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A Georgia Tech researcher manipulates and measures nanodevices based on zinc oxide nanowires fabricated on a flexible polymer substrate. Credit: Gary Meek
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Researchers
at the Georgia Institute of Technology have developed a new class of
electronic logic device in which current is switched by an electric
field generated by the application of mechanical strain to zinc oxide
nanowires.
The
devices, which include transistors and diodes, could be used in
nanometer-scale robotics, nano-electromechanical systems (NEMS),
micro-electromechanical systems (MEMS) and microfluidic devices. The
mechanical action used to initiate the strain could be as simple as
pushing a button, or be created by the flow of a liquid, stretching of
muscles or the movement of a robotic component.
In
traditional field-effect transistors, an electrical field switches – or
“gates” – the flow of electrical current through a semiconductor.
Instead of using an electrical signal, the new logic devices create the
switching field by mechanically deforming zinc oxide nanowires. The
deformation creates strain in the nanowires, generating an electric
field through the piezoelectric effect – which creates electrical charge
in certain crystalline materials when they are subjected to mechanical
strain.
“When
we apply a strain to a nanowire placed across two metal electrodes, we
create a field, which is strong enough to serve as the gating voltage,”
said Zhong Lin Wang, a Regents professor in the Georgia Tech School of Materials Science and Engineering.
“This type of device would allow mechanical action to be interfaced
with electronics, and could be the basis for a new form of logic device
that uses the piezoelectric potential in place of a gate voltage.”
Wang,
who has published a series of articles on the devices in such journals
as Nano Letters, Advanced Materials and Applied Physics Letters, calls
this new class of nanometer-scale device “piezotronics” because they use
piezoelectric potential to tune and gate the charge transport process
in semiconductors. The devices rely on the unique properties of zinc
oxide nanostructures, which are both semiconducting and piezoelectric.
The
transistors and diodes add to the family of nanodevices developed by
Wang and his research team, and could be combined into systems in which
all components are based on the same zinc oxide material. The
researchers have previously announced development of nanometer-scale
generators that produce a voltage by converting mechanical motion from
the environment, and nanowire sensors for measuring pH and detecting
ultraviolet light.
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Georgia Tech researchers measure the performance of a piezo-phototronic device in which a laser changes the conductance of a metal contact attached to a zinc oxide structure. Credit: Gary Meek
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“The
family of devices we have developed can be joined together to create
self-powered, autonomous and intelligent nanoscale systems,” Wang said.
“We can create complex systems totally based on zinc oxide nanowires
that have memory, processing, and sensing capabilities powered by
electrical energy scavenged from the environment.”
Using
strain-gated transistors fabricated on a flexible polymer substrate,
the researchers have demonstrated basic logic operations – including
NOR, XOR and NAND gates and multiplexer/demultiplexer functions – by
simply applying different types of strain to the zinc oxide nanowires.
They have also created an inverter by placing strain-gated transistors
on both sides of a flexible substrate.
“Using
the strain-gated transistor as a building block, we can build
complicated logic,” Wang added. “This is the first time that a
mechanical action has been used to create a logic operation.”
A
strain-gated transistor is made of a single zinc oxide nanowire with
its two ends – the source and drain electrodes – fixed to a polymer
substrate by metal contacts. Flexing the devices reverses their
polarity as the strain changes from compressive to tensile on opposite
sides.
The
devices operate at low frequencies – the kind created by human
interaction and the ambient environment – and would not challenge
traditional CMOS transistors for speed in conventional applications.
The devices respond to very small mechanical forces, Wang noted.
The
Georgia Tech group has also learned to control conductivity in zinc
oxide nanodevices using laser emissions that take advantage of the
unique photo-excitation properties of the material. When ultraviolet
light from a laser strikes a metal contact attached to a zinc oxide
structure, it creates electron-hole pairs which change the height of the
Schottky barrier at the zinc oxide-metal contact.
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Georgia Tech researchers measure the performance of an array of zinc oxide nanodevices fabricated on a flexible polymer substrate. Credit: Gary Meek
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These
conductivity-changing characteristics of the laser emissions can be
used in tandem with alterations in mechanical strain to provide more
precise control over the conducting capabilities of a device.
“The
laser improves the conductivity of the structure,” Wang noted. “The
laser effect is in contrast to the piezoelectric effect. The laser
effect reduces the barrier height, while the piezoelectric effect
increases the barrier height.”
Wang
has called these new devices fabricated by coupling piezoelectric,
photon excitation and semiconductor properties “piezo-phototronic”
devices.
The
research group has also created hybrid logic devices that use zinc
oxide nanowires to control current moving through single-walled carbon
nanotubes. The nanotubes, which were produced by researchers at Duke
University, can be either p-type or n-type.
The
research has been supported by the National Science Foundation (NSF),
the Defense Advanced Research Projects Agency (DARPA), and the U.S.
Department of Energy (DOE). In addition to Wang, the research team
includes Wenzhuo Wu, Yaguang Wei, Youfan Hu, Weihua Liu, Minbaek Lee,
Yan Zhang, Yanling Chang, Shu Xiang, Lei Ding, Jie Liu and Robert
Snyder.
“Our
work with strain-gated devices provides a new approach to logic
operations that performs mechanical-electrical actions in one structural
unit using a single material,” Wang noted. “These transistors could
provide new processing and memory capabilities in very small and
portable devices.”
Original article
