Toward Single Crystal Thin Films of Terthiophene by Directional

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Toward Single Crystal Thin Films of Terthiophene by Directional Crystallization Using a Thermal Gradient Guillaume Schweicher,† Nicolas Paquay,† Claire Amato,† Roland Resel,‡ Markus Koini,‡ Samuel Talvy,§ Vincent Lemaur,|| Jerôme Cornil,|| Yves Geerts,*,† and Gabin Gbabode† †

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Laboratoire de Chimie des Polymeres, Faculte des Sciences, Universite Libre de Bruxelles (ULB), CP 206/1, Boulevard du Triomphe, 1050 Bruxelles, Belgium ‡ Institute of Solid State Physics, Graz University of Technology, Petersgasse 16, 8010 Graz, Austria § Transfers, Interfaces and Processes, Faculte des Sciences Appliquees/Ecole Polytechnique, Universite Libre de Bruxelles (ULB), CP 165/67, Avenue F.D. Roosevelt 50, 1050 Bruxelles, Belgium Service de Chimie des Materiaux Nouveaux, Universite de Mons (UMons), Place du Parc 20, 7000 Mons, Belgium

bS Supporting Information ABSTRACT: A method for the preparation of uniaxially oriented thin films of terthiophene (2050 μm thick) is introduced. It relies on the crystal growth with a Bridgman-type process that decouples nucleation and growth phenomena. An effective thermal gradient of 611.6 °C/mm has been used in which the sample (terthiophene powder deposited on either glass or fluorinated glass substrates) is displaced from a hot zone to a cold zone at a constant rate of 2.55 μm/s. The size and orientation of crystals have been investigated by polarized optical microscopy and X-ray diffraction measurements. A coexistence of two polymorphs of terthiophene has been observed, but optimal gradient conditions enabling the selective crystallization of only the room temperature stable polymorph have been found. Terthiophene films deposited on fluorinated glass substrates and crystallized using the thermal gradient technique show a stronger tendency to polymorphism and random orientation of crystallites for all gradient conditions tested. The monoclinic unit cell (a = 15.410 Å, b = 5.709 Å, c = 26.052 Å, β = 97.77°) of the room temperature phase orients its ab plane parallel to the substrate. Pole figures demonstrate the growth of uniaxially aligned crystals with the [100] and [100] directions along the gradient axis. Finally, a tentative explanation for this peculiar in-plane orientation is given based on crystal morphology calculations.

’ INTRODUCTION For the past two decades, organic electronics has emerged as a new field that relies on the use of organic semiconductors (OSC) in various optoelectronic devices.1 Industrial interest is very high because of the expectation of unprecedented applications, notably in electroluminescent displays, lighting, circuits on flexible substrates, and photovoltaic cells.2 In parallel, fundamental research is also requested. Indeed, in spite of obvious theoretical and experimental progress, a deep understanding of the links between molecular structure, supramolecular organization, and optoelectronic properties is currently lacking.3 An additional difficulty comes from the fact the performances of optoelectronic devices are strongly correlated to their method of fabrication.47 Accordingly, there is a considerable interest in the controlled deposition and crystallization of OSC.8 OSC can generally be divided into two classes of materials: polymers (amorphous, semicrystalline, or polycrystalline) and small molecules (amorphous, polycrystalline, or even single crystalline), displaying different charge transport mechanisms, especially as a function of temperature.3 Thin film fabrication of OSC, either by physical vapor or solvent deposition, induces crystallization upon deposition, leading usually to polycrystalline r 2011 American Chemical Society

films9 for which charge transport is reduced as a result of the presence of structural defects such as vacancies, interstitials, dislocations, and grain boundaries.3,10 On the other hand, single crystals, which are known to exhibit intrinsic electronic properties (e.g. an upper limit for charge transport), require special production conditions.5,9,1114 Moreover, the growth of single crystals from solution or vapor involves little or poor control of orientation, size, and shape.11,12,15 Together with their difficulty of integration into devices, single crystals are not at present of great interest for industrial applications.8 However, some progress has recently been made in the field of single crystal patterning. In particular, Bao et al. have recently managed to pattern R-sexithiophene single crystals with precisely controlled sizes and shapes.16 It is also worth mentioning the work of Hong and Lee, who have succeeded in reproducibly growing single crystals of 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS pentacene) directly in the channel of bottom-contact organic field-effect transistors using a modified drop-cast technique.17 Received: June 20, 2011 Published: June 27, 2011 3663

