Adsorption of Alkanethiols and Biphenylthiols on Au and Ag

Langmuir 2015 31 (10), 3232-3241 ... Structural Investigation of 1,1′-Biphenyl-4-thiol Self-Assembled Monolayers on .... The Journal of Physical Che...
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J. Phys. Chem. B 2001, 105, 4058-4061

Adsorption of Alkanethiols and Biphenylthiols on Au and Ag Substrates: A High-Resolution X-ray Photoelectron Spectroscopy Study K. Heister, M. Zharnikov,* and M. Grunze Angewandte Physikalische Chemie, UniVersita¨ t Heidelberg, Im Neuenheimer Feld 253, D-69120 Heidelberg, Germany

L. S. O. Johansson Department of Physics, Karlstad UniVersity, S-65188 Karlstad, Sweden ReceiVed: December 31, 2000; In Final Form: February 28, 2001

Synchrotron-based high-resolution X-ray photoelectron spectroscopy was applied to monitor the formation of self-assembled monolayers (SAM) of alkanethiols (AT) and biphenylthiols on Au and Ag substrates. Pronounced chemical shifts in the adsorbate- and substrate-related photoemission lines upon SAM formation were observed. Only one sulfur species could be detected in the S 2p spectra of the investigated SAMs, consistent with a thiolate bond. From the fwhm’s of the core level photoemission spectra conclusions on the heterogeneity of the adsorption sites and adsorption geometry can be made. The experimental data imply several (at least two) slightly different adsorption geometries for the AT moieties in AT/Au. Significant final state effects in the C 1s photoemission were found for both the aliphatic and aromatic SAMs.

Self-assembled monomolecular films provide a means to control wetting, adhesion, lubrication, and corrosion on surfaces and interfaces.1,2 Such films are formed by 2D aggregation of long-chain semiflexible organic molecules on a suitable substrate. Generally, these molecules consist of a headgroup that binds strongly to the substrate, a tail group that constitutes the outer surface of the film, and a spacer that connects head and tail. The lateral density and structure of self-assembled monolayers (SAMs) results from a delicate balance between the headgroup-substrate, adsorbate-adsorbate, and intramolecular interactions. Among the balance of forces, the headgroupsubstrate interaction plays often a decisive role, as in the case of the SAMs of alkanethiols (AT) on the (111) surfaces of Au and Ag.1-4 Whereas a c(4 × 2) modulated (x3×x3)R30° lateral packing of AT chains with a lattice constant of ≈5.0 Å and a tilt angle of 27-35° is observed for Au, a (x7×x7)R10.9° 2D arrangement with a lattice constant of ≈4.67-4.77 Å and a chain tilt angle of 10-12° was found in the case of Ag.3,5-8 Less studied (but important for practical applications)9,10 SAMs of biphenylthiols (BPT) exhibit tilt angles of 23° and 18° for the BPT moieties on Au and Ag, respectively.11 For these moieties a herringbone lateral arrangement was proposed.12-14 For BPT/Au molecular mechanics calculations12 and recent X-ray diffraction data15 implied a commensurate (x3×x3)R30° surface lattice. In contrast, neither periodic pattern nor molecularly resolved STM images were observed for this system,16 which could be probably related to the difficulties in the preparation of high-quality BPT SAMs on Au.15,17 The substrate-related differences in packing density and SAM structure are linked with a stronger corrugation of the adsorbatesubstrate surface potential for the chemisorbed sulfur headgroups on Au as compared to Ag (the lattice constants of the both * Corresponding urz.uni-heidelberg.de.

author.

