Synthesis and Characterizations of ZnO Nanostructures for Field Emission Devices

The work presented in this thesis describes the growth and characterizations of ZnO nanostructures with a focus on studying their field emission properties. Successful synthesis of various doped and undoped ZnO nanostructures with different morphologies and compositions on different substrates (zinc foil, silicon, titanium and tungsten) via two different fabrication techniques (i.e. hydrothermal method and thermal evaporation process) were successfully accomplished. The morphological, structural, compositional and optical properties of as synthesized ZnO nanostructures were characterized by various analytical techniques. In addition, plausible growth mechanisms based on the crystallographic properties of the wurtzite hexagonal ZnO and the physical and chemical reactions involved in the growth process have been proposed for all prepared ZnO nanostructures. The influence of the surface morphology on the field emission properties of ZnO nanostructures was also studied. It was found that, the turn-on and threshold voltages of the 2D ZnO nanosheets interconnect to be high compared to that in the ZnO nanorods. In the meantime, ZnO nanorods exhibits much more stable current than nanosheets and this is mainly due to the uniformity (i.e. shape and size) of the ZnO nanorods. The doping also has significant effect on the morphology as well as the electrical properties of the grown ZnO nanostructures and thus the field emission properties. Different ratios of gallium doping led to two different types of morphologies (nanoneedles and multipods). A significant enhancement of the field emission performance of Ga-doped ZnO nanostructures (nanoneedles and multipods) is attained and this is due to the combined effect of the geometrical enhancement factor and the doping effect. Similarly, the In-doped ZnO nanopencils exhibits enhanced emission performance with very low turn on voltage which is related to both the distinctive geometrical configuration and the improvement in the electrical properties (i.e. conductivity and carrier concentration) due to indium doping. Finally, an important goal/objective of this thesis is the application of the ZnO nanorods as alternative high brightness electron cathodes and for their use in harsh environment have been explored and presented. A new approach to fabricate novel a field emitter electron cathode based on ZnO nanostructures is reported. The preliminary results obtained are very encouraging but more experiments are needed before its use in either the field emission display or electron microscopy.