Luminescence enhancement of ZnS:Mn and ZnS:Cu-PVP nanoparticles by laser radiation annealing

Most of the ZnS:M (M: transition metal) nanoparticles synthesized from the S2− source in organic and inorganic precursors cause defects, dangling bonds, and vacancies in the surface of nanoparticles, creating nonradiative recombination centres and reducing the photoluminescence (PL) intensity. ZnS:Mn and ZnS:Cu-PVP nanoparticles were synthesized by different methods with sources of S2− in organic and inorganic precursors of Na2S, Na2S2O3.5H2O or HSCH2COOH (TGA), and PVP. Then, the surface activation methods used ultraviolet radiation at 325 and 337 nm and visible radiation at 532 nm, 632.8 nm, and 650 nm were used to increase the photoluminescence (PL) intensity of the nanoparticles. The results of the XRD pattern analysis showed that the crystal structure and grain size of the samples did not appear to change due to the radiation annealing process. However, the FT-IR spectra showed that samples using the S2− source from the inorganic precursor Na2S, Na2S2O3, exhibited ZnSO4 formation that was attributed to the photochemical phenomena on the surface of the nanoparticles. The samples using S2− source from organic precursor TGA showed the formation of ZnSO4 and polyglycolide [CH2–COO]n (PGA) polymer on the surface of nanoparticles. This was attributed to the polymerization phenomenon. These photochemical and polymerization processes have the effect of reducing the nonradiative recombination processes of the nanoparticle surface defect states and simultaneously increasing the photoluminescence (PL) intensity of the luminescent centres (Mn2+, Cu2+) from 2.0 to 3.6 times for the ZnS:Mn system and 2.7 times for the ZnS:Cu-PVP system in comparison with the samples that were not annealed. The effects of UV and visible radiation, annealing time, and annealing power on the PL intensity of luminescence centres in the nanoparticles were also investigated and explained. The radiation annealing methods for the samples after synthesis are highly important for improving the optical properties of the nanoparticles and for studies on the fabrication of photovoltaic elements, quantum dots, and LEDs.