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Pentacene Nanorails on Au(110) Gregor Bavdek,† Albano Cossaro, Dean Cvetko,† Cristina Africh,‡ Cecilia Blasetti,‡ Friedrich Esch, Alberto Morgante,‡ and Luca Floreano* CNR-INFM Laboratorio Nazionale TASC, BasoVizza SS-14, Km 163.5, I-34012 Trieste, Italy ReceiVed July 5, 2007. In Final Form: September 14, 2007 We studied the molecular orientation of pentacene monolayer phases on the Au(110) surface by means of near-edge X-ray absorption spectroscopy at the carbon K-shell and scanning tunneling microscopy. The highest coverage phase, displaying a (6 × 8) symmetry, is found to be formed by two types of differently oriented molecules mimicking regular arrays of nanorails. Flat-lying molecules, aligned side-by-side with the long molecular axis along the [001] direction, form long crosstie chains extending in the [11h0] direction. In between the adjacent flat chains, additional molecules, tilted by 90° around their molecular axis, line up head-to-tail into rails extending along [11h0]. These molecules are very weakly hybridized with the substrate, as indicated by their lowest unoccupied molecular orbitals, which closely resemble those of the free molecule. The nanorail structure is found to be stable up to 420 K in vacuum and to also remain in place after exposure to air, thus being a template well suited for further self-assembly of organic heterostructures. The tilted quasi-free molecules open the possibility for an optimal lateral π-coupling to other molecules or molecular assemblies.
1. Introduction Optimal charge transport properties in π-conjugated organic materials are obtained by maximizing the overlap between the π* molecular orbitals of adjacent molecules. As a consequence, the conductivity of organic devices based on these materials is strongly anisotropic and limited by the degree of order of the organic film and the mutual orientation of the molecules.1 Selfassembly of organic monolayers on inert substrates is a viable route to fabricate very well ordered thin films down to the monolayer thickness range. However, the good intrinsic transport properties of these active layers are often hampered at the interface with the electrodes, where the molecular orientation is guided by the interaction with the metal surface, yielding large topological defects2 that strongly reduce the performances of organic electronic devices.3 An alternative route is to directly selfassemble the molecules on properly chosen metal electrodes (bottom contact devices) to exploit the substrate anisotropy for driving the molecular orientation in the growing film. The effectiveness of such a mechanism can be further enhanced by choosing uniaxial symmetry molecules, possibly matching the molecular size with the substrate atomic periodicities. Wellordered thin films of flatly oriented sexithienyl4 and sexiphenyl5 have been grown on metal surfaces, indeed. Pentacene (C22H14), one of the molecules with the highest charge mobility,1,6 forms well-ordered monolayers of flat oriented * Corresponding author. Fax: +39-040-226767. E-mail: floreano@ tasc.infm.it. † Also affiliated with Department of Physics, University of Ljubljana, Ljubljana, Slovenia. ‡ Also affiliated with Department of Physics, University of Trieste, Trieste, Italy. (1) Dimitrakopoulos, C. D.; Malenfant, P. R. L. AdV. Mater. 2002, 14, 99. (2) Muck, T.; Fritz, J.; Wagner, V. Appl. Phys. Lett. 2005, 86, 232101. (3) Dholakia, G. R.; Meyyappan, M.; Facchetti, A.; Marks, T. J. Nano Lett. 2006, 6, 2447. (4) Prato, S.; Floreano, L.; Cvetko, D.; De Renzi, V.; Morgante, A.; Modesti, S.; Biscarini, F.; Zamboni, R.; Taliani, C. J. Phys. Chem. B 1999, 103, 7788. (5) Haber, T.; Muellegger, S.; Winkler, A.; Resel, R. Phys. ReV. B 2006, 74, 045419. (6) Kelley, T. W.; Muyres, D. V.; Baude, P. F.; Smith, T. P.; Jones, T. D. Mater. Res. Soc. Proc. 2003, 771, L6.5.1.
