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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 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Gate-Tunable Proximity Effects in Graphene on Layered Magnetic Insulators

The extreme versatility of van der Waals materials derives from their ability to exhibit new electronic properties when assembled in proximity with dissimilar crystals. 1 For example, although graphene is inherently non-magnetic, recent work has reported a magnetic proximity effect in graphene interfaced with magnetic substrates, potentially enabling a pathway towards achieving a high-temperature quantum anomalous Hall effect. Here, we investigate heterostructures of graphene and chromium trihalide magnetic insulators (CrI 3 , CrBr 3 , and CrCl 3 ). Surprisingly, we are unable to detect a magnetic exchange field in the graphene, but instead discover proximity effects featuring unprecedented gate-tunability. The graphene becomes highly hole-doped due to charge transfer from the neighboring magnetic insulator, and further exhibits a variety of atypical gate-dependent transport features. The charge transfer can additionally be altered upon switching the magnetic states of the nearest CrI 3 layers. In conclusion, our results provide a roadmap for exploiting proximity effects arising in graphene coupled to magnetic insulators.

2D magnet↗
DOE OSTI2022

Evidence of Magnon-Mediated Orbital Magnetism in a Quasi-2D Topological Magnon Insulator

Here, we explore spin dynamics in Cu(1,3-bdc), a quasi-2D topological magnon insulator. The results show that the thermal evolution of the Landé $\textit{g}$ factor ($\textit{g}$) is anisotropic: $g_{\text{in-plane}}$ decreases while $g_{\text{out-of-plane}}$ increases with increasing temperature $\textit{T}$. Moreover, the anisotropy of the $\textit{g}$ factor (Δ$\textit{g}$) and the anisotropy of saturation magnetization ($ΔM_s$) are correlated below 4 K, but they diverge above 4 K. We show that the electronic orbital moment contributes to the g anisotropy at lower $\textit{T}$, while the topological orbital moment induced by thermally excited spin chirality dictates the g anisotropy at higher $\textit{T}$. Our work suggests an interplay among topology, spin chirality, and orbital magnetism in Cu(1,3-bdc).

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2024

Spin–Phonon Coupling and Magnetic Transition in an Organic Molecule Intercalated Cr 2 Ge 2 Te 6

The manipulation of spin–phonon coupling in both formations and explorations of magnetism in two-dimensional van der Waals ferromagnetic semiconductors facilitates unprecedented prospects for spintronic devices. The interlayer engineering with spin–phonon coupling promises controllable magnetism via organic cation intercalation. Here, in this study, spectroscopic evidence reveals the intercalation effect on the intrinsic magnetic and electronic transitions in quasi-two-dimensional Cr 2 Ge 2 Te 6 using tetrabutyl ammonium (TBA + ) as the intercalant. The temperature evolution of Raman modes, E g 3 and A g 1 , along with the magnetization measurements, unambiguously captures the enhancement of the ferromagnetic Curie temperature in the intercalated heterostructure. Moreover, the E g 4 mode highlights the increased effect of spin–phonon interaction in magnetic-order-induced lattice distortion. Combined with the first-principle calculations, we observed a substantial number of electrons transferred from TBA + to Cr through the interface. The interplay between spin–phonon coupling and magnetic ordering in van der Waals magnets appeals for further understanding of the manipulation of magnetism in layered heterostructures.

2D magnet↗
DOE OSTI2022

Exchange bias and interface-related effects in two-dimensional van der Waals magnetic heterostructures: Open questions and perspectives

The exchange bias (EB) effect is known as a fundamentally and technologically important magnetic property of a magnetic bilayer film. It is manifested as a horizontal shift in a magnetic hysteresis loop of a film subject to cooling in the presence of a magnetic field. The EB effect in van der Waals (vdW) heterostructures offers a novel approach for tuning the magnetic properties of the newly discovered single-layer magnets, as well as adds a new impetus to magnetic vdW heterostructures. Indeed, intriguing EB effects have recently been reported in a variety of low-dimensional vdW magnetic systems ranging from a weakly interlayer-coupled vdW magnet (e.g., Fe 3 GeTe 2 ) to a bilayer composed of two different magnetic vdW materials (e.g., Fe 3 GeTe 2 /CrCl 3 , Fe 3 GeTe 2 /FePS 3 , Fe 3 GeTe 2 /MnPS 3 , Fe 3 GeTe 2 /CrSe, Fe 3 GeTe 2 /CrOCl, Fe 3 GeTe 2 /CoPc, Fe 5 GeTe 2 /FePS 3 ), to bilayers of two different vdW defective magnets (e.g., VSe 2 /MoS 2 ), or to metallic ferromagnet/vdW defective magnet interfaces (e.g., Fe/MoS 2 ). Despite their huge potential in spintronic device applications, the physical origins of the observed EB effects have remained elusive to researchers. Here, we present here a critical review of the EB effect and associated phenomena such as magnetic proximity (MP) in various vdW heterostructure systems and propose approaches to addressing some of the emerging fundamental questions.

