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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 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]x species (M = Cd or In; x = 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 post-synthetic addition of metal salts for NC photoluminescence emission enhancement. 

The adaptation of colloidal semiconductor nanocrystals (NCs) in applications like displays, photovoltaics, and photocatalysis relies primarily on the core electronic structure of NC materials that give rise to desirable optoelectronic properties like broad absorption and size-tunable emission. However, reduction or oxidation events at localized NC surface sites can greatly affect sample stability and device efficiencies by contributing to NC degradation and carrier trapping. Understanding the local composition, structure, and electrochemical potentials of redox-active NC surface sites continues to present a challenge. In this perspective, we discuss how NC surface reduction, oxidation, and electrostatics contribute to NC electronic properties that include photoluminescence quenching or brightening and shifts in NC band edge potentials, among others. Recent efforts toward combining spectroscopic, electrochemical, and computational methods to characterize redox-active surface sites and trap states are highlighted, including developing methods in the field and future opportunities. 

Nucleophilic 1,2-aminothiol compounds readily reduce typically-insoluble elemental sulfur to polysulfides in both water and nonpolar organic solvents. The resulting anionic polysulfide species are stabilized through hydrogen-bonding interactions with the proximal amine moieties. These interactions can facilitate sulfur transfer to alkenes.