Deposition of Dense Siloxane Monolayers from Water and

Jul 1, 2011 - Richard G. ClosserDavid S. BergsmanStacey F. Bent. ACS Applied Materials & Interfaces 2018 10 (28), 24266-24274. Abstract | Full Text ...
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Deposition of Dense Siloxane Monolayers from Water and Trimethoxyorganosilane Vapor Randall D. Lowe,† Matthew A. Pellow,‡ T. Daniel P. Stack,‡ and Christopher E. D. Chidsey*,‡ †

Department of Chemical Engineering and ‡Department of Chemistry, Stanford University, Stanford, California 94305, United States

bS Supporting Information ABSTRACT: A convenient, laboratory-scale method for the vapor deposition of dense siloxane monolayers onto oxide substrates was demonstrated. This method was studied and optimized at 110 °C under reduced pressure with the vapor of tetradecyltris(deuteromethoxy)silane, (CD 3O)3Si(CH2)13CH3, and water from the dehydration of MgSO4 3 7H2O. Ellipsometric thicknesses, water contact angles, Fourier transform infrared (FTIR) spectroscopy, and electrochemical capacitance measurements were used to probe monolayer densification. The CD3 stretching mode in the FTIR spectrum was monitored as a function of the deposition time and amounts of silane and water reactants. This method probed the unhydrolyzed methoxy groups on adsorbed silanes. Excess silane and water were necessary to achieve dense, completely hydrolyzed monolayers. In the presence of sufficient silane, an excess of water above the calculated stoichiometric amount was necessary to hydrolyze all methoxy groups and achieve dense monolayers. The excess water was partially attributed to the reversibility of the hydrolysis of the methoxy groups.

’ INTRODUCTION We have developed a convenient procedure for vapor-depositing dense siloxane monolayers onto oxide surfaces and a Fourier transform infrared (FTIR) spectroscopic probe for monitoring the hydrolysis of methoxy groups during deposition. Siloxane monolayers have been used in several applications ranging from chromatography1 to the surface attachment of bioanalytical probes.2,3 These monolayers control the properties of oxide surfaces by introducing organic functionality.4 6 This is accomplished using silane monomers with headgroups that bond to the surface and cross-link among themselves and tailgroups that present desired chemical moieties. The ability to investigate and control the underlying interfacial chemistry allows the reproducible deposition of siloxane monolayers of controlled density suitable for specific applications. Siloxane monolayer formation is a multistep process beginning with organosilane monomers possessing one to three hydrolyzable groups, such as chloro or alkoxy groups.7 These hydrolyzable groups react directly with surface hydroxyl groups to form siloxane or other silicon to metal oxide linkages. The hydrolyzable groups also react with water to produce silanols on the organosilanes.8 These silanols can undergo condensation reactions with surface hydroxyl groups or with silanols on neighboring organosilanes to form additional siloxane linkages and water as a byproduct. Monolayers formed from trifunctional silanes, such as the ones used in this study, have been described as two-dimensional network polymers with occasional bonds to the oxide surface.9 Because reaction of the hydrolyzable groups should be necessary to form surface-attached, two-dimensional network polymers, r 2011 American Chemical Society

the amount of water is known to be an important variable for the reproducible deposition of siloxane monolayers from trifunctional silane monomers. With insufficient water available, unhydrolyzed alkoxy groups on the silanes could sterically hinder dense monolayer formation. This would be similar to the case of monofunctional alkyldimethylsilanes where the bulky dimethylsilyl groups prevented tight alkane chain packing.10 Numerous studies have discussed the importance of water in depositing siloxane monolayers from solutions.4,11,12 When sufficient water was present, larger ellipsometric thicknesses10,13 and contact angles14 were measured and indicated the deposition of denser monolayers compared to those in more anhydrous cases. However, those measurements did not provide detailed information about the chemical bonds at the interface. X-ray photoelectron spectroscopy (XPS)10,14 16 and FTIR spectroscopy17,18 have been used to confirm the hydrolysis of Si Cl bonds during solution deposition of trichlorosilanes. Using FTIR spectroscopy, Pasternack et al. found unhydrolyzed ethoxy groups after solution deposition of triethoxysilanes under anhydrous conditions.19 However, the FTIR peaks assigned to residual C H stretching modes of the ethoxy groups were obscured by peaks associated with Si O Si and other C H stretching modes. In this paper, we describe a clearer method of monitoring the hydrolysis of alkoxy groups. Deuteration of hydrolyzable methoxy groups gives a C D stretching mode that is shifted into an unobscured region of the FTIR spectra. Received: April 12, 2011 Revised: June 29, 2011 Published: July 01, 2011 9928

