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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Evaluation of Zinc Oxide: Gallium for High-Speed Thermographic Phosphorescence During Impact Studies

Patrick B Moore (10452029) 06 May 2021 (has links)
Thermographic phosphors are useful compounds to determine temperature, due to their luminescence characteristics being a function of temperature. In this research, Zinc Oxide: Gallium was evaluated for its ability to measure the temperature of an impact event in a drop weight apparatus. Different solids loadings of the phosphor were placed in a sylgard binder and these samples were then excited by a 355 nm laser as they were impacted. Images of the event were captured through two separate filters with a high-speed camera, from which intensity ratios were formed. These intensity ratios correlated to a temperature, revealing the change in temperature of the sample throughout the impact. Initial testing at a repetition rate of 500 kHz provided insignificant data, due to difficulties with timing. The whole impact event was not able to be captured, and the imprecise timing of the drop did not allow for imaging of a specific area of the impact. Moving to a slower repetition rate of 50 kHz, the entire impact was captured on the high-speed camera, showing three separate areas of interest. The first section of this area was where the impact was first initiated, resulting in a temperature increase. Next, there was a temperature decrease, where the energy from the drop weight transitioned to deforming, rather than heating the sample. Lastly, there was a final temperature rise when the sample was fully compressed, but the impact was still occurring. This trend presented itself in all of the samples, supporting the idea that when combined with the intensity ratio method, ZnO:Ga embedded in a sylgard binder is an appropriate method to determine the temperature changes in a high-speed impact event.
2

Investigation of wide-bandgap semiconductors by UV Raman spectroscopy: resonance effects and material characterization

Kranert, Christian 18 December 2014 (has links)
Die vorliegende Arbeit befasst sich mit der Untersuchung von weitbandlückigen Halbleitern mittels Raman-Spektroskopie. Diese wurde vorwiegend unter Verwendung von Licht einer Wellenlänge von 325 nm im ultravioletten Spektralbereich angeregt. Damit konnten zum einen aufgrund eines erhöhten Streuquerschnittes Messungen zur Probencharakterisierung durchgeführt werden, die mit Anregung im sichtbaren Spektralbereich nicht möglich gewesen wären. Zum anderen wurden bei dieser Anregungswellenlänge auftretende Resonanzeffekte untersucht. Dabei werden zwei verschiedene Materialsysteme behandelt: zum einen Kristalle mit Wurtzitstruktur und zum anderen binäre und ternäre Sesquioxide mit Metallionen der III. Hauptgruppe. An den Kristallen mit Wurtzitstruktur wurde die Streuung des Anregungslichts mit Energie oberhalb der Bandlücke an longitudinal-optischen (LO) Phononen untersucht. Die Streuung an einzelnen LO-Phononen wird unter diesen Anregungsbedingungen von einem Prozess dominiert, der eine elastische Streuung beinhaltet, durch die die Impulserhaltung verletzt wird. Es wurde ein Modell aufgestellt, dass zwischen einer elastischen Streuung an der Oberfläche und an Punktdefekten unterscheidet, und mit Hilfe von Experimenten verifiziert. Weiterhin wurde der Einfluss von Ladungsträgern auf die Energie der LO-Phononen untersucht und es wird eine Anwendung dieser Erkenntnisse zur Charakterisierung der Oberfläche von Zinkoxid vorgestellt. An den binären Oxiden des Galliums und Indiums wurden die Energien der Phononenmoden ermittelt und die resonante Verstärkung bei der verwendeten Anregungswellenlänge untersucht. Für das Galliumoxid wurde dabei insbesondere die Anisotropie des Materials berücksichtigt. Für das Indiumoxid wird dargestellt, dass durch die resonante Anregung alle Phononenmoden beobachtet werden können, was insbesondere auch die Bestimmung der Phononenmoden von Dünnschichtproben ermöglicht. Weiterhin waren Mischkristalle des Galliumoxids Untersuchungsgegenstand, in denen das Gallium teilweise durch Indium oder Aluminium ersetzt wurde. Die Phononenenergien wurden in Abhängigkeit der Zusammensetzung ermittelt und der Einfluss von strukturellen Eigenschaften darauf sowie das Auftreten von Phasenübergängen untersucht.
3

Quantum structures in photovoltaic devices

Holder, Jenna Ka Ling January 2013 (has links)
A study of three novel solar cells is presented, all of which incorporate a low-dimensional quantum confined component in a bid to enhance device performance. Firstly, intermediate band solar cells (IBSCs) based on InAs quantum dots (QDs) in a GaAs p-i-n structure are studied. The aim is to isolate the InAs QDs from the GaAs conduction band by surrounding them with wider band gap aluminium arsenide. An increase in open circuit voltage (V<sub>OC</sub>) and decrease in short circuit current (J<sub>sc</sub>) is observed, causing no overall change in power conversion efficiency. Dark current - voltage measurements show that the increase in V<sub>OC</sub> is due to reduced recombination. Electroreflectance and external quantum efficiency measurements attribute the decrease in J<sub>sc</sub> primarily to a reduction in InGaAs states between the InAs QD and GaAs which act as an extraction pathway for charges in the control device. A colloidal quantum dot (CQD) bulk heterojunction (BHJ) solar cell composed of a blend of PbS CQDs and ZnO nanoparticles is examined next. The aim of the BHJ is to increase charge separation by increasing the heterojunction interface. Different concentration ratios of each phase are tested and show no change in J<sub>sc</sub>, due primarily to poor overall charge transport in the blend. V<sub>OC</sub> increases for a 30 wt% ZnO blend, and this is attributed largely to a reduction in shunt resistance in the BHJ devices. Finally, graphene is compared to indium tin oxide (ITO) as an alternative transparent electrode in squaraine/ C<sub>70</sub> solar cells. Due to graphene’s high transparency, graphene devices have enhanced J<sub>sc</sub>, however, its poor sheet resistance increases the series resistance through the device, leading to a poorer fill factor. V<sub>OC</sub> is raised by using MoO<sub>3</sub> as a hole blocking layer. Absorption in the squaraine layer is found to be more conducive to current extraction than in the C<sub>70</sub> layer. This is due to better matching of exciton diffusion length and layer thickness in the squaraine and to the minority carrier blocking layer adjacent to the squaraine being more effective than the one adjacent to the C<sub>70</sub>.
4

ZnO nanoparticles : synthesis of Ga-doped ZnO, oxygen gas sensing and quantum chemical investigation

Hagelin, Alexander January 2011 (has links)
Doped ZnO nanoparticles were synthesized by three different methods – electrochemical deposition under oxidizing conditions (EDOC) , combustion method and wet chemical synthesis – for investigating the oxygen gas sensing response. Ga-doped ZnO was mostly synthesized but also In-doped ZnO was made. The samples were analyzed by XRD, SEM, EDX and TEM. Gas response curves are given alongside with Langmuir fitted curves and data for pure ZnO and Ga-doped ZnO. DFT quantum chemical investigation of cluster models ZnO nanoparticles were performed to evaluate defect effects and oxygen and nitrogen dioxide reactions with the ZnO surface. Defects were investigated by DOS and HOMO-LUMO plots , and are oxygen vacancy, zinc vacancy, zinc interstitial and gallium doping by replacing zinc with gallium. Oxygen and nitrogen dioxide reactions were investigated by computing Mulliken charges, bond lengths, DOS spectra and HOMO-LUMO plots.

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