Selective Growth of Covalent Organic Framework Ultrathin Films on

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Selective Growth of Covalent Organic Framework Ultrathin Films on Hexagonal Boron Nitride Bing Sun, Jing Li, Wei-Long Dong, Mei-Ling Wu, and Dong Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b04410 • Publication Date (Web): 17 Jun 2016 Downloaded from http://pubs.acs.org on June 24, 2016

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The Journal of Physical Chemistry

Selective Growth of Covalent Organic Framework Ultrathin Films on Hexagonal Boron Nitride

Bing Sun,†,‡ Jing Li,†,‡ Wei-Long Dong,†,‡ Mei-Ling Wu,†,‡ Dong Wang†,*

†

Key Laboratory of Molecular Nanostructure and Nanotechnology and Beijing

National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100190, P. R. China ‡

University of CAS, Beijing 100049, P. R. China

1

ACS Paragon Plus Environment


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Abstract Incorporating the intriguing covalent organic framework (COF) into devices and performing their advanced electronic nature are still challenging. Herein, we demonstrate the direct growth of 2D full-conjugated COF ultrathin films on dielectric hexagonal boron nitride (hBN) for the first time and study the carrier transporting characteristics of π-conjugated COF films. Under the optimized solvothermal conditions,

few-layered COF-366

films

with

the covalent connection

of

tetra(p-aminophenyl)porphyrin and terephthaladehyde are selectively fabricated on mechanically exfoliated hBN flakes. COF-366 films on hBN substrate present red-shift absorption edge and decreased band gap compared to the bulk COF powders. The organic field-effect transistor device based on COF-366 ultrathin films demonstrates p-type current modulation with an On/Off ratio of 105 and mobility of 0.015 cm2 V-1 s-1. The present work represents a universal method for COF film growth on dielectric surface, and also provides important insight to the carrier transport of 2D π-conjugated system and potential applications of 2D COFs in electronics.

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1. Introduction Covalent organic frameworks (COFs), a class of porous, crystalline materials,1-2 have drawn remarkable research interests in gas storage and separation,3-4 catalysis,5-7 electrochemical energy storage,8-9 and chemical sensors10 due to their micro- or mesoporosity, high surface area and topologically designable structures. Layered 2D COFs feature the planar π-conjugated system and well-ordered columnar stacking arrangement, which are expected to exhibit anisotropic electrical properties. π-Stacking columns of COFs have been demonstrated to exhibit high carrier mobility,11-13 and great potential applications in electronics.14-17 Inspired by the exceptional high mobility in graphene planes, it is of great interest to sight the intrinsic carrier mobility in π-conjugated backbone of 2D COFs. However, the further understanding of the electric property of 2D COFs faces great challenges. Firstly, the insolubility of COF powders in most of solvents makes it difficult for device or electrode fabrication. Secondly, the randomly oriented microcrystals in COF powders conceals the intrinsic charge transport behaviour.18 Therefore, it is of highly demand to fabricate COF films with well-defined structures. Recently, COF films have been recently developed by growing COF thin film on single-layer graphene (SLG), and other substrate,19-24 as well as assembling 2D COF monolayers from air/water interface.25-27 Although the substrate supported COF films are convenient for investigating the property of COFs, the conductive nature of these substrates can largely interface with the intrinsic electronic measurements of COF 3



