Langmuir 1998, 14, 7435-7449
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Increased Lateral Density in Alkanethiolate Films on Gold by Mercury Adsorption J. Thome, M. Himmelhaus, M. Zharnikov, and M. Grunze* Angewandte Physikalische Chemie, Universita¨ t Heidelberg, Im Neuenheimer Feld 253, D-69120 Heidelberg, Germany Received July 7, 1998. In Final Form: October 5, 1998 We present a method for increasing the lateral chain density in self-assembled monolayers (SAMs) of alkanethiols on polycrystalline gold. This method relies on exposure of the alkanethiolate monolayers to mercury vapor and subsequent reimmersion into the thiol solution. Mercury adsorption on the gold surface induces a structural rearrangement in the alkanethiolate monolayers, as indicated by changes in dichroism in the Fourier transform infrared (FTIR) spectra, in line shape in the sum frequency generation (SFG) spectra, and in the macroscopic wetting behavior of the monolayers. X-ray photoelectron spectroscopy (XPS) data show that saturation of the thiolate samples with mercury occurs after 20-30 min of exposure to air saturated with mercury vapor. For 100 nm evaporated gold films a mercury bulk concentration of 14-16 atom % was determined by energy-dispersive X-ray analysis (EDX). Time-of-flight secondary ion mass spectroscopy (ToF-SIMS) data indicate that after mercury adsorption the monolayers consist of gold thiolate and mercury thiolate molecules. From the FTIR and XPS data we conclude that the mercuryexposed SAMs exhibit an inhomogeneous structure with differently tilted domains. As determined from the IR experiments, the average tilt angle of the alkyl chains in hexadecanethiolate monolayers decreases by ∼16° by mercury adsorption and by an additional ∼3° after reimmersion into the thiol solution. The corresponding changes obtained from near edge X-ray absorption fine structure (NEXAFS) spectra are ∼9° and ∼3°, respectively.
1. Introduction The investigation of self-assembled monolayers (SAMs) formed by chemisorption of alkanethiols on gold surfaces has been a topic of interest over the last 15 years.1-11 By numerous techniques it has been shown that SAMs of alkanethiols above a minimum chain length (n g 12) exhibit a closely packed, well-ordered structure with 2D crystalline domains, that is, with in-plane long-range order. In thiolate SAMs on polycrystalline gold (mainly (111) textured) the sulfur atoms form a (x3 × x3)R30° overlayer with a S-S distance of 5.01 Å, as established by X-ray diffraction.3,4 This relatively large spacing (compared to a S-S distance of 4.77 Å for alkanethiolate SAMs on silver, as determined by X-ray diffraction12) is compensated by the tilting of the alkyl chains to accom* To whom correspondence should be addressed. Fax: +49-6221546199. Phone: +49-6221-542461. E-mail: Michael.Grunze@ urz.uni-heidelberg.de. (1) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152. (2) Ha¨hner, G.; Kinzler, M.; Thu¨mmler, C.; Wo¨ll, Ch.; Grunze, M. J. Vac. Sci. Technol. A 1992, 10, 2758. (3) Camillone, N., III; Chidsey, C. E. D.; Liu, G.-Y.; Scoles, G. J. Chem. Phys. 1993, 98, 3503. (4) Camillone, N., III; Chidsey, C. E. D.; Eisenberger, P.; Fenter, P.; Liang, K. S.; Liu, G.-Y.; Scoles, G. J. Chem. Phys. 1993, 93, 744. (5) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (6) Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. J. Chem. Phys. 1993, 98, 678. (7) Delamarche, E.; Michel, B.; Biebuyck, H. A.; Gerber, Ch. Adv. Mater. 1996, 8, 719. (8) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, CA, 1991. (9) Edinger, K.; Go¨lzha¨user, A.; Demota, K.; Wo¨ll, Ch.; Grunze, M. Langmuir 1993, 9, 4. (10) Strong, L.; Whitesides, G. M. Langmuir 1988, 4, 546. (11) Fenter, P.; Eisenberger, P.; Liang, K. S. Phys. Rev. Lett. 1993, 70, 2447. (12) Fenter, P.; Eisenberger, P.; Li, J.; Camillone, N., III; Bernasek, S.; Scoles, G.; Ramanarayanan, T. A.; Liang, K. S. Langmuir 1991, 7, 2013.
modate for optimum van der Waals interactions between neighboring chains. With IR spectroscopy an average chain tilt angle of 25-30° to the surface normal is obtained.1 Near edge X-ray absorption fine structure (NEXAFS) yields an average chain tilt angle of about 35°.2 The average chain area on gold is 21.6 Å2 on the basis of the assumption of extended chains in the all-trans conformation.1 Interestingly, SAMs of long-chain alkanethiols on the surface of liquid mercury possess a significantly different structure: A recent grazing incidence X-ray diffraction (GIXD) and X-ray reflectivity study by Magnussen et al.13 showed SAMs of alkanethiols of various chain lengths on mercury to be densely packed, but without any in-plane long-range order. The alkyl chains were found to be oriented perpendicular to the mercury surface. The closely packed, defect-free assembly of long-chain thiols on mercury surfaces has also been demonstrated by cyclic voltammetry (CV) experiments on mercury drop electrodes (MDEs).14,15 Demoz and Harrison14 compared their CV results for C16 SAMs on mercury drop electrodes to those for thiolate SAMs on gold electrodes published by other groups (see ref 13) and found the SAMs on mercury to be less permeable for electron tunneling. In this paper we present results for mercury-vaporexposed alkanethiolate SAMs on gold. The sorption of mercury by gold is of fundamental importance both in the mining industry for the separation of gold from nonprecious metals and for numerous applications in analytical and environmental chemistry. Many commercial (13) Magnussen, O. M.; Ocko, B. M.; Deutsch, M.; Regan, M. J.; Pershan, P. S.; Abernathy, D.; Gru¨bel, G.; Legrand, J.-F. Nature 1996, 384, 250. (14) Demoz, A.; Harrison, D. J. Langmuir 1993, 9, 1046. (15) (a) Slowinski, K.; Chamberlain, R. V., II; Miller, C. J.; Majda, M. J. Am. Chem. Soc. 1997, 119 (9), 11910. (b) Slowinski, K.; Chamberlain, R. V., II; Bilewicz, R.; Majda, M. J. Am. Chem. Soc. 1996, 118 (8), 4709.
10.1021/la9808317 CCC: $15.00 © 1998 American Chemical Society Published on Web 12/03/1998
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mercury vapor sensors rely on changes of physical properties (mass, electrical resistivity) of thin gold films induced by the adsorption of trace amounts of mercury.16,17 Ricco et al.17 were the first to study successfully changes of the optical reflectivity of thin gold films by mercury adsorption. In air saturated with mercury vapor at room temperature, they observed two stages for the mercurygold interaction: An initial reflectivity increase is followed, after a few minutes of mercury exposure, by a dramatic decrease in reflectivity below the reflectivity of the unexposed gold film. The initial increase of reflectivity was explained by the physisorption of a few mercury monolayers on the gold surface. The reflectivity decrease was attributed to amalgamation of the gold film, as suggested by microphotographs showing morphology changes of the initially smooth gold film (formation of “wormlike” islands and voids in the gold film). There is still disagreement in the literature on the AuHg phase diagram.18-20 For the solubility of mercury in solid gold at 100 °C a value of 15-17 wt % Hg has been suggested on the basis of lattice-parameter measurements.17 With respect to Au-Hg intermediate phases, there is strong evidence from thermal analysis and dispersive energy-X-ray fluorescence spectroscopy for the existence of Au2Hg, Au2Hg3, and AuHg2.18,19,21 Despite the technological importance of gold amalgams, little is known about the microscopic mechanism of mercury adsorption on gold. A recent in situ study of mercury adsorption on thin epitaxial gold films on muscovite mica by scanning tunneling microscopy (STM)22 revealed the formation of amalgam islands and of pits in the atomically flat (111) terraces of the gold film. Islands and pits both grew from 2 to 30 nm with continuing mercury exposure. A similar microscopic image of mercury adsorption has also been described in two recent in situ atomic force microscopy (AFM) studies of bulk electrodepostion of mercury on gold electrodes.22 Kaciulis et al.24 used X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS) to investigate the depth profile of mercury in thin gold films on silicon substrates. With XPS, which is a more surface-sensitive method, mercury could be detected only up to a depth of 1 nm. A more realistic depth profile was obtained by SIMS, showing mercury in a 5-6 nm thick interphase after 30 min of mercury exposure. The present paper focuses on the changes of molecular structure and macroscopic wetting properties of methylterminated alkanethiolate SAMs on polycrystalline gold induced by exposure to mercury vapor. We have investigated alkanethiolates of various chain lengths after mercury adsorption by contact angle measurements, grazing incidence reflection absorption spectroscopy (IRRAS), sum frequency generation (SFG), near edge X-ray (16) Urba, A.; Kvietkus, K.; Sakalys, J.; Xiao, Z.; Lindquist, Q. Water, Air, Soil Pollut. 1995, 80, 1305. (17) (a) Butler, M. A.; Ricco, A. J.; Baughman, R. J. J. Appl. Phys. 1990, 67, 432. (b) Ricco, A. J. (Sandia National Labs, Microsensor R&D Department, Albuquerque, NM) Private communication. (18) Rolfe, C.; Hume-Rothery, W. J. Less-Common Met. 1967, 13, 1. (19) Hansen, M. Constitution of Binary Alloy; McGraw-Hill: New York, 1958; p 207 and references therein. (20) Gmelins Handbuch der anorganischen Chemie; Verlag Chemie: Weinheim/Bergstrasse, Germany, 1954; Vol. 62, p 812 and references therein. (21) Kita, H.; Kon, T. J. Res. Inst. Catal. 1972, 19, 167. (22) Levlin, M.; Niemi, H. E.-M.; Hautoja¨rvi, P.; Ika¨valko, E.; Laitinen, T. Fresenius J. Anal. Chem. 1996, 335, 2. (23) Yang, X. M.; Tonami, K.; Nagahara, L. A.; Hashimoto, K.; Wie, Y.; Fujishima, A. (a) Surf. Sci. Lett. 1994, 319, L 17; (b) Chem. Lett. 1994, 11, 2059. (24) Battistoni, C.; Bemporad, E.; Galdikas, A.; Kaciulis, S.; Mattogno, G.; Mickevicius, S.; Olevano, V. Appl. Surf. Sci. 1996, 103, 107.
