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Electronic and Geometric Characterization of the L-Cysteine Paired-Row Phase on Au(110) Albano Cossaro,† Silvana Terreni,‡ Ornella Cavalleri,‡ Mirko Prato,‡ Dean Cvetko,†,§ Alberto Morgante,†,| Luca Floreano,*,† and Maurizio Canepa‡ CNR-INFM Laboratorio Nazionale TASC, BasoVizza SS-14, Km 163.5, I-34012 Trieste, Italy, CNISM and Department of Physics, UniVersity of GenoVa, GenoVa, Italy, Department of Physics, UniVersity of Ljubljana, Ljubljana, SloVenia, and Department of Physics, UniVersity of Trieste, Trieste, Italy ReceiVed June 26, 2006. In Final Form: October 5, 2006 We have studied the vapor-phase deposition of L-cysteine on the Au(110) surface by means of synchrotron-based techniques. Relying on a comparison with previous X-ray photoemission analysis, we have assigned the fine structure of the C K-shell X-ray absorption spectra to the nonequivalent carbon bonds within the molecule. In particular, the C1s f σ* transition, where the σ* state is mainly localized on the C-S bond, is shifted well below the ionization treshold, at ∼ -5 eV from the characteristic π* transition line related to carboxylic group. From the polarization dependence of the absorption spectra in the monolayer coverage range, the molecules are found to lay flat on the surface with both the C-S bond and the carboxylic group almost parallel to the surface. We performed in situ complementary surface X-ray diffraction, SXRD, measurements to probe the rearrangement of the Au atoms beneath the L-cysteine molecules. Since the early stage of deposition, L-cysteine domains are formed which display an intermediate fourfold symmetry along [001]. The self-assembly of molecules into paired rows, extending along the [11h0] direction, is fully compatible with our observations, as has been reported for the case of D-cysteine molecules grown on Au(110) [Ku¨hnle, A. et al. Phys. ReV. Lett. 2004, 93, 086101.]
* Corresponding author. Fax: +39-040-226767. E-mail: floreano@ tasc.infm.it. † CNR-INFM Laboratorio Nazionale TASC. ‡ University of Genova. § University of Ljubljana. | University of Trieste.
theoretical14 and experimental developments on enantiospecific adsorption.15,16 Indeed, cysteine adsorption on Au(110) turns out to be an ideal system to study the interplay between moleculesubstrate and molecule-molecule interactions when multifunctional, chiral molecules are considered.12,15 The bare Au(110) surface undergoes spontaneous reconstruction according to the well-known (1 × 2) missing-row structure. Even weak interactions with adsorbates are known to induce large displacements of surface atoms.17 In addition, mobility of the large gold atoms can favor the ordering of the organic overlayer even at room temperature.18 In the case of D-cysteine, paired molecular rows extending along the [11h0] substrate direction have been identified in recent scanning tunneling microscope (STM) investigations.15 At present, density functional theory (DFT) calculations support a model where neutral D-cysteine molecules adsorb in an apparently flat geometry on the surface. In the model, the backbone of every molecule within a row is oriented across the substrate missing rows so that the carboxylic (COOH) group of each molecule faces the carboxylic group of the molecule in the adjacent row.15 In a recent investigation,12 we showed that L-cysteine, at submonolayer coverage, gives rise to a reflection high-energy electron diffraction (RHEED) pattern, that witnesses the formation of an intermediate coverage stage with fourfold periodicity along
(1) Barlow, S. M.; Raval, R. Surf. Sci. Rep. 2003, 50, 201. (2) Uvdal, K.; Bodo¨, P.; Liedberg, B. J. Colloid Interface Sci. 1992, 149, 162. (3) Dodero, G.; De Michieli, L.; Cavalleri, O.; Rolandi, R.; Oliveri, L.; Dacc a`, A.; Parodi, R. Colloids Surf., A 2000, 175, 121 (4) Felice, R.-D.; Selloni, A.; Molinari E. J. Phys. Chem. B 2003, 107, 1151. (5) Gooding, J. J.; Hibbert, D. B.; Yang, W. Sensors 2001, 1, 75. (6) Lee, S. B.; Martin, C. R. Anal. Chem. 2001, 73, 768. (7) Love, J. C.; Estroff, L. A.; Kriebel J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103. (8) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151. (9) For a comprehensive list of references, see Beerbom, M.; Gargagliano, R.; Schlaf, R. Langmuir 2005, 21, 3551. (10) Cavalleri, O.; Gonella, G.; Terreni, S.; Vignolo, M.; Floreano, L.; Morgante, A.; Canepa, M.; Rolandi, R. Phys. Chem. Chem. Phys. 2004, 6, 4042. (11) Cavalleri, O.; Gonella, G.; Terreni, S.; Vignolo, M.; Pelori, P.; Floreano, L.; Morgante, A.; Canepa, M.; Rolandi, R. J. Phys.: Condens. Matter 2004, 16, S2477.
