Characterization of Fluorocarbon Monolayer Surfaces for Direct

We have prepared hydrophobic surfaces by silylating surfaces of molten glass in three different ways. Heptadecafluoro-1,1,2,2,-tetrahydrodecyltriethox...
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Characterization of Fluorocarbon Monolayer Surfaces for Direct Force Measurements Satomi Ohnishi* Department of Polymer Physics, National Institute of Materials & Chemical Research, Tsukuba, Ibaraki 305-8565, Japan

Takao Ishida† Joint Research Center for Atom Technology, National Institute for Advanced Interdisciplinary Research, Tsukuba, Ibaraki 305-8562, Japan

Vassili V. Yaminsky and Hugo K. Christenson Department of Applied Mathematics, Research School of Physical Sciences & Engineering, Australian National University, Canberra, A.C.T. 0200, Australia Received August 31, 1999. In Final Form: November 23, 1999 We have prepared hydrophobic surfaces by silylating surfaces of molten glass in three different ways. Heptadecafluoro-1,1,2,2,-tetrahydrodecyltriethoxysilane (FTE) was either (i) spread on the air-water interface, allowed to polymerize and then deposited as an LB film (at surface pressures of 10, 20, and 35 mN/msdesignated LB10, LB20, and LB35), (ii) allowed to react with the silica surface in a CHCl3 solution (FTE/CHCl3), or (iii) allowed to react with the silica in the undiluted liquid state (FTE/neat). The surfaces thus prepared were scanned by atomic force microscopy; their chemical compositions were analyzed by X-ray photoelectron spectroscopy; wettability studies with water were performed; and adhesion or pull-off forces between two such surfaces in humid air and water were determined. The FTE/neat surface was significantly less stable and less hydrophobic than the other surfaces, although an AFM scans indicated comparable smoothness. Considerable amounts of excess material could be removed from this surface by rinsing with ethanol or water. The FTE/CHCl3 surfaces and the LB10 surfaces were the smoothest, with a mean roughness of ∼0.14 nm, whereas LB20 and LB35 were rougher and showed randomly distributed bulges protruding 2.5-3 nm above the surfaces. All surfaces appeared amorphous and the coverage was similar (90-100%) for all LB surfaces, but lower for FTE/CHCl3 (∼80%), which also showed some loss on rinsing. LB10 was the most hydrophobic, with advancing and receding contact angles of water of 123 and 96°, respectively, that were stable with repeated immersion and retraction. FTE/CHCl3 was less hydrophobic and showed larger hysteresis θa ) 107°, θr ) 60°. The measured pull-off force in humid air was slightly larger for LB10 than for FTE/CHCl3. The pull-off forces in water for LB10 and FTE/CHCl3 were initially similar, but with immersion time the value for LB10 increased and stabilized at a much larger value, whereas that for FTE/CHCl3 remained constant. We conclude that LB deposition at a low surface pressure yields an amorphous surface that is smooth and homogeneous and has optimal hydrophobicity and good stability, whereas deposition at higher pressures give rougher surfaces with more excess material. Nevertheless, there are indications of small amounts of excess material that are slowly removed by water immersion even for deposition at low surface pressures. Adsorption from CHCl3 gives smooth surfaces with large amounts of loosely held material that contributes to a larger contact angle hysteresis and lesser hydrophobicity.

Introduction The preparation of a stable and smooth hydrophobic surface presents a major challenge in the field of direct force measurements. The increasing interest in the attractive forces found between hydrophobic surfaces across aqueous solutions1,2 has especially highlighted the need for a reliable method of preparing suitable hydrophobic surfaces. Many published results on hydrophobic forces are complicated by effects related to changes in * To whom correspondence should be addressed. E-mail: [email protected]. Telephone: +81-298-61-6317. Fax: +81-298-616232. † JRACAT-NAIR; also at PRESTO-Japan Science and Technology corporate. (1) Christenson, H. K. In Modern Approaches to Wettability: Theory and Applications; Schrader, M. E., Loeb, G., Eds., Plenum: New York, 1992. (2) Christenson, H. K.; Yaminsky, V. V. Colloids Surf. 1997, A129130, 67.

surface properties with time and under the influence of electrolytes.3-8 Several reports suggest that the interaction between the more stable surfaces is of considerably shorter range than those obtained with surfaces that lack longterm stability.2,9-11 The frequently observed or inferred presence of vapor or air bubbles between hydrophobic (3) Claesson, P. M.; Blom, C. E.; Herder, P. C.; Ninham, B. W. J. Colloid Interface Sci. 1986, 114, 234. (4) Claesson, P. M.; Christenson, H. K. J. Phys. Chem. 1988, 92, 1650. (5) Christenson, H. K.; Fang, J.; Ninham, B. W.; Parker, J. L. J. Phys. Chem. 1990, 94, 8004. (6) Christenson, H. K.; Claesson, P. M.; Berg, J.; Herder, P. C. J. Phys. Chem. 1989, 93, 1472. (7) Christenson, H. K.; Claesson, P. M.; Parker, J. L. J. Phys. Chem. 1992, 96, 6725. (8) Craig, V. S. J.; Ninham, B. W.; Pashley, R. M. Langmuir 1999, 15, 1562. (9) Wood, J.; Sharma, R. Langmuir 1995, 11, 4797. (10) Hato, M. J. Phys. Chem. 1996, 100, 18530. (11) Christenson, H. K.; Hato, M.; Ohnishi, S. Unpublished data

