Colloidal\nDispersions of Sterically and Electrostatically\nStabilized PbS Quantum Dots: Structure Factors, Second Virial Coefficients,\nand Film-Forming Properties

Electrostatically\nstabilized nanocrystals (NCs) and, in particular,\nquantum dots (QDs) hold promise for forming strongly coupled superlattices\ndue to their compact and electronically conductive surface ligands.\nHowever, studies of the colloidal dispersion and interparticle interactions\nof electrostatically stabilized sub-10 nm NCs have been limited, hindering\nthe optimization of their colloidal stability and self-assembly. In\nthis study, we employed small-angle X-ray scattering (SAXS) experiments\nto investigate the interparticle interactions and arrangement of PbS\nQDs with thiostannate ligands (PbS–Sn<sub>2</sub>S<sub>6</sub><sup>4–</sup>) in polar solvents. The study reveals significant\ndeviations from the ideal solution behavior in electrostatically stabilized\nQD dispersions. Our results demonstrate that PbS–Sn<sub>2</sub>S<sub>6</sub><sup>4–</sup> QDs exhibit long-range interactions\nwithin the solvent, in contrast to the short-range steric repulsion\ncharacteristic of PbS QDs with oleate ligands (PbS-OA). Introducing\nhighly charged multivalent electrolytes screens electrostatic interactions\nbetween charged QDs, reducing the length scale of the repulsive interactions.\nFurthermore, we calculated the second virial (<i>B</i><sub>2</sub>) coefficients from SAXS data, providing insights into how\nsurface chemistry, solvent, and size influence pair potentials. Finally,\nwe explore the influence of long-range interparticle interactions\nof PbS–Sn<sub>2</sub>S<sub>6</sub><sup>4–</sup> QDs\non the morphology of films produced by drying or spin-coating colloidal\nsolutions. The long-range repulsive term of PbS–Sn<sub>2</sub>S<sub>6</sub><sup>4–</sup> QDs promotes the formation of amorphous\nfilms, and screening the electrostatic repulsion by the addition of\nan electrolyte enables the formation of crystalline domains. These\nfindings highlight the critical role of NC–NC interactions\nin tailoring the properties of functional materials made of colloidal\nNCs.