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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 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Extensive hydrogen incorporation is not necessary for superconductivity in topotactically reduced nickelates

Abstract A key open question in the study of layered superconducting nickelate films is the role that hydrogen incorporation into the lattice plays in the appearance of the superconducting state. Due to the challenges of stabilizing highly crystalline square planar nickelate films, films are prepared by the deposition of a more stable parent compound which is then transformed into the target phaseviaa topotactic reaction with a strongly reducing agent such as CaH 2 . Recent studies, both experimental and theoretical, have introduced the possibility that the incorporation of hydrogen from the reducing agent into the nickelate lattice may be critical for the superconductivity. In this work, we use secondary ion mass spectrometry to examine superconducting La 1−x X x NiO 2 / SrTiO 3 (X= Ca and Sr) and Nd 6 Ni 5 O 12 / NdGaO 3 films, along with non-superconducting NdNiO 2 / SrTiO 3 and (Nd,Sr)NiO 2 / SrTiO 3 . We find no evidence for extensive hydrogen incorporation across a broad range of samples, including both superconducting and non-superconducting films. Theoretical calculations indicate that hydrogen incorporation is broadly energetically unfavorable in these systems, supporting our conclusion that extensive hydrogen incorporation is not generally required to achieve a superconducting state in layered square-planar nickelates.

Science & Technology - Other Topics↗
DOE OSTI2026

Unconventional magnon-mediated spin torque enabled by ferroelectric domain engineering in multiferroic BiFeO 3

Spin current provides an energy-efficient approach for manipulating magnetization, when its spin polarization aligns with the magnetization direction. However, conventional spin-source materials possess high crystalline symmetry, restricting spin polarization to be orthogonal to both spin and charge current directions. Here, we overcome this limitation by utilizing the concept of magnon-mediated spin-orbit torque through integration of the insulating multiferroic BiFeO 3 with a conventional spin-source material. We observe that spin polarization generated by conventional spin-source material can excite unconventional magnon polarization due to the interplay between cycloidal antiferromagnetic order and the ferroelectric domain structure in BiFeO 3 . This produces an unconventional magnon torque that allows deterministic, field‑free switching of in‑plane magnetization collinear with the current direction, unattainable with conventional spin-source materials. Our results establish multiferroic-based heterostructure as a symmetry‑engineered magnon spin source, paving the way for low-power spintronic devices.

Liang, Yuhan [Tsinghua Univ., Beijing (China); Cor↗
DOE OSTI2020

The role of faceting and elongation on the magnetic anisotropy of magnetite Fe3O4 nanocrystals

Abstract Fe 3 O 4 nanoparticles are one of the most promising candidates for biomedical applications such as magnetic hyperthermia and theranostics due to their bio-compatibility, structural stability and good magnetic properties. However, much is unknown about the nanoscale origins of the observed magnetic properties of particles due to the dominance of surface and finite size effects. Here we have developed an atomistic spin model of elongated magnetite nanocrystals to specifically address the role of faceting and elongation on the magnetic shape anisotropy. We find that for faceted particles simple analytical formulae overestimate the magnetic shape anisotropy and that the underlying cubic anisotropy makes a significant contribution to the energy barrier for moderately elongated particles. Our results enable a better estimation of the effective magnetic anisotropy of highly crystalline magnetite nanoparticles and is a step towards quantitative prediction of the heating effects of magnetic nanoparticles.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Scalable ultrafast epitaxy of large-grain and single-crystal II-VI semiconductors

A general problem for semiconductor applications is that very slow deposition on expensive single-crystal substrates yields high crystalline quality with excellent electro-optical properties, but at prohibitive costs and throughput for many applications. In contrast, rapid deposition on inexpensive substrates or nanocrystalline films yields low costs, but comparatively inferior crystallinity, carrier transport, and recombination. Here, we present methods to deposit single-crystal material at rates 2-3 orders of magnitude faster than state-of-the-art epitaxy with low-cost methods without compromising crystalline or electro-optical quality. For example, single-crystal CdTe and CdZnTe films that would take several days to grow by molecular-beam epitaxy are deposited in 8 minutes by close-spaced sublimation, yet retain the same crystalline quality measured by X-ray diffraction rocking curves. The fast deposition is coupled with effective n- and p-type in-situ doping by In, P, and As. The epitaxy can be extended to nanocrystalline substrates. For example, we recrystallize thin CdTe films on glass to deposit large grains with low defect density. The results provide new research paths for photovoltaics, detectors, infrared imaging, flexible electronics, and other applications.