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Langmuir Siloxane monolayers have been deposited from dilute solutions20 and from the vapor phase.21 Monolayer deposition from the vapor phase is attractive because there is less siloxane oligomer particle contamination than during solution-phase deposition.22 24 As in solution-phase deposition, the amount of water is an important variable for vapor-phase siloxane monolayer deposition. Vapor-phase deposition with 22,25 33 and without16,34 42 the deliberate addition of water vapor has been reported, but the effect of water addition has not been studied as thoroughly as in the solution case. In this paper, we present a simple method for vapor-depositing siloxane monolayers with controlled amounts of water vapor and show that dense siloxane monolayers are not deposited unless sufficient water is present to hydrolyze the methoxy groups and sufficient silane is present to densify the monolayers. The hydrolysis of tetradecyltris(deuteromethoxy)silane, (CD3O)3Si(CH2)13CH3, is monitored after vapor-phase deposition using FTIR spectroscopy on silicon oxide surfaces. Specifically, we study how the deposition duration and the initial amounts of water and silane reactants affect methoxy group hydrolysis and monolayer completion. Ellipsometric thicknesses, FTIR spectroscopy, water contact angles, and electrochemical capacitance measurements are used to probe monolayer densification.

’ EXPERIMENTAL SECTION Silane Synthesis. Tetradecyltris(deuteromethoxy)silane was prepared from n-tetradecyltrichlorosilane (Gelest) via methanolysis of the trichloro groups following published procedures for making similar alkoxysilanes.43,44 A 2.00 g mass of tetradecyltrichlorosilane (6 mmol) in 15 mL of dry pentane was slowly added to a solution of 0.67 g (19 mmol) of deuterated methanol, CD3OD (99.6+ atom % D, Acros), and 1.85 g (19 mmol) of dry pyridine in 75 mL of dry pentane with stirring under nitrogen, and the resulting solution was cooled in an ice water bath. The solution was allowed to stir overnight at room temperature under a nitrogen atmosphere, filtered to remove pyridinium chloride, and then concentrated by rotary evaporation. The crude product was purified by two successive vacuum distillations. See the Supporting Information for nuclear magnetic resonance (NMR) and FTIR characterization. Surface Cleaning. Double- and single-side-polished Si(100) wafers were used as received or after prior deposition experiments and cleaned in an oxygen plasma (GaLa Instrumente Plasma Prep5) for 10 min prior to deposition using 50% relative power with a dioxygen flow rate of 100 std. mL/min at a pressure of 0.25 mbar. These surfaces were exposed to atmospheric air for approximately 15 min after cleaning to determine effective refractive indices of the substrates with an ellipsometer. This cleaning method resulted in hydrophilic surfaces with water contact angles approaching 0°. Indium tin oxide (ITO) films supported on glass substrates had sheet resistances of 8 12 Ω (Delta Technologies). The ITO surfaces were plasma cleaned with the same procedure as the silicon oxide surfaces. Monolayer Vapor Deposition. Siloxane monolayers were vapordeposited onto silicon oxide and ITO surfaces in O-ring-sealed, glass vacuum desiccators fitted with Teflon stopcocks (Jencons part number 250-048) with internal volumes of about 600 mL (Figure 1). These chambers were oven-dried for a minimum of 4 h at 140 °C in air and cooled for no more than 10 min in air before use. Neat silane (100 μL) was pipetted onto and absorbed into 42.5 mm diameter Whatman filter paper in the bottom of the desiccator. Various masses of MgSO4 3 7H2O (Fisher biochemical grade) were placed in a foil boat in the bottom of the desiccator as a water source for the hydrolysis reaction. Plasma-cleaned surfaces were placed on a metal rack supported above the silane liquid

