David Kirkpatrick

July 2, 2010

Graphene 2.0

Yep, I’m going to be lazy just cop part of the title of this release, well really more of an article than an out-and-out press release. Sounds like a pretty cool graphene transistor with potential real world applications.

The release:

Graphene 2.0: a new approach to making a unique material

June 30, 2010

Since its discovery, graphene—an unusual and versatile substance composed of a single-layer crystal lattice of carbon atoms—has caused much excitement in the scientific community. Now, Nongjian (NJ) Tao, a researcher at the Biodesign Institute at Arizona State University has hit on a new way of making graphene, maximizing the material’s enormous potential, particularly for use in high-speed electronic devices.

Along with collaborators from Germany’s Max Planck Institute, the Department of Materials Science and Engineering, University of Utah, and Tsinghua University, Beijing, Tao created a graphene transistor composed of 13 benzene rings.

The molecule, known as a coronene, shows an improved electronic band gap, a property which may help to overcome one of the central obstacles to applying graphene technology for electronics. Tao is the director of the Biodesign Institute’s Center for Bioelectronics and Biosensors and electrical engineering professor in the Ira A. Fulton Schools of Engineering. The group’s work appears in the June 29 advanced online issue of Nature Communications.

Eventually, graphene components may find their way into a broad array of products, from lasers to ultra-fast computer chips; ultracapacitors with unprecedented storage capabilities; tools for microbial detection and diagnosis; photovoltaic cells; quantum computing applications and many others.

As the name suggests, graphene is closely related to graphite. Each time a pencil is drawn across a page, tiny fragments of graphene are shed. When properly magnified, the substance resembles an atomic-scale chicken wire. Sheets of the material possess exceptional electronic and optical properties, making it highly attractive for varied applications.

“Graphene is an amazing material, made of carbon atoms connected in a honeycomb structure,” Tao says, pointing to graphene’s huge electrical mobility—the ease with which electrons can flow through the material. Such high mobility is a critical parameter in determining the speed of components like transistors.

Producing usable amounts of graphene however, can be tricky. Until now, two methods have been favored, one in which single layer graphene is peeled from a multilayer sheet of graphite, using adhesive tape and the other, in which crystals of graphene are grown on a substrate, such as silicon carbide.

In each case, an intrinsic property of graphene must be overcome for the material to be suitable for a transistor. As Tao explains, “a transistor is basically a switch—you turn it on or off. A graphene transistor is very fast but the on/off ratio is very tiny. ” This is due to the fact that the space between the valence and conduction bands of the material—or band gap as it is known—is zero for graphene.

In order to enlarge the band gap and improve the on/off ratio of the material, larger sheets of graphene may be cut down to nanoscale sizes. This has the effect of opening the gap between valence and conductance bands and improving the on/off ratio, though such size reduction comes at a cost. The process is laborious and tends to introduce irregularities in shape and impurities in chemical composition, which somewhat degrade the electrical properties of the graphene.  “This may not really be a viable solution for mass production,” Tao observes.

Rather than a top down approach in which sheets of graphene are reduced to a suitable size to act as transistors, Tao’s approach is bottom up—building up the graphene, molecular piece by piece. To do this, Tao relies on the chemical synthesis of benzene rings, hexagonal structures, each formed from 6 carbon atoms. “Benzene is usually an insulating material, ” Tao says. But as more such rings are joined together, the material’s behavior becomes more like a semiconductor.

Using this process, the group was able to synthesize a coronene molecule, consisting of 13 benzene rings arranged in a well defined shape. The molecule was then fitted on either side with linker groups—chemical binders that allow the molecule to be attached to electrodes, forming a nanoscale circuit. An electrical potential was then passed through the molecule and the behavior, observed. The new structure displayed transistor properties, showing reversible on and off switches.

Tao points out that the process of chemical synthesis permits the fine-tuning of structures in terms of ideal size, shape and geometric structure, making it advantageous for commercial mass production. Graphene can also be made free of defects and impurities, thereby reducing electrical scattering and providing material with maximum mobility and carrier velocity, ideal for high-speed electronics.

In conventional devices, resistance is proportional to temperature, but in the graphene transistors by Tao et al., electron mobility is due to quantum tunneling, and remains temperature independent—a signature of coherent process.

The group believes they will be able to enlarge the graphene structures through chemical synthesis to perhaps hundreds of rings, while still maintaining a sufficient band gap to enable switching behavior. The research opens many possibilities for the future commercialization of this uncommon material, and its use in a new generation of ultra high-speed electronics.

