Influence of ligand shape and steric hindrance on the composition of the nanocrystal ligand shell

Organic ligands play a key role in the synthesis of colloidal semiconductor nanocrystals or quantum dots. Generally they consist of a functional group and an aliphatic chain, with carboxylic acids, thiols and phosphonic acids as typical examples. The functional group ensures the binding to the nanocrystal surface, while the stability of the dispersion strongly depends on the interactions between the organic chains of the adjacent ligands. A number of studies already addressed the binding strength and the type of binding between the nanocrystal surface and the ligand yet none discuss the effect of the organic chain on the ligand exchange. By means of NMR spectroscopy, we examine the ligand shell composition of CdSe nanocrystals originally capped with oleic acid (OA), when exposed to a linear carboxylic acid. Regardless of chain length, we see a one-to-one exchange between the carboxylic acids. The composition of the ligand shell closely matches that of the ligand mixture in solution, indicating that the ligand shell can be seen as an ideal mixture of both ligands. As a consequence, a mixed ligand shell can easily be prepared by adding a ligand mixture with desired composition to the nanocrystal dispersion. On the other hand, when the CdSe nanocrystals are exposed to a branched carboxylic acid with two long aliphatic chains, like 2-hexyldecanoic acid, the ligand shell mainly consists of OA moieties. We interpret these results using an exchange process where the incoming ligand not only displaces oleic acid but also occupies additional space in the ligand shell to accommodate both aliphatic chains. Hence, given a one-for-one exchange reaction, steric hindrance in a fully packed ligand shell will prevent complete ligand exchange. These results can be very useful in view of producing nanocrystals with lower ligand densities by means of synthesis with these branched carboxylic acids.

Jaar van publicatie:2016