3932
Langmuir 2000, 16, 3932-3936
Molecular Aggregation State of n-Octadecyltrichlorosilane Monolayers Prepared by the Langmuir and Chemisorption Methods Ken Kojio, Atsushi Takahara, Kazuhiko Omote,† 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 July 12, 1999. In Final Form: December 20, 1999 A comparative study in molecular arrangements of the n-octadecyltrichlorosilane (OTS) monolayer prepared by the Langmuir method and the chemisorption methods were carried out on the basis of grazing incidence X-ray diffraction (GIXD) and X-ray reflectivity (XR) measurements. The OTS molecules in the Langmuir OTS monolayer uniformly tilt ca. 8-10° to the surface normal and packed in a hexagonal lattice with the (10) spacing of 0.412 nm. On the other hand, the OTS molecules in the chemisorbed OTS monolayer tilt ca. 15-17° to the surface normal and also crystallite orient randomly in the two-dimensional plane. The average magnitude of the (10) spacing of the chemisorbed OTS monolayer was ca. 0.417 nm. Direct evidence that the packing density of the Langmuir OTS monolayer was higher than that of the chemisorbed OTS monolayer was obtained by GIXD and XR measurements for the first time.
Introduction A monolayer with a highly ordered molecular orientation, high homogeneity of surface, is indispensable for construction of an organic thin film with high functionality such as wettability and lubricating properties. It has been suggested that the organosilane monolayer with a highly ordered structure can be obtained by the Langmuir method.1-8 The electron diffraction (ED) pattern of the n-octadecyltrichlorosilane (OTS) monolayer prepared onto a hydrophilic solid substrate by the Langmuir method exhibited hexagonal crystalline arcs at 293 K. The (10) spacing estimated by the ED pattern was ca. 0.42 nm.1,5 Also, a high-resolution atomic force microscopic (AFM) observation revealed that the methyl end groups of the OTS molecules were packed in a hexagonal manner with a (10) spacing of ca. 0.42 nm.5 The grazing incidence X-ray diffraction (GIXD) measurement gives the lattice spacing of hydrophobic chains of the crystalline organic monolayers.9-16 Kjaer et al. and * To whom correspondence should be addressed. † X-ray Reaserch Laboratory, Rigaku Corporation. (1) Ge, S. R.; Takahara, A.; Kajiyama, T. J. Vac. Sci. Technol. 1994, A12, 2530. (2) Ge, S. R.; Takahara, A.; Kajiyama, T. Langmuir 1995, 11, 1341. (3) Kajiyama, T.; Ge, S. R.; Kojio, K.; Takahara, A. Supramol. Sci. 1996, 3, 123. (4) Takahara, A.; Kojio, K.; Ge, S. R.; Kajiyama, T. J. Vac. Sci. Technol. 1996, A14, 1747. (5) Kojio, K.; Ge, S. R.; Takahara, A.; Kajiyama, T. Langmuir 1998, 14, 971. (6) Takahara, A.; Ge, S. R.; Kojio, K.; Kajiyama, T. Scanning Probe Microscopy of Polymers; ACS Symp. Ser. 694; Ratner, B. D., Tsukruk, V. V., Eds.; American Chemical Society: Washington, DC, 1998; Chapter 12. (7) Kojio, K.; Takahara, A.; Kajiyama, T. Colloids Surf., A, in press. (8) Kojio, K.; Takahara, A.; Kajiyama, T. Silicones and SiliconeModified Materials; ACS Symp. Ser. 729; Clarson S. J., Ed.; American Chemical Society: Washington, DC, 1999; Chapter 22. (9) Kjaer, K.; Als-Nielsen, J.; Helm, C. A.; Laxhuber, L. A.; Mo¨wald, H. Phys. Rev. Lett. 1987, 58, 2224. (10) Kjaer, K.; Als-Nielsen, J.; Helm, C. A.; Tippman-Krayer, P.; Mo¨wald, H. J. Phys. Chem. 1989, 93, 3200. (11) Barton, S. W.; Thomas, B. N.; Flom, E. B.; Rice, S. A.; Lin, B.; Peng, J. B.; Ketterson, J. B.; Dutta, P. J. Chem. Phys. 1988, 89, 2257. (12) Fontaine, P.; Goldmann, M.; Rondelez, F. Langmuir 1999, 15, 1348. (13) Barton, S. W.; Goudot, A.; Rondelez, F. Langmuir 1991, 7, 1029.
