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Scanning Electrochemical Microscope Observation of Defects in a Hexadecanethiol Monolayer on Gold with Shear Force-Based Tip-Substrate Positioning Hiroshi Yamada,* Mitsuko Ogata, and Tohru Koike Department of Applied Chemistry, National Defense Academy, 1-10-20 Hashirimizu, Yokosuka, Kanagawa 239-8686, Japan ReceiVed May 10, 2006. In Final Form: June 28, 2006 Scanning electrochemical microscopy (SECM) was used for imaging of n-hexadecanethiol-modified Au surfaces. In these studies, small defects were observed in the monolayer when a submicrometer electrode was used as an SECM tip, although a cyclic voltammogram of a Au disk electrode showed that the surface of the Au was completely covered with n-hexadecanethiol. The dependence of the SECM images on the potential of the Au electrode was also examined. A comparison of the current at the Au electrode and the tip current in the SECM images showed that direct electron transfer through the monolayer was dominant, rather than electron transfer at the defects. The size of the defects was estimated from the tip current to be 1-100 nm, under the assumption that the defects were small compared to the SECM probe.
Introduction Self-assembled monolayers (SAMs), composed of materials such as alkanethiols and their derivatives, are employed as simple models of biomembranes. Modes of electron transfer between a Au substrate and an electroactive species dissolved in a solution include direct electron tunneling through the monolayer, reaction at the defects in the monolayer and permeation of the species through the monolayer.1 The structure of a monolayer of alkanethiol on a gold substrate has been extensively investigated by scanning tunneling microscopy (STM).2-4 Pinhole-like structures are frequently observed by STM. The holes have been accounted for as regions without thiol molecules, as regions of disordered molecules on the Au, and as depressions in the top layer of the Au surface.3 The present consensus is that the pinhole-like structure consists of depressions in the top layer of the Au surface. True pinholes have been observed in very few instances;2 therefore, according to STM measurements, the alkanethiol monolayer is usually a well-ordered, densely packed structure, practically pinhole free. Although the STM has molecular-level resolution, the area of the STM image is too small to allow a determination of the pinhole density. Whitesides and co-workers investigated the density of defects in an alkanethiol on Au deposited on Si by chemical amplification.5 The pinholes were converted micrometerscale pits in the underlying Si support, which could easily be observed with a scanning electron microscope. Electrochemical methods, such as cyclic voltammetry,6-8 scanning electrochemical microscopy (SECM)7,8 and electro* Address correspondence to this author. E-mail:
[email protected]. (1) Liu, B.; Bard, A. J.; Mirkin, M. V.; Creager, S. E. J. Am. Chem. Soc. 2004, 126, 1485-1492. (2) Edinger, K.; Go¨lzha¨user, A.; Demota, K.; Wo¨ll, C.; Grunze, M. Langmuir 1993, 9, 4-8. (3) Scho¨nenberger, C.; Sondag-Huethorst, J. A. M.; Jorritsma, J.; Fokkink, L. G. J. Langmuir 1994, 10, 611-614. (4) Kim, Y.-T.; Bard, A. J. Langmuir 1992, 8, 1096-1102. (5) Zhao, X.-M.; Wilbur, J. L.; Whitesides, G. M. Langmuir 1996, 12, 32573264. (6) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-3568. (7) Cannes, C.; Kanoufi, F.; Bard, A. J. Langmuir 2002, 18, 8134-8141. (8) Cannes, C.; Kanoufi, F.; Bard, A. J. J. Electroanal. Chem. 2003, 547, 83-91.
