Highly Sensitive Thin-Film Field-Effect Transistor Sensor for Ammonia

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Highly Sensitive Thin-Film Field-Effect Transistor Sensor for Ammonia with the DPP-Bithiophene Conjugated Polymer Entailing Thermally Cleavable tert-Butoxy Groups in the Side Chains Yang Yang, Guanxin Zhang,* Hewei Luo, Jingjing Yao, Zitong Liu, and Deqing Zhang* †

Beijing National Laboratory for Molecular Sciences, Organic Solids Laboratory, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China S Supporting Information *

ABSTRACT: The sensing and detection of ammonia have received increasing attention in recent years because of the growing emphasis on environmental and health issues. In this paper, we report a thin-film field-effect transistor (FET)-based sensor for ammonia and other amines with remarkable high sensitivity and satisfactory selectivity by employing the DPP-bithiophene conjugated polymer pDPPBu-BT in which tert-butoxycarboxyl groups are incorporated in the side chains. This polymer thin film shows p-type semiconducting property. On the basis of TGA and FT-IR analysis, tert-butoxycarboxyl groups can be transformed into the −COOH ones by eliminating gaseous isobutylene after thermal annealing of pDPPBu-BT thin film at 240 °C. The FET with the thermally treated thin film of pDPPBu-BT displays remarkably sensitive and selective response toward ammonia and volatile amines. This can be attributed to the fact that the elimination of gaseous isobutylene accompanies the formation of nanopores with the thin film, which will facilitate the diffusion and interaction of ammonia and other amines with the semiconducting layer, leading to high sensitivity and fast response for this FET sensor. This FET sensor can detect ammonia down to 10 ppb and the interferences from other volatile analytes except amines can be negligible. KEYWORDS: thin-film field-effect transistor, conjugated polymer, ammonia sensor, diketopyrrolopyrrole, thermally cleavable groups

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INTRODUCTION

Various detection methods and sensing strategies for ammonia and amines have been developed. These include mass spectrometry coupled with gas chromatography (GC− MS), chromatography, spectrometry, colorimetry, chemiluminescence, and electrochemistry.6−12 These existing methods, though highly sensitive and selective, still have limitations. For instance, some of them need expensive and cumbersome instrumentation and line of sight is required to read the output, whereas the others require extensive sample preparation prior to analysis. Thus, most of these analytical tools for ammonia and amines are not easily portable and also unwieldy for environmental monitoring.

It is known that ammonia (NH3) is potentially associated with chronic diseases like asthma, severe respiratory inflammations and lung diseases.1,2 The emission of ammonia from fertilizers widely used in agriculture and other industries is believed to play an important role in air pollution, and the presence of ammonia in air can facilitate the formation of hazy weather.3 Therefore, the sensing and detection of ammonia have received increasing attentions in recent years because of the growing emphasis on environmental and health issues.4 Apart from ammonia the sensing and detection of other amines are also important. For instance, putrescine (butane-1,4-diamine) and cadaverine (pentane-1,5-diamine) are salient markers of meat decomposition.4,5 Accordingly, sensitive and selective sensors for these amines can enable both meat providers and consumers to monitor its spoilage for health and economic reasons. © XXXX American Chemical Society

Special Issue: Applied Materials and Interfaces in China Received: August 30, 2015 Accepted: October 1, 2015

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DOI: 10.1021/acsami.5b08078 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Scheme 1. Chemical Structures of pDPPBu-BT and pDPPCOOH-BT and the Design Rationale for Sensing Ammonia and Amines

