Surface-Monolayer-Controlled Molecular Alignment of Short n

Department of Chemistry, Graduate School of Science, The University of Tokyo, Hongo,. Tokyo 113-0033, Japan. Received May 29, 2001. In Final Form: Aug...
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Surface-Monolayer-Controlled Molecular Alignment of Short n-Alkane Multilayers H. Kondoh, F. Matsui,* Y. Ehara, T. Yokoyama, and T. Ohta† Department of Chemistry, Graduate School of Science, The University of Tokyo, Hongo, Tokyo 113-0033, Japan Received May 29, 2001. In Final Form: August 27, 2001 C K-edge near-edge X-ray absorption fine structure studies have been performed on the molecular orientation of short n-alkane (C6H14) multilayers on n-alkanethiolate (C6H13S) monolayers. We have found that the molecular orientation of the n-alkane multilayers is in accordance with that of n-alkanethiolate monolayers on which the multilayers were formed. Monolayers of thiophene also have a significant effect on the alignment of the n-alkane multilayers condensed on the monolayers. The growth manner of the n-alkane films is discussed in terms of the energy balance between surface and interface free energies.

Control of molecular alignment in organic films in a macroscopic scale is important for applications of these materials. In particular, for liquid-crystal displays, the molecular alignment is one of the most crucial parameters to determine the electric-field response. So far, various approaches to control the alignment have been explored. Rubbed polymer films, a quite popular method, have been used in the liquid-crystal display industry as a template for obtaining desired liquid-crystal bulk alignment.1 Microscopic mechanisms for the “rubbing” method have been studied extensively with ellipsometry,2 infrared absorption spectroscopy,3,4 atomic force microscopy,5 nearedge X-ray absorption fine structure (NEXAFS),6,7 second harmonic generation,8 sum-frequency generation,9 and so on. These studies have proved that chains of polymer surfaces are well aligned by rubbing, and the rubbed polymer surface aligns liquid-crystal monolayers through orientational epitaxy. The well-aligned surface monolayer of liquid-crystal molecules thus formed acts as a template for bulk alignment. On the basis of this mechanism it is expected that we can control the bulk alignment of organic films by defining the structure of the first monolayer. Recently, monolayers of alkane-containing molecules, such as alkanethiols self-assembled on metal substrates,

have been studied extensively to understand the selfassembly mechanism. In the course of these studies, it was found that the molecular orientation in the alkanethiol monolayers significantly alters with coverage.10-13 Such a change in molecular orientation will have a decisive influence on the surface property of the monolayer. If additional alkane molecules are adsorbed on the monolayers in the multilayer form, their adsorption structure could be influenced by the structure of the preformed monolayer. Although alkanes are one of the simplest organic molecules, they often play an important role as building blocks in various organic materials such as surfactants, liquid crystals, and polymers. The physicochemical properties of these materials are influenced by including alkyl chains and in particular cases they are dominated by those alkyl chains. In this study, we have investigated the structure of multilayers of hexane (C6H14) adsorbed on hexanethiolate [CH3(CH2)5S] monolayers on Au(111) by using C K-edge NEXAFS. We have also studied the effects of methylthiolate (CH3S) and aromatic carbon species [thiophene (C4H4S) and its derivative] monolayers on the structure of subsequently adsorbed hexane multilayers, and have reported that hexane multilayers follow their alignment to the hexanethiolate monolayers. This observation suggests the possibility of fabricating alignment-controlled organic films based on the self-assembly mechanism.

* Present address: Graduate School of Materials Science, Nara Institute of Science and Technology 8916-5 Takayama, Ikoma, Nara 630 0101, Japan. † Corresponding author. E-mail: [email protected].

