ABSTRACT
Carbon nanotubes (lijima, 1991) are currently being intensively investigated because of their remarkable electronic and mechanical properties. A multi-walled nanotubes can be thought of as graphite sheets wrapped into a coaxial seamless cylinder. In 1993, Iijima's group as well as Bethune's group found that the use of transition-metal catalysts could lead to nanotubes with only a single wall (lijima, 1993; Bethune, 1993). The diameter of each freestanding single-walled carbon nanotube (SWNT) ranges from 0.7 nm to a few tens nanometers with a maximum length of about 1 J..tm. Although theoretical calculations have predicted the stability of a SWNT with diameter as small as 0.4 nm (Sawada, 1992), the existence of free-standing SWNTs with a diameter smaller than that of C60 fullerene (0.7 nm) has been in doubt for quite a while (Ajayan, 1992) because of the extreme curvature and reactivity of these structures. Smaller carbon nanotubes can exist, however, in a spatially confined environment. Carbon nanotubes with diameters of as small as 0.5 nm (Sun, 2000) and 0.4 nm (Qin, 2000) have been observed existing in the centre of multi-walled carbon nanotubes. It is still not clear, however, that whether these small nanotubes are stable in free space. Recently, we have shown that 0.4 nm-sized SWNTs can be produced by means of pyrolysing hydrocarbon molecules in 1 nm-sized channels of AlP04-5 (AF1) single crystals (Tang, 1989; Wang, 2000). These 0.4 nm-sized SWNTs have the same size of the smallest possible fullerene C20 (Prinz bach, 2000). They are stable inside the AF1 channels but not very stable when they are in free standing.
