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Organized Self-Assembled Monolayers from Organosilanes Containing Rigid π-Conjugated Aromatic Segments Johanne Moineau,† Michel Granier, and Gerard F. Lanneau* Laboratoire de Chimie Mole´ culaire et Organisation du Solide, UMR 5637, Universite´ Montpellier 2, case 007 - Place Bataillon, 34095 Montpellier Cedex 05, France Received August 11, 2003. In Final Form: January 10, 2004 The morphology of surfaces modified by π-conjugated arylsilanes depends on various parameters such as the nature and the number of the hydrolyzable functions or the length of the aromatic segment. The grafting of phenyltrichlorosilane and phenyltrimethoxysilane leads to multilayers even when the reactions are carried out at 0 °C, the films obtained from phenyltrichlorosilane being much thicker than the one obtained from phenyltrimethoxysilane. A submonolayer is obtained using phenyltriethoxysilane. Whereas the trifunctional phenyltrichlorosilane forms an inhomogeneous multilayer, the difunctional phenyldichlorosilane (PhSiHCl2) and the monofunctional phenylchlorosilane (PhSiH2Cl) (the SiH bond is not reactive under these experimental conditions) deposit as dense homogeneous monolayers. For these two phenylchlorosilanes, the surface analytical data are similar except for contact angle measurements, which can be explained by a different orientation of the phenyl group at the surface. Concerning the influence of the length of the aromatic segment on the quality of the film, styryltrimethoxysilane and methylstilbenyltrimethoxysilane lead to dense monolayers indicating that adding a short group such as the vinyl group is sufficient to induce an organization between aromatic groups and to achieve a dense monolayer.
Introduction Self-assembled monolayers (SAMs) of organic molecules on solid substrates are becoming increasingly important for various technologies. Areas of possible applications range from surface modifications for wettability control,1 tribology,2 sensor devices,3 or surface patterning.4 Beyond their practical importance, SAMs can serve as well-defined model systems to study the behavior of surfaces. Another point is that although surface science techniques allow one to characterize surfaces with improving lateral spatial resolution,5 in situ studies allow a better understanding of these processes at the molecular level. When functional groups are confined in closely packed molecular arrays, their reactivity often changes.6 For instance, in the case of 4-methyl-4′-mercaptobiphenyl * To whom correspondence should be addressed. E-mail:
[email protected]. Phone: 33 (4) 67 14 39 71. Fax: 33 (4) 67 14 38 52. † Current address: Department of Medicinal Chemistry, The University of Tokushima, Shomachi, Tokushima, 770-8505, Japan. (1) (a) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991. (b) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437463. (c) Ulman, A. Chem. Rev. 1996, 96, 1533-1554. (2) (a) Carpick, R. W.; Salmeron, M. Chem. Rev. 1997, 97, 11631194. (3) (a) Rubinstein, I.; Steinberg, S.; Tor, Y.; Shanzer, A.; Sagiv, J. Nature 1988, 332, 426-429. (b) Kepley, L. J.; Crooks, R. M.; Ricco, A. J. Anal. Chem. 1992, 64, 3191-3193. (c) Mrksich, M.; Sigal, G. B.; Whitesides, G. M. Langmuir 1995, 11, 4383-4385. (d) Huisman, B.-H.; Kooyman, R. P. H.; van Veggel, F. C. J. M.; Reinhoudt, D. N. Adv. Mater. 1996, 8, 561-564. (4) (a) Kumar, A.; Abbott, N. L.; Kim, E.; Biebuyck, H. A.; Whitesides, G. M. Acc. Chem. Res. 1995, 28, 219-226. (b) Xia, Y.; Whitesides, G. M. Adv. Mater. 1995, 7, 471-473. (5) (a) Practical Surface Analysis; Briggs, D., Seah, M. P., Eds.; Wiley: Chichester, 1992. (b) Pursch, M.; Vanderhart, D. L.; Sander, L. C.; Gu, X.; Nguyen, T.; Wise, S. A.; Gajewski, D. A. J. Am. Chem. Soc. 2000, 122, 6997-7011. (6) (a) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151. (b) Allara, D. L. Biosens. Bioelectron. 1995, 10, 771; 1995, 11, 3882. (c) Tour, J. M.; Jones, L.; Pearson, D. L.; Lamba, J. J. S.; Burgin, T. P.; Whitesides, G. M.; Allara, D. L.; Parikh, A. N.; Atre, S. V. J. Am. Chem. Soc. 1995, 117, 9529. See also: Zharnikov, M.; Grunze, M. J. Phys.: Condens. Matter 2001, 13, 11333 and references therein.
