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 Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02466 • Publication Date (Web): 10 Nov 2017 Downloaded from http://pubs.acs.org on November 14, 2017

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Highly sensitive ammonia gas sensor based on single-crystal Poly(3-hexylthiophene) (P3HT) organic field effect transistor SeohyunMun†, Yoonkyung Park†, Yong-Eun Koo Lee† and Myung Mo Sung†* †

Department of Chemistry, Hanyang University, Seoul 04763, Korea

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 two 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

ACS Paragon Plus Environment

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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, carbon and inorganic compounds as ammonia-sensing layers and their applications include environmental ammonia sensing as well as detection of breath ammonia of rat.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, 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 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 sub-ppm 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


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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 output. In another study, ammonia sensitive OFETs were fabricated by employing a p-type DPPbithiophene conjugated polymer containing thermally cleavable side chains, which achieved detection of ammonia down to 10 ppb.17 During thermal annealing of the OFETs, the side chains were cleaved into –COOH and nanopores were produced within the OSC layer, allowing for enhanced interactions with ammonia and faster ammonia diffusion, respectively. Unfortunately, the mobility of the OFETs was compromised after the thermal annealing. While these studies showed promise for sub-ppm ammonia level detection, their practical application may require further performance improvements, especially to enhance the output current signals and ammonia sensitivity. An analysis of the sensing mechanism provides ways to improve the sensor performance. The ammonia sensing mechanism by an OFET-based sensor can be described as a two-stage process.4,15,18 First, ammonia molecules are adsorbed/bound on the surface of the OSC layer. Second, the ammonia contributes to the formation of charge carrier traps and/or the induction of disorder in the electric field, leading to changes in output current, mobility decay and threshold voltage. The ammonia sensitivity of the OFET can be improved by controlling the parameters that affect each stage. The factors affecting the first stage include the surface area of the OSC layer, and the binding efficiency between the ammonia and OSC molecules.14 OSC layers with large surface areas and/or high binding affinities with ammonia can provide OFETs with high ammonia sensitivity.4,15,17 The factor affecting the second stage is the effectiveness of the transduction of the charge carriers generated on the surface of the OSC


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layer into the conducting channel at the organic semiconductor material/insulator interface.19,20 It is known that the active conduction channel at the interface has a thickness of one to several molecular layers.21 It is also known that the defects in the OSC layer can trap mobile charge carriers and reduce the mobility, leading to decrease in output current and limiting the sensing of analytes that create charge traps.22 This suggests that single crystalline OSC layer with no defects can detect a very low concentration of analytes. Therefore, morphology of the OSC layer, including ultrathin layers or nanostructures, provide high surface/volume ratios,4,18 and the use of single crystal OSC materials with high mobility and no defects may be necessary for achieving high ammonia sensitivity. In this study, we developed an OFET-based ammonia sensor by employing an array of single-crystal P3HT nanowires as both the OSC channel and ammonia sensing layer. The single-crystal nanowire channel offers high mobility and a high output current signal due to efficient charge carrier transport in the single crystal. Moreover, the sensing layer (in the form of a nanowire array) offers a very large surface to volume ratio, enabling efficient interaction with ammonia and therefore high sensitivity to ammonia. The P3HT single crystal nanowire OFET showed enhanced mobility and current output by about two orders of magnitude when compared to the P3HT thin film OFET prepared with the same geometry. The P3HT single crystal nanowire OFET could detect ammonia at concentrations ranging from 25 ppm down to as low as 8 ppb, the lowest LOD (limit of detection) reported to date by real-time ammonia sensors.

2. EXPERIMENTAL DETAILS 2.1. Materials and Sample Preparation. P3HT (> 98%) was purchased from Rieke Metals Inc. 1, 2, 4- Trichlorobenzene (TCB),


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octyltrichlorosilane (OTS) and hexane were obtained from Sigma-Aldrich. 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 (LB-nTM) 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 Milli-Q plus system and a Millipore Simplicity system. Heavily doped, p-type Si wafers (resistivity: < 0.005 Ω) with a 200-nm-thick thermally-grown SiO2 layer on the surface were purchased from Namkang Hi-Tech Inc. The SiO2/Si substrates (1x1 cm2) were cut from the SiO2/Si wafers. The wafers were cleaned to remove contaminants by washing with ethanol, drying in a stream of N2, and UV/ozone treatment for 5 min. The substrates were then treated with OTS by dipping the substrates in a 10-mM solution of OTS in hexane for 50 min at room temperature. The P3HT nanowire OFET was fabricated on the 200 nm SiO2/Si substrate using singlecrystal P3HT nanowires as p-type channels and Ag electrodes as source and drain electrodes. First, an array of P3HT nanowires was printed on the prepared substrate by LB-nTM with PUA molds. The masters used for fabrication of the molds were silicon wafers with nanoscale line patterns, which were made by e-beam lithography. The molds were fabricated by casting PUA on them. After UV curing (30 min), the PUA molds were peeled away from the masters. To form arrays of single-crystal organic nanowires, a nanoline-patterned PUA mold (containing 100 nm-wide, 150 nm-deep parallel channels with 600 nm-wide spaces between them) was filled with a P3HT ink solution (2 wt% in 1,2,4-TCB). The mold with the single-crystal organic nanowires was then brought into contact with a substrate surface covered by a thin ethanol layer. The solidified P3HT single-crystal organic nanowires in the


