Recent Submissions

  • Study of III-nitride Nanowire Growth and Devices on Unconventional Substrates

    Prabaswara, Aditya (2019-10) [Dissertation]
    Advisor: Ooi, Boon S.
    Committee members: Ooi, Boon S.; Ohkawa, Kazuhiro; Alshareef, Husam N.; Tchernycheva, Maria
    III-Nitride materials, which consist of AlN, GaN, InN, and their alloys have become the cornerstone of the third generation compound semiconductor. Planar IIINitride materials are commonly grown on sapphire substrates which impose several limitations such as challenging scalability, rigid substrate, and thermal and lattice mismatch between substrate and material. Semiconductor nanowires can help circumvent this problem because of their inherent capability to relieve strain and grow threading dislocation-free without strict lattice matching requirements, enabling growth on unconventional substrates. This thesis aims to investigate the microscopic characteristics of the nanowires and expand on the possibility of using transparent amorphous substrate for III-nitride nanowire devices. In this work, we performed material growth, characterization, and device fabrication of III-nitride nanowires grown using molecular beam epitaxy on unconventional substrates. We rst studied the structural imperfections within quantum-disks-in-nanowire structure grown on silicon and discovered how growth condition could a ect the macroscopic photoluminescence behavior of nanowires ensemble. To expand our work on unconventional substrates, we also used an amorphous silica-based substrate as a more economical substrate for our nanowire growth. One of the limitations of growing nanowires on an insulating substrate is the added fabrication complexity required to fabricate a working device. Therefore, we attempted to overcome this limitation by investigating various possible GaN nanowire nucleation layers, which exhibits both transparency and conductivity. We employed various nucleation layers, including a thin TiN/Ti layer, indium tin oxide (ITO), and Ti3C2 MXene. The structural, electrical, and optical characterizations of nanowires grown on di erent nucleation layers are discussed. From our work, we have established several key processes for transparent nanowire device applications. A nanowire LED emitting at 590 nm utilizing TiN/Ti interlayer is presented. We have also established the growth process for n-doped GaN nanowires grown on ITO and Ti3C2 MXene with transmittance above 40 % in the visible wavelength, which is useful for practical applications. This work paves the way for future devices utilizing low-cost substrates, enabling further cost reduction in III-nitride device fabrication.
  • Biofouling in anaerobic membrane bioreactors: To control or not to?

    Cheng, Hong (2019-10) [Dissertation]
    Advisor: Hong, Peiying
    Committee members: Nunes, Suzana; Moran, Anxelu; Smith, Adam
    Anaerobic membrane bioreactor (AnMBR) serves as a more sustainable form of wastewater treatment. However, biofouling is particularly detrimental to the performanceof AnMBRs. This dissertation focuses on understanding more about the biofouling in nMBR, and to devise strategies to control or make use of these biofoulant layers. First, we aim to investigate the microbial community structure of sludge and biofilm from 13 different AnMBRs. Our findings indicate 20 sludge core genera and 12 biofilm core genera (occurrence ≥ 90% samples) could potentially account for the AnMBR performance. Sloan neutral model analysis indicates the anaerobic microbial consortium between sludge and biofilm is largely affected by stochastic dispersal and migration processes (i.e., neutral assembly), suggesting that the majority of these core genera are not selectively enriched for biofilm formation. Therefore, the second part of this dissertation aims to minimize the growth of the overall bacterial cells attached on the membranes. For this, membranes embedded with zinc oxide (ZnO) and copper oxide (CuO) nanoparticles were examined for their antifouling efficacies. Our findings indicate both CuO and ZnO nanoparticles embedded membranes could delay biofouling formation without significantly triggering the overall expression/abundance of antibiotic resistance genes (ARGs) and metal resistance genes (MRGs) in biofilm. Furthermore, CuO and ZnO nanoparticles could inhibit the expression of quorum sensing associated genes, resulting in lower quorum sensing signal molecules production. Despite the positive results demonstrated from this study as well as those from others, we recognize that no control strategies are likely to achieve total prevention of anaerobic biofouling. Therefore, the last part of this dissertation focuses on exploring the effects of different foulant layers on antibiotic-resistant bacteria (ARB) and ARGs removal. Our findings suggest both ARB and ARGs could be absorbed by membrane foulant. Transmembrane pressures and the foulant layer synergistically affected ARB removal, but the foulant layer is the main factor that contributed to ARG removal through adsorption. Overall, the collective findings could bring new insights to the anaerobic membrane biofouling phenomenon, and offer pragmatic approaches to minimize biofouling without compromising the post-AnMBR effluent quality.
  • Compliant Electronics for Unusual Environments

