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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 289 records · Page 16
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Structure-based enzyme engineering improves donor-substrate recognition of Arabidopsis thaliana glycosyltransferases

Glycosylation of secondary metabolites involves plant UDP-dependent glycosyltransferases (UGTs). UGTs have shown promise as catalysts in the synthesis of glycosides for medical treatment. However, limited understanding at the molecular level due to insufficient biochemical and structural information has hindered potential applications of most of these UGTs. In the absence of experimental crystal structures, we employed advanced molecular modeling and simulations in conjunction with biochemical characterization to design a workflow to study five Group H Arabidopsis thaliana (76E1, 76E2, 76E4, 76E5, 76D1) UGTs. Based on our rational structural manipulation and analysis, we identified key amino acids (P129 in 76D1; D374 in 76E2; K275 in 76E4), which when mutated improved donor substrate recognition than wildtype UGTs. Molecular dynamics simulations and deep learning analysis identified structural differences, which drive substrate preferences. The design of these UGTs with broader substrate specificity may play important role in biotechnological and industrial applications. These findings can also serve as basis to study other plant UGTs and thereby advancing UGT enzyme engineering.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2019

Growth of GaP1 − x − yAsyNx on Si substrates by chemical beam epitaxy

Chemical beam epitaxy has been used to grow layers of GaP1 − xNx, GaP1 − yAsy, and nearly lattice-matched layers GaP1 − x − yAsyNx on Si substrates. To address the issue of antiphase domain generation associated with the growth of polar semiconductors on Si, misoriented Si(001) substrates have been used combined with a carefully designed GaP buffer layer growth. The reflection high-energy electron diffraction pattern exhibits a (2 × 4) surface reconstruction after GaP buffer layer and GaP(As,N) graded layer growth, indicating the good surface quality and planarity of the grown layers. Sample composition was obtained by simultaneous acquisition of Rutherford backscattering spectrometry and nuclear reaction analysis, indicating a linear dependence of N and As mole fractions on the flux of their respective precursor. GaP1 − x − yAsyNx layers grown on Si substrates have a lattice mismatch not larger than ±0.005 for N contents in the range 0.02 < x < 0.05. High-resolution X-ray diffraction reciprocal space maps demonstrate a good crystalline quality. Intense photoluminescence spectra have been measured in all GaP1 − xNx and GaP1 − x − yAsyNx layers, as it is expected for direct bandgap materials. Two wide overlapped emission peaks are observed in all the spectra, most likely related to near bandgap recombination. The position of the higher energy peak for GaP1 − xNx and GaP1 − x − yAsyNx layers has been compared to bandgap energy calculations using the band anticrossing model, showing good agreement.

Ben Saddik, K.↗
DOE OSTI2020

Influence of substrate misorientation on the optical properties of Mg-doped GaN

The GaN substrate miscut angle has a significant effect on the optical properties of Mg-doped GaN grown by metal-organic chemical vapor deposition. We have studied the optical properties of epilayers grown on c-plane GaN wafers with nominal miscut angles of 0.3° and 4° toward the [11¯00] direction. Periodic surface steps are observed in thin films grown on 0.3° miscut substrates. The step separation and height, as well as the surface roughness, increase with layer thickness, leading to a decrease in Mg doping efficiency. For films grown on 4° miscut substrates, step bunching causes the formation of visually observable wavy features on the film surface with strong spatial variations in cathodoluminescence characteristics, indicating a significant effect of surface morphology on the nature of Mg incorporation and p-type characteristics.

Liu, Hanxiao (ORCID:0000000331067542)↗
DOE OSTI2020

Ligand structure and adsorption free energy of nanocrystals on solid substrates

In this paper, we present an investigation on the absorption of alkylthiolated nanocrystals on a solid substrate. We calculate adsorption free energies and report a number of effects induced by the substrate. Nearest neighbor distances and bonding free energies are significantly different than for a free floating case, there is a weakening of bonding free energies among nanocrystals, and the adsorption is manifestly anisotropic, i.e., stronger along certain directions of the nanocrystal core. We contend that this last result accounts for the Bain transition (fcc → bcc) observed in experimental results. We report the presence of vortices induced by the substrate, which explain the increased nearest neighbor distance among nanocrystals, which is in excellent quantitative agreement with experimental results and with the predictions of the Orbifold Topological Model. Implications for the assembly of nanostructures and future experiments are also discussed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Record >10 MV/cm mesa breakdown fields in Al 0.85 Ga 0.15 N/Al 0.6 Ga 0.4 N high electron mobility transistors on native AlN substrates

