Rapid Self-Assembly of Alkanethiol Monolayers on Sputter-Grown Au

Atomically flat, sputter-grown Au(111) films allowed well-ordered alkanethiol ... Physical Structure of Standing-Up Aromatic SAMs Revealed by Scanning...
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Langmuir 2000, 16, 1719-1728

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Rapid Self-Assembly of Alkanethiol Monolayers on Sputter-Grown Au(111) Mitsuo Kawasaki,* Tomoo Sato, Takumi Tanaka, and Kazunori Takao Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Yoshida, Kyoto 606-8501, Japan Received March 16, 1999. In Final Form: October 29, 1999

Atomically flat, sputter-grown Au(111) films allowed well-ordered alkanethiol (exemplified by octanethiol) monolayers to be self-assembled from solution markedly faster and in larger domain sizes than previously reported. An X-ray photoelectron spectroscopy analysis showed that complete monolayer coverage was reached by 0.2-60 min of incubation in 0.1-0.001 mM ethanolic solution at room temperature (∼17 °C), with single-step (0.1 and 0.01 mM) or two-step (0.001 mM) adsorption kinetics. Increasing the temperature to 35 °C was enough to cause a single-step, diffusion-controlled adsorption also from the 0.001 mM solution, yielding the full monolayer coverage in approximately 10 min. Scanning tunneling microscopy (STM) imaging proved that well-ordered islands, with the (x3 × x3)R30° structure more-or-less strongly modulated by the c(4 × 2) superlattice, begin to form at 0.6-0.7 monolayer coverage most likely by homogeneous nucleation and grow rapidly thereafter. This kinetics of ordering requiring the considerably high threshold coverage for the nucleation, but allowing the fast growth of the nuclei was independently confirmed by the infrared reflection absorption spectroscopy. A typical c(4 × 2) domain size at the saturation coverage was estimated to be no less than 10-15 nm, and the structural identity often seemed not to be disrupted even across the etch pits. This superior structural order is reflected on the highest level of molecular resolution achieved by the in-air STM imaging. The expected registry of the (x3 × x3)R30° or c(4 × 2) lattice with that of Au(111) was also confirmed. On Au films that were also sputter-grown but no longer atomically flat, we observed at least by 1 order of magnitude slower self-assembly.

Introduction Over the past decade there has been considerable interest in self-assembled monolayers (SAMs) of organothiols on metal surfaces, most notably alkanethiols on Au(111), which potentially allow one to tailor and optimize the surface properties for a variety of technological applications as well as for fundamental studies of surface phenomena.1-3 The purpose of this paper is to introduce a markedly rapid self-assembly of densely packed alkanethiol monolayers on a sputter-grown, extensively terraced Au(111) film surface. There are several different methods to prepare nominally flat Au(111) substrates suitable for the study of alkanethiol SAMs on gold. Single crystals sputter-cleaned and thermally annealed or flame-annealed have been used in some of the previous scanning tunneling microscopy (STM) studies4-7 as well as for the diffraction7-11 and sumfrequency generation12 studies. For experiments that only * Corresponding author. Tel/fax: (+81)-75-753-5540. E-mail: [email protected]. (1) Ulman, A. J. Mater. Educ. 1989, 11, 205. (2) Ulman, A. Chem. Rev. 1996, 96, 1533. (3) Poirier, G. E. Chem. Rev. 1997, 97, 1117. (4) Edinger, K.; Go¨lzha¨user, A.; Demota, K.; Wo¨ll, Ch.; Grunze, M. Langmuir 1993, 9, 4. (5) Poirier, G. E.; Tarlov, M. J. Langmuir 1994, 10, 2853. (6) Poirier, G. E.; Tarlov, M. J.; Rushmeier, H. E. Langmuir 1994, 10, 3383. (7) Camillone, N.; Eisenberger, P.; Leung, T. Y. B.; Schwartz, P.; Scoles, G.; Poirier, G. E.; Tarlov, M. J. J. Chem. Phys. 1994, 101, 11031. (8) Camillone, N.; Chidsey, C. E. D.; Liu, G.-Y.; Scoles, G. J. Chem. Phys. 1993, 98, 3503. (9) Fenter, P.; Eisenberger, P.; Liang, K. S. Phys. Rev. Lett. 1993, 70, 2447. (10) Fenter, P.; Eberhardt, A.; Eisenberger, P. Science 1994, 266, 1216. (11) Camillone, N.; Leung, T. Y. B.; Scoles, G. Surf. Sci. 1997, 373, 333. (12) Yeganeh, M. S.; Dougal, S. M.; Polizzotti, R. S.; Rabinowitz, P. Phys. Rev. Lett. 1995, 74, 1811.

