Effect of Electrode Roughness On the Capacitive Behavior of Self

Anal. Chem. 2008, 80, 7670–7677

Effect of Electrode Roughness On the Capacitive Behavior of Self-Assembled Monolayers Eugene F. Douglass Jr.,† Peter F. Driscoll,† Deli Liu,‡ Nancy A. Burnham,‡ Christopher R. Lambert,*,† and W. Grant McGimpsey*,† Department of Chemistry and Biochemistry and Department of Physics, Worcester Polytechnic Institute, Worcester, Massachusetts 01609 Analytical gold electrodes were polished mechanically and electrochemically and the true area of the electrode surface was measured by quantitative oxidative/reductive cycling of the electrode. A roughness factor for each electrode was determined from the ratio of the true area to the geometric area. The roughness is fully described by a combination of microscopic roughness (up to tens of nanometers) and macroscopic roughness (on the order of hundreds of nanometers) terms. The electrodes were then derivatized with a self-assembled monolayer (SAM) of dodecanethiol or a thioalkane azacrown and characterized by impedance spectroscopy. The behavior of the electrodes was modeled with either a Helmholtz or Randles equivalent circuit (depending on the SAM used) in which the capacitance was replaced with a constant phase element. From the model, an effective capacitance and an r factor that quantifies the nonideality of the SAM capacitance was obtained. The effective capacitance divided by the roughness factor yields the capacitance per unit true area, which is only a function of microscopic roughness. The relationship between this capacitance and the r factor indicates that microscopic roughness predominantly affects the nonideality of the film while macroscopic roughness predominantly affects the magnitude of the film’s capacitance. Understanding the contribution of the electrode topography to the magnitude and ideality of the SAM capacitance is important in the construction of SAM-based capacitive sensors because it predicts the importance of electrode-electrode variations.

electrochemical measurement of lithium ion binding to the SAM.5 However, defects in SAMs, which can be linked to characteristics of both the substrate (e.g., roughness, grain boundaries) and the film (e.g., alkyl chain length, headgroup size),1 can substantially influence the nanoscale characteristics of the layer, lead to poor reproducibility, and prevent the use of these systems for precise analytical measurements such as those required in surface-based ion sensing.1,6-9 In order to improve the reproducibility and accuracy of these measurements, we undertook a study of the influence of substrate roughness on the electrochemical behavior of monolayer systems. Organic coatings on metals typically act as insulators, and therefore, an electrode modified with a SAM and placed in an electrochemical cell behaves similarly to a parallel-plate capacitor.10-12 Impedance spectroscopy, which analyzes conducting behavior as a function of applied frequencies, can therefore be used to study the capacitive and resistive behavior of substrates modified with SAMs. In this study, we attempt to correlate deviations from ideal capacitive behavior with the degree and scale of surface roughness. For naked metallic electrodes, deviation from ideal capacitive behavior has been shown to be directly related to the degree of nanoscale irregularity/roughness of the electrode with smoother and more homogeneous electrodes exhibiting much closer to ideal capacitive behavior.13 Nanoscale electrode roughness can cause current-density variations in the double layer, making the overall impedance a complex function of local resistances and interfacial capacitances, resulting in their contributions being indistinguishable by ideal fitting.14,15 Nonideality also arises because of chargetransfer effects and diffusion at the metal/electrolyte interface, and these effects are magnified by increased surface roughness.

Self-assembled monolayers (SAMs) of alkanethiolates on gold form well-ordered films and have been widely studied for applications including biosensing, biocompatibility, and photovoltaics.1-4 For example, we recently reported the synthesis of a selective lithium ionophore, its deposition as a SAM on gold, and the

(5) Wanichacheva, N.; Soto, E. R.; Lambert, C. R.; McGimpsey, W. G. Anal. Chem. 2006, 78, 7132–7137. (6) Carvalhal, R. F.; Freire, R. S.; Kubota, L. T. Electroanalysis 2005, 17, 1251– 1259. (7) Ding, S.-J.; Chang, B.-W.; Wu, C.-C.; Chen, C.-J.; Chang, H.-C. Electrochem. Commun. 2007, 9, 1206–1211. (8) Hoogvliet, J. C.; Dijksma, M.; Kamp, B.; Van Bennekom, W. P. Anal. Chem. 2000, 72, 2016–2021. (9) Sadkowski, A.; Motheo, A. J.; Neves, R. S. J. Electroanal. Chem. 1998, 455, 107–119. (10) Boubour, E.; Lennox, R. B. Langmuir 2000, 16, 4222–4228. (11) Boubour, E.; Lennox, R. B. Langmuir 2000, 16, 7464–7470. (12) Boubour, E.; Lennox, R. B. J. Phys. Chem. B 2000, 104, 9004–9010. (13) Macdonald, J. R.; Barsoukov, E. Impedance Spectroscopy: Theory, Experiment and Applications; John Wiley and Sons: Hoboken, NJ, 2005. (14) Jurczakowski, R.; Hitz, C.; Lasia, A. J. Electroanal. Chem. 2004, 572, 355– 366. (15) Kerner, Z.; Pajkossy, T. Electrochim. Acta 2000, 46, 207–211.

