Hydrothermal Synthesis of High-Yield Red Fluorescent Carbon Dots with Ultra-Narrow Emission by Controlled O/N Elements

Red fluorescent carbon dots (r-CDs) with narrow dual emissions (600 nm and 658-683 nm, full width at half-maximums (FWHMs) of 20 nm and 30 nm), fluorescence quantum yield of 41.0%, and yield of 83.3% are prepared by hydrothermal method using o-phenylenediamine as precursor and inorganic oxidant as yield enhancer, and they have graphite nitrate-like structures. The long-wavelength side emission is aggregation-induced emission (AIE). A logarithmic relationship between the AIE wavelength (y) and the concentration (x) (y = 8.853ln(x) + 688.53, R = 0.998) is found. This regularity and the high monochromaticity of AIE are related to the existence of highly ordered structures proved by X-ray diffraction. Its intrinsic emission (FWHM: 20 nm) is the narrowest among the r-CDs prepared by hydrothermal method. The reason is that the decrease of oxygen content makes the FWHMs become narrow, and the decrease of the pyridine nitrogen content and the increase of pyrrole nitrogen content make them narrower further. The Fourier-transform infrared spectra and control experiment prove that oxidative polymerization is a necessary preparation step. The linear relationship between the amount of the oxidant and the CDs yield indicates that the yield can be increased only by increasing the conversion rate of the polymerization process.