formaldehyde gas sensor based on pentacene organic thin-film transistor

5
Formaldehyde gas sensor based on pentacene organic thin-film transistor Yucheng Chen a , Jian Zhong b , and Lin Zhang c School of Optoelectronic Information, University of Electronic Science and Technology of China (UESTC), Chengdu 610054, China a [email protected] (correspondening author), b [email protected], c [email protected] Keywords: PMMA, gas sensor, organic thin-film transistor (OTFT), formaldehyde gas Abstract. Formaldehyde (HCHO) gas sensors based on pentacene active layer and low cost poly (merthyl methacrylate) (PMMA) insulator were fabricated with a structure of bottom contact organic thin-film transistor (OTFT). The OTFT sensor not only presented a remarkable response characteristic in the absence and the presence of HCHO gas with different concentrations, but also exhibited a good repeatability for sensing the HCHO gas. Meanwhile, compared to the device operated in nitrogen circumstance, obvious changes in saturation drain-source current (I DS ) and off-state current were observed when the device exposed to HCHO gas. Also the device performance and sensing mechanisms were discussed. Introduction Organic thin-film transistors (OTFTs) have attracted extensively attention owing to its unique properties, such as low-cost, easy-prepared and large-area fabrication capability [1-3]. Sensors based on OTFTs in recent years have also become an attractive choice because of the development of low cost portable sensing systems with capabilities of multi-parametric responses [4]. OTFTs implemented in sensors open a wealth of opportunity for chemical, biosensing, humidity, vapor and so on [5,6]. On the other hand, indoor air pollution has been regarded as the third generation of air pollution problem, while formaldehyde (HCHO) is regarded as a major cause of indoor air pollution [7]. However, the traditional techniques, such as mass spectroscopy and gas chromatographs, suffered from cost, being time-consuming limitation [8]. To develop low temperature fabrication process and enhance the performance with respect to HCHO sensitivity, a further investigation of HCHO sensors is still quite necessary. Notably, until now, OTFT based HCHO sensors has rarely been investigated. In this work, we used a glass substrate and polymerthylmethacrylate (PMMA) to fabricate bottom contact OTFTs, while pentacene films were used as active layers to detect HCHO gas. Experiment A typical OTFT structure is presented in Fig. 1(a). PMMA was first dissolved in chlorobenzene solvent with a 7 % wt concentration. Then 520 nm PMMA was spun coated on ITO coated glass substrate at a rate of 1000 rpm for 60 s and annealed at 150 o C for 1 h. The capacity of PMMA dielectric was about 5.1 nF/cm 2 . Then, ITO substrates with PMMA dielectrics were transferred into a vacuum chamber to deposit gold source and drain electrodes through a shadow mask with a defined channel length L=150 µm and channel width W=1 cm. Finally, pentacene thin film with a thickness of about 50 nm was deposited at a rate of 0.5 Å/s. Key Engineering Materials Vols. 575-576 (2014) pp 477-480 Online available since 2013/Sep/04 at www.scientific.net © (2014) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/KEM.575-576.477 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 128.42.202.150, Rice University, Fondren Library, Houston, USA-12/11/14,08:55:16)

Upload: lin

Post on 16-Mar-2017

219 views

Category:

Documents


1 download

TRANSCRIPT

Page 1: Formaldehyde Gas Sensor Based on Pentacene Organic Thin-Film Transistor

Formaldehyde gas sensor based on pentacene organic thin-film

transistor

Yucheng Chena, Jian Zhongb, and Lin Zhangc

School of Optoelectronic Information, University of Electronic Science and Technology of China

(UESTC), Chengdu 610054, China

[email protected] (correspondening author), [email protected], [email protected]

Keywords: PMMA, gas sensor, organic thin-film transistor (OTFT), formaldehyde gas

Abstract. Formaldehyde (HCHO) gas sensors based on pentacene active layer and low cost poly (merthyl methacrylate) (PMMA) insulator were fabricated with a structure of bottom contact organic thin-film transistor (OTFT). The OTFT sensor not only presented a remarkable response characteristic in the absence and the presence of HCHO gas with different concentrations, but also exhibited a good repeatability for sensing the HCHO gas. Meanwhile, compared to the device operated in nitrogen circumstance, obvious changes in saturation drain-source current (IDS) and off-state current were observed when the device exposed to HCHO gas. Also the device performance and sensing mechanisms were discussed.

Introduction

Organic thin-film transistors (OTFTs) have attracted extensively attention owing to its unique properties, such as low-cost, easy-prepared and large-area fabrication capability [1-3]. Sensors based on OTFTs in recent years have also become an attractive choice because of the development of low cost portable sensing systems with capabilities of multi-parametric responses [4]. OTFTs implemented in sensors open a wealth of opportunity for chemical, biosensing, humidity, vapor and so on [5,6]. On the other hand, indoor air pollution has been regarded as the third generation of air pollution problem, while formaldehyde (HCHO) is regarded as a major cause of indoor air pollution [7]. However, the traditional techniques, such as mass spectroscopy and gas chromatographs, suffered from cost, being time-consuming limitation [8]. To develop low temperature fabrication process and enhance the performance with respect to HCHO sensitivity, a further investigation of HCHO sensors is still quite necessary. Notably, until now, OTFT based HCHO sensors has rarely been investigated.

