Fabrication of an Interdigitated Array Electrode on ZnSe and Its

School of Chemical Sciences and the Frederick Seitz Materials Research Laboratory, University of Illinois at Urbanas. Champaign, Urbana, Illinois 6180...
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Fabrication of an Interdigitated Array Electrode on ZnSe and Its Application to Electrooptical Measurements Using FT-IR Spectroscopy Rene´e M. Blanchard, Alison R. Noble-Luginbuhl, and Ralph G. Nuzzo*

School of Chemical Sciences and the Frederick Seitz Materials Research Laboratory, University of Illinois at Urbanas Champaign, Urbana, Illinois 61801

An interdigitated array electrode (IDA) is fabricated on an IR-transparent substrate for use in electrooptical measurements using Fourier transform infrared (FT-IR) spectroscopy. The fabrication of the IDA and its unique sampling geometry for transmission spectroscopy is detailed. The gold IDA was patterned on the ZnSe substrate using a photolithographically defined liftoff process. An IR flow cell was modified to enable the substrate containing the IDA to be used for electrooptical measurements in transmission. The utility of the electrooptical cell is demonstrated by application to two model systems. In the first, changes in the conductivity and spectral features of a receptor membrane (Nafion) upon dehydration are measured. In the second, the measurement of the electricfield-induced orientation of a liquid-crystalline film of 4-npentyl-4′-cyanobiphenyl was conducted in a new experimental geometry, with IR transmission normal to the direction of the applied electric field. Difference spectroscopy facilitated the observation of the change in orientation of the liquid crystal with applied potential. These systems demonstrate the general utility of the fabrication scheme described here. It also illustrates its facile adaptation to infrared difference spectroscopy as a means of studying complex phenomena in electrochemical and electrooptical systems.

Electrochemical studies performed in combination with in situ Fourier transform infrared (FT-IR) spectroscopy have typically employed a thin-layer cell in a reflection configuration with the * Corresponding author: (e-mail) [email protected]; (phone) 217 244-0809; (fax) 217 224-2278. 10.1021/ac991293l CCC: $19.00 Published on Web 03/02/2000

© 2000 American Chemical Society

IR radiation sampled in either an external1-5 or internal reflection mode4,5 or an electrode sandwich geometry in a transmission mode.6,7 The first two cell designs (Figure 1a,b) have been used in a three-electrode configuration to characterize species involved in electrochemical processes.1-5,8-11 Studies using a sandwich configuration optical cell (Figure 1c) have been used to examine electrooptical effects, of which the electric-field-induced transitions in liquid crystals provide an important example.6,7,12-21 As shown (1) Bewick, A.; Kunimatsu, K.; Pons, B. S.; Russel, J. W. J. Electroanal. Chem. 1984, 160, 47-61. (2) Mosier-Boss, P. A.; Newbery, R.; Szpak, S.; Lieberman, S. H.; Rovang, J. W. Anal. Chem. 1996, 68, 3277-3282. (3) Li, Z.; Lin, X. J. Electroanal. Chem. 1995, 386, 83-87. (4) Bae, I. T.; Sandifer, M.; Lee, Y. W.; Tryk, D. A.; Sukenik, C. N.; Scherson, D. A. Anal. Chem. 1995, 67, 4508-4513. (5) Neugebauer, H.; Ping, Z. Mikrochim. Acta Suppl. 1997, 14, 125-131. (6) Nakano, T.; Yokoyama, T.; Toriumi, H. Appl. Spectrosc. 1993, 47, 13541366. (7) Gregoriou, V. G.; Chao, J. L.; Toriumi, H.; Palmer, R. A. Chem. Phys. Lett. 1991, 179, 491-496. (8) Osawa, M.; Yoshii, K. Appl. Spectrosc. 1997, 51, 512-518. (9) Port, S. N.; Horswell, S. L.; Raval, R.; Schiffrin, D. J. Langmuir 1996, 12, 5934-5941. (10) Budevska, B. O.; Griffiths, P. R. Anal. Chem. 1993, 65, 2963-2971. (11) Pharr, C. M.; Griffiths, P. R. Anal. Chem. 1997, 69, 4665-4672. (12) Czarnecki, M. A.; Okretic, S.; Siesler, H. W. J. Phys. Chem. B 1997, 101, 374-380. (13) Palmer, R. A.; Gregoriou, V. G.; Fuji, A.; Jiang, E. Y.; Plunkett, S. E.; Connors, L. M.; Boccara, S.; Chao, J. L. In Multidimensional Spectroscopy of Polymers: Vibrational, NMR, and Fluorescence Techniques; Urban, M. W., Provder, T., Eds.; ACS Symposium Series 598; American Chemical Society: Washington, DC, 1995; Chapter 6. (14) Gregoriou, V. G.; Palmer, R. A. Macromol. Symp. 1995, 94, 75-95. (15) Sasaki, H.; Ishibashi, M.; Tanaka, N.; Shibuya, N.; Hasegawa, R. Appl. Spectrosc. 1993, 47, 1390-1393. (16) Sugisawa, H.; Ishibashi, M.; Tanaka, N.; Shibuya, N.; Hasegawa, R. Appl. Spectrosc. 1993, 47, 1390-1393. (17) de Bleijser, J.; Leyte-Zuiderweg, L. H.; Leyte, J. C.; van Woerkom, P. C. M.; Picken, S. J. Appl. Spectrosc. 1996, 50, 167-173. (18) Urano, T. I.; Hamaguchi, H. Chem. Phys. Lett. 1992, 195, 287-292.

