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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 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Understanding Lithium Local Environments in LiMn 0.5 Ni 0.5 O 2 Cathodes: A DFT-Supported 6 Li Solid-State NMR Study

LiMn 0.5 Ni 0.5 O 2 has been plagued by the structural failure driven by the irreversible phase change upon subsequent cycling and the structural degradation caused by the seemingly unavoidable Li/Ni exchange. This begs for a better understanding of the correlation between lithium local structures, synthesis conditions, and overall electrochemical properties of materials. In this study, we use density functional theory (DFT) to assist experimental 6 Li solid-state magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy to understand the nature and change in lithium local structures with extended annealing times (168 vs 15 h) synthesized at 900 °C. To correlate the NMR peak changes after different synthesis conditions, we calculated the NMR spectra of representative low-energy model configurations of LiMn 0.5 Ni 0.5 O 2 composition–zigzag, row, and flower structures–as well as the influence of Ni/Mn ordering and Li/Ni exchange on each spectrum using a supercell that is large enough to allow us to consider the effect of different relative configurations of adjacent transition metal (TM) layers. The analysis based on a combination of the calculated NMR and the experimental spectra suggests that most configurations contributing to the experimental spectra are in fact composed of a blend of configurations higher in energy than the classical well-ordered ground-state structures. Further, the extended annealing of LiMn 0.5 Ni 0.5 O 2 slightly enhances Ni/Mn and Li/Mn orderings while reducing the Li/Ni mixing ratio. This study shows how DFT calculations are crucial in providing a better understanding of the experimental characterization data and therefore local environments and domain structures in oxide materials, such as Li-ion battery cathodes.

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

Strain-Induced Shifts in Defective Graphite Phonon Modes Predicted by Density Functional Theory

Carbon fiber composites have gained attention as a structural material because of their high strength-to-weight ratio, and understanding the effect of defects on reactivity and mechanical properties is important for the longevity and safety of the composite. Although it is known that strain causes the underlying graphitic vibrational modes to redshift, it is not clear how strain may alter reactivity and defect-induced vibrational changes. To investigate the strain-induced phonon changes of defective carbon fiber composites, density functional theory calculations of graphite are used, including intercalated hydrogen and fluorine defects. By comparing changes in the bond lengths, formation energies, and phonon density of states for uniaxially and biaxially strained graphite, strain was found to generally make defect formation more favorable and the specific behavior changes are dependent on the strain direction and defect identity. Specifically, intercalated fluorine phonons are more sensitive to strain than hydrogen intercalation phonons, and strain applied along the zigzag direction alters the calculated properties more than strain along the armchair direction. Finally, these results highlight the importance of understanding the microstructural effect of deviations from the ideal material because small changes in strain or defect type can significantly alter the behavior of the carbon fiber composite core.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Theoretical Study of High Harmonic Generation in Monolayer NbSe 2

High harmonic generation (HHG) is a powerful probe of electron dynamics on attosecond to femtosecond time scales and has been successfully used to detect electronic and structural changes in solid-state quantum materials, including transition-metal dichalcogenides (TMDs). Among TMDs, bulk NbSe 2 exhibits charge density wave (CDW) order below 33 K and becomes superconducting below 7.3 K. Monolayer NbSe 2 also has superconducting and CDW behavior and is therefore interesting as a material whose different structural and electronic properties could be probed via HHG. Here, we predict the HHG response of the pristine 2H and CDW phases of monolayer NbSe 2 using real-time time-dependent density functional theory under the application of a simulated laser pulse excitation. We find that due to the lack of inversion symmetry in both monolayer phases, it is possible to excite even harmonics and that the even harmonics appear as the transverse components of the current response under excitations polarized along the zigzag direction of the monolayer, while odd harmonics arise from the longitudinal current response in all excitation directions. This suggests that the even and odd harmonic responses can be controlled via the polarization of the probing field, opening an avenue for potentially useful applications in optoelectronic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Chemical Vapor Transport Route toward Black Phosphorus Nanobelts and Nanoribbons

Chemical vapor transport (CVT) method is widely used for bulk black phosphorus (BP) fabrication. In this work, we demonstrate that CVT provides a route for the fabrication of BP nanoribbons and nanobelts. This method consists of a two-step procedure, including initial BP column growth using the CVT technique, followed by ultrasonic treatment and centrifugation. The obtained nanostructures preserve BP column dimensions, forming ultralong ribbon-like structures with the length to the width aspect ratio of up to 500. Computational modeling of the growth mechanism of a BP flake is also presented in support of the observed columnar growth. Calculation of the average energy of the molecule in the asymmetric flakes shows that the growth of the structure in the zigzag direction is more energetically favorable than in the armchair direction.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Dirac Fermions and Flat Bands in Phosphorus Carbide Nanotubes: Structural and Quantum Phase Transitions in a Quasi-One-Dimensional Material

