Surface Phase Behavior of n-Alkanethiol Self-Assembled Monolayers

Ionic Strength, Surface Charge, and Packing Density Effects on the Properties of Peptide Self-Assembled Monolayers. Norman Leo , Juan Liu , Ian Archbo...
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1558

Langmuir 1997, 13, 1558-1566

Surface Phase Behavior of n-Alkanethiol Self-Assembled Monolayers Adsorbed on Au(111): An Atomic Force Microscope Study Kaoru Tamada,† Masahiko Hara,* Hiroyuki Sasabe, and Wolfgang Knoll Frontier Research Program, The Institute of Physical and Chemical Research (RIKEN), Wako, Saitama 351-01, Japan Received July 28, 1995. In Final Form: November 25, 1996X n-Alkanethiol (CnH2n+1SH) self-assembled monolayers (SAMs) adsorbed on Au(111) were studied with an atomic force microscope (AFM) to confirm the influence of the lateral interaction between adsorbed thiols on the film morphology. Two experiments were performed: firstly, a study of the domain formation at the initial stage of SAM growth (single component) and, secondly, investigations of the coadsorption phenomenon in mixed SAMs composed of two alkanethiols having different chain lengths. For the kinetics study, Au(111) was immersed into the 10-2 mM ethanol solutions with the single component alkanethiol (C4H9SH, C12H25SH, or C18H37SH), for varying times (1 s to 10 min). In all cases, the film coverage increased as the immersion time became longer, and finally the surface was totally covered with thiols after an immersion time of 3 min or more. Clear island formations were observed in the partially covered C12H25SH and C18H37SH SAMs, while C4H9SH formed meshlike domains. The mixed SAMs were prepared by immersing Au(111) into 1 mM ethanol solutions with mixed alkanethiols (C4H9SH/C18H37SH) of various compositions, Rsoln ) [C4H9SH]/[C18H37SH] ) 1/1 to 100/1, for a time of 1 h. Clear phase separation was observed at Rsoln ) 20/1 and 40/1. Above or below these compositions, the film surface appeared very flat, covered with a nearly single component, C4H9SH or C18H37SH, respectively. This is the first systematic study of the surface phase behavior of alkanethiol SAMs by AFM imaging. It reveals more direct information about the film morphology than previous studies with conventional surface analytical techniques such as X-ray photoelectron spectroscopy, ellipsometry, contact angles, etc.

Introduction Self-assembled monolayers (SAMs) adsorbed on metal surfaces have been studied extensively due to the interest of basic surface science1-15 as well as their technological * Author to whom correspondence should be addressed: e-mail, [email protected]; tel., +81-48-467-9603; fax, +8148-462-4695. † Present address: Department of Molecular Engineering, National Institute of Materials and Chemical Research (NIMC), 1-1 Higashi, Tsukuba, Ibaraki 305, Japan. X Abstract published in Advance ACS Abstracts, January 15, 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) Forkers, J. P.; Laibinis, P. E.; Whitesides, G. M.; Deutch, J. J. Phys. Chem. 1994, 98, 563. (8) 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. (9) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988, 4, 365. (10) Ulman, A.; Evans, S. D.; Shnidman, Y.; Sharma, R.; Eilers, E.; Chang, J. C. J. Am. Chem. Soc. 1991, 113, 1499. (11) Buck, M.; Fischer, J.; Grunze, M.; Tra¨ger, F. Appl. Phys. 1991, A53, 552. (12) Ha¨hner, G.; Wo¨ll, Ch.; Buck, M.; Grunze, M. Langmuir 1993, 9, 195. (13) Strong, L.; Whitesides, G. M. Langmuir 1988, 4, 546. (14) Camillone, N., III; Chidsey, C. E. D.; Li, J.; Scoles, G. J. Chem. Phys. 1993, 98, 3503. Camillone, 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.

