Bistable Switching between Low and High Absorbance States in Oleate-Capped PbS Quantum Dots

We report hysteretic photophysical behavior over a 110 K temperature range in freshly prepared colloidal suspensions of PbS/oleic acid nanocrystals. A bistable regime consisting of both low and high absorbance states of the PbS narrocrystals is observed between 215 and 240 K. The change in absorbance is significant (increasing by at least a factor of 5) and is shown to be entirely nonexcitonic in origin. The absorbance change is correlated with a thermal hysteresis in the photoluminescence quenching rate associated with a sudden switching of the quenching behavior upon cooling below 220 K. Most surprisingly, these effects are temporary, resulting in either the complete destruction of the smallest nanocrystals or stabilization of the larger nanocrystals after many weeks. We attribute this anomalous behavior to a structural phase change in the oleate surface ligands. This phenomenon thus illustrates the potentially large effect that surfaces can have on photophysical properties of PbS nanocrystals. Furthermore it opens up the possibility of surface engineering the photophysical properties of these materials for different applications such as photovoltaics.