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Self-Assembled Monolayers of Semifluorinated Alkaneselenolates on Noble Metal Substrates A. Shaporenko,† P. Cyganik,‡,# M. Buck,‡ A. Ulman,§ and M. Zharnikov*,† Angewandte Physikalische Chemie, Universita¨ t Heidelberg, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany, School of Chemistry, St Andrews University, North Haugh, St Andrews, KY16 9ST, United Kingdom, and Department of Chemistry, Bar-Ilan University, Ramat Gan 52900, Israel Received February 28, 2005. In Final Form: May 4, 2005 Self-assembled monolayers (SAMs) formed from semifluorinated dialkyldiselenol (CF3(CF2)5(CH2)2Se-)2 (F6H2SeSeH2F6) on polycrystalline Au(111) and Ag(111) were characterized by high-resolution X-ray photoelectron spectroscopy, infrared reflection absorption spectroscopy, near edge X-ray absorption fine structure spectroscopy, scanning tunneling microscopy, and contact angle measurements. The Se-Se linkage of F6H2SeSeH2F6 was found to be cleaved upon the adsorption, followed by the formation of selenolate-metal bond. The resulting F6H2Se SAMs are well-ordered, densely packed, and contaminationfree. The packing density of these films is governed by the bulky fluorocarbon part, which exhibits the expected helical conformation. A noncommensurate hexagonal arrangement of the F6H2Se molecules with an average nearest-neighbor spacing of about 5.8 ( 0.2 Å, close to the van der Waals diameter the fluorocarbon chain, was observed on Au(111). The orientation of the fluorocarbon chains in the F6H2Se SAMs does not depend on the substratesthe average tilt angle of these moieties was estimated to be about 21-22° on both Au and Ag.
1. Introduction Control of surfaces properties on microscopic and macroscopic length scales is an important scientific and technological issue. In many cases, this goal can be achieved by the fabrication of functional monomolecular films such as self-assembled monolayers (SAMs).1-4 Being chemically anchored to the substrate, and thus blocking it off the ambient, a SAM gives the surface a new physical and chemical identity which is defined by the identity of the SAM constituents. These can be flexibly designed by a suitable combination of the individual building blocks which are a headgroup that makes the anchoring to the substrate, a tail group that is exposed to ambient, and a spacer that separates head and tail groups. While the surface properties of the entire system are mostly determined by the tail group, the identities of two other building blocks are of importance as well, affecting the quality, packing density, and stability of the film. To design a SAM for a specific application it is important to understand the effect of the individual building blocks on the SAM structure and performance. In particular, for metal and semiconductor (InP, GaAs) substrates, it would be interesting to introduce headgroups which are different from the commonly used thiol.3,4 A prospective alternative is selenium, which has similar chemical properties and the same valence electron configuration as sulfur, being its neighbor in the VIB column of the periodic table. * Corresponding author. E-mail: Michael.Zharnikov@ urz.uni-heidelberg.de. † Universita ¨ t Heidelberg. ‡ St Andrews University. § Bar-Ilan University. # Present address: Physikalische Chemie I, Ruhr-Universitaet Bochum, Universita¨tsstr. 150, 44780 Bochum, Germany. (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.
However, despite these similarities, little work has so far been done on selenol-derived SAMs; just several nonsubstituted aliphatic and aromatic films mostly on Au substrate were investigated.5-17 It has been shown that the selenolate SAMs can be fabricated from both selenol5,6,10-12,14 and diselenide6-9,12,13,15-17 precursors, with the film quality being in some cases superior to the respective thiol analogue (e.g., for the biphenyl selenolate and biphenyl thiolate SAMs).17 In the present work we study a semifluorinated selenolate-based SAM, performing an extensive characterization of the monomolecular films formed from a diselenide compound (CF3(CF2)5(CH2)2Se-)2 (F6H2SeSeH2F6) on polycrystalline Au(111) and Ag(111) substrates. This molecule combines hydrocarbon and fluorocarbon segments, which generally have different molecular volumes with van der Waals diameters of 4.2 and 5.6 Å, respectively.18 The larger volume of a fluorocarbon chain is essentially caused by its helical conformation, which is characteristic of the respective bulk materials such as (5) Samant, M. G.; Brown, C. A.; Gordon, J. G., II. Langmuir 1992, 8, 1615. (6) Dishner, M. H.; Hemminger, J. C.; Feher, F. J. Langmuir 1997, 13, 4788. (7) Bandyopadhyay, K.; Vijayamohanan, K. Langmuir 1998, 14, 625. (8) Huang, F. K.; Horton, R. C., Jr.; Myles, D. C.; Myles, D. C.; Garrell, R. L. Langmuir 1998, 14, 4802. (9) Bandyopadhyay, K.; Vijayamohanan, K.; Venkataramanan, M.; Pradeep, T. Langmuir 1999, 15, 5314. (10) Nakano, K.; Sato, T.; Tazaki, M.; Takagi, M. Langmuir 2000, 16, 2225. (11) Han, S. W.; Lee, S. J.; Kim, K. Langmuir 2001, 17, 6981. (12) Han, S. W.; Kim, K. J. Colloid Interface Sci. 2001, 240, 492. (13) Yee, C. K.; Ulman, A.; Ruiz, J. D.; Parikh, A.; White, H.; Rafailovich, M. Langmuir 2003, 19, 9450. (14) Sato, Y.; Mizutani, F. Phys. Chem. Chem. Phys. 2004, 6, 1328. (15) Monnell, J. D.; Stapleton, J. J.; Jackiw, J. J.; Dunbar, T.; Reinerth, W. A.; Dirk, S. M.; Tour, J. M.; Allara, D. L.; Weiss, P. S. J. Phys. Chem. B 2004, 108, 9834. (16) Shaporenko, A.; Ulman, A.; Terfort, A.; Zharnikov, M. J. Phys. Chem. B 2005, 109, 3898. (17) Shaporenko, A.; Cyganik, P.; Buck, M.; Terfort, A.; Zharnikov, M. J. Phys. Chem. B 2005, 109, 13630. (18) Ulman, A.; Eilers, J. E.; Tillman, N. Langmuir 1989, 5, 1147.
