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Graphene yields secrets to its extraordinary properties
May 15, 2009Applying innovative measurement techniques, researchers from the Georgia Institute of Technology and the National Institute of Standards and Technology (NIST) have directly measured the unusual energy spectrum of graphene, a technologically promising, two-dimensional form of carbon that has tantalized and puzzled scientists since its discovery in 2004.
Published in this week's issue of Science,* their work adds new detail to help explain the unusual physical phenomena and properties associated with graphene, a single layer of carbon atoms arrayed in a repeating, honeycomb-like arrangement.
Graphene's exotic behaviors present intriguing prospects for future technologies, including high-speed, graphene-based electronics that might replace today's silicon-based integrated circuits and other devices. Even at room temperature, electrons in graphene are more than 100 times more mobile than in silicon.
Graphene apparently owes this enhanced mobility to the curious fact that its electrons and other carriers of electric charges behave as though they do not have mass. In conventional materials, the speed of electrons is related to their energy, but not in graphene. Although they do not approach the speed of light, the unbound electrons in graphene behave much like photons, massless particles of light that also move at a speed independent of their energy.
This weird massless behavior is associated with other strangeness. When ordinary conductors are put in a strong magnetic field, charge carriers such as electrons begin moving in circular orbits that are constrained to discrete, equally spaced energy levels. In graphene these levels are known to be unevenly spaced because of the "massless" electrons.
The Georgia Tech/NIST team tracked these massless electrons in action, using a specialized NIST instrument to zoom in on the graphene layer at a billion times magnification, tracking the electronic states while at the same time applying high magnetic fields. The custom-built, ultra-low-temperature and ultra-high-vacuum scanning tunneling microscope allowed them to sweep an adjustable magnetic field across graphene samples prepared at Georgia Tech, observing and mapping the peculiar non-uniform spacing among discrete energy levels that form when the material is exposed to magnetic fields.
The team developed a high-resolution map of the distribution of energy levels in graphene. In contrast to metals and other conducting materials, where the distance from one energy peak to the next is uniformly equal, this spacing is uneven in graphene.
The researchers also probed and spatially mapped graphene's hallmark "zero energy state," a curious phenomenon where the material has no electrical carriers until a magnetic field is applied.
The measurements also indicated that layers of graphene grown and then heated on a substrate of silicon-carbide behave as individual, isolated, two-dimensional sheets. On the basis of the results, the researchers suggest that graphene layers are uncoupled from adjacent layers because they stack in different rotational orientations. This finding may point the way to manufacturing methods for making large, uniform batches of graphene for a new carbon-based electronics.
National Institute of Standards and Technology (NIST)
Graphene apparently owes this enhanced mobility to the curious fact that its electrons and other carriers of electric charges behave as though they do not have mass. In conventional materials, the speed of electrons is related to their energy, but not in graphene. Although they do not approach the speed of light, the unbound electrons in graphene behave much like photons, massless particles of light that also move at a speed independent of their energy.
This weird massless behavior is associated with other strangeness. When ordinary conductors are put in a strong magnetic field, charge carriers such as electrons begin moving in circular orbits that are constrained to discrete, equally spaced energy levels. In graphene these levels are known to be unevenly spaced because of the "massless" electrons.
The Georgia Tech/NIST team tracked these massless electrons in action, using a specialized NIST instrument to zoom in on the graphene layer at a billion times magnification, tracking the electronic states while at the same time applying high magnetic fields. The custom-built, ultra-low-temperature and ultra-high-vacuum scanning tunneling microscope allowed them to sweep an adjustable magnetic field across graphene samples prepared at Georgia Tech, observing and mapping the peculiar non-uniform spacing among discrete energy levels that form when the material is exposed to magnetic fields.
The team developed a high-resolution map of the distribution of energy levels in graphene. In contrast to metals and other conducting materials, where the distance from one energy peak to the next is uniformly equal, this spacing is uneven in graphene.
The researchers also probed and spatially mapped graphene's hallmark "zero energy state," a curious phenomenon where the material has no electrical carriers until a magnetic field is applied.
The measurements also indicated that layers of graphene grown and then heated on a substrate of silicon-carbide behave as individual, isolated, two-dimensional sheets. On the basis of the results, the researchers suggest that graphene layers are uncoupled from adjacent layers because they stack in different rotational orientations. This finding may point the way to manufacturing methods for making large, uniform batches of graphene for a new carbon-based electronics.
National Institute of Standards and Technology (NIST)
Graphene Current Events and Graphene News RSSNew study confirms exotic electric properties of graphene
First, it was the soccer-ball-shaped molecules dubbed buckyballs. Then it was the cylindrically shaped nanotubes. Now, the hottest new material in physics and nanotechnology is graphene: a remarkably flat molecule made of carbon atoms arranged in hexagonal rings much like molecular chicken wire.
Rutgers physicists discover novel electronic properties in two-dimensional carbon structure
Rutgers researchers have discovered novel electronic properties in two-dimensional sheets of carbon atoms called graphene that could one day be the heart of speedy and powerful electronic devices.
Growing geodesic carbon nanodomes
Researchers analyzing the assembly of graphene (sheets of carbon only one atom thick) on a surface of iridium have found that the sheets grow by first forming tiny carbon domes.
Graphite mimics iron's magnetism
Researchers of Eindhoven University of Technology and the Radboud University Nijmegen in The Netherlands show for the first time why ordinary graphite is a permanent magnet at room temperature.
raGraphene and gallium arsenide: two perfect partners find each other
It is the marriage of two top candidates for the electronics of the future, both excentric and extremely interesting: Graphene, one of the partners, is an extremely thin fellow and besides, very young.
Carbon nanotubes could make efficient solar cells
Using a carbon nanotube instead of traditional silicon, Cornell researchers have created the basic elements of a solar cell that hopefully will lead to much more efficient ways of converting light to electricity than now used in calculators and on rooftops.
Researchers design new graphene-based, nano-material with magnetic properties
An international team of researchers has designed a new graphite-based, magnetic nano-material that acts as a semiconductor and could help material scientists create the next generation of electronic devices like microchips.
Camera flash turns an insulating material into a conductor
An insulator can now be transformed to conduct electricity by an ordinary camera flash.
From graphene to graphane, now the possibilities are endless
Ever since graphene was discovered in 2004, this one-atom thick, super strong, carbon-based electrical conductor has been billed as a "wonder material" that some physicists think could one day replace silicon in computer chips.
UCR scientists manipulate ripples in graphene, enabling strain-based graphene electronics
Graphene is nature's thinnest elastic material and displays exceptional mechanical and electronic properties.
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