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Toward Controlled Area Electrode Assemblies: Selective Blocking of Gold Electrode Defects with Polymethylene Nanocrystals Kannan Seshadri,† Ann M. Wilson,‡ Anthony Guiseppi-Elie,*,‡ and David L. Allara*,†,§ Departments of Chemistry and Materials Science, Pennsylvania State University, University Park, Pennsylvania 16802, and Research and Development Department, ABTECH Scientific, Inc., Yardley, Pennsylvania 19067 Received January 13, 1998. In Final Form: December 2, 1998 Using electrochemical and chemical probes, we demonstrate that thin films of crystalline polymethylene (PM), formed via solution decomposition of diazomethane on gold surfaces, exist in the form of a heterogeneous distribution with microscopically small pores at low average PM coverages. For evaporated gold substrates, as the PM loading increases above ∼0.8 µg‚cm-2, a mass density equivalent to an ∼7 nm thick, fully dense planar film, a transition from a micropore to a blocked electrode occurs. This microstructural picture is confirmed by the ability to backfill open areas with alkanethiolates and electropolymerized aniline. Parallel experiments on sputter-deposited gold films show that the blocking threshold occurs at much lower average PM coverages and alkanethiolate chemisorption data suggest that the open pores are approaching the sizes of individual molecules. These results can be rationalized by a more uniform nucleation of PM nanocrystals across sputter-deposited relative to evaporated surfaces. This ability to regulate the conformal deposition of an inert, low dielectric material at the nanometer scale on gold surfaces offers a new way to engineer electrodes with controlled, micropore dielectric barrier structures and “quasi-two-dimensional” nanocomposite films.
Introduction The development of nanometer-scale electrode structures is a rapidly growing field with applications ranging from biology1 to electronic devices.2 In electrochemistry, there is wide interest in applications such as fundamental studies of redox reactions and chemical sensing.3-10 Microscopically structured electrodes are often prepared by partial blocking of electrode surfaces with controlled porosity barrier films including polymers,11 self-assembled monolayers (SAMs),12-16 most notably alkanethiolates on gold,17 and microporous membranes.18-20 These approaches, however, have not yet provided the desired † ‡
Department of Chemistry, Pennsylvania State University. Research and Development Department, ABTECH Scientific,
Inc. § Department of Materials Science, Pennsylvania State University.
(1) Nanofabrication and Biosystems: Integrating Materials Science, Engineering and Biology; Hoch, H. C., Jelinski, L. W., Craighead, H. G., Eds.; Cambridge University Press: Cambridge, UK, 1996. (2) Bard, A. J. In Integrated Chemical Systems: A Chemical Approach to Nanotechnology; John Wiley and Sons: New York, 1994. (3) Microelectrodes: Theory and Applications NATO ASI Ser. E: Applied Sciences - Vol. 197; Montenegro, M. I., Queiro´s, Daschbach, J. L., Eds.; Kluwer: Dordrecht, The Netherlands, 1991. (4) Pletcher, D. In Microelectrodes: Theory and Applications NATO ASI Ser. E: Applied Sciences, Vol. 197; Montenegro, M. I., Queiro´s, Daschbach, J. L., Eds.; Kluwer: Dordrecht, The Netherlands, 1991; pp 3-16. (5) Reller, H.; Kirowa-Eisner, E.; Gileadi, E. J. Electroanal. Chem. 1982, 138, 65. (6) Wightman, R. M. Anal. Chem. 1981, 53, 1125A. (7) Aoki, K., Tanaka, M. J. Electroanal. Chem. 1989, 266, 11. (8) Baranski, A. S. Anal. Chem. 1987, 59, 662. (9) Wightman, R. M.; Wipf, D. O. In Electroanalytical Chemistry: a Series of Advances; Bard, A. J., Ed.; Marcel Dekker: New York, 1989, Vol. 15, p 267. (10) Rolison, D. R. In Ultramicroelectrodes; Fleischmann, M., Pons, S., Rolison, D. R., Schmidt, P. P., Eds.; Datatech Systems: Morgantown, NC, 1987. (11) Imisides, M. D.; Wallace, G. G. Electroanalysis, 1995, 7, 346349.
control and stability for many applications. Microporous membranes tend to be relatively thick with slow diffusion times, and the chemical variations are limited and they are not necessarily conformal to nonplanar electrodes. On the other hand, SAM films are of molecular thickness with wide variations in desirable chemical functionality21-25 and conformal to the electrode, but, problems with stability have been encountered during voltammetric cycling, exposure to light and/or oxygen,26 and exposure to good solvents for the constituent molecules.22,27 In addition, the low operating currents provide difficulties for certain redox reactions.28 Ultrathin (nanometer regime) polymer (12) Finklea, H. O. Organized Monolayers on Electrodes. Electroanalytical Chemistry-A series of advances; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker: New York, 1996; Vol. 19 and references therein. (13) Mandler, D.; Turyan, I. Electroanalysis 1996, 8, 207-213. (14) Finklea, H. O.; Snider, D. A.; Fedyk, J.; Sabatani, E.; Gafni, Y.; Rubinstein, I. Langmuir 1993, 9, 3660-3667. (15) Sabatani, E.; Rubinstein, I.; Maoz, R.; Sagiv, J. J. Electroanal. Chem. 1987, 219, 365. (16) Sabatani, E.; Rubinstein, I. J. Phys. Chem. 1987, 91, 66636669. (17) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (18) Cheng, I. F.; Whiteley, L. D.; Martin, C. R. Anal. Chem. 1989, 61, 762. (19) Penner, R. M.; Martin, C. R. Anal. Chem. 1987, 59, 2625. (20) Cheng, I. F.; Martin, C. R. Anal. Chem. 1988, 60, 2163. (21) Creager, S. E.; Olsen, K. G. Anal. Chim. Acta 1995, 307, 277289. (22) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. J. Am. Chem. Soc. 1990, 112, 4301. (23) Finklea, H. O.; Hanshew, D. D. J. Electroanal. Chem. 1993, 347, 327. (24) Lee, K. A. B. Langmuir 1990, 6, 709. (25) Finklea, H. O. Organized Monolayers on Electrodes. Electroanalytical Chemistry-A series of advances; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker: New York, 1996; Vol. 19 and references therein. (26) Huang, J.; Hemminger, J. C. J. Am. Chem. Soc. 1993, 115, 33423. (27) Groat, K. A.; Creager, S. E. Langmuir 1993, 9, 3668-74. (28) Ohtani, M.; Sunagawa, T.; Kuwabata, S.; Yoneyama, H. J. Electroanal. Chem. 1995, 396, 97-102.
