Optical band gap modulation in functionalized chitosan biopolymer hybrids using absorption and derivative spectrum fitting methods: A spectroscopic analysis

In this study, biopolymer composites based on chitosan (CS) with enhanced optical properties were functionalized using Manganese metal complexes and black tea solution dyes. The results indicate that CS with Mn²⁺-complexes can produce polymer hybrids with high absorption, high refractive index and controlled optical band gaps, with a significant reduction from 6.24 eV to 1.21 eV. The refractive index and optical dielectric constant measurements show that the doped CS films have more charge carriers and traps than those in pure CS films. The Lorentz-Drude model was used to derive several significant optical parameters, and the W-D model was utilized to calculate the optical moments M_-1 changing from 0.35 to 2.13 and M_-3 changing from 0.005 to 0.4. It was shown that the doped samples have larger Urbach energy than pure film, increased from 0.29 to 0.55 eV. Tauc and ASF model was also used to calculate the electronic transitions, band structure, and optical characteristics. Bandgap energy based on Tauc model at m = 2, 1/3, 1/2, and 2/3 are 1.77, 1.54, 1.47, and 1.37 eV, based on ASF model are 1.52, 1.42, 1.69, and 1.47 eV, respectively. As a result of changes in the optical diffraction parameters the optical mobility () changed from 1.67 to 1.27 and optical resistivity from 9.36 × 10^–27 to 4.0 × 10^–29. The dopped samples show an increase in their linear optical susceptibility, third-order nonlinear optical susceptibility and nonlinear refractive indices, changing from 3.165 × 10^–15 to 2.831 × 10^–12 esu. Finally, light propagation velocities, surface resistance, and thermal emissivity were also examined.