J. Phys. Chem. C 2009, 113, 14935–14940
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One-Dimensional Growth of PTCDA Molecular Rows on Si(111)-(23 × 23)R30°-Sn Surfaces Nicoleta Nicoara,* Zheng Wei,† and Jose´ M. Go´mez-Rodrı´guez Departamento de Fı´sica de la Materia Condensada, C-III, UniVersidad Auto´noma de Madrid, E-28049 Madrid, Spain ReceiVed: May 1, 2009; ReVised Manuscript ReceiVed: July 2, 2009
We report an investigation on the adsorption of 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) molecules on Si(111)-(23 × 23)R30°-Sn surfaces using scanning tunneling microscopy (STM) under ultrahigh vacuum. A novel PTCDA structure, consisting of quasi-one-dimensional molecular chains, is obtained at a submonolayer range. A fine balance between intermolecular and molecule-substrate interactions favors and stabilizes the formation of this molecular structure, which extends homogeneously, creating a highly ordered overlayer with regularly spaced rows in a commensurate (43 × 23)R30° PTCDA reconstruction. The visualization of PTCDA molecular orbitals in bias-dependent high resolution STM images allows for identifying the bonding configuration and the molecular adsorption site. Introduction The possibility to integrate organic molecular semiconductors as active components in conventionally used inorganic semiconductor-based devices has motivated interest in these materials in recent years. By exploiting this organic-inorganic approach, the advantages are two-fold. On one hand, one can benefit from well-defined surface structure properties and overall quality of the currently used semiconductors. On the other hand, the wide range of organic molecules, along with their tunable optical and electronic properties, enable the design of new hybrid devices, with enhanced capabilities. While a great amount of research was focused on the deposition of organic molecules on metallic substrates,1 a less extensive and systematic research was reported for organic-inorganic semiconductor surfaces. Well-ordered molecular structures or complex two-dimensional networks have been successfully produced by self-assembly of organic molecules. Approaches based on hydrogen bond formation between selected molecular species,2-9 metal-coordination interaction,10-15 or more recently, covalently bonded structures16,17 successfully resulted in complex supramolecular architectures, mainly on metal surfaces. On semiconductors substrates instead, the controlled deposition of organic materials to produce ordered nanostructures still represents a considerable challenge. An ordered molecular growth on semiconductors is difficult to achieve, as their surfaces are highly reactive due to the presence of chemically active dangling bonds. A strategy to overcome this difficulty and to achieve an improved structural order is to saturate the surface dangling bonds by means of passivation treatments. In the case of GaAs, for instance, surface passivation with selenium or with sulfur has been employed.18-20 For silicon, hydrogen passivated surfaces were obtained either by wet chemical etching methods or by in situ passivation using atomic hydrogen. Both methods usually result in atomically flat surfaces, with an improved molecular order after the deposition of organic material.21-24 An alternative approach to “passivate” * To whom correspondence should be addressed. E-mail: nicoleta.nicoara@ uam.es. † Present address: Max-Planck-Institut fu¨r Mikrostrukturphysik, Weinberg 2, D-06120 Halle, Germany.
semiconductor surfaces can be exploited by adsorbing metal atoms on Si(111) substrates. For these surfaces, the adsorbatesubstrate interaction could be expected to be intermediate in strength between the clean and hydrogen passivated silicon surfaces. A relevant case is the Si(111)-(3 × 3)R30°-Ag surface for which an ordered growth has been obtained for archetype molecules such as C60,25 cobalt phthalocyanine,26 pentacene,27,28 perylene derivatives,29,30 or hydrogen-bonded networks.31 In this study, we use STM to examine the adsorption and growth of 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) molecules on Sn/Si(111) surfaces. Despite the fact that the Si(111)-(23 × 23)R30°-Sn surface is still semiconducting32 after Sn adsorption, factors such as substrate periodicity, corrugation, or reactivity and PTCDA dimensions allow the formation of a quasi-one-dimensional PTCDA structure. STM imaging of molecular orbitals allows us to determine the adsorption site and configuration of the PTCDA molecules. High resolution images resolve the registry of the molecular structure relative to the underlying substrate lattice and show the formation of a commensurate (43 × 23)R30°-PTCDA layer at submonolayer coverage. Experimental Section The STM experiments were performed at room temperature (RT) in an ultrahigh vacuum (UHV) system with a base pressure below 1 × 10-10 Torr, using a home-built STM.33 Si(111)-7 × 7 surfaces were prepared by flashing the samples at approximately 1150 °C, followed by a fast quenching at 850 °C and then a slow cooling down to RT, at a rate of 10 °C/minute. Pure Sn (99.99999%, from Goodfellow) was evaporated from a homemade Ta crucible, heated by electron bombardment. Sn coverage close to 1 ML (1 ML corresponds to 7.84 × 1014 Sn atoms/cm2) was deposited on the Si(111)-7 × 7 reconstructed surface at RT, followed by 2 min annealing at 650 °C. After cooling down the sample at RT, two coexisting phases are obtained: Si(111)-(3 × 3)R30°-Sn and Si(111)-(23 × 23)R30°-Sn. PTCDA molecules (Sigma-Aldrich) were evaporated from a homemade sublimation cell onto the Sn/Si(111) surfaces, at an evaporation rate of ∼0.03 ML/minute. In this study, 1 ML PTCDA corresponds to one molecule per 23 ×
10.1021/jp904051h CCC: $40.75 2009 American Chemical Society Published on Web 07/28/2009
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Figure 1. Room temperature STM image showing the coexistence of (3 × 3)R30° and (23 × 23)R30° phases of a Sn/Si(111) surface (30 × 30 nm, I ) 0.1 nA, V ) +1.8 V).