chemical impedance spectroscopy9 are also employed in the study of the structure of alkanethiol monolayers. When cyclic voltammetry was performed on a long chain alkanethiol-modified Au, the voltammetric response was drastically changed, showing an excellent blocking capability. From the voltammetric response, the average size of the defects and the coverage were estimated.10 Because SECM allows the researcher to observe the local electrochemical reactivity of interfaces, it is widely used for the characterization of the chemical nature of interfaces. Examples of heterogeneous surface reactions studied by SECM include electron transfer11 and mass transfer12 through a bilayer lipid membrane, mass13 and charge transfer14-18 through a liquid/ liquid interface, mass transfer through porous membranes,19 corrosion of a metal surface,20,21 and electron transfer at SAMs.1,7,8,22 Recently, sub-micro/nanometer-sized electrodes were used for high-resolution SECM measurements.23-26 When a sub-micro/nanoelectrode is used as a probe for SECM, the (9) Diao, P.; Guo, M.; Tong, R. J. Electroanal. Chem. 2001, 495, 98-105. (10) Amatore, C.; Save´ant, J. M.; Tessier, D. J. Electroanal. Chem. 1983, 147, 39-51. (11) Amemiya, S.; Ding, Z. F.; Zhou, J. F.; Bard, A. J. J. Electroanal. Chem. 2000, 483, 7-17. (12) Yamada, H.; Matsue, T.; Uchida, I. Biochem. Biophys. Res. Commun. 1991, 180, 1330-1334. (13) Yamada, H.; Akiyama, S.; Inoue, T.; Koike, T.; Matsue, T.; Uchida, I. Chem. Lett. 1998, 147-148. (14) Delville, M.-H.; Tsionsky, M.; Bard, A. J. Langmuir 1998, 14, 27742779. (15) Shao, Y.; Mirkin, M. V. J. Phys. Chem. B 1998, 102, 9915-9921. (16) Barker, A. L.; Unwin, P. R.; Amemiya, S.; Zhou, J.; Bard, A. J. J. Phys. Chem. B 1999, 103, 7260-7269. (17) Shao, Y.; Mirkin, M. V. J. Electroanal. Chem. 1997, 439, 137-143. (18) Strutwolf, J.; Barker, A. L.; Gonsalves, M.; Caruana, D. J.; Unwin, P. R.; Williams, D. E.; Webster, J. R. P. J. Electroanal. Chem. 2000, 483, 163-173. (19) Bath, B. D.; Lee, R. D.; White, H. S. Anal. Chem. 1998, 70, 1047-1058. (20) Garfias-Mesias, L. F.; James, P. I.; Smyrl, W. H. Proc. Electrochem. Soc. 1997, 97-7, 247-256. (21) Garfias-Mesias, L. F.; Alodan, M.; James, P. I.; Smyrl, W. H. J. Electrochem. Soc. 1998, 145, 2005-2010. (22) Forouzan, F.; Bard, A. J.; Mirkin, M. V. Isr. J. Chem. 1997, 37, 155-163. (23) Kranz, C.; Friedbacher, G.; Mizaikoff, B.; Lugstein, A.; Smoliner, J.; Bertagnolli, E. Anal. Chem. 2001, 73, 2491-2500. (24) Macpherson, J. V.; Unwin, P. R. Anal. Chem. 2001, 73, 550-557. (25) Macpherson, J. V.; Jones, C. E.; Barker, A. L.; Unwin, P. R. Anal. Chem. 2002, 74, 1841-1848. (26) Macpherson, J. V.; Mussy, J.-P. G. d.; Delplancke, J.-L. J. Electrochem. Soc. 2002, 149, B306-B313.
