Synthesis of Cd-free blue-emitting ZnTeS/ZnS Quantum dots and study on emission properties affected by defects

Abstract

We reported the facile synthesis and emission properties of Cd-free blue-emitting ZnTeS quantum dots (QDs) affected by defects. The metallic tellurium is hardly transferred to anionic species, thus resulting in poor morphology. To obtain ZnTeS QDs with a uniform spherical shape, we applied superhydride as a strong reducing agent. In the synthesized ZnTeS core sample, dual emission which is rarely observed in typical quantum dots appered at 375 and 475 nm, as a small amount of Te participates in the reaction. The ZnTeS core itself shows very low PL quantum efficiency of less than 3%. To improve PL QY and photostability, the ZnS shell layer were applied on the ZnTeS core by using the zinc acetate and zinc nitrate hexahydrate as zinc precursor. When the Zinc nitrate hexahydrate was used as a Zn precursor at shell coating process, the highest PL QY of 18% which is outstanding achievement among non-Cd based blue-emitting QDs was obtained via drop-wise method. In addition, a strong emission peak at around 440 nm which was not observed from the ZnTeS core showed up in photoluminescence spectrum of ZnTeS/ZnS core/shell structure. To comprehend the origin of defect emission in ZnTeS/ZnS QDs, de-convoluted PL peak component of synthesized ZnTeS/ZnS QDs was compared with those of the ZnS QDs found in the previous report. Furthermore, main peak of ZnTeS/ZnS QDs (~430 nm) turn out to be originated from sulfur vacancy by injecting additional DDT ligand during shell growth process. Interestingly, as the shell growth temperature increased from $220 ^\circ C$ to $300 ^\circ C$, main peak of emission wavelength was significantly shifted. We could tune position of main emission peak as-prepared QDs from near-UV to sky blue by simply varying shell growth temperature.