© Copyright 2003 American Chemical Society
JULY 8, 2003 VOLUME 19, NUMBER 14
Letters Direct Patterning of Poly(p-phenylene vinylene) Thin Films Using Microcontact Printing Ziqi Liang, Kun Li, and Qing Wang* Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802 Received March 23, 2003. In Final Form: May 23, 2003 This Letter describes a convenient means of direct patterning of conjugated polymers on solid substrates by microcontact printing. An amine-containing poly(p-phenylene vinylene) (PPV) was synthesized and printed onto the surface with the SAMs terminating in interchain carboxylic anhydrides. Well-defined PPV micropatterns were characterized by atomic force microscopy and fluorescence microscopy. The interaction between the polymer thin films and the surface was analyzed by grazing angle infrared spectroscopy, X-ray photoelectron spectroscopy, and cyclic voltammetry. The formation of covalent bonds via amidation renders great stability of the resulting functional patterns.
Recently, significant effort has been devoted to the development of conjugated polymers for practical applications.1,2 π-Conjugated polymers combine the optical and electronic properties of semiconductors with advantages of organic materials such as low cost, easy processing, and great opportunities for structural modification.3,4 They are being considered as active components in various types of thin-film organic electronic and optoelectronic devices such as light-emitting diodes (LEDs),5,6 filed effect transistors,7 lasers,8 solar cells,9 and chemical and biological * To whom correspondence should be addressed. E-mail: wang@ matse.psu.edu. (1) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem., Int. Ed. Engl. 1998, 37, 402. (2) Rogers, J. A.; Bao, Z. J. Polym. Sci., Part A: Polym. Chem. 2002, 40, 3327. (3) Hadziioannou, G., van Hutten, P. F., Eds. Semiconducting Polymers: Chemistry, Physics and Engineering; Wiley-VCH: New York, 2000. (4) Skotheim, T. A., Elsenbaumer, R. L., Reynolds, J. R., Eds. Handbook of Conducting Polymers, 2nd ed.; Marcel Dekker: New York, 1998. (5) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Nature 1990, 347, 539. (6) Pei, Q.; Yu, G.; Zhang, C.; Yang, Y.; Heeger, A. J. Science 1995, 269, 1086. (7) Bao, Z.; Rogers, J. A.; Katz, H. E. J. Mater. Chem. 1999, 9, 1895.
sensors.10 While a major effort has been focused on material synthesis to explore new materials with tailored properties, the realization of polymer-based electronics and optoelectronics also depends on developing techniques for processing this new class of materials. A critical element in this emerging technology is the spatial deposition of high definition micron and submicron scale features of active materials.11 Micro- or nanopatterning of conjugated polymers not only is essential to the construction of conductive circuitry but also generates additional functions, such as defining the size and shape of pixels and creating multicolor emission in polymer LEDs12,13 and improving light trapping in polymer photovoltaic devices.14 Microcontact printing (µCP), introduced by Whiteside and co-workers, provides a versatile method of chemically (8) McGehee, M. D.; Heeger, A. J. Adv. Mater. 2000, 12, 1655. (9) Granstro¨m, M.; Petrisch, K.; Arias, A. C.; Lux, A.; Andersson, M. R.; Friend, R. H. Nature 1998, 395, 257. (10) McQuade, D. T.; Pullen, A. E.; Swager, T. M. Chem. Rev. 2000, 100, 2537. (11) Holdcroft, S. Adv. Mater. 2001, 13, 1753. (12) Chang, S.; Bharathan, J.; Yang, Y.; Helgeson, R.; Wudl, F.; Ramey, M. B.; Reynolds, J. R. Appl. Phys. Lett. 1998, 73, 2561. (13) Matterson, B. J.; Lupton, J. M.; Safonov, A. F.; Salt, M. G.; Barns, W. L.; Samuel, I. D. W. Adv. Mater. 2001, 13, 123. (14) Roman, L. S.; Ingana¨s, O.; Granlund, T.; Nyberg, T.; Svensson, M.; Andersson, M. R.; Hummelen, J. C. Adv. Mater. 2000, 12, 189.
