Centralized Excited States and Fast Radiation Transitions Reduce Laser Threshold in Carbon Dots

Abstract As a new type of solution‐processed nano‐laser material, carbon dots (CDs) have shown considerable potential in optical communication, laser displays, micro/nano processing, and biomedicine. Reducing the laser threshold of the gain material is of considerable significance for further development of CDs’ applications in the field of micro/nano lasers. A series of blue‐emissive CDs (B‐CDs) are synthesized by changing the molar ratios of the precursors (citric acid (CA): L‐Cysteine (L‐Cys)). B‐CDs have a structure of carbon nanoparticles with their surface being modified with 5‐oxo‐3,5‐dihydro‐2Hthiazolo [3,2‐a]pyridine‐7‐carboxylic acid (TPCA). The laser can only be generated when the molar ratio of the precursors is between 1:1 and 2:1. With an increase in this ratio, the laser threshold decreases from 341.6 to 165.5 mJ cm −2 . The decrease in the laser threshold is attributed to the increase in the radiation transition rate and centralized sp 3 ‐related excited state levels, which are favorable for light amplification and population inversion. These results will be instructional for the reasonably design of CDs‐based laser materials and prompt their potential use in practical photonics.