Commensurate Growth of Densely Packed PTCDI Islands on the

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Commensurate Growth of Densely Packed PTCDI Islands on the Rutile TiO2(110) Surface Valeria Lanzilotto,†,∥ Giacomo Lovat,† Gonzalo Otero,‡ Laura Sanchez,‡ Maria Francisca López,‡ Javier Méndez,‡ José A. Martín-Gago,‡ Gregor Bavdek,§ and Luca Floreano*,† †

CNR-IOM, Laboratorio TASC, Basovizza SS-14, Km 163.5, I-34149 Trieste, Italy Grupo ESISNA, CSIC-ICMM, C/Sor Juana Inés de la Cruz 3, E-28049 Madrid, Spain § Faculty of Mathematics and Physics, and Faculty of Education, University of Ljubljana, Ljubljana, Slovenia ∥ Department of Chemistry, University of Florence, Via della Lastruccia 3, I-50019 Sesto Fiorentino, Italy ‡

ABSTRACT: We have studied the deposition of perylenetetracarboxylic-diimide, PTCDI, on the rutile (1 × 1)TiO2(110) surface. At variance with other polyaromatic hydrocarbons, like acenes, PTCDI displays a significant interaction with this dielectric substrate. At moderate substrate temperature (∼400 K), first layer molecules aggregate into two-dimensional islands corresponding to a (1 × 5) commensurate phase. According to our surface diffraction, STM, and NEXAFS studies, the substrate accommodates one PTCDI molecule per unit cell, atop each oxygen row. Because of steric repulsion, molecules lie on their long edge, tilted by ∼35° with respect to the surface. This constraint determines a strong π−π coupling between adjacent molecules, resulting into a geometry similar to that reported for acenes on (1 × 1)-TiO2(110), but quite uncommon for perylenes.

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INTRODUCTION Polyaromatic hydrocarbons, PAHs, and their derivatives represent the largest class of small organic molecules for electronic applications, ranging from transistors to photovoltaics. At the nanoscale, the anisotropy of their molecular structure can be further exploited to optimize specific transport properties into layered device architectures. Throughout the development of organic solar cells, perylene-based molecules have always been the most important acceptor materials for nonfullerene based devices.1 By functionalizing different positions of the 12 peripheral atoms, perylene derivatives with significantly different optical, electronic, and morphological properties can be prepared.1−4 As prototypes of all the perylene-derivatives, perylene dianyhdride (PTCDA) and perylene di-imide (PTCDI) have been widely studied in surface science (see Figure 1). Most of these studies focused on their interaction with metal substrates, as representatives of archetypal hybrid junctions in electronic devices, such as Ag,5−9 Au,10−15 and Cu.9,16,17 Only very recently, ultrathin films of PAHs started to be studied on the TiO2(110) surface. This transition metal oxide is widely employed in organic electronics for double purpose. In its stoichiometric form, TiO2 presents a large dielectric constant, very useful for the fabrication of high voltage operating transistors. However, some crystalline surfaces can be easily reduced to become strongly reactive and conductive, a transport property exploited in organic photovoltaics, where crystalline powders are coupled to organic dyes. Although dyesensitized solar cells, DSSCs, rather exploit TiO2 in its anatase © XXXX American Chemical Society

Figure 1. Perylene and its most common peri-modifications: perylenetetra-carboxylic-dianhydride (PTCDA or PDA) and perylene-tetracarboxylic-di-imide (PTCDI or PDI).

form,18,19 the investigation of hybrid interfaces that use the rutile (110) surface, which is almost as reactive as the anatase (101) one,20 would allow a much more detailed study for improved modeling, because of its well-known surface structure and simpler in situ preparation. The rutile (1 × 1)-TiO2(110) surface is characterized by “bridging” oxygen, Obr, rows running along the [001] direction and protruding 1.2 Å out of the surface. The adjacent Obr rows are separated by 6.495 Å, and the Obr atoms are spaced by Received: March 22, 2013 Revised: May 9, 2013

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dx.doi.org/10.1021/jp402852u | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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direction, tilted by an angle of ∼35°, and with negligible distortion of the carbon backbone. The molecular tilting allows side-by-side accommodation of one molecule atop each Obr row, yielding closely coupled molecular domains, much more compact than the PTCDA monolayer phase.41 This geometry, quite uncommon for perylenes, is equivalent to that observed for acenes, and it is observed to allow the accommodation of next layer molecules in a closely π-stacked geometry.