require a relatively small surface area, as is typically the case with STM, Au(111) facets on a small Au ball, prepared by the method originally developed for reflection electron microscopy,13 are also available.14 The most popular Au substrate employed in the majority of the numerous works on alkanethiol SAMs is epitaxial Au films evaporated onto mica, for which the correlation between the surface morphology and the growth condition has been well established.15-20 In contrast sputtered Au films, even though easier to prepare than evaporated ones, have been thought to give much rougher surface morphology short of crystalline area20 and thus have been used only for some restricted purposes to study the SAM structures on the resultant polycrystalline Au surface.21,22 This criterion dictating an inferior crystalline surface of sputtered gold never applies to the film employed in this work, however. As described in detail elsewhere,23,24 our simple dc glowdischarge sputtering in an Ar atmosphere has allowed us to establish quite an easy preparation of atomically flat Au(111) films on mica that favorably compare to the (13) Hsu, T.; Cowley, J. M. Ultramicroscopy 1983, 11, 239. (14) Voets, J.; Gerritsen, J. W.; Grimbergen, R. F. P.; Kempen, H. V. Surf. Sci. 1998, 399, 316. (15) Christopher, E. D.; Chidsey, C. E. D.; Loiacono, D. N.; Sleator, T.; Nakahara, S. Surf. Sci. 1988, 200, 45. (16) Emch, R.; Nogami, J.; Dovek, M. M.; Lang, C. A.; Quate, C. F. J. Appl. Phys. 1989, 65, 79. (17) Putnam, A.; Blackford, B. L.; Jericho, M. H.; Watanabe, M. O. Surf. Sci. 1989, 217, 276. (18) DeRose, J. A.; Thundat, T.; Nagahara, L. A.; Lindsay, S. M. Surf. Sci. 1991, 256, 102. (19) Inukai, J.; Mizutani, W.; Saito, K. Jpn. J. Appl. Phys. 1991, 30, 3496. (20) Golan, Y.; Margulis, L.; Rubinstein, I. Surf. Sci. 1992, 264, 312. (21) Butt, H.-J.; Seifert, K.; Bamberg, E. J. Phys. Chem. 1993, 97, 7316. (22) Scho¨nherr, H.; Vancso, G. Y. Langmuir 1997, 13, 3769. (23) Kawasaki, M.; Uchiki, H. Surf. Sci. 1997, 388, L1121. (24) Kawasaki, M. Appl. Surf. Sci. 1998, 135, 115.

10.1021/la990310z CCC: $19.00 © 2000 American Chemical Society Published on Web 12/28/1999

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Figure 1. ∼1000 × 900 nm2 STM images showing the surface morphologies of (a) atomically flat Au(111) sputter-grown in optimum deposition conditions and (b) polycrystalline Au film grown at room temperature. The maximum height difference measured only 2.5 nm in (a) while 25 nm in (b).

smoothest Au(111) films18 ever grown by vacuum evaporation (cf. Figure 1a). The growth kinetics, molecular order, and domain structures of whatever SAMs on gold cannot be independent of the microscopic structures and crystalline order of the substrate. Thus the method of Au(111) surface preparation must be an important factor influencing the order of the SAMs, as suggested by Tousov and Gorman,25 who addressed the advantage of the Au(111) surface prepared by peeling from mica in producing SAMs with a higher degree of perfection. One naturally expects also that the more perfectly the Au(111) substrate is prepared, the more reliable the experimental characterization of SAM structures and the kinetics of their formation. The level of flatness of our sputter-grown Au(111) is encouraging in this respect, and the ease of preparation and the considerably different physical environment during the film growth24 from that in vacuum evaporation also strengthen the interest in testing its quality as the Au(111) substrate. For practical application of alkanethiol SAMs it is desirable that high-quality monolayers can be grown as fast as possible. Thus the growth kinetics of SAMs and their local defect structures have been studied extensively by using a variety of analytical methods such as ellipsometry and contact angle measurement,26 X-ray photoelectron spectroscopy (XPS),27-29 second harmonic generation,27 infrared reflection absorption spectroscopy (IRRAS),30-33 quartz crystal microbalance,34 atomic force (25) Touzov, I.; Gorman, C. B. J. Phys. Chem. B 1997, 101, 5263. (26) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (27) Buck, M.; Eisert, F.; Fischer, J.; Grunze, M.; Trager, F. Appl. Phys. A. 1991, 53, 552. (28) Ishida, T.; Nishida, N.; Tsuneda, S.; Hara, M.; Sasabe, H.; Knoll, W. Jpn. J. Appl. Phys. 1996, 35, L1710. (29) Ishida, T.; Hara, M.; Kojima, I.; Tsuneda, S.; Nishida, N.; Sasabe, H.; Knoll, W. Langmuir 1998, 14, 2092. (30) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558. (31) Laibinis, P. E.; Whitesides G. M.; Allara, D. L.; Tao, Y.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152. (32) Truong, K. D.; Rowntree, P. A. Prog. Surf. Sci. 1995, 50, 207.