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Figure 1. Schematic illustrations of cells used for spectroelectrochemistry and electrooptic applications in infrared spectroscopy: (a) external reflection cell, (b) internal reflection cell, and (c) sandwich thin-layer transmission cell. The incident IR beam and polarization vector (bold) for each configuration is shown.

in Figure 1, the optics of these cell designs impose significant restrictions on the range of molecular orientations that can be conveniently sampled. An external reflection cell is limited by the boundary-layer optics imposed by the base (working) electrode. The electric field direction is constrained to a strictly collinear orientation with respect to the parallel, Ep, component, which can effectively sample this region (at least for a metal). The sandwich configuration, typically sampled at normal incidence, uses an applied electric field that is parallel to the propagation direction of the IR. This limits sampling to optical polarizations that are transverse to this direction and thus limits the range over which polarized measurements can be made of potential-induced transport or molecular reorientation. Interdigitated array electrodes (IDA)22 constitute yet another electrode configuration that has been used extensively for electrochemical studies23 but has found only limited use in studies of electrooptical24,25 and spectroelectrochemical processes.26 An IDA consists of many parallel bands of electrodes, each separated by a small insulating gap (Figure 2a). In this design, alternating metal bands are connected to give an array of electrode band pairs. The vertical edges and spacing of the electrodes uniquely define the optical boundary conditions of the cell and mediate the types of potential-driven processes occurring in the cell (Figure 2b). For a transmission IR experiment when the radiation is incident at a normal angle, the applied electric field between the electrode bands lies parallel with the plane of the IDA (and substrate) and thus lies perpendicular to the propagation direction of the IR beam. This feature suggests that an electrooptical cell based on an IDA design should enable a much broader analysis of potential(19) Shilov, S. V.; Skupin, H.; Kremer, F.; Gebhard, E.; Zentel, R. Liq. Cryst. 1997, 22, 203-210. (20) Katayama, N.; Czarnecki, M. A.; Satoh, M.; Watanabe, T.; Ozaki, Y. Appl. Spectrosc. 1997, 51, 487-490. (21) Czarnecki, M. A.; Katayama, N.; Ozaki, Y.; Satoh, M.; Yoshio, K.; Watanabe, T.; Yanagi, T. Appl. Spectrosc. 1993, 47, 1382-1385. (22) Chidsey, C. E.; Feldman, B. J.; Lundgren, C.; Murray, R. W. Anal. Chem. 1986, 58, 601-607. (23) Niwa, O. Electroanalysis 1995, 7, 606-613. (24) Wu, S. Y.; Takei, W. J.; Francombe, M. H. Appl. Phys. Lett. 1973, 22, 2628. (25) Oh-E, M.; Kondo, K. Liq. Cryst. 1997, 22, 379-390. (26) Sanderson, D. G.; Anderson, L. B. Anal. Chem. 1985, 57, 2388-2393.

