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A computational investigation of some optoelectronic applications of carbon nanotubes (CNT) is presented, including CNT-based solar cells and biosensors. The results could be used to evaluate the performance of CNT devices and clarify the necessity of further experimental research in this area. A coaxially-gated CNT field-effect transistor (CNFET) forms the basic structure of the devices modeled in this thesis. Diffusive transport is present in long-channel devices, as in our case, while the quantum mechanical effects are mainly present in the form of tunneling from Schottky-barrier contacts at the metal-CNT interfaces. Band-to-band recombination of electron-hole pairs (EHP) is assumed to be the source of electroluminescence. In a first-order approximation, protein-CNT interactions are modeled as the modification of the potential profile along the longitudinal axis of CNTs due to electrostatic coupling between partial charges, in the oxide layer of the CNFET, and the nanotube. The possibility of electronic detection is evaluated. The electroluminescence of the CNT is proposed as an optical detection scheme due to its sensitivity to the magnitude and the polarity of the charge in the oxide. The validity of the model is argued for the given models. A value for the minimum required size of a computational window in a detailed simulation is derived. The structure of an electrostatically gated p-i-n diode is simulated and investigated for photovoltaic purposes. The absorbed power from the incident light and the interaction between the nanotubes is modeled with COMSOL. The results are interpreted as a generation term and introduced to the Drift-Diffusion Equation (DDE). We have observed behavior similar to that in an experimentally-realized device. The performance of CNT-based solar cells under standard AM 1.5 sunlight conditions is evaluated in the form of an individual solar cell and also in an array of such devices.

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. / Thesis (Master, Physics, Engineering Physics and Astronomy) -- Queen's University, 2009-01-08 20:38:56.433

A computational investigation of some optoelectronic applications of carbon nanotubes (CNT) is presented, including CNT-based solar cells and biosensors. The results could be used to evaluate the performance of CNT devices and clarify the necessity of further experimental research in this area. A coaxially-gated CNT field-effect transistor (CNFET) forms the basic structure of the devices modeled in this thesis. Diffusive transport is present in long-channel devices, as in our case, while the quantum mechanical effects are mainly present in the form of tunneling from Schottky-barrier contacts at the metal-CNT interfaces. Band-to-band recombination of electron-hole pairs (EHP) is assumed to be the source of electroluminescence. In a first-order approximation, protein-CNT interactions are modeled as the modification of the potential profile along the longitudinal axis of CNTs due to electrostatic coupling between partial charges, in the oxide layer of the CNFET, and the nanotube. The possibility of electronic detection is evaluated. The electroluminescence of the CNT is proposed as an optical detection scheme due to its sensitivity to the magnitude and the polarity of the charge in the oxide. The validity of the model is argued for the given models. A value for the minimum required size of a computational window in a detailed simulation is derived. The structure of an electrostatically gated p-i-n diode is simulated and investigated for photovoltaic purposes. The absorbed power from the incident light and the interaction between the nanotubes is modeled with COMSOL. The results are interpreted as a generation term and introduced to the Drift-Diffusion Equation (DDE). We have observed behavior similar to that in an experimentally-realized device. The performance of CNT-based solar cells under standard AM 1.5 sunlight conditions is evaluated in the form of an individual solar cell and also in an array of such devices.