10.1021/la0613171 CCC: $33.50 © 2006 American Chemical Society Published on Web 08/02/2006
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regulation of the tip-sample distance is crucial in constructing SECM maps of the electrochemical activities of a sample surface. The tip must be located very close to the surface (a distance less than the diameter of the electrode). Therefore, a tilted or uneven substrate, producing a mechanical contact between the tip and the surface, is a recurring problem. To control the tip-sample distance, various scanning probe microscopies are combined with SECM. Shear force regulation methods,27-33 commonly used for scanning near-field optical microscopy,34 and atomic force microscopy23-26 have also been used with SECM. In early studies of SAMs by SECM, the size of the microelectrode used for the SECM imaging was too large compared to the pinhole or defect: its resolution was ∼10 µm.22 We report here the imaging of n-hexadecanethiol-modified Au surfaces with a submicrometer electrode as a probe for SECM to allow direct observations of the defects in the monolayer on the Au surface. Additionally, we discuss the potential dependence of the current at the pinholes and in a completely covered area. Experimental Section Materials. (Ferrocenylmethyl)trimethylammonium (FA+) perchlorate was synthesized by adding saturated sodium perchlorate solution to a solution of (ferrocenylmethyl)trimethylammonium bromide (purchased from Tokyo Kasei). All the aqueous solutions were prepared with water purified by Milli-Q Jr. (Millipore Co.). Hexadecane thiol (Wako Chemical) was used as received. Preparation of Hexadecanethiol-Modified Gold Microelectrode. A section of lead glass (World Precision Instruments, PG10150-4) was pulled with a capillary puller (Narishige, Tokyo model PC-10), and a gold wire (diameter 30 µm) was inserted in the capillary. The tip was fused by a microforge (Narishige, Tokyo model MF-900). The tip was polished first on a turntable (Narishige, Tokyo model EG-4) and finally with a fine emery paper (Sumitomo 3M, Tokyo #15000) to obtain a disk-shaped electrode. The Au microelectrodes were cleaned thoroughly by dipping the tip of the electrodes in a 60% HNO3 solution for 30 min and then washing them with distilled water for 10 min in a supersonic bath. The clean Au microelectrodes were immersed in a 10 mM hexadecanethiol/ ethanol solution for several hours and then were washed with ethanol and distilled water. Fabrication of SECM Probe. A 30 µm platinum wire was etched electrochemically in a saturated KCl solution in order to sharpen the wire; it was then inserted into the pulled capillary. After the tip was fused to coat the sharpened Pt filament completely within the lead glass, it was polished with 0.05 µm alumina powder (Buehler Micropolish II) on a homemade turntable until the sharpened Pt apex was exposed. The Pt disk radius of the electrode was determined from the steady-state current of FA+ in a voltammogram. The tip and Pt disk radii were 0.1-0.2 µm and 1-3 µm, respectively. After cutting the tip of the microelectrode fabricated above, the 4 mm long tip was glued with epoxy resin to one of the prongs of a crystal quartz tuning fork (Citizen, CFS-308). The tip protruded 2-3 mm past the prong’s end. To excite the mechanical resonance of the fork, it was soldered on a diaphragm of commercially available piezoelectric transducer. The tip of the probe was washed with water in an ultrasonic bath just before the SECM measurement. The tuning (27) Kranz, C.; Gaub, E. H.; Shuhmann, W. AdV. Mater. 1996, 8, 634-637. (28) Lee, Y.; Ding, Z.; Bard, A. J. Anal. Chem. 2002, 74, 3634. (29) Katemann, B. B.; Schulte, A.; Schuhmann, W. Chem.sEur. J. 2003, 9, 2025-2033. (30) James, P. I.; Garfias-Mesias, L. F.; Moyer, P. J.; Smyrl, W. H. J. Electrochem. Soc. 1998, 145, L64-L66. (31) Hengstenberg, A.; Kranz, C.; Schuhmann, W. Chem.sEur. J. 2000, 6, 1547-1554. (32) Yamada, H.; Fukumoto, H.; Yokoyama, T.; Koike, T. Anal. Chem. 2005, 77, 1785-1790. (33) Oyamatu, D.; Kanaya, N.; Mase, Y.; Nishizawa, M.; Matsue, T. Bioelectrochemistry 2003, 60, 115-122. (34) Mulin, D.; Vannier, C.; Bainier, C.; Courjon, D.; Spajer, M. ReV. Sci. Instrum. 2000, 71, 3441-3443.
