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Constructing Thin Polythiophene Film Composed of Aligned Lamellae via Controlled Solvent Vapor Treatment Guanghao Lu,†,‡ Ligui Li,†,‡ Sijun Li,†,‡ Yunpeng Qu,†,‡ Haowei Tang,†,‡ and Xiaoniu Yang*,† State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Renmin Street 5625, Changchun 130022, People’s Republic of China, and Graduate School of the Chinese Academy of Sciences, Beijing 100049, People’s Republic of China ReceiVed October 18, 2008. ReVised Manuscript ReceiVed December 11, 2008 Thin poly(3-butylthiophene) (P3BT) film composed of aligned lamellae attached to the edge of the original film has been achieved via a controlled solvent vapor treatment (C-SVT) method. The polarized optical microscopy operated at both single-polarization and cross-polarization modes has been used to investigate the alignment of the fiber-like lamellae. A numerical simulation method is used to quantitatively calculate angle distributions of the lamellae deviated from the film growth direction. Prepatterned P3BT film edge acts as nuclei which densely initialize subsequent crystal growth by exhausting the materials transported from the partially dissolved film. The growth of new film upon crystallization is actually a self-healing process where the two-dimensional geometric confinement is mainly responsible for this parallel alignment of P3BT crystals. The solvent vapor pressure should be carefully chosen so as to induce crystal growth but avoid liquid instability which will destroy the continuity of the film. The combination of microfabrication technique and C-SVT method provides a novel method to fabricate hierarchical structure within thin polymer film with multiscale morphology via utilizing both up-bottom and bottom-up approaches.
Introduction Constructing specific structures from micro- to nanoscale with controlled hierarchy for thin polymer film has attracted great attention from both academic and industrial communities for its great potentials in, e.g., microelectronics.1 The so-called upbottom approaches, soft lithographic routes, e.g., mold-based microfabrication,1,2 are assumed to be promising low-cost techniques in the fabrication of next-generation polymer integrated circuits. However, it is still a challenge to construct the patterns in nanoscale even with state-of-the-art soft lithographic techniques. On the other hand, although bottom-up approaches such as self-assembly techniques3,4 could bring molecule-scale ordering, it usually suffered from long-range heterogeneity. So a combination of up-bottom and bottom-up approaches gives more opportunities for constructing highly defined structures in a large area.5 Solvent vapor treatment6-11 is known to be an effective method for thin film treatment in terms of morphological requirements. In our previous work we have developed controlled solvent vapor treatment (C-SVT) with well-controlled vapor pressure and pressure increasing rate, which gives the opportunity * To whom correspondence should be addressed. E-mail: xnyang@ ciac.jl.cn. † Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. ‡ Graduate School of the Chinese Academy of Sciences.
(1) Guo, L. J. AdV. Mater. 2007, 19, 495. (2) Ling, M. M.; Bao, Z. Chem. Mater. 2004, 16, 4824. (3) van Hameren, R.; Scho¨n, P.; van Buul, A. M.; Hoogboom, J.; Lazarenko, S. V.; Gerritsen, J. W.; Engelkamp, H.; Christianen, P. C. M.; Heus, H. A.; Maan, J. C.; Rasing, T.; Speller, S.; Rowan, A. E.; Elemans, J. A. A. W.; Nolte, R. J. M. Science 2006, 314, 1433. (4) Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.; Schenning, A. P. H. J. Chem. ReV. 2005, 105, 1491. (5) Fan, H. J.; Werner, P.; Zacharias, M. Small 2006, 2, 700. (6) Iwatsu, F.; Kobayashi, T.; Uyeda, N. J. Phys. Chem. 1980, 84, 3223. (7) Conboy, J. C.; Olson, E. J. C.; Adams, D. M.; Kerimo, J.; Zaban, A.; Gregg, B. A.; Barbara, P. F. J. Phys. Chem. B 1998, 102, 4516. (8) Gregg, B. A. J. Phys. Chem. 1996, 100, 852. (9) Kim, S. H.; Misner, M. J.; Russell, T. P. AdV. Mater. 2004, 16, 2119. (10) Luca, G. D.; Liscio, A.; Maccagnani, P.; Nolde, F.; Palermo, V.; Mu¨llen, K.; Samorı`, P. AdV. Funct. Mater. 2007, 17, 3791. (11) Xing, R.; Luo, C.; Wang, Z.; Han, Y. Polymer 2007, 48, 3574.