E-mail:

Michael.Zharnikov@

surfaces are very similar). It was also suggested that the sulfur headgroup in the AT SAMs is differently hybridized on the two substrates, i.e., sp3 on Au(111) and sp on Ag(111), resulting in substrate-S-C bond angles of ∼104° and ∼180°, respectively.1,4,18 However, there still are uncertainties about the chemical state and the geometrical structure at the SAMsubstrate interface. In particular, both sulfur dimers with two sulfur atoms in nonequivalent adsorption sites and a commensurate (x3×x3)R30° lattice of the sulfur atoms in identical 3-fold hollow sites were suggested for AT/Au, whereas an incommensurate (x7×x7)R10.9° lattice is generally assumed for sulfur in AT/Ag.1,5,8,19,20 The chemical state of the sulfur headgroup on both Au and Ag is not explicitly known and even cleavage of the S-H bond upon the adsorption was questioned recently.21 In comparison to AT SAMs, even less is known about the sulfur headgroups in BPT SAMs on Au and Ag. Considerable theoretical and experimental efforts were made to clarify these issues and, in particular, to identify the adsorbate-substrate bond. Both ab initio and semiempirical cluster calculations for short-chain AT were performed,18,21,22 and structural and spectroscopic experimental methods were applied.5,8,19,20,23,24 Among these methods, X-ray absorption and photoelectron spectroscopy play an important role because they provide local chemical and structural information. Since the excitation of the S 1s core level requires a high photon energy of at least 2473 eV, the XPS characterization of the sulfur headgroups is usually performed on the base of the S 2p emission (BE of 162-163 eV) with a laboratory X-ray source (1254 eV for Mg KR and 1487 eV for Al KR). This approach suffers, however, from the small photoionization cross-section at the respective excitation energies25,26 and the poor energy resolution. In this letter we study the formation of AT and BPT SAMs on Au and Ag substrates (with an emphasis on the S-substrate interface) using an alternative approach, synchrotron-based high-

10.1021/jp010127q CCC: $20.00 © 2001 American Chemical Society Published on Web 04/20/2001

Letters resolution X-ray photoelectron spectroscopy (HRXPS). Variable excitation energy and high-energy resolution of this technique enabled a precise characterization of both the SAM/metal interface and the interior of the SAMs. In particular, we were interested in differences between the XPS peaks ascribed to the Au and Ag core levels of the bulk metal or the clean metal surface and the metal atoms bonded to AT or BPT because no such differences have been measured so far.24 The substrates were prepared by evaporation of 100-300 nm of gold or silver on mica (annealed after the evaporation) or titanium-primed (5 nm) polished single-crystal Si(100) wafers. These films predominantly exhibit a (111) orientation as, e.g., concluded from the characteristic forward-scattering maxima in the angular distributions of the Au 4f and Ag 3d photoelectrons.27 Although the films on Si and mica did not exhibit any differences in their XPS spectra (in contrast to the results of ref 28), we mainly used mica supports having larger (on a scale of 100 µm) atomically flat (111) terraces. The SAMs were formed by immersion of the substrates in an ethanolic 1mM solution of dodecanthiol [C12: CH3-(CH2)11-SH] or 4-methyl4′-mercapto-biphenyl [BPT: CH3-(C6H4)2-SH] for 24 h. C12 was obtained from Fluka Chemicals; BPT was synthesized as described elsewhere.29 After immersion, the samples were carefully rinsed and cleaned with ethanol and blown dry with pure nitrogen. The HRXPS measurements were performed at the synchrotron storage ring MAX II at MAX-Lab in Lund, Sweden, using the D1011 and I311 beamlines. Both beamlines are equipped with a Zeiss SX-700 plane-grating monochromator and a twochamber UHV experimental station with a SCIENTA analyzer. Excitation energies in the range of 140-580 eV were used. For the probed S 2p, C 1s, Au 4f, and Ag 3d core levels a tremendous increase of the photoionization cross-sections occurred at these excitation energies as compared to the respective values for the laboratory X-ray sources. The energy resolution was better than 0.1 eV, which is noticeably smaller than the fwhm’s of the photoemission peaks of this study. Thus, the spectra presented here reflect the intrinsic energy spread of the core-level photoemission process. Energy calibration was performed individually for every spectrum to avoid effects related to the instability of the monochromators. The energy scale is referenced to the pronounced Au 4f7/2 “bulk” peak (83.93 eV) of a reference C12/Au sample, which was attached to the same sample holder as the probed sample. The value of 83.93 eV was derived from independent calibrations to the Fermi edge of a clean Pt foil. Special care was taken to avoid X-ray irradiation-induced damage of the AT and BPT films during the spectra acquisition.30 The spectra were fitted using Doniach-Sunjic peak profiles and a parabolic background. To fit the S 2p3/2,1/2 doublet we used two such peaks with the same fwhm, the standard spin-orbit splitting of ∼1.2 eV (verified by fit), and the branching ratio of 2:1 (S2p3/2/S2p1/2). The resulting accuracy of the binding energies (BE) and fwhm’s reported here is 0.01-0.02 eV. The Au 4f and Ag 3d5/2 spectra of the clean and AT/BPTcovered gold and silver substrates are presented in Figures 1 and 2, respectively. The Au 4f7/2,5/2 emission of the clean gold substrate exhibits two components (83.93 and 83.62 eV with a fwhm of 0.42 eV for 4f7/2), which can be assigned to the gold atoms in the bulk and topmost surface layer, respectively. This assignment is supported by the intensity increase of the surface component with increasing photoelectron take-off angle and by the good agreement of the observed surface core level shift of -0.31 eV with literature values.31-33 Upon formation of both

