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Epitaxial Interfaces in Rubrene Thin Film Heterostructures Luisa Raimondo, Enrico Fumagalli, Massimo Moret, Marcello Campione, Alessandro Borghesi, and Adele Sassella J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp402136f • Publication Date (Web): 18 Jun 2013 Downloaded from http://pubs.acs.org on June 20, 2013

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The Journal of Physical Chemistry

Epitaxial Interfaces in Rubrene Thin Film Heterostructures Luisa Raimondo,1,* Enrico Fumagalli,1Massimo Moret,1Marcello Campione,2 Alessandro Borghesi, 1 Adele Sassella 1 1

Dipartimento di Scienza dei Materiali, Università degli Studi di Milano Bicocca, Via Cozzi 53, I-20125 Milan, Italy, and 2 Dipartimento di Scienze dell'Ambiente e del Territorio e di Scienze della Terra, Università degli Studi di Milano-Bicocca, Piazza della Scienza, I-20126 Milano, Italy

* Corresponding Author. E-mail: [email protected], Tel.: +39 02 64485110-5025, Fax: +39 02 64485400

ACS Paragon Plus Environment

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ABSTRACT

Rubrene (RUB) single crystal displaying the orthorhombic polymorph structure is one of the most promising organic semiconducting material in terms of charge carrier mobility and exciton diffusion length. In view of the development of RUB based devices where structural disorder in the active components would reduce performances, RUB has to be integrated in the form of crystalline thin film either as a single active component or as a part of multilayer heterojunctions. Here, we show how to obtain highly crystalline and oriented RUB thin film heterostructures by growing RUB on top of another organic semiconductor thin film used as templating layer, thus taking advantage of organic epitaxy. A detailed analysis of the heteroepitaxial interface in terms of adhesion energy is presented with a detailed discussion of the epitaxial relationship between RUB overlayer and the layer underneath and of the driving forces leading to organic epitaxy with RUB.

KEYWORDS: organic epitaxy, rubrene thin films, organic semiconductors, organic electronics, epitaxial heterostructures, empirical force-field calculations


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INTRODUCTION The integration of organic semiconductors in the form of thin films in a variety of devices is one of the main topics in the field of organic electronics. The final purpose is to obtain tailored physical properties for improving device performances by tailoring materials, surfaces and interfaces through fundamental research problems. Nonetheless, this represents the starting point for applicative research aimed at understanding how to modify the architecture of the active material for matching its electronic and optical properties to the requisites needed for the final device. For those devices where transport properties must be optimized (e.g. thin film transistors), 1 a high control of molecular arrangement, degree of crystallinity and order of the active layer is strictly required. Organic photovoltaic cells represent an extreme case where both optical and transport properties should be optimized in terms of light harvesting, exciton diffusion, charge carrier separation and transport towards the electrodes 1. In order to get this result, several device architectures could be chosen starting from the single layer junctions of one material or a blend of materials to double layer junctions (donor and acceptor layers). Even more complex architectures can be adopted, like in the case of cascade organic solar cells, 2 where the whole donor layer consists of a multilayer stack of donor materials displaying cascading exciton energies. Obviously, each interface involved achieves a key role in the final device performance. In this respect, crystal engineers have to choose the most promising materials and achieve the complete control of the growth of the most suitable overlayer-substrate couples, possibly by assessing a priori the condition for obtaining proper molecular arrangement and order. In this work, we demonstrate how to obtain highly ordered organic heterostructures based on one of the most representative and promising organic semiconductors, namely rubrene (RUB, hereafter, Figure 1a). RUB single crystals displaying the orthorhombic polymorph structure


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indeed posses the best characteristics in terms of charge carrier mobility 3 and exciton diffusion lenght.4 Thus, the chance of obtaining crystalline RUB thin films displaying the proper polymorph structure is particularly attractive in view of thin film device integration. Crystalline RUB thin films can be directly obtained without using external treatments (such as thermal annealing, etc...) thanks to organic-organic epitaxy, as explicated in a few works reported in the literature

5-8

or implicitly deduced by others

9-14

. The approach of organic-organic epitaxy is

based on the choice of the most suitable substrate with defined surface properties, in terms of symmetry and corrugation (both on a macroscopic and a microscopic scale) 15,16 that can template the growth of a crystalline overlayer. It is straightforward that a solid in the form of single crystal or crystalline thin film has to be chosen as substrate. Both cases have been reported in the literature but with completely different results. Using single crystal substrates leads to the formation of highly crystalline and oriented RUB films

