Keeping electronics cool: Findings on modified form of graphene could have impacts in managing heat dissipation

ScienceDaily (Jan. 9, 2012) — A University of California, Riverside engineering professor and a team of researchers have made a breakthrough discovery with graphene, a material that could play a major role in keeping laptops and other electronic devices from overheating.

Alexander Balandin, a professor of electrical engineering at the UC Riverside Bourns College of Engineering, and researchers from The University of Texas at Austin, The University of Texas at Dallas and Xiamen University in China, have shown that the thermal properties of isotopically engineered graphene are far superior to those of graphene in its natural state.

The research efforts were led by the Professor Rodney S. Ruoff of UT Austin and Balandin, a corresponding author for the paper, "Thermal conductivity of isotopically modified graphene." It was published online Jan. 8 by the journal Nature Materials and will later appear in the print publication.

The results bring graphene -- a single-atom thick carbon crystal with unique properties, including superior electrical and heat conductivity, mechanical strength and unique optical absorption -- one step closer to being used as a thermal conductor for managing heat dissipation in everything from electronics to photovoltaic solar cells to radars.

"The important finding is the possibility of a strong enhancement of thermal conduction properties of isotopically pure graphene without substantial alteration of electrical, optical and other physical properties," Balandin said. "Isotopically pure graphene can become an excellent choice for many practical applications provided that the cost of the material is kept under control."

He added: "The experimental data on heat conduction in isotopically engineered graphene is also crucially important for developing an accurate theory of thermal conductivity in graphene and other two-dimensional crystals."

The research used the optothermal Raman method, a thermal conductivity measuring technique developed by Balandin. In 2008, Balandin and his group members demonstrated experimentally that graphene is an excellent heat conductor. They also developed the first detailed theory of heat conduction in graphene and related two-dimensional crystals.

The work presented in the Nature Materials paper shows that the thermal conductivity of isotopically engineered graphene is strongly enhanced compared to graphene in its natural state.

Naturally occurring carbon materials, including graphene, are made up of two stable isotopes: about 99 percent of 12C (referred to as "carbon 12") and 1 percent of 13C (referred to as "carbon 13"). The difference between isotopes is in the atomic mass of the carbon atoms. The removal of just about 1 percent of carbon 13, also called isotopic purification, modifies the dynamic properties of crystal lattices and affects their thermal conductivity.

The importance of the present research is explained by practical needs for materials with high thermal conductivity. Heat removal has become a crucial issue for continuing progress in the electronics industry, owing to increased levels of dissipated power as the devices become smaller and smaller. The search for materials that conduct heat well has become essential for the design of the next generation of integrated circuits and three-dimensional electronics.

Balandin, who is also founding chair of the materials science and engineering (MS&E) program at UC Riverside, believes graphene will gradually be incorporated into different devices.

Initially, it will likely be used in some niche applications such as thermal interface materials for chip packaging or transparent electrodes in photovoltaic solar cells or flexible displays, he said.

In a few years, it could be used with silicon in computer chips, for example as interconnect wiring or heat spreaders. It also has the potential to benefit other electronic applications, including analog high-frequency transistors, which are used in wireless communications, radar, security systems and imaging.

Balandin and the following researchers contributed to the findings in the Nature Materials paper:

The team at UT Austin, which performed the isotopic purification of graphene, included Ruoff, Shanshan Chen, a post-doctoral fellow, Weiwei Cai a former post-doctoral researcher who is now a professor at the Xiamen University and Columbia Mishra, a graduate student.

The team at UT Dallas, who performed molecular dynamics simulations that compared well with the stronger thermal connectivity of the isotopically engineered graphene, included Kyeongjae Cho, a professor, and Hengji Zhang, graduate student.

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Slippery when stacked: Theorists quantify the friction of graphene

ScienceDaily (Jan. 11, 2012) — Similar to the way pavement, softened by a hot sun, will slow down a car, graphene -- a one-atom-thick sheet of carbon with wondrous properties -- slows down an object sliding across its surface. But stack the sheets and graphene gets more slippery, say theorists at the National Institute of Standards and Technology (NIST), who developed new software to quantify the material's friction.

