Thermodynamics Reveal Potential Ligand-Induced Surface Atom Rearrangement During the Exchange of Oleate for Dodecylphosphonic Acid on CdSe Quantum Dots

The optoelectronic properties of semiconductor nanocrystals, or quantum dots (QD), are dependent on their surface chemistry which is often altered through ligand exchange reactions or growth of inorganic shells after their initial synthesis. Analysis and developed understanding of their surface modification processes are important to finely tune QD properties to their intended application, such as biosensors, solar cells, light-emitting diodes, and (photo)electrocatalysts. To that end, we employed isothermal titration calorimetry (ITC) to investigate the ligand exchange of oleate for dodecylphosphonic acid (DDPA) on CdSe QDs. The thermograms revealed an initial exothermic response follow by a delayed endothermic response upon injection of DDPA into the QD solution. We derived a new model to analyze the ITC isotherms that accounts for the two-step process observed and represents the physical processes occurring. Enthalpy of reaction (ΔH), equilibrium constant (K), and stoichiometry of the reaction (n, or number of binding ligands per QD) were extracted from the ITC analysis, which were then used to determine the entropy (ΔS) and Gibbs free energy (ΔG) for the processes. The initial exothermic response is attributed to the ligand exchange of oleate for dodecylphosphonic acid, and we used quantitative nuclear magnetic resonance spectroscopy to confirm the stoichiometry of the reaction determined by ITC. Interestingly, the endothermic response disappears when the exothermic ligand exchange is only ~76% complete. X-ray diffraction (XRD) revealed a slight increase in strain in the nanocrystal lattice, and Raman spectroscopy revealed a shift in signals corresponding to the surface optical and transverse optical phonons of the DDPA-treated CdSe nanocrystals. The XRD and Raman results indicate that ligand-driven surface atom rearrangement occurs on the QDs and is potentially responsible for the delayed endothermic response observed in ITC in this ligand exchange reaction. This assignment is the first direct experimental observation of ligand-induced surface atom rearrangement on colloidal nanomaterials. The depth of fundamental information afforded by this technique is essential to advance the ability to finely tune QD properties to their applications and will ultimately lead to enhanced efficiency of devices utilizing these low-dimensional semiconductor nanomaterials.