(MMB, CH3-(C6H4)2-SH) on Au(111), it was expected that the interaction of the molecular aromatic backbone with the substrate is stronger than for the alkanethiol, which allows the molecules enough time to rearrange and stand up axially with self-organization, increasing the coverage. Accordingly, SAMs derived from the conjugated rigid rod arenethiols form a highly ordered, overlayer structure on Au(111) as determined by high-impedance scanning tunneling microscopy (STM).6c In addition, these new SAM systems possess novel electrical properties and second-order optical nonlinearities.7 Other systems such as silane-based SAMs represent another possibility to control the coverage of inorganic or organic supports. Their behavior would deviate from the alkanethiol Au(111) case in terms of headgroup bonding structure, steric constraints, or energy parameters. Organosilanes of general formula RSiX3 (where R is an organic functional group and X is an alkoxide or halide) easily form SAMs through chemisorption to surface hydroxyl groups.8 The spontaneous self-assembly of RSiX3 species that occurs at hydroxylated surfaces generally yields homogeneous, well-deposited SAMs, strongly chemisorbed to the substrate. Chemical and physical properties of the SAMs are readily controlled through the proper choice of R. These features prompted the use of organosiloxane SAMs as templates for a variety of advanced applications, including fabrication of nonlinear optics multilayer assemblies,9 alignment of liquid crystals,10 and conducting polymers.11 (7) (a) Dhirani, A.; Lin, P.-H.; Guyot-Sionnest, P.; Zehner, R. W.; Sita, L. R. J. Chem. Phys. 1997, 106, 5249. (b) Dhirani, A.; Zehner, R. W.; Hsung, R. P.; Guyot-Sionnest, P.; Sita, L. R. J. Am. Chem. Soc. 1996, 118, 3319. (8) (a) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92. (b) Silberzan, P.; Leger, L.; Ausserre, D.; Benattar, J. J. Langmuir 1991, 7, 1647. (c) Allara, D. L.; Parikh, A. N.; Rondelez, F. Langmuir 1995, 11, 2357. (d) Fendler, J. H.; Meldrum, F. C. Adv. Mater. 1995, 7, 607. (e) Wang, R.; Baran, G.; Wunder, S. L. Langmuir 2000, 16, 6298-6305. (9) (a) Roscoe, S. B.; Kakkar, A. K.; Marks, T. J.; Malik, A.; Durbin, M. K.; Lin, W. P.; Wong, G. K.; Dutta, P. Langmuir 1996, 12, 4218. (b) Jiang, H. W.; Kakkar, A. K.; Lebuis, A. M.; Zhou, H. T.; Wong, G. K. J. Mater. Chem. 1996, 6, 1075.
10.1021/la030334c CCC: $27.50 © 2004 American Chemical Society Published on Web 03/12/2004
Organized SAMs from Organosilanes
The ability to alter the chemical reactivity of the R pendant groups in aryl moieties through exposure to various sources such as electron beams,12 ion beams,13 and proximal probes14 or conventional masked-based photolithographic processes using UV light15 provides a means to create spatially well-defined reactivity templates. Accordingly, the further characterization of SAMs of arylsilanes and closely related derivatives16 by a variety of techniques is of interest with respect to better defining their potential to serve in various optical and electronic applications. We recently reported a comparative study of alkyl organosilanes.17 The surfaces obtained after grafting of octadecyltrichlorosilane and octadecyldichlorosilane, in cases where cross-linking is possible, are similar with a very dense film of organized alkyl chains in the all-trans configuration. On the other hand, in the case of octadecylchlorosilane where no cross-linking is possible, a less organized film of monografted species is obtained. In the present paper, we describe the chemical behavior of arylchlorosilanes, phenylchlorosilane 4 (C6H5SiH2Cl) and phenyldichlorosilane 5 (C6H5SiHCl2). The surface coatings are compared with those of phenyltrichlorosilane 1 (C6H5SiCl3), which is grafted under the same conditions. The study has been extended to other functionalities and organic groups. The morphology of surfaces modified by π-conjugated arylsilanes depends on various parameters such as the nature and the number of hydrolyzable functions or the length of the aromatic segment. Experimental Section General Procedures. Diethyl ether, tetrahydrofuran (THF), and toluene used as solvents were dried over sodium-benzophenone, triethylamine was dried over potassium hydroxide, and methanol was dried over magnesium-iodine. All solvents were distilled under an inert atmosphere just prior to use. Phenyltrichlorosilane 1, phenyltrimethoxysilane 2, phenyltriethoxysilane 3, phenyldichlorosilane 5, phenylsilane, 4-chlorostyrene, and 4-iodotoluene were commercially available (Sigma-Aldrich, Acros, or Gelest) and used as received. All the reactions were carried out under an inert atmosphere. The synthesis of the monochlorosilane 4 is outlined in Scheme 1, and the method has been described by Ishikawa for the same product and analogous compounds.18 The synthesis of the trimethoxysilane 8 of which the experimental method has been (10) Huang, Z.; Wang, P.