Multi-walled carbon nanotube bands

Fig. 7.10 High-resolution TEM images of multi-walled carbon nanotubes (A) After synthesis at 680°C and (B) after heat treatment at 2600°C in an argon atmosphere for2h. The higher graph itization degree of the graphene planes is dearly visible on the heat-treated sample, (q Raman spectra of the carbon nanotubes before (a) and after (b) heat treatment showing the significant increase in the Ic/Id band ratio.

Qian et al. (2003) studied the Raman spectra of single and multi-walled carbon nanotubes and noted that, in mixtures of these two types, as the percentage of multi-walled carbon nanotubes increased, so the intensity of the G-band of the mixture decreased but the intensity of the D-band increased. This effect is attributed to the increased number of defects within the multi-walled carbon nanotubes as a result of accommodations to maintain these structures and in the kinetics of their formation (more difficult to obtain perfection during growth). 

Carbon nanotubes can be single walled (SWNT) or multi-walled (MWNT). SWNTs can be metallic or semiconducting, depending on their chirality. MWNTs contain several walls, so in combination they will tend to be metallic. In addition, as the band gap of semiconducting tubes varies inversely with their diameter, the larger diameter of MWNTs means that effectively most walls are metallic. 

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