Claim: Carbon peapods 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
Evidence: Carbon peapods can be naturally produced during carbon nanotube synthesis by pulsed laser vaporization. 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.