Langmuir 1996,11, 1341-1346
1341
Phase Separated Morphology of an Immobilized Organosilane Monolayer Studied by a Scanning Probe Microscope Shouren Ge, Atsushi Takahara, and Tisato Kajiyama" Department of Chemical Science a n d Technology, Faculty of Engineering, Kyushu University, Hakozaki 6-10-1,Higashi-ku, Fukuoka 812, J a p a n Received October 6, 1994. I n Final Form: December 20, 1994@ Alkyltrichlorosilanes and [2-(perfluorooctyl)ethyl]trichlorosilane (FOETS) mixed monolayers were prepared on the water subphase and were covalentlyimmobilized onto the silicon wafer surface. Atomic force microscopic observation of the octadecyltrichlorosilane (OTS)/FOETSmixed monolayer revealed that the crystalline OTS formed circular domains of ca. 1-2 pm diameter, which were surrounded by a sealike amorphous FOETS even though molar fraction of OTS was above 75%. The alkylsilane/FOETS mixed monolayers with shorter alkyl chain length such as n-dodecyltrichlorosilane (DDTS) did not show the phase-separated structure. Also, the OTS/FOETS mixed monolayer prepared by a chemisorption method did not show distinct phase separation. Friction force microscopic observation of the OTS/FOETS mixed monolayer revealed that the OTS domain had higher frictional coefficient compared with that of the FOETS matrix. On the basis of scanning viscoelasticitymicroscopic observation, it was revealed that the crystalline OTS domain had higher modulus than that of the amorphous FOETS matrix. Morphological observation of the stearic acid (nonreactive)/FOETS (reactive) mixed monolayer also exhibited the phaseseparated structure. The stearic acid domain in the mixed monolayer was easily extracted by solvent, and then the ghost monolayer of FOETS with holes of ca. 1-3 pm diameter was finally obtained.
Introduction In recent years, organic monolayer and LangmuirBlodgett (LB) film systems have attracted growing attention. These systems are believed to have technological potential in molecular electronics,' nonlinear optics,2 biomedical application^,^ and so on. In order to realize these objects, it is necessary to realize the precise structural and morphological control and, also, environmental stability of the surface structure of the monolayers. The surface properties of the monolayer such as wettability, frictional properties, chemical reactivity, biocompatibility, permeability, charge storage capacity, electrical response, and so on may be controlled by mixing two or more components. Various mixed monolayers prepared by a chemical adsorption from solution4r5(chemisorption method) and LB transfer from the air-water interface6)' (upward drawing method) have been reported. The chemisorption monolayers such as organosilane on silicon and alkanethiol on gold are physically robust, but it is difficult to obtain structurally controlled film by a chemisorption method because the structure formation depends on the random adsorption process from a solution. By utilizing the upward drawing method, it is possible to prepare a monolayer with controlled structure and complete coverage of the substrate. However, these monolayers were thermally and environmentally unstable due to the lack of covalent bond between the monolayer and the substrate. In order to obtain the stable and structurally controlled monolayer, the preparation of an
* To whom correspondence should be addressed. @
Abstract Dublished inAdvanceACS Abstracts. March 1.1995.
(1)S u e , M:J. Mol. Electron. 1986,I , 3. (2)Kunyama, K.;Oishi, Y.; Kajiyama, T. Rep. Prog. Polym. Phys. Jpn. 1994,37,553. (3) Uchida, M.; Tanizaki, T.; Oda, T.; Kajiyama, T. Macromolecules .. 1991,24,3238. (4)Sagiv, J. J. Am. Chem. SOC.1980,102,92. (5)Prime, K. L.; Whitesides, G. M. Science 1991,252, 1164. (6)Baglioni, P.;Dei, L.; Gabrielli, G.; Innocenti, F. M.; Niccolai, A. Colloid Polym. Sei. 1988,266,783. (7)Meyer, E.; Ovemey, R.; Luthi, R.; Brodbeck, D.; Howald, L.; Frommer, J.; Guntherodt, H.-J.; Wolter, 0.;Fujihira. M.: Takano. H.: Gotoh, Y. Thin Solid Films 1992,220,132.
organosilane monolayer by a n upward drawing method was proposed by the author^.^^^ The organosilane monolayer is a novel monolayer system which can be polymerized and immobilized on a substrate surface with hydroxyl groups, and also the two-dimensional surface modification is possible by utilizing the mixed monolayer composed of a n organosilane compound and a nonreactive component such as fatty acid. In this study, the alkyltrichlorosilane4fluoroalkyl)trichlorosilane mixed monolayers were prepared as a twodimensional analogue of phase separation in a polymer blend. The phase-separated structure of the alkyltrichlorosilane/(fluoroalkyl)trichlorosilane mixed monolayer was investigated by use of a n atomic force microscope (AFM), a friction force microscope (FFM), and a scanning viscoelasticity microscope (SVM). The mechanism of the phase separation of the alkyltrichlorosilane/(fluoroalkyl)trichlorosilane mixed monolayer was discussed. Also, the attempt has been made on the two-dimensional surface structure control by utilizing the mixed monolayer composed of reactive and nonreactive components.