* To whom correspondence should be addressed. E-mail: [email protected]. † Department of Chemistry and Biochemistry. ‡ Department of Physics. (1) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1103–1169. (2) Ulman, A. Chem. Rev. 1996, 96, 1533–1554. (3) Wink, T.; van Zuilen, S. J.; Bult, A.; van Bennkom, W. P. Analyst 1997, 122, 43R–50R. (4) Abdelrazzaq Feras, B.; Kwong Raymond, C.; Thompson Mark, E. J. Am. Chem. Soc. 2002, 124, 4796–4803.

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Analytical Chemistry, Vol. 80, No. 20, October 15, 2008

10.1021/ac800521z CCC: $40.75  2008 American Chemical Society Published on Web 09/24/2008

circuit is replaced with the CPE in order to account for nonideal behavior. The presence of a SAM resistance is associated with the movement of water and ions within the layer in response to the AC perturbation.12 This study qualitatively and quantitatively links the scale of substrate roughness to the capacitive magnitude and ideality for these organic films. In addition, a method is illustrated for both modulating and measuring microscopic roughness of a metal electrode using electrochemical polishing.

Figure 1. Complex impedance plots for two types of equivalent circuit models showing ideal (A and C) and nonideal (B and D) capacitive behavior (Rsoln ) solution resistance, RSAM ) SAM resistance, CSAM ) SAM capacitance, CPE ) constant phase element).

Previous work has examined the correlation between electrode roughness and double layer capacitance; however, studies that analyze the effects of substrate roughness on adsorbed monolayer behavior are limited.6,8,9,13–28 Substrate roughness on the scale of the adsorbate, 1-10 nm (referred to here as “microscopic”), is known to cause defects in SAMs.21,27,28 However, larger scale, >10 nm (referred to here as “macroscopic”), roughness apparently does not impact the ordering of SAMs and effectively only increases the area of the coating.21 In order to quantify the capacitive deviations, we have adopted the use of a constant phase element (CPE) as an additional component in ideal capacitive circuits. Plots of the imaginary versus the real components of the impedance for two types of ideal circuits that are commonly used to model SAM behavior are shown in Figure 1.10,11,13,20,22 Curves A and C indicate ideal capacitive behavior for each circuit shown, while curves B and D, respectively, show nonideal behavior similar to that which would be expected if capacitive defects are present in the layers. In the latter case, the ideal capacitor portion of the equivalent (16) Neves, R. S.; De Robertis, E.; Motheo, A. J. Electrochim. Acta 2006, 51, 1215–1224. (17) Piela, B.; Wrona, P. K. J. Electroanal. Chem. 1995, 388, 69–79. (18) Kerner, Z.; Pajkossy, T. Electrochim. Acta 2002, 47, 2055–2063. (19) Zoltowski, P. J. Electroanal. Chem. 1998, 443, 149–154. (20) Brett, C. M. A.; Kresak, S.; Hianik, T.; Brett, A. M. O. Electroanalysis 2003, 15, 557–565. (21) Creager, S. E.; Hockett, L. A.; Rowe, G. K. Langmuir 1992, 8, 854–861. (22) Diao, P.; Jiang, D.; Cui, X.; Gu, D.; Tong, R.; Zhong, B. J. Electroanal. Chem. 1999, 464, 61–67. (23) Guo, L.-H.; Facci, J. S.; McLendon, G.; Mosher, R. Langmuir 1994, 10, 4588–4593. (24) Kondo, T.; Yanagida, M.; Zhang, X.-Q.; Uosaki, K. Chem. Lett. 2000, 964– 965. (25) Leopold, M. C.; Black, J. A.; Bowden, E. F. Langmuir 2002, 18, 978–980. (26) Leopold, M. C.; Bowden, E. F. Langmuir 2002, 18, 2239–2245. (27) Losic, D.; Gooding, J. J.; Shapter, J. G.; Hibbert, D. B.; Short, K. Electroanalysis 2001, 13, 1385–1393. (28) Losic, D.; Shapter, J. G.; Gooding, J. J. Langmuir 2001, 17, 3307–3316.