molecules on crystal metal surfaces,7 among them, on the technologically most relevant Au(111) surface.8,9 However, because of the high pentacene mobility, the next layers usually aggregate into randomly oriented three-dimensional clusters (Stranski-Krastanov growth) with the pentacene molecular axis almost normal to the surface, i.e., each single cluster recovers the natural bulk crystal structure of pentacene.10 A few monolayer phases on Ag,11 Au,12 and Cu,13 which mimic the geometry of the pentacene (110) bulk planes, have been reported to support the growth of some additional flat layer, but their stability is only kinetically limited, and dewetting is expected, like it has been observed on Au(111) already at room temperature.14 For prototypical devices, however, a single flat molecular layer may not be sufficient when grown on the metal electrode, since the optical properties are affected by the substrate charge transfer. Single-layer phases of adsorbed gas15 or molecules16 can instead be important as templates for further growth of heteromolecular systems. The geometry of the first organic layer is thus exploited to drive the orientation of the next growing layers. From the electronic point of view, the first flat layer acts as a buffer layer, enabling the following layers to be exploited for the intrinsic electronic properties of the free molecule. We succeeded to grow a well-ordered monolayer phase of pentacene on Au(110), containing both buffer and quasi-free (7) Ruiz, R.; Choudhary, D.; Nickel, B.; Toccoli, T.; Chang, K.-C.; Mayer, A. C.; Clancy, P.; Blakely, J. M.; Headrick, R. L.; Iannotta, S.; Malliaras, G. G. Chem. Mater. 2004, 16, 4497 and references therein. (8) France, C. B.; Schroeder, P. G.; Forsythe, J. C.; Parkinson, B. A. Langmuir 2003, 19, 1274. (9) France, C. B.; Schroeder, P. G.; Parkinson, B. A. Nano Lett. 2002, 2, 693. (10) Mattheus, C. C.; Dros, A. B.; Baas, J.; Meetsma, A.; de Boer, J. L.; Palstra, T. T. M. Acta Crystallogr. C 2001, 57, 939. (11) Casalis, L.; Danisman, M. F.; Nickel, B.; Bracco, G.; Toccoli, T.; Iannotta, S.; Scoles, G. Phys. ReV. Lett. 2003, 90, 206101. (12) Kang, J. H.; Zhu, X.-Y. Chem. Mater. 2006, 18, 1318. (13) (a) So¨hnchen, S.; Lukas, S.; Witte, G. J. Chem. Phys. 2004, 121, 525. (b) Gavioli, L.; Fanetti, M.; Sancrotti, M.; Betti, M. G. Phys. ReV. B 2005, 72, 035458. (c) Satta, M.; Iacobucci, S.; Larciprete, R. Phys. ReV. B 2007, 75, 155401. (14) Beernink, G.; Strunskus, T.; Witte, G.; Wo¨ll, Ch. Appl. Phys. Lett. 2004, 85, 398. (15) Oehzelt, M.; Grill, L.; Berkebile, S.; Koller, G.; Netzer, F. P.; Ramsey, M. G. Chem. Phys. Chem. 2007, 8, 1707. (16) Stoliar, P.; Kshirsagar, R.; Massi, M.; Annibale, P.; Albonetti, C.; De Leeuw, D. M.; Biscarini, F. J. Am. Chem. Soc. 2007, 129, 6477.