14 SOLAR ENERGY↗
DOE OSTI2021

Modulation of CO 2 adsorption in novel pillar-layered MOFs based on carboxylate–pyrazole flexible linker

Metal–organic frameworks (MOFs) have attracted significant attention as sorbents due to their high surface area, tunable pore volume and pore size, coordinatively unsaturated metal sites, and ability to install desired functional groups by post-synthetic modification. Herein, we report three new MOFs with pillar–paddlewheel structures that have been synthesized solvothermally from the mixture of the carboxylate–pyrazole flexible linker (H 2 L), 4,4-bipyridine (BPY)/triethylenediamine (DABCO), and Zn(II)/Cu(II) ions. The MOFs obtained, namely [Zn II (L)BPY], [Cu II (L)BPY], and [Cu II (L)DABCO], exhibit two-fold interpenetration and dinuclear paddle–wheel nodes. The Zn(II)/Cu(II) cations are coordinated by two equatorial L linkers that result in two-dimensional sheets which in turn are pillared by BPY or DABCO in the perpendicular direction to obtain a neutral three-dimensional framework that shows one-dimensional square channels. The three pillar-layered MOFs were characterized as microporous materials showing high crystalline stability after activation at 120 °C and CO 2 adsorption. All MOFs contain uncoordinated Lewis basic pyrazole nitrogen atoms in the framework which have an affinity toward CO 2 and hence could potentially serve as CO 2 adsorption material. The CO 2 uptake capacity was initially enhanced by replacing Zn with Cu and then replacing the pillar, going from BPY to DABCO. Overall, all the MOFs exhibit low isosteric heat (Q st ) of adsorption which signifies an advantage due to the energy required for the adsorption and regeneration processes.

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

Electrospun porous La–Sr–Co–Ni–O nanofibers for highly sensitive non-enzymatic glucose detection

Glucose biosensors are widely used for clinical, industrial, and environmental applications. Nonenzymatic electrochemical glucose biosensors based on metal oxides with a perovskite structure have exhibited high sensitivity, excellent stability, and cost efficiency. In this work, porous La–Sr–Co–Ni–O (LSCNO) nanofibers, with an ABO 3 -type perovskite structure, were prepared through optimizing the A-site and B-site elements by electrospinning, followed with calcination at 700 °C for 5 h. Characterized by scanning and transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy, fabricated nanofibers were confirmed to be porous and nanosized polycrystalline grains with high crystallinity. A novel La 0.75 Sr 0.25 Co 0.5 Ni 0.5 O 3 -based nonenzymatic electrochemical biosensor was developed, which is sensitive to glucose because of an electrochemically catalytic mechanism, a mediated electron transfer involving Ni(II)/Ni(III) or Co(II)/Co(III), accompanying with gluconic acid complexation. The glucose biosensor presented a linear response in the range of 0.1–1.0 mM with a calibration sensitivity of 924 ± 28 μA mM –1 , a proportion of the variance of 0.9926, and a lower limit of detection of 0.083 mM, respectively, demonstrating an outstanding analytical performance. The biosensor showed no response to the most widely used anionic surfactant, sodium dodecyl sulfate, and low sensitivity to other biomolecules, such as fructose, lactose, galactose, mannose, dopamine, and ascorbic acid. A urine sample was tested by this novel nonenzymatic electrochemical biosensor by standard addition method, suggesting a potential application for clinical test.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Deconstructing proton transport through atomically thin monolayer CVD graphene membranes

