© Copyright 1997 American Chemical Society
JUNE 25, 1997 VOLUME 13, NUMBER 13
Letters Evidence for Cleavage of Disulfides in the Self-Assembled Monolayer on Au(111) Takao Ishida,*,† Shin’ichi Yamamoto,† Wataru Mizutani,‡,| Makoto Motomatsu,† Hiroshi Tokumoto,‡,| Hirofumi Hokari,§ Hiroaki Azehara,§ and Masamichi Fujihira§ Joint Research Center for Atom Technology (JRCAT), Angstrom Technology Partnership (ATP) and National Institute for Advanced Interdisciplinary Research (NAIR), Higashi 1-1-4, Tsukuba, Ibaraki 305, Japan; and Department of Biomolecular Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226, Japan Received November 18, 1996. In Final Form: March 24, 1997X Following the proposal of a new structural model by Fenter et al. (Science 1994, 266, 1216), several reports have indicated the existence of dimers in a self-assembled monolayer (SAM) film on Au (111). We synthesized an asymmetric disulfide with hydrocarbon and fluorocarbon chains, and observed phaseseparated domains in the SAM film with force microscopy after annealing at 100 °C for 8 h. The phase separation clearly shows the cleavage of the S-S bond of the disulfide in the film. Although it cannot be confirmed whether the phase-separated domains consist of exchanged dimers or monomers (thiolates), we obtained new insights into the stability and diffusion of molecules in SAM films.
An interface is a region where different materials meet, and the interaction among the materials implies a wealth of physics and chemistry, and wide applications to industry and daily life. A self-assembled monolayer (SAM) can control the interaction of the interface, because we can alter the macroscopic properties of solid surfaces by designing and synthesizing special SAM molecules. The structure of the alkanethiol SAM was studied using many techniques. The spacing of the molecules was estimated to be 4.97 Å by low-energy electron diffraction.1 Chidsey et al. used low-energy helium diffraction to study the spacing of the end group of docosanethiol and found * Author to whom correspondence should be addressed. E-mail:
[email protected]. † JRCAT-ATP. ‡ JRCAT-NAIR. § Tokyo Institute of Technology. | Permanent address: Electrotechnical Laboratory, Tsukuba, Ibaraki 305, Japan. X Abstract published in Advance ACS Abstracts, May 15, 1997. (1) Strong, L.; Whitesides, G. Langmuir 1988, 4, 546.
S0743-7463(96)02022-7 CCC: $14.00
hexagonal-packed methyl with a lattice distance of 5Å.2 This chain spacing corresponded to the distance of the next-nearest-neighbor gold atoms on a (111) surface, 4.99 Å; therefore, a simple x3 × x3R30° adsorption model was proposed. Molecular resolution images of the film observed by means of scanning tunneling microscopy (STM) showed c(2 × 4) superstructures on the x3 × x3 overlayer.3 Fenter et al. proposed a dimer model based on the X-ray diffraction pattern resulting in a much shorter S-S distance of 2.2 Å.4 The discrepancy in the spacing was explained by introducing a gauche defect in the chain (Figure 1a). Nishida et al. observed, by thermal desorption spectroscopy (TDS), that alkanethiols desorb from Au as dimers.5 Another approach to investigating the dimer model was (2) Chidsay, C.; Liu, G.-Y.; Rowntree, P.; Scoles, G. J. Chem. Phys. 1989, 91, 4421. (3) Delamarche, E.; Michel, B.; Gerber, C.; Anselmetti, D.; Gu¨ntherodt, H.-J.; Wolf, H.; Ringsdorf, H. Langmuir 1994, 10, 2869. (4) Fenter, P.; Eberhardt, A.; Eisenberger, P. Science 1994, 266, 1216. (5) Nishida, N.; Hara, M.; Sasabe, H.; Knoll, W. Jpn. J. Appl. Phys. 1996, 35, L799.