2D magnets↗
DOE OSTI2025

Twist Engineering of Anisotropic Excitonic and Optical Properties of a Two-Dimensional Magnetic Semiconductor

Two-dimensional (2D) van der Waals (vdW) magnetic semiconductors are a new class of quantum materials for studying the emergent physics of excitons and spins in the 2D limit. Twist engineering provides a powerful tool to manipulate the fundamental properties of 2D vdW materials. Here, in this work, we show that twist engineering of the anisotropic ferromagnetic monolayer semiconductor CrSBr leads to bilayer magnetic semiconductors with continuously tunable magnetic moment, dielectric anisotropy, exciton energy, and linear dichroism. We furthermore provide a model for exciton energy in the media with tunable anisotropy. These results advance fundamental studies of 2D vdW materials and open doors to applications to nano-optics, twistronics, and spintronics.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Magnetic Proximity Effects in Iron Germanium Telluride/Platinum Heterostructures

Van der Waals (vdW) magnetic materials have attracted considerable attention for use in spintronic devices such as those controlled by spin–orbit torque (SOT). Such SOT-driven devices are typically fabricated by bringing a vdW magnet in proximity to a spin–charge conversion layer to achieve current-driven magnetization switching. Here, we show that such structures fabricated with iron germanium telluride (FGT) and platinum can exhibit emergent magnetic properties, which we attribute to magnetic proximity effects at the FGT/Pt interface. These changes manifest as increased perpendicular magnetic anisotropy and the emergence of additional magnetization reversal steps as probed by magneto-transport, with the most significant changes appearing in thinner flakes. The behavior was found to be robust and consistently appeared in samples made with crystals from different vendors. In conclusion, our results demonstrate the potential for engineering vdW spintronic systems through magnetic proximity effects.

2D magnet↗
DOE OSTI2020

Atomic defects, functional groups and properties in MXenes

MXenes, a new family of functional two-dimensional (2D) materials, have shown great potential for an extensive variety of applications within the last decade. Atomic defects and functional groups in MXenes are known to have a tremendous influence on the functional properties. In this review, we focus on recent progress in the characterization of atomic defects and functional group chemistry in MXenes, and how to control them to directly influence various properties ( e.g ., electron transport, Li + adsorption, hydrogen evolution reaction (HER) activity, and magnetism) of 2D MXenes materials. Dynamic structural transformations such as oxidation and growth induced by atomic defects in MXenes are also discussed. The review thus provides perspectives on property optimization through atomic defect engineering, and bottom-up synthesis methods based on defect-assisted homoepitaxial growth of MXenes.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Examination of Solvent- Room Temperature Ionic Liquid Interactions using 2D NMR and Computational Chemistry

Rare earth metals (REMs) are the key functioning material found in microelectronics, advanced batteries, and high-efficiency electrical motors. Current methods for producing REMs require high temperatures (i.e. energy intensive) and produce an abundance of hazardous fluoride waste. Room temperature ionic liquids (RTILs) are an ideal alternative electrolyte because they allow for a low-temperature, energy-efficient electrochemical process. RTIL properties such as viscosity and conductivity determine how effective they can be in this process. Since viscosity is directly related to both intra and intermolecular forces, this study sets out to investigate two RTILs-Trihexyltetradecylphosphonium dicyanamide (Cyphos 105) and Trihexyltetradecylphosphonium bistrifimide (Cyphos 109) and their interactions. Results from 2-dimensional nuclear magnetic resonance (2D NMR) spectroscopy show that there is an interaction between the tetradecyl tail and the phosphorus atom of the Cyphos RTIL. In order to establish whether this is an intra or an inter-molecular interaction, DFT-based computational calculations were performed to determine the optimized geometry. Calculations were done both in the gas phase and with propylene carbonate as an augmenting solvent. Preliminary results show that there is minimal intramolecular interaction between the tetradecyl tail and the phosphorus atom of the RTILs, and that the interaction seen in the 2D NMR experiment is likely of an intermolecular nature instead.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Elucidating the Role of Dimensionality on the Electronic Structure of the Van der Waals Antiferromagnet NiPS 3

