Synthesis and Characterization of Carbynoid Structures in Cluster-Assembled Carbon Films - Polyynes: Synthesis, Properties, and Applications - page 31

very sensitive to temperature variations even in a small range. The presence
of two very different decay times may suggest that the faster one is related to
structural rearrangements induced by temperature gradients. The slower
one should be related to accelerated cross-linking processes. Two different
activation energies should correspond to these two regimes.
Considering the formation and deposition process of carbon clusters by
PMCS, it is reasonable to assume that the carbynoid species are formed in
the cluster source prior to deposition. The picture coming out from our
observations is characterized by relatively fragile sp chains that are quite
surprisingly able to survive deposition at kinetic energies per atom, well
above the thermal energy measured to induce the sp rearrangement and sp 2
formation.
One possible mechanism for the stabilization of sp chains in the carbon
matrix may be the formation of joints between the chain ends and sp 2 island
adatoms. Jarrold and co-workers have reported the observation of ball-and-
chain dimers formed by fullerenes linked by sp chains obtained by laser
desorption of fullerene films [44]. The same type of connections could be
formed in our films, rich of fullerene-like fragments [35], thus allowing the
stabilization of the carbynoid species. Different type of stabilization sites
could be present in our films since we observe a metastable decay of the
carbynoid population.
The landing process and the heating process are taking place over two
very different time scales. The landing process takes place on a short time
scale and the kinetic energy can be efficiently dissipated among a huge
amount of degrees of freedom of the substrate and of the cluster itself,
especially for large clusters [45]. On the other hand the thermal heating is
a process where vibrational modes are statistically populated: the time scale
of the process is such that the modes leading to rearrangement can be
efficiently populated.
Our results also address another relevant aspect of carbon clusters: that
is, the shape and the hybridization of the precursor aggregates. As we have
shown, we are depositing particles in a mass range where fullerene-like
shape and sp 2 hybridization should be predominant [46,47]. However, an
accurate analysis of the mass spectra shows that the contribution of odd
clusters is not negligible ( Figure 2.2 ) . This suggests that non-fullerene type
of clusters could be more aboundant than expected even for relatively large
clusters. Another possibility, supported by the studies of Jarrold and
co-workers [44] is that fullerene like clusters can form complexes with the
presence of sp chains.
In any case the survival ability of polyyne and polycumulenes upon
landing indicates that they are somehow protected by the aggregate
where they have been formed, like a kind of ''cushion'' capable of absorbing
and dissipating the deposition energy and to prevent the cross-linking
reactions.