Investigation of extended-gate field-effect transistor pH sensors based on different-temperature-annealed bi-layer MWCNTs-In2O3 films
© Hung et al.; licensee Springer. 2014
Received: 24 July 2014
Accepted: 7 September 2014
Published: 16 September 2014
In this paper, indium (In) films were deposited on glass substrates using DC sputtering method. Multiwalled carbon nanotubes (MWCNTs) and dispersant were dissolved in alcohol, and the mixed solution was deposited on the In films using the spray method. The bi-layer MWCNTs-In2O3 films were annealed at different temperatures (from room temperature to 500°C) in O2 atmosphere. The influences of annealing temperature on the characteristics of the bi-layer MWCNTs-In2O3 films were investigated by scanning electron microscopy, X-ray diffraction pattern, Fourier transform infrared (FT-IR) spectroscopy, and Raman spectroscopy. A separative extended-gate field-effect transistor (EGFET) device combined with a bi-layer MWCNTs-In2O3 film was constructed as a pH sensor. The influences of different annealing temperatures on the performances of the EGFET-based pH sensors were investigated. We would show that the pH sensitivity was dependent on the thermal oxygenation temperature of the bi-layer MWCNTs-In2O3 films.
Carbon nanotubes (CNTs), an important group of nanoscale materials, have received great attention in different fields since their discovery in 1991 by Iijima . Due to their unique structural, electronic, and mechanical properties, CNTs make themselves very attractive materials for a wide range of applications [1–3]. CNTs with their well-defined nanoscale dimensions and unique molecular structure can be used as bridges linking biomolecules to macro/micro-solid-state devices so that bioevent information can be transduced into measurable signals. Among them, chemical and biological sensors  based on CNTs have been the target of numerous investigations because of their simplest chemical composition and atomic bonding configuration even though considerable challenges remain in a specific end use. For that, multiple types of CNT-based chemical sensors have been developed for sensing application. Because single-walled carbon nanotube (SWCNT)-field-effect transistors (FETs) offer several advantages for sensing including the ability to amplify the detection signal with the additional gate electrode, Chen et al. used SWCNT-thin-film transistors (TFTs) as gas sensors to detect methyl methylphosphonate, a stimulant of benchmark threats . Also, Karimi et al. proposed an analytical model of graphene-based solution-gated (SG) FETs to constitute an important step towards development of DNA biosensors with high sensitivity and selectivity . Dong et al. fabricated carbon monoxide (CO) and ammonia (NH3) gas sensors using interdigitated electrodes on Si wafer, and they found that 10 ppm of CO and NH3 could be electrically detected using a carboxylic acid-functionalized single-walled carbon nanotube (C-SWCNT) .
Those researches prove that semiconductor active devices have been developed for sensing application, and SWCNT-FETs offer several advantages for sensing including the ability to amplify detection signals . In the past, CNTs can also be used to investigate as a pH sensor. For example, Kwon et al. fabricated a simple and fast-response pH sensor composed of SWCNTs using a non-vacuum spray method . An ion-sensitive field-effect transistor (ISFET) device is applied to an electrochemical sensing device, and the structure of a separative extended-gate field-effect transistor (EGFET) device has been developed from the ISFET device. Thus, an EGFET device is also a semiconductor active device with a different structure to produce FET isolation from the chemical environment, in which a chemically sensitive membrane is deposited on the end of a signal line extended from the FET gate electrode . The EGFET device's structure also comprises a metal-oxide-semiconductor field-effect transistor (MOSFET) which retains a metal gate electrode and utilizes a signal wire to connect the separative ion sensing films and the field-effect transistor. For that, the EGFET devices can solve the packaging and maintaining problems of ISFET devices, and the EGFET devices can operate at a higher stable condition. The ISFET devices can also be designed from discarded biosensors (the ion sensing films) to save money because they combine two different parts, the sensors and MOSFET. For that, a novel concept combining bi-layer multiwalled carbon nanotubes (MWCNTs)-In2O3 films and EGFET is proposed for pH sensing application. In this study, the bi-layer MWCNTs-In2O3 films were used to fabricate the sensing layer and to catch the ions in the solution and EGFET devices were investigated and used to transport the ions while the EGFET device was active. The bi-layer MWCNTs-In2O3 films were annealed at different temperatures (200°C ~ 500°C), and the effect of annealing temperatures on the characteristics of In2O3 films and on the performances of pH sensors was investigated.
Secondly, 2 mg of as-received MWCNT (Iljin Nanotech Co. Ltd., Seoul, South Korea, average diameter 30 nm) powder with 10 mg of dispersant (type: PVP K30) was ultrasonically dispersed in 10 ml of anhydrous ethanol for 30 min. The solution was spread on In films to form the bi-layer MWCNTs-In films. MWCNT-based suspension was then sprayed on the In-coated glass substrates maintained at 90°C for 40 min by using a portable air spray gun with a distance of 10 cm keeping 3 s with an interval of 1 min for 40 min. The prepared samples were put in the vacuum chamber with 20 mTorr, and N2 with 100 sccm was introduced during the temperature raising process. The composite MWCNTs-In films were annealed at different temperatures, ranging from 200°C to 500°C for 1 h. The surface morphology, microstructure, and cross section of the bi-layer MWCNTs-In2O3 films were characterized by field-emission scanning electron microscopy (FESEM). If the bi-layer MWCNTs-In2O3 films were annealed at a temperature higher than 500°C, the In2O3 (In) films were melted. For that, the bi-layer MWCNTs-In2O3 films could not be annealed at a temperature higher than 500°C. As the temperature was raised to annealing temperature, the chamber was kept at 20 mTorr and O2 with 10 sccm was introduced during the annealing process. The addition of O2 was used to anneal In into In2O3. Fourier transform infrared (FT-IR) spectrum was recorded over the range 400 to 1,000 cm-1 on a Thermo-Nicolet Avatar 370 FT-IR spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) using the KBr pellet method for the inspected In-O phonon vibration mode measurement. X-ray diffraction (XRD) pattern with Cu Kα radiation (λ = 1.5418 Å) was used to find the crystalline structure of In2O3 films, and Raman measurement obtained from red laser (785 nm) was used to examine the chemical composition of MWCNTs.
