The photoluminescence enhancement of Mn²⁺ ions and the crystal field in ZnS:Mn nanoparticles covered by polyvinyl alcohol

The polyvinyl alcohol (PVA)-capped ZnS:Mn nanoparticles with Mn content of 8 mol% and different PVA mass (denoted as ZnS:Mn/PVA) are synthesized by co-precipitation method, in which PVA solution is mixed from the beginning with the initial solutions used to synthesize ZnS:Mn nanoparticles. The microstructures, morphology and average crystalline size of ZnS:Mn/PVA nanoparticles were investigated by X-ray diffraction patterns, high resolution transmission electron microscopy, thermal gravimetric analysis (TGA) and differential thermal gravimetric analysis and Fourier transform infrared absorption spectra. The role of PVA to the photoluminescence (PL) of Mn²⁺ ions in these nanoparticles at 300 K were studied by the PL and photoluminescence excitation (PLE) spectra. The investigated results show that the PVA covering for ZnS:Mn nanoparticles almost is not change the microstructure, morphology, the crystal field and the peak positions in their PL and PLE spectra, but the maximum intensity of peaks increased with PVA mass from 0.2 to 1.0 g. The clear peak positions in PLE spectra show that the energy levels of Mn²⁺ ions were splitted into multiple levels in the ZnS:Mn crystal field, that its strength D_q was caculated. Furthermore, the effect of PVA on the PL enhancement of Mn²⁺ ions in ZnS:Mn/PVA nanoparticles also was explained.