Ligand Shell Thickness of Colloidal Nanocrystals: A Comparison of Small-Angle Neutron and X-ray Scattering

Colloidal nanocrystals (NCs) are coated by an organic ligand shell that imparts colloidal stability, mediates self-assembly, and impacts functional properties. Despite the variety of methods to chemically characterize ligands, common structural characterization techniques like small-angle X-ray scattering (SAXS) and electron microscopy selectively resolve the NC core and can only indirectly infer the structure of ligands. Small-angle neutron scattering (SANS) can directly characterize the ligand shell structure of colloidal NCs, enabled by the unique sensitivity of SANS to organic molecules. In this work, we compare and contrast the information about the NC ligand shell gained directly through SANS and indirectly through SAXS. Monodisperse oleyl-capped PbS NCs were synthesized with varying core sizes (4.8 - 7.4 nm diameter) and solvents (toluene, n-hexane, cyclohexane). We then performed SANS to extract the ligand shell thickness and composition, SAXS to infer the ligand structure from NC interactions, and grazing-incidence SAXS to compare interparticle distances in self-assembled PbS NC superlattices. We observe with SANS that ligands extend up to 15% farther away from the NC surface with increasing core size over the size range studied, attributed to curvature effects that are not captured by the inferred structure from SAXS. We also see that the ligand shell thickness varies with solvent identity due to differences in how solvent molecules penetrate the ligand shell. In a detailed comparison, we demonstrate that SANS, SAXS, and GISAXS reveal distinct but complementary information about the ligand shell, enabling the holistic characterization of the structure-property relationships of NCs from colloid to self-assembled superlattice.