In this work, we used a glass substrate and polymerthylmethacrylate (PMMA) to fabricate bottom contact OTFTs, while pentacene films were used as active layers to detect HCHO gas.

Experiment

A typical OTFT structure is presented in Fig. 1(a). PMMA was first dissolved in chlorobenzene solvent with a 7 % wt concentration. Then 520 nm PMMA was spun coated on ITO coated glass substrate at a rate of 1000 rpm for 60 s and annealed at 150 oC for 1 h. The capacity of PMMA dielectric was about 5.1 nF/cm2. Then, ITO substrates with PMMA dielectrics were transferred into a vacuum chamber to deposit gold source and drain electrodes through a shadow mask with a defined channel length L=150 µm and channel width W=1 cm. Finally, pentacene thin film with a thickness of about 50 nm was deposited at a rate of 0.5 Å/s.

Key Engineering Materials Vols. 575-576 (2014) pp 477-480Online available since 2013/Sep/04 at www.scientific.net© (2014) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/KEM.575-576.477

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 128.42.202.150, Rice University, Fondren Library, Houston, USA-12/11/14,08:55:16)

Page 2: Formaldehyde Gas Sensor Based on Pentacene Organic Thin-Film Transistor

(a) (b)

Fig. 1. (a) Schematic of pentacene based OTFT. (b) AFM images of 50 nm pentacene film.

The devices were stored in an airtight test chamber (about 180 mL) for testing the sensitive characteristics to HCHO gas. The sensitive characteristics of OTFT based sensors were measured with certain concentration of HCHO in high-purity nitrogen (N2) carrier gas. The HCHO and N2

concentrations were introduced into the test chamber by a mass flow controller (MFC) together.

Results and discussions

Many factors in organic active layers, such as the degree of crystallization, the grain boundaries and the surface roughness, could influence the sensing performance of gas. Therefore, we investigated the surface morphology of pentacene sensitive film, Fig. 1(b) shows the 10 µm×10 µm atomic force microscopy (AFM) image of 50 nm thick pentacene film. From the AFM image, large grain sizes

and a large number of grain boundaries of pentacene films can be clearly observed, which is able to enhance sensing ability since the analyte may penetrate the thin film.

(a) (b)

Fig. 2. I-V characteristics curves (a) and transfer IDS-VGS characteristics (b) of the OTFT in N2 and HCHO.

Firstly, the I-V characteristic curve of the OTFT was measured in the dried nitrogen inert atmosphere. Then, the devices were exposed to the saturated vapor of HCHO without N2 gas. A comparison of the device output and transfer characteristics in nitrogen and HCHO (100 ppm) are presented in Figs. 2(a) and (b). An obvious change of IDS can be observed in Fig. 2(a). In Fig. 2 (b), the off state current of the device presents a significant discrepancy in nitrogen and HCHO. Generally, the parameter change are likely caused by hole trapping effect or potential barrier increase at the grain boundaries upon exposure to the HCHO.

478 Recent Highlights in Advanced Materials

Page 3: Formaldehyde Gas Sensor Based on Pentacene Organic Thin-Film Transistor

Fig. 3(a) shows the characteristics of the same pentacene OTFT device exposed to 100 ppm HCHO circumstance. It is obvious that when the transistor exposed to a saturated atmosphere of HCHO, the current IDS decreased in a short time. Once the analyte was removed, the output current dramatically returned to its original value. And then the responses of pentacene OTFT in the absence and in the presence of different concentration of HCHO gas as a function of sensing time were determined, respectively, as displayed in Fig. 3(b).

(a) (b)

Fig. 3. Drain-source transient current variation of the pentacene based OTFT sensor exposed to 100 ppm (a) and in different concentration ranging from 10 to 250 ppm (b) of HCHO gas.

It can be seen that these responses are remarkable and exhibit a good degree of reversibility and repeatability. Similarly, the off state current (VGS = 0 V, VDS = -40 V) of the device presents a significant difference while changing the HCHO gas concentrations: the off state current of the device increases with the increasing of the HCHO gas concentrations.

Besides, it shows in Fig. 3(b) that the recovery can be easily reached once removed HCHO gas. So, we postulate that physisorption caused by adsorption of the HCHO molecules on the pentacene film should be responsible for the fast responses and recovery of OTFT sensors. Generally, the change of source-drain current in OTFT based sensors is due to the electron withdrawing from analytes and organic film by hydrogen bond or gas adsorption (HCHO in this study) for an enhancement of the potential barrier at the boundaries between each grains, or by some kind of doping of the organic material, eventually causing a lowering of output current flowing in the channel region.