Figure 1. (a) Frequency response curves of the tuning fork with microelectrode acquired in air (solid line) and in solution (dashed line). The depth of immersion of the tip was 0.1 mm. (b) Cyclic voltammogram of 1.0 mM FA+ at r ) 0.13 µm microelectrode obtained at 10 mV/s. (c) Approach curves of current (solid line) and output voltage of the tuning fork (dashed line) when the probe tip was moved toward the Au surface and the glass surface. fork and the tip of the electrode were vibrated at its resonance frequency (20-30 kHz), parallel to the sample surface. SECM Apparatus. An SECM system (Hokuto Denko, HV401) was modified to use shear force-based tip positioning. A piezo actuator (Piezomechanik GmbH, model PSt 150/7/20 VS12) was mounted on a stepping motor-driven XYZ stage (Suruga seiki, KS70120LHD) to perform the micromovement of the probe along the Z direction. The stroke of the piezoelectric actuator was 7 µm at an applied voltage of 150 V. The signal of the tuning fork was amplified with a lock-in amplifier (Signal Recovery, Model 7264); then, the signal amplitude was digitized and transferred to a computer (Toshiba Dynabook) with a 16-bit AD/DA board (Interface, AZI3506). The measurement was carried out in a three-electrode configuration. The reference and counter electrodes were composed of Ag/AgCl immersed in saturated KCl solution, and Pt wire (diameter 0.5 mm), respectively. Current-Distance Profile. The substrate was immersed in a 1.0 mM FA+, 0.05 M phosphate buffer (pH 7.5) solution. The tip of the probe was placed above the substrate and set at 0.7 V versus Ag/ AgCl. Resonant frequency voltage was applied to the diaphragm of the piezoelectric transducer. The voltage was adjusted such that the amplitude of the voltage generated by the tuning fork was 0.45 mV. The tip was slowly (0.5 µm/s) approached to the surface by increasing the voltage of the piezo actuator while measuring the output signal of the tuning fork and the tip current until the amplitude of the tuning fork had decreased to 50% of the original value. Scanning Mode for SECM Imaging. We used the standing approach mode,32 which has been described previously, to image the electrochemical activity of the surface. The standing approach mode repeats at each point the approach and retraction of the tip for sampling the current. The lateral movement of the tip is stopped during the data sampling sequence. A starting amplitude of the tuning fork is measured before approaching, and then the tip is lowered to the surface at a rate of 0.5 µm/s until the amplitude of the tuning fork is damped to a level 99% of the starting amplitude. The tip is then retracted by 0.2-0.3 µm to acquire the current. When the tip is moved laterally to the next sampling point with the stepping motor, it vibrates at the beginning and end of the movement as a consequence of the vibration of the stepping motor. Therefore, the tip is retracted
Defects in Hexadecanethiol Monolayers on Au
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Figure 2. Cyclic voltammograms of 1.0 mM FA+ at a hexadecanethiol-covered (solid line) and a bare (dashed line) Au disk (r ) 15 µm) electrode at a scan rate of 10 mV/s. ∼2 µm upward to avoid contact between the tip and the surface during the lateral movement. It takes ∼1 s to get a data point without contacting the surface. In addition, we used two types of potential settings of the tip and the substrate. In the tip collection/substrate generation mode, the substrate potential (Esub) was set at a level more positive than that of the formal potential of the FA+/FA2+ couple, and the potential of the tip (Etip) was set at 0.2 V to collect the FA2+ formed at the substrate. In the tip generation/substrate correction mode, the Esub and Etip were set at 0.2 and 0.8 V, respectively.
Figure 3. (a) SECM image, 40 × 40 µm2, of a Au disk electrode (r ) 15 µm) covered with hexadecanethiol taken with a 0.13 µm Pt microelectrode in 1.0 mM FA+ solution when Esub and Etip were set at 0.8 and 0.2 V, respectively. The image was 80 × 80 data points in size. (b) Single line scan at Y ) 13 µm.
