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Studies of Molecular Orientation and Order in Self-Assembled Semifluorinated n-Alkanethiols: Single and Dual Component Mixtures M.-W. Tsao,*,† C. L. Hoffmann,‡ and J. F. Rabolt† IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose, California 95120
H. E. Johnson 3M Corporate Research, St. Paul, Minnesota 55144
D. G. Castner Department of Chemical Engineering, BF-10 University of Washington, Seattle, Washington 98195
C. Erdelen and H. Ringsdorf Institute for Organic Chemistry, University of Mainz, J. J. Becher Weg 22, D-6500 Mainz, Germany Received March 18, 1997. In Final Form: June 3, 1997X The structure, orientation and morphology of self-assembled monolayers of a semifluorinated nalkanethiol, F(CF2)8(CH2)11SH (F8H11SH), have been investigated by polarized IR, angular dependent XPS, ToF-SIMS, contact angle, and ellipsometric measurements. The orientation of the all trans hydrocarbon segment was found to be tilted much less from the surface normal than the 30° tilt found for octadecanethiol. This has been attributed to the steric constraints imposed by the larger cross section fluorocarbon helices that subsequently are tilted from the surface normal. In addition, studies of dual component mixtures of F8H11SH/F8SH and F8SH/F8H2SH have revealed that competitive adsorption occurs in the former, producing monolayers that are deficient in the shorter F8SH molecules, while in the latter equal representation of both F8SH and F8H2SH molecules are found on the surface due to their similar molecular lengths.
Introduction With increasing interest in the mechanism of selfassembly there have been a number of studies1-6 that have specifically addressed the role of molecular architecture on both the rate of assembly and the resulting structure of the monolayer. Much of this work has focused on alkanethiols on gold1 and alkyltrichlorosilanes on silicon.2 Recently, there have been a number of reports3,4 in which the chemical architecture of the adsorbate has been designed so as to include aromatic groups to introduce structural rigidity into the monolayer. Another approach5,6 that has also been successful is the introduction of conformational rigidity (as compared to electronic rigidity) by the incorporation of perfluoroalkyl groups into linear thiolate molecules. Whereas hydrocarbon chains assume a planar zigzag conformation due to intermolecular stabilization among adjacent chains, perfluorocarbon helices are intramolecularly stabilized and hence † Current address: Materials Science Program, University of Delaware, Newark, DE 19716. ‡ Current address: Charles Evans & Associates, Redwood City, CA 94063. X Abstract published in Advance ACS Abstracts, July 15, 1997.
(1) Bain, C. D.; Troughton; E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (2) Pomerantz, M.; Segmuller, A.; Netzer, L.; Sagiv, J. Thin Solid Films 1985, 132, 153. (3) Tillman, N.; Ulman, A.; Schildkraut, J. S.; Penner, T. L. J. Am. Chem. Soc. 1988, 110, 6136. (4) Tillman, N.; Ulman, A.; Elman, J. Langmuir 1990, 6, 1512. (5) Chidsey, C. E. D.; Lociano, D. Langmuir 1990, 6, 682. (6) Lenk, T. J.; Hallmark, V. M.; Hoffmann, C. L.; Rabolt, J. F.; Castner, D. G.; Erdelen, C.; Ringsdorf, H. Langmuir 1994, 10, 4610.
S0743-7463(97)00293-X CCC: $14.00
will maintain their helical conformation even in isolated chains provided the perfluoroalkyl chains are short (12 CF2 groups or less).7,8 This suggests the interesting possibility that mixtures of two semifluorinated n-alkanethiols (F(CF2)n(CH2)mSH) with different chain lengths could be used to create phase-separated domains that would appear as “pores” whose depth could be determined by the difference in chain length chosen for the two coadsorbed species. This would be in contrast to mixed alkanethiol layers where the flexibility of the hydrocarbon chains dictates the formation of “mushroom-like” structures due to the introduction of gauche bonds in the longer chains at the domain boundary.9 This folding over of the long chains to cover the short chain domains should be prevented in perfluoroalkanethiols due to the helical rigidity resulting from intermolecular stabilization. Work reported recently by Chidsey and Lociano5 and Lenk et al.6 on semifluorinated n-alkanethiols indicated that when fluorocarbon helices are attached to a thiol head group by a short hydrocarbon sequence, the fluorocarbon chain is oriented normal to the surface. It is the purpose of this study to investigate the orientation and order of SA monolayers of a semifluorinated n-alkanethiol that includes the same length fluorocarbon (F(CF2)8-) chains as studied previously but in this case attached to a thiol head group through a long (-(CH2)11-) sequence. In addition, binary mixtures of this molecule F(CF2)8(CH2)11SH (7) Bunn, C. W.; Howells, E. R. Nature 1954, 174, 549. (8) Rabolt, J. F.; Fanconi, B. Polymer 1977, 18, 1258. (9) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L. Porter, M. D. Langmuir 1988, 4, 365.
