Dependence of the Molecular Aggregation State of Octadecylsiloxane

Dec 24, 2004 - Sono Sasaki,‡ Osami Sakata,‡ Hideyuki Otsuka,†,§ and Atsushi Takahara*,†,§. Department of Applied Chemistry, Graduate School ...
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Langmuir 2005, 21, 905-910

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Dependence of the Molecular Aggregation State of Octadecylsiloxane Monolayers on Preparation Methods Tomoyuki Koga,† Masamichi Morita,† Hideomi Ishida,† Hirohiko Yakabe,† Sono Sasaki,‡ Osami Sakata,‡ Hideyuki Otsuka,†,§ and Atsushi Takahara*,†,§ Department of Applied Chemistry, Graduate School of Engineering, Kyushu University, Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan, JASRI, Sayo, Hyogo 679-5198, Japan, and Institute for Materials Chemistry and Engineering, Kyushu University, Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan Received June 11, 2004. In Final Form: September 28, 2004 The molecular aggregation state of octadecylsiloxane monolayers on Si-wafer substrate surfaces prepared from octadecyltrimethoxysilane (OTMS) or octadecyltrichlorosilane (OTS) was investigated on the basis of grazing incidence X-ray diffraction (GIXD), Fourier transform infrared spectroscopy (FT-IR), contact angle measurement, field emission scanning electron microscopy (FE-SEM), and scanning force microscopy (SFM). The OTMS monolayer was prepared by using the chemical vapor adsorption (CVA) method, and the OTS monolayers, which were used as reference samples, were prepared either by chemisorption (OTSS) or by the water-cast method (OTS-W). The GIXD, FT-IR, lateral force microscopic (LFM) measurements, and FE-SEM observation revealed that the alkyl chains in the OTMS monolayers prepared using the CVA method are in an amorphous state at room temperature. According to the LFM measurement, the transition temperature from the hexagonal crystalline phase to the amorphous phase was found to be ca. 333 K for the OTS-S monolayer prepared by the chemisorption method. However, the phase transition was not observed in the OTMS monolayer prepared by the CVA method. Also, the atomic force microscopic (AFM) observation and the contact angle measurement showed that the OTMS monolayer prepared by the CVA method has a uniform surface when compared to the OTS monolayers. These results indicated that organosilane compounds in the monolayer prepared by the CVA method were immobilized on the Si-wafer substrate surface in an amorphous state, which was quite different from the hexagonal crystalline state obtained by the chemisorption and water-cast methods.

Introduction Organosilane compounds have been used as novel surface modifiers because they can be strongly immobilized onto various material surfaces by covalent and multiple hydrogen bonds.1-8 The monolayers prepared from organosilane compounds have attracted much attention because of their technological applications, such as in electronic devices9,10 and biomedical materials.11-14 The LangmuirBlodgett (LB) and chemisorption techniques have been * Corresponding author. E-mail: [email protected]. † Department of Applied Chemistry, Kyushu University. ‡ JASRI. § Institute for Materials Chemistry and Engineering, Kyushu University. (1) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92. (2) Maoz, R.; Sagiv, J. Thin Solid Films 1985, 132, 135. (3) Gun, J.; Sagiv, J. J. Colloid Interface Sci. 1986, 112, 457. (4) Tillman, N.; Ulman, A.; Schildkrau, J. S.; Penner, T. L. J. Am. Chem. Soc. 1988, 110, 6136. (5) Maoz, R.; Sagiv, J.; Degenhardt, D.; Mo¨hwald, H.; Quint, P. Supramol. Sci. 1995, 2, 9. (6) Grange, J. D. L.; Markham, J. L.; Kurkjian, C. R. Langmuir 1993, 9, 1749. (7) Ge, S.; Takahara, A.; Kajiyama, T. J. Vac. Sci. Technol., A 1994, 12, 2530. (8) Kojio, K.; Takahara, A.; Kajiyama, T. Colloid Surf., A 2000, 169, 295. (9) Collet, J.; Vuillaume, D. Appl. Phys. Lett. 1998, 73, 2681. (10) Vuillaume, D.; Boulas, C.; Collet, J.; Davidovits, J. V.; Rondelez, F. Appl. Phys. Lett. 1996, 69, 1646. (11) Stenger, D. A.; Georger, J. H.; Dulcey, C. S., Hickman, J. J.; Rudolph, A. S.; Nielsen, T. B.; McCort, S. M.; Calvert, J. M. J. Am. Chem. Soc. 1992, 114, 8435. (12) Ge, S.; Kojio, K.; Takahara, A.; Kajiyama, T. J. Biomater. Sci., Polym. Ed. 1998, 9, 131. (13) Takahara, A.; Hara, Y.; Kojio, K.; Kajiyama, T. Macromol. Symp. 2001, 166, 271. (14) Takahara, A.; Hara, Y.; Kojio, K.; Kajiyama, T. Colloids Surf., B 2002, 23, 141.