    Almislem, Amani Saleh Saad (2019-09) [Dissertation]
    Advisor: Hussain, Muhammad Mustafa
    Committee members: Hussain, Muhammad Mustafa; Ooi, Boon S.; Bakr, Osman; Ma, Zhenqiang (Jack)
    Compliant electronics are an emerging class of electronics which offer physical flexibility in their structure. Such mechanical flexibility opens up opportunities for wide ranging applications. Nonetheless, compliant electronics which can be functional in unusual environments are yet to be explored. Unusual environment can constitute a harsh environment where temperature and/or pressure is much higher or lower than the usual room temperature and/or pressure. Unusual environment can be an aquatic environment, such as ocean/sea/river/pond, industrial processing related liquid and bodily fluid environment, external or internal for implantable electronics. Finally, unusual environment can also be conditions when extreme physical deformation is anomalously applied to compliant electronics in order to understand their performance and reliability under such extraordinary mechanical deformations. Therefore, in this thesis, three different aspects of compliant electronics are thoroughly studied, addressing challenges of material selection/optimization for unusual environment applications, focusing on electrical performance and mechanical flexible behavior. In the first part, performance of silicon-based high-performance complementary metal oxide semiconductor (CMOS) devices are studied under severe mechanical deformation. Next, a high-volume manufacturing compatible solution is offered to reduce the usage of toxic chemicals in semiconductor device fabrication. To accomplish this, Germanium Dioxide (GeO2) is simultaneously used as transient material and dielectric layer to realize a dissolvable/bioresorbable transient electronic system which can be potentially used for implantable electronics. Finally, wide bandgap semiconductor Gallium Nitride is studied to understand its mechanical flexibility under high temperature conditions. In summary, this research contributes to the advancement of material selection, optimization and process development towards achieving compliant and transient devices for novel applications in unusual environments.
  • Skeletonization of Data for Seismic Inversion, Seismic Imaging and GPS Marker Detection

    Feng, Shihang (2019-09) [Dissertation]
    Advisor: Schuster, Gerard T.
    Committee members: Sun, Shuyu; Zhang, Xiangliang; Lin, Youzuo
    This thesis develops four skeletonization methods for seismic inversion, seismic imaging, and GPS marker detection to improve both their computational efficiency and accuracy. The first two improve the accuracy of the final inverted images by novel skeletonized inversion methods. The third one improves the quality of seismic imaging by employing skeletonized preconditional operators. The fourth one uses skeletonized data for machine learning (ML) identification of GPS markers in drone photos. Three papers are published in top applied-geophysics journals, one paper is submitted and under review, while the fifth paper is now online at ArcXiv. It will soon be submitted to the journal Remote Sensing. 1. To obtain a good starting model for anisotropic full waveform inversion (FWI), the simultaneous inversion of anisotropic parameters vp0 and ε is initially performed using the wave-equation traveltime inversion (WT) method. Then a transmission+reflection wave-equation traveltime and waveform inversion (WTW) method is presented for a vertical transverse isotropic (VTI) medium where both traveltimes and waveforms are inverted for the velocity model. 2. To mitigate the amplitude mismatch problem, multiscale phase inversion (MPI) is presented where the magnitude spectra of the predicted data are replaced by those of the observed data. Moreover, the data are integrated N times in the time domain to boost the low-frequency components. In this case, the skeletonized data are traces with the substituted magnitude spectra so that only the recorded phase data need to be inverted. 3. I have developed a velocity-independent workflow for reconstructing a high-quality zero-offset reflection section from prestack data with a deblurring filter. In this case the Hessian inverse is approximated by its skeletonized representation, also known as the deblurring operator. 4. The GPS markers are only about 0.5×0.5 m2 in size and are difficult to detect manually in the drone images. The marker has a unique hourglass shape and its color is dark. To take advantage of these features, superpixels are used as the skeletonized representations of the targets. Then a superpixel-based classification method is applied to the aerial images.
  • Reversible Formic Acid Dehydrogenation to Hydrogen and CO2 Catalyzed by Ruthenium and Rhodium Complexes

    Guan, Chao (2019-09) [Dissertation]
    Advisor: Huang, Kuo-Wei
    Committee members: Han, Yu; Lai, Zhiping; Zheng, Junrong
    Formic acid (FA) has been considered as one of the most promising materials for hydrogen storage today. The catalytic decarboxylation of formic acid ideally leads to the formation of CO2 and H2, and such CO2/H2 mixtures can be successfully applied in fuel cells. A large number of transition-metal based homogeneous catalysts with high activity and selectivity have been reported for the formic acid decarboxylation. In this presentation, we report ruthenium and rhodium complexes containing an N, N′-diimine ligand for the selective decomposition of formic acid to H2 and CO2 in water in the absence of any organic additives. Among them, the Ru complex could provide a TOF (turnover frequency) of 12 000 h–1 and a TON (turnover number) of 350 000 at 90 °C in the HCOOH/HCOONa aqueous solution. In addition to that, efficient production of high-pressure H2 and CO2 (24.0 MPa (3480 psi)) was achieved through the decomposition of formic acid with no formation of CO by this Ru complex. Moreover, well-defined ruthenium (II) PN3P pincer complexes were also developed for the reversible reaction-hydrogenation of carbon dioxide. Excellent product selectivity and catalytic activity with TOF and TON up to 13,000 h-1 and 33,000, respectively, in a THF/H2O biphasic system were achieved. Notably, effective conversion of carbon dioxide from the air into formate was conducted in the presence of an amine, allowing easy product separation and catalyst recycling.
  • Novel Surface Wave Imaging Methods

