Langmuir 2002, 18, 9307-9311
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Molecular Orientation and Grafting Density in Semifluorinated Self-Assembled Monolayers of Mono-, Di-, and Trichloro Silanes on Silica Substrates Jan Genzer,*,† Kirill Efimenko,† and Daniel A. Fischer‡ Department of Chemical Engineering, North Carolina State University, Raleigh, North Carolina 27695-7905, and Material Science & Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 Received May 7, 2002. In Final Form: September 3, 2002 Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy is used to measure the molecular orientation in semifluorinated self-assembled monolayers (SAMs) prepared by vapor deposition of mono(F3C(CF2)8(CH2)2Si(CH3)2Cl, m-F8H2), di- (F3C(CF2)8(CH2)2Si(CH3)Cl2, d-F8H2), and trichloroorganosilanes (F3C(CF2)8(CH2)2SiCl3, t-F8H2) on flat silica-covered substrates. The average tilt angles (from the sample normal) of the fluorocarbon part, F(CF2)8-, of t-F8H2, d-F8H2, and m-F8H2 measured by carbon K-edge NEXAFS are 10 ( 2°, 35 ( 2°, and 45 ( 3°, respectively. We show that the increase of the tilt angle is associated with the steric hindrance of the methyl groups attached to silicon close to the bonding substrate. We also show that the molecular orientation obtained from the NEXAFS measurements can be used to estimate the grafting densities of the F8H2 molecules on the substrates. We present a simple one-dimensional geometric model to show that the grafting density of m-F8H2 is approximately one-half of that corresponding to the t-F8H2 SAM. Finally, we show that the results of this simple model are in accord with estimates of the two-dimensional fluorine areal density obtained from the fluorine K-edge NEXAFS spectra.
Introduction Formation of self-assembled monolayers (SAMs) on material substrates has now become a routine way to tailor the surface properties of materials.1,2,3 The most commonly used molecules forming SAMs are those bearing either (i) a mercapto group that can be terminally attached to gold or other noble metal surfaces or (ii) a chloro- or alkoxysilane moiety capable of reacting with a hydroxy terminus on the surface.1,2 The application of the latter family of compounds is particularly useful for modifying the surfaces of materials that are covered with a thin (≈several angstroms) native oxide layer. These molecules (R*) bonded to a functionalized Si group exist either in the form of chlorosilanes, e.g., mono- (R*-SiR2Cl), di- (R*SiRCl2), or trichlorosilanes (R*-SiCl3), or alkoxysilanes, e.g., mono- (R*-Si(OR)2Cl), di- (R*-Si(OR)Cl2), or trialkoxysilanes (R*-Si(OR)3), where R is an alkyl. SAMs based on these compounds can be utilized to tailor surface properties of materials, making them suitable for biocompatibility, biosensors, the reduction of corrosion rates, pattern creation (lithographic processes), friction reduction (useful in microelectro-mechanical systems, MEMS, technology), foul resistance, modification of membrane properties, and changes in hydrophobicity.4 One of the outstanding issues concerning the application of SAMs is the organization of the molecules in the SAM. It has long been appreciated that the organization of the SAM molecules does not only influence the wetting properties of the SAMs but it is vital for their stability * To whom correspondence should be addressed: Phone: 919515-2069. E-mail:
[email protected]. † North Carolina State University. ‡ National Institute of Standards and Technology. (1) Ulman, A., An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991. (2) Ulman, A. Chem. Rev. 1996, 96, 1533. (3) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151. (4) Swalen, J. D. et al. Langmuir 1987, 3, 932. Allara, D. L. Biosens. Bioelectr. 1995, 10, 771.
