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Langmuir 1997, 13, 4638-4643
Surface-Conditioning Effect of Gold Substrates on Octadecanethiol Self-Assembled Monolayer Growth Takao Ishida,* Satoshi Tsuneda,† Naoki Nishida, Masahiko Hara, Hiroyuki Sasabe, and Wolfgang Knoll‡ Frontier Research Program, The Institute of Physical and Chemical Research (RIKEN), Wako, Saitama 351-01, Japan Received March 4, 1997. In Final Form: May 2, 1997X The surface-conditioning effect of gold substrates on n-octadecanethiol (ODT) self-assembled monolayer (SAM) growth was studied by X-ray photoelectron spectroscopy (XPS). The gold substrates were immersed in an ethanol solution containing 1 mM ODT for 1 h. The C/Au ratios of the ODT SAMs estimated by XPS formed on gold substrates depended on the amount of contamination on the gold surface. A full-coverage ODT SAM was successfully obtained on the gold substrate after ultraviolet light (UV)/ozone treatment for 1 h and rinsing with pure ethanol for 1 h. XPS measurements indicated that low contamination levels and a sulfur-free gold surface were obtained using this pretreatment and, hence, the adsorption of ODT molecules on gold proceeded efficiently. On the other hand, the C/Au ratio of an ODT SAM on a contaminated gold substrate was less following immersion in an ODT solution for 1 h, because surface contamination may reduce the adsorption rate of ODT molecules on the gold surface. When the gold substrate was rinsed with a chloroform solution for 1 h before SAM formation, the lowest C/Au ratio was obtained. XPS results showed that sulfur atoms quickly adsorbed onto the gold surface after chloroform rinsing and prevented the adsorption of ODT molecules onto the gold surface.
I. Introduction Self-assembled monolayers (SAMs) are attractive because a monolayer film is formed spontaneously by immersion of an appropriate substrate into a solution containing organic molecules such as alkanethiol (CH3(CH2)nSH). SAMs are important from the viewpoint of basic surface science1-19 as well as from the viewpoint * Author to whom correspondence should be addressed. Present address: Joint Research Center for Atom Technology, Angstrom Technology Partnership, 1-1-4 Higashi, Tsukuba, Ibaraki 305, Japan. E-mail:
[email protected]. Telephone: +81-298-54-2633. FAX: +81-298-54-2576. † Present address: Department of Chemical Engineering, School of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169, Japan. ‡ Also at Max-Planck-Institute for Polymer Research, Ackermannweg 10, D-55021 Mainz, Germany. X Abstract published in Advance ACS Abstracts, August 1, 1997. (1) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (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. Bain, C. D.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1989, 28, 506. (3) Bain, C. D.; Evall, J.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7155. Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7164. (4) Laibinis, P. E.; Nuzzo, R. G.; Whitesides, G. M. J. Phys. Chem. 1992, 96, 5097. (5) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M. Langmuir 1992, 8, 1330. (6) Bain, C. D.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1989, 5, 723. Biebuyck, H. A.; Whitesides, G. M. Langmuir 1993, 9, 1766. Biebuyck, H. A.; Bain, C. D.; Whitesides, G. M. Langmuir 1994, 10, 1825. (7) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M.; Deutch, J. J. Phys. Chem. 1994, 98, 563. (8) Ron, H.; Rubinstein, I. Langmuir 1994, 10, 4566. (9) Offord, D. A.; John, C. M.; Griffin, J. H. Langmuir 1994, 10, 761. Offord, D. A.; John, C. M.; Linford, M. R.; Griffin, J. H. Langmuir 1994, 10, 883. (10) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988, 4, 365. (11) Ulman, A.; Evans, S. D.; Shnidman, Y.; Sharna, R.; Eilers, E.; Chang, J. C. J. Am. Chem. Soc. 1991, 113, 1499. (12) Buck, M.; Fischer, J.; Grunze, M.; Trager, F. Appl. Phys. 1991, A53, 552. (13) Ha¨hner, G.; Wo¨ll, Ch.; Buck, M.; Grunze, M. Langmuir 1993, 9, 195. (14) Strong, L.; Whitesides, G. M. Langmuir 1988, 4, 546.
