Gallium nitride-based blue laser diodes

Group III nitride wide band gap semiconductors have recently attracted considerable attention due to their applications for optical devices operating in the blue and UV wavelength regions. Nitride materials are stable at high temperatures and also chemically stable. Analysis of InGaN based blue laser diodes is required for the research and development of future deep UV laser diodes for bio-sensing applications. The conventional multiple quantum wells suffer from inhomogeneous carrier distribution across quantum wells. This inhomogeneous distribution increases the threshold current density of the laser diode. This project addresses the issue through the design of a novel structure for the active region of InGaN blue laser diodes. As a possible replacement for the AlGaN cladding layers optical properties of a 1D photonic crystal was investigated. Part of the work deals with the etching of GaN in SF6 plasma using ICP-RIE systems

Probing And Tuning The Size, Morphology, Chemistry And Structure Of Nanoscale Cerium Oxide

Cerium oxide (ceria)-based materials in the nanoscale regime are of significant fundamental and technological interest. Nanoceria in pure and doped forms has current and potential use in solid oxide fuel cells, catalysis, UV- screening, chemical mechanical planarization, oxygen sensors, and bio-medical applications. The characteristic feature of Ce to switch between the +3 and + 4 oxidation states renders oxygen buffering capability to ceria. The ease of this transformation was expected to be enhanced in the nanoceria. In most the practical scenarios, it is necessary to have a stable suspension of ceria nanoparticles (CNPs) over longer periods of time. However, the existing literature is confined to short term studies pertaining to synthesis and property evaluation. Having understood the need for a comprehensive understanding of the CNP suspensions, this dissertation is primarily aimed at understanding the behavior of CNPs in various chemical and physical environments. We have synthesized CNPs in the absence of any surfactants at room temperature and studied the aging characteristics. After gaining some understanding about the behavior of this functional oxide, the synthesis environment and aging temperature were varied, and their affects were carefully analyzed using various materials analysis techniques such as high resolution transmission electron microscopy (HRTEM), UV-Visible spectroscopy (UV-Vis), and X-ray photoelectron spectroscopy (XPS). When the CNPs were aged at room temperature in as-synthesized condition, they were observed to spontaneously assemble and evolve as fractal superoctahedral structures. The reasons for this unique polycrystalline morphology were attributed to the symmetry driven assembly of the individual truncated octahedral and octahedral seed of the ceria. HRTEM and Fast Fourier Transform (FFT) analyses were used to explain the agglomeration behavior and evolution of the octahedral morphology. Some of the observations were supported by molecular dynamic simulations. Poly (ethylene glycol) (PEG) and ethylene glycol (EG) were used to control the kinetics of this morphology evolution. The ability to control the agglomeration of CNPs in these media stems from the lower dielectric constant and an increased viscosity of the EG and PEG based solvents. CNPs when synthesized and aged in frozen conditions, i.e. in ice, were found to form one dimensional, high aspect ratio structures. A careful analysis has provided some evidence that the CNPs use the porous channels in ice as a template and undergo oriented attachment to form nanorods. When the aging treatment was done near freezing temperature in solution, the nanorods were not observed, confirming the role of channels in ice. When synthesized in aqueous media such as DI water, PEG and EG; CNPs were observed to exhibit a reversible oxidation state switching between +3 and +4. Band gap values were computed from the optical absorption data. The changes in the band gap values observed were attributed to the changes in the oxidation state of CNPs as opposed to the quantum confinement effects, as expected in other nanoparticle systems. The work presented in this dissertation demonstrates, with evidence, that in order to obtain a comprehensive understanding of the properties of nanoscale materials it is of paramount importance to monitor their behavior over relatively longer periods of time under various ambient environments. While the solution based techniques offer a versatility and low cost route to study the fundamental properties of nanomaterials, they suffer some inherent problems such as precursor contamination and uncontrolled chemical reactions. Especially when analyzing the behavior of ceria-based materials for applications like solid oxide fuel cells, a great control in the density and crystalline quality are desired. In order to achieve this, as a first step pure ceria thin films were synthesized using oxygen plasma assisted molecular beam epitaxy (OPA-MBE). The ceria films were analyzed using various in situ and ex situ techniques to study the crystal structure, growth mode and epitaxial quality of the films. It was observed that the epitaxial orientation of the ceria films could be tuned by varying the deposition rate. When the films were grown at low deposition rate (\u3c 8 Å/min) ceria films with epitaxial (200) orientation were observed where as the films grown at high deposition rates (up to 30 Å/min) showed (111) orientation. Theoretical simulations were used to confirm some of the experimental facts observed in both nanoparticles and thin films

