Experimental Evidence for Two-Dimensional Ostwald Ripening in Semiconductor Nanoplatelets

Colloidal semiconductor nanoplatelets are rectangular, quasi-two-dimensional crystallites that are only a few atomic layers thick. In the most heavily studied systems (such as CdE with E = S, Se, or Te), the highly anisotropic nanoplatelet shape forms from an isotropic cubic crystal lattice. This has been difficult to reconcile with standard nanocrystal growth models. Previously, we proposed that nanoplatelets arise due to an intrinsic kinetic instability that enhances growth on narrow crystal facets under surface-reaction-limited conditions. Here, we test this model experimentally by synthesizing small "baby" CdS nanoplatelets and performing ripening experiments. During heating, we observe transitions to thicker nanoplatelet populations that are consistent with two-dimensional Ostwald ripening. We then heat CdSe nanoplatelets in the presence of "baby" CdS nanoplatelets to form CdSe-CdS core-crown nanoplatelets. This indicates that individual nanoplatelets dissolve and transfer their material to thicker nanoplatelets, consistent with the kinetic instability model. Finally, we grow thin films of CdS nanoplatelets by combining "baby" CdS nanoplatelets with cadmium carboxylate and heating on a substrate. This shows that Ostwald ripening may also be exploited as a facile and versatile approach to nanoplatelet growth.