Very Low Energy Vibrational Modes as a Fingerprint of H-Bond

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J. Phys. Chem. C 2008, 112, 14439–14445

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Very Low Energy Vibrational Modes as a Fingerprint of H-Bond Network Formation: L-Cysteine on Au(111) V. De Renzi,*,†,‡ L. Lavagnino,§ V. Corradini,‡ R. Biagi,†,‡ M. Canepa,§ and U. del Pennino†,‡ Dipartimento di Fisica, UniVersità di Modena e Reggio Emilia, V. Campi 213/A, Modena, Italy, CNR-INFM Center for nanoStructures and bioSystems at Surfaces, V. Campi 213/A, Modena, Italy, and CNISM and Dipartimento di Fisica dell’UniVersità di GenoVa, Via Dodecaneso 33, 16146 GenoVa, Italy ReceiVed: March 13, 2008; ReVised Manuscript ReceiVed: June 6, 2008

The ultrahigh vacuum adsorption of cysteine layers on the Au(111) surface has been studied by means of X-ray photoelectron (XPS) and high-resolution energy loss spectroscopies (HREELS). Room-temperature deposition determined the formation of a quite heterogeneous first layer, where both weakly and strongly bound molecules coexist. Deposition at a slightly higher temperature (330 K) led instead to the formation of a homogeneous, self-assembled monolayer made of molecules chemisorbed through a thiolate bond. In the latter case, HREELS measurements have been interpreted in terms of a well-organized H-bond network made of zwitterionic molecules. Two vibrational modes, denoted as N and H modes, respectively, have been identified as distinguishing features of the homogeneous monolayer obtained at 330 K. The N mode lies at 3350 cm-1 and is attributed to a stretching vibration of the N-H · · · O bond. The H mode, observed at 74 cm-1 for full monolayer coverage, is assigned to a collective vibration of the two-dimensional H-bond network. At halfmonolayer coverage, the H mode has been observed at 55 cm-1. This red-shift indicates a coverage dependence of the H-mode frequency, which clearly supports its intermolecular origin. This finding is a nice example of the extreme sensitivity of low-frequency vibrational modes to the details of molecule-molecule interactions. I. Introduction A detailed comprehension of the mechanisms of self-assembly in organic and biological systems is of great relevance in the design of novel organic-based architectures. In any attempt to exploit the bottom-up approach to engineering hybrid interfaces, direct control of noncovalent interactions is extremely important.1 To this regard, a great volume of research on twodimensional (2D) self-assembled structures has been devoted to systems where the main molecule-molecule interactions are of van der Waals and/or π-π types. Monolayers of alchilic2 and aromatic molecules3–5 are representative examples of these categories. The strong directional character of H-bond interactions is essential in driving the self-assembly of many classes of biomolecular aggregates.6 In particular, H-bond interactions are important in molecular recognition phenomena.7 Quite a few examples of H-bond exploitation in the self-assembly of 2D structures are reported in the literature.8–12 In several cases, the geometric structure of the H-bond network is dependent on both layer coverage and substrate temperature.13,14 It has been also shown that the adlayer structure can be tailored by molecular chemical modifications which favor H-bond formation.13 In most cases, the formation of H-bond networks in self-assembled monolayers (SAMs) is inferred from the evaluation of intermolecular distances, carried out by structural probes.8–11,14 On the other hand, vibrational spectroscopy could in principle provide a direct characterization of the intermolecular bond. Recently, spectroscopic fingerprints of H-bond formation in organic three-dimensional (3D) crystals have been obtained by * To whom correspondence should be addressed. † Università di Modena e Reggio Emilia. ‡ CNR-INFM Center for nanoStructures and bioSystem at Surfaces. § CNISM and Dipartimento di Fisica dell’Università di Genova.

