Xylan-Derived Carbon Dots with Tunable Fluorescence for White Light Emitting Diodes

High-value conversion of agroforestry residues into carbon dots (CDs)-based phosphors for white light emitting diodes (WLEDs) suffers from difficulty in adjusting fluorescence from the blue to red region. In this paper, xylan from sugar cane bagasse was utilized as a precursor to fabricate blue (B), green (G), yellow (Y), and red (R) fluorescent CDs with p-phenylenediamine by solvent modulation. The quantum yields of B-, G-, Y-, and R-CDs were 23.5%, 4.5%, 2.2%, and 15.4%, respectively. The formation and tuning mechanism of multicolor CDs were elucidated by structural characterization and density-functional theory (DFT) calculations. The results showed that the increased carbonization of CDs and graft of C═O on the CD surface by xylan passivation are two key reasons for the red-shift of fluorescence emission. WLEDs with color coordinates located at (0.30, 0.31) were successfully prepared by B-, G-, and R-CD/epoxy composites. In addition, the G-CD/epoxy composite demonstrated phosphorescence property and potential application in information security encryption. This study provides new avenues for the high-value utilization of agroforestry residues.