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Cite this: DOI: 10.1039/c5cc04679c Received 6th June 2015, Accepted 24th July 2015

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Few-layer, large-area, 2D covalent organic framework semiconductor thin films† Jeremy I. Feldblyum,a Clara H. McCreery,a Sean C. Andrews,a Tadanori Kurosawa,a Elton J. G. Santos,a Vincent Duong,b Lei Fang,c Alexander L. Ayznerb and Zhenan Bao*a

DOI: 10.1039/c5cc04679c www.rsc.org/chemcomm

In this work, we synthesize large-area thin films of a conjugated, imine-based, two-dimensional covalent organic framework at the solution/air interface. Thicknesses between B2–200 nm are achieved. Films can be transferred to any desired substrate by lifting from underneath, enabling their use as the semiconducting active layer in field-effect transistors.

Bottom-up synthesis represents an attractive method to obtain diverse two-dimensional (2D) materials having chemical and electronic properties currently inaccessible by traditional 2D materials such as graphene or MoS2.1 The synthesis and isolation of 2D materials from bottom-up methods has proven quite challenging, however, and their utilization in thin-film devices has remained almost entirely unexplored.1,2 The difficulties associated with forming and processing these materials arise due to the confluence of properties necessary for device incorporation: the material must be synthesized over large areas, amenable to transfer to device-relevant substrates, supportive of in-plane charge transport, and a semiconductor having easily distinguishable on and off states. Efforts towards such 2D materials have produced polymers having some, but not all of these properties. For example, the successful synthesis of in-plane conjugated 2D organic materials over large areas has been achieved, but only on highly ordered conductive substrates,3 hindering their deployment in semiconductor devices where continuous interfaces with conductors can prohibit device operation. Exfoliation has been applied to 2D layer-type crystalline polymers, yielding solution-processable few-layer4–6 and monolayer7,8 materials, but these polymers have lacked in-plane conjugation. In this work, a

Department of Chemical Engineering, Stanford University, 443 Via Ortega, Stanford, California 94305, USA. E-mail: [email protected] b Department of Chemistry and Biochemistry, University of California Santa Cruz, California 95064, USA c Department of Chemistry, Texas A&M University, College Station, Texas 77843, USA † Electronic supplementary information (ESI) available: Experimental procedures, computational details, additional analytical data. See DOI: 10.1039/ c5cc04679c

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we demonstrate the synthesis and device incorporation of the first processable, in-plane conjugated 2D polymer that can be formed as a thin film over large areas. Our starting point for forming 2D processable, in-plane conjugated films was inspired by previous work in covalent organic frameworks (COFs), pioneered by Yaghi, Matzger, and co-workers.9 COFs are network polymers synthesized from complementary organic small molecules having rigid geometries with multiple endgroups to promote reversible molecule–molecule coupling.10 COFs are typically11 formed as polycrystalline powders with crystalline domain size on the order of tens to hundreds of nanometers.12 While boronic acid condensation is the most common coupling motif for COFs, such COFs do not possess in-plane conjugation. For incorporation into standard backgate geometry field-effect transistors (FETs, the cornerstone device of organic-based electronics, commonly used to probe the electronic characteristics of new semiconducting materials13), charge transport must occur in the plane of the film. Hence, while boronate ester-type COFs can possess out-of-plane charge transport properties,14,15 they would still have to assume an edgeon orientation with respect to the substrate to yield operational FETs, a configuration presenting considerable practical difficulty. Another challenge in fabricating operable FETs from COFs stems from the fact that charge transport in FETs occurs only in the first few molecular layers.16 Electronic percolation within these layers is required for device operation; rough films lacking the requisite lateral electronic connectivity at the substrate–semiconductor interface will not yield working devices. Thus, while three recent examples of COFs having in-plane charge transport capabilities have been reported,17–19 processing into thin films of requisite quality for transistors remains undemonstrated. A processable 2D material capable of in-plane charge transport could contribute both to the fabrication of nanoscale devices for practical applications and to furthering our understanding of the fundamental physics of 2D semiconducting materials. In addition, the structural diversity and functionality of such materials might allow selection and fine-tuning of electronic characteristics. Therefore, we sought to use conjugated building blocks to construct a 2D,

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Fig. 1 (a) Experimental PXRD data (l = 0.414 Å, synchrotron radiation) of polyTB powder and its Pawley refinement against a computationallyderived model of polyTB. (b) FTIR spectra of polyTB and model compound 1 compared with those of their parent monomers. Amine (3300–3500 cm 1) and carbonyl regions (B1650–1700 cm 1) are highlighted.

