Recently, zinc oxide
(ZnO) nanowires have drawn major interest because of their
semiconducting nature and unique optical and piezoelectric
properties. Various applications for ZnO nanowires have been
conceived, including the next generation of field effect
transistors, light emitting diodes, sensors and resonators. ZnO
nanowires are also envisioned as nanogenerators by exploiting the
coupling of semiconducting and piezoelectric properties.
Researchers at the McCormick School of Engineering and Applied
Science at Northwestern University recently performed experiments
and computations to resolve major existing discrepancies about the
scaling of ZnO nanowires elastic properties. These properties are
essential to the design of reliable novel ZnO devices, and the
insight emerging from such studies advances scientific understanding
about atomic structures, which are also responsible for
piezo-electric and piezo-resistive properties.
ZnO nanowires usually have a hexagonal cross-section, with
diameters ranging from 5 to 500 nanometers. Interesting changes in
their properties arise as the diameter of the wires decreases due to
increasing surface-to-volume ratio. Unfortunately, experimental
results reported in the literature on wire elasticity for a given
diameter exhibit a large variability.
"This highlights one of the major challenges in the field of
nanotechnology — the accurate measurement of nanoscale mechanical
properties," says Horacio Espinosa, professor of mechanical
engineering at McCormick. "Indirect measurement techniques and
ill-defined boundary conditions affected mechanical properties
measurements and resulted in problematic inconsistencies."
Espinosa and his group at Northwestern resolved this discrepancy
using a nanoscale material testing system based on
microelectromechanical system (MEMS) technology. The system was used
to perform in-situ electron microscopy tensile testing of nanowire
specimens. Load and displacements were measured electronically while
the deforming material was imaged with atomic resolution.
"Direct atomic imaging was instrumental in assessing the
effectiveness of the test," Espinosa says.
The experimental findings revealed that the elastic stiffness of
ZnO nanowires monotonically increases as their diameter decreases.
Atomic level computational studies were also conducted to identify
the reasons for the observed size effect.
"Our experimental method is the most direct and simplest in terms
of data interpretation," says Bei Peng, a McCormick graduate student
and co-author of the paper. "We feel quite certain on all the
quantities we have measured. Moreover, the fact that the
experimental trends and atomistic predictions agree is quite
rewarding."
In this research article, the reason for the observed size
dependence was also reported.
"Atoms on the surface of the wires are rearranged because they
have fewer neighboring atoms as compared to atoms in the core of the
nanowire," says Ravi Agrawal, a McCormick graduate student and
co-author of the paper. "The resulting surface reconstruction leads
to wire material properties very different to that encountered in
bulk."
This phenomenon has been observed previously for various metallic
nanowires with large surface-to-volume ratios, but the surface
effect was confined to wires with diameters smaller than
approximately 10 nm.
"Due to the ionic character of ZnO, the atoms interact via
electrostatic forces, which are long-range in nature. Therefore, the
size effect is found to be significant up to nanowires with
diameters of about 80 nm," says Eleftherios Gdoutos, an
undergraduate student and co-author of the paper.
"Our research approach based on a combined
experimental-computational investigation of the mechanics of
nanowires is very promising," Espinosa says. "We are currently
employing MEMS devices that allow piezo-electric and piezo-resistive
characterization of semiconducting nanowires. We are also
investigating the effect of the identified atomic surface
reconstruction on polarization and energy bands, which should impact
piezo-electricity and electric conductivity."
The work is published online in the journal Nano Letters. The
paper was authored by Agrawal, Peng, Gdoutos and Espinosa, all from
the McCormick School of Engineering and Applied Science at
Northwestern University.
