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XAS and XPS Characterization of Monolayers Derived from a Dithiol and Structurally Related Disulfide-Containing Polyamides Andrew L. Vance,* Trevor M. Willey,† A. J. Nelson, T. van Buuren, C. Bostedt,‡ Louis J. Terminello, and Glenn A. Fox Chemistry and Materials Science Directorate, Lawrence Livermore National Laboratory, Livermore, California 94550
Mark Engelhard and Don Baer Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352 Received February 14, 2002. In Final Form: July 24, 2002 X-ray absorption spectroscopy and X-ray photoemission spectroscopy have been used to examine sulfurgold bond formation in monolayers derived from a dithiol monomer and related disulfide-containing polyamides. These compounds were designed to allow the molecules to adsorb to gold through two terminal sulfurs, forming surface-attached loops. Element and site-specific density of unoccupied electronic states were probed by X-ray absorption spectroscopy at the C 1s, N 1s, O 1s (K-edge), and S 2p (L2,3-edge) absorption edges. Photoemission measurements of the C 1s, N 1s, O 1s, and S 2p core lines were also used to estimate relative coverage, to confirm layer formation, and to evaluate chemical bonding of the monomer and polyamide to the gold-coated substrates. In the case of the dithiol monomer, the spectroscopic evidence clearly shows that most of the molecules adsorb through a single thiol end. The disulfide-containing precursors, in contrast to the monomer, attach to the surface through both sulfurs to form the anticipated surface-attached loop.
Introduction With the goal of building a technical foundation for the construction of surface-attached molecular devices, we are investigating the formation of surface-attached loops from a variety of dithiol and disulfide precursors. While there is much interest in self-assembled monolayers (SAMs) formed by the adsorption of thiols or disulfides onto a gold surface,1 less attention has been focused on systems derived from R,ω-dithiols. In most cases, dithiols have been found to attach to gold through only one sulfur.2 The free thiol of these “standing” dithiol SAMs is then available for further reactions to form bilayers or to attach additional ions, molecules, or particles to the surface.3 Other dithiols * Corresponding author phone: (925)423-9166; fax: (925)4238772; e-mail:
[email protected]. † Department of Physics, University of California at Davis. ‡ The University of Hamburg. (1) (a) Ulman, A. Chem. Rev. 1996, 96, 1533. (b) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (c) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (d) Bain, C. D.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1989, 5, 723. (e) Gro¨nbeck, H.; Curioni, A.; Andreoni, W. J. Am. Chem. Soc. 2000, 122, 3839. (2) (a) Tour, J. M.; Jones, L.; Pearson, D. L.; Lamba, J. J. S.; Burgin, T. P.; Whitesides, G. M.; Allara, D. L.; Parikh, A. N.; Atre, S. V. J. Am. Chem. Soc. 1995, 117, 9529. (b) Nakamura, T.; Kondoh, H.; Matsumoto, M.; Nozoye, H. Langmuir 1996, 12, 5977. (c) Nakanishi, T.; Ohtani, B.; Shimazu, K.; Uosaki, K. Chem. Phys. Lett. 1997, 278, 233. (d) Rieley, H.; Kendall, G. K.; Zemicael, F. W.; Smith, T. L.; Yang, S. Langmuir 1998, 14, 5147. (e) Duwez, A.-S.; Yu, L. M.; Riga, J.; Pireaux, J.-J.; Delhalle, J. Thin Solid Films 1998, 327-329, 156. (f) Kohli, P.; Taylor, K. K.; Harris, J. J.; Blanchard, G. J. J. Am. Chem. Soc. 1998, 120, 11962. (g) Joo, S. W.; Han, S. W.; Kim, K. Langmuir 2000, 16, 5391. (h) Joo, S. W.; Han, S. W.; Kim, K. J. Phys. Chem. B 2000, 104, 6218. (i) Duwez, A.-S.; Yu, L.-M.; Riga, J.; Delhalle, J.; Pireaux, J.-J. Langmuir 2000, 16, 6569. (j) Leung, T. Y. B.; Gerstenberg, M. C.; Lavrich, D. J.; Scoles, G.; Schreiber, F.; Poirier, G. E. Langmuir 2000, 16, 549.
