no mass; they move at a constant speed
no matter their energy, and they cannot be stopped. Only particles in atom
smashers and cosmic rays behave this
way, and the math that describes graphene electrons is very much like the
math that describes neutrinos, those
elusive, nearly massless particles that
zip through space.
“Boom — all of a sudden we have a
system of quasi-neutrinos,” says Eva
Andrei, a physicist at Rutgers University’s campus in Piscataway, N. J. The only
difference is that graphene electrons
travel at roughly a million meters per
second; neutrinos (and light) travel 300
times that fast.
Discovery after discovery has revealed
the bizarre things these graphene elec-
trons can do. In April in Science, Geim
and his colleagues reported that under
certain conditions, electrons in gra-
phene can adopt a split personality in
which one of their properties (electric
charge) behaves according to the rules of
the everyday world but another property
(spin) behaves according to the other-
world of quantum mechanics. “We are
not used to quantum mechanical effects
happening in our normal life,” says
Castro Neto. “When you find a material
like that, it’s really a treasure.”
Stretching graphene also makes its
electrons do funny things. At the Uni-
versity of California, Berkeley and the
Lawrence Berkeley National Laboratory,
scientists accidentally found that if they
grew graphene atop platinum, the gra-
phene sheet could sprout tiny bubbles on
its surface. Within those bubbles, elec-
trons act as if they are under the influ-
ence of a strong magnetic field. Nobody
is really sure what this means, says team
member Castro Neto, but researchers in
Singapore have managed to create simi-
lar bubbles at will. “Now we can control
at the nanoscale the nature of the elec-
tronic states,” Castro Neto says. “I think
this is going to really generate a revolu-
tion in the way in which we deal with
graphene.”
And all that in just a single layer of gra-
phene. For even more new tricks, scien-
tists are turning to two-layer, or bilayer,
graphene.
Doubled up
When it comes to building new electronic
devices, single-layer graphene suffers
from one huge drawback: It doesn’t have
a “band gap,” or break in the energy levels
that its electrons can occupy. Without a
band gap, scientists can’t turn the flow
of electrons on and off — a crucial part of
any electronic gadget. But adding a second layer of graphene creates such a band
gap, making the bilayer structure more
like a semiconductor in which the flow
of electrons can be controlled instead of
zooming along willy-nilly.
“Unlike single-layer graphene, bilayer
has the possibility of shifting charge
Graphene on top Putting a graphene
sheet atop different substances creates different electronic effects. Atop platinum, the sheet
wrinkles to form tiny nanobubbles (colored
peaks, below). When graphene is put on boron
nitride, the ;ow of electrons is simpli;ed compared with the ;ow on silicon dioxide (right).
Graphene on BN
Boron nitride smooths electron flow
Graphene on Pt
Topography
Charge density
Topography
Charge density
from one layer to another,” says Amir
Yacoby, a physicist at Harvard University. And interactions among the electrons cause other weird and wonderful
physics, Yacoby says, such as the breaking of fundamental symmetries in how
the electrons spin and move. “Several
experiments indicate that interesting
things are happening, but there is really
no good understanding of what is going
on there as of yet,” he says.
As intriguing as bilayer graphene is,
making it isn’t as simple as slapping one
graphene layer atop another. How the
two layers are stacked relative to one
another is crucial for electronic applications, Andrei and her colleagues reported
in March in Physical Review Letters. If
the carbon honeycombs of each layer
are rotated less than 5 degrees relative
to each other, Andrei’s team found, then
they behave as a true bilayer, and can
create the electronic band gap. But if
the honeycombs are offset by about 20
degrees or more, then the graphene layers continue to behave electronically as
two separate layers.
Such research shows how graphene
electrons can be coaxed into acting
however scientists want them to, Andrei
says. “Here we have an external knob to
control the electronic properties,” she
says. “That’s quite exciting.”
Where two layers are good, three might
be even better, and so some researchers
are pushing to make trilayer graphene.
But as with the bilayer, researchers
can’t just throw three graphene sheets
in a pile; the carbon honeycombs have to
line up just so. Usually trilayer graphene
comes in what’s called the ABA form, in
which the honeycombs of the top and
bottom layer mirror each other. The
ABC form, in contrast, slides that top-
most layer over to one side so that the
honeycombs climb like stairsteps.
Nobody has ever gotten the ABA
version of graphene to do anything very
exciting, but a research team led by Tony
Heinz of Columbia University has been
playing around with the ABCs. In a paper
appearing online in May at arXiv.org,
Heinz and his colleagues report making
an electronic band gap appear in ABC
