Flat Lying Pin-Stripe Phase of Decanethiol Self-Assembled

Langmuir , 1998, 14 (23), pp 6693–6698 ... High-resolution scanning tunneling microscope (STM) imaging reveals the formation of a 11.5 × √3 ... S...
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Langmuir 1998, 14, 6693-6698

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Flat Lying Pin-Stripe Phase of Decanethiol Self-Assembled Monolayers on Au(111) R. Staub, M. Toerker, T. Fritz, T. Schmitz-Hu¨bsch, F. Sellam, and K. Leo* Institut fu¨ r Angewandte Photophysik, Technische Universita¨ t Dresden, D-01062 Dresden, Germany Received June 17, 1998. In Final Form: August 11, 1998 Self-assembled monolayers with low molecular surface density of decanethiol on Au(111) have been prepared by annealing of the densely packed (x3 × x3)R30° structure. High-resolution scanning tunneling microscope (STM) imaging reveals the formation of a 11.5 × x3 stripe structure where the molecules are aligned along the substrate surface in double lamellae. We can deduce from the images that the molecules bind to specific sites at the Au(111) surface rather than adsorb as didecane disulfides. For the first time, a lattice structure for an alkanethiol stripe phase can be derived from the obtained real-space data.

Introduction Alkanethiol films on Au(111) are a favorite system for the study of self-assembly processes due to their easy preparation and robust structure (for a review, see refs 1 and 2). However, the precise binding to the surface is not yet known. It is generally assumed that the film growth occurs through breaking of the H-S bond and covalent binding of the thiolate through formation of a Au-S bond. An alternative explanation based on X-ray diffraction data suggests self-assembly of alkanethiols as disulfides.3 Alkanethiol self-assembled monolayers (SAM) are usually prepared by either gas-phase deposition, or by immersion in dilute ethanolic solution. The structural properties for films with the highest possible molecular surface density4 were studied by low-energy electron diffraction (LEED), He diffraction, X-ray diffraction, and scanning probe microscopy. It was found that these films grow commensurate with the Au(111) surface, forming a basic (x3 × x3)R30° 5-8 or one of three possible c(2x3 × 4x3)R30° superstructures.3,9,10 The alkyl chains are tilted against the surface normal by about 30°. For simplicity we will refer to these densely packed structures in the following as the (x3 × x3)R30° structure. According to a widely accepted model the alkanethiols in this structure are bound covalently to Au(111) next nearest neighbor hcp hollow sites (the sites above a gold atom in the second layer),11 although more recent reports have proposed that the chemical binding of the decanethiols in the (x3 × x3)R30° structure is not limited to one site only but involves two different binding sites.12,13 (1) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (2) Delamarche, E.; Michel, B.; Biebuck, H. A.; Gerber, Ch. Adv. Mater. 1996, 8, 719. (3) Fenter, P.; Eberhardt, A.; Eisenberger, P. Science 1994, 266, 1216. (4) The term molecular surface density is herein defined as the number of molecules per area and should not be confused with surface coverage, i.e., the ratio of covered to total substrate surface. (5) Chidsey, C. E. D.; Liu, G.-Y.; Rowntree, P.; Scoles, G. J. Chem. Phys. 1989, 91, 4421. (6) Widrig, C. A.; Alves, C. A.; Porter, M. D. J. Am. Chem. Soc. 1991, 113, 2805. (7) Alves, C. A.; Smith, E. L.; Porter, M. D. J. Am. Chem. Soc. 1992, 114, 1222. (8) Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. J. Chem. Phys. 1993, 98, 678. (9) Camillone, N., III; Chidsey, C. E. D.; Liu, G.; Scoles, G. J. Chem. Phys. 1993, 98, 4234. (10) Poirier, G. E.; Tarlov, G. E. Langmuir 1994, 10, 2853. (11) Sellers, H.; Ulman, A.; Shnidman, Y.; Eilers, J. E. J. Am. Chem. Soc. 1993, 115, 9389.

