MATERIALS ENGINEERING

A game of billiards may never get smaller than this.

Physicists at UC Riverside have demonstrated that graphene -- a one-atom thick sheet of carbon atoms arranged in hexagonal rings -- can act as an atomic-scale billiard table, with electric charges acting as billiard balls.

The finding underscores graphene's potential for serving as an excellent electronic material, such as silicon, that can be used to develop new kinds of transistors based on quantum physics. Because they encounter no obstacles, the electrons in graphene roam freely across the sheet of carbon, conducting electric charge with extremely low resistance.

The research team, led by Chun Ning (Jeanie) Lau, found that the electrons in graphene are reflected back by the only obstacle they meet: graphene's boundaries.

"These electrons meet no other obstacles and behave like quantum billiard balls," said Lau, an assistant professor who joined UCR's Department of Physics and Astronomy in 2004. "They display properties that resemble both particles and waves."

Lau observed that when the electrons are reflected from one of the boundaries of graphene, the original and reflected components of the electron can interfere with each other, the way outgoing ripples in a pond might interfere with ripples reflected back from the banks.

Her lab detected the "electronic interference" by measuring graphene's electrical conductivity at extremely low (0.26 Kelvin) temperatures. She explained that at such low temperatures the quantum properties of electrons can be studied more easily.

"We found that the electrons in graphene can display wave-like properties, which could lead to interesting applications such as ballistic transistors, which is a new type of transistor, as well as resonant cavities for electrons," Lau said. She explained that a resonant cavity is a chamber, like a kitchen microwave, in which waves can bounce back and forth.

In their experiments, Lau and her colleagues first peeled off a single sheet of graphene from graphite, a layered structure consisting of rings of six carbon atoms arranged in stacked horizontal sheets. Next, the researchers attached nanoscale electrodes to the graphene sheet, which they then refrigerated in a cooling device. Finally, they measured the electrical conductivity of the graphene sheet.

Graphene, first isolated experimentally less than three years ago, is a two-dimensional honeycomb lattice of carbon atoms, and, structurally, is related to carbon nanotubes (tiny hollow tubes formed by rolling up sheets of graphene) and buckyballs (hollow carbon molecules that form a closed cage).

Scientifically, it has become a new model system for condensed-matter physics, the branch of physics that deals with the physical properties of solid materials. Graphene enables table-top experimental tests of a number of phenomena in physics involving quantum mechanics and relativity.

Bearing excellent material properties, such as high current-carrying capacity and thermal conductivity, graphene ideally is suited for creating components for semiconductor circuits and computers. Its planar geometry allows the fabrication of electronic devices and the tailoring of a variety of electrical properties. Because it is only one-atom thick, it can potentially be used to make ultra-small devices and further miniaturize electronics.

Study results appear in Science on September 14.

Lau, whose research focuses on nanowires, carbon nanotubes, graphene and other organic molecules, was joined in the research by UCR's Feng Miao, Sithara Wijeratne, Wenzhong Bao, Yong Zhang and Ulas C. Coskun. The research was performed at UCR. Currently, Zhang is at Southwest University, China; Coskun is at Duke University, N.C.

UCR startup funds and the UCR Center for Nanoscale Science and Engineering supported the research.

Graphene, a single-atom-thick sheet of carbon, holds remarkable promise for future nanoelectronics applications. Whether graphene actually cuts it in industry, however, depends upon how graphene is cut, say researchers at the University of Illinois.

Graphene consists of a hexagonal lattice of carbon atoms. While scientists have predicted that the orientation of atoms along the edges of the lattice would affect the material's electronic properties, the prediction had not been proven experimentally.

Now, researchers at the U. of I. say they have proof.

"Our experimental results show, without a doubt, that the crystallographic orientation of the graphene edges significantly influences the electronic properties," said Joseph Lyding, a professor electrical and computer engineering. "To utilize nanometer-size pieces of graphene in future nanoelectronics, atomically precise control of the geometry of these structures will be required."

Lyding and graduate student Kyle Ritter (now at Micron Technology Inc. in Boise, Idaho) report their findings in a paper accepted for publication in Nature Materials online Feb. 15.

To carry out their work, the researchers developed a method for cutting and depositing nanometer-size bits of graphene on atomically clean semiconductor surfaces like silicon.

