Ligand Exchange and Binding at the Surface of PbS Quantum Dots Quantified Using Multimodal Magnetic Resonance.

Semiconductor quantum dots (QDs) show promise for various applications, including biological imaging and photovoltaics. QDs are typically stabilized by surface-bound ligands, which exhibit a dynamic binding equilibrium. This study combines nuclear magnetic resonance (NMR) spectroscopy and diffusometry to quantify the populations and kinetics of oleic acid (OAH) ligand binding to PbS QD surfaces. In addition to quantifying ligand population fractions in bound and free states, our analysis reveals the existence of a third ligand state, which we hypothesize to be the weak coordination of the OAH at (100) sites through the acidic headgroup (-COOH). Thus, bound ligands exist in two subpopulations: weakly bound OAH on (100) facets and strongly bound oleate (OA) on (111) facets. Through variations in temperature and concentration, we assess the changes in ligand populations in different states and determine the energetics of their exchange equilibria. Additionally, using dynamic NMR spectroscopy, we quantify rapid exchange rates (0.09-2 ms) between weakly bound and free OAH ligands as a function of OAH titration concentration and temperature. These findings highlight the complexity of ligand binding mechanisms and enable strategies for precisely tuning QD surface properties, with significant implications for the innovation of next-generation optoelectronic materials.