The Consulate General of France in Atlanta has announced that Berger has been awarded the Chevalier dans L'Ordre des Palmes Académiques for her “exceptional dedication and significant accomplishments in the field of science and education,” says Rami Abi Akl, French attaché for science and technology in Atlanta.

The Palmes Académiques is presented to French citizens and non-citizens who have made significant contributions to French education, science, and culture. The first Palmes Académiques was presented by Napoleon in 1808.

Berger’s “pioneering work in physics, particularly on graphene, has not only advanced scientific knowledge, but also served as an inspiration to others in her field,” Abi Akl says.

In addition to her research and classes at Georgia Tech, Berger is Director of Research at the French National Center for Scientific Research (CNRS), which has been home to 12 Nobel Prize and 10 Fields Medal winners. Berger’s affiliation is with the CNRS International Research Lab, with its main campus at Georgia Tech-Europe in Metz, France, and an affiliated lab at Georgia Tech’s Atlanta campus.

Approximately 50 colleagues from both countries have conducted collaborative research at both Georgia Tech campuses, thanks to Berger’s efforts.

“Her selection for this honor also reflects her remarkable impact on both the American and French scientific communities,” Abi Akl says. “Her collaborative efforts and contributions to scientific research have fostered strong ties between France and the United States, strengthening the bonds of scientific diplomacy.”

“A very big thank you to the French General Consulate in Atlanta for submitting my name for this distinctive honor,” Berger recently shared. “Among other funding agencies and foundations, I am particularly grateful to the French Embassy for their partnership grants that funded travel and helped collaboration between almost 60 faculty members, postdoctoral scholars, and students.”

“I also want to thank Georgia Tech and the School of Physics for their full support,” she added. “All that travel and dedicated lab work wouldn’t have happened without the love and support at home from my husband and our three sons.”

About Claire Berger

Berger was born in Paris, France, and received her Ph.D. from the Université Grenoble Alpes. She joined Georgia Tech in 2001, and she quickly established herself as a noted researcher of the electronic properties of graphene, a material with a flat, two-dimensional structure that is touted as a potential successor to silicon in computer processors.

Berger and School of Physics Regents’ Professor Walter de Heer are working on graphene discoveries that could lead to smaller, more energy-efficient processing that is expected to usher in a new era of quantum and high performance computing.

Walter de Heer welcomed Berger into his lab when she arrived at Georgia Tech, she says. “I want to thank him for being an incredible team leader in this adventure, for his continuous support, his insights, dedication and passion for science.”

Berger co-authored the first article demonstrating the two-dimensional properties of graphene and a possible electronics platform for the material. Berger, de Heer, and School of Physics Professor Phillip First also co-authored the first patent for graphene electronics in 2003.

She is the co-author of more than 200 publications in international journals. From 2014 to 2019, she was among the top one percent most cited researchers in physics.

In good company with another Atlanta Palmes winner

Berger says she was given the letter by the General Consul of Atlanta announcing her award during an event at the Embassy. “I was so surprised by the nomination that I was fumbling trying to find my words. This was a great — and a bit embarrassing — moment at the same time.”

One of her good friends, Bill Moon, is a fellow Palmes Académiques winner for promoting French language instruction at private and public schools in Atlanta and Decatur. “He founded the International Community School in Clarkston, Georgia, a public charter elementary school serving the needs of U.S. and refugee families now living in DeKalb County, and he continues to be active in the service of communities,” Berger says. “To be awarded the same medal as Bill is an incredible honor.”

Platinum is one of the most commonly used catalysts for fuel cells because of how effectively it enables the oxidation reduction reaction at the center of the technology. But its high cost has spurred research efforts to find ways to use smaller amounts of it while maintaining the same catalytic activity.

“There’s always going to be an initial cost for producing a fuel cell with platinum catalysts, and it’s important to keep that cost as low as possible,” said Faisal Alamgir, an associate professor in Georgia Tech’s School of Materials Science and Engineering. “But the real cost of a fuel cell system is calculated by how long that system lasts, and this is a question of durability.

“Recently there’s been a push to use catalytic systems without platinum, but the problem is that there hasn’t been a system proposed so far that simultaneously matches the catalytic activity and the durability of platinum,” Alamgir said.

The Georgia Tech researchers tried a different strategy. In the study, which was published on September 18 in the journal Advanced Functional Materials and supported by the National Science Foundation, they describe creating several systems that used atomically-thin films of platinum supported by a layer of graphene – effectively maximizing the total surface area of the platinum available for catalytic reactions and using a much smaller amount of the precious metal.

Most platinum-based catalytic systems use nanoparticles of the metal chemically bonded to a support surface, where surface atoms of the particles do most of the catalytic work, and the catalytic potential of the atoms beneath the surface is never utilized as fully as the surface atoms, if at all.

Additionally, the researchers showed that the new platinum films that are at least two atoms thick outperformed nanoparticle platinum in the dissociation energy, which is a measure of the energy cost of dislodging a surface platinum atom. That measurement suggests those films could make potentially longer-lasting catalytic systems.

To prepare the atomically-thin films, the researchers used a process called electrochemical atomic layer deposition to grow platinum monolayers on a layer of graphene, creating samples that had one, two or three atomic layers of atoms. The researchers then tested the samples for dissociation energy and compared the results to the energy of a single atom of platinum on graphene as well as the energy from a common configurations of platinum nanoparticles used in catalysts.

“The fundamental question at the heart of this work was whether it was possible that a combination of metallic and covalent bonding can render the platinum atoms in a platinum-graphene combination more stable than their counterparts in bulk platinum used commonly in catalysts that are supported by metallic bonding,” said Seung Soon Jang, an associate professor in the School of Materials Science and Engineering.

The researchers found that the bond between neighboring platinum atoms in the film essentially combines forces with the bond between the film and the graphene layer to provide reinforcement across the system. That was especially true in the platinum film that was two atoms thick.

“Typically metallic films below a certain thickness are not stable because the bonds between them are not directional, and they tend to roll over each other and conglomerate to form a particle,” Alamgir said. “But that’s not true with graphene, which is stable in a two-dimensional form, even one atom thick, because it has very strong covalent directional bonds between its neighboring atoms. So this new catalytic system could leverage the directional bonding of the graphene to support an atomically-thin film of platinum.”

Future research will involve further testing of how the films behave in a catalytic environment. The researchers found in earlier research on graphene-platinum films that the material behaves similarly in catalytic reactions regardless of which side – graphene or platinum – is the exposed active surface.

“In this configuration, the graphene is not acting as a separate entity from the platinum,” Alamgir said. “They’re working together as one. So we believe that if you’re exposing the graphene side, you get the same catalytic activity and you could further protect the platinum, potentially further enhancing durability.”

This research was supported by the National Science Foundation (NSF) under grant Nos. 1103827 and 106913. The content is solely the responsibility of the authors and does not necessarily represent the official views of the sponsoring organizations.

CITATION:  Ji Il Choi, Ali Abdelhafiz, Parker Buntin, Adam Vitale, Alex Robertson, Jamie Warner, Seung Soon Jang and Faisal M. Alamgir, “Contiguous and Atomically-Thin Pt Film with Supra-bulk Behavior Through Graphene-Imposed Epitaxy,” (Advanced Functional Materials, September 2019). http://dx.doi.org/10.1002/adfm.201902274

By heating the ethene in stages to a temperature of slightly more than 700 degrees Celsius -- hotter than had been attempted before – the researchers produced pure layers of graphene on a rhodium catalyst substrate. The stepwise heating and higher temperature overcame challenges seen in earlier efforts to produce graphene directly from hydrocarbon precursors. 

Because of its lower cost and simplicity, the technique could open new potential applications for graphene, which has attractive physical and electronic properties. The work also provides a novel mechanism for the self-evolution of carbon cluster precursors whose diffusional coalescence results in the formation of the graphene layers.

The research, reported as the cover article in the May 4 issue of the Journal of Physical Chemistry C, was conducted by scientists at the Georgia Institute of Technology, Technische Universität München in Germany, and the University of St. Andrews in Scotland. In the United States, the research was supported by the U.S. Air Force Office of Scientific Research and the U.S. Department of Energy’s Office of Basic Energy Sciences.

“Since graphene is made from carbon, we decided to start with the simplest type of carbon molecules and see if we could assemble them into graphene,” explained Uzi Landman, a Regents’ Professor and F.E. Callaway endowed chair in the Georgia Tech School of Physics who headed the theoretical component of the research. “From small molecules containing carbon, you end up with macroscopic pieces of graphene.”

Graphene is now produced using a variety of methods including chemical vapor deposition, evaporation of silicon from silicon carbide – and simple exfoliation of graphene sheets from graphite. A number of earlier efforts to produce graphene from simple hydrocarbon precursors had proven largely unsuccessful, creating disordered soot rather than structured graphene.

