<p dir="ltr">The technological drive of humanity to explore the cosmos, travel at hypersonic speeds, and pursue clean energy solutions requires ceramic scientists and engineers to constantly push materials to their functional, behavioral, and chemical extremes. Ultra-high temperature ceramics, and particularly transition metal carbides, are promising materials to meet the demands of extreme environment materials with their >4000 °C melting temperature and impressive thermomechanical behaviors in extreme conditions. The advent of the 2D version of these transition metal carbides, known as MXenes, added a new direction to design transition metal carbides for energy, catalysis, flexible electronics, and other applications. Toward extreme conditions, although MXenes remain yet unexplored, we believe that the ~1 nm flakes of MXenes gives ceramics scientists and engineers the ability to truly engineer transition metal carbides layer-by-layer at the nanoscale to endure the extreme conditions required by future harsh environment technology. Although MXenes have this inherent promise, fundamental study of their behavior in high-temperature environments is necessary to understand how their chemistry and 2D nature affects the high-temperature stability and phase behavior of MXenes toward application in extreme environments.</p><p dir="ltr">In this dissertation, we investigate the high-temperature phase behavior of 2D MXenes in high temperature inert environments to understand the stability and phase transition behavior of MXenes. In this work, we demonstrate that 1) MXenes’ transition at high-temperatures is to highly textured transition metal carbides is due to the homoepitaxial growth of these phases onto ~1-nm-thick MXenes’ highly exposed basal plane, 2) the MXene to MXene interface plays a major role in the phase behavior of MXenes, particularly toward building layered transition metal carbides using MXenes as ~1-nm-thick building blocks, and 3) Defects are the primary site at which atomic migration begins during phase transition of MXenes into these highly textured transition metal carbides, and these defects can be engineered for different phase stability of MXenes. To do so, we investigate the phase behavior of Ti₃C₂T_<em>x</em>, Ta₄C₃T_<em>x</em>, Mo₂TiC₂T_<em>x</em>, and other MXenes using a combination of <i>in situ</i> x-ray diffraction and scanning transmission electron microscopy and other <i>ex situ</i> methods, such as secondary ion mass spectrometry and x-ray photoelectron spectroscopy, with other methods. By investigating the fundamentals of the high-temperature phase behavior of MXenes, we hope to establish the basic principles behind use of MXenes as the ideal material for application in future extreme environments.</p>

<p dir="ltr">Flash Nanoprecipitation (FNP) is a technique that allows organic nanocarriers (NCs) with core-shell architecture to be prepared reproducibly and at scale. The surface shell may be designed independently of the content in the core. This can allow for encapsulated active ingredients to be delivered to areas of the plant where they naturally would not move to but are needed, the biodistribution becoming a function of NC properties and release of active from the NC. The scalability of FNP is also attractive, since large scale production is ultimately required for commercialization of novel agrochemical solutions. In Chapter 3 scalable NCs encapsulating streptomycin (STP) have been prepared at high encapsulation efficiency (EE) and with controlled release of the antibiotic (< 5%). A surface-similar NC has been shown to translocate (~ 6%) to the roots of citrus trees under controlled conditions after foliar spraying. In vitro efficacy suggests that, if enough NCs containing STP are able to reach the phloem sections of trees where CLas resides at sufficient concentrations under field conditions, then this novel formulation may be able to offer an effective solution for managing the disease. Chapter 4 highlights the challenges in encapsulating weakly hydrophobic fungicides via FNP, the strategies that were employed to module fungicide solubility, and initial quantitative efforts to determine fungicide EE in a reliable and accurate manner. Even without full knowledge about the form in which a particular fungicide, mefentrifluconazole (MFZ), was present in the NCs that were applied to turfgrass during a greenhouse biodistribution test, the novel formulation provided higher MFZ recovery in the lower roots than the conventional treatment 7 days after application. It also presented sustained higher recovery of MFZ on the blades for up to 3 days and after blade clipping at 14 days. These results may indicate that MFZ was present in the vasculature.</p>

