Isostructural Self-Assembled Monolayers. 1. Octadecyl 1-Thiaoligo

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Isostructural Self-Assembled Monolayers. 1. Octadecyl 1-Thiaoligo(ethylene oxides) David J. Vanderah,* Richard S. Gates,† Vitalii Silin, Diana N. Zeiger,‡ John T. Woodward, and Curtis W. Meuse* Biotechnology Division, Chemical Science and Technology Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8313

Gintaras Valincius§ Institute of Biochemistry, Mokslininku 12, Vilnius 2600, Lithuania

Bert Nickel Department of Chemistry, Princeton University, Princeton, New Jersey 08544 Received December 12, 2002 The self-assembled monolayers (SAMs) of a series of octadecyl 1-thiaoligo(ethylene oxide)x disulfides {[S(CH2CH2O)xC18H37]2}, where x ) 4-8, were assembled on gold and characterized by reflection-absorption infrared spectroscopy (RAIRS), spectroscopic ellipsometry (SE), electrochemical impedance spectroscopy (EIS), synchrotron X-ray reflectivity (XRR), and atomic force microscopy (AFM). For x ) 6-8, the EIS data indicate stable, highly ordered, nearly defect-free SAMs that behave as ideal capacitors in an electrochemical environment, comparable to well-packed SAMs of octadecanethiol. For these SAMs, the RAIRS data show that the 1-thiaoligo(ethylene oxide) (TOEO) segments, oriented normal to the substrate, adopt the ordered 7/ helical conformation of the folded-chain crystal polymorph of poly(ethylene oxide) (PEO) and the 2 hydrocarbon segments, tilted ∼30° to the normal, adopt a nearly all-trans conformation. AFM measurements on x ) 6 yield reciprocal lattice points that are consistent with the x3 × x3R30° overlayer structure commonly found in SAMs of long-chain alkanethiols. Within the limits of our measurements, the x ) 6-8 SAMs appear to be isostructural. As a result, SAM thicknesses should increase in a predictable fashion. SE and XRR measurements indicate thickness increases of 0.30 ( 0.05 nm/ethylene oxide (EO) unit, in good agreement with the expected increase of ∼0.28 nm/EO unit as derived from the unit cell dimensions of crystalline PEO. Films in which relative thicknesses can be calculated directly from crystallographic dimensions and are independent of refractive index should be useful materials for surface science metrology.

Introduction Metrology of thin films relies on surface science techniques capable of measurements with spatial resolution at or below 0.1 nm, such as ellipsometry, surface plasmon resonance, neutron and X-ray diffraction, neutron and X-ray reflectivity, X-ray photoelectron spectroscopy, and a variety of scanning probe microscopies.1,2 As the capability of each technique improves, new materials are needed to validate increasing complexities of film molecular structure, component spatial arrangements, and topography. The measurement of film thickness is particularly important. With the exception of microscopies that measure film thickness at a distinct boundary edge, film thickness is not measured directly. For example, ellipsometry measures changes in the amplitude (ψ) and the phase (∆) of plane-polarized light reflected from a substrate through the film. For very thin films, thickness is then calculated (modeled) for each film from these measured * To whom correspondence should be addressed. E-mail: [email protected] (D.J.V.), [email protected]. † Ceramics Division, National Institute of Standards and Technology. ‡ Current address: University of California, School of Dentistry, San Francisco, CA. NIST 1998 Summer Undergraduate Research Fellow (SURF). § NIST Guest Researcher, Biotechnology Division. (1) Deleted in proof. (2) Characterization of Organic Thin Films; Ulman, A., Ed.; Butterworth-Heineman: Boston, 1995.

quantities with assumed values for the refractive index. Because the assumptions might be invalid, this method is not always accurate.3,4 A recent report describing ellipsometer calibration utilizing polymers5 illustrates an interest in organic materials for organic film metrology. However, the materials in that study are still isotropic, thus limiting the refractive index error to one of scaling from a thicker film. No anisotropic organic materials are available for metrology standards below 10 nm. As a result, the uncertainty in many thickness metrology techniques is due to the uncertainty in the film anisotropy and structure. Materials where thickness is independent of structure would facilitate thin film metrology with the potential of lowering the uncertainty of the measurements. Langmuir-Blodgett and self-assembly methods lead to the preparation of highly ordered films. These include films of long-chain n-alkyl carboxylic acids (RCO2H) on water and aluminum oxide; self-assembled monolayers (SAMs) of long-chain silanes (RSiM3), where R g C16 and M ) Cl, OCH3, or OC2H5, on glass and other oxidic surfaces; and SAMs of long-chain alkanethiols on noble metal surfaces.6 Although much is known about these anisotropic films, thickness still depends on structure (number of atoms, tilt, order), and series of even the simplest compounds do (3) Wasserman, S. R.; Whitesides, G. M.; Tidswell, I. M.; Ocko, B. M.; Pershan, P. S.; Axel, J. D. J. Am. Chem. Soc. 1989, 111, 5852. (4) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (5) Richter, A.; Guico, R.; Wang, J. Rev. Sci. Instru. 2001, 72, 3004.

10.1021/la026990e CCC: $25.00 © 2003 American Chemical Society Published on Web 02/19/2003