2. Experimental Section

1. Introduction

(1) Geary, J. M.; Goodby, J. W.; Kmetz, A. R.; Patel, J. S. J. Appl. Phys. 1987, 62, 4100. (2) Hirosawa, I. Jpn. J. Appl. Phys. 1996, 35, 5873. (3) Hasegawa, R.; Mori, Y.; Sasaki, H.; Ishibashi, M. Jpn. J. Appl. Phys. 1996, 35, 2492. (4) Sakamoto, K.; Arafune, R.; Ito, N.; Ushioda, S.; Suzuki, Y.; Morokawa, S. Jpn. J. Appl. Phys. 1994, 33, L1323; J. Appl. Phys. 1996, 80, 431. (5) Pidduck, A. J. et al. J. Vac. Sci. Technol. A 1996, 14, 1723; Kim, Y. B. et al. Appl. Phys. Lett. 1995, 66, 2218; Zhu, Y. M. et al. Appl. Phys. Lett. 1994, 65, 49. (6) Weiss, K.; Wo¨ll, Ch.; Bo¨hm, E.; Fiebranz, G.; Forstmann, G.; Peng, B.; Scheumann, V.; Johannsmann, D. Macromolecules 1998, 31, 1930; Sto¨hr, J.; Samant, M. G.; Cossy-Favre, A.; Dı´az, J.; Momoi, Y.; Odahara, S.; Nagata, T. Macromolecules 1998, 31, 1942. (7) Ouchi, Y.; Mori, I.; Sei, M.; Ito, E.; Araki, T.; Ishii, H.; Seki, K.; Kondo, K. Physica 1995, B208/209, 407. (8) Zhuang, X.; Marrucci, L.; Shen, Y. R. Phys. Rev. Lett. 1994, 73, 1513. (9) Wei, X.; Zhuang, X.; Hong, S.-C.; Goto, T.; Shen, Y. R. Phys. Rev. Lett. 1999, 82, 4256.

The experiments were performed with an ultrahigh vacuum chamber (base pressure: 6 × 10-11 Torr) at the soft-X-ray beam line BL-7A of the Photon Factory.14 The Au(111) substrates were prepared by vacuum deposition of gold onto cleaved mica in another chamber. The Au(111) surfaces were cleaned by repeating cycles of Ar ion sputtering and annealing. The cleanliness was checked with Auger electron spectroscopy and NEXAFS. Selfassembled monolayers of hexanethiolate were prepared on the (10) Poirier, G. Chem. Rev. 1997, 97, 1117. (11) Schreiber, F.; Eberhardt, A.; Leung, T. Y.; Schwartz, P.; Wetterer, S. M.; Lavrich, D. J.; Berman, L.; Fenter, P.; Eisenberger, P.; Scoles, G. Phys. Rev. B 1998, 57, 12476. (12) Kondoh, H.; Kodama, C.; Sumida, H.; Nozoye, H. J. Chem. Phys. 1999, 111, 1175. (13) Kondoh, H.; Saito, N. Matsui, F.; Yokoyama, T.; Kuroda, H.; Ohta, T. J. Phys. Chem. B, in press. (14) Namba, H.; Daimon, H.; Idei, Y.; Kosugi, N.; Kuroda, H.; Taniguchi, M.; Suga, S.; Murata, Y.; Ueyama, K.; Miyahara, T. Rev. Sci. Instr. 1989, 60, 1909.

10.1021/la010783h CCC: $20.00 © 2001 American Chemical Society Published on Web 11/27/2001

Molecular Alignment of Short n-Alkane Multilayers

Figure 1. C K-edge NEXAFS spectra for a monolayer (upper) and a multilayer (lower) of hexane prepared on Au(111) as a function of the X-ray incident angles from surface parallel. The multilayer was condensed at 110 K, whereas the monolayer was prepared at 155 K where no hexane multilayer is formed. clean Au(111) surfaces by vapor deposition of hexanethiol at room temperature with controlling coverages by the deposition time. The methylthiolate monolayer was prepared by exposing a Au(111) substrate to gaseous dimethyl disulfide (CH3SSCH3) at room temperature. The thiophene monolayer was formed by vapor deposition at 180 K. The monolayer-covered Au(111) surfaces were exposed to gaseous hexane by using a gas doser at 110 K to form multilayers of hexane. The temperature was controlled by the balance of cooling with a liquid N2 cryostat and resistive heating. Because the local pressure at the sample position during exposure was not known, we could not define the effective exposure of hexane. We observed blue color for the deposited multilayer films, which means that the thickness of the films was of the order of several thousand angstroms. The X-rays were monochromatized by a plane grating monochromator with an energy resolution of 0.5 eV at the carbon K-edge. The photon energies were calibrated by the lowest-energy 1sfπ* peak of graphite (285.5 eV). C K-edge NEXAFS spectra were taken with the partial electron yield method. Each NEXAFS spectrum was normalized by dividing by the corresponding clean-surface spectrum. Polarization dependence of the NEXAFS spectra was measured to deduce the averaged tilt angle of the molecular axes.