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microscopy (AFM),35,36 and STM.4-7,25,37-40 However, as Bensebaa and co-workers33 pointed out, there have been some conflicting experimental results regarding, in particular, the time required for the ordered monolayer formation. According to the earlier work of Bain and coworkers,26 the kinetics of formation of alkanethiol SAMs on Au(111) is characterized by two distinct phases, the initial, rapid adsorption (completed well within 1 min at thiol concentrations below 1 mM) to 80-90% of the maximum coverage being followed by a slower process of additional adsorption and consolidation that last even many hours. A number of STM studies of alkanethiol SAMs on Au(111),3 especially those which employed high tunneling-junction impedance,5,6,37,39 have also testified that some long incubation period over several hours and even additional annealing are often necessary to grow sufficiently ordered SAMs. In such well-ordered SAMs densely packed in the commensurate (x3 × x3)R30° structure, the c(4 × 2) superlattice, which was independently confirmed earlier by He diffraction8 and grazingincidence X-ray diffraction,9 has been routinely observed by the high-impedance STM imaging. On the other hand, the IR-RAS studies of Truong and Rowntree32 and Bensebaa and co-workers33 suggested a much shorter time scale of 1-10 min for the ordered SAM formation even from a micromolar thiol solution. The preparation of SAMs by the microcontact printing method,41 which is essentially a dry process, may also be counted as another example of rapid SAM formation, requiring the contact time of just ∼10 s between the Au(111) substrate and the stamp inked with thiols. It should be also noted that even the basic chemisorption kinetics of alkanethiols on gold has been given different interpretations, i.e., Langmuir adsorption kinetics27,34 versus diffusion-controlled one.42 There have been some diverse observations also with respect to the long-range structural integrity of the SAMs on Au(111). The previous high-resolution STM studies of solution-deposited SAMs5,6,37,39 suggested that without extended thermal annealing the individual (x3 × x3)R30° or c(4 × 2) domains could be only 5-10 nm across, separated by various kinds of domain boundaries and pinned by the chemisorption-induced vacancy islands (or etch pits) of the Au(111) substrate. On the basis of their He diffraction study, Camillone and co-workers have suggested that the surface order was generally enhanced (the average domain size being increased to ∼20 nm) by using micromolar solutions for preparation of the monolayer, yet the micromolar growth was no guarantee of sample quality.11 Tousov and Gorman showed, on the other hand, that apparently much more uniform, depressionfree SAMs of decanethiol could be grown by vapor-phase deposition on Au(111) peeled from mica.25 Also, though in the overall low-coverage regime where a large fraction of (33) Bensebaa, F.; Voicu, R.; Huron, L.; Ellis, T. H.; Kruus, E. Langmuir 1997, 13, 5335. (34) Karpovich, D. S.; Blanchard, G. J. Langmuir 1994, 10, 3315. (35) Tamada, K.; Hara, M.; Sasabe, H.; Knoll, W. Langmuir 1997, 13, 1558. (36) Xu, S.; Cruchon-Dupeyrat, S. J. N.; Garno, J. C.; Liu, G.-Y.; Jennings, G. K.; Yong, T.-H.; Laibinis, P. E. J. Chem. Phys. 1998, 108, 5002. (37) Delamarche, E.; Michel, B.; Gerber, Ch.; Anselmetti, D.; Gu¨ntherodt, H. J.; Wolf, H.; Ringsdorf, H. Langmuir 1994, 10, 2869. (38) Sondag-Huethorst, J. A. M.; Scho¨nenberger, C.; Fokkink, L. G. J. J. Phys. Chem. 1994, 98, 6826. (39) Scho¨nenberger, C.; Jorritsman, J.; Sondag-Huethorst, J. A. M.; Fokkink, L. G. J. J. Phys. Chem. 1995, 99, 3259. (40) Yamada, R.; Uosaki, K. Langmuir 1998, 14, 855. (41) Larsen, N. B.; Biebuyck, H.; Delamarche, E.; Mchel, B. J. Am. Chem. Soc. 1997, 119, 3017. (42) Schlenoff, J. B.; Li, M.; Ly, H. J. Am. Chem. Soc. 1995, 117, 12528

Alkanethiol Monolayers on Sputter-Grown Au(111)

the surface is left with less-ordered monolayers, Larsen and co-workers showed that the microcontact printing method allowed discrete (x3 × x3)R30° or c(4 × 2) domains to grow to remarkably large sizes of 50-200 nm.41 The significant variation in the experimental results as mentioned above is not at all discouraging but rather suggests that, by optimizing the Au(111) surface preparation and/or the method of SAM deposition, high-quality SAMs with possibly a submicrometer length scale order could be grown on the time scale far less than 1 h. The observation of a rapid self-assembly of densely packed alkanethiol SAMs on our sputter-grown Au(111), within the framework of the standard solution-phase deposition, further solidifies this expectation and provides some new insight into the mechanism of high-quality SAM formation. In this paper we will concentrate on octanethiol (OT) SAMs, which, with the moderate length of the C8 alkyl chain, can be easily ordered in the c(4 × 2) superlattice structure and allow a high-resolution STM imaging at a relatively low tunneling-junction impedance less than 10 GΩ,5,39 the maximum available impedance in our STM microscope. In addition to STM we also used XPS and IR-RAS methods to characterize the kinetics of chemisorption and consolidation of OT SAMs. The consistency of the results obtained by these combined physical methods greatly helps us to avoid erroneous conclusions. We show that almost complete monolayer coverage of OT SAM occurs well within a few minutes e.g., in a 0.01 mM solution at room temperature, with concurrent evolution of the c(4 × 2) superlattice that looks more uniform in larger domain sizes than previously observed for solution-deposited SAMs. This largely contributed to the rather unexpected high resolution achieved in the STM imaging in the ambient atmosphere. Some comparative results obtained for Au substrates that were also sputtergrown but in away from the optimum deposition conditions, and thus no longer atomically flat, are also presented. Experimental Section Preparation of Au(111) Film. The Au(111) film was grown on a freshly cleaved mica (obtained from The Nilaco Corp.), ∼0.1 mm thick and ∼3 × 4 cm2 in area, at ∼300 °C by using the simple dc glow-discharge sputtering method as described in detail elsewhere.23,24 In short the glow discharge was allowed in a pure (99.999%) Ar gas atmosphere of 0.15-0.2 Torr, the corresponding steady-state Ar-gas flow rate being suppressed to ∼1.5 mL/min. The cathode (99.99% gold) voltage was typically ∼0.9 kV (negative), resulting in the total discharge current of ∼8 mA. The corresponding average deposition rate was 40-50 Å/min. As opposed to the conventional vacuum evaporation for which the importance of several hours of substrate preheating at ∼500 °C has been noted,18 the corresponding step in our sputter deposition was only ∼30 min long to wait for the substrate to reach the steady-state deposition temperature of ∼300 °C. Even so, in almost every aspect our sputter-grown Au(111) films favorably compare with the smoothest Au(111) film ever grown by vacuum evaporation. The as-grown Au(111) films, once taken outside the deposition chamber and exposed to the laboratory atmosphere, immediately get contaminated by some hydrocarbon species, as easily detected by XPS. The contamination level was judged to be insignificant for the kinetics of strong chemisorption of alkanethiols onto Au(111) but not for the IR-RAS study as mentioned later. The intentionally roughened Au substrate was prepared also by using the sputter deposition on mica but at room temperature. The substrate was preheated to ∼300 °C in the Ar atmosphere and then cooled back to the room temperature before deposition. The resultant Au film was still (111) oriented (according to an X-ray diffraction analysis), but the surface morphology turned to a rolling-hill structure no longer atomically flat. A couple of STM images that illuminate a large difference in the surface

