Preparation And Properties Of The Rod-like ZnO Based Diluted Magnetic Semiconductors

ZnO is an attractive semiconductor with a direct wide-band gap (3.37eV) and large exciton binding energy (60meV) at room temperature and high chemical stability, excellent piezoelectric, pyroelectric and optoelectronic properties. More importantly, diluted magnetic semiconductors (DMSs) based on ZnO become a good candidate for potential spintronics integrating the excellent optical, electronic and magnetic properties because of their ferromagnetic properties at or above room temperature predicted by theoretical studies, large solubility of magnetic ions and being transparent to visible light. However, up to now, the ferromagnetism of the obtained DMSs based on ZnO is not stable. Experimental reproducibility is poor and the origin of ferromagnetism has no unified theory of interpretation. So it is of great practical significance to study DMSs based on ZnO through experiment and theory.Firstly, Rod-like ZnO crystals were synthesized by hydrothermal method employing zinc acetate dihydrate [Zn(CHsCOO)2·2H2O] and sodium hydroxide (NaOH) as the starting reactants and their growth mechanisms were tentatively elucidated. Moreover the effect of surfactant PAM on the structures and properties of rod-like ZnO crystals were also investigated. The results show that the PAM can hardly influence the morphologies of Rod-like ZnO crystals, but it can alter the stoichiometric ratio and adjust the type and number of the dominant intrinsic defects. The intensities of the violet and blue emissions of the samples increase at first, and then decrease, and the green emission increase with the increase of PAM concentration. All the samples synthesized with the PAM concentration of0%,0.00005%and0.0003%show paramagnetism. When the PAM concentration is0.0001%, the rod-like ZnO crystals exhibit ferromagnetism at room temperature, in which zinc vacancies are the main defects and considered to be the origin of ferromagnetism.Secondly, the structural, magnetic and optical properties of rod-like ZnO doped with Cr, Co and Ni were studied using chromium acetate [Cr(CH3COO)3], cobalt acetate tetrahydrate [Co(CH3COO)2·4H2O] and nickel acetate tetrahydrate [Ni(CH3COO)2·4H2O] as dopants, respectively. Althougt all the rods grow along the preferred direction of [001], they grow larger, the nonuniformity become more obvious and the instrinc defects increase with the increase of doping concentration. The intensities of the visible emission of Cr-doped ZnO and the violet and blue emissions of Co-doped ZnO increase at first, and then decrease with the increase of doping concentration. The intensities of the visible emission of Ni-doped ZnO increase and the green emission of Co-doped ZnO decrease with the increase of doping concentration. All the samples of ZnO doped with0.5%Cr, Co or Ni show paramagnetism. The samples of ZnO doped with1%Cr, Co or Ni exhibit ferromagnetism at room temperature.3%Cr-doped ZnO exhibits paramagnetism because of the antiferromagnetic interaction between Cr ions. The ferromagnetism of3%Co-doped ZnO become weaker comparing with1%Co-doped ZnO due to the antiferromagnetic secondary phase of CO3O4.3%Ni-doped ZnO exhibits antiferromagnetism because antiferromagnetic secondary phase of Ni(OH)2come into being during hydrothermal process.Lastly, the electronic structures, magnetic and optical properties of bulk ZnO and ZnO nanowires undoped or doped with Cr, Co or Ni were investigated by the first-principles calculation based on the density functional theory. The results show that the undoped bulk ZnO and ZnO nanowires exhibit no magnetism and the band gap of ZnO nanowires is wider than that of bulk ZnO because of quantum confinement effect. The Co-doped bulk ZnO exhibits ferromagnetism. The Co-doped ZnO nanowires also show ferromagnetism when the Co atoms substitute for Zn sites in the body of nanowires and its magnetization become larger than that of Co-doped bulk ZnO. Their ferromagnetism originates from the coupling interaction of the electronics between Co3d and O2p. However, the nanowires show no magnetism when the Co atoms substitute for Zn sites on the surface of nanowires. The bulk ZnO and ZnO nanowires doped with Cr or Ni all exhibit no magnetism whenever the Cr or Ni substitutes for Zn in the body or on the surface of nanowires. The defects play an important role in the ferromagnetism of Cr and Ni doped ZnO samples.