III-nitride mainly GaN semiconductors are the most important materials for a wide range of applications, in particular high-power devices, due to the tunable direct bandgap, their chemical, and thermal stability. However, their growth on suitable substrates is still problematic, and low UV GaN efficiency hinders the efforts aimed at improving the performance of emitting devices. This dissertation presents novel growth and device fabrication methods capable of overcoming these issues using different novel strategies. The work reported in this dissertation comprises five parts. The first two parts demonstrate a new low-cost pulsed laser deposition (PLD)-based strategy for large-scale applications. This was developed to grow high-quality dislocation-free GaN NWs epitaxially on any bulk, flexible, or two-dimensional (2D) substrates without a catalyst, irrespective of the lattice mismatch or type of the substrate. As part of the work reported here, Si, p-GaN, Ga2O3, sapphire, graphene, MXene, and transition-metal dichalcogenide (TMD) substrates were utilized. Also, the adopted growth mechanisms are discussed, along with the advanced structural and optical characterizations. Advanced structural and optical characterizations further confirm the growth mechanism and demonstrate the superior optical and structural quality of GaN NWs. In the third part, a novel multiple quantum wells (MQWs)-based structure grown on the NWs is described, indicating that these NWs can be used as a template to grow III-nitride-based devices. In the fourth part of the work, the significance of these GaN NWs is further demonstrated by reporting on the fabrication of a high-performance self-powered broadband photodetector incorporating these NWs hybridized by two perovskite types: organic/inorganic as well as all-inorganic perovskites (CH3NH3PbI3 and CsPbBr3), revealing two different self-powered photodetector characteristics with high photo-responsivity at 0V. In the last part of this work, the focus is given to a new environmentally friendly strategy to enhance the device UV emission efficiency by functionalizing GaN NWs with solution-processed p-MnO quantum dots (QDs) characterized by much wider bandgap energy than that of GaN. The energy transfer mechanism from QDs to NWs is also discussed using different structural and optical characterizations. This novel strategy is based on drop-casting QDs on NWs, which is simple, cost-effective, and applicable for large-scale applications.

No description available.

Most dam break studies require complex numerical simulations and advanced knowledge in hydraulics. A number of programs are available to perform these simulations and the choice regarding the software to be used must be done within the resources available and the complexity of the problem to be addressed. It remains that this choice is also influenced by the experience and by the knowledge of the analyst. This study reports the simulations results of a comparative study performed for three (3) cases of varying complexity with the software NWS FLDWAV and HEC RAS. Thus, the rupture of a dam in a rectangular channel with uniform characteristics showed that both of the software allows obtaining almost identical results. The other two cases associated with the historical failure of Teton Dam (Idaho, United States) and the combined failure of Drummond and Rock Forest dams (Québec, Canada) showed significant differences in dam breach flow rates and maximum tailwater levels. A comparative analysis of the results leads to recommendations intended for users of these softwares.

