Substituent Effects on the Kinetics of Bifunctional Styrene SAM

Jun 9, 2014 - Using substituted styrenes allowed for formation of a covalent C–Si ...... Rincón , L.; Almeida , R. Is the Hammett's Constant Free o...
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Substituent Effects on the Kinetics of Bifunctional Styrene SAM Formation on H‑Terminated Si Esther Frederick, Pearl N. Dickerson, Yu Lin Zhong, and Steven L. Bernasek* Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States S Supporting Information *

ABSTRACT: Self-assembled monolayers (SAMs) on metal and semiconductor surfaces are of interest in electronic devices, molecular and biosensors, and nanostructured surface preparation. Bifunctionalized molecules, where one functional group attaches to the surface while the other remains free for further modification, allow for the rational design of multilayer chemisorbed thin films. In this study, substituted styrenes acted as a model system for SAM formation through an alkene moiety. Substituents ranging from activating to strongly deactivating for aromatic reactions were used to probe the effect of the electronic properties of functionalizing molecules on the rate of SAM formation. Substituted styrene SAMs were formed on hydrogen-terminated p-type Si(100) and n-type Si(111) via sonochemical functionalization. Monolayers were characterized via ellipsometry, IR spectroscopy, contact angle goniometry, and X-ray photoelectron spectroscopy (XPS). Initial rates of reaction for molecules that selectively attached through the alkene were further studied. A linear relationship was observed between the initial rates of surface functionalization and the substituent electron donating/withdrawing ability for the substituted styrenes, as described by their respective Hammett constants. This study provides precedent for applying well quantified homogeneous chemical reaction relationships to reactions at the solid−liquid interface.

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INTRODUCTION

functionalization via rational design based upon molecular properties and chemical behavior. In this study, para-substituted styrenes, specifically 4methylstyrene, styrene, 4-chlorostyrene, 4-cyanostyrene, and 4-trifluoromethylstyrene (Figure 1), were examined. These molecules are well suited for a fundamental study of electronic property effects on the formation and rate of growth of SAMs of aromatic organics on the Si surface, as these substituents range from electron donating to strongly electron withdrawing from aromatic systems.

The functionalization of semiconductor surfaces with organic molecules has attracted a considerable amount of interest due to a wide range of potential applications in microelectronics and sensor design. To overcome the fundamental limits of creating smaller feature sizes for integrated circuits, a bottomup approach utilizing organic molecules chemically bound to silicon surfaces might be used for device fabrication. A variety of techniques have been used to form organic monolayers on Si surfaces via Si−C covalent bonds, including grafting by electrochemical,1 photochemical,2 thermal,3 and sonochemical methods.4 Covalently bonded organic monolayers on Hterminated Si surfaces have been shown to be highly stable, to be resistive to oxidation, and to possess superior electronic passivation.1 Previous work on Si surfaces has shown the formation of covalently bound, well-ordered monolayers consisting of alkyl chains, alkenes, alkynes, and phenyl compounds.5−12 Specifically, when terminal olefins bind to hydrogen-terminated silicon surfaces, a strong Si−C bond is formed, creating a hybrid material which is extremely stable and robust. There is much interest in forming self-assembled monolayers (SAMs) of molecules which possess the ability to undergo further reaction after attachment to the semiconductor surface allowing for highly controlled and versatile bottom-up assembly of nanoscale materials. Furthermore, an understanding of the kinetics of SAM formation will enable surface © 2014 American Chemical Society

Figure 1. Para-substituted styrene derivatives listed from EDG (electron donating group) (CH3) to strongly EWG (electron withdrawing group) (CF3). Received: July 1, 2013 Published: June 9, 2014 7687

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Figure 2. Mechanism of sonochemical functionalization of silicon by aromatic alkenes.