dx.doi.org/10.1021/cg2007793 | Cryst. Growth Des. 2011, 11, 3663–3672

Crystal Growth & Design Although several pieces of current research focus on enhancing the supramolecular order in organic thin films and obtaining large uniaxially oriented domains,8,1825 less attention has been paid to crystallization in a thermal gradient. This technique, well developed in the inorganic semiconductor industry,26,27 decouples nucleation, controlled by undercooling, from crystal growth, controlled by the sample displacement velocity,28 thus increasing the probability of single crystal fabrication. As far as organic materials are concerned, several studies based on this crystallization technique were carried out in the 1970s and 1980s on n-alkanes2932 and polymers.3336 The aim was to afford a deeper understanding of nucleation and growth during polymer processing methods such as molding or extrusion. The growth of single crystals of polar conjugated molecules, such as m-nitroaniline,37,38 4-(N,N-dimethylamino)-3-acetamidonitrobenzene,39 and p-aminobenzoate,40 has also been studied for applications in the field of nonlinear optics. More recent studies involving a thermal gradient with OSC were carried out for purification of small molecules,41 for exploring the effect of recrystallization of poly(3-dodecylthiophene) (P3DDT) under an external field42 or bulk growth by the Bridgman process.43 Let us also mention the work of Brinkmann et al., who succeeded in obtaining highly textured thin films of conjugated polymers using a crystallization method combining the presence of a crystallizable aromatic solvent as an epitaxial template and a thermal gradient.4447 Directional crystal growth in a thin film geometry has commonly been used for the study of faceted growth of crystals,4850 liquid crystals,51 and eutectic mixtures5254 of organic compounds. Surprisingly enough, this method has not been widely applied in the context of organic electronics to grow single crystalline thin films of OSC. In this context, understanding, at a fundamental level, the factors that govern the directional growth of OSC in thin films under the influence of a thermal gradient is of prime importance for the fabrication of single crystalline thin films of OSC with enhanced charge transport properties. Specifically, the role of (i) molecular and crystal structures; (ii) gradient magnitude; (iii) growth rate; (iv) polymorphism and mesomorphism; (v) thickness and geometry of thin films; and (vi) chemical nature of interfaces has to be investigated. In this paper, we report on the directional crystallization of terthiophene (hereafter abbreviated 3T) as a function of thermal gradient parameters (magnitude of the gradient, sample velocity) in a thin film geometry. The influence of the interfaces is also addressed by comparing the results for two different substrates (glass and polymer-treated glass). A detailed structural analysis combining polarized optical microscopy (POM) and X-ray diffraction (specular, pole figures) has been carried out to characterize the shape, size, and orientation (in and out of the plane of the substrate) of the crystals produced by the thermal gradient technique. Finally, a mechanism explaining the self-seeding is proposed.

’ EXPERIMENTAL SECTION 1. Materials. 2,20 : 50 ,2-Terthiophene (3T) has been supplied by Aldrich (purity grade 99%) and used without further purification. FKM (provided by Montefluos) is a fluorinated rubber [(CH2CF2)0.6 (CF2CF(CF3))0.4]n with molecular weight Mw = 70 000.55 2. Thermal Gradient Technique. Description of the Apparatus. The setup, whose schematic representation is given in Figure 1, consists of a Linkam GS350 temperature gradient system heating stage

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Figure 1. Schematic representation (not at scale) of the thermal gradient setup. composed of two independent heating devices separated by a distance of 2.5 mm where the thermal gradient is installed. One is set at a temperature Th above the melting temperature (hot side) and the other at a temperature Tc below the crystallization temperature (cold side) of 3T. The whole system is enclosed in a hermetic metallic container so that the sample is thermally insulated from the laboratory environment. Sample Preparation and Description of the Method. The sample, about 4 mg of 3T deposited on a 20 20 0.16 mm3 precleaned thin glass substrate (Marienfeld cover glasses Cat. No. 0101040) and covered by either the same glass substrate or a FKM-treated one (FKM was dissolved in acetone at a concentration of 10 wt % and spin-coated at 6000 rpm onto the glass substrates), is initially placed entirely at the hot side and is slowly translated to the cold side at a constant speed V until all sample is at the cold side. A 76 26 1 mm3 microscope glass slide (Marienfeld Cat. No. 1000000) is intercalated between the heating stages and the sample—see Figure 1—to ensure a constant displacement velocity of the sample. This yields films with thicknesses in the range 2050 μm (measured with a 025 mm Microrapid etalon micrometer). Cover substrates were removed for X-ray diffraction analysis. About equal amounts of material remained on the two glass substrates (both usable for X-ray diffraction measurements), while, in the case of FKM-treated glass, the material remains stuck on the FKMtreated glass cover. Heat Transfer Simulation. Heat transfer simulation of the experimental setup has been carried out using Comsol Multiphysics software. The input parameters were the dimensions of all elements (heating stages (3.2 3.2 cm2), supporting microscope glass slides, sample— including the glass substrate and cover—with a film thickness of 50 μm, gap between the two heating stages), the temperature of the hot and cold sides, the enthalpy of melting, and the density of 3T and some physical data of naphthalene assumed to be similar to that for 3T (Cpsolid, 1470 J/kg 3 K; Cpliquid, 1715 J/kg 3 K; thermal conductivity solid, 0.333 W/m 3 K; thermal conductivity liquid, 0.1221 W/m 3 K).56,57 Crystal Morphology Calculations. The crystal morphology of 3T has been modeled from the crystal structure determined by van Bolhuis et al.58 using the Morphology module of the Materials Studio package59 (using the Compass force field,60 which has previously been used for crystal structure optimization of a terthiophene derivative61). The attachment energy (Eatt) method developed by Hartman6264 has been used to determine the relative importance of the different crystallographic faces in the crystal morphology: faces with higher Eatt (in absolute value) grow faster and have then less importance in the final crystal shape and vice versa. This methodology is quite efficient to describe the shape of crystals grown from vapor or from the melt, 3664

