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01/23/12 – Graphene is the thinnest material known

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Engineers at Rensselaer Polytechnic Institute
and Rice University Discover How the Extreme Thinness of
Graphene Enables Near-Perfect Wetting
Transparency

Graphene is the thinnest material known to science. The
nanomaterial is so thin, in fact, water often doesn’t even know
it’s there.

Engineering researchers at Rensselaer Polytechnic Institute
and Rice University coated pieces of gold, copper, and silicon
with a single layer of graphene, and then placed a drop of
water on the coated surfaces. Surprisingly, the layer of
graphene proved to have virtually no impact on the manner in
which water spreads on the surfaces.

Results of the study were published Sunday in the journal
Nature Materials. The findings could help inform a new
generation of graphene-based flexible electronic devices.
Additionally, the research suggests a new type of heat pipe
that uses graphene-coated copper to cool computer chips.

The discovery stemmed from a cross-university collaboration
led by Rensselaer Professor Nikhil Koratkar
and Rice Professor Pulickel Ajayan.

“We coated several different surfaces with graphene, and
then put a drop of water on them to see what would happen. What
we saw was a big surprise—nothing changed. The graphene was
completely transparent to the water,” said Koratkar, a faculty
member in the Department of Mechanical, Aerospace, and Nuclear
Engineering and the Department of Materials Science
and Engineering at Rensselaer. “The single layer of
graphene was so thin that it did not significantly disrupt the
non-bonding van der Waals forces that control the interaction
of water with the solid surface. It’s an exciting discovery,
and is another example of the unique and extraordinary
characteristics of graphene.”

Results of the study are detailed in the Nature
Materials paper “Wetting transparency of graphene.” See
the paper online at: http://dx.doi.org/10.1038/NMAT3228

Essentially an isolated layer of the graphite found commonly
in our pencils or the charcoal we burn on our barbeques,
graphene is single layer of carbon atoms arranged like a
nanoscale chicken-wire fence. Graphene is known to have
excellent mechanical properties. The material is strong and
tough and because of its flexibility can evenly coat nearly any
surface. Many researchers and technology leaders see graphene
as an enabling material that could greatly advance the advent
of flexible, paper-thin devices and displays. Used as a coating
for such devices, the graphene would certainly come into
contact with moisture. Understanding how graphene interacts
with moisture was the impetus behind this new study.

The spreading of water on a solid surface is called wetting.
Calculating wettability involves placing a drop of water on a
surface, and then measuring the angle at which the droplet
meets the surface. The droplet will ball up and have a high
contact angle on a hydrophobic surface. Inversely, the droplet
will spread out and have a low contact angle on a hydrophilic
surface.

The contact angle of gold is about 77 degrees. Koratkar and
Ajayan found that after coating a gold surface with a single
layer of graphene, the contact angle became about 78 degrees.
Similarly, the contact angle of silicon rose from roughly 32
degrees to roughly 33 degrees, and copper increased from around
85 degrees to around 86 degrees, after adding a layer of
graphene.

These results surprised the researchers. Graphene is
impermeable, as the tiny spaces between its linked carbon atoms
are too small for water,or a single proton, or anything else to
fit through. Because of this, one would expect that water would
not act as if it were on gold, silicon, or copper, since the
graphene coating prevents the water from directly contacting
these surfaces. But the research findings clearly show how the
water is able to sense the presence of the underlying surface,
and spreads on those surfaces as if the graphene were not
present at all.

As the researchers increased the number of layers of
graphene, however, it became less transparent to the water and
the contact angles jumped significantly. After adding six
layers of graphene, the water no longer saw the gold, copper,
or silicon and instead behaved as if it was sitting on
graphite.

The reason for this perplexing behavior is subtle. Water
forms chemical or hydrogen bonds with certain surfaces, while
the attraction of water to other surfaces is dictated by
non-bonding interactions called van der Waals forces. These
non-bonding forces are not unlike a nanoscale version of
gravity, Koratkar said. Similar to how gravity dictates the
interaction between the Earth and sun, van der Waals forces
dictate the interaction between atoms and molecules.

In the case of gold, copper, silicon, and other materials,
the van der Waals forces between the surface and water droplet
determine the attraction of water to the surface and dictate
how water spreads on the solid surface. In general, these
forces have a range of at least several nanometers. Because of
the long range, these forces are not disrupted by the presence
of a single-atom-thick layer of graphene between the surface
and the water. In other words, the van der Waals forces are
able to “look through” ultra-thin graphene coatings, Koratkar
said.

If you continue to add additional layers of graphene,
however, the van der Waals forces increasingly “see” the carbon
coating on top of the material instead of the underlying
surface material. After stacking six layers of graphene, the
separation between the graphene and the surface is sufficiently
large to ensure that the van der Waals forces can now no longer
sense the presence of the underlying surface and instead only
see the graphene coating. On surfaces where water forms
hydrogen bonds with the surface, the wetting transparency
effect described above does not hold because such chemical
bonds cannot form through the graphene layer.

Along with conducting physical experiments, the researchers
verified their findings with molecular dynamics modeling as
well as classical theoretical modeling.

“We found that van der Waals forces are not disrupted by
graphene. This effect is an artifact of the extreme thinness of
graphene—which is only about 0.3 nanometers thick,” Koratkar
said. “Nothing can rival the thinness of graphene. Because of
this, graphene is the ideal material for wetting angle
transparency.”

“Moreover, graphene is strong and flexible, and it does not
easily crack or break apart,” he said. “Additionally, it is
easy to coat a surface with graphene using chemical vapor
deposition, and it is relatively uncomplicated to deposit
uniform and homogeneous graphene coatings over large areas.
Finally, graphene is chemically inert, which means a graphene
coating will not oxidize away. No single material system can
provide all of the above attributes that graphene is able to
offer.”

A practical application of this new discovery is to coat
copper surfaces used in dehumidifiers. Because of its exposure
to water, copper in dehumidifier systems oxidizes, which in
turn decreases its ability to transfer heat and makes the
entire device less efficient. Coating the copper with graphene
prevents oxidation, the researchers said, and the operation of
the device is unaffected because graphene does not change the
way water interacts with copper. This same concept may be
applied to improve the ability of heat pipes to dissipate heat
from computer chips, Koratkar said.

“It’s an interesting idea. The graphene doesn’t cause any
significant change to the wettability of copper, and at the
same time it passivates the copper surface and prevents it from
oxidizing,” he said.

Along with Koratkar and Ajayan, co-authors of the paper are
Yunfeng Shi, assistant professor in the Department of Materials
Science and Engineering at Rensselaer; Rensselaer
mechanical engineering graduate students Javad Rafiee, Abhay
Thomas, and Fazel Yavari; Rensselaer physics graduate student
Xi Mi; and Rice mechanical and materials engineering graduate
student Hemtej Gullapalli.

This research was supported in part by the Advanced Energy
Consortium (AEC); the National Science
Foundation (NSF); and the Office of Naval Research
(ONR) graphene Multidisciplinary University Research Initiative
(MURI).

For more information on Koratkar’s graphene research at
Rensselaer, visit:

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