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Langmuir 1998, 14, 6158-6166
Microfabrication of Interdigitated Polyaniline/ Polymethylene Patterns on a Gold Surface Yu-Tai Tao,* Kannaiyan Pandian, and Wen-Chung Lee Institute of Chemistry, Academia Sinica, Taipei, Taiwan, Republic of China Received June 26, 1998 Micrometer scale patterns of polymethylene film was prepared by selective formation of polymethylene on a gold surface patterned with an organothiol monolayer as the resist. The monolayer effectively retarded the gold-catalyzed polymerization reaction from diazomethane. The polymethylene film, in the thickness range of 300-400 Å, in turn serves as a stable resist in preventing electrochemical polymerization of polyaniline on the covered area. Selective deposition was thus achieved to give interdigitated patterns of conducting and insulating polymer films.
Introduction Self-assembled monolayers (SAMs) formed from longchain amphiphiles have been the subject of much study in recent years. In many cases, these films provide an interfacial region with well-defined composition, structure, and thickness,1-3 which can serve as an ideal model system in understanding various interfacial phenomena, such as wetting,4 adhesion,5 corrosion,6 catalysis,7 and so on. Great potential was also envisaged with these readily controlled and readily tailored molecular assemblies.8-10 In particular, an alkanethiol monolayer on gold has been thoroughly studied, and many applications have been suggested based on the robust, closely packed, as well as ordered structure of such a monolayer. One application is to use it as a nanoresist to protect the underlying gold surface from oxidative etching11 or block reactions that can be otherwise initiated from the gold surface.12 Furthermore, various techniques were developed to form SAM film patterns on solid surfaces, where, part of the surface is covered with a monolayer and the rest is either uncovered or covered by a second type of monolayer. Patterned SAM film on a surface allows a reaction to proceed selectively on specific areas. This patterning offers great potential to fabricate thin-film electronic devices of diminishing scale. Much progress has been realized in the past several years in preparing patterned monolayers of micron to submicron resolutions.13-18 Among the several techniques that have been utilized for patterning SAMs of organic (1) Ulman, A. Chem. Rev. 1996, 96, 1533 and references therein. (2) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (3) (a) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (b) Bain, C. D.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1989, 28, 506. (4) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87. (5) Lopez, G. P.; Albers, M. W.; Schreiber, S. L.; Carroll, R. N.; Peralta, E.; Whitesides, G. M. J. Am. Chem. Soc. 1993, 115, 5877. (6) (a) Abbott, N.; Kumar, A.; Whitesides, G. M. Chem. Mater. 1994, 6, 596. (b) Haneda, R.; Nishihara, H.; Aramaki, K. J. Electrochem. Soc. 1997, 144, 1215. (7) Ferrence, G. M.; Henderson, J. I.; Kurth, D. G.; Morgenstern, D. A.; Bein, T.; Kubiak, C. P. Langmuir 1996, 12, 3075. (8) Kepley, L. J.; Crooks, R. M.; Ricco, A. J. Anal. Chem. 1992, 64, 3191. (9) Li, Y.; Chailapakul, O.; Crooks, R. M. J. Vac. Sci. Technol. B, 1995, 13, 1300. (10) Kumar, A.; Whitesides, G. M. Science 1994, 263, 60. (11) Xia, Y.; Kim, X. M.; Whitesides, G. M. Chem. Mater. 1995, 7, 2332. (12) Kumar, A.; Biebuyck, H. A.; Abbott, N. L.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 9188.
