Impact of annealing on the structural and optical properties of methylene green nanostructure films prepared by drop casting

• Nanostructured MG films were deposited by drop casting technique.
• The optical constants of MG films were affected by annealing temperature.
• The band gaps of MG film were found to be increase with the annealing temperature.

The methylene green (MG) powder was found to be polycrystalline with the triclinic system. MG films were deposited by drop coating technique. X-ray diffraction and scanning electron microscopy showed that the MG films have nanostructure nature. It is found that the crystal size is increased by annealing. Optical properties of nanostructured MG films were performed in the spectral range from 200 to 2500 nm to determine the optical constants (n and k). The absorption coefficient of the as-deposited film revealed a two indirect allowed optical band gap with values of 1.90 and 3.11 eV, which increased by annealing to 2 and 3.48 eV, respectively. The dispersion of n was discussed in terms of the single oscillator model. The high frequency dielectric constant and the lattice dielectric constant were estimated and found to decreased by annealing.