Long-Range Order in an Organic Overlayer Induced by Surface

Article pubs.acs.org/JPCC

Long-Range Order in an Organic Overlayer Induced by Surface Reconstruction: Coronene on Ge(111) Jesús Martínez-Blanco,*,†,∥ Benjamin Walter,† Arantzazu Mascaraque,‡,§ and Karsten Horn† †

Fritz-Haber Institut der Max-Planck Gesellschaft, 14195 Berlin, Germany Universidad Complutense de Madrid, 28040 Madrid, Spain § Unidad Asociada IQFR (CSIC)−UCM, 28040 Madrid, Spain ‡

ABSTRACT: Using scanning tunneling microscopy, we show that coronene molecules form supramolecular structures upon room-temperature deposition on Ge(111), in full commensurability with the c(2 × 8) reconstruction of the clean surface. The molecules are not adsorbed in a planar manner in the first layer; the balance between lateral intermolecular interactions and directional bonding to the surface forces them into a tilted configuration. Further coronene deposition leads to the formation of large domains of weakly bonded molecules adsorbed on top of the first layer. The ordering of the second layer is dictated by both, the first layer of molecules and the unaltered reconstructed substrate underneath. We provide a detailed analysis of the encountered molecular periodicities and discuss the mechanisms that lead to long-range ordering in these layers.

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fold symmetry.8−10 The coadsorption of coronene with other molecules has been used to give rise to novel mechanisms for intermolecular electronic transport on metals.11 On semiconductor surfaces, a strongly local molecule−substrate interaction often prevents the formation of large ordered islands, which imposes serious limitations for engineering supramolecular structures on these surfaces;12 one way to circumvent this problem is to prepare a nanomesh template previously on the surface.13 An exceptional case is presented by coronene on Ge(001),14 where a weaker interaction strength between coronene molecules and the Ge(001) surface facilitates the formation of large ordered domains of molecules. Generally speaking, the bonding of organic molecules to Ge(001) is much weaker than to Si(001);15 in fact, coronene molecules are bonded covalently to the Si(001) surface dangling bonds and adsorb randomly without forming 2D islands.16 On Si(111), there are also a number of different adsorption configurations, although a certain order is achieved because coronene molecules preferentially adsorb on the unfaulted half of the (7 × 7) unit cell.17 In the present work, we characterize the adsorption of coronene molecules on Ge(111) by means of scanning tunneling microscopy (STM), for coverages up to two molecular layers. Contrary to the case of Ge(001), for very low coverages, it is possible to image single molecules or even small ordered patches of molecules on Ge(111) that presumably condensate at room temperature after diffusing molecules collide and become localized at a nucleation center.

INTRODUCTION The study of molecular adsorption on semiconductor surfaces is an interesting field of fundamental research, and it certainly is the required first step in the strategy of achieving technologically relevant applications of molecular architectures on surfaces.1 In particular, organic semiconductors like those belonging to the family of polycyclic aromatic hydrocarbons (PAHs) have received ample attention due to the relatively high charge mobility observed in the organic crystal.2,3 These molecules can be also used to modify the structural and electronic properties of semiconductor surfaces, not only on large scales but also down to the atomic scale, and a full understanding of the molecule−surface interaction, as well as the intermolecular interactions on the surface, is critical for making controlled use of these systems. Coronene molecules are planar PAHs consisting of seven benzene rings arranged in a “disc” of about 1 nm in diameter (see Figure 1a). They may be also referred to as the smallest “graphene flake”, where the dangling bonds at the perimeter are saturated by hydrogen atoms. Between the coronene molecules, the main source of interaction are the van der Waals forces as they cannot form hydrogen bonds. When adsorbed on surfaces, different structural arrangements are encountered depending on the nature of the surface.4 For metallic surfaces, which have a high density of surface states, coronene molecules tend to adsorb in a planar manner due to the interaction of these states with the molecular π cloud. The delocalized character of the surface states in this case favors a relatively high molecular diffusion and permits the assembly of extensive compact layers. The minimization of the van der Waals intermolecular forces then determines the structure of the molecular superlattice, which is hexagonal5−7 even when the substrate does not have 6© 2014 American Chemical Society

Received: February 13, 2014 Revised: May 11, 2014 Published: May 15, 2014 11699

dx.doi.org/10.1021/jp5015737 | J. Phys. Chem. C 2014, 118, 11699−11703

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We also find that planar adsorption is not compatible with the STM images of molecular islands and show that adjacent molecules are cofacial, forming a finite tilting angle with respect to the surface plane. The ordering of the first molecular layer follows two equivalent periodicities commensurate with the unaltered periodicity of the reconstructed substrate. Contrary to other organic−semiconductor interfaces, no passivation is needed in order to achieve this long-range ordering. Beyond the monolayer, a second layer of molecules is adsorbed, which has a much lower molecular density. We analyze the differences between these two phases and the possible mechanisms for their formation.

