The surface ligands play an important role in the optoelectronic properties of QDs and QD based devices other than solublizing the particles in organic and aqueous phase. Even for photocatalytic applications the incoming chemical species relies on the surface ligands distributed on the surface.

The surface of nanocrystals and their ligand layer present a unique model system for the inorganic-organic interface where the curvature is controlled by their radius. At the infinite radius limit, this surface interface converges to that of widely studied self-assembled monolayers. Beyond the fundamental interest in the nanocrystals surface-ligands region, this inorganic-organic interface plays a critical role in the synthesis of the nanocrystals, where ligands serve to arrest the nanocrystals growth to the nanometer dimensions, to control nanocrystals shape, and to prevent irreversible agglomeration. Ligands also contribute to electronic passivation of the nanocrystals surface and mediate charge transfer to and from the nanocrystals. Last but not least, the ligands determine the solubility of the nanocrystals providing a flexible handle for modifying the environment for their dispersion, either in non-polar or polar solvents, or in different polymer matrices. The importance of this latter aspect cannot be overestimated as it serves as the basis for the broad applicability of nanocrystals in diverse uses enabled via bottom-up chemical manipulations. We take mulimode approach to characterize and probe the ligand densities, binding/exchange kinetics during exhange reaction. In order to decipher the binding motifs and governing factors, we utilize ITC (isothermal titration calorimetry) for the extraction of the thermodynamic parameters such as enthalpy, entropy equilibrium constant, the Gibbs free energy, and the stoichiometric coefficient of the ligand exchange reaction. To complete our analysis we combine a suite of characterization methods such as NMR, FTIR, TGA measurements. In recent past we have extensively used time resolved fluorescence anisotropy and FRET measurements with surface bound fluorophores to probe the surface ligand density for semiconductor QDs with different size and surface curvature. We believe that this work will provide fundamental understanding of the physico-chemical properties of nanocrystals surface, and the achieved insights will pave the way for rational design of nanocrystals surface in optoelectronic and biotechnology applications.

More: Nano Letters 2020, and ACS Nano 2022