Figure 1. (a) Chemical structure of P3BT. (b) Schematic representation of microfabrication with a soft epoxy mold. (c) CPOM image of P3BT film micropatterned and then C-SVT performed. (d) TEM image of newly grown P3BT film as shown in panel c, arrow indicating the growth direction of the lamellae, and the inset is the corresponding selectedarea electron diffraction pattern.
for novel morphology construction or more precise morphology optimization, e.g., precisely controlled crystal growth with specific size and distribution.12-14 Poly(3-butylthiophene) (P3BT) (Figure 1a) has been chosen as an example to carry out the investigation because alkyl(12) Lu, G. H.; Li, L. G.; Yang, X. N. AdV. Mater. 2007, 19, 3594. (13) Lu, G. H.; Li, L. G.; Yang, X. N. Macromolecules 2008, 41, 2062. (14) Lu, G. H.; Li, L. G.; Yang, X. N. Small 2008, 4, 601.
10.1021/la803470u CCC: $40.75 2009 American Chemical Society Published on Web 02/12/2009
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substituted polythiophenes15,16 are excellent p-type semiconductors with wide applications in high-performance organic electronics, e.g., organic field-effect transisitors17,18 and solar cells.19-21 Additionally, the polymorphism of P3BT achieved during C-SVT suggests a wide range of solvent vapor pressure where phase transition may occur, and this transition is crucial for morphological reorganization toward novel structure. We have previously shown that crystallographic form I can be obtained via direct deposition from a solution using o-dichlorobenzene as the solvent and is stable in CS2 vapor as long as the pressure is lower than 0.94 (relative vapor pressure at room temperature13,14), whereas crystallographic form II becomes the stable modification thermodynamically as relative CS2 vapor pressure increased to the range between 0.94 and 0.98.13 In this work, we show that the C-SVT method can be used to induce oriented polymer crystal growth from prepatterned film edge onto naked substrate, and thus a new film is formed as an extension of the original film. This has actually demonstrated a successful combination of up-bottom and bottom-up approaches toward constructing hierarchical structure with multiscale morphology for thin film.
Experimental Section Materials. Regioregular P3BT (97% head-to-tail regiospecific conformation, MW ) 39.4 kDa, PDI ) 2.29) was purchased from Rieke Metals. o-Dichlorobenzene (ODCB, anhydrous, 99%) was purchased from Sigma-Aldrich. Carbon disulfide (CS2) was purchased from Sinopharm Chemical Reagent Co. Ltd. Sample Preparation. P3BT was dissolved in ODCB at 80 °C by stirring for 30 min. After cooling to room temperature, the solution was spin-coated (Laurell spin processor WS-400B 6NPP Lite) onto glass coverslides which were precleaned by ultrasonic treating in acetone and then rinsing in demineralized water. Homogenous films (thickness ca. 200-400 nm) with dimension in order of a square centimeter were obtained. The film was then performed microfabrication using a soft epoxy mold (Figure 1b). For the experiment of C-SVT, the solvent vapor pressure gradient was constructed in a setup we recently developed:13,14 a long glass tube (6 cm in diameter and 120 cm in length) containing liquid solvent CS2 in the bottom. Solvent vapor pressure is given by p ) L/L0 (L0 in this work is 120 cm. L is the distance from the up edge of the setup to the specimen position.) As the vapor pressure gradient in the setup was stably achieved, the pristine films were dipped into this tube and the solvent vapor pressure p ) 0.94-0.96 was chosen in this work while a typical treatment time of 6 h was used. Enough attention was paid to all the processes so as to avoid interrupting too much the already achieved gradient of vapor pressure within the tube during sample placement. After C-SVT, the film was dried in vacuum at room temperature to dissipate residual solvent molecules trapped in the film. Characterizations. Transmission electron microscopy (TEM) was performed on a JEOL JEM-1011 transmission electron microscope operated at 100 kV. Thin films were first floated on deionized water and then transferred onto a copper grid. The samples were dried at room temperature for 24 h before TEM measurements. Optical microscopy (OM), single-polarized optical microscopy (SPOM), and cross-polarized optical microscopy (CPOM) were (15) Chen, S. A.; Ni, J. M. Macromolecules 1992, 25, 6081. (16) Chen, T.-A.; Wu, X.; Rieke, R. D. J. Am. Chem. Soc. 1995, 117, 233. (17) Bao, Z.; Dodabalapur, A.; Lovinger, A. J. Appl. Phys. Lett. 1996, 69, 4108. (18) Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, M. M.; Bechgaard, K.; Langeveld-Voss, B. M. W.; Spiering, A. J. H.; Janssen, R. A. J.; Meijer, E. W.; Herwig, P.; de Leeuw, D. M. Nature 1999, 401, 685. (19) Yang, X. N.; Loos, J.; Veenstra, S. C.; Verhees, W. J. H.; Wienk, M. M.; Kroon, J. M.; Michels, M. A. J.; Janssen, R. A. J. Nano Lett. 2005, 5, 579. (20) Li, G.; Shrotriya, V.; Huang, J.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. Nat. Mater. 2005, 4, 864. (21) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425.
