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Synthesis of a Scalable Two-Dimensional Covalent Organic Framework (COF) by Photon-assisted Imine Condensation Reaction on the Water Surface Soyoung Kim, Hyunseob Lim, Jinho Lee, and Hee Cheul Choi Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00951 • Publication Date (Web): 07 Jul 2018 Downloaded from http://pubs.acs.org on July 8, 2018

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Synthesis of a Scalable Two-Dimensional Covalent Organic Framework (COF) by Photon-assisted Imine Condensation Reaction on the Water Surface Soyoung Kim,†,‡ Hyunseob Lim,§ Jinho Lee†,‡ and Hee Cheul Choi*,†,‡

†Center for Artificial Low Dimensional Electronic Systems, Institute for Basic Science (IBS), Pohang 37673, Republic of Korea

‡Department of Chemistry, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea

§Department of Chemistry, Chonnam National University (CNU), Gwangju, 61186, Republic of Korea

An atomically thin two-dimensional (2D) covalent organic framework (COF) was successfully synthesized via photon-assisted imine condensation reaction within 1 h from the highly uniform and homogeneous precursor solution layer floating on the water surface. The polarity optimization of the precursor solution was the key step for the successful formation of the highquality 2D COF because only the precursor solution consisting of polarity-controlled solvents allows ideal floating on the water surface. The polarity-controlled solution not only prohibits the

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agglomeration of the organic precursors on the water surface but also facilitates the wafer scale and layer number-controllable synthesis of the 2D COF. The resulting 2D COF has a uniform porous structure and highly oriented layered structure along the out-of-plane direction as observed by microscopy analysis and X-ray diffraction, respectively. In addition, we successfully fabricated field effect transistor type polyimine-based COF (pi-COF) electronic devices to demonstrate the prompt electrical responses to photo and water vapor exposure, suggesting the potential applications of the pi-COF in electrical photo- or moisture-detector devices.

Covalent organic frameworks (COFs) have received much attention during the last few decades in many scientific and technological fields as a new class of two-dimensional (2D) or three-dimensional (3D) crystalline polymers possessing multifunctional properties1,2,3 arising from the presence of a few nanometer-scale uniform pores. In particular, the 2D COFs have been regarded as a source for diverse and stable new layered materials because they consist of atomically thin layers covalently bonded in the lateral direction that can be held together in the vertical direction by the out-of-plane van der Waals interactions. Due to this anisotropy of the surface energies in the vertical and lateral crystallographic directions, they are also expected to be isolated or formed into individual atomically thin two-dimensional materials similar to other representative layered materials, such as graphite (to graphene)4, hexagonal boron nitride5 and transition metal dichalcogenides (TMDs)6.7. Therefore, many efforts have been made to make 2D COFs either by ‘top-down (TD)’ approaches or ‘bottom-up (BU)’ approaches as has been attempted for other 2D materials. These approaches include the delamination from bulk COFs as a TD approach8.9 and covalent assembly or surface-confined coupling reactions as BU


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approaches10,11. Especially, a new BU approach to obtain imine based oriented COF film using tert-butyloxycarbonyl containing precursor has been recently reported12. However, the synthesis of large-scale 2D COFs with high quality and the layer number controllability still remains highly challenging due to several limitations as discussed below. Generally, in synthetic processes for the conventional 2D crystalline materials such as graphene or metal dichalcogenides, a solid surface is frequently used as a 2D reaction platform and a catalyst. This kind of reaction platform scheme is highly efficient especially when the precursor reactants are gases13, as the thermally decomposed precursors are adsorbed, rearranged and crystallized into 2D structures on the solid surface14,15. In the case of the COF, on the other hand, such a solid surface platform is not appropriate because the precursors are generally molecular species that should be rearranged in an appropriate manner to create covalent bonds. Compared to the atomic precursors, molecular precursors for COFs are less mobile on the solid surface. In addition, if high thermal energy is introduced to increase the molecular mobility, the precursors could decompose. Therefore, a new surface platform that is appropriate for molecular precursors is needed. A liquid surface formed at a liquid-air or liquid-liquid interface could be a good candidate for such a platform. Recently, multi-layer COF films were successfully synthesized by the COF formation reaction at the water-organic solvent interface for diverse applications, such as molecular separation and dye rejection, etc.16,17,18. However, this method is not appropriate to obtain large-scale, highly uniform and atomically thin COF films. Meanwhile, the water surface at the air-water interface has been suggested to satisfy all required goals. In fact, the synthesis of a mono- or multi-layer COF using a Langmuir-Blodgett (LB) technique was reported19,20. Although the LB technique has demonstrated potential for the successful synthesis of mono- or multi-layer COF materials, it is not an ideal method because the use of highly


