Delicate Surface Reaction of Dialkyl Sulfide Self-Assembled

The surface reaction of ODS on Au(111) was unstable unlike ODT SAM, and it was ..... Minna T. Räisänen , Anna G. Slater (née Phillips) , Neil R. Ch...
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Langmuir 2000, 16, 1703-1710

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Delicate Surface Reaction of Dialkyl Sulfide Self-Assembled Monolayers on Au(111) H. Takiguchi† and K. Sato Faculty of Applied Biological Science, Hiroshima University, 1-4-4 Higashi-Hiroshima, 739-8528, Japan

T. Ishida‡ Joint Research Center for Atom Technology (JRCAT), Angstrom Technology Partnership (ATP), 1-1-4 Higashi Tsukuba, Ibaraki 305-0046, Japan

K. Abe, K. Yase, and K. Tamada* National Institute of Materials and Chemical Research (NIMC), 1-1 Higashi Tsukuba, Ibaraki 305-8565, Japan Received October 16, 1998. In Final Form: October 25, 1999 Self-assembly of n-dioctadecyl sulfide (ODS) on Au(111) has been closely investigated by using X-ray photoelectron spectroscopy (XPS), in which the binding condition of sulfur on Au(111) was determined by the S(2p) XPS peak position, and the surface density and chain conformation of adsorbed molecules were determined by the relative XPS peak intensity, C(1s)/S(2p). The surface reaction of ODS on Au(111) was unstable unlike ODT SAM, and it was changed drastically by small variation of adsorption condition. When adsorption was carried out in 1 mM CH2Cl2 solution at room temperature, ODS molecules mostly formed fully adsorbed SAMs, intact without C-S cleavage. This was evaluated by the C(1s)/S(2p) intensity, which was twice as strong as ODT SAM, and by the S(2p) peak which appeared as a doublet at the position of “unbound” sulfur [S(2p3/2) at ∼163 eV], suggesting “physisorption” of ODS on Au(111). On the other hand, when a different condition for SAM formation was used (e.g., high temperature, long time immersion, or CHCl3 as a solvent), the C(1s)/S(2p) intensity decreased to a value smaller than ODT SAM, and the S(2p) peak was shifted to lower binding energies, the “bound” (162 eV) and “free” (161 eV) sulfur positions. In these SAMs, different surface reactions including carbon-sulfur (C-S) bond cleavage seem to occur rather than nondestructive adsorption. High-resolution atomic force microscope images revealed that ODS SAM, prepared by 24-h immersion in 1 mM CH2Cl2 solution at room temperature, formed a hexagonal lattice with the lattice constant, d ) 0.46 nm, which is nearly equal to the close-packed distance between alkyl chains and totally incommensurate against gold adlattice. Our data suggest a unique self-assembling process of ODS SAM, in which the chain-chain interaction is expected to be more predominant rather than the molecule-substrate interaction unlike ODT SAM.

1. Introduction It is well known that organosulfur compounds such as n-alkanethiols or dialkyl disulfide chemisorb on gold with spontaneous aggregation and form self-assembled monolayers (SAMs).1-3 Many reports exist about alkanethiol or dialkyl disulfide SAMs, in which the structure and property of SAMs were studied with several surface characterization techniques, such as scanning probe microscopy (SPM),4-11 X-ray photoelectron spectroscopy (XPS),12-21 infrared (IR) spectroscopy,22-27 electro* To whom correspondence should be addressed. E-mail: [email protected]. † Present address: Base Technology Research Center, SEIKO EPSON Co., Suwa, Japan. ‡ Present address: PRESTO, Japan Science and Technology Corporation and JRCAT-National Institute for Advanced Interdisciplinary Research. (1) Ulman, A. Chem. Rev. 1996, 96, 1533. (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) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (4) Poirier, G. E.; Tarlov, M. J. Langmuir 1994, 10, 2853. (5) Delamarche, E.; Michel, B.; Gerber, C.; Anselmetti, D.; Guntherodt, H.-J.; Wolf, H.; Ringsdorf, H. Langmuir 1994, 10, 2869. (6) Poirier, G. E.; Pylant, E. D. Science 1996, 272, 1145. (7) Poirier, G. E. Langmuir 1997, 13, 2019.

