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Monomolecular Films of Phthalocyanines: Formation, Characterization, and Expelling by Alkanethiols Thomas Hirsch, Andrey Shaporenko,† Vladimir M. Mirsky,* and Michael Zharnikov*,† Institute of Analytical Chemistry, Chemo- and Biosensors, UniVersity of Regensburg, UniVersita¨tsstrasse 31, 93053 Regensburg, Germany ReceiVed NoVember 24, 2006. In Final Form: January 19, 2007 Adsorption of aluminum-2,3-naphthalocyanine (Al-PC) onto gold (111) substrate from the pure and mixed (with alkanethiols) solutions of the target molecules in ethanol was studied. The resulting films were characterized by X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy. The adsorption from the pure solution resulted in formation of an Al-PC monolayer composed of the strongly inclined molecules. However, a presence of only one molar percent of a thiolated compound (alkanethiol) in the primary solution led to the complete expelling of the Al-PC molecules from the substrate and formation of a one-component alkylthiolate monolayer. The results indicate that an equilibrium formation of mixed monolayers of rodlike thiolated molecules and relatively large planar-geometry molecules, whose interaction with the metal surface is mainly provided by the π-electron system, is difficult to achieve or is in most cases even impossible under equilibrium conditions and requires an introduction of additional anchor moieties (e.g., thiols) into these molecules, as has been demonstrated by successful coadsorption of hexadecanethiol and thiolated tetraphenylporphyrin.
1. Introduction Mixed monolayers containing rodlike (mostly alkanethiols) and comparably large planar-geometry molecules are of interest for different applications, as, e.g., chemical sensors, active templates, or 2D (electro)catalysts.1-4 Such layers can be successfully fabricated by adsorption of thiolated molecules onto gold surface and formation of self-assembled monolayers (SAMs) according to well-known procedures.5-10 However, the affinity of different thiols to gold surface is different and correlates with their insolubility,11 with the adsorption of the alkylthiols being additionally favored by the intermolecular interaction between the neighboring molecules.6 That is why a high excess of the planar-geometry molecules in the primary solution is generally necessary to form heterogeneous mixed SAMs, comprising the alkanethiolate “matrix” and the individually incorporated planar molecules.4 Such mixed SAMs (so-called spreader-bar films), whose surface structure is not influenced by lateral diffusion, were successfully used for the preparation of artificial receptors1-3 and template-defined synthesis of nanoparticles on the substrate surface, serving as an electrode in an electrochemical * Corresponding authors. E-mail: michael.zharnikov@ urz.uni-heidelberg.de;
[email protected]. † Present address: Angewandte Physikalische Chemie, Universita ¨t Heidelberg, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany. (1) Mirsky, V. M.; Hirsch, T.; Piletsky, S. A.; Wolfbeis, O. S. Angew. Chem., Int. Ed. 1999, 38, 1108. (2) Hirsch, T.; Kettenberger, H.; Wolfbeis, O. S.; Mirsky, V. M. Chem. Commun. 2003, 3, 432. (3) Prodromidis, M. I.; Hirsch, Th.; Mirsky, V. M.; Wolfbeis, O. S. Electroanalysis 2003, 15, 1795. (4) Hirsch, T.; Zharnikov, M.; Shaporenko, A.; Stahl, J.; Weiss, D.; Wolfbeis, O. S.; Mirsky, V. M. Angew. Chem., Int. Ed. 2005, 41, 6775. (5) Ulman, A. Chem. ReV. 1996, 96, 1533. (6) Bain, C. D.; Whitesides, G. M. Angew. Chem. 1989, 101, 522. (7) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87. (8) Delamarche, E.; Michel, B. Thin Solid Films 1996, 273, 54. (9) Delamarche, E.; Michel, B.; Biebuych, H. A.; Gerber, C. AdV. Mater. 1996, 8, 719. (10) Poirier, G. E.; Pylant, E. D. Science 1996, 272, 1145. (11) Riepl, M.; Mirsky, V. M.; Novotny, J.; Tvarozek, V.; Rehacek, V.; Wolfbeis, O. S. Anal. Chem. Acta 1999, 392, 77. (12) Li, G.; Fudickar, W.; Skupin, M.; Klyszcz, A.; Draeger, C.; Lauer, M.; Fuhrhop, J.-H. Angew. Chem., Int. Ed. 2002, 11, 1828.
