A tale of tails – A physico-chemical surface study of CdSe NCs ligand exchange reactions from Oleic acid to n-Alkylthiols

The nanocrystals (NCs) surface and their ligand layer have a significant contribution in controlling the NCs size, shape and optoelectronic properties. This leads to their important role in NCs applicability for biological tagging, photocatalysis, electronic devices and more. Yet, the NC surface properties and the interactions with the ligands still remain under-explored.

In this work, we combined several methods for studying both the mechanism and the thermodynamics of a surface ligand exchange reaction on CdSe NCs from oleic acid to various alkylthiols. An exchange mechanism involving a proton migration from the alkylthiol to the released carboxylate ligand was determined by nuclear magnetic resonance spectroscopy (NMR). A complete set of the thermodynamic parameters was extracted from isothermal titration calorimetry (ITC) measurements. We observed a ligand length dependency along with enthalpy-entropy compensation behaviour, associated with inter-chain interactions of the bound ligands and their organization on the NCs. Several NC sizes were studied, which revealed an increase in the spontaneity of the reaction with decreasing NCs size due to their increased reactivity. The temperature dependence of this reaction was also investigated, revealing a non-standard dependency of both the enthalpy and the entropy. This is explained by the differences in the heat capacity of the reaction components, as well as by the ligands’ shell organization. Accompanying thermogravimetric analysis (TGA) verified the exchange ratio based on the ITC measurements.

This work is a starting point for achieving fundamental understanding of the surface physico-chemical properties and their dependence on a variety of SC NC parameters. The insights of the presented research will pave the way for smart NC surface design that can impact NCs applications considering the central role that surface passivation and chemical processing through surface control plays on the NCs functionality.