thiols on gold, the microcontact printing (µCP) technique18 is simple and the most versatile one to fabricate patterns with submicrometer dimensions. In this method, a stamp was prepared by molding a silicone elastomer against a photolithographically or mechanically patterned master. The monolayer pattern is transferred from the stamp, which has been inked with alkanethiol, onto gold substrate through direct physical contact. Many efforts have been made to use patterned SAMs to generate patterned conducting polymer thin films, with an ultimate interest to fabricate microelectronic devices.19 For example, the approach by Rozsnyai and Wrighton20 involved a photochemically patterned monolayer system that selectively deposited conducting polymers [e.g., polyaniline, polypyrrole, and poly(3-methylthiophene)] on the azido group-terminated monolayer surface. In this case, the monolayer served to promote the polymer deposition. In another approach by Gorman et al.,21 a saturated alkyl monolayer served to block the electron transfer from species in solution to the electrode surface and thus inhibited the electrochemical deposition of filmforming material. Thus, patterned polypyrrole film was prepared on an unmodified area on a gold surface patterned with hexadecanethiol monolayer. On the other hand, later study revealed that the selective growth of polyaniline or polypyrrole by the electrooxidative polymerization was hard to achieve on a surface patterned with (13) Abbott, N. L.; Folkers, J. P.; Whitesides, G. M. Science 1992, 257, 1380. (14) Tiberio, R. C.; Craihead, H. G.; Lercel, M.; Lau, T.; Sheen, C. W.; Allara, D. L. Appl. Phys. Lett. 1993, 62, 476. (15) Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir, 1994, 10, 1498. (16) (a) Dulcey, C. S.; Georger, J. H.; Krauthamer, V.; Stenger, D. A.; Fare, T. L.; Calvert, J. M. Science 1991, 252, 551. (b) Calvert, J. M.; Georger, J. H.; Peckerar, M. C.; Pehrsson, P. E.; Schnur, J. M.; Schoen, P. E. Thin Solid Film 1992, 211, 359. (17) (a) Huang, J. Y.; Dahlgren, D. A.; Hemminger, J. C. Langmuir, 1994, 10, 626. (b) Tarlov, M. J.; Burgess, D. R. F., Jr.; Gillen, G. J. Am. Chem. Soc. 1993, 115, 5305. (18) (a) Xia, Y.; Whitesides, G. M. Langmuir 1997, 13, 2059. (b) Kumar, A.; Whitesides, G. M. Appl. Phys. Lett. 1993, 63, 2002. (c) Wilbur, J. L.; Kumar, A.; Kim, E.; Whitesides, G. M. Adv. Mater. 1994, 6, 600. (d) Chou, S. Y.; Krauss, P. R.; Renstrom, P. J. Science 1990, 272, 85. (19) (a) Paul, E. W.; Ricco, A. J.; Wrighton, M. S. J. Phys. Chem. 1985, 89, 9, 1441. (b) Ofer, D.; Crooks, R. M.; Wrighton, M. S. J. Am. Chem. Soc. 1990, 112, 7869. (20) (a) Rozsnyai, L. F.; Wrighton, M. S. J. Am. Chem. Soc. 1994, 116, 5993. (b) Rozsnyai, L. F.; Wrighton, M. S. Langmuir 1995, 11, 3913. (c) Rozsnyai, L. F.; Wrighton, M. S. Chem. Mater. 1996, 8, 309. (21) Gorman, C. B.; Biebuyck, H. A.; Whitesides, G. M. Chem. Mater. 1995, 7, 526.
S0743-7463(98)00767-7 CCC: $15.00 © 1998 American Chemical Society Published on Web 10/13/1998
Microfabrication of Polyaniline/Polymethylene Patterns
simple alkanethiol monolayer, presumably due to the pinholes or defects within a monolayer, as evidenced from cyclic voltammetric studies.22 Instead, it was reported that a monolayer-exposing amino functional group, such as 12-aminododecanethiol, can effectively block the growth of polyaniline. Electrostatic repulsion between the charged termini and the charged monomer species in the solution under the experimental condition was suggested to contribute to the effective blockage.23 The results just presented raise a general question as to the versatility of a thiol monolayer as a resist under various conditions. In addition to the issue of structural defects in the monolayer, the thiolate headgroup in a monolayer is known to undergo exchange with thiols in contact solution,24 to migrate laterally on the surface,25 or to be oxidized, either chemically,11 photochemically,17 or electrochemically,26 to give a weakly adsorbed species. All of these can limit the conditions available in the fabrication of the pattern. Recently we reported that ultrathin films of nonpolar polymethylene, -(CH2)x-, can be prepared on a gold surface by exposing the gold substrate to a diazomethane solution.27 By controlling the concentration and exposure time, films with thickness ranging from several nanometers to a hundred nanometers can be readily prepared. The reaction appears to involve surface-catalyzed decomposition of CH2N2 at gold defect sites to produce methylidene adsorbate and grow into polymer via a free radical propagation process. Although the growth starts from defects and boundaries of the surface, the polymer chains spread to form a continuous film after certain thickness (>20 nm). This continuous film offers a controlled yet much thicker and inert hydrocarbon shield to the gold surface than a monolayer could afford. Here we report