Annealing Temperature-Dependent Dielectric Properties of C₂₂-Quinolinium(TCNQ) Langmuir-Blodgett Films

Abstract

Frequency-dependent dielectric properties of C₂₂-Quinolinium(TCNQ) Langmuir-Blodgett(LB) films were investigated in a frequency range of 10 Hz ∼ 13 MHz along a perpendicular direction to the films. The films were heat-treated in a temperature range of 20 ∼ 240°C. Frequency-dependent dielectric constants show that there are two characteristic dispersions; one is a dispersion occurring near 1 MHz coming from the orientational polarization of the molecules and the other one is probably due to an interfacial polarization below 1 kHz when the annealing temperature is above 80°C. The annealing temperature-dependent dielectric constants show that there are two maxima, one at near 80°C and the other one at near 180°C. The first peak seems to be related to a disorder of the alkyl chains. The second peak may be due to a chemical structure change of the TCNQ-molecules. Several other methods were employed to understand between the dielectric constants and the internal structure change of the films. DSC (differential scanning calorimetry) data of the C₂₂-Quinolinium(TCNQ) molecules shows that there is an endothermic process near 120°C and a weak exothermic process near 180°C. Thickness measurement by ellipsometry shows that there is a thickness drop near 100°C, which is supposed to be due to a softening of the alkyl chains. The film thickness above 120°C becomes around 20 % of the room-temperature value.