<p>In the realm of tunable optical devices, 3D nanostructures with metals and dielectrics have been utilized in a wide variety of practical applications ranging from optical switching to beam-steering devices. 2D materials, on the other hand, have enabled the exploration of truly new physics unattainable with 3D systems due to quantum confinement leading to unique optical properties and enhanced light-matter interactions. Transdimensional materials (TDMs) – atomically thin films of metals – can couple the robustness of 3D nanostructures with the new physics enabled by 2D features. However, the evolution of the optical properties in the transdimensional regime between 3D and 2D is still underexplored. The optical properties of metallic TDMs are expected to show unprecedented tailorability, including strong dependences on the film thickness, composition, strain, and surface termination. They also have an increased sensitivity to external optical and electrical perturbations, owing to their extraordinary light-confinement. Additionally, the small atomic thicknesses may lead to strongly confined surface plasmons and quantum and nonlocal phenomena. The strong tunability and light-confinement offered by TDMs have resulted in a search for atomically thin plasmonic material platforms that facilitate active metasurfaces with novel functionalities in the visible and near infrared (NIR) range. In this research, we explore the plasmonic properties and tailorability of atomically thin titanium nitride (TiN). We experimentally and theoretically study the thickness-dependent optical properties of epitaxial TiN films with thicknesses down to 1 nm to demonstrate confinement induced optical properties. Overall, this research demonstrates the potential of TDMs for unlocking novel optical phenomena at visible and NIR wavelengths and realizing a new generation of atomically thin tunable nanophotonic devices. </p>

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Ilhas de InxGa1-xAs são de grande interesse no desenvolvimento tecnológico de lasers de diodos e diodos emissores de luz. As ilhas de InxGa1-xAs investigadas neste trabalho foram crescidas sobre um substrato semi-isolante de GaAs (001) pelo método de auto-organização usando epitaxia de feixe molecular. Este tipo de ilha, quando isolada e de pequeno tamanho, é considerada um ponto quântico ou sistema zero-dimensional. As amostras foram caracterizadas através do uso da microscopia de força atômica. A densidade e o tamanho dos pontos aumenta com a diminuição da fração molar de In, resultando em uma maior cobertura para o caso de x = O, 25. As características principais dos espectros Raman são os picos que correspondem aos modos LO e TO do substrato de GaAs. Duas estruturas adicionais aparecem no espectro: um pico estreito em 222 cm- 1 e uma banda larga de mais alta energia, que só é resolvida para x = O, 25, centrada em 245 cm-1. O pico em 222 cm-1 é provavelmente devido ao fônon LA(X) do GaAs normalmente proibido, induzido por defeitos. Para identificar a banda larga foi construído um modelo que considera: (i) a frequência Raman do modo tipo- InAs com caráter de LO como constante com a variação de x no InGaAs 3-D; (ii) efeitos de confinamento não afetam a frequência Raman dado ao tamanho dos pontos quânticos das amostras deste trabalho; (iii) A tensão escala com x e o valor máximo ocorre para o composto binário InAs. Este modelo permite prever um intervalo de frequências para os pontos quânticos. O valor medido, 245 cm- 1, está dentro deste intervalo e portanto foi atribuído ao modo tipo-InAs dos pontos quânticos de In0,25Ga0,75As Considerações de simetria reforçam esta designação. Contribuições adicionais de fônons foram consideradas no intervalo de energia de interesse. Para analisar estas contribuições, foi feito um estudo detalhado dos fônons induzidos por desordem em camadas de GaAs, e espalhamento Raman de As cristalino e amorfo. A desordem foi produzida através da erosão por laser e a amostra de As foi formada por um processo de oxidação de um filme de AlAs. Comparação dos espectros Raman permitiu concluir que não houve contribuição de fônons induzidos por desordem no espectro do ponto quântico, seja de GaAs ou arsênio. / InxGa1-x As islands are interesting for use in Laser diode and light-emitting diode technology. The InxGa1-x As islands investigated in this work were grown on semi-insulating (001) GaAs substrates by the self-organization method using molecular beam epitaxy. This type of island, when isolated and of small size, is considered as a quantum dot or zero-dimensional system. The samples were characterized by use of atomic force microscopy. The dot density and size were seen to increase as the In molar fraction decreased, resulting in a large dot­ coverage in the case of x = 0.25. The Raman spectra main features were the peaks corresponding to the LO and TO modes of GaAs-substrate. Second order structures were also present around 520 cm-1 (160 cm-1) for optical (acoustic) vibration of GaAs. Two additional structures appear as a sharp peak at 222 cm- 1 and higher energy broad band, which is resolved only for x = 0.25, at 245 cm- 1. The peak in 222 cm-1 is probably due to the normally forbidden GaAs LA(X) phonon induced by defects. To assign the broad band a model was constructed that considers: (i) the Raman frequency of the InAs-like mode with LO character as constant with x in bulk I nGaAs; (ii) confinement effects for the large dots formed has negligible effects in the quantum dot Raman frequency; (iii) The strain scale with x, the maximun value corresponds to that obtained for InAs. This model allowed to predict a range of frequencies for the dots. The value measured, 245 cm- 1, fit into this range and is, thus, attributed to the InAs-like mode of the In0.25Ga0.75As quantum dots. Selection rules arguments reinforces this assignment. Several additional contributions in the frequency range of interest were considered. In order to analyze those contributions, a detailed study of disorder induced phonons in GaAs, and Raman scattering of As-crystaline and amorphous, was realized. The disorder was produced by laser ablation and the As sample was formed by an oxidation process of an A1As film. Comparison of the Raman spectra allowed to conclude that neither As or GaAs disorder induced phonons contribute to the quantum-dot spectrum.

