Researchers have measured a bandgap of approximately 0.5 electron-volts in 1.4-nanometer bent sections of graphene nanoribbons. The development could provide new direction to the field of graphene electronics, which has struggled with the challenge of creating bandgap necessary for operation of electronic devices.

“This is a new way of thinking about how to make high-speed graphene electronics,” said Edward Conrad, a professor in the School of Physics at the Georgia Institute of Technology. “We can now look seriously at making fast transistors from graphene. And because our process is scalable, if we can make one transistor, we can potentially make millions of them.”

The findings were reported November 18 in the journal Nature Physics. The research, done at the Georgia Institute of Technology in Atlanta and at SOLEIL, the French national synchrotron facility, has been supported by the National Science Foundation Materials Research Science and Engineering Center (MRSEC) at Georgia Tech, the W.M. Keck Foundation and the Partner University Fund from the Embassy of France.

Researchers don’t yet understand why graphene nanoribbons become semiconducting as they bend to enter tiny steps – about 20 nanometers deep – that are cut into the silicon carbide wafers. But the researchers believe that strain induced as the carbon lattice bends, along with the confinement of electrons, may be factors creating the bandgap. The nanoribbons are composed of two layers of graphene.

Production of the semiconducting graphene structures begins with the use of e-beams to cut “trenches” into silicon carbide wafers, which are normally polished to create a flat surface for the growth of epitaxial graphene. Using a high-temperature furnace, tens of thousands of graphene ribbons are then grown across the steps, using photolithography.

During the growth, the sharp edges of trenches become smoother as the material attempts to regain its flat surface. The growth time must therefore be carefully controlled to prevent the narrow silicon carbide features from melting too much.

The graphene fabrication also must be controlled along a specific direction so that the carbon atom lattice grows into the steps along the material’s “armchair” direction. “It’s like trying to bend a length of chain-link fence,” Conrad explained. “It only wants to bend one way.”

The new technique permits not only the creation of a bandgap in the material, but potentially also the fabrication of entire integrated circuits from graphene without the need for interfaces that introduce resistance. On either side of the semiconducting section of the graphene, the nanoribbons retain their metallic properties.

“We can make thousands of these trenches, and we can make them anywhere we want on the wafer,” said Conrad. “This is more than just semiconducting graphene. The material at the bends is semiconducting, and it’s attached to graphene continuously on both sides. It’s basically a Shottky barrier junction.”

By growing the graphene down one edge of the trench and then up the other side, the researchers could in theory produce two connected Shottky barriers – a fundamental component of semiconductor devices. Conrad and his colleagues are now working to fabricate transistors based on their discovery.

Confirmation of the bandgap came from angle-resolved photoemission spectroscopy measurements made at the Synchrotron CNRS in France. There, the researchers fired powerful photon beams into arrays of the graphene nanoribbons and measured the electrons emitted.

“You can measure the energy of the electrons that come out, and you can measure the direction from which they come out,” said Conrad. “From that information, you can work backward to get information about the electronic structure of the nanoribbons.”

Theorists had predicted that bending graphene would create a bandgap in the material. But the bandgap measured by the research team was larger than what had been predicted.

Beyond building transistors and other devices, in future work the researchers will attempt to learn more about what creates the bandgap – and how to control it. The property may be controlled by the angle of the bend in the graphene nanoribbon, which can be controlled by altering the depth of the step.

“If you try to lay a carpet over a small imperfection in the floor, the carpet will go over it and you may not even know the imperfection is there,” Conrad explained. “But if you go over a step, you can tell. There are probably a range of heights in which we can affect the bend.”

He predicts that the discovery will create new activity as other graphene researchers attempt to utilize the results.

“If you can demonstrate a fast device, a lot of people will be interested in this,” Conrad said. “If this works on a large scale, it could launch a niche market for high-speed, high-powered electronic devices.”

In addition to Conrad, the research team included J. Hicks, M.S. Nevius, F. Wang, K. Shepperd, J. Palmer, J. Kunc, W.A. De Heer and C. Berger, all from Georgia Tech; A. Tejeda from the Institut Jean Lamour, CNES – Univ. de Nancy and the Synchrotron SOLEIL; A. Taleb-Ibrahimi from the CNRS/Synchrotron SOLEIL, and F. Bertran and P. Le Fevre from Synchrotron SOLEIL.

This research was supported by the National Science Foundation Materials Research Science and Engineering Center (MRSEC) at Georgia Tech under Grants DMR-0820382 and DMR-1005880, the W.M. Keck Foundation, and the Partner University Fund from the Embassy of France. The content of the article is the responsibility of the authors and does not necessarily represent the views of the National Science Foundation.

CITATION: Hicks, J., A wide-bandgap metal-semiconductor-metal nanostructure made entirely from graphene, Nature Physics (2012). http://dx.doi.org/10.1038/NPHYS2487.

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The basic principle for growing thin layers of graphene on silicon carbide requires heating the material to about 1,500 degrees Celsius under high vacuum. The heat drives off the silicon, leaving behind one or more layers of graphene. But uncontrolled evaporation of silicon can produce poor quality material useless to designers of electronic devices.

"For growing high-quality graphene on silicon carbide, controlling the evaporation of silicon at just the right temperature is essential," said Walt de Heer, a professor who pioneered the technique in the Georgia Tech School of Physics. "By precisely controlling the rate at which silicon comes off the wafer, we can control the rate at which graphene is produced. That allows us to produce very nice layers of epitaxial graphene."

De Heer and his team begin by placing a silicon carbide wafer into an enclosure made of graphite. A small hole in the container controls the escape of silicon atoms as the one-square-centimeter wafer is heated, maintaining the rate of silicon evaporation and condensation near its thermal equilibrium. The growth of epitaxial graphene can be done in a vacuum or in the presence of an inert gas such as argon, and can be used to produce both single layers and multiple layers of the material.

"This technique seems to be completely in line with what people might one day do in fabrication facilities," de Heer said. "We believe this is quite significant in allowing us to rationally and reproducibly grow graphene on silicon carbide. We feel we now understand the process, and believe it could be scaled up for electronics manufacturing."

