Carbon Nanotubes – the
Route Toward Applications
Many potential applications have been proposed for carbon nanotubes,
including conductive and high-strength composites; energy storage and
energy conversion devices; sensors; field emission displays and radiation
sources; hydrogen storage media; and nanometer-sized semiconductor devices,
probes, and interconnects. Some of these applications are now realized
in products. Others are demonstrated in early to advanced devices, and
one, hydrogen storage, is clouded by controversy. Nanotube cost, polydisperity
in nanotube type, and limitations in processing and assembly methods
are important barriers for some applications of single-walled nanotubes.
Despite structural similarity to a single sheet of graphite, which is
a semiconductor with zero band gap, SWNTs my be either metallic or
semiconducting, depending on the sheet direction about which the graphite
sheet plane is rolled to form a nanotube cylinder. This direction in
the graphite sheet plane and the nanotube diameter are obtainable from
a pair of integers (n, m) that denote the nanotube type (1). Depending
on the appearance of a belt of carbon bonds around the nanotube diameter,
the nanotube is either of the arm-chair (n=m), zigzag (n=0 or m=0)
or chiral (any other n and m) variety. All arm-chair SWNTs are metals;
those with n-m=3k where k is a nonzero integer, are semiconductors
with a tiny band gap; and all others are semiconductors with a band
gap that inversely depends on the nanotube diameter (1).
All currently known synthesis methods for SWNTs result in major concentrations
of impurities. Carbon-coasted metal catalyst contaminates the nanotubes
of the HiPco route, and both carbon-coasted metal catalyst and, typically,
~60% forms of carbon other than nanotubes are formed in the carbon-arc
route (11). These impurities are typically removed by acid treatment,
which introduces other impurities, can degrade nanotube length and
perfection, and adds to nanotube cost. Another problem, especially
for electronic devices, is that the usual synthetic routes result in
mixtures of various semiconducting and metallic nanotubes. Metallic
SWNTs can be selectively destroyed by electrical heating, so that only
the semiconducting nanotubes needed for nanotube field-effect transistors
(NT-FETs) survive (14). However, no route to substantial quantities
of SWNTs of one type is yet known.
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