Multilayer Formation of 1,2-Ethanedithiol on Gold ... - ACS Publications

May 10, 2000 - Invoking the fact that surface-enhanced Raman scattering (SERS) is a very sensitive surface-probing technique and that 1,2-ethanedithio...
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Langmuir 2000, 16, 5391-5396

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Multilayer Formation of 1,2-Ethanedithiol on Gold: Surface-Enhanced Raman Scattering and Ellipsometry Study Sang Woo Joo, Sang Woo Han, and Kwan Kim* Laboratory of Intelligent Interface, School of Chemistry and Center for Molecular Catalysis, Seoul National University, Seoul 151-742, Korea Received October 12, 1999. In Final Form: February 3, 2000 Invoking the fact that surface-enhanced Raman scattering (SERS) is a very sensitive surface-probing technique and that 1,2-ethanedithiol (1,2-EDT) is the simplest alkanedithiol molecule, we have analyzed the SER spectra of 1,2-EDT in aqueous gold sol to confirm spectroscopically the feasibility of multilayered film formation for alkanedithiols on the gold surface. We could clearly identify the S-S stretching band that reveals the formation of multilayers of 1,2-EDT on the colloidal gold surface. Surprisingly, the intermolecular disulfide bond appeared to form even at a submonolayer coverage limit, implying that the adsorption of alkanedithiol on gold does not take place homogeneously from the early stage of self-assembly. A separate ellipsometry measurement performed with vacuum-evaporated gold substrates revealed that up to tetralayers could be assembled on gold by 1,2-EDT in n-hexane while a bilayer formed in ethanol.

Introduction The organization of self-assembled monolayers on metal surfaces provides a practical approach for fabricating interfaces with a well-defined thickness and structure.1-3 The most widely studied and well-characterized systems include alkanethiols,4 dialkyl sulfides,5 and dialkyl disulfides6 self-assembled on gold and silver surfaces. In line with this, aliphatic dithiols are usually adsorbed on silver as dithiolates by forming two Ag-S bonds.7,8 Dithiol molecules have been claimed on the contrary to adsorb on gold as monothiolate by forming one single Au-S covalent bond.9 Moreover, Kohli et al.10 recently reported that up to eight covalently attached layers could be formed on gold from solution phase (R,ω)-aliphatic dithiols. The linking chemistry between layers was claimed to be the oxidative formation of a sulfur-sulfur bond. The adsorption characteristics of aliphatic dithiols on gold are in fact dependent on the kind of solvent as well as the concentration and the duration of the self-assembly. For instance, in an investigation of the structure and photooxidation of 1,8-octanedithiol self-assembled on an evaporated gold film, Rieley et al.11 could interpret its X-ray photoelectron spectrum, presuming that the adsorbate was present on gold as a monolayer. * To whom all correspondence should be addressed. Tel. 82-28806651; Fax 82-2-8743704; E-mail [email protected]. (1) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, 1991. (2) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (3) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (4) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152. (5) Joo, T. H.; Kim, K.; Kim, M. S. J. Mol. Struct. 1987, 162, 191. (6) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. Soc. 1987, 109, 733. (7) Kwon, C. K.; Kim, K.; Kim, M. S.; Lee, Y. S. J. Mol. Struct. 1989, 197, 171. (8) Kwon, C. K.; Kim, K.; Kim, M. S.; Lee, S.-B. Bull. Korean Chem. Soc. 1989, 10, 254. (9) Tour, J. M.; Jones, L., II; 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. (10) Kohli, P.; Taylor, K. K.; Harris, J. J.; Blanchard, G. J. J. Am. Chem. Soc. 1998, 120, 11962. (11) Rieley, H.; Kendall, G. K.; Zemicael, F. W.; Smith, T. L.; Yang, S. Langmuir 1998, 14, 5147.

