Spiraling in on Nanosprings

Wrap a nanowire into a helix and what do you get? A nanospring of course. Although wires tens of nanometers in diameter are not actually wrapped to make springs, they are grown that way through a process known as vapor-liquid-solid (VLS) growth mode. VLS growth occurs when a catalyst droplet resting on a surface absorbs wire-building material from a surrounding vapor. Once the concentration of the building material reaches super-saturation in the droplet, a portion of the material is secreted out the droplet base, and a wire gradually forms. Under some circumstances, the material deposition is asymmetrical and the wire develops into a helical nanospring (see image).

Until recently, the mechanism that leads to this asymmetry has been unclear, but now researchers at the University of Idaho have proposed a model that sheds new light on nanowire formation (Dave McIlroy, 208-885-6809, dmcilroy@uidaho.edu). It seems that a small catalyst droplet, roughly the same diameter as a growing nanowire, remains centered on top of the wire, and the resulting growth is linear. If the droplet exceeds the wire diameter, however, its balance atop the structure is precarious and a small perturbation can bump the droplet to one side, abruptly jolting the growth pattern from straight to helical. The researchers confirmed the model by accurately predicting the growth conditions of the world's first boron carbide nanosprings, which they produced in their laboratory.

Nanosprings may someday make highly sensitive magnetic field detectors, perhaps finding application in hard drive read heads. Alternatively, nanosprings could serve as positioners, or even as tiny conventional springs, for nanomachines of the future. (D. N. McIlroy; D. Zhang; Y. Kranov; M. Grant Norton, Applied Physics Letters, 3 September 2001.)