Heteroepitaxy of α-Quaterthiophene on Tetracene Single Crystals

J. Phys. Chem. C 2007, 111, 19009-19014

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Heteroepitaxy of r-Quaterthiophene on Tetracene Single Crystals Marcello Campione,* Luisa Raimondo, and Adele Sassella Department of Materials Science and CNISM, UniVersity of Milano Bicocca, Via Cozzi 53, I-20125 Milano, Italy ReceiVed: July 9, 2007

R-Quaterthiophene thin films were grown on the {001} face of single crystals of tetracene by organic molecular beam epitaxy. The peculiar corrugation of the {001} surface of tetracene, its chirality, and its low symmetry give rise to two phases of the overlayer. The morphology-structure and epitaxial relations of these two phases are investigated on a local scale by atomic force microscopy and on a large scale by optical spectroscopy, revealing the orientation mechanisms driving the formation of this organic-organic heteroepitaxial system of semiconductors.

1. Introduction Organic semiconductors have been demonstrated to be suitable materials to be processed with molecular beam deposition techniques.1 The possibility to use these sophisticated growth techniques for organic semiconductors as for inorganic semiconductors rests on the transfer of the concept of epitaxy into the organic world, with all the necessary modifications and technological consequences.1,2 Organic molecular beam epitaxy (OMBE) has been successfully applied for the growth of epitaxial organic monolayers on a huge variety of inorganic substrates.3 In these “hybrid heterostructures”, the achievement of epitaxy is strictly related to the achievement of possible geometrical coincidences between the lattice points of substrate and overlayer. The prerequisites for a successful growth of an epitaxial organic monolayer on an inorganic atomic surface are surely less restrictive with respect to the case of inorganicinorganic epitaxy; this is substantially justified by the absence of strong covalent bonds between the organic molecule and the substrate atoms and by the large difference between the lattice parameters of substrate and overlayer. Far more subtle mechanisms characterize the epitaxial ordering of organic overlayers on organic substrates, and this is probably the reason why the literature offers only a few examples of such organic-organic epitaxy, even if it would lead to important achievements in both fundamental knowledge and future device applications. In the pioneering works by Ward and co-workers,4 some new mechanisms of epitaxial ordering were investigated and attributed to the ability of morphological features of the surface of organic molecular single crystals (step-edges, ledges, and other extended defects) to drive nucleation of foreign organic crystals.4 Very recently, organic heterostructures have been successfully grown and characterized starting from highly orienting inorganic substrates: R-sexithiophene thin films were grown on (2 0 3)-oriented p-hexaphenyl thin films grown on TiO2(110),5 and p-hexaphenyl thin films were grown on (010)-oriented R-sexithiophene thin films grown on Cu(110)-p(2 × 1)O.6 In these two heterostructures a uniaxial orientation of the organic overlayers is achieved, with molecules lying parallel to the substrate surface. This epitaxial orientation is obtained due to an interaction between molecules which tends to line up their molecular axes. * Corresponding author. E-mail: [email protected].

One of the peculiarities of organic-organic epitaxy is the fact that the lattice parameters of the substrate and overlayer may differ only slightly. Contrary to what happens in the context of organic-inorganic epitaxy, this condition may hinder the achievement of coincidence between substrate and overlayer lattice points. Nonetheless, several incommensurate systems exhibit a high degree of epitaxial order.4c,7 In this peculiar situation, different mechanisms of epitaxial ordering dominate. Recently, it has been demonstrated that organic-organic epitaxy can explicate through mechanisms in which lattice match and surface periodicities play no role.7c Rather, peculiar corrugations of the substrate surface drive the film orientation through crystallo-chemical interactions. These mechanisms have been investigated in R-quaterthiophene (R4T) thin films deposited on potassium hydrogen phthalate,7c a system characterized by a lattice mismatch as large as 18%. Here we use the well-developed {001} surfaces of a single crystal of tetracene (TEN, a widely studied organic semiconductor) for the OMBE deposition of R4T. Three characteristics of this surface motivates a great interest of its use as substrate: (i) the in-plane lattice parameters of TEN are extremely close to those of R4T, ensuring a lattice mismatch as small as 0.5%; (ii) due to the molecular arrangement and the triclinic symmetry, the {001} surface of TEN is characterized by a peculiar corrugation, showing grooves of size comparable with a single R4T molecule, formed by rows of prominent H-atoms along the [1 -1 0] crystallographic direction; (iii) the {001} surface is a chiral surface (plane group p1), the two enatiomers being the (001) and (0 0 -1) surfaces. Characteristics i and ii favor the simultaneous presence of two requirements for the epitaxial growth of organic overlayers on organic substrates: lattice match3d and crystallo-chemical affinity.7c Additionally, characteristic iii potentially confers the possibility to separate or select chiral domains. Atomic force microscopy (AFM) is used here for providing a morphological-structural characterization of the film phases crystallized on TEN crystals; in addition, an AFM highresolution analysis is performed in combination with optical absorption measurements for inferring the epitaxial relation between the crystalline substrate and overlayer. The results show that different phases with markedly different morphological

