Surface Barrier Properties of Self-Assembled Monolayers as Deduced

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Surface Barrier Properties of Self-Assembled Monolayers as Deduced by Sum Frequency Generation Spectroscopy and Electrochemistry Xiaojun Cai and Steven Baldelli* Department of Chemistry, University of Houston, Houston, Texas 77024-5003, United States

bS Supporting Information ABSTRACT: This paper presents a model on the electrochemical behaviors of n-alkanethiol self-assembled monolayers (SAMs) on gold electrodes with different chain length (C10, C12, C16, and C18) using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and sum frequency generation spectroscopy (SFG). The SAMs studied reached their maximum density with a deposition time of ≈72 h according to the kinetics results shown by EIS Bode phase plots. For shorter chain SAMs, C10 and C12 monolayers, the results from CV, EIS, and SFG were consistent with each other. Significant amounts of thiols were removed by reductive desorption of the films for C10 and C12 SAMs, as indicated by decreases in the electrochemical waves and SFG peaks. Surprising differences were observed for longer-chain SAMs made from C16 and C18 monolayers. The CV results suggested that a large amount of the C16 and C18 monolayers chemically desorbed from the gold surface, while the EIS and SFG results illustrated that C16 and C18 layers still remained in the vicinity of the double-layer region near the gold surface and retained their two-dimensional, densely packed ordered structure, respectively. Only small decreases in phase angle were observed from EIS, and all the CH vibrational modes were nearly unchanged in SFG spectra for C16 and C18 SAMs. Furthermore, SFG orientation analysis of the C16 and C18 SAMs revealed a molecular structure for C16 and C18 SAMs after 30 CV cycles similar to that of their original films. Cyclic voltammograms also showed no observable oxidative readsorption waves for C10 and C12 SAMs, while for C16 and C18 monolayers detectable reoxidation peaks appeared. The results from EIS, SFG, and CV strongly suggest that the commonly accepted notion, the formation of aggregates and micelles for alkanethiol SAMs after reductive desorption, does not apply to the long-chain SAMs that were studied. After applying the potential to the long-chain SAMs, both the chemically desorbed and intact SAM molecules maintained molecular structures similar to that of the freshly prepared monolayers. The persistent crystallinity of longer-chain SAMs against negative potentials could be explained by dense packing and much lower solubility of the molecules in aqueous solutions. Implication for corrosion inhibited by the monolayer is discussed.

’ INTRODUCTION Alkanethiol self-assembled monolayers (SAMs) on gold1,2 have been extensively studied and used as model systems due to their ease of preparation, their stability in both vacuum and ambient environment, their well-ordered structure, and the tunability of surface properties by modification of molecular structure and functions.36 Thiolate SAMs desorb under sufficient negative potentials through a one-electron reductive path7 in the following reaction Au SR þ e f Auð0Þ þ SR ðsurfaceÞ

ð1Þ

Auð0Þ þ SR ðsurfaceÞ f Auð0Þ þ SR ðsolutionÞ

ð2Þ

5

The adsorption of thiols on gold is given by

Auð0Þ þ HSR ðsolutionÞ f Au SR þ 1=2H2

ð3Þ

After the cleavage of AuS due to the reductive reaction 1, the chemically desorbed thiolate species dissolve into the bulk electrolyte solution and diffuse away from the surface with various r 2011 American Chemical Society

time scales depending on the solubility and diffusion coefficient of the thiolates. An oxidative readsorption while the applied voltage reverses toward the positive direction is possible as long as the desorbed species remain close enough to the gold surface.8 The one-electron reductive desorption7 of SAMs has been used to characterize the monolayer coverage,9 and also it is useful for controlling and designing the surface properties.10,11 Since Porter and co-workers7 first reported the reductive desorption of alkanethiol SAMs from gold and silver electrodes in an alkaline aqueous solution, many papers on the properties of electrochemical desorption of SAMs have been reported.850 Voltammetric studies of the electrochemical reduction of alkanethiol SAMs by Widrig et al.7 and Kakiuchi et al.17 indicate that the reductive potential of alkanethiol SAMs gradually shifted to the negative direction with increasing the chain length. The chain length Received: March 3, 2011 Revised: July 22, 2011 Published: September 08, 2011 19178

dx.doi.org/10.1021/jp202060n | J. Phys. Chem. C 2011, 115, 19178–19189

The Journal of Physical Chemistry C dependent shift was attributed to the following: (i) the increasing intermolecular interaction between the alkyl chains (about 4.8 kJ mol1 per CH2 unit)51 as the chain length increases;7,22 (ii) the change in adsorption Gibbs energy of alkanethiolates52 (Yamamoto et al.17 pointed out that the contribution of one methylene unit to adsorption Gibbs energy is 2.6 kJ mol1); (iii) the decrease in solubility of the desorbed molecule as the chain length increases;8,46 (iv) a decrease in the fractional drop of the applied potential across the sulfur headgroup as the alkyl chain length increases (a dependence of the potential gradient across the SAM on the chain length);7,50 and (v) a chain length dependence of the permeability toward the electrolyte.8 Porter,33 Morin,12and Kolb53 groups also reported that the reductive potential is dependent on the surface crystallinity of the underlying gold substrate. Porter et al. indicated that the gold sulfur bonding is stronger on Au(110) than on the Au(111) single crystal, which was demonstrated by voltammetric desorption results. They also attributed those differences to the potential of zero charge of Au(111) and Au(110). Fundamental studies of electrochemical stability of alkanethiol SAMs with various chain lengths on gold showed the short-chain alkanethiolate SAMs were removed reductively by the cyclic voltammetry (CV) cycle, while for long-chain alkanethiolate SAMs various behaviors under negative potentials were reported. Some results showed that for the long-chain alkanethiolate SAMs after one cycle of CV the film desorbed from the gold surface, while other studies indicated that even after multicycles of CV sweep the monolayers were still chemically attached to the gold surface. It has been suggested that the different voltammetric behaviors of the long-chain SAMs could be attributed to the different deposition time of the gold substrates in the alkanethiol solutions and quality of gold surface. A variety of techniques have been used to characterize potential induced film transformations in SAMs. Fritsch-Faules et al.43 reported the first direct quantification of a slow loss in coverage of SAMs under potential control as a function of time using ex situ CV. Due to its high resolution, scanning tunneling microscopy (STM) is the most frequently used technique, both in situ and ex situ, to study the potential induced film changes.10,11,26,40,44,45,49 Lennox et al. applied ac impedance spectroscopy to study the potential-induced defects in n-alkanethiol SAMs and described a concept of critical potential,38 a precursor to the electrodesorption process; stability of ω-functionalized SAMs as a function of applied potential was also explored15 using impedance spectroscopy. X-ray photoelectron spectroscopy (XPS)47 was also reported to examine the influence of an electrostatic potential at the goldSAM interface on the electron binding energy of adsorbates. IR spectroscopy35,36,48 was used to characterize the chemical structure change of the potential-swept monolayers. However, few of these techniques were able to accurately reveal the molecular-level changes in the SAMs, which would provide insight into the reductive desorption properties of the monolayers under potential sweeps. In this work, the reductive desorption of different chain length n-alkanethiol SAMs on evaporated gold using cyclic voltammetry, electrochemical impedance spectroscopy, and sum frequency generation is presented. The combined techniques have revealed new details of the reductive desorption of the alkanethiol SAMs. Different desorption behaviors and different states of the SAMs after applying CV negative potentials with various chain lengths of the thiolates were observed. For short-chain (C10 and C12) monolayers, the results from CV, electrochemical impedance spectroscopy (EIS), and sum frequency generation spectroscopy

