From properties as a superconductor to unexpected membrane separation abilities, graphene continues to surprise
When graphene is discovered to have new and sometimes unexpected properties, it quickly adds on potential new applications that it could be used for.
This year we have seen that it actually does become a superconductor, opening up potential as material used in quantum computers. We have also seen graphene surprise even the Nobel Laureate who discovered it by it serving as a membrane for filtering out nuclear waste at nuclear power plants.
Graphene’s Potential as a Superconductor Just Got a Clearer
Illustration: Takashi Takahashi/Tohoku University
Graphene’s property as a conductor is unrivalled. The ballistic transport of graphene—the speed at which electrons pass through a material at room temperature—is so fast that it has surpassed what scientists believed were its theoretical limits. It is at the point now where electrons seem to be behaving like photons in graphene. Whenever this amazing property of graphene as a conductor is mentioned, people wonder if it might make for a good superconductor.
While there has been some research that has suggested that graphene could be made into a superconductor—a material with zero resistance to the flow of electricity—we now have more conclusive proof that it is indeed the case.
In joint research out of Tohoku University and the University of Tokyo in Japan, scientists there have developed a new method for getting graphene to behave as a superconductor, and in so doing have eliminated the chance that what they were observing was the transformation of graphene into a semiconductor.
Takashi Takahashi, a professor at Tohoku University and leader of the research, explained that they took a number of different approaches to ensure that what they were witnessing was graphene becoming a superconductor. In research published in the journal ACS Nano, the researchers were first extremely meticulous about how they fabricated the graphene.
They started with high-quality graphene on a silicon carbide crystal, and controlled the number of graphene sheets. This gave them a well-characterized bilayer graphene, into which they stuffed calcium atoms. So precise was the process hat they could actually ascribe a chemical formula to their sample: C6CaC6. This was an important achievement because having a precise count for the number of Li or Ca atoms determines the amount of donated electrons into graphene, which controls the occurrence of superconductivity.
The researchers’ measurements confirmed that superconductivity did occur with the graphene. Electrical resistivity dropped rapidly at around 4 K (-269 °C), indicative of an emergence of superconductivity. The measurements further indicated that the bilayer graphene did not create the superconductivity, nor did lithium-intercalated bilayer graphene exhibit superconductivity. This meant that the drop in resistance was due to the electron transfer from the calcium atoms to the graphene sheets.
Now that graphene has been made to perform as a superconductor with a clear zero electrical resistivity, it becomes possible to start considering applying graphene into the making of a quantum computer that would use this superconducting graphene as the basis for an integrated circuit.
Unfortunately, like most superconducting materials, the temperature at which graphene reaches superconductivity is too low to be practical. Raising that temperature will be the next step in the research.
Graphene Nanoribbons Increase Their Potential
Image: Patrick Han
Graphene nanoribbons (GNRs) appear to be among the best options for electronics applications because of the each with which it’s possible to engineer a band gap into them. Narrow ones are semiconductors, while wider ones act as conductors. Pretty simple.
With improved methods being developed for manufacturing GNRs that are both compatible with current semiconductor manufacturing methods and can be scaled up, the future would appear bright. But there’s not a lot of knowledge of what happens when you start trying to manipulate GNRs into actual electronic devices.
Now a team of researchers at Tohoku University's Advanced Institute of Materials Research (AIMR) in Japan is investigating what happens when you interconnect GNRs end to end using molecular assembly to form elbow structures, which are essentially interconnection points. The researchers believe that this development provides the key to unlocking GNRs’ potential in high-performance and low-power-consumption electronics.
“Current molecular assemblies either produce straight GNRs (i.e., without identifiable interconnection points), or randomly interconnected GNRs,” said Dr. Patrick Han, the project leader, in press release. “These growth modes have too many intrinsic unknowns for determining whether electrons travel across graphene interconnection points smoothly,” said Han, who added that, “The key is to design a molecular assembly that produces GNRs that are systematically interconnected with clearly distinguishable interconnection points.”
In research published in the journal ACS Nano, the AIMR researchers demonstrated that both the electron and thermal conductivities of two interconnected GNRs should be the same as that of the ends of a single GNR.
“The major finding of this work is that interconnected GNRs do not show electronic disruption (e.g., electron localization that increases resistance at the interconnection points),” said Han in the press release. “The electronically smooth interconnection demonstrates that GNR properties (including tailored band gaps, or even spin-aligned zigzag edges) can be connected to other graphene structures. These results show that finding a way to connect defect-free GNRs to desired electrodes may be the key strategy toward achieving high-performance, low-power-consumption electronics.”
Graphene Has Special Properties for Cleaning Up Nuclear Waste
Image: The University of Manchester
The merits of graphene as a separation membrane medium have long been extolled. The properties that distinguish graphene for these applications are its large surface area, the variability of its pore size and its adhesion properties.
These attractive properties have not gone unnoticed by Andre Geim, who, along with Konstantin Novoselov, won the 2010 Nobel Prize in Physics for their discovery and study of graphene. Geim has dedicated a significant amount of his research efforts since then to the use of graphene as a filtering medium in various separation technologies.
Now Geim and his colleagues at the University of Manchester have found that graphene filters are effective at cleaning up the nuclear waste produced at nuclear power plants. This application could make one of the most costly and complicated aspects of nuclear power generation ten times less energy intensive and therefore much more cost effective.
In research published in the journal Science, Geim and his colleagues at Manchester experimented to see if the nuclei of deuterium—deuterons—could pass through the two-dimensional (2-D) materials graphene and boron nitride. The existing theories seemed to suggest that the deuterons would pass through easily. But to the surprise of the researchers, not only did the 2-D membranes sieve out the deuterons, but the separation was also accomplished with a high degree of efficiency.
“This is really the first membrane shown to distinguish between subatomic particles, all at room temperature,” said Marcelo Lozada-Hidalgo, a post-doctoral researcher at the University of Manchester and first author of the paper, in a press release. “Now that we showed that it is a fully scalable technology, we hope it will quickly find its way to real applications.”
Irina Grigorieva, another member of the research team, added: “It is a really simple set up. We hope to see applications of these filters not only in analytical and chemical tracing technologies but also in helping to clean nuclear waste from radioactive tritium.”