10.1021/la980063j CCC: $18.00 © 1999 American Chemical Society Published on Web 01/15/1999
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films are attractive because of their typical stability, but methods for forming useful films are quite limited. In particular, typical methods involve solution adsorption or spin casting. Unfortunately, these approaches are restricted to a limited number of polymers that are soluble in acceptable solvents. For example, fluoropolymers and polyethylene, which are highly desirable for their inert character, have exceedingly poor solubility properties. Furthermore, solution methods generally do not allow reproducible control of the film morphology and micropore structure, critical characteristics for electrochemical performance, while spin casting does not allow conformal coating for nonplanar electrodes. We recently reported the discovery that highly stable, crystalline, linear polymethylene [PM; -(CH2)x-] can be formed at the nanometer-thickness scale on evaporated gold films via the decomposition of diazomethane (CH2N2).29 Of particular interest is the observation that in the earlier stages of formation, the film grows primarily in the form of clusters which are localized at Au grain boundaries, thus providing a dense, low dielectric constant, conformal cap over these defect regions. The ability to form partial coverage alkanethiolate SAMs on these low PM coverage surfaces further supported a growth mechanism in which PM formation on the Au{111} terraces occurs at much lower rates than at the Au defect sites. At later stages of PM formation, readily observable growth spills over onto the Au{111} terrace regions, eventually leading to a complete coalescence over the surface and blocking of any SAM formation. In all cases, these films are reproducibly prepared with high stability toward degradation by light, oxygen, and typical solvents. These results indicated to us the intriguing possibility of fabricating highly controlled, stable, nanometer-scale structured gold electrode assemblies by carefully regulating the extent of PM growth which would in turn regulate the fraction of the open electrode area and presumably the sizes of the micropores. In particular, the electrode character should consist of blocked grain boundary defect regions with the micropores distributed predominantly across the {111} terraces. In addition, the observation that alkanethiolate SAMs form in the bare gold regions without displacing the deposited polymer,29 offered the eventual possibility of forming chemically functionalized microelectrode arrays from stable, two-component (e.g., SAM + PM) structures. With these possibilities in mind, the current study was carried out with the objective of using electrochemical methods to improve on our current picture of the extent, pattern, and evolution of bare Au surface during the early stages of PM growth on evaporated gold surfaces. The approach was kept very simple and consisted primarily of a systematic series of standard cyclic and differential pulse voltammetry measurements of ferrocene monocarboxylic acid (FMA) in aqueous solution. These data were combined with single wavelength ellipsometry (SWE), infrared spectroscopy (IRS), and atomic force microscopy (AFM) measurements to connect the electrochemical behavior to film structure. FMA was chosen as the electrochemical probe since the electrochemistry of ferrocene is well established as a simple, reversible oneelectron-transfer reaction.30 The monocarboxylic acid derivative was chosen for its considerable solubility in water, both in the neutral state as well as in the monooxidized (ferricenium) state (>5 × 10-3 M).31-33 An aqueous (29) Seshadri, K.; Atre, S. V.; Lee, M.-T.; Tao, Y.-T.; Allara, D. L. J. Am. Chem. Soc. 1997, 119, 9, 4698-4711. (30) Hennig, H.; Gu¨rtler, O. J. Organomet. Chem. 1968, 11, 307. (31) CRC Handbook of Organic Electrochemistry.