EXPERIMENTAL SECTION Materials. All reagents and solvents were reagent grade or better from Aldrich (Milwaukee, WI) or Alfa Aesar (Ward Hill, MA) and used as received. Deionized water from a Millipore (Billerica, MA) Synergy UV system was used for all experiments. General Methods. 1H NMR and 13C NMR spectra were obtained using a 400-MHz Bruker (Billerica, MA) Avance NMR spectrometer and referenced to TMS. Mass spectra were obtained with a Waters (Milford, MA) Micromass model ZMD mass spectrometer using electrospray ionization with a 50/50 acetonitrile/water carrier solvent and nitrogen curtain gas. Synthesis. 11-Mercapto-N,N-(1,4,7,10,13-pentaoxa-16azacyclooctadecyl)-undecanamide (Azacrown). 11-Mercaptoundecanoic acid (1.66 g, 7.60 mmol), monoaza16-crown-6 (2.00 g, 7.60 mmol) and dimethylaminopyridine (0.09 g, 0.76 mmol) were combined in anhydrous dichloromethane (25 mL) and stirred together until completely dissolved. Dicyclocarbodiimide (1.53 g, 7.60 mmol) in dichloromethane (15 mL) was added and the reaction flask capped with a calcium chloride drying tube and allowed to stir overnight at room temperature. The precipitated urea (a white solid) was removed by microsuction filtration and the solvent removed in vacuo yielding a thick oil. This pale yellow crude product was purified by silica gel column chromatography (50:1 dichloromethane/methanol) and 1.43 g of the product was obtained as circular white crystals (41.9%). 1H NMR (CDCl3) δ: 3.65 (m, 24H, Crown), 2.51 (m, 2H, CH2S), 2.34 (t, 2H, COCH2), 1.62 (m, 4H, CH2), 1.33-1.27 (m, 12H, CH2). 13C NMR (CDCl3) δ: 173.9 (CO), 69.9-71.2 (CH2-Crown), 47.2, 49.3 (CNCH2), 34.5 (COCH2), 25.7, 28.8, 29.5, 29.8, 33.5 (CH2), 25.1 (CH2S). MS m/z (fragment): 464.5 (M + 1), 486.5 (M+Na), 502.5 (M + K), 947.9 (2M + Na). Monolayer Preparation. To allow complete film characterization, SAMs were deposited on large, planar gold substrates. Gold surfaces were obtained commercially from Evaporated Metal Films (EMF; Ithaca, NY). The float glass slides (25 mm × 75 mm × 1 mm) are coated with 50 Å of a chromium adhesion layer followed by 1000 Å of gold. Prior to monolayer formation, the slides were cut to the desired size and cleaned by immersion in a piranha solution (70% concentrated sulfuric acid, 30% concentrated hydrogen peroxide) at 90 °C for 10 min (Caution: piranha reacts violently with organic compounds and should not be stored in closed containers). The slides were then washed thoroughly with distilled water, followed by absolute ethanol, and blown dry in a stream of nitrogen. Cleaned slides were immediately placed placed in a 1 mM solution of the azacrown in EtOH for 18 h after which they were washed with ethanol and dried with nitrogen. New films were prepared for each characterization experiment. Contact Angle Goniometry. Sessile drop contact angle measurements were made using a Rame-Hart model 100-00 Analytical Chemistry, Vol. 80, No. 20, October 15, 2008

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goniometer (Netcong, NJ). Measurements were obtained using 1-µL drops of deionized water deposited using a calibrated Eppindorf micropipet. The sessile drop angle was measured using a protractor mounted in the eyepiece. Five measurements were taken per slide for five different samples, and the results were averaged. Ellipsometry. Ellipsometric measurements were obtained with a Manual Photoelectric Rudolf 439L633P ellipsometer (Rudolph Instruments, Fairfield, NJ). The measurements were taken at a 70° angle of incidence using a He/Ne laser (wavelength 632.8 nm). The thickness calculations were obtained with the software package that accompanies Rudolph ellipsometers. A bare gold substrate was used to determine the optical constants of gold, and the results were compared to values previously reported in the literature.29 Values for the extinction coefficient and refractive index of the samples were assumed to be 0 and 1.47, respectively.30 Five measurements were taken per slide for five different samples, and the results were averaged. Grazing Incidence Infrared Spectroscopy. Grazing incidence infrared spectra were obtained using a Thermo Electron (Waltham, MA) Nicolet FT-IR model 6700 spectrometer equipped with a Thermo Nicolet grazing angle accessory and a liquid nitrogen-cooled mercury cadmium telluride detector. The incident IR beam was 75° to the gold substrates. Prior to measurement, the optical path was purged with a stream of nitrogen for 30 min, and purging was continued during the experiments. A clean, bare gold slide was used as the background, and a new background was collected immediately prior to each sample run. The scan range was from 4000 to 1000 cm1, and 64 scans were collected for each sample. Analytical Electrode Construction. Analytical electrodes were fabricated for quantitative electrochemical characterization using 2- and 0.5-mm-diameter gold wires using a procedure similar to that of Maran.31 Electrodes were constructed by soldering a 0.5-cm length of gold wire (99.999%, Aldrich, Milwaukee, WI) to a 15-cm length of copper wire, which was placed within glass tubing, secured and sealed using Torrseal vacuum sealant (Varian Vacuum Technologies, Lexington MA), and cured overnight at room temperature. The gold surface was revealed and brought to a mirror finish by wet polishing with successively higher grit silicon carbide papers (500, 1200, 4000), followed by wet polishing with successively smaller particle size diamond pastes (6, 3, 1, and 1/4 µm) on microcloth pads, all of which were purchased from Struers (Westlake, OH) (2-propanol (Aldrich) was used as the polishing solvent). Between polishing steps the electrodes were sonicated for 30 s in 2-propanol. At the end of the polishing sequence, electrodes were sonicated for 5 min in 2-propanol with decolorizing carbon (Aldrich), sonicated in acetone for 30 s, and stored in 2-propanol. Electrochemical Setup. All voltammetric and impedimetric experiments were conducted using a Gamry Instruments (Warminster, PA) Reference 600 Potentiostat/Galvanostat/ZRA. A threeelectrode setup was used consisting of a standard saturated (29) Ordal, M. A.; Long, L. L.; Bell, R. J.; Bell, S. E.; Bell, R. R.; Alexander, R. W., Jr.; Ward, C. A. Appl. Opt. 1983, 22, 1099–1119. (30) Chechik, V.; Stirling, C. J. M. In The Chemistry of Organic Derivatives of Gold and Silver; Patai, S., Rappoport, Z., Eds.; John Wiley & Sons: New York, 1999; pp 551-640. (31) Antonello, S.; Musumeci, M.; Wayner, D. D. M.; Maran, F. J. Am. Chem. Soc. 1997, 119, 9541–9549.

