Resonant Raman spectroscopy on enriched ¹³C carbon nanotubes

Isotopically enriched single-wall carbon nanotubes with different ¹³C concentrations were investigated by resonant Raman spectroscopy. Linear reductions of the Raman frequencies with an increase of ¹³C concentration are observed for the different nanotube Raman modes, and the effect of the reduced mass variation of the isotope mixture on the phonon frequencies is described through a simple harmonic oscillator model. In addition to the frequency dependence, the Raman linewidths as a function of the ¹³C concentration were also investigated and an expression describing this is presented. We observe an increase in the G band linewidth, as the ¹³C:¹²C ratio approaches unity. Measurements with different excitation energies were performed and the frequency dispersions of the D and G′ bands with laser energy were observed to be the same for ¹²C and ¹³C nanotubes, suggesting no changes in the electronic structure after isotope enrichment. Through analysis of the radial breathing modes in the Raman spectra obtained with different excitation energies, a relation between these modes frequency and the ¹³C nanotubes diameter was also established.