Poly(vinyl alcohol) as a gas accumulation layer for an organic field-effect transistor ammonia sensor

https://doi.org/10.1016/j.snb.2016.12.116Get rights and content

Highlights

  • Typical polar polymer PVA was applied to OFET NH3 sensors for sensitivity improvement.

  • Response of OFET NH3 sensor could be enhanced under pre-bias at gate electrode.

  • Absorption and desorption could be realized under pre-bias with opposite direction.

Abstract

Ammonia (NH3) gas sensors based on organic field-effect transistor (OFET) using poly(vinyl alcohol) (PVA) as a gas accumulation layer were fabricated. The results showed the percentage responses of saturation current (Ion) was 50.3% under 0.5 ppm NH3, which is over one order of magnitude higher than that of pure PMMA dielectric. Also, it showed that there was a remarkable shift in the field-effect mobility after exposed to NH3 gas. Moreover, the percentage responses of Ion would increase to 66.9% and 30.2% under 0.5 ppm and 0.2 ppm NH3 after applied a gate voltage pulse, respectively. The sensing properties of these OFET gas sensors can realize absorption and desorption with opposite gate bias, exhibiting non-volatile states of Ion and VT under the programming-erasing bias as used for memory elements. By analyzing the morphologies of dielectric and organic semiconductor, and the electrical characteristics of OFET sensors, the interaction between NH3 and PVA will be strengthened under the influence of the applied electrical field on the hydroxyl dipoles. The bias-induced re-orientation of the hydroxyl dipoles can modulate the influence of NH3 on the trapping (absorption) and detrapping (desorption) processes of charge carriers at the pentacene/PVA interface.

Introduction

In times of strict environmental requirements and a growing demand for energy and mobility, reliable and low analyte concentration-detecting gas sensing devices are required to determine the dose or concentration of toxic and harmful gases, e.g., sulfur dioxide (SO2), carbon monoxide (CO), hydrogen sulfide (H2S), ammonia (NH3), and nitrogen oxide (NOx) [1], [2], [3], [4]. Among different sensing devices, organic field-effect transistor (OFET) based gas sensors have attracted more attention due to their ability to amplify signal within the device and the highly sensitive nature of organic semiconductor (OSC) [5], [6]. Moreover, device parameters such as saturation current (Ion), off current (Ioff), threshold voltage (VT), sub-threshold slope (SS), field-effect mobility (μ) are all highly sensitive to the adsorbed gases, this multi-parameters detection make OFET as one of the promising candidates for low-cost, portable, sensitive, and selective applications in chemical, biological, and physical monitoring [7], [8], [9].

Ammonia (NH3) is a colorless, pungent gas that is very harmful to the human body. The threshold limit value of ammonia is only 25 ppm for long-term exposure (8 h). Therefore, ammonia sensors are required to monitor its concentration in chemicals, food processing, medical diagnosis and environmental monitoring [10]. Until now, the study of NH3 detection based on OFET sensors are mainly focused on organic semiconductor, many attempts in improving the sensing properties of organic semiconductor layers have been made [11], [12], [13]. For example, L. Chi et al. developed an NH3 gas sensor using ultrathin microstripes as the active layer of OFET, and high sensitivity, fast response/recovery rate, good selectivity, low concentration detection ability, good reversibility, and stability were demonstrated [14]. In addition, the dielectrics of OFETs also play an important role in sensing [15]. Proper dielectric materials or dielectrics surface modification can significantly enhance the performance of OFETs based gas sensor due to an improvement of the organic semiconductor/dielectric interface property or a better crystallization of the upper organic semiconductor [16], [17], [18].

As a typical polymer dielectric, PVA has abundant hydroxyl groups (−OH) attached to polymer chains. Due to the different levels of electronegativity associated with oxygen and hydrogen atoms, the polarity of PVA could be changed by variation of the electrical field [19]. For OFETs with PVA as gate dielectrics, changes in device characteristics during operation have been attributed to surface polarization and trapping inside the gate dielectric, in addition to the typical mechanism of charge trapping at the semiconductor-dielectric interface [20]. Egginger et al. have reported that the ionic impurities in PVA dielectrics could serve as another parameter to modulate the transfer characteristics of OFETs under the influence of applied electrical fields [21]. As previously reported, polar NH3 was randomly adsorbed on OSCs, and then the dipole of NH3 can induce disorder electric fields in the vicinity of OSC molecules, which yield disorder dipole-charge interaction. Such interaction will cause a portion of the mobile charge to be trapped [22], [23]. As the polarization of hydroxyl groups could be changed by variation of the electrical field, when NH3 absorb on PVA as an impurity, applied electrical field must affect the sensitivity. However, the influence of PVA on performance in OFET NH3 gas sensors is still lack of research.

