Relative Ordering between Bright and Dark Excitons in Single-walled Carbon Nanotubes

Abstract

The ordering and relative energy splitting between bright and dark excitons are critical to the optical properties of single-walled carbon nanotubes (SWNTs), as they eventually determine the radiative and non-radiative recombination processes of generated carriers. In this work, we report systematic high-field magneto-optical study on the relative ordering between bright and dark excitons in SWNTs. We identified the relative energy position of the dark exciton unambiguously by brightening it in ultra-high magnetic field. The bright-dark excitonic ordering was found to depend not only on the tube structure, but also on the type of transitions. For the 1st sub-band transition, the bright exciton appears to be higher in energy than its dark counterpart for any chiral species and is robust against environmental effect. While for the 2nd sub-band, their relative ordering was found to be chirality-sensitive: the bright exciton can be either higher or lower than the dark one, depending on the specific nanotube structures. These findings provide new clues for engineering the optical and electronic properties of SWNTs.

Exciton splitting in semiconducting carbon nanotubes in ultrahigh magnetic fields above 300 T

“…2(c), overall spectrum is shown in Ref. 22). This is known as the environment dielectric screening effect on the SWNT exciton, which has been widely discussed.…”

“…The evolution of the split peaks clearly demonstrates that a new peak emerges on the higher energy side of the bright exciton peak, which is consistent with the magnetoabsorption experiment recently carried out up to 186 T using the single-turn coil (STC) technique. [21,22] The evolution of the peak energies and intensities of the splitting absorption spectra were tracked by deconvolution using Lorentzian functions (dashed curves in Fig. 4(b), see also Appendix Sec.…”

“…Therefore, two conditions must be fulfilled to avoid the above problems: (i) the use of a sample with a single chirality without any background absorption, and (ii) the preparation of highly aligned SWNTs, as only B eff contributes to the A-B flux. However, when we used highly aligned single chirality SWNTs, the spectra of the E 22 exciton were not sufficiently split, even for a magnetic field as strong as 186 T; [21,22] therefore, the whole picture of the A-B effect in SWNTs predicted by Ando [2] was not quantitatively discussed. In this paper, we present results from streak spectroscopy measurements of the E 22 transitions performed for highly aligned single chirality SWNTs in magnetic fields up to 370 T.…”

Superemission in vertically-aligned single-wall carbon nanotubes

“…Thus, the authors concluded that the broad background absorption is created by an amorphous carbon fraction present in the SWCNT samples [16]. Takeyama et al [17] performed measurements of the optical absorption cross-section of the SWCNTs. They provided an empirical formula allowing to predict the absorption spectrum of any SWCNT.…”

Strong Light–Matter Coupling in Carbon Nanotubes as a Route to Exciton Brightening

“…However, due to the small diameter of SWCNTs, achieving the required flux density within a tube for magnetic brightening requires magnetic fields of about 5–50 T , or pulsed fields of up to 190 T . Nevertheless, despite lacking practical utility, magnetic brightening is important because it shows that if we are able to shift energetically the bright state below the dark one, the luminescence from the tube increases many fold; the physical mechanism by which the energy of the lowest bright state is reduced is not important.…”