    Liu, Zhaolun (2019-09) [Dissertation]
    Advisor: Schuster, Gerard T.
    Committee members: Peter, Daniel; Santamarina, Carlos; Bruhn, Ronald L.
    I develop four novel surface-wave inversion and migration methods for reconstructing the low- and high-wavenumber components of the near-surface S-wave velocity models. 1. 3D Wave Equation Dispersion Inversion. To invert for the 3D background S-wave velocity model (low-wavenumber component), I first propose the 3D wave-equation dispersion inversion (WD) of surface waves. The results from the synthetic and field data examples show a noticeable improvement in the accuracy of the 3D tomogram compared to 2D tomographic inversion if there are significant 3D lateral velocity variations. 2. 3D Wave Equation Dispersion Inversion for Data Recorded on Rough Topography. Ignoring topography in the 3D WD method can lead to significant errors in the inverted model. To mitigate these problems, I present a 3D topographic WD (TWD) method that takes into account the topographic effects in surface-wave propagation modeled by a 3D spectral element solver. Numerical tests on both synthetic and field data demonstrate that 3D TWD can accurately invert for the S-velocity model from surface-wave data recorded on irregular topography. 3. Multiscale and layer-stripping WD. The iterative WD method can suffer from the local minimum problem when inverting seismic data from complex Earth models. To mitigate this problem, I develop a multiscale, layer-stripping method to improve the robustness and convergence rate of WD. I verify the efficacy of our new method using field Rayleigh-wave data. 4. Natural Migration of SurfaceWaves. The reflectivity images (high-wavenumber component) of the S-wave velocity model can be calculated by the natural migration (NM) method. However, its effectiveness is demonstrated only with ambient noise data. I now explore its application to data generated by controlled sources. Results with synthetic data and field data recorded over known faults validate the effectiveness of this method. Migrating the surface waves in recorded 2D and 3D data sets accurately reveals the locations of known faults.
  • Analyzing and Manipulating Wave Propagation in Complex Structures

    Al Jahdali, Rasha (2019-08-29) [Dissertation]
    Advisor: Wu, Ying
    Committee members: Keyes, David E.; Schuster, Gerard T.; Assouar, Badreddine
    The focus of this dissertation is analyzing and manipulating acoustic wave propagation in metamaterials, which can be used to assist the design of acoustic devices. Metamaterials are artificial materials, which are arranged in certain patterns at a scale smaller than the wavelength and can exhibit properties beyond those naturally occurring materials. With metamaterials, novel phenomena, such as focusing, super absorption, cloaking and localization of ultrasound, are theoretically proposed and experimentally verified. In recent years, a planar version of metamaterials, often called meta-surfaces, has attracted a great deal of attention. Meta-surfaces can control and manipulate the amplitude, phase, and directions of waves. In this dissertation, we conducted a systematic study by deriving the effective medium theories (EMTs), and developing the theoretical and numerical models for our proposed designed metamaterial. Very recently, acoustic meta-surfaces have been used in the design of acoustic lenses, which can achieve various functionalities such as focusing and collimation. In the designs of acoustic lenses, impedance is an important issue because it is usually difficult to make the impedance of the lens equal to that of the environment, and mismatched impedance is detrimental to the performance of the acoustic lens. We developed an EMT based on a coupled-mode theory and transfer matrix method to characterize the propagation behavior and, based on these models, we report two designs of acoustic lenses in water and air, respectively. They are rigid thin plates decorated with periodically distributed sub-wavelength slits. The building block of the acoustic lens in water is constructed from coiling-up spaces, and that of the acoustic lens in air is made of layered structures. We demonstrate that the impedances of the lenses are indeed matched to those of the background media. With these impedance-matched acoustic lenses, we demonstrate acoustic focusing and collimation, and redirection of transmitted acoustic energy by finite-element simulations. In the framework of the hidden source of the volume principle, an EMT for a coupled resonator structure is derived, which shows that coupled resonators are characterized by a negative value of the effective bulk modulus near the resonance frequency and induce flat bands that give rise to the confinement of the incoming wave inside the resonators. The leakage of sound waves in a resonance-based rainbow trapping device prevents the sound wave from being trapped at a specific location. Based on our EMT, we report a sound trapping device design based on coupled Helmholtz resonators, loaded to an air waveguide, to effectively tackle the wave leakage issue. We show that a coupled resonators structure can generate dips in the transmission spectrum by an analytical model derived from Newton’s second law and a numerical analysis based on the finite-element method. We compute the transmission spectra and band diagram from the effective medium theory, which are consistent with the simulation results. Trapping and the high absorption of sound wave energy are demonstrated with our designed device.
  • Asynchronous Task-Based Parallelism in Seismic Imaging and Reservoir Modeling Simulations

