Phase Transition of Alkylsilane Monolayers Studied by Temperature

Langmuir 2007, 23, 8861-8865

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Phase Transition of Alkylsilane Monolayers Studied by Temperature-Dependent Grazing Incidence X-ray Diffraction Tomoyuki Koga,† Koji Honda,‡ Sono Sasaki,§ Osami Sakata,§ and Atsushi Takahara*,†,‡ Institute for Materials Chemistry and Engineering, and Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu UniVersity, 744 Motooka Nishi-ku Fukuoka 819-0395 Japan, and JASRI/SPring-8, Sayo, Hyogo 679-5198, Japan ReceiVed March 6, 2007. In Final Form: June 9, 2007 The phase transition of organosilane monolayers on Si-wafer substrate surfaces prepared from octadecyltrichlorosilane (OTS) or docosyltrichlorosilane (DOTS) was investigated on the basis of grazing incidence X-ray diffraction (GIXD) at various temperatures. The OTS monolayer was prepared by a chemisorption method. The DOTS monolayer was prepared by a water-cast method (DOTS). The GIXD measurement clarified that the OTS monolayer also changed from hexagonal phase to amorphous state above a melting point of otadecyl groups. The GIXD measurements also clarified that the molecular aggregation state of the DOTS monolayer changes from an anisotropic phase to an isotropic phase with an increase in temperature. An estimated linear thermal expansion coefficient of the lattice lengths of a and b of the DOTS monolayer in the rectangular crystalline state assigned a similar value to those of bulk polyethylene with an orthorhombic crystalline lattice. The setting angle of the ab plane of the rectangular DOTS monolayer also showed similar behavior to that of the ab plane of bulk polyethylene.

Introduction Organosilane compounds have been applied as useful surface modifiers because they can be strongly immobilized onto various material surfaces through covalent bonds and multiple hydrogen bonds.1-10 The monolayers prepared from organosilane compounds are very stable to mechanical, thermal, and environmental change and show no exchange reaction. Therefore, the monolayers prepared from organosilane compounds have attracted much attention because of their technological applications.11-16 * [email protected]. † Institute for Materials Chemistry and Engineering, Kyushu University. ‡ Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University. § JASRI/SPring-8. (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. R.; Takahara, A.; Kajiyama, T. J. Vac. Sci. Technol., A 1994, 12, 2530. (8) Kojio, K.; Takahara, A.; Kajiyama, T. Colloids Surf., A 2000, 169, 295. (9) Blinker, C. J.; Schere, G. W. Sol-Gel Science, The Physics and Chemistry of Sol-Gel Processing; Academic Press: San Diego, 1990. (10) Kojio, K.; Takahara, A.; Kajiyama, T. In Fluorinated Surfaces, Coatings, and Films; Castner, D. G., Grainger, D. W., Eds; ACS Symposium Series 787; American Chemical Society: Washington, DC, 2001; Chapter 3, p 31. (11) Ge, S. R.; Kojio, K.; Takahara, A.; Kajiyama, T. J. Biomater. Sci. Polym. Ed. 1998, 9, 131. (12) Takahara, A.; Hara, Y.; Kojio, K.; Kajiyama, T. Macromol. Symp. 2001, 166, 271. (13) Takahara, A.; Hara, Y.; Kojio, K.; Kajiyama, T. Colloid Surf., B 2002, 23, 141. (14) Takahara, A. Dekker Encyclopedia of Nanoscience and Nanotechnilogy, Molecular Assembly Organosilanes, Schwarz, J. A., Contescu, C. I., Putyera, K., Eds.; Dekker Encyclopedia: New York, 2004; p 2031. (15) Koga, T.; Takahara A. Structure and Physicochemical Properties of Polyalkylsiloxane Monolayers Prepared onto the Solid Substrate, In AdVanced Chemistry of Monolayers at Interfaces -Trend of Methodology and Technology; Imae, T., Ed., Elsevier: Amsterdam, 2007; Chapter 8, pp 193-217. (16) Koga, T.; Takahara A. BOTTOM-UP NANOFABLICATION: Supramolecules, Self-Assemblies, and Organized Films: Structure and Properties of Area SelectiVely Assembled Organosilanes; Ariga, K., Nalwa, S. H., Eds.; American Scientific Publishers: Stevenson Ranch, in press.