Langmuir, Vol. 16, No. 4, 2000 1721 morphology between the two types of Au films is shown in Figure 1. The morphology of the film deposited at room temperature is similar to those of evaporated gold films prepared at substrate temperatures below ∼200 °C.17 Preparation of SAMs. Alkanethiols [octanethiol (OT) and 2-naphthalenethiol (NPT) for use in the IR-RAS study] were purchased from Wako Pure Chemical Industry, Ltd., and used without further purification in the form of 0.1-0.001 mM solution in ethanol (guaranteed reagent grade) as received from Nacalai Tesque, Inc. As-deposited Au(111) films were bathed in each solution for desired periods either at room temperature or other controlled temperatures, rinsed by running ethanol, and then dried in air after visible droplets had been blown off. XPS Analysis. All XPS data were taken in an ESCA-750 spectrometer (Shimadzu Corp.) with Mg KR radiation of 1253.6 eV for samples typically ∼6 × 6 mm2 in area. The photoemission angle was fixed at 90°. The binding energies for respective signals were corrected by referring to the peak of the Au4f(5/2) signal from the Au(111) substrate as the common internal reference being placed at the fixed position of 84.0 eV. STM Imaging. All STM images were taken in the ambient atmosphere by using a Nanoscope I microscope (Digital Instruments Inc.). Pt/Ir tips obtained from Digital Instruments Inc. were mechanically cut with a nipper before use so as to gain the optimum resolution. This allowed a single tip to be refreshed repeatedly and to give better resolution than did as-received tips. The microscope was operated under the constant current mode with the minimum set current of 0.1 nA. The sample bias was typically -600 mV with respect to the tip, which in the combination with the minimum set current ensures a tunnelingjunction impedance of 6 GΩ. The scan rates along the x and y directions were 8-13 and 0.033-0.050 Hz, respectively, irrespective of the scan width. The real-time changes in the piezo drive voltages during each scan were sampled at regular time intervals by using a multichannel A-to-D converter and recorded as series of digital data in a personal computer. Height-mapped gray scale STM images were produced from ∼150 × 150 data points at maximum, thereby molecularly resolved images could be obtained for scan widths up to ∼30 nm across. IR-RAS Measurement. IR-RAS spectra were taken by using a FTS-30 FTIR spectrometer (Bio-Rad Laboratories Inc.) with a p-polarized light, the incident angle being fixed at 72° with respect to the sample normal. The incident beam was narrowed down to 8 mm in diameter, which on the sample plane was elongated to ∼25 mm in one direction at the above incident angle. Thus the sample film was sized to ∼20 × 35 mm2 so as to confine the whole elongated light spot inside the film plane. The spectrometer resolution was adjusted to 4 cm-1, thereby the IR-RAS data were collected at 2 cm-1 intervals. In the IR-RAS measurement the choice of a proper reference is vital to gain quantitative information. The as-grown Au(111) film is not appropriate for such a reference, because even submonolayer levels of hydrocarbon contaminants can affect significantly the spectra in the C-H stretch region, most relevant in the characterization of alkanethiol SAMs. We have therefore chosen an Au(111) film covered with NPT SAMs as an alternative reference with no characteristic absorptions in the region of interest (2700-3000 cm-1). It was confirmed by XPS that NPT was as strongly chemisorbed on Au(111) as simple alkanethiols and thus equally capable of replacing the adventitious hydrocarbons.

Results and Discussion Kinetics of Adsorption. For the highly planar Au(111) film as used in this work the XPS method seems to work best of all in estimating the absolute coverage of alkanethiol SAMs. For this purpose we have used the area intensity ratio of S2p to Au4f rather than the C/Au ratio,28,29 because the latter can be easily affected by the presence of adventitious carbon species especially for alkanethiols with relatively short alkyl chains. When the sulfur headgroup of alkanethiol is directly bonded to the Au surface by forming a thiolate, the S2p and Au4f signals are expected to undergo virtually equal intensity attenu-

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Figure 2. (a) Incubation-time dependence of thiol coverage on sputter-grown Au(111) for three different concentrations of octanethiol solution at room temperature (∼17 °C), as expressed by S2p to Au4f area intensity ratio as a function of log incubation time. (b) Examples of S2p and Au4f spectra taken for octanethiol SAMs on Au(111), used to estimate the thiol coverage.