Scheme 1 General scheme for polyTB synthesis and its idealized representation (top). Synthesis of model compound 1 (bottom).

fully conjugated network COF, and develop a method to fabricate thin films of the material compatible with transistor device fabrication. Bisaldehyde bdta and trisamine tapa (Scheme 1) were chosen as the monomers for COF formation. These multifunctional monomers can react with one another via imine condensation, a dynamic covalent coupling reaction that yields conjugated imine bonds between monomer-derived subunits.20 The reversibility of imine condensation enables the formation of crystalline COFs.21 Hence, a fully conjugated 2D crystalline network based on tapa and bdta subunits (hereafter termed ‘‘polyTB’’) can in principle be synthesized. In practice, by incubating bdta and tapa in a N,N-dimethylformamide (DMF) solution with a catalytic amount of acetic acid (Scheme 1), bright red powder is obtained. To assess the crystallinity of this powder, we analyzed it by powder X-ray diffraction (PXRD, Fig. 1a). An idealized model of polyTB was designed in silico based on other COFs in the P6/mmm space group9 (our model was assigned to the P1 space group; see ESI†). Pawley refinement of the experimental data from 0.5 to 3.0 in 2y (l = 0.414 Å, synchrotron radiation) showed that our idealized model provides a good description of the polyTB structure (wRp = 4.43%, wRp without background = 9.18%, Rp = 3.34%). The broad feature extending from B4–6.5 in 2y (d B 4–6 Å, Fig. S5, ESI†), presumably arising from a broad distribution of sheet-to-sheet distances between 2D sheets within the polyTB structure, was excluded from cell refinement. This relative lack of order perpendicular to the plane of each polyTB sheet is unsurprising given that the long alkyl side chains of the bdtaderived subunits may protrude both laterally into the pores of each sheet and vertically out of the plane. While PXRD is sufficient to describe crystalline domains within the bulk polyTB powder, FTIR is necessary as a complimentary tool to assess the extent of functional group conversion. After washing at elevated temperature (see ESI,† Section S1), dry bulk polyTB was examined by grazing incidence FTIR (Fig. 1b). Features originating from both the bdta- and tapa-derived

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subunits are clearly visible and no vibrations in the primary amine region of the spectrum (3300–3500 cm 1) are present, indicating complete consumption of the primary amine groups. However, the peak at 1675 cm 1 suggests the possible presence of free aldehyde. Liu and coworkers18 made a similar observation for their imine-based COF, but after studying a model compound, concluded that the corresponding peak in their COF FTIR spectrum was intrinsic to the COF itself and not indicative of a carbonyl group. To this end, we synthesized a model compound (1, Scheme 1) from bdta and a mono-functional amine and examined it by FTIR (Fig. 1b). In contrast to the previous findings,18 no peak in the carbonyl region was visible for 1. Hence, we conclude that free aldehyde is still present in polyTB powder, likely incorporated as endgroups at the edges of the 2D sheets or from occluded, unreacted bdta or oligomeric species not removed by washing. Optical absorption data are shown in Fig. 2a for polyTB, its parent monomers, and model compound 1 (representing an oligomeric subunit of the extended polyTB framework). The absorption edge (ledge, see Section S1 of the ESI†) for bdta occurs at 561 nm, whereas ledge = 604 nm and 616 nm for 1 and polyTB, respectively. These data suggest a significant increase in the degree of conjugation upon monomer coupling. Coupling of smaller oligomers to form the extended framework provides a smaller additional increase in ledge consistent with previous work22 showing only modest charge delocalization through the N center of triarylamine units. A HOMO–LUMO gap of B2.0 eV can be estimated for polyTB.