Quantification of σX values allowed for the use of benzoic acid ionization as a reference for substituent effects in similar reactions according to the Hammett relationship:

Previously, Rappich et al. examined benzene derivatives, specifically nitro, bromo, chloro, dichloro, and diethylaniline, for surface functionalization of Si(111) via an electrochemical grafting process.13 The benzene rings were directly linked to the Si surface such that the monolayers were terminated by the substituents. The electronic structure of the molecule had observable effects on surface functionalization, with nitro and bromo substituted benzenes forming multilayers and methoxy substituted benzene exhibiting submonolayer coverages. In the present study, monolayer growth of substituted styrenes on Hterminated p-type Si(100) and n-type Si(111) was investigated. Using substituted styrenes allowed for formation of a covalent C−Si linkage to the substrate surface without disrupting the benzene ring. The effect of the electronic nature of the substituents on the kinetics of monolayer growth could then be studied. Multilayer growth was avoided by using a mild, sonochemical method of hydrosilylation as reported by Zhong and Bernasek.4 This approach ensured selective functionalization through the alkene, preventing the second functional group from participating in bonding directly to the surface. In this method, acoustic cavitation activates the hydrogen-terminated silicon surfaces for hydrosilylation selectively with the vinyl group as seen in Figure 2, forming a SAM terminated by a second functional group available for further chemical modification. While there are a number of possible initiation mechanisms,14 the mechanism suggested here includes propagation via radicals as illustrated in Figure 2. In this mechanism, the radical induced via sonication is attacked by the alkene nucleophile4,15 forming a methyl radical intermediate. The organic radical intermediate then abstracts a hydrogen from an adjacent Si atom, forming another surface Si radical available for further reaction. Radical formation on the organic molecule is presumed to be the rate limiting step in this mechanism. Understanding the mechanism becomes important in elucidating the relationship between the molecular properties and reaction rate. In 1937, Hammett proposed a relationship between the electron donating/withdrawing ability of substituents on aromatic rings and the reaction rates of the substituted aromatics.16 Based upon the ionization reaction of meta and para-substituted benzoic acids, a particular substituent X was given the constant σX that quantifies the total electronic effects of the substituent on the reaction rate.17,18 Electron donating groups which destabilized the benzoic acid carboxylate were assigned negative σ values, while electron withdrawing groups had positive σ values as they led to increased stability of the carboxylate. (Ortho substituents were not considered due to the added steric effects in ortho-substituted compounds.) Tables of σX constants have been compiled, including σX+ and σX− for substituent groups that stabilize positive and negative charge, respectively, through resonance.

log(KX /KH) = ρσX

(1)

In this equation, KX is the equilibrium constant for the reaction of the substituted compounds, KH is the equilibrium constant for the unsubstituted compound, and ρ is the slope of the line which gives information about the sensitivity of the reaction to substituent effects. While this relationship was derived based on thermodynamics of acid dissociation, the same equation can be applied to kinetic analyses where the ratio of the reaction rate constants (k) is used instead of equilibrium constants (K) and ρ quantifies the change in charge that occurs during the rate limiting step of the reaction. A positive ρ indicates buildup of negative charge, while a negative ρ indicates that positive charge, or a decrease in negative charge, results during the reaction. Due to the relationship between Gibbs free energy and the equilibrium constant, Hammett type equations are often referred to as linear free energy relationships.17 It is well established that reactions involving para-substituted aromatic radicals have a complex linear free energy relationship due to the extra resonance stabilization felt at the para position.19 A study of the addition of free trichloromethyl radicals to substituted styrenes demonstrated that meta substituted styrenes fit a linear relationship with the σ+ value while para-substituted styrenes did not.20 To deal with radical reactions, attempts have been made to determine a set of σ· constants for substituent effects on radical transition states (see Dust and Arnold21 for a more extensive discussion). Due to the failure of agreement between the various kinetic attempts, Dust and Arnold established a series of σ· constants based upon relative spin delocalization of substituted benzyl radicals as measured by electron spin resonance spectra.21 In the present study, the rates of the substituted styrene functionalization of the silicon surface, resulting from the formation of an aromatic radical, were analyzed using the Hammett equation for radical reactions: log(kX /kH) = ρσ·

(2)

where kX is the reaction rate of the substituted compounds, kH is the reaction rate for the unsubstituted compound, and the slope ρ is related to the reaction susceptibility to substituent effects.

