Structure of Carbon Nanotubes CNTs

CNTs are surely the most important nanostructures both from a fundamental point of view and for future applications. Initially, CNTs were simply be sorted by single-walled carbon nanotube (SWCNT) and multi-walled carbon nanotube (MWCNT) (Fig. 11.2). Paid most attention by scientists and engineers, different types of CNTs have been synthesised via various processes.

11.3.1 Y-shaped

Y-shaped junctions and rings composed of single wall CNTs are proposed as possible nanosized electronic devices exhibiting quantum interference mechanisms. The defect consists of six heptagons, according to the Crespi's rule, localized exactly at the bifurcation area. Fig. 11.3 shows a clear Y-shaped nanotube (Lee et al. 2002), while Fig. 11.4 further shows two Y-shaped junctions form a Y-shaped ring (Grimm et al. 2003).

Helical Chiral Carbon Nanotubes
Fig. 11.2: Left: three types of SWCNT with different chiralities; the difference in structure is shown at the open end of the tubes. a) armchair structure; b) zigzag structure; c) chiral structure (Ajayan et al. 2003)
Equal Thickness Interference Tem
Fig. 11.3: Y-shape nanotube
Fig. 11.4: Y-shape ring with a) equal, b) different ring-arm length
Fig. 11.5: FESEM image of double helical CNTs
Bamboo Cnt Tem
Fig. 11.6: TEM images of CNTs with a bamboo-like structure

Fig. 11.7: (a) All CNTs have a bamboo structure. There are closed tips with no encapsulated catalytic particles (see arrows ®), an open root (see arrow ©), and compartment layers with a curvature directed to the tip (see arrows ©). (b) The CNT has the compartment layers regularly at a distance of about 200 nm. The thickness of the wall increases at the joint of the compartment layers (see the dotted box)

Fig. 11.7: (a) All CNTs have a bamboo structure. There are closed tips with no encapsulated catalytic particles (see arrows ®), an open root (see arrow ©), and compartment layers with a curvature directed to the tip (see arrows ©). (b) The CNT has the compartment layers regularly at a distance of about 200 nm. The thickness of the wall increases at the joint of the compartment layers (see the dotted box)

Fig. 11.8: Hierarchical morphology of SWCNT fibers. (a) outer surface of fiber composed of aligned elementary filaments; (b) elementary filaments of 0.2-2 y m diameter built of packed SWCNT bundles; (c) nanofelt of SWCNT bundles of 10-30 nm diameter; and (d) skin-core model of a SWCNT fiber j)

Fig. 11.8: Hierarchical morphology of SWCNT fibers. (a) outer surface of fiber composed of aligned elementary filaments; (b) elementary filaments of 0.2-2 y m diameter built of packed SWCNT bundles; (c) nanofelt of SWCNT bundles of 10-30 nm diameter; and (d) skin-core model of a SWCNT fiber

Carbon Skin Core
Fig. 11.9: SEM images of multiwalled carbon nanotube rings
Conical Multi Wall Carbon Nanotubes
Fig. 11.10: Cone shape end caps of MWNTs Fig. 11.11: Cylindrical structures

11.3.2 Double Helical

Researchers at Hefei National Laboratory for Physical Sciences at Microscale recently synthesized perfect double helical CNTs by chemical reduction of supercritical CO2 with alkali metals (Chen et al. 2005). As shown in Fig. 11.5, one tube wrapped another, and the tubular structure with an opened cap can be clearly observed. The outer diameter of the wrapped pair is about 220 nm, and diameter of each single CNT is around 110 nm. The helical structure of CNTs is similar to that in protein. It is observed that the double helical CNTs are imaged to have nearly parallel lines; the interlayer separation is about 0.34 nm, consistent with graphitic atomic planes.

11.3.3 Bamboo-like Structure

In a similar experiment described above, bamboo-like structured CNTs were also synthesized. From Fig. 11.6, it is noted that no encapsulated solid particle in the closed compartment is observed. Same structure of CNTs has ever been prepared by thermal chemical vapor deposition (Fig. 11.7) (Lee and Park 2000).

11.3.4 Hierarchical Morphology Structure

The PCS method provides a promising way to produce highly porous SWCNT fibers with exceptional mechanical properties (Neimark et al. 2003). As shown in Fig. 11.8, SWCNT fibers possess a hierarchical structure with several levels of structural organization.

11.3.5 Ring Structured MWCNTs

Rings of typically 0.5 pm in diameter have been observed in carbon nanotube deposits produced catalytically by thermal decomposition of hydrocarbon gas (Fig. 11.9) (Ahlskog and Seynaeve 1999) with an atomic force microscope and a scanning electron microscope. The ring formation was interpreted as single turn coils with a short overlap between the beginning and end of the coiled nanotube, but the toroidal interpretation cannot be ruled out.

11.3.6 Cone Shape End Caps of MWCNTs

Metal-free method some times produce these types of CNTs. Three types of cone shape end caps of MWCNTs have been observed (Fig. 11.10) (Koshio et al. 2002). Image (i) shows the vertex of the cone located on a line extended from the outermost wall of the nanotube. Image (ii) shows a vertex on the center axis of the nanotube, just like a pencil point. Image (iii) shows a vertex located between the center axis and the line extended from the outermost wall of the nanotube. Most compartments have cone-like structures, but several long relatively cylindrical structures (Fig. 11.11) (Wang et al.2004).

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