Electrochemical Evaluation and Enhancement via Heterogeneous

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Langmuir 1997, 13, 2973-2979

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Electrochemical Evaluation and Enhancement via Heterogeneous Exchange of the Passivating Properties and Stability of Self-Assembled Monolayers Derived from the Rigid Rod Arenethiols, X-C6H4-CtC-C6H4-CtC-C6H4-SH (X ) H and F) Robert W. Zehner and Lawrence R. Sita* Searle Chemistry Laboratory, Department of Chemistry, The University of Chicago, 5735 South Ellis Avenue, Chicago, Illinois 60637 Received November 22, 1996. In Final Form: March 17, 1997X The passivating properties of self-assembled monolayers derived from the chemisorption of the rigid rod arenethiols 1a-b on Au(111) have been probed by electrochemical techniques in an aqueous electrolyte. These studies suggest that the pure monolayers of 1a-b possess a high density of unfilled defect sites, potentially as a result of the solvent required for their preparation. However, a heterogeneous “patching” process, which involves both addition of hexadecanethiol at defect sites and exchange with the original monolayer, provides mixed monolayers that have superior passivating properties and which X-ray photoelectron spectroscopy and grazing-angle IR reveal to be still predominantly composed of 1a-b. The ability to reproducibly obtain these highly blocking mixed monolayers in which 1a-b is the primary component now provides a means by which to investigate, at the next level of study, electron transfer/ tunneling through these novel interfacial barriers.

Introduction Self-assembled monolayers (SAMs) derived from the chemisorption of organothiols on Au(111)1 have proven to be excellent model systems in which factors governing the rate of electron transfer/tunneling across interfacial barriers can be probed.2 An understanding of these factors is of considerable importance to a number of scientific and technological problems that range from long-range electron transfer in biological systems to the design and construction of molecular-based electronic devices.3 Along these lines, we recently reported that SAMs derived from the conjugated rigid rod arenethiol 1a form a highly ordered, commensurate 2x3 × x3 overlayer structure on Au(111) as determined by high-impedance scanning tunneling microscopy (STM) in air, thus providing only the second known SAM system to do so.4-7 In addition, it has also been demonstrated that this new SAM system possesses novel electrical rectifying properties and secondX

Abstract published in Advance ACS Abstracts, May 1, 1997.

(1) (a) Ulman, A. An Introduction to Ultrathin Films from LangmuirBlodgett to Self Assembly; Academic Press: San Diego, CA, 1991. (b) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437-463. (2) (a) Li, T. T.-T.; Weaver, M. J. J. Am. Chem. Soc. 1984, 106, 6107. (b) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301. (c) Chidsey, C. E. D. Science 1991, 251, 919. (d) Finklea, H. O.; Hanshew, D. D. J. Am. Chem. Soc. 1992, 114, 3173. (e) Rowe, G. K.; Creager, S. E. J. Phys. Chem. 1994, 98, 5500. (f) Obeng, Y. S.; Laing, M. E.; Freidli, A. C.; Yang, H. C.; Wang, D. N.; Thulstrup, E. W.; Bard, A. J.; Michl, J. J. Am. Chem. Soc. 1992, 114, 9943. (g) Herr, B. R.; Mirkin, C. A. J. Am. Chem. Soc. 1994, 116, 1157. (h) Carter, M. T.; Rowe, G. K.; Richardson, J. N.; Tender, L. M.; Terrill, R. H.; Murray, R. W. J. Am. Chem. Soc. 1995, 117, 2896. (i) Cheng, J.; Sa´ghi-Szabo´, Tossell, J. A.; Miller, C. J. J. Am. Chem. Soc. 1996, 118, 680. (j) Campbell, D. J.; Herr, B. R.; Hulteen, J. C.; Van Duyne, R. P.; Mirkin, C. A. J. Am. Chem. Soc. 1996, 118, 10211. (k) Alleman, K. S.; Weber, K.; Creager, S. E. J. Phys. Chem. 1996, 100, 17050. (l) Finklea, H. O. In Electroanalytical Chemistry; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker, Inc.: New York, 1996; Vol. 19, and references cited therein. (3) (a) Polmeropoulos, E. E.; Sagiv, J. J. Chem. Phys. 1978, 69, 1836. (b) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Israelachvili, J.; McCarthy, T. J.; Murray, R. F.; Rabolt, J. F.; Wynne, K. J. Langmuir 1987, 3, 932. (c) Waldeck, D. H.; Beratan, D. N. Science 1993, 261, 576. (4) Dhirani, A.; Zehner, R. W.; Hsung, R. P.; Guyot-Sionnest, P.; Sita, L. R. J. Am. Chem. Soc. 1996, 118, 3319.

