Synthesis and photoluminescence of water-soluble Mn2+-doped ZnS quantum dots

The water-soluble Mn2+-doped ZnS quantum dots (Mn:ZnS d-dots) were synthesized by using thioglycolic acid (TGA) as stabilizer in aqueous solutions in air, and characterized by X-ray powder diffraction (XRD), UV–vis absorption spectra and photoluminescence (PL) emission spectroscopy. The sizes of Mn:ZnS d-dots were determined to be about 2 nm using XRD measurements and the UV–vis absorption spectra. It was found that the Mn2+ 4T1 → 6A1 emission intensity of Mn:ZnS d-dots significantly increased with the increase of Mn2+ concentration, and showed a maximum when Mn2+ doping content was 1.5%. If Mn2+ concentration continued to increase, namely more than 1.5%, the Mn2+ 4T1 → 6A1 emission intensity would decrease. In addition, the effects of TGA/(Zn + Mn) molar ratio on PL were investigated. It was found that the peak intensity ratio of Mn2+ 4T1 → 6A1 emission to defect-states emission showed a maximum when the TGA/(Zn + Mn) molar ratio was equal to 1.8.