Article pubs.acs.org/Macromolecules
Solvent-Assistant Room Temperature Nanoimprinting-Induced Molecular Orientation in Poly(3-hexylthiophene) Nanopillars Guangzhu Ding, Yangjiang Wu, Yuyan Weng, Weidong Zhang, and Zhijun Hu* Center for Soft Condensed Matter Physics and Interdisciplinary Research, Soochow University, Suzhou 215006, China S Supporting Information *
ABSTRACT: Nanoimprinting has been well explored to create nanostructures and to induce molecular orientation in conjugated polymer thin films. We demonstrate here that large area poly(3-hexylthiophene) (P3HT) nanopillar arrays can be fabricated by a simple, cost-effective nanoimprinting method in a solvent-swollen plasticized state at room temperature. In addition, the solventassistant room-temperature nanoimprinting induces face-on chain alignment in the P3HT nanopillars; i.e., the π−π stacking of P3HT is aligned normal to the substrate, favorable for organic photovoltaic cell applications. The face-on chain alignment solely depends on the diameters of nanopores defined in the nanoimprinting mold, rather than on the initial chain orientation in unprocessed thin films. Furthermore, a critical dimension of ca. 85 nm in diameter is found to be essentially needed to induce the face-on chain alignment within the nanopillars.
■
INTRODUCTION Nanoimprint lithography (NIL) provides a high-resolution and cost-effective lithography by replicating the nanostructures defined on a hard mold into a thermoplastic polymer thin film.1−3 The imprinting resists used in NIL are typically thermoplastic amorphous polymers such as poly(methyl methacrylate), polycarbonate, and polystyrene. Because NIL is essentially a molding technique acting at the nanometer scale, it can be used to directly shape functional soft materials into nanostructures. For example, thin films of semiconducting polymers,4−14 ferroelectric polymers,15,16 proteins,17,18 and gold nanoparticles19 have recently been directly shaped to obtain desirous nanostructures with well-defined morphology. Additionally, when applied to the materials capable to self-organize, NIL is able to control the molecular and crystallographic ordering of imprinted materials due to the effects of selforganization, nanoconfinement, pressure, rheological chain alignment, and so on, provided imprinting conditions are appropriately selected.11−15,20−24 The preferential molecular and crystallographic orientation within the nanostructures have a profound effect on the performance in the organic optoelectronic devices, such as efficient charge transport along specific directions in light-emitting diodes11 and fieldeffect transistors12,25 and lower coercive field in ferroelectric polymer memories.15,26 During the nanoimprinting process, the imprinted materials are typically heated to a temperature above their glass transition temperature or melting point. Some functional materials such as conjugated polymers, however, are easily oxidized and decomposed at elevated temperatures,27,28 potentially detrimental to performance in the targeted organic optoelectronic devices. To overcome the drawback of thermal NIL applied to conjugated polymers, room temperature NIL with the assistant of either solvent vapor or high pressure has been proposed to © 2013 American Chemical Society
obtain desired nanostructures in conjugated polymer thin films.5−9,28,29 The effects of room temperature NIL on the organization of molecules within the fabricated nanostructures are, however, rarely investigated, although the molecular and crystallographic orientation in the nanostructures would have a profound effect on the performance of the organic optoelectronic devices. For example, a face-on chain alignment (π−π stacking orientation normal to the substrate surface) is desirable for organic devices with charge transport normal to the substrate plane (e.g., organic photovoltaic cells and lightemitting diodes).14 In this work, we employ the solvent-assistant room temperature NIL (SART-NIL) as a means to fabricate large area nanopillars in thin films of poly(3-hexylthiophene) (P3HT). The molecular organization was carefully analyzed with grazing incidence wide-angle X-ray diffraction (GIWAXD) coupled with area detectors. The results indicate that the SART-NIL processes reorient the π−π stacking of P3HT chains to a face-on chain alignment, independent of the initial molecular orientation in cast thin films. In addition, a critical diameter of ca. 85 nm was found to be essentially needed to induce the face-on chain alignment within the nanopillars.
