Complex perovskite materials, including Ba(Zn1/3Ta2/3)O3 (BZT), are commonly used to make resonators and filters in communication systems because of their low dielectric loss and high-quality factors (Q). Transition metal additives are introduced (i.e., Ni2+, Co2+, Mn2+) to act as sintering agents and tune their temperature coefficient to zero or near-zero. However, losses in these commercial dielectric materials at cryogenic temperatures increase markedly due to spin-excitation resulting from the presence of paramagnetic defects. Applying a large magnetic field (e.g., 5 Tesla) quenches these losses and has allowed the study of other loss mechanisms present at low temperatures. Work was performed on Fe3+ doped LaAlO3. At high magnetic fields, the residual losses versus temperature plots exhibit Debye peaks at ~40 K, ~75 K, and ~215 K temperature and can be tentatively associated with defect reactions O_i^x+V_O^x→O_i^'+V_O^•, Fe_Al^x+V_Al^"→Fe_Al^'+V_Al^' and Al_i^x+Al_i^(••)→〖2Al〗_i^•, respectively. Peaks in the loss tangent versus temperature graph of Zn-deficient BZT indicate a higher concentration of defects and appear to result from conduction losses.Guided by the knowledge gained from this study, a systematic study to develop high-performance microwave materials for ultra-high performance at cryogenic temperatures was performed. To this end, the production and characterization of perovskite materials that were either undoped or contained non-paramagnetic additives were carried out. Synthesis of BZT ceramic with over 98% theoretical density was obtained using B2O3 or BaZrO3 additives. At 4 K, the highest Q x f product of 283,000 GHz was recorded for 5% BaZrO3 doped BZT. A portable, inexpensive open-air spectrometer was designed, built, and tested to make the electron paramagnetic resonance (EPR) technique more accessible for high-school and university lab instruction. In this design, the sample is placed near a dielectric resonator and does not need to be enclosed in a cavity, as is used in commercial EPR spectrometers. Permanent magnets used produce fields up to 1500 G, enabling EPR measurements up to 3 GHz.

Electron Paramagnetic Resonance (EPR) has facilitated great scientific advancements in many fields, like material science, engineering, medicine, biology, and health. EPR provided the ability to investigate samples on molecular level to detect chemical composition and identify harmful substances like free radicals. This thesis aims to explore current health and diagnostics EPR research and investigate the free radical content in related paramagnetic centers. Examining paramagnetic diagnostic markers of Cancer, Sicklecell disease, oxidative stress, and food oxidation. After exploring current literature on EPR, an experiment is designed and conducted to test seven different coffee samples (Turkish coffee, Espresso Coffee, European Coffee, Ground Arabic Coffee, American Coffee, Roasted Arabic Coffee, and Green Arabic Coffee), using Bruker ELEXSYS E580 spectrometer at x-band and under both room temperature (298 K) and low temperature (106 -113 K). Several microwave powers (1, mW, 0.25 mW, 0.16 mW, 0.06 mW, 0.04 mW) and different modulation frequency (10 G, 5 G, 3 G) are used. The results revealed average g-value was 2.009, highest linewidth was 16.312. Espresso coffee had the highest concentration of radicals, and green Arabic coffee beans had the lowest. Obtained spectra showed signals of Reactive Oxygen Species (ROS) radicals; believed to be result of natural oxidation process, as well as trace amounts of Fe3+ and other transition metals impurities, likely to be naturally found in coffee or resulting from the process of coffee production.

Many important technologies, including electronics, computing, communications, optoelectronics, and sensing, are built on semiconductors. The band gap is a crucial factor in determining the electrical and optical properties of semiconductors. Beyond graphene, newly found two-dimensional (2D) materials have semiconducting bandgaps that range from the ultraviolet in hexagonal boron nitride to the terahertz and mid-infrared in bilayer graphene and black phosphorus, visible in transition metal dichalcogenides (TMDs). These 2D materials were shown to have highly controllable bandgaps which can be controlled by alloying. Only a small number of TMDs and monochalcogenides have been alloyed, though, because alloying compromised the material's Van der Waals (Vdw) property and the stability of the host crystal lattice phase. Phase transition in 2D materials is an interesting phenomenon where work has been done only on few TMDs namely MoTe2, MoS2, TaS2 etc.In order to change the band gaps and move them towards the UV (ultraviolet) and IR (infrared) regions, this work has developed new 2D alloys in InSe by alloying them with S and Te at 10% increasing concentrations. As the concentration of the chalcogens (S and Te) increased past a certain point, a structural phase transition in the alloys was observed. However, pinpointing the exact concentration for phase change and inducing phase change using external stimuli will be a thing of the future. The resulting changes in the crystal structure and band gap were characterized using some basic characterization techniques like scanning electron microscopy (SEM), X-ray Diffraction (XRD), Raman and photoluminescence spectroscopy.