have been designed to attach to gold through both sulfurs, forming chelating monolayers.4 Bis-disulfides derived from thioctic acid have also been shown to form monolayers in which both disulfide ends attach to gold, forming especially stable surface-attached species.5 Since the sulfur-gold bond is relatively weak (ca. 40 kcal/mol vs 87 kcal/mol for a thiol S-H bond1a), monolayers of the spiroalkane dithiols and the thioctic acid derivatives are stabilized by the chelate effect of multiple sulfur-gold bonds. When R,ωdithiols or bis-disulfides are utilized to form monolayers in which both ends are attached to gold, these could be referred to as surface-attached loops. Another approach that could generate surface-attached loops is to expose a disulfide-containing polymer to a gold surface. Cleavage of the disulfide bonds upon attachment to gold would result in loop formation. Monolayers formed from disulfide-containing polymers have been reported, although the details of their adsorption on gold were not the focus of that work.6 By utilizing disulfide-containing polymers, the polymer is broken down to surface-attached monomeric units. This differs from other work in which (3) (a) Bethell, D.; Brust, M.; Schiffrin, D. J.; Kiely, C. J. Electroanal. Chem. 1996, 409, 137. (b) Nakanishi, T.; Ohtani, B.; Uosaki, K. J. Phys. Chem. B 1998, 102, 1571. (c) Brust, M.; Bethell, D.; Kiely, C. J.; Schiffrin, D. J. Langmuir 1998, 14, 5425. (d) He, H. X.; Zhang, H.; Li, Q. G.; Zhu, T.; Li, S. F. Y.; Liu, Z. F. Langmuir 2000, 16, 3846. (e) Deng, W.; Fujita, D.; Yang, L.; Nejo, H.; Bai, C. Jpn. J. Appl. Phys. 2000, 39, L751. (f) Deng, W.; Yang, L.; Fujita, D.; Nejoh, H.; Bai, C. Appl. Phys. A 2000, 71, 639. (g) Chen, S. J. Phys. Chem. B 2000, 104, 663. (4) (a) Shon, Y.-S.; Lee, T. R. Langmuir 1999, 15, 1136. (b) Shon, Y.-S.; Lee, S.; Perry, S. S.; Lee, T. R. J. Am. Chem. Soc. 2000, 122, 1278. (c) Fujihara, H.; Nakai, H.; Yoshihara, M.; Maeshima, T. Chem. Commun. 1999, 737. (5) (a) Lui, H.; Liu, S.; Echegoyen, L. Chem. Commun. 1999, 1493. (b) Liu, S.-G.; Liu, H.; Bandyopadhyay, K.; Gao, Z.; Echegoyen, L. J. Org. Chem. 2000, 65, 3292.
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Scheme 1. Reactions To Form the Dithiol (I) and Disulfide-Containing Polymer (II)
Figure 1. Possible loop formation from I and II.
polymers were bound intact to the gold surface via pendant sulfide or disulfide moieties.7 We are investigating the formation of surface-attached loops from a variety of dithiol and disulfide precursors. This paper concerns the results of our experiments with two of these compounds, an R,ω-dithiol and its related disulfide-containing polyamides. We prepared the compounds from readily available starting materials to give the dithiol monomer (I) and the disulfide-containing polyamide (II) (Scheme 1). The interfacial condensation of the polyamide yielded a polymer with a wide polydispersity as well as a small, but significant, percentage of low molecular weight units such as the [2 + 2] macrocycle shown in Figure 1. In principle, any of these compounds could lead to the same surface-attached species in which both sulfurs attach to gold to form a loop, or the monomer could adsorb through only one sulfur (Figure 1). Using synchrotron-based X-ray absorption spectroscopy (XAS) and X-ray photoemission spectroscopy (XPS), we have probed molecular orientation and the nature of the sulfur-gold bonds formed in our monomer- and polyamidederived monolayers. X-ray absorption probes unfilled states. Features near edges (often referred to as near-edge X-ray absorption spectroscopy or NEXAFS) reveal chemical information near the atoms being probed. Cross-sections of transitions from core levels into such unfilled states often depend on the polarization of the exciting radiation. Utilizing the high degree of linear polarization of synchrotron radiation, the order and orientation of chemical bonds can be determined by rotating the samples with respect to the X-ray beam. Such polarization dependencies in the carbon (6) (a) Tsutsumi, H.; Fujita, K. Electrochim. Acta 1995, 40, 879. (b) Tsutsumi, H.; Okada, K.; Oishi, T. Electrochim. Acta 1996, 41, 2657. (c) Tsutsumi, H.; Takeoka, K.; Oishi, T. J. Colloid Interface Sci. 1997, 185, 432. (d) Tsutsumi, H.; Okada, S.; Oishi, T. Electrochim. Acta 1998, 43, 427. (7) (a) Sun, F.; Grainger, D. W. J. Polym. Sci., Part A 1993, 31, 1729. (b) Lenk, T. J.; Hallmark, V. M.; Rabolt, J. F.; Ha¨ussling, L.; Ringsdorf, H. Macromolecules 1993, 26, 1230. (c) Niwa, M.; Mori, T.; Higashi, N. Macromolecules 1993, 26, 1936.
