Fabrication and Evaluation of Conducting Polymer Nanowire Heterostructures

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Fabrication and Evaluation of Conducting Polymer Nanowire Heterostructures Yevgeny Berdichevsky and Y.-H. Lo Electrical and Computer Engineering Department, University of California, San Diego 9500 Gilman Dr., MC: 0409 La Jolla, CA 92093 ABSTRACT Conducting polymer nanostructures such as nanofibers and nanotubes have potential uses in a variety of applications including electronic and photonic devices and sensors. Conducting polymers have also been used as artificial muscles. In this work, template synthesis method for fabricating solid polypyrrole nanowires and polypyrrole-gold nanowire heterostructures is demonstrated to explore suitability of these structures as nano-artificial muscles or nanoactuators. Polypyrrole nanowires are evaluated in an aqueous electrolyte to see if they retain the ability to expand and contract under electrochemical cycling. Template synthesis is then used to alternatively electroplate gold and electropolymerize polypyrrole in the pores of an alumina membrane to create layered polypyrrole-gold nanowires. INTRODUCTION Template synthesis has been used to synthesize conducting polymer nanowires in nanosized pores of a variety of commercially available and custom-made membranes [1-6]. Potential applications mentioned by these authors include electronic and sensor devices, among others. However, conducting polymers have also been used as artificial muscles, or materials which can expand or contract upon application of an electric field or a pH change[7-8]. There could be numerous potential advantages for scaling the artificial muscle to nanoscale; for example, an improvement in time response due to relaxed diffusion limitation in small structures. Perhaps more importantly, new applications could become possible where artificial muscle fibers are used in devices mimicking the performance of biological nano and microactuators such as motor proteins, cell cilia, or flagella. In our previous work it was shown that polypyrrole (PPy) nanowires doped with dodecylbenzenesulfonate (DBS) can be synthesized in the pores of a commercially available alumina membrane and that the nanowires show electrochemical oxidation/reduction behavior characteristic of mechanical actuation after the template membrane was dissolved [9]. Here, we present the first direct evidence that PPy nanowires function as nanoactuators, and show that the template synthesis technique can be extended to fabricate gold-polypyrrole heterostructure nanowires. EXPERIMENT The procedure to fabricate polypyrrole nanofibers was described in detail in our previous work. As can be seen in Figure 1(a), the top portion of the nanofibers is hollow with very

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Figure 1(a, b). SEM top view of the PPy(DBS) nanowires. On the left (a), the hollow top portion of the nanowires can be clearly seen, on the right (b), the nanowires have been mechanically lapped. thin sidewalls (< 20 nm), and probably unsuitable for mechanical actuation. To remove the hollow tube portion of the nanofiber, the following procedure was used: polypyrrole nanofi