The technique for growing large-area layers of epitaxial graphene was described this week in the Early Edition of the journal Proceedings of the National Academy of Sciences. The research has been supported by the National Science Foundation through the Georgia Tech Materials Research Science and Engineering Center (MRSEC), the Air Force Office of Scientific Research, and the W.M. Keck Foundation.

The paper also describes a technique for growing narrow graphene ribbons, a process de Heer's group has called "templated growth." That technique, which could be useful for making graphene interconnects, was first described in October 2010 in the journal Nature Nanotechnology.

The templated growth technique involves etching patterns into silicon carbide surfaces using conventional nanolithography processes. The patterns serve as templates directing the growth of graphene structures on portions of the patterned surfaces. The technique forms nanoribbons of specific widths without the use of electron beams or other destructive cutting techniques. Graphene nanoribbons produced with these templates have smooth edges that avoid problems with electron scattering.

Together, the two techniques provide researchers with the flexibility to produce graphene in forms appropriate to different needs, de Heer noted. Large-area sheets of graphene may be grown on both the carbon-terminated and silicon-terminated sides of a silicon carbide wafer, while the narrow ribbons may be grown on the silicon-terminated side. Because of different processing techniques, only one side of a particular wafer can be used.

The Georgia Tech research team -- which includes Claire Berger, Ming Ruan, Mike Sprinkle, Xuebin Li, Yike Hu, Baiqian Zhang, John Hankinson and Edward Conrad -- has so far fabricated structures as narrow as 10 nanometers using the templated growth technique. These nanowires exhibit interesting quantum transport properties.

"We can make very good quantum wires using the templated growth technique," de Heer said. "We can make large structures and devices that demonstrate the Quantum Hall Effect, which is important for many applications. We have demonstrated that templated growth can go all the way down to the nanoscale, and that the properties get even better there."

Development of the sublimation technique arose from efforts to protect the growing graphene from oxygen and other contaminants in the furnace. To address the quality concerns, the research team tried enclosing the wafer in a graphite container from which some silicon gas was permitted to leak out.

"We soon realized that graphene grown in the container was much better than what we had been producing," de Heer recalled. "Originally, we thought it was because we were protecting it from contaminants. Later, we realized it was because we were controlling the evaporation of silicon."

Epitaxial graphene may be the basis for a new generation of high-performance devices that will take advantage of the material's unique properties in applications where higher costs can be justified. Silicon, today's electronic material of choice, will continue to be used in applications where high-performance is not required, de Heer said.

Though researchers are still struggling to design nanometer-scale epitaxial graphene devices that take advantage of the material's unique properties, de Heer is confident that will ultimately be done.

"These techniques allow us to make accurate nanostructures and seem to be very promising for making the nanoscale devices that we need," he said. "While there are serious challenges ahead for using graphene in electronics, we have overcome roadblocks before."

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In a paper published in the journal Physical Review B, researchers at the Georgia Institute of Technology and the National Institute of Standards and Technology (NIST) have described a family of seven potential defect structures that may appear in sheets of graphene and imaged examples of the lowest-energy defect in the family.

The defects may arise to help relieve mechanical stress in graphene's carbon-atom honeycomb structure by allowing atoms to spread out and occupy slightly more space. Such stress may arise during the growth of graphene or by stretching the graphene sheet.

"For an engineer interested in the mechanical properties of graphene to create atom-thick membranes, for instance, it would be very important to understand these kinds of properties, which could give rise to plastic deformation of the material," said Phillip First, one of the paper's co-authors and a professor in the Georgia Tech School of Physics. "For instance, it may be that these defects are just one part of the kinetic pathway to failure for a strained sheet of graphene."

For electronic applications, the defects could deflect electrons and cause backscattering that would increase the resistance of the material -- like a rock in a stream slows the flow of water.
However, First says improved growth techniques developed since the defect study began may eliminate that concern.

"With the growth techniques that have now been developed using silicon carbide, we typically do not see these defects," he noted. "The defects occur on material that we know to be of a lower quality because of the growth conditions or substrate preparation."

Defects can appear due to the movement of carbon atoms at high temperatures, explained NIST Fellow Joseph Stroscio. Rearrangements of graphene that require the least amount of energy involve switching from the standard six-member carbon rings to structures containing either five or seven atoms. The NIST researchers have discovered that stringing five- and seven-member rings together in closed loops creates a new type of defect or grain boundary loop in the honeycomb lattice.

According to NIST researcher Eric Cockayne, the fabrication process plays a big role in creating the defects.

"As the graphene forms under high heat, sections of the lattice can come loose and rotate," he said. "As the graphene cools, these rotated sections link back up with the lattice, but in an irregular way. It's almost as if patches of the graphene were cut out with scissors, turned clockwise, and made to fit back into the same place. Only it really doesn't fit, which is why we get these flowers."

So far, only the flower defect, which is composed of six pairs of five- and seven-atom rings, has been observed. Modeling of graphene's atomic structure by the NIST team suggests there might be a veritable bouquet of flower-like configurations. These configurations -- seven in all -- would each possess its own unique mechanical and electrical properties, Cockayne said.

First hopes the team can continue studying the defects, both to learn whether their formation can be controlled and to clarify the role of defects in the material's mechanical properties.

"Graphene is strong and light, so the mechanical properties are of great interest," he noted. "Understanding just how it rips apart is an interesting question that has important implications. But even with these defects, graphene is still spectacularly strong."

Georgia Tech contributions to this work were funded by the Semiconductor Research Corporation (NRI-INDEX) and by the National Science Foundation through the Georgia Tech Materials Research Science and Engineering Center (MRSEC) under grants DMR-0804908 and DMR-0820382.

Mark Esser of NIST also contributed to this article.

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"We can now make very narrow, conductive nanoribbons that have quantum ballistic properties," said Walt de Heer, a professor in the School of Physics at the Georgia Institute of Technology. "These narrow ribbons become almost like a perfect metal. Electrons can move through them without scattering, just like they do in carbon nanotubes."

