Diffusion, Rotation, and Surface Chemical Bond of Individual 2H

Oct 17, 2011 - ... Helen Hölzel , Norbert Jux , Bernd Meyer , M. Alexander Schneider ..... Sybille Fischer , David A. Duncan , Reinhard J. Maurer , J...
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Diffusion, Rotation, and Surface Chemical Bond of Individual 2H-Tetraphenylporphyrin Molecules on Cu(111) Florian Buchner,† Jie Xiao,† Elisabeth Zillner,† Min Chen,† Michael R€ockert,† Stefanie Ditze,† Michael Stark,† Hans-Peter Steinr€uck,† J. Michael Gottfried,†,‡ and Hubertus Marbach*,† †

University Erlangen-N€urnberg, Physical Chemistry II and Interdisciplinary Center for Molecular Materials, Egerlandstr. 3, 91058 Erlangen, Germany ‡ University Marburg, Department of Chemistry, Hans-Meerwein-Str., 35032 Marburg, Germany

bS Supporting Information ABSTRACT: We address the dynamic behavior and the surface chemical bond of 2H-tetraphenylporphyrin (2HTPP) on Cu(111) around room temperature by variable-temperature scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) in ultrahigh vacuum. Between 280 and 345 K, the molecules predominantly display unidirectional diffusion along one of the three densely packed substrate Æ110æ directions, which is attributed to a high site selectivity of the adsorbate substrate bond. Above 305 K, the diffusion direction is found to change occasionally by (120. The activation barriers for the unidirectional diffusion and for rotation of the diffusion direction are determined to 0.71 ( 0.08 and 1.28 ( 0.12 eV, respectively. XPS shows that the iminic nitrogen atoms of 2HTPP interact strongly with the Cu surface. It is postulated that the local bonding situation is similar as in the initial complex (sitting-atop complex), which has previously been observed during the surface-confined in situ metalation of porphyrins.

1. INTRODUCTION The diffusion dynamics and the surface chemical bond of adsorbates on metal surfaces are of fundamental interest in surface science and of particular importance for the bottom-up design of functional molecular architectures.1,2 One example is the self-assembly of molecular building blocks on well-defined substrates to generate thin molecular films with specific functional properties, for example, single-site catalysts.3 Porphyrins are an especially versatile class of such molecular building blocks because they combine a rigid molecular framework with a ligand functionality for many metal ions.4 Porphyrins are well-known because of their omnipresence as active units in nature (examples are iron porphyrin in heme 5 or magnesium porphyrin in chlorophyll6) and also because of their importance in sensor technology.7,8 The occupation of specific adsorption sites on a surface or the self-assembly of extended supramolecular structures requires a sufficient mobility of the adsorbed species. The investigation of surface migration and the determination of the corresponding kinetic parameters for such systems is therefore of great relevance. STM has proven to be a suitable tool to extract this information. Initially, mainly the surface diffusion of atoms or small molecules on metal surfaces9 13 was investigated, but in the past decade also studies on large organic molecules can be found in literature;15 these include decacyclen and hexa-tert-butyl-decacyclene on Cu(111),16 dithioanthracene on Cu(111),17 4-trans-2-(pyrid-4yl-vinyl) benzoic acid on Pd(110),18 C60 on Pd(110),19 tris(2-phenylpyridine)iridium(III) on Cu(111),20 and 2H-tetrapyridylporphyrin (2HTPyP) on Cu(111).21 The latter study is the r 2011 American Chemical Society

first example for the direct observation of an unidirectional motion of porphyrins by STM at 300 K and above. At these temperatures, porphyrins usually diffuse rapidly on the time scale of STM and thus cannot be imaged as individual molecules. It has been suspected in previous work that a specific N Cu interaction may be responsible for the hindered diffusion on Cu(111),21,23 but no unambiguous proof has been presented so far. In case of predominantly unidirectional motion (typically along a highsymmetry axis of the surface), a change of direction of the migration to a symmetry equivalent direction should be possible; such rotations are expected to also play an important role for selfassembly processes. However, to the best of our knowledge, rotations of individual porphyrins have not been reported so far. Herein, we present a detailed investigation of both lateral diffusion and rotation of 2H-tetraphenylporphyrin (2HTPP) on Cu(111) based on time series of STM images at temperatures between 280 and 345 K. Moreover, we show that the “high” diffusion barrier of 2HTPP on Cu(111) is due to the formation of strong coordinative bonds between the iminic N atoms and the substrate.