Lu et al. performed on a Carl Zeiss A1m microscope equipped with an Infinity 4-11 digital camera from Lumenera Co., Canada. Scanning electron microscopy (SEM) was performed on a XL30 ESEM-FEG field emission SEM. A thin layer of Au was presputtered on thin polymer film for SEM measurements. Numerical Simulation. The newly grown P3BT film is composed of numerous oriented P3BT lamellae. We assume that the deviation of these lamellae orientations obeys a normal distribution. So the distribution function of deviation of the lamellae can be described as (1/σ) exp[-Ri2/(2σ2)], where Ri is the deviation angle of the lamella i from the film growth direction and σ is standard deviation. For the convenience, here we assume the film studied is composed of 100 000 lamellae, whose orientation deviation should be between -90 ° and 90 °. A random deviation angle Ri within this range will be generated by computer according to the above-described deviation. Least-squared criterion is used to work out the most appropriate deviation σ and angles according to the experimental results.
Results and Discussion Homogeneous thin P3BT film was prepared on glass substrate via a spin-coating method from its ODCB solution. After drying completely the film was micropatterned using a soft epoxy mold.22 A detailed microfabrication process is schematically given in Figure 1b. The microfabricated film was then exposed to the specific vapor pressure (0.94-0.96) of CS2 for 6 h to carry out the treatment. As shown by the CPOM image in Figure 1c, the naked areas where the P3BT film had been removed during the microfabrication process have been covered by the newly grown film during C-SVT. Intriguingly, those newly developed films appear as bright regions in CPOM image (of Figure 1c) due to birefringence. TEM (Figure 1d) and the corresponding selectedarea electron diffraction (SAED) (Figure 1d, inset) demonstrate that the crystallographic (020) reflection is along the film growth direction, that is, the highly crystalline thin film is composed of P3BT lamellae with preferred growth direction along the b-axis (π-π stacking direction). Moreover, the simultaneously present (hk0) reflections imply that these lamellae of crystallographic form II modification mainly adopt flat-on orientation with respect to the film plane, as observed in our previous publications.12 All these aligned P3BT lamellae have grown from the edges of the previous film. Optical microscopy operated at single- and cross-polarized modes with consecutively rotated angles with respect to the specimen was subsequently used to investigate the detailed morphology of the newly grown P3BT film. A series of digital micrographs were thus recorded via a CCD camera connected to the microscope in the identical settings for image acquisition. As in this work approximately the intensity of light is proportional to the pixel-integrated brightness of the image, the average brightness of the image was determined via integration the gray scale of each pixel of the image (500 pixels × 300 pixels). Correspondingly, the plots of SPOM image brightness versus the angles between polarized light and film growth direction are given in polar coordinates (Figure 2a). It should be noted here that film growth direction is the main lamellae growth direction. All the images for brightness calculation were recorded from the same area (marked with a dashed rectangle in Figure 2b). The appearance of maximum (around 0° and 180°) and minimum (around 90° and 270°) brightness for these SPOM images implies that the absorption of the polarized light by the thin P3BT film composed of parallelly aligned crystals is periodically dependent on the angle between the light polarization and orientation of the crystals. Since the dipole vector for light absorption of conjugated (22) Wang, Z.; Zhang, J.; Xing, R.; Yuan, J.; Yan, D.; Han, Y. J. Am. Chem. Soc. 2003, 125, 15278.
