Highly Sensitive Ammonia Gas Sensor Based on Single-Crystal Poly

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Highly Sensitive Ammonia Gas Sensor Based on Single-Crystal Poly(3-hexylthiophene) (P3HT) Organic Field Effect Transistor Seohyun Mun, Yoonkyung Park, Yong-Eun Koo Lee, and Myung Mo Sung* Department of Chemistry, Hanyang University, Seoul 04763, Korea S Supporting Information *

ABSTRACT: A highly sensitive organic field-effect transistor (OFET)-based sensor for ammonia in the range of 0.01 to 25 ppm was developed. The sensor was fabricated by employing an array of single-crystal poly(3-hexylthiophene) (P3HT) nanowires as the organic semiconductor (OSC) layer of an OFET with a top-contact geometry. The electrical characteristics (field-effect mobility, on/off current ratio) of the single-crystal P3HT nanowire OFET were about 2 orders of magnitude larger than those of the P3HT thin film OFET with the same geometry. The P3HT nanowire OFET showed excellent sensitivity to ammonia, about 3 times higher than that of the P3HT thin film OFET at 25 ppm ammonia. The ammonia response of the OFET was reversible and was not affected by changes in relative humidity from 45 to 100%. The high ammonia sensitivity of the P3HT nanowire OFET is believed to result from the single crystal nature and high surface/volume ratio of the P3HT nanowire used in the OSC layer.

1. INTRODUCTION Real-time monitoring of ammonia is important for environmental and health issues. Ammonia is a common but very toxic and corrosive industrial chemical that should be maintained below its threshold concentrations. It is also an important trace gas in human breath that can be utilized in clinical diagnostics.1−3 To date, real-time ammonia sensors have been developed using organic semiconductors (OSCs), conducting polymers, and carbon and inorganic compounds as ammonia-sensing layers, and their applications include environmental ammonia sensing as well as detection of breath ammonia of rats.4−8 These ammonia sensors are usually electrical sensors and sensors based on organic field-effect transistors (OFETs) that use OSCs as both the transistor channel and sensing layers; the latter have achieved better performance than conventional conductive/resistive sensors.9−11 This is because the transistor configuration offers advantages such as high sensitivity (due to the transistor’s inherent gain mechanism) and low-cost mass production of lightweight, flexible, and large-area devices.12 OFETs using p-type poly-3-hexylthiophene (P3HT)13 and a dialkyl tetrathiapentacene (DTBDT-C6)4 or n-type NDI(2OD)(4tBuPh)-DTYM214 were used for ammonia sensing at room temperature for concentrations between 10 and 100 © 2017 American Chemical Society

ppm, a range that is suitable for environmental monitoring. One of the sensors achieved very high ammonia sensitivity and good reversibility by employing ultrathin microstructured OSC layers.4 Thus, the morphology of the channel/sensing layers is an important factor that influences the interactions between the gas molecules and the OSC layer. Therefore, the morphology determines the gas sensor response characteristics. Several p-type OFET-based ammonia sensors were also reported to detect ammonia concentrations down to subppm levels, the range useful for medical diagnosis.3,15,16 For example, pentacene-based organic thin film transistors (OTFTs) with an ammonia detection range of 0.1−5 ppm3 and P3HT based OFETs with a range of 0.1−25 ppm were reported.16 However, they showed low output current signals and/or small signal changes at low concentrations. In another study, an ammonia receptor molecule was added to an OSC channel layer made of copper or cobalt phthalocyanine to enhance the binding of ammonia to the OSC surface.15 However, the mobility of the OFETs decreased significantly with the addition of the receptor molecules, which further reduced the already low current Received: July 15, 2017 Revised: October 10, 2017 Published: November 10, 2017 13554

DOI: 10.1021/acs.langmuir.7b02466 Langmuir 2017, 33, 13554−13560

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octyltrichlorosilane (OTS), and hexane were obtained from SigmaAldrich. Polyurethane acrylate (PUA; MINS-ERM, Minuta Tech.) was used to prepare UV-curable hard molds. Ethanol (99.9%, Daejung Chemicals Inc.) was used to wash the substrate and as a polar liquid layer during the liquid-bridge-mediated nanotransfer molding (LBnTM) process.23,24 An ammonia solution (30%, Junsei Chemical) was used to prepare ammonia solutions at various concentrations by adding the proper amounts of deionized water from a Millipore MilliQ plus system and a Millipore Simplicity system. Heavily doped, ptype Si wafers (resistivity: 98%) was purchased from Rieke Metals Inc. 1,2,4-Trichlorobenzene (TCB), 13555

