Article pubs.acs.org/Langmuir
Byproduct-Free Route to Aminosiloxane Monolayers on Silicon/ Silicon Dioxide Kiran Khadka,† Nicholas C. Strandwitz,‡ and Gregory S. Ferguson*,†,‡ †
Department of Chemistry and ‡Department of Materials Science & Engineering, Lehigh University, Bethlehem, Pennsylvania 18015, United States S Supporting Information *
ABSTRACT: The chemisorption of N-methyl-aza-2,2,4-trimethylsilacyclopentane from either the solution or the vapor phase produces monolayer films on silicon (oxide) substrates. The formation of a covalent siloxane linkage to the surface by this adsorbate is accompanied by ring opening, which produces no byproduct. The resulting secondary amine reacts with maleic anhydride to produce a carboxylic acid-terminated surface, accompanied by the formation of a stable amide bond. These reactions and their products were characterized by a combination of optical ellipsometry, contact-angle goniometry, and X-ray photoelectron spectroscopy.
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INTRODUCTION Self-assembled monolayers (SAMs)1,2 containing amine functional groups have been used as coupling agents to promote adhesion at silicon (oxide)/polymer interfaces,3−8 for example, in glass-fiber-reinforced composites and the polymer encapsulation of microelectronics.9 Clean reactions with no byproducts that could remain on the modified surfaces would be advantageous in cases for which contamination by the byproduct would be detrimental.10−17 Widely used aminosilanes, such as 3-aminopropyltriethoxysilane (APS), 3-aminopropyldimethylethoxysilane (APDMES), and their trimethoxy derivatives, produce ethanol or methanol as a byproduct upon reaction with surface silanol or with water.11,14,18 In addition, the amine group is sufficiently basic to react, or at least interact, with silanols on the surface.18 Previously, we reported a multistep method for producing an ordered amine-terminated SAM on Si/SiO2 and its subsequent reaction for preparing a well-defined silicon (oxide)/polyimide interface.8 In this article, we describe a one-step preparation of aminecontaining monolayers on Si/SiO2 that produces no byproducts. A cyclic azasilane, N-methyl-aza-2,2,4-trimethylsilacyclopentane, reacts to form a ring-opened product covalently bound to the surface through a siloxane linkage, thereby replacing a silicon−nitrogen bond with a stronger silicon− oxygen bond.12,13 This particular adsorbate was chosen on the basis of two features that aided in interpreting its reactivity: it has only nonhydrolyzable methyl substituents on the silicon atom and its ring-opened product contains a single amine group. Other azasilanes are also commercially available. Most of our studies focused on adsorption from solution, though this compound is sufficiently volatile (b.p. 137 °C) to allow adsorption from the vapor phase as well.19−27 Subsequent © XXXX American Chemical Society
reactivity of the surface-bound amine was tested by examining its reaction with maleic anhydride to form the corresponding maleamide.