and weatherability.1,2,4 The packing within SAMs results from a complex interplay between the molecular nature of the SAM molecules and the type of bonding to the substrate; they both influence the tilt angle on the substrate and ultimately the grafting density of the SAM molecules. The latter issue has recently received much attention. For example, the average tilt angle (from the sample normal) of alkyls (general formula H3C(CH2)x, Hx) terminated with a mercapto group attached to gold (111) is approximately 30-35°.1,2,5 However, when deposited onto a silver (111) covered substrate, the same chemical moiety was found to be oriented almost perpendicular to the sample surface.2,6This distinct behavior has been attributed to the combination of lateral discrimination and electrostatic effects.2 As a result of the different orientation of alkanethiols in the SAMs, the grafting density also varies. Thus, the area per molecule for HS(CH2)xCH3 increases from 0.184 to 0.216 nm2 as one moves from silver (111) to gold (111) bonding environments.2 A detailed study addressing the issue of bonding environment on chain orientation has been published recently. Genzer and co-workers studied the molecular orientation of semifluorinated (SF) alkanes (general formula: F3C(CF2)y(CH2)x, FyHx) attached to polymer backbones (“soft” substrates) and also the orientation in mercapto-based analogues on gold (“hard” substrates).7 The overall conclusion from those experiments was that the average tilt angle of the SF part of the molecules, 〈τF〉, increases with decreasing y and/or increasing x. While the general trend in the behavior of 〈τF〉 on both surfaces was the same, 〈τF〉 on the “soft” surface was consistently about 14° higher. This behavior was attributed to the different bonding environment of the molecules. Genzer and Efimenko recently offered yet another demonstration of the intimate interplay between the grafting density of self-assembling molecules and their tilt angle on the substrate. They (5) Grunze, M. Phys. Scr. 1993, T49, 711. (6) Nemetz, A.; et al. J. Chem. Phys. 1993, 98, 5912. (7) Genzer, J.; et al. Macromolecules 2000, 33, 6068.
10.1021/la025921x CCC: $22.00 © 2002 American Chemical Society Published on Web 10/30/2002
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showed that the molecular orientation of F8H2 and Hx molecules (with x ranging from 8 to 18) attached to flexible substrates fabricated by ultraviolet/ozone treatment of poly(dimethyl siloxane) (PDMS) networks depends on the degree of uniaxial stretching of the (PDMS) substrates.8,9 The attachment of organosilanes to solid substrate is more complex than that of mercapto-terminated alkanes. Depending on the reaction conditions, the chemistry of the organosilane molecule, and the surface structure, a number of various structures, including monolayers and multilayers, are possible.1,2,10,11 While monofunctional organosilane molecules are expected to form only monolayers (because only one hydrolyzable group is present in the molecule), depending on the reaction conditions, diand trifunctional organosilanes are, in principle, capable of forming either monolayers or multilayers. As reported by Fadeev and McCarthy, there are inconsistencies in the literature regarding the assembly of these molecules.10,11 Fadeev and McCarthy presented the to-date most comprehensive study of mono-, di-, and trihydrocarbon-based chlorosilanes.10,11 Using contact angle measurements and X-ray photoelectron spectroscopy, they have established several important conclusions about the growth of organosilane SAMs. Specifically, their studies have revealed the dependence of the wetting properties of these SAMs as a function of the length of the alkane and the importance of the deposition method (vapor vs solution) and have indicated that the kinetics leading to full coverages are rather sluggish (≈days). Because of the nature of the techniques used in refs 10 and 11, only limited information about the orientation of the chains in the monolayer and the grafting densities of the molecules in the SAMs is available. The goal of this work is to investigate the molecular orientation and grafting density of mono-, di-, and trichlorosilane F8H2 molecules. Specifically, we intend to demonstrate that depending on the bonding environment around the silica atoms, the F8H2 molecules adopt different orientations and grafting densities on flat silicacovered substrates.
Genzer et al.
