Biomedical Engineering Reference
In-Depth Information
Fig. 7
Schematics of a thermal CVD furnace (
top
) and basic fl ow of a carbon nanotube growth
process (
bottom
)
decomposed in the furnace at high temperature. The growth reaction is activated by
the presence of a catalyst that can be deposited on the sample's surface (as in the
fi gure) or in the gas mixture fed to the reactor (e.g., injecting ferrocene C
10
H
10
Fe).
The most common carbon sources used in thermal CVD growth of CNTs are mix-
tures of ammonia (NH
3
) and acetylene (C
2
H
2
) while the catalyst metal can be cobalt,
iron, or nickel. The growth proceeds through several steps: fi rst, the catalyst metal
breaks up into islands at high temperatures and forms metal seeds for the reaction;
then, the hydrocarbon gas supplied in the quartz tube decomposes creating fl oating
C and H atoms; the fl oating atoms are then attracted by the catalyst seeds that
become supersaturated and condensate forming ordered tube-shaped graphene
sheets. The catalyst particle can then either stay attached to the substrate (base
growth) or get pushed to the tip of the formed nanotube (tip growth) conferring dif-
ferent properties to the fi nal structure. This type of growth process, although being
relatively inexpensive, forms a randomly distributed array of CNTs with a complex
hierarchical microstructure. Bundles of tubes grow vertically, but the tubes are
intertwined and partially curled at small scales. This lack of complete alignment is
normally overcome by the use of PECVD systems, which are a bit more complex
and expensive, since they need additional constituents like electrodes, pumps, volt-
age supplies, etc.
