Here's a single-walled (5,5) carbon nanotube with arm-chair (as opposed to
zig-zag) chirality. Its size and structure is chosen so that it
can be seamlessly capped by half of a buckyball. Suppose we wanted
to sheath a facing pair of these capped tubes in a larger
nanotube, so that the carbon atoms of the outer tube were
approximately 3.4 Angstroms (graphite's normal inter-layer
spacing) away from the inner tube atoms. Given that this tube
has 5 "armchairs" like this => running around
its circumference, how many
"armchairs of circumference" should the outer tube have?
When that 2-wall internally-capped tube model is constructed, we'll compare it
to the image below of a 15-wall nanotube with a 6-wall internal closure:
The above image is tiny part of a much larger image of multi-walled
carbon nanotubes lying like straight sticks of uncooked spaghetti after
falling out of the box into a pot on the stove.
For more nano-tube and nano-cone models, check out Steffen Weber's
Hop to model number four here.