Materials and Methods. 1H,1H,2H,2H-Perfluorodecyldimethylchlorosilane (m-F8H2) (CAS Registry No. 74612-30-9), 1H,1H,2H,2H-perfluorodecylmethyldichlorosilane (d-F8H2) (CAS Registry No. 3102-79-2), and 1H,1H,2H,2H-perfluorodecyltrichlorosilane (t-F8H2) (CAS Registry No. 78560-44-8) were supplied by Lancaster and used as received. Deionized (DI) water (resistivity > 16 MΩ‚cm) was obtained by using the Millipore water purification system. Heptane, octane, undecane, dodecane, and hexadexane (all HLPC grade) were supplied by Aldrich and were used as received. Single-side-polished, 300-µm-thick silicon wafers with [100] orientation (Virginia Semiconductor, Inc.) were cut into small pieces (≈1 × 1 cm2), placed into an ultraviolet/ ozone (UVO) cleaner (Jelight Company, Model 42, Suprasil lamp),12 and exposed to the UVO treatment for 30 min. This treatment produces a high concentration of the surface -OH groups that serve as attachment points for the chlorosilane molecules at the silica surfaces. A small amount of silane was placed on the bottom of a Petri dish, the silicon wafer was positioned above the silane source (distance ≈1 cm), and the
whole system was closed and kept at ambient conditions. After a predetermined period of time the wafer was taken out of the container, washed thoroughly with ethanol to remove physisorbed silane molecules, and dried with nitrogen. Experimental Techniques. Contact angle experiments were performed using a Rame´-Hart contact angle goniometer (model 100-00) equipped with a CCD camera and analyzed with the Rame´-Hart software.12 The advancing contact angles were read by injecting 6 µL of probing liquid; the receding contact angles were determined by removing 3 µL of probing liquid from the droplet. Each data point reported in the paper represents an average over five measurements on the same sample. The data points have an error better than (1.5°. By combining contact angle measurements using very polar (DI water) and nonpolar (alkanes) liquids, a complete picture of the molecular organization within the SAM can be obtained. While the polar liquid is sensitive to the chain alignment within the SAM, the nonpolar liquid probes mainly the “in-plane” structure of the SAM. Experiments using the near-edge X-ray absorption fine structure (NEXAFS)13,14 spectroscopy were carried out on the NIST/Dow Soft X-ray Materials Characterization Facility at the National Synchrotron Light Source at Brookhaven National Laboratory (NSLS BNL).15 NEXAFS spectroscopy involves the resonant soft X-ray excitation of a K or L shell electron to an unoccupied low-lying antibonding molecular orbital of σ symmetry, σ*, or π symmetry, π*.13 The initial state K-shell excitation gives NEXAFS its element specificity, while the final-state unoccupied molecular orbitals provide NEXAFS with its bonding or chemical selectivity. A measurement of the partial electron yield (PEY) intensity of NEXAFS spectral features thus allows for the identification of chemical bonds and determination of their relative population density within the sample. Moreover, by collecting the PEY NEXAFS spectra at several θ (20° e θ e 90°), where θ is the angle between the sample normal and the polarization vector of the X-ray beam, the surface molecular orientation of the F8H2 SAM molecules on the silicon oxide surfaces can be determined. We express our results on the orientation of the molecules in the SAM in terms of the average tilt angle of the fluorocarbon part, F(CF2)8, of the molecule. We note that the tilt angle determined from NEXAFS represents an average value. There is no straightforward way to discriminate between the case of all chains homogeneously tilted by the same angle and the case of a partially disordered system with a broad distribution of tilt angles. Moreover, due to the nature of the polarization dependencies of the NEXAFS signal intensities, one cannot distinguish between a completely disoriented sample and a sample, whose chains are all tilted by 54.7°, the so-called, “magic angle”.13 An important issue concerning the study of organic materials is the possibility of the sample damage during the characterization with UV light, X-ray, and electron radiation. Semifluorinated materials are particularly sensitive to these effects.16 Hence, a fresh area of the sample was exposed to the X-ray beam spot for each measurement to minimize possible beam damage effects. Moreover, NEXAFS spectra showed no damage effects for at least three consecutive runs taken from the same spot on the sample. The SAM thickness was measured by using variable-angle spectroscopic ellipsometry (VASE) (J. A. Woollam, Co., Inc.).12 The measurements were done at the range of wavelengths ranging from 350 to 1100 nm with the resolution of 10 nm and geometries of φ ) 65, 70, and 76°, where φ is the angle between the sample normal and the incoming laser beam. The thickness of the SAM was evaluated by using WVASE32 software (J. A. Woollam, Co., Inc.)12 by applying the effective medium approximation. Following previous work, we assumed that the monolayer had an index of refraction equal to 1.38.1
(8) Genzer, J.; Efimenko, K. Science 2000, 290, 2130. (9) Efimenko, K.; Genzer, J. Mater. Res. Soc. Symp. Proc., 2002, 710, DD10.3.1. (10) Fadeev, A. Y.; McCarthy, T. J. Langmuir 1999, 15, 3759. (11) Fadeev, A. Y.; McCarthy, T. J. Langmuir 2000, 16, 7268. (12) Certain commercial equipment is identified in this article in order to specify adequately the experimental procedure. In no case does such identification imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the items identified are necessarily the best available for the purpose.
