Luminescence enhancement of Mn doped ZnS nanocrystals passivated with zinc hydroxide

doi:10.1016/j.apsusc.2007.05.067

Applied Surface Science

Volume 253, Issue 24, 15 October 2007, Pages 9330-9335

https://doi.org/10.1016/j.apsusc.2007.05.067 Get rights and content

Abstract

Mn-doped ZnS nanocrystals prepared by solvothermal method have been successfully coated with different thicknesses of Zn(OH)₂ shells through precipitation reaction. The impact of Zn(OH)₂ shells on luminescent properties of the ZnS:Mn nanocrystals was investigated. X-ray diffraction (XRD) measurements showed that the ZnS:Mn nanocrystals have cubic zinc blende structure. The morphology of nanocrystals is spherical shape measured by transmission electron microscopy (TEM). ZnS:Mn/Zn(OH)₂ core/shell nanocrystals exhibited much improved luminescent properties than those of unpassivated ZnS:Mn nanocrystals. The luminescence enhancement was observed with the Zn(OH)₂ shell thickening by photoluminescence (PL) spectra at room temperature and the luminescence lifetime of transition from ⁴T₁ to ⁶A₁ of Mn²⁺ ions was also prolonged. This result was led by the effective, robust passivation of ZnS surface states by the Zn(OH)₂ shells, which consequently suppressed nonradiative recombination transitions.

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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Luminescence enhancement of Mn doped ZnS nanocrystals passivated with zinc hydroxide

Abstract

Introduction

Section snippets

Sample preparation

Formation mechanism of Zn(OH)2 shells

Conclusion

Acknowledgements

Mat. Sci. Eng. C

J. Cryst. Growth

Solid State Commun.

Mater. Chem. Phys.

J. Colloid. Interface Sci.

Mater. Lett.

Physica E

Appl. Phys. Lett.

Mater. Lett.

Opt. Mater.

Appl. Phys. Lett.

J. Appl. Phys.

Formation mechanism of Zn(OH)₂ shells