Optical Properties of Individual and Bundled Single-Walled Carbon Nanotubes

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Anderson, Mitchell
Excitons , Diffusion , Optics , Optical Absorption , Ultrafast , carbon Nanotube
Single-walled carbon nanotubes (SWCNTs) are nearly ideal one-dimensional systems, with aspect ratios exceeding 10$^{5}$:1. The band gap of a SWCNT, determined by the species, covers the majority of the visible spectrum all the way out into the near infrared. However, despite their amazing potential, the vast majority of SWCNT applications are still on the horizon. This is partly because the opto-electronic behaviour of a SWCNT is so sensitive to its physical structure (determined by sample preparation method) and its ambient environment. Structural defects and environmental perturbations introduce extrinsic effects (\textit{e.g.} disorder limited diffusion) which vastly inhibit the realization the intrinsic behaviour. A decade of progress in sample preparation techniques has vastly improved SWCNT properties (\textit{e.g.} increased quantum yield, longer diffusion lengths). The pinnacle of sample purification and isolation culminates in an isolated individual air-suspended SWCNT. These air-suspended SWCNTs are free from substrate effects, encapsulation, and defects caused by post processing. In this thesis, I study the intrinsic exciton dynamics of high quality air-suspended SWCNTs. A diffusion-annihilation model is developed for homogeneously excited SWCNTs which reveals an exciton diffusion constant on the order of 300 cm$^{2}$/s. To strengthen the case for intrinsic diffusion, a temporally and spatially resolved PL experiment is developed which is sensitive to quenching defect sites. Results from several SWCNTs confirm a diffusion constant of $\sim$ 300 cm$^{2}$/s. In order to test the validity of the diffusion-reaction model, direct absorption measurements are performed to ensure that photoluminescence (PL) nonlinearity is not caused by nonlinear optical absorption. The observed strong nonlinearity in PL is not accompanied by a nonlinear absorption signal, indicating that absorption saturation does not play a significant role the observed phenomena. In addition, time-resolved SWCNT PL and absorption studies indicate that the presence of an exciton population does not significantly affect the optical absorption. The observed optical absorption cross-sections for the air-suspended SWCNTs are on the order of 3 $\times$ 10$^{-17}$ cm$^{2}$/atom. This is about 5 times higher than graphene and among the highest observed for SWCNTs.
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