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EXPERIMENTAL SECTION The experiments were carried out with a variable-temperature scanning tunneling microscope (STM-150 Aarhus, SPECS Surface Nano Analysis GmbH) under ultra-high-vacuum conditions (base pressure better than 3 × 10−10 mbar). The Ge sample was cleaned by repeated cycles of Ar+ sputtering and subsequent direct current heating to 875 K until the STM of the surface was showing a sufficiently low defect density. Coronene molecules (99%, Aldrich) were deposited at room temperature (RT) from a Knudsen cell doser that was heated to 390 K. All the STM images shown here were acquired at RT and in constant current mode. The bias voltage in the tunneling junction was applied to the sample; thus, negative (positive) bias voltages were used to probe occupied (unoccupied) electronic states of the sample. For image analysis, we used the WSxM package,18 and in any case, the images were filtered or treated, except for the subtraction of a plane.

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Figure 1. (a) STM image (0.09 nA, +1.8 V, scale bar = 10 nm) of a large area of the surface after depositing a very small amount of coronene molecules, marked by the black arrows. In the inset, a balland-stick model of the coronene molecule is shown. (b) Empty states STM image (bias voltage, +2.0 V) of the Ge(111) surface acquired at constant current (0.05 nA). The unit cell, which is marked in blue, corresponds to the c(2 × 8) periodicity. The inset shows the corresponding filled states image (−2.0 V). Scale bar = 2 nm. (c) 3D ball model of the Ge(111) surface. Ge adatoms and rest atoms are imaged as protrusions when tunneling through empty and filled states, respectively, as shown in panel (b).

RESULTS AND DISCUSSION The STM image of Figure 1a shows two Ge(111) terraces after depositing a very small amount of coronene molecules. Most of the visible area in this image presents two of the three possible equivalent domains corresponding to the well-known c(2 × 8) reconstruction of clean Ge(111), shown in more detail in panels (b) and (c). A majority of the bright protrusions, which we attribute to single coronene molecules, lie on specific sites along the Ge(111) domain borders, as indicated by black arrows in the figure. Molecular diffusion across the pristine reconstructed surface at room temperature probably accounts for the relatively low presence of coronene molecules at the defect-free areas of the terraces. Further deposition of molecules leads to nucleation of small compact islands, as depicted in Figure 2. The process is likely to be facilitated by the diffusion of the molecules in the first place, and the subsequent aggregation at a seed in the form of a surface defect or domain border. At this stage, it is possible to identify the deposited molecules together with the pristine Ge(111) surface in the same image, which permits quantifying the registry between both lattices. In the close-up view of panel (b), we identify every protrusion within the island as a single coronene molecule. For reference, a schematic diagram of the van der Waals isosurface of a planarily adsorbed coronene molecule is superimposed on the STM image using the same scale (down-left in Figure 2b). Placing a flat-lying molecule at the position of every protrusion would lead to an overlap between adjacent molecules. We, therefore, conclude that a planar adsorption configuration is not compatible with the STM data, and thus, the molecular plane must form a finite tilting angle with respect to the surface. Panel (c) of Figure 2

depicts a 3D ball model for the proposed molecular adsorption configuration for this compact phase. Within the same Ge(111) domain, two possible molecular arrangements can be distinguished for sufficiently large islands (≳10 nm2). The island shown in Figure 2b presents these two superlattices, whose periodicities can be easily analyzed with respect to the bulk terminated (1 × 1) surface as being c(2 × 6) and its mirror image c(6 × 2) for the left and the right patches, respectively. The diagrams shown in Figure 2b indicate the (1 × 1) periodicity of the bulk terminated Ge(111) in green and, in black, the superperiodicity of the molecular networks (left and center) and the clean reconstructed surface (right). The molecular arrangement is characterized by having two coronene molecules for every three Ge adatoms along the [110̅ ] direction of the Ge(111) surface. The shortest intermolecular distance (8 Å from center to center) occurs along the direction ±60° off the [11̅0] direction. The molecular density for both superlattices is found to be 1.203 molecules/nm2. As proposed in Figure 2c, along the [11̅0] direction, the molecules are adsorbed alternatively on top of an adatom and in between two adatoms. This commensurability of the molecular layer 11700

