Preferential etching of metallic single-walled carbon nanotubes with small diameter by fluorine gas

We have introduced a new approach to selectively remove metallic single-walled carbon nanotubes (SWCNTs) using a gas phase reaction by fluorine gas, followed by a heat treatment. The metallicity of the treated samples was characterized by the resonant Raman spectra with three wavelengths of 514, 633, and $785\mathrm{nm}$ and UV-visible-NIR absorption spectra. Peaks of metallic SWCNTs with small diameters less than $1.1\mathrm{nm}$ in the radial breathing mode of Raman spectra were greatly suppressed with fluorination and completely disappeared after heat treatment. On the other hand, the semiconducting SWCNTs with small diameters less than $1.1\mathrm{nm}$ were still retained after fluorination. Moreover, the D-band decreased after annealing at $900°\mathrm{C}$, which was associated with the release of fluorine atoms during heat treatment. The removal of metallic SWCNTs was not observed at SWCNTs with diameters greater than $1.1\mathrm{nm}$. The absorption data also demonstrated the similar diameter dependence in the selectivity to the Raman spectra.

Figure 1

Time evolution of FESEM images during fluorination: (a) the raw SWCNTs, fluorinated SWCNTs for (b) 30, (c) 60, and (d) 180 minutes.

Figure 2

Raman spectra at $514\mathrm{nm}$ excitation wavelength. (a) Left panel, RBM regions of the raw SWCNTs which was deconvoluted with several peaks (Lorentzian), fluorinated SWCNTs for 30 minutes, and fluorinated SWCNTs followed by the heat treatment at $600°\mathrm{C}$ and $900°\mathrm{C}$ from the bottom spectrum. The box indicates the range of semiconducting and metallic components determined from Kataura plot. Right panel, G-band regions with same notations to the RBMs. (b) Raman spectra of the purified SWCNTs for comparison.

Figure 3

(a) $\mathrm{F}1\mathrm{s}$, (b) $\mathrm{C}1\mathrm{s}$, and (c) $\mathrm{O}1\mathrm{s}$ XPS spectra for the raw SWCNTs, the fluorinated SWCNTs for 30 minutes, and heat-treated $(900°\mathrm{C})$ SWCNTs after fluorination. (d) Deconvoluted $\mathrm{F}1\mathrm{s}$ XPS spectra for fluorinated SWCNTs.

Figure 4

RBM (left panel) and G-band (right panel) regions in Raman spectra at different excitation wavelengths, (a) $785\mathrm{nm}$ and (b) $633\mathrm{nm}$.

Figure 5

Deconvolution of the G-band for (a) the raw SWCNTs and (b) the heat-treated SWCNTs after fluorination for 30 minutes: excitation wavelengths of $514\mathrm{nm}$. The line at $1524{\mathrm{cm}}^{-1}$ indicates the BWF line shape of $m$-SWCNTs. The metallic and semiconducitng peaks were fitted using BWF and Lorentzian lines. The asymmetric BWF line shape is described by $I\left(\omega \right)=I_0\left\{\right[1+(\omega -{\omega }_{\mathrm{BWF}})∕q\Gamma {]}^2∕1+\left[\right(\omega -{\omega }_{\mathrm{BWF}})∕\Gamma {]}^2\}$, where $1∕q$ is a measure of the interaction of the phonon with a continuum of states, and ${\omega }_{\mathrm{BWF}}$ is the BWF peak frequency at maximum intensity $I_0$ (Ref. 26).

Figure 6

(a) Absorption spectra of the raw SWCNTs (upper line) and the fluorinated SWCNTs for 30 minutes followed by heat treatment (bottom line). Curve fitting of the $S_{22}$ and $M_{11}$ bands after subtraction of the plasmon baseline for (b) the raw SWCNTs and (c) the heat-treated SWCNTs after fluorination for 30 minutes.