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FTIR and AFM Studies of the Kinetics and Self-Assembly of Alkyltrichlorosilanes and (Perfluoroalky1)trichlorosilanesonto Glass and Silicon Reena Banga and Jack Yarwood" Materials Research Institute, Sheffield-Hallam University, Pond Street, Sheffield, S 1 1WB, United Kingdom
Anthony M. Morgan, Brian Evans, and Jaqueline Kells Pilkington plc, Hall Lane, Lathom, Ormskirk, Lancashire, L40 5UF, United Kingdom Received April 24, 1995. In Final Form: August 8, 1995@ Atomic force microscopy (AFM) has been employed to study the adsorption of alkyl-and (perfluoroalky1)trichlorosilane molecules on silica substrates. The effect of surface coverage on the packing of these long chain molecules and the rate of formation of the monolayers on the silica have been investigated and the results compared with those from Fourier transform infrared spectroscopicexperiments. It was found that at low coverage of the silanes on the substrate island formation occurred and that as coverage approached monolayer the islands in the AFM images were in-filled to produce a smooth surface. Above monolayer coverage it has been found that disordered molecular islands were formed. These were not easily detected using FTIR but were clearly identified by AFM.
Introduction Adsorption, by self-assembly, of amphiphilic molecules onto polar substrates leads to the formation of hydrophobic closely packed monomolecular films. Alkyl- and fluoroalkyl-substituted silanes of the general formula RS&, where R = CH3(CH2)17, C F ~ ( C F Z ) ~ C HorZ )CFdCFd7~, (CH2)z;X = C1, OMe, or OEt, are the most commonly used reagents for this reaction. The head group, SiC13, Si(OMe)3, or Si(0Et)s is polar and hydrolyzes in aqueous solution to produce silanol groups which can condense with silanol groups on the surface of silica to form siloxane while exposing the hydrophobic end group CH3 or CF3 to the atmosphere. The most effective reagents for producing hydrophobic surfaces, on reaction with a hydrophilic surface, are the (fluoroalky1)trichloro~i1anes.ll-l~ These have been shown to have water contact angles of 119.4' i 0.7" on silica,1° while OTS (n-octadecyltrichlorosilane, CH3(CH2)17SiC13)has been shown to have a water contact angle of 110', on ~i1ica.l~
OTS monolayers on silica and silicodsilicon oxide substrates have been well quantified by attenuated total reflection infrared spectroscopy, ATR-IR,1-8 reflection infrared s p e c t r o s ~ o p y , ~and ~ - ~by ~ a variety of X-ray techniques and e l l i p s ~ m e t r y . ~Ellipsometry ~ ~ ~ ~ - ~ ~ has shown OTS monolayers to have an approximate thickness of 2.6 nm.17 Infrared spectroscopy results have shown that a monolayer of OTS can be formed on a silica surface, by self-assembly, from a 1 mM solution of the silane in toluene, after 90 min immersion time.21 Many researchers3J8,23have studied the effect of water on the reaction of OTS with silicon or silica and have found that the presence of water in the OTS solutions was required to hydrolyze the silanol groups on the OTS. The absence of this water would reduce the rate of adsorption since the OTS molecules would require water from another source, e.g., the silicodsilica surface for hydrolyzation. F6 [((perfluorohexyl)ethyl)trichlorosilane,CF3(CF&(CH2)2SiC131and F8 [((perhorooctyl)ethyl)trichlorosilane, C F ~ ( C F ~ ) ~ ( C H ~ films ) Z S ~have C ~ Ibeen prepared on silica and silicon substrate^.^^^'^-'^,^^ These have been studied using X-ray reflectivity,13infrared spectroscopy,lOJ1and ellipsometry.12 From ellipsometry results F8 monolayer thicknesses have been measured to be approximately 1.6 nm.12 Infrared and X-ray reflectivity data have shown