newly developed terahertz methods,15,16 which provided access to the collective low-energy (0-100 cm-1) modes. These modes, which are usually not accessible in standard IR spectroscopy, are extremely sensitive to the local molecular environment. In the case of 2D systems, high-resolution electron energy loss spectroscopy (HREELS) should deserve special consideration. HREELS combines high surface sensitivity with the ability to detect very low energy vibrational modes, and it has been widely used for the characterization of adsorbate lattice vibration in the case of relatively simple chemisorbed systems.17–19 It is therefore tempting to exploit the potentialities of HREEL spectroscopy for the study of H-bond formation in the case of the more complex adlayers formed by biomolecules. To this regard, SAMs of amino acids can be considered as particularly well-suited and interesting case studies.8 Among fundamental amino acids, L-cysteine (Cys, HSCH2CH(NH2)COOH) is currently attracting special attention in the study of enantioselectivity processes at surfaces.20,21 Furthermore, cysteine provides the molecular hook between several biomolecules and metal surfaces.22 As shown in Figure 1, three terminal groups are present in the Cys molecule, i.e., a thiol group (SH), a carboxyl group (COOH), and an amino (NH2) group, which are available for H-bond formation. For example, the 3D structure of Cys crystals is formed by cysteine zwitterions (i.e., COO-/NH3+), which interact mainly through intermolecular N-H · · · O hydrogen bonds, though additional hydrogen bonds involving the thiol groups are also present.23 Indeed, the possible zwitterionic character of cysteine in 2D layers at surfaces is an important aspect to be investigated, as it is directly linked to the ability to form H-bond molecular networks. The adsorption of cysteine on metal surfaces has been extensively investigated. Most studies focused on gold surfaces. To this regard, layer deposition from the liquid phase,24,25 under electrochemical control11,26,27 and in an ultrahigh vacuum (UHV)

10.1021/jp802206r CCC: $40.75  2008 American Chemical Society Published on Web 08/26/2008

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Figure 1. Stick-and-ball model of the cysteine molecule in (A) the neutral and (B) zwitterionic forms. Black and light gray balls indicate C and H atoms, respectively. Sulfur, oxygen, and nitrogen are indicated in yellow, red, and blue, respectively.

environment,10,28,29

were considered. The interaction between neutral Cys molecules and the Au surface has been also examined from the theoretical point of view.30 In this paper, we report on the UHV deposition of Cys layers on the Au(111) surface. Previous studies about this system pointed out a very complex scenario. In particular, according to recent scanning tunneling microscopy (STM) investigations,31 the growth of Cys SAM at room temperature (RT) is characterized by a pronounced polymorphism, with the coexistence of several different ordered and disordered phases. The persistence of the herringbone reconstruction typical of bare Au(111) beneath the molecular layer was interpreted as due to weak molecule-substrate interaction. Interestingly, mild thermal annealing (380 K) of the RT deposited adlayer was found to induce the gold-surface deconstruction and the formation of stronger molecule-surface bonds. These findings appear in interesting contrast to what was reported for Cys adsorption on Au(110) surfaces, where molecules apparently form thiolate species already at RT and induce surface atoms mobility.10,20,28,32 In the case of annealed adlayers on the Au(111) surface, two different ordered structures were observed by STM, depending on coverage. Full compact monolayers display a local (3 × 3) geometry and a molecule-molecule distance of about 5 Å. At lower coverage, a complex striped phase, with alternate rows of different brightness was observed, perhaps related to a less dense molecular packing. The Cys/Au(111) interface therefore represents an interesting system for studying the interplay between molecule-substrate and molecule-molecule interactions in SAMs characterized by H-bond networks. In the present work, we investigated the formation of a cysteine adlayer at different temperatures, by means of XPS, ultraviolet photoemission spectroscopy (UPS), and HREELS. We found that at RT the molecule is mainly weakly adsorbed on the substrate and the adlayer is characterized by a quite heterogeneous distribution of phases. Deposition at a slightly higher temperature (330 K) determines a more homogeneous situation, where the molecule is chemisorbed through S-Au bond formation. This fact provided us with an interesting route for investigating the evolution of intermolecular vibrational modes as a function of the adlayer self-organization on the surface. The formation of a highly organized H-bond network at 330 K was clearly singled out by the appearance of specific vibrational modes in the HREEL spectra. In particular, we were able to detect an extremely low frequency mode (in the 0-100 cm-1 region), which we attribute to a lattice vibration of the H-bond network. The frequency of this mode exhibits a clear coverage dependence, and its appearance strongly depends on the adlayer homogeneity. These findings prove that the sensitivity of H-bond lattice vibrations on the detail of the