The high-density films of the (x3 × x3)R30° structure have been studied extensively, and their real space structure is well-known. However, corresponding information is still lacking for low surface density films. The latter are obtained by either annealing of the densely packed (x3 × x3)R30° structure, adequate dosing in gasphase deposition, or immersion in solution for short periods only. LEED,8,14 He diffraction,15,16 and STM17-21 studies revealed the formation of stripe patterns, the so-called pin-stripe structures. It was found that by repeated gasphase deposition and annealing cycles, respectively, the pin-stripe phases can be transformed into the (x3 × x3)R30° structure or the lattice gas phase in a reversible manner.14 In the striped structures, the molecules of any chain length are arranged in rows, where the molecules exhibit the same nearest neighbor distance as in the (x3 × x3)R30° structure. The row spacing, however, shows a dependence on the alkyl chain length.16 For the particular case of decanethiol SAMs, the molecule we are dealing with, two pin-stripe phases with different row spacings could be observed: a 7.5 × x3 phase and a (11 ( 0.5 × x3 phase. We follow here the notation established by Poirier et al.17 where the stripe phases are described by a p × x3 structure. Here p denotes the stripe spacing and x3 the periodicity within the stripes in units of the Au(111) spacing. Both phases seem to be stable at room temperature. The (11 ( 0.5) × x3 phase has the lowest molecular surface density ever observed, whereas the 7.5 × x3 phase is of intermediate surface density. Though these stripe phases have been well characterized in reciprocal space by other authors. Real space structure and exact molecular surface density have not been determined yet. (12) Yeganeh, M. S.; Dougal, S. M.; Polizotti, R. S.; Rabinowitz, P. Phys. Rev. Lett. 1995, 74, 1811. (13) Fenter, P.; Schreiber, F.; Berman, L.; Scoles, G.; Eisenberger, P.; Bedzyk, M. J. Submitted to Surf. Sci. (14) Gerlach, R.; Polanski, G.; Rubahn, H.-G. Appl. Phys. A 1997, 65, 375. (15) Camillone, N., III; Eisenberger, P.; Leung, T. Y. B.; Schwartz, P.; Scoles, G. Poirier, G. E.; Tarlov, M. J. J. Chem. Phys. 1994, 101, 11031. (16) Camillone, N., III; Leung, T. Y. B.; Schwartz, P.; Eisenberger, P.; Scoles, G. Langmuir 1996, 12, 2737. (17) Poirier, G. E.; Tarlov, M. J.; Rushmeier, H. E. Langmuir 1994, 10, 3383. (18) Poirier, G. E.; Tarlov, M. J. J. Phys. Chem. 1995, 99, 10966. (19) Poirier, G. E.; Pylant, E. D. Science 1996, 272, 1145. (20) Kang, J.; Rowntree, P. A. Langmuir 1996, 12, 2813. (21) Yamada, R.; Uosaki, K. Langmuir 1997, 13, 5218.

10.1021/la980717o CCC: $15.00 © 1998 American Chemical Society Published on Web 10/22/1998

