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Influence of Applied Potential on the Impedance of Alkanethiol SAMs Harmonie Sahalov,†,§ Brigid O’Brien,†,§ Kathleen J. Stebe,‡,† Kalina Hristova,† and Peter C. Searson*,† Department of Materials Science and Engineering, Johns Hopkins UniVersity, Baltimore, Maryland 21218 ReceiVed May 14, 2007. In Final Form: June 26, 2007 Self-assembled monolayers are generally considered to behave as dielectric layers with a capacitance that is dependent on the monolayer thickness and the relative permittivity that is determined by the hydrocarbon tail. We show that the impedance response of alkanethiol-modified gold surfaces can be modeled as a parallel network consisting of the capacitance and resistance of the monolayer over a wide potential range. At potentials positive to -0.3 V (Ag/AgCl), the monolayer resistance is greater than 106 Ω cm2; however, at more negative potentials, the monolayer resistance decreases exponentially with potential with an inverse slope of about 250 mV. Over the same potential range, the monolayer capacitance is independent of potential. Although the same behavior is observed on ultrasmooth, templatestripped gold, the resistance at any potential is larger than for evaporated gold. The progressive increase in permeability of the monolayer is associated with an increase in electric field at potentials negative to the potential of zero charge.
Introduction Self-assembled monolayers (SAMs) of alkanethiols are routinely used for patterning surfaces, changing surface wetting properties, or modifying surface chemistry. In many of these applications, the electrical properties of the SAM are critical to performance. For example, alkanethiol monolayers have been explored as molecular resists for etching and electrodeposition.1-3 The electrode potential is a key parameter in electrodeposition and hence the potential dependence of the electrical properties of the SAM are of particular significance. Self-assembled monolayers are usually considered to behave as dielectric layers with a capacitance that is dependent on the monolayer thickness.4-6 The metal/SAM/solution interface is usually modeled as a series resistance due to the solution, leads, and contacts, in series with a capacitor associated with the monolayer. In the absence of electroactive species, the resistance of the monolayer is infinite, and there is no dc current flow across the interface. Any potential perturbation results in a transient current due to charging of the monolayer capacitance. The capacitance of alkanethiol monolayers is inversely dependent on chain length, consistent with the parallel plate capacitor model with C ) 0/d, where is the relative permittivity, 0 is the permittivity of free space, and d is the distance between the two plates. From the monolayer thickness, determined from the length of the molecule and the tilt angle, the relative permittivity is generally taken to be about 2. More recently, it has been suggested that the simple ideally polarizable electrode model of self-assembled monolayers with * To whom correspondence should be addressed. E-mail: searson@ jhu.edu. † Department of Chemical and Biomolecular Engineering. ‡ Department of Materials Science and Engineering. § Both authors contributed equally. (1) Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10 (5), 1498-1511. (2) Pesika, N. S.; Fan, F. Q.; Searson, P. C.; Stebe, K. J. J. Am. Chem. Soc. 2005, 127 (34), 11960-11962. (3) Pesika, N. S.; Radisic, A.; Stebe, K. J.; Searson, P. C. Nano Lett. 2006, 6 (5), 1023-1026. (4) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109 (12), 3559-3568. (5) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310 (1-2), 335-359. (6) Lingler, S.; Rubinstein, I.; Knoll, W.; Offenhausser, A. Langmuir 1997, 13 (26), 7085-7091.