K-edge (and nitrogen K-edge) are used to determine order and orientation in this work. XPS has been used to determine bonding of the thiols and disulfides to the gold surface. Core-level shifts in the XPS spectra are indicative of the chemical state of the emitting atom, and intensities of components of emitted photoelectrons can be used to quantitatively determine chemical composition. Relative intensities of various photoelectrons (e.g., N 1s) between monolayers formed from the monomer and polymer precursors can be used to determine the relative coverage of the surface by the adsorbed species. Experimental Methods Reagents and Materials. All reagents were purchased from commercial sources and were used as received. Au(111)-coated mica substrates were purchased from Molecular Imaging and were hydrogen flame annealed immediately prior to use. Instrumentation. Infrared spectra of the starting materials in KBr pellets were recorded on a Nicolet 560 FT-IR spectrometer. NMR measurements were run at ambient conditions on a Bruker DRX-500 11.7T spectrometer equipped with a Bruker 5 mm inverse HCX probe. An observed pulse of approximately 7 µs was used, and chemical shifts were referenced to the solvent resonance. X-ray photoelectron spectra were obtained using a Physical Electronics Quantum 2000 scanning XPS system with a focused monochromatic Al KR X-ray (1486.7 eV) source for excitation and a spherical section analyzer. The instrument has a 16element multichannel detection system. A 1 mm diameter X-ray beam was used for analysis. The X-ray beam is incident normal to the sample, and the X-ray detector is at 45° away from the normal. The pass energy was 23.5 eV, giving an overall energy resolution of 0.3 eV. The collected data were referenced to an energy scale with binding energies for Cu 2p3/2 at 932.72 ( 0.05 eV and Au 4f7/2 at 84.01 ( 0.05 eV. All spectra were recorded at a base pressure of 5 × 10-10 Torr. Low-energy electrons and argon ions were used for specimen neutralization. X-ray absorption spectra were taken at VUV BL 8.2 of the Stanford Synchrotron Radiation Laboratory (SSRL) at the Stanford Linear Accelerator Center. The beamline uses bend magnet radiation and a spherical grating monochromator.8 XAS experiments were conducted with an energy resolution of ∼0.2 eV at the carbon K-edge. Absorption spectra were recorded using the total electron yield method, as the total current leaving the (8) Tirsell, G. K.; Karpenko, V. P. Nucl. Instrum. Methods 1990, A291, 511.