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Figure 2. Schematic illustration of (a) an IDA and (b) cross-sectional view of one pair of electrode bands of the IDA in a transmission electrooptical cell.

modulated processes since molecular orientations can be studied easily in all directions (if nonnormal incidence angles are used) relative to the applied electric field. An IDA fabricated on an IR-transparent substrate has potential for enabling studies in areas beyond electrooptics since it is a simple extension to adopt these optics to an in situ flow cell in which gas, liquid, and thin-film samples could be studied. Of particular interest to our research is direct measurement, by combined optical and electrical methods, of the partitioning of a gas or liquid (which is flowed through the cell) into a receptor polymer. The use of IDAs in such an application offers many potential advantages, not the least of which is their ability to attain steady state quickly because the characteristic diffusion lengths can be made very small without degrading the S/N of the measurement.22,26,27 The optics of the IDA electrooptical cell present several interesting possibilities that might be exploited in research. First, the gold bands of the IDA establish a boundary layer where a surface-selection rule due to metal must be obeyed.28 Thus we expect that when sampled by IR polarized parallel to the bands (E||; see Figure 2a), regions adjacent to the electrode (out to ∼λ/ 4) will contribute no intensity to the spectra.28,29 The transverse polarization (E⊥, Figure 2a), however, should effectively sample the entire interelectrode region and yield an enhanced contribution from material near the boundary. The cell described in this initial report does not exploit this feature in a significant way since it used an interelectrode spacing larger than the diffraction limit for the frequencies of interest in the mid-infrared region. Such effects will be exploited and form a central focus for studies to be reported in the future, however. Another interesting feature of the IDA optical cell described in this report is the thickness of the thin films that can be examined. The limiting dimensions of the cell sample elements are imposed not only by the electrode spacing but also by the thickness of the Au films used to construct the electrodes. The literature describes numerous applications for IDAs in sensing that suggest interesting possibilities for coordinated (27) Aoki, K.; Morita, M.; Niwa, O.; Tabei, H. J. Electroanal. Chem. 1988, 256, 269-282. (28) Greenler, R. G. J. Chem. Phys. 1966, 44, 310-314. (29) Woodruff, D. P.; Delchar, T. A. Modern Techniques of Surface Science; Cambridge University Press: Cambridge, U.K., 1986; Chapter 9.

studies by vibrational spectroscopy. Among the more intriguing of these applications, IDAs have been used to create moleculebased electronic devices and chemically responsive sensors by combining electroactive polymers with arrays of electrodes.30-33 The narrow gap between electrodes in the IDA is also advantageous for measuring the dynamics of electron conduction in thin films22,27,34-36 or ionic diffusion in polymer films.37,38 A report closer to the applications of interest in this work describes a cell for UV-visible spectroelectrochemistry using a three-electrode configuration.26 These fairly large spacing (50 µm) electrodes were fabricated on a quartz substrate. This work describes the fabrication of IDAs on IR substrates for use in electrooptical measurements using FT-IR spectroscopy. ZnSe was chosen as the substrate because of characteristics such as good chemical resistance (which make them appropriate for use in photolithographic patterning), continuous transmission range (10 000-500 cm-1), and water insolubility.39 The gold IDA was patterned on the ZnSe substrate using a photolithographically defined liftoff40 process. This paper demonstrates a simultaneous measurement of changes occurring in the conductivity of a receptor membrane (Nafion,41,42 a DuPont trademark name for a perfluorinated cation-exchange polymer) and spectral features in response to changes in the gas flow through a cell. In a second application, the measurement of the electric-field-induced orientation (the Freedericksz transition43) of a liquid-crystalline film of 4-n-pentyl-4′-cyanobiphenyl (5CB)44 is demonstrated using the electrooptical cell. EXPERIMENTAL SECTION Materials. Polished ZnSe IR windows (41 mm × 23 mm × 2 mm) were obtained from Wilmad Glass (Buena, NJ). For use in the Beckman-Specac F-05 Cell Mount (Wilmad Glass), ZnSe windows with drilled holes (31 mm center-to-center) were also obtained. A Pt/Ti on glass projection mask (Abtech Scientific, Yardley, PA) was used to pattern the resist defining the IDA and consisted of 100 pairs of interlocking electrode bands 15 µm wide and 5000 µm long, with 15 µm gaps; the configuration used is depicted in Figure 2a. Photoresist, AZ 5214, and 351 basic developer were obtained from Hoechst (Somerville, NJ). Cr metal (used as an Au adhesion layer) was obtained from Aldrich (Milwaukee, WI). Coinage quality gold (>99.99%) was used as (30) Kittlesen, G. P.; White, H. S.; Wrighton, M. S. J. Am. Chem. Soc. 1984, 106, 7389-7396. (31) White, H. S.; Kittlesen, G. P.; Wrighton, M. S. J. Am. Chem. Soc. 1984, 106, 7389-7396. (32) Lonergan, M. C.; Severin, E. J.; Doleman, B. J.; Beaber, S. A.; Grubbs, R. H.; Lewis, N. S. Chem. Mater. 1996, 8, 2298-2312. (33) Freund, M. S.; Lewis, N. S. Proc. Natl. Acad. Sci. U.S.A. 1995, 92, 26522656. (34) Chidsey, C. E. D.; Murray, R. W. Science 1986, 231, 25-31. (35) Feldman, B. J.; Murray, R. W. Anal. Chem. 1986, 58, 2844-2847. (36) Feldman, B. J.; Murray, R. W. Inorg. Chem. 1987, 26, 1702-1708. (37) Nishihara, H.; Dalton, F.; Murray, R. W. Anal. Chem. 1991, 63, 29552960. (38) Cammarata, V.; Talham, D. R.; Crooks, R. M.; Wrighton, M. S. J. Phys. Chem. 1990, 94, 2680-2684. (39) Chantry, G. W. Long-Wave Optics: The Science and Technology of Infrared and Near-Millimetre Waves; Academic Press: London, 1984; Vol. 2, p 630. (40) VLSI Technology; Sze, S. M., Ed.; McGraw-Hill: New York, 1988. (41) Yeo, S. C.; Eisenberg, A. J. Appl. Polym. Sci. 1977, 21, 875-898. (42) Perfluoronated Ionomer Membranes; Eisenberg, A., Yeager, H. L., Eds.; ACS Symposium Series 180; American Chemical Society: Washington, DC, 1982. (43) Freedericksz, V.; Repiewa, A. Z. Phys.ik 1927, 42, 352. (44) Gray, G. W.; Harrison, K. J.; Nash, J. A. Electron. Lett. 1973, 9, 130-131.

