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Langmuir 1999, 15, 2414-2419
Formation of Alkanethiolate Self-Assembled Monolayers on Oxidized Gold Surfaces C. Yan, A. Go¨lzha¨user,* and M. Grunze Angewandte Physikalische Chemie, Universita¨ t Heidelberg, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany
Ch. Wo¨ll Physikalische Chemie I, Ruhr-Universita¨ t Bochum,Universita¨ tsstrasse 150, 44801 Bochum, Germany Received August 18, 1998. In Final Form: January 12, 1999 The formation of self-assembled alkanethiolate monolayers (SAMs) on oxidized gold was studied by X-ray photoelectron spectroscopy (XPS), contact angle measurements, and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. Gold was oxidized by exposure to atomic oxygen in a vacuum. SAMs were prepared by two methods: (a) by immersion of the oxidized substrate into ethanolic solution and (b) via chemical vapor deposition (CVD). After both procedures two sulfur species could be distinguished by XPS, a gold thiolate and a species related to the reaction of the thiol with oxidized gold. Oxygen was found encapsulated under the self-assembled monolayer. The monolayers adsorbed from ethanolic solution were more densely packed and exhibited a smaller molecular tilt than the layers formed by CVD or SAMs prepared on clean gold surfaces.
1. Introduction Self-assembly of a molecular monolayer onto a solid surface is an important method for preparing organic thin films with well-defined structures. These stable, closely packed monolayers on solid substrates can be used for a variety of applications, such as lubrication, adhesion promotion, corrosion inhibition, and microelectronic fabrication. Particular attention has been given to selfassembled monolayers (SAMs) formed by the adsorption of alkanethiols on gold surfaces,1-8 since the chemical inertness of the substrate and its strong interaction with sulfur allow an easy preparation of well-defined SAMs. The atomic nearest neighbor spacings on Ag(111) and Au(111) surfaces are 2.89 and 2.88 Å, respectively. Considering these almost identical lattice constants, one may expect similar thiol chain structures on the closepacked surfaces of these metals. But Fourier transform infrared (FTIR) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy results demonstrate that the alkane chains are tilted ∼30° away from the surface normal on Au(111),9-11 whereas on Ag(111) the chains are less tilted (∼12°) and more densely packed.12 * Corresponding author. Electronic mail: armin.goelzhaeuser@ urz.uni-heidelberg.de. Phone: +49 6221 544921.Fax: +49 6221 546199. (1) Laibinis, P. E. J. Am. Chem. Soc. 1992, 114, 1990. (2) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (3) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7164. (4) Bain, C. D.; Evall, J.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7155. (5) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558. (6) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1995, 43, 437. (7) Walczak, M. M. J. Am. Chem. Soc. 1991, 113, 2370. (8) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3359. (9) Ha¨hner, G.; Kinzler, M.; Thu¨mmler, C.; Wo¨ll, Ch.; Grunze, M. J. Vac. Sci. Technol., A 1992, 10 (4), 2754. (10) Kinzler, M.; Schertel, A.; Ha¨hner, G.; Wo¨ll, Ch.; Grunze, M.; Albrecht, H.; Holzhu¨ter, G.; Gerber, Th. J. Chem. Phys. 1994, 100 (10), 7722. (11) Bierbaum, K.; Kinzler, M.; Wo¨ll, Ch.; Grunze, M.; Ha¨hner, G.; Heid, S.; Effenberger, F. Langmuir 1995, 11, 513. (12) Laibinis, P. E. J. Am. Chem. Soc. 1991, 113, 7152.
