Seed‐Mediated Synthesis Strategy for Monodisperse Ag ₂ Se Colloidal Quantum Dots with Extended Near‐Infrared‐II Emission

Ag₂Se colloidal quantum dots (QDs) are promising as the second near-infrared (NIR-II) emissive materials in biotechnology and photonics applications due to their suitable bandgap and low toxicity. However, achieving precise synthesis control of Ag₂Se QDs remains a significant challenge owing to the ultralow solubility and the fast crystal growth. Herein, a novel seed-mediated synthesis strategy is proposed for Ag₂Se QDs, featuring the initial synthesis of seeds through a hot-injection method and a subsequent slow second growth with continuous addition of precursors. This strategy effectively separates nucleation and growth, yielding monodisperse Ag₂Se QDs with precise size control, as evidenced by the relative standard deviation of 6.9% in size for QDs with a diameter of ≈8 nm. Moreover, depending on the cation exchange strategy with Au atoms to improve the photoluminescence quantum yield (PL QY) of Ag₂Se QDs, this study further fabricates a series of highly luminescent alloyed AgAuSe QDs with extended NIR-II emission from 1010 to 1320 nm and a record absolute PL QY of 44% at 1300 nm. The developed strategy has offered a new opportunity for the precise synthesis control in NIR-II Ag₂Se QDs and pushing the frontier of environmentally friendly QDs into the future optical and optoelectronic applications.