overall accumulated mass can be monitored in real time. When the frequency shift reached around −2,700 Hz at 12,200 s, the desired thickness of the film was reached, and the growth was terminated by switching off the monomer input followed by an immediate solvent rush. The very slight positive drift of frequency during the final solvent rush is at a comparable level with that of the initial rush, indicating that the produced film is stable and cannot be dissolved in the chosen solvent. This illustrates that the frequency shift of −2,700 Hz is resulted from the atom accumulation by irreversible covalent bond formation, corresponding to the acetylenic coupling by the Glaser reaction. To demonstrate the high controllability of growth parameters using the method described here, two sets of experiments (Figure S4) were performed where the monomer concentration and time were used to realize tunable film growth rates and thicknesses, respectively. There are some key advantages of this method that ensure the film quality and differentiate it from other methods. First, compared with the static substrate-catalyzed Glaser reaction, the thickness of the catalyst diffusion layer in continuous flow is stable and controllable. This because the catalyst reaches a stable distribution along the z direction when the diffusion of Cu ions, from the substrate to the solution, and the removal of Cu ions by refreshing flow are in equilibrium. In principle, the thickness of the diffusion layer can be tuned by adjusting the flow rate.36Ratsch M. Ye C. Yang Y. Zhang A. Evans A.M. Börjesson K. All-carbon-linked continuous three-dimensional porous aromatic framework films with nanometer-precise controllable thickness.J. Am. Chem. Soc. 2020; 142: 6548-6553Crossref PubMed Scopus (17) Google Scholar The higher the flow rate, the thinner the diffusion layer becomes, which provides a highly controlled confined space for material growth to shape its aspect ratio. Thus, this method is fundamentally different from the static substrate-catalyzed Glaser reaction, in which the diffusion layer grows thicker with time. Second, the concentration of the reactant in the reaction is stable with time. Generally, in a static reaction, the concentrations of reactants keep decreasing due to consumption, resulting in a reduced reaction rate. However, in our continuous flow setup, the concentration of reactant is constant due to a continuous refreshing input flow, which enables manual control over the reaction rate during the entire process. Furthermore, if the reaction rate is kept low even in the starting phase, there is a higher chance to avoid exponential propagation of the product that usually leads to dendritic growth, which ruins the smoothness of the film. Third, the continuous flow washes away particles that are formed in the bulk solution arising from occasional far diffused Cu ions. Finally, the solid-liquid interfacial reaction produced SBFdiyne-COF films are phsysisorbed to the substrate, thus providing an opportunity for detachment of the film. This in contrast to earlier reports of undetachable chemically bonded films.25Yang Y. Mallick S. Izquierdo-Ruiz F. Schäfer C. Xing X. Rahm M. Börjesson K. A highly conductive all-carbon linked 3D covalent organic framework film.Small. 2021; 17: 2103152Crossref PubMed Scopus (7) Google Scholar,36Ratsch M. Ye C. Yang Y. Zhang A. Evans A.M. Börjesson K. All-carbon-linked continuous three-dimensional porous aromatic framework films with nanometer-precise controllable thickness.J. Am. Chem. Soc. 2020; 142: 6548-6553Crossref PubMed Scopus (17) Google Scholar In short, for the explored method, the film of SBFdiyne-COF was fabricated by controlling the diffusion layer, reactant concentration, and flow environment to conduct a confined growth of 3D material at the interface between the substrate and the solvent flow. After fabrication, the substrate supported SBFdiyne-COF film was taken from the flow cell for characterization. Atomic force microscopy (AFM) was first applied for morphology characterization. Figures 2A–2C show AFM height images of the film. The film exhibits a continuous and uniform state without cracks or defects, indicating a high quality in material continuity and smoothness. In Figure 2A, a clear edge can be seen from the scan of a torn film on the substrate. By extracting a profile along the section at the edge (green line in Figure 2A), the height distribution gives a film thickness of about 100 nm. For a quantitative evaluation of surface roughness, a zoom-in scan was performed in the central area as displayed in Figure 2B. The surface morphology is clearly different from the Cu surface of the QCM chip before the reaction (Figure S5). In more morphological detail, it can be observed that there are some minor height fluctuations on the surface but without any pinholes or large voids (Figure 2B), corroborating the internal continuity within the thin film. The statistic overview over the whole area gives a RMS surface roughness of 4.8 nm, which is considerably smooth compared with the film thickness of 100 nm. The SBFdiyne-COF film, which was so far still physically attached to the QCM chip, was detached from the substrate by carefully rinsing with ethanol. The solvent flow, starting at the edge of the film, gradually peeled it off. Figure 2C shows the AFM image of a partially detached film. The area showing a sharp height contrast represents the detached part of the film, which is rolled up from the substrate initiated by the aforementioned rinsing with ethanol. Thus, the substrate-catalyzed synthesis in continuous flow method allows for detachable low surface roughness films to be made. The AFM characterization shows that the prepared SBFdiyne-COF film is homogeneous in morphology and has low surface roughness. In order to have a stereoscopic and full-scale image of the film, scanning electron microscopy (SEM) was conducted. Figure 3A shows a full view of the film with a 1,000 μm scale featuring high flatness and uniformity that is absent from defects such as wrinkles or holes. The film is attached on the QCM chip as a continuous whole piece without any observable boundaries inside. For a simultaneous observation of the external and internal regions of the film and the substrate, a high magnification zoom-in image was taken of a tear made deliberately (Figure 3B). Here, the bright COF film attached on the dark Cu surface of the QCM chip can be seen. The thickness of the film is equal over the length of the tear, indicating the uniform growth. The film after controlled detachment was also studied under SEM, as shown in Figures 3C and 3D. A closer look at the detached film in Figure 3D indicates that there is no obvious difference to the film before detachment in Figure 3B, illustrating that the detachment process does not lead to structural damage or film quality deterioration. Thus, the SBFdiyne-COF film is robust enough for mechanical post-manipulations such as peeling off and transfer to a new substrate. The noticeable robustness of the film can be ascribed to the covalent bonds reaching out in all three dimensions within the material. This high and isotropic internal continuity of the fabricated SBFdiyne-COF film hints to the possibility of investigating intrinsic electronic properties that exclude interference from continuity breaks caused by gaps or boundaries. To verify the chemical structure of the film, FTIR and Raman spectroscopy were carried out to probe the formation of the new bonds formed in its making. As shown in Figure 4A, the IR absorption at 3,281 cm−1, representing the C–H stretch vibration of terminal C≡C–H is strong for the SBFyne monomer41He J. Wang N. Cui Z. Du H. Fu L. Huang C. Yang Z. Shen X. Yi Y. Tu Z. Li Y. Hydrogen substituted graphdiyne as carbon-rich flexible electrode for lithium and sodium ion batteries.Nat. Commun. 2017; 8: 1172Crossref PubMed Scopus (277) Google Scholar but very weak for the SBFdiyne-COF. This indicates a high conversion of the homocoupling reaction. The comparison of relative absorptions (Figure S6) between the monomer and the COF shows that residual terminal alkyne defects in the film is lower than 9%. In addition, the typical C≡C stretch at 2,105 cm−1 has almost disappeared for the product, due to the increased symmetry of the conjugated diacetylenic linkage. Vibrations that are forbidden in FTIR are visible by Raman spectroscopy. Raman microscopy therefore show the conjugated diacetylenic linkage as a new peak at 2,196 cm−1 (Figure 4B), confirming the formation of the product in the coupling reaction.28Zhang T. Hou Y. Dzhagan V. Liao Z. Chai G. Löffler M. Olianas D. Milani A. Xu S. Tommasini M. et al.Copper-surface-mediated synthesis of acetylenic carbon-rich nanofibers for active metal-free photocathodes.Nat. Commun. 2018; 9: 1140Crossref PubMed Scopus (90) Google Scholar,42Zhou W. Shen H. Zeng Y. Yi Y. Zuo Z. Li Y. Li Y. Controllable synthesis of graphdiyne nanoribbons.Angew. Chem. Int. Ed. Engl. 2020; 59: 4908-4913Crossref PubMed Scopus (35) Google Scholar, 43Zhou W. Shen H. Wu C. Tu Z. He F. Gu Y. Xue Y. Zhao Y. Yi Y. Li Y. Li Y. Direct synthesis of crystalline graphdiyne analogue based on supramolecular interactions.J. Am. Chem. Soc. 2019; 141: 48-52Crossref PubMed Scopus (38) Google Scholar, 44Sakamoto R. Shiotsuki R. Wada K. Fukui N. Maeda H. Komeda J. Sekine R. Harano K. Nishihara H. A pyrazine-incorporated graphdiyne nanofilm as a metal-free electrocatalyst for the hydrogen evolution reaction.J. Mater. Chem. A. 2018; 6: 22189-22194Crossref Scopus (35) Google Scholar The peaks around 1,600 and 1,300 cm−1 belong to the ring stretching and breathing of aromatic moieties. Compared with the SBFyne monomer (Figure 4B), the typical sharp peak at 2,107 cm−1 representing the terminal alkyne group has disappeared, whereas peaks corresponding to aromatic rings remain, illustrating a high conversion of the reaction. Furthermore, data collected at four random positions on the SBFdiyne-COF film show almost identical peaks, indicating the chemical uniformity all over the fabricated film. To determine the presence of residual Cu catalyst in the film, X-ray photoelectron spectroscopy (XPS) was conducted (Figure S7). It was found that the signals from the Cu 2p electrons were barely seen, indicating only trace amounts of Cu in the film. The SBFdiyne-COF has previously been synthesized using homogeneous catalysis and characterized with confirmed polycrystallinity, based on rather broad X-ray diffraction (XRD) scattering signals.25Yang Y. Mallick S. Izquierdo-Ruiz F. Schäfer C. Xing X. Rahm M. Börjesson K. A highly conductive all-carbon linked 3D covalent organic framework film.Small. 2021; 17: 2103152Crossref PubMed Scopus (7) Google Scholar However, the chemically bonded film on the substrate limited further characterizations such as transmission electron microscopy (TEM). Benefiting from the detachability of the film produced here, TEM observation was realized. As shown in Figure 4C, clear lattice patterns over the whole examined area can be observed, illustrating a crystalline state. The SBFdiyne-COF film was polycrystalline as indicated by different orientations of the lattice within the film, with a regional uniform orientation around the size of 10 nm (Figure 4D). Selected area electron diffraction (SAED) was used to complement the TEM observations. A series of concentric diffraction rings were observed, further corroborating the conclusion of the film being polycrystalline (Figure S8A). The lattice distances within a uniform area is lower than 1 nm (Figure 4D), illustrating a dense packing of interpenetrated framework structures. The observed polycrystallinity and interpenetration is consistent with past reports of SBFdiyne-COF synthesized using other methods.25Yang Y. Mal