Polarization dependence of resonant Raman scattering from vertically aligned single-walled carbon nanotube films

We present experimental evidence of drastic changes in low-frequency Raman scattering spectra depending on the polarization of the incident laser with respect to the single-walled carbon nanotube (SWNT) axis. Employing recently developed vertically aligned SWNT films, which have a high density $(1.0\times {10}^{17}{\mathrm{m}}^{-2})$ and a thickness of $5\mu \mathrm{m}$, enabled us to obtain sufficient Raman scattering intensity from the film cross section where bundles of SWNTs are aligned along the same direction, in addition to from the top surface of the film. The measured peaks of the radial breathing mode (RBM) by 1.96, 2.41, and $2.54\mathrm{eV}$ incident lasers are clearly distinguished into 2 groups. One group of peaks is dominant for perpendicular polarization while the other group of peaks is dominant in the case of light polarized parallel to the SWNT axis. The selective vanishing of the perpendicular peaks by adsorption of molecules to the SWNTs along with the resultant change in optical absorption spectrum evidences that the parallel and perpendicular peaks originate from $\Delta \mu =0$ and $\Delta \mu =\pm 1$ excitations of electrons, respectively. The grouping behavior of RBM peaks also causes the drastic spectral variation caused by a change in incident laser power. The unambiguous classification of each RBM peak’s nature presented in this study will allow sounder characterization of SWNTs by the resonant Raman scattering analysis.

Figure 1

A cross-sectional FE-SEM image of a vertically aligned SWNT film at a fractured edge of the quartz substrate taken from tilted angle (top) and horizon (bottom).

Figure 2

Schematical description of relationships between the laser propagation direction $\left(\mathit{k}\right)$, the laser polarization direction $\left(\mathit{e}\right)$, and the SWNT axis direction $\left(\mathit{l}\right)$ in the measurement.

Figure 3

RBM spectra measured by 488, 514.5, and $633\mathrm{nm}$ lasers for different incident configurations [(i)—iv); see text]. G band spectra taken at $488\mathrm{nm}$ are also shown. The RBM spectra were normalized by the corresponding ${\mathrm{G}}^{+}$ height and decomposed into Lorentzian curves by maintaining the FWHM values within a spectrum. Asterisks denote the peaks dominantly observed in parallel polarization condition. The oscillatory line on the baseline denotes differential between experimental spectrum and sum of Lorentzian curves.

Figure 4

RBM spectra taken at $488\mathrm{nm}$ with laser light incident (a) from side and (b) from top of the film. A polarizer was inserted in a scattering light path except the case denoted “-All.” The “X-Y” description represents the polarization directions of incident and scattered light.

Figure 5

The change in intensities of selected RBM peaks divided by the ${\mathrm{G}}^{+}$ band among “from top”–“45°”–“parallel” conditions for (a) 488 and (b) $514.5\mathrm{nm}$. The ordinate was normalized by the values of the “from top” condition.

Figure 6

The change in RBM spectra measured in the “from top” condition by $488\mathrm{nm}$ light. The spectra are (a) the as-synthesized sample and (b) after evacuation by an oil-pump for $1\mathrm{h}$ at $200°\mathrm{C}$. The original spectrum of a different molecule-adsorbed sample (c) is recovered after heating it at $200°\mathrm{C}$ for $1\mathrm{h}$ in a CVD chamber evacuated by an oil-free pump (d). The spectra are normalized by the height of the ${\mathrm{G}}^{+}$ peak. The right panel shows corresponding tangential mode spectra of (a) and (b).

Figure 7

Optical absorption spectra of (b) an adsorbed sample and (c) the same sample after recovery by heating in vacuum at $200°\mathrm{C}$ for $1\mathrm{h}$. In the inset are corresponding Raman spectra taken at $488\mathrm{nm}$ in the “from top” configuration, along with (a) the original spectra before adsorption, to which all spectra are normalized by the height of the ${\mathrm{G}}^{+}$ band. Asterisks indicate the switching noise of the spectrophotometer.

Figure 8

(a) Spectral change of RBM peaks by changing the laser power intensity from $0.75\text{to}2.42\mathrm{mW}$, using $488\mathrm{nm}$ light in the “from top” configuration. The spectra were normalized by the ${\mathrm{G}}^{+}$ band. (b) The intensity variance of each Lorentzian-decomposed RBM peak over the incident laser power. Ordinate values were normalized by those in the case of $0.75\mathrm{mW}$. (c) The relationship between the frequency downshift of RBM peak and that of the ${\mathrm{G}}^{+}$ peak by heating of SWNTs, both from the case of $0.75\mathrm{mW}$.

Figure 9

The locations of several unambiguous $\mathit{e}\Vert \mathit{l}(\Delta \mu =0)$ peaks marked with circles and $\mathit{e}\perp \mathit{l}$ $(\Delta \mu =\pm 1)$ with crosses on a Kataura plot calculated from tight-binding (solid symbols) and GW-method (open symbols). Panels in the upper row represent the plots for the $\Delta \mu =0$ transition while panels in the lower row correspond to $\Delta \mu =\pm 1$.