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From
nanobelts to nanohelices
09 September 2005
A previously-unknown 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.
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.
"This structure provides a new building block for nanodevices," said Zhong
Lin Wang, a Regents professor in the School of Materials Science and
Engineering at the Georgia Institute of Technology. "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."
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 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 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.
 Image
(left): Scanning electron microscope image shows details of "nanohelix"
structures produced from zinc oxide. Color has been added to this image.
(Credit: Science)
"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 which 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.
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.
"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," explained Wang. "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 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. "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 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.
Growth of the nanohelix is terminated by transforming the partially
polar-surface-dominated nanobelt into a non-polar-surface-dominated
single-crystal nanobelt.
"The data suggest that reducing the polar surfaces could be the driving
force behind the formation of the superlattice structure, and the rigid
structural rotation and twist caused by the superlattice results in the
initiation and formation of the nanohelix," Wang explained.
The first dozen batches of nanohelices produced a yield of only about 10
percent, but Wang believes that can be improved over time. Thus far, Wang's
research 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.
"You never know what other structures might be out there that could be added
to this toolbox," he said. "From the richness of this configuration and the
complete properties, this is a unique material that could become the new
material for nanotechnology following carbon nanotubes."
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.
"You can use it for spintronics, biomedical applications and many things you
can make with silicon technology," Wang said. "Zinc oxide is much cheaper
and easier to work with than gallium nitride."
The work was published in
the
September 9, 2005, issue of the journal Science.
Source: Gatech |
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