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Langmuir 2006, 22, 4896-4898
Conducting Block Copolymer for Simple Micro- to Nanopatterns Gyoujin Cho,*,† Minhoon Jung,† Hoetaek Yang,† Jae Hee Song,‡ and Myung Mo Sung§ Department of Chemical Engineering and Nanotechnology Center, Sunchon National UniVersity, 315 Maegok Sunchon, Chonnam, Korea, 540-742, Department of Chemistry, Sunchon National UniVersity, 315 Maegok Sunchon, Chonnam, Korea, 540-742, and Department of Chemistry, Kookmin UniVersity, 861-1, Chongnung-dong, Songbuk-ku, Seoul, 136-702 Korea ReceiVed December 9, 2005. In Final Form: April 4, 2006 To use conducting polymers as substitutes for metals and conventional semiconductors in device fabrication, a cost-effective process for the reproducible deposition of the conducting polymers is needed. In this letter, we report a simple solution casting method for the fabrication of micro- to nanopatterns using the conducting block copolymer, poly(thiophene-block-ethyleneoxide), which shows rectifying characteristics dependent on the pattern width.
Recently, there have been significant consumer-driven demands for the construction of flexible organic and electronic devices with micrometer sizes. In consumer markets, factors such as low cost and massive market applications are important.1 Therefore, simple and reliable methods for the patterning of high-definition micrometer to submicrometer scale features of organic conductors and semiconductors are necessary. A number of conducting polymer patterning techniques have been devised to achieve patterns with micrometer to less than 50 nm dimensions, including photolithography,2 microcontact printing,3 template assisted synthesis,4 scanning electrochemical microlithography,5 electrochemical dip-pen lithography,6 and other “soft lithography” methods.7-9 Although not all of these methods are directly applicable to the construction of micro- to nanometer sized electronic devices, they demonstrate that conducting polymers can offer several advantages over metals and conventional semiconductors in macroelectronic devices. Not the least is their ease of processing, which is pivotal to the fabrication of low cost microelectronic devices. Furthermore, micro- or nano-patterning of conducting polymers is necessary for other functions, such as trapping light to improve the performance of polymer photodiodes10 or increasing the efficiency and controlling light output from microstructured LEDs.11 In practice, however, it had not been feasible to use simple solution casting to prepare conducting polymers for the construction of micrometer-sized circuitry because of their poor solubility and difficulties in retaining their electronic or optical properties in patterns.12 In this letter, a simple method for the construction of micrometer- to submicrometer* To whom correspondence should be addressed. † Department of Chemical Engineering and Nanotechnology Center, Sunchon National University. ‡ Department of Chemistry, Sunchon National University. § Kookmin University. (1) Forrest, P. E.; Gu, G.; Bulovic, V.; Shen, Z.; Forrest, S. R.; Thompson, M. E. IEEE Electron DeVice Magn. 1997, 44, 1188. (2) Jager, E. W. H.; Smela, E.; Inganas, O. Science 2000, 290, 1540. (3) Yu, J. F.; Holdcroft, S. Chem. Commun. 2001, 1274-1275. (4) Marck, C.; Borgwarth, K.; Heinze J. Chem. Mater. 2001, 13, 747. (5) Martin, C. R.; Chem. Mater. 1996, 8, 1739., He, H. X.; Li, C. Z.; Tao, N. J. Apply. Phys. Lett. 2004, 84, 828. (6) Maynor, B. W.; Liu, J. J. Am. Chem. Soc. 2002, 124, 522. (7) Holdcroft, S. AdV. Mater. 2001, 13, 1753., Zhang F.; Nyberg, T.; Inganas, O. Nano Lett. 2002, 2, 1373. (8) Goren, M.; Lennox, R. B. Nano Lett. 2001, 1, 735. Li, Z. F.; Ruckenstein, E. Macromolecules 2002, 35, 9506. (9) Seo, I.; Phyo, M. H.; Cho, G. Langmuir 2002, 18, 7253. Cho, G.; Seo, I.; Jung, S.; Oh, E.; Fung, B. M. Langmuir 2003, 19, 6576. (10) Roman, L. S.; Inganas, O.; Granlund, T.; Nyberg, T.; Svensson, M.; Andersson, M. R.; Hummelen, J. C. AdV. Mater. 2000, 12, 189. (11) Matterson, B. J.; Lupton, J. M.; Safonov, A. F.; Salt, M. G.; Bearns, W. L.; Samel, I. D. W. AdV. Mater. 2001, 13, 123.