5-7

, but this choice is not fully

technologically relevant. On the contrary, randomly oriented RUB films have been obtained on organic buffer layers grown on the top of the most common substrates used in devices 9-14: even though crystalline, the whole heterostructure is rich of defects, grain boundaries, etc. that negatively affect device performances. The best route to follow is, thus, to select highly oriented and crystalline thin films as templating layer, as we propose here, where RUB films are grown on the top of an organic semiconductor thin film, namely α-quaterthiophene (4T, Figure 1a). The choice of 4T is dictated by the already demonstrated epitaxial interface between an orthorhombic RUB single crystal and crystalline 4T overlayers

17

. By means of a combined analysis of the

optical, structural and morphological properties, we demonstrate that RUB thin films are crystalline, displaying the orthorhombic polymorph structure with a specific orientation with respect to the 4T film. Finally, the microscopic approach based on force-field calculation of the


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interfacial energy gives insights into the mechanisms and the requisites for obtaining crystalline RUB on 4T and, in general, into the growth mode of RUB in other epitaxial systems.

EXPERIMENTAL SECTION AND THEORETICAL METHODS The deposition of RUB (Acros Organics, 99%) and 4T (synthesized following the procedure reported in ref 18) was carried out by organic molecular beam epitaxy working under ultra-high vacuum (bare pressure < 2×10-9 mbar) with a Knudsen-type effusion cell and monitoring the film thickness with a quartz crystal microbalance. Temperatures of the cells were 200°C and 170°C for RUB and 4T, respectively. Freshly cleaved (010)-oriented crystals of potassium hydrogen phthalate (KAP) were used as substrates, kept at room temperature during deposition of RUB and 4T. Polarized optical measurements were carried out ex-situ in transmission configuration at normal and oblique incidence in the spectral range from 2 to 4.2 eV (the absorption edge of KAP 19

) using a Perkin-Elmer Lambda900 spectrometer equipped with a depolarizer and Glan-Taylor

polarizers. The light spot size used was about 5 mm2 allowing us to reach a statistically meaningful sampling over a macroscopic sample area. AFM measurements were performed ex-situ with a Nanoscope V MultiMode (Bruker), equipped with a J type piezoelectric scanner. The measurements have been carried out in tapping mode, using silicon nitride tips (NT-MDT) with a force constant of 22÷100 N/m and a tip curvature radius of 10 nm. Images have been collected with a lateral resolution of 512×512 pixels, at a scan rate of 0.7 Hz, and image analysis has been carried out with the Gwyddion software (version 2.29) 20. X-ray diffraction was measured by specular scans (Θ/2Θ mode) performed with a Panalytical X’Pert Pro powder X-ray diffractometer in Bragg-Brentano parafocusing geometry, using Cu Kα


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radiation. In order to suppress the strong diffraction peaks originating from the crystalline KAP substrate, which would hide those originating from the overlayers, the whole RUB/4T heterostructure has been transferred to the surface of a (100) silicon crystal by removing KAP by means of a wet-transfer method

8,21,22

. For this purpose, an amorphous C-film was sputtered on

the sample surface; then, the substrate/organic film/C-film assembly was placed on the surface of distilled water from the substrate side. After a couple of hours, the substrate dissolved and the free insoluble organic film covered with the C-film (easily visible) was found floating on the water surface. Then, the film could be placed on a desirable substrate and let dry in a nitrogen atmosphere before characterization. Simulations of possible epitaxial relationships between a 4T(001) substrate and a RUB(100) overlayer were performed by means of atom-atom empirical potentials

23

exploiting the

capability of Lamarckian genetic algorithms to efficiently explore the configuration space. A monomolecular (200) slice of the RUB orthorhombic polymorph

24

(a = 26.860(10), b =

7.193(3), c = 14.433(5) Å, space group Cmca) comprising 16 RUB molecules was considered. The substrate was built with a bulk terminated 4T(001) slab of the low temperature polymorph (6.085(2), 7.858(2), 30,483(8) Å, β = 91.81(2)°, space group P21/c) 25. The slab comprised 21 x 15 x 1 unit cells along the a, b, and c axis, respectively, giving rise to a total of 1260 molecules. The size of overlayer and substrate models were selected on the basis of previous studies about other

organic-organic

and

organic-inorganic

systems

6,15,17,26-28

.

