Nanotubes can be thought of layers of the conventional graphite structure
rolled up into a cylinder such that the lattice of carbon atoms remains
continuous around the circumference. (
5
) The number of shells varies from one (a single-walled CNT) to as many
as fifty with the spacing between the layers matching closely the layer spacing
in graphite (around 0.34 nm). CNTs are usually 1-50 nm in diameter and typically
a few microns long, although multi-walled nanotubes as long as 2mm have
been grown. The constraints imposed by the continuity of the lattice mean
that, in general, every shell has a different stucture which is chiral due
to a helical twist of the carbon lattice. There are only two types of non-helical
tube - those arranged with the sides of the hexagonal carbon rings parallel
or perpendicular to the axis of the tube, called zig-zag or armchair respectively.
Nanotubes can have a wide range of structural perfection depending on the
synthesis method used to grow them. The defect concentration is known to
have a significant impact on nanotube properties and can lead to curvature
or kinking. Indeed sometimes structures are grown in which the graphite layers
are arranged at an angle to the axis rather than parallel to the tube axis,
an arrangement usually known as a 'herringbone nanofibre'. The term nanofibre
is also applied to larger nanotubes, over a somewhat arbitrary cut-off of
around 50 nm in diameter.
Section of a single walled nanotube shell
Cut-away section of a 3-shell multi-walled nanotube
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