Photostability of Colloidal PbSe and PbSe/PbS Core/Shell Nanocrystals in Solution and in the Solid State

The photostability of semiconductor PbSe nanocrystals (NCs) under different storage conditions and the photoluminescence of PbSe NC films on Si substrates under vacuum and under different atmospheres were investigated. The NCs show a small blue shift in both the emission and absorption spectra when stored in solution. This blue shift is accelerated when the particles are stored in room light compared to when the particles are stored in the dark, which indicates that photooxidation is an important process. The photooxidation process is accelerated by irradiating NC solutions with a 450 W xenon lamp. The photobleaching and appearance of a precipitate are attributed to ligand desorption followed by agglomeration. Reversible O₂-induced luminescence quenching was observed on a PbSe NC film on Si substrates, which could be addressed by two mechanisms:  indirect quenching by alteration of PbSe charging and neutralization dynamics or quenching through direct interaction of O₂ with the exciton. To improve the photostability of PbSe NCs, PbSe/PbS core/shell NCs using (TOP)S (TOP = trioctylphosphine) and TMS₂S (TMS₂ = bis(trimethylsilyl)) as sulfur sources were prepared. However, PbSe/PbS core/shell NCs did not show an increased stability in solution upon irradiation with a xenon lamp when compared with PbSe core NCs. Either the PbS shell is not able to confine the charge carriers or there is incomplete shell passivation with the PbSe core, and as a result these core/shell NCs have comparable stability.