10.1021/la991167c CCC: $19.00 © 2000 American Chemical Society Published on Web 01/17/2000

Characterization of Fluorocarbon Surfaces

surfaces12-14seven at large separationssis difficult to explain unless the surfaces are not as smooth or homogeneous as is often assumed. Stable and smooth hydrophobic substrates would also permit experiments with simple liquids probing the effects of varying the wetting properties of the surface. The two most commonly used substrates in direct force measurements are hydrophilic; atomically smooth muscovite mica and fused silica. Most liquids, whether nonpolar or polar, wet both mica and silica, and a number of interesting surface phenomena are thus not accessible with the surface force apparatus (SFA),15 the MASIF,16 or the interfacial gauge (IG).17 For instruments such as the SFA, the MASIF, or the IG, it is desirable if not essential to use surfaces that are smooth and homogeneous over areas of many square millimeters. (Substantially smaller areas suffice for the atomic force microscope (AFM) and hydrophobic surfaces such as bulk polymers or graphite may be used.) Most efforts at finding a suitable hydrophobic surface have hence concentrated on modification of mica (which is atomically smooth) or silica (fused silica usually shows large-scale surface roughness of a few angstroms). The simplest method of producing a hydrophobic surface is to adsorb a cationic surfactant onto the negatively charged mica or silica surface.18-20 The results of contact angle and surface force measurements with such surfaces show a large variation with the concentration of both surfactant and electrolyte because adsorption equilibrium is shifted either when the surface passes through the threephase contact line or when the surfaces come into contact. Hydrophobic surfaces have also been prepared by adsorbing cationic surfactants from nonaqueous solutions,21-23 but considerable exchange between adsorbed and dissolved species is obviously possible once the surface is immersed in an aqueous medium. Deposition of Langmuir-Blodgett (LB) films (insoluble monolayers) yields surfaces that are more stable toward desorption and dissolution, but they are nevertheless inherently nonequilibrium structures. Detailed examination of the surface morphology in the case of crystalline LB films often reveals the existence of discrete domains, with considerable nonuniformity of surface coverage and resulting surface roughness. Moreover, all types of LB films are usually rapidly destroyed by salt solutions through ion exchange and are particularly vulnerable to removal by three-phase lines, where the molecules can easily migrate to the vapor-liquid interface.24-26 (12) Christenson, H. K.; Claesson, P. M. Science 1988, 239, 390. (13) Parker, J. L.; Claesson, P. M.; Attard, P. J. Phys. Chem. 1994, 98, 8468. (14) Carambassis, A.; Jonker, L. C.; Attard, P.; Rutland, M. W. Phys. Rev. Lett. 1998, 80, 5357. (15) Israelachvili, J. N.; Adams, G. E. J. Chem. Soc., Faraday Trans. 1 1978, 74, 975. Parker, J. L.; Christenson, H. K.; Ninham, B. W. Rev. Sci. Instrum. 1989, 60, 3135. (16) Parker, J. L.; Claesson, P. M. Langmuir 1994, 10, 635. (17) Yaminsky, V. V.; Ninham, B. W.; Stewart, A. M. Langmuir 1996, 12, 2, 836; Yaminsky, V.; Jones, C.; Yaminsky, F.; Ninham, B. W. Langmuir 1996, 12, 3531. (18) Herder, P. C. J. Colloid Interface Sci. 1990, 134, 346. (19) Yoon, R.-H.; Ravishankar, S. A. J. Colloid Interface Sci. 1994, 166, 215. (20) Ke´icheff, P.; Spalla, O. Phys. Rev. Lett. 1995, 75, 1851. (21) Tsao, Y.; Yang, S. X.; Evans, D. F.; Wennerstro¨m, H. Langmuir 1991, 7, 3154. (22) Tsao, Y.; Evans D. F.; Wennerstro¨m, H. Langmuir 1993, 9, 779. (23) Tsao, Y.; Evans D. F.; Wennerstro¨m, H. Science 1993, 262, 547. (24) Abe, K.; Ohnishi, S. Jpn. J. Appl. Phys. 1997, 36, 6511. (25) Eriksson, L. G. T.; Claesson, P. M.; Ohnishi, S.; Hato, M. Thin Solid Films 1997, 300, 240. (26) Yaminsky, V. V.; Nylander, T.; Ninham, B. W. Langmuir 1997, 13, 1746.