dx.doi.org/10.1021/cg2007793 |Cryst. Growth Des. 2011, 11, 3663–3672

Crystal Growth & Design i.e. without external interactions (solvent molecules for example), and thus is relevant to describe crystallization using a thermal gradient. Polarized Optical Microscopy (POM). The thermal gradient apparatus is mounted on a polarized optical microscope Nikon Eclipse 80i so that images can be taken before, during, and after the thermal gradient to obtain qualitative information on the crystallization behavior and also to evaluate the domain size. Specular X-ray Diffraction (sXRD). These measurements were performed on a Bruker D8 Advance diffractometer using Cu KR radiation (λ = 1.5418 Å) equipped with a MRI (Material Research Instruments) heating stage for temperature-dependent measurements. Diffraction patterns were collected in the scattered angular range between 1.6° and 40° with an angular resolution of 0.02° per step and a typical counting time of 10 s per step, using the θ/θ reflection geometry (the source and detector both move from the horizontal sample plane at the same angle θ, that is, in specular reflection conditions). All sXRD patterns are represented as the scattering intensity (in counts or counts per seconds—cps) versus q (in Å1), the momentum transfer, defined as q = 4π/λ sin θ. X-ray Diffraction (XRD) Pole Figures. Pole figures (as well as the specular scans of 3T samples deposited on FKM-treated glass substrates) were measured with a PHILIPS X0 Pert system equipped with an ATC3 cradle using Cr KR radiation (2.291 Å) and a graphite secondary monochromator. Due to the uncertainty of the detailed crystal structure of terthiophene three-dimensional reciprocal space, maps were recorded from the different samples.65 The diffraction patterns were calculated with the software POWDERCELL, and the pole figures were simulated with the software STEREOPOLE.66

’ RESULTS 1. Structural Arrangement of Terthiophene. The crystal structure of 3T has been solved by van Bolhuis et al. from single crystal X-ray diffraction measurements performed at 130 K.58 It is monoclinic with space group P21/c and consists of a lamellar packing of upright 3T molecules with a herringbone packing of neighboring molecules within the lamellae. The relative intensities of diffraction peaks calculated from the determined atomic fractional coordinates are similar to those of the XRD pattern of 3T measured at room temperature, even if peak positions are not exactly the same due to lattice thermal expansion. Hence, the room temperature phase of 3T (hereafter called the LT phase, for low-temperature phase) corresponds to the phase whose crystal structure has been solved. The unit cell parameters of the LT phase at room temperature determined using Lebail refinement67 are a = 15.410(2) Å, b = 5.709(1) Å, c = 26.052(2), and β = 97.77(2)°. Upon heating, the LT phase transforms into another phase (hereafter called the HT phase, for high-temperature phase) just 1 °C below the melting temperature of 3T. The LT phase to HT phase transition has been reproducibly observed by DSC (scanning rate of 1 °C/min; see Supporting Information) and temperature-dependent XRD measurements. In Figure 2, the evolution as a function of temperature of the 002 reflection (d002 = 12.9 Å) of the LT phase is shown. At 95 °C, the intensity of the peak corresponding to the LT phase starts to decrease while the 001 reflection of the HT phase emerges at higher q values (d001 = 12.2 Å). At 97 °C, only the latter reflection is observed, and eventually, at 98 °C, the sample melts, as no diffraction peak is revealed. To the best of our knowledge, the presence of the HT phase has never been reported before. The complete XRD patterns of both the LT and HT phases are given in the Supporting Information, including unit cell dimensions and indexation of the most intensive reflections. The HT phase also reveals a lamellar arrangement of 3T

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Figure 2. Temperature-dependent X-ray diffraction patterns of a 3T powder sample in the (0.450.54 Å1) q-range (first order 00l reflections) upon heating (heating rate: 10 °C/min). The HT phase (d001 = 12.2 Å) is only observed at 95 and 97 °C. At 98 °C, the sample is melted.

molecules (00l reflections up to the fourth order are observed), but molecules are expected to be here tilted by about 19° with the normal to the lamellar plane when comparing the lamellar thickness d001 = 12.2 Å of the HT phase to the d002 = 12.9 Å of the LT phase, in which molecules stand upright with respect to the lamellar plane.58 In the following, the peaks corresponding to the nth orders of the lamellar reflections, which allow for an easy distinction between the two phases, will be denoted n and n0 for the LT and HT phases, respectively (n = 1, 2, ...). 2. Crystallization of Terthiophene Using a Thermal Gradient. 2.1. Thermal Gradient Conditions and Modeling. 3T has been chosen as a model compound for studying the effect of directional crystallization using a thermal gradient on small π-conjugated molecules, as it fulfills some preliminary requirements. Indeed, it has accessible melting and crystallization temperatures (Tmelt = 92 °C and Tcryst = 8384 °C according to DSC measurements—see Supporting Information) with moderate overcooling (Tcryst ≈ 0.98Tmelt). It is also thermally stable up to 220 °C, according to thermogravimetric measurements.68 Finally, as shown in the previous section, its crystal structure is known, which facilitates the investigation of crystal orientation. According to the melting and crystallization temperatures of 3T, two Th Tc couples have been tested, namely 110 °C 60 °C and 100 °C 75 °C, inducing thermal gradients of magnitudes G1 = 20 °C/mm and G2 = 10 °C/mm, respectively (Table 1). In addition, two sample velocities were used, V1 = 2.5 μm/s and V2 = 5 μm/s, thus yielding four conditions, denoted gradij, with i, j = 1 or 2, with i standing for the gradient magnitude (G1 or G2) and j for the sample translation speed (V1 or V2). The 3T sample is cooled at a constant rate C as it proceeds from the hot to the cold side; the latter is calculated from both the gradient magnitude and the sample displacement speed (see Table 1) and readily compared to a DSC cooling rate. All above-mentioned values of gradient magnitude and cooling rate validate the assumption that heat transfer is achieved only by thermal conduction between the heating stages and the sample. However, heat transfer between the mobile hot sample and the surrounding air within the heating device may occur (natural convection). Nevertheless, simulation of heat transfer within the thermal gradient apparatus showed that the presence of convection effects could be discarded. In particular, linear (T, x) profiles have been obtained in the gap between the two heating stages for 3665


