Defect in graphene may present bouquet of possibilities

Flower-like defects in graphene can occur during the fabrication process. The NIST team captured images of one of the defects (figures a and c) using a scanning tunneling microscope. A simulated image from their computer models (figure b) shows excellent agreement.Credit: Cockayne,Stroscio/NIST.

A class of decorative, flower-like defects in the nanomaterial graphene could have potentially important effects on the material's already unique electrical and mechanical properties, according to researchers at the National Institute of Standards and Technology (NIST) and Georgia Tech. In a new paper, the team for the first time describes a family of seven defects that could occur naturally or be induced to occur in graphene, one of which already has been observed.

Graphene is renowned for its strength and conductivity, both of which are a result of its structure. For the most part, graphene is a featureless plane of carbon atoms arranged in a honeycomb lattice.

According to NIST Fellow Joseph Stroscio, defects can appear due to the movement of the carbon atoms at high temperatures when producing graphene by heating silicon carbide under ultrahigh vacuum. The easiest, i.e. requiring the least amount of energy, rearrangements graphene can make are to switch from six-member carbon rings to rings containing five or seven atoms, which keeps all the carbon atoms happy with no unsatisfied bonds. 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 these defects.

"As the graphene forms under high heat, sections of the lattice can come loose and rotate," Cockayne says. "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."

The exceedingly rigid lattice already is stronger than steel, but the defects might allow it a little flexibility, making it even more resilient to tearing or fracturing.

With more experimentation, Cockayne says, researchers should be able to correlate the appearance of defects with variations in growth conditions, which should make it possible to either avoid defects entirely or produce them at will.

Moreover, while the flower defect is composed of six pairs of five- and seven-atom rings, Cockayne and the NIST team's modeling of graphene's atomic structure suggests there might be a veritable bouquet of flower-like configurations. These configurations?seven in all?would each possess their own unique mechanical and electrical properties.

Lasers could produce much sought-after band gaps in graphene

 

This scheme shows graphene illuminated by a laser. By interacting with electrons in graphene, the laser field can produce band gaps in the graphene's electronic structure. Image credit: Luis E. F. Foa Torres.

One of the most often-cited features of graphene ? the two-dimensional crystal lattice made of carbon ? is its unique electronic properties. Many of these electronic properties make graphene attractive as a material for carbon-based electronics. But also among these properties is that graphene is a gapless semiconductor. As physicist Luis Foa Torres explains, the fact that graphene does not have a band gap is its Achilles? heel, which makes it difficult to be incorporated into electronic devices.

?In semiconductors, there is an energy region called the band gap where there are no electronic states available,? Foa Torres, at the National University of Córdoba in Córdoba, Argentina, told PhysOrg.com. ?One says that the carrier density there is zero. If you have your device connected between two electrodes and it has no states available, then the electric current through it can be made very small. A zero-gap semiconductor, also called a gapless semiconductor, is a material where the density of electronic states vanishes at a single point. This is the case for graphene, where the pi and pi* bands touch at a single point, the so-called Dirac point. In practice, they behave as not having a gap at all.

?Lacking a band gap means that graphene cannot be ?switched-off,?? he explained. ?Having ?on? and ?off? currents may encode information such as the 1?s and 0?s necessary for computing and is crucial for active electronic devices such as switches and transistors. This is the reason why having no band gap is one of the main drawbacks hindering many applications of this outstanding material.?

In a new study, Foa Torres and his coauthors have addressed this problem. By analyzing the way that a laser field interacts with electrons in graphene, the researchers have predicted that shining a mid-infrared laser on graphene can produce band gaps in its electronic structure. Further, the researchers predict that the band gaps could be tuned by controlling the laser polarization. As Foa Torres explained, the key to how polarized light "opens up" band gaps in graphene involves electrons interacting with the laser field.

?Imagine an electron moving, say from left to right, into a region illuminated by the laser field,? he said. ?Then what happens is that the electron interacts with the radiation by absorbing or emitting photons. This interaction leads to the electron being reflected or backscattered, as it would have hit a wall: the band gap. In contrast with usual band gaps, this one is dynamically produced by the laser.?

By showing that a laser field could be used to tune the electronic structure of graphene, the study has both fundamental implications and technological applications.

?The interplay between the peculiar electronic structure of graphene and the laser may help to induce exotic states of matter such as topological insulators, materials that are insulators in the bulk but show robust conduction in the surface,? Foa Torres said. ?On the other hand, from an applied point of view, I consider that these laser-induced band gaps could open an avenue for a new breed of optoelectronic devices, devices that transduce optical into electrical signals.?