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significance.16-20 For the latter case, the SAMs are used as a technique for the fabrication of the interfacial layer or for the modification of metal surfaces with organic molecules to construct inorganic-organic supramolecular architectures.19,20 While Langmuir-Blodgett (LB) films are also available for this purpose,21,22 SAMs have advantages such as facile film preparation and long-term stability.1,2,23 In particular, n-alkanethiol SAMs adsorbed on gold from solutions have been recognized as a very simple and widely applicable technique to form wellordered and densely packed monolayers,24-26 in which thiols are believed to be chemisorbed on gold surface as thiolates. The previous studies of alkanethiol SAMs may be classified roughly into two categories: One is the char(15) Fenter, P.; Eisenberger, P.; Liang, K. S. Phys. Rev. Lett. 1993, 70, 2447. Fenter, P.; Eberhardt, A.; Eisenberger, P. Science 1994, 266, 1216. (16) Huang, J.; Dahlgren, D. A.; Hemminger, J. C. Langmuir 1994, 10, 626. (17) 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. (18) Finklea, H. O.; Hanshew, D. D. J. Am. Chem. Soc. 1992, 22, 3173. (19) Ha¨ussling, L.; Ringsdorf, H.; Schmitt, F.-J.; Knoll, W. Langmuir 1991, 7, 1837. Ha¨ussling, L.; Michel, B.; Ringsdorf, H.; Rohrer, H. Angew. Chem., Int. Ed. Engl. 1991, 30, 569. (20) Lang, H.; Duschl, C.; Vogel, H. Langmuir 1994, 10, 197. (21) Ulman, A., Ed. An Introduction to Ultrathin Organic Films; Academic Press, Inc.: Boston, MA, 1991. (22) Overney, R. M.; Meyer, E.; Frommer, J.; Gu¨ntherodt, H.-J.; Fujihira, M.; Takano, H.; Gotoh, Y. Langmuir 1994, 10, 1281. (23) Ge, S.; Takahara, A.; Kajiyama, T. Langmuir 1995, 11, 1341. (24) 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. (25) Poirier, G. E.; Tarlov, M. J. Langmuir 1994, 10, 2853. Poirier, G. E.; Tarlov, M. J.; Rushmeier, H. E. Langmuir 1994, 10, 3383. (26) Delamarche, E.; Michel, B.; Gerber, Ch.; Anselmetti, D.; Gu¨ntherodt, H.-J.; Wolf, H.; Ringsdorf, H. Langmuir 1994, 10, 2869. Sprik, M.; Delamarche, E.; Michel, B.; Ro¨thlisberger, U.; Klein, M. L.; Wolf, H.; Ringsdorf, H. Langmuir 1994, 10, 4116.

