Linear/nonlinear optical properties and dispersion parameters of nanocrystalline indigo organic semiconductor films

Abstract

Films of indigo dye as organic semiconductors have been prepared with different thicknesses (462–885 nm) through thermal evaporation. X-ray diffraction shows a nanocrystalline structure for the films with crystallite size 10.5 ± 0.1 nm. AFM confirms the nanocrystalline structure of the prepared films. The optical properties of the indigo films have been investigated using Swanepoel's method. These films have 2 optical energy gaps (1.67 and 2.61 eV) and Urbach's energy of 0.22 eV. The refractive index has normal behavior to decrease with increasing the wavelength following the Cauchy formula and the single oscillator model. The oscillator energy E o and dispersion energy E d are 2.75 and 4.81 eV, respectively. The lattice dielectric constant ε L is 2.89 while the static dielectric constant ε s is 2.75. The linear/nonlinear optical susceptibility and nonlinear refractive index have similar behavior to increase with increasing the photon energy. • Thermal evaporated films of indigo have a nanocrystalline structure. • Indigo films have two indirect optical transitions. • The optical properties of the indigo films are thickness independent. • The nonlinear refractive index and linear/nonlinear optical susceptibility increase with the photon energy.