ation by whatever kinds of overlayers. Thus the S2p/Au4f intensity ratio should be dependent only on the absolute thiolate coverage irrespective of the alkyl chain length and the level of contamination. More specifically, given that all sulfur atoms are arranged in the (x3 × x3)R30° structure at the full monolayer coverage with the density of 4.6 × 1014 atoms/cm2, the corresponding S2p/Au4f intensity ratio can be readily calculated from the mean free path of photoelectrons in Au (∼13 Å for the kinetic energy of 1200 eV;43 the equivalent Au slab44 contains 7.6 × 1015 Au atoms) and the photoionization cross section45 and instrumental sensitivity factor46 for each signal. The thus calculated, theoretical S2p/Au4f intensity ratio for the saturation coverage was 0.0061. The S2p/Au4f intensity ratio measured as a function of incubation time in OT solutions with three different concentrations at room temperature (∼17 °C) is shown in Figure 2a. The typical XPS spectra from which the data of Figure 2a were obtained are shown in Figure 2b for reference. The main peak of the S2p spectrum [i.e., the spin-orbit split S2p(3/2) subpeak] is located at 162 eV, as expected for thiolate species.29 We have not clearly observed the signal around 161 eV in the whole series of spectra, which Ishida and co-workers recently reported as arising from isolated sulfur without C-S cleavage.29 Figure 2a shows that, except for the 0.001 mM solution, the adsorption of OT SAMs is virtually a single-step process rapidly leading to the saturation, where the S2p/ Au4f intensity ratio takes 0.0063. This is in fair quantita(43) Penn, D. R. J. Electron Spectrosc. 1976, 9, 29. (44) The total XPS intensity from a metal sample with exponentially decreasing contribution of atoms in a deeper position is equivalent to that which is obtained when all the atoms but only within a slab of the mean escape depth contribute equally without any intensity attenuation. (45) Scofield, J. H. J. Electron Spectrosc. 1976, 8, 129. (46) Provided by Shimadzu Corp.

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tive agreement with what was already calculated above, 0.0061, for thiolates arranged in the (x3 × x3)R30° structure. It can be seen that the rising parts of Figure 2a for the three different concentrations, as plotted in the logarithmic time scale, are almost parallel with each other and shifted from each other approximately in proportion to the concentration ratio. (The data for less than 3 s incubation in the 0.1 mM solution should be taken with care, because such a short treatment could not precisely be timecontrolled.) This behavior suggests that the adsorption rate of OT molecules onto our sputter-grown Au(111) is limited by diffusion in solution,42 at least up to the ∼0.6 monolayer coverage where some different kinetics apparently takes over in the case of 0.001 mM solution. By assumption of a typical molecular diffusion constant in ethanol47 of ∼1 × 10-5 cm2 s-1 and a hypothetical effective diffusion layer thickness of ∼0.1 mm48 under convective disruption, the diffusion-limited material transfer rate at the solid/liquid interface becomes ∼1 × 10-11 mol cm-2 s-1 in the case of, e.g., the 0.01 mM solution. At this rate the Au(111) surface would be fully covered by thiolates in about 100 s. This is the same order as that seen in Figure 2a, the 0.01 mM solution leading to the saturation coverage in ∼1 min. Within the framework of XPS analysis, this rate of octanethiol adsorption from 0.01 mM solution is also comparable to that which Ishida and co-workers reported earlier for dodecanethiol (C12) adsorption from 0.01 mM solution on vacuum-evaporated Au(111) based on the C/Au intensity ratio.28 They noted, however, that in the case of octadecanethiol (C18) adsorption even ∼1 h incubation was insufficient to reach the saturation coverage in the same experimental condition. We found, on the other hand, no such strong chain-length dependence as checked by choosing hexadecanethiol (C16), of which the adsorption kinetics paralleled with that shown in Figure 2. Such a diffusion-limited adsorption kinetics is not expected for substrates with the level of contamination that significantly counteracts with the chemisorption of thiols on gold.42 We therefore conclude that the adventitious hydrocarbon species on our sputter-grown Au(111), which are unavoidable in the laboratory atmosphere, are only weakly physisorbed and insignificant for the fast chemisorption kinetics of alkanethiols. In Figure 2a two distinct phases of adsorption, as suggested by Bain and co-workers,26 are certainly (and more clearly) visible for the lowest 0.001 mM solution. This exceptional behavior is strongly temperature sensitive, however, as shown in Figure 3. The initial rising part was scarcely affected by the change of incubation temperature up to 35 °C, while the slow adsorption process was virtually eliminated at 35 °C, causing a single-step adsorption at this low OT concentration as well. The extra incubation time to bring the system to the saturation coverage, as compared to the single-step adsorption at 35 °C, is found to be ∼15 and ∼45 min for 25 and 17 °C, respectively. Though crude, these values suggest that the underlying process may have an apparent activation energy as large as ∼100 kJ/mol. As discussed later in more detail, the alkyl chains of the thiolates are still poorly ordered at ∼0.6 monolayer coverage irrespective of the concentration of the solution from which they are adsorbed. The imperfect alkyl chains somewhat seriously entangled at this moderately high coverage will thus significantly retard further adsorption of thiols depending on the (47) CRC Handbook of Chemistry and Physics, 75th ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 1994; p 6-253. (48) Atkins, P. W. Physical Chemistry; Oxford University Press: Oxford, U.K., 1998; Chapter 29.