Si nanowires have potential applications in a variety of technologies such as micro and nanoelectronics, sensors, electrodes and photovoltaic applications due to their size and specific surface area. Au particle-assisted vapour-liquid-solid or VLS growth method remains the dominant process for Si nanowire growth. A comprehensive kinetic model that addresses all experimental observations and provides a physico-chemical model of the VLS growth method is thus essential. The work done as part of this research is divided into two sections. A steady state kinetic model was first developed for the steady state growth rate of Si nanowires using SiCl4 and SiH4 as precursors. The steady state refers to a balance between the rates of injection and ejection of Si into the Au droplet. This balance results in a steady state supersaturation under which wire growth proceeds. In particular evaporation and reverse reaction of Si from the Au droplet and modes of crystal growth for wire growth have been considered in detail for the first time. The model is able to account for both, the radius independent and radius dependent growth rates reported in the literature. It also shows that the radius dependence previously attributed to purely thermodynamic considerations could also as well be explained just by steady state kinetics alone. Expressions have been derived for the steady state growth rate that require the desolvation energy, activation energy for precursor dissociation and supersaturation prevalent in the particle as inputs for calculation. In order to evaluate this model the incubation and growth of Si nanowires were studied on sapphire substrates in an indigenously built automated MOCVD reactor. Sapphire was chosen as the substrate, as opposed to Si which is commonly used, so as to ensure that the vapour phase is the only source of Si. A classical incubation period for nucleation, of the order of 4-8 minutes, was experimentally observed for the first time. Using the change in this incubation period with temperature a value of 15kT was determined to be the desolvation energy for growth using SiH4. The steady state growth rate of Si nanowires were measured and compared with the predictions of the model using the values of activation energies so determined. The thesis based on the current research work is organized as follows: Chapter 1 introduces the research area followed by a brief outline of the overall work Chapter 2 provides a summary of current literature, and puts the research described in this thesis in perspective. The diameter dependent growth rate of NWs which was initially solely attributed to the Gibbs-Thomson effect is first summarized. Experimental observations to the contrary are then highlighted. These contradictions provided the incentive for the research described in this thesis. Following a summary of the growth rate theories, the experimental observations on incubation available in the literature are summarized. All the other variants of the VLS method are also discussed. Chapter 3 describes the design, construction and working of an indigenously built semi- automated CVD reactor. This CVD reactor was used to conduct the Si NW growth experiments over sapphire substrates. Chapter 4 develops the physical chemistry model for Au catalyzed Si nanowire growth using SiCl4 and SiH4 precursors. The model originated from the contradictions present in the literature over the rate limiting step of the VLS growth mechanism and the steady state growth rate dependence on wire diameter. The development starts with explaining the thermodynamics of the steady state VLS process. The significance of the model lies in the detailed analysis of the all the atomistic process occurring during the VLS growth. In particular the evaporation and reverse reaction of Si from Au-Si droplet is explained in detail and possibly for the first time. Expressions for steady state growth rate by various modes, such as layer by layer growth (LL), by multilayer growth (ML) and growth by movement of a rough interface at the L-S growth interface are derived and presented. Chapter 5 discusses the results which emerge out the kinetic model from the previous chapter. Under a single framework of equations, the model is successful in explaining both the diameter independent and diameter dependent growth of NWs. As one of the major outcomes of the model, the growth rates of Si NWs are predicted and trends in growth rate are found to agree with those experimentally observed. Growth rate dependencies on pressure and temperature are implicitly included in the equations derived. An estimate of supersaturation has been extracted for the first time using the framework of equations. Chapter 6 contains the experimental results of the Si NW growth over sapphire substrates. An incubation period in the order of 3-8 minutes has been observed for Si NW growth on sapphire. The data has been compared with existing literature data and interpreted using classical transient nucleation theory. The incubation period data has been utilized to extract the kinetic parameter, QD, which is the desolvation enegy. These parameters and the measured steady state growth rates have been used to estimate the supersaturation existing in the droplet using the framework developed in chapters 4 and 5. Chapter 7 summarizes the outcome of the current research and highlights the future directions for the research problem addressed in this thesis.

Media use of hurricane graphics to apprise populations vulnerable to severe weather provides a persuasive demonstration of the importance and complexity of visual communication. Surprisingly little research, however, has explored how audiences interpret weather graphics. This study examined whether the general public and the National Weather Service share a common understanding of selected weather related terms and meaning of a NWS informational graphic. Using a coorientation model, general public responses to a questionnaire were compared to definitions prescribed by the NWS. Additionally, the public were asked questions to measure trust of the NWS as a credible and reliable source of severe weather information. Selected broadcast meteorologists were surveyed to measure their opinions of the NWS as well as to measure their perceptions of how the general public would respond to questions relating to knowledge of weather terms and graphics. Results revealed discrepancies between the intent of such graphics and audience interpretations. While the vast majority of respondents recognized the Tropical Cyclone Track Watch/Warning Graphic and understood much of the information it conveyed, study respondents did not seem to remember or understand the meaning of the terms Watch and Warning. While these terms or conditions are only one aspect of the graphic they represent critical information for populations at risk. Additionally, the results of this study indicate that weather forecasting professionals' perceptions of the public's understanding of the graphic are inaccurate. Results also show respondents generally rate the NWS as a reliable and competent agency but they do not consistently rate their local weather providers as well. Weather scientists' foremost concern may be the accuracy of their forecasts, but they also must consider the accuracy of the perceptions of those forecasts if they are to be effective in warning populations at risk of severe weather. These results have sobering implications for both governmental and private sources of emergency communication.

