Langmuir 1996, 12, 5083-5086
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X-ray Photoelectron Spectroscopy Sulfur 2p Study of Organic Thiol and Disulfide Binding Interactions with Gold Surfaces David G. Castner* National ESCA and Surface Analysis Center for Biomedical Problems, Department of Chemical Engineering, Box 351750, University of Washington, Seattle, Washington 98195-1750
Kenneth Hinds and David W. Grainger Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523-1872 Received May 13, 1996X The presence of two sulfur species was detected in X-ray photoelectron spectroscopy (XPS) studies of thiol and disulfide molecules adsorbed onto gold surfaces. These species are assigned to bound thiolate (S2p3/2 binding energy of 162 eV) and unbound thiol/disulfide (S2p3/2 binding energy from 163.5 to 164 eV). These assignments are consistent with XPS data obtained from different thiols (C12, C16, C18, and C22 alkane thiols, a fluorinated thiol, and a cyclic polysiloxane thiol) and different adsorption conditions (solvent type, thiol concentration, temperature, and rinsing). In particular, the use of a poor solvent for thiol adsorption solutions (e.g., ethanol for long chain alkanethiols) and the lack of a rinsing step both resulted in unbound thiol molecules present at the surface of the bound thiolate monolayer. This has implications for recent studies asserting the presence of multiple binding sites for gold-thiolate species in organic monolayers.
Introduction The structure and surface chemistry of organic thin films is a research area relevant to several interfacial processes, including biological events, lubrication, adhesion, wettability, corrosion, electrochemistry, and microelectronic fabrication. To obtain the optimum performance of a material or device in one of these applications, the organic thin film must be prepared with the right type, concentration, and arrangement of functional groups [e.g., ref 1]. Self assembly of a monolayer of molecules from solution onto a solid surface is an important method for preparing organic thin films with well-defined structures.2 The most widely investigated self-assembled monolayer (SAM) systems use sulfur anchor groups (thiol, disulfide, or thioether) and gold surfaces.2-27 Thus, it is important to probe the gold-sulfur bonding mechanism. One technique that provides information about sulfur interactions with gold surfaces is photoelectron spectro* Address correspondence to this author. Phone: 206-543-8094. Fax: 206-543-3778. E-mail:
[email protected]. X Abstract published in Advance ACS Abstracts, September 15, 1996. (1) Ratner, B. D. Surf. Interface Anal. 1995, 23, 521. (2) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (3) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Chem. 1992, 43, 437. (4) Konstadinidis, K.; Zhang, P.; Opila, R. L.; Allara, D. L. Surf. Sci. 1995, 338, 300. (5) Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. J. Chem. Phys. 1993, 98, 678. (6) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988, 4, 365. (7) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558. (8) Bertilsson, L.; Liedberg, B. Langmuir 1993, 9, 141. (9) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. Soc. 1987, 109, 733. (10) 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. (11) Bain, C. D.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1989, 5, 723. (12) Fabianowski, W.; Coyle, L. C.; Weber, B. A.; Granata, R. D.; Castner, D. G.; Sadownik, A.; Regen, S. L. Langmuir 1989, 5, 35. (13) Sun, F.; Castner, D. G.; Grainger, D. W. Langmuir 1993, 9, 3200.