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order optical nonlinearities.8 Accordingly, the further characterization of SAMs of 1, and closely related derivatives,7b by a variety of techniques is of interest with respect to better delineating their potential to serve in various optical and electronic applications. Indeed, in this respect, an important question regarding SAMs of 1 that has remained open concerns the ability of this new class of ordered SAM structure to serve as an efficient tunneling barrier that might find utility in thin-film-based electronic devices, in particular, those that are based on metal-insulator-metal junctions.3a Herein, we now report the results of detailed electrochemical studies that (5) SAMs derived from the chemisorption of n-alkanethiols on Au(111) represent the only other systems for which a highly ordered, commensurate overlayer structure has been unequivocally shown to exist. See: (a) Camillone, N., III; Chidsey, C. E. D.; Liu, G.-Y.; Scoles, G. J. Chem. Phys. 1993, 98, 3503. (b) Poirier, G. E.; Tarlov, M. J. Langmuir 1994, 10, 2853. (6) For SAM systems that are known to form ordered, but incommensurate overlayer structures, see: (a) Liu, G.-Y.; Fenter, P.; Chidsey, C. E. D.; Ogletree, D. F.; Eisenberger, P.; Salmeron, M. J. Chem. Phys. 1994, 101, 4301 and references cited therein. (b) Wolf, H.; Ringsdorf, H.; Delamarche, E.; Takami, T.; Kang, H.; Michel, B.; Gerber, C.; Jaschke, M.; Butt, H.-J.; Bamberg, E. J. Phys. Chem. 1995, 99, 7102. (c) Caldwell, W. B.; Campbell, D. J.; Chen, K.; Herr, B. R.; Mirkin, C. A.; Malik, A.; Durbin, M. K.; Dutta, P.; Huang, K. G. J. Am. Chem. Soc. 1995, 117, 6071. (7) For previous studies of SAMs derived from conjugated arenethiols in which order has been proposed but not unequivocally established, see: (a) Chang, S. C.; Chao, I.; Tao, Y. J. Am. Chem. Soc. 1994, 116, 6792. (b) Tour, J. M.; Jones, L.; Pearson, D. L.; Lamba, J. J. S.; Burgin, T. P.; Whitesides, G. M.; Allara, D. L.; Parikh, A. N.; Atre, S. V. J. Am. Chem. Soc. 1995, 117, 9529. (c) Sabatani, E.; Cohen-Boulakia, J.; Bruening, M.; Rubinstein, I. Langmuir 1993, 9, 2974. (8) Dhirani, A.; Lin, P.-H.; Guyot-Sionnest, P.; Zehner, R. W.; Sita, L. R. J. Chem. Phys. 1997, 106, 5249. (9) For previous electrochemical studies of SAMs of rigid rod arenethiol adsorbates, see ref 7c and (a) Kwan, W. S. V.; Atanasoska, L.; Miller, L. L. Langmuir 1991, 7, 1419. (b) Kwan, V. W. S.; Cammarata, V.; Miller, L. L.; Hill, M. G.; Mann, K. R. Langmuir 1992, 8, 3003.