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films and limit their further applications. More efforts should be made to explore appropriate substrates for COF films growth. Hexagonal boron nitride (hBN) is an excellent layered insulator material with a wide direct band gap (ca. 5.9 eV) and has been used as a reliable dielectric layer in electronic devices.28 The hBN is structurally resembled to graphene, which has been shown previously to be able to help the assembly of 2D organic molecules.29-31 By virtue of the pristine and atomically smooth surface of hBN and the absence of trapped interfacial charge, the electron mobility of the semiconductor layers on hBN surface can be significantly improved.32-33 The recent theoretical work has predicted that the hBN can cause only negligible perturbation to the electronic bands of the supported active layer,34 which is important for us to understand the intrinsic electronic property of COF thin film. Herein, we report the direct growth of ultrathin COF films on the dielectric hBN surface for the first time. COF-366 with tetragonal geometry constructed from covalently coupling of tetra(p-aminophenyl)porphyrin (TAPP) and terephthaladehyde (TPA) is selected as target structure because the imine-based COFs possess fully conjugated networks and the intriguing aromatic structures of porphyrin. Figure 1 shows the selectively growing process of COF film on mechanically exfoliated hBN. By adopting the optimized solvothermal conditions, few-layered (5 – 8 layers) imine-type COF films selectively grown on hBN surface of flakes demonstrate the decreased band gap and also show the p-type current modulation with an Ion/Ioff ratio of 105 and field-effect mobility of 0.015 cm2 V−1 s−1. Our results provide a universal 4


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and promising perspective on the selective growth of COF films on dielectric substrate and shed important insight to the lateral carrier transportation behaviour of 2D π-conjugated systems.

Figure 1 Schematic diagram of selective growth of COF-366 film on hBN surface. The hBN flakes are transferred onto surface of silicon wafer through mechanically exfoliated method. Under the solvothermal treatment in a sealed vessel, COF-366 thin film, which consists of TAPP and TPA connected covalently with imine motifs, selectively grows on hBN surface.

2. Experimental 2.1 Selective growth of COF-366 film on hBN. Selectively solvothermal growth of COF-366 films on hBN proceeded in a sealed tube containing corresponding precursors. All the regents were used as received without further purification. The procedure began with the mechanical exfoliation of hexagonal boron nitride (hBN) single crystals (99.995%) onto degenerately doped silicon substrate covered with a thin layer of oxide (SiO2 or HfO2).32 HfO2 coated on silicon substrate was prepared according to previous work.35 After annealing at 500 5



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o

C for 3 h in ambient atmosphere, the hBN/Oxide/Si slice was added into a cylindrical

tube (18 cm of length, ϕin = 0.8 cm, ϕout = 1.0 cm) which was charged with tetra(p-aminophenyl)porphyrin (TAPP, 97%, TCI Chemicals; 0.51 mg, 0.75 µmol), terephthaladehyde (TPA, TCI Chemicals; 0.20 mg, 1.5 µmol) and a mixture of 1,4-dioxane/ methanol (v/v 3:1, 1 mL, without addition of acid as catalyst) and desaerated with inert argon before sealed. The sealed tube was heated in a 120 oC oven for 24 h. After cooled down naturally to room temperature, the substrate slice was picked out and submerged in THF (10 mL) overnight and finally rinsed by acetone and dried under vacuum. 2.2 General characterization methods Raman spectra and Raman mapping images of COF-366/hBN/Oxide/Si were recorded with a Thermo Scientific DXR Raman spectroscopy (532 nm laser). Fluorescence images were recorded on an Olympus FV1000-IX81 laser scanning confocal microscope. Atomic force microscopy (AFM) was performed by a Bruker Nanoscope IIIa system in tapping mode at ambient temperature. UV-Vis-NIR absorption spectra were recorded on a Shimadzu UV-2600 ultraviolet-visible spectrophotometer with an integrating-sphere accessory. 3.3 Fabrication and measurement of OFET Transistor devices were fabricated in the top-contact device configuration. 60 nm thick gold source and drain electrode were deposited on the COF-366/hBN/Si slice surface through a shadow mask. I–V characteristics of the transistor were recorded on 6


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a Keithley 4200 SCS and a Micromanipulator 6150 probe station in a clean and shielded box at room temperature in air. The field effect mobility (µ) were calculated in the saturation regime by using the following equation: ISD = µCi(W/2L)(VG–VT)2,36 where ISD is the drain current, µ is the field-effect mobility, Ci is the gate dielectric capacitance, W and L are the channel width and length, respectively, and VT is the threshold voltage.