-C.; MacDiarmid, A. G.; Xia, Y.; Whitesides, G. Langmuir 1997, 13, 6480. (11) (a) Collins, R. J.; Shin, H.; DeGuire, M. R.; Heuer, A. H.; Sukenik, C. N. Appl. Phys. Lett. 1996, 69, 860. (b) Bierbaum, K.; Kinzler, M.; Wo¨ll, C.; Grunze, M.; Ha¨hner, G.; Heid, S.; Effenberger, F. Langmuir 1995, 11, 512. (c) Dressick, W. J.; Dulcey, C. S.; Chen, M.-S.; Calvert, J. M. Thin Solid Films 1996, 284/285, 568. (12) Mino, N.; Ozaki, S.; Ogawa, K.; Hatada, M. Thin Solid Films 1994, 243, 374. (13) (a) Schenkel, T.; Schneider, M.; Hattass, M.; Newman, M. W.; Barnes, A. V.; Hamza, A. V.; Schneider, D. H.; Cicero, R. L.; Chidsey, C. E. D. J. Vac. Sci. Technol., B 1998, 16, 3298. (b) Younkin, R.; Berggren, K. K.; Johnson, K. S.; Prentiss, M.; Ralph, D. C.; Whitesides, G. M. Appl. Phys. Lett. 1997, 71, 1261. (14) (a) Brandow, S. L.; Calvert, J. M.; Snow, E. S.; Campbell, P. M. J. Vac. Sci. Technol., B 1997, 15, 1455. (b) Sugimura, H.; Nakagiri, N. J. Am. Chem. Soc. 1997, 119, 9226. (15) (a) Dressick, W. J.; Calvert, J. M. Jpn. J. Appl. Phys. 1993, 32, 5829. (b) Dressick, W. J.; Dulcey, C. S.; Brandow, S. L.; Witschi, H.; Nealey, P. F. J. Vac. Sci. Technol., B 1999, 17, 1432. (16) (a) Suh, D.; Simons, J. K.; Taylor, J. W.; Koloski, T. S.; Calvert, J. M J. Vac. Sci. Technol., B 1993, 11, 2850. (b) Baumann, F.; Deubzer, B.; Geck, M.; Dauth, J.; Sheiko, S.; Schmidt, M. Adv. Mater. 1997, 12, 955. (c) Brandow, S. L.; Chen, M.-S.; Aggarwal, R.; Dulcey, C. S.; Calvert, J. M.; Dressick, W. J. Langmuir 1999, 15, 5429. (17) Granier, M.; Lanneau, G. F.; Moineau, J.; Girard, P.; Ramonda, M. Langmuir 2003, 19, 2691. (18) (a) Kunal, A.; Kawakami, T.; Toyoda, E.; Ishikawa, M. Organometallics 1992, 11, 2708. (b) Reikhsfel’d, V. O. Zh. Obshch. Khim. 1961, 31, 1576; Chem. Abstr. 55, 22098.
Langmuir, Vol. 20, No. 8, 2004 3203 Scheme 1. Reaction Scheme for the Syntheses of Phenylchlorosilane 4 (PhSiH2Cl) and the Hydrolysis Products of 4 and Phenyldichlorosilane 5 (PhSiHCl2) in Moist CCl4
described by Hirao19 and the synthesis of the compound 9 are presented20 in Scheme 2. All the compounds were characterized by classical spectrometric methods (see the Supporting Information). Elementary analyses were performed by the “Service Central d’Analyse du CNRS - France”. Carbon and hydrogen contents were established by high-temperature combustion and IR spectroscopy. Silicon contents were determined by inductively coupled plasma (ICP) atomic emission spectroscopy. Infrared spectra were recorded on a Perkin-Elmer Spectrum 1600 FT-infrared spectrometer. 1H and 13C NMR spectra were recorded on a Bruker ADVANCE DPX-200 MHz spectrophotometer, and 29Si on a Bruker AC-200 MHz spectrometer using deuteriochloroform as the solvent with CHCl3 and TMS, respectively, as references. Mass spectra were measured on JEOL JMS-DW 300 and JEOL JMS-SX 102 instruments. Synthesis of Phenylchlorosilane (PhSiH2Cl), 4. Into a 100 mL Schlenk tube under an inert atmosphere was placed a mixture of 2.35 g (17.5 mmol) of anhydrous CuCl2 and 0.099 g (0.52 mmol) of anhydrous CuI in 20 mL of ether at -10 °C. Next, 0.952 g (8.80 mmol) of phenylsilane in 20 mL of ether was added at -10 °C. The mixture was magnetically stirred at room temperature for 24 h. The resulting mixture was filtered to remove copper salts. The solvent was removed by evaporation, and 4 was obtained as a colorless liquid, 0.78 g (5.46 mmol, 62%, bp760 mmHg ) 156 °C [lit.18: bp760 mmHg ) 154-158 °C]) after distillation. Anal. Calcd for C6H7ClSi: C, 50.52; H, 4.95; Cl, 24.85; Si, 19.69. Found: C, 49.97; H, 4.91; Cl, 24.79; Si, 19.62. Preparation and Characterization of the Dimer (PhSiH2)2O, 6. This product was synthesized by hydrolysis of phenylchlorosilane 4, in CCl4 in air for 2 h. The solvent was removed by evaporation, and 6 was