10.1021/la702004z CCC: $40.75 © 2008 American Chemical Society Published on Web 12/28/2007
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molecules. A buffer layer of flat pentacene, strongly bound to the substrate, hosts tilted pentacene molecules in between that display very little hybridization with the substrate, although in direct contact with it. The tilted pentacenes are well suited to form lateral heterojunctions with other organic complexes, e.g., C60, like in prototypical photovoltaic cells,17,18 as well as for highly anisotropic two-dimensional electron transport devices.19 First, we have studied the phase diagram of pentacene growth on Au(110).20 In the monolayer range, pentacene forms two commensurate phases with (3 × 6) and (6 × 8) symmetry and a coverage ratio of 8 to 9, respectively. We show that the (3 × 6) phase only contains equivalent, perfectly flat molecules that are aligned side-by-side along [001], thus forming crosstie chains extending along the [11h0] direction. These chains are also preserved in the high coverage (6 × 8) phase, but they display a wider separation since new chains are accommodated in between, with the additional molecules aligned head-to-tail along [11h0] and tilted by approximately 90° around their molecular axis. This structure, formed by three molecules per unit cell (two flat and one tilted), forms large domains of nanorails. 2. Experimental Section By complementary X-ray photoemission (XPS) and He atom scattering (HAS), we have previously found that the (3 × 6) phase corresponds to the saturation coverage of the first pentacene monolayer (ML) at Ts g 470 K. The (6 × 8) phase is the saturation coverage at slightly lower temperature (Ts ) 420 K) and can be equally formed by additional deposition on a previously grown (3 × 6) phase. Real-time investigations by HAS and XPS clearly show that the additional molecules are incorporated within the pentacene layer rather than simply added on top of the (3 × 6) phase. To better elucidate the structure of these two phases, we employed a linearly polarized (95%) photon beam at the ALOISA beamline21 of the Elettra Synchrotron (Trieste) to measure the near-edge X-ray absorption fine structure (NEXAFS) at the carbon K-shell ionization threshold. Pentacene is a planar π-conjugated molecule belonging to the D2h symmetry group, so that the electronic transition from s-symmetry core levels to the π* lowest unoccupied molecular orbitals (LUMOs) is polarized perpendicular to the molecular plane. On the contrary, the transition to the σ* orbitals is mainly polarized in the molecular plane. Exploiting this linear dichroism of the C1s excitation to the π* and σ* molecular orbitals, one can determine the molecular orientation by collecting NEXAFS spectra for different orientations of the surface with respect to the linearly polarized photon beam. As shown in Figure 1, NEXAFS spectra have been taken by rotating the sample around the photon beam axis (polar scan), while keeping the grazing angle fixed at 6°. The same polar scans have also been measured on the clean substrate, for a proper normalization of the NEXAFS spectra to the photon flux at the sample. The NEXAFS signal has been collected in partial electron yield by means of a full aperture detector (channeltron) in front of the sample with an electrostatic high-pass filter set to -200 V, in order to reject secondary electrons. The synchronous acquisition of the drain current on the last refocusing mirror (Au coated) allows us to normalize the measured XPS for any fast fluctuations of synchrotron beam. In addition, it also allows us to perform a posteriori an absolute energy calibration. In fact, the drain current displays the typical spectral features of carbon contamination, which have previously been calibrated at the (17) Yoo, S.; Domercq, B.; Kippelen, B. Appl. Phys. Lett. 2004, 85, 5427. (18) Mayer, A. C.; Lloyd, M. T.; Herman, D. J.; Kasen, T. G.; Malliaras, G. G. Appl. Phys. Lett. 2004, 85, 6272. (19) Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, M. M.; Bechgaard, K.; Langeveld-Voss, M. W.; Spiering, A. J. H.; Janssen, R. A. J.; Meijer, E. W.; Herwig, P.; de Leeuw, D. M. Nature 1999, 401, 685. (20) Floreano, L.; Cossaro, A.; Cvetko, D.; Morgante, A. J. Phys. Chem. B 2006, 110, 4908. (21) Floreano, L.; Naletto, G.; Cvetko, D.; Gotter, R.; Malvezzi, M.; Marassi, L.; Morgante, A.; Santaniello, A.; Verdini, A.; Tommasini, F.; Tondello, G. ReV. Sci. Instrum. 1999, 70, 3855.