Modeling

of

the

RUB(100)/4T(001) interface was performed by means of 1240 docking runs with a rigid overlayer free to move and interact with a rigid substrate surface. AutoDock3 package

29

was

used to explore the configuration space without biases over the azimuthal orientations by using 3513 grid points energy maps calculated with the UNI potential 23. Energy minima found with the docking procedure were subsequently improved by full minimization of the most stable


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configurations for each minimum of the energy vs azimuth with program Orient4.6.11

30

using

the UNI potential. In this case, the substrate comprised a slab of 27 × 21 × 4 unit cells along the a, b, and c axis, respectively, for full convergence of intermolecular interactions giving rise to a total of 4536 molecules. The UNI potential was also employed to assess the very small relevance of surface relaxation and the specific surface energy for the RUB(100) and 4T(001) crystal faces in order to estimate the excess energy arising from the heterojunction interface 31-33.

RESULTS AND DISCUSSION a - Morphology and macroscopic properties Figure 1b shows the morphology of a 8 nm nominally thick 4T thin film grown on KAP(010), presenting large and molecularly flat islands with a thickness of 1.5 ± 0.1 nm, as shown in the height profile reported below the AFM image. This value corresponds to the d002 interplanar spacing of the low-temperature monoclinic polymorph of 4T, namely to the height of a single layer of ‘standing’ 4T molecules

25

. The epitaxial relationship between 4T(001) and KAP(010)

has been widely studied and is characterized by 4T[100]||KAP[001]

18,34

. The flat morphology

and the high degree of crystallinity and orientation of 4T film represent at least the necessary starting point for the growth of crystalline RUB. It has to be reminded that this condition is not sufficient since the fundamental requisite for driving the growth of crystalline RUB is dictated by the interfacial energy of the overlayer-substrate interface and by kinetic factors 6, as presented below.


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Figure 1. (a) Molecular structure of 4T and RUB. (b) AFM morphology images of a 5×5 µm2 area of the surface of a 8 nm nominally thick 4T thin film on the (010) surface of KAP single crystal and RUB thin films grown on top of 4T with a nominal thickness of (c) 0.5 nm, (d) 1 nm, and (e) 5 nm. Below each image the cross-sectional profile extracted along the scan line indicated on the respective AFM image is reported.


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On top of similar 4T/KAP samples, RUB films with increasing nominal thickness have been grown. In Figure 1c, 1d, and 1e the AFM images of samples with a nominal thickness of RUB overlayer of 0.5 nm, 1 nm, and 5 nm, respectively, are reported. In Figure 1c, the 4T islands previously grown on KAP appear to be partially covered by separated RUB islands mainly concentrated at the borders of 4T islands. The thickness of these islands is 1.3 ± 0.1 nm or an integer multiple of this value, which approaches the d200 interplanar spacing 1.343 nm of orthorhombic RUB

24

. Thus, this value corresponds to the height of a single layer of RUB

molecules packed in the orthorhombic polymorph with the (100) plane parallel to the substrate surface. The arrangement of RUB molecules in this sample resembles that of epitaxial RUB films grown on other organic substrates, such as RUB or tetracene (TEN) single crystals 5-7, or on crystalline 4T itself by exploiting a different growth mechanism 8. In Figure 1d the 4T islands, still visible, are almost completely covered by a single layer of RUB molecules (with the usual thickness of 1.3 ± 0.1 nm) on top of which a second layer with the same morphology started growing. Finally, in Figure 1e the morphology of the underlying 4T film can no more be observed, being completely hidden by the several molecular layers constituting the RUB film. Consistently with the thinner films, the RUB film consists of successive layers all with the same thickness, corresponding to the height of single (200) layers of the RUB orthorhombic phase parallel to the substrate surface. However, a distinctive feature of this film is easily noticed, namely the presence of a number of holes distributed all over on the whole surface. This kind of ‘hole pattern’ has already been observed for RUB films grown on TEN single crystals, and their origin has been explained as the result of a kinetic driven transformation of amorphous RUB nano-dots initially present on the surface of the film into a crystalline molecular layer after completion of the deposition process 6.