"I don't think anyone expects graphene to behave like a surface of a three-dimensional material, but our simulation for the first time explains the differences at an atomic scale," says NIST postdoctoral researcher Alex Smolyanitsky, who wrote the modeling program and co-authored a new paper about the study. "If people want to use graphene as a solid-state lubricant or even as a part of flexible electrodes, this is important work."

With the capacity to be folded, rolled or stacked, graphene is super-strong and has unusual electronic and optical properties. The material might be used in applications ranging from electronic circuits to solar cells to "greasing" moving parts in nanoscale devices.

Friction is the force that resists the sliding of two surfaces against each other. Studying friction at the atomic scale is a challenge, surmountable in only the past few years. The NIST software simulates atomic force microscopy (AFM) using a molecular dynamics technique. The program was used to measure what happens when a simulated AFM tip moves across a stack of one to four graphene sheets (see image) at different scanning rates.

The researchers found that graphene deflects under and around the AFM tip. The localized, temporary warping creates rolling friction or resistance, the force that exerts drag on a circular object rolling along a surface. Smolyanitsky compares the effect to the sun melting and softening pavement in the state where he got his doctoral degree, Arizona, causing car tires to sink in slightly and slow down. The NIST results are consistent with those of recent graphene experiments by other research groups but provide new quantitative data.

Most significantly, the NIST study shows why friction falls with each sheet of graphene added to the stack (fast scanning also has an effect on the friction). With fewer layers, the top layer deflects more, and the friction per unit of AFM contact force rises. The top surface of the stack becomes less yielding and more slippery as graphene layers are added. By contrast, the friction of three-dimensional graphite-like material is virtually unaffected by deformation and rolling friction, and is due instead to heat created by the moving tip.

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A. Smolyanitsky, J. Killgore, V. Tewary. Effect of elastic deformation on frictional properties of few-layer graphene. Physical Review B, 2012; 85 (3) DOI: 10.1103/PhysRevB.85.035412

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Graphene reveals its magnetic personality

ScienceDaily (Jan. 8, 2012) — Can organic matter behave like a fridge magnet? Scientists from The University of Manchester have now shown that it can.

In a report published in Nature Physics, they used graphene, the world's thinnest and strongest material, and made it magnetic.

Graphene is a sheet of carbon atoms arranged in a chicken wire structure. In its pristine state, it exhibits no signs of the conventional magnetism usually associated with such materials as iron or nickel.

Demonstrating its remarkable properties won Manchester researchers the Nobel Prize in Physics in 2010.

This latest research led by Dr Irina Grigorieva and Professor Sir Andre Geim (one of the Nobel prize recipients) could prove crucial to the future of graphene in electronics.

The Manchester researchers took nonmagnetic graphene and then either 'peppered' it with other nonmagnetic atoms like fluorine or removed some carbon atoms from the chicken wire. The empty spaces, called vacancies, and added atoms all turned out to be magnetic, exactly like atoms of, for example, iron.

"It is like minus multiplied by minus gives you plus," says Dr Irina Grigorieva.

The researchers found that, to behave as magnetic atoms, defects must be far away from each other and their concentration should be low. If many defects are added to graphene, they reside too close and cancel each other's magnetism. In the case of vacancies, their high concentration makes graphene disintegrate.

Professor Geim said: "The observed magnetism is tiny, and even the most magnetized graphene samples would not stick to your fridge.

"However, it is important to reach clarity in what is possible for graphene and what is not. The area of magnetism in nonmagnetic materials has previously had many false positives."

"The most likely use of the found phenomenon is in spintronics. Spintronics devices are pervasive, most notably they can be found in computers' hard disks. They function due to coupling of magnetism and electric current.

"Adding this new degree of functionality can prove important for potential applications of graphene in electronics," adds Dr Grigorieva.