■
EXPERIMENTAL SECTION
Fabrication of Anodic Aluminum Oxide (AAO) Templates. AAO templates were prepared by a two-step anodization method.26 To begin with ultrapure (99.999%) aluminum foils (2 × 2 cm) were cleaned and degreased in acetone by sonication. In order to prepare flat aluminum sheets, the cleaned aluminum foils were pressed at room temperature under a pressure of 60 bar with a presser. The very flat aluminum sheets were then annealed at 450 °C for 5 h. The native Received: August 13, 2013 Revised: October 3, 2013 Published: October 16, 2013 8638
dx.doi.org/10.1021/ma401700d | Macromolecules 2013, 46, 8638−8643
Macromolecules
Article
Figure 1. (a) Schematic of the SART-NIL process. (b) Top-down and cross-sectional SEM images of an AAO template. The diameter of nanopores is ca. 45 nm and depth ca. 170 nm. (c) Top-down and cross-sectional SEM images of a nanoimprinted P3HT thin film produced from the AAO template shown in (b). oxide layer on the surface of aluminum sheets was roughly removed with 1.0 M sodium hydroxide and, after that, electropolished in a solution of perchloric acid/ethanol (1/4, v/v) under a constant electric voltage of 30 V. The first anodization was performed at 0 °C for 12 h. A 0.3 M oxalic acid solution was used as electrolyte, and the applied voltage was 40 V. The first anodic layer of the aluminum sheets was then removed by chemical etching in a mixed solution of chromic acid (1.8 wt %) and phosphoric acid (6 wt %). The second anodization was performed in the conditions as the first one but for 40−70 s. Finally, the pores of the anodic alumina oxide were widened by chemical etching with phosphoric acid solution (5 wt %) at 30 °C. The pore sizes and depths were controlled by the etching duration. To facilitate template separation after nanoimprinting, the AAO templates were coated with a monolayer of perfluorooctyltrichlorosilane deposited from the gas phase as described elsewhere.30 Preparation of P3HT Nanopillars. Regioregular P3HT (Mw 50 000 g mol−1; regioregularity 98%) was purchased from Rieke Metals Inc. and used as received. The polymer was dissolved in chlorobenzene or chloroform at a concentration of 20 mg/mL and filtered with 0.25 μm polytetrafluoroethylene filters. Solvent-swollen thin films of P3HT were obtained by spin coating at a constant spin speed (1600 rpm) for 10 s. The color of thin films will not change if we increase the spin time, indicating that the thickness of the thin films is uniform. After spin coating, the thin films were immediately transferred to a nanoimprinter (Obducat, Eitre 3) and covered with AAO templates. The AAO templates were pressed against the films under pressure (up to 60 bar) at room temperature (23 °C) and held for 15 min. Before releasing the pressure, the stacks were evacuated to solidify the P3HT nanopillars. After releasing the pressure, the AAO templates were easily removed from the samples. Controlled samples were obtained either by simple spin coating in the same conditions or by pressing (60 bar and 23 °C) with a flat silicon wafer coated with a monolayer of perfluorooctyltrichlorosilane. Characterization. The thickness of the homogeneous films was measured by a film thickness measurement instrument (Filmetrics F20) using 1.49 as index of refraction. The morphology of the P3HT nanopillars was characterized with scanning electron microscopy (SEM, Hitachi S-4800) operating at 15 kV. Transmission electronic microscopy (TEM) experiments were performed with a JEOL 1011 TEM with an accelerating voltage of 100 kV. Grazing incidence wide angle X-rays diffraction (GIWAXD) measurements were carried out at the BL14B1 Beamline at the Shanghai synchrotron radiation facility in China. The wavelength and the incident angle of the X-ray beam are 0.123 98 nm and 0.18°, respectively. The distance between the sample center and the two-dimensional charged coupled device (CCD) detector is 280 mm. Data conversion to q space was accomplished by calibration using LaB6 powder.
