Template-Directed Patterning Using Phase-Separated

Mutsuyoshi Matsumoto,*,†,‡ Ken-ichi Tanaka,§ Reiko Azumi,†. Yukishige Kondo,§,| and Norio Yoshino§,|. Nanotechnology Research Institute, Nati...
0 downloads 0 Views 276KB Size
Download PDF
8728

Langmuir 2004, 20, 8728-8734

Template-Directed Patterning Using Phase-Separated Langmuir-Blodgett Films Mutsuyoshi Matsumoto,*,†,‡ Ken-ichi Tanaka,§ Reiko Azumi,† Yukishige Kondo,§,| and Norio Yoshino§,| Nanotechnology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5-2, 1-1-1 Higashi, Tsukuba 305-8565, Japan, CREST, Japan Science and Technology Corporation (JST), Kawaguchi Center Building, 4-1-8, Honcho, Kawaguchi 332-0012, Japan, and Department of Industrial Chemistry, Faculty of Engineering, and Institute of Colloid and Interface Science, Tokyo University of Science, 1-3, Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan Received May 11, 2004. In Final Form: July 19, 2004 The structures of the mixed Langmuir-Blodgett (LB) films of conventional amphiphiles (CAs) and amphiphilic silane-coupling agents (SCAs) were investigated using IR spectroscopy, atomic force microscopy, and friction force microscopy. By using CAs having hydrogenated alkyl chains and SCAs having perfluorinated alkyl chains, phase-separated structures were formed with domains consisting of CAs surrounded by SCAs. The size and shape of the domains depended strongly on the mixed components, the mixing ratios, and the subphase temperature. In particular, usage of a CA having hydrogenated and perfluorinated portions in the hydrophobic group as one of the components led to the formation of nanothreads. When the phase-separated mixed LB films were heated, SCAs formed covalent bonds with the substrates having silanol groups whereas CAs did not have such ability. Rinsing the heat-treated LB films with ethanol selectively removed CAs with the SCA regions intact, resulting in the fabrication of templates. The structures of the templates reflected those of the original phase-separated LB films. LB transfer of amphiphiles on the templates led to the confinement of the amphiphiles in regions with the size and shape delineated by the templates. These results demonstrate that a variety of amphiphiles can be confined two-dimensionally in a designed manner.

Introduction The Langmuir-Blodgett (LB) technique has been used to fabricate ultrathin films with well-defined structures.1 The important feature of mixed LB films is that phase separation of the two components often occurs and leads to the two-dimensional confinement of one component on a micrometer or nanometer length scale, which will be useful for high-density recording and sensing with high sensitivity.2-6 * To whom all correspondence should be addressed. E-mail: [email protected]. † AIST. ‡ JST. § Department of Industrial Chemistry, Tokyo University of Science. | Institute of Colloid and Interface Science, Tokyo University of Science. (1) (a) Kuhn, H.; Mo¨bius, D.; Bu¨cher, H. In Physical Methods of Chemistry; Weisserberger, A., Rossiter, B. W., Eds.; Wiley-Interscience: New York, 1972; Vol. 1, Part IIIB, p 577. (b) Langmuir-Blodgett Films; Roberts, G. G., Ed.; Plenum Press: New York, 1990. (c) Ulman, A. An Introduction to Ultrathin Organic Films -from Langmuir-Blodgett Films to Self-Assembly; Academic Press: San Diego, 1991. (d) Matsumoto, M.; Terrettaz, S.; Tachibana, H. Adv. Colloid Interface Sci. 2000, 87, 147-164. (e) Miller, R., Ed. Special issue: A collection of papers presented at the 9th Int. Conf. Organized Molecular Films. Colloids Surf., A 2002, 198-200. (2) (a) Overney, R. M.; Meyer, E.; Frommer, J.; Brodbeck, D.; Luthi, R.; Howald, L.; Gunterodt, H.-J.; Fujihira, M.; Takano, H.; Gotoh, Y. Nature 1992, 359, 133-134. (b) Frommer, J.; Luthi, R.; Meyer, E.; Anselmetti, D.; Dreier, M.; Overney, R.; Gunterodt, H.-J.; Fujihira, M. Nature 1993, 364, 198. (c) Overney, R. M.; Meyer, E.; Frommer, J.; Guntherodt, H.-J.; Fujihira, M.; Takao, H.; Gotoh, Y. Langmuir 1994, 10, 1281-1286. (3) (a) Duschl, C.; Liley, M.; Vogel, H. Angew. Chem., Int. Ed. Engl. 1994, 33, 1274-1276. (b) Duschl, C.; Liley, M.; Corradin, G.; Vogel, H. Biophys. J. 1994, 67, 1229-1237. (4) (a) Ge, S.; Takahara, A.; Kajiyama, T. Langmuir 1995, 11, 13411346. (b) Takahara, A.; Kojio, K.; Ge, S.-R.; Kajiyama, T. J. Vac. Sci. Technol., A 1996, 14, 1747-1754. (c) Takahara, A.; Hara, Y.; Kojio, K.; Kajiyama, T. Macromol. Symp. 2001, 166, 271-284.