10.1021/la034501k CCC: $25.00 © 2003 American Chemical Society Published on Web 06/12/2003
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and molecularly patterning surfaces on a submicrometer scale.15,16 This technique uses a structured, elastomeric stamp to deliver a molecular “ink” to a substrate upon conformal contact. µCP has been generally used to direct deposition of self-assembled monolayers (SAMs) of thiols and silanes onto coinage metals and silicon substrates, respectively.16,17 More recently, this technique has been extended to stamp biological molecules,18 metal nanoparticles,19 dendrimers,20 and block copolymers21 onto a variety of substrates. The resulting chemically patterned surfaces have been used as templates for the further chemical etching,22 metal deposition,23 living polymerization,24 cell growth,25 and protein adhesion.26 µCP has several advantages over conventional photolithography, including being inexpensive, simple, adaptable to large areas and nonplanar surfaces, and fast. In patterning of conducting polymers, one of the most important aspects of µCP is the fact that it is non-photolithographic, eliminating the likelihood of photochemical damage to the conjugated backbones of polymers and associated physical properties. µCP of SAMs has been used in conjunction with electropolymerization,27,28 chemical vapor deposition,29 and Langmuir-Blodgett (LB)30 techniques to yield conjugated polymer patterns, in which patterned SAMs function as molecular resist layers on substrates. More recently, a conducting polymer complex, poly(3,4ethylenedioxythiophene)-poly(styrenesulfonate), has been deposited onto ITO and gold electrodes by µCP.31 In this Letter, we present results concerning the direct stamping of π-conjugated polymers, poly(p-phenylene vinylene) (PPV), onto a surface containing reactive SAMs. Poly(p-phenylene vinylene) (PPV) and its derivatives are an interesting class of conjugate polymers that have many applications as active components in optoelectronic devices.3,4,8 An amino-substituted PPV (polymer I, Figure 1) was synthesized by a palladium-catalyzed Heck coupling reaction and used as the “ink” in our contact printing. The amino groups are highly reactive toward carboxylic anhydrides of the SAM on the surface, allowing polymer I to be covalently tethered to the surface via the formation of amide bonds. Polymer I is soluble in most common organic solvents, such as THF, chloroform, DMF, and so (15) Kumar, A.; Whitesides, G. M. Appl. Phys. Lett. 1993, 63, 2002. (16) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1998, 37, 550. (17) St. John, P. M.; Craighead, H. G. Appl. Phys. Lett. 1996, 68, 1022. (18) Bernard, A.; Renault, J. P.; Michel, B.; Bosshard, H. R.; Delamarche, E. Adv. Mater. 2000, 12, 1067. (19) Ng, W. K.; Wu, L.; Moran, P. M. Appl. Phys. Lett. 2002, 81, 3097. (20) Li, H.; Kang, D.; Blamire, M. G.; Huck, W. T. S. Nano Lett. 2002, 2, 347. (21) Jiang, X. P.; Zhang, H.; Gourdin, S.; Hammond, P. T. Langmuir 2002, 18, 2607. (22) Huck, W. T. S.; Yan, L.; Stroock, A.; Haag, R.; Whitesides, G. M. Langmuir 1999, 15, 6862. (23) Hidber, P. C.; Nealey, P. F.; Helbig, W.; Whitesides, G. M. Langmuir 1996, 12, 5209. (24) Shah, R. R.; Merreceyes, D.; Husemann, M.; Rees, I.; Abbott, N. L.; Hawker, C. J.; Hedrick, J. L. Macromolecules 2000, 33, 597. (25) Amirpour, M. L.; Ghosh, P.; Lackowski, W. M.; Crooks, R. M.; Pishko, M. V. Anal. Chem. 2001, 73, 1560. (26) Chen, C. S.; Mrksich, M.; Huang, S.; Whitesides, G. M.; Ingber, D. E. Biotechnol. Prog. 1998, 14, 356. (27) Gorman, C. B.; Biebuyck, H. A.; Whitesides, G. M. Chem. Mater. 1995, 7, 526. (28) Roger, J. A.; Bao, Z.; Makhija, A.; Braun, P. Adv. Mater. 1999, 11, 741. (29) Vaeth, K. M.; Jackman, R. J.; Black, A. J.; Whiteside, G. M.; Jensen, K. F. Langmuir 2000, 16, 8495. (30) Bjφrnholm, T.; Greve, D. R.; Reitzel, N.; Hassenkam, T.; Kjaer, K.; Howes, P. B.; Larsen, N. B.; Bφgelund, J.; Jayaraman, M.; Ewbank, P. C.; McCullough, R. D. J. Am. Chem. Soc. 1998, 120, 7643. (31) Granlund, T.; Nyberg, T.; Roman, L. S.; Svensson, M.; Ingana¨s, O. Adv. Mater. 2000, 12, 269.
