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Structural phase transformations in annealed cubic ZnS nanocrystals

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Abstract

The structural changes of cubic ZnS (cZnS) nanocrystals (NCs) doped with 0.2 at.% Mn2+ pulse annealed in vacuum and in air, up to 500 °C, were investigated by multifrequency electron paramagnetic resonance (EPR), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The samples, prepared by a surfactant (Tween20)-assisted liquid–liquid reaction at pH = 6, consist of NCs with a tight size distribution around 3 nm and high crystallinity self-assembled into a stable mesoporous structure. The EPR spectra of the as prepared samples contain only the characteristic lines of the substitutional Mn2+(I) centers. No spectra from Mn2+ ions localized in (hydro)oxidized regions of the NCs surface were observed. The absence of such a surface layer could explain the stability of the cubic (sphalerite) structure observed by XRD and TEM in the investigated cZnS:Mn NCs annealed in vacuum up to 500 °C. The observation of the cubic-hexagonal transformation for the same NCs annealed in air supports the role of such layer in promoting this structural transformation. The narrowing of the EPR spectral lines above 200 °C with the increase in the average size of the cZnS:Mn crystallites was observed. The effect was more pronounced for the sample annealed in air. EPR also revealed the formation of minute amounts of substitutional Mn2+-type centers in a hexagonal ZnO structure at T ~ 300 °C, corresponding to the early stages of the thermally induced oxidation of the cZnS:Mn NCs.

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Acknowledgments

This work was supported by CNCSIS–UEFISCSU, Project Number PNII-IDEI-523/2008. The authors are grateful to D. Zernescu for expert technical assistance.

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  1. National Institute of Materials Physics, POB MG-7, Magurele, Ilfov, 077125, Romania

    D. Ghica, S. V. Nistor, L. C. Nistor, M. Stefan & C. D. Mateescu

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  1. D. Ghica
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  2. S. V. Nistor
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  3. L. C. Nistor
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  4. M. Stefan
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  5. C. D. Mateescu
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Correspondence to D. Ghica.

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Ghica, D., Nistor, S.V., Nistor, L.C. et al. Structural phase transformations in annealed cubic ZnS nanocrystals. J Nanopart Res 13, 4325–4335 (2011). https://doi.org/10.1007/s11051-011-0379-y

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  • DOI: https://doi.org/10.1007/s11051-011-0379-y

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