Posted on 03/24/2008 12:27:09 PM PDT by Red Badger
Graphene is a single planar sheet of sp2-bonded carbon atoms that are densely packed in a honeycomb crystal lattice. It can also be viewed as an atomic-scale chicken wire made of carbon atoms and their bonds. The carbon-carbon bond length in graphene is approximately 1.42 Å. From a physicist point of view, graphene is the basic structural element for all other graphitic materials including graphite, carbon nanotubes and fullerenes. For a chemist, graphene is an infinitely large aromatic molecule, an extension of a family of flat polycyclic aromatic hydrocarbons called graphenes.
University of Maryland physicists have shown that in graphene the intrinsic limit to the mobility, a measure of how well a material conducts electricity, is higher than any other known material at room temperature. Graphene, a single-atom-thick sheet of graphite, is a new material which combines aspects of semiconductors and metals.
Their results, published online in the journal Nature Nanotechnology, indicate that graphene holds great promise for replacing conventional semiconductor materials such as silicon in applications ranging from high-speed computer chips to biochemical sensors.
A team of researchers led by physics professor Michael S. Fuhrer of the university's Center for Nanophysics and Advanced Materials, and the Maryland NanoCenter said the findings are the first measurement of the effect of thermal vibrations on the conduction of electrons in graphene, and show that thermal vibrations have an extraordinarily small effect on the electrons in graphene.
In any material, the energy associated with the temperature of the material causes the atoms of the material to vibrate in place. As electrons travel through the material, they can bounce off these vibrating atoms, giving rise to electrical resistance. This electrical resistance is "intrinsic" to the material: it cannot be eliminated unless the material is cooled to absolute zero temperature, and hence sets the upper limit to how well a material can conduct electricity.
In graphene, the vibrating atoms at room temperature produce a resistivity of about 1.0 microOhm-cm (resistivity is a specific measure of resistance; the resistance of a piece material is its resistivity times its length and divided by its cross-sectional area). This is about 35 percent less than the resistivity of copper, the lowest resistivity material known at room temperature.
"Other extrinsic sources in today's fairly dirty graphene samples add some extra resistivity to graphene," explained Fuhrer, "so the overall resistivity isn't quite as low as copper's at room temperature yet. However, graphene has far fewer electrons than copper, so in graphene the electrical current is carried by only a few electrons moving much faster than the electrons in copper."
n semiconductors, a different measure, mobility, is used to quantify how fast electrons move. The limit to mobility of electrons in graphene is set by thermal vibration of the atoms and is about 200,000 cm2/Vs at room temperature, compared to about 1,400 cm2/Vs in silicon, and 77,000 cm2/Vs in indium antimonide, the highest mobility conventional semiconductor known.
"Interestingly, in semiconducting carbon nanotubes, which may be thought of as graphene rolled into a cylinder, we've shown that the mobility at room temperature is over 100,000 cm2/Vs" said Fuhrer (T. Dürkop, S. A. Getty, Enrique Cobas, and M. S. Fuhrer, Nano Letters 4, 35 (2004)).
Mobility determines the speed at which an electronic device (for instance, a field-effect transistor, which forms the basis of modern computer chips) can turn on and off. The very high mobility makes graphene promising for applications in which transistors much switch extremely fast, such as in processing extremely high frequency signals.
Mobility can also be expressed as the conductivity of a material per electronic charge carrier, and so high mobility is also advantageous for chemical or bio-chemical sensing applications in which a charge signal from, for instance, a molecule adsorbed on the device, is translated into an electrical signal by changing the conductivity of the device.
Graphene is therefore a very promising material for chemical and bio-chemical sensing applications. The low resitivity and extremely thin nature of graphene also promises applications in thin, mechanically tough, electrically conducting, transparent films. Such films are sorely needed in a variety of electronics applications from touch screens to photovoltaic cells.
Fuhrer and co-workers showed that although the room temperature limit of mobility in graphene is as high as 200,000 cm2/Vs, in present-day samples the actual mobility is lower, around 10,000 cm2/Vs, leaving significant room for improvement. Because graphene is only one atom thick, current samples must sit on a substrate, in this case silicon dioxide.
Trapped electrical charges in the silicon dioxide (a sort of atomic-scale dirt) can affect the electrons in graphene and reduce the mobility. Also, vibrations of the silicon dioxide atoms themselves can also have an effect on the graphene which is stronger than the effect of graphene’s own atomic vibrations. This so-called “remote interfacial phonon scattering” effect is only a small correction to the mobility in a silicon transistor, but because the phonons in graphene itself are so ineffective at scattering electrons, this effect becomes very important in graphene.
“We believe that this work points out the importance of these extrinsic effects, and creates a roadmap for finding better substrates for future graphene devices in order to reduce the effects of charged impurity scattering and remote interfacial phonon scattering.” Fuhrer said.
Source: University of Maryland
What they needed it for was a collection of calutrons, to be built at Oak Ridge.
The calutron was the idea of Ernest Lawrence (Lawrence Livermore and Berkeley laboratories are named after him.)
Around 1930, Lawrence had invented the cyclotron; the calutron, and an adaptation of his machine, was capable of separating U235 from U238, one atom at a time! Essentially, it was a mass spectrometer.
The machines needed magnets, really BIG magnets, and the magnets had to be as powerful and efficient as possible. Thus, the silver from Fort Knox.
Ridiculously expensive to build and to operate, the calutron
was one of several approaches to uranium separation, which were tried all at once.
After the war, they sent the silver back to Fort Knox.
Bump.
Next on the agenda: Terahurtz processors..
We’re computin’ now, baby!
There’s an article in the new “Scientific American” about graphene.
IBM Scientists “Quiet” Unruly Electrons in Atomic Layers of Graphite
Marketwatch | March 6, 2008 | Michael Loughran IBM
Posted on 03/06/2008 9:02:41 AM PST by Ernest_at_the_Beach
http://www.freerepublic.com/focus/f-chat/1981391/posts
Getting more from Moore’s Law
BBC News | Tuesday, November 13, 2007 | Jonathan Fildes
Posted on 12/06/2007 10:57:44 AM PST by SunkenCiv
http://www.freerepublic.com/focus/f-chat/1935727/posts
The Charge of the Ultra - Capacitors (Sexy capacitor pics!)
Spectrum / IEEE | Nov 2007 | Joel Schindall
Posted on 11/05/2007 12:14:01 PM PST by Uncledave
http://www.freerepublic.com/focus/f-news/1921279/posts
It’s Super Paper!
ScienceNOW Daily News | 25 July 2007 | Phil Berardelli
Posted on 07/28/2007 3:47:54 AM EDT by neverdem
http://www.freerepublic.com/focus/f-news/1872836/posts
thanks, bfl
Scientists slice graphite into atom-thick sheets
The Register (U.K.) | October 21, 2004 | Lucy Sherriff
Posted on 10/22/2004 3:09:03 AM EDT by Stoat
http://www.freerepublic.com/focus/f-news/1253131/posts
Ah, then, perhaps, we’ll have a processor that can run Vista...
Will need a Terabyte of RAM in ADDITION to the Treahurtz processor to run Vista. BLOATED code
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