De Renzi et al. network structure can provide important clues to the properties of the network itself and more generally for the self-assembling mechanism also in the case of 2D systems. The paper is organized as follows: After a brief description of the experimental setup, the experimental results obtained by application of XPS, UPS, and HREELS are reported in Section III. Subsection IIIA deals with the characterization of the adsorption mechanism of the Cys adlayer at different temperatures. Subsection IIIB reports the results of HREELS measurements addressing the characterization of the vibrational properties of the H-bond network. Eventually, in Section IV, some conclusions and perspectives are drawn. II. Experimental Section All measurements were performed in an ultrahigh vacuum chamber with a base pressure of 7 × 10-11 mbar. The Au sample was cleaned through sputtering-annealing cycles to 730 K. L-Cysteine was evaporated in UHV, after purification, using a source already utilized in previous works28,32 and described in ref 33. Molecular deposition has been performed keeping the sample at room temperature (300 K, RT) or at 330 K. The characterization of the Cys adsorption process at different temperatures is based on the results of X-ray and ultraviolet photoemission (XPS and UPS) and high-resolution electron energy loss (HREELS) spectroscopies. XPS and UPS were performed with an unmonochromatized Mg KR source and a He I lamp, respectively. Electron energy distribution curves were detected with a hemispherical analyzer (Omicron EA125) with an overall energy resolution of 1.0 eV (0.1 eV for UPS). HREELS measurements were performed in specular conditions (θi ) θo ) 55°). The spectrometer (LK ELS5000) routinely ensured an energy resolution of around 28 cm-1 (3.5 meV). In a few measurements, the resolution was enhanced to 12 cm-1 (1.5 meV) in order to get a more detailed view of the quasielastic region of the HREEL spectra. III. Results and Discussion A. Cysteine Layer Formation. Cysteine layer formation was investigated by XPS and UPS at two different temperatures, i.e., 300 K (RT) and 330 K, and for a few values of coverage. In Figure 2, a few S 2p peaks representative of Cys deposition at 330 K (curves a and b) and RT (curves c and d) are shown. At 330 K, the intensity of the core level signal saturates as a function of dose. At the same Cys dose, valence band spectra (see inset in Figure 2) show the vanishing of the well-known gold surface state located just below the Fermi energy level.34 These findings clearly indicate that the XPS intensity saturation corresponds to the formation of a full monolayer at 330 K (ML330 in the following). The (S 2p/Au4f) intensity ratio measured at this coverage is very similar to the one measured on the well-characterized (3 × 3) methilthiolate/Au(111) phase.2 The comparison indicates a similar surface density F of sulfur atoms, i.e., F ) 1/(24) Å-2. This value corresponds to a full monolayer of molecules distributed on the surface at a mean distance of =5 Å, in agreement with previously cited STM observations.31 A detailed analysis of the S 2p core-level line shape provides important clues on the molecular adsorption processes of Cys on Au(111). In Figure 2, all curves are fitted by Voigt-doublet components of different intensities and energies. At 330 K, both spectra display a strongly predominant S1 component at 162.0 ( 0.2 eV binding energy (BE), associated in the literature to molecules chemisorbed through sulfur. Moreover, a very weak S2 component at 164.1 ( 0.2 eV BE, associated to weakly

H-Bond Network Formation

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Figure 3. X-ray photoemission spectra O 1s (A, left panel) and N 1s (B, right panel) core levels of the Cys/Au(111) system, for different coverages and deposition temperatures: (a) 0.5 ML330; (b) ML330; and (c) 1.0 MLERT. Experimental data are shown as filled symbols, while thin and thick lines correspond to the fitting components and the fitting curves, respectively.

Figure 2. X-ray photoemission spectra of the S 2p core level of the Cys/Au(111) system for different coverages and deposition temperatures: (a) 0.5 ML330; (b) 1.0 ML330; (c) 0.2 MLERT; and (d) 1.0 MLERT. Experimental data are shown as empty symbols, while thin and thick lines correspond to the fitting components and the fitting curves, respectively. In the inset, the valence band spectra near the Fermi level region are reported for the clean gold surface and increasing 330 K depositions (0.5 ML330 and 1.0 ML330), showing the progressive decrease of the clean gold surface state intensity.

