Page 59 - Carbon Nanotubes

50 C.-H. KIANG et af.

Fig. 2a. Bundles and individual single-layer carbon nanotubes bridge across a gap in a carbon film.

3. STRUCTURE OF SINGLE-LAYER On the molecular scale, single-layer carbon nano-
CARBONNANOTUBES tubes can be viewed either as one-dimensional crystals
or as all-carbon semi-flexible polymers. Alternatively,
The structure of an ideal straight, infinitely long, one can think of capped nanotubes as extended ful-
single-layer nanotube can be specified by only two lerenes[27]. For example, one can take a Cso molecule
parameters: its diameter, D, and its helicity angle, and add a belt of carbon to form a C,o. By repeating
a, which take on discrete values with small incre- the process, one can make a long tubule of 0.7 nm
ments[2,26]. These atomic scale structural parameters diameter with zero helicity[26]. Likewise, joining belts
can in principle be determined from selected area dif- of 75 edge-sharing benzene rings generates a nanotube
fraction patterns taken from a single tubule. Although of about 6 nm diameter. Nanotubes typically have di-
for multilayer tubes numerous electron diffraction ameters smaller than multilayer tubes. TEM micro-
studies confirming their graphitic structure have been graphs show that single-layer tubules with diameters
published, the very weak scattering from nm diameter smaller than 2 nm are quite flexible, and often are seen
single-layer tubes and their susceptibility to damage by to be bent, as in Fig. 3a. Bends with radii of curvature
a 100-200 keV electron beam make it very difficult to as small as ten nm can be observed.
make such measurements. Iijima was able to show by Tubes with diameters larger than 2 nm usually ex-
diffraction that a single-layer tube indeed had a cylin- hibit defects, kinks, and twists. This is illustrated in
drical graphene sheet structure[2]. Saito reported a the TEM image of several relatively large nanotubes
similar conclusion based on diffraction from a bun- shown in Fig. 3b. The diameter of the tubes seems to
dle of several single-layer tubes[l7]. vary slightly along the tube axis due to radial defor-