2. EXPERIMENTAL SECTION The STM experiments were performed in a two chamber UHV-system at a background pressure in the low 10 10 mbar regime. The microscope is an RHK UHV VT STM 300 with RHK SPM 100 electronics. All given voltages are referred to the Received: July 13, 2011 Revised: October 10, 2011 Published: October 17, 2011 24172

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The Journal of Physical Chemistry C sample and the images have been taken in constant current mode. Moderate filtering (Gaussian smooth, background subtraction) has been applied for noise reduction. The Cu(111) single crystal was purchased from MaTecK, 2HTPP with a specified purity of 98% from Porphyrin Systems GbR. The preparation of the clean Cu(111) surface was done by Ar+-ion sputtering (500 eV) and annealing up to 850 K. The porphyrin layers were prepared by thermal sublimation of the raw material (at a temperature of ∼610 K) with a home-built Knudsen cell onto the Cu substrate held at RT. The Æ110æ axes of Cu(111) were determined in a separate experiment, that is, deposition of nickel on Cu(111) via an electron beam evaporator. This results in hexagonally grown 2D islands (not shown); the monatomic steps are aligned along the close-packed atomic rows of Cu(111). Following subsequent evaporation deposition of 2HTPP, the molecules’ orientation on the copper terraces coincides with these directions. Therefore, the diffusion path of 2HTPP on Cu(111), for example, at room temperature, indicates the crystallographic main directions. The XPS experiments were performed with a Scienta ESCA200 spectrometer equipped with an Al Kα X-ray source (1486.6 eV) and an X-ray monochromator, and a hemispherical energy analyzer (SES-200) was employed. The base pressures of the vacuum systems were below 2  10 10 mbar. All binding energies were referenced to the Fermi edge of the clean metal substrate (Eb  0). Unless otherwise indicated, the photoelectrons were detected at an angle of 70 to the surface normal for increased surface sensitivity. The substrates were Cu, Ag, and Au single crystals (purity >99.999%, purchased from Surface Preparation Laboratory, DE Zaandam, The Netherlands and MaTecK, Germany) with polished (111) surfaces, which were aligned to 98%, Porphyrin Systems GbR) was degassed in ultrahigh vacuum by heating to 420 K for 24 h. 2HTPP was deposited by physical vapor deposition with a source temperature of 638 K, while the substrate surface was kept at 300 K. A quantitative measure of the molecular coverage is given by the unit 1 ML, which is defined as one molecule per atom of the substrate surface. This convention is advantageous in the present case because it is independent of an actual supermolecular order. Note that all spectra shown in Figure 2 were acquired using the same set of instrumental parameters for the X-ray photoelectron spectrometer. Therefore, changes in peak width are related to properties of the adsorbate and not to the instrumental conditions. Compared with previous publications from our laboratory, where a different set of parameters was used, the overall resolution is slightly lower.

3. RESULTS AND DISCUSSION To begin with, we briefly discuss the static adsorption situation (i.e., an individual STM image) of 2HTPP on Cu(111) at very low coverage, along the lines of previous results for the same system at slightly higher coverages.22,23 At 301 K, individual 2HTPP molecules can be identified in the STM images, as evident from Figure 1a. The molecules appear as two elevated parallel rods and a central longish gap in between. Only three different azimuthal orientations are found, with each molecule aligned along one of the three symmetryequivalent densely packed substrate Æ110æ directions (indicated as white arrows in the right lower corner of Figure 1a). The observation of isolated 2HTPPs at 301 K indicates a low mobility compared with, for example, CoTPP on Cu(111)23 and