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R. R. Nair, M. Sepioni, I-Ling Tsai, O. Lehtinen, J. Keinonen, A. V. Krasheninnikov, T. Thomson, A. K. Geim, I. V. Grigorieva. Spin-half paramagnetism in graphene induced by point defects. Nature Physics, 2012; DOI: 10.1038/nphys2183

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'Nanowiggles:' Scientists discover graphene nanomaterials with tunable functionality in electronics

ScienceDaily (Jan. 4, 2012) — Electronics are getting smaller and smaller, flirting with new devices at the atomic scale. However, many scientists predict that the shrinking of our technology is reaching an end. Without an alternative to silicon-based technologies, the miniaturization of our electronics will stop. One promising alternative is graphene -- the thinnest material known to man. Pure graphene is not a semiconductor, but it can be altered to display exceptional electrical behavior. Finding the best graphene-based nanomaterials could usher in a new era of nanoelectronics, optics, and spintronics (an emerging technology that uses the spin of electrons to store and process information in exceptionally small electronics).

Scientists at Rensselaer Polytechnic Institute have used the capabilities of one of the world's most powerful university-based supercomputers, the Rensselaer Center for Nanotechnology Innovations (CCNI), to uncover the properties of a promising form of graphene, known as graphene nanowiggles. What they found was that graphitic nanoribbons can be segmented into several different surface structures called nanowiggles. Each of these structures produces highly different magnetic and conductive properties. The findings provide a blueprint that scientists can use to literally pick and choose a graphene nanostructure that is tuned and customized for a different task or device. The work provides an important base of knowledge on these highly useful nanomaterials.

The findings were published in the journal Physical Review Letters in a paper titled "Emergence of Atypical Properties in Assembled Graphene Nanoribbons."

"Graphene nanomaterials have plenty of nice properties, but to date it has been very difficult to build defect-free graphene nanostructures. So these hard-to-reproduce nanostructures created a near insurmountable barrier between innovation and the market," said Vincent Meunier, the Gail and Jeffrey L. Kodosky '70 Constellation Professor of Physics, Information Technology, and Entrepreneurship at Rensselaer. "The advantage of graphene nanowiggles is that they can easily and quickly be produced very long and clean."

Nanowiggles were only recently discovered by a group led by scientists at EMPA, Switzerland. These particular nanoribbons are formed using a bottom-up approach, since they are chemically assembled atom by atom. This represents a very different approach to the standard graphene material design process that takes an existing material and attempts to cut it into a new structure. The process often creates a material that is not perfectly straight, but has small zigzags on its edges.

Meunier and his research team saw the potential of this new material. The nanowiggles could be easily manufactured and modified to display exceptional electrical conductive properties. Meunier and his team immediately set to work to dissect the nanowiggles to better understand possible future applications.

"What we found in our analysis of the nanowiggles' properties was even more surprising than previously thought," Meunier said.

The scientists used computational analysis to study several different nanowiggle structures. The structures are named based on the shape of their edges and include armchair, armchair/zigzag, zigzag, and zigzag/armchair. All of the nanoribbon-edge structures have a wiggly appearance like a caterpillar inching across a leaf. Meunier named the four structures nanowiggles and each wiggle produced exceptionally different properties.

They found that the different nanowiggles produced highly varied band gaps. A band gap determines the levels of electrical conductivity of a solid material. They also found that different nanowiggles exhibited up to five highly varied magnetic properties. With this knowledge, scientists will be able to tune the bandgap and magnetic properties of a nanostructure based on their application, according to Meunier.

Meunier would like the research to inform the design of new and better devices. "We have created a roadmap that can allow for nanomaterials to be easily built and customized for applications from photovoltaics to semiconductors and, importantly, spintronics," he said.

By using CCNI, Meunier was able to complete these sophisticated calculations in a few months.

"Without CCNI, these calculations would still be continuing a year later and we would not yet have made this exciting discovery. Clearly this research is an excellent example illustrating the key role of CCNI in predictive fundamental science," he said.