© 1997 American Chemical Society
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this approach, it is better to use an asymmetric disulfide with a small modification in one of the alkyl chains, because too much change may cause additional side effects. Of cource, it is difficult to identify domains composed of chains with similar chemical properties. Moreover, if the difference is too small, phase separation may not occur. Phase separation of fluorocarbon and hydrocarbon molecules was observed in Langmuir-Blodgett (LB) films.8-10 Phase separation of SAM films made from a mixture of alkanethiols with different chain lengths was reported by Stranick et al.11 However, phase separation was not observed with molecule A. This does not necessarily mean that the dimers are stable in the film. The negative result might be due to the poor thermal stability of the molecular structure. In order to cause the molecules to interact with each other rather than with the substrate, the SAM film must be annealed to activate the diffusion. The ester bond in the chain of molecule A may be too weak to survive the annealing process. We synthesized the more stable molecule B and successfully observed phase-separated domains. S–(CH2)2–(CF2)5–CF3 S–(CH2)7–CH3 B
Figure 1. (a) Schematic of the dimer model proposed by Fenter et al., based on the S-S bond length 2.2 Å obtained from X-ray diffraction. The larger molecular spacing of 5 Å obtained by LEED and helium diffraction required a gauche defect in the chain. (b) Phase separation of the SAM of the asymmetric disulfide, demonstrating the cleavage of the S-S bonds in the film and providing information regarding the stability of the dimer in the model shown in part a.
demonstrated by Scho¨nherr et al.6 It was believed that SAM films made from thiols and disulfides were structurally the same, since both form thiolates at the end as follows.
CH3(CH2)nSH + Au f CH3(CH2)nS-Au + 2H2
(1)
(CH3(CH2)nS)2 + 2Au f 2CH3(CH2)nS-Au
(2)
In the dimer model, it is assumed that the thiols dimerize on the surface. The SAM films made from disulfides should also have the same structure as the films made from thiols according to the dimer model, because the molecules are dimers from the beginning. The dimer model implies that dimer adsorption is energetically more favored than that in the thiolate form, but the stability of the dimers in the film is not guaranteed. Asymmetric disulfides were used to determine whether cleavage of the S-S bond and/or exchange reaction should take place.7 Asymmetric disulfides consisting of hydrocarbon and fluorocarbon chains were synthesized,6 for example, S–CH2–CH2–O–CO–(CH2)2–(CF2)7–CF3 S–CH2–CH2–O–CO–(CH2)n–CH3 A
When phase separation is observed in the film made from asymmetric dimers, it is concluded that the S-S bond in the dimer should cleave in the SAM film (Figure 1b). With (6) (a) Scho¨nherr, H.; Ringsdorf, H. Langmuir 1996, 12, 3891. (b) Scho¨nherr, H.; Ringsdorf, H.; Jaschke, M.; Butt, H.-J.; Bamberg, E.; Allinson, H.; Evans, S. Langmuir 1996, 12, 3897. (7) Schlenoff, J.; Li, M.; Ly, H. J. Am. Chem. Soc. 1995, 117, 12528.
Asymmetric disulfide B was prepared by iodine oxidation of an equimolar mixture of octanethiol and 2-(perfluorohexyl)ethanethiol12 in ethanol. The resultant mixture of disulfides was separated by reverse-phase high-pressure liquid chromatography (HPLC) using methanol as the eluent. The SAM film was made by dipping a gold substrate into a 0.1 mM ethanol solution of disulfide B for more than 1 day. After the film was taken out, rinsed, and dried, it was annealed at 100 °C for 8 h in air. The survey and S2p X-ray photoelectron spectra (XPS) did not change before and after annealing, which suggested that no oxidation occurred upon annealing in air (Figure 2). The C/Au ratio, which was estimated to be 0.5 using XPS data, was unchanged after annealing at 100 °C, indicating that there was no desorption of molecules. Delamarche et al.13 reported that molecular desorption occurred during annealing in air at 115 °C, and we confirmed by XPS that molecule B was desorbed from the surface by annealing at 120 °C (Data are not shown). The contact angles for water of as-deposited SAMs were measured by a free-standing drop method. Normally, the contact angle of an as-deposited SAM of disulfide B was 112-116°.14 The contact angles of single-component asdeposited alkanethiol SAMs (octanethiol) and the fluoroalkanethiol CF3CF2(CH2)6SH against water were 104(8) Overney, R.; Meyer, E.; Frommer, J.; Brodbeck, D.; Lu¨thi, R.; Gu¨ntherodt, H.-J.; Fujihira, M.; Takano, H.; Gotoh, Y. Nature 1992, 359, 133. (9) Fujihira, M. In Micro/Nanotribology and Its Applications; Bhushan, B., Ed.; NATO ASI Series, Series E: Applied SciencessVol. 330; Kluwer Academic Publishers: Dordrecht, 1997; pp 239-260. (10) Takahara, A.; Kojio, K.; Ge, S.-R.; Kajiyama, T. J. Vac. Sci. Technol. 1996, A14, 1747. (11) Stranick, S.; Parikh, A.; Tao, Y.-T.; Allara, D.; Weiss, P. J. Phys. Chem. 1994, 98, 7636. (12) Dieng, S. Y.; Bertaı¨na, B.; Cambon, A. J. Fluorine Chem. 1985, 28, 341. (13) Delamarche, E.; Michel, B.; Kang, H.; Gerber, C. Langmuir 1994, 10, 4103. (14) This particular film used for measurements of SPM shown in Figure 3 showed a small contact angle of 60° without annealing. The reason for the small measured value might be the degradation of the surface by the contaminants, because this sample was measured after the other part of the film was annealed and observed by AFM. Similar phase separation was observed by annealing in the SAMs which showed large initial contact angles.