S0743-7463(97)00241-2 CCC: $14.00
of industrial application.20-24 Alkanethiol SAMs on gold substrates are the most interesting among the SAMs, because the reaction between the gold and alkanethiol molecules occurs spontaneously even though gold was believed to be an inert material. To date, the properties of the SAMs have been studied using many analysis tools such as X-ray photoelectron spectroscopy (XPS),1-3,5-8,17-19 infrared spectroscopy (IR),4 near-edge X-ray adsorption fine structure (NEXAFS),13 X-ray and helium diffractions,14-16 and scanning probe microscopy (SPM).25-33 XPS is a powerful tool for the investigation of the chemical properties of SAMs, because (15) Carnillone, N., III; Chidsey, C. E. D.; Li, J.; Scoles, G. J. Chem. Phys. 1993, 98, 3503. Carnillone, N., III; Chidsey, C. E. D.; Eisenberger, P.; Fenter, P.; Li, J.; Liang, K. S.; Liu, G.-Y.; Scoles, G. J. Chem. Phys. 1993, 99, 744. (16) Fenter, P.; Eberhardt, A.; Eisenberger, P. Science 1994, 266, 1216. (17) Zubra¨gel, Ch.; Deuper, C.; Schneider, F.; Neumann, M.; Grunze, M.; Schertel, A.; Wo¨ll, Ch. Chem. Phys. Lett. 1995, 238, 308. (18) Walczak, M. M.; Alves, C. A.; Lamp, B. D.; Porter, M. D. J. Electroanal. Chem. 1995, 396, 103. (19) Huang, J.; Dahlgren, D. A.; Hemminger, J. C. Langmuir 1994, 10, 626. (20) Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 1498. Abbott, N. L.; Rolison, D. R.; Whitesides, G. M. Langmuir 1994, 10, 2672. Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 2790. (21) Finklea, H. O.; Hanshew, D. D. J. Am. Chem. Soc. 1992, 22, 3173. (22) Haussling, L.; Ringsdorf, H.; Schmitt, F.-J.; Knoll, W. Langmuir 1991, 7, 1837. Haussling, L.; Michel, B.; Ringsdorf, H.; Rohrer, H. Angew. Chem., Int. Ed. Engl. 1991, 30, 569. (23) Lang, H.; Duschl, C.; Vogel, H. Langmuir 1994, 10, 197. (24) Ulman, A., Ed. An Introduction to Ultrathin Organic Films; Academic: Boston, MA, 1991. (25) Overney, R. M.; Meyer, E.; Frommer, J.; Gu¨ntherodt, H.-J.; Fujihira, M.; Takano, H.; Gotoh, Y. Langmuir 1994, 10, 1281. (26) Ge, S.; Takahara, A.; Kajiyama, T. Langmuir 1995, 11, 1341. (27) Widrig, C. A.; Alves, C. A.; Porter, M. D. J. Am. Chem. Soc. 1991, 10, 2805. Alves, C. A.; Smith, E. L.; Porter, M. D. J. Am. Chem. Soc. 1992, 114, 1222. (28) Poirier, G. E.; Tarlov, M. J. Langmuir 1994, 10, 2853. Poirier, G. E.; Tarlov, M. J.; Rushmeier, H. E. Langmuir 1994, 10, 3383. (29) Delamarche, E.; Michel, B.; Gerber, C.; Anselmetti, D.; Gu¨ntherodt, H.-J.; Wolf, H.; Ringsdorf, H. Langmuir 1994, 10, 2869. Sprik, M.; Delamarche, E.; Michel, B.; Rothlisberger, U.; Klein, M. L.; Wolf, H.; Ringsdorf, H. Langmuir 1994, 10, 4116. (30) Kim. Y.-T.; Bard, A. J. Langmuir 1992, 8, 1096.