10.1021/la050535b CCC: $30.25 © 2005 American Chemical Society Published on Web 07/28/2005
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poly(tetrafluoroethylene), whereas a hydrocarbon chain typically has a planar zigzag conformation as, e.g., in poly(ethylene). Note that, due to intramolecular stabilization, fluorocarbon chains are expected to maintain their helical conformation in a monomolecular film as well.19,20 Until now, different semifluorinated SAMs with thiol and chlorosilane headgroups were fabricated on noble metal and silica substrates, respectively.21-34 Most of these films were found to be well-ordered and densely packed. AFM and X-ray diffraction data revealed an enlarged (relative to alkanethiolate SAMs) intermolecular spacing of ≈5.8 Å,21,22,26,27 which is consistent with the expected helical conformation of the fluorocarbon chains. Additional proof came from infrared spectroscopy, where absorption bands characteristic of the helical conformation were observed.21,26 An enlarged intermolecular spacing was found to be almost independent of the length of the fluorocarbon part. In particular, very similar structures were observed for SAMs formed on Au(111) from semifluorinated alkanethiols (AT) CF3(CF2)n(CH2)2SH with n ) 5, 7, and 11.22 Note, that ref 22 is the single publication where a fluorocarbon chain (n ) 5) of similar length to that of the F6H2SeSeH2F6 of this study was used. Most of the previous results were obtained for molecules containing a longer fluorocarbon moiety. In the following section we describe the 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 Semifluorinated dialkyldiselenol di(3,3,4,4,5,5,6,6,7,7,8,8,8tridecafluoro-1,1,2,2-dihydro-octyl)diselenide, (CF3(CF2)5(CH2)2Se-)2 was synthesized by reacting 0.85 g of NaHSe (Fluka) with 4.74 g (10 mmol) of 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluoro-1,1,2,2dihydrooctyl iodide (Aldrich) in 100 mL of absolute ethanol under nitrogen. The reaction mixture was refluxed for 1 h. After the mixture had cooled to room temperature, the solution was poured on ice-water and the solid was collected. It was filtered and dried in vacuum, dissolved in chloroform, and oxidized with iodine to provide the diselenide. The crude diselenide was chromatographed on silica gel using ligroin 950 as the eluent and was recrystallized from ligroin to provide 3.07 g (72%) of shiny white crystals. 1Η NMR (δ (ppm), CDCl3, TMS): 2.56 (m, 2H), 3.03 (m, 4H). (19) Bunn, C. W.; Howells, E. R. Nature 1954, 174, 549. (20) Rabolt, J. F.; Fanconi, B. Polymer 1977, 18, 1258. (21) Alves C. A.; Porter, M. D. Langmuir 1993, 9, 3507. (22) Liu, G.-Y.; Fenter, P.; Chidsey, C. E. D.; Ogletree, D. F.; Eisenberger, P.; Salmeron, M. J. Chem. Phys. 1994, 101, 4301. (23) Lenk, T. J.; Hallmark, V. M.; Hoffmann, C. L.; Rabolt, J. F.; Castner, D. G.; Erdelen, C.; Ringsdorf, H. Langmuir 1994, 10, 4610. (24) Scho¨nherr, H.; Ringsdorf, H. Langmuir 1996, 12, 3891. (25) Scho¨nherr, H.; Ringsdorf, H.; Jaschke, M.; Butt, H.-J.; Bamberg, E.; Allinson, H.; Evans, S. D. Langmuir 1996, 12, 3898. (26) Tsao, M.-W.; Hoffmann, C. L.; Rabolt, J. F.; Johnson, H. E.; Castner, D. G.; Erdelen, C.; Ringsdorf, H. Langmuir 1997, 13, 4317. (27) Tamada, K.; Nagasawa, J.; Nakanishi, F.; Abe, K.; Hara, M.; Knoll, W.; Ishida, T.; Fukushima, H.; Miyashita, S.; Usui, T.; Koini, T.; Lee, T. R. Thin Solid Films 1998, 329, 150. (28) 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. (29) Tsao, M.-W.; Rabolt, J. F.; Scho¨nherr, H.; Castner, D. G. Langmuir 2000, 16, 1734. (30) Fukushima, H.; Seki, S.; Nishikawa, T.; Takiguchi, H.; Tamada, K.; Abe, K.; Colorado, R.; Graupe, M.; Shmakova, O. E.; Lee, T. R. J. Phys. Chem. B 2000, 104, 7417. (31) Zharnikov, M.; Grunze, M. J. Phys. Condens. Matter. 2001, 13, 11333. (32) Tamada, K.; Ishida, T.; Knoll, W.; Fukushima, H.; Colorado, R.; Graupe, M.; Shmakova, O. E.; Lee, T. R. Langmuir 2001, 17, 1913. (33) Gamble, L. J.; Ravel, B.; Fischer, D. A.; Castner, D. G. Langmuir 2002, 18, 2183. (34) Genzler, J.; Efimenko, K.; Fischer, D. A. Langmuir 2002, 18, 9307.
Langmuir, Vol. 21, No. 18, 2005 8205 The gold and silver substrates were prepared by thermal evaporation of 200 nm of gold or 100 nm of silver (99.99% purity) onto mica or polished single-crystal silicon (100) wafers (Silicon Sense) primed with a 5 nm titanium adhesion layer. The mica substrates were preliminarily annealed at 320 °C for 24 h and kept at this temperature during the evaporation. The evaporated films were polycrystalline, with a grain size of 20-50 nm (Si) or a terrace size of 100-200 nm (mica) as observed by atomic force microscopy. Both grains and terraces predominantly exhibit a (111) orientation.35,36 The SAMs were formed by immersion of freshly prepared substrates into a 5 µmol solution of F6H2SeSeH2F6 in absolute ethanol at room temperature for 24 h. After immersion, the samples were carefully rinsed with pure solvent, blown dry with argon, and, if required, kept for several days in argon-filled glass containers until characterization. No evidence for impurities or oxidative degradation products was found. Note that a µmol concentration of the diselenide and the use of ethanol as the solvent were found to be crucial to produce films of high quality. In particular, a millimolar concentration, used frequently for thiol-derived SAMs, resulted in poor-quality films. As a direct reference to the F6H2SeSeH2F6 films, we studied SAMs formed from the analogous semifluorinated alkanethiol 1H,1H,2H,2H-perfluorooctanethiol CF3(CF2)5(CH2)2SH (F6H2S). This substance (purity 98.5%) was purchased from Apollo Scientific (U.K.) and used without further purification. A 1 mM solution in ethanol was used. The fabricated F6H2SeSeH2F6 films were characterized by X-ray photoelectron spectroscopy (XPS), synchrotron-based highresolution XPS (HRXPS), angle-resolved NEXAFS spectroscopy, Fourier transform infrared reflection absorption spectroscopy (IRRAS), scanning tunneling microscopy (STM), and contact angle measurements. All experiments were performed at room temperature. The F6H2S SAMs were characterized by XPS and HRXPS only. The XPS, HRXPS, and NEXAFS measurements were carried out under UHV conditions at a base pressure better than 1.5 × 10-9 mbar. The spectra acquisition time was selected in such a way that no noticeable damage by the primary X-rays occurred during the measurements.37-40 XPS experiments were performed with a Leybold-Hereaus LHS-12 system and a Mg KR X-ray source. A normal emission geometry was used. The energy resolution was about 0.9 eV. Since the quality of the XPS spectra was inferior to the HRXPS results, XPS data were mostly used to estimate film thickness. HRXPS measurements were carried out at the bending magnet beamline D1011 at the MAX II storage ring of the MAX-lab synchrotron radiation facility in Lund, Sweden. For these measurements, only the films on mica were used. The HRXPS spectra were collected in normal emission geometry at photon energies of 150 eV for the Se 3d range, 350 eV for the S 2p region, and 350 and 580 eV for the C 1s range. In addition, Au 4f and Ag 3d spectra were acquired and the O 1s range was monitored. The binding energy (BE) scale of every spectrum was individually calibrated using the Au 4f7/2 emission line of AT-covered Au substrate at 83.95 eV. The latter energy is given by the latest ISO standard.41 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.36 The energy resolution was better than 100 meV, which is noticeably smaller than the full width at half-maximum (fwhm) of the photoemission peaks addressed in this study. XPS and HRXPS spectra were fitted by symmetric Voigt functions and either a Shirley-type or linear background. To fit (35) Ko¨hn, F. M.S. Thesis, Universita¨t Heidelberg, Heidelberg, Germany, 1998. (36) Heister, K.; Zharnikov, M.; Grunze, M.; Johansson, L. S. O. J. Phys. Chem. B 2001, 105, 4058. (37) Wirde, M.; Gelius, U.; Dunbar, T.; Allara, D. L. Nucl. Instrum. Methods Phys. Res. B 1997, 131, 245. (38) 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. (39) Heister, K.; Zharnikov, M.; Grunze, M.; Johansson, L. S. O.; Ulman, A. Langmuir 2001, 17, 8. (40) Zharnikov, M.; Grunze, M. J. Vac. Sci. Technol. B 2002, 20, 1793. (41) Surface ChemicalAnalysissX-ray Photoelectron Spectrometerss Calibration of the Energy Scales; International Organization for Standardization: Geneva, Switzerland, 2001; ISO 15472:2001.
