Effects of precursor molar ratio and annealing temperature on structure and photoluminescence characteristics of Mn-doped ZnS quantum dots

In our work, we focused on the influence of precursor molar ratio of [S2-]/[Zn2+] and annealing temperature on structure and photoluminescence properties of thiolglycolic acid (TGA)-encapsulated Mn2+-doped ZnS quantum dots synthesized at 80 °C in basic aqueous solutions. The structural, morphological and optical properties were investigated using characterization techniques such as X-ray powder diffraction (XRD), Transmission electron microscope (TEM), Fourier transform infrared spectroscopy (FTIR), UV-vis absorption spectroscopy, photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy. The studied result indicates that the [S2-]/[Zn2+] precursor molar ratio affects to the structure and the Mn2+ 4T1(G) - 6A1(S) emission of the ZnS:Mn2+ QDs. With the increase of the [S2-]/[Zn2+] molar ratio from 0.5:1 to 3.5:1, the relative PL intensity ratio of IOE/IBE increased significantly which is due to the efficiency of energy transfer from the ZnS host to Mn2+ centers. As the molar ratios of [S2-]/[Zn2+] are larger than 1.5:1 the extending orange-red emission beyond 700 nm significantly increases due to the formation of Mn-Mn pairs. Besides that, the result also shows the presence of the compressive strain within the ZnS host. In addition, with the increasing annealing temperature from 100 to 500 °C, the slight redshift in the Mn2+ 4T1(G) - 6A1(S) emission appears in all samples. The Mn2+ ions at the defect sites may also diffuse and form the Mn-Mn pairs within the ZnS host with the increasing annealing temperature. Especially the MnS phase is formed in the ZnS lattice at the annealing temperature of 500 °C.