Chart 1. On the Left: Aluminum-2,3-naphthalocyanine (Al-PC). On the Right: Tetraphenylporphyrin (TMPP); X ) -SH, SO3H
assembly.4 In contrast, the formation of similar heterogeneous films in the case of large aromatic moieties without special anchor groups seems to be more difficult and could so far be only performed under strongly nonequilibrium deposition conditions.13 The aim of this publication is to prove whether a heterogeneous mixed monolayer of thiolated and non-thiolated compounds can be formed under the equilibrium conditions, i.e., by immersion of a metal substrate into the solution containing both species at a high excess of the non-thiolated compound. As test substances, we used 1-mercaptohexadecane (C16) and aluminum-2,3naphthalocyanine (Al-PC). The presence of 68 conjugated bonds in the Al-PC molecule suggests a well-developed π-system, which should interact strongly, for such type of molecules, with the gold substrate at a proper molecular orientation. 2. Experimental Section 1-Mercaptohexadecane (C16) and aluminum-2,3-naphthalocyanine (Al-PC, see Chart 1) were purchased from Sigma-Aldrich and used without further purification. The substrates were fabricated by evaporation of gold (200 nm) on Si(100) wafers precoated with a Ti/Pd adhesion layer (5 nm). The (13) Fudickar, W.; Zimmermann, J.; Ruhlmann, L.; Schneider, J.; Ro¨der, B.; Siggel, U.; Fuhrhop, J.-H. J. Am. Chem. Soc. 1999, 121, 9539.
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Figure 1. Au 4f, C 1s, and N 1s XPS spectra of the SAMs formed from the Al-PC and mixed C16/Al-PC solutions, along with the Cl 2p and Al 2p spectra of the SAMs formed from the Al-PC solution. The relative concentrations of C16 and Al-PC in the primary solutions are given at the respective curves.
Figure 2. C K-edge NEXAFS spectra of the films formed from the Al-PC and mixed C16/Al-PC solutions; the relative concentrations of C16 and Al-PC in the primary solutions are given at the respective curves. Left panel: The spectra acquired at a magic X-ray incidence angle of 55°. Right panel: The differences between the spectra acquired at X-ray incidence angles of 90° and 20°. SAMs were prepared by immersing the substrates in an ethanolic (Baker) solution of C16 and Al-PC for 72 h. The preparation occurred at room temperature and oxygen-free conditions. The concentration of Al-PC in the solution was kept constant at 250 mmol L-1 while that of C16 was varied, being either 25 mmol L-1 (C16:Al-PC ) 1:10) or 2.5 mmol L-1 (C16:Al-PC ) 1:100). An increase of the Al-PC concentration above 250 mmol L-1 was not possible because of the solubility limitation. For simplicity, the films prepared from the Al-PC solution will be denoted the pure Al-PC films further in the manuscript, in contrast to the films prepared from the mixed Al-PC/C16 solutions. The latter films will be denoted C16/Al-PC films. The SAMs were characterized by X-ray photoelectron spectroscopy (XPS) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. The experiments were performed at room temperature and UHV conditions. The XPS and NEXAFS measurements were carried out at the HE-SGM beamline of the synchrotron storage ring BESSY II in Berlin, Germany. In the case of XPS, the energy
resolution was ≈0.40 eV, and the binding energy (BE) scale was referenced to the Au 4f7/2 peak at 84.0 eV. The NEXAFS spectra were collected at the carbon and nitrogen K-edges in the partial electron yield mode with retarding voltages of -150 V and -300 V, respectively. Linear polarized synchrotron light with a polarization factor of ≈84% was used. The incidence angle of the light was varied to monitor the orientational order within the Al-PC/C16 films, following the standard approach.15 The raw NEXAFS spectra were normalized to the incident photon flux. The energy scale was referenced to the π1* resonance of highly oriented pyrolytic graphite at 285.38 eV.14 Other experimental details are described elsewhere.1-4 The results for the C16/Al-PC films were compared with the analogous data for the homogeneous and heterogeneous SAMs formed from 1-mercaptododecane (C12) and tetraphenylporphyrin (TMPP), which, similar to Al-PC, is a large planar rigid molecule with a well-developed π-electron system, but, in contrast to Al-PC, (14) Batson, P. E. Phys. ReV. B 1993, 48, 2608.