Figure 3. Schematic flowchart of procedures used to fabricate the IDA on ZnSe. See text for details.

the evaporation source. Electrically conductive silver epoxy (EpoTek 410E) was obtained from Epoxy-Technology, Inc. (Billerica, MA). Ionomeric thin films of Nafion were prepared using a solution sample (H+ form, 5 wt % solution in a mixture of lower aliphatic alcohols and water) obtained from Aldrich. The liquid crystal, 4′5CB, was obtained from BDH Chemical (Poole, England). IDA Fabrication on ZnSe. The photolithography was carried out in a cleanroom (class 100). Figure 3 shows a schematic of the procedure used for patterning of the IDA on the ZnSe substrate. The photoresist was cast by spin-coating using a Headway photoresist spinner. The ZnSe substrate was flooded with a continuous film of the photoresist solution and the excess then removed by spinning for 30 s at 4000 rpm. The coated substrate (Figure 3a) was heated at 100 °C for 1 min and then allowed to cool. The mask was placed metal side down directly onto the coated ZnSe window (Figure 3b) and then exposed for 4 s at 365 nm (mercury vapor lamp, 16.5 mW/cm2) in soft contact mode using a Karl Suss MJB3 Mask Aligner (Waterbury, VT). The projection mask (Figure 2a) contained a positive image of the structure to be fabricated so it was necessary to invert the contrast of this first exposure. This image reversal was obtained Analytical Chemistry, Vol. 72, No. 7, April 1, 2000