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Table 1. Summary of the Different Gradient Conditions Applied in the Present Studya sample velocity

gradient magnitude

cooling rate

gradient magnitude

cooling rate

entry

Th (°C)

Tc (°C)

V (μm/s)

G (°C/mm)

C (°C/min)

Gsimul (°C/mm)

Csimul (°C/min)

grad11

110

60

2.5

20

3

11.6

1.7

grad12

110

60

5

20

6

11.6

3.5

grad21

100

75

2.5

10

1.5

6

0.9

grad22

100

75

5

10

3

6

1.8

a Gradient magnitude G and cooling rate C are calculated as G = (Th Tc)/x and G = (Th Tc) V/x, with x = 2.5 mm the gap between the hot and cold sides. Gsimul and Csimul correspond to the values calculated from the temperatures extracted from the simulated (T, x) profiles (see text and also Supporting Information).

Figure 3. Calculated thermal profiles along the whole sample (at a position between the two heating blocks) for both gradient conditions studied at a speed of 5 μm/s. The region between the two vertical blue lines corresponds to the gap between the hot (left side) and cold (right side) parts of the heating stage. The vertical green lines delimit the transient regions at both sides of the gap, where the effective temperature is lower (hot side) or higher (cold side) than the applied temperature. Red ellipses surround the predicted temperatures at each border of the gap for the two gradient conditions, which are used to calculate Gsimul and Csimul (see text).

the two Th Tc couples studied (Figure 3), with T the temperature of the 3T sample and x its position along the displacement direction. However, in the vicinity of the gap edges (a couple of millimeters), a nonlinear evolution of T is observed before reaching the temperature of the respective heating stages; these transient regions are wider when the difference between Th and Tc is larger (110 °C 60 °C). This effect is attributed to heat distribution in the thick intercalated microscope glass slide. Hence, the gradient magnitude and cooling rate to which the 3T sample is effectively submitted is different from that previously calculated, since the temperatures at the boundaries of the gap are different from that of the respective heating stages. These values (denoted Gsimul and Csimul) calculated from the temperatures extracted from the (T, x) profiles are also given in Table 1 for comparison. They are smaller than the expected values (around 40% reduction), but their respective magnitude throughout the four gradient conditions is nearly constant. For the sake of consistency, we will in the following, rather, consider Gsimul and Csimul obtained from the calculations. Our simulations

of the temperature profile across the gradient agree well with analytical, modeling, and experimental results obtained for Bridgman-type processes.6974 The comparison with the literature also validates our simulations that do not take into account the heat released at the crystallization front when samples are very slowly laterally displaced, i.e. the so-called quasi-steady-state (QSS) regime.75 Samples crystallized using the thermal gradient technique with the above-mentioned conditions have been systematically analyzed by POM, sXRD, and XRD pole figures in order to determine the size and orientation of crystalline domains and also estimate the effects of gradient parameters. 2.2. Polarized Optical Microscopy. Figure 4 shows the POM images obtained at room temperature before and after the thermal gradient (grad12). Needle-like randomly oriented crystals are typically observed before the thermal gradient. The domain size with uniformly oriented crystals is around 200 μm on average, and the powder-like character is well emphasized by the pronounced variation of birefringence between the domains (presence of brighter and darker zones in Figure 4, left). In contrast, a unique millimeter-size homogeneous crystalline domain is observed for the sample obtained after the thermal gradient, as demonstrated by the uniform color of the image obtained under crossed polarizers. The straight lines splitting the image transversally and vertically correspond to cracks (white arrows in Figure 4, right) which spread after the gradient process when the sample is cooled down to room temperature, most likely due to thermal contraction of the crystal. The same type of image has been reproducibly obtained after the thermal gradient independently of the gradient parameters (see the Supporting Information). 2.3. Specular X-ray Diffraction Measurements. The presence of crystalline monodomains suggested by POM observations is also supported by sXRD measurements. Parts a and c, respectively, of Figure 5 show the sXRD patterns measured at room temperature for samples obtained after thermal gradients of 6 °C/mm (grad21 and grad22) and 12 °C/mm (grad11 and grad12). Parts b and d of Figure 5 are their respective magnifications in the [1.341.70 Å1] q-range, where lie intralayer reflections of the LT phase, whose powder diagram is also shown. Two families of intensive 00l reflections are visible up to the fifth order for all samples, corresponding to the lamellar distances of the LT and HT phases. Sample grad12 is, however, an exception, since only the family of reflections corresponding to the LT phase is observed. Most often, the HT phase remains at room temperature in a metastable state in coexistence with the LT phase after crystallization using the thermal gradient technique, then preventing the formation of a monodomain, strictly speaking. However, it appears that when the sample velocity is 3666



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Figure 4. Polarized optical microscopy images recorded at room temperature for a 3T sample before (left) and after (right) the thermal gradient (grad12: Th = 110 °C, Tc = 60 °C, V = 5 μm/s). White arrows (right) emphasize cracks that appear while cooling the sample down to room temperature. The colored arrow on top (right) indicates the direction of the thermal gradient.

