This is more progress coming out
of the world of graphene research which has been breathtaking. Most important is that we have been quickly
becoming better skilled at producing and manipulating the material and modifying
its behavior.
Its importance is extraordinary and
we can see the day in which it is the principle building block of all technology.
In this case we discover we can
create puddles of opposite charge between the two layers. Sounds like an electrical device to me.
I wonder what all the radio
enthusiasts of the first half of the twentieth century would have thought of
all this.
2 graphene layers may be better than 1
by Staff Writers
measurements show that interactions of the graphene layers with the
insulating substrate material causes electrons (red, down arrow) and electron
holes (blue, up arrow) to collect in "puddles." The differing charge
densities creates the random pattern of alternating dipoles and electon band
gaps that vary across the layers. Credit: NIST
Researchers at the National
The surprising new results reveal that not only does the difference in
the strength of the electric charges between the two layers vary across the
layers, but they also actually reverse in sign to create randomly
distributed puddles of alternating positive and negative charges.
Reported in Nature Physics,*
the new measurements bring graphene a step closer to being used in practical
electronic devices.
Graphene, a single layer of carbon atoms, is prized for its remarkable
properties, not the least of which is the way it conducts electrons at high
speed.
However, the lack of what physicists call
a band gap-an energetic threshold that makes it possible to turn a transistor
on and off-makes graphene ill-suited for digital electronic applications.
Researchers have known that bilayer graphene, consisting of two stacked
graphene layers, acts more like a semiconductor when immersed in an electric field.
According to NIST researcher Nikolai Zhitenev, the band gap may also
form on its own due to variations in the sheets' electrical potential caused by
interactions among the graphene electrons or with the substrate (usually a
nonconducting, or insulating material) that the graphene is placed upon.
NIST fellow Joseph Stroscio says that their measurements indicate that
interactions with the disordered insulating substrate material causes pools of
electrons and electron holes (basically, the absence of electrons) to form in
the graphene layers.
Both electron and hole "pools" are deeper on the bottom layer
because it is closer to the substrate. This difference in "pool"
depths, or charge density, between the layers creates the random pattern of
alternating charges and the spatially varying band gap.
Manipulating the purity of the substrate could give researchers a way
to finely control graphene's band gap and may eventually lead to the
fabrication of graphene-based transistors that can be turned on and off like a
semiconductor.
Still, as shown in the group's previous work**, while these substrate
interactions open the door to graphene's use as a practical electronic
material, they lower the window on speed.
Electrons do not move as well through substrate-mounted bilayer
graphene; however, this may likely be compensated for by engineering the
graphene/substrate interactions.
Stroscio's team plans to explore further the role that substrates may
play in the creation and control of band gaps in graphene by using different
substrate materials.
If the substrate interactions can be reduced far enough, says Stroscio,
the exotic quantum properties of bilayer graphene may be harnessed to create a
new quantum field effect transistor.
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