The role of the annealing temperatures on the structure and optical properties of Rose Bengal thin films

Abstract

Uniform thin films of Rose Bengal, RB, have been successfully prepared by the spin coating technique. Results of thermal analysis for RB films showed its stability up to 514 K. Fourier transform infrared (FTIR) measurements proved no changes occurred in the chemical bonds of RB films upon annealing. X-ray diffraction showed that the powder, the pristine and the annealed (363 K) thin films of RB have amorphous structure. The amorphous pristine films become polycrystalline structure after being annealed at 423 K. Optical properties of pristine and annealed RB films have been investigated using transmittance and reflectance methods. The refractive index (n) and the extinction coefficient (k) of RB films were directly calculated from the absolute values of the transmission and the reflection spectra. Single oscillator parameters and Drude model of free carrier absorption have been applied for the analysis of refractive index dispersion. The optical absorption edge data were analyzed within the frame work of the band-to-band electron transitions theory. Annealing temperatures created variations in the absorption coefficient, the energy gap and the refractive index of RB films. The optical functions and their dependence on the annealing temperatures were directly calculated from the spectral distribution of dielectric constant.