(Invited) Optically Detected Magnetic Resonance of Triplet Excitons in Sorted (6,5) and (7,5) SWCNTs

Single-wall carbon nanotubes (SWCNTs) possess unique electronic and optical properties, strongly depending on their exact chiral structure. Their quasi one-dimensional character results in the formation of strongly bound electron-hole pairs (excitons) that can even be observed at room temperature (i.e. binding energies of the order of several hundred meV).[1] The exciton fine structure of SWCNTs is quite complex, with multiple singlet and triplet excitonic states, of which only one is optically allowed, thereby resulting in extremely low fluorescence (PL) quantum yields. While the singlet excitons have already been investigated thoroughly, little is known about the longer-living triplet excitons. In this work, we report the characterization of triplet excitons in chirality-purified samples of (7,5) and (6,5) SWCNTs by means of optically detected magnetic resonance spectroscopy (ODMR), a technique that measures the influence on the emission of CNTs when making transitions between the spin levels of the triplet excitons. In both chiralities the nanotubes are shown to sustain two types of triplet exciton states, as shown by the symmetry and magnitude of their zero-field splitting parameters in the spin Hamiltonian, which are determined by careful fitting of magnetic-field orientation-dependent ODMR spectra of in-plane aligned SWCNTs. While the triplet excitons only weakly depend on the nanotube environment, an increase in zero-field splitting is found for (6,5) with respect to (7,5) nanotubes, in good agreement with the tighter confinement expected in the narrower-diameter nanotubes. [1] M.S. Dresselhaus et al, Ann. Rev. Phys. Chem. 2007, 58, 719 [2] D. Stich et al, Nature Photon. 2014 , 8,139; J. Palotás et al, ACS Nano 2020, 14, 11254