dx.doi.org/10.1021/jp5015737 | J. Phys. Chem. C 2014, 118, 11699−11703

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Figure 3. (a) STM image (0.16 nA, −2.6 V, scale bar = 50 nm) of Ge(111) after a higher dose of coronene molecules. The numbers indicate how many layers of molecules are present at different regions of the image. In the large terrace, the three possible domain orientations are visible for the adsorbed second layer of molecules (see text for details). (b) Sequence of three consecutive STM images (0.16 nA, −1.85 V, scale bar = 50 nm) acquired on the same area of the sample, where the second layer of molecules is subsequently removed during the tunneling process. The scan direction is from the bottom to the top. The inset shows the first layer of molecules after desorption of the second layer (scale bar = 10 nm).

Figure 2. (a) STM image (0.13 nA, +1.74 V, scale bar = 10 nm) of Ge(111) after deposition of coronene molecules. (b) Topography image (0.11 nA, +2.3 V) on a smaller area marked in (a) by a white rectangle. A schematics of the van der Waals isosurface of a planarily adsorbed coronene molecule has been superimposed. The small coronene islands that condensate on the surface follow two different patterns, having c(2 × 6) and c(6 × 2) periodicities with respect to the bulk terminated Ge(111) surface. The (1 × 1) network is depicted in green in the three diagrams presented below. (c) 3D ball model for the proposed molecular adsorption configuration of coronene molecules on Ge(111).

molecules in three different orientations separated by 120°. For this particular coverage, the second layer is not yet complete, as can be directly seen for the two Ge(111) terraces framed in the scan. The surface here is still partially covered with only one layer of molecules. The adsorption strength of the second layer appears much weaker than in the case of the compact first layer. By scanning repeatedly over the same area at bias voltages around −1.8 V, the tunneling process triggers the desorption of large patches of molecules from the second layer, as revealed in the sequence of STM images shown in Figure 3b, which were acquired consecutively. At that specific bias voltage, the tunneling process is likely to happen in resonance with a molecular orbital, which can facilitate the desorption if the layer is weakly adsorbed. Interestingly enough, the effect seems to be nonlocal; that is, an extensive portion of the molecular layer below the tip is affected so that, on subsequent scans over this area, a significant portion of the first layer underneath becomes directly accessible by the STM. The inset panel at the third image of the sequence shows, in a much greater detail, a region previously covered by two molecular layers that now presents the characteristic periodicity of the first layer of molecules, as described in Figure 2. This compact layer adsorbed directly on top of the substrate is fully stable over time and is largely unperturbed under all scanning conditions utilized. In Figure 4a, we observe the morphology of the second molecular layer in more detail. The spatial resolution allows for a clear imaging of the protrusions that we identify as single coronene molecules. In the gaps left by the second layer of

with the Ge adatom network underneath clearly indicates that the ordering of this phase cannot be governed exclusively by the intermolecular interactions. It is rather the substrate corrugation that imposes this particular arrangement, and the intermolecular forces may have a role in stabilizing the molecular suprastructure. It is this delicate balance between molecule−substrate and molecule−molecule interactions that makes this system quite unique. The molecular bonding to the substrate is sufficiently strong to force the molecules to accommodate according to the adatom network periodicity, but sufficiently weak to prevent a random adsorption as in other systems, such as coronene/Si(001).16 The growth of nanosized islands of organic molecules on semiconductor surfaces has been achieved before, but contrary to the case presented here, the semiconductor surface is typically passivated prior to the molecular deposition in order to limit the high reactivity of the bare substrate.19−23 Increasing the coronene deposition even further, a second layer of molecules starts to settle. In the STM image of Figure 3a, the numbers 1 and 2 label those regions covered with one and two molecular layers, respectively. The majority of the surface is covered by two layers and appears as stripes of 11701