XAS and XPS Characterization of Monolayers sample is proportional to the absorption signal. The sample current was normalized to the incident beam via the current from a clean grid with a freshly evaporated coat of gold. Care was taken to limit X-ray flux to minimize damage to the samples during data collection. All spectra were recorded at a base pressure of ∼2 × 10-9 Torr. At numerous times between samples, the degree of linear polarization of the beam was measured with highly oriented pyrolytic graphite (HOPG) by rotating about two different axes and monitoring the intensity of the C 1s to π* feature in the C K-edge spectra. Results of these measurements obtained throughout data acquisition at the beamline gave 90.1% ((0.8) polarization in the plane of the storage ring. The energy scale of carbon XAS spectra was calibrated to the π* resonance of HOPG set to 285.38 eV. Mass spectra were collected using an Applied Biosystems Voyager DE-STR matrix-assisted laser desorption ionization time-of-flight mass spectrometer with R-cyano-4-hydroxycinnamic acid as the matrix. Gel permeation chromatography was performed by Jordi Associates, Inc., and elemental analysis was performed by Galbraith Laboratories, Inc. Synthesis of the Dithiol Monomer N,N′-Bis(2-mercaptoethyl)-1,3-benzenedicarboxamide (I). 2-Aminoethanethiol hydrochloride (2.50 g, 22.0 mmol) was placed in a 100 mL Schlenk flask that was then flushed with nitrogen. Anhydrous dichloromethane (40 mL) was added via syringe followed by triethylamine (6.2 mL, 44 mmol). The mixture was stirred over an ice bath. Isophthaloyl dichloride (1.83 g, 9.00 mmol) was dissolved in 20 mL of anhydrous dichloromethane and added from a dropping funnel to the reaction flask. The acid chloride was exhausted after 30 min, and the solution was stirred and allowed to warm to room temperature. After 3.5 h, the solution was washed with 50 mL each of 2 N sulfuric acid, water, and 5% sodium bicarbonate. The organic layer was dried over sodium sulfate decanted, and the solvent was removed under rotary evaporation, leaving 1.77 g of a white powder (69% yield). Infrared spectrum: νS-H ) 2535 cm-1, νCdO ) 1634 cm-1. 1H NMR spectrum (DMSO-d6, δ): 8.75 (t, 2H), 8.30 (s, 1H), 7.95 (d, 2H), 7.55 (t, 1H), 3.45 (q, 4H), 2.70 (q, 4H), 2.45 (t, 2H). 13C NMR spectrum (DMSOd6, δ): 167.90, 136.60, 131.80, 130.39, 128.30, 44.86, 25.33. Anal. Calcd for C12H16N2O2S2: C, 50.68; H, 5.67; N, 9.85. Found: C, 50.35; H, 5.72; N, 9.62. Melting point: 122-124 °C. Synthesis of the Disulfide-Containing Polyamide Poly(dithioethyleneiminoisophthaloyliminoethylene) (II). Isophthaloyl dichloride (2.03 g, 10.0 mmol) was dissolved in 100 mL of chloroform. Cystamine dihydrochloride (2.25 g, 10.0 mmol) was dissolved in aqueous sodium hydroxide (100 mL). The aqueous solution was layered onto the chloroform solution and the polymer formed at the interface. A sufficient quantity (ca. 1.0 g) of the polymer was collected, rinsed with water, and dried for 24 h in vacuo. The crude polymer was dissolved in trifluoroacetic acid, precipitated by addition of methanol, collected, and dried in vacuo. Gel permeation chromatography: Mn ) 3385, Mw ) 13 600, Mw/Mn ) 4.02, 7.8% MW < 1000. Infrared spectrum: νCdO ) 1641 cm-1. 1H NMR spectrum (DMSO-d6, δ): 8.82 (t, 2H), 8.33 (s, 1 H), 7.96 (d, 2H), 7.53 (t, 1H), 3.55 (q, 4H), 2.93 (t, 4H). 13C NMR spectrum (DMSO-d , δ): 167.94, 136.53, 131.81, 130.42, 6 128.31, 40.84, 39.03. MALDI-TOF mass spectrometry of MW < 1000 (see Supporting Information for structures): m/z 565.11, 717.13, 847.12, and 999.15. Monolayer Formation. Monolayers were formed by immersing the gold substrates in 1.0 mM solutions of the dithiol (in dichloromethane) or the polyamide (in trifluoroacetic acid) for 24 h. The samples were rinsed thoroughly with clean solvent and dried in a nitrogen stream. Vials containing the samples were closed under the nitrogen stream to prevent oxidation of the sulfurs prior to analysis. Powder Samples. For the XAS and XPS measurements, small amounts of the polyamide and monomer powders were pressed into clean indium metal.