3. Results 3.1. Monolayer and Multilayer of Hexane. Figure 1 shows the polarization dependence of C K-edge NEXAFS spectra for a monolayer and a multilayer of hexane adsorbed on Au(111), where the monolayer is defined as the saturated coverage by the flat-lying molecule. Both in the monolayer and multilayer spectra a broad peak is observed at about 293 eV and most enhanced at the normal incidence (θ ) 90°). Because this peak has been assigned to a 1sfσ*(C-C) resonance,15-19 the -C-C-C- axes are (15) Sto¨hr, J.; Outka, D. A.; Baberschke, K.; Arvanitis, D.; Horsley, J. A. Phys. Rev. B 1987, 36, 2976. (16) Outka, D. A.; Sto¨hr, J.; Rabe, J. P.; Swalen, J. D. J. Chem. Phys. 1988, 88, 4076.

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Figure 2. Curve-fitting results for the C K-edge NEXAFS spectra of hexane multilayer with three X-ray incident angles (15°, 55°, and 90°). Each NEXAFS spectrum was deconvoluted into four Gaussians and one-step functions. Filled curve indicates the σ*(C-C) resonance.

primarily lying down to the substrate both for the monolayer and the multilayer. We observed somewhat complicated structures below the ionization threshold (∼290 eV).20-23 Because it has been found that at least three components exist below the ionization potential (IP) for C1s-excitation spectra of alkane molecules,20-22 the multilayer spectra were curve-fitted with three Gaussian functions for the region below the IP, two asymmetric Gaussians above the IP, and an error function for the transition to the continuum state as shown in Figure 2. The lower-energy region below the IP was well reproduced by summation of three Gaussian functions. The spectral analysis indicates that the three components do not show the same polarization dependence; the 288.5 eV peak is most intense at the grazing incidence (θ ) 15°), whereas the other two peaks stay almost constant in intensity irrespective of the X-ray incidence angle. The 288.5 eV peak has been assigned to a transition to a final state of which the major transition moment lies perpendicular to (17) Ohta, T.; Seki, K.; Yokoyama, T.; Morisada, I.; Edamatsu, K. Phys. Scr. 1990, 41, 150. (18) Hahner, G.; Kinzler, M.; Thummler, C.; Wo¨ll, Ch.; Grunze, M. J. Vac. Sci. Technol. A 1992, 10, 2758. (19) Sto¨hr, J. NEXAFS Spectroscopy; Springer-Verlag Publishing: New York, 1992. (20) Bagus, P. S.; Weiss, K.; Schertel, A.; Wo¨ll, Ch.; Bruun, W.; Hellwig, C.; Jung, C. Chem. Phys. Lett. 1996, 248, 129. (21) Va¨terlein, P.; Fink, R.; Umbach, E.; Wurth, W. J. Chem. Phys. 1998, 108, 3313. (22) Hitchcock, A. P.; Ishii, I. J. Electron Spectrosc. Relat. Phenom. 1987, 42, 11. (23) Weiss, K.; Weckesser, J.; Wo¨ll, Ch. J. Mol. Struct. (THEOCHEM) 1999, 458, 143. They also observed a shoulder at 285.1 eV, which may come from the interaction with metal substrates. In our study, however, certain evidence of the peak was not obtained. We were especially careful around the energy region, because the energy is close to that of π* resonance from contaminated hydrocarbons.

