J. Phys. Chem. C 2008, 112, 12177–12183
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Crystallization of Organic Semiconductor Molecules in Nanosized Cavities: Mechanism of Polymorphs Formation Studied by in Situ XRD Silvia Milita,*,† Chiara Dionigi,‡ Francesco Borgatti,‡ Adina Nicoleta Lazar,‡ William Porzio,§ Silvia Destri,§ Didier Wermeille,| Roberto Felici,| Jens Wenzel Andreasen,⊥ Martin Meedom Nielsen,⊥ and Fabio Biscarini‡ CNR-IMM and CNR-ISMN, Via P.Gobetti 101, 40129 Bologna, Italy, CNR-ISMAC, Via E. Bassini 15, 20133 Milano, Italy, ESRF 6, Rue Jules Horowitz, F-38043 Grenoble, France, and Danmarks Tekniske UniVersitet Nationallaboratoriet for Bæredygtig Energi, FrederiksborgVej 399, P.O. Box 49, 4000 Roskilde, Denmark ReceiVed: February 12, 2008; ReVised Manuscript ReceiVed: May 16, 2008
The crystallization of an organic semiconductor, viz., tetrahexil-sexithiophene (H4T6) molecules, confined into nanosized cavities of a self-organized polystyrene beads template, has been investigated by means of in situ grazing incidence X-ray diffraction measurements, during the solvent evaporation. Thanks to these real time experiments, the phase content and the crystalline domain orientation of H4T6 have been determined, from the onset of the first crystalline molecular assembly to the stable system. The correlation between the bead size dependent crystallization mechanism in this complex system and the enhanced transistor performance has been established. These results, which can be extended to a wide range of organic materials, are useful for developing an attractive sustainable process for fabrication of organic devices with enhanced performance. Introduction The crystallization behavior of active molecules in nanoconfined environments has been drawing increasing attention owing to its potential application in nanotechnologies. The production of new functional materials can take advantage from crystallization processes carried out in complex systems that include emulsions, vesicles, micelles, droplets, and block copolymers.1–3 Special focus is given to unconventional lithography techniques, where nanosized menisci, cavities, and channels can be defined by means of a stamp or lithographic mask brought into intimate contact or in close proximity with the substrate.4,5 The ordering/crystallization of organic semiconductor molecules in confined systems is attractive for their application in electronic devices as organic field effect transistor (OFET). By means of a combined fluidics/printing deposition of bilayer stripes, it has been demonstrated that the charge mobility of self-organized nanostructures is 2 orders of magnitude larger than that of the corresponding spin-cast material.4 This is a result of the better controlsin time and in spacesof supersaturation and precipitation in confined environments leading to the selforganization of the molecular semiconductor into large ordered and oriented domains. Another approach makes use of prepatterned arrays of micro-6 and nanostructures7 to define regions of well-defined shape and size for growth of crystals or highly ordered domains.8 The outcome of the integration of a single crystal approach into large area organic electronics6 introduces the possibility of achieving charge mobilities and device performances considerably superior to those of a thin film technology. Along this direction, we decided to resort to the * Phone: + 39 051 639 9156, E-mail:
[email protected]. † CNR-IMM. ‡ CNR-ISMN. § CNR-ISMAC. | ESRF 6. ⊥ Danmarks Tekniske Universitet Nationallaboratoriet for Bæredygtig Energi.
