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Construction of a Defect-Diminished Fatty Acid Crystalline Monolayer by a Multistep Creep Method Based on Its Area-Creep Characteristics Taishi Kuri,† Fuminobu Hirose,† Yushi Oishi,‡ Kazuaki Suehiro,‡ and Tisato Kajiyama*,† Department of Materials Physics and Chemistry, Graduate School of Engineering, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-81, Japan, and Department of Applied Chemistry, Faculty of Science and Engineering, Saga University, 1 Honjo-machi, Saga 840, Japan Received February 18, 1997. In Final Form: June 17, 1997X A novel preparation method to obtain a defect-diminished fatty acid crystalline monolayer, that is, a multistep creep method, was proposed on the basis of area-creep mechanisms of the monolayer on the water surface. The fatty acid monolayer prepared by the multistep creep method was morphologically homogeneous and mechanically stable even at a high surface pressure, while the monolayer prepared by a conventional continuous compression method without an area-creep process was easily collapsed. The crystallographical regularity of the crystalline monolayer prepared by the multistep creep method was remarkably progressed compared with that of the monolayer prepared by the continuous compression method. Therefore, the multistep creep method is applicable for the construction of the defect-diminished monolayer.
Introduction Since Langmuir-Blodgett (LB) films can provide the desired structure at a molecular level, LB films composed of various functional organic ultrathin films can be used in molecular electronics,1,2 sensors,3,4 optical waveguides,5-7 and so on. For these applications of the LB films, it is necessary to attain the ultimate functional properties of the LB films. However, the LB film cannot exhibit its functional property because of the structural defects that exist in LB films.8 Therefore, construction of the defectdiminished monolayer is required in order to realize the ultimate functional properties of LB films. From the above reasons, many preparation methods have been proposed for the construction of the defect-diminished monolayer.9-12 The monolayer on a water surface, the precursor of the LB film, frequently exhibits area-creep phenomena,13-17 that is, a surface area deteriorates with time at a constant surface pressure. The area-creep behavior of the mono* Author to whom correspondence should be addressed. † Kyushu University. ‡ Saga University. X Abstract published in Advance ACS Abstracts, August 1, 1997. (1) Tieke, B. Adv. Mater. 1990, 2, 222. (2) Hua, Y. L.; Jiang, D. P.; Shu, Z. Y.; Petty, M. C.; Roberts, G. G.; Ahmad, M. M. Thin Solid Films 1990, 192, 383. (3) Fuchs, H.; Ohst, H.; Prass, W. Adv. Mater. 1991, 3, 10. (4) Anzai, J.; Osa, T. Sel. Electrode Rev. 1990, 12, 3. (5) Kuri, T.; Honda, N.; Oishi, Y.; Kajiyama, T. Chem. Lett. 1994, 2223. (6) Hickel, W.; Appel, G.; Lupo, D.; Prass, W.; Scheunemann, U. Thin Solid Films 1992, 210/211, 182. (7) Swalen, J. D. J. Mol. Electron. 1986, 2, 155. (8) Yuda, E.; Uchida, M.; Oishi, Y.; Kajiyama, T. Rep. Prog. Polym. Phys. Jpn. 1989, 32, 151. (9) Kuri, T.; Muto, K.; Oishi, Y.; Suehiro, K.; Kajiyama, T. Chem. Lett. 1997, 417. (10) Kajiyama, T.; Oishi, Y.; Kuri, T. Thin Solid Films 1996, 273, 84. (11) Kajiyama, T.; Kuri, T. Bull. Mater. Sci. 1995, 18, 375 (12) Kuriyama, K.; Kajiyama, T. Bull. Chem. Soc. Jpn. 1993, 66, 2522. (13) Kuri, T.; Oishi, Y.; Kajiyama, T. Bull. Chem. Soc. Jpn. 1994, 67, 942. (14) Kuri, T.; Oishi, Y.; Kajiyama, T. Trans. Mater. Res. Soc. Jpn. 1994, 15A, 567. (15) Tanizaki, T.; Takahara, A.; Kajiyama, T. J. Soc. Rheol., Jpn. 1991, 19, 208. (16) Aveyard, R.; Binks, B. P.; Carr, N.; Cross, A. W.; Gray, G. W.; Kilvert, P. V. A. Colloids Surf. 1992, 65, 29.