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Figure 1. Normalized Se 3d, C 1s, and F 1s HRXPS spectra of F6H2Se/Au and F6H2Se/Ag (open circles) acquired at photon energies of 150, 350, and 750 eV, respectively, along with the corresponding fits (solid lines) and background (dashed lines). The individual components in the C 1s spectrum are assigned to the different carbon atoms, which are successively labeled 1-8 when moving away from the headgroup. The BE positions of some emissions are marked by the dotted lines as a guide to the eye. the Se 3d5/2,3/2 and S 2p3/2,1/2 doublets we used a pair of such peaks with the same fwhm values, branching ratios of 3:2 (3d5/2/3d3/2) and 2:1 (2p3/2/2p1/2), and spin-orbit splittings (verified by fit) of ≈0.86 eV (3d5/2/ 3d3/2) and ≈1.18 eV (2p3/2/2p1/2).42 The fits were carried out self-consistently: the same peak parameters were used for identical spectral regions. The accuracy of the resulting BE and fwhm values is 0.02-0.03 eV. NEXAFS measurements were performed at the HE-SGM beamline of the synchrotron storage ring BESSY II in Berlin, Germany. Spectra acquisition 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 as the primary X-ray source. The energy resolution was ≈0.40 eV. To determine the molecular orientation in SAMs, 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°.43 Raw NEXAFS spectra were normalized to the incident photon flux by division through a spectrum of a clean, freshly sputtered gold sample. For SAMs on Ag substrates, a spectrum of clean silver was subtracted from the raw spectrum before the normalization.28,44 The photon energy (PE) scale was referenced to the pronounced π1* resonance of highly oriented pyrolytic graphite at 285.38 eV.45 IRRAS measurements were performed with a dry-air-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 perdeuterated hexadecanethiolate SAM were used as a reference. STM measurements were carried out in air, using a PicoSPM microscope (Molecular Imaging). For these experiments, only the films on mica were used. In addition, the Au substrates were flame annealed in a butane-oxygen flame prior to SAM preparation. Tips were prepared mechanically by cutting a 0.25 mm Pt/Ir alloy (8:2, Goodfellow) wire. Data were collected in constant-current mode using tunneling currents of 400 and 100 pA and sample biases of -1.2 and 0.5 V for SAMs on Au(111) and Ag(111), respectively. No tip-induced changes were observed. (42) 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. (43) Sto¨hr, J. NEXAFS Spectroscopy; Springer Series in Surface Science 25; Springer-Verlag: Berlin, 1992. (44) Zharnikov, M.; Frey, S.; Heister, K.; Grunze, M. Langmuir 2000, 16, 2697. (45) Batson, P. E. Phys. Rev. B 1993, 48, 2608.
Table 1. Binding Energy Positions (eV) and Full Width at Half-Maximum Values (eV; in Brackets) of the Photoemission Peaks for F6H2Se/Au and F6H2Se/Aga C 1s (8)
(4-7)
(3)
(2)
(1)
Se 3d
F 1s
F6H2Se/Au 293.14 290.90 289.99 284.96 284.15 54.19 688.22 (0.63) (0.79) (0.75) (0.75) (0.75) (0.73) (1.65) F6H2Se/Ag 292.81 290.51 289.71 284.65 284.07 54.24 687.90 (0.78) (0.93) (0.85) (0.85) (0.85) (0.52) (1.72) a
See Figure 1.
Advancing contact angles of Millipore water were measured on freshly prepared samples with a Kru¨ss goniometer model G1. The experiments 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°. Note that with the only exception of STM, all other techniques of this study probed a macroscopic area of the sample, i.e., averaging over the individual molecules and domains occurred.
3. Results 3.1. HRXPS. Normalized Se 3d, C 1s, and F 1s HRXPS spectra of SAMs formed from F6H2SeSeH2F6 on Au(111) and Ag(111) are presented in Figure 1, along with the corresponding fits. The results of fitting and a quantitative analysis of these spectra are shown in Table 1. The Se 3d spectra of both films exhibit a single Se 3d5/2,3/2 doublet, accompanied by a weak Au 5p3/2 emission in the case of Au. The BE position of this doublet (54.15 eV), identical for F6H2Se/Au and F6H2Se/Ag, is distinctly different from that for the bulk diselenides (55.3 eV),13 suggesting the cleavage of the covalent Se-Se bond (through an oxidative addition mechanism) and formation of a selenolate-metal bond upon adsorption of F6H2SeSeH2F6 on Au and Ag. On the basis of this finding, the abbreviation F6H2Se for films formed from F6H2SeSeH2F6 will be used further on. Formation of the selenolate-metal bond requires charge transfer from the substrate, which affects mostly the atoms of the topmost surface layer. This process can be directly monitored in the case of Au since the component related to this layer (83.65 eV) is clearly distinguished and well separated from the component ascribed to the bulk (83.95
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Figure 2. Normalized Au 4f7/2 HRXPS spectra of clean Au and F6H2Se/Au acquired at a photon energy of 350 eV. The decomposition of the former spectrum into the bulk and surface components is shown. Table 2. Full Width at Half-Maximum (eV) of the Au 4f7/2 Surface/Bulk Components for Clean Au and of the Joint Au 4f7/2 Emission for F6H2Se/Au and F6H2S/Aub system\photon energy
350 eV
580 eV
clean Au F6H2Se/Au F6H2S/Au
0.405 0.418 0.440
0.424 0.440 0.456
b See Figure 2. The data for two different photon energies are presented; the higher fwhm value for the higher photon energy is related to an increasing energy spreading of the primary X-ray beam.