Scheme 2. Synthetic Approach for pDPPBu-BT

selectivity, fast response is another important metric for FET sensors. In this paper, we report a polymeric FET-based sensor for ammonia and other amines with remarkable high sensitivity and satisfactory selectivity by employing the DPP-bithiophene conjugated polymer pDPPBu-BT with thermally cleavable tert-butoxy groups in the side chains (Scheme 1). The design rationale is based on the following considerations: (i) recent studies reveal that FETs with thin films of DPP-bithiophene conjugated polymers exhibit reasonably high carrier mobilities;54−60 (ii) the incorporation of − COOH groups in the side alkyl chains is expected to improve the selectivity as ammonia and other amines can react with − COOH groups to form the respective salts which will change the polar environment of charge carriers and thus affect the transporting of charge carriers; (iii) however, the presence of − COOH groups in the side chains will reduce the solubility of the polymer in organic solvents and thus processing with spincoating technique may be difficult. Instead, the tertbutoxycarbonyl (−COOBu-tert) groups are incorporated in the side chains. It is anticipated that −COOBu-tert can be transformed into −COOH by releasing the isobutylene under thermal annealing condition;61,62 (iv) our preliminary result shows that the release of isobutylene accompanies the formation of nanopores within the thin film, which is

In comparison, electronic sensors with electrical current as the output signal offer more advantages.6,12−14 In principle, the sensing and monitoring of the analytes (e.g., NH3) with electronic sensors can be carried out in real time with the as-is sample. Also, they can be readily integrated into electronic circuitry. In recent years, organic and polymeric field-effect transistors (FETs) have received increasing attentions, and their performances have been improved significantly and became comparable to those of amorphous silicon transistors.15−26 Organic and polymeric FETs have been successfully utilized for building chemo/biosensors.27−53 Several groups have reported organic and polymeric FET-based sensors for ammonia.39−53 After optimization of the device structure and thickness of the semiconducting layer, ammonia with concentration down to 0.45 ppm can be detected.53 The sensing mechanism is owing to the formation of defects and traps upon the interaction of ammonia with the semiconducting layer. Thus, selectivity toward ammonia can be a challenge for such FET sensors. Katz and co-workers incorporated tris(pentafluorophenyl)borane (TPFB) into the semiconducting thin film to improve the selectivity and sensitivity for ammonia sensing.53 These organic and polymeric FET sensors for ammonia need further improvements in terms of either sensitivity or selectivity. Apart from the sensitivity and B


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decomposition starts at 210 °C and ends at 265 °C with a weight loss of 10.9%, which corresponds well to the theoretical weight loss (10.8%) of isobutylene from the side chains, leading to the transformation of pDPPBu-BT into pDPPCOOH-BT (Scheme 1). The formation of pDPPCOOH-BT was supported by the FT-IR data. Figure 2 shows the FT-IR spectrum of

detrimental to charge transporting. But, the presence of nanopores within the thin film will facilitate the interaction of − COOH group with ammonia and other amines; accordingly, the sensitivity can be improved and simultaneously fast response can be achieved. The results reveal that the FET sensor with pDPPBu-BT after thermal annealing at 240 °C can sense ammonia down to 10 ppb (v/v) with detectable variation of IDS (the drain−source current) after exposure to ammoniacontaining air for just 5.0 s. The interferences from other volatile solvents except amines can be negligible. Such good selectivity and high sensitivity toward ammonia can be attributed to the generation of −COOH groups and formation of nanopores after thermal annealing of pDPPBu-BT.