© 1997 American Chemical Society

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were undertaken to address this particular issue.9 More specifically, these studies conclude that, while SAMs of 1a and 1b poorly block solution redox couples in aqueous electrolytes, possibly as a result of access to the metal surface via domain boundaries and pinhole defect sites, a heterogeneous exchange process employing hexadecanethiol (HDT) reproducibly provides novel mixed alkanethiol/rigid rod arenethiol monolayers that exhibit dramatically improved passivating properties and stability but which are still predominantly composed of the arenethiol component as revealed by X-ray photoelectron spectroscopy (XPS). In addition, grazing angle Fourier transform infrared spectroscopy (GA-FTIR) supports a structural model for this new mixed system that is comprised of a large component made up of domains of assembled arenethiol adsorbate that are then dispersed by a smaller component consisting of ordered HDT assemblies that reside at domain boundaries. Altogether, these new results with pure and mixed SAMs of 1 may now provide the means by which to investigate, at the next level of study, electron transfer/conduction through the novel interfacial barriers that are presented by these systems. Experimental Section Reagents. The syntheses of 1a and 1b follow previously published procedures and will be reported in full detail elsewhere.10 HDT (Aldrich), potassium chloride (Fisher, ACS reagent), and potassium ferricyanide [K3Fe(CN)6] (ACS reagent) were used without further purification. All metal sputtering sources (Cr-coated tungsten rods (R. D. Mathias) and Au (99.95%) wire (Goldsmith)) were first cleaned in hot nitric acid, washed with water (Barnstead NanoPure, 18 MΩ resistivity), and dried before use. Absolute ethanol, 2-propanol (electronic grade), and dichloromethane (HPLC grade) were used as received. Substrate and SAM Preparation. Polished crystalline silicon wafers (100 orientation) (Silicon Sense) were cleaned by immersion in hot acidic peroxide (75% H2SO4, 25% H2O2) (CAUTION!) for 15 min, rinsed with NanoPure water, and flash dried in refluxing 2-propanol vapors. These cleaned wafers were then immediately placed within a home-built resistive metal evaporator (base pressure 10-6 Torr) where a 15 nm adhesion layer of chromium was first deposited, followed by 150 nm of gold deposited at a rate of ∼3 Å/s. Immediately before use, the Au/Cr/Si substrates were cleaned with hot acidic peroxide for 15 min and then electrochemically cycled (-0.1 to -0.8 V vs SCE in 1 M NaCl) for 5 min. In agreement with observations previously reported by McLendon and co-workers,11 we find that this pretreatment procedure is critical for providing SAMs of HDT with superior passivating properties. After pretreatment, the substrates were rinsed with copious amounts of NanoPure water, ethanol, and hexane and then immediately immersed in a 1 mM solution of the thiol adsorbate in either ethanol (HDT) or dichloromethane (1a-b). Unless otherwise noted, substrates were removed from the adsorbate solution after 18-24 h, rinsed with copious quantities of dichloromethane and 2-propanol, and then spun dry on a spin caster at 3000 rpm for 1 min to leave a surface devoid of any visible film formed during drying. Following a similar procedure, heterogeneous samples were prepared by immersion of SAMs of 1a in a second thiol solution for a specified period of time, followed by an identical wash and drying protocol. Electrochemistry. Cyclic voltammograms were acquired with an EG&G PAR Model 273 (Option 96) potentiostat interfaced to a personal computer. The electrochemical cell comprised a Teflon barrel which was clamped against the SAM/Au substrate to describe a circular working electrode (0.45 cm2 area). A platinum wire was used as the counter electrode, and a saturated calomel electrode (SCE) served as the reference electrode. The electrolyte was aqueous 1 M KCl prepared with Nanopure water. Capacitance measurements were taken from the first voltammogram recorded for a 0.0 to +0.15 V potential cycle. For blocking (10) Hsung, R. P.; Zehner, R. W.; Sita, L. R., in press.

Zehner and Sita studies, a 1 mM solution of K3Fe(CN)6 in aqueous 1 M KCl was employed and the blocking characteristics of the SAM was obtained from the first voltammogram recorded for a -0.15 to +0.6 V potential cycle. Ellipsometry and Contact Angle Measurements. Ellipsometric film thickness measurements were made using a Gaertner Model L116C ellipsometer interfaced to a personal computer. Reference optical constants for the bare Au film were measured for each substrate immediately after electrochemical cleaning. All ellipsometric film thicknesses were calculated assuming an index of refraction of 1.46 for the SAM. The recorded values are the average of recordings taken at four different locations on the substrate. Advancing and receding contact angles were determined with a Ra´me-Hart contact angle goniometer equipped with an electronic pipet and a video image analysis attachment interfaced to a personal computer. Nanopure water was used for all measurements. For advancing measurements, a 2 µL drop of water was deposited onto the SAM surface and the contact angle was measured immediately after advancing movement of the drop ceased. For receding measurements, an additional 2 µL of water was added to an existing drop, the water was withdrawn, and then the contact angle was measured immediately after receding movement of the drop ceased. X-ray Photoelectron Spectroscopy (XPS). XPS spectra were obtained with either a Physical Electronics Model 500LS or Model 5400 electron spectrometer, both of which were equipped with a monochromatic Al KR source. Survey spectra were acquired with a pass energy of 178.5 eV, and high-resolution spectra of the C(1s) and Au(4f) regions were acquired with a pass energy of 17.9 eV. Spectra of the S(2p) region were taken using a Mg KR source with a 35.75 eV pass energy. Relative binding energies between substrates were referenced by overlaying the Au (4f) regions of all samples. Absolute binding energies were determined by calibration against gold and silver references. Grazing Angle Fourier Transform Infrared Spectroscopy (GA-FTIR). Infrared spectra of monolayers were taken with a Nicolet Magna-IR 560 Fourier transform infrared spectrometer fitted with an 85° grazing angle reflectance accessory (SpectraTech) and an internal MCT detector. The reflectance signal was averaged for 4000 scans at 1 cm-1 resolution. A perdeuterated HDT/Au(111) SAM sample was used as the reference. For comparison purposes, isotropic spectra of the bulk materials, 1a and 1b, were taken in the form of KBr pellets on the same instrument in transmission mode.

