Leaving the fullerene road: presence and stability of sp chains in sp 2 carbon clusters and cluster-assembled solids

We report an experimental and theoretical investigation of the growth and structure of large carbon clusters produced in a supersonic expansion by a pulsed microplasma source. The absence of a significant thermal annealing during the cluster growth causes the formation of disordered structures, where sp2 and sp hybridizations coexist for particles larger than ∼90 atoms. Among the various structures, we recognize sp2 closed networks encaging sp chains. This ‘nutshell’ configuration can prevent the fragmentation of sp species upon deposition of the clusters, thus allowing the formation of nanostructured films containing carbynoid species, as shown by Raman spectroscopy. Atomistic simulations confirm that the observed Raman spectra are the signature of the sp/sp2 hybridization typical of isolated clusters and surviving in the film and they provide information about the structure of the sp chains. Endohedral sp chains in sp2 cages represent a novel way in which carbon nanostructures may be organized with potentially interesting functional properties.