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So far, it has been assumed that with decreasing molecular density the thiolates tilt from the almost upright position, as seen in the (x3 × x3)R30° structure, toward the sample surface. This assumption is supported by recent XPS measurements22 on low surface density structures. However, the interrow spacing found for the lowest density stripe phase would not allow the straight fitting of two flat lying thiolates into the stripe spacing. It is thus widely interpreted as adsorption of the alkanethiols as disulfides because of the shorter van der Waals length of the dialkyl disulfide, compared to twice the van der Waals length of a single alkanethiol molecule. In the present article we show, to the best of our knowledge, the first submolecular resolution STM images of the lowest density stripe structure for decanethiol. The data allow us to determine a real space structure of this phase. Furthermore, our images give evidence that the low-density stripe phase does form by adsorption of thiolate units rather than of disulfides. Experimental Section Decanethiol SAMs are obtained by chemisorption from a 3.5 mM ethanolic solution onto Au(111) substrates. The Au(111) substrates are prepared by evaporating Au onto mica at a temperature of 450 °C and a pressure of 5 × 10-7 mbar, similar to the procedure described in ref 23. The Au(111) substrates prepared in this way usually show the x22 × x3 reconstruction, which is characteristic for a clean surface. The Au substrates are then transferred into the decanethiol solution, with immersion times varying between 8 and 144 h. Within this range the immersion period has no visible effect on the monolayer assembly. After thorough rinsing of the decanethiol SAMs with pure spectroscopy grade ethanol, the samples are reloaded into UHV and annealed at approximately 100 °C for about 6 h during bakeout of the load-lock. To obtain the pin-stripe structures, the samples are annealed in UHV at a base pressure of 1 × 10-9 mbar for about 20 min until the temperature reaches ca. 200 °C. The sample surface then shows usually the lowest density pinstripe structure. In some cases, the anneal cycle has to be repeated until only the lowest density pin-stripe structure becomes visible. All STM measurements are carried out with a commercial combined AFM/STM system in UHV.24 The tips used in this study were prepared by electrochemical etching of Pt/Ir wire. Calibration of the STM has been carried out on the 7 × 7 reconstruction of Si(111). The calibration parameters have been checked on highly orientated pyrolitic graphite (HOPG) before and after acquiring the images presented in this paper. Thorough attention has been paid to the distortion of the STM images due to drift. To eliminate these effects, from time to time consecutive scans on a sample area with identifiable surface features have been acquired. Comparison of the corresponding coordinates on these scans allows us to calculate the drift in x- and y-direction and hence enables the reconstruction of the undistorted sample area. This correction results in a skewed STM image, although the originally selected scan area has been rectangular. It should be noted that all images presented here are only slightly low pass filtered.

Figure 1. (a) Large area STM current image showing the pinstripe pattern (U ) 2.0 V; I ) 0.20 nA). The stripes are running perpendicular to the edge on the left side of the Au(111) terrace. (b) Line profile taken along A that gives a stripe spacing of 34.2 Å.

Basic Structure. Figure 1a shows a large scan of the sample area after annealing of the densely packed (x3 × x3)R30° structure. A triangular terrace typical for the Au(111) surface is visible. On the terrace, the stripe pattern of the low-density phase of decanethiol can be seen. The angles formed by the stripes and the edges of the Au(111) terraces are 30°, 90°, and 150°, respectively,

with an error of (3°. We can conclude that the stripes are aligned with the Au〈2 h 11〉 direction, since the edges of the Au(111) terraces run along the 〈1 h 10〉 direction. The distance of the stripes is measured as 34.2 Å (see Figure 1b). At submolecular resolution (Figure 2a), it can be clearly seen that the stripes consist of a pin-stripe pair (with small branches sticking off like ribs from the backbone) rather than a single pin-stripe. In each pair, the two pinstripes appear with different contrast. The lower stripes in Figure 2a are brighter and appear with higher contrast; the upper ones are less corrugated and darker. It should be noted that this is not a scanning or tip effect, but is observed for different scan rotations and tips. The periodicity along each of the two pin-stripes is estimated from the high-resolution scan (Figure 3a) to be 5.0 Å (Figure 3b). Comparison of these line profiles reveals that the dots in the two paired stripes are shifted relative to each other by roughly 1 Å along the stripe direction. For a precise determination of the separation of the pin-stripes within a pair, we have averaged the height profile perpendicular to the rows over about 100 lines, as indicated in Figure 3a. This procedure yields a spacing of about 4.0 Å (Figure 3c). Theoretical studies25 have shown that in both decanethiol and didecyl disulfide molecules, the electron densities of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are localized at the sulfur sites. In STM images, the sulfur atoms should thus appear brighter than the alkyl chains. In our STM

(22) Himmel, H.-J.; Wo¨ll, Ch.; Gerlach, R.; Polanski, G.; Rubahn, H.-G. Langmuir 1997, 13, 602. (23) DeRose, J. A.; Thundat, T.; Nagahara, L. A.; Lindsay, S. M. Surf. Sci. 1991, 256, 102. (24) Omicron GmbH, Taunusstein, Germany.

(25) For the theoretical calculations we used the PM3 method as implemented in HyperChem 5.01 of Hypercube, Inc., Waterloo, Canada. Prior to the orbital calculation, a full structural optimization was performed, resulting in linear decanthiol and zigzag-shaped disulfide molecules, respectively.