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absorption fine structure (NEXAFS), X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray analysis (EDX), and time-of-flight secondary ion mass spectroscopy (ToF-SIMS). Our results clearly indicate that at mercury saturation pressure in air at room temperature the SAMs are sensitive to mercury vapor on a time scale of 20-30 min. A second important aspect of this study is the possibility to increase the packing density of alkanethiolate SAMs by exposure to mercury vapor and subsequent reimmersion in the thiol solution. 2. Experimental Section 2.1. Preparation of Gold Substrates. Polycrystalline gold substrates were prepared by electron beam evaporation of 200 nm of gold (99.99% purity) onto polished single-crystal silicon (100) wafers (Silicon Sense) that had been precoated with a 20 nm titanium adhesion layer. The evaporation was performed at a pressure of 2 × 10-7 Torr and a deposition rate of 0.5 nm/s. 2.2. Chemicals. 1-Dodecanethiol (C12) was obtained from Sigma; 1-pentanethiol (C5), 1-octanethiol (C8), and 1-hexadecanethiol (C16) were obtained from Fluka. 1-Hexadocosanethiol (C26) and perdeuterated hexadecanethiol-d33 were received from G. M. Whitesides (Department of Chemistry, Harvard University). The perdeuterated hexadecanethiol had been synthesized from the perdeuterated hexadecyl bromide. Analytical grade mercury (99.999% purity) was obtained from Fluka. All chemicals were used as received. 2.3. Preparation of Thiolate SAMs. Gold substrates were cut into pieces of 1.5 cm × 2 cm, rinsed with analytical grade ethanol, and immersed for 24 h into 30 mL of a 1 mM thiol solution in analytical grade ethanol. After removal from the thiol solution, the samples were rinsed with pure ethanol and blown dry with pure nitrogen. 2.4. Exposure of the Thiolate SAMs to Mercury Vapor and Additional Thiol Adsorption. The thiolate samples were exposed at room temperature for a period ranging from 5 to 30 min to an atmosphere saturated with mercury vapor. At 293 K the saturation vapor pressure of mercury in air is 0.0017 mbar (0.170 Pa). This corresponds to a mercury vapor concentration of 13.6 mg/m3 of air.25 Our XPS measurements showed saturation of the thiolate samples with mercury after 20-30 min of exposure. The experimental procedure was as follows: A drop of pure mercury was put in a Petri dish which was sealed with Parafilm for several minutes for saturation with mercury vapor. Then the thiolate sample was put in the Petri dish with the mercury drop, which was sealed again with Parafilm. The thiolate samples that had been exposed to mercury vapor were immersed once more in a 1 mM thiol solution in ethanol for 2 h. Each Hg-exposed sample was immersed in a separate solution. As a control experiment, a thiolate sample that had not been exposed to mercury vapor was put in a thiol solution for the same period. 2.5. Atomic Force Microscopy (AFM). AFM data were acquired with a Park Scientific Instruments atomic force microscope in air at room temperature, using a 5 µm scanner and a silicon nitride (Si3N4) tip on a microfabricated cantilever (Ultralever, Park Scientific Instruments) with a nominal spring constant of 0.06 N/m and a theoretical resonance frequency of 19 kHz. Imaging was performed in the contact mode. The applied force was set to about 10 nN. 0.85 × 0.85 µm2 and 0.1 × 0.1 µm2 scans were recorded at several positions of the samples to determine microscopic roughness (rms) from data analysis with the Autoprobe software (Park Scientific Instruments). 2.6. Contact Angles. Water contact angles were measured with a Kru¨ss goniometer Model G 1 on freshly prepared thiolate SAMs under ambient conditions, that is, at room temperature in a water-vapor-saturated atmosphere. Water drops were applied with a micropipet both as sessile drops and as captive drops, that is, with the tip of the pipet attached to the water drop. On both sides of the captive drops, the advancing angle (25) Falbe, J., Regitz, M., Eds. Ro¨ mpp Chemielexikon, 9th ed.; Thieme Verlag: Stuttgart, Germany, 1992; Vol. 5, p 3737.
Alkanethiolate Films on Gold θa was measured. The reported values are the average of at least three measurements at different locations on the sample. Deviations from the average values were not more than (1°. 2.7. Infrared Spectroscopy. Infrared absorption measurements were performed with a dry-air-purged Bio-Rad FTIR spectrometer Model FTS 175C equipped with a liquid-nitrogencooled MCT detector. All spectra were taken in p-polarization with a PIKE advanced grazing angle accessory at a fixed incident angle of 80° to the surface normal. The polarizer consisted of an aluminum wire grid on a KRS 5 window. For all spectra 2000 scans were measured at a spectral resolution of 2 cm-1. The spectra are reported in absorbance units A ) -log R/R0, where R is the reflectivity of the substrate with the monolayer and R0 is the reflectivity of the reference. As gold reference we used a gold substrate covered with a SAM of perdeuterated hexadecanethiol-d33, which protects the gold surface from C-Hcontaining contaminations and absorbs only in the C-D stretching region between 2250 and 2000 cm-1. A difference spectrum of a bare gold reference cleaned with piranha solution (4:1 v/v mixture of 96% H2SO4 and 30% H2O2) and a gold reference covered with a SAM of hexadecanethiol-d33 showed a disordered organic film on the bare gold substrate with asymmetric and symmetric methylene absorption at 2926 and 2854 cm-1, respectively. Exposure of the reference to mercury vapor for 30 min did not change the reflectivity of the gold surface.26 2.8. Determination of Molecular Orientation from the IR Data. To determine quantitatively the average molecular orientation of the alkyl chains, we used a computational 4 × 4 matrix method developed by Allara et al.27,28 The software for the spectra simulations was supplied by Allara and Parikh together with appropriate isotropic optical functions for the alkanethiols. Our calculations are based on a single-chain model (one alkyl chain/unit cell) considering extended chains in the all-trans conformation. It is worth noting that the use of bulk optical functions for simulations of monolayer spectra assumes similar chain packing and conformational defect density in the bulk state and in the monolayer. Optical functions of polycrystalline gold for the C-H stretching region (2800-3000 cm-1) were taken from the literature.29 Since exposure of the gold reference to mercury vapor did not influence the IR reflectivity of the gold surface (see above), we used the optical functions of pure gold films for our calculations. Monolayer thicknesses were calculated from the following incremental values for methylene groups: 1.25 Å for chains oriented perpendicular and 1.1 Å for chains tilted by 25-30° from the surface normal. The accuracy of the tilt and twist angle assignments is about (2°. 2.9. Sum Frequency Spectroscopy (SFG). IR-vis sum frequency signals were obtained by overlapping ultrashort laser pulses of 40 ps duration with wavelengths of λvis ) 532 nm for the visible laser beam and of 2700-3000 cm-1 for the tunable IR laser beam, respectively, on the surface of the samples. The energies of both pulses were about 100 µJ, the bandwidth was 2 cm-1 for the visible beam and 5.5 cm-1 for the IR beam. The samples were put into a nitrogen-purged chamber, accessible for laser radiation. The incident angles with respect to the surface normal were 50° for the visible and 60° for the IR beam, respectively. Both incident beams entered the chamber from the same direction. The IR beam was slightly focused onto the samples with a beam diameter of about 1 mm on the surface, whereas the visible beam remained unfocused with a beam diameter of about 3 mm. Both incident beams were p-polarized. Detection of the p-polarized contribution of the nonlinear signals was performed by spatial separation of signal and pump beams, with band-pass filters, followed by a monochromator and a photomultiplier tube. Data acquisition was done with a boxcar integrator. For interpretation of the obtained spectra it is important to note that sum frequency spectroscopy, as a second-order nonlinear (26) We did, however, observe intensity changes of the C-D stretching vibrations of the perdeuterated hexadecanethiolate monolayer on the gold reference. These changes indicate reorientation of the perdeuterated monolayer but will not be considered any further. (27) Parikh, A. N.; Allara, D. L. J. Chem. Phys. 1992, 96, 927. (28) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 51. (29) Palik, E. D., Ed. Handbook of Optical Constants of Solids; Academic Press: Orlando, FL, 1985; p 286.
