The Different Types of Carbon Nanotubes

It wouldn't be wise to delve into the different types of carbon nanotubes without throwing some light on the carbon nanotubes (CNTs) first. Carbon nanotubes are nanoparticles or molecules made entirely from carbon atoms.

Technically speaking, carbon nanotubes can be called an allotrope of carbon. They take the form of cylindrical carbon molecules. They are possessed with unique properties to make them potentially beneficial in a vast variety of applications in the field of electronics, nanotechnology, optics and other fields of materials science.

These nanotubes are in the shape of hexagonal carbon rings arranged on a flat sheet, with every carbon atom connected to three more carbon atoms via covalent bonds. Hence every carbon ring is attached to the other on the edges, giving carbon nanotubes the shape of a cylindrical tube.

Two major types of carbon nanotubes

Coming to the different types of carbon nanotubes, they can be broadly categorised into multi-walled nanotubes (MWNTs) and single walled nanotubes (SWNTs).

SWNTs - First of the two major types of carbon nanotubes

  • Single walled nanotubes, as evident from their name are made up of just one single layer of hexagonal carbon structure, giving them the form of a solid sheet.

  • Depending on how this solid sheet is wrapped, the resultant nanotube can be described in the form of vector (n, m). The 'n' and 'm' are unit vectors in two different directions on a sheet with a honeycomb structure.

  • The SWNT is termed as a 'zigzag' nanotube if m=0, since the tube's exposed end will lead to creation of alternating dips and points.

  • The SWNT is termed as an 'armchair' nanotube if m=n, since the tube's exposed end will be in the form of alternating low and high plateaus.

  • In case of any other vector, the resulting tube is referred to as a 'Chiral.'

  • The diameter of the single walled nanotubes can be derived from its 'n' and 'm' values.

  • A single walled nanotube normally has a diameter of around 1 nm, and its length is several millimetres, which is many million times of its diameter.

MWNTs - Second of the two major types of carbon nanotubes

Multi walled nanotubes are made up of one or more layers of the hexagonal carbon structures, giving them the appearance of concentric tubes of graphene. The structure of MWNTs can be described using two different models:

  • The first one is the Russian Matryoshka doll model in which multiple SWNTs of different diameters are arranged concentrically inside one another, for instance, a (0,7) SWNT inside a larger (0,18) SWNT.

  • The second one is the parchment model wherein a single sheet of carbon nanotube is rolled up on itself, just like a newspaper or a parchment paper.

Regardless of the MWNT model, the distance between different layers of graphene is generally very small, just about the same as in case of the graphite layers in a pencil, approximately 3.4 A.

The Russian Matryoshka doll structure is more commonly observed in the MWNTs, and its individual SWNT layers can be semiconducting or metallic in nature.

Other mixed variants of MWNTs are possible too, for instance the DWNTs or double walled nanotubes. These mixed types of carbon nanotubes may comprise of two or more different types of concentric carbon nanotubes arranged in different orders, with either random or regular layering.