Monday, 12 August 2013

Researchers discover a tiny twist in bilayer graphene that may solve a mystery

Researchers discover a tiny twist
in bilayer graphene that may solve
a mystery
The Dirac spectrum of bilayer
graphene when the two layers are
exactly aligned (left) shifts with a
slight interlayer twist that breaks
interlayer-coupling and potential
symmetry, leading to a new spectrum
with surprisingly strong signatures in
ARPES data. Credit: Keun Su Kim,
Fritz Haber Institute
Researchers with the U.S.
Department of Energy (DOE)'s
Lawrence Berkeley National
Laboratory (Berkeley Lab) have
discovered a unique new twist to the
story of graphene, sheets of pure
carbon just one atom thick, and in
the process appear to have solved a
mystery that has held back device
development.
Electrons can race through graphene
at nearly the speed of light – 100
times faster than they move through
silicon. In addition to being superthin
and superfast when it comes to
conducting electrons, graphene is
also superstrong and superflexible,
making it a potential superstar
material in the electronics and
photonics fields, the basis for a host
of devices, starting with ultrafast
transistors. One big problem,
however, has been that graphene's
electron conduction can't be
completely stopped, an essential
requirement for on/off devices.
The on/off problem stems from
monolayers of graphene having no
bandgaps – ranges of energy in which
no electron states can exist. Without
a bandgap , there is no way to control
or modulate electron current and
therefore no way to fully realize the
enormous promise of graphene in
electronic and photonic devices.
Berkeley Lab researchers have been
able to engineer precisely controlled
bandgaps in bilayer graphene through
the application of an external electric
field. However, when devices were
made with these engineered
bandgaps, the devices behaved
strangely, as if conduction in those
bandgaps had not been stopped. Why
such devices did not pan out has
been a scientific mystery until now.
Working at Berkeley Lab's Advanced
Light Source (ALS), a DOE national
user facility, a research team led by
ALS scientist Aaron Bostwick has
discovered that in the stacking of
graphene monolayers subtle
misalignments arise, creating an
almost imperceptible twist in the
final bilayer graphene. Tiny as it is -
as small as 0.1 degree - this twist
can lead to surprisingly strong
changes in the bilayer graphene's
electronic properties .
"The introduction of the twist
generates a completely new
electronic structure in the bilayer
graphene that produces massive and
massless Dirac fermions," says
Bostwick. "The massless Dirac
fermion branch produced by this new
structure prevents bilayer graphene
from becoming fully insulating even
under a very strong electric field. This
explains why bilayer graphene has
not lived up to theoretical predictions
in actual devices that were based on
perfect or untwisted bilayer
graphene."
Bostwick is the corresponding author
of a paper describing this research in
the journal Nature Materials titled
"Coexisting massive and massless
Dirac fermions in symmetry-broken
bilayer graphene." Keun Su Kim of
the Fritz Haber Institute in Berlin is
the lead author Other coauthors are
Andrew Walter, Luca Moreschini,
Thomas Seyller, Karsten Horn, and Eli
Rotenberg, who oversees the
research at ALS Beamline 7.0.1.
Monolayers of graphene have no
bandgaps – ranges of energy in which
no electron states can exist. Without
a bandgap, there is no way to control
or modulate electron current and
therefore no way to fully realize the
enormous promise of graphene in
electronic and photonic devices.
Berkeley Lab researchers have been
able to engineer precisely controlled
bandgaps in bilayer graphene through
the application of an external electric
field. However, when devices were
made with these engineered
bandgaps, the devices behaved
strangely, as if conduction in those
bandgaps had not been stopped.
To get to the bottom of this mystery,
Rotenberg, Bostwick, Kim and their
co-authors performed a series of
angle-resolved photoemission
spectroscopy (ARPES) experiments at
ALS beamline 7.0.1. ARPES is a
technique for studying the electronic
states of a solid material in which a
beam of X-ray photons striking the
material's surface causes the
photoemission of electrons. The
kinetic energy of these
photoelectrons and the angles at
which they are ejected are then
measured to obtain an electronic
spectrum.
"The combination of ARPES and
Beamline 7.0.1 enabled us to easily
identify the electronic spectrum from
the twist in the bilayer graphene,"
says Rotenberg. "The spectrum we
observed was very different from
what has been assumed and contains
extra branches consisting of massless
Dirac fermions. These new massless
Dirac fermions move in a completely
unexpected way governed by the
symmetry twisted layers."
Massless Dirac fermions, electrons
that essentially behave as if they
were photons, are not subject to the
same bandgap constraints as
conventional electrons . In their
Nature Materials paper, the authors
state that the twists that generate
this massless Dirac fermion spectrum
may be nearly inevitable in the
making of bilayer graphene and can
be introduced as a result of only ten
atomic misfits in a square micron of
bilayer graphene.
"Now that we understand the
problem, we can search for
solutions," says lead author Kim.
"For example, we can try to develop
fabrication techniques that minimize
the twist effects, or reduce the size
of the bilayer graphene we make so
that we have a better chance of
producing locally pure material."
Beyond solving a bilayer graphene
mystery, Kim and his colleagues say
the discovery of the twist establishes
a new framework on which various
fundamental properties of bilayer
graphene can be more accurately
predicted.
"A lesson learned here is that even
such a tiny structural distortion of
atomic-scale materials should not be
dismissed in describing the electronic
properties of these materials fully
and accurately," Kim says.
Provided by Lawrence Berkeley
National Laboratory

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