10.1021/jp075331d CCC: $37.00 © 2007 American Chemical Society Published on Web 12/01/2007

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features are nucleated, whose epitaxial orientations relate them to the aforementioned orienting mechanism. 2. Methods R4T was synthesized and carefully purified according to a recently optimized procedure,8 whereas commercial TEN was purified by several sublimation steps before use. Single crystals of TEN were grown from the vapor phase following the procedures reported in ref 9 and placed on a glass plate. Its crystal structure belong to the triclinic system P1h with unit cell parameters a ) 6.06 Å, b ) 7.84 Å, c ) 13.01 Å, R ) 77.13°, β ) 72.12°, γ ) 85.79°.10 Close-packed molecules arranged in a herringbone motif form layers parallel to the (001) planes, with spacing d(001) ) 1.21 nm. TEN single crystals are obtained as thin flakes exposing a wide, clean, molecularly flat {001} surface, which does not require cleavage before use. Due to the triclinic symmetry, in TEN single crystals the (001) face can be distinguished from the (0 0 -1) face by observing the crystal morphology.11 The direction of the surface unit cell of each crystal substrate was determined with a mechanical goniometer by polarized optical microscopy exploiting the birefringence and dichroism of the crystals;12 axes directions are determined with an uncertainty of (2°. The R4T thin films were grown in ultrahigh vacuum by OMBE at a base pressure below 5 × 10-10 Torr, with 170 °C source temperature and about 0.3 nm/min deposition rate. The source was a Knudsentype effusion cell with double-heater and double-temperature control; a quartz microbalance installed close to the substrate, kept at room temperature, was used to dose the material.13 Under these typical growth conditions, R4T films crystallize in the lowtemperature polymorph (R4T/LT), a monoclinic system (space group P21/c) having unit cell parameters a ) 6.09 Å, b ) 7.86 Å, c ) 30.48 Å, γ ) 91.8°.14 In the following sections we will always refer to this polymorph structure when dealing with R4T in the solid state. The calculation of lattice mismatch for the interfaces involved was performed with the program EpiCalc.15 AFM measurements were carried out using a Digital Instruments Nanoscope IIIa in intermitting-contact mode, with silicon cantilevers. Absorption measurements were performed in the spectral range from 2 to 6 eV at normal incidence with linearly polarized light by equipping a Perkin-Elmer Lambda 900 spectrometer with a depolarizer and Glan-Taylor calcite polarizers. In order to avoid as much as possible stray light collection,16 light spots of diameter smaller than 1 mm were used. 3. Results The {001} surface of TEN is a surface of type F (flat), as the strongest intermolecular interactions in the crystals run along directions parallel to this plane. This surface is expected to be molecularly flat over large domains. However, as shown in the background of Figure 1a, after OMBE deposition the TEN surface presents regions appearing in dark contrast and giving a substantial roughness. From the cross-sectional profile of the AFM signal, these regions appear about 1 nm deep, which is consistent with the spacing among (001) planes of TEN (1.21 nm10). Hence, one can conclude that the permanence of the crystal in ultrahigh vacuum induces desorption of molecules from the surface, giving rise to the formation of two-dimensional (2D) holes all over the crystal surface, even under room conditions. Desorption of molecules from the surface of molecular crystals kept in vacuum under room conditions is a known phenomenon, recently documented also for R4T.17

