Time Dependent Study of Multiple Exciton Generation in Nanocrystal Quantum Dots

We study the exciton dynamics in an optically excited nanocrystal quantum dot. Multiple exciton formation is more efficient in nanocrystal quantum dots compared to bulk semiconductors due to enhanced Coulomb interactions and the absence of conservation of momentum. The formation of multiple excitons is dependent on different excitation parameters and the dissipation. We study this process within a Lindblad quantum rate equation using the full many-particle states. We optically excite the system by creating a single high energy exciton $E_{SX}$ in resonance to a double exciton $E_{DX}$. With Coulomb electron-electron interaction, the population can be transferred from the single exciton to the double exciton state by impact ionisation (inverse Auger process). The ratio between the recombination processes and the absorbed photons provide the yield of the structure. We observe a quantum yield of comparable value to experiment assuming typical experimental conditions for a $4$ nm PbS quantum dot.Comment: 10 pages, 6 figures. Submitted to the conference "Progress in Nonequilibrium Green's Functions VI Proceedings" at Lund University, Sweden, August 17th - 21st, 2015. To be published in the Journal of Physics: Conference Serie

Optimization Schemes for Efficient Multiple Exciton Generation and Extraction in Colloidal Quantum Dots

Multiple exciton generation is a process in which more than one electron hole pair is generated per absorbed photon. It allows us to increase the efficiency of solar energy harvesting. Experimental studies have shown the multiple exciton generation yield of 1.2 in isolated colloidal quantum dots. However real photoelectric devices require the extraction of electron hole pairs to electric contacts. We provide a systematic study of the corresponding quantum coherent processes including extraction and injection and show that a proper design of extraction and injection rates enhances the yield significantly up to values around 1.6.Comment: 5 pages, accepted by The Journal of Chemical Physic

Transport in serial spinful multiple-dot systems: The role of electron-electron interactions and coherences

Quantum dots are nanoscopic systems, where carriers are confined in all three spatial directions. Such nanoscopic systems are suitable for fundamental studies of quantum mechanics and are candidates for applications such as quantum information processing. It was also proposed that linear arrangements of quantum dots could be used as quantum cascade laser. In this work we study the impact of electron-electron interactions on transport in a spinful serial triple quantum dot system weakly coupled to two leads. We find that due to electron-electron scattering processes the transport is enabled beyond the common single-particle transmission channels. This shows that the scenario in the serial quantum dots intrinsically deviates from layered structures such as quantum cascade lasers, where the presence of well-defined single-particle resonances between neighboring levels are crucial for device operation. Additionally, we check the validity of the Pauli master equation by comparing it with the first-order von Neumann approach. Here we demonstrate that coherences are of relevance if the energy spacing of the eigenstates is smaller than the lead transition rate multiplied by $\hbar$.Comment: 12 pages, 7 figure

Probing Electron Collisions in Nanostructures

This thesis studies the role of interaction between charged particles for transport and optical properties in nanostructures. Simulations are provided for a quantitative description of the system dynamics probed by either current spectroscopy or light absorption. In the first part of the thesis (Part I) the basic theories are described and in the second part of the thesis (Part II) the papers listed below are presented. Paper I studies the effect of different types of electron-electron interaction terms in triple quantum dot transport. Paper II describes a method of calculating the yield for a quantum dot model due to Multiple Exciton Generation (MEG) which is an effect of electron-electron interaction. Paper III suggests optimization schemes for an efficient yield due to Multiple exciton generation in nanocrystal quantum dots. Paper IV describes two dimensional spectroscopy based on phase modulation technique to study the dynamics of ultrafast processes in nanostructures in real time

Analyzing the Dynamics: EU and UN Perspectives on the African Union

This article presents a comprehensive analysis of the dynamics between the European Union (EU) and the United Nations (UN) in their interactions with the African Union (AU). Recognizing the AU's pivotal role in addressing Africa's challenges, the study investigates historical trajectories, policies, and key priorities shaping EU and UN perspectives on the AU. Through a multi-methodological approach, including document review and comparative analysis, it explores areas of convergence, divergence, and potential conflicts in their approaches, shedding light on implications for the AU's development and international relations. Tracing the historical evolution of EU-AU and UN-AU relations, pivotal moments such as the Cotonou Agreement and the establishment of the UN Office to the African Union (UNOAU) are highlighted. Analysis reveals shared objectives in peace, security, sustainable development, and governance, with nuanced differences in emphasis and approach. While areas of alignment exist, particularly in peacekeeping and sustainable development goals, divergences emerge in governance, human rights, and economic relations, posing challenges to collaboration.Case studies, including joint peacekeeping missions in Mali and development initiatives in Eastern Africa, offer insights into successes, challenges, and lessons learned, informing recommendations for future collaborations. Opportunities for enhanced cooperation, such as joint capacity-building initiatives and aligned development programs, are identified.The study contributes to a nuanced understanding of EU and UN perspectives on the AU, providing insights into their implications for Africa's development. Recommendations for improvements in coordination mechanisms, communication channels, and decision-making processes are proposed to enhance effectiveness. Future research avenues include evaluating economic partnerships' impacts and exploring innovative funding models to unlock transformative impact on the African continent. Ultimately, this analysis informs policymakers and international practitioners, fostering constructive partnerships for Africa's stability, prosperity, and sustainable development