recently developed core-shell bead technologies.9,10 In particular, we recently reported a new approach to fabricate OFET from water solution casting of polystyrene (PS) latex beads whose surfaces are decorated with an organic molecular semiconductor, viz., tetrahexil-sexithiophene (H4T6).11 The response of these OFETs, where the organic semiconductor is confined into a self-organized template of PS, is observed to be much larger than those obtained by drop-casting H4T6 solutions. In particular, the performance enhancement in the bead-based FET devices occurs only for bead sizes below a critical diameter, around 150 nm, and in the presence of certain polymorphs. It is therefore important to understand the mechanism upon which the high-mobility polymorphs are formed in the cavities of the bead template, and how this can be controlled. This work reports on the dynamic investigation of the crystallization process of H4T6 in nanosized interconnected cavities defined by the PS template. The crystallization results from the liquid phase evaporation of the colloidal composite of PS-H4T6 drop-cast on a solid substrate. PS nanostructured arrays behave as a template for the organic molecules that crystallize in the interstitial voids (cavities) to form large interconnected domains. We studied the mechanisms driving the crystallization phenomena of organic molecules in this confined system from the analysis of grazing incidence X-ray diffraction (GID) measurements collected in situ12,13 during solvent evaporation. The dynamic experiments allowed us to determine the orientation and phase content from the onset of the first crystalline assembly up to the steady state at room temperature. Aiming at evaluating the contribution of the different forces that drive this template-based crystallization (the geometrical confinement, the molecule-template surface interaction, and the molecule-substrate interactions), we decided to change two of the crucial parameters in sample preparation, i.e., the bead diameter (d ) 150 nm and d ) 270 nm) and the nature of the substrate (hydrophilic and hydrophobic substrates). The infor-
10.1021/jp801434m CCC: $40.75 2008 American Chemical Society Published on Web 07/23/2008
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Figure 1. Sketch of the sample preparation process. The process consists of the colloidal solution preparation (step 1), followed by the separation of the viscous composite from the supernatant solution by means of centrifugation (step 2), the casting of a drop of the colloidal composite on flat substrate, and finally the drop drying (step 3).
mation on the morphology of the colloidal composite thin film deposited over the substrate obtained by atomic force microscopy (AFM) and optical microscopy allows us to complete the process description. Experimental Section Sample Preparation. The sample preparation process is schematically shown in Figure 1. Monodisperse PS and H4T6, whose synthesis is reported in literature,14,15 were used as material sources. Two milligrams of H4T6, previously dissolved in 4 mL of 1:3 acetone/ethanol solution, were mixed with 20 mL of an aqueous PS dispersion, containing 5 mg of PS (d ) 150 ( 4 nm, d ) 270 ( 7 nm). The final mixture resulted in a 5:1 (v/v) water/organic solvent ratio (step 1). In such a mixed solution, H4T6 molecules nucleate in clusters that, because of hydrophobic interaction, are collected by the polymeric beads, and a homogeneous colloidal suspension of H4T6-decorated beads is formed. After centrifugation, a supernatant separates from a highly viscous colloidal suspension (step 2). This
Milita et al. colloidal suspension was drop cast (step 3) on hydrophilic (glass and natively oxidized Si wafer) and hydrophobic (hexamethyldisilazane, HMDS, functionalized Si wafer) substrates. HMDS, which substitutes the CH3 for the OH-terminated SiO2, has been chosen as a surface treatment, because of its high stability in moist conditions, high purity, and short molecular length. Drops of the same initial volume (≈0.05 mL) were deposited on the substrates to allow one to compare the structural evolution during the evaporation process of the different systems. In our experiment, the H4T6starting/PS weight ratio of the starting solution (step 1) has been selected equal to 0.2. It yields comparable values (with an error of 25%) of the weight ratios H4T6adsorbed/PS of the colloidal composite for both PS diameters used, as measured by UV-visible spectroscopy (see Figure 2 in ref 11) In these conditions, we are confident that the same colloidal volume cast on a substrate contained the same amount of H4T6 molecules. X-ray Diffraction Measurements. X-ray diffraction measurements were performed at the ID03 beamline of the European Synchrotron Radiation Facility (ESRF, Grenoble, France). A double-bounce silicon(111) monochromator was used to select the energy of 8 keV from the emission spectrum of the undulator source. The focused beam cross section at the sample was limited by slits to 