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medium is desirable since it greatly reduces complications from solvent effects on the hydrophobic polymer films. In addition to the use of evaporated gold substrates, parallel experiments with sputter-deposited gold electrodes were also conducted since a preponderance of electrochemical studies and practical devices use these types of electrodes. Our results show the onset of very reproducible, sharp transitions from an open, micropore to a blocked electrode at PM mass loadings of ∼0.4 and ∼0.8 µg/cm2 for sputtered and evaporated gold films, respectively. Below these transition threshold coverages, direct molecular access to the active gold surface from the solution permits the filling of these holes with alkanethiolate SAMs and electrochemically polymerized aniline. In general, these observations confirm the previously reported nonuniform lateral growth of PM on the gold surface29 in which the terraces selectively remain relatively open during PM deposition at the grain boundary defects. In more detail, the differing coverage thresholds for the two substrates can be understood in terms of the physical distributions of the polymer across the surfaces as controlled by the densities of grain boundary defects inherent to the two substrates. Overall, these results show a new method for fabricating highly stable, partially blocked gold electrodes with selective blocking of defect regions on the metal surface. Experimental Section 2.1. Materials. Diazald (N-methyl-N-nitroso-p-toluenesulfonamide, obtained from Aldrich Chemical, St. Paul, MN) was used for the preparation of CH2N2.34 Anhydrous ether (Aldrich Chemical, St. Paul, MN), absolute ethanol (Pharmco, Inc.), and ferrocene monocarboxylic acid (Aldrich) were used as received. Hexadecane (99.99%; Aldrich) was dried by agitating an approximately 1:1 mixture with concentrated sulfuric acid in a separatory funnel. The mixture was allowed to stand for 30 min after which the hexadecane was decanted off and immediately passed through a column of activated alumina (Aldrich, grade II). Organic-free, deionized (>18.0 MΩ cm) water was obtained using a Millipore Purification System (Bedford, MA). 2.2. Electrode Preparation. Details of the CH2N2 preparation and PM film formation were previously reported.29 Gold films were prepared using standard thermal evaporation29 and magnetron sputtering methods. Substrates were rigorously cleaned with hot peroxysulfuric acid and/or UV-ozone treatment immediately prior to gold deposition. The evaporated films (∼200 nm Au on ∼10 nm Cr) were deposited onto either 2.5 cm Si wafers or 1.75 cm × 1 cm borosilicate glass substrates and sputtered films (∼100 nm Au on ∼20 nm Ti) onto 4 in. × 4 in. borosilicate glass plates. The sputter deposition was done in an Innotec model VS24C downward flux system (Innotec Group, Simi Valley, CA). The chamber was first evacuated to ∼1.5 × 10-6 Torr and then backfilled with argon to ∼4 mTorr. The plates were first sputter etched at 13.56 MHz with 250 W power for 10 min (this typically raises the substrate temperature to 100 ( 3 °C), and then the Ti and Au depositions were consecutively done at 500 W power. In all cases the substrates were transferred immediately to the CH2N2 solutions at 0 °C. To reduce the possibility of atmospheric contamination, sample exposure to ambient was limited to the time taken to determine the substrate optical functions by ellipsometry (∼2-3 min). After the deposition period, the substrates were removed, washed with pure ether and ethanol, and dried by spinning. The deposition times and CH2N2 concentrations were varied to produce polymethylene films of different coverages according to previous procedures.29 2.3. Ellipsometry, Contact Angles, Atomic Force Microscopy, and Infrared Spectroscopy. Single wavelength ellipsometry (SWE), contact angle, atomic force microscopy (32) Blom, N. F.; Neuse, E. W.; Thomas, H. G. Transition Met. Chem. 1987, 12, 301-6. (33) Faraggi, M.; Weinraub, D.; Broitman, F.; DeFelippis, M. R.; Klapper, M. H. Radiat. Phys. Chem. 1988, 32, 293-297. (34) de Boer, T. J.; Backer, H. J. Org. Synth., Coll. Vol. 1963, 4, 250-253.
744 Langmuir, Vol. 15, No. 3, 1999 (AFM), and infrared reflection spectroscopy (IRS) measurements were made exactly as described earlier.29 Sample-to-sample errors are ∼(0.1 nm for SWE and ∼(2° for wetting. The SWE technique was chosen to monitor the amount of PM at the electrode surfaces because of the relative ease of this measurement compared to direct methods such as forward recoil spectroscopy (FRS) and quartz crystal microgravimetry (QCM), used in our previous study. The ellipsometric data first were converted to thicknesses, dSWE, assuming a model of a planar, dense PM film with a refractive index of 1.50. Because of the complex morphology of the PM films, the dSWE values do not directly reveal the loadings. In our previous report, however, it was established that these dSWE values are directly proportional to the FRS and QCM mass loadings, Γ, for PM on evaporated gold substrates. These calibrations give an average relationship Γ(µg/cm2) ) 0.121 ((0.006)‚dSWE (nm).35 The errors in applying this equation to sputtered gold samples, which have somewhat different PM morphologies than PM/evaporated gold (see later), are estimated to be within ∼(10%. 2.4. Electrochemistry. The electroanalyses were performed using an EG&G PAR model 273 potentiostat/galvanostat together with a data acquisition interface. Aqueous solutions were prepared using 1 × 10-3 M FMA with KCl and KH2PO4 electrolytes, and the pH was adjusted to 7.4. The PM-modified gold electrodes were configured to be the working electrode, and the active geometric solution contact area was exactly defined by attachment of an adhesive-backed polyimide tape (CHR, Connecticut) with a 5 mm diameter window. The reference and counter electrodes were Ag|AgCl|Cl-(3M) and Pt, respectively, and all potentials specified are relative to the reference electrode. Cyclic voltammetry (CV) analysis was performed at 25 °C over the range 0.1-0.5 V with a scan rate of 20 mV/s. Differential pulse voltammetry (DPV) was performed by applying 25 ms wide, 50 mV pulses during the scans. Multiple scan-rate (MSR) CV was performed over 0.0-0.6 V using rates of 10, 20, 50, 100, 200, and 500 mV/s.
Seshadri et al.
Figure 1. Cyclic and differential pulse voltammogram plots (CVs and DPVs, respectively) of ferrocene monocarboxylic acid (FMA) solutions at gold electrodes modified by polymethylene (PM) films at different mass average coverages (Γ). The voltammograms were anodically swept over 0.1-0.5 V at 20 mV s-1 in 1 × 10-3 M FMA in a pH 7.4 solution with KCl electrolyte. The CVs for evaporated and sputtered gold are shown in plots a and b, respectively. Notice the bimodal distribution of CV shapes with respect to low and high coverage regimes. Corresponding DPVs were obtained by imposing 50 mV pulse heights and 25 ms pulse widths on the anodic sweeps. Selected plots of δI vs E in the high PM coverage regime are given in part c (evaporated gold) and part d (sputtered gold).