De Heer discussed recent results of this graphene growth process March 21st at the American Physical Society’s March 2011 Meeting in Dallas. The research was sponsored by the National Science Foundation-supported Materials Research Science and Engineering Center (MRSEC).

First reported Oct. 3 in the advance online edition of the journal Nature Nanotechnology, the new fabrication technique allows production of epitaxial graphene structures with smooth edges. Earlier fabrication techniques that used electron beams to cut graphene sheets produced nanoribbon structures with rough edges that scattered electrons, causing interference. The resulting nanoribbons had properties more like insulators than conductors.

"In our templated growth approach, we have essentially eliminated the edges that take away from the desirable properties of graphene," de Heer explained. "The edges of the epitaxial graphene merge into the silicon carbide, producing properties that are really quite interesting."

The templated growth technique begins with etching patterns into the silicon carbide surfaces on which epitaxial graphene is grown. The patterns serve as templates directing the growth of graphene structures, allowing the formation of nanoribbons and other structures of specific widths and shapes without the use of cutting techniques that produce the rough edges.

In creating these graphene nanostructures, de Heer and his research team first use conventional microelectronics techniques to etch tiny "steps" -- or contours -- into a silicon carbide wafer whose surface has been made extremely flat. They then heat the contoured wafer to approximately 1,500 degrees Celsius, which initiates melting that polishes any rough edges left by the etching process.

Established techniques are then used for growing graphene from silicon carbide by driving the silicon atoms from the surface. Instead of producing a consistent layer of graphene across the entire surface of the wafer, however, the researchers limit the heating time so that graphene grows only on portions of the contours.

The width of the resulting nanoribbons is proportional to the depth of the contours, providing a mechanism for precisely controlling the nanoribbon structures. To form complex structures, multiple etching steps can be carried out to create complex templates.

"This technique allows us to avoid the complicated e-beam lithography steps that people have been using to create structures in epitaxial graphene," de Heer noted. "We are seeing very good properties that show these structures can be used for real electronic applications."

Since publication of the Nature Nanotechnology paper, de Heer's team has been refining its technique. "We have taken this to an extreme -- the cleanest and narrowest ribbons we can make," he said. "We expect to be able to do everything we need with the size ribbons that we are able to make right now, though we probably could reduce the width to 10 nanometers or less."

While the Georgia Tech team is continuing to develop high-frequency transistors -- perhaps even at the terahertz range -- its primary effort now focuses on developing quantum devices, de Heer said. Such devices were envisioned in the patents Georgia Tech holds on various epitaxial graphene processes.

"This means that the way we will be doing graphene electronics will be different," he explained. "We will not be following the model of using standard field-effect transistors (FETs), but will pursue devices that use ballistic conductors and quantum interference. We are headed straight into using the electron wave effects in graphene."

Taking advantage of the wave properties will allow electrons to be manipulated with techniques similar to those used by optical engineers. For instance, switching may be carried out using interference effects -- separating beams of electrons and then recombining them in opposite phases to extinguish the signals.

Quantum devices would be smaller than conventional transistors and operate at lower power. Because of its ability to transport electrons with virtually no resistance, epitaxial graphene may be the ideal material for such devices, de Heer said.

"Using the quantum properties of electrons rather than the standard charged-particle properties means opening up new ways of looking at electronics," he predicted. "This is probably the way that electronics will evolve, and it appears that graphene is the ideal material for making this transition."

De Heer's research team hopes to demonstrate a rudimentary switch operating on the quantum interference principle within a year.

Epitaxial graphene may be the basis for a new generation of high-performance devices that will take advantage of the material's unique properties in applications where higher costs can be justified. Silicon, today's electronic material of choice, will continue to be used in applications where high-performance is not required, de Heer said.

"This is an important step in the process," he added. "There are going to be a lot of surprises as we move into these quantum devices and find out how they work. We have good reason to believe that this can be the basis for a new generation of transistors based on quantum interference."

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That material, the focus of the 2010 Nobel Prize in physics, is graphene -- a fancy name for extremely thin layers of ordinary carbon atoms arranged in a "chicken-wire" lattice. These layers, sometimes just a single atom thick, conduct electricity with virtually no resistance, very little heat generation -- and less power consumption than silicon.

With silicon device fabrication approaching its physical limits, many researchers believe graphene can provide a new platform material that would allow the semiconductor industry to continue its march toward ever-smaller and faster electronic devices -- progress described in Moore's Law. Though graphene will likely never replace silicon for everyday electronic applications, it could take over as the material of choice for high-performance devices.

And graphene could ultimately spawn a new generation of devices designed to take advantage of its unique properties.

Since 2001, Georgia Tech has become a world leader in developing epitaxial graphene, a specific type of graphene that can be grown on large wafers and patterned for use in electronics manufacturing. In a recent paper published in the journal Nature Nanotechnology, Georgia Tech researchers reported fabricating an array of 10,000 top-gated transistors on a 0.24 square centimeter chip, an achievement believed to be the highest density reported so far in graphene devices.

In creating that array, they also demonstrated a clever new approach for growing complex graphene patterns on templates etched into silicon carbide. The new technique offered the solution to one of the most difficult issues that had been facing graphene electronics.

"This is a significant step toward electronics manufacturing with graphene," said Walt de Heer, a professor in Georgia Tech's School of Physics who pioneered the development of graphene for high-performance electronics. "This is another step showing that our method of working with epitaxial graphene grown on silicon carbide is the right approach and the one that will probably be used for making graphene electronics."

Unrolled Carbon Nanotubes

For de Heer, the story of graphene begins with carbon nanotubes, tiny cylindrical structures considered miraculous when they first began to be studied by scientists in 1991. De Heer was among the researchers excited about the properties of nanotubes, whose unique arrangement of carbon atoms gave them physical and electronic properties that scientists believed could be the foundation for a new generation of electronic devices.

