Carbyne ( a.k.a. Linear Acetylenic Carbon - LAC) is a monovalent allotrope of carbon with three non-bonded electrons, having the chemical structure (−C≡C−)_n in a (long) repeating chain, with alternating single and triple bonds.

In the same way that graphene is the ultimate 2D carbon system, a single, one-atom-thick carbon layer (often isolated from graphite) carbyne represents the ultimate 1D carbon system - a 'chain' with a diameter of only one atom.

Since the 1960s, numerous laboratory studies have claimed to have observed (and/or created) carbyne.

Theoretical calculations show that carbyne chains would, per density, be the strongest material known. Considerably exceeding the tensile strength of diamond, graphene and fullerenes.

So far, its existence has been confirmed in short, self-linked (endohedral) chains.

Because of the lack of truly isolated carbyne chains for experimentation, its chemical and electronic properties are as yet unknown, and so are restricted to theoretical models. Some researchers assert that it cannot stably exist in any but the shortest of chains. The limit so far observed is around 40 atoms.

Further reading :

Wikipedia and Carbyne: from the elusive allotrope to stable carbon atom wires arXiv, 2018