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In order to confirm the molecular packing inferred from the analysis of the surface morphology, X-ray diffraction measurements have been carried out on the heterostructure with a 5 nm nominally thick RUB film. In Figure 2, all the observed diffraction peaks could be easily indexed and all of them were found to unambiguously correspond either to the (00l) family of planes of the 4T crystalline phase or to the (h00) family of planes of the RUB orthorhombic phase, thus confirming the results from the analysis of AFM images. Moreover, the relative integrated diffraction intensities of the two families of peaks nicely correspond to those expected from the ratio between the nominal thicknesses of the two layers. It is also important to notice that the full width at half maximum of the diffraction peaks for RUB film is noticeably less than the ones for 4T films, thus giving evidence of a higher crystallinity of RUB overlayer with respect to 4T underneath.

Figure 2. X-ray diffractogram collected on a RUB thin-film with a nominal thickness of 5 nm grown on top of a 8 nm thick 4T film after wet-transfer of the whole heterostructure on a Si(100) substrate. The 00l peaks arise from the 4T substrate, while the h00 peaks are due to the RUB overlayer. The silicon 200 peak (labeled Si 200 in the diffractogram) is also visible. For fully characterizing the heterostructures from a macroscopic point view, we used polarized optical spectroscopy in transmission configuration at normal and oblique incidence. The use of


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linearly polarized light is extremely important in order to determine the optical anisotropy (if any) of the films, which suggests the presence of a crystalline phase. Moreover, by analyzing the spectral response on the basis of the known optical response of the materials building the heterostructure, both the out-of-plane and in-plane orientation can be obtained. Figure 3 shows the evolution with nominal thickness of the normal incidence absorption spectra of RUB films grown on the top of a 4T/KAP heterostructure (dotted line) as detected under linearly polarized light along and perpendicular to KAP[001] (β =0° and 90° hereafter, where β is the angle between the electric field direction of light and KAP[001]). These two directions are those giving the maximum optical anisotropy. The presence of optical anisotropy even for the thinner film is a clear signature of the presence of a crystalline phase. In particular, the typical optical response of crystalline RUB can be more clearly observed in the spectra collected for β = 90° (Figure 3a), where 4T thin film is almost transparent 34,35 (see for comparison the dotted line). Indeed, a broad peak centered at about 3.8 eV increases in intensity and becomes more and more structured with increasing thickness, showing three main features at about 3.7, 3.8, and 3.9 eV. For β = 0° (Figure 3b), the optical response of RUB can not be easily detected being overlapped to that of 4T. Nonetheless, for higher RUB thicknesses, the strong absorption peak at 3.68 eV attributed to 4T 34,35 increases in intensity, shifts towards higher energy (at 3.72 eV for 5 nm thick RUB film), becomes sharper, and, finally, a further broad peak at about 3.9 eV arises. The whole behavior at high energy gives clearly the optical signature of crystalline RUB, affecting more and more the spectrum as RUB thickness increases. The optical response of RUB single crystal is indeed characterized by the presence of strong and polarized optical transitions at about 3.7, 3.8 and 3.9 eV

17,36

. Looking at the low energy spectral range, two peaks at 2.49 and 2.66 eV can be clearly

detected under any light polarization for the 2.5 and 5 nm thick RUB film spectra (Figure 3). These two peaks are typically observed along any direction of crystalline RUB but they display


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an absorption coefficient about one order of magnitude lower than the higher energy peaks; this is why they can be detected just in the thickest films. Note that they are also detectable in the spectra of the 1 nm thick RUB film (not shown here), but with a very low signal to noise ratio. It is known that single crystals display another absorption peak at lower energy, i.e. at 2.33 eV, not detected under the present experimental conditions (and sensitivity). This result suggests two pieces of information. On one hand, since this peak is always present in amorphous RUB films 37,38

