© Copyright 1998 American Chemical Society
MARCH 3, 1998 VOLUME 14, NUMBER 5
Letters Molecular Aggregation State of n-Octadecyltrichlorosilane Monolayer Prepared at an Air/Water Interface Ken Kojio, Shouren Ge, Atsushi Takahara, and Tisato Kajiyama* Department of Materials Physics and Chemistry, Graduate School of Engineering, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan Received January 13, 1997. In Final Form: November 14, 1997 Molecular arrangement of polymerized n-octadecyltrichlorosilane (OTS, CH3(CH2)17SiCl3) monolayer transferred onto hydrophilic substrate by an upward drawing method was investigated with a transmission electron microscope (TEM) and an atomic force microscope (AFM). The electron diffraction (ED) pattern of the OTS monolayer revealed that the OTS molecules were regularly arranged in a hexagonal array with a (10) spacing of ca. 0.42 nm. The high-resolution AFM image of the OTS monolayer in a scan area of 10 × 10 nm2 exhibited the individual methyl group of which packing was a hexagonal array in a similar molecular arrangement concluded on the basis of the ED study. Also, the point defect in the crystalline OTS monolayer was successfully observed for the first time.
Introduction Organosilane compounds have been applied as coupling agents and surface treatment agents because the organosilane compounds are immobilized onto the material surface by strong chemical interaction. These monolayers are very stable to mechanical, thermal, and environmental attacks.1-4 Two methods have been proposed for the preparation of organosilane monolayers: a chemical adsorption from a solution (chemisorption method)1-3 and a Langmuir transfer from an air/water interface (upward drawing method, which is similar to the “vertical dipping method” where the substrate is immersed perpendicular to air/water interface before spreading a solution).4-8 When * Author to whom correspondence should be addressed. (1) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92. (2) Tillman, N.; Ulman, A.; Penner, T. L. Langmuir 1989, 5, 101. (3) Maoz, R.; Sagiv, J.; Degenhardt, D.; Mo¨hwald, H.; Quint, P. Supramol. Sci. 1995, 2, 9. (4) Ge, S. R.; Takahara, A.; Kajiyama, T. J. Vac. Sci. Technol. 1994, A12 (4), 2530. (5) Ge, S. R.; Takahara, A.; Kajiyama, T. Langmuir 1995, 11, 1341. (6) Takahara, A.; Kojio, K.; Ge, S. R.; Kajiyama, T. J. Vac. Sci. Technol. 1996, A14 (3), 1747.
the organosilane molecules are immobilized onto the material surface with the chemisorption method, the formation of an organosilane monolayer depends on random adsorption processes from a solution. Therefore, it is quite difficult to control the molecular aggregation state. In contrast, when the organosilane monolayer prepared by an upward drawing method is polymerized on the water surface at a certain surface pressure and then immobilized onto the silicon wafer substrate, the surface structure of this monolayer on the water surface is controllable by forming a monolayer at air/water interface. The electron diffraction (ED) studies of the organic monolayers enable crystallographical analyses such as the crystal system and lattice spacing of the monolayers.9 Also, atomic force microscopic (AFM) studies allow us a direct observation of molecular arrangement and structural defect in the monolayer. In our previous studies of (7) Kajiyama, T.; Ge, S. R.; Kojio, K.; Takahara, A. Supramol. Sci. 1996, 3, 123. (8) Ariga, K.; Okahata, Y. J. Am. Chem. Soc. 1989, 111, 5618. (9) Kajiyama, T.; Oishi, Y.; Uchida, M.; Morotomi, N.; Ishikawa, J.; Tanimoto, Y. Bull. Chem. Soc. Jpn. 1992, 65, 864.
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the fatty acid monolayers, the high-resolution AFM observation revealed that the hexagonal array of lignoceric acid (LA, CH3(CH2)22COOH) molecules was locally disordered10 and an edge dislocation was clearly imaged in the two-dimensional crystal lattice.11 Therefore, we anticipate similar information to be revealed about the OTS monolayer prepared by the upward drawing method. In this study, a two-dimensional molecular aggregation state and the point defect for the OTS monolayer prepared by the upward drawing method from the air-water interface were investigated on the basis of transmission electron microscope (TEM) and high-resolution AFM. Experiment Monolayer Preparation. n-Octadecyltrichlorosilane (OTS, CH3(CH2)17SiCl3, PCR Co., Ltd.) was used to prepare the monolayer. OTS was purified via distillation in vacuo. A toluene solution of OTS was prepared with a concentration of 2 × 10-3 M. Toluene was refluxed with sodium and distilled under atmospheric pressure just prior to use. The OTS solution was spread on the pure water surface at a subphase temperature of 293 K. Surface pressure-area (π-A) isotherm was measured with a computer-controlled homemade Langmuir trough. In order to polymerize and form the defect-diminished monolayer, the spread OTS molecules were kept on the water subphase under a certain constant surface pressure for 30 min. The polymeric monolayer formed on the water subphase was transferred onto the silicon wafer substrate surface by the upward drawing method and immobilized onto the substrate with Si-OH groups. In order to obtain a clean silicon wafer substrate with Si-OH groups, the silicon wafer substrate was pretreated by heating at 773 K for 1 h in a ceramic oven and then immersed in a mixed solution of concentrated H2SO4 and 30% H2O2 (70/30 v/v) at 363 K for 1 h. Characterization of Monolayer. In order to study the molecular aggregation state of the monolayer with TEM, the monolayer should be transferred onto the substrate without any change of the aggregation or crystallographic structure of the monolayer on the water subphase.9 Therefore, the monolayer was transferred onto the collodion-covered electron microscope grids (200 mesh) covered with an evaporated hydrophilic SiO layer by the upward drawing method. The ED pattern was obtained with TEM (Hitachi H-7000), which was operated at an acceleration voltage of 75 kV, a beam current of 0.5 µA, and an electron beam spot size of 10 µm diameter. The high-resolution AFM image of the monolayer was observed with AFM (SPA 300, Seiko Instruments Industry, Co., Ltd., Japan) in water at room temperature, using a 1 µm × 1 µm scanner and a silicon nitride tip attached to a cantilever with a spring constant of 0.021 N m-1. In water, the effect of adhesive force between tip and sample surface due to adsorbed water can be eliminated in AFM imaging. AFM was operated in the constant height mode.