Over the past three decades, significant progress in the development of organic light-emitting diodes (OLEDs) has been achieved, enabling OLEDs to become a main component in state-of-the-art displays and next generation solid-state lighting. As this technology advances, it is highly desirable to further improve the device efficiency and operational stability to drive the success of OLEDs in future display and lighting applications. This dissertation aims at developing novel organic emitting materials covering visible and near-infrared (NIR) emissions for efficient and table OLEDs. Firstly, a series of tetradentate Pd(II) complexes, which have attractive phosphorescent aggregate emission performance especially at high brightness level in device settings, have been developed. The luminescent lifetime of Pd(II) complex aggregates was demonstrated to be shorter than 1 μs with a close-to-unity photoluminescence quantum yield. Moreover, a systematic study regarding structure-property relationship was conducted on four tetradentate Pd(II) complexes, i.e., Pd3O3, Pd3O8-P, Pd3O8-Py2, and Pd3O8-Py5, featuring aggregate emission. As a result, an extremely efficient and stable OLED device utilizing Pd3O8-Py5 was achieved. It demonstrated a peak external quantum efficiency (EQE) of 37.3% with a reduced efficiency roll-off retaining a high EQE of 32.5% at 10000 cd m-2, and an estimated LT95 lifetime (time to 95% of the initial luminance) of 48246 h at 1000 cd m-2. Secondly, there is an increasing demand for NIR OLEDs with emission spectra beyond 900 nm to expand their applications in biometric authentication, night vision display, and telecommunication, etc. A stable and efficient NIR Pt(II) porphyrin complex named PtTPTNP-F8 was developed, and exhibited an electroluminescent spectrum at 920 nm. By carefully choosing the host materials, an PtTPTNP-F8 based NIR OLED achieved a EQE of 1.9%. Furthermore, an PtTPTNP-F8 OLED fabricated in a stable device structure demonstrated extraordinary operational stability with LT99 of >1000 h at 20 mA cm-2. Lastly, a series of imidazole-based blue Pt(II) complexes were developed and studied. Results indicated that structural modification of ligand molecules effectively tuned the emission spectral wavelength and bandwidth. Two blue complexes, i.e., Pt2O2 P2M and Pt2O2-PPy5-M, emitting at 472 and 476 nm respectively, exhibited narrow-band emission spectra with a full width at half maximum of 16 nm.

Organic materials have emerged as an attractive component of electronics over the past few decades, particularly in the development of efficient and stable organic light-emitting diodes (OLEDs) and organic neuromorphic devices. The electrical, chemical, physical, and optical studies of organic materials and their corresponding devices have been conducted for efficient and stable electronics. The development of efficient and stable deep blue OLED devices remains a challenge that has obstructed the progress of large-scale OLED commercialization. One approach was taken to achieve a deep blue emitter through a color tuning strategy. A new complex, PtNONS56-dtb, was designed and synthesized by controlling the energy gap between T1 and T2 energy states to achieve narrowed and blueshifted emission spectra. This emitter material showed an emission spectrum at 460 nm with a FWHM of 59 nm at room temperature in PMMA, and the PtNONS56-dtb-based device exhibited a peak EQE of 8.5% with CIE coordinates of (0.14, 0.27). A newly developed host and electron blocking materials were demonstrated to achieve efficient and stable OLED devices. The indolocarbazole-based materials were designed to have good hole mobility and high triplet energy. BCN34 as an electron blocking material achieved the estimated LT80 of 12509 h at 1000 cd m-2 with a peak EQE of 30.3% in devices employing Pd3O3 emitter. Additionally, a device with bi-layer emissive layer structure, using BCN34 and CBP as host materials doped with PtN3N emitter, achieved a peak EQE of 16.5% with the LT97 of 351 h at 1000 cd m-2. A new neuromorphic device using Ru(bpy)3(PF6)2 as an active layer was designed to emulate the short-term characteristics of a biological synapse. This memristive device showed a similar operational mechanism with biological synapse through the movement of ions and electronic charges. Furthermore, the performance of the device showed tunability by adding salt. Ultimately, the device with 2% LiClO4 salt shows similar timescales to short-term plasticity characteristics of biological synapses.

