Ligand Exchange Equilibrium at Quantum Dot Surfaces in Polar and Aqueous Solvent Environments

Our group and many others have contributed to an increasingly detailed understanding of the surface chemistry of representative compound semiconductor quantum dots, including metal chalcogenides, pnictides, and halide perovskites with effective bandgaps spanning much of the visible and infrared spectrum. Such particles are frequently stabilized by organic molecules with nucleophilic or anionic headgroups that coordinate surface atoms. To date, the greatest understanding has been achieved in non-polar or less polar solvents in which charge balance helps to simplify the range of surface reactions that must be considered. Techniques such as NMR and infrared spectroscopy, mass spectrometry, and isothermal titration calorimetry have helped to reveal the chemistry at such surfaces, including the importance of ligand interactions that can govern the composition and local organization of the surface monolayer.<br/><br/>It is critically important to extend the understanding of such nanocrystal surfaces to polar solvents, including aqueous solution required for biological applications and polar organic mixtures favored for storable nanocrystal inks for electronic devices. However, polar solvents and buffers introduce additional challenges for fundamentals studies due to charge stabilization, acid-base equilibria, protic buffer components, oxidation, and the need for core/shell structures to achieve good photoluminescence quantum yield.<br/><br/>I will describe our group’s efforts to quantify ligand association and exchange reactions on nanocrystal surfaces in polar and aqueous solutions, including coordination of quantum dots by imidazoles, thiolates, and polymeric ligands, and prospects for extending these studies to additional classes of materials including magnetic oxides and halide perovskites. I will also emphasize the advantages and limitations of isothermal titration calorimetry as a tool to study the surfaces of colloidal nanostructures and compare the binding of different ligand architectures in polar environments.