, its absence implies that the heterostructure is completely crystalline. On the other hand, in

the crystal this peak originates from a dipole moment transition polarized along RUB[100] 36: its absence from the normal incidence absorption spectra is a further confirmation that RUB domains exhibit (100) contact face over macroscopic areas, as indeed suggested by AFM and fully confirmed by XRD. This is assessed by means of oblique incidence absorption spectroscopy, as shown in Figure 4, where the peak at 2.33 eV arises in the spectrum by selecting p-polarization under oblique incidence. Note that contributions to the optical response from the known photooxidation of RUB 37,38 are negligible as checked by collecting the same absorption spectra several days after the growth of the heterostructures, in particular for 1, 2.5 and 5 nm thick RUB films. On similar crystalline films, indeed, oxidation is demonstrated to be limited and restricted to the very first molecular layers.39


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Figure 3. Normal incidence absorption spectra for β = 90° (a) and 0° (b) for increasing RUB thickness (0.5, 1.0, 2.5 and 5.0 nm). The dotted line refers to the spectrum of the bare 4T(001)/KAP(010) heterostructure.


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Figure 4. Normal incidence absorption spectrum of a 5 nm thick RUB film on 4T(001)/KAP(010) heterostructure for β = 0° (dashed line) together with oblique incidence absorption spectra (full line) collected by selecting KAP(100) (and thus 4T(010)) as plane of incidence, p-polarization and 60° as angle of incidence.

Notwithstanding the complete out-of-plane orientation, the in-plane orientation of RUB crystalline domains with respect to the 4T(001) underneath is not unique. By taking into consideration the optical response of RUB single crystals

36

, the high energy peak of crystalline

RUB centered at about 3.7 eV is detectable for light polarized along RUB[001], whilst the peaks at about 3.8 and 3.9 eV along RUB[010]. These absorption peaks in Figure 3 possess similar intensity: this is particular evident in the β = 90° spectrum, not affected by the strong absorption of 4T. The coexistence of these peaks in the β = 90° spectrum represents direct evidence of the presence of several in-plane RUB azimuthal orientations. By performing simulations of the absorption spectra of the heterostructure (see Supporting Information) by using the dielectric tensors of 4T and RUB single crystal reported in the literature

36,40

, the spectral lineshape

observed can be reasonably obtained by taking into consideration the presence of two


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“macroscopic” azimuthal domains at θ = 0° and 90° with similar weight, where θ is the angle between RUB[010] and 4T[100].

b - Microscopic Properties and Epitaxy In order to find and study the true epitaxial relationship between the two materials and the microscopic orientation mechanisms, empirical force-field atom-atom potential simulations were used. This approach has been already assessed as a powerful tool for predicting and justifying the occurrence of specific azimuthal orientations in terms of adhesion energy of the deposit-substrate interface

6,15,17,26-28,41-43

. The results of calculations for the RUB(100)/4T(001) interface are

reported in Figure 5a, in terms of adhesion energy averaged over the number of RUB molecules defining the overlayer island. As observed in previous papers

6,15,17,26-28

, the dominant

interactions are those between atomic arrangements emerging at the substrate and overlayer surfaces. These interactions are responsible for organic-organic heteroepitaxy due to their fast decay with distance

15

, even in the absence of good matching of lattice parameters, i.e. of

commensurism. In the present study, the potential energy of the heterostructure interface appears to be strongly dependent on the azimuthal orientation of RUB(200) islands with respect to the 4T(001) substrate (Figure 5a). Azimuthal orientations corresponding to well defined minima of the potential energy for the substrate/overlayer interaction are located by the adopted docking. In Figure 5a data are symmetry averaged to span the 0°÷180° range owing to the C2 symmetry of the RUB molecules; eight peaks corresponding to favorable orientations are present, clearly showing further symmetry of the configuration space about θ = 90° (see later). The isolated dots at the bottom of each peak are the best estimate of the adhesion energy for each azimuthal angle obtained by full minimization after the docking trials. Two quasi symmetrical and sharp peaks are characterized by the lowest adhesion energy of -34.6 kJ/mol per molecule in the overlayer at


