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/

Suggested searches

carbon

Turning up the heat on quantum mechanics

Scientists have made a startling prediction about the quantum world that seems to show that simply taking the temperature of certain types of quantum systems at frequent intervals causes such systems to break one of the hard and fast rules of thermodynamics.

Anyone who has dabbled in quantum mechanics will know just how slippery is the atomic and sub-atomic world of probability wave-functions where particles eddy and swirl like waves.

Gershon Kurizki

Gershon Kurizki

One of the underlying rules of the quantum world is the Time-Energy Uncertainty Principle. Wrapped up in this apparently simple phrase is the notion that it is impossible to know both the precise duration of any process and its exact energy cost in an atomic or subatomic particle with 100 % certainty; the very act of observing one or the other somehow disturbing the counterpart property.

The quantum world is spooky, to say the least.

Quantum systems run hot and cold when you take their temperature regularly (Credit: Gershon Kurizki)

Quantum systems run hot and cold when you take their temperature regularly (Credit: Gershon Kurizki)

Now, the laws of thermodynamics are apparently irrefutable, after all they allow sceptics to see straight through the claims of those inventors who claim perpetual motion machines, they allow us to build power stations, and ultimately they will take us to the ends of the universe.

One law reveals that the interaction between a large heat source and a cluster of smaller systems will, on average, move progressively towards thermal equilibrium – hot moves to cold to even out the temperature, in other words; this is the so-called zero’th law of thermodynamics. But, it ain’t necessarily so in the quantum world claim Weizmann chemists Gershon Kurizki, Noam Erez and Goren Gordon of the Weizmann Institute in Rehovot, Israel, working with Mathias Nest of Potsdam University, Germany. They have shown that an ensemble of quantum systems in thermal contact with a large heat source could buck this thermodynamic trend.

Their predictions suggest that such a quantum ensemble could actually heat up even if it is hotter than a neighbouring large heat source or if it is colder, it could get colder still, but only under certain conditions. The scientists showed that if the energy of these systems is measured repeatedly, both systems and large heat source will undergo a temperature increase or decrease, and this change depends only on the rate of measurement, not on the results of the measurements themselves.

In the classical world, a thermometer does not interfere with the laws of thermodynamics no matter how hot or cold a system nor how often the thermometer is read, but taking the temperature of a quantum system somehow decouples it from the neighbouring heat source. This decoupling, followed by recoupling of the two when measurement ceases, introduces energy (at the expense of the measuring apparatus) into the systems and the heat source alike, and so heats them up. Depending on whether the measurements are repeated at short or long intervals, it should be possible to heat up or cool down the systems.

The predicted effects may be the key to developing novel heating and cooling schemes for microscopic solid-state devices, such as quantum computer chips or in allowing ultrafast temperature control for fast optical measurements in the chemistry laboratory.

Further reading

Nature, 2008, 452, 724-727
http://dx.doi.org/10.1038/nature06873

Weizmann Institute Quantum Optics Group homepage
http://www.weizmann.ac.il/chemphys/gershon/

Mathias Nest homepage
http://tcb16.chem.uni-potsdam.de/nest/

Suggested searches

thermodynamics
quantum mechanics

Want optical chips with that?

Ever-smaller and ever-faster microelectronics devices with increased storage space, more communications and other functions, and much-reduced battery usage, are part of the incentive behind research into photonic crystals. These materials are bringing us closer to a technologically viable optical transistor that will form the building blocks of future optoelectronics that use photons instead of electrons to process information.

The optical properties of photonic crystals vary in a regular pattern on a scale of hundreds of nanometres. This physical structure means that light entering a photonic crystal can be controlled. For instance, a photonic crystal can transmit light of one particular wavelength, and block all others.

Rana Biswas

Rana Biswas

The simplest material of this kind has a layered structure, like a film of oil on water. Such one-dimensional structures are used as mirrors, non-reflective coatings, and paints whose colours change with the viewing angle. While nature does not exactly abound with photonic crystals, the natural gemstone, opal, is a photonic crystal, which is what gives it its unique shifting and shimmering colours. Synthetic photonic crystals have been on the science agenda since the nineteenth century, but it is only with the advent of modern fabrication techniques that designer 3D photonic crystals have become attainable.

Optical computers aside, there is a second thread woven into the fabric of photonic crystal research – telecommunications.

Now, researchers at the US Department of Energy’s Ames Laboratory have come up with what might be the perfect way to sort and distribute vast quantities of data through optical fibres. The new technology is based on a filter constructed from a three-dimensional photonic crystal and could allow multiple wavelength channels to be carried along the same stretch of optical fibre without loss and without error. The so-called add-drop filter could ultimately give us the all-optical transmission links that require no electronic components along the route.

There are up to 160 wavelength channels travelling through an optical fibre at the same time, explains Ames physicist Rana Biswas, That means a lot of dialogue is going on simultaneously. He adds that as data is carried along the fibre it is necessary to drop off individual wavelength channels at different points.

When the data being transported in multiple frequency channels over an optical fibre comes to a receiving station, you want to be able to pick off just one of those frequencies and send it to an individual end user, explains Biswas, That’s where these 3D photonic crystals come into play. The same filter technology will also allow optimal use of the fibre’s bandwidth.

The idea of add-drop filters was first conceived in the 1990s, but work focused on 2D photonic crystals until now. The Ames team created a 3D photonic crystal device that contains an entrance waveguide and an exit waveguide for channelling light, which means there is none of the light intensity loss seen with 2D photonics.

There is still at least one hurdle to jump before the 3D add-drop filter can be used in fibre optic communications and that is to scale down the device to the wavelengths of light used in Internet communications – 1.5 micrometres. That remains a big challenge confesses Biswas.

Further reading

Rana Biswas homepage
http://cmp.ameslab.gov/personnel/biswas/bio.html

Feature on photonic crystals from ICT Results
http://cordis.europa.eu/ictresults/index.cfm/section/news/tpl/article/id/89575

Suggested searches

photonic crystals
fibre optics
optical fibres