When amphiphiles that can form covalent bonds with the substrates are used as one of the components of the phase-separated LB films, templates are formed, reflecting the phase-separated structures. The validity of this methodology has been proved by using the mixed LB films of an amphiphilic thiol and a conventional amphiphile (CA)3 and those of amphiphilic silane-coupling agents (SCAs) and CAs.4,5a The mixed LB films have phaseseparated structures with domains consisting of CA surrounded by the thiol or SCAs covalently bonded with the substrates after appropriate treatments for the latter. The domain regions can be selectively removed to give void regions where the bare surface of the substrates is exposed. Adsorption of another thiol or SCA on the void regions from liquid phases gives self-assembled monolayers. The resulting films show special properties for the adsorption of biomolecules and vesicles.3b,4b,c We have demonstrated that amphiphiles can be introduced selectively onto the void regions using the LB technique.5a Modification of solid surfaces with fluorinated molecules has been investigated extensively because of the characteristic properties of fluorinated surfaces such as low friction, high insulation, and chemical inertness.2,4-6 As a result of the weak van der Waals attraction of fluorocarbons, fluorinated surfaces are endowed with both hydrophobicity and lipophobicity. This feature leads to the occurrence of phase separation in mixed LB films of hydrogenated and perfluorinated amphiphiles where (5) (a) Matsumoto, M.; Tanaka, M.; Azumi, R.; Manda, E.; Tachibana, H.; Kondo, Y.; Yoshino, N. Mol. Cryst. Liq. Cryst. 1997, 294, 31-34. (b) Matsumoto, M.; Tanaka, K.; Kondo, Y.; Yoshino, N. Chem. Lett. 2002, 970-971. (c) Matsumoto, M.; Tanaka, K.; Azumi, R.; Kondo, Y.; Yoshino, N. Langmuir 2003, 19, 2802-2807. (6) (a) Jacobi, S.; Chi, L. F.; Fuchs, H. J. Vac. Sci. Technol., B 1996, 14, 1503-1508. (b) Fang, J.; Knobler, C. M. Langmuir 1996, 12, 13681374. (c) Iimura, K.; Shiraku, T.; Kato, T. Langmuir 2002, 18, 1018310190.

10.1021/la0488250 CCC: $27.50 © 2004 American Chemical Society Published on Web 08/26/2004