    AlOnazi, Amani (2019-08-26) [Dissertation]
    Advisor: Keyes, David E.
    Committee members: Knio, Omar M.; Hadwiger, Markus; Ltaief, Hatem; Badia, Rosa
    The components of high-performance systems continue to become more complex on the road to exascale. This complexity is exposed at the level of: multi/many-core CPUs, accelerators (GPUs), interconnects (horizontal communication), and memory hierarchies (vertical communication). A crucial task is designing an algorithm and a programming model that scale to the same order of the HPC system size at multiple levels. This trend in HPC architecture more critically affects memory-intensive appli- cations than compute-bound applications. Accomplishing this task involves adopting less synchronous forms of the mathematical algorithm, reducing synchronization in the computational implementation, introducing more SIMT-style concurrency at the finest level of system hierarchy, and increasing arithmetic intensity as the bottleneck shifts from number of floating-point operations to number of memory accesses. This dissertation addresses these challenges in scientific simulation focusing in the dominant kernels of a memory-bound application: sparse solvers in implicit model- ing, and I/O in explicit reverse time migration in seismic imaging. We introduce asynchronous task-based parallelism into iterative algebraic preconditioners. We also introduce a task-based framework that hides the latency of I/O with computation. This dissertation targets two main applications in the oil and gas industry: reservoir simulation and seismic imaging simulation. It presents results on multi- and many- core systems and GPUs on four Top500 supercomputers: Summit, TSUBAME 3.0, Shaheen II, and Makman-2. We introduce an asynchronous implementation of four major memory-bound kernels: Algebraic multigrid (MPI+OmpSs), tridiagonal solve (MPI+OpenMP), Additive Schwarz Preconditioned Inexact Newton (MPI+MPI), and Reverse Time Migration (StarPU/StarPU+MPI and CUDA).
  • Study of ultraviolet AlGaN nanowires light-emitting diodes

    Priante, Davide (2019-08) [Dissertation]
    Advisor: Ooi, Boon S.
    Committee members: Ooi, Boon S.; Ohkawa, Kazuhiro; Schwingenschlögl, Udo; Mi,Zetian
    Ultraviolet (UV) group III-Nitride-based light emitters have been used in various applications such as water purification, medicine, lighting and chemical detection. Despite attractive properties such as bandgap tunability in the whole UV range (UV-C to UV-A), high chemical stability and relative low cost, the low quantum efficiency hamper the full utilization. In fact, external quantum efficiencies of UV devices are below 10 % for emission wavelength shorter than 350 nm. This thesis aims to show alternative solutions to such problems by employing nanowires (NWs) structures, and target the eventual application of reliable and high power NWs-based light-emitting devices, enabling large-scale production using the established silicon foundry processes. Here, we present the improvement of injection current and optical power of AlGaN NWs LEDs by involving a metal bilayer thin film with a dual purpose: eliminate the potential barrier for carrier transport, and inhibit the formation of silicide. We then study the AlGaN/GaN UV LED design to optimize the device structure and improve the LED performance. We compared multiple devices having different active region and graded layers’ thicknesses. Improvement on the output power was achieved for larger p-AlGaN graded layer and thinner p-GaN contact layer structure due to the better hole injection and lower p-GaN absorption. The junction temperature of AlGaN-based NWs LEDs on metal bi-layer and silicon is also presented as a crucial parameter affecting the device efficiency, chromaticity and reliability. In this regard, by using the forward-voltage and peak-shift method we extracted the junction temperature values and confirmed the better heat dissipation in NWs grown on metal substrate. Finally, the origin of single and ensemble NWs current injection and injection efficiency are studied by treating the AlGaN NWs with KOH solution. Measurements based on conductive atomic force microscopy enabled a fast feedback cycle without fabricating the device. Despite the NWs technology is still at its infancy compared to the matured planar, we believe that this research effort will give important insight in advancing the AlGaN NWs devices for future industrial employment.
  • Designing Surfaces for Enhanced Water Condensation and Evaporation

    Jin, Yong (2019-08) [Dissertation]
    Advisor: Wang, Peng
    Committee members: Nunes, Suzana Pereira; Lai, Zhiping; Wang, Zuankai
    With the increasing pressure of providing reliable potable water in a sustainable way, it is important to understand water phase change phenomena (condensation and evaporation) as the water phase change is involved in many processes such as membrane distillation and solar still which can be a feasible choice of supplementing the present potable water access. In the present thesis, we first elucidate the role of wettability of water condensation substrate by combining the droplet growth dynamics and droplet population evolution. The results show that wettability has a negligible effect on water condensation rate in an atmospheric environment. After confirming the role of substrate wettability, we provide a quantitative analysis of the effect of substrate geometry on water condensation in the atmospheric environment. The analysis can help to predict the efficiency of water condensation rate with a given substrate of a certain geometry with the aid of computational simulation tools. The results show that water condensation can be increased by 40% by rationally designing the geometry of the condensation surface. However, the condensation rate in the atmospheric environment is relatively slow due to the presence of non-condensable gas. In order to increase the condensation rate, a relatively pure vapor environment is desired, in which condensed water will be the major heat transfer barrier. Coalescence induced jumping of condensed droplets on superhydrophobic surfaces is an interesting phenomenon to help faster removal of condensed droplets. However, it is still not clear how to optimize the overall heat transfer efficiency by condensation on such surfaces. We observed an interesting phenomenon on a superhydrophobic nano-cones array, on which water preferentially condenses within larger cavities among the nanocones. Droplets growing form larger cavities have larger growth rate. This finding can possibly provide a solution to optimizing heat transfer efficiency. Finally, a nylon-carbon black composite is prepared by electrospinning to enhance water evaporation under solar radiation. The composite shows an interesting light absorption property. In a wet state, the composite can absorb around 94% of the incident sunlight. The composite also shows strong mechanical and chemical stability. Thus, the composite is considered to be a practical candidate to be applied in the solar distillation process.
  • The role of NAC transcription factors in responses of plants to heat and salt stresses