a solution resistance in series with a monolayer capacitance is not adequate to describe the electrical behavior. Impedance spectroscopy measurements have indicated that there is a conductive pathway parallel to the monolayer capacitance and that this parallel resistance is associated with ion permeability of the SAM.7,8 However, rationalizing results in this field is complicated by the fact that virtually all experiments have been performed in a narrow potential range around 0 V (Ag/AgCl). The range of stability of thiol monolayers is bounded at negative potentials by reductive desorption of the thiol according to R-SAu + H+ + e- f R-SH + Au.5,9,10 Reductive desorption is characterized by a peak in voltammograms where the peak potential is dependent on the chain length n with a slope of -20 mV/n.5 Since reductive desorption occurs at negative potentials, there is a broad potential range of more than 1.5 V where SAMs are stable. As noted above, most prior studies of the electrical properties of SAMs have been performed near 0 V (Ag/AgCl), and hence, the potential dependence of the electrical properties of SAMs are, at present, unknown. In this paper, we report on the influence of applied potential on the impedance of alkanethiols on gold. We show that the impedance response can be modeled as a parallel network consisting of the SAM capacitance and resistance over a potential range of more than 1.5 V. Although the SAM capacitance remains constant from positive potentials to the onset of reductive desorption, the SAM resistance exhibits two characteristic regimes. At potentials positive to -0.3 V (Ag/AgCl), the resistance is very large and independent of potential, whereas at more negative potentials, the resistance decreases exponentially with potential. These results show that alkanethiols become progressively more permeable as the onset of reductive desorption is approached and highlight the importance of understanding the influence of the applied potential on the electrical properties of SAMs. Experimental Methods SAMs were formed on gold films evaporated on silicon wafers (Montco Silicon Technologies, Inc.) or template-stripped gold (see (7) Janek, R. P.; Fawcett, W. R.; Ulman, A. J. Phys. Chem. B 1997, 101 (42), 8550-8558. (8) Protsailo, L. V.; Fawcett, W. R. Electrochim. Acta 2000, 45 (21), 34973505. (9) Yang, D. F.; Wilde, C. P.; Morin, M. Langmuir 1996, 12 (26), 6570-6577. (10) Yang, D. F.; Wilde, C. P.; Morin, M. Langmuir 1997, 13 (2), 243-249.
10.1021/la701398u CCC: $37.00 © 2007 American Chemical Society Published on Web 08/18/2007
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Figure 1. Current-voltage curves for bare gold and C18H37SH (ODT) on gold in PBS at a scan rate of 100 mV s-1. below). Self-assembled monolayers were formed with the following alkanethiols: octadecanethiol (ODT, 98.5%, Aldrich), 1-nonanethiol (95%, Aldrich), and 1-tetradecanethiol (98%, Aldrich). The gold substrates used were first cleaned with ethanol, dried with N2, and then incubated for 24 h in a 3-5 mM thiol solution prepared with ethanol (200 Proof, ACS/USP grade). All glassware was soaked in 18 M sulfuric acid overnight and rinsed with distilled and deionized water prior to use. After incubation, the samples were rinsed with ethanol to remove any unbound thiols, dried with N2, and subsequently placed in a three-electrode Teflon cell with a 3 M Ag/AgCl reference electrode (Ueq ) 0.200 V vs SHE) and a platinum mesh counter electrode positioned parallel to the gold surface. All potentials are reported versus the Ag/AgCl reference. All experiments were performed in phosphate buffered saline, PBS (10 mM phosphate and 150 mM sodium chloride), pH 7.4 (Sigma Aldrich). In further experiments evaluating the effect of surface roughness on the impedance response, octadecanethiol SAMs were formed on template-stripped gold (TSG).11-13 The TSG substrates were fabricated by evaporating 50 nm gold onto a p-Si (001) wafer (University Wafers). Wafers were cleaned with distilled water, followed by acetone and isopropanol prior to use. After evaporation, an epoxy (EPO-TEK 364, Epoxy Technology, Billerica, MA) was placed on the gold surface followed by a glass slide. The sample was then cured in an oven at 120 °C for 15 min. To limit any contamination of the gold surface, the silicon wafer was removed to expose the TSG substrate immediately prior to incubation of the SAM. The surface roughness was characterized using atomic force microscopy (AFM, Agilent 5500 AFM/SPM microscope). Electrochemical impedance spectroscopy was performed with a computer-controlled potentiostat (Solartron 1286) connected to an impedance analyzer (Solartron 1260). An rms perturbation of 20 mV was superimposed on the applied potential and the impedance measured over the frequency range from 0.1 Hz to 100 kHz. All experiments were conducted at room temperature. The impedance spectra were analyzed using a complex nonlinear least fit squares immittance fitting program (Z-plot, Scribner Associates). All data were analyzed using an equivalent circuit with a series resistance Rs comprising the contacts and the solution resistance in series with a constant phase element Z ) 1/Q(iω)n in parallel with the resistance of the monolayer Rm. In all cases, the exponent n was very close to 1, and we take Q ) Cm, the capacitance of the monolayer. All experiments were repeated three times.