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RESULTS AND DISCUSSION Synthesis. As depicted in Scheme 2, pDPPBu-BT was prepared by the Stille coupling reaction between 2BrDPPBu and 4,4′-didodecyl-[2,2′-bithiophene]-5,5′-diyl)bis(trimethylstannane). The synthesis of 2BrDPPBu started from 3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)dione (DPP), which was transformed into DPPBu after reaction with tert-butyl bromoacetate and K2CO3 in DMF according to previous report.63 Then, reaction of DPPBu and N-bromosuccinimide (NBS) yielded 2BrDPPBu in 88% yield. The Stille coupling of 2BrDPPBu and 4,4′-didodecyl-[2,2′bithiophene]-5,5′-diyl)bis(trimethylstannane) was successfully carried out in the presence of Pd2(dba)3 and P(o-tolyl)3 as the catalyst and ligand, respectively. pDPPBu-BT was precipitated out from the reaction mixture after addition of methanol. The precipitate was subjected to Soxhlet extraction with methanol, hexane, acetone, and chloroform to remove the respective monomers and oligomers. The resulting residues were collected and dried under vacuum to afford pDPPBu-BT in 73% yield. The molecular weight of pDPPBu-BT was measured with gel permeation chromatography (GPC) in 1,2,4-trichlorobenzene using polystyrene as a standard. Weight-average molecular weight (Mw) and number-average molecular weight (Mn) of pDPPBu-BT were determined to be 25294 and 12004, respectively, with a PDI (polydispersity index) of 2.11. pDPPBu-BT shows poor solubility in other solvents except in hot o-dichlorobenzene, 1,2,4-trichlorobenzene, and 1,1,2,2tetrachloroethane. The chemical structure of pDPPBu-BT was confirmed by 1H NMR, 13C NMR, and elemental analysis (see the Experimental Section). Thermal Behavior and Removal of tert-Butoxy Groups in the Side Chains. Thermogravimeric analysis (TGA) was performed with pDPPBu-BT. As shown in Figure 1, pDPPBuBT exhibits three-step thermal decomposition. The first

Figure 2. FT-IR spectra of pDPPBu-BT before (black) and after (blue) thermal treatment at 240 °C for 30 min.

pDPPBu-BT and that after thermal treatment at 240 °C. The strong FT-IR signal at 1745 cm−1 owing to the CO in tertbutoxycarbonyl became weak and a new weak signal at 1729 cm−1 emerged. Simultaneously, the signal at 1155 cm−1 which can be ascribed to the C−O in tert-butoxycarbonyl disappeared after thermal treatment. These FT-IR data agree with the elimination of isobutylene and the transformation of tertbutoxycarbonyl groups to −COOH ones after thermal treatment at 240 °C. HOMO/LUMO Energy Levels. The absorption spectra of the solution and thin film of pDPPBu-BT were recorded. As shown in Figure 3a, the solution of pDPPBu-BT shows two absorption bands at 394 and 658 nm. In comparison, the absorption spectrum of the thin film was red-shifted. The absorptions at 394 and 658 nm in solution were shifted to 420 and 722 nm, respectively, for the thin film. Such absorption spectral shifts may be ascribed to the intermolecular interactions within thin film of pDPPBu-BT, which may entail the intermolecular electron-donor and acceptor interactions, according to previous reports.58,59 Thin film of pDPPBu-BT after thermal annealing at 240 °C for 30 min shows rather similar absorption spectrum (see Figure 3a). As discussed above, the thermal treatment at 240 °C can lead to the transformation of pDPPBu-BT into pDPPCOOH-BT by removal of tert-butoxy group. pDPPBu-BT and pDPPCOOHBT possess the same conjugated backbone, thus it is understandable that they show rather similar absorption spectrum. On the basis of the onset absorption of the thin film of pDPPBu-BT the optical bandgap (Egopt) was estimated to be 1.29 eV. Cyclic voltammgram of pDPPBu-BT was measured in the form of thin film. As depicted in Figure 3b, pDPPBu-BT exhibits one quasi-reversible oxidation wave. The redox potential of ferrocene was measured under the same condition, and the respective onset oxidation potential (Eonsetox) of the thin film was measured to be 0.51 V by reference to the redox potential of ferrocene/ferrocenium (Fc/Fc+) (see Figure 3b). HOMO energy of pDPPBu-BT was estimated to be −5.31 eV by using the following equation: HOMO = −(Eonsetox + 4.8) eV. Because no reliable reduction waves were recorded in our measurements, the LUMO energy of pDPPBu-BT was estimated with the HOMO and Egopt energies to be −4.02 eV.