Guided by a theoretical approach, the researchers reasoned that the path from ethene to graphene would involve formation of a series of structures as hydrogen atoms leave the ethene molecules and carbon atoms self-assemble into the honeycomb pattern that characterizes graphene. To explore the nature of the thermally-induced rhodium surface-catalyzed transformations from ethene to graphene, experimental groups in Germany and Scotland raised the temperature of the material in steps under ultra-high vacuum. They used scanning-tunneling microscopy (STM), thermal programed desorption (TPD) and high-resolution electron energy loss (vibrational) spectroscopy (HREELS) to observe and characterize the structures that form at each step of the process.

Upon heating, ethene adsorbed onto the rhodium catalyst evolves via coupling reactions to form segmented one-dimensional polyaromatic hydrocarbons (1D-PAH). Further heating leads to dimensionality crossover – one dimensional to two dimensional structures – and dynamical restructuring processes at the PAH chain ends with a subsequent activated detachment of size-selective carbon clusters, following a mechanism revealed through first-principles quantum mechanical  simulations.  Finally, rate-limiting diffusional coalescence of these dynamically self-evolved cluster-precursors leads to condensation into graphene with high purity.

At the final stage before the formation of graphene, the researchers observed nearly round disk-like clusters containing 24 carbon atoms, which spread out to form the graphene lattice. “The temperature must be raised within windows of temperature ranges to allow the requisite structures to form before the next stage of heating,” Landman explained. “If you stop at certain temperatures, you are likely to end up with coking.”

An important component is the dehydrogenation process which frees the carbon atoms to form intermediate shapes, but some of the hydrogen resides temporarily on, or near, the metal catalyst surface and it assists in subsequent bond-breaking process that lead to detachment of the 24-carbon cluster-precursors.  “All along the way, there is a loss of hydrogen from the clusters,” said Landman. “Bringing up the temperature essentially ‘boils’ the hydrogen out of the evolving metal-supported carbon structure, culminating in graphene.”

The resulting graphene structure is adsorbed onto the catalyst. It may be useful attached to the metal, but for other applications, a way to remove it will have to be developed. Added Landman: “This is a new route to graphene, and the possible technological application is yet to be explored.”

Beyond the theoretical research, carried out by Bokwon Yoon and Landman at the Georgia Tech Center for Computational Materials Science, the experimental work was done in the laboratory of Professor Renald Schaub at the University of St. Andrews and in the laboratory of Professor Ueli Heiz and Friedrich Esch at the Technische Universität München. Other co-authors included Bo Wang, Michael König, Catherine J. Bromley, Michael-John Treanor, José A. Garrido Torres, Marco Caffio, Federico Grillo, Herbert Frücht, and Neville V. Richardson.

The work at the Georgia Institute of Technology was supported by the Air Force Office of Scientific Research through Grant FA9550-14-1-0005 and by the Office of Basic Energy Sciences of the U.S. Department of Energy through Grant FG05-86ER45234. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the sponsoring organizations.

CITATION: Bo Wang, et al., “Ethene to Graphene: Surface Catalyzed Chemical Pathways, Intermediates, and Assembly,” (Journal of Physical Chemistry C). http://dx.doi.org/10.1021/acs.jpcc.7b01999

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Writer: John Toon

Using carbon atoms deposited on graphene with a focused electron beam process, Fedorov and collaborators have demonstrated a technique for creating dynamic patterns on graphene surfaces. The patterns could be used to make reconfigurable electronic circuits, which evolve over a period of hours before ultimately disappearing into a new electronic state of the graphene. Graphene is also made up of carbon atoms, but in a highly-ordered form.

Reported in the journal Nanoscale, the research was primarily supported by the U.S. Department of Energy Office of Science, and involved collaboration with researchers from the Air Force Research Laboratory (AFRL), supported by the Air Force Office of Scientific Research. Beyond allowing fabrication of disappearing circuits, the technology could be used as a form of timed release in which the dissipation of the carbon patterns could control other processes, such as the release of biomolecules.

“We will now be able to draw electronic circuits that evolve over time,” said Andrei Fedorov, a professor in the George W. Woodruff School of Mechanical Engineering at Georgia Tech. “You could design a circuit that operates one way now, but after waiting a day for the carbon to diffuse over the graphene surface, you would no longer have an electronic device. Today the device would do one thing; tomorrow it would do something entirely different.”

The project began as a way to clean up hydrocarbons contaminating the surface of the graphene. But the researchers soon realized they could use it to create patterns, utilizing the amorphous carbon produced via electron beam “writing” as a dopant to create negatively-charged sections of graphene.

The researchers were initially perplexed to discover that their newly-formed patterns disappeared over time. They used electronic measurements and atomic force microscopy to confirm that the carbon patterns had moved on the graphene surface to ultimately form a uniform coverage over an entire graphene surface. The change usually occurs over tens of hours, and ultimately converts positively-charged (p-doped) surface regions to surfaces with a uniformly negative charge (n-doped) while forming an intermediate p-n junction domain in the course of this evolution.

“The electronic structures continuously change over time,” Fedorov explained. “That gives you a reconfigurable device, especially since our carbon deposition is done not using bulk films, but rather an electron beam that is used to draw where you want a negatively-doped domain to exist.”

Graphene consists of carbon atoms arranged in a tight lattice. The unique structure provides attractive electronic properties that have led to widespread study of graphene as a potential new material for advanced electronics applications.

But graphene still consists of carbon atoms, and when patterns are deposited on the surface with ordinary carbon atoms, they begin slowly migrating over the graphene surface. The speed at which the atoms move around can be adjusted by varying the temperature or by fabricating structures that direct the movement of the atoms. The carbon atoms can also be “frozen” into a fixed pattern by using a laser to convert them to graphite – another form of carbon.

“There are multiple ways to modulate the dynamic state, through changing the temperature because that controls the diffusion rate of carbon, by directing the atomic flow, or by changing the carbon phase,” Fedorov said. “The carbon deposited through the focused electron beam induced deposition (FEBID) process is linked to graphene very loosely through van der Waals interactions, so it is mobile.”

Beyond the potential security applications for disappearing circuits, Fedorov sees the possibility of simplified control mechanisms that would use the diffusing patterns to turn processes off at preset intervals. The technique might also be used to time the release of pharmaceuticals or other biomedical processes.

“You could write information in ones and zeroes with the electron beam, use the device to transfer information, and then two hours later the information will have disappeared,” he said. “Instead of relying on complex control algorithms that a microprocessor has to execute, by changing the dynamic state or the electronic system itself, your program could become very simple. Perhaps there could be certain activated, triggered processes that could benefit from this type of behavior in which the electronic state changes continuously over time.”

Fedorov and his collaborators have so far shown only the ability to create simple patterns of charged domains in the graphene. Their next step will be to use their p-n junctions to create devices that would operate for specific periods of time.

Fedorov admits that this dynamic carbon patterning could pose a challenge for electrical engineers accustomed to static devices that perform the same functions day after day. But he thinks that some will find useful applications for this new phenomena.

“We have made a critical step in discovery and understanding,” he said. “The next step will be to demonstrate a complicated and unique application which would otherwise be impossible to do with a conventional circuit. That would bring a whole new level of excitement to this.”

Songkil Kim, a post-doctoral fellow in Fedorov group, was a lead researcher in this work assisted by Georgia Tech’s graduate students M. Russell and M. Henry. Other collaborators on the project include S. S. Kim, R. R. Naik, and A. A. Voevodin from the U.S. Air Force Research Laboratory and S. S. Jang, and V. V. Tsukruk from the School of Materials Science and Engineering at Georgia Tech.

This research was supported by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), under Award DE-SC0010729 and by the Air Force Office of Scientific Research (AFOSR) through BIONIC Center Award FA9550-09-1-0162. The comments and conclusions are those of the authors and do not necessarily reflect the official views of the DOE or AFOSR.

CITATION: S. Kim, et al., “Dynamic modulation of electronic properties of graphene by localized carbon doping using focused electron beam induced deposition,” (Nanoscale 7, 14946-14952, 2015). http://dx.doi.org/10.1039/c5nr04063a

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The atom-thin sheet of carbon is touted not just for its electrical properties but also for its physical strength and flexibility. The bonds between carbon atoms are well known as the strongest in nature, so a perfect sheet of graphene should withstand just about anything. Reinforcing composite materials is among the material’s potential applications.

But materials scientists know perfection is hard to achieve. Researchers Jun Lou at Rice and Ting Zhu at Georgia Tech have measured the fracture toughness of imperfect graphene for the first time and found it to be somewhat brittle. While it's still very useful, graphene is really only as strong as its weakest link, which they determined to be "substantially lower" than the intrinsic strength of graphene.

