Improved Manufacturing Processes for 2D Materials Continue to Advance
Gaining greater control on both the quality and quantity of graphene production should support its commercialization
Graphene and other two-dimensional materials have some pretty astounding properties when measured in a laboratory. However, when you try to reproduce these properties in 3D devices not only do you see those properties recede but sometimes they are not even possible in regular real-world environments.
Of course, it’s not enough to simply maintain those attractive properties in real-world devices, but you also need to have a process that is scalable and economical. Over the last three months, we have some significant research in achieving these two goals of scalability and the preservation of the properties of 2D materials in the 3D world.
Graphene Fibers Promise Functional Textiles
Researchers at Rensselaer Polytechnic Institute (RPI) have developed a new, layered structure for graphene that promises the ability to transfer the properties of high quality 2D graphene sheets into 3D macroscopic structures.
In the graphene-based structures they fabricated they were able to achieve a mechanical strength for the 3D material while maintaining the thermal and electrical properties that makes graphene so attractive in its 2D form. The resulting graphene fibers that the RPI researchers developed could be useful in thermal management for high power electronics, structural composites, flexible and stretchable electrodes for energy storage, sensors, and membranes.
“The highly thermally conductive and mechanical strong fibers can also be used as additives or fillers for the fiber-reinforced ceramic/glass/polymer composites to further increase the mechanical strength, thermal and electrical conductivity of the composites,” said Jie Lian, one of the researchers and the lead author in the paper, in an e-mail interview. “Further possibilities may be explored for thermal actuators and constructing fabric/textiles structures by integrating other functional materials, such as for thermal energy storage.”
In the research, which was published in the journal Science, the graphene fiber was fabricated by a scalable wet spinning process in which different sized sheet structures can be applied to construct macroscopic structures (papers, fibers, tubes, and fabrics). This novel technique represents a new approach for reassembling 2D sheet structures into 3D macroscopic structures with greatly enhanced physical properties.
The discovery of the highly thermally conductive and mechanically strong fibers using different size graphene sheets is unprecedented for the conventional fiber technology, according to Lian.
In future research, Lian and his colleagues will continue to explore the large-scale manufacturing of graphene fibers by wet spinning process or combining with additive manufacturing for large-scale components. They will also be pursuing the use of these graphene fibers in functional textiles and explore their applications for thermal energy storage, thermal management and structure components.
Gaining Control of Graphene Nanoribbons’ Production
Graphene nanoribbons are held out as the best hope for the use of graphene in digital electronics. Their potential in electronic computing stems from their width: narrow ribbons serve as semiconductors and wide ones act as conductors.
Researchers at the University of Wisconsin-Madison have developed a production technique for creating these different width graphene nanoribbons that is compatible with semiconductor manufacturing methods and can be scaled up to bulk production.
The manufacturing technique is based on a “bottom-up” approach to nanomaterial manufacturing in which the graphene nanoribbons self assemble into a desired form as opposed to “top-down” approaches, like lithographic techniques, in which material is cut away to produce the final product.
In research published in the journal Nature Communications, the researchers grew the graphene on a conventional germanium semiconductor wafer.
"Graphene nanoribbons that can be grown directly on the surface of a semiconductor like germanium are more compatible with planar processing that's used in the semiconductor industry, and so there would be less of a barrier to integrating these really excellent materials into electronics in the future," said Michael Arnold, an associate professor UW-Madison, in a press release.
Of course, this is not the first time that someone thought to use a bottom-up manufacturing technique for growing graphene on a substrate. But previous attempts were restricted to growing the graphene on a metal substrate. And even the nanoribbons that were produced in this way did not have the necessary length to be useful for electronic applications.
The basic process the UW-Madison researchers employed was chemical vapor deposition (CVD) in which graphene is grown from vaporous precursor on a metal substrate, like copper, in a furnace and then later peeled off the substrate. But they put a twist into the CVD process that made all the difference: they started the process with methane. With this little wrinkle, the methane attaches to the surface of germanium substrate and breaks down into various hydrocarbons. It is these hydrocarbons that begin reacting with each other to start forming the graphene.
The other key to the process was slowing the growth rate of the graphene. This made it possible to lengthen and make narrower the nanoribbons simply by decreasing the amount of methane in the CVD furnace chamber.
"What we've discovered is that when graphene grows on germanium, it naturally forms nanoribbons with these very smooth, armchair edges," Arnold said in the release. "The widths can be very, very narrow and the lengths of the ribbons can be very long, so all the desirable features we want in graphene nanoribbons are happening automatically with this technique."
Pulsing the Voltage Tunes Graphene
By providing pulses of an electrical charge in the solution-based exfoliation of graphene from graphite rather than a steady voltage, researchers at National Cheng Kung University in Taiwan have found a way to tune both the electrical and mechanical properties of graphene in bulk process manufacturing.
One of the identified obstacles hindering graphene’s commercialization has been the slow development of manufacturing techniques that maintain its attractive properties but are scalable. To date, the mechanical exfoliation of graphite, the so-called Scotch-Tape method since you essentially use an adhesive to pull away thin layers from the graphite, is the best way to produce graphene with all the attractive mechanical and electrical properties. When solution-based techniques have been used in the past, the graphene is not in the form that has been proven to possess the material’s remarkable properties, like conductivity.
If a scalable, solution-based process for the exfoliation of graphene could be developed that preserved all the attractive qualities that mechanically exfoliated graphene possesses, then that would be a game changer.
“Whilst electrochemistry has been around for a long time it is a powerful tool for nanotechnology because it’s so finely tunable,” said Mario Hofmann, a researcher at National Cheng Kung University in Taiwan, in a press release. “In graphene production we can really take advantage of this control to produce defects.”
While pulsing the voltage was the key mechanism for tuning the defects of the graphene to ensure certain properties, to know whether they had achieved their desired tuning the researchers depended on observing the transparency of the solution.
In the research that was published in the journal Nanotechnology, the Taiwan-based researchers tested the quality of their graphene as a transparent conductor where it could be used as a replacement for indium tin oxide in displays. The resistance of their graphene films (at 50 percent transparency) was 30 times that of other graphene-based transparent conductors.
The researchers will continue to look at how altering the duration of the voltage pulses changes the exfoliation process to gain better control of the quantity of the material produced as well as the quality of the graphene.
Making 2D Materials Viable in the Real World
For all the amazing capabilities of 2D materials possess they hardly amount to anything if you can’t duplicate them outside of the controlled environments of a research lab. If air and ambient temperatures render devices made from these materials useless, it would all seem to be for naught.
To overcome this rather significant shortcoming, researchers have been investigating various passivation techniques used in semiconductor manufacturing, which involve putting a light coat of a protective oxide as a shell to protect against corrosion.
Now researchers at the University of Manchester in the U.K. have combined these passivation techniques with a number of other manufacturing techniques to produce 2-D materials that can survive in real-world environments.
“This is an important breakthrough in the area of 2-D materials research, as it allows us to dramatically increase the variety of materials that we can experiment with using our expanding 2-D crystal toolbox,” said Roman Gorbachev, who led the research, in a press release.