Protonation Effects on the Branching Ratio in Photoexcited Single-Walled Nanotube Dispersions

The ensemble PL quantum yield for raw single-walled carbon nanotubes (SWNTs) dispersed in sodium ctiolate (SC) is ∼5 times greater than that for the same raw SWNTs dispersed in sodium dodecyl sulfate (SDS) and ∼10 times greater than the quantum yield of purified SWNTs dispersed In SC. Absorbance and Raman spectra indicate that purified SC-dispersed SWNTs and raw SDS-dispersed SWNTs are hole-doped by protonation. Experiments comparing PL emission efficiency using E ₂ and E ₁ excitation show that protonation significantly affects the E ₂ → E ₁ relaxation process, which has typically been assumed to occur with unit efficiency. The E ₂ → E ₁ relaxation is 5 times more efficient in producing E ₁ PL when SWNTs are unprotonated and protected by the SC surfactant. The results provide clear evidence that extrinsic factors, such as residual acids and the specific nature of SWNT-surfactant and SWNT-solvent interactions, can significantly affect measured SWNT luminescence quantum yields.