Synthesis and characterization of CdS x Se 1- x alloy quantum dots with composition-dependent band gaps and paramagnetic properties.

Ternary CdS x Se 1- x alloy quantum dots (QDs) and CdS x Se 1- x /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 x Se 1- x 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 x Se 1- x core QDs and CdS x Se 1- x /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 x Se 1- x nanocrystals were adjustable by varying the ratio of S:Se in the CdS x Se 1- x core and were in the range of 1.96 eV to 2.32 eV. Hence, when composition x 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 x Se 1- x /ZnS core/shell QDs displayed a small redshift. Electron paramagnetic resonance (EPR) measurements for manganese-doped CdS x Se 1- x /ZnS nanocrystals revealed paramagnetic properties for both low and high Mn 2+ doping levels.