Ilhas de InxGa1-xAs são de grande interesse no desenvolvimento tecnológico de lasers de diodos e diodos emissores de luz. As ilhas de InxGa1-xAs investigadas neste trabalho foram crescidas sobre um substrato semi-isolante de GaAs (001) pelo método de auto-organização usando epitaxia de feixe molecular. Este tipo de ilha, quando isolada e de pequeno tamanho, é considerada um ponto quântico ou sistema zero-dimensional. As amostras foram caracterizadas através do uso da microscopia de força atômica. A densidade e o tamanho dos pontos aumenta com a diminuição da fração molar de In, resultando em uma maior cobertura para o caso de x = O, 25. As características principais dos espectros Raman são os picos que correspondem aos modos LO e TO do substrato de GaAs. Duas estruturas adicionais aparecem no espectro: um pico estreito em 222 cm- 1 e uma banda larga de mais alta energia, que só é resolvida para x = O, 25, centrada em 245 cm-1. O pico em 222 cm-1 é provavelmente devido ao fônon LA(X) do GaAs normalmente proibido, induzido por defeitos. Para identificar a banda larga foi construído um modelo que considera: (i) a frequência Raman do modo tipo- InAs com caráter de LO como constante com a variação de x no InGaAs 3-D; (ii) efeitos de confinamento não afetam a frequência Raman dado ao tamanho dos pontos quânticos das amostras deste trabalho; (iii) A tensão escala com x e o valor máximo ocorre para o composto binário InAs. Este modelo permite prever um intervalo de frequências para os pontos quânticos. O valor medido, 245 cm- 1, está dentro deste intervalo e portanto foi atribuído ao modo tipo-InAs dos pontos quânticos de In0,25Ga0,75As Considerações de simetria reforçam esta designação. Contribuições adicionais de fônons foram consideradas no intervalo de energia de interesse. Para analisar estas contribuições, foi feito um estudo detalhado dos fônons induzidos por desordem em camadas de GaAs, e espalhamento Raman de As cristalino e amorfo. A desordem foi produzida através da erosão por laser e a amostra de As foi formada por um processo de oxidação de um filme de AlAs. Comparação dos espectros Raman permitiu concluir que não houve contribuição de fônons induzidos por desordem no espectro do ponto quântico, seja de GaAs ou arsênio. / InxGa1-x As islands are interesting for use in Laser diode and light-emitting diode technology. The InxGa1-x As islands investigated in this work were grown on semi-insulating (001) GaAs substrates by the self-organization method using molecular beam epitaxy. This type of island, when isolated and of small size, is considered as a quantum dot or zero-dimensional system. The samples were characterized by use of atomic force microscopy. The dot density and size were seen to increase as the In molar fraction decreased, resulting in a large dot­ coverage in the case of x = 0.25. The Raman spectra main features were the peaks corresponding to the LO and TO modes of GaAs-substrate. Second order structures were also present around 520 cm-1 (160 cm-1) for optical (acoustic) vibration of GaAs. Two additional structures appear as a sharp peak at 222 cm- 1 and higher energy broad band, which is resolved only for x = 0.25, at 245 cm- 1. The peak in 222 cm-1 is probably due to the normally forbidden GaAs LA(X) phonon induced by defects. To assign the broad band a model was constructed that considers: (i) the Raman frequency of the InAs-like mode with LO character as constant with x in bulk I nGaAs; (ii) confinement effects for the large dots formed has negligible effects in the quantum dot Raman frequency; (iii) The strain scale with x, the maximun value corresponds to that obtained for InAs. This model allowed to predict a range of frequencies for the dots. The value measured, 245 cm- 1, fit into this range and is, thus, attributed to the InAs-like mode of the In0.25Ga0.75As quantum dots. Selection rules arguments reinforces this assignment. Several additional contributions in the frequency range of interest were considered. In order to analyze those contributions, a detailed study of disorder induced phonons in GaAs, and Raman scattering of As-crystaline and amorphous, was realized. The disorder was produced by laser ablation and the As sample was formed by an oxidation process of an A1As film. Comparison of the Raman spectra allowed to conclude that neither As or GaAs disorder induced phonons contribute to the quantum-dot spectrum.

