Thermodynamics of the Adsorption of Cadmium Oleate to Cadmium Sulfide Quantum Dots and Implications of a Dynamic Ligand Shell

Adsorbed surface ligands play an important role in determining the chemical and physical properties of colloidal semiconductor nanocrystals. Most particularly, these ligands influence the optical properties of these nanocrystals. For instance, the luminescence of type II-VI quantum dots has been shown to decrease as metal carboxylates are stripped from the surface. To gain a better understanding of the thermodynamics and equilibria that influence the optical properties of colloidal quantum dots, we studied the adsorption energies of aliphatic cadmium carboxylates to the surfaces of cadmium sulfide quantum dots. Direct calorimetric measurements of the adsorption energies of such ligands have previously proven to be challenging because they are tightly adsorbed to the quantum dot surface. Here, we show that tetrahydrofuran can be used as a coordinating solvent, allowing cadmium oleates to more easily be stripped from the surface, thereby creating a dynamic ligand equilibrium. Taking advantage of this dynamic equilibrium, ligand-deficient cadmium sulfide quantum dots were prepared, and the adsorption energy of adding cadmium oleate to the surface was measured via isothermal titration calorimetry. Quantum chemical calculations were performed to investigate the binding energy of the cadmium complex with tetrahydrofuran and to calculate the adsorption energy of cadmium oleate to the surface of cadmium sulfide in the absence of a coordinating solvent. Additionally, a modified Ising-model-based simulation was used to estimate the enthalpic and entropic contributions of interligand interactions to the reaction thermodynamics, which play a significant role in describing the quantum dot surface. Lastly, the effects of a dynamic ligand shell on the optical properties of quantum dots were studied, suggesting that a static ligand shell provides higher quantum yields.