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Figure 3. Incident-angle dependence of peak intensity of the σ*(C-C) resonance for the hexane multilayer shown in Figure 2. The intensity with respect to that for 55° is plotted as a function of the incident angle. Solid curves indicate simulated curves assuming that the transition moment is tilted by 74°, 79°, and 84° from the surface normal and the linear-polarization factor of the X-ray used here was 0.90.

the -C-C-C- plane.20,21 On the basis of this assignment, the polarization dependence of this peak indicates that the carbon backbone plane predominantly lies parallel to the substrate. The monolayer spectra shown in Figure 1 exhibit a clear splitting into two peaks at 287.5 and 289.0 eV in the lowerenergy region.23 This splitting might come from interaction of the molecule with the substrate. Because in the lyingdown configuration one-half of the hydrogen atoms of hexane directly interact with the substrate but the other half face to vacuum, the (C-H)*-character state is energetically split into two, a strongly interacting state and a less interacting one. The polarization dependence of these structures is similar to that of the 288.5-eV peak in the multilayer spectra. If the transition moments of these peaks orient perpendicular to the -C-C-C- plane, the plane is almost parallel to the substrate. The flatlying configuration of n-alkane monolayers has been found on Au(111)24-26 and other metal surfaces.27-30 It seems difficult to deduce orientation angles quantitatively from analyses of the lower-energy structures, because they consist of a couple of transitions to differentsymmetry final states20-22 and their peaks appear at photon energies close to each other, resulting in serious ambiguity in the peak analysis. Therefore, we analyzed polarization dependence quantitatively only for the σ*(C-C) resonance (black-filled peak in Figure 2), which seems to be much less influenced by artificial effect in the peak analysis. Figure 3 shows the polarization dependence of the σ*(C-C) resonance for the multilayer of hexane. The plotted data are well reproduced by a calculated curve assuming that the -C-C-C- molecular axes are tilted (24) Uosaki, K.; Yamada, R. J. Am. Chem. Soc. 1999, 121, 4090. (25) Xie, Z. X.; Xu, X.; Tang, J.; Mao, B. W. J. Phys. Chem. B 2000, 104, 11719. (26) Yamamoto, M.; Sakurai, Y.; Hosoi, Y.; Ishii, H.; Kajikawa, K.; Ouchi, Y.; Seki, K. J. Phys. Chem. 2000, 104. (27) Firment, L. E.; Somorjai, G. A. J. Chem. Phys. 1977, 66, 2901. (28) Firment, L. E.; Somorjai, G. A. J. Chem. Phys. 1978, 69, 3940. (29) Weckesser, J.; Fuhrmann, D.; Weiss, K.; Wo¨ll, Ch.; Richardson, N. V. Surf. Rev. Lett. 1997, 2, 209. (30) Yoshimura, D.; Ishii, H.; Ouchi, Y.; Ito, E.; Miyamae, T.; Hasegawa, S.; Okudaira, K. K.; Ueno, N.; Seki, K. Phys. Rev. B 1999, 60, 9046.

Figure 4. Coverage dependence of C K-edge NEXAFS spectra for hexanethiolate monolayers on Au(111). The full monolayer of hexanethiolate on Au(111) (θ ) 0.33 with respect to the surface Au atoms) is defined as 1 ML.

by 79° from the surface normal. The error to this analysis is estimated to be about (5°. A similar analysis32 for the monolayer indicated that the tilting angle is 75 ( 5°. The incomplete flat-lying angle might be caused by a nonnegligible amount of steps in the gold substrate deposited on mica, not by an intrinsic molecular orientation. 3.2. Monolayer of Hexanethiolate. Figure 4 shows the polarization dependence of C K-edge NEXAFS spectra for hexanethiolate monolayers adsorbed on Au(111) with a series of different coverages. At low coverages below 0.5 monolayer (ML), the σ*(C-C) resonance peak at 293 eV is most enhanced at the normal incidence (θ ) 90°), which indicates a lying-down orientation. The lying-down hexanethiolate molecules on Au(111) has been observed at low coverages by a molecular-resolution scanning tunneling microscopy.12 The low-energy-region spectra below the IP resemble those of the hexane monolayer, which suggests a flat-on configuration for the -C-C-C- plane of the alkyl chain. As the coverage is increased above 0.5 ML, the polarization dependence decreases and almost disappears at around 0.65 ML. Although the absence of (31) Yamamoto, M.; Sakurai, Y.; Hosoi, Y.; Ishii, H.; Kajikawa, K.; Ouchi, Y.; Seki, K. J. Phys. Chem. 2000, 104, 7370. (32) In the monolayer adsorbates, another continuum state appears above the Fermi level which is usually located below the ionization threshold of the adsorbates (see ref 19). Therefore, another error function with a lower-energy position (287.8 eV) was introduced into this curvefitting analysis.

