Investigation of the structural ,optical and electrical properties of plasma polymerized N,N,3,5 tetramethylaniline thin films

Abstract

Plasma polymerized N, N, 3, 5 tetramethylaniline (PPTMA) thin films were deposited on to glass substrates at room temperature by a capacitively coupled parallel plate reactor. The structural and optical properties of as deposited, heat treated and iodine (I) doped PPTMA thin films of different thicknesses were characterized by scanning electron microscopy, elemental analysis (EA), infrared (IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and ultraviolet-visible (UV-Vis) spectroscopy. The surfaces of the PPTMA thin films were observed to be smooth and pinhole free. The energy dispersive x-ray observations indicate the presence of C, N, O in as deposited PPTMA and I doped PPTMA samples. The structural analyses by IR spectroscopy reveal that PPTMA thin films are formed with certain amount of conjugation, which modifies on heat treatment and iodine doping. The transition in PPTMA thin film demonstrates the presence of an increased degree of conjugation in the resulting films. The XPS analyses show that C, N, and O are present on the surface of the PPTMA thin film and possible groups C-H, C=C, C-N, and C=O are found in the PPTMA structure. The atomic ratio N/C of the PPTMA thin film bulk and surface are found to be 0.12 and 0.20 by EA and XPS, respectively. Allowed direct transition (Eqd) and indirect transition (Eqi) energy gaps were determined from Tauc plots. The direct energy gap of as deposited PPTMA thin films is 2.8 eV and that of iodine doped PPTMA thin films is 2.6 eV. The change of energy gap indicates that there is a charge transfer complex arising between the host PPTMA and the iodine. It is observed that Eqd increases a little, Eqi decreases, on heat treatment of PPTMA. The current density-voltage characteristics of as deposited PPTMA and iodine doped PPTMA thin films have been investigated at different temperatures. It is observed that space charge limited conduction is operative in the higher voltage region in iodine doped PPTMA thin films. From the experimental studies the carrier mobility, the free carrier density, and the total trap density of as deposited PPTMA are found to be about 1.3 10-13 m2 V-1s-1, 2.0 1021 m-3 and 4.0 1030 m-3, respectively and those of iodine doped PPTMA are about 1.5 10-12 m2 V-1s-1, 3.7 1022 m-3 and 1.2 1027 m-3, respectively . At lower temperatures, the available thermal energy is not sufficient to ionize the traps, so hopping mobility is observed. While at higher temperatures, the higher values of activation energy indicate a transition from a hopping process to a distinct energy level process. The conductivity of iodine doped PPTMA thin film is higher than that of as deposited PPTMA thin film. The alternating current (ac) capacitance and electrical conductance of PPTMA thin films were measured as functions of frequency (100 < f < 105 Hz) and temperature (300 < T< 450 K). The ac electrical conductivity is more dependent on temperature in the low frequency region than that in the high frequency region. In PPTMA thin films the conduction may be dominated by hopping of carriers between the localized states at low temperatures and thermally excited at the high temperatures. The activation energies are estimated to be about 0.05 eV in the low temperature and 0.23 eV in the high temperature. Dielectric constant decreases with the increase of frequency but dielectric loss increases with increasing frequency with a minimum in the low frequency region. The temperature dependence of the Cole-Cole diagram shows the existence of distribution of dielectric relaxation times in the PPTMA thin films.