Langmuir, Vol. 14, No. 26, 1998 7437 optical technique, is very sensitive to the breakdown of inversion symmetry. Therefore, a certain resonance is observable only if inversion symmetry is violated. For a well-ordered alkanethiolate monolayer on gold with all alkyl chains in the all-trans conformation, inversion symmetry breaks down only for the terminal methyl group as well as for the methylene groups adjacent to the sulfur headgroup and to the methyl group, respectively, whereas the inner methylene groups of the alkyl chains show local inversion symmetry.30 Therefore, the spectrum is dominated by the resonances of the terminal methyl group and the terminal methylene groups, whereas the inner methylene modes vanish for a well-ordered film. Further, for SFG spectra from metal substrates the line shape depends strongly on interference effects between the nonresonant substrate signal and the contributing resonances.31 Therefore, as a function of the phase shift between nonresonant and resonant nonlinear optical contributions, the resonances may enhance or diminish the constantly emitted substrate signal. Resonances of a certain vibrational mode may shift in phase by π due to a change of orientation of their dynamical dipole moments with respect to the surface normal. Hence the signal change from constructive to destructive interference for a certain resonance indicates a change of orientation of the respective molecular groups. 2.10. X-ray Photoelectron Spectroscopy (XPS). The XPS experiments were done with a Leybold Heraeus LH 12 XPS spectrometer equipped with a Mg KR radiation source (Ephot ) 1253.6 eV) typically operated at 240 W. All spectra were acquired at a photoelectron takeoff angle of 0° relative to the surface normal. For the element spectra, the step width was set at 0.1 eV and the analyzer pass energy at 23.4 eV. The energy resolution of the instrument was about 1 eV. Survey spectra were measured at a pass energy of 100 eV. The following numbers of scans were routinely accumulated: C 1s, 10 scans; Au 4f, 2 scans; Hg 4f, 5 scans; S 2p, 50 scans. All intensities were determined after subtraction of a linear background. C 1s, Au 4f, and Hg 4f intensities were calculated by numerical integration using standard software. S 2p intensities were determined by deconvolution into two Gaussian peaks, as expected from the 2p1/2/ 2p3/2 spin-orbit splitting of 1.2 eV. At longer exposure to ultrahigh vacuum (UHV), partial desorption of mercury occurred both from gold substrates and from alkanethiolate monolayers on gold (after 3 h at 2 × 10-9 mbar, approximately 25%, as judged by the decrease of the Hg 4f signal). To minimize mercury desorption and for better reproducibility of the Hg 4f/Au 4f intensity ratio, all spectra were acquired at the same pressure of 6 × 10-9 mbar and after approximately the same exposure time to UHV (ca. 30 min). To infer on the thickness of the monolayers, the intensity ratios I(C 1s)/I(Au 4f) before mercury exposure (a), after mercury exposure (b), and after reimmersion in the thiol solution (c) were compared for alkanethiols of various chain lengths. The ratio is given by
IC(1s) IAu(4f)
)
NC σC(1s) TC(1s) λC(1s) 1 - e-dC/λC NAu σAu(4f) TAu(4f) λAu(4f) e-dCS/λAu
where I is the integrated peak area, N is the concentration of an atom, σ is the ionization cross-section of an atom (Scofield factor), T is the experimentally determined transmission function of the instrument for photoelectrons of a given kinetic energy, λc is the escape depth of the C 1s photoelectrons through the carbon layer, λAu is the escape depth of the Au 4f photoelectrons through the thiolate layer, dc is the thickness of the carbon layer, and dcs is the thickness of the thiolate layer. For the long-chain alkanethiolate monolayers on gold (before mercury exposure) the escape depth λ and the alkyl chain density are known from the literature, and the following equation holds: (30) Guyot-Sionnest, P.; Hunt, J. H.; Shen, Y. R. Phys. Rev. Lett. 1987, 59, 1597. (31) Bain, C. D.; Davies, P. B.; Ong, T. H.; Ward, R. N. Langmuir 1991, 7, 1563.
7438 Langmuir, Vol. 14, No. 26, 1998 IC(1s) IAu(4f)
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1 - e-dC/λC e-ddc/λAu
)k
k is an instrument-specific constant. For the calibration of the instrument, C 1s/Au 4f intensity ratios were determined for alkanethiolate monolayers of various chain lengths. Values for λ are based on λ(Au 4d5/2) ) 26 Å for normal emission of the photoelectrons32 and interpolated for C 1s and Au 4f using the expression λ ∝ Ekin0.67 reported by Laibinis et al.:33 λ(Au 4f), 31 Å; λ(C 1s), 27 Å. 2.11. Energy-Dispersive X-ray Microscopy (EDX). The EDX measurements were carried out by the Freudenberg Forschungsdienste KG (Microscopy Department), 69465 Weinheim, Germany. The instrument was a JEOL JSM 35C scanning microscope operated at a base pressure of 10-5 mbar. The distance between the electron source and the sample was 40 mm, and the electrons were accelerated toward the sample by a voltage of 25 kV. The emitted X-rays were detected at a photon takeoff angle of 44.8° from the surface with a Drakor Micro Z system equipped with a Si/Li detector. For silicon the Si KR line was measured. The amount of gold was determined from the Au LR1 line (E ) 9.713 keV). To determine the amount of mercury, the overlapping Au MR, Hg MR, and Au Mγ lines were fitted according to the amount of gold calculated from the Au LR1 line. For 2D elemental mapping of the samples, the electron beam was rastered over areas of 100 µm × 100 µm. All samples were kept in vacuum for about 30 min. 2.12. Time-of-Flight Secondary Ion Mass Spectroscopy (ToF-SIMS). The ToF-SIMS measurements were performed at the ETH, Zu¨rich, Switzerland, by using a Physical Electronics PHI 7200 ToF-SIMS spectrometer. The data were collected at high mass resolution ((M/∆M) ) 7500) to distinguish mass overlaps of different species with a similar nominal mass. The Cs+ primary ion gun was operated in the pulsed mode using a pulse rate of 10 kHz, a pulse duration of approximately 1 ns, and a primary ion energy of 8 kV. A primary ion dose of 1011 ions/cm2 was applied. The negative-ion spectra were calibrated with CH-, OH-, C2H-, and Au-. 2.13. Near Edge X-ray Absorption Fine Structure (NEXAFS). The NEXAFS experiments were carried out under UHV conditions in a multitechnique analysis chamber34 with a base pressure of about 5 × 10-10 mbar. The measurements were performed at the synchrotron radiation facility BESSY (Berlin) at the monochromator HE-TGM 2 (High Energy Toroidal Grating Monochromator).35 The energy resolution was better than 0.8 eV at the C 1s edge. All spectra were taken in the partial-yield mode with a retarding voltage of -150 V. The angle of incidence of the X-ray photons was varied from 90° (the E vector in the surface plane: normal incidence) to 20° (the E vector near the surface normal: grazing incidence) to monitor orientational order within the investigated films. The resonant photoexcitation process strongly depends on the relative orientation of the light polarization with respect to the vector orbital under consideration. Therefore, the dependence of the intensity of a characteristic NEXAFS resonance on photon incidence angle provides information on the average orientation of the corresponding molecular orbitals in the object of interest. The raw NEXAFS spectra were normalized to the incident photon flux dividing by a spectrum of a clean, freshly sputtered gold sample. For absolute energy calibration the simultaneously measured photoemission current signal of a carbon covered gold grid with a characteristic resonance at about 285 eV was used. This resonance was separately calibrated by using the NEXAFS spectra of a graphite sample (HOPG). The significant π*resonance of HOPG was set to 285.38 eV.36 The size of the synchrotron light spot was approximately 5 × 1 mm2 and 10 × (32) Harder, P.; Dahint, R.; Grunze, M.; Whitesides, G. M.; Laibinis, P. E. J. Phys. Chem. 1998, 102, 426. (33) Laibinis, P. E.; Bain, C. D.; Whitesides, G. M. J. Phys. Chem. 1991, 95, 7017. (34) Schaible, M.; Petersen, H.; Braun, W.; Koch, E. E. Rev. Sci. Instrum. 1989, 60, 2172. (35) Bernstorff, S.; Braun, W.; Mast, M.; Peatman, W.; Schro¨der, T. Rev. Sci. Instrum. 1989, 60, 2097. (36) Batson, P. E. Phys. Rev. B 1993, 48, 2608.
Figure 1. Time dependence of mercury sorption by long-chain alkanethiolate monolayers on gold and by bare gold substrates, as determined by XPS: Hg 4f/Au 4f intensity ratio as a function of exposure time to mercury vapor for C16 and C12 monolayers on gold and for bare gold substrates. There was no difference between C16 and C12 monolayers. Error bars are given as the difference between the maximum and minimum values of the intensity ratio. The XPS data show saturation of the thiolate samples with mercury after 20-30 min of exposure. 1 mm2 at the incidence angles of 90° and 20°, respectively. The acquisition time of every spectrum was about 5 min. It took about 30 min to transfer the samples from air to the main UHV chamber and to adjust their position for the NEXAFS measurements.