Abstract published inAdvance ACSAbstracts, October 15,1995. (1)Netzer, L.;Iscovici, R.; Sagiv, J. Thin Solid Films 1983,100, 67-76. (2)Cohen, S.R.; Naaman, R.; Sagiv, J. J . Phys. Chem. 1986,90, 3054-3056. (3)Angst, D.L.;Simmons, G. W. Langmuir 1991,7, 2236-2242. (4)Maoz, R.; Sagiv, J. J . Colloid Interface Sci. 1984,100,465-495. (5)Brzoska, J. B.; Azouz, I. B.; Rondelez, F. Langmuir 1994,10, (18)Le Grange, J. D.; MarKham, J. L.; Kurkjian, C. R. Langmuir 4367-4373. 1993,9,1749-1753. (6)Cheng, S.S.;Scherson, D. A,; Sukenik, C. N. J . Am. Chem. SOC. (19)Berquier, J.-M.; Fernandes, A.-C.; Chartier, P.; Arribart, H. SPIE 1992,114,5436-5437. Fourier Transform Spectrosc. 1989,1145,300-301. (7)Gun, J.; Sagiv, J. J . Colloid Interface Sci. 1986,112, 457-472. (20)Mathauer, K.; Frank, C. W. Langmuir 1993,9,3446-3451. ( 8 ) Brandiss, S.; Margel, S. Langmuir 1993,9,1232-1240. (21)Kallury, K. M. R.; Thompson, M.; Tripp, C. P.; Hair, M. L. Langmuir 1992,8,947-954. (9)Banga, R.;Yanvood, J.; Morgan, A. M. Langmuir 1996,11 (2), 618-619. (22)Le Grange, J . D.; Markham, J. L.; Kurkjian, C. R. Antec. 1993, 1148-1 151. (10)Tada, H.; Nagayama, H. Langmuir 1994,10, 1472-1476. (11)Tripp, C. P.; Veregin, R. P. N.; Hair, M. L. Langmuir 1993,9, (23)Tripp, C. P.; Hair, M. L. Langmuir 1992,8, 1120-1126. 3518-3522. (24)McGovern, M. E.;Kallury, K. M. R.; Thompson, M. Langmuir 1994,10,3607-3614. (12)Geer,R.;Stenger,D.A.;Chen,M.S.;Calvert,J.M.;Shashidhar, R. Langmuir 1994,10, 1171-1176. (25)Offord, D.A.;Griffin, J. H. Langmuir 1993,9,3015-3025. (13)Wasserman, S.R.;Whitesides, G. M.; Tidswell, I. M.; Ocko, B. (26)Tidswell, I. M.; Rabadeau, T. A.; Pershan, P. S.; Kosowsky, S. M.; Pershan, P. S.; Axe, J. D. J . Am. Chem. SOC.1989,llI, 5852-5861. D.J. Chem. Phys. 1991,95,2854-2861. (27)Tidswell, I. M.; Ocko, B. M.; Pershan, P. S. Phys. Reu. B 1990, (14)Chaudhury, M. K.; Owen, M. J. Langmuir 1993,9,29-31. 41,1111-1128. (15)Yoshino, N.; Yamamato, Y.; Seto, T.; Tominaga, S.-I.; Kawase, (28)Silberzan, P.;LBger, L.; Benattar, J. J.Langmuir 1991,7,1647T. Bull. Chem. SOC.Jpn. 1993,66, 472-476. (16)Yoshino,N.;Yamamato,Y.;Teranaka,T. Chem.Lett.1993,8211651. 824. (29)Wei, M.; Bowman, R. S.; Wilson, J. L.; Morrow, N. R. J . Colloid (17)Wasserman, S.R.; Tao, Y.-T.; Whitesides, G. M. Langmuir 1989, Interface Sci. 1993,157,154-159. (30)Lindner, E.;Arias, E. Langmuir 1992,8, 1195-1198. 5,1074-1087. @
0743-746319512411-4393$09.00/0 0 1995 American Chemical Society
4394 Langmuir, Vol. 11, No. 11, 1995
that monolayers of F8 form on glass exposing the CF3 groups.1o Lindner and Arias30formed monolayers of F8 and F6 on glass and quartz from 0.17%solutions in Freon113, under dry nitrogen conditions, by immersing the substrate into the silane solution for %to 24 h. The monolayers were seen to have formed after 5 min immersion time. Atomic force microscopy has been used to produce molecularly resolved images of various LangmuirBlodgett films on a variety of s ~ b s t r a t e s ~ and l - ~ ~of organomercaptan monolayers on g ~ l d . ~ AFM ~ - ~ 'images of monolayers of OTS on cleaved mica have also been p r o d ~ c e d . ~Nakagawa ~ - ~ ~ et al.38showed