Straintronics: Engineers create piezoelectric graphene

Straintronics: Engineers create piezoelectric graphene
This illustration shows lithium atoms (in red) adsorbed to a layer of graphene to create electricity when the graphene is bent, squeezed or twisted. Credit: 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 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 . 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 .

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.

Listen to Stanford engineers Evan Reed and Mitchell Ong discuss their piezoelectric graphene. Credit: ACS Nano

"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 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.

Explore further

Graphene's piezoelectric promise

Provided by Stanford School of Engineering
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Mar 16, 2012
Although they mention they could control chemical reactions, they don't suggest this could be used to make nanomechanical devices; they could fold up the graphene into nanomechanical parts maybe; and even make them functional?

Of course, they could be just trying to be minimal and conservative.

Mar 16, 2012
all dandy, but it aint piezoelectric until a real slice of doped graphene is tested. Granted simulations have are maturing to a point that more and more usefull extrapolations can be done, but the proof is in eating the pudding not in the puddingsimulation.

Mar 16, 2012
they could fold up the graphene into nanomechanical parts maybe;

Piezoelectric effects aren't that big. They're usually less than 0.1 percent (i.e. after the voltage is applied the material is less than 0.1 percent shorter/longer depending on thy type of geometry of the material)

Mar 16, 2012
Well, o.k, they could use these piezo to make nano-Stms. There was other recent breakthroughs in making micron level at least piezos; but, this is even more exciting from at least that standpoint.

I mean to point out that Stms can pick and up and place and even do some nano-chemistry; but, for them to do anything on the scale of what Eric Drexler envisioned(and Richard Feynman) . . . well, you wouldn't see a macroscopic product anytime soon(maybe millions of years). But, if you make a molecular scale Stm . . . well, I've been wondering for decades now why they havn't done that. Recent work on making Piezos shows that we needed to make a breakthrough. Now, we can make nano-piezos!

Mar 16, 2012
On the other hand, on that scale, a little force goes much further.

Mar 16, 2012
Well, o.k, they could use these piezo to make nano-Stms

They already have. I worked with a company that makes high precision piezo motors on a project (REMROB - microrobotics in an electron microscope)...along with some other research institutes in germany.
The guy who owns the company brought an STM with him for 'show and tell' what one could do with his motors (though we used his motors for the gripper and robot motion in the project).

The freaky thing is: STMs are incredibly sensitive to vibrations. They are huge. You put them in vacuum apparatus in rooms isolated from the rest of the building around it, with extra laminar airflow an whatnot. But with his motors you could make that thing SMALL. And when you halve the size you decreases susceptibility to vibrations by a factor of 8.

Point of the story: The STM he showed us needed no vacuum chamber and was 5x5x5cm large. It operated on the desk of the meeting room.

Needless to say: jaws dropped around the table.

Mar 16, 2012
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Mar 20, 2012
The effect would make the graphene sheet curl. Even a 0.1% effect would be huge as it would cause major movement at the sub micron scale. This could work with coating one side of a nanotube too. Keeping the coating atoms on one side of the sheet may be tough though as it is a pretty thin layer to penetrate to the other side.

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