Barrier to faster graphene devices identified and suppressed

This single-atom-thick honeycomb of carbon atoms is lighter than aluminum, stronger than steel and conducts heat and electricity better than copper. As a result, scientists around the world are trying to turn it into better computer displays, solar panels, touch screens, integrated circuits and biomedical sensors, among other possible applications. However, it has proven extremely difficult to reliably create graphene-based devices that live up to its electrical potential when operating at room temperature and pressure.

Now, writing in the Mar. 13 issue of the journal Nature Communications, a team of Vanderbilt physicists reports that they have nailed down the source of the interference inhibiting the rapid flow of electrons through graphene-based devices and found a way to suppress it. This allowed them to achieve record-levels of room-temperature electron mobility – the measure of the speed that electrons travel through a material – three times greater than those reported in previous graphene-based devices.

According to the experts, graphene may have the highest electron mobility of any known material. In practice, however, the measured levels of mobility, while significantly higher than in other materials like silicon, have been considerably below its potential.

"The problem is that, when you make graphene, you don't get just graphene. You also get a lot of other stuff," said Kirill Bolotin, assistant professor of physics, who conducted the study with Research Associate A.K.M. Newaz. "Graphene is extraordinarily susceptible to external influences so the electrical fields created by charged impurities on its surface scatter the electrons traveling through the graphene sheets, making graphene-based transistors operate slower and heat up more."

A number of researchers had proposed that the charged impurities that are omnipresent on the surface of graphene were the main culprits, but it wasn't completely certain. Also, several other theories had been advanced to explain the phenomenon.

"Our study shows without question that the charged crap is the problem and, if you want to make better graphene devices, it is the enemy that you need to fight," Bolotin said.

At the same time, the experiment didn't find evidence supporting one of the alternative theories, that ripples in the graphene sheets were a significant source of electron scattering

In order to get a handle on the mobility problem, Bolotin's team suspended sheets of graphene in a series of different liquids and measured the material's electric transport properties. They found that graphene's electron mobility is dramatically increased when graphene is submerged in electrically neutral liquids that can absorb large amounts of electrical energy (have large dielectric constants). They achieved the record-level mobility of 60,000 using anisole, a colorless liquid with a pleasant, aromatic odor used chiefly in perfumery.

"These liquids suppress the electrical fields from the impurities, allowing the electrons to flow with fewer obstructions," Bolotin said.

Now that the source of the degradation in electrical performance of graphene has been clearly identified, it should be possible to come up with reliable device designs, Bolotin said.

According to the physicist, there is also a potential advantage to graphene's extraordinary sensitivity to its environment that can be exploited. It should make extremely sensitive sensors of various types and, because it is made entirely of carbon, it is biocompatible and so should be ideal for biological sensors.

Provided by Vanderbilt University (news : web)

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Using graphene foam to detect subtle traces of hazardous gases, explosives

A student in the Department of Mechanical, Aerospace, and Nuclear Engineering at Rensselaer, Yavari’s sensor opens the door to a new generation of gas detectors for use by bomb squads, defense and law enforcement officials, as well as in industrial settings. For this innovation, Yavari has been named the winner of the 2012 $30,000 Lemelson-MIT Rensselaer Student Prize. He is among the three 2012 $30,000 Lemelson-MIT Collegiate Student Prize winners announced today.

“Innovating solutions to the challenges of tomorrow requires a certain kind of individual—one who is ready and willing to take calculated risks and seize promising opportunities. These architects of change push forward the state of the art, and can affect progress on a global scale,” said Rensselaer President Shirley Ann Jackson. “Fazel Yavari, with his creative exploitation of graphene to create a promising new gas sensor, is a stellar example of such an architect of change. We congratulate him, and applaud all of the winners and finalists of the Lemelson-MIT Collegiate Student Prize for innovating a bolder, brighter future.”

Yavari is the sixth recipient of the Lemelson-MIT Rensselaer Student Prize. First given in 2007, the prize is awarded annually to a Rensselaer senior or graduate student who has created or improved a product or process, applied a technology in a new way, redesigned a system, or demonstrated remarkable inventiveness in other ways.

“This year’s Lemelson-MIT Collegiate Student Prize winners and finalists from MIT, RPI, and UIUC are helping to fulfill the country’s need for innovation. These students’ passion for invention and their ideas will improve people’s lives around the world,” states Joshua Schuler, executive director of the Lemelson-MIT Program. “We applaud their accomplishments that will also undoubtedly inspire future generations of inventors.”

Graphene-Powered Gas Detection

With his project, titled “High Sensitivity Detection of Hazardous Gases Using a Graphene Foam Network,” Yavari overcomes the shortcomings that have prevented nanostructure-based gas detectors from reaching the marketplace.

