Structure-property relationships in fluorescence of carbon dots from premixed ethylene flames

Structure-property relationships are essential for the efficient synthesis of fluorescent carbon dots by combustion-based methods. In this work, the structure-property relationships in fluorescence of carbon dots from premixed ethylene flames are investigated at different flame temperatures by applying both characterization methods and density functional theory (DFT) calculations. It is found that the fluorescence peak is blue-shifted and the quantum yield increases with higher flame temperature for carbon dots synthesized within the range of 1680-1770 K. Higher flame temperature can produce larger carbon dots with larger polycyclic aromatic hydrocarbons (PAHs) and their derivatives inside. The -OH content and the C-O/C=O ratio are higher in the high temperature sample. As precursors of carbon dots, PAH derivatives with C-O have larger HOMO-LUMO gaps compared with PAH derivatives with C=O, leading to the blue shift of the 1it -> it* transition in the UV-Vis spectra. It can be concluded that the conventional size effect is not the dominant factor controlling the blue-shifted fluorescence peak of carbon dots synthesized with higher flame temperature range of 1680-1770 K. More -OH in carbon dots from higher temperature flames can possibly lead to the higher quantum yield. The higher C-O/C=O ratio in carbon dots from higher flame temperature is likely to be the dominant factor governing the blue-shifted peaks in both UV-Vis and fluorescence spectra since it can enlarge the HOMO-LUMO gap and cause a blue shift of the 1it -> it* transition.