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Figure 1. Chemical structure of polymer I.
Figure 2. (A) Fluorescent image and (B) tapping-mode AFM image of the printed PPV micropatterns.
forth. Its weight-average molecular weight, measured by gel-permeation chromatography (GPC) in THF with polystyrene as the standard, is 30 kDa with a polydispersity of 2.13. Elastomeric poly(dimethylsiloxane) (PDMS) stamps with 10 µm line features were fabricated by pouring a 10:1 mixture of Sylgard 184 elastomer/curing agent (Dow Corning, Midland, MI) over a photolithographically prepared silicon master.15 SAMs of 16-mercaptohexadecanoic acid (MHA) were prepared on a Au surface by immersion in a 1 mM solution of the thiol in absolute ethanol, and then acid groups were activated with trifluoroacetic anhydride to generate reactive SAMs terminated with interchain carboxylic anhydrides.32,33 The PDMS stamp was treated by air plasma and then inked with a THF solution of polymer I (0.1 M with respect to the monomer (32) Yan, L.; Marzolin, C.; Terfort, A.; Whitesides, G. M. Langmuir 1997, 13, 6704. (33) Yan, L.; Zhao, X.; Whitesides, G. M. J. Am. Chem. Soc. 1998, 120, 6179.
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Figure 3. GA-FTIR spectra of (A) the SAM presenting carboxylic acids and (B) the stamped PPV thin films.
unit). After evaporation of solvent, the PDMS stamp was blown dry under a stream of nitrogen and was brought into conformal contact with the carboxylic anhydride activated substrate for about 30 min at room temperature. After removal of the stamp, the substrate was subsequently rinsed with THF and ethanol and sonicated in CH2Cl2 to remove physisorbed materials before drying with nitrogen. Figure 2A shows a fluorescence optical micrograph of printed PPV thin films, indicating that the pattern exists over a large scale, 1 cm × 1 cm (not shown in full). Fluorescence images taken after storage of the patterned films in air for several weeks indicated no change in appearance. Atomic force microscopy (AFM) experiments were performed on the printed surfaces to determine layer thickness and coverage. Line analysis in AFM (Figure 2B) suggests that the average thickness of the patterned PPV thin films is approximately 7-8 nm, which is on the order of four “monolayers” of polymer I. Interestingly, this height is almost identical to the thickness of the chemisorbed PPV films which were prepared by immersing Au substrates with anhydride SAMs into THF solutions of polymer I. Several factors in the stamping process were examined to determine optimal conditions. A simple, but crucial, step in carrying out the printing is “inking” the surface of the stamp. When printing with polymer I/THF, the poor wetting characteristics of the untreated, hydrophobic PDMS stamp yield extremely nonuniform patterns. This can be attributed to the polar nature of the polymer with amino groups on side chains. Oxidation of the PDMS stamp turns out to be necessary in order to transfer a well-defined, continuous thin film of polymer I onto the reactive SAMs. The more hydrophilic, oxidized PDMS is wetted better by polymer I; better wetting leads to more efficient transfer of molecules to the stamp during the inking process.34 Similar results were observed with the stamping of charged or highly polar molecule inks onto the surface.16,35 Different contact times have been investigated. Short stamping contact times (1 h), the stamp tends to adhere to the surface, making it difficult to remove. We found that the optimal stamping contact time is around 30 min. This condition is markedly different from the case of stamping of small molecule inks, which works best at contact times around 30 s to 1 min.16 This is presumably a consequence of the high molecular weight of the polymer, which greatly influences the kinetics of the reaction. The interaction between polymer I and the SAM was analyzed by grazing angle Fourier transform infrared spectroscopy (GA-FTIR) and X-ray photoelectron spectroscopy (XPS). A flat, unstructured stamp was made on a clean Si wafer and inked by polymer I/THF solution. Under the same printing conditions, polymer I was stamped onto the surface having SAMs of interchain carboxylic anhydrides. The samples were thoroughly rinsed and sonicated after stamping. Figure 3 gives the GA-FTIR spectra of (A) the SAMs of carboxylic acids and (B) the PPV films formed by contact printing. The spectrum of the SAMs of