© 1997 American Chemical Society
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(F8H11SH in our notation) and shorter semifluorinated n-alkanethiols F(CF2)8(CH2)2SH (F8H2SH) and F(CF2)8C(O)N(H)(CH2)2SH (F8SH) have also been investigated by FTIR, ToF-SIMS, angular dependent XPS, ellipsometry, and contact angle measurements. Experimental Section A. Synthesis of C8F17(CH2)11SH (1, F8H11SH). All starting materials were obtained from commercial sources and used without further purification. Perfluorooctyl iodide (27.39g, 50 mmol), ω-undecylenyl alcohol (7.14g, 42 mmol), and benzoyl peroxide (0.38 g) were mixed in a glass ampule. The mixture was degassed and the ampule was sealed under vacuum. It was then placed in an oven at 100 °C for 24 h. The product, C8F17CH2CHI(CH2)9OH (2), was used without additional purification. Lithium aluminum hydride (LAH, 2.0g, 53 mmol) was dissolved in 100 mL of tetrahydrofuran (THF) in a 500 mL three-neck flask cooled in an ice bath. Product 2 (16.6g, 23 mmol) was also dissolved in 100 mL of THF, and the resulting solution was added dropwise to the LAH/THF solution via an addition funnel. After addition was complete, the flask was removed from the ice bath and the mixture was heated at reflux overnight. The flask was then again placed in an ice bath, and Freon 113 (100 mL) was added via an addition funnel. About 10-20 mL of water was added dropwise through the addition funnel to quench excess LAH; then the mixture was acidified with 10% aqueous sulfuric acid. The freon phase was washed three times with water, dried with MgSO4, filtered, and freed from solvent on a rotary evaporator. The residue was purified by Kugelrohr distillation to give C8F17(CH2)11OH (3) in 70% yield. Alcohol 3 was converted to C8F17(CH2)11Br (4) by heating for 7 h at 110 °C under a stream of hydrogen bromide.10 The reaction was followed to completion by gas chromatography, and Kugelrohr distillation gave 4 in 82% yield. The conversion of bromide 4 to thiol 1 was carried out via the Bunte salt reaction.11,12 Bromide 4 (1.92g, 3 mmol), was dissolved in 5 mL of ethanol in a 25 mL flask. Sodium thiosulfate (0.48g, 3 mmol) was dissolved in 5 mL of water, and the resulting solution was added to the ethanol solution. The mixture was heated at reflux overnight. Hydrolysis was carried out by adding 3 mL of 10% aqueous sulfuric acid and refluxing for 7 h. The thiol product could be seen separating as a heavier liquid phase. Thiol 1 was recovered by extraction with ether, washed with water three times, dried with MgSO4, filtered, and freed from solvent. Kugelrohr distillation gave a 74% yield of thiol, with purity on the order of 98% by gas chromatography. The remaining 2% consists of C8F17C11H25 and C8F17C9H18CHdCH2. Neither of these two impurities would interfere with the adsorption of F8H11SH on gold. B. Substrate Preparation and Film Formation. Glass microscope slides (3 in. × 1 in.) used as substrates were cleaned by acid etching followed by degreasing in isopropyl alcohol vapor. They were then dried under a warm nitrogen flow. Vapor deposition of 200 Å of chromium under vacuum (5 × 10-6 Torr) was followed by evaporation of 2000 Å of gold. The deposition rate for chromium was 4-5 Å/s, while that for gold was 8-10 Å/s. Gold substrates prepared by this method were then dipped in 0.1 mg/mL thiol solutions. Thiol deposition from ethanol and dichloromethane solutions was tested with no significant difference in the film properties detected. Usually, a dipping time of 2 h was utilized unless otherwise specified. All samples were rinsed with an excess of the deposition solvent and then dried by a dry nitrogen flow before any measurements were made. C. Characterization. FTIR. Infrared measurements were made on an IBM/Bruker IR98 vacuum FTIR operated in the grazing incidence reflection mode. All data were taken at 4 cm-1 resolution with the accumulation of 8000 scans. Gold surfaces containing self-assembled deuterated octadecanethiol were used for all IR references since this procedure minimizes the amount of airborne contaminants that settle on the bare gold surface. In addition, the use of deuterated octadecanethiol minimizes any spectral interference from vibrational bands from the reference. (10) Reid, E. E.; Ruhoff, J. R.; Burnett, G. Org. Synth. Colloids 1943, 2, 246. (11) Milligan, B.; Swan, J. M. Rev. Pure Appl. Chem. 1962, 12, 72. (12) Distles, H. Angew. Chem. 1967, 79, 520.
