New Insights\nin Luminescence and Quenching Mechanisms\nof Ag<sub>2</sub>S Nanocrystals through Temperature-Dependent Spectroscopy

Bright near-infrared-emitting\nAg<sub>2</sub>S nanocrystals\n(NCs)\nare used for in vivo temperature sensing relying on a reversible variation\nin intensity and photoluminescence lifetime within the physiological\ntemperature range. Here, to gain insights into the luminescence and\nquenching mechanisms, we investigated the temperature-dependent luminescence\nof Ag<sub>2</sub>S NCs from 300 to 10 K. Interestingly, both emission\nand lifetime measurements reveal similar and strong thermal quenching\nfrom 200 to 300 K, indicating an intrinsic quenching process that\nlimits the photoluminescence quantum yield at room temperature, even\nfor perfectly passivated NCs. The low thermal quenching temperature,\nbroadband emission, and multiexponential microsecond decay behavior\nsuggest the optical transition involves strong lattice relaxation,\nwhich is consistent with the recombination of a Ag<sup>+</sup>-trapped\nhole with a delocalized conduction band electron. Our findings offer\nvaluable insights for understanding the optical properties of Ag<sub>2</sub>S NCs and the thermal quenching mechanism underlying their\ntemperature-sensing capabilities.