Isostructural Self-Assembled Monolayers

not result in isostructural films. For example, the homologous series CnH2n+1, C(n+1)H2(n+1)+1, C(n+2)H2(n+2)+1, etc., is not isostructural because of the different methyl group geometries for the odd and even chains and different degrees of order for n e 18.6 Furthermore, all even or all odd series CnH2n+1, C(n+2)H2(n+2)+1, C(n+4)H2(n+4)+1, etc., might not give isostructural SAMs, for n < 17 or 18, because of differing amounts of gauche defects along each alkyl chain. The probability of gauche defects increasing near both the hydrocarbon-Au interface and the hydrocarbon-air interface was suggested in a recent study on octadecanethiol (ODT).7 The possibility of preparing isostructural films, with constant conformational structure along an axis normal to the substrate was suggested from our earlier work on the SAMs of HS(CH2CH2O)6R, where R ) C10 or C18, on gold.8 Reflection-absorption infrared spectroscopy (RAIRS) data indicated that the 1-thiahexa(ethylene oxide) (THEO) segment of these SAMs adopts the 7/2 helical conformation of the folded-chain crystal (FCC) polymorph of poly(ethylene oxide) (PEO).9 The THEO segment is oriented normal to the substrate as the result of a nearly perfect match between the cross-sectional area of the helix, 21.38 Å2, and the packing density, 21.4 Å2/thiolate,10 of a x3 × x3R30° adlayer on Au. The THEO atom positions along the helix axis correlate to comparable positions along the c axis of the unit cell of PEO. We reasoned, therefore, that compounds in the series HS(CH2CH2O)xR, where R represents an n-alkyl chain and x ) 3-10, that have the corresponding 1-thiaoligo(ethylene oxide) (TOEO) segments adopt the same conformation as when x ) 6 would be isostructural. Because the helix axis is normal to the substrate and correlates with the c axis of the unit cell, the SAM thickness should change in a predicable fashion of ∼0.28 nm/ethylene oxide (EO) unit.11 As an initial test of this hypothesis, we prepared a series of HS(CH2CH2O)xR SAMs, where R ) C10H21 and x ) 4-8.12 Infrared spectroscopic ellipsometry (IRSE) and spectroscopic ellipsometry (SE) data showed, however, that these SAMs, assembled from 100% ethanol, were not isostructural over x or any subset of x. The IRSE data showed that the structure of the TOEO segments varied. For x ) 4, the TOEO segment adopted a nearly all-trans extended conformation. For x ) 8, a disordered SAM was obtained in which the TOEO segment was a mixture of helical and nonhelical conformations. For x ) 5 and 7, the TOEO segments were mixtures of helical and nonhelical conformations, with the former predominating. Only for x ) 6 was the TOEO segment in essentially a single helical phase, within the limits of the IRSE measurements. The increased order found for the SAMs of HS(CH2CH2O)6C18H378 suggested that an analogous series, with the longer C18H37 alkyl segment, might be isostructural over x or some subset of x. We report here on the synthesis and characterization of a series of SAMs of octadecyl 1-thiaoligo(ethylene oxide)x disulfides [S(CH2CH2O)xC18H37]2, where x ) 4-8, hereafter referred to as (EO)4C18 to (EO)8C18, respectively.13 (6) An Introduction to Ultrathin Organic Films from LangmuirBlodgett to Self-Assembly; Ulman, A., Ed.; Academic Press: Boston, 1991. (7) Mountain, R.; Hubbard, J. B.; Meuse, C. W.; Simmons, V. J. Phys. Chem. B 2001, 105, 9503. (8) Vanderah, D. J.; Meuse, C. W.; Silin, V.; Plant, A. L. Langmuir 1998, 14, 6916. (9) Kobayashi, M.; Sakashita, M. J. Chem. Phys. 1992, 96, 748. (10) Laibinis, P.; Bain, C. D.; Nuzzo, R. G.; Whitesides, G. M. J. Phys. Chem. 1995, 99, 7663. (11) Takahashi, Y.; Tadokoro, H. Macromolecules 1973, 23, 672. (12) Vanderah, D. J.; Pham, C. P.; Springer, S. K.; Silin, V.; Meuse, C. W. Langmuir 2000, 16, 6527.

Langmuir, Vol. 19, No. 7, 2003 2613 Scheme 1

Materials and Methods14 Synthesis. The compounds (EO)4C18 to (EO)8C18 in this study were synthesized as outlined in Scheme 1. The synthesis of corresponding thiols was analogous to that reported earlier,8 with final conversion to the disulfides using well-known procedures.15 Structural assignments were made from proton nuclear magnetic resonance and high-resolution mass spectrometry data.16 Sample purity (>98%) was determined from thin-layer chromatography (TLC) analysis (one spot by TLC). Sample Preparation. All RAIRS, electrochemical impedance spectroscopy (EIS), and SE experiments used substrates prepared on silicon(100) wafers (Silicon Inc., Boise, ID) initially coated with chromium (∼2 nm) and then with gold (∼200 nm) by magnetron sputtering (Edwards Auto 306, U.K.) at a base pressure of ∼1.3 × 10-6 mbar, as described previously.8 The monolayers were prepared by immersing the gold substrates in ∼0.25 × 10-3 mol/L solutions of the disulfide in 99.5% ethanol for 16-24 h. Immediately before each measurement, the substrates were removed from the thiol solution, extensively washed with 99.5% ethanol, and then dried in a stream of dry nitrogen (N2). The AFM measurements were carried out on (EO)6C18 SAMs, prepared as just described, on 150-nm Au(111) on freshly cleaved mica (Molecular Imaging, Tempe, AZ). The X-ray reflectivity (XRR) measurements on the multilayers used Au-on-silicon substrates with chromium and Au thicknesses of ∼1 and 7.8 nm, respectively, with the latter determined from the X-ray reflectivity data. To obtain the multilayers, the substrates were immersed in the thiol solutions (0.25 mM) for longer times (>8 days) and then, without being washed, gently dried (N2). The XRR measurements on the (EO)6C18 SAMs were made on single-crystal Au(111) samples that were thoroughly washed and dried, as described above. Visible Spectroscopic Ellipsometry (SE). Multiple-wavelength ellipsometric measurements were performed on a J. A. Woollam Co., Inc. (Lincoln, NE) M-44 spectroscopic ellipsometer aligned at a nominal incidence angle of ∼70° from the surface normal. The exact incidence angle, which varied slightly from day to day, was determined by the modeling software (WVASE from J. A. Woollam Co.). Prior to the SE measurements, the samples were removed from the disulfide solution, sonicated in 99.5% ethanol for ∼2 min,17 rinsed (99.5% ethanol), and then dried in a stream of dry N2. Four measurements at different (13) SAMs prepared with the corresponding thiols appear to be identical with those from the disulfides, with small differences in the kinetics of SAM formation. The data in this report are for the disulfides because of chemical stability considerations of potential standard materials. (14) Certain commercial materials, instruments, and equipment are identified in this manuscript to specify the experimental procedure as completely as possible. In no case does such identification imply a recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials, instruments, or equipment identified are necessarily the best available for the purpose. (15) McAllen, D. T.; Cullum, T. V.; Dean, R. A.; Fidler, F. A. J. Am. Chem. Soc. 1951, 73, 3627. (16) HRMS: (EO)6C18 and (EO)7C18 were characterized by HRMS as thiols as reported earlier.8 (EO)4C18: HR FAB M+calcd for C52H106O8S2, 922.7329; found, 922.7350; accompanied by [M + Na]+ at 945.4. (EO)5C18: HR CI [M + 1]+ calcd for C56H115O10S2, 1011.7932; found, 1011.7849; accompanied by [M + Na]+ at 1033.9. (EO)8C18: HR FAB [M + 1]+ calcd for C68H139O16S2, 1247.5972; found, 1247.5888; accompanied by [M + Na]+ at 1297.8. 270 MHz 1H NMR: δ 2.88 (relative to tetramethylsilane), t, J ≈ 6.5 Hz; [S(CH2CH2O)xC18H37], x ) 4-8.

