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Simultaneous axial and radial epitaxies have many applications. However, until recently they eluded researchers. Scientists, including three from Oak Ridge National Laboratory, have now demonstrated the realization of simultaneous axial and radial epitaxies by growing ZnO microtowers through a regrowth technique of repeating the same growth cycle (loading the source material, heating, growing, cooling, exposing to air, and repeating) 2-10 times inside a well-controlled tube-furnace system. The ZnO microtowers thus fabricated are hybrid nano/microstructures that possess both nanoscale (at the tip) and microscale (at the stem) features.

Epitaxial nanowire superlattices and cables exhibit unique photonic and electronic properties, suggesting potential applications ranging from nano-barcodes to polarized nanoscale light-emitting diodes. The ZnO microtowers that have been fabricated offer an advance in the complexity and functionality of nanowire building blocks for nanoscience and nanotechnology. The nanoscale feature is important in retaining nanoscopic properties, while the microscale “seeable and touchable” feature allows the facile manipulation and integration of nanostructures (whisker tip) into desired devices for property measurements and applications. For example, the microscale section of the ZnO whiskers can be easily picked up from the growth substrates using sharp tweezers, and can be placed with a degree of control into any location or glued onto a substrate using silver paste. It could then be used for property measurements as a probe or field emitter (these experiments are currently under way). Future work will also focus on doping these whiskers with transition metals or rare-earth elements, in order to control or tune their optical and electrical properties.

This research was sponsored by the University of Georgia
Research Foundation, the US Office of Naval Research,
Oak Ridge National Laboratory, and DOE,
Office of Science’s Office of Basic Energy Science

Adv. Mater. 2009, 21, 890-896