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Figure 1. (a) Constant current STM image of 2HTPP on Cu(111) at low coverage, obtained at 301 K (scale bar = 10 nm). The space-filling model in the insert exhibits the proposed intramolecular saddle-shape conformation and depicts the orientation relative to the Cu surface atoms (indicated in orange) along the surface Æ110æ directions. (b,c) Subsequently recorded images of the same scan area: the position of the molecules in panel b are highlighted in panel c with crosses; the dashed white lines indicate the directions of the 1D diffusion (scale bar = 3 nm). (d) Pseudo 3D STM image of a single 2HTPP and (e) the average STM frame of the corresponding STM time lapse movie (37 images at 20 s), with the elongated shape emphasizing the unidirectional diffusion. [(a e) U = 1.49 V, I = 30 pA].

both CoTPP and 2HTPP on Ag(111),24 which cannot be imaged as isolated molecules around 300 K. In previous work, it has been suggested that diffusion of 2HTPP on Cu(111) is hindered by a specific N Cu interaction.21,23 To obtain direct information about the bonding situation, we performed XPS measurements in the N 1s binding energy region for 2HTPP on Cu(111), Ag(111), and Au(111) and for 2HTPP multilayers. (Note that the multilayer spectra do not depend on the choice of the substrate.) The spectra in Figure 2 show two peaks, which have been previously assigned to secondary aminic nitrogen ( NH , at higher binding energy) and iminic nitrogen ( Nd).25 For the nearly unperturbed molecules in the multilayer, the peak separation is 2.1 eV. For 2HTPP monolayers on Ag(111) and Au(111), for which the molecule substrate interaction is known to be weak, 23,24,26 this value changes only slightly to 2.0 eV. On Cu(111), however, the peak separation shrinks to only 1.5 eV, mainly because the peak related to iminic nitrogen shifts to higher binding energy by 0.65 eV. These results indicate that preferably the iminic nitrogen atoms interact with the Cu surface. A reduction of the peak separation has previously been found when an adsorbed porphyrin reacts with a metal atom to form an initial complex (or sitting-atop complex).25 In such a complex, 24173

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Figure 2. N 1s XP spectra of 2HTPP multilayers (top) (intensity of the signal multiplied by 0.5), 2HTPP monolayers on Au(111) and Ag(111), and 2HTPP on Cu(111) at a coverage of 0.022 ML. The spectra were taken with monochromatized Al Kα radiation at an electron collection angle of 70 relative to the surface normal.

the intact porphyrin molecule coordinates to a neutral metal atom such that the metal atom sits on one side of the porphyrin plane and the two N H hydrogen atoms on the other. Previous DFT calculations showed that iminic nitrogen forms shorter bonds to the metal center than secondary aminic nitrogen (1.96 vs 2.21 Å in the case of Zn), which indicates that the metal atom is mainly coordinated by the iminic nitrogen atoms.25 For adsorbed 2HTPP on Cu(111), the bonding situation is apparently very similar, as indicated by the substantial peak shift for iminic N. Therefore, we propose that 2HTPP forms strong localized bonds to the Cu surface through the iminic nitrogen atoms. This leads to a specific bonding situation with identical adsorption sites for the porphyrin molecules, as is also confirmed by the peak widths of the spectra in Figure 2. The individual peaks in the multilayer spectrum have a full width at half-maximum (fwhm) of 1.0 eV. On Au(111) and Ag(111), the fwhm increases to ∼1.5 eV, which is attributed to inhomogeneity broadening, resulting from the fact that the N atoms are located at a variety of different adsorption sites. This is expected, because the molecules form longrange ordered structures, in which the location of an individual molecule is determined by intermolecular interactions and not by the substrate.23,24,26 On Cu(111), however, the fwhm is reduced to 1.0 eV, the same value as in the multilayer. This is in line with our conclusion of strong adsorbate substrate interactions, which ensure that the molecules bind to identical adsorption sites, leading to reduced inhomogeneity broadening. Therefore, our XPS results provide an experimental basis for previous speculations that the “high” diffusion barrier of 2HTPP

