Coordinated Anions Tune Z-Type Ligand Displacement from Colloidal CdSe and InP Nanocrystal Surfaces

Neutral metal salts coordinate to the surfaces of colloidal semiconductor nanocrystals (NCs) by acting as Lewis acid acceptors for the NC surface anions. This ligand coordination has been associated with increased emission due to the passivation of surface hole traps. Here, variation of the anionic ligands of metal salts is used to study anion effects on metal complex Lewis acidity and surface coordination at CdSe and InP NCs. To resolve dynamic ligand exchange processes, the tetracarbonylcobaltate anion, [Co(CO)₄]^-, is used as a monoanionic ligand for which IR spectroscopy can readily identify displacement of neutral M[Co(CO)₄]_<i>x</i> species (M = Cd or In; <i>x</i> = 2 or 3, respectively) upon addition of neutral donor ligands. Notably, although Cd[Co(CO)₄]₂ is more Lewis acidic than cadmium oleate, the former is more readily displaced from the NC surfaces. Lewis acidity and X-type anion exchange are, therefore, factors to be considered when performing postsynthetic addition of metal salts for NC photoluminescence emission enhancement.