For Foa Torres and his coauthors, the next crucial step is experimental verification.

?Experimental verification of our findings is one of the main driving forces of our project,? he said. ?With the aim of paving the way for experimentalists to be able to verify them, we have performed a very fine tuning of parameters such as laser frequency, amplitude, etc. During the last months we received very valuable feedback from top-level experimental groups in the US and Spain that are interested in our proposal. As always, there are surely still some issues to solve before it turns into reality but one should go step by step. The door is now open, we are just entering a promising terra incognita.?

 

IBM creates first graphene based integrated circuit

 

Schematic illustration of a graphene mixer circuit. Image credit: Yu-Ming Lin/Science, 10.1126/science.1204428

Taking a giant step forward in the creation and production of graphene based integrated circuits, IBM has announced in Science, the fabrication of a graphene based integrated circuit on a single chip. The demonstration chip, known as a radio frequency "mixer" is capable of producing frequencies up to 10 GHz, and demonstrates that it is possible to overcome the adhesion problems that have stymied researchers efforts in creating graphene based IC's that can be used in analog applications such as cell phones or more likely military communications.

IBM and others had previously demonstrated that it was possible to create a graphene based transistor; this latest research takes that technology one step further by marrying the transistor and other electronics on a single chip to create a full-fledged integrated circuit.

The graphene circuits were started by growing a two or three layer graphene film on a silicon surface which was then heated to 1400°C. The graphene IC was then fabricated by employing top gated, dual fingered graphene FET?s (field-effect transistors) which were then integrated with inductors. The active channels were made by spin-coating the wafer with a thin polymer and then applying a layer of hydrogen silsequioxane. The channels were then carved by e-beam lithography. Next, the excess graphene was removed with an oxygen plasma laser, and then the whole works was cleaned with acetone. The result is an integrated circuit that is less than 1mm² in total size.

The chip produces output signals with mixed frequencies, hence its name, and while the prototype developed is not expected to be used in an actual device, future chips using the new technology are expected to be used in wireless communications. Since a major source of the funding for the research has been provided by the U.S. Defense Advanced Research Projects Agency (DARPA) the first uses of such a chip in an application is likely to be for secretive communications between airborne military pilots; the ultra high frequencies generated make it ideal for such secure applications.

Getting the graphene to adhere to other electronic components was by all accounts the most difficult part of the whole process, and apparently took the team, led by Phaedon Avouris, all of a year to accomplish; but it was clearly well worth the effort as research teams the world over continue to search for an eventual replacement to silicon in integrated chips, which by most accounts will hit its limits within the next few years.

More information: Wafer-Scale Graphene Integrated Circuit, Science 10 June 2011: Vol. 332 no. 6035 pp. 1294-1297. DOI: 10.1126/science.1204428

ABSTRACT
A wafer-scale graphene circuit was demonstrated in which all circuit components, including graphene field-effect transistor and inductors, were monolithically integrated on a single silicon carbide wafer. The integrated circuit operates as a broadband radio-frequency mixer at frequencies up to 10 gigahertz. These graphene circuits exhibit outstanding thermal stability with little reduction in performance (less than 1 decibel) between 300 and 400 kelvin. These results open up possibilities of achieving practical graphene technology with more complex functionality and performance.

 

Graphene optical modulators could lead to ultrafast communications

 

This is a schematic illustration of the graphene-based optical modulator. A layer of graphene (black fishnet) is placed on top of a silicon waveguide (blue), which is used as an optical fiber to guide light. Electrical signals are sent in from the side of the graphene to alter the amount of photons the graphene absorbs. Credit: Graphic by Ming Liu, UC Berkeley

Scientists at the University of California, Berkeley, have demonstrated a new technology for graphene that could break the current speed limits in digital communications.

The team of researchers, led by UC Berkeley engineering professor Xiang Zhang, built a tiny optical device that uses graphene, a one-atom-thick layer of crystallized carbon, to switch light on and off. This switching ability is the fundamental characteristic of a network modulator, which controls the speed at which data packets are transmitted. The faster the data pulses are sent out, the greater the volume of information that can be sent. Graphene-based modulators could soon allow consumers to stream full-length, high-definition, 3-D movies onto a smartphone in a matter of seconds, the researchers said.

"This is the world's smallest optical modulator, and the modulator in data communications is the heart of speed control," said Zhang, who directs a National Science Foundation (NSF) Nanoscale Science and Engineering Center at UC Berkeley. "Graphene enables us to make modulators that are incredibly compact and that potentially perform at speeds up to ten times faster than current technology allows. This new technology will significantly enhance our capabilities in ultrafast optical communication and computing."