chemistry,28-31 electron, He or X-ray diffractions.32-35 On the other hand, although monosulfide is listed as one of the sulfur-containing compounds reactive with gold as (8) Delamarche, E.; Michel, B.; Kang, H.; Gerber, C. Langmuir 1994, 10, 4103. (9) 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. (10) Tamada, K.; Hara, M.; Sasabe, H.; Knoll, W. Langmuir 1997, 13, 1558. (11) Nelles, G.; Scho¨nherr, H.; Jaschke, M.; Wolf, H.; Schaub, M.; Ku¨ther, J.; Tremel, W.; Bamberg, E.; Ringsdorf, H.; Butt, H.-J. Langmuir 1998, 14, 808. (12) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. Soc. 1987, 109, 733. (13) Sun, F.; Grainger, D. W.; Castner, D. G.; Leach-Scampavis, D. K. Macromolecules 1994, 27, 3053. (14) Castner, D. G.; Hinds, K.; Granger, D. W. Langmuir 1996, 12, 5083. (15) Bain, C. D.; Whitesides, G. M. J. Phys. Chem. 1989, 93, 1670. (16) Laibinis, P. E.; Bain, C. D.; Whitesides, G. M. J. Phys. Chem. 1991, 95, 7017. (17) Walczak, M. M.; Alves, C. A.; Lamp, B. D.; Porter, M. D. J. Electroanal. Chem. 1995, 396, 103. (18) Buck, M.; Eisert, F.; Fischer, J.; Grunze, M.; Tra¨ger, F. J. Appl. Phys. 1991, A53, 552. (19) Ja¨ger, B.; Schu¨rmann, H.; Mu¨ller, H. U.; Himmel, H.-J.; Neumann, M.; Grunze, M.; Wo¨ll, Ch. Z. Phys. Chem. 1997, 202, 263. (20) Biebuyck, H. A.; Bain, C. D.; Whitesides, G. M. Langmuir 1994, 10, 1825.

10.1021/la981450w CCC: $19.00 © 2000 American Chemical Society Published on Web 12/31/1999

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well as thiol and disulfide, the characteristics of monosulfide SAM are not understood completely. Recently, several research groups have investigated the reaction of monosulfide with gold, especially, concerning carbon-sulfur (C-S) bond cleavage reactions.36-45 Zhong and co-workers have studied the adsorption reaction of n-dibutyl sulfide on gold in ethanol solution with XPS, IR and volumetric methods.37-39 They concluded that dibutyl sulfide adsorbed onto gold with C-S cleavage in one of side chains and formed butanethiolate SAM, which is identical with the SAM prepared from butanethiol. However, in their latest publication, they admitted a mistake in their original work.40 The SAM they used originally was contaminated with thiol or disulfide impurities. After purification of monosulfide samples, no evidence of C-S cleavage was found. The problem of thiol impurities for studies of monosulfide SAMs was already pointed out by Troughton et al. in the first publication about monosulfide SAMs.36 Several other research groups have subsequently studied this problem.41,42 Trevor et al. investigated alkanethiol, dialkyl disulfide, and dialkyl sulfide SAMs with two-laser mass spectrometry (L2MS)41 and found that there was an impurity effect on mass spectra for dialkyl sulfide SAMs. In time-of-flight secondary ion mass spectrometry (TOFSIMS) of the monosulfide SAMs, impurity-generated alkanethiol and dialkyl disulfide mass numbers were obtained. Jung et al. also investigated the effect of thiol (21) Ishida, T.; Nishida, N.; Tsuneda, S.; Hara, M.; Sasabe, H.; Knoll, W. Jpn. J. Appl. Phys. 1996, 35, L1710; Ishida, T.; Hara, M.; Kojima, I.; Tsuneda, S.; Nishida, N.; Sasabe, H.; Knoll, W. Langmuir 1998, 14, 2092. (22) Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. J. Electron Spectrosc. Relat. Phenom. 1990, 54/55, 1143. (23) Nuzzo, R. G.; Korenic, E. M.; Dubois, L. H. J. Chem. Phys. 1990, 93, 767. (24) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558. (25) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikn, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152. (26) Laibinis, P. E.; Nuzzo, R. G.; Whitesides, G. M. J. Phys. Chem. 1992, 96, 5097. (27) Tamada, K.; Nagasawa, J.; Nakanishi, F.; Abe, K.; Ishida, T.; Hara, M.; Knoll, W. Langmuir 1998, 14, 3264. (28) Strong, L.; Whitesides, G. M. Langmuir 1988, 4, 546. (29) Lamp, B. D.; Hobara, D.; Porter, M. D.; Niki, K.; Cotton, T. M. Langmuir 1997, 13, 736. (30) Imabayashi, S.-I.; Hobara, D.; Kakiuchi, T.; Knoll, W. Langmuir 1997, 13, 4502. (31) Sato, Y.; Ye, S.; Haba, T.; Uosaki, K. Langmuir 1996, 12, 2726; Ye, S.; Sato, Y.; Uosaki, K. Langmuir 1997, 13, 3157. (32) Carnillone, N. III; Chidsey, C. E. D.; Li, J.; Scoles, G. J. Chem. Phys. 1993, 97, 3503. (33) 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, 97, 744. (34) Lavrich, D. J.; Wetterer, S. M.; Bernasek, S. L.; Scoles G. J. Phys. Chem. 1998, B102, 3456. (35) Fenter, P.; Eberhardt, A.; Eisenberger, P. Science 1994, 266, 1216. (36) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988, 4, 365. (37) Zhong, C.-J.; Porter, M. D. J. Am. Chem. Soc. 1994, 116, 11616. (38) Weisshaar, D. E.; Walczak, M. M.; Porter, M. D. Langmuir 1993, 9, 323. (39) Zhong, C.-J.; Porter, M. D. Anal. Chem. 1995, 67, 709A. (40) Zhong, C.-J.; Brush, R. C.; Anderegg, J.; Porter, M. D. Langmuir 1999, 15, 518. (41) Trevor, J. L.; Lykke, R. L.; Pellin, M. J.; Hanley, L. Langmuir 1998, 14, 1103. (42) Jung, Ch.; Dannenberger, O.; Xu, Y.; Buck, M.; Grunze, M. Langmuir 1998, 14, 1103. (43) Beulen, M. W. J.; Huisman, B.-H.; van der Heijden, P. A.; van Veggel, F. C. J. M.; Simons, M. G.; Biemond, E. M. E. F.; de Lange, P. J.; Reinhoudt, D. N. Langmuir 1996, 12, 6170. (44) Beulen, M. W. J.; Bu¨gler, J.; Lammerink, B.; Geurts, F. A. J.; Biemond, Ed M. E. F.; van Leerdam, K. G. C.; van Veggel, F. C. J. M.; Engbersen, J. F. J.; Reinhoudt, D. N. Langmuir 1998, 14, 6424. (45) Hagenhoff, B.; Benninghoven, A.; Spinke, J.; Liley, M. Knoll, W. Langmuir 1993, 9, 1622.

