Theoretical Investigation Of MgO And ZnO-based Nanomaterials

Compared with bulk materials, one-dimensional nanowires possess more controllable parameters, such as the components, the radical size, the axial and radical orientation, and the characteristics of the surface, to modulate their properties. The manipulation of the properties could be realized through monitoring these parameters of the one-dimensional nanowires. Of many kinds of one-dimensional materials, MgZnO nanowires have attracted much attention due to the special properties of ZnO and MgO. On one hand, ZnO is a typical wide gap semiconductor (Eg=3.37eV) with the exciton binding energy of60meV, which is much higher than the thermal active energy (26meV) that provided by room temperature. These provide ZnO a sound prospect application in ultra-violet optoelectric devices at room and higher temperature. On the other hand, the band gap of MgO is as high as7.8eV and the radius of Mg2+and Zn2+are so close that the substitution of Zn by Mg in the system do not induce obvious lattice mismatch. Based on the controllable parameters of nanowires and the properties of MgO and ZnO, the MgZnO alloy nanowires attracted many research interests. It’s expected that the band gap of the alloy system could be regulated from3.37eV to7.8eV continuously by changing the concentration of Mg and structural parameters of a nanowire. Accordingly, the one-dimensional material that could be worked at any range of the ultra-violet area could be realised. Although many valuable results had achieved by the experiment research, some inherent mechanisms are still unknown which requires theoretical investigations.In this thesis, we mainly investigated the structure and electronic structural properties of the MgZnO alloy nanowires, ZnO/MgZnO radial and axial heterostructure nanowires by using the density functional theory. Our results would be helpful in understanding the regulation rules of the concentration of Mg and the structural parameters to the band gap of the nanowires. Meanwhile, the regulation rules may provide a reference to synthesis of the MgZnO nanowires with the desired band gaps. Besides, we also investigate the structural characters and electronic properties of MgO monolayer and Y2O3stabilized ZrO2(YSZ) films. Two new kinds of monolayers are founded. For the YSZ that used as an oxygen detector in nuclear coolant device, the performance of the detectors with diffierent orientations are tested.This thesis contains six chapters. The first chapter mainly presents the experimental summary of the structures and properties of MgZnO alloys, ZnO/MgZnO radial and axial heterostructure nanowires. The modulation of the electronic structures and optical properties by the Mg concentration and structural parameters are mainly shown. Then the electronic properties of MgO films and the oxygen migration behavior in YSZ are briefly introduced from the experimental and theoretical researches.In the second chapter, the density functional theory and the computational codes that used in our calculations are briefly introduced.In the third chapter, we mainly discussed the modulation trends of the structures, electronic structures and optical properties by the concentration of Mg and nanowire sizes for MgZnO alloy nanowires. Take the alloy nanowire with the biggest radius (D=3.23nm) for example, it’s found that the Mg atoms tend to distribute discretely and uniformly in the MgxZn1-xO nanowires with low concentration of Mg (x<30%). It is consistent with the fact that MgO phase was not observed in experiments. Through the calculation of the binding energy of the wurtzite and cubic alloy nanowires, we found that the phase transition from wurtzite to cubic structure occurs at x=0.65, which is a little higher than that of MgZnO film in experiment (x=0.62). For the modulation of the electronic structure, the band gap could be modulated by the component of Mg by by lifting the CBM and pushing down the VBM simultaneously. It was found that the band gap increase from3.37to4.35eV linearly as the component of Mg increase from0to30%. The blue shift of optical absorption edge is also observed as the concentration of Mg increases.The structures and electronic properties of the ZnO/MgZnO radial and axial heterostructure nanowires were discussed in the fourth chapter. For the ZnO/MgZnO radial heterostructure (core shell) nanowire, the concentration of Mg and the size of MgZnO shell modulate the band gap simultaneously but with different ways. The VBM is mainly dominated by Mg atoms while the CBM is mainly affected by the size of shell through the modulation of Zn-O bond length in core. Furthermore, the modulation trend by these parameters can be expressed by an experimental equation well. The distribution of electrostatic potential along the radial direction is also discussed at last. For the ZnO/MgZnO axial heterostructure nanowire, the distribution behavior of Mg atoms and the stability of the nanowires are studied. Be similar to the core shell nanowire, both of Mg and ZnO quantum well could modulate the band gap of the axial heterostructure nanowire.In the fifth chapter, the structure and electronic structures of the MgO-(111) and (100) monolayer and belts are introduced. For the MgO(100) monolayer, it is found that the structures characterizing with either rumpling or wave-like features are more stable than the planar sheet. The band gaps modulated by the external uniform strain are also studied for the two kinds of monolayers. Besides, the graphane-like MgO(111) nanobelt with zigzag edge shows semimetal character, while the armchair (111) nanobelt and (100) nanobelt show semiconductor property. The band gaps of (111) and (100) belts could be modulated from2.75to2.95eV and3.75to3.95eV respectively.In the sixth chapter, we focus on the behavior of Y2O3stabilized ZrO2(YSZ) as the oxygen detector in the nuclear coolant device. The results show that both of the YSZ-(100) and YSZ-(110) films could be used in detecting the oxygen concentration. The energy barrier of O2-enters into the YSZ-(100) film from the PbBi alloy liquid and the migration behavior of O2-in the film are also shown finally.