| Magnesium oxide (MgO) is a wide-band-gap material having a band gap at about 7.6 eV at room temperture. The advantage of MgO is toxic-free, low-cost, temperature resistance, anti-corrosion, high transmittance, and widely distributed on Earth. The large value of the exciton binding energy of MgO (80 meV) is advantageous for excitonic light emitters. Micro/nano MgO crystals have importeant applications on optics, catalysis, magnetism, mechanics and chemical industry. In plasma display panels (PDPs), submicro-sized MgO crystals are used on top of the regular MgO thin-film protective layer. However, some aspects of micro/nano MgO crystals have still no yet been understood, such as the ultraviolet (UV) and visible optical characteristics, and the excitonic effects. Micro/nano MgO crystals have different types of topological feature, including low-coordinated corners, edges, and steps on the surfaces of nanocrystallites and interfaces between the terraces of two different nanocrystallites. Nowadays, nanoscience and optoelectronic devices have motivated increasingly active research on the wide-band-gap materials, which implies the importance for reaserch on the micro/nano MgO crystals.Micro/nano MgO crystals emit 5.3 eV (235 nm) UV light, and it has been found that the emission of the 5.3 eV light can be used as an indicator of strong electron exoemission. The physics of the recombination processes however is not understood, while the UV optical properties of MgO crystals and the effect of doping are relatively unknown and require further study. We therefore investigated the generation and diffusion of excitons in MgO crystals, by exciting the crystals by electron-beam at a scanning electron microscope (SEM) and vacuum ultraviolet (VUV) radiation of a synchrotron, and measuring the cathode luminescence (CL) spectra, excitation and emission properties of the MgO crystals, both doped and undoped. The different types of MgO crystals, such as undoped MgO, MgO:F, MgO:Si, MgO:Sc and MgCaO, used in a PDP, enable us to study the optical properties in more detail.The results show that excitons play an important role in the diffusion of energy inside MgO crystals, resulting in 5.3 eV (235 nm) emission from impurity centres at room temperature, and of 5.76-5.85 eV emission from the 5C-surfaces at 16 K. The observed results lead us to the conclusion that there is a clear separation between two different excitation-diffusion paths. Bulk excitons (6C) can be generated at 7.7 eV and then diffuse towards the surface, emitting 5.76-5.85 eV radiation. Surface excitons on the other hand can be excited directly at 5.7 eV, then probably diffuse along edges to corners, finally emitting 400-500 nm light.In PDPs, the speed of addressing is determined by the discharge formation time Tf, and by the statistical variation time Ts of the appearance of priming electrons. As the 5C surface excitons are triplets, they have a relatively long lifetime and can lose their energy almost only by Auger emission of electrons. Therefore, the intermediate creation of the 5C surface excitons might play an important role in the process of delayed electron emission from micro/nano MgO crystals. These electrons are very important for the priming of the addressing discharge in a PDP.Nano MgO crystals are applied in state of the art core-shell colloidal quantum dots (QDs). The ZnO-MgO core-shell QD is one of the promising active nanostructures, thanks to the UV lightemitting at the near-band-edge emission (NBE), while the stability and performance of ZnO nanocrystals is improved by the MgO passivation. Such a shell passivation could reduce the surface-related defect states as well as confine the charge carriers into the core region due to the band offset potentials, resulting in more efficient and photostable luminescent nanocrystals. Transmission electron microscopy (TEM), UV/Vis absorption spectroscopy, UV-Vis-NIR spectrophotometer, atomic force microscope (AFM), and X-ray diffractometer (XRD) have been utilized to analyze and characterize the nanostucture and optical properties. The results show clear evidence of the NBE, based on nano MgO or ZnO, which is important to develop UV-QLED device in the future.In conclusion, this dissertation focuses on the diffusion of exciton energy inside the micro/nano MgO, which results the UV emission at about 5.7-5.8 eV and 5.3 eV, while the broadband visible light is only excited by surface self-trapped excitons. The ralation between exo-electrons emission and surface excitons have been elaborated. The impurities in MgO play an important role in the UV emssion at around 5.3 eV. Nano MgO crystals are applied in core-shell QDs, which are important to develop ZnO and MgO based UV-QLED device in the future. |
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