© Copyright 2002 American Chemical Society
JANUARY 8, 2002 VOLUME 18, NUMBER 1
Letters Soft-Lithography-Mediated Chemical Vapor Deposition of Architectured Carbon Nanotube Networks on Elastomeric Polymer Hou Tee Ng,† Maw Lin Foo,†,‡ Aiping Fang,† Jun Li,*,§,| Guoqin Xu,† Stephan Jaenicke,† Lap Chan,⊥ and Sam Fong Yau Li*,† Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Institute of Materials Research & Engineering, 3 Research Link, Singapore 117602, and Research & Development, Chartered Semiconductor Manufacturing, 60 Woodlands Industrial Park D, Street 2, Singapore 738406 Received June 4, 2001. In Final Form: August 13, 2001 The ability to develop highly site-selective and in situ orientation-controlled growth of carbon nanotubes on novel substrates such as elastomeric polymers may provide new opportunities in both fundamental research and practical applications. A soft-lithography-mediated approach has been used in combination with the surface wetting manipulation to selectively immobilize solution-based catalyst precursors for growing highly regular microarrays of multiwalled carbon nanotubes with controlled morphologies on elastomeric poly(dimethylsiloxane) substrates. The thermal shrinking property of poly(dimethylsiloxane) at elevated temperatures allows the possibility of fabricating three-dimensionally complex intertwined networks of carbon nanotubes, which may be utilized as a basic matrix for integration with flexible polymeric frameworks to fabricate flexible nanodevices, such as highly sensitive electrochemical and chemical gas sensors.
Carbon nanotubes (CNTs), due to their unique electronic, optical,and mechanical properties,1-3 may provide * To whom correspondence should be addressed. E-mails:
[email protected] and
[email protected]. † Department of Chemistry, National University of Singapore. ‡ Present address: Department of Chemistry, Princeton University, Princeton, NJ 08544. § Institute of Materials Research & Engineering. | Present address: ELORET Corp., NASA AMES Research Center, MS229-1, Moffett Field, CA 94035. ⊥ Research & Development, Chartered Semiconductor Manufacturing. (1) Dresselhaus, M. S.; Dresselhaus, G.; Eklund, P. C. Science of Fullerenes and Carbon Nanotubes; Academic Press: New York, 1996. (2) Ebbessen, T. W. Carbon Nanotubes: Preparation and Properties; CRC Press: Boca Raton, FL, 1997. (3) Saito, R.; Dresselhaus, M. S.; Dresselhaus, G. Physical Properties of Carbon Nanotubes; World Scientific: New York, 1998.
the route to nanotechnology advancements and technologically useful applications.4 The successes of these exploits, however, rely significantly on the efficient syntheses and smart manipulations of these CNTs. Various approaches of synthetic routes have been successfully reported, such as the conventional arc-discharge, laser ablation, and the more recent thermal- and/or plasma-assisted chemical vapor deposition (CVD). The latter may provide a viable choice for direct integration with electronic devices on either solid-state substrates or as stand-alone nanodevices. One of the important criteria to consider is the ability to achieve site selective or localized growth, which has been recently demonstrated by CVD techniques using (4) Toma´nek, D.; Enbody, R. Science and Application of Nanotubes; Kluwer Academic: New York, 2000.
10.1021/la0108095 CCC: $22.00 © 2002 American Chemical Society Published on Web 10/16/2001
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Figure 1. An overview of the procedural steps incorporating soft lithography and CVD techniques to obtain highly localized growth of CNTs on elastomeric PDMS and Si(001) substrates. The former was prepared by thoroughly mixing 10:1 (by weight) of the siloxane base oligomer and its Pt-based curing agent (Dow Corning Sylgard Elastomeric 184) and curing the mixture at 100 °C for 4 h. In the present work, substrates of physical dimensions 10 mm by 10 mm with a thickness of 3.5 mm were used. Magnetron sputtering (Denton Vacuum Inc., Moorestown, NJ) of Ni and surface wettability manipulation via OPT (Trion RIE, Trion Technology, Tempe, Arizona) were performed under conditions which induced no topographical modification to the substrates.
