Controlled synthesis of high quality type-II/type-I CdS/ZnSe/ZnS core/shell1/shell2 nanocrystals.

Using phosphine-free and "green" chalcogen precursors, controlled synthesis of high quality CdS/ZnSe/ZnS core/shell1/shell2 nanocrystals has been successfully carried out using different sized CdS nanocrystals as cores. The properties and structures of the synthesized nanocrystals were characterized by absorption spectroscopy, photoluminescence (PL) spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), and inductively coupled plasma mass spectroscopy (ICP-MS). By changing CdS core sizes and/or ZnSe shell thicknesses, the PL range of CdS/ZnSe core/shell nanocrystals could be adjusted from 500 nm to 630 nm with type-II optical characteristics. The PL quantum yields (QYs) of these nanocrystals were 50-60% after the growth of thick ZnS shells, their full width at half maximum (FWHM) was kept below 50 nm during the entire growth process, and the total Cd content was reduced to about 1% in atomic ratio. TEM images showed narrow size distributions and XRD results demonstrated the zinc blende structure of CdS was retained following subsequent growth of ZnSe and ZnS shells. More than 2 g of high quality CdS/ZnSe/ZnS nanocrystals were successfully prepared in a large scale synthesis with the use of only low-cost, green, and environmentally friendly reagents.