Supramolecular Gelation of Cadmium Oleate in the Synthesis of Nanocrystals for Applications in Photonics and Optoelectronics

Abstract

Cadmium oleate is widely used as a cation precursor in the synthesis of cadmium chalcogenide nanocrystal quantum dots (QDs) for a broad range of photonic and optoelectronic applications. Cd oleate can noncovalently assemble to form a supramolecular coordination gel, or metallogel, in solvents commonly used to disperse oleate-capped QDs. The gelation severely impedes the purification of oleate-capped QDs from excess Cd oleate, resulting in a gelled product that cannot be reliably characterized or used in further synthesis reactions. Here, we investigate the Cd oleate gel to gain insights into its viscoelastic properties and behavior under conditions relevant to QD synthesis, purification, and storage. We then examine how to effectively mitigate gelation by adding oleylamine as an additional ligand to disrupt noncovalent assembly. We synthesize PbS/CdS core/shell QDs via cation exchange as a case study to illustrate gelation of the reaction product and further demonstrate how this issue can be resolved through a better understanding of the supramolecular gel.