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sensitive instruments and the difficulty of the accurate control of the number of layers reduce the reaction efficiency and give rise to limitations for practical applications. Hence, we propose a new strategy for the synthesis of the 2D COF on the water surface that does not involve complicated processes but rather is carried out by a simple photo reaction under sunlight using a well-controlled solvent mixture system that allows the almost perfect spreading of molecular precursors on the water surface. Recently, we reported photon-assisted reaction method for the synthesis of bulk COF-5 in the solution phase21, during which there is no liquid convection that could destabilize the liquid surface during the synthesis with the negligible increase of the reaction temperature. These phenomena ensure the stability of the water surface during the photon-assisted reaction, thus motivating us to apply the photon-assisted reaction to the reaction on the water surface. We chose the polyimine-based COF (pi-COF) as a target compound to demonstrate the photonassisted synthesis of the 2D COF on the water surface because is known that imine condensation can be achieved by the photoactivated Schiff-base dehydration of 1,3,5-Tris(4-aminophenyl)benzene (TAPB) and terephthalaldehyde (PDA), both of which are good candidates for the backbone of a COF system. Although the photon-assisted imine condensation reaction itself has been widely studied in the different fields of fundamental organic synthesis22,23,24, its applications to the formation of functional materials (especially in 2D structures) are quite rare. In this paper, we report the successful application of the photon-assisted condensation reaction to synthesize a 2D pi-COF film, which is not only rapid (~1 h) and scalable (limited only by the size of the reaction vessel, demonstrated size in this study as high as a diameter of 4 inches) but also controls the number of layers at the single atom thickness level by appropriate adjustment of the polarity, amount, and concentration of the precursor solution. In addition, we successfully


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fabricated 2D-pi-COF thin film transistor-type electronic devices which show the photo- and humidity-sensitive electrical current changes. As a preliminary test, we first attempted the photon-assisted synthesis of bulk pi-COF powder to check the likelihood of the successful execution of our strategy. A substantial amount of piCOF (light-promoted pi-COF: Lp-pi-COF) was obtained as precipitates only after 3 hours of reaction time, which is ~25 times faster than the conventional solvothermal method. Briefly, the reaction was carried out by irradiating the vial containing the reaction solution with sunlight at wavelengths of 200~2500 nm (Oriel Class AAA Solar simulator, Newport, AM 1.5G illumination with an intensity of 50 mW/cm2), which is similar to our previous work21 except for the usage of a different light source and of acetic acid, which is the necessary catalyst for the initiation of the imine condensation. The solar simulator light source was chosen due to the large-area irradiation that is essential for the synthesis of large-scale pi-COF film and intensive emission of 280 ~ 480 nm light which is efficiently absorbed to the precursor solution. (Figure S1) Note that there are immediate precipitations after adding acetic acid which is not crystalline product but amorphous imine containing aggregates which are generated by catalyst-activated partial imine formation between precursors25. However, these aggregates are non-crystalline and disordered amorphous polymer due to the absence of proper energy to achieve reversible dynamic covalent bond formation which is the key process for the formation of ordered COFs26. By irradiating intensive photon energy, we could obtain well-ordered crystalline COF within 3 h, which refers that photon energy not only accelerates imine condensation reaction by fast excitation of precursor molecules, but also facilitate transformation from immediately generated amorphous imine containing precipitates to crystalline COF through reversible dynamic imine condensation reaction, as similar phenomenon by thermal energy activation has been reported by