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Figure 6. Schematic illustration for molecular alignment of n-alkane multilayers formed on alkanethiolate monolayers with three different structures. The multilayers duplicate the molecular orientation of the monolayers.

Figure 7. C K-edge NEXAFS spectra from hexane multilayers formed on a methylthiolate monolayer on Au(111) with a (3 × 4)-4CH3S structure. The (3 × 4) monolayer was formed by dissociative adsorption of CH3SSCH3 at room temperature and subsequent cooling to 110 K (ref 29). Figure 5. C K-edge NEXAFS spectra from hexane multilayers formed on hexanethiolate monolayers on Au(111) with different monolayer coverages.

polarization dependence may seem to suggest an ordered structure with an orientation angle of 55° (magic angle), scanning tunneling microscopy and He atom diffraction studies showed that the thiolate molecules adsorbed on Au(111) form a disordered structure at this coverage region.11,12 There is no ordering in two-dimensional arrangement of the molecules. Thus, the lack of polarization dependence at this coverage region is interpreted in terms of the result from significant disordering in the molecular orientation. Further increase in coverage up to more than 0.8 ML gives rise to formation of an ordered structure with the molecular axes standing up. The thiolate molecules in this structure are arranged twodimensionally into a commensurate “c(4 × 2)” structure.10-12 Quantitative analyses for the polarization dependence of the σ*(C-C) resonance indicated an alignment angle of 35 ( 5° from the surface normal, which is consistent with the tilted angles (32-34°) for short alkanethiolates (Cn; n < 14) determined by the grazing incidence X-ray diffraction method.33 Such coverage dependence of the molecular orientation has also been observed for alkanethiolates on other metal surfaces.13 3.3. Multilayer of Hexane Formed on Hexanethiolate Monolayer. Next we investigated the effects of preformed haxanethiolate monolayers on the molecular orientation of hexane multilayers subsequently formed on the monolayers. NEXAFS spectra of hexane multilayers on hexanethiolate-monolayer-covered Au(111) surfaces with three different monolayer coverages are depicted in Figure 5. The monolayers with coverages of 0.36, 0.66, and 1.0 ML used here exhibited lying-down, disordered, and standing-up orientations, respectively. In the hexane (33) Fenter, P.; Eberhardt, A.; Liang, K. S.; Eisenberger, P. J. Chem. Phys. 1997, 106, 1600.

multilayer formed on the lying-down monolayer (top), alkane chains are aligned lying-down to the surface, whereas in the standing-up monolayer (bottom) the multilayer has a standing-up orientation as clearly evidenced by the opposite polarization dependence. Furthermore, the multilayer formed on the disordered phase (middle) shows no polarization dependence, indicating disordering of the orientation. Quantitative analyses for the polarization dependence of the σ*(C-C) resonance indicate that the orientation angles in hexane multilayers are in good agreement with those of the corresponding monolayers (0.36 ML: 73 ( 5° for monolayer and 75 ( 5° for multilayer, 1.0 ML: 35 ( 5° both for monolayer and multilayer). Thus, the molecular orientation in hexane multilayers follows that of preformed hexanethiolate monolayers as illustrated in Figure 6. 3.4. Multilayer of Hexane Formed on Methylthiolate Monolayer. The methylthiolate monolayer exhibits (x3 × x3)R30° low-energy electron diffraction pattern at room temperature. Cooling of this monolayer to 110 K gives rise to transformation of the (x3 × x3)R30° structure to a (3 × 4) structure with forming tetramers.34 The (x3 × x3)R30° lattice is often observed for self-assembled monolayers of alkanethiols (Cn; n g 4) on Au(111) and is very similar to that in the (001) plane of alkane bulk crystals. In contrast, the (3 × 4) surface lattice is significantly different from the (x3 × x3)R30° lattice, although in both cases the surfaces consist of the CH3 end groups. With the use of this (3 × 4) monolayer, we studied the effect of surface lattice on the orientation of the multilayer on the CH3-terminated monolayer. Figure 7 depicts the polarization dependence of C-K NEXAFS spectra for a hexane multilayer prepared on the (3 × 4)CH3S monolayer at 110 K. Hexane molecules no longer form a vertically aligned phase but a considerably tilted phase. The average tilt angle of the molecular axis is 65 ( 10° from the surface normal. Therefore, the (3 × 4) (34) Kondoh, H.; Nozoye, H. J. Phys. Chem. B 1999, 103, 2585.