(12) Gonella, G.; Terreni, S.; Cvetko, D.; Cossaro, A.; Mattera, L.; Cavalleri, O.; Rolandi, R.; Morgante, A.; Floreano, L.; Canepa, M. J. Phys. Chem. B 2005, 109, 18003. (13) Ku¨hnle, A.; Linderoth, T. R.; Hammer, B.; Besenbacher, F. Nature (London) 2002, 415, 891. (14) Sljivancanin, Z.; Gothelf, K. V.; Hammer, B. J. Am. Chem. Soc. 2002, 124, 14789. (15) Ku¨hnle, A.; Molina, L. M.; Linderoth, T. R.; Hammer, B.; Besenbacher, F. Phys. ReV. Lett. 2004, 93, 086101. (16) Greber, T.; Sljivancanin, Z.; Schillinger, R.; Wider, J.; Hammer, B. Phys. ReV. Lett. 2006, 96, 056103. (17) Cossaro, A.; Cvetko, D.; Bavdek, G.; Floreano, L.; Gotter, R.; Morgante, A.; Evangelista, F.; Ruocco, A. J. Phys. Chem. B 2004, 108, 14671. (18) Prato, S.; Floreano, L.; Cvetko, D.; De Renzi, V.; Morgante, A.; Modesti, S.; Biscarini, F.; Zamboni, R.; Taliani, C. J. Phys. Chem. B 1999, 103, 7788.
1. Introduction Self-assembled monolayers (SAMs) of amino acids are extensively investigated, aiming at the possibility of introducing sophisticated functionalities at metal surfaces.1 Cysteine [HSCH2-CH(NH2)-COOH] is capable of forming chemical bonds with metal surfaces, and in particular with gold, through its thiol side chain.2-4 Cysteine is often involved in the modification of electrodes for recognition purposes5 and immobilization of peptides and proteins at surfaces and is important in the fabrication of hybrid devices.6 In addition, the cysteine-gold interaction is linked to other thiol-bonded molecules, and this study can, at the same time, shed light and rely on a wide range of investigations, for example, those on the formation of alkanethiolate SAMs.7,8 Following early research,9 we first concentrated on cysteine adsorption from solution.10 More recently, we moved to investigate cysteine self-assembly from the gas phase, under ultra-high-vacuum conditions.11,12 The report of chiral recognition effects13 for cysteine adsorption on Au(110) has stimulated
10.1021/la061833r CCC: $33.50 © 2006 American Chemical Society Published on Web 11/15/2006
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the [001] direction. The RHEED pattern was related to the pairedrow phase reported by STM for the D-cysteine case.15 Highresolution X-ray photoemission spectroscopy (HRXPS) measurements, taken under the same experimental conditions of RHEED, showed that the fourfold phase is most likely formed by molecules in the zwitterionic form [HS-CH2-CH(NH3+)COO-].12 In this article, we advance a model of this adsorbateinduced phase, derived from combination of surface X-ray diffraction (SXRD) and near edge X-ray absorption fine spectroscopy (NEXAFS) at the C K-edge. SXRD measurements offer new insight into the coverage evolution of the cysteine/Au interface, allowing a detailed comparison with STM investigations.15 State-of-the-art NEXAFS spectra, obtained as a function of photon polarization, display a rich structure with sharp preedge features, not reported in the literature studies of cysteine (that focused on the solid phase only19,20) and alkanethiols as well.21 We discuss these new features in terms of the adsorption configuration, through correlation with our previous HRXPS study12 and in the framework of the building-block principle.22,23 