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Smith, B. J. et al.25.The detailed experimental procedure and experimental scheme are described in the Supporting Information (Scheme S1). Then, the bulk Lp-pi-COF powder was characterized by X-ray diffraction (XRD), Fourier transform-infrared spectroscopy (FT-IR) and thermogravimetric analysis (TGA). The XRD pattern matches well with the previously reported results23 as well as with the calculated results obtained using the Reflex and Dmol3 modules in the Accelrys Materials Studio 7.0 software, with refined cell parameters of a=b=37.25 Å and c=3.52 Å (Figure S2b). The most intense peak observed at 2θ = 2.76 ο corresponds to the (100) crystal plane which is marked by a red line in the optimized structure of the pi-COF (inset of Figure S2a). The other three peaks observed at 2θ = 4.8ο, 5.5ο and 7.3ο are related with the (110), (200) and (210) planes, respectively, and the inset of Figure S2a shows the (001) diffraction peak corresponding to the interlayer distance of the pi-COF powder. The TGA analysis reveals that the Lp-pi-COF powder has high thermal stability up to 400°C, which is higher than that of the TAPB (350°C) and PDA (110°C) precursors, indicating the formation of robust covalent organic frames (Figure S3). The successful demonstration of the photon-assisted synthesis of bulk Lp-piCOF powder strongly supports our hypothesis that the pi-COF can be an appropriate candidate for the 2D COF formation via photon-assisted approach. To examine the effect of light source on the reaction, we attempted the synthesis of the pi-COF film in the dark and under natural lab light. The reaction time and other experimental procedures were kept the same except for the use of the light energy source. Figure S4a display the FT-IR spectra of obtained products, which show not only imine vibrations but also remaining amine and carbonyl vibrations, indicating incomplete imine condensation. Also, we did XRD measurement to confirm the crystallinity of obtained products in the dark and under lab light condition. The obtained products show amorphous structures in both cases. (Figure S4b) This result confirms that the appropriate light


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source is essential to obtain crystalline pi-COF films in a short time. Moreover, to check the effect of thermal energy generated from solar simulator, we measured the temperature increase of the precursor solution for the bulk synthesis and the water surface for the film synthesis during light irradiation. The temperature increase by the light irradiation is negligible, as the change is less than 3°C for 24 h irradiation in both case. Synthesis of the 2D Lp-pi-COF film For the synthesis of the 2D Lp-pi-COF film on the water surface, a designated volume of the precursor solution was dropped carefully on the distilled water surface in a reaction glass vessel (diameter ≈ 12.5 cm), and then the sample was exposed to sunlight for 1 h using a solar simulator (AM 1.5G illumination with an intensity of 50 mW/cm2, Oriel 1 kW). The detailed experimental procedure and experimental scheme are described in the Supporting Information. The formation of a multi-layer Lp-pi-COF film on the water surface can be recognized by the naked eye under natural lab light. Then, the floating Lp-pi-COF product film was manually transferred onto a substrate by carefully scooping the film using the target substrate such as silicon, silicon oxide, quartz or mica selected depending on the purpose of the further examination of the film (Figure S5). On the other hand, a specially designed substrate holder was used to transfer the atomically thin Lp-pi-COF mono-layer film from the water to the target substrate surface by lifting up the substrate (Figure S6) because the formation of the atomically thin mono-layer is difficult to detect by the naked eye under natural lab light. The detailed experimental procedure for the synthesis and transfer of the 2D Lp-pi-COF film is described in the Supporting Information. The overall synthesis process is summarized in Figure 1a. The optical image of the mono-layer 2D Lp-pi-COF film transferred onto a SiO2/Si substrate (Figure 1b) shows a large-scale and clean surface without any noticeable defects or wrinkles. The surface of the mono-layer 2D Lp-pi-COF