Results and Discussion As noted previously, the arenethiol 1a has been shown to self-assemble on Au(111) to form a densely packed chemisorbed monolayer that possesses a highly ordered commensurate structure.4 In addition, since the measured ellipsometric film thickness of this SAM and SAMs of 1b are close to the predicted value (cf. 1a, 22 ( 4 Å; 1b, 24 ( 5 Å; calcd, 21 Å),12 a structure in which the conjugated rigid rod adsorbates adopt a herringbone close packed arrangement and are aligned nearly normal to the gold surface is assumed. Taken together, a structural picture for SAMs of 1a-b emerges that is nearly identical to that suggested by Rubinstein and co-workers7c for SAMs derived from the p-terphenylmercaptan adsorbate, C6H5C6H4-C6H4-SH (TPM), and which predicts that SAMs of 1a-b should, in fact, be capable of forming a barrier that is at least as passivating as SAMs of HDT, the latter of which possesses a similar film thickness (cf. 20 Å for HDT vs 22 Å for 1a). In apparent agreement with this, Rubinstein and co-workers7c reported that SAMs of TPM, with a calculated thickness of only 14 Å, were more passivating than SAMs of octadecanethiol (ODT) (calculated thickness 24 Å). Unfortunately, this result is somewhat misleading in that the ODT SAMs used in this previous study were, as noted by the authors, not particularly passivating toward blocking of the Fe(CN)64-/3redox couple when compared to results obtained by others

Passivating Properties of SAMs

with SAMs of HDT.11,13 Furthermore, even taking into consideration differences in electrode size among the various studies, one reaches the conclusion that SAMs of TPM are, in fact, not very good barriers for faradaic reactions when compared to SAMs derived from long-chain n-alkanethiols, such as HDT. Finally, it is important to note that Rubinstein and co-workers7c attempted to improve the passivating properties of TPM SAMs via a subsequent treatment with ODT that might potentially repair defect sites through the formation of a mixed TPM/ ODT monolayer. Surprisingly, however, no apparent changes in the properties of the TPM SAMs were noted after this treatment, leading these authors to speculate that the TPM SAM structure might be too stable and densely packed to allow for ODT adsorption. Given these previous results with TPM SAMs, then, it was not clear at the outset what the passivating properties of SAMs of 1a-b might be like when compared to those of well-defined and highly passivating HDT SAMs. Electrochemical capacitance has proven to be a good measure of the macroscopic permeability of a SAM toward solvated ions, and as such, it can be taken as an indicator of the SAM’s integrity.2l,13,14 Briefly, if a SAM is impermeable, it should function as an ideal capacitor with the capacitance being given by the relation

C ) 0/deff where C is the capacitance per unit area,  is the dielectric constant of the film, 0 is the permittivity of free space, and deff is the SAM film thickness. If the monolayer is impermeable, the charging current observed in the cyclic voltammetry experiment will be independent of potential during the linear potential scans. As a SAM structure becomes more permeable to ions, however, a larger charging current and capacitance will be encountered. Finally, at extreme electrochemical potentials, the large electric field across the SAM can be expected to lead to, at some particular potential value, a failure of the barrier properties of the SAM that is characterized by ion permeation followed by electron-transfer chemistry at the metal-monolayer interface. Shown in Figure 1 is a comparison of cyclic voltammograms obtained for scans from 0.0 to +0.15 V and back to 0.0 V vs SCE in 1 M KCl for SAMs of 1a and HDT. As can be seen, the charging current for the former is significantly larger than that of the latter and this correlates with the large difference in their calculated capacitance values (cf. C ) 5 ( 2 µF‚cm-2 for SAMs of 1a vs C ) 1.2 ( 0.1 µF‚cm-2 for SAMs of HDT). Since both 1a and HDT are expected to have similar dielectric properties and molecular lengths, their SAMs should possess nearly identical capacitance values if their barrier properties are the same. The observation that the capacitance for SAMs of 1a is, at a minimum, more than double that for SAMs of HDT implies that the former are more permeable, most likely as a result of a higher density of defect sites. The larger variance in the measured capacitance values for SAMs of 1a relative to that for (11) Guo, L.-H.; Facci, J. S.; McLendon, G.; Mosher, R. Langmuir 1994, 10, 4588. (12) The predicted thickness of SAMs of 1a is measured from the outer edge of the van der Waal surface of the terminal aryl hydrogen atom to the Au surface and assuming a Au-S bond length of 2.4 Å. (13) (a) Finklea, H. O.; Avery, S.; Lynch, M.; Furtsch, T. Langmuir 1987, 3, 409. (b) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (c) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682. (14) (a) Finklea, H. O.; Snider, D. A.; Fedyk, J. Langmuir 1990, 6, 371. (b) Bilewicz, R.; Majda, M. Langmuir 1991, 7, 2794. (c) Groat, K. A.; Creager, S. E. Langmuir 1993, 9, 3668. (d) Badia, A.; Back, R.; Lennox, R. B. Angew. Chem., Int. Ed. Engl. 1994, 33, 2332.

