Characterization of Single Quantum Dot Blinking: Dwell Time Statistics and Electrochemical Control

This thesis addresses the observed fluorescence intermittency of single semiconductor nanocrystals, so called Quantum Dots (QDs), which is also referred to as blinking. Despite continuous excitation their fluorescence is randomly interrupted by dark periods that can last over several minutes. Especially the extraction of power law dwell time statistics in bright and dark states indicates very complex underlying processes that are not fully understood to date. Here two approaches are followed to reveal the nature of the blinking mechanism. One addresses the common threshold method for extraction of power law dwell times. Its performance is tested with simulations to a broad range of experimentally determined parameters. Strong deviations are found between input and extracted statistics dependent on input parameters themselves. A comparison with experimental data does not support the assignment of power law statistics for the bright state and indicates the existence of distinct blinking mechanisms. The second approach directly aims at the nature of the dark state, which is mostly attributed to charges in the QD or trap states in its vicinity. A method is developed to detect charging processes on single QDs with their fluorescence. Electrochemistry is combined with confocal microscopy also allowing evaluations of excited state lifetimes and emission spectra. Reduction and oxidation of the QD bands are successfully observed as a quenching of QD fluorescence. Single QD observations identify two independent blinking mechanisms, that are assigned to positive and negative charging. Positive charging is not only observed after hole injection but also the extraction of excited electrons. Three additional quenching mechanisms are identified, two of which are assigned to trap relaxation. Differences between two substrate electrodes demonstrate the importance of the substrate material

Characterization of Single Quantum Dot Blinking: Dwell Time Statistics and Electrochemical Control

This thesis addresses the observed fluorescence intermittency of single semiconductor nanocrystals, so called Quantum Dots (QDs), which is also referred to as blinking. Despite continuous excitation their fluorescence is randomly interrupted by dark periods that can last over several minutes. Especially the extraction of power law dwell time statistics in bright and dark states indicates very complex underlying processes that are not fully understood to date. Here two approaches are followed to reveal the nature of the blinking mechanism. One addresses the common threshold method for extraction of power law dwell times. Its performance is tested with simulations to a broad range of experimentally determined parameters. Strong deviations are found between input and extracted statistics dependent on input parameters themselves. A comparison with experimental data does not support the assignment of power law statistics for the bright state and indicates the existence of distinct blinking mechanisms. The second approach directly aims at the nature of the dark state, which is mostly attributed to charges in the QD or trap states in its vicinity. A method is developed to detect charging processes on single QDs with their fluorescence. Electrochemistry is combined with confocal microscopy also allowing evaluations of excited state lifetimes and emission spectra. Reduction and oxidation of the QD bands are successfully observed as a quenching of QD fluorescence. Single QD observations identify two independent blinking mechanisms, that are assigned to positive and negative charging. Positive charging is not only observed after hole injection but also the extraction of excited electrons. Three additional quenching mechanisms are identified, two of which are assigned to trap relaxation. Differences between two substrate electrodes demonstrate the importance of the substrate material

Characterization of Single Quantum Dot Blinking: Dwell Time Statistics and Electrochemical Control

This thesis addresses the observed fluorescence intermittency of single semiconductor nanocrystals, so called Quantum Dots (QDs), which is also referred to as blinking. Despite continuous excitation their fluorescence is randomly interrupted by dark periods that can last over several minutes. Especially the extraction of power law dwell time statistics in bright and dark states indicates very complex underlying processes that are not fully understood to date. Here two approaches are followed to reveal the nature of the blinking mechanism. One addresses the common threshold method for extraction of power law dwell times. Its performance is tested with simulations to a broad range of experimentally determined parameters. Strong deviations are found between input and extracted statistics dependent on input parameters themselves. A comparison with experimental data does not support the assignment of power law statistics for the bright state and indicates the existence of distinct blinking mechanisms. The second approach directly aims at the nature of the dark state, which is mostly attributed to charges in the QD or trap states in its vicinity. A method is developed to detect charging processes on single QDs with their fluorescence. Electrochemistry is combined with confocal microscopy also allowing evaluations of excited state lifetimes and emission spectra. Reduction and oxidation of the QD bands are successfully observed as a quenching of QD fluorescence. Single QD observations identify two independent blinking mechanisms, that are assigned to positive and negative charging. Positive charging is not only observed after hole injection but also the extraction of excited electrons. Three additional quenching mechanisms are identified, two of which are assigned to trap relaxation. Differences between two substrate electrodes demonstrate the importance of the substrate material

Post-Mortem Investigations of Fluorinated Flame Retardants for Lithium Ion Battery Electrolytes by Gas Chromatography with Chemical Ionization

Using flame retardants (FRs) in lithium ion battery (LIB) electrolytes is usually a tradeoff between electrochemical performance and electrolyte flammability. Fluorinated FRs are a promising class of FRs which are currently under investigation. During this work, three FRs originating from triethyl phosphate with varying degree of fluorination were investigated regarding their electrochemical stability on cathode (LiNi0.33Co0.33Mn0.33O2, NCM) and anode (graphite) in half cells. During long-term cycling, changes in performance were observed. Especially on the anode side the FR addition showed a decrease in performance in comparison to the standard electrolyte (DEC/EC 1:1, 1M LiPF6). The electrolytes containing the three FRs were extracted from the cells and analyzed regarding their changes in composition and structural degradation. The decomposition products were investigated by gas chromatography (GC) with electron impact (EI) ionization and mass selective (MS) detection. To obtain more information with regard to the identification of unknown decomposition products further GC‐MS experiments with positive chemical ionization (PCI) and negative chemical ionization (NCI) were performed. Twelve different volatile organic decomposition products were identified. These decomposition products can be subdivided regarding their basic structure. Ether based, carbonate based and phosphate based fluorinated and non-fluorinated decomposition products were identified. Furthermore, possible formation pathways for all groups of decomposition products were postulated taking existing literature into account

Electrolyte Extraction—Sub and Supercritical CO2

This chapter reports on experiments aimed at investigating the capability of pressurized carbon dioxide to extract the electrolyte from commercial available LIBs on a laboratory scale. Two different phase conditions of carbon dioxide (subcritical and supercritical) and two different extraction (static and dynamic) have been considered and analyzed for their strengths and weaknesses. Furthermore, the addition of co-solvents is examined with regard to their contribution to higher recovery rates. After reporting the optimized extraction method, the extracted electrolyte was analyzed by gas and ionic chromatography methods for potential de-composition products and their relative amount