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calomel reference electrode (SCE), a platinum wire counter electrode, and a gold working electrode. The electrochemical cell was assembled within a sealed four-neck flask, and the electrolyte was purged with nitrogen for 10 min before experiments. Impedance spectra were recorded using a 0 VSCE dc voltage and a 5 mV ac perturbation from 100 kHz to 1 Hz. Electrochemical Polishing. Electrochemical polishing and cleaning was accomplished by cycling a mechanically polished electrode between the redox potentials of gold. At ∼+1.0 VSCE, an oxide layer is known to form at the gold surface, which is then removed at ∼+0.6 VSCE.32,33 By successive cycling through these potentials, oxide layers are applied and removed, smoothing the surface through removal of a layer of gold atoms. A pH 7.0, 0.1 M phosphate buffer electrolyte was used for this procedure and was prepared from 0.1 M sodium monohydrogen phosphate and 0.1 M sodium dihydrogen phosphate solutions in nitrogen purged, deionized water. The potential range for polishing was -0.1 to +1.2 VSCE, and the scan rate was 50 mV/s. Every five scans, an impedance measurement was taken in order to characterize the capacitive behavior at each stage of the polishing process. Determination of Electrode True Area and Roughness. The true area of an electrode can be determined by the amount of oxide formed at the gold electrode.6,8 The amount of charge needed to reduce the chemisorbed oxygen layer on polycrystalline gold has been determined to be 390 µC/cm2 in pH 7.0, 0.1 M phosphate buffer using a scan rate of 50 mV/s.6 By integrating the area of the voltammetric oxide reduction peak and dividing by 390 µC/cm2, the true area of the electrode can be determined electrochemically. The roughness of an electrode is defined by the ratio of the true to the geometric surface area, given by eq 1.6,8,24

roughness )

Atrue Ageometric

(1)

Monolayer Preparation on Analytical Electrodes. Monolayers were prepared by immersing the freshly polished electrodes in 1 mM ethanolic solutions of the target adsorbate (commercially obtained dodecanethiol or the azacrown) for two days. Following monolayer formation, the electrodes were extensively rinsed with ethanol and deionized water and dried with nitrogen prior to characterization. Electrochemical Desorption. Electrochemical desorption of SAMs was accomplished by cycling a coated electrode through the reduction potential of the gold-sulfur bond (∼-1.0 VSCE).1 The applied potential was cycled between +0.1 and -1.5 VSCE until reproducible voltammagrams were obtained, indicating complete film desorption. The electrolyte used for this procedure was a 0.5 M potassium hydroxide solution in nitrogen purged, deionized water.20 Impedance Spectroscopy Analysis. Impedance spectroscopy involves applying a sinusoidal ac potential with a dc offset potential. The capacitive components of the system studied give sinusoidal current responses that are 90° out of phase with the ac perturba(32) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundementals and Applications; Wiley: New York, 1980. (33) Bockris, J.; Khan, S. Surface Electrochemistry: A Molecular Level Approach; Plenum Press: New York, 1993.

tion and resistive components give current responses in phase with the perturbation. Dividing the Fourier transform of the voltage by the Fourier transform of the current gives the impedance response over a frequency range, given by eq 2.13,34

Z(ω) )

E(ω) I(ω)

(2)

By fitting this response to an ideal system, the capacitive and resistive contributions can be deconvoluted and an appropriate equivalent circuit identified in order to model the system. Nonideal capacitive behavior manifests itself in complex impedance plots as a nonvertical line in the case of a Helmholtz circuit and an arc for Randle’s type systems, as shown in Figure 1.13,34 This nonideal behavior can be represented by a CPE, which is a complex impedance with a phase angle (Φ) between 90° (ideal capacitor) and 0° (ideal resistor). The impedance of a constant phase element (ZCPE) is defined in eq 3, where Y0 represents the capacitancetype quantity (CPE), which is determined by model fitting; j is the square root of -1; ω is the frequency; and R (CPE ideality factor) is a fractional term related to the phase angle (Φ) by eq 4.13,15,34

ZCPE )

1 Y0(jω)R

R)

Φ 90

(3)

(4)

Assuming that the CPE represents a distribution of capacitances across an electrode surface, eqs 5 and 6 have been proposed to yield an averaged capacitance from the CPE circuit parameters for the CPE-Helmholtz and CPE-Randles circuits (shown in Figure 1), respectively.13,14,35 R

C)

Y0 -1 Rsoln

(5)

R

C)

Y0 -1 -1 RSAM + Rsoln

(6)