Figure 1. AFM images showing the morphological evolution with coverage of R4T thin films epitaxially grown on the {001} surface of TEN single crystals (a and b). Nominal thicknesses are 1 nm (a) and 10 nm (b). The cross-sectional profiles below each image are taken along the scan line joining the white marks; the orientation of the substrate surface unit cell is reported in each image. Black arrows in panel a indicate 3D R4T islands. (c) 5 × 5 µm2 AFM image of the same film of b in a different area showing the layer-by-layer homoepitaxial growth of R4T. The image is superimposed with the instrument error signal in order to mark the profile of monomolecular steps. (d) 7.5 × 7.5 nm2 high-resolution AFM image collected on the R4T film in panel b; the orientation of the surface unit cell is indicated. (e) Structural model showing the arrangement of molecules at the heterostructure interface between the TEN substrate and the R4T film; the two views are taken along the bR4T axis and along a direction orthogonal to the aTEN axis of the equilibrium crystal structures of the two materials (refs 10 and 14). The interlayer spacing for the structures is indicated on the right.

A small fraction of the AFM image reported in Figure 1a is covered with R4T aggregates formed during the deposition by OMBE of 1 nm thick film. The substrate surface induces the aggregation of R4T molecules in three-dimensional (3D) oriented needles, which appear in bright contrast in the image. However, these needles, which reach a height of 100 nm, coexist with another phase of flat islands (indicated by arrows in Figure 1a), with an average height of 4 nm (see the cross-sectional profile). The observation of films with such a small amount of deposited material but already more than one monolayer thick (1 nm ) 0.7 monolayers of R4T) suggests a 3D nucleation mechanism.

Heteroepitaxy of R-Quaterthiophene on Tetracene

Figure 2. (a) AFM image of an enlarged area (50 × 50 µm2) around Figure 1b. Inset: 2D Fourier transform of the image contrast showing in reciprocal space the preferential orientation of needle-like crystallites. (b) Structural model showing the molecular epitaxial arrangement of the R4T molecules within the needles; the equilibrium surface unit cells of TEN and R4T are indicated in green and in red, respectively. H-atoms of TEN molecules protruding from the (001)TEN surface are highlighted in light red.

The observation of 10 nm thick films of R4T on TEN, deposited under exactly the same conditions, shows that flat islands grow forming a uniform layer, whereas needles increase their length. Figure 1b reports a 20 × 20 µm2 AFM image collected on such a film. The orientation of needles grown on TEN is better visualized in images collected on a larger scale: a 50 × 50 µm2 image is reported in Figure 2a, representing an enlarged area around that of Figure 1b. Needles are observed to be oriented along two specific directions, forming angles of 6.8° ( 0.5° and 66.2° ( 0.8° to aTEN. The 2D Fourier transform of this image, reported in the inset, helps to better define the angular range between the two preferential directions of the needles. This range falls within 50° and 70°. A direct counting of the needles appearing in AFM images has revealed that 60% of them lie at 6.8° to the aTEN directions. Voids are observed in the region nearby each needle (dark contrast), probably due to material desorption favored by the formation of grain boundaries at the interface between needles and islands.17 When analyzing the R4T uniform layer, a thickness close to the nominal one is detected (see the cross-sectional profile of Figure 1b), and on top of that, 2D homoepitaxial R4T islands are nucleated. Indeed, the signal profile shows layers 1.5 nm in height, which

