Yes / Existing works on energy harvesting wireless systems often assume a constant conversion efficiency for the energy harvester. In practice, the conversion efficiency often varies with the input power. In this work, based on a review of existing energy harvesters in the literature, a heuristic expression for the conversion efficiency as a function of the input power is derived by curve fitting. Using this function, two example energy harvesters are used to analyze the realistic performances of wireless relaying and wireless energy transfer. Numerical results show that the realistic performances of the wireless systems could be considerably different from what predicted by the existing analysis.

Chapter 1: The unique chemical and biological properties of porphyrins have led to increased interest in the development of porphyrin-based materials. Metal organic frameworks (MOFs) can act as a scaffold for the immobilization of porphyrins in desired arrangements. The crystalline nature of MOFs allows for control over spatial arrangement of porphyrins and the local environment of the porphyrin molecules. This opens up the possibility of conducting systematic studies aimed at exploring structure-property relationships. Several strategies for the design and synthesis of porphyrin-based frameworks have been developed over the last two decades, such as, the pillared-layer strategy, construction of nanoscopic metal-organic polyhedrals (MOPs), post-synthetic modification, etc. These strategies provide an opportunity to engineer porphyrin-based MOFs that can target a specific application or serve as multi-functional assemblies. Porphyrin-based MOFs provide a tunable platform to perform a wide variety of functions ranging from gas adsorption, catalysis and light harvesting. The versatile nature of these frameworks can be exploited by incorporating them in multi-functional assemblies that mimic biological and enzymatic systems. Nano-thin film fabrication of porphyrin-based MOFs broadens their application range, making it possible to use them in the construction of photovoltaic and electronic devices. Chapter 2: The reaction of zirconium salts with meso-tetracarboxyphenylporphyrin (TCPP) in the presence of different modulators results in the formation of a diverse set of metal-organic frameworks (MOFs), each displaying distinct crystalline topologies. However, synthesis of phase-pure crystalline frameworks is challenging due to the concurrent formation of polymorphs. The acidity and concentration of modulator greatly influence the outcome of the MOF synthesis. By systematically varying these two parameters, selective framework formation can be achieved. In the present study, we aimed to elucidate the effect of modulator on the synthesis of zirconium-based TCPP MOFs. With the help of powder X-ray diffraction (PXRD) and scanning electron microscopy (SEM), modulator candidates and the optimal synthetic conditions yielding phase-pure PCN-222, PCN-223 and MOF-525 were identified. 1H NMR analysis, TGA and N2 gas adsorption measurements were performed on select MOFs to gain insight into the relationship between their defectivity and modulator properties. Chapter 3: Singlet-singlet energy transfer in PCN-223(free-base), a highly stable Zr-MOF based on meso-tetrakis(4-carboxyphenyl)porphyrin was investigated, using diffuse reflectance spectroscopy, steady-state emission spectroscopy, time-correlated single photon counting (TCSPC) spectroscopy and nanosecond transient absorption spectroscopy. The effects of the surrounding media and temperature on the excited-state properties of PCN-223(fb) were explored to understand the mechanistic aspects of energy transfer. Stern-Volmer photoluminescence quenching of PCN-223(fb) suspensions was performed to extract quenching rate constants and gain insight into the efficiency of energy transfer. Chapter 4: The fourth chapter of this thesis is adapted from chapter 14 of the book "Elaboration and Applications of Metal-Organic Frameworks" authored by Jie Zhu, Shaunak Shaikh, Nicholas J Mayhall and Amanda J Morris. This chapter summarizes the fundamental principles of energy transfer in MOFs and provides an overview of energy transfer in lanthanide-Based luminescent MOFs, Ru/Os-Based MOFs, porphyrin- and metalloporphyrin-based MOF