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Defect-Free Polymer Multilayers Prepared via Chemoselective Immobilization Mi-Kyoung Park,† Dong-Chan Lee, Yongye Liang, Gan Lin, and Luping Yu* Department of Chemistry and The James Frank Institute, The UniVersity of Chicago, 929 E. 57th Street, Chicago, Illinois 60637 ReceiVed NoVember 20, 2006. In Final Form: January 11, 2007 In this paper, we investigated electrochemical properties of polymer multilayers on gold substrates using impedance spectroscopy. The multilayer was prepared by chemoselective ligation between aldehyde- and oxyamine-functionalized polymers via a layer-by-layer approach. The impedance spectra in a buffer solution in the absence of redox species revealed the formation of highly impermeable and defect-free films. The dielectric thickness of the polymer film, which is proportional to the reciprocal of capacitance, linearly increased as the number of deposition layer increased. The defect area of the polymer multilayer was obtained using the faradaic impedance with redox species. The surface coverage of eight polymer layers was determined to be 99.99%. Thus, the layer-by-layer deposition via chemoselective ligation offers a new way to prepare a highly insulating and defect-free polymer layer with finely tunable capacitance as a function of the number of deposition layers.
Introduction Modification of metal and semiconductor surfaces with welldefined, defect-free organic thin films is a key step to the realization of nanoelectronics, molecular electronics, and bioelectronics. The highly ordered and defect-free organic thin films, which are impermeable to ions or electrons (i.e., ionic or electric insulating), can provide the ideal surfaces for passivation of various metal surfaces1-3 and resists for soft lithography4 and be integrated as an element of molecular electronics such as gate insulators for nano-FET5 and molecular capacitors.6-8 These films also have been utilized for biomembrane applications as smooth platforms on solid substrates.9-11 In recent years, the self-assembly techniques have been used to fabricate insulating thin organic films. Among the selfassembled monolayers (SAMs), alkanethiol monolayers on metal surfaces (e.g., gold, silver, copper) are the most extensively studied because of their ease of preparation and high degree of order in organization.12-14 It has been reported that long-chain alkanethiol (n > 11) monolayers on mercury and polycrystalline gold † Present address: Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany.
(1) Zamborini, F. P.; Crooks, R. M. Langmuir 1998, 14, 3279-3286. (2) (a) Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 9022. (b) Jennings, G. K.; Munro, J. C.; Yong, T.-H.; Laibinis, P. E. Langmuir 1998, 14, 6130-6139. (3) McGuiness, C. L; Shaporenko, A.; Mars, C. K.; Uppili, S.; Zharnikov, M.; Allara, D. L. J. Am. Chem. Soc. 2006, 128, 5231. (4) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 550. (5) Collet, J.; Vuillaume, D. Appl. Phys. Lett. 1998, 73, 2681. (6) Yu, H.-Z.; Morin, S.; Wayner, D. D. M.; Allongue, P.; Henry de Villeneuve, C. J. Phys. Chem. B 2000, 104, 11157. (7) (a) Rampi, M. A.; Schueller, O. J. A.; Whitesides, G. M. Appl. Phys. Lett. 1998, 72, 1781. (b) Haag, R.; Rampi, M. A.; Holmlin, R. E.; Whitesides, G. M. J. Am. Chem. Soc. 1999, 121, 7895. (8) Budianto, Y.; Aoki, A.; Miyashita, T. Macromolecules 2003, 36, 8761. (9) Steizie, M.; Weissmu¨ller, G.; Sackmann, E. J. Phys. Chem. 1993, 97, 2974. (10) (a) Hillebrandt, H.; Wiegand, G.; Tanaka, M.; Sackmann, E. Langmuir 1999, 15, 8451. (b) Hilebrandt, H.; Tanaka, M.; Sackmann, E. J. Phys. Chem. B 2002, 106, 477. (11) Lingler, S.; Rubinstein, I.; Knoll, W.; Offenha¨usser, A. Langmuir 1997, 13, 7085. (12) (a) Bain, C. D.; Whitesides, G. M. Science 1988, 240, 62. (b) Strong, L.; Whitesides, G. M. Langmuir 1988, 4, 546. (c) Yan, L.; Huck, W. T. S.; Whitesides, G. M. J. Macromol. Sci., Polym. ReV. 2004, C44, 175. (13) Dubois, L. H.; Nuzzo, R. G. Annu. ReV. Phys. Chem. 1992, 43, 437. (14) Ulman, A. Chem. ReV. 1996, 96, 1533.