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Figure 1. Kinetics of C18 SAM formation: Bode phase plot for C18 SAMs at various deposition times: 4, 38, 43.5, 51.5, 53.5, 61, and more than 72 h. The inset shows the phase angle of Z at 0.1 Hz vs the deposition time of a Au slide in the 1 mM ODT solutions.

(SFG) all indicated the removal of the films to a large extent from the surfaces. The most intriguing observation was for the longchain (C16 and C18) SAMs. Surface monolayers of the longchain species appeared to be removed by CV but appeared intact by EIS and SFG. The phase angle results from EIS illustrated that C16 and C18 layers still retained their ion insulator properties after the potential sweeps. SFG results clearly showed that C16 and C18 SAMs remained densely packed and well-ordered under negative potentials. The higher solubility of the desorbed molecules in aqueous solution and the relatively low intermolecular interactions for short-chain SAMs contribute to their instability compared to long-chain SAMs.

’ EXPERIMENTAL SECTION Materials. The alkanethiols, 1-octanethiol (97+ %), 1-dodecanethiol (98+ %), and 1-octadecanethiol (98%), were purchased from Sigma Aldrich and used as received; 1-hexadecanethiol (95%) was obtained from Fluka and used without further purification. Also, absolute ethyl alcohol from Aaper and sodium hydroxide (NaOH, 97%) purchased from EM Science were used as received. The aqueous solution of sodium hydroxide was prepared in 18.2 MΩ cm water obtained by purification of distilled water with a Millipore Milli-Q system. Monolayer Preparation. All alkanethiol SAMs were prepared using freshly evaporated polycrystalline gold surfaces. Thin gold films for SAM adsorption were deposited on the highly polished side of silicon (100) substrates by thermal evaporation at a pressure of 1 105 Torr in a vacuum chamber. A 10 nm Cr layer was deposited with a deposition rate of 1.0 Å/s prior to Au deposition to improve adhesion to the Si wafer. Gold of 99.99% purity was deposited to a thickness of 100 nm at a rate of 1.0 Å/s. The gold substrate was then rinsed with ethanol and then immersed in a 1 mg/mL alkanethiol solution of absolute ethanol for more than 72 h with the staining jar sealed with parafilm, covered with aluminum foil, and placed in a dark cabinet. The deposition time of the gold slides in thiol solutions was based on EIS kinetics study, as shown in Figure 1. This plot shows the time evolution of C18 SAM formation on gold surfaces presented using a Bode phase plot. The inset illustrated the phase angle vs the dipping time of gold slides in ODT solutions. 19179

dx.doi.org/10.1021/jp202060n |J. Phys. Chem. C 2011, 115, 19178–19189

The Journal of Physical Chemistry C

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Figure 2. Three representative cycles of cyclic voltammograms of an evaporated Au electrode coated with (a) C10 thiol SAM and (b) C12 thiol SAM, measured in 100 mM NaOH aqueous solution. Sweep rate: 50 mV s1. For a clearer view of the evolution of the CV curves as the number of cycle increases, not all the CV cycles are presented.

The phase angle results for C18 SAMs, with various deposition times, 4, 38, 43.5, 51.5, 53.5, 61, and more than 72 h, demonstrate that after 72 h for the C18 SAM the phase angle at 1 Hz reached a value of 88°, an indication of a defect-free monolayer.54,55 Thus a deposition time of 72 h was applied to all the SAM preparations. An in situ Surface Plasma Resonance56 kinetics study also showed that it took more than 70 h for the ODT SAM signal to reach a steady state. Staining jars were cleaned using a 12% micro-90 (International Products) cleaning solution and rinsed thoroughly and sonicated in Milli-Q. To minimize artifacts originated from different morphologies of the Au surface, all samples were prepared using one batch of thermal evaporated gold substrates. Electrochemistry. All electrochemistry measurements were carried out using a one-compartment, three-electrode cell driven by a 263A Princeton Applied Research Potentialstat/Galvanostat in combination with a PAR M5210 lock-in amplifier which is controlled by the PowerSuite software version 4.7. The gold substrate with an alkanethiol SAM on it was used as the working electrode. The counter electrode was a platinum wire, and the reference electrode was a saturated calomel electrode (SCE). The three-electrode cell contains 100 mM NaOH water solution. Cyclic voltammetric measurements were conducted at a scan rate of 50 mV s1. The scan range for all SAMs was from open circuit potential (approximately 0.2 V) to 1.35 V. The potential was initially scanned to the negative-going direction to reduce the SAMs and then scanned reversibly to reoxidize the desorbed thiolates. EIS measurements were applied to the SAMs before and after the CV scans and were performed at open circuit potential (OCP). The amplitude of the applied sinusoidal signal was 10 mV. The scan range of frequency was from 100 kHz to 0.1 Hz. The impedance data were fit to a suitable equivalent circuit (EC) by ZSimpWin software using complex nonlinear leastsquares fitting (CNLS). All the potentials reported here are relative to the reference electrode, SCE. The electrolyte solution was deaerated by bubbling N2 gas for at least 15 min before each experiment. The cell was kept under a small nitrogen overpressure during all electrochemical measurements. All Au working electrodes with the same geometry are used in these studies. The area of the working electrode was 1.69 cm2. Sum Frequency Generation Spectroscopy. SFG experiments were conducted ex situ in air at room temperature before

and after applying the sweeping potentials to the monolayers by a home-built setup in a copropagating optical geometry as described previously.57 A visible beam having a wavelength of 532 nm was generated via second harmonic generation of the 1064 nm beam through a nonlinear crystal KTiOPO4 (KTP); the tunable IR having a range of 20004000 cm1 was produced via an optical parametric generation/optical parametric amplification (OPG/OPA) system. The incident angles of the visible and IR beams to the sample surface normal are 50° and 60°, respectively. The scan rate was 1 cm1/s with a repetition rate of 20 Hz. All the spectra were obtained in the CH stretching region from 2750 to 3050 cm1 using a ppp polarization combination with an average of five scans for each spectrum. The SFG spectra were curve fit by Origin 7.5 according to the following equation ISFG µ jχNR