2.959 Å along the rows. Indeed, the large corrugation and strong anisotropy of this substrate have been tentatively exploited to drive the adsorption geometry of different kinds of organic molecules, from small ones (benzene,21 pryridine,22 2-butanol,23 terephthalic acid24−26) to relatively large heteroaromatics (such as phthalocyanines27,28 and porphyrins29), polyconjugated oligomers (sexiphenyl,30 sexithiophene31), fullerenes,32,33 and acenes.34−36 Recently, a few studies of deposition on TiO2(110) of perylene37 and some derivatives have been reported, such as PTCDA,38−41 functionalized PTCDI,42 di-indeno perylene, DIP,43 and functionalized perylenes.44 Apart from the direct bonding through specific PAH functionalization, as in ref 44 for the case of a simple van der Waals interaction with TiO2(110), the lattice spacing between the protruding Obr rows (∼6.5 Å) is well suited to host the aromatic units of PAHs. The acenes have shown a common self-assembly mechanism on the rutile (1 × 1)-TiO2(110) surface, indeed. Naphthalene,34 anthracene,35 and pentacene36 were found to lie more or less parallel to the surface with the major molecular axis azimuthally oriented along the substrate [001] direction. We recently gathered additional information for the case of pentacene by investigating the electronic and structural properties with complementary techniques.36 Pentacene aggregates into a long-range ordered molecular overlayer, where molecules are coupled side-by-side to form stripes running along the [11̅0] direction (i.e., transverse to the atomic rows). Molecules within the stripes strictly preserve the substrate periodicity along the [11̅0] direction by tilting the molecular plane around the long axis about 25° off the surface. The molecular overlayer is incommensurate along the opposite [001] direction, where the interstripe spacing is simply dictated by the pentacene coverage (i.e., by the chemical potential), yielding a long-range order slightly beyond a 6-fold periodicity at the saturation of the monolayer. Notwithstanding a larger width of the molecular plane, perylenes in the first layer are also found to lie almost flat on the TiO2(110) surface and with the long molecular axis oriented along to the Obr rows, with the possible exception of DIP43 and functionalized perylenes.44 In particular, a microscopic study of PTCDA deposition unveiled the formation of a commensurate phase in the monolayer range, which has been interpreted in terms of an interaction of the anhydride O atoms with the Ti atoms underneath resulting in significant distortion (bending like an arch) of the molecular structure.41 This molecular configuration is much different from the one expected for PTCDI, where DFT calculations predict the molecule to stay atop the Obr rows, slightly tilted around the long axis.42 In this work, we present a detailed structural study on the adsorption of PTCDI on the (1 × 1)-TiO2(110) surface revealing how its adsorption geometry is quite similar to that observed for the acenes, in spite of a much different mechanism of growth. PTCDI domain formation proceeds via island growth due to a significant interaction with the substrate, which pins the molecules into a unique structural arrangement that is commensurate with the substrate. By surface diffraction, we observe the emergence of a (1 × 5) molecular superstructure. The competition of the substrate interaction with a strong but anisotropic molecular attraction (side-by-side) prevents the formation and coalescence of large islands, and a large density of domain walls is always observed. By NEXAFS we observe that molecules adsorb with their long axis parallel to the [001]