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Gas-phase plasma polymerization of small hydrocarbon or silane molecules may be used to provide a hydrophobic layer on mica. This layer is not tightly bound to the surface, and force measurements with such surfaces show that the polymer film is rapidly destroyed.27,28 Spin-coating of polymer films onto mica surfaces29 has also been used. One of the most widely employed methods of rendering mica or silica hydrophobic involves chemical reaction with the surface, usually with silane derivatives.9,13,14,30-37 This technique has been extensively applied in surface preparation for direct force measurements,13,14,34-37 and many such surfaces have been shown to have good stability. Alkylsilanes or fluoroalkylsilanes react under suitable conditions with hydroxyl groups present on silica surfaces to form covalent bonds,30-32 although the actual extent of bonding to the surface in some cases has been questioned.32 In the case of mica, which lacks surface hydroxyls,33 an initial water-vapor plasma treatment must be undertaken to create suitable binding sites.35 The reaction takes place in the gas phase with low-molecular weight silanes, although solvents such as chloroform or cyclohexane are more suitable for low-vapor-pressure compounds. Silylated surfaces, however, are prone to hydrolysis,38 and the large variability in the results of force measurements indicates a number of largely unresolved problems. There is often evidence of substantial amounts of excess, polymerized material that is not incorporated into an ordered monolayer.39 Spectroscopic techniques that depend on average surface coverage may not identify the presence of such undesired aggregates. Recent research suggests that optimum stability can only be achieved by a combination of polymerization and covalent bonding to the surface.9,40 With this in mind, we have undertaken a systematic approach toward the preparation and characterization of such a type of hydrophobic surface which we believe fulfils the necessary criteria for use in accurate surface force measurements. Our method is based on previous work by Wood and Sharma as well as Ge et al.41 In this paper, we report on the preparation and characterization of fluorocarbon monolayer surfaces using a fused glass substrate. We have used the same fluorinated silane, (heptadecafluoro-1,1,2,2,-tetrahydrodecyl)triethoxysilane, to make hydrophobic surfaces by three different methodssLB-deposition, adsorption from solution, and adsorption from the neat liquid. In addition, limited studies were carried out on surfaces prepared by a gasphase reaction with the fluorosilane. We have used AFM scanning, X-ray photoelectron spectroscopy (XPS), contact angle studies, and adhesion measurements to obtain a detailed picture of the structure and stability of the (27) Proust, J.-E.; Perez, E.; Segui, Y.; Montalan, D. J. Colloid Interface Sci. 1988, 126, 629. (28) Parker, J. L.; Claesson, P. M.; Wang, J.-H.; Yasuda, H. K. Langmuir 1994, 10, 2766. (29) Kohler, J. A.; Ulbricht, M.; Belfort, G. Langmuir 1997, 13, 4162. (30) Maoz, R.; Sagiv, J. J. Colloid Interface Sci. 1984, 100, 465. (31) Tripp, C. P.; Hair, M. L. Langmuir 1991, 7, 923. (32) Tripp, C. P.; Hair, M. L. Langmuir 1992, 8, 1120. (33) Nakagawa, T.; Ogawa, K.; Kurumizawa, T. Langmuir 1994, 10, 525. (34) Rabinovich, Ya. I.; Derjaguin, B. V. Colloids Surf. 1988, 30, 243. (35) Parker, J. L.; Cho, D. L.; Claesson, P. M. J. Phys. Chem. 1991, 93, 6321. (36) Yaminsky, V. V. Colloids Surf. A 1997, 129-130, 415. (37) Rabinovich, Y. I.; Yoon, R.-H. Langmuir 1994, 10, 1903. (38) Kessel, C. R.; Granick, S. Langmuir 1991, 7, 532. (39) Trau, M.; Murray, B. M.; Grant, K.; Grieser, F. J. Colloid Interface Sci. 1992, 148, 182. (40) Wood, J.; Sharma, R. Langmuir 1994, 10, 2307. (41) Ge, S.; Takahara, A.; Kajiyama, T. J. Vac. Sci. Technol. 1994, A12, 2530. Kajiyama, T.; Ge, S.; Kojio, K.; Takahara, A. Supramol. Sci. 1996, 3, 123.

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Figure 1. Surface pressure-area (π-A) isotherm of heptadecafluoro(1,1,2,2,-tetrahydrodecyl)triethoxysilane(FTE) spread on 10-2 M nitric acid subphase (pH 2).

