Flat-packed particles

Graphene, a Manchester University discovery, is a material comprising sheets of carbon just one atom thick; graphene is like a single layer of graphite. However, it was the discovery that it has some peculiar electronic properties because of the existence of massless quasiparticles that has led to an explosion of interest in this material. Some researchers suggest that ultimately it will become the material that gives us a post-silicon world in computing.

Now, US scientists have made the first observation of the energy bands of complex particles within graphene known as plasmarons. This small step is an important one in understanding graphene and using it to develop devices for that future of ultrafast chemical computers.

At Berkeley Lab’s Advanced Light Source, an international team led by Aaron Bostwick and Eli Rotenberg have shown that these composite plasmaron particles are vital in generating graphene’s unique properties. “Graphene’s true electronic structure can’t be understood without understanding the many complex interactions of electrons with other particles.”

The electric charge carriers in graphene are negative electrons and positive holes, which in turn are affected by plasmons, oscillations in the density of the material that travel like sound waves through a sea of electrons. A plasmaron is “simply” a charge carrier coupled to a plasmon. “Although plasmarons were proposed theoretically in the late 1960s, and indirect evidence for them has been found, our work is the first observation of their distinct energy bands in graphene, or indeed in any material,” Rotenberg says. The team reported details of their findings in the journal Science in May.

Top: graphene structure. Bottom: a theoretical model of plasmaron interactions in graphene, sheets of carbon one atom thick.

The relationships between charge carriers, plasmons, and plasmarons will be important in the development of plasmonics, the architecture analogous to electronics in conventional silicon semiconductor circuitry. An important aspect of studying these relationships is to produce flat graphene sheets; graphene is usually rumpled like unmade bed linen. “One of the best ways to grow a flat sheet of graphene is by heating a crystal of silicon carbide,” Rotenberg explains, “and it happens that our German colleagues Thomas Seyller from the University of Erlangen and Karsten Horn from the Fritz Haber Institute in Berlin are experts at working with silicon carbide. As the silicon recedes from the surface it leaves a single carbon layer.”

With flat graphene sheets in hand, the team used a beam of low-energy, or soft, X-rays to analyse the materials. The resulting data provided them with an image of the electronic bands created by the electrons themselves. Even from the initial experiments, the team suspected graphene’s behaviour was more complicated than simple theory would suggest and seemed to hint at the existence of bare electrons. Since bare electrons cannot exist, the researchers postulated the fuzziness in their image was due to charge carriers emitting plasmons. Additional experiments with graphene sheets isolated from their support material revealed that electrons detached by the X-rays can leave behind either an ordinary hole or a hole bound to a plasmon – a plasmaron, explains Rotenberg.

“By their nature, plasmons couple strongly to photons, which promises new ways for manipulating light in nanostructures, giving rise to the field of plasmonics,” Rotenberg says. “Now we know that plasmons couple strongly to the charge carriers in graphene, which suggests that graphene may have an important role to play in the merging fields of electronics, photonics, and plasmonics on the nanoscale.”

Links

Science, 2010, 328, 999-1002
Eli Rotenberg homepage

Carbon chipped

An international team of researchers has developed a new magnetic carbon material that not only acts as a semiconductor but is also magnetic and could help scientists develop the next generation of microelectronic devices.

The new carbon material is based on graphene, which resembles graphite, the form of carbon found in pencil “lead”, but which exists as single sheet-like layers resembling nanoscopic chicken wire fencing. Graphene was first created by scientists in Manchester five years ago and is not only 200 times stronger than steel but because its electrons are highly mobile it has unique electro-optical properties. As such, some researchers think that graphene is the natural successor to silicon and could lead to the advent of spintronic devices that exploit electron spin and charge in computer memory and data processing.

Black balls are the carbon atoms in the sheet, white balls are hydrogen atoms (Credit: Puru Jena/VCU)

Now, researchers from the Virginia Commonwealth University, USA, Peking University in Beijing, China, the Chinese Academy of Science in Shanghai, and Tohoku University in Sedai, Japan have used computer modelling to design a chemical cousin of graphene, which they call graphone. Experiments with the new material confirm the electromagnetic properties predicted by the computer models.

The team points out that while the properties of graphene can be modified relatively easily by introducing “defects” into its structure or by saturating it with hydrogen atoms, it has not proven easy to make it magnetic.

