Ultrasmall,deformation-free,single-crystal nanohelixesanosprings of piezoelectric ZnO are reported.The nanohelixes are made of ~12 nm nanowires and have a uniform mean diameter of ~30 nm.The growth follows a hexagonal screw-coiling model,in which the growth of the nanowire is led by the Zn-terminated(0001)front surface due to self-catalysis.A sequential and periodic 60deg rotation in growth direction among the six equivalent directions of<0111>in an ordered and equally spaced distance results in the formation of the nanohelix.The sequential change in growth direction is to reduce the electrostatic interaction energy caused by the(+-){0111} polar surfaces of the nanowire,analogous to the charge model of an RNA molecule.
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