commonly used to prepare monolayers on a solid substrate. However, these techniques inevitably form monolayer defects and crystalline domains because the monolayer formation process in these techniques is dominated by the crystallization of organosilane molecules.11,12 In recent years, the chemical vapor adsorption (CVA) method has attracted much attention as a better technique for the preparation of organosilane monolayers.15-20 Sugimura et al. reported that the organosilane monolayers prepared by the CVA method possess remarkable uniform surfaces without defects or aggregates,21,22 and these uniform surfaces are suitable for a microfabrication process based on the photolithography of nanofilms. The physicochemical properties of the organosilane monolayers are governed by the molecular aggregation state.7,8,23-25 Although many studies have been conducted on the applications of organosilane monolayers prepared by the (15) Tada, H.; Nagayama, H. Langmuir 1994, 10, 1472. (16) Tada, H. J. Electrochem. Soc. 1995, 142, L11. (17) Tada, H.; Shimoda, K.; Goto, K. J. Electrochem. Soc. 1995, 12, L230. (18) Koga, T.; Otsuka, H.; Takahara, A. Chem. Lett. 2002, 1196. (19) Koga, T.; Morita, M.; Sakata, H.; Otsuka, H.; Takahara, A. Int. J. Nanosci. 2002, 1, 419. (20) Takahara, A.; Sakata, H.; Morita, M.; Koga, T.; Otsuka, H. Compos. Interfaces 2003, 10, 489. (21) Hozumi, A.; Ushiyama, K.; Sugimura, H.; Takai, O. Langmuir 1999, 15, 7600. (22) Hozumi, A.; Yokogawa, Y.; Kameyama, T.; Sugimura, H.; Hayashi, K.; Shirayama, H.; Takai, O. J. Vac. Sci. Technol., A 2001, 19, 1812. (23) Kajiyama, T.; Ge, S.; Kojio, K.; Takahara, A. Supramol. Sci. 1996, 3, 123. (24) Kojio, K.; Ge, S.; Takahara, A.; Kajiyama, T. Langmuir 1998, 14, 971. (25) Takahara, A.; Kojio, K.; Kajiyama, T. Ultramicroscopy 2002, 91, 203.