materials, and nonporphyrinic, organic chromophore-based MOFs. / Master of Science / Metal Organic frameworks (MOFs) composed of Zirconium-oxo clusters connected through meso-tetra(4-carboxyphenyl)porphyrin (TCPP) linker molecules have emerged as promising solid-state materials because of their unique structural features and diverse applications. Although these MOFs have demonstrated great potential over the years, synthesizing them in phase-pure form has proven to be very challenging as they are susceptible to polymorphism. Syntheses of these frameworks often result in phase mixtures and have poor reproducibility. To address, this issue, we conducted a systematic exploration of the synthetic parameter landscape to identify reaction conditions for the synthesis of phase-pure Zirconium-based porphyrin MOFs, and to gain deeper insights into the factors governing the formation of these MOFs. We also investigated the defectivity of pristine Zr-TCPP MOFs using a variety of techniques, including 1H NMR spectroscopy, thermogravimetric analysis (TGA), inductively coupled plasma mass spectrometry (ICP-MS), and Nitrogen gas adsorption/desorption measurements. The long-term goal of this project is to use phase-pure Zr-based porphyrin MOFs as model systems to study energy transfer in three dimensional structures. To achieve this goal, we characterized the photophysical properties of PCN-223(fb) (a Zr-based porphyrin MOF) using a variety of techniques including steady-state photoluminescence spectroscopy, time-resolved photoluminescence spectroscopy, nanosecond transient absorption spectroscopy and femtosecond transient absorption spectroscopy. Understanding the mechanistic aspects of energy transfer in PCN-223(fb) can pave the way for the design of a new generation of solar energy conversion devices.

Over the past fifteen years, advances in Micro-Electro-Mechanical Systems (MEMS) technology have enabled rapid development of wireless sensor networks (WSNs). A WSN consists of a large number of sensor nodes that are typically powered by batteries. Each sensor node collects useful information from its environment, and forwards this data to a base station through wireless communications. Applications of WSNs include environmental monitoring, industrial monitoring, agriculture, smart home monitoring, military surveillance, to name a few. Due to battery constraint at each sensor node, a fundamental challenge for a WSN is its limited operational lifetime -- the amount of time that the network can remain operational before some or all of the sensor nodes run out of battery. To conserve energy and prolong the lifetime of a WSN, there have been active research efforts across all network layers. Although these efforts help conserve energy usage and prolong network lifetime to some extent, energy and lifetime remain fundamental bottlenecks and are the key factors that hinder the wide-scale deployment of WSNs. This dissertation addresses the energy problem of a WSN by exploiting a recent breakthrough in wireless energy transfer (WET) technology. This breakthrough WET technology is based on the so-called magnetic resonant coupling (MRC), which allows electric energy to be transferred from a source coil to a receive coil without any plugs or wires. The advantages of MRC are high energy transfer efficiency even under omni-direction, not requiring line-of-sight (LOS), and being robust against environmental conditions. Inspired by this enabling WET technology, this dissertation focuses on applying MRC to a WSN and on studying how to optimally use this technology to address lifetime problem for a WSN. The goal is to fundamentally remove lifetime bottleneck for a WSN. The main contributions of this dissertation are summarized as follows: 1. Single-node Charging for a Sparse WSN. We first investigate how MRC can be applied to a WSN so as to remove the lifetime performance bottleneck in a WSN, i.e., allowing a WSN to remain operational forever. We consider the scenario of a mobile wireless charging vehicle (WCV) periodically traveling inside the sensor network and charging each sensor node's battery wirelessly. We introduce the concept of renewable energy cycle and offer both necessary and sufficient conditions for a sensor node to maintain its renewable energy cycle. We