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locations on the substrate were obtained for each sample, and to reduce noise, 3000 revolutions of the analyzer were accumulated. For reproducibility, thicknesses were determined relative to a SAM on Au as described earlier.8 Thicknesses of the (EO)4C18 to (EO)8C18 SAMs were determined similarly but, in this case, relative to the SAM of (C18D37S)2. The optical constants of the gold were determined using a two-layer model for the SAM of (C18D37S)2, for which we assumed a thickness of 2.3 nm and a refractive index of 1.45. The optical constants for the Au were then used to determine the SAM thicknesses, assuming a refractive index of 1.45 for all compounds. A two-layer model is adequate here because the assumed refractive index of 1.45 is reasonable for both the TOEO segment, for example, 1.4530 (Aldrich Handbook, #18,198-6, Milwaukee, WI), and the hydrocarbon segment,6 and small differences in the refractive indexes of these two segments of the SAMs cannot be separated from the thickness measurement error. Modeling and thickness calculations were carried out using the WVASE software. Reflection-Absorption Infrared Spectroscopy (RAIRS). The RAIRS data were obtained using a Nicolet Magna-IR model 570 Series II spectrometer (Thermo Nicolet, Madison, WI) with a model FT-85 (85° grazing angle) Spectra-Tech external reflection accessory (Thermo Spectra-Tech, Shelton, CT). The spectrometer utilized a KBr beam splitter and mercury cadmium telluride (MCT/A) detector. The 85° grazing angle accessory contained an integral polarizer that ensured that only p-polarized light reached the detector. Prior to the RAIRS measurements, the monolayer samples were removed from the thiol solution, thoroughly rinsed (99.5% ethanol), and then dried in a stream of dry N2. Spectra were acquired at 4 cm-1 resolution between 4000 and 700 cm-1 as a summation of 1000 scans using HappGenzel apodization and no zero filling. Background spectra (Ro) were taken using [CD3(CD2)17S]2 SAMs on gold. The sample spectra (R) were acquired under identical equipment conditions and compared to background spectra to obtain spectra of -log(R/Ro) versus wavenumber. Electrochemical Impedance Spectroscopy Measurements (EIS). The EIS measurements were obtained using a Solartron electrochemical impedance system (model 1286 potentiostat, model 1250 frequency response analyzer, computer, and software) (Farnborough, U.K.). The EIS data were fitted with ZView software (Scribner Associates, Southern Pines, NC) to the models discussed later (vide infra). The EIS spectra were obtained for frequencies between 0.5 and 65 000 Hz with five logarithmically distributed measurements per decade and are presented in complex capacitance form, that is, plots of Im Y/ω vs Re Y/ω, where Y is the admittance and ω is the angular frequency (ω )2πf, where f is the frequency in hertz). The working electrode (SAM-coated Au on Si wafer) was attached to a polycarbonate 2-mL cell with a 6-mm-i.d. Kalrez O-ring. For precise determination of the surface area of the working electrode exposed to the solution, the working solution was replaced with a solution containing 0.05 M CuSO4 and 0.1 M H2SO4, and Cu was deposited (red contrast) by electrodeposition at E ) -0.4 V for 20 min. The working electrode surfaces were found to be between 0.28 and 0.33 cm2. A Ti plate with a surface area of approximately 4 cm2 and a Ag/AgCl/KClsat microelectrode M-401F (Microelectrodes, Inc., Bedford, NH) were used as the auxiliary and reference electrodes, respectively. The distance between the reference electrode tip (diameter ∼1 mm) and the working electrode plane varied from 1.0 to 2.5 mm. All measurements were carried out at 0 V bias vs the reference electrode at 21 ( 1 °C in 100 mM potassium phosphate buffer solutions containing 200 mM KCl and an uncompensated solution resistance between 10 and 15 Ω. The values of the uncompensated solution resistance are consistent with the buffer/salt bulk resistivity, which, measured separately, was 27.8 ( 0.3 Ω‚cm. In the potential range from -0.5 to +0.5 V (vs Ag/AgCl/KClsat), the SAM capacitance was invariant with respect to bias. We found no influence of (17) SE and surface plasmon resonance (SPR) data for the SAMs of (EO)6C18,8 as well as the SAMs of octadecanethiol (ODT) (ref 40 therein), indicated thicknesses greater than all structural and conformational possibilities, suggesting the presence of physisorbed material or bilayers/ multilayers. SE and SPR data immediately after washing and/or brief sonication of these SAMs suggest the complete removal of physisorbed materials.