2. Experimental Section Measurements have been performed at the Aloisa beamline of the Elettra Synchrotron (Trieste, Italy).24 A grating-crystal monochromator coupled to a wiggler/undulator insertion device yields 1011-12 photon/s in the 140-8000 eV energy range. The experimental chamber hosts several rotatable detectors for both photons and photoelectrons in ultra-high-vacuum conditions (1 × 10-10 mbar).24,25 A specially designed six degrees of freedom manipulator allows sample positioning and orientation with a reproducibility better than 10 µm and 0.005°, respectively. The sample temperature can be varied in the 150-1200 K range. The Au(110) surface has been prepared by low-energy Ar+ sputtering (1 keV) and annealing up to ∼700 K, where the (1 × 2) f (1 × 1) order-disorder phase transition takes place,26 as checked by RHEED. This procedure yields the largest domains of the (1 × 2)-Au(110) missing-row phase. Organic molecular beam deposition of L-cysteine has been accomplished by means of a differentially pumped source. The Knudsen PBN crucible has been typically operated at ∼400 K corresponding, under our geometric conditions, to deposition rates of ∼0.1 ML/min. The sample temperature during deposition has been kept in the range 300-350 K. HRXPS spectra have been routinely taken to check the cleanliness of the substrate before deposition and the cysteine coverage, calibrated by measurement of the S2p spectrum, according to the guidelines outlined in ref 12. HRXPS was also used to characterize the X-ray beam irradiation damage of the adsorbed phase.12 Closely similar XPS and diffraction patterns have been obtained by room-temperature deposition followed by postgrowth annealing to 350 K. SXRD patterns have been collected at fixed grazing incidence (0.4°), using an energy-resolved Si photodiode detector (Eurysis). Radial scans across the in-plane diffraction peaks have been measured in order to monitor the evolution of the substrate reconstruction for different stages of deposition. The radial scans were taken by scanning (19) Kaznacheyev, K.; Osanna, A.; Jacobsen, C.; Plashkevych, O.; Vahtras, O.; A° gren, H.; Carravetta, V.; Hitchcock, A. P. J. Phys. Chem. A 2002, 106, 3153. (20) Zubavichus, Y.; Shaporenko, A.; Grunze M.; Zharnikov, M. J. Phys. Chem. A 2005, 109, 6998. (21) Zharnikov, M.; Grunze, M. J. Phys.: Condens. Matter 2001, 13, 11333. Zharnikov, M.; Frey, S.; Heister, K.; Grunze, M. J. Electron Spectrosc. Relat. Phenom. 2002, 124, 15. (22) Sto¨hr, J. NEXAFS Spectroscopy; Springer-Verlag: Berlin, 1992. (23) Hasselstro¨m, J.; Karis, O.; Weinelt, M.; Wassdahl, N.; Nilsson, A.; Nyberg, M.; Petterson, L. M. G.; Samant, M. G.; Sto¨hr, J. Surf. Sci. 1998, 407, 221. (24) Floreano, L.; Naletto, G.; Cvetko, D.; Gotter, R.; Malvezzi, M.; Marassi, L.; Morgante, A.; Santaniello, A.; Verdini, A.; Tommasini, F.; Tondello, G. ReV. Sci. Instrum. 1999, 70, 3855; a presentation of the beamline can be found at http://www.tasc.infm.it/research/aloisa/scheda.php. (25) Gotter, R.; Ruocco, A.; Morgante, A.; Cvetko, D.; Floreano, L.; Tommasini, F.; Stefani, G. Nucl. Instrum. Methods Phys. Res., Sect. A 2001, 467-468, 1468. (26) Cvetko, D.; Lausi, A.; Morgante, A.; Tommasini, F.; Prince, K. C. Surf. Sci. 1992, 269/270, 68.