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EXPERIMENTAL SECTION

Preparation of Si Samples. Si(100) wafers (p-type, 0.001−0.005 6-8 Ω·cm, University Wafer) were diced into ∼1 in. × 1.5 in. samples, and n-type Si(111) (6-8 Ω·cm, Institute of Electronic Materials Technology, Poland) were diced into ∼1 in. × 1 in. squares. Samples were cleaned by sequential sonication in acetone, methanol, and water (HPLC grade, 15 min each) to eliminate surface contaminants. The 7688

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Corp.) using a helium−neon laser (633 nm) source at a fixed incident angle of 70°. The refractive index of the silicon and organic layer was taken to be 3.85 and 1.5, respectively. The ellipsometer was calibrated using a manufacturer provided standard reference sample before each measurement to ensure that ellipsometer height was consistent from sample to sample. At least six different points on the surface were measured to ensure uniformity across the sample. The standard deviation for a single sample was typically between 0.2 to 0.6 Å. Contact angle goniometry was performed to characterize the quality of the SAM and to further characterize the functionalization process. In this case, a drop of Millipore water (∼2 μL) was placed on the sample surface to form a sessile drop from which the contact angle was measured by using a Theta Optical tensiometer (KSV Instruments, Finland) at room temperature and humidity. On each sample surface, the experiment was carried out at least six times to ensure uniformity across the sample. Where applicable, attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) for molecular characterization of the SAMs was carried out on a Nicolet 6700 FT-IR spectrometer with a liquid nitrogen cooled mercury cadmium telluride (MCT) detector (Thermo Electron Corp.), using a commercial VeeMax II variable angle reflection unit with germanium ATR crystal and wire grid polarizer (Pike Technologies). The crystal was cleaned with isopropyl alcohol before the background reference was collected. Scans were manually corrected via a linear baseline and then ATR corrected and smoothed.