Development of surface micromachined Aluminum Nitride air-bridges for piezoelectric MEMS/NEMS applications by Metal Organic Vapor Phase Epitaxy techniques

Group III-nitrides have attracted considerable attention for piezoelectric Micro/Nano electromechanical (MEMS/NEMS) applications due to their excellent bio compatibility, well developed growth techniques for high quality thin films and structural stability at high temperatures when compared to the commonly used piezoelectric metal oxides. Among the group III-nitrides Aluminum Nitride (AlN) possess superior material properties such as highest piezoelectric coefficient and good mechanical properties. Growth techniques for fabricating group III-nitride MEMS/NEMS by metal organic vapor phase epitaxy (MOVPE) techniques have involved sacrificial layers such as epitaxial group III-nitrides/ alloys, nanocrystalline films and porous interlayers. However, the material properties of the MOVPE grown films on the amorphous sacrificial layers such as silicon oxide have not been adequately investigated to evaluate potential MEMS/NEMS devices such as piezoelectric micro/ nanofluidic channels.;This work demonstrates a process for the fabrication of Aluminum nitride (AlN) thin film air-bridges using MOVPE techniques on silicon templates. Micro-FTIR techniques were used to study the crystallographic orientation of the AlN thin film air-bridges with lateral dimensions as low as 100 mum. FTIR results also show that the wet etching process to remove the underlying sacrificial layer also improves the material properties of the AlN films on SiOx. The study indicates that AlN air-bridges are polycrystalline in nature and are preferentially c-axis oriented after wet etching. Lateral field excitation of the piezoelectric films and laser Doppler vibrometer techniques were combined to investigate the piezoelectric response of the AlN films on the sacrificial layer. Lateral field excitation of the AlN films grown on the amorphous sacrificial layer shows that the AlN films exhibit piezoelectric properties. The displacement of the AlN air-bridges obtained by lateral field actuation is around 1 nm over an air-gap of 130 nm after the removal of the sacrificial layer. However, the mismatch in the coefficient of thermal expansion between the substrate and thin films induces significant residual stress in the heterostructure. The AlN air-bridges on silicon substrate exhibit fracture due to the tensile residual stress exceeding the fracture limit

Size dependency variation in lattice parameter and valency states in nanocrystalline cerium oxide

A correlation between the particle size and the lattice parameter has been established in nanocerium oxide particles (3-30 nm). The variation in the lattice parameter is attributed to the lattice strain induced by the introduction of Ce3+ due to the formation of oxygen vacancies. Lattice strain was observed to decrease with an increase in the particle size. Ce3+ ions concentration increased from 17% to 44% with the reduction in the particle size