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Figure 1. (a) Schematic illustration of a P3HT single-crystal nanowire OFET and its interaction with ammonia. (b) SEM surface image of P3HT single-crystal nanowires. (c) SEM perspective image of the nanowires. (d) SAED pattern of a P3HT single-crystal nanowire. previous measurement was removed and the chamber was open to ambient air for about 10 min. To clear the ammonia molecules from the chamber, 2 mL of an ammonia solution of the desired concentration was placed in the chamber, and 10 min was allowed for the atmosphere to equilibrate before the output curves of the OFETs were measured. The IDS values at a VDS of −50 V were taken from the curves, and the ratios of Iair to Iammonia (Iair/Iammonia) were calculated, where Iair is IDS under ambient air (ammonia free) and Iammonia is under an ammonia-containing atmosphere. For each P3HT OFET, a calibration curve was constructed by plotting the Iair/Iammonia value with respect to ammonia level. The reversibility of the ammonia sensing by the P3HT OFETs was determined by using two different methods. In the first method, a forward calibration (i.e., a calibration by increasing the ammonia concentration) was conducted. Subsequently, a reverse calibration (i.e., a calibration by decreasing ammonia concentration back to 0 ppm) was also conducted. The reversibility was determined by how the Iair/ Iammonia values from forward and reverse calibrations were matched at each ammonia level selected for calibration. In the second method, the chamber ammonia level was oscillated between two values, i.e., no ammonia (ambient air) and 10 ppm. The variation of the Iair/Iammonia values was monitored for up to five oscillating cycles. Like the second reversibility method, the chamber atmosphere was oscillated between ambient air and water-saturated air. The water−saturated air was prepared by placing 2 mL of deionized water in the chamber. The I−V curves of the OFETs were monitored up to four oscillating cycles.

thermal evaporation through a shadow mask. A channel with a length of 90 μm and a width of 990 μm was formed in accordance with the dimension of the mask. Figure 1a shows a schematic illustration of the prepared nanowire OFET in which the channel region contains nanowires that are parallel to the channel length direction. It also demonstrates that interactions of ammonia with the nanowire channel layer occur during the ammonia sensing process as ammonia molecules can access the P3HT nanowires through their top and side surfaces. Figure 1b,c present SEM images of the P3HT nanowires, revealing parallel-aligned 100 nm wide 150 nm high nanowires that are separated equally by a 600 nm wide space. The channel of the nanowire OFET was found to contain 300 nanowires, which was determined by analyzing the optical images. The P3HT nanowires were single crystals as demonstrated by very wellordered, bright Bragg diffraction spots in the selective-area electron diffraction (SAED) pattern of a P3HT nanowire shown in Figure 1d. The SAED pattern was recorded perpendicular to the nanowire long axis that corresponds to the [010] lattice direction. The lattice distance along the nanowire long axis and that along the nanowire short axis are 16.6 and 7.8 Å, respectively. These results are consistent with the previously reported values for orthorhombic P3HT crystal structures.23 For comparison, the P3HT thin film OFET with the same top-contact geometry was also fabricated. The P3HT thin film was 150 nm-thick as demonstrated by the SEM images (Figure S1). Note that, in the case of thin film OFETs, ammonia molecules can access only the top surface of the channel. The electrical performance of the two types of the OFETs (i.e., with single-crystal P3HT nanowire arrays and the amorphous P3HT thin film as OSC channel layers) were investigated. Figure 2a,b presents typical drain current−drain

3. RESULTS AND DISCUSSION The ammonia-sensitive P3HT nanowire OFET was fabricated with a top-contact geometry using an array of P3HT nanowires as an OSC layer. First, the nanowire array was printed on an SiO2(200 nm)/Si substrate by means of liquid-bridge-mediated nanotransfer molding (LB-nTM).24 Next, the source (S) and drain (D) electrodes were defined over the nanowire array by depositing two 150 nm-thick silver electrodes by means of 13556

