Phonon Spectromicroscopy of Carbon Nanostructures with Atomic Resolution

The vibrational properties of single-wall carbon nanotubes have been probed locally with atomic-scale resolution by inelastic electron tunneling spectroscopy with a low-temperature scanning tunneling microscope. The high spatial resolution has allowed the unraveling of changes in the local phonon spectrum related to topological defects. We demonstrated that the radial breathing mode is suppressed within tube segments of lengths below $\sim 3\mathrm{n}\mathrm{m}$, and that in the cap region phonon modes characteristic of the fullerene hemisphere are emerging. Phonon spectromicroscopy should lead to a better understanding of the mechanisms that limit the transport of heat or electrical charge inside nanostructured carbon materials.

Figure 1

(a) IETS-STM spectra of eight different isolated SWCNTs, obtained at 6 K as the $d^{2}I/d{V}^2$ lock-in signal with a bias modulation of 10 meV at a frequency of 2.7 kHz in the energy range of the RBM. Each spectrum is labeled by the $(n,m)$ pair of the respective tube. The spectra have been arranged such that the tube diameter increases from the top to the bottom. (b) Energy of the RBM of isolated SWCNTs plotted against the inverse diameter. The experimentally observed peak energies in the IETS and the theoretically calculated phonon energies are shown as solid squares and open circles, respectively.

Figure 2

Changes in the local vibrational properties of a SWCNT associated with the presence of local defects. (a) Topographic image of a (17,14) tube containing an intramolecular tube junction and a closed ending. The transitions between the corresponding regions are highlighted by black dashed lines. The white dotted line indicates the sampling positions along the tube axis where the IETS spectra were taken. (b) Color scale map of the lock-in signal ($d^{2}I/d{V}^2$) as a function of energy and position along the tube axis.

Figure 3

Scanning tunneling spectra displaying the electronic LDOS at different regions of the (17,14) tube depicted in Fig. 2a. The spectra were acquired at the tube body (spectrum a), the neck region (spectrum b), and the tube cap (spectrum c). The electronic LDOS at the cap exhibits a peak at $\sim -0.2\mathrm{V}$, which is attributed to a resonant state (RS).

Figure 4

Eigenvector characteristics of a cap-specific eigenmode at $\sim 59\mathrm{m}\mathrm{e}\mathrm{V}$ in a finite-length (40 nm) carbon nanotube capped by ${\mathrm{C}}_{60}$ hemispheres. This mode has finite radial ($u_r$) and zero tangential ($u_t$) displacements in the cap region (0–3.5 Å), akin to the characteristics of the RBM of a molecule. The nature of this mode transforms from a radial to a tangential character in the interior of the tube, a consequence of the resonance between the RBM of the ${\mathrm{C}}_{60}$ hemispherical cap and a bulk mode.