The carbon allotrope family welcomes a new member
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The carbon allotrope family welcomes a new member
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ollowing fullerenes, carbon nanotubes, graphene, and graphdiyne, a new carbon molecule—cyclo[18]carbon—has recently joined the carbon allotrope family. This new allotrope consists of 18 sphybridized carbon atoms interconnected in a ring form. Due to their high reactivity, purely sp-hybridized cyclo-carbons had never been isolated before, leading to past controversy over their existence and structures. A joint research team led by Leo Gross of IBM Research–Zürich, as well as Przemyslaw Gawel and Harry L. Anderson of the University of Oxford, has settled the dispute. They successfully synthesized and revealed the molecular structure of cyclo[18]carbon. This work was published recently in Science (doi:10.1126/science. aay1914). The collaboration was initiated three years ago when the Anderson team approached Gross about making cyclo-carbons by leveraging IBM’s expertise for atom manipulation. “The route that was successful in the end seemed like a long shot to me, and I was happily surprised that it worked,” Gross says. The researchers adopted a voltagepulse decarbonylation strategy to prepare cyclo[18]carbon. They first synthesized cyclocarbon oxide C24O6 (a precursor) and sublimed it onto a Cu(111) surface partially coated with NaCl bilayers. The substrate was kept at 10 K to stabilize the oxide and loaded into a combined scanning tunneling microscope/ atomic force microscope (STM/AFM)
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(a) The voltage-pulse decarbonylation process converted cyclocarbon oxides (C24O6, C22O4, and C20O2) to cyclo[18]carbon. (b) Atomic force microscope image of cyclo[18]carbon. The two bright lopes at the lower part are individual CO molecules. Credit: Science.
instrument. Owing to the atomic resolution of the STM/AFM, the researchers were able to trigger chemical reactions with one precursor molecule at a time by applying 3 V voltage between the STM probe and the precursor molecule for a few seconds. This process eliminated the CO moieties of C24O6 and successfully synthesized cyclo[18]carbon after detaching all six CO moieties. By analyzing the contrast of the AFM images, the researchers concluded that cyclo[18]carbon consists of alternating triple and single carbon–carbon bonds, in contrast to a ring merely composed of double bonds. The synthesized carbon rings have not been separated from the copper substrate due to the high reactivity of cyclocarbons. “It cannot be stable at room temperature in the presence of potential reaction partners,” Gross says. Changshui Huang of the Chinese Academy of Sciences, whose research
group studies graphene and graphdiyne, says this work offers a powerful tool to address the challenge of high reactivity of cyclo
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