Electron-Hole Asymmetry in Single-Walled Carbon Nanotubes Probed by Direct Observation of Transverse Quasi-Dark Excitons

TL;DR

This study directly observes transverse dark excitons in single-walled carbon nanotubes, revealing how electron-hole asymmetry varies with nanotube structure and affects exciton properties.

Contribution

It provides the first direct observation of spin-singlet transverse dark excitons and quantifies electron-hole asymmetry in SWNTs through combined experimental and theoretical analysis.

Findings

Electron-hole asymmetry lifts degeneracy of transverse excitons.

Spectral weight transfer depends on nanotube chirality.

Quantitative evaluation of band asymmetry parameters.

Abstract

We studied the asymmetry between valence and conduction bands in single-walled carbon nanotubes (SWNTs) through the direct observation of spin-singlet transverse dark excitons using polarized photoluminescence excitation spectroscopy. The intrinsic electron-hole (e-h) asymmetry lifts the degeneracy of the transverse exciton wavefunctions at two equivalent K and K' valleys in momentum space, which gives finite oscillator strength to transverse dark exciton states. Chirality-dependent spectral weight transfer to transverse dark states was clearly observed, indicating that the degree of the e-h asymmetry depends on the specific nanotube structure. Based on comparison between theoretical and experimental results, we evaluated the band asymmetry parameters in graphene and various carbon nanotube structures.