carboxylic acids shows two CdO stretching bands at 1745 and 1720 cm-1, which arise from the free and hydrogen bonded carboxylic acids, respectively.36 We attribute the new absorption bands appearing around 1530 cm-1 in Figure 3B to amide I (CdO stretching) and amide II (NsH mixed mode) vibrations.37 These spectral signatures are evidence that the polymer films are covalently attached to the surface through the formation of amide groups. The hydrocarbon region of the spectra provides additional support for the deposition of the polymer. In Figure 3B, the absorption peaks at 3370 cm-1 correspond to the NsH stretch of amides and amines. Peaks at 3040 cm-1 represent the CsH stretching mode characteristic for aromatic groups. The methylene vibrations at 2923 and 2860 cm-1 are significantly broader than those of the SAMs. The XPS survey spectrum of the PPV film shows a strong photoelectron peak for N(1s) at 400.3 eV, further confirming the presence of amide bonds in the films. In addition, the intensities (i.e. photoelectron counts) of the Au peaks in the film are much lower than those of the SAMs, which also suggests the coverage of the surface with a thin layer of PPV. The ratio of the intensities of the photoelectron peaks for N(1s) and O(1) provides semi-
(34) Lahiri, J.; Ostuni, E.; Whitesides, G. M. Langmuir 1999, 15, 2055. (35) Martin, B. D.; Brandow, S. L.; Dressick, W. J.; Schull, T. L. Langmuir 2000, 16, 9944.
(36) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558. (37) Pretsch, E.; Bu¨hlmann, P.; Affolter, C. Structure Determination of Organic Compounds; Springer: New York, 2000.
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Figure 4. Cyclic voltammograms of the stamped PPV thin film and the SAM presenting carboxylic acids on gold substrates in CH3CN. n-Bu4NBF4 was used as the supporting electrolyte, and the scan rate was 100 mV/s.
quantitative information about the yield of the reaction.32 The estimated yield of the reaction of polymer I and the SAMs of interchain anhydrides is about 15% on the basis of the ratios of N(1s)/O(1s). The excess functional groups of the polymer can be used to immobilize other functional species, such as inorganic nanoparticles and conducting polymers of different electron affinities. This provides us a facile route to the construction of patterned multilayer heterostructures, which are extremely interesting from the viewpoint of numerous applications. The introduction of covalent bonds via amidation renders great stability of the pattern. For example, the PPV patterned film has been treated ultrasonically in THF for several hours and the micropatterns remain intact. The cyclic voltammograms of the stamped PPV thin films and the SAMs on gold substrates are shown in Figure 4. Two typical cathodic
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waves of the solid-state PPVs were observed for the stamped films at -1.37 and -2.45 V, which correspond to the reduction of the delocalized π-conjugated backbone.38 The voltammograms can be recycled consecutively over 30 times without any loss of the signal, showing that amide formed by reactions between amines and carboxylic anhydrides is stable on the film. In conclusion, this work extends the microcontact printing technique to the formation of robust patterned thin films of conjugated polymers. To our knowledge, this is the first report of direct printing of π-conjugated polymers using µCP. We believe that this method can be readily applied to other classes of conducting polymers and construction of complex multiple level heterostructures. Direct patterning of conducting polymers will provide us an effective and rapid method for routine production of functional patterns for applications in optoelectronics. Applications of this new method in polymeric light-emitting devices are currently underway. Acknowledgment. This work was supported by The Pennsylvania State University and the Commonwealth of Pennsylvania through the Lehigh/Penn State Center for Optical Technologies (Grant No. 21-166-0014). The authors thank Professor Michael Pishko and Mr. WonGun Koh for their help with fluorescence microscopy. Supporting Information Available: The synthetic procedures and characterization data for polymer I. XPS survey spectra of the SAMs and the stamped PPV films. This material is available free of charge via the Internet at http:// pubs.acs.org. LA034501K (38) Chan, W. K.; Yu, L. Macromolecules 1995, 28, 6410.