3. Results 3.1. X-ray Photoelectron Spectroscopy (XPS) and Energy-Dispersive X-ray Microscopy. Mercury Sorption. Our first objective was to investigate ex situ, using XPS, the diffusion of mercury vapor through the thiolate SAMs into the gold substrates as a function of time. As described in the Experimental Section, we exposed hexadecanethiolate (C16) and dodecanethiolate (C12) monolayers on gold and as reference bare gold substrates to an atmosphere saturated with mercury vapor for periods from 5 to 30 min. Figure 1 displays the time dependence of the intensity ratio Hg 4f/Au 4f for the thiolate monolayers on gold and for bare gold substrates. There was no significant difference between C16 and C12 monolayers. The plot clearly shows a similar time dependence of mercury sorption for gold substrates covered with closely packed thiolate monolayers and for naked gold substrates. According to the XPS data, the mercury concentration on the clean gold and on the thiolate-covered gold reaches a saturation limit after 20-30 min. The thiolate monolayers on gold and the bare gold substrates were exposed to mercury vapor under identical conditions. After any exposure time the Hg 4f/Au 4f intensity ratio is higher for the thiolate samples than for the bare gold substrates, as shown in Figure 1. The higher Hg 4f/Au 4f intensity ratio for the thiolate samples correlates with a stronger attenuation of the Au 4f signal by mercury adsorption, as shown in Figure 2: After 5 min of mercury exposure, the Au 4f substrate signal of C12 and C16 monolayers on gold is attenuated by nearly 25% (a) compared to 15-18% for bare gold substrates (b). (There was no change of the contamination layer on the gold substrates, as verified by the C 1s intensity.) After longer exposure times, we observe no further attenuation of the Au 4f intensity. It is important to note that for the mean free path (MFP) λ of Hg 4f photoelectrons in gold films a value of about 1 nm has been reported by Brundle and
Alkanethiolate Films on Gold
Figure 2. Au 4f XPS data of a C12 monolayer on gold (a) and a bare gold substrate (b) before and after 5 min of exposure to mercury vapor in air. There was no change of the C 1s intensity of the C12 monolayer and of the carbon contamination layer on the bare gold substrate, as checked by the C 1s data.
Roberts.37 The absolute escape depth, that is, the distance from which 95% of the photoelectrons have been inelastically scattered, is about 3 λ (ca. 3 nm). Given an average (37) Brundle, C. R.; Roberts, M. W. Chem. Phys. Lett. 1973, 18, 380.
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atomic spacing in Au-Hg alloys of about 2.9 Å,18 this means that only Hg 4f photoelectrons from the top 10 atomic layers of the amalgamated gold film can be detected with XPS. Mercury diffusion into the bulk of the gold film will not result in a further increase of the Hg 4f XPS signal. Qualitatively speaking, the higher Hg 4f/Au 4f intensity ratio and the stronger attenuation of the Au 4f signal suggest a higher mercury surface concentration of the thiolate samples compared to the bare gold substrates, possibly due to mercury-sulfur interactions. However, from our XPS data at fixed photoelectron takeoff angle, it is not possible to determine quantitatively the stoichiometric ratio of gold and mercury: In situ STM studies of mercury adsorption on gold films by Levlin et al.22 and in situ AFM studies by Yang et al.23 have shown preferential mercury diffusion at grain boundaries and defects of the gold film. This means that as soon as mercury diffuses into the gold film, there will be an inhomogeneous lateral distribution of gold and mercury atoms and a timedependent depth profile of mercury. To determine quantitatively the average atomic concentration of mercury, C16 monolayers on gold, and for comparison also bare gold substrates, were investigated by energy-dispersive X-ray analysis (EDX). In contrast to XPS, which is a surface-sensitive method, EDX probes the bulk of the gold film. From the EDX data an average mercury bulk concentration of 14-16 atom % was calculated for 100 nm gold films. Within the accuracy of the EDX measurements ((10%) there was no difference between bare gold substrates and C16 samples exposed to mercury vapor. From 100 µm elemental distribution maps (not shown) the macroscopic Hg distribution looks homogeneous. Since the lateral resolution of the EDX microscope is only approximately 1 µm, this is not in contradiction with the inhomogeneous microscopic Hg distribution reported by Levlin et al.22 and Yang et al.23 Figure 3 displays a XPS spectrum of a C16 monolayer on gold after 30 min of exposure to mercury vapor in air. In contrast to XPS data of air-exposed alkanethiolate SAMs on silver and copper from the literature,1 we observe no O 1s peak (530-535 eV) and no oxidized sulfur species. The XPS binding energies of the thiolate SAMs and of bare gold substrates before and after mercury exposure are listed in Table 1. All spectra were acquired at exactly the same pass energy (23.4 eV), and except for the C 1s binding energy of C12 SAMs (284.8-284.7 eV), no fluctuations of binding energies for identically prepared samples were observed. Exposure to mercury vapor shifts the Au 4f binding energies of all thiolate samples and of bare gold substrates by +0.1 eV, as shown in Figure 4a for a C16 sample (Au 4f7/2 from 84.0 to 84.1 eV). Positive binding energy shifts of Au 4f have also been reported by Rodriguez et al.38 for surface alloys of gold with zinc, another group IIb metal. The energy shift after mercury exposure can be explained by charge transfer from the Hg adatoms to the more electronegative gold (electronegativity according to Pauling: Au, 2.5; Hg, 2.0) by amalgam formation. With respect to mercury-sulfur interactions, we observe a negative shift of 0.2 eV in the fitted S 2p spectra of the thiolate SAMs after mercury adsorption, as shown in Figure 4b (S 2p3/2 from 162.0 to 161.8 eV. The fwhm (full width at half-maximum) of the fitted S 2p3/2 und S 2p1/2 peaks is, however, unchanged after mercury adsorption: fwhm ) 1.0 eV). The Hg 4f binding energy seems to be slightly shifted to a higher binding energy by e0.1 eV due to thiol adsorption (Hg 4f7/2 from 99.9 to 100.0 eV). These (38) Rodriguez, J. A.; Hrbek, J. J. Vac. Sci. Technol., A 1993, 11, 1998.
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Figure 3. Representative XPS survey spectrum of a C16 monolayer on gold after 30 min of exposure to mercury vapor. Table 1. XPS Binding Energies for Monolayers of Hexadecanethiol (C16) and Dodecanethiol (C12) on Gold as Well as for Bare Gold Substrates before and after Exposure to Mercury Vapor binding energies (eV) C 1s treatment
Au 4f7/2
Hg 4f7/2
C16
C12
C12 and C16 monolayers on gold (a) as deposited (b) after 30 min in Hg vapor (c) after reimmersion in thiol solution bare gold substrates (with organic contamination layer) (a) as kept in air (b) after 30 min in Hg vapor (c) after immersion in thiol solution
84.0 84.1 84.1
100.0 100.0
284.9 284.7-284.8 285.0
284.7-284.8 284.7 284.9
84.0 84.1 84.1
99.9 100.0
opposite energy shifts suggest formation of Hg thiolate molecules on the gold surface. The shift of the S 2p core level to a lower binding energy results from the lower electronegativity of Hg compared to Au (see above). A partial conversion of Au thiolate to Hg thiolate is not surprising given the comparable Hg-S and Au-S bond strengths (Hg-S and Au-S binding energy: ∼200 kJ/ mol;39 for Au thiolate, see also ref 8). However, from our XPS data we cannot quantify the fractions of thiol molecules bound to gold and mercury atoms, respectively. The most striking feature in the spectra of the thiolate SAMs is the negative shift of the C 1s core level energy by 0.1-0.2 eV after 20-30 min in Hg vapor, as shown in Figure 4c and d. For C16 monolayers (Figure 4c) and for octadecanethiolate (C18) monolayers (not shown), we observe a shift from 284.9 to 284.8 eV, and for C12 monolayers (Figure 4d), the shift is from 284.8 to 284.7 eV. To ensure that this difference in the C 1s position is not due to problems of energy calibration of our detector, we verified the position of the 4d and 4p3/2 substrate peaks. Since for the C 1s measurements the samples were exposed (39) Weast, R. C., Astle, M. J., Beyer, W. H., Eds. CRC Handbook of Chemistry and Physics, 66th ed.; CRC Press: Boca Raton, FL, 1986; pp F174-F184.
∼284.0 ∼284.0 285.0 (C16) 284.9 (C12)
S 2p3/2 162.0 161.8 161.8
161.8
to X-rays only for 5 min, X-ray-induced degradation of the SAMs can also be excluded. After reimmersion of the samples in a thiol solution, the C 1s core level energies are shifted to higher binding energies by 0.2 eV, as depicted in Figure 4c and d. For C16 monolayers (Figure 4c) and C18 monolayers (not shown), we observe a shift from 284.8 to 285.0 eV, and for C12 monolayers (Figure 4d), the shift is from 284.7 to 284.9 eV. The Au 4f, Hg 4f, and S 2p binding energies show no detectable shift after reimmersion. We also checked the binding energies of C16 samples that were prepared by immersing previously amalgamated gold substrates in the thiol solution. As listed in Table 1, the Au 4f, C 1s, and S 2p binding energies were identical for both preparation procedures. At present the C 1s binding energy shifts cannot be explained unambiguously, since XPS binding energies depend on numerous initial state effects and final state relaxation effects.40 Core hole screening by the metal surface is expected to shift the C 1s binding energies to lower energies. In NEXAFS spectra at the C 1s edge, a Rydberg resonace R/σ at a lower excitation energy has (40) Bagus, P. Private communication.