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as follows.45 In the previously exposed region, the diazoquinone sensitizer in the photoresist was converted to a substituted indenecarboxylic acid, which is soluble in the basic developing solution. The exposed sample was heated for 1 min at 115 °C and allowed to cool (Figure 3c). This induces the thermal decarboxylation of the photoproduct in the exposed regions of the photoresist to the less soluble indene, which is inert to further photochemical or thermal processing. The coated substrate was then flood exposed, without the use of a projection mask, for 12 s (Figure 3d). This converts the previously unexposed photoresist to the soluble photoproducts. The exposed resist was developed in a 5:1 mixture of DI water and 351 basic developer (Figure 3e). The substrates were soaked at ambient temperature in the developing solution with gentle agitation until the developing was complete (∼10 s). The patterned ZnSe substrates were then coated with 50 Å of chromium (as an adhesion promoter) followed by (for the specific data shown here) either 1000 (Nafion flow cell) or 3000 (5CB electrooptics) Å of gold (0.5-1 Å/s) using thermal evaporation at a pressure of ∼1 × 10-6 Torr (Figure 3f). Liftoff of the undesired metal and photoresist was achieved by soaking the metallized substrate in acetone with agitation by sonication (Figure 3g). Electrooptical Cell. For the electrode configuration shown in Figure 2a, the leads were connected as depicted with one set of bands held at electrical ground and the other set used to modulate the cell potential. Electrical connections were made to the IDA using copper wire (26 gauge) leads attached to the lead pads using electrically conductive silver epoxy. After curing at 60 °C for 4 h in an oven, the silver epoxy was insulated by coating with a 5-min epoxy. The exposed Cu wire was fitted with Teflon tubing to provide insulation. A Beckman Specac flow cell (Wilmad Glass) was modified by attaching a BNC coaxial cable connector to the bottom of the front plate of the cell. The Cu leads from the IDA were soldered to the pin and outer casing of the BNC connector. This increased the stability of the leads coming from the IDA and facilitated connection to the external electronics. The ZnSe with the fabricated IDA and a second ZnSe window with drilled holes (for gas flow) were sandwiched in the cell as shown in Figure 4. The outside of the optical cell window was partially masked to allow the IR beam to pass through the IDA region of the substrate only. Before cell assembly and application of material to the ZnSe with fabricated IDA, it was cleaned using a UV/ozone exposure,46 followed by rinsing with hexane, ethanol, water, and isopropyl alcohol, and dried with a stream of N2. Electrooptical Measurements. A Bio-Rad (Cambridge, MA) FTS 6000 spectrometer was used for all IR measurements. The spectrometer was equipped with a KBr beam splitter and a ceramic source. The collected interferograms were single-sided (asymmetric), and a triangular apodization algorithm was used to weight the interferogram. All sample measurements were made using a transmission geometry with the optical cell in the sample compartment. A Wavetek function generator was used to apply ac or dc voltages to the electrode bands of the IDA. For the Nafion study, the 5 wt % solution was spin-coated on the IDA for 30 s at 2000 rpm. The films obtained were ∼1000 Å (45) AZ Product Bulletin, Mechanism and Lithographic Evaluation of Image Reversal in AZ 5214 Photoresist. AZ Division of Hoechst, Somerville, NJ. (46) Jeon, N. L.; Finnie, K.; Branshaw, K.; Nuzzo, R. Langmuir 1997, 13, 33823391.

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Figure 4. Schematic illustration of electrooptical flow cell: (a) expanded view and (b) cross-sectional view.

thick, determined using surface profilometry. The gas-flow region in the electrooptical cell was assembled using a 1-mm-thick Teflon shim, which also served as a seal. The film was hydrated or dehydrated by flowing N2 gas containing various amounts of water vapor through the flow cell. The flow rate was metered by using a controlled 0.5 psi head pressure over ambient. For the Nafion membrane conductivity measurement, an ac voltage of 1.0 V peak-to-peak and 0 V dc offset ((0.5 V) at 35 kHz was applied and the resulting ac current recorded using a multimeter. The FT-IR spectra were collected using a deuterated triglycine sulfate (DTGS) detector at 5 kHz with a 1.2-kHz electronic filter and a spectral resolution of 4 cm-1. The background spectra consisted of 64 scans of the Nafion membrane after equilibration with the humidified N2 gas flowing through the cell. The flow-dependent spectra were acquired over 30 min using a kinetics scheme with a 10-s time resolution. The humidified N2 was switched to dry N2 (to dehydrate the Nafion film) after 2 min of data collection. For the liquid crystal experiments, 5CB (which forms a nematic phase at room temperature) was used as a prototypical example of a liquid-crystalline material. A 1-µL drop of 5CB was placed in the center of the ZnSe with fabricated IDA and then spread over the entire electrode array using a second ZnSe window placed directly on top without using a spacer. This assembly must be carried out in a cleanroom to rigorously exclude dust particles, which generally are of several micrometers in size and thus would prevent sealing the cell against the electrode bands. The sand-

