Infrared-Active Vibrational Modes of Single-Walled Carbon Nanotubes

The IR-active vibrational modes of single-walled carbon nanotubes have been observed by optical transmission through thin films of bundled nanotubes. Because IR-active chemical functional groups, e.g., $-\mathrm{COOH}$, $-\mathrm{OH}$, might be attached to the tube walls and contribute additional spectral features, we have also studied the effects of chemical purification and long-term high-temperature vacuum annealing on the IR spectrum. Through comparison with theory, we are able to assign much of the sharp stucture observed in our IR spectra.

Figure 1

Difference spectra for raw and purified SWNT samples annealed at various temperatures: (a) raw soot SWNT material, (b) after purification, (c) after purification and $\mathrm{HTVA}=1100°\mathrm{C}$, (d) after purification and $\mathrm{HTVA}=1400°\mathrm{C}$. Spectrum (e) is that of a sample with $\mathrm{HTVA}=1400°\mathrm{C}$ and a polynomial fit to a smooth background (dotted line). Inset: the entire spectrum of a sample exposed to $\mathrm{HTVA}=1100°\mathrm{C}$. The asterisk (*) indicates a triplet, which is a spectral artifact.

Figure 2

Spectra of a purified SWNT sample with $\mathrm{HTVA}=1400°\mathrm{C}$. Calculated 1st order-allowed $q=0$ modes of SWNTs in the diameter range of 1.2–1.6 nm from current work and Jishi et al. are also shown [13].

Figure 3

Top three panels: calculated 2-phonon DOS for three selected tubes [(10,10), (15,0), (14,7)]. Bottom panel: experimental IR spectrum ($\mathrm{HTVA}=1400°\mathrm{C}$).