that there were pin holes in the OTS films from several nanometers to 100 nm in diameter, in monolayers formed by selfassembly from a solvent mixture. Israelachvili et al.40 studied the adsorption of OTS onto mica, by AFM. They discovered that the OTS forms monolayers on the mica by nucleating isolated domains, whose fractal dimensions increase with increased surface coverage. Alkyl- and (fluoroalkylkhlorosilane monolayers on silica and silicon have been characterized by AFM.41-46 Fujii et al.41 silanized single crystal silicon with OTS and with F8. Molecularly resolved AFM images of these samples showed that the alkyl chains ofthe molecules were slanted slightly to the surface normal and that the molecules occupied areas of 0.43 & 0.07 and 0.50 f 0.08 nm2, respectively. Yoshino4*has produced AFM images of a (fluoroalky1)trialkoxysilane on glass. He found that this silane forms double and triple monolayers on the substrate. Grunze et al.43used AFM to probe OTS monolayers on silicon. It was stressed that in order to obtain reproducible, good quality films, the samples should be prepared under class 100 clean room conditions. They found that OTS monolayers formed on silicon, by self-assembly, first by growth of large islands and then by in-filling with smaller islands until the film is complete. Yoon et al.44have produced submono- and monolayers of OTS on silica plates. All solutions and films were prepared in a glovebag filled with dry nitrogen gas. Initially, from the AFM images, the OTS was seen, a t low coverage, to form patches of 20 x 40 nm2. As the surface coverage increased, the number of patches increased until a smooth surface was formed. The size ofthe patches did not vary greatly with increased coverage. At 25-30% coverage they believed that the silica was covered completely with OTS and that the molecules lie flat on the surface, producing a water contact angle of (31) Peltonen, J. P. K.; He, P.; Rosenholm, J. B. J.Am. Chem. Soc. 1992,114, 7637-7642. (32) Weisenhorn,A. L.; Romer, D. U.; Lorenzi,G. P. Langmuir 1992, 8,3145-3149. (33) Weisenhorn,A. L.; Egger, M.; Ohnesorge,F.; Gould, S. A,; Heyn, S.-P.: Hansma. H. G.: Sinsheimer. R. L.: Gaub. H. E.: Hansma. P. K. Langmuir mi, 7 , 8-12.
(34)Meyer, E.; Howlad, L.; Overney, R. M.; Heinzelmann, H.; Frommer,J.;Giintherodt,H.-J.;Wagner, T.; Scheir,H.; Roth, S. Nature
1991,349, 398-400. (35) Pan, J.;Tao, N.; Lindsay, S. M. Langmuir 1993,9,1556-1560. (36) Alves, C. A.i Porter, M. D. Langmuir 1993, 9, 3507-3512. (37) Liu, G.-Y.; Salmerson, M. B. Langmuir 1994, 10, 367-370. (38) Nakagawa, T.; Ogawa, K. Lungmuir 1994, 10, 525-529. (39) Okusa, H.;Kurihara, K.; Kunitake, T. Langmuir 1994,10,8-12. (40) Schwartz, D. K.; Steinberg, S.; Israelachvili, J.;Zasadzinski, Z. A. N. P h y ~REV. . Lett. 1992, 69 (231, 3354-3357. (41) Fujii, M.; Sugisawa, S.; Fukada, K.; Kato, T.; Shirakawa, T.; Seimiya, T. Langmuir 1994, 10, 984-987. (42) Yoshino, N. Chem. Lett. 1994, 735-736. (43) Bierbaum, K.; Grunze, M. Adhesion Soc. 1994, 213-216. (44) Flinn, D. H.; Guzonas,D. A.;Yoon, R.-H.Colloids Surf.A. 1994, 87, 163-176. (45) Rabinovich,Y.-I.; Yoon, R.-H. Langmuir 1994,10, 1903-1909. (46) Siedlecki,C. A,; Eppell, S. J.; Marchant, R. E. J.Biomed. Mater. Res. 1994, 28, 971-980. (47) Banga, R.; Yanvood, J.;Morgan, A. M.; Evans, B.; Kells, J.Thin Solid Films, in press. (48) Sperline, R. P.; Muralidharan, S.;Freiser, H. Langmuir 1987, 3, 198.
