Organic electrochemical transistors based on a dielectrophoretically aligned nanowire array
© Choi et al; licensee Springer. 2011
Received: 5 November 2010
Accepted: 14 April 2011
Published: 14 April 2011
In this study, we synthesized an organic electrochemical transistor (OECT) using dielectrophoresis of a carbon nanotube-Nafion (CNT-Nafion) suspension. Dielectrophoretically aligned nanowires formed a one-dimensional submicron bundle between triangular electrodes. The CNT-Nafion composite nanowire bundles showed p-type semiconductor characteristics. The drain-source current decreased with increasing gate voltage. The nanowire bundles showed potential as pH sensor because the drain-source current ratio varied linearly according to the gate voltage in pH buffers.
Recently, there has been significant research in the area of organic thin-film transistors (OTFTs), because of the many benefits of organic semiconductors, such as structural flexibility, low temperature processing, and low cost [1–7]. Organic electrochemical transistors (OECTs), a subset of OTFTs, have been considered as sensors because of their ability to operate in aqueous environments with relatively low voltages and their integration with microfluidics. Furthermore, one can to get information on additional dimensions using gate-induced modulation, compared with two-terminal devices [5–12]. In particular, OECTs, formed using one-dimensional nanostructures, such as nanotubes and nanowires, are more attractive for use as chemical and biological sensors because of their large surface-to-volume ratio, light weight, and controllable transport properties [10–13].
In this article, we report the fabrication of CNT composite nanowires with Nafion, a well-known proton conductor [21, 22] and the use of CNT-Nafion composite nanowires as electrochemical transistors in various pH buffers.
Results and discussion
We fabricated organic chemical transistors based on CNT-Nafion composite nanowires using dielectrophoresis. These composite nanowires had p-type semiconductor characteristics in aqueous media, and the drain-current to gate voltage ratio was proportional to the buffer pH. Because the synthesis of nanowire bundles occurred at electrodes with an applied electric field, and various organic materials have the potential to form composites with CNT, one can synthesize an individually addressable CNT composite nanowire array.
CNT-Nafion nanowires were synthesized between cantilever electrodes that were fabricated using a traditional MEMS technique. These electrodes were fabricated using a standard lift-off process. A gold layer (2000 Å) was deposited with a chrome layer (200 Å) as an adhesion layer using an e-beam evaporator on a silicon substrate covered with 1 μm of low-stress silicon nitride using low-pressure chemical vapor deposition (LPCVD). For the cantilever structure, the silicon nitride was etched using standard reactive ion etching (RIE), and the silicon was etched using isotropic wet etching using RSE-200 etchant. The SWNTs with 1.0-1.2 nm diameters and lengths of 5-20 μm were purchased from Ilgin Nanotech, and a SWNT-COOH suspension was prepared by oxidizing the CNTs in a strong acid with sonication . Nafion was purchased from Aldrich and was used without purification. The CNT-Nafion solutions were prepared by combining 3 μL Nafion solution and 200 μL CNT-COOH suspension with sonication for 10 min.
The CNT-Nafion solution was placed on the cantilever electrodes, and an AC voltage of 1 MHz and 10 V peak-to-peak was applied. The SWNTs and monomers were aligned between the cantilever electrodes by the dielectrophoretic force. The SWNT-Nafion solution was removed partially while maintaining the AC electric field and the SWNT-Nafion nanowire bundles were synthesized as the remaining solution evaporated.
Figure 1 shows a schematic of the electrochemical transistors, which consisted of two Au electrodes connected by CNT-Nafion nanowires and a remote Ag/AgCl gate electrode immersed in an electrolyte droplet. The electrochemical transistors were characterized in pH buffers using Samchun Chemical at room temperature using a semiconductor analyzer (HP4156A, Hewlett-Packard).
energy dispersive X-ray spectroscopy
low-pressure chemical vapor deposition
organic electrochemical transistor
organic thin film transistors
reactive ion etching
scanning electron microscope
single-walled carbon nanotube
vascular endothelial growth factor.
This study was supported by the Mid-career Researcher program through NRF grant funded by the MEST (No. 2009-0085377), the World Class University program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (R31-2008-000-10105-0), and Development of Intelligent Robot Technology for Total Clinical System based (10024733) under the Industrial Source Technology Development Programs of the MKE of Korea.
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