Chemically realistic quasi-one-dimensional (1D) materials in which Dirac Fermions and highly degenerate flat bands coexist intrinsically at the Fermi level are exceedingly rare, while representing a highly desirable platform for correlated and topological quantum phenomena. Here, in this work, using specialized symmetry-adapted first-principles calculations we predict a new class of nanomaterials─phosphorus carbide nanotubes (P 2 C 3 NTs)─obtained by rolling monolayer P 2 C 3 , a two-dimensional material shown in a previous letter to host “double Kagome bands”. Both armchair and zigzag P 2 C 3 NTs are stable at room temperature and feature the rare coexistence of Dirac crossings and multiple flat bands at the Fermi level inherited from the underlying honeycomb–Kagome lattice, with the flat bands resilient to elastic deformations. Under large strain, the structure transforms from honeycomb–Kagome to “brick-wall”, accompanied by multiple coupled structural and quantum phase transitions. We also uncover localized edge states, spin splitting from vacancies and dopants, and strain-tunable magnetism. Together, these results establish P 2 C 3 NTs as a chemically specific and mechanically tunable 1D material platform with potential applications in quantum hardware and spintronics.

carbon nanotubes↗
DOE OSTI2022

Atomic Edge-Guided Polyethylene Crystallization on Monolayer Two-Dimensional Materials

In this report we combine an advanced synthesis of two-dimensional (2D) materials (MoSe 2 ) having well-defined atomic edge configurations with ab initio and atomistic molecular dynamics (MD) simulations to study how atomic edges interact with polyethylene (HDPE) chains in a dilute solution assembly process. Our results reveal that Mo-terminated zigzag (Mo-ZZ) edges act as preferred nucleation sites and strongly interact with HDPE chains. The HDPE chains align in parallel with the Mo-ZZ edges and form arrays of lamellae that are perpendicular to the edges. Interestingly, atomic edge configurations are observed to dramatically change such interactions. The crystallization discrepancy at different edges was demonstrated on the same piece of MoSe 2 with different types of edges. The ab initio and MD simulations between n-alkane (n = 5 and 25), a segment of HDPE, and MoSe 2 suggest that the atomic structures of MoSe 2 can affect their interactions with n-alkane chains. Following the Mo-ZZ edge preferred nucleation principle, controlled long-range alignment of HDPE lamellae can be realized by creating multilayer MoSe 2 with parallel atomic steps. This research opens a pathway toward an atomic level understanding of polymer–2D nanomaterial interactions. It also bridges the gap between atomic-level and long-range mesoscopic structures and introduces a novel strategy for long-range structural control.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Visible Out-of-plane Polarized Luminescence and Electronic Resonance in Black Phosphorus

Black phosphorus (BP) is unique among layered materials because of its homonuclear lattice and strong structural anisotropy. While recent investigations on few-layer BP have extensively explored the in-plane (a, c) anisotropy, much less attention has been given to the out-of-plane direction (b). Here, the optical response from bulk BP is probed using polarization-resolved photoluminescence (PL), photoluminescence excitation (PLE), and resonant Raman scattering along the zigzag, out-of-plane, and armchair directions. An unexpected b-polarized luminescence emission is detected in the visible, far above the fundamental gap. PLE indicates that this emission is generated through b-polarized excitation at 2.3 eV. The same electronic resonance is observed in resonant Raman with the enhancement of the A g phonon modes scattering efficiency. These experimental results are fully consistent with DFT calculations of the permittivity tensor elements and demonstrate the remarkable extent to which the anisotropy influences the optical properties and carrier dynamics in black phosphorus.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Enhanced Exciton-to-Trion Conversion by Proton Irradiation of Atomically Thin WS 2