Fig. 2 (a) UV-Vis diffuse reflectance data for tapa, bdta, compound 1, and bulk polyTB. Data were offset along the y-axis for ease of visualization and units converted to Kubelka–Munk (see ESI†). (b) UV-Vis absorbance spectrum of polyTB film compared with those of 1, tapa, and bdta deposited on glass. Data were offset along the y-axis to ease visualization.

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COFs are generally obtained as bulk powders unsuitable for integration as semiconductors in FETs, where high quality thin films having intimate contact with dielectric substrates are required. While solution-processable, exfoliated COFs have been demonstrated,4–6 isolated sheets showed broad thickness distributions and had lateral dimensions in the hundreds of nm to single mm range, too small for conveniently patterning multiple devices over cm-sized substrates. Attempts at direct growth of COF films on SiO2 substrates immersed in reacting solutions of tapa and bdta in DMF with a catalytic amount of acetic acid yielded films too rough for FET fabrication. However, reacting tapa and bdta in a covered petri dish for 2 days in ambient conditions yielded a highly reflective red film at the solution/air interface (Section S1 of the ESI,† ‘‘polyTB Thick Film Synthesis’’). This film could be picked up from the surface of the solution with tweezers or substrates such as glass or silicon wafers (Videos S1 and S2, ESI†) and rinsed by transferring multiple times to petri dishes containing fresh DMF; films are held at the DMF/ air interface by surface tension (Video S2, ESI†). Atomic force microscopy (AFM) showed that these films are composed of lacey, web-like structures roughly 50 nm thick (Fig. 3a). We often observed particle-like features affixed to the films as well. The interfacially-grown films were examined by FTIR, grazing incidence X-ray scattering, UV-Vis, and photoelectron spectroscopy in air (PESA). Grazing incidence X-ray scattering of a dry film (Fig. S6, ESI†) shows a single reflection attributable to the film at Q = 0.218 Å 1, greater than that of the most intense reflection of the bulk powder (Q = 0.192 Å 1). A shift to higher Q (Q = 0.211 Å 1) is also observed after drying bulk polyTB powder (Fig. S7, ESI†), which occurs concomitantly with significant peak broadening suggestive of a reduction in crystalline order. Hence, we attribute the shift to higher Q for the film to a reduction in crystallinity. To determine whether the COF film exhibited a preferred orientation with respect to the substrate, we performed

Fig. 3 (a) AFM image of ‘‘thick’’ polyTB film formed by using monomer precursor solution. (b) AFM image of ‘‘thin’’ film formed by using polyTB mother liquor (after powder has formed) for film growth. Red lines in images indicate sections used for height profiles.