23-Sn unit cell. All of the STM images were measured in the constant current mode using the WSxM software. The voltage bias was applied to the sample with respect to the tip.34 Results and Discussion Si(111)-(23 × 23)R30°-Sn Substrate. The exposure of a Si(111)-7 × 7 surface to a certain amount of Sn and further annealing results in the formation of different Sn/Si(111) phases depending on the final Sn coverage on the sample. For ∼1 ML Sn, (3 × 3)R30°-Sn and (23 × 23)R30°-Sn phases coexist at RT. Typical STM images (see Figure 1) show almost defect-free domains of the coexisting phases on large scale areas. Surfaces with exclusively 3 × 3 or 23 × 23 periodicities can be obtained by depositing precise amounts of Sn, i.e., 1/3 ML and 13/12 ML, respectively; however, the coexistence of both phases was not critical in this work. The present study mainly focuses on the adsorption of PTCDA on the Si(111)(23 × 23)R30°-Sn surface (23-Sn in the following). Currently, the accepted structural model for the 23-Sn phase is that proposed by Ichikawa and Cho.35 The schematic model consistent with the reported relaxed atom positions is shown in Figure 2a. According to this model, the 13 Sn atoms within the unit cell are distributed in a two layer-structure on top of the Si(111) surface. The topmost layer consists of two Sn adatom pairs, labeled Sn-I and Sn-II, which differ in both vertical and lateral positions. The Sn-II pair appears slightly lower in comparison to the Sn-I pair, and has the atoms closer to one another regarding the lateral position as shown in Figure 2a. A high resolution STM image (Figure 2b) and the superimposed model of the 23-Sn unit cell (only the topmost layer is shown) allow attributing the brighter protrusions to the Sn-I pair (green spheres) and the less bright protrusions to the Sn-II pair (blue spheres). The difference in the apparent height between both adatoms pairs measured from STM is ∼0.6 Å, in agreement with previous experimental results and first-principles calculations.35 Submonolayer PTCDA Coverage on Si(111)-(23 × 23)R30°-Sn Phase. PTCDA (Figure 2c) deposition on the 23-Sn surface was performed in several steps up to halfmonolayer coverage. A series of topographic STM images as illustrated in Figure 3 shows the evolution of sample morphology with increasing PTCDA coverage. At 0.2 ML PTCDA (Figure 3a), the adsorbates are imaged as bright protrusions
Figure 2. (a) Structural model of the Si(111)-(23 × 23)R30°-Sn surface, adapted from ref 35. The dashed line represents a symmetry axis of the 23-Sn unit cell. (b) High resolution STM image of the Si(111)-(23 × 23)R30°-Sn surface. The bright and less bright protrusions in the image correspond to the Sn adatom pairs of the topmost layer, as shown by the two superimposed 23-Sn unit cells (5.0 × 4.0 nm, I ) 0.1 nA, V ) +2.2 V). (c) Chemical structure of PTCDA.