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CVA method, the relationship between the aggregation structure of the organosilane monolayers and their physicochemical properties still remains unclear. In the present study, the authors investigated the molecular aggregation state and the molecular motion of the octadecylsiloxane monolayers by using grazing incidence X-ray diffraction (GIXD) in conjunction with Fourier transform infrared (FT-IR) spectroscopy, lateral force microscopy (LFM), and field emission scanning force microscopy (FESEM). A relationship between the surface physicochemical properties and the molecular aggregation states of the octadecylsiloxane monolayers was studied using atomic force microscopy (AFM) and a contact angle measurement. Experimental Section Materials and Sample Preparation. Octadecyltrichlorosilane (OTS, Chisso Co., Ltd.) and octadecyltrimethoxysilane (OTMS, Gelest Co., Ltd.) were used as organosilane compounds in this study. The OTS was used for the preparation of the reference samples for the investigation of the molecular aggregation state in the OTMS monolayers. The OTS and OTMS were purified by vacuum distillation. Toluene (Nacalai Tesque, Inc.) was refluxed with sodium for 6 h and distilled, and bicyclohexyl (Tokyo Kasei, Co., Ltd.) was dried with molecular sieves. Ethanol (Wako Pure Chemical Industries, Co., Ltd.), concentrated H2SO4 (Nacalai Tesque, Inc.), and 30% H2O2 (Santoku Chemical Industries, Co., Ltd.) were used as received without any further purification. Water for the contact angle measurement and for the preparation of the organosilane monolayer was purified with the NanoPure Water system (Millipore, Inc.). The substrates used in this study were Si-wafers from the Sumitomo Mitsubishi Silicon Corp., Co., Ltd. (one side polished, thickness of 0.5 mm) and Ferrotech Silicon, Co., Ltd. (one side polished, thickness 3 mm; or both sides polished, thickness 2 mm). The wafers were soaked in a mixed solution of concentrated H2SO4 and 30% H2O2 (70/30, v/v) at 363 K for 1 h. The treated Si-wafers were then rinsed with distilled water and ethanol. After they were dried in vacuo, the substrates were irradiated clean by a vacuum ultraviolet ray (VUV, λ ) 172 nm, a Xe excimer lamp, Ushio Electric, UER20-172) for 10 min at a pressure of 15 mmHg. The organosilane monolayers were prepared using three different procedures. The OTMS monolayers were prepared using the CVA method.18-20 The substrates were placed together with 0.2 mL of an organosilane compound into a 65 mL Teflon container in a N2 atmosphere. The container was then sealed with a cap and another stainless container. The stainless container was placed in an oven that was maintained at 423 K for 2 h. After the heating treatment, the substrates were taken from the container and were immediately rinsed with ethanol. Finally, the substrates were dried in vacuo. Some OTS monolayers were prepared onto the Si-wafer substrate from a 5 mM OTS bicyclohexyl solution using the chemisorption method in a N2 atmosphere at room temperature 25,26 for 1 h. Also, some OTS monolayers were prepared onto the Si-wafer substrate by using the water-cast method from a 5 mM toluene solution.27 Micropatterned samples for the investigations of the thicknesses, nanotribological behavior, and packing density of the octadecyl group were prepared using the photolithography method.18-21 Figure 1 shows the photolithography process of fabrication of the micropatterned organosilane monolayer. The first organosilane monolayer [OTMS monolayer prepared by CVA method (OTMS) or OTS monolayer prepared by water-cast method (OTS-W)] prepared on the Si-wafer surface was placed in an evacuated vacuum chamber. The sample was then covered with a photomask (Toyo Precision Parts MFG. Co., Ltd., 20 mm × 20 mm square, 4 µm width Cr pattern, 2 µm width slit with a 20 mm line length) to prepare for irradiation. The samples (26) Kojio, K.; Takahara, A.; Omote, K.; Kajiyama, T. Langmuir 2000, 16, 3932. (27) Kojio, K.; Tanaka, K.; Takahara, A.; Kajiyama, T. Bull. Chem. Soc. Jpn. 2001, 74, 1.