The impact of downsizing on survivors: from organizational commitment and job insecurity point of view (the case of Ethio Telecom, Addis Ababa)

Given the face of globalization, most organizations involved in the implementation 0(' downsizing as one of the management strategies. However, the way being exercised has a profound impact on survivors from layoff and on objective 0(' downsizing. The objective 0(' this study is to determine how the perceptions of downsizing in relation to procedural and distributive justice affect the level of organizational commitment and job insecurity of survivors. To achieve the objective of the study data were collected from employees of Ethio Telecom, who survive from downsizing, using questionnaire and unstructured interview. A total of 252 survivors from two zones were included using simple random sampling technique. Both descriptive and binary logistic regression methods were employed as a tool for the empirical analysis using statistical package for social sciences (SPSS 16.0). The study reveals that perception of organizational injustice in both procedural and distributive justices have an impact on survivor's commitment and feeling job insecurity. Procedural justice has found a strong predictor for the outcome variables. While distributive justice has become the second predictor that has an influence on components of organizational commitment. However, this study did not found any relationship between distributive justice and feeling of job insecurity with p-value of 0.532. In general, this study concludes that perception of survivors towards downsizing implementation affects level 0(' commitment and job security. Based on these, it is recommended that Ethio Telecom better to practice an organizational justice while making decisions and also advisable to adopt the involvement of employees in the process to get a positive work related behavioral outcomes. Keywords: Downsizing, Survivors, Layoffs, Procedural justice, Distributive Justice, Job Insecurity, Organizational CommitmentJimma Universit

New incoherent scatter radar measurement techniques and data analysis methods

AbstractThis dissertation presents new incoherent scatter radar measurement techniques and data analysis methods. The measurements used in the study were collected by connecting a computer-based receiver to the EISCAT (European Incoherent SCATter) radar on Svalbard. This hardware consists of a spectrum analyzer, a PCI-bus-based programmable digital I/O card and a desktop computer with a large-capacity hard disk. It takes in the 70-MHz signal from the ESR (Eiscat Svalbard Radar) signal path and carries out down-conversion, AD conversion, quadrature detection, and finally stores the output samples effective sampling rate is 1 MHz, large enough to span all the frequency channels used in the experiment. Hence the total multichannel signal was stored instead of separate lagged products for each frequency channel, which is the procedure in the standard hardware. This solution has some benefits including elimination of ground clutter with only a small loss in statistical accuracy. The capability of our hardware in storing the incoherent scatter radar signals directly allows us to use very flexible and versatile signal processing methods, which include clutter suppression, filtering, decoding, lag prole calculation, inversion and optimal height integration. The performance of these incoherent scatter radar measurement techniques and data analysis methods are demonstrated by employing an incoherent scatter experiment that applies a new binary phase code. Each bit of this code has been further coded by a 5-bit Barker code. In the analysis, stochastic inversion has been used for the first time in decoding Barker-coded incoherent scatter measurements, and this method takes care of the ambiguity problems associated with the measurements. Finally, we present new binary phase codes with corresponding sidelobe-free decoding filters that maximize the signal-to-noise ratio (SNR) and at the same time eliminate unwanted sidelobes completely.Original papersOriginal papers are not included in the electronic version of the dissertation.Lehtinen, M., Markkanen, J., Väänänen, A., Huuskonen, A., Damtie, B., Nygrén, T., & Rahkola, J. (2002). A new incoherent scatter technique in the EISCAT Svalbard Radar. Radio Science, 37(4), 3-1-3–14. https://doi.org/10.1029/2001RS002518Damtie, B., Nygrén, T., Lehtinen, M. S., & Huuskonen, A. (2002). High resolution observations of sporadic-E layers within the polar cap ionosphere using a new incoherent scatter radar experiment. Annales Geophysicae, 20(9), 1429–1438. https://doi.org/10.5194/angeo-20-1429-2002Damtie, B., Lehtinen, M. S., & Nygrén, T. (2004). Decoding of Barker-coded incoherent scatter measurements by means of mathematical inversion. Annales Geophysicae, 22(1), 3–13. https://doi.org/10.5194/angeo-22-3-2004Lehtinen, M. S., Damtie, B., & Nygrén, T. (2004). Optimal binary phase codes and sidelobe-free decoding filters with application to incoherent scatter radar. Annales Geophysicae, 22(5), 1623–1632. https://doi.org/10.5194/angeo-22-1623-2004OsajulkaisutOsajulkaisut eivät sisälly väitöskirjan elektroniseen versioon.Lehtinen, M., Markkanen, J., Väänänen, A., Huuskonen, A., Damtie, B., Nygrén, T., & Rahkola, J. (2002). A new incoherent scatter technique in the EISCAT Svalbard Radar. Radio Science, 37(4), 3-1-3–14. https://doi.org/10.1029/2001RS002518Damtie, B., Nygrén, T., Lehtinen, M. S., & Huuskonen, A. (2002). High resolution observations of sporadic-E layers within the polar cap ionosphere using a new incoherent scatter radar experiment. Annales Geophysicae, 20(9), 1429–1438. https://doi.org/10.5194/angeo-20-1429-2002Damtie, B., Lehtinen, M. S., & Nygrén, T. (2004). Decoding of Barker-coded incoherent scatter measurements by means of mathematical inversion. Annales Geophysicae, 22(1), 3–13. https://doi.org/10.5194/angeo-22-3-2004Lehtinen, M. S., Damtie, B., & Nygrén, T. (2004). Optimal binary phase codes and sidelobe-free decoding filters with application to incoherent scatter radar. Annales Geophysicae, 22(5), 1623–1632. https://doi.org/10.5194/angeo-22-1623-2004Academic dissertation to be presented, with the permission of the Faculty of Science of the University of Oulu, for public discussion in Raahensali (Auditorium L10), Linnanmaa, on 16 April, 2004, at 12 o’clock noon.Abstract This dissertation presents new incoherent scatter radar measurement techniques and data analysis methods. The measurements used in the study were collected by connecting a computer-based receiver to the EISCAT (European Incoherent SCATter) radar on Svalbard. This hardware consists of a spectrum analyzer, a PCI-bus-based programmable digital I/O card and a desktop computer with a large-capacity hard disk. It takes in the 70-MHz signal from the ESR (Eiscat Svalbard Radar) signal path and carries out down-conversion, AD conversion, quadrature detection, and finally stores the output samples effective sampling rate is 1 MHz, large enough to span all the frequency channels used in the experiment. Hence the total multichannel signal was stored instead of separate lagged products for each frequency channel, which is the procedure in the standard hardware. This solution has some benefits including elimination of ground clutter with only a small loss in statistical accuracy. The capability of our hardware in storing the incoherent scatter radar signals directly allows us to use very flexible and versatile signal processing methods, which include clutter suppression, filtering, decoding, lag prole calculation, inversion and optimal height integration. The performance of these incoherent scatter radar measurement techniques and data analysis methods are demonstrated by employing an incoherent scatter experiment that applies a new binary phase code. Each bit of this code has been further coded by a 5-bit Barker code. In the analysis, stochastic inversion has been used for the first time in decoding Barker-coded incoherent scatter measurements, and this method takes care of the ambiguity problems associated with the measurements. Finally, we present new binary phase codes with corresponding sidelobe-free decoding filters that maximize the signal-to-noise ratio (SNR) and at the same time eliminate unwanted sidelobes completely