different surfaces, and we will compare the relative merits of the different preparation methods. Subsequent papers will describe the results of force measurements with some of these surfaces. Materials and Methods Materials. The fused glass substrates, Pyrex rods or plates, were melted in a propane-oxygen flame. The molten glass was rinsed with pure water in order to remove possible ionic components from the glass surface and avoid any organic contamination, and tested for uniform wetting by water. Heptadecafluoro(1,1,2,2,-tetrahydrodecyl)triethoxysilane was purchased from Gelest Inc. and used without further purification. Chloroform (ACS grade, Aldrich) and nitric acid were used as received. Ethanol (Wako Pure Chemical Inc., Tokyo, or CSR Australia) was distilled under a nitrogen atmosphere. The water was distilled and processed through a Millipore UHQ unit. Surface Preparation. LB Method. We followed the method for deposition of octadecyltriethoxysilane (OTE) monolayers on mica reported by Wood and Sharma.9,40 The pH of the subphase was adjusted to 2 with nitric acid and a 3 mM chloroform solution of FTE spread over the surface. Although the main features of the surface-pressure-area (π-A) isotherm for the FTE monolayer were essentially the same as that reported by Ge et al. for the chlorosilane analogue heptadecafluoro-1,1,2,2,-tetrahydrodecyl)trichlorosilane (FOETS),41 the plateau at around 10-20 mN/m was more pronounced as the ethoxy group polymerizes more slowly than the chloro group (Figure 1).40 We deposited the FTE monolayer at pressures just before the plateau (10 mN/m), just after the plateau (20 mN/m), and just before the monolayer collapse (35 mN/m). The FTE monolayer on the air-water interface was kept at these surface pressures for 30 min before deposition. The monolayers were transferred onto the molten glass substrate by upward drawing (5 mm/min). Adsorption Method. FTE was adsorbed on the substrates from CHCl3 solution (30 mM) and from neat FTE liquid for 1 h. For a comparison of the surface coverage and chemical composition with XPS, fluorocarbon surfaces were also prepared by adsorption from vapor. Molten glass surfaces were exposed to FTE vapor for 1 h at 70 ° C in a desiccator. All surfaces were annealed in a dry nitrogen atmosphere at 100 °C for 2 h after deposition/adsorption. Prior to some measurements (indicated where relevant), the surfaces were rinsed in distilled ethanol and then dried in nitrogen. AFM Imaging. The AFM system used in this study was a Nanoscope IIIa equipped with 12 µm × 12 µm and 130 µm × 130 µm scanners (Digital Instruments, Inc.). Triangular, 200 µmlong cantilevers, with nominal spring constants of 0.12 N/m and pyramidal silicon nitride tips were used. Images were acquired at a rate of 14-28 lines/s, requiring roughly 15-25 s of collection time per image. Topographic images and friction force images of the surfaces were taken simultaneously with an applied force of about 0.1 nN. The surface roughness was estimated from AFM images by using the “roughness” function of the Nanoscope III software. XPS Measurements. High-resolution XPS spectra were recorded using an ESCALAB 220iXL system (VG Scientific Inc.) with a monochromatic Al-KR X-ray source (1486.6 eV). The binding energy was calibrated using the Au (4f7/2) peak energy

Ohnishi et al. (84.0 eV) as the energy standard. The X-ray power, the pass energy of the analyzer and the takeoff angle of photoelectrons were set at 180 W, 20 eV, and 90°, respectively. The energy resolution of this system is less than 0.6 eV, as estimated by the Ag (3d5/2) peak width under our measurement conditions. All the peaks were resolved by using the spectral processing program in the XPS system. The coverage of the monolayer was estimated from the XPS peak areas and the value was normalized by the sensitivity factor of each element; 1.00, 4.26, and 0.817 for C(1s), F(1s), and Si(2p), respectively. We used mainly the 300 nm SiO2/ Si substrates (10-20 Ωcm, p-type, B doped) prepared by thermal oxidation instead of glass substrates for XPS measurements because glass substrates, being insulators, often charge up and make measurement impossible. We have confirmed using specially prepared samples that the XPS spectra of fluorocarbon monolayers on SiO2/Si substrates are almost identical to that on glass substrate using a different XPS system with fine neutralizer (Shimazu AXIS-165). Contact Angle Measurements. Advancing and receding contact angles were measured by the Whilhelmy balance method42 at room temperature. The surface tension was measured with a high-sensitivity electromicrobalance. The monolayers were deposited and immobilized on molten Pyrex glass rods (length ) 2 cm; diameter ) 2 mm). The contact angle hysteresis curves were obtained at a stage driving speed of 20 µm/s. Adhesion Measurements. Adhesion forces were measured with the interfacial gauge (IG). A detailed description of this instrument has been given previously.17 This instrument permits the force vs separation curves for two surfaces to be determined for arbitrary, smooth materials. One surface is rigidly connected to the base of the instrument via a piezoelectric drive, and the other is mounted on a cantilever spring of a piezoelectric bimorph which acts as the force/deflection sensor. The sensor is positioned on a motorized microtranslation stage which permits one surface to be moved with respect to the other to adjust their initial separation. A magnetic transducer with a magnet attached to the cantilever allows fine control of the separation in the range of up to several micrometers. The bimorph deflection and jumps occurring at the onset of spring instabilities were monitored with a computer. Pull-off forces were calculated from the measured jump-out distances and the cantilever spring constant (258 N/m). The speed at which the surfaces approach and separate in the absence of interaction between them in these experiments was about 12 nm/s. The substrates used in the IG experiments were molten Pyrex glass spheres with radii of approximately 2 mm, formed by melting 2 mm glass rods. The radii of curvature of the spheres were measured with a microscope with a travelling reticule (estimated error ( 0.02 mm).

