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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 1,657 records · Page 92
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Comprehensively Strengthened Metal‐Oxygen Bonds for Reversible Anionic Redox Reaction

Introducing anionic redox in layered oxides is an effective approach to breaking the capacity limit of conventional cationic redox. However, the anionic redox reaction generally suffers from excessive oxidation of lattice oxygen to O 2 and O 2 release, resulting in local structural deterioration and rapid capacity/voltage decay. Here, a Na 0.71 Li 0.22 Al 0.05 Mn 0.73 O 2 (NLAM) cathode material is developed by introducing Al 3+ into the transition metal (TM) sites. Thanks to the strong Al–O bonding strength and small Al 3+ radius, the TMO 2 skeleton and the holistic TM–O bonds in NLAM are comprehensively strengthened, which inhibits the excessive lattice oxygen oxidation. The obtained NLAM exhibits a high reversible capacity of 194.4 mAh g —1 at 20 mA g —1 and decent cyclability with 98.6% capacity retention over 200 cycles at 200 mA g —1 . In situ characterizations reveal that the NLAM experiences phase transitions with an intermediate OP4 phase during the charge–discharge. Theoretical calculations further confirm that the Al substitution strategy is beneficial for improving the overlap between Mn 3d and O 2p orbitals. Importantly, this finding sheds light on the design of layered oxide cathodes with highly reversible anionic redox for sodium storage.

25 ENERGY STORAGE↗
DOE OSTI2023

Tailoring Mid–Gap States of Chalcogenide Glass by Pressure–Induced Hypervalent Bonding Towards the Design of Electrical Switching Materials

Phase change memory (PCM) and ovonic threshold switching (OTS) materials using chalcogenide glass are essential elements in advanced 3D memory chips. The mid–gap states, induced by the disorder and defects in the glass, are the physical mechanisms of the electrical switching behavior, while the origin of these trap states is still under debate and the medium–range clusters that break the global octet rule, such as over–coordinated atoms, are known to be responsible in various glass. Here, it is discovered that a large fraction of over–coordinated clusters fails to generate mid–gap states, which are probably caused by hypervalent bonding, a multi–centered covalent bond participated by delocalized lone–pair electrons. This is confirmed by the pressure–driven simulations of amorphous GeSe models, in which it is found that octahedral motifs and hypervalent bonds prevent the over–coordinated medium–range clusters from providing excessive electrons. In practical applications, compatible dopants can be used to change the number of hypervalent bonds, thus controlling the number of mid–gap states and consequently the performance of PCM and OTS materials. Finally, these results reveal the origin of mid–gap states in chalcogenide glasses, enabling extensive control in the development of pioneering electrical switching materials.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Strain‐Driven Stabilization of a Room‐Temperature Chiral Multiferroic with Coupled Ferroaxial and Ferroelectric Order

Noncollinear ferroic materials are sought after as testbeds to explore the intimate connections between topology and symmetry, which result in electronic, optical, and magnetic functionalities not observed in collinear ferroic materials. For example, ferroaxial materials have rotational structural distortions that break mirror symmetry and induce chirality. When ferroaxial order is coupled with ferroelectricity arising from a broken inversion symmetry, it offers the prospect of electric-field-control of the ferroaxial distortions and opens up new tunable functionalities. However, chiral multiferroics, especially ones stable at room temperature, are rare. A strain-stabilized, room-temperature chiral multiferroic phase in single crystals of BaTiS 3 is reported here. Using first-principles calculations, the stabilization of this multiferroic phase having P6 3 space group for biaxial tensile strains exceeding 1.5% applied on the basal ab-plane of the room temperature P6 3 cm phase of BaTiS 3 is predicted. The chiral multiferroic phase is characterized by rotational distortions of TiS 6 octahedra around the long c-axis and polar displacement of Ti atoms along the c-axis. Furthermore, the ferroaxial and ferroelectric distortions and their domains in P6 3 -BaTiS 3 are directly resolved using atomic resolution scanning transmission electron microscopy. Landau-based phenomenological modeling predicts a strong coupling between the ferroelectric and the ferroaxial order making P6 3 -BaTiS 3 an attractive test bed for achieving electric-field-control of chirality.

