Influence of Group Modification at the Edges of Carbon Quantum Dots on Fluorescent Emission

We present a detailed investigation on the effect of functional group modulation at the edges of carbon quantum dots (CQDs) on the fluorescence from the CQDs. The CQDs attached by N, S, and P elements are synthesized via pyrolysis of a mixture of citric acid and NH3H2O, H2SO4, and H3PO4, respectively. Thus, part of -COOH at the edges of CQDs can be converted into -C=O and functional groups such as -NH2, -SO2, -HSO3, and -H2PO4 can connect to the carbon bonds. We find that the formation of the N/S/P-CQDs can reduce the amount of -COOH that attaches to the edges of sp(2)-conjugated pi-domains located at centers of these CQDs. This effect can result in the reduction of the non-radiative recombination for electronic transition in these CQDs. As a result, the quantum yield (QY) for fluorescence from the CQDs can be efficiently enhanced. We demonstrate experimentally that the QYs for N/S/P-CQDs can reach up to 18.7%, 29.7%, and 10.3%, respectively, in comparison to 9% for these without functional group modulation. This work can provide a practical experimental approach in improving the optical properties of fluorescent CQDs.