Gallium
nitride (GaN) and zinc oxide (ZnO) are among the most technologically
relevant semiconducting materials. Gallium nitride is ubiquitous today
in optoelectronic elements such as blue lasers (hence the blue-ray disc)
and light-emitting-diodes (LEDs); zinc oxide also finds many uses in
optoelectronics and sensors.
In
the past few years, though, nanostructures made of these materials have
shown a plethora of potential functionalities, ranging from
single-nanowire lasers and LEDs to more complex devices such as
resonators and, more recently, nanogenerators that convert mechanical
energy from the environment (body movements, for example) to power
electronic devices. The latter application relies on the fact that GaN
and ZnO are also piezoelectric materials, meaning that they produce
electric charges as they are deformed.
In a paper published online in the journal Nano Letters, Horacio Espinosa,
the James N. and Nancy J. Farley Professor in Manufacturing and
Entrepreneurship at the McCormick School of Engineering and Applied
Science at Northwestern University, and Ravi Agrawal, a graduate student
in Espinosa’s lab, reported that piezoelectricity in GaN and ZnO
nanowires is in fact enhanced by as much as two orders of magnitude as
the diameter of the nanowires decrease.
“This
finding is very exciting because it suggests that constructing
nanogenerators, sensors and other devices from smaller nanowires will
greatly improve their output and sensitivity,” Espinosa said.
“We
used a computational method called Density Functional Theory (DFT) to
model GaN and ZnO nanowires of diameters ranging from 0.6 nanometers to
2.4 nanometers,” Agrawal said. The computational method is able to
predict the electronic distribution of the nanowires as they are
deformed and, therefore, allows calculating their piezoelectric
coefficients.
The
researchers’ results show that the piezoelectric coefficient in 2.4
nanometer-diameter nanowires is about 20 times larger and about 100
times larger for ZnO and GaN nanowires, respectively, when compared to
the coefficient of the materials at the macroscale. This confirms
previous computational findings on ZnO nanostructures that showed a
similar increase in piezoelectric properties. However, calculations for
piezoelectricity of GaN nanowires as a function of size were carried out
in this work for the first time, and the results are clearly more
promising as GaN shows a more prominent increase.
“Our
calculations reveal that the increase in piezoelectric coefficient is a
result of the redistribution of electrons in the nanowire surface,
which leads to an increase in the strain-dependent polarization with
respect to the bulk materials,” Espinosa said.
The
findings by Espinosa and Agrawal may have important implications for
the field of energy harvesting as well as for fundamental science. For
energy harvesting, where piezoelectric elements are used to convert
mechanical to electrical energy in order to power electronic devices,
these results point to an advantage in reducing the size of the
piezoelectric elements down to the nanometer scale. Energy harvesting
devices built from small-diameter nanowires should in principle be able
to produce more electrical energy from the same amount of mechanical
energy than their bulk counterparts.
In
terms of fundamental science, these results further previous
conclusions that matter at the nanoscale has different properties. It is
clear now that by tailoring the size of nanostructures, their
mechanical, electrical and thermal properties can be tuned as well.
“Our
focus remains on understanding the fundamental principles governing the
behavior of nanostructures as a function of their size,” Espinosa and
Agrawal say. “One of the most important issues that needs to be
addressed is to obtain experimental confirmation of these results, and
establish up to what size the giant piezoelectric effects remain
significant.”
Espinosa
and Agrawal hope their work will spur new interest in the
electromechanical properties of nanostructures, both from theoretical
and experimental standpoints, in order to clear the path for the design
and optimization of future nanoscale devices.