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EXPERIMENTAL SECTION Surface diffraction by He scattering and X-ray absorption spectroscopy experiments have been performed at the ALOISA beamline installed in the Elettra Synchrotron (Trieste). Scanning tunneling microscopy measurements have been performed at the ESISNA group of the Instituto de Ciencia de Materiales de Madrid (ICMM- CSIC) by means of a commercial room temperature STM (Omicron head, driven by a Nanotec electronics). In order to guarantee the experimental reproducibility, several TiO2(110) samples (from Mateck company) have been employed and exchanged among the three experimental chambers. PTCDI was deposited on samples with different degree of oxygen reduction (from yellow transparent to black samples) without detecting any significant variation of the film growth from the structural and morphological point of view. Further details about sample cleaning and ordering can be found in ref 36. PTCDI powder (Alfa-Aesar, Product N. 44098) was evaporated from homemade Knudsen cells (boron nitride) with a thermocouple in direct contact with the powder. We filled the crucibles with the powder as-received; thus, before deposition, the cells were outgassed at a temperature slightly higher than the operating one in order to remove completely any residual contamination from molecular fragments or precursors. We always used low deposition rates on the samples in the range 10−30 min/ML, corresponding to a temperature of ∼670 K. He atom scattering, HAS, has been performed at the HASPES endstation of the ALOISA branchline. This technique exploits thermal energy neutral He atoms to probe the surface symmetry and defect density in real time during molecular deposition at different substrate temperature. At variance with X-ray and electron diffraction techniques, HAS is a nonpenetrating and nonperturbing probe well suited to study the deposition of organic molecules on TiO2(110) that are both sensitive to radiation damage (see ref 45 for a direct comparison of the three techniques). As a consequence, we defined the optimal protocol of PTCDI deposition by HAS measurements. Within the endstation of the ALOISA beamline, a RHEED system is employed to check the consistency of the PTCDI phase preparation with the HAS one. The STM UHV chamber in Madrid is equipped with a LEED system for the same purpose. Near-edge X-ray absorption fine-structure spectroscopy, NEXAFS, experiments have been performed at the carbon Kedge by collecting the partial electron yield with a channeltron facing the sample and a polarized grid in between (negative bias of −250 V) to filter out low energy secondary electrons. At ALOISA, the orientation of the surface with respect to the linear polarization of the photon beam is changed while keeping a constant grazing angle α. The polarization can be changed from transverse magnetic, TM, to transverse electric, TE, by a rotation around the photon beam axis of the polar angle θ from 90° to 0°, which allows us to determine the B



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average tilt angle γ of the molecular plane with respect to the surface, according to the symmetry of the molecular orbitals.45 The rectangular lattice of the substrate further allows us to determine the orientation of the molecular plane with respect to the substrate symmetry by changing the azimuthal orientation ϕ of the surface in TE polarization (see details in ref 46).

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RESULTS AND DISCUSSION Surface Diffraction. The film growth mode was first studied by monitoring in real time the He-atom reflectivity of the surface during PTCDI deposition. At room temperature, RT, the reflectivity simply vanishes according to an exponential slope (see Figure 2) that is indicative of a disordered, possibly

Figure 3. Surface diffraction patterns taken for 1 ML of PTCDI on the TiO2(110) surface. Upper panel: HAS diffraction scans taken at RT along the [001] direction for the clean substrate (filled markers) and 1 ML of PTCDI (open markers) deposited at TS = 420 K. The PTCDI film shows a peak belonging to a 5-fold periodicity. Both scans are shown in a log intensity scale; the scan measured on the PTCDI film is amplified by a factor 100. Lower panel: RHEED diffraction pattern taken with the beam transverse to the [001] direction. Aside the substrate (0,0) and (0, ±1) Bragg peaks, additional streaks of a 5-fold periodicity can be appreciated. Figure 2. Intensity of the HAS specular reflectivity (0,0) during deposition at different substrate temperatures. The exponential intensity decay is emphasized by the log intensity scale in the main graph. The inset (linear intensity scale) shows the emergence of a weak reflectivity oscillation as the substrate temperature is increased.

angular accuracy (±0.01°) of the HAS apparatus, the extra peaks can be unequivocally associated with a commensurate 5fold periodicity. We have seldom detected additional 5-fold peaks of higher diffraction order. The spacing associated with the 5-fold periodicity (14.8 Å) is compatible with the long side of PTCDI (14.4 Å vdW radius8) suggesting that molecules are aligned head-to-head parallel to the [001] direction, like in the monolayer saturation phase of pentacene.36 The PTCDI length, shorter than the pentacene one (16.2 Å vdW radius48), corresponds exactly to the smaller periodicity of the molecular superstructure: we pass from a quasi-6-fold periodicity for pentacene to a 5-fold periodicity for PTCDI. Like in the case of pentacene, we never observe the emergence of additional diffraction peaks along the [11̅0] substrate direction. For comparison, we applied the same protocol of growth in the ALOISA endstation, where the surface symmetry can be probed by RHEED. Although poorly quantitative, RHEED has a sensitivity to point defects much lower than HAS. As shown in the lower panel of Figure 3, the 5-fold superlattice is weak, but fully developed along the [001] direction. No extra peaks are detected along the [11̅0] direction, but the substrate ones. As a consequence we can conclude that the monolayer phase of PTCDI displays a two-dimensional, 2D, commensurate (1 × 5) symmetry. According to the STM images shown in the next paragraph, we can associate the overall low intensity of the diffraction patterns along the [001] direction with a large

