  The formation of ZnO nanorings and nanohelices with large polar surfaces
observed in experiments [Nano Lett. 3, 1625 (2003); J. Am. Chem. Soc. 126, 6703
(2004)] is shown to be a result of the competition between elastic energy,
spontaneous polarization-induced surface energy, volume energy, and
defect-induced energy. It is found that nanorings and nanohelices observed in
experiments are stable and energetically favorable structures.