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Figure 3. As in Figure 2a, but for varied incubation temperatures for a fixed thiol concentration of 0.001 mM.

concentration of the solution and the incubation temperature. This is the most natural explanation for the twostep adsorption kinetics observed in the 0.001 mM solution at relatively low temperatures. Then the above-noted apparent activation energy of ∼100 kJ/mol may be associated with some dynamic motion of such disordered thiolates that helps additional thiols in solution to find their way to open adsorption sites. The results indicate, however, that the thiol concentration of 0.01 mM is high enough at room temperature to make such a thermal assistance apparently insignificant for completion of the full monolayer coverage. It is expected that occurrence of the saturation coverage corresponding to the (x3 × x3)R30° structure requires the hydrocarbon backbones to be sufficiently ordered concurrently, thus yielding an almost complete SAM structure. We next investigate this point in detail by STM imaging, thereby clarifying also at what coverage and how well-ordered SAMs begin to evolve. Evolution of Highly Ordered SAMs. Figures 4 and 5 show a couple of most impressive series of STM images that clearly demonstrate the nucleation of highly ordered SAMs. A relatively wide-scan image shown in Figure 4a, which was taken at ∼0.6 monolayer coverage (corresponding to 15 s incubation in 0.01 mM solution), shows the presence of some etch pits from place to place, but only in very limited areas could small ordered islands be detected, as exemplified by the high-resolution image presented in Figure 4b. The bright spots resolved within each uniaxially elongated island ∼5 nm long have ∼5 Å spacing along the elongation axis, which suggests that these islands consist of 2-4 rows of thiolates constrained in the (x3 × x3)R30° lattice. It is also important to note that the nucleation of these small islands is not spatially correlated with the etch pits, none of which are seen within the scan window of Figure 4b. Soon after the ∼0.7 monolayer coverage is reached, a number of two-dimensional islands, isotropically grown to 10-40 nm across, show up as stainlike patches in widescan STM images; see Figure 5a. That these islands (∼1.7 Å higher than the surrounding region) represent highly ordered (x3 × x3)R30° or c(4 × 2) domains of thiolates was verified by the high-resolution images taken inside an arbitrary one of the islands, as exemplified by Figure 5b. Note in addition that the etch pits are located both inside and outside the ordered islands without any strong implication that they might have acted as a preferential nucleation site. Furthermore, both Figures 4 and 5 show that the nucleation of the ordered SAMs is not at all spatially correlated with the atomic steps of the Au(111)

Figure 4. (a) Wide-scan (180 × 167 nm2) and (b) high-resolution (15.5 × 14.3 nm2) STM images taken at ∼0.6 monolayer coverage of thiols. All images were taken with -600 mV sample bias and 0.1 nA tunneling current corresponding to 6 GΩ junction impedance. Ordered domains of thiolates can be observed in very limited areas and in small sizes often uniaxially elongated.

Figure 5. As in Figure 4 but for an increased coverage of ∼0.75 monolayer. Stainlike patches (∼1.7 Å higher than surrounding regions) appearing in the wide-scan (155 × 143 nm2) image (a) represent highly ordered domains of thiolates, inside which the high-resolution (15.5 × 14.3 nm2) image (b) clearly resolves the c(4 × 2) superlattice.

substrate either. These facts suggest that the ordered domains were formed on each atomic terrace by a sort of homogeneous nucleation out from the disordered phase of chemisorbed thiols. The STM images of discrete 2D islands as shown in Figure 5a also give us a good hint as to what the typical domain size is like for the SAMs grown on our sputtergrown Au(111). Here we need to discriminate two broad categories of domain boundaries, i.e., gross boundaries that are easily visible even in ∼100 nm length scale images

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Figure 6. ∼100 nm length scale images (118 × 109 nm2) taken in the saturation coverage regime showing traces of the line boundaries formed by coalescence of 2D islands. Images were taken for SAMs prepared by (a) 2 min immersion in 0.01 mM solution and (b) only 15 s immersion in 0.1 mM solution.

versus molecularly thin boundaries detected only by highresolution imaging. The latter type of boundary is associated with a translational or rotational lattice mismatch between two adjacent (x3 × x3)R30° or c(4 × 2) domains.5 Figure 5b suggests that these molecular-scale boundaries (examined later in a little more detail) are likely absent within each discrete 2D island, which can be thus molecularly homogeneous over the length of 10-40 nm spanning the whole island. This already gives a considerably large domain size. The 2D islands grow further (some new islands also form) in the higher coverage regime to eventually coalesce with each other, with or without the mutual lattice mismatch. In the latter case the two islands can merge into a single, much larger isomorphous domain, though such a fortunate coalescence would be a rather minor event especially for the low-symmetry c(4 × 2) domains having a high degree of positional as well as rotational degeneracy.5 Traces of the line boundaries formed by the coalescence of the 2D islands were certainly visible in some of the ∼100 nm length scale images taken in the saturation coverage regime, as shown in Figure 6. The maximum domain size as enclosed by these coalescence-induced boundaries in Figure 6a (imaged for the SAM deposited by 2 min incubation in 0.01 mM solution) can easily exceed ∼50 nm. Figure 6b is for the SAM grown much faster by only 15 s immersion in 0.1 mM solution. The average domain size became noticeably smaller in this preparation condition, but the change is not particularly significant for the large difference (10 times) in the chemisorption rate, and even the smallest domains here still have the size of ∼20 nm. c(4 × 2) Superlattice and Molecular-Scale Boundary Structures. As expected, we have observed highly ordered structures of the OT SAMs all over the Au(111) surface at or near the saturation coverage irrespective of the OT concentrations (0.1-0.001 mM) examined. They invariably revealed the basic x3 × x3 structure moreor-less strongly modulated by the c(4 × 2) superlattice. The couple of typical c(4 × 2) patterns observed in the