Results and Discussion X-ray absorption spectra reveal the local bonding environment around specific atoms and hence the chemical state of these atoms. When the sample is rotated in the X-ray beam, polarization effects are observed as intensity
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modulations of the peaks in the spectra. Differences in spectra taken at various angles of incidence allow determination of average orientation of the molecules.9,10 Through the dipole approximation, the intensity of a resonance is proportional to the square of the scalar product of the electric field of the X-rays and the direction of the final state orbital when exciting from s-like core levels. Hence, when the electric field of the radiation is parallel to the direction of the final state orbital, the resonance into this orbital is strong, and the absorption signal at the resonance is strong. Conversely, when the electric field of the radiation is orthogonal to the direction of the final state orbital, the resonance is greatly attenuated. These linear dichroism effects disappear when the molecules are oriented totally randomly or when the final state orbitals are oriented with respect to the surface near arcsin(x(2/3)) (54.7°), often referred to as the magic angle.10 For this reason, angles chosen and presented here are 55°, as well as 90° (normal) and 20° (grazing), which are both 35° from the magic angle. Figure 2 shows carbon K-edge spectra for the monomer and polyamide monolayers at various angles of incidence as well as the differences of these spectra. Features typical to carbon-containing organic molecules are present in all carbon absorption spectra and are related to the distinct environments of carbon atoms in the molecule.9 The peak at 285.1 eV arises from CdC π bonds and is assigned to a transition from the C 1s level into a π* orbital of the central phenyl ring in each molecule. The C 1s (C-NH f π*CdC) transition at 286.9 eV is visible only as a shoulder. The next resonance at 288.0 eV arises primarily from the C 1s to π* transition in carbonyl groups. At this energy, additional π* states in the phenyl ring and rydberg resonances in the alkyl chain carbon atoms near the ends of the molecules contribute to the intensity.9b Features at 288.8 eV in the polyamide as well as features from 290 to 300 eV that are not present in monomer spectra are due to a small amount of trifluoroacetic acid still present in the polyamide sample, confirmed by XPS via a small F 1s photoelectron peak that was not present in the monomer. Other transitions from C 1s into σ* states make up the broad features from ∼290 to ∼310 eV. Continuing with Figure 2, spectra taken at three angles of incidence are shown: grazing at 20 and 55° and normal at 90°. Carbon spectra are normalized to the step edge at ∼320 eV. The bottom panes show the differences in spectra by subtracting the spectra taken at 20° from the spectra taken at 90 and 55°. To determine orientation of these monolayers, the phenyl ring π* resonance at 285.1 eV is used. Additionally, the CdO π* and C-C σ* resonances prove useful. In the monomer spectra, the aromatic π* feature is slightly more intense at normal incidence than at grazing. The CdO π* feature at 288 eV is somewhat more intense at grazing than at normal incidence. C-C σ* features are faintly more intense at grazing incidence. This indicates (9) (a) Giebler, R.; Schulz, B.; Reiche, J.; Brehmer, L.; Wuhn, M.; Woell, Ch. Langmuir 1999, 15, 1291. (b) Bagus, P. S.; Weiss, K.; Schertel, A.; Woell, Ch.; Braun, W.; Hellwig, C.; Jung, C. Chem. Phys. Lett. 1996, 248, 129. (c) Sto¨hr, J.; Samant, M. G.; Lu¨ning, J.; Callegari, A. C.; Chaudhari, P.; Doyle, J. P.; Lacey, J. A.; Lien, S. A.; Purushothaman, S.; Speidell, J. L. Science 2001, 292, 2299-2302 (10) (a) Sto¨hr, J. NEXAFS Spectroscopy; Springer-Verlag: Berlin, Heidelberg, 1992. (b) Sto¨hr, J.; Outka, D. A. Phys. Rev. B 1987, 13, 7891. (c) Imanishi, A.; Isawa, K.; Matsui, T.; Tsuduki, T.; Yokoyama, T.; Kondoh, H.; Kitajima, Y.; Ohta, T. Surf. Sci. 1998, 407, 282. (d) Harder, P.; Bierbaum, K.; Woell, Ch.; Grunze, M. Langmuir 1997, 13, 445. (e) Himmelhaus, M.; Gauss, I.; Buck, M.; Eisert, F.; Woell, Ch.; Grunze, M. J. Electron Spectrosc. Relat. Phenom. 1998, 92, 139. (f) Frey, S.; Stadler, V.; Heister, K.; Eck, W.; Zharnikov, M.; Grunze, M.; Zeysing, B.; Terfort, A. Langmuir 2001, 17, 2408.