Monomolecular Films of Phthalocyanines
Figure 3. N K-edge NEXAFS spectra of the films formed from the Al-PC and mixed C16/Al-PC solutions; the relative concentrations of C16 and Al-PC in the primary solutions are given at the respective curves. The spectra were acquired at a magic X-ray incidence angle of 55°. contains several thiol anchors (see Chart 1).4 The C12/TMPP monolayers were prepared in a similar way as the C16/Al-PC films.4
3. Results and Discussion XPS spectra of the pure Al-PC films and the SAMs prepared from the mixed Al-PC/C16 solutions are presented in Figure 1. In the spectra of the pure Al-PC films (bottom curves), characteristic emissions of all the elements comprising the AlPC molecule, including those related to nitrogen, chlorine, and aluminum, could be found. The C 1s XPS spectrum of these films can be decomposed into two components, related to the aromatic core (the main peak) and C-N groups (the high BE shoulder), in accordance with the chemical composition of the Al-PC molecule. The C and N K-edge NEXAFS spectra of the pure Al-PC film are presented in Figures 2 and 3, respectively (bottom lines). These spectra exhibit the characteristic absorption resonances of naphthalenes (≈285 eV) and pyridine-like moieties (≈400 eV and ≈410 eV).15 In addition to these characteristic resonances and the broad σ-resonances at higher photon energies, a low intense π-resonance related to the CdO moiety (288.7 eV) is observed in the C K-edge NEXAFS spectrum, suggesting that the Al-PC film is slightly contaminated. This contamination stemmed, presumably, from the Au substrate, which was exposed to ambient during its transfer from the evaporation chamber to the immersion solution. Presumably, the contamination was not completely removed upon the absorption of the Al-PC molecules. The spectra presented in the left panel of Figure 2 and Figure 3 were acquired at the so-called magic angle of X-ray incidence, 55°. At this particular orientation, NEXAFS spectra are not affected by the molecular orientation of the Al-PC film.15 The information on the molecular orientation can, however, be derived from the entire set of the NEXAFS spectra acquired at different angles of X-ray incidence, 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 (15) Sto¨hr, J. NEXAFS Spectroscopy; Springer Series in Surface Science 25; Springer-Verlag: Berlin, 1992.
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to the molecular orbital of interest (so-called linear dichroism in X-ray absorption).15 Generally, a high intensity is observed if the direction of the E-vector coincides with the direction of the transition dipole moment of the molecular orbital under consideration. A fingerprint of the molecular orientation is the difference between the spectra acquired at the normal (90°, E-vector is parallel to the sample surface) and grazing (20°, E-vector is almost perpendicular to the sample surface) incidence of X-rays. Such a difference spectrum for Al-PC/Au is presented in the right panel of Figure 2 (bottom curve). In this spectrum, the difference peaks related to the π*- and σ*-resonances of the Al-PC molecule have negative and positive signs, respectively. Taking into account that the π*- and σ*-orbitals are perpendicular to and coplanar with the molecular plane of the Al-PC molecule, respectively, we can then conclude that these molecules are predominantly oriented parallel to the substrate surface. The average angle between the Al-PC molecules and the surface was estimated to be about 40°. On the basis of the XPS data (the C1s and Au4f intensities), assuming an exponential attenuation of the photoemission signals, and taking the attenuation lengths reported in ref 16, the effective thickness of the Al-PC film was estimated at about 2 nm. This value correlates with the size of the Al-PC molecule, suggesting that this film represents mainly a monolayer. Taking into account the obtained average tilt angle of the Al-PC molecules in the film (40° with respect to the substrate surface), we can suggest that the molecules are mostly adsorbed in a diagonal orientation, so that one bond of the naphthalene substituent is placed parallel to the metal surface. The inclined orientation of the Al-PC molecules