Natural dyes of mulberry leaf enriched with carbon dots as greener methodology to improve the optical properties of chitosan biopolymer

This study presents an innovative, environmentally friendly, and cost-effective approach to enhancing the optical properties of the chitosan (CS) biopolymer. By utilizing a natural dye extracted from autumn-fallen mulberry leaves (MuL) which serves as a sustainable precursor naturally enriched with carbon quantum dots (CQDs); this work illustrates a successful fabrication of polymer composite films via simple casting technique. The structural, optical, and optoelectronic characteristics of the pristine and composite films were extensively evaluated. Fourier-transform infrared (FTIR) analysis confirmed that MuL dye enriched with CQD contains abundant NH, OH, and C=O functional groups. The naturally MuL dye enriched with CQDs exhibited distinct blue photoluminescence with an emission peak at 461 nm and a quantum yield of 1.76%, showcasing their viability for eco-friendly optical uses. Ultraviolet–visible (UV–Vis) spectroscopy investigation on MuL dye enriched with CQD particles illustrated clear peaks associated π → π* and n → π*. The FTIR study of the films confirmed strong physical interactions and hydrogen bonding between the abundant functional groups present in the MuL dye enriched with CQD particles and the CS matrix. UV–Vis study revealed that enriching the CS matrix with the MuL dye enriched with CQDs significantly shifted the absorption edge to lower photon energies. Most notably, the electronic band gap (Eg) drastically decreased from 5.45 eV in pristine CS down to 2.1 eV in the doped composites. The doping process increased the refractive index from 1.16 to 1.22 and enhanced the dielectric constant. Advanced modeling using the Wemple–DiDomenico (WDD), Tauc, and Absorption Spectra Fitting (ASF) methods provided deeper insights into dispersion energies and the nature of the electronic transitions. The study successfully connected these macroscopic optical parameters, such as phase velocity, to microscopic properties like charge carrier mobility. Other key parameters in optical properties including; N/m*, ε∞, τ, μopt, ρopt, and ωp were evaluated, while the WDD model provided energies of dispersion (Ed), oscillator (Eo), and static index of refraction (no). Optoelectronic parameters such as thermal emissivity (εth) and sheet resistance (Rs) from Kirchhoff’s function were evaluated and studies as a function of wavelength. The figure of merit (FOM) main peak appeared was used to separate the region of photons of electromagnetic radiation that are inefficient to transfer the electrons from the valence band to conduction band.