Optimal Control of UV-Induced Curing Processes for Layer-by-Layer Manufacturing of Composites

Composite materials are becoming viable solutions for making safe, yet lighter and more fuel-efficient vehicles in the automotive industry. However, conventional thermal-based composite manufacturing methods are energy intensive. Potential alternatives are radiation-based curing processes which lend themselves to layer-by-layer additive processes that are suitable for making thick structural parts. This dissertation documents an investigation into ultraviolet (UV) induced curing and layering processes including schemes for their optimization and control. First, a curing process model is developed that is comprised of the coupled cure-kinetics and thermal evolution for a cationic polymerization of a single layer of material. This model is then extended to the process of concurrent layering and curing of multiple layers. The model for processing multiple layers is characterized as a multi-mode hybrid system that switches modes both when the UV source is turned off and when a new layer is added. A computational framework is outlined for determining the optimal sequence of switching times that gives a minimal cure level deviation across all layers subjected to the multi-mode hybrid system model of the process. For validation purposes, a one layer material with two mode has been considered. Comparison of the hardness of a sample cured with optimal switching time versus another sample cured for a longer time showed similar hardness values while using energy/total time. To improve the interlaminar shear strength, the effect of in-situ consolidation pressure on the inter-laminar shear strength of the final product is assessed experimentally. Using the optimal time sequence, a fiber-reinforced composite is made with in-situ consolidation and curing. The results showed that thick composite parts fabricated with in-situ consolidation and UV curing process, with the optimal sequence, showed increased inter-laminar shear strength with increases of the consolidation pressure up to a certain point. An increase in consolidation pressure beyond this point decreased the interlaminar-shear strength. Scanning electron microscopy (SEM) is used to investigate the effect of compaction on the microstructure of the final cured product. For online control, first, a nonlinear model predictive control (NMPC) scheme is outlined for UV-induced acrylate-based curing of a single layer thick composite part. Then, the model is extended for switching nonlinear model predictive control (SNMPC) for layer-by-layer curing process. The key characteristic is that the processes model switches when a new layer is added to the existing layer. Open loop optimal control is used to determine the optimal layering time and temperature profile which give a nearly uniform cure distribution of a thick composite material. Once the temperature trajectory and optimal time sequences are found, the SNMPC is implemented for online control. The objective is to determine theoretical optimal behavior which is then used for online SNMPC for tracking the reference temperature distribution. To demonstrate the effectiveness of the proposed approach a three-layer fiber-reinforced resin is considered and results show a very good agreement between the reference temperature distribution and SNMPC