Figure 1. Scattering geometry for the NEXAFS spectra. The grazing incidence is kept at 6° while the sample is rotated around the polar angle R. The polar scans are taken for two surface azimuthal orientations, namely, with the scattering plane in the [11h0] and [001] directions (upper and lower panels, respectively). The chemical structure of pentacene (C22H14) is shown on the right side of the figure. first vibrational resonance of the C1s f π* transition (287.40 eV) by simultaneous acquisition of NEXAFS spectra from CO in a windowless in-line gas ionization cell.21 The (3 × 6) and (6 × 8) phases for NEXAFS measurements have been prepared in situ at the ALOISA beamline, according to the protocols described in more detail in ref 20. Scanning tunneling microscopy (STM) images have been taken on a (6 × 8) phase prepared in the ALOISA experimental chamber that has been transferred to a separate chamber, equipped with a variabletemperature Omicron STM. Images taken just after insertion into the STM chamber showed a dominant 8-fold periodicity along the [001] direction with a minority component of 6-fold periodicity. We attribute the residual (3 × 6) domains to a nonoptimal preparation procedure within the ALOISA chamber, where the surface phase was simply checked by visual inspection of reflection high-energy electron diffraction (RHEED) patterns. Within the STM chamber, we have also employed low-energy electron diffraction (LEED) to check the preservation of the surface phase after exposure to air. We observed that prolonged exposure to air over a few hours increased the diffuse background, smearing the diffraction pattern. The appearance of neat LEED peaks of the eighth order was observed after simple annealing beyond 390 K, without affecting the relative population of 6-fold and 8-fold domains in the STM images. This suggests that water adsorption is the main contamination that occurred. Residual contamination is observed to affect the pentacene overlayer only at higher temperature. In fact, we have never recovered a clear (3 × 6) pattern upon sample annealing, as opposed to what is routinely observed during in-vacuum preparation and annealing. STM images taken after annealing to 470 K showed the presence of a large density of small molecular fragments. On the contrary, the (3 × 6) monolayer phase is found to be stable up to ∼ 550 K, when the sample is prepared and annealed in vacuum (see Figure 2 of ref 20).
3. Results and Discussion In the NEXAFS spectra of the free pentacene molecule (gas phase), the π* region below the ionization threshold is characterized by two groups of narrow resonances at about 284 and 285.8 eV photon energy, corresponding to the C1s transitions
Pentacene Nanorails on Au(110)
Figure 2. Polar NEXAFS scans measured on the (3 × 6) phase, by changing the polarization angle every 10°, from R ) 0° (ppolarization) to R ) 90° (s-polarization). Top: sample oriented with the [001] direction along the photon beam axis. The polarization changes from s (lower curve) to p (upper curve) Middle: sample oriented with the [11h0] direction along the photon beam axis. Bottom: integrated intensity of the π /2 NEXAFS resonance as a function of the polarization orientation R. The data points have been fitted according to the function described in the text.
into the LUMO and LUMO+1 states, respectively.22 The broad σ* symmetry resonances are located at hν g 290 eV. In the NEXAFS spectra of the (3 × 6) phase, shown in Figure 2, the resonances due to the contributions of inequivalent C atoms appear to be strongly hybridized, since the transition lines cannot be individually resolved and only the LUMO and LUMO+1 resonances, although overlapped, can still be distinguished. By studying the polarization dependence of the NEXAFS spectra, a strong dichroism is observed: the π* transitions display the maximum intensity when the electric field is almost normal to the surface (polar angle R ) 0°, p-polarization), and their intensity vanishes when the electric field is parallel to the surface (R ) 90°, s-polarization). The reverse behavior is observed for the σ* symmetry resonances. The same dichroism is observed for any azimuthal orientation of the surface with respect to photon beam axis. We can conclude that the (3 × 6) phase contains only one type of pentacene molecules, which lie flat on the Au(110) surface. A more quantitative analysis can be performed by fitting the intensity of the π* transition as a function of the polarization angle. Given the scattering geometry, the π-symmetry of the LUMO and the twofold substrate symmetry, the intensity is described by the function A(cos2 R cos2 γ + sin2 R sin2 γ).23 The (22) Alagia, M.; Baldacchini, C.; Betti, M. G.; Bussolotti, F.; Carravetta, V.; Ekstro¨m, U.; Maariani, C.; Stranges, S. J. Chem. Phys. 2005, 122, 124305. (23) Stohr, J. NEXAFS Spectroscopy; Springer-Verlag: Berlin, 1992; Paragraph 9.4.2.