Results and Discussion 3.1. Electrochemical Analysis at PM-Covered, Evaporated Gold Electrodes. 3.1.1. Cyclic and Differential Pulse Voltammetry. Figure 1a shows the overlay of current (I) vs. voltage plots obtained from different evaporated gold electrodes, each of different mass average coverage (Γ). The reported data are the second of two scans. For Γ e 0.44 µg/cm2, the behavior is nearly indistinguishable from that of bare gold. With increasing Γ, I continually decreases until the current finally diminishes to near zero at Γ ) 3-5 µg/cm2. The near unity values of the cathodic to anodic peak current ratios, ICmax/ IAmax, indicates reversible cycling.36 The CV shapes indicate an evolution from linear, semiinfinite diffusion (current peaks) at zero and low Γ to radial diffusion (shallow peaks and current plateaus) at high Γ. The cathodic and anodic peak potential separations, ∆Ep, shown in Figure 2 (solid line), remain constant for Γ < 0.9 µg/cm2 with a sharp increase up to Γ ∼ 2.4 µg/cm2.37 This behavior qualitatively mimics the collapse of the voltammograms with increasing Γ (Figure 1a). The low coverage ∆Ep value of ∼0.063 V is close to the theoretical value of 0.059 V for Nernstian behavior. However, for Γ > 0.9 µg/ cm2 the increasing deviation from ideal behavior suggests the onset of a small degree of apparent electrochemical quasi-reversibility and is consistent with the transition from a planar to a pinhole microarray electrode.37,38 Since
our previous work establishes that PM nucleates initially at grain boundary defect sites,29 known to be of higher surface energy than the predominant {111} terraces of evaporated gold,39,40 the data of Figure 2 thus suggest that the apparent quasi-reversible character may be
(35) The previous QCM and FRS measurements agree on average within ∼5% (ref 29). (36) Greef, R.; Peat, R.; Peter, L. M.; Pletcher, D. Robinson, J. In Instrumental Methods in Electrochemistry; Ellis Horwood Ltd., Chichester, England, 1985; Chapter 6. (37) One must be cautious in interpreting these trends because of the difficulties in determining accurate redox potentials as the voltammograms approach a sigmoidal shape; for example, see: Tokuda, K.; Morita, K.; Shimizu, Y. Anal. Chem. 1989, 61, 1763-8.
(38) Parry, E. P., Osteryoung, R. A. Anal. Chem. 1964, 37, 1634. (39) It has been established from zero-charge potential measurements on gold that the highest surface energies are found for the high-index planes around the (111) point in the stereographic triangle [(554), (332), etc.], the typical planes found at grain boundaries at {111} textured surfaces. Also, the atoms in defect regions have lower coordination numbers and higher energies as calculated from Madelung constants and interparticle attraction, than those on {111} terraces. (See: Hamelin, A.; Lecoeur, J. Surf. Sci. 1976, 57, 771-4.)
Figure 2. Plots of the energetics parameter, ∆Ep ) Ecp - Eap, taken from the CV data in Figure 1, vs PM coverage (Γ) for evaporated (b) and sputtered (9) gold electrodes.
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associated with the fractional contribution of Au{111} terraces to the overall electrochemistry. A set of DPV experiments (50 mV, 25 ms wide pulses applied over an anodic voltage sweep of +0.1 to +0.5 V) were run to examine the high Γ regime behavior in more detail. Figure 1c shows the incremental current outputs (δI) over each voltage pulse. We note two observations. First, over the entire range of Γ (0-5.20 µg/cm2), the peak widths at half-maximum height are ∼90 mV. This value is close to the ideal value of 3.52RT/F ) 90.4 mV38,41 for a fully reversible one-electron-transfer probe and shows that FMA is behaving as expected. Second, while there is a sharp decrease in δI to ∼0 for Γ g 2.35 µg/cm2, Emax (corresponding to δICmax) remains approximately constant. This shows that PM coverage has only small effects on the overall electrochemical process. 3.1.2. Multiple Scan Rate Cyclic Voltammetry. These measurements were performed in order to analyze for the effective electrode areas as a function of Γ. For any given PM coverage, the observed values of the E°′ potential and the ratio of cathodic to anodic peak currents were found to be generally independent of the scan rate (data not shown). These observations support the approximation of the electrochemistry as that of a diffusion limited, reversible electrochemical process36 with the role of PM as an insulating blocking layer on the electrode surface. The increase of PM coverage primarily just serves to shift the diffusion profiles from linear, semi-infinite to radial. By use of limiting diffusion models, the above data can be interpreted in more detail. While models have been developed14 for radial diffusion, they are based on electrode structures that are much more regular than ours.42 Therefore, we chose to analyze the complete data set according to a linear, semi-infinite diffusion model using the Randles-Sevcik equation:36
ICmax ) (2.69 × 105)n3/2 F A D01/2 ν1/2 C0
(1)
where ICmax ) cathodic peak current, n ) electron stoichiometry coefficient, F ) Faraday’s constant, A ) electrode area, D0 ) electroactive species diffusion coefficient, ν ) voltage scan rate, and C0* ) electroactive species bulk concentration. While this approximation makes the analysis more tractable, it does render the results to be somewhat qualitative, particularly at high Γ. When C0 and D0* are constant for all the scan rates over a given sample, eq 1 reduces to ICmax ) Rν1/2, where R is a constant. Consistent with this relationship, plots of ICmax vs ν1/2 were observed to be quite linear (usually 95-99% confidence; data not shown) for a range of samples with Γ ) 0-5.2 µg/cm2. This behavior indicates direct access of the solution-phase electroactive species to the gold surface, as opposed to molecular diffusion through the film. The latter is not expected since diffusion of an aqueous, solvated electroactive species through a highly hydrophobic, crystalline matrix would be highly unfavor(40) Porter and co-workers also have shown that the defect regions are more active than the terraces in the underpotential deposition of Pb on gold surfaces (Walczak, M. M.; Alves, C. A.; Lamp, B. D.; Porter, M. D. J. Electroanal. Chem. 1995, 396, 103-114.) (41) Ferrocene electrochemistry does not always follow a simple oneelectron process. For example, Bond et al (Bond, A. M.; McLennan, E. A.; Stojanovic, R. S.; Thomas, F. G. Anal. Chem. 1987, 59, 2853-2860])report ferrocene at saturated concentrations in aqueous media show significant deviations of the cathodic to anodic current ratios (iC/iA) from unity at high scan rates. These observations can be associated with weak adsorption of the electroactive species. In our case, FMA was used as the electroactive reagent, and the concentrations were held well below saturation values, thus reducing the possibility of adsorption effects.
