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Unique Orientation of Organic Epitaxial Thin Films: The Role of Intermolecular Interactions at the Interface and Surface Symmetry Luisa Raimondo,*,† Massimo Moret,† Marcello Campione,‡ Alessandro Borghesi,† and Adele Sassella† † ‡
Dipartimento di Scienza dei Materiali, Universita degli Studi di Milano Bicocca, Via Cozzi 53, I-20125 Milano, Italy Dipartimento di Scienze Geologiche e Geotecnologie, Universita degli Studi di Milano Bicocca, Piazza della Scienza 4, I-20126 Milano, Italy
bS Supporting Information ABSTRACT: The investigation of the mechanisms driving the growth of single crystalline organic thin films is a fundamental issue of organic electronics in view of the optimization of transport properties in thin film-based devices. In this paper, a complete azimuthal order of heteroepitaxial domains of R-quaterthiophene grown by organic molecular beam epitaxy on tetracene single crystal substrates is assessed by oblique incidence absorption spectroscopy and fully validated by empirical force field calculations. The effect of the many-body interactions combined with the symmetry of the substrate and deposit surfaces involved in the heterojunction is demonstrated to drive toward this relevant achievement, which unveils new strategies for controlling the growth of single crystalline organic thin films.
’ INTRODUCTION The fine control of the interfaces embedded in multilayer devices in terms of morphology and structure represents a key factor in the field of opto-electronics. In particular, within the framework of organic electronics exploitation of organicorganic epitaxy by using crystalline substrates (both insulating and semiconducting) has been revealed strategic and successful in order to obtain highly ordered and oriented heterojunctions14 even though the mechanisms responsible for the orientation effect of the organic substrate upon the organic overlayer are far to be completely understood. In view of thin film based technology, the growth of single crystalline thin films displaying a unique azimuthal orientation with respect to the substrate represents the most desired event for avoiding structural disorder, which is the first cause of reduced device performances.5 Thus, finding a strategy for selecting “a priori” suitable depositsubstrate couples that can give a unique azimuthal orientation of overlayer domains represent a fundamental task, not yet deeply investigated in the field of organic electronics. Here, we report on a paradigmatic case of organicorganic heteroepitaxy between two semiconductors, namely R-quaterthiophene (R-4T) and tetracene (TEN), as overlayer and single crystal substrate, respectively. We demonstrate by means of oblique incidence optical spectroscopy that the R-4T uniform r 2011 American Chemical Society
layer growing on TEN substrate possesses a unique azimuthal orientation thus revealing its single crystalline nature over a sample area of a few millimeters squared in size. This result is discussed and rationalized in terms of adhesion energy of the R-4T nuclei growing on TEN substrate by means of empirical force field calculations and by analyzing the role of the symmetry of the surfaces constituting the interface of the heterostructure. Finally, we propose a strategy for selecting the most suitable substratedeposit couples for obtaining uniquely oriented epitaxial thin films.