“Graphene has exceptional physical properties, but to use it in real applications, we have to understand the useful strength of large-area graphene, which is controlled by the fracture toughness,” said Zhu, who is an associate professor in the Woodruff School of Mechanical Engineering at Georgia Tech.

The researchers reported in the journal Nature Communications the results of tests in which they physically pulled graphene apart to see how much force it would take. Specifically, they wanted to see if graphene follows the century-old Griffith theory that quantifies the useful strength of brittle materials.

It does, said Lou. "Remarkably, in this case, thermodynamic energy still rules," he said.

Imperfections in graphene drastically lessen its strength – with an upper limit of about 100 gigapascals (GPa) for perfect graphene previously measured by nanoindentation – according to physical testing at Rice and molecular dynamics simulations at Georgia Tech. That's important for engineers to understand as they think about using graphene for flexible electronics, composite material and other applications in which stresses on microscopic flaws could lead to failure.

The Griffith criterion developed by a British engineer during World War I describes the relationship between the size of a crack in a material and the force required to make that crack grow. Ultimately, A.A. Griffith hoped to understand why brittle materials fail.

Graphene, it turns out, is no different from the glass fibers Griffith tested.

"Everybody thinks the carbon-carbon bond is the strongest bond in nature, so the material must be very good," Lou said. "But that's not true anymore, once you have those defects. The larger the sheet, the higher the probability of defects. That's well known in the ceramic community."

A defect can be as small as an atom missing from the hexagonal lattice of graphene. But for a real-world test, the researchers had to make a defect of their own – a pre-crack – they could actually see. "We know there will be pinholes and other defects in graphene," he said. "The pre-crack overshadows those defects to become the weakest spot – so I know exactly where the fracture will happen when we pull it.

"The material resistance to the crack growth – the fracture toughness – is what we're measuring here, and that's a very important engineering property," he said.

Just setting up the experiment required several years of work to overcome technical difficulties, Lou said. To suspend it on a tiny cantilever spring stage similar to an atomic force microscopy (AFM) probe, a graphene sheet had to be clean and dry so it would adhere (via van der Waals force) to the stage without compromising the stage movement necessary for the testing. Once mounted, the researchers used a focused ion beam to cut a pre-crack less than 10 percent of the width into the microns-wide section of suspended graphene. Then they pulled the graphene in half, measuring the force required.

While the Rice team was working on the experiment, Zhu and his team performed computer simulations to understand the entire fracture process.

“We can directly simulate the whole deformation process by tracking the motion and displacement with atomic-scale resolution in fairly large samples so our results can be directly correlated with the experiment,” said Zhu. “The modeling is tightly coupled with the experiments.”

The combination of modeling and experiment provides a level of detail that allowed the researchers to better understand the fracture process – and the tradeoff between toughness and strength in the graphene. What the scientists have learned in the research points out the importance of fabricating high quality graphene sheets without defects – which could set the stage for fracture.

“Understanding the tradeoff between strength and toughness provides important insights for the future utilization of graphene in structural and functional applications,” Zhu added. “This research provides a foundational framework for further study of the mechanical properties of graphene.”

Lou said the techniques they used should work for any two-dimensional material. "It's important to understand how defects will affect the handling, processing and manufacture of these materials," he said. "Our work should open up new directions for testing the mechanical properties of 2-D materials."

Co-authors of the paper are graduate students Peng Zhang, Lulu Ma, Phillip Loya and Yongji Gong, and former graduate students Cheng Peng and Jiangnan Zhang, all at Rice; Feifei Fan and Zhi Zeng, graduate students at Georgia Tech; Zheng Liu, an assistant professor at Nanyang Technological University, Singapore, with a complimentary appointment at Rice; Pulickel Ajayan, Rice's Benjamin M. and Mary Greenwood Anderson Professor in Materials Science and Nanoengineering and of Chemistry; and Xingxiang Zhang, a professor at Tianjin Polytechnic University, China.

Lou is an associate professor of Materials Science and Nanoengineering and of Chemistry at Rice. The Welch Foundation, the National Science Foundation, the U.S. Office of Naval Research and the Korean Institute of Machinery and Materials supported the research.

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Research reported this week shows that electrical resistance in nanoribbons of epitaxial graphene changes in discrete steps following quantum mechanical principles. The research shows that the graphene nanoribbons act more like optical waveguides or quantum dots, allowing electrons to flow smoothly along the edges of the material. In ordinary conductors such as copper, resistance increases in proportion to the length as electrons encounter more and more impurities while moving through the conductor.

The ballistic transport properties, similar to those observed in cylindrical carbon nanotubes, exceed theoretical conductance predictions for graphene by a factor of 10. The properties were measured in graphene nanoribbons approximately 40 nanometers wide that had been grown on the edges of three-dimensional structures etched into silicon carbide wafers.

“This work shows that we can control graphene electrons in very different ways because the properties are really exceptional,” said Walt de Heer, a Regent’s professor in the School of Physics at the Georgia Institute of Technology. “This could result in a new class of coherent electronic devices based on room temperature ballistic transport in graphene. Such devices would be very different from what we make today in silicon.”

The research, which was supported by the National Science Foundation, the Air Force Office of Scientific Research and the W.M. Keck Foundation, was reported February 5 in the journal Nature. The research was done through a collaboration of scientists from Georgia Tech in the United States, Leibniz Universität Hannover in Germany, the Centre National de la Recherche Scientifique (CNRS) in France and Oak Ridge National Laboratory – supported by the Department of Energy – in the United States.

For nearly a decade, researchers have been trying to use the unique properties of graphene to create electronic devices that operate much like existing silicon semiconductor chips. But those efforts have met with limited success because graphene – a lattice of carbon atoms that can be made as little as one layer thick – cannot be easily given the electronic bandgap that such devices need to operate.

De Heer argues that researchers should stop trying to use graphene like silicon, and instead use its unique electron transport properties to design new types of electronic devices that could allow ultra-fast computing – based on a new approach to switching. Electrons in the graphene nanoribbons can move tens or hundreds of microns without scattering.

“This constant resistance is related to one of the fundamental constants of physics, the conductance quantum,” de Heer said. “The resistance of this channel does not depend on temperature, and it does not depend on the amount of current you are putting through it.”

What does disrupt the flow of electrons, however, is measuring the resistance with an electrical probe. The measurements showed that touching the nanoribbons with a single probe doubles the resistance; touching it with two probes triples the resistance.

“The electrons hit the probe and scatter,” explained de Heer. “It’s a lot like a stream in which water is flowing nicely until you put rocks in the way. We have done systematic studies to show that when you touch the nanoribbons with a probe, you introduce a method for the electrons to scatter, and that changes the resistance.”

The nanoribbons are grown epitaxially on silicon carbon wafers into which patterns have been etched using standard microelectronics fabrication techniques. When the wafers are heated to approximately 1,000 degrees Celsius, silicon is preferentially driven off along the edges, forming graphene nanoribbons whose structure is determined by the pattern of the three-dimensional surface. Once grown, the nanoribbons require no further processing.

The advantage of fabricating graphene nanoribbons this way is that it produces edges that are perfectly smooth, annealed by the fabrication process. The smooth edges allow electrons to flow through the nanoribbons without disruption. If traditional etching techniques are used to cut nanoribbons from graphene sheets, the resulting edges are too rough to allow ballistic transport.

“It seems that the current is primarily flowing on the edges,” de Heer said. “There are other electrons in the bulk portion of the nanoribbons, but they do not interact with the electrons flowing at the edges.”

The electrons on the edge flow more like photons in optical fiber, helping them avoid scattering. “These electrons are really behaving more like light,” he said. “It is like light going through an optical fiber. Because of the way the fiber is made, the light transmits without scattering.”

The researchers measured ballistic conductance in the graphene nanoribbons for up to 16 microns. Electron mobility measurements surpassing one million correspond to a sheet resistance of one ohm per square that is two orders of magnitude lower than what is observed in two-dimensional graphene – and ten times smaller than the best theoretical predictions for graphene.

“This should enable a new way of doing electronics,” de Heer said. “We are already able to steer these electrons and we can switch them using rudimentary means. We can put a roadblock, and then open it up again. New kinds of switches for this material are now on the horizon.”

Theoretical explanations for what the researchers have measured are incomplete. De Heer speculates that the graphene nanoribbons may be producing a new type of electronic transport similar to what is observed in superconductors.  

“There is a lot of fundamental physics that needs to be done to understand what we are seeing,” he added. “We believe this shows that there is a real possibility for a new type of graphene-based electronics.”

Georgia Tech researchers have pioneered graphene-based electronics since 2001, for which they hold a patent, filed in 2003. The technique involves etching patterns into electronics-grade silicon carbide wafers, then heating the wafers to drive off silicon, leaving patterns of graphene.

