Browsing by Subject "lifetime"
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Item Energy Transfer Dynamics and Dopant Luminescence in Mn-Doped CdS/ZnS Core/Shell Nanocrystals(2012-11-13) Chen, Hsiang-YunMn-doped II-VI semiconductor nanocrystals exhibit bright dopant photoluminescence that has potential usefulness for light emitting devices, temperature sensing, and biological imaging. The bright luminescence comes from the 4T1?6A1 transition of the Mn2+ d electrons after the exciton-dopant energy transfer, which reroutes the exciton relaxation through trapping processes. The driving force of the energy transfer is the strong exchange coupling between the exciton and Mn2+ due to the confinement of exciton in the nanocrystal. The exciton-Mn spatial overlap affecting the exchange coupling strength is an important parameter that varies the energy transfer rate and the quantum yield of Mn luminescence. In this dissertation, this correlation is studied in radial doping location-controlled Mn-doped CdS/ZnS nanocrystals. Energy transfer rate was found decreasing when increasing the doping radius in the nanocrystals at the same core size and shell thickness and when increasing the size of the nanocrystals at a fixed doping radius. In addition to the exciton-Mn energy transfer discussed above, two consecutive exciton-Mn energy transfers can also occur if multiple excitons are generated before the relaxation of Mn (lifetime ~10^-4 - 10^-2 s). The consecutive exciton-Mn energy transfer can further excite the Mn2+ d electrons high in conduction band and results in the quenching of Mn luminescence. The highly excited electrons show higher photocatalytic efficiency than the electrons in undoped nanocrystals. Finally, the effect of local lattice strain on the local vibrational frequency and local thermal expansion was observed via the temperature-dependent Mn luminescence spectral linewidth and peak position in Mn-doped CdS/ZnS nanocrystals. The local lattice strain on the Mn2+ ions is varied using the large core/shell lattice mismatch (~7%) that creates a gradient of lattice strain at various radial locations. When doping the Mn2+ closer to the core/shell interface, the stronger lattice strain softens the vibrational frequency coupled to the 4T1?6A1 transition of Mn2+ (Mn luminescence) by ~50%. In addition, the lattice strain also increases the anharmonicity, resulting in larger local thermal expansion observed from the nearly an order larger thermal shift of the Mn luminescence compared to the Mn-doped ZnS nanocrystals without the core/shell lattice mismatch.Item Measurement and model assessment of fluorescence lifetime sensing in multiply scattering media(Texas A&M University, 2005-08-29) Kuwana, EddyThe generation and propagation of fluorescence light within biological tissue offers the potential for biomedical diagnostics and analyte sensing. Arising from an exogenous fluorescent dye injected as a contrast agent or immobilized in a polymer implant, the fluorescent decay kinetics can be sensitive to the tissue??s biochemical environment, providing quantitative in vivo information of the confined tissue site. The impact of light propagation and decay kinetics upon the measured signals is important for consideration, simply because tissue scatters light, giving rise to nanosecond photon time-of-flights that are comparable to fluorescence relaxation kinetics. The goal of this study is to develop a time-dependent model describing (i) the generation of fluorescence from dyes exhibiting multi-exponential or more complex kinetics and (ii) its propagation in scattering media. In the preliminary study, fluorescence lifetime spectroscopy is investigated in tissue-like scattering solution. Two fluorescent dyes, 3,3-diethylthiatricarbocyanine iodide (DTTCI) and Indocynanine Green (ICG), which exhibit distinctly different lifetimes and each exhibits single-exponential decay kinetics, were employed. Measurements of phase-modulation as a function of modulation frequency were made at varying concentration ratios of the two dyes to experimentally simulate fluorescence multi-exponential decay kinetics in non-scattering and scattering solutions. The results suggest that frequency-domain measurements of fluorescent decay kinetics along with models of light propagation may be enhanced by scatter in order to probe kinetics more sensitively than in non-scattering solutions. The next study involved fluorescence lifetime sensing in scattering and non-scattering solutions with a pH sensitive dye, Carboxy Seminaphthofluorescein-1 (C-SNAFL-1), which is known to exhibit multi-exponential decay kinetics. The results demonstrate accurate pH sensing in scattering solution via fluorescence kinetics using a simplified propagation model incorporating an average lifetime. Finally, fluorescence lifetime sensing in immobilized systems were investigated. C-SNAFL-1 was immobilized in poly(ethylene glycol) (PEG) microparticles that were immersed in buffered polystyrene solutions. The results demonstrate the ability to perform pH sensing with fluorescence lifetime without the confounding effect of fluorophore loading or the use of 'reference' measurement within multiply scattering systems. In addition, the stability of the immobilized fluorescence sensor and the reliability of fluorescence lifetime measurement verify the prospect of this technology for implantable purposes.