sized conducting polymer patterns while retaining their electrical properties is described. The method is based upon the use of a block copolymer. Theoretically, if one unit of the conducting block copolymer is very attractive to the hydrophilic surface while the other unit is repulsive to the surface, the conducting polymer patterns can be simply fabricated on a pre-patterned hydrophilic surface. Therefore, simply controlling the sizes of the pre-patterns via microcontact printing can afford the conducting polymer patterns. If a substrate is pre-patterned with both hydrophobic and hydrophilic areas using the microcontact printing method, one block of a medium hydrophobic conducting block copolymer could be deposited beside the hydrophobic patterns on the surface due to the strong interaction between the hydrophilic block of the conducting block copolymer and the hydrophilic patterns on the surface. This method uses the substrate-segment interactions that significantly influence the microdomain ordering in diblock copolymer films13 for the construction of micrometer- to submicrometer sized conducting polymer patterns. We prepared poly(thiophene-b-ethyleneoxide) (PTH-b-PEO) using a simple synthetic scheme. A solution of 1 mol of poly(ethylene glycol) methyl ether (Aldrich; Mw ) 5000) and triethylamine (Aldrich) was dissolved into 5 mL of dried chloroform. A total of 1 mol of distilled 2-thiophenecarbonyl chloride (Aldrich) was dissolved in 50 mL of dry THF at 0 °C. The chloroform solution was slowly added to the THF solution at 0 °C. The temperature of mixtures was increased to 25 °C and further reacted for 12 h. After 12 h of reaction, the mixture was concentrated and poured into diethyl ether to precipitate the resulting 2-thiophenecarboxylic acid poly(ethylene glycol) ester (THPEO). THPEO was recrystallized from a mixture of diethyl ether and chloroform in 30% yield. 1H NMR [CDCl3] δ vs TSM: 7.82 (dd, J ) 3.77 Hz, 1.27 Hz, 1H), 7.57 (dd, J ) 4.96 Hz, 1.27 Hz, 1H), 7.11 (dd, J ) 4.96 Hz, 3.77 Hz, 1H), 3.8-3.5 (m, 312 H). IR (KBr) νc ) 0: 1710 cm-1. For this study, we prepared poly(thiophene-b-ethyleneoxide) (PTH-b-PEO) from copolymerization of thiophene and poly(ethylene glycol)-thiophene using FeCl3 as an oxidant under dried chloroform for 12 h. The resulting dark blue solution was evaporated at room temperature under reduced pressure. The resulting solids were redisolved in distilled THF, and then the THF solution was filtered to remove the undissolved residues. The filtrate was then evaporated again (12) Nguyen, M. T.; Leclerc, M.; Diaz, A. F. Trends Polym. Sci. 1995, 3, 186. Jeo, J.; Lee, J. K.; Hong, J. K.; Baeck, J. S.; Epstein, A. J.; Jang, K. S.; Suh, J. S.; Oh, E. J. Macromolecules 1998, 31, 479. (13) Devereaux, C. A.; Baker, S. M.; Macromolecules 2002, 35, 1921.
10.1021/la0533361 CCC: $33.50 © 2006 American Chemical Society Published on Web 04/25/2006
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Langmuir, Vol. 22, No. 11, 2006 4897
Figure 1. AFM images of pre-patterned substrate with 1 µm width (a) and regioselectively fabricated conducting polymer on prepatterned substrate with 1 µm width on 5 × 5 µm2 area (b) and 30 × 30 µm2 area (c).
