  We report the experimental and theoretical investigation of the growth and of
the 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 roughly 90 atoms.
Among different 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 characteristic of the isolated clusters and surviving in
the film and 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 potential interesting functional
properties.