substrates form defect-free, closely packed, and two-dimensional crystalline structures and exhibit electronic and ionic insulating properties.7,15-17 However, SAMs on common substrates such as evaporated gold show pinholes and domain boundaries due to the steps and irregularities in the substrate, resulting in the formation of defects in the monolayers.18-21 A remedy to reduce the effect of surface morphology of substrates on the quality of the film is the preparation of multilayer organic films that are able to cover the defect sites with repeated deposition. Several techniques have been developed for the preparation of these organic multilayers. The Langmuir-Blodgett (LB) technique has been used to fabricate insulating polymer films for applications in a ultrathin polymer film capacitor8 and as polymer cushions for lipid membranes9,10 However, the LB technique requires special equipment and has limitations with respect to substrate size and topology as well as quality and stability of the film. Recently, layer-by-layer (LbL) self-assembly based on the alternating physisorption of oppositely charged polyelectrolytes was developed by Decher and co-workers.22,23 This technique offers a simple, inexpensive, and versatile way to prepare welldefined multilayers with various charged species including polyelectrolytes, biomaterials, and inorganic nanoparticles.24 Furthermore, the LbL multilayers show self-healing properties, which is presumably linked to the charge overcompensation during the deposition step and makes the method tolerant to the defects.24-26 However, polyelectrolyte multilayers are highly (15) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682. (16) Demoz, A.; Harrison, D. J. Langmuir 1993, 9, 1046. (17) Boubour, E.; Lennox, R. B. Langmuir 2000, 16, 4222. (18) Creager, S. E.; Hockett, L. A.; Rowe, G. K. Langmuir 2002, 8, 854. (19) Guo, L.-H.; Facci, J. S.; McLendon, G.; Moscher, R. Langmuir 1994, 10, 4588. (20) Losic, D.; Shapter, J. G.; Gooding, J. J. Langmuir 2001, 17, 3307. (21) Diao, P.; Jiang, D.; Cui, X.; Gu, D.; Tong, R.; Zhong, B. L. J. Electroanal. Chem. 1999, 464, 61. (22) (a) Decher, G.; Hong, J.-D. Makromol. Chem., Macromol. Symp. 1991, 46, 321. (b) Decher, G.; Hong, J.-D. Ber. Bunsen-Ges. Phys. Chem. 1991, 95, 1430. (23) Decher, G. Science 1997, 277, 1232. (24) Bertrand, P.; Jonas, A.; Laschewsky, A.; Legras, R. Macromol. Rapid Commun. 2000, 21, 319. (25) Kleinfeld, E. R.; Ferguson, G. S. Chem. Mater. 1996, 8, 1575. (26) Elbert, D. L.; Herbert, C. B.; Hubbell, J. A. Langmuir 1999, 15, 5355.
10.1021/la0633785 CCC: $37.00 © 2007 American Chemical Society Published on Web 03/03/2007
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Figure 1. Structures of functional polymers used in this study.
permeable to monovalent or divalent ions of supporting electrolytes because of the presence of charges within the layers.27,28 Therefore, new methods are needed in order to fabricate defect-free and insulating organic films for practical applications. Recently, a chemoselective layer-by-layer deposition approach has been developed.29 This approach is based on a highly chemoselective and kinetically facile reaction between carbonyl compounds (aldehydes and ketones) and alkyloxyamine. It was shown that this approach generates covalently bonded, robust, smooth, and end-functionalizable polymer multilayers. In this work, we investigated ionic insulating properties of polymer multilayers prepared from oxyamine-functinalized and aldehyde-functionalized polymers on the gold electrode using electrochemical impedance spectroscopy. Impedance spectroscopy is an ac technique based on the response of a circuit to an alternating current or voltage as a function of frequency.30 It provides a noninvasive means of characterizing the electrical properties of materials and their interfaces with electronically conducting electrodes. This paper describes electrical properties including capacitance and resistance of polymer multilayers as a function of number