J. Phys. Chem. C, Vol. 111, No. 51, 2007 19011 corresponds to the spacing between adjacent (002) planes of R4T (d002 ) 1.53 nm),14 which enclose molecular monolayers with molecules tilted by 25° (see the structure sketched in Figure 1e). All these results are in perfect agreement with a previous study of the homoepitaxial growth of R4T achieved using (001)oriented single crystals of R4T/LT.18 Figure 1c shows a 5 × 5 µm2 AFM image of the same sample as in Figure 1b, where the 2D R4T layers grown on top of the 3D R4T layer grown on TEN are observed. An impressive aspect of these results is that the epitaxial growth of R4T is not hindered by the roughness of the TEN surface, which is still observable in the dark background of this image. A final assessment of the heteroepitaxial growth of R4T on TEN is achieved by a high-resolution AFM study of the homoepitaxial layers observed. Figure 1d reports a 7.5 × 7.5 nm2 AFM image collected on an R4T layer of the sample reported in Figure 1, parts b and c; the image contrast is characterized by a regular disposition of bright spots arranged in a pseudohexagonal lattice. If a centered unit cell is chosen, the best-fit parameters extracted from the image are a ) 6.9 Å, b ) 8.1 Å, and γ ) 87°, with an uncertainty on estimated values of 3%, and the orientation of the surface unit cell axes is consistent with the epitaxial relation aR4T | aTEN. These experimental unit cell parameters are to be compared with the parameters of the equilibrium unit cell of the {001} surface of R4T, which are a ) 6.1 Å, b ) 7.9 Å, and γ ) 90°.14 A structural model showing the molecular arrangement in the heterostructure is sketched in Figure 1e. The epitaxial relation of the R4T grown on TEN just assessed by AFM analysis on a local scale can be confirmed on a macroscopic scale by means of optical spectroscopy. In particular, normal incidence absorption spectra have been collected with polarized light on the same R4T/TEN(001) systems analyzed by AFM, in the spectral range where both crystalline TEN and R4T show a strong optical response. Figure 3a shows the absorption spectra of the complete heterostructure collected with the electric field E of the incoming light parallel and perpendicular to the aTEN axis. Under linearly polarized light, the known anisotropy of the TEN crystals is observed. Indeed, in the low-energy range of the spectra, the peak series at 2.39, 2.60, 2.80, and 2.96 eV and at 2.47, 2.63, 2.81, and 2.98 eV with | aTEN- and ⊥ aTEN-polarized incident light, respectively, coincide with the known spectral response of crystalline TEN;19 for comparison, the spectra of one of the as-grown TEN crystal used as substrates are shown in Figure 3b. These bands are attributed to the transitions originating from the molecular transition polarized along the short molecular axis M, which in the solid splits into two polarized Davydov excitonic components.20 Below the absorption edge of R4T (2.6 eVssee the reference absorption spectra of an R4T crystal in the upper part of Figure 3b), the line shape of spectra collected on TEN before and after R4T deposition coincides; however, since the spectra have not been corrected for reflectivity, different absorbance values have been observed due to the lower reflectance of the heterostructure (affected by the R4T reflectivity21) with respect to that of the bare TEN crystal.22 At higher energy, evident modifications can be observed, better evidenced in the inset of Figure 3a, where the absorption spectra as a function of the angle of polarization β indicating the electric field direction with respect to the aTEN axis are reported in the region of low TEN absorbance (from 3.2 to 4 eV). The E | aTEN-polarized spectrum (β ) 0°) of the heterostructure reveals the presence of a small peak at about 3.7 eV. For values of β between 0° (E | aTEN) and 90° (E ⊥

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Figure 3. (a) Normal incidence absorption spectra of the complete heterostructure analyzed by AFM in Figure 1b, collected with E | aTEN and E ⊥ aTEN. Inset: absorption spectra in a reduced spectral range for more values of the polarization angle β indicating the electric field direction with respect to the aTEN axis. (b) Normal incidence absorption spectra of a 120 nm thick TEN crystal and of a 400 nm thick R4T crystal as taken on the accessible face ((001) for both cases) with the electric field parallel and perpendicular to the a-axis of the unit cells.

aTEN), the intensity of this peak decreases progressively to zero, reaching a match with the shape of the as-grown TEN crystal. From the known optical response of R4T single crystals (Figure 3b), we can attribute this absorption structure to the transition to the stronger bu state, originating from the first molecular transition of the R4T, which is polarized along the aR4T axis.23-25 In the E ⊥ aTEN-polarized spectrum, no evident spectral features are present different from those in the optical response of the as-grown TEN crystal. This is due to the low absorption coefficient of R4T when E ⊥ a4T in this spectral range.23 At energy above 4 eV, R4T spectral bands originating from higher energy molecular transitions are superimposed to those of TEN crystals, increasing the absorption but creating no distinctive new features for any polarizations. The similarities of the optical behavior of R4T films grown on TEN(001) and R4T single crystals in the spectral range of strong anisotropy of R4T is hence consistent with the optical response expected for the (001) surface of R4T and clearly suggests that R4T films grow heteroepitaxially on TEN, with aR4T aligned to aTEN, in complete agreement with the epitaxial relation assessed by the AFM analysis. 4. Discussion A fundamental tool for the study of heteroepitaxial systems is the analysis of the geometrical lattice mismatch between the substrate and overlayer surface. By observing the equilibrium lattice parameters of TEN and R4T, one can readily realize the perfect metrical match between the lattice parameters in the (001) planes. Indeed, the lattice parameters of the two surfaces are practically identical (a ) 6.06 Å and b ) 7.84 Å for TEN,

Campione et al.

