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.