Results and Discussion Figure 1 shows voltammograms for bare gold and ODTmodified gold in PBS. For the ODT-modified gold surface, the current is negligible over a wide potential range from the positive (11) Wagner, P.; Hegner, M.; Guntherodt, H. J.; Semenza, G. Langmuir 1995, 11 (10), 3867-3875. (12) Gupta, P.; Ulman, A.; Fanfan, S.; Korniakov, A.; Loos, K. J. Am. Chem. Soc. 2005, 127 (1), 4-5. (13) Losic, D.; Shapter, J. G.; Gooding, J. J. Langmuir 2001, 17 (11), 33073316.
Figure 2. Impedance spectra for a C18H37SH (ODT) SAM on gold in PBS at (a) 0 and (b) -0.5 V. The solid lines correspond to nonlinear least-squares fits from which we calculate the monolayer resistance and capacitance. The inset shows the equivalent circuit used to analyze the impedance spectra.
potential limit to about -0.9 V. The current onset at about -0.9 V is followed by a peak at -1.1 V due to reductive desorption of the thiol: R-S-Au + H+ + e- f R-SH + Au.5,9,10,14 Figure 2 shows typical impedance spectra for an ODT monolayer on gold in PBS at 0 and -0.5 V. The impedance spectra show the characteristic response of a resistance Rs in series with a parallel RC network corresponding to the monolayer resistance Rm and capacitance Cm.15 The solid lines correspond to nonlinear least-squares fits from which we can extract the values of Rm and Cm. At 0 V, the slope of the Bode plot is -0.98 and the maximum phase angle is very close to 90°. At 0.1 Hz, the impedance response is purely capacitive and hence we can only place a lower limit on the resistance of the monolayer of about 106 Ω cm2. The impedance response at -0.5 V, however, shows important differences compared to the impedance at 0 V. At low frequency, the phase angle decreases and the magnitude of the impedance approaches a constant value. These features are characteristic of an ideal parallel plate capacitor in parallel with a resistance. From nonlinear least-squares fits to the spectra, we can extract the capacitance and resistance of the SAM over the potential range of interest. Figure 3a shows the capacitance versus applied potential. Over a wide potential range from 0.4 to -0.9 V, the monolayer capacitance Cm is 0.83 ( 0.04 µF cm-2, independent of potential. This is in good agreement with values reported in the literature measured at 0 V.5,6 The monolayer capacitance was highly reproducible with an average value of 0.82 ( 0.04 µF cm-2 at 0 V. Since the SAM capacitance is much smaller than the double layer capacitance at a bare gold surface, the change in surface charge density over this potential range is about 1.5 µC cm-2, much smaller than over the same potential range at an unmodified surface. (14) Pesika, N. S.; Stebe, K. J.; Searson, P. C. Langmuir 2006, 22 (8), 34743476. (15) Macdonald, D. D. Transient techniques in electrochemistry; Plenum Press: New York, 1977.
Impedance of Alkanethiol SAMs
Figure 3. (a) Capacitance and (b) resistance versus applied potential for bare gold and a C18H37SH (ODT) SAM on evaporated gold. The capacitance was obtained from fitting the full spectrum at each potential.