Figure 4. Results of the misfit calculation for the interface between R4T and TEN(001), calculated with EpiCalc for the R4T(001) surface (a), the R4T(010) surface (b and d), and the R4T(0 -1 0) surface (c and e) as contact plane. The azimuthal angle θ corresponds to the angle between aR4T and aTEN. Panels a-c represent the curves obtained with the actual unit cell parameters of substrate and overlayer, whereas in panels d and e a 1 × 2 supercell of the overlayer is considered. The lattice vectors of the overlayer are obtained by multiplying the transformation matrix (reported below each peak) by the vector [aTEN, bTEN].

and a ) 6.09 Å and b ) 7.86 Å for R4T); however, the two surface unit cells have a different symmetry originating from the monoclinic versus triclinic structure of R4T and TEN: the surface unit cell is therefore rectangular for R4T and rhombic for TEN, with γ ) 85.79°. On the basis of these geometric characteristics, one may expect the R4T{001}|TEN(001) interface energy to be very close to the energy of the homoepitaxial R4T{001}|R4T(001) interface, and then giving rise to a layerby-layer type of growth, as expected and actually observed in the growth of R4T.18 Nonetheless, the nucleation mechanism of R4T on TEN(001) is 3D. This may be ascribed to a major role played by the different symmetry of the two crystals on the interface energies. Figure 4a reposts the results of the misfit calculation for the R4T{001}|TEN(001) interface. The presence of minima in the output of the calculation (the adimensional potential V/V0) identifies the azimuth angle θ at which some coincidence between the lattice points of the substrate and overlayer occurs (in Figure 4a this is the angle between aR4T and aTEN; positive values correspond to counterclockwise rotations of the overlayer with respect to the substrate). Due to the extremely high accordance between the overlayer and substrate lattice parameters, the only predictable coincidence falls at θ ) 0.0°. However, since the symmetry of the two lattices is different, this coincidence is not very robust, as can be seen by the relatively weak minimum of the output function, which is 0.8, whereas for more favorable cases it is expected to reach values as high as 0.5. In any case, the epitaxial relation experimentally found by AFM analysis on the uniform R4T layer and by optical

Heteroepitaxy of R-Quaterthiophene on Tetracene spectroscopy on the entire film corresponds to the one predicted by misfit calculation. All these arguments are valid only if possible relaxation phenomena at the interface are neglected. In this respect, the high-resolution AFM analysis (Figure 1d) reveals a rhombic unit cell for R4T with γ ) 87°. Since the small difference between the experimental value and the expected one (87° vs 90°) is comparable with the experimental error, we are not sufficiently confident to establish if the surface lattice of the crystalline R4T film is strained or corresponds with the equilibrium one. However, we must note that a similar highresolution study performed recently on the surface of a homoepitaxial R4T film did not show the presence of such a distortion.26 Hence, these results suggest the presence of a pseudomorphic phase of R4T and eventually support the textural order of the film and the azimuthal orientation of crystalline domains. The epitaxial relation concerning needle-like crystallites is less straightforward. The needle-like morphology is quite recurrent in the framework of OMBE deposition of R4T, and its presence is the signature of a contact plane parallel to the {010} crystallographic planes, the axes of needles being parallel to the aR4T axis.7c The results of the misfit calculation for the R4T{010}|TEN(001) interface are summarized in Figure 4, parts b and c, which report the results for the (010) and (0 -1 0) orientations of R4T by considering the actual unit cell of the overlayer. Two couples of minima are identified: θ ) -2.4°, -13.8°, and θ ) -6.0°, +5.4°. If a 1 × 2 supercell of the overlayer is considered, the strongest coincidence is found, occurring at azimuths θ ) -11.7° and θ ) +3.3°. All these orientations must be compared with those experimentally observed, +6.8° and +66.2°, for the needle main axis with aTEN; in addition, since in principle we are not able to distinguish between the (010)- and (0 -1 0)-oriented needles, the supplementary angles -173.2° and -113.8° have to be considered. Among the calculated azimuths, only the one at θ ) +5.4° for the (0 -1 0)-oriented needles is in accordance, within the experimental error, with one of the two experimentally observed orientations. This is not sufficient for a thorough explanation of the mechanism of epitaxial ordering of this system. We can then follow another route, neglecting the role of lattice match and focusing on possible molecular epitaxial mechanisms. By analyzing deeply the (001)TEN surface, one can realize that it exhibits a particular corrugation. This surface is lined up with hydrogen atoms (see the model in Figure 2b) arranged in such a way to form well-defined channels along the [1 -1 0]TEN direction. The presence of such H-atom rows on molecularly flat surfaces has been recently demonstrated to give rise to a new mechanism of epitaxial ordering of crystalline overlayers of rodlike organic semiconductors.7c Following that model, (0 (1 0)-oriented R4T domains achieve their energetically favored configurations when the long molecular axes are oriented parallel to the direction of the H-rows. Since in R4T crystals the axes of molecules are all parallel one another, such an orientation effect has very strong consequences. The result is depicted in Figure 2b: if R4T molecules arrange with their long axes exactly parallel to the [1 -1 0]TEN direction, the (010) needles would be oriented at +10.15° to aTEN, and (0 -1 0) needles at +59.85° (or more properly -120.15°) to aTEN, i.e., extremely close to the experimentally observed values. The discrepancy of ≈5° between experimental and calculated angles can be ascribed to a preferential alignment of molecules parallel to a direction not exactly coincident with [1 -1 0]TEN, but rotated by ≈5° in opposite directions for the two domains. Figure 5