    Alshareef, Nouf Owdah Hameed (2019-08) [Dissertation]
    Advisor: Tester, Mark
    Committee members: Blilou, Ikram; Pain, Arnab; Balazadeh, Salma
    Soil salinity and heat stress are two major abiotic stresses affecting plant growth and yield. Transcription factors (TFs) are key regulators in stress responses. They link stress sensing with many tolerance mechanisms by translating stress signals into changes in gene expression that ultimately contribute to stress tolerance. The NAC (NAM, ATAF and CUC) TF family have been found to be involved in responses to biotic and abiotic stresses. In this PhD project, the role of NAC TFs in response to heat and salt stress was studied in the model system Arabidopsis thaliana (Arabidopsis), and in two agriculturally relevant species, Solanum lycopersicum (tomato) and Chenopodium quinoa (quinoa). Plants have the ability to acquire thermotolerance if they are pre-exposed to a mild, non-lethal high temperature. The maintenance of acquired thermotolerance for several days is known as thermomemory. Here we investigated the role of NAC TFs in thermotolerance. The expression profiles of 104 Arabidopsis NAC TFs were measured and compared between primed and unprimed plants. Some NACs with a distinctive expression pattern in response to thermopriming were selected for further phenotypic analysis. Knock-out (KO) mutants of the ATAF1 gene showed an enhanced thermomemory phenotype compared with wild type plants (WT) and from this work, the functions of the ATAF1 gene were studied further. RNAseq co-expression analyses of ATAF1 overexpressor and ataf1 KO plants found that ANAC055 expression was co-regulated with that of ATAF1. JUBGBRUNNEN1 (JUB1) is another NAC TF involved in responses to heat, drought and salinity. In this study, the role of AtJUB1 overexpression in salinity was investigated in tomato plants. AtJUB1 overexpression resulted in higher proline levels and improved maintenance of water content and biomass in AtJUB1-overexpressing plants grown hydroponically under salinity compared with WT plants. Quinoa has recently gained much attention because of its high nutritional value and high tolerance to several stresses including drought and salinity. NAC TFs are hypothesized to play a major role in quinoa’s tolerance to abiotic stresses. In this study, the NAC TFs family were identified and investigated in the genome of quinoa. 107 NAC TF genes were identified and their transcriptional responses to different stresses including salt, drought and heat were investigated.
  • Identification of MALAT1 as a PRC2-Ezh1 Associated lncRNA Essential for Epigenetic Control of Skeletal Muscle Adaptation and Plasticity

    El Said, Nadine H. (2019-08) [Dissertation]
    Advisor: Orlando, Valerio
    Committee members: Froekjaer Jensen, Christian; Tester, Mark; Rinn, John
    Polycomb Proteins (PcG) are chromatin proteins that control the maintenance of “transcriptional memory” and cell identity by fixing the repressed state of developmentally regulated genes. This function has been linked to interaction with RNA moieties, in particular long non-coding RNAs (lncRNAs). However, specificity of PcG-RNA interactions has been controversial (Beltran et al., 2016; Chen Davidovich, Leon Zheng, Karen J. Goodrich, & Thomas R. Cech, 2013). In this study we took advantage of recent work published from our lab reporting about a novel and reversible mechanism regulating genome wide Ezh1-PRC2 activation in mouse skeletal muscle cells in response to atrophic stress (Bodega et al., 2017). Using this physiological, in vivo tool we could identify a functional dynamic crosstalk between Malat1 (Metastasis Associated Lung Adenocarcinoma Transcript 1) and PRC2-Ezh1 complex. By combining immuno-fluorescence, biochemistry, epigenomics, ChIRP, DNA and RNA immunoprecipitation we identified a novel pathway in which Malat1 plays a role in compartmentalization, assembly and activity of PRC2 in chromatin, allowing epigenetic plastic response to atrophic stress and recovery. We conclude that Malat1 is an essential partner for PRC2-Ezh1 adaptive function in skeletal muscle cells.
  • Traffic Monitoring and MAC-Layer Design for Future IoT Systems

