Dual roles of a transparent polymer film containing dispersed N-doped carbon dots: A high-efficiency blue light converter and UV screen

We report the synthesis of a transparent plastic material with very high performance ultraviolet (UV)-blocking and blue-light (440 nm) emission. Polyvinyl alcohol (PVA) was employed as a transparent plastic matrix that disperses N-doped carbon dots (N-CDs) prepared via hydrothermal treatment of citric acid, ethylene diamine, and HCl solution. Luminescence of these N-CDs is excitation-independent and the quantum yield (QY) is maximal at an excitation wavelength range of 350-370 nm, in the UV-A radiation segment of the solar spectrum. By encapsulating N-CDs in a polyvinyl alcohol (PVA) matrix, the absolute QY achieves 91%, which is higher than in aqueous solution. As the particle concentration increases in PVA matrix or in solution, UV absorbance increases and QY decreases. On the other hand, UV absorbance of the film is proportional to thickness with no appreciable deterioration in QY, a quality that is beneficial for UV light absorption as well as conversion into blue light. We propose that this N-CD/PVA composite is an ideal transparent plastic for UV-ray shielding, as well as being useful for blue light emission, composed only of eco-friendly nontoxic elements. The high UV absorption and strong luminescence of N-CDs are beneficial also for other applications that include anti-counterfeiting (secret inks) and greenhouse sheathing that simultaneously blocks UV-rays while generating blue light to promote plant growth.