Monday, 22 July 2013

Electronics: Graphene Makes a Magnetic Switch

Electronics: Graphene Makes a
Magnetic Switch
July 18, 2013 — Tiny nanoribbons of
carbon could be used to make a
magnetic field sensor for novel
electronic devices.
Researchers in Singapore have
designed an electronic switch that
responds to changes in a magnetic
field. The device relies on graphene,
a strong and flexible electricity-
conducting layer of carbon atoms
arranged in a honeycomb pattern.
Seng Ghee Tan of the A*STAR Data
Storage Institute, along with
colleagues at the National University
of Singapore, used theoretical models
to predict the properties of their
proposed device, known as a
magnetic field-effect transistor.
The transistor is based on two
nanoribbons of graphene, each just a
few tens of nanometers wide, which
are joined end to end. The atoms
along the edges of these nanoribbons
are arranged in an 'armchair'
configuration -- a pattern that
resembles the indented battlements
of castle walls. If these edges were
in a zigzag pattern, however, the
material would have different
electrical properties.
One of the nanoribbons in the team's
transistor acts as a metallic
conductor that allows electrons to
flow freely; the other, slightly wider,
nanoribbon is a semiconductor.
Under normal conditions, electrons
cannot travel from one nanoribbon to
the other because their quantum
wavefunctions -- the probability of
where electrons are found within the
materials -- do not overlap.
A magnetic field, however, warps the
distribution of electrons, changing
their wavefunctions until they overlap
and allowing current to flow from
one nanoribbon to the other. Using
an external field to change the
electrical resistance of a conductor in
this way is known as a
magnetoresistance effect.
The team calculated how electrons
would travel in the nanoribbons
under the influence of a 10-tesla
magnetic field -- the rough equivalent
of that produced by a large
superconducting magnet -- at a range
of different temperatures.
Tan and colleagues found that larger
magnetic fields allowed more current
to flow, and the effect was more
pronounced at lower temperatures.
At 150 kelvin, for example, the
magnetic field induced a very large
magnetoresistance effect and current
flowed freely. At room temperature,
the effect declined slightly but still
allowed a considerable current. At
300 kelvin, the magnetoresistance
effect was approximately half as
strong.
The researchers also discovered that
as the voltage across the
nanoribbons increased, the electrons
had enough energy to force their way
through the switch and the
magnetoresistance effect declined.
Other researchers recently produced
graphene nanoribbons with
atomically precise edges, similar to
those in the proposed design. Tan
and his colleagues suggest that if
similar manufacturing techniques
were used to build their device, its
properties could come close to
matching their theoretical
predictions.
The A*STAR-affiliated researchers
contributing to this research are from
the Data Storage Institute

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