Detecting trace amounts of hazardous gases present within air is a critical safety and health consideration in many different situations, from industrial manufacturing and chemical processing to bomb detection and environmental monitoring. Conventional gas sensors are either too bulky and expensive, which limits their use in many applications, or they are not sensitive enough to detect trace amounts of gases. Also, many commercial sensors require very high temperatures in order to adequately detect gases, and in turn require large amounts of power.

Researchers have long sought to leverage the power of nanomaterials for gas detection. Individual nanostructures like graphene, an atom-thick sheet of carbon atoms arranged like a nanoscale chicken-wire fence, are extremely sensitive to chemical changes. However, creating a device based on a single nanostructure is costly, highly complex, and the resulting devices are extremely fragile, prone to failure, and offer inconsistent readings.

Yavari has overcome these hurdles and created a device that combines the high sensitivity of a nanostructured material with the durability, low price, and ease of use of a macroscopic device. His new graphene foam sensor, about the size of a postage stamp and as thick as felt, works at room temperature, is considerably less expensive to make, and still very sensitive to tiny amounts of gases. The sensor works by reading the changes in the graphene foam’s electrical conductivity as it encounters gas particles and they stick to the foam’s surface. Another benefit of Yavari’s device is its ability to quickly and easily remove these stuck chemicals by applying a small electric current.

The new graphene foam sensor has been engineered to detect the gases ammonia and nitrogen dioxide, but can be configured to work with other gases as well. Ammonia detection is important as the gas is commonly used in industrial processes, and ammonia is a byproduct of several explosives. Nitrogen dioxide is also a byproduct of several explosives, as well as a closely monitored pollutant found in combustion exhaust and auto emissions. Yavari’s sensor can detect both gases in quantities as small as 0.5 parts-per-million at room temperature.  

Provided by Rensselaer Polytechnic Institute (news : web)

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Molecular graphene heralds new era of 'designer electrons'

"The behavior of electrons in materials is at the heart of essentially all of today's technologies," said Hari Manoharan, associate professor of physics at Stanford and a member of SLAC's Stanford Institute for Materials and Energy Sciences, who led the research. "We're now able to tune the fundamental properties of electrons so they behave in ways rarely seen in ordinary materials."

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Pictured is a version of molecular graphene in which the electrons are tuned to respond as if they're experiencing a very high magnetic field (red areas) when none is present. Scientists from Stanford and SLAC National Accelerator Laboratory calculated the positions where carbon atoms in graphene should be to make its electrons believe they were being exposed to a magnetic field of 60 Tesla, more than 30 percent higher than the strongest continuous magnetic field ever achieved on Earth (a 1 Tesla magnetic field is about 20,000 times stronger than the Earth?s). The researchers then used a scanning tunneling microscope to place carbon monoxide molecules (black circles) at precisely those positions. The electrons responded by behaving exactly as expected -- as if they were exposed to a real field. Credit: Manoharan Lab, Stanford/SLAC

Their first examples, reported today in Nature, were hand-crafted, honeycomb-shaped structures inspired by graphene, a pure form of carbon that has been widely heralded for its potential in future electronics. Initially, the electrons in this structure had graphene-like properties; for example, unlike ordinary electrons, they had no mass and traveled as if they were moving at the speed of light in a vacuum. But researchers were then able to tune these electrons in ways that are difficult to do in real graphene. This video is not supported by your browser at this time.

This video shows carbon monoxide molecules (black circles) being moved one at a time by a scanning tunneling microscope into the hexagonal "molecular graphene" arrangement. The molecules repel the free-flowing electrons (yellow-orange) on the copper surface, forcing them into a honeycomb pattern. Credit: Manoharan Lab, Stanford/SLAC

To make the structure, which Manoharan calls molecular graphene, the scientists use a scanning tunneling microscope to place individual carbon monoxide molecules on a perfectly smooth copper surface. The carbon monoxide repels the free-flowing electrons on the copper surface and forces them into a honeycomb pattern, where they behave like graphene electrons.

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This graphic shows the effect that a specific pattern of carbon monoxide molecules (black/red) has on free-flowing electrons (orange/yellow) atop a copper surface. Ordinarily the electrons behave as simple plane waves (background). But the electrons are repelled by the carbon monoxide molecules, placed here in a hexagonal pattern. This forces the electrons into a honeycomb shape (foreground) mimicking the electronic structure of graphene, a pure form of carbon that has been widely heralded for its potential in future electronics. The molecules are precisely positioned with the tip of a scanning tunneling microscope (dark blue). Image credit: Hari Manoharan / Stanford University.