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Figure 1. Schematic of the glass vacuum chamber that was used for siloxane monolayer vapor depositions onto oxide surfaces from liquid silane and MgSO4 3 7H2O reactants. An incomplete monolayer with unhydrolyzed methoxy groups is shown in the schematic in the middle. A dense, completely hydrolyzed monolayer deposited in the presence of sufficient silane and MgSO4 3 7H2O is shown on the right. The drawing on the right is adapted from ref 8. Copyright 1991 Academic Press and hydrated salt in the glass vacuum chamber. The desiccator was then evacuated through a rubber hose on a glass vacuum line with a liquid nitrogen trapped mechanical pump for approximately 60 s. The final pressure at the trap was 1 Torr. The Teflon valve on the desiccator was closed, and the chamber was placed in a 110 °C preheated oven (Forma Scientific) for various periods of time from 1 to 24 h. After deposition, the valve was opened to ambient air to return the chamber to atmospheric pressure and remove the samples. Ashurst et al. used a related but more elaborate setup.25 Their chamber was modified with water and silane reactant inputs from external sources similar to other systems that have been used to introduce water during vapor depositions. Treatment of Monolayers with Methanol Vapor. Substrates onto which monolayers had been previously vapor-deposited were placed in a clean, dry desiccator chamber. The glass desiccator was evacuated to 1 Torr, and the Teflon valve was closed as in the monolayer deposition experiments. A rubber septum was then placed on the Teflon valve. The septum was then punctured with a needle on a syringe containing pure deuterated methanol or a premixed deuterated methanol water liquid solution. The Teflon valve on the vacuum desiccator was then opened, which evacuated the contents of the syringe into the desiccator under vacuum. The Teflon valve was then closed, and the rubber septum and syringe were removed from the top of the desiccator. The desiccator was then placed into a preheated 110 °C oven for 12 h. Surface Characterization. FTIR spectra were obtained with a Bruker Vertex 70 spectrometer using a KBr beam splitter and a deuterated triglycine sulfate (DTGS) detector. Spectra were collected in transmission mode through the silicon with the samples at the silicon Brewster angle (74°)45 using p-polarized light from a wire-grid polarizer (Specac KRS-5). For each spectrum, 1024 scans were collected at 4 cm 1 resolution. Background spectra were collected from freshly oxygen plasma cleaned, double-side-polished Si(100) crystals in the same orientation. Ellipsometry measurements were performed using a Gaertner L116 ellipsometer at a 70° angle of incidence. Effective real and imaginary refractive indices of the Si(100) substrates were determined immediately after plasma cleaning but before siloxane monolayer vapor deposition. Measurements after vapor deposition, rinsing with toluene and isopropyl alcohol, and drying in a stream of nitrogen were used to determine monolayer thicknesses. A film refractive index of 1.46 was used for all thickness calculations. At least three measurements were performed on each sample to obtain average substrate refractive indices or sample thicknesses. Advancing sessile drop water contact angles were measured with a Rame-Hart model 100 goniometer using water from a four-bowl Millipore purification system. Water droplets were dispensed from a syringe with a 9929

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Langmuir flat-tipped needle. Fresh water was obtained from the Millipore system for each measurement session. At least two measurements were made on each sample on both sides of the droplets and averaged. Cyclic voltammetry experiments were conducted using either a Bioanalytical Systems model CV-50W or a Pine Instrument Co. model AFCBP1 potentiostat. ITO surfaces with an exposed area of 0.21 cm2

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were used as the working electrodes in a three-electrode electrochemical cell. Platinum wire was used as the counter electrode, and all potentials measured were with respect to a Ag/AgCl/saturated KCl reference electrode. Cyclic voltammograms were collected over a scan range of 0.1 to +0.4 V versus the reference electrode (Figure S3 in the Supporting Information). The reported electrochemical capacitances were calculated by averaging the magnitudes of the currents measured at 0.1 V during the forward and reverse scans and then dividing by both the scan rate and the electrode area. Measurements were performed under ambient laboratory conditions using an electrolyte solution of 0.1 M NaClO4 in water.

’ RESULTS Time Dependence of the Ellipsometric Thickness, C D Stretch, and Water Contact Angle. Figure 2a is a plot of

Figure 2. Time dependence of 100 μL of (CD3O)3Si(CH2)13CH3 monolayer vapor deposition onto silicon oxide without and with 0.5 g of MgSO4 3 7H2O present monitored using the (a) ellipsometric thickness, (b) integrated area of the CD3 FTIR symmetric stretch, and (c) water contact angle. The lines are guides to the eye. Error bars represent 1 standard deviation.

ellipsometric thickness versus deposition time for monolayers vapor-deposited from 100 μL of tetradecyltri(deuteromethoxy)silane, without and with 0.5 g of MgSO4 3 7H2O present. At each time point, the ellipsometric thicknesses are greater with the hydrated salt than without. A limiting thickness is reached at 12 h in both cases, but at different values: 14.1 Å without MgSO4 3 7H2O and 18.2 Å with MgSO4 3 7H2O. Figure 3 shows that a peak is present at 2073 cm 1 in the FTIR spectrum after a 12 h deposition with 100 μL of silane but without MgSO4 3 7H2O. This peak is assigned to the symmetric CD3 stretching mode of the deuteromethoxy groups46,47 and is used to monitor methoxy group hydrolysis. With 0.5 g of MgSO4 3 7H2O present during deposition, the CD3 absorbance is no longer observed. Figure 2b shows the integrated absorbance of the CD3 mode at various deposition times. The CD3 absorbance is observed at all time points without MgSO4 3 7H2O present, although it decreases with increasing deposition time until reaching a limiting value at about 4 h. In contrast, the CD3 absorbance is not observed at any time point with 0.5 g of MgSO4 3 7H2O. Figure 2c is a plot of the water contact angle versus deposition time. Before deposition, the freshly plasma cleaned silicon oxide surfaces exhibit water contact angles approaching zero. After deposition, water contact angles are greater than 100° in all cases after 1 h, but are lower for monolayers vapor-deposited without MgSO4 3 7H2O than for those deposited with MgSO4 3 7H2O. The contact angles plateau after 12 h at 104° without MgSO4 3 7H2O and 109° with MgSO4 3 7H2O. C H Stretch. Figure 3 also shows the C H stretching region for siloxane monolayers formed by 12 h vapor depositions.

