DEFECT EVOLUTION AND STRUCTURAL IMPROVEMENT IN LOW ENERGY ION IRRADIATED CARBON NANOTUBES: MICROSCOPIC AND SPECTROSCOPIC STUDIES

The capability of graphitic networks to reorganize their structures under irradiation of energetic particles provides the tool for nano-engineering of carbon nanotubes (CNTs). We have studied the er effect of 30 keV N+ ion irradiation with three different fluences 10(12), 10(13), and 10(14) ions/cm(2) on the structural and spectroscopic properties of single-walled carbon nanotubes (SWNTs) and multiwalled carbon nanotubes (MWNTs). Irradiation-induced structural defects and coalescence of the nanotubes are studied using high-resolution transmission electron microscopy (HRTEM). Upon irradiation, some of the radial breathing modes in Raman spectra disappear due to conversion from single-walled to multiwalled structure. We observed a systematic change in intensity of the intermediate frequency mode (IFM) with increasing dose of ion-irradiation and these IFM modes are attributed to structural defects of SWNTs. Dramatic improvement in the intensity ratio G-band to D-band (at similar to 1335 cm(-1)) for ion fluence of 10(13) ions/cm(2) indicates improved graphitic structure as a result of reconstruction. Similarly, X-ray Photoelectron spectroscopy studies show improvement in the amount of sp(2) carbon upon 10(13) ions/cm(2) N+ ion irradiation dose. At a higher dose (10(14) N+ ions/cm(2)), vacancy and bent structures having Stone-Wales defects were observed in HRTEM, whereas MWNTs show formation of surface hillock like protrusions leading to formation of fullerene-like structures.