’ EXPERIMENTAL AND COMPUTATIONAL METHODS TEN single crystals were grown as thin flakes exposing (001) surface, and few millimeters squared in size, by means of physical vapor transport6 under a nitrogen flow and then placed on fused silica plates. R-4T was synthesized and purified according to the procedure reported in ref 7. R-4T thin films were grown by organic molecular beam epitaxy on TEN(001) at room temperature and at a base pressure e5 1010 Torr with a deposition rate around 0.3 nm/min, as measured by a quartz microbalance.8 Received: December 10, 2010 Revised: February 25, 2011 Published: March 16, 2011 5880
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Polarized optical absorption measurements were carried out using a Perkin-Elmer Lambda 900 spectrometer with a depolariser and Glan-Taylor polarisers. To avoid stray-light collection from cracks and holes within TEN single crystal substrates, the samples were masked over a spot of a few millimeters squared in size. Modeling of the depositsubstrate interface has been performed through calculations based on atomatom empirical potentials exploiting genetic algorithms. A slab of R-4T lowtemperature polymorph9 (a = 6.085(2), b = 7.858(2), c = 30.483(8) Å, β = 91.81(2)°, space group P21/c) comprising a single d002 layer with 35 molecules interacting with a bulkterminated TEN(001) surface was used. The TEN(001) substrate surface10 (a = 6.0565(9), b = 7.8376(11), c = 13.0104(18) Å, R = 77.127(2), β = 72.118(2), γ = 85.792(2)°, space group P1) was modeled with a slab of 21 17 2 unit cells along the a, b, and c axis, respectively, giving rise to a total of 1428 molecules. The choice of cluster and substrate slab size has been made on the basis of previous tests carried out to validate our modeling scheme on other organicorganic systems.11 Simulations were performed by means of docking runs with a rigid deposit crystallite free to move and interact with a rigid substrate surface. AutoDock3 package12 performing Lamarckian genetic algorithm docking runs permits to efficiently explore the configuration space without biases over the azimuthal orientations. A simulation box comprised 3513 grid points with a sampling grid of 0.217 Å for mapping the interaction potential between substrate and deposit. A total of 1309 docking runs with the UNI empirical force field13 were performed providing a satisfactory sampling statistics of the in-plane orientations of the deposit crystallites. Energy minima values found with the docking procedure were subsequently refined by full minimization of the more stable configurations with program Orient4.6.1114 using the same interaction potentials and a substrate slab of 27 21 4 unit cells along the a, b, and c axis, respectively, for better convergence of the potentials. The UNI potential13 was also employed to assess the very small relevance of surface relaxation and the specific surface energy for the TEN(001) and R-4T(001) crystal faces in order to estimate the excess energy arising from the heterojunction interface.
’ RESULTS The multiscale analysis performed by atomic force microscopy (AFM) and normal incidence optical spectroscopy reported in refs 15 and 16 shows that the R-4T/TEN heterostructure is characterized by the presence of a pseudomorphic phase of R-4T constituted by upright-standing molecules forming a uniform crystalline layer with a structure close to that of the lowtemperature polymorph of crystalline R-4T,9 with R-4T(001) as contact plane and R-4T[100] almost parallel to TEN[100]. Because of the polarity of the a-axis of the (001) surface of R-4T, this epitaxial relationship is fulfilled by two orientations of the R-4T(001) domains related by a 180°-rotation about the normal to the substrate surface (compare the interface structures sketched in Figure 1a,b). Even if these two 180°-rotated domains are distinct, the simple geometrical models17,18 and experimental tools adopted15 are blind to their difference in azimuthal orientation. In particular, AFM does not reach sufficient lateral resolution for distinguishing submolecular details discriminating between 180°-rotated domains. On the other hand, the inadequacy of normal incidence optical spectroscopy is related to the
Figure 1. (a,b) Sketch of the interface structure of the two 180°-rotated domains of R-4T on TEN fulfilling the same epitaxial relation. A schematic representation of the relative orientations of dTEN and dR-4T as deduced from the literature and from the optical measurements in (d) is also reported. (c) Cartesian reference frame adopted in this work for the optical measurements. (d) Oblique incidence absorption spectra collected for different angles of incidence on a 10 nm thick R-4T thin film grown on TEN(001) by selecting p-polarized light and xz plane as plane of incidence. Absorption spectra for j = 0°,j > 0°, and j < 0° are respectively reported as black, blue, and red lines. Note that in order to make clearer the angle of incidence dependence of R-4T spectral response, all the spectra have been shifted to the same value (zero) at 2.2 eV. (Inset) Zoom of the complete set of absorption spectra in the spectral range of the peculiar optical response of the R-4T thin film for different j from 40° to þ50° in steps of 10°. 5881
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Figure 2. (a) Summary of empirical force field calculations showing the adhesion energy normalized to a single R-4T molecule contacting the substrate for different in-plane azimuthal orientations of a R-4T(002) island (low-temperature polymorph); the asterisks below the minima indicate the refined azimuth and energy values obtained after full minimization. (Inset) Sketch of R-4T and TEN unit cells (yellow and blue, respectively) for the definition of the azimuthal angle θ, adopted in this work. (b,c) Representation of the R-4T(001) overlayer arrangement on TEN(001) for θ = 3.8° and þ178.2°, respectively. (d) Details of depositsubstrate molecular interactions represented by means of Hirshfeld surfaces for the substrate molecules and a stick model for the bottom thienyl groups of the deposit cluster of molecules for θ = 3.8°. Red regions correspond to close contacts between terminal H-atoms of deposit molecules and TEN molecules of the substrate surface.