Banga et al. Marchant et al.46 have shown that OTS forms smooth monolayers on a glass substrate, with a roughness greater than that ofthe usual mica substrate. They believe this is achieved by.the entropy-driven alignment reaction of the OTS molecules through the formation of intermolecular siloxane linkages, Si-0-Si. This means that not all the OTS molecules need to be bonded to the glass substrate beneath. No AFM images of alkyl- and (perfluoroalkyl)silanes, adsorbed onto industrial glass, have been published previously. Neither has degree of orientational order been determined as a function of the surface coverage, on this substrate, particularly for coverages greater than monolayer. In these studies, we have silanized precleaned Pilkington float glass on the non-tin side using OTS and F8 from organic solution. Two sets of samples were prepared: one set under class 100 clean room conditions and the other in the open atmosphere. AFM images of these surfaces have been produced in order to determine the degree of packing with increased surface coverage. The roughness average value, R,, for each image has also been obtained, using the Nanoscope I11 AFM software, to provide a measure of surface roughness. R, is the arithmetic average value of the distance of the sample profile from the center line, along the whole sample length. The center line is drawn by the software so that the sum of the areas of the profile above the center line is equal to the sum of the areas below the center line. Fourier transform infrared attenuated total reflection spectroscopic (FTIR-ATR)data for similar systems on siliconhave been studied and are compared with the AFM results. It should be stressed here that the information obtained from ATR experiments will not be exactly the same as that obtained from AFM experiments. This is because the areas probed by the two techniques are very different (mm2 for ATR and nm2 for AFM) and also because the substrates used for the different experiments are not the same (siliconfor ATR and float glass for AFM). The reason why the AFM experiments have been conducted is to obtain information on the finer topographical structure of the adsorbed silane films.
Experimental Section Chemicals. OTS (n-octadecyltrichlorosilane, CH3(CH2)17SiC13,95%)HPLC grade toluene (99.9%),HPLC grade 2-propanol (99.9%), and HPLC grade methanol (99.9%) were obtained from Aldrich Chemical Co. These chemicals were used as received. F8 [((perfluorooctyl)ethyl)trichlorosilane, CF3(CF2)7(CH&Sic131 and Freon-113 were provided by Pilkington plc. These chemicals were predried over molecular sieve before use. Preparation of Substrates. Silicon ATR prisms, with 4550 internal reflections, were cut from commercial, single crystal, p-doped, 0.5 mm silicon at Durham University and were used for the infrared measurements. Float glass, provided by Pilkington plc, was used for the AFM measurements. The float glass is manufactured by floatation on hot tin. The non-tin side of the glass was analyzed here, since it is atomically smoother than the tin side. Both types of substrate were precleaned by refluxing in hot 2-propanol for 4 h and then stored in a desiccator before use. The silicon prisms were checked for contamination prior to coating, using FTIR-ATR. Silicon and glass substrates can be compared as both have similar surface properties as far as the reactions of OTS and F8 with these surfaces are concerned, i.e., 5 silanol groupshm2.8 Preparation of Silane Solutio,ns. 1mM solutions of OTS in toluene were prepared in a fume cupboard under atmospheric conditions, at room temperature. These were used immediately, and fresh solutions were prepared for each experiment. 0.5 mM solutions of F8 in Freon-113 were prepared in a dry nitrogenfilled glovebox to prevent the silane from polymerizing. Again these solutions were used immediately. Preparation of Silane Films. The submono- and monolayers of OTS and F8 were self-assembled on the substrates by
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Integrated intensity of CH2 stretching bandslcm-1
Integrated intensity of CF2 stretching bandslcm-1
.
21
1 I1
0.5
0.2
1
o ~ l l l l l l l l l l l , l l l l , l l l l l , 30
80
90
120
Cumulative Dip timelminutes
I
1
01 0
I
5
I
1
1
1
I
,
I
,
I
,
I
10 15 20 25 30 35 40 45 50 55 60 Cumulative dip timelminutes
Figure 1. The integrated area under the v(CH2)peaks versus the cumulative dip time in the OTS solution.
Figure 2. The integrated area under the v(CF2)peaks versus the cumulative dip time in the F8 solution.
