Thermodynamics and Modeling of Collective Effects in the Organic Ligand Shell of Colloidal Quantum Dots

ConspectusColloidal nanocrystals are an interesting platform for studying the surface chemistry of materials due to their high surface area/volume ratios, which results in a large fraction of surface atoms. As synthesized, the surfaces of many colloidal nanocrystals are capped by organic ligands that help control their size and shape. While these organic ligands are necessary in synthesis, it is often desirable to replace them with other molecules to enhance their properties or to integrate them into devices. Traditionally, these ligand exchanges have been studied using ¹H NMR. Recently, isothermal titration calorimetry has proven itself to be a highly versatile measurement technique, yielding insights into the thermodynamics of the reaction, including the enthalpy and entropy of the reaction, that are inaccessible via ¹H NMR. The most common technique for analyzing ligand exchange reactions has been to model these data with one-site and two-site Langmuir isotherm models. Unfortunately, a detailed analysis of ¹H NMR and isothermal titration calorimetry data simultaneously demonstrates that these simple models are inadequate for understanding ligand reactions on the surfaces of colloidal nanocrystals.In this Account, we illustrate that the collective effects of the aliphatic chains of the organic ligands on the surfaces of colloidal nanocrystals dictate much of the reaction thermodynamics and how we have manipulated the thermodynamics of ligand exchange reactions by modulating the geometry and length of the organic ligands, the shape of the underlying nanocrystal, and the size of the nanocrystal. One of the main contributions of our body of work is the implementation of a modified Ising model, which accounts for nearest neighbor interactions, or collective effects, between the surface ligands and can be used to fit self-consistent thermodynamic parameters to describe the ligand exchange reactions. Using this model, we reveal the entropic and enthalpic factors of both the head binding group and the tail group that drive exchange reactions. In particular, we demonstrate the significant interligand interactions and the effect that ligand geometry and length have on these interactions. Further, we have shown that as the size of the nanocrystal increases, the interactions between the organic ligands become much stronger, and we have provided evidence that structural differences are present in the solvated ligand shell based on the ligand length. We also demonstrate in the case of ligand exchanges on cadmium selenide quantum dots that the crystal facet has very little impact on the thermodynamics of the ligand exchange using (100) and (111) faceted quantum dots. These findings rely critically on using a composition dependent model. We believe that this model or another accounting for these collective effects is critical for accurately analyzing the thermodynamics of the organic ligand shells for the field moving forward.