Chalcogenide↗
DOE OSTI2025

Magnetic Switching in Monolayer 2D Diluted Magnetic Semiconductors via Spin‐to‐Spin Conversion

Abstract The integration of 2D van der Waals (vdW) magnets with topological insulators or heavy metals holds great potential for realizing next‐generation spintronic memory devices. However, achieving high‐efficiency spin–orbit torque (SOT) switching of monolayer vdW magnets at room temperature poses a significant challenge, particularly without an external magnetic field. Here, it is shown field‐free, deterministic, and nonvolatile SOT switching of perpendicular magnetization in the monolayer, diluted magnetic semiconductor (DMS), Fe‐doped MoS 2 (Fe:MoS 2 ) at up to 380 K with a current density of ≈7 × 10 4 A cm −2 . The in situ doping of Fe into monolayer MoS 2 via chemical vapor deposition and the geometry‐induced strain in the crystal break the rotational switching symmetry in Fe:MoS 2 , promoting field‐free SOT switching by generating out‐of‐plane spins via spin‐to‐spin conversion. An apparent anomalous Hall effect (AHE) loop shift at a zero in‐plane magnetic field verifies the existence of z spins in Fe:MoS 2 , inducing an antidamping‐like torque that facilitates field‐free SOT switching. This field‐free SOT application using a 2D ferromagnetic monolayer provides a new pathway for developing highly power‐efficient spintronic memory devices.

Chen, Siwei [Department of Mechanical Engineering ↗
DOE OSTI2025

Discovery of an Intrinsic Antiferromagnetic Semiconductor EuSc 2 Te 4 With Magnetism‐Driven Nonlinear Transport

Magnetic topological materials have recently emerged as a promising platform for studying quantum geometry by the nonlinear transport in thin film devices. In this work, an antiferromagnetic (AFM) semiconductor EuSc₂Te₄ as the first bulk crystal that exhibits quantum geometry-driven nonlinear transport is reported. This material crystallizes into an orthorhombic lattice with AFM order below 5.2 K and a bandgap of less than 50 meV. The calculated band structure aligns with the angle-resolved photoemission spectroscopy spectrum. The AFM order preserves combined space-time inversion symmetry but breaks both spatial inversion and time-reversal symmetry, leading to the nonlinear Hall effect (NLHE). Nonlinear Hall voltage measured in bulk crystals appears at zero field, peaks near the spin-flop transition as the field increases, and then diminishes as the spin moments align into a ferromagnetic order. This field dependence, along with the scaling analysis of the nonlinear Hall conductivity, suggests that the NLHE of EuSc₂Te₄ involves contributions from quantum metric, in addition to extrinsic contributions, such as spin scattering and junction effects. Furthermore, this NLHE is found to have the functionality of broadband frequency mixing, indicating its potential applications in electronics. This work reveals a new avenue for studying magnetism-induced nonlinear transport in magnetic materials.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Fractional Skyrmion Tubes in Chiral‐Interfaced 3D Magnetic Nanowires

Magnetic skyrmions are chiral spin textures with rich physics and great potential for unconventional computing. Typically, skyrmions form in bulk crystals with reduced symmetry or ultrathin film multilayers involving heavy metals. Here, the formation of fractional Bloch skyrmion tubes at room temperature is demonstrated by 3D printing ferromagnetic double‐helix nanowires with two regions of opposite chirality. Using X‐ray microscopy and micromagnetic simulations, it is shown that the coexistence of vortex and anti‐parallel spin states induces the formation of fractional skyrmion tubes at zero magnetic fields, minimizing the energy cost of breaking the coupling between geometric and magnetic chirality. Control over zero‐field states is also demonstrated, including pure vortex, or mixed skyrmion‐vortex states, highlighting the magnetic reconfigurability of these 3D nanowires. This work shows how interfacing chiral geometries at the nanoscale can enable advanced forms of topological spintronics.

