High-Temperature Growth of Gallium Nitride Using the Ammonothermal Method with Ammonium Chloride Mineralizer

Gallium nitride (GaN) has become an important semiconductor for the optoelectronics and power electronics fields in the pursuit of high efficiency devices. However, the lack of a natural native GaN substrate has forced growth of GaN devices on foreign substrates such as sapphire, silicon carbide, and silicon. To further enhance efficiency and develop devices with longer lifetimes, the number of defects present in devices must be reduced. The development of a native GaN substrate of high crystalline quality would directly enable defect reduction. The ammonothermal method of GaN growth has shown significant promise as a technique for the production of high quality GaN crystals of industrially significant size (crystals on the order of centimeters in the largest dimension). The ammonothermal method is a solvothermal method that uses a mineralizer (here ammonium chloride) with supercritical ammonia to transport GaN from a source material from one temperature zone to grow a seed crystal in another temperature zone. High pressures, high temperatures, and the presence of a highly corrosive chemistry make development of an economical growth reactor challenging. This body of work outlines the development of a growth reactor capable of high temperature ammonothermal growth of GaN using ammonium chloride mineralizer.Initial development of the ammonothermal reactor required identification of suitable reactor materials. A materials stability study was conducted by exposing samples of materials to the ammonothermal environment and measuring mass loss as well as any chemical or mechanical changes that occurred. An Inconel 625 alloy reactor was employed, although the reactor itself was somewhat susceptible to corrosion from the ammonothermal environment. The study yielded a subset of materials that may be suitable for use as gaskets and other single use items which include niobium, molybdenum, titanium, vanadium, tungsten, gold, and platinum. Alloys of molybdenum and cobalt may also be useful. High strength titanium-zirconium-molybdenum (TZM) was also identified as a corrosion resistant material and was selected for reactor design.A TZM reactor was then designed and fabricated. Subsequent high pressure, high temperature tests indicated that TZM was essentially inert and growth of GaN crystals followed. All GaN growth was accomplished at or above 650°C using seed crystals grown by hydride vapor phase epitaxy. Seeds were characterized by micrometer measurements for growth thickness, x-ray diffraction (XRD) for crystalline quality, and secondary ion mass spectrometry (SIMS) for impurity concentrations. The growth quality appeared to match the seed quality as measured by XRD. Growth coloration ranged from slightly gray to green or yellow with growth rates up to 191 µm/day. Most seeds exhibited significant faceting at the edges of the sample, forming semipolar planes. SIMS was performed on a couple of samples which indicated oxygen concentrations of ~1018 cm¬-3

Planar and Three-Dimensional Printing of Conductive Inks

Printed electronics rely on low-cost, large-area fabrication routes to create flexible or multidimensional electronic, optoelectronic, and biomedical devices1-3. In this paper, we focus on one- (1D), two- (2D), and three-dimensional (3D) printing of conductive metallic inks in the form of flexible, stretchable, and spanning microelectrodes

Cardiorespiratory Fitness and Attentional Control in the Aging Brain

A growing body of literature provides evidence for the prophylactic influence of cardiorespiratory fitness on cognitive decline in older adults. This study examined the association between cardiorespiratory fitness and recruitment of the neural circuits involved in an attentional control task in a group of healthy older adults. Employing a version of the Stroop task, we examined whether higher levels of cardiorespiratory fitness were associated with an increase in activation in cortical regions responsible for imposing attentional control along with an up-regulation of activity in sensory brain regions that process task-relevant representations. Higher fitness levels were associated with better behavioral performance and an increase in the recruitment of prefrontal and parietal cortices in the most challenging condition, thus providing evidence that cardiorespiratory fitness is associated with an increase in the recruitment of the anterior processing regions. There was a top-down modulation of extrastriate visual areas that process both task-relevant and task-irrelevant attributes relative to the baseline. However, fitness was not associated with differential activation in the posterior processing regions, suggesting that fitness enhances attentional function by primarily influencing the neural circuitry of anterior cortical regions. This study provides novel evidence of a differential association of fitness with anterior and posterior brain regions, shedding further light onto the neural changes accompanying cardiorespiratory fitness

Structural conservation of an ancient tRNA sensor in eukaryotic glutaminyl-tRNA synthetase