Template-Directed Patterning Using LB Films

domains consisting of hydrogenated amphiphiles are surrounded by fluorinated species in monolayer LB films. The adsorption characteristics of tobacco mosaic virus,2b bovine serum albumin,4b,c,6b and γ-globulin4c on the phaseseparated LB films have been investigated. Phase-separated structures of mixed LB films of hydrogenated and perfluorinated amphiphiles have been used to fabricate templates. One of the important aspects is that the original phase-separated structures are reflected in the structures of the templates. This indicates that the structures of the templates can be designed by controlling the original phase-separated structures of the mixed LB films. In this study, we demonstrate the availability of the template-directed patterning using phase-separated LB films. We investigate the phaseseparated structures of the mixed LB films of hydrogenated and perfluorinated amphiphiles using IR spectroscopy, atomic force microscopy (AFM), and friction force microscopy (FFM). Phase-separated structures can be controlled by the variations in the mixed amphiphilic species, the mixing ratios, and the subphase temperature. In particular, using a CA having hydrogenated and perfluorinated portions in the hydrophobic group as one of the components leads to the formation of nanothreads. Templates can be fabricated from the phase-separated LB films by heating and rinsing. The voids can be filled with another amphiphile using the LB technique. The amphiphiles introduced in the templates are confined in regions with the size and shape delineated by the templates fabricated from the phase-separated LB films. Experimental Section Chemicals. The amphiphiles used in this study and the abbreviations are shown below. The numbers following “F” and “H” are the lengths of the perfluorocarbon and the hydrocarbon, respectively. Eicosanoic acid (H19COOH) was purchased from Acros Organics. (Heptadecafluoro-1,1,2,2-tetrahydrodecyl)triethoxysilane (F8H2SiEt) and (heptadecafluoro-1,1,2,2-tetrahydrodecyl)trichlorosilane (F8H2SiCl) were obtained from Gelest, Inc. Octadecylamine (H18Am) was purchased from Tokyo Kasei Kogyo Co., Ltd. Eicosanol (H20OH) and 12,12,13,13,14,14,15, 15,16,16,17,17,18,18,19,19,19-heptadecafluorononadecanoic acid (F8H10COOH) were obtained from Wako Pure Chem. Ind., Ltd. These chemicals were used as received. (Heneicosafluoro-1,1,2,2tetrahydrododecyl)trimethoxysilane (F10H2SiMe) and (heptadecafluoro-1,1,2,2-tetrahydrodecyl)trimethoxysilane (F8H2SiMe) were synthesized as described in the literature.7 N-[p-(p-Octylphenylazo)phenyloxy]hexanoic acid (H8AzH5COOH) was synthesized as follows: 4-hydroxy-4′-octylazobenzene was synthesized by the coupling reaction of 4-octylaniline and phenol. The azo dye was refluxed for 30 min in butanol in the presence of potassium tert-butoxide and dicyclohexyl-18-crown-6 and reacted with ethyl 6-bromohexanoate overnight under reflux. The reaction mixture was put in water, extracted with chloroform, washed, dried, and recrystallized from methanol. The obtained ester was dissolved in ethanol and refluxed with KOH(aq) for 2 h. The reaction mixture was acidified with HCl(aq), filtered, washed with water, and dried. H8AzH5COOH was recrystallized from CCl4. The spreading solvents shown below were of spectroscopic grade. Hexane and chloroform were purchased from Dojindo. Tetrahydrofuran (THF) was obtained from Wako Pure Chem. Ind., Ltd. Monolayer Measurements. All the monolayer measurements were done using a Lauda Filmwaage (FW-1) at 20 °C unless otherwise stated. The size of the trough available for the spreading was 15 × 50 cm2. The spreading solvents were hexane for H19COOH, H18Am, and H20OH; chloroform for H8AzH5COOH; and hexane/THF (99:1 v/v) for F8H10COOH. Mixed spreading solutions of F10H2SiMe, F8H2SiEt, F8H2SiMe, and F8H2SiCl with the CAs were prepared using the same solvent (7) Yoshino, N.; Yamamoto, Y.; Hamano, K.; Kawase, T. Bull. Chem. Soc. Jpn. 1993, 66, 1754.

Langmuir, Vol. 20, No. 20, 2004 8729

Figure 1. Surface pressure-area isotherms of H19COOH, F10H2SiMe, and their mixtures with mixing ratio r at 20 °C. for dissolving the CAs. All the spreading solutions containing SCA were prepared under a nitrogen atmosphere. A spreading solution at a concentration of 1.0 × 10-3 M was spread on an aqueous subphase purified by passing through a Milli-Q filter. The molecules were compressed at a speed of (0.4-1.2) × 10-2 nm2 molecule-1 min-1 after 10 min of evaporation time. The surface pressure-area isotherms depended on the amount of spread solutions. The Langmuir film was transferred at 10 mN m-1 using the vertical dipping method at a withdrawal speed of 4 mm min-1 onto Si wafers with oxidized surfaces for transmission IR spectroscopic measurements and AFM and FFM observations. The Si wafers were kept in aqueous NH4OH and H2O2 at 353 K for 10 min and rinsed with water before use. Characterization. IR spectra of single-layer LB films were measured using a Perkin-Elmer Spectrum 2000 FTIR at an angle of incidence of 45° to avoid the effect of interference. The spectrometer was purged with nitrogen gas to minimize the amount of water vapor present in the sample chamber. The spectra were recorded at a 4-cm-1 resolution by coadding 256 scans in the 3200-800 cm-1 region. AFM observations were made using a Seiko SPA300 microscope in a noncontact mode using a silicon tip with a resonant frequency of 127 kHz and a spring constant of 15 N m-1 unless otherwise stated. AFM observations in a contact mode and FFM observations were made using a Si3N4 tip with a spring constant of 0.93 N m-1.

