Preparation and characterization of plasma-polymerized diphenyl thin films

Abstract

A capacitively coupled plasma polymerization system was designed and fabricated for plasma polymer deposition. Transparent and light yellow colour plasma-polymerized diphenyl (PPDP) thin films were deposited by a dry and flawless process using the fabricated plasma polymerization system. The surface morphology of the PPDP film has been studied by scanning electron spectroscopy (SEM) and was observed to be uniform, pinhole and fracture-free. The studies of the PPDP using the elemental analysis (EA), xray photoelectron microscopy (XPS), Auger electron spectroscopy (AES) and infrared (IR) spectroscopic analyses indicate that the PPDP is chemically and structurally different from that of the monomer diphenyl. The empirical formula of the PPDP film is C9.oJH4.0l01.06N0.46. The chemical characteristics of the PPDP thin films were studied by AES and XPS techniques and similar result was found. It is found that the surface and bulk atomic concentrations of the constituent elements of the PPDP film are different. The IR analyses reveal that cyclizationlaggregation by conjugation occurs in the PPDP structure on heat treatment which is partially relieved on aging. The differential scanning calorimetric (DSC) investigations identify the different phase transitions in the asdeposited and heat-treated PPDP. The thermogravimetric analyses (TGA) have shown that the thermal degradation of PPDP occurs in two different steps. The optical properties have been evaluated by ultraviolet-visible (UV-Vis) spectroscopic studies of as-deposited, heat-treated and aged (as-deposited and heattreated) PPDP thin films. The bandgap energies are about 3.59 and 4.08 eV for unaged and aged PPDP films respectively. The optical band gap does not change much on heat treatment but increases on aging. Direct and indirect transitions are identified and these energy gaps are calculated for the PPDP films. The current density-voltage (J-V) characteristics of as-deposited and heat-treated PPDP films have been studied in the voltage region from 0.2 to 15.0 V. The Poole- Frenkel conduction mechanism is observed to be operative in both type of PPDP samples. The thermally activated conduction process in different as-deposited and heat-treated PPDP films were examined from 298 to 523 K. The activation energies (tl.E) suggest that electrons and/or holes from traps and/or sublevels are active in the low temperature region and in the high temperature region ions from the material are involved in the conduction process. Xl The alternate current (ac) conduction mechanism of the as-deposited and heat-treated PPDP thin films is observed to be dominated by hopping of carriers between the localized states up to 300 and 350 K, respectively below 103 Hz. The thermally excited carriers from energy levels within the band gap in the vicinity of high temperature are responsible for conduction above 103 Hz. The dielectric constant is dependent on frequency above 303 and 343 K in the as-deposited and heat-treated PPDP respectively. The a and ~ relaxation processes were observed to be superposed at around 300 K in as-deposited PPDP whereas in the heat-treated PPDP only the a relaxation was observed. For different relaxation processes, the ilE values are estimated to be about 0.45 and 0.67 eY for the asdeposited and 1.35 eY for heat-treated PPDP. The distribution of relaxation time was observed in these materials. The thermally stimulated depolarization current (TSDC) studies have been performed at different polarizing temperatures (298, 323 and 348 K) and polarizing voltages (5.0 and 10.0 Y) on as-deposited and heat-treated PPDP films. The origin of TSDC current in the PPDP films is due to depolarization of dipoles. The ilE values obtained from the initial rise part of the TSDC are found to be in the range 0.20-0.47 eY and 0.16-0.24 eY for the as-deposited and heat-treated PPDP films, respectively. The low temperature ilE value is comparable to that calculated in the direct current (dc) measurement. This indicates that the conduction process in the TSDC may be same as that in the case of dc conduction in the low temperature region. A preliminary effort has been made to see the suitability of PPDP thin films as a new material for moisture and ammonia gas sensor device. The PPDP films show good response toward moisture in comparison to ammonia gas.