Result and Discussion Characterization of the Submicrometer SECM Probe. To check the performance of the SECM probe, we measured the frequency response, cyclic voltammogram, and current-distance profiles. Figure 1a shows the amplitude of the output signal of the tuning fork as a function of the driving frequency, in air (solid line) and in a solution (dash line), with the microelectrode tip glued along one side of the prong. The depth of immersion of the tip was 0.1 mm. A tuning fork without the microelectrode tip has a Q factor of 2400 in air. When a Pt microelectrode tip was glued along one side of the prong, the Q factor was reduced to ∼200. Since the tip of the microelectrode vibrates in a solution to detect the shear force between the tip and a sample, horizontal vibration may affect the Faraday current for a submicrometer electrode. When the amplitude of the tuning fork was larger than 10 mV, a significant increase (typically, an ∼10% increase compared to the original value) in the Faraday current was observed. Therefore, the applied voltage of the piezoelectric transducer was adjusted to be the amplitude of the tuning fork at 0.45 mV. Figure 1b shows the cyclic voltammogram for 1.0 mM FA+ on the Pt submicrometer electrode. From the steadystate current, we calculated the electrode radius to be 0.13 µm. Figure 1c shows the current-distance profiles (solid line) obtained as the probe approached a Au surface and a glass surface. The amplitude of the tuning fork was simultaneously measured (dashed line). The current was normalized by dividing it by the current obtained at a distance far from the surface. A digital simulation35 was carried out to reproduce the current-distance profile (solid squares). The distance between the tip and the substrate was determined by shifting the experimental current-distance curves to fit the theoretical curves. At a distance of 0.14 µm, the amplitude of the tuning fork suddenly decreased, a result indicating that the insulator part of the tip was very close to the surface. Since the radius of the insulator part of the microelectrode was relatively large, the electroactive part (Pt) could not approach more closely than 0.14 µm to the surface. SECM Imaging of a 30 µm Au Electrode Covered with Hexadecanethiol. The excellent blocking characteristics of a long-chain alkanethiol were confirmed by cyclic voltammetry. (35) Shiku, H.; Takeda, T.; Yamada, H.; Matsue, T.; Uchida, I. Anal. Chem. 1995, 67, 312-317.
Figure 4. Line profiles of the current acquired with substrate generation/tip collection mode (a) and substrate collection/tip generation mode (b) taken with a 0.13 µm Pt microelectrode in 1.0 mM FA+, and a topographic profile (c) obtained simultaneously with the current profile shown in panel b.
Figure 2 shows cyclic voltammograms of a bare Au electrode and a hexadecanethiol-covered Au electrode in a 1.0 mM FA+ solution. Although we noted a large decrease in the oxidation current for FA+, a small oxidation current was still present with the hexadecanethiol-covered Au electrode. This oxidation current is considered to be the sum of the currents originating in a direct electron transfer through the monolayer and a reaction at pinholes or defects.1 A passivate film with pinholes on a electrode considered as the array of a nano/micrometer electrode36-38 and at a nanometer-sized electrode,39 sigmoidal voltammetric response was observed. Therefore, if sigmoidal response was observed in the voltammogram of the hexadecanethiol-covered electrode, the average size of the pore and the density of the pinhole can (36) Bilewicz, R.; Majda, M. J. Am. Chem. Soc. 1991, 113, 5464-5466. (37) Bilewicz, R.; Majda, M. Langmuir 1991, 7, 2794-2802. (38) Bilewicz, R.; Sawaguchi, T.; Chamberlain, R. V., II; Majda, M. Langmuir 1995, 11, 2256-2266. (39) Penner, R. M.; Heben, M. J.; Longin, T. L.; Lewis, N. S. Science 1990, 250, 1118-1121.