Vanderah et al. solution deaeration (purified argon, 20 min of intense purging with one to two bubbles per second, followed by a stream of argon above the solution surface during the measurements) on the EIS data. For direct comparison, EIS measurements were made on SAMs of ODT. In all experiments, the background current did not exceed 10 nA. Atomic Force Microscopy (AFM). SAMs of (EO)6C18 and ODT were imaged via atomic force microscope (PicoScan Molecular Imaging, Phoenix, AZ) using the deflection signal in contact mode. A 200-µm-long silicon nitride tip with a nominal spring constant of 0.32 N/m was used (Veeco, Woodbury, NY). Pairs of images that include up and down scans with both rightto-left and left-to-right data were used to minimize the effects of thermal drift.18 Data were collected at several different orientations of the molecular lattice to the AFM scan direction. Images that gave three discernible peaks in the FFT were used, although only two reciprocal lattice vectors are required to determine the lattice structure. The AFM was calibrated to the hexagonally symmetric mica lattice that had a spacing of 0.45 nm between rows (0.52-nm lattice constant) and a 60° angle between reciprocal lattice vectors. The standard deviation in the row spacing was 0.015 nm and in the angle between reciprocal lattice vectors was 2.6°. X-ray Reflectivity (XRR). The X-ray reflectivity experiments were conducted at the National Synchrotron Light Source (NSLS) at beamline X10b. A first set of experiments was performed on the (EO)6C18 to (EO)8C18 SAMs. Unwashed samples (soaked in the various 0.25 mM disulfide solutions for 8 days) were probed after solvent removal in a gentle stream of dry N2. The substrates were exposed to monochromatic X-rays (λ ) 0.1123 nm), and the reflected intensities were recorded as a function of the momentum transfer qz. Here, qz ) 4π/λ sin(2θ/2) is determined by the detector scattering angle (2θ). AFM measurements on the same samples indicated multilayer island formation (data not shown). Using X-rays, a series of Bragg reflections were observed. The Bragg positions were analyzed in terms of the layering constant ΛSL. Here, qz ) L × 2π/ΛSL, where L is the Miller index denoting the order of the Bragg reflection. The second set of experiments was performed on an (EO)6C18 SAM on single-crystal Au(111) with λ ) 0.087 nm. The sample was prepared by immediately placing the Au crystal, thoroughly cleaned in an ultrahigh-vacuum surface preparation system [repeated Ar sputtering and heating cycles (1000 K)] in the 0.25 mM solution. After 4 days, the sample was removed and rinsed thoroughly with ethanol.

Results and Discussion Film uniformity and defects of the (EO)4C18 to (EO)8C18 SAMs were assessed using EIS measurements in an aqueous electrolyte environment. The complex capacitance spectra of the (EO)4C18 to (EO)8C18 SAMs and equivalent circuit models are shown in Figure 1. The (EO)6C18 to (EO)8C18 SAMs yield spectra (c-e, respectively) that are single-semicircular (part of a single semicircle) over the frequency range between 0.5 and 65 000 Hz with Re Y/ω f 0 at low frequencies, whereas the (EO)4C18 and (EO)5C18 SAMs yield spectra (1a and b, respectively) that are multiple-semicircular and/or exhibit other features and Re Y/ω does not converge to 0 at low frequencies. The spectra in Figure 1 indicate the (EO)6C18 to (EO)8C18 SAMs are similar dielectric layers that differ significantly from the (EO)4C18 and (EO)5C18 SAMs. The spectral features of the (EO)6C18 to (EO)8C18 SAMs suggest, and experimental EIS data fitting19 confirms, that their electrochemical behavior might be modeled as an equivalent circuit consisting of a single RsolCSAM (time constant) element, where Rsol is the electrolyte solution resistance and CSAM (18) Woodward, J. T.; Schwartz, D. K. J. Vac. Sci. Technol. B 1998, 16, 51-53. (19) See Supporting Information on the fitting of the EIS data. The source and magnitude of the deviations seen in the high-frequency experimental points from the fitted lines in Figure 1 are discussed.


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Figure 1. (a-e) Complex capacitance plots of the (EO)4C18 to (EO)8C18 SAMs, respectively. The squares are the experimental points; the lines are the model-generated curves. The frequency range is from 0.5 Hz to 65 000 Hz. In the upper right corner are the equivalent electrical circuits (model a and model b) used to fit the EIS data. Rsol is the solution resistance; CSAM is the the constant phase element of the SAM; Cdef is the the capacitance of the defect region; and Rdef is the resistance of the defect region, reflecting possible electron transfer through the defects and/or resistivity of solution entrapped in the pinholes. The arrows in a-e indicate the direction of increasing frequency.

is the constant phase element (CPE) or capacitance of the interface. In the absence of any frequency dispersion of capacitance and with an exponent (R) of the CPE equal to 1.00 ( 0.00, model a predicts a perfect semicircular (or part thereof) EIS spectrum, in the coordinates Im Y/ω versus Re Y/ω, with Re Y/ω f 0 and Im Y/ω f CSAM at low frequencies. The position of the high-frequency terminal point of the Im Y/ω versus Re Y/ω plot depends entirely on the time constant of the circuit RsolCSAM, and in the extreme case when the solution resistance is negligible, all data points tend to cluster at the Re Y/ω ) 0, Im Y/ω ) CSAM point. Indeed, these spectral characteristics are observed in the EIS spectra of the (EO)6C18 to (EO)8C18 SAMs (Figure 1c-e). In Figure 1c-e, only 12-15 of the data points, out of the 50 data point set, noticeably deviate from Re Y/ω ) 0, indicating almost ideal capacitive behavior for these films. The experimental data fit the simple RsolCSAM model and give CSAM values for the (EO)6C18 to (EO)8C18 SAMs (Table 1). Importantly, the CPE for the (EO)6C18 to (EO)8C18 SAMs

Table 1. Experimental and Calculated Specific Capacitance Values of Au Electrodes Covered with the (EO)4C18 to (EO)8C18 SAMs capacitance (µF/cm2) compound

experimental

calculated

Ra

[S(CH2CH2O)4C18H37]2 [S(CH2CH2O)5C18H37]2 [S(CH2CH2O)6C18H37]2 [S(CH2CH2O)7C18H37]2 [S(CH2CH2O)8C18H37]2

∼0.820 ∼0.816 0.608 ( 0.004 0.596 ( 0.007 0.565 ( 0.008

0.663 0.635 0.613 0.589 0.567

0.99 0.98 1.00 1.00 1.00

a R values correspond to the constant phase element C SAM (Figure 1, model b). For (EO)4C18 and (EO)5C18 SAMs, the constant phase element Cdef exhibited significantly lower R values. Typically, these values were found to be in the range from 0.45 to 0.79. The Cdef values of these SAMs were about 10-20% of the CSAM value and varied significantly from sample to sample.