In this latest work, described in the May 8 advanced online publication of the journal Nature, researchers were able to tune the graphene electrically to absorb light in wavelengths used in data communication. This advance adds yet another advantage to graphene, which has gained a reputation as a wonder material since 2004 when it was first extracted from graphite, the same element in pencil lead. That achievement earned University of Manchester scientists Andre Geim and Konstantin Novoselov the Nobel Prize in Physics last year.

Zhang worked with fellow faculty member Feng Wang, an assistant professor of physics and head of the Ultrafast Nano-Optics Group at UC Berkeley. Both Zhang and Wang are faculty scientists at Lawrence Berkeley National Laboratory's Materials Science Division.

"The impact of this technology will be far-reaching," said Wang. "In addition to high-speed operations, graphene-based modulators could lead to unconventional applications due to graphene's flexibility and ease in integration with different kinds of materials. Graphene can also be used to modulate new frequency ranges, such as mid-infrared light, that are widely used in molecular sensing."

Graphene is the thinnest, strongest crystalline material yet known. It can be stretched like rubber, and it has the added benefit of being an excellent conductor of heat and electricity. This last quality of graphene makes it a particularly attractive material for electronics.

"Graphene is compatible with silicon technology and is very cheap to make," said Ming Liu, post-doctoral researcher in Zhang's lab and co-lead author of the study. "Researchers in Korea last year have already produced 30-inch sheets of it. Moreover, very little graphene is required for use as a modulator. The graphite in a pencil can provide enough graphene to fabricate 1 billion optical modulators."

It is the behavior of photons and electrons in graphene that first caught the attention of the UC Berkeley researchers.

Shown is a scanning electron microscope (SEM) image magnifying the key structures of the graphene-based optical modulator. (Colors were added to enhance the contrast). Gold (Au) and platinum (Pt) electrodes are used to apply electrical charges to the sheet of graphene, shown in blue, placed on top of the silicon (Si) waveguide, shown in red. The voltage can control the graphene's transparency, effectively turning the setup into an optical modulator that can turn light on and off. (Ming Liu image)

The researchers found that the energy of the electrons, referred to as its Fermi level, can be easily altered depending upon the voltage applied to the material. The graphene's Fermi level in turn determines if the light is absorbed or not.

When a sufficient negative voltage is applied, electrons are drawn out of the graphene and are no longer available to absorb photons. The light is "switched on" because the graphene becomes totally transparent as the photons pass through.

Graphene is also transparent at certain positive voltages because, in that situation, the electrons become packed so tightly that they cannot absorb the photons.

The researchers found a sweet spot in the middle where there is just enough voltage applied so the electrons can prevent the photons from passing, effectively switching the light "off."

"If graphene were a hallway, and electrons were people, you could say that, when the hall is empty, there's no one around to stop the photons," said Xiaobo Yin, co-lead author of the Nature paper and a research scientist in Zhang's lab. "In the other extreme, when the hall is too crowded, people can't move and are ineffective in blocking the photons. It's in between these two scenarios that the electrons are allowed to interact with and absorb the photons, and the graphene becomes opaque."

In their experiment, the researchers layered graphene on top of a silicon waveguide to fabricate optical modulators. The researchers were able to achieve a modulation speed of 1 gigahertz, but they noted that the speed could theoretically reach as high as 500 gigahertz for a single modulator.

While components based upon optics have many advantages over those that use electricity, including the ability to carry denser packets of data more quickly, attempts to create optical interconnects that fit neatly onto a computer chip have been hampered by the relatively large amount of space required in photonics.

Light waves are less agile in tight spaces than their electrical counterparts, the researchers noted, so photon-based applications have been primarily confined to large-scale devices, such as fiber optic lines.

"Electrons can easily make an L-shaped turn because the wavelengths in which they operate are small," said Zhang. "Light wavelengths are generally bigger, so they need more space to maneuver. It's like turning a long, stretch limo instead of a motorcycle around a corner. That's why optics require bulky mirrors to control their movements. Scaling down the optical device also makes it faster because the single atomic layer of graphene can significantly reduce the capacitance ? the ability to hold an electric charge ? which often hinders device speed."

Graphene-based modulators could overcome the space barrier of optical devices, the researchers said. They successfully shrunk a graphene-based optical modulator down to a relatively tiny 25 square microns, a size roughly 400 times smaller than a human hair. The footprint of a typical commercial modulator can be as large as a few square millimeters.

