Fabrication of Thiol-Terminated Surfaces Using Aromatic Self

Oct 5, 2004 - 69120 Heidelberg, Germany, and School of Chemistry, St Andrews UniVersity, North Haugh,. St Andrews, KY16 9ST, United Kingdom...
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16806

J. Phys. Chem. B 2004, 108, 16806-16810

Fabrication of Thiol-Terminated Surfaces Using Aromatic Self-Assembled Monolayers Y. Tai,† A. Shaporenko,† H.-T. Rong,†,‡ M. Buck,§ W. Eck,† M. Grunze,† and M. Zharnikov*,† Angewandte Physikalische Chemie, UniVersita¨t Heidelberg, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany, and School of Chemistry, St Andrews UniVersity, North Haugh, St Andrews, KY16 9ST, United Kingdom ReceiVed: March 23, 2004; In Final Form: July 27, 2004

Self-assembled monolayers (SAMs) formed from [1,1′-biphenyl]-4,4′-dimethanethiol (BPDMT) and [1,1′;4′,1′′terphenyl]-4,4′′-dimethanethiol (TPDMT) on Au were characterized by X-ray photoelectron spectroscopy (XPS), high-resolution XPS, infrared reflection absorption spectroscopy, near-edge X-ray absorption fine structure spectroscopy, and water contact angle measurements. The results of all experimental techniques suggest the formation of densely packed and highly oriented SAMs for both BPDMT and TPDMT, with a slightly higher packing density and a smaller molecular inclination in TPDMT/Au. All molecules were found to be bound to the substrate via the thiolate link, i.e., by one of the thiol groups, whereas the second thiol group is located at the SAM-ambient interface. This suggests that aromatic dithiols are well-suited for the fabrication of thiol-terminated SAMs. Such films are of particular importance for molecular electronics, since the thiol group has a high affinity to metals and can be used as a chemical link between a metal nanowire and the molecule.

1. Introduction Self-assembled monolayers (SAMs), which are well-ordered and densely packed monomolecular organic films, enable to a large extent the control over surface properties of materials on macroscopic and microscopic length scales. SAMs are usually composed of rodlike molecules, which are chemically bound to the substrate via their headgroup while the molecular chain (spacer) points away from the substrate.1,2 Then, the functionality at the end of the spacer (end group) mainly defines the surface properties of the organic film. For many applications, including future molecular electronics and the fabrication of defect deficient metal films on organic substrates, it is highly desirable to form well-defined SAMs with a thiol termination. Due to the high affinity to metals, the thiol groups can serve as nucleation centers for the growth of metal films or metal wires at the SAM-ambient interface. This may prevent the diffusion of metal atoms into the film, which is usually accompanied by deterioration of the molecular arrangement. A highly suitable SAM constituent is a dithiol, with one thiol serving as the headgroup and another one as the tail group. However, if both thiol groups bind to the substrate and a bridgeor looplike construction is formed, the concept of a thiolterminated SAM is not realized. Even if the fraction of the respective moieties is low, the resulting SAM is no more welldefined and cannot serve as a platform for the applications. The formation of bridge- or looplike constructions is presumably connected to the flexibility of the molecular backbone, as for example in alkanethiols.1-3 On the contrary, a rigid molecular backbone, such as an oligophenyl or oligophenyleneethynylene, may render bridging unlikely, unless it occurs via formation of dimers or higher oligomers with S-S bonds.

In this study, we make use of this advantage and report on the fabrication of highly oriented thiol-terminated SAMs formed from [1,1′-biphenyl]-4,4′-dimethanethiol (HSCH2(C6H4)2CH2SH, BPDMT) and [1,1′;4′,1′′-terphenyl]-4,4′′-dimethanethiol (HSCH2(C6H4)3CH2SH, TPDMT) on gold. The methylene linkage between the thiol group and oligophenyl backbone was introduced to achieve favorable packing conditions on Au, as has been shown for SAMs of biphenyl-substituted alkanethiols CH3(C6H4)2(CH2)nSH and 4,4′-terphenyl-substituted alkanethiols C6H5(C6H4)2(CH2)nSH with odd n.4-8 Note that, whereas the fabrication of densely packed TPDMT SAMs on Au has already been reported,9 there is a controversy concerning the preparation of high-quality SAMs of dithiols with a biphenyl backbone. On one hand, the preparation of SAMs of 4,4′-dimercaptobiphenyl has been reported, even though the presented experimental data do not allow an unequivocal conclusion on the film quality.10,11 On the other hand, it was claimed that organodithiols with a short oligophenyl backbone, i.e., BPDT and BPDMT, do not form well-oriented layers on Au(111).9 Our goal is to clarify this contradiction and to provide information on the structure and packing density of BPDMT and TPDMT SAMs on gold. Note that beside the preparation of thiol-terminated films, the assembly of the BPDMT and TPDMT moieties on metal substrates is of practical interest in view of specific applications in molecular electronics.12-14 In the following section we describe the experimental procedure and techniques. The results are presented in section 3, followed by a discussion and a summary in sections 4 and 5, respectively.