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film was further examined by atomic force microscopy (AFM) (Figure 1c). Its thickness was measured to be ~ 0.7 nm, indicating a one-atomic layer thickness of the pi-COF19,20. The height distribution measured by AFM reveals that the Lp-pi-COF film has a highly smooth surface (root mean square roughness (Rz,COF) = 0.275 nm), which is comparable to that of the bare SiO2/Si substrate (Rz,SiO2 = 0.227 nm). These observations demonstrate that the photon-assisted process is a promising approach for the synthesis of the 2D COF on the water surface. The reaction time is ~4 times faster than that of the recently reported single-layer COF formation by a mechanical force-driven method using an LB trough20. It should be noted that the proper solvent combination, i.e., the volume ratio of 1,4-dioxane, mesitylene, chloroform and acetic acid, is the most important key factor for obtaining a uniform and atomically thin film on the water surface because only the solvent mixture at the specific ratio guarantees the homogeneous spreading of the precursor solution on the water surface. If the mixed solvent with a different volume ratio was used, the precursor solutions are either self-agglomerated on the water surface or sank down into water instead of evenly spreading on the water surface. These behaviors are believed to arise from the polarity effect of the precursor solution. To visualize the effect of the solvent combination more intuitively, videos (see supporting video 1-3) were recorded during the solution dropping process for various solvent combinations. For clear visualization, a small amount of phthalocyanine was added to dye the dropping precursor solution a green color. The precursor solution prepared only with chloroform results in the agglomeration on the water surface due to the nonpolar nature of the solution (Figure 1d and supporting video 1), which shows a similar result to that obtained when a mixed solvent without acetic acid was used (Figure S7). The appropriately controlled solvent combination results in the formation of a uniform and stable layer of the precursor solution floating on the water surface (Figure 1e) due to


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the suitable polarity of the solution (supporting video 2). If the polarity of the solution is higher than that of the appropriately controlled solvent combination by increasing the portion of acetic acid, the solution does not float on the water surface but sinks down into water (Figure 1f and supporting video 3). Moreover, the role of various solvents, such as mesitylene and the acid catalyst in the COF synthesis, is also important for obtaining robust and highly crystalline products during the dynamic self-healing process of the COF25,27. Without such consideration of the solvent polarity, such as in the previously reported cases of the liquid-air interface system19,20, it is difficult to achieve a homogeneous spreading of the precursor solution due to high immiscibility between chloroform and water, requiring the use of complex and sensitive procedures including in situ surface force monitoring with sensitive control of mechanical force to increase the uniformity and grain size of the product. Layer number control of the 2D-Lp-pi-COF film While the accurate control of the number of layers is another important issue in the synthesis of two-dimensional materials, the uniform control of the number of layers of the 2D COF in a large area by the bottom-up approach has not been reported to date mainly due to the lack of understanding and lack of a technique that allows the controlled formation of a homogeneous precursor solution layer on the platform surface. In our system, we demonstrate that the number of 2D Lp-pi-COF layers can be controlled by simply changing the total volume or concentration of the precursor solution. In short, the number of layers has a general tendency to linearly increase (1) as a function of the volume of the reaction precursor solution at a constant concentration and (2) as a function of the concentration of the precursors. Figure 2a shows the AFM images of the 2D Lp-pi-COF films with the height profiles. All the AFM images show a uniform and clean surface with the wafer-scale size (insets of all the AFM images in Figure 2a).


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First, the two AFM images at the top of Figure 2a show the results for the mono- and tri-layer Lp-pi-COF synthesized with 10 and 30 µL of precursor solutions (2.5 mM TAPB and 3.75 mM PDA), respectively, exhibiting the correlation of the number of layers to the volume of the precursor solution. The heights of each film are 0.75 and 1.44 nm, corresponding to the mono and tri layers of the 2D Lp-pi-COF films, respectively. We note that the interlayer distance of the pi-COF is ~0.36 nm and that the height of a mono-layer of a 2D material could be measured to be larger than expected due to the different interaction forces between the probing tip-substrate and probing tip-2D materials28. Second, when the concentration of the precursor solution is increased by a factor of 2 (5 mM TAPB and 7.5 mM PDA), the number of layers of the resulting Lp-pi-COF film is also doubled (the two AFM images at the bottom of Figure 2a). Figure 2b shows the linear correlation between the loading volume of the precursor solution and the number of the obtained pi-COF layers for the two different precursor concentrations. The selected concentrations and loading volumes of the precursor solution are optimized through numerous trials to produce a continuous film close to the 4-inch wafer size. Additionally, it should be noted that a critical minimum volume of the precursor solution is necessary to fully cover the water surface. Thus, when the amount of the precursor solution is too small to cover the whole reaction vessel, the correlation between the concentration and the number of layers is only applicable to the smaller size films. For example, if we use 5 µL of TAPB (5 mM) and PDA (7.5 mM) precursor solution instead of 10 µL, a mono-layer film is still obtained but at the scale of only a few micrometers due to the deficient amount of the precursor solution that is not sufficient to cover the reaction vessel (diameter ≈ 12.5 cm) (Figure S8). Additional AFM images with height profiles according to the various loading volumes of the different concentrations are shown in the Supporting Information (Figure S9). To the best of our knowledge, this is the first