study an optimization problem, with the objective of maximizing the ratio of the WCV's vacation time over the cycle time. For this problem, we prove that the optimal traveling path for the WCV is the shortest Hamiltonian cycle and uncover a number of important properties. Subsequently, we develop a near-optimal solution by a piecewise linear approximation technique and prove its performance guarantee. This first study shows that network lifetime bottleneck can be fundamentally resolved by WET. 2. Multi-node Charging for a Dense WSN. We next exploit recent advances in MRC that allows multiple sensor nodes to be charged at the same time, and show how MRC with multi-node charging capability can address the scalability problem associated with the single-node charging technology. We consider a WCV that periodically travels inside a WSN and can charge multiple sensor nodes simultaneously. Based on the charging range of the WCV, we propose a cellular structure that partitions the two-dimensional plane into adjacent hexagonal cells. We pursue a formal optimization framework by jointly optimizing the traveling path of the WCV, flow routing among the sensor nodes, and the charging time with each hexagonal cell. By employing discretization and a novel Reformulation-Linearization Technique (RLT), we develop a provably near-optimal solution for any desired level of accuracy. Through numerical results, we demonstrate that our solution can indeed address the scalability problem for WET in a dense WSN. 3. Bundling Mobile Base Station and Wireless Energy Transfer: The Pre-planned Path Case. Our aforementioned work is based on the assumption that the location of base station is fixed and known in the WSN. On the other hand, it has been recognized that a mobile base station (MBS) can offer significant advantages over a fixed one. But employing two separate vehicles, one for WET and one for MBS, could be expensive and hard to manage. So a natural question to ask is: can we bundle WET and MBS on the same vehicle? This is the focus of this study. Here, our goal is to minimize energy consumption of the entire system while ensuring that none of the sensor nodes runs out of energy. To simplify the problem, we assume that the path for the vehicle is given a priori. We develop a mathematical model for this problem. Instead of studying the general problem formulation (called CoP-t), which is time-dependent, we show that it is sufficient to study a special subproblem (called CoP-s), which only involves space-dependent variables. Subsequently, we develop a provable near-optimal solution to CoP-s with the development of several novel techniques including discretizing a continuous path into a finite number of segments and representing each segment with worst-case energy bounds. 4. Bundling Mobile Base Station and Wireless Energy Transfer: The Unconstrained Path Case. Based on our experience for the pre-planned path case, we further study the problem where the traveling path of the WCV (also carrying the MBS) can be unconstrained. That is, we study an optimization problem that jointly optimizes the traveling path, stopping points, charging schedule, and flow routing. For this problem, we propose a two-step solution. First, we study an idealized problem that assumes zero traveling time, and develop a provably near-optimal solution to this idealized problem. In the second step, we show how to develop a practical solution with non-zero traveling time and quantify the performance gap between this solution and the unknown optimal solution to the original problem. This dissertation offers the first systematic investigation on how WET (in particular, the MRC technology) can be exploited to address lifetime bottleneck of a WSN. It lays the foundation of exploring WET for WSNs and other energy-constrained wireless networks. On the mathematical side, we have developed or applied a number of powerful techniques such as piecewise linear approximation, RLT, time-space transformation, discretization, and logical point representation that may be applicable to address a broad class of optimization problems in wireless networks. We expect that this dissertation will open up new research directions on many interesting networking problems that can take advantage of the WET technology. / Ph. D.

