Langmuir 1996, 12, 5509-5511
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Photocontrolled Formation of Hydroxyl-Bearing Monolayers and Multilayers Rochael J. Collins,† In Tae Bae,† Daniel A. Scherson,† and Chaim N. Sukenik*,†,‡ Departments of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106, and Bar Ilan University, Ramat Gan, Israel Received February 20, 1996. In Final Form: September 10, 1996X Siloxane-anchored, nitrate-bearing, monolayer films have been fabricated and have been used as the precursors for the photochemical creation of uniformly hydroxylated monolayer surfaces. The surface hydroxyl groups have been shown to be reactive toward phosphorylation and, more importantly, they can serve as the base layer for addition of a subsequent siloxane-anchored monolayer. This approach provides an important new tool for monolayer transformation and allows for photocontrol of organic multilayer construction.
In the study of self-assembled monolayers (SAMs), the seminal work of Sagiv and co-workers1 described the conversion of an olefin-terminated SAM into an alcoholbearing surface as the key to the creation of self-assembled, covalently bound, organic multilayer assemblies (Scheme 1a). While highly innovative in concept, the approach was plagued by the fact that the olefin hydroboration did not afford the quantitative yields needed to optimize such a strategy. Hence, the water contact angle of the alcohol surface was higher than that which would be expected for a uniformly hydrophilic surface. Nevertheless, the OHbearing surface was a suitable base for the construction of modest multilayer assemblies. An improvement was subsequently reported,2 involving a more effective multilayer construction system based on the LiAlH4 reduction of methyl esters to alcohols as depicted in Scheme 1b. This approach succeeded in making better wetted surfaces and in constructing assemblies of up to 25 layers.2b Its primary disadvantage was the reliance on a very aggressive and air/moisture sensitive reagent that leaves a significant inorganic residue on the substrate surface (which must be manually cleaned) after each reduction cycle. Evidence was also presented2b for a small amount of incomplete reduction as revealed by IR spectral features attributable to unreacted ester groups within the multilayer structure. * Author to whom correspondence should be addressed, at Bar Ilan University. † Case Western Reserve University. ‡ Bar Ilan University. X Abstract published in Advance ACS Abstracts, October 15, 1996. (1) Netzer, L.; Sagiv, J. A New Approach to Construction of Artificial Monolayer Assemblies. J. Am. Chem. Soc. 1983, 105, 674. (b) Sagiv, J. Organized Monolayers by Adsorption. 1. Formation and Structure of Oleophobic Mixed Monolayers on Solid Surfaces. J. Am. Chem. Soc. 1980, 102, 92. (c) A recent, improved, re-examination of Sagiv’s “olefin to alcohol” multilayer construction approach can be found in: Heid, S.; Effenberger, F. Self-Assembled Mono- and Multilayers of Terminally Functionalized Organosilyl Compounds on Silicon Substrates. Langmuir 1996, 12, 2118-2120. (2) (a) Pomerantz, M.; Segmuller, A.; Netzer, L.; Sagiv, J. Coverage of Si by Self-Assembling Monolayers and Multilayers as Measured by IR, Wettability and X-ray Diffraction. Thin Solid Films 1985, 132, 153-162. (b) Tillman, N.; Ulman, A.; Penner, T. L. Formation of Multilayers by Self-Assembly. Langmuir 1989, 5, 101. (3) For some recent examples of SAM photochemistry see: (a) Dulcey, C. S.; Georger, J. H.; Chen, M. S.; McElvany, S. W.; O’Ferrall, C. E.; Benezra, V. I.; Calvert, J. M. Photochemistry and Patterning of SelfAssembled Monolayer Films Containing Aromatic Hydrocarbon Functional Groups. Langmuir 1996, 12, 1638-1650. (b) Delamarche, E.; Sundarababu, G.; Biebuyck, H.; Michel, B.; Gerber, Ch.; Sigirist, H.; Wolf, H.; Ringsdorf, H.; Xanthopoulos, N.; Mathieu, H. J. Immobilization of Antibodies on a Photoactive Self-Assembled Monolayer on Gold. Langmuir 1996, 12, 1997-2006. (c) Rozsnyai, L. F.; Wrighton, M. S. Seletive Deposition of Conducting Polymers via Monolayer Photopatterning. Langmuir 1995, 11, 3913.