randomly distributed over the surface. The observed protrusions have similar shape and size and they are assigned to single PTCDA molecules. They can be found either isolated or assembled into short chains as highlighted by the ovals marked on the image. The observed chains are composed of molecules with the same orientation. With an additional exposure, the length and the density of these molecular chains increase, as can be seen in Figure 3b, which corresponds to PTCDA coverage of 0.35 ML. For PTCDA coverage close to 0.5 ML (Figure 3c), the molecular rows extend homogeneously over the entire surface, forming a highly ordered structure with regularly spaced molecular rows. Even though the space between rows would be sufficient for the adsorption of an additional molecule; such an arrangement was not observed for this coverage. Adsorption Geometry. The adsorption geometry of PTCDA on Si(111)-(23 × 23)R30°-Sn surfaces is inferred from high resolution bias-dependent STM images where intramolecular features are clearly resolved, as shown in Figure 4. Despite the fact that the overall appearance of molecular features in STM images can be significantly altered by interaction with the substrate, comparison between the resolved intramolecular features and shapes of specific molecular orbitals of the free molecule may provide information on the adsorption geometry and molecular configuration. In STM images corresponding to occupied states of the sample (Figure 4a, V ) -2.0 V), each molecule within a row is characterized by several distinct features. The distribution of five lobes resolved on one side of the molecule clearly resembles the LUMO (lowest unoccupied molecular orbital) of the free PTCDA molecule. As shown in the inset of Figure 4a, LUMO can be described by two groups of five maxima, each of them located symmetrically along the large molecular axis and separated by a nodal plane. The intramolecular features observed in STM images recorded at positive bias compares well with the LUMO+2 of the free PTCDA (Figure 4b, V ) +1.5 V). The resolved intramolecular features and their similarities to the shape of LUMO and
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Figure 3. Representative STM images of the Si(111)-(23 × 23)R30°-Sn surface after successive exposures to PTCDA. The bright spots partially covering the substrate are assigned to individual molecules. (a) At 0.2 ML, either isolated molecules or short molecular chains are observed, V ) +2.0 V, I ) 1.0 nA. (b) At 0.35 ML, the length and the density of molecular rows increase, V ) +2.2 V, I ) 0.1 nA. (c) At 0.5 ML, highly ordered molecular rows extend over the surface, V ) +2.2 V, I ) 0.1 nA. In all cases, two orientations of PTCDA rows can be observed for a single substrate domain. Notice the arrows indicating the substrate direction. Images size: 50 × 40 nm.
Figure 4. Bias-dependent STM images of 0.5 ML PTCDA on Si(111)-23-Sn surface. The resolved intramolecular features (highlighted by blue circles marked on top of one molecule in the row) resemble the free PTCDA LUMO and LUMO+2 shown in the insets of parts (a) and (b), respectively. The molecular orbitals were calculated within density functional theory using the SIESTA method,36,37 as detailed in ref 38. (a) V ) -2.0 V, (b) V ) +1.5 V. Images size: 8.0 × 8.0 nm, I ) 1.0 nA. (c) Proposed structural model for the PTCDA adsorption geometry: a1 and a2 represent the substrate lattice vectors. The dashed line represents the PTCDA long axis and indicates its orientation relative to the [112j] substrate direction. (d) Schematic drawing of the proposed commensurate (43 × 23) PTCDA structure on 23-Sn surface. b1 and b2 represent the PTCDA unit cell vectors. (e) STM image showing two preferential orientations of PTCDA relative to the substrate domain (10 × 10 nm, V ) +2.0 V, I ) 0.1 nA).
LUMO+2 allow us to identify the PTCDA long axis, as indicated in Figure 4a by the solid line. The observed molecular shape suggests a rather planar adsorption of PTCDA, i.e., the molecular plane is parallel to the substrate. Since STM data cannot provide accurate information related to this, a slightly tilted adsorption configuration is not excluded. Further information related to the adsorption site and molecular orientation of PTCDA on 23-Sn surface can be deduced from these high resolution STM images, where both the molecular shape and the 23-Sn substrate lattice are simultaneously resolved.