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EXPERIMENTAL SECTION
General. P-type (boron) silicon (100) wafers (725 ± 25 μm, 1− 100 Ω·cm resistivity) were used as received from Akrion Systems. NMethyl-aza-2,2,4-trimethylsilacyclopentane (>95%) was used as received from Gelest. Toluene (99.9%, Omni-Solv) and ethanol (190 proof, Koptec) were used as received. Compressed N2 was used as received from Praxair. Tetrahydrofuran (THF, 99%, Mallinckrodt) was purified and dried with a Pure Solv system (Innovative Technology). Hydrogen peroxide (30%), phosphoric acid (85%), sodium phosphate monobasic (ACS grade), sodium phosphate dibasic (ACS grade), and sodium carbonate (ACS grade) were used as received from Fisher Scientific. Sodium phosphate tribasic (Mallinckrodt), sodium acetate (Sigma), and sodium bicarbonate (EM) were used as received (all ACS grade). Water was purified with a Millipore Simplicity UV system (18.2 MΩ·cm). Hydrochloric acid (HCl, 38%) and sulfuric acid (95%) were used as received from Macron. Solution-Phase Adsorption of N-Methyl-aza-2,2,4-trimethylsilacyclopentane. Silicon wafers were cut into ∼1 × 2 cm2 pieces, cleaned in piranha solution, and then rinsed with deionized 18.2 MΩ· cm water and blown dry with a stream of N2. (Caution! Piranha solution, a 7:3 (v/v) mixture of concentrated H2SO4 and 30% H2O2, reacts violently with organic material and should be handled caref ully.) An ∼0.03 M adsorbate solution was prepared in a 20 mL scintillation vial by dissolving N-methyl-aza-2,2,4-trimethylsilacyclopentane (0.10 g, 0.70 mmol) in 20 g of toluene. The solution was used within 15 min of its preparation. A clean Si/SiO2 substrate was placed with its polished side facing downward and tilted against the side of the vial into the Received: December 8, 2016 Revised: January 23, 2017 Published: January 25, 2017 A
DOI: 10.1021/acs.langmuir.6b04415 Langmuir XXXX, XXX, XXX−XXX
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electrons was used to hold the nonmetallic (Si) samples at a constant potential for an accurate assessment of the binding energy. Spectra were analyzed with Casa XPS software, and photoemission was modeled using a linear background and the minimum number of peaks necessary to provide an adequate fit of the data. For quantification, areas under the peaks (or envelopes) were corrected by applying the Scofield cross sections for photoionization (C 1s, 1.00; N 1s, 1.80) as relative sensitivity factors.31
adsorbate solution. Aside from the time-dependent study, the samples were allowed to react for at least 16 h. The sample was then removed, rinsed with toluene and then ethanol for at least five times each, and then rinsed with deionized 18.2 MΩ·cm water and blown dry with a stream of nitrogen. After rinsing with water, beads of water on the surface would typically fall off when the sample was held vertically. Samples with droplets that adhered, indicating inhomogeneous surfaces, were discarded; this occurred on less than ∼5% of the samples. Vapor-Phase Adsorption of N-Methyl-aza-2,2,4-trimethylsilacyclopentane. A Cambridge Nano Savannah 100 model ALD was used to deliver the azasilane onto piranha-cleaned silicon wafers.29 The vapor-phase treatments were performed with samples inside the ALD vacuum chamber held at 80 °C with the polished side of the wafers facing up. Three pulses of water for 0.1 s each with 5 s between each pulse were first used, followed by two pulses of azasilane for 2 s each and 10 s between pulses, inside the ALD chamber. Immediately after these treatments, the thickness of the added layer and contact angles on the surface were measured. Secondary Reaction with Maleic Anhydride. The aminosiloxane surface was treated with a freshly prepared 3.0 M solution of maleic anhydride (2.94 g, 30.0 mmol) in anhydrous THF (10 mL). Aside from initial kinetics studies, samples were allowed to react for at least 6 days to ensure complete reaction. After treatment, the sample was rinsed with THF and then ethanol, at least five times each, and rinsed with 18.2 MΩ·cm deionized water and dried with a stream of nitrogen. Variable-Angle Spectroscopic Ellipsometry. The thickness of added layers was determined by measuring the ellipsometric parameters, Ψ and Δ, for at least three locations using a J.A. Woollam V-VASE instrument. Data were collected between 350 and 800 nm at 50 nm intervals with angles of incidence of 60 and 70°. Thicknesses were calculated by fitting these parameters to an optical model using WVASE 32R analysis software, and the averages of the measurements are reported. The model consisted of silicon of optically infinite thickness terminated by a native oxide measured after cleaning with piranha solution. The layer formed on top of native oxide was modeled as a Cauchy layer to determine its thickness. Contact-Angle Measurements and Titrations. Buffered aqueous solutions (0.01 M) were used for measuring contact angles of water, except