© 1997 American Chemical Society
Surface-Conditioning Effect of Gold Substrates
the XPS spectrum provides much useful information such as that regarding the chemical states,1-3,5-7,17-19 the amount of the element on the surface, the thickness of the monolayers,1-3,5-8 etc. For SAM formation, it is very important to remove contaminants from the gold surface, using ultraviolet light (UV)/ozone treatment, immersion into piranha solution, and rinsing with an organic solvent such as chloroform or acetone. UV/ozone treatment is particularly wellknown as a simple and effective pretreatment method to remove the contaminants from the surfaces of not only gold substrates but other materials.34 Ron and Rubinstein8 reported that the gold surface is oxidized by this UV/ozone treatment and that the gold oxide species were removed in an ethanol solution based on the results of the XPS and electrochemical measurements. They also proposed that UV/ozone treatment is useful in SAM formation because gold oxide species act as catalysts in the formation of gold-sulfur bonds. King35 also reported that the gold surface is oxidized at about 1.7 nm after the UV/ozone treatment for 1 h. We consider that further study is essential because the number of studies concerned with pretreatment of the gold surface is few. We recently demonstrated using atomic force microscopy (AFM)33 and XPS36 that short-alkyl-chain thiols are adsorbed onto gold at a higher rate than are long-alkylchain thiols, which is contrary to Bain et al.’s result.2 Such a growth kinetics of the SAMs should be strongly related to the surface condition before SAM formation. However, it is difficult to discuss the amount of element and chemical states by the AFM technique. Thus, we carried out XPS analysis as one of the most sensitive and quantitative methods. In addition, XPS is useful for the investigation of the kinetics and interfacial properties of SAMs, although almost all recent XPS studies on n-alkanethiol SAMs on gold have been concerned mainly with full-coverage SAMs. In the present study, we have studied the effect of the conditioning of the gold substrate on the growth of n-octadecanethiol (ODT) SAMs using the XPS technique. We have found that the amount of ODT molecules adsorbed onto the gold surface depends on the degree of contaminants of the surface. As a result of this work, it was concluded that a combination of UV/ozone treatment and pure ethanol rinsing is the best method to reduce the surface contaminants. On the other hand, it was also found that sulfur atoms adsorbed on the active site of the gold surface prevent the adsorption of ODT molecules, because these sulfur atoms adsorb very quickly after chloroform rinsing and form thiolates while the level of contaminants by carbon and oxygen decreased. II. Experimental Section Gold Deposition. Gold substrates were prepared by thermal vacuum evaporation using an Edwards Auto-306 system.36 Gold deposition was carried out at a pressure of less than 4 × 10-7 Torr at room temperature. The thickness of the gold films was adjusted to 50 nm using a quartz monitor during the deposition. Conditions of the Gold Substrates. We compared the gold substrates under different conditions: (1) untreated gold substrates obtained immediately after gold deposition (hereafter called as-deposited gold); (2) untreated gold films kept in air for (31) Stranick, S. J.; Parikh, A. N.; Tao, Y.-T.; Allara, D. L.; Weiss, P. S. J. Phys. Chem. 1994, 98, 7636. (32) Mizutani, W.; Motomatsu, M.; Tokumoto, H. Thin Solid Films 1996, 273, 70. (33) Tamada, K.; Hara, M.; Sasabe, H.; Knoll, W. Langmuir 1997, 13, 1558. (34) Takahagi, T.; Nagai, I.; Ishitani, A.; Kuroda, H.; Nagasawa, Y. J. Appl. Phys. 1988, 64, 3516. (35) King, D. E. J. Vac. Sci. Technol. A 1995, 13, 1247. (36) Ishida, T.; Nishida, N.; Tsuneda, S.; Hara, M.; Sasabe, H.; Knoll, W. Jpn. J. Appl. Phys. 1996, 35, L1710.