In addition to de Heer, the paper’s authors included Jens Baringhaus, Frederik Edler and Christoph Tegenkamp from the Institut für Festkörperphysik, Leibniz Universität, Hannover in Germany; Edward Conrad, Ming Ruan and Zhigang Jiang from the School of Physics at Georgia Tech; Claire Berger from Georgia Tech and Institut Néel at the Centre National de la Recherche Scientifique (CNRS) in France; Antonio Tejeda and Muriel Sicot from the Institut Jean Lamour, Universite de Nancy, Centre National de la Recherche Scientifique (CNRS) in France; An-Ping Li from the Center for Nanophase Materials Sciences at Oak Ridge National Laboratory, and Amina Taleb-Ibrahimi from the CNRS Synchotron SOLEIL in France.

This research was supported by the National Science Foundation (NSF) Materials Research Science and Engineering Center (MRSEC) at Georgia Tech through award DMR-0820382; the Air Force Office of Scientific Research (AFOSR); the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy, and the Partner University Fund from the Embassy of France. Any conclusions or recommendations are those of the authors and do not necessarily represent the official views of the NSF, DOE or AFOSR.

CITATION: Jens Baringhaus, et al., “Exceptional ballistic transport in epitaxial graphene nanoribbons,” (Nature 2013). (http://dx.doi.org/10.1038/nature12952).

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With antennas made from conventional materials like copper, communication between low-power nanomachines would be virtually impossible. But by taking advantage of the unique electronic properties of the material known as graphene, researchers now believe they’re on track to connect devices powered by small amounts of scavenged energy.

Based on a honeycomb network of carbon atoms, graphene could generate a type of electronic surface wave that would allow antennas just one micron long and 10 to 100 nanometers wide to do the work of much larger antennas. While operating graphene nano-antennas have yet to be demonstrated, the researchers say their modeling and simulations show that nano-networks using the new approach are feasible with the alternative material.

“We are exploiting the peculiar propagation of electrons in graphene to make a very small antenna that can radiate at much lower frequencies than classical metallic antennas of the same size,” said Ian Akyildiz, a Ken Byers Chair professor in Telecommunications in the School of Electrical and Computer Engineering at the Georgia Institute of Technology. “We believe that this is just the beginning of a new networking and communications paradigm based on the use of graphene.”

Sponsored by the National Science Foundation, the research is scheduled to be reported in the journal IEEE Journal of Selected Areas in Communications (IEEE JSAC). In addition to the nanoscale antennas, the researchers are also working on graphene-based nanoscale transceivers and the transmission protocols that would be necessary for communication between nanomachines.

The communications challenge is that at the micron scale, metallic antennas would have to operate at frequencies of hundreds of terahertz. While those frequencies might offer advantages in communication speed, their range would be limited by propagation losses to just a few micrometers. And they’d require lots of power – more power than nanomachines are likely to have.

Akyildiz has studied nanonetworks since the late 1990s, and had concluded that traditional electromagnetic communication between these machines might not be possible. But then he and his Ph.D. student, Josep Jornet – who graduated in August 2013 and is now an assistant professor at the State University of New York at Buffalo – began reading about the amazing properties of graphene. They were especially interested in how electrons behave in single-layer sheets of the material.

“When electrons in graphene are excited by an incoming electromagnetic wave, for instance, they start moving back and forth,” explained Akyildiz. “Because of the unique properties of the graphene, this global oscillation of electrical charge results in a confined electromagnetic wave on top of the graphene layer.”

Known technically as a surface plasmon polariton (SPP) wave, the effect will allow the nano-antennas to operate at the low end of the terahertz frequency range, between 0.1 and 10 terahertz – instead of at 150 terahertz required by traditional copper antennas at nanoscale sizes. For transmitting, the SPP waves can be created by injecting electrons into the dielectric layer beneath the graphene sheet.

Materials such as gold, silver and other noble metals also can support the propagation of SPP waves, but only at much higher frequencies than graphene. Conventional materials such as copper don’t support the waves.

By allowing electromagnetic propagation at lower terahertz frequencies, the SPP waves require less power – putting them within range of what might be feasible for nanomachines operated by energy harvesting technology pioneered by Zhong Lin Wang, a professor in Georgia Tech’s School of Materials Science and Engineering.

“With this antenna, we can cut the frequency by two orders of magnitude and cut the power needs by four orders of magnitude,” said Jornet. “Using this antenna, we believe the energy-harvesting techniques developed by Dr. Wang would give us enough power to create a communications link between nanomachines.”

The nanomachines in the network that Akyildiz and Jornet envision would include several integrated components. In addition to the energy-harvesting nanogenerators, there would be nanoscale sensing, processing and memory, technologies that are under development by other groups. The nanoscale antenna and transceiver work being done at Georgia Tech would allow the devices to communicate the information they sense and process to the outside world.

“Each one of these components would have a nanoscale measurement, but in total we would have a machine measuring a few micrometers,” said Jornet. “There would be lots of tradeoffs in energy use and size.”

Beyond giving nanomachines the ability to communicate, hundreds or thousands of graphene antenna-transceiver sets might be combined to help full-size cellular phones and Internet-connected laptops communicate faster.

“The terahertz band can boost current data rates in wireless networks by more than two orders of magnitude,” Akyildiz noted. “The data rates in current cellular systems are up to one gigabit-per-second in LTE advanced networks or 10 gigabits-per-second in the so-called millimeter wave or 60 gigahertz systems. We expect data rates on the order of terabits-per-second in the terahertz band.”

The unique properties of graphene, Akyildiz says, are critical to this antenna – and other future electronic devices.  

“Graphene is a very powerful nanomaterial that will dominate our lives in the next half-century,” he said. “The European community will be supporting a very large consortium involving many universities and companies with an investment of one billion euros to conduct research into this material.”

The researchers have so far evaluated numerous nano-antenna designs using modeling and simulation techniques in their laboratory. The next step will be to actually fabricate a graphene nano-antenna and operate it using a transceiver also based on graphene.

“Our project shows that the concept of graphene-based nano-antennas is feasible, especially when taking into account very accurate models of electron transport in graphene,” said Akyildiz. “Many challenges remain open, but this is a first step toward creating advanced nanomachines with many applications in the biomedical, environmental, industrial and military fields.”

The research described here was supported by the National Science Foundation under award number CCF-1349828. Any opinions or conclusions are those of the authors and do not necessarily reflect the official views of the NSF.

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Patterning of graphene is key for device fabrication. We report a way to increase or reduce the number of layers in epitaxial graphene grown on the C-face (000-1) of silicon carbide by the deposition of a 120 nm to 150nm-thick silicon nitride mask prior to graphitization. In our process we find that areas covered by a Si-rich SiN mask have three more layers than non-masked areas. Conversely N-rich SiN decreases the thickness by three layers. In both cases the mask decomposes before graphitization is completed. Graphene grown in masked areas show good quality as observed by Raman, AFM and transport data. By tailoring the growth parameters selective graphene growth has been obtained.

A low temperature, controllable and stable method has been developed to dope graphene films using self-assembled monolayers (SAM) that modify the interface of graphene and its support substrate. Using this concept, a team of researchers at the Georgia Institute of Technology has created graphene p-n junctions – which are essential to fabricating devices – without damaging the material’s lattice structure or significantly reducing electron/hole mobility.

The graphene was grown on a copper film using chemical vapor deposition (CVD), a process that allows synthesis of large-scale films and their transfer to desired substrates for device applications. The graphene films were transferred to silicon dioxide substrates that were functionalized with the self-assembled monolayers.

Information about creating graphene p-n junctions using self-assembled monolayers was presented on November 28, 2012 at the Fall Meeting of the Materials Research Society. Papers describing aspects of the work were also published in September 2012 in the journals ACS Applied Materials & Interfaces and the Journal of Physical Chemistry C. Funding for the research came from the National Science Foundation, through the Georgia Tech Materials Research Science and Engineering Center (MRSEC) and through separate research grants.

“We have been successful at showing that you can make fairly well doped p-type and n-type graphene controllably by patterning the underlying monolayer instead of modifying the graphene directly,” said Clifford Henderson, a professor in the Georgia Tech School of Chemical & Biomolecular Engineering. “Putting graphene on top of self-assembled monolayers uses the effect of electron donation or electron withdrawal from underneath the graphene to modify the material’s electronic properties.”

The Georgia Tech research team working on the project includes faculty members, postdoctoral fellows and graduate students from three different schools. In addition to Henderson, professors who are part of the team include Laren Tolbert from the School of Chemistry and Biochemistry and Samuel Graham from the Woodruff School of Mechanical Engineering.  The project team also includes Hossein Sojoudi, a postdoctoral fellow, and Jose Baltazar, a graduate research assistant.