* Corresponding author. Tel: +49-6221-544921. Fax: +49-6221546199. E-mail: [email protected]. † Universita ¨ t Heidelberg. ‡ Present address: SusTech GmbH & Co. KG, Petersenstr. 20, 64287 Darmstadt, Germany. § St Andrews University.

BPDMT15 and TPDMT were synthesized via 4,4′-bis(bromomethyl)biphenyl and 4,4′′-bis(bromomethyl)-p-terphenyl, respectively, which were obtained from 4,4′-dimethylbiphenyl by photobromination with NBS/AIBN16 and from p-terphenyl using a standard bromomethylation procedure.17,18 Both dibro-

2. Experimental Section

10.1021/jp0402380 CCC: $27.50 © 2004 American Chemical Society Published on Web 10/05/2004

Fabrication of Thiol-Terminated Surfaces mides were converted to the corresponding dithiols by reaction with an excess of thiourea in ethanol under reflux and subsequent alkaline hydrolysis in boiling water.19 After acidification with HCl and washing with water and acetone, the products were extensively purified by 3-fold recrystallization from toluene under nitrogen and final sublimation in vacuo. No disulfide or other impurities could be found by 1H and 13C NMR spectroscopy, EI mass spectrometry, and elemental analysis. The gold substrates were prepared by thermal evaporation of 100 nm of gold (99.99% purity) onto polished single-crystal silicon (100) wafers (Silicon Sense), which had been precoated with a 5 nm titanium adhesion layer. Such evaporated films are standard substrates for thiolate SAMs. They are polycrystalline, with a grain size of 20-50 nm, as observed by atomic force microscopy. The grains predominantly exhibit a (111) orientation, which is, in particular, supported by the observation of the corresponding forward-scattering maxima in the angular distributions of the Au 4f photoelectrons20 and by the characteristic binding energy shift of the Au 4f surface component.21 The SAMs were formed by immersion of freshly prepared substrates into a 1 mM BPDMT or TPDMT solution in tetrahydrofuran (stabilized with 0.1% hydroquinone) at room temperature for 24 h. After immersion, the samples were carefully rinsed with chloroform and blown dry with argon. No evidence for impurities or oxidative degradation products was found by XPS and infrared spectroscopy. The BPDMT and TPDMT films were characterized by X-ray photoelectron spectroscopy (XPS), high-resolution XPS (HRXPS), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, infrared reflection absorption spectroscopy (IRRAS), and contact angle measurements. All experiments were carried out at room temperature. The XPS, HRXPS, and NEXAFS measurements were performed under UHV conditions at a base pressure better than 1.5 × 10-9 Torr. The XPS and NEXAFS experiments were carried out at the HE-SGM beamline of the synchrotron storage ring BESSY II in Berlin, Germany. The HRXPS experiments were performed at the beamline D1011 at the MAX II storage ring of the MAX-lab synchrotron radiation facility in Lund, Sweden. The time for the spectroscopic measurements was selected in such a way that no noticeable damage by the primary X-rays occurred.22-25 The collection of the XPS spectra was carried out using either synchrotron light or a Mg KR X-ray source and a VG CLAM 2 analyzer. The spectra acquisition was carried out in normal emission geometry with an energy resolution varying from ≈0.6 to ≈0.9 eV, depending on the photon energy. For each sample, a wide scan spectrum and the C 1s, O 1s, S 2p, and Au 4f narrow scan spectra were measured. The HRXPS spectra were acquired in normal emission geometry at photon energies of 350 and 580 eV for the C 1s range and 350 eV for the S 2p region, respectively. A SCIENTA analyzer was used. The energy resolution was better than 100 meV, which is noticeably smaller than the fwhms of the photoemission peaks addressed in this study. Thus, these fwhms are very close to the natural widths of the respective XP lines. The energy scale of both XPS and HRXPS spectra was referenced to the Au 4f7/2 peak at a binding energy of 84.0 eV.26 The spectra were fitted by symmetric Voigt functions and a Shirley-type background. To fit the S 2p3/2,1/2 doublet, we used two peaks with the same full width at half-maximum (fwhm), the standard26 spin-orbit splitting of ≈1.18 eV (verified by fit), and a branching ratio of 2 (S 2p3/2/S 2p1/2). The fits were performed self-consistently: the same fit parameters were used for identical spectral regions.

