Page 103 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

Nanomaterials Fabrication 89

Acc.V Spot Magn Det WD 2 µm
30.0 kV 3.0 15000x SE 18.6 Hivac

Figure 3.45 SEM image of vapor-grown carbon nanofibers.

“twist” of the nanotube. The chirality in turn affects the conductance of
the nanotube, its density, its lattice structure, and other properties. A
SWNT is considered metallic if the value n – m is divisible by three. For
example, an armchair tube is metallic in character. Otherwise, the nan-
otube is semiconducting [174]. Environment also has an effect on the
conductance of a tube. Due to its highly delocalized electrons, it is pos-
sible for a nanotube to accept electrons from or donate electrons to its
environment [175, 176]. Molecules such as O 2 and NH 3 can change the
overall conductance of a tube.
Multiwalled carbon nanotubes (MWNTs) range from double-walled
NTs to carbon nanofibers. Carbon nanofibers are the extreme of multi-
walled tubes (Figure 3.45). They are thicker and longer than either
2
SWNTs or MWNTs, having a cross-sectional area of approximately 500 Å
and are between 10 to 100
m in length. They have been used exten-
sively in the construction of high-strength composites [177].

Synthesis of single-walled carbon nanotubes. A range of methodologies
have been developed to produce nanotubes in sizeable quantities, includ-
ing arc discharge, laser ablation, high-pressure carbon monoxide (HiPco),
and vapor-liquid-solid (VLS) growth. It is worth noting that the latter is
often referred to as chemical vapor deposition (CVD), however, this is not
strictly correct. All of these processes take place in a vacuum or at low
pressure with process gases, although VLS growth can take place at