To tune the electrons' properties, the researchers repositioned the carbon monoxide molecules on the surface; this changed the symmetry of the electron flow. In some configurations, electrons acted as if they had been exposed to a magnetic or electric field. In others, researchers were able to finely tune the density of electrons on the surface by introducing defects or impurities. By writing complex patterns that mimicked changes in carbon-carbon bond lengths and strengths in graphene, the researchers were able to restore the electrons' mass in small, selected areas. This video is not supported by your browser at this time.

Molecular Manipulation. An individual carbon monoxide molecule (black/red) on the copper surface (yellow atoms) is positioned by the scanning tunneling microscope tip (comprised of iridium atoms, magenta). To manipulate the molecule, the quantum mechanical tunneling conditions are changed until the tip exerts an attractive force upon the molecule. The tip drags the molecule from its initial position to a desired final position. Once the molecule reaches its final position, the tunneling conditions are changed back to normal conditions suitable for imaging rather than manipulation. This process is repeated hundreds of times to create a molecular graphene lattice.

"One of the wildest things we did was to make the electrons think they are in a huge magnetic field when, in fact, no real field had been applied," Manoharan said. Guided by the theory developed by co-author Francisco Guinea of Spain, the Stanford team calculated the positions where carbon atoms in graphene should be to make its electrons believe they were being exposed to magnetic fields ranging from zero to 60 Tesla, more than 30 percent higher than the strongest continuous magnetic field ever achieved on Earth. The researchers then moved carbon monoxide molecules to steer the electrons into precisely those positions, and the electrons responded by behaving exactly as predicted – as if they had been exposed to a real field. This video is not supported by your browser at this time.

Tunable Pseudomagnetic Field. This video shows the progression of different molecular graphene structures that produce "phantom" magnetic fields of 0, 15, 30, 45 and 60 Tesla. The strongest continuous magnetic field actually achieved on Earth is 45 Tesla. (A 1 Tesla magnetic field is about 20,000 times stronger than the Earth's.) Each particular arrangement of carbon monoxide molecules (black circles) on a copper surface causes the copper's surface electrons (yellow-orange) to behave as if they're experiencing a very high magnetic field, although none is actually present.

"Our new approach is a powerful new test bed for physics," Manoharan said. "Molecular graphene is just the first in a series of possible designer structures. We expect that our research will ultimately identify new nanoscale materials with useful electronic properties."

Provided by SLAC National Accelerator Laboratory (news : web)

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Straintronics: Engineers create piezoelectric graphene

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.

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

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.

Provided by Stanford School of Engineering

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Scientists produce graphene using microorganisms

Chemical reduction of graphene oxide (GO) flakes is widely used for the synthesis of graphene. In this process, the critical stage of reducing GO flakes into graphene requires the exposure of the GO to hydrazine. This reduction process has fundamental limitations for large scale production; in particular because of the hydrazine vapor is highly toxic.

The method developed by the Toyohashi Tech team was inspired by a recent report showing that graphene oxide behaves as a terminal electron acceptor for bacteria, where the GO is reduced by microbial action in the process of breathing or electron transport. Notably, the Toyohashi Graphene Research Group method is a hybrid approach, where chemically derived graphene oxide flakes are reduced by readily available microorganisms extracted from a river bank near the Tempaku Campus of Toyohashi University of Technology, Aichi, Japan. Raman scattering measurements showed that the GO flakes had indeed been reduced.

The approach offers a low-cost, highly efficient, and environmentally friendly method for the mass production of high quality graphene for the electronics industry.

More information: Y. Tanizawa et al. Microorganism mediated synthesis of reduced graphene oxide films, IOP Journal of Physics: Conference Series (in press), http://iopscience. … rg/1742-6596

Provided by Toyohashi University of Technology

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New method offers control of strain on graphene membranes

A group of physicists from the University of Arkansas and other institutions have developed a technique that allows them to control the mechanical property, or strain, on freestanding graphene, sheets of carbon one-atom thick suspended over the tops of tiny squares of copper. By controlling the strain on freestanding graphene, they also can control other properties of this important material.

“If you subject graphene to strain, you change its electronic properties,” said physics professor Salvador Barraza-Lopez. Strain on freestanding graphene causes the material to behave as if it is in a magnetic field, even though no magnets are present, a property that scientists will want to exploit -- if they can control the mechanical strain.