structure and optical response of crystalline R-4T. Indeed, the strong dipole moment transition of R-4T (labeled dR-4T
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henceforth, see Figure 1a) lies in the R-4T(010) plane with a tilt angle of about 30° with respect to the normal to R-4T(001), which is the contact plane between film and substrate.15,16 Thus, at normal incidence absorption measurements are only sensitive to the projection of dR-4T onto R-4T(001), so that it can not distinguish the optical response of 180°-rotated domains. On the contrary, such information can be obtained by means of absorption spectroscopy at oblique incidence since, by changing the angle of incidence (j), the coupling between dR-4T and the light wave-vector within the crystal can be enhanced or suppressed (when light wave-vector is mostly transverse or longitudinal to dR-4T, respectively).19 The presence of either a single crystalline orientation or two rotational domains therefore is easily recognizable in the thin film spectra by the complete absence or persistence, respectively, of the main peak, even under the experimental configuration matching the longitudinal condition. Figure 1d reports the oblique incidence absorption spectra collected on a 10 nm thick R-4T thin film grown on a TEN(001) single crystal substrate by selecting the plane normal to TEN[010] as plane of incidence (xz plane, see Figure 1c) and p-polarized light. The xz plane also individuates the R-4T(010) plane.15,16 The known dependence on j of the intensity of the lowest energy transition of TEN single crystals20 (in the following, the related dipole moment transition is referred to as dTEN, see Figure 1a) is observable in the low energy spectral range. On the contrary, the absorption peak attributable to the presence of the R-4T overlayer is detected at about 3.6 eV (in a spectral range where the TEN single crystal is transparent even at oblique incidence), with an intensity strongly dependent on j. In particular, for negative j (as defined in Figure 1c), the intensity of the peak increases, suggesting that a better coupling between light and dR-4T is reached; on the contrary, intensity decreases for increasing positive j, it almost disappears for j = þ50°, and then it slightly increases again for j > þ50° (spectra not shown here). The evident asymmetry of the spectral response of the R-4T film for positive and negative j gives the first important piece of information: the population of the two 180°-rotated domains must be largely different. Moreover, the complete disappearance of the peak when j = þ50° provides evidence of the unique orientation of the film, this optical behavior being the one expected for a single crystal.19 From this condition and by taking into account refraction at the air/R-4T interface, we deduce that dR-4T is tilted by about þ30° with respect to the z axis, as drawn in Figure 1a, which is in agreement with previous independent experimental findings.16 A complete and coherent picture of the interface can be drawn on the basis of the known orientation of dTEN:20 both dR-4T and dTEN lie into the xz plane, forming an angle of about 60° to each other. The aR-4T and aTEN axes have therefore a specific relation of their senses, thus establishing that the most favorable orientation of R-4T[100] versus TEN[100] is for the azimuthal rotation θ, as defined in the inset of Figure 2a, of about 0°. Finally, it is worth to underline that by taking into account the low thickness of the film, the limited signal-to-noise ratio in the spectral range of interest, and the calculated dielectric tensor of R-4T,21 we can estimate that under the present experimental conditions at least 95% of the domains constituting the R-4T overlayer display the same azimuthal orientation θ ∼ 0°. This means that we can consider the overlayer as possessing a single crystalline nature. In order to gain insights on the microscopic orientation mechanism, we performed empirical force field atomatom simulations of the R-4T(001)||TEN(001) interface, a useful tool for predicting and explaining the occurrence of specific azimuthal 5882
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The Journal of Physical Chemistry C orientations in terms of adhesion energy of the depositsubstrate interface.4,11,2225 The results are reported in Figure 2a in terms of adhesion energy averaged over the number of R-4T molecules considered in the simulations. First of all, the interface energy of the heterostructure is strongly dependent on the azimuthal orientation of the R-4T(001) crystal slab with respect to TEN(001). The adhesion energy shows only four well-defined energy minima in the range 0°/360° centered at θ = 72.8, 178.2, 252.7, and 356.2° (in the following indicated as 3.8°). The latter is the absolute minimum, it matches the azimuthal orientation observed experimentally by means of oblique absorption measurements, and