Figure 1. The photolithography process of the fabrication of a micropatterned organosilane monolayer. were irradiated for 15 min with VUV light generated by a Xe excimer lamp to photodecompose the first organosilane monolayer.28-32 The patterned sample was sonicated for 5 min in ethanol and then dried in vacuo. The second organosilane monolayer [OTS monolayer prepared by chemisorption method (OTS-S)] was immobilized on the photodecomposed area, which was coated by Si-OH groups as a residue of photodecomposed organosilane monolayer. Finally, the obtained micropatterned organosilane monolayer with first and second organosilane monolayer phases was rinsed in bicyclohexyl, toluene, and ethanol, and then dried in vacuo. GIXD Measurement. The GIXD measurement was carried out at a BL13XU beam line of SPring-8 using an incident X-ray with the wavelengths λ of 0.123 or 0.128 nm. In the GIXD measurement, a strong diffraction was observed on the ultrathin film when the incident angle (ai) to the sample was below a critical angle (ac).3,33 Under this condition, the X-ray underwent total external reflection and penetrated into the sample as evanescent waves. The ac of organosilane was ca. 0.15°, and, to analyze the structure of the organic ultrathin films, the GIXD measurement was conducted at ai ) 0.10°. Diffractions from the sample were detected in the in-plane direction by a scintillation counter. A (28) Dressick, W. J.; Calvert, J. F. Jpn. J. Appl. Phys. 1993, 32, 5829. (29) Sugimura, H.; Ushiyama, K.; Hozumi, A.; Takai, O. Langmuir 2000, 16, 885. (30) Saito, N.; Wu, Y.; Hayashi, K.; Sugimura, H.; Takai, O. J. Phys. Chem. B 2003, 107, 664. (31) Hayashi, K.; Saito, N.; Sugimura, H.; Takai, O.; Nagagiri, N. Langmuir 2002, 18, 7469. (32) Uosaki, K.; Quayum, M. E.; Nihonyanagi, S.; Kondo, T. Langmuir 2004, 20, 1207. (33) Kaganer, V. M.; Brezesinski, G.; Mo¨hwald, H.; Howes, P. B.; Kjaer, K. Phys. Rev. 1999, 59, 2141.

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Figure 2. The GIXD profiles of the organosilane monolayers: (a) OTMS monolayer, (b) OTS-S monolayer, and (c) OTS-W monolayer.

Figure 3. FT-IR spectra for the organosilane monolayers: (a) OTMS monolayer, (b) OTS-S monolayer, and (c) OTS-W monolayer. sample cell with a polyimide window was attached to a sample stage. Helium gas was passed through the cell during the GIXD measurement to prevent the sample from oxidation. A 0.2 mm thick aluminum foil was set in front of the sample to control the intensity of the incident X-ray beam. The GIXD profiles were fitted using a Lorentzian function on a linear background. All GIXD observations were carried out at room temperature. FT-IR Measurement. The FT-IR spectra were recorded at a resolution of 0.5 cm-1 with a Spectrum One (Perkin Elmer, Inc.) spectrometer, which was equipped with a mercurycadmium-tellurium detector. A p-polarized beam struck the sample at an incident angle of 74°.5 As substrates for the FT-IR measurements, both-sides-polished Si-wafer substrates were used to reduce the influence of fringes in the spectra.27 To obtain the spectra with high signal-to-noise ratios, 1024 scans were collected. FE-SEM Observation. The micropatterned samples were observed using a S-4300SE (Hitachi Co. Ltd) with accelerating voltages (Vacc) of 3 kV. The samples were observed without performing conductive metal coatings.32 AFM Observation and LFM Measurement. The monolayer thickness and surface morphology were evaluated by SFM observations. The AFM (TMX-2100, Veeco Inc.; SPA400, Seiko Instruments Inc.) was operated with a constant force mode under an ambient atmosphere at room temperature. It used a 20 µm × 20 µm scanner and a silicon nitride tip on a cantilever with a spring constant of 0.09 N m-1. The imaging force was in the range of 1-3 nN and was done in the air at room temperature with 50%-60% humidity. The thickness of the monolayer was estimated to measure the height difference between the monolayer surface and bare Si-wafer surface by AFM. The 10 AFM images were collected to calculate a roughness value as a number average. The monolayer’s surface roughness was calculated by using the root-mean-square (RMS) roughness, and all of the roughness values were calculated from AFM images of a 5 µm × 5 µm square. Temperature-dependent LFM images of the organosilane monolayer were obtained with AFM (E-sweep, Seiko Instruments Inc.) at a pressure of 10-3 Pa or under and a constant normal force. It used a 20 µm × 20 µm scanner and a silicon nitride tip on a cantilever with a spring constant of 0.09 N m-1. The imaging repulsive force was 0.1 nN. Static and Dynamic Contact Angle Measurement. The surface free energy was determined from the static contact angles of water and methylene iodide, which were based on Owens and Wendt’s method.34 The static and dynamic contact angles of the monolayers against water were measured by a handmade system (34) Owens, D. K.; Wendt, R. C. J. Appl. Polym. Sci. 1969, 13, 1741.

at 293-301 K. The dynamic advancing (θA), receding (θR), and sliding (R) contact angles were recorded while the water droplet slipped from the tilted substrate surfaces.

