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."
Posted October 15th,
2008