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layer is strongly related to the mechanical properties, because this reflects the aggregation structure of the monolayer on the water surface.18-20 Hence, analyses of the area-creep mechanisms of the monolayers must give us good information on how to construct the defectdiminished monolayer. Area-creep behavior of the fatty acid crystalline monolayer on the water surface has been discussed.13,14 The area-creep behavior of the crystalline monolayer was classified mainly into two regions on the basis of the magnitudes of the surface pressure of the monolayer.13 At a low surface pressure, the vacancies among the crystalline domains in the monolayer were fully filled up with areacreep time, and therefore, the crystallographical regularity of the monolayer progressed without the collapse of the monolayer. On the other hand, at a high surface pressure, the fatty acid molecules in the monolayer were rearranged in an initial stage of the area-creep, and then, the monolayer collapsed in a long time range of the areacreep. The collapse of the monolayer after the long time area-creep at a high surface pressure was caused by the mechanical instability of the monolayer because of its structural heterogeneity. However, the crystallographical regularity in the monolayer prepared after the area-creep at a high surface pressure was more progressed than the case of the monolayer prepared after the area-creep at a low surface pressure. Therefore, it can be expected from the classification of the area-creep phenomena that the stepwise alternating compression and area-creep method with a gradual increase of the surface pressure after a sufficient structural relaxation of the monolayer at each step might provide a mechanically stable and defectdiminished fatty acid monolayer. In this study, the defect-diminished fatty acid crystalline monolayer was prepared by the multistep creep method and the molecular aggregation in the monolayer was (17) Vollhardt, D.; Retter, U.; Siegel, S. Thin Solid Films 1991, 199, 189. (18) Kajiyama, T.; Kozuru, H.; Takashima, Y.; Oishi, Y.; Suehiro, K. Supramol. Sci. 1995, 2, 107. (19) Kajiyama, T.; Oishi, Y.; Uchida, M.; Takashima, Y. Langmuir 1993, 9, 1978. (20) Kajiyama, T.; Oishi, Y.; Uchida, M.; Morotomi, N.; Ishikawa, J.; Tanimoto, Y. Bull. Chem. Soc. Jpn. 1992, 65, 864.
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Scheme 1. Preparation Process of Stearic Acid Crystalline Monolayer by the Multistep Creep Method
Figure 1. Schematic representation of water level maintainer.
examined with a transmission electron microscope (TEM) and an atomic force microscope (AFM). Experimental Section Monolayer Preparation by Multistep Creep Method. Stearic acid (CH3(CH2)16COOH, chromatographical reference quality) was used without further purification. A benzene with spectroscopic quality was used as solvent. A benzene solution of stearic acid was prepared with a concentration of 3.0 × 10-3 M (1 M ) 1 mol‚dm-3). Subphase water was purified by the Milli Q-II system (Millipore Co., Ltd.). The sample solution was spread on the pure water surface at a subphase temperature (Tsp) of 293 K. Since a Tsp of 293 K is below the crystalline relaxation temperature (TRc) and the melting temperature (Tm) of the stearic acid monolayer (TRc ) 298 K, Tm ) 317 K),20 stearic acid molecules form a fusing-oriented crystalline monolayer.18,19 The dimensions of the trough were 404 mm in length, 150 mm in width, and 5 mm in depth. A subphase water level was maintained by a home-made water level maintainer as shown in Figure 1. The surface pressure was measured by the Wilhelmy method. Compression and area-creep measurements of the monolayer were carried out with a microprocessor-controlled film balance system (FSD-20, USI System Co., Ltd.). The crystalline monolayers of stearic acid were prepared by both the continuous compression and the multi-step creep methods. Scheme 1 shows the preparation process of the stearic acid crystalline monolayer by the multistep creep method. The stearic acid monolayer was compressed to a surface pressure of 10 mN‚m-1 at a constant rate of an area change of 1.1 × 10-3 nm2‚molecule-1‚s-1. After the monolayer was compressed, the variation of the monolayer area was measured to evaluate the quasi-equilibrium state, while maintaining the monolayer at the surface pressure of 10 mN‚m-1. Then, the monolayer was further compressed to 13 mN‚m-1 at a constant rate of 5.5 × 10-5 nm2‚molecule-1‚s-1, and again, area-creep at 13 mN‚m-1 was measured to classify the quasi-equilibrium state. By the stepwise compression mentioned above, the monolayer was finally compressed to a surface pressure of 24 mN‚m-1. Electron Microscopic Observation. The stearic acid monolayer on the water surface was transferred onto a hydrophilic SiO substrate (static water contact angle θ ) 30°), on which the monolayer could be transferred without any phase or crystallographical change.20 The substrate was prepared by vapordeposition of SiO onto a Formvar-covered21 electron microscope grid (200-mesh). The monolayer was transferred onto the substrate by a vertical dipping method at a transfer rate of 1.0 mm‚s-1. Bright field electron micrographs and electron diffraction (ED) patterns were taken with a Hitachi H-7000 transmission electron microscope (TEM), which was operated at an acceleration voltage of 75 kV, a beam current of 0.5 µA, and an electron beam (21) Fereshtehkhou, S.; Neuman, R. D.; Ovalle, R. J. Colloid Interface Sci. 1986, 109, 385.