Figure 3. Carbon deuterium and carbon hydrogen stretching regions of the p-polarized Brewster angle transmission FTIR spectra of monolayers vapor-deposited onto silicon oxide for 12 h from 100 μL of (CD3O)3Si(CH2)13CH3 without and with 0.5 g of MgSO4 3 7H2O present. 9930

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Figure 5. Electrochemical capacitance versus scan rate during cyclic voltammetry after 12 h depositions of 100 μL of (CD3O)3Si(CH2)13CH3 onto ITO electrodes without and with 0.5 g of MgSO4 3 7H2O present. The squares and triangles represent the capacitances measured after two and three repeated 12 h depositions, each with 100 μL of fresh silane and 0.5 g of fresh MgSO4 3 7H2O. Error bars represent 1 standard deviation.

Figure 4. (a) Ellipsometric thickness versus mass of MgSO4 3 7H2O added for 12 h depositions onto silicon oxide with various initial volumes of (CD3O)3Si(CH2)13CH3. (b) Integrated FTIR symmetric CD3 stretching absorbance versus mass of MgSO4 3 7H2O added for 12 h depositions with various initial volumes of the silane. The lines are guides to the eye. Error bars represent 1 standard deviation.

The intensities of the peaks in the 2800 3000 cm 1 range are lower for depositions performed without MgSO4 3 7H2O than for those with MgSO4 3 7H2O. In addition, the peak frequencies for the symmetric and asymmetric methylene stretches shift down 3 cm 1 from 2854 and 2924 cm 1 without MgSO4 3 7H2O to 2851 and 2921 cm 1 with 0.5 g of MgSO4 3 7H2O. Effect of the Mass of MgSO4 3 7H2O. The plateau of the ellipsometric thickness at long deposition time with 100 μL of silane varies with the mass of MgSO4 3 7H2O (Figure 4a, circles). As the mass of MgSO4 3 7H2O is increased, the ellipsometric thickness increases until a plateau is reached with 0.5 g of MgSO4 3 7H2O. Figure 4b shows the integrated absorbance of the CD3 symmetric stretch versus the mass of MgSO4 3 7H2O. The mass required to decrease the CD3 stretch to zero matches the mass required to reach the plateau in ellipsometric thickness. Effect of the Amount and Surface Area of the Silane Source. To test whether the amount of silane limits monolayer growth, experiments were performed with varying volumes of silane. Above 50 μL of silane, the ellipsometric thickness is independent of the amount of silane and follows the dependence on MgSO4 3 7H2O described above (Figure 4a). However, with only 10 μL of silane, the thickness is equal to the thickness observed with 100 μL of silane in the absence of MgSO4 3 7H2O but does not increase with added MgSO4 3 7H2O. Reduced CD3 absorbances are observed for e50 μL of silane with 0.05 g of MgSO4 3 7H2O present as shown in Figure 4b. With 0.5 g of MgSO4 3 7H2O, the CD3 absorbance is reduced to the level of noise for all silane volumes. To test for any role of the surface area of the liquid silane, its exposed area was reduced by at least 1000-fold by placing the