can be caused by a relatively strong contribution of the intermolecular interaction (stacking) of Al-PC as compared to the interaction with the substrate. Under definite circumstances (see below), the energy gain associated with a dense, SAM-like molecular packing of the Al-PC moieties (inclined geometry) can prevail over the energy gain obtained at the optimal interaction of the molecules with the substrate, which occurs at their inplane orientation, since the latter geometry involves a loose molecular packing. Note that an inclined stacking has been observed previously for similar molecules, e.g., on the (001) surface of alkali halide,29 on stepped sapphire surface,30 and on silicon dioxide surface.31 Additionally, the deviation from the parallel-to-the-substrate orientation, which is often observed in the case of organic molecular beam deposition of naphthalocyanines in ultrahigh vacuum (UHV),32 can be caused by interaction of chlorine atom, standing out of the phthalocyanine plane,33 with gold in a similar way as in ref 34. An important factor, which can contribute to the formation of the inclined phase, hindering the optimal interaction of the AlPC molecules with the substrate, is surface contamination. Whereas, under the UHV conditions, the substrate is usually completely cleaned from contamination before the molecular deposition, it is slightly contaminated in the case of deposition from solution (see above), since it was exposed to ambient, even though for a short time, before the immersion. In the case of chemisorption of dissolved molecules, so-called self-cleaning, i.e., complete removal of contamination upon the adsorption occurs, as it, e.g., happens for alkanethiols.35 In contrast, presumably, contamination persists to some extent in the case of the comparably weak (π-d) bonding, which is characteristic of the adsorption of phthalocyanines. The situation changed crucially as soon as the Au substrates were immersed into the mixed Al-PC/C16 solutions. In spite of the strong Al-PC excess (by factors of 10 and 100), the respective (16) Lamont, C. L. A.; Wilkes, J. Langmuir 1999, 15, 2037.
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Figure 4. Adsorption of either Al-PC or alkanethiol leads to formation of a molecular monolayer. However, a simultaneous adsorption of both components from a mixed Al-PC/alkanethiol solution results in a monolayer of alkylthiolate only, even at a high (by factors of 10 and 100) excess of Al-PC in the primary solution.
Figure 5. C K-edge NEXAFS spectra of the films formed from the TMPP and mixed C12/TMPP solutions; the relative concentrations of C12 and TMPP in the primary solution are given at the respective curves; the most prominent resonances of C12 and TMPP are assigned. Left panel: The spectra acquired at a magic angle of 55°. Right panel: The differences between the spectra acquired at X-ray incidence angles of 90° and 20°.
XPS and NEXAFS spectra in Figures 1-3 are characteristic of the one-component C16 SAM and do not exhibit any features related to the Al-PC molecules. In particular, the C 1s XPS spectra of both C16/Al-PC films in Figure 1 exhibit a relatively sharp emission at about 285.0 eV, which is characteristic of the intact alkanethiolate SAMs,17,18 whereas no emissions were observed in the N 1s, O 1s, Cl 2p, and Al 2p ranges. In the S 2p XPS spectra (not shown), a characteristic doublet at 162.0 eV (S2p3/2)18 related to the thiolate headgroup of C16 appeared. The effective thickness of both films prepared from the mixed AlPC/C16 solutions was estimated at about 18.9 Å, which is the expected value for the C16 SAM on Au.18 The C K-edge NEXAFS spectra of both C16/Al-PC films in Figure 2 also exhibit characteristic absorption resonances of well-ordered aliphatic SAMs: a mixed C-H*/Rydberg resonance at 287.7 eV15,19-22 and C-C and C-C′ σ* resonances at (17) Heister, K.; Zharnikov, M.; Grunze, M.; Johansson, L. S. O. J. Phys. Chem. B 2001, 105, 4058. (18) Heister, K.; Johansson, L. S. O.; Grunze, M.; Zharnikov, M. Surf. Sci. 2003, 529, 36. (19) Bagus, P. S.; Weiss, K.; Schertel, A.; Wo¨ll, Ch.; Braun, W.; Hellwig, H.; Jung, C. Chem. Phys. Lett. 1996, 248, 129. (20) Weiss, K.; Bagus, P. S.; Wo¨ll, Ch. J. Chem. Phys. 1999, 111, 6834.
