ABSTRACT
FIG. 15 Thefirstpublished electron micrographs of a nanotube rope where the spacing between the tubes is about 0.3 nm and the triangular lattice pitch is about 1.7 nm. (From Ref. 192.)
(c) Single-Wall Nanotubes. Single-wall nanotubes (SWNTs) are normally produced as extensive tangled mats [192,193] or course filaments/ribbons of nanotube ropes [194]. Isolated SWNTs are generally rare, although Dai et al. [195] and Kong et al. [196] have recently developed methods for producing significant numbers of such nanotubes. A nanotube rope is shown in Fig. 15 where the cross-section is observed to reveal individual SWNTs of similar diameter assembled on a regular triangular lattice by van der Waals forces [192]. The uniform nanotube diameter and regular lattice structure means the nanotube ropes are, in effect, crystallites. A brief survey of the literature indicates that these nanotube crystallites are normally 5-35 nm in diameter and can be longer than 100 ]um [192,193,197,198]. Recent scanning tunneling microscopy (STM) studies [199] suggest that larger nanotube ropes (20-100 nm) may consist of interwoven crystallites, although no further evidence exists to confirm this finding. Other STM studies have shown that individual SWNTs within a crystallite may twist as they attempt to conform to the complex potential field arising from neighboring nanotubes [200]. SWNT diameters normally range from 1 to 2 nm, with the averages being —1.4 nm for conventional production methods, and somewhat larger at — 1.7 nm for SWNTs produced from catalytic decomposition of hydrocarbons [194]. The smallest SWNT observed to date is 0.7 nm [201], while SWNTs as large as 4-5 nm have been reported [194,195,202]. Following a review of the literature, Lambin et al. [203] conclude that the diameter and chirality of SWNTs within a crystallite are distributed to some extent. Single-wall nanotubes are believed to possess fewer defects compared to their multiwall counterparts [192], although defects have been observed both directly and indirectly [204,205].