<p>Functional oxide-based thin films have attracted much attention owing to their broad applications in modern society. The multifunction tuning in oxide thin films is critical for obtaining enhanced properties. In this dissertation, four new nanocomposite thin film systems with highly textured growth have been fabricated by pulsed laser deposition technique. The functionalities including ferromagnetism, ferroelectricity, multiferroism, magnetoelectric coupling, low-field magnetoresistance, transmittance, optical bandgap and dielectric constants have been demonstrated. Besides, the tunability of the functionalities have been studied via different approaches.</p> <p>First, varies deposition frequencies have been used in vertically aligned nanocomposite BaTiO₃:YMnO₃ (BTO:YMO) and BaTiO₃:La_0.7Sr_0.3Mn₃(BTO:LSMO) thin films. In both systems, the strain coupling effect between the phases are affected by the density of grain boundaries. Increasing deposition frequency generates thinner columns in BTO:YMO thin films, which enhances the anisotropic ferromagnetic response in the thin films. In contrast, the columns in BTO:LSMO thin films become discontinuous as the deposition frequency increases, leading to the diminished anisotropic ferromagnetic response. Coupling with the ferroelectricity in BTO, the room temperature multiferroic properties have been obtained in these two systems.</p> <p> Second, the impact of the film composition has been demonstrated in La_0.7Ca_0.3MnO₃ (LCMO):CeO₂thin film system, which has an insulating CeO₂in ferromagnetic conducting LCMO matrix structure. As the atomic percentage of the CeO₂increases, enhanced low-field magnetoresistance and increased metal-to-insulator transition temperature are observed. The thin films also show enhanced anisotropic ferromagnetic response comparing with the pure LCMO film.</p> <p> Third, the transition metal element in Bi₃MoM_TO₉(M_T, transition metals of Mn, Fe, Co and Ni) thin films have been varied. The thin films have a multilayered structure with M_T-rich pillar-like domains embedded in Mo-rich matrix structure. The anisotropic magnetic easy axis and optical properties have been demonstrated. By the element variation, the optical bandgaps, dielectric constants as well as anisotropic ferromagnetic properties have been achieved. </p> <p> The studies in this dissertation demonstrate several examples of tuning the multifunctionalities in oxide-based nanocomposite thin films. These enhanced properties can broaden the applications of functional oxides for advanced nanoscale devices.</p><br>

<p dir="ltr">Since before the Apollo era, the rocket propulsion sector has been a key player in developing standards of cleanliness and compatibility when designing, building, and operating systems with toxic propellants. The advent of hypergols and the widespread use of propellants like N₂O₄, Mixed Oxides of Nitrogen (MON), and hydrazine have forced new standards to be developed to meet the ever-growing need for safety when working with dangerous substances. These systems have only continued to grow more complex and many propellant combinations remain toxic and corrosive to most substances as the industry seeks the optimal methods for deriving the most efficient, highest performing, and generally more capable. ASTM International and other standards organizations carry on documenting standards for cleaning and passivation to ensure safe use today to meet the needs of the ever-expanding propulsion industry.</p><p dir="ltr">This thesis aims to determine the feasibility of using Raman spectroscopy as a method of characterizing interactions between metals and propellants. First, a background of knowledge regarding the spectroscopic method, propellants, and industry practices was researched and current areas of possible application were identified. The passivation and propellant storage phases of system lifecycles were determined to be the scope and target for experimentation. A multilevel passivation study consisting of exposing three metal types to different concentrations of nitric acid for various durations was conducted to begin developing a greater understanding of the applicability of and the techniques required to make Raman spectroscopy work as a complement to the ASTM passivation verification tests. Lessons learned from this and a short-duration compatibility study with MON and similar metal samples were documented and will be used for a larger scale and longer duration compatibility study in conjunction with NASA White Sands Test Facility (WSTF). The buildup of safe and adequate facilities for such a study was undertaken, completed, and documented in this work.</p><p dir="ltr">The results of testing in this thesis suggest the promising and desirable non-destructive and minimally invasive features of Raman spectroscopy have the potential to be used extensively in the propulsion sector. Suggestions for developing key techniques and methods for this application are developed and outlined as they were learned throughout the study's conduction.</p>