Crystalline silicon covers more than 85% of the global photovoltaics industry and has sustained a nearly 30% year-over-year growth rate. Continued cost and capital expenditure (CAPEX) reductions are needed to sustain this growth. Using thin silicon wafers well below the current industry standard of 160 µm can reduce manufacturing cost, CAPEX, and levelized cost of electricity. Additionally, thinner wafers enable more flexible and lighter module designs, making them more compelling in market segments like building-integrated photovoltaics, portable power, aerospace, and automotive industries. Advanced architectures and superior surface passivation schemes are needed to enable the use of very thin silicon wafers. Silicon heterojunction (SHJ) and SHJ with interdigitated back contact solar cells have demonstrated open-circuit voltages surpassing 720 mV and the potential to surpass 25% conversion efficiency. These factors have led to an increasing interest in exploring SHJ solar cells on thin wafers. In this work, the passivation capability of the thin intrinsic hydrogenated amorphous silicon layer is improved by controlling the deposition temperature and the silane-to-hydrogen dilution ratio. An effective way to parametrize surface recombination is by using surface saturation current density and a very low surface saturation density is achieved on textured wafers for wafer thicknesses ranging between 40 and 180 µm which is an order of magnitude lesser compared to the prevalent industry standards. Implied open-circuit voltages over 760 mV were accomplished on SHJ structures deposited on n-type silicon wafers with thicknesses below 50 µm. An analytical model is also described for a better understanding of the variation of the recombination fractions for varying substrate thicknesses. The potential of using very thin wafers is also established by manufacturing SHJ solar cells, using industrially pertinent processing steps, on 40 µm thin standalone wafers while achieving maximum efficiency of 20.7%. It is also demonstrated that 40 µm thin SHJ solar cells can be manufactured using these processes on large areas. An analysis of the percentage contribution of current, voltage, and resistive losses are also characterized for the SHJ devices fabricated in this work for varying substrate thicknesses.

Nanocrystalline (NC) materials are of great interest to researchers due to their multitude of properties such as exceptional strength and radiation resistance owing to their high fraction of grain boundaries that act as defect sinks for radiation-induced defects, provided they are microstructurally stable. In this dissertation, radiation effects in microstructurally stable bulk NC copper (Cu)- tantalum (Ta) alloys engineered with uniformly dispersed Ta nano-precipitates are systematically probed. Towards this, both ex-situ and in-situ irradiations using heavy (self) ion, helium ion, and concurrent dual ion beams (He+Au) followed by isochronal annealing inside TEM were utilized to understand radiation tolerance and underlying mechanisms of microstructure evolution in stable NC alloys. With systematic self-ion irradiation, the high density of tantalum nanoclusters in Cu-10at.%Ta were observed to act as stable sinks in suppressing radiation hardening, in addition to stabilizing the grain boundaries; while the large incoherent precipitates experienced ballistic mixing and dissolution at high doses. Interestingly, the alloy exhibited a microstructure self-healing mechanism, where with a moderate thermal input, this dissolved tantalum eventually re-precipitated, thus replenishing the sink density. The high stability of these tantalum nanoclusters is attributed to the high positive enthalpy of mixing of tantalum in copper which also acted as a critical driving force against atomic mixing to facilitate re-precipitation of tantalum nanoclusters. Furthermore, these nanoclusters proved to be effective trapping sites for helium, thus sequestering helium into isolated small bubbles and aid in increasing the overall swelling threshold of the alloy. The alloy was then compositionally optimized to reduce the density of large incoherent precipitates without compromising on the grain size and nanocluster density (Cu-3at.%Ta) which resulted in a consistent and more promising response to high dose self-ion irradiation. In-situ helium and dual beam irradiation coupled with isochronal annealing till 723 K, also revealed a comparable microstructural stability and enhanced ability of Cu-3Ta in controlling bubble growth and suppressing swelling compared to Cu-10Ta indicating a promising improvement in radiation tolerance in the optimized composition. Overall, this work helps advancing the current understanding of radiation tolerance in stable nanocrystalline alloys and aid developing design strategies for engineering radiation tolerant materials with stable interfaces.