The surface physicochemical properties of organic ultrathin films such as the organosilane monolayers and alkane thiol selfassembled monolayers strongly depend on the molecular aggregation state on the substrate surfaces. It is well-known that the various physicochemical properties of ultrathin films depend on their phase transition behavior. Gurau et al.17 and Liu et al.18 revealed the phase transition of the octadecylamine monolayer prepared by the Langmuir-Blodgett (LB) technique and the single lipid bilayer based on sum frequency generation spectroscopy, respectively. Osman et al.19 have reported the phase transition behavior of octadecyltrimethylammonium chloride monolayer prepared on a mica substrate by Fourier transformation infrared (FT-IR) spectroscopy, X-ray diffraction study, differential scanning calorimetry, and thermogravimetric analysis. In addition, our group has revealed the relationship between thermal molecular motion and the molecular aggregation state of organosilane monolayers with long alkyl chains prepared at the air/water interface by electron diffraction (ED), FT-IR spectroscopy, and lateral force microscopy (LFM).20,21 In the course of our studies of organosilane monolayers, we have clarified the phase transition behavior and the surface frictional properties of long alkyl chains in organosilane monolayers. The rectangular crystalline phase of the organosilane monolayer transformed to the amorphous phase via the hexagonal crystalline phase. Furthermore, the surface lateral force decreased with the phase transition process due to the decrease of shear strength at the surface region. However, detailed investigation of the thermal expansion of the crystalline lattice in organosilane monolayer during the phase transition has not yet been achieved due to the thermal decomposition of samples in the high-temperature region. Clarifying the thermal expansion behavior of organosilane monolayer is necessary to understand (17) Gurau, M. C.; Castellana, E. T.; Albertorio, F.; Kataoka, S.; Lim, S.-M.; Yang, R. D.; Cremer, P. S. J. Am. Chem. Soc. 2003, 125, 11166. (18) Liu, J.; Conboy, J. C. J. Am Chem. Soc. 2004, 126, 8994. (19) Osman, M. A.; Seyfang, G.; Suter, U. W. J. Phys. Chem. B 2000, 104, 4433. (20) Kojio, K.; Takahara, A.; Kajiyama, T. Langmuir 2000, 16, 9314. (21) Takahara, A.; Kojio, K.; Kajiyama, T. Ultramicroscopy 2002, 91, 203.