3D, growth. By increasing the substrate temperature, we observe the emergence of a weak plateau at TS = 380 K, which becomes a well-defined oscillation at TS = 420 K, although with a very small amplitude (see inset of Figure 2). The weak increase of the He reflectivity at the oscillation maximum witnesses the smoothing of the surface roughness due to the formation of an ordered overlayer. We conventionally associate the completion of the first monolayer with the coverage corresponding to the oscillation maximum. The overall low intensity of the He reflectivity might be attributed to static disorder (domain walls, point defects, or 3D islands) as well as to thermal vibrations of the molecules (Debye−Waller attenuation). The latter aspect can totally hamper the detection of the HAS diffracted peaks also in the case of a very long-range ordered phase, as very recently found for fullerene molecules on TiO2(110).33 As a matter of fact, new diffraction features emerged in the HAS patterns only upon cooling the sample to room temperature after deposition. In a coverage range of 1 ± 0.2 ML, the 1-dimensional HAS diffraction scans along the [001] substrate direction have systematically revealed the appearance of new peaks aside the (0,0) specular one (see Figure 3). Thanks to the extremely high C



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Figure 4. Series of STM images recorded in constant current mode. (a) 0.2 ML PTCDI deposited at RT. The bright rows running from the top left to the bottom right corners correspond to the rows of Ti atoms. PTCDI molecules appear as bright ovals oriented along the [001] direction (I = 19 pA, V = 636 mV). (b) Detail of the (1 × 5) PTCDI island selected by the square in panel a. The green lines superimposed to the image correspond to the substrate Ti rows . The bright lobes are clearly seen to stay in between, i.e., atop the Obr rows. (c) ∼0.4 ML PTCDI annealed to 420 K after deposition at RT (I = 120 pA, V = 1360 mV). (d) At a coverage exceeding the monolayer (∼1.2 ML), islands display a large density of 2nd layer molecules, for which spacing and orientation become less regular (I = 26 pA, V = 1610 mV).

On the contrary, PTCDI growth is always dominated by a stronger interaction with the substrate that keeps the molecules in register with the substrate lattice. Although detected only beyond ∼3/4 ML, the constant observation of 5-fold symmetry peaks strongly supports a mechanism of growth via island nucleation and coalescence. This mechanism is also different from that driving the formation of the chemisorbed brick-wall c(2 × 6)-PTCDA phase, which sets in upon precipitation of physisorbed meandering stripes, when the PTCDA coverage approaches the monolayer.41 Scanning Tunnelling Microscopy. To obtain a deeper insight into the growth process, we characterized by STM the deposition for a coverage ranging from 0.2 to 1.5 ML. In STM images (see Figure 4), PTCDI molecules appear as bright ovals elongated in the [001] direction. At low coverage, the substrate Ti atoms are clearly observed on the uncovered surface areas, as bright rows running along the [001] direction.47 Figure 4 shows the formation of compact molecular islands since the early stages of deposition. The island inner structure is preserved as the coverage approaches the saturation of the first layer, but we never observed the full coalescence into large domains when approaching the monolayer coverage. Within the STM apparatus we could not evaporate PTCDI following the same

density of defects, as well as with a poor degree of correlation among the PTCDI islands in the [001] direction. When comparing the monolayer phase of PTCDI with that of pentacene, a strong similarity of the geometric structure emerges, whereas the mechanisms of growth, hence the forces driving the molecular orientation, are very different. The small size of the (1 × 5) unit cell (∼6.5 Å × 14.8 Å) indicates a very dense monolayer phase. As the PTCDI molecular width exceeds the spacing between adjacent Obr rows, we might assume that the molecules are tilted with respect to the surface, like observed for pentacene.36 Pentacene growth is driven by a strong side-by-side attraction and a relatively weaker head-to-head molecular repulsion.36 The pentacene interaction with the substrate is very weak and simply dictated by the large corrugation associated with the Obr rows that confines the molecules within the troughs (templated growth), thus yielding a 1D commensurate side-by-side coupling of the molecules. The smooth corrugation of the surface charge density along the troughs is not sufficient to pin the molecules into a commensurate spacing along the [001] direction, not even into a metastable intermediate stage. The close similarity of pentacene to the anthracene STM reports35 lets us assume an identical behavior for the shorter acenes too. D