These equations were used to extract capacitance values from CPE circuit fittings as Y0, although this form of capacitance has units that depend on R and is therefore not comparable between different systems. Atomic Force Microscopy. Atomic force microscopy (AFM) imaging was performed in intermittent-contact mode with an Autoprobe M5 AFM (Veeco Metrology, Santa Barbara, CA). A commercial silicon AFM tip (Mikromasch NSC11 A) was used, and its radius was measured to be 83 nm with 9% accuracy by imaging a δ function grating (Mikromasch TGT01).36 Images were processed by flattening to remove the background slope. WSxM (34) Gabrielli, C. Centre National de la Recherche Scientifique; Paris, 1998. (35) Shervedani, R. K.; Mozaffari, S. A. Anal. Chim. Acta 2006, 562, 223–228. (36) Thoreson, E. J.; Burnham, N. A. Rev. Sci. Instrum. 2004, 75, 1359–1362.

Scanning Probe Microscopy software was used to render the images in 3D.37 RESULTS AND DISCUSSION SAM Characterization. SAMs of alkanethiols on gold are commonly shown to form well-ordered surface films using various characterization techniques including electrochemistry, contact angle goniometry, infrared spectroscopy, and ellispometry.38 In order to establish that SAMs of the azacrown formed well-ordered monolayers, the films were completely characterized on goldcoated slides. Contact angle measurements of 51.9° (±1.9°) indicated partial wetting of the surface, as expected for a crown ether terminal group.39 Ellipsometric measurements produced a thickness of 1.36 ± 0.68 nm, and electrochemical analysis gave capacitance values of 3.02 ± 0.43 µF/cm2 for monolayers of the azocrown. Combining ellipsometric thickness data with average capacitance data, the dielectric constant of the azacrown SAM was calculated to be 4.6 (using eq 7, discussed below), which is much closer to that of polyethylene (2.3) than water (80), indicating that there is minimal water present in the layer and therefore minimal defects.40 Additionally, the saturated hydrocarbon infrared stretches of the azacrown SAM (2925 and 2859 cm-1) (data not shown) were close to those of liquid polyethylene (2928 and 2856 cm-1), which leads us to believe the SAM exhibits semicrystalline packing.41 The lack of highly crystalline packing is expected as the azocrown has a bulky headgroup, which prohibits a close arrangement of the alkyl chains that would be seen in monolayers composed of a compound with a small headgroup such as dodecanethiol. The compilation of characterization data on bulk films indicates relatively well ordered monolayer formation considering that the azocrown contains a bulky headgroup. Electrochemical Polishing. Analytical electrodes were always prepared in an identical manner (as described in the Experimental Section), although small differences in the construction and mechanical polishing of each electrode can be expected to result in deviations in electrochemical behavior. Electrodes were oxidatively cycled repeatedly, and their true area was determined as described in the Experimental Section. Roughness was calculated from the ratio of the true area to the geometric area (eq 1). Plots of the electrode roughness versus number of oxidative cycles, Figure 3, show each polishing profile divided into three stages. The first stage is a rapid increase in the apparent roughness, the second, a rapid decrease in the roughness, and the third, a slow decrease in the roughness that approaches a constant value. The first stage corresponds to the oxidative removal of contaminants from the electrode; the presence of impurities on the surface would impede the initial formation of an oxide layer resulting in a reduced magnitude of current as the oxide is removed and an apparent increase in roughness.8,33 Following contaminant re(37) Horcas, I.; Fernandez, R.; Gomez-Rodriguez, J. M.; Colchero, J.; GomezHerrero, J.; Baro, A. M. Rev. Sci. Instrum. 2007, 78, . 13705/013701013705/013708. (38) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559–3568. (39) Flink, S.; Van Veggel, F. C. J. M.; Reinhoudt, D. N. J. Phys. Chem. B 1999, 103, 6515–6520. (40) Weast, R. C., Ed. CRC Handbook of Chemistry and Physics, 55th ed.; CRC Press Inc.: Cleveland, OH, 1975. (41) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 5145– 5150.


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Table 1. Summary of Dodecanethiol SAM Characterization Resultsa electrode capacitance (µF/cm2) CPE ideality factor roughness Figure 2. Structure of Azacrown.

Figure 3. Electrode roughness versus polishing cycles for electrodes A-E.

moval, a maximum roughness factor is observed after which only electrochemical polishing takes place. Impedance measurements, taken every five scans for three selected electrodes, confirm initial contaminant removal, followed by electrochemical polishing (data not shown). The impedance spectra were fit using a Helmholtz circuit (Figure 1) with a CPE in place of the capacitor. Fitting results show that the capacitance and the CPE ideality factor both increase significantly during the initial cleaning phase and plateau at the same point as the maximum roughness is observed. Given the mechanism previously described,8 this procedure facilitates polishing the surface on the molecular scale and it is therefore unlikely that roughness on a much larger scale is significantly influenced by oxidative cycling. In other words, electrochemical polishing can only eliminate small-scale (“microscopic” 1-10 nm) imperfections of the electrode surface, and larger scale (“macroscopic” >10 nm) defects must be removed first by mechanical polishing in order to prepare uniformly smooth substrates for monolayer deposition. In this study, electrochemically cleaned and polished electrodes were functionalized with different SAMs to investigate the relationship between substrate roughness and nonideal capacitive behavior of SAMs. Although the absolute values of capacitance for electrodes A-E were different, the relative values of capacitance for both dodecanethiol and the azacrown functionalized electrodes were the same; i.e., electrode C always had the largest capacitance and electrode E the smallest. Self-Assembled Monolayers. Dodecanethiol was deposited on each of the cleaned and polished electrodes, and the resulting films were characterized by impedance spectroscopy. The spectra were fit using either a Helmholtz or Randles circuit with a CPE, 7674


