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Integration and Response of Organic Electronics with Aqueous Microfluidics Takao Someya,*,†,| Ananth Dodabalapur*,†,‡ Alan Gelperin,†,§ Howard E. Katz,*,† and Zhenan Bao† Bell Laboratories, Lucent Technologies, Murray Hill, New Jersey 07974, The University of Texas at Austin, Austin, Texas 78758, and Monell Chemical Senses Center, Philadelphia, Pennsylvania 19104 Received January 8, 2002. In Final Form: April 1, 2002 We have successfully integrated microfluid flow channels, defined by surface energy differences, with thin-film organic field-effect transistors. The device performance is stable for specific semiconductors with water flowing over the channel region and the contacts protected by hydrophobic surface films, indicating that organic semiconductors are not necessarily degraded by water even when carrying current. In addition, a substantial response from dilute organic solutes, as low as 1 ppm in an aqueous environment, can be observed, with some degree of semiconductor-solute specificity.
Introduction Organic and polymer semiconductors have the required electronic properties to transduce ambient chemical information to electronic information in a circuit.1-8 In many cases, including biological systems, the “ambient” is an aqueous solution. However, water is often thought to be detrimental to organic semiconductor devices such as thin-film organic field-effect transistors (OFETs),9,10 and it is expected that significant effort will be needed to protect such devices from water, even in the vapor phase, if they are to be implemented into “plastic electronic” circuits. Here we show, somewhat counterintuitively, that it is possible to operate such devices when the channel is under a pool of liquid water. Furthermore, we describe the successful integration of microfluid flow channels with OFETs through the definition of regions of low surface energy around the semiconductor-metal contacts. With this arrangement, water flow is directed perpendicular to the electronic current flow direction with the fluid in * To whom reprint requests should be addressed. E-mail:
[email protected] and
[email protected]. † Lucent Technologies. ‡ The University of Texas at Austin. § Monell Chemical Senses Center. | Also with Columbia University, Department of Electrical Engineering, New York, NY. (1) Lundstrom, I.; Erlandsson, R.; Frykman, U.; Hedborg, E.; Spetz, A.; Sundgren, H.; Welin, S.; Winquist, F. Nature 1991, 352, 47. (2) Doleman, B. J.; Severin, E. J.; Lewis, N. S. Proc. Natl. Acad. Sci. U.S.A. 1998, 95, 5442. (3) Gardner, W.; Bartlett, P. N. Electronic Noses: Principles and Applications; Oxford University Press: Oxford, 1999. (4) Gelperin, A.; Dawson, J. L.; Cazares, S. M.; Seung, H. S. In Electronic Noses and Sensor Array Based Systems; Hurst, W. J., Ed.; Technomic: Lancaster, PA, 1999; p 263. (5) Matzger, A. J.; Lawrence, C. E.; Grubbs, R. H.; Lewis, N. S. J. Comb. Chem. 2000, 2, 301. (6) Crone, B. K.; Dodabalapur, A.; Gelperin, A.; Torsi, L. A.; Katz, H. E.; Lovinger, A. J.; Bao, Z. Appl. Phys. Lett. 2001, 78, 2229. (7) Jones, R. M.; Bergstedt, T. S.; McBranch, D. W.; Whitten, D. G. J. Am. Chem. Soc. 2001, 123, 6726. (8) Guiseppi-Elie, A.; Wallace, G.; Matsue, T. Chemical and Biological Sensors Based on Electrically Conducting Polymers. In Handbook of Conducting Polymers, 2nd ed.; Skotheim, T. A., Elsenbaumer, R. L., Reynolds, J. R., Eds.; Marcel Dekker: New York, 1998. (9) Sheats, J. R.; Antoniadis, H.; Hueschen, M.; Leonard, W.; Miller, J.; Moon, R.; Roitman, D.; Stocking, A. Science 1996, 273, 884. (10) deLeeuw, D. M.; Simenon, M. M. J.; Brown, A. R.; Einerhand, R. E. F. Synth. Met. 1997, 87, 53.
contact with the active region of the OFET device. Devices that are active under standing water are also active under flowing water. Tests of several combinations of semiconductors and dissolved hydroxy and carboxy compounds, at concentrations as low as 1 ppm, reveal that there is some solute-dependent specificity to the electrical response, even in the absence of defined receptors attached to the semiconductors. Thus, the use of arrays of OFETs and novel information-processing algorithms could eventually permit analyte identification, provided that the “underwater” current can be sufficiently stabilized. Experimental Setup for OFETs under Standing Water. Figure 1 shows an OFET with a water droplet covering the active region. Because moisture is considered very detrimental to the performance of organic semiconductor based devices,9,10 the operation of an organic transistor with the active layer immersed in water is problematic. A principal mechanism of OFET performance degradation is water-mediated electrochemical decomposition at high electric fields. In an OFET, the electric fields can be very high near the source (S) and drain (D) contacts,11 although they can be modest at regions between these contacts, depending on the device geometry. Also, high fields can promote charge injection into water from the contacts, resulting in a large parallel ionic current flow that can overwhelm current flow in the active channel, making sensing, and even switching, impossible. We solve these problems by confining the water or an aqueous solution to the central part of the active channel, above the organic semiconductor layer but away from the S and D contacts, as shown in Figure 1. The contact regions were coated with a hydrophobic material, such as a fluorinated polymer, that repels water. Even though the semiconductor film surfaces themselves are also fairly hydrophobic, the fluorinated surface provides significant surface energy contrast. Another surface treatment that proved valuable for the microfluidics channel experiment (see below) was to affix a methylphenylsiloxane adhesion promotion layer to the oxide dielectric surface, by treatment of the silicon substrates with a 5% solution of dimethoxymethylphenylsilane in a hydrocarbon solvent at 70 °C. By employment of these methods to define the (11) Chwang, A. B.; Frisbie, C. D. J. Phys. Chem. B 2000, 104, 12202.