for pH values below 2. The buffers were prepared according to a literature method:30 pH 0−1, HCl; pH 2−3, phosphoric acid/sodium phosphate monobasic; pH 4−5, acetic acid/sodium acetate; pH 6−8, sodium phosphate monobasic/sodium phosphate dibasic; and pH 9−11, sodium bicarbonate/sodium carbonate. The pH of the solutions was verified with an Orion 2 star benchtop pH meter and a VWR symphony probe. The probe was calibrated with standard buffers (Fluka, as received) at pH 4.0, 7.0, and 10.0 on each day of use. Advancing contact angles were measured with a Rame-Hart NRL model 100 goniometer, with the sample inside an environmental chamber (model 2030). Kimwipes soaked with deionized water were placed in the wells of the chamber to maintain the humidity at nearly 100%. Samples were allowed to equilibrate in this atmosphere for at least 10 min before contact angles were measured. For each sample, a minimum of five measurements on both sides of independent drops at five different spots were made, and their average is reported. Error bars indicate values within one standard deviation of the average. X-rays Photoelectron Spectroscopy (XPS). A Thermo Scientific K-Alpha instrument located in the Department of Physics and Astronomy at Rutgers University was used to perform all of the XPS measurements. The spectrometer utilizes monochromatized X-rays from an Al Kα source, with detection using a concentric hemispherical electron analyzer with a six-channel electron detector, allowing the accuracy of relative peak positions to within less than 0.1 eV. Highresolution scans were performed with a pass energy of 50 eV, an energy step size of 0.1 eV, and a dwell time at each energy of 50 ms. For survey spectra, these parameters were 200 eV, 1 eV, and 50 ms, respectively. Reported binding energies were calibrated on the basis of the Au 4f7/2 peak of a clean gold sample. Because of the intensity of the X-ray beam, a dual-beam flood gun producing low-energy Ar+ ions and
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RESULTS AND DISCUSSION The immersion of clean silicon (Si/SiO2) wafers in an ∼0.03 M solution of N-methyl-aza-2,2,4-trimethylsilacyclopentane in toluene resulted in the adsorption of a thin layer of aminosiloxane at the surface. The cyclic azasilane may react directly with silanol groups on the surface, as shown in Figure 1. It is also possible, however, that it may react initially with
Figure 1. Ring-opening adsorption of N-methyl-aza-2,2,4-trimethylsilacyclopentane on Si/SiO2 to form a monolayer film.
water in solution or near the surface, followed by condensation to form siloxane linkages to the substrate.32 The kinetics of adsorption were measured by monitoring the contact angles of both neutral and acidic water (pH 1.9) as a function of time during the adsorption (Figure 2). In these experiments, a
Figure 2. Advancing contact angle of neutral (filled circles) and acidic water, pH 1.9 (open circles), as a function of the time the Si/SiO2 substrate was immersed in an ∼0.03 M solution of N-methyl-aza-2,2,4trimethylsilacyclopentane in toluene at room temperature.
separate sample was pulled from the adsorbate solution, and the contact angles were measured for each set of points at a given time of immersion. The gradual divergence of the contact angles at neutral and low pH is consistent with the addition of secondary amines, which are Brønsted bases, in the ring-opened product upon adsorption (Figure 1). Limiting values of the contact angles were reached within approximately 5 h. The eventual hysteresis on these surfaces (after 5 h) was ∼20° (neutral pH) and 13° (pH, 1.9), consistent with some reconstruction of the surface under the drop of probe liquid (vide infra). The thickness of the adsorbed layer, measured ellipsometrically, increased immediately (≤1 s of immersion) to ∼3 Å and then remained approximately constant. This thickness is lower than would be expected for a compact B
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Langmuir monolayer of the siloxane product in a chain-extended conformation (∼8 Å) and instead indicates an absence of order in the monolayer film. Disorder in the monolayer could result, for example, from hydrogen bonding between the secondary amine of the product and silanol groups on the surface of the Si/SiO2.17,18 Atomic force micrographs (AFM images) of the silicon wafer before and after chemisorption of the monolayer showed no significant differences in surface topography, indicating the addition of a uniform film rather than large isolated islands of the adsorbate (Supporting Information). To examine the Brønsted basicity of the modified surface in more detail,33 we measured the contact angle of water as a function of pH between 1 and 11 (Figure 3) on samples
Figure 3. Contact-angle titration of a monolayer film prepared by adsorption on Si/SiO from a 0.03 M solution of N-methyl-aza-2,2,4trimethylsilacyclopentane in toluene for at least 16 h.