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mode of thin films is generally determined by the solidsolid and solid-vacuum interfacial free energies. When “A”-terminated surface is covered by a monolayer with “B”-terminated surface, the free energy change ∆γ is described as follows,

∆γAfB ) γB + γA-B - γA

(1)

where γA and γB are surface free energies of the Aterminated surface and the B-terminated surface, respectively, and γA-B is a A/B interfacial energy mainly arising from strains at the A/B interface. If ∆γAfB < ∆γAfA, the B-terminated surface is formed on the A-terminated surface. Because ∆γAfA is zero, the condition for growth of a monolayer of which the surface is different from that of the underlying layer is

∆γAfB < 0

Figure 8. C K-edge NEXAFS spectra from hexane multilayers formed on monolayers of thiophene and its derivative on Au(111); (upper) a flat-lying monolayer of thiophene which was prepared by exposing to gaseous thiophene at 180 K, and (lower) a tilted (45 ( 10°) monolayer of thiophene oligomer which was prepared by X-ray irradiation to thiophene multilayer and subsequent heating to room temperature to remove unreacted thiophene monomer (ref 35).

CH3-terminated monolayer does not induce the CH3-CH3 interface that is necessary for formation of the vertically aligned phase. 3.5. Multilayer of Hexane Formed on AromaticCarbon Monolayer. Thiophene (C4H4S) monolayers were also used as a preformed monolayer to investigate the effect of aromatic-carbon monolayers on the structure of hexane multilayer. A thiophene full-monolayer on Au(111) was prepared by vapor deposition at 180 K. This monolayer shows a flat-on adsorption configuration.35 X-ray irradiation on a thiophene multilayer induces oligomerization of thiophene.36 This X-ray-induced species adsorbs on Au(111) with its molecular plane being tilted from surface parallel by 45 ( 10°.35 Figure 8 shows C K-edge NEXAFS spectra from hexane multilayers formed on the flat-on thiophene monolayer and the tilted thiophene-derivative monolayer. Polarization dependence for these multilayers is opposite to each other; the hexane multilayer on the flat-on monolayer exhibits an averaged alignment angle of 70 ( 10° from the surface normal, whereas that on the tilted monolayer shows 43 ( 10° from the surface normal. The alignment of the aromatic carbon molecules also has a significant effect on the molecular alignment of hexane multilayer. 4. Discussion The film growth of organic molecules on a metal substrate is not straightforward, depending on the substrate temperature, deposition speed, and so on. Here, we discuss the growth mode of thin films with a simplified thermodynamical model, assuming the layer-by-layer growth and neglecting the kinetic factors. The growth (35) Nambu, A.; Kondoh, H.; Ehara, Y.; Yokoyama, T.; Ohta, T., manuscript in preparation. (36) Baumgartner, K. M.; Volmer-Uebing, M.; Taborski, J.; Bauerle, P.; Umbach, E. Ber. Bunsen-Ges. Phys. Chem. 1991, 95, 1488.