Carbon nanotubes still have attractive properties, but the ability to grow them consistently -- and to incorporate them in high-volume electronics applications -- has so far eluded researchers. De Heer realized before others that carbon nanotubes would probably never be used for high-volume electronic devices.

But he also realized that the key to the attractive electronic properties of the nanotubes was the lattice created by the carbon atoms. Why not simply grow that lattice on a flat surface, and use fabrication techniques proven in the microelectronics industry to create devices in much the same way as silicon integrated circuits?

By heating silicon carbide -- a widely-used electronic material -- de Heer and his colleagues were able to drive silicon atoms from the surface, leaving just the carbon lattice in thin layers of graphene large enough to grow the kinds of electronic devices familiar to a generation of electronics designers.

That process was the basis for a patent filed in 2003, and for initial research support from chip-maker Intel. Since then, de Heer's group has published dozens of papers and helped spawn other research groups also using epitaxial graphene for electronic devices. Though scientists are still learning about the material, companies such as IBM have launched research programs based on epitaxial graphene, and agencies such as the National Science Foundation (NSF) and Defense Advanced Research Projects Agency (DARPA) have invested in developing the material for future electronics applications.

Georgia Tech's work on developing epitaxial graphene for manufacturing electronic devices was recognized in the background paper produced by the Royal Swedish Academy of Sciences as part of the Nobel Prize documentation.

The race to find commercial applications for graphene is intense, with researchers from the United States, Europe, Japan and Singapore engaged in well-funded efforts. Since awarding of the Nobel to a group from the United Kingdom, the flood of news releases about graphene developments has grown.

"Our epitaxial graphene is now used around the world by many research laboratories," de Heer noted. "We are probably at the stage where silicon was in the 1950s. This is the beginning of something that is going to be very large and important."

Silicon "Running Out of Gas"

A new electronics material is needed because silicon is running out of miniaturization room.

"Primarily, we've gotten the speed increases from silicon by continually shrinking feature sizes and improving interconnect technology," said Dennis Hess, director of the National Science Foundation-sponsored Materials Research Science and Engineering Center (MRSEC) established at Georgia Tech to study future electronic materials, starting with epitaxial graphene. "We are at the point where in less than 10 years, we won't be able to shrink feature sizes any farther because of the physics of the device operation. That means we will either have to change the type of device we make, or change the electronic material we use."

It's a matter of physics. At the very small size scales needed to create ever more dense device arrays, silicon generates too much resistance to electron flow, creating more heat than can be dissipated and consuming too much power.

Graphene has no such restrictions, and in fact, can provide electron mobility as much as 100 times better than silicon. De Heer believes his group has developed the roadmap for the future of high-performance electronics -- and that it is paved with epitaxial graphene.

"We have basically developed a whole scheme for making electronics out of graphene," he said. "We have set down what we believe will be the ground rules for how that will work, and we have the key patents in place."

Silicon, of course, has matured over many generations through constant research and improvement. De Heer and Hess agree that silicon will always be around, useful for low-cost consumer products such as iPods, toasters, personal computers and the like.

De Heer expects graphene to find its niche doing things that couldn't otherwise be done.

"We're not trying to do something cheaper or better; we're going to do things that can't be done at all with silicon," he said. "Making electronic devices as small as a molecule, for instance, cannot be done with silicon, but in principle could be done with graphene. The key question is how to extend Moore's Law in a post-CMOS world."

Unlike the carbon nanotubes he studied in the 1990s, de Heer sees no major problems ahead for the development of epitaxial graphene.

"That graphene is going to be a major player in the electronics of the future is no longer in doubt," he said. "We don't see any real roadblocks ahead. There are no flashing red lights or other signs that seem to say that this won't work. All of the issues we see relate to improving technical issues, and we know how to do that."

Making the Best Graphene

Since beginning the exploration of graphene in 2001, de Heer and his research team have made continuous improvements in the quality of the material they produce, and those improvements have allowed them to demonstrate a number of physical properties -- such as the Quantum Hall Effect -- that verify the unique properties of the material.

"The properties that we see in our epitaxial graphene are similar to what we have calculated for an ideal theoretical sheet of graphene suspended in the air," said Claire Berger, a research scientist in the Georgia Tech School of Physics who also has a faculty appointment at the Centre National de la Recherche Scientifique in France. "We see these properties in the electron transport and we see these properties in all kinds of spectroscopy. Everything that is supposed to be occurring in a single sheet of graphene we are seeing in our systems."

Key to the material's future, of course, is the ability to make electronic devices that work consistently. The researchers believe they have almost reached that point.

"All of the properties that epitaxial graphene needs to make it viable for electronic devices have been proven in this material," said Ed Conrad, a professor in Georgia Tech's School of Physics who is also a MRSEC member. "We have shown that we can make macroscopic amounts of this material, and with the devices that are scalable, we have the groundwork that could really make graphene take off."

Reaching higher and higher device density is also important, along with the ability to control the number of layers of graphene produced. The group has demonstrated that in their multilayer graphene, each layer retains the desired properties.

"Multilayer graphene has different stacking than graphite, the material found in pencils," Conrad noted. "In graphite, every layer is rotated 60 degrees and that's the only way that nature can do it. When we grow graphene on silicon carbide, the layers are rotated 30 degrees. When that happens, the symmetry of the system changes to make the material behave the way we want it to."

Epitaxial Versus Exfoliated

Much of the world's graphene research -- including work leading to the Nobel -- involved the study of exfoliated graphene: layers of the material removed from a block of graphite, originally with tape. While that technique produces high-quality graphene, it's not clear how that could be scaled up for industrial production.

While agreeing that the exfoliated material has produced useful information about graphene properties, de Heer dismisses it as "a science project" unlikely to have industrial electronics application.

"Electronics companies are not interested in graphene flakes," he said. "They need industrial graphene, a material that can be scaled up for high-volume manufacturing. Industry is now getting more and more interested in what we are doing."

De Heer says Georgia Tech's place in the new graphene world is to focus on electronic applications.