lithographic5,6 or template-directed approaches.7-11 However, these approaches relied on rigid solid-state substrates such as silicon (Si) and alumina to achieve CNT growth which may limit its wider scope of applications. Hence, it would be interesting and challenging to investigate the possibility of performing in situ site-selective growth on novel substrates such as polymers and its direct integration with elastomeric polymers to fabricate practical devices. We show here the combination of a non-photolithographic approach, i.e., soft-lithography-mediated growth,12,13 and surface wettability manipulation of an elastomeric polymer to achieve in situ highly site-selective growth of multiwalled (MW) CNTs on elastomeric substrates patterned with conveniently available, low-cost, and solution-based organometallic catalyst precursors. We have extended the present approach, coupled with the thermal-shrinking properties of the polymeric substrate at elevated temperatures, to fabricate three-dimensional (5) Fan, S. S.; Chapline, M. G.; Franklin, N. R.; Tombler, T. W.; Cassell, A. M.; Dai, H. J. Science 1999, 283, 512. (6) Kong, J.; Soh, H. T.; Cassell, A. M.; Quate, C. F.; Dai, H. J. Nature 1998, 395, 878. (7) Che, G.; Lakshmi, B. B.; Fisher, E. R.; Martin, C. R. Nature 1998, 393, 346. (8) Jirage, K. B.; Hulteen, J. C.; Martin, C. R. Science 1997, 278, 655. (9) Nishizawa, M.; Menon, V. P.; Martin, C. R. Science 1995, 268, 700. (10) Parthasarathy, R. V.; Martin, C. R. Nature 1994, 369, 298. (11) Martin, C. R. Science 1994, 266, 1961. (12) Cassell, A. M.; Franklin, N. R.; Tombler, T. W.; Chan, E. M.; Han, J.; Dai, H. J.J. Am. Chem. Soc. 1999, 121, 7975. (13) Huang, S. M.; Mau, A. W. H.; Turney, T. W.; White, P. A.; Dai, L. M. J. Phys. Chem. B 2000, 104, 2193.
(3D) complex intertwined CNT networks that may serve as a free-standing “membrane”. Figure 1 shows an overview of five different procedural schemes to achieve highly selective localized in situ CVD growth of MWCNTs on micropatterned substrates as well as 3D complex intertwined free-standing networks, whereby elastomeric poly(dimethylsiloxane) (PDMS) was used as an example of a flexible substrate in the present study due primarily to its chemical inertness, ease of use, and characteristic high hydrophobicity.14 These properties have facilitated subsequent immobilization of the solutionbased precursors to designated locations and thus siteselective growth of CNTs. We fabricated the elastomeric PDMS substrates using the recently developed softlithography15 technique by replica micromolding from a master having integrated relief patterns. This technique has been demonstrated16 and allowed us to obtain highly uniform and regular arrays of relief features ranging from the micrometer to nanometer length scales at a relatively minimal cost. In process flow A (Figure 1, scheme A-1), we deposited periodic square patterns of ∼50 nm thickness of metallic nickel (Ni) (as the catalyst) through a shadow mask onto featureless blank PDMS or Si(001) substrates, which were used for comparisons with CNTs grown from the solutionbased precursors. In process flows B (Figure 1, schemes (14) Clarson, S. J.; Semlyen, J. A. Siloxane Polymers; PTR Prentice Hall: Englewood Cliffs, NJ, 1993. (15) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1998, 37, 550. (16) Xia, Y.; McClelland, J. J.; Gupta, R.; Qin, D.; Zhao, X. M.; Sohn, L. L.; Celotta, R. J.; Whitesides, G. M. Adv. Mater. 1997, 9, 147.
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B-3 and B-4) and C (Figure 1, scheme C-5), we used PDMS replicas having patterned relief features as our supporting substrates instead. Since PDMS is highly hydrophobic (having a low surface free energy ∼21.6 × 10-3 J m-2),14 we were able to direct the solution-based organometallic precursor (0.5 M aqueous solution of nickel(II) acetate tetrahydrate (Ni(OCOCH3)2‚4H2O) was used in the present study) into designated locations as depicted in process flows A and B. We manipulated the surface wetting properties (or surface free energy) by subjecting the substrates to oxygen plasma treatment (OPT). The treated samples thus formed an ultrathin silica-like layer17 (