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one molecule every one and a half unit cells (×1.5 periodicity). This mismatch between the two networks forces the molecules in the second layer to slightly squeeze into groups of three molecules to better accommodate on the first layer network. In this way, the molecular environment for all second layer molecules becomes almost identical. Note also that the two sublattices created by the molecules in the first layer for each of the three domains of Ge(111) (c(2 × 6) and c(6 × 2) as indicated in Figure 2b) will serve as a template for the adsorption of stripes of second layer molecules oriented in the same direction. This is the reason why we observe three different stripe orientations, and not six. According to this model, the arrangement of the molecules in the second layer is entirely defined by the substrate and the first layer of molecules, which act as a host structure,28,29 and the intermolecular interactions between second layer molecules are irrelevant here. This view is compatible with the relatively large distance between these molecules along the stripes (≲1.6 nm). Also, this value is large enough to be consistent with a planar adsorption geometry, so the STM data cannot rule out this possibility. For comparison with the single layer phase described in Figure 2, this second layer phase has an overall density of 1.428 molecules/nm2; thus, the second layer contributes only 0.226 molecules/nm2 to the molecular density of the bilayer. Although it is possible to study with the STM technique thicker films of molecules beyond the bilayer,30 it becomes increasingly difficult due to an ever wider tunneling gap and a higher probability of functionalizing the tip with molecules for thicker films. Therefore, other techniques, such as low-energy electron diffraction (LEED) or near-edge X-ray adsorption fine structure (NEXAFS), would be more suitable to quantify the geometrical arrangement of the molecules as their epitaxial growth approaches the 3D crystal situation,31,32 although the formation of amorphous films of coronene is also not impossible.33

Figure 4. (a) Detailed STM image (0.08 nA, −2.6 V, scale bar = 10 nm) taken on the second layer, where it is possible to distinguish individual coronene molecules and the first layer of molecules underneath. (b) Ball model of the proposed adsorption registry between the two layers of molecules and the substrate. (c) Constant current (0.1 nA) STM image of an area with two layers of molecules, where the bias voltage was switched during the scan from −0.1 V (no molecular states are probed) to −1.44 V and back to −0.1 V. For reference, a schematics of the van der Waals isosurface of a planarily adsorbed coronene molecule has been superimposed (orange color molecule). The inset is provided to help in identifying the clean Ge periodicity revealed in the part of the image acquired at −0.1 V. In white, the Ge(111) surface unit cell.

molecules, the corrugation of the first layer is accessible. In order to find out the actual registry of these molecules with respect to the substrate, we performed the experiment shown in Figure 4c, where the bias voltage was first set low enough (−0.1 V) to avoid any molecular states to contribute significantly to the tunneling current. In this way, the pristine Ge(111)−c(2 × 8) is revealed (we see through the double layer of molecules). Further below in the same image, the bias voltage was changed abruptly to −1.44 V so that the molecules can be discerned, and further down again back to −0.1 V. This experiment provides a direct quantification of the adsorption registry of the molecules. As a result of this analysis, we conclude that the molecular stripes shown in Figure 3a and, in more detail, in Figure 4a are oriented along the [11̅0] direction of Ge(111), and therefore, it is obvious that we should find three different stripe orientations, corresponding to the three equivalent rotational domains of the Ge(111) surface. Note also that the image presented in Figure 4c clearly demonstrates that the c(2 × 8) periodicity of the clean surface remains intact, which indicates that the Ge adatom reconstruction remarkably survives the molecular adsorption. The adsorption of other molecules such as C60 have been proven to strongly distort the surface reconstruction.24−27 A tentative ball model for the adsorption geometry of the second layer is proposed in Figure 4b. Along the stripes of the second layer (orange color molecules), we find one molecule for every two unit cells of the Ge(111) substrate, i.e., a (×2) periodicity. However, the molecules in the second layer are obviously in direct contact with the molecules in the first layer, whose registry with the substrate in this direction amounts to

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CONCLUSIONS Coronene on Ge(111) provides a unique organic−inorganic semiconductor interface in the sense that long-range order in the first layer is obtained without the need to passivate the surface. STM experiments for the single and bilayer of coronene on Ge(111) show that the c(2 × 8) reconstruction is preserved upon molecular adsorption. The first layer crystallizes by the aggregation of the molecules in two different equivalent sublattices in each of the three rotational domains of Ge(111) and in full commensurability with its c(2 × 8) reconstruction. A strong interaction with the adatom network of the substrate accounts for this commensurability, and this balance between lateral interactions and directional substrate bonding induces a tilted configuration of the molecules. The first layer acts as an adsorption template for the second layer, which is weakly bound, as demonstrated by tip-induced desorption of the molecules in the second layer. The longrange ordering of the molecules in the second layer can be rationalized in terms of their interaction with the molecules in the first layer.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 11702