The ultra-wide bandgap of Al-rich AlGaN is expected to support a significantly larger breakdown field compared to GaN, but the reported performance thus far has been limited by the use of foreign substrates. In this Letter, the material and electrical properties of Al 0.85 Ga 0.15 N/Al 0.6 Ga 0.4 N high electron mobility transistors (HEMT) grown on a 2-in. single crystal AlN substrate are investigated, and it is demonstrated that native AlN substrates unlock the potential for Al-rich AlGaN to sustain large fields in such devices. We further study how Ohmic contacts made directly to a Si-doped channel layer reduce the knee voltage and increase the output current density. High-quality AlGaN growth is confirmed via scanning transmission electron microscopy, which also reveals the absence of metal penetration at the Ohmic contact interface and is in contrast to established GaN HEMT technology. Two-terminal mesa breakdown characteristics with 1.3 μm separation possess a record-high breakdown field strength of ~11.5 MV/cm for an undoped Al 0.6 Ga 0.4 N-channel layer. The breakdown voltages for three-terminal devices measured with gate-drain distances of 4 and 9 μm are 850 and 1500 V, respectively.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Molecular beam homoepitaxy of N-polar AlN on bulk AlN substrates

N-polar AlN epilayers were grown on the N-face of single-crystal bulk AlN substrates by plasma-assisted molecular beam epitaxy. A combination of in situ thermal deoxidation and Al-assisted thermal desorption at high temperature aided in removing native surface oxides and impurities from the N-polar surface of the substrate enabling successful homoepitaxy. Subsequent epitaxial growth of the AlN layer on the in situ cleaned substrates, grown in a sufficiently high Al droplet regime, exhibited smooth surface morphologies with clean and wide atomic steps. KOH etch studies confirmed the N-polarity of the homoepitaxial films. Secondary ion mass spectrometry profiles show Si and H impurity concentrations below the noise levels, whereas O and C impurity concentrations of [Formula: see text] and [Formula: see text] atoms/cm 3 are observed, respectively. Although the structural defect densities are low, they interestingly appear as inversion domains of different dimensionalities.

Singhal, Jashan↗
DOE OSTI2022

Synthesis of antiferromagnetic Weyl semimetal Mn 3 Ge on insulating substrates by electron beam assisted molecular beam epitaxy

The antiferromagnetic kagome semimetals Mn 3 X (X = Ge, Sn, Ga) are of great interest due to properties arising from their Berry curvature, such as large anomalous Nernst and anomalous Hall coefficients, and spin to charge conversion efficiencies at ambient temperatures. However, the synthesis of epitaxial thin films of Mn 3 Ge in the desired hexagonal phase has been challenging because they do not wet insulating substrates, necessitating the use of a metallic buffer layer. Furthermore, a ferrimagnetic tetragonal phase also forms readily under typical growth conditions, interfering with hexagonal phase properties. We have synthesized atomically smooth and continuous epitaxial thin films of hexagonal Mn 3 Ge directly on insulating LaAlO 3 (111) substrates using electron beam assisted molecular beam epitaxy, using a three-step process that mitigates the formation of the tetragonal phase. The anomalous Nernst coefficient is found to be more than six times larger in our films than in sputtered thin films of Mn 3 Ge and significantly larger than that of Fe. Our approach can be used to grow thin layers of kagome materials, without interference from a buffer layer in transport properties, and may be applicable to a broader range of materials with large surface energies that do not grow readily on insulating substrates.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Carbon nanotube substrates enhance SARS-CoV-2 spike protein ion yields in matrix-assisted laser desorption–ionization mass spectrometry

Nanostructured surfaces enhance ion yields in matrix-assisted laser desorption–ionization mass spectrometry (MALDI-MS). The spike protein complex, S1, is one fingerprint signature of Sars-CoV-2 with a mass of 75 kDa. Here, we show that MALDI-MS yields of Sars-CoV-2 spike protein ions in the 100 kDa range are enhanced 50-fold when the matrix–analyte solution is placed on substrates that are coated with a dense forest of multi-walled carbon nanotubes, compared to yields from uncoated substrates. Nanostructured substrates can support the development of mass spectrometry techniques for sensitive pathogen detection and environmental monitoring.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2023

