Ultrasmall Pb:Ag2S Quantum Dots with Uniform Particle Size and Bright Tunable Fluorescence in the NIR‐II Window

Ag2S 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 Ag2S crystal, which causes plenty of cation deficiency and crystal defects, may be responsible mainly for the low photoluminescence quantum yield (PLQY) of Ag2S QDs. Herein, a cation‐doping strategy is presented via introducing a certain dosage of transition metal Pb2+ ions into Ag2S nanocrystals to mitigate this intrinsic shortcoming. The Pb‐doped Ag2S QDs (designated as Pb:Ag2S 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:Ag2S 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 Ag2S QDs and modulation of their optical properties. Both the crystal structure and optical properties of narrow bandgap semiconductor Ag2S QDs have a revolutionary change upon the doping of transition metal Pb2+ ions. The ultrasmall size (≈2.7–2.8 nm), bright tunable emission (975–1242 nm), excellent stability and decent biocompatibility of the Pb‐doped Ag2S QDs render this material a promising application prospect in in vivo fluorescence‐based imaging.