Multiple Families of Magic-Sized CdSe Nanocrystals with Strong Bandgap Photoluminescence via Noninjection One-Pot Syntheses

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Abstract

Three families of colloidal CdSe magic-sized nanocrystals (MSNs) exhibiting bright bandgap photoluminescence (PL) with narrow full width at half-maximum (fwhm) on the order of ca. 10 nm were synthesized in pure form. This noninjection one-pot synthetic approach uses cadmium acetate dihydrate (Cd(OAc)2·2H2O) and elemental selenium as Cd and Se source compounds, respectively, while a fatty acid as surface ligands and 1-octadecene (ODE) as the reaction medium. All of these chemicals were loaded at room temperature in a reaction flask, with low acid-to-Cd and high Cd-to-Se feed molar ratios; the growth of the CdSe MSNs was carried out at 120 −240 °C. This synthetic approach allows long growth/annealing periods at high temperature and thus results in high-quality CdSe MSNs exhibiting strong bandgap PL; furthermore, this ready approach features high synthetic reproducibility and large-scale production. The reason for the low acid-to-Cd feed molar ratio is argued to be related to the low activity of the Cd precursor in the form of Cd(OAc)x(OOC−(CH2)n−CH3)2−x, which releases Cd slowly. The reason for the high Cd-to-Se feed molar ratio is also addressed to help prevent the dissociation of the formed CdSe MSNs. Furthermore, short ligand and low temperature favors the formation of small MSNs, while long ligand and high temperature favors large MSNs. Regarding our synthetic approach, these MSN families can coexist together with one regular nanocrystal ensemble which is larger in size; in such a case, they develop independently from different nuclei; the small MSN families can dissociate into monomers to feed the formation of other MSN families and/or the regular nanocrystal ensemble. The three MSN families exhibit sharp bandgap absorption at 395, 463, and 513 nm, within the size range of 1.7−2.2 nm by diffusion ordered NMR spectroscopy (DOSY).

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