samples were immersed in a 3:1 v/v mixture of conc. H2SO4/30% H2O2 at 80 °C for 30 min, followed by rinsing with copious amount of Millipore water (18.2 MΩ·cm) to remove any organic contaminants. Clean samples were stored in Millipore water for up to 3 days before use. The samples were etched and hydrogen terminated by immersion in Ar-purged 40% NH4F solution for 8 min followed by an additional quick rinse in Ar-purged Millipore water. All solvent purging was done using UPC grade argon (Airgas). The hydrophobic H-terminated samples were dried under argon and used immediately for functionalization. Sonochemical Hydrosilylation of Self-Assembled Monolayers on p-Type Si(100). For the hydrosilylation, five different styrene derivative molecules were used. 4-Methylstyrene (99%), styrene (99%), 4-chlorostyrene (97%), 4-cyanostyrene (99%), and 4trifluoromethylstyrene (98%) were purchased from Alfa-Aesar or Fischer, and used without further purification. H-terminated silicon samples were introduced to a 50 mL glass weighing bottle and sealed with a rubber septum. The sealed container was connected to a highvacuum/argon Schlenk line via a needle through the rubber septum and partially immersed in an ultrasonic bath (model 1510, Branson) filled with ice. The container was pumped and backfilled with argon gas three times before connecting the system to a bubbler and leaving under argon environment by continuously purging with argon. For ntype samples, functionalization proceeded by injecting 2 mL of Arpurged styrene molecule (20% v/v in mesitylene) into the bottle to sufficiently cover the entire sample surface. Immediately after, the glass vessel was partially immersed in an ultrasonic bath (model 1510, Branson) and subjected to sonication at ice bath temperature for a fixed period of time ranging from 1 to 60 min. For p-type samples, 2 mL of Ar-purged mesitylene was introduced into the bottle to sufficiently cover the entire sample surface and sonicated for 1 min. Immediately after, 0.5 mL of the desired styrene was injected into the vessel and subjected to sonication at ice bath temperature for a fixed period of time ranging from 1 to 60 min. Functionalization via the ntype method was also done on p-type wafers for select styrenes to ensure that changing the method did not alter the observed rate. The second method was then chosen as it reduced the amount of styrene waste. Ice bath temperature was chosen to maintain constant temperature throughout the reaction. After sonication, the functionalized samples were sequentially rinsed by sonication in degassed anhydrous toluene, dichloromethane (DCM), and tetrahydrofuran (THF) (5 min each). For toluene and DCM sonication, the wafers were removed from the previous reaction or rinse flask and transferred to a clean flask before being placed back under an argon environment. The THF rinse was not done under an argon environment. Surfaces were manually rinsed before each sonication with a few pipet washes of the degassed solvent. A volume of 10 mL of degassed anhydrous toluene was used for a final pipet rinse, and the surface was blown dry with nitrogen. The surfaces were characterized immediately after this process. Characterization of SAMs. X-ray photoelectron spectroscopy (XPS) was used to confirm SAM formation and to characterize the styrene−silicon bond. XPS was performed with a Mg Kα X-ray source (1253.6 eV) and an electron spectrometer (VG ESCALAB MkII) operated in constant analyzer energy mode with a pass energy of 100 and 20 eV for survey and high-resolution scans, respectively. The photoelectron takeoff angle was 15° with respect to the sample normal, and a base pressure of ∼10−9 Torr was maintained throughout the XPS analysis. The samples were placed in a load-lock chamber for initial evacuation and then transferred to the main chamber. Spectra were calibrated using the Si 2p1/2 peak set to 99.2 eV and the Si 2p3/2 peak set to 99.79 eV. The integrated area ratio of Si 2p1/2/Si 2p3/2 was fixed at 0.5. Curve-fitting of the core XPS lines was carried out with a 70% Gaussian/30% Lorentzian function. All 2p peaks were fit with a Shirley background subtraction while 1s lines were fit with a Linear background subtraction. Ellipsometry was used to measure the coverage, to determine the rate of SAM formation, of the substituted styrenes on the Hterminated Si(100) or Si(111) surfaces. Ellipsometric measurements were made with an LSE Stokes ellipsometer (Gaertner Scientific

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RESULTS AND DISCUSSION Monolayer Formation and Functionalization through the Alkene. Ellipsometry and goniometry measurements from the 60 min sonication times demonstrated successful monolayer formation (Table 1). Full coverage was achieved Table 1. Average Contact Angles and Coverages after 60 min Reaction on p-Type H−Si(100) sample

Θaverage (deg)

coverage after 60 min (Å)

methyl styrene trifluoro chloro cyano Si−H

68 ± 2 63 ± 2 72 ± 1 58 ± 3 56 ± 3 44 ± 8

9.9 ± 0.6 9.2 ± 0.4 11.4 ± 0.2 11.1 ± 0.2 10.8 ± 0.6

at 60 min, as values of 10−11 Å for monolayer thickness agreed with published values for similar monolayers.3,22 The contact angle trend also demonstrated successful surface deposition, with lower contact angles for hydrophilic amino-, cyano-, and chlorostyrene SAMs which expose hydrophilic groups at the surface, and a higher contact angle for the hydrophobic trifluoromethylstyrene and methylstyrene SAMs. XPS data, and where possible ATR-FTIR spectra, provided further evidence for successful SAM deposition. All XPS spectra after 60 min reaction times showed peaks for carbon, atmospheric oxygen, and silicon (Table 2). The C 1s peak around 285−285.5 eV in each spectrum was attributed to adventitious carbon by comparison with an XPS spectra of a clean SiO2 and H−Si wafer containing only adventitious carbon. The second carbon peak around 286−286.5 eV was attributed to remaining mesitylene solvent on the sample by comparison to an XPS spectra of a p-type H−Si(100) wafer that had been sonicated for 60 min in mesitylene. Styrene molecules with a noncarbon para substituent exhibited a third carbon peak due to the carbon bound to the substituent. In molecules without noncarbon substituents, the styrene 7689

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Table 2. XPS Peak Positions for All Elements Present in Each Sample after 60 min Reaction sample