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“domino” structure,14,51 as also reported for PTCDA.16,49,53 In particular, the “domino” structure mimics the molecular arrangement in the (102) plane of the PTCDA crystals, which is the direction of stacking (growth) of next sheets of almost coplanar molecules.54 In the present case, the π−π intermolecular coupling and the substrate interaction are strong enough to overcome the head-to-side hydrogen interaction and to keep PTCDI molecules straightly aligned along the [001] direction with a spacing as small as 14.8 Å . In this regard, it is not surprising that second layer molecules mimic the stacking geometry of the first one, as reported for pentacene on several monolayer phases of much different density.55 However, in absence of the direct interaction with the substrate, which pins the molecules to the same spacing of the atomic rows, the π-coupling of 1st to 2nd layer molecules is not strong enough to preserve the first layer periodicity, and the orientation of 2nd layer molecules is irregular, as shown in panel d of Figure 4. NEXAFS. In order to complete the structural investigation of this phase, we made use of polarization dependent NEXAFS measurements for determining the geometric orientation of the adsorbed molecules. In the K-shell spectrum of a planar πconjugated molecule, like PTCDI, the π*-resonances are associated with electronic transitions into unoccupied molecular orbitals (LUMOs) with off-plane symmetry, whereas the σ*-resonances are associated with transitions into LUMOs that lie within the molecular plane. We have thus determined the molecular orientation by collecting NEXAFS spectra for different orientations of the surface with respect to the linear polarization of the photon beam. The grazing incidence is kept at 6° while the sample is rotated around the beam axis to change its polar orientation, from TM (p-polarization, θ = 90°) to TE (s-polarization, θ = 0°). The TE spectra are further measured for two surface azimuthal orientations, namely, with the scattering plane in the [001] and [11̅0] directions. Figure 5 shows the carbon K-edge NEXAFS spectra taken for the aforementioned surface orientations on the (1 × 5) monolayer phase and on a multilayer. First of all, one observes a striking similarity of the LUMO resonances (e.g., in TM polarization) of the monolayer with those taken in the multilayer, that are also fully consistent (albeit better resolved) with those reported in the literature for PTCDI and PTCDA multilayers.39,40,56 This is indicative of a negligible perturbation of the molecular orbital symmetry and density of states notwithstanding the very compact and strictly commensurate packing of PTCDI. When looking at the polarization dependence for the monolayer phase (top panel in Figure 5), a strong intensity decrease of the π*-resonances is observed (and concurrent increase of the σ-symmetry ones), upon switching from TM to TE polarization (polar dichroism). In TE polarization, the π*-resonances almost disappear when the substrate [11̅0] direction is brought in the scattering plane, indicating that the electric field (oriented along the [001] direction) lies in the nodal plane of the π-symmetry molecular orbitals. The azimuthal dichroism of the π* resonances is also accompanied by a small shift to higher photon energy of the main σ* resonance. The latter azimuthal dichroism might be associated with the uniaxial anisotropy of PTCDI,57 thus suggesting that the molecules are oriented with their long axis parallel to the [001] direction, in full agreement with STM indications. As a consequence, the polar dichroism ITE/ITM of the π* resonances when the scattering plane is oriented parallel to the [001] direction is entirely associated with a tilt of the