Results and Discussion Structures of Mixed Films of H19COOH and F10H2SiMe. The structures of the Langmuir and the LB films of SCAs have been studied. According to the literature,8,9 hydrolysis of SCAs on the water surface depends on pH and the hydrolyzable group of the SCAs. Under neutral conditions, trichloro derivatives are hydrolyzed whereas triethoxy derivatives remain almost unhydrolyzed. We consider that the reactivity of trimethoxy derivatives should be between those of trichloro and triethoxy derivatives. For simplicity, the Langmuir and LB films will be described by using the names of the spread chemical species irrespective of the extent of the hydrolysis. We study the structures of mixed Langmuir and LB films of H19COOH and F10H2SiMe. Figure 1 shows surface pressure-area isotherms of H19COOH, F10H2SiMe, and their mixtures. The phase transition of pure H19COOH that appears at about 26 mN m-1 becomes less pronounced with decreasing fraction of H19COOH. The surface pressure rises at about 0.6 nm2 for pure F10H2SiMe and increases gradually by compression, resulting in an expanded film. Considering that perfluorotetradecanoic acid forms a condensed film at about 0.25 nm2,5c the Langmuir film of F10H2SiMe should be (8) Ariga, K.; Okahata, Y. J. Am. Chem. Soc. 1989, 111, 5618-5622. (9) Iimura, K.; Suzuki, N.; Kato, T. Bull. Chem. Soc. Jpn. 1996, 69, 1201-1211.

8730

Langmuir, Vol. 20, No. 20, 2004

Matsumoto et al.

Figure 3. IR transmission spectra in the range 3000-2800 cm-1 of single-layer LB films of H19COOH, F10H2SiMe, and their mixtures with mixing ratio r, fabricated at 20 °C.

Figure 2. AFM images (25 × 25 µm2) of single-layer mixed LB films of H19COOH and F10H2SiMe with mixing ratios r of (A) 3/1, (B) 1/1, and (C) 1/3, fabricated at 20 °C.

unstable. We consider that F10H2SiMe is miscible with water to some extent and that some of the F10H2SiMe molecules gradually disperse in the water on the basis of the finding that the molecular area decreased gradually when the waiting time after the spreading was extended. We do not discuss the additivity rule for the surface pressure-area isotherms of the mixtures of H19COOH and F10H2SiMe because the Langmuir film of F10H2SiMe is unstable. AFM is a powerful tool for investigating the morphology of ultrathin films such as LB films. In particular, AFM has been used to study the morphology of LB films with phase-separated structures at the micrometer and nanometer levels2,4-6 and the morphology of the LB films whose structures can be changed by external stimuli.10 Figure 2 shows typical AFM images of the mixed LB films of H19COOH and F10H2SiMe. All the mixed films show phase-separated structures with hexagonal domains of the size of 1-6 µm. It is reported that the domains consist (10) (a) Matsumoto, M.; Tachibana, H.; Sato, F.; Terrettaz, S. J. Phys. Chem. B 1997, 101, 702-704. (b) Terrettaz, S.; Tachibana, H.; Matsumoto, M. Langmuir 1998, 14, 7511-7518. (c) Matsumoto, M.; Miyazaki, D.; Tanaka, M.; Azumi, R.; Manda, E.; Kondo, Y.; Yoshino, N.; Tachibana, H. J. Am. Chem. Soc. 1998, 120, 1479-1484. (d) Tachibana, H.; Yamanaka, Y.; Matsumoto, M. J. Phys. Chem. B 2001, 105, 10282-10286. (e) Matsumoto, M.; Nakazawa, T.; Azumi, R.; Tachibana, H.; Yamanaka, Y.; Sakai, H.; Abe, M. J. Phys. Chem. B 2002, 106, 11487-11491.