coating is able to lower the glass transition temperature and the viscosity of the polymers during the NIL process, beneficial to the mobility of polymer molecule.27 The thickness of the homogeneous P3HT thin films after drying in vacuum is about 50 nm. Examples of top-down and cross-sectional SEM images of the AAO mold and P3HT nanopillars are shown in Figures 1b and 1c, respectively, confirming that uniform arrays of P3HT nanopillars can be obtained by the SART-NIL method. The AAO mold shown in Figure 1b contains regular and hexagonally packed nanopores bearing a diameter of ∼45 nm and a center-to-center distance (period) of ∼100 nm. The depth of the nanopores is about 170 nm. The diameter and period of the obtained P3HT nanopillars precisely replicate the dimensions of the nanopores, whereas the height is about 75 nm and is smaller than the depth of the nanopores. A residual layer of ∼20 nm thick beneath the nanopillars is typically observed from the cross-sectional SEM image. Molecular Orientation in P3HT Nanopillars. It is wellknown that the molecular orientation in P3HT thin films depends strongly on the processing conditions such as casting solvents, casting methods, and substrate−polymer interactions. For example, P3HT thin films cast from slowly evaporated solvents (e.g., chlorobenzene) predominantly exhibit edge-on orientation,13,14,31 i.e., π−π stacking direction being aligned along the surface plane. In contrast, P3HT thin films cast from fast evaporating solvents (e.g., chloroform) have some degree mixture of edge-on and face-on orientation.32−34 To probe the influence of initial molecular orientation on the chain organization in the nanopillars obtained by the SART-NIL process, we first focus on the P3HT thin films obtained from chlorobenzene, which is a slowly evaporated solvent. Figures 2a and 2b show the two-dimensional (2D) GIWAXD images of an unprocessed P3HT thin film and a nanoimprinted thin film with arrays of nanopillars (∼45 nm in diameter), respectively, using chlorobenzene as solvent. An incidence angle of 0.18°, above the critical angle of the P3HT polymer (0.16°),35 was chosen to probe the orientation throughout the entire film. Here, we define the diffraction vector qxy and qz pointing along and normal to the substrate plane. The peaks at q = 3.8 nm−1 and q = 16.8 nm−1 correspond to the (100) plane and (010) plane reflections of P3HT,13,36,37 respectively. Apparently, (h00) and (0k0) reflections appear in the qz and qxy, respectively, indicating that edge-on chain alignment dominates in the unprocessed P3HT thin films (Figure 2a). This is in consistent with the previously reported results.13,14,31,38 In the GIWAXD image of nanoimprinted P3HT thin films with arrays of nanopillars, apart from the strong (h00) and (0k0) reflections along the qz and qxy directions, additional reflections of (010) plane and (100)
■
RESULTS AND DISCUSSION Fabrication of P3HT Nanopillars. The SART-NIL process, shown schematically in Figure 1a, consists of preparing P3HT thin films by spin coating for very short time and immediate nanoimprinting at room temperature. The residual solvent in the P3HT thin film resulting from short time spin 8639
dx.doi.org/10.1021/ma401700d | Macromolecules 2013, 46, 8638−8643
Macromolecules
Article
evaporation) as a comparative study. Figure 3a shows the 2D image of GIWAXD of an unprocessed P3HT thin film obtained
Figure 2. Two-dimensional GIWAXD images of an unprocessed P3HT thin film (a) and a nanoimprinted P3HT thin film bearing nanopillar arrays (b), using chlorobenzene as solvent. The onedimensional GIWAXD intensity profiles integrated along the qz direction (c) and along the qxy direction (d). The inset graph of (c) is the amplified views of the (300) and (010) peaks.
Figure 3. Two-dimensional GIWAXD images of an unprocessed P3HT thin film (a) and a nanoimprinted P3HT thin film bearing nanopillar arrays (b), using chloroform as solvent. The onedimensional GIWAXD intensity profiles integrated along the qz (c) and the qxy (d) directions. The inset graph of (c) is the amplified views of the (300) and (010) peaks.