ð2Þ

þ χR

j ¼ χNR þ

ð2Þ 2

∑q

2 Aq ωIR ωq þ iΓq ð4Þ

where χNR and χR are the nonresonant susceptibility attributed to the metal surface and resonant susceptibility attributed to the adsorbed SAM, respectively. Aq, ωIR, ωq, and Γq are the amplitude, the scanning IR frequency, the qth resonant vibrational frequency of the adsorbates, and the line width, respectively. Prior to the measurements, the original SAM samples were rinsed thoroughly with ethanol; the SAM samples after 30 cycles of CV and EIS measurements were rinsed with copious Milli-Q water. All samples were blow dried with nitrogen gas.

’ RESULTS AND DISCUSSION Cyclic Voltammetry. Figure 2 and Figure 3 show reductive desorption CVs of C10, C12, C16, and C18 alkanethiol SAMs, respectively, measured in 100 mM NaOH water solution with the sweeping potential ranging from OCP to 1.35 V vs SCE. At more negative, potentials the interference of hydrogen evolution occurs as presented in the voltammograms. For each film, 30 CV scans were performed. In Figure 2, two cathodic waves corresponding to reductive desorption of SAMs were observed for C10 and C12 SAMs. 19180



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Figure 3. Three representative cycles of cyclic voltammograms of an evaporated Au electrode coated with (a) C16 thiol SAM and (b) C18 thiol SAM, measured in 100 mM NaOH water solution. Sweep rate: 50 mV s1. Not all the CV cycles are presented, for a clearer view of the evolution of the CV.

When the potential was swept in the positive direction, no significant anodic peak corresponding to the oxidative readsorption of the desorbed thiolates was observed. It has been suggested that the absence of the anodic peak for C10 and C12 SAMs indicates that a fast desorption and diffusion process occurred such that under the present sweep rate conditions the desorbed watersoluble thiolates diffused away from the electrode surface before the oxidative readsorption.8 Thus, a chemically irreversible redox process was observed. The absence of anodic peaks for both C10 and C12 SAMs on evaporated Au surfaces in this study is somewhat different from some previous observations,13,17 where on Au(111) surfaces the oxidative peaks were observed. The origin of the difference in the oxidation behavior for the SAMs is not clear; however, the surface morphology, or the surface roughness,8 might be the possible factor that resulted in the different oxidative response of the desorbed thiolates. For the first reductive sweep for both C10 and C12 SAMs, multiple unresolved peaks appeared: one is a broad peak near 1.15 V for C10 and C12 SAMs, with a shoulder on its positive potential side, and the second one is a sharp peak at around 1.25 V. The reductive peaks for both C10 and C12 SAMs decreased dramatically in the second CV cycle, indicating a major loss of the monolayers during the first cycle of the CV scans. As the number of scans increased, the reductive peaks of both SAMs decreased monotonically, and the peak positions shifted in the positive direction. The decrease of the reductive current for each SAM implies that fewer and fewer thiol molecules are chemisorbed to the surface as the number of scans increases, which was further supported by EIS and SFG results as will be discussed later. The gradual anodic shift of the reductive potential, Ep, in the successive CV cycles as the scan number increased could arise from the decrease of the surface coverage, Γ, and a shift of potential of zero charge (PZC) to more negative values as Γ decreased.58 Porter and co-workers also related the shift of the reductive desorption peak to the anodic position with the decrease of surface coverage to the penetration of electrolyte ions in the SAM.9,12,59 In the 1960s, a detailed theoretical study of the kinetic behavior of a simple one electron reaction driven by a linear potential sweep was performed by Conway et al.60,61 It revealed both qualitatively and quantitatively the dependence of peak potential, shape, and the half-width on the magnitude and nature (repulsive or attractive) of the lateral interactions in the chemisorbed layer.

Their results indicate that as surface coverage decreases the attractive interaction between the adsorbed molecules results in a decrease in desorption energy of the film and hence a shift of the reductive peak potential to more positive values. For C16 and C18 SAMs, the cyclic voltammograms also showed multiple waves as illustrated in Figure 3. Researchers have explored the possible origins of the multiple waves. It was reported that multiple waves showing in the voltammogram might be associated with the polycrystallinity of the asevpaporated gold surface.26 The appearance of multiple waves in the reductive/oxidative CV cycle of alkanethiolate SAMs on the gold surface was also observed by other groups8,12,13,17,26,33,40,50 and has been ascribed to different binding sites,13,26,33,50 different crystallinity of gold substrate,26,33 or the difference in the adsorbed states, i.e., physisorption and chemisorption.36,42 Specifically, the Porter group33 reported that the reductive desorption potential is dependent on the surface crystallinity of the underlying gold substrate. They indicated that the goldsulfur bonding is stronger on Au(110) than on Au(111) single crystal. The voltammetric desorption results indicated that AuS bonding is stronger at the Au(110) single crystal than at the Au(111) single crystal. They also attributed those differences to potential of zero charge of Au(111) and Au(110). The measurements by Morin et al.12 further indicate a correlation between the surface charge density and PZC of a clean Au electrode and the potential of the thiol reduction peak. They suggested that the different PZC values for the different crystallographic orientation of Au could be used to predict the peak position of the reductive desorption of SAMs. The PZC values of the Au(111), Au(100), Au(311), Au(110), and Au(210) faces are 0.28, 0.14, 0.02, 0.02, and 0.10 V, respectively.53 Thus, a range of values of the reductive potential for a SAM on different Au crystalline faces would be expected. The as-evaporated gold used in these experiments is a polycrystalline Au. These two groups12,13 also reported that the presence of multiple waves in voltammograms of alkanethiolate SAMs on Au(111) surfaces was chain length dependent. It was observed13 that as the chain length increased from C4 to C11 multiple waves appeared in both the reductive and oxidative currents. The splitting of both the reductive and oxidative peaks was also observed for C14, C15, and C16 SAMs on Au(111). The pH value of a solution was found to affect the appearance, disappearance, and shape of the multiple waves as well.8 19181


