Resonance Raman study of linear carbon chains formed by the heat treatment of double-wall carbon nanotubes

We study the resonance behavior of the unusual Raman feature known as the coalescence-inducing mode (CIM), observed at $\sim 1850{\mathrm{cm}}^{-1}$, in samples of double-wall carbon nanotubes annealed at high temperatures. Resonance Raman spectra taken with different laser energies show that the intensity of the CIM band exhibits a maximum around $2.20\mathrm{eV}$. By comparing the resonance Raman experimental results with first-principles calculations for the vibrational frequency and the energy gap, we propose that the CIM feature is associated with short carbon chains with an odd number of atoms, interconnecting the nanotube surfaces.

Figure 1

(a) Raman spectrum of undoped DWNTs heat treated at $1500°\mathrm{C}$ where we can see the $G$ band, the CIM band, and the two combination modes 2oTO and iTOLA with the intensity multiplied by 10. (b) First-order CIM bands obtained for $E_{\mathrm{laser}}=2.20\mathrm{eV}$ at different locations on an undoped sample heat treated at $1500°\mathrm{C}$. The spectra are fit with a sum of Lorentzians and their frequencies are displayed in ${\mathrm{cm}}^{-1}$. (c) Frequency vs $E_{\mathrm{laser}}$ for all Lorentzian peaks used to fit the CIM feature.

Figure 2

Resonance Raman profile for the CIM feature of (a) the undoped and (b) the B-doped DWNT samples subjected to $1500°\mathrm{C}$ and $1300°\mathrm{C}$ heat treatment, respectively. The arrows indicate the intensity maxima of the electronic resonance with $E_{\mathrm{laser}}$.

Figure 3

(a) Molecular model obtained during MD calculations at $3000\mathrm{K}$, revealing the presence of carbon threads that form prior to the full coalescence of the nanotubes. The ends of each chain (highlighted in gray) are fixed to a hexagonal carbon network. (b) Molecular model considering an alternative framework where the C5 chain interconnects the inner nanotubes of two coalescing DWNTs.