20(v) × 300(h) µm2. The bare substrates were mounted horizontally on a vertical six-circle diffractometer and properly aligned to control with high accuracy ((0.01°) of the incident angle of the X-ray beam on the sample surface. The colloidal bead sample was drop cast on the aligned bare substrate and the monochromatic beam impinged on the sample with an incidence angle Ri ) 1 ( 0.01°. At this angle, the beam footprint on the surface (≈1 × 0.3 mm2) is fully included in the film. Diffraction patterns in grazing incidence (GI) reflection were recorded in situ, during the crystallization process for a time span of 48 h from the deposition, at intervals of 1 min. The high photon flux available at ID03 allows one to detect weak signals in a very short acquisition time (few seconds) and so to follow the crystallization onset. To limit the X-ray radiation damage, the sample has been exposed to the beam only for a few seconds by inserting a fast shutter close to the presample slits position. Indeed, along the acquisition sequence, we verified that diffraction signal intensities did not vary after changing the region impinged by the beam (i.e., by slightly moving the sample perpendicularly to the beam direction). This indicates that no relevant structural modifications should be ascribed to the X-ray beam damage. A 2D CCD detector of 132 mm diameter and pixel size of 64.445 µm was placed normal to the incident beam direction and at ≈170 mm from the sample. In this way, a range of scattering angles up to 24° could be collected, allowing one to simultaneously identify the crystalline phases, to determine the crystallite texturing and their evolution during the evaporation process. For each sample, the calibration of the sample-camera distance was done by independent measurements of the scattering angles in high resolution symmetrical scan (ω/2θ) by using a point detector. The 2D images have been analyzed by means of the software FIT2D.16 Optical and Atomic Force Microscopy. The morphological characteristics of the resulting samples have been observed ex situ by optical and atomic force microscopy. A Nikon epiilluminated optical microscope, ECLIPSE LV series, was used for the characterization of the systems at the micrometer scale. Images were taken in bright field. The AFM measurements were carried out in semicontact mode in air using SMENA (NT-
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Figure 2. AFM images of the central part of dried drops cast on HMDS-Si of colloidal composite with (a) PS beads of d ) 150 nm, (b) PS beads of d ) 270 nm, (c) H4T6/PS beads of d ) 150 nm, and (d) H4T6/PS beads of d ) 270 nm.
MDT). Silicon cantilevers (series NSG10 and CSG10) were used with resonance frequency of 260 kHz and force constant of 45 N. For each sample, images of size lengths from 5 to 40 µm and 512 × 512 pixels were realized with a frequency of 1-2 lines per second. Results and Discussion Drop Drying Process. For all systems, the drop evaporation leads, after complete drying, to the formation of a deposit with a ridge of higher mass concentration at the perimeter of the initial drop and a thin film in the center. This indicates that the evaporation dynamics is governed by the same mechanism observed for the evaporation of the simpler polystyrene sphere-H2O suspension.17 Such a mechanism is linked to the most general “coffee-ring” formation, experimentally and theoretically studied,18–22 and can be explained as follows. During the drying process the contact line or rim of the drop remains pinned, and due to the highest evaporation rate that occurs at the thinner edge, the solution is concentrated close to the center of the drop. In order to replenish the liquid removed by evaporation at the edge, a radial flow from the inner to the outer regions sets in the drop. This flow transfers the solute (PS beads and H4T6 in this case) to the contact line by convection and accounts for the inhomogeneous radial concentration of the deposit. Optical images, as the ones monitored by CCD camera during the XRD experiments, are shown in the Supporting Information. During the evaporation process, regardless of the substrate, the PS beads self-organize in a close-packed structure. Figure 2 shows AFM images of the dried deposit on HMDS-Si, for the initial solution of colloidal suspension of PS of d ) 150 nm (Figure 2a) and d ) 270 nm (Figure 2b). Large domains of ordered beads are observed for both colloidal suspensions of 150 and 270 nm, together with the presence of few defects (vacancies, dislocations, boundaries). When the colloidal composite contains H4T6 the regular packing at short range is preserved (Figure 2c,d), although the ordered domains are smaller and defect density higher. This reduced long-range order
Figure 3. 2D diffraction images recorded 40 min after deposition on HMDS-Si of colloidal composite containing PS of (a) d ) 150 nm, (b) d ) 270 nm, and (c) of H4T6 in acetone solvent. The thin vertical white line is the shadow of part of the beam stopper.