The phenomenon of transition in a laminar flow has been a topic of continued interest for many years. Recent experiments in shear flows have revealed a series of instabilities that lead to breakdown to turbulence. We have completed an analysis of the mechanisms which drive the primary (TS wave) and secondary instabilities in plane Poiseuille flow. This was accomplished by studying the solutions of linear primary and secondary stability theory with energy methods. We found that primary instability occurred when the viscous stresses overpowered dissipative forces near the channel walls. For the secondary instability, we saw that the TS wave catalyzes the instability and then mediates the transfer of brge amounts of energy from the mean flow into the three-dimensional disturbance, thus driving the instability. In addition, we have compiled an extensive catalog of the loc!l.l energy and vorticity field distributions which result from each instability. / Master of Science

The synthesis, characterization and photophysical properties of three Zr-based Metalorganic frameworks (MOFs) assembled from 2,6-anthracenedicarboxylic acid (2,6-ADCA, 2,6- MOF) and 1,4-anthracenedicarboxylic (1,4-ADCA, 1,4-MOF), and 9,10-anthracenedicarboxylic acid (9,10-ADCA, 9,10-MOF) are described. The crystal structure of the 9,10-MOF was elucidated by synchrotron powder X-ray diffraction (PXRD) analysis and is isostructural with the well-known UiO-66 framework. The 2,6-MOFs also form highly crystalline, octahedral-shaped structures and was characterized by PXRD. Le Bail refinement of the powder pattern revealed that the 2,6-MOF also has UiO-type crystal structure. Conversely, incorporation of the 1,4-ADCA ligand results in large rod-shaped crystals. The excited-state properties of the MOFs were examined using steadstate diffuse reflectance, steady-state emission spectroscopy and time-correlated single photon counting (TCSPC) spectroscopy and are compared to those of the corresponding ligand in solution. Both the unique fluorescent properties of the ligand as well as individual framework structure, result in distinctive luminescent behavior and dictate the extent of intermolecular interactions. Specifically, the 2,6-MOF displays monomeric emission with a fluorescence lifetime (t) of 16.6 ± 1.1 and fluorescence quantum yield (Ff). On the other hand, the 1,4-MOF displays both monomeric and excimeric emission, with corresponding lifetime values of 7.5 ± 0.01 and 19.9 ± 0.1, respectively and a quantum yield of 0.002 ± 0.0001. The propensity for photon upconversion through sensitized triplet-triplet annihilation (TTA-UC) was probed in the three anthracene-based MOFs. The MOFs were surface-modified with Pd(II) mesoporphyrin IX (PdMP) as the triplet sensitizer. Upconverted emission from the 9,10-MOF was observed, with a quantum efficiency (FUC) of 0.46 % and a threshold intensity (Ith) of 142 mW/cm2 . The variation of the spacing between the anthracene units in the MOFs was found to have significant impact on TTA-UC. As a result, upconverted emission is only displayed by the 9-10-MOF. The distance between anthracene linkers in the 2,6-MOF are too large for TTA to occur, while the short distances in the 1,4-MOF inhibit upconversion through competitive excimer formation. To further explore the effects of chromophore spacing on energy transfer processes, a series of zinc-based mixed-ligand MOF were constructed from Zn(II) tetrakis(4- carboxyphenyl)porphyrin (ZnTCPP) and pyrazine, 2,2′-bipyridine (pyz) or 4,4′-bipyridyl (bpy) or 1,4-di(4-pyridyl)benzense (dpbz), comprising ZnTCPP/Zn paddlewheel layers. Across this series, the porphyrin spacing was approximately 6 Å, 11 Å and 16 Å for pyz, bpy and dpbz, respectively. The photophysical properties of the MOFs were explored using stead-state diffuse reflectance spectroscopy and steady-state and time-resolved emission spectroscopies. Florescence quenching studies examined the correlation between porphyrin spacing and efficiency of energy transfer. / Ph. D.

Surface plasmon resonance has become a widely investigated phenomenon in the past few years. Initially descriptive of light interactions with metallic films, research has branched out to encompass the nanoparticles as well. Generation of the maximum surface plasmon resonance for nanostructures is based on the resonance condition that the oscillatory behavior of the 'free' electrons on the surface of the particle become equivalent to the frequency of the excitation light; for films this required a specific geometry. Metallic nanoparticles have also interested researchers because of their unique optical properties. Depending on the metal, observations of quenching as well as fluorescence enhancement have been reported. Based on the phenomenon of surface plasmon resonance as well as the properties of metallic nanoparticles, this research reports the interaction of gold and silver nanoparticles in an aqueous dye solution. Our research is the basis for developing an optical sensor used for water treatment centers as an alarm mechanism. Due to the inefficiency of the fluorophore used in similar optodes, sufficient fluorescence was not obtained. With the addition of the nanoparticles, we hoped to observe the transfer of energy from the nanoparticle to the fluorophore to increase the overall intensity, thereby creating a sufficient signal. Using the excitation theories discovered by Raman, Mie, and Forster and Dexter as our foundation, we mixed a strongly fluorescent dye with gold nanoparticles and aagain with silver nanoparticles. After taken measurements via fluorescence spectroscopy, absorption spectroscopy, and photoluminescence excitation, we observed that the silver nanoparticles seemed to enhance the fluorescence of the dye while the gold nanoparticles quenched the fluorescence. / Master of Science

