Photophysics of carbon nanotubes in organic polymer-toluene dispersions: Emission and excitation satellites and relaxation pathways

We have prepared toluene-polymer dispersions of single-walled carbon nanotubes (SWNTs) containing only one or a few specific semiconducting $\left(n,m\right)$ species. These have large helical angles and manifest a high discrimination against larger-diameter SWNTs, metallic species, and nanotube aggregates. Such $\left(n,m\right)$-narrowed dispersions proved especially valuable for identification of low-intensity photoluminescence (PL) features as well as relaxation pathways, e.g., for the optically active $E_{22}$ and $E_{11}$ excitons, in specific nanotubes. Among other features, PL excitation and emission satellites at $\sim \left(E_{11}+95\right)$ and $\sim \left(E_{11}-200\right)\text{meV}$, respectively, were resolved. We show that the $E_{22}$ exciton primarily decays $\left(\ge 80\%\right)$ to the optical $E_{11}$ exciton. A minor decay pathway likely involves relaxation into “dark” deep excitonic states, which weakly emit at $\sim 40$ and $\sim 140\text{meV}$ below $E_{11}$. In addition, we report PL polarization measurements, supporting assignment of transverse $E_{12,21}$ photoexcitations, and PL quantum yields for $\left(n,m\right)$ nanotubes in toluene-polymer dispersions.

Figure 1

(Color online) Absorption spectra of HiPco and PLV nanotubes sonicated in toluene-POF and left in the dispersion after ultracentrifugation. Optical path is 10 mm. The $\left(n,m\right)$ assignment of $E_{11}$ and $E_{22}$ absorption peaks is supported by the PL data (see below). No background correction was applied to the spectra. The increase in absorption below $\sim 550\text{nm}$ is due to the POF polymer.

Figure 2

(Color online) (a) PL map (emission intensity vs excitation and emission wavelength) of a $\left(n,m\right)$-narrowed dispersion of PLV nanotubes in toluene-POF. Emitting $\left(n,m\right)$ species are indicated at $E_{22}\text{−}{E}_{11}$ excitation-emission peaks. $E_{11}\text{−}{E}_{11}$ PL peaks are overlaid on the Rayleigh scattering line due to a very small Stokes shift. Dashed lines denote phonon-coupled $\left(E_{11}+G\right)$ and $\left(E_{11}+G^{\prime }\right)$ and transverse $E_{12,21}$ photoexcitations. Dotted lines show correlations between the major PL peaks and emission satellites ${\text{DE}}_1$ and ${\text{DE}}_2$ for (9,7) tubes (see text for details). Note that the PL intensity (color bar) is logarithmically scaled. (b) Photoluminescence excitation (PLE) spectra of (9,7) tubes at emission wavelengths of 1350 $\left(E_{11}\right)$ and 1580 $\left({\text{DE}}_1\right)$. $G$- and $G^{\prime }$-phonon-coupled PLE satellites are indicated for both $E_{11}$ and $E_{22}$ excitations. The asterisk indicates a contribution of the (8,7)-${\text{DE}}_1$ satellite (see the PL map). (c) Emission spectra of (9,7) tubes at $E_{11}$ (1346 nm) and $E_{22}$ (800 nm) excitations.

Figure 3

(Color online) PL map of HiPco nanotubes dispersed in toluene-POF and comprising five emitting $\left(n,m\right)$ species with large helical angles. Notations of the PL peaks follow those in Fig. 2. The dotted line ${\text{DE}}_1$ indicates emission satellites ${\text{DE}}_1$ excited via $E_{11}$ transitions. These satellites are also clearly seen at $E_{22}$ excitations of (8,6), (8,7), and (9,7) nanotubes.

Figure 4

(Color online) PL maps of (a) PLV and (b) HiPco nanotubes in the $E_{11}$ excitation region. Dashed lines $\mathit{S}$ and ${\mathit{E}}_{11}-\mathit{G}$ indicate PL excitation and $G$-phonon-coupled emission satellites at $\sim \left(E_{11}+95\right)$ and $\sim \left(E_{11}-200\right)\text{meV}$, respectively. A superposition of PLE and emission satellites in (b) produces a characteristic crossed stripes pattern, where some crossings are marked with open circles (see text).

Figure 5

(Color online) PL polarization factor vs emission and excitation wavelengths for HiPco nanotubes in toluene-POF (see Fig. 3). The main $E_{22}\text{−}{E}_{11}$ excitation-emission peaks of five emitting $\left(n,m\right)$ nanotubes and ${\text{DE}}_1$ emission satellites of (8,6), (8,7), and (9,7) nanotubes are marked with black and white open squares, respectively. Spots with a low polarization factor (open circles) correspond to transverse $E_{12,21}$ excitations (Ref. 9). White open circles indicate ${\text{DE}}_1$ emission satellites of (8,7) and (9,7) nanotubes excited via $E_{12,21}$ transitions.

Figure 6

(Color online) Absorption of PLV nanotubes in toluene-POF combined with a PLE spectrum $\left({\lambda }_{\text{emis}}=1350\text{nm}\right)$ of the dominant (9,7) nanotubes. The PLE intensity is scaled to an equal area of (9,7)-$E_{11}$ peaks in the both spectra.

Figure 7

Efficiency of $E_{22}\to E_{11}$ decay estimated for $\left(n,m\right)$ nanotubes in toluene-POF. Open squares, circles, and a triangle correspond to HiPco, PLV, and CoMo-CVD samples, respectively, and are plotted in order of increasing nanotube diameter. Mean values were estimated for (7,5) and (7,6) as well as for (8,6) and (8,7) nanotubes in HiPco dispersion because of their overlapping $E_{22}$ absorption bands (see Fig. 1).

Figure 8

Schematic diagram of experimentally observed one-photon optical absorption (doubled solid lines) and emission (single solid lines) transitions in semiconducting SWNTs. Phonon-coupled excitation and emission transitions are lettered with $+G$, $+G^{\prime }$, and $-G$, respectively. Transverse $E_{12,21}$ excitations and PLE satellites $S$ (see text) are omitted. Dotted lines indicate possible relaxation pathways, including minor ones that couple the optical $E_{22}$ and $E_{11}$ excitonic states to proposed dark (weakly emitting) ${\text{DE}}_2$ and ${\text{DE}}_1$ states. Radiative recombination of the $E_{11}$ exciton is relatively effective (${\phi }_{\text{PL}}\sim 1.1\%$ in toluene-POF). The other emission transitions shown here are very weak by comparison.