The Remarkable Properties of Graphene Continue to Expand
Manufacturing techniques improve but we’re still finding out what graphene is capable of doing
While a fair amount of graphene research is now centered around improving manufacturing techniques—and deservedly so—we are still at the point at which an equal amount of research is dedicated to just finding out all its properties. We know about its unparalleled conductivity in which electrons can move through it so that they behave almost like photons. And we’ve even been able to engineer its less desirable properties out of it, such as lacking a band gap, so that we can use it for stopping and starting the flow of electrons to enable its use in digital logic applications.
Graphene Can Become a Superconductor
Now an international research team from Canada and Germany have discovered when graphene is “decorated” with lithium atoms it behaves like a superconductor.
Just as a bit of background, superconductors conduct electricity without resistance and without dissipating energy. They are distinct from ordinary materials in which electrons repel each other. Instead in superconductors the electrons form pairs known as Cooper pairs. These Cooper pairs then flow together through the material without resistance. Phonons, the mechanism that facilitates these pairings of electrons’ are vibrations in lattice crystalline structures.
In a research published in the online journal arXiv, the international research team were able to produce this superconductivity in graphene by growing layers of graphene on silicon-carbide substrates, then depositing lithium atoms onto the graphene in a vacuum at 8 K, creating a version of graphene known as “decorated” graphene.
Now that the researchers have been able to demonstrate that they make graphene into a superconductor they believe that this new property for graphene could lead to a new generation of superconducting nanoscale devices. Some prominent researchers in the field have suggested that this work could usher in the production of nanoscale superconducting quantum interference devices and single-electron superconductor quantum dots.
A Simple Twist Changes Graphene’s Fate
While joint research between a team at Rice University and a researcher in Russia remains only in computer models, it does suggest that by simply twisting graphene it’s possible to dramatically change its electrical properties.
In research published in the Journal of Physical Chemistry Letters, the international research team were able to produce in graphene something called the flexoelectric effect in which a material exhibits a spontaneous electrical polarization brought on by a strain.
When graphene is laid out in a flat plane all its atoms have a balanced electrical charge. However, when you begin to put a curve into that plane, the electron clouds begin to compress on the concave side of the curve and stretch on the convex side. This alters the dipole moment, which measures the overall polarity of the polarity and determines how polarized atoms interact with external electric fields.
“While the dipole moment is zero for flat graphene or cylindrical nanotubes, in between there is a family of cones, actually produced in laboratories, whose dipole moments are significant and scale linearly with cone length,” said Boris Yakobson, who led the research, in a press release.
Yakobson believes that this research could help with a number of engineering issues with graphene.
“One possibly far-reaching characteristic is in the voltage drop across a curved sheet,” he said. “It can permit one to locally vary the work function and to engineer the band-structure stacking in bilayers or multiple layers by their bending. It may also allow the creation of partitions and cavities with varying electrochemical potential, more ‘acidic’ or ‘basic,’ depending on the curvature in the 3-D carbon architecture.”
Engineering a Bit More Selectivity Into Graphene Biosensors
The attractive properties that makes graphene so attractive for biosensors is its large surface-to-volume ratio—it’s entirely surface—and its conductivity. What this means is that when any molecule comes in contact with its surface, the change of conductivity of the material is pretty clear.
While this should sell anyone on the idea of using graphene as a biosensor material, the problem has been that graphene changes its conductivity for just about any molecule that it comes in contact with. It’s not very selective.
Researchers at Helmholtz-Zentrum Berlin for Material and Energy in Germany have set out to change that by developing a method for both measuring and controlling the thickness of an organic compound bound to a graphene layer.
The researchers achieved this by electrochemically treating the graphene with an organic solution that grafts itself to the surface of the graphene. The organic molecules serve as a kind of mounting bracket on which only the molecules being targeted will attach themselves.
“Thanks to these molecules, the graphene can now be employed for detecting various substances similar to how a key fits a lock,” explained Marc Gluba, one of the researchers, in a press release.
While other research teams have reported the ability to achieve similar selectivity, those attempts depended on graphene flakes that were so small they suffered from a lot of edge effects, which can detrimentally impact the electronic and magnetic properties of the material and reduce the effectiveness of the sensor.
Instead the German researchers were able to fabricate large sheets of graphene several square centimeters in size, significantly reducing the damaging edge effects.
The ultimate aim of the researchers is use graphene in a lab-on-a-chip device with which a range of medical diagnoses can be carried out with a single drop of blood.