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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 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Twisting and Rippling of a Two-Dimensional Kagome Lattice: Silica on Au(111)

We report the intrinsic properties of two-dimensional (2D) SiO 2 were revealed by forming the material on inert Au(111). Growth by SiO deposition enabled formation of a crystalline phase consisting of two linked sheets of six-membered rings of tetrahedral [SiO 4 ] building units. The bases of the tetrahedra form an isostatic 2D kagome lattice. The weak interaction with Au allowed a new corrugation of the 2D material to be detected: ripples with a ~ 4 nm periodicity that help stabilize the 2D crystalline layer. On the atomic scale, substantial distortions from ideal hexagonal rings were observed even though crystalline defects were extremely rare. The ring distortions were reproduced in theoretical models that showed that they were associated with a twisting of the 2D kagome lattice, which stabilizes the material when subjected to disturbances. The twisting and rippling impart 2D silica with the flexibility to adapt to strain and changes in the crystallographic direction without introducing defects.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Moiré-Assisted Strain Transfer in Vertical van der Waals Heterostructures

Strain provides a powerful method to study 2D monolayers and to tune their properties. The same approach also has great potential for van-der-Waals (vdW) heterostructures. However, we need to understand how strain can be applied to vertically stacked vdW structures, for which strain transfer from one layer to the next remains little explored. In our experiment, we fabricated vertical heterostructures consisting of transition metal dichalcogenides (TMDCs) monolayers that were deposited on a flexible substrate. These TMDC heterostructures allowed us to read out separately the strain in each monolayer by photoluminescence measurements. We find that, in TMDC heterostructures with large twist angles (>5°), strain transfer is limited. However, for aligned heterostructures with small twist angles (≤5°), near unity strain transfer efficiency is observed. Here, we correlate this finding with the moiré domains formed in the aligned heterostructures by reconstruction.

2D materials↗
DOE OSTI2021

Heterostructured Lepidocrocite Titanate-Carbon Nanosheets for Electrochemical Applications

Lepidocrocite-type titanates that reversibly intercalate sodium ions at low potentials (~0.6 V vs Na/Na + ) are promising anode candidates for sodium-ion batteries. However, large amounts of carbon additives are often used to improve their electrical conductivity and overcome poor cycling performance in the electrode composites. To ameliorate electronic transport issues of lepidocrocite titanate (K 0.8 Ti 1.73 Li 0.27 O 4 , KTL) in sodium-ion batteries, we have designed and synthesized heterostructures of exfoliated lepidocrocite-type titanium oxide (LTO) nanosheets with alternating carbon layers via a solution-based self-assembly approach. Positively charged dopamine (Dopa) was used as the carbon precursor and intercalated between negatively charged exfoliated titania nanosheets through electrostatic interaction. Dopa-intercalated LTO was then annealed under argon to form conductive carbon layers between titania sheets. The carbon content in the heterostructures was controlled by modifying the self-assembly conditions (i.e., pH, stirring duration, and Dopa-to-LTO ratio). Electrodes were prepared using carbonized heterostructures (LTO-C) without adding more carbon to the composites and tested in sodium half-cell configurations. Further, higher capacities and improved capacity retention over 250 cycles and lower impedance were observed, as the carbon content of LTO-C heterostructures was increased from 0% (LTO nanosheets) to 30%. These results indicate that the self-assembly approach for 2D heterostructured electrode materials is a promising strategy to overcome electronic transport limitations of layered transition-metal oxides and improve their electrochemical performance for next-generation energy storage applications.

25 ENERGY STORAGE↗
DOE OSTI2022

The exceedingly strong two-dimensional ferromagnetism in bi-atomic layer SrRuO 3 with a critical conduction transition

In recent years, few-layer or even monolayer ferromagnetic materials have drawn a great deal of attention due to the promising integration of two-dimensional (2D) magnets into next-generation spintronic devices. The SrRuO 3 monolayer is a rare example of stable 2D magnetism under ambient conditions, but only weak ferromagnetism or antiferromagnetism has been found. The bi-atomic layer SrRuO3 as another environmentally inert 2D magnetic system has been paid less attention heretofore. Here we study both the bi-atomic layer and monolayer SrRuO 3 in (SrRuO 3 ) n /(SrTiO 3 ) m (n = 1, 2) superlattices in which the SrTiO 3 serves as a non-magnetic and insulating space layer. Although the monolayer exhibits arguably weak ferromagnetism, we find that the bi-atomic layer exhibits exceedingly strong ferromagnetism with a T c of 125 K and a saturation magnetization of 1.2 μ B /Ru, demonstrated by both superconducting quantum interference device (SQUID) magnetometry and element-specific X-ray circular dichroism. Moreover, in the bi-atomic layer SrRuO 3 , we demonstrate that random fluctuations and orbital reconstructions inevitably occurring in the 2D limit are critical to the electrical transport, but are much less critical to the ferromagnetism. Our study demonstrates that the bi-atomic layer SrRuO 3 is an exceedingly strong 2D ferromagnetic oxide which has great potentials for applications of ultracompact spintronic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2022

