Nanohelix Structure Is the New Building Block

A previously unknown zinc oxide (ZnO) 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, according to researchers at the Georgia Institute of Technology (Atlanta).

Based on a superlattice composed of alternating single-crystal "stripes" just a few nanometers wide (Figure ), 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. Information about the growth and analysis of the new structures was reported in the journal Science.

"This structure provides a new building block for nano devices," said Zhong Lin Wang, a Regents professor in the college's School of Materials Science and Engineering. "From them, we can make resonators, place molecules on their surfaces to create frequency shifts — and because they are piezoelectric, make electromechanical couplings. This adds a new structure to the toolbox of nanomaterials."

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.

With their superlattices composed of many near-parallel single-crystal stripes, each about 3.5 nm wide and offset about 5°, the nanohelices are very different from the nanosprings and nanorings of ZnO reported by the same research group in Science in 2004. 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 µm, with diameters of 300-700 nm and widths of 100-500 nm. The nanohelices exist in both right- and left-handed versions, with production split ~50/50 between the two directions.

"This is a brand new structure, which shows a new growth model for nanomaterials," Wang said. "But from the properties point of view, these are like the earlier nanobelts in having semiconducting and piezoelectric properties, which makes them good for electromechanical coupling."

However, unlike the earlier single-crystal nanosprings that are elastic, the nanohelices are rigid and retain their shape even when cut apart. "When we first saw these structures, we were amazed by their perfection," said Wang, who is also director of Georgia Tech's Center for Nanoscience and Nanotechnology. "Once you form a nanohelix, it is perfectly uniform."

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. ZnO powder is positioned inside an alumina tube in a horizontal high-temperature tube furnace. Under vacuum, the material is heated to ~1000°C, at which point an argon carrier gas is introduced. Heating continues until the furnace reaches ~1400°C. The nanohelix structures form on a polycrystalline aluminum oxide (Al₂O₃) substrate in the furnace.

"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," Wang explained. "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."

Heating the ZnO 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. "By the time the carrier gas is introduced, the crystal orientation is fixed, but the structures must continue to grow," Wang explained. "Introducing the carrier gas initiates a transition to the superlattice structure."

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 because of rigid structural alteration, Wang said. The superlattice nanobelt is a periodic, coherent, epitaxial and parallel assembly of two alternating stripes of ZnO crystals oriented with their C axes perpendicular to one another. Growth of the nanohelix is terminated by transforming the partially polar-surface-dominated nanobelt into a non-polar-surface-dominated single-crystal nanobelt.