prepared by adsorption for at least 16 h. At low pH (∼1−2), the surface was relatively hydrophilic, consistent with the protonation of the secondary amines to form charged ammonium ions. Between pH 2 and 6, the contact angle increased from ∼43 to ∼67°, consistent with a gradual shift in the surface population from the protonated to unprotonated forms of the amine. The breadth of contact-angle titration curves is usually larger than for solution titrations because of neighboring group effects (e.g., electrostatic, H-bonding). Between pH 7 and 11, the contact angle increased only modestly. In the course of these studies, we found that samples stored in pure water or in a closed, air-filled vial showed no significant change in contact angle measured with neutral water over about 1 week.15−18,32,34,35 To characterize the composition of the surface, X-ray photoelectron spectra were collected at takeoff angles of 90 and 30° between the plane of the surface and the detector. Measurements made at 30° are more sensitive to the outermost atoms of the sample, whereas those at 90° more fully sample the composition throughout the interfacial region. A survey scan revealed the presence of Si, O, C, and N near the surface, consistent with expectations for these samples. High-resolution spectra in the C 1s and N 1s regions (Figure 4) were also consistent with the product shown in Figure 1. We assign the peak at ∼285 eV in the C 1s region to a combination of photoemission from aliphatic carbons and those bound to the silicon atoms of the aminosiloxanes.36−38 The peak at ∼286 eV is consistent with carbon bound to amine or protonated amine.36−38 The minor peak at 287.6−287.9 eV is assigned to impurities on the amine surface.39 No peak corresponding to carbamate carbon at 289 eV (or higher binding energy) was
Figure 4. High-resolution XPS spectra in the C 1s (left) and N 1s (right) regions for a sample prepared by the adsorption of N-methylaza-2,2,4-trimethylsilacyclopentane from a 0.03 M solution in toluene onto Si/SiO2. The takeoff angle was 90° (top) or 30° (bottom).
observed.40,41 We assign the primary photoemission in the N 1s region (399.4−399.5 eV) to the secondary amine of the product.36−38,40,41 A small component at ∼401.4−401.5 eV is assigned to protonated or hydrogen-bonded amine.36−38,40,41 The ratio of amine carbon to amine nitrogen is ∼2 for data collected at both takeoff angles, consistent with the expected product. Chemical derivatization is a valuable method for verifying the assignments of chemical structure by connecting spectroscopic measurements of composition to the characteristic reactivity of the proposed product structure. To examine the chemical reactivity of the aminosiloxane surface, in particular, its nucleophilicity relevant to its potential use as a coupling agent,4,6,7 we studied its reaction with a model electrophile, maleic anhydride (Figure 5). Organic anhydrides are common reactants in polymerization and coupling reactions, particularly those producing polyamides and polyimides.7,8 The progress of this reaction was followed by measuring the contact angle of water at pH 10.0 as a function of time, with reported values measured on stable drops after ∼1−2 min of placement. The surface gradually became more hydrophilic as the reaction proceeded until after 6 days it was wet by this buffer (Figure 6). We caution that these measurements do not necessarily reflect C
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monolayer,42 as the hydrophilic carboxylate anion would be expected to predominate at this pH. During these studies, we noticed that after the application of a drop of probe liquid on the acid-terminated surface the contact angles required some time to become constant enough to measure reproducibly. The initial angles decreased over a period of ∼1−2 min; examples of these data are shown in Figure 7. This dynamic behavior indicates that the surfaces reconstruct under basic drops as ionization of the acid occurs, and the charged carboxylic anion migrates to the monolayer/ water interface.19 This reconstruction of the surface under basic drops was reversible, with the surface recovering its original wettability once those drops were removed. A contact-angle titration, measured on stable drops, was consistent with an acidterminated surface as the product, with the onset of ionization occurring at pH ∼4−5 (Figure 8).42
Figure 5. Reaction of the aminosiloxane surface with maleic anhydride.