Even at such a small size, graphene packs a punch in bandwidth capability. Graphene can absorb a broad spectrum of light, ranging over thousands of nanometers from ultraviolet to infrared wavelengths. This allows graphene to carry more data than current state-of-the-art modulators, which operate at a bandwidth of up to 10 nanometers, the researchers said.

"Graphene-based modulators not only offer an increase in modulation speed, they can enable greater amounts of data packed into each pulse," said Zhang. "Instead of broadband, we will have 'extremeband.' What we see here and going forward with graphene-based modulators are tremendous improvements, not only in consumer electronics, but in any field that is now limited by data transmission speeds, including bioinformatics and weather forecasting. We hope to see industrial applications of this new device in the next few years."

Electronic life on the edge: Scientists discover the edge states of graphene nanoribbons

A scanning tunneling microscope determines the topography and orientation of the graphene nanoribbons on the atomic scale. In spectroscopy mode, it determines changes in the density of electronic states, from the nanoribbon's interior to its edge. Credit: Crommie et al, Lawrence Berkeley National Laboratory

As far back as the 1990s, long before anyone had actually isolated graphene ? a honeycomb lattice of carbon just one atom thick ? theorists were predicting extraordinary properties at the edges of graphene nanoribbons. Now physicists at the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab), and their colleagues at the University of California at Berkeley, Stanford University, and other institutions, have made the first precise measurements of the "edge states" of well-ordered nanoribbons.

A graphene nanoribbon is a strip of graphene that may be only a few nanometers wide (a nanometer is a billionth of a meter). Theorists have envisioned that nanoribbons, depending on their width and the angle at which they are cut, would have unique electronic, magnetic, and optical features, including band gaps like those in semiconductors, which sheet graphene doesn't have.

"Until now no one has been able to test theoretical predictions regarding nanoribbon edge-states, because no one could figure out how to see the atomic-scale structure at the edge of a well-ordered graphene nanoribbon and how, at the same time, to measure its electronic properties within nanometers of the edge," says Michael Crommie of Berkeley Lab's Materials Sciences Division (MSD) and UC Berkeley's Physics Division, who led the research. "We were able to achieve this by studying specially made nanoribbons with a scanning tunneling microscope."

The team's research not only confirms theoretical predictions but opens the prospect of building quick-acting, energy-efficient nanoscale devices from graphene-nanoribbon switches, spin-valves, and detectors, based on either electron charge or electron spin. Farther down the road, graphene nanoribbon edge states open the possibility of devices with tunable giant magnetoresistance and other magnetic and optical effects.

Crommie and his colleagues have published their research in Nature Physics, available May 8, 2011 in advanced online publication.

The well-tempered nanoribbon

"Making flakes and sheets of graphene has become commonplace," Crommie says, "but until now, nanoribbons produced by different techniques have exhibited, at best, a high degree of inhomogeneity" ? typically resulting in disordered ribbon structures with only short stretches of straight edges appearing at random. The essential first step in detecting nanoribbon edge states is access to uniform nanoribbons with straight edges, well-ordered on the atomic scale.

Hongjie Dai of Stanford University's Department of Chemistry and Laboratory for Advanced Materials, a member of the research team, solved this problem with a novel method of "unzipping" carbon nanotubes chemically. Graphene rolled into a cylinder makes a nanotube, and when nanotubes are unzipped in this way the slice runs straight down the length of the tube, leaving well-ordered, straight edges.

Graphene can be wrapped at almost any angle to make a nanotube. The way the nanotube is wrapped determines the pitch, or "chiral vector," of the nanoribbon edge when the tube is unzipped. A cut straight along the outer atoms of a row of hexagons produces a zigzag edge. A cut made at a 30-degree angle from a zigzag edge goes through the middle of the hexagons and yields scalloped edges, known as "armchair" edges. Between these two extremes are a variety of chiral vectors describing edges stepped on the nanoscale, in which, for example, after every few hexagons a zigzag segment is added at an angle.

These subtle differences in edge structure have been predicted to produce measurably different physical properties, which potentially could be exploited in new graphene applications. Steven Louie of UC Berkeley and Berkeley Lab's MSD was the research team's theorist; with the help of postdoc Oleg Yazyev, Louie calculated the expected outcomes, which were then tested against experiment.