Origins of epitaxial macro-terraces and macro-steps on GaN substrates

Localized lattice distortions in GaN substrates can serve as nucleation sites for epitaxial macro-steps and macro-terraces. These detrimental macro-scale features give rise to optically hazy homoepitaxial GaN surfaces. After nucleating, these macro-features grow laterally along the surface and coalesce, leading to significant coverage of the wafer surface. Dot-core GaN substrates consisting of a periodic array of cores were used as a defect-engineered system, where dislocations are intentionally concentrated at the cores. The high density of threading dislocations at the cores induced localized lattice distortions. Here, these distortions are associated predominantly with lattice tilt on the order of hundreds of arcsec across ~0.5 mm laterally along the wafer surface. The resulting macro-features that nucleated at these localized distorted sites were made up of macro-terraces with lengths ranging ~30–~150 μm and macro-step heights ranging ~200–~400 nm. Another source of localized distortion was threading screw dislocations or GaN nanopipes that resulted in spiral growth and hillock formation. Based on x-ray topography and optical microscopy measurements, we speculate that the coalescence of hillocks evolves into macro-terraces and macro-steps. While previous studies focused on the substrate miscut as a means to control macro-feature formation, we show that localized lattice tilt from defects is another important contributor to macro-feature formation.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Interband cascade light-emitting diodes grown on silicon substrates using GaSb buffer layer

Interband cascade light-emitting diodes (ICLEDs) offer attractive advantages for infrared applications, which would greatly expand if high-quality growth on silicon substrates could be achieved. Here, this work describes the formation of threading dislocations in ICLEDs grown monolithically on GaSb-on-Silicon wafers. The epitaxial growth is done in two stages: the GaSb-on-Silicon buffer is grown first, followed by the ICLED growth. The buffer growth involves the nucleation of a 10-nm-thick AlSb buffer layer on the silicon surface, followed by the GaSb growth. The AlSb nucleation layer promotes the formation of 90° and 60° interfacial misfit dislocations, resulting in a highly planar morphology for subsequent GaSb growth that is almost 100% relaxed. The resulting GaSb buffer for growth of the ICLED has a threading dislocation density of ~10 7 /cm 2 after ~3 μm of growth. The fabricated LEDs showed variations in device performance, with some devices demonstrating comparable light–current–voltage curves to those for devices grown on GaSb substrates, while other devices showed somewhat reduced relative performance. Cross-sectional transmission electron microscopy observations of the inferior diodes indicated that the multiplication of threading dislocations in the active region had most likely caused the increased leakage current and lower output power. Enhanced defect filter layers on the GaSb/Si substrates should provide more consistent diode performance and a viable future growth approach for antimonide-based ICLEDs and other infrared devices.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2024

Superconducting (Ba,K)Fe 2 As 2 epitaxial films on single and bicrystal SrTiO 3 substrates

The realization of single crystal and bicrystal films of superconducting materials is of great interest because they allow the investigation of the intragrain performance as well as the understanding of potential limitations in the grain boundary transparency. For many years, the realization of a high-quality (Ba,K)Fe 2 As 2 film has been challenging. Here, the realization of (Ba,K)Fe 2 As 2 epitaxial thin films on single crystal SrTiO 3 (001) and [001]-tilt-type SrTiO 3 bicrystal substrates with high superconducting properties is demonstrated. The epitaxial growth of (Ba,K)Fe 2 As 2 was enabled by implementing an undoped BaFe 2 As 2 buffer layer between the SrTiO 3 substrate and (Ba,K)Fe 2 As 2 film. The film exhibits a high T c of 38.0 K and an extremely high J c of 14.3 MA/cm 2 at 4.2 K. Artificial grain boundaries of (Ba,K)Fe 2 As 2 were also successfully achieved on bicrystals with misorientation angles up to 36.8° by the same preparation methods. The artificial grain boundaries exhibited an identical T c of 38.0 K and an excellent transfer of the grain orientation from the bicrystal substrates with high crystallinity comparable to that of the high-quality Ba(Fe,Co) 2 As 2 films. Furthermore, this enables the investigation of the intrinsic (Ba,K)Fe 2 As 2 grain boundary nature, which will clarify its potential for superconducting applications, like Josephson junctions, wires, and magnets.