also resulted in increased surface roughness. This is more evident for the smaller diameter PS beads, where the presence of the semiconductor material increases the roughness of the film from 30 to 45 nm. AFM does not allow us to resolve the presence of H4T6 around the beads or in the interstitial cavities. However, we infer the presence of H4T6 from the enhanced disorder, as this behavior can be driven by the larger size distribution of beads in the case of H4T6/PS composite, with respect to the pristine beads. The images shown are of the central thinner regions of the deposits, since the enhanced roughness at the edges prevented high-resolution images of this region. The topographical analysis of regions, where the substrate was partially covered, allowed us to estimate that the film consists mainly of three layers of nanobeads. Crystallization Kinetics and Polymorphs Formation. By comparing the crystallization behavior of the systems obtained with the PS beads of the two different diameters, and regardless of the chemical nature of the substrate, two main features were observed: (1) The crystallization time depends on the bead diameter and increases with it. As an example, we report in Figure 3 the diffraction images recorded 40 min after the drop casting on the HMDS functionalized substrate of the colloidal solution with bead diameters of 150 (Figure 3a) and 270 nm (Figure 3b). Interestingly, in the 150 nm composite the solvent is consistently evaporated and stable crystallites form, as indicated by the presence of Debye rings. On the contrary, in the 270 nm
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Figure 4. Optical images under polarized light of H4T6 crystallized film alone (a) and in the presence of PS beads of 150 nm (b) on HMDS functionalized silicon.
system nothing but diffuse halos, produced by the PS beads, were recorded, indicating that the onset of crystallites has not yet occurred. The difference in evaporation/crystallization rate can be ascribed to the different surface areas per unit volume of PS beads coated by the solution, which almost doubles as the diameter decreases from 270 to 150 nm. The larger the surface for the smaller diameter, the shorter the evaporation time. This result is in agreement with previous data, which indicate that the PS beads enhance evaporation of water compared to pure water.17 (2) H4T6 crystallizes in different crystallographic phases, reported as Meso (M), Red (R), and Yellow (Y) phases.23–25 The amounts of different phases contributing to the completely dried/crystallized films differ for the two bead sizes. When the system has reached the steady state in the deposits formed with 270 nm PS beads, the dominant phase is the Y phase, whereas in those with 150 nm the M phase prevails. These results can be explained considering that the Y phase is thermodynamically more stable and is expected to be favored in a very slow evaporation process, where the molecules have time to organize in the lowest energy configuration. On the other hand, the M phase is kinetically favored, and is preferentially formed under fast evaporation condition. The relation between evaporation rate and phase composition in the film is confirmed by the complete absence of the Y phase in a dried film obtained by drop casting of a solution of H4T6 in acetone (without PS beads), which is characterized by very fast evaporation (Figure 3c). The different crystallite morphologies of the samples in Figures 3a and c, as observed from their optical images under polarized light (Figure 4a,b), indicate that the crystallization mechanism is strongly affected by the presence of the bead template. Moreover, we have to consider that in a highly confined system, the molecular mobility will be reduced, thus preventing
Milita et al. the transition from M to Y phase, similarly to what is observed for other allotropic systems.26 The quite different confinement of the colloidal solution in the two PS diameter systems can be estimated by simple geometrical considerations. Indeed, in the closed packed lattices, tetrahedral, Tc, and octahedral, Oc, cavities coexist. The number of Tc is twice that of Oc, and hypothetical spherical guests of diameter ∼0.11d and ∼0.20d, where d is the diameter of the host (PS) sphere, can fill into these cavities. For the same initial PS weight, the total volume of the cavities of both sizes of beads is conserved: smaller PS beads generate a larger number of smaller cavities (V150 ) 53 nm3 versus V270 ) 310 nm3) and this difference plays an important role in determining the H4T6 phase composition. We now focus on the 150 nm beads because their use as active layer in organic FET has been reported to yield working devices.11 Comparison of the diffracted images at different times after drop deposition provides the time evolution of the film structure during the evaporation process. Figure 5 shows a sequence of 2D diffraction images from the colloidal composite containing PS d ) 150 nm, recorded at 40 and 190 min after the drop deposition on glass (SiO2) substrate (Figure 5a,b, respectively). In spite of the overlapping of several Debye rings at high scattering angles, three phases can be easily identified by considering their characteristic peaks at scattering angles below 10°. However, we underline that the low scattering intensity of R phase reflections at small angles makes its analysis more difficult. The plot of the diffracted intensities integrated along the Debye rings versus the scattering angle is reported in Figure 5c. The spectra are vertically shifted for clarity. In the first stage of the crystallization (Figure 5a), the M and the R