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impurity on dialkyl sulfide SAMs through the kinetics of second harmonic generation (SHG).42 Some issues were still obscure because of an unavoidable quantity of thiol impurities; however, they came to a similar conclusion that dialkyl monosulfide adsorbed on gold intact (without C-S cleavage) and formed less densely packed and less ordered SAMs than alkanethiol SAMs. Beulin et al. maintained quite different results for dilakyl monosulfide SAMs in their series of studies.43,44 They claimed that large receptor molecules (β-cyclodextrin) with long dialkyl sulfides may form more regular layers on gold than that with thiols because of advantageous multiple points of attachment. They presented TOF-SIMS spectra as evidence of intact adsorption of dialkyl monosulfide on gold and reported a shift of the S(2p) XPS peak to the “bound” sulfur position (∼162 eV).44 This is inconsistent with Zhong’s results.40 These inconsistencies may stem from the different molecular structures or different SAM preparation conditions used in each of the studies. Considering the lower reactivity of the monosulfide group with gold, such details may change the adsorption mechanism completely for monosulfide SAMs. Troughton et al. noted that monosulfide SAMs have certain problems in experimental reproducibility,36 and in fact similar problems also were found in our study. However, if an unstable surface reaction is an essential characteristic of these molecules, it is quite important to confirm actual state systematically with well-defined systems. In this study, surface reaction of dioctadecyl monosulfide (ODS) on Au(111) was carefully monitored by S(2p) XPS spectrum in comparison with octadecanethiol (ODT) SAM. Because bulk dialkyl sulfides have a S(2p3/2) peak at ∼163 eV,44 at the different position from thiolate on gold (∼162 eV), “unbound” and “bound” sulfurs in monosulfide SAMs can be distinguished by the S(2p) XPS peak position and the progress of adsorption reaction of monosulfides can be followed by the S(2p) signals even if adsorption kinetic data are poorly reproducible.36 The existence of C-S cleavage can be reconfirmed with relative XPS peak intensities of C(1s) against S(2p). If ODS molecules form densely packed SAMs with double chains, the relative intensity of C(1s)/S(2p) should be simply twice as strong as that of ODT SAMs. Although there is no doubt that dialkyl sulfides are able to adsorb on gold without C-S cleavage as evaluated by TOF-SIMS, it is still unclear that all molecules uniformly adsorb intact on gold.41-43 Both ODS and ODT SAMs were prepared in varied adsorption conditions (different immersion time, temperature, and solvent) to achieve substantial comparison between the two SAMs independently of SAM preparation condition. Atomic force microscopy (AFM) was used to determine the lattice structure of ODS SAMs. Dynamic contact angle (DCA) measurement also was used to evaluate surface density and mobility of adsorbed molecules on gold. 2. Experimental Section Au(111) Substrate and Monolayer Preparation. Octadecanethiol [CH3(CH2)17SH, ODT; >98%], and n-dioctadecyl sulfide [(CH3(CH2)17)2S, ODS; >98%] was purchased from Tokyo Chemical Industry (Tokyo, Japan). ODS was carefully recrystallized in the mixed solution of ethanol and n-hexane several times to minimize the impurity effect. Purity of ODS was determined by NMR and X-ray crystallographic analysis. The level of impurities was lower than the detection limit. The solutions were prepared with ODS crystals grown for X-ray crystallography. ODT was used as received, and the solutions were prepared just before use to avoid dimerization to disulfide in solution. CH2Cl2

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Table 1. Relative Peak Intensities of C(1s) and S(2p) XPS Signals in ODT and ODS SAMs Formed at Various Adsorption Conditions on Au(111) SAM preparation condition (solvent/time/temperature) ODT SAM ODS SAM