(Ba,K)Fe2As2↗
DOE OSTI2025

Substrate effects on phase formation and interfacial stability in superconducting vanadium silicide thin films

Thin films of vanadium silicide (V-silicide) in the A15 cubic phase (V 3 Si) are promising for superconducting quantum devices due to their high transition temperature and potential compatibility with scalable semiconductor fabrication. However, solid-phase synthesis often yields secondary silicide phases that degrade performance. Here, in this study, we investigate the influence of substrate properties using two silicon-on-insulator architectures: one with a polycrystalline HfO 2 buried layer (group A) and the other with amorphous SiO 2 (group B). Both systems exhibit superconductivity consistent with V 3 Si formation, yet structural analysis reveals mixed-phase films in both cases. Crucially, only group A maintains an atomically sharp and chemically stable interface, a prerequisite for phase purity. Prolonged annealing in group A reduces the unwanted V 5 Si 3 phase but also leads to the emergence of Si-rich VSi 2 , likely due to localized substrate degradation. Preliminary atomic-resolution imaging suggests that HfO 2 crystallinity may promote local phase-selective nucleation. These findings highlight the importance of substrate design in promoting phase control and maintaining interfacial integrity in superconducting silicides.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2026

The mechanism of hydroxyapatite coatings degradation at high substrate temperatures

Physical vapor deposition methods used for hydroxyapatite (HA) coatings typically require elevated substrate temperatures and post-deposition annealing to induce crystallization. However, such thermal treatments can degrade both the mechanical integrity and bioactivity of the coating, particularly when substrate temperatures exceed 500 °C. The mechanisms underlying these phenomena remain insufficiently understood. In this study, HA thin films were deposited on silicon and Ti6Al4V substrates using pulsed laser deposition and were systematically characterized to elucidate these mechanisms. XPS and SIMS analyses revealed a temperature-dependent loss of OH − and PO 4 3− groups, an increased Ca/P ratio, and the formation of interfacial oxides, all of which contribute to weakened adhesion. To clarify the temperature-dependent decline in bioactivity, protein adsorption behavior was analyzed using a Kramers-type kinetic framework; the fitted desorption kinetics indicate that coatings deposited near ∼500 °C provide the most stable protein attachment, whereas higher temperatures accelerate desorption due to dehydroxylation and carbonate substitution. Together, these findings provide mechanistic insight into the thermal degradation of HA coatings and offer a framework for optimizing deposition parameters to preserve stoichiometry, adhesion, and bioactivity for long-term biomedical applications.

Kylychbekov, Salizhan [Univ. of Oxford (United Kin↗
DOE OSTI2019

Functionally critical residues in the aminoglycoside resistance-associated methyltransferase RmtC play distinct roles in 30S substrate recognition

Methylation of the small ribosome subunit rRNA in the ribosomal decoding center results in exceptionally high-level aminoglycoside resistance in bacteria. Enzymes that methylate 16S rRNA on N7 of nucleotide G1405 (m 7 G1405) have been identified in both aminoglycoside-producing and clinically drug-resistant pathogenic bacteria. Using a fluorescence polarization 30S-binding assay and a new crystal structure of the methyltransferase RmtC at 3.14 Å resolution, here we report a structure-guided functional study of 30S substrate recognition by the aminoglycoside resistance-associated 16S rRNA (m 7 G1405) methyltransferases. We found that the binding site for these enzymes in the 30S subunit directly overlaps with that of a second family of aminoglycoside resistance-associated 16S rRNA (m 1 A1408) methyltransferases, suggesting that both groups of enzymes may exploit the same conserved rRNA tertiary surface for docking to the 30S. Within RmtC, we defined an N-terminal domain surface, comprising basic residues from both the N1 and N2 subdomains, that directly contributes to 30S-binding affinity. In contrast, additional residues lining a contiguous adjacent surface on the C-terminal domain were critical for 16S rRNA modification but did not directly contribute to the binding affinity. The results from our experiments define the critical features of m 7 G1405 methyltransferase–substrate recognition and distinguish at least two distinct, functionally critical contributions of the tested enzyme residues: 30S-binding affinity and stabilizing a binding-induced 16S rRNA conformation necessary for G1405 modification. Our study sets the scene for future high-resolution structural studies of the 30S-methyltransferase complex and for potential exploitation of unique aspects of substrate recognition in future therapeutic strategies.