obtained after distillation at reduced pressure as a colorless liquid (57%, bp3 mmHg ) 110 °C [lit.21: bp7 mmHg ) 121-122 °C]). Anal. Calcd for C12H14OSi2: C, 62.55; H, 6.12; O, 6.94; Si, 24.38. Found: C, 62.41; H, 6.13; O, 7.01; Si, 24.42. Synthesis of 4-Vinylphenyltrimethoxysilane, 8. Into a 250 mL two-necked round-bottomed flask equipped with a condenser and an addition funnel under an inert atmosphere were placed 3.43 g (141 mmol) of magnesium turnings and some THF to cover the turnings. Drops of 1,2-dibromoethane were added to activate the magnesium (effervescence). Next, 14.97 g (108 mmol) of 4-chlorostyrene in 70 mL of THF was added dropwise, and the solution became gray-green. Then, the mixture was magnetically stirred for 1 h under reflux. After cooling to room temperature, the mixture was added to a solution of 38.5 g (246 mmol) of chlorotrimethoxysilane in 55 mL of THF at -40 °C and magnetically stirred at room temperature for 12 h. Pentane was added to precipitate the magnesium salts, which were eliminated by filtration. Then, 6 mL (148 mmol) of methanol and 20.5 mL (148 mmol) of triethylamine were added dropwise to the solution; ammonium salts formed instantly, and the mixture was magnetically stirred at room temperature for 12 h. After (19) (a) Hirao, A.; Hatayama, T.; Nagawa, T. Macromolecules 1987, 20, 242. (b) Lewis, D. W. J. Org. Chem. 1958, 23, 1893. (20) Corriu, R.; Hesemann, P.; Lanneau, G. J. Chem. Soc., Chem. Commun. 1996, 1845. For a recent review on catalytic arylation reactions, see: Bolm, C.; Hildebrand, J. P.; Muniz, K.; Hermanns, N. Angew. Chem., Int. Ed. 2001, 40, 3285. (21) Petrov, A. D.; Nikishin, G. I. Zh. Obshch. Khim. 1956, 26, 1233; Chem. Abstr. 50, 14515.
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Scheme 2. Reaction Scheme for the Synthesis of 4-Vinylphenyltrimethoxysilane 8 and 4-Methylstilbenyl-trimethoxysilane 9
elimination of the ammonium salts by filtration and washing with pentane, the solvent was removed by evaporation and 8 was obtained after distillation under a vacuum as a colorless liquid, 11.13 g (49.68 mmol, 46%, bp0.1 ) 60 °C [lit.19: bp0.1 mmHg ) 66.5 °C]); 3,5-diterbutylcatechol was added to the mixture before distillation to prevent the polymerization of the styrene function. Anal. Calcd for C11H16O3Si: C, 58.89; H, 7.19; O, 21.40; Si, 12.52. Found: C, 58.74; H, 7.21; O, 21.36; Si, 12.43. Synthesis of 4-Methylstilbenyl-trimethoxysilane, 9. Into a 50 mL two-necked round-bottomed flask equipped with a condenser under an inert atmosphere were placed 3.052 g (14 mmol) of 4-iodotoluene 12, 3.003 g (13.4 mmol) of 4-vinylphenyltrimethoxysilane 8, 0.027 g (0.12 mmol) of palladium(II) acetate, 0.16 g (0.54 mmol) of tri-orthotolylphosphine (TOP), and 5 mL (35.87 mmol) of triethylamine in 20 mL of toluene. Then, the mixture was magnetically stirred under reflux for 12 h. After cooling to room temperature, the mixture was filtered under an inert atmosphere on silanized silica to remove the ammonium salts, using toluene as the eluent. The solvent was removed by evaporation, and 9 was obtained as a yellow solid, 3.11 g (9.9 mmol, 74%, mp ) 194 °C) after recrystallization in toluene/ pentane at -10 °C. Anal. Calcd for C18H22O3Si: C, 68.75; H, 7.05; O, 15.26; Si, 8.93. Found: C, 68.69; H, 7.09; O, 15.13; Si, 8.94. Contact Angle Measurements. The measurements were performed on a Kru¨ss Contact Angle Meter G1 goniometer. About 10 drops of HPLC water were deposited onto the surface using a microsyringe, and a reading was taken 10 s after deposition. The temperature was held at 25 °C. Ellipsometric Measurements. The measurements were made on a Plasmos SD 2300 ellipsometer with a rotating analyzer as a modulating element. The laser source was a helium/neon laser with a wavelength of 632.8 nm. The angle of incidence was 70°, and the spot size was about 2 mm. The substrate was oxidized silicon. The refractive index is 1.46 for silica and 3.83 for silicon. A refractive index of 1.50 was used for the phenylsilane layers, 1.55 for the styrylsilane layers, and 1.62 for the 4-methylstilbenylsilane layers. Atomic Force Microscopy (AFM). The atomic force microscope used for these experiments was a commercial optical deflection microscope (stand-alone configuration for large samples, Dimension 3100 with Nanoscope IIIa Digital Instruments) operated under