3. Results and discussion 3.1 Selective growth of COF films on hBN The structure of COF films formed on hBN surface is firstly confirmed by using confocal Raman spectra. As shown in Figure 2, there is only one peak at 1362 cm–1 indexed as the vibration mode of substrate (hBN) in the range from 1000 to 2000 cm–1. By comparing the spectrum of COF films with that of corresponding monomers (curve d with curves b and c), the typical Raman bands at 1691 and 1702 cm–1 ascribing to the aldehyde stretching vibration of TPA are absent in the case of COF films. On the other hand, the Raman peak at 1328 cm–1 corresponding to the stretching vibration of phenyl groups linking with amino groups and wagging mode of these amino groups is remarkably reduced in the Raman spectrum of COF films (at 1330 cm–1)

27

. Particularly, two bands located at 1570 and 1625 cm–1 are very

different from the vibration mode of phenyl and porphin ring (at 1548 and 1586 cm–1) of TAPP 37, which are corresponding to the newly stretching vibration of C=N bond 7



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between phenyl group of TPA and phenyl group of TAPP. Therefore, the results from Raman spectra confirm the characteristic details of imine-connected structure. The COF film structure is also further confirmed by X-ray photoelectron spectra (XPS, ESI# Figure S4). The emerging new peak at 398.6 eV (N1s) attributed to C=N indicates the formation of imine motifs in COF films.

Figure 2 Typical Raman spectra of (a) hBN, (b) TPA, (c) TAPP, and (d) COF films on hBN.

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Figure 3 (a) Optical image of selected region for Raman mapping. Raman mapping images of (b) hBN on SiO2 surface at Raman shift of 1362 cm–1 and COF-366/hBN at (c) 1570 cm–1 and (d) 1625 cm–1. Fluorescence images of (e) hBN (The profile of hBN flake is outlined by green dash line) and (f) COF-366 on hBN surface. (Exciting laser: λex = 488 nm, emission range: λem = 600 – 700 nm).

The selective growth of COF-366 films on hBN surface is characterized by using Raman mapping and fluorescence imaging., The Raman mapping image of hBN at 1362 cm–1 (Figure 3b) profiles the original shape of the exfoliated hBN flake (Figure 3a). The hBN flake covered with COF-366 films (Figure 3c and 3d) shows the same shape feature as the original hBN sheet. No remarkable Raman signals are found in bared silicon surface. For the fluorescence images of hBN flake before and after solvothermal processing, the remarkable fluorescent emission in the range of 600 – 700 nm excited by the laser of 480 nm also reveals the outline of hBN flake (Figure 3e, f). Furthermore, the selective growth of COF films on hBN is independent of the chemical nature of oxide layer (ESI# Figure S5) and the graphene-like hexagonal lattice of hBN is integrant for the orientation of COF-366 films. Such results may be attributed to the favorite nucleation of COFs on hBN at the lower monomer concentration. In the presence of substrate, the monomers in solution have the tendency to absorb orderly paralleling with the substrate surface due to the inductive effect of specific substrate, especially for the lattice interface of layered materials.38 9



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With the interacting difference of organic molecules with hBN and silicon surface, monomers are preferential to adsorbing onto hBN surface via van der Waals interaction. Meanwhile, the homogeneous nucleation of COFs in solution slows down at the lower concentration conditions. With a solid-surface-induced self-cure dynamics, COFs are liable to laterally extend on the surface.39 As a result, the conjugated molecular structures can reversibly expend along the solid-solution interface. When the monomer concentration below 0.75 mM for TAPP and 1.5 mM for TPA, it favors the selective growth of COF films on hBN. In the case of a higher monomer concentration (5 mM of TAPP), the nucleation in solution occurs and the selective growth of COF films on hBN surface is disturbed since the insolubale microcrystalline COF particles tend to adsorb indiscriminately onto substrates. Additionally, solvent components are also vital to regulate the selective growth of COFs. When the solvent is replaced by pure dioxane or dioxane/methitylene mixture, the COF films can spread on whole wafer surface. Under the optimized solvothermal conditions, our preliminary results show that the selective growth of COFs on hBN surface can be also applied to other Schiff-base typed COFs.

