Structural phase transformations in annealed cubic ZnS nanocrystals

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.