ambient conditions. Roughness measurements were performed in the Tapping mode. The samples were rinsed with ethanol and wiped with clean-room paper before imaging, the images were flattened, and no other filtering was done before analyzing roughness. In Situ Attenuated Total Reflection (ATR) Measurements. ATR infrared spectra were recorded on a FTIR PerkinElmer 2000 spectrometer equipped with a narrow band liquid nitrogen cooled MCT detector and a thermoregulated ATR flow cell. The sample compartment was purged with dry air. All the spectra were registered at a resolution of 1 cm-1, and 128 scans were accumulated. Experimental conditions are described elsewhere.17 Silica Surface Pretreatment. The internal reflection element (IRE) used in these experiments was a trapezoidal single crystal from a two-sides-polished p-type (1,0,0) silicon wafer (45°, 72 × 10 × 1 mm3), and some pieces of silicon wafer disk (2 × 1 cm2) with the same characteristics were also used. Pretreatment conditions were described previously.17 General Disk Grafting Procedure. The silanization solution used is a 10-2 M solution of silane in CCl4 dried over molecular sieves just prior to use. The grafting reaction is performed in a 50 mL Schlenk tube under an inert atmosphere and thermoregulated at 15 °C for 24 h without stirring. Some other
Table 1. Surface Analytical Data for Graftings of Silanes 1, 2, and 3 on Oxidized Silicon Wafers over 24 h at 15 °C
PhSiCl3, 1 PhSi(OMe)3, 2 PhSi(OEt)3, 3
contact angle
roughness (nm)
ellipsometry (nm)
72° ( 3 82° ( 3 57° ( 4
1.26 0.19 0.22
4.4 ( 0.5 1.6 ( 0.1 0.5 ( 0.1
graftings are performed at 0 °C for 24 and 72 h in the cases of phenyltrichlorosilane 3 and phenyltrimethoxysilane 4. Then, the wafers are successively rinsed with CCl4, ethanol, and chloroform under ultrasonic stirring to remove the solution containing the excess of silane and the physisorbed organosilane molecules.
Results and Discussion Influence of the Functional Group on the Silicon Atom. In grafting reactions, the hydrolyzable functions most often used on silicon are chloro or alkoxy groups (methoxy or ethoxy).1 We have analyzed the surfaces obtained after grafting of phenyltrichlorosilane 1, phenyltrimethoxysilane 2, and phenyltriethoxysilane 3 under the same experimental conditions. The results are reported in Table 1. AFM measurements show that the layer of phenyltrichlorosilane 1 is inhomogeneous contrary to the layers obtained with the two other organosilanes 2 and 3. A root-mean-square (rms) value of 1.27 nm for 1 indicates a strong roughness, whereas the other surfaces are flat (rms ) 0.19 nm for methoxysilane 2, rms ) 0.22 for ethoxysilane 3, and rms ) 0.15 nm before grafting). Contact angle values indicate that the surfaces are relatively different since angles vary from 57° to 82°. Compared to the values of 63° and 69° obtained by Dulcey, Calvert et al.20 for, respectively, phenyl and benzyl films prepared from the corresponding trichlorosilanes, it seems that surfaces treated with phenyltrichlorosilane 1 or phenyltrimethoxysilane 2 are totally covered, in contrast to the phenyltriethoxysilane 3 modified surface which seems to be only partially covered. Ellipsometry measurements indicate that the thickness of the deposited films is highly dependent on the hydrolyzable function at the silicon atom. It increases as this function becomes more reactive. This seems to indicate that chloro or methoxy groups are not convenient for generating monolayers since the thicknesses (respectively 4.5 and 1.7 nm) are larger than the expected value (0.7 nm) for a silane with the phenyl group in the normal direction to the surface. In contrast, with phenyltriethoxysilane, an incomplete monolayer is obtained (0.5 nm thick). All these data indicate that, at that grafting temperature (15 °C), the two more reactive silanes 1 and 2 deposit as multilayers and phenyltriethoxysilane 3 as a homogeneous submonolayer (no island observed by AFM). That means phenyltriethoxysilane is not reactive enough. In a study by in situ FTIR-ATR, the spectrum acquired after rinsing of the surface shows an intensive band at 2976 cm-1 (Figure 1) which indicates that a large amount of ethoxy groups remain on the surface. These voluminous groups prevent the approach of other silane molecules. That can be an explanation of the incomplete monolayer.
