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Structure of Carbon Nanotubes

Structurally speaking, carbon nanotubes can be termed as long and thin cylinders of carbon, discovered in the year 1991. What's special about these large macromolecules is their uniqueness in terms of their size, shape and physical properties. One can perceive them as graphite sheets or hexagonal carbon lattices, rolled in the form of cylinders.

More on the structure of carbon nanotubes

The structure of carbon nanotubes is defined by a layer of carbon atoms, bonded with each other in a way that they create a honeycomb or a hexagonal mesh. This carbon layer of one-atom-thickness is referred to as graphene. It's wrapped up in a cylindrical shape and bonded together to create what we call a carbon nanotube.

Carbon nanotubes can either have just a single outer carbon wall (in case of single walled carbon nanotubes), or many of them (in a way that there are multiple carbon cylinders of varying diameters inside a single carbon cylinder; known as multi walled carbon nanotubes). The variance in the structure of carbon nanotubes can have a major impact on their thermal and electrical properties.

Single walled carbon nanotubes' structure

The single walled carbon nanotubes' structure can be in any of the following three distinct designs:

  • Chiral
  • Armchair and
  • Zig-Zag

The exact design of the SWNT depends on the manner in which the graphene has been rolled into a cylindrical shape. For instance, think about rolling a simple A4 sheet of paper from its edge - that can be one particular design, while a different one can be when you roll the same A4 sheet of paper from its corner.

In case of single walled carbon nanotubes, their structure is defined or represented through a pair of indices (n, m) referred to as the chiral vector (Ch), and the chiral angle θ.

The chiral vector can be described based on these translational indices (n, m) and unit vectors a1 and a2, as shown in the equation below:

Ch = na1 + ma2

Here 'n' and 'm' are integers that represent the number of steps taken along the zig-zag carbon bonds in a hexagonal lattice.

The degree of twisting in the carbon nanotube is determined by the chiral angle θ. This angle is the angle θ existing between the vectors a1 and Ch. This angle can fall anywhere in the range of 0o ≤ |θ| ≤ 30o.

There can be two limiting cases based on these carbon bonds' geometry around the circumference of the carbon nanotube, corresponding the achiral tubes:

  • Zig-zag, in which θ = 0o and
  • Armchair, in which θ = 30o

Apart from these two, the carbon nanotube is referred to as chiral if 0o < |θ| < 30o. In the chiral vector's terms, a carbon nanotube is zig-zag when (n, 0) and armchair when (n, m).

Multi walled carbon nanotubes' structure

Now talking about the multi walled carbon nanotubes' structure, they can be of two kinds:

Russian doll model, in which a carbon nanotubes consists of another carbon nanotubes inside it (the outer nanotube's diameter being larger than the inner one). For example, a (0, 7) single walled nanotube inside a larger (0, 16) single walled nanotube.

The parchment model, in which a single carbon nanotube or grapheme sheet is wrapped up around itself a number of times, just like a rolled-up scroll of parchment paper or newspaper.

The properties of single walled nanotubes and multi walled nanotubes are quite similar to each other, however in case of latter, the outer walls can protect the inner nanotubes from various chemical interactions with the outside materials. Owing to their structure, multi walled carbon nanotubes have a higher tensile strength compared to the single walled carbon nanotubes.

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