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Figure 1. C 1s XPS spectra of BPDMT/Au and TPDMT/Au acquired at a photon energy of 600 eV. The spectra are decomposed into a main emission peak assigned to the aromatic backbone and a shoulder at high binding energy (see the text).

The acquisition of the NEXAFS spectra was carried out at the carbon K-edge in the partial electron yield mode with a retarding voltage of -150 V. Linearly polarized synchrotron light with a polarization factor of ≈82% was used. The energy resolution was ≈0.40 eV. The incidence angle of the light was varied from 90° (E-vector in surface plane) to 20° (E-vector near surface normal) in steps of 10-20° to monitor the orientational order of the organic films.27 The raw NEXAFS spectra were normalized to the incident photon flux by division through a spectrum of a clean, freshly sputtered gold sample. The energy scale was referenced to the pronounced π1* resonance of highly oriented pyrolytic graphite (HOPG) at 285.38 eV.28 Infrared absorption measurements were performed with a dryair-purged Bio-Rad FTIR spectrometer Model FTS 175C equipped with a liquid nitrogen cooled MCT detector. All spectra were taken using p-polarized light incident at a fixed angle of 80° with respect to the surface normal. The spectra were measured at a resolution of 2 cm-1 and 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. Substrates covered with a SAM of perdeuterated hexadecanethiol were used as a reference. Advancing and receding contact angles of Millipore water were measured on freshly prepared samples with a Kru¨ss goniometer Model G1. The measurements were performed under ambient conditions with the needle tip in contact with the drop. At least three measurements at different locations on each sample were made. The averaged values are reported. Deviations from the average were less than (1°. 3. Results 3.1. XPS and HRXPS. C 1s XPS spectra and S 2p HRXPS spectra of BPDMT/Au and TPDMT/Au are presented in Figures 1 and 2, respectively. The spectra were acquired at photon energies of 600 eV (C 1s) and 350 eV (S 2p). The C 1s spectra for both SAMs show a main emission peak at a binding energy of 284.3 eV assigned to the aromatic backbone and a shoulder at ≈1.2 eV higher binding energy. Similar shoulders were observed previously for different aromatic SAMs and alternatively assigned to the carbon atom bound to the sulfur headgroup or to shake-up processes.29-33 On the basis of our previous XPS

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Figure 3. S 2p HRXPS spectrum of BPDMT/Au acquired at a photon energy of 580 eV. The spectrum is decomposed into the components associated with the thiolate headgroup and the thiol tail group (dashed lines).

Figure 2. S 2p HRXPS spectra of BPDMT/Au and TPDMT/Au acquired at a photon energy of 350 eV. The spectra are decomposed into the components associated with the thiolate headgroup and the thiol tail group (dashed lines).

and HRXPS measurements on different aromatic SAMs and HOPG (the shoulder is observed for this material as well),7,21,32,33 we use the latter assignment. Note, that upon normalization to the same intensity of the main emission peak, the C 1s spectra of BPDMT/Au and TPDMT/Au practically coincide with the only difference that this peak for BPDMT/Au is slightly broader than that for TPDMT/Au (1.45 eV vs 1.4 eV). The nonnormalized spectra in Figure 1 exhibit a higher total intensity for TPDMT/Au as compared to BPDMT/Au, in accordance with the molecular compositions. Assuming an exponential attenuation of the XPS signal and using standard34,35 attenuation lengths for the Au 4f and C 1s emissions, we estimated the thickness of the BPDMT and TPDMT films as 15.5 ( 0.5 and 20.0 ( 0.5 Å. Comparing these values to the molecular lengths of BPDMT and TPDMT (16.1 and 20.4 Å, respectively), a smaller molecular inclination, i.e., a higher packing density in TPDMT/Au than in BPDMT/Au, can be assumed. The S 2p spectra of BPDMT/Au and TPDMT/Au exhibit two S 2p3/2,1/2 doublets at 162.06 eV (S 2p3/2) and 163.3 eV, which are assigned to the thiolate-type sulfur bound to the metal surface21,36 and to the thiol tailgroup,24 respectively. For both SAMs, the intensity of the thiolate component is noticeably lower than that of the thiol component, which is understandable considering the attenuation of the thiolate signal by the hydrocarbon overlayer. In accordance with the molecular compositions, the thiolate signal is smaller for TPDMT/Au as compared to BPDMT/Au, whereas the thiol signals are very similar in intensity. If corrected for the signal attenuation, the intensity ratio of thiol and thiolate components is close to 1:1 within several percents both for BPDMT/Au and TPDMT/Au. All these findings demonstrate that bridgelike constructionss if present at allsare below the detection limit of XPS, and therefore the SAMs are indeed terminated by thiol groups. Additional evidence is provided by the change of the S 2p spectra upon variation of the primary photon energy or takeoff angle of photoelectrons. The intensity ratio of the thiol to the thiolate components increases with increasing surface sensitivity, either at increasing takeoff angle (not shown) or at decreasing photon energy. This is in full accordance with the assumed structural model of BPDMT/Au and TPDMT/Au. An example of the photon energy effect is given in Figure 3, where the S 2p HRXPS spectrum of BPDMT/Au acquired at a photon energy