In the Rare-earth-Tri-telluride family, (RTe3s) [R=La, Ce, Nd, Sm, Gd, Tb, Dy, Er, Ho, Tm] the emergence of Charge Density Waves, (CDW) has been under investigation for a long time due to broadly tunable properties by either chemical substitution or pressure application. These quasi 2D Layered materials RTe3s undergo Fermi Surface Nesting leading to CDW instability. CDWs are electronic instabilities found in low-dimensional materials with highly anisotropic electronic structures. Since the CDW is predominantly driven by Fermi-surface (FS) nesting, it is especially sensitive to pressure-induced changes in the electronic structure. The FS of RTe3s is a function of p-orbitals of Tellurium atoms, which are arranged in two adjacent planes in the crystal structure. Although the FS and electronic structure possess a nearly four-fold symmetry, RTe3s form an incommensurate CDW.This dissertation is structured as follows: Chapter 1 includes basic ideas of Quantum materials, followed by an introduction to CDW and RTe3s. In Chapter 2, there are fundamentals of crystal growth by Chemical Vapor Transport, including various precursors, transport agent, temperature gradient, and rate of the reaction. After the growth, the crystals were confirmed for lattice vibrations by Raman, for composition by Energy Dispersive Spectroscopy; crystal structure and orientation were confirmed by X-ray Diffraction; magnetic ordering was established by Vibrating sample measurement. Detailed CDW study was done on various RTe3s by Raman spectroscopy. The basic mechanism and instrumentations used in these characterizations are explained in Chapter 3. Chapter 4 includes experimental data for crystal growth and results of these characterizations for Parent RTe3s. Chapter 5 includes fundamental insights on Cationic alloying of RTe3s, along with one alloy system’s crystal growth and characterization. This work tries to explain the behavior of CDW by a Temperature-dependent Raman study of RTe3s established the CDW transition temperature accompanied by Phonon softening; Angle-resolved Raman data confirming the nearly four-fold symmetry; thickness-dependent Raman spectroscopy resulting in the conclusion that as thickness decreases CDW transition temperature increases. Also, CDW transition is analyzed as a function of alloying.

Nanoholes on the basal plane of graphene can provide abundant mass transport channels and chemically active sites for enhancing the electrochemical performance, making this material highly promising in applications such as supercapacitors, batteries, desalination, molecule or ion detection, and biosensing. However, the current solution-based chemical etching processes to manufacture these nanoholes commonly suffer from low process efficiency, scalability, and controllability, because conventional bulk heating cannot promote the etching reactions. Herein, a novel manufacturing method is developed to address this issue using microwave irradiation to facilitate and control the chemical etching of graphene. In this process, microwave irradiation induces selective heating of graphene in the aqueous solution due to an energy dissipation mechanism coupled with the dielectric and conduction losses. This strategy brings a remarkable reduction of processing time from hour-scale to minute-scale compared to the conventional approaches. By further incorporating microwave pretreatment, it is possible to control the population and area percentage of the in-plane nanoholes on graphene. Theoretical calculations reveal that the nanoholes emerge and grow by a repeating reduction–oxidation process occurring at the edge-sites atoms around vacancy defects on the graphene basal plane. The reduced holey graphene oxide sheets obtained via the microwave-assisted chemical etching method exhibit great potentials in supercapacitors and electrocatalysis. Excellent capacitive performance and electrocatalytic activity are observed in electrochemical measurements. The improvements against the non-holey counterpart are attributed to the enhanced kinetics involving ion diffusion and heterogeneous charge transfer.

Wide bandgap semiconductors are of much current interest due to their superior electrical properties. This dissertation describes electron microscopy characterization of GaN-on-GaN structures for high-power vertical device applications. Unintentionally-doped (UID) GaN layers grown homoepitaxially via metal-organic chemical vapor deposition on freestanding GaN substrates, were subjected to dry etching, and layers of UID-GaN/p-GaN were over-grown. The as-grown and regrown heterostructures were examined in cross-section using transmission electron microscopy (TEM). Two different etching treatments, fast-etch-only and multiple etches with decreasing power, were employed. The fast-etch-only devices showed GaN-on-GaN interface at etched location, and low device breakdown voltages were measured (~ 45-95V). In comparison, no interfaces were visible after multiple etching steps, and the corresponding breakdown voltages were much higher (~1200-1270V). These results emphasized importance of optimizing surface etching techniques for avoiding degraded device performance. The morphology of GaN-on-GaN devices after reverse-bias electrical stressing to breakdown was investigated. All failed devices had irreversible structural damage, showing large surface craters (~15-35 microns deep) with lengthy surface cracks. Cross-sectional TEM of failed devices showed high densities of threading dislocations (TDs) around the cracks and near crater surfaces. Progressive ion-milling across damaged devices revealed high densities of TDs and the presence of voids beneath cracks: these features were not observed in unstressed devices. The morphology of GaN substrates grown by hydride vapor-phase epitaxy (HVPE) and by ammonothermal methods were correlated with reverse-bias results. HVPE substrates showed arrays of surface features when observed by X-ray topography (XRT). All fabricated devices that overlapped with these features had typical reverse-bias voltages less than 100V at a leakage current limit of 10-6 A. In contrast, devices not overlapping with such features reached voltages greater than 300V. After etching, HVPE substrate surfaces showed defect clusters and macro-pits, whereas XRT images of ammonothermal substrate revealed no visible features. However, some devices fabricated on ammonothermal substrate failed at low voltages. Devices on HVPE and ammonothermal substrates with low breakdown voltages showed crater-like surface damage and revealed TDs (~25µm deep) and voids; such features were not observed in devices reaching higher voltages. These results should assist in developing protocols to fabricate reliable high-voltage devices.