X-ray microscopy↗
DOE OSTI2026

Toward Tunable Magnetic Dirac Semimetals: Mn Doping of Cd 3 As 2

Magnetic impurities provide a route toward increasing functionality in electronic materials, often enabling new device concepts and architectures. In the case of topological semimetals, dilute magnetic doping presents a particularly attractive approach for inducing a Dirac to Weyl phase change via time reversal symmetry breaking. However, efforts to realize changes in the electronic structure have been limited by challenges in incorporating magnetic impurities into crystals with sufficiently high electron mobilities to detect them via transport or spectroscopic techniques. Here, we demonstrate incorporation of Mn into Cd 3 ⁢As 2 Dirac semimetal thin films grown by molecular beam epitaxy (MBE). Using As-rich growth conditions and [001] oriented thin films, Mn compositions of >10% are achieved. Films contain uniform distributions of Mn with no evidence of secondary phases and exhibit electron mobilities greater than 10 000–30 000 cm 2 /Vs up to 5% Mn. An evolution in the magnetization behavior along with the emergence of a second quantum oscillation frequency at low Mn concentrations provide preliminary evidence of Mn-induced changes in the electronic structure that are consistent with a Weyl phase. This work demonstrates the potential of magnetically doping topological semimetal thin films and a pathway for synthesizing them.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Electrostatic Superlattices Beyond 1:1 Stoichiometry

ABSTRACT Exotic nanoparticle superstructures can be accessed by harnessing nanoparticle softness and charge regulation, features often viewed as obstacles to structural control. Here, we show that regulated charge mismatch in polymer‐grafted nanoparticles enables the assembly of high‐stoichiometry cubic superlattices. By co‐tuning grafting density, particle size, and bulk composition, we realize ionic‐lattice analogues, such as and , as well as single‐component and superlattices without atomic counterparts. The superlattice has recently been identified theoretically as a photonic band‐gap lattice. These phases emerge from a 1:1 “parent” lattice when local charge neutrality cannot be satisfied, driving either progressive interstitial filling or reorganization into a larger basis. For instance, the systematic occupation of ZnS tetrahedral sites yields , while ligand‐swapping symmetry breaking converts CsCl into . Upon heating, the assemblies exhibit reversible lattice contraction and pronounced negative thermal expansion. Furthermore, the energetic penalty for defects increases with nanoparticle size, facilitating the scalable production of high‐quality, open superlattices for photonic applications.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Ferroelectric Domain Wall Motion in Freestanding Single-Crystal Complex Oxide Thin Film

Ferroelectric domain walls in single-crystal complex oxide thin films are found to be orders of magnitude slower when the interfacial bonds with the heteroepitaxial substrate are broken to create a freestanding film. This drastic change in domain wall kinetics does not originate from the alteration of epitaxial strain; rather, it is correlated with the structural ripples at mesoscopic length scale and associated flexoelectric effects induced in the freestanding films. In contrast, the effects of the bond-breaking on the local static ferroelectric properties of both top and bottom layers of the freestanding films, such as domain wall width and spontaneous polarization, are modest and governed by the change in epitaxy-induced compressive strain.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Quasi‐Binary Transition Metal Dichalcogenide Alloys: Thermodynamic Stability Prediction, Scalable Synthesis, and Application

Abstract Transition metal dichalcogenide (TMDCs) alloys could have a wide range of physical and chemical properties, ranging from charge density waves to superconductivity and electrochemical activities. While many exciting behaviors of unary TMDCs have been demonstrated, the vast compositional space of TMDC alloys has remained largely unexplored due to the lack of understanding regarding their stability when accommodating different cations or chalcogens in a single‐phase. Here, a theory‐guided synthesis approach is reported to achieve unexplored quasi‐binary TMDC alloys through computationally predicted stability maps. Equilibrium temperature–composition phase diagrams using first‐principles calculations are generated to identify the stability of 25 quasi‐binary TMDC alloys, including some involving non‐isovalent cations and are verified experimentally through the synthesis of a subset of 12 predicted alloys using a scalable chemical vapor transport method. It is demonstrated that the synthesized alloys can be exfoliated into 2D structures, and some of them exhibit: i) outstanding thermal stability tested up to 1230 K, ii) exceptionally high electrochemical activity for the CO 2 reduction reaction in a kinetically limited regime with near zero overpotential for CO formation, iii) excellent energy efficiency in a high rate Li–air battery, and iv) high break‐down current density for interconnect applications. This framework can be extended to accelerate the discovery of other TMDC alloys for various applications.