As shown in Figure 3a, the capacitance begins to increase at -1.0 V. Comparison to Figure 1 shows that the onset of the capacitance increase agrees very well with the onset of reductive desorption in the voltammogram. The capacitance increases from about 1 to about 10 µF cm-2 at -1.2 V, close to the value measured on bare gold. The double layer capacitance for bare gold, also shown in the figure, is about 30 µF cm-2 in the potential range from 0 to -1.2 V. Figure 3b shows the SAM resistance Rm versus applied potential. From 0.4 to -0.3 V, the monolayer resistance is greater than 106 Ω cm2, as expected for a good dielectric layer. However, at more negative potentials, from -0.3 to -1.0 V, the resistance decreases exponentially with potential. The inverse slope is 268 mV decade-1 corresponding to a decrease of about 3 orders of magnitude from almost 107 to less than 104 Ω cm2 at the onset of reductive desorption. The average inverse slope for ODT on evaporated gold was 257 ( 14 mV decade-1. This behavior is similar to the behavior of a solid-state diode where the conductivity is low over a fixed potential range (reverse bias) but increases exponentially (forward bias) beyond a characteristic potential. The decrease in impedance indicates that the ODT monolayer becomes progressively more leaky at more negative potentials. The exponential dependence suggests that there is an energy barrier associated with ion penetration that is dependent on the magnitude of the electric field across the monolayer. At -1.0 V, the resistance of the ODT monolayer converges with the charge-transfer resistance associated with the unmodified gold substrate resulting in no obvious discontinuity associated with reductive desorption. With the exception of voltammetry, which has been widely used to study reductive desorption, there have been surprisingly few reports of the potential dependence of the electrical properties of self-assembled monolayers. Electrochemical quartz crystal microbalance experiments have revealed a decrease in mass that correlates with the reductive desorption peak in voltammograms,
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but no change in mass was seen at positive potentials prior to the onset of reductive desorption.16 Anderson and Gatin17 observed no significant changes in polarization modulation Fourier transform infrared absorption spectra for ODT SAMs at 0.5, 0, and -0.5 V. We propose the following explanation for the exponential decrease on Rm with applied potential shown in Figure 3. At potentials positive to the potential of zero charge of the ODTmodified surface, the gold has a positive surface charge and the solution at the interface with the ODT monolayer has a negative charge of equal magnitude. The thiol molecules have a dipole moment due to the large electronegativity of the sulfur headgroup in comparison to the components of the alkane chain. If the thiol were not chemisorbed to the gold surface, the electric field would still align the thiol dipole with the sulfur toward the surface. At potentials negative to the potential of zero charge, however, the electric field across the monolayer would flip the thiol dipole if the sulfur headgroup were not tethered to the surface. Since the thiol is tethered to the surface through a chemical bond, a negative electric field across the monolayer could result in a conformation change of the thiol in the vicinity of the surface where the dipole moment is largest. The induced conformational change would be expected to be most significant at domain boundaries and other defects where the ordering and hence steric hindrance is reduced. The conformational changes, in turn, allow ions to penetrate into the monolayer resulting in a decrease in the resistance. Electrical measurements are particularly sensitive to short circuit pathways, and hence, it is not surprising that this effect has not been detected by vibrational spectroscopy techniques. The determination of the potential of zero charge for thiolmodified gold has been the subject of some debate in the literature. Iwami et al.18 reported a value of -0.48 V (Ag/AgCl) for ODT on gold evaporated on mica in 0.1 M NaClO4 using contact angle measurements. Sondag-Huethorst and Fokkink19-21 reported a value of -0.45 V (Ag/AgCl) for ODT on Au evaporated onto mica in 10 mM K2SO4 using the Wilhelmy plate method. Pope and Buttry22 reported a value of -0.40 for undecanethiol on evaporated gold in NaNO3 using the Stark effect. Becka and Miller23 reported a value of about -0.4 V for undecanethiol on sputtered gold in CF3COONa from capacitance-voltage curves. The onset of the decrease in SAM resistance at -0.3 V shown in Figure 3 is close to the values of the potential of zero charge for ODT-modified gold surfaces reported in the literature. As the potential is shifted negative to the potential of zero charge, the field exerts a torque on the headgroup region of the ODT resulting in conformational changes that may be largest at sites such as domain boundaries where there are more configurational degrees of freedom. This in turn would result in ion penetration into the monolayer and a decrease in the resistance. As the potential is shifted further negative the field strength is increased, and the conformational changes would also be expected to increase, spreading inward from a domain boundary toward the center of the domain. (16) Kawaguchi, T.; Yasuda, H.; Shimazu, K.; Porter, M. D. Langmuir 2000, 16 (25), 9830-9840. (17) Anderson, M. R.; Gatin, M. Langmuir 1994, 10 (6), 1638-1641. (18) Iwami, Y.; Hobara, D.; Yamamoto, M.; Kakiuchi, T. J. Electroanal. Chem. 2004, 564 (1-2), 77-83. (19) Sondag-Huethorst, J. A. M.; Fokkink, L. G. J. Langmuir 1992, 8 (10), 2560-2566. (20) Sondag-Huethorst, J. A. M.; Fokkink, L. G. J. Langmuir 1995, 11 (6), 2237-2241. (21) Sondag-Huethorst, J. A. M.; Fokkink, L. G. J. J. Electroanal. Chem. 1994, 367 (1-2), 49-57. (22) Pope, J. M.; Buttry, D. A. J. Electroanal. Chem. 2001, 498 (1-2), 75-86. (23) Becka, A. M.; Miller, C. J. J. Phys. Chem. 1993, 97 (23), 6233-6239.