How Water Attacks MXene

Two-dimensional (2D) transition metal carbides and nitrides (MXenes) have shown outstanding performances in electrochemical energy storage and many other applications. However, the stability of MXene remains a concern, especially its quick degradation in aqueous solutions under ambient conditions. Here, we report on the water/Ti 3 C 2 O 2 -MXene interfacial chemistry from first-principles molecular dynamics simulations at room temperature. Surprisingly, we find that the water molecules can attack the basal plane of Ti 3 C 2 O 2 and pull the surface Ti atoms out, thereby reconstructing the surface. Further, by tracking close encounters of water molecules and surface Ti atoms on the basal plane of Ti 3 C 2 O 2 , we show that the attack is initiated by the chemisorption of a water molecule on a surface Ti atom, followed by the breaking of Ti–C bonds and deprotonation of the water molecule, leading to the formation of Ti–OH on the Ti 3 C 2 O 2 surface and a hydronium ion in the aqueous phase. Our finding highlights the susceptibility of Ti 3 C 2 O 2 MXene to water attack, supporting recent experimental observations. Furthermore, we demonstrate that preventing close encounters of water molecules and the surface Ti atoms is key to the stability of the basal plane and can be realized by negatively charging the surface (thereby reorienting the O atoms of water away from the surface) or converting the surface O to -OH groups (thereby shifting the water layer further away from the surface). Our insights and approach highlight the importance of the reactivity of water when interfacing with 2D materials such as MXenes.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Van der Waals heterostructures

The integration of dissimilar materials into heterostructures has become a powerful tool for engineering interfaces and electronic structure. The advent of two-dimensional (2D) materials brought unprecedented opportunities for novel heterostructures in the form of van der Waals stacks, laterally stitched 2D layers, and more complex layered and 3D architectures. This Primer provides an overview of state-of-the-art methodologies for producing such van der Waals heterostructures, focusing on the two fundamentally different strategies, top-down deterministic assembly and bottom-up synthesis. For both approaches, successful techniques, advantages, and limitations are discussed. As important as the fabrication itself is the characterization of the resulting engineered materials, for which a range of analysis techniques covering structure, composition, and emerging functionality are highlighted. Examples of the properties of artificial van der Waals structures include optoelectronics and plasmonics, twistronics, and unique functionality arising from the generalization of van der Waals assembly from 2D- to 3D-crystalline components. Lastly, current issues of reproducibility, limitations, and opportunities for future breakthroughs in terms of enhanced homogeneity, interfacial purity, feature control, and ultimately orders of magnitude increased complexity of van der Waals heterostructures are discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2022

A Novel $\sqrt {19} $ × $\sqrt {19} $ Superstructure in Epitaxially Grown 1T–TaTe 2

The spontaneous formation of electronic orders is a crucial element for understanding complex quantum states and engineering heterostructures in 2D materials. A novel $\sqrt {19} $ × $\sqrt {19} $ charge order in few-layer-thick 1T-TaTe 2 transition metal dichalcogenide films grown by molecular beam epitaxy, which has not been realized, is report. The photoemission and scanning probe measurements demonstrate that monolayer 1T-TaTe 2 exhibits a variety of metastable charge density wave orders, including the $\sqrt {19} $ × $\sqrt {19} $ superstructure, which can be selectively stabilized by controlling the post-growth annealing temperature. Moreover, it is found that only the $\sqrt {19} $ × $\sqrt {19} $ order persists in 1T-TaTe 2 films thicker than a monolayer, up to 8 layers. Furthermore, the findings identify the previously unrealized novel electronic order in a much-studied transition metal dichalcogenide and provide a viable route to control it within the epitaxial growth process.

