Stochastic Formation of Quantum Defects in Carbon Nanotubes

Small perturbations in the structureof materials significantlyaffect their properties. One example is single wall carbon nanotubes(SWCNTs), which exhibit chirality-dependent near-infrared (NIR) fluorescence.They can be modified with quantum defects through the reaction withdiazonium salts, and the number or distribution of these defects determinestheir photophysics. However, the presence of multiple chiralitiesin typical SWCNT samples complicates the identification of defect-relatedemission features. Here, we show that quantum defects do not affectaqueous two-phase extraction (ATPE) of different SWCNT chiralitiesinto different phases, which suggests low numbers of defects. Forbulk samples, the bandgap emission (E-11) of monochiral(6,5)-SWCNTs decreases, and the defect-related emission feature (E-11*) increases with diazonium salt concentration and representsa proxy for the defect number. The high purity of monochiral samplesfrom ATPE allows us to image NIR fluorescence contributions (E-11 = 986 nm and E-11* = 1140 nm) on the single SWCNTlevel. Interestingly, we observe a stochastic (Poisson) distributionof quantum defects. SWCNTs have most likely one to three defects (forlow to high (bulk) quantum defect densities). Additionally, we verifythis number by following single reaction events that appear as discretesteps in the temporal fluorescence traces. We thereby count singlereactions via NIR imaging and demonstrate that stochasticity playsa crucial role in the optical properties of SWCNTs. These resultsshow that there can be a large discrepancy between ensemble and singleparticle experiments/properties of nanomaterials.

Automated summary

Small perturbations in material structures can significantly affect their properties. By modifying single wall carbon nanotubes (SWCNTs) with quantum defects using diazonium salts, the photophysics of the nanotubes can be determined. The presence of multiple chiralities in SWCNT samples complicates the identification of defect-related emission features. This study shows that quantum defects do not affect aqueous two-phase extraction of different SWCNT chiralities, indicating a low number of defects. The results demonstrate the importance of stochasticity in the optical properties of SWCNTs and highlight the discrepancy between ensemble and single particle experiments/properties of nanomaterials.