CH2Cl2/24 h/r.t. CHCl3/48 h/r.t. CH2Cl2/24 h/r.t. CH2Cl2/30 min/r.t. CH2Cl2/1 h/r.t. CH2Cl2/3 h/r.t. CH2Cl2/6 h/r.t. CH2Cl2/12 h/r.t. CH2Cl2/24 h/r.t. CH2Cl2/9 days/r.t. CH2Cl2/24 h/35 °C CHCl3/24 h/r.t. CHCl3/48 h/60 °C

peak position (eV) S(2p) C(1s) 162 161, 162 162.7 163.0 163.0 161, 162, 162.8 161, 162, 162.8 161, 162, 162.8 161, 162, 162.7 161, 162, 162.7 161, 162, 162.7 161, 162, 162.7 161, 162, 162.7

285.0 284.8 284.7 284.3 284.3 284.3 284.4 284.6 284.6 283.9 284.2 283.9 283.9

relative peak intensitya C(1s)/Au(4f)c S(2p)/Au(4f)d C(1s)/S(2p)

nb

0.13 ( 0.01 0.10 0.114 ( 0.002 0.048 0.045 0.061 0.052 0.061 0.068 0.047 ( 0.003 0.057 0.035 ( 0.001 0.064 ( 0.005

5 1 3 1 1 1 1 1 1 2 1 2 2

0.008 ( 0.001 0.007 0.0035 ( 0.0001 0.0017 0.0016 0.0023 0.0019 0.0035 0.0066 0.0095 ( 0.0023 0.0054 0.013 ( 0.001 0.013 ( 0.002

16 ( 3 14.5 33 ( 2 28 28 27 27 17 10 5 ( 1.5 10.5 2.7 ( 0.1 5(1

comments thiolate SAM double chains

]

kinetic data (a series of experiments)

a The relative peak intensities in the table are averaged value of these experiments. b The number of independent XPS measurements. The errors in estimation of each C(1s) and Au(4f7/2) peak areas are less than 2%. d The error in estimation of each S(2p) peak area is less than 5%.

c

and CHCl3 in HPLC grade (99.9%) were used as a solvent without further purification. Gold was thermally deposited on a freshly cleaved mica surface at 350 °C under vapor pressure of 4 × 10-8 to 1 × 10-7 Torr (Veetech Japan Co. Ltd., Ibaraki, Japan). The deposition rate of gold was controlled at 0.1 nm/s. The mica was preheated at 550 °C for 6-9 h before deposition, and the deposited gold was heated at 550 °C for 3 h for annealing in the chamber. The grain size of epitaxially grown Au(111) obtained by the abovementioned procedure was 200-500 nm in diameter. The substrate was removed from vacuum chamber just before use and immediately immersed into the ODT or ODS solution within 10 min after exposure to air. The detailed experimental conditions for SAM preparation are described in the section of each characterization technique. At the designated time, the substrates were quickly removed from solution and rinsed with absolute solvent, and dried with dry N2 flow. X-ray Photoelectron Spectroscopy. XPS measurements were carried out using an ESCALAB 220 iXL system (VG Scientific Inc.) with a monochromatic Al-kR X-ray source (1486.6 eV). The binding energies were corrected using a Au(4f7/2) peak energy (84.0 eV) as an energy standard. The pass energy of the analyzer was set at 20 eV. Fitting of XPS peaks was performed using the spectra processing program in the XPS system, and the XPS peak position and intensity were determined by these fitting surces. The SAM preparation conditions (solvent/immersion time/temperature) for ODT and ODS SAMs are listed in Table 1. The reproducibility of SAM structures was also confirmed by repeating independent measurements, and the numbers of measurements are listed in Table 1. Atomic Force Microscopy. The AFM system used in this study was a commercially available NanoScope IIIa (Digital Instruments, Inc., Santa Barbara, CA). The measurements were performed in the contact mode (30-µm scanner) in air at room temperature. A Si3N4 cantilever with a spring constant of 0.38 N/m was used for molecular resolution imaging (scanning rate, 10-30 Hz). All images (512 × 512 pixels) were collected in the “height mode”, and the applied force was minimized during the AFM imaging to adjust the “set point voltage” to the lower limit.10 Calibration of the AFM scanner has been done by both grating pattern (x-, y- and z-directions) and mica images (x- and y-directions). The ODS and ODT SAMs used for AFM imaging were prepared by 24-h immersion of Au(111)/mica substrates in 1 mM CH2Cl2 solution at room temperature. DCA Measurements. DCA measurements were performed using a Wilhelmy plate method with a Cahn balance (DCA 322 model).46,47 In this study, the advancing and receding speeds were set at 20 µm/s. The ODT and ODS SAMs for DCA measurements were prepared by 24-h immersion of Au(111)/ (46) Lander, L. M.; Siewierski, L. M.; Brittain, W. J.; Vogler, E. A. Langmuir 1993, 9, 2237. (47) Uyama, Y.; Inoue, H.; Ito, K.; Kishida, A.; Ikada, Y.J. Colloid Interface Sci. 1991, 141, 275.

