"We used the epitaxial relationship between ZnO and GaN [these two materials are lattice-matched] to make sure the GaN nanotubes are indeed single-crystalline," Peidong Yang of the University of California, Berkeley told nanotechweb.org. "These nanotubes are pinhole-free, semiconducting and photoluminescent: they combine all the nice features of semiconductor nanowires and carbon nanotubes with added advantages."

To make the tubes, Yang and colleagues prepared arrays of single-crystal ZnO nanowires on sapphire wafers using a vapour deposition process developed in their laboratory. Then they placed the array of ZnO nanowires in a reaction tube suitable for GaN chemical vapour deposition. Following deposition of GaN from a mix of trimethylgallium and ammonia, the scientists treated the sample with 10% hydrogen in argon: this etched away the ZnO templates.

The epitaxial casting technique exploits the fact that ZnO and GaN have the same crystal structure and similar lattice constants: this means that GaN can grow onto the template in the form of a single crystal. In this way, Yang and the team made GaN nanotubes with inner diameters of 30-200 nm and wall thicknesses of 5-50 nm.

According to Yang, unlike carbon nanotubes, which have either semiconducting or metallic forms, all the GaN nanotubes are semiconducting. What's more, they have interesting optical properties, as they can readily emit at ultraviolet and blue wavelengths, and it's relatively easy to functionalize the surfaces of the tubes.

"Compared with other semiconductor nanowires, the GaN nanotubes have additional internal surface area that would be advantageous for chemical sensing/separation," he added.

GaN nanotubes could have applications in nanoscale optoelectronic devices, and in chemical and biological sensing/separation. The epitaxial casting growth technique itself, meanwhile, may find a use in preparing nanotubes of materials such as silicon, gallium arsenide and cadmium selenide.

The scientists reported their work in Nature.