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Abstract

The optical properties of single-walled carbon nanotubes (SWCNTs) are governed by near-infrared (NIR) excitons that are stable at room temperature owing to the quasi one-dimensional nanotube nature. Emission from additional states, such as trions and luminescent oxygen defects, can be induced by distinct protocols, but may also occur unintentionally. While trions and luminescent oxygen defects are of different physical origin, both appear at distinct energies that are red-shifted compared to the emission of the mobile excitons and can provide insights into the energy landscape and excited state dynamics of SWCNTs. A trion is a three-particle state of an exciton bound to a positive or negative charge carrier and is formed in the presence of excess charges. Conversely, emission from luminescent lattice defects is of excitonic character. Distinct configurations of covalently bound oxygen defects create trapping potentials below the exciton state, allowing luminescent decay of localized excitons. To date, both trions and oxygen defects have mostly been investigated in dispersed SWCNTs. However, applications in optoelectronic devices usually rely on nanotube networks in contact with other materials that can significantly affect the optical properties of trions and luminescent defects. A better understanding of, and strategies for, the reproducible and robust formation of trions and luminescent oxygen defects in SWCNT networks are therefore required. Trions are commonly reported in the emission spectra of redoxchemically, electrochemically or electrostatically charged nanotubes. Precise control of trion formation and emission relative to excitons might be achieved by exploiting the high sensitivity of trions towards their dielectric environment. This thesis investigates the influence of different gate dielectric materials on trions and excitons in networks of semiconducting (6,5) SWCNTs that are incorporated into field-effect transistors (FETs). At a given charge carrier density, variations in photo-luminescence (PL) intensities and energies depending on the gate dielectric material, but also trion polarity, are observed. Analysis of the corresponding electrical FET properties identifies charge localization by the dielectric environment as a major factor for the polarity-dependent reduction of exciton quenching, emission blue-shift and trion formation. While trions can be induced reversibly in SWCNTs through charge accumulation, luminescent oxygen defects usually alter the emissive properties of carbon nanotubes permanently as a result of a chemical reaction of the carbon lattice. In this thesis, a photocatalytic approach is employed to functionalize (6,5) SWCNTs deposited on the transition metal oxides TiOx and ZnOx with spatial precision. Optical excitation of these reactive oxidic nanotube environments in the presence of trace amounts of water and oxygen initiates the defect functionalization of adjacent SWCNTs. The emission energies of the two resulting emission bands correspond well to the previously reported ether-d and epoxide-l oxygen defect configurations. The defect PL characteristics are strongly influenced by the surface and dielectric properties of the underlying oxidic substrates, as oxygen-functionalized SWCNTs on TiOx are brighter than on ZnOx and pristine nanotubes on glass.