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Figure 1. Cyclic voltammograms (first scan) for a gold electrode covered by a monolayer of 1a (solid line) and HDT (dashed line) in aqueous 1 M KCl (0.45 cm2 electrode area).

SAMs of HDT can then be viewed as a reflection of a greater difficulty in controlling the density of these defect sites from sample to sample. It can also be mentioned that the barrier properties of SAMs of 1a and HDT both fail in 1 M KCl between -0.4 and -0.6 V vs SCE, and this failure causes irreversible damage to the monolayers, as indicated by significantly larger charging currents and capacitance values being observed after this negative scan. On scanning to more extreme positive potentials, the barrier properties of SAMs of 1a fail between +0.5 and +0.7 V in 1 M KCl which is a much lower limit than that obtained for SAMs of HDT (cf. a failure that occurs at ca. +1.2 V). In this case, failure also results in apparent damage to the monolayer of 1a; however, the observed rise in charging current and capacitance as a function of the number of repetitive scans to +0.8 V vs SCE is much less than that in the negative direction. Finally, the capacitance and stability of SAMs of 1b are, qualitatively, nearly identical to those of SAMs of 1a. The ability of a monolayer to block electron transfer between the gold surface and an electron donor or acceptor in solution has also been shown to be a good measure of its defectiveness.2l,8a,11,13,14 Here, the diffusion-limited currents measured during the cyclic voltammetry experiment at partially blocked electrodes are a function of the distribution of residual pinholes in the monolayer and their size.15 In fact, it has been demonstrated that these pinholes can effectively function as a microelectrode array.16 Figure 2 shows cyclic voltammograms of monolayer-covered and “bare” gold electrodes in a solution containing 1 mM K3Fe(CN)6 in 1 M KCl. The voltammogram for bare gold (dashed line) shows the oxidation and reduction peaks typically seen for this redox couple at a metal electrode. On the other hand, the voltammogram recorded using a HDT SAM-covered gold electrode (dotted line) reveals that this SAM structure efficiently blocks electron transfer, in agreement with previous studies. In contrast to both of these extremes, however, the voltammogram for a gold electrode covered with a SAM of 1a (solid line) indicates that the SAM of this adsorbate apparently has a fairly high defect density that is in keeping with its large capacitance value. In order to understand the possible origin of the higher defect density in SAMs of 1a versus those of HDT, it is (15) Amatore, C.; Save´ant, J. M.; Tessier, D. J. Electroanal. Chem. 1983, 147, 39. (16) Finklea, H. O.; Snider, D. A.; Fedyk, J.; Sabatani, E.; Gafni, Y.; Rubinstein, I. Langmuir 1993, 9, 3660.

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Figure 2. Cyclic voltammograms for a “bare” gold electrode (dashed line) and for gold electrodes covered by a monolayer of 1a (solid line) and HDT (dotted line) in aqueous 1 M KCl containing 1 mM K3Fe(CN)6 (0.45 cm2 electrode area).

important to comment on the differences in their methods of preparation. As is routinely performed, the HDT SAMs used in this study were prepared from a 1 mM solution of this adsorbate in ethanol. Although this results in a SAM with superior passivating characteristics, it is important to note that recent studies indicate that, due to the poor solvating properties of ethanol for long-chain alkanethiols, the SAM that is obtained is likely composed of a mixture of densely-packed physisorbed and chemisorbed material.17 Furthermore, in this case, it is also likely that the strong driving force for adsorption that is provided by this poor solvent is effective in filling defect sites with adsorbed material. A much better solvent for the adsorbate should lead to a SAM with a higher density of defect sites even though it might contain a higher percentage, or consist exclusively of, chemisorbed material. This idea is supported by our observation of inferior passivating properties, both capacitance and blocking, for SAMs of HDT that are prepared from 1 mM solutions of this adsorbate in dichloromethane. A return to the original properties, however, can be achieved by taking this initially prepared sample and soaking it overnight in an ethanolic solution of HDT (1 mM). In contrast to SAMs of HDT, those of 1a are prepared using a solution of this adsorbate in dichloromethane. Use of this solvent is mandated by the apparent insolubility of 1a in ethanol.18 Accordingly, we believe that, due to the better solubility of 1a in dichloromethane, there is a large reduction in the driving force to fill defect sites in the SAM structure with either physisorbed or chemisorbed material, and as a result, poorer passivating properties are realized. Another alternative suggestion, however, is that, due to the rigid-rod nature of 1, the molecular subunits in the SAM derived from this class of adsorbate are not flexible enough to cover defect sites as (17) Castner, D. G.; Hinds, K.; Grainger, D. W. Langmuir 1996, 12, 5083. (18) Interestingly, Rubinstein and co-workers7c prepared SAMs of TPM using water as the “solvent”.