C 1s

cyano chloro

285.4 ± 0.07 285.4 ± 0.1

286.4 ± 0.07 286.4 ± 0.4

methyl styrene trifluoro-methyl

285.06 ± 0.03 285.2 ± 0.03 285.3 ± 0.03

286.13 ± 0.07 286.3 ± 0.17 286.0 ± 0.08

SiO2 H−Si mesitylene

285.5 ± 0.03 285.1 285.38 ± 0.07

287.4 286.5 ± 0.2

O 1s

Si 2p3/2

Si 2p1/2

Si minor

SiO2

290.4 ± 0.3 289.6 ± 0.2

533.2 ± 0.03 532.9 ± 0.1

99.20 99.20

99.79 99.79

97.5 ± 0.03 97.5 ± 0.03

103 ± 0.04 103.1 ± 0.08

293.0 ± 0.1

532.7 ± 0.04 532.8 ± 0.03 532.9 ± 0.03

99.20 99.20 99.20

99.79 99.79 99.79

97.5 ± 0.08 97.5 ± 0.02 97.6 ± 0.07

102.3 ± 0.08 103 ± 0.02 102.9 ± 0.2

N/A N/A 532.8 ± 0.01

99.20 99.20 99.20

99.79 99.79 99.79

97.5 97.4 97.5 ± 0.02

103.57

N 1s/Cl 2p/F 400.3 ± 0.1 p1/2, 203.3 ± 0.1; p3/2, 201.7 ± 0.1

688.8 ± 0.02; 686.6 ± 0.05

102.3 ± 0.3

the asymmetric and symmetric stretch of the C−H bonds.28 Additionally, the high contact angle values for the methylstyrene monolayer relative to the styrene monolayer indicated the presence of a bulky hydrophobic group, providing further support for methyl termination of the monolayer. Cyanostyrene. Evidence for cyanostyrene attachment through the alkene was obtained from XPS. A 400.3 eV value was observed for the N 1s binding energy (see Supporting Information SI 6). The N 1s binding energy for the cyanostyrene monolayer agrees well with physisorbed −CN substituted organics, suggesting no strong interaction between the CN and the silicon surface in the present case. The measured N 1s binding energy value is also in good agreement with the N 1s binding energy of 401 eV for CN bound to the Si surface through the carbon.29 Therefore, cyanostyrene is believed to bind selectively through the alkene. Further evidence that the cyanostyrene did not bond through the nitrogen was provided by Tao et al.30 in a study that observed a significantly lower N 1s binding energy of 398.7 eV for benzonitrile chemisorbed to Si(111) through the nitrogen. Additionally, the low contact angle for the cyanostyrene surface relative to the styrene also supports monolayer termination by a hydrophilic cyano group. Overall, results from the 60 min functionalization time period indicated that monolayers were successfully formed. A combination of IR spectroscopy, XPS and contact angle goniometry studies indicated that the methyl, cyano, chloro, and styrene selectively chemisorbed through the alkene while trifluoromethylstyrene predominantly chemisorbed through the alkene. Kinetics of SAM Growth. To investigate the effect of the para substituent on the kinetics of styrene functionalization of the Si surface, a series of samples were reacted by sonication for various times in solution. Ellipsometry, XPS, and contact angle measurements were made for surfaces sonicated in the presence of the functionalizing solution for 1, 5, and 10 min. This time range provided a reasonably linear initial rate for the reaction. After the initial time period, the reaction slowed down with increasing surface coverage and leveled off at 1 monolayer coverage after 60 min (see Figure 3 and Supporting Information SI 7 for data). Initial rates were determined by monitoring the monolayer thickness as a function of time using ellipsometry and XPS. Even carefully calibrated ellipsometry measurements present difficulties for determination of submonolayer surface coverage due to uncertainties in molecular orientation and refractive index of the adsorbed material. It is important to note, however, that the initial rates are determined here from the difference in ellipsometric measurements at various time points. Therefore, when taking the difference, the errors present in every

