Reaction Chemistry and Ligand Exchange at Cadmium−Selenide Nanocrystal Surfaces

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Abstract
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.

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