Direct Observation of Dark Excitons in Individual Carbon Nanotubes: Role of Local Environments

TL;DR

This study provides direct evidence of dark excitons in individual carbon nanotubes by using magnetic fields to reveal their presence and quantify their energy splitting, highlighting environmental effects on excitonic properties.

Contribution

First direct observation and quantification of dark excitons in individual carbon nanotubes, demonstrating environmental influence on excitonic fine structure.

Findings

Dark excitons can be brightened by magnetic fields

Dark-bright exciton splitting is 1-4 meV

Local environment affects excitonic properties

Abstract

We report the direct observation of the spin-singlet dark excitonic state in individual single-walled carbon nanotubes through low-temperature micro-photoluminescence spectroscopy in magnetic fields. A magnetic field up to 5 T, applied along the nanotube axis, brightened the dark state, leading to the emergence of a new emission peak. The peak rapidly grew in intensity with increasing field at the expense of the originally-dominant bright exciton peak and finally became dominant at fields $>$3 T. This behavior, universally observed for more than 50 nanotubes of different chiralities, can be quantitatively explained through a model incorporating the Aharonov-Bohm effect and intervalley Coulomb mixing, unambiguously proving the existence of dark excitons. The directly measured dark-bright splitting values were 1-4 meV for tube diameters 1.0-1.3 nm. Scatter in the splitting value emphasizes the role of the local environment surrounding a nanotube in determining the excitonic fine structure of single-walled carbon nanotubes.