Ulman13 attributes this structural difference to a larger surface corrugation potential of the (111) lattice in the case of gold, which results in a more site selective adsorption. However, it has also to be considered that silver is much more reactive than gold and forms an oxide under ambient conditions. As discussed in a previous paper,14 the surface oxide on silver is reduced and has an autocatalytic effect on the self-assembly process. Thus an investigation of the adsorption of alkanethiols on oxidized gold surfaces can be helpful in understanding the differences and similarities in the self-assembly process on Ag and Au and their surface oxides. Various methods have been applied to oxidize gold. King15 produced a thin layer of Au2O3 by exposing gold to UV/ozone in air at 25 °C. This layer was reported to be stable in UHV and against water and ethanol rinses. Krozer and Rodahl16 performed oxidation of Au(111) by UV/ozone in UHV and reported the simultaneous formation of a chemisorbed oxygen species and Au2O3. Bigelow17 used dc air corona discharge-induced modification of gold foil to produce Au2O3. Ron et al. prepared Au2O3 by exposure to an oxygen plasma at 10-1 Torr18 and by UV/ ozone treatment under atmospheric conditions.19 It is well-known that atomic oxygen is much more reactive toward metallic surfaces than molecular oxygen. Canning et al.20 produced atomic oxygen through molecular dissociation of O2 over a hot metallic filament in UHV. Pireaux et al.21 oxidized an Au(111) crystal by dc reactive sputtering in UHV and produced Au2O3. A different (13) Ulman, A. Chem. Rev. 1996, 96, 1533. (14) Himmelhaus, M.; Gauss, I.; Buck, M.; Eisert, F.; Wo¨ll, Ch.; Grunze, M. J. Electron. Spectrosc. Relat. Phenom. 1998, 92, 139. (15) King, D. E. J. Vac. Sci. Technol., B 1995, 13 (3), 1247. (16) Krozer, A.; Rodahl, M. J. Vac. Sci. Technol., A 1997 15 (3), 1704. (17) Bigelow, R. W. Appl. Surf. Sci. 1988, 32, 122. (18) Ron, H.; Rubinstein, I. Langmuir 1994, 10, 4566. (19) Ron, H.; Matlis, S.; Rubinstein, I. Langmuir 1998, 14, 1116. (20) Canning, N. D. S.; Outka, D.; Madix, R. J. Surf. Sci. 1984, 141, 240. (21) Pireaux, J. J.; Liehr, M.; Thiry, P. A.; Delrue, J. P.; Caudano, R. Surf. Sci. 1984, 141, 221.
10.1021/la981054d CCC: $18.00 © 1999 American Chemical Society Published on Web 02/27/1999
Formation of Alkanethiolate SAMs
approach was performed by Zeppenfeld and co-workers: 22-24 They exposed an Au(111) single crystal at high temperatures (∼800 °C) to a high oxygen pressure (∼1 bar). In a series of STM studies, they observed a restructuring of the Au(111) surface after oxygen chemisorption. A (x3×x3)R30° structure and a long-range hexagonal superstructure with a periodicity varying between 60 and 80 Å was reported. So far no general agreement on the formation of selfassembled monolayers on oxidized gold has been reached. Nuzzo and Allara25 could not detect any adsorption of disulfides on oxygen-plasma-pretreated gold. Similar observations were reported by Bain et al.2 They suspect that the formation of metastable surface gold oxide inhibits the assembly of good SAMs on oxygen-plasma-cleaned gold. On the other hand, Hickman et al. reported a successful formation of disulfide monolayers on oxygenplasma-treated gold surfaces.26 Ron and Rubinstein18 also produced oriented dodecanethiolate monolayers on plasmapreoxidized gold surfaces and found an encapsulated oxygen layer under the thiol film. In this paper, we describe the characterization of SAMs formed by the chemisorption of alkanethiols (HS(CH2)nCH3) on a gold surface that was oxidized by exposure to atomic oxygen, produced by dissociation of O2 over a heated Pt filament in UHV. The alkanethiolate self-assembled monolayers were prepared through adsorption from ethanol solution and via chemical vapor deposition.
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Figure 1. O 1s and Au 4f X-ray photoelectron spectra of oxidized gold: (a) clean gold surface and (b) after 5 min, (c) 15 min, and (d) 20 min of exposure to atomic oxygen (generated by dissociation of O2 over a hot Pt filament) in UHV. The arrows show the small oxide shoulders of the Au 4f lines.