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molds were also transferred to specific positions on the SiO2 substrate.23 Then, 150 nm-thick silver source (S) and drain (D) electrodes were deposited by thermal evaporation of Ag using a shadow mask that has a rectangular space (channel) with a length of 90 µm and a width of 990 µm between the openings for the two electrodes. The P3HT thin film OFET was fabricated in the same way as the P3HT nanowire OFET except the P3HT thin film was used as a p-type channel layer instead of the P3HT nanowire array. The P3HT thin film was formed on the substrate by spin coating of the 3 wt% P3HT solution in 1,2,4-TCB at 3000 rpm for 100 s. 2.2. Sample Characterization. The morphology of the P3HT nanowires and thin films were characterized using a scanning electron microscope (SEM, Hitachi S4800) operated at 15 kV and an optical microscope (ICS-305B, Sometech). The crystallinity of the nanowires was determined by selective-area electron diffraction (EM 912 Omega) with a transmission electron microscope operating at 120 kV. The current–voltage (I–V) properties of the OFETs were measured with a semiconductor parameter analyzer (HP 4155C, Agilent Technologies). The field-effect mobility (µ) and threshold voltage (Vth) were calculated in the saturation regime (VDS = 50V) by using IDS = (WCi /2L)µ(VGS - Vth)2, where Ci is the capacitance per unit area of the gate dielectric layer (15 nF/cm2), and W and L are the channel width and length, respectively. The P3HT nanowire OFET or thin film OFET was placed inside a gas-tight gas chamber (volume: 153.86 cm3), and the I–V curves of the OFETs were measured under ambient air (relative humidity 45%, 26 °C) and under air containing various levels of ammonia (0.007, 0.008, 0.01, 0.03, 0.05, 0.1, 0.3, 0.5, 1, 5, 10, 25 ppm). The ammonia level in the chamber atmosphere was controlled by using 2 mL of an ammonia solution of various concentrations


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as follows. The concentration of the ammonia solution to produce the ammonia vapor at desired concentration was calculated using Henry’s law using 5.9×10-1 mol/m3 Pa at 298.15K as the Henry’s solubility constant (Hcp) for water as solvent.25 The ammonia solution of the calculated concentration was then prepared to produce the ammonia vapor for calibration. For instance, 10.2 ppm concentration of ammonia solution was prepared to make 10ppm ammonia in gas phase. Each time the ammonia level was changed, the ammonia solution in the chamber for 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 -50V 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 5 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


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by placing 2 mL of deionized water in the chamber. The I–V curves of the OFETs were monitored up to 4 oscillating cycles.

3. RESULT 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 (LBnTM).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 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. Figures 1b and 1c 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 well-ordered, 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 Å and 7.8 Å, respectively. These results are consistent with the previously reported values for


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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.


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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.


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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. Figures 2a and 2b present typical drain current-drain 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. Figures 2c and 2d present 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 cm2V-1s-1 vs. 0.008 cm2V-1s-1 and 106 vs. 104, respectively. A larger output signal usually results from 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.


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Figure 2. The 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.


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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. Figures 3a and 3b show the output curves (IDS vs. VDS) for the OFETs at a constant gate voltage of -50V 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 saturat ion region (VDS = -50V) (Figure 3c). Here, the drain current at each different ammon ia level (Iammonia) was normalized with respect to the current in the ammonia-free amb ient 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 t hat Iair/Iammonia increases with increasing ammonia concentration for both OFETs, but it s change rate and the change relative to the ammonia concentration was greater for t he nanowire OFETs. For instance, the Iair/Iammonia values were 11.8 and 3.83 at 25 pp m ammonia for the nanowire OFET and the film OFET, respectively, indicating that t he nanowire OFET is more sensitive to ammonia by a factor of 3. The P3HT nanow ire 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 the ir higher mobility (~ 100 times) and the larger surface/volume ratio (~ 5 times) of th


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e 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 OF ET was also supported by its higher sensitivity despite its 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 se nsitivity of the nanowires can be further enhanced by increasing the surface area of t he nanowires in the channel area, for instance by increasing the number of nanowires in the channel, or by reducing the thickness of each nanowire. We observed that both calibration curves were non-linear 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 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.


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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 = -50V. (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.


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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 single-crystal nanowire P3HT OFETs can be used repeatedly for reliable ammonia sensing.


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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.


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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 were 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 4 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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Figure 5. Interference effect of water vapor on OFET device characteristics: single-crystal P3HT nanowire OFETs.


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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 topcontact 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 ppm to 25 ppm. Their drain currents decreased with increasing ammonia concentration. The ammoniasensing 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 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


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and OSC molecules within the OSC layer. The design concept for the P3HT nanowire OFETbased ammonia sensor may be applicable to OFET-based sensors for other chemical species.

■ ASSOCIATED CONTENT

Supporting Information SEM images for P3HT Thin film; OFET device characteristics on interference effect of water vapor (PDF)

■ 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.

■ ACKNOWLEDGMENT

This

work

was

supported

by

the

Creative

Materials

Discovery

Program

(2015M3D1A1068061) through the National Research Foundation of Korea (NRF) funded


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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), and by the Samsung Research Funding Center of Samsung Electronics under Project Number SRFC-MA1401-05.


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Highly Sensitive Ammonia Gas Sensor Based on Single-Crystal Poly

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