Selective proton (H + ) permeation through the atomically thin lattice of graphene and other 2D materials offers new opportunities for energy conversion/storage and novel separations. Practical applications necessitate scalable synthesis via approaches such as chemical vapor deposition (CVD) that inevitably introduce sub-nanometer defects, grain boundaries and wrinkles, and understanding their influence on H + transport and selectivity for large-area membranes is imperative but remains elusive. Using electrically driven transport of H + and potassium ions (K + ) we probe the influence of intrinsic sub-nanometer defects in monolayer CVD graphene across length-scales for the first time. At the micron scale, the areal H + conductance of CVD graphene (~4.5-6 mS cm -2 ) is comparable to that of mechanically exfoliated graphene indicating similarly high crystalline quality within a domain, albeit with K + transport (~1.7 mS cm -2 ). However, centimeter-scale Nafion|graphene|Nafion devices with several graphene domains show areal H + conductance of ~339 mS cm -2 and K + conductance of ~23.8 mS cm -2 (graphene conductance for H + is ~1735 mS cm -2 and for K + it is ~47.6 mS cm -2 ). Using a mathematical-transport-model and Nafion filled polycarbonate track etched supports, we systematically deconstruct the observed orders of magnitude increase in H + conductance for centimeter-scale CVD graphene. The mitigation of defects (>1.6 nm), wrinkles and tears via interfacial polymerization results in a conductance of ~1848 mS cm -2 for H + and ~75.3 mS cm -2 for K + (H + /K + selectivity of ~24.5) via intrinsic sub-nanometer proton selective defects in CVD graphene. We demonstrate atomically thin membranes with significantly higher ionic selectivity than state-of-the-art proton exchange membranes while maintaining comparable H + conductance. Our work provides a new framework to assess H + conductance and selectivity of large-area 2D membranes and highlights the role of intrinsic sub-nanometer proton selective defects for practical applications.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Green mechanochemical synthesis of imine-linked covalent organic frameworks for high iodine capture

Covalent organic frameworks (COFs) have emerged as promising adsorbents for radioiodine capture. However, the conventional solvothermal synthesis contradicts the principles of green chemistry due to the use of hazardous solvents, long reaction times (typically 3 days), and high reaction temperatures. Here, to address these issues, we demonstrate for the first time the green, efficient, liquid-assisted mechanochemical synthesis of COF adsorbents for high iodine capture. The ball milling synthesis was performed with various liquid additives at ambient temperature, yielding six imine-linked COFs of diverse pore sizes and functionalities in merely 1 hour. Notably, one representative COF exhibited high crystallinity and a remarkable surface area of 1387 m 2 g –1 in just 1 minute of ball milling. When tested as adsorbents for static iodine vapor capture at 75 °C, four mechanochemically synthesized COFs exhibited outstanding iodine adsorption capacities of 6.4–7.1 g g –1 , comparable to or exceeding those of solvothermally synthesized COF and most reported COF adsorbents. Furthermore, Fourier transform infrared and X-ray photoelectron spectroscopy corroborated the charge transfer between iodine and COF adsorbents. This mechanochemical methodology will pave the path for green, rapid, facile, and scalable synthesis of COFs, revealing great prospects for applications in environmental remediation and beyond.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Exploring Cs 2 AgIn x Bi 1− x Cl 6 double perovskites for optoelectronics: insights from theoretical and photophysical approaches

Lead-free halide double perovskites (HDPs) have become attractive materials for optoelectronic applications owing to their nontoxicity, structural stability, and germane photoelectric properties. In this work, we report the synthesis of high-quality In-alloyed Cs 2 AgIn x Bi (1−x) Cl 6 nanocrystals (NCs) using the antisolvent recrystallization method and comprehensively investigate the effects of In alloying on the structural, morphological, optoelectronic, and temperature-dependent photoluminescence (TDPL) properties using the state-of-the-art experimental and computational tools. Both XRD and Raman spectroscopy analyses confirmed the synthesis of highly crystalline Cs 2 AgIn x Bi (1−x) Cl 6 materials, which exhibited cubic morphology, as confirmed by TEM analysis. Room-temperature photoluminescence (PL) measurements revealed a drastic increase in the intensity above 75% In concentration with dual emission, whereas the time-resolved PL (TR-PL) results show an increase in the average lifetime values with an increase in In content, suggesting that the materials have excellent optical properties and hence are suitable candidates for optoelectronics. The TDPL measurements revealed the smallest Huang–Rhys factor (18.6) for the Cs 2 AgIn x Bi (1−x) Cl 6 (x = 0.9) sample, indicating weak exciton–phonon coupling in this composition. When deployed in the fabrication of a photodetector device, the Cs 2 AgIn x Bi (1−x) Cl 6 (x = 0.9) sample exhibited significantly enhanced photoresponsivity and a faster response time, confirming its potential for photodetector applications. Complementary DFT calculations showed that In alloying modifies the band structure of Cs 2 AgIn x Bi (1−x) Cl 6 . Our results provide valuable insights for designing multifunctional Cs 2 AgIn x Bi (1−x) Cl 6 -based materials for next-generation energy and optoelectronic devices.