Alkanethiolate Films on Gold
Figure 4. XPS binding energy shifts by mercury adsorption: (a) Au 4f7/2 substrate peak before (1) and after (2) mercury adsorption; (b) fitted S 2p spectrum of a C12 monolayer on gold before (1) and after (2) mercury adsorption; (c and d) C 1s peaks of C16 (c) and C12 (d) monolayers as deposited on gold (1), after mercury adsorption (2), and after reimmersion into the thiol solution (3). The binding energies are listed in Table 1.
been observed for monolayers with a low coverage.40,41 This demonstrates a change in the electronic properties (41) Dannenberger O.; Weiss, K.; Himmel, H.-J.; Ja¨ger, B.; Buck, M.; Wo¨ll, Ch. Thin Solid Films 1997, 307, 183.
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of the film as a function of density and/or lateral order. Hence, the data can be interpreted in terms of a positive shift of the C 1s core level energy with increasing packing density. If the shift of the C 1s binding energy was caused by a positive charging of the dielectric carbon layer, we would also expect a broadening of the C 1s peaks. The fwhm (full width at half-maximum) of the C 1s peaks is, however, unchanged after reimmersion in the thiol solution. XPS Intensities. After exposure to mercury vapor, there is no significant change of the C 1s intensity for any of the investigated alkanethiolates (C12, C16, and C18). The attenuation of the Au 4f substrate signal by mercury adsorption amounts to 22-25% and does not show any dependence on the chain length of the preadsorbed thiol within the accuracy of the XPS data. This is not surprising, since the amount of carbon and sulfur on the gold surface is expected to be unchanged after mercury adsorption. After reimmersion in the thiol solution, an increase of the C 1s/Au 4f intensity ratio by 5-12% is observed for all studied alkanethiolates (C12, C16 and C18) without any systematic chain length dependence. The Hg 4f/Au 4f intensity ratio remains unchanged, which rules out a change in attenuation of the Au 4f signal by mercury.42 3.2. Time-of-Flight Secondary Ion Mass Spectroscopy (ToF-SIMS). To identify the thiolate species in the monolayers after mercury adsorption, C16 monolayers were investigated by time-of-flight secondary ion mass spectroscopy (ToF-SIMS). The C16 gold thiolate molecule has a theoretical mass of 454.1997 amu. Mercury consists of the isotopes 198Hg (10%), 199Hg (17%), 200Hg (23%), 201Hg (13%), 202Hg (30%), and 204Hg (7%). The most prominent isotopes of the C16 mercury thiolate molecule are therefore expected at the masses 457.2011 and 459.2034 amu (isotopes 200Hg and 202Hg). The fragment ion C16H33S- that is formed by cleavage of the metalsulfur bond should be found at 257.2328 amu. Figure 5 shows negative-ion SIMS spectra in the mass regions 240270 amu (Figure 5a) and 440-460 amu (Figure 5b). The experimental and theoretical masses of the fragments are listed in Table 2. Mercury thiolate could be detected only in the negative-ion spectra, so the positive-ion spectra of this mass region are not shown. In Figure 6a the peaks at masses 257.2297, 256.2325, and 255.2275 amu are assigned to C16H33S-, C16H32S-, and C16H31S-, respectively. In Figure 5b the peaks at 453.2018, 454.2004, and 455.2063 amu are assigned to C16H32SAu-, to the parent ion of C16 gold thiolate, C16H33SAu-, and to C16H34SAu-, respectively. As suggested above, the peaks at 457.1974 and 459.1410 amu are assigned to parent ions of C16 mercury thiolate (isotopes 200Hg and 202Hg). The peaks at 457.8905 and 458.8785 amu can be tentatively assigned to Au2S2- and Au2S2H-, respectively. Qualitatively, the spectra give evidence that after exposure to mercury vapor the monolayers consist of two thiolate species, gold thiolate and mercury thiolate. Because of the matrix effect in ToF-SIMS experiments, it is, however, not possible to quantify the amount of gold thiolate and mercury thiolate in the monolayer. 3.3. Macroscopic Wetting Properties. To detect defects at the film-air interface, water contact angles were measured on C16 and C12 monolayers on gold before (42) The mercury adsorbed on the gold surface from the vapor phase partly diffuses into the gold film. If we assume mobility of mercury atoms in the gold film, the following scenario could happen during immersion of the sample into the thiol solution: Mercury atoms could diffuse from the bulk to the surface of the gold film to form mercury thiolate. This would result in a higher mercury concentration on the gold surface and increase the attenuation of the Au 4f signal by mercury.
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Figure 5. Negative-ion SIMS spectra of a C16 monolayer on gold after exposure to mercury vapor: (a) mass region of the C16 fragment ions C16H33S- (257 amu), C16H32S- (256 amu), and C16H31S- (255 amu); (b) mass region of the C16 gold thiolate ion (C16H33Au-, 454 amu) and of the C16 mercury thiolate ion (C16H33S200Hg-, 457 amu; C16H33S202Hg-, 459 amu). The experimental and theoretical masses are listed in Table 2.
and after exposure to mercury vapor. As described in detail in the Experimental Section, contact angles were taken both from sessile drops and as advancing angles of captive drops. For the C16 and C12 SAMs without mercury, average advancing angles θa of 112-113° were recorded, which is in good agreement with the literature.1,6,43 After 30 min of exposure to mercury vapor, the advancing angles are lowered by about 5° from 112-113° to 107-108°. Sessile drop angles decrease from 103° to 100°. The wettability of a surface is influenced by numerous factors, among them microscopic and macroscopic surface roughness. To make sure the observed decrease of contact angle is not caused by microscopic roughening, we (43) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321.
investigated the morphology of the thiolate samples before and after 30 min of exposure to mercury vapor by atomic force microscopy (AFM). 0.85 µm and 0.1 µm scans (256 pixel × 256 pixel) were recorded at several positions on the samples in the contact mode. Image analysis shows no significant changes in microscopic surface roughness rms (rms ) 12-15 Å) after mercury adsorption. After reimmersion into the thiol solution, advancing contact angles increase again from 107-108° to the initial value of 112-113° and sessile drop angles from 100° to 103°. This increase in hydrophobicity indicates that by reimmersion in the thiol solution a homogeneous, closely packed methyl surface is reformed. The structural implications of the wetting behavior will be discussed later on in reference to the IRRAS and XPS data.
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Table 2. Atomic Mass Unit Values of a C16 Monolayer on Gold after Exposure to Mercury Vapor fragment
theor mass
exp mass
C16H31SC16H32SC16H33SC16H32SAuC16H33SAuC16H34SAuC16H33S198HgC16H33S200HgAu2S2Au2S2HC16H33S202Hg-
255.2170 256.2249 257.2328 453.1918 454.1997 455.2076 455.1996 457.2011 457.8774 458.8853 459.2034
255.2275 256.2325 257.2297 453.2018 454.2004 455.2063 455.2063 457.1974 457.8905 458.8785 459.1410
a Masses of the elements: H, 1.0079 amu; 12C, 12.0000 amu; 32S, 31.97207 amu; Au, 196.9666 amu; 198Hg, 197.9668 amu; 200Hg, 199.9683 amu; 202Hg, 201.9706 amu.
3.4. Infrared Spectroscopy. Figure 6a shows the IR reflection absorption (IRRAS) data of a C16 monolayer on gold in the C-H stretching region, as assembled on the gold substrate (1), after 30 min of exposure to mercury vapor (2), and after reimmersion of the Hg-exposed sample in the thiol solution (3). Figure 6b depicts the corresponding IR data of a C12 monolayer on gold. Mode assignments for the C-H stretching vibrations of alkanethiolate monolayers on polycrystalline gold have been reported in detail in the literature1,44,45 and will not be repeated here. For the calculation of integrated intensities and line widths, the overlapping bands were resolved into the vibrational modes, using Bio-Rad software. The C-H stretching modes were assumed to be a mixture of Gaussian/Lorentzian line shapes. A leastsquares routine was applied to optimize the fit. The obtained band positions and full widths at half-maximum are listed in Table 3. The method used to determine the molecular orientation of the alkyl chains has been described in the Experimental Section. The inset in Figure 6 introduces a conventional coordinate system to define the tilting and twisting direction of the alkyl chains. Given that in IRRAS experiments only the projection of the dynamic dipoles onto the z axis contributes to absorption, the choice of the x and y axes is arbitrary and hence independent of any crystallographic orientation of the substrate. All x and y directions are equivalent (uniaxial symmetry). The tilting direction is defined to describe the orientation of the terminal methyl group with respect to the surface normal for an alkyl chain with an even number of carbon atoms (e.g. C16): For θ ) +34° and ψ ) 0°, the C-CH3 bond is oriented along the surface normal. After 30 min of exposure to mercury vapor, we observe the following effects in the C-H stretching region: (1) pronounced intensity changes of all methyl and methylene stretching vibrations46 and (2) line broadening of the methylene stretching vibration at 2919 cm-1. Note that the band positions of the asymmetric and symmetric methylene stretching vibration at 2919 and 2851 cm-1 for the C16 monolayers and at 2920 cm-1 and 2850-2851 cm-1 for the C12 monolayers are unchanged after mercury exposure. These band positions are characteristic of extended chains with a low concentration of gauche defects. (44) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558. (45) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (46) The absolute band intensities in the presented spectra are specific for our optical equipment and should not be compared to those for spectra acquired with other spectrometers. Even minor differences in the effective incident angle on the sample and in the numerical aperture of the IR beam will result in large differences in absolute spectral intensities.
