The two-step thermochemical growth of ZnS:Mn nanocrystals and a study of luminescence evolution

In this work we report a new thermochemical method for the synthesis of ZnS:Mn nanocrystals. Zn(NO3)2 and Na2S2O3 were used as the precursors and Mn(NO3)2 was the source of impurity. Thioglycerol (TG,C3H8O2S) was also used as the capping agent and the catalyst of the reaction. Na2S2O3 is a heat sensitive material which releases S species upon heating. Consequently, the reaction proceeds in temperatures higher than room temperature. The reaction was done in two steps. In the first step, the precursors were heated at 96 °C for an hour without TG. In the second step, TG was injected to the solution and the heating process was continued for longer heating durations. A fast growth occurred in the first 10 min after the addition of TG, resulting in a sample with a band edge located at 4.0 eV. The growth was followed by elimination of the sample's scattering and emergence and increase of the luminescence during the heating process. Transmission electron microscopy and x-ray diffraction analyses demonstrated round shaped cubic phase ZnS:Mn nanocrystals with an average size around 3.0 nm. The luminescence emerged from about 15 min after the addition of TG. The emission was located at around 585 nm, demonstrating the Mn incorporation inside the particles. The luminescence intensity increased with time and saturated during the second step of the heating process. Finally, we propose a model explaining the formation and changes in the scattering and luminescence characteristic of the ZnS:Mn nanoparticles. The model is based on the separation of the nucleation and growth steps of the synthesis in the first and second steps of the reaction. This separation directly affects the achievement of the luminescent ZnS:Mn nanocrystals.