Lasing and ion beam doping of semiconductor nanowires

Semiconductor nanowires exhibit extraordinary optical properties like highly localized light emission, efficient waveguiding and light amplification. Even the stimulation of laser oscillations can be achieved at optical pumping, making nanowires promising for optoelectronic applications. For successful integration into future devices, three major key challenges have to be faced: 1) the understanding of the fundamental properties, 2) the modification of the emission characteristics and 3) the investigation of the efficiency-limiting factors. All key challenges are addressed in this thesis: 1) The fundamental properties of CdS nanowire have been investigated to uncover the size limits for photonic nanowire lasers. Laser oscillations were observed at room temperature and the emission characteristics were correlated to the morphology, which allowed the determination of a minimum diameter and length necessary for lasing. 2) The emission characteristics of ZnO nanowires have been successfully modified by ion beam doping with Co. The structural investigations revealed a good recovery of the ion induced damage in the crystal lattice. Optical activation of the implanted Co ions was achieved and an intense intra-3d-emission confirmed successful modification. 3) The temporal decay of excited luminescence centers strongly depends on the interplay of luminescent ions and defects, thus offering an approach to investigate the efficiency-limiting processes. Mn implanted ZnS nanowires were investigated, as the temporal decay of the incorporated Mn ions can be described by a Förster energy transfer model modified for nanostructures. The defect concentration was varied systematically by several approaches and the model could successfully fit the transients in all cases. The emission properties of Tb implanted ZnS nanowires were investigated and the temporal decay of the intra-4f-emission could also be fitted by the model, proving its accuracy for an additional element.Halbleiter-Nanodrähte besitzen außergewöhnliche Eigenschaften wie hochlokalisierte Lichtemission, exzellente Wellenleitung und Lichtverstärkung. Sogar Laseroszillationen können unter optischer Anregung stimuliert werden, wodurch Nanodrähte vielversprechend für optoelektronische Anwendungen sind. Für eine zukünftige Integration als Bauteile sind jedoch drei Herausforderungen zu überwinden: 1) Verständnis der fundamentalen Eigenschaften, 2) Modifikation der Emissionscharakteristik und 3) Untersuchung der Effizienz-limitierenden Faktoren. Alle Herausforderungen werden in dieser Arbeit angesprochen: 1) Die fundamentalen Eigenschaften anhand von CdS Nanodrähten untersucht. Laseroszillationen konnten bei Raumtemperatur beobachtet werden. Durch die Korrelation der Emissionscharakteristik mit der Morphologie konnten das Minimum des Durchmesser und der Länge für photonische Nanolaser bestimmt werden. 2) Die Emissionseigenschaften von ZnO Nanodrähten wurden gezielt über Ionenstrahl-Dotierung mit Co modifiziert. Strukturelle Untersuchungen zeigten ein gutes Ausheilen des strahleninduzierten Schaden im Kristallgitter. Die eingebrachten Co-Ionen konnten erfolgreich optisch aktiviert werden und zeigen eine intensive intra-3d-Emission. 3) Das zeitliche Abklingen der angeregten Leuchtzentren hängt stark von der Interaktion dieser mit Defekten ab, so dass darüber die Untersuchung der Effizienz-limitierenden Faktoren ermöglicht wird. Das zeitliche Abklingen von Mn in ZnS Nanodrähten wurde untersucht und die Transienten können mit einem modifizierten Förster-Energietransfer-Modell beschrieben werden. Die Defektkonzentration wurde systematisch durch mehrere Ansätze variiert. Das Modell konnte die Messdaten in allen Fällen gut wiedergeben. Die Emissionseigenschaften Tb implantierter ZnS Nanodrähte wurden charakterisiert und das zeitliche Abklingen der intra-4f-Emission kann ebenfalls mithilfe des Modells beschrieben werden, wodurch dieses für ein weiteres Element bestätigt wird

Defect induced changes on the excitation transfer dynamics in ZnS/Mn nanowires

Transients of Mn internal 3d5 luminescence in ZnS/Mn nanowires are strongly non-exponential. This non-exponential decay arises from an excitation transfer from the Mn ions to so-called killer centers, i.e., non-radiative defects in the nanostructures and is strongly related to the interplay of the characteristic length scales of the sample such as the spatial extensions, the distance between killer centers, and the distance between Mn ions. The transients of the Mn-related luminescence can be quantitatively described on the basis of a modified Förster model accounting for reduced dimensionality. Here, we confirm this modified Förster model by varying the number of killer centers systematically. Additional defects were introduced into the ZnS/Mn nanowire samples by irradiation with neon ions and by varying the Mn implantation or the annealing temperature. The temporal behavior of the internal Mn2+ (3d5) luminescence is recorded on a time scale covering almost four orders of magnitude. A correlation between defect concentration and decay behavior of the internal Mn2+ (3d5) luminescence is established and the energy transfer processes in the system of localized Mn ions and the killer centers within ZnS/Mn nanostructures is confirmed. If the excitation transfer between Mn ions and killer centers as well as migration effects between Mn ions are accounted for, and the correct effective dimensionality of the system is used in the model, one is able to describe the decay curves of ZnS/Mn nanostructures in the entire time window

Rede zur hundertjährigen Feier der Geburt Schiller's : am zehnten November 1859 in der St. Peters-Kirche zu Zürich

gehalten von Friedr. Vische

A Comparative Analysis of Plant-Based Milk Alternatives Part 2: Environmental Impacts

Human food production is the largest cause of global environmental changes. Environmental benefits could be achieved by replacing diets with a high amount of animal-sourced foods with more plant-based foods, due to their smaller environmental impacts. The objective of this study was to assess the environmental impacts of the three most common plant-based milk alternatives (PBMAs)—oat, soy, and almond drink—in comparison with conventional and organic cow milk. Life cycle assessments (LCA) were calculated by the ReCiPe 2016 midpoint method, in addition to the single issue methods “Ecosystem damage potential” and “Water scarcity index”. PBMAs achieved lower impact values in almost all 12 of the calculated impact categories, with oat drink and the organic soy drink being the most environmentally friendly. However, when LCA results were expressed per energy and by the protein content of the beverages, the ranking of the beverages, in terms of their environmental impacts, changed greatly, and the results of PBMAs approached those of milk, particularly with regard to the protein index. The study highlights the importance of considering a broader range of impact categories when comparing the impacts of PBMAs and milk