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demonstration of the systematic control of the number of layers of a COF film in a bottom-up approach, especially using a solvent mixture system. Characterization of the 2D-Lp-pi-COF film Various spectroscopic and microscopic analyses were carried out to investigate the chemical structure and crystallographic orientation of the 2D Lp-pi-COF film. To confirm the successful formation of new C-N chemical bonds through the dehydration reaction between TAPB and PDA upon the photon-assisted imine condensation reaction, Fourier transform infrared (FT-IR) and Raman spectra were obtained first for the 2D Lp-pi-COF films (Figure 3). We note that since the spectral intensity from a mono-layer of the 2D Lp-pi-COF is too weak to identify the vibrational modes in the structure from the FT-IR and Raman spectra, a multi-layer pi-COF film was used for a more effective detection of the vibrational modes. The black and red lines in Figure 3a are the FT-IR spectra of the TAPB and PDA precursors, respectively, in which the primary amine stretching bands appear at 3434 cm-1 and 3355 cm-1, and the carbonyl C=O stretching band appears at 1689 cm-1. However, these bands disappeared in the FT-IR spectra of both the Lp-pi-COF powder (blue) and the multi-layer film (green), but the imine stretching bands were observed at 1621 cm-1, 1596 cm-1 and 1563 cm-1. Consequently, the appearance of imine bonds implies the successful photo condensation between TAPB and PDA. Furthermore, the latter two bands can be assigned as quadrant stretching vibrations related to the substituted conjugated benzenes of the pi-COF structure29. The Raman spectra also provide evidence of imine formation in the Lp-pi-COF film. The band appearing at 1590 cm-1 indicates the imine stretching mode, and the disappearance of the bands at 1358 cm-1 and 1692 cm-1 signifies the successful condensation between the amine and carbonyl functional groups, which is well matched with that of the bulk Lp-pi-COF (Figure 3b) 30,31.


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High-resolution transmission electron microscope (HR-TEM) images were taken to obtain detailed crystallographic information and to reveal the presence of pore structures of the 2D Lppi-COF film (Figure 4a, b). Although the atomic resolution of the TEM images could not be obtained due to the low stability of the 2D Lp-pi-COF film against the electron beam irradiation, the periodic pore structures were confirmed, as shown in Figure 4b. The diameter of the dark holes in Figure 4b is ~3.9 nm, in agreement with the theoretical pore size of ~3.83 nm (inset of Figure 4a). A cross-section TEM measurement was also carried out to confirm the crystallographic structure along the out-of-plane direction. The hexa-layer Lp-pi-COF film was used to confirm the interlayer spacing, and the cross-sectional TEM specimen was prepared by chromium coating followed by the focused ion beam (FIB) milling process (Figure S10, more details regarding the sample preparation and characterization are provided in the Supporting Information). The cross-sectional image in Figure 4c shows the layered structure of the Lp-piCOF film between chromium and SiO2, which is also clearly observed in the energy-filtered TEM (EF-TEM) images taken for C and N 1s (Figure S11). The stripe pattern parallel to the surface of SiO2 observed in Figure 4d indicates that the Lp-pi-COF layers are well stacked along the direction of the normal surface substrate. The interlayer distance is 0.37 nm according to the line profile of the image contrast (Figure 4e), which is also well matched with the (001) lattice spacing extracted from the XRD pattern of the bulk Lp-pi-COF (Figure S2). The grazing incidence wide angle X-ray scattering (GI-WAXS) analysis is used to obtain the surfacesensitive structural information for the crystal and reveals the stacking orientation of the Lp-piCOF films. The GI-WAXS patterns (Figure 4f-h) were obtained with the incidence angle (θi) varying from 0.21 to 0.16ο. A ring-shaped pattern was predominantly observed when the higher incidence angle (θι = 0.21ο) was used (Figure 4f). A more surface-sensitive condition (θi = 0.16ο)