Results and Discussion Figure 1 shows the π-A isotherm for the OTS monolayer on the pure water surface at a subphase temperature of 293 K. The π-A isotherm showed a steep increase in surface pressure with a decrease in surface area. The molecular occupied area, that is, the limiting area, was determined to be 0.24 nm2 molecule-1 from the π-A isotherm measurement. This value will be discussed later. The molecular aggregation state in the OTS monolayer was investigated on the basis of the ED pattern observation. Figure 2 shows the ED pattern of the OTS monolayer which was transferred onto a hydrophilic SiO substrate at a surface pressure of 15 mN m-1 at 293 K. The ED pattern exhibited hexagonal crystalline arcs which were broadened along an azimuthal direction.4 This indicates (10) Kajiyama, T.; Oishi, Y.; Hirose, F.; Shuto, K.; Kuri, T. Langmuir 1994, 10, 1297. (11) Kajiyama, T.; Oishi, Y.; Suehiro, K.; Hirose, F.; Kuri, T. Chem. Lett. 1995, 241.
Figure 1. Surface pressure-area (π-A) isotherm for OTS monolayer on the water subphase at the temperature of 293 K.
Figure 2. Electron diffraction (ED) pattern of an OTS monolayer which was transferred onto a hydrophilic SiO substrate by the upward drawing method at a surface pressue of 15 mN m-1 at the subphase temperature of 293 K.
that the crystalline OTS domains are gathered without any complete sintering at the domain interfaces. Also, the ED pattern showed that the (10) spacing of the OTS crystal is ca. 0.42 nm. The (10) spacing of the OTS monolayer agrees with that of the stearic acid (SA) monolayer. However, a change in the ED pattern of the OTS monolayer during a compression process was completely different from that for the SA monolayer. The ED pattern of the SA monolayer at 0 mN m-1 was a crystalline Debye ring and those at 24 and 40 mN m-1 were crystalline hexagonal spots. This change of the ED pattern from the crystalline Debye ring to the crystalline hexagonal spots during a compression process of the SA monolayer apparently indicates that the SA crystalline domains on the water surface are fused or recrystallized at the monolayer domain interface owing to sintering characteristics caused by compressive strain or stress, resulting in the formation of the larger two-dimensional crystalline monolayer domains.12,13 The ED patterns of the OTS monolayer at surface pressures of 10, 15, 35, and 50 mN m-1 showed crystalline hexagonal arcs. The size of OTS crystalline domain formed right after spreading solution on the water subphase was 1-2 µm in diameter by AFM.14 These results indicate that the crystallographical axes of the OTS monolayer domains are not completely aligned along a certain crystalline direction in a 10 µm diameter where electron beam irradiated; in other words, the broad crystallographical orientation of the OTS monolayer domains did not occur. Since the OTS molecules were polymerized on the water subphase, molecular aggregation (12) Kajiyama, T.; Oishi, Y.; Uchida, M.; Morotomi, N.; Kozuru, H. Langmuir 1992, 8, 1563. (13) Kajiyama, T.; Oishi, Y.; Uchida, M.; Takashima, Y. Langmuir 1993, 9, 1978. (14) Kojio, K.; Ge, S. R.; Takahara, A.; Kajiyama, T. Manuscript in preparation.
