Structural, optical and electrical properties of the thin films of N-benzylaniline synthesized by plasma polymerization method

Abstract

The plasma polymerized N-benzylaniline (PPNBA) thin films of different thicknesses have been synthesized using N-benzylaniline (NBA) as a precursor onto glass substrate by a parallel plate capacitively coupled AC (f = 50 Hz) plasma system. All the films were modified by heat treatment in the temperature range of 373 ‒ 573 K for 30 min and also by iodine doping for 2 hours to compare their properties with those of the as-deposited films. From the X-ray diffraction studies it is confirmed that the PPNBA thin films are amorphous in nature. Fourier transform infrared spectroscopy revealed that the chemical structure of these films is moderately changed from that of the monomer owing to the elimination of some benzyl groups, but the conjugated nature due to C=C stretching of the quinoid and benzenoid units are retained in the polymer backbone. The films contain mostly carbon and nitrogen, which is confirmed by energy dispersive X-ray analysis. However, the presence of oxygen is considered to be incorporated from the reactor or atmosphere. The surface morphology of the thin films is mosaic-like structure, which is formed by the uniform distribution of unequal nanosized clusters over the smooth background of the film surface. The average roughness and root mean square roughness of the surface of the films are found to be in the order of nm as examined by atomic force microscopy. The cluster size is increased with a little shift of conjugated band to higher wavenumber region in the heated and iodine doped films, which make the films smoother, more intense absorption and good quality as compared to the as-deposited films. As-deposited films are found thermally more stable than monomer as observed by thermo-gravimetric analysis and thickness dependence thermal behavior is also found. The absorbance spectra taken at room temperature by ultraviolet-visible (UV-Vis) spectroscopy reveal that the PPNBA thin films have high absorbance and double peaks in ultraviolet region due to ‒* transition. UV-Vis absorption spectra are utilized to elucidate absorption coefficient, allowed direct and indirect transition energy gaps, Urbach energy, steepness parameter, extinction co-efficient, and skin depth. The refractive index, dielectric constant, dissipation factor, and optical conductivity are also determined. All the optical parameters of PPNBA thin films are varied with the change of film thickness. The thermal and optical properties are influenced by the modification of PPNBA thin films with heat treatment and iodine doping as the results of structural change. The aluminum (Al)/PPNBA/Al sandwich type structure was used to explore the electrical properties of as-deposited and iodine doped films of various thicknesses over the temperature range of 298 to 423 K. The current density-voltage (J‒V) characteristics in the voltage region from 0.3 to 100 V revealed that the dependence of J on V obeyed Ohm’s law in the lower voltage (< 10 V) region, while the charge transport phenomena appeared to be non-Ohmic in the higher voltage (> 10 V) region. The Schottky type conduction mechanism was identified as the most probable mechanism of carrier transport in the higher voltage region. The dielectric constant of the films is low (< 15) and was observed to decrease with the increase of frequency in the region of 100 Hz to 100 MHz but increased with the increase of film thickness. The AC electrical conductivity was increased with the increase of frequency for all thicknesses of films due to the hopping conduction mechanism. Electrical properties were enhanced with doping of iodine in films due to the formation of charge transfer complexes. These results encourage the use of the PPNBA thin films in optoelectronic devices.