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its various physicochemical properties, which depend on the phase transition behavior. In order to investigate the two-dimensional molecular aggregation structure of organic ultrathin films at the air/liquid or the air/solid interface, many researchers have tried a surface X-ray diffraction and a X-ray reflectivity measurement.5,21-41 In particular, a grazing incidence X-ray diffraction (GIXD) measurement has attracted much attention as a technique for investigating the transformation of molecular aggregation state of organic ultrathin films, because we can directly obtain a variety of information about the two-dimensional aggregation state of the surface of materials.5,21,23,25,29,37-42 Carino et al. clarified the crystallization process of long alkyl chains of organosilane monolayer at the air/water interface by a real-time GIXD.41,42 Kaganer et al. reported the temperature-dependent phase transition behavior of octadecanol monolayer at air/water interface.37 Huang et al. evaluated the thermal expansion behavior of a watersupported perfluoro-n-eicosane monolayer by the GIXD.25 In the present study, the authors investigated the phase transition behavior of organosilane monolayers with long alkyl chains immobilized on Si-wafer substrate surfaces by GIXD measurement at various temperatures using a synchrotron beam as an X-ray source. Experimental Section Materials. Octadecyltrichlorosilane [CH3(CH2)17SiCl3, OTS, Chisso Co., Ltd.] and docosyltrichlorosilane [CH3(CH2)21SiCl3, DOTS, Shin-Etsu Chemical Co., Ltd.] were used as organosilane compounds in this study. The OTS and DOTS 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, (22) Kaganer, V. M.; Mo¨hwald, H.; Dutta, P. ReV. Mod. Phys. 1999, 71, 779. (23) Kuzmenko, I.; Rapaport, H.; Kjaer, K.; Als-Nielsen, J.; Weissbuch, I.; Lahav, M.; Leiserowitz, L. Chem. ReV. 2001, 101, 1659. (24) Tidswell, I. M.; Ocko, B. M.; Pershan, P. S.; Wasserman, S. R.; Whitesides, G. M. Phys. ReV. B 1990, 41, 1111. (25) Huang, Z.; Schlossman, M. L.; Acerro, A. A.; Zhang, Z.; Lei, N.; Rice, S. A. Langmuir 1995, 11, 2742. (26) Magnussen, O. M.; Ocko, B. M.; Deutsch, M.; Regan, M. J.; Pershan, P. S.; Abernathy, D.; Gru¨bel, G.; Legrand, J.-F. Nature (London) 1996, 384, 250. (27) Maoz, R.; Mattis, S.; DiMasi, E.; Ocko, B. M.; Sagiv, J. Nature (London) 1996, 384, 150. (28) Bo¨hm, C.; Leveiller, F.; Jacquemain, D.; Mo¨nwald, H.; Kjaer, K.; AlsNielsen, J.; Weissbuch, I.; Leiserowitz, L. Langmuir 1994, 10, 830. (29) Genson, K. L.; Holzmuller, J.; Villacencio, O. F.; McGrath, D. V.; Vaknin, D. V.; Tsukruk, V. V. J. Phys. Chem. B 2005, 109, 20393. (30) Melzer, V.; Weidemann, G.; Vollhardt, D.; Brezesinski, G.; Wagner, R.; Struth, R.; Mo¨hwald, J. Phys. Chem. B 1997, 101, 4752. (31) Kago, K.; Fu¨rst, M.; Katsuoka, H.; Yamaoka, H.; Seki, T. Langmuir 1999, 15, 2237. (32) Kago, K.; Seki, T.; Schu¨cke, R. R.; Mouri, E.; Matsuoka, H.; Yamaoka, H. Langmuir 2002, 18, 3875. (33) Mouri, E.; Furuya, Matsumoto, K.; Matsuoka, H. Langmuir 2004, 20, 8062. (34) Matsuoka, H.; Furuya, Y.; Kaewasaiha, P.; Mouri, E.; Matsumoto, M. Langmuir 2005, 21, 6842. (35) Weissbuch, I.; Baxter, O. N. W.; Cohen, S.; Cohen, H.; Kjaer, K.; Howea, P. B.; Als-Nielsen, J.; Hanan, G. S.; Schubert, U. S.; Lehn, J.-M.; Leiserowitz, L.; Lahav, M. J. Am. Chem. Soc. 1998, 120, 4850. (36) Yim, H.; Foster, M. D.; Engelking, J.; Menzel, H.; Ritcy, A. M. Langmuir 2000, 16, 9792. (37) Kaganer, V. M.; Brezesinski, G.; Mo¨hwald, H.; Howes, P. B.; Kjaer, K. Phys. ReV. E 1999, 59, 2141. (38) Kojio, K.; Takahara, A.; Omote, K.; Kajiyama, T. Langmuir 2000, 16, 3932. (39) Kojio, K.; Tanaka, K.; Takahara, A.; Kajiyama, T. Bull. Chem. Soc. Jpn. 2001, 74, 1. (40) Koga, T.; Morita, M.; Ishida, H.; Yakabe, H.; Sasaki, S.; Sakata, O.; Otsuka, H.; Takahara, A. Langmuir 2005, 21, 905. (41) Carino, S. R.; Tostmann, H.; Underhill, R. S.; Logan, J.; Weerasekera, G.; Culp, J.; Davidson, M.; Duran, R. S. J. Am. Chem. Soc. 2001, 123, 767. (42) Carino, S. R.; Underhill, R. S.; Tostmann, H. S.; Skolnik, A. M.; Logan, J. L.; Davidson, M. R.; Culp, J. T.; Duran, R. S. Langmuir 2003, 19, 10514.