A

B

C

D

E

2.2 0.94 2.4

2.1 0.93 2.9

2.4 0.93 3.3

2.0 0.93 2.7

1.9 0.93 2.7

a Standard deviation, capacitance e0.1 of three measurements; CPE ideality factor, e0.01 of three measurements; roughness, e0.05 of five measurements.

Figure 4. Experimental azacrown impedance data fit with a Randles circuit (red) and a CPE-modified Randles circuit (blue).

and eq 5 or 6 was used to extract SAM capacitance in addition to CPE ideality factors (R) from the fitting results. A summary of the impedance results for dodecanethiol SAMs is provided in Table 1. Solution and SAM resistance data for all monolayermodified electrodes are included as Supporting Information. Capacitance values obtained are larger than those typically seen for the same type of films due to a greater frequency of defects in the layer, allowing water penetration and resulting in an increase in the dielectric constant of the bulk film.12,39,42 A higher frequency of defects is indicated by a relatively low CPE R factor for this type of SAM, which has been shown to directly correlate to defect frequency.22 Additionally, the larger capacitance may also be the result of a greater effective area than the geometric area alone indicates, as discussed in more detail below. All electrodes were subjected to reductive desorption followed by electrochemical cleaning and polishing. The azacrown was deposited on each electrode, and the resulting films were characterized by impedance spectroscopy. Impedance data were fit to a Randle’s circuit with a CPE. These SAMs have bulky head groups (see Figure 2) allowing penetration of water that is not a result of film defects, and therefore, another resistive component is needed in the model circuit. The inclusion of a CPE element in place of a capacitor dramatically improved the Randles circuit fitting to the experimental data, as shown in Figure 4 where both types of fits are provided. The impedance fit and eqs 1 and 6 were used to extract the SAM CPE ideality factor (R), the substrate roughness, and the SAM capacitance, and these values are presented in Table 2. (42) Flink, S.; Boukamp, B. A.; van den Berg, A.; van Veggel, F. C. J. M.; Reinhoudt, D. N. J. Am. Chem. Soc. 1998, 120, 4652–4657.

Table 2. Summary of Azacrown SAM Characterization Resultsa

Table 3. Summary Azacrown Characterization Results on Unpolished Electrodesa

electrode capacitance (µF/cm2) CPE ideality factor roughness

electrode

A

B

C

D

E

3.5 0.95 2.4

3.8 0.93 3.1

4.2 0.93 3.8

3.4 0.92 3.1

2.6 0.90 3.8

capacitance (µF/cm2) CPE ideality factor roughness

F

G

H

I

2.9 0.85 4.6

2.5 0.89 3.6

2.7 0.90 4.0

2.4 0.87 4.5



Trends in Capacitive Behavior. Analysis of the impedance data for dodecanethiol and the azacrown on the same set of electrodes leads to the identification of several trends that allow for the correlation of capacitance to electrode characteristics. Both sets of SAM’s show a similar trend in the capacitance magnitude, with the exception of electrodes A and B.

defects are apparent in the CPE ideality factor. Deviations between the total substrate roughness and the SAM capacitance trends can be explained in terms of different levels of microscopic roughness (see below). Because electrode A consistently has the lowest roughness factor (i.e., lowest true surface area), it may be expected to have the lowest capacitance. However, this is not the case and our interpretation of the relative contributions of macroscopic and microscopic roughness to capacitance leads to the conclusion that, for electrode A, the microscopic roughness is minimal and the larger capacitance arises because of the relatively large contribution of the macroscopic roughness. An examination of the relative CPE ideality factors for SAMs formed on the five electrodes confirms minimal defects for SAMs on electrode A (close to ideal capacitive behavior), indicating minimal microscopic roughness.

C > A > B > D > E (dodecanethiol capacitance) C > B > A > D > E (azacrown capacitance) The trends in SAM capacitive magnitude are electrode dependent, indicating that the characteristics of the electrode determine the absolute capacitance. SAMs formed on electrode C consistently show the greatest capacitance. However, it is unlikely that this is simply due to defects in the film, since ideality factors for SAMs on electrode C are consistently in the median of those seen on other electrodes (see Tables 1 and 2). Therefore, this capacitive trend is likely an effect of increased coating area which is directly related to capacitance by eq 7 (where 0 is the permittivity of vacuum, r is the dielectric constant of the layer, A is the area, and d is the thickness of the layer). C)

ε0εrA d

(7)