Hemmat, Zahra↗
DOE OSTI2020

Perovskite-Derivative Valleytronics

Halide perovskites are revolutionizing the renewable energy sector owing to their high photovoltaic efficiency, low manufacturing cost, and flexibility. Their remarkable mobility and long carrier lifetime are also valuable for information technology, but fundamental challenges like poor stability under an electric field prevent realistic applications of halide perovskites in electronics. Here, it is discovered that valleytronics is a promising route to leverage the advantages of halide perovskites and derivatives for information storage and processing. The synthesized all-inorganic lead-free perovskite derivative, Cs 3 Bi 2 I 9 , exhibits strong light-matter interaction and parity-dependent optically addressable valley degree of freedom. In this work, robust optical helicity in all odd-layer-number crystals with inversion symmetry breaking is observed, indicating excitonic coherence extending well beyond 11 layers. The excellent optical and valley properties of Cs 3 Bi 2 I 9 arise from the unique parallel bands, according to first principles calculations. This discovery points to new materials design principles for scalable valleytronic devices and demonstrates the promise of perovskite derivatives beyond energy applications.

2D materials↗
DOE OSTI2021

Whole-Voltage-Range Oxygen Redox in P2-Layered Cathode Materials for Sodium-Ion Batteries

Oxygen-redox of layer-structured metal-oxide cathodes has drawn great attention as an effective approach to break through the bottleneck of their capacity limit. However, reversible oxygen-redox can only be obtained in the high-voltage region (usually over 3.5 V) in current metal-oxide cathodes. Here, we realize reversible oxygen-redox in a wide voltage range of 1.5-4.5 V in a P2-layered Na 0.7 Mg 0.2 [Fe 0.2 Mn 0.6 $\square$0.2]O 2 cathode material, where intrinsic vacancies are located in transition-metal (TM) sites and Mg-ions are located in Na sites. Mg-ions in the Na layer serve as "pillars" to stabilize the layered structure during electrochemical cycling, especially in the high-voltage region. Intrinsic vacancies in the TM layer create the local configurations of "$\square$-O-$\square$", "Na-O-$\square$" and "Mg-O-$\square$" to trigger oxygen-redox in the whole voltage range of charge-discharge. Additionally, time-resolved techniques demonstrate that the P2 phase is well maintained in a wide potential window range of 1.5-4.5 V even at 10 C. It is revealed that charge compensation from Mn- and O-ions contributes to the whole voltage range of 1.5-4.5 V, while the redox of Fe-ions only contributes to the high-voltage region of 3.0-4.5 V. The orphaned electrons in the nonbonding 2p orbitals of O that point toward TM-vacancy sites are responsible for reversible oxygen-redox, and Mg-ions in Na sites suppress oxygen release effectively.

25 ENERGY STORAGE↗
DOE OSTI2021

Unraveling Strain Gradient Induced Electromechanical Coupling in Twisted Double Bilayer Graphene Moiré Superlattices

Moiré superlattices of 2D materials with a small twist angle are thought to exhibit appreciable flexoelectric effect, though unambiguous confirmation of their flexoelectricity is challenging due to artifacts associated with commonly used piezoresponse force microscopy (PFM). For example, unexpectedly small phase contrast (≈8°) between opposite flexoelectric polarizations is reported in twisted bilayer graphene (tBG), though theoretically predicted value is 180°. We report a methodology is developed to extract intrinsic moiré flexoelectricity using twisted double bilayer graphene (tDBG) as a model system, probed by lateral PFM. For small twist angle samples, it is found that a vectorial decomposition is essential to recover the small intrinsic flexoelectric response at domain walls from a large background signal. The obtained threefold symmetry of commensurate domains with significant flexoelectric response at domain walls is fully consistent with the theoretical calculations. Incommensurate domains in tDBG with relatively large twist angles can also be observed by this technique. A general strategy is provided here for unraveling intrinsic flexoelectricity in van der Waals moiré superlattices while providing insights into engineered symmetry breaking in centrosymmetric materials.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Reaching the Excitonic Limit in 2D Janus Monolayers by In Situ Deterministic Growth

