Reaction chemistry and ligand exchange at cadmium-selenide nanocrystal surfaces

The surface chemistry of cadmium selenide nanocrystals, prepared from tri-n-octylphosphine selenide and cadmium octadecylphosphonate in tri-n-octylphosphine oxide, was studied with 1H and {1H}31P NMR spectroscopy as well as ESI-MS and XPS. The identity of the surface ligands was inferred from reaction of nanocrystals with Me3Si-X (X = -S-SiMe3, -Se-SiMe3, -Cl and -S-(CH2CH2O)4OCH3)) and unambiguous assignment of the organic byproducts, O,O'-bis(trimethylsilyl)octadecylphosphonic acid ester and O,O'-bis(trimethylsilyl)ocatdecylphosphonic acid anhydride ester. Nanocrystals isolated from these reactions have undergone exchange of the octadecylphosphonate ligands for -X as was shown by 1H NMR (X = -S-(CH2CH2O)4OCH3) and XPS (X = -Cl). Addition of free thiols to as prepared nanocrystals results in binding of the thiol to the particle surface and quenching of the nanocrystal fluorescence. Isolation of the thiol-ligated nanocrystals shows this chemisorption proceeds without displacement of the octadecylphosphonate ligands, suggesting the presence of unoccupied Lewis-acidic sites on the particle surface. In the presence of added triethylamine, however, the octadecylphosphonate ligands are readily displaced from the particle surface as was shown with 1H and {1H}31P NMR. These results, in conjunction with previous literature reports, indicate that as-prepared nanocrystal surfaces are terminated by X-type binding of octadecylphosphonate moieties to a layer of excess cadmium ions.