Ultrasmall Pb:Ag₂ S Quantum Dots with Uniform Particle Size and Bright Tunable Fluorescence in the NIR-II Window

Ag₂ S quantum dots (QDs) are well-known near-infrared fluorophores and have attracted great interest in biomedical labeling and imaging in the past years. However, their photoluminescence efficiency is hard to compete with Cd-, Pb-based QDs. The high Ag⁺ mobility in Ag₂ S crystal, which causes plenty of cation deficiency and crystal defects, may be responsible mainly for the low photoluminescence quantum yield (PLQY) of Ag₂ S QDs. Herein, a cation-doping strategy is presented via introducing a certain dosage of transition metal Pb²⁺ ions into Ag₂ S nanocrystals to mitigate this intrinsic shortcoming. The Pb-doped Ag₂ S QDs (designated as Pb:Ag₂ S QDs) present a renovated crystal structure and significantly enhanced optical performance. Moreover, by simply adjusting the levels of Pb doping in the doped nanocrystals, Pb:Ag₂ S QDs with bright emission (PLQY up to 30.2%) from 975 to 1242 nm can be prepared without altering the ultrasmall particle size (≈2.7-2.8 nm). Evidently, this cation-doping strategy facilitates both the renovation of crystal structure of Ag₂ S QDs and modulation of their optical properties.