Results and Discussion AFM Observations. The AFM scans in Figure 2 show the morphology of the surfaces deposited at different surface pressures: 10, 20, and 35 mN/m, hereafter called LB10, LB20, and LB35. In the case of LB10, the maximum height difference between the brightest and darkest part of the image was estimated as 0.9 nm with cross-sectional profile, which is less than the length of FTE molecule (1.2 nm).41 The mean roughness is estimated to be ca. 0.14 nm. No clear domain structure or domain boundaries were observed in the large scanning area image of LB10, suggesting that the surface of LB10 is a homogeneous monolayer. As reported for 2-(perfluorooctyl)ethyltrichlorosilane monolayers,41 the FTE monolayer should be amorphous as the fluoroalkyl chain length is too short for crystallization. The AFM image of LB20 shows some protrusions (imaged as small white blobs) which are 2.5-3 nm in height. These objects appearing on the surface deposited at pressures above 10 mN/m are likely to be polymerized FTE aggregates projecting out from the monolayer by (42) Wilhelmy, L. Ann. Phys. 1863, 199, 177.

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Figure 2. AFM images of FTE surfaces deposited by LB method at different surface pressures (10, 20, and 35 mN/m), and the height profiles cross-sectioned shown by the dashed line in the images. The area of each image is 4 µm × 4 µm. Surface roughness of each image is Rmax ) 0.910 nm, Ra ) 0.143 nm for LB10; Rmax ) 2.952 nm, Ra ) 0.197 nm for LB20; and Rmax ) 3.482 nm, Ra ) 0.346 nm for LB35. An AFM image of the molten glass substrate did not show any features (Rmax < 0.2 nm, Ra < 0.1 nm). These images obtained after rinsing with ethanol exhibited features similar to those before rinsing, suggesting that the small blobs observed in LB20 and LB35 are not physisorbed airborne contaminants.

compression. The mean surface roughness of ∼0.2 nm is also larger than for LB10, suggesting a more uneven surface. The AFM image of LB35 shows a number of particles distributed across the surface. Their height is 2.5-3 nm, as for the small protrusions observed on LB20, while their diameter of 100 nm is three times larger than that of the protrusions on LB20. The mean surface roughness (0.35 nm), excluding the contribution of the particles is twice as large as that of LB10. It is clear that the amount of protruding FTE molecules increases with the surface pressure of the air-water interface. In fact, in SFA measurements between similarly prepared surfaces on mica, we usually observe repulsive forces acting between FTE surfaces deposited at pressures higher than 15 mN/ m.11 This could be associated with a steric repulsion stemming from such surface protrusions. It thus appears that deposition at 10 mN/m provides optimum conditions for the formation of a uniform and homogeneous monolayer without excess material on the surface. The topography of the surfaces prepared by adsorption is compared with that of surfaces prepared by the LB method in the left column of Figure 3. No notable features were observed in these images except for a slightly bumpy structure in the image of FTE/CHCl3. The mean surface roughness of these surfaces was 0.13-0.15 nm, indicating that the surfaces prepared by retraction have comparable smoothness to the surface of LB10. The right column of Figure 3 shows images of the frictional force distribution recorded simultaneously with topographic imaging. The image of LB10 reveals no variation in the frictional force across the surface, while ripplelike patterns are observed in the image of FTE/ CHCl3. The patterns could be interpreted in either of the following two ways: (I) It has been reported that the fluorocarbon regions show a larger frictional force than the hydrocarbon region on fluorocarbon/hydrocarbon phase-separated LB films or self-assembled monolayers (43) Overney, R.; Meyer, E.; Brodbeck, D.; Lu¨thi, R.; Gu¨ntherodt, H.-J.; Fujihira, M.; Takano, H.; Gotoh, Y. Nature 1992, 359, 133. (44) Takahara, A.; Kojio, K.; Ge, S.-R.; Kajiyama, T. J. Vac. Sci. Technol. 1996, A14, 1747. (45) Ishida, T.; Yamamoto, S.; Mizutani, W.; Motomatsu, M.; Tokumoto, H.; Hokari, H.; Azehara, H.; Fujihira, M. Langmuir 1997, 13, 3261.

Figure 3. AFM topographic images (right column) and the corresponding friction force images (left column) of FTE surfaces prepared by LB method and two different adsorption methods. The area of each image is 5 µm × 5 µm. Topographic images obtained from vertical deflection of the cantilever (right) and force distribution images obtained from lateral deflection of the cantilever (left) recorded simultaneously with an applied force of about 0.1 nN. The surface roughness of each topographic image is Rmax ) 0.993 nm, Ra ) 0.115 nm for LB10; Rmax ) 2.422 nm, Ra ) 0.156 nm for FTE/ CHCl3; and Rmax ) 1.705 nm, Ra ) 0.133 nm for FTE/neat. The banding observed in the force distribution images is due to mechanical vibration.

