The regions depict folds in graphene, whereas the green regions are relatively flat domains. The “hills and valleys” present in the electron cloud can act as speed bumps preventing the flow of charge through graphene. Ideally, for high-performance electronics, one would like a midwestern topography: completely flat, which would appear all green. Image credit: Brian J. Shulz and Christopher J. Patridge, University of Buffalo. |
A research team led by University at Buffalo chemists has used
synchrotron light sources to observe the electron clouds on the surface
of graphene, producing a series of images that reveal how folds and
ripples in the remarkable material can harm its conductivity.
The
research, scheduled to appear June 28 in Nature Communications, was
conducted by UB, the National Institute of Standards and Technology
(NIST), the Molecular Foundry at Lawrence Berkeley National Laboratory
(Berkeley Lab), and SEMATECH, a global consortium of semiconductor
manufacturers.
Graphene,
the thinnest and strongest material known to man, consists of a single
layer of carbon atoms linked in a honeycomb-like arrangement.
Graphene’s
special structure makes it incredibly conductive: Under ideal
circumstances, when graphene is completely flat, electric charges speed
through it without encountering many obstacles, said Sarbajit Banerjee,
one of the UB researchers who led the study in Nature Communications.
But conditions are not always optimal.
The
new images that Banerjee and his colleagues captured show that when
graphene is folded or bent, the electron cloud lining its surface also
becomes warped, making it more difficult for an electric charge to
travel through.
“When
graphene is flat, things just kind of coast along the cloud. They don’t
have to hop across anything. It’s like a superhighway,” said Banerjee,
an assistant professor of chemistry. “But if you bend it, now there are
some obstacles; imagine the difference between a freshly paved highway
and one with construction work along the length forcing lane changes.
“When
we imaged the electron cloud, you can imagine this big fluffy pillow,
and we saw that the pillow is bent here and there,” said Banerjee, whose
National Science Foundation CAREER award provided the primary funding
for the project.
To
create the images and understand the factors perturbing the electron
cloud, Banerjee and his partners employed two techniques that required
use of a synchrotron: scanning transmission X-ray microscopy and near
edge X-ray absorption fine structure (NEXAFS), a type of absorption
spectroscopy. The experiments were further supported by computer
simulations performed on computing clusters at Berkeley Lab.
“Using
simulations, we can better understand the measurements our colleagues
made using X-rays, and better predict how subtle changes in the
structure of graphene affect its electronic properties,” said David
Prendergast, a staff scientist in the Theory of Nanostructures Facility
at the Molecular Foundry at Berkeley Lab. “We saw that regions of
graphene were sloped at different angles, like looking down onto the
slanted roofs of many houses packed close together.”
Besides
documenting how folds in graphene distort its electron cloud, the
research team discovered that contaminants that cling to graphene during
processing linger in valleys where the material is uneven. Such
contaminants uniquely distort the electron cloud, changing the strength
with which the cloud is bound to the underlying atoms.
Graphene’s
unusual properties have generated excitement in industries including
computing, energy and defense. Scientists say that graphene’s electrical
conductivity matches that of copper, and that graphene’s thermal
conductivity is the best of any known material.
But
the new, UB-led study suggests that companies hoping to incorporate
graphene into products such as conductive inks, ultrafast transistors
and solar panels could benefit from more basic research on the
nanomaterial. Improved processes for transferring flat sheets of
graphene onto commercial products could greatly increase those products’
efficiency.
“A
lot of people know how to grow graphene, but it’s not well understood
how to transfer it onto something without it folding onto itself,”
Banerjee said. “It’s very hard to keep straight and flat, and our work
is really bringing home the point of why that’s so important.”
Dotted lines show distinctive regions of graphene that are sloped at different angles. Soft X-rays paint a bird’s-eye view of the electron cloud of graphene. Image credit: Brian J. Shulz and Christopher J. Patridge, University of Buffalo. |
“Graphene
is going to be very important in electronics,” said PhD candidate Brian
Schultz, one of three UB graduate students who were lead authors on the
Nature Communications paper. “It’s going to be one of the most
conductive materials ever found, and it has the capability to be used as
an ultrahigh-frequency transistor or as a possible replacement for
silicon chips, the backbone of current commercial electronics.
“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,” Schultz continued. “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.”
Other research partners offered the following insight into the significance of the findings:
Dan
Fischer, NIST Material Measurement Laboratory, leader, Synchrotron
Methods Group: “The NEXAFS results indicating that performance-damaging
contaminants cling to graphene during processing highlights the
importance of chemically sensitive advanced synchrotron measurement
method developments for promoting innovation and industrial
competiveness in commercial applications of nanotechnology.”
Pat
Lysaght, SEMATECH Front End Processes, senior member technical staff:
“We place a premium on the power of collaboration, and this is a great
example of the benefits associated with that philosophy. The unique
expertise of each of the four collaborative entities has come together
to forge a new understanding of subtle functionalization variations of
surface graphene atoms. Our findings represent another important step
toward potential industrial applications such as low-cost broadband
radio frequency (RF) devices, and correlation of NEXAFS with Raman
spectroscopy which may enhance monitoring capabilities for graphene as a
replacement for large area organic LED displays.”
Synchrotron
imaging was conducted at the Canadian Light Source in Saskatchewan in
Canada and at the National Synchrotron Light Source (NSLS ) at
Brookhaven National Laboratory in New York State. NEXAFS was measured at
the NIST soft X-ray beamline of the NSLS.
Portions
of this work conducted at the Molecular Foundry were supported by the
Department of Energy Office of Science. The Molecular Foundry is one of
five DOE Nanoscale Science Research Centers (NSRCs), premier national
user facilities for interdisciplinary research at the nanoscale.
Together the NSRCs comprise a suite of complementary facilities that
provide researchers with state-of-the-art capabilities to fabricate,
process, characterize and model nanoscale materials, and constitute the
largest infrastructure investment of the National Nanotechnology
Initiative.
Additional
support for the project came from a New York State Energy Research and
Development Authority (NYSERDA) grant disbursed through Graphene
Devices, a Western New York start-up company exploring ways to optimize
production of graphene using processes that Banerjee and UB colleagues
invented.