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enables the measurement of the signal from the Lp-pi-COF film, which is observed as a bright spot at qz = 2.08 as shown in Figure 4 h. These results indicate that the Lp-pi-COF film has a periodicity of 0.39 nm along the direction normal to the surface, which is in good agreement with the lattice d-spacing obtained from the TEM measurements. Fabrication of the 2D Lp-pi-COF film electronic devices and the electrical sensing tests upon light and water exposure One of the major advantages of the 2D Lp-pi-COF is its geometrical suitability and high compatibility for device fabrication processes. Therefore, we investigated the electrical properties of the 2D Lp-pi-COF film by fabricating it into field effect transistor (FET)-type electronic devices with a channel length of 30 µm of channel length (inset of Figure 5a). For this method, a 2D Lp-pi-COF film was transferred onto a SiO2/Si substrate, and then metal electrodes (chromium: 5 nm then gold: 20 nm) were deposited with a specific pattern using a shadow mask. The detailed experimental procedure is described in the Supporting Information. The electrical conductance of the Lp-pi-COF film was 2 × 10-9 S as calculated from the Ids-Vds curve in Figure 5a. The non-constant slope change observed in the Ids-Vds curve suggests the non-Ohmic contacts between the COF film and the metal electrodes. We fabricated 52 devices to confirm the conductance of COF, the 43 devices show similar electrical property. In addition, we examined 1) the photoresponse and 2) the humidity response of the 2D Lp-pi-COF device. Similar to the pi-COF bulk powder, the pi-COF film shows a broad light absorption property up to 500 nm and an emission peak at approximately 600 nm, signifying its potential response to light at a wide range of wavelengths (Figure S12). The electrical current change upon light irradiation was monitored by measuring the current changes at the constant bias voltage of 10 V. The current magnitude was increased by a factor of 10 when laser-driven light (λrange = 170 ~ 2100 nm) was


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irradiated, demonstrating the outstanding photo-response property of the 2D Lp-pi-COF and indicating its potential for use as an organic photo detector. Since a COF is also known as a good gas- or vapor-adsorbent due to its well-ordered and uniform porous structure, the 2D Lp-pi-COF electronic devices were tested as gas or vapor sensors. We tested the electrical conductance changes of the 2D Lp-pi-COF upon exposure to water vapor. The details of the experimental procedure and a photograph of the experimental setup are provided in the Supporting Information (Figure S13). Figure 5c shows the Ids-Vds curves obtained before (black) and after (blue) exposure to water vapor. The drain current drastically increased from 400 nA to 100 µA at Vds = 20 V. The real-time sensing experiment also reveals the large current change (Iwater/Ivacuum = ~103) due to the water vapor (Figure 5d). Moreover, we did same experiment using empty device without any current channel. Figure S14a is I-V characteristic of empty device which show non-linear and saturated curve with 10-10 current level. Also, there were no current changes when we exposed the light and water vapor, respectively. (Figure S14b, c) From this result, we proved that the chemiresistive property is the intrinsic property of 2D Lp-pi-COF film. Although further details regarding the mechanism responsible for the current change are still under investigation, we suggest that hydrogen bonding between the imine groups of the pi-COF and the water molecules would play a critical role in the increase of the conductance. Although the mechanism of the conductance change is not fully understood yet, our results suggest that the 2D COF can be potentially used as a highly sensitive electronic sensor for molecules in the gas phase. In summary, we have demonstrated the light-promoted one-pot rapid synthesis of a large area and layer number-controllable 2D pi-COF film on the water surface from a uniformly spread precursor solution prepared using a properly controlled solvent combination. The resulting 2D


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Lp-pi-COF film has a uniform and clean surface with a 4-inch wafer-size scale for which the number of layers can be controlled quite accurately from mono- to multi-layers. In addition, the lateral and layered structures of the Lp-pi-COF film were directly confirmed by the HR-TEM measurement for the first time. In particular, the highly oriented stacking structure along the outof-plane direction was clearly observed from the cross-section TEM analysis and GI-WAXS measurements, and the measured interlayer distance corresponds to the interlayer distance of the general π−π stacked layered material. In addition, we fabricated 2D Lp-pi-COF electronic devices to measure electrical conductivity and confirmed the potential of the 2D Lp-pi-COF for use in photo- and humidity-responsive electronic devices. We believe that our findings provide new insights for the photon-assisted reactions on the water surface, which will contribute to the development of novel synthetic methods for various additional two-dimensional COFs and provide the opportunity to develop potential applications for COF films in an electronic device form for many purposes.