Results and Discussion Aggregation Structure of Organosilane Monolayers. Figure 2 shows the GIXD profiles of the organosilane monolayers: (a) the OTMS monolayer prepared by the CVA method, (b) the OTS monolayer prepared by the chemisorption method (OTS-S), and (c) the OTS monolayer prepared by the water-cast method (OTS-W). In Figure 3a, no crystalline diffraction peak was observed, indicating that the octadecyl groups in the OTMS monolayer were immobilized on the Si-wafer substrate in a disordered state. On the other hand, crystalline diffraction peaks were observed at qxy,max ) 14.9-15.2 nm-1 in Figure 3b and c. In our previous reports,8,23,26 electron diffraction (ED) of the OTS monolayer prepared by the LangmuirBlodgett (LB) method gave a (10) diffraction of the hexagonal crystalline lattice with a 0.42 nm spacing. Furthermore, the (10) spacing of the OTS-W monolayer was smaller than that of the OTS-S monolayer. These results apparently indicate that the OTS molecules in the monolayer prepared by the water-cast method are tightly packed in comparison with those prepared by the chemisorption method. The conformation of octadecyl groups in the organosilane monolayers was investigated using FT-IR spectroscopy. Figure 3 illustrates the FT-IR spectra for the organosilane monolayers: (a) the OTMS monolayer, (b) the OTS-S monolayer, and (c) the OTS-W monolayer. The peaks observed at 2916.5-2924.6, 2849.3-2855.5, and 2962.32963.4 cm-1 were, respectively, assigned to antisymmetric [νa(CH2)], symmetric [νs(CH2)], and antisymmetric [νa(CH3)] bands for the alkylene chains and terminal methyl groups of OTS and OTMS molecules.35,36 It has been reported that the peak positions of the νa(CH2) and νs(CH2) bands of the octadecyl groups in the hexagonal crystalline lattice are observed at 2915-2918 and 28462850 cm-1. They are also observed at greater than 2921 (35) Snyder, R. G.; Strauss, H. K.; Ellger, C. A. J. Phys. Chem. 1982, 86, 5145. (36) MacPhail, R. A.; Strauss, H. L.; Snyder, R. G.; Ellger, C. A. J. Phys. Chem. 1984, 88, 334.

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Figure 6. Relative lateral forces of each organosilane monolayer.

Figure 4. FE-SEM images of (a) the OTMS/OTS-S micropatterned monolayer surface and (b) the OTS-S/OTS-W micropatterned monolayer surface.

Figure 7. Temperature dependence of the lateral force of the OTMS/OTS-S micropatterned monolayer measured at a scanning rate of 10 µm s-1 in vacuo.

Figure 5. LFM images of (a) the OTMS/OTS-S micropatterned monolayer surface in the air at room temperature and (b) the OTS-S/OTS-W micropatterned monolayer surface in air at room temperature.

cm-1 and greater than 2852 cm-1 in the case of the amorphous alkylsilane monolayer, which has molecular chains with gauche-rich conformations.5,7,8 This suggested the gauche-rich conformation of octadecyl groups in the OTMS monolayer. On the other hand, Figure 4b and c revealed that the OTS monolayers prepared by the chemisorption and the water-cast methods were in a hexagonal crystalline state. The aggregation state differences among the organosilane monolayers with different preparation methods were investigated using FE-SEM observations. The difference in brightness in the FE-SEM image means that the amount of the emitted secondary electrons in the region

