Nanotube

Nanotubes are cylindrical nanostructures with diameters on the order of a few nanometers and lengths that can extend to micrometers. The term most often refers to carbon nanotubes, which are formed from rolled graphene sheets. Carbon nanotubes are categorized as single-walled (SWCNTs) or multi-walled (MWCNTs) depending on whether a single graphene cylinder or multiple concentric cylinders are present. Other materials can form nanotube-like structures, but CNTs remain the most extensively studied.

Key properties of carbon nanotubes include exceptionally high tensile strength and stiffness, an extremely high aspect ratio, and excellent electrical and thermal conductivities along the tube axis. Whether a CNT behaves as a metal or a semiconductor depends on its chirality and diameter, which are determined by how the graphene sheet is rolled.

Synthesis and structure: CNTs are typically produced by arc discharge, laser ablation, or chemical vapor deposition. Post-synthesis purification and sorting by chirality are common challenges. The structure is described by a chiral vector (n,m) that defines the tube's diameter and electronic properties.

Applications: CNTs are used to reinforce polymers and ceramics, in nanoelectronic components, conductive films, and energy storage devices such as batteries and supercapacitors. They also serve as sensors and in nanomechanical systems. Research continues into scalable production, purification, and biocompatibility for potential medical uses.

History: Carbon nanotubes were discovered in 1991 by Sumio Iijima. Since then, they have been the subject of extensive study due to their unique properties and potential impact across materials science, electronics, and energy technologies.