Langmuir, Vol. 13, No. 17, 1997 4639
Figure 1. Survey-XPS spectra of the gold surface: (a) asdeposited gold; (b) UV + EtOH gold; (c) chloroform gold; (d) contaminated gold. 1 week after the gold deposition (hereafter called contaminated gold); (3) UV/ozone treatment for 1 h, followed by rinsing the gold film with pure ethanol solution for 1 h (hereafter we call UV + EtOH gold) [UV/ozone pretreatments were carried out using a low-pressure mercury-vapor lamp (λ ) 185 and 253.7 nm)36]; (4) gold substrates rinsed with pure chloroform for 1 h (hereafter called chloroform gold). The contact angles against the water were measured by the free-standing method before and after the SAM formation. SAM Formation. n-Octadecanethiol (ODT) was purchased from Aldrich. Ethanol solutions containing ODT at a concentration of 1 mM were used. Adsorption of ODT was performed in a glass beaker for 1 h at room temperature. After removal from the solution, the gold substrates were rinsed with pure ethanol to remove the multilayer. To avoid further contamination on the SAM surface, the SAMs were quickly inserted into the XPS analysis chamber. XPS Measurements. XPS spectra were recorded using an ESCALAB MarkII system (VG Scientific Inc.) with a Mg KR X-ray source.36 The vacuum pressure of the XPS system was less than 1 × 10-9 Torr. The binding energies were corrected by use of the Au(4f7/2) peak (84.0 eV) as an energy standard. The pass energy of the analyzer and the take-off angle of the photoelectrons were set at 20 eV and 90°, respectively. The amount of ODT molecules on the gold substrate was estimated using the C/Au ratio. We measured three SAM samples in each surface condition.
III. Results Surface Conditions of Gold before SAM Formation. Figure 1 shows the survey-XPS spectra of gold substrates before SAM formation. For the gold substrates described above except for contaminated gold, (Figure 1d), it was clearly seen that the C 1s peaks at 284.3 eV attributed to surface contamination were small. Table 1 lists the relative atomic ratios of the elements on the gold surface after these pretreatments. In the cases of UV + EtOH gold and chloroform gold, the amounts of carbon and gold were about 30% and 60%, respectively.36 For the chloroform gold, the amount of sulfur on the gold surface was 3.5%, which was much higher than that of other gold substrates. This percentage was also higher than that of fully covered ODT SAM.36 For the asdeposited gold, the amount of carbon was a little higher
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Ishida et al.
Table 1. Relative Atomic Ratios of Elements on the Gold Surface before SAM Formationa surface conditions element as-deposited UV + EtOH contaminated chloroform gold oxygen carbon sulfur chlorine
58.1 3.6 38.3 0
64.7 4.8 30.5 0
45.3 10.6 43.4 0.7
63.9 0 31.6 3.5 1.0
a We measured three samples in each condition. In each condition, the deviation of the atomic ratios of the gold, carbon, and oxygen is about (2%.
Figure 3. XPS spectra in the O 1s region for the gold substrates: (a) as-deposited gold; (b) after UV/ozone treatment for 1 h; (c) UV + EtOH gold; (d) chloroform gold.
Figure 2. XPS spectra in the S 2p region for the gold substrate before SAM formation: (a) as-deposited; (b) contaminated gold; (c) chloroform gold. The S 2p spectrum of the UV + EtOH gold is almost the same as in a.