Polythiophene Film Via SolVent Vapor Treatment
Langmuir, Vol. 25, No. 6, 2009 3765 2 1 1 I ) 2 A1 sin 2θ + A12 sin2 2θ cos ∆φ ) 2 2 1 2 2 A sin 2θ(1 + cos ∆φ) 2 1
(
)
where I is the light intensity of the composed light eventually passed through P2, θ is the angle between P1 and the electric field direction of e-light in the crystal
θ ) 180°n/2 w I ) Imin θ ) (360°n + 180°)/4 w I ) Imax
Figure 2. (a) Dependence of SPOM image brightness of newly grown P3BT film (area marked with a dashed rectangle in panel b) on the angles between polarized light and film growth direction. (b) A example SPOM image (0°), from which the image for calculation was cropped. (c) Dependence of CPOM image brightness of newly grown P3BT film (the same area as panel a) on the angles between polarization of incident light and film growth direction. (d) Scheme demonstrates light propagation between two polarizers (P1 and P2) inserted by an optically uniaxial crystal.
polymer P3BT is parallel to the plane of the thiophene ring, which is perpendicular to the growth direction of P3BT crystals, the parallel orientation (0° or 180°) between the polarized light and the film growth direction causes minimum absorption so the maximum of image brightness. In contrast, the perpendicular orientation (90° or 270°) results in maximum absorption. These dipole components absorbing polarized light may be corresponding to P3BT fiber slightly tilted from the direction perpendicular to substrate, which projects the conjugated backbone of P3BT on the film plane and results in somewhat of an overlap with the incident polarized light. This result again confirms that these newly grown P3BT lamellae are mainly oriented according to film growth direction. Figure 2c gives the brightness of the CPOM image versus the directions of the polarized light with respect to the growth direction of the crystals. The maximum or minimum brightness of the image repeats every 90°, that is, there are four repeats of this transition within a complete 360° rotation. Since all the CPOM data are collected from the same area as that of the SPOM images, the newly grown P3BT film marked in Figure 2b, this periodical change of the image brightness should be related to the birefringence of those parallelly aligned crystals. Generally, the propagation route of a light between a pair of cross polarizers inserted by an optical uniaxial crystal can be schematically illustrated in Figure 2d. Polarized light (amplitude A1) from polarizer P1 is resolved into two polarized light components: o-light (amplitude Ao) and e-light (amplitude Ae) during propagation in a uniaxial crystal. At last, o-light and e-light are further resolved, and only the component along the polarizer P2 can be passed through P2. The following equation can be obtained:
and ∆φ is phase difference between two polarizer lights (with amplitude Ao2, and Ae2, respectively). So correspondingly, the light that comes out from the second polarizer has minimum intensity I at θ ) 0°, 90°, 180°, 270°, 360°, whereas maximum intensity I is at θ ) 45°, 135°, 225°, 315°. Figure 2c clearly shows that the appearance of maximum and minimum brightness of the image obeys the rule mentioned above. Although this result has already proved that newly grown thin film is composed of parallelly oriented lamellae on the whole, a quantitative fitting of angle distributions of the lamellae deviated from the film growth direction is needed. For convenience, hereby we assume the newly grown film is composed of 100 000 lamellae and each one is labeled with i(i ) 1-100 000), where deviation angle Ri is between lamella i and the film growth direction. Statistically, the light intensity upon transmission through the cross-polarizers can be described as n
I∼
∑ (1 + cos ∆φ) sin2 2(θ + Ri) i)1
The distribution function could be described as (1/σ) exp[-Ri2/ (2σ2)], where σ is the standard deviation. Furthermore, light absorption should be considered as the light propagation within materials undergoes both absorption and refraction processes. So the equation above could be rewritten as n
I∼
∑ g(θ + Ri)(1/σ) exp[-Ri2/(2σ2)](1 + i)1
