Quantum Dot Size Dependence on Ligand Exchange Driving Forces and Insights into the Structure of the Solvated Ligand Shell

As synthesized, nanocrystal surfaces are typically covered by coordinating organic ligands. Prior to commercial applications, treatments are often utilized to exchange these native ligands for new ligands that can enhance the nanocrystal properties, such as electrical conductivity or luminescence. Understanding the thermodynamics of these ligand exchanges is critical for optimizing performance and gaining insight into reaction mechanisms. Previous work has shown that the geometry and length of a ligand’s aliphatic chain can influence the thermodynamics of exchange, which is attributed to inter-ligand interactions. In this report, the relationship between nanocrystal size and degree of inter-ligand interactions was studied using the reaction between indium phosphide quantum dots and zinc chloride. Using isothermal titration calorimetry and quantitative 1H NMR, alongside a modified Ising model simulation, we determined that inter-ligand interactions increase as the nanocrystal size increases, changing the thermodynamics of the ligand exchange reaction. Support for a phase transition in stearate ligand shells was indicated through our analysis and in powder X-ray diffraction data. The structure of the solvated ligand shell was also studied using X-ray diffraction, results from which suggest that the spacing between aliphatic ligands may increase as the ligand shell is solvated.