Figure 6. IR reflection absorption (IRRAS) spectra of a C16 monolayer (a) and a C12 monolayer (b) on gold in the C-H stretching region with treatment of the samples as in Figure 4c and d. The band positions and line widths of the vibrational modes are listed in Table 3. The inset introduces a Cartesian substrate coordinate system1 used to define the molecular orientation of an all-trans alkyl chain in the thiolate monolayers on gold in terms of molecular tilt θ and molecular twist ψ.
We start our analysis with the methylene mode intensities and then discuss the methyl bands. After mercury vapor exposure, the methylene modes of the C16 monolayers (Figure 6a) and the C12 monolayers (Figure 6b) are
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Table 3. IRRAS Data for C16 and C12 Monolayers on Gold before and after Exposure to Mercury Vapor: Band Positions and Full Widths at Half-Maximum (fwhm) of the C-H Stretching Modese sample treatment C16 monolayer on golda (a) as deposited (b) after 30 min in Hg vapor (c) after reimmersion in the thiol solution C12 monolayer on goldb (a) as deposited (b) after 30 min in Hg vapor (c) after reimmersion in the thiol solution
CH3, asym ip
CH3, sym FR
CH2, asym
CH3, sym
CH2, sym
2964-2965c/9d 2965/9 2964-2965/9
2938/11 2938/10 2938/10
2919/14 2919/21 2919/22
2878/8 2879/8 2878/9
2851/10 2851/14 2851/12
2965c/8d 2966/9 2965/8
2938/9 2938/9 2938/9
2920/15 2920/21 2920/26
2879/7 2879/8 2879/8
2850/10 2851/12 2850/12
a The spectra are shown in Figure 6a. b The spectra are presented in Figure 6b. c Band position (cm-1). d fwhm ) full width at halfmaximum (cm-1). All band widths were obtained from deconvolution of the overlapping bands into the C-H stretching modes, as described in the text. e The following abbreviations are used: asym ) asymmetric; sym ) symmetric; ip ) in-plane; FR ) Fermi resonance component.
much weaker and their dichroic ratio I(CH2, asym)/I(CH2, sym) is reduced. On the basis of the best agreement for the methylene modes between measured and simulated spectra, we obtain an average tilt angle θ of 11° for Hgexposed C16 monolayers and θ ∼ 15° for Hg-exposed C12 monolayers, compared to θ ) 26-28° for the monolayers on gold before exposure to mercury vapor. From the intensity ratio I(CH2, asym)/I(CH2, sym), which is independent of the chain tilt angle, the average twist angle ψ of the alkyl chains was determined. It decreases from ψ ) 52° to ψ ∼ 45° both for C16 and C12 monolayers. A twist angle of 45° is equivalent to an isotropic twisting of the alkyl chains. We stress, however, that the molecular orientation calculated from the IR data is only a macroscopic average. We do not know the microscopic lateral distribution of gold and mercury atoms on the substrate surface, and from the IR band intensities it is not possible to distinguish between homogeneous untilting of the chains and a laterally inhomogeneous structure with tilted and perpendicular oriented chains. A structural model for the monolayers will be derived in the Discussion. With respect to the methyl stretching modes, the spectra exhibit the following changes: After exposure to mercury vapor, the in-plane asymmetric methyl stretch at 2965 cm-1 is by far the strongest methyl mode, whereas the symmetric methyl stretch at 2878 cm-1 and its Fermi resonance at 2938 cm-1 are reduced in intensity. This holds for both C16 and C12 monolayers. We find much better agreement with our spectra simulations for a tilt angle of -11° (C-CH3 bond inclined to the surface normal by 41° for ψ ) 45°) than for a tilt angle of +11° (C-CH3 bond inclined to the surface normal by 26° for ψ ) 45°). The pronounced intensity changes of the methyl modes compared to those of alkanethiolate monolayers on gold (average inclination of the C-CH3 bond to the surface normal for θ ) 26° and ψ ) 52°: 17°) suggest that mercury adsorption causes not only untilting of chains but also a structural rearrangement of the chains with metal-S-C binding geometries different from that on the gold surface. Upon reimmersion in the thiol solution, the average molecular tilt angle is further reduced: There is a small further decrease of the methylene mode intensities and an increase of all methyl mode intensities. This effect is much stronger for C12 monolayers (Figure 6b) than for C16 monolayers (Figure 6a). The best agreement for the methylene modes with simulated spectra yields an average tilt angle θ ) 9° for C16 and θ ) 10° for C12 monolayers. From the methyl mode intensities we derive again a negative tilting direction, as defined by the coordinate system in Figure 6. The average twist angle is unchanged after reimmersion (45°). Apart from the intensity changes, mercury adsorption induces a line broadening of the methylene stretching
vibrations (Table 3). This increase in the fwhm (full width at half-maximum) is much greater for the asymmetric mode, which has been considered to be more sensitive to structural perturbations.47 Broadening of line shapes is observed when the same vibrational transition occurs at slightly different frequencies for different molecular oscillators in an ensemble,27 either by intramolecular disordering, that is, conformational defects, or by intermolecular effects in the adsorbate. In the case of significant conformational disordering, we would expect a blue shift of the methylene modes. The band positions of the methylene stretching vibrations are, however, unchanged after mercury exposure. Further evidence for the conformational order of the monolayers comes from the SFG data, which will be presented in the following paragraph. Apart from conformational disorder, a line broadening of C-H stretching vibrations can be caused by a lateral inhomogeneity of the adsorbate (static line broadening) or by twisting-rotational mobility of the alkyl chains (dynamic line broadening),48 as known for isolated alkane molecules in alkane urea clathrates48,49 and for the rotator phase of crystalline alkanes.50,51 However, free rotation of the alkyl chains can be ruled out by their negative tilt angle. In a rotator phase there should be no preferential tilting direction of the alkyl chains. (In addition, infrared spectra as opposed to Raman spectra show no significant line broadening for rotator phases: Casal et al. reported only a slight change in line shape for the asymmetric CH2 stretching vibration of a hexadecane urea clathrate compared to the pure hexadecane crystal.49 Similarly, Busico, Ferraro, and Vacatello observed no significant changes in the C-H stretching region for the rotator phase transition of lithium hexadecanoate.51) We therefore attribute the broadening of the methylene bands to an inhomogeneous local environment of the alkyl chains due to the formation of intermolecular defects by mercury adsorption on the gold surface. 3.5. Sum Frequency Generation (SFG). Figure 7 shows sum frequency spectra of C16 monolayers on gold before (1) and after (2) exposure to mercury vapor as well as after subsequent reimmersion into the hexadecanethiol solution (3). The spectrum of the monolayer before exposure to mercury vapor represents the characteristic line shape of sum frequency spectra taken from well-ordered alkanethiolate monolayers on polycrystalline gold sur(47) Gericke, A.; Mendelsohn, R. Langmuir 1996, 12, 758. (48) Snyder, R. G.; Scherer, J. R.; Gaber, B. P. Biochim. Biophys. Acta 1980, 601, 47. (49) Casal, H. L.; Mantsch, H. H.; Cameron, D. G.; Snyder, R. G. J. Chem. Phys. 1982, 77, 2825. (50) Nielsen, J. R.; Hathaway, C. E. J. Mol. Spectrosc. 1963, 10, 366. (51) Busico, V.; Ferraro, A.; Vacatello, M. J. Chem. Phys. 1986, 84, 471.
