Nanotechnology’s Wonder Material: Synthesis of Carbon Nanotubes - Hierarchical Nanostructures for Energy Devices - page 32

type system enables a rapid ramp of the substrate temperature, which can
mitigate unwanted thermally activated processes (i.e. Ostwald ripening of
catalyst particles). Hart et al. 17,18 pioneered this type of reactor for CNT
growth by using a doped silicon susceptor as a Joule heating resistor. Several
other designs also take advantage of inductive heating or focused laser
irradiation, implementing laser-assisted CVD (LACVD). 19,20
Besides heating uniformity, another technical difference between hot- or
cold-wall reactors in the CVD growth of CNTs is the degree and duration of
gas-phase reactions (GPR). In a hot-wall CVD reactor, the temperature of the
gas feedstock becomes increased due to heat diffusion from the wall. Con-
sequently, gas-phase reactions begin well before the carbon precursors ac-
tually reach the surface of growth substrates. As we will discuss, the effects of
this feedstock preconditioning can be significant in the growth kinetics
of CNTs.
PECVD systems for CNT growth look similar to the thermal CVD systems,
except they contain an additional plasma generator (Figure 3.2B). 21 Methods
that can generate plasma for CNT growth include RF capacitance and
microwaves. Due to the aggressive reactivity of gas precursors in plasma,
most PECVD systems use a cold-wall reactor and low process pressure. The
most important feature of PECVD is that reactive carbon precursors gener-
ated by collision with energetic electrons in plasma enable CNT growth at
remarkably lower temperatures. For instance, Hofmann et al. 22 demon-
strated that multi-walled CNTs can grow at 120 1C by PECVD, compared to
general growth temperatures (600-900 1C) by thermal CVD.
While PECVD lowers the thermal budget of CNT growth processes, the
plasma can also cause structural defects on the growing nanotubes. Alter-
natively, the plasma generator can be installed at an upstream position to
avoid such damage. This type of CVD is called remote PECVD (Figure 3.2C). 23
Zhang et al. 24 demonstrated the reproducible growth of vertically aligned
SWCNT arrays by a remote PECVD. Radicals generated in the remote plasma
zone are likely to evolve quickly by colliding with other gas molecules and
the chamber wall. This complicated reaction path can obscure the role of a
specific gas feedstock, but it can mitigate the plasma damage without losing
the advantages of PECVD. Researchers can also achieve similar types of
growth enhancement by flowing gases through a high-temperature flow
cell. 17 In this case, thermal energy replaces plasma to pre-condition the
carbon precursors.

d n 3 r 4 n g | 6

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3.2.1.2 Catalyst Nanoparticles
In principle, a catalyst accelerates chemical reaction kinetics by lowering
energetic barriers, neither modifying thermodynamics nor being consumed
during reactions. Carbon nanotube growth in a CVD-type process almost
always uses a catalyst and thus represents an example of catalytic CVD
(CCVD). Particle-based heterogeneous catalysis is an essential part for the
low temperature growth of CNTs, and it has been consistently demonstrated