Synthesis and Characterization of PbS/ZnS Core/Shell Nanocrystals

We demonstrate a synthetic method to add a ZnS shell, with controlled thickness, to PbS nanocrystals using Zn oleate and thioacetamide as Zn and S precursors. The ZnS shell reaction is self-limiting and deposits approximately a monolayer of ZnS per shell reaction without causing the PbS nanocrystals to Ostwald ripen. The reaction is self-limiting because the sulfur precursor, thioacetamide, is less reactive toward the PbS/ZnS core/shell nanocrystal surface as compared to the Zn oleate precursor present in the reaction solution. To increase the ZnS shell thickness beyond a monolayer, subsequent ZnS shell reactions are modified by adding the thioacetamide 10 minutes before the Zn oleate. This gives the thioacetamide time to react at the PbS/ZnS core/shell nanocrystal surface before the Zn oleate is added. High angle annular dark field scanning transmission electron microscopy (HAADF-STEM) shows most ZnS shells lack crystalline order. However, select core/shell nanocrystals have epitaxial crystalline (zinc-blende) ZnS shells or crystalline (zinc-blende) shells with no obvious epitaxial relationship to the PbS core. The PbS core 1Sh–1Se absorbance and photoluminescence peak energies redshift upon shell addition due to relief of a ligand-induced tensile strain and wave function leakage into the shell. The photoluminescence quantum yield decreases after ZnS shell addition likely due to nonradiative defect states at the core/shell interface.