Eco-friendly fabrication of flag blue flower-derived CQD-doped chitosan films with tunable optical and photoluminescent properties

This study explores the influence of carbon quantum dots (CQDs) synthesized from flag blue (FB) flower dyes on the optical band gap reduction of chitosan (CS) biopolymer. The resulting CQDs were characterized using fluorescence spectroscopy, XPS, XRD, HR-TEM, FTIR, and UV-Vis spectroscopy. Fluorescence measurements revealed a pronounced emission peak at 415 nm when excited at 330 nm, with a quantum yield of 7.41%. The HR-TEM and XRD analyses confirm the successful formation of uniformly dispersed, nanosized CQDs. XPS analysis was conducted to determine the elemental composition and surface functionalities of the CQDs. The characteristic absorption peak of the FB dye vanished in the CQDs spectrum, indicating successful carbonization and transformation of the dye molecules into carbonaceous nanostructures. The interactions of CQDs with the functional groups of the CS host polymer were examined through FTIR spectroscopy. Shifts and intensity changes in the FTIR bands of the CS polymer upon incorporation of CQDs demonstrated the formation of strong intermolecular interactions between chitosan and CQDs derived from FB dye. UV-Vis spectroscopy revealed that the optical properties of the films could be tailored by adjusting the CQDs concentration. Essential optical parameters-absorption edge, refractive index (n), dielectric loss (ε_i), energy gap (Eg), Urbach energy (Eu), extinction coefficient (k), plasma energy (Ψ), Fermi energy (E_f), and Penn energy (Ep)-were determined for both pure CS and composite films. The doped CS films exhibited a reduced optical band gap of 2.55 eV, indicating enhanced optoelectronic performance. The Wemple-DiDomenico (W-D) model further supported the modification of electronic transitions, confirming that the incorporation of CQDs effectively tunes the optical behavior of CS films for potential optoelectronic applications.