Changes without Exchanges: Modification of Native Custom Glycolic Ligands on Colloidal Cadmium Chalcogenide Quantum Dots

Abstract

Alteration of surface-bound molecules (ligands) in colloidal quantum dot (QD) samples has traditionally relied on removal and replacement of native ligands with new moieties tailored for a specific purpose (altered solubility, particle−particle separation distance, etc.). This process, known as the ligand exchange, has proven to be successful and invaluable in many respects but is limited in that it can often be finicky to accomplish in solution and can introduce new surface trap defects that may negatively affect the optoelectronic properties. Here, we carry out a start-to-finish colloidal QD synthesis and subsequent ligand alteration without removal of the ligands used in the synthesis of particles, an approach we call on-particle ligand modification (OPLM). Specifically, we develop a new preparation method for cadmium chalcogenide QDs utilizing 9,10-dihydroxystearic acid, a fatty acid glycol, as the sole ligating species. In our OPLM procedure, we are able to treat these colloidal particles with an oxidant capable of cleaving the surface-bound glycol group to the corresponding aldehydes, verified with FTIR spectroscopy, thereby shortening the ligand length by roughly half without ever removing the original ligating head groups. The absorbance and fluorescent properties of the sample remain largely preserved throughout the OPLM process and both pre-and post-treatment samples are colloidally stable. We suggest that this OPLM approach is extended to further QD and ligand systems as an alternative to ligand exchange procedures when applicable, with this report serving as a proof of principal.