Figure 1. S(2p) XPS spectra of (a) ODS bulk crystal and (b) ODT and (c) ODS SAMs formed by 24-h immersion in 1 mM CH2Cl2 solutions at room temperature. The peak positions of S(2p3/2) are (a) 162.8, (b) 162.0, (c) 162.7 eV, respectively. mica substrates in 1 mM CH2Cl2 solutions at room temperature. We used a modified technique to measure asymmetric Au-thiol SAM plates (surface: Au-thiol SAMs, back: cleaved mica, substrate size: 1 cm × 2 cm × 50 µm), in which the contact angle of SAM surface was estimated by simple numerical calculation from the force F acting on the sample plate measured with a electrobalance. For the calculation, the perfect wetting (θ ) 0) of the freshly cleaved mica surface was assumed. The details of this technique is presented in the literature.48 Because surface roughness can have an impact on the hysteresis in contact angles, gold films were deposited carefully under the same condition, and the morphology of gold surface of each plate was ascertained by AFM imaging after DCA measurements.48 All DCA measurements were performed with pure water at 22 °C.

3. Results and Discussions XPS Data of ODT and ODS SAMs. The S(2p) XPS spectra of ODS bulk crystals, ODT SAMs and ODS SAMs, are shown in Figure 1, and the C(1s) XPS spectra of both SAMs are shown in Figure 2. The ODT and ODS SAMs were prepared by 24-h immersion of Au(111)/mica substrates in 1 mM CH2Cl2 solutions at room temperature. Because the S(2p) peak appears as a doublet caused by the spin-orbit splitting [S(2p3/2), S(2p1/2)] even when one sulfur species exists on the surface, we extend all discussion for sulfur-binding energy with the peak position (48) Abe, K.; Takiguchi, H.; Tamada, K. Langmuir, in press.

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Figure 2. C(1s) XPS spectra of (a) ODT and (b) ODS SAMs formed by 24-h immersion in 1 mM CH2Cl2 solutions at room temperature. The peak positions of C(1s) are (a) 285, (b) 284.7 eV, respectively.

of S(2p3/2) to avoid any confusion. As shown in Figure 1a, ODS bulk crystals exhibited a doublet S(2p) band at 162.8 eV, which can be regarded as an “unbound” sulfur position.44 The ODT SAMs exhibited a doublet S(2p) peak at 162 eV (Figure 1b), at the position of “bound” sulfur, that is consistent with the XPS data reported.13-21 We performed five independent measurements with ODT SAMs prepared under the same conditions and obtained good reproducibility for both peak position and intensities. For ODT SAM, the relative peak intensity of carbon C(1s) against Au(4f7/2) [C(1s)/Au(4f)] and sulfur S(2p) against Au(4f7/2) [S(2p)/Au(4f)] were constant, 0.13 and 0.008, respectively; and the relative peak intensity of carbon C(1s) against sulfur S(2p) [C(1s)/S(2p)] was estimated to be 16 ( 3. Unlike ODT SAMs, the XPS signal of ODS SAMs was poorly reproducible. The binding condition of sulfur was changed drastically by even small variations in the adsorption conditions. However, the S(2p) peak mostly appeared as a doublet at ∼162.7 eV (Figure 1c), at the position of “unbound” sulfur, originating from the intact monosulfide group, when the SAM was prepared by the condition described above (24-h immersion in CH2Cl2 at room temperature). When the S(2p) peak was detected at this “unbound” position, the relative peak intensity of S(2p)/Au(4f) was always twice as small as that of ODT SAM (0.0035), and the peak intensity of C(1s)/Au(4f) was almost the same as that of ODT SAM (0.114). Consequently, the relative peak intensity of C(1s)/S(2p) was estimated to be roughly twice as large as that of ODT SAM (33 ( 2). This result suggests a double-chain conformation of ODS molecules on the surface, resulting from ODS adsorption without C-S cleavage. This result, intact adsorption of monosulfide on gold, is consistent with the latest report by Porter et al. with phenyl ethyl sulfide purified by preparative scale gas chromatography (PrepGC).40 As shown in Figure 2, the C(1s) peak of ODT SAM appeared at 285 eV in a manner similar to previously reported results,20,21 whereas the C(1s) peak of ODS SAM was observed at 284.7 eV, a slightly lower binding energy than ODT SAM. The surface charge generated by X-ray irradiation during XPS measurements cannot be discharged completely on densely packed films and caused the shift of XPS signals to higher binding energy.20,21 Following this interpretation, the ODS SAM, which exhibited the lower binding energy, is expected to form

Takiguchi et al.

Figure 3. Change of S(2p) XPS signals by the immersion time: (a) 1 h, (b) 3 h, (c) 12 h, (d) 24 h, in 1 mM CH2Cl2 solution at room temperature. The S(2p) peak was shifted from 163 eV to lower binding energy, 161 and 162 eV, when the immersion time increased.