Results and Discussion

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Figure 2. (a) Molecular resolution STM height image (U ) 2.0 V; I ) 0.20 nA) showing the bright pin-stripe pairs with the alkyl chains lying flat on the substrate surface. (b) The line profiles A and B show the 2.5 Å shift between adjacent pairs.

data, we hence assign the dots in the stripes to sites of sulfur atoms and the ribs to alkyl chains. The images indicate that the alkyl chains are aligned with the substrate surface. Sulfur Binding Sites. Along the pin-stripes the thiol molecules show a nearest neighbor distance of 5.0 Å. Across the pin-stripe pair, however, the nearest neighbor distance is roughly 4 Å. This value results from the 4 Å pin-stripe pair spacing and the 1 Å shift. The spacing of the molecules along the stripes would be in agreement with the model for the (x3 × x3)R30° structure, where it is assumed that alkanethiols chemisorb as thiolates at next nearest neighbor Au(111) 3-fold hollow sites.11 However, the 4 Å nearest neighbor distance observed across the pin-stripe pair is in contradiction to that model. This implies two possibilities: (i) The molecules bind as thiolates to nonequivalent Au(111) sites in each stripe of a pair. (ii) The molecules are adsorbed as didecyl-disulfides. For the latter case, structure calculations26 have been performed that reveal the typical sulfur-sulfur distance in different dialkyl disulfide molecules as 2.02-2.05 Å. Assuming that in STM images of didecyl disulfide the separation of the electron density maxima is imaged rather

Figure 3. (a) Submolecular resolution STM height image (U ) 2.0 V; I ) 0.20 nA). (b) The line profiles A and B show the shift between the pin-stripes of each pair. (c) Profile C gives the lateral spacing of the pin-stripe pair. This profile has been averaged over the area marked by the dashed rectangle.

than the pure S-S bond length,27 one obtains a slightly larger distance of the bright spots in the STM images, still far away from the 4 Å observed in our data.28 At the same time, one would not expect to image the two sulfur atoms with considerable contrast difference due to the (26) In addition to the didecyl disulfide, we included a dimethyl disulfide in our calculations. This enabled us to check the results of the semiempirical PM3 method by ab initio restricted Hartree-Fock calculations with the 6-31G(d) basis set, done by means of the Gaussian94W program of Gaussian, Inc., Pittsburgh, PA. The deviations found are smaller than 0.02 Å.

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symmetrical distribution of the electron densities. This leads us to the conclusion that the alkanethiol molecules are covalently bound as thiolates to different Au(111) sites in each stripe of a pair, rather than being adsorbed as disulfides. In the following we discuss this assumption and propose a model deduced from our data. On the basis of the theoretical work by Sellers et al.11 and Beardmore et al.,29 we take into consideration on-top sites as well as 3-fold hollow sites and bridge sites for the chemical binding of thiolates to the Au(111) surface. Ontop sites are defined as positions of a gold atom on the Au(111) surface. For the hollow sites one has to distinguish between the sites that are atop a gold atom in the second layer (which we refer to as hcp hollow sites), and the sites atop a gold atom in the third layer (fcc hollow sites). Bridge sites are the positions between top sites. Our findings for the pin-stripe pair separation of about 4 Å and the shift of the intrarow corrugation of about 1 Å are basically consistent with three different binding configurations for which one would expect a stripe-pair separation of 4.3 Å and a parallel displacement of 0.8 Å: (i) As a first possibility, they would be consistent with the binding of the sulfur atoms in one stripe to on-top positions and in the other stripe to hcp hollow sites. (ii) A second configuration of the S-Au binding sites consistent with our measurements would be the binding of one molecule at an hcp hollow site and the other molecule at an fcc hollow site. (iii) The third possibility for the binding sites would be at on-top and fcc hollow site positions. However, there is no consistent configuration involving bridge sites. For sp hybridization of the sulfur atom, both Sellers et al.11 and Beardmore et al.29 find the hcp hollow site to be energetically favored. For sp3 hybridization of the sulfur atom, according to Sellers et al.,11 again the hcp hollow site is energetically preferred, whereas Beardmore et al.29 locate the minimum binding energy at the fcc hollow site. If one considers only the covalent binding energy, one would thus reach an energy minimum only when the sulfur atoms in the stripe pair would bind to equivalent sites. However, we can exclude this possibility from both the structural data and the different contrast pattern that is observed for each sulfur row of the stripe pair. Though it is not possible to conclusively identify the binding positions from our STM images, it is clearly observed that the sulfur-gold binding occurs at two different sites of the Au(111) surface for each stripe in a pair. The fact that one could not reach a minumum of the covalent binding energy for such mixed site schemes suggests that the overall binding energy could be minimized by a gain of van der Waals energy. A definite clarification would require further advanced quantum chemistry calculations. Structure Model. Figure 4 shows the superposition of a modeled Au(111) surface and the decanethiol stripe phase structure. In this representation, the Au(111) surface is arranged such that the bright sulfur atoms of the lower stripe pair are at on-top sites and the darker ones at hollow sites that could be of either type hcp or fcc. (27) This is a valid assumption since for a bias of +2 V applied to the sample the Fermi level of the tip is well in the HOMO-LUMO gap of the decanethiol molecule. Thus electron tunneling through the molecule should occur mainly through the front orbitals. As the HOMO and LUMO are localized around the sulfur atom, we can assign the bright spots to the binding sites of the sulfur to the Au(111) surface. (28) We found an electron density maximum separation of 3.1, 3.2, and 2.1 Å for HOMO, LUMO, and the total charge density, respectively. The calculations were performed with PM3 (see ref 25). (29) Beardmore, K. M.; Kress, J. D.; Bishop, A. R.; Grønbech-Jensen, N. Synth. Met. 1997, 84, 317.

