Strong reduction of exciton-phonon coupling in single-wall carbon nanotubes of high crystalline quality: Insight into broadening mechanisms and exciton localization

Carbon nanotubes are quantum sources whose emission can be tuned at telecommunication wavelengths by appropriately choosing the diameter. Most applications require the smallest possible linewidth. Therefore, the study of the underlying dephasing mechanisms is of utmost interest. Here, we report on the low-temperature photoluminescence (PL) of high crystalline quality individual single-wall carbon nanotubes synthesized by laser ablation (L-SWNTs) and emitting at telecommunication wavelengths. A thorough statistical analysis of their emission spectra reveals a typical linewidth one order of magnitude narrower than that of most samples reported in the literature. The narrowing of the PL line of L-SWNTs is due to a weaker effective exciton-phonon coupling subsequent to a weaker localization of the exciton. These results suggest that exciton localization in SWNTs not only arises from interfacial effects, but that the intrinsic crystalline quality of the SWNT plays an important role.

Figure 1

(a), (c) Temporal traces of the PL emission of two different SWNTs consisting of 60 consecutive spectra (1 s integration time for each spectrum). (b), (d) Example of a spectrum extracted from the trace in (a) and (c) (indicated by the white arrows). The power density used here is 60 $\mathrm{kW}/{\mathrm{cm}}^2$.

Figure 2

FWHM distribution measured for 43 individual SWNTs. The data reported on the histogram are obtained by picking the minimum FWHM in the temporal traces such as those showed in Fig. 1. The most probable values are around 500 $\mu \mathrm{eV}$. The low-values tail is cut off by the optical resolution of the setup at 250 $\mu \mathrm{eV}$ (gray dashed line). Inset: Dependence of FWHM on emission energy.

Figure 3

PL spectra of L-SWNTs : (a) and (b) are the tubes of Fig. 1. (a), (b), and d) were acquired with an integration time of 60 s and show small phonon wings on either side of the ZPL. (c) Broader profile with an asymmetric shape (integration time 20 s). In red, fits to the experimental data using the nonperturbative exciton-phonon coupling model (see the main text and the Supplemental Material). Insets: Magnified view on the phonon sidebands.

Figure 4

In black, two typical PL spectra of L-SWNT. In red, PL spectra of CoMoCAT SWNT with the same type of line profile. (a) Example of sharp ZPL emerging from the phonon wings. Inset: Zoom on the phonon wings. (b) Example of triangular asymmetric shape. Inset: Zoom on the L-SWNT PL spectra to highlight the asymmetric shape. The energy axes are shifted in order to superimpose the PL peaks.