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Present Address

(18) Horcas, I.; Fernández, R.; Gómez-Rodríguez, J.; Colchero, J.; Gómez-Herrero, J.; Baró, A. WSXM: A Software for Scanning Probe Microscopy and a Tool for Nanotechnology. Rev. Sci. Instrum. 2007, 78, 013705. (19) Uder, B.; Ludwig, C.; Petersen, J.; Gompf, B.; Eisenmenger, W. STM Characterisation of Organic Molecules on H-Terminated Si(111). Z. Phys. B 1995, 97, 389−390. (20) Keeling, D.; Oxtoby, N.; Wilson, C.; Humphry, M.; Champness, N.; Beton, P. Assembly and Processing of Hydrogen Bond Induced Supramolecular Nanostructures. Nano Lett. 2003, 3, 9−12. (21) Theobald, J.; Oxtoby, N.; Phillips, M.; Champness, N.; Beton, P. Controlling Molecular Deposition and Layer Structure with Supramolecular Surface Assemblies. Nature 2003, 424, 1029−1031. (22) Zhang, H.; Li, Y.; Li, B.; Fang, X.; Ji, H.; Wang, B.; Zeng, C.; Hou, J. Templated Growth of Quasi-One Dimensional Molecular Structures on Si(111)-(4× 1)-In Surface. Surf. Sci. 2009, 603, L70− L73. (23) Baris, B.; Luzet, V.; Duverger, E.; Sonnet, P.; Palmino, F.; Cherioux, F. Robust and Open Tailored Supramolecular Networks Controlled by the Template Effect of a Silicon Surface. Angew. Chem. 2011, 123, 4180−4184. (24) Xu, H.; Chen, D.; Creager, W. C60-Induced Reconstruction of the Ge(111) Surface. Phys. Rev. B 1994, 50, 8454−8459. (25) Wirth, K.; Zegenhagen, J. STM Study of the Adsorption of Single C60 Molecules on the Ge(111)-c(2 × 8) Surface. Surf. Sci. 1996, 351, 13−23. (26) Wirth, K.; Zegenhagen, J. Defect Creation on Ge(111)-c(2× 8) by Thermal C60 Deposition Studied with STM. Phys. Rev. B 1997, 56, 9864−9870. (27) Kidd, T.; Aburano, R.; Hong, H.; Gog, T.; Chiang, T. Structural Determination of the C60/Ge(111) Interface via X-ray Diffraction. Surf. Sci. 1998, 397, 185−190. (28) Griessl, S.; Lackinger, M.; Jamitzky, F.; Markert, T.; Hietschold, M.; Heckl, W. Incorporation and Manipulation of Coronene in an Organic Template Structure. Langmuir 2004, 20, 9403−9407. (29) Schull, G.; Douillard, L.; Fiorini-Debuisschert, C.; Charra, F.; Mathevet, F.; Kreher, D.; Attias, A. Single-Molecule Dynamics in a Self-Assembled 2D Molecular Sieve. Nano Lett. 2006, 6, 1360−1363. (30) Nicoara, N.; Custance, O.; Granados, D.; García, J.; GómezRodríguez, J.; Baró, A.; Méndez, J. Scanning Tunnelling Microscopy and Spectroscopy on Organic PTCDA Films Deposited on Sulfur Passivated GaAs(001). J. Phys.: Condens. Matter 2003, 15, 2619−2629. (31) Zimmermann, U.; Karl, N. Epitaxial Growth of Coronene and Hexa-Peri-Benzocoronene on MoS2(0001) and Graphite(0001): A LEED Study of Molecular Size Effects. Surf. Sci. 1992, 268, 296−306. (32) Lukas, S.; Söhnchen, S.; Witte, G.; Wöll, C. Epitaxial Growth of Pentacene Films on Metal Surfaces. ChemPhysChem 2004, 5, 266− 270. (33) Cho, K.; Shimada, T.; Sakurai, M.; Koma, A. Effect of Growth Temperature and Substrate Materials on Epitaxial Growth of Coronene. J. Appl. Phys. 1998, 84, 268−274.

