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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 361 records · Page 20
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Better Catalysts through Microscopy: Mesoscale M1/M2 Intergrowth in Molybdenum–Vanadium Based Complex Oxide Catalysts for Propane Ammoxidation

Catalysis research has transformed from the predominantly empirical field to one where it is possible to control the catalytic properties via characterization and modification of the atomic-scale active centers. Many phenomena in catalysis, such as synergistic effect, however, transcend the atomic scale and also require the knowledge and control of the mesoscale structure of the specimen to harness. Our paper, we use our discovery of atomic-scale epitaxial interfaces in molybdenum vanadium based complex oxide catalysts systems (i.e., MoVMO, M = Ta, Te, Sb, Nb, etc.) to achieve control of the mesoscale structure of this complex mixture of very different active phases. We can now achieve true epitaxial intergrowth between the catalytically critical M1 and M2 phases in the system that are hypothesized to have synergistic interactions, and demonstrate that the resulting catalyst has improved selectivity in the initial studies. Finally, we highlight the crucial role atomic scale characterization and mesoscale structure control play in uncovering the complex underpinnings of the synergistic effect in catalysis.

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

Structure prediction of epitaxial inorganic interfaces by lattice and surface matching with Ogre

We present a new version of the Ogre open source Python package with the capability to perform structure prediction of epitaxial inorganic interfaces by lattice and surface matching. In the lattice matching step, a scan over combinations of substrate and film Miller indices is performed to identify the domain-matched interfaces with the lowest mismatch. Subsequently, surface matching is conducted by Bayesian optimization to find the optimal interfacial distance and in-plane registry between the substrate and the film. For the objective function, a geometric score function is proposed based on the overlap and empty space between atomic spheres at the interface. The score function reproduces the results of density functional theory (DFT) at a fraction of the computational cost. The optimized interfaces are pre-ranked using a score function based on the similarity of the atomic environment at the interface to the bulk environment. Final ranking of the top candidate structures is performed with DFT. Ogre streamlines DFT calculations of interface energies and electronic properties by automating the construction of interface models. The application of Ogre is demonstrated for two interfaces of interest for quantum computing and spintronics, Al/InAs and Fe/InSb.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Implications of the fractional charge of hydroxide at the electrochemical interface

Rational design of materials that efficiently convert electrical energy into chemical bonds will ultimately depend on a thorough understanding of the electrochemical interface at the atomic level. Towards this goal, the use of density functional theory (DFT) at the generalized gradient approximation (GGA) level has been applied widely in the past 15 years. In the calculation of electrochemical reaction energetics using GGA-DFT, it is frequently implicitly assumed that ions in the Helmholtz plane have unit charge. However, the ion charge is observed to be fractional near the interface through both a capacitor model and through Bader charge partitioning. In this work, we show that this spurious charge transfer can be effectively mitigated by continuum charging of the electrolyte. Finally, we show that, similar to hydronium, the observed fractional charge of hydroxide is not due to a GGA level self-interaction error, as the partial charge is observed even when using hybrid level exchange–correlation functionals.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Electrical Breakdown of Suspended Mono- and Few-Layer Tungsten Disulfide via Sulfur Depletion Identified by in Situ Atomic Imaging

The high bias behavior of mono- and few-layer WS2 is explored by in situ aberration corrected transmission electron microscopy (AC-TEM). The suspended WS2 devices were found to undergo irreversible breakdown at sufficiently high biases, however simultaneous to the removal of WS2 was the accompanying formation of few-layer graphene decorated with W and WS2 nanoparticles. The breakdown of few-layer WS2 resulted in the formation of facetted S-depleted WS2 tendrils along the vaporization boundary, which were found to exhibit lattice contraction indicative of S depletion, alongside pure W phases incorporated into the structure, with the interfaces imaged at atomic resolution. The combination of observing; graphitization of the amorphous carbon surface residue, W nanoparticles, and S depleted WS2 phases, following the high-bias WS2 disintegration all indicate a thermal Joule heating breakdown mechanism over an avalanche process. The observation of graphene formation, and the role the thin amorphous carbon layer has in the pre-breakdown behavior of the device, demonstrate the importance of employing encapsulated heterostructure device architectures that exclude residues.