The Journal of Physical Chemistry C A recent study addressing the origin of the double peak from the Buck group62 combined the in situ SHG and CV. The advantage of SHG is that it is essentially sensitive to the faradaic part only, while CV probes both faradaic and capacitive processes. The study clearly evidenced that neither the splitting of the desorption peak nor the splitting of the readsorption peak arose from the separation of Faradaic and capacitive processes. The appearance of the double peak was suggested to originate from intermolecular interactions.62 In contrast to the shorter-chain SAMs, C16 and C18 SAMs exhibited markedly different behaviors from those of C10 and C12 SAMs in the CV sweeps, as shown in Figure 3. Unlike the short-chain SAMs with no detected oxidative waves, peaks did appear in the anodic sweeps for C16 and C18 SAMs, providing the evidence that some of the desorbed thiolate molecules of the longer-chain SAMs readsorbed onto the gold surface, while the potential went to the less cathodic voltages in the CV cycles. These observations are in good agreement with previous studies of the chain length dependent oxidative readsorption of the alkanethiol SAMs.7,8,13,31,46,50,63 The change in the behavior of oxidative readsorpiton as the chain length increased was explained by the dramatic decrease in solubility of the desorbed thiolates in aqueous solutions7,8,31,46,50,63 and the decrease of diffusion coefficients13 of the thiols as the chain length increases. The solubilities of C10, C12, C16, and C18 thiols in water were estimated using the method64 established by Wakita et al. and were 5.7 106, 3.5 107, 1.6 109, and 6.5 1011 mg/mL, respectively. The values of the estimated solubility showed a five-order of magnitude decrease in solubility for C18 thiol molecules compared to that of C10 thiol molecules. A simple estimate of the diffusion coefficient change of the thiol molecules as the chain length changes by applying Fick’s law gives the relative diffusion coefficient of C12, C16, and C18 to C10 values of 0.93, 0.82, and 0.78, respectively. The differences in magnitude of solubility and diffusion coefficient of the thiols with different chain length suggest that these might be the major factors affecting the reoxidative behavior of alkanethiol SAMs. Morin et al.8 also demonstrated that the solubility of the thiol is the most important factor in the oxidative readsorption process by examining the reductive desorption/oxidative redeposition of C4, C9, and C16 SAMs from the Au(111) surface in 0.1 M KClO4 solutions at different pH values. Kinetics studies further demonstrated that the very low solubility of ODT, as well as other long-chain alkanethiols, desorbed in aqueous solutions prevents any significant loss of material near the surface by diffusion into the solution in a short time (e.g., 1 s).50,63 Consistent with the chain length dependent peak potential of SAMs reported by other groups as described previously, the reductive peak potentials for longer-chain SAMs, C16 and C18 SAMs, are located at a more cathodic position compared to those of shorter-chain monolayers. It is possible that one of the reductive peaks for C16 and C18 SAMs overlapped with or is at a potential more negative than hydrogen evolution. For the C18 SAM, in the first cathodic potential sweep, there was no well-resolved reductive peak showing in the voltammogram, only a small wave overlapped with hydrogen evolution presented in the range of 1.2 to 1.3 V. One small oxidative peak at 0.87 V presented in the voltammogram in the following positive-going potential sweep. One of the reductive desorption peaks of C18 SAM might be located at a potential more negative than that of the hydrogen evolution and is out of the scan range. Similar to C10 and C12 SAMs, as the number of scans increased,

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the anodic shift of the reductive wave for both C16 and C18 SAMs was observed. For the anodic sweeps, a gradual negative shift of the oxidative potential for both C16 and C18 SAMs was also shown in the voltamograms (see Supporting Information), which indicates that the desorbed thiolates tended to be more easily oxidized as more thiols desorbed during subsequent CV cycles. Note that in each cycle the anodic stripping peak area was smaller than the cathodic stripping peak area, indicating only part of the desorbed species redeposited on gold and the net effect for each cycle was still the loss of chemically bound thiolates. It is apparent in the final cycle of the C16 and C18 voltammograms that the reductive currents of both SAMs decreased dramatically compared to those in the first cycles, indicating that a large amount of the long-chain films also lost their chemical bond to the gold surface even with the partial reoxidation of the desorbed species in the positive-going sweeps. Quantitative estimations of the surface coverage of the monolayers studied by integration of the reductive current were complicated by the overlap of the multiple reductive desorption waves and the overlap of the cathodic waves with hydrogen evolution. However, estimations base on the peak current65 showed that after 30 cycles of CV scans approximately 90% of the monolayer was removed for both C10 and C12 SAMs, and nearly 70% of the SAMs were removed for C16 and C18 monolayers, respectively. A similar trend of the SAM loss (to about 70% of the first scan) for the reductive desorption of the C16 SAM on Au(111) surfaces was reported by the Kakiuchi group,10 while others8 showed that about 10% of the C16 monolayer desorbed after the first cycle and remained almost unchanged in the following cycles. The change in the properties of the SAMs was also examined by the change of double layer capacitance of the monolayers calculated using the CV results. Densely packed SAMs are wellknown to be both ion and electron insulators. Desorption or partial desorption of the film from the electrode would result in the dielectric constant change in the medium between the gold electrode and the electrolyte solution and thus the change in effective capacitance of the double layer. The double layer capacitance of the monolayer coated electrode and bare electrode is estimated from the double layer charging current assuming a perfect capacitor model65 using the following equation Ia Ic dE Cdl ¼ υCdl ¼ ð5Þ dt 2 The double layer capacitances of C10, C12, C16, and C18 SAMs and of those after CV cycles were estimated and listed in Table 1. Surprisingly, the double layer capacitance results for the SAMs studied showed that after 30 CV sweeps the Cdl for C10 and C12 are both close to that of the bare Au, indicating that C10 and C12 films lost their ion/electron barrier function, while for C16 and C18 SAMs, the Cdl values after the potential sweeps were 10 and 5 μF, which were just slightly different from those of their original films, indicating that the monolayers after the applied potential sweeps were still within the double layer region. Recall that the reductive desorption of the SAMs consists of two processes, as shown in eqs 1 and 2, respectively, the break of the AuS bond and the dissolution and diffusion of the desorbed species into the bulk electrolyte solution. The dramatic difference between the change in double layer capacitance for the shortchain SAMs and long-chain SAMs might be the consequence of a dramatic difference in the time scale of the dissolution and 19182



