Study of the optical and AC electrical properties of plasma polymerized benzonitrile thin flims

Abstract

Plasma polymerization technique was applied to deposit plasma polymerized benzonitrile (PPBN) thin films from benzonitrile(BN) monomer at room temperature by a parallel plate capacitively coupled glow discharge reactor. It is seen from the FTIR spectrum of the PPBN thin films that the peak at 1595 cm-1 for C=N stretching is observed in lower intensity than that is observed in BN. The peaks for aromatic C=C bands at 1560 and 1455 cm-1 are retained in PPBN. The C-C=N deformation vibration at 545 cm-1 was also observed in PPBN spectrum, indicating that a certain amount of C≡N bonds remained unreacted. These observations indicate that plasma polymerization technique has modified the basic structure of the monomer. The thermal analyses suggest that the thermal stability temperature, Ts of BN and PPBN is about 378 and 500 K respectively. The values of Egd vary from 2.80 to 2.47 eV, and those of Egi vary from 2.20 to 1.73 eV as the thickness vary from 233 to 352 nm for PPBN thin films. From the Ultraviolet visible spectroscopic analyses it is reveled that as the thickness of the films increases, some fragmentation/crosslinking may develop within the bulk of the material with increasing deposition time and as a consequence lower energy gaps are observed. The decrease in Eg with increasing thickness is assumed to appear due to the merge of defect states and generation of sublevels at the end of the valance band and conduction band. AC conductivity, σac, increases as the frequency increases. The frequency exponent, n of PPBN thin films correspond to Debye-type in the lower frequency region and in the high frequency region correspond to the relaxation process other than Debye type. The value of the ac activation energy and the increase of σac(ω), with the increase of frequency confirms that hopping conduction is the dominant current transport mechanism in PPBN films. The dielectric constant ( ) decreases slowly with increasing frequency and the values of  lie between 9 and 24 at different temperatures in the low frequency region (10 kHz). The loss tangent is found to increase with the increase in frequency having a loss peak around 105 Hz.