Investigation On The Controlled Synthesis, Doping And Tailored Properties Of ZnO Nanostructured Materials

ZnO is a low cost and environmentally friendly semiconductor with a wide direct bandgap (3.37 eV) and a large exciton binding energy (60 meV) at room temperature, which makes it a promising material for applications in light-emitting diodes (LEDs), ultraviolet photodetectors, solar cells, field-effect transistors (FETs), sensors, spintronics and so on. Tailored properties of ZnO can be readily achieved by the successful doping of selected element into the host matrix.Recently low-dimensional ZnO nanomaterials have attracted much attention due to their unique morphologies, structures, and the resulting materials properties. Although great efforts have been made, controlled synthesis and doping of ZnO nanomaterials with tailored properties is challenging and the deep understanding of doping in ZnO nanomaterials is still limited.In this regard, the work was mainly focused on the controlled synthesis and doping of ZnO nanostructured materials for desired properties. The work included:1. Low-dimensional ZnO-based nanostructured materials were prepared by a thermal evaporation method. The effects of Cd doping on the growth of ZnO-based nanomaterials was investigated.2. Nearly monodisperse and well crystallized ZnO nanocrystals were synthesized via a hot injection method in solutions. The developed method was also applied to synthesize other oxide nanocrystals. Insights revealed on the basis of pure oxide nanocrystals were considered to be useful for doping in ZnO nanocrystals.3. Mg doped ZnO nanocrystals with different Mg concentrations, defined shapes and tunable optical properties were prepared by a hot injection method. The Mg doped ZnO tetrapods and ultrathin nanowires were carefully characterized. The core regions of the tetrapods had cubic zinc-blende structure while the branching arms had typical hexagonal wurtzite structure. All ultrathin nanowires had single crystalline nature with wurtzite structure.4. The doping effects of Mg on the shapes, structures and properties of the resulting doped nanocrystlas were studied, and the growth process of doped ZnO nanocrystals was monitored. Based on the experimental results, it was suggested that the initial growth seeds at the primary growth stages with different structures were responsible for the formation of doped nanocrystals with defined shapes. 5. To further confirm the doping effects of Mg ions on the seeds at the primary growth stage, a seeded growth experiment was designed. The result showed that the introduction of Mg dopant ions significantly influenced the growth of the host lattices by modifying the crystalline phase of the ZnO seeds, leading to the formation of doped nanocrystals with different morphologies.6. We demonstrated that stearate ions were the surface ligands for both pure ZnO nanocrystals and the Mg doped ZnO nanocrystals. The surface ligands played an important role for the synthesis of oxide nanocrystals, which significantly slowed the reactions. Both surface ligands and solvents were not the key factors for the shape evolution of doped ZnO nanocrystals.7. The doping procedure based on the Mg-doped ZnO nanocrystals was readily extended to ZnO nanocrystals doped by Cd, Mn or Ni, demonstrating the dopant-induced shape evolution of colloidal nanocrystals could be applicable in other oxide systems.