of hydrogenated species and that the other part is comprised of fluorinated species in the phase-separated structures of the mixed LB films.2,4-6 We assume that the domains observed in this study also consist of H19COOH and that the other part is comprised of F10H2SiMe. FFM has also been used to study the phase-separated structures of ultrathin films.2a,c,4a,b,5b,c,6a It is reported that friction is smaller on the hydrocarbon region than on the fluorocarbon region of the phase-separated structures of the mixed LB films of perfluorinated and hydrogenated amphiphiles.2a,c,4b,5b,c,6a We used FFM for the investigation of the structures of the mixed LB films of H19COOH and F10H2SiMe. Friction was smaller on the domain part than on the rest, supporting the assumption that the domains consist of H19COOH with the rest comprised of F10H2SiMe. The domain region was higher than the rest by 1-2 nm. The molecular lengths of H19COOH and F10H2SiMe were estimated to be about 2.6 nm and about 1.8 nm, respectively, by the Corey-Pauling-Koltun model. This suggests that the F10H2SiMe molecules are tilted in the mixed films. The domain size decreases slightly with a decrease in fraction of H19COOH in the region of mixing ratio r of 3/1 to 1/3. When the mixing ratio r of H19COOH to F10H2SiMe was reduced to 1/9, the domain size was much smaller. The area fractions of the domains were 0.9, 0.7, and 0.4 for the mixing ratios r of 3/1, 1/1, and 1/3, respectively. Considering that the cross section of perfluorocarbon is larger than that of hydrocarbon, the observed area fraction of the domains is larger than that expected from the mixing ratio and the cross-sectional areas of the two components. This suggests that not all the F10H2SiMe molecules spread were transferred as LB films. These results are consistent with the results of the surface pressure-area isotherms in that the Langmuir film of F10H2SiMe is unstable. IR spectroscopy has been used for studying the molecular orientation in ultrathin films.11 In particular, the peak intensities of the CH2 antisymmetric vibration mode νa(CH2) and symmetric vibration mode νs(CH2) depend strongly on the orientation of hydrocarbons. Figure 3 shows the region of the CH stretching vibration of the IR transmission spectra of the mixed LB films of H19COOH (11) (a) Rabolt, J. F.; Burns, F. C.; Schlotter, N. E.; Swalen, J. D. J. Chem. Phys. 1983, 78, 946. (b) Kimura, F.; Umemura, J.; Takenaka, T. Langmuir 1986, 2, 96-101. (c) Umemura, J.; Kamata, T.; Kawai, T.; Takenaka, T. J. Phys. Chem. 1990, 94, 62-67. (d) Shimomura, M.; Song, K.; Rabolt, J. F. Langmuir 1992, 8, 887-893. (e) Fujimoto, Y.; Ozaki, Y.; Kato, T.; Matsumoto, N.; Iriyama, K. Chem. Phys. Lett. 1992, 196, 347-352. (f) Gericke, A.; Huhnerfuss, H. J. Phys. Chem. 1993, 97, 12899-12908. (g) Azumi, R.; Matsumoto, M.; Kuroda, S.; Crossley, M. J. Langmuir 1995, 11, 4495-4498. (h) Kawai, T. Bull. Chem. Soc. Jpn. 1997, 70, 771-775. (i) Ren, Y.; Iimura, K.; Ogawa, A.; Kato, T. J. Phys. Chem. B 2001, 105, 4305-4312.

Template-Directed Patterning Using LB Films

Langmuir, Vol. 20, No. 20, 2004 8731

antisymmetric stretching mode. M is perpendicular to P when the hydrocarbon is in the all-trans conformation. M is in the X′-Y′ plane and has an angle ψ with the coordinate X′. In the coordinates (X′, Y′, Z′), the elements of M are expressed as follows:

M ) (M cos ψ, M sin ψ, 0)

(2)

The unit vectors X′, Y′, and Z′ are represented in the coordinates (X, Y, Z) as follows:

Figure 4. Space coordinates for expressing the orientation of amphiphiles.

X′ ) (cos θ cos φ, cos θ sin φ, -sin θ)

(3)

Y′ ) (-sin φ, cos φ, 0)

(4)

Z′ ) (sin θ cos φ, sin θ sin φ, cos θ)

(5)

By substituting eqs 3-5 into eq 2, we obtain the expression of M in the coordinates (X, Y, Z) as follows:

M ) (M cos ψ cos θ cos φ M sin ψ sin φ, M cos ψ cos θ sin φ + M sin ψ cos φ, -M cos ψ sin θ) (6) Absorbance A of the CH2 symmetric and antisymmetric stretching bands in the transmission spectra of the LB films is expressed as follows: Figure 5. Intensities of νa(CH2) and νs(CH2) of single-layer mixed LB films of H19COOH and F10H2SiMe as a function of the area fraction of the domains in the AFM images.

and F10H2SiMe. The bands of the CH3 antisymmetric vibration mode νa(CH3), CH2 antisymmetric vibration mode νa(CH2), and symmetric vibration mode νs(CH2) appear at 2960, 2917, and 2850 cm-1, respectively. The band positions of the latter two modes in this study show that H19COOH has an all-trans conformation at all the mixing ratios studied.11f The band intensity decreases with decreasing mixing ratio of H19COOH. However, the intensities are larger than those expected from the mixing ratios and the cross-sectional areas of the two components. This corresponds to the finding that the area fractions of the domains are larger than those expected from the mixing ratios and the cross-sectional areas of the two components. The area fraction of the domains and the band intensities of the IR spectra are discussed quantitatively. We neglect the contribution of F10H2SiMe to the intensities of the νa(CH2) and νs(CH2) bands because the LB film of F10H2SiMe gives no significant signals in Figure 3. We assume a constant tilt angle θ of the H19COOH carbon backbone with respect to the normal of the film and the absence of in-plane anisotropy of the orientation of H19COOH. The area fraction of the domains, AF, is expressed as follows:

AF ) daH19COOH/cos θ

(1)

Here, d is the density of H19COOH in the film and aH19COOH is the cross section of H19COOH. In Figure 4, we introduce Cartesian coordinates (X, Y, Z). The X-Y plane is chosen as the surface of the substrate, and the Z axis is perpendicular to the substrate. The vector P indicates the direction of the chain axis of the hydrocarbon of H19COOH and has a polar angle of θ. Space coordinates (X′, Y′, Z′) were also introduced to express the orientation of H19COOH in the LB films. The Y′ axis is parallel to the X-Y plane, and the Z′ axis has the same direction as P. M is the transition moments of the CH2 symmetric or

A ) dkM2{(cos2 β + 1)(cos2 θ + 1) + 2 sin2 β sin2 θ} (7) Here, k is a constant and β is the refractive angle of the incident light at the air-film interface. The angle β is obtained using Snell’s law:

n1 sin β′ ) n2 sin β

(8)

Here, n1, n2, and β′ are the refractive index of air, the refractive index of the film, and the incident angle, respectively. By substituting n1 )1.0, n2 )1.5, and β′ ) 45° in eq 8 and using eq 7, we obtain the following expression:

A ) dk′(cos2 θ + 1.7)

(9)

Here, k′ is a constant. It is to be noted that both AF and A depend on d in a linear fashion, whereas the dependences of AF and A on θ are different from each other. The intensities of νa(CH2) and νs(CH2) of single-layer mixed LB films of H19COOH and F10H2SiMe are plotted as a function of the area fraction of the domains in the AFM images (Figure 5). The intensities of the two bands give straight lines passing through the origin. Considering that AF and A depend on θ in different manners, the tilt angle of H19COOH should be constant irrespective of the molecular density. Quantitative analyses of the intensities assigned to CF2 stretching vibration modes were not attempted because of the small intensities. Variations in Phase-Separated Structures of Mixed Films of SCA and CA. The effect of the hydrolyzable group of SCA on the structures of the mixed LB films with H19COOH is investigated. Figure 6 shows the AFM images of the mixed LB films of H19COOH with F8H2SiEt, F8H2SiMe, and F8H2SiCl. All the mixed LB films exhibit phase-separated structures with circular domains of the size 1-7 µm. Area fractions of the domains are 0.9, 0.8, and 0.3 for the 1:1 mixed LB films of H19COOH with F8H2SiEt, F8H2SiMe, and F8H2SiCl, respectively. The former two area fractions of the domains are larger than those expected from the mixing ratios and cross-

8732

Langmuir, Vol. 20, No. 20, 2004

Figure 6. AFM images (25 × 25 µm2) of single-layer 1:1 mixed LB films of H19COOH with (A) F8H2SiEt, (B) F8H2SiMe, and (C) F8H2SiCl, fabricated at 20 °C.

sectional area of the two components, suggesting the unstability of the former two mixed Langmuir films and/ or the incomplete transfer of the SCA portions of the two mixed films. On the other hand, the calculation of the area fraction of H19COOH in the mixed LB films on the basis of the molecular areas of H19COOH and F8H2SiCl gives the value of about 0.4, which is not very different from the area fraction of the domains of 0.3. These results show that phase-separated structures depend strongly on the hydrolyzable group of SCA. Figure 7 shows the AFM images of the 1:1 mixed LB films of CA and SCA with variations in the chemical species. All the mixed LB films have phase-separated structures with CA domains surrounded by fluorinated SCA. In other words, it is possible to confine hydrogenated CA in the domains of mixed LB films irrespective of the hydrolyzable group and the presence of π electrons. Furthermore, the size and shape of the domains can be controlled by the combination of the mixed amphiphiles. Effect of Subphase Temperature on the PhaseSeparated Structures of Mixed LB Films. Phaseseparated mixed LB films of EA and 10F2S3E (EA/ 10F2S3E ) 1:1) were fabricated at subphase temperatures of 10, 20 and 30 °C. The AFM image of a single-layer mixed LB film fabricated at 20 °C resembled the image shown in Figure 6A in terms of the size and area fraction of the domains of EA. When the subphase temperature

Matsumoto et al.