Creating n-type and p-type doping in graphene – which has no natural bandgap – has led to development of several approaches. Scientists have substituted nitrogen atoms for some of the carbon atoms in the graphene lattice, compounds have been applied to the surface of the graphene, and the edges of graphene nanoribbons have been modified. However, most of these techniques have disadvantages, including disruption of the lattice – which reduces electron mobility – and long-term stability issues.

“Any time you put graphene into contact with a substrate of any kind, the material has an inherent tendency to change its electrical properties,” Henderson said. “We wondered if we could do that in a controlled way and use it to our advantage to make the material predominately n-type or p-type. This could create a doping effect without introducing defects that would disrupt the material’s attractive electron mobility.”

Using conventional lithography techniques, the researchers created patterns from different silane materials on a dielectric substrate, usually silicon oxide. The materials were chosen because they are either strong electron donors or electron acceptors. When a thin film of graphene is placed over the patterns, the underlying materials create charged sections in the graphene that correspond to the patterning.

“We were able to dope the graphene into both n-type and p-type materials through an electron donation or withdrawal effect from the monolayer,” Henderson explained. “That doesn’t lead to the substitutional defects that are seen with many of the other doping processes. The graphene structure itself is still pristine as it comes to us in the transfer process.”

The monolayers are bonded to the dielectric substrate and are thermally stable up to 200 degrees Celsius with the graphene film over them, Sojoudi noted. The Georgia Tech team has used 3-Aminopropyltriethoxysilane (APTES) and perfluorooctyltriethoxysilane (PFES) for patterning. In principle, however, there are many other commercially-available materials that could also create the patterns.

“You can build as many n-type and p-type regions as you want,” Sojoudi said. “You can even step the doping controllably up and down. This technique gives you control over the doping level and what the dominant carrier is in each region.”

The researchers used their technique to fabricate graphene p-n junctions, which was verified by the creation of field-effect transistors (FET). Characteristic I-V curves indicated the presence of two separate Dirac points, which indicated an energy separation of neutrality points between the p and n regions in the graphene, Sojoudi said.

The group uses chemical vapor deposition to create thin films of graphene on copper foil. A thick film of PMMA was spin-coated atop the graphene, and the underlying copper was then removed. The polymer serves as a carrier for the graphene until it can be placed onto the monolayer-coated substrate, after which it is removed.

Beyond developing the doping techniques, the team is also exploring new precursor materials that could allow CVD production of graphene at temperatures low enough to permit fabrication directly on other devices. That could eliminate the need for transferring the graphene from one substrate to another.

A low-cost, low-temperature means of producing graphene could also allow the films to find broader applications in displays, solar cells and organic light-emitting diodes, where large sheets of graphene would be needed.

“The real goal is to find ways to make graphene at lower temperatures and in ways that allow us to integrate it with other devices, either silicon CMOS or other materials that couldn’t tolerate the high temperatures required for the initial growth,” Henderson said. “We are looking at ways to make graphene into a useful electronic or opto-electronic material at low temperatures and in patterned forms.”

This material is based on work supported by the National Science Foundation (NSF) under Grants CHE-0822697, CHE-0848833 and CMMI-0927736 and the Georgia Tech Materials Research Science and Engineering Center (MRSEC). The content of this article is solely the responsibility of the authors and does not necessarily represent the official views of the NSF.

CITATIONS: Sojoudi, Hossein, Creating Graphene p-n Junctions Using Self-Assembled Monolayers, ACS Applied Materials & Interfaces, dx.doi.org/10.1021/am301138v and Baltazar, Jose, Facile Formation of Graphene P-N Junctions Using Self-Assembled Monolayers, The Journal of Physical Chemistry C, dx.doi.org/10.1021/jp3045737.

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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 study also found that after the material is produced, its structural and chemical properties continue to evolve for more than a month as a result of continuing chemical reactions with hydrogen.

Understanding the properties of graphene oxide – and how to control them – is important to realizing potential applications for the material. To make it useful for nano-electronics, for instance, researchers must induce both an electronic band gap and structural order in the material. Controlling the amount of hydrogen in graphene oxide may be the key to manipulating the material properties.

“Graphene oxide is a very interesting material because its mechanical, optical and electronic properties can be controlled using thermal or chemical treatments to alter its structure,” said Elisa Riedo, an associate professor in the School of Physics at the Georgia Institute of Technology. “But before we can get the properties we want, we need to understand the factors that control the material’s structure. This study provides information about the role of hydrogen in the reduction of graphene oxide at room temperature.”

The research, which studied graphene oxide produced from epitaxial graphene, was reported on May 6 in the journal Nature Materials. The research was sponsored by the National Science Foundation, the Materials Research Science and Engineering Center (MRSEC) at Georgia Tech, and by the U.S. Department of Energy.

Graphene oxide is formed through the use of chemical and thermal processes that mainly add two oxygen-containing functional groups to the lattice of carbon atoms that make up graphene: epoxide and hydroxyl species. The Georgia Tech researchers began their studies with multilayer expitaxial graphene grown atop a silicon carbide wafer, a technique pioneered by Walt de Heer and his research group at Georgia Tech. Their samples included an average of ten layers of graphene.

After oxidizing the thin films of graphene using the established Hummers method, the researchers examined their samples using X-ray photo-emission spectroscopy (XPS). Over about 35 days, they noticed the number of epoxide functional groups declining while the number of hydroxyl groups increased slightly. After about three months, the ratio of the two groups finally reached equilibrium.

“We found that the material changed by itself at room temperature without any external stimulation,” said Suenne Kim, a postdoctoral fellow in Riedo’s laboratory. “The degree to which it was unstable at room temperature was surprising.”

Curious about what might be causing the changes, Riedo and Kim took their measurements to Angelo Bongiorno, an assistant professor who studies computational materials chemistry in Georgia Tech’s School of Chemistry and Biochemistry. Bongiorno and graduate student Si Zhou studied the changes using density functional theory, which suggested that hydrogen could be combining with oxygen in the functional groups to form water. That would favor a reduction in the epoxide groups, which is what Riedo and Kim were seeing experimentally.

“Elisa’s group was doing experimental measurements, while we were doing theoretical calculations,” Bongiorno said. “We combined our information to come up with the idea that maybe there was hydrogen involved.”

The suspicions were confirmed experimentally, both by the Georgia Tech group and by a research team at the University of Texas at Dallas. This information about the role of hydrogen in determining the structure of graphene oxide suggests a new way to control its properties, Bongiorno noted.

“During synthesis of the material, we could potentially use this as a tool to change the structure,” he said. “By understanding how to use hydrogen, we could add it or take it out, allowing us to adjust the relative distribution and concentration of the epoxide and hydroxyl species which control the properties of the material.”

Riedo and Bongiorno acknowledge that their material – based on epitaxial graphene – may be different from the oxide produced from exfoliated graphene. Producing graphene oxide from flakes of the material involves additional processing, including dissolving in an aqueous solution and then filtering and depositing the material onto a substrate. But they believe hydrogen plays a similar role in determining the properties of exfoliated graphene oxide.

“We probably have a new new form of graphene oxide, one that may be more useful commercially, although the same processes should also be happening within the other form of graphene oxide,” said Bongiorno.

The next steps are to understand how to control the amount of hydrogen in epitaxial graphene oxide, and what conditions may be necessary to affect reactions with the two functional groups. Ultimately, that may provide a way to open an electronic band gap and simultaneously obtain a graphene-based material with electron transport characteristics comparable to those of pristine graphene.

“By controlling the properties of graphene oxide through this chemical and thermal reduction, we may arrive at a material that remains close enough to graphene in structure to maintain the order necessary for the excellent electronic properties, while having the band gap needed to create transistors,” Riedo said. “It could be that graphene oxide is the way to arrive at that type of material.”

Beyond those already mentioned, the paper’s authors included Yike Hu, Claire Berger and Walt de Heer from the School of Physics at Georgia Tech, and Muge Acik and Yves Chabal from the Department of Materials Science and Engineering at the University of Texas at Dallas.

This research was supported by the National Science Foundation under grants CMMI-1100290, DMR-0820382 and DMR-0706031, and by the U.S. Department of Energy’s Office of Basic Energy Sciences under grants DE-FG02-06ER46293 and DE-SC001951. The content is solely the responsibility of the principal investigators and does not necessarily represent the official views of the National Science Foundation or the Department of Energy.

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Chemical doping is routinely used in conventional three-dimensional semiconductors to control the density of electron carriers that are essential to the operation of devices such as transistors. But graphene, a semi-metal available in sheets just one atom thick, has properties very different from traditional materials such as silicon -- though researchers say doping will still be needed for producing electronic devices.