Figure 4. Carbon K-edge NEXAFS spectra of BPDMT/Au and TPDMT/Au acquired at different X-ray incidence angles along with the difference between the spectra collected at X-ray incidence angles of 90° and 20° (bottom curves). For direct comparison of the linear dichroism, the difference spectrum for TPDMT/Au was divided by the intensity ratio of the π1* resonances in the 55° spectra of TPDMT/Au and BPDMT/Au. The dashed lines represent the zero level of the difference spectra.

of 580 eV is presented. The relative intensity of the thiolate component in this spectrum is noticeably higher than that in the respective spectrum acquired at a photon energy of 350 eV (Figure 2, upper panel). Note that besides the above-mentioned findings, the S 2p HRXPS spectra in Figures 2 and 3 provide information on the homogeneity of the adsorption sites in BPDMT/Au and TPDMT/Au, since the deviation of the fwhm of the S 2p3/2 and S 2p1/2 peaks from the natural width of these emissions is representative for the inhomogeneity effects. Comparing the S 2p3/2,1/2 fwhms for the thiolate headgroups in BPDMT/Au (0.69 eV) and TPDMT/Au (0.63 eV) with the smallest S 2p3/2,1/2 fwhm observed for the thiolate SAMs (0.50 eV),21,33 we can conclude that the 2D lattices of the thiolate headgroups in the BPDMT and TPDMT films on Au(111) contain several different absorption sites. Interestingly, the S 2p3/2,1/2 fwhms for the thiol tail groups in BPDMT/Au (0.84 eV) and TPDMT/Au (0.77 eV) are slightly higher than the corresponding, above-mentioned values for the thiolate headgroups in these films. 3.2. NEXAFS Spectroscopy. C K-edge NEXAFS spectra of BPDMT/Au and TPDMT/Au acquired at different X-ray incident angles are presented in Figure 4, along with the difference between the spectra collected at X-ray incidence angles of 90° and 20° (bottom curves). The spectra of both films are very similar, even though the intensities of the absorption resonances are higher for the TPDMT film. The spectra are