Figure 6. Advancing contact angle of basic water (pH 10.0) as a function of the time of immersion of a Si/SiO2 substrate bearing an aminosiloxane monolayer in a 3 M solution of maleic anhydride in THF.
the kinetics of the chemical reaction occurring at the surface because the contact angle may not be a linear function of the mole fraction of product on the surface. Nonetheless, the time required to reach the limiting wettability is a useful measure of the amount of time required for the reaction to reach completion. The large decrease in contact angle at high pH is consistent with conversion to a carboxylic acid-terminated
Figure 8. Contact-angle titration of a surface formed by treatment of the amine-terminated monolayer with maleic anhydride for at least 6 days.
The product of this amidation reaction was characterized by XPS.38,43,44 A high-resolution C 1s spectrum at a 90° takeoff
Figure 7. Change in the contact angle of aqueous buffer (pH 10.0) immediately after the application of a droplet on aminosiloxane samples that had been treated with maleic anhydride for (a) 4, (b) 5, (c) 6, and (d) 7 days. Each color represents an individual drop, monitored over time. Contact angles of 10° or below were considered to be wetting. D
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Langmuir angle could be fit by three peaks centered at ∼284.4, ∼ 285.5, and 286.6 eV, assigned to the noncarbonyl carbon bound to Si, C, and N, respectively, and at ∼288.8 eV, assigned to carbonyl carbons of the amide and carboxylic acid groups (Figure 9).36−38 Data collected in this region at a 30° takeoff angle were
rinsed with toluene and then ethanol at least 5 times each and then rinsed with deionized (18.2 MΩ·cm) water and blown dry with a stream of nitrogen. The thickness of the adsorbed layer, measured ellipsometrically, was ∼3 Å, consistent with values found for adsorption from solution. The contact angle of neutral water was 69°, and that of acidic water (pH 1.9) was 46°. This difference is somewhat smaller than that measured on samples adsorbed from solution (72 and 44°). To verify the composition of the surface, X-ray photoelectron spectra were collected at takeoff angles of 90 and 30°. A survey scan contained photoemission from Si, C, N, and O, consistent with the expected product of adsorption (Figure 1). Highresolution spectra in the C 1s and N 1s regions (Figure 10)
Figure 9. High-resolution XPS spectra in the C 1s (left) and N 1s (right) regions for a sample prepared by the reaction of an aminosiloxane surface with maleic anhydride (3 M in THF) for 12 days. The takeoff angle was 90° (top) or 30° (bottom).
similar, though the photoemission from carbonyl carbons was less prominent than found at the higher takeoff angle. We interpret this difference as an indication that the carboxylic acid groups tend to lie beneath the noncarbonyl carbons, which could be a consequence of hydrogen bonding or may simply reflect the configuration of lowest interfacial free energy. Data collected in the N 1s region were similar at both 90 and 30° takeoff angles, with a large peak at ∼400.0 eV due to the amide nitrogen (and any unreacted amine) and a very small peak at ∼402.5 eV due to protonated residual amine,36−38,40,41 comprising ∼10% of the total nitrogen photoemission (Figure 9). For comparison, we also examined the adsorption of Nmethyl-aza-2,2,4-trimethylsilacyclopentane from the vapor phase onto Si/SiO2. Clean substrates were pulsed in an ALD chamber first with water vapor and then with N-methyl-aza2,2,4-trimethylsilacyclopentane, which resulted in the adsorption of an aminosiloxane film at the surface. Prior to measurements of thickness and wettability, the samples were
Figure 10. High-resolution XPS spectra in the C 1s (left) and N 1s (right) regions for a sample prepared by the vapor adsorption of Nmethyl-aza-2,2,4-trimethylsilacyclopentane onto Si/SiO2. The takeoff angle was 90° (top) or 30° (bottom).
