Core/shell nanocrystals are attractive for optoelectronic applications because they have superior properties as compared to simple core nanocrystals, such as improved chemical stability, increased photoluminescence quantum yield (QY), reduced Auger recombination rates, as well as larger Stokes shifts. In particular, PbS/ZnS core/shell nanocrystals are appealing because ZnS shells typically increase nanocrystal photoluminescence QYs and are less toxic than the usual CdS shells used on PbS nanocrystals. Additionally, PbS/ZnS core/shell particles are of scientific interest since the band alignment between the PbS core and the ZnS shell is currently unknown. Nevertheless, PbS/ZnS core/shell nanocrystals have not been previously produced due to their tendency to Ostwald ripen at moderate temperatures. Here we present the first demonstration of PbS/ZnS core/shell nanocrystals using a low temperature (below 80 °C) colloidal synthesis with thioacetamide and zinc oleate as precursors.<br/><br/>We have characterized the PbS/ZnS core/shell nanocrystals both structurally and optically. Transmission electron microscopy along with energy-dispersive X-ray spectroscopy demonstrate that submonolayer to multiple monolayers of ZnS can be deposited on the PbS cores depending on the reaction conditions. Upon ZnS shell formation the nanocrystal absorption and photoluminescence peak energies both redshift and the photoluminescence QY decreases. In addition, time-resolved photoluminescence indicates that the PbS/ZnS core/shell nanocrystals of these sizes are Type I, and that non-radiative traps form upon shell deposition which diminish the photoluminescence QY. Furthermore, using X-ray diffraction we have demonstrated that the excitonic transitions red shift with shell formation due to the hydrostatic pressure the ZnS shell applies to the PbS core. Finally, we show that these PbS/ZnS core/shell nanocrystals can be transferred via a ligand exchange into an aqueous buffer solution for potential applications in biological imaging.

Synthesis and characterization of PbS/ZnS core/shell nanocrystals

254th National Meeting (2017)

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