Incidence of Quantum Confinement on Dark Triplet Excitons in Carbon Nanotubes

TL;DR

This study investigates how quantum confinement affects dark triplet excitons in carbon nanotubes, revealing their energy structure, phosphorescence behavior, and spin relaxation times, with implications for optoelectronic and quantum information applications.

Contribution

It provides the first observation of triplet state phosphorescence in SWCNTs and quantifies the singlet-triplet gap dependence on nanotube diameter, advancing understanding of exciton states.

Findings

Observed phosphorescence from triplet states in SWCNTs.

Determined singlet-triplet energy gap varies with nanotube diameter.

Measured spin-relaxation times under high microwave power.

Abstract

Photophysics of single-wall carbon nanotubes (SWCNTs) is intensively studied due to their potential application in light harvesting and optoelectronics. Excited states of SWCNTs form strongly bound electron-hole pairs, excitons, of which only singlet excitons participate in application relevant optical transitions. Long-living spin-triplet states hinder applications but they emerge as candidates for quantum information storage. Therefore knowledge of the triplet exciton energy structure, in particular in a SWCNT chirality dependent manner, is greatly desired. We report the observation of light emission from triplet state recombination, i.e. phosphorescence, for several SWCNT chiralities using a purpose-built spectrometer. This yields the singlet-triplet gap as a function of SWCNT diameter and it follows predictions based on quantum confinement effects. Saturation under high microwave power (up to 10 W) irradiation allows to determine the spin-relaxation time for triplet states. Our study sensitively discriminates whether the lowest optically active state is populated from an excited state on the same nanotube or through F\"orster exciton energy transfer from a neighboring nanotube.