"We are not really trying to compete with these other groups," he said. "We are really trying to create a practical electronic material. To do that, we will have to do many things right, including fabricating a scalable material that can be made as large as a wafer. It will have to be uniform and able to be processed using industrial methods."

Resolving Technical Issues

Among the significant technical issues facing graphene devices has been electron scattering that occurs at the boundaries of nanoribbons. If the edges aren't perfectly smooth -- as usually happens when the material is cut with electron beams -- the roughness bounces electrons around, creating resistance and interference.

To address that problem, de Heer and his team recently developed a new "templated growth" technique for fabricating nanometer-scale graphene devices. The technique involves etching patterns into the silicon carbide surfaces on which epitaxial graphene is grown. The patterns serve as templates directing the growth of graphene structures, allowing the formation of nanoribbons of specific widths without the use of e-beams or other destructive cutting techniques. Graphene nanoribbons produced with these templates have smooth edges that avoid electron-scattering problems.

"Using this approach, we can make very narrow ribbons of interconnected graphene without the rough edges," said de Heer. "Anything that can be done to make small structures without having to cut them is going to be useful to the development of graphene electronics because if the edges are too rough, electrons passing through the ribbons scatter against the edges and reduce the desirable properties of graphene."

In nanometer-scale graphene ribbons, quantum confinement makes the material behave as a semiconductor suitable for creation of electronic devices. But in ribbons a micron or so wide, the material acts as a conductor. Controlling the depth of the silicon carbide template allows the researchers to create these different structures simultaneously, using the same growth process.

"The same material can be either a conductor or a semiconductor depending on its shape," noted de Heer. "One of the major advantages of graphene electronics is to make the device leads and the semiconducting ribbons from the same material. That's important to avoid electrical resistance that builds up at junctions between different materials."

After formation of the nanoribbons, the researchers apply a dielectric material and metal gate to construct field-effect transistors. While successful fabrication of high-quality transistors demonstrates graphene's viability as an electronic material, de Heer sees them as only the first step in what could be done with the material.

"When we manage to make devices well on the nanoscale, we can then move on to make much smaller and finer structures that will go beyond conventional transistors to open up the possibility for more sophisticated devices that use electrons more like light than particles," he said. "If we can factor quantum mechanical features into electronics, that is going to open up a lot of new possibilities."

Collaborations with Other Groups

Before engineers can use epitaxial graphene for the next generation of electronic devices, they will have to understand its unique properties. As part of that process, Georgia Tech researchers are collaborating with scientists at the National Institute of Standards and Technology (NIST). The collaboration has produced new insights into how electrons behave in graphene.

In a recent paper published in the journal Nature Physics, the Georgia Tech-NIST team described for the first time how the orbits of electrons are distributed spatially by magnetic fields applied to layers of epitaxial graphene. They also found that these electron orbits can interact with the substrate on which the graphene is grown, creating energy gaps that affect how electron waves move through the multilayer material.

"The regular pattern of magnetically-induced energy gaps in the graphene surface creates regions where electron transport is not allowed," said Phillip N. First, a professor in the Georgia Tech School of Physics and MRSEC member. "Electron waves would have to go around these regions, requiring new patterns of electron wave interference. Understanding this interference would be important for some bi-layer graphene devices that have been proposed."

Earlier NIST collaborations led to improved understanding of graphene electron states, and the way in which low temperature and high magnetic fields can affect energy levels. The researchers also demonstrated that atomic-scale moiré patterns, an interference pattern that appears when two or more graphene layers are overlaid, can be used to measure how sheets of graphene are stacked.

In a collaboration with the U.S. Naval Research Laboratory and University of Illinois at Urbana-Champaign, a group of Georgia Tech professors developed a simple and quick one-step process for creating nanowires on graphene oxide.

"We've shown that by locally heating insulating graphene oxide, both the flakes and the epitaxial varieties, with an atomic force microscope tip, we can write nanowires with dimensions down to 12 nanometers," said Elisa Riedo, an associate professor in the Georgia Tech School of Physics and a MRSEC member. "And we can tune their electronic properties to be up to four orders of magnitude more conductive."

A New Industrial Revolution?

Though graphene can be grown and fabricated using processes similar to those of silicon, it is not easily compatible with silicon. That means companies adopting it will also have to build new fabrication facilities -- an expensive investment. Consequently, de Heer believes industry will be cautious about moving into a new graphene world.

"Silicon technology is completely entrenched and well developed," he admitted. "We can adopt many of the processes of silicon, but we can't easily integrate ourselves into silicon. Because of that, we really need a major paradigm shift. But for the massive electronics industry, that will not happen easily or gently."

He draws an analogy to steamships and passenger trains at the dawn of the aviation age. At some point, it became apparent that airliners were going to replace both ocean liners and trains in providing first-class passenger service. Though the cost of air travel was higher, passengers were willing to pay a premium for greater speed.

"We are going to see a coexistence of technologies for a while, and how the hybridization of graphene and silicon electronics is going to happen remains up in the air," de Heer predicted. "That is going to take decades, though in the next ten years we are probably going to see real commercial devices that involve graphene."

This article originally appeared in Research Horizons, Georgia Tech's research magazine.

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“I am very pleased and encouraged that our research todevelop epi-graphene for electronics is recognized already in this early stage.This will certainly stimulate others to join us in this important endeavor,”said de Heer.

De Heer and his lab at Georgia Tech are known worldwide asthe first to conceptualize the use of graphene for electronics, back in 2001.Currently de Heer’s lab is working on developing epitaxial graphene as areplacement for silicon in electronics.

“Because epi-graphene may be able to surpass the speedlimitations of silicon, while also allowing for less heat to be generated in asmaller chip, we believe that graphene shows great promise in being able toreplace silicon in electronics for applications such as ultra-high frequencyelectronics, where these attributes will be needed most,” said de Heer.

“Walt de Heer is aglobal leader in graphene research, and we congratulate him on this latestrecognition of his important work,” said Georgia Tech President G.P. “Bud” Peterson.  “The interdisciplinary research that he and his colleagues aredoing at Georgia Tech has the potential to dramatically change the electronicsindustry by enabling the use of this promising material in future generationsof high-performance electronic devices.”