Figure 5. Specular X-ray diffraction patterns measured at 30 °C for 3T samples obtained after a thermal gradient of 6 °C/mm ((a) whole [0.35 2.80 Å1] q-range and (b) represented in the [1.341.70 Å1] q-range) and 12 °C/mm ((c) whole [0.352.80 Å1] q-range and (d) represented in the [1.341.70 Å1] q-range). For parts b and d, the X-ray diffraction pattern of a 3T powder sample (LT phase) is also shown for comparison. Note that the 30 reflection (indicated with a blue arrow) is absent for the grad12 sample in part d, while it is clearly present for the three other samples crystallized using the thermal gradient technique.

high (5 μm/s), nucleation of the HT phase can be limited (sample grad22), as shown by the much lower intensity of n0 reflections compared to n ones (see Figure 3b, for example), or even suppressed (sample grad12). The much higher intensity of the 00l reflections (either of the LT or HT phase) observed for samples obtained after the gradient compared to the case of powder samples (for which approximately the same amount of material was analyzed), and

the fact that only these reflections are visible in the whole sXRD diagram for each sample, indicates a high preferential orientation of the lamellar planes parallel to the substrate plane for both the LT and HT phases, that is, a preferential orientation of 3T molecules perpendicularly to the substrate (LT phase) or slightly leaning from its normal direction (HT phase). However, the (106) reflection (of high intensity for powder samples in the LT phase) is observed for all samples obtained after thermal 3667



Figure 6. (a) Schematic representation of the pole figures measurement and pole figures measured at 30 °C of the (b) {211} (q = 1.39 Å1) Bragg peak for a 3T powder sample (background is 250 counts, white; maximum intensity is 1200 counts, black) and the (c) {211} (q = 1.39 Å1) and (d) {400} (q = 1.67 Å1) Bragg peaks for a 3T sample crystallized using the gradient technique (grad12, background is 250 counts, white; maximum intensity is 2800 counts, black). The black arrows in part c indicate low intensity spots, presumably originating from other reflections at nearby 2θ angles (see text).

gradient, though with very small intensity. We attribute this to some mosaicity of the corresponding reticular planes, as these should only be tilted by an angle of around 15° from the 00l planes (i.e., from the substrate surface), assuming a single crystalline sample. sXRD measurements are then consistent with POM observations, by indicating a unique orientation of crystalline domains. In particular, sample grad12 is of utmost interest, as it only exhibits the LT phase. Nevertheless, this conclusion is only limited to the direction perpendicular to the substrate (outof-plane), as a result of the specular configuration. We have further performed XRD pole figures measurements so that we can access the crystallite orientation in the plane of the substrate (in-plane). The two extreme cases are biaxially textured films (a single direction for the out-of-plane orientation and a single direction for the in-plane orientation of the crystallites) on the one hand and fiber structures (a single direction for the out-ofplane orientation but a random in-plane orientation of crystallites) on the other hand; the latter are quite commonly encountered in thin films of lamellar stacking π-conjugated compounds.7680 2.4. X-ray Diffraction Pole Figures. Figure 6a shows a schematic representation of the pole figures measurements, Figure 6b shows the XRD pole figure of the {211} reflection (q = 1.39 Å1) measured for a sample before thermal gradient, and Figure 6c and d shows the pole figures of the {211} and {400} reflections (q = 1.67 Å1 for the latter) obtained for a sample crystallized after a thermal gradient (sample grad12). These two particular reflections have been chosen because they are of high intensity in the powder sample (see Figure S3a in the Supporting Information) and also because they are related to the intralayer arrangement of 3T molecules. As expected, a random in-plane orientation of crystallites is revealed for samples without a thermal gradient, as shown by a wide intensity distribution of the {211} reflection along with azimuthal and meridional rotations of the sample. This result is

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Figure 7. Specular X-ray diffraction patterns measured at 30 °C for a 3T powder sample (blue) and 3T samples crystallized using the thermal gradient technique (grad12) deposited on glass substrates (red) and FKM-treated glass substrates (black). “/” indicates reflections which could not be indexed with the unit cell parameters of the LT or HT phase.

consistent with POM observations and sXRD measurements. When the sample is crystallized with a thermal gradient, the intensity of both the {211} and {400} reflections is located at a number of symmetry related discrete positions (four for the former and two for the latter). This points to two antiparallel orientations of crystallites for each family of reflections (the crosses on selected intensity spots—the two upper ones for {211} and the lower one for {400}; see Figure 6c and 6d—in the pole figures indicate one of the two orientations for both reflections). These results clearly emphasize an in-plane uniaxial orientation of crystallites for the grad12 sample. Some intensity spots, yet with much lower magnitude, are also visible at other discrete locations on the {211} pole figure (black arrows in Figure 6c), which likely come from the diffraction of nearby reflections such as (210) or (302), occurring at q = 1.37 Å1 and q = 1.39 Å1, respectively. 2.5. Influence of the Substrate. In order to investigate the influence of the substrate when samples are crystallized using the thermal gradient technique, 3T samples on FKM-treated glass substrates (see the Experimental Section) were also considered, using the same gradient conditions as those for samples on glass substrates (see Table 1); they were also analyzed by POM observations and by sXRD and XRD pole figure measurements. Large-size homogeneous crystalline domains separated by cracks were observed by POM, as for samples deposited on glass substrates (see Supporting Information). However, sXRD measurements reveal a much larger tendency of these samples to polymorphism, which is detrimental to the formation of a monodomain, and also a much more powder-like orientation of crystallites, as shown in Figure 7, where sXRD patterns of the 3T powder sample and samples crystallized using a thermal gradient on glass and FKM-treated glass substrates are superimposed. A preferential orientation of 00l planes parallel to the substrate surface is still dominant for the sample deposited on a FKMtreated glass substrate. However, coexistence of both LT and HT phases prevails while only the LT phase is observed for the sample deposited on glass substrate with these gradient conditions (grad12). Furthermore, the observation of several intralayer reflections of the LT phase, yet with much lower intensity compared to 00l reflections, evidences the presence of an appreciable amount of randomly oriented domains. This is also supported by XRD pole figures measurements which show a less good in-plane 3668