eV) in the Au 4f7/2 HRXPS spectrum of the clean Au substrate (Figure 2). Note that the assignment is supported by the decrease in intensity of the surface component with increasing kinetic energy of the photoelectrons16 and by the good agreement of the observed surface core level shift of -0.31 eV with literature values.36,46-48 Upon adsorption of F6H2SeSeH2F6, the surface Au 4f7/2 component shifts to higher binding energy and, thus, merges with the bulk component, as seen in the respective spectrum of F6H2Se/Au in Figure 2. A fingerprint of the adsorbate-induced shift of the surface component is the fwhm of the joint Au 4f7/2 emission, given for F6H2Se/Au in Table 2, where the respective data for F6H2S/Au are also presented for comparison. At both primary photon energies used, the fwhm is smaller for F6H2Se/Au than for F6H2Se/Au (by 0.02 eV), which suggests a larger adsorbate-induced shift of the surface component for selenolate as compared to thiolate on Au. The respective shifts were estimated at +0.29 and +0.27 eV, respectively. Apart from the monitoring of the selenolate-metal bond formation, information on the respective bonding configuration could be obtained. For this purpose, the fwhm of the Se 3d5/2 and Se 3d3/2 components was considered (see Table 1). Since the instrumental spreading was negligible in our experiment, the value of fwhm is characteristic of the inhomogeneity of the bonding configurations (e.g., a distribution of the adsorption sites) for the selenolate headgroups. The smallest observed Se 3d5/2,3/2 fwhm for selenol-derived SAMs is 0.51 eV,16 which can be considered as a tentative reference for a single adsorption site. A (46) Culbertson, R. J.; Feldman, L. C.; Silverman, P. J.; Boehm, H. Phys. Rev. Lett. 1981, 47, 675. (47) Citrin, P. H.; Wertheim, G. K.; Baer, Y. Phys. Rev. B 1983, 27, 3160. (48) Hsieh, T. C.; Shapiro, A. P.; Chiang, T.-C. Phys. Rev. B 1985, 31, 2541.
Figure 3. C 1s HRXPS spectra of F6H2Se/Au (thick solid lines) acquired at photon energies of 150, 350, and 750 eV, along with the corresponding fits (thin solid lines). The individual components are assigned to the different carbon atoms, which are successively numbered from 1 to 8 when moving away from the headgroup. The spectra are normalized to the intensity of the “CF3” peak (peak 8).
relatively large Se 3d5/2,3/2 fwhm for F6H2Se/Au (0.73 eV) suggests a coexistence of several different adsorption sites for this system. In contrast, the Se 3d5/2,3/2 fwhm for F6H2Se/Ag (0.52 eV) is very close to the ultimate value of 0.51 eV16 so that a single adsorption site for the selenolate headgroups can be tentatively assumed for this film. The C 1s HRXPS spectra of F6H2Se/Au and F6H2Se/ Ag in Figure 1 exhibit five distinguished emissions of different intensity, which can be clearly assigned to different carbon atoms along the F6H2 chain.23,28,32 Moving from the headgroup, we successively numbered the carbon atoms by 1-8 and put the numbers at the respective components in Figure 1 (the BE positions and fwhm of these components are given in Table 1). According to this assignment, peak 8, peaks 4-7 and 3, and peaks 1 and 2 correspond to the CF3 tailgroup, CF2 chain part, and CH2 chain part, respectively, which is supported by the intensity increase of peaks 1, 2, and 3 (to a lesser extent) with increasing kinetic energy of the photoelectrons as shown in Figure 3. The lower BE of peak 3 as compared to that of peaks 4-7 is related to the effect of the adjacent hydrocarbon chain while the higher BE of peak 2 as compared to that of peak 1 is related to the effect of the adjacent fluorocarbon chain. The BEs of the carbon atoms 4-7 seem to be very close to each othersthe respective component represents a well-separated peak, which is only slightly broader as compared to other individual emissions. In contrast to the C 1s spectra, F 1s HRXPS spectra of F6H2Se/Au and F6H2Se/Ag in Figure 1 exhibit only one emission peak related to the fluorocarbon chain. No individual components could be distinguished within this peak in accordance with previous data.28,32 The HRXPS data for the F6H2Se SAMs can be compared with those for the F6H2S films. Normalized S 2p, C 1s, and F 1s HRXPS spectra of F6H2S/Au and F6H2S/Ag are presented in Figure 4, along with the corresponding fits. Whereas the C 1s and F 1s spectra are quite similar to those of the F6H2Se SAMs, the S 2p spectra contain not
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Figure 4. Normalized S 2p, C 1s, and F 1s HRXPS spectra of F6H2S/Au and F6H2S/Ag (open circles) acquired at photon energies of 150, 350, and 750 eV, respectively, along with the corresponding fits (solid lines) and a background (dashed lines). The individual components in the C 1s spectrum are assigned to the different carbon atoms, which are successively numbered from 1 to 8 when moving away from the headgroup. The BE positions of some emissions are marked by the dotted lines as a guide to the eye.
