Repulsive, Densely Packed Ligand-Shells Mediate Interactions between PbS Nanocrystals in Solution

The surface capping ligands on nanocrystals (NCs) impart colloidal stability and determine the organization into ordered solids when cast from solution. Here, we use small-angle neutron scattering (SANS) to measure the solution structure factor in dispersions of fully passivated, oleate-capped PbS NCs up to concentrations of 16 v/v%. Fitting the data by assuming an attractive square well interaction potential between NCs yields a repulsive core diameter larger than the physical NC core diameter. This repulsive core stems from a densely packed region of the ligand-shell near the NC surface. The spatial extent of the repulsive core increases with temperature as ligand motion increases, becoming repulsive farther out in the ligand-shell, while the portion of ligand-shell outside of the repulsive region remains attractive with a strength of ∼1 k B T . SANS provides a molecular interpretation of the fundamental interparticle interactions in colloidal nanocrystal suspensions, ultimately relevant to understanding the self-organization of nanoscale materials.