that patterns of nonpolar and insulating polymethylene film could be prepared on a gold surface masked with patterns of thiol monolayers generated by µCP. These films provide an insulating layer that is stable to extensive electrochemical cycling and chemical etching conditions that are typically used to remove a thiolate monolayer. This offers another organic resist system that works in the submicron region. These insulating polymethylene film patterns in turn can serve as a resist to allow fabrication of patterns of conducting polyaniline film intercalated with the insulating polymethylene pattern. Experimental Section Materials. Gold (99.99% purity) and chromium (99.99%) metals were obtained from Johnson Matthey Company (Ward Hill, MA). One-side polished silicon wafers were purchased from Semiconductor Processing (Boston, MA). The stamp used in contact printing was fabricated with poly(dimethylsiloxane) (PDMS) as described in the literature.18 Dial (N-methyl-Nnitroso-p-toluene sulfonamide, obtained from Aldrich Chemical, St. Paul, MN) was used for the preparation of diazomethane. The concentration of diazomethane was determined by titrating with benzoic acid. N-Hexadecanethiol was obtained from Aldrich. N-Eicosanethiol was prepared from N-eicosanol by a standard method with bromide. N-Hexadecyloxybiphenylmethanethiol was synthesized as described before.28 The thiol(22) Sayre, C. N., Collard, D. M. Langmuir 1997, 13, 714. (23) (a) Nishizawa, M.; Miwa, Y.; Matsue, T.; Uchida, I. J. Electrochem. Soc. 1993, 140, 1650. (b) Nishizawa, M.; Shibuya, M.; Sawaguchi, T.; Matsue, T.; Uchida, I. J. Phys. Chem. 1991, 95, 9042. (24) (a) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am Chem. Soc. 1990, 112, 4301. (b) Collard, D. M.; Fox, M. A. Langmuir, 1991, 7, 1192. (25) Sayre, C. N.; Collard, D. M. J. Mater. Res. 1997, 7, 909. (26) Widrig, C. A.; Chung, C. Porter, M. D. J. Electroanal. Chem. 1991, 310, 335. (27) Seshadri, K.; Atre, S. V.; Tao, Y. T.; Lee, M. T.; Allara, D. L. J. Am. Chem. Soc. 1997, 119, 4698.
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Figure 1. Ellipsometric thickness of polymethylene during film growth as a function of immersion time for different monolayer-covered surfaces, in comparison with that for a bare gold surface. terminated aniline dimer derivative, 4-(5-mercaptopentylamino) diphenylamine, was prepared from 4-aminodiphenylamine.29 Aniline (Merck, Germany) was distilled over zinc dust to eliminate the oxidized impurities and stored under nitrogen atmosphere in the dark prior to use. All other chemicals were obtained as received from commercial sources. All solutions were freshly prepared before starting the experiments. Ellipsometry. Ellipsometry measurements were done at a single wavelength of 632.8 nm with a Rudolph AutoEL-II Ellipsometer (Rudolph Instruments, Fairfield, NJ) at 70° angle of incidence. An index of refraction of 1.47 was assumed for the polymethylene in calculating the thickness. Scanning Electron Microscopy. Scanning electron microscopy (SEM) studies were performed with a JEOL scanning electron microscope (model JSM-5400, JEOL Ltd., Tokyo, Japan). An accelerating voltage of 2.0 kV was used for all images. Atomic Force Microscopy. The atomic force microscopy (AFM) studies were performed with an Autoprobe AFM (Park Scientific Instrument) equipped with a 10 µm scan head. For the topographic studies, the images were taken in contact mode. All images were acquired in air with a Si3N4 cantilever with a radius of 0.6 µm. Reflectance Infrared (IR) Spectroscopy. The absorption spectra were recorded in single reflection mode with a grazing angle of 86°, using Digilab FTS60 Fourier transform infrared (FTIR) spectrometer (Bio-Rad, Cambridge, MA), which was equipped with an MCT detector. Spectra were obtained with p-polarized light. Substrate Preparation. The gold substrates were prepared by depositing ∼50 Å chromium on a 2-in. silicon wafer, followed by depositing 2000 Å of gold. The deposition was carried out at a pressure of ∼3 × 10-7 Torr using an Ulvac cryopumped evaporator at a rate of ∼10 Å/s. Patterning. A 1 mM solution of thiol in ethanol was used to prepare the ink solution. The stamp, prepared against a lithographed silicon wafer master according to published procedure,18 was inked with thiol solution with a cotton bud. The (28) Chang, S. C.; Chao, I.; Tao, Y. T.; J. Am. Chem. Soc. 1994, 116, 6792. (29) The compound was prepared by selective alkylation at the primary amino group of 4-aminodiphenylamine with 5-bromopentylthioacetate. The thioester group obtained was reduced by lithium aluminum hydride to yield the thiol derivative. The product was purified by column chromatography and gave a mp of 67-68 °C; m/z 286(M+), 1H NMR(CDCl ): 7.10 (dd, 2H), 6.92 (d, 2H), 6.75 (d, 2H), 6.69 (t, 1H), 3 5.40 (br s, 1H), 3.11 (t, 2H), 2.55 (q, 2H), 1.65 (m, 4H, 1.54 (m, 2H), 1.34 (t, 1H).
