Zhong L. Wang is, in a way, a lord of the nanorings. By coaxing a zinc oxide nanobelt--a long, thin ribbon composed of alternating layers of Zn2+ and O22--to coil up Slinky-style, he and his coworkers at Georgia Institute of Technology have prepared the first freestanding, seamless, single-crystal nanorings out of ZnO [Science, 303, 1348 (2004)].
Wang, a professor of materials science and engineering, says the structures could be used to make semiconducting and piezoelectric-based nanoscale components that are biocompatible.
The group uses a solid-vapor process to make the rings, which account for 20 to 40% of the total ZnO material formed. The structures vary in size, with diameters of 1–4 µm, widths of 0.2–1 µm, and thicknesses of 10–30 nm. Since submitting the paper, Wang says, the group has been able to fine-tune the process to produce more uniform rings. And now they are also able to manipulate individual nanorings.
The rings form when ZnO nanobelts spontaneously coil up. Over time, the coil's loops, which may be as few as five or as many as 100, become sintered together into a single crystal.
Wang thinks that long-range electrostatic interactions drive the ring-forming process. By coiling, the structures neutralize the polar charges at the edges of the belts, decreasing the overall electrostatic energy. He adds that indium oxide and lithium carbonate dopants are key to producing these belts. Without the dopants, the nanobelts form stringy masses.
Jun Liu, the department manager of chemical synthesis and nanomaterials at Sandia National Laboratories, describes the work as "a very good example of the high level of structural control that can be accomplished. This will not only enrich our understanding of nanostructure growth but also motivate us to think harder on new applications for future nanodevices."