Figure 4. C(1s) XPS signals at the same surface as Figure 3, at the immersion time, (a) 1 h, (b) 3 h, (c) 12 h, (d) 24 h, in 1 mM CH2Cl2 solution at room temperature. The C(1s) peak was shifted from 284.3 to 284.6 eV when the immersion time increased.

less dense films than ODT SAM. We detected no oxygen and oxidized carbon or sulfur peaks for both SAMs. 49 As described above, the surface reaction of ODS on Au(111) is not stable, and it occasionally gives quite different XPS signals even for the films prepared at the same adsorption condition. Kinetic data shown in Figures 3 and 4 are examples. In Figure 3, the S(2p) signal appeared at about 163 eV by short immersion time (1 h, a); however, when the immersion time increased, the S(2p) was shifted to lower binding energy, at about 161 and 162 eV. These signals can be divided into three pairs of doublets, “unbound sulfur” (163 eV), “bound sulfur” (162 eV), and “free sulfur” or “atomic sulfur” (161 eV), respectively.14,21,40,50 Thiolate and several other adsorbed sulfur species, such as thiophenes, have the binding energy at 162 eV, so that the binding energy of 162 eV cannot be a conclusive evidence of formation of thiolate.51 However, (49) Schoenfisch, M. H.; Pemberton, J. E. J. Am. Chem. Soc. 1998, 120, 4502. (50) Weisshaar, D. E.; Walczak, M. M.; Porter, M. D. Langmuir 1993, 9, 323.

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Figure 5. AFM images of ODS SAMs on Au(111) at different immersion times: (a) 1 min, (b) 10 min, (c) 24 h in 1 mM CH2Cl2 solutions (1.25 × 1.25 µm). The growth of dendritic domains was observed at the initial stage of SAM growth (a), and the area fraction of the domains phases increased as the immersion time increased (b). Finally, the entire surface was covered with adsorbed ODS (c); however, small amounts of defect phases were still remaining at the edge of gold grains unlike ODT SAM.

at least it is clear that chemical state of sulfur changed by reaction with gold with the passage of time. These peak positions of sulfur are similar to the result of β-CD monosulfide SAMs reported by Beulen et al.43,44 As shown in Figure 4, these ODS SAMs exhibited the C(1s) peaks at relatively lower binding energy (284.3-284.6 eV) than the ODS SAM shown in Figure 2. The relative peak intensity of C(1s)/S(2p) is nearly constant for ODS SAMs prepared by less than 12-h immersion in CH2Cl2, and is nearly the same value as ODS SAM with double chains shown in Figure 1c. The C(1s)/S(2p) value tends to decay by additional immersion (more than 24 h) with the shift of S(2p) to lower binding energies (see Table 1). We attempted 9-day immersion for ODS SAM in CH2Cl2 at room temperature and found that the C(1s)/S(2p) intensity was never as high as that of ODT SAMs. As described in the Introduction, there has been a deep concern for the effects of thiol impurities in monosulfide SAMs36-45; however, the lower values for the C(1s)/S(2p) intensity of ODS SAMs suggest the reaction we monitored was not a simple exchange of physisorbed monosulfides for thiol impurities on the surface. We investigated the XPS peak positions and intensities of ODS SAMs prepared at different adsorption conditions to obtain further information for ODS adsorption reaction, and obtained the results listed in Table 1. When a slightly higher temperature (35 °C) was used for SAM formation, the S(2p) peaks appeared at lower binding energies (161 and 162 eV), and the C(1s) peak was shifted to smaller binding energy (284.2 eV). In these SAMs, the relative peak intensities of C(1s)/S(2p) was less than ODT SAMs. Similar results were obtained in ODS SAMs prepared in CHCl3 solutions, in which the density of alkyl chains were certainly much less than fully adsorbed SAM. Because good reproducibility was obtained for each SAM (we performed two independent measurements for each SAM), this is not a simple scatter of data but suggests that different surface reactions occur for different adsorption conditions. Although the ODT SAM also exhibited a shift of the S(2p) and C(1s) peak positions for the adsorption in CHCl3, no significant change of C(1s)/S(2p) intensity was detected within experimental error. As described with the kinetic data, the change of C(1s)/S(2p) values in ODS