Tsao et al. In this paper, multiple IR spectra are often shown on a single graph with their baselines offset at a constant interval. Unless specified otherwise, all spectra on the same graph are plotted with the same intensity scale in this work. X-ray Photoelectron Spectroscopy. The X-ray photoelectron spectroscopy (XPS) experiments were done on a Surface Science Instruments X-probe spectrometer (SSI, Mountain View, CA). This system has a monochromatic Al KR X-ray source (hν ) 1486.6 eV), hemispherical analyzer, and resistive strip, multichannel detector. The XPS binding energies (BE) were referenced by setting the BE of the Au 4f7/2 peak to 84.0 eV. The highresolution C1s and S2p spectra were acquired at an analyzer pass energy of 50 eV. XPS elemental compositions were obtained using a pass energy of 150 eV. At this pass energy the transmission function of the spectrometer was assumed to be constant.13 The peak areas were normalized by the number of scans, points per electronvolt, Scofield’s photoionization crosssections,14 and sampling depth. The sampling depth was assumed to vary as KE0.7, where KE is the kinetic energy of the photoelectrons.13 XPS data were acquired at nominal photoelectron takeoff angles of 0°, 55°, and 80°. The takeoff angle was defined as the angle between the surface normal and the axis of the analyzer lens system. For determination of a compositional depth profile (CDP), the solid acceptance angle of the analyzer lens was decreased from its normal value of 30° to 6° × 12° by placing an aperture over the analyzer lens to improve the depth resolution at each takeoff angle.15 The regularization method of Tyler et al.16 was used to generate a CDP from elemental compositions measured at each takeoff angle. The mean free paths used to generate the CDPs were calculated from the equations given by Seah and Dench.17 Time-of-Flight Secondary Ion Mass Spectrometry. The timeof-flight secondary ion mass spectrometry (ToF-SIMS) data were acquired using a Model 7200 Physical Electronics instrument (PHI, Eden Prairie, MN). The 8 keV Cs+ ion source was operated at a current of 1.5 pA and a pulse width of 0.9 ns. Data were acquired over a mass range from m/z ) 0 to 2000 for both positive and negative secondary ions. The ion beam was moved to a new spot on the sample for each spectrum. The total ion dose used to acquire each spectrum was less than 2 × 1012 ions/cm2. The area of analysis for each spectrum was 0.01 mm2. The secondary ions were extracted into a two-stage reflectron time-of-flight mass analyzer with a potential of 3 kV. A secondary ion focusing lens between the analyzer entrance and drift region was held at 1 kV, promoting high angular acceptance and good transmission of ions. The band pass of the analyzer is 100 eV, and an independent adjustable grid voltage (deceleration) allows energy focusing to be performed. The ions were postaccelerated to 10 kV and converted to charge pulses by a stacked pair of chevron-type multichannel plates (MCP). The signals were detected using a 256 stop time-to-digital converter (TDC) with a 156 ps time resolution. The mass resolution (m/∆m) for both positive and negative secondary ions was typically between 6000 and 10000. The mass scale for the negative secondary ions was calibrated using the CF (30.9984), CF3 (68.9952), C2F5 (118.9920), and C3F7 (168.9888) peaks. The mass scale for the positive secondary ions was calibrated using the CF, CF2 (49.9968), CF3, C2F5, and C3F7 peaks. The fit between the expected and observed masses for both positive and negative calibration ions was less than 10 ppm. Using this mass calibration procedure typically resulted in agreement between observed mass and the expected mass of less than 50 ppm for secondary ions over the entire mass range.
Results and Discussion A. F8H11SH. FTIR. Since the F8H11SH used in this study has the same length fluorocarbon chain as the F8SH previously studied in detail, the assignments of bands in the isotropic spectrum (Figure 1) are straightforward.6 (13) Application note from Surface Science Instruments, Mountain View, CA, 1987. (14) Scofield, J. H. J. Electron. Spectrosc. Relat. Phenom. 1976, 8, 129. (15) Tyler, B. J.; Castner, D. G.; Ratner, B. D. J. Vac. Sci. Technol. 1989, A7, 1646. (16) Tyler, B. J.; Castner, D. G.; Ratner, B. D. Surf. Interface Anal. 1989, 14, 443. (17) Seah, M. P.; Dench, W. A. Surf. Interface Anal. 1979, 1, 2.
Semifluorinated n-Alkanethiols
Figure 1. IR spectra (1800-500 cm-1) of isotropic F(CF2)12(CH2)12H (top) and isotropic F8H11SH (middle). The bottom trace is the grazing incidence IR spectrum of the F8H11SH monolayer on gold.
Figure 2. Polarized (E⊥) IR spectra in the CH stretching region of H(CH2)18SH (- - -) and F8H11SH (s).