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Journal Reference:

Eduardo Costa Gir?o, Liangbo Liang, Eduardo Cruz-Silva, Antônio Filho, Vincent Meunier. Emergence of Atypical Properties in Assembled Graphene Nanoribbons. Physical Review Letters, 2011; 107 (13) DOI: 10.1103/PhysRevLett.107.135501

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Imperfections may improve graphene sensors

Although they found that graphene makes very good chemical sensors, researchers at Illinois have discovered an unexpected "twist" -- that the sensors are better when the graphene is "worse" -- more imperfections improved performance.

"This is quite the opposite of what you would want for transistors, for example," explained Eric Pop, an assistant professor of electrical and computer engineering and a member of the interdisciplinary research team. "Finding that the less perfect they were, the better they worked, was counter intuitive at first."

The research group, which includes researchers from both chemical engineering and electrical engineering, and from a startup company, Dioxide Materials, reported their results in the November 23, 2011 issue of Advanced Materials.

"The objective of this work was to understand what limits the sensitivity of simple, two-terminal graphene chemiresistors, and to study this in the context of inexpensive devices easily manufactured by chemical vapor deposition (CVD)," stated lead authors Amin Salehi-Khojin and David Estrada.

The researchers found that the response of graphene chemiresistors depends on the types and geometry of their defects.

"Nearly-pristine graphene chemiresistors are less sensitive to analyte molecules because adsorbates bind to point defects, which have low resistance pathways around them," noted Salehi-Khojin, a research scientist at Dioxide Materials and post-doctoral research associate in the Department of Chemical and Biomolecular Engineering (ChemE) at Illinois. "As a result, adsorption at point defects only has a small effect on the overall resistance of the device. On the other hand, micrometer-sized line defects or continuous lines of point defects are different because no easy conduction paths exist around such defects, so the resistance change after adsorption is significant."

"This can lead to better and cheaper gas sensors for a variety of applications such as energy, homeland security and medical diagnostics" said Estrada who is a doctoral candidate in the Department of Electrical and Computer Engineering.

According to the authors, the two-dimensional nature of defective, CVD-grown graphene chemiresistors causes them to behave differently than carbon nanotube chemiresistors. This sensitivity is further improved by cutting the graphene into ribbons of width comparable to the line defect dimensions, or micrometers in this study.

"What we determined is that the gases we were sensing tend to bind to the defects," Pop said. "Surface defects in graphene are either point-, wrinkle-, or line-like. We found that the points do not matter very much and the lines are most likely where the sensing happens."

"The graphene ribbons with line defects appear to offer superior performance as graphene sensors," said ChemE professor emeritus and Dioxide Materials CEO Richard Masel. "Going forward, we think we may be able engineer the line defects to maximize the material's sensitivity. This novel approach should allow us to produce inexpensive and sensitive chemical sensors with the performance better than that of carbon nanotube sensors."

Pop is also affiliated with the Beckman Institute for Advanced Science and the Micro and Nanotechnology Laboratory at Illinois. Additional authors of the paper, Polycrystalline Graphene Ribbons as Chemiresistors," include Kevin Y. Lin, Myung-Ho Bae, and Feng Xiong. This work was supported by Dioxide Materials, by ONR grants N00014-09-1-0180 and N00014-10-1-0061, and the NDSEG Graduate Fellowship (D.E.).

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Amin Salehi-Khojin, David Estrada, Kevin Y. Lin, Myung-Ho Bae, Feng Xiong, Eric Pop, Richard I. Masel. Polycrystalline Graphene Ribbons as Chemiresistors. Advanced Materials, 2011; DOI: 10.1002/adma.201102663

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'Graphene earns its stripes': New nanoscale electronic state discovered on graphene sheets

Researchers from the London Centre for Nanotechnology (LCN) have discovered electronic stripes, called 'charge density waves', on the surface of the graphene sheets that make up a graphitic superconductor. This is the first time these stripes have been seen on graphene, and the finding is likely to have profound implications for the exploitation of this recently discovered material, which scientists believe will play a key role in the future of nanotechnology. The discovery is reported in Nature Communications, 29th November.