silane into a glass capillary sealed at one end and held with the open end upward during deposition. Experiments using 10 μL of silane with 0.05 g of MgSO4 3 7H2O and 100 μL of silane with 0.5 g of MgSO4 3 7H2O yield ellipsometric thicknesses and CD3 integrated absorbances equal to the values when the silane is absorbed into filter paper. Treatment of Monolayers with Methanol Vapor. To assess the reversibility of the hydrolysis of the methoxy groups, completely hydrolyzed, partial monolayers deposited from 10 μL of silane and 0.05 g of MgSO4 3 7H2O for 12 h were treated with a large excess of pure CD3OD or mixtures of CD3OD and H2O vapor at 110 °C for 12 h.48 After treatment with pure CD3OD vapor, CD3 stretching is observed with an integrated absorbance of 3.1  10 4 cm 1 and an ellipsometric thickness of 14 Å. Exposure to 10:1 and 3:1 molar ratios of CD3OD/H2O result in CD3 integrated absorbances of 2.3  10 4 and 1.1  10 4 cm 1, respectively. Negligible CD3 absorbance is observed after 12 h of exposure to 1:1 CD3OD/H2O. Electrochemical Capacitance. To assess the permeability of monolayers formed by silane vapor deposition, cyclic voltammetry was performed on conductive ITO samples that were plasma-cleaned and vapor-deposited with the silane in the same manner. Figure 5 is a plot of the electrochemical capacitance versus scan rate determined by cyclic voltammetry. Capacitances are highest for monolayers deposited without the addition of MgSO4 3 7H2O. When monolayers are deposited in the presence of 0.5 g of MgSO4 3 7H2O, the measured capacitances decrease. A further decrease in the capacitances is observed for surfaces exposed to a second 12 h deposition with 100 μL of fresh silane and 0.5 g of fresh MgSO4 3 7H2O. The decrease in capacitance is accompanied by an increase in the intensity of the asymmetric methylene stretch by less than 10% in the FTIR spectrum after the second deposition with no discernible changes to the peak positions or widths of the methylene stretching modes (Figure S4 in the Supporting Information). The ellipsometric thicknesses and the water contact angles are also unchanged after the second deposition. Exposure of the surfaces to three repeated 12 h depositions with fresh silane and fresh MgSO4 3 7H2O results in the same capacitance as two repeated depositions, suggesting 9931

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Langmuir that a minimum limiting value of the capacitance is reached after two repeated depositions.

’ DISCUSSION Measurements of ellipsometric thickness show that incomplete monolayers are formed if insufficient MgSO4 3 7H2O is present. Dense monolayers are deposited with 100 μL of silane and 0.5 g of MgSO4 3 7H2O as inferred from the plateau of the ellipsometric thickness at 18.2 Å independent of further increases in deposition time, amount of silane, or amount of MgSO4 3 7H2O. This vapor-deposited thickness is consistent with literature values which range from 18 to 20 Å for monolayers solutiondeposited from tetradecyltrichlorosilane.14,15 We infer that dense monolayers form only when sufficient water to hydrolyze all methoxy groups is present and sufficient silane is present for the time required to densify the monolayer. When sufficient silane is present, incomplete monolayers can nonetheless result from the steric crowding of unhydrolyzed methoxy groups or from insufficient water to form additional siloxane bonds. FTIR spectroscopy was used to monitor the hydrolysis of methoxy groups during deposition. From the observation of CD3 stretching in FTIR spectra measured on monolayers prepared in the absence of sufficient MgSO4 3 7H2O, we infer that unhydrolyzed methoxy groups remain on some of the adsorbed silanes. These may prevent monolayer completion. The decrease of the CD3 integrated absorbance with increased deposition time without MgSO4 3 7H2O present (Figure 2b) suggests that methoxy groups are hydrolyzed over time by adventitious water in the chamber or on the silicon oxide surfaces. However, adventitious water is insufficient to hydrolyze all methoxy groups as indicated by the residual CD3 absorbance for 100 μL of silane. The deliberate addition of a water source is necessary to deposit dense monolayers. Increasing the mass of MgSO4 3 7H2O above the approximately 0.5 g required for complete removal of the methoxy groups on the surface yields no increase in the ellipsometric thickness and provides further support for the possibility that unreacted methoxy groups are responsible for incomplete monolayer deposition. The hydrolysis of methoxy groups and completion of the monolayer are correlated with an observed shift of methylene stretching to lower frequency. It is known that, as the packing of alkyl chains becomes denser, the positions of the symmetric and asymmetric methylene stretches shift to lower frequency.49 52 In the absence of MgSO4 3 7H2O, the methylene stretching frequencies are characteristic of liquidlike packing of alkyl chains. With the addition of sufficient MgSO4 3 7H2O for complete hydrolysis, the methylene stretching frequencies shift to lower frequency and are similar to previous reports53,54 for dense monolayers deposited from solutions of alkyltrichlorosilanes of similar chain length. This further supports our conclusion that incomplete monolayers are formed without sufficient MgSO4 3 7H2O present. Water contact angle measurements on monolayers deposited from 100 μL of silane with sufficient MgSO4 3 7H2O are equal to those measured for dense alkylsiloxane monolayers deposited from solution15 and provide additional evidence for improved packing of the monolayers. Lower contact angles indicative of less dense monolayers are measured without deliberate addition of MgSO4 3 7H2O. The amount of silane required to form a dense monolayer when there is sufficient MgSO4 3 7H2O is approximately 100 μL