Figure 1. From top to bottom: S2p, C1s, and N1s XPS spectra taken at a coverage of ∼1 ML with a photon energy of 500 eV. Raw spectra are shown without background subtraction. The spectra taken from a freshly prepared surface (open circles) are compared with spectra taken after seven consecutive NEXAFS measurements at the C K-shell (filled circles). The spectra from the pristine SAM are vertically shifted by a constant offset for the sake of clarity. the photon energy, i.e., the parallel momentum transfer, at fixed scattering geometry. Azimuthal scans at fixed photon energy of 7 keV have been also measured across the in-plane fractional order peaks to check the domain correlation length along the substrate [11h0] direction. To probe the vertical rearrengement of the Au atoms beneath the cysteine overlayer, we have also taken out-of-plane XRD data at 7 keV, by scanning the rods of some integer and fractional peaks at suitable coverage. NEXAFS measurements (at the carbon K-edge) have been taken in partial electron yield mode by means of a channeltron electron multiplier equipped with a high pass filter set to 200 eV. The sample has been kept at a photon grazing incidence of 6°, varying the polar and azimuthal orientation of the surface with respect to the direction of the linearly polarized electric field. An absolute calibration of the photon energy has been obtained by collecting absorption spectra of the C1s f π* transition at 287.40 eV from CO gas phase in the Aloisa ionization cell.24 Like many organic molecules, the cysteine SAMs are subject to radiation damage, when probed by soft X-rays. In particular, the irradiation causes fast desorption of weakly bound molecules and slower damaging of the SAM, ascribed to molecular fragmentation.12 In general, the cysteine SAM on the (110) surface of Au demonstrated greater stability than on the (111) surface against both irradiation and thermal annealing (compare refs 10 and 11 with ref 12). Due to the large number of secondary electrons produced at the ionization threshold, NEXAFS experiments are more damaging than XPS and XRD ones (here, we perform XRD at a photon energy lower than the Au L-shell ionization threshold). In Figure 1, we show the HRXPS spectra taken from a freshly prepared surface together with those taken after the measurement of polar-dependent NEXAFS spectra at the C K-shell (seven spectra, taking 12 min for each one). As can be seen, the S2p component at 161.2 eV due to cysteine decomposition11 is just starting to appear in the irradiated sample. In the C1s spectrum, a slight decrease of the COO- component at 288.2 eV can be detected. Finally, the N1s component of neutral cysteine at 399.5 eV is slightly enhanced in the irradiated sample. The very small variations observed in the C1s and N1s spectra are possibly due to irradiation-induced evaporation of weakly bound zwitterionic
Characterization of L-Cysteine on Au(110)
Figure 2. In-plane XRD scans as a function of the parallel momentum transfer q| (h, k ) q|/bh,k with bh,k being the Au(110) surface reciprocal lattice parameters). (a) Radial scans along the [001] direction for increasing exposure of cysteine. The indicated coverage was calibrated after XPS measurements according to the guidelines of ref 12. The intensity of quarter-integer peaks increases with coverage. The position slightly shifts, whereas the half-integer peak gradually disappears. (b) Deconvolution into two components of the (0, -3/2) peak profile measured at ∼3/4 ML along the [001] direction. (c) (0, -7/4) peak profile along the [1-10] direction at ∼3/4 ML. molecules.12 In any case, the resulting overall shape changes are too small to affect the NEXAFS data analysis and the related interpretation.
3. Results and Discussion 3.1. In-Plane XRD. Radial scans in the [001] direction were measured as a function of exposure, to enlighten the lifting of the (1 × 2) reconstruction and obtain quantitative information on the assembly of the adsorbate-induced phase.12 Representative scans are shown in Figure 2(upper panel). Within the coverage range spanned in the figure, the molecules were found to be strongly bound to gold through their sulfur termination.12 Quarter-integer diffraction peaks were detected already in the early stages of deposition. While their intensities increase with coverage, the intensities of the half-integer peaks decrease. The half-integer peaks display two subcomponents, as is shown in detail in panel b of Figure 2. The narrow component retains the same width as the bare surface peak up to disappearance. The corresponding correlation length, ∼500 Å along the [001] direction, is attributed to uncovered regions of the substrate, which preserve the correlation of the missing-row long-range order. The broad component is associated with the domains of the fourfold L-cysteine phase. The fourfold periodicity fits the spacing of the paired-row phase observed by STM for D-cysteine molecules.15 The average correlation length of the fourfold domains in the [001] direction amounts to Λ×4 ≈ 32-35 Å, as estimated from the width of quarter-integer peaks and of the broad component of half-integer peaks. This estimate fits very well to the lateral width of two pairs of cysteine rows, as previously reported by STM.15 From the width of azimuthal scans of the quarter-integer peaks (an example is reported in panel c of Figure 2), a correlation length of 200 Å is found along the [11h0] direction, again in excellent agreement with STM measurements. We can therefore infer that L-cysteine forms long molecular rows elongated in the [11 h0]
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Figure 3. NEXAFS scans (at the carbon K-edge) measured for transverse electric polarization at ∼1 ML. The sample has been kept at a photon grazing incidence of 6°, varying the azimuthal orientation of the surface with respect to the direction of the linearly polarized electric field.
