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

Abstract

In this work we report a new thermochemical method for the synthesis of ZnS:Mn nanocrystals.Zn(NO3)2 andNa2S2O3 were used as the precursorsand Mn(NO3)2 was the source ofimpurity. 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. Thereaction was done in two steps. In the first step, the precursors were heated at96 °C for an hour without TG. In the second step, TG was injected to the solution and theheating process was continued for longer heating durations. A fast growth occurredin the first 10 min after the addition of TG, resulting in a sample with a bandedge located at 4.0 eV. The growth was followed by elimination of the sample’sscattering and emergence and increase of the luminescence during the heating process.Transmission electron microscopy and x-ray diffraction analyses demonstrated roundshaped cubic phase ZnS:Mn nanocrystals with an average size around 3.0 nm. Theluminescence emerged from about 15 min after the addition of TG. The emissionwas located at around 585 nm, demonstrating the Mn incorporation inside theparticles. The luminescence intensity increased with time and saturated during thesecond step of the heating process. Finally, we propose a model explaining theformation and changes in the scattering and luminescence characteristic of theZnS:Mn nanoparticles. The model is based on the separation of the nucleation andgrowth steps of the synthesis in the first and second steps of the reaction. Thisseparation directly affects the achievement of the luminescent ZnS:Mn nanocrystals.