Graphene is a material made up of a single sheet of carbon atoms just one atom thick, and is found in the marks made by a graphite pencil. Graphene has remarkable physical properties and therefore has great technological potential, for example, in transparent electrodes for flat screen TVs, in fast energy-efficient transistors, and in ultra-strong composite materials. Scientists are now devoting huge efforts to understand and control the properties of this material.

The LCN team donated extra electrons to a graphene surface by sliding calcium metal atoms underneath it. One would normally expect these additional electrons to spread out evenly on the graphene surface, just as oil spreads out on water. But by using an instrument known as a scanning tunneling microscope, which can image individual atoms, the researchers have found that the extra electrons arrange themselves spontaneously into nanometer-scale stripes. This unexpected behavior demonstrates that the electrons can have a life of their own which is not connected directly to the underlying atoms. The results inspire many new directions for both science and technology. For example, they suggest a new method for manipulating and encoding information, where binary zeros and ones correspond to stripes running from north to south and running from east to west respectively.

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K.C. Rahnejat, C.A. Howard, N.E. Shuttleworth, S.R. Schofield, K. Iwaya, C.F. Hirjibehedin, Ch. Renner, G. Aeppli, M. Ellerby. Charge density waves in the graphene sheets of the superconductor CaC6. Nature Communications, 2011; 2: 558 DOI: 10.1038/ncomms1574

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Researching graphene nanoelectronics for a post-silicon world

ScienceDaily (Nov. 10, 2011) — Copper's days are numbered, and a new study at Rensselaer Polytechnic Institute could hasten the downfall of the ubiquitous metal in smart phones, tablet computers, and nearly all electronics. This is good news for technophiles who are seeking smaller, faster devices.

As new generations of computer chips continue to shrink in size, so do the copper pathways that transport electricity and information around the labyrinth of transistors and components. When these pathways -- called interconnects -- grow smaller, they become less efficient, consume more power, and are more prone to permanent failure.

To overcome this hurdle, industry and academia are vigorously researching new candidates to succeed traditional copper as the material of choice for interconnects on computer chips. One promising candidate is graphene, an atom-thick sheet of carbon atoms arranged like a nanoscale chicken-wire fence. Prized by researchers for its unique properties, graphene is essentially a single layer of the graphite found commonly in our pencils or the charcoal we burn on our barbeques.

Led by Rensselaer Professor Saroj Nayak, a team of researchers discovered they could enhance the ability of graphene to transmit electricity by stacking several thin graphene ribbons on top of one another. The study, published in the journal ACS Nano, brings industry closer to realizing graphene nanoelectronics and naming graphene as the heir apparent to copper.

"Graphene shows enormous potential for use in interconnects, and stacking up graphene shows a viable way to mass produce these structures," said Nayak, a professor in the Department of Physics, Applied Physics, and Astronomy at Rensselaer. "Copper's limitations are apparent, as increasingly smaller copper interconnects suffer from sluggish electron flows that results in hotter, less reliable devices. Our new study makes a case for the possibility that stacks of graphene ribbons could have what it takes to be used as interconnects in integrated circuits."

The study, based on large-scale quantum simulations, was conducted using the Rensselaer Computational Center for Nanotechnology Innovations (CCNI), one of the world's most powerful university-based supercomputers.

Copper interconnects suffer from a variety of unwanted problems, which grow more prominent as the size of the interconnects shrink. Electrons travel through the copper nanowires sluggishly and generate intense heat. As a result, the electrons "drag" atoms of copper around with them. These misplaced atoms increase the copper wire's electrical resistance, and degrade the wire's ability to transport electrons. This means fewer electrons are able to pass through the copper successfully, and any lingering electrons are expressed as heat. This heat can have negative effects on both a computer chip's speed and performance.

It is generally accepted that a quality replacement for traditional copper must be discovered and perfected in the next five to 10 years in order to further perpetuate Moore's Law -- ;an industry mantra that states the number of transistors on a computer chip, and thus the chip's speed, should double every 18 to 24 months.