protocol of growth set up by HAS; rather, we performed postgrowth annealing after room temperature, RT, deposition. The latter procedure slightly improves the island ordering and domain size, as can be seen by comparison of the largest islands in panel a of Figure 4, taken after RT deposition of ∼0.2 ML PTCDI, with the islands in panel c of Figure 4, taken after annealing to 420 K a ∼0.4 ML film deposited at room temperature. The island extension, in terms of number of molecules, is larger along the [11̅0] direction, which we may attribute to an anisotropic surface diffusion (faster along the [001] direction), as well as to a stronger side-by-side interaction of PTCDI. Islands do nucleate on terraces according to a conventional model of condensation from the vapor phase. For intermediate coverage (0.2−0.4 ML), we observe the coexistence of molecular islands and surface defects in the uncovered portion of TiO2(110) surface, such as oxygen vacancies (dark spots) and hydroxyl groups (small bright spots). We have not observed any specific interaction of the island nucleation with the surface defects, whereas the functionalized PTCDI molecules used in ref 42 rather displayed a preferential anchoring to surface defects. From comparison, we might attribute the latter behavior to a direct interaction of the surface defects with the hexyl chains of the functionalized PTCDI.42 From the analysis of images where the substrate Ti rows are discriminated (panel b of Figure 4), we can identify the adsorption site. In particular, molecules are strictly coupled side-by-side and sit atop adjacent Obr rows. Adjacent molecules display the same STM contrast, thus confirming the absence of a superlattice periodicity along the [11̅0] direction, as indicated by surface diffraction. The preference for Obr rows as adsorption site was predicted for the case of single molecules on the surface,42 but with an intermolecular spacing along the [11̅0] direction twice the observed one of 6.5 Å . This value is lower than the expected 8−9 Å width of a PTCDI molecule, thus suggesting a molecular tilting around the major axis. The STM topographic images do not allow us to discriminate the matching of PTCDI with the substrate atoms along the [001] direction. However, considering that a unique PTCDI island aggregation geometry is observed, we can unequivocally associate the 5-fold periodicity observed by diffraction with the island intermolecular spacing along the Obr rows. The large density of point and line defects, as well as the poor degree of spatial correlation among the islands in the [001] direction, are fully consistent with the faintness of the surface diffraction features. Contrary to the c(2 × 6) phase of PTCDA, the topographic appearance of the (1 × 5) phase of PTCDI seems to exclude a chemical bond to the substrate, whereas it indicates a strong π−π interaction between adjacent molecules, as reported for the monolayer phase of pentacene.36 Looking at the adsorption of PTCDI on different substrates like graphite and MoS2,49 Ag/ Pt(111),6 Pt(100),50 NaCl(001),51 Au(111),13,14,52 and Ag/ Si(111),8 first layer PTCDI molecules have been reported to self-assemble in three main structures named “domino”, “canted”, and “brick-wall”.14 These phases are characterized by a flat lying adsorption geometry, where the reciprocal azimuthal orientation is governed by intermolecular hydrogen bonds of different strength, mostly depending on the molecular separation. Starting from a low density phase, where molecules line up in adjacent rows longitudinally displaced (brick-wall phase, like the c(2 × 6)PTCDA-TiO2(110)41), molecules are canted as they get closer, eventually yielding a compact E



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Figure 6. Geometric model of the (1 × 5) PTCDI phase after STM and NEXAFS. Molecules are found atop the Obr rows and tilted by 35°. Molecules are let to slide down into the troughs according to ref 42.

the tilted geometry is consistent with the asymmetric appearance of the molecules as well as with NEXAFS, where the larger measured tilt angle is to be associated with the strong intermolecular π-coupling resulting from the much denser (1 × 5) phase. The PTCDI matching with the substrate along the [001] direction could not be determined from our topographic images; however, the predicted faster diffusion along the [001] direction is consistent with the observed anisotropic size of the (1 × 5) islands. The latter ones are more extended across the rows, where it is easier to capture molecules moving forth and back along the rows. Our experimental evidence clearly shows that a larger computational effort is necessary to quantitatively address the self-assembling of PTCDI on TiO2(110). In particular, the strict commensurability of the islands that is independent of the coverage, together with the very dense packing and the straightness of the molecular alignment along the rows, points to a relatively strong interaction with the substrate. This behavior is in striking contrast with that observed for pentacene36 and anthracene,35 where, in front of an equivalent side-by-side packing geometry, the substrate corrugation along the [001] direction is not able to pin the molecules and the commensurability is only observed in one dimension, across the rows. In general, the interaction between an aromatic molecule and the substrate atoms is found to be maximized by matching the lattice spacing and the distance between adjacent benzene rings.58,59 When molecules are also tilted around the major axis, one must additionally consider the interaction of the hydrogen atoms at the rim of the molecules with the substrate atoms. In the case of pentacene, the aromatic rings are spaced by 2.45 Å which is also the distance between the hydrogen atoms in the multiple peri-positions (see Figure 7). Obviously this size does not fit the lattice spacing of 2.959 Å along the [001] direction,

Figure 5. NEXAFS taken at the C K-edge in the monolayer phase for two opposite orientations of the surface with respect to the X-ray polarization (TM and TE polarization), and for two different azimuthal orientations in TE polarization. Top panel: (1 × 5) monolayer phase, corresponding to an equivalent thickness of ∼2 Å. Bottom panel: multilayer corresponding to an equivalent thickness of ∼15 Å.

molecular plane around the long axis. For a π-symmetry plane and 2-fold symmetry, the tilt angle γ can be obtained from the intensity ratio of the π*-resonances taken in the two opposite polarizations as45 ITE/ITM = tan 2 γ

