Symmetry Breaking In The Formation Of ZnO And MgO Nanostructures

In recent years, with the developments of quantum field theory, string theory and theory of elementary particles, symmetry and symmetry breaking have been realized to be more and more important, and have been introduced in many other fields, including those researches at micro-scale or cosmoscopic-scale. Yet at meso-scale, little study focusing on symmetry and symmetry breaking has been done till now. On the other hand, the study of nanocrystal is an interdisciplinary field crossing materials, crystallography and physics at meso-scale. If theories of symmetry and symmetry breaking could be introduced into the investigation of nanocrystals, epochal results would be drawn out to check the generality of the theories about symmetry and symmetry breaking. Whatever the outcome is, it will be a scientific method innovation to introduce symmetry breaking into exploring the mechanism for nanocrystals.ZnO nanocrystals with different morphologies and MgO nanowires along<110> direction have been obtained in our experiments. Based on deep theoretical analyses and experiment results support, it can be concluded that symmetry breaking dictated the formation of these nanocrystals. Thus, not only the spontaneous symmetry breaking, but also the dynamical symmetry breaking have been proved to play their roles in crystalliation of nanocyrstals. Two new mechanisms have been proposed to explain the formation of ZnO nanocrystals and MgO nanowires, respectively. Some puzzling structures of nanocrystals have been explained reasonably by these mechanisms based on symmetry and symmetry breaking.Additionally, systemic symmetry, which describes both the symmetry of lattice and the symmetry of morphology for a crystal particle, is defined here and introduced into our work firstly.The main conclusions of our work are summarized as follows:1. Symmetry breaking did take place at meso-scale, and it could provide the critical clue to link some phenomena which were taken as isolated before.2. Systemic symmetry, which describes both the symmetry of lattice and the symmetry of morphology for a crystal particle, was proved to be the subject of symmetry breaking.3. Symmetries of some ZnO nanocrystals were proved to obey the law of spontaneous symmetry breaking during the formation processes. While, the third land growth model of ZnO tetrapod, which had been widely accepted, was proved to be false.4. MgO nanowire obtained in our experiment was proved to obey the law of dynamical symmetry breaking during its formation processes. While fluctuation of local net charges was found to be the dynamical factor which could induce the dynamical symmetry breaking during the formation of MgO nanowires with<110> orientation.5. Sometimes, crystallization at meso-scale is completely different from those processes discovered by conventional crystallographic theories, for that symmetry breaking possesses the flexible priority over other principles to be satisfied during the formation of some nanocrystals.6. Symmetry breaking in the formation of nanocrystals is different from symmetry breaking during Landau phase transition, for that the former describes the transition for localized particle systems, and it is based on geometry; while the latter describes the transition from a non-localized particle system to a localized particle system, and it is based on statistics.