Experimental Section Materials. Octadecyltrichlorosilane (OTS), n-hexadecyltrichlorosilane (HDTS),n-dodecyltrichlorosilane(DDTS),[2-(perfluorooctyl)ethylltrichlorosilane(FOETS), and stearic acid (SA) were used to prepare the mixed monolayers. The chemical structures of these components are shown in Figure 1. Organosilanes were purified by vacuum distillation. Benzene used as solvent was refluxed with sodium wire and distilled under atmospheric pressure to remove trace water. Organic silanes could be polymerized and covalently bonded with silanol groups on the silicon wafer surface, while SA is nonreactive to organosilanes and the silicon wafer surface. Substrate. In order to obtain a clean silicon wafer surface with SiOH groups, the silicon substrate was pretreated as described below. n-Doped, test grade, polished silicon wafers were cut into smaller strips and then cleaned in a detergent (8) Ge, S.R.; Takahara, A,; Kajiyama, T. Rept. Prog. Polym. Phys. Jpn. 1993,36,221. (9)Ge. S.R.: Takahara., A.:. Kaiivama, T. J.Vac. Sei. Technol. 1994, -.
A 12 (4),'2530.'
0743-7463/95/2411-1341$09.00/00 1995 American Chemical Society
Ge et al.
1342 Langmuir, Vol. 11, No. 4, 1995
b
1. CH3(CH2)1 1SiCl3
n-dodecyltrichlorosilane(DDTS)
(alkyltrichlorosilane CI L and O A H
2. CH3(CH2)1sSiC13 n-hexadecyltrichlorosilane (HDTS)
1
fluoroalkyltrichlorosilane)
3. CH3(CH2)17SiC13 n-octadecyltrichlorosilane (OTS) 4. CH3(CH2) 6COOH stearic acid (SA)
-n
Polymerization on the water surface
E
5 . CF3(CF2)7CH2CH2SiC13
E
(2-(perfluorooctyl)ethyl]trichlorosilane (FOETS) Figure 1. Chemical structures of various componentsused to
prepare the mixed monolayers. solution (Scat 20-X,Co., Ltd., ca. 5% in water) by brushing with a soft camel-hair brush, rinsed with distilled water, and finally heated in a ceramicovenfor 1h at 773K. Then they were covered with native oxide a few nanometers thick. Normally, silicon oxide, Si02 has a hydrophilic surface covered by silanols, SiOH, which can be used to immobilizethe organic silane monolayers to the substrate. Above 440 K, the SiOH groups on the silicon wafer substrate can be condensed almost completely to form hydrophobicsiloxane linkages Si-0-Si. In order to dissociate the siloxanebridges Si-0-Si on the substratesurface, the silicon substrate was placed into a mixed solution of concentrated H2SO4 and 30% H202 (70:30(v/v)). After the silicon substrate in the mixture solution was heated for 1h at 363 K and also it was cooled to room temperature, the silicon substrate was immediately rinsed with large amounts of pure water purified by the Milli-&I1system (MilliporeCo. Ltd.). The silicon substrates were stored in pure water until use. The silicon substrate was used within 2days after its cleaning treatmentmentioned above, in order to avoid recontamination of the substrate surface. The clean substrate was completely wetted by water (e, = 0 "). Monolayer Preparation. A series of mixture solutions of OTWOETSin benzendl,l,2-trichloro-l,2,2-trifluoroethane (TCTFE) (Ubenzen$UTC'J"E = OTS/FOETS molar ratio) solvent were prepared with a concentration of 2.0x M. OTSand FOETS were added to solvent in a nitrogen-filled glovebox because organosilanes react with moisture and polymerizewhen exposed to air. The solutions of organosilane compound were spread on the pure water surface at a subphase temperature of 293 K. Surface pressure-area (n-A) isotherms were measured with a microprocessor-controlledfilm balance system (FSD-20, Sanesu Keisoku Co., Ltd.). In order to polymerizethe monolayers through polymerization reaction, they were kept on the water surface under certain surface pressure for 30min. The monolayerswere transferred by the upward drawing method and immobilized onto the substrate surface with Si-OH groups. Scanning Force Microscopic Observation. Topographic images of the monolayer surfaces were taken with atomic force microscopy (AFM, SPA300, Seiko Instruments Industry Co., Japan). The AFM was operated under the constant force mode, in air at room temperature, using a 20pm x 20pm scanner and a silicon nitride tip on a cantilever with a small spring constant of 0.022N m-l. The imaging forcewas in a repulsive range from 0.1 to 1nN. The friction force microscopic (FFM) observation was carried out by using the same equipment as for the AFM observation. Both the vertical and torsional motion of the cantilever were detectedby the reflectedlaser beam. Thevertical and torsional motionswere proportionalto the normal force(AFM image) and the frictional force (FFM image), respectively. In order to obtain the maximum FFM output voltage (torsional motion),the cantilever scanned alongthe vertical direction to its axis. The two-dimensional image of dynamic viscoelastic functions was obtainedby utilizing a forced oscillationAFM (scanning viscoelasticitymicroscope,SVM).l0 The magnitude of strain was (lO)Kajiyama, T.; Tanaka, IC; Ohki, I.; Ge, S. R.;Yoon, J. S.; Takahara, A. Macromolecules 1994,27, 7932.