(2)

Let us consider the three cases of the n-alkane film growth: (1) on a standing-up monolayer, (2) on a disordered monolayer, and (3) on a flat-lying monolayer. 1. The Growth Manner on a Standing-up Monolayer. For the standing-up phase of the thiol monolayer, its surface consists of methyl (CH3) end groups, but for the flat-lying phase it consists predominantly of methylene (CH2) groups. Thus, for the flat-lying monolayer growth on the CH3-terminated standing-up phase, the following free-energy change should be considered.

∆γCH3fCH2 ) γCH2 + γCH3-CH2 - γCH3

(3)

Although these values depend on carbon number of n-alkane, it was found that γCH3 is smaller than γCH2 irrespective of carbon number.37 Because γCH3-CH2 is positive, ∆γCH3fCH2 > 0. This means that a flat-lying monolayer does not grow on the standing-up monolayer. On the other hand, for the disordered monolayer growth, the following energy should be considered.

∆γCH3fd ) γd + γCH3-d - γCH3 = γl + γCH3-l - γCH3 (4) where γd is the surface energy of the disordered phase and γCH3-d is the CH3-disordered interfacial energy. γd and γCH3-d are approximated by surface energy of liquid (γl) and CH3-liquid interfacial energy (γCH3-l), respectively.38 On the basis of the surface-tension study,37 we can estimate γCH3-l ) 7.9 mJ/m2 and γCH3 ) 27.1 mJ/m2 for hexane. The γl value decreases with the temperature rise: 19.4 mJ/m2 at 10 °C and 17.9 mJ/m2 at 25 °C for hexane.39 Therefore, ∆γCH3-d ∼ 0 at room temperature and becomes negative only at higher temperatures, meaning that the formation of disordered phase on a standing-up monolayer surface takes place at high temperatures greater than RT. Eventually, the CH3-terminated surface induces formation of CH3-terminated surface, where the free-energy change ∆γCH3fCH3 is zero, resulting in vertical preferential alignment of the alkyl (37) Ocko, B. M.; Wu, X. Z.; Sirota, E. B.; Sinha, S. K.; Gang, O.; Deutsch, M. Phys. Rev. E 1997, 55, 3164. (38) The interface energy for disordered monolayers cannot be determined unless the degree of disordering is defined. From the NEXAFS spectra, however, the “disordered” phase shows complete disordering. Therefore, the interface energies for liquid are used to stand for the disordered phases. This substitution would not cause a significant effect on the estimated interface energy difference if we properly consider the temperature dependence of the interface energies of liquid. (39) Jasper, J. J. J. Phys. Chem. Ref. Data 1972, 1, 841.

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chain. Repetition of such a process causes formation of the vertically aligned multilayer on the CH3-terminated monolayer. Epitaxial growth may be essential for formation of the vertically aligned multilayer, because the (3 × 4)-CH3S/Au(111) surface, which has a completely different surface lattice from that in bulk alkane, does not allow the formation of a vertically aligned phase on it. 2. The Growth Manner on a Disordered Monolayer. Two cases are considered here in the same way as above: standing-up-multilayer/disordered monolayer and flat-lying-multilayer/disordered monolayer. For these cases, the following free-energy change should be taken into account:

∆γdfCH3 = γCH3 + γCH3-l - γl and

∆γdfCH2 = γCH2 + γCH2-l - γl

(5)

From the free-energy values for hexane, it is deduced that ∆γdfCH2 > ∆γdfCH3 > 0. This indicates neither the standingup phase nor the flat-lying phase grows on the disordered monolayer. 3. The Growth Manner on a Flat-lying Monolayer. In the standing-up multilayer/flat-lying monolayer, the following free-energy change should be considered.