J. Phys. Chem. B, Vol. 105, No. 19, 2001 4059

Figure 1. Au 4f HRXPS spectra of clean and C12/BPT-covered gold substrate. For the spectrum of C12/Au the effect of the photon energy variation is demonstrated. The fwhm’s of the Au 4f7/2,5/2 peaks are indicated.

Figure 2. Ag 3d5/2 HRXPS spectra of clean and C12/BPT-covered Ag substrate. The effect of the take-off angle variation is shown. The fwhm’s of the Ag 3d5/2 peaks are indicated.

the AT or BPT SAMs a significant upward BE shift of the surface component occurs, resulting in spectra in which the bulk and surface (shifted by about -0.06 eV) components can only be derived from the peak fitting procedure assuming a constant BE position of the bulk component. This component can be directly emphasized by increasing the kinetic energy of the photoelectrons, which results in the decrease of the fwhm of the Au 4f7/2 peak for AT/Au from ∼0.55 eV to ∼0.42 eV (bottom spectrum in Figure 1), the latter value is identical to the fwhm of the surface and bulk Au 4f7/2 components for the clean gold surface. In the case of the clean silver surface, only one Ag 3d5/2 peak with a BE of 368.25 eV and a fwhm of ∼0.37 eV is discernible in the respective XP spectrum (Figure 2), which is in agreement with literature data32,34 and implies that the BEs of the surface and bulk components for the Ag(111) surface are very close. From an analysis of the spectra with increasing photoelectron take-off angle (a fwhm of 0.39 eV at 40° and 0.43 eV at 70°) the BE shift of the surface component can be estimated to be about +0.05 ( 0.03 eV.

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Figure 3. C 1s and S 2p HRXPS spectra of AT/Au and AT/Ag. The BEs and fwhm’s of the S 2p3/2 and C 1s peaks are indicated.