For comparison, a spectrum of a crystalline semifluorinated n-alkane, F12H12, is included. The strong bands at 1244, 1205, and 1150 cm-1 have been assigned to both the asymmetric, νa(CF2), and symmetric, νs(CF2), stretching vibrations of the CF2 groups and consequently have their change in dipole moment perpendicular to the fluorocarbon helical axis. The strong shoulder in the vicinity of 1220 cm-1 has been assigned to a vibration that involves stretching of the CC bonds and a bending of CCC angles, ν(CC) + R(CCC). Correspondingly, the dipole moment change for this vibration is parallel to the fluorocarbon helical axis. In addition, the weak bands at 1372 and 1333 cm-1, whose positions are a function of fluorocarbon chain length and can be seen to shift to 1350 and 1320 cm-1 in the F12H12 spectrum, were attributed to axial CF2 stretching vibrations with a corresponding change in dipole moment parallel to the helical axis. Characteristic bands attributable to the hydrocarbon portion of the F8H11SH chain are found at 1470 cm-1 (CH2 scissoring vibration, δ(CH2)) and 720 cm-1 (CH2 rocking vibration, r(CH2)), both of which have a change in dipole moment that is parallel to the hydrocarbon axis. When the F8H11SH is adsorbed on gold, the CH stretching region shown in Figure 2 provides a good insight into the conformational order of the hydrocarbon portion of the F8H11SH molecule. By comparison with the spectrum of n-octadecanethiol, C18SH, which contains CH stretching bands from both CH2 (νa(CH2) at 2918 cm-1,
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νs(CH2) at 2850 cm-1) and CH3 (νa(CH3) at 2965 cm-1, νs(CH3) at 2878 cm-1 ), the spectrum of adsorbed F8H11SH contains only two bands, νa(CH2) at 2918 cm-1 and νs(CH2) at 2850 cm-1 since there are no methyl end groups present in the latter. The location of the bands is significant since Snyder et al.18 have shown that as the conformational order of the hydrocarbon chain increases the νa(CH2) and νs(CH2) vibrational frequencies approach 2918 and 2850 cm-1, respectively. Hence, the H11 portion of F8H11SH appears to assume an ordered planar zigzag conformation. Another interesting piece of information comes from considering the relative intensities of the CH stretching modes of the two adsorbed species. It is wellknown that the chain axis of the C18SH is tilted about 30° relative to the surface normal. If one assumed that the H11 portion of the F8H11SH were also tilted by this amount, then one would expect the νa (CH2) intensity of the latter to be 11/17 (64%) of that found for C18SH. However, this is not the case. As shown in Figure 2 the intensity of νa(CH2) and νs(CH2) in the F8H11SH is approximately 50% of that observed for C18SH, indicating that the H11 portion assumes a different tilt than 30°. Since the spectra shown in Figure 2 were obtained in grazing incidence and hence the electric field was oriented normal to the substrate, the only way to account for this lower than expected intensity is to allow for the H11 portion of the F8H11SH molecule to be more normal to the surface; i.e., its tilt is less than 30°. This undoubtedly is caused by the additional interactions that result from the packing of F8 fluorocarbon helices (at the end of the H11 sequence) into a lattice. If the hydrocarbon portion of the F8H11SH chain is tilted less than the C18SH chain, then one might expect this to have some effect on the orientation of the F8 helix. In studies of the F8SH it was clearly shown by NEXAFS, FTIR and XPS that the fluorocarbon chain was oriented normal to the substrate.6 By comparing the IR spectra of both F8H11SH in the bulk (Figure 1, middle) and after chemisorption on gold (Figure1, lower), remembering that the latter was obtained with E⊥ to the surface, it is possible to get some indication of the orientation of the F8 chain by comparing the relative intensities of the axial CF2 stretches at 1373 and 1335 cm-1 with the bands at 1245, 1205, and 1150 cm-1, which are CF2 vibrations with their change in dipole moment perpendicular to the fluorocarbon axis. Upon inspection, it is clear that the F8 chain is highly oriented toward the surface normal, but by comparison with the spectrum of the F8SH shown in the lower portion of Figure 3, it becomes apparent that there is a slight tilt due to the packing constraints dictated by the H11 portion attached to the gold surface. XPS Characterization. As concluded from previous results obtained for F8SH,6 the SAMs prepared with F8H11SH have excellent uniformity and reproducibility, as shown by the low variability in the XPS composition measurements. Averages of two locations on two different samples at a photoelectron takeoff angle of 55° gave the following atomic percentages and standard deviations: Au, 16.8 ( 0.2; F, 48.9 ( 0.6; C, 33.6 ( 0.7; S, 0.8 ( 0.1. The oxygen concentration was at or below the XPS detection limits (