Fabrication of Thiol-Terminated Surfaces dominated by the intense π1* resonance of the phenyl rings at ≈285 eV, which is accompanied by the respective π2* resonance at ≈288.8 eV, the R*/C-S* resonance at ≈287.8 eV, and several broad σ* resonances at higher photon energies (the assignment has been performed in accordance with refs 9, 32, 37, and 38). The spectra of both BPDMT/Au and TPDMT/Au exhibit a pronounced linear dichroism, i.e., a dependence of the absorption resonance intensity on the incidence angle of X-rays, which suggests good orientational order in these SAMs. The dichroism is additionally highlighted by the 90-20° difference spectra (bottom curves), the amplitude of the difference peaks being a fingerprint of molecular orientation. The sign of the difference peaks in Figure 4 suggests an upright orientation of the oligophenyl backbone both in BPDMT/Au and TPDMT/Au. The larger amplitude of the difference peak for TPDMT/Au as compared to BPDMT/Au implies a higher orientational order (or a smaller molecular tilt) in the former film. Apart from these qualitative conclusions, average tilt angles of the oligophenyl backbone in BPDMT/Au and TPDMT/Au can be derived by numerical evaluation of the NEXAFS data,27 which is slightly modified for the case of aromatic SAMs (see refs 39 and 40 for details). A herringbone packing of the oligophenyl backbones with a twist angle of 32° close to that for bulk aromatic compounds has been assumed.8,41-43 For the evaluation, the π1* resonance as the most intense one in the spectra has been selected; the transition dipole moment (TDM) of this resonance is oriented perpendicular to the ring plane. The derived values of the average tilt angle of the oligophenyl backbone in BPDMT/Au and TPDMT/Au are 21.5° and 19.3°, respectively; the accuracy of these values is (3-5° (this is just a general accuracy of the NEXAFS experiment and data evaluation). The derived tilt angles are only slightly larger than the respective values for SAMs of biphenyl-substituted alkanethiols CH3(C6H4)2CHnSH (n-odd) and 4,4′-terphenylsubstituted alkanethiols C6H5(C6H4)2CH2SH on Au (17° and 11.2°, respectively).4,7 Thus, the attachment of the thiol tailgroup to the oligophenyl backbone does not result in a noticeable loss of orientational order or a large increase in molecular inclination. Comparing BPDMT/Au and TPDMT/Au, a slight decrease in molecular inclination with increasing length of the oligophenyl chain becomes evident. Taking the derived average tilt angles and the length of the BPDMT and TPDMT molecules (see above), we can estimate the thickness of BPDMT/Au and TPDMT/Au at 15.1 and 19.4 Å, respectively, which correlate rather well with the XPS-derived values (15.5 and 20.0 Å, respectively). 3.3. IRRAS. IR spectra of bulk BPDMT in KBr and BPDMT/ Au are presented in Figure 5 (the respective spectra of TPDMT look very similar; see also ref 9). In the region of low wavenumbers, characteristic absorption modes of the oligophenyl chain are exhibited. Comparison of the bulk and film spectra shows that only the modes with a transition dipole moment (TDM) oriented parallel to the ring plane and the molecular axis of the biphenyl unit (at 1497 and 1005.9 cm-1) are presented in the latter spectrum. The modes with a TDM oriented perpendicularly to the ring plane and the molecular axis of the biphenyl unit, including the most intense mode at 824.4 cm-1 (see the bulk spectrum), are absent for BPDMT/ Au. Considering the selection rules for IR spectroscopy at metal surfaces, the observed difference implies an orientational order in BPDMT/Au with an approximately upright orientation of the biphenyl moieties. Note that the presented IR spectrum of BPDMT/Au differs significantly from the previously published

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Figure 5. IR spectra of bulk BPDMT in KBr (bottom curve, right vertical scale) and BPDMT/Au (upper curve, left vertical scale).

spectrum of poorly oriented BPDMT/Au,9 in which the in-plane mode at 824.4 cm-1 is clearly visible. 3.4. Contact Angle Measurements. The water contact angles are 76° (advancing) and 71° (receding) for BPDMT/Au and 76° (advancing) and 71.5° (receding) for TPDMT/Au. These angles are similar to previously reported values for biphenyldithiol, which were 67° (advancing) and 62° (receding) in one case10 and 81° (advancing) and 40° (receding) in another.11 The contact angle hysteresis is rather small for both BPDMT/Au and TPDMT/Au, which suggests that the SAM-ambient interface is rather smooth. As follows from the contact angle values given above, there is no noticeable difference in the wetting properties of BPDMT/Au and TPDMT/Au. Most likely, this is related to the comparably high quality of both SAMs. Note, that an increase in the water contact angles and a slight decrease in the contact angle hysteresis with increasing length of the oligothiophene chain have been recently reported for SAMs formed from a series of oligothiophene dithiols.11 4. Discussion The results of all experimental techniques consistently imply that BPDMT and TPDMT molecules form highly oriented and densely packed SAMs on polycrystalline Au(111) substrates. The molecules are bound to the substrate via a thiolate link by one of the thiol groups, whereas the second thiol group is located at the SAM-ambient interface. Bonding of the BPDMT and TPDMT molecules by both thiol groups simultaneously, which would be associated with flat-lying adsorbate or the formation of a bridge-like construction, can be excluded on the basis of the spectroscopic data. Also, the formation of dimers or higher oligomers with S-S bonds can be excluded, since the experimental data corroborate the preparation of monomolecular films. Thus, highly oriented thiol-terminated SAMs can be fabricated both with terphenyl and biphenyl backbones. While there is an agreement for the terphenyl thiols between the present and published work9 concerning the structural quality of the layer, our biphenyl SAMs differ from the reported ones.9 In this respect, BPDMT appears to be very similar to SAMs of 1,4-benzenedimethanethiol (BDMT), in which the structural quality ranges from densely packed to strongly oxidized, depending on the preparation conditions.11,44,45 A similar dependence has also been reported for other systems such as carboxylic acid terminated alkane thiols.46 The preparation conditions are the most obvious difference between the present and published work,9 since different solvents, i.e., tetrahydrofuran and chloroform, were used. Our observation that a noticeably