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Paul-Drude-Institut der Festkörperelektronik, Berlin, Germany. Tel.: +49 30 20377266. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.M. thanks Ministerio de Ciencia y Tecnologiá (MAT201021153-C03-02) for financial support. A.M. and B.W. acknowledge the hospitality of the Fritz Haber Institute during their stay. We gratefully acknowledge the support of the Deutsche Forschungsgemeinschaft under SPP 1459 “Graphene” through project DE 1679/3-1.

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REFERENCES

(1) Leftwich, T.; Teplyakov, A. Chemical Manipulation of Multifunctional Hydrocarbons on Silicon Surfaces. Surf. Sci. Rep. 2008, 63, 1−61. (2) Hummer, K.; Ambrosch-Draxl, C. Electronic Properties of Oligoacenes from First Principles. Phys. Rev. B 2005, 72, 205205. (3) Koller, G.; Berkebile, S.; Oehzelt, M.; Puschnig, P.; AmbroschDraxl, C.; Netzer, F.; Ramsey, M. Intra- and Intermolecular Band Dispersion in an Organic Crystal. Science 2007, 317, 351−355. (4) Thayer, G.; Sadowski, J.; Meyer zu Heringdorf, F.; Sakurai, T.; Tromp, R. Role of Surface Electronic Structure in Thin Film Molecular Ordering. Phys. Rev. Lett. 2005, 95, 256106. (5) Uemura, S.; Sakata, M.; Taniguchi, I.; Hirayama, C.; Kunitake, M. In situ Observation of Coronene Epitaxial Adlayers on Au(111) Surfaces Prepared by the Transfer of Langmuir Films. Thin Solid Films 2002, 409, 206−210. (6) Lackinger, M.; Griessl, S.; Heckl, W.; Hietschold, M. Coronene on Ag(111) Investigated by LEED and STM in UHV. J. Phys. Chem. B 2002, 106, 4482−4485. (7) Hietschold, M.; Lackinger, M.; Griessl, S.; Heckl, W.; Gopakumar, T.; Flynn, G. Molecular Structures on Crystalline Metallic Surfaces − From STM Images to Molecular Electronics. Microelectron. Eng. 2005, 82, 207−214. (8) Schuerlein, T.; Schmidt, A.; Lee, P.; Nebesny, K.; Armstrong, N. Large Molecule Epitaxy on Single Crystal Metals, Insulators and Single Crystal and MBE-Grown Layered Semiconductors. Jpn. J. Appl. Phys. 1995, 34, 3837−3845. (9) Seidel, C.; Ellerbrake, R.; Gross, L.; Fuchs, H. Structural Transitions of Perylene and Coronene on Silver and Gold Surfaces: A Molecular-Beam Epitaxy LEED Study. Phys. Rev. B 2001, 64, 195418. (10) Richardson, N. Adsorption-Induced Chirality in Highly Symmetric Hydrocarbon Molecules: Lattice Matching to Substrates of Lower Symmetry. New J. Phys. 2007, 9, 395. (11) Chen, L.; Li, H.; Wee, A. T. S. Delocalized π State between Molecules through a Surface Confined Pseudodihydrogen Bond. Phys. Rev. Lett. 2010, 105, 226103. (12) Koma, A. Molecular Beam Epitaxial Growth of Organic Thin Films. Prog. Cryst. Growth Charact. Mater. 1995, 30, 129−152. (13) Chen, W.; Wee, A. Self-Assembly on Silicon Carbide Nanomesh Templates. J. Phys. D: Appl. Phys. 2007, 40, 6287−6299. (14) Martínez-Blanco, J.; Mascaraque, A.; Dedkov, Y.; Horn, K. Ge(001) as a Template for Long Range Assembly of π-Stacked Coronene Rows. Langmuir 2012, 28, 3840−3844. (15) Tao, F.; Bernasek, S.; Xu, G. Electronic and Structural Factors in Modification and Functionalization of Clean and Passivated Semiconductor Surfaces with Aromatic Systems. Chem. Rev. 2009, 109, 3991−4024. (16) Suzuki, T.; Sorescu, D.; Jordan, K.; Levy, J.; Yates, J., Jr. The Chemisorption of Coronene on Si(001)-2 × 1. J. Chem. Phys. 2006, 124, 054701. (17) Martínez-Blanco, J.; Klingsporn, M.; Horn, K. Selective Adsorption of Coronene on Si(111)-(7× 7). Surf. Sci. 2010, 604, 523−528. 11703

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