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 (1)H and {(1)H}(31)P NMR spectroscopy as well as ESI-MS and XPS. The identity of the surface ligands was inferred from reaction of nanocrystals with Me(3)Si-X (X = -S-SiMe(3), -Se-SiMe(3), -Cl and -S-(CH(2)CH(2)O)(4)OCH(3))) 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 (1)H NMR (X = -S-(CH(2)CH(2)O)(4)OCH(3)) 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 (1)H and {(1)H}(31)P 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.