Biomedical Engineering Reference
In-Depth Information
CVD nanotube growth is believed to promise an enhanced control
of the fabricated CNTs better than the other techniques such as arc
discharge and laser ablation.
The choice of the carbon feedstock gas is one of the key elements
to the growth of high-quality CNTs containing no defects and very
low content of amorphous carbon. It was demonstrated by Dai and
coworkers [125] that the chemical and textural properties of the
catalyst materials dictate the yield and quality of the SWCNTs by
using methane as feed gas in the CVD growth. A catalyst consisting
of Fe/Mo bimetallic species supported on a sol-gel derived alumina-
silica multilayer material produces individual SWCNTs and bundle
of SWCNTs with a surface area of 200 m
2
/g, grown at 900°C in a
quartz tube furnace. The diameters of the SWCNTs are dispersed
in the range of 0.7-3 nm with a peak at 1.7 nm. The yield of the
nanotubes is up to a value of 45 wt.%, e.g., 1 g of catalyst yields
0.45 g of SWCNTs.
Another aspect of the CVD technique is its ability to synthesize
aligned arrays of CNTs with controlled diameter and length. The
synthesis of well-aligned and straight CNTs on a variety of substrates
has been accomplished by the use of plasma-enhanced chemical
vapor deposition (PECVD), where the plasma is excited by a RF or
DC source [126-128], or a microwave source [129-133]. Ren
.
[127] have grown aligned CNTs onto substrates coated by nickel
catalyst by using ammonia as catalytic gas and acetylene as carbon
source gas. A DC power generates the required PECVD plasma and
a tungsten hot filament assists the dissociation of the reactive gases
and supply heat to the substrate for the growth of nanotubes. Penza
et al
et al
. [30] demonstrated that RF-PECVD is a valid deposition system
to grow networked films onto alumina substrates differently coated
by catalysts of Co and Fe to promote synthesis of dense MWCNTs for
enhanced gas sensing applications. A review of the CNTs synthesis
by PECVD method has been proposed by Meyyappan
et al
. [159].
Bower
. [129, 130] showed that in microwave plasma-enhanced
CVD (MPECVD), the alignment of the CNTs results from the self-bias
that is imposed on the surface of the catalyzed substrate from the
microwave plasma. Dai and coworkers [134] have grown CVD-based
array of aligned CNTs for studying the field emission properties. Su
et al
et al
. [135] showed the CVD-growth of SWCNTs for scale-up at high
catalyst productivity.
