Carbon peapods whose molecules are amorphous carbon can be naturally produced although C60 fullerene impurities are formed the fullerenes within a nanotube can be stabilized at a diameter difference of 0.34 nm or less
Carbon peapod is a hybrid nanomaterial consisting of spheroidal fullerenes encapsulated within a carbon nanotube. C60 molecules normally form amorphous carbon when heated to 10001200 C under ambient conditions; when heated to such a high temperature within a carbon nanotube, they instead merge in an ordered manner to form another SWNT, thus creating a double-wall carbon nanotube. C60 fullerene impurities are formed during the annealing treatment and acid purification, and enter the nanotubes through defects or vapor-phase diffusion. Fullerenes within a nanotube are only stabilized at a diameter difference of 0.34 nm or less, and when the diameters are nearly identical, the interacting energy heightens to such a degree (comparable to 0.1 GPa) that the fullerenes become unable to be extracted from the SWNT even under high vacuum. 