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Figure 4. Overlay of the Au(111) surface on the pin-stripe structure shown in Figure 3. Along the stripe direction there is perfect match of the Au(111) layer with the sulfur sites; perpendicular to the stripe direction the mismatch is about 3%.

Figure 4 clarifies the shift of the corrugation pattern of adjacent stripe pairs that is measured in Figure 2 to be 2.5 Åsexactly half the sulfur nearest neighbor distance along the stripes. Since we observe the same contrast pattern in each pair of parallel orientated stripes, we conclude that the alkanethiols are bound to equivalent Au(111) sites in corresponding stripes. This shift of half the Au(111) next nearest neighbor distance implies that the periodicity of the stripe pattern must be an (n + 1/2) multiple (n being an integer) of the Au(111) nearest neighbor distance of 2.884 Å. Our measurement for the stripe spacing gives a value of 34.2 Å (Figure 1), which is 11.85 times the Au(111) nearest neighbor distance. The deviation to the (n + 1/2) multiple is also visible in Figure 4 as a mismatch of the Au(111) surface overlay with the upper stripe pair. The closest integer value for n corresponding to our measurements would be 11, leading to an interrow spacing of 11.5 × 2.884 Å ) 33.2 Å. The deviation to this value is about 3%. Such a deviation could easily stem from calibration errors due to noncorrectable distortion of the scans that can be caused by creep of the piezo scanner and can reach up to 5%. Our interpretation is further supported by an analysis of the FFT spectrum for a large area scan that includes two differently orientated domains of the stripe structure. In this case, the FFT spectrum of the original scan can be corrected for distortion since the angle between the two orientations (60°) and the periodicity along the stripes (4.996 Å) is well-known. In the corrected FFT spectrum (depicted in Figure 5) the unit mesh is indicated for the two domains. This figure demonstrates unambiguously that the unit mesh is monoclinic. For the angle γ we measure 94.5 ( 0.5°; with the given value of 4.996 Å for the unit vector a we can estimate b to 33.1 ( 0.2 Å. Our result for the stripe phase periodicity is in excellent agreement with recent findings of LEED studies14 that revealed a 33.2 Å spacing. The c(23 × x3) surface unit cell given in that work can be redrawn and is then equivalent to ours, i.e., monoclinic. Earlier He diffraction studies16 gave a value for the stripe spacing of 11 ( 0.5 times the Au(111) unit vector corresponding to 31.7 ( 1.4 Å. Within the error limits also this result is in agreement with our value for the stripe periodicity. However, it should be noted that the authors state that their structure exhibits a rectangular surface mesh, in contradiction to

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Figure 6. Real space structure model for the 11.5 × x3 pinstripe structure of decanethiol.