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Figure 5. Schematic representation of diffusion near the substrate with substrate generation/tip collection mode (a) and substrate collection/tip generation mode (b), and the resulting line profiles of SECM. Table 1. Height of Current Peaks and Calculated Radii of Pinholes in Figure 2 position (X (µm), Y (µm))
peak current (pA)
radius (nm)
(25, 29.5) (20.5, 7) (13, 12.5) (12, 23.5) (23, 26) (22.5, 32) (31, 25.5) (28.5, 25) (33.5, 25.5) (26, 27.5) (19, 17) (22.5, 5) (31, 22.5)
16.6 10.1 8.4 6.9 4.7 4.5 4.4 3.4 3.3 3.3 3.1 2.6 2.6
66.2 40.3 33.5 27.5 18.7 17.9 17.5 13.6 13.2 13.2 12.4 10.4 10.4
be obtained from the shift of a half-wave potential with respect to a reversible value and a limiting current. To look for the trace of sigmoidal response of the voltammogram, the expanded voltammogram of the solid line in Figure 2a is given in the Supporting Information. Since the differential of the current (Figure 1b in the Supporting Information) has a peak around the formal potential of the FA+/FA2+ couple, the trace of sigmoidal response exists in the voltammogram. However, the fraction of steady-state current was too small to analyze the average size and density of the pinholes. From the cyclic voltammetric response, we cannot derive the ratio of the currents in these two pathways. Therefore, we obtained a 40 × 40 µm2 SECM image of the hexadecanethiol-adsorbed Au electrode in the substrate generation/tip collection mode. Figure 3a shows an SECM image of this electrode when Esub and Etip were set at 0.8 and 0.2 V, respectively. The circular Au area was barely observable in the SECM image; most of the Au surface was covered with thiol. However, several regions with a high reduction current were present. These regions are considered to be defects or pinholes in the monolayer. Interestingly, although the reduction currents at the pinholes differed, most of the pinholes appeared to be of a similar size. Figure 3b shows a single line scan at Y ) 13 µm. Comparing the currents at the glassy and completely covered regions, we found a very small reduction current in the latter region. From the current profile (Figure 3b), we judged the apparent radius of the pinhole to be 3-4 µm, a size which seemed too large when compared with the radius reported for a pinhole observed by STM.2 To further evaluate the apparent size of the pinhole, we obtained an SECM image with the substrate
Figure 6. (a) Single line scans obtained at the region where several pinholes were observed when Esub varied between 0.5 and 0.8 V. (b) Dependence of the height of the current peaks observed in each of the single scans on Esub.
collection/tip generation mode after having scanned with the substrate generation/tip collection mode. Figure 4 shows the line profiles of the currents acquired with the substrate generation/tip collection mode (a), and the substrate collection/tip generation mode (b). The sharp peak at X ) 25 µm and the broad peak from X ) 20 to 35 µm were observed. The broad peak is caused by the direct electron transfer through the monolayer since the Au part was located from X ) 20 to 35 µm. With the former mode, the apparent size of the pinhole was 4 µm, but, with the latter mode, it was less than 1 µm. Since a data point was acquired at 0.5 µm intervals, and since the radius of the tip was larger than the pinhole, the actual size of the pinhole could not be obtained. This discrepancy in the observed size of the pinhole between the two scanning modes is explained as follows. Since the SECM image was obtained with the substrate generation/tip collection mode, oxidized FA+ diffused spherically to the bulk solution. The current was accumulated at a distance of 0.3∼0.4 µm from the surface. Therefore, the oxidized FA+, which formed at the pinhole, was collected with the tip located several micrometers away from the pinhole, and the resulting profile shows a broad peak (Figure 5a). Estimation of the Radius of the Pinhole. In the previous section, we showed that the apparent size of the pinhole observed with the substrate generation/tip collection mode is much larger than the actual size of the pinhole. If we assume that the electron transfer at the pinhole is diffusion-limited, and that all of the oxidized FA+ is collected with the tip when the tip is located just