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Figure 7. Sum frequency spectra of C16 monolayers on gold: as deposited (1); after mercury adsorption (2); and after reimmersion into the thiol solution (3). The dots represent the experimental data; the solid lines are fits to the experimental data.
faces.30,31,52 As mentioned above, the dominance of the various methyl resonances and the almost complete lack of methylene modes indicate the high degree of order within the monolayer. As will be substantiated in a forthcoming publication, the resonance at 2862 cm-1 can be assigned to the symmetric stretching vibration of the terminal methylene groups. Its constructive interference indicates an opposite orientation of its dynamic dipole moment relative to the surface normal as compared to that of the methyl resonances, which interfere destructively with the nonresonant substrate contribution. This difference in orientation is in agreement with a tilt angle of 28-30° for well-ordered alkanethiolate monolayers on gold. After exposure to mercury vapor, the spectrum is still dominated by the methyl resonances, although the nonvanishing intensity of the asymmetric methylene vibration at 2918 cm-1 indicates the presence of gauche defects. It is important to note that the density of these gauche defects within the film is not high enough to remove the anisotropy of the terminal methylene and methyl groups, so that the monolayer can still be assumed to be laterally ordered. In agreement with the IR data, the magnitude of the different methyl modes has changed, indicating a reorientation of the molecules. Starting from an average tilt angle θ ) 28° before mercury exposure, the reduced symmetric methyl stretching mode and the increased in-plane asymmetric methyl stretching mode are consistent with either an increased positive tilt angle > 30° or a negative tilt angle with a magnitude < 30°. The former possibility can be ruled out by the behavior of the terminal symmetric methylene stretching vibration, which shows a change in orientation, as indicated by the destructive interference (52) Ong, T. H.; Davies, P. B.; Bain, C. D. Langmuir 1993, 9, 1836.
with the nonresonant substrate signal. This change can be explained by a change of the tilt direction from a positive to a negative tilt angle. Further, its intensity is reduced, as it should be for an all-trans alkyl chain with a tilt angle < 30° due to the IR selection rules on metal surfaces. Altogether, the SFG spectrum corroborates a negative tilt angle close to the surface normal for the C16 monolayer after exposure to mercury vapor. These results for C16 films are in good agreement with those found for docosanethiolate (C22) monolayers on gold, which show the same direction in reorientation due to mercury sorption.53 Minor differences in the spectra may be caused by the different chain lengths thatsin the case of C22s might lead to a stronger response of the methylene modes for comparable distributions of gauche defects. After reimmersion into the hexadecanethiol solution, the asymmetric methylene stretching vibration has vanished and the monolayer exihibits the same quality of conformational order as before mercury exposure. 3.6. Near Edge X-ray Absorption Fine Structure (NEXAFS). NEXAFS spectroscopy provides information complementary to the infrared absorption spectroscopy data. In the present case NEXAFS spectroscopy has been used to probe transitions of C 1s electrons into the lowest unoccupied molecular orbitals (LUMOs) of C16. Since these antibonding states are spatially directed and related to the specific chemical bonds between the molecular constituents, NEXAFS spectroscopy is sensitive both to the nature of the chemical bonds and to the orientation of the C16 molecules. Three characteristic resonances are clearly visible in the spectra, namely a R* resonance at ≈287.7 eV sup(53) Himmelhaus, M.; Buck, M.; Grunze, M. Chem. Phys. Lett., submitted.
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Figure 8. Series of NEXAFS spectra at different angles of incidence R for C16 monolayers on gold: (a) as deposited (reference); (b) after exposure to mercury vapor; (c) after reimmersion in the thiol solution.
posedly corresponding to the transitions of the C 1s electrons into Rydberg states below the ionization edge,54 a C-C σ* resonance at 293.4 eV, and a weak C-C′ resonance at 301.6 eV. The R* peak is essentially caused by two transitions into Rσ and Rπ orbitals at respectively 287.4 and 288.1 eV,54 which could not be resolved with the energy resolution of our experment. Both orbitals are spatially extended and oriented perpendicular to the molecular axis of the alkyl chains, with the transition dipole moment (TDM) of the Rσ orbital located in the C-C-C plane and the TDM of the Rπ orbital perpendicular to this plane. The transition moments of the molecular orbitals corresponding to the C-C σ* and the C-C′ resonances are directed along the alkyl chain axis. It should, however, be noted that the R* assignment for the resonance at ≈287.7 eV is somewhat unclear at present. This assignment is based on calculations for an isolated propane (C3H8) molecule,54 and it is not known if the same assignment is correct for the significantly longer alkanethiolate molecules in a densely packed film. The assignment of the resonance at ≈287.7 eV is, however, of no importance for this particular study. Only the direction of the corresponding TDM, which is well-known, is relevant for the determination of the average tilt angle of the alkyl chains from angular-dependent NEXAFS experiments. In Figure 8 a series of NEXAFS spectra taken at different X-ray incident angles R are presented for an untreated C16 reference sample (a), a C16 sample exposed to mercury vapor (b), and a C16 sample reimmersed in the thiol solution after exposure to mercury vapor (c). The characteristic dependence of the NEXAFS spectra for wellordered alkanethiolate layers on the incidence angle is clearly seen in all three sets of spectra (Figure 8a-c). To determine the average tilt angle of the alkyl chains with respect to the surface normal, we used a procedure (54) Bagus, P. S.; Weiss, K.; Schertel, A.; Wo¨ll, Ch.; Braun, W.; Hellwig, H.; Jung, C. Chem. Phys. Lett. 1996, 248, 129.
that relies on difference spectra.55 The difference spectra calculated from the spectra in Figure 8 are displayed in Figure 9. This method requires calibration against a reference. As reference we have taken a monolayer of octadecyltrichlorosilane (OTS), having an average tilt angle of 7-8° with respect to the substrate normal.56 With this reference we obtain an average tilt angle of ∼35° for the untreated C16 sample. This value is in very good agreement with previous NEXAFS results for monolayers of alkanethiols on gold.2,57 The spectra in Figure 8b and c exhibit, however, more pronounced changes of the resonance intensities with the angle of incidence as compared with those for the untreated C16 sample. Quantitative analysis of the difference spectra yields an average tilt angle of 26° for the mercury-exposed C16 sample (Figure 9b) and of 23° for the reimmersed C16 sample (Figure 9c). Hence, from the NEXAFS data we obtain a decrease of the tilt angle by ∆θ ) 9° after exposure to mercury vapor and by ∆θ ) 12° after mercury exposure and reimmersion in the thiol solution. This change in molecular orientation is in fair agreement with the IR data. From the IR spectra we calculate a decrease of chain tilting by ∆θ ) 16° after exposure to mercury vapor and by ∆θ ) 17° for the reimmersed C16 sample, as compared with the untreated C16 sample. There is also a discrepancy between the NEXAFS and the IR data with respect to the absolute chain orientation. This issue will be adressed in detail in the next section. 4. Discussion From our combined IRRAS, SFG, NEXAFS, XPS, and wetting behavior study we will derive a schematic model (55) Kinzler, M.; Schertel, A.; Ha¨hner, G.; Wo¨ll, Ch.; Grunze, M.; Albrecht, H.; Holzhu¨ter, G.; Gerber, Th. J. Chem. Phys. 1994, 100, 7722. (56) Bierbaum, K. Ph.D. Thesis, Universita¨t Heidelberg, 1995. (57) Ha¨hner, G.; Wo¨ll, Ch.; Buck, M.; Grunze, M. Langmuir 1993, 9, 1955.
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Figure 9. Difference spectra for C16 monolayers on gold: (a) as deposited (reference); (b) after exposure to mercury vapor; (c) after reimmersion in the thiol solution.
for the structural changes in the alkanethiolate monolayers on gold induced by mercury adsorption and reimmersion in the thiol solution. Our XPS data imply saturation of the alkanethiolate samples with mercury after 20-30 min. At first sight, it is surprising that long-chain alkanethiolate monolayers on gold show a similar time dependence of mercury sorption as that for bare gold substrates with a disordered, not closely packed carbon contamination layer. It should be noted, however, that mercury vapor consists of single atoms with an atomic radius of about 1.4 Å and can penetrate the alkanethiolate film easier through defects or at domain boundaries. In addition, mercury adsorption on the gold surface creates further defects in the monolayer, which probably make the SAM more permeable for further diffusion. It is generally accepted for monolayers of long-chain alkanethiols on polycrystalline gold that the sulfur atoms form a (x3 × x3)R30° overlayer with a S-S spacing of 5.01 Å. Tilting of the alkyl chains reduces the interchain distance to ∼4.5 Å, which corresponds to the equilibrium chain distance in alkane crystals. Our IR data for C16 and C12 monolayers yield an average tilt angle of 26-28°, which is consistent with previous IR investigations.1,44 From our NEXAFS data for C16, we obtain, however, a larger average tilt angle of about 35°, which is in its turn in very good agreement with previous NEXAFS studies.2,57 The observed difference between the IR and NEXAFS results represents a well-known problem and is presumably related to the basic limitations of both methods. In the case of IR spectroscopy the determination of the chain orientation is based on an idealized optical model. For the related calculations extended alkyl chains in the alltrans conformation are usually considered and the optical functions of bulk crystals are used for the monolayers. It is, however, not clear whether the molecular packing and the conformational defect density are exactly the same in the monolayer and in the bulk state.