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Figure 2. C K-edge absorption spectra for the monomer and polyamide samples on Au/mica substrates. Table 1. Summary of Core-Level Binding Energies (eV) sample monomer polymer monomer/Au/mica polymer/Au/mica
C 1s
N 1s
O 1s
S 2p3/2,1/2
Au 4f7/2
284.8 287.7 284.8 287.6 284.9 287.8
399.6
531.6
399.5
531.0
399.6
84.1
284.7 287.4 290.5
399.3
531.3 532.8 533.2 530.8 533.1
163.4 164.6 163.3 164.5 see Table 2 see Table 2
84.1
disordered monolayers that (according to the aromatic and C-C σ* resonances) have molecules on average tilted slightly more than 54.7° from the surface or that are more “standing up” than “lying down”. The polyamide spectra, however, have linear dichroism that is larger and different than that of the monomer. The C 1s (CdC) f π*CdC transition is relatively much stronger at grazing incidence, where the electric field of the synchrotron radiation is nearly perpendicular to the surface. At normal incidence, where the electric field of the radiation is in the plane of the surface, this π* resonance is weak. This indicates that the plane of the phenyl ring is nearly parallel to the surface. The CdO π* resonance shows a polarization dependence with strongest features at grazing incidence, and C-C σ* features have slightly more intensity at normal incidence. Nitrogen NEXAFS (not shown) supports the carbon spectra with a strong pre-edge excitation from the N 1s into the CdO at grazing incidence. These polarization dependencies indicate that the polymer molecules, on average, lie nearly flat on the surface. Thus, carbon NEXAFS reveals that the monomer precursor forms a less ordered, more upright monolayer where the molecules are tilted on average more than 54.7° from the surface. Polyamide-derived monolayers, on the other hand, have order with the molecules oriented nearly parallel to the surface. High-resolution C 1s, N 1s, O 1s, and S 2p3/2,1/2 corelevel XPS spectra were obtained for the powders and the monolayers of the monomer and polyamide samples. Binding energies are summarized in Table 1. The spectra of the powder samples were used to evaluate the effects
Figure 3. Deconvolved high-resolution S 2p3/2,1/2 core-level spectra.
of surface attachment on the chemical structure of the compounds. Shifts indicative of sulfur-gold adsorption were observed in the S 2p3/2,1/2 spectra (Figure 3), while the carbon, nitrogen, and oxygen spectra showed little qualitative variation upon monolayer formation. The C 1s, N 1s, O 1s, and S 2p3/2,1/2 (powder and monolayers) spectra and a table summarizing the XPS compositional analysis and elemental ratios for the powder samples and monolayers are available in Supporting Information. The C 1s core-level spectra for the powder samples show two components indicative of C-H (284.8 eV) and CdO (287.8 eV). These features are also present in the spectra of the monomer and polyamide monolayers on gold. This indicates that attachment of these molecules to the goldcoated substrates has little effect on the chemical structure
XAS and XPS Characterization of Monolayers
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Table 2. Summary of Curve Fitting Results
sample
S 2p3/2,1/2 bound sulfur (eV)
% total peak area
S 2p3/2,1/2 unbound sulfur (eV)
monomer polyamide mono/Au/mica poly/Au/mica
162.08 163.25 162.07 163.24
57.6 90.7
of the central phenyl ring or the carbonyl groups in either molecule. The N 1s core-level spectra are all centered at 399.6 eV and represent the N-H bonding. Line intensity variations are related to monomer and polymer coverage on the Au-coated substrates. The O 1s core-level spectra for the monomer and polyamide powders have binding energies of 531.6 and 531.0 eV, respectively, and represent the carbonyl moieties. In the case of the monomer monolayer, the O 1s spectrum exhibits an additional component at 532.7 eV that is assigned to the formation of sulfinates and/or sulfonates (see below). In addition, both the monomer and the polyamide spectra have a peak at ∼533.2 eV that indicate the presence of -OH species (i.e., water). Deconvolution of the S 2p3/2,1/2 spectra reveals the spinorbit pairs for both bound and unbound sulfurs (Figure 3). Standard Gaussian-Lorentzian curve-fitting with Shirley background subtraction was performed. Specifically, the peaks were fit using the S 2p3/2,1/2 doublet with a 0.56 area ratio and a splitting of 1.17 eV. The curve fitting results are summarized in Table 2. The spectra for each monolayer clearly deconvolute into two S 2p3/2,1/2 spin-orbit pairs representing the bound and unbound sulfurs.2d,11 In the monomer-derived monolayer, about half (57.6%) of the sulfurs are bound to gold while the other half are either unbound (36%) or present as sulfinates and/or sulfonates (6.4%). In