Molecular Interactions at Cadmium Selenide Nanocrystal Surfaces

The synthesis of n-alkylamine-bound CdSe-NH2Rʹ nanocrystals from carboxylateterminated CdSe-Cd(O2CR)2/HO2CR requires the removal of acidic impurities prior to the addition of primary amine. Otherwise, the formation and subsequent tight binding of n-alkylammonium carboxylate ion pairs prevents quantitative removal of carboxylate species. Dimethylcadmium and diethylzinc were used as reagents to deprotonate acidic impurities, which either causes methylation (with a surface density of 0.04−0.22 nm−2) and photoinduced reduction of the nanocrystal core or X-type ligand exchange with ethyl species, respectively. The acid-scavenged nanocrystals could be completely isolated from displaced carboxylate ligands (≤ 0.01 carboxylates nm-2). In addition to traditional selective precipitation procedures, gel permeation and silica chromatography were investigated as alternative purification methods for the isolation of CdSe-NH2Rʹ. Both demonstrated no improvement compared to the more convenient precipitation process.

Thin films fabricated from CdSe-NH2C4H9 show little to no grain growth upon thermal annealing at 250 ºC, maintaining domains (~10 nm) despite complete desorption of n-butylamine from the nanocrystal surface above 150 ºC.