at room temperature under reduced pressure. The resulting powder was kept under vacuum and used without further treatment. The prepared PTH-b-PEO shows the number-average molecular weights of the PTH and PEO blocks being 6000 and 5000, respectively, and polydispersity of 1.3 using GPC with polystyrene reference. The PTH-b-PEO is soluble in most solvents even in water due to its amphiphilic character. When simply dropping an aqueous solution of PTH-b-PEO on the substrate pre-patterned with micrometer (1 µm width) to submicrometer sized (600 nm width) patterns of hydrophobic alkyl groups, which were fixed onto the substrate using the micro-contact printing technique,14 the solution was retained selectively on the hydrophilic regions of the surface. As water evaporates with time, the droplet sizes gradually decreased, eventually leading to polymer solidification to form regular arrays within the hydrophilic microto sub-micropatterns. By simply providing pre-patterned substrates with various sizes and shapes, micro- to nanopatterns of conducting polymer can be simply fabricated. Figures 1 and 2 show AFM images of patterns with 1 µm and 600 nm widths, respectively, before and after the regioselective fabrication of conducting polymer (PTH-b-PEO). We can observe that the conducting polymers are regioselectively deposited along the way of the bare substrate in a large area due to the strong interaction between PEO units of PEOb-PTH and the hydroxyl group on the surface of the substrate. (14) Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 1499.
Figure 2. AFM images of pre-patterned substrate with 600 nm width (a) and regioselectively fabricated conducting polymer on pre-patterned substrate with 600 nm width on 5 × 5 µm2 area (b) and 30 × 30 µm2 area (c).
The topology of the patterned PEO-b-PTH is like packed rough pebbles. To prove the connectivity of the patterns, the electrical properties of patterned PTH-b-PEO with 1 µm and 600 nm widths were investigated using AFM with an Au coated tip. As shown in Figure 3, one side of patterned conducting polymer was coated by an Au film, which was connected with a Pt wire into a source meter (KEITHLEY 2004). The Au coated AFM tip was also connected to the source meter through a Pt wire. The load force of the gold coated tip was maintained at 20 nN to avoid damage to the tip and the sample. We measured at least 10 different patterns of the deposited conducting polymer for each sample. The i-V characteristics of both the self-assembled monolayer and the conducting polymer patterns with 1 µm and 600 nm widths are shown respectively in Figure 4. The small differences of these curves among other contact points suggest that no PEO-b-PTH patterns with 1 µm and 600 nm widths were broken. Although the curve for self-assembled monolayers is essentially horizontal, indicating almost perfect insulation, the conducting polymer patterns show unsymmetrical i-V curves depending on the pattern width (Figure 4). In other words, although the current is almost linear for the thin films of PEO-b-PTH (Figure 4a) as we expect for general conducting
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Letters
Figure 3. Schematic illustration for the measurement of I-V characteristics of patterned conducting polymer.
Figure 4. Current-voltage curves obtained conducting polymer films (a) and patterns with 1 um width (b) and 600 nm width (c).
polymers, a current deflection from the simple ohmic behavior is apparent for the patterns of PEO-b-PTH with 600 nm widths when sweeping the bias positive. This indicates that the rectifying characteristics are more pronounced when the width of patterns is narrower. These results may suggest that the orientation of patterned PEO-b-PTH would be changed depending on the width of patterns, and the molecular orientation will be directly related to its electrical properties. Therefore, the patterns can be used not only as conductive wires and electrodes for macroelectronic devices but also as active circuit elements. More complete studies are underway to better understand the relationships between the orientation of PEO-b-PTH patterns and electrical properties with varying widths of patterns and controlling the chain length of PTH block in PEO-b-PTH. In summary, with designed conducting block copolymer (PEOb-PTH), we can simply fabricate micrometer- to nanometer-
sized patterns through the spin coating method on a pre-patterned substrate using self-assembled monolayers because the PEO block units in the block copolymer can be strongly attracted to the hydrophilic surface while the PEO block repels the hydrophobic surface. Furthermore, since the width of PEO-b-PTH patterns can effect the orientation of the conducting polymer and consequently electrical properties, the electronic devices can be simply constructed via induced orientation of PTH block units in PEO-b-PTH. It is possible that this technique may solve the intrinsic problems of conducting polymers and can be applied directly to construct microelectronic devices. Acknowledgment. This work was supported by the Regional Research Center at Sunchon National University for which we are grateful, and G.C. thanks to Dr. Dustin James at Rice University for reviewing the manuscript. LA0533361