In all organisms, aminoacyl tRNA synthetases covalently attach amino acids to their cognate tRNAs. Many eukaryotic tRNA synthetases have acquired appended domains, whose origin, structure and function are poorly understood. The N-terminal appended domain (NTD) of glutaminyl-tRNA synthetase (GlnRS) is intriguing since GlnRS is primarily a eukaryotic enzyme, whereas in other kingdoms Gln-tRNAGln is primarily synthesized by first forming Glu-tRNAGln, followed by conversion to Gln-tRNAGln by a tRNA-dependent amidotransferase. We report a functional and structural analysis of the NTD of Saccharomyces cerevisiae GlnRS, Gln4. Yeast mutants lacking the NTD exhibit growth defects, and Gln4 lacking the NTD has reduced complementarity for tRNAGln and glutamine. The 187-amino acid Gln4 NTD, crystallized and solved at 2.3 Å resolution, consists of two subdomains, each exhibiting an extraordinary structural resemblance to adjacent tRNA specificity-determining domains in the GatB subunit of the GatCAB amidotransferase, which forms Gln-tRNAGln. These subdomains are connected by an apparent hinge comprised of conserved residues. Mutation of these amino acids produces Gln4 variants with reduced affinity for tRNAGln, consistent with a hinge-closing mechanism proposed for GatB recognition of tRNA. Our results suggest a possible origin and function of the NTD that would link the phylogenetically diverse mechanisms of Gln-tRNAGln synthesis

Isolation, Cloning and Structural Characterisation of Boophilin, a Multifunctional Kunitz-Type Proteinase Inhibitor from the Cattle Tick

Inhibitors of coagulation factors from blood-feeding animals display a wide variety of structural motifs and inhibition mechanisms. We have isolated a novel inhibitor from the cattle tick Boophilus microplus, one of the most widespread parasites of farm animals. The inhibitor, which we have termed boophilin, has been cloned and overexpressed in Escherichia coli. Mature boophilin is composed of two canonical Kunitz-type domains, and inhibits not only the major procoagulant enzyme, thrombin, but in addition, and by contrast to all other previously characterised natural thrombin inhibitors, significantly interferes with the proteolytic activity of other serine proteinases such as trypsin and plasmin. The crystal structure of the bovine α-thrombin·boophilin complex, refined at 2.35 Å resolution reveals a non-canonical binding mode to the proteinase. The N-terminal region of the mature inhibitor, Q16-R17-N18, binds in a parallel manner across the active site of the proteinase, with the guanidinium group of R17 anchored in the S1 pocket, while the C-terminal Kunitz domain is negatively charged and docks into the basic exosite I of thrombin. This binding mode resembles the previously characterised thrombin inhibitor, ornithodorin which, unlike boophilin, is composed of two distorted Kunitz modules. Unexpectedly, both boophilin domains adopt markedly different orientations when compared to those of ornithodorin, in its complex with thrombin. The N-terminal boophilin domain rotates 9° and is displaced by 6 Å, while the C-terminal domain rotates almost 6° accompanied by a 3 Å displacement. The reactive-site loop of the N-terminal Kunitz domain of boophilin with its P1 residue, K31, is fully solvent exposed and could thus bind a second trypsin-like proteinase without sterical restraints. This finding explains the formation of a ternary thrombin·boophilin·trypsin complex, and suggests a mechanism for prothrombinase inhibition in vivo

Digestion processes and elemental analysis of oxide and sulfide solid electrolytes

Detailed elemental analysis is essential for a successful development and optimization of material systems and synthesis methods. This is especially relevant for Li- and Na-containing compounds, found in state-of-the-art and next-generation battery systems. Their materials’ properties and thus the final device performance strongly depend on the crystal structure, the stoichiometry, and defect chemistry, e.g., influencing charge carrier concentration and activation energies for vacancy transport. However, a detailed quantitative analysis of light elements in a heavy matrix, featuring a broad range of solubilities and vapor pressures, is often difficult and associated with large uncertainties and thus neglected in favor of just reporting the stoichiometry as “weighed in.” In this work, we report several approaches to digest and dissolve various oxide and sulfide-based materials, used in next-generation Li batteries, for elemental analysis via optical emission spectroscopy. These include the most common solid electrolytes Li-La-Ti–O, a perovskite material (LLTO), and Li-La-Zr-O which has garnet structure (LLZO). Additionally, a facile thermal digestion process is reported for a surrogate sulfide solid electrolyte (Na2S). The digestion procedures reported here are suitable for almost any laboratory environment and, when applied, will improve understanding of the synthesis-structure–property correlations needed to advanced batteries with all solid-state configurations