Graphene,
a single-atom-thick layer of carbon, has spawned much research into its unique
electronic, optical, and mechanical properties. Now, researchers at Massachusetts
Institute of Technology (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 Nano
Letters.
Like
graphene, the new material has electronic properties that are known as 2D 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 graphene, 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 materials.
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.”