Barton et al. evaluated the molecular arrangement of the lipid, fatty acid9,10 and normal alcohol11 monolayers at the air/water interface by GIXD measurement. Fontaine et al. reported on the influence of headgroup cross-linking on chain packing of the n-octadecyltriethoxysilane (OTE) monolayer on water subphases with acidic and basic pH to neutral.12 Barton et al. reported that the molecular occupied area in the OTS monolayer at the air/water interface was calculated to be 0.203 nm2 molecule-1.13 Furthermore, it was revealed by Sagiv et al. and Tidswell et al. that the (10) spacing of the OTS monolayer prepared on a silicon wafer substrate by the chemisorption method was 0.425 nm.14,15 However, no attempt has been made on the direct comparison of the molecular aggregation state of the OTS monolayers prepared by the Langmuir and chemisorption methods. In this study, the molecular arrangements of the OTS monolayers prepared on a silicon wafer substrate by the Langmuir and the chemisorption methods were investigated on the basis of the GIXD and X-ray reflectivity (XR) measurements. Experimental Section n-Octadecyltrichlorosilane (OTS, CH3(CH2)17SiCl3, Shin-Etsu Chemical, Ltd., Co.) was used to prepare the monolayer. OTS was purified by vacuum distillation. A toluene solution of OTS was prepared with a concentration of 3 × 10-3 M. Toluene was refluxed with sodium and distilled under atmospheric pressure because the trichlorosilyl group of OTS is sensitive to moisture. The toluene solution of OTS was spread on the pure water surface at a subphase temperature of 293 K. The surface pressure-area (π-A) isotherm was obtained with a computer-controlled homemade Langmuir trough. To make the polymeric monolayer form in a well-regulated state, the spread molecules were kept on the water subphase at a surface pressure of 20 mN m-1 for 15 min. After the polymeric monolayer formed on the water subphase, it was transferred onto the silicon wafer substrate by the upward drawing method and then immobilized onto the substrate with Si-OH groups. To obtain a clean silicon wafer substrate with (14) Maoz, R.; Sagiv, J.; Degenhardt, D.; Mo¨hwald, H.; Quint, P. Supramol. Sci. 1995, 2, 9. (15) Tidswell, I. M.; Rabedeau, T. A.; Pershan, P. S.; Kosowsky, S. D.; Flokers, J. P.; Whitesides, G. M. J. Chem. Phys. 1991, 95, 2854. (16) Frydman, E.; Cohen, H.; Maoz, R.; Sagiv, J. Langmuir 1997, 13, 5089.