To control the mechanical strain, University of Arkansas researchers developed a new experimental approach. Physicists Peng Xu, Paul Thibado and students in Thibado’s group examined freestanding graphene membranes stretched over thin square “crucibles,” or meshes, of copper. They performed scanning tunneling microscopy with a constant current to study the surface of the graphene membranes. This type of microscopy uses a small electron beam to create a contour map of the surface. To keep the current constant, researchers change the voltage as the scanning tunneling microscope tip moves up and down, and the researchers found that this causes the freestanding graphene membrane to change shape.

“The membrane is trying to touch the tip,” Barraza-Lopez said. They discovered that the electric charge between the tip and the membrane influences the position and shape of the membrane. So by changing the tip voltage, the scientists controlled the strain on the membrane. This control becomes important for controlling the pseudo-magnetic properties of graphene.

In conjunction with the experiments, Barraza-Lopez, Yurong Yang of the University of Arkansas and Nanjing University, and Laurent Bellaiche of the University of Arkansas examined theoretical systems involving graphene membranes to better understand this new-found ability to control the strain created by the new technique. They verified the amount of strain on these theoretical systems and simulated the location of the scanning tunneling microscopy tip in relation to the membrane. While doing so, they discovered that the interaction of the membrane and tip depends upon the tip’s location on the freestanding graphene. This allows scientists to calculate the pseudo-magnetic field for a given voltage and strain.

“If you know the strain, you can use theory and compute how big the pseudo-magnetic field may be,” said Barraza-Lopez. They found that because of the boundaries created by the square copper crucible, the pseudo-magnetic field swings back and forth between positive and negative values, so scientists are reporting the maximum value for the field instead of a constant value.

“If you were able to make the crucibles triangular, you would be closer to having non-oscillating fields,” Barraza-Lopez said. “This would bring us closer to using this pseudo-magnetic property of graphene membranes in a controlled way.”

The researchers report their findings in Physical Review B Rapid Communications.

More information: PRB 85, 121406(R) (2012)

Provided by University of Arkansas (news : web)

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Researchers find simple and cheap way to mass-produce graphene nanosheets

Graphene, which is made from graphite, the same stuff as "lead" in pencils, has been hailed as the most important synthetic material in a century. Sheets conduct electricity better than copper, heat better than any material known, are harder than diamonds yet stretch.

Scientists worldwide speculate graphene will revolutionize computing, electronics and medicine but the inability to mass-produce sheets has blocked widespread use.

A description of the new research will be published the week of March 26 in the online Early Edition of the Proceedings of the National Academy of Sciences.

Jong-Beom Baek, professor and director of the Interdisciplinary School of Green Energy/Advanced Materials & Devices, Ulsan National Institute of Science and Technology, Ulsan, South Korea, led the effort.

"We have developed a low-cost, easier way to mass produce better graphene sheets than the current, widely-used method of acid oxidation, which requires the tedious application of toxic chemicals," said Liming Dai, professor of macromolecular science and engineering at Case Western Reserve and a co-author of the paper.

Here's how:

Researchers placed graphite and frozen carbon dioxide in a ball miller, which is a canister filled with stainless steel balls. The canister was turned for two days and the mechanical force produced flakes of graphite with edges essentially opened up to chemical interaction by carboxylic acid formed during the milling.

The carboxylated edges make the graphite soluble in a class of solvents called protic solvents, which include water and methanol, and another class called polar aprotic solvents, which includes dimethyl sulfoxide.

Once dispersed in a solvent, the flakes separate into graphene naonsheets of five or fewer layers.

To test whether the material would work in direct formation of molded objects for electronic applications, samples were compressed into pellets. In a comparison, these pellets were 688 times better at conducting electricity than pellets yielded from the acid oxidation of graphite.

After heating the pellets at 900 degrees Celsius for two hours, the edges of the ball-mill–derived sheets were decarboxylated, that is, the edges of the nanosheets became linked with strong hydrogen bonding to neighboring sheets, remaining cohesive. The compressed acid-oxidation pellet shattered during heating.

To form large-area graphene nanosheet films, a solution of solvent and the edge-carboxylated graphene nanosheets was cast on silicon wafers 3.5 centimeters by 5 centimeters, and heated to 900 degrees Celsius. Again, the heat decarboxylated the edges, which then bonded with edges of neighboring pieces. The researchers say this process is limited only by the size of the wafer. The electrical conductivity of the resultant large-area films, even at a high optical transmittance, was still much higher than that of their counterparts from the acid oxidation.

By using ammonia or sulfur trioxide as substitutes for dry ice and by using different solvents, "you can customize the edges for different applications," Baek said. "You can customize for electronics, supercapacitors, metal-free catalysts to replace platinum in fuel cells. You can customize the edges to assemble in two-dimensional and three-dimensional structures."

Provided by Case Western Reserve University (news : web)

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