all equal 1.00 ( 0.00. This confirms that these SAMs behave as ideal capacitors, similarly to well-packed ODT SAMs.20-22

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Assessment of structural models for these SAMs is possible from a comparison of calculated and experimental specific capacitance (capacitance per unit area) values (Table 1). The calculation of specific capacitance values is carried out using experimental data and certain assumptions. Because the SAMs are composed of alkyl and oligo(ethylene oxide) (EO) segments, the SAMs are best represented, for these calculations, as two capacitors in series with a total specific capacitance CSAM defined by

CSAM ) (CALK-1 + CEO-1)-1

(1)

where

CALK )

0ALK dALK

(2)

CEO )

0EO dEO

(3)

In these equations, CALK and CEO are the specific capacitances of the corresponding alkyl and EO segments;23 0 is the vacuum permittivity; and and d are the dielectric constant and thickness of the corresponding segment, respectively. Four parameters are needed to calculate the electrode specific capacitance with eq 1: ALK, EO, dALK, and dEO. Three of these parameters are obtained readily. On the basis of structural models derived from the RAIRS data, discussed below, the established values for dALK and ALK of ODT,24 2.3 nm and 2.10, respectively, yield a specific capacitance of 0.81 µF/cm2 for CALK, in good agreement with the specific capacitance of 0.80 ( 0.03 µF/cm2 for ODT SAMs (our data; data not shown). The dEO values are estimated as 1.67, 1.95, and 2.22 nm for (EO)6C18 to (EO)8C18, respectively, from the total expected thicknesses8,23 and the unit cell dimensions of crystalline PEO.11 However, EO is not known. The direct substitution of the dielectric constant of bulk PEO (PEO) for EO is precluded for the following reasons. First, at room temperature, PEO is strongly dependent on frequency below 104 Hz,25,26 whereas the (EO)6C18 to (EO)8C18 SAMs are described adequately by the simple RsolCSAM model, with R ) 1.00, over the entire frequency (20) Diao, P.; Cui, J. X.; Gu, D.; Tong, R.; Zhong J. Electroanal. Chem. 1999, 464, 61. (21) Lindholm-Sethson, B. Langmuir 1996, 12, 3305. (22) Finklea, H. O., Electroanalytical Chemistry; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker: New York, 1996; Vol. 1, p 110. (23) All of the atoms contribute to the thickness, dielectric constant, and specific capacitance values for each of the (EO)xC18 SAMs. For convenience in the calculation of the dielectric constants, we take advantage of the additivity of these components by considering the sulfur atom and the C18H37 components as being directly bonded to allow the use of the well-established values of the ODT thickness (dODT ) 2.3 nm) and dielectric constant (ODT ) 2.1) in these calculations. Thus, the subscript “ALK” refers to this ODT equivalent, and “EO” refers then to the (CH2CH2O)x segments only. Because the measured thicknesses (Table 2) are in good agreement with the structural model proposed here, and earlier,8 for the (EO)6C18 to (EO)8C18 SAMs, the calculated specific capacitance values are approached with calculated thicknesses. Expected thicknesses (dSAM) were calculated as the sums of dODT + dEO, where dODT ) 2.3 ( 0.1 nm.24 The dEO values were calculated as a product of 1/7 of 2.0 nm (c axis of PEO unit cell containing seven EO units)11 times x (number of EO units) + 0.04 nm (longer C-S bond). Thus, the expected thicknesses for (EO)6C18 to (EO)8C18 are 4.0, 4.3, and 4.6 nm, respectively, (0.1 nm. Specific capacitance values for the (EO)4C18 and (EO)5C18 SAMs were calculated in a similar fashion with dEO values derived from dSAM ) 3.4 and 3.7, respectively. (24) Peterlinz, K. A.; Georgiadis, R. Langmuir 1996, 20, 4731. (25) Fanggao, C.; Saunders: G. A.; Lambson, E. F.; Hampton, R. N.; Carini, G.; Di Marco, G.; Lanza M. J. Polym. Sci. B 1996, 34, 425. (26) Porter, C. H.; Boyd, R. H. Macromolecules 1971, 4, 589.

