Langmuir 1997, 13, 2027-2032
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Functionalization of Alkylsiloxane Monolayers via Free-Radical Bromination Murray V. Baker* and Jason D. Watling Department of Chemistry, University of Western Australia, Nedlands, Western Australia 6907, Australia Received September 30, 1996. In Final Form: January 2, 1997X When hexadecylsiloxane monolayers are immersed in solutions of bromine in carbon tetrachloride and irradiated with a tungsten lamp, C-H bonds near the surface of the monolayer undergo free-radical bromination. The bromine introduced into the monolayers can easily be substituted by small nucleophiles. Properties of brominated hexadecylsiloxane monolayers are generally similar to those of monolayers prepared from 16-bromohexadecyltrichlorosilane, but with minor differences arising due to different amounts and distributions of bromine within the two types of monolayers.
Introduction Self-assembled monolayers have many potential applications in modern technology. Uses of self-assembled monolayers range from the control of properties of surfaces1,2 (adhesion, wettability, etc.) to the fabrication of new materials for sensors3 and nonlinear optical devices.4 Development in these fields depends upon the availability of self-assembled monolayers bearing reactive functional groups at their surfaces. These reactive functional groups are necessary to elaborate monolayers into multilayer films (for new materials) or anchor molecular fragments to the monolayer surface (for device applications). For practical applications, alkylsiloxane monolayers are of particular interest because they are physically and chemically robust.5 These monolayers are usually prepared by reaction of alkyltrichlorosilanes with substrates bearing hydrated oxide surfaces.5,6 Compared to many other types of monolayers, however, alkylsiloxane monolayers are experimentally difficult to prepare. This difficulty arises, at least in part, because the water that is required for the formation of alkylsiloxane monolayers5-7 also promotes the formation of oligomeric siloxanes.5,8 Adsorption of these oligomeric siloxanes to the surface of the substrate or an assembling monolayer can prevent the formation of high-quality self-assembled alkylsiloxane monolayers. In practice, the preparation of alkylsiloxane monolayers is reasonably straightforward when the alkyl group of the alkyltrichlorosilane precursor is an unbranched, saturated hydrocarbon chain. Moaz and Sagiv6 developed procedures for preparing self-assembled monolayers from longchain alkyltrichlorosilanes in the presence of relatively large amounts of water, but these procedures do not work well when alkyltrichlorosilanes with short alkyl chains are used. Whitesides and his co-workers5 prepared selfassembled monolayers from alkyltrichlorosilanes containX Abstract published in Advance ACS Abstracts, February 1, 1997.
(1) Tidswell, I. M.; Rabedeau, T. A.; Pershan, P. S.; Folkers, J. P.; Baker, M. V.; Whitesides, G. M. Phys. Rev. B 1991, 44, 869-879. (2) Wasserman, S. R.; Biebuyck, H.; Whitesides, G. M. J. Mater. Res. 1989, 4, 886-892. (3) Hickman, J. J.; Ofer, D.; Laibinis, P. E.; Whitesides, G. M.; Wrighton, M. S. Science 1991, 252, 688-691. (4) Yitzchaik, S.; Marks, T. J. Acc. Chem. Res. 1996, 29, 197-202. (5) Wasserman, S. R.; Tao, Y.-T.; Whitesides, G. M. Langmuir 1989, 5, 1074-1087. (6) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92-98. (7) Silberzan, P.; Leger, L.; Ausserre, D.; Benattar, J. J. Langmuir 1991, 7, 1647-1651. (8) Ulman, A. Adv. Mater. 1990, 2, 573-582.
