New Insights in Luminescence and Quenching Mechanisms of Ag₂S Nanocrystals through Temperature-Dependent Spectroscopy

Bright near-infrared-emitting Ag₂S nanocrystals (NCs) are used for in vivo temperature sensing relying on a reversible variation in intensity and photoluminescence lifetime within the physiological temperature range. Here, to gain insights into the luminescence and quenching mechanisms, we investigated the temperature-dependent luminescence of Ag₂S NCs from 300 to 10 K. Interestingly, both emission and lifetime measurements reveal similar and strong thermal quenching from 200 to 300 K, indicating an intrinsic quenching process that limits the photoluminescence quantum yield at room temperature, even for perfectly passivated NCs. The low thermal quenching temperature, broadband emission, and multiexponential microsecond decay behavior suggest the optical transition involves strong lattice relaxation, which is consistent with the recombination of a Ag⁺-trapped hole with a delocalized conduction band electron. Our findings offer valuable insights for understanding the optical properties of Ag₂S NCs and the thermal quenching mechanism underlying their temperature-sensing capabilities.