© 1997 American Chemical Society

AFM Study of SAMs on Au

acterization of SAMs by macroscopic techniques such as X-ray photoelectron spectroscopy (XPS), ellipsometry, and contact angles,1-3,5-7 which give the “averaged” film thickness and physical properties of SAMs over a few millimeters of the surface. Another is the characterization of SAMs by microscopic characterization techniques such as scanning probe microscopy (SPM).24-31 An enormous number of SPM studies with atomic scale resolution (∼Å) has been performed to confirm the ordered structures of alkanethiols on Au(111). It has been reported that thiol molecules exhibit a hexagonal (x3×x3)R30° structure24,25 and several variants of a (4×2) superlattice structure.25,26 A “pinstripe” pattern with a p×x3 unit cell (8 e p e 10) in films with a lower packing density25 and a formation of defect “holes”,31 found by scanning tunneling microscope (STM) studies, have also been discussed. The various diffraction measurements such as electron, X-ray, and helium diffractions13-15 have also been used to determine the lattice constants of SAMs. Furthermore, infrared spectroscop (IR)4,32 and near-edge X-ray absorption fine structure (NEXAFS)12 have been employed to determine the tilt angles of alkyl chains in densely packed SAMs (30-35° with respect to the surface normal). In this study, we observed the surface phase behavior of alkanethiol SAMs with an atomic force microscope (AFM), which is classified as a “mesoscopic” study in the sense of spatial resolution. We have focused on two topics at issue; one is whether the domain formation appears at the initial stage of SAM growth, and another is whether SAMs of coadsorbed mixtures of two kinds of alkanethiols exhibit phase separation. There have been several studies which discussed the existence of a phase separation in mixed SAMs experimentally3-5 and theoretically,7 but no clear evidence of this has as yet been reported.33 Buck et al. reported that the film growth of alkanethiols on gold exhibited the Langmuir adsorption kinetics.11 This result supports the random adsorption of the alkanethiols on the gold surface. The competitive adsorption phenomena in mixed SAMs composed of two alkanethiols, however, exhibited the remarkable preference of longer chain alkanethiols in adsorption,4,5,7 whereby the significant tendency to form the single-component phase rather than the mixed phase was also revealed.5,7,8 These results of the mixed SAMs indicate that the lateral interaction between adsorbed thiols promotes the adsorption process, although the existence of the phase separation was not confirmed by the contact angle data.3-5 The surface phase behavior in SAMs is probably determined by the balance of two forces: the molecularmolecular lateral interaction and the molecular-substrate vertical interaction.7,21 It is of interest for a better understanding to compare the phase behavior of SAMs with that of monolayers at the air/water interface (Langmuir films), which has been studied more extensively with fluorescence microscopy34-37 and Brewster angle microscopy.38-40 The significant difference between the two (Langmuir films and SAMs) is the strength of the (27) Kim, Y.-T.; Bard, A. J. Langmuir 1992, 8, 1096. (28) Liu, G.-Y.; Salmeron, M. B. Langmuir 1994, 10, 367. (29) Bucher, J.-P.; Santesson, L.; Kern, K. Langmuir 1994, 10, 979. (30) Delamarche, E.; Michel, B.; Kang, H.; Gerber, Ch. Langmuir 1994, 10, 4103. (31) Scho¨nenberger, C.; Sondag-Huethorst, J. A. M.; Jorritsma, J.; Fokkink, L. G. J. Langmuir 1994, 10, 611. (32) Dubois, L. H.; Zegarski, R.; Nuzzo, R. G. J. Chem. Phys. 1993, 98, 678. (33) Recently, the phase separated images in CH3(CH2)15SH/ CH3O2C(CH2)15SH mixed films were reported by Stranick et al. They utilized STM to characterize inhomogeneous surfaces: Stranick, S. J.; Parikh, A. N.; Tao, Y.-T.; Allara, D. L.; Weiss, P. S. J. Phys. Chem. 1994, 98, 7636. (34) Mo¨hwald, H. Thin Solid Films 1988, 159, 1 and references therein. (35) Peters, R.; Beck, K. Proc. Natl. Acad. Sci. U.S.A. 1983, 80, 7187.

