Synthesis of Carbynoid Structures by the Combustion Flame Method - Polyynes: Synthesis, Properties, and Applications - page 91

For n 4.4565 nm was calculated. It was suggested that each layer

of carbyne was constituted from 2 to 4 carbon chains.
In order to understand the atomic nucleation processes, let us recall
some theoretical concepts concerning the kinetic processes of film growth.
The process can be understood as a competition between nucleation and
step propagation; there are only three modes of crystals growth of material A
on a substrate B (layer, island, island plus layer) depending on their relative
surface energies

¼ 35, C O ¼

B . A study by Bauer in 1958 and reviewed by
Venables [49] shows that the initial growth of A on B is of island type if
A >

A and

B the initial growth is layer).
In the combustion flame method, the nucleation process leading to
growth of carbon films (sp 1 ,sp 2 or sp 3 ) results from a high gaseous flow at
slightly higher than the atmospheric pressure which arrives on the substrate
surface and which contains the combustion species including carbon atoms,
hydrogen atoms, hydrocarbon free radicals and ionized particles. The
carbon atoms from the vapor arriving on the substrate surface can either
nucleate a new island on the terraces by meeting another free atom, or walk
to the step edges causing a propagation of such steps. In the initial growth,
these carbon atoms have low bonds on the surface and may evaporate, or
may alternatively start the nucleation chain of small clusters. The high
concentration of carbon atoms that arrive is due to the incident gaseous
flow which decreases the re-evaporation rate. In that situation, many of
carbon adatoms will bond together before they reach the step and create a
nucleation site, which opens up another channel for the loss of adatoms.
The role of hydrogen atoms in chemical vapor deposition (CVD) carbon
film growth has been studied by many authors [50,51]. It generally proceeds
as follows: hydrogen atoms incident on the surface abstract hydrogen to
produce vacant sites, where growth species can then stick to the carbon
layer. Gas-phase hydrogen atoms may also produce condensable carbon
radicals by reactions with hydrocarbons and, by impinging on the surface,
they may create surface radicals and refill vacant sites by adsorption. Once
adsorbed, atomic hydrogen may also stabilize the carbon chains. A study of
Cataldo [52] has shown that carbon chains of any length must be terminated
by end groups to ensure stability. The simplest end groups are hydrogen
atoms. Some zigzag-like polycumulene carbon chains can be formed as a
result of a simple poly-recombination reaction of short-chain clusters with
an odd number of carbon atoms [45]. In the initial growth, the sp chains are
short, metastable and have the tendency to undergo cross-linking reactions
to form the sp 2 phase or onion carbon, tube-like structures [4] or sp 3 .
Depending on the deposition conditions (surface temperature, condensation
of carbon atoms on the substrate surface, incident angle etc.) the carbon
chains can recombine by Baeyer's mechanism [53] to form a system of
aromatic rings.
One drawback of the combustion flame method is the non-uniformity of
the flame, with radical variations in velocity, temperature and, particularly,

B (if A <