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(Figure 4a), which is 2 orders of magnitude above the calculated amount needed to coat all surfaces in the deposition chamber with a dense monolayer of the silane.55 The volatile silane is presumably depleted by hydrolysis and condensation reactions in the reservoir of liquid silane, leading to cross-linked siloxanes with low vapor pressure. The lifetime of the silane source must be sufficiently large to provide volatile silane for the time required for monolayer densification. This is illustrated by experiments that use 10 μL of silane with 0.05 and 0.5 g of MgSO4 3 7H2O. In those experiments, complete methoxy group hydrolysis is observed, but ellipsometric thicknesses are measured that are similar to those of monolayers deposited from 100 μL of silane without MgSO4 3 7H2O. Experiments with the silane liquid source contained in a glass capillary, rather than being absorbed into filter paper, do not result in observable differences in the monolayers, suggesting that the surface area of the silane source is not limiting its lifetime. To further assess the lifetime of the silane source, silicon oxide and ITO surfaces were subjected to two repeated 12 h vapor depositions. Fresh reactants, 100 μL of silane and 0.5 g of MgSO4 3 7H2O, were used in each deposition. Exposure to a second deposition results in an increase in the intensity of the asymmetric methylene stretch of less than 10% with no discernible changes to the methylene stretching peak positions or widths, the ellipsometric thicknesses, or the water contact angles of monolayers deposited onto silicon oxide surfaces. The work of Hong et al.26 motivated us to pursue a more sensitive measure of monolayer densification. They reported no changes in the ellipsometric thicknesses or water contact angles measured on vapor-deposited monolayers by increasing the deposition time beyond 12 h. However, an increase in the blocking of atomic layer deposition was observed on monolayers deposited for longer than 12 h. Electrochemical capacitance measurements were performed to test the resistance of vapor-deposited siloxane monolayers on ITO to ion permeation. The measured capacitances decrease with increasing scan rate (Figure 5). We attribute this decrease to a decrease in the time for permeation with higher scan rates.56 Electrochemical capacitances are highest for monolayers deposited without MgSO4 3 7H2O, from which we infer that these monolayers are most permeable to ions in solution. This observation agrees with the ellipsometric, FTIR spectroscopic, and water contact angle measurements indicating incomplete monolayers. The denser monolayers formed with 0.5 g of MgSO4 3 7H2O are more resistant to ion permeation. Exposing monolayers formed with 0.5 g of MgSO4 3 7H2O to a second 12 h deposition with 100 μL of fresh silane and 0.5 g of fresh MgSO4 3 7H2O results in even greater resistance to ion permeation. We infer that the electrochemical capacitance is a more sensitive probe for monolayer densification than the other measurements used because a difference in the electrochemical capacitance is clearly observed after the second deposition while only a subtle increase in the asymmetric methylene stretching intensity in the FTIR spectrum is observed with no discernible changes to the methylene stretching peak positions or widths, ellipsometric thicknesses, or water contact angles. The lowest capacitance measured, 2.0 μF 3 cm 2, is higher than the value of 1.2 μF 3 cm 2 for thiol monolayers of the same chain length on gold.51 We infer that vapor-deposited siloxane monolayers on ITO contain more pinhole and gauche defects that allow ion permeation than their thiol counterparts on gold. Previous reports of capacitance values measured on siloxane 9932

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Langmuir monolayers solution-deposited onto electrode surfaces vary widely57 59 from 1.1 to 24 μF 3 cm 2, highlighting the sensitivity of the monolayer packing in siloxane monolayers to the details of the deposition method. We also find that an excess of water above the calculated stoichiometric minimum amount of water is required to achieve complete hydrolysis and dense monolayers. For example, the complete hydrolysis of 100 μL of silane (0.27 mmol) should stoichiometrically require 1.5 times as much water (0.40 mmol).60 However, we observe in Figure 4 that dense monolayer formation requires approximately 0.5 g of MgSO4 3 7H2O, which, upon partial dehydration at 110 °C,61 63 yields 9 mmol of water, or about 20 times as much water as required for complete hydrolysis of the silane. What determines the amount of water required to form dense monolayers when sufficient silane is present? Equilibrium effects might explain the excess of water required to achieve complete hydrolysis. Methanol generated as a byproduct of hydrolysis reactions accumulates in the deposition chamber over time and could drive the reverse methoxylation reaction. We have confirmed the reversibility of the hydrolysis of methoxy groups by exposing completely hydrolyzed, partial monolayers to pure CD3OD and CD3OD/H2O vapor mixtures and observing the CD3 stretch in the FTIR spectra. We attribute the reappearance to methoxylation of free silanols on the surface. The CD3 stretch is observed after exposure to CD3OD/H2O vapor mixtures with molar ratios of 3:1 and higher but not with a 1:1 ratio. From these data, we estimate that the equilibrium for methoxy group hydrolysis is mildly favorable with an equilibrium constant of approximately 4.64 This is in agreement with determinations of the equilibrium constant for alkoxy group hydrolysis in solution on the order of 10.65 70 While the equilibrium argument predicts the qualitative trend of increased density with increased water, the equilibrium constant we calculate from reversing the hydrolysis does not quantitatively explain the data. For instance, with 50 μL of silane and 0.05 g of MgSO4 3 7H2O, the methanol to water ratio is expected to be 0.6, well below the level required to inhibit the hydrolysis reaction (Table S1 in the Supporting Information). One possibility is that water release from the MgSO4 3 7H2O is rate limiting. In that case, the liquid silane may remove most of the water from the vapor phase with the result that the silanes in the monolayer remain methoxylated until the liquid silane is exhausted and monolayer growth terminates. Increasing the mass of MgSO4 3 7H2O and the amount of water released per unit time might maintain a greater fraction of free silanols in the monolayer at early time and promote monolayer growth.