it corresponds to average adhesion energy of 234 meV per molecule in the cluster. The secondary minima present adhesion energies higher by about 15 to 45 meV with respect to the absolute minimum. Assuming that critical nuclei are formed on the surface by diffusion and collisions of molecules, according to Boltzmann’s principle the lower energy azimuthal orientation at θ = 3.8° is preferentially populated.26 This can be demonstrated as follows. The overlayer arrangement corresponding to θ = 178.2° displays the lowest energy difference in adhesion energy ΔEm (∼15 meV per molecule) with respect to θ = 3.8°. These two azimuthal orientations of the R-4T overlayer correspond to the aforementioned 180°-rotated domains (compare the overlayer arrangement for these two azimuths in Figure 2b,c). By assuming the 0.05 population ratio deduced by optical spectroscopy for these two orientations in favor of θ = 3.8°, the statistical weight exp(ΔE/kT), where ΔE = mΔEm and m is the number of molecules in the critical nucleus, fulfills the aforementioned ratio for m = 5. Since molecular dynamics simulations of R-4T on potassium hydrogen phthalate showed that (001) critical nuclei are composed by more than 20 molecules,26 a population fraction larger than 95% in favor of the more stable azimuth is expected, thus demonstrating that the presence of the less stable orientation is negligible. By comparing the calculated adhesion energy as a function of the azimuthal angle θ and the specific surface energies for the R4T(001) and TEN(001) faces, we can also evaluate the excess energy per unit area γ* required to create the R-4T(001)|| TEN(001) heteroepitaxial interface. Indicating with γsub, γdep, and βadh the specific surface energies for substrate, deposit, and the new interface, respectively, one gets γ* = γsub þ γdep βadh.27 Calculation of γsub and γdep with unrelaxed surfaces is a good approximation of the real situation (we found negligible relaxations for close-packed faces such as those containing the herringbone motifs of R-4T and TEN), while the ultrahigh vacuum environment during growth is a reasonable approximation for surfaces at equilibrium in the presence of their own vapors. We obtained γsub = 4.6 meV/Å2 for TEN(001) and γdep = 5.9 meV/Å2 for R-4T(001), which gives γ* = 0.62 meV/Å2 for the azimuthal orientation θ = 3.8° and γ* = 1.2 meV/Å2 for θ = 178.2°. Such low values of the specific interfacial excess energy indicate a good wettability and the propensity for R-4T to nucleate and grow epitaxially on the TEN(001) surface with a clear preference for the azimuthal orientation θ = 3.8°.
’ DISCUSSION The previous findings justify the presence of a most favorable orientation in terms of adhesion and interfacial energy. Looking at epitaxy matrix for the most favored azimuthal
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orientation θ = 3.8 °, "
1:01 0:01
0:05 1:00
#
the epitaxial relations corresponding to commensurism or point-on-line epitaxy can be ruled out (see Supporting Information).22,23 Nonetheless, the occurrence of the so-called line-on-line registry can be deduced looking at the coincidence of TEN[110] and R-4T[110] directions, as discussed in ref 16 (see below). Since the condition of line-on-line epitaxy would be fulfilled by both domains related by 180°-rotation with respect to the normal to the substrate surface, one further step is still required for fully justifying the peculiar domain selection observed in our system. To this purpose, (i) we develop a microscopic model of the interface at the molecular level, and consequently (ii) we clarify the role of the symmetry of the surfaces involved in the interface. i. Microscopic Model of the Interface. The adhesion energies plotted in Figure 2a are values averaged over the 35 molecules comprising the simulation deposit cluster interacting with the TEN substrate. The value of the calculated average energy for the best configuration at θ = 3.8° analyzed in greater detail permits to evaluate the contribution of each molecule in the cluster, finding a significant variation of such individual molecular adhesion energies ranging from 202 to 260 meV (see Supporting Information Figure S1). One can discern regions where adhesion of upright-standing R-4T molecules is more effective thanks to a stronger interaction with the substrate. Therefore it can be supposed that nucleation is not equally probable at all points on the surface, and this can be of great relevance for nucleation whenever critical nuclei are made of few molecules.26 The distribution of adhesion energies within the deposit cluster shows no molecules in a repulsive regime, but only more or less favorable interactions. By further partitioning over single atoms the adhesion energy of deposit molecules, the terminal H-atom of the thienyl group closest to the substrate surface is demonstrated to be in a repulsive regime with all other