S0743-7463(96)00465-9 CCC: $12.00
scopy, using either laboratory X-ray sources (XPS) or soft X-ray synchrotron sources (SXPS). By monitoring the S2p3/2 binding energy (BE) of the self-assembling molecules containing thiol, disulfide, or thioether anchors evidence for bond formation between the sulfur atom and the gold surface can be obtained. Typical S2p3/2 BEs for unbound alkanethiols and dialkyl disulfides are between 163 and 164 eV.9-17 After self assembly of these molecules onto the gold surface the S2p3/2 BE decreases to 162 eV.9-21 This BE shift has generally been interpreted as the formation of a gold-thiolate bond.3 On the basis of the S2p3/2 BE, the gold-thiolate bonds formed from thiol and disulfide molecules are indistinguishable. Although definitive proof of the sulfur bonding site on the Au(111) surface does not exist, it seems most likely to be the threefold Au hollow.5 This is consistent with studies of methanethiolate species chemisorbed onto W, Ru, Ni, Cu, and Ag single-crystal surfaces.28-33 On a few surfaces thiolate species were (14) Sun, F.; Grainger, D. W.; Castner, D. G. J. Vac. Sci. Technol., A 1994, 12, 2499. (15) Sun, F.; Castner, D. G.; Mao, G.; McKeown, P.; Grainger, D. W. J. Am. Chem. Soc. 1996, 118, 1856. (16) Wang, W.; Castner, D. G.; Grainger, D. W. Supramol. Sci., in press. (17) Sun, F.; Grainger, D. W.; Castner, D. G.; Leach-Scampavia, D. K. Macromolecules 1994, 27, 3053. (18) Lenk, T. J.; Hallmark, V. M.; Hoffmann, C. L.; Rabolt, J. F.; Castner, D. G.; Erdelen, C.; Ringsdorf, H. Langmuir 1994, 10, 4610. (19) Evans, S. D.; Goppert-Beraducci, K. E.; Urankar, E.; Gerenser, L. J.; Ulman, A.; Snyder, R. G. Langmuir 1991, 7, 2700. (20) Hutt, D. A.; Leggett, G. J. J. Chem. Phys. 1996, 100, 6657. (21) Walczak, M. M.; Alves, C. A.; Lamp, B. D.; Porter, M. D. J. Electroanal. Chem. 1995, 396, 103. (22) Whitesides, G. M.; Ferguson, G. S.; Allara, D. L.; Scherson, D.; Speaker, L.; Ulman, A. Crit. Rev. Surf. Chem. 1993, 3, 49. (23) Fenter, P.; Eberhardt, A.; Eisenberger, P. Science 1994, 266, 1216. (24) Yeganeh, M. S.; Dougal, S. M.; Polizzoti, R. S.; Rabinowitz, P. Phys. Rev. Lett. 1995, 74, 1811. (25) Zubragel, Ch.; Deuper, C.; Scheider, F.; Neumann, M.; Grunze, M.; Schertel, A.; Woll, Ch. Chem. Phys. Lett. 1995, 238, 308. (26) Graham, R. L.; Bain, C. D.; Biebuyck, H. A.; Laibinis, P. E.; Whitesides, G. M. J. Chem. Phys. 1993, 97, 9456. (27) Laibinis, P. E.; Bain, C. D.; Whitesides, G. M. J. Chem. Phys. 1991, 95, 7017.
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detected in lower coordination sites than the hollow sites (e.g., bridge sites).28,29 In all cases where the S2p3/2 BE was measured it was observed to decrease in the order unbound thiol or disulfide > thiolate in hollow site > thiolate in low-coordination site.28,29 Recently it has been proposed, on the basis of grazing incidence X-ray diffraction (GIXD)23 and sum-frequency generation (SFG)24 studies, that two different sulfur species are present in alkanethiol SAMs. The bonding sites proposed for these two sulfur atoms were threefold hollow and bridge positions. A recent photoemission study has been interpreted to support this proposal.25 However, two problems exist with the Zubragel et al.25 photoemission study. First the S2p3/2 BEs are not consistent with the sulfur environments proposed by the GIXD results. Second, the solubility of CH3(CH2)21SH in ethanol, the solvent used by Zubragel et al. to prepare their SAMs, is limited.10 On the basis of results previously reported in the literature, it appears the two sulfur species detected by Zubragel et al. are bound and unbound C22 thiol. Further evidence for this assignment is given in the Results and Discussion. Experimental Methods Preparation of the Self-Assembled Monolayers. A ∼2000 Å layer of gold (99.999%) was thermally evaporated onto silicon wafers precoated with a ∼200 Å adhesive underlayer of Cr. The freshly prepared gold substrates were then directly immersed into 1-28 mM solutions of alkanethiols {CH3(CH2)n-1SH designated CnSH, where n ) 12, 16, 18, or 22} for 16 or 48 h at room temperature or 36 °C. C12SH, C16SH, and C18SH were purchased commercially from Aldrich, and C22SH was synthesized. The C22SH synthesis followed that published previously for analogous alkanethiols and disulfides.15 The samples were then stored in sealed vials under nitrogen until they were placed into the XPS system, typically within 30 h of removing the samples