4370
Langmuir 2005, 21, 4370-4375
Odd-Even Effects in Photoemission from Terphenyl-Substituted Alkanethiolate Self-Assembled Monolayers A. Shaporenko,† M. Brunnbauer,‡ A. Terfort,‡ L. S. O. Johansson,§ M. Grunze,† and M. Zharnikov*,† Angewandte Physikalische Chemie, Universita¨ t Heidelberg, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany, Anorganische und Angewandte Chemie, Universita¨ t Hamburg, 20146 Hamburg, Germany, and Department of Physics, Karlstad University, Universitetsgatan 1, S-65188 Karlstad, Sweden Received September 28, 2004 Self-assembled monolayers (SAMs) formed from 4,4′-terphenyl-substituted alkanethiols C6H5(C6H4)2(CH2)nSH (TPn, n ) 1-6) on polycrystalline (111) gold and silver substrates have been characterized by synchrotron-based high-resolution X-ray photoelectron spectroscopy. The intensities, binding energy positions, and width of most photoemission lines exhibited pronounced odd-even effects, i.e., systematic and periodic variation, depending on either odd or even number of the methylene units in the aliphatic linker of the TPn molecules. The detailed analysis of these effects provides important information on the bonding and arrangement of the chemisorbed sulfur headgroups in the TPn films and balance of the structural forces in alkanethiolate SAMs.
1. Introduction Self-assembled monolayers (SAMs) are two-dimensional (2D) polycrystalline films of semirigid molecules that are chemically anchored to a suitable substrate. These systems allow tailoring of surface properties such as wetting, adhesion, lubrication, corrosion, and biocompatibility.1-5 Practical applications of SAMs rely upon the understanding of the assembling process and knowledge of their structure and properties. In this context, three essential parts of a SAM constituent are of importance: a headgroup that binds strongly to the substrate, a tailgroup that constitutes the outer surface of the film, and a chainlike spacer that separates headgroups and tailgroups. For given SAM constituents, the properties of the SAM depend on its structure and packing density. These, in turn, result from the complex interplay of intermolecular and headgroup-substrate interactions,6 provided that the tail groups are weakly interacting.7 In the case of alkanethiolate (AT) SAMs on noble metal substrates,3,4 the headgroup-substrate interaction plays a predominant role in the balance of structural forces. This conclusion is supported by a variety of experimental data, including recent measurements on semifluorinated,6-9 biphenyl-substituted (CH3(C6H4)2(CH2)nSH, * Corresponding author urz.uni-heidelberg.de). † Universita ¨ t Heidelberg. ‡ Universita ¨ t Hamburg. § Karlstad University.
BPn),6,8,10-12 and terphenyl-substituted (C6H5(C6H4)2(CH2)nSH, TPn)13 ATs on Au and Ag substrates. The respective experiments suggested that there is a significant driving force to keep sp3 and sp bonding configurations of the sulfur headgroups on Au(111) and Ag(111), respectively, corresponding to substrate-S-C bond angles of ≈104° (Au) and ≈180° (Ag) (see also refs 14 and 15). This energetically preferable geometry of the metal-S-C bond affects the orientation and the packing density of the bi- and terphenyl moieties in the BPn and TPn SAMs on Au and Ag. Both these parameters were found to be dependent on the length of the short (n ) 1-6) aliphatic linker, so that a more upright orientation and a higher packing density were observed for an odd number of methylene units on Au and an even number on Ag,8,10,11,13 as schematically shown in Figure 1. Obviously, energy minimization is achieved by keeping the optimum hybridization of the sulfur headgroup, rather than by a denser packing of the oligophenyl moieties, which could require a significant distortion of the bonding configuration of the chemisorbed sulfur. The observed odd-even effects suggest that the BPn and TPn SAMs are highly correlated systems, which has been directly demonstrated for the BPn films (n ) 1-4) using high-resolution X-ray photoelectron spectroscopy (HRXPS).16 In this study, we apply HRXPS to characterize
(Michael.Zharnikov@
(1) Ulman, A. An Introduction to Ultrathin Organic Films: Langmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991. (2) Ulman, A. Chem. Rev. 1996, 96, 1533. (3) Thin films: self-assembled monolayers of thiols; Ulman A., Ed.; Academic Press: San Diego, CA, 1998. (4) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151. (5) Schreiber, F. J. Phys.: Condens. Matter 2004, 16, R881. (6) Zharnikov, M.; Grunze, M. J. Phys.: Condens. Matter 2001, 13, 11333. (7) Frey, S.; Shaporenko, A.; Zharnikov, M.; Harder, P.; Allara, D. L. J. Phys. Chem. B 2003, 107, 7716. (8) Zharnikov, M.; Frey, S.; Rong, H.; Yang, Y. J.; Heister, K.; Buck, M.; Grunze, M. Phys. Chem. Chem. Phys. 2000, 2, 3359.
