A new route to very stable water-soluble ultra-small core/shell CdSe/CdS quantum dots

A new synthetic route to water-soluble ultra-small CdSe quantum dots (QDs) is proposed based on using a mixed complex of Cd(II) with ammonia and mercaptoacetate (MA) anions as a stabilizer and Na 3 SeO 3 as a stable and non-volatile selenium precursor that can be reduced in situ to Se 2− by MA. The method is facile, reproducible and allows gram-scale production of CdSe QDs in the form of powders, uniform and smooth films and concentrated solutions with a CdSe content up to 20 m%. At a molar MA/Cd(II) ratio of 2.25 the ultra-small CdSe QDs are characterized by an average size of around 2 nm and a narrow size distribution, as well as by a stability towards oxidation by ambient oxygen and under light illumination. A combined Raman and XPS study shows that the stability of the CdSe QDs arises from a thin layer of CdS on the QD surface formed during the post-synthesis thermal treatment. An increase in a MA/Cd(II) ratio to 4.00 results in the formation of a species that by its spectral characteristic resembles earlier reported “magic-size” (CdSe) 19 clusters. Upon heating this species transforms into CdSe QDs with a broadened size distribution.