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vertical interaction and the rate of lateral diffusion. Our purpose is to confirm the influence of the lateral interaction on the film morphology of SAMs and furthermore, if clear domain formations can be observed in SAMs, to model them by kinetic and/or thermodynamic theories where applicable. In this study, we did not utilize STM41,42 but AFM for the observation of domains. This is because AFM seems to be more suitable for the topological characterization of nonconductive films, especially for the discrimination of the alkyl chain length (the height of SAM domains). The position of the STM tip during imaging of alkanethiol SAMs has been a matter of argument until quite recently;31,43-45 one claim was that the STM tip must contact the thiolate headgroups to obtain the required current for imaging,24,31 and another was that the STM tip is not in contact with the surface.25 This argument has just been settled by the recent studies of azobenzenethiols with high-gap-impedance STM.46 The topographic differences correlated to the tail groups revealed that the topographic contours in SAM images were generated at the monolayer episurface.46 For all that, it is still imprudent to determine the height of nonconductive films through the tunneling distances. On the contrary, the AFM technique has been employed extensively for the topological characterization of nonconductive films.22,23,47 The deformation of surfaces by scanning is the only concern for the AFM measurement.28 In this study, we resolved this problem by fine adjustment of the height of the cantilever, by which the applied forces were minimized to avoid crushing of domains by scanning. Experimental Section Monolayer Preparation. Au(111)/mica substrates were prepared by the epitaxial growth of a 100 nm gold film onto freshly cleaved mica sheets. Gold was thermally deposited on mica which was preheated to 580 °C48 for 3 h in a vacuum chamber (Vieetech Japan Co. Ltd.). Deposition rates were controlled to 0.5-1 Å/s under a vacuum pressure of 10-7-10-8 Torr. After deposition, the substrates were annealed at the same temperature (580 °C) in a vacuum chamber for 1.5 h. This procedure produced an atomically flat Au(111) surface with single crystal grains measuring 500-1000 nm in diameter. Au(111) surfaces were removed from the vacuum chamber immediately before use and immersed into freshly prepared alkanethiol solutions within 10 min after exposure to air. Several Au(111)/mica substrates were prepared simultaneously in order to ensure identical experimental conditions for each sample. n-Alkanethiols used here were purchased from Aldrich (1-butanethiol (C4H9SH), 1-octadecanethiol (C18H37SH)) and Wako Pure Chemical Industries, Ltd. (1-dodecanethiol (C12H25SH)), respectively. (36) Miyano, K.; Tamada, K. Langmuir 1992, 8, 160. Miyano, K.; Tamada, K. Langmuir 1993, 9, 508. Tamada, K.; Miyano, K. Jpn. J. Appl. Phys. 1994, 33, 5012. (37) Tamada, K.; Kim, S.; Yu, H. Langmuir 1993, 9, 1545. (38) Ho¨nig, D.; Mo¨bius, D. J. Phys. Chem. 1991, 95, 4590. (39) He´non, S.; Meunier, J. Rev. Sci. Instrum. 1991, 62, 936. (40) Tamada, K.; Minamikawa, H.; Hato, M.; Miyano, K. To be submitted. (41) Stranick, S. J.; Parikh, A. N.; Tao, Y.-T.; Allara, D. L.; Weiss, P. S. J. Phys. Chem. 1994, 98, 7636. (42) Iwakabe, Y.; Kondo, K.; Oh-hara, S.; Mukoh, A.; Hara, M.; Sasabe, H. Langmuir 1994, 10, 3202. (43) Han, T.; Beebe, T. P., Jr. Langmuir 1994, 10, 2705. (44) Edinger, K.; Go¨lzha¨user, A.; Demota, K.; Wo¨ll, Ch.; Grunze, M. Langmuir 1993, 9, 4. (45) Du¨rig, U.; Zu¨ger, O.; Michel, B. Phys. Rev. B. 1993, 48, 1711. (46) Wolf, H.; Ringsdorf, H.; Delamarche, E.; Takami, T.; Kang, H.; Michel, B.; Gerber, Ch.; Jaschke, M.; Butt, H.-J.; Bamberg, E. J. Phys. Chem. 1995, 99, 7102. Takami, T.; Delamarche, E.; Michel, B.; Gerber, Ch.; Wolf, H.; Ringsdorf, H. Langmuir 1995, 11, 3876. (47) Ohnishi, S.; Hara, M.; Furuno, T.; Sasabe, H. Biophys. J. 1992, 63, 1425. Ohnishi, S.; Hara, M.; Furuno, T.; Okada, T.; Sasabe, H. Biophys. J. 1993, 65, 573. (48) It corresponds to the temperature of the electric heater which contacts the sample holder from the back side, so that the temperature of the mica surface must be 100-200 °C lower than that of the heater.

1560 Langmuir, Vol. 13, No. 6, 1997 (1) Kinetics Studies. For the kinetics studies, ethanol solutions containing a single component alkanethiol (C4H9SH, C12H25SH, or C18H37SH) at a concentration of 10-2 mM were prepared by 100 times dilution of freshly prepared 1 mM alkanethiol solutions. The SAMs were prepared simultaneously by immersing each Au(111)/mica substrate (1 cm × 1 cm) into a separate glass weighing bottle containing 5 mL of alkanethiol solutions for different immersion times (1 s to 1 h) at room temperature.49 The immersion time was chosen following the procedure described by Bain et al.2 According to their study, which employed ellipsometry and contact angles,2 nearly 90% of bare gold surfaces was covered with C18H37SH by 10 min of immersion in 10-2 mM ethanol solutions. At the designated time, the substrates were quickly removed from the solutions and immediately rinsed with absolute ethanol to remove droplets of thiol solution before drying the surfaces in a steam of N2 in the same manner as reported in previous papers.2 All AFM imaging was accomplished within 1 day of sample preparation. (2) Mixed SAMs. Mixed SAMs were formed by exposing the Au(111) surfaces to ethanol solutions containing two kinds of alkanethiols having different chain lengths (C4H9SH/C18H37SH). The thiol compositions (mole fraction of two thiols) in solution were varied widely, Rsoln ) [C4H9SH]/[C18H37SH] ) 1/1 to 100/1, while the total concentration of thiols in solution was kept constant at 1 mM. Adsorption was carried out in separate glass weighing bottles with 5 mL mixed solutions for an immersion time of 1 h at room temperature. The substrates were rinsed with absolute ethanol and dried in a stream of N2 in the same manner as the kinetic measurements. According to the previous reports,4,8 which employed XPS, ellipsometry, contact angles, etc., the mixed SAMs (C4H9SH/C18H37SH coadsorbed films) from ethanol solution are expected to be formed in the the composition range 10/1 e Rsoln e 100/1. AFM Imagings. The AFM system used in this study was a commercially available NanoScope II (Digital Instruments, Inc., Santa Barbara, CA). The measurements were performed in air at room temperature by use of a Si3N4 cantilever (V-shaped and 200 µm long) with a spring constant of 0.12 N/m. All images (400 × 400 pixels) were collected in the “height mode” of a NanoScope II, which kept the force constant. The applied force during the AFM imaging was minimized to adjust the “setpoint voltage” to the lower limit for imaging, which corresponds to setting the height of the cantilever as far from the surface as possible.47 All images were taken at the scanning rate 3.474.34 Hz and were processed using only a “planefit” program in NanoScope II.