(9) Frey, S.; Heister, K.; Zharnikov, M.; Grunze, M., Tamada, K.; Colorado, R., Jr.; Graupe, M.; Shmakova, O. E.; Lee, T. R. Isr. J. Chem. 2000, 40, 81. (10) Rong, H. T.; Frey, S.; Yang, Y. J.; Zharnikov, M.; Buck, M.; Wu¨hn, M.; Wo¨ll, Ch.; Helmchen, G. Langmuir 2001, 17, 1582. (11) Azzam, W.; Cyganik, P.; Witte, G.; Buck, M.; Wo¨ll, Ch. Langmuir 2003, 19, 8262. (12) Cyganik, P.; Buck, M.; Azzam, W.; Wo¨ll, Ch. J. Phys. Chem. B 2004, 108, 4989. (13) Shaporenko, A.; Brunnbauer, M.; Terfort, A.; Grunze, M.; Zharnikov, M. J. Phys. Chem. B 2004, 108, 14462. (14) 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. (15) Walczak, M. M.; Chung, C.; Stole, S. M.; Widrig, C. A.; Porter, M. D. J. Am. Chem. Soc. 1991, 113, 2370. (16) Heister, K.; Rong, H.-T.; Buck, M.; Zharnikov, M.; Grunze, M.; Johansson, L. S. O. J. Phys. Chem. B 2001, 105, 6888.
10.1021/la040118j CCC: $30.25 © 2005 American Chemical Society Published on Web 04/09/2005
Self-Assembled Monolayer Orientation
Figure 1. Sketch of a TPn molecule (a) and schematic drawings of the orientation and packing of the TPn molecules in the respective SAMs on Au(111) (b) and Ag(111) (c) for an odd and an even number of the methylene entities in the aliphatic linker. Note that for clarity, the large intermolecular distances for the case of unfavorable packing are exaggerated.
TPn SAMs (n ) 1-6) on polycrystalline (111) gold and silver substrates. By doing so, we extend our previous work on these films,13 providing detailed quantitative information about the parameters of the characteristic photoemission peaks and odd-even effects in photoemission from these systems. On one hand, this information represents further support for the conclusions made in our previous publications. On the other hand, it provides a deeper insight into the properties of the TPn SAMs and, in particular, the SAM-substrate interface in these systems. Note, that except for ref 13, only a few studies on TPn SAMs (n e 3), and only on Au, have been published so far.17-23 In the following section we describe our experimental procedure and techniques. The results are presented and briefly discussed in section 3. An extended analysis of the data is given in section 4, followed by a summary in section 5. 2. Experimental Section The synthesis of the TPn compounds is described elsewhere.19,23-25 The gold and silver substrates were prepared by thermal evaporation of 300 nm of gold or silver (99.99% purity) onto mica. The fabricated films were annealed at 450-500 °C for 1 h in a vacuum and flame annealed (only Au) before the SAM fabrication. The resulting metal films are polycrystalline, with a terrace size of 1000 nm, as observed by atomic force microscopy. The terraces predominantly exhibit an (111) orientation, which, in the case of Au, is corroborated by the angular distributions of the Au 4f photoelectrons26 and by the characteristic binding (17) Ishida, T.; Choi, N.; Mizutani, W.; Tokumoto, H.; Kojima, I.; Azehara, H.; Hokari, H.; Akiba, U.; Fujihira, M. Langmuir 1999, 15, 6799. (18) Ishida, T.; Mizutani, W.; Akiba, U.; Umemura, K.; Inoue, A.; Choi, N.; Fujihira, M.; Tokumoto, H. J. Phys. Chem. B 1999, 103, 1686. (19) Ishida, T.; Mizutani, W.; Choi, N.; Akiba, U.; Fujihira, M.; Tokumoto, H. J. Phys. Chem. B 2000, 104, 11680. (20) Ishida, T.; Mizutani, W.; Tokumoto, H.; Choi, N.; Akiba, U.; Fujihira, M. J. Vac. Sci. Technol., A 2000, 18, 1437. (21) Fuxen, C.; Azzam, W.; Arnold, R.; Witte, G.; Terfort, A.; Wo¨ll, Ch. Langmuir 2001, 17, 3689. (22) Arnold, R.; Azzam, W.; Terfort, A.; Wo¨ll, Ch. Langmuir 2002, 18, 3980. (23) Ishida, T.; Mizutani, W.; Aya, Y.; Ogiso, H.; Sasaki, S.; Tokumoto, H. J. Phys. Chem. B 2002, 106, 5886. (24) Himmel, H.-J.; Terfort, A.; Wo¨ll, Ch. J. Am. Chem. Soc. 1998, 120, 12069. (25) Brunnbauer, M. Ph.D. Thesis, Universita¨t Hamburg, 2003.