S0743-7463(96)00156-4 CCC: $12.00
We report herein a new methodology for the creation of hydroxyl-bearing, siloxane-anchored, self-assembled monolayers and multilayers. Analogous to previously reported phototransformations of SAMs,3 this work explores the photolysis of a nitrate-bearing SAM as a route to surface hydroxyl formation which does not require sensitive reagents or have to contend with inorganic residues. This photoconversion allows the photopatterning of film functionalization and/or of subsequent multilayer construction. Experimental Section Formation of Nitrate-Bearing Self-Assembled Monolayer. The nitrate-bearing, siloxane-anchored, monolayer is made from the corresponding alkyl nitrate-trichlorosilane (Cl3Si-C16H32-ONO2), which is derived from commercially available ω-undecenyl alcohol. The ω-undecenyl alcohol was converted to ω-hexadecenyl bromide as previously reported.4 ω-Hexadecenyl nitrate was made by reacting the alkyl bromide with AgNO3 (1:2 molar ratio) in CH3CN at 60 °C overnight. After cooling, the solution was reduced to half of its original volume on a rotary evaporator, and it was filtered and partitioned between diethyl ether and water. The combined ether extracts were washed with aqueous NH4Cl, water, and saturated aqueous NaCl, and then dried over Na2SO4. The ether was removed on a rotary evaporator, and the product was purified by flash chromatography (hexanes/SiO2), 75% yield: 1H NMR (CDCl3) δ 1.2-1.5 (m, 22 H), 1.67 (m, 2 H), 2.01 (m, 2 H), 4.40 (t, J ) 7 Hz, 2 H), 4.90 (m, 2 H), 5.76 (m, 1 H); 13C NMR (CDCl3) δ 22.61, 25.61, 26.71, 28.94, 29.15, 29.39, 29.51 (2C), 29.64 (4C), 33.79, 73.16, 113.90, 139.88; IR (neat, thin film sandwich) 3076, 2925, 2854, 1633, 1466, 1441, 1370, 1279, 992, 909, 863, 758, 722, 699 cm-1. The ω-hexadecenyl nitrate was hydrosilylated in CHCl3 solution (pressure tube, 40 °C) using chloroplatinic acid catalyst. Typically, ω-hexadecenyl nitrate (2 g) was mixed with CHCl3 (2 mL) and HSiCl3 (3 mL) and 20 µL of catalyst solution (4% in isopropyl alcohol) and allowed to react overnight. Purification of 16-(trichlorosilyl)hexadecanyl nitrate involved Kugelrohr distillation (140 °C, 3 µmHg); yield 30%: 1H NMR (CDCl3) δ 1.2-1.4 (m, 26 H), 1.67 (m, 2 H), 1.70 (m, 2 H), 4.40 (t, J ) 7 Hz, 2 H); 13C NMR (CDCl3) δ 22.22, 24.28, 25.61, 26.72, 28.99, 29.09, 29.36, 29.48 (2C), 29.60 (4C), 30.27, 31.79, 73.44; IR (neat, thin film sandwich) 2926, 2854, 1633, 1466, 1371, 1279, 979, 863, 761, 722, 693, 588, 565 cm-1. Monolayer films of the 16-(trichlorosilyl)hexadecanyl nitrate were deposited onto piranha solution-cleaned (70:30 H2SO4:H2O2, 80 °C) Si(100) wafers from 20 mM solutions in dicyclohexyl under a nitrogen atmosphere for periods of time ranging from 7 h at 22 °C to 4 days at 2 °C. After deposition, wafers were cleaned by sonication and wiping with methylene chloride and ethanol. Sessile drop water contact angle measurements (Rame-Hart Model 100; reported values are (2°) and XPS analysis (PerkinElmer ESCA 5400; Al KR source; 10-9 Torr; take-off angle 45°; peak positions referenced to C(1s) at 284.75 eV) were used to (4) Balachander, N.; Sukenik, C. N. Monolayer Transformation by Nucleophilic Substitution. Langmuir 1990, 6, 1621.