The feasibility of the formation of multilayers for dithiols on gold is usually deduced from the ellipsometric thickness data.9,10 The most compelling spectroscopic evidence of multilayer formation will be the identification of the S-S stretching band. In fact, we have recently reported the observation of the S-S stretching band, albeit weak, by the surface-enhanced Raman scattering (SERS) for p-xylene-R,R′-dithiol dissolved in aqueous gold sol.12 To further clarify the adsorption characteristics of dithiols on gold, we herein report the SER spectral analysis of 1,2-ethanedithiol (1,2-EDT) in gold sol, taking advantage of the facts that Raman spectroscopy is suitable for the identification of S-S stretching bands as well as that SERS provides high spectral resolution and spectral sensitivity even at a submonolayer coverage limit.13 We chose 1,2EDT as adsorbate on the grounds that the molecule does not possess any fundamental mode around the usual S-S stretching frequency region5,14 and that a large SER enhancement is expected for the S-S stretching band once a multilayer or at least a bilayered film is formed on the gold surface since there are only two methylene units in 1,2-EDT. For a more reliable analysis of SER spectra in gold sol, the feasibility of the formation of multilayers on the vacuum-evaporated gold was also examined by means of ellipsometry. Experimental Section The gold sol was prepared following the documented procedures.15 Namely, 133.5 mg of KAuCl4 (Aldrich) was initially dissolved in 250 mL of water, and the solution was brought to the boil. A solution of 1% sodium citrate (25 mL) was then added to the KAuCl4 solution under vigorous stirring, and boiling was continued for ca. 20 min. The resulting Au sol solution was stable for several weeks. The chemical 1,2-ethanedithiol (1,2-EDT) was purchased from Aldrich and used as received. To 0.1-1 mL of Au sol solution, 10-3-10-5 M ethanolic solution of 1,2-EDT was added dropwise to the final concentration of 10-4-10-8 M using a micropipet; the purple gold sols became bluish green by the (12) Joo, S. W.; Han, S. W.; Kim, K. J. Phys. Chem. B 1999, 103, 10831. (13) Chang, R. K., Furtak, T. E., Eds.; Surface Enhanced Raman Scattering; Plenum Press: New York, 1982. (14) Freeman, S. K. Applications of Laser Raman Spectroscopy; WileyInterscience: New York, 1974; p 232. (15) Lee, P. C.; Meisel, D. J. Phys. Chem. 1982, 86, 3391.

10.1021/la991331w CCC: $19.00 © 2000 American Chemical Society Published on Web 05/10/2000

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Joo et al. Table 1. Spectral Data and Vibrational Assignments of 1,2-EDTa SERS OR neat 301 396

