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Promise and challenge on the road to practical graphene electronics

Three recent news items illuminate the promise and challenge on the road to practical graphene electronics. In the first, IBM announced a new speed record for an experimental graphene transistor that looks exceptionally promising for processing analog signals. It was produced on a “diamond-like carbon” substrate using standard semiconductor manufacturing processes. From “IBM shows smallest, fastest graphene processor“, by Agam Shah:

… The transistor has a cut-off frequency of 155GHz, making it faster and more capable than the 100GHz graphene transistor shown by IBM in February last year, said Yu-Ming Lin, an IBM researcher.

The research also shows that high-performance, graphene-based transistors can be produced at low cost using standard semiconductor manufacturing processes, Lin said. That could pave the way for commercial production of graphene chips, though Lin could not say when manufacturing of such chips would begin.

Commercialized graphene transistors will provide a performance boost in applications related to wireless communications, networking, radar and imaging, said Phaedon Avouris [winner for experimental work, 1999 Feynman Prize in Nanotechnology], an IBM fellow. Graphene is a single-atom-thick layer of carbon atoms structured in a hexagonal honeycomb form.

The transistor was developed as part of research IBM is conducting for the U.S. Department of Defense’s DARPA (Defense Advanced Research Projects Agency) program to develop high-performance RF (radio frequency) transistors. Avouris said the military has considerable interest in graphene transistors.

The flow of electrons is faster on graphene transistors than conventional transistors, which enables faster data transfers between chips, Lin said. That makes it promising technology for applications such as networking that require communications at fast speeds and high frequencies.

Graphene transistors may be able compute faster than conventional transistors, but are not ideal for PCs yet, Lin said. Because of the lack of energy gap in natural graphene, graphene transistors do not possess the on-off ratio required for digital switching operations, which makes conventional processors better at processing discrete digital signals.

By contrast, the continuous energy flow makes graphene better at processing analog signals, Lin said. Graphene’s high electron speed allows for faster processing of applications in analog electronics where such a high on-off ratio is not needed. …

In a second development, researchers have found that atomic vacancies in graphene can give rise to magnetic properties that were entirely unexpected because carbon has no d or f electrons. PhysOrg.com points to this University of Maryland news release “UMD Scientists Make Magnetic New Graphene Discovery“:

University of Maryland researchers have discovered a way to control magnetic properties of graphene that could lead to powerful new applications in magnetic storage and magnetic random access memory.

The finding by a team of Maryland researchers, led by Physics Professor Michael S. Fuhrer of the UMD Center for Nanophysics and Advanced Materials is the latest of many amazing properties discovered for graphene. …

In their new graphene discovery, Fuhrer and his University of Maryland colleagues have found that missing atoms in graphene, called vacancies, act as tiny magnets — they have a “magnetic moment.” Moreover, these magnetic moments interact strongly with the electrons in graphene which carry electrical currents, giving rise to a significant extra electrical resistance at low temperature, known as the Kondo effect. The results appear in the paper “Tunable Kondo effect in graphene with defects” published this month in Nature Physics [abstract]. …

Fuhrer thinks that if vacancies in graphene could be arranged in just the right way, ferromagnetism could result. “Individual magnetic moments can be coupled together through the Kondo effect, forcing them all to line up in the same direction,” he said.

“The result would be a ferromagnet, like iron, but instead made only of carbon. Magnetism in graphene could lead to new types of nanoscale sensors of magnetic fields. And, when coupled with graphene’s tremendous electrical properties, magnetism in graphene could also have interesting applications in the area of spintronics, which uses the magnetic moment of the electron, instead of its electric charge, to represent the information in a computer.

“This opens the possibility of ‘defect engineering’ in graphene – plucking out atoms in the right places to design the magnetic properties you want,” said Fuhrer.

The third item reports that graphene transistors may solve one of the major problems associated with silicon electronics—graphene transistors could cool themselves rather than have to spend energy dissipating heat with fans or water cooling. From “Graphene transistors could cool themselves“, by Isaac Leung:

Researchers with the University of Illinois have found graphene transistors have a nanoscale cooling effect which reduces their temperature.

Mechanical science and engineering professor William King and electrical and computer engineering professor Eric Pop led the team, which published the findings in the 3 April advance online edition of the journal Nature Nanotechnology [abstract]. …

The research team used an atomic force microscope tip as a temperature probe to make the first nanometer-scale temperature measurements of a working graphene transistor.

The measurements revealed that thermoelectric cooling effects can be stronger at the areas where the graphene touches the metal contacts, and this effect overpowers resistive heating, actually lowering the temperature of the transistor.

This self-cooling effect means that graphene-based electronics could require little or no cooling, allowing even greater energy efficiency and increasing graphene’s attractiveness as a silicon replacement.

The University of Illinois News Bureau adds a few additional details. From “Self-cooling observed in graphene electronics“, by Liz Ahlberg:

… The measurements revealed surprising temperature phenomena at the points where the graphene transistor touches the metal connections. They found that thermoelectric cooling effects can be stronger at graphene contacts than resistive heating, actually lowering the temperature of the transistor.

“In silicon and most materials, the electronic heating is much larger than the self-cooling,” King said. “However, we found that in these graphene transistors, there are regions where the thermoelectric cooling can be larger than the resistive heating, which allows these devices to cool themselves. This self-cooling has not previously been seen for graphene devices.” …

“Graphene electronics are still in their infancy; however, our measurements and simulations project that thermoelectric effects will become enhanced as graphene transistor technology and contacts improve ” said Pop, who is also affiliated with the Beckman Institute for Advanced Science, and the Micro and Nanotechnology Laboratory at the U. of I. …

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