The effect of oxygen on the microwave-assisted synthesis of carbon quantum dots from polyethylene glycol

Fluorescent carbon quantum dots (CQDs) were synthesized using polyethylene glycol (PEG) as both the carbon source and solvent in various atmospheres including air, nitrogen, carbon dioxide and oxygen by following a microwave irradiation method. The CQDs were characterized by performing ultravioletvisible absorption spectroscopy, Fourier transform infrared spectroscopy, transmission electron microscopy, and fluorescence spectroscopy. We found that different reaction atmospheres yielded significantly different reaction rates and properties of the CQDs. In particular, including oxygen in the synthesis of the CQDs clearly shortened the reaction time, significantly enhanced the photoluminescence (PL) intensity and quantum yield (PLQY) of the CQDs, and extended their average lifetime. The PLQY and fluorescence lifetime of the CQDs were improved from 5.78% and 2.4 ns, respectively, when synthesized in a medium open to air to 7.84% and 3.0 ns, respectively, when synthesized in an O-2 atmosphere. The CQDs synthesized using microwave irradiation showed strong blue fluorescence under ultraviolet light.