Figure 1. (a) OR spectrum of neat 1,2-EDT. (b) SER spectrum of 2 × 10-5 M 1,2-EDT in aqueous gold sol. (c) OR spectrum of neat diethyl disulfide. The spectral region between 2400 and 1700 cm-1 was omitted due to the absence of any information. addition of 1,2-EDT. The chemicals otherwise specified were reagent grade, and triply distilled water of resistivity greater than 18.0 MΩ‚cm was used in making aqueous solutions. Raman spectra were obtained using a Renishaw Raman system model 2000 spectrometer equipped with an integral microscope (Olympus BH2-UMA). The 632.8 nm from a 17 mW air-cooled He/Ne laser (Spectra Physics model 127) was used for the excitation source of the Au sol SERS experiment; an interference filter (CVI Laser Corp.) was put in front of the He/Ne laser source to reject many unwanted plasma lines. Raman scattering was detected with 180° geometry using a peltier cooled (-70 °C) CCD camera (400 × 600 pixels). A glass capillary (KIMAX-51) with an outer diameter of 1.5-1.8 mm was used as a sampling device. The laser beam was focused onto a spot approximately 2 µm diameter with an objective microscope of the order of ×20. Data acquisition times were usually 270 s for the gold sols. The holographic grating (1800 grooves/mm) and the slit allowed the spectral resolution to be 1 cm-1. The Raman band of a silicon wafer at 520 cm-1 was used to calibrate the spectrometer, and the accuracy of the spectral measurement was estimated to be better than 1 cm-1. The Raman spectrometer was interfaced with an IBM-compatible PC, and the spectral data were analyzed using a Renishaw WiRE software v. 1.2 based on the GRAMS/ 32C suite program (Galactic). Ellipsometric measurements were made for 1,2-EDT layers self-assembled on gold substrates. Initially, gold substrates were prepared by resistive evaporation of titanium (Aldrich, >99.99%) and gold (Aldrich, >99.99%) at ∼10-6 Torr on batches of glass slides, cleaned previously by sequentially sonicating in ethanol, hot piranha solution (1:3 H2O2(30%)/H2SO4), and distilled deionized water. Titanium was deposited prior to gold to enhance adhesion of the gold to the substrate. After the deposition of ≈200 nm of gold, the evaporator was back-filled with nitrogen. The gold substrates were immersed subsequently in a 1,2-EDT solution in n-hexane or methanol or ethanol. After removing the substrates from the solution, they were rinsed with excess solvent and then dried in a N2 gas stream. The ellipsometric thickness of such self-assembled 1,2-EDT films was estimated using a Rudolph Auto EL II optical ellipsometer. The measurement was performed using a 632.8 nm line of He/Ne laser incident upon the sample at 70°. The ellipsometric parameters, ∆ and Ψ, were determined for both the bare clean substrate and the selfassembled film. The so-called DafIBM program supplied by Rudolph Technologies was employed to determine the thickness values, assuming that the refractive index of the organic film was 1.45. At least four different sampling points were considered in order to obtain averaged thickness values.

Results and Discussion The ordinary Raman (OR) spectrum of neat 1,2-EDT is shown in Figure 1a. The SER spectrum of 1,2-EDT

636 666 719 739 815 847 vw 895 941 974 2558 2923

Ag solb (4 × 10-6 M) 418 612 646 688 715 825

Au sol (2 × 10-5 M) 339 417 505 629 672 726 825

900 930 2548 2899

assignmentc CCS def (T) CCS def (G) ν (SS) CS stretch (G) CS stretch (G) CS stretch (T) CS stretch (T) CSH bend CH2 rock (G) CSH bend CH2 rock (G) CH2 rock (T) ν (SH) CH2 asym stretch

a All data given in cm-1. b Taken from ref 7. We observed separately that the SER peak positions in silver sol were independent of the adsorbate concentration. Abbreviation: vw (very weak). c G and T refer to the gauche and trans conformers, respectively, with respect to the C-C bond of 1,2-EDT. See ref 7.

obtained at the bulk concentration of 2 × 10-5 M in gold sol is shown in Figure 1b. The positions of the peaks appearing in Figure 1a,b and their proper assignments are listed in Table 1. For comparison, the positions of peaks appearing in the SER spectrum of 1,2-EDT in silver sol are also listed in Table 1. We already reported7 that 1,2-EDT was chemisorbed dissociatively on the colloidal silver surface by rupture of the two S-H bonds, implying that the 1,2-ethanedithiolate anion formed upon adsorption was bound to silver atom(s) via its two sulfur atoms. Such adsorption behavior was independent of the surface coverage. It is remarkable that a very distinct peak is observed at 505 cm-1 in the Au sol SER spectrum in Figure 1b. Its counterpart was not identified at all in the infrared or OR spectra of 1,2-EDT in neat or anion states, nor in the SER spectrum in silver sol. Consulting the vibrational spectral data reported for dialkyl disulfides,5,6,14 the peak has to be attributed to the S-S stretching vibration. For instance, the S-S stretching band of diethyl disulfide appears distinctly at 508 cm-1 in its OR spectrum as shown in Figure 1c. For a more clear presentation, we have performed several control experiments. First of all, any disulfide species was not detected from 1H NMR spectroscopy for 1,2-EDT saturated D2O as well as for 1 M 1,2-EDT in CD3OD or C6D6, all of which had been left for 2 days in an ambient condition. Besides, the S-S stretching band was not detected at all, at least for 2 days, in the OR spectra of 1,2-EDT solution in water, methanol, and ethanol. Furthermore, at least for 1 day, we could not detect any OR peak assignable to disulfide or oxidized sulfur species for an aqueous solution containing 1,2-EDT, sodium citrate, and KCl; the pH and ionic strength of the aqueous solution were nearly the same as those of the Au sol solution. Since the concentration of 1,2-EDT was as large as ∼0.1 M, we could clearly identify the OR peaks of 1,2-EDT. The present observation does not necessarily mean that any disulfide or oxidized sulfur species are not produced at all from 1,2-EDT. Nonetheless, we can presume that the amount of oxidized species, if produced, will be too small to detect with Raman spectroscopy. On the other hand, it may be worth mentioning that, in a SER spectrum of 1-ethanethiol in Au sol, we cannot identify any S-S stretching band. This suggests that the possibility of Au surface catalyzed formation of S-S bond

