Deformation-free single-crystal nanohelixes of polar nanowires

Ultrasmall, deformation-free, single-crystal nanohelixes/nanosprings 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 60° 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 ±{011̄1} polar surfaces of the nanowire, analogous to the charge model of an RNA molecule.