Engineering the Surface of PbS Quantum Dots—Insights from Solution NMR and Small-Angle Neutron Scattering

PbS quantum dots (QDs) possess tunable absorbance and emission in the near-infrared spectrum and are of interest for next-generation photovoltaic and photodetector devices. Like all QDs, PbS QDs are coated in organic ligands which enable solution processability, passivate defect sites, and guide self-assembly into ordered superlattices. A common method of synthesizing PbS QDs uses an excess of PbCl₂ to produce PbS-eCl QDs with record monodispersity and improved air stability. Although prior studies of PbS-eCl QDs suggest that they are coated by a PbCl₂ shell and passivated by oleic acid ligands, a recent study by Green, Wilson, and coworkers demonstrated that PbS-eCl QDs are actually passivated by oleylammonium-chloride.¹ Building upon this study, we refined a ligand exchange method for these QDs and found that the hallmark PbCl₂shell of PbS-eCl QDs is highly sensitive to the presence of excess PbCl₂ during ligand exchange.<br/><br/>We detail our findings studying the surface of eCl-PbS QDs, including our ligand exchange methodology, characterization of the PbCl₂ shell, and evidence of defect passivation from photoluminescence quantum yield (PL QY) measurements. We observe that the native oleylammonium-chloride ligands of PbS-eCl QDs can be exchanged with an alkylamine-carboxylic acid pair. From 1D and 2D NMR experiments, we find that the new ligands bind in dynamic equilibrium with the surface and completely displace the native ligands. However, small-angle neutron scattering (SANS) measurements reveal that the PbCl₂ shell is absent from PbS-eCl if the excess PbCl₂ remaining from the synthesis is removed from solution prior to ligand exchange. Interestingly, we measure a PL QY &gt; 50% for PbS-eCl synthesized both with and without the PbCl₂ shell a few weeks after synthesis, but further experiments are needed to assess the stability of the QDs. Our study indicates that despite their complex surface chemistry, eCl-PbS QDs can be engineered with precision through careful washing and ligand exchange methodologies.<br/><br/>¹ Green, P. B. <i>et al.</i> PbS Nanocrystals Made Using Excess Lead Chloride Have a Halide-Perovskite-Like Surface. <i>Chem. Mater.</i> <b>33</b>, 9270–9284 (2021).