(1)

which yields γ = 35° ± 2°. The same behavior is observed in the multilayer film although with a reduced degree of dichroism, both polar and azimuthal (bottom right panel of Figure 5). From comparison with STM (panel d in Figure 4), we attribute the reduced dichroism to an increase of disorder rather than to an average increase of the tilt angle (contrary to the case of 2−3 layers of pentacene36). We can compare the structure of the (1 × 5) phase, as drawn in Figure 6, with previously reported theoretical calculations for the equilibrium geometry of an isolated molecule on the TiO2(110) surface.42 The latter DFT study neglected any intermolecular interaction, constrained the PTCDI to keep the long axis parallel to the [001] direction, and neglected possible molecular relaxation; nonetheless, a preferential absorption atop the Obr rows was predicted. The most favorable adsorption site was achieved by letting the molecule tilt around the long axis by 22° and “to slide down” into the trough by a lateral displacement of 1.3 Å. Apart from the lateral displacement that cannot be appreciated in our STM images,

Figure 7. Comparison of the distance between hydrogen atoms in different positions of a polyaromatic complex. F



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Notes

and pentacene never displays the emergence of a commensurate ordering, indeed. Concerning PTCDI, the distance of ∼3.1 Å between the hydrogens in the ortho-positions60 can easily fit the substrate periodicity, but the distance ≤2.0 Å between the hydrogens in bay position would result into a major mismatch. As a matter of fact, whatever scheme involving the interaction with the substrate of the C−H terminations at the PTCDI rim is not sufficient to explain the formation of the (1 × 5) PTCDI island since the early stage of deposition; otherwise, it should be effective for PTCDA too. One must assume an additional role played by the hydrogen terminated imide groups, even if a direct chemical bonding (as reported for pyridine at oxygen vacancies61) seems to be excluded since (i) the molecular D2h symmetry is preserved, as witnessed by the NEXAFS azimuthal dichroism, and (ii) the LUMO resonances located on the imide carbons are closely similar to those of a PTCDI multilayer.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the Italian MIUR through PRIN project DESCARTES N. 2010BNZ3F2 and by the Spanish research projects MAT2011-26534 and CSD20070041.

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(1) Li, C.; Wonneberger, H. Perylene Imides for Organic Photovoltaics: Yesterday, Today, and Tomorrow. Adv. Mater. 2012, 24, 613−636. (2) Chai, S.; Wen, S.-H.; Han, K.-L. Understanding ElectronWithdrawing Substituent Effect on Structural, Electronic and Charge Transport Properties of Perylene Bisimide Derivatives. Org. Electron. 2011, 12, 1806−1814. (3) Facchetti, A. π-Conjugated Polymers for Organic Electronics and Photovoltaic Cell Applications. Chem. Mater. 2011, 23, 733−758. (4) Huang, C.; Barlow, S.; Marder, S. R. Perylene-3,4,9,10tetracarboxylic Acid Diimides: Synthesis, Physical Properties, and Use in Organic Electronics. J. Org. Chem. 2011, 76, 2386−2407. (5) Rohlfing, M.; Temirov, R.; Tautz, F. S. Adsorption Structure and Scanning Tunneling Data of a Prototype Organic-Inorganic Interface: PTCDA on Ag(111). Phys. Rev. B 2007, 76, 1−16. (6) Aït-Mansour, K.; Treier, M.; Ruffieux, P.; Bieri, M.; Jaafar, R.; Fasel, R.; Gro, O. Template-Directed Molecular Nanostructures on the Ag/Pt(111) Dislocation Network. J. Phys. Chem. C 2009, 113, 8407− 8411. (7) Glöcker, K.; Seidel, C.; Soukopp, A.; Sokolowski, M.; Umbach, E.; Bo, M.; Berndt, R.; Schneider, W. Highly Ordered Structures and Submolecular Scanning Tunnelling Microscopy Contrast of PTCDA and DM-PBDCI Monolayers on Ag(111) and Ag(110). Surf. Sci. 1998, 405, 1−20. (8) Swarbrick, J. C.; Ma, J.; Theobald, J. A.; Oxtoby, N. S.; O’Shea, J. N.; Champness, N. R.; Beton, P. H. Square, Hexagonal, and Row Phases of PTCDA and PTCDI on Ag-Si(111) √3× √3R30°. J. Phys. Chem. B 2005, 109, 12167−12174. (9) Manandhar, K.; Parkinson, B. A. Photoemission and STM Study of the Morphology and Barrier Heights at the Interface between Perylene and Noble Metal (111) Surfaces. J. Phys. Chem. C 2010, 114, 15394−15402. (10) Schmitz-Hübsch, T.; Fritz, T.; Staub, R.; Back, A.; Armstrong, N. R.; Leo, K. Structure of 3,4,9,10-Perylene-TetracarboxylicDianhydride Grown on Reconstructed and Unreconstructed Au(100). Surf. Sci. 1999, 437, 163−172. (11) Mannsfeld, S.; Toerker, M.; Schmitz-Hübsch, T.; Sellam, F.; Fritz, T.; Leo, K. Combined LEED and STM Study of PTCDA Growth on Reconstructed Au(111) and Au(100) Single Crystals. Org. Electron. 2001, 2, 121−134. (12) Nicoara, N.; Roman, E.; Gomez-Rodriguez, J. M.; Martin-Gago, J. A.; Mendez, J. Scanning Tunneling and Photoemission Spectroscopies at the PTCDA/Au(111) Interface. Org. Electron. 2006, 7, 287− 294. (13) O’Shea, J. N.; Saywell, A.; Magnano, G.; Perdigão, L. M. a.; Satterley, C. J.; Beton, P. H.; Dhanak, V. R. Adsorption of PTCDI on Au(111): Photoemission and Scanning Tunneling Microscopy. Surf. Sci. 2009, 603, 3094−3098. (14) Mura, M.; Silly, F.; Briggs, G. a. D.; Castell, M. R.; Kantorovich, L. N. H-Bonding Supramolecular Assemblies of PTCDI Molecules on the Au(111) Surface. J. Phys. Chem. C 2009, 113, 21840−21848. (15) Manandhar, K.; Sambur, J. B.; Parkinson, B. A. Morphologies, Structures, and Interfacial Electronic Structure of Perylene on Au(111). J. Appl. Phys. 2010, 107, 063716 (8p). (16) Schmidt, A.; Schuerlein, T. J.; Collins, G. E.; Armstrong, N. R. Ordered Ultrathin Films of Perylenetetracarboxylic Dianhydride (PTCDA) and Dimethylperylenebis(dicarboximide) (Me-PTCDI)

