Carbon nanotubes are cylindrical fullerenes with very high length to…

The idea

A carbon nanotube is a fullerene in which the carbon cage is rolled into a long, hollow cylinder instead of a closed ball. The cylinder wall is a curved sheet of hexagonal carbon rings, with each carbon bonded to three others, and the ends may be capped by curved rings containing five-membered rings. Nanotubes are typically nanometres in diameter but can be micrometres or millimetres long, so the length to diameter ratio is extremely large, often thousands to one. This high aspect ratio, together with strong covalent bonding along the tube and delocalised electrons, gives nanotubes exceptional tensile strength, stiffness and electrical conductivity. They are therefore used in composite materials, nanoscale electronics and as molecular wires.

What earns the marks

• A carbon nanotube is a fullerene whose carbon framework forms a hollow cylinder rather than a closed sphere.
• The cylinder wall is made of hexagonal rings of carbon atoms, with each carbon covalently bonded to three others.
• The tube is very long compared with its diameter, giving a very high length to diameter ratio.
• The high length to diameter ratio and strong covalent bonding along the tube give nanotubes high tensile strength and stiffness.
• Delocalised electrons allow nanotubes to conduct electricity, so they can act as nanoscale wires.
• Nanotubes are nanoscale in diameter, typically a few nanometres, while their length can be far greater.

Common mistakes and how to put them right

• Confusing nanotubes with graphene sheets.

  Put it right by stating that a nanotube is a rolled-up cylinder, whereas graphene is a single flat layer one atom thick.

• Describing nanotubes as solid rods of carbon.

  Put it right by stating that they are hollow cylinders with a carbon framework wall.

• Claiming nanotubes have a low length to diameter ratio.

  Put it right by stating that the ratio is very high because the diameter is nanoscale while the length is much greater.

Exam tips

• Use the phrase 'length to diameter ratio' explicitly when explaining why nanotubes are described as having a very high aspect ratio.
• When comparing nanotubes with other carbon allotropes, refer to the cylindrical shape and the three bonds per carbon atom.
• Support statements about strength or conductivity by linking them to the covalent bonding along the tube and the delocalised electrons.