Dielectric Response of Aligned Semiconducting Single-Wall Nanotubes

We report measurements of the full intrinsic optical anisotropy of isolated single-wall carbon nanotubes (SWNTs). By combining absorption spectroscopy with transmission ellipsometry and polarization-dependent resonant Raman scattering, we obtain the real and imaginary parts of the SWNT permittivity from aligned semiconducting SWNTs dispersed in stretched polymer films. Our results are in agreement with theoretical predictions, highlighting the limited polarizability of excitons in a quasi-1D system.

Figure 1

(a) Background-corrected SANS and SAXS intensity versus $q$ for an unstretched 0.035% HPCO film, where the (straight) red line is a $-1$ power law. The inset shows an AFM image of DNA-wrapped SWNTs on mica. (b) 2D NIR fluorescence plot (excitation versus emission) for the $(7,5)$ SWNT feature in a stretched 0.035% CoMoCat film ($S=0.5$) with the incident light polarized along the stretch direction and (c) with the excitation polarized normal to the stretch direction.

Figure 2

UV-visible-NIR absorbance spectra as a function of photon energy and wavelength for a stretched 0.035% HPCO SWNT-PAA film ($S=0.8$), where the black trace is the prestretch absorption spectrum. The indicated angle is between the incident polarization direction and the alignment axis, where $\Delta$ is the measurement increment.

Figure 3

(a) Polarization-dependent ($VV$) resonant Raman scattering from the stretched HPCO sample in Fig. 2 (751.4 nm excitation) for the RBM, $D$ band, and $G$ band. (b) Angular dependence of the normalized peak intensity in (a) with fits to Eq. (3).

Figure 4

(a) Intrinsic permittivity of HPCO SWNTs with a number of different chiralities evident in the $S_{11}$ and $S_{22}$ transitions (gray horizontal bands). Upper (lower) traces denote parallel (normal) to the SWNT symmetry axis and black curves are fits to the absorption spectra with Eq. (5). The real (imaginary) part is shown as solid (dotted) traces, where the light curves are from transmission ellipsometry. (b) An identical plot for CoMoCat SWNTs. The inset of (a) shows the anisotropy of the $\pi$ plasmon and the inset of (b) shows the $(6,5)$ SWNT response extracted from the fit to Eqs. (5, 6).