<p dir="ltr">In recent years, the world has witnessed a growing trend of record high temperatures, heat waves, and extreme weather events due to climate change. Thus, there is an urgent need to develop technologies that enhance quality of life while mitigating further contributions to climate change. Radiative cooling, a passive cooling technique, offers a promising solution to this challenge. Nature serves as a vast, largely unexplored source of inspiration, with various biological systems utilizing radiative cooling to thrive in extreme environments. This work looks at what can be learned from nature to better develop radiative cooling technologies.</p><p dir="ltr">While nanoparticle-based coatings and biologically-inspired nanocellulose-based structures have shown promise in radiative cooling, each has its limitations. Nanocellulose-based structures exhibit high mechanical strength but lower solar reflectance due to UV absorption. On the other hand, nanoparticle-based coatings require a high volume of nanoparticles, resulting in brittleness. This work introduces a dual-layer system comprising a cellulose-based substrate and a thin nanoparticle-based radiative cooling paint, maximizing both radiative cooling potential and mechanical strength. The relationship is studied between thickness and reflectance of the top coating layer with a consistent thickness of the bottom layer. The saturation point is identified and used to determine the optimal thickness for the top-layer. With the use of cotton paper painted with a 125 microns BaSO⁴-based layer, the cooling performance is enhanced to 149.6 W/m² achieved by the improved total solar reflectance from 80% to 93%.</p><p dir="ltr">Looking at another source of biological inspiration, radiative cooling potential of the white shell of the <a href="" target="_blank"><i>Sphincterochila</i></a><i> zonata</i> desert snail is investigated through experimental techniques, revealing a remarkable 90.8% total solar reflectance and 0.88 sky window emissivity, which is achieved through nanoscale features and layered platelet-like morphologies. This is a record high for a biological system. The porosity, nanostructure, and material composition are analyzed, and compared to relative biological systems in other white shells, including those living in the same Negev desert and highly contrasting ocean dwellers. Structural analysis demonstrates layered platelet-like morphologies that optimize for light scattering in solar wavelengths. We investigate the shell's porosity, nanostructure, and material composition through comparison with other species’ shells in the Negev desert and marine environments. Through this, we gain inspiration from <i>Sphincterochila zonata</i> to develop our own radiative cooling technologies.</p><p dir="ltr">In weight-sensitive applications, thin and lightweight radiative cooling paints are crucial, but achieving high solar reflectance remains a challenge. Using inspiration of the layered structure seen in desert snails, this research introduces ultrawhite <a href="" target="_blank">hBN</a>-Acrylic paints that achieve a remarkable solar reflectance of 97.9% with only 150 µm thickness and 0.029 g/cm² weight. The unique properties of hexagonal boron nitride (hBN), including a high refractive index and nanoplatelet morphology, enable a combination of Mie and Rayleigh scattering, while a 44.3% porosity enhances refractive index contrast. Field tests demonstrate that hBN-Acrylic paints provide full daytime cooling under direct sunlight, reducing temperatures by 5-6℃ below ambient.</p><p dir="ltr">Furthermore, biodegradable chitosan-hBN films are introduced as a promising advancement in sustainable cooling technology. These films, composed of up to 60% hBN nanoplatelets within a chitosan matrix, offer flexibility, mechanical robustness, and significant cooling potential. Preliminary results show that these films achieve high solar reflectance and maintain structural integrity, with further potential for optimization through nanoplatelet alignment techniques like hot pressing. By integrating bio-inspired and synthetic approaches, this work contributes to the broader goal of developing sustainable, high-performance materials for passive cooling.</p>