were consistent with the product shown in Figure 1, as well as with that produced by adsorption from solution. We assign the peak at ∼285 eV to a combination of photoemission from aliphatic carbons and those bound to the silicon atoms of the aminosiloxanes. The peak at ∼286.4 eV is consistent with carbon bound to amine or protonated amine.37 Photoemission at 287.6−287.9 eV that was assigned to impurities on the sample adsorbed from solution (Figure 4) was absent in the spectrum of the vapor-phase sample (Figure 10). HighE
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(5) O’leary, L. E.; Johansson, E.; Lewis, N. S.; Chemical Stability of Organic Monolayers Formed in Solution. In Functionalization of Semiconductor Surfaces; Tao, F.; Bernasek, S. L., Eds.; John Wiley & Sons: Hoboken, NJ, 2012; pp 339−399. (6) Onclin, S.; Ravoo, B.; Reinhoudt, D. Engineering silicon oxide surfaces using self-assembled monolayers. Angew. Chem., Int. Ed. 2005, 44, 6282−6304. (7) Haensch, C.; Hoeppener, S.; Schubert, U. S. Chemical modification of self-assembled silane based monolayers by surface reactions. Chem. Soc. Rev. 2010, 39, 2323−2334. (8) Lee, M.-T.; Ferguson, G. S. Stepwise synthesis of a well-defined silicon (oxide)/polyimide interface. Langmuir 2001, 17, 762−767. (9) Aswal, D. K.; Lenfant, S.; Guerin, D.; Yakhmi, J. V.; Vuillaume, D. Self assembled monolayers on silicon for molecular electronics. Anal. Chim. Acta 2006, 568, 84−108. (10) Semiconductor Industry Association - International Technology Roadmap for Semiconductors (ITRS) http://www.semiconductors. org/clientuploads/directory/DocumentSIA/ITRS_2011ExecSum.pdf (accessed 3/24/2016, 2016). (11) McGovern, M. E.; Kallury, K. M. R.; Thompson, M. Role of solvent on the silanization of glass with octadecyltrichlorosilane. Langmuir 1994, 10, 3607−3614. (12) Arkles, B.; Pan, Y.; Larson, J.; Berry, H. D. Cyclic Azasilanes: Volatile Coupling Agents for Nanotechnology. In Silanes and Other Coupling Agents; Mittal, K., Ed.; VSP (Brill), 2004; Vol. 3, pp 179−191. (13) Maddox, A. F.; Matisons, J. G.; Singh, M.; Zazyczny, J.; Arkles, B. Single molecular layer adaption of interfacial surfaces by cyclic azasilane ″click-chemistry. MRS Online Proc. Libr. 2015, 1793, 35−40. (14) White, L. D.; Tripp, C. P. Reaction of (3-aminopropyl) dimethylethoxysilane with amine catalysts on silica surfaces. J. Colloid Interface Sci. 2000, 232 (2), 400−407. (15) Vandenberg, E. T.; Bertilsson, L.; Liedberg, B.; Uvdal, K.; Erlandsson, R.; Elwing, H.; Lundström, I. Structure of 3-aminopropyl triethoxy silane on silicon oxide. J. Colloid Interface Sci. 1991, 147, 103−118. (16) Howarter, J. A.; Youngblood, J. P. Optimization of silica silanization. Langmuir 2006, 22, 11142−11147. (17) Kim, J.; Seidler, P.; Wan, L. S.; Fill, C. Formation, structure, and reactivity of amino-terminated organic films on silicon substrates. J. Colloid Interface Sci. 2009, 329, 114−119. (18) Smith, E. A.; Chen, W. How to prevent the loss of surface functionality derived from aminosilanes. Langmuir 2008, 24, 12405− 12409 and references cited therein.. (19) Ferguson, G. S.; Chaudhury, M. K.; Biebuyck, H. A.; Whitesides, G. M. Monolayers on disordered substrates: self-assembly of alkyltrichlorosilanes on surface-modified polyethylene and poly (dimethylsiloxane). Macromolecules 1993, 26, 5870−5875. (20) Chaudhury, M. K.; Whitesides, G. M. Direct