Flower-like defects in graphene can occur during the fabrication process. The NIST team captured images of one of the defects (figures a and c) using a scanning tunneling microscope. A simulated image from their computer models (figure b) shows excellent agreement. Image: Cockayne,Stroscio/NIST
A class of decorative, flower-like defects in the
nanomaterial graphene could have potentially important effects on the
material’s already unique electrical and mechanical properties, according to
researchers at the National Institute of Standards and Technology (NIST) and
Georgia Tech. In a new paper, the team for the first time describes a family of
seven defects that could occur naturally or be induced to occur in graphene,
one of which already has been observed.
Graphene is renowned for its strength and conductivity, both
of which are a result of its structure. For the most part, graphene is a
featureless plane of carbon atoms arranged in a honeycomb lattice.
According to NIST Fellow Joseph Stroscio, defects can appear
due to the movement of the carbon atoms at high temperatures when producing
graphene by heating silicon carbide under ultrahigh vacuum. The easiest, i.e.
requiring the least amount of energy, rearrangements graphene can make are to
switch from six-member carbon rings to rings containing five or seven atoms,
which keeps all the carbon atoms happy with no unsatisfied bonds. The NIST
researchers have discovered that stringing five and seven member rings together
in closed loops creates a new type of defect or grain boundary loop in the honeycomb
According to NIST researcher Eric Cockayne, the fabrication
process plays a big role in creating these defects.
“As the graphene forms under high heat, sections of the
lattice can come loose and rotate,” Cockayne says. “As the graphene
cools, these rotated sections link back up with the lattice, but in an
irregular way. It’s almost as if patches of the graphene were cut out with
scissors, turned clockwise, and made to fit back into the same place, only it
really doesn’t fit, which is why we get these flowers.”
The exceedingly rigid lattice already is stronger than
steel, but the defects might allow it a little flexibility, making it even more
resilient to tearing or fracturing.
With more experimentation, Cockayne says, researchers should
be able to correlate the appearance of defects with variations in growth
conditions, which should make it possible to either avoid defects entirely or
produce them at will.
Moreover, while the flower defect is composed of six pairs
of five- and seven-atom rings, Cockayne and the NIST team’s modeling of
graphene’s atomic structure suggests there might be a veritable bouquet of
flower-like configurations. These configurations—seven in all—would each
possess their own unique mechanical and electrical properties.