component of the C 1s spectra could not be quantitatively distinguished from mesitylene and adventitious carbon. Evidence of SiO2 was present in all of the Si 2p XPS spectra of substituted styrene monolayers on p-type Si(100). A minor peak at 97.5 eV was observed in all Si 2p spectra (see Supporting Information SI 1). The mode of surface attachment was studied using the binding energy measured by XPS of the nonalkene substituent, contact angle values, and with ATR-FTIR spectra where possible. Trifluoromethylstyrene. Fluorine 1s XP spectra exhibited two peaks (see Supporting Information SI 2). The peak at 688.8 eV comprised about 93% of the total area, and the smaller peak occurred at 686.6 eV. These measurements were compared to values published by Mitsuya and Sato for fluorinated organic compounds adsorbed onto a fluorine terminated Si(111) surface.23 The F 1s peak at 688.8 eV is in good agreement with the published 689.0 eV F 1s value for a fluorine terminated alkyl monolayer on Si(111). The small peak at 686.6 eV closely corresponds to F 1s binding energies where the fluorine interacts directly with the Si; 687.0 eV for Si−F2 and 685.5 eV for Si−F.23 The presence and relative abundance of the two F 1s peaks in the XPS data suggest that the trifluoromethylstyrene reacts primarily through the alkene moiety with a small amount of surface contamination occurring from a fluorine−silicon interaction. High contact angle measurements further demonstrate that the surface is more hydrophobic when functionalized with trifluoromethylstyrene than when functionalized by styrene or methylstyrene. As C−F terminated surfaces are highly hydrophobic,24 the observed contact angle is likely due to the predominance of CF3 terminated portions of the surface. Chlorostyrene. XPS exhibited Cl 2p1/2 and 2p3/2 binding energies of 203.3 and 201.7 eV (see Supporting Information SI 3), respectively, which closely match the published Cl 2p3/2 value of 201 eV in solid chlorobenzene and in 4-chloroaniline bound to Si(111) through the nitrogen.25,26 The Cl peak position is significantly higher than the published value for Cl directly bound to Si (199.5 eV27). Thus, the XPS data indicates that chlorostyrene bonds selectively through the alkene. Additionally, the contact angle, lower than styrene, suggests that the monolayer is terminated by the Cl group. Methylstyrene. As the styrene component could not be quantitatively distinguished from mesitylene and adventitious carbon, the C 1s XPS region could not be used to confirm selective attachment through the alkene (see Supporting Information SI 4). IR spectra taken in similar experiments done on n-type Si(111) wafers indicated a clear presence of the additional methyl group with distinct sharp peaks at 2923 and 2854 cm−1 (see Supporting Information SI 5) corresponding to 7690

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Table 3. Initial Rate Values for the Functionalization of pType H−Si(100) with Styrenes Listed in Order from Weakly Electron Donating (methyl) to Strongly Electron Withdrawing (trifluoro)

Figure 3. Initial rate determination for methylstyrene on p-type H− Si(100).