10.1021/la9909042 CCC: $19.00 © 2000 American Chemical Society Published on Web 03/04/2000
Molecular Arrangements of OTS
Langmuir, Vol. 16, No. 8, 2000 3933
Figure 1. Schematic representation of the scattering geometries at the sample surface: (a) in-plane diffraction mode; (b) specular reflectivity mode; (c) definition of the rotation angle, φ, of the sample in the GIXD measurement. Si-OH groups, the silicon wafer substrate was immersed in a mixed solution of concentrated H2SO4 and 30% H2O2 (70/30 v/v) at 363 K for 1 h. Bicyclohexyl used as a chemisorption solvent was passed twice through an alumina column at 293 K. The chemisorbed OTS monolayer was prepared onto the silicon wafer substrate by immersing in a bicyclohexyl solution of 5.0 × 10-3 M for 2 min. Then the silicon wafer was withdrawn and rinsed with clean toluene for ca. 30 s; the chemisorption and rinse processes were repeated twice.16 To investigate the molecular arrangement of the OTS monolayers, GIXD studies were carried out for the monolayers prepared onto the silicon wafer substrate by the Langmuir and chemisorption methods. Figure 1 shows the schematic representation of the scattering geometries at the sample surface. A monochromator was set next to the exit of the X-ray source (Ultra X18, Rigaku, Ltd., Co.). The sample was placed on the sample stage of a three-axis goniometer for surface X-ray diffraction (ATX-G, Rigaku, Ltd., Co.). A monochromatic X-ray beam was irradiated to the surface at the incident angle, Ri, of 0.2° for the in-plane diffraction study. Also, the diffracted X-ray was detected by a sintilation counter at the takeoff angle, Rf. The wave vector was defined as qxy ()4π(sin θ)/λ), where θ and λ were the angle and the wavelength of the X-ray (Cu KR, λ ) 0.154 nm), respectively. Also, the anisotrpy of the orientation of the OTS monolayers in the two-dimensional plane was evaluated by GIXD measurement at various rotation angles, φ, of the sample as shown in Figure 1c. Figure 1c shows the schematic representation of the definition of the rotation angle, φ, of the sample in the GIXD measurement. The φ was defined as the angle between the incident direction of the X-ray and the drawing direction in the Langmuir method. To evaluate the film thickness of the monolayer, XR measurement was carried out. The angular dependence of the specular reflectivity was measured by a series of RRi-2RRi scans. The reflected beam was detected by a scintillation counter after the
Figure 2. GIXD data for the OTS monolayers prepared on the silicon wafer substrate by the Langmuir method (a) and the chemisoprtion method (b) measured at various rotation angles, φ, at 293 K. slit. Since the results obtained after second GIXD and XR scans were the same as the first scans of GIXD and XR data, it can be inferred that the damage induced by irradiated X-ray beam did not occur through GIXD and XR measurements in this study.
Results and Discussion Figure 2a shows GIXD data for the OTS monolayers prepared on the silicon wafer substrate by the Langmuir method (Langmuir OTS monolayer) measured at various rotation angles, φ at 293 K. The Langmuir OTS monolayer was transferred onto the silicon wafer substrate at a surface pressure of 20 mN m-1. Since the background of a silicon wafer substrate showed linear dependence, the data were fitted by using a Lorentzian peak shape on a linear sloping background. In our previous report, the ED study revealed that the (10) spacing of the hexagonal crystalline lattice of the OTS monolayer was ca. 0.42 nm.1,5 Therefore, it seems reasonable to consider that the peaks observed in Figure 2a can be assigned to the (10) spacing of the hexagonal crystalline lattice of the OTS monolayer. The peaks assigned to the (10) spacing of the hexagonal
3934
Langmuir, Vol. 16, No. 8, 2000
Kojio et al.
Figure 3. Rotation angle, φ, dependence of the (10) spacing of the OTS monolayers prepared by the Langmuir method and the chemisorption method obtained from Figure 2.