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Figure 4 Topographic images of exfoliated hBN flakes on SiO2 surface (a) in large and (b) amplified scale; COF-366 film/hBN (c) in large and (e) amplified scale, respectively. (e) Cross-section analysis of the same hBN flake before (the upper red curve) and after (the bottom olive curve) solvothermal processing. 3.2 AFM analysis The surface characteristics of COF film on hBN are measured by using atomic force microscopy (AFM). COF-366 thin films can smoothly spread on the pristine hBN interface although there are few particles distributing on the surface (Figure 4c). The average roughness of COF-366 films on hBN surface is evaluated as 0.24 nm, which is comparable to that of hBN (0.19 nm). The thickness of the ultrathin film on hBN in Figure 4 is measured by AFM at the same location before and after solvothermal procedure and evaluated as 2.46 ± 0.5 nm. Statistical analysis over 50 samples shows that the average thickness of the ultrathin films is in the range of 2.34 to 4.56 nm. Particularly, the film thickness is independent of the selected hBN flakes. With prolonging the reaction time to 48 h, there is no significant increase of the thickness of COF films. Based on the AFM observation, the flat COF ultrathin films imply that the in-plane full conjugated ultrathin films spread on hBN substrate. According to the previous work,12 the interlayer distance of square layers in COF-366 stacking along the (001) direction is evaluated as 5.64 Å. It is reasonable to conclude that the ultrathin COF-366 films are fabricated by 5 – 8 layers onto hBN surface. 3.3 Optical absorption spectra and electronic property 11



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Absorption spectrum of COF-366 film on hBN (Figure 5) is recorded for investigating its electronic properties. The COF film reveals the intense Soret band around 390 nm and the typical Q-band absorption ranging from 500 to 700 nm, which is dominated by contributions from tetrapyrrole circle. The absorption edge can reach 960 nm which is remarkably red-shifted comparing with that of TAPP monomer and, more importantly, the COF-366 powders synthesized under similar solvothermal condition (ESI# Figure S6). The HOMO-LUMO gap is evaluated as 1.34 eV on the basis of the absorption spectrum, which is lower than that of corresponding powders (evaluated as 1.47 eV, ESI# Figure S6). The red-shifting absorption edge and decreased HOMO-LUMO gap also indicate the ordered assembly of the lateral extended π-conjugated films on hBN substrate. Considering the limited chemical hybridization between hBN and COF film,34 the existence of dielectric hBN substrate cause very weak perturbation to the electronic properties of COF-366 films, while the intimate contact at the interface of COF-366 and dielectric hBN layer is desirable to facilitate the incorporation of the COF films as semiconductors into organic field-effect transistor (OFET) devices.