De Heerearned a doctoral degree in physics from the University of California - Berkeleyin 1986. He worked at the École Polytechnique Fédérale de Lausanne inSwitzerland from 1987-1997.

Currently aRegents' Professor of Physics at the Tech, de Heer directs the EpitaxialGraphene Laboratory in the School of Physics and leads the Epitaxial GrapheneInterdisciplinary Research Group at the Georgia Tech Materials Research Scienceand Engineering Center.

De Heer andhis research groups have made significant contributions to several areas innanoscopic physics. In 1995, de Heer’s research turned to carbon nanotubes,showing that they are excellent field emitters with potential application toflat panel displays. In 1998, he discovered that carbon nanotubes are ballisticconductors, which is a key property for graphene-based electronics.

In 2001, hiswork on nanopatterned epi-graphene electronics led to the development ofgraphene-based electronics. This project was funded by Intel Corporation in 2003and by the National Science Foundation (NSF) in 2004. His paper, UltrathinEpitaxial Graphite: Two-Dimensional Electron Gas Properties and a Route TowardsGraphene-Based Electronics, published in the Journal of Physical Chemistry,laid the experimental and conceptual foundation for graphene-based electronics.De Heer holds the first patent for graphene-based electronics that wasprovisionally filed in June 2003.

The 2010 physics prize was awarded for producing, isolating, identifying and characterizing graphene, a single atomic layer of carbon whose unique properties make the material attractive for electronic applications. Scientists at the University of Manchester were recognized for their work on graphene sheets peeled from blocks of graphite.

The work of the Georgia Tech group, headed by Professor Walt de Heer in the Georgia Tech School of Physics, was recognized by the Royal Swedish Academy of Sciences in its scientific background document on the physics prize. De Heer's group pioneered epitaxial techniques for growing large-scale graphene sheets by heating wafers of silicon carbide to drive off the silicon, leaving a thin layer of graphene.

The technique, which is now being used by research groups at companies such as IBM, has practical applications in large-scale production of electronic devices. On Oct. 3, the group published a paper in the journal Nature Nanotechnology describing a new technique used to produce an array of 10,000 graphene transistors.

"We believe that our technique, or one very much like it, will ultimately be used to manufacture future generations of graphene-based electronic devices," said de Heer. "Using techniques that are suitable for scaling up for mass production, we can grow graphene in the patterns that we need for electronic devices."

The Georgia Tech group holds a patent, filed in 2003, on fabricating electronic devices from these graphene layers.

Georgia Tech is home to a Materials Research Science and Engineering Center (MRSEC), funded by the National Science Foundation (NSF) and including collaborators from the University of California-Berkeley, University of California-Riverside and University of Michigan. The foundation focus of the center is research and development of epitaxial graphene.

"The unique properties of graphene portend considerable promise for future electronic and optical devices," said Dennis Hess, the center's director. "If graphene is to serve as a viable successor to silicon-based microelectronic devices and circuits, large scale production on a suitable substrate is required. Proof of concept of this approach has already been demonstrated by the fabrication of a 10,000 epitaxial graphene transistor array by Walt de Heer and his collaborators. This achievement is a significant advance toward realizing carbon-based electronics for the 21st century."

The Georgia Tech team also collaborates with researchers at the National Institute of Standards and Technology (NIST) on characterizing the unique properties of graphene. That work has led to several recent important papers, in journals such as Science and Nature Physics. The latter described for the first time how the orbits of electrons are distributed spatially by magnetic fields applied to layers of epitaxial graphene.

On Oct. 3 in the advance online publication of the journal Nature Nanotechnology, de Heer and collaborators described the development of a new "templated growth" technique for fabricating nanometer-scale graphene devices. The method addresses what had been a significant obstacle to the use of this promising material in future generations of high-performance electronic devices.

The technique involves etching patterns into the silicon carbide surfaces on which epitaxial graphene is grown. The patterns serve as templates directing the growth of graphene structures, allowing the formation of nanoribbons of specific widths without the use of e-beams or other destructive cutting techniques. Templated nanoribbon growth addresses the edge roughness that causes electron scattering.

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The technique involves etching patterns into the silicon carbide surfaces on which epitaxial graphene is grown. The patterns serve as templates directing the growth of graphene structures, allowing the formation of nanoribbons of specific widths without the use of e-beams or other destructive cutting techniques. Graphene nanoribbons produced with these templates have smooth edges that avoid electron-scattering problems.

"Using this approach, we can make very narrow ribbons of interconnected graphene without the rough edges," said Walt de Heer, a professor in the Georgia Tech School of Physics. "Anything that can be done to make small structures without having to cut them is going to be useful to the development of graphene electronics because if the edges are too rough, electrons passing through the ribbons scatter against the edges and reduce the desirable properties of graphene."

The new technique has been used to fabricate an array of 10,000 top-gated graphene transistors on a 0.24 square centimeter chip – believed to be the largest density of graphene devices reported so far.

The research was reported Oct. 3 in the advance online edition of the journal Nature Nanotechnology. The work was supported by the National Science Foundation, the W.M. Keck Foundation and the Nanoelectronics Research Initiative Institute for Nanoelectronics Discovery and Exploration (INDEX).

In creating their graphene nanostructures, De Heer and his research team first use conventional microelectronics techniques to etch tiny "steps" – or contours – into a silicon carbide wafer. They then heat the contoured wafer to approximately 1,500 degrees Celsius, which initiates melting that polishes any rough edges left by the etching process.

They then use established techniques for growing graphene from silicon carbide by driving off the silicon atoms from the surface. Instead of producing a consistent layer of graphene one atom thick across the surface of the wafer, however, the researchers limit the heating time so that graphene grows only on the edges of the contours.

To do this, they take advantage of the fact that graphene grows more rapidly on certain facets of the silicon carbide crystal than on others. The width of the resulting nanoribbons is proportional to the depth of the contour, providing a mechanism for precisely controlling the nanoribbons. To form complex graphene structures, multiple etching steps can be carried out to create a complex template, de Heer explained.