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Figure 5. AFM images and line scans for (a) evaporated gold and (b) TSG. The z scale for both images is 7.32 nm.
Figure 4. (a) Capacitance and (b) resistance versus applied potential for C9H19SH, C14H29SH, and C18H37SH on evaporated gold.
Taking the potential of zero charge as -0.3 V, at the onset of reductive desorption at -1.0 V, the electric field strength across the ODT SAM (taking d ) 2 nm) is 3.5 × 106 V cm-1. This is larger than the breakdown field for silicon (about 3 × 105 V cm-1) and similar to the field required to induce electroporation in mammalian cells (1 × 106 to 2 × 106 V cm-1).24 Haag et al. reported on the breakdown voltage of alkanethiol SAMs on various metals in contact with a mercury drop modified with the same alkanethiol.25 For Hg-SAM/SAM-Ag junctions, the breakdown voltage was determined from the bias where the current dramatically increased. The breakdown field was found to increase with increasing chain length, reaching a maximum value of 8 × 106 V cm-1 for C16H33SH thiols. The breakdown field for Hg-SAM/SAM-Au junctions was reported to be about 4 × 106 V cm-1. The excellent agreement between the breakdown field obtained for Hg-SAM/SAM-Au junctions, and our results provides further support for the location of the potential of zero charge of thiol-modified gold surfaces at about -0.3 V. In related work, Boubour and Lennox26 reported on phase angle versus frequency plots of alkanethiol SAMs on gold in 50 mM K2HPO4 solution. Qualitatively, they observed that at a critical potential of about -0.35 V for hexadecanethiol the phase angle at 1 Hz decreased dramatically. This same effect can be seen by comparison of the impedance spectra in Figure 2. The critical potential was interpreted as the onset of formation of pinhole-like defects in the monolayers. However, if such defects were responsible for the decrease in phase angle then there would be an obvious discontinuity in the resistance of the monolayer in response to an applied potential beyond the critical potential. From our quantitative analysis of the impedance spectra, we show that this is not the case and that the SAM resistance decreases exponentially with potential. (24) Weaver, J. C. IEEE Trans. Dielectr. Electr. Insul. 2003, 10 (5), 754-768. (25) Haag, R.; Rampi, M. A.; Holmlin, R. E.; Whitesides, G. M. J. Am. Chem. Soc. 1999, 121 (34), 7895-7906. (26) Boubour, E.; Lennox, R. B. J. Phys. Chem. B 2000, 104 (38), 9004-9010.
Influence of Chain Length. Figure 4a shows the influence of chain length on the SAM capacitance. The average capacitance at 0 V for 1-nonanethiol (C9H19SH) is 1.46 ( 0.04 µF cm-2, and for 1-tetradecanethiol (C14H29SH) it is 1.15 ( 0.14 µF cm-2. From our results, the monolayer capacitance is inversely proportional to chain length with a slope of 0.060 (µF cm-2)-1/n and an intercept of 0.19 (µF cm-2)-1. These values are in excellent agreement with results reported by Porter et al.4 and Lingler et al.6 Due to the decrease in hydrophobic interactions, the capacitance increase associated with the onset of reductive desorption shifts to more positive potentials with decreasing chain length. The dependence of the onset potential for thiol desorption on chain length is -27 mV/n, in good agreement with a value of -20 mV/n reported for the dependence of the peak potential on chain length.5 The field strength at the onset of reductive desorption, taking the potential of zero charge as -0.3 V, is 4.0 × 106 V cm-1 for C9H18SH (d ) 1.0 nm) and 3.8 × 106 V cm-1 for C14H38SH (d ) 1.6 nm) SAMs, slightly larger than the breakdown field for an ODT (C18H37SH) SAM of 3.5 × 106 V cm-1. Since our measurements were obtained at 100 mV intervals, the difference is within experimental error. Figure 4b shows the monolayer resistance Rm versus applied potential for different chain lengths. In the potential range from +0.4 to -0.3 V, the SAM resistance is greater than 106 Ω cm2 for all three chain lengths. At potentials negative to -0.3 V, the resistance decreases exponentially with potential as described above for ODT. These results suggest that the potential of zero charge is independent of chain length. At any given potential in the range from -0.3 V to the onset of reductive desorption, the resistance increases with