Defect engineering of van der Waals semiconductors has been demonstrated as an effective approach to manipulate the structural and functional characteristics toward dynamic device controls, yet correlations between physical properties with defect evolution remain underexplored. Here, using proton irradiation, we observe an enhanced exciton-to-trion conversion of the atomically thin WS 2 . The altered excitonic states are closely correlated with nanopore induced atomic displacement, W nanoclusters, and zigzag edge terminations, verified by scanning transmission electron microscopy, photoluminescence, and Raman spectroscopy. Density functional theory calculation suggests that nanopores facilitate formation of in-gap states that act as sinks for free electrons to couple with excitons. The ion energy loss simulation predicts a dominating electron ionization effect upon proton irradiation, providing further evidence on band perturbations and nanopore formation without destroying the overall crystallinity. This study provides a route in tuning the excitonic properties of van der Waals semiconductors using an irradiation-based defect engineering approach.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2023

Ultrafast Optomechanical Strain in Layered GeS

Strong coupling between light and mechanical strain forms the foundation for next-generation optical micro- and nano-electromechanical systems. Such optomechanical responses in two-dimensional materials present novel types of functionalities arising from the weak van der Waals bond between atomic layers. In this paper, by using structure-sensitive megaelectronvolt ultrafast electron diffraction, we report the experimental observation of optically driven ultrafast in-plane strain in the layered group IV monochalcogenide germanium sulfide (GeS). Surprisingly, the photoinduced structural deformation exhibits strain amplitudes of order 0.1% with a 10 ps fast response time and a significant in-plane anisotropy between zigzag and armchair crystallographic directions. Rather than arising due to heating, experimental and theoretical investigations suggest deformation potentials caused by electronic density redistribution and converse piezoelectric effects generated by photoinduced electric fields are the dominant contributors to the observed dynamic anisotropic strains. Our observations define new avenues for ultrafast optomechanical control and strain engineering within functional devices.

2D materials↗
DOE OSTI2022

Density Functional Theory Study of Controllable Optical Absorptions and Magneto-Optical Properties of Magnetic CrI 3 Nanoribbons: Implications for Compact 2D Magnetic Devices

A chromium triiodide (CrI 3 ) monolayer has an interesting ferromagnetic ground-state. In this work, we calculate band structures and magnetic moments of tensile-strained and bent zigzag CrI 3 nanoribbons with density functional theory. The edge iodine atoms form flat low-lying conduction bands and couple with chromium atoms ferromagnetically, while the non-edge iodine atoms weakly couple antiferromagnetically. Narrow CrI 3 nanoribbons have two locally stable magnetic moment orientations, namely out-of-plane and in-plane (along the nanoribbon periodic direction) configurations. This enables four magnetization states in CrI 3 nanoribbons, including two out-of-plane ones (up and down) and two in-plane ones (forward and backward along the nanoribbon periodical direction), increasing the operating controllability. Based on the one-dimensional Ising spin chain model, the spin correlation length of the narrow CrI 3 nanoribbon is estimated as about 10 Å at its estimated Curie temperature of 27 Kelvin, which is lower than the measured 45 Kelvin of the monolayer CrI 3 . The optical absorption and magneto-optical properties of CrI 3 nanoribbons are investigated with many-body perturbation GW-BSE (Bethe-Salpeter equation), including magnetic dichroism, Faraday and magneto-optical Kerr effects. Here, the low-energy dark excitons are mainly from transitions between electrons and holes with unlike spins and are non-Frenkel-like, while the bright excitons have mixed spin configurations. The intrinsic lifetime of excitons can be over one nanosecond, suitable for quantum information processes. Tensile strains and bending manifestly modulate the absorption spectra and magneto-optical properties of CrI 3 nanoribbons within a technologically important photon energy-range of ~1.0-2.0 eV. The CrI 3 nanoribbons can be used in 1D or 2D magnetic storage nanodevices, tunable magnetic optoelectronics, and spin-based quantum information controls.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Pressure-Induced Chemical Bonding Effects on Lattice and Magnetic Instabilities in Antiferromagnetic Insulating CaMn 2 Sb 2

Exotic quantum phenomena often emerge near an electronic delocalization transition (EDT) from an antiferromagnetic insulating phase to a strongly correlated metallic state under pressure. We report the pressure-induced structural and magnetic evolution of the antiferromagnetic insulator CaMn 2 Sb 2 . Single-crystal X-ray diffraction reveals a first-order phase transition near 5.4 GPa from a trigonal P-3m1 structure to a monoclinic P2 1 /m phase accompanied by a ∼7% volume collapse. Residual electron density analysis at intermediate pressures reveals charge localization along Mn–Sb chains, signaling electronic instability preceding the structural transition. Bonding analysis indicates anisotropic Mn–Sb orbital reconfiguration under pressure, driving a distorted square-pyramidal geometry. Neutron scattering confirms the transition and identifies a pressure-induced incommensurate magnetic order, distinct from the ambient antiferromagnetic state. In the monoclinic phase, zigzag Mn chains exhibit antiferromagnetic coupling along the ac-plane, enabled by enhanced orbital overlap. These results establish CaMn 2 Sb 2 as a model system for studying the coupling of structural distortion, charge redistribution, and magnetic order in layered Mn pnictides under pressure.

