such small structures. Arc discharge also requires a large amount of
power and is operated at a high temperature, making it an unfavorable
method for the production of large quantities of nanotubes.
9.6.3.2 Laser ablation
Laser ablation uses a high-energy laser beam to vaporize a carbon source.
The carbon source is sealed off in a quartz tube and heated to temperatures ranging from a few hundred degrees to just over a thousand degrees
Celsius. The tube is filled with an inert gas that flows toward a cooled
collector at its end. As the laser vaporizes the carbon source, the flow of
inert gas carries it down the tube, where it deposits. The nanotubes are
then collected from the deposits and purified.
In both arc discharge and laser ablation, the vaporized carbon is released
as low molecular weight carbon, which coalesces to form larger molecular
weight structures. The carbon begins to condense very quickly after
vaporization. Using metal catalyst particles doped into the carbon source
prevents carbon from closing into a cage structure and decreases the
amount of carbon deposited in nonnanotube form by aggregating with
the carbon quickly. This results in more single-walled nanotubes being
formed when a metal catalyst is used. Nanotube length can be controlled
by changing the time the carbon spends in the high temperature area of
the growth chamber, which can be controlled by using heavier or lighter
gases and altering the flow rate. Research is undergoing into changing
growth conditions, which may make laser ablation a viable option for the
large-scale production of nanotubes.
9.6.3.3 Chemical vapor deposition
Chemical vapor deposition relies on the decomposition of a carboncontaining gas, such as methane, ethane, benzene, acetylene, or ethylene,
which then aggregates on a catalyst or substrate to form nanotubes.
Studies have shown that gases with saturated carbon bonds produce
structures with more walls than those grown from unsaturated gases.
Therefore, methane and ethane are preferred for single-walled carbon
nanotube growth, and ethylene, benzene, and acetylene are preferred for
multiwalled carbon nanotube growth. Because high temperatures create
more high-quality crystals by reducing defects and increasing crystallization, the source gas must also be one that does not thermally decompose into amorphous carbon and reduce the purity of nanotubes. The
amorphous carbon can accumulate on the aggregation sites and inactivate them from forming nanotubes. The aggregation sites are typically
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374
power and is operated at a high temperature, making it an unfavorable
method for the production of large quantities of nanotubes.
9.6.3.2 Laser ablation
Laser ablation uses a high-energy laser beam to vaporize a carbon source.
The carbon source is sealed off in a quartz tube and heated to temperatures ranging from a few hundred degrees to just over a thousand degrees
Celsius. The tube is filled with an inert gas that flows toward a cooled
collector at its end. As the laser vaporizes the carbon source, the flow of
inert gas carries it down the tube, where it deposits. The nanotubes are
then collected from the deposits and purified.
In both arc discharge and laser ablation, the vaporized carbon is released
as low molecular weight carbon, which coalesces to form larger molecular
weight structures. The carbon begins to condense very quickly after
vaporization. Using metal catalyst particles doped into the carbon source
prevents carbon from closing into a cage structure and decreases the
amount of carbon deposited in nonnanotube form by aggregating with
the carbon quickly. This results in more single-walled nanotubes being
formed when a metal catalyst is used. Nanotube length can be controlled
by changing the time the carbon spends in the high temperature area of
the growth chamber, which can be controlled by using heavier or lighter
gases and altering the flow rate. Research is undergoing into changing
growth conditions, which may make laser ablation a viable option for the
large-scale production of nanotubes.
9.6.3.3 Chemical vapor deposition
Chemical vapor deposition relies on the decomposition of a carboncontaining gas, such as methane, ethane, benzene, acetylene, or ethylene,
which then aggregates on a catalyst or substrate to form nanotubes.
Studies have shown that gases with saturated carbon bonds produce
structures with more walls than those grown from unsaturated gases.
Therefore, methane and ethane are preferred for single-walled carbon
nanotube growth, and ethylene, benzene, and acetylene are preferred for
multiwalled carbon nanotube growth. Because high temperatures create
more high-quality crystals by reducing defects and increasing crystallization, the source gas must also be one that does not thermally decompose into amorphous carbon and reduce the purity of nanotubes. The
amorphous carbon can accumulate on the aggregation sites and inactivate them from forming nanotubes. The aggregation sites are typically
CHAPTER 9: Introduction to Functional Nanomaterials
374