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ordering than samples on glass substrates (not shown). Several reflections could not be indexed in the sXRD pattern of sample deposited on a FKM-treated glass substrate (noted as “/”), which then suggests the presence of additional polymorphs in coexistence with LT and HT phases.

’ DISCUSSION 1. Decoupling Nucleation and Growth. The aim of this work is to assess whether a decoupling of nucleation (triggered by the thermal gradient) and growth (monitored by the sample displacement) could be effective for OSC in a thin film geometry and could ultimately lead to a single crystalline sample (growth of a unique nucleus). Previous works document the growth of single crystals of organic compounds by the vertical Bridgman technique in a double-walled ampule via a selective self-seeding mechanism.40,43,81,82 However, the cause of the selectivity of a single seed was not explained. Lovinger et al., in their pioneering works on the directional crystallization of rods of polyethylene oxide using a thermal gradient, cast some more light on the origin of the self-seeding mechanism.28,33 Indeed, the authors have derived the expression of the nucleation rate as a function of the gradient parameters (gradient magnitude, sample velocity) and have suggested that optimal values could be found so that the nucleation rate should be sufficiently decreased to promote single crystal growth (total number of primary nuclei formed equal to 1). The results obtained for 3T clearly show the decoupling of nucleation and growth, which lead to highly textured films with in-plane uniaxial orientation of crystallites. It appears that crystallization homogeneously occurs along a line perpendicular to the gradient direction (crystallization front), as suggested by the large domain size observed by POM along this direction. To the best of our knowledge, the present work constitutes the first experimental evidence of the production of such highly textured films on large scale (mm-size domains) of a π-conjugated molecule using the thermal gradient crystallization technique. Previous works have either not explicitly determined the crystal orientation of thin films of OSC41,83 or have focused on the production of single crystal ingots of OSC by Bridgmantype techniques.43,84 They have in common the use of a hot wire or a hot ring to melt samples with poor control of the gradient conditions. We emphasize here the importance of using tailored gradient profiles combined with a specific displacement rate for the control of the crystal morphology. 2. Polymorphism Selection. The clear effect of the variation of gradient parameters is the LT phase selective nucleation evidenced by sXRD measurements. If we consider grad12 and grad22 samples for which a lower amount of HT phase has been observed, it appears that the cooling rate is higher for these conditions, that is 3.5 °C/min and 1.8 °C/min, respectively, and that the nucleation of HT phase is suppressed for the fastest cooling (grad12). However, it has been shown that grad11 samples exhibit a much higher amount of HT phase in coexistence with the LT phase than grad22 samples while the cooling rate is nearly the same in both experiments (see Table 1). This shows that it is more the higher sample velocity rather than the effective cooling rate that kinetically hampers the formation of HT phase nuclei. In addition, when the cold side temperature is low enough, so that the HT phase is thermodynamically unstable, nucleation of the HT phase is made highly improbable, as observed for the grad12 sample. The suppression of the HT phase by appropriate gradient conditions is quite relevant, as it is

Figure 8. Schematic representation of the 3T unit cell orientation with respect to the gradient direction for 3T samples crystallized using the thermal gradient technique: (a) side view showing the two possible unit cell orientations along the gradient direction; (b) top view.

known that the presence of polymorphism can be detrimental to charge transport.85 3. Influence of Interfaces. The introduction of a FKM polymer layer between the film and the substrate amplifies the tendency of 3T samples crystallized with the thermal gradient technique to exhibit several phases in coexistence, thus preventing the formation of monodomains. Here, the physical and chemical nature of the substrate has been changed when replacing glass substrates by FKM-treated glass substrates, so that the exact reason for this behavior is difficult to know and should be investigated by a systematic study with different substrates.55,8688 However, these results clearly emphasize that the substratematerial interface also plays a significant role in the nucleation and growth of 3T crystallites in addition to gradient parameters. 4. Crystal Orientation and Self-Seeding Mechanism. Finally, by combining sXRD and pole figure measurements, it is possible to determine the orientation of the unit cell of 3T with respect to the gradient direction for the grad12 sample. The former experiments indicate that the reciprocal vector c* is perpendicular to the substrate plane (observation of 00l planes only); that is, the (a, b) plane is parallel to the substrate plane. When looking at the pole figure of the 400 reflection (Figure 6d), it is clear that the intensity spots are aligned with the black dot situated at the north pole (gradient direction). This indicates that the in-plane component of the reciprocal vector a* (hence the a axis) is parallel to the gradient direction. The unit cell is oriented with either the [100] or [100] crystallographic directions along the thermal gradient direction. These two unit cell orientations are schematically represented in Figure 8 together with a view of the solidification front of 3T molecules during crystallization by the thermal gradient technique. Sirringhaus and co-workers,19 who studied the crystallization of pentacene using a zone casting technique (similar to the thermal gradient technique but with crystallization obtained from solution and not from the melt), also found that the a axis of the pentacene unit cell was parallel to the zone-casting (direction of displacement of the sample). Their assumption was that the crystal facet with the fastest growth speed preferentially grew along the zone-casting direction. Inspired by this work, we have modeled the 3T crystal morphology (LT phase) to calculate the fastest and slowest growing crystallographic orientations on the basis of the relative attachment energies.6264 Table 2 displays the latter values for the most dominant faces of the equilibrium crystal shape, which is shown in Figure 9 (a detailed indexation of the crystal faces can be found in 3669