atoms being far enough to experience the attractive regime. The major contribution to the overall attractive molecular adhesion energy comes from the thienyl closer to the interface and, specifically, to the two closest C-atoms and to the S-atom. These three atoms alone greatly overcome the unfavorable interaction involving the aforementioned closest H-atom; all the remaining atoms of each R-4T molecule experience a net attractive energy, falling off as r6, with distance r from the substrate. This interpretation can be graphically summarized as in Figure 2d through the use of Hirshfeld surfaces28,29 representing the corrugation for the total electron density of the substrate surface (showing furrows along the TEN[110]) and at the same time giving a visual impression of the closest atomatom contacts just described. The red regions evidence close approaches between TEN molecules at the interface and terminal H-atoms of the deposit molecules. In the past, it has been suggested the important role of structural analogies of surface atoms in the planes of contact of the two materials on deciding whether organicorganic epitaxy can occur and eventually which azimuthal orientations are observed.30 We studied other organicorganic systems1,11,24,25 and proposed that furrows or rows of protruding atoms (usually hydrogen) are responsible for the occurrence of epitaxy in cases where no simple geometric criteria were able to explain these findings. In the present case, the TEN(001) surface can be seen 5883
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as exhibiting well-defined furrows parallel to the [110] direction accommodating the most protruding thienyl groups of the R-4T overlayer, running along the [110] direction.16 Finally, the fact that each R-4T molecule interacts in a slightly different manner with the substrate, even in the presence of some kind of epitaxy (here line-on-line), shows how the outcome of organicorganic heteroepitaxy can only be explained by analyzing local intermolecular interactions. ii. Role of Surface Symmetry. Given the microscopic model of the interface, it is useful to add some insights on the role of symmetry in determining specific epitaxy conditions, an important topic not deeply investigated yet in the literature on organicorganic heteroepitaxial systems. In the framework of the general theory developed under the name of bicrystallography,3133 the point group symmetry of the substrate and deposit contact planes determine the number n of possible symmetry-equivalent orientations for each independent azimuthal angle through the relation:34 n¼
#Ssub #ðSsub ∩ Sdep Þ
ð1Þ
where #Ssub and #Sdep are the number of elements in the sets Ssub and Sdep of symmetry operations of the two-dimensional (2D) point group of the substrate and deposit surfaces, respectively, and #(Ssub ∩ Sdep) is the number of common symmetry operations of the two 2D point groups. On the basis of eq 1, a strategy can be found for selecting material pairs that give n = 1, even if this is just a necessary, not sufficient, condition for achieving a unique orientation. One way is having a substrate contact plane with the lowest point group symmetry, p1; another one is choosing substrateoverlayer pairs with contact surfaces exhibiting the same set of point group symmetry operations. Noteworthy, due to the long-range nature of intermolecular interactions, the surface symmetry is not determined only by the arrangement of topmost atoms; rather, large moieties of the superficial molecules participate to interface interactions, modifying the overall projected surface symmetry. Applying eq 1 to the present case, one has first to determine the plane group symmetry of the interface involved in our heterostructure, the previous results of modeling of interface interactions must be taken into account. While the TEN(001) surface has unambiguously a p1 plane group symmetry (point group symmetry 1), deriving from the P1 space group of the crystal, the symmetry of the R-4T(001) surface changes depending on the number of atoms we consider active in the surface. If only H-atoms participated to the interactions with the substrate, the surface symmetry would result cmm2. However, since all the atoms of the terminal thienyl moiety participate to the interface interactions, the plane group of the R-4T(001) surface reduces to pg (point group symmetry m). Therefore, for R-4T on TEN, one has #Ssub = 1 and #Sdep = 2, obtaining n = 1, that is, only one symmetry-equivalent orientation for each independent azimuthal angle can be predicted on TEN(001). As our experimental and computational results evidence, only one deposit configuration is indeed selected during nucleation, based on its adhesion energy and Boltzmann statistics. Hence, once the nuclei have enlarged enough to give coalescence, only one domain orientation is created thus minimizing the detrimental effects of grain boundaries.
