Photobrightening and photodarkening in PbS quantum dots

Fluorescence spectroscopy is utilized to investigate photodarkening and photobrightening behaviors in PbS quantum dots (QDs) subjected to various environmental conditions. We are able to separate contributions from charge trapping to a long-lived optically dark state (single particle fluorescence blinking) and irreversible photooxidation to the overall photodarkening behavior. Both processes produce effects that are potentially detrimental for emission-based technological applications. Charge trapping is the dominant mechanism on short time scales (< 3 s), exhibits no particle size- or environmental-dependence, is reversible, and is an order of magnitude faster compared to CdSe QDs. Photooxidation is the dominant mechanism on long time scales (50-100 s), is strongly dependent on particle size and environmental atmosphere, and results in irreversible decreases in emission intensity, large blue shifts of emission maximum, and increases in particle size distribution.