Synthesis and Characterization of Carbynoid Structures in Cluster-Assembled Carbon Films - Polyynes: Synthesis, Properties, and Applications - page 16

source. As a result of the relatively low kinetic energy of the clusters (below
0.2 eV/atom) they are not subjected to relevant fragmentation during the
deposition, so that the films obtained retain the memory of the precursor
clusters structure. The use of a supersonic beam makes also possible to
exploit aerodynamic effects to reduce the beam divergence (leading to high
deposition rates) and to control the clusters mass distribution.
In situ Raman spectroscopy on the films has confirmed the presence
of polyynes and polycumulenes in a nanostructured sp 2 matrix. These sp
hybridized linear carbon structures are metastable even in UHV at room
temperature. When exposed to oxygen, the carbon network structure rap-
idly evolves towards a more common mainly sp 2 amorphous phase, with
only a small residual amount of sp linear aggregates [23].
A deeper understanding of the stability of sp carbon structures and of
their role in the nanostructured carbon network (abundance, stabilization
mechanisms, interconnectivity) would provide a new insight in the physics
and chemistry of linear carbon chains and it would address the long-sought
opportunity of combining carbon building blocks with different hybridiza-
tion to produce new forms of carbon with tailored structural and functional
properties [1,24].
Raman spectroscopy is one of the techniques of choice for the study of
carbon-based materials and for the identification of carbyne [5,25]. The
presence of a Raman band at roughly 2100 cm 1 , generated by a stretching
of the carbon triple bond, is admitted to be one of the strongest arguments
in favor of the presence of carbynes [5,9]. Raman spectra of carbynoid
materials are characterized by G and D bands in the 1200-1700 cm 1 range
as in amorphous carbon and by the presence of the band in the 2100 cm 1
region [5,9]. For carbyne-rich materials this band should have an intensity
comparable or even greater than the amorphous contribution.
In the following we present a Raman spectroscopy characterization of sp
carbon chains embedded in a nanostructured carbon thin film deposited by
SCBD. The evolution of sp carbon chains when exposed to different inert or
reactive atmospheres (He, N 2 ,H 2 and dry air), or when annealed in vacuum
at various temperatures up to 200 C, has been followed by monitoring the
variation of the Raman peaks associated with polyynes and polycumulenes
in order to achieve detailed information about the chemical and thermo-
dynamic stability of these species.
2.2 EXPERIMENTAL
A supersonic beam is schematically described as a gas stream expanding
very rapidly from a high pressure region (source), through a nozzle, to a low
pressure region. The characteristics of the beam are mainly determined by
the size and shape of the nozzle and by the pressure difference between the
two regions [26]. Compared to effusive beams used in molecular beam