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

Mapping 1D Confined Electromagnetic Edge States in 2D Monolayer Semiconducting MoS 2 Using 4D-STEM

Here, four-dimensional (4D) scanning transmission electron microscopy is used to study the electric fields at the edges of 2D semiconducting monolayer MoS2. Sub-nanometer 1D features in the 2D electric field maps are observed at the outermost region along zigzag edges and also along nanowire MoS-terminated MoS 2 edges. Atomic-scale oscillations are detected in the magnitude of the 1D electromagnetic edge state, with spatial variations that depend on the specific periodic edge reconstructions. Electric field reconstructions, along with integrated differential phase contrast reconstructions, reveal the presence of low Z number atoms terminating many of the uniform edges, which are difficult to detect by annular dark field scanning transmission electron microscopy due to its limited dynamic range. Density functional theory calculations support the formation of periodic 1D edge states and also show that enhancement of the electric field magnitude can occur for some edge terminations. The experimentally observed electric fields at the edges are attributed to the absence of an opposing electric field from a nearest neighbor atom when the electron beam propagates through the 2D monolayer and interacts. These results show the potential of 4D-STEM to map the atomic scale structure and fluctuations of electric fields around edge atoms with different bonding states than bulk atoms in 2D materials, beyond conventional imaging.

2D materials↗
DOE OSTI2024

Giant Anisotropic Magnetoresistance in Few-Layer α-RuCl 3 Tunnel Junctions

The spin–orbit-assisted Mott insulator α-RuCl 3 is proximate to the coveted quantum spin liquid (QSL) predicted by the Kitaev model. In the search for the pure Kitaev QSL, reducing the dimensionality of this frustrated magnet by exfoliation has been proposed as a way to enhance magnetic fluctuations and Kitaev interactions. Here, in this work, we perform angle-dependent tunneling magnetoresistance (TMR) measurements on ultrathin α-RuCl 3 crystals with various layer numbers to probe their magnetic, electronic, and crystal structures. We observe a giant change in resistance, as large as ~2500%, when the magnetic field rotates either within or out of the α-RuCl 3 plane, a manifestation of the strongly anisotropic spin interactions in this material. In combination with scanning transmission electron microscopy, this tunneling anisotropic magnetoresistance (TAMR) reveals that few-layer α-RuCl 3 crystals remain in the high-temperature monoclinic phase at low temperatures. It also shows the presence of a zigzag antiferromagnetic order below the critical temperature T N ≃ 14 K, which is twice the one typically observed in bulk samples with rhombohedral stacking. Our work offers valuable insights into the relation between the stacking order and magnetic properties of this material, which helps lay the groundwork for creating and electrically probing exotic magnetic phases such as QSLs via van der Waals engineering.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Lattice Anisotropy-Driven Reduction of Phonon Velocities in Black Phosphorus

Phonon dynamics and transport determine how heat is utilized and dissipated in materials. In 2D systems for optoelectronics and thermoelectrics, the impact of nanoscale material structure on phonon propagation is central to controlling thermal conduction. Here, we directly observe in-plane coherent acoustic phonon propagation in black phosphorus (BP) using ultrafast electron microscopy. We identify a significant reduction of the group velocities in directions intermediate to the armchair and zigzag lattice directions. Using a machine learning-based model with an >8000 atom supercell, we find that this slowing results from the mixing of in-plane transverse and longitudinal acoustic phonons and is independent of broken symmetries of edge reconstructions. In conclusion, this work demonstrates how coherent phonon transport is sensitive to propagation direction in the lattice plane.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Modes of Disorder in Poly(carbon monofluoride)

Poly(carbon monofluoride), or (CF) n , is a layered fluorinated graphite material consisting of nanosized platelets. In this work, we present experimental multidimensional solid-state NMR spectra of (CF)n, supported by density functional theory (DFT) calculations of NMR parameters, which overhauls our understanding of structure and bonding in the material by elucidating many ways in which disorder manifests. We observe strong 19 F NMR signals conventionally assigned to elongated or “semi-ionic” C–F bonds and find that these signals are in fact due to domains where the framework locally adopts boat-like cyclohexane conformations. We calculate that C–F bonds are weakened but are not elongated by this conformational disorder. Exchange NMR suggests that conformational disorder avoids platelet edges. We also use a new J-resolved NMR method for disordered solids which provides molecular-level resolution of highly fluorinated edge states. Strings of consecutive difluoromethylene groups at edges are relatively mobile. Topologically distinct edge features, including zigzag edges, crenellated edges, and coves are resolved in our samples by solid-state NMR. Disorder should be controllable in a manner dependent on synthesis, affording new opportunities for tuning the properties of graphite fluorides.

