Substitution-induced structural transformation in Mn-doped ZnS nanorods studied by positron annihilation spectroscopy

Zinc sulphide nanorods were doped with manganese ions and the structural changes were monitored through positron annihilation measurements. X-ray diffraction and high-resolution transmission electron microscopy revealed a transformation from hexagonal to cubic structure, and we observed characteristic changes in the measured positron lifetimes, their intensities and Doppler broadened lineshapes. These features were explained on the basis of positron trapping and subsequent annihilation at the nanorod surfaces. Further incorporation of manganese resulted in an increase of the diameter of the nanorods, and the effect was reflected in the decreasing lifetime of positrons. At sufficiently high concentration of manganese ions, Mn-Mn clusters were formed. The results are compared with those obtained for ZnS nanoparticles where vacancies within the grains trapped a significant fraction of positrons.