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, 2009

Surveillance and wireless data networks

Filed under: Technology — Tags: , , , , , — David Kirkpatrick @ 12:04 pm

This is a pretty ingenious use of wireless data networks. And a pretty cheap method of imaging into rooms and buildings.

From the link:

Researchers at the University of Utah say that the way radio signals vary in a wireless network can reveal the movement of people behind closed doors. Joey Wilson and Neal Patwari have developed a technique called variance-based radio tomographic imaging that processes the signals to reveal signs of movement. They’ve even tested the idea with a 34-node wireless network using the IEEE 802.15.4 wireless protocol, the protocol for personal area networks employed by home automation services such as ZigBee.

The basic idea is straightforward. The signal strength at any point in a network is the sum of all the paths the radio waves can take to get to the receiver. Any change in the volume of space through which the signals pass, for example caused by the movement of a person, makes the signal strength vary. So by “interrogating” this volume of space with many signals, picked up by multiple receivers, it is possible to build up a picture of the movement within it.

The physics arXiv blog post did offer a bit of a caveat:

How might such cheap and easy-to-configure monitoring networks be used if they become widely available? What’s to stop next door’s teenage brats from monitoring your every move, or house thieves choosing their targets on the basis that nobody is inside?

August 19, 2009

Even more invisibility cloak news

Filed under: Science — Tags: , , , , — David Kirkpatrick @ 1:35 pm

Via KurzweilAI.net — I’ve long blogged on invisibility cloak technology, but it seems there’s been a real spate of news lately. In fact I’ve already posted the original release for the news in this article.

Here’s the latest:

Active invisibility cloaks could work at many wavelengths
EE Times, Aug. 18, 2009

Active cloaking devices can use destructive interference, similar to noise-cancelling headphones, to render invisible areas up to 10 times larger than the wavelength of light being disguised and over large regions of space, University of Utah researchers have found.

The researchers predict that engineers will be able to use their method to create active invisibility cloaks that could shield submarines from sonar, planes from radar, and buildings from earthquakes.

 
Read Original Article>>

August 16, 2009

More cloaking news

Filed under: Science — Tags: , , , , — David Kirkpatrick @ 11:03 pm

This isn’t really on my typical topic of invisibility cloaks, but it is a very interesting cloaking technology.

The release:

A new cloaking method

This is not a ‘Star Trek’ or ‘Harry Potter’ story

IMAGE: Graeme Milton, a distinguished professor of mathematics at the University of Utah, is the senior author of two newly published studies outlining the numerical and theoretical basis for a new…

Click here for more information. 

SALT LAKE CITY, Aug. 17, 2009 – University of Utah mathematicians developed a new cloaking method, and it’s unlikely to lead to invisibility cloaks like those used by Harry Potter or Romulan spaceships in “Star Trek.” Instead, the new method someday might shield submarines from sonar, planes from radar, buildings from earthquakes, and oil rigs and coastal structures from tsunamis.

“We have shown that it is numerically possible to cloak objects of any shape that lie outside the cloaking devices, not just from single-frequency waves, but from actual pulses generated by a multi-frequency source,” says Graeme Milton, senior author of the research and a distinguished professor of mathematics at the University of Utah.

“It’s a brand new method of cloaking,” Milton adds. “It is two-dimensional, but we believe it can be extended easily to three dimensions, meaning real objects could be cloaked. It’s called active cloaking, which means it uses devices that actively generate electromagnetic fields rather than being composed of ‘metamaterials’ [exotic metallic substances] that passively shield objects from passing electromagnetic waves.”

Milton says his previous research involved “just cloaking clusters of small particles, but now we are able to cloak larger objects.”

IMAGE: These images are from animated computer simulations of a new method — developed by University of Utah mathematicians — for cloaking objects from waves of all sorts. While the new…

Click here for more information. 

For example, radar microwaves have wavelengths of about four inches, so Milton says the study shows it is possible to use the method to cloak from radar something 10 times wider, or 40 inches. That raises hope for cloaking larger objects. So far, the largest object cloaked from microwaves in actual experiments was an inch-wide copper cylinder.

A study demonstrating the mathematical feasibility of the new cloaking technique – active, broadband, exterior cloaking – was published online today in the journal Optics Express. A related paper was published online Aug. 14 in Physical Review Letters.

Milton conducted the studies with Fernando Guevara Vasquez and Daniel Onofrei, both of whom are assistant professors-lecturers in mathematics. The research was funded by the National Science Foundation and the University of Utah.

Cloaking: From Science Fiction to Science

Cloaking involves making an object partly or completely invisible to incoming waves – sound waves, sea waves, and seismic waves, but usually electromagnetic waves such as visible light, microwaves, infrared light, radio and TV waves.

