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We present changes in the first and second order Raman spectra of multiwalled carbon nanotubes ͑MWNTs͒ functionalized with oxygenated groups. The oxygen groups were introduced onto the nanotube surface through two strong acid purification routes: ͑1͒ reflux in concentrated ͑70%͒ HNO 3 acid for 4 h at 80°C and ͑2͒ ultrasonification in 3 HNO 3 ͑70% ͒ :1H 2 SO 4 ͑98%͒ for 8.5 h. Raman spectroscopy, using two laser excitation wavelengths ͑514.5 and 632.8 nm͒, x-ray photoelectron spectroscopy, and thermal gravimetric analysis were employed to study the evolution of the products. All the techniques revealed a higher degree of functionalization for scheme 2 compared to scheme 1. Charge transfer phenomena were manifested by a shift of the C1s core level towards higher binding energies. We found that the intensity of both the D and G energy Raman modes if normalized to the second order mode D * mode follows similar trends upon acid treatments. We interpret this result together with the observed dispersion of G mode as an indication that the G mode in carbon nanotubes is defect induced in a double resonant process. Both acid schemes cause an upshift of D and G Raman modes, due to intercalation of acid molecules, exerting pressure on the sp 2 structure and an electron transfer from the states in MWNTs to the oxygen atoms.

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Near edge x-ray absorption fine structure ͑NEXAFS͒ spectroscopy has been employed to obtain the temperature dependent evolution of the electronic structure of acid treated carbon nanotubes, which were further modified by dielectric barrier discharge plasma processing in an ammonia atmosphere. The NEXAFS studies were performed from room temperature up to 900°C. The presence of oxygen and nitrogen containing functional groups was observed in C K edge, N K edge, and O K edge NEXAFS spectra of the multiwalled carbon nanotubes. The N K edge spectra revealed three types of * features, the source of which was decisively identified by their temperature dependent evolution. It was established that these features are attributed to pyridinelike, NO, and graphitelike structures, respectively. The O K edge indicated that both carbonyl ͑C v O͒, * ͑CO͒, and ether CO -C, * ͑CO͒, functionalities were present. Upon heating in a vacuum to 900°C the * ͑CO͒ resonances disappeared while the * ͑CO͒ resonances were still present confirming their higher thermal stability. Heating did not produce a significant change in the * feature of the C K edge spectrum indicating that the tabular structure of the nanotubes is essentially preserved following the thermal decomposition of the functional groups on the nanotube surface.

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The effect of nitrogen incorporation on the bonding structure of hydrogenated carbon nitride films Electron cyclotron resonance deposition, structure, and properties of oxygen incorporated hydrogenated diamondlike amorphous carbon films