"By using the silicon carbide to provide the template, we can grow graphene in exactly the sizes and shapes that we want," he said. "Cutting steps of various depths allows us to create graphene structures that are interconnected in the way we want them to be."

In nanometer-scale graphene ribbons, quantum confinement makes the material behave as a semiconductor suitable for creation of electronic devices. But in ribbons a micron or more wide, the material acts as a conductor. Controlling the depth of the silicon carbide template allows the researchers to create these different structures simultaneously, using the same growth process.

"The same material can be either a conductor or a semiconductor depending on its shape," noted de Heer, who is also a faculty member in Georgia Tech’s National Science Foundation-supported Materials Research Science and Engineering Center (MRSEC). "One of the major advantages of graphene electronics is to make the device leads and the semiconducting ribbons from the same material. That's important to avoid electrical resistance that builds up at junctions between different materials."

After formation of the nanoribbons – which can be as narrow as 40 nanometers – the researchers apply a dielectric material and metal gate to construct field-effect transistors. While successful fabrication of high-quality transistors demonstrates graphene's viability as an electronic material, de Heer sees them as only the first step in what could be done with the material.

"When we manage to make devices well on the nanoscale, we can then move on to make much smaller and finer structures that will go beyond conventional transistors to open up the possibility for more sophisticated devices that use electrons more like light than particles," he said. "If we can factor quantum mechanical features into electronics, that is going to open up a lot of new possibilities."

De Heer and his research team are now working to create smaller structures, and to integrate the graphene devices with silicon. The researchers are also working to improve the field-effect transistors with thinner dielectric materials.

Ultimately, graphene may be the basis for a generation of high-performance devices that will take advantage of the material's unique properties in applications where the higher cost can be justified. Silicon will continue to be used in applications that don't require such high performance, de Heer said.

"This is another step showing that our method of working with epitaxial graphene on silicon carbide is the right approach and the one that will probably be used for making graphene electronics," he added. "This is a significant new step toward electronics manufacturing with graphene."

In addition to those already mentioned, the research has involved M. Sprinkle, M. Ruan, Y Hu, J. Hankinson, M. Rubio-Roy, B. Zhang, X. Wu and C. Berger.

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Reported Sept. 9 in the journal Nature, the new research raises several intriguing questions about the fundamental physics of this exciting material and reveals new effects that may make graphene even more powerful than previously expected for practical applications.

Led by NIST Fellow Joseph Stroscio, the research team included scientists from the Georgia Institute of Technology, the University of Maryland, Seoul National University, and the University of Texas at Austin.

Graphene is one of the simplest materials -- a single-atom-thick sheet of carbon atoms arranged in a honeycomb-like lattice -- yet it has many remarkable and surprisingly complex properties. Measuring and understanding how electrons carry current through the sheet is a key to achieving its technological promise in wide-ranging applications, including high speed electronics and sensors.

For example, the electrons in graphene act as if they have no mass and are almost 100 times more mobile than in silicon. Moreover, the speed with which electrons move through graphene is not related to their energy, unlike materials such as silicon where more voltage must be applied to increase their speed, which creates heat that is detrimental to most applications.

To fully understand the behavior of graphene's electrons, scientists must study the material under an extreme environment of ultra-high vacuum, ultra-low temperatures, and large magnetic fields. Under these conditions, the graphene sheet remains pristine for weeks.

NIST has recently constructed the world’s most powerful and stable scanning-probe microscope, with an unprecedented combination of low temperature (as low as 10 millikelvin, or 10 thousandths of a degree above absolute zero), ultra-high vacuum, and high magnetic field. In the first measurements made with this instrument, the international team has used its power to resolve the finest differences in the electron energies in graphene, atom-by-atom.

"Going to this resolution allows you to see new physics," said Young Jae Song, a postdoctoral researcher who helped develop the instrument at NIST and make these first measurements.

And the new physics the team saw raises a few more questions about how the electrons behave in graphene than it answers.

Because of the geometry and electromagnetic properties of graphene's structure, an electron in any given energy level populates four possible sublevels, called a "quartet." Theorists have predicted that this quartet of levels would split into different energies when immersed in a magnetic field, but until recently there had not been an instrument sensitive enough to resolve these differences.

"When we increased the magnetic field at extreme low temperatures, we observed unexpectedly complex quantum behavior of the electrons," said NIST Fellow Joseph Stroscio.

What is happening, according to Stroscio, appears to be a "many-body effect" in which electrons interact strongly with one another in ways that affect their energy levels.

One possible explanation for this behavior is that the electrons have formed a "condensate" in which they cease moving independently of one another and act as a single coordinated unit.

The new experiments also showed surprising stability in the quartet states, an issue that warrants further study, said Phillip First, a professor in Georgia Tech's School of Physics and one of the study's co-authors.

"The experiment shows that these magnetic configurations become especially stable when any one of the quartet states is completely filled with electrons, which indicates the importance of many-body correlations," he said. "However, the most surprising thing is the observation of new stable states that occur when a quartet state is exactly half filled. That's pretty remarkable, and we still need an explanation."

Graphene has attracted strong interest as a potential material for future electronic devices, and this new work reinforces that expectation.

"If our hypothesis proves to be correct, it could point the way to the creation of smaller, very-low-heat producing, highly energy efficient electronic devices based upon graphene," said Shaffique Adam, a postdoctoral researcher who assisted with theoretical analysis of the measurements.

In addition to First, Georgia Tech researchers contributing to the paper included Walt de Heer, Yike Hu and David Torrance. The research was supported in part by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD)(KRF-2006-214-C00022), the National Science Foundation (DMR-0820382 [MRSEC], DMR-0804908, DMR-0606489), the Welch Foundation and the Semiconductor Research Corporation (NRI-INDEX program).

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In the Aug. 8 advance online edition of the journal Nature Physics, researchers from the Georgia Institute of Technology and the National Institute of Standards and Technology (NIST) describe for the first time how the orbits of electrons are distributed spatially by magnetic fields applied to layers of epitaxial graphene.