Letters
Figure 2. XPS spectra of the asymmetric disulfide SAMs of the (a) survey and (b) S2p regions. The solid line corresponds to the as-deposited film, and the dashed line to the film annealed at 100 °C. We could not detect oxygen in either the as-deposited or the annealed films. In the S2p spectra (b), no clear peak at around 167 eV (the sulfoxide peak appears at this position) was observed in the annealed film.
108° and 108 -112°, respectively. These values are close to the contact angle of 108° obtained on the asymmetric disulfide SAM after annealing, supporting the speculation that the phase-separated film is almost fully covered with the two components. The film was then observed by means of force microscopy, as shown in Figure 3. Steps and terraces originating from Au (111) are imaged in Figure 3a, and slightly depressed round domains of 20-200 nm in size can be observed on the terraces. We could not obtain the molecular resolution of this particular film. However, we reproducibly observed molecular images on the higher domains of the annealed films by scanning tunneling microscopy (STM), which was not possible on the lower topographic parts. In the case of STM, individual molecular imaging of alkanethiol SAM films was possible. It should be noted that molecular imaging of the fluorocarbon thiol single-component film was impossible with STM under the conditions that we tried, although we obtained molecular images of fluoro alkanethiols by force microscopy.15 However, the lattice image is produced only by the periodicity of the crystal averaged over the contact area of the probe, and it is difficult to produce a real molecular resolution image of an organic crystal in contactmode force microscopy. In the phase-separated domains, the short fluorocarbon molecules may be disordered. However, we believe that, in the case of nanometer-scale (15) Motomatsu, M.; Mizutani, W.; Nie, H.-Y.; Tokumoto, H. Thin Solid Films 1996, 281-282, 548.
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domains, it is difficult to obtain a lattice image by force microscopy even when the molecules are ordered. The height difference of the domain is about 3 Å. The friction force was recorded simultaneously and plotted in Figure 3b. The friction is larger in the depressed region in Figure 3a and is insensitive to the steps of the substrate. The ratio of the large-friction area to the other area is almost 1:1. Then, stiffness data were recorded at the same position by modulating the sample vertically and detecting the transferred modulation amplitude using a lock-in amplifier. The contrast of the stiffness was confirmed by vibrating the cantilever with magnetic force modulation.16 In Figure 3c, hard regions appear bright while soft parts are dark. The hard domains perfectly overlap the highfriction domains. The microcantilever used here has a spring constant of 0.1 N/m. Quantitative analysis of the stiffness is now in progress. The fluorocarbon/hydrocarbon phase-separated LB film was studied by means of force microscopy, and Fujihira et al. concluded that, in contrast to our expectation, the fluorocarbon region shows a larger friction force than the hydrocarbon part.8 A historical review of the problem is given elsewhere.9 In the latest report, Takahara et al. also reached the above conclusion.10 The SAM film made from a short fluorinated thiol on Au (111) was investigated by force microscopy, and the tilt angle of the molecules was estimated to be 56-76° to the surface normal,15 which is larger than the tilt angle of alkanethiols, typically around 30° on gold. The difference in the tilt angles accounts for the 3 Å height difference between the domains, if we assume that the lower topographic regions consist of fluorocarbon chains. This tilt angle is discussed later. The SAM film made from a 1:1 mixture of molecule B and a symmetric disulfide with both chains consisting of fluorocarbon was measured. After the same annealing process, the area fraction of the large-friction domains increased. It is, therefore, concluded that the large-friction area was the fluorocarbon-rich region. We confirmed the concentration of fluorocarbon chains in the domains by scanning surface potential microscopy.17 The structure of the fluorocarbon SAM film was reported by Alves and Porter18 and Liu et al.19 They estimated the molecular tilt angle of the single component SAMs made from CF3(CF2)n(CH2)2SH to be 20° (n ) 7) and 12° (n ) 11). As far as we know, shorter molecules such as CF3(CF2)5(CH2)2SH or annealed SAM films have not been investigated. We consider that the tilt angle is dependent on the molecular length due to the dipole-dipole interaction. We calculated the dipole moment of CF3(CF2)n(CH2)2SH using MOPAC (PM3) and found that the dipole is parallel to the molecular axis with a strength of 2.45 D for n ) 5, 2.24 D for n ) 7, and 2.27 D for n ) 11. While the van der Waals interaction increases with n, the contribution from the dipole interaction has a smaller dependence on n. Therefore, the ratio of the two interactions changes with n, and the shorter the molecules are, the larger the contribution of the dipole interaction is. (16) Florin, E.