14 SOLAR ENERGY↗
DOE OSTI2020

Molecular beam epitaxy of the magnetic Kagome metal FeSn on LaAlO 3 (111)

Materials with Kagome layers are expected to give rise to rich physics arising from band structures with topological properties, spin liquid behavior, and the formation of Skyrmions. Until now, most work on Kagome materials has been performed on bulk samples due to difficulties in thin film synthesis. Here, by using molecular beam epitaxy, layered Kagome-structured FeSn films are synthesized on the (111) oriented LaAlO3 substrate. Both in situ and ex situ characterizations indicate that these films are highly crystalline and c-axis oriented, with atomically smooth surfaces. The films grow as disconnected islands, with lateral dimensions on the micron meter scale. By patterning Pt electrodes using a focused electron beam, the longitudinal and transverse resistance of single islands have been measured in magnetic fields. Our work opens a pathway for exploring mesoscale transport properties in thin films of Kagome materials and related devices.

36 MATERIALS SCIENCE↗
DOE OSTI2020

The role of iron in magnetic damping of Mg(Al,Fe) 2 O 4 spinel ferrite thin films

© 2020 Author(s). We have investigated magnesium aluminum ferrite thin films with a range of iron concentrations and identified the optimal iron content to obtain high crystalline quality thin films with the low magnetic damping required for spin current-based applications. Epitaxial MgAl 2-x FexO4 films with 0.8 < x < 2.0 were grown by pulsed laser deposition on single crystal MgAl2O4 substrates and were characterized structurally and magnetically. We find that the x = 1.5 composition minimizes the room-temperature magnetic damping with a typical Gilbert damping parameter of α eff = 1.8 × 10-3. This minimized damping is governed by a competition between the more robust magnetic ordering with increased iron content, x, and the more defective structure due to larger film-substrate lattice mismatch with increased iron content. The temperature-dependent magnetization curves indicate that Tc is suppressed below room temperature for iron content x ≤ 1.2 and eventually suppressed entirely for x = 0.8. X-ray magnetic circular dichroism results indicate that for all x the magnetic moment is dominated by Fe 3 + cations distributed in a 60:40 ratio on the octahedral and tetrahedral sites, with minimal contribution from Fe 2 + cations. Films with x = 1.4-1.6 exhibit very strong ferromagnetic resonance and low Gilbert damping with α eff = (1.8-6) × 10-3, making them ideal candidates for microwave and spintronic applications.

36 MATERIALS SCIENCE↗
DOE OSTI2020

In-situ fabrication and transport properties of (111) Y 2 Ir 2 O 7 epitaxial thin film

We report on the successful fabrication of (111) oriented thin films of pyrochlore iridate Y 2 Ir 2 O 7 by means of solid phase epitaxy. Essentially, we developed an entirely in situ annealing protocol, which is superior to the conventional ex-situ routine that requires multi-hour annealing to stabilize the proper pyrochlore structure. The morphological quality of the films has been confirmed by x-ray diffraction, reflectivity, and reciprocal space mapping, demonstrating their high crystallinity with a pure pyrochlore phase and an expected epitaxial relation to the substrate. Strikingly, below the magnetic phase transition, the temperature dependence of resistivity shows a power-law behavior with the power exponent of similar to 3/2, characteristic of a Weyl semimetal in the presence of impurities and electron-electron correlations. Finally, below 10K, the magneto-transport measurements reveal the emergence of a non-saturated negative magnetoresistance up to 9T with a small "valley" around zero field. These findings are further interpreted in relation to the weak anti-localization effect and the non-collinear antiferromagnetic ordering on the Ir sublattice.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Sputtered ferroelectric aluminum scandium boron nitride (Al 1−x−y B x Sc y N)/ n -GaN heterostructures

This work demonstrates ferroelectric switching in magnetron sputtered Al 1−x−y B x Sc y N/ n -GaN heterostructures. Using high power impulse magnetron sputtering, a silicon doped n -GaN bottom electrode with an electron concentration of 6.0 × 10 19 cm −3 is grown on c-plane sapphire. Al 1−x−y B x Sc y N films are prepared on the GaN surface with Al:B:Sc ratios that produce tensile, lattice matched, or compressive epitaxial strains. X-ray diffraction shows that lattice matched and compressively strained Al 1−x−y B x Sc y N compositions are pseudomorphic, while partial relaxation is observed for tensilely strained Al 1−x−y B x Sc y N/ n -GaN heterostructures. Electrically, the Al 1−x−y B x Sc y N/ n -GaN stacks show robust hysteresis; the P–E loops are fully saturated with both lattice matched and compressively strained Al 1−x−y B x Sc y N compositions exhibiting remanent polarization values of 135 μC/cm 2 . For comparison, Al 1−x−y B x Sc y N films are also prepared on metal organic chemical vapor deposition and single crystal GaN substrates to extend strain and morphology trends to more common substrate types. This report validates that sputter deposition is a feasible technique for fabricating strain-tunable ferroelectric III–N heterostructures with high crystalline fidelity and smooth surface morphologies.

