Zinc-Reduced Carbon Quantum Dots as Color-Conversion Materials in Blue Light-Emitting Diodes

Carbon quantum dots (CQDs) were prepared by a thermal cracking method; amino-modified CQDs (NH2-CQDs) were synthesized by a hydrothermal method, and zinc-reduced CQDs (Zn-CQDs) were synthesized by a mild method. Optical properties and stability are two key issues for further applications of CQDs. In this study, we report a simple and effective method to simultaneously improve the optical properties and stability of CQDs. We generated Zn-CQDs by reacting zinc powder with CQD solution, which was operated under mild conditions and completed in a short time. These three materials were placed in an air environment for 48 h. The results show significantly improved moisture resistance of Zn-CQDs compared to the other two materials. The exciton binding energy of Zn-CQDs was calculated by the temperature-dependent fluorescence emission spectra to be 208.7 meV, which is much higher than those of 49.9 meV for CQDs and 46.6 meV for (NH2)-CQDs. This indicates that Zn-CQDs exhibit more stable internal exciton states, a higher degree of crystal ordering, and better moisture resistance. Due to the good moisture resistance and excellent photoluminescence properties of Zn-CQDs, their solid powders were used as color conversion materials for making fluorescence-excited LED devices. Finally, a blue light-emitting LED with a stable and bright color was obtained.