36 MATERIALS SCIENCE↗
DOE OSTI2020

NaGaS 2 : An Elusive Layered Compound with Dynamic Water Absorption and Wide‐Ranging Ion‐Exchange Properties

Abstract Most ternary sulfides belonging to the MGaS 2 structure‐type have been known for many years and are well‐characterized. Surprisingly, there have been no reports of the NaGaS 2 composition, which contains Na, a monovalent cation slightly larger in size than Li, found in LiGaS 2 , a compound known for its non‐linear optical properties. Now it is demonstrated for the first time that the unique reversible water absorption in NaGaS 2 has resulted in its absence from previous reports owing to difficulties encountered when characterizing this compound by SC XRD. The layered structure of this compound coupled with uniquely easy migration of water molecules between the layers allows for ion exchange with 3d and 5f metal cations. Some cations, for example, Ni 2+ , facilitate exfoliation of the layers, providing a facile synthetic route to a new class of 2D chalcogenide materials and furthermore demonstrating that NaGaS 2 can readily uptake uranyl species from aqueous solutions.

Klepov, Vladislav V.↗
DOE OSTI2020

NaGaS 2 - an Elusive Layered Compound with Dynamic Water Absorption and Wide-Ranging Ion Exchange Properties

Most ternary sulfides belonging to the MGaS 2 structure-type have been known for many years and are well-characterized. Surprisingly, there have been no reports of the NaGaS 2 composition, which contains Na, a monovalent cation slightly larger in size than Li, found in LiGaS 2 , a compound known for its non-linear optical properties. Herein we demonstrate for the first time that the unique reversible water absorption in NaGaS 2 has resulted in its absence from the literature due to the difficulties that one encounters when characterizing this compound by SC XRD. The layered structure of this compound coupled with uniquely easy migration of water molecules between the layers allows for ion exchange with 3d and 5f metal cations. We established that some cations, e.g. Ni 2+ , facilitate exfoliation of the layers, providing a facile synthetic route to a new class of 2D chalcogenide materials and, in addition, demonstrating that NaGaS 2 can readily uptake uranyl species from aqueous solutions.

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

Experimental and Theoretical Studies of the Surface Oxidation Process of Rare–Earth Tritellurides

Recent studies have established Van der Waals (vdW) layered and 2D rare-earth tritellurides (RTe 3 ) as superconductors and near room-temperature charge density wave (CDW) materials. Their environmental stability raises natural concern owing to aging/stability effects observed in other tellurium-based layered crystals. Here, the results establish the stability and environmental aging characteristics of these RTe 3 systems involving a variety of metals such as La, Nd, Sm, Gd, Dy, and Ho. The atomic force microscopy (AFM) and scanning electron microscopy (SEM) results show that all the RTe 3 sheets oxidize to form thin TeO x layers that are primarily confined to the surface, edges, and grain boundaries. Time-resolved in situ Raman spectroscopy measurements are used to understand the kinetics of the oxidization process for different lanthanide metal cations and establish their relative stability/resilience to oxidization. Overall results indicate that the vdW layers show higher air stability as the 4f electron number decreases going from Ho to La, resulting in the most stable LaTe 3 compared to the least stable HoTe 3 . Comprehensive quantum mechanical simulations reveal that environmental degradation originates from a strong oxidizing reaction with O 2 molecules, while humidity (H 2 O) plays a negligible role unless Te vacancies are present. Moreover, the simulations explain the effects of 4f electrons on the work function and Te vacancies formation, which directly impact the aging characteristics of RTe 3 layers. Interestingly, optical and electrical measurements show that the CDW response is still observed in aged RTe 3 layers owing to the presence of underlying pristine/nonoxidized RTe 3 layers, except CDW transition temperatures increase due to the thickness effect. Overall results offer the first in-depth environmental aging studies on these materials, which can be applied to engineer and design their chemical stability, surface properties, and overall CDW characteristics.

36 MATERIALS SCIENCE↗
DOE OSTI2022

The role of temperature on defect diffusion and nanoscale patterning in graphene

Graphene is of great scientific interest due to a variety of unique properties such as ballistic transport, spin selectivity, the quantum hall effect, and other quantum properties. Nanopatterning and atomic scale modifications of graphene are expected to enable further control over its intrinsic properties, providing ways to tune the electronic properties through geometric and strain effects, introduce edge states and other local or extended topological defects, and sculpt circuit paths. The focused beam of a scanning transmission electron microscope (STEM) can be used to remove atoms, enabling milling, doping, and deposition. Utilization of a STEM as an atomic scale fabrication platform is increasing; however, a detailed understanding of beam-induced processes and the subsequent cascade of aftereffects is lacking. Here, we examine the electron beam effects on atomically clean graphene at a variety of temperatures ranging from 400 to 1000 °C. We find that temperature plays a significant role in the milling rate and moderates competing processes of carbon adatom coalescence, graphene healing, and the diffusion (and recombination) of defects. The results of this work can be applied to a wider range of 2D materials and introduce better understanding of defect evolution in graphite and other bulk layered materials.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Spin–orbit coupling proximity effect in MoS 2 /Fe 3 GeTe 2 heterostructures

