Multi walled carbon nanotubes can appear either in the form of a coaxial assembly of SWNT similar to a coaxial cable, or as a single sheet of graphite rolled into the shape of a scroll.The diameters of MWNT are typically in the range of 5 nm to 50 nm. The interlayer distance in MWNT is close to the distance between graphene layers in graphite.MWNT are easier to produce in high volume quantities than SWNT. However, the structure of MWNT is less well understood because of its greater complexity and variety. Regions of structural imperfection may diminish its desirable material properties.
The challenge in producing SWNT on a large scale as compared to MWNT is reflected in the prices of SWNT, which currently remain higher than MWNT.SWNT, however, have a performance of up to ten times better, and are outstanding for very specific applications.
Fullerenes and carbon nanotubes (CNTs) are two closely related carbon materials. While fullerenes have bucky-ball structure, CNTs are stripes of graphite rolled up seamlessly into tubes (cylinders). The carbon atoms in a nanotube are arranged in hexagons, similarly to the arrangement of atoms in a sheet of graphite. The electronic properties are fully determined by its helicity (chirality) and diameter. They can have both metallic and semiconducting properties. The typical dimensions of a single wall CNT are: 1 nm in diameter and length of few micrometers. On the other hand, multi-walled CNTs can have diameters up to 100 nm. Recently, super long nanotubes with length of around 1 cm were successfully synthesized.
CNTs are produced by a variety of methods. The most common methods include chemical vapor deposition (CVD), electric arc-discharge, laser ablation of a carbon target, etc. Aligned (forest-like) nanotubes can also be synthesized. Aligned CNTs provide a well-defined structure for some applications. For example, high power density supercapacitors can be built using locally aligned nanotube electrodes.
CNTs play important role in the developing field of nanotechnology. Their excellent electronic transport properties make them good candidates for building blocks in nanoelectronics. The high aspect ratio of nanotubes is favorable in applications based on field emission, like flat panel displays and lamps. Furthermore, the strong mechanical properties and high thermal stability of CNTs improve the properties of matrix materials such as polymers or ceramics. Nanotubes have also been used as an alternative to currently used fillers (e.g. carbon black) to facilitate electrostatic dissipation by increasing the conductivity of polymers. Other studies have been directed towards improving the conductivity of already conducting polymers, thus resulting in a more conductive material.
As already mentioned, the properties of CNTs are fully determined by their exact atomic structure. Thus, in order to build a precise nanotube-based nanoelectronic device with well-defined properties, it is crucial to control the positioning and the atomic (electronic) structure (helicity) of nanotubes already in the growth phase. Some major hurdles still need to be overcome in this field. However, there are many applications where CNT networks are used instead of individual nanotubes. In these cases the properties of the whole nanotube network are determinative. These applications are very promising and a long line of nanotube-based materials and devices are already in the pipeline.
Related reading: Copper Oxide Nanoparticles ruthenium metal powders