from solution. Monolayers were also prepared using poly[methyl(mercaptopropyl)siloxane] oligomers (PMMPS)17 and CF3(CF2)7C(O)N(H)(CH2)2SH (F8 thiol).18 X-ray Photoelectron Spectroscopy. The XPS experiments were done on Surface Science Instruments X-probe and M-probe spectrometers using monochromatic Al KR X-ray sources (hν ) 1486.6 eV). The BE scales for the monolayers on gold were referenced by setting the Au4f7/2 BE to 84.0 eV. The highresolution S2p and C1s spectra were acquired with an analyzer pass energy of 50 eV. All XPS data were acquired at a nominal photoelectron takeoff angle of 55°, where the takeoff angle is defined as the angle between the surface normal and the axis of the analyzer lens. Further details of the XPS analysis procedures are given elsewhere.13-17 Three to five spots on two or more replicates of each SAM were examined. The compositional data are averages of the values determined at each analysis spot. The S2p high-resolution spectra are the sum of the scans taken at all spots. The summing of S2p spectra was done to increase the spectral signal-to-noise ratio while minimizing the time the samples were exposed to the X-ray source. This should minimize the possibility of sample degradation by the incident X-rays or exiting photoelectrons.26 C22SH is a solid at room temperature and was pressed onto double-sided tape and inserted into the XPS system for analysis. The bulk C22SH powder sample required the use of a low-energy (28) Mullins, D. R.; Lyman, P. F. J. Phys. Chem. 1993, 97, 9226 and 12008. (29) Rufael, T. S.; Huntley, D. R.; Mullins, D. R.; Gland, J. L. J. Phys. Chem. 1995, 99, 11480. (30) Fernandez, A.; Espinos, J. P.; Gonzalez-Elipe, A.R.; Kerkar, M.; Thompson, P. B. J.; Ludecke, J.; Scragg, G.; de Carvalho, A. V.; Woodruff, D. P.; Fernandez-Garcia, M.; Conesa, J. C. J. Phys.: Condens. Matter 1995, 7, 7781. (31) Bao, S.; McConville, C. F.; Woodruff, D. P. Surf. Sci. 1987, 187, 133. (32) Prince, N. P.; Seymour, D. L.; Woodruff, D. P.; Jones, R. G.; Walter, W. Surf. Sci. 1989, 215, 566. (33) Harris, A. L.; Rothberg, L.; Dhar, L.; Levinos, N. J.; Dubois, L. H. J. Chem. Phys. 1991, 94, 2438.
Figure 1. XPS S2p spectra for C16SH, F8 thiol, and PMMPS films adsorbed onto gold surfaces. The C16SH and F8 thiol peaks were fit using one S2p doublet with a 2:1 area ratio and a splitting of 1.2 eV. The PMMPS peaks were fit using two S2p doublets with 2:1 area ratios and splittings of 1.2 eV. The position of the S2p3/2 peaks assigned to bound thiolate and unbound thiol species are shown. electron flood source for charge compensation. The BE scale for this sample was referenced by setting the C1s BE to 285.0 eV, the value observed for the C1s BE from the C22SH film on gold.
Results and Discussion The XPS elemental compositions for alkanethiol SAMs rinsed in solvent after assembly exhibited an exponential increase in the carbon signal and an exponential decrease in both the gold and sulfur signals with increasing length of the alkyl chains, in agreement with previous studies.20,27 For the unrinsed C18SH and C22SH SAMs, the gold and carbon concentrations differed significantly from the corresponding rinsed concentrations. The direction of these changes was consistent with additional, unbound thiols present at the surface of the bound alkanethiolate monolayer. These unbound thiol molecules could be either lying on top of the SAM or partially penetrating into the SAM. Oxygen was also detected by XPS on all the SAMs. Typically, the oxygen concentration was close to the detection limits of our XPS instruments (ca. 0.3 atom %). No oxidized sulfur species (S2p BE > 166 eV) such as sulfonate were detected by XPS on any of the samples. For the SAMs prepared from C12SH, C16SH, and C18SH solutions and then rinsed in pure solvent, only one sulfur species was detected by XPS. The S2p spectra acquired for these SAMs had a doublet structure due to the presence of the S2p3/2 and S2p1/2 peaks. All spectra could be fit using a 2:1 peak area ratio and a 1.2 eV splitting, as shown for the C16SH SAM in Figure 1a. The
Thiol and Disulfide Molecules on Gold Surfaces
Figure 2. XPS S2p spectra of solid C22SH and C22SH molecules adsorbed onto gold and rinsed with ethanol. To properly fit the experimental C22SH SAM spectrum two S2p doublets with 2:1 area ratios and splittings of 1.2 eV were required. These two sulfur species are assigned to bound alkanethiolate and unbound alkanethiol molecules.