phases have been detected, with the former being largely the dominant one. The occurrence of the Y phase is observed only at almost 190 min after deposition (Figure 5b,c). Even though a quantitative analysis from the present diffraction data is beyond the purpose of the present work, the evolution of the crystallization can be promptly accounted for by means of simple arguments. On the basis of crystal energy calculations, coupled with both differential scanning calorimetry (DSC) analysis and XRD structure determination, the relative stability of the three phases has been evaluated as Y > R>M.11,23–25 It is evident that the kinetic factor prevails at first, i.e., formation of both the M and R phases, whereas the Y phase arises from both the M and R phases when the thermodynamic contribution is dominating (Figure 5). Thin Film Texturing. From the analysis of the distribution of the diffracted intensity along a Debye ring, the orientation of the corresponding lattice planes can be deduced: a homogeneous intensity distribution indicates random orientation of the crystallites, whereas a discrete distribution along certain directions indicates their preferential orientations. An estimate of orientation spread with respect to the preferential one is given by the fwhm (full width half maximum) of the azimuthal intensity profile along the Debye ring. The crystallites of the M and Y phases result in a strongly oriented system, while those of the R phase are almost randomly oriented. The fact that the intensity of two most intense reflections, the (100) of the M and the (001) of the Y phases (marked by arrows in Figures 5b), are concentrated in the vertical position on the 2D detector indicates that their crystalline domains are mainly orientated with the (100) and (001) planes, respectively, nearly parallel to the substrate. By taking the structure model of both phases23–25 into account, these orientations correspond
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Figure 6. Zoom of 2D images of colloidal composite cast with d ) 150 nm, (a) from the center after 220 min, and from the edge (b) after 220 min and (c) 240 min. The thin vertical white line is the shadow of part of the beam stopper.
Figure 5. 2D images of colloidal composite containing PS d ) 150 nm recorded at (a) 40 and (b) 190 min after deposition on glass substrate. The thin vertical white line is the shadow of part of the beam stopper. (c) The diffracted intensity integrated over the Debye ring is plotted as a function of the scattering angle.
to a preferential arrangement of the molecules with their long axis at about 60° and 45° from the surface substrate for the M and Y phase, respectively. The pronounced overlap of the conjugated orbitals of adjacent molecules, almost standing up, favors the charge transfer in a direction parallel to the substrate, required for FET working in planar geometry. On the other hand, the molecular axis of the R phase has never been observed to be preferentially oriented. As a consequence, the R phase allows a less efficient charge transfer with respect to both M and Y phase orientations.11 The different thickness of the PS/H4T6 on the middle and at the edge of the drop induced by the solvent evaporation provides for depth-sensitive information. Indeed in our experimental
conditions, the penetration depth has been estimated27 to be on the order of ten micrometers and all the film thickness, both on edge and on the central region, is probed by the beam and contributes to the scattered intensity: on the edge, the structural information is averaged over few micrometers, while on the central region over the submicrometer film closer to the substrate interface. In order to illustrate this point, we recorded images diffracted by different regions of the drops (center and edge). By shifting the sample perpendicular to the incident beam (lateral size 0.3 mm), diffraction images by different positions on the drop, whose ridge typical size is several millimeters, have been recorded. As indicated in the experimental section, the size of the footprint of the beam along the incident direction (≈1 mm) was smaller than the typical size of the inner part of the drop. In order to be sure that the information came only from the inner (or the edge) region, before recording the last images of the sequence, we took care to remove the edge deposit along the beam direction. This local information contributes to describing the crystallization mechanism. Figure 6 shows images obtained 4 h after casting with the beam impinging on the center (Figure 6a) and on the edge (Figure 6b) of the dried deposit. The appearance of the different phases along the depth of the film is explained by the kinetics of solvent evaporation from the filled pores. If the evaporation rate, at the air/composite interface, is faster than the solvent capillary flow, crystallites form in the proximity of the surface. The solvent flows up to the surface and disorders the orientation of the crystallites. In this scheme the presence of randomly oriented crystallites, of R and M phases, is ascribed to the crystallization at the drop/ air interface, while the oriented crystallites of the M and Y
12182 J. Phys. Chem. C, Vol. 112, No. 32, 2008 phases form at the substrate interface. The crystallite orientation is more pronounced in the central (thinner) part of the drop than at the edge. However, a degree of molecular orientation exists through the film thickness. This molecular orientation is due to the molecule-substrate interaction, which propagates to the molecules further away from the substrate. The slightly higher molecular misorientation (