∆γCH2fCH3 ) γCH3 + γCH2-CH3 - γCH2

(6)

This free-energy change significantly depends on alkylchain length, because the difference in the free energy between the CH3- and the CH2-terminated surfaces increases with carbon number; the CH3-terminated surface becomes more stable than the CH2-terminated one for longer alkanes.37 The CH2/CH3 interfacial energy γCH2-CH3 can be estimated by using the following equation by Adamson40:

γCH2-CH3 = γCH2 + γCH3 - 2x(γCH2 ‚ γCH3)

(7)

If the surface energy values given for long alkane (γCH3 ∼ 21 mJ/m2 for C36 and γCH2 ∼ 33 mJ/m2 for polyethylene)41 are used, we obtain γCH2-CH3 ∼ 1.3 mJ/m2. From these values, eq 6 yields ∆γCH2fCH3 < 0, indicating that the long alkane prefers to form a standing-up multilayer with a CH3-terminated surface even though it is condensed on a flat-lying monolayer. In fact, a vertically aligned multilayer of C36 is formed on a Cu substrate,17 although the first monolayer of n-alkane adsorbed on metal substrates usually adopts a flat-lying geometry.27-30 The (40) Adamson, A. W. Physics and Chemistry of Surfaces, 3rd ed.; Wiley: New York, 1976. The Adamson’s original equation was γCH2-CH3 Z γCH2 + γCH3 - 2x(γCH2d ‚EγCH3d), where γCH2d and γCH3d are interfacial energies contributed from dispersion forces. Because the intermolecular interactions between n-alkane molecules are mainly associated with the dispersion forces, γCH2d and γCH3d can be approximated to γCH2 and γCH3, respectively. (41) Fox, H. W.; Zisman, W. A. J. Coll. Sci. 1952, 7, 428.

high stability of the CH3-terminated surface compared with the CH2-terminated one surpasses the CH2/CH3 interface strain. In hexane, however, the difference between γCH2 and γCH3 is much smaller than that for C36 and it is inferred to be comparable with γCH2-CH3 from the difference between γCH2-l and γCH3-l for hexane (γCH2-l γCH3-l ) 1.3 mJ/m2).42 Therefore, hexane might be close to the critical carbon number where ∆γCH2fCH3 ) 0 is fulfilled. At the critical carbon number, the alignment behavior changes from a monolayer-controlled regime to a selfcontrolled regime. In this sense, the monolayer-controlled molecular alignment of n-alkane multilayer is applicable to short alkanes. Finally, we are commenting briefly on the aromaticring monolayers. According to the contact-angle measurements of benzene liquid against a single crystal (001) face of C36 (CH3-terminated surface) and a polyethylene (CH2terminated surface), γCH3-C6H6 is much larger than γCH2-C6H6 (by 9.4 mJ/m2),41 which indicates that aromatic(conjugated π)-ring species such as benzene prefer to attach to CH2 groups rather than CH3 groups. The larger π-CH2 affinity induces alignment of alkane chain along the ring plane to maximize the π-CH2 interaction. Indeed, the hexane multilayer formed on the flat-lying monolayer of thiophene exhibited lying-down orientation as shown in Figure 8. Furthermore the tilted monolayer of the thiophene derivative (45 ( 10°) causes a tilted alignment of hexane with a very similar tilt angle (43 ( 10°), which supports the π-CH2-induced alignment. This result suggests a possible application for the control of molecular alignment. Because we can change the tilt angle of the aromatic ring in the monolayer continuously by the molecular density, aromatic-ring monolayers will provide the control ability of molecular alignment in n-alkane multilayers. 5. Conclusions We have investigated molecular orientation of multilayers of hexane formed on self-assembled monolayers of alkylthiolate on Au(111) with different structures. Polarization dependence of C K-edge NEXAFS revealed that the molecular alignment in the hexane multilayer well follows that in the monolayer upon which the multilayer is condensed. The monolayer-controlled molecular alignment is also observed for hexane multilayers formed on thiophene and its derivative monolayers. This behavior is interpreted in terms of the interfacial free energy change coming from surface tension and interface strain by the terminating layer. Acknowledgment. The authors are grateful for the financial support of the Ministry of Education, Sports and Culture (Grant 11640576). This research was performed under the approval of Photon Factory Program Advisory Committee (PF-PAC 99G078). H. K. acknowledges financial support of Toyota Physical and Chemical Research Institute. LA010783H (42) Based on the interfacial free energies for n-alkanes reported in ref 37, we estimated γCH3-l ) 7.9 mJ/m2 and γCH2-l ) 9.2 mJ/m2 for hexane, yielding γCH2-l - γCH3-l ) 1.3 mJ/m2.