Nanotubes can be formed from many materials. A carbon nanotube consists of one or more sheets of graphite rolled into a tube. A carbon nanotube has a typical inner diameter of ~1nm. The length of carbon nanotubes can approach 1mm. A nanotube with one graphene sheet is called a single-wall nanotube (SWNT). A nanotube with more than one graphitic sheet is a multi-wall nanotube (MWNT). Each sheet can be metallic or semiconducting based on its chirality.
Nanotubes can be created by laser ablation of graphitic targets, arc discharge from graphitic electrodes, or chemical vapor deposition (CVD). The most promising fabrication method for applications is CVD since it allows precise placement of nanotubes. In the CVD nanotube process, catalyst material is deposited onto a wafer and patterned by lithography. A hydrocarbon gas such as methane is streamed across the wafer at ~900 degrees Celsius. Nanotubes grow from the catalyst material.
Nanotubes have many exciting properties such as ballistic transport, a Young's Modulus greater than 1TPa, and a huge aspect ratio. However, applications of nanotubes are held back by fabrication problems. Chirality and position of tubes are very difficult to control.