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θ = 6° and 174°. Although these peaks are sharp, the eight candidate configurations for the full energy minimization give azimuth values within few tenths of a degree and adhesion energy within 0.5 kJ/mol, but the relative positions of overlayer and substrate were different and belonging to four groups. This feature has already been recognized and seems to be a general one for organic-organic epitaxy 27: a single macroscopic epitaxial relation can correspond to several distinct configurations at the molecular level due to the presence of many close energy minima in the very complex energy landscape describing the overlayer/substrate interface. The next energy minima are represented by four poorly resolved peaks differing by 1÷2 kJ/mol per molecule compared to the previous ones and centered at about θ = 83°, 86°, 94°, and 97°. This distribution could be symptomatic of frustration of the overlayer in finding the best azimuth around θ = 90°, a feature than can be partly induced by the actual size of the simulated system. The minimized configurations with θ = 82.9° and 97.1° lie at about -33.5 kJ/mol while those at θ = 86.3° and 93.7°correspond to an adhesion energy of about -32.5 kJ/mol. Finally, other minima, definitely less relevant for describing the present potential epitaxial relationship, are present at about 34° and 145° with -30.6 kJ/mol per molecule. All the minimized configurations exhibit a tilt angle between substrate and overlayer surfaces not exceeding 3°.


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Figure 5. (a) Adhesion energy vs. azimuth θ. The dots below each peak indicate energy values after full minimization following the docking runs. Energy has been normalized to a single RUB molecule contacting the substrate. (b) Sketch of the epitaxial interface for two azimuthal orientations of RUB domains (θ = 6°, and 83°). The unit cell for both 4T layer and RUB domains are reported in cyan and green, respectively. The pink straight line shows the coincidence of the RUB and 4T directions.


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Finally, it is worth noting that empirical force-field calculations provide values of θ = 6° (174°) for the absolute minima and θ = 83° (97°) for the next energy minima, that are in fairly good agreement with θ = 0°, and 90°,which are the two “macroscopic” orientations deduced by optical spectroscopy. These energy minima lead to an epitaxial relation close to RUB||4T for the rotational domains with θ = 6° (RUB[021]||4T[110]) and 83° (RUB[021]||4T[1-10]). Note that similar results have been reported for the “inverted” heterostructure 17. Since there is almost a coincidence of non-primitive reciprocal lattice vectors, the epitaxy is line-on-line

45,46

as

observed for organic-inorganic 45,46 as well as organic-organic heterojunctions 15,17,47 and recently also for RUB thin films deposited on (001) surface of TEN single crystal 5.

c – Microscopic model of the interface A typical feature of organic-organic heterojunctions is that each molecule of the overlayer island interacts in a different manner with the substrate compared to the other molecules, a natural condition for line-on-line epitaxy. The average adhesion energy reported in Figure 5a can be expanded to distinguish the contributions of each molecule in the overlayer. For θ = 6° individual molecular adhesion energies ranges from -17.3 to -43.3 kJ/mol, while for θ = 83° the range is from -19.6 to -42.2 kJ/mol. There are thus RUB molecules whose adhesion is stronger than the average while other molecules are less tightly bound to the substrate, although there are no overlayer molecules in a destabilizing regime. This can have a dramatic influence during the nucleation which is hence not equally probable at all sites on the surface. By further partitioning over single atoms the contributions to the adhesion energy, few of the outmost H-atoms of the RUB phenyl groups are in a destabilizing regime, with all other atoms far enough from the substrate surface to stabilize the heterojunction, the major stabilizing contributors being the


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phenyl carbon atoms closer to the interface. Similarly, for the 4T(001) substrate the most exposed H, C, and S atoms provide the major contributions to the interaction with the overlayer through a balance of attractive and repulsive forces. Given the previous considerations, the coincidence of the RUB and 4T directions, sketched in Figure 5b, can be easily explained. The 4T(001) surface exhibits furrows parallel to the [110] direction with a repetition distance of 4.81 Å lined with the most protruding H-atoms (arranged in an approximate compact hexagonal packing) of the terminal thienyl groups. Similar considerations apply to the RUB(100) surface; in this case the [021] direction displays the deepest trenches produced by the pair of most protruding H-atoms (arranged according to a distorted simple hexagonal packing) belonging to ancillary phenyls, the repetition distance being 5.09 Å. The coincidence of direction and periodicity of the main surface corrugations of the substrate and overlayer can be considered the driving force for the epitaxial orientation of the overlayer, as indeed shown for other heteroepitaxial systems

