ZnO nanocones and nanoplatelets: synthesis and characterization

Abstract

Nanowire structure plays an important role in the development of nanotechnology. However, further study shows that the shape of nanowires may not be the ideal morphology for some applications such as solar cells and sensors. Thus, the purpose of this thesis is to find a low cost and high yield approach to the synthesis of other morphologies of nanostructures in order to further improve the performance of these nanodevices.

To this end, a chemical approach has been extended to the synthesis ZnO nanocones and platelets. With UV illumination, the synthesis of ZnO nanocones was achieved on GaN films on sapphire and gold films on silicon substrates. Both TEM and XRD results show that as-grown ZnO nanocones are single crystals. The formation of ZnO nanocones could be explained by the absorption process of photons. The UV light induced thermal gradient modifies the heat distribution as well as the reagent transport. The chemical reaction system is kinetically limited and results in ZnO nanocones. If the UV light is blocked, the ZnO nanowires result. In addition, the density of ZnO nanocones is higher than ZnO nanowires grown without UV illumination.

By this chemical approach, ZnO platelets could also be obtained on GaN films deposited by PLD, whose c-axis is parallel to the surface of the substrate. The diameters and the thickness of the platelets depend on the quality and thickness of GaN film. TEM results illustrate that the obtained ZnO platelets are single crystals grown along the <0 1 1 0> direction within the {0 0 0 1} planes. Relative growth rates of various facets were altered by the presence of [1 0 0] textured GaN film. The suppression of the growth along c axis can also be achieved by citrate anions as a structure-directing agent to adsorb selectively on ZnO basal planes. Electrical measurement shows that the resistance of ZnO platelets is about 20-40 GΩ¸ and it is higher than that of ZnO nanowires. Piezoelectric potential calculation results also indicate that the piezoelectric potential is higher than for ZnO nanowires with the same external applied stress.

These procedures and results demonstrate an easy and low cost way to fabricate ZnO nanocones and platelets, which may aid the utilization of nanostructures in solar cells, sensors and other applications to further improve their performance.