How may I convert a multiwalled nanotube from amorphous to crystal?

I would like to try to convert a multiwalled nanotube with diameter of about 40 nm to a crystal but when I use build and crystal, it converts it to two separate figures and on four corners of crystal; It does not appear as one set inside the nanotube. Please advise if there is any solution for this to create a crystal structure with preserving the integrity of the system.

Sincerely,

Mahdi