Optical transition and electrical conduction mechanisms in plasma polymerized 2,6-diethylaniline thin films

Abstract

Deposition of the plasma polymerized 2, 6 diethylaniline [(C2H5)2C6H3 NH2] (PPDEA) thin film is carried out by using a capacitively coupled glow discharge reactor after optimizing the plasma deposition parameters. Thermal, surface morphological, structural and optical properties of as-deposited PPDEA thin films are compared with those of the heat treated, aged and iodine doped as-deposited ones by using different characterization techniques. The electrical properties of as-deposited and iodine doped as-deposited PPDEA thin films are also investigated. The thermal analyses reveal that PPDEA is thermally stable up to about 580 K whereas, the thermal stability is increased up to about 650 K after heat treatment at 573 K. Iodine doping has not caused any significant effect on thermal stability. The glass transition temperature (Tg), is found about 275 and 290 K and the heat capacity (ΔCp) at Tg is calculated out to be about 121.8 and 357 J/kg-K for as-deposited and heat treated PPDEA thin films respectively. Scanning electron microscopy shows uniform and pinhole free surface for both of the asdeposited and heat treated (573 K) PPDEA thin films and the surface become much smoother after iodine doping. The Electron dispersive X-ray analysis has detected C, N and O in asdeposited and heat treated PPDEA thin films and iodine in iodine doped ones. Fourier transform infrared spectroscopic analyses have revealed the retention of aromatic ring structure and ethyl group of the starting monomer in PPDEA structure along with some rearrangement/cross-linking due to plasma polymerization technique. Heat treatment caused some structural rearrangement and iodine doping has showed a little increase/decrease in band intensity and shift in few absorption bands which suggests modification of bond length. The allowed direct and indirect optical transition energy gaps (Eg) of as-deposited PPDEA thin films of different thicknesses are found about 3.63 and 2.23 to 2.38 eV respectively. The Eg values decrease after heat treatment and iodine doping and also with the doping period but not changed appreciably after aging. The other optical parameters (Urbach energy, steepness parameter and extinction coefficient) of as-deposited PPDEA thin films are also changed after heat treatment and iodine doping but are not changed appreciably after aging. Iodine doping increased the direct current electrical conductivity of as-deposited PPDEA thin films of different thicknesses at different temperatures and the electrical conduction mechanism of Schottky type dominating in as-deposited PPDEA thin films is changed to Poole-Frenkel after iodine doping which might have resulted due to the charge transfer complex formation through donor type PPDEA and acceptor type iodine. The increase in activation energy, (ΔE) from low to high temperature region of both types of (as-deposited and iodine doped asdeposited) PPDEA thin films may be attributed due to a transition from a hopping regime to a regime dominated by distinct energy levels. The higher value of ΔE of as-deposited PPDEA thin films compared to that of the iodine doped ones most possibly be due to the extra carrier generation which ensues the lower energy to activate the carriers to conduct through the PPDEA thin films. The alternating current (AC) electrical conductivity (σac) of both types of PPDEA thin films increases as frequency increases for all the temperatures scanned. The values of frequency exponent, n of both types of PPDEA thin films corresponds Debye-type and other than Debye type relaxation process in the lower and higher frequency region respectively. The temperature dependence of σac of both types of PPDEA thin films suggest hopping of carriers between the localized states. The behavior of dielectric constant (ε'), of both types of PPDEA thin films indicates space charge or interfacial polarization and dipolar polarization at lower and higher frequency regions respectively. At higher temperature region (373 to 398 K) the increase of maximum value of loss tangent, tanδ with increasing temperature implies increase of carriers by thermal activation. The increase in σac, ε' and tanδ after iodine doping can be explained on the basis of the formation of charge transfer complex.