Langmuir, Vol. 21, No. 10, 2005 4371 energy (BE) shift of the Au 4f surface core level.27 The SAMs were prepared by immersion of the freshly prepared substrates into a 1 mM TPn solution in THF at room temperature for 24 h. After immersion, the samples were carefully rinsed with pure solvent, blown dry with argon, and kept for several days in argonfilled glass containers until the characterization at the synchrotron. No evidence for impurities or oxidative degradation products was found. The quality of the SAMs was confirmed by XPS, NEXAFS spectroscopy, ellipsometry, and contact angle measurements.13 Along with the TPn films, several reference systems, such as nonsubstituted ATs and SAMs formed from 1,1′;4′,1′′terphenyl-4-thiol (TPT, C6H5(C6H4)2SH)28 were characterized. In particular, these systems, along with the clean Au and Ag, were used to calculate the normalization constants, necessary for the film thickness evaluation. The organic monolayers were characterized by HRXPS. The experiments were performed at the bending magnet beamline D1011 at the MAX II storage ring of the MAX-lab synchrotron radiation facility in Lund, Sweden. This beamline is equipped with a Zeiss SX-700 plane-grating monochromator and a twochamber ultrahigh vacuum experimental station with a SCIENTA analyzer. Special care was taken to avoid X-ray beam damage during spectra acquisition.29-32 The spectra were collected 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. In addition, Au 4f and Ag 3d spectra were acquired and the O 1s range was monitored. The energy resolution was better than 100 meV. The BE scale of every spectrum was individually calibrated using the Au 4f7/2 emission line of ATcovered Au substrate at 83.95 eV. The latter value is given by the latest ISO standard.33 It is very close to a value of 83.93 eV, which has been obtained by us for Au 4f7/2 using a separate calibration to the Fermi edge of a clean Pt foil.27 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 standard34 spin-orbit splitting of ≈1.18 eV (verified by fit), and a branching ratio of 2 (S 2p3/2/S 2p1/2). The accuracy of the resulting BE/fwhm values is 0.01-0.02 eV.
3. Results C 1s and S 2p HRXPS spectra of TPn/Au (Figure 2) and TPn/Ag (Figure 3) suggest the formation of contaminationfree and densely packed SAMs on both substrates. The S 2p spectra exhibit a single S 2p3/2,1/2 doublet at ≈162.0 eV (S 2p3/2) commonly assigned to the thiolate-type sulfur bonded to the metal surfaces,14,27 with no evidence for disulfides, alkyl sulfides, or oxidative products. There is, however, a weak trace of atomic sulfur (≈161.0 eV for S 2p3/2)35 in some of the spectra. The C 1s spectra show a slightly asymmetric emission peak at a BE of 284.1-284.2 eV, which can be tentatively considered as a superposition of an intense symmetric peak assigned to the aromatic backbone and a small shoulder at ≈0.6 eV higher BE. Similar shoulders were observed previously for different aromatic SAMs and alternatively assigned to the carbon (26) Ko¨hn, F. Diploma Thesis, Universita¨t Heidelberg, Heidelberg, Germany, 1998. (27) Heister, K.; Zharnikov, M.; Grunze, M.; Johansson, L. S. O. J. Phys. Chem. B 2001, 105, 4058. (28) Frey, S.; Stadler, V.; Heister, K.; Eck, W.; Zharnikov, M.; Grunze, M.; Zeysing, B.; Terfort, A. Langmuir 2001, 17, 2408. (29) Wirde, M.; Gelius, U.; Dunbar, T.; Allara, D. L. Nucl. Instrum. Methods Phys. Res., Sect. B 1997, 131, 245. (30) Ja¨ger, B.; Schu¨rmann, H.; Mu¨ller, H. U.; Himmel, H.-J.; Neumann, M.; Grunze, M.; Wo¨ll, Ch. Z. Phys. Chem. 1997, 202, 263. (31) Heister, K.; Zharnikov, M.; Grunze, M.; Johansson, L. S. O.; Ulman, A. Langmuir 2001, 17, 8. (32) Zharnikov, M.; Grunze, M. J. Vac. Sci. Technol., B 2002, 20, 1793. (33) Surface chemical analysis-X-ray photoelectron spectrometersCalibration of the energy scales, ISO 15472:2001. (34) Moulder, J. F.; Stickle, W. E.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Chastian, J., Ed.; PerkinElmer Corp.: Eden Prairie, MN, 1992. (35) Yang, Y.-W.; Fan, L.-J. Langmuir 2002, 18, 1157.