’ CONCLUSIONS In summary, we showed that the addition of water was necessary during siloxane monolayer vapor deposition to hydrolyze methoxy groups and form dense monolayers. Water was deliberately introduced to vapor depositions through the dehydration of MgSO4 3 7H2O. We developed a probe to measure the hydrolysis of deuterium-labeled methoxysilanes. This allowed the monitoring of CD3 stretching by FTIR spectroscopy as a function of time, the mass of MgSO4 3 7H2O, and the amount of silane. With sufficient silane present, ellipsometric thicknesses consistent with dense monolayers were measured when a 10 20-fold excess of water above the stoichiometric amount was present. Lower frequency methylene stretching modes,

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higher water contact angles, and lower electrochemical capacitances indicating denser monolayers were achieved when enough silane and water were present. Electrochemical capacitance measurements were the most sensitive to changes in the density of the siloxane monolayers. The excess water required was at least in part due to the reversibility of the hydrolysis of the methoxy groups. We provide a simple laboratory method to deposit dense siloxane monolayers using optimized amounts of silane and water and repeated deposition cycles if needed. We are currently using this method to deposit mixed, azide-terminated monolayers onto oxide surfaces, which can be further functionalized with a broad array of groups by the Cu(I)-catalyzed azide alkyne cycloaddition reaction.

’ ASSOCIATED CONTENT

bS

Supporting Information. NMR and FTIR spectra of tetradecyltris(deuteromethoxy)silane, cyclic voltammogram and FTIR spectrum of a siloxane monolayer after two repeated 12 h vapor depositions, time dependence of MgSO4 3 7H2O dehydration, and table of expected CH3OH:H2O ratios in the deposition chamber based on stoichiometry. This material is available free of charge via the Internet at http://pubs.acs.org/.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by funding from Helicos BioSciences Corp. We acknowledge NSF Grant CHE-0639053 for support of FTIR equipment. We also acknowledge NSF Grant DMR-0213618 to the Center on Polymer Interfaces and Macromolecular Assemblies for use of the plasma cleaner and ellipsometer. Useful discussions with Dr. Erin Artin and Dr. Tim Harris are acknowledged. ’ REFERENCES (1) Sander, L. C.; Lippa, K. A.; Wise, S. A. Anal. Bioanal. Chem. 2005, 382, 646–668. (2) Devaraj, N. K.; Miller, G. P.; Ebina, W.; Kakaradov, B.; Collman, J. P.; Kool, E. T.; Chidsey, C. E. D. J. Am. Chem. Soc. 2005, 127, 8600–8601. (3) Sassolas, A.; Leca-Bouvier, B. D.; Blum, L. J. Chem. Rev. 2007, 108, 109–139. (4) Ulman, A. Chem. Rev. 1996, 96, 1533–1554. (5) Schwartz, D. K. Annu. Rev. Phys. Chem. 2001, 52, 107–137. (6) Onclin, S.; Ravoo, B. J.; Reinhoudt, D. N. Angew. Chem., Int. Ed. 2005, 44, 6282–6304. (7) Plueddemann, E. P. Silane Coupling Agents, 2nd ed.; Plenum Press: New York, 1991. (8) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett Films to Self-Assembly; Academic Press: San Diego, CA, 1991. (9) Brzoska, J. B.; Azouz, I. B.; Rondelez, F. Langmuir 1994, 10, 4367–4373. (10) Angst, D. L.; Simmons, G. W. Langmuir 1991, 7, 2236–2242. (11) Vallant, T.; Kattner, J.; Brunner, H.; Mayer, U.; Hoffmann, H. Langmuir 1999, 15, 5339–5346. (12) Krasnoslobodtsev, A. V.; Smirnov, S. N. Langmuir 2002, 18, 3181–3184. 9933