lattice of the Langmuir OTS monolayers were observed at qxy,max from 15.1 to 15.3 nm-1 at various rotational angles, φ. The peak positions apparently depend on φ. Figure 3 shows the rotation angle, φ, dependence of the (10) spacing of the Langmuir OTS monolayer obtained from Figure 2a. It was shown that the (10) spacing of the Langmuir OTS monolayer showed a minimum of 0.412 nm at φ ) 0-15° and then the (10) spacing increased with an increase in φ. Thus, the (10) spacing clearly depends on the rotation angle, φ. It is considered that the rotation angle, φ, dependence of the (10) spacing of the OTS monolayer occurred by the tilting of the alkyl chain of the OTS molecule. Also, since the apparent (10) spacing increases with an increase in tilting angle, it seems reasonable to conclude that the (10) spacing of the hexagonal lattice of the Langmuir OTS monolayer is 0.412 nm of minimum value in Figure 3. The effect of the tilting of the alkyl chain of the amphiphilic molecule on the diffraction peak position was reported.10 On the assumption that the OTS molecules uniformly tilt in the region irradiated by X-ray beam, there are two models of hexagonal lattice orientation. Figure 4a shows the model structures (model I and model II) for the hexagonal lattice with molecular tilting and Figure 4b shows the tilting angle dependence of the relative (10) spacing between minimum and maximum (10) spacings for the OTS monolayer in hexagonal crystal lattice of the (10) spacing with 0.412 nm. Relative (10) spacing was defined as the ratio between the two spacings in model I and model II. The OTS molecules in model I and model II tilt toward (11) and (1h 1), respectively. In this speculation, the van der Waals radius of the alkyl chain of the OTS molecule was constant at various tiliting angles, t. The (10) spacings of the Langmuir OTS monolayer at rotation angles of 15° and 75° were ca. 0.412 and 0.417 nm, respectively, as shown in Figure 3. That is, the relative (10) spacing for the Langmuir OTS monolayer was ca. 1.012. In this study, it is not clear that both model I and model II are appropriate for the molecular orientation model of the Langmuir OTS monolayer. However, as shown in Figure 4, the tilt angle t of the alkyl chains of the OTS monolayer is ca. 8-10° for both model I and model II, when the relative (10) spacing is 1.012. Therefore, it seems reasonable to consider from Figure 4 that the OTS molecules in the Langmuir OTS monolayer tilt ca. 8-10° to the surface normal as shown in model I and/or model II on the assumption that the molecular tilting of the OTS monolayer was uniformly tilted in the region irradiated by the X-ray beam. Furthermore, since the (10) spacing of the Langmuir OTS monolayer measured at φ ) 0-15°
Figure 4. (a) Model structures for the hexagonal lattice with molecule tilting. (b) Tilting angle, t, dependence of the relative (10) spacing for the OTS monolayer in the hexagonal crystal lattice of the (10) spacing with 0.412 nm.
was smaller than that measured at φ ) 90°, it can be considered that the OTS molecules in the Langmuir OTS monolayer almost tilt along the drawing direction. It is inferred that the tilting of alkyl chain in the Langmuir OTS monolayer occurred at the drawing process from the air/water interface onto the silicon wafer substrate. Figure 2b shows GIXD data for the OTS monolayer prepared onto the silicon wafer substrate by the chemisorption method (chemisorbed OTS monolayer) measured at various rotation angles, φ, at 293 K. The peaks assigned to the (10) spacing of the hexagonal lattice were measured at around qxy,max ) 15.1 nm-1. The chemisorbed OTS monolayers remained at constant peak positions with an increase in φ, in contrast to the Langmuir OTS monolayer. The (10) spacing of the chemsorbed OTS monolayer was a constant magnitude of ca. 0.417 nm at various rotaion angles, φ, as shown in Figure 3, and also, this magnitude was larger than the (10) spacing of the Langmuir OTS monolayer. The comparison of packing density of the Langmuir OTS monolayer with the chemisorbed OTS monolayer will be discussed later with the result of the film thickness obtained by XR measurement. To evaluate the film thickness and electron density distribution along the film thickness direction of the Langmuir OTS and chemisorbed monolayers, an XR measurement was carried out. Figure 5a shows the XR and the fitting curves for the Langmuir OTS and chemisorbed monolayers. Figure 5b also shows an Rqz4 vs qz plot for both monolayers. Successive data sets are displaced by a factor of 10. The first minimum might arise from a destructive interference between X-ray reflections from the top and bottom regions of the OTS monolayer. The
Molecular Arrangements of OTS
Langmuir, Vol. 16, No. 8, 2000 3935
Figure 6. Schematic representation of the molecular aggregation states of (a) the Langmuir OTS monolayer and (b) the chemisorbed monolayer.