4372
Langmuir, Vol. 21, No. 10, 2005
Figure 2. Normalized C 1s (left panel) and S 2p (right panel) HRXPS spectra of TPn/Au acquired at a photon energy of 350 eV (points). The fits of the spectra are drawn by solid lines. The C 1s spectra are tentatively decomposed into the main emission and the higher BE shoulder. For comparison, the data for TPT/ Au are also presented.
Figure 3. Normalized C 1s (left panel) and S 2p (right panel) HRXPS spectra of TPn/Ag acquired at a photon energy of 350 eV (points). The fits of the spectra are drawn by solid lines. The C 1s spectra are tentatively decomposed into the main emission and the higher BE shoulder. For comparison, the data for TPT/ Ag are also presented.
atom bonded to the sulfur headgroup or to a shake-up excitation in the aromatic matrix.16,28,36-38 In addition, in our case, this shoulder can be ascribed to the aliphatic linker of the TPn molecule, even though there was a strong attenuation of the respective signal at the given photon energies. The analysis of the C 1s spectra and comparison of the derived relative intensities of the main and additional peaks with theoretical estimates contradict the R-carbon and aliphatic linkage assignments, favoring, thus, shake-up excitation assignment. The intensities of the Au 4f, Ag 3d, C 1s, and S 2p emissions exhibit pronounced odd-even effects, i.e., systematic and periodic variation, depending on either odd or even number of the methylene units in the aliphatic linker of the TPn molecules. An example is given by Figures 4 and 5, in which the Au 4f, Ag 3d, and S 2p data are presented. Apart from the general decrease of the intensities with increasing attenuation of the respective signal (at increasing n), there are pronounced zigzag deviations from the general trend at a level of (5-10%. (36) Go¨lzha¨user, A.; Panov, S.; Schertel, A.; Mast, M.; Wo¨ll, Ch.; Grunze, M. Surf. Sci. 1995, 334, 235. (37) Whelan, C. M.; Barnes, C. J.; Walker, C. G. H.; Brown, N. M. D. Surf. Sci. 1999, 425, 195. (38) Whelan, C. M.; Smyth, M. R.; Barnes, C. J. Langmuir 1999, 15, 116.
Shaporenko et al.
Figure 4. The intensity of the Au 4f7/2 (top panel) and Ag 3d5/2 (bottom panel) emissions for TPn/Au and TPn/Ag, respectively.
Figure 5. The intensity of the S 2p doublet for TPn/Au (top panel) and TPn/Ag (bottom panel), respectively.
Figure 6. The effective thickness of TPn/Au (full circles) and TPn/Ag (hollow circles) derived on the basis of the C 1s/Au 4f and C 1s/Ag 3d intensity ratios. For the evaluation, the attenuation lengths reported in ref 39 were used. The error bars for the thickness values are (1 Å.
The observed zigzag changes are inverse for Au and Ag substrates, i.e., a larger attenuation is observed for an odd number of the methylene units in TPn/Au and an even number of these entities in TPn/Ag, in accordance with the schemes presented in Figure 1. In addition to the intensity data, the effective thicknesses of the TPn films were estimated. The respective values are given in Figure 6. They were derived on the basis of the C 1s/Au 4f and C 1s/Ag 3d intensity ratios for a photon energy of 580 eV,
Self-Assembled Monolayer Orientation
Langmuir, Vol. 21, No. 10, 2005 4373
Figure 7. Binding energy position of the S 2p3/2 emission (top panel) and fwhm of the S 2p3/2,1/2 components (bottom panel) for TPn/Au.