    Odat, Enas M. (2019-08) [Dissertation]
    Advisor: Shamma, Jeff S.
    Committee members: Shihada, Basem; Al-Naffouri, Tareq Y.; Pavel, Lacra
    The advances in the technology and the emergence of low complexity intelligent devices result in the evolution of the Internet-of-Things (IoT). In most IoT application scenarios, billions of things are interconnected together using standard communication protocols to provide services for different applications in the healthcare industry, smart cities, transportation, and food supply chain. Despite their advantage of connecting things anywhere, anytime, and anyplace, IoT presents many challenges due to the heterogeneity, density, the power constraints of things, and the dynamic nature of the network that things might connect and disconnect at any time. All of these increase the communication delay and the generated data, and it is thereby necessary to develop resource management solutions for the applications in IoT. One of the most important resources is the wireless channel, which is a shared resource; thus, it is necessary for the nodes to have methods that schedule channel access. This thesis considers the problem of distributed sensing and channel access in the context of IoT systems, where a set of selfish nodes competes for transmission opportunities. In the channel access part, a memory-one channel access game is proposed to reduce the collision rate, to enhance the cooperation among the nodes, and to maximize their payoffs by optimizing their channel access probabilities, based on the channel state in the previous time step. To overcome the communication cost overhead in the network and to solve the problem efficiently, the nodes use distributed learning algorithms. Next, the problem is extended to include energy constraints on the transmission decisions of the nodes, where each one of them has a battery of finite capacity, which is replenished by an energy-harvesting process. This constrained problem is solved using energy-aware channel access games under different scenarios of perfect and imperfect information. In the distributed sensing part, a traffic-monitoring system, integrated into a WSN, is proposed as a potential application to implement the channel access solution. This system maximizes the privacy of the sensed traffic by using low-cost and low-power sensor devices that integrate passive infrared sensors (PIR) and ultrasonic range finders. To estimate the parameters required to solve the real-time monitoring problem (vehicle detection, classification, and speed estimation), the measurements of these sensors are analyzed using a set of optimized machine-learning algorithms. The selection of these algorithms is due to the continuous variation of the sensed environment over time, the lack of the system state dynamic models, and the limitation in the resources.
  • Understanding the Molecular Basis of Thermopriming in Plants

    Serano, Natalia Lorena Gorron (2019-08) [Dissertation]
    Advisor: Mahfouz, Magdy
    Committee members: Blilou, Ikram; Saikaly, Pascal; Benhamed, Moussa
    Plants acclimate to the changing environmental conditions by adjusting their molecular responses at different molecular levels including genome, epigenome, transcriptome, metabolome, and proteome levels to ensure survival. Plants adapt to abiotic stresses by establishing a ‘stress memory’ of previous exposures to mild stresses. Stress memory helps plants to develop tolerance and survive recurring exposures to the stress conditions. This memory establishes a new cellular state that differs from the state of unexposed naïve plants. This process is known as priming. Priming and the stress memory give the plants the possibility to acclimate to different biotic and abiotic stress conditions. The acquisition and maintenance of the stress-memory are two separate processes and crucial for successful tolerance to subsequent stress conditions. Priming promises to improve plant performance under severe stress conditions and enhance food production. Therefore, understanding the molecular basis of heat stress priming and stress-induced memory is of vital importance to plant biology. In this thesis, I investigated the role of transcriptional, post-transcriptional and metabolomic regulation controlling plant responses to heat stress, one of the major abiotic stresses affecting agriculture. I designed and established a heat stress priming strategy which reveals that heat stress-induced priming leads to the establishment of heat stress memory that permits plants to survive lethal temperatures. In this thesis, I analyzed the genome-wide differential gene expression, the alternative splicing patterns and regulation, and the reprogramming of the metabolic homeostasis that reprogram the establishment of the heat stress priming and stress-memory. I identified a set of candidate genes and metabolites playing key roles in the establishment of heat stress-induced memory. Intriguingly, it was possible also to establish a link between alternative splicing patterns and heat stress-induced memory. Subsequently, the knowledge of heat stress priming in Arabidopsis was translated into tomato crop plants, to improve their heat stress tolerance. This work enhances our understanding of the molecular basis of heat stress priming, and the establishment of heat stress memory, at transcriptional, post-transcriptional, and metabolomic levels. These findings can be translated into crop species to improve their survival under recurring heat stress conditions to improve world agriculture and food security.
  • Supervirtual Interferometry and Seismic Imaging of the Oludvai Gorge, Tanzania

    Lu, Kai (2019-08) [Dissertation]
    Advisor: Schuster, Gerard T.
    Committee members: Mai, Paul Martin; Afifi, Abdulkader; Hanafy, Sherif M.; Chavez- Perez, Sergio
    ABSTRACT This dissertation presents three novel applications of supervirtual interferometry (SVI) to seismic data: 3D refraction SVI, reection SVI, and the combination of machine learning and SVI for automatic picking of SVI arrival times. The most important scienti c nding of this dissertation is that I applied refraction SVI to seismic data recorded over the Olduvai Basin, Tanzania to more than double the source-receiver o set of pickable refraction arrivals. This allows us, for the rst time, to delineate the depth of the bedrock in the Olduvai Basin. The maximum basin thickness is around 405 m, which has important palaeoantropological signi cance in that hominin palaeoenvironments in the Olduvai Basin possibly date back to 4 Ma, almost doubling the age estimated from the previous coring logs. The seismic results also map synsedimentary normal faults and trace individual strata at depth for the Olduvai Basin.
  • Reactivity and Properties of the PN 3P Pincer Platform Insights from Computations and Spectroscopy