The bad news is that electronic designers working with graphene won't be able to simply apply what they've been doing with three-dimensional semiconductors -- which would translate to vastly degraded material quality for graphene. The good news, according to the study, is that graphene doping can be combined with other processes -- and need be applied only to the edges of nanoscale structures being fabricated.

"We are learning how to manipulate these two-dimensional sheets of carbon atoms to get some very unusual results that aren't available with any other material," said James Meindl, director of Georgia Tech's Nanotechnology Research Center, where the research was conducted. "Doping graphene to try to influence its properties is important to being able to use it effectively."

Details of the research were published online in the journal Carbon on October 29th. The research was supported by the Semiconductor Research Corporation (SRC), the Defense Advanced Research Projects Agency (DARPA) through the Interconnect Focus Center, and the National Science Foundation (NSF).

Because graphene sheets contain so few atoms by area, the substitution of elements such as oxygen or nitrogen for carbon atoms in the lattice -- as in conventional doping -- detracts from the high electron mobility and other properties that make the material interesting. So the researchers are rethinking the doping process to take advantage of graphene's unique properties.

"When we work with a three-dimensional semiconductor, we embed the dopant species in the bulk material and then fabricate it into a device," said Kevin Brenner, a graduate research assistant in the Georgia Tech School of Electrical and Computer Engineering. "With graphene, we will dope the material as we process it and fabricate it into devices or interconnects. Doping may be done as part of other fabrication steps such as plasma etching, and that will require us to reinvent the whole process."

Using sheets of exfoliated graphene, Brenner and collaborators Raghu Murali and Yinxiao Yang evaluated the effectiveness of two different techniques: edge passivation by coupling electron-beam lithography with a common resist material, and adsorption from coating the surface of the material. They found that the edge treatment, which chemically reacts with defects created when the material is cut, was a thousand times more efficient at producing carriers in the graphene sheets than the surface treatment.

"We will only be working with the edges of the material," Brenner explained. "That will allow us to leave the center pristine and free of defects. Using this approach, we can maintain very high mobilities and the special properties of graphene while creating very high carrier densities."

Because of the two-dimensional nature of the graphene, controlling the edge chemistry can provide control over the bulk properties of the sheet. "At nanoscale dimensions, the edge atoms tend to dominate over surface adsorption techniques," he added. "With a seven nanometer by seven nanometer graphene device, passivating just one edge C-atom provides the doping equivalent of covering the entire surface."

For doping the edge of a graphene structure, the team applied a thin film of hydrogen silsesquioxane (HSQ), a chemical normally used as a resist for etching, then used electron beam lithography to cross-link the material, which added oxygen atoms to the edges to create p-type doping. The resist and electron beam system combined to provide nanometer-scale control over where the chemical changes took place.

Doping treatment could also be applied using plasma etching, Brenner said. Controlling the specific atoms used in the plasma, or conducting the etching process in an environment containing specific atoms, could drive those atoms into the edges where they would serve as dopants.

"Anytime you create an edge, you have created a location where you can passivate using a dopant," he added. "Instead of needing to embed it in the surface, you can just take the edge that is already there and passivate it with oxygen, nitrogen, hydrogen or other dopant. It could be almost an effortless process because the doping can be done as part of another step."

Beyond fabricating electronic devices, Nanotechnology Research Center scientists are interested in using graphene for interconnects, potentially as a replacement for copper. As interconnect structures become smaller and smaller, the resistivity of copper increases. Edge-doped graphene sheets exhibit a trend of increasing doping with reduced dimensions, possibly becoming more conductive as their size shrinks below 50 nanometers, making them attractive for nanoscale interconnects.

Armed with basic information about graphene doping, the researchers hope to now begin producing devices to study how graphene actually performs.

"Now that we have made a start at understanding how to dope the material, the next step is to begin putting this into nanoscale devices," Brenner said. "We want to see what kind of performance we can get. That may tell us where graphene's niche could be as an electronic material."

Meindl, who has worked with silicon since the dawn of integrated circuits, says it's too early to predict where graphene will ultimately find commercial applications. But he says the material's properties are too interesting not to explore.

"The chances are that something very interesting and unique will develop from the use of graphene," he said. "But we don't yet have the ability to predict what we will be able to do with this new material."

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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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Nature article located at http://www.nature.com/news/2010/101118/full/news.2010.620.html

“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.

Graphene is anticipated to have potential applications in electronics to build semiconductors beyond the limits of silicon-based technology. It also offers promising applications for higher performance solar cells, LCD screens and photon sensors. Now that graphene has been identified and found to be stable in ultra-thin sheets, research efforts to understand it more thoroughly and to produce it in large quantities have ballooned. Yet, graphene is still at the development stage, and its commercialization pathway remains to be determined.

To kick-off their work on graphene innovation, Youtie and Shapira have been undertaking field work in two of the world’s leading centers for graphene development: the University of Manchester and Georgia Tech. As acknowledged by the Nobel Committee for Physics when it awarded its 2010 Prize to Manchester physicists Andre Geim and Konstantin Novoselov, Manchester is the site of seminal work on graphene, including the first laboratory production of graphene sheets. Georgia Tech is the site of a National Science Foundation-funded Materials Science and Engineering Center (MRSEC) focused on research and development on epitaxial graphene. Youtie’s and Shapira’s project seeks to understand similarities and differences in the plans, programs and approaches to commercialize graphene-related applications in both locations. This includes examination of both the strategies for research and development and those for fostering commercialization in terms of external partnerships in the metropolitan regions of Manchester and Atlanta, elsewhere in the country, and internationally. In addition to field work, Youtie and Shapira also are undertaking analyses of publications, patents, funding, and corporate activities in graphene.

Over the coming year, Youtie and Shapira plan to expand their research focus to other locations in the United States and around the world where graphene research and commercialization clusters are emerging. Although graphene’s full impacts may take many years to materialize, the results of Youtie’s and Shapira’s research will provide real-time insights to researchers, companies, policymakers and other stakeholders keen to understand how research in specific nanotechnology domains moves into early applications, what barriers and concerns are raised, and how these are being addressed.

Youtie’s and Shapira's pilot project has received travel funding from a UK-US Collaboration Development Award (CDA) of the British Embassy and British Consulates in the United States, with supplementary support through CNS-ASU and the Manchester Institute for Innovation Research.

This article is from Scientific Computing

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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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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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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“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.

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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Information on the ways in which attaching metal contacts affects electron transport in graphene will be important to scientists studying the material -- and to designers who may one day fabricate electronic devices from the carbon-lattice material.

"Graphene devices will have to communicate with the external world, and that means we will have to fabricate contacts to transport current and data," said Mei-Yin Chou, a professor and department chair in the School of Physics at the Georgia Institute of Technology. "When they put metal contacts onto graphene to measure transport properties, researchers and device designers need to know that they may not be measuring the instrinsic properties of pristine graphene. Coupling between the contacts and the material must be taken into account."

Information on the effects of metal contacts on graphene was reported in the journal Physical Review Letters on February 19th. The research was supported by the U.S. Department of Energy, and involved interactions with researchers at the National Science Foundation (NSF)-supported Materials Research Science and Engineering Center (MRSEC) at Georgia Tech.

Using large-scale, first-principles calculations done at two different NSF-supported supercomputer centers, the Georgia Tech research team -- which included postdoctoral fellows Salvador Barraza-Lopez and Mihajlo Vanevic, and assistant professor Markus Kindermann -- conducted detailed atomic-level calculations of aluminum contacts grown on graphene.

The calculations studied two contacts up to 14 nanometers apart, with graphene suspended between them. In their calculations, the researchers allowed the aluminum to grow as it would in the real world, then studied how electron transfer was induced in the area surrounding the contacts.

"People have been able to come up with phenomenological models that they use to find out what the effects are with metallic contacts," Chou explained. "Our calculations went a few steps farther because we built contacts atom-by-atom. We built atomistically-resolved contacts, and by doing that, we solved this problem at the atomic level and tried to do everything consistent with quantum mechanics."

Because metals typically have excess electrons, physically attaching the contacts to graphene causes a charge transfer from the metal. Charge begins to be transferred as soon as the contracts are constructed, but ultimately the two materials reach equilibrium, Chou said.

The study showed that charge transfer at the leads and into the freestanding section of the material creates an electron-hole asymmetry in the conductance. For leads that are sufficiently long, the effect creates two conductance minima at the energies of the Dirac points for the suspended and clamped regions of the graphene, according to Barraza-Lopez.

"These results could be important to the design of future graphene devices," he said. "Edge effects and the impact of nanoribbon width have been studied in significant detail, but the effects of charge transfer at the contacts may potentially be just as important."