-L.; Radmacher, M.; Fleck, B.; Gaub, H. Rev. Sci. Instrum. 1994, 65. (17) The scanning surface potential microscopy measurements of phase-separated LB films made of hydrocarbon and fluorocarbon were described in the following papers: Fujihira, M.; Kawate, H.; Yasutake, M. Chem. Lett. 1994, 2224. Fujihira, M.; Kawate, H. Thin Solid Films 1994, 242, 163. The principle of measurements was described in this paper: Yokoyama, H.; Saito, K.; Inoue, T. Mol. Bio Electron. Bioelectron. 1992, 3, 79. (18) (a) Alves, C. A.; Porter, M. D. Langmuir 1993, 9, 3507. (b) Alves, C. A.; Smith, E. L.; Porter, M. D. J. Am. Chem. Soc. 1992, 114, 1222. (19) Liu, G.; Fenter, P.; Eberhardt, A.; Chidsey, C. E. D.; Ogletree, D. F.; Eisenberger, P.; Salmeron, M. J. Chem. Phys. 1994, 101, 4301.
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Figure 3. Force micrographs of the phase-separated SAM film. (a) AFM topographic image and (b) friction force distribution recorded simultaneously with an applied force of about 0.1 nN. (c) Stiffness image recorded at the same position with the sample modulating technique afterward. Domains showing larger friction (bright in part b) and a harder property (bright in part c) overlap completely. In part a, they appeared depressed (darker) by about 3 Å. The topographic structures originating from the gold substrates do not affect the friction or stiffness. The domain showing a larger friction force should be attributed to the film with the fluorocarbon rather than the hydrocarbon chains (see text).
The dipole-dipole interaction in the SAM film is readily derived by assuming that the two dipoles have the same strength and the same orientation as
φ(r,θ) ∝ p2(1 - 3 sin2 θ)/r3
(3)
where p is the strength of the dipole moment of the molecule, r is the distance between the two dipoles, and θ is the tilt angle of the dipoles measured from the surface normal. At a small tilt angle, θ < 35.2°, the potential is positive, indicating that the interaction is repulsive. As long as the van der Waals interaction is dominant, the molecular structure is not affected significantly, but in the case of short molecules, the dipole-dipole interaction cannot be neglected, and in order to reduce the potential energy, the tilt angle θ should be increased. Comparison of the peaks in the range from 1100 to 1400 cm-1 of the infrared spectra indicates that the tilt angle of the fluorocarbon chain in the film increased by annealing.20 Quantitative analysis is now in progress. We have further data to support the large tilt angle of the short fluorocarbon SAM. Figure 4 shows C1s XPS spectra of the disulfide SAMs. Lenk et al.21 reported the XPS spectrum of a SAM made from the amide thiol CF3(CF2)7C(O)N(H)(CH2)2SH and estimated the tilt angle to be almost normal to the substrate. They described that this molecule has an amide group which promotes intermolecular attractive interaction. In the XPS spectrum shown in Figure 3 of ref 21, the CH2 peak (around 284.6 eV) is very weak, indicating that the CH2 is shielded deep inside the film because of the small molecular tilt angle. We measured the C1s XPS spectrum of the SAM prepared from the symmetric fluorocarbon disulfide (CF3(CF2)5(CH2)2)S)2 (Figure 4c) and detected a strong CH2 peak. This is explained by the large tilt angle and the presence of CH2 near the film surface. It should be noted that no CsO or CdO peaks22 were observed at around (20) We measured the grazing-angle IR spectra of this SAM film with p-polarized light with an incident angle of 80°. After annealing, we observed an increase in the intensities of the peaks at 1148 and 1245 cm-1, which were attributed to vibration peaks of CF2 group (perpendicular to the chain axis), suggesting an increase in the tilt angle of the fluorocarbon chain (see refs 6a and 18a). (21) Lenk, T. J.; Hallmark, V. M.; Hoffmann, C. L.; Rabolt, J. F.; Castner, D. G.; Erdelen, C.; Ringsdorf, H. Langmuir 1994, 10, 4610. (22) Chatain, J., Ed.; Handbook of X-ray Photoelectron Spectroscopy: Perkin-Elmer Corporation: 1993.