Figure 5. (a) Large area STM current image of 54.5 × 52.5 nm that shows two domains of the pin-stripe phase. The FFT spectrum presented in (b) is corrected for distortion using the given intrarow periodicity of 4.996 Å and the 60° angle between the two orientations of the stripe phase. It reveals a monoclinic unit mesh for the stripe phase.

our findings. In the studies quoted above, it was discussed that decanethiols can only form stripe structures of this spacing if they adsorb as disulfides; otherwise, there would be an overlap. In fact, the spacing of 33.2 Å is shorter than twice the van der Waals length of the decanethiol molecule (16.95 Å16). If one takes the lateral distance for the stripe pair of about 4 Å and the van der Waals radius for sulfur of 1.85 Å into account, a perpendicular orientation of the alkyl chains to the stripe would result in a overlap of approximately 1.5 Å. However, our images undoubtedly show that the alkyl chains are not exactly perpendicular to the stripe direction, but exhibit a slight inclination of approximately 4-5°. On the basis of this observation, we modeled a structure for the 11.5 × x3 stripe structure, which is depicted in Figure 6. We found that a tilt of 4.5° results in interdigitated chain end groups and eliminates the overlap. The model consists of a lamella structure with a monoclinic surface mesh that is skewed by 4.3° and contains two molecules. The molecular

surface density corresponds to 82.8 Å2 per molecule, just 26.1% of the saturation surface density in the (x3 × x3)R30° structure. Our STM images reveal that for the 11.5 × x3 stripe structure the alkyl chains are aligned along the substrate surface. In contrast, for the densely packed (x3 × x3)R30° structure the chains are directed upright with a tilt of 30° against the substrate normal. This observation is somehow surprising, since one would expect for chemisorbed molecules that the tilt angle is determined by the chemical bond to the substrate. Two possible reasons are: First, the force to change the bend angle around the sulfur atom is low. The minimization of the van der Waals energy for the flat conformation exceeds the bending energy of the molecule and thus the molecules adopt for low molecular surface densities the stripe phase structure with the alkyl chains lying flat on the substrate surface. Second, the S-Au bond is not located at the hcp hollow site, as assumed to be the case for the (x3 × x3)R30° phase, but at the fcc hollow site and the top site, respectively. It could be possible that for these positions the sulfur atom hybridizes differently than for the hcp hollow site and the bend angle around the sulfur favors in these cases the alkyl chains lying flat on the substrate surface. Another surprising observation for the stripe structure is the spacing between sulfur atoms of a pin-stripe pair: It is lower than the nearest neighbor distance observed for the (x3 × x3)R30° highest molecular surface density structure for alkanethiols. It is conceivable that the upright standing alkyl chains do not allow closer packing, because they would otherwise overlap. Such steric effects do not exist for the flat lying chains that bend to opposite directions from the double row sulfur backbone. Conclusions In conclusion, we have presented the first submolecular resolution STM images of a decanethiol self-assembled stripe structure on Au(111). We show that the structure is formed by decanethiol monomers rather than dialkyl disulfides, with the alkyl chains lying flat on the substrate surface. We ascribe the flat lying conformation to an effect of minimization of the overall van der Waals and covalent binding energy in low molecular surface density alkanethiol self-assembled monolayers.

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Along with FFT analysis the STM data reveal a 11.5 × x3 stripe structure, in agreement with recent LEED results. The two-dimensional surface cell is monoclinic and consists of two molecules. Our high-resolution STM images allow definition of a real space structure model for the 11.5 × x3 stripe pattern. The unit cell vectors of the model structure are a ) 4.99 Å, b ) 33.26 Å, and the angle

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δ ) 94.7°. This results in a surface molecular surface density of 82.8 Å2/molecule. Acknowledgment. This work has been supported by the Bundesministerium fu¨r Forschung und Technologie (contracts 13N6362 and 13N7169). LA980717O