Nayak's recent work, published in the journal ACS Nano, is titled "Effect of Layer Stacking on the Electronic Structure of Graphene Nanoribbons." When cut into nanoribbons, graphene is known to exhibit a band gap -- an energy gap between the valence and conduction bands -- which is an unattractive property for interconnects. The new study shows that stacking the graphene nanoribbons on top of each other, however, could significantly shrink this band gap.

"The optimal thickness is a stack of four to six layers of graphene," said Neerav Kharche, first author of the study and a computational scientist at CCNI. "Stacking more layers beyond this thickness doesn't reduce the band gap any further."

The end destination, Nayak said, is to one day manufacture microprocessors -- both the interconnects and the transistors -- entirely out of graphene. This game-changing goal, called monolithic integration, would mean the end of the long era of copper interconnects and silicon transistors.

"Such an advance is likely still many years into the future, but it will certainly revolutionize the way nearly all computers and electronics are designed and manufactured," Nayak said.

Along with Nayak and Kharche, contributors to this study were: former Rensselaer physics graduate student Yu Zhou; Swastik Kar, former Rensselaer physics research assistant professor; and Kevin P. O'Brien of Intel Corporation.

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Neerav Kharche, Yu Zhou, Kevin P. O’Brien, Swastik Kar, Saroj K. Nayak. Effect of Layer Stacking on the Electronic Structure of Graphene Nanoribbons. ACS Nano, 2011; 5 (8): 6096 DOI: 10.1021/nn200941u

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Researching graphene nanoelectronics for a post-silicon world

ScienceDaily (Nov. 10, 2011) — Copper's days are numbered, and a new study at Rensselaer Polytechnic Institute could hasten the downfall of the ubiquitous metal in smart phones, tablet computers, and nearly all electronics. This is good news for technophiles who are seeking smaller, faster devices.

As new generations of computer chips continue to shrink in size, so do the copper pathways that transport electricity and information around the labyrinth of transistors and components. When these pathways -- called interconnects -- grow smaller, they become less efficient, consume more power, and are more prone to permanent failure.

To overcome this hurdle, industry and academia are vigorously researching new candidates to succeed traditional copper as the material of choice for interconnects on computer chips. One promising candidate is graphene, an atom-thick sheet of carbon atoms arranged like a nanoscale chicken-wire fence. Prized by researchers for its unique properties, graphene is essentially a single layer of the graphite found commonly in our pencils or the charcoal we burn on our barbeques.

Led by Rensselaer Professor Saroj Nayak, a team of researchers discovered they could enhance the ability of graphene to transmit electricity by stacking several thin graphene ribbons on top of one another. The study, published in the journal ACS Nano, brings industry closer to realizing graphene nanoelectronics and naming graphene as the heir apparent to copper.

"Graphene shows enormous potential for use in interconnects, and stacking up graphene shows a viable way to mass produce these structures," said Nayak, a professor in the Department of Physics, Applied Physics, and Astronomy at Rensselaer. "Copper's limitations are apparent, as increasingly smaller copper interconnects suffer from sluggish electron flows that results in hotter, less reliable devices. Our new study makes a case for the possibility that stacks of graphene ribbons could have what it takes to be used as interconnects in integrated circuits."

The study, based on large-scale quantum simulations, was conducted using the Rensselaer Computational Center for Nanotechnology Innovations (CCNI), one of the world's most powerful university-based supercomputers.

Copper interconnects suffer from a variety of unwanted problems, which grow more prominent as the size of the interconnects shrink. Electrons travel through the copper nanowires sluggishly and generate intense heat. As a result, the electrons "drag" atoms of copper around with them. These misplaced atoms increase the copper wire's electrical resistance, and degrade the wire's ability to transport electrons. This means fewer electrons are able to pass through the copper successfully, and any lingering electrons are expressed as heat. This heat can have negative effects on both a computer chip's speed and performance.

It is generally accepted that a quality replacement for traditional copper must be discovered and perfected in the next five to 10 years in order to further perpetuate Moore's Law -- ;an industry mantra that states the number of transistors on a computer chip, and thus the chip's speed, should double every 18 to 24 months.

