  Carbon allotropes built from rings of two-coordinate atoms, known as
cyclo[n]carbons, have fascinated chemists for many years, but until now they
could not be isolated or structurally characterized, due to their high
reactivity. We generated cyclo[18]carbon (C18) using atom manipulation on
bilayer NaCl on Cu(111) at 5 Kelvin by eliminating carbon monoxide from a
cyclocarbon oxide molecule C24O6. Characterization of cyclo[18]carbon by
high-resolution atomic force microscopy revealed a polyynic structure with
defined positions of alternating triple and single bonds. The high reactivity
of cyclocarbon and cyclocarbon oxides allows covalent coupling between
molecules to be induced by atom manipulation, opening an avenue for the
synthesis of other carbon allotropes and carbon-rich materials from the
coalescence of cyclocarbon molecules.
