Tuning Spin-Orbit Coupling in (6,5) Single-Walled Carbon Nanotube Doped with sp3 Defects

Single-walled carbon nanotubes (SWCNTs) doped with sp3 defects are a promising class of optoelectronic materials with bright tunable photoluminescence and demonstrated single-photon emission. Here we perform a density functional theory (DFT) study, complemented by experiment, of a variety of sp3 defects attached to (6,5) SWCNT, with the goal of tuning spin-orbit coupling (SOC) by introduction of a heavy atom in the defect structure. For four recently synthesized tubes containing aryl-based defects, both computation and electron spin resonance indicate that a single unpaired electron is introduced to the system. Both the DFT bandstructure and photoluminescence measurements suggest a defect-induced gap state 0.2-0.3 eV above the valence band maximum of the nanotube. Additional theoretical analysis indicates that the spin density is localized around the sp3 site with no increase of the spin-orbit splitting in the near-gap states (a measure of SOC in the excited state) even when a Pd atom-containing ligand is introduced. We then theoretically model additional metal-containing defects with decreased metal-tube separation. We determine that forming a direct sp3 carbon-metal bond is necessary for tuning SOC.