Studies of the optical properties and DC electrical conduction mechanism of plasma polymerized cyclohexane thin films

Abstract

In this work, plasma polymerized cyclohexane (PPCH) thin films were synthesized at room temperature on to glass substrates by using a capacitively coupled parallel plate glow discharge system. The surface morphology and elemental analysis of PPCH thin films were investigated by Field Emission Scanning Electron Microscopy (FESEM) and Energy Dispersive X-ray (EDX) Analysis. From SEM analysis, a smooth, flawless and pinhole free surface of PPCH thin films was observed. No significant change was observed between the surface of the PPCH thin films of different thicknesses. The EDX analysis indicated the presence of prominent percentage of carbon in PPCH thin films with a small percentage of oxygen. The structural analyses were performed by Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR). Chemical structures of PPCH thin films by ATR-FTIR revealed that the polymerized films are structurally different to some extent compared with the monomer. The films are hydrocarbon-rich with presence of oxygen in the form of O-H and C-O functional groups. Thermal analyses were performed by DTA/TGA. From thermal analysis, it is observed that below the temperature 510 K and 530 K, the rate of weight loss is very slow in both air and N2 atmosphere respectively and are thermally stable. Above 510 K weight loss takes place in different stages in air and that in N2 takes place in one stage reaction above 530 K. The optical properties of PPCH thin films were investigated by UV-visible spectroscopic analysis. The absorption coefficients, at various wavelengths were calculated using UV-vis data. The direct transition energy (Eg(d)) and indirect transition energy (Eg(i)) were obtained by Tauc formula. Eg(d) of PPCH thin films is found between 2.97 and 3.61 eV for different thicknesses and Eg(i) is found to be 1.83 to 2.31 eV. The current density-voltage (J-V) characteristics of PPCH revealed that in the low voltage region, the conduction current obeys Ohm’s law while in higher voltage region the conduction current shows non-Ohmic behavior. Schottky conduction mechanism is dominant in PPCH thin films in the non-Ohmic region. It is found that at low temperature region, activation energies are about 0.14±0.05 and 0.16±0.05 eV for applied voltages 10 and 50 V respectively and at the higher temperature region, activation energies are about 0.57 ±0.05 and 0.83±.05 eV for applied voltages 10 and 50 V respectively.