dipping the substrate into the silane solution for various times, Two sets of OTS and F8 samples were produced for AFM analysis: one set under class 100 clean room conditions and the other under atmospheric conditions. The FTIR-ATR samples were prepared in the open atmosphere. After each dipping stage the OTS-coated samples were washed with chloroform, with methanol, and then with chloroform again, to remove any excess silane deposits. The F8-coated sampleswere washed with Freon113 and gently “buffed”with a lens tissue to remove any excess silane deposits. Measurement of Substrates and of Silane Films. FTIRATR measurements were made on a Mattson Polaris Fourier transform infrared spectrometer, using a signal gain of 4, 256 scans per sample, a resolution of 4 cm-l, and zero filling. OTS films prepared on a silicon ATR prism at cumulative dip times
time, which corresponds to monolayer coverage of the silicon. This agrees with the results of Tripp et a1.21From our dichroic ratio measurements it was showng that as the surface coverage of the OTS on the silicon increases, the orientational order of the OTS on the silicon increases. At low coverage, below 90 min dipping time, more “liquidlike’’ gauche conformations of the alkyl chain are seen while at higher coverage more trans conformations of the alkyl chain are seen. F8 was deposited from solution in freon, as explained in the Experimental Section, onto a silicon ATR crystal. The area under the antisymmetric v(CF2) peaks in each of the FTIR spectra was integrated and plotted against cumulative dip time in the silane solution (see Figure 2). The plot approaches a line parallel to the concentration axis at about 6-8 min dipping time. This corresponds to the time required to form a monolayer and agrees with the results of Lindner and Arias.30The coverage of the silicon increases very quickly with time and almost reaches monolayer after 1 min dip time. The areas occupied by the OTS and the F8 molecules were also determined from similar FTIR-ATR experiments to be 0.244 f0.001 and 0.300 f0.001nm2,respectively. The difference in the areas occupied by the two silanes is due to the fact that OTS molecules contain straight chains while the F8 molecules form helixes on the silicon surface. The helixes occupy a larger area than the straight chains, and therefore the F8 molecules occupy a larger area than the OTS molecules. These FTIR results will now be compared with the AFM images produced. Atomic Force Microscopy. Image of Pilkington Float Glass before Silane Treatment. The AFM image of the non-tin side of Pilkington float glass can be seen in Figure 3. The surface roughness value, R,, ofthis surface is 0.254 k 0.01 nm. This is rougher than mica but still smooth enough for the measurement of the thin films described here. Images of OTS-Treated Pilkington Float Glass. When analyzing the AFM images, the results from the infrared data are used for assigning degree of surface coverage, i.e., 20 and 40 min dipping times correspond to submonolayer coverage, 90 min dipping time corresponds to monolayer coverage, and 120 min dipping time corresponds to a time greater than that required for monolayer formation.
of 0, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85,90,95, 100, 105, 110, 120 min, in the silane solution, and F8
films prepared on a silicon ATR prism at cumulative dip times of0.25, 0.5, 0.75, 1, 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 ,10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 30, 60 min, in the silane solution were analyzed using FTIR-ATR. In situ experiments of the adsorption of OTS
and F8 on silicon were also carried out to determine the areas occupied by the silane molecules on the silicon substrate. These experiments were performed using a liquid micro-ATR cell. Atomic force microscopy measurements were made using a Nanoscope 111, (Digital Instruments Inc., Santa Barbara, CAI. The D scanning head (maximum scanning range 15 x 15 pm2) and the 100 pm cantilever tip were used in normal scanning mode. The AFM was vibration table-mounted to eliminate external vibrational noise. Samplesof approximately 1 cm2 area were examined on the non-tin side of bare float glass and on the non-tin side of silane-coatedfloat glass. OTS films on float glass at dip times of 20,40,90, 120 min in the silane solution and F8 films on float glass at dip times of 0.25,5,8,30min in the silane solution were examined by AFM for both the clean room and atmosphere prepared samples. Several images, at various magnifications, were obtained for each sample to ensure reproducibiity.
