High-field magnetooptical behavior of polymer-embedded single-walled carbon nanotubes

We report a study of the magnetophotoluminescence (PL) of single-walled carbon nanotubes embedded in polymer films. The magnetic-field dependence of the intensity and emission energy is used to deduce the splitting and mixing of the bright and dark exciton states. It is shown that the presence of strain in the films causes a considerable increase in the bright–dark exciton splitting. The unstrained nanotubes show considerably smaller exciton splittings than reported previously and demonstrate an unexpected magnetic suppression of the PL intensity which can only be observed once the Aharanov-Bohm phase splitting is significantly larger than the bright–dark exciton splitting. Functional fitting of the data also allows the diameter dependence of the Aharanov-Bohm (AB) phase-induced energy shifts to be accurately measured. Evidence is also presented for the existence of a zero-field AB splitting due possibly to spin-orbit interaction effects.

Figure 1

(Color online) Parts (a) and (b) show PLE maps from Ti:sapphire excited tube species for both film samples at 2 K. The red crosses represent the positions of the $\left(n,m\right)$ indexed SWNT peaks in MEHPPV-solvent solution. Arrows indicate the shifts in the transitions due to strain. These shifts are shown on an energy scale in part (c). Part (d) shows the dependence of the $E_{11}$ transition on the nanotube chiral angle $\theta$.

Figure 2

(Color online) The PL peak of the (8,7) species in the unstrained drop-cast MEHPPV film, as measured between 0 and 32 T. The lower figures show the separate behaviors of the amplitude and emission energy.

Figure 3

(Color online) PL intensity for a range of nanotube species observed in the unstrained drop-cast MEHPPV film as a function of field. The use of polarized light allows the study of SWNTs oriented either parallel or perpendicular to the magnetic field.

Figure 4

(Color online) Dark–bright exciton splittings as deduced for light polarized parallel to the SWNT axis from fitting to Eq. (5).

Figure 5

(Color online) The values determined for the AB shift parameter, $\alpha$, measured for different orientations for the different samples, compared with theoretically predicted values [Eq. (1)]. The geometrical sum of the parallel and perpendicular components, ${\alpha }_{\text{total}}$, is found to be in good agreement with the theoretically predicted values.

Figure 6

(Color online) The PL spectra at 280 K from the stretched MEH-PPV films aligned parallel to the magnetic field studied from 0 to 32 T, excited at 820 nm. The inset shows the magnetic-field dependence of the peak shifts for the (9,8) peak, measured at 2, 70, and 280 K, compared with the predictions of Eq. (6) and the values of $\alpha$ and ${\Delta }_x$ deduced previously.