(13) Sto¨hr, J. NEXAFS Spectroscopy; Springer-Verlag: Berlin, 1992. (14) See for example: Zharnikov, M. et al. Phys. Chem. Chem. Phys. 2000, 2, 3359. Frey, S.; et al. Isr. J. Chem. 2000, 40, 81. Bagus, P. S.; et al. Chem. Phys. Lett. 1996, 248, 129. (15) For detailed information about the NIST/Dow Soft X-ray Materials Characterization Facility at NSLS BNL, see: http:// nslsweb.nsls.bnl.gov/nsls/ pubs/newsletters/96-nov.pdf. (16) Ja¨ger, B.; et al. Phys. Chem. 1997, 202, 263. Wirde, M.; et al. Nucl. Instr. Methods Phys. Res. B 1997, 131, 245. Zharnikov, M.; et al. Phys. Chem. Chem. Phys. 1999, 1, 3163.
Experimental Section
Molecular Orientation and Grafting Density
Langmuir, Vol. 18, No. 24, 2002 9309 Table 1. Molecular Properties of SF SAMs SAM
thickness,a nm
〈τF〉,b deg
m-F8H2 d-F8H2 t-F8H2
0.9 1.4 1.65
45 ( 3 35 ( 3 10 ( 2
σF8H2,c ∆IPEY,F,d nm-2 au σF8H2/∆IPEY,Fe 1.59 2.14 3.09
0.045 0.064 0.105
35.3 33.4 29.4
a Thickness measured by VASE (assuming n 1 F8H2 ) 1.38). Average tilt of the F(CF2)8 measured by NEXAFS spectroscopy. c Grafting density of F8H2 calculated from 〈τ 〉. d Fluorine K-edge F jump. e Calculated from σF8H2 and ∆IPEY,F given in the table.
b
Figure 1. Advancing (top bar) and receding (bottom bar) contact angles of deionized water (DIW, surface tension σ ) 72.8 mJ/m2), hexadecane (HxD, σ ) 27.6 mJ/m2), dodecane (DoD, σ ) 25.5 mJ/m2), undecane (UnD, σ ) 24.8 mJ/m2), octane (Oc, σ ) 22.0 mJ/m2), and heptane (Hp, σ ) 20.2 mJ/m2) on m-F8H2 (a), d-F8H2 (b), and t-F8H2 (c). SAMs prepared by vapor deposition for 30 min (white), 2 h (light gray), and 2 days (dark gray). The contact angles have an error better than (1.5°.