Roughness, by definition, increases the area of the naked metal electrode. Thus, it is conceivable that electrode roughness could also increase interfacial area of the SAM increasing the coated electrodes’ effective area and capacitance. If this is true, then monolayers deposited on the roughest electrode would have the largest capacitance, which in fact is what was observed (electrode C). However, this relationship is not born out across the entire electrode series. C > E > B > D > A (gold substrate roughness factor) The roughness factor gives a measure of the total roughness, which is the product of macroscopic and microscopic roughness on the substrate. Macroscopic roughness has been shown not to influence SAM packing.21 We propose that macroscopic roughness primarily influences the area of the SAMs rather than the degree of order in the film and, therefore, is directly related to the capacitance rather than the CPE ideality factor. Microscopic roughness has been shown to create defects in SAMs and influence their organization.21 We propose that microscopic roughness primarily results in disorder of the SAMs and these

A > B > C > D > E (capacitive ideality factors of azacrown) Conversely, electrode E has a high roughness value, but SAMs deposited on electrode E consistently give the lowest capacitance values. The smaller capacitance indicates a low relative macroscopic roughness for electrode E, but because the total roughness is high, the contribution of microscopic roughness is large. This reasoning is supported by the capacitive ideality trend shown above in which SAMs formed on electrode E show the greatest frequency of defects, resulting from a substrate with significant microscopic roughness. In order to further investigate microscopic roughness, SAMs of the azacrown were deposited on a different set of electrodes that had not been electrochemically polished. Unpolished electrodes are expected to have significant microscopic roughness resulting in SAMs with a large number of defects and therefore a low capacitive ideality factor when examined with impedance spectroscopy. The characterization results for monolayers on unpolished electrodes are summarized in Table 3. As expected, impedance measurements demonstrate that substrates with significant microscopic roughness will result in adsorbed SAMs with a high degree of defects as evidenced by a decreased ideality factor. These results for unpolished electrodes strongly support the conclusions drawn in the previous discussion of electrode E. Atomic Force Microscopy. To confirm that electrochemical polishing removes roughness on a microscopic scale, and to further support that microscopic roughness causes CPE-type behavior in SAMs, an AFM study was conducted on electrochemically polished and unpolished electrodes. AFM measurements were made on several different scales to adequately distinguish “macroscopic” and “microscopic” features. Analytical Chemistry, Vol. 80, No. 20, October 15, 2008

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Figure 5. The 500 × 500 nm AFM images of unpolished (left) and electrochemically polished (right) electrodes.

Imaging on a macroscopic scale (5 × 5 µm) revealed no significant differences between the two surfaces (data not shown). Electrochemical polishing should only remove defects on a much smaller scale, so the macroscopic similarity of the two surfaces is expected. Imaging on a smaller scale (500 × 500 nm) revealed significant differences in the unpolished and polished topographies, as shown in Figure 5. On the unpolished electrode (Figure 5, left) well-defined, deep “canyons” are present, which are caused by the grain boundaries of polycrystalline gold. These grain boundaries have been shown to cause defects in SAMs.21,28 On the polished electrode (Figure 5, right), the same well-defined, deep “canyons” are absent. Electrochemical polishing therefore reduces the effects of gold grain boundaries by removing microscopic roughness associated with these boundaries (effectively smoothing these regions out). Capacitance and Microscopic Roughness. Macroscopic and microscopic substrate roughnesses have different effects on adsorbed self-assembled monolayers. Macroscopic roughness is defined as roughness that affects the total area of the coated electrode and thus the film capacitance, independent of layer ideality. Microscopic roughness is defined as roughness that affects both the film capacitance and ideality. It is impossible to directly distinguish these effects since the observed capacitance (COBS) is a function of both macroscopic (RMACRO) and microscopic (RMICRO) roughness as shown in eq 8. COBS ) C(RMACRO, RMICRO)

(8)

Given that RMACRO is defined as only influencing the area of the coated electrode, it only affects the area term in eq 7 and is therefore directly proportional to the observed capacitance (COBS). Therefore, the role of RMACRO in eq 8 can be specifically defined as shown in eq 9. COBS ) C(RMICRO)RMACRO

(9)

Total substrate roughness (provided by electrochemical measurements) is a combination of the macroscopic and microscopic roughnesses, which compound each other (eq 10). RTOTAL ) RMACRO × RMICRO

(10)

Combining eq 9 and eq 10, a form of capacitance is obtained that cancels out the area effects of roughness. As eq 11 illustrates, for a given SAM-coated electrode, this capacitance per unit true area is only a function of the microscopic roughness. COBS ⁄ RTOTAL ) 7676

C(RMICRO)RMACRO C(RMICRO) ) RMACRORMICRO RMICRO

(11)


Figure 6. Relationship between dodecanethiol SAM capacitance per unit true area of the substrate and CPE ideality factor (red circles, experimental data; blue square, literature value included for comparison, not used in linear data fit).

