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With the support from several institutions including CAS, Prof. Wang and colleagues have realized the self-assembly of superlattice nanohelices from the zinc oxide nanobelt. The nanohelices, which get their shape from twisting forces created by a small mismatch between the stripes, are produced using a vapor-solid growth process at high temperature. At first the zinc-oxide powder with a wurtzite-crystal structure was heated to 1000°C in a vacuum before introducing an argon carrier gas. Nanohelix structures were formed on a polycrystalline aluminium oxide substrate when the materials were further heated to 1400°C.
"The key difference between growing nanohelices and the earlier types of nanobelt is that we control raising the temperature and when we introduce the carrier gas," said Wang. "With the earlier structures, we introduced the carrier-gas flow at the beginning. With these nanohelices, we only introduce the carrier gas when the temperature reaches a certain level. That allows formation to begin in a vacuum, which is the key to controlling the helix formation."