Then they used a scanning tunneling microscope to probe the electronic structure of the graphene with atomic-scale resolution.

"From this emerged a clear picture that edges with so-called zigzag orientation exhibited a strong edge state, whereas edges with armchair orientation did not," said Lyding, who also is affiliated with the university's Beckman Institute and the Micro and Nanotechnology Laboratory.

"We found that pieces of graphene smaller than about 10 nanometers with predominately zigzag edges exhibited metallic behavior rather than the semiconducting behavior expected from size alone," Lyding said. "This has major implications in that semiconducting behavior is mandatory for transistor fabrication."

Unlike carbon nanotubes, graphene is a flat sheet, and therefore compatible with conventional fabrication processes used by today's chipmakers. But, based on the researchers' experimental results, controlled engineering of the graphene edge structure will be required for obtaining uniform performance among graphene-based nanoelectronic devices.

"Even a tiny section of zigzag orientation on a 5-nanometer piece of graphene will change the material from a semiconductor into a metal," Lyding said. "And a transistor based on that, will not work. Period."

The Office of Naval Research and the National Science Foundation funded the work.

Nearly 2,000 years ago, the discovery of paper revolutionized human communication. Now researchers at Northwestern University have fabricated a new type of paper that they hope will create a revolution of its own -- and while it won't replace your notepad, this remarkably stiff and strong yet lightweight material should find use in a wide variety of applications.

In a paper to be published July 26 in the journal Nature, researchers led by Rod Ruoff, John Evans Professor of Nanoengineering in the Robert R. McCormick School of Engineering and Applied Science, report on the development of graphene oxide.

Ruoff's research team was the first to develop graphene-based composite materials, which was reported in Nature last year. Graphene -- a sheet of carbon only one atom thick -- has the potential to serve as the basis of an entirely new class of materials.

"The mechanical, thermal, optical and electrical properties of graphene are exceptional," says Ruoff. "For example, the stiffness and strength of these graphene-like sheets should be superior to all other materials, with the possible exception of diamond."

To form the graphene oxide paper, the group oxidized graphite to create graphite oxide, which falls apart in water to yield well-dispersed graphene oxide sheets. After filtering the water, the team was able to fabricate pieces of graphene oxide 'paper' more than five inches in diameter and with thicknesses from about one to 100 microns, in which the individual micron-sized graphene oxide sheets are stacked on top of each other.

"I have little doubt that very large-area sheets of this paper-material could be made in the future," Ruoff notes.

In addition to their superior mechanical properties as individual sheets, the graphene oxide layers stack well, which could be key to the development of other materials.

"You can imagine that these microscale sheets may be stacked together and chemically linked, allowing us to further optimize the mechanical properties of the resulting macroscale object," Ruoff says. "This combination of excellent mechanical properties and chemical tunability should make graphene-based paper an exciting material."

Of further interest are the electrical properties of the graphene oxide paper in comparison to graphene sheets. "When we oxidize the graphene sheets to create graphene oxide, the material goes from being an electrical conductor to an electrical insulator," Ruoff says. "This is an important step and in the future it will be possible to tune the material as a conductor, semiconductor or insulator. One will be able to control the electrical properties without sacrificing exceptional mechanical properties."

Ruoff sees a wide variety of application for graphene oxide paper, including membranes with controlled permeability, and for batteries or supercapacitors for energy applications. Graphene oxide paper could also be infused to create hybrid materials containing polymers, ceramics or metals, where such composites would perform much better than existing materials as components in, for example, airplanes, cars, buildings and sporting goods products.

The development of this paper-like material is the latest of several recent advancements by Ruoff's team in launching the new field of graphene-based materials. In a paper in the July issue of Nano Letters, the group reported that graphene sheets could be embedded into glass films to make them electrically conductive. These transparent thin films could find applications in solar cells or a variety of transparent electronics such as electronic paper and flexible color screens. The processing of these films may provide a cheaper alternative to the widely used indium tin oxide coatings that are typically used as the transparent conductive film.

In addition to Ruoff, other authors on the Nature paper are Dmitry Dikin, Sasha Stankovich, Eric Zimney, Richard Piner, Geoffrey Dommett, Guennadi Evmenenko and SonBinh Nguyen. For the Nano Letters paper, this group of researchers was also joined by coauthors Supinda Watcharotone, and from the National Cheng Kung University in Taiwan, Shang-En Wu, Shu-Fang Chen and Chuan-Pu Liu.