Results and Discussion 1. AFM Images of SAM Growth at the Initial Stage. Figures 1-3 are typical AFM images of C12H25SH, C18H37SH, and C4H9SH, respectively, adsorbed on Au(111) from the 10-2 mM ethanol solutions at different immersion times. In all cases, the substrates are partially covered with the thiols after an immersion time of 1 min or less. In these AFM images, domain formation is obvious. In Table 1, we collected the height of domains obtained from the cross section of the original AFM images of Figure 1b, Figure 2b, and Figure 3b (10 s immersion). The height of domains nearly agreed with the theoretical film thickness when a molecular tilt of 30° was assumed. This result suggests that these biphasic films were composed of the condensed thiol phase (so-called “liquid” or “solid” phase) and bare gold surface (so-called “gas” phase), i.e., the brighter part in the images corresponds to the condensed phase, while the darker part corresponds to the gas phase.50 The area fraction of the condensed phase increased as the immersion time became longer, and finally, the whole surfaces were covered with the con(49) The glass weighing bottles were cleaned in acid bath (concentrated H2SO4) for a week and rinsed with distilled, deionized water and absolute ethanol in an ultrasonic bath. They were further rinsed with the thiol solutions once before use to avoid fluctuation of solution concentration by adsorption on glass walls. All glassware used in this study was washed in a same manner. (50) Adamson, A. W., Ed. Physical Chemistry of Surfaces; John Wiley & Sons, Inc.: New York, 1991.

Tamada et al. Table 1. Height of n-Alkanethiol Domains (CnH2n+1SH) CnH2n+1SH

film thickness (theoretical value),a nm

domain height,b nm

n ) 18 n ) 12 n)4

2.3 1.7 0.8

2.1-2.3 1.7-2.0 1.0-1.4

a Film thickness expected theoretically for a close-packed monolayer tilted 30° from the normal to the surface. b Height of domains obtained experimentally from the AFM images.

densed phase at an immersion time of 3 min or longer. The adsorption rate of alkanethiols estimated by the AFM images was consistent with the reference value,2 although, of course, the adsorption rate may not precisely be determined from the images. Those features of SAM growth are not affected by rinsing time (1 s to 10 min). Note that the alkanethiol SAMs exhibit domain formation, in spite of the Langmuir-type adsorption kinetics suggesting random adsorption on Au(111). This indicates a rearrangement of the adsorbed thiols during the adsorption process and, furthermore, that the lateral interaction between adsorbed molecules was large enough to induce such molecular rearrangements. Evidently the lateral diffusion of thiol molecules on Au(111) and/or the selective exchanges of thiol molecules between surface and solution took place within the duration of the experiment. Considering the mobility of adsorbed molecules on the solid, these domains may be formed on the physisorption stage, that is, before the covalent bonding between sulfur and gold atoms (i.e., chemisorption) is completed.51 Let us now look at the detailed film morphology. For the C12H25SH SAMs, the surface deposit was already inhomogeneous after 1 s of immersion (Figure 1a); however, the outline of the domains was not clear and their heights (0.6-0.8 nm) were much lower than the expected film thickness (1.7 nm, see Table 1). The clear island formation appeared after 10 s of immersion (Figure 1b), and the domain size was estimated to be 20-40 nm in diameter. Over 50% of the Au(111) surface was covered with the condensed phase after an immersion time of 30 s (Figure 1c), whereupon the domains coalesced and turned into the continuous phase. The shape of the domains differed with differing alkyl chain length. The C18H37SH SAMs exhibit smaller islands than the C12H25SH SAMs (Figure 2b), while the C4H9SH formed the continuous meshlike domains (Figure 3b). The similarity of the behavior of this morphology to the thermodynamic behavior of Langmuir films52 invites comparison, feasible only if these SAMs could be regarded as in equilibrium, i.e., if a rapid equilibration could proceed for physisorbed thiols resulting in an equilibrium even in the chemisorbed monolayers. According to the AFM images at the same immersion time (see parts a and b of Figures 1-3), the adsorption rate of the shorter chain thiols appeared rather faster than that of the longer ones at the initial stage of SAM growth (C4H9SH > C12H25SH > C18H37SH), as opposed to the previous reports.2,21 Ulman suggested the kinetics is due to the fact that van der Waals interactions are a function of the chain length.21 We propose the discrepancy in the reports may be caused by the different characterization techniques. They determined the adsorption kinetics by use of the contact angles,2,21 while we did that (51) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. Soc. 1987, 109, 733. Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J. Am. Chem. Soc. 1987, 109, 2358. (52) Toshev, B. T.; Platikanov, D.; Scheludko, A. Langmuir 1988, 4, 189. McConnell, H. M.; Keller, D.; Gaub, H. J. Phys. Chem. 1986, 90, 1717.