measurement of interfacial interactions between semispherical lenses and flat sheets of poly(dimethylsiloxane) and their chemical derivatives. Langmuir 1991, 7, 1013−1025. (21) Chaudhury, M. K.; Whitesides, G. M. Correlation between surface free energy and surface constitution. Science 1992, 255, 1230− 1232. (22) Ferguson, G. S.; Chaudhury, M. K.; Sigal, G. B.; Whitesides, G. M. Contact adhesion of thin gold films on elastomeric supports: cold welding under ambient conditions. Science 1991, 253, 776−778. (23) Chaudhury, M. K.; Owen, M. J. Correlation between adhesion hysteresis and phase state of monolayer films. J. Phys. Chem. 1993, 97 (21), 5722−5726. (24) Haller, I. Covalently attached organic monolayers on semiconductor surfaces. J. Am. Chem. Soc. 1978, 100, 8050−8055. (25) Jonsson, U.; Olofsson, G.; Malmqvist, M.; Rönnberg, I. Chemical vapor deposition of silanes. Thin Solid Films 1985, 124, 117−123. (26) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. Fundamental studies of the chemisorption of organosulfur compounds on gold(111). Implications for molecular self-assembly on gold surfaces. J. Am. Chem. Soc. 1987, 109, 733−740.
resolution spectra collected at both takeoff angles in the N 1s region are also included in Figure 10. We assign the primary photoemission at 399.4 eV36−38,40,41 to the secondary amine of the expected product. Photoemission arising from protonated amine was absent for both takeoff angles.
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CONCLUSIONS The cyclic azasilane, N-methyl-aza-2,2,4-trimethylsilacyclopentane, reacts at the surface of Si/SiO2 from the solution or the vapor phase to produce an aminosiloxane monolayer without the formation of byproduct(s). Adsorption from solution produced a highly functionalized film bearing reactive secondary amines that could be used as anchors for further surface modification. Treatment with maleic anhydride led to the formation of the corresponding amide-carboxylic acid in high yield. Both XPS spectra and the pH dependence of the advancing contact angle of water on both the amine- and carboxylic acid-terminated surfaces were consistent with the structures of the expected products.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b04415. Atomic force microscopy images of a clean silicon wafer and a clean silicon wafer bearing a monolayer absorbed from a solution of N-methyl-aza-2,2,4-trimethylsilacyclopentane (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Gregory S. Ferguson: 0000-0002-7510-3464 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge the Pennsylvania Infrastructure Technology Alliance (PITA, PA-DCED 000055676) for the support of this research and the National Science Foundation (CHE-0923370) for the funding to purchase a spectroscopic ellipsometer. We thank Gelest (Morrisville, PA) for the cyclic azasilane used in this study and Akrion Systems (Allentown, PA) for their donation of silicon wafers. We also thank Ling Ju for assistance with the ALD experiments and Professor Xiaoji Xu for help with the AFM measurements.
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REFERENCES
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DOI: 10.1021/acs.langmuir.6b04415 Langmuir XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.langmuir.6b04415 Langmuir XXXX, XXX, XXX−XXX