Koga et al. Inc.), and 30% H2O2 (Santoku Chemical Industries, Co., Ltd.) were used as received without further purification. Water for preparation of the organosilane monolayer was purified with the NanoPure Water system (Millipore, Inc.). The Si-wafer substrates used in this study were purchased from Sumitomo Mitsubishi Silicon Corporation, Co., Ltd. (thickness: 0.5 mm) and Ferrotech Silicon, Co., Ltd. (thickness: 3 mm). The Si wafers were immersed in a mixed solution of concentrated H2SO4 and 30% H2O2 (70/30, v/v) at 363 K for 1 h.7 The treated Si-wafers were then rinsed with distilled water and ethanol. After they were dried in vacuo, the substrates were irradiated a vacuum ultraviolet ray (VUV, λ ) 172 nm, a Xe excimer lamp, Ushio Electric, UER20-172) to photodecompose an organic contaminant at the surface of Si-wafer substrates for 10 min at a pressure of 15 mmHg.40,43-50 Sample Preparation. The OTS monolayer was prepared onto the Si-wafer substrate from a 5 mM OTS bicyclohexyl solution using a chemisorption method in N2 atmosphere at room temperature for 1 h.38 The Si-wafer substrate was rinsed with the bicyclohexyl, toluene, and ethanol. Finally, the samples were dried in vacuo. The DOTS monolayer was prepared on the Si-wafer substrate by using a water-cast method39,40 from a DOTS toluene solution. Water droplets with a total volume of 0.8 mL were put onto a Si-wafer with an area of ca. 5.0 cm2. The 5 mM DOTS toluene solution of 5.0 µL was subsequently spread in order to completely cover the water surface with the DOTS molecules. The DOTS monolayer was finally obtained after water evaporation at room temperature in air. Atomic Force Microscopic Observation. The monolayer thickness was evaluated by atomic force microscopy (AFM). The AFM (SPA-400, SII NanoTechnology Inc.) was operated with a length force mode under an ambient atmosphere at room temperature. AFM measurements were carried out using a 20 µm × 20 µm scanner and a Si3N4 tip on a cantilever with a spring constant of 0.09 N m-1. The tip loads were in the range 0.1-1.0 nN, and AFM images were obtained in the air at room temperature with 50-60% humidity. The thickness of the monolayer was estimated by measuring the height difference between the monolayer surface and the bare Si-wafer surface with silanol groups (Si-OH) prepared by the VUV photolithography method.30 The 10 number data were collected in order to calculate a thickness as a number average. In-Plane Grazing Incidence X-ray Diffraction Measurement. The in-plane GIXD measurement was carried out at a BL13-XU beam line40,51,52 of SPring-8 (JAPAN) using an incident X-ray with wavelengths λ of 0.1277 nm (DOTS monolayer) or 0.1117 nm (OTS monolayer). In the GIXD measurement, a strong diffraction was observed on the ultrathin film when the incident angle (Ri) to the sample was below a critical angle (Rc).53 Under this condition, the X-ray underwent total external reflection and penetrated into the sample as evanescent waves. The Rc values of the alkylsilane monolayer are ca. 0.12° for λ ) 0.1277 nm and ca. 0.11° for λ ) 0.1117 nm, respectively. Therefore, in order to analyze the in-plane structure of the organosilane monolayer, the GIXD measurements were performed at Ri ) 0.10° for λ ) 0.1277 nm and Ri ) 0.09° for λ ) 0.1117 nm, respectively. Diffractions from the sample surface (43) Koga, T.; Otsuka, H.; Takahara, A. Chem. Lett. 2002, 31, 1196. (44) Koga, T.; Morita, M.; Sakata, H.; Otsuka, H.; Takahara, A. Int. J. Nanosci. 2002, 1, 419. (45) Takahara, A.; Sakata, H.; Morita, M.; Koga, T.; Otsuka, H. Compos. Interfaces 2003, 10, 489. (46) Hozumi, A.; Ushiyama, K.; Sugimura, H.; Takai, O. Langmuir 1999, 15, 7600. (47) Dressick, W. J.; Calvert, J. F. Jpn. J. Appl. Phys. 1993, 32, 5829. (48) Sugimura, H.; Ushiyama, K.; Hozumi, A.; Takai, O. Langmuir 2000, 16, 885. (49) Saito, N.; Wu, Y.; Hayashi, K.; Sugimura, H.; Takai, O. J. Phys. Chem. B 2003, 107, 664. (50) Hayashi, K.; Saito, N.; Sugimura, H.; Takai, O.; Nagagiri, N. Langmuir 2002, 18, 7469. (51) Sakata, O.; Furukawa, Y.; Goto, S.; Mochizuki, T.; Uruga, T.; Takeshita, K.; Ohashi, H.; Ohata, T.; Matsushita, T.; Takahashi, S.; Tajiri, H. Surf. ReV. Lett. 2003, 10, 543. (52) Yakabe, H.; Sasaki, S.; Sakata, O.; Takahara, A.; Kajiyama, T. Macromolecules 2003, 36, 5905. (53) Dosch, H. Critical Phenomena at Surfaces and Interfaces, EVanescent X-Ray and Neutron Scattering; Springer-Verlag: Dusseldorf, 1992.