Understanding thermal transport across length scales lays the foundation to developing high-performance electronic devices. Although many experiments and models of the past few decades have explored the physics of heat transfer at nanoscale, there are still open questions regarding the impact of periodic nanostructuring and coherent phonon effects, as well as the interaction of strain and thermal transport. Thermomechanical effects, as well as strains applied in flexible electronic devices, impact the thermal transport. In the simplest kinetic theory models, thermal conductivity is proportional to the phonon group velocity, heat capacity, and scattering times. Periodic porous nanostructures impact the phonon dispersion relationship (group velocity) and the boundaries of the pores increase the scattering times. Strain, on the other hand, affects the crystal structure of the lattice and slightly increases the thermal conductivity of the material under compression. Intriguingly, applying strain combined with the periodic porous structures is expected to influence both the dispersion relation and scattering rates and yield the ability to tune thermal transport actively. But often these interrelated effects are simplified in models.<br><br>This work evaluates the combination of structure and strain on thermal conductivity by revisiting some of the essential methods used to predict thermal transport for a single layer of graphene with a periodic porous lattice structure with and without applied strain. First, we use the highest fidelity method of Non-Equilibrium Molecular Dynamics (NEMD) simulations to estimate the thermal conductivity which considers the impact of the lattice structure, strain state, and phononic band structure together. Next, the impact of the geometry of the slots within the lattice is interrogated with Boltzmann Transport Equation (BTE) models under a Relaxation Time Approximation. A Monte Carlo based Boltzmann Transport Equation (BTE) solver is also used to estimate the thermal conductivity of phononic crystals with varying pore geometry. Dispersion relations calculated from continuum mechanics are used as input here. This method which utilizes a simplified pore geometry only partially accounts for the effects of scattering on the pore boundaries. Finally, a continuum level model is also used to predict the thermal conductivity and its variations under applied strain. As acoustic phonon branches tend to carry the most heat within the lattice, these continuum models and other simple kinetic theories only consider their group velocities to estimate their impact on phonon thermal conductivity. As such, they do not take into account the details of phonon transport across all wavelengths.<br><br>By comparing the results from these different methods, each of which has different assumptions and simplifications, the current work aims to understand the effects of changes to the dispersion relationship based on strain and the periodic nanostructures on the thermal conductivity. We evaluate the accuracy of the kinetic theory, ray tracing, and BTE models in comparison to the MD results to offer a perspective of the reliability of each method of thermal conductivity estimation. In addition, the effect of strain on each phononic crystal with different pore geometry is also predicted in terms of change to their in-plane thermal anisotropy values. To summarize, this deeper understanding of the nanoscale thermal transport and the interrelated effects of geometry, strain, and phonon band structure on thermal conductivity can aid in developing lattices specifically designed to achieve the required dynamic thermal response for future nano-scale thermoelectric applications.

<p>Plasmonics with the capability to harness electromagnetic waves at a nanoscale can be utilized for multitude of applications in ultra-compact miniature optical devices. Plasmonic metasurfaces which are artificially designed sub-wavelength structures have gained unprecedented interest in being able to engineer and effectively modulate the amplitude and phase of the incident wave. Introducing randomness to such plasmonic metasurfaces can also advance possibilities for extraordinary wave manipulation. Hence, by exploiting the plasmonic response of the ordered and disordered metasurfaces, we can design high performance devices for nanoscale optics.</p> <p>Aiming to provide a holistic solution to the current device limitations and bio-compatibility, my research focuses on non-toxic and environment-friendly coloration using plasmonic disordered metasurfaces. These structures generate a broad range of long-lasting colors in reflection that can be applied to real-life artistic or technological applications with a spatial resolution on the order of 0.3 mm or less. Moreover, my research also deals with the possibility of even concentrating energy in the smallest phase-space volume in optics in the form of coherent radiation through designing nanolasers. The study of carrier dynamics and photophysics of the gain media can be extremely beneficial towards the practicability of these lasers. This work elucidates the evolution of different competing mechanisms for coherent lasing. The dynamic study and experimental demonstration of these devices and respective materials can therefore provide a novel aspect to fundamental and applied research.</p>

