Strong exciton-phonon interaction assisting simultaneous enhancement of photoluminescence and Raman scattering from suspended carbon nanotubes

We have observed intensity enhancement in exciton photoluminescence and $G$-band Raman scattering of suspended pristine semiconducting carbon nanotubes (CNTs) under resonant excitation while preventing surrounding dielectric Coulomb screening and emission quenching effects. Photoluminescence excitation (PLE) microspectroscopy for the isolated CNTs shows that the PL and Raman scattering are simultaneously enhanced when the excitation photon energy is equal to the sum of the energies of the lowest-energy bright exciton and a longitudinal optical (LO) phonon. This simultaneous enhancement is attributed to a large density of states of the lowest-energy bright exciton strongly interacting with the LO phonon at the Γ point. The resonant excitation is a key to understanding the excitation and emission processes of CNTs and to designing optimal photonic nanostructures applicable to efficient CNT-based light-emitting devices.

Figure 1

(a) Scanning electron microscope image of a CNT suspended over a $\mathrm{Si}{\mathrm{O}}_2$ trench. (b) PL spectra of another suspended CNT with no electron-beam exposure as a function of the position along a $\mathrm{Si}{\mathrm{O}}_2$ trench. The PL spectrum is highlighted at the center position of the CNT. The excitation wavelength and incident power are 780 nm and 3 mW, respectively.

Figure 2

PLE spectrum map of a vacuum-suspended CNT under short-wavelength laser excitation. (b) Exciton PL intensity at an emission wavelength of 1176.1 nm as a function of the excitation photon energy and wavelength. The solid curve shows a fitted double Lorentzian function. The incident laser power is 3 mW.

Figure 3

(a) PLE spectrum map of a CNT under longer-wavelength laser excitation. The slanted lines indicate Raman scattering peaks assigned to $G^{+}, M$, and $G^{\prime }$ bands. Peak intensities of (b) exciton PL and (c) $G^{+}$-band Raman scattering as a function of the excitation energy and wavelength. The dashed-dotted, solid, and broken curves for the PL intensity are theoretically calculated with a damping rate of 4, 12, and 20 meV, respectively. The broken curve for the Raman scattering intensity is a fitted Lorentzian function with a damping rate of 4 meV. (d) $G^{+}$-band Raman-scattering and exciton PL spectra as a function of the photon energy and wavelength of scattered light or PL. The incident laser power is 0.5 mW.

Figure 4

PLE spectrum and peak intensities of exciton PL and $G^{+}$-band Raman scattering as a function of the excitation energy for (a)–(c) a CNT with a chirality of (8,7) and (d)–(f) a CNT with a chirality of (9,7). The solid lines in the PL intensities are guides for the eye. The broken curves in the Raman scattering intensities are fitted Lorentzian functions with a linear offset and a damping rate of 5 meV. The incident laser power is 2 mW.

Figure 5

Exciton band in a chiral CNT with excitation and relaxation processes of (a) exciton PL and (b) resonant $G^{+}$ band Raman scattering. The solid and broken parabolic curves indicate first-band bright ($E_{1u}^{11}$) and dark ($E_{1g}^{11}$) excitons with zero angular momentum, respectively. The dashed-dotted curves indicate dark exciton bands with a positive angular momentum. The dark exciton bands with a negative angular momentum are not shown for simplicity. Each diagram shows two cases of the relaxation through LO phonons near the Γ or $K$ points. The horizontal axis indicates the total momentum including one-dimensional translational momentum and angular momentum.