Results and Discussion JWIR-ATR Spectroscopy. OTS submono- and monolayers were deposited from solution in toluene, as explained in the Experimental Section, onto a silicon ATR prism. The area under the symmetricand antisymmetric v(CH2)peaks in each of the FTIR spectra was integrated, and this was plotted against cumulative dip time in the silane solution (see Figure 1). The plot approaches a line parallel to the concentration axis at about 90 min dipping
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Figure 3. An atomic force microscope image of t h e non-tin side of Pilkington float glass. Table 1. R, Values for OTS-CoatedFloat Glass ~
dipping time/min
conditions
Rdnm f 0.01 nm
20 40 90 120 20 40 90 120
clean room clean room clean room clean room open laboratory open laboratory open laboratory open laboratory
0.109 0.131 0.066 0.100 0.246 0.188 0.056 0.101
Parts a, c-e of Figure 4 depict three-dimensional AFM images of OTS on the non-tin side of Pilkington float glass. The images are presented as three-dimensional line plots in order to demonstrate the surface features of each sample. The vertical scale is given in the top right-hand corner of each figure. The “peaks” and “troughs” in the images correspond to higher peaks and lower troughs in the samples. The Ra values for the samples were calculated using the computer software, and the results are reported in Table 1. Some of the features in the sample images have been analyzed using the section analysis mode in the software. The images corresponding to these section analyses can be seen in parts b and f of Figure 4. The image is in the bottom left-hand corner of each figure. The cross section, along the white line in the image, is shown in the top left-hand corner of each figure. The height of the bright spots in the cross section of each image is shown in the box in the bottom right-hand corner next to vert distance, and the diameter of each spot is shown next to horiz distance. The brighter areas in each image represent peaks on the sample, and the darker areas represent troughs on the sample. Figure 4a is an AFM image of OTS on float glass, prepared under clean room conditions, and removed from the silane solution after 20 min dippingtime. In the image bright spots can be seen. These are islands at submonolayer coverage, of a height less than that of an OTS monolayer, i.e., less than 2.6 nm. The heights of some of the peaks can be seen in Figure 4b, which shows a section analysis of the image. A good indication of surface roughness is the Ra value. The Ra value for the image in Figure 4a is 0.109 nm. This value is less than the Ra value of the float glass substrate. Figure 4c depicts the AFM image of OTS on float glass, prepared under clean room conditions, and removed from
the silane solution after 40 min dipping time. Since poor resolution was obtained in the 10nm x 500 nm x 500 nm image, the 10 nm x 10pm x 10pm image is shown. Again islands can be seen of less than OTS monolayer height. The Ra value for the image in Figure 4c is 0.13 1nm. This value is again less than that of the float glass substrate. The Ra value for the sample prepared in the clean room is less than that for the sample dipped for 20 min, which indicates an increase in smoothness as the film builds up. Figure 4d is an AFM image of an OTS monolayer on float glass, after 90 min dipping time in silane solution. The Ra value obtained for this image is 0.066 nm. This is indicative of increased smoothness as compared to the submonolayer images and the glass substrate. Figure 4e depicts an AFM image of OTS on float glass, prepared under clean room conditions, and removed from the silane solution after 120 min dipping time (a time greater than that necessary to produce a monolayer). Islands of less than OTS monolayer height can be seen in the section analysis of the image (Figure 40. The Ravalue for the image in Figure 4e is 0.100 nm. This is indicative of a rougher surface than the monolayer but a smoother surface than the glass substrate. From the AFM images of the samples prepared under open laboratory conditions we have discovered that, apart from the monolayer sample, the Ra values are greater than those for the correspondingsamples prepared in the clean room, (see Table 1). We have no particular explanation for the difference, and previous researchers, studying the AFM images of OTS on silicon, have found that reproducibility of samples was only found in those samples prepared under class 100clean room condition^.^^ Thereforeno detailed discussion of the AFM data obtained from the samples prepared in the open laboratory has been given. From the AFM images of samples prepared under clean room conditions, it is concluded that at below monolayer coverage, at 20 and 40 min dippingtimes, island formation occurs, which gradually in-fills to produce a smooth monolayer a t 90 min dipping time. The increased smoothness is seen from the decreasing Ra value. This agrees with what was found by Yoon et al.45 However, a t dipping times above those required for monolayer formation, (120 min), island formation and an increased roughness average value is again determined from the AFM image. The islands in the images correspondingto below and above monolayer coverage are of a height less than monolayer. These are thought to be due to orientationally disordered OTS molecules. For the submonolayer samples this is in agreement with the infrared data. However,the infrared dichroic data for the sample dipped in the OTS solution for 120 min shows the film to be orientationally ordered. This appears to disagree with the AFM data. The difference is due to the fact that in the infrared experiment the whole sample, including the ordered monolayer is sampled, giving a result averaged over the beam size: We would not detect islands of disordered OTS molecules on top of the ordered layer with the infrared experiment except in that such deposition may contribute to the statistical variations observed in the dichroic ratio results. We believe that these disordered isolated islands form on top of the ordered OTS monolayer at 120 min dipping time. (If multilayers were formed these would be detected in the FTIR-ATR experiment.) So we deduce, both from FTIR and AFM, that no integral multilayers are formed. From the infrared data it can be seen that the OTS monolayer takes up to 90 min to form. The AFM data support this since a smooth surface is not seen in the AFM images before 90 min dipping time. Images of F8-Treated Pilkington Float Glass. As for the OTS samples, when analyzing the AFM samples the
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Langmuir, Vol. 11, No. 11, 1995 4397
Figure 4. Atomic force microscope images of OTS submono- and monolayers on the non-tin side of float glass at the following dipping times: (a,b) 20 min (clean room conditions), (c) 40 min (clean room conditions), (d) 90 min (clean room conditions), (e,O 120 min (clean room conditions).