Fan, Ye↗
DOE OSTI2015

Interface engineering in epitaxial growth of layered oxides via a conducting layer insertion

There is a long-standing challenge in the fabrication of layered oxide epitaxial films due to their thermodynamic phase-instability and the large stacking layer number. Recently, the demand for high-quality thin films is strongly pushed by their promising room-temperature multiferroic properties. Here, we find that by inserting a conducting and lattice matched LaNiO{sub 3} buffer layer, high quality m = 5 Bi{sub 6}FeCoTi{sub 3}O{sub 18} epitaxial films can be fabricated using the laser molecular beam epitaxy, in which the atomic-scale sharp interface between the film and the metallic buffer layer explains the enhanced quality. The magnetic and ferroelectric properties of the high quality Bi{sub 6}FeCoTi{sub 3}O{sub 18} films are studied. This study demonstrates that insertion of the conducting layer is a powerful method in achieving high quality layered oxide thin films, which opens the door to further understand the underline physics and to develop new devices.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Revealing the semiconductor–catalyst interface in buried platinum black silicon photocathodes

Nanoporous 'black' silicon semiconductors interfaced with buried platinum nanoparticle catalysts have exhibited stable activity for photoelectrochemical hydrogen evolution even after months of exposure to ambient conditions. The mechanism behind this stability has not been explained in detail, but is thought to involve a Pt/Si interface free from SiOx layer that would adversely affect interfacial charge transfer kinetics. In this paper, we resolve the chemical composition and structure of buried Pt/Si interfaces in black silicon photocathodes from a micron to sub-nanometer level using aberration corrected analytical scanning transmission electron microscopy. Through a controlled electrodeposition of copper on samples aged for one month in ambient conditions, we demonstrate that the main active catalytic sites are the buried Pt nanoparticles located below the 400-800 nm thick nanoporous SiOx layer. Though hydrogen production performance degrades over 100 h under photoelectrochemical operating conditions, this burying strategy preserves an atomically clean catalyst/Si interface free of oxide or other phases under air exposure and provides an example of a potential method for stabilizing silicon photoelectrodes from oxidative degradation in photoelectrochemical applications.

14 SOLAR ENERGY↗
DOE OSTI2020

Vector potential and surface magnetic field in magnetoelectric antiferromagnetic materials

A general formula for the average vector potential of bulk periodic systems is proposed and shown to set the boundary conditions at magnetic interfaces. Here, for antiferromagnetic materials, the study reveals a unique relation between the macroscopic potential and the orientation-dependent magnetic quadrupole as a result of the different crystalline and magnetic symmetries. In particular, at surfaces and interfaces of a truncated bulk without inversion and time-reversal symmetries, the average vector potential exhibits a discontinuity which results in an interfacial magnetic field. In general, however, due to the surface and interface electronic and atomic relaxations, additional magnetization may result. For the experimentally observed magnetoelectric antiferromagnets, in particular, our symmetry analysis suggests that the relaxation effects could well be a system response to the presence of such a potential discontinuity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Thermal effects on transducer material for heat assisted magnetic recording application

Heat Assisted Magnetic Recording (HAMR) is a promising technology for next generation hard disk drives with significantly increased data recording capacities. In HAMR, an optical near-field transducer (NFT) is used to concentrate laser energy on a magnetic recording medium to fulfill the heat assist function. The key components of a NFT are transducer material, cladding material, and adhesion material between the cladding and the transducer materials. Since transducer materials and cladding materials have been widely reported, this paper focuses on the adhesion materials between the Au transducer and the Al{sub 2}O{sub 3} cladding material. A comparative study for two kinds of adhesion material, Ta and Cr, has been conducted. We found that Ta provides better thermal stability to the whole transducer than Cr. This is because after thermal annealing, chromium forms oxide material at interfaces and chromium atoms diffuse remarkably into the Au layer and react with Au to form Au alloy. This study also provides insights on the selection of adhesion material for HAMR transducer.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Electrical and structural characterizations of crystallized Al{sub 2}O{sub 3}/GaN interfaces formed by in situ metalorganic chemical vapor deposition