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Table 2. Attachment Energies and Corresponding Percentage of Total Facet Area of the Most Dominant Faces in the Crystal Shape of 3Ta hkl

dhkl (Å)

Eatt (kJ/mol)

% total facet area

{100}

15.1

145

42.8

{002}

12.8

148

42.3

{110}

5.3

462

3.9

{011}

5.5

464

8.0

{111} {020}

5.2 2.8

464 654

3.0

{hkl} indicates all symmetry related planes (monoclinic P21/c symmetry) corresponding to a (hkl) crystallographic plane (for example {110} = {(110), (110), (110), (110)}. a

Figure 9. Schematic representation of the nucleation mechanism inducing the alignment of the [100] direction parallel to that of the gradient.

Figure S6 of the Supporting Information). It appears that {100} and {002} are the slowest growing faces, as indicated by their lowest Eatt (in absolute value) compared to that of the other faces. This is well depicted in the final crystal shape, where the two faces represent around 85% of the total surface of the crystal (last column of Table 2). All other relevant faces which form the edges of the crystal ([010] direction) grow much faster and thus have reduced importance in the final crystal morphology. In particular, the {020} face is even not observed in the equilibrium crystal morphology, as it grows too fast. If we neglect the (002) and (002) faces that are parallel to the glass plates (3T molecules preferentially stand upright on the glass substrates, very likely as a result of reduced interfacial tension in this configuration89), the slowest growing faces are (100) and (100) and are oriented along the gradient direction. This observation contrasts with the hypothesis of Sirringhaus and co-workers. However, our results can be rationalized on the basis of the recent work of Ward et al., who have shown that the growth of glycine nanocrystals in cylindrical pores of around 20 nm diameter and 3 mm height leads to a crystal orientation with a fast growth direction aligned with the pore direction (long axis of the cylinders).90 It was hypothesized that confinement favors a growth orientation that permits nuclei to achieve critical size more competitively when their fast growth axis is unimpeded by pore walls, i.e. in the pore direction. If we assume that a similar selection mechanism takes place when nucleation occurs in a thin slice at the coldest extremity of the film, the fastest growing directions of the nuclei align within the plane of the slice, i.e.

perpendicular to the gradient direction. For 3T, [010] corresponds to the fastest growing direction and the crystal grows steadily, when the sample is moved from the hot side to the cold side, along the [100] (or [100]) direction parallel to the gradient direction, as depicted in Figure 9. We note also that the self-selection of the LT phase, discussed in the Experimental Section , occurs for the grad12 sample, i.e. the sample that experienced the largest gradient magnitude and fastest displacement rate, thus affording the fastest cooling rate. Under these conditions, the thickness of the nucleation slice is considerably reduced, corroborating our tentative explanation. Further work, well beyond the scope of the current paper, will aim at proving the hypothesis of Ward et al. using various confinement geometries, notably a funnel-shaped geometry for the safe production of a single seed,91 since currently we do not know whether the presence of crystals with [100] and [100] growth directions occurs from several nucleation grains or results from a growth mechanism.

’ CONCLUSIONS A systematic study of the crystal structure and microstructure of 3T powder samples and samples crystallized using the thermal gradient technique has been undertaken. The two key conclusions are that nucleation and growth can be decoupled for OSC crystallizing from the melt in a temperature gradient and that these conditions lead to the generation of highly textured thin films with uniaxial in-plane orientation of crystallites. The third important conclusion is that adequate gradient conditions allow the selective growth of a single polymorphic form. The fourth, and somewhat more anticipated, conclusion deals with the substantial role that the chemical nature of interfaces plays on crystal orientation and polymorphism. The fifth conclusion concerns the alignment of the unit cell with the reciprocal vector c* normal to the substrate and the [100] direction parallel to the gradient direction. It is hypothesized that the geometry of the system and the temperature profile induce a preferential fast growth direction perpendicular to the gradient direction. This hypothesis, which rationalizes the current results, however, needs further verifications. The gradient technique developed here with the idea to fabricate single crystal thin films for organic electronics could easily be applied to nonlinear optics, where the availability of single crystal thin films is highly desirable too.9294 The ability to induce reproducibly, by engineering the growth conditions, the occurrence of a single pure crystalline phase among two or more polymorphs should also be of great interest in the case of pharmaceutical compounds.95 ’ ASSOCIATED CONTENT

bS

Supporting Information. Additional information on the simulations of heat transfer in the thermal gradient apparatus, DSC heating/cooling cycle performed at a rate of 1 °C/min, complete X-ray powder diffraction patterns of both LT and HT phases, additional polarized optical microscopy images of samples crystallized using the thermal gradient technique and deposited on glass or FKM-treated glass substrates, and equilibrium crystal morphology visualized using the Material Studio package. This material is available free of charge via the Internet at http://pubs.acs.org. 3670