Zehner and Sita

Figure 3. Comparison of the (a) capacitance in aqueous 1 M KCl and (b) blocking characteristics in 1 M KCl containing 1 mM K3Fe(CN)6 of a gold electrode covered by a monolayer of HDT (dashed lines) with a gold electrode covered by a monolayer of 1a that has been subsequently immersed in a 1 mM ethanolic solution HDT for 24 h (solid lines) (0.45 cm2 electrode area).

well as the much more conformationally mobile HDT molecule potentially can. In this scenario, rigid domain boundaries between ordered assemblies of 1a might then expose the underlying gold surface to a greater extent than in the more fluid HDT SAM structures. Finally, at this point, it must be mentioned that in addition to the electrochemical studies of TPM SAMs mentioned above, Miller and co-workers9a have investigated the passivating properties of SAMs prepared from chloroform/trifluoroacetic acid solutions of thiophenol-terminated oligoimides that also form a class of rigid-rod adsorbate. In this case, the passivating properties of these SAMs are qualitatively quite similar to those obtained for SAMs of 1. The ability to enhance the passivating properties of a HDT SAM prepared from a dichloromethane solution by subsequent adsorption from an ethanolic solution suggested that a similar “patching” process might work in the case of SAMs of 1a as well.7c,19 As Figure 3 demonstrates, soaking a gold electrode covered with a SAM of 1a in an ethanolic solution of HDT (1 mM) for 24 h dramatically improves both the capacitance (cf. C ) 2.2 ( 0.2 µF‚cm-2) and the blocking characteristics of the monolayer assembly (parts a and b of Figure 3, respectively). Indeed, even brief periods of immersion in the HDT solution result in substantial improvements in the passivating properties of SAMs of 1a as revealed by cyclic voltammograms of samples that were treated in this fashion for only 5 and 30 min (parts a and b of Figure 4, respectively). Finally, these patched samples of SAMs of 1a showed a remarkable improvement in stability toward repetitive cycling between 0.0 and +0.8 V (vs SCE) in 1 M KCl as Figure 5 documents. Once again, similar (19) For heterogeneous patching of a structurally unrelated nanoporous SAM assembly, see: Yang, Z. P.; Engquist, I.; Kauffmann, J.M.; Liedberg, B. Langmuir 1996, 12, 1704.

Passivating Properties of SAMs

Figure 4. Blocking characteristics in 1 M KCl containing 1 mM K3Fe(CN)6 of gold electrodes covered by a monolayer of 1a that have been subsequently immersed in a 1 mM ethanolic solution of HDT for (a) 5 min and (b) 30 min (0.45 cm2 electrode area).

Figure 5. Comparison of the stabilities (as measured by capacitance values) of a monolayer of 1a (filled circles) and a monolayer of 1a that has been subsequently immersed in a 1 mM ethanolic solution of HDT for 24 h (filled squares) to continous scanning for 0.0 to +0.80 V and back to 0.0 V vs SCE. Lines are aids to the eye only.

improved properties for SAMs of 1b can also be obtained by this heterogeneous patching process. An important question that arises with the heterogeneous patching of SAMs of 1a is: what is the composition of the resulting monolayers? If the improved passivating properties simply arise from the addition of HDT into defect sites and domain boundaries, then the new mixed monolayers should still be significantly composed of molecular subunits of 1. However, it is well known that SAMs of n-alkanethiols are prone to undergo facile selfand heterogeneous exchange with thiol adsorbates in solution.2b,l Accordingly, it is conceivable that the improved properties shown in Figures 3-5 are a result of a mixed monolayer structure that is substantially derived from HDT through an exchange of arenethiol adsorbate by HDT. To address this issue, a variety of macroscopic