<p dir="ltr">Metamaterials are artificially engineered nanoscale systems with a three-dimensional repetitive arrangement of certain components, and present exceptional optical properties for applications in nanophotonics, solar cells, plasmonic devices, and more. Self-assembled oxide-metal vertically aligned nanocomposites (VANs), with metallic phase as nanopillars embedded in the matrix oxide, have been recently proposed as a promising candidate for metamaterial applications. However, precise microstructural control and the structure-property relationships in VANs are still in high demand. Thus, by employing multiple approaches for structural design, this dissertation attempts to investigate the mechanisms of nanostructure evolutions and the corresponding optical responses.</p><p dir="ltr">In this dissertation, the precise control over the nanostructures has been demonstrated through morphology tuning, nanopillar orderings, and strain engineering. Firstly, Au, a well-known plasmonic mediator, has been selected as the metallic phase that forms nanopillars. Based on the previously proposed strain compensation model which describes the basic formation mechanism of VAN morphology, two oxides were then considered: La_0.7Sr_0.3MnO₃(LSMO) and CeO₂. In the first two chapters of this dissertation, LSMO was considered due to its similar lattice (a_LSMO= 3.87 Å, a_Au= 4.08 Å) and its enormous potential in nanoelectronics and spintronics. Deposited on SrTiO₃ (001) substrate through pulsed laser deposition (PLD), LSMO-Au nanocomposites exhibit ideal VAN morphology as well as promising hyperbolic dispersions in response to the incident illuminations. By substrate surface treatment of annealing at 1000°C, and variation of STO substate orientations from (001), to (111) and (110), the improved and tunable in-plan orderings of Au nanopillars have been successfully achieved. In the third chapter, a new oxide-metal VAN system of <a href="" target="_blank">CeO₂</a>-Au (a_CeO2= 5.411 Å, and a_CeO2/= 3.83 Å) has been deposited. The intriguing 45° rotated in-plan epitaxy presents an unexpected update to the strain compensation model, and tuning of Au morphology from nanopillars, nanoantennas, to nanoparticles also shows an effective modulation of the LSPR responses. COMSOL simulations have been exploited to reveal the relationships between Au morphologies and optical responses. In the last chapter, the two VAN systems of LSMO-Au and CeO₂-Au have been combined to form a complex layered VAN thin film. Investigations into the strain states, the nature of complex interfaces, and the according hybrid properties, show dramatic possibilities for further strain engineering. In summary, this dissertation has provided multiple routes for highly tailorable oxide-metal nanocomposite designs. And the two proposed material systems present great potential in optical metamaterial applications including biosensors, photovoltaics, super lenses, and more.</p>

<p dir="ltr">For reliable, facile, and user-friendly, solution-based synthesis of materials, the colloidal nanocrystal route has proven to be the method of choice for so many. The tunability that this process renders its users---from choice of precursors, solvent systems, and reaction conditions including temperature, pressure, and precursor addition order---is truly second to none. In their simplest form, these nanomaterials are usually comprised of an inorganic core of the desired material and an outer layer of surface-stabilizing molecules called ligands. These ligands provide colloidal stability and allow for the solution-processing of these materials for downstream usage in devices such as light-emitting diodes and photovoltaics, for example. In this thesis, the study and use of colloidal nanomaterials of Cu(In,Ga)(S,Se)₂ (CIGSSe), IIA-IVB-S₃ (including BaZrS₃ and SrZrS₃), alkaline earth polysulfides (IIAS_x; IIA = Sr, Ba; x = 2, 3), and other materials like Cu₂GeS₃ and Cu₂BaSnS₄, for studies into the formation, colloidal stability, and fabrication into solar cells was performed.</p><p dir="ltr">More specifically, an experimental protocol was developed to fabricate high-quality CIGSSe nanoparticles with carbonaceous residues that are substantially reduced from traditional pathways. Traditional methods for synthesizing colloidal CIGS NPs often utilize heavy, long-chain organic species to serve as surface ligands which, during annealing in a Se/Ar atmosphere, leave behind an undesirable carbonaceous residue in the film. In an effort to minimize these residues, N-methyl-2-pyrrolidone (NMP) was used as an alternative surface ligand. Through the use of the NMP-based synthesis, a substantial reduction in the number of carbonaceous residues was observed in selenized films. Additionally, the fine-grain layer at the bottom of the film, a common