Figure 5. (a) X-ray reflectivity (XR) and fitting curves of the OTS monolayers prepared onto silicon wafer substrate by the Langmuir and the chemisorption methods. (b) Rqz4 vs qz plots for the both monolayers. Successive data sets are displaced by a factor of 10. Table 1. Fitting Parameters for the OTS Monolayer Prepared by the Langmuir Method and the Chemisorption Method LCH2-SiO, nm LSiO-SiO, nm LSiO-Si, nm FCH2/FSi FSiO-silane/FSi FSiO-substrate/FSi FSi/FSi σair-CH2, nm σCH2-SiO, nm σSiO-SiO, nm σSiO-Si, nm
Langmuir
chemisorbed
2.35 0.40 3.30 0.45 0.98 0.98 1.00 0.49 0.19 0.10 0.10
2.25 0.40 2.30 0.44 0.98 0.98 1.00 0.55 0.37 0.10 0.10
magnitude of qz for the first minimum in the XR curve was defined as qz,min. The film thickness, L, could be calculated by the equation L ) π/qz,min. The film thicknesses of the Langmuir OTS and chemisorbed monolayers were evaluated to be 2.34 and 2.25 nm, respectively. Table 1 shows the magnitudes of the electron density, F, thickness of each layer, L, and mean square roughness, σ, for the both OTS monolayers calculated based on the fitting analysis. The thicknesses of alkyl chain layer, LCH2-SiO, of the Langmuir OTS and chemisorbed monolayers were 2.35 and 2.25 nm, respectively. These magnitudes corresponded well to film thicknesses calculated from the qz,min, as mentioned above. Then, it seems likely that the film thickness of the Langmuir OTS monolayer was thicker than that of the chemisorbed one. Besides the reflectivity of the chemisorbed OTS monolayer was definitely smaller than that of the Langmuir one once the qz goes beyond 3.5 nm-1 as shown in parts a and b of Figure 5. Furthermore, the two Kiessig fringes were clearly observable only for the
Langmuir OTS monolayer, in paticular, in Figure 5b. These results make it clear that the Langmuir OTS monolayer is superior to the chemisorbed OTS monolayer in terms of the monolayer ordering. Since the tilt angle and film thickness of the Langmuir OTS monolayer were ca. 8-10° and ca. 2.34 nm from GIXD and XR measurements, respectively, the film thickness of the OTS monolayer orienting to the surface normal can be calculated to be ca. 2.36 nm. On the other hand, the film thickness of the chemisorbed OTS monolayer was ca. 2.25 nm by XR measurement. Therefore, it is inferred from these results that the OTS molecules in the chemisorbed OTS monolayer tilt ca. 15-17° to the surface normal on the assumption that the OTS molecules are in an alltrans conformation in the Langmuir and chemisorbed OTS monolayers. The peak position assigned to the (10) spacing of the hexagonal lattice in the GIXD data of the chemisorbed OTS monolayer showed constant magnitude at qxy ) 15.1 nm-1, as mentioned above. These results suggest that the chemisorbed OTS monolayer was formed with crystallite randomly orienting in the two-dimesional plane, in which the OTS molecules in the chemisorbed OTS monolayer tilt ca. 15-17° to the surface normal. It is inferred that the crystallite of the OTS monolayer was formed by nucleation and growth of self-similar islands (fractal aggregation).17,18 As mentioned above, it is concluded that the (10) spacing of the Langmuir OTS monolayer was ca. 0.412 nm. In contrast, the average magnitude of the (10) spacing of the chemisorbed OTS monolayer was ca. 0.417 nm. Therefore, it seems reasonable to conclude that the packing density of the OTS monolayer prepared by the Langmuir method was higher than that prepared by the chemisorption method. Figure 6 shows the schematic representaion of the molecular aggregation states of (a) the Langmuir OTS monolayer and (b) the chemisorbed one. The OTS molecules in the Langmuir OTS monolayer uniformly tilted ca. 8-10° to the surface normal, and the (10) spacing of the Langmuir OTS monolayer was ca. 0.412 nm. On the (17) Schwartz, D. K.; Steinberg, S.; Israelachvili, J.; Zasadzinski, A. N. Phys. Rev. Lett. 1992, 69, 3354. (18) Bierbaum, K.; Grunze, M.; Baski, A. A.; Chi, L. F.; Schrepp, W. Fuchs, H. Langmuir 1995, 11, 2143.
