Structural, electrical and optical properties of diethanolamine thin films synthesized by plasma polymerization technique

Abstract

The materials formed using plasma polymerization method are vastly different from conventional polymers and constitute a new kind of material. Diethanolamine, an organic compound has been chosen to deposit plasma polymerized diethanolamine (PPDEOA) thin films of different thicknesses using a capacitively coupled glow discharge reactor at an optimum condition. Different properties such as surface morphological, thermal, structural, optical and electrical (direct and alternating current) (DC and AC) properties have been investigated. The surface morphology of PPDEOA thin films of various thicknesses of different magnifications was observed to be agglomerates/mosaic-like structure. The films were uniform, defect free and pinhole free. The mass (%) of the elements changes with the film thickness as analyzed by energy dispersive X-ray analysis. The average roughness and root mean square roughness values are 0.706 -0.997 nm and 1.0 to 1.3 nm, respectively, as observed by atomic force microscope. PPDEOA thin films are thermally stable up to 570 K in air environment. The structural difference between the monomer and the PPDEOA was analyzed by Fourier transform infrared spectroscopy. The direct transition energy gap, (Eg(d)) and indirect transition energy gap, (Eg(i)) of as-deposited PPDEOA thin films are 3.15 to 2.83 eV and 1.98 to 1.34 eV, respectively and these change with the film thickness. After heat treatment of the PPDEOA thin films, the Eg(d) and Eg(i) become 3.09 to 2.5 eV and 1.88 to 1.04 eV, respectively. The Eg(d) and Eg(i) of iodine doped PPDEOA thin films are 2.95 to 2.81 eV and 1.90 to 1.69 eV, respectively. Urbach energy, steepness parameter, refractive index and extinction coefficient were also evaluated for as-deposited PPDEOA. All optical parameters change with film thickness. The samples were configured as aluminum/PPDEOA/aluminum sandwich type in exploring the electrical (DC and AC) properties of PPDEOA thin films. The values of the power index of the relation J ∝ Vn, n are found to be 0.53 ˂ n ˂ 1.1, represented Ohmic nature in the lower voltage region (˂ 15 V) and in the higher voltage region (˃ 15 V), it is in between 1.3 ˂ n ˂ 4.5 that exhibited non-Ohmic behavior. The thickness dependence of xxii current density in the non-Ohmic region and by comparing the theoretical and experimental values of Schottky and Poole-Frenkel coefficients, the Schotkky type conduction mechanism is found in the PPDEOA thin films. In the Ohmic region, the activation energies for PPDEOA thin films are correspondingly 0.06 – 0.14 eV and 0.23 – 0.69 eV in the lower and higher temperatures. The activation energies in the non-Ohmic region are 0.12 to 0.19 eV and 0.52 to 0.74 eV in the low and high temperatures, respectively. The dielectric constant, 𝜀 increases with the decrease of frequency for PPDEOA thin films of all thicknesses and that increases with the increase of temperature. The AC electrical conductivity, σac increases with the increase of frequency. The frequency exponent, s values are 0.32 to 1.0 and 1.0 to 1.8 in the lower and higher frequencies, respectively. Those indicate the presence of hopping conduction in the lower frequency of PPDEOA thin films. The activation energies are found to be 0.01 to 0.17 eV. The loss tangent, tanδ increases with the increase of frequency firstly and then decreases after reaching a maximum loss. The 𝜀 , σac, and tanδ change with the film thickness.