Figure 1. TGA curve of pDPPBu-BT with a heating rate of 10 °C min−1. C


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Figure 3. (a) Absorption spectra of the solution and pristine/annealed thin films of pDPPBu-BT; (b) cyclic voltammogram of pDPPBu-BT thin film and the solution of ferrocene in CH3CN.

Figure 4. (a) Transfer and (b) output characteristics for the FET with pDPPBu-BT after thermal annealing at 100 °C.

Fabrication of FETs and Semiconducting Performance. FETs with the bottom-gate and bottom-contact (BGBC) geometry were fabricated with conventional techniques (see Experimental Section). Briefly, a solution of pDPPBu-BT in odichlorobenzene (o-DCB) was spin-coated to form thin-film onto the OTS (n-octadecyltrichlorosilane) modified SiO2 (300 nm) surfaces, and drain-source (D-S) gold contacts were fabricated by photolithography. The semiconducting performance of the as-prepared FET and those after thermal annealing at different temperatures were investigated. On the basis of the respective transfer and output curves, thin films of pDPPBu-BT exhibit typical p-type semiconducting properties as anticipated from the HOMO/LUMO energy levels of pDPPBu-BT. As an example, Figure 4 shows the transfer and output curves for the FET with thin film of pDPPBu-BT after thermal annealing at 100 °C. Table 1 summarizes the performance data of FETs based on thin film of pDPPBu-BT before and after thermal annealing at

groups and the transformation of pDPPBu-BT into pDPPCOOH-BT. The charge mobility decreased to 5.6 × 10−4 cm2 V−1 s−1 after thermal annealing 240 °C for 30 min under vacuum condition. Thermal annealing did not affect the Ion/off (see Table 1). The low charge mobilities of thin films of pDPPBu-BT before and after thermal annealing can be ascribed to the low packing order of polymer chains and poor thin-film morphology. Grazing incidence X-ray diffraction (GIXRD) and atomic force microscopy (AFM) were utilized to characterize thin films of pDPPBu-BT before and after thermal annealing at 240 °C. As shown in Figure S1, a signal at 2θ = 3.49° was detected in GIXRD pattern of the as-prepared thin film of pDPPBu-BT. Additional diffractions at 2θ = 6.98, 9.79, and 14.01° were rather weak and diffused. These data indicate that the lamella packing order of alkyl chains of pDPPBu-BT is low. Moreover, no diffractions corresponding to pi-pi interactions of conjugated backbone were observed. After thermal treatment at 240 °C even the diffraction at 2θ = 3.49° became weak and diffused (see Figure S1b). These GIXRD data agree well with the fact that thin film of pDPPBu-BT exhibits low hole mobility in comparison with other DPP-based conjugated polymers reported previously.54−59,64−68 Figure 5 shows the AFM images of the thin film of pDPPBuBT before and after thermal treatment at 240 °C. The pDPPBu-BT before heating showed relatively uniform morphology. However, the morphology altered significantly after thermal treatment. Pinholes with sizes around a hundred nanometers were observed within the film (see Figures 5c, d). We reason that the release of gaseous isobutylene induced by thermal treatment may perturb the interchain packing and thinfilm morphology under heating condition. It is expected that the emission of gaseous isobutylene can induce the formation of pinholes within the heated thin films. Such poor thin-film morphology is consistent with the observation that hole mobility decreased after thermal treatment of the thin film at

Table 1. Mobility (μh), Current on/off Ratio (Ion/off), and Threshold Voltage (Vth) of FETs with Thin Films of pDPPBu-BT at Different Annealing Temperatures μh (cm2 V−1 s−1)a

T (°C) rt 100 120 240

3.6 1.4 1.0 2.2

× × × ×

−4

10 10−3 10−3 10−4

(1.5 (4.4 (2.5 (5.6

× × × ×

Ion/off −3

10 ) 10−3) 10−3) 10−4)