’ CONCLUSIONS We have shown by means of optical spectroscopy at oblique incidence that the R-4T overlayer grows on TEN single crystals
with a single azimuthal orientation, which is the most desirable event for device integration. Such an orientation reflects line-online epitaxy, which nonetheless admits two 180°-rotated R-4T domains on the TEN substrate. Only by the use of force field calculations for modeling the local intermolecular interactions we could successfully justify the selection of a unique orientation. The paper results in a twofold general conclusion. First, the analysis of local surface characteristics, such as furrows, rows, protruding atoms, which reflect the local intermolecular interactions, has a precise physical meaning that makes it a powerful and reliable tool. Second, from a more general point of view, a possible strategy for predicting when depositsubstrate couples can give a unique orientation of organicorganic heterostructures can be proposed. A necessary requisite is imposed by surface symmetry; eq 1 should predict if only one symmetryequivalent orientation is allowed for each independent azimuthal angle. Nonetheless, the actual selection of a unique orientation is only ensured by its significantly favorable adhesion energy with respect to competitive orientations to be checked by modeling of the local intermolecular interactions at the interface.
’ ASSOCIATED CONTENT
bS
Supporting Information. Calculation of the epitaxy matrix. Individual molecular adhesion energy of the R-4T cluster. This material is available free of charge via the Internet at http:// pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Tel.: þ39 02 64485025. Fax: þ39 02 64485400.
’ ACKNOWLEDGMENT This work was supported by Fondazione Cariplo (Grants 2007/5205 and 2009/2551). Silvia Trabattoni is kindly acknowledged for R-4T synthesis and purification. ’ REFERENCES (1) Campione, M.; Sassella, A.; Moret, M.; Papagni, A.; Trabattoni, S.; Resel, R.; Lengyel, O.; Marcon, V.; Raos, G. J. Am. Chem. Soc. 2006, 128, 13378–13387. (2) Koller, G.; Berkebile, S.; Krenn, J. R.; Netzer, F. P.; Oehzelt, M.; Haber, T.; Resel, R.; Ramsey, M. G. Nano Lett. 2006, 6, 1207–1212. (3) de Oteyza, D. G.; Krauss, T. N.; Barrena, E.; Sellner, S.; Dosch, H.; Osso, J. O. Appl. Phys. Lett. 2007, 90, 243104. (4) Kasemann, D; Wagner, C.; Forker, R.; Dienel, T.; Mullen, K.; Fritz, T. Langmuir 2009, 25, 12569–12573. (5) Horowitz, G.; Hajlaoui, M. E. Adv. Mater. 2000, 12, 1046–1050. (6) Laudise, R. A.; Kloc, Ch.; Simpkins, P. G.; Siegrist, T. J. Cryst. Growth 1998, 187, 449. (7) Trabattoni, S.; Laera, S.; Mena, R.; Papagni, A.; Sassella, A. J. Mater. Chem. 2004, 14, 171. (8) Campione, M.; Cartotti, M.; Pinotti, E.; Sassella, A.; Borghesi, A. J. Vac. Sci. Technol., A 2004, 22, 482. (9) Siegrist, T.; Kloc, Ch.; Laudise, L. A.; Katz, H. E.; Haddon, R. C. Adv. Mater. 1998, 10, 379–382. (10) Holmes, D.; Kumaraswamy, S.; Matzger, A. J.; Vollhardt, K. P. C. Chem.—Eur. J. 1999, 5, 3399–3412. (11) Haber, T.; Resel, R.; Thierry, A.; Campione, M.; Sassella, A.; Moret, M. Physica E 2008, 41, 133–137. 5884
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