observation of solution-processed films from organic media, was observed to exhibit a larger average grain size and increased chalcopyrite character over those of traditionally prepared films, presumably as a result of the reduced carbon content, allowing for superior growth. As a result, a gallium-free CuIn(S,Se)₂ device was shown to achieve power-conversion efficiencies of over 11% as well as possessing exceptional carrier generation capabilities with a short-circuit current density (J_SC) of 41.6 mA/cm², which is among the highest for the CIGSSe family of devices fabricated from solution-processed methods. It was shown that pre-selenized films of sulfide nanoparticles instead of selenide nanoparticles performed better as solar cells. While the exact mechanism is still under debate, it appears that the growth phase during selenization, which varies depending on the chalcogen present in the starting material plays an important role.</p><p dir="ltr">The IIA-IVB-S₃ system is just beginning to emerge as a material system shown to be capable of solution-based synthesis methods. This is primarily due to the extremely high oxophilicity of the IVB elements, Ti, Zr, and Hf, necessitating that extreme care and judicial use of inert environments be used to synthesize these materials via solution-based methods. In the IIA-IVB-S₃ system exists some of the chalcogenide perovskites, including BaZrS₃, which are expected to have similar electronic properties to the well-known, high-performing halide perovskites, albeit much more stable, making them attractive prospects as novel semiconductor materials for optoelectronic applications. This work builds upon recent studies to show a general synthesis protocol, involving the use of carbon disulfide insertion chemistry to generate highly reactive precursors, that can be used towards the colloidal synthesis of numerous nanomaterials in the IIA-IVB-S₃ system, including BaTiS₃, BaZrS₃, BaHfS₃, α-SrZrS₃ and α-SrHfS₃. Additionally, we establish a method to reliably control the formation of the BaZrS₃ perovskite, a complication seen in previous literature where BaZrS₃ appears to exist as two different phases when synthesized via colloidal methods. The utility of these nanomaterials is also assessed via the measurement of their absorption properties and in the form of highly stable colloidal inks for the fabrication of homogenous, crack-free thin films of BaZrS₃. In addition to the chalcogenide perovskites, the IIA-S system was also explored to better understand the solution-based formation of these materials and how the control of IIA polysulfides can be achieved. We show that the synthesis of these materials is strongly correlated to the reaction temperature and that the length of the S_n^2- oligomer chain is the dependent variable. We also report on the synthesis of a previously unreported polymorph of SrS₂ which appears to take on the <i>C2/c</i> space group, the same as BaS₂.</p><p dir="ltr">Finally, some discussion is also provided on the use of transmission electron microscopy (TEM) to analyze the crystal structure of materials. Some tips and techniques used throughout this thesis are summarized in this section.</p>

<p dir="ltr">This doctoral thesis investigates how wave phenomena, including light and acoustic waves, can be harnessed to characterize and manipulate fluids, suspensions, and nanoparticles. It explores light-matter interactions and their role in material characterization, leveraging the complex refractive index as a material fingerprint. Additionally, it examines acoustic wave interactions to enhance particle separation and manipulation in fluid media.</p><p dir="ltr">The research introduces a portable Schlieren imaging system for real-time detection of refractive index gradients in pharmaceutical solutions, providing insights into heterogeneity and diffusion during thawing. A novel method based on Rayleigh-Sommerfeld diffraction theory is developed to size and determine the refractive index of sub-micron particles from holographic data, enabling precise particle characterization. Enhanced filtration performance in fiber filters is demonstrated using standing acoustic waves, with observed efficiency improvements through different fiber arrangements. Finally, the thesis presents Spectral Interferometric SCATtering (SiSCAT) microscopy, a label-free system that combines interferometry and wavelength-dependent scattering to achieve chemically dependent nanoparticle characterization. </p><p dir="ltr">These findings advance the fields of biophysics, materials science, and nanotechnology, offering innovative tools for material and particle analysis.</p>

