Facile One‐Pot Microwave‐Assisted Synthesis and Ultrafast Spectroscopic Characterization of Nitrogen–Sulfur‐Codoped Carbon Quantum Dots

We report an eco-friendly, cost-effective, one-pot microwave-assisted synthesis of nitrogen and sulfur co-doped carbon quantum dots (N,S-CQDs) using citric acid and thiourea in formamide without surface passivation. The N,S-CQDs were characterized by HRTEM, FE-SEM, XRD, EDX, FTIR, Raman, and XPS, confirming monodispersed spherical particles of 4.8 nm with an amorphous carbon phase containing oxygen, nitrogen, and sulfur. The comprehensive photophysical studies of N,S-CQD employed by steady state and different time-resolved spectroscopic techniques (TCSPC, Ultrafast Time-Resolved Fluorescence Up-Conversion and Femtosecond Transient Absorption techniques). These N,S-CQDs show broad UV-visible to near-infrared absorption with peaks near 300 and 400 nm and emit strong blue photoluminescence at 360 nm excitation, with a quantum yield of ~8.4%. Time-resolved spectroscopy (TCSPC, fluorescence up-conversion, transient absorption) reveals multiexponential carrier relaxation with time constants from 0.5 ps to > 500 ps, including a 360 ps rise component and three distinct decay components, indicating complex fluorescence driven by surface defects. Ultrafast decay components correspond to thermal cooling of hot excitons, while later decays relate to carrier trapping at surface states. The tunable optical properties and carrier dynamics make N,S-CQDs promising for optoelectronic applications such as LEDs, sensors, and photodetectors, with further enhancement possible through surface engineering and defect control.