Colloidal Structure and Interactions of Quantum Dots Characterized by Neutron and X-Ray Scattering

The self-assembly and aggregation of colloidal particles such as quantum dots (QDs) are driven by complex interparticle interactions and dynamics. Experimentally studying these interactions is challenging due to the nanoscale and hybrid nature of QDs, which are composed of an inorganic core coated in an organic ligand shell. To address this gap, we performed complementary small-angle neutron scattering (SANS) and small-angle X-ray scattering (SAXS) measurements to investigate the colloidal ligand shell structure and interactions of PbS QDs and inform their self-assembly behavior.<br/><br/>We consider PbS QDs with core sizes between 4.8-7.4 nm, dispersed in toluene, hexane, and cyclohexane solvents. We probe the colloidal structure of both the inorganic core and organic ligand shell through SANS contrast matching experiments, where solvent deuteration is varied to resolve each component. After fitting appropriate models to the SANS data using Bayesian methods, we find a statistically significant correlation of the QD ligand shell thickness with core size. This correlation persists despite nearly identical passivation for all samples, confirmed by H¹-NMR. We also find significant differences in the solvent penetration into the ligand shell for each of the solvents studied. To quantify the colloidal interactions of the PbS QDs, we extract the second virial coefficient from SAXS measurements of colloids with varying concentrations. We then study how the ensemble-scale properties of a QD colloid translate to self-assembly behavior by comparing our measurements to self-assembled films of PbS QDs.<br/><br/>Our work leverages complementary SANS and SAXS measurements to study the colloidal structure and interactions of PbS QDs and inform their self-assembly behavior. In addition to thoroughly characterizing colloidal PbS QDs, we demonstrate the utility of these tools and analyses for studying structure-property relationships in other self-assembling colloidal systems of interest.