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Langmuir, Vol. 20, No. 8, 2004 3205
Figure 1. In situ ATR-FTIR absorbance spectrum of the phenyltriethoxysilane-modified silica surface. Table 2. Surface Analytical Data for Graftings of Silanes 1 and 2 on Oxidized Silicon Wafers over 24 and 72 h at 0 °C contact angle
roughness (nm)
ellipsometry (nm)
24 h
24 h 72 h
24 h
72 h
Figure 2. Infrared spectra of phenylchlorosilane 4 in solution in CCl4. Scheme 3. Redistribution Reactions of Phenylalkoxysilanes in the Presence of a Catalyst Such as a Nucleophile or a Metallic Complex
72 h
PhSiCl3, 1 78° ( 8 76° ( 3 0.18 0.15 3.8 ( 0.1 4.1 ( 0.6 PhSi(OMe)3, 2 73° ( 3 82° ( 5 0.16 0.14 1.3 ( 0.1 1.95 ( 0.2
A means to reduce the reactivity of silanes 1 and 2 is to decrease the reaction temperature. An attempt to obtain a monolayer with these organosilanes has been carried out at 0 °C over 24 and 72 h. The results obtained under these temperature conditions are reported in Table 2. Here, the roughness measured by AFM (rms varies from 0.14 to 0.18 nm) shows that all the surfaces are flat even using phenyltrichlorosilane over 3 days and demonstrate that reactivity is highly dependent on grafting temperature.22 By ellipsometry, we can see that all the films are thicker (from 1.35 to 4.21 nm) than the expected thickness for a phenylsiloxane monolayer. Even at 0 °C, a monolayer cannot be obtained indicating that temperature is not the most important parameter in obtaining phenyl monolayers using trichloro or trimethoxy derivatives. For phenyltrichlorosilane, the thickness of the film does not vary after 24 h and is almost the same as that observed at 25 °C indicating that the deposition is finished after 1 day, whereas in the case of phenyltrimethoxysilane, films are thinner and the thickness increases from 1.3 to 1.95 nm indicating that the reaction rate is slower. Influence of the Number of Functional Groups. Another way to obtain thin films is to change the number of reactive functions to limit or prevent cross-linking reactions. Previously, we have shown that octadecylchlorosilane and octadecyldichlorosilane behave as monofunctional and difunctional silanes, respectively, since the Si-H bonds are stable under the grafting reaction conditions;17 that is, only Si-Cl bonds are reactive which was the necessary condition to study the influence of the number of reactive functions on the quality of the film avoiding steric hindrance around the silicon atom of the silane. Here, we also decided to use chloro groups as reactive functions because phenylmethoxysilanes are very difficult to synthesize. The synthesis of such products requires the use of catalysts such as nucleophiles or metallic complexes. Unfortunately, these catalysts induce redistribution reactions leading finally to phenylsilane and phenyltrimethoxysilane (Scheme 3).23 For this reason, we have synthesized phenylchlorosilane 4 in the same way as octadecylchlorosilane (Scheme 1) (22) Brzoska, J. B.; Azouz, I. B.; Rondelez, F. Langmuir 1994, 10, 4367. (23) (a) Itoh, M.; Inoue, K.; Ishikawa, J.; Iwata, K. J Organomet. Chem. 2001, 629, 1. (b) Minge, O.; Mitzel, N. W.; Schmidbaur, H. Organometallics 2002, 21, 680.
