Linear Carbon: From 1D Carbyne to 2D Hybrid sp-sp2 Nanostructures Beyond Graphene

Carbon is a versatile element forming a wide variety of allotropic forms and nanostructures, in which sp-, sp2-, and sp3-hybridization give rise to linear, planar and three-dimensional systems. In addition, other more exotic and low dimensional phases have been found and produced, including quasi-zero- and quasi-one-dimensional systems as fullerenes, carbon clusters, nanodiamonds, nanotubes, and truly 2D materials like graphene and related systems, whose peculiar properties are of great interest in the fields of physics, chemistry, nanoscience, and nanotechnology, materials science and engineering. Here, we focus on linear carbon structures as functional building blocks for novel sp-sp2 hybrid carbon structures, including 2D crystals beyond graphene. Carbon atoms with sp-hybridization ideally form an infinite chain, the so-called “carbyne”, while finite systems are carbon atomic wires (CAWs) terminated with suitable end groups. CAWs show tunable electronic and optical properties between the ideal polyynic (-C≡C-) and cumulenic (=C=C=) structures, corresponding to a semiconducting or metallic behavior and offering an appealing opportunity for developing tailored functional nanostructures. In addition, CAWs can be integrated in 2D systems such as hybrid sp– sp2-carbon nanostructures (e.g., graphyne and graphdiyne) with still widely unexplored potential in science and technology.