contrast to the monomer, over 90% of the sulfurs in the polyamide-derived monolayer are bound to the gold surface. Conclusions XAS and XPS have been used to characterize the chemical interaction between gold surfaces and monolayers derived from either a dithiol monomer or disulfidecontaining polyamides. These experiments differentiate between surface-attached loops on gold and simple monolayers in which one sulfur in each molecule binds to the surface (i.e., nonloops). In the case of the dithiol monomer monolayer, the spectroscopic evidence clearly shows that the molecules adsorb predominantly through only one sulfur. The disulfide-containing polyamides, in contrast to the monomer, attach to the surface through both sulfurs to form the anticipated surface-attached loop. In the NEXAFS spectra, spectral features vary with the angle of incidence of the polarized exciting radiation. XAS polarization dependence measurements indicate a higher degree of ordering in the polyamide-derived monolayer with the phenyl rings oriented nearly parallel to the gold surface. The monomer-derived monolayer shows only very slight polarization dependence, indicative of more upright, disordered aromatic moieties. XPS studies showed that the only significant changes in the chemical structure of the compounds occurred in the sulfurs upon attachment to the gold surface. (11) Castner, D. G.; Hinds, K.; Grainger, D. W. Langmuir 1996, 12, 5083.
163.44 164.61 163.34 164.51 163.89 165.06 163.92 165.09
% total peak area
S 2p3/2,1/2 sulfinate/sulfonate (eV)
% total peak area
36.0
167.84 169.01
6.4
9.3
The surface coverage of the monolayer formed from the polyamide is less than that of the dithiol monomer monolayer, as might be expected because of the increased surface area of the loop versus the standing monomer. That the polyamides preferentially form loops while the monomer attaches primarily through only one sulfur may be attributed to the fact that the sulfurs of the disulfide adsorb to the gold simultaneously while the sulfurs of the monomer are at separate ends of the molecule. At the concentrations used to form these systems, the monomer is present in large excess over what is required to form a monolayer. There may simply be too little space for both sulfurs to access the surface for adsorption. In contrast, as the polyamides (with either the polymers or low molecular weight macrocycles) approach the gold surface, their adjacent disulfide sulfurs are positioned to effectively “unzip” the polyamide as it is exposed to the gold, leaving surface-attached loops. The fact that the monomer had a small percentage of molecules bound through both sulfurs means it may yet be possible to manipulate reaction conditions (e.g., concentration) to favor loop formation. We are pursuing further studies of these and similar compounds and are especially interested in determining the role of disulfide-containing macrocycles in the formation of surface-attached loops. The results obtained from the experiments presented here demonstrate the importance of designing monolayer precursors that will bind to surfaces in a predictable manner. In the case of surface-attached loops, simply preparing compounds with terminal binding moieties does not guarantee loop formation. Other factors such as anticipated surface coverage, solution concentration, and steric interactions must be taken into account to produce a desired surface-attached system. We are currently working to design and characterize monolayers derived from monomers incorporating features that will significantly enhance loop formation. Acknowledgment. We thank the SSRL VUV staff (Jan Luning, Curtis Troxel, and Hal Thomkins) for their assistance during these experiments and Sharon Shields and Matilda Fone of LLNL for mass spectrometry. This work was performed under the auspices of the U.S. Department of Energy by University of California, Lawrence Livermore National Laboratory under Contract W-7405-ENG-48. Work was conducted at the Stanford Synchrotron Radiation Laboratory, which is supported by the U.S. Department of Energy under Contract DE-AC03-76SF00515. Work was also conducted at the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. Supporting Information Available: Sulfur L2,3-edge absorption spectra for monomer and polymer precursor powders
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and monolayers on Au(111) (including spectrum of oxidized sulfur for comparison), C 1s, N 1s, and O 1s XPS spectra, a table summarizing the XPS compositional analysis and elemental ratios for the powder samples and monolayers, as well as probable structures of the low MW materials present in the polyamide.
Vance et al. This material is available free of charge via the Internet at http://pubs.acs.org.
LA025631G