Using a lump of graphite, a piece of Scotch tape and a silicon wafer, Cornell researchers have created a balloonlike membrane that is just one atom thick -- but strong enough to contain gases under several atmospheres of pressure without popping.

And unlike your average party balloon -- or even a thick, sturdy glass container -- the membrane is ultra-strong, leak-proof and impermeable to even nimble helium atoms.

The research, by former Cornell graduate student Scott Bunch (now an assistant professor at the University of Colorado), Cornell professor of physics Paul McEuen and Cornell colleagues, could lead to a variety of new technologies -- from novel ways to image biological materials in solution to techniques for studying the movement of atoms or ions through microscopic holes.

The work was conducted at the National Science Foundation-supported Cornell Center for Materials Research.

Graphene, a form of carbon atoms in a plane one atom thick, is the strongest material in the world, with tight covalent bonds in two dimensions that hold it together even as the thinnest possible membrane. It's also a semimetal, meaning it conducts electricity but changes conductivity with changes in its electrostatic environment.

Scientists discovered several years ago that isolating graphene sheets is as simple as sticking Scotch tape to pure graphite, then peeling it back and re-sticking it to a silicone dioxide wafer. Peeled back from the wafer, the tape leaves a residue of graphite anywhere from one to a dozen layers thick -- and from there researchers can easily identify areas of single-layer-thick graphene.

To test the material's elasticity, the Cornell team deposited graphene on a wafer etched with holes, trapping gas inside graphene-sealed microchambers. They then created a pressure differential between the gas inside and outside the microchamber. With a tapping atomic force microscope, which measures the amount of deflecting force a tiny cantilever experiences as it scans nanometers over the membrane's surface, the researchers watched the graphene as it bulged in or out in response to pressure changes up to several atmospheres without breaking.

They also turned the membrane into a tiny drum, measuring its oscillation frequency at different pressures. They found that helium, the second-smallest element (and the smallest testable gas, since hydrogen atoms pair up as a gas), stays trapped behind a wall of graphene -- again, even under several atmospheres of pressure.

"When you work the numbers, you would expect that nothing would go through, so it's not a scientific surprise," said McEuen. "But it does tell you that the membrane is perfect" -- since even an atom-sized hole would allow the helium to escape easily.

Such a membrane could have all kinds of uses, he added. It could form a barrier in an aquarium-like setup, for example, allowing scientists to image biological materials in solution through a nearly invisible wall without subjecting the microscope to the wet environment. Or, researchers could poke atomic-sized holes in the membrane and use the system to study how single atoms or ions pass through the opening.

"This could serve as sort of an artificial analog of an ion channel in biology," McEuen said -- or as a way to measure the properties of an atom by observing its effect on the membrane.

"You're tying a macroscopic system to the properties of a single atom," he said, "and that gives opportunities for all kinds of single atom sensors."

The paper's co-authors are Cornell physics graduate students Arend van der Zande and Jonathan Alden; postdoctoral researcher Scott Verbridge; and professors Jeevak Parpia and Harold Craighead.

Airtight containers are not always so airtight. As any child will discover the day after a birthday party, even a tightly tied helium balloon will leak its gas out over the course of many hours. Now scientists have come up with a supremely efficient barrier that lets nothing in or out.

As described in a recent issue of the journal Applied Physics Letters, this new wrapping material is made of graphene, a natural carbon fabric that is only a single-atomic-layer thick.

A related form of carbon, graphite, is used in pencils. At the microscopic level, graphite consists of billions of two-dimensional sheets of carbon atoms. The fact that these sheets are only loosely attached to each other is what makes graphite such a good lubricant. Graphene is what you get when you take the layers of graphite one at a time. Such little slivers of carbon -- imagine chicken wire made of carbon atoms -- can hardly be seen, and therefore graphene was only discovered a few years ago.

Now, because of graphene’s interesting properties -- such as the fact that electrons flow through it without much energy loss -- it has become a hot topic among physicists. One of the most impressive properties is its mechanical strength, which is surprising since it is so thin.