5,6,15-18

. It is worth noting that the

5.8% misfit between RUB and 4T can be accommodated by the growing islands with the introduction of defects for relieving part of the strain. Since the present simulations can not take into account these relaxation phenomena, they provide the highest limit of the adhesion energy of the heterojunction, with possibly slight differences in the azimuthal orientations. The trenches running along the substrate can also have a dramatic influence on the diffusion processes of the deposited molecules during the nucleation and growth of the overlayer islands 48. The same observation has been used previously to rationalize the RUB(100) on 4T(001) epitaxy 17

, where the mutual orientations between the two crystalline surfaces were indeed similar to the

present ones. Finally, it is worth noting that by taking into account the combination between substrate and overlayer surface symmetry 15, we should expect one rotational domain equivalent per symmetry


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for each orientation, since the bulk terminated 4T(001) surface and the overlayer RUB(100) surface display p1 and p2gg symmetry plane group symmetry, respectively. On the contrary, isoenergetic ±θ azimuthal orientations (the pairs 6°/174° 83°/97° and 86°/94° of azimuth angles) can be seen in Figure 5a. Within each pair, the two configurations are in principle different due to the p1 plane group symmetry of the 4T(001) surface, which could potentially be able to select a single overlayer orientation. Based on the previous microscopic model of the interface unexpected symmetry-related findings are thus easy to be justified. In general, due to the decay length of intermolecular interactions, the effective surface symmetry is not determined only by the arrangement of the topmost atoms but, on the contrary, moieties of the superficial molecules can significantly participate to the interactions, determining the overall effective surface symmetry. In this peculiar case, the height difference of thienyl groups emerging at the 4T(001) surface amounts to only 5 pm, so that, for all practical aspects, 4T(001) can be considered of pg symmetry as also described in a previous paper about 4T on RUB epitaxy

17

. Owing to this

feature, all azimuthal angles θ have a pseudo-equivalent configuration at -θ since the pseudoglide line runs parallel to the direction 4T[100]. For obtaining the data reported in Figure 5a, this pseudo symmetry has not been imposed, but the distribution of configurations found spontaneously exhibits a pg symmetry. Comparing this result with the 4T(001)/TEN(001) epitaxy 15

, where the role of surface symmetry on determining the number of rotational domains has been

stressed, it is clear that the actual “amount” of substrate surface corrugation is relevant to define the effective surface symmetry. The lowest possible surface symmetry, i.e. p1 of this study and of ref 15, is potentially able to select a single overlayer orientation. However, in the presence of a very low roughness as in the present case it can be ineffective due to the higher pseudo-symmetry of the surface or, in other terms, to the quasi isoenergetic azimuthal configurations that arise.


20

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d – Concluding remarks on epitaxial rubrene interfaces From the adhesion energy for the favorable azimuthal orientations and the specific surface energies for the 4T(001) and RUB(100) surfaces it is possible to evaluate the excess energy per unit area γ* required to create the heteroepitaxial interface, γ* = γsub + γover - βadh, where γsub, γover, and βadh are the specific surface energies for substrate, overlayer and the interface adhesion energy, respectively

49,50

; γsub and γover were calculated with non relaxed surfaces, as we found

negligible relaxations for close-packed faces such as those containing the herringbone motifs of 4T(001) and RUB(100)

31,32

. From the values γsub = 90.2 erg/cm2, γover = 73.8 erg/cm2, βadh =

110.6 erg/cm2 (θ = 6°) and 107.6 erg/cm2 (θ = 97°) one obtains γ* = 53.4 and 56.4 erg/cm2. Analyzing the “inverted” interface (4T on RUB(001) single crystal): it is interesting to note that the same epitaxial relation as well similar adhesion energy (βadh is ~ 110.4 erg/cm2 for the absolute minima) has been determined

17

. Despite these strong similarities, the morphology of

the overlayer reveals different growth modes. In the present case, a 2D growth mode is observable, whilst for the “inverted” interface a 3D ones was found. This result is not really surprising: indeed, when heterogeneous nucleation is considered, the most relevant aspect is to find the conditions under which the development of 2D islands is favored

49,50

. This can be

evaluated by considering the difference 2γover - βadh, which quantify the wettability of the substrate surface by the overlayer. This difference is about 40 erg/cm2 for the RUB(100)/4T(001) system and much higher for 4T(001)/RUB(100) (about 70 erg/cm2). Being this a positive value, 2D nucleation is possible only if the supersaturation is higher than a threshold value given by the product of the wettability times the surface area per molecule in the nucleus. Furthermore, since βadh exhibits a weak dependence on substrate, the previous condition holds for other epitaxial systems (see Table 1). It is known that OMBE ensures relatively high supersaturations and this


21


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represents a main prerequisite towards the layer-by-layer growth of organic semiconductors, as in the present heterostructure. However, in the “inverted” interface, 3D growth of the overlayer is observed, possibly due to a non-sufficient level of supersaturation achieved in the sublimation of 4T.