infrared data are taken into account, i.e., 15 s and 5 min dipping times correspond to submonolayer coverage, 8 min dipping time correspondsto monolayer coverage, and 30 min dipping time corresponds to a time greater than that required to form a monolayer. Parts a, c-e of Figure 5 depict three-dimensional AFM images of F8 on float glass. Again, as for the OTS samples, the peaks and toughs in the images represent peaks and troughs in the samples. Due to the reproducibility problems encountered when preparing samples outside of the clean room, the F8 samples prepared in the open laboratory are not described or shown here.
The Ra values for the samples were calculated, and the results are reported in Table 2. As for the OTS samples, some of the features in the sample images have been analyzed using the section analysis mode in the software. These images are shown in parts b and f of Figure 5. Figure 5a is an AFM image of an F8 film a t submonolayer coverage, prepared under clean room conditions, and removed from the silane solution after 15 s dipping time. From Figure 5b it can be seen that this sample has islands of lower than F8 monolayer height, i.e., less than 1.6 nm. The Ra value for the image in Figure 5a is 0.176.
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Figure5. Atomic forcemicroscopeimages of F8 submono- and monolayers on the non-tin side of float glass at the followingdipping times: (a,b) 15 s (clean room conditions), ( c ) 5 min (clean room conditions), (d) 8 min (clean room conditions), (e,f) 30 min (clean room conditions). Table 2. R, Values for FS-Coated Float Glass dipping time/min conditions R h m f 0.01 nm 0.25 clean room 0.176 5 0.096 clean room 8 clean room 0.070 30 clean room 0.124 0.25 0.968 open laboratory 5 0.174 open laboratory 8 0.088 open laboratory 30 open laboratory 0.175 Therefore the sample prepared under clean room con&tions has a surface roughness less than that of the glass substrate. The AFM image of an F8 submonolayer film, prepared
under clean room conditions, and removed from the silane solution after 5 min dipping time, can be seen in Figure 5c. The R a value of this sample is 0.096 nm. This is indicative of increased smoothness as compared to the sample dipped for 15 s. This image again has islands of lower height than an F8 monolayer. The AFM image of a monolayer of F8 on float glass, prepared under clean room conditions, and removed from the silane solution after 8 min dipping time, can be seen in Figure 5d. The Ravalue obtained for this sample, 0.070 nm, is less than those obtained for the submonolayer samples, which indicates an increased smoothness in this sample. After 30 min dipping time, a time greater than
Langmuir, Vol. 11, No. 11, 1995 4399
Alkyl- and (PerfZuoroalky1)trichlorosilanes that required to produce a monolayer of F8 on float glass, a few islands of approximately monolayer height and less can be seen in the AFM image of the sample prepared under clean room conditions (see parts e and f of Figure 5 ) . The R, value obtained for this sample was 0.124 nm, which indicates a n increase in roughness as compared with the monolayer sample. The AFM images of the F8 samples prepared in the clean room a t coverages corresponding to submonolayer (15 s and 5 min dipping time) have islands of less than monolayer height. These are thought to be due to disordered F8 chains. At monolayer coverage, 8 min dip time, the islands have been filled in and a smooth surface of a lower roughness than the glass the substrate is produced. The smoothness of the surface can be seen from the decreasing R, values. The clean room image of the F8 film produced at a dip time of 30 min shows islands of monolayer height and less. Again, as for the OTS case, it is believed that these are due to disordered F8 molecules depositing over the ordered F8 monolayer. From the infrared data it can be seen that the F8 monolayer builds up very quickly. This can also be seen from the AFM images. After only 15 s dipping time, the glass surface is seen to be fairly well covered with F8 islands which then in-fill to produce a monolayer at 8 min dipping time. The rates of adsorption of the two silanes onto silicon or glass were expected to be dependent on many conditions including water content in the solvent and atmospheric humidity. Since it was the F8 solution that was predried and not the OTS solution, and the F8 monolayer formation time was faster than the OTS monolayer formation time, the adsorption of these silanes can be said to be system inherent. The reasons for this include the fact that if the OTS solutions had been predried and treated in the same way as the F8 solutions, then the rate of adsorption of the OTS onto the substrates would have been reduced. Therefore the dipping time for formation of the OTS monolayer would have been much larger than it was determined to be here. The reason why the F8 