An artist’s rendering of an array of pressurized graphene membranes. A CU-Boulder team recently discovered that graphene has surprisingly high adhesion properties, findings that may help lead to the development of new graphene-based mechanical devices like gas separation membranes. Image: Victor Tzen and Rex Tzen
Graphene, considered the most exciting new material under
study in the world of nanotechnology, just got even more interesting, according
to a new study by a group of researchers at the University of Colorado
The new findings—that graphene has surprisingly powerful
adhesion qualities—are expected to help guide the development of graphene
manufacturing and of graphene-based mechanical devices such as resonators and
gas separation membranes, according to the CU-Boulder team. The experiments
showed that the extreme flexibility of graphene allows it to conform to the
topography of even the smoothest substrates.
Graphene consists of a single layer of carbon atoms chemically
bonded in a hexagonal chicken wire lattice. Its unique atomic structure could
some day replace silicon as the basis of electronic devices and integrated
circuits because of its remarkable electrical, mechanical, and thermal
properties, says Assistant Professor Scott Bunch of the CU-Boulder mechanical
engineering department and lead study author.
A paper on the subject was published online in Nature Nanotechnology. Coauthors on the
study included CU-Boulder graduate students Steven Koenig and Narasimha Boddeti
and Professor Martin Dunn of the mechanical engineering department.
“The real excitement for me is the possibility of
creating new applications that exploit the remarkable flexibility and adhesive
characteristics of graphene and devising unique experiments that can teach us
more about the nanoscale properties of this amazing material,” Bunch says.
Not only does graphene have the highest electrical and thermal
conductivity among all materials known, but this “wonder material”
has been shown to be the thinnest, stiffest, and strongest material in the
world, as well as being impermeable to all standard gases. It’s newly
discovered adhesion properties can now be added to the list of the material’s
seemingly contradictory qualities, says Bunch.
The CU-Boulder team measured the adhesion energy of graphene
sheets, ranging from one to five atomic layers, with a glass substrate, using a
pressurized “blister test” to quantify the adhesion between graphene
and glass plates.
Adhesion energy describes how “sticky” two things
are when placed together. Scotch tape is one example of a material with high
adhesion; the gecko lizard, which seemingly defies gravity by scaling up
vertical walls using adhesion between its feet and the wall, is another.
Adhesion also can play a detrimental role, as in suspended micromechanical
structures where adhesion can cause device failure or prolong the development
of a technology, says Bunch.
The CU research, the first direct experimental measurements of
the adhesion of graphene nanostructures, showed that so-called “van der
Waals forces”—the sum of the attractive or repulsive forces between molecules—clamp
the graphene samples to the substrates and also hold together the individual
graphene sheets in multilayer samples.
The researchers found the adhesion energies between graphene
and the glass substrate were several orders of magnitude larger than adhesion
energies in typical micromechanical structures, an interaction they described
as more liquid-like than solid-like, says Bunch.
The CU-Boulder study was funded primarily by the National
Science Foundation and the Defense Advanced Research Projects Agency.
The importance of graphene in the scientific world was
illustrated by the 2010 Nobel Prize in physics that honored two scientists at Manchester University
Andre K. Geim and Konstantin Novoselov, for producing, isolating, identifying,
and characterizing graphene.
There is interest in exploiting graphene’s incredible
mechanical properties to create ultrathin membranes for energy-efficient
separations such as those needed for natural gas processing or water
purification, while graphene’s superior electrical properties promise to
revolutionize the microelectronics industry, says Bunch.
In all of these applications, including any large-scale
graphene manufacturing, the interaction that graphene has with a surface is of
critical importance and a scientific understanding will help push the
technology forward, he said.