plane are discernible along the qz direction and qxy direction (Figure 2b), respectively. These results indicate that face-on chain alignment is induced in the P3HT nanopillars by the SART-NIL process. To better reveal the change of chain alignment before and after the SART-NIL process, onedimensional integrated intensity profile of the GIWAXD images along the qz and qxy directions are also shown in Figures 2c and 2d, respectively. The positions of all the reflection peaks do not change before and after the SART-NIL process. The intensities of diffractions in the nanoimprinted P3HT thin films are, however, significantly increased, compared to that of the unprocessed thin films. The enhancement of diffraction intensity might result from the enhanced crystallization or reorientation during the SART-NIL process.14 In addition, stronger (010) and (100) peaks along the qz direction and qxy direction, respectively, can be found in the nanoimprinted thin films with arrays of P3HT nanopillars. This indicates that face-on molecular orientation is induced in the arrays of nanopillars, in addition to edge-on chain alignment in the residual layer. This edge-on chain orientation observed in the nanoimprinted thin films, i.e., (h00) and (0k0) reflections appearing in the qz and qxy, respectively, arises primarily from the effect of residual layer beneath the nanopillar structures, in agreement with previous reports.13,14,36 Therefore, the SARTNIL process can not only transfer topographical nanostructures from the template to a P3HT thin film with high fidelity but also induce the polymer chains orientation transition from edge-on to face-on alignment. The signals of GIWAXD measurements do not change if the samples are rotated around the normal direction of thin film, indicating that chains alignment displays a random distribution in the in-plane direction for unprocessed thin film and nanopillars. To explore the influence of molecular orientation in the original P3HT thin films on the chain alignment in the nanopillars due to SART-NIL process, we turn our attention to the films which are cast from chloroform solvent (fast
from chloroform solution. The (h00) and (0k0) diffraction planes are simultaneously present in the qz and qxy directions, indicating that mixture of edge-on and face-on chain orientation exists in the unprocessed thin film, in consistent with the results of previous reports.32−34 The 2D GIWAXD image of the nanoimprinted P3HT thin films bearing nanopillar arrays of ∼45 nm diameter (Figure 3b) is very similar to that of the unprocessed thin films shown in Figure 3a. Small differences between the two images are, however, discernible in the onedimensional integration intensity profiles along the qz and qxy directions shown in Figures 3c and 3d, respectively. In the qz direction, the intensity ratio between the (010) and (100) reflection planes of unprocessed thin film is 0.023, being smaller than that of the nanoimprinted thin film (0.035). Along the qxy direction, the ratio between the (010) and (100) reflection planes decreases from 0.57 to 0.42 after nanoimprinting. This indicates that the proportion of face-on molecular orientation is increased after nanoimprinting. The enhancement of face-on chain orientation should be contributed by the P3HT nanopillars. And thus, the chains in the nanopillars take a face-on orientation. We note here that the limited pressure (up to 60 bar) applied to the samples during the SART-NIL process would not give rise to either the phase transition or the preferential chain orientation in the P3HT thin films. This is revealed by the constant amounts and intensity ratios of diffractions obtained from unprocessed P3HT thin films and pressed thin films with a flat silicon wafer in the same conditions as the SART-NIL process (Figures S1 and S2 in the Supporting Information). Thus, the SART-NIL confinement process can not only achieve the regular P3HT nanopillars but also induce the preferential face-on molecular orientation. In order to further study the effect of confinement dimension on the molecular alignment, 8640
dx.doi.org/10.1021/ma401700d | Macromolecules 2013, 46, 8638−8643
Macromolecules
Article