of layers. The defect area in the film was also investigated in the presence of a redox couple in the electrolyte solution. The results revealed that the multilayers prepared via LbL chemoselective ligation are highly defect-free and insulating ultrathin films. Experimental Section Materials. Polymers containing aminooxy and formyl side groups (P-ONH2 and P-CHO)29c and 11-mercapto-2-undecanone29a were synthesized as described in previous papers (Figure 1). Octanethiol (Aldrich), potassium hexaferricyanide (Acros), potassium hexaferrocyanide (Acros), and sodium chloride (Fisher) were used as received. A phosphate buffer solution was prepared with Na2HPO4 and NaH2PO4 (0.01 M, pH 7.50). Millipore water (18 MΩ) was used throughout the experiments. Film Preparation. Silicon wafers were cleaned by sonication in acetone, ethyl alcohol, and deionized water for 10 min each, followed by dipping in piranha solution (70:30 H2SO4/H2O2) for 10 min and thoroughly rinsed with deionized water (CAUTION: pirhana solution is a strong oxidizing agent and reacts violently with organic materials). The silicon wafers were then coated with 9 nm of chromium (deposition rate of 0.5 Å/s) followed by 100 nm of gold (deposition rate of 1.0 Å/s) using thermal evaporation under high vacuum ((1 - 3) × 10-6 Torr). (27) Krasemann, L.; Tieke, B. Langmuir 2000, 16, 287. (28) Han, S.; Lindholm-Sethson, B. Electrochim. Acta 1999, 45, 845. (29) (a) Chan, E. W. L.; Yu, L. Langmuir 2002, 18, 311. (b) Chan, E. W. L.; Lee, D.-C.; Ng, M.-K.; Wu, G.; Lee, K. Y. C.; Yu, L. J. Am. Chem. Soc. 2002, 124, 12238. (c) Lee, D.-C.; Chang, B.-J.; Morales, G. M.; Jang, Y. A.; Ng, M.-K.; Heller, S. T.; Yu, L. Macromolecules 2004, 37, 1849. (d) Lee, D.-C.; Chang, B.-J.; Yu, L.; Frey, S. L.; Lee, K. Y. C.; Patchipulusu, S.; Hall, C. Langmuir 2004, 20, 11297. (30) Macdonald, J. R. Impedance Spectroscopy; Wiley: New York, 1897.
Park et al. Freshly prepared gold-evaporated substrates were first cleaned with piranha solution and rinsed with copious amounts of water and ethyl alcohol. The substrates were then dipped into an ethanolic solution of 11-mercapto-2-undecanone and 1-octanethiol (1:1, 10 mM) for 18 h. They were then removed, washed thoroughly in ethanol, and dried in a nitrogen gas stream. Ketone-modified gold substrates were immersed in a methylene chloride solution of the oxyamine-substituted polymer (P-ONH2, 2 mg/mL) for 2 h at room temperature. The substrates were washed with methylene chloride and then dried with nitrogen gas. The substrates then were subsequently immersed in a solution of aldehydesubstituted polymer (P-CHO, 2 mg/mL) for 30 min at room temperature to form the second polymer layer. Repetition of alternative dipping in oxyamine- and aldehyde-derivatized polymers (30 min each) gave the corresponding multilayer films. Instrumentation. Electrochemical experiments were carried out with an Autolab PGSTAT12 potentiostat (Eco Chemie B.V., The Netherlands). A homemade Teflon cell with standard three-electrode systems was used. A Ag/AgCl electrode was used as the reference electrode, a platinum foil was used as the counter electrode, and a gold-coated silicon wafer was used as the working electrode (geometry area of ∼0.76 cm2). For impedance measurements, an FRA module was used to apply a sinusoidal voltage signal of variable frequency to the electrodes. The measurements were conducted in the frequency range of 0.1 Hz to 50 kHz with an amplitude of 10 mV. All experimental data were fitted to an appropriate equivalent circuit using FRA software (Eco Chemie B.V., The Netherlands). Ellipsometric film thickness measurements were made on a Gaertner model L116C single-color optical ellipsometer equipped with a HeNe laser at 632.8 nm and interfaced to a personal computer. Measurements were made at an incident angle of 70°. The real and imaginary indices of refraction were measured at 10 or more locations on each substrate. An average value of these results was used to determine the thickness of resulting films.
