Alteration of Optical and Photoluminescence Properties of Biomass-Derived Carbon Quantum Dots: Engineering Large Stokes Shift

Carbon quantum dots (CQDs) have attracted considerable interest as functional luminescent materials for energy conversion and optoelectronic applications. In this study, a low-cost approach was developed for the synthesis of nitrogen-doped CQDs (NCQDs) from waste biomass using polyethyleneimine as the nitrogen precursor. The structural and optical properties of the synthesized NCQDs were systematically investigated as a function of synthesis temperature. The average particle sizes were 3.65, 3.25, and 2.72 nm for synthesis temperatures of 160, 180, and 200 °C, respectively. Energy-dispersive X-ray spectroscopy confirmed the presence of C, O, N, Mg, K, Cl, and Na in the synthesized materials. Nitrogen incorporation produced a pronounced modification of the Stokes shift over the investigated excitation range of 300–480 nm. The maximum Stokes shift of 286.07 nm was obtained for the NCQDs synthesized at 160 °C under 300 nm excitation. Compared with the corresponding pristine CQDs, nitrogen doping increased the Stokes shift by 16.85–47.03 nm, with the largest enhancements of 29.51, 42.17, and 47.03 nm observed at 400 nm excitation for synthesis temperatures of 160, 180, and 200 °C, respectively. The corresponding optical band gaps were 2.64, 2.60, and 2.86 eV, respectively. These results demonstrate that polyethyleneimine-assisted nitrogen incorporation provides an effective strategy for tuning the optical and photoluminescence properties of biomass-derived CQDs, particularly their Stokes shift, highlighting their potential for further investigation in luminescent and wavelength-conversion applications.