Facile synthesis of high-quality ZnS, CdS, CdZnS, and CdZnS/ZnS core/shell quantum dots: characterization and diffusion mechanism

Binary CdS and ZnS and ternary CdZnS alloy quantum dots (QDs) were synthesized via a simple, inexpensive, and reproducible route using sulfur, cadmium stearate, and zinc stearate as precursors and N-oleoylmorpholine as the reaction medium and solvent. Both binary and ternary QDs exhibited a narrow size distribution and high crystallinity as confirmed TEM and HRTEM images. The alloy QDs exhibited excellent composition-dependent optical properties and a narrow full-width at half maximum of 19-21 nm. UV-visible absorbance and photoluminescence (PL) emission spectra of the CdZnS QDs showed a blue shift during growth, indicating the formation of alloy QDs. ZnS shells were successively coated onto the alloy core via decomposition of zinc diethyldithiocarbamate at a relatively low temperature. The CdZnS/ZnS core/shell QDs obtained showed a significant increase in size and exhibited strong band edge emission with a significant increase in PL quantum yield. XRD patterns revealed that all the QDs had a zinc blende structure. The QD diffraction peaks gradually shifted to higher angle in the order CdS < CdZnS < CdZnS/ZnS < ZnS. The mechanism for the synthesis of CdZnS alloy and CdZnS/ZnS core/shell QDs is discussed.