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By "unzipping" carbon nanotubes, regular edges with differing chiralities can be produced between the extremes of the zigzag configuration and, at a 30-degree angle to it, the armchair configuration. Credit: Hongjie Dai, Stanford University, and Michael Crommie et al, Lawrence Berkeley National Laboratory

Chenggang Tao of MSD and UCB led a team of graduate students in performing scanning tunneling microscopy (STM) of the nanoribbons on a gold substrate, which resolved the positions of individual atoms in the graphene nanoribbons. The team looked at more than 150 high-quality nanoribbons with different chiralities, all of which showed an unexpected feature, a regular raised border near their edges forming a hump or bevel. Once this was established as a real edge feature ? not the artifact of a folded ribbon or a flattened nanotube ? the chirality and electronic properties of well-ordered nanoribbon edges could be measured with confidence, and the edge regions theoretically modeled.

Electronics at the edge

"Two-dimensional graphene sheets are remarkable in how freely electrons move through them, including the fact that there's no band gap," Crommie says. "Nanoribbons are different: electrons can become trapped in narrow channels along the nanoribbon edges. These edge-states are one-dimensional, but the electrons on one edge can still interact with the edge electrons on the other side, which causes an energy gap to open up."

Using an STM in spectroscopy mode (STS), the team measured electronic density changes as an STM tip was moved from a nanoribbon edge inward toward its interior. Nanoribbons of different widths were examined in this way. The researchers discovered that electrons are confined to the edge of the nanoribbons, and that these nanoribbon-edge electrons exhibit a pronounced splitting in their energy levels.

"In the quantum world, electrons can be described as waves in addition to being particles," Crommie notes. He says one way to picture how different edge states arise is to imagine an electron wave that fills the length of the ribbon and diffracts off the atoms near its edge. The diffraction patterns resemble water waves coming through slits in a barrier.

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Graphene nanoribbons are narrow sheets of carbon atoms only one layer thick. Their width, and the angles at which the edges are cut, produce a variety of electronic states, which have been studied with precision for the first time using scanning tunneling microscopy and scanning tunneling spectroscopy. Credit: Crommie et al, Lawrence Berkeley National Laboratory

For nanoribbons with an armchair edge, the diffraction pattern spans the full width of the nanoribbon; the resulting electron states are quantized in energy and extend spatially throughout the entire nanoribbon. For nanoribbons with a zigzag edge, however, the situation is different. Here diffraction from edge atoms leads to destructive interference, causing the electron states to localize near the nanoribbon edges. Their amplitude is greatly reduced in the interior.

The energy of the electron, the width of the nanoribbon, and the chirality of its edges all naturally affect the nature and strength of these nanoribbon electronic states, an indication of the many ways the electronic properties of nanoribbons can be tuned and modified.

Says Crommie, "The optimist says, 'Wow, look at all the ways we can control these states ? this might allow a whole new technology!' The pessimist says, 'Uh-oh, look at all the things that can disturb a nanoribbon's behavior ? how are we ever going to achieve reproducibility on the atomic scale?'"

Crommie himself declares that "meeting this challenge is a big reason for why we do research. Nanoribbons have the potential to form exciting new electronic, magnetic, and optical devices at the nanoscale. We might imagine photovoltaic applications, where absorbed light leads to useful charge separation at nanoribbon edges. We might also imagine spintronics applications, where using a side-gate geometry would allow control of the spin polarization of electrons at a nanoribbon's edge."

Although getting there won't be simple -- "The edges have to be controlled," Crommie emphasizes -- "what we've shown is that it's possible to make nanoribbons with good edges and that they do, indeed, have characteristic edge states similar to what theorists had expected. This opens a whole new area of future research involving the control and characterization of graphene edges in different nanoscale geometries."

Shining light on graphene sensors

Shining light on graphene sensors

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A light-sensitive graphene/polymer heterostructure.

National Physical Laboratory, together with an international team of scientists, have published research showing how light can be used to control graphene's electrical properties. This advance is an important step towards developing highly sensitive graphene-based electronic devices.

Graphene is an extraordinary two-dimensional material made of a single atomic layer of carbon atoms. It is the thinnest material known to man, and yet is one of the strongest ever tested.

It has unique properties which make it a very exciting material for a huge range of applications from high-speed electronics and solar cells, to super-sensors capable of detecting single molecules of toxic gases.

It is able to act as a sensor because its entire structure is exposed to its surroundings, and it reacts to any molecules that touch its surface. This reaction causes graphene's electrical properties to alter, i.e. it senses the molecules' presence.

In their paper published in the Journal of Advanced Materials, the team show that when graphene is coated with light-sensitive polymers its unique electrical properties can be precisely controlled and therefore exploited.

The polymers also protect graphene from contamination.

Light-modified graphene chips have already been used at NPL in ultra-precision experiments to measure the quantum of the electrical resistance.