than that of UV + EtOH or chloroform gold. For the contaminated gold, the amount of carbon was the highest at 43.4%. Before SAM deposition, the contact angles of all the gold surface against the water were about 60 ( 3° and did not depend on the surface conditions. These results indicated that the adsorbed contaminants are not ordered. We utilize AFM to check the surface roughness; however, no significant roughness change was observed before and after each treatment in this study.37 Figure 2 shows the XPS spectra in the S 2p region. For as-deposited gold substrates, clear peaks attributed to the sulfur species were not observed (Figure 2a). The spectrum of the UV + EtOH gold was almost the same as that in Figure 2a. For the contaminated gold, a very small peak at around 168 eV attributed to the sulfoxide species was observed (Figure 2b). On the other hand, for the chloroform gold, a clear peak at 162.3 eV attributed to the thiolate species was observed (Figure 2c). The peak was clear, and the amount of sulfur atoms on the gold surface was larger than that of the fully covered ODT SAM, as shown in Table 1. Thus, it is likely that such contaminated thiolate species were present on the gold surface before SAM deposition. Figure 3 shows the XPS spectra in the O 1s region. For (37) We will describe a further study using AFM and contact angle measurements by the following title: Tsuneda, S.; Ishida, T.; Nishida, N.; Hara, M.; Sasabe, H.; Knoll, W. Tailoring of Highly-Ordered SelfAssembled Monolayers on Pretreated Polycrystalline Gold Surfaces. To be submitted.
Figure 4. XPS spectra in the C 1s region for the gold substrates: (a) as-deposited gold; (b) contaminated gold; (c) UV+ EtOH gold; (d) chloroform gold.
the as-deposited gold (Figure 3a), a broad peak attributed to adsorbed oxygen was observed at around 533 eV. After UV/ozone treatment (Figure 3b), a broad structure was observed in the lower binding energy region. The peak at the lower binding energy at 530.7 eV was assigned to the lattice oxygen of gold oxide.35 Such an assignment of the O 1s peaks is generally accepted when a metal oxide species such as indium tin oxide was measured by the XPS method.38 By immersion of the gold substrate in pure ethanol solution for 1 h after UV/ozone treatment (Figure 3c), the magnitude of the lower binding energy peak which was assigned to the lattice oxygen of the gold oxide decreased; thus, the gold oxide species were removed by the ethanol solution. For the chloroform gold, a clear peak was not observed in the O 1s region (Figure 3d). Figure 4 shows the XPS spectra in the C 1s region. In all cases, the peaks were observed at around 284.3 eV with a broad shoulder at the higher binding energy.36 It is difficult to determine the exact nature of the species on the gold before SAM formation, because there was no clear difference among these spectra. For the contaminated (38) Ishida, T.; Kobayashi, H.; Nakato, Y. J. Appl. Phys. 1993, 73, 4334. Ishida, T.; Kouno, H.; Kobayashi, H.; Nakato, Y. J. Electrochem. Soc. 1994, 141, 1357.
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chloroform gold, the C/Au ratio was 1.75-1.8. This result indicates that if the gold substrates were immersed long enough into ODT solution, the C/Au ratios of the SAMs did not depend on the level of surface contaminants. The contact angles against water of all the SAMs were more than 108°, which is a typical value of ODT SAMs,24 except for chloroform gold SAM. The contact angles of the SAMs prepared from chloroform gold gave a lower value (98-100°) even after immersion into ODT solution for 24 h. IV. Discussion
Figure 5. Survey-XPS spectra of the ODT SAMs prepared using different pretreatments of gold films: (a) UV + EtOH gold; (b) as-deposited gold; (c) contaminated gold; (d) chloroform gold. All the gold substrates were immersed in an ethanol solution containing 1 mM ODT for 1 h. Table 2. Average of the C/Au Ratios of SAMs Formed by Immersion of Gold Substrates in an Ethanol Solution Containing 1 mM ODT for 1 ha surface conditions C/Au a
as-deposited
UV + EtOH
contaminated
chloroform
1.56 ( 0.1
1.78 ( 0.1
1.43 ( 0.2
1.12 ( 0.5
In all the conditions, we measured three samples.