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Figure 5. Structural model of the interface between the (001) face of TEN and a 4T needle as viewed along the molecular axes of 4T molecules (corresponding to the [52 0 21] crystallographic direction).

shows a sketch of the molecular arrangement at the interface between an R4T needle and the deposition surface of TEN. It is interesting to note how the corrugation of the (010) surface of R4T nicely matches that of the (001) surface of TEN, under the epitaxial relation [52 0 21]R4T | [1 -1 0]TEN. Differently to what was observed for R4T on (010)-oriented potassium hydrogen phthalate, where the corrugation given by H-atoms of the substrate surface is characterized by two symmetrical equivalent directions originating four preferential orientations of R4T needles,7c here only one crystallographic direction of the substrate surface is marked by well-defined H-rows so that only two preferential orientations of R4T needles are generated. However, a slight overpopulation of needles oriented at +6.8° to aTEN is observed (60% of the total). The reason of that may be found in the additional stabilization caused by lattice match for needles oriented at +5.4° (see Figure 4c). However, needles oriented at +6.8° are predicted by molecular epitaxy to contact the substrate with the (010)R4T plane (see Figure 2b), whereas needles oriented at +5.4° are predicted by misfit calculation to contact the substrate with the (0 -1 0)R4T plane (see Figure 4c). Then, another rationale is on the basis of the selectivity of the (001)TEN surface for the nucleation of R4T needles and must be searched in the chiral properties of this surface in relation with those of the overlayer. Indeed, the two domains of (010)and (0 -1 0)-oriented R4T are two reflection domains with p2 plane symmetry,14 which is consistent with 2D chiral systems. The symmetry of the (001)TEN surface (p1) is in principle sufficient for selecting the nucleation of a unique chiral domain of R4T; however, a selectivity of 60% is observed. We point out that the effect of chirality is expected to be active also during the adsorption and diffusion of single R4T molecules, since oligothiophenes with an even number of thiophene rings are chiral objects when confined in two dimensions. Finally, it must be noted that from the results of optical spectroscopy the epitaxial relation between the TEN(001) surface and the uniform film of R4T with (001) as contact plane can be inferred, but no information can be obtained on the needle-like crystallites. This aspect is strictly related to the contact plane shown by the needles, being parallel to the {010} crystallographic planes as previously mentioned. Light impinging on the R4T (010) or (0 -1 0) face gives access to the transverse excitation of the transition to the bu exciton state, corresponding to an absorption coefficient of about 106 cm-1, 1 order of magnitude higher than the usual almost longitudinal configuration with (001) as the accessible face.24,27 It is straightforward to understand that 100 nm thick needles do not allow light to be transmitted, hence give no contribution to the optical response. 5. Conclusions Following common rules accepted in the field of epitaxial growth, a low lattice mismatch is the fundamental requirement