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ing either long or short alkyl groups, using a procedure involving only traces of water in the silanizing solutions. Both routes to self-assembled monolayers are less successful when the alkyl component of the silane bears a functional group. When functional groups are present in the silane, self-assembled films often form with greater than monolayer thickness and require lengthy washing procedures involving hot solvents,9 alkaline detergents,10 and even mechanical abrasion of the monolayer surface with a soft brush.11 In addition to this problem, the range of functional groups which may be chosen is limited, since many groups (e.g., amines, alcohols, carboxylic acids) are not compatible with the trichlorosilane group. We have developed a new route to the preparation of alkylsiloxane monolayers that contain reactive functional groups. This route begins with hexadecylsiloxane monolayers, which are easily prepared by treatment of silicon substrates with hexadecyltrichlorosilane according to published procedures.5,6 These monolayers contain a densely-packed array of fully trans-extended hexadecyl chains.5,8 We have found12 that irradiation of hexadecylsiloxane monolayers in the presence of bromine results in free-radical bromination of some of the C-H bonds near the surfaces of the monolayers. In this paper, we describe in detail the bromination procedure and compare the physical and chemical characteristics of brominated hexadecylsiloxane monolayers with those of monolayers assembled from 16-bromohexadecyltrichlorosilane.13 Experimental Section Dichloromethane was distilled from anhydrous CaCl2. Hexadecane (Aldrich) and dimethylformamide (DMF) were percolated through alumina (Fluka, neutral, Type 507C). Dimethyl sulfoxide (DMSO) was distilled from CaH2 and percolated through alumina. Ethanol was redistilled prior to use. Hexadecyltrichlorosilane (Hu¨ls America) was distilled under vacuum prior to use. 16Bromohexadecyltrichlorosilane was prepared according to the procedure of Balachander and Sukenik.14 (9) Offord, D. A.; Griffin, J. H. Langmuir 1993, 9, 3015-3025. (10) Tillman, N.; Ulman, A.; Elman, J. F. Langmuir 1989, 5, 10201026. (11) Tillman, N.; Ulman, A.; Penner, T. L. Langmuir 1989, 5, 101111. (12) Baker, M. V.; Watling, J. D. Tetrahedron Lett. 1995, 36, 46234624. (13) Throughout this paper, monolayers prepared by photochemical bromination of headecylsiloxane monolayers are referred to as “brominated hexadecylsiloxane monolayers”, and monolayers prepared from 16-bromohexadecyltrichlorosilane are referred to as “16-bromohexadecylsiloxane monolayers”.
© 1997 American Chemical Society
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Measurement of Contact Angles. Contact angles were measured using a home-built goniometer with RameHart scope attachment. A Matrix Technologies micro Electrapette was used to dispense drops of probe liquid. A 1-5 µL drop of liquid was formed at the tip of the pipet dispenser. The dispenser was then lowered until the drop came in contact with the surface of the sample. While the drop was examined through the scope, liquid was added to the drop until its edges advanced across the surface uniformly, and when the drop edges came to rest, the advancing contact angle was measured. Some liquid was then withdrawn from the drop so that the drop edges contracted. When the drop edges came to rest, the receding contact angle was measured. Ellipsometry. Ellipsometric measurements were made using a Gaertner L117 production thin film ellipsometer fitted with a He-Ne laser (λ ) 6328 Å), with an angle of incidence of 70° and with the compensator set to +45°. Analyzer and polarizer readings were made in zones 2 and 4.15 These readings were used to calculate film thickness according to the algorithm of McCrackin.15 A computer program written by S. R. Wasserman (Harvard University) and modified by J. D. Watling (University of Western Australia) was used to do the calculations. Fourier Transform Infrared Spectroscopy. Monolayers to be characterized by infrared spectroscopy were formed on silicon wafers that had been polished on both sides. Fourier transform infrared (FTIR) spectra were obtained using a Bruker 66 FTIR single beam spectrophotometer equipped with a He-Ne laser and liquidnitrogen-cooled MCT detector. All measurements were taken in transmission mode, with the beam orientated normal to the surface of the sample. The spectrophotometer was maintained under constant nitrogen purge. After introduction of each new sample, and prior to acquisition of spectra, the sample chamber was purged with nitrogen for 20 min at a rate such that the volume of nitrogen introduced each minute was approximately equal to the volume of the chamber. For each sample, single beam spectra consisted of 10 000 coadded scans and were recorded at 4 cm-1 resolution. Immediately prior to acquisition of spectra for a given monolayer, a single beam spectrum was recorded for a freshly cleaned bare silicon substrate, and this spectrum was used as a background spectrum. For a given monolayer, an absorbance spectrum was obtained by subtraction of the single beam background spectrum from the single beam spectrum for the substrate bearing the monolayer. X-ray Photoelectron Spectroscopy. X-ray photoelectron spectra were obtained using a modified AEI DS100 X-ray photoelectron spectrometer. All spectra were obtained using a non-monochromatized Al KR line fixed collinear anode source. The spectrometer has a typical resolution of 1.5 eV, with anode voltage and power settings of 15 kV and 15 mA beam current, respectively. Typical operating pressure was 10-8 Torr. Survey scans were preformed at a 70° (“magic”) takeoff angle with a pass energy of 50 eV. In all cases, the observed relative intensities were determined from experimental peak areas and were normalized with atomic and instrument sensitivity factors). Deconvolution of complex peaks was performed using Kaleidagraph, with Gaussian lineshapes used to describe individual components of peaks. Assembly of Monolayers. Hexadecane was saturated with water by gentle stirring with 1/20 of its volume of deionized water in a glass bottle for 3 days. About 20 mL (14) Balachander, N.; Sukenik, C. N. Langmuir 1990, 6, 1621-1627. (15) McCrakin, F. L.; Passaglia, E.; Stromberg, R. R.; Steinberg, H. L. J. Res. Natl. Bur. Stand., Sect. A 1963, 67, 363-367.