Table 3. Average of the C/Au Ratios of SAMs Formed by Immersion of Gold Substrates in an Ethanol Solution Containing 1 mM ODT for 24 ha surface conditions C/Au a
as-deposited
UV + EtOH
contaminated
chloroform
1.75 ( 0.1
1.8 ( 0.1
1.8 ( 0.1
1.2 ( 0.2
In all the conditions, we measured three samples.
and chloroform gold substrates, the C-S species must be present on the gold surface, because the presence of the sulfur species was confirmed by the S 2p spectra (Figure 2b and c). Surface Conditions of Gold after SAM Formation. Figure 5 shows the survey-XPS spectra when the gold substrates were immersed in an ethanol solution containing 1 mM ODT for 1 h. The C 1s peak intensities were increased in all cases except for SAM prepared on chloroform gold. All the C 1s spectra exhibited one peak at 285 eV, as described previously.36 Table 2 lists the average C/Au ratios of the SAMs formed on gold substrates by immersion for 1 h. It was clearly seen that for SAMs prepared under these three conditions, the C/Au ratio of the SAMs depended on the percentage of carbon contaminants on the gold surface (cf. Table 1). In contrast, for the SAM prepared on chloroform gold, the C/Au ratio was smaller than that of the SAMs prepared under the other three conditions (1.12 ( 0.5). Table 3 displays the C/Au ratios of the ODT SAMs after immersion in an ethanol solution containing 1 mM ODT for 24 h. For the SAMs, except for that prepared on
First, we will discuss the relationship between the amount of carbon contamination and the C/Au ratio of the SAMs after immersion of the gold substrate in 1 mM ODT solution for 1 h. This concentration is sufficiently high to form a fully covered ODT SAM as described by Bain et al.2 We have already reported that the contaminants must be removed from the surface of the gold substrate at an earlier time in the SAM adsorption process.36 Therefore, we expected that the C/Au ratios of the SAMs would not depend on the amount of contaminants. However, by comparing the results shown in Tables 1 and 2, it was clear that the C/Au ratios of the ODT SAMs depended on the amount of carbon contamination except in the case of chloroform gold. We believe that surface contamination affects not only the C/Au ratio but also molecular ordering and tilt angles. Although such a difference was not detected by contact angle measurements as described before, the C/Au ratio clearly changed. We observed a C 1s peak shift with changing coverage.39 We believe that this peak shift phenomenon is strongly related to the presence of an alkanethiol SAM new phase (so-called striped phase).40 The reason for the binding energy shift of the C 1s peak is explained as follows. When X-ray irradiates the fullcoverage SAM, the photoelectron emission process induces positive charges on the SAM surface and such positive charges cannot be discharged easily, because full-coverage SAM is acting as a good insulating layer. However, if the coverage was changed, the charge separation between the gold and the SAM surface would be broken, resulting the C 1s peak move to lower position. In this case, the binding energy of ODT SAMs changed from 285.0 to 284.3 eV.36 Furthermore, in molecules oriented parallel to surface, the C 1s peak position shifted to a lower position (283.9 eV, data are not shown), because the C-Au interaction is expected to be stronger. Himmel et al. recently observed similar C 1s peak shift, supporting our consideration.41 Our consideration and concept are consistent with the similar XPS analysis concerned with the silicon oxide/ silicon interface of the metal oxide semiconductor.42 A further detailed study using SPM will be reported elsewhere. The level of carbon contaminants was decreased by the UV/ozone treatment for 1 h and ethanol rinsing for 1 h. For the as-deposited gold, the amount of contamination was a little higher than that of UV + EtOH gold, because there is a possibility of contamination during sample transfer between the deposition chamber and the immersion solution. By comparing the C 1s spectra (Figure 4a-c), it is difficult to identify the exact species on the gold before SAM formation, because there was no clear difference among these spectra. The surface contaminants on as(39) Ishida, T.; Mizutani, W.; Tokumoto, H. Unpublished data. (40) Poirier, G. E.; Pylant, E. D. Science 1996, 272, 1145. (41) Himmel, H.-J.; Wo¨ll, Ch.; Gerlach, R.; Polamski, G.; Rubahn, H.-G. Langmuir 1997, 13, 602. (42) Iwata, S.; Ishizaka, A. J. Appl. Phys. 1996, 79, 6653.