3936
Langmuir, Vol. 16, No. 8, 2000
other hand, the chemisorbed OTS monolayer was formed with crystallite randomly orienting in the two-dimesional plane, in which the OTS molecules tilt ca. 15-17° to the surface normal and the (10) spacing of the chemisorbed OTS monolayer was ca. 0.417 nm. Seimiya et al. reported that the occupied area for the OTS monolayer prepared by the chemisorption method was 0.23 ( 0.02 nm2 molecule-1 on the basis of highresolution atomic force microscopy (AFM).19 In our previous report, it has been reported that the occupied area for the OTS monolayer prepared by the Langmuir method was 0.20 nm2 molecule-1 by high-resolution AFM observation.5 Also, the methyl end groups of the Langmuir OTS monolayer in the area of 10 nm2 were clealy observed in comparison with those of the chemisorbed one in highresolution AFM observations. These results apparently indicate that the OTS molecules of the Langmuir OTS monolayer are more closely packed in the hexagonal lattice than in that of the chemisorbed OTS monolayer. As mentioned above, direct evidence that the molecules in the Langmuir OTS monolayer were more densely packed than those in the chemisorbed monolayer and, also, the ordering of the Langmuir OTS monolayer was higher than that of the chemisorbed monolayer could be obtained by GIXD and XR measurements for the first time. The chemisorbed OTS monolayer was formed by random adsorption of the OTS molecules from a bicyclohexyl solution. On the other hand, in the case where the OTS monolayer was prepared at the air/water interface and then subsequently transferred onto the silicon wafer by the upward drawing method (the Langmuir method), the OTS molecules in the all-trans conformation easily oriented perpendicular to the monolayer surface, because (19) Fujii, M.; Sugisawa, S.; Fukuda, K.; Kato, T.; Seimiya, T. Langmuir 1995, 11, 405.
Kojio et al.
the OTS molecules were two-dimensionally crystallized at the air/water interface of two-dimensional plane. Also, the OTS molecules were mechanically compressed by the barrier of the Langmuir trough during the compression process. As the result, it is considered that the formation of a much densely packed monolayer with high ordering was achieved. Therefore, it seems reasonable to conclude that the OTS monolayer with higher packing density can be obtained through preparation by the Langmuir method. Conclusion The comparative study in molecular arrangements of the Langmuir OTS monolayer and chemisorbed monolayer was carried out on the basis of GIXD and XR measurements. The OTS molecules in the Langmuir OTS monolayer uniformly tilt ca. 8-10° to the surface normal and packed in a hexagonal lattice with (10) spacing of 0.412 nm. The OTS molecules in the chemisorbed OTS monolayer tilt ca. 15-17° to the surface normal and also crystallite in random orientation in the two-dimensional plane. The average magnitude of the (10) spacing of the chemisorbed OTS monolayer was ca. 0.417 nm. Direct evidence that the packing density of the Langmuir OTS monolayer was higher than that of the chemisorbed OTS monolayer was obtained by GIXD and XR measurements for the first time. Acknowledgment. This study was partially supported by a Research Fellowship of the Japan Society for the Promotion of Science for Young Scientists and by a 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. LA9909042
