EDTA-assisted hydrothermal synthesis, characterization and photoluminescent properties of Mn2+-doped ZnS

•Hydrothermal synthesis of EDTA-assisted ZnS:Mn2+ nanoparticles with 3-4 nm.•Well-defined quantum confinement effect Eg (NPs) = 4.59 eV > Eg (bulk) = 3.64 eV.•Investigation of fixed blue region with the red shift in yellow-orange region.•Origin of the additional luminescence observed.

In this paper, undoped ZnS and Mn2+-doped ZnS nanocrystals were synthesized through a facile EDTA-assisted hydrothermal method. The as-synthesized powder samples were systematically characterized by employing the following characterization technique such as X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM), UV-visible optical absorption and photoluminescence (PL) spectroscopy. X-ray diffraction pattern revealed the presence of material in single phase with average crystallite size of about 3 nm and the material remained cubic over the whole Mn solid solution range. Formation of ultrafine, spherical and homogeneous dispersed nanoparticles with size 4 nm was confirmed by HRTEM analysis. Absorption shoulders of the samples were blue-shifted as compared to bulk ZnS (3.6 eV) with decrease in the energy band gap as the Mn concentration increases. The room temperature photoluminescence (PL) spectra of Mn2+-doped ZnS nanocrystalline showed extra peaks in yellow-orange and red region in comparison of pure ZnS. Mn induced PL was suggested with the significant enhancement of the PL intensity in ZnS:Mn nanocrystalline due to Mn incorporation. The red shift in the yellow-orange emission peak can be attributed to the change in band structure due to the formation of ZnS:Mn alloy with increase in Mn2+ concentration. The yellow-orange emission peak corresponds to the 4T1(excited)-6A1(ground) transition of Mn2+ ion in Td symmetry in the ZnS host lattice. The emission peak in the red region may be due to Mn2+ d-d transitions in (Zn Mn)S matrix as some of the nearest neighbors of Mn2+ are now predominantly S atoms due to their random positioning nature in the nanocrystallite and Mn-Mn interaction at high Mn2+ concentration. This type of doped semiconductors with multi-band emission can be made bioactive when they are linked with suitable biomolecules.