Investigations of the optical properties and charge transport mechanism in the plasma polymerized methacrylate-vinyl acetate composite thin films

Abstract

Plasma polymerized (PP) methacrylate (MA) and vinyl acetate (VA) composite thin films with various monomer compositions were deposited onto glass substrate under low-pressure (0.25 Torr) dynamic plasma. The same plasma conditions were used to deposit PPMA and PPVA homopolymer thin films. The thicknesses of the thin films are observed to vary between 117 to 213 nm depending on the monomer ratio. The field emission scanning electron microscopy images show that the surface morphology of the composite films is more immaculate, smooth, and pinhole-free compared to those of homopolymers. The energy dispersive X-ray study shows that the films contain mainly carbon and oxygen with 26.09 − 37.20 at% and 35.03 − 40.10 at%, respectively. The appearance of some broad absorption bands in the FTIR spectra indicates structural changes in the PP films caused by the degradation of monomers or reorganization of monomer molecules during the plasma polymerization. Optical properties of the composite films as observed from the UV-visible spectra were dependent on the monomer composition to some extent. Thus, the optical band gap can be tuned by changing the monomer ratio, where the direct band gap varies from 2.72 to 3.15 eV, whereas the indirect band gap varies from 1.70 to 2.35 eV. The highest band gaps is observed with MA:VA = 3:1, where the films show about 86% transmittance in the visible range (550 − 800 nm). Other optical parameters like absorbance, absorption coefficient, refractive index, steepness parameter, and extinction coefficient were also examined to study the suitability of these composite films in various electronic and optoelectronics applications. Thermogravimetric analysis and differential scanning calorimetry results appear that composite films are found to be more thermally stable (up to 612 K) compared to the homopolymer thin films. The current density-voltage (J-V) characteristics of PP(MA-VA) thin films of varying monomer ratios as well as thickness-dependent films have been studied over the temperature range of 298 to 373 K. The thickness-dependent composite films dominate the space charge limited conduction (SCLC) mechanism in the non-Ohmic region. The J values of the composite films gradually increase with elevating VA monomer density and temperature. The activation energy at room temperature for the Ohmic region varies from 0.013 to 0.019 eV and it varies from 0.019 to 0.260 eV in the non-Ohmic region.