ELSEVIER Synthetic Metals 97 (1998) 23-29 Crossover from Mott to Efros-Shklovskii variable-range-hopping conductivity in conducting polyaniline M. Ghosh, A. Barman, S.K. De *, S. Chatterjee Department of Materials Science, Indian Associationfor the Cultivation ofScience, Jadayur, Calcutta 700 032, India Received 9 April 1998; accepted 17 June 1998 Abstract Electrical resistance and magnetoresistance of the HCl-doped polyaniline (PANI) in aqueous ethanol have been investigated at low temperature down to 1.8 K and in magnetic field up to S T. The weaker temperature dependence of resistivity characterized by the ratio, pr= p( 1.8 K) lp(300 K) indicates that a better homogeneity and less disorder can be obtained by protonation with HCI in ethanol media. The samples with resistivity ratio lying in the range 10’1p,1 l@’ exhibit a crossover from Mott to Efros-ShklovsIcii variable-range-hopping (VRH) conduction below 10 K. The Coulomb gap energy has been calculated and is small (0.22-0.04 meV) . 0 1998 Elsevier Science S.A. All rights reserved. Kepvords: Polyaniline; Variable-range-hopping conductivity 1. Introduction The low temperature transport properties of disordered semiconductors andinsulators aregoverned by variable range hopping (VRH) betweenthe localized states. Mott andDavis [ l] were the first to predict the key relation for the temper- ature dependence of conductivity for noninteracting carriers and for a constant density of states near the Fermi energy or a slowIy varying function of energy. Shklovskii and Efros [ 2] pointed out that, because of long range Coulomb inter- actionsbetweenlocalized electron states, the density of states near the Fermi energy tendsto zero which yields a parabolic Coulomb gap. The gap in density of states hasbeenobserved by tunnelling [ 3J and photoemission measurements [ 4,5]. The Efros-Shklovskii (ES) hopping theory is valid at low temperaturedependingon the size of the gap. Mott’s theory has been successfully used in many different insulators. In the last few yearsthe crossover between Mott andES hopping regimesas a function of temperaturehasbeen found experi- mentally, particularly in some doped inorganic semiconduc- tors [ 6-81. Very recently, it has been observed that some conducting polymers also show crossover from the Mott to ES VRH conduction with a small Coulomb gap [ 91. * Corresponding author. Tel.: + 91 33 473 4971; fax: + 91 33 473 2805; e-ma% msskd@iacs.emet.in The transport properties of conducting polymers are mainly determined from the disorder-induced localization. The disorder arises during the synthesis and processing and alsofrom the partial crystallinity andinhomogeneous doping. However, in the last few years methods have beendeveloped for producing more homogeneous and less disordered con- ducting polymers. Doped polyaniline is one of the most attractive materials due to its environmental stability and dopability by protonation. The electrical resistivity of poly- aniline doped with HCl preparedby the conventional syn- thetic route is relatively small [ 10-121. Recently, it hasbeen shown that the improved quality and conductivity can be obtained in PANI doped with HCl in aqueous ethanol [ 131. The temperature dependence of resistivity of conducting polymers follows Mott’s VRH conduction mechanismin three dimensions [ 14-161. However, conventional HCl- doped PANI samples follow Mott’s law in one dimension [ 17,181. The resistivity of such PANl samples increases by several orders of magnitude as the temperature is lowered, indicating the presence of extensive disorder and the forma- tion of inhomogeneous metallic islands. The electronic prop- erties of conducting polymers are strongly influenced by the polymerization condition. Hence the transport properties of PANl protonated with HCl in aqueous alcohol medium will be more interesting. In this paper, we report the temperature dependence of resistivity of HCI-doped PAN1 samples pre- pared in ethanol withdifferent concentrations+ 0379-6779/98/$ - see front matter 0 1998 Elsevier Science S.A. All rights reserved. PIlSO379-6779(98)00105-2