I synthesized Carbon Quantum Dots (CQDs) using microwave irradiation (700 W, 1 h) with citric acid as the precursor. The resulting material formed a brownish residue at the bottom of the container instead of the expected black powder CQDs reported in the literature. However, the product still exhibits green fluorescence. Could this color difference indicate incomplete carbonization or another issue? How can I improve the synthesis to obtain the desired black CQDs?