Besides the observed molecular shape, Sn adatoms corresponding to the highest adatoms pair, labeled Sn I in Figure 2a, are resolved next to each molecule. The other Sn-II pair is not visible, but it is assumed to be hidden by the molecule positioned on top. This is schematically shown by superimposing the atomic model of the 23-Sn surface to the STM image (Figure 4b). For simplicity, only the substrate topmost layer, consisting of Sn-I and Sn-II adatom pairs is considered. The 23 × 23-Sn unit cell and the symmetry axis along the [112j] substrate direction can be directly determined according to this repre-
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Figure 5. STM images (a, b, c) of 0.5 ML PTCDA on 23-Sn substrate. Each image corresponds to one of the three substrate domains. Within each substrate domain there are two preferential orientations of the molecule related by mirror symmetry relative to [112j] the substrate direction (50 × 50 nm, V ) +2.2 V, I ) 0.1 nA). (d) Schematic model according to STM data showing the orientations of PTCDA molecular rows with respect to the three symmetry directions of the 23-Sn substrate.
sentation. A derived experimental model corresponding to the PTCDA adsorption on Si(111)-23-Sn surface is shown in Figure 4c. The long PTCDA axis previously determined is found to be 80° oriented relative to the [112j] direction of the substrate or 20° relative to the 23-Sn unit cell a1 vector. Since STM data, in general, cannot yield a direct evidence of molecular adsorption site, from the present results it can be suggested that the corner oxygen atoms in the PTCDA dianhydride group are located very close to the Sn-II adatom pair. Within the rows, the molecules adopt a relatively side-by-side orientation with a separation of 13.3 ( 0.5 Å parallel to the [2j11] substrate direction, in close agreement with the lattice spacing of the 23Sn substrate of 13.28 Å. For the spacing between rows, the measured distance is 26.5 ( 0.5 Å which corresponds to twice the substrate lattice spacing. According to the proposed adsorption geometry and the measured lattice parameters, it is determined that PTCDA at 0.5 ML coverage form a commensurate (43 × 23) reconstruction with the unit cell defined by b1 and b2 vectors and one molecule per unit cell, as shown in Figure 4d. Two different orientations of PTCDA molecules can be observed on a single substrate domain as displayed in Figure 4e. The two differently oriented molecules observed in the images have an equivalent adsorption site, with the oxygen atoms close to the Sn-II adatom pair. The angle formed between the PTCDA long axis and the substrate direction, the symmetry axis of the substrate unit cell, is +80° and -80°, respectively. Thus, the mirror plane symmetry with
respect to the [112j] direction explains the two differently oriented molecules. Figure 5 shows three large scale STM images, obtained for 0.5 ML PTCDA coverage on the 23-Sn surface. Each of the images represents one of the three 23Sn domains of the substrate, 120° rotated with respect to each other, as evidenced by the marked arrows. Within each substrate domain, there are two preferential orientations of the molecule, resulting from the mirror symmetry relative to the [112j] substrate direction. By considering the three domains of the 23-Sn substrate and two equivalent adsorption geometries within one substrate domain, a total of six orientations of PTCDA rows result for a coverage of 0.5 ML on the 23-Sn substrate. This is schematically shown in Figure 5d, where molecular rows running parallel to the three equivalent symmetry directions of the 23-Sn substrate are shown. Bonding Mechanism. The finding that PTCDA molecules do not self-assemble into highly ordered compact islands with a herringbone structure, as in the case of PTCDA deposition on inert surfaces,1 and form instead molecular chains indicates a significant molecule-substrate interaction. At the same time, at RT, the molecules have sufficient thermal energy to diffuse on the surface in order to reach specific adsorption sites. Furthermore, the side-by-side orientation of molecules even at the lowest coverages could be indicative of the existence of an attractive intermolecular interaction. This interaction could be related to the permanent quadrupole moment of the free PTCDA molecule,39 which may favor the formation of hydrogen bonds
