Engineering nitrogen-doped carbon quantum dots: Nitrogen content-controlled dual-phase emission behavior

Nitrogen doping is a widely used method for enhancing the performance of carbon quantum dots (CQD). However, the precise relationship between nitrogen content and emission spectra remains unclear when preparing high-performance nitrogen-doped CQD ( N -CQD). This study systematically investigates the effects of nitrogen content on the crystalline structure, optical properties, and electronic band structure of N -CQD. Citric acid was used as the carbon source, and ethylenediamine monohydrate was used as the nitrogen source, with their ratio controlled to hydrothermal synthesized N -CQD with N/C ratios ranging from 0 to 0.4. Notably, when the N/C ratio increases from 0 to 0.2, the N -CQD exhibits redshifted emission with excitation dependence. However, when the N/C ratio rises from 0.2 to 0.4, the N -CQD shows blueshifted emission with excitation-independence. We define it as the dual-phase emission behavior of N -CQD attributed to the transition of doping sites from graphitic nitrogen to pyridine nitrogen with increased nitrogen content. DFT calculations indicate that different doping sites influence electron transfer in N -CQD, resulting in distinct optical behaviors. Importantly, this work comprehensively explains the relationship between nitrogen content and the emission behavior of N -CQD for the first time, providing crucial insights for refining the theoretical framework of N -CQD.