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CONCLUSIONS The adsorption of PTCDI on the (1 × 1)-TiO2(110) surface represents a particular case where the head-to-side intermolecular H-bonds do not play a significant role in the molecular self-assembly mechanism. The commensurate (1 × 5) PTCDI phase, observed from the early island nucleation stage up to the completion of the first layer, is due to the interplay among three different factors: the geometrical constraints imposed by the substrate anisotropy, the π−π intermolecular attraction, and a significant molecule-to-substrate interaction. The substrate corrugation along the [11̅0] direction favors an orientation of the molecules on the surface with their long axis parallel to the [001] direction and tilted around the long axis. The molecular tilt angle is tuned by the π−π coupling between molecules lying in adjacent troughs. They have been found to be tilted by ∼35° with respect to the surface plane, thus preserving the substrate periodicity along the [11̅0] direction. The resulting film structure is quite uncommon for PTCDI, where the imide groups together with the hydrogen terminations are responsible for other types of arrangements (i.e., canted, brick-wall, and domino phases14). The orientation geometry of the (1 × 5)-PTCDI film on TiO2(110) is very similar to that found for the monolayer phase of pentacene on TiO2(110), even if there are some differences. Contrary to the case of acenes,35,36 PTCDI displays an additional significant interaction with the substrate along the [001] direction, which pins the molecules in a strictly straight head-to-head alignment along the Obr rows with a close and commensurate spacing (14.8 Å). In general, the (1 × 1)-TiO2(110) surface is found to be very effective to drive the growth of PAHs. A common aspect of both the 1D-commensurate monolayer phase of acenes and the 2D-commensurate phase of PTCDI is the direct π-coupling between adjacent molecules favored by the substrate anisotropy. This artificial intermolecular packing is expected to have relevant effects on the optical properties of the PAH overlayer.62 Thanks to its thermal stability, the (1 × 5)-PTCDI phase also represents a suitable template to taylor the growth of next layer molecules with improved intermolecular π-overlap and molecular packing, which are beneficial to the performances of n-type organic transistors in terms of both charge mobility63 and environmental stability.64

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REFERENCES

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Commensurate Growth of Densely Packed PTCDI Islands on the

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