GIWAXD images of nanopillars with diameters larger than ∼85 nm, only (h00) reflections in the qz direction and (010) reflection in the qxy direction are observed (Figure 4f,h), indicating sole edge-on molecular orientation which is similar to the chains alignment of unprocessed P3HT thin film. Obviously, there is a critical dimension of ca. 85 nm in diameter for P3HT nanopillars to induce face-on chains alignment during SART-NIL process. Therefore, the preferential face-on molecular and crystallographic alignment within P3HT nanopillars mainly depends on the confinement dimension of nanopores defined on the nanoimprinting mold. To understand the existence of a critical size for the preferential face-on chain orientation in the P3HT nanostructures, we have carried out some supplemental experiments. The cross-sectional SEM images of the nanoimprinted P3HT thin films bearing nanopillars are taken and shown in Figure S3 of the Supporting Information. The existence of residual layers is a common feature of all the samples. The thickness of the residual layer lies in the range of 16−20 nm and does not apparently vary among the samples. We have previously demonstrated that the residual layer can be inhibited if the dimensions of cavities defined on the mold are much larger than the polymer film in thermal NIL process.39 Although the depth of nanopores (ca. 170 nm) in the AAO template is much higher than the thickness of P3HT thin film (ca. 50 nm) in our experiments, it is very hard to inhibit the residual layer. The existence of residual layer in the SART-NIL process may result from the concentration gradient of solvent in the normal direction of the thin film. It has been reported that the existence of residual layer beneath the mold strongly influences the crystal orientation within the nanostructures in flexible polymer systems.39 The orientation of polymer crystals in the nanoimprinted nanostructures with low aspect ratio is hardly controlled due to heterogeneous nucleation and propagation of lamellar crystals through the residual layer. In the nanostructures with high aspect ratio, however, preferential orientation of polymer crystals can be found due to fast growth of lamellar crystals along one crystallographic axis and “gate effect” of nanopores in the templates.40 The fast growth direction of polymer lamellar crystals is typically aligned parallel to the nanopores. Thermal NIL-induced edge-on and vertical (the backbone of chains normal to the substrate) molecular orientations in P3HT nanopillars were also reported by other groups.13,35 The possible reason for the different chains alignment is the differentia of NIL parameters, such as nanoimprint temperature and nanopore geometries of mold, which may result in diverse polymer chains organization. The preferential face-on chain alignment in the P3HT nanopillars produced by SART-NIL can be rationalized based on the knowledge of the microstructures of P3HT thin films.20 Crystallizable conjugated polymers, such as P3HT, are incline to form rod-like crystals with their π−π stacking direction along the rod axis.41 In our case of P3HT thin films obtained by spin coating for short time, the length of rod-like crystals lies in the range of 65−105 nm whereas the width in the range of 10−18 nm, as observed from the TEM image (Figure S4 in the Supporting Information). When the diameters of nanopores in the AAO templates are close to or smaller than the lengths of the rod-like P3HT crystals during the SART-NIL process, the long axis of basic rod-like crystals is aligned parallel to the nanopore axis for entropic and geometrical reasons.20 Thus, the π−π stacking direction is extended along the axis of P3HT nanopillars, and
the SART-NIL was carried out with AAO templates of different nanopore diameters. The SEM and GIWAXD images of nanoimprinted P3HT thin films bearing nanopillars of different diameters (from ∼60 to ∼130 nm) fabricated by SART-NIL process are shown in Figure 4. The solvent used here is chlorobenzene. The
Figure 4. Left column: top-down SEM images of nanoimprinted P3HT thin films bearing nanopillars of different diameters, (a) 60, (c) 85, (e) 100, and (g) 130 nm. Right column: the corresponding 2D GIWAXD images of nanoimprinted P3HT thin films bearing nanopillars of 60 (b), 85 (d), 100 (f), and 130 nm (h).
diameters of nanopillars can be easily controlled by the pore sizes of AAO mold. The SEM images (Figures 4a,c,e,g) show that all the standing P3HT nanopillars exhibit regular and hexagonally packed distribution, confirming an excellent fidelity of the SART-NIL process. For the P3HT nanopillars with diameters less than ∼85 nm, the 2D images of GIWAXD show (h00) and (0k0) diffraction planes in the qz direction, along with the obvious (100) and (010) reflections in the qxy direction (Figures 4b,d), indicating that the face-on molecular organization is prevalent in P3HT nanopillars. However, for the 8641
dx.doi.org/10.1021/ma401700d | Macromolecules 2013, 46, 8638−8643
Macromolecules the preferential face-on orientation is achieved in the nanopillars, as shown in Figure 5a. The residual layer film
■
REFERENCES