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at the gold sol surfaces as proposed by Fenter et al.16 is very low not only for 1-ethanethiol but also for 1,2-EDT. On these grounds, the SER spectra taken in the Au sol are supposed to indicate that multilayers can be assembled for 1,2-EDT on the colloidal gold surface. The other obvious difference between the SER spectra of 1,2-EDT in gold and silver sols is that the S-H stretching peak is identified, albeit weakly, at 2548 cm-1 in the gold sol SER spectrum while the peak is completely absent in the silver sol SER spectrum. As can be noticed in Table 1, the peak positions of the C-S stretching bands are also substantially different between the SER spectra taken in gold and silver sols. Recalling our earlier report,7 the OR peaks at 636 and 666 cm-1 in Figure 1a (see also Table 1) have to be attributed to the C-S stretching vibration of the gauche conformer around the C-C bond of 1,2EDT. On the other hand, the OR peaks at 719 and 739 cm-1 have to be assigned to the C-S stretching vibration of the trans conformer. As is indicated in Table 1, the four peaks at 612, 646, 688, and 715 cm-1 appearing in the Ag sol SER spectrum could be correlated respectively with the OR peaks at 636, 666, 719, and 739 cm-1. Invoking the fact that when a mercaptan adsorbs on a silver surface its C-S stretching frequency red-shifts by 20-50 cm-1 from the value for neat liquid,7,8,17,18 the substantial red shift of the above four bands was caused by the formation of two Ag-S bonds upon adsorption on silver as dithiolate. In the SER spectrum of 1,2-EDT in gold sol (Figure 1b), three C-S stretching peaks are identified at 629, 672, and 726 cm-1 (see Table 1). One C-S stretching band is thus considerably red-shifted upon adsorption on gold (i.e., from 719 to 672 cm-1) even though the amount of red shift of the other two bands is far less than is the case on silver. This can be understood by recalling that one thiol group of 1,2-EDT is deprotonated to bind to gold and the other thiol group is pendent with respect to the gold surface. We have to mention that the SER peaks described herein are all to have nothing to do with any oxidized sulfur species. Although recent studies have shown that organothiol monolayers on silver and gold oxidize to sulfates and sulfonates under the presence of excess oxidizers,10,19 any typical oxidized sulfur peaks such as νs(SO42-), νas(SO42-), and νs(SO2) which are expected to appear at ∼875, ∼1040, and ∼1145 cm-1, respectively,19 are never identified in all SER spectra (vide supra). On the other hand, according to our separate control experiment, we could not identify any peak due to SO42- in a SER spectrum taken after dissolving a 9:1 mixture of 1,2-EDT and potassium sulfate in a Au sol. This implies that the adsorption strength of sulfate to gold is much weaker than that of 1,2-EDT. On these grounds, the SER peak appearing, for instance, at 610-630 cm-1 cannot be ascribed to δ(SO42-);19 rather, it has to be attributed to the C-S stretching vibration. For 1,2-EDT on gold, the multilayer formation seems thus to be more favorable than the oxidation of thiol groups at least under the absence of excess oxidizers. Another noticeable feature in the Au sol SER spectrum is that the C-S stretching band of the trans conformer is more intense than that of the gauche conformer. In the case of the Ag sol experiment, the gauche conformer dominated over the trans form, however. This can also be understood by presuming different structures of 1,2-EDT on Au and Ag. Invoking the fact that 1,2-EDT adsorbs on Ag as dithiolate, (16) Fenter, P.; Eberhardt, A.; Eisenberger, P. Science 1994, 266, 1216. (17) Bryant, M. A.; Pemberton, J. E. J. Am. Chem. Soc. 1991, 113, 8284. (18) Lee, T. G.; Kim, K.; Kim, M. S. J. Phys. Chem. 1991, 95, 9950. (19) Schoenfisch, M. H.; Pemberton, J. E. J. Am. Chem. Soc. 1998, 120, 4502.