Figure 8. Binding energy position of the S 2p3/2 emission (top panel) and fwhm of the S 2p3/2,1/2 components (bottom panel) for TPn/Ag.
assuming an exponential attenuation of the photoemission signal with standard39 attenuation lengths for the respective emissions (the necessary constants were determined using several reference samples of known thickness). In the same way as for the individual emissions (see Figures 4 and 5), there are pronounced zigzag deviations from the general trend at a level of (5-7%, with the changes being inverse for Au and Ag substrates. In particular, a higher effective thickness is observed for an odd number of the methylene units in TPn/Au and an even number of these entities in TPn/Ag, once more in full accordance with the schemes presented in Figure 1. Also, the values of the effective thickness correlate quite well with the previously published XPS and ellipsometry data.13 Note that XPSderived effective thicknesses in ref 13 were calculated using the same approach as in the present study. The intensity of the S 2p signal in Figure 5 mimics those of the Au 4f and Ag 3d emissions in Figure 4. However, in contrast to the latter emissions, the S 2p signal intensity results from the interplay of two different effects. On one hand, the packing density of the sulfur headgroups varies with n due to the either favorable or unfavorable packing conditions (see Figure 1). On the other hand, the attenuation of the S 2p signal, governed by the effective film thickness, varies in the opposite way; i.e., there is a larger attenuation for a densely packed film. Obviously, the odd-even variation in the headgroups’ density is overcompensated by the changes of the effective film thickness. A similar effect was observed for the BPn SAMs.16 Apart from the intensity, parameters of the S 2p doublet are of importance, since they provide valuable information on the SAM-substrate interface. The BE position of the S 2p3/2 emission and the fwhm of the S 2p3/2,1/2 components for TPn/Au and TPn/Ag are presented in Figures 7 and 8, respectively. The BE position exhibits periodical changes with varying n, which on silver are opposite to those on gold (the effect is small but noticeable). The higher packing density corresponds to a higher BE of the S 2p3/2,1/2 doublet, whereas the lower packing density corresponds to a lower BE of this spectral feature. The most logical explanation of this effect is that the headgroup-substrate bond is somewhat strained in the case of the unfavorable packing conditions (even n for Au and odd n for Ag), due to the direct competition between the optimization of the in-
termolecular interaction (favoring a dense packing) and the lowest energy bonding configuration of the sulfur headgroups. Such a strain might affect the exact geometry of the S-metal bond, which will result in a change of the binding energy position of the respective S 2p core level, as observed experimentally. Alternatively, the S 2p BE variation can be related to the difference in exact bonding geometries in different adsorption sites on the (111) Au and Ag surfaces, which, however, seems to be less probable due to the superposition of several different sites for most of the TPn SAMs (see below). Indirect information on the adsorption site/sites is given by the S 2p fwhm. This parameter exhibits pronounced zigzag changes with varying n, which are, in contrast to all other parameters, synchronous for TPn/Au and TPn/ Ag. Since the experimental energy resolution is noticeably smaller than the natural spreading of the photoemission peaks, the fwhm is characteristic of the adsorbate system. Under these conditions, the S 2p3/2,1/2 fwhm is a fingerprint of the inhomogenity of the adsorption sites for the sulfur headgroups in TPn/Au and TPn/Ag, since the broadening of the S 2p3/2,1/2 components with respect to the value observed for a single adsorption site can only result from a superposition of several S 2p3/2,1/2 emissions with slightly different BEs (related to different adsorption sides). In all TPn films, the inhomogeneity seems to be higher on Ag (0.60 eV e fwhm e 0.67 eV) than on Au (0.50 eV e fwhm e 0.55 eV). These values have to be compared to the smallest value of the S 2p3/2,1/2 fwhm observed in thiolderived SAMs (0.50 eV),16,27 which can be considered as the width of the S 2p components for a system with a single adsorption site. This comparison implies the similar situation (i.e., a single adsorption site) for the thiolate headgroups in TPn/Au with even n (fwhm ≈ 0.50-0.51 eV). In the case of TPn/Au with odd n (fwhm ≈ 0.54-0.55 eV) and TPn/Ag (0.60 eV e fwhm e 0.67 eV), a coexistence of several different adsorption sites is suggested. In contrast to the above considered parameters, the BE position of the main C 1s emission exhibits zigzag changes, which are not completely systematic over the entire range of n. A similar situation occurred for BPn/Au.16 Since the reason for this behavior is not clear, we prefer to exclude the respective data from the analysis and hope that further work will clarify the situation. At the same time, the C 1s fwhm exhibits systematic odd-even variation over the entire range of n, as shown in Figure 9. Interestingly, the observed changes on silver are opposite to those on gold,
(39) Lamont, C. L. A.; Wilkes, J. Langmuir 1999, 15, 2037.
