Using a combination of sophisticated computer modeling and
advanced materials analysis techniques at synchrotron laboratories, a research
team led by the University at Buffalo (UB) including NIST, the Molecular
Foundry at Lawrence Berkeley National Laboratory, and SEMATECH has demonstrated
how some relatively simple processing flaws can seriously degrade the otherwise
near-magical electronic properties of graphene.
Their new paper demonstrates how both wrinkles in the
graphene sheet and/or chance contaminants from processing—possibly hiding in
those folds—disrupt and slow electron flow across the sheet. The results could
be important for the design of commercial manufacturing processes that exploit
the unique electrical properties of graphene. In the case of contaminant
molecules at least, the paper also suggests that heating the graphene may be a
simple solution.
Graphene is under intense study because of a combination of
outstanding properties. It’s extremely strong, conducts heat very well, and has
high electrical conductivity while being flexible and transparent. Graphene’s
electrical properties, however, apparently depend a lot on flatness and purity.
Using X-rays, the UB team produced images that show the
electron “cloud” lining the surface of graphene samples—the structure
responsible for the high-speed transit of electrons across the sheet—and how
wrinkles in the sheet distort the cloud and create bottlenecks. Spectrographic
data showed anomalous peaks in some regions that also corresponded to
distortions of the cloud. NIST researchers, using their dedicated materials
science beam line at the National Synchrotron Light Source (NSLS), contributed
a sensitive analysis of spectroscopic data confirming that these peaks were
caused by chemical contaminants that adhered to the graphene during processing.
Significantly, the NIST synchrotron methods group was able
to make detailed spectroscopic measurements of the graphene samples while
heating them, and found that the mysterious peaks disappeared by the time the
sample reached 150 C. This, according to Dan Fischer, leader of the NIST group,
showed both that those particular disturbances in the electron cloud were due
to contaminants, and that there is a way to get rid of them. “They’re not
chemical bound, they’re just physically absorbed on the surface, and that’s an
important thing. You have a prescription for getting rid of them,” Fischer
says.
“When graphene was discovered, people were just so
excited that it was such a good material that people really wanted to go with
it and run as fast as possible,” says Brian Schultz, one of three UB
graduate students who were lead authors on the paper, “but what we’re
showing is that you really have to do some fundamental research before you
understand how to process it and how to get it into electronics.”
“This is the practical side of using graphene,”
agrees Fischer, “It has all these remarkable properties, but when you go
to actually try to make something, maybe they stop working, and the question
is: why and what do you do about it? These kinds of extremely sensitive,
specialized techniques are part of that answer.”