Synthesis and properties of confined carbyne and beyond

Carbyne, a one-dimensional carbon allotrope characterized by sp¹ hybridization, has attracted significant attention due to its unique structure and exceptional properties. In principle, carbyne is an infinite linear carbon chain, or a long linear carbon chain that its properties remain independent of its length. Despite being proposed a century ago, the existence of carbyne has been a subject of controversy, primarily because of its extreme instability and strong chemical reactivity. So far the longest end-capped polyyne and the carbon nanotube-confined linear carbon chain comprise up to 68 and 6000 carbon atoms, respectively. In this review, general synthesis methods for confined linear carbon chains, i.e., confined carbyne, are outlined, with a particular focus on the chronological development of routes towards carbyne. In addition, the structure and properties of the carbon chains unraveled by theoretical calculations and various Raman spectroscopy are discussed in detail. Finally, the current challenges in the synthesis and property-engineering of sp¹-hybridized carbon but not limited to confined carbyne are addressed, offering insights into potential future directions for both fundamental research and applications.