Both experiments in the lab and simulations carried out in a computer have now shown that graphene sheets can sustain high pressure and act as ideal containers. Physicists in the theory group of Francois Peeters at the University of Antwerp in Belgium have performed studies of how graphene sheets can hold gases within a tiny balloon structure. If any atom could escape from a nanoscopic bag it would be helium. As one of the noble gases, helium is chemically inert and might be able to wiggle past any atomically-thin enclosure. But, according to Antwerp researcher Ortwin Leenaerts, helium is unable to escape.

An experiment at Cornell in the laboratory of Harold Craighead, with a sheet of graphene stretched across a tiny bottle holding gas, has shown that indeed the gas, even high-pressure gas, is kept in. (This work was published in the journal Nano Letters).

Leenearts says that graphene, aside from its electrical properties, can contribute to a number of nanotech products. Examples include tiny pressure sensors: depending on the pressure of a gas in a tiny bottle, the graphene “stopper” of such a bottle would vibrate at telltale frequencies. A tiny patch of graphene could serve as a nanoresonator: clamped on two sides, the graphene would vibrate at radio frequencies. An electrical signal could be sent in, causing the graphene to act as a tiny radio antenna. Graphene might even be used as an artificial membrane. Formed into a nanoscopic vesicle, almost like an artificial cell, the graphene could contain medicine that might be released in the body in a time-release manner.

Researchers in New York are reporting development of the world's thinnest balloon, made of a single layer of graphite just one atom thick.

This so-called graphene sealed microchamber is impermeable to even the tiniest airborne molecules, including helium. It has a range of applications in sensors, filters, and imaging of materials at the atomic level, they say.

Paul L. McEuen and colleagues note that membranes are fundamental components of a wide variety of physical, chemical and biological systems, found in everything from cellular compartments to mechanical pressure sensing.

Graphene, a single layer of graphite, is the upper limit: A chemically stable and electrically conducting membrane just one atom thick. The researchers wanted to answer whether such an atomic membrane would be impermeable to gas molecules and easily incorporated into other devices.

Their data showed that graphene membranes were impermeable to even the smallest gas molecules. These results show that single atomic sheets can be integrated with microfabricated structures to create a new class of atomic scale membrane-based devices. We envision many applications for these graphene sealed microchambers, says McEuen. These range from hyper-sensitive pressure, light and chemical sensors to filters able to produce ultrapure solutions.

Northwestern University researchers have developed a process that promises to lead to the creation of a new class of composite materials -- "graphene-based materials."

The method uses graphite to produce individual graphene-based sheets with exceptional physical, chemical and barrier properties that could be mixed into materials such as polymers, glasses and ceramics.

The Northwestern team, led by materials scientist and physical chemist Rod Ruoff and composed of chemists, physicists and engineers, reports the results of their research in the July 20 issue of the journal Nature.

"This research provides a basis for developing a new class of composite materials for many applications, through tuning of their electrical and thermal conductivity, their mechanical stiffness, toughness and strength, and their permeability to flow various gases through them," said Ruoff, professor of mechanical engineering in the McCormick School of Engineering and Applied Science. "We believe that manipulating the chemical and physical properties of individual graphene-based sheets and effectively mixing them into other materials will lead to discoveries of new materials in the future."

The Northwestern team's approach to its challenge was based on chemically treating and thereby "exfoliating" graphite to individual layers. Graphite is a layered material of carbon with strong chemical bonds in the layers but with moderately weak bonds between the layers. The properties of the individual layers have been expected to be exceptional because the "in-plane" properties of graphite itself are exceptional, but until now it was not possible to extract such individual layers and to embed them as a filler material in materials such as polymers, and particularly not by a scalable route that could afford large quantities.

There are approximately one million metric tons of graphite sold annually around the world, and there are roughly 800 million metric tons of untapped natural graphite that could be mined and used in the future, according to the U.S. Geological Survey. Graphite is used in a wide variety of applications such as those related to friction (brake linings are one example), in gaskets, as a lubricant, and as an electrode material in the making of steel.

Ruoff has worked and published extensively on other novel carbon materials such as fullerenes ("buckyballs") and carbon nanotubes. His Northwestern research team included SonBinh Nguyen, professor of chemistry; chemist and postdoctoral fellow Sasha Stankovich; physicist and research assistant professor Dmitry Dikin; physicist and research scientist Richard Piner; and graduate students Eric Zimney, Geoffrey Dommett and Kevin Kohlhaas. Professor Eric Stach of Purdue University assisted in analyzing transmission electron microscopy images acquired by the Northwestern team.