Huge values of room-temperature dielectric constants in pellets of poly(3-methylthiophene)
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C. Pereira and A.A. Correa Departamento de Quı´mica, Universidade Federal de Sa˜o Carlos, Caixa Postal 676, CEP 13560-970, Sa˜o Carlos, S.P., Brazil (Received 4 November 2003; accepted 16 March 2004)
Room temperature data of impedance and phase angle in pellets of electrochemically synthesized ClO4− doped poly(3-methylthiophene) (P3MT) were analyzed assuming the sample being represented by a parallel resistor-capacitor (RC) circuit or by a series RC circuit. The last assumption proved to be the correct one, and to confirm it we use the sample as the RC component of a resistor-capacitor-inductor series resonator. We discuss the possibility of this RC series behavior to be due to a charge-density wave characteristic also evidenced from the huge values of the low-frequency dielectric constant of the system. I. INTRODUCTION
Conducting polymers are materials of increasing importance for the fabrication of electronic and optoelectronic devices. Conducting polymers exhibit novel properties not typically available in other materials. These novel properties make possible a number of applications including polymer light-emitting diodes (LEDs), conducting polymers as electrochemical materials, polymer photodetectors, and polymer photovoltaic cells.1 Among the conducting polymers, the polythiophene derivative poly(3-methylthiophene) (P3MT) has been used with great versatility in a number of applications. The electrochemical synthesis of this polymer allows endless possibilities due to great number of variables that can be changed in the synthesis process, giving rise to polymers with different properties. According to Tourillon,2 responsible for the first electrochemical synthesis,3 the working electrode can be Pt, Au, or glass coated with SnO2 or In2O3. A tin-oxide (TO) electrode has also been used.4 The electrolytic medium typically consists of an organic solvent like acetonitrile, dichloromethane, or benzoitrile. The supporting electrolyte is in general a salt of BF4−, ClO4−, PF6−, or SO3CF3−. The films can be grafted on the electrode with either a controlled potential or a controlled current. The molarity of the monomer and the temperature are also synthesis variables giving different results concerning the electrical properties.5 The
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0260 2068
http://journals.cambridge.org
J. Mater. Res., Vol. 19, No. 7, Jul 2004 Downloaded: 13 Mar 2015
oxidized polymer can be reversibly reduced. In the reduced form, P3MT was used in field-effect devices.6 In the oxidized state, it was used as a solar cell in a P3MT/CuInSe2 junction.7 In a recent article,8 we have shown room-temperature weak ferromagnetic behavior in pellets of partially reduced P3MT. We have also shown evidence of metamagnetic behavior in the same system. The stability of P3MT at ambient conditions is a subject far from being clarified. According to Tourillon,2 it is stable in the oxidized and reduced forms at ambient conditions. But the influence of light, oxygen,6 and
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