BE of the S2p3/2 peak was 161.9 eV, consistent with the sulfur atoms bound to the gold surface as a thiolate species. This is in agreement with previous results reported by several research groups.9-21 In particular, no detectable intensity is present in the BE region above 164 eV for any of these three SAMs. This means that unbound thiol molecules (S2p1/2 BE of ca. 165 eV) are not detected by XPS. This observation is not limited to SAMs formed via adsorption of alkanethiols. Fluorinated thiols, as reported previously,18 also have XPS S2p spectra that are consistent with the presence of only bound thiolate species. The S2p spectrum of F8 thiol fit with a doublet having a 2:1 area ratio, and a 1.2 eV splitting is shown in Figure 1b. Again the S2p/3/2 BE of 161.9 eV is representative of bound thiolate. The results for rinsed C22SH SAMs depended on the solvent used to prepare and rinse the monolayers. For isooctane, a good solvent for C22SH, similar results to those for the C12SH to C18SH SAMs rinsed in ethanol were obtained. The predominant, if not only, sulfur species detected was bound thiolate for C22SH SAMs prepared and rinsed with isooctane. Likewise C22SH SAMs prepared and rinsed with toluene and chloroform solvents only exhibited bound thiolate species. For ethanol, which is a poor solvent for C22SH, the S2p spectrum obtained could no longer be fit with a single doublet having a 2:1 area ratio and a 1.2 eV splitting. The additional intensity above 163 eV in this spectrum requires two doublets (each with area ratios of 2:1 and splittings of 1.2 eV) to properly fit the data (see Figure 2). A recent study by Zubragel et al.25 has also reported the presence of two sulfur species for a C22SH monolayer assembled from ethanol, which they attributed to the presence of two bound thiolate species. They assigned the sulfur species with a S2p3/2 BE of 161.8 eV to a bound thiol in the threefold hollow and the second, higher BE sulfur species (163.1 eV) to a bound thiolate in a lower coordination site. There are two problems with this assignment. First, for surfaces where detailed studies of the S2p BE have been done for thiolates and sulfur atoms in multiple binding sites, the lower coordination sites all have lower S2p BEs than the hollow site.28,29 Second, the observed BE of the second sulfur species is identical to the BEs detected for solid C22SH (Figure 2) and thiol groups that are not bound to the gold surface (e.g., PMMPS, Figure 1c). Additional supporting evidence for the assignment of unbound thiol species in PMMPS monolayers has been reported previously.17 It includes angle-dependent XPS observations and the ability to derivatize the PMMPS monolayers by the in situ attachment of hydrocarbon, fluorocarbon, and ether chains to the unbound thiols in PMMPS monolayers. Previous studies
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Figure 3. XPS S2p spectrum of unrinsed C18SH adsorbed onto gold from a 28 mM chloroform solution. Two S2p doublets with 2:1 area ratios and splittings of 1.2 eV were used to peak fit the experimental spectrum. The signal from unbound C18SH molecules was significantly larger than the signal from the bound C18 thiolate layer.