(SAM).43-45 If the outermost surface were composed of varying amounts of exposed fluorocarbon, hydrocarbon

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Ohnishi et al. Table 1. Estimated Surface Coverage from F(1s)/Si(2p) Peak Area Ratios of FTE Surfaces before and after Rinsing with Ethanol peak area

Figure 4. Survey-XPS spectra of FTE surfaces without rinsing with ethanol. All surfaces were annealed at 100 °C for 2 h after deposition. No components other than those present in the FTE molecules and the substrate were observed even before rinsing. The main features of spectra were not changed by rinsing with ethanol but the F(1s)/Si(2p) peak area ratios decreased as shown in Table 1.

and unreacted ethoxy moieties, different frictional forces would be detected across the surface. (II) Another interpretation is concerned with conformational changes of molecules in the monolayer. Such molecular motion of molecules covalently bound to the substrate could produce stick-slip of the AFM tip on scanning, resulting in the formation of the ripplelike patterns observed. The image of FTE/neat also shows different frictional forces across the surface. Assuming that the brightest areasthe large-friction areascorresponds to the fluorocarbon-rich region, the surface coverage of FTE molecules would be lower than for the other two surfaces. Since the topographic images of these surfaces show no obvious features the second of the above two interpretations seems to be reasonable for this surfacesi.e., conformational changes on scanning. Although the origin of the frictional-force distribution on each surface remains to be elucidated, it depends strongly on the preparation method, even though the different surfaces were of comparable smoothness. The images reveal that the LB surface deposited at 10 mN/m provides the most uniform frictional force distribution. XPS Measurements. The chemical composition of the surfaces was measured with XPS. To investigate the stability of the film, we compared XPS data with the surfaces before and after rinsing in pure ethanol for 30 min. Figure 4 shows the survey spectra of the surfaces. The F(1s), O(1s), Si(2s), and Si(2p) peaks were observed at 688, 533, 151, and 103 eV, respectively.46 Five or six C(1s) peaks appeared at 285-295 eV, which will be discussed in detail below, with narrow scan spectra of the C(1s) region between 280 and 300 eV. The Si(2p) peak is attributed to the silica substratesthe contribution from (46) Lenk, T. J.; Hallmark, V. M.; Hoffmann, C. L.; Rabolt, J. F.; Castner, D. G.; Erdelen, C.; Ringsdorf. H. Langmuir 1994, 10, 4610.

sample name

F(1s)

O(1s)

Si(2p)

F(1s)/ Si(2p)

LB10 LB10 rinsed LB20 LB20 rinsed LB35 LB35 rinsed FTE/CHCl3 FTE/CHCl3 rinsed FTE/neat FTE/neat rinsed

44056 44793 38760 49371 47430 30868 32770 19738 38963 36034

46231 45737 39215 50880 49655 36236 42620 33734 27316 48730

9405 9707 7848 10221 10330 7430 8643 6806 5853 9872

4.68 4.61 4.94 4.83 4.59 4.15 3.79 2.90 6.66 3.65

estimated surface coverage (%) 97 96 102 100 95 86 78 60 138 75

the silane silicon is by comparison very small. We estimated the relative amounts of FTE in the surface layers by using the F(1s)/Si(2p) peak area ratios (Table 1). Since the value of F(1s)/Si(2p) is the largest and does not change much after rinsing, we have defined the standard of comparison as the rinsed LB20 surface (100%). It was obvious that the initial coverage of the LB surfaces is more than 95% and that only the coverage of LB35 decreases after ethanol rinsing. As the AFM images of LB35 have suggested, some of FTE monomers are expelled from the monolayer at 35 mN/m. These would not bind covalently to the substrate and easily come off on rinsing. The surface coverage of both the FTE/CHCl3 and FTE/ neat surfaces decreases on rinsing, suggesting that the surfaces prepared by adsorption are less stable than those prepared by deposition. The percent coverage after rinsing is also much lower than for the deposited surfaces. It is interesting to compare the conclusions drawn from the friction force images (Figure 3) with the XPS data. The FTE/neat surface shows a much more heterogeneous frictional force distribution than FTE/CHCl3 despite the larger amount of FTE, both before and even after rinsing. This pattern in the friction force image is hence most likely associated with molecular movement during the scan rather than with differences in the frictional forces due to chemical heterogeneity. The molecular movement on the FTE/neat surface is larger than on the FTE/CHCl3 surface because many molecules are not covalently bound and come off on rinsing. For comparison, FTE surfaces prepared by vapor adsorption showed a surface coverage of only 55%, decreasing to 45 ( 5% after rinsing. The features of C(1s) spectra (and an AFM image) were similar to those of FTE/ CHCl3. Therefore, the properties of the surface prepared by vapor adsorption are expected to be the same as those of FTE/CHCl3 but with a lower surface coverage. Figure 5 shows the C(1s) spectra of the surfaces after rinsing with ethanol. We observed five or six peaks in the C(1s) spectra, as mentioned. At higher binding energies, of 290-295 eV, we detected three peaks which can be assigned to fluorocarbon groups.41,45 The 293.5 eV peak is attributed to a CF3 species. The strong peak at 291.5 eV was resolved into two peaks and assigned to two CF2 species. The major peak at 291.5 eV is CF2 in the fluorinated alkyl chains. The shoulder peak at 290.6 eV is a (-CH2-C*F2-) species. The peaks at around 285 eV are attributed to alkyl groups attached to fluorinated alkyl chains. For LB10 and LB20, the broad peak at 285 eV was separated into two peaks. Since these two peaks are nearly equal in area, it could be interpreted as the peak of a methylene carbon next to fluorocarbon (-C*H2-CF2-) at 285.5 eV and the peak of a methylene carbon next to