cos ∆φ) sin2 2(θ + Ri) where factor g(θ + Ri) is the polarized absorption factor (see Figure 2a). Least-squared criterion was used to work out σ (Figure 3). For comparison, another two simulated CPOM profiles are given in Figure 3a with 1.5- and 2-fold σ, which correspond to averaged deviation 〈|R|〉 ) 17.58°, 22.77°, respectively. Figure 3b is the calculated angle distribution with averaged deviation 〈|R|〉 ) 12.47°, which is obtained from the fitted σ and defined as n
〈|R|〉 )
∑ |Ri|/n i)1
The newly grown film is highly crystalline, and its crystallization process could be illuminated in terms of nucleation and growth. As has already been well demonstrated, the pristine P3BT film directly spin-coated from ODCB solution typically adopts form I structure. However, the SAED pattern given in Figure 1d (inset) reveals that the P3BT crystals in the newly grown film adopt form II modification. Therefore, phase transition has to take place so as new P3BT crystals could grow. As has already been demonstrated in our previous publications,12,13 the crystallographic transition from modification I to II does happen upon treating by CS2 vapor, which is actually the thermodynamics-
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Figure 3. (a) Experimental CPOM intensity (black) and the numerical simulation CPOM intensity with the most appropriate deviation of the angles between the polarization of incident light and film growth direction (red, 〈|R|〉 ) 12.47° with σ2 ) 0.0692) on. For comparison, the simulated CPOM intensity for the higher deviation angles at (green) 〈|R|〉 ) 17.58° with σ2 ) 0.1556, and (blue) 〈|R|〉 ) 22.77° with σ2 ) 0.2767 are added. (b) The distribution of deviation angles of lamellae from film growth direction, as calculated from simulated CPOM intensity at (red, with 0.0692 and 〈|R|〉 ) 12.47°) the most fitted curve with the experimental result.
driven process since form I P3BT crystals are typically the kinetically determined modification.12,13 As disclosed by a series of SEM images shown in Figure 4, the patterned film edge acts as nuclei which induce crystal growth of P3BT into lamellae during C-SVT. The fast Fourier transform (FFT) pattern (Figure 4c, inset) confirms the newly grown film is composed of parallelly aligned P3BT lamellae with the similar width, which is consistent with OM or POM observations shown above. During the growing process of these lamellae, branching is observed from time to time as indicated by the arrow 1 in Figure 4d. Occasionally, a few lamellae even may bend and divert from the main growth direction, which is perpendicular to the film edge. However, for both the bent and some branched cases, where during the growth of these lamellae they will inevitably encounter their neighbors, the crystal growth will be restrained and eventually terminated
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by the neighbor lamellae due to a spatially confined environment as shown in arrow 2 of Figure 4d. As a consequence, only those lamellae adopting the harmonious growth direction govern the main crystallization process and contribute to the major part of the newly grown film, which is therefore mainly composed of closely packed lamellae with identically parallel orientation perpendicular to the film edge. We would like to emphasize again that these lamellae are densely packed with each other and thoroughly covered the substrate when they pass through, forming a continuous and crystallographically aligned film. The formation of the unique morphology of the film implies that the growth of these P3BT lamellae is strictly on the surface of the substrate upon crystallization. This is because in the process of C-SVT, the pristine P3BT film remaining on the substrate is partially dissolved and achieves P3BT solution, which gradually allows P3BT chains to diffuse to the naked region of the substrate. As has been already reported previously,12,13 P3BT film will crystallize into spherulites upon a treatment in CS2 vapor. However, due to the dense nucleus at the film edge, many lamellae grow up simultaneously, which causes spatial confinement to each other from their neighbors, and only the development using a straight route can survive. This