The production of amorphous regions in carbon nanotubes by 140 keV He ion irradiation

The production of amorphous regions in carbon nanotubes irradiated with 140 keV He ions was studied using Raman spectroscopy and transmission electron microscopy (TEM). Intensity ratios of Raman D to G bands (ID/IG) initially increase and then decrease as a function of ion fluence at all investigated irradiation temperatures (room temperature, 200 and 400 °C). The critical ion fluences corresponding to the maximum in ID/IG ratios increase with increasing irradiation temperature because of the enhanced defect annealing. The displacement per atom (dpa) values, consistent with a maximum in ID/IG ratios, are determined to be 0.15 dpa at room temperature and 200 °C, and 0.3 dpa at 400 °C. TEM examination of all irradiated specimens supports Raman results indicating that the maximum in ID/IG correlates to the formation of amorphous regions. The study shows that after formation of amorphous regions at high fluences, ID/IG ratio can be no longer used to measure amorphous/graphitic content in CNTs.