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.