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Figure 7. Two distinct c(4 × 2) superlattice patterns observed in high-resolution STM images: (a) 20.5 × 19.0 nm2; (b) 10.2 × 9.4 nm2. Rectangular c(4 × 2) unit cells are indicated inside a digitally magnified image in the inset of (a) and in the lower part of (b).

previous high-impedance STM studies5,6,37,39 have been found also in this work. Figure 7 shows two particularly well-resolved images exhibiting these typical patterns. In Figure 7a the x3 × x3 structure is modulated by the formation of two distinct like pairs, one brighter than the other corresponding to ∼0.3 Å apparent height difference. The rectangular c(4 × 2) unit cell with the dimensions of 8.6 × 10.0 Å2, satisfactorily describing this structure, is illustrated in the inset. Within the capability of our STM imaging system the ∼20 nm wide scan window was almost the upper limit to resolve the detailed molecular arrangement. Over this length scale Figure 7a exhibits no discernible molecular-scale boundaries but etch pits. Furthermore, an identical superlattice structure appears to persist even into the etch pit located in the upper right corner. We also stress that the etch pits were often located in the interior of otherwise homogeneous domains. In another c(4 × 2) pattern shown in Figure 7b one can more easily locate the rectangular c(4 × 2) unit cell. The simple rectangular unit cell of the c(4 × 2) lattice should contain four thiolates, 4/4 on the corners, 2/2 on the longer edges, and 2 in the interior. The image shown in Figure 7b resolves those on the corners in a particularly distinct way, others being somewhat elongated in different directions. This may possibly reflect the specific orientations of the respective alkyl chains within the unit cell, but the exact interpretation of this image structure is beyond the scope of this paper. It should be also noted that no matter what the origin of the c(4 × 2) superstructuresplane alternation of the all-trans hydrocarbon backbones8 or disulfide formation10sthe height-mapped STM images represent a relatively small perturbation or modulation of the commensurate (x3 × x3)R30° structure, ∼0.5 Å in the overall height modulation. Thus even if different images taken in different positions look significantly different in the apparent image features, it does not necessarily point to any serious polymorphism in the real SAM structure. Including the images shown in Figure 7, the 10-20 nm wide scan windows that ensure a sufficient molecular resolution were insufficient to encompass the majority of

Alkanethiol Monolayers on Sputter-Grown Au(111)

Figure 8. Examples of molecular-scale domain boundaries observed in high-resolution (15.5 × 14.3 nm2) STM images: (a) straight-line translational boundary with a finite width; (b) rotational boundary separating two c(4 × 2) domains rotated by 60° from each other. Dashed line shows the symmetry axis.

the individual domains. We therefore conclude that the molecularly homogeneous domain size is no less than 1015 nm on the average and can be comparable to that constrained by the coalescence-induced boundaries. In fact the molecular-scale domain boundaries observed by the high-resolution imaging invariably gave us the impression that they were originated from the coalescence between adjacent domains. Figure 8 shows some common examples of such molecular-scale boundaries observed in this work. In Figure 8a one can see a straight-line boundary with a finite (∼10 Å) width as expected for the translational boundaries; the translational lattice mismatch across the boundary is illustrated in the upper part of the image. An example of rotational boundary is shown in Figure 8b. The two c(4 × 2) unit cells on both sides of the boundary exhibit the expected rotation from each other by 60°, and the symmetry axis indicated by the broken line reasonably fits the boundary line. Finally, a potentially more serious question regarding our high-resolution STM images is whether the tipsample interaction played any significant role in producing the observed image contrast. We used the relatively large tunneling current of 0.1 nA, but the overall tunnelingjunction impedance of 6 GΩ is thought to be sufficient to ensure a nondestructive imaging for the C8 alkyl chains.5,39 The required minimum junction impedance for nondestructive imaging is strongly sensitive to the length of the alkyl chain, and our imaging condition for the OT SAMs is probably comparable to that for C10-C12 alkyl chains under the junction impedance of the order of ∼100 GΩ. Even so we still need to think of the tip-induced structural transformation that Touzov and Gorman suggested recently;25 the hexagonal (x3 × x3)R30° arrangement in their decanethiol (C10) SAMs could be transformed to a more stable c(4×2) superlattice under the influence of the STM tip even at the junction impedance exceeding ∼100 GΩ (1 V bias and ∼10 pA set-point current). In our present study the degree of the c(4 × 2) superlattice modulation significantly varied for different images as mentioned already, and in some cases only the basal (x3 × x3)R30°

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Figure 9. (a) STM image (20.5 × 19.0 nm2) taken across a (111) facet step of which the direction is specified by broken line. Two axes of the rectangular c(4 × 2) unit cell are indicated by arrowheaded lines, a and b, which are oriented in the expected azimuths of 30 and 120° with respect the step edge. (b) 10.2 × 9.4 nm2 image taken locally across a round step, providing good resolution both on upper and lower terraces and uncovering interterrace structural correlation of (x3 × x3)R30° structures. Interterrace lattice misalignment is noticed in the C-D direction across the step.