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considerable steric hindrance will exist between the methylene hydrogens and the surface specifically when the 1,2-ethanedithiolate species on Ag assumes a trans form. On the other hand, a trans form would be energetically more favorable than a gauche form for multilayers to be assembled on gold. These SER spectral features suggest that 1,2-EDT is adsorbed on the surface of gold as monothiolate by forming one single Au-S bond while the molecule is adsorbed on silver as dithiolate forming two Ag-S bonds. Nonetheless, the origin of the different adsorption mechanism of 1,2-EDT on gold and silver is uncertain at the moment. The nature of the metal-sulfur bond has been studied by Sellers et al.20 using ab initio geometry optimization calculations for HS and CH3S on gold and silver. We have recently reported the molecular dynamics simulation study for benzenethiolate and benzyl mercaptide on Au(111) to examine the packing structures of the adsorbates.21 These kinds of theoretical investigations may be helpful to elucidate the different adsorption characteristics of 1,2-EDT on gold and silver; we plan to perform such calculations in the near future. To see whether the S-S linkage could form prior to the formation of a full-covered monolayer as well as whether the S-S stretching band were intensified upon increase in the bulk concentration of 1,2-EDT, a series of SER spectra have been taken as a function of the concentration of 1,2-EDT in gold sol. In parts a and b of Figure 2 are shown the SER spectra of 1,2-EDT taken at 8 × 10-8-4 × 10-4 M concentrations in the wavenumber regions of 3000-2500 and 1500-200 cm-1, respectively. According to the TEM measurement, the average diameter of gold particles was 50 nm. Assuming that the adsorbate was oriented perpendicularly with respect to the gold surface, the concentration of 1,2-EDT required for monolayer coverage was estimated to be 9.0 × 10-6 M. In fact, the SER spectral pattern is seen to depend on the bulk concentration of 1,2-EDT. Even at 8 × 10-8 M concentration, the νas(CH2) band appeared distinctly at 2894 cm-1. The position of the νas(CH2) band varied, however, from 2894 to 2901 cm-1 at 4 × 10-4 M concentration. (The substantial red shift by as much as 20 cm-1 of the νas(CH2) mode in the SER spectrum compared with that in the neat OR spectrum may be associated with the multilayer formation of 1,2-EDT on gold. In the SERS study of n-alkanethiols on Ag and Au, Bryant and Pemberton17 observed that the position of the νas(CH2) band shifts considerably to lower frequencies as the chain length increases.) On the other hand, at concentrations lower than 2 × 10-5 M, the ν(SH) band was hardly detected. On the contrary, the band became gradually more distinct upon increase in the bulk concentration from 2 × 10-5 to 8 × 10-5 M; further increase in the bulk concentration slowed the growth of the band, however. Concomitantly, the position of the ν(SH) band was blue-shifted gradually from 2548 cm-1 at 2 × 10-5 M to 2552 cm-1 at 8 × 10-5 M. The ν(SH) band appeared eventually at 2557 cm-1 at 4 × 10-4 M concentration, which is hardly different from that (i.e., 2558 cm-1) observed for neat 1,2-EDT. The most noteworthy feature in Figure 2b is the prominent appearance of the S-S stretching band even at a concentration corresponding to the submonolayer coverage limit. Although the band is seen weakly at 498 cm-1 at 8 × 10-8 M concentration, it is distinctly observed at 505 cm-1 at 2 × 10-7 M. In Figure 3 is shown the variation of the SER peak intensities of the ν(SH) and ν(SS) bands drawn with (20) Sellers, H.; Ulman, A.; Shnidman, Y.; Eilers, J. E. J. Am. Chem. Soc. 1993, 115, 9389. (21) Jung, H. H.; Won, Y. D.; Shin, S.; Kim, K. Langmuir 1999, 15, 1147.