Energy harvesting using piezoelectric nanomaterials provides an opportunity for advancement towards self-powered systems. Self-powered systems are a new emerging technology, which allows the use of a system or a device that perform a function without the need for external power source like for example, a battery or any other type of source. This technology can for example use harvested energy from sources around us such as ambient mechanical vibrations, noise, and human movement, etc. and convert it to electric energy using the piezoelectric effect. For nanoscale devices, the size of traditional batteries is not suitable and will lead to loss of the concept of “nano”. This is due to the large size and the relatively large magnitude of the delivered power from traditional sources. The development of a nanogenerator (NG) to convert energy from the environment into electric energy would facilitate the development of some self-powered systems relying on nano- devices. The main objective of this thesis is to fabricate a piezoelectric Zinc Oxide (ZnO) NGs for low frequency (˂ 100 Hz) energy harvesting applications. For that, different types of NGs based on ZnO nanostructures have been carefully developed, and studied for testing under different kinds of low frequency mechanical deformations. Well aligned ZnO nanowires (NWs) possessing high piezoelectric coefficient were synthesized on flexible substrates using the low temperature hydrothermal route. These ZnO NWs were then used in different configurations to demonstrate different low frequency energy harvesting devices. Using piezoelectric ZnO NWs, we started with the fabrication of sandwiched NG for hand writing enabled energy harvesting device based on a thin silver layer coated paper substrate. Such device configurations can be used for the development of electronic programmable smart paper. Further, we developed this NG to work as a triggered sensor for wireless system using foot-step pressure. These studies demonstrate the feasibility of using ZnO NWs piezoelectric NG as a low-frequency self-powered sensor, with potential applications in wireless sensor networks. After that, we investigated and fabricated a sensor on PEDOT: PSS plastic substrate either by one side growth technique or by using double sided growth. For the first growth technique, the fabricated NG has been used as a sensor for acceleration system; while the fabricated NG by the second technique has worked as anisotropic directional sensor. This fabricated configurations showed stability for sensing and can be used in surveillance, security, and auto-mobil applications. In addition to that, we investigated the fabrication of a sandwiched NG on plastic substrates. Finally, we demonstrated that doping ZnO NWs with extrinsic element (such as Ag) will lead to the reduction of the piezoelectric effect due to the loss of crystal symmetry. A brief summary into future opportunities and challenges are also presented in the last chapter of this thesis.

Firstly, this dissertation proposes a new characterization and classification method for daily sky conditions by using the daily sky clearness index (KD) and the daily probability of persistence (POP-KD) that can be derived from ground-based irradiance measurement data. Quality of daily solar irradiance is characterized by a newly proposed parameter, POP-KD. This characterized daily quality is varying and uncertain at the middle level of the quantity, but high and more certain at very high and low quantity levels. In addition, the proposed characterization method shows interesting results for KD and POP-KD: a statistical consistency for multiple years and similarity for their seasonal trends. The classification results also indicate an existence of dominant classes, and transitions between the dominant classes are significant for all locations. This dissertation also generates annual synthetic sequences of KD and POP-KD using a Markov approach. The generated sequences show statistical similarities with observed sequences. Secondly, this dissertation proposes methodologies to estimate day-ahead solar irradiance using the National Weather Service (NWS) sky cover forecast. For model development, this paper splits up a direct estimation process from the sky cover forecast to solar irradiance into two stages: forecast verification and cloud-to-irradiance conversion. Uncertainty for each stage and for the overall estimation process is quantified. NWS forecast uncertainty (about 20%) is identified as the main source of uncertainty for the overall process. In addition, verification of the sky cover forecast shows approximately 20% overestimated bias at days with a high irradiance level. Thus, the NWS sky cover forecast needs to be adjusted based on the type of day. This dissertation also proposes a conversion equation relating daily quantity of cloud information and daily quantity of solar irradiance. The proposed conversion equation achieves accuracy with simplicity. Five day-ahead solar irradiance quantity estimation methods are proposed in this dissertation. The proposed methods incorporate different schemes for dealing with the bias discovered in the cloud forecast. The observed data are regularly found within the 95% confidence intervals of the estimated values. Estimation results demonstrate the effectiveness of the conditional adjustment schemes at different irradiance levels. Lastly, this dissertation proposes a methodology to estimate day-ahead solar irradiance using fluctuation information of the NWS sky cover forecast. POP-KD was used as a parameter for the quality of daily solar irradiance. POP-KD efficiently represents the quality of daily solar irradiance. In addition, POP-KD indicates the probability that solar irradiance variability is within the ramp rates of common generators in power systems at a certain photovoltaic penetration level. This dissertation also proposes a new equation for the conversion from cloud fluctuation information to daily quality of surface solar irradiance. The proposed equation achieves accuracy. The proposed day-ahead solar irradiance quality estimation method is based on fluctuation information provided by the NWS sky cover forecast. This method uses a normalization approach to relate fluctuation of cloud forecast and fluctuation of cloud observation. The observed data are regularly found within the 95% CIs of the estimated values. / Ph. D.