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tilt angle γ of pentacene with respect to the substrate surface is found to be 0° with an indetermination of about (5° (due to the uncertainty of the absolute polar angle and to the linear polarization that does not exceed 95%). Apart from the absolute molecular tilt angle, the observed disappearance of the π* resonances at 90° polar angle also implies that the LUMOs preserve their free-molecule symmetry, as opposed to the case of the first monolayer of pentacene adsorbed on Ag24 and Cu,25 where larger rehybridization has been reported. As a consequence, we can infer that the planar geometry of the pentacene molecule is not significantly distorted by the substrate charge transfer, which causes the LUMO broadening. By knowing that the (3 × 6) phase corresponds to the saturation of the first layer20 and that the molecules are laying flat, a model of this monolayer phase can be drawn by simple geometric arguments. In fact, the 8.65 × 24.47 Å2 size of the unit cell can only host a single flat pentacene, which is ∼15-16 Å long and ∼5.5 Å wide.10 The (3 × 6) phase is thus formed by flat molecules aligned side-by-side into chains that extend along the [11h0] direction. The lateral separation of the molecules within the chains is 8.65 Å, and the chains are spaced by 24.47 Å: a crosstie geometry results. The NEXAFS spectra of the (6 × 8) phase display the same π* resonances as the (3 × 6) phase, when the polar scan is taken with the [001] direction oriented along the photon beam axis (see upper panel of Figure 3). This means that the same kind of molecules of the (3 × 6) phase are also present in the (6 × 8) one. However, when the polar scan is measured with the photon beam along the [11h0] direction, the (6 × 8) phase displays a dichroic behavior different from that of the [001] direction. When the electric field is normal to the surface (R ) 0°), the π* resonances are still the same as those measured along [001] and for the (3 × 6) phase. These resonances decrease in their intensity as the polar angle R increases, but they do not disappear when the electric field is parallel to the surface (see middle panel of Figure 3). Most importantly, a clear change of the π* shape is observed. At R ) 90°, the LUMO and LUMO+1 resonances are much narrower and clearly separated, indicating that they are associated with pentacene molecules that are differently oriented and less hybridized with the substrate. The study of the azimuthal dependence of these π* resonances with the electric field parallel to the surface (Figure 4) reveals that the intensity of the LUMO and LUMO+1 transitions completely vanishes when the [001] direction is parallel to the photon beam, and it reaches the maximum when the [11h0] direction is parallel to the photon beam. As a consequence, the second type of pentacene must be oriented with the molecular plane normal to the surface (because their π* intensity is maximum when R ) 90°) and parallel to [11h0] (because their π* intensity vanishes when the photon beam is parallel to [001]). Additional pentacenes accommodated in the (6 × 8) phase are thus found to be tilted off the surface and aligned along the [11h0] direction. A more detailed analysis of the LUMO states of these molecules reveals that they closely resemble the LUMO of bulk molecules. For comparison, we have prepared a thicker film of ∼4 ML pentacene deposited at 150 K on a previously prepared (3 × 6) phase. The low substrate temperature reduces molecular diffusion, preventing the surface dewetting by cluster growth, and results in a disordered film. The NEXAFS spectrum taken on this film (24) Ka¨fer, D.; Witte, G. Chem. Phys. Lett. 2007, 442, 376. (25) Ferretti, A.; Baldacchini, C.; Calzolari, A.; Di Felice, R.; Ruini, A.; Molinari, E.; Betti, M. G. Phys. ReV. Lett. 2007, 99, 046802.