Results and Discussion Multilayer Assembly of P-CHO and P-ONH2 on a Gold Electrode. First, a gold electrode was modified with a selfassembled monolayer of a mixture of 11-mercapto-2-undecanone and 1-octanethiol. The ketone-terminated alkanethiol reacts selectively with aminooxy groups on the polymer to form a stable oxime bond.29a 1-Octanethiol was used to provide a necessary space between the ketones on the surface. The thickness of mixed SAMs determined by using ellipsometry was around 8.0 ( 2 Å based on the refractive index of 1.43. The polymer multilayer was then immobilized on the ketone-functionalized gold electrode by alternating deposition of an oxyamine-funcitonalized polymer (P-ONH2) and an aldehyde-functionalized polymer (P-CHO) at room temperature. Here, an n-layer denotes the number of deposited layers starting with n ) 1, where P-ONH2 was deposited. As shown in the previous papers, the formation of multilayers is driven by oxime bond formation between oxyamine and aldehyde or ketone.29 The ellipsometric thickness showed a linear increase in the thickness after three cycles of deposition (Figure 2). The calculated thickness of each layer from the slope was 29.9 ( 9 Å. This value is consistent with the theoretical thickness of about 30 Å for the polymers estimated by molecular mechanics calculations with an assumption of extended side groups.29c Surface topographies of the polymer multilayers were investigated by using atomic force microscopy (AFM). AFM images revealed a rough grained structure (Rrms ) 1.88 nm) for the bare gold surface. After the deposition of eight layers of the polymers, the films patched up the grains and generated a very smooth surface (Rrms ) 0.742 nm; see Supporting Information). Impedance Spectroscopy. 1. In the Absence of Redox Species. Electrochemical impedance spectra of the mixed SAMs and
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Figure 2. Ellipsometric thickness of multilayers of P-ONH2 and P-CHO on a ketone SAM modified gold substrate. The odd layer numbers correspond to P-ONH2 deposition, and the even layer numbers correspond to P-CHO adsorption.
polymer multilayer deposited on gold electrodes were measured at 0.0 V (vs Ag/AgCl) with an amplitude of 10 mV in a phosphate buffer solution (10 mM, pH 7.5). Impedance responses of the films were plotted in terms of the absolute value of complex impedance, |Z|, and the phase shift, φ, as a function of frequency (Bode plots). Impedance spectra of the multilayers show interesting features: (1) At 1 Hz < f 3.
number of deposited layers. The plot of dn/ versus the number of layers shows a linear increase for more than three layers of deposition, which is consistent with the ellipsometric thickness data. The dielectric constant , estimated from the slope of linear part of the dn/ vs the number of layers and marked with a dotted line, was found to be 3.40. This is a reasonable value compared with that of poly(p-phenylene vinylene)s (PPVs), ≈ 3-4 from the literature.37 This result clearly shows that the capacitance of the multilayer films can be finely tuned by changing the number of deposited layers. Furthermore, it indicates the possibility of building an organic capacitor with nanometer thickness and without current leakage after a deposition of as small as two layers of polymer. 2. In the Presence of Redox Species. In order to obtain quantitative information on insulating properties of polymer multilayers, electrochemical impedance spectra were determined in the presence of a pair of redox molecules, [Fe(CN)6]3-/4-, in buffer solution. The data were collected at +0.2 V versus Ag/ AgCl, which is the formal potential of the redox couple. The Nyquist plots of the impedance spectra of bare gold electrode (inset) and one, three, seven, and eight layers of multilayers are shown in Figure 6. The Nyquist plot of the bare gold electrode represents a combination of a slightly depressed semicircle at high frequencies and a straight line oriented at 45° with respect to each axis at low frequencies. The semicircle portion corresponds to the electron-transfer limited process, and the linear part (37) Esteghamatian, M.; Popovic, Z. D.; Xu, G. J. Phys. Chem. 1996, 100, 13716.
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charge-transfer resistance can be converted into the exchange current I0 following
Rct )
RT nFI0
(1)
and the apparent heterogeneous electron-transfer rate constant, kapp, can be obtained using39,40
I0 ) nFAkappC
Figure 6. Impedance spectra for one layer (1), three layers (O), seven layers (b), and eight layers (0) of polymer deposited on the gold electrode obtained in solutions containing 1 mM each of Fe(CN)63-/4- with 0.1 M NaCl buffered with 10 mM Na2HPO4 (pH 7.5). All the spectra were recorded at +0.2 V vs Ag/AgCl. The impedance spectrum of bare gold is shown in the inset. Table 2. Fitting Parameters Obtained for One Layer, Three Layers, Seven Layers, and Eight Layers (Equivalent Circuit III) of Polymer Deposited on the Gold Substrate in Solutions Containing 1 mM Each of Fe(CN)63-/4- with 0.1 M NaCl Buffered with 10 mM Na2HPO4 (pH 7.5)a Rs , Ω one layer three layers seven layers eight layers
76.0(0.89) 70.7(0.78) 75.8(2.12) 76.8(1.16)
Rct, MΩ cm2 CCPE, µF/cm2 0.199(1.32) 0.295(1.05) 0.535(1.99) 0.710(0.824)
1.35(1.82) 0.853(1.40) 0.176(2.93) 0.153(1.30)
R 0.976(0.202) 0.974(0.151) 0.978(0.291) 0.976(0.133)
a
The data were collected at the formal potential of the redox couple (+0.2 V vs Ag/AgCl). The percent errors from the fit for each element are given in parentheses.
