| Diluted magnetic semiconductors (DMSs), which are fabricated by doping semiconductors with magnetic elements, are attracting a great deal of interests because of their potential applications in spintronic devices. ZnO-based DMSs were theoretically predicted to be good candidates for room temperature ferromagnetism, and these predictions also initiated intensive research on ZnO DMSs with or even without transition metal (TM) doping. However, the origin of intrinsic ferromagnetism in these systems is still under controversy. This thesis presents magnetism investigation on ZnO DMSs doped or co-doped with dopants Co and Ga. The ZnO DMSs samples are prepared by different methods, such as the thermal decomposition, the sol-gel route and pulsed laser deposition (PLD). Experimental measurements together with first-principles calculations are used to investigate the room temperature ferromagnetic mechanism of ZnO DMSs samples.Nanocrystalline Co-doped ZnO sample was synthesized by the thermal decomposition that exhibits ferromagnetism above the room temperature. It was found that Co ions with a valence 2+ in the sample indeed substituted Zn ions in the wurtzite ZnO lattice. The Co K-edge XANES together with the absorption spectra indicated the presence of oxygen vacancies surrounded the Co center. An electron trapped in the oxygen vacancy constituted an F-center, and the electron occupied an orbital which overlaps the d shells of both Co neighbors, leading to a ferromagnetic coupling between the two Co ions. Finally, the Co-doped ZnO sample showed the intrinsic ferromagnetism above the room temperature.The origin of high temperature ferromagnetism in (Co, Ga)-codoped ZnO, prepared by the sol-gel method, has been investigated. It was found that Co ions substitute Zn sites, while Ga ions were located at octahedral interstitial sites together with one O vacancy. The strong hybridization between the charge carriers in the Co-3d and Ga-4p states and electronic polarization for surrounding O ions at Co ions were detected. Finally, the Ga-4p electrons merged with conduction band and polarized O ions acted as medium for an indirect exchange between the Co ions, which could be the origin of ferromagnetism in the (Co, Ga)-codoped ZnO.Thin films of pure ZnO and Ga-doped ZnO were prepared by pulsed laser deposition. It is found that Ga ions were located at interstitial sites with three plus valences in Ga-doped ZnO film, and the large bulk resistance of films rules out the existence of carrier-mediated ferromagnetism in the present films. On the basis of first principles calculations, it was found that the Zn vacancies at O-terminated (0001) surface led to a high number of 2p holes in the O valance band, which might be the FM source of pure ZnO film. With the Ga doping, the O vacancies at O-terminated (0001) surface resulted in the spin polarization of Ga-4s electrons. Thus, the Ga exchange splitting further induced the polarity of O and Zn atoms surrounding the Ga atom.The phase transformation path for wurtzite ZnO nanorods were studied using the combined techniques of high pressure synchrotron X-ray diffraction measurements and first-principles calculations. It was found that the transformation path for the WZ-to-RS transition occurred at about 12GPa for ZnO nanorods was governed by first a dramatic deformation along the [0001] direction with little change along the vertical direction, and then the deformation was observed along the two directions together.High-pressure behaviors of bulk and nanocrystalline SnO2 samples have been studied by in situ high-pressure synchrotron radiation X-ray diffraction measurements at ambient temperature. It was found that the smaller the nanocrystal SnO2, the higher the onset transition pressure for the rutile-to-cubic phase transformation in SnO2. The enhancement of the transition pressure was mainly due to the surface energy differences between the rutile and cubic phases and/or the enhancement of energy band gap when the SnO2 crystallite size gets smaller.
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