Luminescence enhancement of Mn doped ZnS nanocrystals passivated with zinc hydroxide
Introduction
Zinc sulfide, as an important type of II-VI group semiconductor with its wide band gap energy of 3.67 ev at room temperature, is particularly suitable for use as luminescent host materials for a very large variety of dopants. It has been extensively studied for a variety of applications, such as electroluminescent devices, photoconductors, optical coatings, solid state solar window layers, photo catalysts, and so on [1], [2], [3]. There have been numerous research reports about the structural and luminescent properties of doped ZnS, such as ZnS doped with manganese [4], copper [5], silver [6] and europium [7], [8]. The doping ions act as recombination centers for the excited electron–hole pairs and result in strong and characteristic luminescence. However, with the decrease of particle size, extremely high surface-to-volume ratio causes the surface states to act as luminescent quenching centers. Hence, the passivation of surface is of crucial importance for the applications of this type of luminescent semiconductor nanomaterials. In order to get a better passivation, inorganically passivated (or core/shell structured) nanocrystals have been developed and shown dramatically enhanced properties [9]. Photoenhanced luminescence has been observed in ZnS/CdS/ZnS quantum dot quantum well [10], ZnS:Mn/ZnS nanoparticles [11] and ZnS:Mn/ZnO nanophosphors [12]. Photo-oxidation of the ZnS nanocrystals surface in the presence of oxygen and water, speculated by Bol and Meijerink [13], would led to the formation of ZnSO4 or Zn(OH)2 which can serve as passivating layers on ZnS nanocrystals surface. Besides, in 1987, A. Henglein group reported that when a Cd(OH)2 layer was deposited on the CdS nanocrystals [14], the fluorescence quantum efficiency was increased by 50%, and their luminescence-stability was enhanced by 2000 times.
Compared with the unpassivated nanocrystals, the photoluminescence of the nanocrystals with a core/shell structure was enhanced, which is usually interpreted as the surface passivation effect of the nanocrystals inhibits the nonradiative recombination, thus improves photoluminescent properties.
With these in mind, we aimed to prepare Mn-doped ZnS nanocrystals by solvothermal method and coat the nanocrystals with different thicknesses of Zn(OH)2 shells through precipitation reaction. The impact of Zn(OH)2 shells on the luminescent properties of the ZnS:Mn nanocrystals was investigated.
Section snippets
Sample preparation
All the reactants and solvents used in this study were analytical grade and used without any further purification.
In a typical synthesis process, 0.008 mol zinc acetate [Zn(CH3COO)2·2H2O], 0.004 mol thioacetamide [CH3CSNH2] and 4 × 10−5 mol manganese acetate [Mn(CH3COO)2·4H2O] were put into a Teflon-lined stainless steel autoclave of 72 ml capacity, and then the autoclave was filled with a mixture solvent of ethylenediamine and deionized water (in 1:1 volume ratio) to 80% of its total volume. After
Formation mechanism of Zn(OH)2 shells
According to the viewpoints of H.C. Warad et al. [15] and L.L. Sun et al. [16], the formation mechanism of Zn(OH)2 shells on the surface of ZnS:Mn nanocrystals can be explained as follows: First, when the Zn(NO3)2 aqueous solution was added into the ZnS:Mn suspension, the Zn2+ ions (dissociated from Zn(NO3)2) were attracted on the surface of ZnS:Mn nanocrystals, by reason of the existence of the S2− dangling bonds on the surface of ZnS:Mn nanocrystals (Fig. 1a). Then, Zn(OH)2 was produced as
Conclusion
In summary, Mn-doped ZnS nanocrystals prepared by solvothermal method had been successfully coated with different thicknesses of Zn(OH)2 shells. The luminescent properties of ZnS:Mn/Zn(OH)2 with different thicknesses of Zn (OH)2 shells were well studied. With the increasing of the thickness of the Zn (OH)2 shell, the number of surface Mn2+ ions decreased and the PL intensity of ZnS:Mn/Zn(OH)2 was significantly enhanced. The luminescent decay curves were fitted well with the fitting ones by
Acknowledgements
We thank the National Science Foundation of China (NSFC 50672089) and the Excellent Young Teachers Program of MDE, P.R.C. ([2003] 355) for financial support.
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2017, Journal of LuminescenceCitation Excerpt :Work on developing the methods of increasing ZnS:Mn nanoparticles luminescence efficiency is actively underway. Jiang and coworkers in Ref. [44] were able to increase the PL intensity of ZnS:Mn nanoparticles by coating them with Zn(OH)2 shell. Bol and Meijerink.