direction. Moreover, the fact that we always observed an even number of rows within the domains suggests that the rows of L-cysteine are coupled in pairs (this is also consistent with the reported formation of chiral molecular pairs already in the early deposition stage13). As the coverage approaches the ML, the half-integer peaks weaken; the quarter-integer peaks weaken as well, but at the same time, they also shift toward the closest integer peak. We have verified that, for exposure exceeding the monolayer, all the surface-related diffraction peaks rapidly vanish. The fourfold diffraction pattern found at submonolayer coverage corresponds to an intermediate deposition stage where the domains formed by two chains of coupled cysteine rows coexist with uncovered surface domains. The gold rows of the Au (1 × 2) structure apparently act to laterally constrain the cysteine chains. At intermediate coverage, the adsorbed phase appears practically commensurate with the substrate. We interpret the shift of the quarter-integer peaks in terms of a partial release of the compressive strain within the cysteine domains. Our findings suggest the tendency toward a poorly ordered overlayer structure, most likely incommensurate, as soon as the whole substrate surface gets completely covered. Note that the width of quarter-integer peaks remains practically unchanged, indicating that the correlation length of the adsorbatecovered domains along the [001] direction never exceeds the typical lateral extent of two pairs of coupled rows (e.g., four cysteine rows) also in the incommensurate stage. This fact suggests that the underlying gold layer inhibits a full relaxation of the cysteine overlayer. 3.2. NEXAFS. To understand the orientation of the cysteine backbone with respect to the Au(110) surface, we measured absorption spectra at the carbon K-edge as a function of the surface orientation with respect to the linearly polarized X-ray beam. In Figure 3, we show an extended set of azimuthal scans measured at fixed, transverse electric polarization (TE or s-polarization) on a high-coverage phase (almost 1 ML) displaying fourfold periodicity, after mild annealing at 350 K. Note that, in this coverage regime, the double-lobe pair structure, characterized at low coverage in ref 13, has a negligible weight, as nicely demonstrated in the STM pictures of ref 15. All the spectra display features which are common to all R-amino acids:19,20 a sharp resonance at 288.6 eV (which will
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Figure 4. Comparison between the NEXAFS spectra taken at ∼1 ML in transverse magnetic, TM, and transverse electric, TE, polarization (filled and open circles, respectively). Both spectra have been taken at a fixed grazing angle of 6° with the photon beam along the [001] direction. The deconvolution of the spectra into individual components is also shown (see text for details); the energy position of the deconvoluted peaks is reported in the inset table.
be denoted as C4 in the following), a steplike behavior in the 286-292 eV region, and a relatively broad peak at about 290291 eV (C5). The narrow resonance C4 is the well-known π* transition related to the carboxyl group.19,20,27-29 The broad peak C5 is commonly assigned to a σ* (C-N) resonance.19,20 Another sharp feature is observed at 283.4 eV (C1), very far from the π* resonance, in the so-called pre-edge region. It was never observed in experiments on solid-state cysteine.19,20 A neat shoulder can be appreciated at about 287.4 eV (C3), with high intensity when the electric field is parallel to the [001] azimuth. Finally, broad and less-defined structures are appreciable in the 295-305 eV range. The set of azimuthal scans was complemented by measurements in transverse magnetic polarization (TM or p-polarization), as shown in Figure 4. By comparison with the s-polarization spectrum for the same azimuthal orientation, a neat polarization dependence can be appreciated that looks particularly sharp in the case of the peak C1 at 283.4 eV and the shoulder C3 at 287.4 eV, and even in the case of the small peak C5 at 290-291 eV. These observations clearly suggest that the molecules exhibit a preferential orientation. To obtain more insight, we attempted a fit to the spectra. According to calculations27 and after HRXPS meaurements,12 we might have expected three different ionization thresholds, associated with the nonequivalent C atoms in the molecule,22 occurring at energy positions a few electronvolts larger than, or coincident with, the π* resonance. The measurements did not allow resolution of the individual steps. According to a wellestablished analysis procedure,19 we decided to introduce a single broadened edge. After this choice, the whole ensemble of spectra were reproduced by a minimum set of eight peaks. Four peaks, of symmetric line shape,22 were used to reproduce the three pre-edge structures and the π* resonance. Simulation of the highenergy part of the spectra required four asymmetric structures.22 (27) Plashkevych, O.; Carravetta, V.; Vahtras, O.; A° gren, H. Chem. Phys. 1998, 232, 49.
