Anomalous Size-Dependent Decay of Low-Energy Luminescence from PbS Quantum Dots in Colloidal Solution

We report on an anomalous size dependence of the room-temperature photoluminescence decay time from the lowest-energy state of PbS quantum dots in colloidal solution, which was found using the transient luminescence spectroscopy. The observed 10-fold reduction in the decay time (from similar to 2.5 to 0.25 mu s) with the increase in the quantum dots' diameter is explained by the existence of phonon-induced transitions between the in-gap state-whose energy drastically depends on the diameter-and the fundamental state of the quantum dots.