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charcterization techniques were employed. Thus, ellipsometry provided an apparent film thickness value of 29 ( 4 Å after a 24 h treatment that was significantly thicker than that obtained for the pure SAM of 1a (cf. 22 ( 4 Å). However, since repeated washing with both polar and nonpolar solvents had no effect on this measured thickness, the presence of an overlayer of physisorbed material on the SAM can be ruled out. Hence, the increase in the SAM’s apparent film thickness is more in keeping with an increase in surface coverage by an adsorbate that most likely arises as a result of a densification of the monolayer that occurs with the filling of defect sites.21 Advancing contact angles for water, θadv, also increase from a value of 79 ( 3° for the pure SAM of 1a to a value of 91 ( 3° for the mixed monolayer after a 24 h treatment indicating that the latter are significantly more hydrophobic, presumably as a result of inclusion of HDT into the monolayer structure. Finally, this increase in hydrophobicity is accompanied by a decrease in the hysteresis defined by the difference between θadv and the receding contact angles for water, θrec, which can be taken as further indication of a less defective SAM structure that results upon patching.1a In order to examine more closely the composition of the heterogeneously patched SAMs of 1, X-ray photoelectron spectroscopy (XPS) was utilized.22,23 To begin, a series of XPS spectra for a patched monolayer of 1a were recorded at varying takeoff angles to provide a rough depth profile of its composition.22 The data revealed an increase in intensity of the combined S(2p3/2) and S(2p1/2) peaks (binding energies 161.5 and 162.7 eV, respectively) relative to that of the C(1s) peak (284 eV binding energy) as the takeoff angle was increased. This trend supports the notion that the sulfur atoms in the patched monolayer are located beneath the carbon atoms, next to the gold surface, rather than being associated with physisorbed HDT material that is forming an overlayer on the original SAM of 1. Next, as Figure 6 reveals, a comparison of XPS spectra for pure SAMs of HDT and 1a, and a patched monolayer of 1a after a 24 h treatment, exhibit diagnostic differences in the C(1s) peaks that can be used to assess the composition of the latter. More specifically, XPS spectra of SAMs of HDT (solid line) and 1a (dashed line) show a 0.7 eV difference in their C(1s) binding energies (cf. BE (HDT) 284.8 eV vs BE (1) 284.0 eV) which is in excellent agreement with the values already reported for HDT SAMs24 and for those of an arenethiol adsorbate that is closely related to 1.7b The C(1s) peaks observed in the XPS spectra for SAMs of 1a that have been treated with a solution of HDT for 24 h (dotted line) are sandwiched between these two values. As Figure 6 further indicates, this C(1s) peak can be satisfactorily fit with a linear combination of the two reference spectra for SAMs of HDT and 1a, with the three variable parameters used in this curve-fitting being relative intensity, absolute intensity, and baseline level. Using this analysis, a plot of relative SAM composition as a function of time was obtained that reveals that either a rapid addition of HDT to, or exchange of 1a by HDT in, the original SAM structure occurs rapidly within the first 30 min. In fact, if this inclusion of HDT were to occur solely by an exchange process, then it would (20) (a) Collard, D. M.; Fox, M. A. Langmuir 1991, 7, 1192. (b) Schlenoff, J. B.; Ly, H. J. Am. Chem. Soc. 1995, 117, 12528. (c) Peterlinz, K. A.; Georgiadis, R. Langmuir 1996, 12, 4731. (d) Allara, D. L.; Nuzzo, R. G. Langmuir 1995, 1, 45-52.(21) (a) Azzam, R. M. A.; Bashara, N. M. Ellipsometry and Polarized Light; North-Holland: New York, 1987. (b) Smith, T. J. Opt. Soc. Am. 1968, 58, 1069. (22) Watts, J. F. An Introduction on Surface Analysis by Electron Spectroscopy; Oxford University Press: Oxford, 1990. (23) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321-335.

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Zehner and Sita

Figure 7. Comparison of the grazing-angle infrared spectra of 1a (a) and 1b (b), showing the greatly enhanced intensity of the phenyl stretching band just above 1500 cm-1.

Figure 6. (a) Comparisons of the C(1s) photoelectron peaks from the XPS spectra for a SAM of HDT (solid line), a SAM of 1a (dashed line), and a SAM of 1a that has been subsequently immersed in a 1 mM ethanolic solution of HDT for 24 h (dotted lines). (b) A best fit (dashed line) for the C(1s) photoelectron peak of a SAM of 1a that has been subsequently immersed in a 1 mM ethanolic solution of HDT for 24 h (solid line) that was constructed from taking linear combinations of the XPS spectra for a SAM of HDT (30% relative composition) and a SAM of 1a (70% relative composition).

