Excitons and Peierls Distortion in Conjugated Carbon Nanotubes

Abstract

We investigate coupled excitonic and vibrational effects in carbon nanotubes using a time-dependent Hartree-Fock approach. The results reveal intricate details of excited-state dynamics. In the ground state, spontaneous uneven distribution of the pi electrons over the bonds (i.e., Peierls dimerization) is observed throughout the entire nanotube, particularly in large-radius CNTs. However, we demonstrate that vibrational relaxation following photoexcitations leads to substantial local distortion of the tube surface, overriding the Peierls dimerization. This mutually affects the electronic system, resulting in localized states (self-trapped excitons). These phenomena critically control photoinduced dynamics and charge transport in nanotube materials.

Cross-polarized excitons in carbon nanotubes

“…The amplitudes vanish at the tube edges, which reflects excitonic scattering (reflection) at the ends (39). Such exciton delocalization patterns are identical to those observed in other quasi-1D materials (23,24,38,40). Another important characteristic of the electronic excitation is the exciton coherence size L C (maximal distance between electron and hole along the tube axis).…”

“…All molecular systems are oriented along the z-direction, have a finite length of 9-12 nm, and comprise several repeat units. The nanotube lengths were chosen to be significantly larger than the diameter of the tube with characteristic exciton sizes [Յ 5 nm (23,24)]. For these conditions, the finite size of 1D systems is expected to reproduce well the properties in the infinite-size limit (24).…”

“…Theoretical studies (13)(14)(15), followed by spectroscopic experiments (16)(17)(18), have unambiguously revealed that the photophysics of SWNTs is dominated by excitons with typical binding energies of 0.2-0.5 eV (19)(20)(21)(22)(23)(24)(25), depending on the tube diameter and chirality. Electronic correlation effects give rise to several excitonic bands associated with each transition between van Hove peaks.…”

“…This methodology has been applied to calculate optical properties of a variety of conjugated molecular materials (39,40). Recently, we used this technique to investigate a number of excited-state phenomena in SWNTs, including anharmonic coherent phonon dynamics (18), quantification of exciton-phonon coupling constants (41), effects of Peierls distortion and exciton self-trapping (23,24), and characterization of triplet (25) and high-energy excitonic transitions (E 33 and E 44 ) (42). Here, we focus on bright, dark, and weakly allowed excited states related to low-lying excitonic bands by disentangling longitudinal and circumferential components of excitations.…”

Triplet State Absorption in Carbon Nanotubes: A TD−DFT Study

“…All these phenomena are typical features of quasi-one-dimensional materials 20 such as conjugated organic 21 and organometallic 22 polymers, and mix-valence chains 23 . In particular, recent studies reveal many common electronic features observed in spectra of CNTs and conjugated polymers 7,9,13,24,25 . Figure 1 schematically shows the essential electronic states contributing to photophysics of these materials providing common comparison baseline.…”