Theoretical investigation of the cathodoluminescence spectra of Co-doped ZnO nanowires

The morphology, structural and optical properties of the Co2+ ions doped in ZnO nanowires were investigated by S. Geburt et al. (2013) [1]. The cathodoluminescence CL spectrum is composed of the ZnO near-band-edge NBE (ultraviolet emission), green emission band attributed to the recombination of carriers at intrinsic ZnO defects and sharp red emission assigned to the Co2+ intra-3d luminescence. The photoluminescence excitation PLE spectrum shows the direct transitions from the ground state of Co2+ occupying Td site to the higher excited states. The visible region provides important information about the electronic structure of the Co2+ doped zinc oxide nanowires. This work investigates an interpretation of the origin of the ultraviolet NBE and discusses the defects involved in the broad green emission band. A theoretical crystal-field analysis of the visible lines, associated to the Co2+ ion transition occupying Td site symmetry in ZnO host crystal, is also realized which leads to a satisfactory correlation between experimental and calculated energy levels. The electronic structure was compared to the structure reported for Co2+ ion doped in ZnO nanocrystalline (nanorods, nanoparticles) and bulk crystals. The quantum confinement effect of ZnO nanowires is interpreted.