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Figure 3. Polar NEXAFS measured on the (6 × 8) phase. Top: sample oriented with the [001] direction in the scattering plane. Middle: sample oriented with the [11h0] direction in the scattering plane. An oval is superimposed to the spectra in s-polarization to put in evidence the appearance of marked π* resonances, which are absent in the spectra taken along the [001] direction. See text for explanation. Bottom: integrated intensity of the π /2 NEXAFS resonance as a function of the polarization orientation R.
Figure 4. Azimuthal NEXAFS scans taken on the (6 × 8) phase in s-polarization (R ) 90°). The azimuthal orientation φ of the surface with respect to the electric field is indicated for each spectrum.
is shown in the upper panel of Figure 5 overimposed to the gas-phase spectrum taken from ref 22. Apart from a slight broadening, all of the narrow features characteristic of the free pentacene LUMO and LUMO+1 states are also present in the film with the same intensity ratio. Moreover, it is also very similar to the NEXAFS spectrum of the (6 × 8) phase taken in s-polarization and with the electric field along [001] (photon beam along the [11h0] direction in our geometry), as shown in the middle panel of Figure 5. For a quantitative comparison, we
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Figure 5. Analysis of NEXAFS spectra (open symbols) taken on different pentacene phases. Spectra have been measured at the same grazing angle of 6° and with the photon beam along the [11h0] direction. For direct comparison, the spectrum of the free molecule from ref 22 is overimposed on each graphic (shaded curve). The deconvoluted components (see text for details) are shown (thin solid lines) together with the best fit curve (thick solid line). Each spectral component has been labeled according to the corresponding photon energy location in the top panel. Top: NEXAFS spectrum taken on a 4 ML thickness film deposited at 150 K on the (3 × 6) monolayer phase. Spectrum taken in p-polarization (R ) 0°). Middle: NEXAFS spectrum taken on the (6 × 8) phase. Spectrum taken in s-polarization (R ) 90°), with the electric field oriented along the [001] direction. Bottom: NEXAFS spectrum taken on the (3 × 6) phase. Spectrum taken in p-polarization (R ) 0°).
have fitted the spectra by means of Lorentzian curves that have been simultaneously convoluted with a Gaussian broadening. Without any constraint to the fitting parameters, a Gaussian broadening of 310-330 meV has been obtained. In the figure, only the deconvoluted peaks are shown in order to allow a proper evaluation of the intrinsic line width. Apart from a general broadening of the spectrum, the (6 × 8) phase differs from the 4 ML film by the complete disappearance of the spectral feature f at the high-energy side of the LUMO+1 state. The lowest energy feature a of the LUMO is also strongly reduced. According to ab initio calculations, both features a and f stem from the central edge-atoms of pentacene,22 i.e., our findings are consistent with the hypothesis of a molecule lying on its long edge. The sharp features characteristic of the free molecule completely disappear in the molecules adsorbed flat on the surface, as can be better appreciated in the lower panel of Figure 5, where the NEXAFS spectrum of the (3 × 6) phase in p-polarization is shown. A very broad spectrum is observed, where only a coarse distinction between LUMO and LUMO+1 states can be made, resembling similar findings on the monolayer phase of pentacene on Ag(111).24,26 In the present case, we have observed that the π symmetry is also preserved for the flat molecules (both
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Figure 6. The (3 × 6) phase (upper panel) is formed exclusively by flat-laying molecules (one molecule per unit cell), which align into chains extending in the [11h0] direction. The (6 × 8) phase (lower panel) also contains flat chains as the (3 × 6) phase, but is separated by chains of tilted molecules, forming regular arrays of nanorails (three molecules per unit cell). The corresponding unit cells are indicated by rectangles (thick lines).