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CD3OD), 0.400 g of CD3OD and 0.020 g of H2O (10:1 molar ratio), 0.330 g of CD3OD and 0.055 g of H2O (3:1 molar ratio), and 0.216 g of CD3OD and 0.108 g of H2O (1:1 molar ratio). (49) Snyder, R. G.; Strauss, H. L.; Elllger, C. A. J. Phys. Chem. 1982, 86, 5145–5150. (50) Snyder, R. G.; Maroncelli, M.; Strauss, H. L.; Hallmark, V. M. J. Phys. Chem. 1986, 90, 5623–5630. (51) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559–3568. (52) Cicero, R. L.; Linford, M. R.; Chidsey, C. E. D. Langmuir 2000, 16, 5688–5695. (53) Hoffmann, H.; Mayer, U.; Krischanitz, A. Langmuir 1995, 11, 1304–1312. (54) Tillman, N.; Ulman, A.; Schildkraut, J. S.; Penner, T. L. J. Am. Chem. Soc. 1988, 110, 6136–6144. (55) A dense siloxane monolayer should exhibit an approximate coverage of 5  1014 molecules/cm2 on the basis of each alkylsilane occupying an area of about 20 Å2.15 Therefore, 100 μL (0.27 mmol) of the silane should be enough to cover 3  105 cm2 with a monolayer. The upper limit of surface area inside the silane vapor deposition chamber is estimated to be approximately 103 cm2. (56) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682– 691. (57) Hillebrandt, H.; Tanaka, M. J. Phys. Chem. B 2001, 105, 4270– 4276. (58) Kulkarni, S. A.; Vijayamohanan, K. P. Surf. Sci. 2007, 601, 2983– 2993. (59) Chen, C.; Hutchison, J. E.; Postlethwaite, T. A.; Richardson, J. N.; Murray, R. W. Langmuir 1994, 10, 3332–3337. (60) The calculation of 0.4 mmol of water to hydrolyze 0.27 mmol (100 μL) of the silane assumes a water:trimethoxysilane stoichiometric ratio of 1.5:1, which is derived from the overall reaction of complete trimethoxysilane hydrolysis (requiring water) followed by complete condensation (releasing water) to form a polysiloxane network. The expected upper limit for the water:trimethoxysilane stoichiometry is 3:1 from complete hydrolysis of the methoxy groups and no water-producing condensation to form siloxanes. In this case, 0.8 mmol of water would be needed to hydrolyze the silane. (61) Experiments were conducted to determine the amount of water released from the dehydration of MgSO4 3 7H2O, at 110 °C. Specifically, 0.5 g of MgSO4 3 7H2O was placed into the same deposition chamber and 110 °C oven used in the vapor deposition experiments for various periods of time. The measured mass difference was used to determine how much water was liberated (Figure S5 in the Supporting Information). Our experimental determination of the hydrated salt releasing about 4.5 water molecules per formula unit agrees well with previous reports.62,63 In addition, loss of vacuum in the chamber was observed after the dehydration of masses of MgSO4 3 7H2O slightly greater than 1.0 g as expected from the measured dehydration stoichiometry. (62) Paulik, J. J.; Paulik, F.; Arnold, M. Thermochim. Acta 1981, 50, 105. (63) Emons, H.; Ziegenbalg, G.; Naumann, R.; Paulik, F. J. Therm. Anal. 1990, 36, 1265. (64) The equilibrium constant for hydrolysis was estimated by interpolation of the measured CD3 absorbances after exposure to CD3OD/H2O mixtures. An interpolation was performed between the 10:1 and 3:1 CD3OD/H2O data points to estimate the ratio where an integrated CD3 absorbance of 1.55  10 4 cm 1, half the absorbance after exposure to pure CD3OD, should be measured. The result of this interpolation was a CD3OD:H2O molar ratio of 4.3:1, which is the equilibrium constant for the reaction RSiOCD3 + H2O T RSiOH + CD3OH. It is assumed that all available free silanols that would participate in the equilibrium reaction would be methoxylated after a 12 h treatment with excess pure CD3OD. (65) Ro, J. C.; Chung, I. J. J. Non-Cryst. Solids 1989, 110, 26–32. (66) Rankin, S. E.; McCormick, A. V. Macromolecules 2000, 33, 7743–7750. 9934

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(67) van Beek, J. J.; Seykens, D.; Jansen, J. B. H. J. Non-Cryst. Solids 1992, 146, 111–120. (68) Rankin, S. E.; Sefcík, J.; McCormick, A. V. Ind. Eng. Chem. Res. 1999, 38, 3191–3198. (69) Sanchez, J.; McCormick, A. J. Phys. Chem. 1992, 96, 8973–8979. (70) Sefcík, J.; Rankin, S. E.; Kirchner, S. J.; McCormick, A. V. J. NonCryst. Solids 1999, 258, 187–197.

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