Mn SUBSTITUTION EFFECTS AND ASSOCIATED DEFECTS IN ZnO NANOPARTICLES STUDIED BY POSITRON ANNIHILATION

Abstract

Nanocrystalline ZnO particles substituted with different concentrations (0–30%) of Mn were synthesized by using a modified ceramic route and characterized by X-ray diffraction, transmission electron microscopy, selected area electron diffraction and energy dispersive X-ray analysis methods. Positron lifetime and coincidence Doppler broadening measurements were used as probes to identify the vacancy-type defects present in them and monitor the changes while doping. The predominant positron trapping center in the undoped ZnO is identified as the trivacancy-type cluster V Zn + O + Zn , which is negatively charged, and it transformed to the neutral divacancy V Zn + O on doping with Mn 2+ ions. The intensity of the defect-specific positron lifetime component got reduced initially indicating partial occupancy of the vacancies by the doped cations but then recovered on further doping due to the additional Zn vacancies created as a result of the increasing strain introduced by the Mn ions of larger radius. The creation of a new phase ZnMn 2 O 4 thereafter changed the course of variation of the annihilation parameters, as the positrons got increasingly trapped in the vacancies at the tetrahedral and octahedral sites of the spinel nanomanganite.