≈293.4 eV and ≈301.6 eV.19,23 These resonances show pronounced linear dichroism (see the right panel of Figure 2), which is characteristic of the well-ordered aliphatic SAMs.19,23,24 The average tilt angle of the aliphatic chains in both C16/Al-PC films was estimated at 32° with respect to the substrate normal, which is the typical value for alkanethiloate SAMs on Au.23-25 Most important, no characteristic resonances of the Al-PC molecule were observed in both C and N K-edge NEXAFS spectra of the C16/Al-PC films, as shown in Figures 2 and 3, respectively. A low-intense feature at a photon energy of 285 eV in the C K-edge spectra is frequently observed for alkanethiolate SAMs and is alternatively assigned to a contamination or an excitation into alkane-metal orbitals.26,27 The N K-edge NEXAFS spectra for both C16/Al-PC films represent identical smooth and structureless curves, without any features related to the excitation from the N1s core level to the nitrogen-derived unoccupied molecular orbitals. Thus, we conclude that both C16/Al-PC films represent well-ordered and densely packed C16 SAMs, which do not contain any Al-PC molecules within the detection limit of XPS and NEXAFS spectroscopy (several % of monolayer). We would like to stress once more that the molar ratio of Al-PC and C16 in the mixed solutions used for the substrate coating was as high as 10:1 or even 100:1. The deposition time was 72 h, which is above the characteristic time required for the formation of a well-ordered molecular monolayer.28 Therefore, one can expect that the composition of the adsorbate film corresponds to the thermodynamical equilibrium. (21) Va¨terlein, P.; Fink, R.; Umbach, E.; Wurth, W. J. Phys. Chem. 1998, 108, 3313. (22) Scho¨ll, A.; Fink, R.; Umbach, E.; Mitchell, G. E.; Urquhart, S. G.; Ade, H. Chem. Phys. Lett. 2003, 370, 834. (23) Outka, D. A.; Sto¨hr, J.; Rabe, J. P.; Swalen, J. D. J. Chem. Phys. 1988, 88, 4076. (24) Zharnikov, M.; Grunze, M. J. Phys.: Condens. Matter 2001, 13, 11333. (25) Ha¨hner, G.; Kinzler, M.; Thu¨mmler, C.; Wo¨ll, Ch.; Grunze, M. J. Vac. Sci. Technol. 1992, 10, 2758. (26) Bierbaum, K.; Kinzler, M.; Wo¨ll, Ch.; Grunze, M.; Ha¨hner, G.; Heid, S.; Effenberger, F. Langmuir 1995, 11, 512. (27) Witte, G.; Weiss, K.; Jakob, P.; Braun, J.; Kostov, K. L.; Woell, Ch. Phys. ReV. Lett. 1998, 80, 121. (28) Schlenhof, J. B.; Li, M.; Ly, H. J. Am. Chem. Soc. 1995, 117, 12528. (29) Yanagi, H.; Kouzeki, T.; Ashida, M. J. Appl. Phys. 1993, 73, 3812. (30) Osso, J. O.; Schreiber, F.; Kruppa, V.; Dosch, H.; Garriega, M.; Alonso, M. I.; Cerdeira, F. AdV. Funct. Mater. 2002, 12, 455. (31) Osso, J. O.; Schreiber, F.; Alonso, M. I.; Garriega, M.; Barrena, E.; Dosch, H. Org. Electron. 2004, 5, 135. (32) Leznoff, C. C.; Lever, A. B. P. Phthalocyanines: Properties and Applications, VCH: New York, 1989. (33) Weber, A.; Reino¨hl, U.; Bertagnolli, H. Bessy Jahresbericht; BESSY GmbH: Berlin, 1998. (34) Shi, Z.; Lipkowski, J. J. Electroanal. Chem. 1996, 403, 225.
