Nanohelix Structure Provides New Building Block for Nanoscale Piezoelectric Devices

A previously-unknown zinc oxide nanostructure that resembles the helical configuration of DNA could provide engineers with a new building block for creating nanometer-scale sensors, transducers, resonators and other devices that rely on electromechanical coupling.

New nanohelix structures could provide engineers with a new building block for creating nanometer-scale sensors, transducers, resonators and other devices that rely on electromechanical coupling.

Based on a superlattice composed of alternating single-crystal “stripes” just a few nanometers wide, the “nanohelix” structure is part of a family of nanobelts – tiny ribbon-like structures with semiconducting and piezoelectric properties – that were first reported in 2001. 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.

With their superlattices composed of many near-parallel single-crystal stripes each about 3.5 nanometers wide and offset about five degrees, the nanohelices are very different from the nanosprings and nanorings of zinc oxide reported by the same research group. Nanosprings are composed of a single crystal whose shape is governed by balancing the electrostatic forces created by opposite electrical charges on their edges with the elastic deformation energy of the entire structure.

The nanohelices reach lengths of up to 100 microns, with diameters from 300 to 700 nanometers and widths from 100 to 500 nanometers. The nanohelices exist in both right- and left-handed versions, with production split approximately 50-50 between the two directions.

However, unlike the earlier single-crystal nanosprings which are elastic, the nanohelices are rigid and retain their shape even when cut apart. The nanohelices are formed using a simple process similar to the one used for fabricating other nanobelts. However, changing the growth conditions leads to entirely different structures.

Zinc oxide (ZnO) powder is positioned inside an alumina tube in a horizontal high-temperature tube furnace. Under vacuum, the material is heated to approximately 1,000 degrees Celsius, at which point an argon carrier gas is introduced. Heating continues until the furnace reaches approximately 1,400 degrees. The nanohelix structures form on a polycrystalline aluminum oxide (Al2O3) substrate in the furnace.

Heating the zinc oxide powder in a vacuum leads to formation of structures with polar surfaces. When the carrier gas is introduced, the growth changes to minimize the polar surfaces, creating the superlattice structure with mismatches at the crystalline interfaces. The nanohelices begin and end with conventional single-crystal nanobelt structures.

Formation of a nanohelix is initiated from a single-crystal stiff nanoribbon that is dominated by polar surfaces. An abrupt structural transformation of the single-crystal nanoribbon into stripes of the superlattice-structured nanobelt leads to the formation of a uniform nanohelix due to rigid structural alteration, Wang said. The superlattice nanobelt is a periodic, coherent, epitaxial and parallel assembly of two alternating stripes of zinc oxide crystals oriented with their c-axes perpendicular to one another. Transforming the partially polar-surface-dominated nanobelt into a non-polar-surface-dominated single-crystal nanobelt terminates growth of the nanohelix.

The first dozen batches of nanohelices produced a yield of only about 10 percent, but the researchers believe that can be improved over time. Thus far, the team has produced nearly 20 different zinc oxide nanostructures, including nanobelts, aligned nanowires, nanotubes, nanopropellor arrays, nanobows, nanosprings, nanorings, nanobowls and others. And there may yet be other structures discovered.

A wideband semiconductor, zinc oxide also has interesting piezoelectric and optical properties, can produce ultraviolet laser emissions and shows electroluminescence at room temperature. Those properties make it potentially useful in many applications.

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