X
v: I.....
v: Preparation of the monolayer by Langmuir-Blodgett technique
I
substrate
Silicon wafer
I
Immobilization on the glass or silicon wafer substrate
Figure 2. Schematic representation of the polycondensation on the water surface and immobilization process of the alkyltrichlorosilane monolayer on the s i m e r surface.
modulated sinusoidally by applying the sinusoidal voltage generated by a frequency generator (OSC). The modulation frequency was 5 kHz which was below the resonance frequency of the piezoscanner. The z-sensitivity of the piezoscanner was 4.46 nm V-l for a 20 pm x 20 pm scanner. The measurement of the surface dynamic viscoelasticity was carried out in air at 293 K under the repulsive force of 0.021 nN.
Results and Discussion Phase-Separated Structure of the Alkyltrichlorosilane/(Fluoroalky1)trichlorosilane Mixed Monolayers. Figure 2 shows the schematic representation of the polycondensation reaction on the water surface and the immobilization process for the alkyltrichlorosilane monolayer onto the silicon wafer surface. The chlorine groups of organosilane on the water surface were substituted by hydroxy groups (the top portion of Figure 2). At a certain surface pressure, the hydroxy groups in an organosilane molecule reacted with those in the adjacent molecules in the case of the highly condensed monolayer, resulting in the formation of a polymerized monolayer (the second part of Figure 2). The polymerized monolayer was easily transferred onto a glass plate or silicon wafer by the upward drawing method and the residual hydroxyl groups could be covalently bonded with silanol groups on the silicon wafer surface (the middle and bottom parts of Figure 2). Figure 3 shows the n-A isotherms for the OTS and FOETS monolayer and also, the OTSB'OETS (50/50 molar) mixed monolayer on the water surface at Tspof 293 K. The molecular occupied area (the limiting area) of 0.28 nm2
Langmuir, Vol. 11, No. 4, 1995 1343
Morphology of Organosilane Monolayers
6o
t
0
0.2
0.4
0.6
0.8
1
Area t nm2 molecule-'
Figure3. Surface pressure-area (n-A)isotherms for the OTS, FOETS,and OTS/FOETS (50/50) mixed monolayer on the pure water surface at 293 K. molecule-' for the OTSFOETS (50/50 molar) mixed monolayer is almost equal to the average of the molecular occupied area for the OTS (0.24 nm2 molecule-l) and the FOETS (0.31nm2molecule-l) monolayer considering the molar fraction of OTS and FOETS. The OTSFOETS mixed monolayer can be transferred onto the silicon wafer substrate over a wide surface pressure range. The transfer ratio of the OTSPOETS mixed monolayer was ca. 1.0 a t the surface pressure of 25 mN m-l. This indicates that the substrate surface is almost completely covered with the mixed monolayer immobilizedto the substrate surface. Parts a-c of Figure 4 show the AFM images (scanned area 5 x 5 pm2)of the OTSPOETS (25/75), OTSFOETS (5060) and OTSFOETS (75/25) mixed monolayers, respectively, which were transferred onto the silicon wafer substrate by the upward drawing method a t the surface pressure of 25 mN m-l. The AFM image is given in a top-view presentation. The brighter and the darker portions correspond to the higher and the lower regions of the monolayer surface, respectively. The OTSFOETS mixed monolayers were in a phase-separated state as shown in Figure 4 and circular flat-topped domains of ca. 1-2pm diameter were observed in a island-like assembly. The z-dimensional on the monolayer surface became clearer by three-dimensional view. Parts a-c of Figure 5 show the three-dimensional images of the OTSFOETS (25/75), (50/50), and (75/25) mixed monolayers, respectively. The cylinder scallop-like domains are clearly observed for these mixed monolayers, even if OTS content is 75 mol %. The area fraction of the circular flat-topped domains was in good agreement with the area fraction of OTS component which was calculated on the basis of the limiting area (the molecular occupied area) of n-A isotherm. In other words, the OTSFOETS mixed monolayer formed a phase-separated structure