19014 J. Phys. Chem. C, Vol. 111, No. 51, 2007 for the ordering of the overlayer. When this requirement is lacking, organic epitaxy may explicate through different mechanisms relying on favored molecular interactions between substrate and overlayer. The R4T/TEN heterostructure is a system characterized by a very low lattice mismatch. However, the AFM and optical characterization of the organic heterostructure constituted by R4T deposited on TEN{001} has revealed the presence of two different crystalline phases, one phase of standing upright molecules forming a uniform layer, and one phase of flat-lying molecules arranged in needle-like aggregates. The former presents an epitaxial relation consistent with common epitaxial rules based on the minimization of lattice mismatch (this condition is achieved when aR4T | aTEN), whereas the latter present an epitaxial relation which reflect a crystallochemical directed ordering in which R4T molecules in {010}oriented crystallites align parallel to the [1 -1 0]TEN direction. These results demonstrate that molecular epitaxial mechanisms are active independently of the degree of lattice match; additionally, the chirality of the TEN surface in combination with molecular epitaxy gives rise to a selectivity of 60% for the nucleation of one reflection domain of {010}-oriented R4T. These observations have crucial consequences on the possibility to drive the growth of organic nanostructures; in other words, the choice of the substrate and overlayer materials to be combined in multilayer structures must be carried out after an accurate analysis not only of the lattice mismatch but also of the symmetry and corrugation of the surfaces and interfaces involved. The example of the R4T/TEN heterostructure individuates in the presence of H-atom rows forming chiral channels on the substrate surface the principal crystallographic characteristic driving the orientation of overlayers of rodlike molecules. It will be of particular interest to study the epitaxial relations and to verify if an increment of the selectivity is achieved when less symmetric organic molecules are deposited on TEN. As a final remark, some details of the high-resolution AFM analysis suggest the presence of a pseudomorphic (and then chiral) phase of (001)-oriented R4T. Further investigations, probing, e.g., the emission properties of such a heterostructure, could provide clearer evidence in favor of this conclusion, which may have important consequences also in the field of molecular recognition and sensoring. Acknowledgment. The authors are grateful to Silvia Caprioli for her help in performing AFM measurements. One of the authors (L.R.) thanks Sovvenzione Globale Ingenio for financial support given by Fondo Sociale Europeo, Ministero del Lavoro e della Previdenza Sociale and Regione Lombardia. References and Notes (1) Forrest, S. R. Chem. ReV. 1997, 97, 1793.