Figure 5 (a) Normalized UV-Vis-NIR absorption spectrum of COF-366 thin films 12


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growing on hBN surface, and (b) the corresponding Tauc’s plot. 3.4 Electronic property of COF films Figure 6a illustrates the typical bottom-gate top-contact OFET architecture to investigate the lateral electronic properties of COF-366 ultrathin films. High-k HfO2 layer coated on heavily n-doped silicon is employed as the gate substrate. As the first few molecular layers at the semiconductor-dielectric interface play the most important roles in OFET devices,40 the ultrathin COF film is prospective as semiconductor layer. The typical transfer characteristic of the COF-366 ultrathin film measured under ambient conditions demonstrates p-type current modulation with operating gate voltage to –60 V (Figure 6b), although a lower current (less than 10 nA) is yielded on applying series of voltages between source and drain (ESI# Figure S7). An Ion/Ioff ratio of approximate 105 is measured and the threshold voltage is evaluated as –9.6 V. As control experiment, OFET measurements are also carried out based on COF-366/HfO2/Si wafers without hBN flakes. The HfO2 surface is covered with discrete COF-366 particles (AFM, ESI# Figure S8). Under the same OFET operating conditions, a weak current (< 0.3 nA) is measured with the gate voltage increasing to –60 V (ESI# Figure S9). It indicates that flat hBN surface is not only necessary for selectively growth of ultrathin COF films, but also provides an essential platform for enhanced OFET measurement. The field-effect mobility of COF-366 films on hBN surface in the saturated regime is calculated as 0.015 cm2 V–1 s–1. The typical OFET devices display the carrier mobility in the range of 0.01 – 0.02 cm2 V–1 s–1, which 13



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increases remarkably by two orders of magnitude comparing with that of directly growing COF films onto HfO2/Si surface (3.4 × 10–4 cm2 V–1 s–1, ESI# Figure S9).

Figure 6 (a) Schematic illustration of the proposed bottom-gate top-contact OFET device based on COF-366 ultrathin films on hBN surface. (b) Typical transfer characteristics for the OFET device with the source-drain voltage (VSD = –5 V).

The enhanced carrier mobility of COF films in the present of hBN may be attributed to the 2D lateral assembly of π-conjugated COF-366 layers on hBN surface as the lateral channel and the improved semiconductor/dielectrics interfacial properties. Nevertheless, the carrier mobility of COF-366 film is still lower than the previous result (8.1 cm2 V–1 s–1) of COF-366 powders.12 For COF-366 powders, Yaghi et al points out that the extended planar π-electron system allows close interlayer distances, facilitates the electronic transport through π-π columns and results in the high charge carrier mobility value. For ultrathin COF-366 films involved in our OFET devices, the gate-field induced charge carriers are confined in 14


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few-layered channel and transported along π-conjugated plane. The huge discrepancy of carrier mobility for powder sample in previous work and ultrathin film may be attributed to the anisotropic carrier transportation behavior of 2D COFs, asides from difference in the employed characterization techniques. The extended π-conjugated layer of COF-366 ultrathin films covering the whole channel is essional to carrier transport, thus the carrier mobility is also considerably subject to the defects and domain boundaries that can interrupt conjugation (Figure S10). Further optimizing the reaction conditions to obtain larger planar domains with fewer defects is required to improve the lateral carrier mobility of COF films and further extend their potential applications in organic electronics.

4. Conclusions In summary, we reported a universal methodology for the selective growth of ultrathin COF films onto the surface of mechanically exfoliated hBN flakes through a convenient solvothermal method. Few-layered (5 – 8 layers) π-conjugated COF-366 sheets were confirmed to laterally extend on hBN surface. The few-layered ultrathin films could act as lateral transferring channels of charge carriers in the OFET device. The p-type current modulation was observed within operating bottom-gated voltage range and the carrier mobility of COF-366 films was yielded to 0.015 cm2 V–1 s–1 with an On/Off ratio of 105. The compatibility of dielectric hBN to support the formation of layered π-conjugated COFs is expected to stimulate further attention for lateral 15



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electron transportation along 2D π-systems.

ASSOCIATED CONTENT Supporting Information Description of further characterization of COF-366 powders and ultrathin film on hBN surface, including Raman, XPS, UV-visible absorption spectra, AFM, OFET measurements and GIXRD.

AUTHOR INFORMATION Corresponding Author: *E-mail: [email protected] Tel: +86-10-82616953 Notes: The authors declare no competing financial interests.

Acknowledgements This work was supported by National Natural Science Foundation of China (21433011, 21127901, and 91527303), and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB12020100).

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Selective Growth of Covalent Organic Framework Ultrathin Films on

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