dipping time for monolayer formation was less than that of the OTS monolayer is most likely due to the relative rate of hydrolysis of the silane molecules. The rate ofhydrolysis of the Sic1 groups on the F8 molecules is faster than the rate ofhydrolysis ofthe Sic1 groups on the OTS molecules due to the greater electron-withdrawing effect of the CF3 and CF2 groups on the F8 molecules compared with the electron-withdrawing effect of the CHB and CH2 groups on the OTS molecules. This greater electron-withdrawing effect in the F8 molecules means that the C1 groups on the F8 silicon atoms are substituted faster than the C1groups on the OTS Si atoms (both SN2 reactions with OH- as the nucleophile). Because the F8 molecules were hydrolyzed faster than the OTS molecules, condensation of the silane silanol groups with the substrate silanol groups occurred sooner for F8 than for OTS. To determine if material was being moved by the AFM tip, certain features observable in the AFM image were imaged at various magnifications. The shape of these features were seen to remain constant, while the size of the features varied accordingly with the magnification of the image. The R, values obtained for all the silane-coated samples (except the F8-coated sample dipped in the open laboratory for 15 s) were smaller than the R, value obtained for the float glass substrate. This smoothing effect has been interpreted in the l i t e r a t ~ r eto~be ~ due to the entropydriven cross-linkage of silane (OTS)chains to form siloxane
links, Si-0-Si, so that not all the silane (OTS)molecules are adsorbed onto the glass substrate. Therefore the silane smooths the substrate surface. Similar silane film formation appears to have occurred with the samples here.
Conclusions n-Octadecyltrichlorosilane (OTS) and ((perfluoroocty1)ethy1)trichlorosilane (F8)monolayers have been formed by the self-assembling monolayer technique on silicon and on float glass substrates. They have been characterized by FTIR-ATRspectroscopy and by atomic force microscopy. Fourier transform infrared spectroscopicdata have shown that monolayers of OTS form after 90 min immersion time of the substrate in silane solution and that F8 monolayers form after 8 min immersion time in the silane solution. The AFM samples prepared in the open laboratory for both OTS and F8 had larger R, values than the samples prepared in the clean room. No explanation as to why there is a discrepancy between the two sets of results has been given here, but it has already been shown that reproducible samples (OTS on silicon) were only produced if clean room conditions were used for their p r e p a r a t i ~ n . ~ ~ The OTS images showed island formation at submonolayer coverage and then in-fillingto produce a smooth monolayer surface (shown by the decrease in R, value), a t about 90 min dipping time, which agrees with previous and with the infrared data reported in this and our previous paper. However, the images presented in this paper show a better distinction between different surface coverages than those presented by Yoon et a1.44 The submonolayer islands are assumed to be due to orientationally disordered OTS molecules forming on the substrate surface. At dipping times greater than those required to produce a monolayer, island formation and a greater R, value was also seen, which has not been observed in any previous studies. Again this is attributed to the fact that orientationally disordered OTS molecules deposit on top of an ordered OTS monolayer. Such islands could not be detected using FTIR spectroscopy as both the ordered monolayer and the disordered islands were sampled, giving a resulting spectrum as the average over the whole sample. (Multilayer formation, which would be detected by the FTIR-ATR technique, does not occur here.) The F8 images, observed here for the first time on float glass, showed island formation at submonolayer coverage and then in-filling to produce a smooth monolayer (shown by the decrease in R, values), a t 8 min dipping time. The time for F8 monolayer formation on float glass, determined here by AFM and FTIR-ATR data, agrees with that determined by Lindner and Arias30 from surface energy data, of F8 on glass and quartz. The submonolayer islands are assumed to be due to disordered F8 molecules forming on the substrate. Above monolayer dipping times, islands and a n increase in the R, value are seen which are again attributed to molecules depositing over the monolayer. Both infrared and AFM data show that the fluorinated silanes form monolayers much faster than the nonfluorinated silanes under similar conditions. When considering the time for the formation of the monolayers of F8 and OTS on silica substrates, the AFM data are in excellent agreement with the FTIR data. Acknowledgment. The authors wish to acknowledge the financial support received from EPSRC and from Pilkington plc. LA950319A