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Figure 2. SER spectra of 1,2-EDT at 8 × 10-8-4 × 10-4 M in gold sol, in the wavenumber regions of (a) 3000-2500 and (b) 1500-200 cm-1. The peak intensities are normalized with respect to those of νas(CH2) bands in each SER spectra.

Figure 3. SER peak intensities of the ν(S-H) and ν (S-S) bands in Figure 2 drawn as a function of the concentration of 1,2-EDT in gold sol.

respect to that of the νas(CH2) band as a function of the bulk concentration of 1,2-EDT in gold sol. It can be noticed from the figure that the relative ν(SS) band intensity attains a plateau value at 8 × 10-5 M concentration. This may indicate that multilayers do not grow ceaselessly beyond a certain limit of bulk concentration; on the other hand, in ellipsometry measurement on flat gold surfaces, the thickness reached a plateau value after a certain period of self-assembly (vide infra). As mentioned above, the intensity of the ν(SH) band attains a quasi-plateau value at the same bulk concentration albeit that it grows further at higher bulk concentrations. These observations indicate that the S-S bond begins to form from the early stage of the self-assembly of 1,2-EDT on gold (between the adsorbed species on gold and the free 1,2-EDT species in the solution phase). More S-S bonds are formed at bulk concentrations above 2 × 10-5 M, but considering the features of the variation of the SER peak positions of the ν(SH) and νas(CH2) bands, the resulting multilayered film seems to possess a more disordered structure such that

occurrence of an intermolecular H-bond involving the terminal -SH group is infeasible. This may be associated with the fact that the multilayers do not grow homogeneously over the colloidal gold surface. The continuing growth of the ν(SH) band suggests on the other hand that the 1,2-EDT layer on gold should be an inhomogeneous multilayer instead of assuming a close-packed structure. This implies that the growth of multilayers of 1,2-EDT on gold may take place along with the interdigitation of free 1,2-EDT22 as well as the polydisulfide formation between metal particles.23 Invoking the fact that the pH of the gold sol was ∼3, any pendent SH group, if it were present, would not have been deprotonated. As mentioned above, the ν(SH) band was barely detectable in the SER spectra taken at any bulk concentration lower than 2 × 10-5 M. The absence of the ν(SH) band is thought, however, not to dictate that the adsorbates are present as dithiolate species on the gold surface. If 1,2-EDT were adsorbed on gold exclusively as dithiolate at a submonolayer coverage limit, it is difficult to rationalize the distinct appearance of the S-S stretching band in the SER spectrum taken at 2 × 10-7 M. There is no doubt that, in forming a S-S bond between the adsorbed species and the free 1,2-EDT dissolved in the solution phase, the adsorbate possessing one pendent SH group will have an obvious advantage over that anchored on gold as dithiolate. Hence, at least at a bulk concentration above 2 × 10-7 M, the adsobate anchored on the gold surface is supposed to have a monothiolate species. That the ν(SH) band is barely detectable in the SER spectra taken at 2 × 10-7-2 × 10-5 M may be associated with the fact that only a limited number of free thiol units are available for the adsorbed species owing to the formation of the intermolecular S-S bonds. At an extremely low surface coverage limit, however, we cannot completely ignore the possible presence of dithiolate species on gold since the peak positions of the C-S stretching bands observed in gold sol at 8 × 10-8 M are highly comparable (22) Partil, V.; Sastry, M. Langmuir 1998, 14, 2707. (23) Brust, M.; Bethell, D.; Schiffrin, D. J. Adv. Mater. 1995, 7, 795.

