Synthesis and characterization of type IIZnSe/CdS core/shell nanocrystals

High-quality ZnSe/CdS core/shell nanocrystals, exhibiting a type II carrier localization regime, were fabricated via a traditional pyrolysis of organometallic precursors. The two-step synthesis involved fabrication of 4.5-6 nm ZnSe seeds followed by a subsequent deposition of the CdS shell. An efficient spatial separation of electrons and holes between the core and the shell was observed for heterostructures containing more than three monolayers of CdS, which was primarily evidenced by the spatially indirect emission tunable from 480 to 610 nm for a fixed core diameter. Because of a large (type II) offset of band edges at the core/shell interface, fabricated nanocrystals exhibited a relatively low spectral overlap between emission and absorption profiles, with associated Stokes shifts of up to 110 nm. The quantum yield of as-prepared samples was 12-18% and was further improved to 20% after purification of nanocrystals through multiple hexane/methanol extractions. Novel properties of synthesized ZnSe/CdS nanocrystals as well as their applicability to practical realizations in areas of biomedical imaging, solar sells, and quantum dot-based lasers are discussed.