Pb‐Doped Ag 2 Se Quantum Dots with Enhanced Photoluminescence in the NIR‐II Window

Ag 2 Se quantum dots (QDs) as an effective biological probe in the second near‐infrared window (NIR‐II, 1000–1700 nm) have been widely applied in bioimaging with high tissue penetration depth and high spatiotemporal resolution. However, the ions deficiency and crystal defects caused by the high Ag + mobility in Ag 2 Se crystals are mainly responsible for the inefficient photoluminescence (PL) of Ag 2 Se QDs. Herein, a tailored route is reported to achieve controllable doping of Ag 2 Se QDs in which Ag is exchanged by Pb via cation exchange (CE), which is unattainable by direct synthetic methods. The Pb‐doped Ag 2 Se QDs (denoted as Pb:Ag 2 Se QDs) present fire‐new optical features with significantly enhanced PL intensity of 4.2 folds. Photoelectron spectroscopy confirms that Pb acts as an n‐type dopant for Ag 2 Se QDs and therefore the electronic impurities provide additional carriers to fill the traps. Moreover, the general validity of this method is demonstrated to convert different sized Ag 2 Se into Pb:Ag 2 Se QDs, so that a wide range of NIR‐II PL with high intensity is obtained. The bright NIR‐II emission of Pb:Ag 2 Se QDs is further successfully performed in lymphatic system mapping.