Carbon Quantum Dot Derived from the Extracted Natural Dye of Hybrid Poplar Waste Leafs as an Original Progress to Enhance the Optical Properties of PVA Polymer: Solvothermal Synthesis and Spectroscopic Characterization

This research demonstrates a green and facile single-step solvothermal method for the synthesis of carbon quantum dots (CQDs) using natural dye obtained from hybrid poplar (HP) leaf waste. The prepared CQDs were thoroughly characterized by FTIR, 1 H and 13 C NMR, UV–Vis absorption, and fluorescence emission spectroscopies. These spectroscopic techniques confirm the presence of abundant functional groups (–OH, –COOH, –C=O) on the surface of CQDs. These CQDs were subsequently incorporated into a polyvinyl alcohol (PVA) matrix to fabricate nanocomposite films and examine their impact on the structural, optical, and optoelectronic properties of the PVA polymer. X-ray diffraction (XRD) analysis revealed a gradual reduction in the crystallinity of PVA with increasing CQD content, indicating a disruption of polymer chain ordering and the formation of more amorphous phases. The integration of CQDs induced a progressive red-shift in the absorption spectra and significantly reduced the direct optical bandgap from 6.35 to 2.87 eV and the indirect optical bandgap from 5.85 to 2.5 eV, as estimated from Tauc plots. The Wemple–DiDomenico (W-DD) single oscillator model further confirmed enhanced interband electronic transitions, as evidenced by an increase in dispersion energy ( E d ) from 0.343 to 0.911 eV and a concurrent decrease in oscillator energy ( E o ) from 5.817 to 2.601 eV with increasing CQD concentration. Furthermore, the nanocomposite films exhibited markedly improved nonlinear optical (NLO) performance. Third-order nonlinear susceptibility (χ 3 ) increased by more than three orders of magnitude, from 9.74× 10 −17 to 1.32 × 10 −13 esu , while the nonlinear refractive index ( n2 ) rose from 0.027 to 0.167. These results were accompanied by enhancements in dielectric constants, refractive index, plasma frequency (from 2.89 × 10 15 to 3.78 × 10 15 rad/s), effective carrier concentration (from 2.90 × 10 57 to 4.96 × 10 57 ), and Fermi energy (from 0.698 to 0.997 eV), indicating a significant modification in charge carrier dynamics and light–mater interactions within the PVA matrix.