This thesis describes investigations into the photophysical properties of luminescent materials and their application in optoelectronic devices such as light emitting diodes and photodetectors. The materials used were all solution processable because of the interest in low cost processing of organics. I have investigated the photophysics of 1,4,5,8,9,12-hexamethyltriphenylene, a triphenylene derivative which has its luminescence enhanced by the addition of methyl groups. These groups change the planar shape of the triphenylene molecule into a twisted one, changing the symmetry of the molecule and increasing its dipole moment in absorption and emission by ~4 fold. This increased its rate of radiative deexcitation by ~20 times. In addition, the twisted shape of the molecule prevents intermolecular interactions and concentration effects from affecting the luminescence. This results in an efficient solid-state photoluminescence quantum yield of 31%. This thesis also includes an investigation into phosphorescent polymer dendrimers, designed to have suitable viscosities in solution for inkjet printed OLED applications. A photophysical study of the intra-chain aggregation effects on the luminescence was undertaken in both homopolymers and copolymers with high energy gap spacer units. Using double dendrons to increase the steric protection of the luminescent cores, the best homopolymers achieved 12.1% external quantum efficiency (39.3 cd/A) at 100 cd/m² brightness and the best co-polymer achieved 14.7% EQE (48.3 cd/A) at 100 cd/m². This compares favourably with 11.8% EQE for the best phosphorescent polymer and 16% for the best solution processed dendrimer OLED previously reported. Finally I have applied a solution processed enhancement layer to silicon photodiodes to enhance their ultraviolet response. Using a blend of materials to give favourable absorption and emission properties, 61% external quantum efficiency was achieved at 200 nm, which is better than the 20-30% typical for vacuum deposited lumogen enhancement layers used commercially.

Transparent polycrystalline aluminum oxide is a promising optical material, particularly in applications that require ballistic protection. However, the rhombohedral crystal structure of alumina limits its transparency due to birefringent scattering. One method of reducing birefringent scattering is to align the particles along the same crystallographic direction, minimizing the refractive index mismatch. This dissertation explores the use of high aspect-ratio platelet-morphology alumina powder in order to process a crystallographically aligned polycrystalline alumina part, with improved optical properties. The optimal hot-pressing parameters of non-pre-aligned platelet alumina were explored, showing that a low pre-load pressure (0MPa), a high maximum temperature (1800°C), a low maximum pressure (10MPa), and a long isothermal hold time (>5hrs) yields dense, transparent parts. These parameters resulted in samples with a high in-line transmission (>65%) despite a large grain size (>60μm). This is due to a high degree of crystallographic orientation, which minimizes the refractive index mismatch between grains, reducing birefringent scattering. Pre-alignment resulted in a further increase in crystallographic orientation, indicating that the pre-alignment procedure effectively aligns the platelets along the same crystallographic orientation. However, pre-alignment resulted in a minimal improvement in optical properties due to the pre-aligned platelets decreasing the densification. Mechanical properties were characterized, resulting in a flexure stress and Vickers hardness of approximately 175MPa and 17GPa, respectively. These low mechanical properties are due to the large grain size. The Vickers hardness was also characterized along different alignment/hot-pressing directions, showing that the hardness matches that of sapphire along corresponding crystallographic directions. Modifications to the Rayleigh-Gans-Debye model were made, accounting for crystallographic orientation. The modified model more closely matches the experimental optical data, illustrating the importance of accounting for crystallographic alignment. This dissertation emphasizes the importance of characterizing optical losses in transparent ceramics and how they relate to the microstructure, as well as the significance of crystallographic alignment in a birefringent transparent ceramic like alumina.