Equation 11 demonstrates that experimentally measured capacitance and substrate roughness values can be used to determine a form of capacitance that is only dependent on microscopic roughness. This capacitance per unit true area can then be used to indirectly evaluate relative microscopic roughness. Capacitance per Unit True Area and CPE Ideality. As discussed above, both the CPE ideality factor and the capacitance per unit true area of a monolayer depend only on microscopic substrate roughness. Therefore, CPE ideality and capacitance per unit true area are both functions of microscopic roughness and can be directly related by eq 12. COBS ⁄ RTOTAL ≈ F(R)

(12)

An examination of the relationship between the capacitance per unit true area and CPE ideality factor for SAMs of dodecanethiol confirms this direct relationship, as shown in Figure 6. The plot demonstrates that, as the CPE ideality factor increases, the capacitance per unit true area increases in a linear fashion. The increase in CPE ideality indicates a decrease in the amount of defects in the layer due to an overall decrease in the microscopic roughness of the electrode. As predicted by eq 11, this decrease in microscopic roughness leads to an increase in the capacitance per unit true area of the monolayer. Additionally, more support for the direct relationship proposed is given by examining other impedance studies of similar SAMs.12 As the CPE ideality factor approaches unity, RMICRO approaches unity and the capacitance per unit true area approaches a real quantity representing an ideal unit capacitance. Near-ideal dodecanethiol films can be used as an approximation of this ideal unit capacitance and to evaluate the validity of the proposed relationship. It has been shown that near-defect-free dodecanethiol SAMs (R ) 0.986 ± 0.001) have a reproducible capacitance of 1.44 ± 0.1 µF/cm2.12 This capacitance value supports a direct relationship as the linear fit shown in Figure 6 predicts a capacitance of 1.40 µF/cm2 for a SAM with a capacitive ideality of 0.986. However, in order to make this comparison, it is assumed that the capacitance value obtained from the literature is corrected for true area where all effects of RMACRO have been removed. This assumption can be justified because a significantly smaller scale mechanical polishing grit was

chemical polishing has been shown to increase the CPE ideality. Circuit models that include a CPE have been widely used in previous studies to examine self-assembled monolayer systems that have nanotechnology applications. The use of a CPE fitting element indicates disorder in SAMs, which can lead to variation from sample to sample. SAM disorder manifests itself as a frequency dependence of the capacitance that hampers capacitive detection as it is impossible to directly measure a pure capacitance from such systems. In addition, the presence of defects often inhibits layer functioning as active head groups are not effectively organized. Therefore, understanding the source of capacitive nonideality and being able to attenuate it is important to improve reproducibility, ease of capacitance determination, and optimize film functioning for various applications that use SAMs. Figure 7. Relationship between azacrown SAM capacitance per unit true area of the substrate and capacitive ideality factor (open red circles, experimental data; solid red circle, experimental data not used in the linear fit).

used in the study referenced (0.05 µm for the referenced study compared to 0.25 µm in these experiments) which removed virtually all macroscopic substrate roughness.12 A similar relationship is observed for monolayers of the azacrown. In Figure 7, a plot of the capacitance per unit true area versus the CPE ideality factor for SAMs, the azacrown on both electrochemically polished and unpolished electrodes is shown along with a fit for the linear portion of the data. Once again, the direct relationship indicates that the capacitance per unit true area for a monolayer-modified substrate is only dependent on microscopic roughness and is independent of large-scale (macroscopic) defects. We note that there is a limit on the maximum microscopic roughness possible on an individual electrode, and therefore, the linear relationship will not continue once this limit is reached when all features are on the scale characteristic of the polishing process. The point in Figure 7 with the lowest CPE ideality (0.85, solid red circle in the plot) is seen to deviate significantly from the direct relationship, likely due to the microscopic roughness having reached a maximum for this unpolished electrode, and was not included in the fit. Additionally, by extrapolating to R ) 1 (a defectfree layer), an ideal unit capacitance value for azacrown SAMs of 2.0 µF/cm2 is obtained. This ideal value for SAMs of the azacrown is reasonable for a monolayer with a bulky crown ether terminal group that would be expected to have a larger capacitance than an ideal SAM of a simple alkanethiol (1.44 ± 0.1 µF/cm2 for dodecanethiol).12 We have directly related the CPE ideality factor to microscopic roughness though a capacitance per unit true area and electro-

CONCLUSIONS Cyclic voltammetry can be used to clean and polish a gold electrode on the molecular scale, and the polishing process can be monitored simultaneously. For SAM-coated electrodes, substrate roughness can be divided into two categories: macroscopic, influencing solely the capacitance; and microscopic, influencing both the layer ideality and the capacitance. A method has been developed that quantitatively relates microscopic roughness to the constant phase element ideality of SAMs through measurements of the SAM capacitance per unit true area. As a result, it is possible to modulate monolayer ideality through the electrochemical polishing procedure investigated. This control is especially important for SAMs with bulky head groups, which appear to be more susceptible than simple alkanethiol SAMs to the effects of microscopic roughness. In addition, this type of analysis can be used to determine the effects of substrate roughness on overlying coatings and clarify the source of CPE-type behavior that has been often observed but poorly defined. ACKNOWLEDGMENT The authors gratefully acknowledge financial support from the U.S. Army Medical Research and Materiel Command. SUPPORTING INFORMATION AVAILABLE Solution and SAM resistance data for all monolayer-modified electrodes. This information is available free of charge via the Internet at http://pubs.acs.org.

Received for review March 11, 2008. Accepted July 25, 2008. AC800521Z


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Effect of Electrode Roughness On the Capacitive Behavior of Self

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