SAM cannot be explained by film density, or rather, the progress of C-S bond cleavage seems to be a more appropriate explanation for this phenomenon. The ODS SAM never approaches a film structure identical with the ODT SAM, even if C-S bond cleavage takes the place of nondestructive adsorption. The much smaller C(1s)/S(2p) value for ODS molecules suggests the possiblity that both alkyl chains are lost in these SAMs. Surface Morphology and Lattice Structure of ODS SAM. Figure 5 shows ex situ AFM images of ODS SAM during the growth. When gold substrate was immersed into 1.0 mM CH2Cl2 solution for 1 min, the surface was partially covered with domains shaped like “dendrite”,52 which can be regarded as a type of stable aggregation form for ODS. These dendritic domains could be desorbed by 1-h rinsing in absolute CH2Cl2, suggesting physisorption of ODS molecules at this stage. ODT domains are never desorbed completely by this rinsing procedure with CH2Cl2. Adsorption kinetics of ODS SAMs estimated by surface coverage with domains in AFM images were poorly reproducible, as with the XPS results, and they are distinguished easily from that of alkanethiols exhibiting good reproducibility with the same Au(111) surface and experimental procedure.10 Figure 6 shows molecular resolution AFM images of ODT and ODS SAMs prepared by 24-h immersion in 1 mM CH2Cl2 solutions at room temperature. The ODT SAM exhibited a two-dimensional hexagonal lattice with lattice constant of 0.50 ( 0.02 nm (Figure 6a), consistent with the literature.4-11 The ODS SAM exhibited two-dimensional crystalline structures as well; however, the observed lattice (hexagonal lattice) was significantly smaller than that of ODT SAM, exhibiting a lattice constant of 0.46 ( 0.02 nm (Figure 6b). The different molecular lattices of ODT and ODS SAMs seem to be correlated with the different assembling processes of these SAMs. The ODT SAM exhibits the hexagonal lattice with a lattice constant of 0.50 nm, which is consistent with sulfur adsorption at the 3-fold hollow site. The ODS SAM has a molecular lattice with a lattice constant of 0.46 nm, which is nearly equal to the close-packed structure of alkyl chains. This structure is totally incommensurate with the gold adlattice, i.e, the chain-chain interaction seems to be more

(51) Ishida, T.; Choi, N.; Mizutani, W.; Tokumoto, H.; Kojima, I.; Azehara, H.; Hokari, H.; Akiba, U.; Fujihira, M. Langmuir 1999, 15, 6799..

(52) Miller, A.; Knoll, W.; Mo¨hwald, H. Phys. Rev. Lett. 1986, 56, 2633; Suresh, K. A.; Nittmann, J.; Rondelez, F. Prog. Colloid Polym. Sci. 1989, 79, 184.

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Figure 6. Molecular resolution AFM images (20 × 20 nm) and the corresponding 2D Fourier transform of (a) ODT and (b) ODS SAMs on Au(111) prepared by 24-h immersion in 1 mM CH2Cl2 solutions. ODT SAM exhibited a 2D hexagonal lattice with lattice constant of 0.50 ( 0.01 nm (a), whereas ODS SAM showed a hexagonal lattice with significantly smaller lattice constant of 0.46 ( 0.01 nm (b).

Dialkyl Sulfide SAMs on Au(111)

Figure 7. DCA hysteresis loops for (a) ODT and (b) ODS SAMs on Au(111) (1, first run; 2, second run). All these loops were measured at wetting and dewetting rates of 20 µm/s with pure water at 22 °C.

predominant for formation of ODS SAMs than the molecule-substrate interaction.27 It is not clear whether this 0.46-nm lattice structure is composed of ODS molecules with only double chains, free from thiol impurity and the C-S bond cleavage. However, the lattice constant is significantly different from that of thiolate SAMs (0.50 nm). Dynamic Contact Angles. Figure 7 shows DCA hysteresis loops for ODT and ODS SAMs on Au(111) prepared by 24-h immersion in 1 mM CH2Cl2 solutions. The advancing angles of both ODT and ODS SAMs were quite high (ODT: θa ) 112 ( 1 °, ODS: θa ) 110 ( 1°) as expected from the high-resolution AFM images (Figure 6), which show a highly ordered molecular lattice composed of methylene terminal groups. The receding angles of ODS and ODT SAMs were quite different. The ODS SAM exhibited a receding angle 7° lower than that of the ODT SAM (ODT: θr ) 102 ( 1 °, ODS: θr ) 95 ( 1 °). When the surface is covered homogeneously with singlecomponent molecules and the influence of gold roughness is negligible,36,47 the hysteresis data (∆θ ) θa - θr) should be relevant to the mobility of adsorbed molecules on surface during the wetting and dewetting process.47 In general, thiol SAMs, strongly chemisorbed on gold by site-to-site reaction, exhibit remarkably small hysteresis (∼10°)27,54 compared with organosilane SAMs (∼20°),55-57 which include a relatively large number of physisorbed molecules in the films. As an another example, quite large hysteresis (15-50°) was reported for cationic surfactant films that adsorbed on mica by ionic interaction.58,59 If this interpretation is applicable to the ODS system, the large (53) Christenson, H. Colloid Surf. A 1997, 129/130, 67. (54) Tamada, K.; Nagasawa, J.; Nakanishi, F.; Abe, K.; Hara, M.; Knoll, W.; Ishida, T.; Fukushima, H.; Miyashita, S.; Usui, T.; Koini, T.; Lee, T. R. Thin Solid Films 1998, 150, 327. (55) Rabinovich, Y. I.; Derjaguin, B. V. Colloid Surf. 1988, 30, 243. (56) Rabinovich, Y. I.; Yoon, R. H. Langmuir 1994, 10, 1903. (57) Wood, J.; Sharma, R. J. Adhes. Sci. Technol. 1995, 9, 1075. (58) Christensen, H.; Claesson. P. M. Science 1988, 239, 390