1 1 1 1

× × × ×

3

10 103 103 103

to to to to

1 1 1 1

Vth (V) × × × ×

4

10 104 104 104

11−29 2−14 0−6 −10−15

The mobilities were provided in “average (highest)” form, and the data were obtained on the basis of more than 20 different devices.

a

different temperatures. Hole mobility of the as-prepared transistor was measured to be 1.5 × 10−3 cm2 V−1 s−1, and it increased to 4.4 × 10−3 cm2 V−1 s−1 after thermal annealing at 100 °C. As stated above thermal annealing at 240 °C of pDPPBu-BT can lead to thermal elimination of tert-butoxy D


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were fabricated with thin films of pDPPBu-BT that were thermally treated at 240 °C for 30 min. Figure 6a shows the transfer characteristics of the FET upon exposure to air. Obviously, the transfer characteristics remained almost unaltered. However, the transfer characteristics of the FET varied obviously after exposure to different concentrations of ammonia as shown in Figure 6b. The on-current (IDS) decreases after exposure to different concentrations of ammonia (see Figure 6b). For instance, the on-current decreases by 8.1% after exposure to 100 ppb of ammonia, and 15% after exposure to 1.0 ppm of ammonia (see Figure S2). This reduction of oncurrent (IDS) is still detectable even when the concentration of ammonia is as low as 10 ppb (see Figure 6c). Figures 6d show the variation of on-current (IDS) vs the exposure time in response to different concentrations of ammonia. The plot indicate that (i) this FET senor toward ammonia is extremely sensitive and ammonia with concentration down to 10 ppb can be detected, (ii) the sensing response is rather fast and the IDS can reach saturation after exposure to ammonia within a few seconds, and (iii) the decrease of IDS is larger when the FET is allowed to expose to higher concentration of ammonia, but the current decrease tends to be saturated after exposure more concentrated ammonia. The FET was also examined after exposure to other volatile solvents including triethylamine (Et3N), piperidine, 1,4diaminobutane (putrescine), dichloromethane, ethanol, ethyl acetate, hexane, acetone and hydrochloric acid. Figure 7 shows the decrease of IDS after exposure to these gaseous analytes. Clearly, the decrease of IDS is rather minor after exposure of this FET to dichloromethane, ethanol, ethyl acetate, hexane, acetone and hydrochloric acid with concentrations higher than 1000 ppm. But, large reduction of IDS is yielded after exposure to triethylamine (Et3N), piperidine and 1, 4diaminobutane (putrescine) as to ammonia (see Figure 7).

Figure 5. AFM images of pDPPBu-BT thin-film on OTS-modified SiO2/Si substrate (a, b) before and (c, d) after thermal annealing at 240 °C.

240 °C. However, as it will be discussed below, the existence of these pinholes is beneficial for improving the sensitivity and response of the FET sensor for ammonia and other amines. FET Sensor for Ammonia and Amines. In the following, we demonstrate the application of FETs with thin-films of pDPPBu-BT after thermal annealing in the selective and sensitive detection of ammonia and other amines. The gaseous analytes used in the experiments were prepared by controllable dilution with air and their concentrations were presented in the v/v form. The FETs employed for the sensing experiments

Figure 6. (a) Transfer characteristics for FET of pDPPCOOH-BT before and after exposure to air flow; (b) transfer characteristics for the FET of pDPPCOOH-BT after exposure to different concentrations of NH3 (0−1000 ppm); (c) variation of IDS for FET of pDPPCOOH-BT after exposure to 10 ppb NH3; inset shows the variation of IDS after exposure to air flow; (d) the variation of IDS vs time for FET of pDPPCOOH-BT after exposure to different concentrations of NH3 (0−1000 ppm). E


