It was hard to understand how a graphene sheet - a
featureless, flat sheet of carbon atoms - lying on an equally
featureless iridium surface, somehow converted itself into a kind of
muffin tin that formed “muffins” made from newly
arrived iridium atoms. The muffins were equally spaced and of equal
size.
 | | Graphene has proven a difficult material for researchers to tame. Peter Feibelman 's computational simulation suggests an explanation for why iridium atoms (colored green) nest regularly atop a base of graphene (dark-colored atoms) grown over an iridium substrate. Peter's image of the orderly nanoscopic metallic arrangement may provide insights to other scientists. His paper on the work was published in Physical Review B online. |
Graphene flakes are notoriously difficult to work with. Still,
they are stronger than diamond, better heat-shedders and conductors
than silicon, and thought to have great potential in the worlds of
microelectronics and sensors. If only they could be tamed.
Imagining a whole new set of possible applications, people
wanted to know why the orderly metallic array self-created itself.
“At the outset,” writes Sandia researcher
Peter Feibelman, who created the explanatory simulation published last
week in Physical Review B, “this seemed quite a
mystery.”
The mystery started in 2005, when a German team discovered the
new wrinkle in the battle to harness graphene but had difficulty in
explaining the reaction.
A graphene flake lying atop an iridium crystal unexpectedly
caused new iridium atoms, deposited atop the flake, to arrange
themselves into cluster arrays, stable even as its temperature reached
400 to 500 kelvin.
Sherlock Holmes himself, looking for clues to why the iridium
quantum dots so mysteriously attached, would have found little to go on.
The iridium support layer was flat as could be. The same was
true of the graphene layer that formed on top of it, which sported
neither hooks nor ports for nanoparticle docking.
Graphite itself - merely a group of sheets of graphene - is so
slippery it can be used as a lubricant. Why would nanodots attach to
the completed graphene layer instead of just sliding away?
Even granted an attachment mechanism, why would newly
introduced iridium atoms form a moiré - a regular, ordered
array - atop the graphene instead of a planar second surface - a
sandwich where the iridium was the bread and graphene the meat?
The explanation for the template effect would be almost
impossible to see by direct examination.
But Feibelman’s computational simulations produced a
plausible explanation.
The simulation suggest that in regions where half the graphene
flake’s carbon atoms sit directly above iridium atoms of the
underlying crystal, iridium atoms added on top of the graphene flake
make it buckle. These regions do not occur randomly, and in fact form
the regular array needed to explain the nanodot moiré.
The buckling weakens tight links between the
graphene’s neighboring carbon atoms, freeing them to attach
to the added iridium atoms. Furthermore, buckling not only allows the
carbon atoms that buckle upward to capture deposited iridium atoms, but
also causes the carbon atoms that buckle down to attach firmly to the
metal below, explaining the remarkable thermal stability of the nanodot
arrays.
This orderly nanoscopic arrangement appeals to scientists
trying to understand aspects of catalysis, Feibelman says. The atoms
that make up tiny nanodots are expected to be in direct contact with
inserted materials, important for speeding up desirable chemical
reactions. The regular arrangement of the nanodots makes the science
relatively simple, because every catalyst particle is the same and sits
in the same environment.
“The rigorous periodicity of the nanodot arrays is a
huge advantage compared to amorphous or ‘glassy’
arrangements where everything has to be described
statistically,” says Feibelman.
Similar quantum dot arrangements on electrically insulating
graphene could keep information packets separate and
“addressable” for data storage, or provide superior
conditions for quantum computing.
Sandia is a multiprogram laboratory operated by Sandia
Corporation, a Lockheed Martin company, for the U.S. Department of
Energy’s National Nuclear Security Administration. With main
facilities in Albuquerque, N.M., and Livermore, Calif., Sandia has
major R&D responsibilities in national security, energy and
environmental technologies, and economic competitiveness.
Posted 30th April 2008
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