structure could also be imaged without noticeable changes of the structure upon repeated scanning. This latter result suggests that the tip-sample interaction was not particularly serious in our imaging condition. We were nevertheless not able to set the junction impedance any greater than ∼10 GΩ, so we can still not totally rule out the possibility that we potentially imaged a SAM surface that had already undergone some tip-induced structural modification. We will come back to this issue later in conjunction with the kinetics of ordering as monitored by the IR-RAS spectra. Epitaxial Relationship with Au(111). Another point we have not explicitly addressed yet is the epitaxial registry of the OT SAMs with the underlying Au(111) lattice. To clarify this aspect it is useful to refer to the images taken by scanning across a step edge and to see the structural correlation between the upper and lower terraces and also the azimuthal relationship to the step edge direction. In the latter case, the step needs to run straight along one of the three equivalent crystallographic (nearest neighbor) directions, as is the case with the (111) facet steps relatively easy to find on the sputter-grown Au(111).24,25 The (x3 × x3)R30° structure, as its name stands, has to have the two unit cell axes rotated by 30 and 90°, respectively, with respect to such facet steps. Equivalently, the rectangular c(4 × 2) unit cell has the two axes rotated by 30 and 120°, respectively, from the nearest-neighbor direction of Au(111). The STM image shown in Figure 9a indeed reproduces this proper azimuthal relationship between a typical c(4 × 2) domain and the step edge. Figure 9b shows another step-related image but taken locally across a round step (not a facet step), which fortunately gives good resolution both on the upper and

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Figure 10. Series of IR-RAS spectra taken for OT monolayers as a function of incubation time in 0.01 mM solution. θ represents the apparent surface coverage of the OT monolayer estimated from the S2p/Au4f vs incubation time profile shown in Figure 2. Structural ordering is manifested among others by evolution of a sharp peak growing at 2878 cm-1, which is associated with the symmetric stretching mode of terminal methyl groups.

lower terraces to infer the interterrace structural correlation. With the help of two straight lines overlaid in the image, one can see that the two (x3 × x3)R30° structures on the upper and lower terraces align in the A-B direction but not in the C-D direction. This kind of misalignment across a monatomic step in a single-crystal domain of Au(111) is typical for the (x3 × x3)R30° structure, as one can easily confirm by using the lattice of circles. Together with the proper azimuthal orientation verified in Figure 9a, this satisfactorily testifies to the expected epitaxial registry of the OT SAMs with the Au(111) lattice. Kinetics of Ordering Examined by IR-RAS. The IR-RAS spectra in the C-H stretch region give clearer insight into the extent of orientational ordering of the hydrocarbon backbones in alkanethiol SAMs, because of the selection rule that only vibrational modes whose transition dipoles have a major component along the surface normal are IR-RAS active. Thus the relative intensities of a series of vibrational modes in the C-H stretch region must be considerably altered in the course of the orientational ordering. Besides, the structural ordering can also be manifested by significant peak narrowing and/or peak shift in the relevant signals.32,33 Figure 10 shows a series of IR-RAS spectra taken for OT SAMs as a function of incubation time in 0.01 mM solution. The approximate thiol coverage (θ) for each incubation time, as inferred from Figure 2, is also specified for each spectrum. A broad absorption centered around ∼2930 cm-1 (probably associated with the antisymmetric C-H stretching mode of the methylene groups according to the standard peak assignments in the C-H stretch region30,31) predominates the spectra for fractional coverage of OT up to θ ∼ 0.6. This fact, together with the lack of any sharp signal associated with the terminal methyl group, indicates that the SAM at this stage is strongly disordered as a whole. We expect that the methyl group,

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which is located in the outermost position in whatever ordered SAMs, gives some distinct peaks as a result of the orientational ordering. Indeed, a large change in the spectra due to the symmetric C-H stretch of the methyl group [νs(CH3)] shows up in the considerably high coverage regime above θ ∼ 0.7, where a sharp νs(CH3) peak grows steadily at 2878 cm-1. This change is also accompanied by the growth of the Fermi-resonance splitting component of νs(CH3) at 2938 cm-1 and by the sharpening of the antisymmetric C-H stretch of the methyl group at 2965 cm-1. These spectral changes are completed, however, as soon as the full monolayer coverage is reached in just a few minutes. The IR-RAS spectra taken at θ ∼ 1.0 also allow us to figure out in what orientations the OT molecules are arranged in the saturation coverage regime, based on the transition-dipole analysis similar to that described by Nuzzo and co-workers.30,31 Our analysis led us to conclude a minor tilt angle of ∼5° for the C8 alkyl chains in our OT SAMs. In this nearly upright orientation, the variation in the twist angle produces minor changes in the IR-RAS spectra. The result of the IR-RAS study is fully consistent with that of the STM imaging. Both results testify that the well-ordered OT monolayers begin to evolve at the relatively high threshold coverage of 0.6-0.7 monolayer and grow rapidly thereafter. This consistency is particularly encouraging in view of some technical problems associated with STM; in particular the question of the tip-sample interaction mentioned earlier. The possibility of a tip-induced transformation from the (x3 × x3)R30° arrangement to the c(4 × 2) superlattice is still not clearly addressed, as they both represent a well-ordered phase. However, the more extreme event that the tip-sample interaction could also be involved in the ordering process itself can be excluded on the basis of the IR-RAS data. Effect of Substrate Morphology. Since the method of SAM preparation employed in this work is not special but the standard solution-phase deposition, the rapid selfassembly of highly ordered monolayers should be attributed primarily to the high-quality Au(111) substrate. In fact the identical solution-phase deposition of OT SAMs on the Au film sputter grown at room temperature reproduced a much slower self-assembly kinetics similar to that mentioned in the Introduction. As shown in Figure 1b the film surface sputter grown at room temperature is polycrystalline-like and no longer atomically flat, but the overall surface morphology is analogous to that reported for evaporated gold films prepared at relatively low (