Hotly debated carbon ring allotrope reveals its structure

Cyclo[18]carbon. Artistic representation of AFM data of a cyclo[18]carbon molecule, with the determined molecular structure fading in. Credit: IBM Research

Bonding matters. It explains the difference between diamond and pencil lead (graphite) – both pure carbon – but one has each carbon bonded to another four carbons, while the layers of graphite have carbons bonded to just three other carbons in a hexagonal honeycomb lattice. A question that has plagued researchers for a long time, is could a form of carbon exist where the atoms bond to just two other carbons, and if so, how would they bond?

“This was really debated,” says Leo Gross, a researcher at IBM Zurich in Switzerland. “There were people proposing this and that; papers coming out that contradicted and then again contradicted previous works – it was clearly an open question.”

Not any more. In recent work that demonstrates the frontiers of scanning probe microscopy capabilities, a collaboration led by Gross, and Przemyslaw Gawel and Harry L. Anderson at Oxford University in the UK, have for the first time isolated pure carbon rings, as well as imaging them with sufficient resolution to characterize their bonding structure.

Scanning probe precision

Researchers have pondered the existence of cyclocarbons since the 1960s, long before “nanocarbons” hit the limelight. But along with their posited existence came two theories for the bonding in cyclocarbons: a “cumulenic” ring of even double bonds; or a “polyynic” ring of alternating single and triple bonds, and consequently alternating bond lengths. While there have been glimpses of gas-phase cyclocarbons these have been too fleeting to pin down what is going on between the carbon atoms.

The IBM Zurich and Oxford University researchers produced a cyclocarbon of 18 carbon atoms from precursor molecules that are effectively C₁₈ cyclocarbons with additional molecular C-O groups “masking” the pure carbon allotrope. They focused on C₁₈ because the precursors proved less complicated to synthesize. In addition, Hückel’s rule - devised by Erich Hückel in 1931 to explain the delocalized electrons in benzene and derivative “aromatic” molecules in terms of quantum mechanics – predicted that C_18­ would also have delocalized aromatic bonding, making it more stable.

Previously people had attempted to burn off masking molecules by heating or illuminating the precursors, but a conversation at a conference spawned the idea to bring the precision of scanning tunnelling microscopy (STM) to the approach. First invented in the mid-1980s, STMs use the highly sensitive tunnelling current to map samples with atomic precision. The IBM Zurich group had devised ways of manipulating atoms with STMs and the Oxford University researchers were keen to apply the techniques for isolating cyclocarbon. However, despite the mutual enthusiasm to try the idea, as Gross tell Physics World, “It was a new type of reaction triggered to get rid of the CO masking groups - it was not clear this would work.”

By ramping up the STM voltage from 0.2 V to 3 V for a few seconds the researchers successfully removed the extra C-O groups to form pure cyclocarbon molecules. The isolated cyclocarbons proved very reactive, so that they could easily react with other molecules, which as lead author of the Science paper IBM Zurich’s Katherina Kaiser points out “actually happened very often.” While posing practical complications for experiments that already pushed STM precision to its limits, this reactivity meant that the researchers could demonstrate how pushing two cyclocarbons together would cause them to fuse with a covalent bond between them. This additional  capability brings the IBM Zurich team a step closer to their goals in developing designer molecules for single-electron circuits that enable ultralow-power electronics and possibly neuromorphic computing.

Seeing is believing

The real surprise came when the researchers then examined their samples with an atomic force microscope functionalized with a CO molecule at the tip for enhanced resolution. The atomic force microscope (AFM) came soon after the development of STM, and again uses a probe with a nano-sharp tip but this time to feel across the sample a little like the needle of a vinyl record player, which means it can image non-conducting samples.

Kaiser describes how the resolution revealed details as fine as the increasingly even brightness for the more planar cyclocarbons as they removed CO groups from the precursor. What they saw when all CO groups were removed were nonagons clearly indicating triple bonds at the bright corners of the nonagon with longer single bonds inbetween, in contrast to Hückel’s prediction. “We were surprised and delighted,” adds Gross. “because any other result would have been hard to interpret.”

Gross explains that they are familiar with the inert bilary NaCl surface the cyclocarbons formed on. Several other molecules they produced on this surface have had very similar structures to the gas phase so there is reason to expect little impact from the surface on the cyclocarbon structure.

“This work brings us a novel experimental fact about an sp-hybridized carbon allotrope,” says Hiroyuki Isobe, a researcher at the University of Tokyo in Japan, who was not involved in the work but has also pioneered new allotropes of carbon. “In a cyclic form on salt, the molecule adopts a polyynic conjugation and can be manipulated by AFM for unique chemical reactions. This study deepens our understanding of the important element, carbon, and stimulates our interest about the uniqueness of cyclic conjugated systems.”

Full details are reported in Science.

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