range (0.5-65 000 Hz) (Figure 1c-e and model a). Second, the relatively high conductivity of bulk PEO25 (4.3 × 10-10 S/cm), believed to be the main reason for the dielectric constant’s dispersion below 104 Hz, would dramatically change the EIS spectra, if present in EO segments of the SAMs described here, from single semicircles to a combination of either a semicircle and a line or multiple semicircles. This is not observed. The semicircular EIS spectra for (EO)6C18 to (EO)8C18 SAMs (Figure 1c-e) do not deviate from the Re Y/ω f 0 and Im Y/ω f CSAM point at low frequencies, indicative of ideal capacitive behavior. Calculations using eq 1, the estimated CALK values, and the above calculated dEO thicknesses23 yield a single frequency invariant value of EO of 4.8 ( 0.227 for all three (EO)6C18 to (EO)8C18 SAMs close to the high-frequency value of 5.2 for PEO.25 The semicircular complex capacitance plots and low value of the dielectric constant indicate that these SAMs are nearly defect-free, highly uniform films, with the EO segments devoid of dipole solvent (water) and ionic species, that is, the constituents that cause the frequency dispersion in PEO. Importantly, the EIS responses for the (EO)6C18 to (EO)8C18 SAMs were stable for 2-3 h under continuous contact with the solution. The stability of these SAMs is undoubtedly due to their highly ordered, uniform, low-defect structures. In contrast, the (EO)4C18 and (EO)5C18 SAMs exhibit more complex spectra over the same frequency range (Figure 1a and b, respectively) and are better described by models with additional parallel capacitive and resistive elements (Figure 1, model b) that account for the presence of defects, such as pinholes and collapse sites, in a dielectric film.28-30 For these SAMs, the CPE R values are lower than 1.00 (Table 1), and the specific capacitance values are about 20% higher than expected, assuming (1) that they are isostructural with the (EO)6C18 to (EO)8C18 SAMs with thickness values of 3.4 and 3.7 nm, respectively,23 and (2) that EO ) 4.8. These data suggest that the (EO)4C18 and (EO)5C18 SAMs are less ordered (see RAIRS discussion below) than the (EO)6C18 to (EO)8C18 SAMs and, when exposed to the aqueous electrolyte solution, are penetrated by solvent (water) and/or ionic species, as was concluded for oligo(ethylene oxide)-terminated SAMs.31 Penetration of polar molecules and ion charge carriers into the SAM would increase the dielectric constant and decrease the effective thickness of the layers by further increasing disorder or causing the formation of collapsed sites on the surface. Also noteworthy, the response from (EO)4C18 and (EO)5C18 SAMs exhibited a slow drift of the spectral features with time (10-15% SAM capacitance increase within 1 h), characteristic of less ordered SAMs containing more defects and/or penetration by solvent (water). The RAIRS and spectroscopic ellipsometry (SE) data for the (EO)4C18 to (EO)8C18 SAMs are shown in Figures (27) SE thicknesses are slightly higher than the calculated thicknesses for the (EO)6C18 to (EO)8C18 SAMs. EO ) 5.4 ( 0.3 using dEO ) dSAM (Table 2) - 2.3 nm (dODT) values. (28) Glazier, S. A.; Vanderah, D. J.; Plant, A.l.; Bayley, H.; Valincius, G.; Kasianowicz, J. J. Langmuir 2000, 16, 10428. (29) Stora, T.; Lakey, J. H.; Vogel, H. Angew. Chem., Int. Ed. 1999, 38, 389. (30) The spectra in Figure 1a and b can also be modeled by an equivalent circuit consisting of a solution resistance and two parallel RC elements connected in series (see Raguse, B.; Braach-Maksvytis, V.; Cornell, B. A.; King, L. G.; Osman, P. D. J.; Pace, R. J.; Wieczorek, L. Langmuir 1998, 14, 648 and ref 28). Both of these models represent degenerate circuits that adequately describe the set of experimental points. In our opinion, model b better reflects the physical picture of the surface, which is covered with a nearly uniform dielectric layer containing a small number of structural defects, possible collapsed sites, and/or pinholes. (31) Zolk, M.; Eisert, F.; Pipper, J.; Herrewerth, S.; Eck, W.; Buck, M.; Grunze, M. Langmuir 2000, 16, 5848.


Langmuir, Vol. 19, No. 7, 2003 2617

Figure 2. RAIRS spectra of the (EO)6C18 to (EO)8C18 SAMs from 1400 to 900 cm-1.

Figure 3. RAIRS spectra of the (EO)6C18 to (EO)8C18 SAMs from 3050 to 2750 cm-1. Spectra offset for clarity.

Table 2. SE Thickness Data of the (EO)4C18 to (EO)8C18 SAMs structure

d ((0.05 nm)

[S(CH2CH2O)4C18H37]2

3.68


3.96


4.13


4.41


4.73

δ 0.28 0.17 0.28 0.32

2-5 and Table 2, respectively. As for the EIS spectra, simple inspection of these data clearly indicates that the (EO)6C18 to (EO)8C18 SAMs are similar and differ significantly from the (EO)4C18 and (EO)5C18 SAMs. Figure 2 shows that only four absorption bands at 1345, 1243, 1118, and 966 cm-1 are found for the (EO)6C18 to (EO)8C18 SAMs in the region from 1400 to 900 cm-1. These bands, which correspond to those vibrations with transition moments parallel to the chain axis (symmetry-designated A2 series), indicate that the TOEO segments are in the 7/2 helical conformation of the folded-chain crystal polymorph of PEO.9 The absence of bands at 1360, 1280, 1234, 1149, 1116, 1061, and 946 cm-1, which correspond to those vibrations with transition moments perpendicular to the chain axis (symmetry-designated E1 series9), indicate that the TOEO segments are perpendicular to the substrate.8 The orientation of the EO segments in the (EO)7C18 and (EO)8C18 SAMs is also inferred by spectral characteristics nearly identical to those of the (EO)6C18 SAM, which was quantitatively analyzed earlier (Figure 5 of ref 8 and literature cited therein). RAIRS data for (EO)6C18 to (EO)8C18 SAMs in the C-H stretch region from 3050 to 2750 cm-1 are shown in Figure 3. The prominent band at 2892 cm-1 indicates the presence of the helical OEO segment.32 These data, together with those found in the 1400 to 900 cm-1 region and the earlier EIS data, indicate that the (EO)6C18 to (EO)8C18 SAMs adopt nearly single-phase, helical TOEO segments. In addition, the similarity between the spectra in Figure 3 and those quantitatively analyzed earlier8 indicates that the C18 alkyl segments are ordered [strong, sharp band at 2917 cm-1 (CH2 asymmetric stretch)] in an essentially all-trans configuration tilted at ∼30° to the normal. RAIRS spectra of the SAMs of (EO)4C18; (EO)5C18; and, for direct comparison, (EO)6C18 from 1400 to 900 cm-1 (32) Miyazawa, T.; Fukushima, K.; Ideguchi, Y. J. Chem. Phys. 1962, 37, 2764.



and from 3050 to 2750 cm-1 are shown in Figures 4 and 5, respectively. The spectral differences between the SAMs of (EO)4C18 and (EO)5C18 and the (EO)6C18 SAM in Figure 4 are similar to those observed for the HS(CH2CH2O)4-8C10H21 SAMs,12 where nonhelical conformations were indicated by the attenuation or complete absence of the bands indicative of a helical conformation and/or the presence of higher-wavenumber absorptions near the 1118 cm-1 band. The bands at 1345, 1243, 1118, and 966 cm-1 indicate that some of the (EO)5C18 SAM adopts the helical conformation, but these bands are

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Vanderah et al.