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Figure 3. (a) High-resolution nonfiltered and (b) low pass-filtered AFM images for a crystalline OTS monolayer on a scan area of 10 × 10 nm2. The monolayer was prepared on a silicon wafer substrate by the upward drawing method at a surface pressure of 15 mN m-1 at the subphase temperature of 293 K. Note the periodic arrangement of the molecules with a hexagonal array. (c) Magnification of the marked zone shown in (b).
rearrangement of the OTS molecules was fairly difficult and the sintering behavior at the monolayer-domain interface might not easily proceed over a wide range of the OTS monolayer during a surface compression process. Parts a and b of Figure 3 show the nonfiltered highresolution and the low pass-filtered high-resolution AFM images for the crystalline OTS monolayer transferred onto a silicon wafer substrate by the upward drawing method at a surface pressure of 15 mN m-1 at 293 K. The (10) spacing was estimated to be ca. 0.42 nm on the basis of the two-dimensional fast Fourier transform (2D-FFT) of the image shown in Figure 3a. This magnitude agreed well with the (10) spacing which was determined from the ED pattern for the crystalline OTS monolayer shown in Figure 2. Therefore, the coincidence of the (10) spacings calculated on the basis of the ED pattern and the highresolution AFM image apparently shows that the higher
portions (the bright dots) in the AFM images of parts a and b of Figure 3 represent the individual methyl group of the OTS molecule in the monolayer. The occupied areas evaluated from the ED pattern and the 2D-FFT of the high-resolution AFM image were 0.20 nm2 molecule-1. The magnitude of the occupied area is slightly different from that of 0.24 nm2 molecule-1 determined from the π-A isotherm shown in Figure 1. The cross sectional area of a linear alkyl chain is 0.19 nm2 molecule-1 and the one for the OTS monolayer on the water subphase is 0.20 nm2 molecule-1 based on X-ray diffraction measurements.15 The molecular cross sectional areas evaluated from the ED pattern and the 2D-FFT of the high-resolution AFM image agree well with that from the X-ray study mentioned above. On the other hand, the (15) Barton, S. W.; Goudot, A.; Rondelez, F. Langmuir 1991, 7, 1029.
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occupied area evaluated from the π-A isotherm was larger than the cross-sectional area of the linear alkyl chain or the OTS molecule. The difference in magnitudes of crosssectional areas indicates that a certain amount of defects and/or irregular packing region of alkyl chains exist in the OTS monolayer, resulting in an increase in an apparent limiting area of the OTS molecules based on the π-A isotherm. Indeed, the AFM observation performed for the OTS monolayer at micrometer scale (10 × 10 µm2) showed mesoscopic holes among crystalline domains.14 Since the OTS monolayer is a polymeric monolayer, it is easily expected that the defect and an amorphous region might be induced due to difficulties in molecular rearrangements during the water surface compression. It was reported on the basis of the high-resolution AFM image that the occupied area for the OTS monolayer prepared by a chemisorption method was 0.227 ( 15 nm2 molecule-1.16 The magnitude of short range ordering for the n-octadecyltriethoxysilane (OTE) monolayer prepared by the chemisorption method was 0.8 nm.17 Also, it was revealed from AFM observation at the micrometer scale (10 × 10 µm2) that the OTS monolayer prepared by the chemisorption method has a homogeneous surface.18,19 Though the OTS monolayer prepared by the upward drawing method has mesoscopic holes in the monolayer, as mentioned above, a high-resolution AFM image clearly revealed that the ordering of the OTS molecules was in a range of about 10 nm. Therefore, it seems reasonable to conclude that the crystalline phase of the OTS monolayer prepared by the upward drawing method is much closely packed than that prepared by the chemisorption method. (16) Fujii, M.; Sugisawa, S.; Fukuda, K.; Kato, T.; Seimiya, T. Langmuir 1995, 11, 405. (17) Xiao, X. D.; Liu, G.; Charych, D. H.; Salmeron, M. Langmuir 1995, 11, 1600. (18) Maoz, R.; Matlis, S.; DiMasi, E.; Ocko, B. M.; Sagiv, J. Nature 1996, 384, 150. (19) Frydman, E.; Cohen, H.; Maoz, R.; Sagiv, J. Langmuir 1997, 13, 5089.
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The image of Figure 3c corresponds to a magnification (3 × 3 nm2) of a zone analogous to the marked region shown in Figure 3b. Figure 3c apparently exhibits a point defect in the two-dimensional crystal lattice. Since the OTS molecule forms three equivalent reactive hydroxy groups by hydrolysis of chlorine groups, the OTS monolayer is polymerizable. This character might easily produce the point defect in the OTS monolayer in comparison with the case for the nonpolymerizable and crystallizable monolayer. However, it is fairly difficult to estimate the point defect density in the OTS monolayer, because no information about molecular arrangements in a region wider than 100 nm2 was obtained with AFM. However, it seems reasonable to consider that the point defect density in the monolayer depends on the preparation conditions of the monolayer on the water subphase and can be varied by controlling the rates of hydrolysis and polymerization. Conclusion TEM and high-resolution AFM analyses revealed that the crystalline OTS monolayer prepared by the upward drawing method was in the crystalline state, with OTS molecules closely packed in a hexagonal unit cell. The OTS monolayer prepared by an upward drawing method has a more highly oriented structure than that prepared by the chemisorption method, even though defect of the OTS monolayer was observed. Furthermore, the point defect in the crystalline OTS monolayer was successfully observed for the first time. Acknowledgment. This work was partially supported by Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists and by Grantin-Aid for COE Research and Scientific Research on Priority Areas, “Electrochemistry of Ordered Interfaces” (No. 282/09237252), from Ministry of Education, Science, Sports and Culture of Japan. LA970040P