bound species with intact SH groups,28,35,36 is also observed. We conclude therefore that at this temperature most of the Cys molecules chemisorb on the gold substrate through a thiolate bond. Deposition at RT temperature leads to a different scenario. Curves c and d show the S 2p core levels of 0.2 and 1.0 MLERT Cys, respectively. Here, we define one monolayer equivalent at RT (MLERT) as corresponding to the same amount of molecules per unit surface as that of the complete ML330. For low coverage, only the S1 component at 162.0 ( 0.2 eV BE is detected. With an increase of coverage, the S2 component at 164.1 ( 0.2 eV BE instead becomes predominant. In curve d, the S2 component is greater (by about a factor of 2) than the S1 one, which indicates a large contribution from weakly bound molecules. At RT, therefore, thiolate chemisorption is prevalent only at an early stage of adlayer formation, while weak adsorption becomes predominant with increasing coverage. This can be explained by considering that thiolate chemisorption preferentially occurs at steps and/or defects, while terraces are mainly covered by weakly adsorbed species. These findings were confirmed by the analysis of the C 1s core level (not shown): the spectra of the RT and 330 K monolayers showed fairly different lineshapes. The 330 K spectra were similar to those found on chemisorbed layers on the Au(110) surface.28 The

MLERT spectrum was instead characterized by a high-intensity component located at 186 eV BE, associated to the S-bonded carbon atoms of weakly bound molecules,35 and shifted by 1.2 eV with respect to the corresponding component observed at 184.8 eV in the ML330 spectrum. The results of XPS core-level analysis therefore clearly showed that for RT deposition most of the layer is formed by weakly adsorbed molecules, in agreement with the information deduced from STM observations.31 Increasing the deposition temperature to 330 K determines the formation of a thiolate chemisorbed layer. The 330 K temperature was therefore high enough to avoid the formation of weakly adsorbed layers, leading to the completion of a thiolate full monolayer. As will be shown later, these findings were also confirmed by the HREELS spectra (see Figure 4). A comprehensive characterization of the adlayer needs also to establish the molecular chemical state. Cysteine is known to adsorb mainly in a zwitterionic state on several surfaces, such as, for example, Au(110) and Cu(100).10,28,37 The adsorption of neutral molecules was reported on the Pt(111) surface.38 Information on the molecular charge state can be derived by a combined analysis of the XPS oxygen and nitrogen core levels and of the HREEL vibrational spectra. O 1s and N 1s spectra representative of RT and 330 K deposited layers are shown in Figure 3, together with their best-fit curves. The core-level shapes appear essentially the same for all coverages and deposition temperatures. Concerning O 1s, on the left panel, the largely predominant component at 531.3 ( 0.2 eV BE (O1) is assigned to oxygen in the carboxylate group (COO-).39 The low-intensity component at 532.5 ( 0.2 eV BE (O2) is assigned to the two oxygens of the carboxylic group (HO-CdO). The N 1s spectra (right panel of Figure 3) present only one relatively broad feature at 401.5 eV, which is assigned in the literature to the NH3+ group.28,35,39 No evidence of a component due to the NH2 group, which is usually observed at around 399.5 eV,28,35 was found for any deposition temperature and coverage. On the whole, XPS analysis indicated the cysteine zwitterionic form (i.e., COO-/NH3+), though a minor percentage of COOH groups was likely present on the surface. These findings are substantially confirmed by HREEL spectra in the so-called fingerprint region. HREELS data obtained under conditions corresponding to the XPS spectra of previous figures

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Figure 4. HREEL spectra (fingerprint region) of the Cys/Au(111) system for different coverages and deposition temperatures: (a) 0.5 ML330; (b) 1.0 ML330; and (c) 1.0 MLERT. The main features discussed in the text are indicated by arrows along with their frequencies. Spectra are taken in specular conditions and with an energy resolution of less than 30 cm-1.