Written by Richard Harth
Biodesign Institute Science Writer

October 2, 2008

The time has come for flexi display tech

I’ve blogged on flexible display technology before (such as here in the middle of three news bits) and this is some exciting news from researchers at Sony and the Max Planck Institute.

The release:

Flexi display technology is now

Rigid television screens, bulky laptops and still image posters are to be a thing of the past as new research, published today, Thursday, 2 October, in the New Journal of Physics, heralds the beginning of a technological revolution for screen displays.

Screen display technology is taking a significant step forward as researchers from Sony and the Max Planck Institute demonstrate the possibility of bendable optically assessed organic light emitting displays for the first time, based on red or IR-A light upconversion.

The paper, ‘Annihilation Assisted Upconversion: All-Organic, Flexible and Transparent Multicolour Display’, makes feasible the design of computers that can be folded up and put in your pocket, the mass-production of moving image posters for display advertising, televisions which can be bended to view or, even, newspaper display technology which allows readers to upload daily news to an easy-to-carry display contraption.

All organic, upconversion multicolour displays have significant advantages when compared to the traditional technology used for projection displays and televisions. Namely UC displays are:

 

  • All-organic − transparent and flexible
  • Ultra low excitation intensity (red or IR)– less than 15 mWcm-2
  • Emissive display – no speckles
  • Coherent or non-coherent excitation allowed
  • High efficiency – at the moment ca. 6 %
  • Fast response times – ca. 1 µs up to 500 µs on request (LCDs have ms)
  • Almost unlimited viewing angle – up to the total internal reflection angle
  • Tailoring of emitted colours realised even when using the same excitation source
  • Multilayer Displays
  • Size limited only by the size of the substrates

 

With LCD-based projection displays, the liquid crystal acts as a filter for the light being shone through so when coherent excitation is used (e.g. laser diodes) the problems with speckles are serious. For this organic emissive UC displays, the organic molecules themselves emit non-coherent light in 4 (all directions) to produce an image.

Sony announced the development of flexible OLED display screens in 2006 but glitches such as size and resolution limitations, and the difficulty of structuring the organic compounds so as not to be distorted when bent, have stopped designs coming to market. This new technology for optically excited organic emissive displays hasn’t got this problem and gives further opportunities for new applications.

The research published today concludes through the use of a new structure and unique combinations for the organic compounds within viscous polymeric matrix, that there need be no size or resolution limitations for the new screens.

The researchers conclude, “To the best of our knowledge we demonstrate for the first time a versatile colour all-organic and transparent UC-display. The reported displays are also flexible and have excellent brightness.”

 

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There is a small film of a prototype screen in action available.

Update — Technology Review covers this story here.

September 10, 2008

Nanonets improve solar, too

Filed under: Science, Technology — Tags: , , , , , — David Kirkpatrick @ 8:20 am

I’ve blogged on nanonets and how they improve electronics and energy applications. Here’s a Technology Review story with more detail on how nanonets improve solar energy.

And as a bonus, with picutures!

From the second link:

One problem with solar cells is that they only produce electricity during the day. A promising way to use the sun’s energy more efficiently is to enlist it to split water into hydrogen gas that can be stored and then employed at any time, day or night. A cheap new nanostructured material could prove an efficient catalyst for performing this reaction. Called a nanonet because of its two-dimensional branching structure, the material is made up of a compound that has been demonstrated to enable the water-splitting reaction. Because of its high surface area, the nanonet enhances this reaction.

Researchers led by Dunwei Wang, a chemist at Boston College, grew the nanonets, creating structures made up of branching wires of titanium and silicon. Last year, researchers at the Max Planck Institute, in Germany, showed that titanium disilicide, which absorbs a broad spectrum of visible light, splits water into hydrogen and oxygen–and can store the hydrogen, which it absorbs or releases depending on the temperature. Other semiconducting materials have been tested as water-splitting catalysts but have proved unstable.

Nanonets, structures made up of branching titanium and silicon wires, are flat yet have a high surface area, making them more efficient at using solar energy to split water into oxygen and hydrogen fuel. The top image shows a nanonet magnified 50,000 times. At bottom, a flexible nanonet rolls up when poked by the tip of a scanning tunneling microscope. Both images were taken with a tunneling electron microscope.

Net reaction: Nanonets, structures made up of branching titanium and silicon wires, are flat yet have a high surface area, making them more efficient at using solar energy to split water into oxygen and hydrogen fuel. The top image shows a nanonet magnified 50,000 times. At bottom, a flexible nanonet rolls up when poked by the tip of a scanning tunneling microscope. Both images were taken with a tunneling electron microscope.