In the future similar polymers could be used to effectively 'translate' information from their surroundings and influence how graphene behaves. This effect could be exploited to develop robust reliable sensors for smoke, poisonous gases, or any targeted molecule.

Creating a pure spin current in graphene

Creating a pure spin current in graphene

Graphene is a material that has the potential for a number of future applications. Scientists are interested in using graphene for quantum computing and also as a replacement for electronics. However, in order to realize these graphene applications, a solid understanding of how spin current works in graphene is important.

One of the goals is to achieve pure spin current in graphene. “Pure spin current is a spin current with zero charge current, which means electrons with different spins travel toward opposite directions,” K S Chan tells PhysOrg.com via email. Chan is a professor at the City University of Hong Kong. Working with Zijing Lin, a professor at the University of Science and Technology of China in Hefei, and, Qingtian Zhang, a student of the CityU-USTC joint Ph.D. program, Chan studied adiabatic pumping in graphene as a way to generate spin current. Their work is published in Applied Physics Letters: “Spin current generation by adiabatic pumping in monolayer graphene.”

“Spin current is an important tool of studying spins in graphene,” Chan explains. “With spin current, you can create polarization in a particular region, and you can study the behavior of the spin in that particular region.” Chan points out that spin current is important in the development of a graphene quantum computer. Additionally, he points out that graphene is the material of choice for spintronics, which some hope will be able to replace electronics.

“Spintronic devices are believed to be faster and consume less power than electronic devices,” Chan continues. Understanding how spin works in graphene could be an important part of making a breakthrough in spintronics. Chan and his colleagues use a method called adiabatic quantum pumping to generate spin current for study.

Chan describes the technique: “[Adiabatic pumping] is a quantum phenomenon in which a DC current is generated without a DC voltage. Two AC voltages are applied to the graphene and a DC charge current can be generated through adiabatic quantum pumping. Adiabatic means the rates of change of the voltages are very slow in comparison with the speed at which the electrons travel through the graphene structure.”

On top of that, the team created asymmetry between electrons with different spin using the ferromagnetic proximity effect. “A ferromagnetic thin film is deposited on graphene. Electrons with different spins under the ferromagnetic layer will have different energies and therefore respond differently to adiabatic pumping,” Chan says. As a result of these different responses, pure spin current is generated, with different spins traveling in opposite directions. “What is so special about the present method is that a pure spin current can be generated at some Fermi energy without an external magnetic field, which is important for making nanosized devices.”

Fundamentally, the work done by Chan and his colleagues show that it is possible to generate pure spin current in graphene without magnetic field. This could lead to more practical applications in quantum computing and perhaps, later, spintronics. The next step, though, is to learn how spin current can be detected.

“Spin current is difficult to detect,” Chan explains. “It’s not like the charge current which can be easily measured by a voltmeter.” He admits that there are other important issues that need to be studied regarding spin in graphene, but Chan points this out: “To develop graphene spintronic devices, we need to know how to measure the spin current in graphene.”

New transistors: An alternative to silicon and better than graphene

New transistors: An alternative to silicon and better than graphene

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This is a digital model showing how molybdenite can be integrated into a transistor. Credit: Credit: EPFL

Smaller and more energy-efficient electronic chips could be made using molybdenite. In an article appearing online January 30 in the journal Nature Nanotechnology, EPFL's Laboratory of Nanoscale Electronics and Structures (LANES) publishes a study showing that this material has distinct advantages over traditional silicon or graphene for use in electronics applications.

A discovery made at EPFL could play an important role in electronics, allowing us to make transistors that are smaller and more energy efficient. Research carried out in the Laboratory of Nanoscale Electronics and Structures (LANES) has revealed that molybdenite, or MoS₂, is a very effective semiconductor. This mineral, which is abundant in nature, is often used as an element in steel alloys or as an additive in lubricants. But it had not yet been extensively studied for use in electronics.

100,000 times less energy

"It's a two-dimensional material, very thin and easy to use in nanotechnology. It has real potential in the fabrication of very small transistors, light-emitting diodes (LEDs) and solar cells," says EPFL Professor Andras Kis, whose LANES colleagues M. Radisavljevic, Prof. Radenovic et M. Brivio worked with him on the study. He compares its advantages with two other materials: silicon, currently the primary component used in electronic and computer chips, and graphene, whose discovery in 2004 earned University of Manchester physicists Andre Geim and Konstantin Novoselov the 2010 Nobel Prize in Physics.