’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT The research leading to these results has received funding from the European Community’s Seventh Framework Program (FP7/2007-2013) under Grant Agreement No. 212311 of the ONE-P project, the Belgian Federation Science Policy Office (PAI-SF2), and the Belgian National Fund for Scientific Research (FNRS—Research Fellow Ph.D. Grant for G.S.). We would like to thank Anne de Wit, Pierre Colinet, and Benoît Haut for their help with the heat transfer simulations and Linkam for technical assistance with the hot plate. J.C. is an FNRS research fellow. ’ REFERENCES (1) Forrest, S. R.; Thompson, M. E. Chem. Rev. 2007, 107, 923–925. (2) Shaw, J. M.; Seidler, P. F. IBM J. Res. Dev. 2001, 45, 3–9. (3) Coropceanu, V.; Cornil, J.; da Silva Filho, D. A.; Olivier, Y.; Silbey, R.; Bredas, J. L. Chem. Rev. 2007, 107, 926–952. (4) Klauk, H. Organic electronics; Wiley-VCH: Weinheim, 2006. (5) Bao, Z.; Locklin, J. J. Organic field-effect transistors; CRC: 2007. (6) Menard, E.; Meitl, M. A.; Sun, Y.; Park, J. U.; Shir, D. J. L.; Nam, Y. S.; Jeon, S.; Rogers, J. A. Chem. Rev. 2007, 107, 1117–1160. (7) Anthony, J. E. Chem. Rev. 2006, 106, 5028–5048. (8) Liu, S.; Wang, W. M.; Briseno, A. L.; Mannsfeld, S. C. B.; Bao, Z. Adv. Mater. 2009, 21, 1217–1232. (9) Reese, C.; Bao, Z. Mater. Today 2007, 10, 20–27. (10) Horowitz, G.; Hajlaoui, M. E. Synth. Met. 2001, 122, 185–189. (11) Reese, C.; Bao, Z. J. Mater. Chem. 2006, 16, 329–333. (12) Mannsfeld, S. C. B.; Sharei, A.; Liu, S.; Roberts, M. E.; McCulloch, I.; Heeney, M.; Bao, Z. Adv. Mater. 2008, 20, 4044–4048. (13) De Boer, R. W. I.; Gershenson, M. E.; Morpurgo, A. F.; Podzorov, V. Phys. Status Solidi (a) 2004, 201, 1302–1331. (14) Yamao, T.; Miki, T.; Akagami, H.; Nishimoto, Y.; Ota, S.; Hotta, S. Chem. Mater. 2007, 19, 3748–3753. (15) Briseno, A. L.; Mannsfeld, S. C. B.; Ling, M. M.; Liu, S.; Tseng, R. J.; Reese, C.; Roberts, M. E.; Yang, Y.; Wudl, F.; Bao, Z. Nature 2006, 444, 913–917. (16) Liu, S.; Mannsfeld, S. C. B.; Wang, W. M.; Sun, Y. S.; Stoltenberg, R. M.; Bao, Z. Chem. Mater. 2009, 21, 15–17. (17) Hong, J.-P.; Lee, S. Angew. Chem., Int. Ed. 2009, 48, 3096–3098. (18) Virkar, A. A.; Mannsfeld, S.; Bao, Z.; Stingelin, N. Adv. Mater. 2010, 22, 3857–3875. (19) Duffy, C. M.; Andreasen, J. W.; Breiby, D. W.; Nielsen, M. M.; Ando, M.; Minakata, T.; Sirringhaus, H. Chem. Mater. 2008, 20, 7252– 7259. (20) DeLongchamp, D. M.; Kline, R. J.; Jung, Y.; Germack, D. S.; Lin, E. K.; Moad, A. J.; Richter, L. J.; Toney, M. F.; Heeney, M.; McCulloch, I. ACS Nano 2009, 3, 780–787. (21) Nishizawa, T.; Lim, H. K.; Tajima, K.; Hashimoto, K. J. Am. Chem. Soc. 2009, 131, 2464–2465. (22) Wu, Y.; Liu, P.; Ong, B. S.; Srikumar, T.; Zhao, N.; Botton, G.; Zhu, S. Appl. Phys. Lett. 2005, 86, 142102. (23) Di, C.; Yu, G.; Liu, Y.; Guo, Y.; Sun, X.; Zheng, J.; Wen, Y.; Wu, W.; Zhu, D. Chem. Mater. 2009, 21, 4873–4879. (24) van Breemen, A. J. J. M.; Herwig, P. T.; Chlon, C. H. T.; Sweelssen, J.; Schoo, H. F. M.; Setayesh, S.; Hardeman, W. M.; Martin, C. A.; de Leeuw, D. M.; Valeton, J. J. P. J. Am. Chem. Soc. 2006, 128, 2336–2345. (25) Lengyel, O.; Hardeman, W. M.; Wondergem, H. J.; de Leeuw, D. M.; van Breemen, A. J. J. M.; Resel, R. Adv. Mater. 2006, 18, 896–899. (26) Rudolph, P.; Kakimoto, K. MRS Bull. 2009, 34, 251–258.

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