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Table 1. Double Layer Capacitance of the SAM Coated Au Electrodes before and after 30 CV Cycles Estimated Using Charging Current double layer capacitance (Cdl, μF) SAMs

before CV cycles

after CV cycles

C10

6

57

C12

18

52

C16

7

10

C18

1

bare gold

5 55

diffusion process between the short-chain molecules and the long-chain molecules. To further explore the monolayer structure change of the monolayers after the applied potential, SFG experiments were carried out; the SAM permeability of ions and electrons, thus the quality of monolayers, was also examined using EIS. Electrochemical Impedance Spectroscopy. Alternating current electrochemical impedance spectroscopy (AC EIS) is a very useful technique to assess the quality of SAMs with high resolution without damaging the monolayers. The most prominent advantage of EIS is that a Bode phase vs frequency plot is not affected by true (vs geometric) area differences or variations in parameters such as sweep rate.55,66 EIS was applied to a bare gold surface and the SAM modified gold surface. In a Bode phase plot, the phase angle (j) of a pure capacitor is 90°; the angle of a pure resistor is 0°; and the angle of a pure inductor is 90°.66 This analysis holds in the low frequency (f) range (1 Hz < f < 103 Hz) where diffusion dominates. Thus, in this region a SAM with a j g 88° is considered to act as a pure capacitor, and a SAM with a j e 87° behaves like a capacitor parallel with a resistor, having a certain amount of leakage of current due to the defects in the film.54,55 The Bode phase angle at 1 Hz is used for the diagnostic of the SAM quality as in this region the diffusion-related phenomena occur,55,67 and the change in phase angle was demonstrated to be more sensitive than the change in capacity in revealing the insulating character of a SAM.55 Figure 4 shows the Bode phase plot of the bare gold electrode and electrodes modified by C10, C12, C16, and C18 alkanethiol SAMs. In this figure, the increase in phase angle as the alkyl chain length increased is apparent, indicating that the longer-chain SAMs tend to be densely packed and have fewer defects. As listed in Table 2, the phase angle at a frequency of 1 Hz for C10 is 85°, which is less than 88°, showing that the monolayer formed by the C10 thiol has some defects in the monolayer allowing ion transportation through the film and thus the current leakage. The phase angle increased to 88°, 89°, and 89° for C12, C16, and C18, respectively, while the phase angle of bare Au without blocking film is 51°. These data reveal that C12, C16, and C18 thiols all essentially form densely packed monolayers which exhibit phase angles of g88°, indicating that these SAMs were nearly perfect ion barriers. To examine the effect of reductive potentials on the monolayers, EIS was also applied to the films after 30 cycles of CV sweeps from OCP to 1.35 V. The EIS spectra of the all the SAMs before and after applying the cathodic potential sweeps were shown in Figure 5. From the previous CV results, even for the longer-chain SAMs, C16 and C18 monolayers, the magnitude of the reductive current, or the integrated Faradic charge

Figure 4. Bode phase plot of a bare gold electrode and electrodes modified by C10, C12, C16, and C18 alkanethiol SAMs. All the SAMs were prepared using a deposition time of more than 72 h in the corresponding 1 mM thiol ethanol solution. The impedance spectrum was obtained at open circuit potential in 100 mM NaOH.

Table 2. Phase Angle Changes in SAMs before and after CV Cycles at a Frequency of 1 Hz SAMS

C10

C12

C16

C18

bare Au

before CV (deg)

85

88

89

89

52

after CV (deg)

71

59

83

87

-

under the reductive peak, clearly indicated that a large amount of the monolayers lost their chemical bond to the gold surface. It is possible that some thiolate molecules remained adsorbed for the C18 SAM due to the more negative location of the reductive waves than that of the hydrogen evolution. For the shorter-chain SAMs, C10 and C12, it was found from the CV that most of the SAMs chemically detached from the gold surface. On the basis of these observations, and considering the mechanisms proposed for the reductive desorption of thiolate SAMs, it is expected that the low frequency phase angle of C10 and C12 monolayers would decrease dramatically to an angle that is close to that of bare gold, and C16 and C18 monolayers would also lose their ion insulator character and therefore induce a significant drop in the phase angle. Surprisingly, the EIS Bode phase results for C16 and C18 SAMs did not agree with these expectations of the change of phase angle for these monolayers: the angle decreased from 89° to 83° for C16 and from 89° to 87° for C18. The results for C10 and C12 self-assemblies were consistent with their CV behaviors: the phase angle changed from 85° to 71° for C10 and 88° to 59° for C12, which were listed in Table 2 and illustrated in Figure 5. The electrochemical property change in C10 and C12 SAMs due to the applied CV potential sweeps is obviously linked to the desorption of the films from the gold electrodes, which renders the SAMs permeable to ions and water. The unexpected small variation of the phase angles of C16 and C18 SAMs from those of the original films reveals that the chemical bond of sulfur and gold no longer exists to a large extent. For the SAM molecules after the CV cycles as indicated by the CV results, the chemically desorbed C16 and C18 thiolates, especially C18 thiolates, were still excellent ion insulators with little indication of defects after the negative potential sweep. This suggests that those chemically desorbed molecules and chemically intact molecules for C16 and C18 SAMs would still 19183



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Figure 5. Impedance Bode phase spectra of (a) C10, (b) C12, (c) C16, and (d) C18 n-alkanethiol SAMs before and after 30 cycles of CV sweeps from OCP to 1.35 V.

maintain their 2D network similar to the original densely packed monolayers due to the hindered diffusion process of the desorbed species away from the electrode arising from the dramatically decreased solubility (5 orders smaller for C18 than that of C10) of the long-chain species compared to those of the shorterchain molecules and the increased chainchain interactions. The decrease in diffusion coefficient as the chain length increases also plays a role in impeding the diffusion of desorbed thiolates from the surface. It is well-known that poorly soluble compounds dissolve slowly. The dissolution rate (Dr)64 of a solute described by the Noyes and Whitney equation is Dr ¼