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Figure 2. Electrical characteristics of P3HT OFETs. Output curves (IDS vs VDS) of (a) single-crystal P3HT nanowire OFET and (b) P3HT thin film OFET. Corresponding drain current−gate voltage (IDS−VGS) transfer curves (VDS = −50 V) of (c) P3HT nanowire OFET and (d) P3HT thin film OFET.

voltage (IDS−VDS) output curves for the P3HT nanowire OFET and the P3HT thin film OFET, respectively. Both OFETs were well-modulated in accordance with the gate voltage (VGS) as typical p-type transistors, and they exhibited saturation behavior. Figure 2c,d presents drain current−gate voltage (IDS−VGS) transfer curves of the two OFETs. The field-effect mobility (μ) and threshold voltage (Vth) were calculated in the saturation regime (VDS = −50 V) by plotting the square root of the drain current versus the gate voltage using IDS = (μWCi/2L)(VGS − Vth)2. Here, Ci is the capacitance/unit area of the gate dielectric layer (200 nm thick SiO2, 15 nF/cm2) and W and L are the channel width and length, respectively. The single-crystal P3HT nanowire OFET showed much better device performance than the P3HT thin film OFET. The field effect mobility and the on/off current ratio of the nanowire OFET were ∼100 times greater than those of the thin film OFET, i.e., 0.93 vs 0.008 cm2 V−1 s−1 and 106 vs 104, respectively. A larger output signal usually results from a larger mobility, and a larger on/off ratio should lead to larger signal changes in the presence of signal quenchers. Therefore, better ammonia sensing performance is expected by the nanowire OFET than by the thin film OFET. Note that the output current of the nanowire OFET can be further enhanced if the channel width of the OFET was increased by including more nanowires in the channel area. The ammonia sensing behaviors of the two types of OFETs were investigated. Each of the OFETs were located in a sealed glass chamber and were then exposed to various concentrations of ammonia by placing ammonia−water solutions with the desired fractions of ammonia in the chamber. The current− voltage properties of the OFETs under each different condition

were measured. Figure 3a,b shows the output curves (IDS vs VDS) for the OFETs at a constant gate voltage of −50 V when exposed to ammonia at concentrations from 0.01 ppm up to 25 ppm. There were recognizable decreases in the drain currents for both of OFETs when they were exposed to ammonia, although a larger decrease was observed in the case of the single-crystal nanowire OFETs. An ammonia response curve (calibration curve) in the concentration range from 0 to 25 ppm was constructed for each type of OFET by using the drain currents in saturation region (VDS = −50 V; Figure 3c). Here, the drain current at each different ammonia level (Iammonia) was normalized with respect to the current in the ammonia-free ambient air (Iair). Note that the ratio, Iair/Iammonia, can represent the ammonia sensitivity of the OFETs at each ammonia concentration.4 The constructed calibration curves show that Iair/Iammonia increases with increasing ammonia concentration for both OFETs, but its change rate and the change relative to the ammonia concentration was greater for the nanowire OFETs. For instance, the Iair/Iammonia values were 11.8 and 3.83 at 25 ppm ammonia for the nanowire OFET and the film OFET, respectively, indicating that the nanowire OFET is more sensitive to ammonia by a factor of 3. The P3HT nanowire OFET was capable of detecting ammonia at a concentration down to 8 ppb. The higher sensitivity of the nanowire OFET than the film OFET can be explained by their higher mobility (∼100 times) and the larger surface/volume ratio (∼5 times) of the OSC layer, as well as the P3HT channel’s single crystallinity that allows no defects and therefore less initial charge traps. The superior performance of the nanowire OFET was also supported by its higher sensitivity despite its 13557

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Figure 3. Ammonia sensitive output characteristics of (a) single-crystal P3HT nanowire OFETs and (b) P3HT thin film OFETs at VGS = −50 V. (c) Ammonia sensing calibration curves based on output current ratio, Iair/Iammonia, for the nanowire OFETs and the thin film OFETs. The current was measured with a constant VGS and VDS, and both were set to −50 V.