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Figure 4. (a) Ellipsometric thicknesses of 1,2-EDT layers selfassembled on vacuum-evaporated gold substrates in 1 mM 1,2EDT solution in methanol, ethanol, and n-hexane for a duration of 25 h. (b) Same as (a) but dipping first in a hexane medium followed by dipping in a ethanol medium.

to those observed in the SER spectra of 1,2-EDT in silver sol. (As can be seen in Table 1, the ν(CS) bands appear at 612, 688, and 715 cm-1 in the silver sol SER spectrum. Their counterparts appear at 619, 672, and 719 cm-1 in the gold sol spectrum taken at 8 × 10-8 M while they are observed at 628, 674, and 729 cm-1 at 2 × 10-7 M.) At 8 × 10-8 M concentration, the ν(SS) band was seen very weakly at 498 cm-1, and this differs by as much as 7 cm-1 from that observed at the concentration above 2 × 10-7 M. This may also imply that the disulfide present on gold at 8 × 10-8 M is not the same kind present at 2 × 10-7 M. The present SERS observation dictates that a brushlike polydisulfide could form on the gold sol particles. Considering the fact that the relative ν(SS) band intensity attained a plateau value at 8 × 10-5 M concentration, the length of polydisulfide should not be extensive, however. At the moment, it is difficult to assess the number of disulfide bond formed on the gold sol surfaces. The 1,2EDT species remaining in the aqueous gold sol medium was hardly determined using 1H NMR spectroscopy due to its low resolution limit; in a near future, we plan to measure the number of hydrogen gas which is presumably produced along with the disulfide linkages. It would also be intriguing that the SER band at 505 cm-1 in Figure 1b, attributed to the S-S stretching vibration, is at least 3 times weaker than the S-S stretching band at 508 cm-1 in the OR spectrum of diethyl disulfide (Figure 1c) when their intensities are normalized with respect to the C-H stretching bands. Although one has to take account of the surface enhancement factors, the low intensity of the S-S stretching band in the SER spectrum of 1,2-EDT may be due to imperfect multilayer coverage on the monolayer. On the basis of the electromagnetic enhancement mechanism,13 the low intensity of the S-S stretching band may also be associated with the long distance of the -SSmoiety from the sol surface and/or the quite parallel alignment of the S-S bond with respect to the sol surface. We have to mention that the SER spectral features of 1,2-EDT on gold are quite similar to those of p-xyleneR,R′-dithiol.12 Nonetheless, the S-S stretching band was much weaker in the SER spectrum of p-xylene-R,R′-dithiol than that of 1,2-EDT. The relatively larger intensity in 1,2-EDT may be associated with the fact that the S-S bond formed by 1,2-EDT will be much closer than that formed by p-xylene-R,R′-dithiol to the sol surface since there are only two methylene units in 1,2-EDT. In addition, the comparatively lower intensity of the S-S stretching

Figure 5. Plausible adsorbate structures of 1,2-EDT on gold (a) at low and (b) high surface coverage limit. For a reference, the structure of 1,2-EDT on silver is drawn in (c).

