Intrinsic exciton dynamics from single air-suspended semiconducting single-walled carbon nanotubes
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Semiconducting single-walled carbon nanotubes (S-SWCNTs) have direct band gaps with a range of 0.5 to 2 eV depending on the SWCNT chirality. The photoluminescence (PL) quantum efficiency and the carriers’ radiative lifetime have been previously studied but neither of them have been confirmed due to the large variation resulting from ensemble averaging, environmental effects, SWCNT defects, and SWCNT bundles. For example, quantum efficiency was estimated to be 0.01% to 7% and radiative lifetime was estimated or calculated to be 10 to 100 ns. In this thesis, we study absorption cross section, PL quantum efficiency and exciton relaxation dynamics from single air-suspended S-SWCNTs and extract “intrinsic” S-SWCNT properties. The photo-excited carriers are electron-hole pairs (called excitons) in a SWCNT due to the strong Coulomb interactions in the nm-scale system. We selected relatively bright and less defected S-SWCNTs on our samples for investigation. For each SSWCNT, the tube length, orientation, absorption and emission spectra were recorded. Experimentally, we observed that PL from a single S-SWCNT increases linearly at low excitation intensity (linear regime) and saturates at higher intensity (saturation regime). We also studied the exciton relaxation dynamics on each S-SWCNT by femtosecond excitation correlation (FEC) spectroscopy and resolved two relaxation time constants which were independent of the excitation intensity. We compare the simulation results based on a stochastic model to the experimental data and extract essential parameters including S-SWCNT unitless absorption coefficient (typically 0.02 to 0.06), PL quantum efficiency (typically 7 to 20 %) and exciton relaxation time constants. We observed very fast nonlinear exciton-exciton annihilation rate (>(2 ps)^−1) in a typical 5 μm-long S-SWCNTs. The exciton dynamics were consistent from 4 different S-SWCNTs in the saturation regime and the average total exciton number per pulse per tube in this saturation regime ranges from 2 to 12. Compared to past work, the results (PL saturation curves and FEC data) between S-SWCNTs are very consistent which supports our belief that we are studying “intrinsic properties”. We found a higher absorption coefficient, and higher PL quantum efficiency of S-SWCNTs compared to previous work. We also observe very fast nonlinear exciton-exciton annihilation in a relatively longer S-SWCNT and at lower exciton numbers.