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Figure 2. AFM micrographs of monolayer surfaces after exposure to diazomethane for 15 min on (a) monolayer of biphenyl derivative, III, (b) monolayer of aniline dimer, IV; and (c) monolayer of hexadecanethiol, I. For comparison, the film grown on a bare gold surface for 1.5 min (∼300 Å) is shown in (d). excess thiol and solvent were blown dry with a gentle flow of nitrogen. After the gold substrate was brought in contact with the stamp for 1 min, it was separated and placed immediately in a freshly prepared ethereal solution of diazomethane for an appropriate amount of time. The experimental details regarding the polymer growth, crystallinity, and stability were described previously.27 Electrochemical Measurement. The electropolymerization studies were performed with a BAS 100B electrochemical analyzer system (Bioanalytical Systems, Inc., West Lafayette, IN). A three-electrode system with single-compartment cell was used. The arrangement and design of working electrode are similar to that reported by Porter.26 The gold-coated substrate was used as working electrode with an exposed area of 0.66 cm2. (The substrate was connected to the cell with an elastic “O”ring.) For reflection-absorption infrared (RAIR) studies of electropolymerized film, a 2-in. gold-coated silicon wafer with an exposed area of 11.3 cm2 was used as the working electrode. A spiral platinum wire and an Ag/AgCl (3 M NaCl) electrode were used as the counter and reference electrodes, respectively. Patterned Polyaniline Growth. The thiol monolayerpolymethylene patterned substrate was placed in a singlecompartment cell that contains 0.1 M aniline in 0.85 M H2SO4 and 0.25 M NaHSO4 solution, and the solution was deoxygenated by purging with pure argon gas for 10 min prior to the cycling. Electropolymerization was carried out by potential cycling between -0.2 and 0.85 V versus Ag/AgCl. The cycling was terminated at 0.4 V versus Ag/AgCl, so that the film is at the reduced state (emeraldine form).30 All solutions were deoxygenated by purging with pure argon gas for 10 min prior to the cycling. Then, the substrate was washed with ultrapure water and dried with a slow stream of nitrogen.
was prepared on a gold surface that is patterned with a monolayer assembly by the µCP method. The effect of a closely packed SAM in blocking the growth of polymethylene chain was examined first. Four SAM systems were compared for the effectiveness as the nanoresist; namely, n-hexadecanethiol (I, C16H33SH), n-eicosanethiol (II, C20H41SH), 4-hexadecyloxyphenyl-4′-benzylthiol (III, C16H33OC6H4C6H4CH2SH),28 and 4-(5-mercaptopentylamino)diphenylamine (IV, C6H4NHC6H4NH(CH2)5SH).31
Results and Discussion
(30) Mattoso, L. H. C.; Faria, R. M.; Bulhoes, L. O. C.; MacDiarmid, A. G. Polymer 1994, 35, 5104. (31) A detailed structural study of this dimer aniline monolayer will be published elsewhere.
Preparation of Patterned Polymethylene Using Patterned SAM as Resist. The polymethylene pattern
Compounds I and II were compared for the effect of chain length. Compound III was included to assess the effect of additional biphenyl moiety in terms of the blocking efficiency. Compound IV was examined for the effect of an aniline dimer moiety in blocking the polymethylene growth, whereas the same group, being identified as a major intermediate in the chain growth during electropo-
Microfabrication of Polyaniline/Polymethylene Patterns
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Figure 3. AFM micrographs of patterned aniline dimer (IV) monolayer surface after etching by CN-/O2 for 30 min: (a) top view; (b) 3-dimensional view.
lymerization of aniline,32 can serve as the initiation site of aniline polymerization (vide infra). The substrates, either a bare gold surface or a gold surface covered with an SAM film, were dipped into diazomethane solutions of various concentrations for different lengths of time. The growth of polymethylene was followed by RAIR and ellipsometry measurements. It was found that all four monolayers greatly retarded the growth rate of polymethylene on the gold surface. Nevertheless, some differences in the blocking effect did appear upon longer exposure time. Figure 1 shows the representative growth curves, as determined by ellipsometry, on different surfaces at a concentration of 0.02 M for diazomethane at 0 °C. It can be seen that a rapid growth on bare gold occurred in the first several minutes, reaching ∼ 350 Å in 3 min and slowed with time and reaches about 600 Å after 25 min of immersion. Under the same condition, a negligible amount of polymethylene (10 min), a substantial growth (∼50 Å) was found on the n-hexadecanethiol-covered surface. More than 200 Å of polymethylene were accumulated eventually after 25 min. In contrast, on the n-eicosanethiol monolayer-covered surface,