only the doublet related to the thiolate headgroup (≈162.0 eV for S 2p3/2)36 but an additional component at ≈161.0 eV (S 2p3/2) alternatively assigned to physisorbed thiol (without the cleavage of the S-C bond)49,50 and atomic sulfur.51 Also, a close examination of the C 1s spectra exhibits an enlarged intensity of the component 1-2 for F6H2S/Au (C-C or C-H contamination) and a strange shape of this emission for F6H2S/Ag. In the latter film, there is also a small C-F contamination, as exhibited by a low intense peak at 287 eV. Thus, the quality of the F6H2S films seems to be somewhat inferior to that of the F6H2Se SAMs, even though the contamination and additional thiol-derived species are minor and can only be detected by HRXPS. Apart from these defects, the thicknesses of F6H2Se and F6H2S films are quite similar. With the use of different evaluation procedures, involving both the absolute and relative intensities of the individual emissions,52 and the assumption of a standard exponential attenuation of the photoemission signal,53 the thickness was determined to be around 9.1-9.6 Å for all SAMs of the present study. No noticeable difference in thickness between the films on Au and Ag or between F6H2Se and F6H2S was observed. 3.2. IRRAS. IRRAS spectra of F6H2Se/Au and F6H2Se/ Ag in the range of the C-F stretching modes are depicted in Figure 5. While the C-H stretching region does not yield a noticeable signal, in accordance with previous results for thiol-derived semifluorinated SAMs with the same length of the hydrocarbon chain,28,30 the C-F stretching region exhibits pronounced stretching and bending modes of the CF2 groups (see refs 23 and 28 for the exact assignments). Absorption bands at ≈1363 and ≈1321 cm-1 are identified as “axial CF2” stretching vibrations with a strong component of the dynamic dipole moment along the helical axis.23 These characteristic modes for the helical conformation of the fluorocarbon chain23 are commonly observed in thin organic films containing these entities.21,23,26-28 In our case, these modes (49) Ishida, T.; Hara, M.; Kojima, I.; Tsuneda, S.; Nishida, N.; Sasabe, H.; Knoll., W. Langmuir 1998, 14, 2092. (50) Himmelhaus, M.; Gauss, I.; Buck, M.; Eisert, F.; Wo¨ll, Ch.; Grunze, M. J. Electron. Spectrosc. Relat. Phenom. 1998, 92, 139. (51) Yang, Y.-W.; Fan, L.-J. Langmuir 2002, 18, 1157. (52) Thome, J.; Himmelhaus, M.; Zharnikov, M.; Grunze, M. Langmuir 1998, 14, 7435. (53) Lamont, C. L. A.; Wilkes, J. Langmuir 1999, 15, 2037.
Figure 5. IRRAS spectra of F6H2Se/Au and F6H2Se/Ag. The characteristic absorption bands are indicated. The positions of these bands for F6H2Se/Au are marked by the dotted lines as a guide to the eye.
are clearly present both for F6H2Se/Au and F6H2Se/Ag, which implies that the fluorocarbon part of the F6H2Se molecules in the densely packed layers on the Au and Ag substrates has the expected helical conformation. The intense absorption bands in the 1150-1250 cm-1 region are also characteristic of fluorocarbon entities. The pronounced bands at 1246, 1214, and 1146 cm-1 have, in contrast to the axial CF2 stretches at 1375 and 1346 cm-1, a significant contribution from the asymmetric CF2 stretching vibration with a dynamic dipole moment perpendicular to the helical axis.23 Hence, the relative strength of the axial and asymmetric absorption bands parallel and perpendicular to the helical axis is a fingerprint of the average tilt angle of the fluorocarbon chains. Comparing the relative strengths for F6H2Se/Au and F6H2Se/Ag, we found quite similar intensity ratios, with a slightly lower value for the former film, which suggests a slightly larger molecular inclination of the fluorocarbon moieties. Note that apart from the common features discussed above, the details of the spectral shapes of the IRRAS spectra of F6H2Se/Au and F6H2Se/Ag are somewhat different. Due to the complexity of the spectra
Self-Assembled Monolayers on Noble Metals
Figure 6. Carbon K-edge NEXAFS spectra of F6H2Se/Au acquired at X-ray incidence angles of 90°, 55°, and 20°, along with the difference between the 20° and 90° spectra. The dashed line corresponds to zero. The characteristic absorption resonances are indicated.
a detailed structural interpretation based on these differences is, however, difficult. 3.3. NEXAFS. In NEXAFS experiments, core level electrons (e.g., C 1s for a C K-edge spectrum) are excited into nonoccupied molecular orbitals, which are characteristic for specific bonds, functional groups, or an entire molecule. The photon energy positions of the respective absorption resonances provide then a clear signature of these entities. In addition, the molecular orientation can be derived from NEXAFS data since the cross-section of the resonant photoexcitation process depends on the orientation of the electric field vector of the linearly polarized synchrotron light with respect to the molecular orbital of interest (so-called linear dichroism in X-ray absorption).43 Carbon K-edge NEXAFS spectra of F6H2Se/Au and F6H2Se/Ag acquired at X-ray incidence angles of 90°, 55°, and 20° are presented in Figures 6 and 7, respectively, along with the difference between the 20° and 90° spectra. The spectra of F6H2Se/Au and F6H2Se/Ag look very similar with respect to the occurrence of excitations, while intensity differences are mostly related to the different normalization procedures. The spectra contain two absorption edges at ≈287.8 and ≈294.0 eV related to the C1s f continuum excitations for carbon atoms bonded to hydrogen and fluorine, respectively. The spectra are dominated by the pronounced resonances at ≈292.5, ≈295.5, and ≈299 eV related to the transitions from the C1s state to the C-F σ*, C-C σ*, and C-F’ σ* orbitals of the fluorocarbon part, respectively.23,28,34,54,55 The corresponding transition dipole moments (TDM) are believed to be oriented almost perpendicular (C1s f C-F σ*) or along the chain axis (C1s f C-C σ*), respectively.23,54-56 (54) Ohta, T.; Seki, K.; Yokoyama, T.; Morisada, I.; Edamatsu, K. Phys. Scr. 1990, 41, 150. (55) Castner, D. G.; Lewis, K. B.; Daniel, A. F.; Ratner, B. D.; Gland, J. L. Langmuir 1993, 9, 537. (56) Ha¨hner, G.; Kinzler, M.; Wo¨ll, Ch.; Grunze, M.; Scheller, M. K.; Cederbaum, L. S. Phys. Rev. Lett. 1991, 67, 851; 1992, 69, p 694 (erratum).
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Figure 7. Carbon K-edge NEXAFS spectra of F6H2Se/Ag acquired at X-ray incidence angles of 90°, 55°, and 20°, along with the difference between the 20° and 90° spectra. The difference spectrum is normalized to the π1* intensity ratio of the 55° spectra of F6H2Se/Ag and F6H2Se/Au, for direct comparison to the respective difference spectrum of F6H2Se/ Au. The dashed line corresponds to zero.
