Pure carbon ring created and imaged at last


Rebecca Pool

Friday, August 16, 2019 - 10:45
Image: Artistic 3D representation of AFM data (at increased tip-sample distance) [IBM Research]
In a feat that has eluded researchers for more than fifty years, researchers from the University of Oxford and IBM Research have created an 18-carbon ring and characterised it using atomic force microscopy.
The new form of carbon, cyclocarbon, joins other carbon forms including buckyballs and carbon nanotubes and provides fresh insight into the reactivity and structure of the molecule, which has been long-debated amongst chemists.
“Our goal was to synthesize, stabilize and characterize cyclocarbon,” says Katharina Kaiser from the University of Oxford. “And for the first time, we have succeeded in stabilizing and imaging a ring of 18 carbon atoms.” 
From left to right, precursor molecule C24O6, intermediates C22O4 and C20O2 and the final product cyclo[18]carbon C18 created on surface by dissociating CO masking groups using atom manipulation. The bottom row shows atomic force microscopy (AFM) data using a CO functionalized tip. [Credit: IBM Research]
Cyclocarbons are allotropes consisting of covalent carbon bonds arranged into a ring formation.
To generate cyclocarbon, researchers from the Anderson Research Group at Oxford generated an 18-carbon ring - the smallest cyclocarbon that’s considered thermodynamically stable - from a stable precursor molecule, C24O6, adsorbed on a salt surface at 5 degrees Kelvin.
Then using a home-built STM/AFM system at IBM, Leo Gross and IBM colleagues were able to apply voltage pulses that removed carbon monoxide molecules to form the carbon ring as well as characterise the structure at high resolution.
The researchers functionalised the AFM tip with carbon monoxide to boost resolution, with the image contrast allowing the researchers to identify the C18 as polyynic, comprising alternating triple and single bonds.
This result resolves decades-long theoretical debate on whether this from of carbon is polyynic or cumulenic, with repeated double bonds.
Cyclo[18]carbon. Bottom: artistic 3-dimensional representation of AFM data (at increased tipsample distance); top: molecular structure model. [IBM Research]
Cyclocarbons have been notoriously difficult to isolate as the molecular rings are highly reactive; previous attempts provided only transient glimpses of cyclocarbons in the gas phase. 
As Kaiser and colleagues, write in Science: “These attempts have given tantalizing glimpses of cyclocarbon in the gas phase.”
“Cyclocarbons may coalescence to form fullerenes, and gas-phase electronic spectra of C18, formed by laser ablation of graphite, have been measured,” adds Kaiser. “But these studies did not reveal whether the structures are cumulenic or polyynic.”
The research highlights potential new avenues for creating other elusive carbon allotropes by manipulating atoms and developing new carbon-rich materials.
“Our results provide direct experimental insights into the structure of a cyclocarbon and open the way to create other elusive carbon-rich molecules by atom manipulation,” says Kaiser.
Initial studies of the properties of the cyclocarbon also suggest that it acts as a semiconductor, opening the door to molecular-scale transistors.
Research is published in Science.
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