Figure 4. C1s XPS spectra of the disulfide SAMs: (a) asdeposited asymmetric disulfide SAM; (b) annealed asymmetric disulfide SAM; (c) as-deposited SAM prepared from symmetric disulfide with only the fluorocarbon chain (CF3(CF2)5(CH2)2S)2. The intensity of the peak corresponding to CH2 (around 284.6 eV) is much stronger than that of the SAM prepared from the fluoro amide thiol.21 No clear peak was observed at around 287-288 eV, suggesting that no oxidation of carbon occurred by annealing.
287-288 eV in Figure 4b, suggesting the absence of carbon oxidation during annealing. Recent STM studies revealed the presence of a new striped phase of molecules in the low-coverage alkanethiol SAM.23 The topographically lower region (larger-friction region) seems to correspond to the striped phase. However, since we observed no desorption of molecules (see Figure 4a and b)), the above interpretation is unlikely because such a phase was observed only in the lowcoverage SAM. (23) Poirier, G. E.; Pylant, E. D. Science 1996, 272, 1145.
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Schlenoff et al.7 studied the molecular exchange between the film and the solution using molecules radiolabeled with 35S. In our case, the exchange reaction should take place in the film, because the film was annealed in air. Since we did not observe any domains before annealing, the initial film was microscopically uniform. Provided the asymmetric disulfides B adsorbed as they were, the hydrocarbon/fluorocarbon chain should have at least one chain of the other kind at the nearest neighbor position. Phase-separated domains (Figure 3) of about 200 nm in size were therefore produced as a result of molecular diffusion of that length in the surface. The diffusion coefficient can be derived from a systematic study of the domain size, annealing time, and temperature, and from computer simulations. Assuming Brownian motion, we can roughly estimate the diffusion coefficient at 100 °C to be of the order of 10 -18 m2 s-1. Our data proved that the S-S bonds in disulfides are cleaved on the surface, although we cannot conclude whether the phase-separated domains consist of monomers or symmetric dimers generated from the exchange reaction. There are other approaches to the dimer/monomer problem. TDS measurement revealed desorption from SAM films in dimer form.5,24 Nishida et al.24 reported that two desorption peaks were observed at low and high temperatures. Having analyzed the desorption peaks, they concluded that low- and high-temperature desorption peaks correspond to dimers and monomers, respectively. However, the amount of monomers and dimers is difficult to estimate by TDS, because the decomposition and dimerization of the molecules might occur at elevated temperature. High-resolution XPS seems to be useful for investigating the presence of the dimers. For example, Zubra¨gel et al.25 (24) Nishida, N.; Hara, M.; Sasabe, H.; Knoll, W. Jpn. J. Appl. Phys. 1996, 35, 5866.
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obtained two sets of S2p peaks at around 163 eV, whose positions differ by about 1.3 eV. Because the splitting of 1.3 eV is too large to assign to the difference in binding sites (the energy difference in the on-top and hollow sites is estimated to be about 0.26 eV 26), they concluded that two different S species exist on Au.25 However, Castner et al.27 pointed out the multilayer effect in the XPS result of Zubra¨gel et al.25 In Figure 2b, the S2p spectra showed a combined peak (S2p1/2 and S2p3/2) attributed to one species. Our further XPS study found that the spectral shape changed slightly after annealing. However, we do not believe that we detected the formation of monomers from the dimers by annealing. Other effects should be considered; for example, Schlenoff et al. reported that defects such as steps in the substrate play an important role in the binding energy.7 In conclusion, phase separation of molecules in the SAM film made from asymmetric disulfide was observed by force microscopy. We attributed the area showing larger friction and stiffness to the domain consisting of fluorocarbon molecules. Acknowledgment. The authors gratefully acknowledge I. Kojima and Y. Saito (National Institute of Materials and Chemical Research) for their helpful suggestions and experimental assistance in the XPS measurements. This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) of Japan. LA962022Y (25) Zubra¨gel, Ch.; Deuper, C.; Schneider, F.; Neumann, M.; Grunze, M.; Schertel, A.; Wo¨ll, Ch. Chem. Phys. Lett. 1995, 238, 308. (26) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, CA, 1991; p 289. (27) Castner, D. G.; Hinds, K.; Grainger, D. W. Langmuir 1996, 12, 5083.