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volatile analytes. To our delight, the FET displays remarkably sensitive and selective response toward ammonia and volatile amines. This FET sensor can detect ammonia down to 10 ppb and the interferences from other volatile analytes except amines can be negligible. Moreover, the variation of the IDS is rather fast upon exposure to ammonia and other amines, which will enable the detection with this FET to be carried out in real time. Such high sensitivity and satisfactory selectivity can be attributed to the incorporation of tert-butoxycarboxyl groups in the side chains of pDPPBu-BT; thermal treatment results in the elimination of isobutylene and the transformation of tertbutoxycarboxyl groups into −COOH ones which will endow the FET sensor with good selectivity toward ammonia and amines, whereas the elimination of gaseous isobutylene accompanies the formation of nanopores with the thin film, which will facilitate the diffusion and interaction of ammonia and other amines with the semiconducting layer, leading to high sensitivity and fast response for this FET sensor.

Figure 7. Variation of IDS after exposure to different gases and solvent vapors: 1, CH2Cl2 (3066 ppm); 2, EtOH (1200 ppm); 3, ethyl acetate (1010 ppm); 4, hexane (20200 ppm); 5, acetone (12300 ppm); 6, HCl (3000 ppm); 7, Et3N (10 ppm); 8, piperidine (10 ppm); 9, 1,4diaminobutane (10 ppm); 10, NH3 (10 ppm) under the same conditions.

Therefore, this FET sensor shows satisfactory selectivity toward ammonia and volatile amines. Moreover, this FET sensor can be reuseable. As shown in Figure S3, the on-current decreases by 15% after exposure to 1.0 ppm of ammonia. However, after further annealing at 80 °C for 1.0 h under vacuum, the IDS current was almost restored. Further exposure to ammonia led to the reduction of IDS again (see Figure S3). The transfer curve of the FET with the as-prepared thin film of pDPPBu-BT kept almost unaltered after exposure to ammonia as shown in Figure S4. Accordingly, the sensing ability of the FET with the thermally treated thin film of pDPPBu-BT should be owing to the reaction of ammonia (and other amines) with −COOH groups in pDPPCOOH-BT. This hypothesis is corroborated by the FT-IR data. Figure S5 shows the IR spectra of the thermally treated thin film of pDPPBu-BT and that after exposure to ammonia. The FT-IR signal at 1729 cm−1, because of the −COOH group in pDPPCOOH-BT, was shifted to 1650 cm−1, indicating the formation of ammonium carboxylate which may act as additional traps for charge carriers. In fact, hole mobility of the FET with the thermally treated thin film of pDPPBu-BT decreases after exposure to ammonia as depicted in Figure S6. For instance, hole mobility is reduced from the initial 2.65 × 10−4 cm2 V−1 s−1 to 2.38 × 10−4 cm2 V−1 s−1 after treating the FET with 100 ppb ammonia. This may explain the selectivity of this FET sensor toward ammonia and other volatile amines. The high sensitivity and quick response may be ascribed to the formation of nanopores within the thermally treated thin films. Although such pinholes are detrimental to charge transporting, they can facilitate the diffusion and interaction of ammonia and other amines with the semiconducting layer.