wiched assembly was placed in the electrooptical cell. The cell, when initially sealed, showed hysteresis in the alignment of the 5CB. For the data shown here, the alignment was brought to a steady-state limit by repeated application of 10-15 V. This treatment led to completely reversible polarization behavior. A dc voltage was then applied to the electrode bands and infrared spectra collected using a liquid nitrogen-cooled narrow-band mercury cadmium telluride (MCT) detector. The data were collected with a spectral resolution of 4 cm-1 using a modulation of 20 kHz and a 5-kHz low-pass filter. Single-beam spectra consisted of 1024 co-added scans. A rotatable KRS-5 Au wire grid polarizer was used for polarization-dependent measurements. Several different types of background spectra were used. Singlebeam spectra of the unfilled IDA cell were used to calculate an absolute spectrum of the 5CB. The IDA cell loaded with the liquid crystal and held at 0 V was used to calculate the difference spectra of the 5CB at various applied dc voltages. RESULTS AND DISCUSSION Lithographic patterning methods, while optimized typically for use on Si substrates, can be used to construct materials microstructures and devices on a variety of other materials.47 Chemically sensitive substrate materials do pose special challenges, however. For example, the frequently harsh reagents or gas-phase processes needed to effect the etching of a deposited material sometimes can lead to unacceptably high levels of damage to the substrate. In the present instance, we have achieved the needed patterning easily by using a liftoff process40 that obviates the need for a Au wet-etching process step, many of which would also etch the ZnSe substrate. The contrast inverted resist used here works extremely well for micrometer-scale patterning of Au via liftoff and patterns with a 25-nm feature size have been achieved using imprint lithography with a liftoff process.48 An IDA is shown in Figure 5 at various stages of the liftoff process. The electrode dimensions were verified using profilometry and found to be 1000 Å tall with 15-µm bands separated by 15-µm spaces. Examination by optical microscopy did not show any evidence of substrate damage due either to the resist developer solution or from the solvents used to effect the Au liftoff. The IDA pattern was derived from a simple design for which projection masks are commercially available. We note, for the readers’ interest, that more complex layouts can be patterned at this size scale using custom CAD files and projection masters printed on a commercial high-resolution printer.49 Having constructed the IDA and cell assembly, we tested their functioning using two prototypical measurements. The first of these is water binding by an ionomeric polymer, Nafion. The second example described here is electrooptical measurements on the liquid-crystalline material, 5CB. Each is discussed in turn. Measurement of Conductivity and IR Response of a Nafion Ionomer Membrane during Dehydration. A thin film of Nafion with a mass coverage of ∼1000 Å was applied to the IDA. The polymer was cast to completely cover both the electrodes and the interelectrode spaces. The electrical properties of this assembly were found, as expected, to be quite simple (being (47) Wohltjen, H. Anal. Chem. 1984, 56, 87A-103A. (48) Chou, S. Y.; Krauss, P. R.; Renstrom, P. J. Science 1996, 272, 85-87. (49) Qin, D.; Xia, Y.; Whitesides, G. M. Adv. Mater. 1996, 8, 917-919.

Figure 5. Gold IDA on ZnSe during liftoff. In the top frame, the ZnSe with pattered photoresist and blanket gold coverage is depicted. The middle frame shows the gold partially peeled off after a short period of sonication in acetone. The finished IDA is pictured in the third frame; gold electrodes (yellow) are spaced at 15 µm on the ZnSe substrate (dark).

closely approximated by an RC circuit50). In an RC circuit, with an applied ac voltage at high frequency (v . 1/RC), the current is determined by the bulk resistance. At low frequencies, the (50) Diefender, A. J. Principles of Electronic Instrumentation, 2nd ed.; Saunders: Philadelphia, PA, 1979; Chapter 2.

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Figure 7. Change in the Nafion spectrum upon dehydration. The difference spectra shown correspond to 1-min time intervals during the time of the dry N2 gas flow.

Figure 6. Frequency response curve of hydrated Nafion. The ac voltage was 200 mV peak-to-peak with a 0 V dc offset ((100 mV).

current is determined by the interfacial capacitance. The boundary between these two limiting electrical response regimes is given by the RC time constant. As shown in Figure 6, this limiting behavior was observed for a hydrated Nafion receptor membrane. The RC time constant determined from the current measurement as a function of the frequency of the applied ac voltage is ∼2 × 10-4 s. The IDA flow cell was used to simultaneously interrogate spectroscopic and electrical changes that occur upon dehydration of the Nafion film. The dehydration was accomplished by flowing a dry stream of N2 through the cell containing the hydrated Nafion. In this experiment, the Nafion membrane conductivity was measured by applying an ac voltage of 35 kHz (which is . 1/RC for the system) and monitoring the current as the Nafion was dehydrated. Coincident with these measures, vibrational spectra of the membrane were sequentially measured (as difference spectra) and the changes observed were computed relative to the initial hydrated state. As expected, a significant decrease was seen in the membrane ac conductivity during the time course of this experiment (vide infra). These changes correlate sensibly with the hydration state of the membrane as observed via the spectroscopy. The difference spectra depicted in Figure 7, computed as the ratio of the hydrated Nafion background and the time-dependent spectra, show that the Nafion becomes increasingly dehydrated over time. The shift to lower frequencies in the band centered at 1210 cm-1, containing contributions of the antisymmetric CF2 stretch and overlapping SO3- antisymmetric stretch,51 is the most notable feature found in the difference spectra. The changes seen in the SO3- symmetric stretch, involving a shift to higher frequency and band broadening upon dehydration, are revealed by the difference bands appearing between 1040 and 1080 cm-1.51-54 The broadening (and shift) of the ether band closest (51) Falk, M. In Perfluoronated Ionomer Membranes; Eisenberg, A., Yeager, H. L., Eds.; ACS Symposium Series 180; American Chemical Society: Washington, DC, 1982; Chapter 8. (52) Lowry, S. R.; Mauritz, K. A. J. Am. Chem. Soc. 1980, 102, 4665-4667. (53) Cable, K. M.; Mauritz, K. A.; Moore, R. B. J. Polym. Sci. B 1995, 33, 10651072.