Layered two-dimensional (2D) magnet/semiconductor heterostructures combine spintronic and optoelectronic properties of constituent materials, leading to new magneto-optical and magnetoelectric phenomena such as spontaneous emission of helical light and enhanced Zeeman splitting in single photon emission. While prior focus was mostly on the magnetic proximity effect, where properties of 2D magnets are transferred to nonmagnetic 2D materials, the inverse effect of 2D semiconductors altering 2D magnets is much less understood. Here, we fabricated and studied van der Waals (vdW) heterostructures of 2D magnet Fe 3 GeTe 2 (FGT) and 2D semiconductor MoS 2 . With reflectance magnetic circular dichroism, we found that the coercive field of MoS 2 -covered FGT reduces compared with uncovered FGT, agreeing well with our first-principles calculations. With its strong spin–orbit coupling (SOC), MoS 2 effectively alters the crystal field of the adjacent FGT and its magnetic anisotropy. Furthermore, an unconventional two-step hysteresis loop emerges in MoS 2 /FGT as a result of the superposition of two regions of FGT: at the interface and away from the interface. Our experimental elucidation of the SOC proximity effect that MoS 2 exerts on FGT provides fundamental understanding for the rational development of 2D magnet/semiconductor heterostructures.

2D materials↗
DOE OSTI2021

Giant Transport Anisotropy in ReS 2 Revealed via Nanoscale Conducting-Path Control

The low in-plane symmetry in layered 1T'-ReS 2 results in strong band anisotropy, while its manifestation in the electronic properties is challenging to resolve due to the lack of effective approaches for controlling the local current path. In this work, we reveal the giant transport anisotropy in monolayer to four-layer ReS 2 by creating directional conducting paths via nanoscale ferroelectric control. By reversing the polarization of a ferroelectric polymer top layer, we induce conductivity switching ratio of >1.5×10 8 in the ReS 2 channel at 300 K. Characterizing the domain-defined conducting nanowires in an insulating background shows that the conductivity ratio between the directions along and perpendicular to the Re-chain can exceed 5.5×10 4 in monolayer ReS 2 . Theoretical modeling points to the band origin of the transport anomaly, and further reveals the emergence of a flat band in few-layer ReS 2 . Our work paves the path for implementing the highly anisotropic 2D materials for designing novel collective phenomena and electron lensing applications.

2D van der Waals materials↗
DOE OSTI2025

Structural stability, elemental ordering, and transport properties of layered ScTaN 2

Ternary transition metal (TM) nitrides have gained significant attention in thin film research due to their promising properties for a broad range of applications. Particularly, some of the ternary TM nitrides have been predicted to adopt layered structures that make them interesting for thermoelectric conversion and quantum materials applications. Unfortunately, synthesis of TM ternary nitride films by physical vapor deposition often favors disordered 3D structures rather than the predicted 2D-like layered structure. In this study, we investigate the structural interplay in the Sc-Ta-N ternary system using a combinatorial approach. Combinatorial libraries S⁢c 𝑥 ⁢T⁢a 1−𝑥 ⁢N are synthesized following a two-step method: First, deposit film precursors by cosputtering and then process the resulting 3D-structured samples with rapid thermal annealing. Synchrotron grazing-incidence wide-angle x-ray scattering on films annealed at 1200 ⁢°⁢C for 20 min leads to the nucleation of ScTaN 2 layered structure (𝑃⁢6 3 /𝑚⁢𝑚⁢𝑐) near stoichiometry. We find that the layered structure can accommodate large off-stoichiometry in the Ta-rich region (𝑥 < 0.5), facilitated by the alloying with quasi-isostructural Ta 5 ⁢N 6 compound that exists on a composition tie line at 𝑥 = 0. While focusing on ScTaN 2 , we estimate the long-range order parameter in near-stoichiometric films to be 0.86, corresponding to a fraction of Sc/Ta antisites of 7%. Transport measurements on ScTaN 2 reveal a nearly temperature-independent high carrier density (10 21 c⁢m −3 ), suggesting a heavily doped semiconductor or semimetallic character, consistent with a small positive Seebeck coefficient of +19 µV/K. The carrier mobility at 2 K is relatively small (9.5c⁢m 2 V −1 s −1 ) and the residual-resistivity ratio is minor, suggesting that electrical conduction is dominated by defects or disorder. Measured magnetoresistance suggests possible weak antilocalization at 2 K. This paper highlights the interplay between ScTaN 2 and Ta 5 ⁢N 6 crystal structures in stabilizing layered materials, emphasizes the importance of cation order/disorder for potential tunable alloys, and suggests that ScTaN 2 is a promising platform for exploring electronic properties.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Understanding and tuning magnetism in van der Waals-type metal thiophosphates

