Resonance Raman scattering studies in ${\mathrm{Br}}_2$-adsorbed double-wall carbon nanotubes

The vibrational and electronic properties of ${\mathrm{Br}}_2$-adsorbed double-wall carbon nanotubes (DWNTs) were investigated by resonance Raman scattering. Special attention was given to distinguish the behavior between S/M and M/S outer/inner semiconducting (S) and metallic (M) tubes. By using three laser excitation energies 2.33, 1.96, and $1.58\mathrm{eV}$, resonance Raman spectra were obtained for the DWNTs before and after bromine adsorption, and also for the $\mathrm{Br}\text{−}\mathrm{Br}$ molecular resonance, thereby facilitating the study of charge transfer between the DWNTs and the bromine. It was found that ${\mathrm{Br}}_2$ molecules act as acceptors and that metallic nanotubes are specially sensitive to the presence of ${\mathrm{Br}}_2$ molecules even when they constitute the inner tubes of DWNTs.

Figure 1

Transmission electron microscope (TEM) images of pristine double-wall carbon nanotubes (a) before and (b) after ${\mathrm{Br}}_2$ adsorption. (c) and (d) show, respectively, scanning electron microscope (SEM) images, for both pristine and ${\mathrm{Br}}_2$-doped DWNTs.

Figure 2

(Color online) Calculated $E_{ii}$ values for semiconducting (S) (open circles) and metallic (M) (open squares) tubes vs radial breathing mode frequency ${\omega }_{\mathrm{RBM}}$ (bottom horizontal axis) and nanotube diameter (top horizontal axis). The open and solid symbols, respectively, stand for pristine and for 0.04 electron/carbon atom deficient bromine doped nanotubes (Refs. 18, 21). The three horizontal lines stand for the 1.58, 1.96, and $2.33\mathrm{eV}$ laser energies used for excitation of the Raman spectra. The family behavior is indicated by the $2n+m$ values for the $E_{22}^S$ and $E_{11}^M$ transitions (see text). We indicate in the upper part of the figure the frequency domain where the main contributions of the inner and outer tubes to the RBM spectra are expected based on the diameter distribution of the sample and the spectrometer limitations at low wave numbers. The calculations for doped tubes were performed only for the diameter range compatible with our samples.

Figure 3

Raman spectra taken with $E_{\mathrm{laser}}=2.33\mathrm{eV}$ for pristine (a) and ${\mathrm{Br}}_2$-adsorbed (b) DWNTs in the RBM spectral region.

Figure 4

Raman spectra taken with $E_{\mathrm{laser}}=1.96\mathrm{eV}$ for pristine (a) and ${\mathrm{Br}}_2$-adsorbed (b) DWNTs in the RBM spectral region.

Figure 5

Raman spectra taken with $E_{\mathrm{laser}}=1.58\mathrm{eV}$ for pristine (a) and ${\mathrm{Br}}_2$-adsorbed (b) DWNTs in the RBM spectral region. The sharp peaks near 170 and $272{\mathrm{cm}}^{-1}$ marked with an asterisk are experimental artifacts. The inserts show the lineshape analyses for the spectral region of $250–280{\mathrm{cm}}^{-1}$.

Figure 6

Raman spectra taken with $E_{\mathrm{laser}}=1.58\mathrm{eV}$ for pristine and ${\mathrm{Br}}_2$-adsorbed DWNTs in the region of the $G$ band. The inset shown the line shape analysis for the pristine sample.

Figure 7

Raman spectra taken with $E_{\mathrm{laser}}=1.96\mathrm{eV}$ for pristine and ${\mathrm{Br}}_2$-adsorbed DWNTs in the region of the $G$ band. The inset shows the line shape analysis for the pristine sample.

Figure 8

Raman spectra shown in the left panel are taken with $E_{\mathrm{laser}}=2.33\mathrm{eV}$ for pristine and ${\mathrm{Br}}_2$-adsorbed DWNTs in the region of the $G$ band. The figures on the right show the line shape analysis for the pristine (lower panel) and the doped (upper panel) samples.

Figure 9

Dependence of the ${\omega }_{G^{+}}-{\omega }_{G^{-}}$ splitting on reciprocal diameter. The solid lines come from the dependence of the splitting of SWNTs on $1∕d_t^2$ found in Ref. 25 and the points are our experimental data obtained from Figs. 6, 7, 8.

Figure 10

The line shape analysis for the Raman peaks from the ${\mathrm{Br}}_2$ intercalant species from the Raman spectrum taken at $E_{\mathrm{laser}}=2.33\mathrm{eV}$. The dashed line is the contribution from the nanotubes close to $200{\mathrm{cm}}^{-1}$. The inset shows the second and third harmonic modes of the $\mathrm{Br}\text{−}\mathrm{Br}$ stretching mode.

Figure 11

In situ Raman spectra of ${\mathrm{Br}}_2$-adsorbed DWNTs subjected to different thermal annealing temperatures during heating (in steps of $25°\mathrm{C}$) and cooling (in steps of $100°\mathrm{C}$). The Raman spectra were excited by $E_{\mathrm{laser}}=2.33\mathrm{eV}$.

Figure 12

$G$-band and $G$’-band spectra of pristine, ${\mathrm{Br}}_2$-adsorbed, and thermally treated $(600°\mathrm{C})$ ${\mathrm{Br}}_2$-adsorbed DWNTs. The Raman spectra were excited by $E_{\mathrm{laser}}=2.33\mathrm{eV}$.