Optical redshift in the Raman scattering spectra of Fe-doped multiwalled carbon nanotubes: Experiment and theory

Electron doping of arc-grown multiwalled carbon nanotubes coated with iron nanoparticles is established by the redshift of the graphite peak and a larger redshift of the defect peak in the Raman spectra of doped samples compared to the corresponding peak positions in undoped samples. This is unlike the blue Raman shift usually observed in defect-induced double-Raman-resonance studies. On doping, the defect peak splits into two peaks. One has approximately the same dispersion (50 cm(-1) eV(-1)) as the defect peak of undoped samples. The other peak has a very large dispersion (110 cm(-1) eV(-1)). We show that the above observations are consequences of drastic changes in the electronic band structure of the graphitic systems under doping. Experimental observations of the splitting of the defect peak into two and larger dispersions of one of the peaks are explained via double-Raman-resonance processes studied through detailed theoretical calculations of electronic and phonon band structures based on a first principles ab initio method.