able. Further, if it did occur, it would cause the CVs to vary strongly with the detailed structure and thickness of the growing PM clusters, contrary to the data. Two distinct regimes of slopes were observed: (1) Rhigh ) 2.53 ((0.04) µA/(mV s-1)0.5 for Γ ) 0.0-0.85 µg/cm2 and (2) Rlow ) 0.170 ((0.016) µA/(mV s-1)0.5 for Γ g 1.46 µg/ cm2, a near-zero slope. Equation 1 was applied to these data with C0* ) FMA solution concentration. For the Γ ) 0 sample, setting A ) AGEOM ) experimental geometric area ) 0.1963 cm2 results in D0 ) 3.7 × 10-6 cm2 s-1. This value is in good agreement with the value of 3 × 10-6 cm2 s-1 reported for FMA in 0.1 M NaH2PO4/HClO4 solution.43 This D0 value then was applied to estimate values of A from eq 1 for different PM coverage samples.44 At low Γ, where linear diffusion holds, we can expect these areas to be reliable. However, at increasing coverage, as diffusion shifts to radial, eq 1 will become less applicable and the values of A will become more approximate. Keeping in mind this constraint, the areas were converted to apparent % electrode passivation using the expression {[AGEOM A)]/AGEOM} × 100. The results, presented in Figure 3a, show that A varies quite nonlinearly with Γ and that the onset of electrode passivation occurs at Γ ∼ 0.5 µg/cm2 with a crossover region between ∼1 and 1.5 µg/cm2. While this trend must be considered approximate, the onset of a threshold region is very clear. The heterogeneity of the PM distribution across the electrode surface, as observed above, should be reflected in surface-sensitive properties such as wetting. Using experimental contact angles of water in conjunction with the approximation of Cassie’s equation,29,45 the fraction of open geometric area on the gold surface as a function of Γ was estimated. Films with Γ > 2.42 µg/cm2 show a contact angle of ∼103°, the value expected for pure -(CH2)xsurfaces. However, the angles approach a limiting value of ∼85° at Γ ) 0 (caused by the presence of adventitious organic contamination on the otherwise bare gold surface). (42) Our experimental system of a topographically rough PM surface with cluster heights of ∼20-60 nm for coverages of 0.6-1.2 µg/cm2 is expected to show complex electrochemical diffusion profiles. However, it is instructive to make a few simple analyses in terms of a uniformly planar macrodisk electrode model with generalized, hemispherical diffusion profiles. First, it has been reported (Shoup, D.; Szabo, A. J. Electroanal. Chem. 1984, 160, 27) that for isolated electrode areas of radius r, this model requires the electroactive areas each to be embedded in an insulating annulus of radius >2r. For a centripetal (concentric) PM growth initiated at the periphery of a Au grain of ∼25-50 nm radius (appropriate for our evaporated surfaces) the macrodisk model would thus only be applicable when r e 8-16 nm, equivalent to >90% passivation. Second, using the standard relationship Ilim ) 4rFD0C0* (Reinmuth, W. H. J. Am. Chem. Soc. 1957, 79, 6358), where, Ilim ) limiting sigmoidal voltammogram current (other quantities as for eq 1), our data in the sigmoidal region (ΓJ 4.8 µg/cm2; ν ) 10 mV s-1) lead to r ∼ 0.04 nm, a physically unrealistic result (see: Tokuda, K.; Morita, K.; Shimizu, Y. Anal. Chem. 1989, 61, 1763-8). Third, the comprehensive approach of Amatore et al. (Amatore, C.; Saveant, J. M.; Tessier, D. J. Electroanal. Chem. 1983, 147, 39-51), with a ∼2.2 × 10-3 cm s-1 e-transfer rate constant (estimated from the data of Nicholson, R. S. Anal. Chem. 1965, 37, 1351-5), predicts a sigmoidal CV for the Γ ) 5.2 µg/cm2 PM film (∼90% passivation), as observed. From this, the theory further predicts r ∼ 4-5 nm, a not unreasonable upper limit. (43) Cass, A. E. G.; Davis, G.; Francis, G. D.; Hill, H. A. O.; Aston, W. J.; Higgins, I. J.; Plotkin, E. V.; Scott, L. D. L.; Turner, A. P. F. Anal. Chem. 1984, 56, 667. (44) Assuming a linear diffusion profile, the following equation can be applied to the DPV data (Bard, A. J.; Faulkner, L. R. In Electrochemical Methods -Fundamentals and Applications; John Wiley and Sons: New York, 1980, Chapter 5): δICmax ) nF A D01/2C0* ([(1 - σ)/(1 + σ)]/(π δτ)1/2), where, δICmax ) peak current over the complete DPV voltage scan, δτ ) pulse width (25 ms), σ ) e(nF/RT)‚∆E/2 with ∆E ) voltage pulse amplitude (50 mV) and the other variables defined as in eq 1. Given the conditions of constant T, C0*, δτ, ∆E, and D0, the values of A calculated are within (5% of those given in Figure 3a. (45) Cassie, A. B. D. Discuss. Faraday Soc. 1948, 3, 11-16. Adamson, A. W. Physical Chemistry of Surfaces, 5th ed.; John-Wiley & Sons: New York, 1990; pp 385-389.
