Copper-grown graphene circuits. (Photo: Zhengtang Luo) |
New
research from the University of Pennsylvania demonstrates a more
consistent and cost-effective method for making graphene, the
atomic-scale material that has promising applications in a variety of
fields, and was the subject of the 2010 Nobel Prize in Physics.
As
explained in a recently published study, a Penn research team was able
to create high-quality graphene that is just a single atom thick over
95% of its area, using readily available materials and manufacturing
processes that can be scaled up to industrial levels.
“I’m
aware of reports of about 90%, so this research is pushing it closer to
the ultimate goal, which is 100%,” said the study’s principal
investigator, A.T. Charlie Johnson, professor of physics. “We have a
vision of a fully industrial process.”
Other
team members on the project included postdoctoral fellows Zhengtang Luo
and Brett Goldsmith, graduate students Ye Lu and Luke Somers and
undergraduate students Daniel Singer and Matthew Berck, all of Penn’s
Department of Physics and Astronomy in the School of Arts and Sciences.
The group’s findings were published on Feb. 10 in the journal Chemistry of Materials.
Graphene
is a chicken-wire-like lattice of carbon atoms arranged in thin sheets a
single atomic layer thick. Its unique physical properties, including
unbeatable electrical conductivity, could lead to major advances in
solar power, energy storage, computer memory and a host of other
technologies. But complicated manufacturing processes and
often-unpredictable results currently hamper graphene’s widespread
adoption.
Producing
graphene at industrial scales isn’t inhibited by the high cost or
rarity of natural resources – a small amount of graphene is likely made
every time a pencil is used – but rather the ability to make meaningful
quantities with consistent thinness.
One
of the more promising manufacturing techniques is CVD, or chemical
vapor deposition, which involves blowing methane over thin sheets of
metal. The carbon atoms in methane form a thin film of graphene on the
metal sheets, but the process must be done in a near vacuum to prevent
multiple layers of carbon from accumulating into unusable clumps.
The
Penn team’s research shows that single-layer-thick graphene can be
reliably produced at normal pressures if the metal sheets are smooth
enough.
“The
fact that this is done at atmospheric pressure makes it possible to
produce graphene at a lower cost and in a more flexible way,” Luo, the
study’s lead author, said.
Whereas
other methods involved meticulously preparing custom copper sheets in a
costly process, Johnson’s group used commercially available copper foil
in their experiment.
“You could practically buy it at the hardware store,” Johnson said.
Other
methods make expensive custom copper sheets in an effort to get them as
smooth as possible; defects in the surface cause the graphene to
accumulate in unpredictable ways. Instead, Johnson’s group
“electropolished” their copper foil, a common industrial technique used
in finishing silverware and surgical tools. The polished foil was
smooth enough to produce single-layer graphene over 95% of its surface
area.
Working
with commercially available materials and chemical processes that are
already widely used in manufacturing could lower the bar for commercial
applications.
“The overall production system is simpler, less expensive, and more flexible” Luo said.
The
most important simplification may be the ability to create graphene at
ambient pressures, as it would take some potentially costly steps out of
future graphene assembly lines.
“If
you need to work in high vacuum, you need to worry about getting it
into and out of a vacuum chamber without having a leak,” Johnson said.
“If you’re working at atmospheric pressure, you can imagine
electropolishing the copper, depositing the graphene onto it and then
moving it along a conveyor belt to another process in the factory.”
This research was supported by Penn’s Nano/Bio Interface Center through the National Science Foundation.