Abstract Named after the two‐faced Roman god of transitions, transition metal dichalcogenide (TMD) Janus monolayers have two different chalcogen surfaces, inherently breaking the out‐of‐plane mirror symmetry. The broken mirror symmetry and the resulting potential gradient lead to the emergence of quantum properties such as the Rashba effect and the formation of dipolar excitons. Experimental access to these quantum properties, however, hinges on the ability to produce high‐quality 2D Janus monolayers. Here, these results introduce a holistic 2D Janus synthesis technique that allows real‐time monitoring of the growth process. This prototype chamber integrates in situ spectroscopy, offering fundamental insights into the structural evolution and growth kinetics, that allow the evaluation and optimization of the quality of Janus monolayers. The versatility of this method is demonstrated by synthesizing and monitoring the conversion of SWSe, SNbSe, and SMoSe Janus monolayers. Deterministic conversion and real‐time data collection further aid in conversion of exfoliated TMDs to Janus monolayers and unparalleled exciton linewidth values are reached, compared to the current best standard. The results offer an insight into the process kinetics and aid in the development of new Janus monolayers with high optical quality, which is much needed to access their exotic properties.

2D materials↗
DOE OSTI2021

Hysteresis-Free Nanoparticle-Reinforced Hydrogels

The elastic storage and release of mechanical energy has been key to many developments throughout the history of mankind. Resilience, absent hysteresis, has been an elusive goal to achieve, particularly at large deformations. Using a low-crosslink-density polyacrylamide hydrogel at 96% water content having hyperbranched silica nanoparticles (HBSPs) as the major junction points, a hysteresis-free material is realized. The fatigue-free characteristic of these composite hydrogels is evidenced by the invariance of the stress–strain curves at strain ratios of 4, even after 5000 cycles. At a strain ratio of 7, only a 1.3% hysteresis is observed. A markedly increased strain-ratio-at-break of 11.5 is observed. Finally, the unique attributes of these resilient hydrogels are manifested in the high-fidelity detection of dynamic deformations under cyclic loading over a broad range of frequencies, difficult to achieve with other materials.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Topological Antiferromagnetic Van der Waals Phase in Topological Insulator/Ferromagnet Heterostructures Synthesized by a CMOS-Compatible Sputtering Technique

Breaking time-reversal symmetry by introducing magnetic order, thereby opening a gap in the topological surface state bands, is essential for realizing useful topological properties such as the quantum anomalous Hall and axion insulator states. In this work, a novel topological antiferromagnetic (AFM) phase is created at the interface of a sputtered, c-axis-oriented, topological insulator/ferromagnet heterostructure—Bi 2 Te 3 /Ni 80 Fe 20 because of diffusion of Ni in Bi 2 Te 3 (Ni-Bi 2 Te 3 ). The AFM property of the Ni-Bi2Te3 interfacial layer is established by observation of spontaneous exchange bias in the magnetic hysteresis loop and compensated moments in the depth profile of the magnetization using polarized neutron reflectometry. Analysis of the structural and chemical properties of the Ni-Bi2Te3 layer is carried out using selected-area electron diffraction, electron energy loss spectroscopy, and X-ray photoelectron spectroscopy. These studies, in parallel with first-principles calculations, indicate a solid-state chemical reaction that leads to the formation of Ni=Te bonds and the presence of topological antiferromagnetic (AFM) compound NiBi 2 Te 4 in the Ni-Bi 2 Te 3 interface layer. The Neél temperature of the Ni-Bi 2 Te 3 layer is ≈ 63 K, which is higher than that of typical magnetic topological insulators (MTIs). The presented results provide a pathway toward industrial complementary metal-oxide-semiconductor (CMOS)-process-compatible sputtered-MTI heterostructures, leading to novel materials for topological quantum devices.

36 MATERIALS SCIENCE↗