on the water surface and was transferred onto the silicon wafer surface quantitatively. Figure 6a shows the height profile along the line in the AFM image of Figure 4b. The height of circular flattopped domains was higher by 1.1-1.3 nm than the surrounding flat monolayer. Figure 6b shows the schematic representation of the cross section of the immobilized OTSFOETS mixed monolayer. Since the difference in molecular length between OTS and FOETS is ca. 1.3 nm on the assumption of a trans conformation of alkyl group, it is apparent that the higher, circular domain and the surrounding flat regions are composed of OTS and FOETS molecules, respectively. The schematic representation of Figure 6b apparently corresponds to the conclusion reached from Figures 4 and 5 . Generally, polymer blends or copolymers are expected to show a surface enrichment of one component in order
to minimize interfacial free energy between the surface and the exterior environment. Even for compatibleblends, the mean field theory requires a surface enrichment of one component in order to minimize interfacial free energy." Therefore, it is very diffcult to control the surface structure of polymer blend films systematically on the basis of only the bulk composition. The definite surface compositioncorresponding to the feed composition and the stable surface structure of the organosilane monolayers indicate that each monolayer is well anchored to the substrate surface. Though a scanning operation was done repeatedly for AF'M observationwith arepulsive force larger than N, the OTS and FOETS monolayers were not damaged by a tip. Therefore, it seems reasonable to conclude that the remarkably stable surface and strong cohesion of these monolayers to the substrate are due to both the wide-area two-dimensional polymerization of these organosilane monolayers and the formation of covalent bonds between OTS or FOETS molecules and the substrate surface. As shown in Figures 4 and 5, OTS formed circular domains even if the molar percent of OTS was 75%. It is apparent from the electron diffraction pattern in Figure 4c that the OTS domain was in a crystalline state, since the magnitude of spacing corresponds to the (10) one of the OTS m ~ n o l a y e r Therefore, .~ the crystallizationof OTS molecules may be an important factor for the phase separation. In order to confirm the above mentioned conclusion, the alkylsilane with various alkyl chain lengths was used to prepare a n alkylsilane/(fluoroalkyl)silane mixed monolayer. Figure 7 shows the AFM images of the HDTSFOETS (50/50) and the DDTSFOETS (50/50) mixed monolayers which were transferred onto the silicon wafer surface by the upward drawing method a t a surface preseure of 25 mN m-l at 293 K. The domain morphology was incomplete for the HDTSFOETS mixed monolayer because the crystallizability of HDTS molecules is lower and its polymerization reaction is faster than OTS due to the shorter alkyl chain length of HDTS. Moreover, the domain was not observed in the case of the DDTSFOETS mixture, since a shorter alkyl chain effect for a phaseseparated state was more enhanced. Therefore, the AFM observations of Figures 4, 5, and 7 apparently indicate that the crystallization of alkylsilane plays a n important role in the phase separation process of the alkylsilane/ (fluoroalky1)silane mixed monolayer. Also, the phase-separated structure might be formed due to the difference in surface free energy between O W and FOETS monolayers. FOETS has a higher spreading coefficient against water than that of OTS. When FOETS molecules are spread on the water surface, a rapid initial spreading of FOETS occurs accompanying fast polycondensation reaction of FOETS molecules. The difference in spreading coefficients between OTS and FOETS molecules causes phase separation, and then the domain of OTS with a lower spreading coefficient is generally formed. Therefore, it can be concluded that the crystallization of OTS molecules and the faster spreading rate of FOETS molecules, resulting in a formation of an amorphous matrix, were important factors for the phase separation of alkyltrichlorosilane/(fluoroalkyl)trichlorosilane mixed monolayers. On the other hand, as shown in Figure 8, a clear phaseseparated structure was not observed for the OTSFOETS mixed monolayer prepared by the chemisorption method, because the formation of the chemisorption monolayer proceeds through a molecularly random adsorption pro(11)Jones, R.A. L.; Kramer, E.J. Polymer 1993,34,115.