© 1996 American Chemical Society
5510 Langmuir, Vol. 12, No. 23, 1996
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Scheme 1. Covalent Multilayer Construction
Scheme 2. Monolayer-Based Nitrate Photochemistry
characterize all surfaces. Spectroscopic ellipsometric measurements (J. A. Woollam) of monolayer and multilayer structures were performed following calibration with a 25 nm SiO2 on Si standard. Data for each thickness measurement were collected at angles of 75, 76, 77° and at wavelengths of 300-800 nm (10 nm increments), yielding 150 pairs of ψ and ∆ values from which each reported thickness ((0.1 nm) was calculated using V.A.S.E. software provided by J. A. Woollam. Transformations of the Nitrate-Bearing Monolayer. Photolysis of the nitrate-terminated monolayer was accomplished by mounting the coated Si wafer in a quartz tube and covering the sample with a thin film of HPLC grade isopropyl alcohol. The tube was purged with argon and placed within 15 mm of a 450 W, medium pressure Hanovia lamp (manufacturer specified output in the 220-320 nm range of 27-29 W) in a water-cooled jacket. Typical photolysis times were 6-7 min. Phosphorylation of monolayer hydroxyl groups was performed using a procedure analogous to that reported for thiol-anchored OH-bearing monolayers.5 Typically, the photolyzed SAM was treated with a mixture of triethylamine, dry THF, and POCl3 for 2 h at room temperature. Photopatterning of the nitrate functionalized monolayer was accomplished by interposing a stainless steel grid with 290 µm circular features between the coated Si substrate (wetted with HPLC isopropyl alcohol) and a piece of UV grade fused silica. The assembly was mounted parallel to the UV source and irradiated. Imaging of the patterned monolayer was accomplished using a Hitachi S-4500 SEM with field emission tip at 1 keV. In Situ Attenuated Total Reflectance-Fourier Transform Infrared (ATR-FTIR) Analysis of Photolysis Product and of Multilayer Formation. ATR-FTIR experiments were conducted with a Si 50 × 20 × 3 mm parallelepiped prism using an IBM IR-98 spectrometer. Due to space constraints in the IR sample chamber, a much smaller and less intense UV source was used for these experiments (UVP Model UVG-54 lamp with an intensity (at 254 nm) of 2.2 mW at 75 mm). The ATR crystals were coated with deposition solutions which were 0.7 mM 16(trichlorosilyl)hexadecanyl nitrate in dicyclohexyl for 9-12 h. The liquid flow-through cell6 and the UV source were placed 15 cm away from each other in the nitrogen-purged FTIR chamber. The flow cell was rinsed with CHCl3 and then nitrogen-dried before a reference spectrum was collected. The deposition of the nitrate-bearing monolayer was accomplished by filling the cell with the trichlorosilane solution without removing the cell from the FTIR chamber. After deposition, the cell was emptied, rinsed (5) Bertilsson, L.; Liedberg, L. Infrared Study of Thiol Monolayer Assemblies on Gold: Preparation, Characterization, and Functionalization of Mixed Monolayers. Langmuir 1993, 9, 141. (6) Analogous to that described in: Cheng, S. S.; Scherson, D. A.; Sukenik, C. N. In Situ FTIR-ATR Spectroscopy of Carboxylate-Bearing, Siloxane-Anchored, Self-Assembled Monolayers: A Study of Carboxylate Reactivity and Acid-Base Properties. Langmuir 1995, 11, 1190-1195.
with fresh dicyclohexyl and then CHCl3, and nitrogen-dried before collecting the spectrum of the deposited film. Photolysis of the nitrate functionality was accomplished by filling the flow cell with isopropyl alcohol and irradiating, without removing the crystal from the IR chamber. Due to the reduced intensity of this light source and the fact that its output is primarily at longer wavelength than that of the Hanovia lamp (while the for the alkyl nitrate chormophore in isopropyl alcohol solvent is 50 times greater at 220 nm than at 254 nm), irradiation times of 7 h were needed to complete the phototransformation. After the photolysis was complete, the cell was rinsed with fresh isopropyl alcohol and nitrogen-dried before the spectrum was collected. These steps were repeated to build a second layer.
Results and Discussion The essential chemistry of our approach to the creation of hydroxyl-bearing siloxane-anchored SAMs is summarized in Scheme 2. The application of nitrate photolysis to hydroxyl formation has been reported by Binkley et al.7 for alkyl nitrate derivatives of sugars. We confirmed this for simple, straight-chain, alkyl nitrates by a model study of the photolysis of n-octyl nitrate in isopropyl alcohol solution which showed quantitative photoconversion to octanol.8 The corresponding transformation of the alkyl nitratebearing surface was studied by a variety of tools. It was possible to confirm the complete disappearance of the nitrate functionality by monitoring the loss of the N(1s) signal at 408 eV and the loss of the contribution of the nitrate ester at 534 eV to the O(1s) signal in the XPS. In addition, FTIR spectra of the nitrate-bearing film deposited on a Ge ATR crystal showed the expected asymmetricNO2 stretch at 1633 cm-1 and symmetric-NO2 stretch at 1279 cm-1; these signals disappeared upon irradiation. The lack of substantial change in the methylene stretching region of the FTIR spectrum (2900-3000 cm-1) and the small decrease (0.15 nm) in ellipsometrically measured film thickness upon irradiation were both consistent with the preservation of the overall monolayer structure, despite terminal functional group interconversion. Further evidence for the installation of surface OH functionality was found in the decrease of the water contact (7) Binkley, R. W.; Koholic, D. J. Photolysis of Nitrate Esters. Photochemically Initiated Inversion of Configuration. J. Org. Chem. 1979, 44, 2047. (8) Octyl nitrate was prepared from octyl bromide as described above for ω-hexadecenyl nitrate. Photolyses were done on 2.8 mM solutions in HPLC grade isopropyl alcohol. Analyses were done using a HPModel 5890-5971A GC-MS using a 12 m HP1 (methylsilicone) capillary column and using hexadecane as an internal standard.
