Carbon nanotubes (CNTs) are allotropes of carbon with a cylindrical nanostructure. There are two predominant varieties of CNTs: multi-walled carbon nanotubes (MWCNTs) and single-walled carbon nanotubes (SWCNTs). MWCNTs have a number of layers of graphite sheets rolled up into concentric cylinders. They sometimes have diameters starting from 2 to 100 nanometers and lengths as much as a number of centimeters.