Interband Recombination Dynamics in Resonantly Excited Single-Walled Carbon Nanotubes

Wavelength-dependent pump-probe spectroscopy of micelle-suspended single-walled carbon nanotubes reveals two-component dynamics. The slow component (5–20 ps), which has not been observed previously, is resonantly enhanced whenever the pump photon energy coincides with an absorption peak and we attribute it to interband carrier recombination, whereas we interpret the always-present fast component (0.3–1.2 ps) as intraband carrier relaxation in nonresonantly excited nanotubes. The slow component decreases drastically with decreasing $p\mathrm{H}$ (or increasing ${\mathrm{H}}^{+}$ doping), especially in large-diameter tubes. This can be explained as a consequence of the disappearance of absorption peaks at high doping due to the entrance of the Fermi energy into the valence band, i.e., a 1D manifestation of the Burstein-Moss effect.

Figure 1

(a) Linear absorption spectrum for first (${\mathrm{E}}_1{\mathrm{H}}_1$) and second (${\mathrm{E}}_2{\mathrm{H}}_2$) subband transitions in semiconducting SWNTs and first subband transitions in metallic SWNTs. (b) Linear absorption spectra in the ${\mathrm{E}}_1{\mathrm{H}}_1$ range for $p\mathrm{H}=5.5$, 5.0, and 4.5. (c) Photoluminescence spectrum taken with an excitation wavelength of 632 nm.

Figure 2

(a) Pump-probe data at two wavelengths, corresponding to first and second subband transitions. (b) Maximum transmission change vs pump fluence.

Figure 3

(a) Linear absorption. The numbers correspond to the energies at which pump-probe measurements were made. (b) The peak value of $\Delta T/T_0$ and (c) the ratio of slow to fast components vs photon energy. (d)–(g) Pump-probe data taken at absorption peaks and valleys.

Figure 4

$p\mathrm{H}$-dependent pump-probe data for different wavelengths. The $p\mathrm{H}$ dependence becomes stronger for smaller photon energies (or larger-diameter tubes).