represent replacement of approximately a quarter to a third of the arenethiol adsorbates after the first 24 h (e.g., Figure 6b showing a SAM composition of 30% HDT after 24 h). Finally, after this initial period, further slow exchange of 1a by HDT apparently continues so that after 12 days, the original arenethiol comprises only 30% of the monolayer. A more detailed picture of the addition/exchange process can be obtained from grazing-angle infrared spectra taken at various points in the patching process. By comparison of the increase in intensity of the characteristic C-H stretching modes of HDT (2700-3100 cm-1) with the decrease of the phenyl stretching mode (mode 19a)24 for the aromatic thiol at 1518 cm-1, relative rates for removal of the original arenethiol monolayer and addition of HDT can be obtained. A similar method was previously applied by Allara and Nuzzo20d to measure the extent of exchange of carboxylic acids at an aluminum surface. For this study, SAMs of 1b were utilized since, as Figure 7 indicates, fluorine substitution greatly enhances the transition dipole of this longitudinal phenyl stretching mode, thereby, increasing the intensity of its absorption by at least an order of magnitude. The course of exchange of SAMs of 1b by HDT can be followed by the data presented in Figures 8-10. After 30 min of exchange (Figure 8), it can be seen that the C-H stretching modes of HDT have already grown in significantly; however, they are shifted to higher energy relative to those of a well-ordered HDT monolayer. This shift can be attributed to a more disordered, liquid-like state of the (24) Varsanyi, G. Assignments for Vibrational Spectra of Seven Hundred Benzene Derivatives; Adam Hilger: London, 1974.

Figure 8. Grazing-angle infrared spectra of the C-H stretching region of a monolayer of 1b after (from bottom to top) initial preparation and 33 min, 21 h, 8 days, and 20 days immersion in an ethanolic 1 mM HDT solution.

alkanethiol adsorbates.13b By comparison, spectra taken after an overnight exchange show a smaller amount of additional increase in intensity of the alkane C-H stretching modes (Figure 8), but now this change is accompanied by a shift of the peaks to lower energy, thereby, indicating the formation of a quasi-crystalline, well-packed n-alkane assembly. Longer exchange times result in a much slower rate of additional HDT incorporation which continues over a period of weeks and results in IR spectra that resemble even more closely that of a pure HDT monolayer (Figure 8). Finally, it is important to note that, during the exchange process, the primary peak in the phenyl stretching region at 1518 cm-1 shows a corresponding decrease in intensity as a function of the time of immersion (Figure 9). However, here it is important to note that, after overnight immersion (21 h), it has only decreased to ∼85% of its original height, and after 10 days or more, it still retains ∼40-50% of its original height. As Figure 10 reveals, a qualitative comparison of the kinetics of HDT addition and arenethiol removal show the two to be closely coupled with the final values for the HDT and arenethiol components being close to those obtained from other studies involving the selfand heterogeneous exchange of n-alkanethiol SAMs.2b,21 In addition, the relatively close agreement of the XPS data, which gives a measure of the proportion of each moiety on the surface, and the IR data, which measures the actual change in surface concentration, indicates that

Passivating Properties of SAMs

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Conclusion

Figure 9. Grazing-angle infrared spectra of the phenyl stretching region of a monolayer of 1b after (from bottom to top) initial preparation and 33 min, 21 h, 8 days, and 20 days immersion in an ethanolic 1 mM HDT solution.

Figure 10. Comparison of the decrease in intensity of the phenyl stretching region (filled circles) with the increase in intensity of the C-H stretching region (open triangles) as a function of time of immersion in an ethanolic 1 mM HDT solution. Lines are aids to the eye only.

the patching process of monolayers of 1 is indeed primarily that of heterogeneous exchange.

Self-assembled monolayers of the arenethiols 1 on Au(111) appear to possess relatively poor passivating properties when compared to those of SAMs of HDT. A possible origin for this difference may lie with the difference in the methods used for the SAM preparations. More specifically, the use of dichloromethane in the former case and ethanol in the latter most likely provides SAMs of 1 that have a higher density of vacant defect sites relative to HDT SAMs. In addition, it is possible that the more rigid rod like nature of the molecular subunits in these SAMs are less effective than the more conformationally flexible n-alkane chains of HDT in providing surface coverage at domain boundaries. However, heterogeneous patching of SAMs of 1 with an ethanolic solution of HDT for short periods of time (e.g., 5 min to 24 h), provides mixed 1/HDT monolayers that are composed primarily of the original arenethiol material, but which now possess superior passivating properties and stability as determined by electrochemical techniques in aqueous electrolytes. Significantly, grazing angle Fourier transform IR data support a structural model for these new mixed 1/HDT monolayers that is comprised of a large component made up of domains of assembled arenethiol adsorbate that are then dispersed by a smaller component consisting of order HDT assemblies that reside at domain boundaries. Thus, the ability to reproducibly obtain these highly mixed monolayers in which 1 is the primary component now provides a means by which to investigate, at the next level of study, electron transfer/tunneling through these novel interfacial barriers. Studies along these lines are now in progress. Acknowledgment. This work was supported in part by the MRSEC program under the National Science Foundation (DMR-9400379) for which we are grateful. L.R.S. is a Beckman Young Investigator (1995-1997) and a Camille Dreyfus Teacher-Scholar (1995-2000). We wish to also thank Dr. Steven Wasserman at the Argonne National Laboratory for assistance with obtaining and analyzing the XPS data. LA962035B