The transition to turbulence of plane wakes is characterized by the development of the velocity-fluctuation field from a spectrum of weak random background noise in the initial laminar wake to a nearly featureless broad spectrum of intense fluctuations within the turbulent wake. This transition has also been described as a sequence of instabilities and wave-wave interactions. In the initial small-amplitude stage,. a narrow, but continuous, band of dominant instability modes centered near the most unstable mode, known also as the fundamental mode, grow exponentially at rates that can be calculated from the linearized Navier-Stokes equations. As these modes grow, the nonlinear terms become more important and cannot be neglected anymore. The effect of these terms is to introduce wave-wave interactions that lead to quadratic energy transfer between the different spectral components of the velocity-fluctuation field. While the consequences of these interactions, such as broadening of the power spectra, have been observed in many experiments, the characteristics of these interactions have only been examined in limited cases. Previous measurements of the auto-bispectrum showed that three-wave interaction processes are important in the transitioning wake. However, quantification of these processes can only be obtained from measurement of the nonlinear energy transfer rates resulting from the nonlinear wave-wave interactions. Such quantification is very important for understanding the effects of the different mechanisms involved in the transition and final breakdown to turbulence. An understanding of these mechanisms and their effects can then be used to control the transition by enhancing certain mechanisms and reducing the role of others through external excitation. In this work, quantitative estimates of the auto-bispectrum, linear and quadratic coupling coefficients and the resulting energy transfer rates between the interacting waves at different locations are presented in controlled and natural transitions of the plane wake. The results show that, in both natural and controlled transitions, the underlying nonlinear dynamics are similar. Basically, nonlinear interactions between the instability modes result in energy transfer to harmonic bands as well as low-frequency difference components. These components play an important role in the transfer of energy to the sidebands and the valleys between the peaks. The results also show that, while energy-transfer rates in natural transition are lower than in controlled transition, the random nature of wave excitation in natural transition causes energy transfer to a band of low-frequency components which leads to energy transfer to many sidebands and results in a spectrum that differs dramatically from the one obtained in the controlled case where two instabilities are excited. / Master of Science

An electropolymerizable zinc porphyrin carrying eight entities of peripheral bithiophene, 4 was newly designed and synthesized. In this design, the bithiophene entities were separated by a biphenyl spacer to minimize ground state interactions perturbing porphyrin π-electronic structure. By multi-cyclic voltammetry, thin-films of 4 were formed on transparent FTO electrode and were characterized by optical, electrochemical and STM measurements. Further, the ability of zinc porphyrin in 4 to axially coordinate phenyl imidazole functionalized fullerene, C60Im both in solution and on the film interface was performed and characterized. Fluorescence quenching of zinc porphyrin both in solution and in the film was observed upon binding of C60Im. Femtosecond transient absorption studies revealed excited state charge separation for the dyad in solution wherein the measured rate of charge separation, kCS and charge recombination, kCR were found to be 2 x 1010 s−1 and 1.2 x 109 s−1, respectively. In contrast, transient absorption studies performed on the dyad in the film were suggestive of energy transfer with minimal contributions from electron transfer. The present study brings out the importance of modulating photochemical reactivity of donor-acceptor dyad in film as compared to that in solution. The electro- and photocatalytic reduction of molecular nitrogen to ammonia (nitrogen reduction reaction, NRR) is of broad interest as an environmentally- and energy-friendly alternative to the Haber–Bosch process for agricultural and emerging energy applications. Herein, we review our recent findings from collaborative electrochemistry/surface science/theoretical studies regarding transition metal oxides, oxynitrides and sulfides as NRR catalysts. We found that, for all metal oxides and oxynitrides specifically, there is no Mars–van Krevelen mechanism and that the reduction of lattice nitrogen and N2 to NH3 occurs by parallel reaction mechanisms at O-ligated metal sites without incorporation of N into the oxide lattice. Additionally, the results highlight the importance of both O-ligation and the importance of N in stabilizing the transition metal cation in an intermediate oxidation state, for effective N≡N bond activation. For transition metal sulfides, various exfoliation treatments are known to yield Sulfur vacancies and DFT calculations corroborate N2 binding to S-vacancies, with substantial π-backbonding to activate dinitrogen. Most of our NRR catalysts were selective to ammonia production without appreciable competing production of H2.