Conclusion

In conclusion, pentacene-based OTFT sensors have been produced to detect HCHO gas. Meanwhile, our fabricated HCHO sensors in this work can be seen the reversibility and repeatability are remarkable in the absence and in the presence of different concentrations in HCHO gas. Such results indicated pentacene-based OTFTs are able to effectively monitor HCHO gas.

Acknowledgement

This work was financially supported by the Fundamental Research Funds for the Central Universities (ZYGX2010J060) and the National Science Foundation of China (61177032). The submission is intended for the 2nd International Congress on Advanced Materials

Key Engineering Materials Vols. 575-576 479

Page 4: Formaldehyde Gas Sensor Based on Pentacene Organic Thin-Film Transistor

References

[1] C. D. Dimitrakopoulos, P. R. L. Malenfant, Organic thin film transistors for large area electronics, Adv. Mater. 14 (2002) 99-117.

[2] S. Chung, S. O. Kim, S. K. Kwon, C. Lee, Y. Hong, All-inkjet-printed organic thin-film transistor inverter on flexible plastic substrate, IEEE Electron Device Lett. 32 (2011) 1134-1136.

[3] A. N. Sokolov1, M. E. Roberts, Z. N. Bao, Fabrication of low-cost electronic biosensors, Material Today. 12 (2009) 12-20.

[4] L. Torsi, A. Dodabalapur, Sabbatini, P. G. Zambonin, Multi-parameter gas sensors based on organic thin-film-transistors, Sensors and Actuators B. 67 (2000) 312-316.

[5] J. T. Mabeck, G. G. Malliaras, Chemical and biological sensors based on organic thin-film transistors, Anal Bioanal Chem. 384 (2006) 343-353.

[6] Z. T. Zhu, J. T. Mason, R. D. Dieckmann, G. G. Malliaras, Humidity sensors based on pentacene thin-film transistors, Appl. Phys. Lett. 81 (2002) 4643-4645.

[7] Y. Suzuki, N. Nakano, K. Suzuki, Portable sick house syndrome gas monitoring system based on novel colorimetric reagents for the highly selective and sensitive detection of formaldehyde, Environ. Sci .Technol. 37 (2003) 5695-5700.

[8] S. Risticevic, V. H. Niri, D. Vuckovic, J. Pawliszyn, Recent developments in solid-phase microextraction, Anal. Bioanal. Chem. 393 (2009) 781-795.

480 Recent Highlights in Advanced Materials

Page 5: Formaldehyde Gas Sensor Based on Pentacene Organic Thin-Film Transistor

Recent Highlights in Advanced Materials 10.4028/www.scientific.net/KEM.575-576 Formaldehyde Gas Sensor Based on Pentacene Organic Thin-Film Transistor 10.4028/www.scientific.net/KEM.575-576.477

DOI References

[1] C. D. Dimitrakopoulos, P. R. L. Malenfant, Organic thin film transistors for large area electronics, Adv.

Mater. 14 (2002) 99-117.

http://dx.doi.org/10.1002/1521-4095(20020116)14:2<99::AID-ADMA99>3.0.CO;2-9 [2] S. Chung, S. O. Kim, S. K. Kwon, C. Lee, Y. Hong, All-inkjet-printed organic thin-film transistor inverter

on flexible plastic substrate, IEEE Electron Device Lett. 32 (2011) 1134-1136.

http://dx.doi.org/10.1109/LED.2011.2156757 [3] A. N. Sokolov1, M. E. Roberts, Z. N. Bao, Fabrication of low-cost electronic biosensors, Material Today.

12 (2009) 12-20.

http://dx.doi.org/10.1016/S1369-7021(09)70247-0 [4] L. Torsi, A. Dodabalapur, Sabbatini, P. G. Zambonin, Multi-parameter gas sensors based on organic thin-

film-transistors, Sensors and Actuators B. 67 (2000) 312-316.

http://dx.doi.org/10.1016/S0925-4005(00)00541-4 [5] J. T. Mabeck, G. G. Malliaras, Chemical and biological sensors based on organic thin-film transistors,

Anal Bioanal Chem. 384 (2006) 343-353.

http://dx.doi.org/10.1007/s00216-005-3390-2 [6] Z. T. Zhu, J. T. Mason, R. D. Dieckmann, G. G. Malliaras, Humidity sensors based on pentacene thin-film

transistors, Appl. Phys. Lett. 81 (2002) 4643-4645.

http://dx.doi.org/10.1063/1.1527233 [7] Y. Suzuki, N. Nakano, K. Suzuki, Portable sick house syndrome gas monitoring system based on novel

colorimetric reagents for the highly selective and sensitive detection of formaldehyde, Environ. Sci . Technol.

37 (2003) 5695-5700.

http://dx.doi.org/10.1021/es0305050 [8] S. Risticevic, V. H. Niri, D. Vuckovic, J. Pawliszyn, Recent developments in solid-phase microextraction,

Anal. Bioanal. Chem. 393 (2009) 781-795.

http://dx.doi.org/10.1007/s00216-008-2375-3