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The deconvolution of the NEXAFS profiles is reported in Figure 4. The energy position of the deconvoluted peaks is reported in the inset table. Note that the intensity of C2 is very small and possibly coupled to the step edge. Therefore, the attribution of C2 to a resonance is dubious. In the absence of realistic calculations on the chemisorbed system, we attempted to assign the NEXAFS peaks relying on comparison with calculations and experiments on related systems, by applying the so-called building-block approach. Within this approximation, the NEXAFS spectrum of a large molecule can be regarded as the linear combination of the spectroscopical features characteristic of the individual chemical groups (blocks) forming the molecule.22,23 Apart from the plain assignment of C4 to the π* resonance of carboxyl, comparison with calculations,27 data on powders,20 and observations on many related systems23,30,31 gives good confidence to the assignment of the highest-energy feature, C8, to a σ*-shaped resonance stemming from the carboxyl group. This attribution is supported by a comparison of the polarization behavior of C8 and C4. As suggested by a simple inspection of the NEXAFS spectra (both polar and azimuthal plots) and confirmed by the fit, C8 displays its minimum intensity where C4 gets maximum.23 The polar polarization dependence of π* indicates that the plane of the carboxyl group is tendentially parallel to the surface. Calculations for a few “free” zwitterionic amino acids show that absorption features related to the methyl carbon (Cβ) strongly depend on the residual part of the amino acid.19,27 This fact, intuitively conceivable, is well-represented in survey studies.19,20 Regarding specifically free cysteine, two Cβ resonances were calculated below the π* peak.27 In NEXAFS experiments on powders, a well-defined shoulder was detected at 287.3 eV, the position of the C3 peak, and assigned to a σ(Cβ-S) resonance.19,20 These findings, together with the observation that C1 and C3 display the same spatial orientation (as shown in Figures 4 and 5), support the assignment of C1 and C3 peaks to σ* resonances stemming from the Cβ carbon. Indeed, the C1 separation from the π* peak looks very large. We must consider that, upon adsorption, the -SH group of the free molecules is directly involved in the molecule-surface bond12 and is likely substituted by a -SAux complex. In this respect, we observed a large photoemission chemical shift toward the low binding energy side in both the C1s and S2p XPS spectra of thiolate cysteine, as compared with weakly bound molecules.12 The XPS chemical shift was particularly large (about 1.5 eV) in the case of the spectral subcomponent assigned to the Cβ atom.12 Further, we note that density functional theory calculations of the Cys/gold interface (specific, however, to the Au(111) surface) predict a lengthening of the Cβ-S bond;4 this effect could be even more pronounced on the Au(110) surface. It seems therefore fully plausible to correlate C1 with a redistribution of charge in the C-S bond, as originated by the formation of the thiolate species. Particular conditions of screening by metal electrons due to the specific adsorption configuration of the molecule cannot be excluded as well. According to our assignment, the sharp polarization character of C1 in Figure 4 indicates that the C-S bond is parallel to the surface. The azimuthal dependence of C1, (28) Tzvetkov, G.; Koller G.; Zubavichus, Y.; Fuchs, O.; Casu, M. B.; Heske, C.; Umbach, E.; Grunze, M.; Ramsey, M. G.; Netzer, F. P. Langmuir 2004, 20, 10551. (29) Petoral, R. M.; Uvdal K. J. Phys. Chem. B 2003, 107, 13396. (30) Jones, G.; Jones, L. B.; Thibault-Starzyk, F.; Seddon, E. A.; Raval, R.; Jenkins, S. J.; Held, G. Surf. Sci. 2006, 600, 1924. (31) Zubavichus, Y.; Shaporenko, A.; Grunze M.; Zharnikov, M. J. Phys. Chem. B 2006, 110, 3420.
