New structural variant of carbon made of pentagons

Researchers from the US and China have discovered a structural variant of carbon called ‘penta-graphene’ - a very thin sheet of pure carbon that resembles the pentagonal pattern of the tiles which pave the streets of Cairo.

Unlike most forms of carbon, which are made of hexagonal building blocks and sometimes interspersed with pentagons, carbon structures composed only of pentagons were, until now, unknown. Writing in the journal Proceedings of the National Academy of Sciences, the researchers ask, “Is it possible to have materials made exclusively of carbon pentagons and if so will they be stable and have unusual properties?”

Qian Wang, a professor at Peking University and an adjunct professor at Virginia Commonwealth University (VCU), explained that she was dining in a restaurant with her husband when she noticed artwork on the wall depicting pentagon tiles from the streets of Cairo.

“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,’” she recalled. “He did, and it turned out that this structure is so beautiful yet also very simple.”

The researchers simulated the synthesis of penta-graphene - a two-dimensional carbon allotrope composed entirely of pentagons - using computer modelling. According to their calculations, “the new carbon polymorph is not only dynamically and mechanically stable, but also can withstand temperatures as high as 1000 K”, the authors said.

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

The authors of the paper include Puru Jena, a distinguished professor in VCU’s Department of Physics; Qian Wang, a professor at Peking University and an adjunct professor at VCU; and Jian Zhou, a postdoctoral researcher at VCU.

Penta-graphene’s mechanical strength is derived from a rare property known as negative Poisson’s ratio, said the researchers, as a result of its unique atomic configuration. The way the material stretches is particularly unusual, with Wang noting, “If you stretch graphene, it will expand along the direction it is stretched but contract along the perpendicular direction. However, if you stretch penta-graphene, it will expand in both directions.”

And while graphene is a conductor of electricity, penta-graphene is a semiconductor. Jena explained, “When you take graphene and roll it up, you make what is called a carbon nanotube, which can be metallic or semiconducting. Penta-graphene, when you roll it up, will also make a nanotube, but it is always semiconducting.”

Although penta-graphene is currently only theoretical, Jena believes the eventual synthesis of the material will “open up an entirely new branch of carbon science”. The versatility of the penta-graphene and its derivatives is expected to have applications in electronics, biomedicine, nanotechnology and more.

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