On the Effect of Initial Conditions on Rayleigh-Taylor Mixing

An experimental investigation of the effects of Initial Conditions (ICs) on Rayleigh-Taylor Instability (RTI) is performed using theWater Channel facility at Texas A&M University. Hot and cold water (with a temperature difference of ~ 5-8 °C) selected as working fluids are unstably stratified initially. The resulting Atwood number for this instability is of the order of 10^-3. In this dissertation, effect of the composition of the initial perturbations generated by a flapper mechanism is studied. Using the servo controlled flapper system, initial wavelengths are varied between 2-8 cm and phase angles within 0-180°, and the dependence of ensemble averaged mixing width in the linear and nonlinear stages of growth on low Atwood number Rayleigh-Taylor mixing is studied. The Interaction of multiple (up to 11) modes of the IC is studied by varying the wavelengths and phase angles that are generated at the interface of these fluids. High resolution Planar Laser Induced Fluorescence (PLIF) images of the flow field indicate that changing the phase angle results in the leaning phenomenon, i.e. the leaning of bubbles and spikes with respect to gravitational acceleration, and relative to each other. Experimental measurement of total mixing width, quality of molecular mixing and scalar dissipation rate are performed using ensemble averaging technique. The results indicate that nonlinear mode coupling of the initial modes affects the rate of mixing as well as the transition to turbulence. Molecular mixing measurements indicate that the molecular mixing rate depends upon the ICs. Particle Image Velocimetry (PIV) data indicates that late-time velocity profiles of the mixing layer depend upon ICs. While the density power spectra indicate independence of ICs in the inertial range, the turbulent velocity statistics indicate that increasing the number of modes of the IC quickens the transition to turbulence. It also results in greater mixing, which reduces the density gradients as the flow evolves with time. Thus, the memory of the ICs is lost sooner with increasing number of initial modes

Characteristics and Outcomes of Dementia Patients Who Receive Inpatient Palliative Care Consultation

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/162727/2/jgs16521_am.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/162727/1/jgs16521.pd

Experimental Investigation of the Effect of Initial Conditions on Rayleigh-Taylor Instability

An experimental study of the effect of initial conditions on the development of Rayleigh Taylor Instabilities (RTI) at low Atwood numbers (order of 10-4) was performed in the water channel facility at TAMU. Initial conditions of the flow were generated using a controllable, highly reliable Servo motor. The uniqueness of the study is the system’s capability of generating the required initial conditions precisely as compared to the previous endeavors. Backlit photography was used for imaging and ensemble averaging of the images was performed to study mixing width characteristics in different regimes of evolution of Rayleigh-Taylor Instability (RTI). High-speed imaging of the flows was performed to provide insights into the growth of bubble and spikes in the linear and non-linear regime of instability development. RTI are observed in astrophysics, geophysics and in many instances in nature. The vital role of RTI in the feasibility and efficiency of the Inertial Confinement Fusion (ICF) experiment warrants a comprehensive study of the effect of mixing characteristics of RTI and its dependence on defining parameters. With this broader objective in perspective, the objectives of this present investigation were mainly threefold: First was the validation of the novel setup of the Water channel system. Towards this objective, validation of Servo motor, splitter plate thickness effects, density and temperature measurements and single-mode experiments were performed. The second objective was to study the mixing and growth characteristics of binary and multi-mode initial perturbations seeking an explanation of behavior of the resultant flow structures by performing the first ever set of such highly controlled experiments. The first-ever set of experiments with highly controlled multi-mode initial conditions was performed. The final objective of this study was to measure and compare the bubble and spike velocities with single-mode initial conditions with existing analytical models. The data derived from these experiments would qualitatively and quantitatively enhance the understanding of dependence of mixing width on parametric initial conditions. The knowledge would contribute towards a generalized theory for RTI mixing with specified dependence on various parameters, which has a wide range of applications. The system setup was validated to provide a reliable platform for the novel multi-modal experiments to be performed in the future. It was observed that the ensemble averaged mixing width of the binary system does not vary significantly with the phase-difference between the modes of a binary mode initial condition experiment, whereas it varies with the amplitudes of the component modes. In the exponential and non-linear regimes of evolution, growth rates of multi-mode perturbations were found to be higher than the component modes, whereas saturation growth rates correspond to the dominant wavelength. Quadratic saturation growth rate constants, alpha were found to be about 0.07 ± 0.01 for binary and multi modes whereas single-mode data measured alpha about 0.06 ± 0.01. High-speed imaging was performed to measure bubble and spike amplitudes to obtain velocities and growth rates. It was concluded that higher temporal and spatial resolution was required for accurate measurement. The knowledge gained from the above study will facilitate a better understanding of the physics underlying Rayleigh-Taylor instability. The results of this study will also help validating numerical models for simulation of this instability, thereby providing predictive capability for more complex configurations