phenomenon somehow resembles the transcrystalline growth frequently observed in some polymer blends during thermal treatment, where dense nucleating occurs at the two-phase interface.23,24 As it has been shown that the direction of parallelly oriented P3BT lamellae in the newly grown film is mainly perpendicular to the film edge where they grow, the profile of this edge, if it is not straight anymore microscopically, might influence the growth direction of P3BT lamellae. Correspondingly, a variety of film edges with interesting motifs were prepared and then exposed to the solvent vapor. As observed by bright-field and cross-polarized optical microcopy shown in Figure 5, for the film patterned with convex or concave edge profiles (Figure 5, parts a and b), the newly grown P3BT film has the same width
Figure 4. SEM micrographs showing the thin P3BT film composed of highly aligned lamellae. (a) An overview. (b) A zoom-in micrograph of panel a showing the densely aligned nuclei, which are actually the edge of the original film. (c) A zoom-in micrograph and corresponding fast Fourier transform (FFT) pattern (inset) showing aligned lamellae. (d) A zoom-in micrograph showing the growth frontier of the lamellae. The arrows indicate the growth direction of the P3BT film.
Polythiophene Film Via SolVent Vapor Treatment
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Figure 5. Solvent-induced formation of highly aligned P3BT lamellae associated to the original film with different edge profiles, demonstrating a solvent-assisted film self-healing process. (a and c) OM images. (b and d) Cross-polarized images, and the cross-polarization directions are shown in the images.
from the edge of the original film, while their orientations are still parallel to the normal of the curved film edge. The different brightness observed by the cross-polarized microscopy is resulted from the various angles between the incident polarization light and lamellae orientation associated to the curved edge. In particular, as shown in Figure 5, parts c and d, a P3BT film was first patterned with the holes within the film. Upon solvent vapor treatment, P3BT lamellae grew from the film edges and encountered each other; eventually the naked regions were recovered by the newly grown P3BT film. This special case actually shows an additional capability of this C-SVT, self-healing associated to the thin polymer film with initial small broken areas. It has been well addressed that the crystal growth rate of any polymer is governed by either the chain diffusion rate within the matrix or migration to the crystal frontier. However, both of them are strongly influenced by the amount of solvent absorbed to the original film. Correspondingly, the solvent vapor pressure should play a crucial role in determining the formation of these P3BT lamellae. In this work, the vapor pressure p between 0.94 and 0.96 is used to induce parallel crystal growth since this pressure is enough to induce P3BT to be partially dissolved. The thus obtained solution allows P3BT chains to gradually diffuse to the naked area of the substrate from the film edge and crystallize into parallelly aligned P3BT lamellae. However, liquid instability25-29 was observed at higher vapor pressure p > 0.98, which results in the destroyed film in terms of homogeneity. Therefore, to extend this novel method to other systems, enough attention should be paid to choose an appropriate vapor pressure to avoid liquid instability and preserve film uniformity. Figure 6 schematically gives morphological evolution of the patterned P3BT film upon solvent vapor treatment at various (23) Ishida, H.; Bussi, P. Macromolecules 1991, 24, 3569. (24) Cho, K.; Kim, D.; Yoon, S. Macromolecules 2003, 36, 7652. (25) Chandrasekhar, S. Hydrodynamic and Hydromagnetic Stability; Dover: New York, 1981. (26) Que´re´, D.; di Meglio, J.-M.; Brochard-Wyart, F. Science 1990, 249, 1256. (27) Tidswell, I. M.; Rabedeau, T. A.; Pershan, P. S.; Kosowsky, S. D. Phys. ReV. Lett. 1991, 66, 2108. (28) Thiele, U.; Mertig, M.; Pompe, W. Phys. ReV. Lett. 1998, 80, 2869. (29) Gau, H.; Herminghaus, S.; Lenz, P.; Lipowsky, R. Science 1999, 283, 46.