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Figure 5. XPS spectra in the C(1s) region of FTE surfaces. All surfaces were annealed at 100 °C for 2 h after deposition. The C(1s) peaks were resolved into 293.4 ( 0.3, 291.2 ( 0.3, 162.4 ( 0.2, 290.3 ( 0.3, 286.4 ( 0.1, 285.5 ( 0.1, and 284.5 ( 0.1 eV with full widths at half-maximum (fwhm) of 1.0 ( 0.2, 1.1, 1.0 ( 0.1, 1.3 ( 0.1, 1.3 ( 0.3, and 1.3 ( 0.1 eV.

a silane (-C*H2-Si-) at 284.5 eV.47 The broad peak of LB35 was resolved into three peaks at 286.5, 285.5, and 284.5 eV. It has been reported that a peak at around at 287 eV may be assigned to the C-O group.48 Because C-O peaks are not observed in LB10 and LB20, suggesting the absence of carbon oxidation during the heat treatment, the peak at 286.5 eV is probably due to C-O groups of unpolymerized residual ethoxy groups (CH3-C*H2-O).49 In the LB35 monolayer the peak intensity at 284.5 eV is larger than that at 285.5 eV. Since the peak at 284.5 eV increases as the peak at 286.5 eV does, residual ethoxy groups should also affect the peak intensity. In this case, it should be methyl carbon of residual ethoxy moiety (C*H3-CH2-O-).49 Thus, the peak at 284.5 eV observed in LB35 can be assigned to the methyl carbon of residual ethoxy moiety (C*H3-CH2-O-) overlapping with the peak of a methylene carbon adjacent to silane (-C*H2Si-). The same three peaks are found in the C(1s) spectra of the adsorbed surfaces. Obviously, a larger number of unreacted ethoxy groups remain on the FTE/CHCl3 surface compared to the FTE/neat surface. On both surfaces, the amount of unreacted ethoxy groups decreased slightly after rinsing, probably due to hydrolysis of the ethoxy groups by residual water in the ethanol. The C(1s) spectrum of the FTE/neat surface is similar to that of LB35, suggesting that most of the ethoxy groups have been converted to silanol (Si-OH) by reaction with water or to siloxane (SiO-Si) by reaction with other FTE molecules. To detect the main chemical species on these two surfaces, we estimated the contact angle of water using the sessile (47) Cotton, C.; Glidle, A.; Beamson, G.; Cooper, J. M. Langmuir 1998, 14, 5139. (48) Chatain, J., Ed. Hand book of X-ray photoelectron Spectroscopy; Perkin-Elmer Corporation: Norwalk, CT, 1993. (49) Tsukruk, V. V.; Luzinov, I.; Julthongpiput, D. Langmuir 1999, 15, 3029.

drop method. LB35 showed a contact angle of more than 90° and FTE/neat one of less than 90°, and it thus seems likely that the LB35 surface is mainly composed of siloxane, while the FTE/neat surface is composed of silanol. The XPS data show the coverage and the chemical composition of the surfaces but it is also possible to gain information on the orientation of FTE molecules at the surface. There have been reports on the orientation of thiols possessing identical, partially fluorinated alkyl chains.45,50,51 As for the peak of hydrocarbons (around 285 eV), the CH2 peaks of LB10 and LB20 are much smaller than those of the FTE/CHCl3 surface, indicating that the CH2 groups of LB10 and LB20 are shielded deep inside the film because of the low (with respect to the normal) molecular tilt angle. The stronger CH2 peak at 285 eV of the FTE/CHCl3 surface is due to the large tilt angle and consequent presence of CH2 near the outermost surface. Theoretically, the intensity of the photoelectron I(n) is related to the tilt angle θ by

I(n) ) exp(-dn(cos θ)/λ)

(1)

where d is the distance between carbon atoms of alkyl chain (ca. 1.3 Å), n is the carbon atom number counting from the surface, and λ is the attenunation length of a photoelectron in the monolayer (ex. 32.6 Å).52-54 Therefore, the relative ratio of the peak intensity of CF2 to CF3 is derived as (50) Alves, C. A.; Porter, M. D. Langmuir 1993, 9, 3507. (51) Liu, G.; Fenter, P.; Chidsey, C. E. D.; Ogletree, D. F.; Eisenberger, P.; Salmeron, M. J. Chem. Phys. 1994, 101, 4301. (52) Briggs, D.; Seah, M. P. Practical Surface Analysis by Auger and X-ray photoelectron Spectroscopy; Wiley and Sons: Chichester, England, 1983. (53) Bain, C. D.; Whitesides, G. M. J. Phys. Chem. 1989, 93, 1670. (54) Biebuyck, H. A.; Bain, C. D.; Whitesides, G. M. Langmuir 1994, 10, 1825.

