A new nanoscale structure of carbon has joined the family. With no curve at all, carbon atoms form a flat sheet known as graphene, and a positive curvature creates the soccer ball-like structure sometimes called buckyballs. For decades scientists have theorized that a third variation should exist – a structure with negative curvature, known as schwarzite. An international team has now found a way to create these structures, which may have unusual electrical, magnetic and optical properties.

Common as it is, carbon is far from dull, especially when laid out in 2D forms. Graphene is famously strong and an excellent conductor of heat and electricity, and can find different uses when rolled up into buckyballs or curled into nanotubes. But since the 1990s, it's been hypothesized that another form should be possible – that with a negative curve. It can be difficult to picture what that looks like, but rather than bending outwards as a sphere, it bends inwards like a saddle.

Although researchers have been trying to create these structures out of carbon for decades, it turns out other scientists have solved the mystery without realizing there was a mystery to be solved. Now an international team, headed up by chemists at UC Berkeley, has identified the structures they were creating as the long-sought schwarzites.

The scientists who had created the structures had been calling them zeolite-templated carbons (ZTCs). That's because they were formed inside the pores of zeolites, crystalline silicon dioxide minerals. These researchers were investigating the structures for strange properties that might make them useful.

Realizing these ZTCs were schwarzites, the UC Berkeley team then developed the recipe for making them. A vapor of carbon-containing molecules is first injected into the pores of a zeolite, where the carbon gathers on the inner walls. As it forms a 2D sheet similar to graphene, the surface begins to pull inwards, creating the telltale negative curve of a schwarzite. Finally, the zeolite is dissolved away, leaving the carbon structure ready for harvesting.

"These negatively-curved carbons have been very hard to synthesize on their own, but it turns out that you can grow the carbon film catalytically at the surface of a zeolite," says Efrem Braun, an author of the study. "But the schwarzites synthesized to date have been made by choosing zeolite templates through trial and error. We provide very simple instructions you can follow to rationally make schwarzites and we show that, by choosing the right zeolite, you can tune schwarzites to optimize the properties you want."

The team says that the schwarzites could have a range of applications. Their ability to carry large amounts of electrical charge could make for better capacitors, while their large surface area could see them used as catalysts for creating petroleum, and the empty space inside them could make for useful storage vessels of atoms and molecules.

The researchers plan to continue investigating the properties of these carbon structures, as well as checking different zeolites to determine whether other forms can be made.

"The experimental validation of schwarzites thus completes the triumvirate of possible curvatures to graphene; positively curved, flat, and now negatively curved," says Braun.

The research will be published in the journal Proceedings of the National Academy of Sciences. The team demonstrates the formation of the schwarzite in the animation below.

Source: UC Berkeley