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

Gas-Phase Synthesis of Coronene through Stepwise Directed Ring Annulation

Molecular beam experiments together with electronic structure calculations provide the first evidence of a complex network of elementary gas-phase reactions culminating in the bottom-up preparation of the 24π aromatic coronene (C 24 H 12 ) molecule–a representative peri-fused polycyclic aromatic hydrocarbon (PAH) central to the complex chemistry of combustion systems and circumstellar envelopes of carbon stars. The gas-phase synthesis of coronene proceeds via aryl radical-mediated ring annulations through benzo[e]pyrene (C 20 H 12 ) and benzo[ghi]perylene (C 22 H 12 ) involving armchair-, zigzag-, and arm-zig-edged aromatic intermediates, highlighting the chemical diversity of molecular mass growth processes to polycyclic aromatic hydrocarbons. Furthermore, the isomer-selective identification of five- to six-ringed aromatics culminating with the detection of coronene is accomplished through photoionization and is based upon photoionization efficiency curves along with photoion mass-selected threshold photoelectron spectra, providing a versatile concept of molecular mass growth processes via aromatic and resonantly stabilized free radical intermediates to two-dimensional carbonaceous nanostructures.

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

Direct Observation of Circularly Polarized Nonlinear Optical Activities in Chiral Hybrid Lead Halides

Circularly polarized light emission is a crucial application in imaging, sensing, and photonics. However, utilizing low-energy photons to excite materials, as opposed to high-energy light excitation, can facilitate deep-tissue imaging and sensing applications. The challenge lies in finding materials capable of directly generating circularly polarized nonlinear optical effects. In this study, we introduce a chiral hybrid lead halide (CHLH) material system, R/S-DPEDPb 3 Br 8 ·H 2 O (DPED = 1,2-diphenylethylenediammonium), which can directly produce circularly polarized second harmonic generation (CP-SHG) through linearly polarized infrared light excitation, exhibiting a polarization efficiency as high as 37% at room temperature. To understand the spin relaxation mechanisms behind the high polarization efficiency, we utilized two models, so-called D’yakonov–Perel’ (DP) and Bir–Aronov–Pikus (BAP) mechanisms. The unique zigzag inorganic frameworks within the hybrid structure are believed to reduce the dielectric confinement and exciton binding energy, thus enhancing spin polarization, especially in regions with a high excitation pump fluence based on the DP mechanism. In the case of low excitation pump fluence, the BAP mechanism dominates, as evidenced by the observed decrease in the polarization ratio from CP-SHG measurement. Using density functional theory analysis, we elucidate how the distinctive 8-coordination environment of lead bromide building blocks effectively suppresses spin–orbit coupling at the conduction band minimum. This suppression significantly diminishes spin-splitting, thereby slowing the spin relaxation rate.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

MnSi 2 Te 4 : A van der Waals Antiferromagnetic Semiconductor with Large Negative Magnetoresistance

Magnetism in van der Waals semiconductors offers significant potential for fundamental research on low-dimensional magnetism and the development of high-performance two-dimensional spintronic devices. Here, we report the growth, physical properties, and first-principles calculations of a new dual-octahedral transition metal chalcogenide (DTMC) MnSi 2 Te 4 . MnSi 2 Te 4 features a layered structure with an intralayer heterostructure, where the metal octahedra and nonmetal dimeric octahedra form zigzag chains alternately. Property characterization reveals that MnSi 2 Te 4 is a collinear G-type antiferromagnetic semiconductor, with a Néel temperature T N of 18.6 K and a significant unsaturated negative magnetoresistance (NMR) reaching −42.5% at 9 T and 100 K. First-principles calculations on the electronic band structure demonstrate that the large NMR primarily originates from the spin splitting due to parity-time symmetry breaking. This study not only discovers a new member of DTMCs with a unique crystal structure and large NMR, but also establishes a promising platform for investigating next-generation spintronic devices.

Liao, Ke [Chinese Academy of Sciences (CAS), Beiji↗