LUMOs disappear in s-polarization, R ) 90°), thus two simple Guassians can effectively account for the intensity of the LUMO and LUMO+1 states in the angular dependence analysis of the (3 × 6) phase. The same analysis can be performed in the (6 × 8) phase when the photon beam is oriented along the [001] direction (upper panel of Figure 3), where only flat molecules are probed. Because of the exact match of the (3 × 6) LUMO spectral region with that of the free molecule, it was not possible to disentangle the contribution of the flat molecule from the tilted ones in the polar scans of the (6 × 8) phase with the photon beam along the [11h0] direction. As a consequence, Stohr’s angular analysis cannot be applied, and simply the intensity variation of the overall LUMO+1 state has been reported in the lower panel of Figure 3. The relative population of the two kind of molecules in the (6 × 8) phase can be roughly estimated from the comparison of the intensity of the LUMO resonances at R ) 90° and 0°, i.e., 1:∼2.2-2.3. From the XPS measurements of the relative coverage of the (3 × 6) and (6 × 8) phases (8 to 9), we notice that the latter must contain three molecules within its unit cell.20 We can conclude that the (6 × 8) phase is formed by one tilted pentacene every two flat pentacenes. The geometry of the (6 × 8) can be easily deduced by increasing the spacing between the adjacent chains of flat molecules (crossties) from 24.47 to 32.62 Å in order to host the additional chains of tilted molecules. The doubling of the periodicity along [11h0] (from 8.65 to 17.3 Å) can be associated with the tilted molecules being aligned headto-tail along [11h0], i.e., they lay down on their edge. A drawing of this geometry can be seen in the lower panel of Figure 6, where we have assumed an arbitrary register with a deconstructed (1 × 1) substrate. In fact, the flat adsorption on transition metal surfaces of pentacene27,28 and other polycyclic aromatic hydro(26) Pedio, M.; Doyle, B.; Mahne, N.; Giglia, A.; Borgatti, F.; Nannarone, S.; Henze, S. K. M.; Temirov, R.; Tautz, F. S.; Casalis, L.; Hudej, R.; Danisman, M. F.; Nickel, B. Appl. Surf. Sci. 2007, 254, 103. (27) Chen, Q.; McDowall, A. J.; Richardson, N. V. Langmuir 2003, 19, 10164. (28) Wang, Y. L.; Ji, W.; Shi, D. X.; Du, S. X.; Seidel, C.; Ma, Y. G.; Gao, H.-J.; Chi, L. F.; Fuchs, H. Phys. ReV. B 2004, 69, 075408. (29) Witte, G.; Ha¨nel, K.; Busse, C.; Birkner, A.; Woll, Ch. Chem. Mater. 2007, 19, 4228.
Figure 7. Upper panel: STM image (100 × 100 nm2 area) of a mixed (6 × 8) and (3 × 6) region, as obtained by ex-situ preparation (ALOISA chamber) and insertion into the STM chamber. The bright rows correspond to the crosstie chains of flat molecules running along the [11h0] direction. Two domains displaying 6-fold and 8-fold periodicity along [001] are indicated by the A and B labels, respectively. Imaging conditions: Vtip ) -0.2V, I ) 0.2 nA, room temperature. Central panel: two line scans along the [001] direction have been taken in the A and B domains (thick bright lines in the upper panel), that put in evidence the 6-fold and 8-fold periodicity, respectively. Lower panel: 60 × 60 nm2 image, where single flat pentacene molecules are resolved in both the (3 × 6) and (6 × 8) phases (labeled A and B, respectively). Domains A-B, A′, and B′′ are grown on different terraces that are separated by monatomic steps of the Au(110) substrate. As a guide to the eye, thick circles enclose groups of three molecules.