Over the past two decades, significant progress in two-dimensional (2D) materials has invigorated research in condensed matter and material physics in low dimensions. While traditionally studied in three-dimensional systems, magnetism has now been extended to the 2D realm. Recent breakthroughs in 2D magnetism have attracted substantial interest from the scientific community, owing to the stable magnetic order achievable in atomically thin layers of the van der Waals (vdW)-type layered magnetic materials. These advances offer an exciting platform for investigating related phenomena in low dimensions and hold promise for spintronic applications. Consequently, vdW magnetic materials with tunable magnetism have attracted significant attention. Specifically, antiferromagnetic metal thiophosphates MPX 3 (M = transition metal, P = phosphorus, X = chalcogen) have been investigated extensively. These materials exhibit long-range magnetic order spanning from bulk to the 2D limit. The magnetism in MPX 3 arises from localized moments associated with transition metal ions, making it tunable via substitutions and intercalations. In this review, we focus on such tuning by providing a comprehensive summary of various metal- and chalcogen-substitution and intercalation studies, along with the mechanism of magnetism modulation, and a perspective on the development of this emergent material family.

42 ENGINEERING↗
DOE OSTI2026

Final Technical Report for DE-SC0021049: Manipulating interfacial reactivity with atomically layered heterostructures

This final technical report summarizes the work accomplished in this DOE Early Career Research Program project that has established moiré superlattice materials and two-dimensional (2D) heterostructures as a highly tunable platform for controlling heterogeneous charge transfer (ET) kinetics at solid-liquid interfaces. By precisely engineering van der Waals heterostructures of atomically thin 2D materials, particularly bilayer and trilayer graphene with controlled twist angles, this project demonstrated systematic control of interfacial charge transfer rates spanning three orders of magnitude. This research addresses fundamental questions about how electronic structure, charge localization, and atomic layer-dependent properties govern charge transfer at electrochemical interfaces, with broad implications for energy conversion, electrocatalysis, and next-generation electrochemical devices.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Strong Magnetocrystalline Anisotropy Arising from Metal–Ligand Covalency in a Metal–Organic Candidate for 2D Magnetic Order

Layered metal–organic frameworks are promising candidates for new two-dimensional magnets, as their synthetic programmability of these materials can provide a route to diverse structural and electronic properties. However, such framework materials typically lack the heavy elements that engender magnetocrystalline anisotropy in the monolayer ferromagnets reported to date. Alternative sources of magnetic anisotropy are therefore needed in these materials. Here, we report the synthesis of single crystals of the framework material (NMe 4 ) 2 [Fe 2 L 3 ] (H 2 L = 3,6-dichloro-2,5-dihydroxybenzoquinone) and evaluate the angular dependence of its magnetic properties. Oriented-crystal magnetization measurements reveal strong uniaxial anisotropy, where the easy axis is aligned with the crystallographic c axis. While the spin carriers of this structure are isotropic $S = ^5/_2$ Fe III metal centers and $S = ^1/_2$ organic linkers, the anisotropy energy of the framework material is comparable to that of reported 2D ferromagnets. Density functional theory calculations indicate that the observed magnetocrystalline anisotropy arises from ligand-to-metal charge transfer that enhances the magnetic anisotropy of the otherwise isotropic Fe centers, suggesting that metal–ligand covalency can be utilized as a general additive for the development of 2D magnets. These results in this study show the possibility for (NMe 4 ) 2 [Fe 2 L 3 ] to retain magnetic order down to the 2D monolayer limit. In addition, the combination of large magnetic anisotropy and semiconducting character in (NMe 4 ) 2 [Fe 2 L 3 ] highlights its potential as a new 2D magnetic semiconductor.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