is greater than that in the other regions.29 Bitterman et al. discovered that a different molecular packing in chemically homogeneous organic monolayers can be identified by FE-SEM observations.37 In the case of chemically homogeneous organic monolayers, the amount of emitted secondary electrons depends on the packing density of molecules in the monolayer. Figure 4 shows the FE-SEM images of (a) the OTMS/OTS-S micropatterned monolayer surface and (b) the OTS-S/OTS-W micropatterned monolayer surface. In Figure 5, the OTMS, OTSS, and OTS-W phases can be easily distinguished by strong contrasts. The brightness of the image was in the order of (from brightest to darkest): OTS-W > OTS-S > OTMS. Taking into account the FE-SEM results, it is considered that the packing density magnitude of the monolayer is in the order of (from highest to lowest): OTS-W > OTS-S > OTMS. This is consistent with the GIXD results. The relationship between the molecular aggregation state and the molecular motion of the organosilane monolayers was investigated using LFM measurements. Figure 6 illustrates the LFM images of (a) the OTMS/ OTS-S micropatterned monolayer surface in the air at room temperature and (b) the OTS-S/OTS-W micropatterned monolayer surface in the air at room temperature. Figure 6 shows the relative magnitude of the lateral force of each organosilane monolayer. The LFM results indicate (37) Bittermann, A. G.; Jacobi, S.; Chi, L. F.; Fuchs, H.; Reichelt, R. Langmuir 2001, 17, 1872.

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Figure 8. AFM images of (a) the OTMS monolayer surface, (b) the OTS-S monolayer surface, (c) the OTS-W monolayer surface, and the line profiles of the white line part in the AFM images [(d) OTMS, (e) OTS-S, (f) OTS-W].

that the magnitude of the lateral force corresponds to the packing density of the organosilane molecules. In one of our previous studies, we revealed that the lateral force of the crystalline alkylsilane domain in the (crystalline/ amorphous) mixed alkylsilane monolayer was larger than that of the amorphous alkylsilane matrix.38 Because the alkylsilane monolayer was in an amorphous state at room temperature, its molecular motion was activated in comparison with crystalline monolayers. This means that a smaller sheer force can cause molecular bending and/or a conformational change of alkyl chains in the amorphous monolayer. Thus, it is considered that the lateral force of the OTMS monolayer with an amorphous state showed the smallest value. A remarkable difference in the molecular immobilization states of the organosilane monolayers was also confirmed by temperature-dependent LFM measurements. Figure 7 shows the temperature dependence of lateral force for the OTMS/OTS-S micropatterned monolayer at a scanning rate of 10 µm s-1 in vacuo. The lateral force of the OTS-S monolayer decreased at ca. 333 K. Previous temperaturedependent study of the OTS monolayer prepared by the LB method with ED and FT-IR25,38 revealed that the aggregation state of the OTS monolayer prepared by the LB method changed from the hexagonal crystalline phase to the amorphous phase at 333 K. When the OTS monolayer prepared by the LB method becomes an amorphous state, the molecular motion of the alkyl chain is activated in comparison with the crystalline state.38 Similar behavior was observed for the OTS-S monolayer. The lateral force of the OTS-S monolayer, which was in the hexagonal crystalline state at room temperature, started to decrease at 333 K. On the other hand, a clear decrease of the lateral force of the OTMS monolayer prepared by the CVA method was not observed between 303 and 373 K. This result indicates that the OTMS monolayer prepared by the CVA method does not have a phase transition temperature between 303 and 373 K. These results support the conclusion that OTMS molecules were immobilized on the Si-wafer substrate surface with a very different state as compared to OTS-S molecules. Surface Morphology and Wettability of Organosilane Monolayers. The surface morphology and thickness of the organosilane monolayers were evaluated by (38) Kojio, K.; Takahara, A.; Kajiyama, T. Langmuir 2000, 16, 9314.