corresponds to the diffusion process. The diameter of the semicircle corresponds to the electron-transfer resistance at the electrode surface.30 This system can be described by the Randles circuit (III in Figure 4), which consists of electrolyte resistance Rs in series with a parallel combination of the Faradaic impedance Zf and the constant phase element. The faradaic impedance Zf is represented by a charge-transfer resistance (Rct) in series with the Warburg impedance (Zw), which describes the diffusion of the species to the surface.38 A charge-transfer resistance Rct ) 104.9 Ω cm2 and a Warburg impedance of 172.3 Ω s-1/2 cm2, which yields a diffusion coefficient of 0.19 × 10-6 cm2 s-1, for a bare gold electrode were obtained by fitting the impedance spectrum to the Randles circuit. After the deposition of polymer multilayers on the gold electrode the linear mass transfer region at low frequency is not observed, suggesting that the layer on the gold electrode is forming a barrier that prevents direct approach of the redox molecules to the electrode surface. Also, the radius of semicircles in the Nyquist plot increased as the number of deposited layers increased, which indicates that the charge-transfer resistance increased. The results of fitted data for multilayers using a Randles circuit (III in Figure 4, excluding the Warburg impedance elements) are summarized in Table 2. The Rct value for one layer is 0.199 × 106 Ω cm2 and increases to 0.71 × 106 Ω cm2 for eight layers of polymer on the gold electrode. The (38) Protsalio, L. V.; Fawcett, W. R.; Russell, D.; Meyer. R. L. Langmuir 2002, 18, 9342. (39) Bandyopadhyay, K.; Vijayamohanan, K.; Shekhawat, G. S.; Gupta, R. P. J. Eletroanal. Chem. 1998, 447, 11. (40) Pardo-Yissar, V.; Katz, E.; Lioubashevski, O.; Willner, I. Langmuir 2001, 17, 1110.
(2)
where R is the gas constant, T is the temperature (K), F is Faraday’s constant, n is the number of transferred electrons per molecules of the redox probe (n ) 1 in this case), A is the electrode area (cm2), and C is the bulk concentration of the redox probe (mol cm-3). The kapp values were found to be 1.33 × 10-6, 8.99 × 10-7, 4.96 × 10-7, and 3.74 × 10-7 cm s-1 for one layer, three layers, seven layers, and eight layers of polymer, respectively. The apparent rate constant for eight layers is comparable with that of SAMs of hexadecanethiol (HDT) on an atomically flat gold substrate and is 10 times lower when compared to SAMs of HDT on an evaporated gold on glass substrate that had similar roughness with our case.20 Typical polyelectrolyte multilayers have a charge-transfer resistance of a few thousand Ω cm2 (on the order of 10-5 cm s-1) because of the presence of charges within the layers.40 Previously, the post-treatment of heat-induced cross-linking via formation of amide bonds in the polyelectrolytes multilayers has been performed in order to produce a passivating and stable film. An Rct of 0.1 × 106 Ω cm2 (kapp ) 5.3 × 10-7 cm s-1) was reported after heating the films (nine bilayers) at 215 °C,41 which is lower than that of the chemoselective multilayer in the current investigation. These results clearly showed that the chemoselective layer-by-layer approach is a method to prepare an ultrathin, defect-free, and ionic insulating film even on common substrates such as evaporated gold. Defect Area of Polymer Multilayers. The polymer coverage on the gold electrode (θ) was obtained using a theory that has been developed by Finklea et al., based on the impedance response of the monolayer-covered electrode treated as a microarray electrode.42 According to their study, both the real and imaginary components of the faradaic impedance are related to ω-1/2 (ω ) 2π f ) for a total pinhole area fraction less than 0.1. At high frequencies, the individual microelectrode has a nearly isolated diffusion profile and the expression for the faradaic impedance is
Zf′(ω) )
Rct σ σ + + 1 - θ xω (1 - θ)xω
(3)
and at low frequencies, where all the pinholes show overlapping diffusion profiles, the expression becomes
Zf′(ω) )
Rct σrax0.72D σ + + 1 - θ xω 1-θ
(4)
where
σ)
x2(RT/F) FACxD
(5)
ra is the radius of the pore, A is the electrode geometrical area, (41) Harris, J. J.; DeRose, P. M.; Bruening, M. L. J. Am. Chem. Soc. 1999, 121, 1978. (42) Finklea, H. O.; Snider, D. A.; Fedyk, J.; Sabatani, E.; Gafni, Y.; Rubinstein, I. Langmuir 1993, 9, 3660.
