Optical and AC electrical properties of plasma polymerized o-methoxyaniline thin films

Abstract

Plasma polymerized o-methoxyaniline (PPOMA) thin films of different thickness were prepared in glow discharge plasma using a parallel plate capacitively coupled polymerization reactor. The PPOMA thin films shows smooth, flawless and homogeneous surface together with the content of carbon (63 %) oxygen (24 %), nitrogen (8 %) and others (5 %). Fourier transform infrared spectrum reveals a decrease of absorption intensity and disappearance of some absorption peaks for PPOMA as compared to that of OMA, indicating the polymer formation from the monomer. The differential thermal analysis showed a peak at around 350 and 700 K which may be due to the removal of water content and the breaking of bonds in the PPOMA respectively. The thermogravimetric analysis shows the onset of weight loss from 540 K, which may be due to the loss of low molecular mass hydrocarbon gases. The ultraviolet-visible spectroscopy shows that the allowed indirect and direct band gaps for as deposited PPOMA thin films of 100 – 300 nm thickness have the values in the range 1.65-1.76 eV and 2.94-3.30 eV, respectively. Both the energy gaps showed a slight increase for 100 – 125 nm and above this range the gaps remain almost constant. From the plot of alternate current electrical conductivity (σac) versus frequency, the value of ‘n’ is calculated using the relation n  ac ()  A . In the low frequency range (0.1-1 kHz) the value of ‘n’ is 1 which indicates Debye type conduction and in the higher frequency range (1- 100 kHz) ‘n’ is > 1 which indicates non-Debye type conduction. The observed mechanism of carrier conduction in PPOMA films at low frequencies is due to hopping of carriers between the localized states. At the low frequency region (0.1 - 20 kHz), the change of dielectric constant, ′ at low temperature exhibits the nature of ‘static’ dielectric constant and the decrease of ′ at higher temperature may be due to the interfacial polarization. The loss factor, tan increases with the increase in frequency having a peak above 100 kHz for all the PPOMA films of different thickness. Cole-Cole plot between real and imaginary dielectric constant exhibits single relaxation mechanism in PPOMA thin films and the sample become more resistive with the increase in film thickness and temperature.