746 Langmuir, Vol. 15, No. 3, 1999
Figure 3. Plots of the percent passivation or electrode area coverage by PM films on evaporated (a) and sputtered (b) gold electrodes. The squares (9, with a solid line) represent values calculated from the MSRCV data. The open circles (O, with a dotted line) represent estimates derived from wetting measurements. For details see text.
Applying the Cassie relation to our reported wetting data,29 we estimate percent covered gold areas of 8-10, 48-55, and 85-100% for Γ values of ∼0.12-0.61, ∼3.0, and ∼4.8 µg/cm2, respectively. These percent coverages are plotted as an overlay (dotted line) in Figure 3a where one observes a good qualitative correlation between the wetting behavior and electrochemical data. 3.2. Substrate Effects: Comparisons of PM-Covered, Sputtered, and Evaporated Gold Electrodes. 3.2.1. Substrate and PM Film Morphologies. Previous studies have established that the evaporated gold films typically exhibit a strong {111} texture, with variably sized grains ranging from ∼10-20 nm to up to the micrometer scale, depending primarily upon the substrate temperature and background pressure.46-48 Typical AFM images of evaporated and sputtered Au films prepared under our standard conditions are shown in parts a and c of Figure 4. Note the contrast. The sputtered film exhibits a high density of spherical deposits, each tens of nanometers in diameter while the evaporated film is much smoother. Parts b and d of Figure 4 show images of evaporated and sputtered Au with PM films at Γ ) 0.73 and 0.91 µg/cm2, respectively. Notice that the PM clusters on the evaporated substrate are significantly more localized than those on the sputtered substrate, which shows an apparent higher density of film nucleation and growth sites. More detailed comparisons are difficult because of the difficulty of isolating the PM and sputtered Au grain structures. Despite the different morphologies, however, over the complete range of PM coverages examined the two types of gold substrates give similar Γ values for similar PM deposition times and conditions. 3.2.2. Infrared Spectroscopy: The PM Structures. Figure 5 shows the C-H stretching region IRS spectra for the PM on evaporated29 and sputtered Au. The main features consist of the symmetric (d+) and antisymmetric (d-) stretches for the CH2 unit which appear at ∼28512854 and 2920-2924 cm-1, respectively. The smaller feature at 2963 cm-1 is attributed to the CH3 group. The shoulder at ∼2895 cm-1 is typically observed in polyeth(46) Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J. Am. Chem. Soc. 1987, 109, 2358-2368. (47) DeRose, J. A.; Thundat, T.; Nagahara, L. A.; Lindsay, S. M. Surf. Sci. 1991, 256, 102-108. (48) Chidsey, C. E. D.; Loiacono, D. N.; Sleator, T.; Nakahara, S. Surf. Sci. 1988, 200, 45-66.
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ylene spectra, but its origin is unclear.49 We focus on the d+, d- spectra. For the lowest coverages (0.15-0.51 µg/ cm2), both sets of spectra show similar d+, d- peak frequencies of 2853-2854 and 2922 cm-1. Single crystals of polyethylene [-(CH2-CH2)x-, chemically equivalent to PM] are reported50 to have values of 2849 and 29172918 cm-1, respectively, whereas conformationally disordered chains typically show values of ∼2856 and ∼2928 cm-1, respectively.51 On this basis, the observed peak frequencies indicate that the chains at these low coverages contain a larger degree of gauche defects than the bulk crystalline state, but not sufficient to constitute a liquidlike disordered state. Such defects typically could include tgt or gtg′ conformational sequences. As the coverages increase we note opposite trends in the spectral peak shifts: ∼2 cm-1 decreases for PM/(evaporated Au) and ∼2 cm-1 increases for the PM/(sputtered Au). These reflect corresponding increases and decreases in chain conformational ordering for PM on evaporated and sputtered substrates, respectively.52 On the basis of the AFM observation that sputtered Au surfaces show a higher defect density than evaporated Au, one expects many more initiation sites for PM nucleation29 and consequently more rapid coalescence of the PM film on the sputtered surface. This could be expected to lead to correspondingly smaller, less ordered PM crystallites than those on evaporated Au. The low-frequency region of the PM/(sputtered Au) spectrum, presented in Figure 6b, shows the typical -CH2- features seen for crystalline polyethylene, a scissors deformation doublet centered at ∼1468 cm-1 and rocking modes at 720 and 730 cm-1.29 The simultaneous presence of crystallinity, shown by these features, and conformational disorder, shown by the C-H stretching features (Figure 5b), could arise from the PM existing as a mixture of crystalline and disordered chains and/or extensive populations of disordered regions in otherwise crystalline chains, e.g., the presence of substantial lamellar chain folds in a crystalline polymer. 3.2.3. Electrochemistry. Data for the sputter-deposited Au electrodes are given along side the evaporated Au data in Figures 1-3. The qualitative similarities of the sets of CV and DPV (Figure 1) data indicate that the overall electrochemical behavior of the two electrodes is similar. In addition, nearly identical to the evaporated electrode case, the MSRCV data show linear ICmax vs ν1/2 plots (data not shown) with two distinct regimes of slopes: (1) Rhigh ) 2.87 ((0.03) µA/(mV s-1)0.5 for Γ ) 0.0-0.48 µg/cm2 and (2) Rlow ) 0.054 ((0.041) µA/(mV s-1)0.5 for Γ g 1.46 µg/ cm2, a near-zero slope. Applying eq 1 to the bare electrode MSRCV data yields D0 ) 4.6 × 10-6 cm2 s-1, somewhat higher than 3.7 × 10-6 cm2 s-1 obtained for bare evaporated gold. Several comparisons of the evaporated and sputtered electrode data are of interest. (49) Nielson, J. R.; Holland, R. F., J. Mol. Spectrosc. 