Radiation effects on theDtoGRaman intensities of carbon nanotubes

Abstract

For carbon nanotubes irradiated with 140 keV He ions, the intensity ratio of the $D$ to $G$ Raman modes initially increases and then decreases with increasing ion fluence. A model is proposed to describe the competing effects between the reduction in crystal size, which increases the $D$ band intensity with increasing fluence as a long-range order effect, and the reduction in the number of sixfold rings available for $D$ mode vibration, which decreases the $D$ band intensity with increasing fluence as a short-range order effect. The latter effect dominates when the length of the short-range order becomes comparable to the unit-cell dimensions as a result of ion bombardment. This suggests that the appearance of the maximum $D$ to $G$ intensity ratio coincides with the onset of structural amorphization. The model is supported experimentally by examination of irradiated nanotubes with transmission electron microscopy, and theoretically by Raman calculation based on density functional perturbation theory. A universal formula is proposed to describe $D/G$ behaviors under arbitrary ion irradiation.