“The new material we are predicting – graphone – makes graphene magnetic simply by controlling how much hydrogen is put on graphene,” explains VCU’s Puru Jena. “One of the important impacts of this research is that semi-hydrogenation provides us a very unique way to tailor magnetism,” adds team member Qiang Sun, “The resulting ferromagnetic graphone sheet will have unprecedented possibilities for the applications of graphene-based materials.”

The team explains that graphene undergoes a transition from its original “metallic” state to semiconductor when all the carbon valencies are fully hydrogenated, to make graphane. However, density functional theory predicted that half hydrogenation (to make graphone) would result in a ferromagnetic semiconductor with a small indirect gap. This they confirmed experimentally.

“From graphene to graphane and to graphone, the system evolves from metallic to semiconducting and from nonmagnetic to magnetic. Hydrogenation provides a novel way to tune the properties with unprecedented potentials for applications,” the team says.

Further Reading
Nano Lett, 2009, in press

Chips are down

Graphene is a modified form of the all-carbon pencil lead material graphite and is being touted as the material of choice for a future generation of computer chips to augment, or even usurp, silicon. Now, three research teams have devised new approaches to handling graphene that could accelerate development of this material.

Carbon has several allotropes – same element, different forms. Graphite is the stuff of pencil lead and exists as layer upon layer of hexagonally patterned chicken wire type sheets with a carbon at each vertex. Diamond is the hardest known materials and exists as a robust tetrahedrally bonded network of carbon atoms. Fullerenes and nanotubes are tiny spheres, spheroids, and tubes. Amorphous carbon, which has a mixture of the trivalent and tetravalent bonded carbon atoms. Graphene is akin to single layers of graphite.

Nitin P. Padture

Nitin P. Padture

Andre Geim and colleagues at The University of Manchester and colleagues focused on experimental measurements of the intriguing electronic properties of graphene after theoreticians had predicted them. Now, research teams around the globe are further investigating this intriguing substance. However, processing graphene is not without limitations. As such, various efforts have focused on ways to simplify the handling of the material.

Rod Ruoff and his colleagues at the University of Texas at Austin have found a way to disperse chemically modified graphene in a wide variety of organic solvents. This could open the door to developing graphene in conductive films, polymer composites, ultracapacitors, batteries, paints, inks and plastic electronics, the team says.

Rod Ruoff

Rod Ruoff

By using ‘solubility parameters’ ubiquitously applied by industry to determine the solvents most likely to dissolve certain materials or to create good colloids, we have developed a set of solubility parameters for chemically modified graphenes, explains Ruoff. We believe that this approach will have exceptional utility for technology transition in use of colloidal suspensions of graphene sheets.

Tomás Palacios

Tomás Palacios

In parallel, but unconnected work, a team at Ohio State University, led by Nitin Padture are developing a technique for mass producing computer chips made from graphene. Graphene has huge potential, it’s been dubbed the new silicon, says Padture, but there hasn’t been a good process for high-throughput manufacturing it into chips.

Graphene’s chickenwire structure

Graphene’s chickenwire structure

He and his colleagues have found a way to mesh the graphene fabrication process with standard microelectronics manufacturing methods. In their first series of experiments, the team stamped high-definition features just ten graphene layers thick on to a silicon oxide substrate, making this a potential mass-production method.

A graphene frequency multiplier (Photo by Donna Coveney)

A graphene frequency multiplier (Photo by Donna Coveney)

In other work to be published in the May issue of Electron Device Letters, MIT researchers, led by Tomás Palacios, have built an experimental graphene chip known as a frequency multiplier. Frequency multipliers are widely used in telecommunications and computing applications. However, current technology suffers from noise interference that requires energy-intensive filtering. The graphene frequency multiplier system has but a single transistor and so, these researchers say, efficiently produces a very clean output that needs no filtering.

Further reading

Nano. Lett., 2009, in press
http://dx.doi.org/10.1021/nl803798y

Adv Mater, 2009, 21, 1243-1246
http://dx.doi.org/10.1002/adma.200802417

Nanoscience and Technology Lab
http://bucky-central.me.utexas.edu/

Nitin P. Padture homepage
http://www.matsceng.ohio-state.edu/faculty/padture/padturewebpage/

Tomás Palacios homepage
http://web.mit.edu/tpalacios/

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