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Introduction
The last 30 years have seen carbon materials and nanostructures playing a relevant and increasing role in science and technology. The discovery of fullerenes, nanotubes, and conductive polymers and the advent of graphene are some examples of fundamental milestones. The existence of other exotic forms of carbon and the foreseen potential of developing novel systems make this period “The era of carbon allotropes”, as proposed by A. Hirsch in 2010.[1] In fact, among carbon materials, the case of the “lacking allotrope” consisting of sp-hybridized carbon atoms found a significant interest in the past[2] and its importance is expected to increase progressively in the future, in view of recent theoretical predictions and achievements of new hybrid sp–sp2 materials developed and investigated in the last 10 years.[3,4]
Following the commentary by Hirsch, in Fig. 1 we report the roadmap of the most relevant nanostructures in the field of sp-carbon. The history of carbon materials has been enlightened by a number of Nobel prizes such as those awarded to R. Curl, H. Kroto, and R. Smalley for the discovery of fullerenes in 1996[5] to A. Heeger, A. MacDiarmid, and H. Shirakawa for conducting polyacetylene in 2000[6] and to A. Geim and K. Novoselov for research on graphene in 2010.[7] It is interesting to notice that the serendipitous discovery of fullerene in 1985 was indeed stimulated by sp-carbon since H. Kroto was truly looking for short linear carbon chains relevant for astrophysics and astrochemistry research investigating the presence of carbon aggregates in the interstellar medium.[8]
Figure 1.
The path of some of the most relevant sp2 and sp–sp2 carbon nanostructures discovered and/or fabricated.
[Figure omitted. See PDF]
While the research of sp-carbon actually dates back to the end of the IXX century focusing on the quest for a new carbon allotrope called carbyne, recent achievements and the research on graphene pushed the interest on sp-carbon as possible novel nanostructures. Initially, linear carbon chains of increasing lengths, carbon rings, cages, sheets, and other kind of carbon...