Researchers at Virginia Commonwealth University (VCU) and universities in China and Japan have discovered a new structural variant of carbon called “penta-graphene”— a fourth form of carbon, joining fullerene, the nanotube and graphene.

Penta-graphene is a very thin sheet of pure carbon that has a unique structure inspired by a pentagonal (five-sided) pattern of tiles and that appears to be dynamically, thermally and mechanically stable.

“The three last important forms of carbon that have been discovered were. Each one of them has unique structure. Penta-graphene will belong in that category,” “Other forms of carbon are usually made of hexagonal building blocks, sometimes interspersed with pentagons. Penta-graphene would also be a unique two-dimensional carbon allotrope composed exclusively of pentagons,” said the paper’s senior author, Puru Jena, Ph.D., distinguished professor in the Department of Physics in VCU’s College of Humanities and Sciences.

Might outperform graphene in some applications

The researchers simulated the synthesis of penta-graphene using computer modeling. The results suggest that the material might outperform graphene in certain applications because it would be mechanically stable, possess very high strength, and be capable of withstanding temperatures of up to 1,000 degrees Kelvin.

“You know the saying, diamonds are forever? That’s because it takes a lot of energy to convert diamond back into graphite,” Jena said. “This will be similar.”

Penta-graphene has several interesting and unusual properties, Jena said. For example, penta-graphene is a semiconductor, whereas graphene is a conductor of electricity.

“When you take graphene and roll it up, you make what is called a carbon nanotube which can be metallic or semiconducting,” Jena said. “Penta-graphene, when you roll it up, will also make a nanotube, but it is always semiconducting.”

The way the material stretches is also highly unusual, the researchers said. “If you stretch graphene, it will expand along the direction it is stretched, but contract along the perpendicular direction,” Wang said. “However, if you stretch penta-graphene, it will expand in both directions.”

The material’s mechanical strength, derived from a rare property known as Negative Poisson’s Ratio, may hold especially interesting applications for technology, the researchers said, with applications in electronics, biomedicine, nanotechnology, and more.

The next step, Jena said, is for scientists to synthesize penta-graphene. “Once you make it, it [will be] very stable. So the question becomes, how do you make it? In this paper, we have some ideas. Right now, the project is theoretical. It’s based on computer modelling, but we believe in this prediction quite strongly. And once you make it, it will open up an entirely new branch of carbon science. Two-dimensional carbon made completely of pentagons has never been known.”

An accidental discovery in a restaurant

Qian Wang, Ph.D., a professor at Peking University and an adjunct professor at VCU, was dining in a restaurant in Beijing with her husband when she noticed artwork on the wall depicting pentagon tiles from the streets of Cairo.

“I told my husband, “Come, see! This is a pattern composed only of pentagons,’” she said. “I took a picture and sent it to one of my students, and said, ‘I think we can make this. It might be stable. But you must check it carefully.’ He did, and it turned out that this structure is so beautiful yet also very simple.”

The researchers’ paper, “Penta-Graphene: A New Carbon Allotrope,” will appear in the journal Proceedings of the National Academy of Sciences, and is based on research that was launched at Peking University and VCU. Along with Jena and Wang, the paper’s authors include Shunhong Zhang, Ph.D candidate, from Peking University; Jian Zhou, Ph.D., a postdoctoral researcher at VCU; Xiaoshuang Chen, Ph.D., from the Chinese Academy of Science in Shanghai; and Yoshiyuki Kawazoe, Ph.D., from Tohoku University in Sendai, Japan.

Abstract of Penta-graphene: A new carbon allotrope

A 2D metastable carbon allotrope, penta-graphene, composed entirely of carbon pentagons and resembling the Cairo pentagonal tiling, is proposed. State-of-the-art theoretical calculations confirm that the new carbon polymorph is not only dynamically and mechanically stable, but also can withstand temperatures as high as 1000 K. Due to its unique atomic configuration, penta-graphene has an unusual negative Poisson’s ratio and ultrahigh ideal strength that can even outperform graphene. Furthermore, unlike graphene that needs to be functionalized for opening a band gap, penta-graphene possesses an intrinsic quasi-direct band gap as large as 3.25 eV, close to that of ZnO and GaN. Equally important, penta-graphene can be exfoliated from T12-carbon. When rolled up, it can form pentagon-based nanotubes which are semiconducting, regardless of their chirality. When stacked in different patterns, stable 3D twin structures of T12-carbon are generated with band gaps even larger than that of T12-carbon. The versatility of penta-graphene and its derivatives are expected to have broad applications in nanoelectronics and nanomechanics.