spot diameter of 2 µm. TEM observations were carried out at 293 K, corresponding to Tsp at which the monolayer was prepared on the water surface. Pt-carbon was vapor-deposited onto the monolayer samples with a shadowing angle of 23° for the bright field electron micrographs. Evaluation of Crystallographical Regularity of the Monolayer. Crystallographical regularity of the monolayer was evaluated on the basis of the magnitudes of crystallographical distortion (Dlat) and continuity (Llat) in a direction along the monolayer surface, that is, the hk direction.22 Dlat corresponds to the root mean square value of the differential rate between the positions of ideal crystalline lattice and real crystalline lattice and Llat the crystalline lattice length contributing to the electron diffraction coherency. These values were quantitatively evaluated by a modified single line method23 based on the Fourier analysis of the ED profiles. AFM Observation of Crystalline Monolayer. For AFM observations,22,24 the stearic acid monolayer prepared on the water surface was transferred onto a freshly cleaved mica (Okabe Mica) by the vertical dipping method. The transfer ratio of the monolayer was unity.25 This means that the mica substrate is completely covered with the monolayer. The AFM images of the monolayers were obtained with a SFA300 (Seiko Instruments, Inc.) in air, using a silicon nitride tip on a cantilever with a spring constant of 0.027 N‚m-1. The applied force upon scanning was ∼10-10 N. In order to reduce the noise component in raw AFM images, a digital filtering treatment for the Fouriertransformed image was carried out by keeping only the spatial frequencies corresponding to the spots of the Fourier-transformed image.
Results and Discussion Effect of Water Level Maintainer for Preparation of Monolayer. Figure 1 shows the schematic representation of the home-made water level maintainer for the trough. The water in vessel 2 was pumped up from vessel 1. As the excess volume of water in vessel 2 overflows into vessel 1, the height of the water level in vessel 2 is kept constant. Hence, the height of the water level in the trough can be kept constant through a connecting tube between the trough and vessel 2. Figure 2 shows the time dependence of the surface pressure (a) and the change of the height of the water level in the trough, ∆h (b), with and without the water level maintainer at Tsp of 293 K. In the case of the trough without the water level maintainer, the surface pressure and the height of the water level in the trough decreased gradually due to water evaporation. On the other hand, in the case of the trough with the water level maintainer, the surface pressure and the height of the water level remained constant within the variations of (0.2 mN‚m-1 and (0.2 mm, respectively. Therefore, it is expected that (22) Oishi, Y.; Hirose, F.; Kuri, T.; Kajiyama, T. J. Vac. Sci. Technol. A 1994, 12, 2971. (23) Hofmann, D.; Walenta, E. Polymer 1987, 28, 1271. (24) Kajiyama, T.; Oishi, Y.; Hirose, F.; Shuto, K.; Kuri, T. Langmuir 1994, 10, 1297. (25) Kuri, T.; Oishi, Y.; Kajiyama, T. Langmuir 1995, 11, 366.