Characterization of L-Cysteine on Au(110)
Figure 5. Azimuthal dependence of the C1, C3, and C5 NEXAFS peaks, as obtained from the fit to the spectra of Figure 3.
reported in Figure 5, suggests that the C-S bonds of most of the molecules are likely oriented about 30-40° from the [001] direction. The identification of the spectral counterparts of the CR-N bond is not straightforward. Experiments on solid-state cysteine locate a σ* (CR-NH3+) resonance at about 290-291 eV,19,20 in good agreement with the C5 peak. Such an assignment would also be consistent with findings on related systems.32,33 It appears therefore reliable to assign C5 to the C-N bond, which is oriented at 10-20° from the [11h0] direction, according to its azimuthal dependence (Figure 5). Some caution on such a conclusion is suggested from calculations of ref 27, which indicate contributions of the CR bond to pre-edge intensity as well, eventually partially affecting the C3 intensity. Further, analysis on a related system (glycine/Cu(110)) suggests for C5 a possible overlapping of C-N and C-C features.23 It is interesting to note that C3 and C5 present the same sharp polar dependence in Figure 4. Therefore, independent from the actual assignment to C3 or C5, we can conclude that the C-N bond is parallel to the surface. The determination of the intensity and energy position of C6 and C7, in particular, C6, is critical, since they are strongly coupled to the position and intensity of the step edge feature. Considering their large width and energy position, C6 and C7 can be assigned to σ*-shaped resonances, most likely associated with the C-C bonds.19,20,23,31,32 It is not possible to obtain clearcut information from the azimuthal dependence of C6 and C7, while we observe that C6 displays a larger spectral weight in the direction perpendicular to the surface. The corresponding bond orientation suggests assignment of this structure to the Cβ-CR bond. In fact, if we consider that both C-S and C-N are parallel to the surface and admit that the molecule is not distorted too much upon adsorption,4 the Cβ-CR bond must point out of the surface. Our interpretation of NEXAFS findings implies a flat adsorption geometry, in agreement with STM suggestions.15 A (32) Messer, B. M.; Cappa, C. D.; Smith, J. D.; Wilson, K. R.; Gilles, M. K.; Cohen R. C.; Saykally, R. J. J. Phys. Chem. B 2005, 109, 5375. (33) Gordon, M. L.; Cooper, G.; Morin, C.; Araki, T.; Turci, C. C.; Kaznacheev, K.; Hitchcock, A. P. J. Phys. Chem. A 2003, 107, 6144.
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Figure 6. Top and side views of a three-dimensional model of the paired-row phase of L-cysteine on Au(110). Hydrogen atoms are omitted for clarity. An unreconstructed (1 × 1) substrate as been considered (no substrate relaxation has been considered in this simple model).
representative model is presented in Figure 6. We have assumed that the C-N bond is associated with the C5 NEXAFS features, of which the azimuthal orientation is at about 10-20° from the [11h0] direction (see Figure 5). In the figure, we have assumed the molecular configuration, as calculated for the free molecule in ref 4. Some subtle adsorption-induced strains may be expected,16 also, in view of recent findings of vibrational spectroscopy experiments in the solid phase.34 The sulfur adsorption site, purely indicative, has been chosen in accordance with the model of ref 15. In the most stable configuration identified in DFT calculations of ref 15, where neutral apolar molecules were considered, cysteine pairs were found to interact through OsH‚‚‚O hydrogen bonds, involving the carboxyl groups. From the discussion of NEXAFS data presented above, it appears difficult to extract a clear-cut fingerprint of the molecular charge state, although the C5 peak was identified as the marker of the CR-NH3+ bond in a recent survey on solid-state R-amino acid samples.20 Rather, our model relies on our recent highresolution XPS investigation, where the fourfold phase was associated with the zwitterionic cysteine moiety.12 Note that the zwitterionic form was also invoked to interpret a recent experiment on UHV-deposited SAMs of L-cysteine on Au(111).35 Considering zwitterionic molecules, in Figure 6 the COOgroup of a molecule faces the NH3+ group of both the opposite molecule in the adjacent row and the consecutive molecule in the same row. The geometry of Figure 6 suggests a correlation between the zwitterionicity and the orientation of L-cysteine in the molecular network. Note that L-cysteine molecules exhibit zwitterionic character in both known solid-state phases, e.g., the monoclinic and orthorombic phases. NsH‚‚‚O hydrogen bonds between adjacent molecules play a major role in the crystal packing.34,36,37 (34) Pawlukojc, A.; Leciejevicz, J.; Ramirez-Cuesta, A. J.; Nowicka-Scheibe, J. Spectrochim. Acta 2005, A61, 2474. (35) Shin, T.; Kim, K.; Lee, C.; Shin, S. K.; Kang, H. J. Phys. Chem. B 2003, 107, 11674. (36) Go¨rbitz, C. H.; Dahlus, B. Acta Crystallogr. 1996, C52, 1756. (37) Moggach, S. A.; Allan, D. R.; Clark, S. J.; Gutmann, M. J.; Paarsons, S.; Pulham, C. R.; Sawyer, L. Acta Crystallogr. 2006, B62, 296.