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Figure 8. (a) Response of membrane conductivity, and (b) absorbance of 1206-cm-1 difference band, to the dehydration of the Nafion film.

to the sulfonate group53 (∼970 cm-1) is also readily apparent in the time-dependent spectra. Data demonstrating the changes seen in the membrane conductivity over time are shown in Figure 8a. Again, as expected, the conductivity is observed to decrease over time as water is lost from the membrane.55,56 Comparing the changes seen in the membrane conductivity with a diagnostic spectroscopic indicator, such as the intensity of the 1206-cm-1 difference band (Figure 8b), a qualitative, but not direct, correlation is observed. We see, (54) Nandan, D.; Pushpa, K. K.; Kartha, V. B.; Wahi, P. K.; Iyer, R. M. Indian J. Chem. 1994, 33A, 395-400. (55) Yeo, R. S.; Yeager, H. L. In Modern Aspects of Electrochemistry; Conway, B. E., White, R. E., Bockris, J. O′M., Eds.; Modern Aspects Series of Chemistry 16; Plenum: New York, 1985; Chapter 6, p 480. (56) Anantaraman, A. V.; Gardner, C. L. J. Electroanal. Chem. 1996, 414, 115120.

Figure 9. Schematic depiction of a diffusion model for water loss in the Nafion membrane.

for example, that the conductivity changes more rapidly than does the intensity of the 1206-cm-1 difference band. The magnitude of the latter must largely follow the changes occurring due to the loss of water from the membrane matrix. These same changes must underlie the kinetic behaviors seen in the conductivity data as well. The magnitude of the kinetic differences can be estimated (albeit crudely) by fitting the data shown in Figure 8a and b to a simple exponential rate process and extracting an apparent firstorder rate constant for each. Such fits, while qualitative, suggest that the rate of the conductivity change is at least a factor of 2 faster than is the change seen in spectra. As discussed below, we believe it is the spectroscopic data that best reflect the water content of the membrane. If so, then this implicitly establishes that a more complex relationship must define the hydration dependence of the membrane conductivity. We discuss this issue below, and we consider the more precise nature of rate processes measured in the spectroscopic data. It is possible in principle to fit the water loss from the membrane to a simple diffusion model following the boundary conditions in Figure 9. For this model, one assumes that the water is incorporated into the membrane uniformly across its 1000-Å mass coverage (running from the ZnSe crystal to the N2 gas interface). The dry gas sweeping through the flow cell removes H2O lost from the membrane and so this portion of the rate process is modeled as being irreversible. This analysis also assumes, perhaps rather simplistically, that the apparent diffusivity of the H2O is single-valued for all degrees of hydration. This model would require a somewhat more complex treatment of the data than is the case of the single-exponential approximation. One expects the early time behavior to approximate an exponential decay, however, and there seems little reason to construct an analysis for the data given in this example that incorporates the full treatment of the diffusion characteristics. It is clear, even with a simplified kinetic analysis, that the conductivity-based measurement probes sensitivities that are not linearly correlated with the membrane H2O content. Many possibilities exist for what the structural underpinnings of these might be, but most likely would reflect the fact that the structure of the membrane is microscopically heterogeneous. Electric-Field-Induced Reorientation of a Liquid-Crystalline Material. In a second application, which illustrates the utility of this cell design, a thin film of the nematic liquid crystal 5CB was applied to the IDA and its orientation equilibrated to a steady

Figure 10. Schematic depiction of electric-field-induced transition in 5CB. The molecular axis of the liquid crystal is depicted as a stiff rod. With a potential greater than the threshold, the 5CB aligns with the applied field. Electrode strips represent a pair of electrode bands in the IDA. Direction of incoming IR and polarization vectors is defined.