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EXPERIMENTAL SECTION

Materials and Characterization Techniques. Chemicals were purchased from Alfa-Aesar and Sigma-Aldrich, and used without further purification. Solvents and other common reagents were obtained from Beijing Chemical Co. DPPBu (Scheme 2) was synthesized according to the reported literature.63 1 H NMR and 13C NMR spectra were measured on a Bruker AVANCE III 400 and 500 MHz spectrometers. Elemental analysis of carbon, hydrogen, sulfur and nitrogen was performed on a Carlo Erba model 1160 elemental analyzer. UV−vis absorption spectra were measured with JASCO V-570 UV−vis spectrophotometer. FT-IR spectra were measured with TENSOR-27 (Bruker). Gel permeation chromatography (GPC) analysis was performed on an PL-GPC 220 high temperature chromatograph at 150 °C equipped with a IR5 detector; polystyrene was used as the calibration standard and 1,2,4trichlorobenzene as eluent; the flow rate was 1.0 mL/min. TGA-DTA measurements were carried out on a SHIMADZU DTG-60 instruments under a dry nitrogen flow, heating from room temperature to 550 °C, with a heating rate of 10 °C/min. Cyclic voltammetric measurements were carried out in a conventional three-electrode cell using a Pt working electrode, a Pt counter electrode and a Ag/AgCl (saturated KCl) reference electrode on a computer-controlled CHI660C instruments at room temperature; the scan rate was 100 mV s−1, and n-Bu4NPF6 (0.1 M) in CH3CN was used as the supporting electrolyte. For calibration, the redox potential of ferrocene/ferrocenium (Fc/Fc+) was measured under the same conditions. The onset oxidation was presented by reference to the redox potential of ferrocene/ferrocenium (Fc/Fc+). The X-ray diffraction data were obtained at beamline BL14B1 of the Shanghai Synchrotron Radiation Facility (SSRF) at a wavelength of 1.2398 Å; the beamline BL14B1 is based on bending magnet and a Si (111) double crystal monochromator was employed to monochromatize the beam; the size of the focus spot is about 0.5 mm and the end station is equipped with a Huber 5021 diffractometer; NaI scintillation detector was used for data collection. Atomic force microscopy (AFM) images of the thin films were obtained on a NanoscopeIII AFM (Digital instruments) operating in tapping mode. AFM samples and microscopic images were identical to those used in organic field-effect transistors. Synthesis of 2BrDPPBu. DPPBu (528.4 mg, 1.0 mmol) and Nbromosuccinimide (NBS) (445.0 mg, 2.5 mmol) were dissolved in 30 mL of anhydrous CHCl3. The reaction mixture was stirred at room temperature under N2 atmosphere for 4.0 h. Then, 25 mL of water was added, and the mixture was extracted with 25 mL of CHCl3 for three times. The organic layer was dried with MgSO4 and concentrated by rotary evaporation. The crude product was purified by column chromatography with CH2Cl2/petroleum ether (60−90 °C) (v/v, 1/ 1) as eluent to give 2BrDPPBu as purple-red solids (610 mg, 88%). 1H

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CONCLUSION In this paper, we report a DPP-containing conjugated polymer pDPPBu-BT in which tert-butoxycarboxyl groups are incorporated in the side chains. TGA and FT-IR data indicate that the tert-butoxy groups in the side chains are thermally cleavable, and the elimination of isobutylene leads to transformation of pDPPBu-BT into pDPPCOOH-BT (see Scheme 1) after thermal treatment of thin film pDPPBu-BT at 240 °C. Thin film of pDPPBu-BT shows relatively low hole mobility due to the low packing order of polymer chains and poor thin film morphology on the basis of XRD and AFM characterizations. The semiconducting performance of the FET with the thermally treated thin film of pDPPBu-BT was investigated in the presence of different concentrations of ammonia and other F