Results and discussion
Wavelengths of I D and I G peaks and calculation value of I D / I G ratio under different treatment temperatures
Figure 6 also shows that the D band and the G band had apparent changes in their intensities as the oxidation temperature was increased, and the intensity of the D band peak at approximately 1,620 cm-1 increased with increasing oxidation temperature. The R = ID/IG ratio, where I corresponds to the peak area of the Lorentzian functions, allows us to estimate the relative extent of structural defects. Table 1 shows that the ID/IG ratio of the 200°C-annealed sample was equal to the value of as-received tubes. This result suggests that no oxidation happens on the MWCNTs under this condition. As the oxidation temperature was increased to 300°C, the ID/IG ratio decreased to 0.484. The removal of defective tubes (some amorphous carbon layers, sp3 carbon, and other impurities) and improvement of disordered carbon are the reasons . Therefore, as the oxidation temperature was further increased from 300°C to 500°C, the ID/IG ratio induced an increase from 0.484 to 0.92. As the MWCNTs are annealed in oxygen atmosphere, the increase in ID/IG ratio is believed to be caused by the enhancement of surface defects and embedment of oxygen atoms.
Sensitivity of MWCNTs/In 2 O 3 sensing layer as a function of thermal treatment temperature
In this study, XRD patterns showed that as the annealing temperature was equal to and higher than 400°C, only the In2O3 phase was clearly observed in the bi-layer MWCNTs-In2O3 films. The composite MWCNT-In2O3 electrode was used in the EGFET devices to enhance the performance of pH sensors. From the Raman spectra, as the oxidation temperature was further increased from 300°C to 500°C, the ID/IG ratio (R) induced an increase from 0.484 to 0.92. The increase in R values was believed to be caused by the enhancement of surface defects and embedment of oxygen atoms. The variation of the reference voltage for the MWCNTs/In2O3 electrode in the EGFET devices without thermal treatment did not show linear dependence on the low pH value of the buffer solution. The reference voltage of the MWCNTs/In2O3 electrode after thermal treatment was almost linearly dependent on the pH value of the buffer solution. It was found that the superior sensitivity characteristic of the MWCNT/In2O3 films in the EGFET devices was 36.43 mV/pH while the thermal treatment temperature was 400°C.
S-CH was born in Taipei, Taiwan. After graduating in electrical engineering from the University of Alabama, Huntsville, he returned to Taiwan and worked at MATRA (France) branch in Taiwan as an electrical engineer responsible for constructing the first subway in Taiwan in 1992. He is now an associate professor at Shih Chien University, Kaohsiung, Taiwan. Much of Hung's research interests has been in the field of one-dimensional nanostructures including the design, fabrication, and characterization of optoelectronic materials for device applications. He is also in the field of carbon nanotubes for pH sensor application and announced in 2012 to 2014.
N-JC graduated from the Department of Physics, National Cheng Kung University in 1988. He obtained his master's degree and PhD degree from the Department of Optics and Photonics, National Central University in 1992 and 1999. After obtaining his master's degree, he joined the Digital Signal Processing Division of Chunghua Telecom Laboratories in 1994 as an assistant researcher. He is currently an assistant professor in the Institute of Photonics and Communications at National Kaohsiung University of Applied Sciences, Kaohsiung, Taiwan. His research interests involve physics education, optical information processing, optical metrology, image processing, and 3-D optical profilometry.
C-FY gained his bachelor's, master's, and PhD degrees in 1976, 1988, and 1993, respectively, from the Department of Electrical Engineering of Cheng Kung University. After obtaining his PhD degree, Yang entered the Department of Electronic Engineering, Chinese Air Force Academy and since February 2000 as a professor at the Chinese Air Force Academy, Taiwan. In February 2004, he became a professor of Chemical and Materials Engineering at National University of Kaohsiung (NUK). His current research interests are focused on fine ceramics, microwave ceramics, dielectric thin films, optical materials, transparent conducting oxides, solar cell materials, applications of carbon nanotubes, microwave antennas, and microstrip filters.
Y-PL was born in Taiwan. He got his master's degree in the Graduate Institute of Electro-Optical Engineering and Department of Electronic Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan. While furthering his graduate program, Lo focuses his research on composite structures with carbon nanotube nanomaterials for pH sensor application.
This work was supported by the National Science Council of Taiwan under grant nos. MOST-103-2221-E-158-005 and NSC 102-2221-E-020-020 and the Shih Chien University, Kaohsiung Campus, under contract number USC-103-05-05013. The authors would like to thank Bohr-Ran Huang for the equipment support.
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