DAðCs CÞ hV

ð6Þ

where D and A are the diffusion coefficient and the solute surface area, respectively; Cs and C are concentrations of the solute at the surface and in the bulk medium, respectively; V is the volume of the dissolution medium; and h is a constant that depends upon the viscosity and the degree of agitation of the medium. With the assumption that the concentration of the solute at the surface is proportional to solubility (Sw) and that in the bulk solvent is assumed to be near zero, the equation became Dr ¼

DASw hV

ð7Þ

As mentioned previously, the diffusion coefficient of the thiol molecules varies slightly as the chain length changes. Considering that the experimental conditions of each system are the same, the dissolution rate of the thiol is approximately proportional to its solubility. Therefore, the dissolution rate of C16 and C18 molecules would be roughly 104 and 105 times slower than that of C10 molecules in the electrolyte solutions. It is commonly accepted that the monolayers formed by the long-chain alkanethiols on gold are at least semicrystalline.51,6870 Due to the much lower dissolution rate compared to the short-chain (C10 and C12) molecules, most likely the solid-like monolayers of C16 and C18 on gold surfaces after the reductive potential sweeps remained in the solid state and did not dissolve and diffuse into the bulk electrolyte solution in the time scale of the experiments. Similarly, the melting points for C10, C12, C16, and C18 thiols are 26 °C, 7∼9 °C, 18 °C, and 2431 °C, respectively. The solid characteristic of the long-chain SAMs emphasizes the SAu bonds. To further examine the state of the SAMs and to lend evidence to our suggestion of the long-chain SAM structure after the CV cycles, SFG, the specifically interfacial sensitive second-order nonlinear vibrational technique, was applied to the films studied, which will be described later. Capacitance of the SAMs was extracted from EIS curve fitting using the equivalent circuit illustrated in Figure 6 and revealed increasing barrier properties with increasing chain length, as indicated in Table 3. The details of the equivalent circuit and fitting 19184



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Figure 6. Equivalent circuit for SAMs, C10, C12, C16, and C18 monolayers, before and after 30 cycles of CV sweeps from OCP to 1.35 V.

Table 3. Summary of Calculated Capacitance Density (μF cm2) Values of the SAMs from EIS Curve Fitting Results total interfacial capacitance density (μF cm2) SAMs

before CV

after CV

C10

1.63

11.79

C12

1.41

17.14

C16

1.27

6.46

C18

0.79

1.04

bare Au

55.34

results for all the equivalent circuit elements are found in the Supporting Information. The total interfacial capacitance for C10, C12, C16, and C18 SAMs is 1.63, 1.41, 1.27, and 0.71 μF cm2, respectively. The low capacitance values for all the SAMs indicated that all the alkanethiols formed ordered films and were good ion insulators. Moreover, the decrease in capacity from 1.63 to 0.71 μF cm2 as the alkanethiol chain length increased from C10 to C18 also revealed that the longer-chain thiols formed more densely packed SAMs compared to the shorter-chain thiols. The calculated capacitances for C10, C12, C16, and C18 from EIS curve fitting compare favorably with those found in the literature.7,14,38,41,55,67 Widrig et al. obtained a capacitance of a C18 SAM of ∼1.7 μF cm2;7 Fokkink and co-workerss showed a value of ∼0.7 μF cm2 for a C18 SAM;41 and the typical capacitances for C12, C14, C16, and C18 SAMs in 0.5 M H2SO4 measured by the Lynch group were in the range of 15 μF cm2.14 The capacitance of a C16 SAM determined from a cyclic voltammogram was ca. 1.2 μF cm1,55 in good agreement with the capacitance of C16, 1.27 μF cm2, calculated from EIS fitting. The capacitance (1.63 μF cm2) obtained for the C10 SAM also agreed well with Ulman group’s result, which demonstrated that a C10 SAM was impermeable to ions (Cdl = 1.61 ( 0.09 μF cm2 in 0.01 M NaClO4) at the PZC of Au(111);67 Lennox et al. indicated a capacity of 1.55 ( 0.01 μF cm2 for a C10 SAM.38 The calculated capacities of the SAM modified electrodes after 30 CV sweeps were 11.79, 17.14, 6.46, and 1.04 μF cm2 for C10, C12, C16, and C18 SAMs, respectively. The EIS results for C10 and C12 SAMs after applying the negative potentials are consistent with those of the CV. The capacity of C10 and C12 increased to a value which is comparable to a reported double layer capacity of a polycrystalline gold electrode, ∼16 μF cm2, using CNLS for the EIS experimental data,67 which was also in agreement with the double layer capacitance results of the

polycrystalline gold from the Skoog group.71 The increase in the interfacial capacity of C10 and C12 SAM modified gold electrodes indicates that most of the monolayers desorbed from the electrodes by applying the potential sweeps. These results are consistent with what has been revealed by cyclic voltammetry experiments as discussed earlier. The most intriguing feature of the change in capacity values of SAMs after the CV treatments is that after 30 cycles of potential sweeps the capacity of C16 and C18 covered gold electrodes changed little and remained low, 6.46 and 1.04 μF cm2, respectively, indicating that water and/or ions were still mostly impermeable to the chemically desorbed thiolate structure. In another word, the monolayers remained in the vicinity of the electrode and retained its densely packed structure within the double layer region. Morin et al.36,48 also observed no noticeable change in the IR spectrum of the alkane chains after a CV sweep using differential reflectance IR spectroscopy. To support our observations and to gain further insight into the behavior of the thiol molecules with various chain lengths after being chemically detached from the gold surface by applying sufficient negative potentials, a highly interfacial sensitive nonlinear optical vibrational technique, SFG, was applied to the SAMs studied.