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7

CF2/CF3 )

∑ I(n)/I(1) n)2

(2)

Table 2. The CF2/CF3 Peak Area Ratios of FTE Surfaces before and after Rinsing with Ethanol peak area of C(1s) sample name

CF3

CF2(-CF2)

CF2(-CH2)

CF2/CF3

LB10 LB10 rinsed LB20 LB20 rinsed LB35 LB35 rinsed FTE/CHCl3 FTE/CHCl3 rinsed FTE/neat FTE/neat rinsed

571.3 602.9 490.2 674.7 611.8 493.8 205.7 447.6 609.7 499.1

2919.8 3035.1 2744.9 3462.5 3251.7 2150.1 1450.7 2105.8 3062.4 2471.6

513.8 501.3 296.2 513.5 208.8 432.0 266.4 500.5 432.5 431.5

6.01 5.87 6.20 5.90 5.66 5.23 5.82 7.20 5.73 5.82

Although one should be cautious in using these calculated values, it would be reasonable to say that a larger value means a larger tilt angle, indicating that the molecules tend to lie down to the substrate. Table 2 shows the values of CF2/CF3 ratio for each of the surfaces. For most of the surfaces these values are between 5 and 7. It should be noted that CF2/CF3 for LB surfaces is decreased by rinsing with ethanol although the surface coverage does not change much, particularly in the case of LB10 and LB20. In contrast to that, CF2/CF3 ratio of the surface prepared by the adsorption method is increased by rinsing with ethanol. The decrease in CF2/CF3 ratio of the LB surfaces could be associated with orientational change of molecules or removal of physisorbed molecules on rinsing (both changes should induce a decrease in the average tilt angle of molecules on the surface). It has been reported that fluorinated alkanethiols (CF3(CF2)n(CH2)2SH; n ) 5, 7, 11) on gold form a hexagonal lattice with a nearestneighbor distance of 5.7 ( 0.2 Å.50,51 In the case for LB10 and LB20, FTE molecules were deposited at 0.2 and 0.3 nm2/molecule, close to the molecular area. Moreover, it was also reported that frictional force images of these fluorinated alkanethiol monolayers reproducibly showed different lattice constant depending on applied load.50 Therefore, it is possible that FTE molecules deposited by the LB method tend to stand up on the substrate on rinsing. The increase in the CF2/CF3 ratio of the surfaces prepared by the adsorption method could be due to more molecules tending to lie down on the substrate because of depletion of the monolayer. An alternative explanation would involve a greater fraction of randomly oriented, nonbonded FTE molecules physisorbed in the monolayer. Contact Angle Measurements. Figure 6 shows Wilhelmy-plate wetting curves and the corresponding dynamic contact angles of water on the surfaces. The contact angles on the LB10 surface indicated stable hydrophobicity. Note that the average values of advancing and receding contact angles of second and fourth cycles almost overlap. The measured average advancing and receding contact angles were 123 and 96°, respectively. Even after 11 immersion-retraction cycles the contact angles decreased only slightly, by 1.2° for the advancing and by 0.1° for the receding. Only a minor further decrease of these contact angles was observed after 5 days of immersion in water (less than 3° for both the advancing and receding angles). After 2 h of immersion in ethanol, the contact angles increased slightly. It may be due to ethanol removing contaminants or ethanol changing the orientation of the molecules on the surface. This is in accord with the XPS results showing a change in the average tilt angle of molecules after rinsing. The contact angles of the FTE/CHCl3 surface changed in the first three cycles. The advancing contact angles increased from 107 to 109° after the first cycle, then started decreasing to 107° again. The measured average values of advancing and receding contact angles were 107 and 60°, respectively. Similar results for the advancing contact angles were reported with FTE surfaces prepared by adsorption from a different solvent (1,1,2-trichloro-1,2,2trifluoroethane).55 The advancing and receding contact angles decreased by less than 1° after five cycles. These changes in the contact angles suggest that the FTE/CHCl3

where cos θFTE and cos θsubstrate are the contact angles on a fully covered FTE surface and a fused glass substrate, respectively. With θFTE ) 126° and θsubstrate ) 0°, the contact angles for LB10 (fFTE ) 0.97) and FTE/CHCl3 (fFTE ) 0.78) are calculated as 122.7 and 104°, which are close to the measured contact angles. The measured contact angle of the FTE/neat surface was, however, much smaller than the calculated angle, probably due to residual hydroxyl groups and random orientation of molecules on the surface. Adhesion Measurements. The normalized pull-off forces Fad/R were measured with the interfacial gauge (IG) using surfaces prepared by each of the different methods. In dry air, the range of measured pull-off forces was larger than in humid air-120 ( 20 mN/m for both LB10 and FTE/CHCl3. Figure 7 shows the dependence of the pull-off forces measured with the LB10 and FTE/CHCl3 surfaces in humid air on the time in contact. There was no obvious time dependence beyond 25 s. Fad/R was also

(55) Yoshino, N.; Yamamoto, Y.; Hamano, K.; Kawase, T. Bull. Chem. Soc. Jpn. 1993, 66, 1754.

(56) Adamson, A. W. Physical Chemistry of Surfaces; Wiley and Sons: New York, 1990.

surface is less stable in the original form than the LB surface, although it eventually stabilizes, perhaps after extra material has been washed away. The contact angle hysteresis is also almost two times larger than for the LB surface, possibly due to lower surface coverage and unreacted ethoxy moieties on the surface. The FTE/neat surface shows an advancing contact angle