Energietransfer in funktionalisierten II-VI-Halbleiternanodrähten und kolloidalen Quantenpunkten

Das zeitliche Besetzungsverhalten angeregter Zustände ist durch den Energietransfer zu benachbarten Energieniveaus bestimmt. In dieser Arbeit wurden Halbleiternanodrähte und -quantenpunkte mit Hilfe der zeitaufgelösten Photolumineszenzspektroskopie hinsichtlich ihres Energietransfers untersucht. Die...

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Bibliographic Details
Main Author: Kaiser, Uwe
Contributors: Heimbrodt, Wolfram (Prof. Dr.) (Thesis advisor)
Format: Doctoral Thesis
Published: Philipps-Universität Marburg 2014
Online Access:PDF Full Text
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The importance of semiconductor nanostructures for technological applications has rapidly grown in the last decades. Nowadays, nanostructures with highly efficient photoluminescence properties can be synthesized. The understanding and control of the photoluminescence dynamics in such structures is of major importance for potential optical applications. In most materials the dynamic of excitation energy is determined by an energy transfer from the excited donor to an adjacent acceptor. A description of this energy transfer in low dimensional nanostructures is given by a modified version of the original Förster model. In this work semiconductor nanowires as well as quantum dots were investigated by time resolved spectroscopy to yield information about the population of different energetic states. ZnS nanowires with a diameter of 100nm and a length of several micrometers were implanted with manganese and terbium, respectively. Both ions show a slowly decaying luminescence, which is highly non-exponential due to the energy transfer to nonradiative centers within the nanowires. A systematical variation of doping concentration, ambient temperature and excitation power yields a complete understanding of the energy transfer in the framework of the modified Förster model. The realization of a microscopic time resolved setup enables a comparison of the decay characteristics of single nanowires with an ensemble of wires. Remarkably, the luminescence decay of a single wire is the same as the one of many nanowires due to the uniformity of the wires and the high number of donor-acceptor pairs within a single wire. Therefore, the ensemble measurements can be compared to an especially developed Monte Carlo simulation of the microscopic energy transfer in a single wire. The results of the simulation are in good agreement with the fitting of the modified Förster model. In addition to the time resolved measurements different optical techniques were used to investigate the unique features of the ZnS nanowires. Resonant Raman scattering experiments of single nanowires show a high angular dependence regarding the orientation of the wire and the polarization of the incident and the scattered light. This polarization dependence underlines the unique morphology of the nanowires. Additionally, photoluminescence excitation spectroscopy discloses the exact transitions within the variety of terbium states and yields information on the energy transfer from the ZnS host material. Colloidal semiconductor quantum dots with attached dye molecules were investigated by time resolved measurements to gain a better understanding of the transfer characteristics in such conjugates. Quantum dots show a slowly decaying luminescence with a lifetime in the range of 100ns. By manganese doping of these quantum dots their lifetime can be enhanced by several orders of magnitude, for the manganese luminescence itself exhibits a decay time in the millisecond range. For the use of colloidal quantum dots in biological sensor applications a functionalization with ionsensitive dyes is required. A tuning of the spectral overlap between the quantum dot emission and the dye absorption leads to an efficient energy transfer. This transfer results in a tremendous enhancement of the luminescence lifetime of the dye, yielding dye molecules with similar luminescence spectra but significantly different decay times. Time resolved measurements not only enable to distinguish between these different types of dyes, but furthermore, allow the determination of the exact ratio of several dyes in a given solution, which is the basis of temporal multiplexing. Most biological sensing applications require the correct knowledge of the quantum dot dye conjugate, especially the exact number of dye molecules per quantum dot is of great importance for quantitative determinations. As the binding of molecules to the polymer shell of the quantum dot underlies a certain statistic, this statistical distribution has to be taken into account for a correct characterization of the energy transfer. The consideration of a Poisson distributed number of dye molecules per quantum dot leads to a good description of the experimentally determined decay of the quantum dot luminescence. Additionally, the fitting within the energy transfer model results in unique values for the average number of dye molecules per quantum dot as well as for the transfer rate between the quantum dot and a single dye molecule. In this work the importance of the energy transfer for semiconductor nanostructures is revealed. A better understanding of the transfer characteristics is of great significance for an advancement of optical devices. Additionally, the energy transfer can be used to enhance current sensor concepts, which extends the application area of this material class.