Ultrafast Optical Spectroscopy of Micelle-Suspended Single-Walled Carbon Nanotubes

TL;DR

This study uses ultrafast optical spectroscopy to investigate carrier dynamics in micelle-suspended single-walled carbon nanotubes, revealing new insights into relaxation processes and the influence of environmental pH.

Contribution

It provides the first observation of a slow carrier relaxation component linked to radiative recombination in these nanotubes, influenced by pH levels.

Findings

Ultrafast decays at second van Hove singularities are due to intraband relaxation.

A newly observed slow relaxation component is enhanced at interband resonances.

Slow relaxation depends on solution pH, indicating environmental effects.

Abstract

We present results of wavelength-dependent ultrafast pump-probe experiments on micelle-suspended single-walled carbon nanotubes. The linear absorption and photoluminescence spectra of the samples show a number of chirality-dependent peaks, and consequently, the pump-probe results sensitively depend on the wavelength. In the wavelength range corresponding to the second van Hove singularities (VHSs), we observe sub-picosecond decays, as has been seen in previous pump-probe studies. We ascribe these ultrafast decays to intraband carrier relaxation. On the other hand, in the wavelength range corresponding to the first VHSs, we observe two distinct regimes in ultrafast carrier relaxation: fast (0.3-1.2 ps) and slow (5-20 ps). The slow component, which has not been observed previously, is resonantly enhanced whenever the pump photon energy resonates with an interband absorption peak, and we attribute it to radiative carrier recombination. Finally, the slow component is dependent on the pH of the solution, which suggests an important role played by H$^+$ ions surrounding the nanotubes.