’ SUM FREQUENCY GENERATION Figure 7 shows the SFG ppp spectra of C10, C12, C16, and C18 SAMs on Au surfaces in air before and after 30 cycles of CV scans, respectively. Curve fitting of the spectra was performed using the method described previously. The SFG fitting results for CH3 modes, symmetric stretching (r1+), antisymmetric stretching (r), and Fermi resonance (r2+), for C10, C12, C16, and C18 SAMs before and after CV sweeps are listed in the Supporting Information. Three typical modes39,7274 of the alkanethiol SAM on gold appeared in the ppp-polarized SFG spectra in the 27503050 cm1 region for all the SAMs before the CV cycles and the potentialswept C16 and C18 SAMs. All the vibrational modes showed dips in the ppp spectra due to the interference of the resonant signal with the nonresonant background from the gold surface.75 The resonances at 28702875 cm1 for C10, C12, C18, and C16 SAMs, respectively, are attributed to the symmetric (r1+) CH stretches of the methyl (CH3) group; the vibrational modes at 29252935 cm1 are assigned to the Fermi resonances (r2+(FR)) between the symmetric mode and the overtone of bending mode of methyl CH; and the mode at approximately 2960 cm1 for the SAMs studied is assigned to the antisymmetric (r) CH stretch of the methyl group. Signals due to methylene modes were negligible for all the freshly prepared SAMs studied, which suggests that the thiolates adopt an all-trans chain conformation. In straight alkane chains, methylene groups are distributed symmetrically along the chain, and therefore they were SFG inactive; when the alkane chains are not ordered, gauche defects of the methylene reduce the centrosymmetry of the ordered methylene unit and result in the appearance of νCH2 modes.39,7681 The intensity of the CH2 modes is determined by the combination of the number of gauche defects and the average orientation of those groups. The SFG results indicate that all the SAMs prepared were well ordered, which is consistent with the EIS results presented previously and other alkanethiol SAM studies.3 The spectra of the SAMs after applying 30 cycles of CV showed that for the short-chain SAMs, C10 and C12 monolayers, the intensity of all characteristic CH3 peaks decreased dramatically, 19185



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Figure 7. SFG ppp spectra of (a) C10, (b) C12, (c) C16, and (d) C18 thiol SAMs before and after 30 cycles of CV sweeps.

and CH2 modes at around 2845 cm1(symmetric CH stretching) and 2910 cm1 (Fermi resonance) presented in the spectra, indicating the disordered layer structure for the potential swept shorter-chain SAMs. This is in good agreement with the previous CV and EIS results and further provided evidence that C10 and C12 thiolates desorbed to a large extent and diffused away from the gold surface after multiple cycles of CV sweeps, leaving a low density/coverage disordered film. An interesting and surprising phenomenon was observed for C16 and C18 SAMs, the long-chain monolayers, after the CV potential sweeps. SFG results showed that no obvious change in terms of peak position and peak intensity was observed for the CH3 modes in the spectra for C16 and C18 SAMs before and after the CV cycles. It is well-known82 that the intensity of the resonant SFG signal is proportional to the square root of the number of molecules contributing to the SF signal and their average orientation, which is shown in the following equation ISF µ jχð2Þ j2 ¼ N 2 jj2

ð8Þ

where χ(2) is the second-order susceptibility; N is the number of molecules contributing to the SF signal; and is the molecular hyperpolarizability averaged over all molecular orientations on the surface. The nearly unchanged SFG intensities observed for the CH vibrational modes for the potential-swept C16 and C18 SAMs clearly indicate that the monolayers after the potential

sweeps retained the same monolayer structure as the freshly formed SAMs, i.e., the same molecular density and the same average orientation of the terminal CH3 functional groups, which will be discussed later. The C16 and C18 monolayers were still highly ordered on the gold surface even after 30 cycles of CV sweeps, which strongly supports observations by EIS. An in situ SHG study of reductive desorption of a C16 SAM on a gold/mica surface under a CV cycle ranging from 0.3 to 1.3 V by Buck et al.19 also concluded that the SAM remained largely intact after the CV cycle. The overshoot observed in the SHG desorption signals for a C22 thiolate SAM62 also suggested that in the cathodic sweep the thiols still affect the capacity of the layer. A previous SFG study83 of reductive desorption of ODT on zinc surfaces in a 0.5 M H2SO4 solution illustrated the unchanged spectrum of the monolayer after applying a CV cycle from 1.0 to 1.8 V. Orientation analysis was performed by a well-established model for organic layer on metal surfaces with the assumptions that the local symmetry of the CH3 group is treated as C3v and the interface of air and the monolayer is rotationally isotropic about the z-axis or surface normal. A distribution of a δ-function was also assumed considering the average tilt angle of the terminal CCH3 axis. The Fresnel factor model84,21,85,86 assumed that the nonlinear polarization is induced by the fields in the substrate just beneath the thin layer; i.e., a two-layer model was assumed. The refractive indices of gold57 used for IR, visible beam, and SF 19186


The Journal of Physical Chemistry C beam were 2.046 + 21.3i, 0.402 + 2.54i, and 1.426 + 1.846i, respectively. The refractive indices for air and the alkanethiol were 1 and 1.45,87 respectively; the local field factor of the interface layer, n, was estimated with the Lorentz model given by the Shen group.88 The intensity ratio of the symmetric and antisymmetric was simulated89 by assuming a Raman depolarization ratio of 0.053.90 The estimated intensity ratios of the SAMs studied and the simulated intensity ratio of the symmetric mode and antisymmetric mode of the CH3 group versus the tilt angle are presented in the Supporting Information. The average tilt angle of the terminal CH3 functional groups obtained for all the SAMs studied before the CV cycles was near 30°, which is consistent with the reports that alkanethiols with more than 10 carbons in the chain chemisorbed at gold form densely packed, ordered monolayers with the chain tilted ∼30° from the surface normal.51,69,91 The average tilt angle for C10 and C12 monolayers after the potential sweeps had values of ∼50° and ∼40°, respectively, showing that the monolayers were disordered after the reductive desorption, while for the long-chain SAMs the average tilt angle still remained near 30° even after 30 cycles of the potential sweeps. The SFG average tilt angle results for C16 and C18 layers indicate that for both of the SAMs, before and after applying the potential cycles, the monolayers remained well ordered. It is very interesting comparing the SFG results for the longchain SAMs with the results obtained from CV and EIS. SFG and EIS indicated the intact C16 and C18 monolayers, while CV results clearly showed that C16 and C18 SAMs chemically desorbed from the gold surface to a large extent. Specifically, the decrease in the area of the reductive peak for the final CV scan for each SAM compared to that of the first CV cycle illustrated that a large amount of SAM molecules lost their chemical bond to Au. Only a small fraction of the SAM recovered its chemical bond to Au. The combined results suggest that those chemically desorbed C16 and C18 thiolates induced by the potential sweeps remained in the same structure as the original SAM thiolates. The most likely interpretation is that for C16 and C18 SAMs both the interplay of the strong lateral interaction and much lower solubility (recall that the estimated solubility of the alkanethiols is 5.7 106, 3.5 107, 1.6 109, and 6.5 1011 mg/mL for C10, C12, C16, and C18 thiols, respectively) of the desorbed species prevented the desorbed thiolates from diffusing away from the gold surface; i.e., after applying the potentials the chemically desorbed but still physically adsorbed species remained in the double layer region in the same configuration as the freshly made SAMs. Limited diffusion into solution allows the long-chain SAMs to maintain their barrier properties. The dissolution rate is proportional to the diffusion coefficient and solubility. For all the SAMs studied, the diffusion coefficients (D) for the molecules of each thiol species are similar, ≈106 cm2 s1,92 while the solubility of C16 and C18 is 104 and 105 times smaller, respectively, than that of C10. Thus, the dissolution rate of C16 and C18 would be 104 and 105 times smaller than that of the shorter-chain SAM, which is believed to be the major cause of the different behaviors of the short-chain and long-chain SAMs after potential sweeps. It takes 104∼105 times longer for the long-chain molecules to dissolve and diffuse away from the surface. Furthermore, the more densely packed monolayers for C16 and C18, as suggested by EIS and SFG results, are more solid-like compared to the shorter-chain SAMs. Apparently, even after the partial reductive desorption of