(1) Chou, S. Y.; Krauss, P. R.; Renstrom, P. J. Appl. Phys. Lett. 1995, 67, 3114−3116. (2) Chou, S. Y.; Krauss, P. R.; Renstrom, P. J. Science 1996, 272, 85− 87. (3) Guo, L. J. Adv. Mater. 2007, 19, 495−513. (4) Behl, M.; Seekamp, J.; Zankovych, S.; Sotomayor Torres, C. M.; Zentel, R.; Ahopelto, J. Adv. Mater. 2002, 14, 588−591. (5) Pisignano, D.; Persano, L.; Raganato, M. F.; Visconti, P.; Cingolani, R.; Barbarella, G.; Favaretto, L.; Gigli, G. Adv. Mater. 2004, 16, 525−529. (6) Mele, E.; Di Benedetto, F.; Persano, L.; Cingolani, R.; Pisignano, D. Nano Lett. 2005, 5, 1915−1919. (7) Mele, E.; Camposeo, A.; De Giorgi, M.; Di Benedetto, F.; De Marco, C.; Tasco, V.; Cingolani, R.; Pisignano, D. Small 2008, 4, 1894−1899. (8) Di Benedetto, F.; Camposeo, A.; Pagliara, S.; Mele, E.; Persano, L.; Stabile, R.; Cingolani, R.; Pisignano, D. Nat. Nanotechnol. 2008, 3, 614−619. (9) Pagliara, S.; Camposeo, A.; Mele, E.; Persano, L.; Cingolani, R.; Pisignano, D. Nanotechnology 2010, 21, 215304. (10) Kim, M.-S.; Kim, J.-S.; Cho, J. C.; Shtein, M.; Guo, L. J.; Kim, J. Appl. Phys. Lett. 2007, 90, 123113−123116. (11) Zheng, Z.; Yim, K.-H.; Saifullah, M. S. M.; Welland, M. E.; Friend, R. H.; Kim, J.-S.; Huck, W. T. S. Nano Lett. 2007, 7, 987−992. (12) Hu, Z.; Muls, B.; Gence, L.; Serban, D. A.; Hofkens, J.; Melinte, S.; Nysten, B.; Demoustier-Champagne, S.; Jonas, A. M. Nano Lett. 2007, 7, 3639−3644. (13) Aryal, M.; Trivedi, K.; Hu, W. ACS Nano 2009, 3, 3085−3090. (14) Hlaing, H.; Lu, X.; Hofmann, T.; Yager, K. G.; Black, C. T.; Ocko, B. M. ACS Nano 2011, 5, 7532−7538. (15) Hu, Z.; Tian, M.; Nysten, B.; Jonas, A. M. Nat. Mater. 2009, 8, 62−67. (16) Liu, Y.; Weiss, D. N.; Li, J. ACS Nano 2010, 4, 83−90. (17) Amsden, J. J.; Domachuk, P.; Gopinath, A.; White, R. D.; Dal Negro, L.; Kaplan, D. L.; Omenetto, F. G. Adv. Mater. 2010, 22, 1746−1749. (18) Brenckle, M. A.; Tao, H.; Kim, S.; Paguette, M.; Kaplan, D. L.; Omenetto, F. G. Adv. Mater. 2013, 25, 2409−2414. (19) Yu, X.; Pham, J.; Subramani, C.; Creran, B.; Yeh, Y.-C.; Du, K.; Patra, D.; Miranda, O. R.; Crosby, A. J.; Rotello, V. M. Adv. Mater. 2012, 24, 6330−6334. (20) Hu, Z.; Jonas, A. M. Soft Matter 2010, 6, 21−28. (21) Okerberg, B. C.; Soles, C. L.; Douglas, J. F.; Ro, H. W.; Karim, A.; Hines, D. R. Macromolecules 2007, 40, 2968−2970. (22) Cheng, J. Y.; Ross, C. A.; Smith, H. I.; Thomas, E. L. Adv. Mater. 2006, 18, 2505−2521. (23) Thebault, P.; Niedermayer, S.; Landis, S.; Chaix, N.; Guenoun, P.; Daillant, J.; Man, X.; Andelman, D.; Orland, H. Adv. Mater. 2012, 24, 1952−1955. (24) Kim, S.; Lee, J.; Jeon, S.; Lee, H. H.; Char, K.; Sohn, B. Macromolecules 2008, 41, 3401−3404. (25) Mele, E.; Lezzi, F.; Polini, A.; Altamura, D.; Giannini, C.; Pisignano, D. J. Mater. Chem. 2012, 22, 18051−18056. (26) Wu, Y.; Gu, Q.; Ding, G.; Tong, F.; Hu, Z.; Jonas, A. M. ACS Macro Lett. 2013, 2, 535−538.
beneath nanopillars still remains the same molecular orientation as the unprocessed film. When the diameters of nanopores are much larger than the lengths of the rod-like P3HT crystals, the orientation of rod-like crystals is unaffected by the nanopores and remains the same as the thin films because of the mismatching of size (Figure 5b). As a consequence, there is a critical dimension to induce face-on chains alignment for P3HT nanopillars during the SART-NIL process.
■
CONCLUSIONS Summing up, the SART-NIL provides a simple and costefficient approach to the massive fabrication of nanopillar arrays of conjugated polymers which are easily oxidized and decomposed at elevated temperatures. According to the GIWAXD observations, a preferential face-on chains alignment within the P3HT nanopillars can be induced by the nanoimprint process. The chain alignments (π−π stacking orientation normal to the surface) within the nanopillars is immune to the orientation of the unprocessed thin films but depends mainly on the confinement dimension of the mold. A critical diameter of ca. 85 nm which is equal to or smaller than the length of P3HT rod-like crystals is needed to induce the favorable alignment within nanopillars. These results demonstrate the SART-NIL is a powerful tool for controlling both nanostructures and molecular alignment in polymer materials. This technique should provide new insight and guidance for understanding the structure−property relationships of organic semiconducting devices. ASSOCIATED CONTENT
S Supporting Information *
GIWAXD results of an unprocessed film and a pressed film with bare mold; the cross-sectional SEM images of nanoimprinted P3HT thin films with nanopillars of different diameters; the TEM image of a unprocessed P3HT thin film showing the dimensions of rod-like crystals. This material is available free of charge via the Internet at http://pubs.acs.org.