The researchers modeled aluminum, but believe their results will apply to other metals such as copper and gold that do not form chemical bonds with graphene. However, other metals such as chromium and titanium do chemically alter the material, so the effects they have on electron transport may be different.

Beyond the new information provided by the calculations, the research further proposes quantitative models that can be used under certain circumstances to describe the impact of the contacts.

"Earlier models had been based on physical insights, but nobody really knew how faithfully they described the material," Kindermann said. "This is the first calculation to show that these earlier models apply under certain circumstances for the systems that we studied."

Data from the study may one day help device designers engineer graphene circuits by helping them understand the effects they are seeing.

"When we modify graphene, we need to understand what changes occur as a result of adding materials," added Chou. "This is really fundamental research to understand these effects and to have a numerical prediction for what is going on. We are helping to understand the basic physics of graphene."

This research was supported by Department of Energy grant DE-FG02-97ER45632. Comments and conclusions in this article are those of the researchers and do not necessarily reflect the views of the Department of Energy.

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By applying a commercially-available spin-on-glass (SOG) material to graphene and then exposing it to electron-beam radiation, researchers at the Georgia Institute of Technology created both types of doping by simply varying the exposure time. Higher levels of e-beam energy produced p-type areas, while lower levels produced n-type areas.

The technique was used to fabricate high-resolution p-n junctions. When properly passivated, the doping created by the SOG is expected to remain indefinitely in the graphene sheets studied by the researchers.

"This is an enabling step toward making possible complementary metal oxide graphene transistors," said Raghunath Murali, a senior research engineer in Georgia Tech's Nanotechnology Research Center.

A paper describing the technique appeared February 10, 2010 in the journal Applied Physics Letters. The research was supported by the Semiconductor Research Corporation and the Defense Advanced Research Projects Agency (DARPA) through the Interconnect Focus Center.

In the new doping process, Murali and graduate student Kevin Brenner begin by removing flakes of graphene one to four layers thick from a block of graphite. They place the material onto a surface of oxidized silicon, then fabricate a four-point contact device.

Next, they spin on films of hydrogen silsesquoxane (HSQ), then cure certain portions of the resulting thin film using electron beam radiation. The technique provides precise control over the amount of radiation and where it is applied to the graphene, with higher levels of energy corresponding to more cross-linking of the HSQ.

"We gave varying doses of electron-beam radiation and then studied how it influenced the properties of carriers in the graphene lattice," Murali said. "The e-beam gave us a fine range of control that could be valuable for fabricating nanoscale devices. We can use an electron beam with a diameter of four or five nanometers that allows very precise doping patterns."

Electronic measurements showed that a graphene p-n junction created by the new technique had large energy separations, indicating strong doping effects, he added.

Researchers elsewhere have demonstrated graphene doping using a variety of processes including soaking the material in various solutions and exposing it to a variety of gases. The Georgia Tech process is believed to be the first to provide both electron and hole doping from a single dopant material.

Doping processes used for graphene are likely to be significantly different from those established for silicon use, Murali said. In silicon, the doping step substitutes atoms of a different material for silicon atoms in the material’s lattice.

In the new single-step process for graphene, the doping is believed to introduce atoms of hydrogen and oxygen in the vicinity of the carbon lattice. The oxygen and hydrogen don't replace carbon atoms, but instead occupy locations atop the lattice structure.

"Energy applied to the SOG breaks chemical bonds and releases hydrogen and oxygen which bond with the carbon lattice," Murali said. "A high e-beam energy converts the whole SOG structure to more of a network, and then you have more oxygen than hydrogen, resulting in a p-type doping."

In volume manufacturing, the electron beam radiation would likely be replaced by a conventional lithography process, Murali said. Varying the reflectance or transmission of the mask set would control the amount of radiation reaching the SOG, and that would determine whether n-type or p-type areas are created.

"Making everything in a single step would avoid some of the expensive lithography steps," he said. "Gray-scale lithography would allow fine control of doping across the entire surface of the wafer."

For doping bulk areas such as interconnects that do not require patterning, the researchers simply coat the area with HSQ and expose it to a plasma source. The technique can make the nanoribbons up to 10 times more conductive than untreated graphene.

Because HSQ is already familiar to the microelectronics industry, the one-step approach to doping could help integrate graphene into existing processes, avoiding a disruption of the massive semiconductor design and fabrication system, Murali noted.

Over the past two years, researchers in the Nanotechnology Research Center had observed changes caused by application of HSQ during electrical testing. Only recently did they take a closer look at what was happening to understand how to take advantage of the phenomenon.

For the future, they'd like to better understand how the process works and whether other polymers might provide better results.

"We need to have a better understanding of how to control this process because variability is one of the issues that must be controlled to make manufacturing feasible," Murali explained. "We are trying to identify other polymers that may provide better control or stronger doping levels."

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The current-carrying and heat-transfer measurements were reported by a team of researchers from the Georgia Institute of Technology. The same team had previously reported measurements of resistivity in graphene that suggest the material's conductance would outperform that of copper in future generations of nanometer-scale interconnects.

"Graphene nanoribbons exhibit an impressive breakdown current density that is related to the resistivity," said Raghunath Murali, a senior research engineer in Georgia Tech's Nanotechnology Research Center. "Our measurements show that these graphene nanoribbons have a current carrying capacity at least two orders of magnitude higher than copper at these size scales."

Measurements of thermal conductivity and breakdown current density in narrow graphene nanoribbons were reported June 19 in the journal Applied Physics Letters. The research was supported by the Semiconductor Research Corporation/DARPA through the Interconnect Focus Center and by the Nanoelectronics Research Initiative through the Institute for Nanoelectronics Discovery and Exploration (INDEX).

The unique properties of graphene -- which is composed of thin layers of graphite -- make it attractive for a wide range of potential electronic devices. Murali and his colleagues have been studying graphene as a potential replacement for copper in on-chip interconnects, the tiny wires that are used to connect transistors and other devices on integrated circuits. Use of graphene for these interconnects, they believe, would help extend the long run of performance improvements in integrated circuit technology.

"Our measurements show that graphene nanoribbons have a current carrying capacity of more than 10^8 amps per square centimeter, while a handful of them exceed 10^9 amps per square centimeter," Murali said. "This makes them very robust in resisting electromigration and should greatly improve chip reliability."

Electromigration is a phenomenon that causes transport of material, especially at high current density. In on-chip interconnects, this eventually leads to a break in the wire, which results in chip failure.

"We are learning a lot of new things about this material, which will lead researchers to consider other potential applications," said Murali. "In addition to the high current carrying capacity, graphene nanoribbons also have excellent thermal conductivity."

Because heat generation is a significant cause of device failure, the researchers also measured the ability of the graphene nanostructures to conduct heat away from devices. They found that graphene nanoribbons have a thermal conductivity of more than 1,000 watts per meter Kelvin for structures less than 20 nanometers wide.

"This high thermal conductivity could allow graphene interconnects to also serve as heat spreaders in future generations of integrated circuits," said Murali.

To study the properties of graphene interconnects, Murali and collaborators Yinxiao Yang, Kevin Brenner, Thomas Beck and James Meindl began with flakes of multi-layered graphene removed from a graphite block and placed onto an oxidized silicon substrate. They used electron beam lithography to construct four electrode contacts, then used lithography to fabricate devices consisting of parallel nanoribbons of widths ranging between 16 and 52 nanometers and lengths of between 0.2 and 1 micron.

The breakdown current density of the nanoribbons was then studied by slowly applying an increasing amount of current to the electrodes on either side of the parallel nanoribbons. A drop in current flow indicated the breakdown of one or more of the nanoribbons.

In their study of 21 test devices, the researchers found that the breakdown current density of graphene nanoribbons has a reciprocal relationship to the resistivity.

Because graphene can be patterned using conventional chip-making processes, manufacturers could make the transition from copper to graphene without a drastic change in chip fabrication.

"Graphene has very good electrical properties," Murali said. "The data we have developed so far looks very promising for using this material as the basis for future on-chip interconnects."

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In a paper published in the June 2009 issue of the IEEE journal Electron Device Letters, researchers at the Georgia Institute of Technology report detailed analysis of resistivity in graphene nanoribbon interconnects as narrow as 18 nanometers.

The results suggest that graphene could out-perform copper for use as on-chip interconnects -- tiny wires that are used to connect transistors and other devices on integrated circuits. Use of graphene for these interconnects could help extend the long run of performance improvements for silicon-based integrated circuit technology.

"As you make copper interconnects narrower and narrower, the resistivity increases as the true nanoscale properties of the material become apparent," said Raghunath Murali, a research engineer in Georgia Tech's Microelectronics Research Center and the School of Electrical and Computer Engineering. "Our experimental demonstration of graphene nanowire interconnects on the scale of 20 nanometers shows that their performance is comparable to even the most optimistic projections for copper interconnects at that scale. Under real-world conditions, our graphene interconnects probably already out-perform copper at this size scale."