using polymer monolayers anchored to gold surfaces with alkyl disulfide side chains have also shown S2p3/2 BEs of ca. 164 eV for the unbound disulfides, close to the 163.5 eV BE observed for the unbound thiol species in Figures 1 and 2.13-16 Thus, it appears the most likely assignment for the high BE sulfur species from the C22SH SAM prepared and rinsed in ethanol is unbound thiol that is located at the surface of the first monolayer. Since the C22SH SAM analyzed by Zubragel et al.25 was prepared with ethanol, it is likely the second sulfur species they assign to bound thiolate in a low coordination site is actually unbound thiol. Their measured BE for this species is consistent with those shown in Figures 1 and 2 for unbound thiol. To further confirm that the sulfur species with a S2p3/2 BE near 163.5 eV is unbound thiol, samples prepared without the final solvent rinsing step were examined. This should enhance the amount of unbound thiol remaining on top of the SAM. Two solutions were compared: 3 mM C22SH in ethanol and 28 mM C18SH in chloroform. The elemental compositions and S2p spectra for both samples indicated significant quantities of unbound thiol present in the unrinsed samples. The S2p spectrum from the unrinsed C18SH film is shown in Figure 3 with a peak fit using two doublets having area ratios of 2:1 and splittings of 1.2 eV. For this sample the amount of unbound thiol detected is significantly higher than the amount of bound thiolate. This is probably due to the high thiol solution concentration (28 mM) used and the location of the unbound thiol (above the bound thiolate). The S2p spectrum for the unrinsed C22SH sample also showed the presence of unbound thiol and bound thiolate species. Thus, from the evidence presented here and previous results from the literature it is apparent that the most consistent interpretation of S2p BEs observed from films deposited from thiol solutions onto gold surfaces is a bound thiolate species with a S2p3/2 BE of ca. 162 eV and an unbound thiol species with a S2p3/2 BE of ca. 163.5 eV. This interpretation is not consistent with the recent X-ray scattering results of Fenter et al.,23 which indicated the presence of bound thiolate species in hollow and bridge sites in alkanethiol SAMS on gold surfaces. At present we do not have a good explanation for this difference. One possibility is the existence of a model structure with only one bound thiolate species that was not considered in the previous X-ray diffraction studies yet still consistent with the diffraction data. However, the previous X-ray diffraction study searched a wide range of models, so this is unlikely. Another possibility is that two bound thiolates could be present in alkyl SAMS, but XPS cannot detect the difference in BEs between the two binding sites of
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these thiolates. On the basis of previous XPS results for thiolates and sulfur atoms on other metal surfaces,28,29 this also does not seem likely. However, a recent XPS study by Walczak et al.21 suggests the difference in S2p3/2 BEs for alkanethiolates bound to terrace versus step sites on a gold surface is only 0.2 eV, so for gold surfaces the difference in XPS S2p3/2 BEs of thiolates bound to different sites may be smaller than that observed on other metal surfaces. Also, the recent SFG study provides another explanation, that the two different sites for bound thiolate on gold could be two different hollow sites instead of a bridge and hollow site.24 On a fcc(111) surface one set of hollow sites has a second layer atom directly below it while the other set of hollow sites does not. The XPS S2p3/2 BEs for thiolates bound to the two types of hollow sites may not be distinguishable, but GIXD and SFG can readily distinguish between these two hollow sites. In any event, it would be desirable to do additional high-resolution SXPS experiments for sulfur species on gold surfaces to explore in more detail the effect of binding site on the S2p3/2 BE. Conclusions The results of this study along with previous results from the literature indicate only one bound thiolate species
Castner et al.
is detectable by XPS after adsorption of thiol or disulfide molecules onto gold surfaces. The S2p3/2 BE of this bound thiolate species is 162 eV. The presence of a second sulfur species is also detected by XPS for some adsorbed thiol and disulfide molecules under certain conditions. XPS experiments on solid C22SH, variation of the sulfur signal with the photoelectron takeoff angle, unrinsed films, and functionalization of the absorbed films are all consistent with the assignment of this second species to an unbound thiol or disulfide within or on top of the thiolate adlayer. The observed S2p3/2 BEs of the unbound thiols and disulfides are between 163.5 and 164 eV. The results of this study emphasize the importance of selecting a proper solvent for the adsorption and rinsing of SAMs on gold surfaces. Acknowledgment. Support is gratefully acknowledged from a joint National Science Foundation/EPRI grant (NSF MSS-9212496 and EPRI RP-8017, D.W.G.), a National Science Foundation grant (DMR-9357439, D.W.G.), and a National Institutes of Health grant (RR01296, D.G.C.). LA960465W