(ribosomal RNA) ribosome RNA methylation↗
DOE OSTI2023

Adhesion and tribological characteristics of modified polyurethane coating on composite substrate

In the present study, glass-epoxy composites were used as a substrate to study the performance of modified polyurethane coating. Two-layer coating (180–200 µm) wherein the epoxy primer as the first coat and polyurethane as the top coat was applied using an air-assisted spray gun to increase the service life of the composite. The analysis of coating was performed using gloss, contact angle, FTIR, TGA, tensile test and SEM. The bonding strength between coating and substrate was evaluated by cross-hatch cut, 180 deg peel strength and pull-off adhesion tests. Scratch, Taber abrasion and Pin-on-disc tests were also performed to evaluate the wear resistance. In addition, a rain erosion test was performed to check the stability of the coating. From the above tests, it is concluded that the applied coating well adhered to the composite substrate. The data generated from different tests can be used to estimate the service life of polymer composites and coatings.

Mushtaq, Mohammed↗
DOE OSTI2025

High-Rate Sputter Deposition of Ultrathick Boron Carbide Coatings on Rolling Spherical Substrates

Amorphous boron carbide (B 4 C) is a promising next-generation ablator material for inertial confinement fusion. However, the deposition of ultrathick B4C coatings with submicron-scale density uniformity on spherical substrates, as required for ablator shell fabrication, remains challenging. Here, in this study, we use direct current magnetron sputtering to deposit B 4 C onto 2-mm-diameter Si spheres rolling in a dish-shaped substrate holder. High deposition rates of ∼3 μm/h are achieved. Ultrathick films have a columnar microstructure, with the column width determined by the density of nodular defects. Nodular defect nucleation is dominated by the pickup of particulates from the holder during substrate rolling. We demonstrate the fabrication of a hollow B 4 C spherical shell with a wall thickness of 80 μm. Also demonstrated is the laser machining of holes in the B 4 C coating, which is necessary for both the chemical removal of the Si template and the attachment of a fusion fuel fill tube.

Inertial confinement fusion targets↗
DOE OSTI2021

Tuning plasma parameters to control reactive species fluxes to substrates in the context of plasma catalysis

The key reactive plasma-produced gas phase species responsible for the enhanced conversion of chemicals in plasma catalysis compared to thermal catalysis have to date not been identified. This outstanding question is mainly due to the inherent large variety of plasma-produced species and the challenge of controlling and measuring the flux of each constituent of the cocktail of reactive species to a (catalytic) substrate. In this paper, we explore the possibility to control the dominant reactive species fluxes, relevant for plasma-catalysis, to a substrate in the effluent of an RF driven Ar-O 2 plasma jet. The absolute species densities of the major reactive species (O, O 2 (a 1 Δ g ), O 3 and ions) were quantified by molecular beam mass spectrometry (MBMS) to assess the possibility of using treatment distance, O 2 admixture concentration, plasma dissipated power, RF modulation frequency and duty cycle as well as the feed gas flow rate to alter the dominant species densities. Selected experimental results were also compared with a pseudo-1D plug flow model. The short-lived and long-lived species can be effectively separated by changing the treatment distance and the RF modulation frequency. Furthermore, adjusting the O 2 admixture concentration enables to change the ratio of the O 2 (a 1 Δ g ) and O 3 density. The changes in the trend of ion and O flux were found to be very similar for nearly all investigated parameters. Nonetheless the gas flow rate was able to significantly change the ratio of the O and ion density in the plasma jet effluent. Here, the impact of the surface-dependent loss probability and boundary layer reactions on the species flux to a substrate and how this qualitatively relates to the MBMS density measurements is further addressed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2022

Time-resolved electron energy distribution functions at the substrate during a HiPIMS discharge with cathode voltage reversal

Abstract The time-dependent plasma properties of a high-power impulse magnetron sputtering plasma are investigated which include a positive ‘kick’ pulse on the sputtering target 2 μ s after the main negative pulse, this reversing the voltage on the cathode. At a substrate 15 cm distant from the magnetron, the time-dependent electron energy distribution function (EEDF), plasma potential, potential commute time and plasma diffusion properties are measured using a single Langmuir probe. Results show that the positive pulse on the target expels plasma and raises the plasma potential across the chamber on the order of 1 to 2 μ s, which is the time scale of the electron diffusion. The EEDF at the substrate fits a Druyvesteyn distribution during the main negative pulse rising slightly in average energy over time. The distribution is still Druyvesteyn and at the very start of the positive pulse, but then loses the higher energy electrons and drops in average electron energy as the positive pulse progresses. A Boltzmann equation solver, BOLSIG+, was used to predict the EEDF at the substrate during the positive pulse and it agrees best with the measurements assuming a value of 0.2 Td for the E / N (electric field/gas number density).

Physics↗