With formation of the C12 or BPT SAM broadening of the Ag 3d5/2 peak occurs (Figure 2). In particular, the fwhm of this peak for AT/Ag increases to 0.43 and 0.50 eV at take-off angles of 0° and 40°, respectively (for BPT/Ag very similar values were found). There also is a slight downward BE shift of the Ag 3d5/2 emission at larger take-off angles after SAM formation, e.g., a displacement by -0.06 eV at a take-off angle of 50°. A self-consistent fit of the Ag 3d5/2 spectra by two peaks of same fwhm gives a BE shift of the surface component of about -0.11 ( 0.03 eV. Thus, whereas the BE difference of the Au 4f/Ag 3d5/2 bulk and surface components is distinctly different for the clean Au and Ag surfaces, similar BE differences are observed for both substrates after the AT/BPT adsorption. This implies a similarity in the character of the headgroup-substrate bond for the AT and BPT SAMs on both substrates. The significant BE shift of the Au4f/Ag3d surface components with respect to their positions for the clean substrates stems presumably from the chemical shift due to the chemisorption of the AT and BPT molecules. Note, that the difference in the BE shifts of surface components for the clean Au and Ag substrates can be related to the differences in the electronic structure of the two metals and to the (x3 × 23) surface reconstruction of the clean Au(111) surface35 which is lifted upon SAM formation.36 The S 2p and C 1s spectra of the AT and BPT SAMs are presented in Figures 3 and 4, respectively. All S 2p spectra exhibit only one S 2p doublet with a BE of ∼161.9 eV (S 2p3/2) and a fwhm of the individual S 2p3/2,1/2 peaks of 0.53-0.58 eV. The exceptionally large fwhm of 0.73 for BPT/Au is caused by the limited quality of this particular sample: Independent measurements performed by us on other BPT/Au and BPT/Ag samples at the SRRC in Taiwan gave a smaller (by ∼0.03 eV) fwhm of the S 2p3/2,1/2 peaks for BPT/Au as compared to that for BPT/Ag. Thus, the real S 2p3/2,1/2 fwhm for BPT/Au should be ∼0.50 eV. The occurrence of only one S 2p doublet suggests that a single sulfur species is present in the AT and BPT SAMs on both substrates. Considering the conclusions of former XPS studies3,30,37 and the large BE difference with respect to unbound thiols (∼163.5 eV for S 2p3/2)24,38 the single species found in our spectra is identified as a thiolate. The BE difference to unbound thiols stems from the chemical shift due to dissociation of the S-H bond and formation of a sulfur-metal bond and

Letters

Figure 4. C 1s and S 2p HRXPS spectra of BPT/Au and BPT/Ag. The BEs and fwhm’s of the S 2p3/2 and C 1s peaks are indicated (see text for the fwhm of the S 2p peaks for BPT/Au).

from screening of the S 2p core hole by the substrate electrons. The BEs and fwhms of the S 2p3/2,1/2 peaks for the films on Ag are slightly smaller than those for the films on Au. While the BE difference for the two substrates may be related to a stronger S-substrate bonding on Ag,3,39 the fwhm distinction can be associated with the generally assumed (at least for AT)3-8 superposition of nonequivalent adsorption sites for the sulfur headgroups on Ag and the equivalency of the adsorption sites on Au. For AT/Au this equivalency is, however, not absolute: The larger S 2p3/2,1/2 fwhm for AT/Au (0.54 eV) as compared to the corresponding values for BPT/Au (0.50 eV, as assumed) and for biphenyl-substituted AT on gold (0.50 eV)40 suggests slight differences in the adsorption geometry of the AT moieties in AT/Au. These differences can be associated with the wellknown c(4 × 2) modulation of the commensurate (x3×x3)R30° packing of AT molecules on Au(111). Note, that a S 2p3/2,1/2 fwhm of 0.50 eV should be representative for the fully commensurate (x3×x3)R30° surface lattice, taking into account that such a lattice was found for the high-density BPT film on Au(111)12,15 and considering that a value of 0.50 eV is the smallest one observed for thiol-derived SAMs.40 The C 1s spectra of the AT and BPT SAMs on both substrates exhibit a single emission peak with identical fwhms of 0.760.77 eV. In the case of BPT SAMs, this peak is accompanied by a shoulder at ∼1 eV higher BE, which might be alternatively assigned to the methyl tailgroup, the carbon atom bonded to the sulfur headgroup or to shake-up processes.9,41,42 The latter assignments is most likely as implied by measurements on a series of the biphenyl-substituted AT SAMs: The intensity and BE position of the higher BE shoulder correlate with the density of the aromatic matrix.40 The positions of the C 1s emission for AT and BPT SAMs differ noticeably, which is related to the saturated and unsaturated character of the alkyl and biphenyl moieties, respectively. Another feature of the C 1s spectra in Figures 3 and 4 is the very similar BEs of the C 1s emission lines for BPT/Au and BPT/Ag and the different BEs of these peaks for AT/Au and AT/Ag. Considering the similarity of the S 2p BEs for all investigated SAMs, this difference cannot stem from different screening of the C 1s hole by the substrate electrons, but might be related to the final state effects in the alkyl matrix, which