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angle-resolved X-ray absorption measurements at the carbon K-edge (see Section S1 and Fig. S8 of the ESI†). By analyzing the angular dependence of the absorption intensity, we determined a mean orientation of 391 with respect to the surface for sp2 carbon species in the COF backbone. Hence, the film has a weak preference for face-on orientation relative to the substrate plane, (see ESI,† Fig. S8 caption for further discussion). Despite the reduction in crystallinity for the film after drying, the FTIR spectrum is nearly identical to that of the polyTB bulk powder (Fig. S9, ESI†). UV-Vis data of films rinsed thoroughly with DMF and deposited on glass are shown in Fig. 2b. ledge = 616 nm (2.0 eV) for the film, consistent with that determined for bulk polyTB powder, and still higher than those of spin-cast films of the parent bdta and tapa monomers. HOMO and LUMO levels of 5.5 and 3.5 eV, respectively, were estimated from PESA (Fig. S10, ESI†). While the formed films are relatively thin, their roughness precluded incorporation into FETs. By utilizing diluted mother liquor isolated after bulk polyTB synthesis as our film growth medium (see ESI,† ‘‘polyTB Thin Film Synthesis’’), we were able to form smooth, thin films at the solution–air interface. Film growth could be detected by eye, as the surface of the dark red mother liquor solution acquired a green-tinted sheen as soon as film of any thickness was present (Fig. S11, ESI†). The films, while delicate, could still be transferred to fresh DMF using Si substrates without dissolution or fracture within the edges of the substrate used for transfer. Film thickness could be controlled by isolating films at different stages of growth; longer incubation times led to thicker films. The thinnest of the collected films was 1.8 nm thick with an average roughness of 0.2 nm (Fig. 3b), while the thickest was 29 nm, also with an average roughness of 0.2 nm (Fig. S12a and b, ESI†). The maximum thickness obtainable by this growth was not explored. Strikingly, the thinnest films appear to be composed of single layers by AFM; sharp, well-defined steps found in one film indicate the beginnings of growth of a subsequent layer, also of 1.8 nm thickness (Fig. S12c, ESI†). Similar step heights were also found at the surface of thicker films (Fig. S12b, ESI†), suggesting that each growing layer is of a similar thickness. Given that a 1.8 nm step height is greater than that expected for an atomically thin 2D monolayer, the possibility still exists that each growth layer consists of one or two molecular layers (see Section S5 of the ESI† for further discussion). Although the thickness of films grown from polyTB mother liquor approaches molecular dimensions, they are faint red and highly reflective when deposited on glass. Even the thinnest films share optical absorption and FTIR characteristics similar to those of thicker polyTB films (Fig. S9 and S13, ESI†). This simple method for forming nanometer-thin, large-area polyTB films enabled us to use the material as the semiconductor active layer in a thin-film FET. After washing with fresh DMF, a polyTB thin film was transferred to a degenerately doped (n++), 2  2 cm Si wafer with a 300 nm oxide layer by lifting the wafer underneath the film. After drying the film in air, 40 nm thick Au source and drain top contacts were deposited through a shadow mask, defining a top-contact transistor with channel length and width of 50 and 4000 mm, respectively.

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open questions regarding film formation, stability, and electronic performance remain to be addressed before advanced applications using these materials may be realized. Acknowledgment is made to the Donors of the American Chemical Society Petroleum Research Fund for support of this research (#54335-ND10). Use of the Advanced Photon Source at Argonne National Laboratory was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract no. DE-AC02-06CH11357.

References Fig. 4 Representative transfer plot of thin-film transistor employing a polyTB thin film active layer.

Applying a potential between source and drain (VSD) up to 100 V yielded very low current (o10 pA). However, on applying an additional gate voltage (VG), the source-drain current (ISD) rose by two orders of magnitude (Fig. 4). Both transfer (ISD vs. VG) and output (ISD vs. VSD) plots showed characteristics consistent with those expected for organic FETs (Fig. 4 and Fig. S14, ESI,† respectively). Calculating in the linear regime,13 we obtained a mobility of 3.0  10 6 cm2 V 1 s 1. We subsequently fabricated and measured 15 additional devices, yielding an average mobility of 3.0  10 6 cm2 V 1 s 1 and an average on/off ratio of 850 (all device metrics for individual devices are provided in Table S1 of the ESI†). The relatively low mobility13 of these COF/polyTB transistors may be attributable to defects in the thin film, suggesting that improvements in both chemistry and film preparation are needed to yield higher mobility devices. Calculations based on crystallite size (see Section S7 of the ESI†) suggest that charge carriers passing across a 50 mm transistor channel must traverse a minimum of B750 grain boundaries. Internal defects (as discussed previously) may further contribute to reducing the measured mobility. Finally, the loss in crystallinity upon film drying could contribute to torsional and other conformational defects that can interrupt conjugation. Utilizing more planar branching monomers in place of tapa and optimizing reaction conditions to yield larger crystalline domains might lead to COF films with fewer electronic defects. Recent studies suggest 2D network polymers may possess electronic characteristics of fundamental interest such as Dirac cones.23 Furthermore, such materials might be functionalized easily without modifying the backbone, enabling control of electronic properties without disrupting thin-film morphology. Functionalization without changes in morphology may enable complex device architectures such as 2D p–n junctions24 or transistor sensors with high analyte sensitivity and selectivity. However, while we report a simple proof of principle herein,

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Few-layer, large-area, 2D covalent organic framework semiconductor thin films.

In this work, we synthesize large-area thin films of a conjugated, imine-based, two-dimensional covalent organic framework at the solution/air interfa...
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