16810 J. Phys. Chem. B, Vol. 108, No. 43, 2004 poorer quality of the BPDMT and TPDMT films is obtained, when ethanol is used as solvent instead of tetrahydrofuran (see section 2), is fully consistent with this explanation. Besides this difference, additional factors, such as the substrate cleanliness and grain size, can also play a role. An impressive example of such an effect has been provided recently by the example of butanethiol films on Au(111): the structural order in the SAMs was found to be extremely sensitive to the quality of the substrate.47 Another aspect that can be of importance is the temperature during immersion. As has been shown for SAMs of biphenylthiols and ω-biphenylalkane thiols, this temperature affects long-range ordering and domain formation to a large extent.8,48,49 Comparing the BPDMT and TPDMT films, we found that the latter is characterized by a smaller molecular tilt (or by a higher orientational order) and a higher packing density. The reason for this difference is a stronger intermolecular interaction in the case of TPDMT/Au. It is well-known that this interaction plays a primary role in the balance of structural forces in thioaromatic SAMs.38 In particular, a successive decrease in molecular tilt with increasing length of the aromatic backbone was observed for SAMs of thiophenol, 1,1′-biphenyl-4-thiol, and 1,1′;4′,1′′-terphenyl-4-thiol on Au and Ag.32 Also, it has been reported recently that SAMs comprised of longer p-phenylene systems are oriented more closely to the surface normal as compared to films formed of shorter p-phenylene system.11 5. Conclusion Densely packed and highly ordered SAMs of BPDMT and TPDMT were fabricated on polycrystalline Au(111), with a slightly higher orientational order and packing density in the case of TPDMT/Au. All molecules were found to be bound to the substrate via the thiolate link provided by one of the thiol groups, whereas the second thiol group is located at the SAMambient interface. These results suggest that aromatic dithiols are well-suited for the fabrication of thiol-terminated SAMs and, probably, SAMs with other terminations. Both molecules with biphenyl and terphenyl backbone can be successfully used for this purpose. As has been shown in this study, the introduction of a suitable aliphatic linker into the aromatic molecular backbone can be of advantage to prepare high-quality SAMs. Acknowledgment. We thank G. Helmchen for support of the BPDMT synthesis, L. S. O. Johansson (Karlstad University) for the cooperation at MAX-lab, Ch. Wo¨ll (Universita¨t Bochum) for providing us with experimental equipment for the XPS and NEXAFS measurements, and the BESSY II and MAX-lab staff for assistance during the experiments at the synchrotrons. This work has been supported by the Deutsche Forschungsgemeinschaft (JA 883/4-1 and JA 883/4-2), German BMBF (GRE1HD and 05KS4VHA/4), Access to Research Infrastructure action of the Human Potential Program of the European Community, and the Fonds der Chemischen Industrie. References and Notes (1) Ulman, A. An Introduction to Ultrathin Organic Films: LangmuirBlodgett to Self-Assembly; Academic Press: New York, 1991. Ulman, A. Chem. ReV. 1996, 96, 1533. (2) Thin films: self-assembled monolayers of thiols; Ulman, A., Ed.; Academic Press: San Diego, CA, 1998. (3) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151. (4) Zharnikov, M.; Frey, S.; Rong, H.; Yang, Y. J.; Heister, K.; Buck, M.; Grunze, M. Phys. Chem. Chem. Phys. 2000, 2, 3359. (5) Rong, H. T.; Frey, S.; Yang, Y. J.; Zharnikov, M.; Buck, M.; Wu¨hn, M.; Wo¨ll, Ch.; Helmchen, G. Langmuir 2001, 17, 1582. (6) Frey, S.; Rong, H.-T.; Heister, K.; Yang, Y.-J.; Buck, M.; Zharnikov, M. Langmuir 2002, 18, 3142. (7) Shaporenko, A.; Brunnbauer, M.; Terfort, A.; Grunze, M.; Zharnikov, M. J. Phys. Chem. B 2004, 108, 14462. (8) Azzam, W.; Cyganik, P.; Witte, G.; Buck, M.; Wo¨ll, Ch. Langmuir 2003, 19, 8262.

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