increasing chain length. The inverse slopes for the C9H19SH and C14H29SH SAMs are 206 ( 27 and 255 ( 27 mV decade-1, respectively. These values are comparable to the average value for C18H37SH of 257 ( 14 mV decade-1. Influence of Surface Morphology. Figure 5 shows AFM images for evaporated gold and template-stripped gold. A comparison of the line scans illustrates a considerable difference in surface morphology between the two substrates. The rms roughness for the evaporated gold is 1.329 nm µm-2, whereas the rms roughness for TSG is 0.140 nm µm-2. Figure 6 shows impedance spectra for ODT on templatestripped gold. In general, the impedance spectra show the same features as for ODT on evaporated gold, with the characteristic features of a parallel RC network. At 0 V, the slope of the Bode plot is -1.0 and the maximum phase angle is 90° characteristic of an ideal capacitor. At -0.5 V, the phase angle at low frequencies decreases, illustrating a decrease in the monolayer resistance.
Impedance of Alkanethiol SAMs
Figure 6. Impedance spectra for C18H37SH (ODT) on TSG in PBS at (a) 0 and (b) -0.5 V. The solid lines correspond to nonlinear least-squares fits.
Comparison of the impedance spectra for ODT on the two different substrates shows that the dispersion is lower for TSG. Increasing surface roughness is known to increase the dispersion behavior at the metal/solution interface.27 The potential dependence of the capacitance and resistance of ODT monolayers on TSG are shown in Figure 7. The capacitance is independent of potential from +0.5 to -1.0 V but then increases and approaches the value for bare gold at more negative potentials. The magnitude of the capacitance at potentials positive to the reductive desorption potential is the same for both evaporated gold and TSG, indicating that the surface morphology has no influence on the capacitance. The resistance of ODT SAMs on TSG also shows the same general features as on evaporated gold. The resistance at positive potentials is very large but at -0.3 V decreases exponentially with potential. However, the resistance of the ODT SAM on TSG remains about an order of magnitude larger than on evaporated gold down to -1.0 V beyond which the resistance on both substrates is indistinguishable. These results suggest that the same mechanism is responsible for the exponential decrease in resistance seen at more negative potentials. The inverse slopes average of ODT on TSG is 206 ( 24 mV decade-1 close to the value on evaporated gold. The increased resistance on the TSG indicates that ODT monolayers on this substrate are inherently less leaky or permeable (27) Macdonald, J. R. Impedance spectroscopy: emphasizing solid materials and systems; Wiley: New York, 1987.
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Figure 7. (a) Capacitance and (b) resistance versus applied potential for C18H37SH (ODT) SAMs on TSG and evaporated gold.
to ions than monolayers formed on evaporated gold. Thus, topology induced defects must play a key role in determining the resistance of SAMs through a decreased density of domain boundaries within the monolayer.
Summary Analysis of the impedance response of alkanethiol-modified gold surfaces over a wide potential range can be modeled as a solution resistance in series with a parallel network consisting of the monolayer capacitance and the monolayer resistance. The capacitance is independent of potential over a wide range down to the onset of reductive desorption. At potentials positive to -0.3 V (Ag/AgCl), the monolayer resistance is greater than 106 Ω cm2. At more negative potentials the resistance decreases exponentially with potential with an inverse slope of about 250 mV. Experiments on template-stripped gold show the same behavior although the resistance in the exponential regime is larger than for evaporated gold. It is proposed that the exponential decrease in resistance is related to the increasing positive electric field at potentials negative to the potential of zero charge that induces configurational changes in the monolayer. Acknowledgment. This work was supported by the JHU MRSEC (NSF Grant No. DMR05-20491). The authors acknowledge Audrey Medford and Aleksandar Radisic for preliminary experiments on the impedance response of selfassembled monolayers. LA701398U