As to the hydrocarbon part, only a weak feature at ≈288 eV, alternatively assigned to the C1s excitations into predominantly Rydberg states,57,58 valence C-H orbitals,43 or mixed valence/Rydberg states,59 is discernible in the spectra (the orbitals related to this resonance are oriented perpendicular to the alkyl chains axis).19,56,61 The characteristic C-C σ* and C-C′ σ* resonances of the hydrocarbon part at ≈293 and ≈302 eV, respectively,56,19,61 overlap with the strong features related to the fluorocarbon part and are, therefore, indistinguishable. The positions of the resonances related to the fluorocarbon part and the entire spectral shape in Figures 6 and 7 are very similar to the calculated NEXAFS spectra of poly(tetrafluoroethylene) (PTFE) assuming the standard 13/6 or 15/7 helical conformations of the fluorocarbon chains (it is difficult to distinguish between these two conformations on the basis of NEXAFS data).33 The respective calculations, performed with self-consistent spherical muffin-tin potentials and a full multiplescattering formalism, show a strong dependence of the widths, positions, and intensities of peaks in the NEXAFS spectra on the fluorocarbon chain conformation. Thus, we have further strong evidence that the fluorocarbon chains in the F6H2Se SAMs retain the 13/6 or 15/7 helical conformation of the respective bulk materials. Note that 15/7 helix has been reported to be the stable PTFE structure at room temperature (>19 °C), while the 13/6 helix is claimed to be the stable form at lower temperatures.62,63 Note also that, generally, the helical conforma(57) Bagus, P. S.; Weiss, K.; Schertel, A.; Wo¨ll, Ch.; Braun, W.; Hellwig, C.; Jung, C. Chem. Phys. Lett. 1996, 248, 129. (58) Weiss, K.; Bagus, P. S.; Wo¨ll, Ch. J. Chem. Phys. 1999, 111, 6834. (59) Va¨terlein, P.; Fink, R.; Umbach, E.; Wurth, W. J. Chem. Phys. 1998, 108, 3313. (60) Ha¨hner, G.; Kinzler, M.; Thu¨mmler, C.; Wo¨ll, Ch.; Grunze, M. J. Vac. Technol. A 1992, 10, 2758. (61) Outka, D. A.; Sto¨hr, J.; Rabe, J. P.; Swalen, J. D. J. Chem. Phys. 1988, 88, 4076. (62) Farmer, B. L.; Eby, R. K. Polymer 1981, 22, 1487.
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Figure 8. The angular dependence of the C-F σ* resonance intensity ratio I(θ)/I(90°) for F6H2Se/Au (filled triangles) and F6H2Se/Ag (open triangles), along with the best theoretical fits according to ref 43 (solid and dotted lines, respectively) and the resulting values of the tilt angle.
tion of fluorocarbon chains occurs via rotation about the C-C bonds, which arises from dipolar repulsion between the 1,3-diaxial C-F bonds.64 The respective twist angle is about 12-15° per C-C bond, and the dihedral angle between adjacent planes of three neighboring carbon atoms is 160-165°.19,65,66 The absorption resonances related to the fluorocarbon part of the F6H2Se molecules exhibit a pronounced linear dichroism, i.e., an intensity dependence on the angle of the X-ray incidence. This behavior implies a high orientational order in the F6H2Se films. The resonances with a TDM oriented along the fluorocarbon chain (e.g., C-F σ*) show an intensity increase with increasing angle of the incident X-ray beam, whereas those with a TDM oriented perpendicular to the fluorocarbon chain axis (e.g., C-C σ*) exhibit an opposite behavior. This suggests a predominantly perpendicular orientation of the fluorocarbon moieties in the F6H2Se SAMs. The exact values of the average tilt angle can be derived by a numerical evaluation of the NEXAFS data, analyzing angular dependence of the resonance intensities.28,43,55 For this evaluation, the C-F σ* resonance at 292 eV was chosen because of its high intensity and its separation from the other spectral features. To avoid normalization problems, not the absolute intensities but the intensity ratios I(θ)/ I(90°) were analyzed,43 where I(θ) and I(90°) are the intensities of the C-F σ* resonance at X-ray incidence angles of θ and 90°, respectively. The angular dependences of this intensity ratio for F6H2Se/Au and F6H2Se/Ag are presented in Figure 8, along with the best theoretical fits according to ref 43. These fits give similar average tilt angles of the fluorocarbon moieties in the F6H2Se SAMs on gold and silver, namely 31.4° for F6H2Se/Au and 29.7° for F6H2Se/Ag. It should be noted, however, that these values were obtained assuming that the molecular plane of the CF2 entities is exactly perpendicular to the fluorocarbon chain axis.23 In reality, these planes are slightly tilted toward the helix axis with a tilt angle depending on the helix parameters. Assuming a standard 15/7 helix with a twist of about 13-15° per C-C bond for the fluorocarbon parts in the F6H2Se SAMs,21,65-67 we derive a tilt angle of 8-9° between the normal of the CF2 (63) Farmer, B. L.; Eby, R. K. Polymer 1981, 26, 1944. (64) Dixon, D. A.; Van-Catledge, F. A. Int. J. Supercomput. Appl. 1988, 2, 62. (65) Clark, E. S.; Muus, L. T. Z. Kristallogr. 1962, 117, 119. (66) Piseri, L.; Powell, B. M.; Dolling, G. J. Chem. Phys. 1973, 58, 158.
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planes and the chain axis. Exactly this angle is derived from NEXAFS spectra of vertically standing fluorocarbon chains if one assumes that the CF2 molecular plane is perpendicular to the fluorocarbon chain axis. The real tilt angle of these moieties (0°) is then obtained by a subtraction of the tilt angle of the CF2 planes from the above value. The situation becomes more complex for a system of almost vertically standing helical chains (as in the present case), but the simple angle subtraction procedure can be used as a rough approximation, which results in tilt angles of 22.4° and 20.7° for the fluorocarbon chains in F6H2Se/Au and F6H2Se/Ag, respectively. The accuracy of these values is estimated to be (5°. It is related to the uncertainty of the exact geometry of the helical fluorocarbon chain and the simple angle subtraction procedure used to correct for the tilt of the CF2 planes with respect to the chain axis. The obtained values can be compared with literature data for thiol-derived semifluorinated SAMs. Whereas no such data for F6H2S films are available, the average tilt angle for SAM with a longer fluorocarbon moiety was reported as ≈20° for F8H2SH/Au,21 ≈12° for F12H2SH/ Au,22 and ≈12.5° for F10H2SH/Au28swe have specially selected SAMs with the (CH2)2 linker, since the length of the hydrocarbon part affects the inclination of the fluorocarbon moiety.28,30 In a similar way, the inclination of this moiety can also depend on its own length, which may explain the observed difference between the average tilt angle of the fluorocarbon chain in F8H2SH/Au as compared to those of F10H2SH/Au and F12H2SH/Au. Note that our value for F6H2Se/Au (22.4°) is very close to that reported for F8H2S/Au (20°).21 The single observable resonance of the short hydrocarbon linker in the F6H2Se films at ≈288 eV (we will name it R* resonance) also exhibits linear dichroism, which is, however, quite weak. Because of this weakness, a quantitative evaluation of the respective angular dependence could not be performed, but only a general statement could be made. Considering that the TDM of the R* resonance is oriented perpendicular to the hydrocarbon chain,19,56,61 we can conclude a predominantly perpendicular orientation of the (CH2)2 moiety in the F6H2Se SAMs, in accordance with general expectations. 3.4 STM. STM data for F6H2Se/Au are summarized in Figure 9 which shows images taken at different spatial resolution along with a line profile and a drawing of the unit cell. Larger scale images (Figure 9a) reveal that the formation of the F6H2Se monolayer is not paralleled by the appearance of substrate islands as was observed in the previous STM studies of alkaneselenolate (decaneselenolate and dodecaneselenolate)15 and aromatic selenolate (benzeneselenolate6 and biphenylselenolate17) SAMs on Au(111). Since both the F6H2Se and alkaneselenolate molecules are linked to the substrate by the same -CH2Se moiety, and thus, the headgroup-substrate interactions should be quite similar, the observed difference in the substrate morphology between the respective systems is presumably related to the effect of bulky fluorocarbon spacer, i.e., to changes in the intermolecular interactions caused by the partial fluorination of the alkyl chain. Another interesting feature is a slight but clearly visible modification of the step edges upon the F6H2Se adsorption. Whereas the bare Au(111) surface showed triangular facets formed by steps running along 〈110〉 directions, the step edges became sawtooth-like after the adsorption of F6H2Se (see Figure 9a). This effect results from a small(67) Wunderlich, B. Macromolecular Physics; Academic Press: New York and London, 1973.