Abstract The sustained interest in investigating magnetism in the 2D limit of insulating antiferromagnets is driven by the possibilities of discovering, or engineering, novel magnetic phases through layer stacking. However, due to the difficulty of directly measuring magnetic interactions in 2D antiferromagnets, it is not yet understood how intra layer magnetic interactions in insulating , strongly correlated, materials can be modified through layer proximity. Herein, the impact of reduced dimensionality in the model van der Waals antiferromagnet NiPS 3 is explored by measuring electronic excitations in exfoliated samples using Resonant Inelastic X‐ray Scattering (RIXS). The resulting spectra shows systematic broadening of NiS 6 multiplet excitations with decreasing layer count from bulk down to three atomic layers (3L). It is shown that these trends originate from a decrease in transition metal‐ligand and ligand–ligand hopping integrals, and by charge‐transfer energy evolving from Δ = 0.83 eV in the bulk to 0.37 eV in 3L NiPS 3 . Relevant intralayer magnetic exchange integrals computed from the electronic parameters exhibit a decrease in the average interaction strength with thickness. This study underscores the influence of inter layer electronic interactions on intra layer ones in insulating magnets, indicating that magnetic Hamiltonians in few‐layer insulating magnets can greatly deviate from their bulk counterparts.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Magnetism at the interface of MoSe 2 /V 2 O 5 heterostructures

In this study, we report the existence of magnetism in a heterostructure made up of an MoSe 2 transition metal dichalcogenide monolayer and a V 2 O 5 substrate. Using density functional theory simulations, we find that ferromagnetic ordering can be found in the MoSe 2 /V 2 O 5 heterostructure even though the individual components are nonmagnetic. The role of van der Waals interactions is examined.

2D magnet↗
DOE OSTI2020

Proximity Spin–Orbit Torque on a Two-Dimensional Magnet within van der Waals Heterostructure: Current-Driven Antiferromagnet-to-Ferromagnet Reversible Nonequilibrium Phase Transition in Bilayer CrI 3

The recently discovered two-dimensional magnetic insulator CrI 3 is an intriguing case for basic research and spintronic applications since it is a ferromagnet in the bulk but an antiferromagnet in bilayer form, with its magnetic ordering amenable to external manipulations. Using the first-principles quantum transport approach, we predict that injecting unpolarized charge current parallel to the interface of the bilayer-CrI 3 /monolayer-TaSe 2 van der Waals (vdW) heterostructure will induce spin–orbit torque and thereby drive the dynamics of magnetization on the first monolayer of CrI 3 in direct contact with TaSe 2 . By combining the calculated complex angular dependence of spin–orbit torque with the Landau-Lifshitz-Gilbert equation for classical dynamics of magnetization, we demonstrate that current pulses can switch the direction of magnetization on the first monolayer to become parallel to that of the second monolayer, thereby converting CrI 3 from antiferromagnet to ferromagnet while not requiring any external magnetic field. We explain the mechanism of this reversible current-driven nonequilibrium phase transition by showing that first monolayer of CrI 3 carries current due to evanescent wave functions injected by metallic transition metal dichalcogenide TaSe 2 , while concurrently acquiring strong spin–orbit coupling via such a proximity effect, whereas the second monolayer of CrI 3 remains insulating. Furthermore, the transition can be detected by passing vertical read current through the vdW heterostructure, encapsulated by a bilayer of hexagonal boron nitride and sandwiched between graphite electrodes, where we find a tunneling magnetoresistance of ≃240%.

