Distinguishing defect induced intermediate frequency modes from combination modes in the Raman spectrum of single walled carbon nanotubes

Understanding of the origin of some of the intermediate frequency modes (IFMs) in the Raman spectrum of carbon nanotubes has remained controversial in the literature. In this work, through controlled introduction and elimination of defects in carbon nanotubes, we attempt to isolate the IFMs due to structural defects from that of the combination modes in single walled carbon nanotubes (SWCNTs). Our investigations on pristine and defect engineered SWCNTs using ion-irradiation, thermal annealing, and laser processing show systematic changes in the IFMs in the range 400-1200cm(-1) and its manipulation with the processing parameters. In particular, we found that the intensity of IFM at 929 cm(-1) scale up with the increasing defect concentration, while that at 668cm(-1) follows opposite behavior. New IFM peaks were observed upon the creation of a controlled amount of structural defects through 30 keV N+ ion irradiation. Elimination of defects through vacuum annealing results into reduction of intensity of some IFMs identified as defect related, while the intensity of characteristic combination modes correspondingly increases. Our results show that the IFMs observed at 709, 805, 868, 926, and 1189cm(-1) are due to structural defects in the SWCNTs, while those in the range 400-550cm(-1) and at 669cm(-1) are due to the combination modes. Our x-ray photoelectron spectroscopy analysis on ion irradiated SWCNTs supports the Raman results. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3692070]