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on Cu(111) is due to the interaction of the iminic nitrogen atoms with the surface Cu atoms.23 A similar interpretation was used to explain the comparable behavior of 2HTPyP on Cu(111).21 The proposed intramolecular conformation of 2HTPP on Cu(111) is discussed in detail in ref 23 and is very similar to that deduced for 2HTPyP on the same surface from NEXAFS data,27 with a ∼10 twist angle of the phenyl legs. A corresponding model is shown as insert in Figure 1a, with an underlying densely packed Æ110æ substrate row. This geometry is in line with the proposed strong N Cu interaction, which forces an in-plane tilting of the phenyl legs and induces a decreased distance of the porphyrin plane to the surface. We next concentrate on the dynamic behavior of 2HTPP on Cu(111). Figure 1b,c shows two subsequently recorded STM images of the same surface region (time interval: ∼265 s). Both images show three molecules with identical orientation. However, the position of two of the molecules has changed in a very specific way. (Note that the positions of the molecules in Figure 1b are indicated by white crosses in Figure 1c.) The positional changes represent 1D displacements along one of the high-symmetry directions of the substrate, along which the molecules are aligned (indicated with dashed-dotted lines). This behavior was followed up by acquiring time series of STM images at 301 K. Figure 1d shows one single pseudo-3D image of an individual 2HTPP, and Figure 1e shows an average frame extracted from a time lapse movie of 37 images. The average frame appears as an elongated version of the two parallel protrusions of the single frame, which confirms the unidirectional motion of 2HTPP on Cu(111). To study the unidirectional diffusion behavior in more detail, STM movies were acquired at different temperatures between 280 and 345 K. (See the STM movies in the Supporting Information.) A high tunneling resistance (U ≈ 1.5 V, I ≈ 30 pA, R ≈ 50 GΩ) was applied to avoid tip-induced effects. Diffusion measurements in literature are usually carried out at tunneling resistances in the range of 1 10 GΩ.16 19,21 Under these “mild“ tunneling conditions, no indications were found that the lateral molecular motion is influenced by the STM measurement process. Because in our experiments the tunnel resistance is even higher, that is, the tip substrate interaction is lower, significant influence of the STM measurement on the diffusion properties can be practically ruled out. In addition no influence upon changing image size, scanning speed, and the fast scan direction (vertical/horizontal) was observed. The 2HTPP coverage was kept sufficiently low, such that a significant contribution of intermolecular interactions can be ruled out. Each movie comprises roughly 50 images with an acquisition time of 20 40 s/image, allowing for the analysis of roughly 2500 diffusion events. The position of each molecule was marked in the images with a specific color. Figure 3a d presents the average frames (superposition of the image series) at four temperatures. The resulting traces of the individual molecules allow us to analyze the temperaturedependent diffusion behavior. At 280 K (Figure 3a), the 2HTPP molecules are almost completely confined to their adsorption sites; that is, they do not move on the time scale of our experiment, as evidenced by the circular or only slightly elongated dots. Upon increasing the temperature to 301 K (Figure 3b), solely unidirectional trajectories are observed along one of the three highsymmetry axes of the surface. At 315 K (Figure 3c), the diffusion length has further increased and occasionally the direction of the trajectories changes by (120, that is, to one of the other highsymmetry substrate axes. This trend intensifies at higher temperatures (330 K in Figure 3d), with the diffusion lengths and 24174

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Figure 3. (a d) Average frames of STM movies of 2HTPP on Cu(111) between 280 and 330 K (scale bar = 10 nm). The positions of the molecules were color-coded in each single STM image and then superimposed over the corresponding STM average frames [(a) U = 1.51 V, I = 29 pA and (b d) U = 1.49 V, I = 30 pA]. (e) Arrhenius plot of the hopping rates vs temperature. (f) Arrhenius plot of the rotational change of the diffusion direction.