Cloaking things from visible light long has been a staple of science fiction, from invisible Romulan Bird of Prey warships in “Star Trek” to cloaking devices in books, games, films and shows like “Harry Potter,” “Halo,” “Predator,” and “Stargate.”

In recent years, scientists devised and tested various cloaking schemes. They acknowledge practical optical cloaking for invisibility is many years away. Experiments so far have been limited to certain wavelengths such as microwaves and infrared light, and every method tried so far has limitations.

Compared with passive cloaking by metamaterials, the new method – which involves generating waves to protect or cloak an object from other waves – can cloak from a broader band of wavelengths, Milton says.

“The problem with metamaterials is that their behavior depends strongly on the frequency you are trying to cloak from,” he adds. “So it is difficult to obtain broadband cloaking. Maybe you’d be invisible to red light, but people would see you in blue light.”

Most previous research used interior cloaking, where the cloaking device envelops the cloaked object. Milton says the new method “is the first active, exterior cloaking” technique: cloaking devices emit signals and sit outside the cloaked object.

Videos Simulate How Cloaking Method Works

The new studies are numerical and theoretical, and show how the cloaking method can work. “The research simulates on a computer what you should see in an experiment,” Milton says. “We just do the math and hope other people do the experiments.”

The Physical Review Letters study demonstrates the new cloaking method at a single frequency of electromagnetic waves, while the Optics Express paper demonstrates how it can work broadband, or at a wide range of frequencies.

In Optics Express, the mathematicians demonstrate that three cloaking devices together create a “quiet zone” so that “objects placed within this region are virtually invisible” to incoming waves. Guevara Vasquez created short videos of mathematical simulations showing a pulse of electromagnetic or sound waves rolling past an object:

 

     

  • In one video, with the kite-shaped object uncloaked, the wave clearly interacts with the object, creating expanding, circular ripples like when a rock is thrown in a pond. 

     

  • In the second video, the object is surrounded by three point-like cloaking devices, each of which emits waves that only propagate a short distance. Those points and their emissions resemble purple sea urchins. As the passing waves roll by the cloaking devices, waves emitted by those devices interfere with the passing waves. As a result, the passing waves do not hit the cloaked object and there are no ripples.

 

Milton says the cloaking devices cause “destructive interference,” which occurs when two pebbles are thrown in a pond. In places where wave crests meet, the waves add up and the crests are taller. Where troughs meet, the troughs are deeper. But where crests cross troughs, the water is still because they cancel each other out.

The principle, applied to sound waves, is “sort of like noise cancelation devices you get with headphones in airplanes if you travel first class,” Milton says.

Protecting from Destructive Seismic and Tsunami Waves

“We proved mathematically that this method works when the wavelength of incoming electromagnetic radiation is large compared with the objects being cloaked, meaning it can cloak very small objects,” Milton says. “It also can cloak larger objects.”

Because visible light has tiny wavelengths, only microscopic objects could be made invisible by the new method.

“The cloaking device would have to generate fields that have very small wavelengths,” Milton says. “It is very difficult to build antennas the size of light waves. We’re so far from cloaking real-sized objects to visible light that it’s incredible.”

But imagine incoming waves as water waves, and envision breakwater cloaking devices that would generate waves to create a quiet zone that would protect oil rigs or specific coastal structures against incoming tsunami waves. Or imagine cloaking devices around buildings to generate vibrations to neutralize incoming seismic waves.

“Our method may have application to water waves, sound and microwaves [radar],” including shielding submarines and planes from sonar and radar, respectively, and protecting structures from seismic waves during earthquakes and water waves during tsunamis, Milton says. All those waves have wavelengths much larger than those of visible light, so the possible applications should be easier to develop.

“It would be wonderful if you could cloak buildings against earthquakes,” Milton says. “That’s on the borderline of what’s possible.”

The new method’s main disadvantage “is that it appears you must know in advance everything about the incoming wave,” including when the pulse begins, and the frequencies and amplitudes of the waves within the pulse, Milton says. That might require placement of numerous sensors to detect incoming seismic waves or tsunamis.

“Even though cloaking from light is probably impossible, it’s a fascinating subject, and there is beautiful mathematics behind it,” Milton says. “The whole area has exploded. So even if it’s not going to result in a ‘Harry Potter’ cloak, it will have spinoffs in other directions,” not only in protecting objects from waves of various sorts, but “for building new types of antennas, being able to see things on a molecular scale. It’s sort of a renaissance in classical science, with new ideas popping up all the time.”

 

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A video showing an object uncloaked and cloaked as a wave passes may be seen and downloaded from http://vimeo.com/6092319 or as separate videos from http://vimeo.com/5406253 (no cloaking) and http://vimeo.com/5406236 (with cloaking).

University of Utah Public Relations