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Figure 1. The overall synthesis process and observation of the surface of the obtained 2D Lp-piCOF film with photographs of the water surface after deposition of the three different precursor solutions. (a) Experimental scheme and molecular structure of the Lp-pi-COF synthesized by dehydration between TAPB and PDA precursors upon the photon-assisted imine condensation on the water interface. (b) Optical microscopy and (c) Atomic force microscopy (AFM) images of the 2D Lp-pi-COF film transferred on a SiO2/Si substrate. The white dotted line in (b) indicates the boundary edges of the transferred 2D Lp-pi-COF film. The inset black graphs in (c) are the height profiles of the mono-layer 2D Lp-pi-COF film. (d-f) Photographs of the water surface after dropping the precursor solutions prepared in only chloroform (d), in properly controlled solvent mixture (e), and in mixed solvents with excessive acetic acid (f).


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Figure 2. Number of layers in the 2D Lp-pi-COF film by changing the loading volume and concentration of the precursor solution. (a) AFM images and height profiles of mono-, bi-, triand hexa-layered 2D Lp-pi-COF films obtained from fixed loading volumes (10 µL and 30 µL) of the two different precursor concentrations ((1) TAPB (2.5 mM) PDA (3.75 mM), (2) TAPB (5 mM) and PDA (7.5 mM)). (b) Correlation between the number of layers of the 2D Lp-pi-COF film and the loading volume of the precursor solution for the two different concentration conditions ((TAPB (2.5 mM) and PDA (3.75 mM)) (orange) (TAPB (5 mM) and PDA (7.5 mM) (blue)).


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Figure 3. Spectroscopic analysis of the 2D Lp-pi-COF film (a) Fourier transform infrared (FTIR) and (b) Raman spectra of precursors (TAPB (black), PDA (red)) and Lp-pi-COF powder (blue) and film (green) showing the appearance of the imine functional group, which is the main product of the successful reaction of the amine and carbonyl functional groups.


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Figure 4. Lateral and vertical structure analysis of the Lp-pi-COF film (a, b) TEM images of the porous structure of the 2D Lp-pi-COF film at different magnifications. Insets are (a) a calculated structure of the pi-COF and (b) a magnified pore image of the film. (c) Layered structure of a 2D Lp-pi-COF film between chromium and SiO2 substrates, (d) magnified high resolution TEM image and (e) line profile corresponding to the red boxed region in (d). (f-h) Grazed incidence wide angle X-ray scattering (GI-WAXS) images of the Lp-pi-COF film on the SiO2/Si substrate showing different diffraction signals according to the change in the incidence angle.


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Figure 5. Electronic device fabrication of the 2D Lp-pi-COF film and the electrical sensing experiment upon light and water exposure. (a) IV characteristic curve of the 2D multi-layer Lppi-COF film. The inset shows a photograph of the tested device. (b) On-off switching behavior upon light irradiation of the 2D Lp-pi-COF film device tested in (a) as a function of time at a bias voltage of 10 V. (c) I-V curve of the 2D multi-layer Lp-pi-COF film measured in argon (black) and water (blue) environments. The inset shows a photograph of the tested device. (d) On-off switching behavior upon moisture exposure to the 2D multi-layer Lp-pi-COF film device tested in (c) as a function of time at a bias voltage of 20 V.


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website.