carbons27,29 is known to be driven by the matching between the substrate lattice and the aromatic rings of the molecule. However, pentacene register with the Au(110) surface has not been reported yet. To confirm this peculiar geometry determined from spectroscopic analysis, we performed additional STM measurements for a complementary, direct probing of the surface topography. As shown in the upper panel of Figure 7, the (6 × 8) and (3 × 6) domains appear structurally equivalent, as composed by the same chains of flat pentacene (crosstie chains), but with a different interchain spacing. The observed side-by-side alignment of molecules in the flat chains is consistent with similar STM images taken on the (3 × 6) phase and originally attributed to a multilayer planar phase.30 If not aware of the relative coverage by independent measurements, one would erroneously estimate a higher coverage for the (3 × 6) phase than for the (6 × 8) one (larger spacing between flat chains). Although individual molecules within the crosstie chains can be resolved in our measurements (lower panel of (30) Guaino, Ph.; Carty, D.; Huges, G.; McDonald, O.; Cafolla, A. A. Appl. Phys. Lett. 2004, 85, 2777.
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Figure 7), we have not been able to directly image the nanorails of tilted pentacene in between the flat chains. Instead, the rail region appears dark and devoid of structure in the STM images (see the dark lines running aside the bright rows of the crosstie chains in Figure 7), notwithstanding the large spacing periodicity of 32.6 Å between the crosstie chains, the width of which is only 15-16 Å.10,31 This can be tentatively explained by two effects: (1) the reduced hybridization with the substrates results in a reduced conductivity on the edge of the molecule (nodal plane of π-symmetry LUMOs), and (ii) the strong fluctuations of the tilt angle due to the tip-molecule interaction prevent a proper imaging of the molecule. Scanning across the chains always resulted in a large blurring because of an enhancement of the tilting fluctuations. We have not been able to freeze the dynamical fluctuations of the tilted molecules down to 100 K. Apparent discrepancies between the periodicity observed at room temperature by STM and that determined by diffraction techniques have been reported also for the ML phase of sexithiophene on Au(110),4 and they might be equally attributed to dynamical fluctuations of the sexithiophene tilt angle (typical phonon frequencies are in the terahertz range), being faster than the STM sampling frequency. The tilting of pentacene around its long molecular axis might eventually occur for very dense layers also on Au(111),9 as a precursor for the accommodation of the next layer in a herringbone structure (that is, the bulk structure of crystalline pentacene, and shows only small variations among its known polymorphs10). This mechanism has been reported for the first monolayer of sexiphenyl on Au(111),32 where next layers can be accommodated (31) Endres, R. G.; Fong, C. Y.; Yang, L. H.; Witte, G.; Wo¨ll, Ch. Comput. Mater. Sci. 2004, 29, 362.
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in a herringbone structure,5 like what is often observed on inert substrates33 or templated metal surfaces.15 In the present case, what is remarkable is that the pentacene tilting is also accompanied by an azimuthal reorientation of the long axis by 90° to form an assembly of linear chains. This peculiar geometry leaves large enough spacing between nanorails to allow lateral π-coupling with molecules that are not hybridized with the substrate, although in direct contact with the Au atoms beneath. This system turns out to be ideally suited to study the correlation between the molecule-substrate hybridization and the charge-transfer efficiency. In addition, heteromolecules to be coupled with pentacene nanorails would be accommodated on the pentacene crossties, possibly preventing the quenching of the guest molecular orbitals by the Au electrons. This system appears to be a good candidate for the templated growth of prototypical heterojunctions characterized by very fast charge-transfer rates between the linked donor-acceptor (thanks to the nanorail tilted molecules), as well as between the active film and the substrate (through the crosstie molecules). The high degree of order of the (6 × 8) phase is also expected to further enhance the efficiency of a possible heterojunction.18 Finally, this monolayer phase takes advantage of the known stability of pentacene devices against environmental contamination,34 as confirmed by its persistence after exposure to air for a few hours, and a relatively good thermal stability (up to ∼420 K in vacuum). LA702004Z (32) France, C. B.; Parkinson, B. A. Appl. Phys. Lett. 2003, 82, 1194. (33) Resel, R. Thin Solid Films 2003, 433, 1. (34) Knipp, D.; Muck, T.; Benor, A.; Wagner, V. J. Non-Cryst. Solids 2006, 352, 1774.
