Understanding and Controlling the Colloidal Stability of CdSe Nanoplatelets by Solvation Force Engineering

The interaction and steric stability of colloidal nanocrystals are determined by the interplay of various contributions, including van der Waals, dipole-dipole, and solvation forces. Recent simulations have unveiled that the solvation force dictates the colloidal stability of two-dimensional nanomaterials with no experimental validation. Here, we introduce optical-pump THz-probe spectroscopy (OPTP) as a novel approach to track the colloidal aggregation of two-dimensional nanoplatelets. We show that far below the saturation concentration previously determined by scattering methods, OPTP can already report nanoscale aggregations by sensitively probing the short-range free carrier photoconductivity arising from internanoplatelet electronic coupling. Combining the OPTP and molecular dynamics simulations allows us to (1) confirm that increasing the nanoplatelet base facet area results in enhanced solvation force and thus aggregation tendency, and (2) demonstrate that the attraction between the nanoplatelets increases with their chain length for n-alkane solvents. Finally, we extend our simulations to study the shape of the interaction that can be tuned via the isomer of the solvent molecules. Our results provide not only a new sensitive tool to probe the aggregation effects of semiconducting colloidal particles but also fundamental insights into the critical parameters to engineer colloidal stability.