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Polythiophene Nanowires

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Communications1218Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 1999 0935-9648/99/1410-1218 $ 17.50+.50/0 Adv. Mater. 1999, 11, No. 14was dried only to about 30 % of its weight before the crosslinking step. Adrop of 1 M MgSO4was added for the crosslinking, and after about 15 minthe electrode was equilibrated with aqueous 0.25 M MgSO4for at least 2 h.The PPy was grown on the polymer gel as described above. The unswelledPEDOT-PSS/PPy electrode was prepared by electropolymerizing pyrrole onan uncrosslinked PEDOT-PSS-coated electrode from acetonitrile solutioncontaining 0.25 M pyrrole and 0.1 M LiClO4at a constant potential of0.85 V vs. an Ag/AgCl electrode. The PPy-coated electrode was preparedby growing PPy on a gold electrode under the same conditions as in the caseof PEDOT-PSS/PPy electrodes.The electrochemical cell in the supercapacitor geometry for the cyclicvoltammetric and the chrono-potentiometric studies was a two-electrodeset-up, with two identical electrodes face to face and parallel, in aqueous1MNa2SO4. For the swelled-polymer electrodes, i.e., in the cases wherehigh power-density was applied the electrodes were at a fixed separation of1 mm, whereas in other cases, the distance was about 1 cm. The cells werein ambient atmospheric conditions. The chrono-potentiometric studies ofgalvanostatic charging±discharging were carried out in the potential rangeof 0 V to 0.8 V. For calculation of the energy and power densities, the firstdischarge curve after equilibration of the cell at 0.8 V was analyzed, and thedry mass of the polymeric material on a single electrode was considered.The mass of the PEDOT-PSS was determined from the weight of the disper-sion of known concentration that was applied to the electrode. The mass ofPPy was calculated from the coulomb of charge passed for its electropoly-merization, assuming 2.25 electrons per pyrrole unit are needed for the poly-merization of PPy at 25 % doping level.Received: January 14, 1999Final version: June 7, 1999±[1] G. S. Attard, P. N. Bartlett, N. R. B. Coleman, J. M. Elliott, J. R.Owen, J. H. Wang, Science 1997, 278, 838.[2] C. Niu, E. K. Sichel, R. Hoch, D. Moy, H. Tennent, Appl. Phys. Lett.1997, 70, 1480.[3] Handbook of Conducting Polymers (Eds: T. Skotheim, R. L. Elsen-baumer, J. R. Reynolds), Marcel Dekker, New York 1998.[4] Electroactive Polymer Electrochemistry (Ed: M. E. G. Lyons), Plenum,New York 1994.[5] P. Novµk, O. Inganäs, R. Bjorklund, J. Electrochem. Soc. 1987, 134,1341.[6] S. Ghosh, O. Inganäs, Synth. Met., in press.[7] S. Ghosh, J. R. Rasmusson, O. Inganäs, Adv. Mater. 1998, 10, 1097.[8] S. Sarangapani, B. V. Tilak, C.-P. Chen, J. Electrochem. Soc. 1996, 143,3791.[9] B. E. Conway, J. Electrochem. Soc. 1991, 138, 1539.[10] C. Arbizzani, M. Mastragostino, L. Meneghello, R. Paraventi, Adv.Mater. 1996, 8, 331.[11] A. Rudge, J. Davey, I. Raistrick, S. Gottesfeld, J. P. Ferraris, J. PowerSources 1994, 47, 89.[12] A. F. Burke, presented at 36th Power Sources Meeting, Cherry Hill,NJ, June 6±9, 1994.[13] 8th Int. Seminar on Double Layer Capacitors and Similar EnergyStorage Devices, Deerfield Beach, FL, December 7±9, 1998.[14] Q. Zhong, D. Innis, K. Kjoller, V. B. Elings, Surf. Sci. Lett. 1993, 290,L688.[15] B. E. Conway, J. Power Sources 1996, 63, 255.[16] J. R. Miller, in Proc. ECS Symposium on Supercapacitors (Eds: F.Delnick, M. Tomkiewicz), Electrochemical Society, Pennington, NJ1996, p.246.[17] M. Liu, S. J. Visco, L. C. D. Jonghe, J. Electrochem. Soc. 1991, 138,1891.[18] N. Oyama, T. Tatsuma, T. Sato, T. Sotomura, Nature 1995, 373, 598.Polythiophene Nanowires**By Thomas Bjùrnholm,* Tue Hassenkam,Daniel R. Greve, Richard D. McCullough,Manikandan Jayaraman, Steve M. Savoy, Chris E. Jones,and John T. McDevittDevices based on conjugated polymers are becomingcommercially available in the form of light emittingdiodes,[1]proto-type electronic polymer circuits of conju-gated polymers are presently being demonstrated to work[2]and lasers are being developed.[3]All these applicationsmake use of the polymer samples as solution cast thin films.Since the electronic and optical properties of the filmsare inherently related to their local structure, it is vitallyimportant, for the performance of the devices, to achieve agreater control of the self-assembly process that takes thepolymers from a disordered state in solution into a semi-ordered solid state. The desire to make nanometer scale de-vices for future electronic applications further emphasizesthe need for such nanoscale control.Here we report on the formation of polythiophene nano-wires. The work is a continuation of previously reportedstudies[4]of Langmuir-Blodgett (LB) films of amphiphilicpolythiophenes which are substituted by alternating hydro-philic and hydrophobic side groups along the backbone(1 see Fig. 1A). Langmuir films of 1 form readily, and iso-thermic compression (Fig. 1B) leads to densely packedmonolayers in which the polythiophene backbones arep-stacked parallel to the water surface as indicated inFigure 1A. The stacking of the polymers is highly ordered.Adjacent polymers of 1 are displaced one thiophene unitalong the backbone, thereby allowing the alkyl chain ofone polymer to fill the void space between the alkyl chainson the adjacent polymer.[4]From X-ray diffraction data,the domain size of the highly ordered p-stacked polymerhas been estimated to be 6 nm in the stacking direction.[4]The average length of the polymers is £ 40 nm (i.e. £ 50 re-peat units).[4,5]As described below, monolayers of 1 spontaneously foldinto wire-like structures when compressed beyond the col-lapse point on the LB-trough. These wires are microns long_______________________±[*] Prof. T. Bjùrnholm, T. Hassenkam, Dr. D. R. GreveDepartment of Chemistry, University of CopenhagenFruebjergvej 3, DK-2100 Copenhagen (Denmark)Prof. R. D. McCullough, Dr. M. JayaramanDepartment of Chemistry, Carnegie Mellon UniversityPittsburgh, PA 15213-3890 (USA)Dr. S. M. Savoy, C. E. Jones, Prof. J. T. McDevittDepartment of Chemistry and BiochemistryThe University of Texas at AustinAustin, TX 78712 (USA)[**] We gratefully acknowledge funding from the following institutions:The Danish Research Councils (TB), NATO-CRG(TB,RDM),NSF(JTM RDM(CHE 9807707)), ONR(JTM), and AFOSR(JTM).We thank Allen J. Bard for use of his Langmuir-Blodgett trough dur-ing T.B.'s sabbatical in Austin.with nano-scale cross sections. Bundles of the ªnano-wiresºcan be manipulated to fill the gap between gold electrodesand


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