<p> Optical metamaterials have triggered extensive studies driven by their fascinating electromagnetic properties that are not observed in natural materials. Aside from the extraordinary progress, challenges remain in scalable processing and material performance which limit the adoption of metamaterial towards practical applications. The goal of this dissertation is to design and fabricate nanocomposite thin films by combining nitrides with a tunable secondary phase to realize controllable multi-functionalities towards potential device applications. Transition metal nitrides are selected for this study due to the inherit material durability and low-loss plasmonic properties that offer stable two-phase hybridization for potential high temperature optical applications. Using a pulsed laser deposition technique, the nitride-metal nanocomposites are self-assembled into various geometries including pillar-in-matrix, embedded nanoinclusions or complex multilayers, that possess large surface coverage, high epitaxial quality, and sharp phase boundary. The nanostructures can be further engineered upon precise control of growth parameters. </p><p> This dissertation is composed of a general review of related background and experimental approaches, followed by four chapters of detailed research chapters. The first two research chapters involve hybrid metal (Au, Ag) - titanium nitride (TiN) nanocomposite thin films where the metal phase is self-assembled into sub-20 nm nanopillars and further tailored in terms of packing density and tilting angles. The tuning of plasmonic resonance and dielectric constant have been achieved by changing the concentration of Au nanopillars, or the tuning of optical anisotropy and angular selectivity by changing the tilting angle of Ag nanopillars. Towards applications, the protruded Au nanopillars are demonstrated to be highly functional for chemical bonding detection or surface enhanced sensing, whereas the embedded Ag nanopillars exhibit enhanced thermal and mechanical stabilities that are promising for high temperature plasmonic applications. In the last two chapters, dissimilar materials candidates beyond plasmonics have been incorporated to extend the electromagnetic properties, include coupling metal nanoinclusions into a wide bandgap semiconducting aluminum nitride matrix, as well as inserting a dielectric spacer between the hybrid plasmonic claddings for geometrical tuning and electric field enhancement. As a summary, these studies present approaches in addressing material and fabrication challenges in the field of plasmonic metamaterials from fundamental materials perspective. As demonstrated in the following chapters, these hybrid plasmonic nanocomposites provide multiple advantages towards tunable optical or biomedical sensing, high temperature plasmonics, controllable metadevices or nanophotonic chips.</p><div><br></div>

<p dir="ltr">Conducting polymers have garnered enormous attention due to their unique properties, including tunable chemical structure, high flexibility, solution processability, and biocompatibility. They hold promising applications in flexible electronics, renewable energies, sensing, and healthcare. Despite notable progress in conducting polymers over the past few decades, most of them still suffer from complicated synthesis routes, limited processability, low electrical conductivity, and poor ambient stability compared to their inorganic counterparts. Additionally, the susceptibility of conducting polymers to high temperatures makes them not applicable in real-life electronics. To address the challenges of developing high-performance and stable conducting polymers, we present two key approaches: dopant innovation for polymer-dopant interaction engineering and the discovery of new conjugated polymer hosts. From the perspective of dopant design, we first utilize cross-linkable chlorosilanes (C-Si) to design thermally and chemically stable conductive polymer composites. C-Si can form robust siloxane networks and simultaneously<i> </i>dope the host conjugated polymers. Besides, we have introduced a new class of dopants, namely aromatic ionic dopants (AIDs). The use of AIDs allows for the separation of doping and charge compensation, two processes involved in molecular doping, and therefore leads to highly efficient doping and thermally stable doped systems. We then provide insights into the design of novel conjugated polymer hosts. Remarkably, we have developed the first thermodynamically stable n-type conducting polymer, n-doped Poly (3,7-dihydrobenzo[1,2-b:4,5-b′]difuran-2,6-dione) (n-PBDF). n-PBDF is synthesized from a simple and scalable route, involving oxidative polymerization and reductive doping in one pot in the air. The n-PBDF ink is solution processable with excellent ink stability and the n-PBDF thin film is highly conductive, transparent, patternable, and robust. In addition, precise control over the doping levels of n-PBDF has been achieved through chemical doping and dedoping. By tuning the n-PBDF thin films between highly doped and dedoped states, the system shows controllable conductivity, optical properties, and energetics, thereby offering potential applications in a variety of organic electronics. Overall, this research advances the fundamental understanding of molecular doping and offers insights for the development of high-conductivity, stable conducting polymers with tunable properties for next-generation electronics.</p>