One-Dimensional Growth of PTCDA Molecular Rows between molecules within rows. According to the proposed adsorption model, we have determined a distance of 2.5 Å between O and H atoms, as indicated by the dotted lines in Figure 4d. This distance is consistent with the formation of C-H · · · O bonds between molecules in close vicinity, further stabilizing the chain growth. Systems containing CdO and C-H groups in near vicinity are known to manifest a great affinity for the formation of hydrogen bonds. This is also the case for PTCDA, whose herringbone structure in the (102) unit cell in the bulk crystal is mostly determined by the molecular quadrupole-quadrupole interaction and hydrogen bonds between the O and H atoms of neighboring molecules.39 Thus, a fine balance between intermolecular (direct or substrate-mediated) and molecule-substrate interactions allow the formation of one-dimensional PTCDA rows. On the basis of the intramolecular contrast, resolved for individual molecules within rows, it has been inferred that PTCDA occupy equivalent adsorption sites on the 23-Sn substrate, with the oxygen atoms of the dianhydride group preferentially adsorbed in the proximity of Sn-II adatom pairs. The present results could be compared to previously reported photoemission spectroscopy results on the interfaces resulting from In or Sn deposition on PTCDA surfaces.40-42 Studies investigating the In/PTCDA interface using photoemission spectroscopy found that In atoms highly react with oxygen atoms in the carbonyl group (CdO), inducing a high density of interface states in the PTCDA band gap. In addition, DFT calculations show significant charge transfer from In to O atoms, as a result of a new In-O bond formation.42 Studies of Sn/PTCDA interface using XPS and UPS reported similar results.40 The authors report a chemical reaction between Sn and O atoms in the CdO group, the process of which is accompanied by a substantial charge transfer. According to these reports, we may suggest a possible Sn-O bond formation, since the O atoms of the CdO group are located close to the Sn-II adatoms pair as inferred from STM results. The proposed bonding mechanism could imply a charge transfer from Sn atoms into the PTCDA molecular states (CdO group) that would account for the observed shift in energy of the PTCDA molecular orbitals, i.e., the observation of the LUMO at highly negative sample biases, while the LUMO+2 is observed at positive sample biases. Evidence of charge transfer from surface to molecules has also been found in a related system, i.e., PTCDA on Si(111)-(3 × 3)R30°Ag.29 For that system, however, the formation of coexisting herringbone and square phases indicates that intermolecular forces dominate over the molecule-substrate interaction. In our present case, to gain further insight on this charge transfer issue photoelectron spectroscopy measurements would be desirable. We expect that the present work will stimulate further research in this direction. Conclusions STM has been used to characterize the initial stage of PTCDA growth on the Si(111)-(23 × 23)R30°-Sn surface. Our results show that a well-ordered PTCDA structure can be obtained on a semiconducting surface. The registry of the PTCDA molecule was determined from high resolution STM images where the intramolecular features and Si(111)-(23 × 23)-Sn substrate lattice are simultaneously resolved. We propose that the molecular anchoring process may be the result of an Sn-O bond formation which stabilizes the molecular rows and promotes the commensurate (43 × 23)-PTCDA structure at 0.5 ML coverage. This system, for which unprecedented one-dimensional structure has been obtained, may be used as