Baker and Watling
of water-saturated hexadecane was carefully decanted into a 25 mL sample vial, the vial was moved into a nitrogenfilled drybox, and 1 drop (ca. 40 µmol) of the appropriate alkyltrichlorosilane was added. The vial was capped, shaken briefly, and removed from the drybox, and the silanizing solution was used immediately. Silicon wafers (100 orientation, Silicon Sense Inc.) were cut into 3 cm × 1 cm strips, placed into 25 mL sample vials, and covered with freshly-prepared 1:1 mixture of 30% hydrogen peroxide and concentrated sulfuric acid. (CAUTION: this mixture reacts violently with many materials and should be handled with care.) After 1-2 h, the liquid was decanted from the sample vials and replaced with deionized water, and this rinsing process was repeated 10 times. The substrates were then removed (Teflon-coated tweezers), blown dry under a stream of nitrogen, and within 2 min were placed in freshly prepared silanizing solutions. After 3 min the wafers were removed and rinsed successively in 25 mL vials of CH2Cl2 and ethanol, before being rinsed under a stream of ethanol and blown dry under a stream of nitrogen. Solution-Phase Bromination of Hexadecylsiloxane Monolayers. In a typical bromination experiment, a silicon substrate bearing a hexadecylsiloxane monolayer was placed face down in a solution of bromine (20 mM) in carbon tetrachloride in a 100 mL Pyrex round bottom flask under nitrogen. The sample was irradiated for 6 h, using a 150 W tungsten lamp positioned 4 cm below the bottom of the flask. During this time, the heat from the lamp warmed the solution to about 50 °C. After irradiation, the sample was removed, washed with ethanol, and dried under a stream of nitrogen. Vapor-Phase Bromination Experiments. In a typical bromination experiment, a silicon substrate bearing a hexadecylsiloxane monolayer was placed face down in a two-neck 250 mL Pyrex round bottom flask. One neck of the flask was sealed with a rubber septum, and the other neck was connected to a vacuum line. The flask was evacuated to a pressure of 0.01 Torr, 10 µL of bromine was introduced to the flask via syringe, and the sample was irradiated as described above. After irradiation, the bromine was removed under vacuum and the flask was backfilled with nitrogen. The brominated wafer was removed, washed with ethanol, and dried under a stream of nitrogen. Functional Group Transformations in Monolayers. Displacement of Bromide by Azide, Sulfide, and Thiocyanate. Silicon substrates bearing brominated hexadecylsiloxane and 16-bromohexadecylsiloxane monolayers were immersed in 0.15 M solutions of NaN3, Na2S, or KSCN in DMF or DMSO in a round bottom flask under nitrogen. The solutions were gently stirred, with care being taken to prevent the Teflon-coated stirrer bar from coming into contact with the substrates. After 4 days the wafers were removed, washed with ethanol, and blown dry under a stream of nitrogen. Mercury-Assisted Replacement of Bromide by Hydroxide.16 A mixture of HgO (2.15 g) and 60% HClO4 (2.5 mL) in dimethoxyethane (20 mL) was gently warmed until all the HgO was dissolved. Distilled water (2.0 mL) was added, followed by a substrate bearing a brominecontaining monolayer, and the mixture was stirred gently at room temperature for 3 h. The substrate was then removed, washed with ethanol, and blown dry under a stream of nitrogen. Attempted Displacement of Bromide by Alkylamines and Alkanethiols. Silicon substrates bearing bromine-containing monolayers were immersed in solu(16) McKillop, A.; Ford, M. E. Tetrahedron 1974, 30, 2467-2475.
Functionalization of Alkylsiloxane Monolayers
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Figure 1. Idealized structures for (a) a hexadecylsiloxane monolayer, (b) a brominated hexadecylsiloxane monolayer, and (c) a 16-bromohexadecylsiloxane monolayer. Table 1. Contact Angles and Thicknesses of Hexadecylsiloxane, Brominated Hexadecylsiloxane, and 16-Bromohexadecylsiloxane Monolayers water contact angles (deg)
hexadecane contact angles (deg)
monolayer
advancing
receding
advancing
receding
thickness (Å)
hexadecylsiloxane brominated hexadecylsiloxane 16-bromohexadecylsiloxane
114 82 88
108 65 78
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