10.1021/la020026z CCC: $22.00 © 2002 American Chemical Society Published on Web 06/01/2002
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Figure 1. The basic structure of the organic semiconductor device and the configuration of the analyte delivery system and recording electrodes during an experiment. In (A) is shown the cross-sectional illustration: Thin film organic layers (1015 nm) were deposited in a vacuum sublimation system on highly doped silicon wafers covered with 100 nm thick silicon oxide. Silicon wafers were contacted by gold that functioned as a gate electrode, while silicon oxide served as the gate dielectric layer. The region between the two dashed lines in (B) is the organic active layer. The channel length L and width W of organic transistors were 1.6 mm and 5.0 mm, respectively. Fluorochemical-coated layers were applied to prevent water from touching the electrodes. Surfaces of organic active layers of field-effect transistors were in direct contact with water. Analyte was introduced into the water through a glass micropipet.
proper interfacial interactions, FET devices made with many organic semiconductors were shown to operate under water. The Response to Water. The electrical current flowing between source and drain was measured as a function of time and water coverage after applying a S-D voltage. When the active layer pentacene was exposed to water, current decreased, but only by 16% (Figure 2A). The response was reversible and reproducible. The difference between initial and final currents that persists on repeated cycling occurs even in the absence of water and could be explained by a trap-filling process that is slow compared to the time scale of the experiments; filling these traps requires that holes first overcome a potential barrier. On removing the gate voltage, the traps are slowly discharged, and turning the device on once again results in a restored initial current. Many organic semiconductors were found to be robust enough to operate under water for times on the order of hours, or dozens of cycles; however, a few showed irreversible degradation under water (Figure 2B). Possible water-induced degradation mechanisms include the formation of ionic current channels between the electrodes and delamination of the semiconductor from the dielectric, and these would have to be avoided through improved semiconductor adhesion and film continuity before technologically valuable systems could be realized. Besides pentacene, materials responding well under water include R-sexithiophene (R6T), dihexyl R6T (DHR6T), and copper
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Figure 2. The response of OFETs to water. (A) The drain current of the pentacene device was monitored as a function of time after S-D voltage application. The device was biased with a gate voltage Vg of -25 V and a drain-source voltage Vds of -25 V so that channel regions had field-induced charge with densities of 1013 cm-2. When the semiconductor was exposed to water at t ) 60 s, the drain current suddenly decreased. The change of current was easily measured from the reference data without water (dashed lines) by subtraction. When water was removed at t ) 120 s, the current level recovered. Similar procedures were repeated using the same device. (B) The current of a PTPTP device as a function of time, where the thiophene ring and phenylene are abbreviated by T and P, respectively.26 When water was put on the surface at t ) 80 s, the current decreased suddenly and became steady. This reduction of the current did not reverse after removing water at t ) 110 s or on further addition/removal of water. (C) The water response in devices of R6T, DHR6T, CuPc, and pentacene.
phthalocyanine (CuPc). Their water responses can be seen in Figure 2C. As a preliminary lifetime test, a channel of a copper phthalocyanine FET was kept under a continuously present and occasionally replenished water droplet in a water-rich atmosphere. The device gave an output current that drifted within 10% of the mean when tested intermittently over 8 h, until mechanical failure occurred due to the replenishment process. The Response to Analytes in Nonflowing Solutions. Analytes were investigated as dilute aqueous solutions. For example, when lactic acid was delivered to the CuPc device, a reduction of current was observed (Figure 3A). The degree of S-D current reduction was correlated with the analyte concentration. A clear response was observed for a 10 µM analyte, demonstrating that the sensitivity in the micromole range is successfully realized. The square roots of drain currents Id in CuPc devices are plotted as a function of gate voltage Vg in Figure 3B. Measurements were performed without water, with water, and with water plus analyte. The data show that both water and analyte affect primarily the field-effect mobility rather than the threshold voltage. Other combinations of semiconductors and analytes were also investigated. R6T was responsive to glucose at concentrations in the millimole range, whereas CuPc was not responsive at all. CuPc was very sensitive to lactic acid, which was detected at concentrations of