are shown in Figure 4. The vibrational spectra of both 330 K and RT monolayers (curves b and c) are characterized by an intense feature peaked at around 1600-1610 cm-1, associated to the νas(COO-) stretching mode (at 1600-1620 cm-1 37,40). This peak showed an evident broadening toward the higher energy side, influenced by the δas(NH3+) asymmetric bending mode (at 1630-1650 cm-1 37,40). The predominance of (COO-) groups is also confirmed by the feature observed at 787 cm-1, which is associated to the symmetric bending mode δsym(COO-).40 Moreover, the feature centered at 1420-1440 cm-1 can also be attributed to the symmetric νsym(COO-) stretching mode (usually observed at 1400-1420 cm-1), though a contribution from the δ(CH2) mode (localized at around 1440 cm-1 9) is also expected. On the other hand, a shoulder is observed at 1720 cm-1, which is attributed to the ν(COOH) mode.37 It indicates the presence of a small percentage of COOH species for both 330 K and RT monolayers, in agreement with XPS findings. As far as the vibrational frequencies of the amino group are concerned, an univocal determination of the molecular charge state is difficult, due to the subtle differences between the NH3+ and NH2 modes. Nevertheless, some conclusions can be drawn for both the RT and 330 K monolayer spectra. In these spectra, the above-mentioned feature at 1630 cm-1 and another feature at 1506 cm-1 are observed. According to the literature, these frequencies correspond to the δas(NH3+) and δsym(NH3+) modes, respectively, whereas in the case of the NH2 group, the bending modes are located at 1570 and 1465 cm-1.37 HREEL data therefore suggest, in agreement with XPS results, the prevalence of the NH3+ species. A careful look at the 0.5 ML330 spectrum (curve a in Figure 4) reveals some differences with respect to the full ML330 spectrum. In particular, the feature located at 1600-1610 cm-1 is less intense. Furthermore, the peak at 1506 cm-1, previously attributed to the δsym(NH3+) mode, almost disappears. The intensity decrease of these modes is likely an indication of a different geometrical orientation of the Cys molecule in the lowcoverage 330 K regime.17 The observed variations could therefore indicate some differences in the structural properties

De Renzi et al.

Figure 5. HREEL spectra (stretching region) of the Cys/Au(111) system for different coverages and deposition temperatures: (a) 0.5 ML330; (b) 1.0 ML330; and (c) 1.0 MLERT. Spectra are taken in specular conditions and with an energy resolution of less than 30 cm-1. The main features discussed in the text are indicated by arrows along with their frequencies. The small peak observed at 2575 cm-1 in the RT spectrum (curve c) is associated to the SH stretching mode, indicating, in agreement with XPS, the presence of intact thiol groups. The peak is instead absent in the 330 K spectra, confirming that cleavage of the SH bond occurs at higher temperature. The weakness of the SH mode is in accord with previous observations on adsorbed thiols.24,43

of the half and full ML330. These arguments are in agreement with the recent STM measurements on annealed Cys layers which showed variations of the adlayer structure as a function of coverage.31 This point will be further discussed after the presentation of HREELS data in the quasi-elastic region. B. Characterization of the H-Bond Network. We have shown in the previous section that a slight variation of the substrate deposition temperature induces the transition from a heterogeneous interface, where a large part of the molecules are weakly adsorbed, to a more homogeneous layer made of thiolate chemisorbed species. These findings make the Cys/ Au(111) interface a system suited for the investigation of intermolecular vibrational modes as a function of the adlayer self-assembly. Important information on the interaction between adjacent molecules is provided by inspection of the frequency regions where the stretching modes of CH, NH, and OH are expected. In Figure 5, representative data of this stretching region are shown. All spectra are characterized by an intense asymmetric feature peaked at around 2960 cm-1, which is attributed to the CH2 and CH symmetric and asymmetric stretching modes.40 Differences can be observed in the highfrequency side of this feature for the different spectra. At RT (curve c), a broad tail at higher frequency is observed, above which a tiny peak localized at 3335 cm-1 can be detected. Spectra taken at 330 K (curves a and b) show a marked increase of this peak, indicated as a N-mode in the following, combined with a decreased intensity of the high-frequency tail. Examination of the literature brings us to the conclusion that the frequencies of the NH3+, NH2, and OH stretching modes are apparently very sensitive to the local conditions; i.e., they may be strongly affected by molecule-molecule and moleculesubstrate interactions. Furthermore, the formation of intermolecular hydrogen bonds is known to conspicuously broaden both the NH and OH stretching bands of zwitterionic compounds in the solid state.42 Indeed, the literature frequency values of NH3+

