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.