This dissertation presents the development of temperature sensors which employ a fiber-optic probe consisting of single crystal YB3BAlB5BOB12B (YAG) fiber with a phosphor of short length grown directly onto one end using the laser heated pedestal growth method. The response of all the crystalline temperature sensors derives from the temperature-dependent decay time of fluorescence. Yb3+P ions served as the fluorescer, while the addition of various rare-earth codopants (i.e., NdP3+ and ErP3+) with YbP3+ provided an additional path in the form of phonon assisted energy transfer. With the additional nonradiative decay path, the temperature sensors exhibited a more desirable response. A thermally compensated fluorescence decay rate fiber optic temperature sensor was demonstrated for the first time experimentally to the best of our knowledge to make accurate surface temperature measurements. Overall, this novel technique is envisioned to aid in the perpetual challenge of precise surface temperature measurements in comparison to current methods, with the emphasis in the area of rapid thermal processing of semiconductors.

Un-doped, Tb3+ as well as Yb3+ doped ZnO nanocrystals with different concentrations of RE3+ (Tb3+, Yb3+) ions were successfully synthesized via sol-gel method to produce rare earth activated zinc oxide nanophosphors. The phosphor powders were produced by drying the precursor gels at 200˚C in ambient air. Based on the X-ray diffraction results, it was found that the pure and RE3+ doped ZnO nanophosphors were highly polycrystalline in nature regardless of the incorporation of Tb3+ or Yb3+ ions. Moreover, the diffraction patterns were all indexed to the ZnO Hexagonal wurtzite structure and belong to P63mc symmetry group. The Raman spectroscopy confirmed the wurtzitic structure of the prepared samples. Elemental mapping conducted on the as prepared samples using Scanning electron microscope (SEM) equipped with energy dispersive X-ray spectrometer (EDX) revealed homogeneous distribution of Zn, O, and RE3+ ions. The high resolution transmission electron microscope (HR-TEM) analyses indicated that the un-doped and RE3+ doped samples were composed of hexagonal homogeneously dispersed particles of high crystallinity with an average size ranging from 4 to 7 nm in diameter, which was in agreement with X-ray diffraction (XRD) analyses. ZnO:Tb3+ PL study showed that among different Tb3+ concentrations, 0.5 mol% Tb3+ doped ZnO nanoparticles showed clear emission from the dopant originating from the 4f-4f intra-ionic transitions of Tb3+ while the broad defects emission was dominating in the 0.15 and 1 mol% Tb3+doped ZnO. Optical band-gap was extrapolated from the Ultraviolet Visible spectroscopy (UV-Vis) absorption spectra using TAUC‟s method and the widening of the optical band-gap for the doped samples as compared to the un-doped sample was observed. The PL study of ZnO:Yb3+ samples was studied using a 325 nm He-Cd laser line. It was observed that the ZnO exciton peak was enhanced as Yb3+ions were incorporated in ZnO matrix. Furthermore, UV-VIS absorption spectroscopic study revealed the widening of the band-gap in Tb3+ doped ZnO and a narrowing in the case of Yb3+ doped ZnO system. X-ray photoelectron spectroscopy demonstrated that the dopant was present in the doped samples and the result was found to be consistent with PL data from which an energy transfer was evidenced. Energy transfer mechanism was evidenced between RE3+ and ZnO nanocrystals and was discussed in detail. / Physics / M.Sc. (Physics)