Carbon nanotubes: From macromolecules to nanotechnology

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Abstract
The discovery of fullerenes (1) provided exciting insights into how architectures built from pure carbon units can result in new symmetries and structures with remarkable physical properties. Carbon nanotubes represent the most striking example (2). A carbon nanotube can be considered as the ultimate fiber, reflecting highly organized, near ideal sp2 bonded carbon structure. The organization of the hexagonal honeycomb carbon lattice into cylinders with helical arrangement of hexagonal arrays has created a very unusual macromolecular structure that is by far the most superior carbon fiber ever made. The discovery of nanotubes happened in 1991 when Dr. Sumio Iijima of NEC corporation found these tiny needles on electrodes used to prepare fullerenes (3). What he first observed were multiwalled nanotubes (MWNTs) consisting of coaxial cylindrical units (4) (Fig. 1); single-walled nanotubes (SWNTs), which often bundle up via Van der Waals forces into larger assemblies of ropes, came along a few years later (5, 6) (Fig. 1). Several methods exist today to synthesize these structures, including electric arc-discharge (4, 7), laser ablation (8), and catalytic chemical vapor deposition (9). In all of these, carbon vapor is made to condense into tubular structures, with or without the presence of catalysts, which are mostly nanoparticles of transition metals. The diameter of SWNTs range narrowly between 1 and 2 nm and that of MWNTs are 2–25 nm. They both run microns in length, providing large aspect ratios. Transmission electron microscopy images showing a SWNT (Top), organized SWNT …