Carbon Nanotubes FAQ

As with the topic of nanotechnology, there is so much to say, that it seemed quixotic to try and embody it in a page, but here goes my attempt at an FAQ. I will add to this often, as there is a lot to be said. If what I am saying does not make sense or if you have corrections/updates, please contact me.

• General Questions
• Synthesis - Where Do Carbon Nanotubes Come From?
• Electronic (electrical, thermal) Properties
• Mechanical Properties

General Questions

What is a nanotube? A nanotube earns its name because it's diameter falls into the range of a nanometer. There are nanotubes made of a variety of materials, e.g. carbon, silicon carbide, boron nitride, and even peptides, just to name a few.

What is a carbon nanotube? Generically, it's just a tube made of carbon with a diameter measured in nanometers; to be deserving of the prefix "nano", it should really be about 0.1 to 100 nm in diameter. There are two main types of carbon nanotube, single-wall and multi-wall.

What is a single-wall carbon nanotube (SWCNT or SWNT)? Graphite is made up of planes of carbon that form a hexagonal lattice, much like sheets of chicken wire. If you take one of these planes/sheets and roll them into a tube so that the overlapping hexagons line-up with each other, you have a single-wall carbon nanotube. These tubes are generally around 1-2 nm in diameter, similar to human DNA (~2 nm), and usually range from hundreds of nanometers to many microns in length. Note: I will use "SWNT" in the rest of this FAQ.

What is a "(10,10)" SWNT? It turns out that the way in which the sheet is rolled correlates to the electronic properties of the tube. Researchers quantify this by using the basis vector (see below) to describe which hexagon overlaps with the starting hexagon. For example, if you printed the image below on a transparent page and overlapped the "(0,0)" hexagon with the "(8,0)" hexagon, you would end up with an (8,0) SWNT.

Basis vector for graphite sheet.	Some sample vectors

Zigzag vs. armchair

What is a multi-wall carbon nanotube (MWCNT or MWNT)? This structure consists of many concentric shells, akin to a Russian matreshka doll (see picture below). They typically have diameters of tens of nanometers, and are microns long, up to hundreds of microns.

Russian matreshka doll

Which is better, MWNT or SWNT? That depends on what you want. SWNTs are structurally simpler and generally considered to contain less defects than MWNTs, which is good for transport of electrons; as a result, some see ultimately more promise for SWNTs in electronic and thermal applications. Because of their concentric shells, it's not surprising that MWNTs are stiffer than SWNTs, which can be good or bad, depending upon your requirements. MWNTs are currently much easier and inexpensive to produce, which makes them more commercially viable, but production of SWNTs continues a slow but steady march towards mass production.