4642 Langmuir, Vol. 13, No. 17, 1997
deposited and UV + EtOH golds consisted of mainly carbon and oxygen species judging from the S 2p spectrum which exhibits no sulfur peak (Figure 2a). On the other hand, for the contaminated gold, a small peak in the S 2p region including the C-S species around 286 eV was observed (cf. Figure 2b). However, the sulfoxide species can be removed during adsorption of ODT molecules very easily.43 Thus, the presence of small amounts of sulfoxide species may have little influence on the SAM formation process. The C/Au ratios of ODT SAMs also depended on the amounts of oxygen contaminants (cf. Table 1). However, we concluded that such adsorbed oxygen does not influence the ODT adsorption for the following reason. Most of the oxygen was attributed to adsorbed oxygen species. It seems that the adsorbed oxygen can be removed easily during ODT adsorption because the O 1s peak attributed to adsorbed oxygen almost disappeared after immersion of the gold substrate in ODT solution.44 For all cases, surface contaminants on the gold surface were replaced with ODT molecules, because (1) only one peak which is attributed to C-C bonds was observed in the C 1s region,36 (2) a decrease in the amounts of carbon occurred when we used butanethiol (C4H9SH) having a very short alkyl chain as reported in our previous paper,36 and (3) the O 1s peak attributed to adsorbed oxygen almost disappeared during the adsorption of ODT.44 The reason for the decrease in the C/Au ratios of SAMs was due to surface contamination, which prevented the adsorption of ODT molecules on the gold surface. By considering the above argument, we think that the influence of the adventitious carbon on the SAMs is very small. Our further studies concerned with the relationship between UV/ozone treatment and SAM molecular ordering will be described in a separated paper.37 However, if the gold substrates were immersed in an ODT solution for 24 h, the C/Au ratios of SAMs did not depend on the pretreatment conditions except for chloroform gold, as shown in Table 3. Therefore, we concluded that the effect of such carbon or oxygen contaminant layers on the gold surface became negligible in the case of longterm immersion. Ron and Rubinstein8 suggested that gold oxide species which were produced by UV/ozone treatment can be removed by ethanol, and hence, a clean gold surface was obtained. However, the thickness of the gold oxide layer after UV/ozone treatment is expected to be less than 0.5 nm (Figure 3b) because there was no additional Au 4f peaks on the spectrum at higher binding energy.35 The oxide species were removed by immersion of the gold substrate in pure ethanol for 1 h (Figure 3c), which agreed with the results reported by Ron and Rubinstein.8 No clear peak around the binding energy of 160-170 eV in the S 2p region (Figure 2a) indicated that a sulfur-free gold surface was obtained using this treatment. Ron and Rubinstein8 also argued that the gold oxide species act as a catalyst to form gold-sulfur bonds and a SAM was formed on the gold oxide layer.17 However, our XPS results demonstrated that when a gold substrate with a gold oxide was immersed into a 1 mM ODT solution for 1 h, the gold oxide species were almost completely removed after SAM formation. Therefore, we concluded that gold oxide is not a strong catalyst for gold-sulfur bond formation and SAM was not formed on the gold oxide layer. (43) Tarlov, M. J.; Burgess, D. R. F.; Gillen, G. J. Am. Chem. Soc. 1993, 115, 5305. (44) Ishida, T.; Hara, M.; Sasabe, H.; Knoll, W. Unpublished data. For the ODT SAM, the O 1s peak almost disappeared when the gold substrate was immersed in a low concentration of ODT (1 × 10-2 mM) for 5 min. However, we consider that the gold surface was not fully covered by the ODT molecules in this time period.
Ishida et al.