H-Bond Network Formation and NH2 stretching modes are quite scattered in the 2800-3400 cm-1 region;24,37,40,44 the OH stretching mode, which is observed at around 3500 cm-1 for isolated species, is known to shift to lower frequencies in the case of intermolecular H-bond formation.19 We therefore attribute the spectral intensity observed in the frequency region above 3000 cm-1 to a distribution of the stretching modes of the NH and N-H · · · O bonds, whose frequency depends on the molecular local environment and on the details of the molecule-molecule interactions. In the case of RT adsorption (curve c), the broadness of the spectral distribution clearly indicates a pronounced heterogeneity of the molecule’s local environment, in agreement with the reported enhanced polymorphism of the layer.31 This heterogeneity corresponds to the presence of several H-bond configurations, characterized by different lengths and orientations. For deposition at 330 K instead, the concomitant decrease of the rightside tail of the ν(CH) modes above 3000 cm-1 and the strong increase of the N-mode are signs of a more narrow distribution of the NH and N-H · · · O stretching mode frequencies. This clearly indicates that deposition at 330 K produces more homogeneous layers. We therefore attribute the N-mode peak to a stretching mode associated to a single N-H · · · O configuration within a highly ordered and homogeneous network formed at 330 K. Subtle differences between the CH stretching mode regions of 0.5 ML330 and 1 ML330 spectra are likely due to the different geometrical orientations of the molecule as a function of coverage. The well-organized layers obtained at 330 K can be therefore considered as ideal candidates to explore the lowenergy vibrational properties of H-bond networks. Indeed, important information on the vibrational properties of the adlayer network is gained by analysis of the low-energy region (0-400 cm-1) of HREEL spectra. In this region, vibrational modes of different origin are expected: (a) stretching modes of species involved in the molecular bonding to the substrate; (b) torsional and skeleton modes of the molecule, and (c) low-energy modes related to H-bond networks, which have been reported to lie in the 0-200 cm-1 region for several amino acids41 and in particular at around 70 and 150 cm-1, for serine and cysteine crystals.15 The HREEL spectra of the 330 K and RT layers in the 0-400 cm-1 region are reported in Figure 6. The spectra showed interesting features. We want to draw particular attention to the pronounced peak, denoted as the H mode, occurring at 74 cm-1 in the 1.0 ML330 spectrum (curve b). Most interestingly, the H peak was absent (or so broad and weak as to be completely hidden in the elastic tail) in the RT spectrum. To this regard, the H mode showed the same behavior as the N mode at 3335 cm-1. In fact, both H and N modes had appreciable intensities and well-defined frequencies in the 330 K deposited layers (see also later), while these were absent or extremely small in the case of RT deposition. This finding indicates that the H mode is also extremely sensitive to the heterogeneity and disorder of the adlayer, i.e., by molecule-molecule interactions. Thus, it is likely not an internal (skeleton) mode of the molecule (which should not be strongly affected by the molecular environment), but rather a vibration of the intermolecular bonds. We therefore attribute this feature to a mode of the highly organized H-bond network formed at 330 K. This attribution is in agreement with the previously cited far-infrared spectroscopy results on the frequency of H-bond modes in serine and cysteine crystals.15 Further proof and insight in this assignment is derived from the analysis of the 0.5 ML330 spectra

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Figure 6. HREEL spectra of the Cys/Au(111) system in the quasielastic region for different coverages and deposition temperatures: (a and a′) 0.5 ML330; (b) 1.0 ML330; and (c) 1.0 MLERT. Spectra are taken in specular conditions and with an energy resolution of less than 30 cm -1. For 0.5 ML330, two spectra are reported as taken with different frequency resolutions: the shaded curve a has the same resolution of the spectra taken at high coverage, while curve a′ corresponds to an enhanced resolution of 12 cm-1. Comparison of the two curves clearly shows that by changing the resolution, the H feature results were broadened but not shifted in frequency. The frequency shift of the H feature on passing from 0.5 ML330 (curves a and a′) to 1.0 ML330 (curve b) is therefore a true physical feature, not attributable to the different resolutions of the spectra.