Results and Discussion Figure 1 shows the results of contact angle measurements on the m-F8H2 (a), d-F8H2 (b), and t-F8H2 (c) SAMs. The measurements were performed with DI water and a series of alkanes. Because of its hydrophilic nature, the water molecules remain on top of the hydrophobic substrate and are thus sensitive to the chain alignment within the SAM. On the other hand, being much more hydrophobic than water, the nonpolar alkanes can in principle penetrate into the SAM, thus revealing information about the “in-plane” structure of the SAM. There are three general trends in the data shown in Figure 1. First, when going from m-F8H2 to t-F8H2, the contact angle for each liquid increases. Considering that the surface energy of a CF3 group is lower than that of the CF2 moiety, this behavior reveals that there are more CF2 groups exposed to the surface in m-F8H2 molecules relative to t-F8H2. This trend thus suggests that the chain tilt from the surface normal increases in the following trend: t-F8H2 < d-F8H2 < m-F8H2. Second, the time dependence of the contact angles provides a first glimpse at the kinetics of the formation of the SF SAMs. For m-F8H2 and t-F8H2, there is mostly a steady increase in the contact angle with increasing time, although the contact angles values of the 2-h and 2-day data are approximately the same (within the experimental error). The thickness measurements also show that the SAM thickness initially increases and reaches an “equilibrium” value at about 2 h of deposition. These values are reported in Table 1. The wetting data for the d-F8H2 samples reveal that the highest contact angle is measured for samples prepared by vapor deposition for 2 h. With further increase of the deposition time, the contact angle decreases. We speculate that this behavior is associated with the formation of multilayers, a trend reported earlier by Fadeev and McCarthy.10,11 In fact, ellipsometry measurements also detect an increase in thickness in these
samples. Overall, the time dependence of the contact angle data shows that the “equilibrium” density within the SAM is achieved roughly after about 2 h of deposition. Fadeev and McCarthy reported that the time to reach equilibrium concentration in alkane-based siloxanes is about 2 days.11 We attribute the faster kinetics in the SF alkanes to stronger intermolecular van der Waals forces relative to the normal alkanes. Third, for each F8H2 sample, the contact angles of alkanes decrease systematically with decreasing surface tension of the probing liquid. Interesting is the behavior of the contact angle hysteresis ()difference between the advancing and the receding contact angles). The hysteresis in water contact angle on samples deposited for 30 min and 2 h decreases in the following manner: t-F8H2 > d-F8H2 > m-F8H2, similar to normal alkanes.10,11 We speculate that this behavior is associated with the molecular order within the SAM. Note that there is an increase in the contact angle hysteresis for the d-F8H2 SAM prepared by vapor deposition for 2 days. We attribute this observation to the formation of d-F8H2 multilayers, already mentioned earlier. We examined the chemistry (including bond densities) and molecular orientation of the chlorosilane molecules on the SAM surfaces by using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Figure 2 shows the carbon K-edge NEXAFS spectra from the m-F8H2 (a), d-F8H2 (b), and t-8H2 SAMs (c) deposited from vapor for 2 h. The NEXAFS data were collected at various angles ranging from θ ) 20° (“glancing geometry”) to θ ) 90° (“normal geometry”), where θ denotes the angle between the sample normal and the direction of the electric vector of the X-ray beam. The preedge and postedge in the NEXAFS spectra were normalized to 0 and 1, respectively. Several characteristic peaks can be identified in each set of the PEY NEXAFS spectra. These correspond to the 1s f σ* transitions associated with the C-H (E ) 287.5 eV), C-F (E ) 292.0 eV), and C-C (E ) 295.5 eV) bonds (depicted with the vertical dashed lines in Figure 2). From the PEY NEXAFS spectra, several general observations can be made regarding the molecular organization of the F8H2 chains in the SAMs. Considering that intensities originating from the 1s f σ/C-F and 1s f σ/C-C transitions change with varying angle θ (as θ increases, the intensity corresponding to σ* of the C-F bond increases while that of the C-C bond decreases), we conclude that the molecules are oriented. The angular dependence of the 1s f σ/C-F and 1s f σ/C-C intensities is the weakest for the m-F8H2 samples, increases for the d-F8H2 sample, and is the strongest for the t-F8H2 SAM. Following the method proposed by Outka and co-workers,17 the PEY NEXAFS spectra were fitted to a series of Gaussians, a step corresponding to the excitation edge of carbon, and a background. The resulting intensities of the 1s f σ/C-F and 1s f σ/C-C transitions are plotted in Figure 3. In Figure 3 we plot the normalized PEY NEXAFS intensities vs sin2 (θ) from 1s f σ/C-F (closed circles) and (17) Outka, D. et al. J. Chem. Phys. 1994, 88, 4076.