Table 1. Static Contact Angles of the Organosilane Monolayers monolayer θH20/deg θCH2I2/deg γsd/mJ m-2 γsh/mJ m-2 γs/mJ m-2 OTMS OTS-S OTS-W

103.5 108.4 107.5

72.0 72.5 105.9

21.0 21.2 19.8

0.8 0.3 0.4

21.8 21.5 20.2

Table 2. Dynamic Contact Angles of the Organosilane Monolayers monolayer

θA/deg

θR/deg

θA - θR/deg

R/deg

OTMS OTS-S OTS-W

92.0 96.0 101.5

88.5 86.0 82.1

3.5 10.0 16.8

19.0 21.7 28.2

atomic force microscopic (AFM) measurements. Figure 8 shows the AFM images of (a) the OTMS monolayer surface, (b) the OTS-S monolayer surface, (c) OTS-W monolayer surface, and line profiles of the white line part in the AFM images [(d) OTMS, (e) OTS-S, and (f) OTS-W]. The AFM images revealed that the OTMS monolayer possessed highly uniform surfaces (RMS < 0.2 nm) and that there were no domains and defects on the OTMS monolayer. On the other hand, some crystalline domains of OTS molecules and defects were observed on the OTS-S and OTS-W monolayer surfaces (OTS-S, RMS ) 0.3-0.5 nm; OTS-W, 0.2-0.4 nm). The thicknesses of the monolayers estimated by the AFM observation of the micropatterned samples were ca. 2.0, 2.1, and 2.3 nm for OTMS, OTS-S, and OTS-W, respectively. There is a relationship between the molecular aggregation state and the surface morphology of organosilane monolayers. It is considered that the inhomogeneous surfaces of the OTS-S and OTS-W monolayers might be due to the formation of OTS microcrystals on the Si-wafer substrate surfaces.26,27,39,40 Conversely, the OTMS monolayer formed highly homogeneous aggregation structures because of the disordered OTMS molecules. The relatively lower thickness of the OTMS monolayer also suggested disordered octadecyl groups in the OTMS monolayer. The highly uniform surface of the OTMS monolayer showed a characteristic wetting behavior in the contact angle measurements. Tables 1 and 2 display the static and dynamic contact angles of the alkylsilane monolayers, (39) Helmy, R.; Fadeev, A. Y. Langmuir 2002, 18, 8924. (40) Bautista, R.; Hartmann, N.; Hasselbrink, E. Langmuir 2003, 19, 6590.

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respectively. In all of the static contact angle results, there were no significant differences among any of the samples. However, a remarkable difference was observed in hysteresis, θA - θR, of the dynamic contact angle measurements. The OTMS monolayer prepared by the CVA method exhibited a small θA - θR value in comparison with the OTS-S and OTS-W monolayers. Thus, it is considered that the small θA - θR value of the OTMS monolayer was associated with the highly uniform surface of the OTMS. It is considered that the relatively large θA - θR magnitude of the OTS-S and OTS-W monolayers is ascribed to the surface heterogeneity, which includes domains and defects on the surface. Conclusion The molecular aggregation state in either crystalline or amorphous organosilane monolayers with octadecyl groups was studied using GIXD, FT-IR, LFM, temperature-dependent LFM, contact angle measurement, AFM, and FE-SEM. The monolayers prepared by the chemisorption and water-cast methods were in hexagonal crystalline states at room temperature. Conversely, the monolayer prepared by the CVA method was in an amorphous state. Temperature-dependent LFM revealed that the OTMS monolayer prepared by the CVA method had no clear phase transition point when compared to the OTS-S monolayer, which shows a phase transition at ca.

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333 K. The aggregation structure of the OTMS monolayer prepared by the CVA method influenced the morphology and wettability. The small hysteresis in the dynamic contact angle was due to the homogeneous molecular aggregation structure. Because the aggregation state of organosiloxane monolayer is cleared, it would be expected to control the surface physicochemical property of the organosiloxane monolayer for an application in electronic devices, biomedical materials, etc. Acknowledgment. This work was partially supported by a Grant-in-Aid for the 21st century COE Program “Functional Innovation of Molecular Informatics” from the Ministry of Education Culture, Science, Sports, and Technology of Japan. T.K. acknowledges the financial support by Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists. The synchrotron radiation X-ray diffraction experiments were performed at SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) as Nanotechnology Support Project of The Ministry of Education, Culture, Sports, Science, and Technology, Japan (Proposal 2002B0227- ND1-np/BL-13XU). The FESEM observation was performed using S-4300SE (Hitachi Co. Ltd.) at the Collabo-station II, Kyushu University. LA048544S