Phase Transition of Alkylsilane Monolayers

Figure 1. GIXD profiles of the OTS monolayer on Si-wafer substrate surface at the temperatures of 305 and 339 K.

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Figure 2. GIXD profile of DOTS monolayer at 304 K.

were detected in the in-plane direction by a scintillation counter. A 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 oxidizing by irradiation of synchrotron beam. The GIXD profiles were fitted using a Lorentzian function with a linear background. The scattering vector is defined as qxy ()(4π/λ) sinθ) where θ is the Bragg angle. The Si-wafer substrate was controlled by the original sample stage. The surface temperature of Si-wafer substrates was monitored during the measurements. The GIXD measurement was performed at a temperature range from 305 to 390 K.

Results and Discussion Evaluation of the Monolayer Thickness. The thickness of the monolayers was estimated by the AFM observation. The height differences of the micropatterned OTS/Si-OH and DOTS/ Si-OH monolayers were ca. 2.1 nm and ca. 2.9 nm, respectively. The calculated molecular lengths of OTS and DOTS were assumed to be in an all-trans state and were 2.35 and 2.87 nm, respectively. It therefore seems likely that the DOTS molecules in the monolayer prepared by a water-cast method were oriented almost normal to the surface. On the other hand, the thickness of the OTS monolayer was relatively small compared with the molecular length of OTS molecules in the all-trans state. The authors have reported that the OTS monolayers prepared by the chemisorption method are slightly tilted to the surface normal on the assumption that the OTS molecules are in an all-trans conformation in the chemisorbed OTS monolayers.38 Thus, the relatively small thickness of the OTS monolayer compared with DOTS can be attributed to the tilting of OTS molecules in the monolayers. Grazing Incidence X-ray Diffraction Measurements of OTS Monolayer at High Temperature. Figure 1 shows the GIXD profiles of the OTS monolayer on the Si-wafer substrate surface in the temperature range 305-339 K. In Figure 1, the (10) reflection of the OTS monolayer observed at 305 K [d(10) ) 0.431 nm] disappears at 339 K. It seems that the disappearance of the (10) diffraction of the OTS monolayer is attributed to a process of going from the hexagonal phase to the amorphous state. This finding is in good agreement with the estimated melting point of octadecyl groups in the OTS monolayer reported by Kojio et al.20 Grazing Incidence X-ray Diffraction Measurements of DOTS Monolayer at Various Temperatures. To clarify the crystalline structure of the DOTS monolayer at room temperature, GIXD measurement using synchrotron radiation was carried out. Figure 2 shows a GIXD profile of a DOTS monolayer prepared on a Si-wafer substrate surface measured at 304 K. The GIXD profile of the DOTS monolayer displays three diffraction peaks overlapped on an amorphous halo. Two diffraction peaks at 2θ

Figure 3. Top views of crystalline lattices of docosyl molecules in the DOTS monolayers.