Detailed experimental procedure, PXRD data, TGA analysis, SEM images, TEM analysis and characterization (PDF), precursor solution deposition videos (AVI) AUTHOR INFORMATION Corresponding Author *Tel: +82 54 279 2130. Fax: +82 54 279 3399. E-mail address: [email protected] Author Contributions S. Kim and H.C. Choi designed the overall experiments and analyzed the data together with H. Lim, and S. Kim and J. Lee performed the sensing experiments of the 2D-Lp-pi-COF. All the authors contributed to the discussion of the results. The manuscript was written with contributions by all the authors. All the authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT We are grateful for the use of the Grazing incidence wide angle X-ray scattering (GI-WAXS) at 3C Beamline of the Pohang Accelerator Laboratory (PAL), Korea. We also thank the national institute for nanomaterials technology (NINT) for the use of the focus ion beam (FIB) equipment and the high-resolution transmission electron microscopy (HR-TEM) measurements.


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ABBREVIATIONS pi-COF, polyimine-based COF (pi-COF); Lp-pi-COF, light-promoted pi-COF; REFERENCES

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SYNOPSIS


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Figure 1. The overall synthesis process and observation of the surface of the obtained 2D Lp-pi-COF film with photographs of the water surface after deposition of the three different precursor solutions. (a) Experimental scheme and molecular structure of the Lp-pi-COF synthesized by dehydration between TAPB and PDA precursors upon the photon-assisted imine condensation on the water interface. (b) Optical microscopy and (c) Atomic force microscopy (AFM) images of the 2D Lp-pi-COF film transferred on a SiO2/Si substrate. The white dotted line in (b) indicates the boundary edges of the transferred 2D Lp-pi-COF film. The inset black graphs in (c) are the height profiles of the mono-layer 2D Lp-pi-COF film. (d-f) Photographs of the water surface after dropping the precursor solutions prepared in only chloroform (d), in properly controlled solvent mixture (e), and in mixed solvents with excessive acetic acid (f). 195x207mm (300 x 300 DPI)


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Figure 2. Number of layers in the 2D Lp-pi-COF film by changing the loading volume and concentration of the precursor solution. (a) AFM images and height profiles of mono-, bi-, tri- and hexa-layered 2D Lp-pi-COF films obtained from fixed loading volumes (10 µL and 30 µL) of the two different precursor concentrations ((1) TAPB (2.5 mM) PDA (3.75 mM), (2) TAPB (5 mM) and PDA (7.5 mM)). (b) Correlation between the number of layers of the 2D Lp-pi-COF film and the loading volume of the precursor solution for the two different concentration conditions ((TAPB (2.5 mM) and PDA (3.75 mM)) (orange) (TAPB (5 mM) and PDA (7.5 mM) (blue)). 181x254mm (300 x 300 DPI)


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Figure 3. Spectroscopic analysis of the 2D Lp-pi-COF film (a) Fourier transform infrared (FT-IR) and (b) Raman spectra of precursors (TAPB (black), PDA (red)) and Lp-pi-COF powder (blue) and film (green) showing the appearance of the imine functional group, which is the main product of the successful reaction of the amine and carbonyl functional groups. 138x200mm (300 x 300 DPI)


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Figure 4. Lateral and vertical structure analysis of the Lp-pi-COF film (a, b) TEM images of the porous structure of the 2D Lp-pi-COF film at different magnifications. Insets are (a) a calculated structure of the piCOF and (b) a magnified pore image of the film. (c) Layered structure of a 2D Lp-pi-COF film between chromium and SiO2 substrates, (d) magnified high resolution TEM image and (e) line profile corresponding to the red boxed region in (d). (f-h) Grazed incidence wide angle X-ray scattering (GI-WAXS) images of the Lp-pi-COF film on the SiO2/Si substrate showing different diffraction signals according to the change in the incidence angle. 206x254mm (300 x 300 DPI)


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Figure 5. Electronic device fabrication of the 2D Lp-pi-COF film and the electrical sensing experiment upon light and water exposure. (a) IV characteristic curve of the 2D multi-layer Lp-pi-COF film. The inset shows a photograph of the tested device. (b) On-off switching behavior upon light irradiation of the 2D Lp-pi-COF film device tested in (a) as a function of time at a bias voltage of 10 V. (c) I-V curve of the 2D multi-layer Lp-pi-COF film measured in argon (black) and water (blue) environments. The inset shows a photograph of the tested device. (d) On-off switching behavior upon moisture exposure to the 2D multi-layer Lp-pi-COF film device tested in (c) as a function of time at a bias voltage of 20 V. 254x185mm (300 x 300 DPI)


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