The determination of the absolute molecular orientation of extended alkyl chains from NEXAFS data suffers from two experimental problems: The experiments at the C 1s absorption edge are performed in the partial yield mode with a retarding voltage of -150 V, that is, collecting secondary electrons in the kinetic energy range 150-325 eV. For electrons with such a kinetic energy the mean free path is about 10 Å, compared to a C16 monolayer thickness of 19 Å on gold. This means that the NEXAFS data overemphasize the contribution of the upper chain part in which the density of gauche defects is higher than that for the alkyl chains on average. The second disadvantage of NEXAFS is related to the usually used fixed detector position in the angular-dependent measurements. As a rule, the X-ray incident angle is varied by rotating the sample, the detector position being fixed. Such a rotation of the sample changes the angle between the sample surface and the detector and hence also the effective pathway of the detected secondary electrons in the monolayer. In our opinion grazing incidence X-ray diffraction (GIXD) is the most reliable method to determine the average chain orientation. By GIXD an average tilt angle of 30.3° for C18 monolayers and of 32.5° for C12 monolayers was determined.11 This value is approximately the average of the tilt angles obtained from IR spectroscopy and NEXAFS, respectively. In the following paragraph, we will use the IR data to discuss the orientational changes by mercury adsorption, but we will give the NEXAFS data in parentheses. After 30 min of exposure to mercury vapor, the IR data show a change of the average chain tilt angle to θ ) -11° for C16 monolayers (∆|θ|IR ) 15°, ∆|θ|NEXAFS ) 9°) and to θ ) -15° for C12 monolayers. The IR results indicate the same direction of chain reorientation as the SFG data for C16 films, which also corroborate a negative tilt angle close to the surface normal. To our knowledge, alkanethiolate monolayers on liquid mercury have not been studied by
7448 Langmuir, Vol. 14, No. 26, 1998
infrared spectroscopy. As mentioned in the Introduction, X-ray reflectivity measurements showed the alkyl chains in these monolayers to be oriented perpendicular to the substrate surface (θ ) 0°).13 Therefore the reduced chain tilting of the alkanethiolate monolayers on gold after mercury adsorption suggests the formation of mercury thiolate, which can, however, not be quantified from the IRRAS data. A chain reorientation in alkanethiolate monolayers on gold by chemical modification of the substrate has also been reported by Jennings and Laibinis. They investigated alkanethiolate monolayers on underpotentially deposited silver/gold substrates, that is, gold substrates with an underpotentially deposited layer of silver on top.58 Their IR data indicate that the structure of the SAMs on underpotentially deposited silver/gold is not a composite of the SAM structures on the bulk metals. The average chain tilting calculated from the IR data is θ ∼ 20°. The average orientation of the alkyl chains and terminal methyl groups after exposure to mercury vapor suggests a change in the binding geometry of the sulfur headgroup. In the common structural model for alkanethiolate films on Au(111), the headgroups adopt a Au-S-C bond angle of ∼110°.8 Laibinis et al. have reported a negative chain tilt angle of -13°, which is similar to the macroscopic tilt angle of the mercury-exposed SAMs on Au(111), for alkanethiolate monolayers on Ag(111).1 An ab initio geometry optimization by Sellers et al.59 and a SFG study by Harris et al.60 have shown that the methyl group in methanethiolate on Ag(111) is oriented vertically. This implies an Ag-S-C bond angle of 180° if we assume an “on top” adsorption site.59 Within this model, and assuming also that the sulfur atom occupies “on top” sites on the Au/Hg alloy surface, our IR and SFG data for the alkanethiolate films would be consistent with vertically oriented metal-S-C bonds formed by mercury adsorption. However, we feel that such a simplified model based on the assumption of a specific adsorption site of the sulfur atom is not suitable to derive an adsorption geometry on the Au/Hg alloy surfaces. Not only are the chemical composition and homogeneity of the alloy surface not known, but also a multicenter bonding of the sulfur atom and a possible reconstruction of the surface (as described recently for alkanethiols on a Cu(111) surface61) would invalidate the assumptions used to derive metal-S-C bond angles. From the negative-ion SIMS spectra, we infer that after mercury adsorption the monolayers consist of two thiolate species: gold thiolate and mercury thiolate. Due to the matrix effect, it is, however, not possible to quantify the two species. From the band intensities of the C-H stretching vibrations in the IR and sum frequency spectra, only a macroscopic average of the chain orientation is obtained. The band intensities are consistent both with a laterally homogeneous structure with identical tilting of all chains and with a laterally inhomogeneous structure with differently tilted domains. However, the behavior of the methylene modes in the IR and SFG spectra favors a laterally inhomogeneous structure. In the IR spectra, we observe a significant line broadening of the asymmetric (58) Jennings, G. K.; Laibinis, P. E. (a) J. Am. Chem. Soc. 1997, 119, 5208; (b) Langmuir 1996, 12, 6173. (59) Sellers, H.; Ulman, A.; Shnidman, Y.; Eilers, J. E. J. Am. Chem. Soc. 1993, 115, 9389. (60) Harris, A. L.; Rothberg, L.; Dubois, L. H.; Levinos, N. J.; Dhar, L. Phys. Rev. Lett. 1990, 64, 2086. (61) Imanishi, A.; Isawa, K.; Matsui, F.; Tsuduki, T.; Yokoyama, T.; Kondoh, H.; Kitajima, Y.; Ohta, T. Surf. Sci. 1998, 407, 282.
Thome et al.
CH2 stretching band to a fwhm (full width at halfmaximum) of about 20 cm-1 despite unchanged band positions of the methylene stretching modes. Their positions (CH2 asym, 2919 cm-1; CH2 sym, 2851 cm-1) are characteristic of extended chains with a low percentage of gauche defects. Similarly, the SFG spectra show a certain increase in intensity of the methylene modes although the methyl modes indicate a high degree of conformational order. These effects can be explained by formation of well-ordered domains surrounded by defectrich domain boundaries, resulting in a laterally inhomogeneous structure. For bare gold films on mica exposed to mercury vapor, the in situ AFM study by Levlin et al.22 has shown the formation of a microscopically inhomogeneous amalgam structure on the initially atomically flat (111) terraces. Unfortunately, STM was not available to investigate the microscopic structure of the alkanethiolate monolayers. Evaporated gold films on silicon (100) are mainly (111) textured.8 A possible reason for the domain structure in the monolayers could be that after mercury adsorption the substrate surface exhibits various crystallographic orientations of the amalgamated gold substrate. From the domain structure the wetting behavior after mercury adsorption can be understood. The advancing angles θa ) 112-113° measured for C16 and C12 monolayers on gold before mercury exposure are consistent with a closely packed methyl surface. The lowering of θa by 5° to 107-108° after 30 min of exposure to mercury vapor can be explained by the creation of defects in the monolayer. The formation of defect-rich domain boundaries possibly also accounts for the negative shift of the C 1s core level energy in the XPS spectra from 284.9 to 284.8 eV for C16 and C18 monolayers and from 284.8 to 284.7 eV for C12 monolayers. After reimmersion in the thiol solution, there is clear evidence for additional thiol adsorption from the XPS data: The C 1s/Au 4f intensity ratio is increased by 5-12% without the Hg 4f/Au 4f intensity ratio being changed. The average chain tilt angle further decreases to θ ) 9° for C16 monolayers and θ ) 10° for C12 monolayers, as determined from the IR data. From the inhomogeneous structural model the XPS data can be explained by an increase of the lateral chain density by thiol adsorption in the holes at domain boundaries and/or in less closely packed domains with perpendicular chain orientation. That the enhanced packing density results in a decrease of gauche defects can be seen from the SFG data, which show almost lacking methylene resonances after reimmersion. Further evidence for the filling of void space in the monolayer comes from the wetting behavior. The advancing water angles reach the initial value 112-113°, which indicates that a closely packed methyl surface is reformed. The positive shift of the C 1s core level energy of the alkyl chains by 0.2 eV is probably also caused by the higher lateral chain density. 5. Conclusion We have shown that the lateral density of alkanethiolate SAMs on polycrystalline gold can be increased by exposure to mercury vapor and subsequent reimmersion into the thiol solution. XPS data indicate saturation of the thiolate samples with mercury after 20-30 min of exposure to air saturated with mercury vapor. From IRRAS, SFG, and XPS data we conclude that by mercury adsorption the SAMs rearrange to a laterally inhomogeneous structure with differently tilted domains. The average orientation
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of the alkyl chains obtained from IR and SFG data implies a change in the binding geometry of the sulfur headgroup. As a general trend our results show that the lateral density of SAMs can be influenced by chemical modifications of the substrate. The increase of chain density by mercury adsorption offers the opportunity to study the influence of molecular packing density on macroscopic properties of organic surfaces, such as wettability and lubrication. Acknowledgment. We would like to thank M. Buck (Angewandte Physikalische Chemie, Universita¨t Heidelberg), P. Bagus, and A. J. Ricco (Sandia National Labs, Albuquerque, NM) for helpful discussions, G. Ha¨hner and I. Pfund (Laboratory for Surface Science and Technology, ETH Zu¨rich, Switzerland) for carrying out the ToF-SIMS
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measurements, M. Zolk and I. Bo¨hm (Angewandte Physikalische Chemie, Universita¨t Heidelberg) for their assistance in performing the SFG experiments, D. L. Allara and A. N. Parikh (Pennsylvania State University) for providing software for the spectra simulations, G. M. Whitesides (Department of Chemistry, Harvard University) for supplying the perdeuterated hexadecanethiol, and the Freudenberg Forschungsdienste KG (Weinheim, Germany) for recording the EDX data. This work was supported by the Office of Naval Research (ONR Grant No. N00014-94-1-0492), the Deutsche Forschungsgemeinschaft (DFG), and Fonds der Chemie. J.T. acknowledges a Ph.D. scholarship from the Landesgraduiertenfo¨rderung (LGFG). LA9808317