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Finally, it is worthwhile to mention that, in ref 15, and in Figure 6 as well, the L-cysteine domains are taken in registry with an unreconstructed (1 × 1) substrate. However, the periodicity observed in the SXRD radial scans of Figure 2 (which probe the substrate atoms) support an adsorbate-induced reconstruction of the substrate beneath the molecules. In fact, we also collected out-of-plane SXRD data (rod scans) for some fractional and integer peaks at the intermediate fourfold stage at 3/4 ML (data not shown). From a first analysis, where we assumed a 20-25% residual (1 × 2) missing-row phase, we found that the rod scans of both bulk and reconstruction peaks clearly require the lifting of the missing-row reconstruction for substrate atoms beneath the molecules. However, a simple bulk-terminated (1 × 1) Au geometry proved inadequate to fit the rod scans and one has to consider the relaxation of the Au atoms within the cell. A rather good agreement with the rod scans was obtained by introducing correlated lateral displacements of the atoms, according to the chiral coupling of the L-cysteine in opposite rows. Due to the large number of fitting parameters, this analysis cannot yield reliable details of the substrate reconstruction; rather, we captured an overview of the topmost layer atomic rearrangement that resulted in consistency with the geometry of the molecular phase.
4. Summary We have followed the evolution of L-cysteine domains grown on Au(110) by vacuum deposition. From a morphological point of view, the domain symmetry and correlation length are consistent with the formation of molecular rows extending along the [11h0] direction. The rows are always coupled in pairs yielding a quasi-fourfold periodicity along the [001] direction. By studying the polarization dependence of NEXAFS at the carbon K-shell,
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we have associated the spectroscopic features with molecular bonds. In particular, a very large shift to lower binding energy has been found for the σ* symmetry molecular orbital of the C-S bond (-5 eV with respect to the π* resonance at 288.6 eV). According to our multitechnique investigation, we propose a geometrical model, where cysteine molecules bind to the substrate through their thiol group and the azimuthal orientation is correlated with their charge state, as derived by former XPS measurements.12 The molecular-row phase is assumed to be made of zwitterionic molecules azimuthally oriented in such a way that the carboxyl group COO- of one molecule in a row is adjacent to the NH3+ group of both one consecutive molecule in the same row and its chiral twin molecule in the adjacent row (180° azimuthal relative orientation). Further investigation is necessary for a full assessment of the phase structure. In particular, detailed checks of the subtle rearrangements of Au atoms beneath the molecular rows, suggested by preliminary rod scan measurements, would be highly desirable, as well as supplementary studies to definitely assess the charge state of the molecules. Acknowledgment. This project has been cofinanced by the University of Trieste, the University of Genova, and MIUR (PRIN 2003028141). Financial support from FIRB “Molecular NanoDevices” is also acknowledged. We thank Ranieri Rolandi and Lorenzo Mattera for their continuous support to the experiment and for discussions. Antonio Gussoni (CNR-IMEM) and Paolo Pelori are warmly acknowledged for help in the realization of the differentially pumped source. Stefano Amore is acknowledged for assistance in the early stages of the experiments. LA061833R