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NMR (400 MHz, CDCl3) δ 8.46 (d, J = 4.2 Hz, 2H), 7.22 (d, J = 4.2 Hz, 2H), 4.70 (s, 4H), 1.45 (s, 18H). 13C NMR (101 MHz, CDCl3) δ 167.04, 160.85, 139.09, 135.07, 131.92, 131.28, 119.34, 107.93, 83.35, 44.29, 28.13. MALDI-TOF: 686.6 (M + ). Anal. Calcd for C26H26Br2N2O6S2: C, 45.49; H, 3.82; N, 4.08; S, 9.34. Found: C, 45.32; H, 3.75; N, 4.06; S, 9.17. Synthesis of pDPPBu-BT. 2BrDPPBu (100 mg, 0.15 mmol) and (4,4′-didodecyl-[2,2′-bithiophene]-5,5′-diyl)bis(trimethylstannane) (121 mg, 0.15 mmol) were taken into a Schlenk tube under nitrogen atmosphere with 10 mL of anhydrous toluene. Then tris(dibenzylideneacetone)dipalladium(0) (Pd2(dba)3) (2.6 mg, 0.003 mmol) and tri(o-tolyl)phosphine [P(o-tol)3] (7.9 mg, 0.024 mmol) were added in one portion. The tube was charged with nitrogen through a freeze−pump−thaw cycle for three times. The mixture was stirred for 48 h at 100 °C under nitrogen. The resulting mixture was poured into methanol and the dark precipitate was filtered. The polymer was purified by Soxhlet extraction using methanol, acetone, hexane and chloroform sequentially. The residue was collected and dried under vacuum to afford polymer (110 mg, 73%). 1H NMR (500 MHz, CDCl2CDCl2, 373.2k) δ 8.82−8,72 (m, 2H), 7.37−7.34 (m, 2H), 7.15−7.08 (m, 2H), 4.86 (m, 4H), 2.89 (m, 2H), 2.66−2.62 (m, 2H), 1.78 (m, 4H), 1.56−1.34 (m, 54H), 0.95 (m, 6H). 13C NMR (125 MHz, CDCl2CDCl2, 373.2k) δ 166.95, 161.04, 136.37, 135.49, 129.31, 127.49, 127.05, 82.91, 44.59, 31.83, 30.25, 29.56, 29.43, 29.21, 28.19, 22.54, 13.89. Mw/Mn (GPC) = 25.3/12.0 kg mol−1, PDI = 2.11. Anal. Calcd for (C58H78N2O6S4)n: C, 67.66; H, 7.83; N, 2.72; O, 9.32; S, 12.46. Found: C, 67.34; H, 7.77; N, 2.72; S, 12.35. Fabrication of FET Devices. Bottom-gate/bottom-contact FETs were fabricated. A heavily doped n-type Si wafer and a layer of dry oxidized SiO2 (300 nm, with roughness lower than 0.1 nm and capacitance of 11 nF cm−2) were used as a gate electrode and gate dielectric layer, respectively. The drain−-source (D-S) gold contacts were fabricated by photolithography. The substrates were first cleaned by sonication in acetone and water for 5.0 min and immersed in piranha solution (2:1 mixture of sulfuric acid and 30% hydrogen peroxide) for 20 min. This was followed by rinsing with deionized water and isopropyl alcohol for several times, and it was blow-dried with nitrogen. Then, the surface was modified with n-octadecyltrichlorosilane (OTS). After that, the substrates were cleaned in nhexane, CHCl3 and isopropyl alcohol. The films of pDPPBu-BT were fabricated by spin-coating its o-dichlorobenzene solution (3 mg/mL) at 2000 rpm. The annealing process was carried out in vacuum for 30 min at each temperature. Field-effect characteristics of the devices were determined using a Keithley 4200 SCS semiconductor parameter analyzer. The mobility of the OFETs in the saturation region was extracted from the following equation:

IDS =

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present research was financially supported by NSFC (21372226, 21422207) and the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB12010300). The authors also thank beamline of BL14B1 of the Shanghai Synchrotron Radiation Facility (SSRF) for providing the beamline.

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REFERENCES

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W μCi(VGS − Vth)2 2L

Where IDS is the drain electrode collected current; L and W are the channel length and width, respectively; μ is the mobility of the device; Ci is the capacitance per unit area of the gate dielectric layer; VGS is the gate voltage, and Vth is the threshold voltage. The Vth of the device was determined by extrapolating the (IDS, sat)1/2 vs. VGS plot to IDS = 0.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b08078. GIXRD patterns, variation of IDS for FETs of pDPPBuBT thin film and the thermally treated thin film after exposure to ammonia, FT-IR spectra of the thermally treated thin film before and after exposure to ammonia, and copies of 1H NMR and 13C NMR spectra (PDF) G


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Highly Sensitive Thin-Film Field-Effect Transistor Sensor for Ammonia

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