Campione et al. (2) Herman, M. A.; Richter, W.; Sitter, H. Epitaxy; Springer-Verlag: Berlin, 2004. (3) (a) Hillier, A. C.; Ward, M. D. Phys. ReV. B 1996, 54, 14037. (b) Lopinski, G. P.; Wayner, D. D. M.; Wolkow, R. A. Nature 2000, 406, 48. (c) Basu, R.; Guisinger, N. P.; Greene, M. E.; Hersam, M. C. Appl. Phys. Lett. 2004, 85, 2619. (d) Hooks, D. E.; Fritz, T.; Ward, M. D. AdV. Mater. 2001, 13, 227. (e) Lukas, S.; Witte, G.; Woll, Ch. Phys. ReV. Lett. 2002, 88, 28301. (f) Eremtchenko, M.; Schaefer, J. A.; Tautz, F. S. Nature 2003, 425, 602. (g) Rosei, F.; Schunack, M.; Naitoh, Y.; Jiang, P.; Gourdon, A.; Laegsgaard, E.; Stensgaard, I.; Joachim, C.; Besenbacher, F. Prog. Surf. Sci. 2003, 71, 95. (4) (a) Carter, P. W.; Ward, M. D. J. Am. Chem. Soc. 1993, 115, 11521. (b) Bonafede, S. J.; Ward, M. D. J. Am. Chem. Soc. 1995, 117, 7853. (c) Mitchell, C. A.; Yu, L.; Ward, M. D. J. Am. Chem. Soc. 2001, 123, 10830. (5) Koller, G.; Berkebile, S.; Krenn, J. R.; Netzer, F. P.; Oehzelt, M.; Haber, T.; Resel, R.; Ramsey, M. G. Nano Lett. 2006, 6, 1207. (6) Oehzelt, M.; Koller, G.; Ivanco, J.; Berkebile, S.; Haber, T.; Resel, R.; Netzer, F. P.; Ramsey, M. G. AdV. Mater. 2006, 18, 2466. (7) (a) Le Moigne, J.; Kajzar, F.; Thierry, A. Macromolecules 1991, 24, 2622. (b) Da Costa, V.; Le Moigne, J.; Oswald, L.; Pham, T. A.; Thierry, A. Macromolecules 1998, 31, 1635. (c) Campione, M.; Sassella, A.; Moret, M.; Papagni, A.; Trabattoni, S.; Resel, R.; Lengyel, O.; Marcon, V.; Raos, G. J. Am. Chem. Soc. 2006, 128, 13378. (8) Trabattoni, S.; Laera, S.; Mena, R.; Papagni, A.; Sassella, A. J. Mater. Chem. 2004, 14, 171. (9) Laudise, R. A.; Kloc, Ch.; Simpkins, P. G.; Siegrist, T. J. Cryst. Growth 1998, 187, 449. (10) Holmes, D.; Kumaraswamy, S.; Matzger, A. J.; Vollhardt, K. P. C. Chem. Eur. J. 1999, 5, 3399. (11) Cuppen, H. M.; Graswinckel, W. S.; Meekes, H. Cryst. Growth Des. 2004, 4, 1351. (12) Vrijmoeth, J.; Stok, R. W.; Veldman, R.; Schoonveld, W. A.; Klapwijk, T. M. J. Appl. Phys. 1998, 83, 3816. (13) Campione, M.; Cartotti, M.; Pinotti, E.; Sassella, A.; Borghesi, A. J. Vac. Sci. Technol., A 2004, 22, 482. (14) Siegrist, T.; Kloc, Ch.; Laudise, R. A.; Katz, H. E.; Haddon, R. C. AdV. Mater. 1998, 10, 379. (15) Hillier, A. C.; Ward, M. D. Phys. ReV. B 1996, 54, 14037. (16) Sassella, A.; Borghesi, A.; Spearman, P.; Tavazzi, S. Eur. Phys. J. B 2002, 28, 385. (17) Campione, M.; Sassella, A.; Moret, M.; Marcon, V.; Raos, G. J. Phys. Chem. B 2005, 109, 7859. (18) (a) Sassella, A.; Campione, M.; Papagni, A.; Goletti, C.; Bussetti, G.; Chiaradia, P.; Marcon, V.; Raos, G. Chem. Phys. 2006, 325, 193. (b) Sassella, A.; Campione, M.; Raimondo, L.; Tavazzi, S.; Borghesi, A.; Goletti, C.; Bussetti, G.; Chiaradia, P. Surf. Sci. 2007, 601, 2571. (19) Bree, A.; Lyons, L. E. J. Chem. Soc. 1960, 5206. (20) Davydov, A. S. Theory of Molecular Excitons; Plenum Press: New York, 1971. (21) Tavazzi, S.; Borghesi, A.; Campione, M.; Laicini, M.; Trabattoni, S.; Spearman, P. J. Chem. Phys. 2004, 120, 7136. (22) Kalinowki, J.; Godlewski, J.; Stizza, S.; Davoli, I.; Mancini, G. Mol. Cryst. Liq. Cryst. 1989, 166, 233. (23) Tavazzi, S.; Borghesi, A.; Laicini, M.; Spearman, P. J. Chem. Phys. 2004, 121, 8542. (24) Laicini, M.; Spearman, P.; Tavazzi, S.; Borghesi, A. Phys. ReV. B 2005, 71, 0452121. (25) Sun, H.; Zhao, Z.; Spano, F. C.; Beljonne, D.; Cornil, J.; Shuai, Z.; Bre´das, J. L. AdV. Mater. 2003, 15, 818. (26) Sassella, A.; Borghesi, A.; Campione, M.; Tavazzi, S.; Goletti, C.; Bussetti, G.; Chiaradia, P. Appl. Phys. Lett 2006, 89, 261905. (27) Weiser, G.; Mo¨ller, S. Phys. ReV. B 2002, 65, 045203.