One of molybdenite's advantages is that it is less voluminous than silicon, which is a three-dimensional material. "In a 0.65-nanometer-thick sheet of MoS₂, the electrons can move around as easily as in a 2-nanometer-thick sheet of silicon," explains Kis. "But it's not currently possible to fabricate a sheet of silicon as thin as a monolayer sheet of MoS₂." Another advantage of molybdenite is that it can be used to make transistors that consume 100,000 times less energy in standby state than traditional silicon transistors. A semi-conductor with a "gap" must be used to turn a transistor on and off, and molybdenite's 1.8 electron-volt gap is ideal for this purpose.

Better than graphene

In solid-state physics, band theory is a way of representing the energy of electrons in a given material. In semi-conductors, electron-free spaces exist between these bands, the so-called "band gaps." If the gap is not too small or too large, certain electrons can hop across the gap. It thus offers a greater level of control over the electrical behavior of the material, which can be turned on and off easily.

The existence of this gap in molybdenite also gives it an advantage over graphene. Considered today by many scientists as the electronics material of the future, the "semi-metal" graphene doesn't have a gap, and it is very difficult to artificially reproduce one in the material.

Graphene and 'spintronics' combo looks promising

Graphene and 'spintronics' combo looks promising

A team of physicists has taken a big step toward the development of useful graphene spintronic devices. The physicists, from the City University of Hong Kong and the University of Science and Technology of China, present their findings in the American Institute of Physics' Applied Physics Letters.

Graphene, a two-dimensional crystalline form of carbon, is being touted as a sort of "Holy Grail" of materials. It boasts properties such as a breaking strength 200 times greater than steel and, of great interest to the semiconductor and data storage industries, electric currents that can blaze through it 100 times faster than in silicon.

Spintronic devices are being hotly pursued because they promise to be smaller, more versatile, and much faster than today's electronics. "Spin" is a quantum mechanical property that arises when a particle's intrinsic rotational momentum creates a tiny magnetic field. And spin has a direction, either "up" or "down." The direction can encode data in the 0s and 1s of the binary system, with the key here being that spin-based data storage doesn't disappear when the electric current stops.

"There is strong research interest in spintronic devices that process information using electron spins, because these novel devices offer better performance than traditional electronic devices and will likely replace them one day," says Kwok Sum Chan, professor of physics at the City University of Hong Kong "Graphene is an important material for spintronic devices because its electron spin can maintain its direction for a long time and, as a result, information stored isn't easily lost."

It is, however, difficult to generate a spin current in graphene, which would be a key part of carrying information in a graphene spintronic device. Chan and colleagues came up with a method to do just that. It involves using spin splitting in monolayer graphene generated by ferromagnetic proximity effect and adiabatic (a process that is slow compared to the speed of the electrons in the device) quantum pumping. They can control the degree of polarization of the spin current by varying the Fermi energy (the level in the distribution of electron energies in a solid at which a quantum state is equally likely to be occupied or empty), which they say is very important for meeting various application requirements.

Shining light on graphene sensors

Shining light on graphene sensors

Enlarge

A light-sensitive graphene/polymer heterostructure.

National Physical Laboratory, together with an international team of scientists, have published research showing how light can be used to control graphene's electrical properties. This advance is an important step towards developing highly sensitive graphene-based electronic devices.

Graphene is an extraordinary two-dimensional material made of a single atomic layer of carbon atoms. It is the thinnest material known to man, and yet is one of the strongest ever tested.

It has unique properties which make it a very exciting material for a huge range of applications from high-speed electronics and solar cells, to super-sensors capable of detecting single molecules of toxic gases.

It is able to act as a sensor because its entire structure is exposed to its surroundings, and it reacts to any molecules that touch its surface. This reaction causes graphene's electrical properties to alter, i.e. it senses the molecules' presence.

In their paper published in the Journal of Advanced Materials, the team show that when graphene is coated with light-sensitive polymers its unique electrical properties can be precisely controlled and therefore exploited.

The polymers also protect graphene from contamination.

Light-modified graphene chips have already been used at NPL in ultra-precision experiments to measure the quantum of the electrical resistance.

In the future similar polymers could be used to effectively 'translate' information from their surroundings and influence how graphene behaves. This effect could be exploited to develop robust reliable sensors for smoke, poisonous gases, or any targeted molecule.

Damaging graphene to create a band gap

Damaging graphene to create a band gap

(PhysOrg.com) -- "Graphene offers a lot of interesting potential applications for nanoelectronics," Florian Banhart tells PhysOrg.com, "but there is no band gap. This is a well-known problem. Without the band gap, switching as needed in electronic devices is difficult."