J. Phys. Chem. C, Vol. 113, No. 33, 2009 14939 an organic template to further functionalize silicon-based semiconductor surfaces. Acknowledgment. Financial support from Spain’s MEC under Grant No. MAT2007-60686 is gratefully acknowledged. References and Notes (1) Tautz, F. S. Prog. Surf. Sci. 2007, 82, 479. (2) Bo¨hringer, M.; Morgenstern, K.; Schneider, W.-D.; Berndt, R.; Mauri, F.; De Vita, A.; Car, R. Phys. ReV. Lett. 1999, 83, 324. (3) Barth, J. V.; Weckesser, J.; Cai, C.; Gu¨nter, P.; Bu¨rgi, L.; Jeandupeux, O.; Kern, K. Angew. Chem., Int. Ed. 2000, 39, 1230. (4) Lorenzo, M. O.; Baddeley, C. J.; Muryn, C.; Raval, R. Nature 2000, 404, 376. (5) Theobald, J. A.; Oxtoby, N. S.; Phillips, M. A.; Champness, N. R.; Beton, P. H. Nature 2003, 424, 1029. (6) Sto¨hr, M.; Wahl, M.; Galka, C. H.; Riehm, T.; Jung, T. A.; Gade, L. H. Angew. Chem., Int. Ed. 2005, 44, 7394. (7) Perdiga˜o, L. M. A.; Perkins, E. W.; Ma, J.; Staniec, P. A.; Rogers, B. L.; Champness, N. R.; Beton, P. H. J. Phys. Chem. B 2006, 110, 12539. (8) Chen, W.; Li, H.; Huang, H.; Fu, Y.; Zhang, H. L.; Ma, J.; Wee, A. T. S. J. Am. Chem. Soc. 2008, 130, 12285. (9) Treier, M.; Nguyen, M.-T.; Richardson, N. V.; Pignedoli, C.; Passerone, D.; Fasel, R. Nano Lett. 2009, 9, 126. (10) Messina, P.; Dmitriev, A.; Lin, N.; Spillmann, H.; Abel, M.; Barth, J. V.; Kern, K. J. Am. Chem. Soc. 2002, 124, 14000. (11) Stepanow, S.; Lingenfelder, M.; Dmitriev, A.; Spillmann, H.; Delvigne, E.; Lin, N.; Deng, X.; Cai, C.; Barth, J. V.; Kern, K. Nat. Mater. 2004, 3, 229. (12) Clair, S.; Pons, S.; Brune, H.; Kern, K.; Barth, J. V. Angew. Chem., Int. Ed. 2005, 44, 7294. (13) Stepanow, S.; Lin, N.; Payer, D.; Schlickum, U.; Klappenberger, F.; Zoppellaro, G.; Ruben, M.; Brune, H.; Barth, J. V.; Kern, K. Angew. Chem., Int. Ed. 2007, 46, 710. (14) Me´ndez, J.; Caillard, R.; Otero, G.; Nicoara, N.; Martı´n-Gago, J. A. AdV. Mater. 2006, 18, 2048. (15) Schlickum, U.; Decker, R.; Klappenberger, F.; Zoppellaro, G.; Klyatskaya, S.; Ruben, M.; Silanes, I.; Arnau, A.; Kern, K.; Brune, H.; Barth, J. V. Nano Lett. 2007, 7, 3813. (16) Grill, L.; Dyer, M.; Lafferentz, L.; Persson, M.; Peters, M. V.; Hecht, S. Nat. Nanotechnol. 2007, 2, 687. (17) Matena, M.; Riehm, T.; Sto¨hr, M.; Jung, T. A.; Gade, L. H. Angew. Chem., Int. Ed. 2008, 47, 2414. (18) Hirose, Y.; Forrest, S. R.; Kahn, A. Phys. ReV. B 1995, 52, 14040. (19) Tenne, D. A.; Park, S.; Kampen, T. U.; Das, A.; Scholz, R.; Zahn, D. R. T. Phys. ReV. B 2000, 61, 14564. (20) Nicoara, N.; Custance, O.; Granados, D.; Garcı´a, J. M.; Go´mezRodrı´guez, J. M.; Baro´, A. M.; Me´ndez, J. J. Phys.: Condens. Matter 2003, 15, S2619. (21) Chen, Q.; Rada, T.; Bitzer, T.; Richardson, N. V. Surf. Sci. 2003, 547, 385. (22) Gustafsson, J. B.; Moons, E.; Widstrand, S. M.; Johansson, L. S. O. Surf. Sci. 2004, 572, 23. (23) Sazaki, G.; Fujino, T.; Sadowski, J. T.; Usami, N.; Ujihara, T.; Fujiwara, K.; Takahashi, Y.; Matsubara, E.; Sakurai, T.; Nakajima, K. J. Cryst. Growth 2004, 262, 196. (24) Vaurette, F.; Nys, J. P.; Grandidier, B.; Priester, C.; Stievenard, D. Phys. ReV. B 2007, 75, 235435. (25) Upward, M. D.; Moriarty, P.; Beton, P. H. Phys. ReV. B 1997, 56, R1704. (26) Upward, M. D.; Beton, P. H.; Moriarty, P. Surf. Sci. 1999, 441, 21. (27) Guaino, P.; Carty, D.; Hughes, G.; Moriarty, P.; Cafolla, A. A. Appl. Surf. Sci. 2003, 212-213, 537. (28) Teng, J.; Wu, K.; Guo, J.; Wang, E. Surf. Sci. 2008, 602, 3510. (29) Gustafsson, J. B.; Zhang, H. M.; Johansson, L. S. O. Phys. ReV. B 2007, 75, 155414. Gustafsson, J. B.; Zhang, H. M.; Moons, E.; Johansson, L. S. O. Phys. ReV. B 2007, 75, 155413. (30) Swarbrick, J. C.; Ma, J.; Theobald, J. A.; Oxtoby, N. S.; O’Shea, J. N.; Champness, N. R.; Beton, P. H. J. Phys. Chem. B 2005, 109, 12167. (31) Swarbrick, J. C.; Rogers, B. L.; Champness, N. R.; Beton, P. H. J. Phys. Chem. B 2006, 110, 6110. (32) Ottaviano, L.; Profeta, G. L.; Petaccia, L.; Nacci, C. B.; Santucci, S. Surf. Sci. 2004, 554, 109. (33) Custance, O. PhD Thesis, Universidad Auto´noma de Madrid, 2002. (34) STM data were acquired and processed with WSxM, free software downloadable from http://www.nanotec.es, see: Horcas, I.; Ferna´ndez, R.; Go´mez-Rodrı´guez, J. M.; Colchero, J.; Go´mez-Herrero, J.; Baro, A. M. ReV. Sci. Instrum. 2007, 78, 013705.
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