Figure 6. RAIRS peak intensities for the (EO)6C18 to (EO)8C18 SAMs from 1400 to 900 cm-1 normalized per EO unit.

attenuated relative to those for (EO)6C18. The higherwavenumber bands near the 1118 cm-1 band at 1130 and 1145 cm-1 are assigned as C-O stretching bands. The former is indicative of less ordered/amorphous configurations of PEO,33 and the latter is consistent with SAMs that have the OEO segment largely in an all-trans conformation.34 In the (EO)4C18 spectrum, the 1145 cm-1 band is dominant with the 1130 cm-1 band as a small shoulder, suggesting a predominantly all-trans conformation, whereas, in the (EO)5C18 spectrum, the 1145 and 1130 cm-1 bands are comparable in intensity, indicating a more equal ratio of the TOEO structures in these two conformations. The gradual change from the nearly all helical OEO conformation in the (EO)6C18 SAMs to essentially no helical OEO conformation in the (EO)4C18 SAMs is also evident in Figure 5 by the gradual disappearance of the 2892 cm-1 band. The four bands at 1345, 1243, 1118, and 966 cm-1 (Figure 2) are due solely to the conformation of the TOEO segment. Perfectly identical helical SAMs should, therefore, exhibit constant absorbance increases per EO unit. The bands in Figure 2 were integrated, and the absorbance per EO unit normalized to the average. Figure 6 shows that the absorbance per EO unit is relatively constant for each band. Small variations of approximately 2% to approximately 4% are seen for the 1345, 1243, and 1118 cm-1 bands, with the larger variations seen for the smallest 966 cm-1 band, where small baseline differences can amplify differences in the integration ratios. Thickness differences (δ) for the (EO)6C18 to (EO)8C18 SAMs should increase in a predictable way because the TOEO segments are helical, are oriented normal to the substrate, correlate with the c axis of the unit cell of PEO, and differ only in the number of EO units. We expect δ to be ∼0.28 nm/EO unit [helix axis ≡ c axis (PEO unit cell) ) 1.948 nm, and 1.948 nm/unit cell ÷ 7 EO units/unit cell ) 0.28 nm/EO unit]. The average δ for (EO)6C18 to (EO)8C18 is 0.30 ( 0.05 nm (Table 2), in good agreement with our expectations. Also noteworthy, δ for (EO)5C18 to (EO)6C18 of 0.17 ( 0.05 nm is significantly less than 0.28 nm, consistent with the RAIRS data and indicating a structure for the (EO)5C18 SAM in which the TOEO segment is not all in the helical conformation. The average δ for the [S(CH2CH2O)6-8C18H37] units was confirmed by X-ray reflectivity (XRR) data. Figure 7a shows the XRR Bragg reflections for unwashed/unsonicated (EO)6C18 to (EO)8C18 samples (see Experimental Section), indicating thick multilayers (>50 nm) for all three compounds with some decrease in order with increasing EO length. The sharp width of the Bragg reflections for (33) Dissanayake, M. A. K. L.; Frech, R. Macromolecules 1995, 28, 5312. (34) Harder, P.; Grunze, M.; Dahint, R.; Whitesides, G. M.; Laibinis, P. E. J. Phys. Chem. 1998, 102, 426.

Figure 7. (a) X-ray reflectivity data for the (EO)6C18 to (EO)8C18 multilayers (sharp peaks). The broad oscillations originate from the Au film. (b) The inset shows the identified Bragg positions. The solid line is a linear fit; its slope is equivalent to 2π/ΛSL.

(EO)6C18 indicates a total thickness of g100 nm. Layering constants, ΛSL, calculated from the Bragg reflections are consistent for multilayers of a bilayer XYYX repeat unit, where X ) C18H37 and Y ) S(EO)x segments, as we might expect for amphiphilic compounds. Linear regression analysis of the identified peaks (Figure 7b) give precise ΛSL values of 3.89, 4.15, and 4.45 nm for (EO)6C18 to (EO)8C18, respectively. The ΛSL values are consistently lower than the SE thicknesses of the SAMs (Table 2), but their average difference (δ) per EO unit of 0.28 ( 0.02 nm is in good agreement with that found from the SE data. The high quality of the diffraction pattern for the (EO)6C18 sample should allow a quantitative analysis of the diffraction intensities by modeling details of the electron density distribution down to interatomic distances (work in progress). This should permit the determination of whether the systematically reduced thickness in these multilayers is due to packing, an additional tilt of the molecule, a compression of the helix, or a combination of all three. After rinsing/sonication, no Bragg reflections were observed, indicating that the multilayers are physisorbed to the SAMs.17 The strong intensity oscillations due to the sputtered gold film prohibit a detailed analysis of the weak contribution resulting from the SAM. However, this analysis is possible using single-crystalline Au(111) as the substrate. XRR data for (EO)6C18 on Au(111) are shown in Figure 8. Multiplication of the intensities by qz (qz4, Figure 8 inset) compensates for the overall (Fresnel-like) intensity decay. The solid lines indicate the positions of the expected intensity maxima assuming qz ) 2π/d × n ) 1.64 nm-1, where n denotes the order of the maximum and d ) 3.82 ( 0.07 nm. In this measurement, d is the distance from the sulfur atom to the top hydrogen of the terminal methyl group. Therefore, the (EO)6C18 SAM thickness is, after inclusion of the Au-S bond distance, either 4.01 ( 0.07 nm, if the sulfur atoms are located in the 3-fold hollow site (Au-S ) 0.19 nm35), or 4.06 ( 0.07 nm, if the sulfur atoms are located on bridge sites (Au-S ) 0.24 nm35). Regardless of the location of the sulfur atoms, either of the XRR determined SAM thicknesses are in excellent agreement with our calculated SAM thickness of 4.0 nm23 for (EO)6C18 and in good agreement with the SE value (Table 2). The persistence of the intensity (35) Sellers, H.; Ulman, A.; Shnidman, Y.; Eilers, J. E. J. Am. Chem. Soc. 1993, 115, 9389.


Langmuir, Vol. 19, No. 7, 2003 2619

Figure 8. X-ray reflectivity of (EO)6C18 obtained on singlecrystal Au(111). Upper right inset is the same data multiplied by qz4 to compensate for the overall decay. Solid lines indicate peak positions as expected for a film thickness, from the sulfur atom to the furthest hydrogen on the methyl group, of d ) 3.82 nm (qz ) 2π/d × n, n ) 1, 2, 3, etc.). Figure 10. AFM image of an (EO)6C18 SAM on Au(111) on mica and corresponding FFT (inset). The arrows in the FFT point to the reciprocal lattice points of the SAM. Other bright spots near the axes vary with the scan direction and are attributed to periodic noise. The reciprocal lattice points are consistent with the x3 × x3R30° adlayer structure commonly found in SAMs of long-chain alkanethiols on Au(111).