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The adsorption energy of alkanethiols on gold is close to 50 kJ mol-1,5,35 which corresponds to 20 kT at room temperature. Geometrical considerations give the ratio of the areas occupied by one Al-PC and one C16 molecule in the pure monolayers of these compounds as R ) 450 A2/21.4 A2 ) 21. Therefore, one Al-PC molecule occupies the same area as about 21 C16 molecules. The total adsorption energy of 21 C16 molecules is about 420 kT. This is an overestimation, since one has to take into account also the exact shape of the vacant place on gold surface after the desorption of an Al-PC molecule and possible hindrances for adsorption of C16 at the boundary of the vacant place. However, even if the real amount of the C16 molecules occupying the area of one Al-PC molecule is not 21 but only 5, their total binding energy is still high enough to explain a replacement of Al-PC by C16 at 100 times lower concentration of C16 in the coating solution. The only way to prevent the replacement is increasing the binding energy of planar geometry molecules. It can be achieved by their chemical modification, e.g., by the introduction of anchor thiol groups. An example is given in Figure 5, in which the C K-edge NEXAFS spectra of the films formed from the thiolated tetraphenylporphyrin (TMPP) and mixed C12/TMPP solutions are presented, along with the respective difference (90°-20°) curves.4 A continuous variation of the spectra and difference curves with the solution composition (C12:TMPP) is observed, which assumes the formation of a mixed monolayer of variable composition. As the fingerprints of these two molecules in the C12/TMPP film, π1* resonance of TMPP at 285.1 eV and C-H/R* resonance of C12 at 287.7 eV can be taken. According to the experimental data, a near equimolar ratio of both types of molecules on the surface requires more then 100-fold excess in the molar ratio of these species in the bulk solution.4 However, even at the 100-fold excess of TMPP in the primary solution, a pronounced signature of this molecule is clearly observed in the NEXAFS spectrum of the resulting film (see Figure 5). This is definitely not the case for the Al-PC molecule; no signature of these moieties could be found in the respective spectrum of the film, after the adsorption from a mixed C16/Al-PC solution with the 100-fold excess of Al-PC. Note that, according to our experience, C12 and C16 behave quite similarly with respect to their codeposition with planar molecules, so that the C16/Al-PC and C12/TMPP systems can be directly compared.
4. Summary The results of the XPS and NEXAFS experiments show that an ordered phthalocyanine film can be formed on the Au(111) substrate by adsorption from organic solution. The molecules in (35) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103.
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this film are noticeably inclined, which suggests that, under the conditions of our experiment, i.e., at adsorption of Al-PC from ethanolic solution, the intermolecular interaction prevails over the interaction of these molecules with the metal substrate, mediated by their π-electron system. An important factor, which probably contributes to the formation of the inclined phase, hindering the optimal interaction with the substrate, is surface contamination, which is difficult to avoid within the immersion procedure involving the externally prepared substrates, and which cannot be completely removed by the adsorbing molecules in the case of the comparable weak (π-d) bonding. The results indicate that the formation of a mixed monolayer from alkanethiols and phthalocyanines at equilibrium conditions, i.e., by a long-time coadsorption from a solution containing both components, can be impossible, at least at the excess of the phthalocyanines in the primary solution by a factor of up to 100. A further increase of the concentration ratio of the reagents is not always possible. For example, in the studied case, the maximal concentration ratio of C16 and Al-PC was limited by the solubility constraints for the latter molecule. Putting in the general context, the results for the C16/Al-PC system indicate that the formation of mixed monolayers consisting of chainlike thiolated molecules and molecules, whose adsorption is only governed by interactions of their π-systems with the metal substrate, is difficult to achieve or is in most cases even impossible at the equilibrium conditions. Of course, this assumption does not exclude the formation of such monolayers at nonequilibrium conditions, for example by a short time coadsorption of both constituents or by controlled partial replacement of non-thiolated adsorbates by thiolated ones.13 At the same time, the formation of mixed monolayers of strongly adsorbing chainlike molecules and weakly adsorbing, planar moieties with the thermodynamic control of the ratio of both constituents in the resulting film can be easily performed as far as the weakly adsorbing moiety is equipped with additional anchors such as, e.g., thiol groups. This conclusion is important for the choice of the best strategy for the bottom-up fabrication of nanostructured monolayers containing two or more moieties. The approach can be used for the fabrication of different functional films, serving as chemical sensors, biosensors, protective coatings, electrochemical templates for in-situ synthesis of 3D nanoobjects, or further assembly of ordered multilayer structures, etc. Acknowledgment. We thank M. Grunze and O. S. Wolfbeis for the support of this work, Ch. Wo¨ll (Universita¨t Bochum) for providing us with the equipment for the NEXAFS measurements, and the BESSY II staff for the assistance during the experiments. This work was supported by the Volkswagenstiftung (Project I/78 749) and the Deutscher Forschungsgemeinschaft (ZH 63 9/2). LA0634239