4374
Langmuir, Vol. 21, No. 10, 2005
Figure 9. The fwhm of the C 1s emission from TPn/Au (top panel) and TPn/Ag (bottom panel), respectively.
which is in contrast to the behavior of the S 2p3/2,1/2 fwhm. Presumably, the inhomogeneity of the adsorption sites plays a minor role further away from the SAM-substrate interface, but some other effects are of importance. Note, that the C 1s spectra acquired at a photon energy of 350 eV are predominantly originated from the upper part of the TPn films due to a small probing depth of HRXPS at a given kinetic energy of photoelectrons (≈60 eV). 4. Discussion The HRXPS spectra exhibit pronounced odd-even effects in the intensity, BE position, and fwhm of the relevant photoemission lines for TPn/Au and TPn/Ag. The observed odd-even changes are in some cases close to the experimental error, yet their systematic character over the entire range of n suggests the reliability of the observed effects. The odd-even changes in the intensity of the Au 4f, Ag 3d, C 1s, and S 2p emissions support the hypothesis that the headgroup-substrate interaction plays a predominant role in the balance of structural forces in AT SAMs. These changes are related to the variation of the packing density in the TPn SAMs with the number of methylene groups in the aliphatic linker. This effect is explained by the strong dependence of the bending potentials in the metal-S-C bond on the deviation of the metal-S-C bond angle from its lowest energy value of ≈104° (Au) and ≈180° (Ag), respectively. If the molecular orientation given by these bond angles is unfavorable for the optimal packing, there is a noticeable strain on the metal-S-C bond, resulting in its distortion. The respective change of the exact bonding geometry is reflected in the change of the binding energy position of the S 2p doublet related to the thiolate headgroup. The observed odd-even variation of the packing density and molecular inclination is closely related to the conformation of the aliphatic linker in the TPn molecules. Theoretically, there can be gauche defects in this linker, which will allow the optimization of the π-stacking at any metal-S bond angle and any length of the aliphatic chain (at least for n > 2). But exactly the opposite is observed experimentally, there is no π-stacking optimization at an unfavorable choice of the aliphatic linker, which is the case for all TPn films of this study (n e 6). We have not systematically studied TPn (and BPn) SAMs with larger n, but there are some indications that the conformational
Shaporenko et al.
mobility of the aliphatic linker destroys the odd-even variation of the packing density at a sufficiently large n. An interesting observation is the dominance of the attenuation over the packing density effect for the S 2p HRXPS signal. This emission is frequently used as a fingerprint of the packing density in SAM-like systems while the impact of the attenuation is mostly neglected. Our results suggest that the attenuation of the headgroup signal has to be always taken into account as far as one tries to link the intensity of this signal to the packing density of the SAM constituents. Important information is provided by the S 2p3/2,1/2 fwhm. The observed odd-even changes suggest a single adsorption site for all thiolate headgroups in TPn/Au with even n and a coexistence of several different adsorption sites in TPn/Au with odd n and all TPn/Ag. Comparing the latter systems, a larger inhomogeneity is exhibited in the case of TPn/Ag. This can be understood by taking into account the weaker corrugation of the bonding energy hypersurface for the chemisorbed sulfur headgroups on Ag as compared to Au,2,40 which gives more freedom for the arrangement of the headgroups on the former substrate. In particular, the difference in the bonding energy corrugation is believed to be the main reason for the different packing arrangement of AT chains in nonsubstituted AT SAMs on Au and Ag. A c(4 × 2) modulated, commensurate (x3 × x3)R30° lateral packing with a lattice constant of ≈5.0 Å is observed for Au, whereas an incommensurate (x7 × x7)R19.1° 2D arrangement with a lattice constant of ≈4.67-4.77 Å is found for Ag.2,4,14,41-45 The HRXPS values of the S 2p3/2,1/2 fwhm are 0.54 and 0.58 eV for AT/Au and AT/Ag, respectively.27 The difference from the ultimate value of 