2D magnetic materials↗
DOE OSTI2025

Effect of Ni deposition on CrI 3 monolayer: First-principles study

In our recent experimental work, we demonstrated that Ni thin film deposition can be used to tune the surface magnetic states in bulk CrI 3 . Here, we present the results of a computational study of Ni deposition on a CrI 3 monolayer. In particular, we compare structural, electronic, and magnetic properties of three different systems: a pristine CrI 3 monolayer, a CrI 3 monolayer with partial Ni coverage, and a CrI 3 monolayer with full Ni coverage. The corresponding cells used in our calculations are Cr 2 I 6 , Cr 2 I 6 Ni 1 , and Cr 2 I 6 Ni 3 . In Cr 2 I 6 Ni 1 , the Ni atom is positioned over the hollow site, which corresponds to the lowest energy configuration. In Cr 2 I 6 Ni 3 , the three Ni atoms are positioned over the single hollow site, and over the two Cr sites. Our calculations indicate that the partial coverage of CrI 3 monolayer with Ni retains the semiconducting nature of the parent compound, but results in a strong reduction of the energy band gap. At the same time, the full coverage of CrI 3 monolayer with Ni results in a metallic transition, with a highly spin-polarized (possibly half-metallic) electronic structure. The magnetic alignment of the CrI 3 monolayer with the deposited Ni is ferrimagnetic, due to the anti-aligned magnetic moments of Cr and Ni atoms. While the partial coverage of the CrI 3 monolayer with Ni has a moderate impact on the lattice parameter of the parent compound, full coverage by Ni results in a significant increase of the lattice constant. For both partial and full coverages, the lowest energy configuration corresponds to the Ni atoms being approximately aligned with the out-of-plane coordinate of the surface iodine layer. In addition, in the full coverage mode, the Cr layer exhibits a significant shift away from the Ni atoms, effectively “detaching” the Ni / I atomic layer from the rest of the cell.

2D magnetism↗
DOE OSTI2025

Giant Modulation of Magnetoresistance in a Van Der Waals Magnet by In‐Plane Current Injection

Efficient magnetization control is a central issue in magnetism and spintronics. Particularly, there are increasing demands for manipulation of magnetic states in van der Waals (vdW) magnets with unconventional functionalities. However, the electrically induced phase transition between ferromagnetic-to-antiferromagnetic states without external magnetic field is yet to be demonstrated. Here, the current-induced magnetic phase transition in a vdW ferromagnet Fe 5 GeTe 2 is reported. Based on magneto-transport measurements and theoretical analysis, it is demonstrated that transition in the interlayer magnetic coupling occurs through vertical voltage drop between layers induced by current which is attributed to high anisotropy of the resistivity caused by the vdW gaps. Such magnetic phase transition results in giant modulation of the longitudinal magnetoresistance from 5% to 170%. The electrical tunability of the magnetic phase in Fe 5 GeTe 2 with current-in-plane geometry opens a path for electric control of magnetic properties, expanding the ability to use vdW magnets for spintronic applications.

2D magnet↗
DOE OSTI2025

Charge Density Wave and Ferromagnetism in Intercalated CrSBr

In materials with 1D electronic bands, electron–electron interactions can produce intriguing quantum phenomena, including spin-charge separation and charge density waves (CDW). Most of these systems, however, are non-magnetic, motivating a search for anisotropic materials where the coupling of charge and spin may affect emergent quantum states. Here, in this study, chemical intercalation of the van der Waals magnetic semiconductor CrSBr yields Li 0.17(2) (tetrahydrofuran) 0.26(3) CrSBr, which possesses an electronically driven quasi-1D CDW with an onset temperature above room temperature. Concurrently, electron doping increases the magnetic ordering temperature from 132 to 200 K and switches its interlayer magnetic coupling from antiferromagnetic to ferromagnetic. The spin-polarized nature of the anisotropic bands that give rise to this CDW enforces an intrinsic coupling of charge and spin. The coexistence and interplay of ferromagnetism and charge modulation in this exfoliatable material provide a promising platform for studying tunable quantum phenomena across a range of temperatures and thicknesses.