number of directional changes further increasing. At 345 K, the molecular diffusion was fast on the STM time scale, and thus the unequivocal tracking of the molecules became difficult. In particular, changes in the images already occurred on a time scale shorter than that required to obtain one image, and the displacements were partially larger than the scan area. These data are therefore not included in the kinetic analysis described below. For the quantitative analysis, we follow the procedure described by Eichberger et al.21 In a first step, the molecular displacements between consecutive images were evaluated for each temperature. These data enable the determination of the mean square displacement and correspondingly the hopping rate1 h(T) = /(2t) (λ is the jump length, i.e., the lattice constant of 2.55 Å along the Æ110æ directions and t is the corresponding time interval) for each of the investigated temperatures. The Arrhenius plot (ln(h) vs 1/T) of the hopping rate in Figure 3e shows a linear dependence, consistent with the general Arrhenius expression for surface diffusion h(T) = Am exp[ Em/(kBT)] .1,16,18

From the slope of the linear fit, the migration barrier for diffusion of 2HTPP on Cu(111) was determined to Em = 0.71 ( 0.08 eV, with an attempt frequency of Am = 1010.9(1.4 s 1. These observations are in line with previous results for 2HTPyP on Cu(111). The slightly larger migration barrier of Em = 0.96 ( 0.09 eV21 can be attributed to an additional substrate interaction contribution of the nitrogen atoms in the pyridyl side groups of 2HTPyP. We conclude that the contribution from the iminic nitrogen atoms dominates the diffusion behavior. For 2HTPP on Cu(111), the temperature dependence of the rotation of the migration direction by (120 was analyzed between 305 and 330 K. (At lower T, the rotation events were too infrequent.) Even though the statistical basis is rather small with 30 events, the corresponding Arrhenius plot for the rotation rate r(T) = Ar exp[ Er/(kBT)] allows a reasonable linear fit (Figure 3f) and yields a rotational activation barrier Er = 1.28 ( 0.12 eV and an frequency factor of Ar = 1017.0(1.8 s 1. The much higher barrier for rotation than for unidirectional motion is 24175

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Figure 4. (a f) STM images of 2HTPP on Cu(111) acquired at 345 K. The significant deviations of the observed features from the expected two longish protrusions per molecule are a direct indication for the diffusion on the time scale of image acquisition (scan velocity = 1.65 μm/s, U = 1.49 V, I = 30 pA); for details see the text. Scale bar in panel a represents 3 nm and applies to a f.

attributed to the very pronounced site specificity of the N Cu interaction in combination with a complex change of the molecular conformation during the rotation event. Whereas the frequency factor of the diffusion is in the range of molecular vibrations, the corresponding prefactor of the rotation appears to be unreasonable high at first glance. However, one has to keep in mind that in transition-state theory the frequency factor does not directly reflect a molecular vibration but rather an entropy gain of the transition state. We speculate that additional degrees of freedom (e.g., frustrated azimuthal rotation) account for this gain. For the desorption of “large” organic molecules (like the 2HTPP investigated here), similar or even higher frequency factors were reported in literature.28 31 So far, rotational hopping of porphyrins has been found only when the molecules are embedded in a closely packed supramolecular framework. Examples are zinc-octaethylporphyrin on Cu(111) with an rotational activation energy of 0.17 eV32 and zinc-tetrakis(di-tert-butylphenyl)porphyrin with an activation energy of 0.24 eV.33 Because both are metal complexes in which the nitrogen atoms coordinate already to the Zn central ion, no such pronounced site specificity as found for 2HTPP is expected. As a result, the rotational dynamics of both metal Zn porphyrin complexes is most likely governed by lateral molecule molecule rather than by molecule substrate interactions.14,21 Therefore, 2HTPP on Cu(111) represents the first example of the rotation of individual porphyrins on a pristine surface and for the evaluation of the rotation barrier. As mentioned above, the STM images collected at 345 K were excluded from the kinetic analysis because the image-to-image molecular displacement partially exceeded the scan area. (See the STM movie in the Supporting Information.) However, the respective constant-current images (Figure 4) show other

interesting features resulting from molecular diffusion during imaging. The superimposed space-filling models with the white arrow indicate the diffusion direction. In Figure 4a, the appearance of 2HTPP is dominated by an elongated protrusion and a shorter one with the typical length of a single static molecule. This appearance is attributed to a molecule diffusing a certain distance from the left upper corner to the right lower corner of the scan area: Because the image is taken from the top to the bottom with the horizontal direction being the fast scanning direction, initially only the upper part of the diffusing molecule is seen. Only after the diffusion is disrupted, also the lower part is seen such that the whole molecule is imaged. In Figure 4b, two vertically diffusing molecules are observed. The left one appears shortened because it diffuses against the slow scanning direction; the right one appears elongated because it diffuses along the slow scanning direction. The images in Figure 4c f are presenting similar examples, which can be interpreted analogously.

