Rings of Carbon Atoms: How Scientists Assembled a Molecule from 88 Links express
On the surface of ordinary table salt, a ring of carbon atoms grows, like a bracelet assembled link by link. Only here the links are not metallic but carbon, and they are joined not by a jeweler but by the tip of a scanning microscope. Physicists have learned to weave such bracelets up to 88 atoms long and test how they conduct electricity.
Each ring is a cyclocarbon molecule. It has no side branches, only a closed chain of atoms. To build a large ring, scientists first made small building blocks of 20 and 22 atoms, then fused them together with the same microscopic tip. The result was rings of 42, 44, 46, 66, and 88 atoms.
The difference between rings with atom counts divisible by four (20, 44) and their neighbors (22, 42) repeats each time, but it weakens as the rings grow. For the 88-atom ring, it is barely visible. This means that very large carbon rings lose their aromaticity and behave like ordinary conducting wires.
Such molecular bracelets could become the basis for wires just one atom thick. If we learn to link them into chains and control the current, we could build electronic circuits assembled literally from individual molecules.
Original abstract
as written by the authors arXiv:2603.02040
Molecular rings of N carbon atoms, that is, cyclo[N]carbons, or C$$_N$$, can be formed by tip-induced chemistry [1-7]. Because of their monocyclic geometry, cyclocarbons are fundamentally important for testing theoretical models of aromaticity [8-11]. Here, we synthesized large cyclo[N]carbons, with N up to 88, by tip-induced chemistry on a NaCl surface and studied their aromaticity by measuring their transport gaps by scanning tunnelling spectroscopy. We first generated C$$_{20}$$ and C$$_{22}$$, and then fused multiple cyclocarbons [5-7] by means of atom manipulation, obtaining C$$_{42}$$, C$$_{44}$$, C$$_{46}$$, C$$_{66}$$ and C$$_{88}$$. In agreement with predictions obtained using a finely tuned density functional [12-15] and large active space approximate configuration interaction calculations executed on quantum hardware [16, 17], we observe a substantially smaller transport gap for C$$_{20}$$ (N = 4n) compared to C$$_{22}$$ (4n+2), and for C$$_{44}$$ (4n) relative to C$$_{42}$$ (4n+2). In larger cyclocarbons, the oscillation of the transport gap between anti-aromatic N = 4n and aromatic N = 4n+2 cyclocarbons becomes smaller, and is expected to eventually vanish with increasing N, indicating non-aromaticity. Our experimental results show that aromaticity persists at N = 42, and theory predicts ring currents comparable in magnitude to that of benzene in cyclocarbons of this size. In the future, such large cyclocarbons could be used to study conductance, quantum interference, and the effects of aromaticity in single atomic carbon wires and circuits.
The 20-atom carbon ring is the smallest one ever assembled and measured. Its diameter is less than a nanometer, tens of thousands of times thinner than a human hair.
In Ted Chiang's story "Understand," the protagonist creates ultra-thin structures by manipulating atoms — much like the way carbon rings are assembled in this work.