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Figure 9. STM images of F6H2Se on Au(111) taken at different spatial resolution (a-d). The inset in (d) shows the respective FFT spectrum. In (e), the height profile along line D in image (d) is shown. In (f), a schematic drawing of the film structure with a nearest-neighbor distance of 5.8 Å is presented. This arrangement corresponds to the commensurate c(7×7) superstructure. The average nearest-neighbor distance of 5.8 ( 0.2 Å and the angle R ) 59 ( 2° were calculated from the cross-sectional height profiles taken along the directions marked by the lines A, B, and C in (d) for a set of five different images.
scale reorientation of step edges from the original 〈110〉 to 〈211〉 directions as marked in Figure 9b. A similar, but even more extensive, change in the step orientation was reported in our recent study of biphenyl selenolate SAMs on Au(111) surface.17 However, it is not solely the effect of the selenolate headgroup, since such a reorientation of the substrate step edges has been observed neither for alkanene selenolate15 nor for benzene selenolate6 monolayers on Au(111). A proper balance of the headgroupsubstrate and intermolecular interactions seems to be crucial. This view is corroborated by studies on biphenyl based thiols where, depending on the details of molecular structure and film preparation, such a change in step direction was also observed.68 The high-resolution STM images presented in Figure 9, parts c and d, show that F6H2Se forms well-ordered structures on Au(111) with domains of about 3-10 nm in diameter. Besides the well-known, 2.4 Å deep, depressions due to monatomic steps in the substrate, additional ones, 1-1.5 Å deep, were also observed (see cross-section D in Figure 9e), which is significantly larger than contrast (68) Cyganik, P.; Buck, M.; Wo¨ll, C., J. Phys. Chem. B, in press.
variations seen for molecules within a unit cell.69 The origin of the latter depressions is not clear at present, but based on very similar features observed for benzeneselenolate17 and alkaneselenolate15 monolayers on Au(111) we tentatively assign these depressions to missing substrate atoms. A convolution of electronic and geometric effects might cause the deviation from the geometric height of a step in the Au surface. Details of the molecular packing of the F6H2Se film on Au are visible in the high-resolution STM image in Figure 9d. The F6H2Se molecules form a hexagonal structure rotated by 30° with respect to the Au(111) substrate (see the FFT spectrum as the inset of Figure 9d) with an average nearest-neighbor spacing of about 5.8 ( 0.2 Å. This is noticeably larger than distances of 4.9 and 5.2 Å in slightly distorted (by 3%) hexagonal (x3×x3)R30° assembly of alkaneselenolates on Au(111) as concluded from X-ray diffraction5 and STM15 experiments. The increase in the nearest-neighbor distance in structure formed by semifluorinated alkaneselenolates as compared (69) Cyganik, P.; Buck, M.; Azzam, W.; Wo¨ll, C. J. Phys. Chem. B 2004, 108, 4989.
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Shaporenko et al. Table 3. Advancing (θadv) and Receding (θrec) Water Contact Angles for F6H2Se/Au and F6H2S/Ag Au Ag
θadv
θrec
118° 116°
108° 106°
was observed for semifluorinated SAMs (F8H2) on silica.34 Note that all these values are similar to that for poly(tetrafluorethylene), which is 116° (θadv).70 The difference between the advancing and receding contact angles, i.e., the contact angle hysteresis, is only 10° for both F6H2Se/Au and F6H2Se/Ag, which suggests a quite smooth film-ambient interface. The hysteresis is close to the previously reported values of 8-11° for semifluorinated alkanethiolate SAMs.23,30 4. Discussion
Figure 10. STM images of F6H2Se on Ag(111) taken at different spatial resolution (a-b). In (c), the height profile along the line C in the image (b) is shown.
to nonfluorinated ones is justified by the larger van der Waals diameter of the perfluorocarbon moiety (5.6 Å) as compared to that of the hydrocarbon chain (4.2 Å).18 A similar molecular packing was also observed in semifluorinated alkanethiol SAMs on Au(111).21,22,32 In particular, the formation of a hexagonal structure rotated by 30° with respect to the Au(111) substrate and showing the nearestneighbor distance of 5.7 ( 0.2 Å was reported for the analogous thiolate-based system F6H2S/Au.22 Within the experimental error, this structure is identical to that reported here for F6H2Se on Au(111). It corresponds either to an incommensurate packing or a superstructure close to a “commensurate” c(7×7) arrangement, schematically shown in Figure 9f. Note, however, that we did not directly observe the latter superstructure in our STM images, but rather saw a homogeneous hexagonal packing. One possible explanation is that differences in the tunneling conditions for different adsorption sites of the F6H2S molecules on Au(111) are small in comparison with the signal variation related to the domain boundary network so that the different adsorption sites within the c(7×7) superstructure cannot be resolved by STM. Whereas high-quality molecular resolution images were acquired for F6H2Se/Au, we have not succeeded in collecting such images for F6H2Se/Ag. Some low-resolution STM images for the latter system are presented in Figure 10. It can be seen that the surface created after the adsorption of F6H2Se is quite rough (see the cross-section in Figure 10c), which hindered molecularly resolved imaging. 3.5. Contact Angle Measurements. The values of the advancing (θadv) and receding (θrec) water contact angles for F6H2Se/Au and F6H2Se/Ag are given in Table 3. These values are very close for both substrates and characteristic of well-ordered semifluorinated SAMs. In particular, θadv and θrec for F8H2S/Au were found to be 114.5° and 106.5°, respectively;23 θadv and θrec for F10H2S/Au were reported to be 116° and 105°, respectively;30 and θadv of 114-116°