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the monolayers, the 2D crystal maintained their whole patch layer structure without dissolving and diffusing away from the surface. In fact, C18 is solid at room temperature (MP ≈ 2431 °C). The shift of the reductive wave to a region which overlaps with the hydrogen evolution or even more negative to hydrogen evolution as the chain length increases is also an important contribution for the longer-chain SAMs maintaining their ordered layer structures due to the increased number of molecules that still chemically bind, or anchor, to the gold surface. There was no evidence for the long-chain SAMs in this study that aggregates or micelles formed after the reductive desorption and no evidence that the orientation of the longer-chain SAMs changed48 since these would show a much different SFG spectrum with lower overall intensity and varied peak ratio. One question arose from the study, why these physically adsorbed species did not chemically readsorb to the gold surface after the potential swept anodically. A possible explanation is that while some of the desorbed thiolates do reoxidize to SAMs most of the desorbed thiolates formed disulfides, RSSR. Disulfides remain physically adsorbed on the gold surface since their readsorption rate is much slower than thiols. This interpretation is consistent with that of Thom et al.62 where the AuS bonds form but not from a flat lying thiol. It is well-known that after the reductive desorption of the longchain alkanethiol SAMs the product is the free thiol/thiolate or the corresponding diakyl disulfide.7,93 The adsorption of disulfides on gold was reported to follow different mechanisms from that of thiols with multiple steps, as illustrated by the following equations93,94 RSSR þ Au þ e ðAuÞ f RSAu þ RS RS þ Au f RSAu þ e ðAuÞ

ð9Þ ð10Þ

The adsorption of disulfides occurs in a cathodic process (eq 9) and a following anodic process (eq 10). It was reported that the reductive potentials for diakyl disulfides to adsorb were more negative than 1.5 V,93,95 which was out of the CV scan range, and the electrochemical reductive adsorption of the generated disulfide would not occur in the CV scans. The spontaneous adsorption of disulfides onto gold most likely would not happen either in the time scale of our experiments considering that it was reported that the rate of the spontaneous adsorption of disulfide on gold was much slower (43%) than that of thiols.96 The disulfides would stay physically adsorbed on the surface and maintained the insulator property of the layer. From these observations, the ideas of electrochemical stability and monolayer protection are different from the traditionally presented model. While many inhibitors are grouped into cathodic or anodic inhibitors and some are considered barrier inhibitors, the results here suggest a dual role of protection, however not necessarily at the same time. While the initial adsorption of the thiol binds to cathodic sites, i.e., the bare metal, they can be effectively removed by sufficiently negative potential. This limiting cathodic inhibition apparently does not reduce their barrier inhibiting properties. As indicated by SFG and EIS, the longerchain alkanethiols still retain good blocking, while their cathodic protection, by lack of chemical bonding to the surface, is lost. Thus the films are stable even in the cathodic region despite losing chemisorption. A major reason is simple: they are solid at this temperature and crystalline by virtue of self-assembly. This suggests the crystallinity is an important property of the corrosion 19187


The Journal of Physical Chemistry C protection which also limits dissolution into solution. An interesting idea from SAMs is then the following: can there be a balance between the crystallinity and the fluid nature that the films are in a liquid crystalline phase where the protection of a barrier is retained as well as mobility for self-healing like properties.

’ CONCLUSIONS Exploring the reductive desorption of alkanethiol SAMs on the gold surface using CV, EIS, and SFG revealed some new features of the electrochemical properties of the monolayers with the increase of the alkane chain length. For the short-chain SAMs, C10 and C12 layers, the results from all the techniques indicated that the monolayers desorbed largely from the gold surface and that the desorbed thiolates diffused away from the surface after the applied CV sweeps. For the long-chain SAMs, the characterization of the electrochemical properties of sulfurgold bonding in SAMs by CV indicated the significant loss of the surface coverage of the C16 and C18 SAMs, while the EIS and SFG results demonstrated that the ion and electron insulation structure of the films and the conformational structure of the monolayers remained almost the same as that of the well-ordered SAMs. On the basis of these observations, we propose that for the long-chain alkane SAMs, such as C16 and C18 SAMs, even though some of the sulfurgold bonds break after the CV sweeps, the low solubility of the desorbed species and the interaction between desorbed and intact molecule chains impede the diffusion of the desorbed thiolates away from the surface. Furthermore, part of the reductively desorbed products formed disulfides in the oxidative process and maintained physically adsorbed on the gold surface. Therefore, the desorbed species remained in the double layer region of the gold electrode, and the 2D compact structure of the molecules remained almost unchanged. The different responses of the long-chain and short-chain alkanethiol SAMs under negative potential sweeps are closely associated with the negative shift of the reductive potential to even more negative to hydrogen evolution, the different solubility of the desorbed thiolates in aqueous solutions, and the different intermolecular interactions as the chain length changes. ’ ASSOCIATED CONTENT

bS

Supporting Information. (A) Estimation of the solubility of thiols in aqueous solvent. (B) CV results of the surface coverage of the monolayers estimated for the first cycle and the last cycle. (C) EIS equivalent circuit and fitting results. (D) SFG spectrum curve fitting results and orientation analysis. This material is available free of charge via the Internet at http:// pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel.: 832-842-8844. Fax: 713-7432709.

’ ACKNOWLEDGMENT We are indebted to Professor Thomas Randall Lee's group for providing us all the evaporated gold slides used in the experiments. Financial support from NSF-DMR #0856009 is also gratefully acknowledged.

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Surface Barrier Properties of Self-Assembled Monolayers as Deduced

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