Figure 7. AFM images (25 × 25 µm2) of single-layer 1:1 mixed LB films of (A) H8AzH5COOH and F10H2SiMe, (B) H18Am and F10H2SiMe, and (C) H20OH and F8H2SiEt, fabricated at 20 °C.

was 10 °C, domains of slightly smaller sizes than those in Figure 6A were observed. When the subphase temperature was as high as 30 °C, the size of the domains was much larger than those in Figure 6A. The size of some of the domains was larger than 10 µm. These results show that an increase in the subphase temperature gives rise to an increase in the size of the domains. Similar features were reported for mixed LB films of octadecanoic acid and perfluorinated amphiphilic carboxylic acid.6c The temperature dependence of the domain structures may be relevant to larger mobilities of molecules at higher subphase temperatures when the spreading solvent is present on the subphase. The evolution of the domain structures may be quenched after the evaporation of the solvent, resulting in larger domains at higher subphase temperatures.6c Formation of Nanostructures in the Mixed Films. The free energy change of mixing molecules A and B is given as follows:

∆GAB ) ∆HAB - T∆SAB

(10)

Here, ∆GAB, ∆HAB, and ∆SAB are the free energy change, the enthalpy change, and the entropy change, respectively. Phase separation occurs when ∆GAB is positive. We consider that phase separation occurs in the present study

Template-Directed Patterning Using LB Films

Langmuir, Vol. 20, No. 20, 2004 8733

Figure 9. Scheme of template-directed patterning using phaseseparated LB films.

Figure 8. AFM images (2.5 × 2.5 µm2) of single-layer mixed LB films of F8H10COOH and F8H2SiEt with mixing ratios r of (A) 1/3 and (B) 1/14, fabricated at 20 °C.

because the homo interaction between the hydrogenated species is much larger than the hetero interaction in the case of mixed Langmuir and LB films of hydrogenated and fluorinated amphiphiles. The variations in the hydrophobic groups will lead to the variations in the relative strength of the homo and hetero interactions. By using a CA having both hydrogenated and fluorinated portions in the hydrophobic group, the homo interaction will decrease to a value similar to that of the hetero interaction. The cohesive force between the CA molecules will also decrease. This methodology provides us with a means to control the phase-separated structures. Figure 8 shows the AFM images of the mixed LB films of F8H10COOH and F8H2SiEt. Nanothreads are formed in the LB films with mixing ratios of F8H10COOH to F8H2SiEt between 1/3 and 1/19. A closer look at the images shows that circular domains of the size 30-60 nm are connected with each other to form the nanothreads. The small size of the circular domains should be due to the controlling of the enthalpy terms originating from the hydrophobic groups. The reason the nanodomains are connected with each other is not clear at present. Formation of threads is also reported for the mixed LB films of long-chain fatty acids and perfluorinated amphiphilic carboxylic acid.6c Nanothreads are oriented along the dipping direction because of flow orientation during transfer12 when the fraction of F8H10COOH is small. Template-Directed Patterning using Phase-Separated LB Films. The above results show that the structures of phase-separated LB films can be controlled by the selection of the mixed amphiphiles and the mixing ratios. Figure 9 shows the scheme of template-directed patterning using phase-separated LB films. When the phase(12) (a) Minari, N.; Ikegami, K.; Kuroda, S.; Saito, K.; Saito, M.; Sugi, M. J. Phys. Soc. Jpn. 1989, 58, 222-231. (b) Sugi, M.; Minari, N.; Ikegami, K.; Kuroda, S.; Saito, K.; Saito, M. Thin Solid Films 1989, 178, 157-164. (c) Ikegami, K.; Mingotaud, C.; Delhaes, P. Phys. Rev. E 1997, 56, 1987-1997.

