Investigation of structural and optoelectronic properties in dye-doped MC biopolymer films

Biodegradable polymers modified with natural additives are gaining increasing attention for sustainable optoelectronic applications. In this work, a natural dye extracted from Cosmos sulphureus Cav. (CSC) flowers was incorporated into methylcellulose (MC) biopolymer films via a casting technique to enhance their optical properties. The dye-doped MC films were characterized to evaluate structural, morphological, and optical changes using established spectroscopic and imaging techniques. FTIR analysis confirmed strong hydrogen bonding between MC and CSC functional groups, primarily involving OH and NH groups. This interaction suggests good chemical compatibility and uniform dispersion of the dye within the polymer matrix. XRD results revealed an increased amorphous phase in doped samples, with a significant decrease in crystallinity, corroborated by a rise in Urbach energy from 0.327 to 0.463 eV. SEM imaging revealed distinct changes in surface morphology, with the dye-doped films exhibiting increased roughness and varied surface features compared to the smooth texture of the pristine MC, indicating effective dye incorporation and microstructural interaction. UV–Visible spectroscopy showed a red-shift in the absorption edge from 6.3 eV in pure MC to 2.24 eV in the highest doped sample (MCCS3), indicating effective band gap reduction. The refractive index increased from 1.15 to 1.18 with dye loading, indicating a denser optical medium and increased electronic polarizability. Simultaneously, the optical electronegativity decreased from 2.171 to 2.157, suggesting greater ease of electronic transitions. Optical dispersion parameters were analyzed using the Wemple-DiDomenico (WDD) model, oscillator energy ($E_o$) dropped from 6.26 to 2.63 eV and dispersion energy ($E_d$) from 1.54 to 0.71 eV. Other derived parameters such as effective mass, plasma frequency, mobility, and Verdet constants support the suitability of the films for optical limiting and magneto-optical applications. This study presents a novel, eco-friendly approach to tailoring the optical properties of biodegradable polymers, offering low-cost, sustainable solutions for future green technologies.