Theoretical examination of defect structures and spin Hamiltonian parameters of manganese (II)- and cobalt (II)-doped ZnO nanowires

Abstract

This study theoretically investigates the defect structures of Mn2+ at 3 and 10 at. % as well as those of Co2+ at 5 at. % in ZnO nanowires by analyzing their experimental spin Hamiltonian parameters (g factors, hyperfine structure constants A, and zero-field splitting D) based on perturbation formulae in trigonally distorted tetrahedral 3 d5 and 3 d7 clusters, respectively. Unlike the trigonal distortion of the host Zn2+ site in ZnO nanowires, the impurities Mn2+ and Co2+ underwent displacements ΔZ (≈0.0223 and 0.0233 Å for Mn2+, and 0.040 and 0.043 Å for the two Co2+ centers) away from the ligand oxygen triangles along the C3 axis at 3 and 10 at. %, and at 5 at. %, respectively. In light of the significant covalent interactions of the two Co2+ centers, their contributions to the spin Hamiltonian parameters due to the charge transfer mechanism need to be included in the theoretical calculations. The defect structures and spectral properties are also discussed in view of the different crystalline morphologies of the doped samples, which indicate that the values of ΔZ of the Mn2+ centers decreased in low-dimensional materials, whereas those of the Co2+ centers remained nearly constant owing to strong covalence.