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Figure 1. AFM images of C12H25SH SAMs on Au(111) produced by different immersion times in 10-2 mM ethanol solutions: (a) 1 s; (b) 10 s; (c) 30 s; (d) 3 min. The terraces observed in all images (0.5-1 µm in diameter) correspond to the atomically-flat Au(111) single crystalline surfaces. In the images of the inhomogeneous films (a-c), the brighter part on Au(111) corresponds to the condensed thiol phase (so-called “liquid” or “solid” phase), while the darker part corresponds to the dilute phase (so-called “gas” phase or bare Au(111) surface). For immersion times of 3 min or longer ((d)), the surfaces are almost fully covered with the liquid or solid phase.

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Figure 2. AFM images of C18H37SH SAMs on Au(111) after different immersion times in 10-2 mM ethanol solutions: (a) 1 s; (b) 10 s; (c) 30 s; (d) 3 min. The explanation of images followed that in Figure 1.

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Figure 3. AFM images of C4H9SH SAMs on Au(111) after different immersion times in 10-2 mM ethanol solutions: (a) 1 s; (b) 10 s; (c) 30 s; (d) 3 min. The explanation of images follows that in Figure 1.

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Figure 4. Typical STM image of C12H25SH SAMs on Au(111) at the immersion time of 10 min. Imaging conditions: Pt/Ir tip, 1000 mV (tip positive), 0.1 nA, in air. The image shows the nearly hexagonal lattice having a nearest-neighbor distance of 0.5 nm.

by the AFM imaging. The contact angles mainly detect the molecular ordering at the alkyl chain ends (gaucheand trans-conformations), so that is sensitive to the solid-

Tamada et al.

liquid phase transition, while we characterize the liquidgas transition by a true spatial method. Thus, the adsorption stage investigated is different at each case. Our result suggests that the initial stage of SAM growth is the diffusion-controlled process unlike the later adsorption stage, and as is generally known, the diffusion favors the shorter chain molecules.3,37,53 We also took STM images of C12H25SH SAMs to confirm the molecular-level structure at the initial stage of SAMs growth. The STM observation was performed at room temperature with Pt/Ir tips at a tunnel gap bias voltage of 800-1200 mV (tip negative) and a constant tunneling current of 0.1-0.5 nA (NanoScope II). In the partially covered SAMs (Figure 1a-c), no clear images were observed probably due to the thermal fluctuation of molecules. The regular patterns of the nearly hexagonal lattice (nearest-neighbor spacing ∼0.5 nm) were obtained only in the fully covered SAMs, which had been immersed for 10 min and longer (Figure 4). These data imply that the condensed phase changes from liquid to solid after the surfaces are fully covered with thiol molecules in air. This result is identified with the molecular ordering process reported by Ha¨hner et al.,12 in which the entangled alkyl chains, produced by a first rapid adsorption, were gradually straightened at the slow, second step of adsorption. We intend to make further experiments for this solid-liquid phase transition in combination with diffraction13-15 or IR.4,32

Figure 5. AFM images of the mixed SAMs coadsorbed from the ethanol solution at various thiol composition for 1 h: (a) Rsoln ) ([C4H9SH]/[C18H37SH]) ) 20/1; (b) Rsoln ) 40/1. In the images, the brighter islands correspond to the C18H37SH phase, while the matrix sea corresponds to the C4H9SH phase.

