Electrical properties of C22-quinolinium(TCNQ) and polyamic acid alkylamine salts Langmuir–Blodgett films

Abstract

Electrical properties of C22-quinolinium(TCNQ) and polyamic acid alkylamine salts Langmuir–Blodgett (LB) films have been investigated through a study of surface pressure (π)–area (A) isotherms, current–voltage(I–V) characteristics, frequency-dependent dielectric responses, and thermally-stimulated currents (TSC). Through-plane I–V characteristics of C22-quinolinium(TCNQ) LB films show an anomalous behavior of breakdown near the electric-field strength of 106 V/cm. Frequency-dependent dielectric constants of the thermally annealed C22-quinolinium(TCNQ) LB films show that there are two characteristic dispersions in a frequency range of 10–13 Hz; one is a dispersion occurring near 1 MHz coming from the orientational polarization of the molecules and the other is probably due to an interfacial polarization below 1 kHz when the annealing temperature is above 80°C. The annealing temperature-dependent dielectric constants of the C22-quinolinium(TCNQ) LB films show that there are two maximum; one at near 80°C and the other at near 180°C. The first peak seems to be related to a disorder of alkyl chains. In the I–V characteristics of Z-type polyamic acid alkylamine salts (PAAS) LB films measured along the through-plane at room temperature, ohmic conduction has been observed below around 105 V/cm, which is similar to that of C22-quinolinium(TCNQ) LB films. Non-ohmic conduction has been observed above around 105 V/cm and the conduction mechanism can be explained by Schottky effect. Frequency-dependent dielectric responses of the PAAS LB film show that there is a dispersion at 104 Hz when the annealing temperature reaches about 100°C. Measurements of thermally-stimulated currents of those LB films were performed from room temperature to about 250°C and the temperature was increased at a rate of 0.2°C/s linearly. TSC peaks are observed at about 70°C in the arachidic acid LB films, and at about 70 and 160°C in the PAAS LB films. They indicate that the peak at 70°C is resulted from the alkyl group, and the other peak at 160°C is probably due to CO group in PAAS molecules.