Surface Chemistry of CdTe Quantum Dots Synthesized in Mixtures of Phosphonic Acids and Amines: Formation of a Mixed Ligand Shell

Solution nuclear magnetic resonance (NMR) spectroscopy is used to analyze the surface chemistry of CdTe quantum dots (QDs) synthesized in the presence of tetradecylphosphonic acid and oleylamine. The resulting CdTe QDs have a mixed capping, composed of tightly bound phosphonate anhydride and amine ligands. The overall ligand density is relatively low, which concurs with a low Cd excess in the QDs. We find that upon addition of oleic acid, neither amine or phosphonate anhydride ligands are released from the QDs. On the other hand, the addition of phosphonic acids to CdTe QDs partially capped by oleic acid moieties through a high temperature ligand exchange process leads to the ready, one-to-one release of oleic acid. A full thermodynamic analysis of the phosphonate/carboxylate exchange is however not possible since the exchange appears to be modified by the forced ligand exchange used to replace phosphonate by carboxylate ligands