<p>  We utilize thermoreflectance (TR) thermal imaging to experimentally study heat transport and reliability of micro to nano-scale devices. TR imaging provides 2D thermal maps with sub-micron spatial resolution. Fast thermal transients down to 50 ns resolution can be captured. In addition, finite element modeling is carried out to better understand the underlying physics of the experiment. We describe four main applications; 1) Development of a full-field thermoreflectance imaging setup with a variable optical (laser) heating source as a general characterization tool. We demonstrate the setup’s sensitivity to extract anisotropic<br> thermal conductivity of thin flms and evaluate its sensitivity for detecting buried (below the surface) defects in 3D integrated circuits. This method provides a low-cost noncontact alternative to destructive defect localization methods. It also doesn’t require any special sample<br> preparations. 2) Physics of localized electromigration-failures in metallic interconnects is investigated. One can distinguish two separate mechanisms responsible for electromigration depending on the current density and temperature gradient. 3) Thermal transport in silicon near sub-micron electrical heaters is studied. Quasiballistic and hydrodynamic (fluid-like) behavior is observed at room temperature for different device sizes and geometries. 4) Temperature-dependent thermoreflectance coefcient of phase-change materials is characterized. We focus on tungsten (W) doped VO₂ (W_0.02V_0.98O₂) compound, which experiences an insulator-to-metal transition (IMT) at ≈33 °C. Strong TR-signal non-linearity is observed at the IMT temperature. This non-linearity is used to localize the phase-change boundary with resolutions down to ≈0.2 µm. TR full-feld imaging enables a simple and fast characterization complementing near-feld microscopy techniques. <br>  </p>

<p dir="ltr">In this dissertation, we investigate the potential of two-dimensional (2D) transition metal carbides, known as MXenes, as precursor materials for the development of ultra-high temperature ceramics (UHTCs), with a focus on Ti₃C₂T_<em>x</em> MXene. MXenes are distinguished by their unique combination of 2D structure, high surface area, and chemically active basal planes, making them ideal candidates for a wide range of high-performance applications. This study focuses on the phase transformation, grain growth, surface texturing, and electrocatalytic behavior of Ti₃C₂T_<em>x</em> MXene films when subjected to high-temperature annealing, along with their role as sintering aids in UHTCs.</p><p dir="ltr">We present the transformation of 2D Ti₃C₂T_<em>x</em> flakes into ordered vacancy carbides of three-dimensional (3D) TiC_y phases at temperatures above 1000°C. Using X-ray diffraction and ex-situ annealing (up to 2000°C in a tube furnace and spark plasma sintering), we investigate the resulting nano-lamellar and micron-sized cubic grain morphologies. Single-flake Ti₃C₂T_<em>x</em> films retain a lamellar morphology after annealing, while multi-layer clay-like MXene transforms into irregular cubic grains.</p><p dir="ltr">In addition to investigating the structural evolution, we examine the influence of cationic intercalation on grain growth and texture. Specifically, Ca²⁺ ions lead to highly templated growth along the (111) crystal plane, significantly altering carbon diffusion and metal atom migration during annealing. We show that this preferential growth influences properties with hydrogen evolution reactions (HER) as an example functionality. We observe that with Ca²⁺-intercalated Ti₃C₂T_<em>x</em> films, exhibit an overpotential of 594 mV and a current density of -13 mA/cm² due to increased surface area and dominant texturing.</p><p dir="ltr">Additionally, we investigate the use of MXenes in self-assembly with ceramic materials such as ZrB₂, facilitated by optimizing zeta potentials. MXenes, with their functionalized hydrophilic surfaces and negative zeta potentials, serve as sintering aids and reinforcements in UHTC composites. The introduction of Ti₃C₂T_<em>x</em> to ZrB₂ enables improved sinterability, achieving 96% relative density compared to 89% for pure ZrB₂. Furthermore, the addition of MXenes leads to a core-shell microstructure with (Zr,Ti)B₂ solid-solution interfaces, enhanced mechanical properties such as a 36% increase in hardness, and reductions in oxygen content. These findings establish MXenes as promising materials for the development of advanced UHTCs, suitable for extreme environments.</p><p dir="ltr">Through a combination of experimental techniques, and theoretical estimations, and advanced characterizations, this dissertation provides critical insights into the role of MXenes in both phase transformation and mechanical reinforcement, thereby laying the foundation for future studies and opening new avenues for applications of MXene derived carbides and the design of high-performance UHTCs.</p>