■
ACKNOWLEDGMENTS
This work was financially supported by the National Natural Science Foundation of China (Nos. 91027040, 21074084, and 21204058), the National Basic Research Program of China (No. 2012CB821500), the Natural Science Foundation of Jiangsu Province of China (No. BK2010213), a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and the BL14B1 Beamline at the Shanghai Synchrotron Radiation Facility in China.
Figure 5. Schematic drawing of the molecular orientation in nanoimprinted P3HT thin films bearing nanopillars produced by SART-NIL. The diameter of nanopillars is close to or smaller than the critical size (ca. 85 nm) in (a) and is larger than the critical size in (b).
■
■
Article
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (Z.H.). Notes
The authors declare no competing financial interest. 8642
dx.doi.org/10.1021/ma401700d | Macromolecules 2013, 46, 8638−8643
Macromolecules
Article
(27) Voicu, N. E.; Ludwigs, S.; Crossland, E. J. W.; Andrew, P.; Steiner, U. Adv. Mater. 2007, 19, 757−761. (28) He, X.; Gao, F.; Tu, G.; Hasko, D.; Huttner, S.; Steiner, U.; Greenham, N. C.; Friend, R. H.; Huck, W. T. S. Nano Lett. 2010, 10, 1302−1307. (29) He, X.; Gao, F.; Tu, G.; Hasko, D. G.; Huttner, S.; Greenham, N. C.; Steiner, U.; Friend, R. H.; Huck, W. T. S. Adv. Funct. Mater. 2011, 21, 139−146. (30) Pallandre, A.; Glinel, K.; Jonas, A. M.; Nysten, B. Nano Lett. 2004, 4, 365−371. (31) Lilliu, S.; Agostinelli, T.; Pires, E.; Hampton, M.; Nelson, J.; Macdonald, J. E. Macromolecules 2011, 44, 2725−2734. (32) 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, 685−688. (33) Yang, H.; Lefevre, S. W.; Ryu, C. Y.; Bao, Z. N. Appl. Phys. Lett. 2007, 90, 172116. (34) Serban, D. A.; Greco, P.; Melinte, S.; Vlad, A.; Dutu, C. A.; Zacchini, S.; Lapalucci, M. C.; Biscarini, F.; Cavallini, M. Small 2009, 5, 1117−1122. (35) Chen, D.; Zhao, W.; Russell, T. P. ACS Nano 2012, 6, 1479− 1485. (36) Kim, J. S.; Park, Y.; Lee, D. Y.; Lee, J. H.; Park, J. H.; Kim, J. K.; Cho, K. Adv. Funct. Mater. 2010, 20, 540−545. (37) Yuan, Y.; Zhang, J.; Sun, J.; Hu, J.; Zhang, T.; Duan, Y. Macromolecules 2011, 44, 9341−9350. (38) Hartmann, L.; Djurado, D.; Florea, I.; Legrand, J.; Fiore, A.; Reiss, P.; Doyle, S.; Vorobiev, A.; Pouget, S.; Chandezon, F.; Ersen, O.; Brinkmann, M. Macromolecules 2013, 46, 6177−6186. (39) Hu, Z.; Baralia, G.; Bayot, V.; Gohy, J.-F.; Jonas, A. M. Nano Lett. 2005, 5, 1738−1743. (40) Steinhart, M.; Göring, P.; Dernaika, H.; Prabhukaran, M.; Gösele, U.; Hempel, E.; Thurn-Albrecht, T. Phys. Rev. Lett. 2006, 97, 027801. (41) Hugger, S.; Thomann, R.; Heinzel, T.; Thurn-Albrecht, T. Colloid Polym. Sci. 2004, 282, 932−938.
8643
dx.doi.org/10.1021/ma401700d | Macromolecules 2013, 46, 8638−8643
