Study On The Synthesis And Physical Properties Of ZnO-based Diluted Magnetic Semiconductors

The ZnO-based diluted magnetic semiconductors (DMSs) doped with different magnetic ions (Eu3+, Sm3+, Mn2+, Fe3+, Co2+ and Ni2+) were synthesized by sol-gel method and hydrothermal synthesis. The morphology, crystalline structures, photoluminescence, magnetic properties and magnetoresistance were studied systemically in the films, powder and nanometer structure samples. The crystalline structures of the film materials are highly c-axis oriented wurtzite symmetry. The effect of synthesis conditions, such as sintering ambience, temperature and the concentration of doped ions, on the highly oriented crystalline growth were studied. Among these conditions, the sintering ambience is the most important factor, which affected the oriented growth of the films. The decrease of oxygen concentration in the sintering ambience strongly improve the crystal grow along the c-axis. In addition, the increases of the sintering temperature also enhance the properties mentioned above, while the increases of the doped ions concentration. The surface of the film samples is cubic-like grains in the samples sintered in vacuum, while the samples are flake grains in the samples sintered in air. The diameters of the grains are less than 100nm for all samples. No valence change occurred for the doped and host ions (Eu3+, Sm3+, Mn2+, Fe3+, Co2+, Ni2+, Zn2+ and O2-)during the synthesis process. The lowest transmittance of the ZnO: (Eu, Mn, Co, Fe and Ni) films (absorption edge) were observed between 356nm and 369nm, which corresponds to band gap (Eg=3.34-3.46ev) of ZnO semiconductor. A number of regular interference bands can be found in the Zn1-xEuxO (0.005≤x≤0.15) films, which can be attributed to the change of the interface property of the films doped with Eu ions. Additionally, the absorption band corresponding to d-d transitions can be found in the visible region in ZnO:Co films. The transmittance of the films decreased with the increase of the doped ions concentration. The transmittance of the 15 layers films is higher than 90% in the Zn1-xEuxO films, while the others are about 60% in visible region. Not only absorption peaks at 296nm, 394nm and 466nm belonging to Eu3+, but also a broad band with the maximum at 378nm belonging to ZnO could be observed in the Luminescence spectra of Zn1-xEuxO (0.005≤x≤0.15) films. Excitation into the 7F0→5L6 transition line of Eu3+ at 394nm yields the characteristic emissions of Eu3+ corresponding to 5D0→7FJ (J=1, 2, 3, and 4) transitions, which is dominated by the 5D0→7F2 transition at 612nm. Excitation into the ZnO band gap at 378nm, the 5D0→7F2 transition can also be observed. This indicates that there is an energy transfer from ZnO host to Eu3+ ion. The ferromagnetic behavior is found in the Zn0.9Co0.1O film. The ferromagnetic behavior presents a Br =0.21emu/g and Hc =327Oe below 230K. The Zn0.9Eu0.1O film resembles a Curie-Weiss paramagnetic behavior between 4~400K, while presents an obvious magnetization enhancement at low temperature. The Zn0.9Mn0.1O, Zn0.9Ni0.1O and Zn0.9Co0.1O films give paramagnetic properties above 80K. Resistivity dependence of temperature for the films Zn0.9M0.1O (M= Eu, Mn, Co, Fe and Ni) is typical semiconductor trends, in which resistivity increase with decreasing temperature. The maximum MR of 14.53% occurred at 110K under 2T applied magnetic field in the Zn0.9Eu0.1O films. The MR of Zn1-xTMxO films also were studied under different magnetic field, temperature, and doped ions concentrations. The solubility of the magnetic ions in powder samples is less than that in films. The ZnO films doped with Mn2+, Fe3+, Co2+ and Ni2+ are paramagnetic above 80K. Unlike the films samples, the ferromagnetic behavior cannot be observed in the Zn0.9Co0.1O powder samples, indicating the magnetic properties of the DMSs strongly depend on the synthesis conditions.The DMSs (ZnO:TM) can be successfully assembled into the channel of MCM-41 and AAO template by sol-gel method. The wurtzite symmetry of the MCM-41 pores was not changed while pore diameter became smaller after the DMSs entered the pores. The size of the DMSs particles is less than 100nm in the AAO channels. The Zn0.9Co0.1O-MCM-41 presents a superparamagnetic behavior between 80k and 300K, while Zn0.9TM0.1O-MCM-41 (TM=Mn, Fe and Ni) samples are paramagnetic at the same temperature region. The AAO-ZnO:TM (TM=Mn, Fe, Co and Ni) samples are also paramagnetic between 80K and 300K. The shape of the product changed from particle to rod-like can be controlled by pH values of the solution in hydrothermal process. The cubic-like ZnO:Co particles was found when the pH=10~11, while rod-like product was obtained when the pH=14. When pH=5, the rod-like ZnO:RE(RE=Eu,Sm)nanoparticles can be obtained by hydrothermal synthesis. The ZnO:Co nanocrystal obtained is superparamagnetic at 80K, while ZnO:RE(RE=Eu,Sm)are weak paramagnetic at the same temperature. The zinc citrate single crystal with a new layer structure was synthesized under the hydrothermal condition. To our knowledge, the current compound is the only example with a non-molecular structure within the metal citrate complexes known to date, which have discrete molecular structures.