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hysteresis of ODS SAM can be regarded as evidence of the existence of physisorbed molecules in the film, or surface inhomogeneity caused by C-S bond cleavage. Self-Assembling Mechanism of ODS on Au(111). The ODS molecules are self-assembled on Au(111) surface by a different adsorption mechanism than that for thiol SAMs, in which the molecule-molecule (chain-chain) interaction seems to be the more predominant mechanism for SAM formation. As confirmed by XPS and AFM measurements, ODS molecules adsorbed on gold with much slower adsorption kinetics than ODT molecules (ca. 60% of surface coverage was achieved by 10-min immersion in 1 mM CH2Cl2 solution, in Figure 5),42 and most ODS molecules are held on the surface by “physisorption”. Poor reproducibility of adsorption kinetics at the initial stage of SAM growth is regarded as typical for physisorbed surfaces, in which uncontrollable experimental factors such as small fluctuation of temperature or degree of surface contamination affect molecular adsorption appreciably. Because the C(1s)/S(2p) peak intensity of ODS SAMs is twice as strong as that of ODT SAMs, there is no doubt that ODS molecules are able to adsorb on gold intact without C-S cleavage. On the other hand, it may also be true that different surface reactions including carbon-sulfur (C-S) bond cleavage take the place of nondestructive adsorption due to experimental conditions. This complication of surface reaction is what enables us to discriminate monosulfide SAM from thiol or disulfide SAMs. In other words, the simple and homogeneous surface reaction proposed for thiol SAMs is unique,60 considering that most chemical reactions have sidereactions to some extent. Zhong et al. found that the binding condition of sulfur in monosulfide SAMs was changed by an Ar-etching process.40 It is reasonable that parameters related to the molecule-molecule interaction, e.g., alkyl chain length or surface roughness (gold grain size or degree of surface contamination), directly influenced surface reaction of monosulfide, because the position of the sulfur group on gold (whether sulfur is on an active site or not) is determined by the molecular assembling process during physisorption. For our experiment with contaminated amorphous gold, in which gold was thermally deposited on glass without temperature control and kept in air for more than a week before SAM formation, almost no adsorption of ODS could be detected by TOFSIMS measurements, despite using the same SAM preparation condition (24-h immersion in 1 mM CH2Cl2 solution at room temperature). Because the adsorption coefficient of monosulfide may be influenced by detailed experimental conditions, the effect of thiol or disulfide impurities on adsorption of monosulfide should not be considered the same in each system as long as these are competitive reaction.36-42 Hagenhoff et al. detected no signals corresponding to thiol or disulfide impurities in their TOF-SIMS spectrum of monosulfide SAM with long alkyl chains, although they used the monosulfide sample without purification.45 Beulen et al. also reported in their study that long dialkyl sulfides can form well-packed and stable SAMs and short dialkyl sulfides cannot form wellpacked and stable SAMs.44 Thus, it may not be appropriate to assume a universal conclusion for the adsorption mechanisms of monosulfide SAMs, regardless of these details. (59) Claesson, P. M.; Blom, C. E.; Herder, P. C.; Ninham, B. W. J. Colloid Interface Sci. 1986, 114, 234. (60) There have been several arguments concerning dimerization of thiols (formation of disulfides) for alkanethiol SAMs. Kluth, G. J.; Carraro, C.; Maboudian, R. Phys. Rev. B 1999, 59, R10449. Reference 35.

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Conclusion We characterized the adsorption mechanism and film structure of ODS SAM on Au(111) with XPS, AFM, and DCA, in comparison with ODT SAM. We found that ODS molecules can adsorb on Au(111) intact with double-chain conformation without C-S cleavage and that the adsorption mechanism of ODS SAM is different from that of ODT SAM. ODS SAM is also capable of a completely different surface reaction route depending on the experimental conditions. Poor reproducibility of adsorption kinetics makes the surface reaction of ODS look more complicated; however, it can be regarded as one of essential characteristics of ODS SAMs and is related to the low reactivity of monosulfide group with gold. The ODS molecules are self-assembled on Au(111) in a different

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manner from thiol SAMs, in which the molecule-molecule (chain-chain) interaction is more predominant for SAM formation than the molecule-substrate interaction. Acknowledgment. The authors gratefully acknowledge Dr. H. Akiyama in NIMC for helping with purification of ODS molecules, and Dr. I. Kojima in NIMC for providing us opportunities for XPS measurements. We are grateful to Dr. M. Beulen and Prof. F. C. J. M. van Veggel in University Twente for valuable discussions. We thank S. Kra¨mer and Prof. W. Knoll for TOF-SIMS measurements. We also thank Dr. E. T. Foley in JRCAT-ATP for kind suggestions. LA981450W