1344 Langmuir, Vol. 11, No. 4, 1995
Ge et al.
(a) (25175) (b) (50150) ( c ) (75125) Figure 4. AFM imagesof the OTSFOETS (25175)(a),OTSFOETS(50/50) (b),and OTSFOETS(75/25) (c)mixed monolayerswhich were transferred onto the silicon wafer substrate by the upward drawing method at the surface pressure of 25 mN m-l at 293 K. The electron diffraction pattern of the OTSFOETS (75125) mixed monolayer was inserted in Figure 4c.
g p - p 8 0
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1
2 3 Lateral distance / Pm
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(4 (b) Figure 7. AFM images of the HDTSFOETS (50150)(a) and DDTSFOETS (50/50)(b) mixed monolayerswhich were transferred onto the silicon wafer substrate by the upward drawing method at the surface pressure of 25 mN m-l at 293 K. Figure 5. Three-dimensionalAFM images (5 pm x 5 pm) of the OTSPOETS (25/75)(a),OTS/FOETS (50/50)(b),and OTS/ FOETS (75125)(c) mixed monolayers which were transferred onto the silicon wafer surface by the upward drawing method at the surface pressure of 25 mN m-l at 293 K. cess. The comparison among Figures 4,7, and 8indicates the monolayer formation on the water surface is better for surface structure control than the chemisorption method.
FFM Observation of the OTS/FOETSMixed Monolayer. Parts a and b of Figure 9 show the friction force microscopic (FFM) image and the frictional force curve along the line mentioned in the upper part of Figure 9a for the OTSLFOETS(50150)mixed monolayer, respectively. The FFM image reflects the local frictional coefficient on the surface. Since the exact contact area of a tip against the surface could not been estimated, the FFM image was
Langmuir, Vol. 11, No. 4, 1995 1345
Morphology of Organosilane Monolayers
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Figure 8. AFM image of the OTSROETS (50/50) mixed monolayer prepared by the chemisorption method.
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Figure 10. (a)AFM image for the OTS/FOETS(50/50)mixed monolayer and (b) image of real part of dynamic modulus for the monolayer record with (a) simultaneously by the SVM technique.
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Scan distance I nm
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Figure 9. (a) FFM image of the OTS/FOETS (50/50)mixed monolayerprepared on the siliconwafer by the upward drawing method at surface pressure of 25 mN m-l at 293 K and (b) friction curve along the line in Figure 9a.
expressed as the magnitude of output voltage from the photodiode, which is proportional to the magnitude of frictional force. The frictional force curve showed the contrast reversal of friction depending on the scanning direction. The bright part in Figure 9a corresponds to the region with the high frictional coefficient, while the dark part does to that with the low frictional coefficient. Figure 9b exhibits that the OTS domain has a higher frictional coefficient compared with the FOETS matrix because OTS has a larger cohesive force than FOETS. This result corresponds to the fact that poly(tetrafluor0ethylene) (PTFE) shows a lower frictional coefficient compared with polyethylene (PE).12
Measurementof Surface DynamicViscoelasticity of the OTS/FOETS Mixed Monolayer. The immobilized organosilane monolayers were stable compared with the monolayer of conventional amphiphilic molecules as mentioned before, because of the presence of a covalent bond between substrate and monolayer. Since mechanical stability is required for the surface viscoelastic measurement, the organosilane monolayer is suitable for the surface viscoelasticmeasurement. The cyclic compressive strain must be applied under a repulsive force region for the surface dynamic viscoelastic measurement.1° In the case of the mechanically unstable stearic acid monolayer, the monolayer was completely swept out during first scanning, then the image of viscoelasticity could not be obtained. The modulated force was detected by the deflection of the cantilever. The deflection signal was (12)Pooley, C. M.;Tabor, D.Proc. R.Soc. London,A 1972,329,251.
