Zinc Oxide Clusters High-pressure Phase Change Of Molecular Dynamics Simulation

ZnO has a wonderful prospect of application due to a broad rangeof interesting properties. Owing to the small size of the material particlesand large surface area, ZnO nano-materials have surface andinterface effects, quantum size effect, volume effect, quantumtunneling effect, high dispersion and other characteristics. It has abetter performance than ZnO bulks in the optical, electrical, magnetic,chemistry. ZnO exists in a hexagonal wurtzite structure (B4) at normalconditions of temperature and pressure, transforming into the rocksaltstructure when the external pressure is up to9GPa. It has been foundthat the transition pressures observed for nanocrystals exceed the bulktransition pressures and this has been attributed to the surface effects.For two-dimensional nano-plate, one-dimensional nanowires andnanorods molecular dynamics simulations, the fivefold coordinationstructure occurs as the intermediate structure, but for thezero-dimensional ZnO nanocrystal, it has never be further investigated.In this thesis, we use molecular dynamics simulations to study ZnOnanocrystals with different surface morphologies at high-pressure. In thisway, we want to investigate the phase transition mechanism fromwurtzite to rocksalt structure, which will be helpful to the developmentof new ZnO materials.We have studied the pressure-induced structural transition of ZnOnanocrystals using constant pressure molecular dynamics simulations forfinite system. We have observed the transition from the fourfoldcoordination wurtzite to the sixfold coordination rocksalt structure, andthe process of transition is strongly dependent on the morphology of thenanocrystals. It is found that the perfect faceted ZnO nanocrystalsundergo wurtzite to rocksalt transition with a perfect fivefold h-MgOstructure as the intermediate status. The fivefold coordination structure is transformed from wurtzite structure by a simple axial compressionalong the c direction The fivefold coordination structure to B1structuraltransition is formed through changing of bond angle from120degree to90degree in the (001) and (00-1) plane with nucleation on thenanocrystal surface and growth by sliding of parallel crystal planes. Butfor the faceted ones without perfect surface structure, as the number ofthe atoms removed from the (001) and (00-1) surface edge increases,the local morphology will become more similar to spherical. Thenanocrystal will receive equal stress from every direction and it will bemore difficult to compress the structure along only c axis as the perfectfaceted ZnO nanocrystal. In this situation, only partial structureexperiences intermediate fivefold coordination structure or even nointermediate fivefold coordination structure exists dependent on thesurface disorder level. The ZnO nanocrystals with spherical and othermorphology are all experiencing once structure transition andtransforming to the rocksalt structure with single domain finally.