The experimental data show that F6H2SeSeH2F6 forms contamination-free, well-ordered, and densely packed F6H2Se SAMs on Au(111) and Ag(111) substrates. On the basis of the HRXPS data, the quality of these SAMs was found to be slightly superior to the respective thiolderived films (F6H2S), where traces of additional fluorocarbon and thiol-derived species were observed. Driven by the formation of the selenolate-metal bond, the Se-Se linkage of F6H2SeSeH2F6 gets cleaved upon adsorption on both Au and Ag, analogous to other diselenide compounds.8,13,16,17 This was shown by the analysis of the HRXPS data, using both headgroup and substrate emissionssthe headgroup emission in F6H2Se/ Au and F6H2Se/Ag exhibited the characteristic BE of the metal selenolate while the Au 4f7/2 component related to the topmost layer of the substrate shifted by +0.29 eV upon adsorption of F6H2Se on Au(111) due to the chargetransfer associated with the selenolate-gold bond formation. This shift is somewhat larger than the one for the thiol analogue (F6H2S) which indicates that the seleniumgold bond is stronger than the sulfur-gold one. This is in full agreement with most of the previously published data, including STM results by Dishner et al.,6 the substitution experiments by Huang et al.,8 electrochemistry data by Sato and Mizutani,14 and our recent HRXPS data for alkylselenides.16 In particular, the adsorption of diphenyl diselenide (DBDSe) on Au(111) was found to be more favorable than that of benzenethiolate (BT) by 0.7 kcal/ mol8 which is very small compared to the sulfur-gold interaction energy in AT/Au (44 kcal/mol),71 but, nevertheless, sufficient to drive the substitution of BT by DBDSe.8 In accordance with the dense molecular packing, the fluorocarbon moieties in the F6H2Se SAMs have an upright orientation, as evidenced by the NEXAFS data. They have the expected helical conformation as inferred from the IRRAS, NEXAFS, and STM data, building a 13/6 or 15/7 helix typical for the respective bulk materials. The average tilt angle of these moieties was determined to be 22.4° (Au) and 20.7° (Ag), which is rather close to the respective values in semifluorinated alkanethiolate SAMs (see section 3.3). The conformation of the hydrocarbon part could not be derived, which is not surprising considering that it consists of only two methylene units. Similar to the case of semifluorinated alkanethiolates, the packing density of the F6H2Se films was found to be governed by the bulky fluorocarbon part. For F6H2Se/ Au, a hexagonal arrangement of the F6H2Se molecules (70) Brandup, J., Immergut, E. H., Eds.; Polymer Handbook, 3rd ed.; John Wiley & Sons: New York, 1989. (71) Dubois, L. H.; Nuzzo, R. G. Ann. Phys. Chem. 1992, 43, 437.
Self-Assembled Monolayers on Noble Metals
with an average nearest-neighbor spacing of about 5.8 ( 0.2 Å was observed by STM. The latter value is very close to the van der Waals diameter of the fluorocarbon chain having a helical conformation (5.6 Å).18 The difference between 5.8 and 5.6 Å is presumably related to the abovediscussed tilt of the fluorocarbon moieties in the F6H2Se films. The observed molecular arrangement corresponds to several different adsorption sites of the selenolate headgroups on Au(111) substrate, which correlates with the HRXPS data for F6H2Se/Au showing quite broad Se 3d5/2,3/2 emissions (see section 3.1). We assume that this broadening stems from superposition of the photoemission signals for different adsorption sites which can be associated with different bonding geometry and, consequently, with different binding energy of the photoelectrons. In contrast to F6H2Se/Au, we have not succeeded in molecular resolution STM imaging of F6H2Se/Ag. However, considering all other experimental data, we believe that the poor quality of the STM images in this particular case does not reflect an inferior film quality but is rather related to difficulties in STM imaging of SAMs on this specific substrate. Examination of the literature shows that Ag, in general, seems to be a less favorable substrate for STM than Au. Only very few studies on aliphatic thiolderived SAMs presenting high-quality, molecularly resolved STM images on Ag(111) exist.72,73 Except for the STM data, the parameters of the F6H2Se films were found to be quite similar on both Au and Ag, which seems to be a general property of selenolate-derived SAMs, considering that similar behavior was previously observed for nonsubstituted aliphatic and aromatic selenolates.16,17 Note, however, that thiol-derived semifluorinated SAMs with a short hydrocarbon part also do not show much difference on Au and Ag.28 In contrast, for semifluorinated thiols with a long hydrocarbon part, there is a substrate dependent difference in the orientation of the hydrocarbon chains, whereas the fluorocarbon parts are inclined in a similar way independent of the substrate (but dependent on the length of the hydrocarbon part).28,30,74 (72) Heinz, R.; Rabe, J. P. Langmuir 1995, 11, 506. (73) Dhirani, A.; Hines, M. A.; Fisher, A. J.; Ismail, O.; GuyotSionnest, P. Langmuir 1995, 11, 2609. (74) Zharnikov, M.; Frey, S.; Rong, H.; Yang, Y. J.; Heister, K.; Buck, M.; Grunze, M. Phys. Chem. Chem. Phys. 2000, 2, 3359.
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5. Conclusions Using F6H2SeSeH2F6 as a model substance, we have shown that high-quality semifluorinated alkaneselenolate SAMs can be fabricated on Au(111) and Ag(111) substrates from a diselenide precursor. Driven by the formation of the selenolate-metal anchor, the Se-Se bond is cleaved upon adsorption. The resulting F6H2Se SAMs are wellordered, densely packed, and contamination-free. The packing density of the F6H2Se SAMs is governed by the bulky fluorocarbon part, which retains the 13/6 or 15/7 helical conformation of the respective bulk materials. In accordance with the van der Waals diameter of this moiety, a noncommensurate hexagonal arrangement of the F6H2Se molecules with an average nearest-neighbor spacing of about 5.8 ( 0.2 Å was observed on Au(111) by STM. On Ag(111), no molecularly resolved images were obtained, which is explained by the difficulty in the STM imaging of SAMs on this particular substrate. Apart from the STM data, the structural parameters of the F6H2Se films were found to be quite similar on Au and Ag. In particular, the values of the average tilt angle of the fluorocarbon chain in F6H2Se/Au (22.4°) and F6H2Se/Ag (20.7°) are very similar. The HRXPS indicates that the selenium-gold bond in semifluorinated alkaneselenolates is stronger than the sulfur-gold one in the analogous thiol-derived SAMs, which is in accordance with most of the previously published data on other thiolate- and selenolate-based systems. For the selenium-silver bond no conclusion on the relative bond strength could be derived. Acknowledgment. We thank M. Grunze for the support of this work, Ch. Wo¨ll (Universita¨t Bochum) for providing us with the experimental equipment for the NEXAFS measurements, L. S. O. Johansson (Karlstad University) for the cooperation at MAX-lab, and the BESSY II and MAX-lab staff for their assistance during the synchrotron-based experiments. This work has been supported by the German BMBF (05KS4VHA/4), European Community (Access to Research Infrastructure action of the Improving Human Potential Program), the Fonds der Chemischen Industrie, the Leverhulme Trust, and the Scottish Higher Education Funding Council (SHEFC). LA050535B