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Genzer et al.
Figure 3. Normalized PEY NEXAFS intensities vs sin2 (θ) from 1s f σ/C-F (closed circles) and 1s f σ/C-C (open circles) transitions for m-F8H2 (a), d-F8H2 (b), and t-F8H2 (c). The solid lines were obtained by fitting the experimental data using the “modified building block” model method described in refs 18-20.
Figure 2. Carbon K-edge PEY NEXAFS spectra from m-F8H2 (a), d-F8H2 (d), and t-F8H2 (c) samples collected at various sample orientations with respect to the X-ray beam, θ, ranging from 20° (glancing geometry) to 90° (normal geometry). The preedge and postedge in each spectrum has been normalized to 0 and 1, respectively. The dashed lines in the figures indicate the positions of the 1s f σ* transitions associated with the C-H (E ) 287.8 eV), C-F (E ) 292.0 eV), and C-C (E ) 295.3 eV) bonds.
1s f σ/C-C (open circles) transitions for m-F8H2 (a), d-F8H2 (b), and t-F8H2 (c). A more detailed analysis presented recently18,19,20 can be used to elucidate the average angular orientation of the rigid fluorocarbon helix F(CF2)8, 〈τF〉, within the F8H2 SAM. The results for 〈τF〉 are shown in Table 1 and the corresponding NEXAFS intensities are denoted with solid lines in Figure 3. The data of the NEXAFS analysis reveal that the tilt of the F8H2 systematically increases as t-H8H2 < d-F8H2 < m-F8H2. The F8H2 chains in the t-F8H2 SAM deviate only slightly from the surface normal (〈τF〉 ) 10 ( 2°).21 By changing the bonding environment of the F8H2 molecules from SiCl3 to SiCH3Cl2, 〈τF〉 increases to 35 ( 3°. Further replacement of one more chlorine by methyl results in Si(CH3)2Cl, whose average chain tilt angle as measured by NEXAFS is 45 ( 3°. One can see that there is a systematic increase in the chain tilt with increasing number of the methyl groups close to the substrate. As mentioned previously, from the work done on alkanethiol SAMs, we know that the chain tilt is larger when deposited on gold relative to that prepared on silver substrates. Correspondingly, the grafting density of the alkane molecules in SAMs on silver is higher than that in SAMs on gold. Figure 4 shows schematically the arrangement of the F8H2 molecules in the t-F8H2 (a) and the m-F8H2 (b) SAMs. We use our data on 〈τF〉 to estimate the grafting densities of the F8H2, σF8H2, in the various SF SAMs. We consider a very simple one-dimensional model of the chain organization in the SAM. From Figure 4
σF8H2 ≈ s-2 )
[
]
cos(〈τF〉) d
2
(1)
(18) Genzer, J. et al. Macromolecules 2000, 33, 1882. (19) Genzer, J. et al. Langmuir 2000, 16, 1993. (20) Genzer, J.; Fischer, D. A.; Kramer, E. J. J. Appl. Phys., in press.
Molecular Orientation and Grafting Density
Langmuir, Vol. 18, No. 24, 2002 9311
Figure 5. Fluorine K-edge PEY NEXAFS spectra from m-F8H2, d-F8H2, and t-F8H2 samples collected at θ ) 50°. The insets shows the method of determining the intensity at the edge jump, ∆IPEY,F ()difference between the postedge and preedge intensities). Figure 4. Schematic illustrating the molecular organization of t-F8H2 (a) and m-F8H2 (m) on silica substrates. Also shown is the geometry used to evaluate the distance between two neighboring F8H2 molecules, s, from the SF mesogen average tilt angle, 〈τF〉, and the mesogen diameter, d.