Letters
Figure 1. Correlation of thickness (90% confidence) of nitratefunctionalized SAM with its wetting behavior after photolysis to a hydroxyl-functionalized SAM.
angle of the nitrate-bearing SAM (before photolysis: 81 adv., 67 rec.; after photolysis: 31 adv., 18 rec.). An interesting correlation (Figure 1) was observed between the ellipsometrically measured thickness of samples of the alkyl nitrate film from various depositions and the hydrophilicity of the OH-bearing surface obtained upon photolysis: the closer film thickness approached the 2.5 nm thickness calculated (using PCMODEL software) for a fully extended O-Si-C16H32-ONO2 chain, the more hydrophilic the resulting OH surface. Films of 2.0 nm yielded poorly hydrophilic OH surfaces with advancing water contact angles of 47°, while films of 2.4 nm gave rise to the most hydrophilic surfaces with advancing water contact angles of 31°. Presumably, the better packed nitrate-bearing films provide more uniformly hydroxylated surfaces which are also less prone to surface restructuring.2b Given the sensitivity of SAM deposition to small changes in ambient conditions, films of 2.4 nm thickness or greater were only rarely obtained. Many replicate depositions routinely yielded films with thicknesses in the 2.0-2.2 nm range and comparable water contact angles. Additional insight was obtained by studying the chemistry of the OH-bearing surface. Reaction with POCl3 gave a surface with the expected phosphate functionality (strong P(2p) XPS signal at 134 eV). Also, its application to multilayer construction was demonstrated. The construction of a trilayer assembly was monitored by ellipsometry. Each layer of the nitratosilane added approximately 2.25 nm to the overall thickness (nitrate-terminated assemblies were measured at 2.26 ( 0.06, 4.49 ( 0.08, and 6.94 ( 0.12 nm, for one, two, and three layers; photolyzed films measured 2.14 ( 0.06, 4.40 ( 0.08, and 6.70 ( 0.11 nm, respectively). In situ ATR-FTIR on a Si internal reflection element provided direct spectroscopic monitoring of the construction of a bilayer assembly. Figure 2 shows the difference spectra of the deposited nitrate monolayer before and after photolysis for two cycles. The asymmetric-NO2 stretch is easily seen at 1632 cm-1 in the nonphototransformed films; in fact, the methylene stretching band doubles its intensity (ratio of CH2 stretch at 2920 cm-1 for layer 2/layer 1 ) 2.03) upon addition of the second layer. Quantitation of the nitrate absorbance was somewhat less precise, presumably due to overlap with spectral features attributable to background water. We have also demonstrated the ability to photopattern these OH-bearing surfaces by irradiation of a nitrate monolayer through a mask. Direct SEM visualization of the photopatterned surface was not possible since the
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Figure 2. In situ FTIR-ATR monitoring of nitrate photolysis and second layer attachment.
Figure 3. SEM image of phosphorylated photopatterned surface.
nitrate functionality is not sufficiently stable to the electron beam. However, by subjecting the patterned nitrate surface to the phosphorylation reaction prior to SEM analysis, sufficient contrast is achieved.9 This phenomenon is believed to be derived from differences in the surface free energies (and their ability to adsorb contaminants) of the phosphate and nitrate terminal functional groups.10 The SEM image of a sample prepared in such a manner is shown in Figure 3. The bright features, having a diameter of 280 µm, represent the photolyzed/phosphate functionalized regions and are separated by 50 µm dark areas of nonphototransformed nitrate-functionalized regions. Further applications of these OH-bearing surface films and their photopatterning capabilities are currently under investigation in our laboratories. Acknowledgment. We gratefully acknowledge the loan of photochemical equipment from Professor Robert G. Salomon (CWRU, Chemistry). We thank Dr. Yongwoo Lee for preliminary experiments and Professors Arthur H. Heuer and Mark R. DeGuire (CWRU, Material Science and Engineering) for many useful discussions. Financial support was provided by the Air Force Office of Scientific Research. LA9601566 (9) The specificity of the phosphorylation reaction was demonstrated by a control experiment using an unirradiated nitrate-functionalized monolayer and POCl3. The water contact angle of this sample remained unchanged and analysis by XPS showed the nitrate N(1s) signal at 408 eV and no signal in the P(1s) region. (10) Lopez, G. P.; Biebuyck, H. A.; Whitesides, G. M. Scanning Electron Microscopy Can Form Images of Patterns in Self-Assembled Monolayers. Langmuir 1993, 9, 1513.
