Synthesis and Photoluminescence of Nanocrystalline ZnS:Mn²⁺

Abstract

The influence of the synthesis conditions on the properties of nanocrystalline ZnS:Mn 2+ is discussed. Different Mn 2+ precursors and different ratios of the precursor concentrations [S 2-]/[Zn 2+ ] were used. The type of Mn 2+ precursor does not have an effect on the luminescence properties in the synthesis method described. On going from an excess of [Zn 2+ ] to an excess of [S 2-] during the synthesis, the particle diameter increases from 3.7 to 5.1 nm, which is reflected by a change in the luminescence properties. Photoluminescence measurements also showed the absence of the ZnS defect luminescence around 450 nm when an excess [S 2-] is used during the synthesis. This effect is explained by the filling of sulfur vacancies. The ZnS luminescence is quenched with an activation energy of 62 meV, which is assigned to the detrapping of a bound hole from such a vacancy.Over the past few years, considerable interest in the novel optical and electrical properties of doped semiconductor nanocrystals (NC) has emerged. 1-5 These structures are interesting from a physical and chemical point of view mainly because several of their properties are very different from those of bulk materials. 3 Especially, the significant sizedependent shift in the band gap has attracted much attention. This so-called quantum-size effect allows one to tune the emission and excitation wavelengths of a nanocrystal by tuning the crystal radius r. A quite good first-order approximation to calculate the energy of the band gap is given by the Brus equation. 1 In the case of zinc blende ZnS, the bulk values of all the materials parameters are known. 6 For nanocrystalline ZnS this results in a relation between the particle radius r, in nanometers, and the band gap E, in electronvolts, as follows:Manganese-doped materials represent a class of phosphors that have already found their way into many applications. The 4 T 1 f 6 A 1 transition within the 3d 5 configuration of the divalent manganese ion (Mn 2+ ) has been studied extensively and its orange-yellow luminescence in ZnS is well documented. This luminescence was also observed in nanocrystalline ZnS:Mn 2+ 7,8 and applications have already been suggested. 9-11 Different types of Mn 2+ centers are present in nanocrystalline ZnS:Mn 2+ . 12,13 The orange luminescence originates from Mn 2+ ions on Zn 2+ sites, where the Mn 2+ is tetrahedrally coordinated by S 2-.Previous workers have always used equal concentrations of Zn 2+ and S 2-precursors in the synthesis of these ZnS: Mn 2+ nanocrystals. This letter will focus on the effect of the synthesis conditions on several properties of these nanocrystals. Two different Mn 2+ precursors were used and the influence of the ratio of [Zn 2+ ] to [S 2-] was investigated. The experimental comparison will include measurements of the diameter, reflectivity, temperature-dependent photoluminescence (PL) emission and excitation as well as luminescence lifetimes. From these results, new insights into the ZnS-related luminescence are obtained and a qualitative...

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