Figure 6. Schematic illustration showing the dependence of formation of the highly aligned P3BT lamellae on the vapor pressure used during the treatment. Pt is the onset point of vapor pressure to dissolve the original form I P3BT film, whereas Pd is the dissolving pressure of form II P3BT film in CS2 vapor.
vapor pressures. For a low vapor pressure in the range of [0, Pt], no obvious change in morphology occurs as the phase transition from form I to II cannot be carried out below critical vapor pressure Pt for transformation, which is around 0.94 for P3BT. In the pressure range [Pt, Pd], where Pd is around 0.98 for P3BT as determined previously,13 newly grown film composed of parallelly aligned P3BT lamellae could be achieved. For the vapor pressure higher than Pd, the whole film will be dissolved and developed into inhomogeneous liquid film due to the instability of P3BT solution on the substrate. The whisker-like polythiophene crystal in the form I30 modification, which is one of the most excellent p-type polymer semiconductors broadly used in various fields, e.g., field-effect (30) Ihn, K. J.; Moulton, J.; Smith, P. J. Polym. Sci., Part B: Polym. Phys. 1993, 31, 735.
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transistors (FET)31-33 and polymer-based solar cells,19,34 has been intensively investigated. In contrast, the P3BT lamellae with crystallographic modification II was recently reported by us.12 Therefore, the form II P3BT lamellae, although their crystal size is larger than that of conventional whisker crystals, are less studied. However, the larger crystal size should be beneficial to the enhanced electronic properties. We have also demonstrated that crystallographic form II can be transformed to form I without apparent morphology change13 upon heating to ca. 160 °C. In this work, we use C-SVT to achieve a thin film composed of parallelly aligned lamellae, which might be potentially applied in the fields such as polarized absorption, polarized emission, and anisotropic electrical conduction.
Conclusion Thin P3BT film composed of aligned lamellae associated to the edge of prepatterned film has been achieved via C-SVT. The (31) Kline, R. J.; McGehee, M. D.; Kadnikova, E. N.; Liu, J.; Fre´chet, J. M. J.; Toney, M. F. Macromolecules 2005, 38, 3312. (32) Yang, H.; Shin, T. J.; Yang, L.; Cho, K.; Ryu, C. Y.; Bao, Z. AdV. Funct. Mater. 2005, 15, 671. (33) Kim, D. H.; Jang, Y.; Park, Y. D.; Cho, K. J. Phys. Chem. B 2006, 110, 15763. (34) Li, L. G.; Lu, G. H.; Yang, X. N. J. Mater. Chem. 2008, 18, 1984.
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anisotropic growth of P3BT lamellae has been verified by TEM, SEM, SPOM, and CPOM. A numerical simulation method is used to quantitatively calculate the angle distribution of lamellae orientation deviated from the film growth direction. The formation of these aligned lamellae within the thin film is mainly due to the spatial confinement of the lamellae with their neighbors during the growth. The appropriate solvent vapor pressure should be chosen so as to carry out the development of these lamellae but prevent the liquid instability which will completely destroy the homogeneity of the film. To the best of our knowledge, the film growth upon solvent vapor treatment is a novel method for achieving oriented lamellae in the thin film. The combination of micropattern technique and C-SVT has actually demonstrated a novel fabrication method using both up-bottom and bottom-up approaches, which could be potentially employed to fabricate hierarchical structure with multiscale ordering in the thin film. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Grant Nos. 20604029, 20874100). X.Y. thanks the Fund for Creative Research Groups (Grant No. 50621302) for financial support. We thank Professor Yanchun Han and Mr. Rubo Xing for the assistance in microfabrication. LA803470U