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Figure 4. Time dependences of molecular occupied area in stearic acid crystalline monolayer at various surface pressures during multistep creep measurement.
Figure 2. Time dependences of surface pressure (a) and water level in trough (b).
Figure 3. Molecular occupied area with maintaining time at the constant surface pressure during the area-creep for the crystalline monolayer of stearic acid at 293 K.
the water level maintainer is effective for the long time necessary to prepare the monolayer on the water surface by the multistep creep procedure. Area-Creep Behavior of Stearic Acid Crystalline Monolayer during the Multistep Creep Process. Figure 3 shows the variation of the molecular occupied area for the stearic acid crystalline monolayer with maintaining time at each constant surface pressure at Tsp 293 K. The straight line shown by an arrow in Figure 3 represents the calculated molecular occupied area on the basis of the ED pattern of the stearic acid monolayer transferred onto the substrate at 10 mN‚m-1 at 293 K. The surface area of the monolayer at 15 and 24 mN‚m-1 decreased continuously with the creep time, perhaps owing to the collapse of the monolayer. On the other hand, the surface area for the cases of surface pressures of 1, 5, and 10 mN‚m-1 decreased gradually with creep time and almost remained constant after a long area-creep time. Also, in the case of the area-creep experiment of 10 mN‚m-1, the monolayer morphology was still homogeneous even after about 1 × 104 s of area-creep, and the
molecular occupied area became comparable to the calculated molecular occupied area. Furthermore, it had been reported that the crystallographical regularity of the monolayer was remarkably progressed by the areacreep method at 10 mN‚m-1 at 293 K.13 This apparently indicates that the area-creep at 10 mN‚m-1 at 293 K is useful for the construction of a defect-diminished crystalline monolayer. Hence, a surface pressure of 10 mN‚m-1 was determined as the pressure of the first step in the multistep creep procedure. Figure 4 shows the time dependence of molecular occupied area for the stearic acid crystalline monolayer at various surface pressures during the multistep creep measurements. The molecular occupied area of the monolayer became fairly constant after the long time areacreep even at surface pressures higher than 15 mN‚m-1, where the surface area of the monolayer without areacreep decreased remarkably by the collapse of the monolayer as well as the area-creep behavior, as shown in Figure 3. The area-creep behaviors at every surface pressure during the multistep creep process, which keep constant surface area even after the long time area-creep as shown in Figure 4, show the completion of the structural relaxation of the monolayer at each surface pressure. Hence, it seems from Figure 4 that the structural relaxation of the monolayer by the area-creep at each surface pressure was almost completed. Figure 5 shows the area-creep behaviors and the bright field images of the stearic acid crystalline monolayers at 24 mN‚m-1 and Tsp ) 293 K. The monolayers were prepared by (1) the multistep creep method and (2) the continuous compression method. The continuous compression method is a conventional one used to prepare the monolayer on the water surface. Then, the monolayer was compressed to the desired surface pressure at a constant compressing rate. In the case of the monolayer prepared by the continuous compression method, the surface area of the monolayer decreased strikingly with creep time and the monolayer morphology became heterogeneous at the creep time of 6 × 103 s. This remarkable decrease of the monolayer surface area might be attributed to the localized collapse of the monolayer as shown by the rough surface morphology. In contrast, the surface area of the monolayer prepared by the multistep creep method decreased very slightly with creep time and remained almost constant after the long time area-creep. The magnitude of the surface area of the monolayer after the multistep creep was comparable to that of the calculated surface area of the monolayer on the basis of its ED profile, indicating the closed packing of stearic acid molecules in the monolayer. Further, the electron micrographs of the monolayer surface showed that the monolayer was still
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Figure 5. Area-creep behaviors and bright field images of stearic acid crystalline monolayers prepared by (1) the multistep creep method and (2) the continuous compression method. Table 1. Crystallographical Regularity of Monolayer Prepared by Continuous Compression and Multistep Creep Methods preparation method continuous compression multisteep creep polyethylene (single crystal)
pressure crystallographical crystallographical mN‚m-1 distortion, Dlat/% continuity, Llat/nm 24
5.0
6.2
10
3.9
10.9
24
3.1
22.9