Ce travail de thèse porte sur l’étude des propriétés optiques de nanofils InP et d’hétérostructures nanofils InAs/InP épitaxiés sur substrat silicium. Ce travail de thèse a été réalisé principalement dans le cadre du projet ANR «INSCOOP». / This thesis is focused upon the experimental investigation of optical properties of InAs/InP NW heterostructures by means of photoluminescence (PL) spectroscopy. First, it was demonstrated that the host-substrate may have significant impacts on the optical properties of pure InP NWs, as due to the strain, created by the difference in the LTECs of the NWs and the host-substrate, as due to some other surface effects. Next, the optical properties of such nanowire heterostructures as quantum rod (QRod) and radial quantum well (QWell) NWs were investigated. The features of obtained spectra were explained using theoretical simulation of similar NW heterostructures. The polarization properties of single InP NWs, InAs/InP QWell-NWs, InAs/InP QRod-NWs and ensemble of the InAs well ordered NWs were studied at different temperatures. Further, we report on the evidences of the strain-induced piezoelectric field in WZ InAs/InP QRod-NWs. Finally, PL QE of NW heterostructures and their planar analogues are measured by means of a PL setup coupled to an integrating sphere. In general, the obtained knowledge of the optical and mechanical properties of pure InP NWs and InAs/InP NW heterostructures will improve understanding of the electrical and mechanical processes taking place in semiconductor NW heterostructures and will serve for the fabrication of future nanodevice applications.

In this thesis I present a study of nanowires, and, in particular, I apply EBIC microscopy for investigation of their electro-optical properties. First, I describe details of the EBIC analytical technique together with a brief historical overview of the electron microscopy, the physical principles of the EBIC, its space resolution, parameters defining the signal amplitude, and the information we can acquire concerning defects, electric fields, etc. Then I focus on the characterization of LEDs based on GaN nanowires, which were analyzed in a cross-section and in a top view configurations. The EBIC measurements were correlated with micro-electroluminescence mapping. Further, I address the fabrication and measurement of nanowire-based InGaN/GaN LEDs detached from their original substrate. I present the EBIC measurements of individual nanowires either cut from their substrate and contacted in a planar geometry or kept standing on supphire substrate and cleaved to reveal the horizontal cross-section.The next part of this thesis is dedicated to an EBIC study of irregular Si nanowire array-based solar cells, and then of the regular nanowire array devices. The current generation was analyzed on a submicrometer scale. Finally, I discuss the fabrication and EBIC measurements of GaN nanowires grown on Si substrate. In particular, I show that the p-n junction was induced in the Si substrate by Al atom diffusion during the nanowire growth. / Dans cette thèse je me propose d’étudier des nano-fils, et en particulier d’utiliser la technique EBIC pour explorer leurs propriétés électro-optiques. Je décris d’abord les détails de la technique d’analyse EBIC avec un bref retour historique sur la microscopie électronique, le principe physique de l’EBIC, sa résolution spatiale, les paramètres conditionnant l’amplitude du signal, et les informations que l’on peut en tirer sur le matériau en termes de défauts, champ électrique, etc. Je m’intéresse ensuite à la caractérisation de LEDs à nano-fils à base de GaN, qui ont été observés par EBIC, soit en coupe soit en vue plane (depuis le haut des fils). Les mesures EBIC sont comparées à celles de micro-électroluminescence. Plus loin j’adresse la fabrication et la mesure de nano-fils à base de GaN séparés de leur substrat d’origine. Je présente les mesures EBIC de nano-fils uniques entiers, puis de nano-fils en coupe horizontale.La partie suivante de la thèse traite d’étude EBIC des cellules solaires à base de nano-fils Si ayant d’abord une géométrie aléatoire, puis une géométrie régulière. La génération de courant dans ces cellules solaires est analysée à l’échelle submicronique. A la fin du manuscrit je discute la fabrication et les mesures EBIC de fils GaN épitaxiés sur Si. Je montre en particulier qu’une jonction p-n est enduite dans le substrat Si par la diffusion d’Al lors de la croissance de nanofils.

