Variable Surface Termination and Ligand Passivation of Lead Sulfide Nanocrystals Synthesized with Excess Lead Chloride

The surface termination and ligand passivation of semiconducting nanocrystals (NCs) impact the stability, optical properties, and self-assembly of NCs. In this work, we definitively characterize the surface of lead sulfide (PbS) NCs synthesized from excess PbCl2. With a combination of small-angle neutron scattering (SANS), photoluminescence, and 1D and 2D NMR experiments, we show that the surface termination of PbS NCs depends on the presence of PbCl2 during ligand exchange. When excess PbCl2 is removed prior to ligand exchange, PbS[RNH3+Cl–] NCs are obtained, which are terminated by a monolayer of PbClx on the {100}PbS facet and passivated by oleylammonium chloride ligands. On the other hand, when excess PbCl2 remains in solution during ligand exchange, lead oleate forms and attaches to the {111}PbS facets of PbS[RNH3+Cl–] NCs, creating PbS@PbClx NCs. PbS@PbClx NCs are coated in an epitaxial layer of PbClx on both the {100}PbS and {111}PbS facets, making them 0.3–0.4 nm larger on average than PbS[RNH3+Cl–] NCs with identical absorption wavelengths. Additionally, PbS@PbClx NCs have a consistently higher photoluminescence quantum yield and longer photoluminescence lifetimes than PbS[RNH3+Cl–] NCs. This study clarifies the surface structure of PbS NCs synthesized from excess PbCl2, highlighting ligand exchange strategies and reconciling observations from across the literature.