molecule

ellipsometry rate

XPS rate

methylstyrene styrene chlorostyrene cyanostyrene trifluoromethylstyrene

0.050 0.047 0.051 0.059 0.039

0.053 0.032

expected from the XPS evidence suggesting that it does not exclusively bond through the alkene moiety and therefore may bind by a different pathway than the other substituted styrenes. Though the linear relationship between initial growth rate and Hammett constant provides evidence that monolayer formation through the alkene moiety is controlled by the electronic properties of the second functional group, some details from the data should be further discussed. First, electron donating groups (EDGs) are expected to stabilize the radical intermediate, leading to a faster initial rate, while electron withdrawing groups (EWGs) are expected to destabilize the intermediate, leading to slower initial rates. Second, all the substituted styrenes exhibited faster rates than styrene did, indicating that all substituents led to radical stabilization. Therefore, a plot of the rates should exhibit a linear relationship with a negative slope and styrene in the middle. In fact, literature reports for homogeneous reactions of free trichloromethyl radicals with substituted styrenes that proceed through a radical transition state exhibited the expected negative slope.20 A possible explanation for the present results is that both of these observations may be ascribed to captodative stabilization. This is the stabilization of a radical through the combined effects of EWG (captor) and EDG (donor) substituents.31 In this case, the location of the styrene radical between the Si donor and an EWG captor substituent may lead to captodative stabilization. The increased stabilization causes the EWG/EDG combination to exhibit a faster rate than both a nonsubstituted styrene and an EDG/EDG combination. The positive slope of the Hammett plot is therefore postulated to be due to the captodative effect. Experiments with substrates less electronegative than the substituents were used to probe the validity of invoking the captodative effect. As Si p-type wafers are less electron donating than their n-counterparts, the captodative stabilization from EDG−EWG interaction would diminish when switching from n-type to p-type wafers. Therefore, if captodative stabilization is occurring, an increase in rate should be seen when switching to more electron donating n-type Si. As Si wafers, regardless of dopant, are still electron donating with respect to the styrene, the Hammett slope is still expected to be positive. Experiments with n-type Si(111) were carried out in an attempt to probe the captodative effect (see Supporting Information SI 10 for coverage data for 60 min reactions). Relative rates measured by ellipsometry show a small suppression of the rates on the p-type wafers except for the case of trifluoromethylstyrene (Table 4). The small effect on rate observed here is likely due to the low concentration of dopant present. Further experiments with more heavily doped n and p-type Si surfaces with the same crystal structures should be carried out to explore this idea more fully.

measurement cancel in determination of the coverage differences and the subsequent initial rate determination. Initial rates for trifluoromethylstyrene and chlorostyrene functionalization were also obtained via XPS analysis to confirm the use of ellipsometry to determine reaction rates. Initial rates were determined from the increase in heteroatom signal (see Supporting Information SI 8 for example of F 1s intensity increase) relative to the total intensity of the Si signal for the sample. Rates could not be obtained using XPS analysis for molecules without noncarbon substituents, as it was impossible to use the C 1s signal to quantitatively differentiate the styrene peak from mesitylene and adventitious carbon adsorbed on the surface (Supporting Information SI 4). Additionally, XPS measurements of the N 1s signal lacked the sensitivity at low coverage for determination of an initial rate. Using both ellipsometry and XPS for the rate determination overcomes limitations inherent to the individual techniques. In particular, the ability to reproduce the rate measured by ellipsometry using XPS peak intensities demonstrates that ellipsometry is valid for determining initial rates of reaction for these systems. The use of ellipsometry allows for measurement of initial rates in systems that could not be determined via XPS due to difficulties in deconvolution of complex multicomponent spectra or in measurement of substituents with low sensitivity in XPS. To compare relative initial rates, the time and thickness measurements (for both ellipsometry and XPS) were normalized by setting the 1 min measurement to t = 0 min and the initial coverage to zero by subtracting the 1 min coverage from the other coverage values. Initial rates were further normalized to 1 monolayer by dividing all measurements by the final coverages at 60 min (see Supporting Information SI 9A for table and SI 9B for graphs depicting data pre- and post-normalization). All of the initial rate data fit a straight line with a minimum R2 value of 0.94 except the cyanostyrene which had an R2 value of 0.86. The calculated standard error of the slopes was negligible, ranging from 0.002 to 0.009. All substituted styrenes that bound selectively through the alkene increased the reaction rate, in the order cyano > chloro > methyl > styrene. The trifluoromethylstyrene, which showed another minor mode of attachment, exhibited the lowest initial rate. Initial rates (Table 3) were determined from the first three data points for each substituted styrene (see Figure 3 for an example). To determine whether the substituent effect on the radical transition state followed a Hammett free energy relationship, the log of the substituted styrene rates relative to styrene were plotted versus the sigma radical constant.21 The styrene, cyanostyrene, methylstyrene and chlorostyrene exhibited a linear trend with respect to the radical Hammett constant (Figure 4). The trifluoromethylstyrene did not fit the trend, as 7691

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Figure 4. Hammett plot of substituted styrene rates vs σ· on p-type H−Si(100).