above the pinhole, theoretical tip current (itip) dependence on the distance between the tip and the substrate (d) can be simulated. (see Figure 2 in the Supporting Information). Although, when the distance between the pinhole and the tip is small, itip is increased by redox cycling effect, itip would be almost equal to the oxidation current at the pinhole (at d/rpinhole ) 5, itip/ 4nFcDrpinhole ) 1.08). Since the current was accumulated at a distance of 0.3-0.4 µm from the surface, we should be able to estimate the actual pinhole radius from the peak current at the tip. Table 1 shows the list of the observed current peaks and the size of the pinholes found in Figure 2. Wo¨ll and co-workers reported a defect with a diameter of ∼25 nm within a docosanethiol film observed by STM.2 The size of
Defects in Hexadecanethiol Monolayers on Au
the pinholes estimated by the above procedure were similar to those in their report. The sum of the peak current in Table 1 corresponds to the current caused by electron transfer at the defect. The sum of the peak current was ∼70 pA, and the current at the Au electrode was ∼400 pA. Therefore, ∼80% of the current at the Au electrode was caused by a direct electron transfer through the monolayer. The number of pinholes observed within an area of 30 µm on the Au electrode varied in each experiment, and the SECM image in Figure 2a is an example containing relatively many pinholes. Dependence of the Tip Current on Esub. Figure 6a shows single line scans obtained at a region where several pinholes present with Esub varied from 0.5 to 0.8 V. The tip currents (itip) at the pinholes and at the baseline rose with increases in Esub. Figure 6b shows the dependence of the height of each of the current peaks on the potential of the substrate. If all of the oxidized FA+ at the pinhole is collected with the tip, the potential dependence of the tip current should be identical with the voltammogram of the electrode having the same radius as that of the pinhole. The theoretical voltammograms of electrodes with radii of 13, 8.6, and 4.3 nm (assuming k0 ) 1 cm/s 22) are shown in Figure 6b. The apparent electron-transfer rate at nanometer-sized electrodes and molecular gates incorporated in passivating monolayers enhanced by a double-layer effect when Ru(NH3)63+/ Ru(NH3)62+ was used as redox couple.36-39 To discuss the dependence of the apparent electron-transfer rate on the size of the pinholes, detailed current dependence on the potential is needed. The potential dependence of the single line scans at the area where no pinholes were found was also measured (Figure 7a). The tip current in this case increased with increasing Esub. The average of the tip currents at each single line and the current of the substrate (isub) were plotted against Esub (Figure 7b). Since the potential dependence on itip is similar to that on isub, most of the current at the Au electrode was caused by a direct electron transfer through the monolayer. The density of defects in the alkanethiol/Au/Si system has been investigated by chemical amplification.5 The pinholes were converted to micrometer-scale pits in an underlying Si support.
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Figure 7. (a) Single line scans obtained at a region where no pinhole was observed when Esub was changed from 0.6 to 0.9 V. (b) Dependence of the average of itip (from X ) 10 µm to 40 µm) in each single scan (solid square) and isub (open circle) on Esub.
By counting the pits on the Si support, a density of 5 pits/mm2 was reported. The number of pinholes observed in the SECM image (7.1 × 10-4 mm2) was 1-10; therefore, the density is 1.4 × 103-1.4 × 104 pits/mm2, a value which is too large compared to that found by chemical amplification. This difference may be caused by the fact that we used a rough Au surface polished with fine emery paper. Observation of a larger area of a smooth Au surface is essential to accurately determine the density of the pinholes in the monolayer. Supporting Information Available: Expanded cyclic voltammogram of 1.0 mM FA+ at a hexadecanethiol-covered Au disk (r ) 15 µm), its differential of the current and theoretical tip current (itip) dependence on the distance between the tip and the surface (d). This material is available free of charge via the Internet at http://pubs.acs.org. LA0613171