Nayak's recent work, published in the journal ACS Nano, is titled "Effect of Layer Stacking on the Electronic Structure of Graphene Nanoribbons." When cut into nanoribbons, graphene is known to exhibit a band gap -- an energy gap between the valence and conduction bands -- which is an unattractive property for interconnects. The new study shows that stacking the graphene nanoribbons on top of each other, however, could significantly shrink this band gap.

"The optimal thickness is a stack of four to six layers of graphene," said Neerav Kharche, first author of the study and a computational scientist at CCNI. "Stacking more layers beyond this thickness doesn't reduce the band gap any further."

The end destination, Nayak said, is to one day manufacture microprocessors -- both the interconnects and the transistors -- entirely out of graphene. This game-changing goal, called monolithic integration, would mean the end of the long era of copper interconnects and silicon transistors.

"Such an advance is likely still many years into the future, but it will certainly revolutionize the way nearly all computers and electronics are designed and manufactured," Nayak said.

Along with Nayak and Kharche, contributors to this study were: former Rensselaer physics graduate student Yu Zhou; Swastik Kar, former Rensselaer physics research assistant professor; and Kevin P. O'Brien of Intel Corporation.

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Journal Reference:

Neerav Kharche, Yu Zhou, Kevin P. O’Brien, Swastik Kar, Saroj K. Nayak. Effect of Layer Stacking on the Electronic Structure of Graphene Nanoribbons. ACS Nano, 2011; 5 (8): 6096 DOI: 10.1021/nn200941u

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Innovation is step toward digital graphene transistors

ScienceDaily (Sep. 7, 2011) — Researchers are making progress in creating digital transistors using a material called graphene, potentially sidestepping an obstacle thought to dramatically limit the material's use in computers and consumer electronics.

Graphene is a one-atom-thick layer of carbon that conducts electricity with little resistance or heat generation. After its discovery in 2004 -- which earned a Nobel Prize in physics -- it was touted as a potential replacement for silicon, possibly leading to ultrafast devices with simplified circuits that might be less expensive to manufacture.

However, graphene's luster has dulled in recent years for digital applications as researchers have discovered that it has no "band gap," a trait that is needed to switch on and off, which is critical for digital transistors.

"The fact that graphene is a zero-band-gap material by nature has raised many questions in terms of its usefulness for digital applications," said Purdue doctoral student Hong-Yan Chen.

Electrons in semiconductors like silicon exist at two energy levels, known as the valence and conduction bands. The energy gap between these two levels is called the band gap. Having the proper band gap enables transistors to turn on and off, which allows digital circuits to store information in binary code consisting of sequences of ones and zeroes.

Chen has led a team of researchers in creating a new type of graphene inverter, a critical building block of digital transistors. Other researchers have created graphene inverters, but they had to be operated at 77 degrees Kelvin, which is minus 196 Celsius (minus 320 Fahrenheit).

"If graphene could be used in digital applications, that would be really important," said Chen, who is working with Joerg Appenzeller, a professor of electrical and computer engineering and scientific director of nanoelectronics at Purdue's Birck Nanotechnology Center.

The Purdue researchers are the first to create graphene inverters that work at room temperature and have a gain larger than one, a basic requirement for digital electronics that enables transistors to amplify signals and control its switching from 0 to 1.

Findings were detailed in a paper, "Complementary-Type Graphene Inverters Operating at Room-Temperature," presented in June during the 2011 Device Research Conference in Santa Barbara, Calif.

Thus far graphene transistors have been practical only for specialized applications, such as amplifiers for cell phones and military systems. However, the new inverters represent a step toward learning how to use the material to create graphene transistors for broader digital applications that include computers and consumer electronics.

To create electronic devices, silicon is impregnated with impurities to change its semiconducting properties. Such "doping" is not easily applicable to graphene. However, the researchers have potentially solved this problem by developing "electrostatic doping," which makes it possible for graphene inverters to mimic the characteristics of silicon inverters.

Electrostatic doping is induced through the electric field between metal gates, which are located 40 nanometers away from graphene channels. The doping can be altered by varying the voltage, enabling researchers to test specific doping levels.

