Strong localization in conducting Langmuir–Blodgett films of quasi-one-dimensional charge-transfer complexes

The temperature dependence of intragrain conductivity of the Langmuir–Blodgett (LB) films of (C16H33–TCNQ)0.4(C17H35–DMTTF)0.6 has been studied by means of measuring the surface acoustic waves (SAW) attenuation in a piezocrystal resonator covered by the LB film. [(C16H33–TCNQ)0.4(C17H35–DMTTF)0.6 denotes the surface-active charge-transfer complex of hexadecyltetracyanoquinodimethane (C16H33–TCNQ) and heptadecyldimethyltetrathiofulvalene (C17H35–DMTTF)]. We have found that the intragrain conductivity decreases with decreasing temperature, following σ∝exp(−1/T1/2) law. According to the theory of electron transport in quasi-one-dimensional disordered systems (Q1D DS) proposed by E.P. Nakhmedov et al. [Sov. Phys. Solid State 31 (1989) 368] and developed by Z.H. Wang et al. [Phys. Rev. B 43 (1991) 4373], this temperature behaviour of the conductivity points to the disorder-induced electron localization within the film grains. Fitting our experimental data to the relations proposed in these references, allowed us to evaluate the electron relaxation times due to scattering by phonons and impurities. Based on structural parameters of the conducting bilayer, we have also estimated the Fermi velocity and density of states at the Fermi level, which enabled us to determine the mean free path and localization length in the film studied.