2D magnets↗
DOE OSTI2025

A green degumming approach for lignocellulosic kenaf bast using ethanolamine deep eutectic solvent

Traditional chemical degumming methods for lignocellulosic bast fibers pose substantial challenges, especially when dealing with kenaf bast, which has a high lignin content. Here, these challenges include severe pollution and low efficiency, etc. Therefore, this study developed a novel, green, and efficient degumming approach for lignocellulosic kenaf bast, leveraging choline chloride-ethanolamine (ChCl-ETA) alkaline deep eutectic solvent (DES). The efficacy of the ChCl-ETA DES based degumming method was validated through comprehensive chemical composition analysis, scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), thermogravimetric analysis (TG), and two-dimensional (2D) nuclear magnetic resonance (NMR) spectroscopy. Subsequently, the optimal degumming conditions, with a choline chloride to ethanolamine ratio of 1:6 and a degumming temperature of 120 °C, were determined based on the results of the residual gum content. Significantly, the recycled ChCl-ETA DES mixtures could maintain a degumming capacity (revealed by the residual gum content of degummed fibers) comparable to that of the conventional double-boiling method for at least five degumming cycles without purification. Given its low-pollution process and high efficiency, this developed method holds great potential for industrial-scale applications of lignocellulosic bast fibers.

Choline chloride↗
DOE OSTI2023

NMR investigation of proton transport in polybenzimidazole/polyphosphoric acid membranes prepared via novel synthesis route

Here, in this work, we present a molecular-level view of the structural changes in polybenzimidazole (PBI) membranes doped with phosphoric acid (PA) generated by a novel membrane fabrication technique. The modified PA doped membranes displayed unprecedented ionic conductivity at elevated temperatures, comparable to the starting PBI gel membranes prepared by the commonly used PPA process, while also exhibiting enhanced mechanical properties. To elucidate the cause of these effects, we used multi-nuclear ( 1 H, 13 C, 31 P) 1D PFG, MAS and CPMAS, and 2D HETCOR magnetic resonance (NMR) techniques to characterize the structure and dynamics of the modified film and the original gel PBI membranes. 1 H diffusivity measurements show significantly enhanced proton diffusivity, both in magnitude and in lower activation energy, which were consistent with its high ionic conductivity despite the lower PA content compared to the original gel membrane. CPMAS experiments further substantiated that the location of these distinct phosphate environments as being close to the polymer network. Finally, the phosphate groups in the modified membrane were revealed to be strongly bounded to each other and to the PBI polymer backbone in contrast to the combination of weakly and strongly bounded groups of the original gel membrane.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2020

Mild Acetylation and Solubilization of Ground Whole Plant Cell Walls in EmimAc: A Method for Solution-State NMR in DMSO-d6

Lignocellulosic biomass is mainly composed of polysaccharides and lignin. The complexity and diversity of the plant cell wall polymers makes it difficult to isolate the components in pure form for characterization. Many current approaches to analyzing lignocellulose structure, which involve sequential extraction and characterization of the resulting fractions, are time-consuming and labor-intensive. The present study describes a new and facile system for rationally derivatizing and dissolving coarsely ground plant cell wall materials. Using ionic liquids (EmimAc) and dichloroacetyl chloride as a solvent/reagent produced mildly acetylated whole cell walls without significant degradation. The acetylated products were soluble in DMSO-d6 from which they can be characterized by solution-state two-dimensional nuclear magnetic resonance (2D NMR) spectrometry. A distinct advantage of the procedure is that it realizes the dissolution of whole lignocellulosic materials without requiring harsh ball-milling, thereby allowing the acquisition of high-resolution 2D NMR spectra to revealing structural details of the main components (lignin and polysaccharides). The method is therefore beneficial to understanding the composition and structure of biomass aimed at its improved utilization.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Anionic Effects on Concentrated Aqueous Lithium Ion Dynamics

In this article, the structural dynamics, chemical reactivity, anisotropy, diffusivity, viscosity, and density were measured for concentrated lithium salt solutions, including lithium chloride (LiCl), lithium bromide (LiBr), lithium nitrite (LiNO 2 ), and lithium nitrate (LiNO 3 ), with methyl thiocyanate as an infrared vibrational probe molecule, using two-dimensional infrared spectroscopy (2D IR), nuclear magnetic resonance (NMR) spectroscopy, and viscometry. 2D IR, NMR, and viscosity results show that LiNO 2 exhibits longer correlation times, lower diffusivity, and nearly four times greater viscosity when compared to the other lithium salt solutions of the same concentration, suggesting that nitrite anions may strongly facilitate structure formation via strengthening water-ion interactions, directly impacting bulk solution properties at sufficiently high concentrations. Additionally, the LiNO 2 and LiNO 3 solutions show significantly reduced chemical reactivity with respect to lithium cations coordinating with the methyl thiocyanate when compared to the lithium halide salts.

2D IR↗