Scientists report graphene breakthroughs

The explosion of scientific research into the properties and potential benefits of graphene since the material was first identified nearly two decades ago continues to yield substantial results.

Ultrathin materials made of a single layer of atoms have riveted scientists' attention since the isolation of graphene.

Among the exciting advances made by researchers since that time include the discovery that stacking individual sheets of the carbon-based 2D materials and sometimes twisting them at a slight angle to each other activates new properties, from superconductivity to magnetism.

The carbon-based wonder material is believed to have almost limitless potential. It is the thinnest material known to man, so thin that it is considered two-dimensional. Graphene is also the strongest material ever measured, 100-300 times stronger than steel; the best conductor of electricity, approaching the performance of a superconducting material at room temperature; the best conductor of heat, even better than diamond which is also a carbon material; and nearly transparent (when in a perfect single layer of carbon atoms).

What is equally impressive, according to the Graphene Council, is what happens when graphene is combined with other elements and molecules to create new hybrid materials that benefit from the material's extreme properties.

Ongoing investigations are fueling dramatic growth in the market for graphene, which currently totals about 10,000 metric tons annually. But that yearly output is expected to soon mushroom to some 300,000 metric tons, according to the council, as manufacturers refine production methods and scale up operations.

The Graphene Council, an independent organization based in New Bern, N. C., monitors roughly 300 companies that produce graphene or graphene-related products, of which some 100 are currently commercially viable. The organization also tests and certifies graphene products and production methods.

Numerous applications for graphene and related new materials, meanwhile, are being identified almost daily, according to the council.

Self-organizing graphene oxide

In mid-September, teams of researchers working on opposite sides of the globe reported graphene-related breakthroughs in separate sectors that could lead to numerous beneficial applications.

Scientists at the University of New South Wales in Australia Sept. 16 reported on their research involving graphene oxide that shows potential for applications in water filtration and energy storage.

Graphene oxide is made up of a single layer of carbon atoms with oxygen atoms attached. In nature, the oxygen atoms are connected to the graphene in a rather chaotic way. At elevated temperatures, however, the researchers found that the oxygen atoms will re-arrange themselves to form more organized structures with the graphene atoms. And this reorganization drastically improves various properties of the compound, such as electrical conductivity.

"Imagine scattering a big box of LEGO bricks on your living room floor," wrote the Graphene Council's Terry Barkan in reporting on the breakthrough. "A few days later you come back, and they have magically formed nicely organized piles by themselves. A team of researchers at UNSW have observed something just like that, but under a very powerful microscope, ... looking at atoms, not toy bricks."

A paper on the pioneering research, led by Rakesh Joshi, Ph. D., at UNSW is published in the September issue of "Materials Today." It reports on how the UNSW team successfully observed the oxygen reorganization phenomenon for the first time in real life, using cutting-edge electron microscopy. While common microscopes use light to create a magnified image, electron microscopes use electrons. With this type of microscope, which uses electrons instead of light to create a magnified image, it is possible to observe single atoms by magnifying what you're looking at by a factor of 1 million.

The paper's first author, Tobias Foller, a Ph. D. student in Joshi's group, said he first read about the temperature method that enhances the properties of graphene oxide without changing the chemical structure in a paper by researchers at Massachusetts Institute of Technology.

"I was immediately fascinated. Reading more, I noticed a significant amount of research was using this phenomenon to fine-tune the properties of GO (graphene oxide) for a wide range of possible applications. But none of these studies showed a direct observation of the mechanism – they assumed it was driving these enhancements, but (they) didn't actually demonstrate it," he told reporters.

Foller decided to investigate the phenomenon.

While the first promising results began to form, Priyank Kumar – the first author on the MIT paper – joined UNSW as a Scientia Lecturer in Engineering and collaborated on the study.

"Now that we understand this mechanism and have seen how it actually plays out in real life, we can control the properties of (graphene oxide) more precisely," Joshi said. "This all adds up to a key finding that gives us a deeper understanding of the properties of GO – and it might play a key role in bringing it a step closer to real-world applications such as sustainable water filtration, hydrogen generation and many more."

The study in Materials Today is also co-authored by leading scientists in electron microscopy, including UNSW's Associate Professor Shery Chang and Gwan-Hyoung Lee from Seoul National University, as well as the university's hydrogen generation experts, Professor Rose Amal and Rahman Daiyan.

Ferroelectric breakthrough

Researchers at the Massachusetts Institute of Technology's Materials Research Laboratory and Japan's National Institute for Materials Science, meanwhile, reported demonstrating that when two single sheets of boron nitride are stacked parallel to each other, this 2D material becomes ferroelectric, in which positive and negative charges in the material spontaneously head to different sides or poles.

Reporting this summer in the journal "Science," the team said the results of their work with the material, though unexpected, is an important advance. Boron nitride is also known as "white graphene" because its atomic structure is similar to that of graphene.

According to the researchers, the new material, which works via a mechanism that is completely different from existing ferroelectric materials, could have many applications.

"Wide varieties of physical properties have already been discovered in various 2D materials. Now we can easily stack the ferroelectric boron nitride with other families of materials to generate emergent properties and novel functionalities," said Pablo Jarillo-Herrero, the Cecil and Ida Green Professor of Physics at MIT and leader of the research. Jarillo-Herrero is also affiliated with MIT's Materials Research Laboratory.

In addition to Jarillo-Herrero, co-authors of the paper are Kenji Yasuda, an MIT postdoc; Xirui Wang, an MIT graduate student in physics; and Kenji Watanabe and Takashi Taniguchi of the National Institute for Materials Science in Japan.

Among potential applications of the new ultrathin ferroelectric material is denser memory storage. For example, switching the polarization of the material could be used to encode ones and zeros – digital information – and that information will be stable over time. Furthermore, it won't change unless an electric field is applied. In its research paper, the team reports a proof-of-principle experiment showing this stability.

Because the new material is only billionths of a meter thick – it's one of the thinnest ferroelectrics ever made – it also could allow much denser computer memory storage.

The researchers further found that twisting the parallel sheets of boron nitride at a slight angle to each other resulted in yet another "completely new type of ferroelectric state," Yasuda said. This general approach, known as "twistronics," was pioneered by the Jarillo-Herrero group, which used it to achieve unconventional superconductivity in graphene.

The team said the new ultrathin ferroelectric material is also exciting because it involves new physics. The mechanism behind how it works is completely different from that of conventional ferroelectric materials.

The discovery also suggests that other ferroelectrics could be produced by applying the same technique to other materials with atomic structures that are similar to boron nitride, they added.

The researchers are currently working to that end and have reported some promising results.

The Jarillo-Herrero lab is a pioneer at manipulating and exploring ultrathin, two-dimensional materials like graphene.