0.50 eV is presumably related to the c(4 × 2) modulation of the commensurate packing in AT/Au and to the incommensurate lattice in AT/Ag. Thus, the difference in the corrugations of the bonding energy hypersurface for the chemisorbed sulfur headgroups on (111) Au and Ag can be responsible for the synchronous character of the S 2p3/2,1/2 fwhm changes in TPn/Au and TPn/Ag with varying n. The identity and distribution of the adsorption sites in the TPn SAMs can be distinctly different on these two substrates. Our results do not provide any information on the identity of the adsorption site/sites. Note, that the headgroup hybridization cannot be exclusively associated with the symmetry of the adsorption site (e.g., an sp3 hybridization with a 3-fold hollow site). A second important contribution to the hybridization is the electronic structure of the substrate, governing a specific overlap of the headgroup’s orbitals with the frontier orbitals of the substrate. Recent studies favor a top adsorption site for methanethiol and short chain AT/Au,46,47 although previous results suggested either 3-fold hollow2,40,48 or bridge49,50 positions. At the same time, there are indications for a (40) Sellers,H.; Ulman, A.; Shnidman, Y.; Eilers, J. E. J. Am. Chem. Soc. 1993, 115, 9389. (41) Fenter, P.; Eisenberger, P.; Li, J.; Camillone, N., III; Bernasek, S.; Scoles, G.; Ramanarayanan, T. A.; Liang, K. S. Langmuir 1991, 7, 2013. (42) Camillone, N., III; Chidsey, C. E. D.; Liu, G.; Scoles, G. J. Chem. Phys. 1993, 98, 3503; J. Chem. Phys. 1993, 98, 4234. (43) Fenter, P.; Eisenberger, P.; Liang, K. S. Phys. Rev. Lett. 1993, 70, 2447; Fenter, P.; Eberhardt, A.; Eisenberger, P. Science 1994, 266, 1216. (44) Poirier, G. E.; Tarlov, M. J. Langmuir 1994, 10, 2853. (45) Rieley, H.; Kendall, G. K.; Jones, R. G.; Woodruff, P. Langmuir 1999, 15, 8856. (46) Kondoh, H.; Iwasaki, M.; Shimada, T.; Amemiya, K.; Yokoyama, T.; Ohta, T.; Shimomura, M.; Kono, S. Phys. Rev. Lett. 2003, 90, 0661021. (47) Roper, M. G.; Skegg, M. P.; Fisher, C. J.; Lee, J. J.; Dhanak, V. R.; Woodruff, D. P.; Jones, R. G. Chem. Phys. Lett. 2004, 389, 87.
Self-Assembled Monolayer Orientation
chain-length-dependent displacement of the adsorption site,47,51,52 so that the results obtained for the short-chain ATs are not necessary representative for the long chains. Presumably, there is a coexistence of several different adsorption sites in the latter systems.52,53 Also, for AT/Ag, a combination of several adsorption sites was assumed, even though the 3-fold hollow site seems to be favored.45,51,54 The behavior of the S 2p3/2,1/2 fwhm in the TPn/SAMs correlates with the behavior in the BPn films (n ) 1-4) on Au and Ag.16 This refers not only to the odd-even effects but also to the values of the fwhm. They vary from 0.50 to 0.55 eV for BPn/Au and from 0.59 to 0.75 eV for BPn/ Ag, which practically coincides with the TPn values. This agreement indicates a similarity in the arrangement of the BPn and TPn molecules in the respective SAMs on (111) Au and Ag substrates. Considering also the behavior of the S 2p3/2,1/2 BE (see above) and the similarity of the molecular inclination in the analogous (the same n and the same substrate) BPn8,10 and TPn13 SAMs, we suggest that the attachment of the additional ring to the biphenyl spacer (by going from BPn to TPn) does not affect the structure and packing density in the respective SAMs to a noticeable extent. Obviously, the stronger intermolecular interaction, which is associated with the longer terphenyl spacer, is not enough to overcome the energetic contribution related to the optimal bonding configuration of the sulfur headgroup. In context of the BPn SAMs, recent STM measurements on these systems should be mentioned.11,12 For BPn/Au (n ) 1-6), these experiments suggest only two basic structures, which are alternatively adopted with n changing between odd and even. The unit cell of odd-numbered SAMs is described by an oblique (2x3 × x3)R30° structure and contains two molecules. In contrast, the evennumbered SAMs are described by a much larger, rectangular (5x3 × 3) structure with eight molecules per unit cell and occupying an area per molecule larger by 25% compared to n ) odd.12 The (5x3 × 3) arrangement requires presumably several different adsorption sites on Au(111) surface, which contradicts the