1961, 6, 394. (50) Painter, P. C.; Runt, J.; Coleman, M. M.; Harrison, I. R. J. Polym. Sci., Polym. Phys. Ed. 1977, 15, 1647-1654. (51) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 5145-5150. (52) One also can note qualitatively from Figure 5 that the band intensities of PM films with similar Γ values are significantly lower for sputtered than for evaporated substrates. This effect could arise from differences in orientation of the CH2 groups, and local electromagnetic field effects (see: Tao, Y.-T.; Hietpas, G. D.; Allara, D. L. J. Am. Chem. Soc. 1996, 118, 6724-6735) arising from the inhomogeneous density distributions of PM which differ on the two surfaces. For evaporated substrates we speculate that the PM chains lie near parallel to the surface (see ref 29) with an average 45° chain axis twist, a configuration which would result in a near maximum IR signal for the combined d+, d- modes. Since the increased disorder of the PM chains on sputtered relative to evaporated Au indicates a relatively more random orientation of the CH2 units, one expects a relative decrease in the d+, d- band intensities.
Controlled Area Electrode Assemblies
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Figure 4. Tapping mode atomic force microscope images taken with a 1.1-µm scanner. The image sizes are 500 nm × 500 nm and the gray scale represents the height topography, with black and white representing the lowest and highest features, respectively. (a) Bare, evaporated gold; note the average grain size is ∼100 nm. (b) PM-modified, evaporated gold for a coverage of Γ ) 0.73 µg/cm2; note how the clusters appear isolated between the grains on the gold surface. (c) Bare sputter-deposited gold; the average grain size is ∼20 nm. (d) PM-modified, sputter-deposited gold for Γ ) 0.91 µg/cm2; the PM film is much more uniformly distributed over the surface than for the evaporated gold substrate in part b. For details see text.
Figure 5. Infrared spectra in the C-H stretching region for PM films at different mass average coverages on evaporated (a) and sputter-deposited (b) gold substrates. Note that the spectrum of the 0.12 µg/cm2 film on the evaporated substrate has been scaled by a factor of 3.
Figure 6. Low-frequency region infrared spectra of the same samples as in Figure 5. The CH2 scissors deformation at 1465 and 1475 cm-1 and the CH2 rocking modes at 719 and 730 cm-1 are depicted.
The onset of the current blocking transition (Figures 1 and 3) with increasing PM coverage occurs at lower Γ values for the PM/(sputtered Au) electrodes. This behavior is consistent with the wetting trends (Figure 3).53 Further support for morphology effects is reflected by the different ∆Ep values of ∼0.065 and ∼0.075 V for the bare evaporated and sputtered surfaces, respectively, as
seen in Figure 2. Comparison with the theoretical value36 of 0.058 V expected for a reversible one-electron-transfer FMA reaction32 could indicate that the sputtered surface electrochemistry has some intrinsic degree of quasireversible character. The ∆Ep vs coverage plots (Figure 2) show that the onset of a ∆Ep increase does not occur for PM/(evaporated Au) until Γ > 0.85 µg/cm2 whereas sharp increases occur at the lowest coverages for PM/ (sputtered Au). The above results are quite consistent with a PM growth mechanism in which nucleation initiates in the high surface energy sites of the grain boundary regions and spills out onto the lower energy terraces, described in one limit as a radial or centripetal growth. The higher, more uniform distribution of high-surface energy defect sites on the sputtered relative to the evaporated surface, inferred from the AFM images shown in Figure 4, thus leads to a more uniform PM distribution on the sputtered surface with faster coalescence as coverage increases. 3.3. Stability of PM-Modified Gold Electrodes. The stability of two PM films, 3.33 µg/cm2 on evaporated Au and 0.91 µg/cm2 on sputtered Au, was assessed by (1) 300 CV cycles (0.0-0.6 V for 5 h) at 20 mV s-1 and (2) repeated cycling over -1.2 to +1.2 V followed by CV evaluation with FMA over the range 0.0-0.6 V. The results of both tests on both samples showed no detectable changes in the electrode properties. These results indicate that the PM/Au films are quite robust, a critical feature for application as electrode barrier materials. 3.4. Electrode modification: formation of ultrathin nanocomposite films. 3.4.1. Alkanethiolate/PM Composites. Our previous report29 showed that open areas in the PM films can be functionalized by solution self-assembly of alkanethiols with no apparent displacement of the initial PM film. On the basis of this phenomenon, we have estimated the fraction of open area in PM/Au films by using IRS to count alkanethiolates adsorbed by exposure to ethanolic solutions of HS(CH2)15CO2CH3.54 The spectrum of the pure methylester thiol SAM has been reported previously and is well understood.55 The (53) Note that the contact angle hysteresis for PM/(sputtered Au) is consistently higher by as much as 30° than for PM/(evaporated Au), as expected from the increased surface roughness for the former. (54) The procedure involved the introduction of PM-modified substrates into 2 mM solutions of the thiol in ethanol. The substrates were taken out after a 24-h immersion period and washed sequentially in ethanol, hexanes, water, and ethanol, under sonication, and then spectra were taken. (55) Nuzzo, R. G.; Dubois, L. H.; Allara; D. L. J. Am. Chem. Soc. 1990, 112, 558-569.
