Multi-walled Carbon Nanotubes

Multi-walled Carbon Nanotubes

The discovery of carbon nanotubes (CNTs) in 1991 has stimulated intensive research to characterize their structure and to determine their physical properties both by direct measurement and through predictive methods using modeling techniques. Many of their fundamental and remarkable properties are now well-known, and their exploitation in a wide range of applications forms a large part of the research effort now in progress. For example, their electronic properties have already had a significant impact on field emission applications in various electronic devices, while their geometry offers a new and exciting means of precision-controlled drug delivery. However, it is their extraordinary mechanical properties (exceptionally high tensile strength and stiffness) that has aroused interest and promoted research into the fabrication of nanotube composite materials. The credit for discovering multi-walled carbon nanotubes is given to Sumio Iijima, who in 1991 reported on carbon microtubulii found as by-products in the Krätschmer electronic arc discharge reactor used for fullerene synthesis. Multi-walled carbon nanotubes can be distinguished clearly from single-walled and double-walled carbon nanotubes based on their larger diameter and their Raman spectrum. The distinction between MWCNTs and carbon nanofibers relies on the well-defined structure of carbon nanotubes: any elongated carbonaceous object with diameter below ∼500 nm that does not feature a carbon nanotube structure is a carbon nanofiber (CNF). Multi-walled carbon nanotubes (MWCNTs) are elongated hollow cylindrical nanoobjects made of sp2 carbon. Their diameter is 3–30 nm and they can grow several cm long, thus their aspect ratio can vary between ten and ten million. The wall thickness of a MWCNT is fairly constant along the axis, and therefore the inner channel is straight. This channel is not directly accessible from the outside because the ends of perfect MWCNTs are capped by half fullerene spheres; however, it can be accessed by opening the nanotube using, e.g., oxidation, milling or ion beam treatment.