Figure 4. Single-crystal P3HT OFET reversibility tests by (a) monitoring the response in the forward direction (increasing the ammonia concentration) and then in the reverse direction (decreasing the ammonia concentration). (b) Results for oscillating the ammonia concentration between 0 and 10 ppm for 4 cycles. The current was measured with a constant VGS and VDS, and both were set to −50 V.

nanowires in the channel or by reducing the thickness of each nanowire. We observed that both calibration curves were nonlinear with upward curvatures, indicating that the interactions between ammonia and OSC molecules become less efficient with an increase in ammonia concentration. It seems that the

significantly lower channel surface area (about 1/8) as compared to those of the thin film OFET. Considering the factors that affect the ammonia sensing mechanism described above, the ammonia sensitivity of the nanowires can be further enhanced by increasing the surface area of the nanowires in the channel area, for instance by increasing the number of 13558

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showed excellent reversibility upon multiple exposures to ammonia, suggesting that they can be used repeatedly for reliable ammonia sensing. Moreover, humidity did not affect the ammonia sensing characteristics of the two OFETs. All these results demonstrate the high potential of the developed P3HT nanowire OFET as a sensitive ammonia sensor for environmental monitoring and/or medical diagnosis. The excellent ammonia-sensing characteristics of the P3HT nanowire OFET are believed to result from its single crystal nature and the high surface/volume of the P3HT nanowire used in the OSC layer. The latter allows easy access of ammonia to the surface of the OSC layer and effective transduction of the charge carriers produced by interactions between ammonia and OSC molecules within the OSC layer. The design concept for the P3HT nanowire OFET-based ammonia sensor may be applicable to OFET-based sensors for other chemical species.

surface region occupies only a small portion of the OSC layer even for the nanowire OFET case. Further increases in surface/ volume of the nanowire seem to be necessary for achieving better linearity. The reversibility of ammonia-sensing by the single-crystal nanowire P3HT OFETs was tested in two ways as shown in Figure 4. As shown, the two ammonia sensors demonstrated excellent reversibility. The Iair/Iammonia values increased and decreased according to the levels of ammonia and returned to the same initial values after multiple exposures to air containing different levels of ammonia. These results show that singlecrystal nanowire P3HT OFETs can be used repeatedly for reliable ammonia sensing. Environmental humidity varies depending on time and place. Moreover, exhaled breath contains lots of moisture and is close to saturation. Effective ammonia sensors should work regardless of humidity level. The influence of water vapor on the output characteristics of the single crystal of P3HT OFETs was investigated under a chamber atmosphere that was oscillated between ambient air and water-saturated air. There were no noticeable changes in the output currents even after four oscillating cycles as shown in Figure 5. This result indicates that the humidity changes would not interfere with the ammonia sensing characteristics of the two OFETs under current experimental conditions (RH 45−100%).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b02466. SEM images for P3HT thin film; OFET device characteristics on interference effect of water vapor. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Myung Mo Sung: 0000-0002-2291-5274 Author Contributions

M.M.S. conceived of and designed the experiments. S.H.M. performed the experiments. Y.P., S.H.M., Y.-E.K.L., and M.M.S. cowrote the paper. Notes

The authors declare no competing financial interest.

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Figure 5. Interference effect of water vapor on OFET device characteristics: single-crystal P3HT nanowire OFETs.

ACKNOWLEDGMENTS This work was supported by the Creative Materials Discovery Program (2015M3D1A1068061) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, & Future Planning. This work was also supported by a grant from the National Research Foundation of Korea (NRF), funded by the Korean government (MSIP; No. 2014R1A2A1A10050257).

4. CONCLUSION An OFET-based ammonia sensor with high sensitivity was developed by employing an array of single-crystal P3HT nanowires as a p-type semiconductor layer of an OFET with a top-contact geometry. It showed much better device performance when compared to the P3HT thin film OFET with the same geometry, probably due to the single crystal nature of the nanowires. The mobility and the on/off current ratio of the nanowire OFET were ∼100 times greater and the output current was ∼50 times greater than those of the thin film OFET. The I−V curves of the OFETs revealed ammonia sensitive behavior when the two P3HT OFETs were exposed to various concentrations of ammonia ranging from 0.01 to 25 ppm. Their drain currents decreased with increasing ammonia concentration. The ammonia-sensing calibration curves, constructed by plotting Iair/Iammonia vs ammonia concentration, clearly demonstrated the superior ammonia sensitivity of the nanowire OFET to that of the thin film OFET. The P3HT OFETs

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