Isotope Engineering of Carbon Nanotube Systems

The synthesis of a unique isotope engineered system, double-wall carbon nanotubes with natural carbon outer and highly ${}^{13}\mathrm{C}$ enriched inner walls, is reported from isotope enriched fullerenes encapsulated in single-wall carbon nanotubes (SWCNTs). The material allows the observation of the $D$ line of the highly defect-free inner tubes that can be related to a curvature induced enhancement of the electron-phonon coupling. Ab initio calculations explain the inhomogeneous broadening of inner tube Raman modes due to the distribution of different isotopes. Nuclear magnetic resonance shows a significant contrast of the isotope enriched inner SWCNTs compared to other carbon phases and provides a macroscopic measure of the inner tube mass content. The high curvature of the small diameter inner tubes manifests in an increased distribution of the chemical shift tensor components.

Figure 1

Raman spectra of DWCNTs with ${}^{\mathrm{Nat}}\mathrm{C}$ and ${}^{13}\mathrm{C}$ enriched inner tubes at 676 nm excitation. (a) RBM, (b) $D$ and $G$ mode Raman responses. Arrows and circles indicate the $D$ and $G$ modes of inner and outer tubes, respectively.

Figure 2

RBMs of some inner tubes for 676 nm excitation with $0.5{\mathrm{cm}}^{-1}$ spectral resolution. (a) ${}^{\mathrm{Nat}}\mathrm{C}$, (b) ${{}^{13}\mathrm{C}}_{0.25}$. 1–4 mark the experimental spectrum, the spectrum after deconvolution by the spectrometer response, a simulated spectrum with an extra Gaussian broadening, and a calculated spectrum (see text), respectively.

Figure 3

$G$ modes of DWCNT with ${}^{\mathrm{Nat}}\mathrm{C}$ (a) and 89% ${}^{13}\mathrm{C}$ (b) enriched inner walls at 488 nm excitation. Circles and arrows indicate the nonshifting components of the outer and the shifting components of the inner tubes, respectively. Dashed curves show the deconvolution of the observed spectra.

Figure 4

NMR spectra normalized by the sample mass, taken with respect to tetramethylsilane (TMS) shift: (a) static spectrum for nonenriched SWCNT enlarged by a factor of 15, (b) static, and (c) MAS spectra of ${}^{13}\mathrm{C}_{0.89}\mathrm{\text{−}}\mathrm{DWCNT}$. Smooth curve is a CSA powder pattern simulated with literature parameters [10]. Asterisks show the spinning sidebands at 8 kHz.