42 ENGINEERING↗
DOE OSTI2025

Metal-organic chemical vapor deposition of MgGeN2 films on GaN and sapphire

MgGeN2 films were synthesized using metal-organic chemical vapor deposition on GaN/c-sapphire templates and c-plane sapphire substrates. Energy-dispersive x-ray spectroscopy was used to estimate the cation composition ratios. To mitigate magnesium evaporation, the films were grown at pyrometer temperature 745 °C with a wafer rotation speed of 1000 rpm. Growth rates were determined by fitting energy-dispersive x-ray spectroscopy spectra to film thicknesses using NIST DTSA-II software. The thickness estimates determined by this method were consistent with scanning transmission electron microscopy measurements done for selected samples. Scanning electron microscopy images revealed faceted surfaces indicative of a tendency toward three-dimensional growth. X-ray diffraction spectra confirmed that the films were highly crystalline and exhibited preferential orientation in alignment with the substrate. Atomic force microscopy measurements show that film thicknesses are consistent across samples grown on both GaN templates and sapphire substrates, with typical roughnesses around 10 nm. Transmittance spectra of films grown on double-side-polished sapphire substrates yielded band gaps of 4.28 ± 0.06 eV for samples exhibiting close-to-ideal stoichiometry. Comparison of the measured spectra with ab initio calculations are in good agreement both near the bandgap and at higher energies where excitation is into higher-lying bands. These findings provide insight into the growth and characterization of MgGeN2, contributing to the development of this material for potential applications in optoelectronics and power electronics.

Hu, Chenxi (ORCID:0000000236299660)↗
DOE OSTI2023

Optical conductivity and vibrational spectra of the narrow-gap semiconductor FeGa 3

Intermetallic narrow-gap semiconductors have been intensively explored due to their large thermoelectric power at low temperatures and a possible role of strong electronic correlations in their unusual thermodynamic and transport properties. Here we study the optical spectra and vibrational properties of FeGa 3 . The optical conductivity indicates that FeGa 3 has a direct band gap of ≈ 0.7 eV, consistent with density functional theory (DFT) calculations. Most importantly, we find a substantial spectral weight also below 0.4 eV, which is the energy of the indirect (charge) gap found in resistivity measurements and ab initio calculations. We find that the spectral weight below the gap decreases with increasing temperature, which indicates that it originates from the impurity states and not from the electronic correlations. Interestingly, we did not find any signatures of the impurity states in vibrational spectra. The infrared and Raman vibrational lines are narrow and weakly temperature dependent. The vibrational frequencies are in excellent agreement with our DFT calculations, implying a modest role of electronic correlations. Narrow Mössbauer spectral lines also indicate high crystallinity of the sample.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Near-half-metallic state in the half-Heusler PtMnSb film on a III-V substrate

The interplay between half-metallic ferromagnetism and spin-orbit coupling within the inversion-symmetry-broken structure of half Heuslers provides an ideal platform for various spintronics functionalities. Taking advantage of good lattice matching, it is highly desired to epitaxially integrate promising Heuslers into III-V semiconductor-based devices. PtMnSb is one of the first half Heuslers predicted to be an above-room-temperature half-metal with large spin orbit coupling, however, its half-metallicity and potential as a spintronics material has remained elusive due to lack of high quality samples. Here, in this study, we demonstrate epitaxial growth of single crystal PtMnSb(001) film on GaSb(001) substrates using molecular beam epitaxy. Direct observation of the band structure via angle-resolved photoemission spectroscopy and many-body perturbation theory within the quasi-particle self-consistent GW approximation (QPGW) reveal that PtMnSb hosts rather a near-half-metallic state with both spin bands crossing the Fermi level and with high spin polarization over 90%. Temperature dependence of magnetization also shows an anomalous enhancement below 60 K, which can be associated with the development of such a near-half-metallic state at low temperatures. Epitaxial growth of high crystalline PtMnSb on a III-V paves the way for systematic clarification of its spin transport properties with fine-tuning of strain in heterostructure devices.

36 MATERIALS SCIENCE↗