The optical and electronic properties of quantum dots (QD) are related to their surface chemistry; analysis and developed understanding of their surface modifications are important to finely tune QD properties to their intended application. 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. 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 data, 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 as 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 of the QDs is potentially responsible for the delayed endothermic response observed in ITC in this ligand exchange reaction. This assignment would be the first direct experimental observation of ligand-induced surface atom rearrangement on colloidal nanomaterials.

Thermodynamics reveal potential ligand-induced surface atom rearrangement during the exchange of oleate for dodecylphosphonic acid on CdSe quantum dots

ACS Spring Meeting 2021

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