Letters has a denser lateral packing in AT/Ag than in AT/Au. In particular, so-called intersite antiscreening,43 i.e., a transfer of the electron density from the excited molecules to its neighbors might be of importance. Note, that a positive shift of the C 1s peak with increasing lateral packing density was also observed for AT SAMs on Hg-modified Au substrates.44 In summary, synchrotron-based HRXPS was applied to monitor the formation of AT and BPT SAMs on Au and Ag substrates. Pronounced chemical shifts in the adsorbate- and substrate-related photoemission lines upon the AT/BPT adsorption manifest a strong, presumably thiolate-type bonding of the AT and BPT molecules to the substrates. Only one sulfur species was found in all investigated SAMs. At the same time, the obtained spectra are consistent with a superposition of the different adsorption sites in the films on Ag, and imply several (at least two) slightly different adsorption geometries in AT/ Au in full agreement with the c(4 × 2) modulation of the commensurate (x3×x3)R30° surface lattice. A pronounced difference in the BEs of the C 1s emissions for AT/Au and AT/Ag and a shake-up excitation for BPT/Au and BPT/Ag were observed and attributed to final state effects. The presented HRXPS data can be taken as the experimental reference for theoretical work. Acknowledgment. We thank the DAAD and the BMBF for financial support, G. Albert for preparing the Au and Ag substrates, H.-T. Rong and M. Buck for providing us with the BPT substance, R. Klauser for assistance at the SRRC, D. L. Adams for supplying us with his fitting program FitXPS 1.06, and the MAX-LAB crew for support during beamtimes. References and Notes (1) Ulman, A. An Introduction to Ultrathin Organic Films: LangmuirBlodgett to Self-Assembly; Academic Press: New York, 1991; Chem. ReV. 1996, 96, 1533. (2) Thin films: self-assembled monolayers of thiols; Ulman, A., Ed.; Academic Press: San Diego, 1998. (3) 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. (4) Zharnikov, M.; Frey, S.; Rong, H.; Yang, Y.-J.; Heister, K.; Buck, M.; Grunze, M. Phys. Chem. Chem. Phys. 2000, 2, 3359. (5) Fenter, P.; Eisenberger, P.; Liang, K. S. Phys. ReV. Lett. 1993, 70, 2447; Fenter, P.; Eberhardt, A.; Eisenberger, P. Science 1994, 266, 1216. (6) Camillone, N., III; Chidsey, C. E. D.; Liu, G.; Scoles, G. J. Chem. Phys. 1993, 98, 3503; J. Chem. Phys. 1993, 98, 4234. (7) Poirier, G. E.; Tarlov, M. J. Langmuir 1994, 10, 2853. (8) Rieley, H.; Kendall, G. K.; Jones, R. G.; Woodruff, P. Langmuir 1999, 15, 8856. (9) Geyer, W.; Stadler, V.; Eck, W.; Zharnikov, M.; Go¨lzha¨user, A.; Grunze, M. Appl. Phys. Lett. 1999, 75, 2401. (10) Eck, W.; Stadler, V.; Geyer, W.; Zharnikov, M.; Go¨lzha¨user, A.; Grunze, M. AdV. Mater. 2000, 12, 805. (11) Frey, S.; Stadler, V.; Heister, K.; Zharnikov, M.; Grunze, M.; Zeysing, B.; Terfort, A. Langmuir 2001, 17, 2408.

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