Table 1. Excess energy per unit area γ* (erg/cm2) required to create the heteroepitaxial interface between RUB(100) overlayer and other organic semiconductors. substrate

γsuba

βadhb

γ*

4T(001)

90.2c

110.6

53.4

107.6

56.4

TEN(001)

81.7d

105.6e

41.6

RUB(100)

73.8

147.6

0

a

γsub(erg/cm2) has been calculated by means of UNI potential 23

b

βadh (erg/cm2) has been evaluated by means of empirical force field calculations.

c

Taken from ref 31.

d

Taken from ref 32.

e

Taken from ref 6.

Conclusions Organic epitaxy is assessed as the most successful strategy for growing crystalline and oriented RUB films on organic semiconductor surfaces. In particular, RUB grows on top of 4T(001)/KAP displaying the most promising polymorph structure, i.e. the orthorhombic one, and showing (100) as contact plane, that is the most developed face even in single crystals. The epitaxial relation between these organic semiconductor crystals has been determined by means of force field


22

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calculations, whose results are in complete agreement with the macroscopic orientations deduced by means of polarized optical spectroscopy. The 2D growth mode of RUB thin films observed for this heteroepitaxial system has been discussed on the basis of the calculation of the energy required to create the interface, also in comparison with the “inverted” heterostructure.

ACKNOWLEDGMENT This work was supported by Fondazione Cariplo (Grant n. 2009/2551).

Supporting Information Available. Simulation of the absorption spectra. This material is available free of charges via the Internet at http://pubs.acs.org.

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TOC


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Figure 1. (a) Molecular structure of 4T and RUB. (b) AFM morphology images of a 5×5 µm² area of the surface of a 8 nm nominally thick 4T thin film on the (010) surface of KAP single crystal and RUB thin films grown on top of 4T with a nominal thickness of (c) 0.5 nm, (d) 1 nm, and (e) 5 nm. Below each image is reported the cross-sectional profile extracted along the scan line indicated on the respective AFM image. 80x171mm (300 x 300 DPI)


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Figure 2. X-ray diffractogram collected on a RUB thin-film with a nominal thickness of 5 nm grown on top of a 8 nm thick 4T film after wet-transfer of the whole heterostructure on a Si(100) substrate. The 00l peaks arise from the 4T substrate, while the h00 peaks are due to the RUB overlayer. The silicon 200 peak (labeled Si 200 in the diffractogram) is also visible. 60x46mm (300 x 300 DPI)



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Figure 3. Normal incidence absorption spectra for β = 90° (a) and 0° (b) for increasing RUB thickness (0.5, 1.0, 2.5 and 5.0 nm). The dotted line refers to the spectrum of the bare 4T(001)/KAP(010) heterostructure. 141x249mm (300 x 300 DPI)


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Figure 4. Normal incidence absorption spectrum of a 5 nm thick RUB film on 4T(001)/KAP(010) heterostructure for β = 0° (dashed line) together with oblique incidence absorption spectra (full line) collected by selecting KAP(100) (and thus 4T(010)) as plane of incidence, p-polarization and 60° as angle of incidence. 63x50mm (300 x 300 DPI)



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Figure 5. (a) Adhesion energy vs. azimuth θ. The dots below each peak indicate heterostructures after full minimization following the docking runs. Energy has been normalized to a single RUB molecule contacting the substrate. (b) Sketch of the epitaxial interface for two azimuthal orientations of RUB domains (θ = 6°, and 83°). The unit cell for both 4T layer and RUB domains are reported in cyan and green, respectively. The pink straight line shows the coincidence of the RUB and 4T directions. 80x115mm (300 x 300 DPI)


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88x34mm (300 x 300 DPI)


Epitaxial Interfaces in Rubrene Thin Film Heterostructures - The

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