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coverage on the gold electrode θ (%) was obtained using the following equation,37
θ (% ) ) (1 - σ/β) × 100
Figure 7. Real part of the faradaic impedance plotted as a function of ω-1/2, where the solid lines are slopes in the high-frequency region: one layer (9), three layers (4), seven layers (b), and eight layers (0) of polymer deposited on gold electrode modified with the ketone thiol. The solution resistance and interfacial capacitance have been subtracted from the impedance data shown in Table 2.
C is the concentration of redox molecules, and D is the diffusion coefficient of the redox molecules. The faradaic impedance was obtained by subtracting the solution resistance (Rs in Table 2) and the double-layer capacitance (CCPE in Table 2) from the total impedance. It is worth mentioning that the solution resistance in series with the faradaic impedance should be subtracted from the impedance data and the doublelayer capacitance that is parallel with the faradaic impedance should be subtracted from the admittance data. The solution resistance for the conversion is obtained from impedance data at the highest frequencies. The mathematical removal of the interfacial capacitance from the total impedance introduced some error into the faradaic impedance. However, this error introduced by subtraction of the interfacial capacitance does not significantly affect the defect parameters. Plots of the real faradaic impedance Zf′ as a function of ω-1/2 for one, three, seven, and eight layers on the gold electrode are shown in Figure 7. The faradaic impedance plots show features similar to those for an ideal microelectrode. They exhibit two linear domains at high and low frequencies. The deviations from the ideal microarray behavior may be due to the nonuniform distribution of pinholes in the polymer multilayer. The polymer
where β is the slope of the Zf′ versus ω-1/2 plot in the highfrequency region. The value of σ was obtained experimentally with a bare electrode in the same solution, which was 172.6 Ω s-1/2 cm2. The coverage for polymer films of one, three, seven, and eight layers was found to be 99.96%, 99.97%, 99.98%, and 99.99%, respectively. It is noted that the surface coverage for the low number of deposition layers (up to four layers) was varied within an error range depending on the quality of the evaporated gold substrates. However, after deposition of six to eight polymer layers, the surface coverage was always found to be ∼99.99%. These results clearly show that polymer multilayers prepared via chemoselective ligation form highly insulating, defect-free polymer films on the gold electrode.
Conclusions The present work demonstrates that polymer multilayers prepared via chemoselective ligation are highly insulating and defect-free. Impedance spectroscopy of polymer multilayers shows that the ultrathin polymer films behave as an ideal dielectric material and can be described by the Helmholtz ideal capacitor model. The frequency exponent is R > 0.99 after deposition of the second polymer layer, suggesting a very smooth surface of polymer films. The dielectric thickness d/ shows a linear increase for more than three layers. The surface coverage of eight polymer layers was determined to be 99.99% using impedance spectroscopy in the presence of a redox couple. Acknowledgment. We gratefully acknowledge the financial supports of the National Science Foundation and the NSF MRSEC program at the University of Chicago and AFOSR. UC-Argonne Nanoscience Consortium provided partial support of this research. Supporting Information Available: AFM images of the bare gold substrate and after deposition of eight layers of P-CHO/P-NH2 and cyclic voltammograms of a bare gold substrate and six layers of polymer deposited on the gold substrate in solutions containing 1 mM each of Fe(CN)63-/4- with 0.1 M NaCl buffered with 10 mM Na2HPO4 (pH 7.5). This material is available free of charge via the Internet at http://pubs.acs.org. LA0633785
