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

Abstract Ag 2 Se 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 Ag 2 Se 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 Ag 2 Se 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 Ag 2 Se 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 Ag 2 Se 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 Ag 2 Se QDs and pushing the frontier of environmentally friendly QDs into the future optical and optoelectronic applications.