Electron localization in conducting langmuir-blodgett films

Abstract

The temperature dependence of the intracrystallite conductivity of Langmuir-Blodgett films of the (C16H33-TCNQ)0.4(C17H35-DMTTF)0.6 charge-transfer complex (CTC) is studied by measuring the surface acoustic wave attenuation in a piezoelectric delay line coated with such a film. (C16H33-TCNQ)0.4(C17H35-DMTTF)0.6 is a surface-active CTC made from a 1.5: 1 mixture of heptadecyl-dimethyltetrathiafulvalene (C17H35-DMTTF) and hexadecyl-tetracyanoquinodimethane (C16H33-TCNQ). The temperature dependence of the intracrystallite conductivity is found to have a maximum at TMD=193.5 K. Above TMD, the conductivity of the films is metallic (∂σ/∂T<0), while below this temperature it obeys a law that is close to the one-dimensional Mott law. The decrease in the conductivity with decreasing temperature at T<TMD is shown to be related to the localization of electron states in the quasi-one-dimensional system under study and to be caused by the presence of impurities and defects in the TCNQ chains, along which a charge is transferred. The detected variation in the conductivity with temperature below TMD is found to qualitatively and quantitatively agree with the model of localization in a weakly disordered quasi-one-dimensional system proposed earlier by Nakhmedov, Prigodin, and Samukhin. Fitting the experimental results to the theoretical dependences obtained in the framework of this model allows us to find the electron-phonon and electron-impurity scattering times. The structural parameters of the conducting layer are used to estimate the density of states at the Fermi level and the Fermi velocity in the films. With these values, the mean free path and the localization length in the films under study are determined.