The investigation of the defect structures for Co2+ in ZnO microwires, thin films and bulks

•The defect structures for Co2+ in ZnO microwires, thin films and bulks are theoretically investigated.•Co2+ suffer the trigonal distortions away from the ligand triangles along the C3 axis due to Jahn-Teller effect.•The contributions of the charge-transfer (CT) mechanism are taken into account.•The defect structures and spectral properties of the three ZnO:Co2+ systems are discussed.

The defect structures for Co2+ in ZnO microwires, thin films and bulks are theoretically investigated by analyzing the electron paramagnetic resonance (EPR) parameters (zero-field splitting D, g factors g// and g⊥ and hyperfine structure constants A// and A⊥) for trigonally distorted tetrahedral 3d7 clusters in a consistent way. The impurities Co2+ are found to suffer the displacements (≈0.046, 0.044 and 0.045 Å) away from the ligand triangles along the C3 axis in ZnO microwires, thin films and bulks, respectively, which considerably reduce the trigonal distortions of the Zn2+ sites in the hosts. Apart from the contributions from the conventional crystal-field (CF) mechanism, those from the charge-transfer (CT) mechanism are important and should be included in the EPR analysis. The defect structures and spectral properties of the three ZnO:Co2+ systems are discussed.