This illustration shows lithium atoms (red) adhered to a graphene lattice that will produce electricity when bent, squeezed or twisted. Conversely, the graphene will deform when an electric field is applied, opening new possibilities in nanotechnology. Illustration: Mitchell Ong, Stanford School of Engineering

In
what became known as the ‘Scotch tape technique,” researchers first
extracted graphene with a piece of adhesive in 2004. Graphene is a
single layer of carbon atoms arranged in a honeycomb, hexagonal pattern.
It looks like chicken wire.

Graphene
is a wonder material. It is one-hundred-times better at conducting
electricity than silicon. It is stronger than diamond. And, at just one
atom thick, it is so thin as to be essentially a two-dimensional
material. Such promising physics have made graphene the most studied
substance of the last decade, particularly in nanotechnology. In 2010,
the researchers who first isolated it shared the Nobel Prize.

Yet,
while graphene is many things, it is not piezoelectric.
Piezoelectricity is the property of some materials to produce electric
charge when bent, squeezed or twisted. Perhaps more importantly,
piezoelectricity is reversible. When an electric field is applied,
piezoelectric materials change shape, yielding a remarkable level of
engineering control.

Piezoelectrics
have found application in countless devices from watches, radios and
ultrasound to the push-button starters on propane grills, but these uses
all require relatively large, three-dimensional quantities of
piezoelectric materials.

Now, in a paper published in the journal ACS Nano,
two materials engineers at Stanford have described how they have
engineered piezoelectrics into graphene, extending for the first time
such fine physical control to the nanoscale.

Straintronics

“The
physical deformations we can create are directly proportional to the
electrical field applied and this represents a fundamentally new way to
control electronics at the nanoscale,” said Evan Reed,
head of the Materials Computation and Theory Group at Stanford and
senior author of the study. “This phenomenon brings new dimension to the
concept of  ‘straintronics’ for the way the electrical field strains—or deforms—the lattice of carbon, causing it to change shape in predictable ways.”

“Piezoelectric
graphene could provide an unparalleled degree of electrical, optical or
mechanical control for applications ranging from touchscreens to
nanoscale transistors,” said Mitchell Ong, a post-doctoral scholar in
Reed’s lab and first author of the paper.

Using
a sophisticated modeling application running on high-performance
supercomputers, the engineers simulated the deposition of atoms on one
side of a graphene lattice—a process known as doping—and measured the
piezoelectric effect.

They
modeled graphene doped with lithium, hydrogen, potassium and fluorine,
as well as combinations of hydrogen and fluorine and lithium and
fluorine on either side of the lattice. Doping just one side of the
graphene, or doping both sides with different atoms, is key to the
process as it breaks graphene’s perfect physical symmetry, which
otherwise cancels the piezoelectric effect.

The results surprised both engineers.

“We
thought the piezoelectric effect would be present, but relatively
small. Yet, we were able to achieve piezoelectric levels comparable to
traditional three-dimensional materials,” said Reed. “It was pretty
significant.”

Designer piezoelectricity

“We
were further able to fine tune the effect by pattern doping the
graphene—selectively placing atoms in specific sections and not others,”
said Ong. “We call it designer piezoelectricity
because it allows us to strategically control where, when and how much
the graphene is deformed by an applied electrical field with promising
implications for engineering.”

While
the results in creating piezoelectric graphene are encouraging, the
researchers believe that their technique might further be used to
engineer piezoelectricity in nanotubes and other nanomaterials with
applications ranging from electronics, photonics, and energy harvesting
to chemical sensing and high-frequency acoustics.

“We’re
already looking now at new piezoelectric devices based on other 2D and
low-dimensional materials hoping they might open new and dramatic
possibilities in nanotechnology,” said Reed.

Listen to Reed and Ong talk about their work with ACS Nano

Engineered Piezoelectricity in Graphene

Source: Stanford University