separated mixed LB films of CA and SCA are heated, the SCA molecules will form covalent bonds with the substrates and with each other, resulting in network structures between the SCA molecules. Subsequent treatment of the mixed LB films with solvent selectively removes the CA molecules, yielding the templates. The surface of the hydrophilic substrates is exposed where CA molecules used to reside whereas the hydrophobic surface of the SCA monolayers remains unchanged. The transfer of a monolayer onto the template by an upward stroke will lead to a selective transfer of the monolayer on the hydrophilic surface because the molecules in the monolayer are transferred with the hydrophilic group oriented to the template. Figure 10A shows the AFM image of a 1:1 mixed LB film of H19COOH and F10H2SiMe after heat treatment and rinsing with ethanol. The AFM image of the asdeposited LB film is shown in Figure 2A. The region where domains used to reside in the as-deposited film is lower than the rest by about 1 nm, showing that the H19COOH molecules were selectively removed by rinsing with ethanol. This leads to the exposure of the hydrophilic surface of the Si-wafer substrate. When a monolayer of H19COOH was transferred on this template by the LB method, a phase-separated structure similar to the one shown in Figure 2B was restored (Figure 10B). The results of the AFM observations were confirmed using IR spectroscopy. Figure 11 shows IR transmission spectrum of the as-deposited 1:1 mixed LB film of H19COOH and F10H2SiMe, that of the template, and that of the restored film. In the as-deposited film, absorption bands of the CH2 antisymmetric vibration mode νa(CH2) and CH2 symmetric vibration mode νs(CH2) are evident. These bands are absent in the IR spectrum of the template. After the monolayer of H19COOH is transferred on the template, these bands are evident with the intensities the same as those in the IR spectrum of the as-deposited film. This shows that the phase-separated structure of the mixed LB film of H19COOH and F10H2SiMe was restored by these procedures with the structure almost the same as those of the as-deposited film in terms of the film morphology and molecular orientation. However, SCAs should be completely hydrolyzed, followed by dehydration to form not only covalent bonds with the substrates but also network structures with each other. When the restored film was rinsed with

8734

Langmuir, Vol. 20, No. 20, 2004

Matsumoto et al.

Figure 12. AFM image (25 × 25 µm2) of the film fabricated by transferring a monolayer of H8AzH5COOH on the template shown in Figure 10A.

Figure 10. AFM images (25 × 25 µm2) of (A) the template fabricated from a single-layer 1:1 mixed LB film of H19COOH and F10H2SiMe fabricated at 20 °C, followed by heat treatment and rinsing with ethanol, and (B) the film fabricated by transferring a monolayer of H19COOH on the template.

transferred on the hydrophilic region of the template, giving a structure similar to those shown in Figures 2B and 10B, though the domains consist of H8AzH5COOH in Figure 12. It is to be noted that the structure shown in Figure 12 is different from that of the as-deposited 1:1 mixed LB film of H8AzH5COOH and F10H2SiMe shown in Figure 7A. On the other hand, when a monolayer of H19COOH was transferred on a template fabricated from the 1:1 mixed LB film of H8AzH5COOH and F10H2SiMe, we observed a structure similar to that shown in Figure 7A. Similar results were obtained for other templates and other amphiphiles for transfer. These results indicate the availability of template-directed patterning. In other words, amphiphiles can be confined in regions with the size and shape delineated by the templates fabricated from the phase-separated LB films. Conclusions

Figure 11. IR transmission spectrum of an as-deposited 1:1 mixed LB film of H19COOH and F10H2SiMe fabricated at 20 °C, that of the template, and that of the restored film.

ethanol, the template was restored. Transfer of a monolayer of H19COOH on the template also restored the morphology of the original film. Repetition of these procedures always gave similar structures, showing that the template can be reused. These results show that the scheme shown in Figure 9 is valid in the present system. The scheme in Figure 9 shows that the molecules present in the domain region can be replaced by different amphiphiles. This process was investigated by AFM. Figure 12 shows the AFM image of the film fabricated by transferring a monolayer of H8AzH5COOH on the template shown in Figure 10A. H8AzH5COOH molecules were

This study demonstrates the availability of templatedirected patterning using phase-separated LB films. The structures of the templates can be designed by controlling the original phase-separated structures. By tuning the homo and hetero interactions of the two components, nanothreads are formed in the phase-separated LB films. This demonstrates that the size and the shape of the domains can be controlled to form unique structures by tuning the homo and hetero interactions. Amphiphiles can be introduced on the templates, resulting in the confinement of the amphiphiles in regions with the size and shape delineated by the templates. These results demonstrate that a variety of amphiphiles can be confined two-dimensionally in a designed manner. This technique will be important in the fabrication of patterned materials on the micrometer or nanometer length scale for applications to memories, switches, and sensors. Acknowledgment. We are grateful to Dr. E. Manda for synthesizing H8AzH5COOH. This work was partly supported by a Grant-in-Aid for Science Research (No. 16655060) from the Ministry of Education, Culture, Sports, Science and Technology of Japan. LA0488250