Banhart, a scientist at the University of Strasbourg in Strasbourg, France, believes that there is a solution to this problem. “Everyone tries to solve this problem, trying to create different properties in order to create a band gap. Our solution is doping with metal atoms attached to reconstructed defects in the graphene.”

Working with Ovidiu Cretu and Julio Rodríguez-Manzo at the University of Stasbourg, and with Arkady Krasheninnikov at the University of Helsinki, Risto Nieminen at Aalto University in Finland and Litao Sun at Southeast University in Nanjing, China, Banhart developed a method to modify the properties of graphene. The group’s work is published in Physical Review Letters: “Migration and Localization of Metal Atoms on Strained Graphene.”

“The idea is to be able to attach something to the surface of the graphene, changing some of the properties to get a band gap,” Banhart explains. By creating reconstructed defects, we can enhance the activity of the graphene and attach metal atoms firmly, possibly producing a band gap.”

Banhart and his colleagues created graphene layers that were then damaged. “We used an electron beam to damage the graphene,” Banhart says. “For this paper, we used tungsten atoms to bond to the graphene. The defects we created made it possible for the tungsten atoms to be trapped by the defects, creating stable bonds.”

Reconstructed defects increase the activity seen in graphene, making bonding to other atoms possible. “The graphene surface is normally rather inert,” Banhart explains, “but defects such as pentagonal or heptagonal rings enhance its activity. We saw enhanced chemical activity with the graphene.”

Even though Banhart and his colleagues hope that this work will lead to the eventual creation of nanoelectronic devices made with graphene, he points out that they were unable to show definitive evidence of band gap creation. “There is no evidence that we did create a band gap,” he admits. “But perhaps tungsten is not ideal. We used it because it is large, and easy to see with the electron microscope when trapped by the graphene.”

Banhart says that the tungsten has served its purpose, showing that it is possible to attach metal atoms to graphene with the help of defects on the graphene’s surface. He also points out that their recent work shows that it is possible to use this technique to modify graphene’s properties locally. “We have shown that our method might be used in the future to control graphene’s electronic properties better.”

The next step is to try to trap other atoms using defects in graphene. Banhart would also like to do more tests on the electronic properties of graphene doped in this manner. “It would be good to do more tests of graphene,” he says. “With more experiments, we should be able to begin to model the electronic structure of graphene more accurately. Once we better understand the properties of graphene, we should be able to better manipulate them so that we can get a band gap, and so that we can use them in nanoelectronic devices.”

Physicists use graphene to decode DNA

Physicists use graphene to decode DNA

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This is the cover of Physics World. Credit: Physics World

Genome sequencing will have a profound effect on our understanding of genetic biology and could usher in a day when doctor and patient are able to review individual genome sequences to fully personalise medical treatment.

As the X PRIZE FOUNDATION begins to receive nominations for its $10m prize for the first privately funded company that can accurately sequence 100 genomes in 10 days for less than $10,000 per genome, the science writer Philip Ball looks at the latest advances towards success in December Physics World's lead feature.

The baton, once firmly in the hands of chemists and biologists, has been grabbed by physicists around the world since the mid-1990s when David Deamer from the University of California, Santa Cruz imagined threading a DNA strand through a tiny pore -- reading out the chemical bases strung along the strand as it passes through. His idea was that in a salt solution, the number of dissolved ions passing through the pore would vary depending on which base was sitting in the pore.

Over the past decade, scientists have sought means to use Deamer's technique with far greater control of the pore and the movement of DNA through the pore, while also contemplating how the technique can be turned into a handy device that could be used in doctors' surgeries worldwide.

Initial thoughts were towards the use of a silicon-nitride nanopore but researchers have found the material a little too thick, meaning that more than one nucleotide -- the structural units that make up DNA -- can be in the pore at any one time.

Now, however, graphene -- one-atom thick sheets of carbon that led to this year's Nobel Prize for Physics -- is generating huge excitement as a possible DNA sequencing material following the work of three independent research groups earlier this year.

The teams -- based at the universities of Delft, Pennsylvania and Harvard -- have each drawn DNA through a nanopore drilled into graphene. As the materials is so much thinner than silicon nitride, the teams are reported to believe that graphene may be a "game changer".

Whether for the physicists it's the lure of a $10m prize, the joy of basic research, or the satisfaction of designing a technique that could revolutionize medicine, it looks like graphene -- already dubbed a "wonder material on account it being ultrathin, ultrastrong and a great electrical conductor -- could be adding one more string to its already powerful bow.