Table 4. Comparison of Initial Rates on n-Type H−Si(111) versus p-Type H−Si(100) sample

relative rates (Å/s) n-type H−Si(111)

relative rates (Å/s) p-type H−Si(100)

methylstyrene styrene chlorostyrene cyanostyrene trifluoromethylstyrene

0.052 0.049 0.056 0.060 0.051

0.050 0.047 0.051 0.059 0.039

Figure 5. Hammett plot for substituted styrenes on n-type H−Si(111).

A σ· Hammett plot for the n-type wafers also displayed a linear free energy relationship (Figure 5). The corroboration of this result for n-type wafers substantiates the ability to apply well quantified homogeneous reactivity relationships to their less-studied surface reaction counterparts. Also of interest, the n-type H−Si(111) wafers did not display an SiO2 peak in the Si 2p region after 60 min reactions with styrene (see Supporting Information SI 11A and SI 11B). This was contrary to what was observed on the H−Si(100) p-type wafers that did exhibit an SiO2 peak after the 60 min reaction. The difference in amount of SiO2 present is likely due to differences in crystal surface structure. In a study by Niwano et al., using NH4F to H-terminate SiO2 had different effects on a Si(111) surface than a Si(100) surface.32 The H-terminated Si(111) surface was smooth, atomically flat and consisted of monohydride Si−H. On the other hand, the H-terminated Si(100) surface exhibited both monohydride and dihydride moieties when exposed to NH4F for 7 min. When exposed for 14 min, the coverage of monohydride species increased, but results suggested that small Si(111) facets were also generated. Additionally, Popoff et al. have demonstrated with high resolution XPS of freshly prepared organic monolayers on H−Si(111) that lack of a SiO2 peak may not indicate absence of

SiO2. Oxidation of Si(111) was observed in the O 1s region before being observed in the Si 2p region, indicating that the Si 2p region could act as a true measurement of Si oxidation only when a significant amount of SiO2 was present.33 The literature suggests that the presence of SiO2 in the current study on ptype Si(100) and not on the n-type Si(111) was likely due to NH4F H-termination causing defects on Si(100) and not on Si(111). As surface defects allow for oxygen diffusion and therefore faster oxidation of the H−Si; the defect containing H−Si(100) surface had a significant and detectable SiO2 presence while the relatively defect free H−Si(111) did not have enough SiO2 to be detected in the Si 2p region. Evidence of early oxidation in the O 1s region was not detected using the ESCA instrument in the current study. Overall, the linear free energy correlation for the observed functionalization rates suggests that it may be possible to predict growth rates of monolayers for other substituted styrene molecules based solely on the radical Hammett constant of the functional group. Future work is needed to fully study the effect of EWGs on the reaction rate. Using different substrates with greater difference in electron donating capability is also of interest in future work, to determine how altering the surface property affects the growth rates and linear free energy 7692

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relationship. This study sets an important precedent for the application of well quantified relationships known from homogeneous chemistry to the not as well explored dynamics of reactions at a solid surface.

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CONCLUSIONS

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ASSOCIATED CONTENT

The sonochemical method for hydrosilylation resulted in the predominantly selective attachment through the alkene for a series of para-substituted styrenes with substituent electronic properties ranging from EDG (CH3) to strongly EWG (CF3) chemisorbed to both p-type and n-type H−Si. Results from this study show that substituent effects on the rate of surface attachments have a predictable quantitative Hammett relationship. This information can potentially be used to control the formation of similar monolayers. This study sets precedence for the application of well quantified relationships known from homogeneous chemistry to the less explored dynamics of chemical reactions at a solid surface.

S Supporting Information *

Additional experimental details and figures as described in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the National Science Foundation, Division of Chemistry, CHE-1213216. P.N.D. acknowledges the support of NSF-IGERT Award DGE0903661. E.F. acknowledges the NSF-GRFP for funding and useful discussions with Zach Detweiler, Pat Donnelly, Conor Thomas, Julia Kalow, and Prof. Jeffrey Schwartz.

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REFERENCES

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