Analysis of photoluminescence from solubilized single-walled carbon nanotubes

The functional form of the photoluminescence (PL) line shape from individual single-walled carbon nanotube (SWNT) species is found to contain a significant Lorentzian component and the Stokes shift is observed to be very small $(<8\mathrm{meV})$, which suggests an excitonic dephasing mechanism that is largely decoupled from surrounding solvent and surfactant molecules. The PL quantum yield (PLQY) of two SWNT species is determined to be $\sim 5\times {10}^{-4}$, and it is suggested that this is lower than the “true” value due to quenching of the PL in bundles by metallic tubes. Time-resolved PL measurements reveal a dominant, luminescence lifetime component of $130\mathrm{ps}$ that, when combined with a predicted natural radiative lifetime of $\sim 20\mathrm{ns}$, suggests that the true PLQY is $\sim 6.5\times {10}^{-3}$. Finally, deconvoluted PL excitation spectra are produced for eight SWNT species, and the appearance of a higher-energy excitonic subband is discussed.

Figure 1

(Color online) 2D PL spectrum of aqueous SWNTs. Images (a) and (b) are contiguous, but the intensity scale on image (b) is enhanced by a factor of 15 compared to that of image (a). The encircled areas, marked B and C, contain resonant PL peaks from SWNTs that have been excited to the $B_0$ and $C_0$ excitons, respectively.

Figure 2

Resonant (upper) and nonresonant (lower) PL spectra from an aqueous suspension of HiPco SWNTs. Both spectra contain transitions from eight SWNT species, each of which have been fitted with Voigt profiles (shaded) to illustrate the individual SWNT PL components.

Figure 3

The absorption spectrum of HiPco SWNTs suspended in water using SDS surfactant $(1\mathrm{wt}\%)$. The emission and excitation energy ranges that define the axes in Fig. 1 are highlighted.

Figure 4

A comparison of PL and absorption spectra within the PL energy range defined in Fig. 3. Also shown are three of the Voigt profiles corresponding to the PL contributions from the (7,5), (6,5), and (8,3) tubes. Values of the Stokes shifts for these SWNT species are calculated from the energies of their Voigt PL contributions and the positions of the corresponding absorption peaks.

Figure 5

Instrument response function (IRF) and luminescence decay profile from (7,5) SWNTs. The solid line corresponds to the IRF convoluted with a biexponential decay function. The inset table presents the corresponding lifetimes and yields of each function used to fit the PL from the (6,5), (7,5), and (8,3) SWNTs.

Figure 6

A section of the fitted % absorption spectrum containing transitions to the $B_0$ states of semiconducting SWNTs. The Voigt profiles that contribute to the fitted spectrum are offset from zero by a linear background correction (freely variable in the fitting procedure) that estimates the contribution of both residual carbonaceous impurities and the SWNT $\pi$-plasmon resonance to the optical density.

Figure 7

Normalized excitation spectra for the (7,5), (6,5), (8,3), and (6,4) SWNTs. PL due to excitation of the $B_0$ and $C_0$ excitons is clearly observed. The peak labeled $B_1$ is a subpeak associated with $B_0$ for all observed SWNT species.

Figure 8

Showing the linear relationship between $\Delta ∕E\left(B_0\right)$ and tube diameter, where $\Delta =E\left(B_1\right)-E\left(B_0\right)$.