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solid such as in amorphous carbon, carbon black, soot, cokes, glassy carbon
etc [2-4].
Solids based on sp hybridization, although subject of intense experi-
mental efforts, seem to be the most elusive of the different carbon families
[5]. The existence of linear chains of carbon atoms linked by alternating
single and triple bonds (polyyne) or double bonds (polycumulene) with
stabilizing molecular complexes at the end of the chains, has been
recognized in interstellar molecular clouds and can be artificially produced
by different chemical routes [6-8].
Polyyne and polycumulene chains are very fragile and reactive: exposure
to oxygen and/or water completely destroys these species [5-9]. Isolated
carbon chains have been extensively studied only in the gas phase [10-12] or
by means of matrix-isolation spectroscopy at very low temperature [13-15].
The high reactivity of unsaturated sp chains and their tendency to undergo
chain-chain cross-linking reaction causing the evolution towards an sp
2
phase [7] generated a deep skepticism about the possibility of assembling sp
carbon chains to form a pure carbon solid [5].
Despite their fragility, the existence of an elemental carbon solid formed
solely by sp chains known as ''carbyne'' has been proposed by many authors
and it has been the subject of controversy [16]. Several results have been
published mainly reporting on the crystallographic characterization of this
hypothetical material, but no compelling evidence of the existence of the
carbyne solid has been provided so far [5].
sp chains synthetic routes are based either on the high pressure and high
temperature modification of carbon-based solids [17], or on chemical strate-
gies aiming at the elimination of substituents from a linear organic molecule
to end the naked linear carbon backbone [7,18]. This latter strategy includes
catalytic dehydropolymerization of acetylene [19], dehydrohalogenation of
chlorinated polyacetylene [19], air promoted coupling reaction of dicopper
acetylide [20], and electrochemical reductive carbonization of poly(tetra-
fluoroethylene) [9].
The material obtained with the ''chemical'' approach is formed by
carbon chains separated by reaction byproducts (e.g. alkali metal fluoride)
preventing cross-linking reactions between adjacent carbynoid chains and
decomposition in a reactive environment [9]. These systems are conceptually
similar to the matrix-isolated chains where polyyne and polycumulene spe-
cies are kept isolated and protected form chain-chain and chain-moisture
interaction [21].
Recently, we have shown the possibility of growing a pure carbon
amorphous solid containing a significant amount of carbynoid structures
by supersonic carbon cluster beam deposition (SCBD) at room temperature
and in an ultra-high vacuum (UHV) [22].
SCBD is a technique consisting in the production of a supersonic beam
of inert gas seeded by carbon clusters (covalent aggregates with masses
ranging from tens to thousands of atoms) by means of an appropriate
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