Page 62 - Carbon Nanotubes

52 C.-H. KIANG et al.

Fig. 2c. Aggregated single-layer nanotubes from soot produced by co-vaporizing Co and Bi.

mation. Classical mechanical calculations show that [38,42,43] of single-layer nanotubes, very little char-
the tubes with diameters greater than 2 nm will deform acterization beyond TEM microscopy and diffraction
radially when packed into a crystal[23], as indicated has been possible to date, due to the difficulty in sep-
by TEM images presented by Ruoff et al.[22]. The arating them from the myriad of other carbon struc-
stability of nanotubes is predicted to be lowered by tures and metal particles produced by the relatively
the defects[28]. This may account for the observation primitive synthetic methods so far employed. Recently,
that smaller tubes often appear to be more perfect be- it was reported that the Raman spectrum of a sample
cause, with their higher degree of intrinsic strain, containing Co-catalyzed nanotubes showed striking
smaller tubes may not survive if defective. The most features that could be correlated with theoretical pre-
likely defects involve the occurrence of 5- and 7-fold dictions of the vibrational properties of single-layer
rings. These defects have discrete, specific effects on tubules[44]. Kuzuo et al. [45] were able to use trans-
the tube morphology, and can give changes in tube di- mission electron energy loss spectroscopy to study the
ameter, bends at specific angles, or tube closure, for electronic structure of a bundle of single-layer nano-
example[29,30,3 11. By deliberately placing such de- tubes selected by focusing the electron beam to a 100-
fects in specific locations, it would be possible in prin- nm diameter circular area. Features of the spectra
ciple to create various branches and joints, and thus obtained were shifted and broadened compared to the
to connect nanotubes together into elaborate 3-D corresponding features for graphite and multilayer
networks[32,33]. nanotubes. These changes were tentatively interpreted
Despite a wealth of theoretical work on the elec- to be effects of the strong curvature of the nanotube
tronic structure [26,34-411, and vibrational properties wall.