Raman spectra and thermal stability analysis of $0.4\mathrm{nm}$ freestanding single-walled carbon nanotubes

The thermal stability of ultrasmall $0.4\mathrm{nm}$ single-walled carbon nanotubes (SWNTs) are studied by means of Raman-scattering measurements under a vacuum. The $0.4\mathrm{nm}$ SWNTs are very stable when they are confined inside the channels of the $\mathrm{Al}\mathrm{P}{\mathrm{O}}_4\text{−}5$ zeolite crystal. When these SWNTs are extracted from the channels into free space, however, they become thermally unstable because of the strong curvature effect. The in situ Raman-scattering measurement under $1\times {10}^{-5}\mathrm{mbar}$ shows all three structures of the $0.4\text{−}\mathrm{nm}$-sized SWNTs are destroyed between $730\text{to}790\mathrm{K}$, a temperature range much lower than that of large-sized SWNTs. The (5,0) tube is only destroyed after the temperature reaches $790\mathrm{K}$ and seems slightly more stable than the other two structures: the (3,3) and (4,2) tubes. A reference measurement under UHV conditions confirms that the $0.4\mathrm{nm}$ SWNTs are destroyed after the same thermal treatment indicating that the structural degradation is determined by the curvature effect other than oxidation.

Figure 1

(Color online) (a) Raman spectra of freestanding SWNTs (b) SWNTs confined inside AFI crystals at $300\mathrm{K}$. The inset shows the magnified spectra of RBMs for the freestanding SWNTs (a) and the confined SWNTs (b). The RBMs are fitted by three Lorenzian peaks at 499, 538, and $576{\mathrm{cm}}^{-1}$ for the freestanding tubes and at 510, 550, and $580{\mathrm{cm}}^{-1}$ for confined SWNTs, respectively.

Figure 2

(Color online) Raman spectra in a RBM region at temperature range from $300\text{to}870\mathrm{K}$. Three distinguishable RBM peaks are fitted by Lorenzian components at $499{\mathrm{cm}}^{-1}$, $538{\mathrm{cm}}^{-1}$, and $576{\mathrm{cm}}^{-1}$ from $300\text{to}670\mathrm{K}$. RBM components of (4,2) and (3,3) tubes merge into the tails of the RBM component of the (5,0) tube at $730\mathrm{K}$. No distinguishable RBM exists at a temperature higher than $790\mathrm{K}$.

Figure 3

(Color online) Variation of Raman spectra of SWNTs from $300–870\mathrm{K}$. The inset shows the enlarged $G$ band at different temperatures. The dot and arrow denote the peak position of two components in the asymmetric $G$ band showing downshift and broadening with elevating temperature. The two components merge into a single symmetric peak at a temperature higher than $670\mathrm{K}$.

Figure 4

(Color online) (a) Spectra change under an isotherm condition at $870\mathrm{K}$ for different heating durations. Spectrum at $300\mathrm{K}$ after the temperature is cooled down is at the bottom. (b) Spectra of SWNTs@AFI at $870\mathrm{K}$ (top) and at $300\mathrm{K}$ (bottom) after heating at $870\mathrm{K}$ under a vacuum of $1\times {10}^{-5}\mathrm{mbar}$. (c) Spectra at $300\mathrm{K}$ of freestanding SWNTs before (above) and after (below) heating at $870\mathrm{K}$ under a vacuum of $1\times {10}^{-10}\mathrm{mbar}$.