Chemistry Reference
In-Depth Information
An alternative method for catalyst delivery to the surface relies on vapor
phase transport in a so-called floating catalyst method. Catalyst precursors,
such as ferrocene (Fe(C
5
H
5
)
2
) and iron chloride (FeCl
3
), get vaporized and
then mixed with process gases. When this mixture of gases reaches the hot
zone of the CVD furnace, catalyst particles precipitate on the substrate in situ
and immediately initiate CNT growth. This method combines catalyst
preparation and CNT growth processes, simplifying the overall synthesis
processes. It also enables the continuous production of CNTs, whereas
conventional CVD is a batch process that needs replacement of the sub-
strate. The drawback is that the floating catalyst method can cause con-
tamination of the CVD chamber since the catalyst gets deposited everywhere
in the reactor. For the same reason, catalyst patterning on a desired area of a
substrate is dicult to achieve.
Lastly, catalyst nanoparticles can be formed in situ during heat treatment
of a thin catalyst film (Figure 3.4). Thin film deposition methods, such as
sputtering and electron beam evaporation, can create an extremely thin (less
than 10 nm) catalyst film on a substrate. When annealed at a high tem-
perature, the metal film minimizes its surface energy by breaking into dis-
crete islands and dewetting on the underlying oxide support. Researchers
have adopted this method widely because of its simplicity in preparation and
the ability to change the resulting catalyst size by controlling the initial film
thickness. The spontaneous particle formation, however, produces a range
of catalyst size distributions and consequently, nanotubes with a range of
different diameters. The Ostwald ripening process changes the particle size
distribution even more;
39
therefore, the dynamics of catalyst evolution also
represents one of the critical parameters for CNT growth control.
An area density of nanotubes is proportional to that of the catalyst par-
ticles. Interestingly, an increased catalyst density leads to an abrupt change
in the collective morphology of the nanotubes. As the inter-nanotube dis-
tance decreases, mechanical interactions start to work and the crowded-out
nanotubes start to support each other, resulting in self-aligned nanotube
arrays. The growth direction lies perpendicularly to the substrate surface, so
the nanotubes of this particular structure are called vertically aligned carbon
nanotubes (VACNTs). Because of their near-straight alignment, VACNTs are
invaluable templates for device fabrication as well as for fundamental
research on CNT growth mechanisms.
d
n
3
r
4
n
g
|
6
.
3.2.1.3 Process Gases
Process gases for CVD growth of CNTs contain a carbon precursor, a carrier
gas, and sometimes a reducing gas. Light hydrocarbon gases are the most
common carbon sources owing to their resistance to non-catalytic carbon
polymerization. Several studies showed that alcohols in a vapor phase could
also be used as a carbon feedstock. Hydrogen (H
2
) gas is often present in the
process feed gases to reduce the oxidized catalyst as well as for adjusting the
chemical equilibrium of the gas-phase reaction. Argon (Ar) and helium (He)
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