Controlled growth of ZnS:Mn nanophosphor in porous silica matrix

Abstract

The development of nanophosphors of desired sizes and properties for various practical applications and its growth in quantitative amounts inside the pores of an inorganic matrix is presented. By doing so, nanophosphors get surface passivated and are stabilized against environmental attacks. Accordingly, in the present study, the growth parameters for ZnS:Mn nanophosphors were systematically studied inside a SiO2 gel matrix, which can act as a capping agent as well. The samples were prepared using the sol-gel technique, followed by annealing at different temperatures to remove the trapped fluid inside the amorphous silica cage. Two categories of samples with lower (3.11×10−4) and higher (1.5×10−1) ZnS∕SiO2 molar ratios were studied. The x-ray diffraction and scanning electron microscopy observations show that upon annealing, the nanocrystals grow in size and undergo a phase transition from cubic to hexagonal at temperatures between 700 and 900°C. This is one of the very few known reports published on nano hexagonal ZnS formation. The observed phase transition is possibly the combined effect of the high-temperature (∼900°C) and annealing-related compressive stress induced on the nano-ZnS by the silica cage. There has been formation of an intermediate metastable phase of the zinc silicate at annealing temperatures around 700°C. The particle size distribution and emission properties were correlated using the optical absorption and photoluminescence (PL) results. The unannealed cubic nano-ZnS:Mn samples gave a broad PL, peaking at ∼585nm, whereas the samples annealed at 900°C for 5h gave a narrow and sharp PL at ∼590nm. This is attributed to the more efficient T14→A16 transitions of Mn in the resultant hexagonal nano-ZnS matrix.