) 17.55° (qxy ) 15.01 nm-1) and 2θ ) 19.60° (qxy ) 16.75 nm-1) can be assigned to the (11) and (20) diffraction of a rectangular crystalline lattice with 0.42 and 0.40 nm spacing.20 A diffraction peak at 2θ ) 18.0° (qxy ) 15.39 nm-1) then suggested the (10) diffraction of an oblique crystalline lattice, which is a more anisotropic structure compared with that of a rectangular crystalline lattice. This structure has a (10) spacing of 0.408 nm. These results indicate that the two-dimensional crystalline lattice of the DOTS monolayer consists of a mixture of the rectangular crystalline lattice and the oblique crystalline lattice as well as an amorphous phase. Figure 3 provides a top view of the rectangular and oblique crystalline lattices of docosyl groups in the DOTS monolayers. Datta and co-workers have reported that the Langmuir-Blodgett monolayer of fatty acid molecules with long alkyl chains such as those of heneicosanoic acid molecules formed a rectangular crystalline phase at air/ water interfaces, as determined by GIXD measurement.54 Peng et al. have shown using GIXD measurement that a rectangular crystalline lattice is present in a multilayer LB film that consists of slightly tilted cadmium arachidate molecules.55,56 The variation of the crystalline structure of organosilane monolayers during the phase transition was investigated on the (54) Datta, A.; Kmetko, J.; Richter, A.G.; Yu, C.-J.; Dutta, P.; Chung, K. S.; Bai, J. M. Langmuir 2000, 16, 1239. (55) Peng, J. B.; Barnes, G. T.; Gentle, I. R.; Foran, G. J. J. Phys. Chem. B 2000, 104, 5553. (56) Peng, J. B.; Foran, G. J.; Barnes, G. T.; Gentle, I. R. Langmuir 2003, 19, 4701.

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Koga et al.

Figure 5. Temperature dependence of lattice lengths a and b of the rectangular crystalline lattice of DOTS monolayer at 304, 308, 313, 318, and 323 K.

Figure 4. (a) the GIXD profiles of the DOTS monolayer on the Si-wafer substrate surface, and (b) the lattice spacings of rectangular (11), rectangular (20), oblique (10), and hexagonal (10) phases at the temperatures of 304, 308, 313, 318, 323, and 328 K.

basis of in-plane GIXD measurements conducted at various temperatures. Figure 4 shows (a) the GIXD profiles of the DOTS monolayer on the Si-wafer substrate surface and (b) the lattice spacings of rectangular (11), rectangular (20), oblique (10), and hexagonal (10) phases at the temperatures of 304, 308, 313, 318, 323, and 328 K. The lattice spacings of rectangular crystalline lattice of the DOTS monolayer at 304 K were similar to the values of rectangular crystalline lattice of OTS monolayer prepared on the Si-wafer substrate at 223 K evaluated by Kojio et al. using ED measurement [d(11) ) ca. 0.40 nm, and d (20) ) ca 0.42 nm].20 As shown in Figure 4, the lattice spacings of rectangular and oblique crystalline lattices increased with increases in temperature. The diffractions originated from the rectangular and oblique crystalline lattice eventually transformed a diffraction peak originating from the hexagonal crystalline lattice. The ED patterns of the OTS monolayer prepared by the LB method at 243 and 324 K exhibited (10) spacing of the hexagonal lattice with ca. 0.42 nm.20 It is therefore considered that the change in GIXD profiles from 305 to 352 K is due to the phase transition of the DOTS monolayer from the anisotropic phases, which are rectangular and oblique crystalline states, to the isotropic phase, which is the hexagonal crystalline state. It seems that increasing lattice spacings of the DOTS monolayer from 304 to 318 K is due to thermal expansion of the crystalline lattice of the DOTS molecules in a process of phase transition from the rectangular crystalline lattice to a hexagonal one with the activation of thermal molecular motion of docosyl groups. The lattice spacing of d(11) and d(20) of the rectangular crystalline lattice varied to almost the same value at 323 K. These results indicate that two kinds of isotropic lattice structures changed to an anisotropic lattice structure during the phase transition. The increment of d(10) spacing of the oblique crystalline lattice is larger than that of d(11) and d(20) spacings of the rectangular crystalline lattice. The molecular number per unit cell of the oblique crystalline lattice is one, which is a half of that of the rectangular crystalline lattice. Thus, it is reasonable to consider that the large increment of the oblique crystalline lattice is due

