Synthesis and characterization of CdS Se alloy quantum dots with composition-dependent band gaps and paramagnetic properties.

Ternary CdS Se alloy quantum dots (QDs) and CdS Se /ZnS core/shell nanocrystals exhibiting composition dependent band gaps have been successfully synthesized. The ZnS shell was doped with 0.1% and 5% of paramagnetic manganese ions so as to be used as a fluorescent/paramagnetic bi-functional probe. Energy-dispersive X-ray spectroscopy (EDS) measurements confirmed the presence of Cd, S, and Se in CdS Se nanocrystals with the atomic ratios of Cd, S, and Se which are well consistent with our synthetic ratios. Wide angle X-ray diffraction (WAXD) indicated that the crystal structures of the CdS Se core QDs and CdS Se /ZnS core/shell QDs were zinc blende phases. Both dynamic light scattering (DLS) and transmission electron microscopy (TEM) revealed that the as-synthesized nanocrystals had a narrow size distribution and high crystallinity. The band gaps of CdS Se nanocrystals were adjustable by varying the ratio of S:Se in the CdS Se core and were in the range of 1.96 eV to 2.32 eV. Hence, when composition was changed from 0 to 1, the fluorescence color of the nanocrystals varied from red to green. After shell coverage, the ternary alloy QDs exhibited a superior photoluminescence (PL) quantum yield up to 57%. In comparison with the alloy core QDs, the PL emission peaks of the CdS Se /ZnS core/shell QDs displayed a small redshift. Electron paramagnetic resonance (EPR) measurements for manganese-doped CdS Se /ZnS nanocrystals revealed paramagnetic properties for both low and high Mn doping levels.