Photoluminescence and Photodetecting Properties of the Hydrothermally Synthesized Nitrogen-Doped Carbon Quantum Dots

Carbon quantum dots (CQDs) have attracted more and more attention as the representative of a new generation of photoluminescence (PL) and photodetecting materials due to their unique optoelectrical properties. However, the formation mechanism of the CQDs as well as the origin of the PL from the CQDs are still open questions to be issued. Here, we report our recent progress on the synthesis of the nitrogen-doped carbon quantum dots (N-CQDs) with a high photoluminescence quantum yield (PLQY) of 97.4% by adjusting the hydrothermal synthesis parameters with citric acid (CA) and ethylenediamine (EDA) as the precursors. The detailed structure and properties indicate that N-CQDs are synthesized by dehydration, condensation, and carbonization, and the PL is attributed to the synergistic effect of the carbogenic core and the surface/molecule state. With the above progress, an all-carbon-based ultraviolet (UV) photodetector is fabricated based on the N-CQDs/graphene hybrid composites, which exhibits a significant negative photoconductivity phenomenon. A maximal negative responsivity up to 2.5 × 10⁴ AW^–1 in the UV region has been observed, which was attributed to the two competing mechanisms. One is the oxygen adsorption and photodesorption induced negative photoresponse, while the other is the surface defects in N-CQDs related positive photoconducting. Our work reveals the mechanisms driving force behind the positive and the negative photoconductance phenomenon of photodetectors based on CQDs, which not only contributes to further understanding of the fluorescent and photoresponse mechanisms of CQDs, but also promotes the application potential of CQDs in the field of photodetection and nano-optoelectronic sensors.