Beyond resistivity improvement, graphene interconnects would offer higher electron mobility, better thermal conductivity, higher mechanical strength and reduced capacitance coupling between adjacent wires.

"Resistivity is normally independent of the dimension -- a property inherent to the material," Murali noted. "But as you get into the nanometer-scale domain, the grain sizes of the copper become important and conductance is affected by scattering at the grain boundaries and at the side walls. These add up to increased resistivity, which nearly doubles as the interconnect sizes shrink to 30 nanometers."

The research was supported by the Interconnect Focus Center, which is one of the Semiconductor Research Corporation/DARPA Focus Centers, and the Nanoelectronics Research Initiative through the INDEX Center.

Murali and collaborators Kevin Brenner, Yinxiao Yang, Thomas Beck and James Meindl studied the electrical properties of graphene layers that had been taken from a block of pure graphite. They believe the attractive properties will ultimately also be measured in graphene fabricated using other techniques, such as growth on silicon carbide, which now produces graphene of lower quality but has the potential for achieving higher quality.

Because graphene can be patterned using conventional microelectronics processes, the transition from copper could be made without integrating a new manufacturing technique into circuit fabrication.

"We are optimistic about being able to use graphene in manufactured systems because researchers can already grow layers of it in the lab," Murali noted. "There will be challenges in integrating graphene with silicon, but those will be overcome. Except for using a different material, everything we would need to produce graphene interconnects is already well known and established."

Experimentally, the researchers began with flakes of multi-layered graphene removed from a graphite block and placed onto an oxidized silicon substrate. They used electron beam lithography to construct four electrode contacts on the graphene, then used lithography to fabricate devices consisting of parallel nanoribbons of widths ranging between 18 and 52 nanometers. The three-dimensional resistivity of the nanoribbons on 18 different devices was then measured using standard analytical techniques at room temperature.

The best of the graphene nanoribbons showed conductivity equal to that predicted for copper interconnects of the same size. Because the comparisons were between non-optimized graphene and optimistic estimates for copper, they suggest that performance of the new material will ultimately surpass that of the traditional interconnect material, Murali said.

"Even graphene samples of moderate quality show excellent properties," he explained. "We are not using very high levels of optimization or especially clean processes. With our straightforward processing, we are getting graphene interconnects that are essentially comparable to copper. If we do this more optimally, the performance should surpass copper."

Though one of graphene's key properties is reported to be ballistic transport -- meaning electrons can flow through it without resistance -- the material's actual conductance is limited by factors that include scattering from impurities, line-edge roughness and from substrate phonons -- vibrations in the substrate lattice.

Use of graphene interconnects could help facilitate continuing increases in integrated circuit performance once features sizes drop to approximately 20 nanometers, which could happen in the next five years, Murali said. At that scale, the increased resistance of copper interconnects could offset performance increases, meaning that without other improvements, higher density wouldn't produce faster integrated circuits.

"This is not a roadblock to achieving scaling from one generation to the next, but it is a roadblock to achieving increased performance," he said. "Dimensional scaling could continue, but because we would be giving up so much in terms of resistivity, we wouldn't get a performance advantage from that. That's the problem we hope to solve by switching to a different materials system for interconnects."

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Published in the May 15, 2009 issue of the journal Science, the work adds new detail to help explain the unusual physical phenomena and properties associated with graphene, a single layer of carbon atoms arrayed in a repeating, honeycomb-like arrangement.

"Our experiments directly measured the energy spectrum of graphene with unprecedented precision and show that the unique multilayer epitaxial graphene grown in the Georgia Tech laboratory of Walt de Heer behaves remarkably like independent graphene sheets," said Philip N. First, an associate professor in the Georgia Tech School of Physics and one of the paper's co-authors. "This effective single-layer behavior is due to small rotations between the graphene sheets that dramatically reduce the interlayer atomic interactions. Because the measurements showed only very small surface potential fluctuations and long times between scattering events, it could be that this multilayer material is one of the best places to study many properties of 'single-layer' graphene."

The research was funded by the National Science Foundation, the Semiconductor Research Corporation through the Nanoelectronics Research Initiative INDEX Program, and by the W.M. Keck Foundation.

Graphene's exotic behaviors present intriguing prospects for future technologies, including high-speed, graphene-based electronics that might replace today's silicon-based integrated circuits and other devices. Even at room temperature, electrons in graphene are more than 100 times more mobile than in silicon.

Graphene apparently owes this enhanced mobility to the curious fact that its electrons and other carriers of electric charges behave as though they do not have mass. In conventional materials, the speed of electrons is related to their energy, but not in graphene. Although they do not approach the speed of light, the research team found that unbound electrons in graphene behave much like photons, massless particles that also move at a speed independent of their energy.

This weird massless behavior is associated with other strangeness, the researchers found. When ordinary conductors are put in a strong magnetic field, charge carriers such as electrons begin moving in circular orbits that are constrained to discrete, equally spaced energy levels. In graphene these levels are known to be unevenly spaced because of the "massless" electrons.

The Georgia Tech/NIST team tracked these massless electrons in action, using a specialized NIST instrument to zoom in on the graphene layer at a billion times magnification, tracking the electronic states while at the same time applying high magnetic fields. The custom-built, ultra-low-temperature and ultra-high-vacuum scanning tunneling microscope allowed them to sweep an adjustable magnetic field across graphene samples prepared at Georgia Tech, observing and mapping the peculiar non-uniform spacing among discrete energy levels that form when the material is exposed to magnetic fields.

The team developed a high-resolution map of the distribution of energy levels in graphene. In contrast to metals and other conducting materials, where the distance from one energy peak to the next is uniformly equal, this spacing is uneven in graphene.

The researchers also probed and spatially mapped graphene's hallmark "zero energy state," a curious phenomenon where the material has no electrical carriers until a magnetic field is applied.

The measurements also indicated that layers of graphene grown and then heated on a substrate of silicon carbide behave as individual, isolated, two-dimensional sheets. On the basis of the results, the researchers suggest that graphene layers are uncoupled from adjacent layers because they stack in different rotational orientations. This finding may point the way to manufacturing methods for making large, uniform batches of graphene for a new carbon-based electronics.

The research team included David L. Miller, Kevin D. Kubista, Ming Ruan, Walt A. de Heer and Philip N. First of Georgia Tech's School of Physics, and Gregory M. Rutter and Joseph A. Stroscio of the Center for Nanoscale Science and Technology at NIST.

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The Laboratory will focus its efforts on the development of new materials to serve as the successors to silicon in the semiconductor industry. Specifically, the development of graphene - which holds tremendous promise as an electronic material - will be the initial core of research and development at the Center.

NSF funding will be $8.1 million for six years of research and development. The MRSEC office suite will be housed in the Georgia Tech's new Marcus Nanotechnology Research Center Building.

"This is an exciting time for graphene research," said Dennis Hess, director of the Georgia Tech MRSEC. "Our studies may allow the manufacture of microelectronic devices and integrated circuits based on graphene. The Georgia Tech team, in conjunction with external partners, has already pioneered the use of epitaxial graphene to achieve such goals. Georgia Tech Physics Professors Walt de Heer, Phil First and Ed Conrad are worldwide leaders in the growth and characterization of epitaxial graphene. We look forward to additional innovative discoveries from our Center over the next few years."

The Laboratory will be a cross-disciplinary effort utilizing the talent and resources of Georgia Tech and four additional institutions: University of California Berkeley, University of California Riverside, Alabama A & M and the University of Michigan. Georgia Tech will initially have 13 faculty members involved in the Laboratory's efforts, with five additional members representing the partner schools. Collaborations are already in place with several companies and national laboratories within the U.S. and abroad.

Graphene, a sheet of carbon only one-atom thick, holds the potential to become the core material for computer processors in electronics, which continue to become smaller in size. Silicon, comparatively, has fundamental limitations that inhibit operation in ever-shrinking devices used in microelectronics, optics and sensors.

Georgia Tech will develop the fundamental science and technology to maximize graphene's potential as a component in future electronics technologies. In addition, the Center will provide the core curriculum, train a diverse workforce and develop the future academic and industrial leaders needed for this new direction in the semiconductor industry.

An industrial advisory board is being assembled for the Center, which will include representatives from leading electronics companies.

"This new MRSEC complements Georgia Tech's multiple programs and investments in nanotechnology extremely well," said Professor Mark Allen, senior vice provost for Research and Innovation. "Much of the work will take place in our Nanotechnology Research Center, a new facility dedicated to research into both inorganic and organic nanoscience and nanotechnology. We look forward to enabling the next generation of graphene electronics through the efforts of the researchers in this new MRSEC."