band for p-xylene-R,R′-dithiol may reflect that the extent of multilayer formation by p-xylene-R,R′-dithiol is far smaller than that by 1,2-EDT at least in the aqueous gold sol medium. The feasibility of the formation of multilayers of 1,2EDT on the gold surface was further examined by measuring the ellipsometric thickness of 1,2-EDT layers on vacuum-evaporated gold substrates. In Figure 4a are shown the ellipsometric thicknesses of 1,2-EDT layers self-assembled on vacuum-evaporated gold substrates in 1 mM 1,2-EDT solution in methanol, ethanol, and nhexane. It is seen that the thickness of the self-assembled layer is dependent on the kind of solvent. After a prolonged immersion (24 h) in 1 mM methanol and ethanol solutions, the thicknesses of the 1,2-EDT layer attained 0.7 and 0.9 nm, respectively, while in a 1 mM n-hexane solution the value was as large as 1.8 nm. Figure 4b displays that the thickness of the 1,2-EDT layer can be adjusted by changing the kind of solvent. Assuming that the monothiolate, 1,2EDT -1, is tilted by 30° from the surface normal as is the case of aliphatic thiolate on Au(111),24 the thickness of a full-covered 1,2-EDT monolayer is estimated to be 0.45 nm, using the known bond lengths, bond angles, van der Waals atomic radii, and the approximate distance between the sulfur atom and the gold surface (0.15 nm).25 Although the present estimate is somewhat rough, it suggests that a one-and-half-layered 1,2-EDT monolayer is formed on the gold substrate in 1 mM methanol solution. In contrast, (24) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152. (25) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321.

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the measured ellipsometric thickness suggests that a tetralayered 1,2-EDT film is presumably formed in 1 mM n-hexane solution while a bilayer is formed in 1 mM ethanol solution. The origin of the solvent dependence is unclear. A separate experiment dictated, however, that the lower thickness values observed in methanol and ethanol had nothing to do with the greater solubility of oxygen in a polar solvent. We have also to mention that the growth rate of multilayers was insensitive to the concentration of 1,2-EDT at least in 1-10 mM ethanolic and hexane solutions. Since the solubility of 1,2-EDT in water was very low, we in fact added ethanol solution of 1,2-EDT to the Au sol solution to obtain the SER spectra. Consulting the ellipsometry measurement, it is then conjectured that at best one single disulfide bond might have formed on the gold sol surface. On these grounds, the plausible structures of 1,2-EDT on gold at both low and high surface coverage limits are depicted in parts a and b of Figure 5, respectively; for comparison, the structure of 1,2-EDT on silver is also drawn in Figure 5c. We have to mention, however, that a direct comparison between the ellipsometric data on planar gold surface and the SERS data on colloidal gold surface may not be appropriate. Nonetheless, it would be informative to recall that the adsorbate structures of alkanethiolates self-assembled on flat gold surfaces differ little from those anchored on gold nanoparticles,26 albeit that the adsorption characteristics of dithiols may depend greatly on the detailed morphology of the metal substrates. (26) Hostetler, M. J.; Stokes, J. J.; Murray, R. W. Langmuir 1996, 12, 3604.

Joo et al.

To elucidate the adsorption behavior of 1,2-EDT on gold more firmly, we are thus currently performing a combined analysis with ellipsometry, quartz crystal microbalance, scanning probe microscopy, FT-IR spectroscopy, and SERS in various solvents such as n-hexane and ethanol. Summary and Conclusion We have clearly demonstrated by observing the distinct S-S stretching band that multilayered films of aliphatic dithiols can be assembled on a gold surface. It is, however, indeterminate from the SER spectral features alone how many layers are assembled on the colloidal gold surface. Nonetheless, a separate ellipsometry measurement revealed that up to tetralayers could be assembled on gold at least in n-hexane solution of 1,2-EDT. To elucidate more firmly the adsorption charactersitics of 1,2-EDT on gold, we are currently performing a combined analysis with ellipsometry, quartz crystal microbalance, scanning probe microscope, FT-IR spectroscopy, and SERS using a vacuum-evaporated gold film as substrate for the selfassembly of 1,2-EDT in various solvents such as n-hexane, methanol, and ethanol. Acknowledgment. This work was supported by the Korea Science and Engineering Foundation (KOSEF) through the Center for Molecular Catalysis at Seoul National University. Authors also acknowledge KOSEF for providing a grant to purchase noble chemicals through the Interdisciplinary Research Program (Grant 1999-2121-001-5). S.W.J. and S.W.H. acknowledge the Korea Research Foundation for providing the BK21 fellowship. LA991331W