Credit: © 2006 AIP

Electron emission from a field emitter is distinct from that from a hot filament because it is driven by an electric field rather than thermal excitation. ZnO nanowires show promise as ‘cold’ field emitters, owing to their high mechanical strength and the relatively low electron binding energy of this material. The size and shape of ZnO nanowires are critical to their electron-emission characteristics; namely, high current density and low turn-on voltage. As these properties vary depending on the growth conditions, controlled studies are necessary to determine the optimum tip geometry for field emission.

Y. H. Yang and colleagues1 of Zhongshan University, Guangzhou, China, used thermal chemical vapour transport to self-assemble ZnO nanostructures of different shapes on 50–150-nm-thick films of amorphous carbon. Scanning electron microscope images revealed the formation of ‘conical’, ‘pencil-like’ and ‘inverted-T’ morphologies. The synthesis was carried out without metal catalysts, thus simplifying the process and reducing the possibility of metal contamination of the nanowires. The threshold electric fields for inducing electron emission from the conical, pencil-like and inverted-T structures were 2.4, 3.7 and 6.9 V μm-1, respectively.

These threshold electric fields are comparable to, or less than those reported for carbon nanotubes and other metallic/semiconducting nanowires that are produced using more-complex, catalytic-based syntheses. These results confirm that the field-emission characteristics of ZnO nanowires depend markedly on their shape, and that conical structures, with tip radii of 20 nm, are the most promising for practical devices.