Biomedical Engineering Reference
In-Depth Information
CNTs have very high chemical stability and can be chemically functionalized: that is, it is
possible to attach a variety of atomic and molecular groups to their ends [12].
The current challenge is to control the synthesis of these nanostructures in ways that
allow the potential applications of CNTs to be explored. For example, growing patterned
and aligned nanotubes with controlled structure, dimension, and morphology.
12.2.2
Synthesis of Carbon Nanotubes
From a synthesis point of view, there were two significant advances in the early stage after
Iijima's first discovery, the MWCNTs. One was the discovery, in 1993, of SWCNTs by Iijima
and coworkers [14] as well as by Bethune and coworkers [15]. The other milestone,
reached in 1996, was a relatively high-yield synthesis of SWCNTs, which allowed investi-
gations of physical properties to be carried out. Following these advances, considerable
effort has been made toward high-quality and large-scale synthesis of SWCNTs and
MWCNTs with controlled structures. Clearly, future developments in nanotube-based sci-
ence and technology will rely on the highly controlled synthesis of nanotube materials.
The principal methods of CNT synthesis are arc discharge, laser ablation, and chemical
vapor deposition (CVD). While the first two methods use high-energy input to release the
carbon atoms from carbon-containing precursor molecules, CVD relies on carbon atom-
ization via catalytic decomposition of carbon precursors on the surface of transition metal
particles. Compared with arc and laser methods that produce powdered samples, the
CVD method allows scale-up of production to an industrial level and production of CNTs
in a predictive fashion with controlled length, positions, and orientations on substrates.
Significant progress has been made by Dai and coworkers [16,17] by developing a novel
CVD method to produce high-quality individual SWCNTs on isolated catalyst particles on
substrates. This technique yields large numbers of SWCNTs at specific locations and opens
up new possibilities for integrated nanotube systems. We will describe recent progress in
the CVD process.
During CVD synthesis, it is possible to form several different carbonaceous products,
including MWCNTs, SWCNTs, amorphous carbon, and metal particles encapsulated by
graphitic shells. Control over the CNT structure and growth is achieved through the vari-
ation of a series of experimental parameters such as process temperature, gas mixtures,
pressure, flow rates, and catalyst materials. Among these, one of the most important
parameters is the specific catalyst material employed [18]. The catalytic materials used
are generally transition metals, for example iron, cobalt and nickel or their alloys, at the
nanometer scale, either in oxide or metallic forms or as mixtures. The important proper-
ties of metals for nanotube formation include their ability to catalytically decompose
gaseous carbon-containing molecules, possessing a modest melting temperature, and the
solubility of carbon and the carbon diffusion rate in the metal. Various methods have
been employed to produce catalytic particles, including the precipitation of metal salts
(nitrates, sulfates, and chlorides), metal-organic, or organometallic precursors on a sup-
port, followed by drying, calcining and grinding, and sputtering [19]. Extensive mecha-
nistic studies have been conducted to determine the dependence of the CNT diameter
and structure on the catalyst particle size. One of the challenges is to predetermine the
catalyst size and explore its relationship to the resulting nanotubes. This will be discussed
in Section 12.2.3.
Many research groups have contributed to the use of the CVD method to grow high
quality and controlled SWCNTs. For example, Dai and coworkers synthesized high-qual-
ity SWCNTs from catalyst islands patterned directly on substrates. Further, the SWCNTs
thus synthesized bridge adjacent catalytic islands from pillar to pillar [19] (Figure 12.2).