(curves a and a′). If our attribution is correct, a feature similar to the H mode should be also observed for lower coverage. Indeed, in curve a (0.5 ML330 taken with fwhm 28 cm-1 (3.5 meV)), a feature at very low frequency is observed as a shoulder along the tail of the elastic peak. In order to disentangle this feature from the elastic tail, we performed measurements at the highest resolution attainable in our apparatus. As seen in curve a′ (fwhm 12 cm-1 (1.5 meV), see caption for details) a peak centered at 55 cm-1 was clearly observed. This feature is red-shifted by 19 cm-1 (e.g., 2.4 meV) with respect to the 1.0 ML330 H mode. This shift can be explained as follows. Recent STM work showed a strong dependence on coverage of the structure of the mildly annealed (380 K) Cys/Au(111) interface.31 Though the experimental conditions were not exactly the same as in our case, the results of ref 31 are consistent with the small differences we observed in the fingerprint and stretching regions of the 0.5 ML330 and 1.0 ML330 spectra and suggest a dependence on the geometry of the 330 K layers on coverage. In our picture, the low-frequency feature observed in the 0.5 ML330 is then also attributed to a collective vibrational mode of the H-bond network (H mode) and its red-shift explained by the differences in the packing and/or relative orientation of adjacent molecules for different coverage. A precise description of this mode in term of atomic displacements is beyond our actual capability and calls for both detailed structural investigations and theoretical modeling. We thus qualitatively describe the H mode as a collective vibration of the H-bonds network and attribute its coverage dependence to variations in the H-bond length and orientation. The observed coverage dependence indeed shows

14444 J. Phys. Chem. C, Vol. 112, No. 37, 2008 that this feature is extremely sensitive to the details of molecular organization on the surface. To complete the picture, it is also worth discussing the other main features observed in the low-energy region. Both the ML330 and MLRT spectra in Figure 6 showed a well-resolved peak at 240 cm-1, which was also present, though less intense, in the 0.5 ML330 spectrum. Following the literature45,46 this peak is attributed to the S-Au stretching mode of the chemisorbed molecule, in agreement with XPS results. We should note that the 240 cm-1 peak appears more intense in the RT-monolayer spectrum than in the 330 K one. This fact may indicate a different molecular orientation in the two layers, with the Au-S bond oriented more perpendicular to the surface at RT. On the other hand, contributions to the RT peak from other vibrational modes should also be considered. Moreover, a very well defined peak was observed in the RT spectrum at 144 cm-1, which we attribute to the stretching mode between the thiol group and the substrate atoms in the weakly adsorbed species. Indeed, the S-Au mode is expected to shift at lower frequency for weakly adsorbed species, as for instance in the case of dialkyl sulfide47 and disulfide molecules,45 where it was found at 205 and 129 cm-1, respectively. For 330 K deposition, a small peak is also observed at 155 cm-1. In this case, XPS results excluded a significant presence of weakly adsorbed molecules. We tentatively assign this peak, on the basis of far-infrared spectra,15,41 to another collective mode of the highly organized H-bond network. However, a contribution from skeleton modes of the single molecule cannot be excluded. Eventually, the feature observed in all spectra at 360 cm-1 is attributed to a CCN deformation mode of the molecule.37 IV. Conclusions The Cys/Au(111) interface has been experimentally studied as a model system in which H-bond molecular interaction plays a fundamental role. Cys adsorption has been studied by XPS and HREELS, showing that at RT the molecule is mainly weakly adsorbed on the surface (though at low coverage chemisorption on the step edge can be envisaged), while already at 330 K chemisorption through S-Au bonding prevails. This clear spectroscopic evidence substantially confirms and clarifies the mechanism of Cys adlayer formation on the Au(111) surface previously proposed by A. Ku¨hnle et al. on the basis of STM measurements.31 The evolution of the vibrational properties of the system has been studied on passing from the quite heterogeneous layer deposited at RT to the more homogeneous and ordered interface obtained for a compact layer at 330 K. In the latter case, a sharp feature (N mode) localized at 3335 cm-1, attributed to a stretching mode of the N-H · · · O bond, and an extremely low-frequency feature (H mode) at 74 cm-1, assigned to a collective mode, are proposed as fingerprints of the formation of a highly organized H-bonding network. The ability to measure small variations in the H-mode frequency as a function of coverage demonstrates the great potentiality of HREEL spectroscopy, used at its highest performance level, to investigate the low-energy collective modes of 2D organic layers laterally self-organized through noncovalent bonds. Indeed, our work proves, in agreement with terahertz spectroscopy results on 3D aminoacid crystals, that these low-frequency modes are extremely sensitive (more than the high frequency stretching) to the molecular local environment, and their frequency variations may therefore be used to characterize H-bond networking. More generally, we believe this approach, combined with a theoretical effort for an accurate description of low-frequency modes, could provide novel contributions toward a more detailed

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