  Electrostatically stabilized nanocrystals (NCs) and, in particular, quantum
dots (QDs) hold promise for forming strongly coupled superlattices due to their
compact and electronically conductive surface ligands. However, studies on the
colloidal dispersion and interparticle interactions of electrostatically
stabilized sub-10 nm NCs have been limited, hindering the optimization of
colloidal stability and self-assembly. In this study, we employed small-angle X
ray scattering (SAXS) experiments to investigate the interparticle interactions
and arrangement of PbS QDs with thiostannate ligands (PbS-Sn2S64-) in polar
solvents. The study reveals significant deviations from ideal solution behavior
in electrostatically stabilized QD dispersions. Our results demonstrate that
PbS-Sn2S64- QDs exhibit long-range interactions within the solvent, in contrast
to the short-range steric repulsion characteristic of PbS QDs with oleate
ligands (PbS-OA). Introducing highly charged multivalent electrolytes screens
electrostatic interactions between charged QDs, reducing the length scale of
the repulsive interactions. Furthermore, we calculate the second virial (B2)
coefficients from SAXS data, providing insights into how surface chemistry,
solvent, and size influence pair potentials. Finally, we explore the influence
of long-range interparticle interactions of PbS-Sn2S64- QDs on the morphology
of films produced by drying or spin-coating colloidal solutions. The long-range
repulsive term of PbS-Sn2S64- QDs promotes the formation of amorphous films,
and screening the electrostatic repulsion by addition of an electrolyte enables
the formation of a crystalline film. These findings highlight the critical role
of NC-NC interactions in tailoring the properties of functional nanomaterials.