2. Experimental Section 2.1. Substrates. Gold films (1000 Å), evaporated on polished silicon wafers, were used as substrates. A 10 Å layer of titanium was deposited on the clean wafer prior to the gold evaporation in order to improve the adhesion. All substrates were cut into 10 mm × 15 mm pieces. Polycrystalline gold sheets (99.99%) as well as an Au(111) single crystal were used as reference substrates. 2.2. Chemicals. Octadecanethiol (MC-18, GC 98%) was obtained from Aldrich. 1-Decanethiol (MC-10, >95%) and heptanethiol (MC-7) were obtained from Fluka. Absolute ethanol (>99.8%) was obtained from Riedel-deHaen. All the chemicals were used as received. 2.3. Oxidation with Atomic Oxygen in UHV. A hot platinum filament (0.125 mm diameter, 99.99% purity, 3.0 W power consumption) was used to dissociate research grade oxygen in UHV. The distance between the filament and the sample surface was adjustable. Heating of the Pt filament up to 1100 °C did not result in the deposition of platinum onto the sample. Prior to oxidation, the gold surfaces were extensively sputtered with Ar+ ions until no carbon and oxygen contaminations were detected by XPS. Subsequently 2 × 10-6 mbar of oxygen was admitted through a leak valve while the hot Pt filament (T ) 1100 °C) was positioned in front of the gold surface at a distance of ∼4 mm. After this procedure was applied for 15 min, the chemical composition of the substrate surface was analyzed by XPS. 2.4. Preparation of Self-Assembled Thiolate Monolayers from Solution. The samples were immersed into a 1 mM alkanethiol solution for 22-24 h at room temperature. After removal from the thiol solution, the samples were rinsed with absolute ethanol, blown dry with pure N2, and immediately transferred into UHV for analysis. 2.5. Thiol Adsorption via CVD. MC-18 was evaporated from a Knudsen cell equipped with a heating wire and a thermocouple. (22) Huang, L.; Chevrier, J.; Zeppenfeld, P.; Cosma, G. Appl. Phys. Lett. 1995, 66 (8), 935. (23) Huang, L.; Zeppenfeld, P.; Chevrier, J.; Comsa, G. Surf. Sci. 1996, 352-354, 285. (24) Chevrier, J.; Huang, L.; Zeppenfeld, P.; Comsa, G. Surf. Sci. 1996, 355, 1. (25) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (26) Hickman, J. J.; Laibinis, P. E.; Auerbach, D. I.; Zou, C.; Gardner, T. J.; Whitesides, G. M.; Wrighton, M. S. Langmuir 1992, 8, 357.
Figure 2. O 1s and Au 4f XP spectra before and after adsorption of MC-18 self-assembled monolayers on oxidized gold. (a) clean gold, (b) oxidized gold, (c) after chemical vapor deposition of MC-18, (d) after adsorption of MC-18 from ethanol solution. The cell was separated from the preparation chamber by a valve. A manipulator allowed us to position the Knudsen cell (100 µm aperture) at a distance of ∼10 mm from the substrate. To evaporate octadecanethiol, a temperature of 75-80 °C was applied. With the sample at room temperature, 50-60 s of exposure was sufficient to adsorb a thiol monolayer. 2.6. X-ray Photoelectron Spectroscopy. XP spectra were acquired using a Leybold Heraeus LH 12 spectrometer, equipped with non-monochromatic Mg KR (1253.6 eV) and Al KR (1486.6 eV) sources. The spectra were recorded at normal emission with an experimental resolution of 1.0 eV. The typical operating pressure was 20 h) air exposures. To compare these findings to those of the more commonly used plasma oxidation, we have also exposed gold substrates to an ozone plasma in air. Compared to the case for the oxide investigated in this paper, the O 1s binding energy of the oxide generated by the plasma is ∼1 eV higher, and more carbon contaminants are found on the surface. Immersion of the plasma-treated gold into an ethanolic solution of thiols leads to the formation of SAMs which were virtually indistinguishable from films adsorbed on bare gold substrates.31 After the adsorption of alkanethiols on the oxide prepared by exposure to atomic oxygen, changes in the O 1s and Au 4f XP signals reveal a change of the nature of the oxygen layer on the gold. Two sulfur species are found; one of them can be unambiguously assigned to gold thiolate. The second species is not due to radiation damage, as we have confirmed in a control experiment. This species does not originate from contamination, as has been shown by the CVD preparation where solvent and rinsing effects can be ruled out. The intensity of the second species increases with the amount of oxygen present on the surface, after adsorption from solution as well as after (30) Gmelin Handbook of Inorganic and Organometallic Chemistry, 8th ed.; Gold Supplement Volume B1; Springer-Verlag: New York, 1992. (31) Yan, C.; Go¨lzha¨user, A.; Grunze, M.; Wo¨ll, Ch. In preparation.
Yan et al.