4. CONCLUSIONS In conclusion, our combined STM and XPS study shows that 2HTPP forms a strong coordinative bond to the Cu(111) surface, mainly via the iminic nitrogen atoms. As a result of this highly site-specific adsorbate substrate bond, the molecules undergo unidirectional diffusion along densely packed rows of substrate atoms in high-symmetry directions. This motion was followed by STM in the temperature range from 280 to 345 K. Above 305 K, changes of the diffusion directions by (120, that is, to one of the other symmetry-equivalent directions, were observed. From Arrhenius plots, the activation barriers for the unidirectional diffusion and the rotation of the diffusion directions were determined to be 0.71 and 1.28 eV, with preexponential factors of 1010.9 and 1017.0 s 1, respectively. These 24176

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The Journal of Physical Chemistry C kinetic parameters provide valuable information for the control of the self-assembly of large organic molecules on flat surfaces and thus for the engineering of nanoscale materials.

’ ASSOCIATED CONTENT

bS

Supporting Information. STM quick time movies, which were acquired in the temperature interval between 280 and 345 K. This material is available free of charge via the Internet at http:// pubs.acs.org

’ AUTHOR INFORMATION Corresponding Author

*E-Mail: [email protected]. Phone: +49 9131 85-27316. Fax: +49 9131 85-28867.

’ ACKNOWLEDGMENT We thank the German Science Foundation (DFG) for financial support through SFB 583 and the Excellence Cluster “Engineering of Advanced Materials” granted to the University of Erlangen-N€urnberg. ’ REFERENCES (1) Barth, J. V. Surf. Sci. Rep. 2000, 40, 75–149. (2) De Feyter, S.; De Schryver, F. C. Chem. Soc. Rev. 2003, 32, 139–150. (3) Barth, J. V. Surf. Sci. 2009, 603, 1533–1541. (4) Gottfried, J. M.; Marbach, H. Z. Phys. Chem. 2009, 223, 53–74. (5) Kadish, K. M., Smith, K. M., Guilard, R. The Porphyrin Handbook; Academic Press: San Diego, 2000; Vol. 4. (6) Kadish, K. M., Smith, K. M., Guilard, R. The Porphyrin Handbook; Academic Press: San Diego, 2003; Vol. 13. (7) Rakow, N. A.; Suslick, K. S. Nature 2000, 406, 710–713. (8) Guillaud, G.; Simon, J.; Germain, J. P. Coord. Chem. Rev. 1998, 178 180, 1433–1484. (9) Ganz, E.; Theiss, S. K.; Hwang, I.-S.; Golovchenko, J. Phys. Rev. Lett. 1992, 68, 1567–1570. (10) Swartzentruber, B. S. Phys. Rev. Lett. 1996, 76, 459–462. (11) Zambelli, T.; Trost, J.; Wintterlin, J.; Ertl, G. Phys. Rev. Lett. 1996, 76, 795–798. (12) Linderoth, T. R.; Horch, S.; Lægsgaard, E.; Stensgaard, I.; Besenbacher, F. Phys. Rev. Lett. 1997, 78, 4978–4981. (13) Barth, J. V.; Brune, H.; Fischer, B.; Weckesser, J.; Kern, K. Phys. Rev. Lett. 2000, 84, 1732–1735. (14) Briner, B. G.; Doering, M.; Rust, H.-P.; Bradshaw, A. M. Science 1997, 278, 257–260. (15) Rosei, F.; Schunack, M.; Naitoh, Y.; Jiang, P.; Gourdon, A.; Laegsgaard, E.; Stensgaard, I.; Joachim, C.; Besenbacher, F. Prog. Surf. Sci. 2003, 71, 95–146. (16) Schunack, M.; Linderoth, T. R.; Rosei, F.; Lægsgaard, E.; Stensgaard, I.; Besenbacher, F. Phys. Rev. Lett. 2002, 88, 1561021–156102-4. (17) Kwon, K.-Y.; Wong, K. L.; Pawin, G.; Bartels, L.; Stolbov, S.; Rahman, T. S. Phys. Rev. Lett. 2005, 95, 166101-1–166101-4. (18) Weckesser, J.; Barth, J. V.; Kern, K. J. Chem. Phys. 1999, 110, 5351–5354. (19) Weckesser, J.; Barth, J. V.; Kern, K. Phys. Rev. B 2001, 64, 161403-1–161403-4. (20) Yokoyama, T.; Takahashi, T.; Shinozaki, K. Phys. Rev. B 2010, 82, 155414-1–155414-5. (21) Eichberger, M.; Marschall, M.; Reichert, J.; Weber-Bargioni, A.; Auw€arter, W.; Wang, R. L. C.; Kreuzer, H. J.; Pennec, Y.; Schiffrin, A.; Barth, J. V. Nano Lett. 2008, 8, 4608–4613.