Figure 9. Schematic illustration of the isostructural (EO)6C18 to (EO)8C18 SAMs on Au.

oscillations to higher orders indicates a well-defined structure. The EIS, RAIRS, SE, and XRR data suggest that the (EO)6C18 to (EO)8C18 SAMs are isostructural, highly uniform, nearly defect-free dielectric layers with the conformation and tilt for each segment as illustrated in Figure 9. The structural model proposed for these isostructural SAMs closely matches the bulk X-ray structure for similar oligo(ethylene oxide) n-alkyl ethers, where the Au-S plane in the SAMs and the layer-crystal end-group plane of the latter are equivalent.36 Figure 9 denotes oligo(ethylene oxide) segments whose order, conformation, cross-sectional area, and orientation, as indicated by the above data and discussion, locate the hydrophobic (C18) segments relative to one another essentially the same as the 3-fold hollow sites of Au(111). The conformational and lateral ordering of the C18 segments should, therefore, be similar to that found for ODT directly bound to the Au. Lateral order was determined on an (EO)6C18 SAM with AFM on Au(111). The AFM image (Figure 10) shows both areas of molecular order and disordered regions. Domain sizes are on the order of 10 nm in diameter, similar to that seen on ODT SAMs (our data, data not shown, and Fenter et al.37). Fast Fourier transforms (FFTs) of the images (36) Craven, J. R.; Hao, Z.; Booth, C. J. Chem. Soc., Faraday Trans. 1991, 87, 1183. (37) Fenter, P.; Eisenberger, P.; Liang, K. S. Phys. Rev. Lett. 1993, 70, 2447.

(Figure 10 inset) yield peaks corresponding to the spacing between rows of molecules in the ordered regions of the SAM. Analysis of the FFTs yields row spacings of 0.434 ( 0.020 nm (0.501-nm lattice vector) and a standard deviation in the angles between reciprocal lattice vectors of 4.6°. This is in good agreement with the lattice vector of 0.500 nm for a x3 × x3R30° adlayer on an Au(111) surface as seen in scattering experiments37-39 and AFM studies40-42 on long-chain alkanethiol SAMs. The spread in angles is somewhat larger than expected (4.6° versus 2.6° for the mica calibration). This raises the possibility that the lattice is distorted and not hexagonally symmetric. If this were the case, we would expect to see either one of angles between the lattice vectors larger than 60° or the opposite, with two larger and one smaller angle, depending on how the lattice had distorted. In fact, we see both in our data, indicating that there is no distortion of the lattice from hexagonal symmetry. The larger spread in the angles might be due to the AFM tip distorting the lattice during the imaging, or it might simply reflect an increased uncertainty in the positions of the lattice spots due to the shorter range of the (EO)6C18 SAM relative to the longer-range order of the mica lattice. These direct lateral order measurements confirm the order suggested from the EIS [(EO)6-8C18 SAMs; near ideal capacitors] and RAIRS (C18 segment essentially identical to ODT SAMs8) data. Additional measurements to further validate the structural model presented in Figure 9 will be the subject of future reports. (38) Chidsey, C. E. D.; Liu, G.-Y.; Rowntree, P.; Scoles, G. J. Phys. Chem. 1989, 91, 4421. (39) Strong, L.; Whitesides, G. M. Langmuir 1988, 4, 546. (40) Alves, C. A.; Smith, E. L.; Porter, M. D. J. Am. Chem. Soc. 1992, 114, 1222. (41) Pan, J.; Tao, N.; Lindsay, S. M. Langmuir 1993, 9, 1556. (42) Liu, G.-Y.; Salmeron, M. B. Langmuir 1994, 10, 367.

2620


Conclusions RAIRS, SE, and EIS data show that the SAMs of the amphiphilic disulfides [S(CH2CH2O)xC18H37]2 on gold differ over the range of x ) 4-8. Significantly, however, the SAMs appear to be isostructural for x ) 6-8. For this smaller range of x, the RAIRS data show that the TOEO segment adopts the ordered helical conformation, oriented normal to the substrate, of the folded-chain crystal polymorph of PEO, with the alkyl segment in an essentially all-trans conformation canted ∼30° to the normal. SE and XRR measurements indicate thickness increases of ∼0.30 ( 0.05 nm/EO unit, in accord with that expected (0.28 nm/EO unit) on the basis of the unit cell of crystalline PEO and the orientation of the TOEO segment. EIS data indicate that these SAMs behave as ideal capacitors in an electrochemical environment, indicating that they are homogeneous and nearly defect-free. A low dielectric constant of 4.8 ( 0.2 for all three SAMs that, notably, does not vary with frequency down to 0.5 Hz supports the isostructural nature of these SAMs and indicates that the TOEO segments are devoid of water or ionic species. In contrast, the RAIRS, SE, and EIS data indicate that, for x < 6, the SAMs are less ordered and are similar to those found for SAMs with a shorter alkyl group reported earlier.12 The EIS data indicate that these SAMs do not

Vanderah et al.

behave as ideal capacitors in an electrochemical environment. For the isostructural (EO)6C18 to (EO)8C18 SAMs, the thickness differences result from the number of EO units, along an axis normal to the substrate, that correlate with -C-C-O- positions in the crystallographic unit cell of crystalline PEO. As a result, the optical constants are independent of the film thickness. The structural features and macroscopic properties of these SAMs appear to render these films useful for a variety of surface science measurement techniques. Acknowledgment. We express our appreciation to John J. Kasianowicz for critical input into the preparation of this manuscript and M. F. Danisman for contributing to the X-ray measurements. The DOE, under Grant DEFG02-93ER45503, supported the work at Princeton and at the NSLS. Supporting Information Available: Details of the fitting of the EIS data, including a discussion of the source and magnitude of the deviations seen in the high-frequency experimental points from the fitted lines in Figure 1. This material is available free of charge via the Internet at http://pubs.acs.org. LA026990E

Isostructural Self-Assembled Monolayers. 1. Octadecyl 1-Thiaoligo

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