The research team also found that these electron orbits can interact with the substrate on which the graphene is grown, creating energy gaps that affect how electron waves move through the multilayer material. These energy gaps could have implications for the designers of certain graphene-based electronic devices.

"The regular pattern of energy gaps in the graphene surface creates regions where electron transport is not allowed," said Phillip N. First, a professor in the Georgia Tech School of Physics and one of the paper’s co-authors. "Electron waves would have to go around these regions, requiring new patterns of electron wave interference. Understanding such interference will be important for bi-layer graphene devices that have been proposed, and may be important for other lattice-matched substrates used to support graphene and graphene devices."

In a magnetic field, an electron moves in a circular trajectory -- known as a cyclotron orbit -- whose radius depends on the size of the magnetic field and the energy of electron. For a constant magnetic field, that's a little like rolling a marble around in a large bowl, First said.

"At high energy, the marble orbits high in the bowl, while for lower energies, the orbit size is smaller and lower in the bowl," he explained. "The cyclotron orbits in graphene also depend on the electron energy and the local electron potential -- corresponding to the bowl -- but until now, the orbits hadn’t been imaged directly."

Placed in a magnetic field, these orbits normally drift along lines of nearly constant electric potential. But when a graphene sample has small fluctuations in the potential, these "drift states" can become trapped at a hill or valley in the material that has closed constant potential contours. Such trapping of charge carriers is important for the quantum Hall effect, in which precisely quantized resistance results from charge conduction solely through the orbits that skip along the edges of the material.

The study focused on one particular electron orbit: a zero-energy orbit that is unique to graphene. Because electrons are matter waves, interference within a material affects how their energy relates to the velocity of the wave -- and reflected waves added to an incoming wave can combine to produce a slower composite wave. Electrons moving through the unique "chicken-wire" arrangement of carbon-carbon bonds in the graphene interfere in a way that leaves the wave velocity the same for all energy levels.

In addition to finding that energy states follow contours of constant electric potential, the researchers discovered specific areas on the graphene surface where the orbital energy of the electrons changes from one atom to the next. That creates an energy gap within isolated patches on the surface.

"By examining their distribution over the surface for different magnetic fields, we determined that the energy gap is due to a subtle interaction with the substrate, which consists of multilayer graphene grown on a silicon carbide wafer," First explained.

In multilayer epitaxial graphene, each layer's symmetrical sublattice is rotated slightly with respect to the next. In prior studies, researchers found that the rotations served to decouple the electronic properties of each graphene layer.

"Our findings hold the first indications of a small position-dependent interaction between the layers," said David L. Miller, the paper's first author and a graduate student in First's laboratory. "This interaction occurs only when the size of a cyclotron orbit -- which shrinks as the magnetic field is increased -- becomes smaller than the size of the observed patches."

The origin of the position dependent interaction is believed to be the "moiré pattern" of atomic alignments between two adjacent layers of graphene. In some regions, atoms of one layer lie atop atoms of the layer below, while in other regions, none of the atoms align with the atoms in the layer below. In still other regions, half of the atoms have neighbors in the underlayer, an instance in which the symmetry of the carbon atoms is broken and the Landau level -- discrete energy level of the electrons -- splits into two different energies.

Experimentally, the researchers examined a sample of epitaxial graphene grown at Georgia Tech in the laboratory of Professor Walt de Heer, using techniques developed by his research team over the past several years.

They used the tip of a custom-built scanning-tunneling microscope (STM) to probe the atomic-scale electronic structure of the graphene in a technique known as scanning tunneling spectroscopy. The tip was moved across the surface of a 100-square nanometer section of graphene, and spectroscopic data was acquired every 0.4 nanometers.

The measurements were done at 4.3 degrees Kelvin to take advantage of the fact that energy resolution is proportional to the temperature. The scanning-tunneling microscope, designed and built by Joseph Stroscio at NIST's Center for Nanoscale Science and Technology, used a superconducting magnet to provide the magnetic fields needed to study the orbits.

According to First, the study raises a number of questions for future research, including how the energy gaps will affect electron transport properties, how the observed effects may impact proposed bi-layer graphene coherent devices -- and whether the new phenomenon can be controlled.

"This study is really a stepping stone in long path to understanding the subtleties of graphene's interesting properties," he said. "This material is different from anything we have worked with before in electronics."

In addition to those already mentioned, the study also included Walt de Heer, Kevin D. Kubista, Ming Ruan, and Markus Kinderman from Georgia Tech and Gregory M. Rutter from NIST. The research was supported by the National Science Foundation, the Semiconductor Research Corporation and the W.M. Keck Foundation. Additional assistance was provided by Georgia Tech's Materials Research Science and Engineering Center (MRSEC).

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“The symposium brings together engineers and scientists fromaround the world to discuss recent progress and future trends in the rapidlydeveloping science and technology of epitaxial graphene,” said Walt de Heer,Regents’ Professor in Georgia Tech’s Schoolof Physics and a pioneer in graphene-based electronics. “The symposium willcover a broad range of epitaxial graphene on silicon carbidetopics, including surface science and growth, transport, opticalproperties, chemistry, devices andtheory.  The discussions during this symposium will help to establish thefuture directions of epitaxial graphene science and technology.”

The symposium was first held in 2009 and is expected to be ayearly gathering. This year 130 attendees are expected. In addition to scientistsfrom Georgia Tech, researchers from institutions such as the University ofCalifornia, the National Institute of Standards and Technology,  the French National Center forScientific Research (CNRS), the German Max Planck Institute, the Japanese NTTlabs  and  several representatives from industry will be in attendance.

So far, the substance has shown great promise in being amaterial that can conduct electricity with little resistance without many ofthe problems that carbon nanotubes have exhibited, such as difficulties withplacing them and building them into wires. In addition, research suggests thatepitaxial graphene may offer much greater speed and performance over silicon.

Scientists at the symposium will discuss the recent resultsof their research and will likely plan future scientific endeavors in thisfield.