small value of the S 2p3/2,1/2 fwhm observed for n ) even in the present study. A possible explanation can be an adsorbate-mediated reconstruction of the substrate, which may easily occur.47 The Au(111) surface is susceptible to a reconstruction, which is evidenced by a (x3 × 23) surface reconstruction of the clean Au(111),55 which is assumed to be lifted upon SAM formation.56 In the case of BPn/Au and TPn/Au with even n, the reconstruction might be triggered by sterical (48) Gro¨nbeck, H.; Curioni, A.; Andreoni, W. J. Am. Chem. Soc. 2000, 122, 3839. (49) Vargas, M. C.; Giannozzi, P.; Selloni, A.; Scoles, G. J. Phys. Chem. B 2001, 105, 9509. (50) Hayashi, T.; Morikawa, Y.; Nozoye, H. J. Chem. Phys. 2001, 114, 7615. (51) Floriano, P. N.; Schlieben, O.; Doomes, E. E.; Klein, I.; Janssen, J.; Hormes, J.; Poliakoff, E. D.; McCarley, R. L. Chem. Phys. Lett. 2000, 321, 175. (52) Fisher, D.; Curioni, A.; Andreoni, W. Langmuir 2003, 19, 3567. (53) Kato, H. S.; Noh, J.; Hara, M.; Kawai, M. J. Phys. Chem. B 2002, 106, 9655. (54) Hutt, D. A.; Cooper, E.; Legget, G. J. Surf. Sci. 1998, 397, 154. (55) Wo¨ll, Ch.; Chiang, S.; Wilson, R. J.; Lippel, P. H. Phys. Rev. B 1989, 43, 7988. (56) Yeganeh, M. S.; Dougal, S. M.; Polizzotti, R. S.; Rabinowitz, P. Phys. Rev. Lett. 1995, 74, 1811.
Langmuir, Vol. 21, No. 10, 2005 4375
constrains provided by the unfavorable orientation of the aliphatic linker. 5. Summary SAMs of 4,4′-terphenyl-substituted alkanethiolates on polycrystalline (111) gold and silver substrates were characterized by synchrotron-based HRXPS. The intensities, BE positions, and fwhm’s of almost all characteristic photoemission lines exhibited pronounced odd-even effects, i.e., systematic and periodic variation depending on the odd or even number of the methylene units in the aliphatic linker of the TPn molecules. The observed odd-even intensity changes are associated with the systematic variation of the packing density in the TPn SAMs, related to either favorable or unfavorable orientation of the terphenyl moieties, governed by the metal-S-C bond angle and the length of the aliphatic linker (either odd or even n). This supports the hypothesis on the predominant role of the headgroup-substrate interaction and, in particular, the bonding configuration of the chemisorbed sulfur in the balance of the structural forces in AT SAMs. The respective bond is, however, straining and distorted to some extent, if its orientation (at a given length of the aliphatic chain) hinders the optimal packing of the SAM constituents. The odd-even variation of the packing density in the TPn SAMs is accompanied by the change in the identity and distribution of the adsorption sites for the thiolate headgroups. The results imply only one adsorption site for all thiolate headgroups in TPn/Au with even n and a coexistence of several different adsorption sites in TPn/ Au with odd n. In TPn/Ag, a coexistence of several different adsorption sites occurs at both odd and even n, even though their exact distribution changes in a systematic way going from an odd to even n. The experimental data correlate quite well with the previously published results on biphenyl-substituted alkanethiolates on polycrystalline (111) gold and silver substrates.16 Thus, the attachment of the additional ring to the biphenyl spacer (by going from BPn to TPn) does not affect the structure and packing density in the respective SAMs. The stronger intermolecular interaction, provided by the longer terphenyl spacer, is still not enough to overcome the energetic contribution related to the optimal hybridization of the sulfur headgroup. The obtained results demonstrate the potential of synchrotron-based HRXPS for the characterization of thin organic films. SAMs are obviously strongly correlated systems, so that a variety of “fine” effects in photoemission can be expected. The monitoring of these effects by the advancing spectroscopic techniques can provide valuable information, which is difficult to access with a standard laboratory approach. Acknowledgment. We thank the MAX-lab staff and, in particular, Alexei Preobrajenski and Ralf Nyholm for the assistance and Manfred Buck and Piotr Cyganic (St. Andrews University) for useful discussions. This work has been supported by the 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. LA040118J
