NIST 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. Image: NIST
Researchers at the National Institute of Standards and
Technology (NIST) have shown that the electronic properties of two layers of
graphene vary on the nanometer scale. 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
Graphene 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 makes graphene ill-suited for digital
Researchers have known that bilayer graphene, consisting of
two stacked graphene layers, acts more like a semiconductor when immersed in an
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
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.
* G. Rutter, S. Jung, N. Klimov, D. Newell, N. Zhitenev and
J. Stroscio. Microscopic
polarization in bilayer graphene. Nature Physics. Published online
April 24, 2011.
** “See the Jan. 19, 2011, Tech Beat article
Graphene Devices May Have a Bumpy Ride” at www.nist.gov/public_affairs/tech-beat/tb20110119.cfm#graphene.