"This will allow us to find the sweet spot for operating the device," Chen said.

Further work will be needed to integrate the prototype into a working graphene circuit for digital applications.

The research is based at the Birck Nanotechnology Center in Purdue's Discovery Park.

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Purdue University. The original article was written by Emil Venere.

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Unexpected adhesion properties of graphene may lead to new nanotechnology devices

ScienceDaily (Aug. 24, 2011) — Graphene, considered the most exciting new material under study in the world of nanotechnology, just got even more interesting, according to a new study by a group of researchers at the University of Colorado Boulder.

The new findings -- that graphene has surprisingly powerful adhesion qualities -- are expected to help guide the development of graphene manufacturing and of graphene-based mechanical devices such as resonators and gas separation membranes, according to the CU-Boulder team. The experiments showed that the extreme flexibility of graphene allows it to conform to the topography of even the smoothest substrates.

Graphene consists of a single layer of carbon atoms chemically bonded in a hexagonal chicken wire lattice. Its unique atomic structure could some day replace silicon as the basis of electronic devices and integrated circuits because of its remarkable electrical, mechanical and thermal properties, said Assistant Professor Scott Bunch of the CU-Boulder mechanical engineering department and lead study author.

A paper on the subject was published online in the Aug. 14 issue of Nature Nanotechnology. Co-authors on the study included CU-Boulder graduate students Steven Koenig and NarasimhaBoddeti and Professor Martin Dunn of the mechanical engineering department.

"The real excitement for me is the possibility of creating new applications that exploit the remarkable flexibility and adhesive characteristics of graphene and devising unique experiments that can teach us more about the nanoscale properties of this amazing material," Bunch said.

Not only does graphene have the highest electrical and thermal conductivity among all materials known, but this "wonder material" has been shown to be the thinnest, stiffest and strongest material in the world, as well as being impermeable to all standard gases. It's newly discovered adhesion properties can now be added to the list of the material's seemingly contradictory qualities, said Bunch.

The CU-Boulder team measured the adhesion energy of graphene sheets, ranging from one to five atomic layers, with a glass substrate, using a pressurized "blister test" to quantify the adhesion between graphene and glass plates.

Adhesion energy describes how "sticky" two things are when placed together. Scotch tape is one example of a material with high adhesion; the gecko lizard, which seemingly defies gravity by scaling up vertical walls using adhesion between its feet and the wall, is another. Adhesion also canplay a detrimental role, as in suspended micromechanical structures where adhesion can cause device failure or prolong the development of a technology, said Bunch.

The CU research, the first direct experimental measurements of the adhesion of graphene nanostructures, showed that so-called "van der Waals forces" -- the sum of the attractive or repulsive forces between molecules -- clamp the graphene samples to the substrates and also hold together the individual graphene sheets in multilayer samples.

The researchers found the adhesion energies between graphene and the glass substrate were several orders of magnitude larger than adhesion energies in typical micromechanical structures, an interaction they described as more liquid-like than solid-like, said Bunch.

The CU-Boulder study was funded primarily by the National Science Foundation and the Defense Advanced Research Projects Agency.

The importance of graphene in the scientific world was illustrated by the 2010 Nobel Prize in physics that honored two scientists at Manchester University in England, Andre K. Geim and Konstantin Novoselov, for producing, isolating, identifying and characterizing graphene.

There is interest in exploiting graphene's incredible mechanical properties to create ultrathin membranes for energy-efficient separations such as those needed for natural gas processing or water purification, while graphene's superior electrical properties promise to revolutionize the microelectronics industry, said Bunch.

In all of these applications, including any large-scale graphene manufacturing, the interaction that graphene has with a surface is of critical importance and a scientific understanding will help push the technology forward, he said.

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Journal Reference:

Steven P. Koenig, Narasimha G. Boddeti, Martin L. Dunn, J. Scott Bunch. Ultrastrong adhesion of graphene membranes. Nature Nanotechnology, 2011; DOI: 10.1038/nnano.2011.123

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