Nitrogen-doped carbon dots synthesized from lychee exocarp for LED down-conversion applications

Carbon dots (CDs) materials are commonly used as color conversion materials for light-emitting diode (LED) display. However, their poor resistance to water, oxygen and chemical reagents limits their applications in LED down-conversion coatings. To facilitate further processing and enhance their application, it is essential to encapsulate CDs with a solid substrate to preserve the original optical properties of CDs. In this study, lychee exocarp was used as the carbon source to prepare CDs via a solvothermal method. The as-synthesized CDs were deliberately doped with various nitrogen sources, including urea, ammonia and melamine (MA), to regulate the color emission behavior of CDs. Additionally, a widely-used water-soluble polyvinyl alcohol (PVA) was applied serving as the solid phase dispersion matrix for the CDs, resulting in CDs/PVA films with excellent fluorescence and biocompatibility. The morphological, structural, and physicochemical properties of the nitrogen-doped CDs were systematically investigated using transmission electron microscopy (TEM), fluorescence (FL) spectrum, and other methods. The synthesized CDs exhibit multi-color fluorescence upon UV excitationand demonstrate outstanding photostability. The absolute quantum yield of the CDs is relatively high, reaching up to 7.20%. This research results suggest that the synthesized CDs have potential applications in LED down-conversion.