This thesis contains three essays in the field of real-time econometrics, and more particularly<p>forecasting.<p>The issue of using data as available in real-time to forecasters, policymakers or financial<p>markets is an important one which has only recently been taken on board in the empirical<p>literature. Data available and used in real-time are preliminary and differ from ex-post<p>revised data, and given that data revisions may be quite substantial, the use of latest<p>available instead of real-time can substantially affect empirical findings (see, among others,<p>Croushore’s (2011) survey). Furthermore, as variables are released on different dates<p>and with varying degrees of publication lags, in order not to disregard timely information,<p>datasets are characterized by the so-called “ragged-edge”structure problem. Hence, special<p>econometric frameworks, such as developed by Giannone, Reichlin and Small (2008) must<p>be used.<p>The first Chapter, “The impact of macroeconomic news on bond yields: (in)stabilities over<p>time and relative importance”, studies the reaction of U.S. Treasury bond yields to real-time<p>market-based news in the daily flow of macroeconomic releases which provide most of the<p>relevant information on their fundamentals, i.e. the state of the economy and inflation. We<p>find that yields react systematically to a set of news consisting of the soft data, which have<p>very short publication lags, and the most timely hard data, with the employment report<p>being the most important release. However, sub-samples evidence reveals that parameter<p>instability in terms of absolute and relative size of yields response to news, as well as<p>significance, is present. Especially, the often cited dominance to markets of the employment<p>report has been evolving over time, as the size of the yields reaction to it was steadily<p>increasing. Moreover, over the recent crisis period there has been an overall switch in the<p>relative importance of soft and hard data compared to the pre-crisis period, with the latter<p>becoming more important even if less timely, and the scope of hard data to which markets<p>react has increased and is more balanced as less concentrated on the employment report.<p>Markets have become more reactive to news over the recent crisis period, particularly to<p>hard data. This is a consequence of the fact that in periods of high uncertainty (bad state),<p>markets starve for information and attach a higher value to the marginal information content<p>of these news releases.<p>The second and third Chapters focus on the real-time ability of models to now-and-forecast<p>in a data-rich environment. It uses an econometric framework, that can deal with large<p>panels that have a “ragged-edge”structure, and to evaluate the models in real-time, we<p>constructed a database of vintages for US variables reproducing the exact information that<p>was available to a real-time forecaster.<p>The second Chapter, “Real-time nowcasting of GDP: a factor model versus professional<p>forecasters”, performs a fully real-time nowcasting (forecasting) exercise of US real GDP<p>growth using Giannone, Reichlin and Smalls (2008), henceforth (GRS), dynamic factor<p>model (DFM) framework which enables to handle large unbalanced datasets as available<p>in real-time. We track the daily evolution throughout the current and next quarter of the<p>model nowcasting performance. Similarly to GRS’s pseudo real-time results, we find that<p>the precision of the nowcasts increases with information releases. Moreover, the Survey of<p>Professional Forecasters does not carry additional information with respect to the model,<p>suggesting that the often cited superiority of the former, attributable to judgment, is weak<p>over our sample. As one moves forward along the real-time data flow, the continuous<p>updating of the model provides a more precise estimate of current quarter GDP growth and<p>the Survey of Professional Forecasters becomes stale. These results are robust to the recent<p>recession period.<p>The last Chapter, “Real-time forecasting in a data-rich environment”, evaluates the ability<p>of different models, to forecast key real and nominal U.S. monthly macroeconomic variables<p>in a data-rich environment and from the perspective of a real-time forecaster. Among<p>the approaches used to forecast in a data-rich environment, we use pooling of bi-variate<p>forecasts which is an indirect way to exploit large cross-section and the directly pooling of<p>information using a high-dimensional model (DFM and Bayesian VAR). Furthermore forecasts<p>combination schemes are used, to overcome the choice of model specification faced by<p>the practitioner (e.g. which criteria to use to select the parametrization of the model), as<p>we seek for evidence regarding the performance of a model that is robust across specifications/<p>combination schemes. Our findings show that predictability of the real variables is<p>confined over the recent recession/crisis period. This in line with the findings of D’Agostino<p>and Giannone (2012) over an earlier period, that gains in relative performance of models<p>using large datasets over univariate models are driven by downturn periods which are characterized<p>by higher comovements. These results are robust to the combination schemes<p>or models used. A point worth mentioning is that for nowcasting GDP exploiting crosssectional<p>information along the real-time data flow also helps over the end of the great moderation period. Since this is a quarterly aggregate proxying the state of the economy,<p>monthly variables carry information content for GDP. But similarly to the findings for the<p>monthly variables, predictability, as measured by the gains relative to the naive random<p>walk model, is higher during crisis/recession period than during tranquil times. Regarding<p>inflation, results are stable across time, but predictability is mainly found at nowcasting<p>and forecasting one-month ahead, with the BVAR standing out at nowcasting. The results<p>show that the forecasting gains at these short horizons stem mainly from exploiting timely<p>information. The results also show that direct pooling of information using a high dimensional<p>model (DFM or BVAR) which takes into account the cross-correlation between the<p>variables and efficiently deals with the “ragged-edge”structure of the dataset, yields more<p>accurate forecasts than the indirect pooling of bi-variate forecasts/models. / Doctorat en Sciences économiques et de gestion / info:eu-repo/semantics/nonPublished