whereas phenyldichlorosilane 5 and phenyltrichlorosilane 1 were purchased from a chemical supplier. Hydrolysis of the two silanes 4 and 5 in moist carbon tetrachloride (Scheme 1) has been carried out to check that the Si-H bonds are not reactive. The presence in 29Si NMR of a triplet centered at -27.2 ppm with a coupling constant of 224 Hz for the product of hydrolysis of compound 4 and a doublet centered at -46.8 pip with a coupling constant of 259 Hz for the product of hydrolysis of compound 5 is proof that only the SiCl bonds are reactive. Other classical analytical techniques confirm that compounds 6 and 7 are the hydrolysis products of silanes 4 and 5, respectively. As we can see in Figure 2, the IR spectrum of phenylchlorosilane 4 exhibits a strong band at 2178 cm-1 (νSi-H) and weak bands at 1591 cm-1 (νCdC) and above 3000 cm-1 (νC-H). Graftings of phenylchlorosilane 4 and phenyldichlorosilane 5 have been monitored by in situ FTIR-ATR spectroscopy following the evolution of the strong stretching vibration band of the Si-H bond (Figure 3). In both cases, the intensity of this band (2180 cm-1 for the monochlorosilane 4 and 2210 cm-1 for the dichlorosilane 5) does not change since the silane in the solution is in very large excess, and this concentration remains constant throughout the reaction. A new stretching vibration band of the Si-H bond (2165 and 2179 cm-1, respectively) appears and stabilizes at the end of the grafting. The position of this new band is very similar to that of the corresponding hydrolysis compound (2168 cm-1 for 6 and 2178 cm-1 for 7), which indicates that only the Si-Cl bonds are reactive. Thereby, phenylchlorosilane 4 can be considered as a monofunctional silane, phenyldichlorosilane 5 as a difunctional silane, and phenyltrichlorosilane obviously as a trifunctional silane. Table 3 presents results obtained from wettability, AFM, and ellipsometry analysis of surfaces modified by compounds 1, 4, and 5. As seen above, PhSiCl3 deposits as a rough multilayer. On the other hand, silanes 4 and 5 covalently react onto oxidized silicon leading to a flat dense monolayer. The rms values are very weak (0.16 and 0.24 nm, respectively), and a thickness of 0.7 nm is almost as high as the expected value for a dense monolayer in which phenyl groups are perpendicular to the surface (0.8 nm). Contact angles are higher than those obtained for phenyltrichlorosilane (here
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Figure 3. Infrared spectra showing the evolution of the SiH vibration bands during the grafting reaction of phenylchlorosilane 4 (a) and phenyldichlorosilane 5 (b). Table 3. Surface Analytical Data for Graftings of Silanes 1, 4, and 5 on Oxidized Silicon Wafers over 24 h at 15 °C
PhSiCl3, 1 PhSiHCl2, 5 PhSiH2Cl, 4
contact angle
roughness (nm)
friction coefficient
ellipsometry (nm)
72° ( 3 92° ( 2 78° ( 4
1.26 0.24 0.16
0.147 0.144 0.137
4.4 ( 0.5 0.7 ( 0.1 0.7 ( 0.1
or found in the literature24), meaning that phenyl groups lean out of the surface, but relatively different (78° for the monochlorosilane 4 and 92° for the dichlorosilane 5), indicating probably a different orientation of the phenyl group because of the possible cross-linking in the case of the phenyldichlorosilane 5. The phenyl group can be bent, the ring remaining or not in a plane perpendicular to the surface. When this ring is in the plane, the surface energy is higher, which involves a higher contact angle value. For this reason, we suppose that in the case of the grafting of phenyldichlorosilane 5, the phenyl groups are perpendicular to the surface whereas they are bent in the case of the monochlorosilane 4. The lateral force microscopy (LFM)-AFM study exhibits friction coefficient values which characterize similar surfaces (0.147 for 1, 0.144 for 5, and 0.137 for 4). These values are much higher than those obtained for the corresponding octadecylchlorosilane (µ e 0.011). This can be explained by the fact that the strong interactions between these modified surfaces and the Si3N4 tip directly depend on the stiffness of the phenyl groups, which are much more rigid entities than the octadecyl long chain. The similar values obtained for the mono-, di-, and trifunctional silanes 4, 5, and 1 show that the friction forces do not depend on the organization of the phenyl groups in the film. (24) Dulcey, C. S.; Georger, J. H.; Chen, M. S.; McElvany, S. W.; Oferall, C. E.; Benezra, V. I.; Calvert, J. M. Langmuir 1996, 12, 1638.
