New material shares many of graphene's unusual properties

Apr 24, 2012 by David L. Chandler
PhD candidate Shuang Tang, left, and Institute Professor Mildred Dresselhaus. Photo: Dominick Reuter

Graphene, a single-atom-thick layer of carbon, has spawned much research into its unique electronic, optical and mechanical properties. Now, researchers at MIT have found another compound that shares many of graphene’s unusual characteristics — and in some cases has interesting complementary properties to this much-heralded material.

The material, a thin film of bismuth-antimony, can have a variety of different controllable characteristics, the researchers found, depending on the ambient temperature and pressure, the material’s thickness and the orientation of its growth. The research, carried out by materials science and engineering PhD candidate Shuang Tang and Institute Professor Mildred Dresselhaus, appears in the journal .

Like graphene, the new material has electronic that are known as two-dimensional Dirac cones, a term that refers to the cone-shaped graph plotting energy versus momentum for electrons moving through the material. These unusual properties — which allow electrons to move in a different way than is possible in most materials — may give the bismuth-antimony films properties that are highly desirable for applications in manufacturing next-generation electronic chips or thermoelectric generators and coolers.

In such materials, Tang says, electrons “can travel like a beam of light,” potentially making possible new chips with much faster computational abilities. The electron flow might in some cases be hundreds of times faster than in conventional silicon chips, he says.

Similarly, in a thermoelectric application — where a temperature difference between two sides of a device creates a flow of electrical current — the much faster movement of electrons, coupled with strong thermal insulating properties, could enable much more efficient power production. This might prove useful in powering satellites by exploiting the temperature difference between their sunlit and shady sides, Tang says.

Such applications remain speculative at this point, Dresselhaus says, because further research is needed to analyze additional properties and eventually to test samples of the material. This initial analysis was based mostly on theoretical modeling of the bismuth-antimony film’s properties.

Until this analysis was carried out, Dresselhaus says, “we never thought of bismuth” as having the potential for Dirac cone properties. But recent unexpected findings involving a class of materials called topological insulators suggested otherwise: Experiments carried out by a Ukrainian collaborator suggested that Dirac cone properties might be possible in bismuth-antimony films.

While it turns out that the thin films of bismuth-antimony can have some properties similar to those of , changing the conditions also allows a variety of other properties to be realized. That opens up the possibility of designing electronic devices made of the same material with varying properties, deposited one layer atop another, rather than layers of different .

The material’s unusual properties can vary from one direction to another: Electrons moving in one direction might follow the laws of classical mechanics, for example, while those moving in a perpendicular direction obey relativistic physics. This could enable devices to test relativistic physics in a cheaper and simpler way than existing systems, Tang says, although this remains to be shown through experiments.

“Nobody’s made any devices yet” from the new material, Dresselhaus cautions, but adds that the principles are clear and the necessary analysis should take less than a year to carry out.

“Anything can happen, we really don’t know,” Dresselhaus says. Such details remain to be ironed out, she says, adding: “Many mysteries remain before we have a real device.”

Joseph Heremans, a professor of physics at Ohio State University who was not involved in this research, says that while some unusual properties of bismuth have been known for a long time, “what is surprising is the richness of the system calculated by Tang and Dresselhaus. The beauty of this prediction is further enhanced by the fact that system is experimentally quite accessible.”

Heremans adds that in further research on the properties of the bismuth-antimony material, “there will be difficulties, and a few are already known,” but he says the properties are sufficiently interesting and promising that “this paper should stimulate a large experimental effort.”

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4 / 5 (1) Apr 25, 2012
There may be clues to practical applications of this in work done at Los Alamos about 15 years ago or more: Vacuum deposited Bismuth-Zinc-Magnesium in a multilayer sandwich seems to approach "perfect" conduction too ... in the interface between the materials.

The real question would be is Bismuth (and the rest) prolific enough here on Earth for real world, macro applications?
not rated yet Apr 25, 2012
i really like your comment/is earth the only real world.
1 / 5 (1) Apr 25, 2012
"we never thought of bismuth" as having the potential for Dirac cone properties
It's hard to believe, as the bismuth is common part of most topological insulators known for physicists. Such sentence is a typical journalism, when some finding is presented as a more fundamental, than it really is. The existence of Dirac cones has been predicted in many bismuth related materials.
1 / 5 (1) Apr 25, 2012
To explain this subject for laymans we can imagine, that the topological insulators (like the bismuth antimonide) are behaving with respect to electrons like the hydrophobic sponge - the electrons are expelled into its cavities and remain unmovable there - only the surface of sponge is covered with film of electrons in similar way, like the Teflon foam soaked with mercury. The idea therefore is to make a thin layer of that material, similar to graphene, where the motion of all electrons will be constrained to the surface layer of their liquid. These electrons are strongly repulsing mutually there, their repulsive forces are compensated mutually and as the result these electrons tend to move in fast ballistic way like the electrons within superconductors - just in localized islands corresponding the tips of Dirac cones.