We will now discuss the effect of chloroform treatment and sulfur contamination. For the SAM prepared on chloroform gold, the C/Au ratios of ODT SAM were very small even after immersion in an ethanol solution containing 1 mM ODT. In this study, thiolate species on the gold surface (cf. Figure 2c) where the atomic ratio of sulfur was 3.5% were observed. This ratio is sufficient for covering all the active sites of the gold surface because the atomic ratio of sulfur in all elements detected by XPS is less than 2% when ODT SAM is fully covered on the gold surface. It is not likely that chloroform molecules or chlorine atoms prevented the adsorption of ODT. We detected the presence of chlorine atoms on the gold surface after chloroform rinse (cf. Tables 1 and 3). However, the amount of chlorine atoms was too small to cover all the sites of the gold surface. Therefore, we assumed that the sulfur atoms on the chloroform-treated gold surface originated from the atmosphere after the chloroform rinsing, because the number of sulfur species in the chloroform is negligible.45 We observed the same phenomenon for the gold substrate rinsed with methylene chloride, suggesting that this phenomenon is not characteristic only of the chloroform gold. We assume that thiolate species on the gold surface reduce the adsorption rate of ODT because of reducing the rate of the replacement. Recently, we studied this replacement process in detail by SPM, and our results are described elsewhere.46 Finally, here we briefly discuss the gold-sulfur binding. It has been believed in the results of previous studies that most alkanethiols are chemisorbed on Au. However, we confirmed the presence of weakly adsorbed alkanethiol dimers on gold,47 as related to the dimer model proposal by Fenter et al.16 Schlenoff et al. also suggested that most ODT molecules adsorb weakly on Au.48 Therefore, the gold-sulfur binding properties should be corrected based on the recent findings, if these findings are right. However, in this study, we did not refer to such binding properties because it was difficult to confirm the presence of the dimers using conventional XPS with the Mg KR X-ray source. Moreover, recently, the difficulty of observing the dimerization of the alkanethiols even using highresolution XPS was also reported by Castner et al.49 V. Conclusion We have studied the effect of the surface condition of gold on ODT SAM formation using the XPS method, leading to the following conclusions. 1. The C/Au ratios of ODT SAMs prepared by the immersion of the gold substrate in a 1 mM ODT solution for 1 h depended on the amount of surface carbon contamination before SAM formation. When the gold substrates were treated in a UV/ozone atmosphere and then rinsed with pure ethanol for 1 h, the C/Au ratio of ODT SAM on gold substrate was the greatest. This is because surface contaminants were removed by this UV/ ozone and ethanol rinse treatment and, hence, the adsorption of ODT molecules on gold occurred efficiently. (45) We have already asked several companies about the presence of such sulfur impurities in chloroform. However, the amount of such sulfur impurities was negligibly small. Thus, sulfur adsorption from the chloroform cannot be considered here. (46) Nishida, N.; Hara, M.; Sasabe, H.; Knoll, W. Jpn. J. Appl. Phys. 1997, 36, 2379. (47) Nishida, N.; Hara, M.; Sasabe, H.; Knoll, W. Jpn. J. Appl. Phys. 1996, 35, 5866. Nishida, N.; Hara, M.; Sasabe, H.; Knoll, W. Jpn. J. Appl. Phys. 1996, 35, L799. (48) Schlenoff, J. B.; Li, M.; Ly, H. J. Am. Chem. Soc. 1995, 117, 12528. (49) Castner, D. G.; Hinds, K.; Grainger, D. W. Langmuir 1996, 12, 5083.
Surface-Conditioning Effect of Gold Substrates
2. When the gold substrate was treated with a chloroform solution for 1 h before SAM formation, the C/Au ratios of the ODT SAM were the lowest. XPS results demonstrated that thiolate species appeared on the active sites of the gold surface very quickly. Thus, it was concluded that these sulfur atoms prevent the adsorption of the ODT molecules.
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Acknowledgment. We gratefully acknowledge A. Nakao for her helpful suggestions and experimental assistance in the XPS measurements. We also thank Drs. K. Tamada, K. Kajikawa, W. Mizutani, and H. Tokumoto for stimulating discussions. LA970241T