    Munkerup, Kristin (2019-08) [Dissertation]
    Advisors: Huang, Kuo-Wei; Tung
    Committee members: Cavallo, Luigi; Tung, Vincent; Sølling, Theis I
    Abstract: Pincer compounds are organometallic complexes with intriguing tunable reactivities. In this work we explore their unique properties and reactivities through spectroscopic and computational investigations, with a focus on the PN3P pincer platform. First, we conducted a computational study on five pincer complexes with stereogenic phosphine arms that have multiple well-defined rotamers. Significant energy differences could be found between the lowest and highest energy rotamer in each set of pincer complexes. All rotamers for reactant, transition state, and product, were evaluated in a reaction energy profile of a CO2 reduction by a pincer nickel hydride, and we found that this reaction could be found either favorable or unfavorable, depending on the choice of rotamer. A software to generate rotamers has been developed and applied to the work presented in this part. The zwitterionic aromatic resonance form has a large contribution in the dearomatized PN3P* nickel pincer complexes, which is demonstrated by the imine arm's ability to act as an organic σ-donor, similar to NHC catalysts. Related to this property, as well as the pincer compound's ability to undergo metal-ligand cooperation catalysis, is the basicity (or acidity) of pincer ligand spacer arms. Therefore, we have determined the Brønsted basicity of the imine arm in three PN3P* nickel pincer complexes in THF. The relative basicity was found to be strongly influenced by the X ligand trans to the PN3P* ligand, and less by alkyl groups on phosphine donor arms. Finally, we explored the reactivity between a PN3P* rhodium carbonyl pincer complex and dioxygen at room temperature in solution, and at elevated temperature in the solid state. Intriguingly, the singlet PN3P* rhodium carbonyl complex reacts with the triplet dioxygen both in solution and in the solid state to afford oxidation on the ligand backbone. This is possible due to the ligands ability to do a single-electron transfer to dioxygen. The solid state reaction was studied with in situ rhodium K-edge X-ray absorption spectroscopy under dioxygen flow, where an isobestic point was observed, and simulation studies support formation of a Rh-O2 adduct. In situ FTIR studies in a static dioxygen environment revealed that the PN3P* rhodium carbonyl complex is able to facilitate the incorporation of O2 into CO and CO2.
  • SPECTRUM MANAGEMENT FOR FUTURE GENERATIONS OF CELLULAR NETWORKS

    Randrianantenaina, Itsikiantsoa (2019-08) [Dissertation]
    Advisor: Alouini, Mohamed-Slim
    Committee members: Alouini, Mohamed-Slim; Ghanem, Bernard; Shihada, Basem; Dahrouj, Hayssam; Bennis, Mehdi
    The demand for wireless communication is ceaselessly increasing in terms of the number of subscribers and services. Future generations of cellular networks are expected to allow not only humans but also machines to be immersively connected. However, the radio frequency spectrum is already fully allocated. Therefore, developing techniques to increase spectrum efficiency has become necessary. This dissertation analyzes two spectrum sharing techniques that enable efficient utilization of the available radio resources in cellular networks. The first technique, called full-duplex (FD) communication, uses the same spectrum to transmit and receive simultaneously. Using stochastic geometry tools, we derive a closed-form expression of an upper-bound for the maximum achievable uplink ergodic rate in FD cellular networks. We show that the uplink transmission is vulnerable to the new interference introduced by FD communications (interference from the downlink transmission in other cells), especially when the disparity in transmission power between the uplink and downlink is considerable. We further show that adjusting the uplink transmission power according to the interference power level and the channel gain can improve the uplink performance in full-duplex cellular networks. Moreover, we propose an interference management technique that allows a flexible overlap between the spectra occupied by the downlink and uplink transmissions. The flexible overlap is optimized along with the user-to-base station association, the power allocation and the channel allocation in order to maximize a network-wide utility function. The second spectrum sharing technique, called non-orthogonal multiple access (NOMA), allows a transmitter to communicate with multiple receivers through the same frequency-time resource unit. We analyze the implementation of such a scheme in the downlink of cellular networks, more precisely, in the downlink of fog radio access networks (FogRANs). FogRAN is a network architecture that takes full advantage of the edge devices capability to process and store data. We propose managing the interference for NOMA-based FogRAN to improve the network performance by jointly optimizing user scheduling, the power allocated to each resource block and the division of power between the multiplexed users. The simulation results show that significant performance gains can be achieved through proper resource allocation with both studied spectrum sharing techniques.
  • Analysis and Optimization of Massive MIMO Systems via Random Matrix Theory Approaches

