Vacancy-type defects and their evolution under Mn substitution in single crystalline ZnO nanocones studied by positron annihilation

Bipyramidal-shaped single crystalline nanocones of ZnO, doped with Mn ²⁺ ions up to different concentrations, were synthesized through a solvothermal route and characterized by X-ray diffraction and transmission electron microscopy. The compositional analysis was also carried out by energy-dispersive analysis of X-rays (EDAX). Positron annihilation studies were carried out to extract information on the vacancy-type defect clusters and their evolution under doping, which may have a major influence on the physical properties of the material. In the undoped ZnO, trivacancy-type defects of the type V _Zn+O+Zn are present. Doping by Mn ²⁺ ions reduced them to divacancies (V _Zn+O) as a result of the ion-vacancy complex formation. These were indicated by the measured positron lifetimes and coincidence Doppler broadening measurements. An interesting observation is the reduction in base diameters of the nanocones at high (1 atom %) dopant concentrations, an effect of increased strain due to occupancy of Zn ²⁺ vacancy sites by Mn ²⁺ ions of slightly larger radius. As the diameters of the grains reduce to below the thermal diffusion lengths of positrons, significant number of annihilation events seemed to result from the surfaces of the nanocones. The intercrystalline regions also gave a favorable site for the formation and "pick-off" annihilation of orthopositronium atoms.