Figure 11. (a) Infrared spectrum of 5CB for the perpendicular polarization acquired with 5CB on the ZnSe IDA referenced to the empty cell, (b) perpendicular and (c) parallel polarized difference spectra of 5CB with dc electric field applied to the IDA referenced to the 0-V state.

state (as described above). Applying an electric field to the electrodes affects a realignment of the 5CB molecular axes within the IDA. When the electric field applied to the liquid crystal is greater than the threshold for the transition (Eth), a gross reorientation of the molecules occurs along the electric field lines.7,18,57,58 A schematic depiction of this transition is shown in Figure 10. The present cell design allows a broad study to be made of liquid crystal electrooptical and surface effects. We will describe such studies in future reports,59 but now discuss spectra in Figure 11 which illustrate the nature of the information that can be obtained. The absolute spectrum of a film of 5CB on the ZnSe IDA is shown in Figure 11a. The spectrum shows the intensities of bands appearing with components of their transition moments aligned perpendicular to the electrode bands. The spectra obtained for the parallel and perpendicular polarizations are approximately the same. Prominent absorbances are observed in the hydrocarbon region (3200-2800 cm-1) for the various aromatic and alkyl hydrocarbon modes. The CN stretching mode appears at 2226 cm-1, and contributions from in-plane phenyl CC stretching modes are seen at 1606 and 1494 cm-1. After application of a potential large enough to induce the reorientation of the liquid crystal, the change in intensity can be (57) Kaito, A.; Wang, Y. K.; Hsu, S. L. Anal. Chim. Acta 1986, 189, 27-40. (58) Toriumi, H.; Sugisawa, H.; Watanabe, H. Jpn. J. Appl. Phys. 1988, 27, L935L937. (59) Noble-Luginbuhl, A. R.; Blanchard, R. M.; Nuzzo, R. G. J. Am. Chem. Soc. in press.

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measured easily in directions both along and opposed to the electric field lines (shown in Figure 11b and c). To collect the difference spectra for each polarization, the ratio of the singlebeam spectrum collected at each applied voltage to that of a background of the liquid crystal collected at 0 V was taken. Therefore, the changes in intensity observed in these spectra correspond to changes in the projections of specific transition moments due to the applied electric field as projected along a specific optical axis. These spectra are a highly diagnostic indicator of the net anisotropy of both the medium and the motions occurring in response to the applied field. The net optical anisotropy we see at a potential greater than the transition threshold is in fact quite large. At 15.5 V, for example, the CN peak area in the perpendicular difference spectrum is 39% of the magnitude of the absolute spectrum measured at this polarization. The intensity changes seen in the parallel polarization spectrum are significantly smaller and of the opposite sign, thus suggesting a complex, anisotropic reorientation of the liquid crystal director (relative to both the plane of the IDA boundaries and cell windows which serve to confine the 5CB) as it aligns with the applied electric field. The peak areas of the CN stretching absorbance, when plotted against the voltage applied to the electrode band for each polarization (Figure 12), define the threshold voltage where the liquid crystal begins aligning with the applied electric field. The data, when viewed on an expanded scale, suggest motions of the liquid crystal at potentials as low as 1 V, which only begins to orient to a significant degree at applied potentials exceeding 3 V (which corresponds to an electric field of 2000 V/cm in the IDA). The change in the intensities of the difference bands seen with increasing voltage did not saturate in the range of the voltages studied. We note that the behaviors seen here are highly sensitive to the confinement dimensions defined by the Au band (here being 300 nm). We have used this cell design to study the electrooptical dynamics of films that are as thin as 40 nm, a value that is of the order of 25 molecule lengths in size.59

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Figure 12. Plot of the integrated peak intensity of the CN stretching band at 2226 cm-1 as a function of the strength of the applied dc electric field. Data are plotted for both polarization difference spectra referenced to the 0-V state.

Taken together, the model demonstrations suggest a significant utility for this cell design. The means of microfabrication used are quite simple and easily adapted to a general chemical laboratory setting. These protocols should also be easily adaptable to other sensitive substrate materials which might be competitively etched along with the metals used to form the electrodes. We will describe more specific applications of this system in timeresolved studies of electrooptical dynamics in future papers. ACKNOWLEDGMENT This research was supported in part by the Defense Advanced Research Projects Agency and the Office of Naval Research (N00014-96-1-0490), the National Science Foundation (CHE9626871), and the Department of Energy through the Seitz Materials Research Laboratory (DEFG02-91-ER45439). Received for review November 10, 1999. Accepted February 1, 2000. AC991293L