748 Langmuir, Vol. 15, No. 3, 1999
Figure 7. Low-frequency region infrared spectra of gold substrates modified by exposure to an ethanolic solution of HS(CH2)15COOCH3. The plots on the left (a) correspond to evaporated gold substrates with Γ ) 0, 0.44, and 1.16 µg/cm2; the right-hand plots (b) correspond to sputtered gold substrates with Γ ) 0, 0.48, and 0.91 µg/cm2.
low-frequency region56 spectra of PM/(evaporated Au) and (PM + thiol)/(evaporated Au) films are shown in Figure 7a, for examples of Γ ) 0.44 and 1.16 µg/cm2 PM coverages. The near lack of perturbation of the initial PM spectral features indicates that the thiol has not displaced any detectable amount of PM from the surface. Note that the relative intensities and spacings of the twist-wag progression bands between 1150 and 1350 cm-1 for the backfilled alkanethiolate chains are quite similar to those of the pure SAM spectrum. From this we conclude that the backfilled chains are predominantly in all-trans conformations, similar to those of the pure SAM. On the basis of the reasonable assumption that the alkanethiol orientation is constant (see below), analysis of the CdO stretching intensities (1745 cm-1) relative to those of a pure thiol/Au film leads to estimates of 70 and 40% open substrate areas, respectively.57 Inspection of Figure 3 shows that these values agree with the estimates from CV data within ∼(10%. This shows that a significant fraction of the total open area in the PM/(evaporated Au) film consists of regions sufficiently large to support formation of correlated ensembles of alkanethiolate adsorbates, likely regions of 2-3 nm diameter or more (∼ four to six chains across). Attempts to perform similar analyses for PM/(sputtered Au) samples were precluded by the reproducible observation of strongly perturbed thiolate spectra. Data are shown in Figure 7b for PM coverages of 0.48 and 0.91 µg/cm2. First, one can note that the overall spectral intensity of the pure thiol monolayer is greater for the sputtered relative to the evaporated gold substrate.58 Second, the 0.48 µg/cm2 PM coverage spectrum shows perturbations of the chain wag-twist progression and the ester C-O-C stretching features between ∼1150 and 1350 cm-1. Finally, the 0.91 µg/cm2 coverage spectrum is markedly different (56) The C-H spectra are not reported as the strong C-H modes of the PM interfere with the thiol component. (57) However, note that the intensities of the progression bands appear to increase with increasing PM coverage (Figure 7a), in contrast to the decrease of the CdO stretching peak. The source of this contrary effect is not clear but may be due to chain orientation effects. Since the transition moment of the progression series is parallel to the chain axis, this would imply a more vertical alignment of the alkanethiolate chains as the PM pore sizes decrease. (58) The origin of this effect is unclear but could be a combination of chain orientation effects and local electromagnetic field effects induced by the atomic-level surface roughness of the sputtered surface. Further studies are in progress to clarify this issue. Also, see footnote 52.
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Figure 8. Schematic illustrating the suggested relative dimensions of the isolated microscopic pores on evaporated and sputter-deposited Au PM-covered electrodes as the PM coverages approach the coalescence (blocking) limit. The pictures show how the smaller pore structures on the sputtered surface do not allow molecular organization of domains of alkanethiolate chains to occur, compared to the case of the evaporated surface at similar PM mass average coverages. For details see text.
from that of the pure thiol monolayer. The 1745 cm-1 Cd O peak is broadened on the low-frequency side, but its presence does indicate that thiol has been adsorbed into spaces within the PM film. More striking is the observation that the wag-twist progression is gone while an intense peak appears at 1263 cm-1. While further work is needed to clarify these results in detail, they generally seem to indicate that the open regions in the PM/(sputtered Au) films have very different character(s) than those of PM/ (evaporated Au). From Figure 3b, the fraction of open area can be estimated as 1-1.5 µg/cm2, equivalent to the amount of PM in 11-16 nm, fully dense films. When combined with the AFM data, a centripetal type of growth mechanism emerges in which the initial PM film nucleates in Au grain boundary defect sites and grows inward on the terraces. This mechanism is consistent with the observation that the ability to chemisorb alkanethiols and electropolymerize aniline at the open Au regions decreases with PM coverage. Comparisons of the data for evaporated and sputtered gold electrodes indicate that the PM growth on the sputtered surfaces involves a more rapid coalescence and that the micropores approach molecular dimensions more rapidly than those for the evaporated surfaces. These results are consistent
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with a substantially higher fraction of grain boundary defect regions on the sputtered relative to the evaporated surfaces. Both surfaces lead to PM films that withstand repeated electrochemical cycling. This attribute, combined with the ability to functionalize the nanometer-scale pores by electropolymerization or molecular self-assembly, points to the potential of these films as a means for forming new types of ultrathin film nanocomposites. Acknowledgment. The authors acknowledge the support of ABTECH Scientific, Inc., and the Defense Advanced Research Projects Agency. LA980063J