Here, we have demonstrated that phenylchlorosilane 4 or phenyldichlorosilane 5 grafts on oxidized silicon as a dense monolayer, which was the goal of the investigation. However, these chlorosilanes are very air sensitive and are therefore difficult to handle. It would be interesting to use less sensitive silanes that behave like the two silanes 4 and 5. Alkoxy groups such as ethoxy or methoxy are less sensitive than chloro, but we cannot synthesize phenylmonoalkoxysilane or phenyldialkoxysilane; phenyltrimethoxysilane leads to multilayered films, and phenyltriethoxysilane to an incomplete monolayer. Influence of the Length of the Aromatic Entity. An important parameter in obtaining a monolayer with alkyltrichlorosilanes is the length of the alkyl chain. We have studied this parameter in the case of the aromatic pattern. We choose to use silanes with methoxy functions and the least possible sterically hindered organic group. We have synthesized styryltrimethoxysilane 8 and pmethylstilbenyltrimethoxysilane 9 (Scheme 2) and grafted them on oxidized silicon. The results of the characterization of the obtained surfaces are summarized in Table 4. All the films are flat. Contact angle measurements indicate that the hydrophobicities of the surfaces are similar and in the expected order of magnitude for surfaces covered by phenyl groups. According to ellipsometry, contrary to the case of phenyltrimethoxysilane in which a multilayered organic film (1.6 nm thick) forms, a thickness corresponding to the monolayer is obtained for the two other trimethoxysilanes 8 and 9 (1.06 and 1.7 nm, respectively); if we consider a monolayer of vinylphenylpolysiloxane or p-methylstilbenylpolysiloxane, in which the planes of the aromatic skeletons are perpendicular to the surface, theoretical values of 1.04 or 1.64 nm, respectively, are calculated. These thicknesses show that the monolayers are very dense and demonstrate that the length of the aromatic entity is an essential parameter in obtaining a densely packed monolayer; however, it is not necessary to have a very long aromatic system. A study is now in progress to generate functionalized surfaces with rigid spacers and to check the accessibility of the introduced functions. Conclusion The morphology of surfaces modified by π-conjugated arylsilanes depends on various parameters such as the number and the nature of the hydrolyzable functions or the length of the aromatic segment. The reactivity of the hydrolyzable function is an important parameter since the grafting of phenyltrichlorosilane and phenyltrimethoxysilane leads to multilayers even when the reaction is carried out at 0 °C, the film obtained from phenyltrichlorosilane being much thicker than the one obtained from phenyltrimethoxysilane, whereas an incomplete monolayer is obtained using phenyltriethoxysilane. Therefore, chloro or methoxy functions appear to be too reactive and the ethoxy function not sufficiently reactive for trifunctional phenylsilanes.
Organized SAMs from Organosilanes
We also have shown that the number of functions plays an important role. This study has been carried out using SiCl bonds as the reactive functionality owing to the synthesis of the mono- and difunctional precursors. When the trifunctional phenyltrichlorosilane forms an inhomogeneous multilayer, the difunctional phenyldichlorosilane, PhSiHCl2, and the monofunctional phenylchlorosilane, PhSiH2Cl (the SiH bond is not reactive in these experimental conditions), deposit as dense homogeneous monolayers. For these two phenylchlorosilanes, the surface analytical data are similar except for contact angle measurements, whereas the hydrophobicity is higher for the difunctional silane, which can be explained by a different orientation of the phenyl groups at the surface. Replacing a chlorine atom by a hydrogen atom in phenyltrichlorosilane is enough to obtain a dense homogeneous monolayer, but it is not easy to synthesize such dichlorosilanes bearing an organic function on the phenyl group. Finally, we have studied the influence of the length of the aromatic segment on the quality of the film. Grafting styryltrimethoxysilane or methylstilbenyltrimethoxysilane leads to dense monolayers, indicating that adding a short group like the vinyl group is sufficient to induce an
Langmuir, Vol. 20, No. 8, 2004 3207
organization between aromatic groups and to achieve a dense monolayer. These results are very interesting for further studies about more sophisticated aromatic silanes. The trimethoxy group seems to be the best hydrolyzable function to obtain a dense monolayer providing that the aromatic segment is long enough (styryl or stilbenyl). In cases where a monolayer is not obtained with the trimethoxy derivative, it is possible to choose the dichloro- or monochloro-silanes as an alternative although these precursors are more difficult to synthesize. Acknowledgment. The authors are pleased to thank Dr. J. Durand, from the Institut Europe´en des Membranes (Montpellier), and Michel Ramonda from the Laboratoire de Microscopie a` Champ Proche, Universite´ Montpellier 2, for helpful advice regarding the ellipsometry and AFM analyses. Supporting Information Available: Spectroscopic data for compounds 4, 6, 8, and 9; preparation and characterization of compound 7. This material is available free of charge via the Internet at http://pubs.acs.org. LA030334C
