Left, an atomic force microscope image of the suspended graphene membrane on the copper mesh. On the right, a scanning tunneling microscope image with atomic resolution taken on the suspended graphene membrane. The researchers were able to use the scanning tunneling microscope to control the shape, and therefore the electronic properties, of the graphene membrane. Courtesy image.

Graphene
could be the superhero of materials—it’s light, strong and conducts
heat and electricity effectively, which makes it a great material for
potential use in all kinds of electronics. And because it’s made from
carbon atoms, graphene is cheap and plentiful. Its electric and
mechanical properties also affect one another in unique ways. But before
freestanding graphene can live up to its potential, scientists need to
be able to control these properties.

A
group of physicists from the University of Arkansas and other
institutions have developed a technique that allows them to control the
mechanical property, or strain, on freestanding graphene, sheets of
carbon one-atom thick suspended over the tops of tiny squares of copper.
By controlling the strain on freestanding graphene, they also can
control other properties of this important material.

“If
you subject graphene to strain, you change its electronic properties,”
said physics professor Salvador Barraza-Lopez. Strain on freestanding
graphene causes the material to behave as if
it is in a magnetic field, even though no magnets are present, a
property that scientists will want to exploit — if they can control the
mechanical strain.

To
control the mechanical strain, University of Arkansas researchers
developed a new experimental approach. Physicists Peng Xu, Paul Thibado
and students in Thibado’s group examined freestanding graphene membranes
stretched over thin square “crucibles,” or meshes, of copper. They
performed scanning tunneling microscopy with a constant current to study
the surface of the graphene membranes. This type of microscopy uses a
small electron beam to create a contour map of the surface. To keep the
current constant, researchers change the voltage as the scanning
tunneling microscope tip moves up and down, and the researchers found
that this causes the freestanding graphene membrane to change shape.

“The
membrane is trying to touch the tip,” Barraza-Lopez said. They
discovered that the electric charge between the tip and the membrane
influences the position and shape of the membrane. So by changing the
tip voltage, the scientists controlled the strain on the membrane. This
control becomes important for controlling the pseudo-magnetic properties
of graphene.

In
conjunction with the experiments, Barraza-Lopez, Yurong Yang of the
University of Arkansas and Nanjing University, and Laurent Bellaiche of
the University of Arkansas examined theoretical systems involving
graphene membranes to better understand this new-found ability to
control the strain created by the new technique. They verified the
amount of strain on these theoretical systems and simulated the location
of the scanning tunneling microscopy tip in relation to the membrane.
While doing so, they discovered that the interaction of the membrane and
tip depends upon the tip’s location on the freestanding graphene. This
allows scientists to calculate the pseudo-magnetic field for a given
voltage and strain.

“If
you know the strain, you can use theory and compute how big the
pseudo-magnetic field may be,” said Barraza-Lopez. They found that
because of the boundaries created by the square copper crucible, the
pseudo-magnetic field swings back and forth between positive and
negative values, so scientists are reporting the maximum value for the
field instead of a constant value.

“If
you were able to make the crucibles triangular, you would be closer to
having non-oscillating fields,” Barraza-Lopez said. “This would bring us
closer to using this pseudo-magnetic property of graphene membranes in a
controlled way.”

The researchers report their findings in Physical Review B Rapid Communications.

Full
details of this work are available online (PRB 85, 121406(R) (2012)).
Scientists involved in the research are from the University of Arkansas,
Nanjing University, École Centrale Paris, Quingdao University and
Missouri State University.

Source: University of Arkansas