    Boukhedimi, Ikram (2019-08-01) [Dissertation]
    Advisor: Alouini, Mohamed-Slim
    Committee members: Alouini, Mohamed-Slim; Laleg-Kirati, Taous-Meriem; Al-Naffouri, Tareq Y.; Kammoun,Alba; Li, Yonghui
    By endowing the base station with hundreds of antennas and relying on spatial multiplexing, massive multiple-input-multiple-output (MIMO) allows impressive advantages in many fronts. To reduce this promising technology to reality, thorough performance analysis has to be conducted. Along this line, this work is focused on the convenient high-dimensionality of massive MIMO's corresponding model. Indeed, the large number of antennas allows us to harness asymptotic results from Random Matrix Theory to provide accurate approximations of the main performance metrics. The derivations yield simple closed-form expressions that can be easily interpreted and manipulated in contrast to their alternative random equivalents. Accordingly, in this dissertation, we investigate and optimize the performance of massive MIMO in di erent contexts. First, we explore the spectral e ciency of massive MIMO in large-scale multi-tier heterogeneous networks that aim at network densi cation. This latter is epitomized by the joint implementation of massive MIMO and small cells to reap their bene ts. Our interest is on the design of coordinated beamforming that mitigates cross-tier interference. Thus, we propose a regularized SLNR-based precoding in which the regularization factor is used to allow better resilience to channel estimation errors. Second, we move to studying massive MIMO under Line-of-Sight (LoS) propagation conditions. To this end, we carry out an analysis of the uplink (UL) of a massive MIMO system with per-user channel correlation and Rician factor. We start by analyzing conventional processing schemes such as LMMSE and MRC under training-based imperfect-channel-estimates, and then, propose a statistical combining technique that is more suitable in LoS-prevailing environments. Finally, we look into the interplay between LoS and the fundamental limitation of massive MIMO systems, namely, pilot contamination. We propose to analyze and compare the performance using single-cell and multi-cell detection methods. In this regard, the single-cell schemes are shown to produce higher SEs as the LoS strengthens, yet remain hindered by LoS-induced interference and pilot contamination. In contrast, for multi-cell combining, we analytically demonstrate that M-MMSE outperforms both single-cell detectors by generating a capacity that scales linearly with the number of antennas, and is further enhanced with LoS.
  • Anatase Titanium Dioxide with Exposed {001} Facets as a Support for Molecular Catalysts: Surface Characterization and Application in Photocatalysis

    Jeantelot, Gabriel (2019-08) [Dissertation]
    Advisor: Basset, Jean-Marie
    Committee members: Cavallo, Luigi; Manchon, Aurelien; Astruc, Didier
    A specific allotrope of titanium dioxide (anatase) was synthesized with a highly anisotropic morphology ({001}-anatase) dominated by the {001} facet (81%). its surface chemistry after dehydroxylation was studied by 1H NMR and FT-IR. Influence of surface fluorides on surface chemistry was also studied by 1H NMR, FT-IR and DFT. Full attribution of the IR and NMR spectra of anatase with dominant {001} facets could be provided based on experimental data and further confirmed by DFT. Our results showed that chemisorbed H2O are still present on anatase after dehydroxylation at 350°C, and that the type of surface hydroxyls present on the {001} facet is dependent on the presence of fluorides. They also provided general insight on the nature of surface species on both fluorinated and fluorine-free anatase. The use of vanadium oxychloride (VOCl3) allowed determining the accessibility of the various OH groups spectroscopically observed. A platinum complex, (CH3)2Pt(COD), is then grafted via surface organometallic chemistry (SOMC) on morphology-controlled Anatase TiO2 to generate single, isolated Pt atoms on TiO2 nano-platelets. The resulting material is characterized by FT-IR, High resolution scanning transmission electron microscopy (HRSTEM), NMR, and XAS, and then used to perform photocatalytic water splitting. The photocatalyst with SOMC-grafted Pt shows superior performance in photocatalytic hydrogen evolution and strongly suppresses backwards reaction of H2 and O2 forming H2O under dark conditions, compared to photocatalyst prepared by standard wet impregnation at the same Pt loading. However, single Pt atoms on this surface also rapidly coalesce into nanoparticles under photocatalytic conditions. It was also found that adsorbtion of carbon monoxide gas at room temperature also triggers the aggregation of Pt single atoms into nanoparticles. A detailed mechanism is investigated for the mobility of Pt in the formation of its carbonyls using density functional theory (DFT) calculations.
  • Tomographic Measurements of Turbulent Flow through a Contraction

    Mugundhan, Vivek (2019-08) [Dissertation]
    Advisor: Thoroddsen, Sigurdur T
    Committee members: Samtaney, Ravi; Farooq, Aamir; Knio, Omar; Armann Gylfason
    We investigate experimentally the turbulent flow through a two-dimensional contraction. Using a water tunnel with an active grid we generate turbulence at Taylor microscale Reynolds number Reλ ~ 250 which is advected through a 2.5:1 contraction. Volumetric and time-resolved Tomo-PIV and Shake-The-Box velocity measurements are used to characterize the evolution of coherent vortical structures at three streamwise locations upstream of, and within the contraction. We confirm the conceptual picture of coherent large-scale vortices being stretched and aligned with the mean rate of strain. This alignment of the vortices with the tunnel centerline is stronger compared to the alignment of vorticity with the large-scale strain observed in numerical simulations of homogeneous turbulence. We judge this by the peak probability magnitudes of these alignments. This result is robust and independent of the grid-rotation protocols. On the other hand, while the point-wise vorticity vector also, to a lesser extent, aligns with the mean strain, it principally remains aligned with the intermediate eigen-vector of the local instantaneous strain-rate tensor, as is known in other turbulent flows. These results persist when the distance from the grid to the entrance of the contraction is doubled, showing that modest transverse inhomogeneities do not significantly affect these vortical-orientation results.

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