(a) Carbon nanotubes (CNTs) could be viewed as single-atom layer thick graphene sheets rolled into a cylinder. Different directions of rolling determine CNTs' properties. (b) Schematic diagram showing a carbon nanotube's lifetime during chemical vapor deposition synthesis. Transition metals (blue structure) serve as catalysts, critical to elongate the CNT (left), until the carbon concentration on the catalyst surface becomes so abundant that the nanoparticle gets encapsulated by graphitic or amorphous carbon, forming a "cap" at the end of the cylinder and ending the growth of the CNT (right). (c) Environmental transmission electron microscope images of a CNT taken at different times during growth. The CNT contains a cobalt nanoparticle on its top end, a typical feature of tip-growth.