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Figure 6. AFM image and the cross section in the image of the C4H9SH/C18H37SH mixed SAMs (Rsoln ) 40/1).

2. AFM Images of Mixed SAMs. The film morphology of coadsorbed SAMs from the C4H9SH/C18H37SH mixed solutions was studied with AFM at the thiol composition in solution, Rsoln ) [C4H9SH]/[C18H37SH] ) 1/1, 10/1, 20/1, 40/1, 100/1. The mixed SAMs at Rsoln ) 1/1 and 10/1 exhibited homogeneous film morphology, in which no domains due to phase separation were observed. The film

morphology changed drastically at Rsoln ) 20/1 and 40/1, in which clear domains were observed as shown in Figure 5. No domain structures appeared at Rsoln ) 100/1. These AFM images are consistent with the coadsorption phe(53) Landau, L. D.; Lifshitz, E. M. Fluid Mechnics; Pergamon Press: Elmsford and New York, 1959.

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nomena in the mixed SAMs reported previously3-5,7,8 whereby the film composition in the mixed SAMs changed abruptly at the sharp boundary of an intermediate range of Rsoln, above and below which, nearly single component SAMs were formed due to the strong intermolecular interaction between the same molecules. On the basis of this explanation, the homogeneous films at Rsoln ) 1, 10/1 and at Rsoln ) 100/1 correspond to a single component SAMs composed of C4H9SH or C18H37SH, respectively, and the inhomogeneous films at Rsoln ) 20/1, 40/1 correspond to the C4H9SH/C18H37SH coadsorbed SAMs. In Figure 5, the number of domains changed significantly according to the Rsoln value, which justifies a possibility that the brighter islands were the C18H37SH phase, while the matrix sea was the C4H9SH phase. A cross section of the mixed SAMs at Rsoln ) 40/1, shown in Figure 6, reveals the height of domains (∆d ) 1.0-1.4 nm) to be in agreement with the difference of the film thickness between C4H9SH and C18H37SH (see Table 1), consistent with our assumption. The fact that the shape and size of domains are quite similar to those of the single component C18H37SH SAMs (Figure 2b) may also suggest these islands were composed of nearly single component C18H37SH.54 These domain sizes, 10 - 20 nm across, are identical to those estimated by IR and contact angles.3-5,7 In this paper, we have resolved the long standing issue concerning the phase separation in the mixed SAMs using a true spatial method. We have been studying the immersion time and solution concentration dependence of the film morphology in the mixed SAMs as well as (54) Recently, our research group succeeded in obtaining the XPS data which supports the phase separation in those mixed films. Ishida, T.; Hara, M.; Sasabe, H.; Knoll, W. To be submitted.

Tamada et al.

investigating the kinetic contribution to the coadsorption phenomena. The result will be reported in a separate paper.55 The influence of alkyl chain length on coadsorption phenomena will also be discussed in it. Conclusions The adsorption kinetics of single component SAMs and the coadsorption phenomena for the mixed SAMs on Au(111) were studied with AFM. The AFM images revealed the clear domain formations at the initial stages of SAM growth, which implies the rearrangement of thiols on the surface due to the lateral interaction between adsorbed thiols. It was also confirmed that the initial stage of adsorption was the diffusion-controlled process; the adsorption rate of shorter chain molecules was rather faster than that of the longer chain ones. For the study of the mixed SAMs, the clear domain formations due to the phase separation were determined in the AFM images, while the problem of the film equilibrium is still open.7 For a better understanding of adsorption phenomena of thiols, it may be very important to understand the following two processes separately: the lateral diffusion of thiol on the surface and the exchange of thiols between the surface and in solution. It is also necessary to develop an in situ technique for direct observation. Acknowledgment. We thank Professor M. Grunze of Angewandte Physikalische Chemie in Germany for providing us copies of references and valuable discussions. LA950630S (55) Tamada, K.; Hara, M.; Sasabe, H.; Knoll, W. To be submitted.