ZipA Binds to FtsZ with High Affinity and Enhances the Stability of FtsZ Protofilaments

A bacterial membrane protein ZipA that tethers FtsZ to the membrane is known to promote FtsZ assembly. In this study, the binding of ZipA to FtsZ was monitored using fluorescence spectroscopy. ZipA was found to bind to FtsZ with high affinities at three different (6.0, 6.8 and 8.0) pHs, albeit the binding affinity decreased with increasing pH. Further, thick bundles of FtsZ protofilaments were observed in the presence of ZipA under the pH conditions used in this study indicating that ZipA can promote FtsZ assembly and stabilize FtsZ polymers under unfavorable conditions. Bis-ANS, a hydrophobic probe, decreased the interaction of FtsZ and ZipA indicating that the interaction between FtsZ and ZipA is hydrophobic in nature. ZipA prevented the dilution induced disassembly of FtsZ polymers suggesting that it stabilizes FtsZ protofilaments. Fluorescein isothiocyanate-labeled ZipA was found to be uniformly distributed along the length of the FtsZ protofilaments indicating that ZipA stabilizes FtsZ protofilaments by cross-linking them

Powerful sequence similarity search methods and in-depth manual analyses can identify remote homologs in many apparently "orphan" viral proteins

The genome sequences of new viruses often contain many "orphan" or "taxon-specific" proteins apparently lacking homologs. However, because viral proteins evolve very fast, commonly used sequence similarity detection methods such as BLAST may overlook homologs. We analyzed a data set of proteins from RNA viruses characterized as "genus specific" by BLAST. More powerful methods developed recently, such as HHblits or HHpred (available through web-based, user-friendly interfaces), could detect distant homologs of a quarter of these proteins, suggesting that these methods should be used to annotate viral genomes. In-depth manual analyses of a subset of the remaining sequences, guided by contextual information such as taxonomy, gene order, or domain cooccurrence, identified distant homologs of another third. Thus, a combination of powerful automated methods and manual analyses can uncover distant homologs of many proteins thought to be orphans. We expect these methodological results to be also applicable to cellular organisms, since they generally evolve much more slowly than RNA viruses. As an application, we reanalyzed the genome of a bee pathogen, Chronic bee paralysis virus (CBPV). We could identify homologs of most of its proteins thought to be orphans; in each case, identifying homologs provided functional clues. We discovered that CBPV encodes a domain homologous to the Alphavirus methyltransferase-guanylyltransferase; a putative membrane protein, SP24, with homologs in unrelated insect viruses and insect-transmitted plant viruses having different morphologies (cileviruses, higreviruses, blunerviruses, negeviruses); and a putative virion glycoprotein, ORF2, also found in negeviruses. SP24 and ORF2 are probably major structural components of the virionsd

What the CERAD Battery Can Tell Us about Executive Function as a Higher-Order Cognitive Faculty

Executive function (EF) is believed to control or influence the integration and application of cognitive functions such as attention and memory and is an important area of research in cognitive aging. Recent studies and reviews have concluded that there is no single test for EF. Results from first-order latent variable modeling have suggested that little, if any, variability in cognitive performance can be directly (and uniquely) attributed to EF; so instead, we modeled EF, as it is conceptualized, as a higher-order function, using elements of the CERAD neuropsychological battery. Responses to subtests from two large, independent cohorts of nondemented elderly persons were modeled with three theoretically plausible structural models using confirmatory factor analysis. Robust fit statistics, generated for the two cohorts separately, were consistent and support the conceptualization of EF as a higher-order cognitive faculty. Although not specifically designed to assess EF, subtests of the CERAD battery provide theoretically and empirically robust evidence about the nature of EF in elderly adults