In this work, we used PVA dielectric layer as the gas accumulation layer to achieve sub-ppm sensitivity OFET-based NH3 gas sensors, and their properties on NH3 sensitivity were studied. By analyzing the electrical parameters of OFET NH3 sensor, it was found that NH3 sensitivity of these transistors could be enhanced by a pre-bias at gate electrode. Moreover, these gas sensors can realize absorption and desorption under the negative and positive gate bias. The results of this research may improve our understanding of design strategy of OFET-based gas sensors using polar materials.

Section snippets

Materials

PVA (Mw ∼146–186 kDa, 99+% hydrolyzed), PMMA (average Mw ∼120 kDa) and anisole were purchased from Sigma-Aldrich and all the materials were used without further purification.

Device preparation

The electrical performance characteristics of the OFET gas sensors were investigated using a bottom gate, top contact transistor configuration, and the structures of devices A, B and C were shown in Fig. 1. The OFETs were fabricated on indium tin oxide glass substrates. Prior to the deposition of dielectric layers, the

Results and discussion

The sensing properties of these devices to NH3 were tested. Fig. 2 depicts the transfer and output curves of OFETs of devices A, B, and C. When 10 ppm NH3 is introduced, both the saturation current and threshold voltage of all the OFETs deteriorate compared to the status that without NH3. It is obvious that the sensor based on device B holds the best sensing properties, a more significantly change in Ion and VT than device A that with only PMMA dielectric. Note that, the Ion decrease of device B

Conclusions

In summary, we fabricated the OFET NH3 gas sensors by using PVA dielectric, and reported the sensing property of PVA under a bias voltage. The sensor based on PMMA/PVA dielectric Exhibits 66.9% and 30.2% percentage change under 0.5 ppm and 0.2 ppm NH3 after a gate voltage pulse. The results showed that the orientation of hydroxyl groups attached to polymer chains in PVA dielectrics is crucial to determining the magnitude of response. This PVA-dielectric OFET sensor can realize both absorption and

Acknowledgements

This research was funded by the This research was funded by the National Natural Science Foundation of China (NSFC) (Grant No. 61675041, 51503022), the Foundation for Innovation Research Groups of the National Natural Science Foundation of China (Grant No. 61421002) and Science program of Education Commission of Chongqing (Grant No. KJ1601112). Also, this work was sponsored by Science & Technology Department of Sichuan Province via Grant No. 2016HH0027.

Shijiao Han received his B. A. Degree from the Yingcai Honors College of UESTC in 2012. He has been studying for his Ph.D. Degree at State Key Laboratory of Electronic Thin Film & Integrated Devices (SKLETFID) & UESTC since 2012, where his main research interests are in OFETs and OFET based sensors.

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    Shijiao Han received his B. A. Degree from the Yingcai Honors College of UESTC in 2012. He has been studying for his Ph.D. Degree at State Key Laboratory of Electronic Thin Film & Integrated Devices (SKLETFID) & UESTC since 2012, where his main research interests are in OFETs and OFET based sensors.

    Xinming Zhuang received his B. A. Degree from School of Optoelectronic Information at UESTC in 2014. He has been studying for his Master Degree at SKLETFID & UESTC since 2014, where his main research interests are in OFETs and OFET based sensors.

    Yiming Jiang received his B. A. Degree from School of Optoelectronic Information at UESTC in 2015. He has been studying for his Master Degree at SKLETFID & UESTC since 2015, where his main research interests are in OFETs and OFET based sensors.

    Xin Yang got his Ph. D. degree from Graduate School of Material Science and Technology at Sichuan University in 2013. He is faculty of Co- innovation Center for Micro/nano Optoelectronic Materials and Devices of Chongqing University of Arts and Sciences and his major is in solar energy materials and electrochemistry.

    Lu Li got his Ph.D. degree from SKLETFID & UESTC in 2012. Now, he is the professor of Chongqing University of Arts and Sciences and deputy director of Co-Innovation Center for Micro/nano Optoelectronic Materials and Devices majoring in optoelectronic materials and devices.

    Junsheng Yu got his Ph. D. degree from Graduate School of Bio-Applications & System Engineering at Tokyo University of Agriculture and Technology in 2001. He is Professor of SKLETFID & UESTC majoring in organic photoelectronic and electronic materials and devices.

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