CVD (cf. Tables 1 and 2). Hence, this second species must rise from thiol molecules that react with the gold oxide. Although a complete determination of the film’s stoichiometry is difficult because the spatial distribution of the atoms within the film is not known, we can assume that the atoms of the second sulfur species are found in the neighborhood of the oxygen atoms. Thus, we can use the XPS intensities32 to determine the stoichiometric ratio between the second species and the remaining oxygen to be ∼0.4 ((0.1). Parallel to the development of the second sulfur species, we also find an increase of both the C 1s and S 2p intensities with the amount of oxide present on the surface (cf. Tables 1 and 2). The C 1s signal did not show any sign of an interaction with oxygen, and the contact angle still indicated a hydrophobic surface. We thus conclude that the remaining oxygen is buried under the thiol layer. This result is similar to the findings of Ron and Rubinstein,18 who used electrochemical stripping to find oxygen encapsulated under a dodecanethiolate monolayer. It is well-known that upon adsorption from ethanolic solution the thiols can replace weakly adsorbed substrate contaminations.33 In the above experiment however, oxygen and two sulfur species could still be detected on the surface after thiol adsorption. This is of particular interest, as the oxide without adsorbed thiol is removed from the gold surface by ethanol. The amount of encapsulated oxygen detected after thiol adsorption from ethanolic solution suggests that the thin organic film prevents the partial dissolution of the oxide layer observed in pure ethanol. Hence the formation of the thiol film must occur faster than this process. The measured water contact angles of 120° and 115° indicate densely packed monolayers12 on the oxidized gold. The intensity variation of the NEXAFS spectra also shows that the alkyl chains are less tilted (R ) 18° after adsorption from solution, R ) 28° after CVD) than on clean gold (R ) 35°). The surface oxygen thus increases the alkanethiolate saturation coverage, resulting in a denser packing of the monolayer. The combination of the small alkyl chain tilt angle and the buried oxygen is responsible for the large values of the overlayer thickness listed in Tables 1 and 2. On the basis of the available XPS data, the thickness of the thiol film and the remaining oxygen cannot be unambiguously determined.34 It is however consistent with the XPS and the NEXAFS data that an alkanethiol monolayer with a molecular tilt of 18° and, hence, a thickness of the thiol film of ∼27 Å covers the buried oxygen. Microscopic changes of the gold surface after oxygen treatment have been investigated by Huang et al.22,23 with STM. Their data show an oxygen-induced restructuring of the gold surface. Although their oxidization procedure differs from ours, it is plausible that a restructuring is also present on our oxidized surface. Such a complex surface topography can cause the adsorption of alkanethiols on different sites. However, the accurate relationship between the oxide surface structure and thiol adsorption geometry has to be investigated in future work. 5. Conclusion We can summarize that the self-assembly process of alkanethiols is strongly influenced by the presence of oxide (32) Corrected for the photoelectron’s excitation probability. See: Scofield, J. H. J. Electron Spectrosc. Relat. Phenom. 1976, 8, 129. (33) Buck, M.; Eisert, F.; Fischer, J.; Grunze, M.; Tra¨ger, F. Appl. Phys. 1991, A53, 552. (34) An estimation of the thiol thickness based on the attenuation of the O 1s signal from the buried oxygen is not reliable because some oxygen may dissolve before thiol adsorption and thus the unattenuated O 1s intensity is not known.
Formation of Alkanethiolate SAMs
on the gold surface. On gold oxide, formed by exposure to atomic oxygen in UHV, self-assembled monolayers form from ethanolic thiol solution as well as via chemical vapor deposition. Two different sulfur species could be distinguished on these surfaces by XPS. They were assigned to gold thiolate and a species related to the reaction of the thiol’s sulfur headgroup with the oxide. Encapsulated oxygen was found under the self-assembled monolayer, and more thiols adsorb on the gold oxide than on a clean gold surface. The thiol layers adsorbed from solution are more densely packed and reveal a smaller molecular tilt
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(18°) than the vapor-deposited monolayers (28°) and SAMs on clean gold (35°). Acknowledgment. We are grateful to the Fond der Chemischen Industrie for general financial support. The authors also thank M. Zharnikov for stimulating discussions and the German Ministry of Education and Research (BMBF) for supporting part of this work by Grant no. 05 644VHA9. LA981054D