the grafting density values obtained from the tilt angles further supports the feasibility of the simple onedimensional model. Conclusions
where s is the distance between two neighboring Si atoms and d is the diameter of the F(CF2)8 helix. Table 1 lists the values of σF8H2 calculated from the experimental 〈τF〉 using d ) 0.56 nm.22 From the data, one can see that there is a 2-fold decrease in the grafting density associated with changing the bonding environment from SiCl3 to Si(CH3)2Cl. We note that these grafting densities are consistent with the values reported in the literature for an array of densely packed alkyldimethylsilyl groups (≈0.32-0.38 nm2)23 and molecules in trichlorosilane-based SAMs (≈0.20 nm2).24 NEXAFS spectroscopy also offers a relative measure of the concentration of the F8H2 groups on the substrate. Specifically, we can use the difference between the preedge and postedge signals in the PEY NEXAFS spectra at the fluorine K-edge, ∆IPEY,F, as a measure of the concentration of fluorine per area of the incident beam in the sample.13 Figure 5 shows the fluorine K-edge PEY NEXAFS spectra collected at θ ) 50° from t-F8h2, d-F8H2, and m-F8H2 samples. Only the preedge of each spectrum was normalized to 0. By inspecting the spectra in Figure 5, one can see that the areal concentration of F8H2 decreases in the following manner: t-F8H2 > d-F8h2 > m-F8H2. The values of ∆IPEY,F obtained from the fluorine K-edge NEXAFS spectra are listed in Table 1. There is a consistency between the σF8H2 and ∆IPEY,F values; for example, the ratios σF8H2/∆IPEY,F (column 6 in Table 1) are almost the same for all samples studied. Considering that NEXAFS provides a relative measure of the molecule grafting density in two dimensions (the spot size of the X-ray beam on the sample during the NEXAFS experiments, ≈1 mm2, is much larger than the area occupied by a single F8H2 molecule on the surface), the agreement between the relative concentrations estimated from the edge-jump in the fluorine K-edge NEXAFS spectra and (21) We note that previously we have reported that a value of 〈τF〉 ) 4 ( 2° for t-F8H2 SAM (ref 18). We believe that our current results do not deviate dramatically from those reported previously. (22) Chidsey, C. E. D.; Loiacano, D. N. Langmuir 1990, 6, 682. (23) Claudy, P. et al. J. Chromatogr. 1985, 329 (331). (24) Wasserman, S. R., et al. J. Am. Chem. Soc. 1989, 111, 5852.
Several conclusions can be made from the present work. First, our results indicate that the kinetics of SF SAM formation during vapor deposition is faster that that of normal organosilane hydrocarbons. We speculate that this faster kinetics is a consequence of stronger intermolecular interaction between the F8H2 moieties, relative to simple hydrocarbons. Second, results of NEXAFS spectroscopic experiments reveal that the molecular organization in F8H2 SAMs depends critically on the bonding environment around the silicon group. Specifically, while trichlorosilane-based molecules are oriented almost perpendicular to the silica substrate, the average tilt angle of F8H2 increases with increasing number of methyl groups attached to silicon. This observation can be rationalized by considering steric hindrance present in the latter cases. The molecular orientation elucidated from the NEXAFS spectroscopy measurements can be used to estimate the grafting densities of the F8H2 molecules on the substrates. Analysis using a simple one-dimensional geometric model reveals that the grafting density of m-F8H2 is approximately one-half of that corresponding to the t-F8H2 SAM. We have also demonstrated the self-consistency of these results. Specifically, we have shown that the grafting densities of F8H2 determined from the one-dimensional chain tilt model are in accord with those that can be inferred from the relative edge-jumps in the fluorine K-edge NEXAFS spectra. Acknowledgment. This research was funded by the Camille Dreyfus Foundation, 3M, and NACE International. NEXAFS spectroscopic experiments were carried out at the National Synchrotron Light Source, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Division of Materials Sciences and Division of Chemical Sciences. Funding for acquiring the variable-angle spectroscopic ellipsometry has been generously provided by the National Science Foundation through the Instrumentation for Materials Research Program, Grant No. DMR-9975780. LA025921X
