Unique Carbon Allotrope Offers Path to Neuromorphic Computing

Scientists have isolated cyclocarbons for the first time, and aim to use these molecules to build new materials

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Cyclo[18]carbon molecular structure model.
In a cyclocarbon molecule, each carbon atom has two neighboring atoms. Together, they form a ring of 18 carbon atoms.
Image: IBM Research

For the first time, scientists have successfully isolated and characterized a unique carbon allotrope that they expect will lead to the creation of entirely new types of materials.

Carbon is one of the elements on the periodic table that can take many structural forms and still remain carbon. These different forms of carbon are known as allotropes and each one has different properties, ranging from the hardness of diamonds to the extraordinary electrical and thermal properties of graphene.

Researchers from IBM Zurich and the University of Oxford have produced a stable version of a carbon allotrope that has long been discussed and theorized, but which remained elusive until now. It’s called a cyclocarbon. The carbon atoms in cyclocarbons have just two neighbors, and, when linked together, take the shape of a ring.

This is different than other types of carbon allotropes, like fullerenes, carbon nanotubes, and graphene, which have three neighboring carbon atoms. The team’s work is described today in the journal Science

No one knew for certain if a cyclocarbon could be made or exactly what kind of structure it would take. There was some published evidence of their existence in gas form, but because of their extreme reactivity, cyclocarbons did not remain stable enough to be isolated and characterized.

artistic 3-dimensional representation of AFM data (at small tipsample distance)This artist’s rendering of a cyclocarbon molecule is based on data collected through atomic force microscopy.Image: IBM Research

Now that a cyclocarbon has been isolated and imaged, scientists can begin to characterize its properties and determine its usefulness.

“We will see what applications the molecule can be used for in the future,” said Leo Gross, a researcher at IBM Zurich and one of the coauthors of the paper.

At the moment, what the researchers know about the cyclocarbon is that it has a relatively high electric conductivity, according to Gross.

“What makes the molecule appealing to us is that we can use it to create larger carbon-rich structures by atom manipulation on insulating films,” said Gross. “In the future, we aim to build molecular devices that function based on single electron transfer. Such devices could be used for computations with extremely low power dissipation, possibly as devices for neuromorphic computing.”

Before discussions of potential applications in neuromorphic computing could begin in earnest, the researchers had to work for three years to get to the point where they could isolate a stabilized version of the molecule.

“The cyclocarbon oxide molecules are unstable compounds as they are light- and temperature-sensitive and can explode on heating, which we observed on sublimation,” said Lorel Scriven, a doctoral student at Oxford. “We had to coordinate closely with the IBM team to synthesize and ship the cyclocarbon oxides to Zurich immediately before they were to be measured so that they could be sublimed before they decomposed.”

The researchers were able to stabilize the molecule at low temperatures (5 kelvins) on a very inert substrate (sodium chloride). This eliminated the ingredients necessary for the molecule to react and rendered it relatively stable.

However, the key to making it all work was to find a precursor molecule that was stable enough to be prepared and that can be converted by atom manipulation into the desired cyclocarbon, according to Katharina Kaiser,  a researcher at IBM Zurich.

The Oxford and IBM researchers intend to continue looking for new carbon allotropes related to cyclocarbons. Eventually, they aim to use these structures to create new materials.

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