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(22) Rojas, G.; Chen, X.; Bravo, C.; Kim, J.-H.; Kim, J.-S.; Xiao, J.; Dowben, P. A.; Gao, Y.; Zeng, X. C.; Choe, W.; Enders, A. J. Phys. Chem. C 2010, 114, 9408–9415. (23) Buchner, F.; Zillner, E.; R€ockert, M.; Gl€aßel, S.; Steinr€uck, H.-P.; Marbach, H. Chem.—Eur. J. 2011, 17, 10226–10229. (24) Buchner, F.; Kellner, I.; Hieringer, W.; G€orling, A.; Steinr€uck, H.-P.; Marbach, H. Phys. Chem. Chem. Phys. 2010, 12, 13082–13090. (25) Shubina, T. E.; Marbach, H.; Flechtner, K.; Kretschmann, A.; Jux, N.; Buchner, F.; Steinr€uck, H. P.; Clark, T.; Gottfried, J. M. J. Am. Chem. Soc. 2007, 129, 9476–9483. (26) Brede, J.; Linares, M.; Kuck, S.; Schw€obel, J.; Scarfato, A.; Chang, S.-H.; Hoffmann, G.; Wiesendanger, R.; Lensen, R.; Kouwer, P. H. J.; Hoogboom, J.; Rowan, A. E.; Br€oring, M.; Funk, M.; Stafstr€om, S.; Zerbetto, F.; Lazzaroni, R. Nanotechnology 2009, 20, 275602 1–275602 10. (27) Auw€arter, W.; Klappenberger, F.; Weber-Bargioni, A.; Schiffrin, A.; Strunskus, T.; Woll, C.; Pennec, Y.; Riemann, A.; Barth, J. V. J. Am. Chem. Soc. 2007, 129, 11279–11285. (28) Zacharia, R.; Ulbricht, H.; Hertl, T. Phys. Rev. B. 2004, 69, 155406-1–155406-7. (29) Roos, M.; Breitruck, A.; Hoster, H.; Behm, J. Phys. Chem. Chem. Phys. 2009, 12, 818–822. (30) Tait, S. L.; Dohnalek, Z.; Campbell, C. T.; Bruce, D. K. J. Chem. Phys. 2005, 122, 194708-1–194708-13. (31) Tait, S. L.; Dohnalek, Z.; Campbell, C. T.; Bruce, D. K. J. Chem. Phys. 2005, 125, 234308-1–234308-15. (32) Wahl, M.; St€ohr, M.; Spillmann, H.; Jung, T. A.; Gade, L. H. Chem. Commun. 2007, 1349–1351. (33) Wintjes, N.; Bonifazi, D.; Cheng, F.; Kiebele, A.; St€ohr, M.; Jung, T.; Spillmann, H.; Diederich, F. Angew. Chem., Int. Ed. 2007, 46, 4089–4092.

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