Figure 6. Temperature dependence of the setting angle φ of the ab plane of the rectangular unit cell for the DOTS monolayer.

to the low molecular packing density of the oblique crystalline lattice compared with the rectangular one. Figure 5 shows the temperature dependence of lattice lengths of the DOTS monolayer. The lattice lengths of a and b, which were estimated from the (11) and (20) spacing of a rectangular crystalline lattice, of the DOTS monolayer were formed to increase with increasing temperature. In order to discuss the thermal expansion behavior of the DOTS monolayer, we estimated a linear thermal expansion coefficient, R, of the rectangular crystalline lattice of the DOTS monolayer. An expansion ∆L of lattice length is given by

∆L ) RL∆T where L is the lattice length of a crystalline organosilane monolayer at the temperature, T (K). The R of the DOTS monolayer could be calculated on the basis of Figure 6. The lattice length a in the rectangular crystalline lattice of the DOTS monolayer expanded slightly with increases in temperature [Ra(DOTS) ) 7.2 × 10-4 K-1]. On the other hand, the thermal expansion behavior of the lattice length b in the rectangular crystalline lattice was almost zero. The evaluated expansion behavior of the lattice length of a DOTS monolayer in the rectangular crystalline state was in good agreement with the reported linear thermal expansion behavior of lattice lengths a and b of bulk polyethylene in the orthorhombic crystalline phase.57 This agreement is due to the similar molecular packing state and the chemical structure of docosyl groups in the rectangular DOTS monolayer with long alkyl chains in bulk polyethylene in an orthorhombic crystal. (57) Tashiro, K.; Ishino, K.; Ohta, T. Polymer 1999, 40, 3469.

Phase Transition of Alkylsilane Monolayers

The similar manner of thermal expansion behavior between the DOTS monolayer and polyethylene was also shown in the temperature dependence of the setting angle φ of the unit cell. Figure 6 shows the temperature dependence of the setting angle φ of the ab plane of the rectangular unit cell for the DOTS monolayer. A two-dimensional crystal is in a hexagonal form when φ ) 60°. The value of φ decreased above 318 K toward 60°, which is considered to be consistent with the concept of the rotational transition of a planar zigzag plane having more or less radial symmetry around the molecular axis.

Conclusion The temperature-dependent GIXD measurements were carried out at various temperatures in order to clarify the phase transition behavior of organosilane monolayers prepared on the Si-wafer surface. Detailed investigation of the phase transition and thermal expansion of crystalline lattice of organosilane monolayers was achieved by GIXD measurement at various temperatures. The OTS monolayer changed from the hexagonal phase to the amorphous state with increases in temperature. This finding is in good agreement with the melting point of octadecyl groups. The crystalline lattice of the DOTS monolayer also expanded as the temperature increased. The crystalline state of the DOTS monolayer changed from the rectangular and oblique phases to the hexagonal phase with an increase in temperature. The linear

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thermal expansion behavior of the lattice lengths a and b and a rotation behavior of setting angle φ of the ab plane of the rectangular crystalline state of the DOTS monolayer exhibited a similar tendency to bulk polyethylene with the orthorhombic crystalline lattice due to their similar crystalline structures. It is expected that detailed knowledge of the phase transition behavior of organosilane monolayers at high temperature is key to clarifying the various temperature-dependent surface physicochemical properties of crystalline organic ultrathin films. 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, Sports, Science and Technology of Japan. T. K. and K. H. acknowledge 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 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/BL13-XU, 2003A0606-ND1-np/BL13-XU, 2003B0178-ND1d-np-Na/BL13-XU, and 2004A0319-ND1d-npNa/BL13-XU, 2004B0257-Nd1d-np/BL13-XU). LA7006588