filtered by a band-pass filter (BPF) to obtain dynamic modulus and simultaneously filtered by a low pass filter (LPF) to obtain an AFM image. Figure 10a shows the AFM image for the OTS/FOETS (50/50molar) mixed monolayer. The AFM image under this experimental condition showed the same resolution compared with the normal AFM image as shown in Figure 4. Figure 10b showed the image on the real part of the dynamic modulus for the OTS/FOETS (50/50)monolayer recorded simultaneously with the AFM image shown in Figure loa. The linear response of force against the magnitude of dynamic modulation strain was observed up to the modulation amplitude of ca. 2 nm. The image of the real part of modulus was obtained under the dynamic modulation along the z-axisof 0.89 nm. The brighter and the darker portions corresponded to the higher and the lower values of the apparent elastic modulus on the monolayer surface, respectively. Since the OTS region is in a crystalline state a t 293 K, it is reasonable that the circular flat-topped OTS domains have a higher modulus than that of the amorphous FOETS matrix.
Two-Dimensional Surface Structure Control by Utilizing the Mixed Monolayer Composed of a Reactive and Nonreactive Components. The phaseseparated monolayer can be prepared from FOETS and the nonpolymerizable, crystallizable amphiphile such as stearic acid (SA). Figure 11shows the AFM image of the SALFOETS (50150) mixed monolayer (a) and that afker being extracted with hexane (b). The SNFOETS mixed monolayer was in a phase-separated state in a similar fashion to the OTS/FOETS monolayer. The circular flattopped domains of ca. 1-3 pm in diameter are surrounded by a sealike flat FOETS region. I t is apparent from Figure 11that the circular flat-topped domains were preferentially extracted with hexane. Therefore, it seems reasonable to conclude that the circular domains are composed of SA molecules. Since the covalent bond cannot be formed between the hydrophilic part of SA and the substrate, SA molecules are easily extracted with hexane. On the other hand, the FOETS matrix could not be extracted with hexane because FOETS molecules were immobilized on the substrate by Si-0-Si covalent bond. It was confirmed from the electron diffraction pattern inserted in Figure 1la that the SA domains were in a crystalline state. Then, this indicates that a phase-separated state, composed of SA domains and the FOETS matrix, might be selectively formed from the crystallization of nonpolymerizable SA. The two-dimensional surface structure control such as the formation and the removal of the circular flat-topped domains of a few micrometers in diameter can be achieved by utilizing the mixed monolayer composed of a reactive
1346 Langmuir, Vol. 11, No.4, 1995
Ge et al.
~~
0 nm
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~
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(b)
(a)
Figure 11. (a)AFM image of the SA/FOETS (50150)mixed monolayerand (b)AFM image after being extracted with hexane. ED pattern was inserted in Figure l l a .
and nonreactive components as mentioned above. Then the patterned surface with high surface free energy (bare Si wafer) and low surface free energy (FOETS) can be obtained after the removal of SA. If the protein or inorganic crystal can be intentionally immobilized in the hole as shown in Figure l l b , it might be useful for microbioreactor, biosensor, biological, or memory devices. The intentionally formed holes can be modified by, for example, the chemisorption of other silane compounds schematically as shown in Figure 12 and patterned surfaces with different chemical and physical properties were formed. The modification of hole by other organosilane molecules has been done and will be published elsewhere.l3
upward drawing method
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Substmte(glass, Si-wafer, stainless steel, polymer) components B by solvent extraction
chemisorption method
Conclusion The alkylsilane/(fluoroalkyl)silane mixed monolayers prepared by a n upward drawing method showed a phaseseparated structure. The surface structure and properties were characterized by AFM, FFM, and SVM. Formation of phase-separated structure was governed by factors such as the difference in the crystallinity and the surface free energy between alkyltrichlorosilane and (fluoroalky1)silane. The FFM observation of the OTSB'OETS mixed monolayer revealed that the OTS domain had higher frictional coefficient compared with the FOETS matrix. The result of the SVM observation revealed that the crystalline OTS domain had a higher modulus than that of the amorphous FOETS matrix. The SA/FOETS mixed monolayer with a phase-separated structure can be modified and patterned surfaces with different chemical and physical properties can be formed. These surfaces
Mixed monolayer surface
L
C in solution
L
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Figure 12. Schematic representationof the surface structure control by utilizing the mixed monolayer composed of reactive and nonreactive components. may find some use in the biologic field such as patterning of protein adsorption.
Acknowledgment. The authors thank Professor S. Shinkai and Mr. S. Tsutsui of the Shinkai Chemirecognics Project, Research Development Corporationof Japan, for making the apparatus available for our present work. This research was supported in part by a grant from the Ministry of Education, Science and Culture, Japan. LA9407828