Al{sub 2}O{sub 3} films were grown in situ by metalorganic chemical vapor deposition at 900 °C on GaN of both Ga- and N-face polarities. High-resolution transmission electron microscopy revealed that the Al{sub 2}O{sub 3} films were crystalline and primarily γ-phase. The Al{sub 2}O{sub 3}/Ga-GaN and Al{sub 2}O{sub 3}/N-GaN interfaces were both atomically sharp, and the latter further exhibited a biatomic step feature. The corresponding current-voltage (J-V) characteristics were measured on a metal-Al{sub 2}O{sub 3}-semiconductor capacitor (MOSCAP) structure. The leakage current was very high when the Al{sub 2}O{sub 3} thickness was comparable with the size of the crystalline defects, but was suppressed to the order of 1 × 10{sup −8} A/cm{sup 2} with larger Al{sub 2}O{sub 3} thicknesses. The interface states densities (D{sub it}) were measured on the same MOSCAPs by using combined ultraviolet (UV)-assisted capacitance-voltage (C-V), constant capacitance deep level transient spectroscopy (CC-DLTS), and constant capacitance deep level optical spectroscopy (CC-DLOS) techniques. The average D{sub it} measured by CC-DLTS and CC-DLOS were 6.6 × 10{sup 12} and 8.8 × 10{sup 12} cm{sup −2} eV{sup −1} for Al{sub 2}O{sub 3}/Ga-GaN and 8.6 × 10{sup 12} and 8.6 × 10{sup 12 }cm{sup −2} eV{sup −1} for Al{sub 2}O{sub 3}/N-GaN, respectively. The possible origins of the positive (negative) polarization compensation charges in Al{sub 2}O{sub 3}/Ga-GaN (Al{sub 2}O{sub 3}/N-GaN), including the filling of interface states and the existence of structure defects and impurities in the Al{sub 2}O{sub 3} layer, were discussed in accordance with the experimental results and relevant studies in the literature.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Unusual zwitterionic catalytic site of SARS–CoV-2 main protease revealed by neutron crystallography

The main protease (3CL M pro ) from SARS–CoV-2, the etiological agent of COVID-19, is an essential enzyme for viral replication. 3CL M pro possesses an unusual catalytic dyad composed of Cys 145 and His 41 residues. A critical question in the field has been what the protonation states of the ionizable residues in the substrate-binding active-site cavity are; resolving this point would help understand the catalytic details of the enzyme and inform rational drug development against this pernicious virus. Here, we present the room-temperature neutron structure of 3CL M pro , which allowed direct determination of hydrogen atom positions and, hence, protonation states in the protease. We observe that the catalytic site natively adopts a zwitterionic reactive form in which Cys 145 is in the negatively charged thiolate state and His 41 is doubly protonated and positively charged, instead of the neutral unreactive state usually envisaged. The neutron structure also identified the protonation states, and thus electrical charges, of all other amino acid residues and revealed intricate hydrogen-bonding networks in the active-site cavity and at the dimer interface. The fine atomic details present in this structure were made possible by the unique scattering properties of the neutron, which is an ideal probe for locating hydrogen positions and experimentally determining protonation states at near-physiological temperature. Our observations provide critical information for structure-assisted and computational drug design, allowing precise tailoring of inhibitors to the enzyme's electrostatic environment.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Atomic-Scale Control of Magnetism at the Titanite-Manganite Interfaces

Complex oxide thin-film heterostructures often exhibit magnetic properties different from those known for bulk constituents. This is due to the altered local structural and electronic environment at the interfaces, which affects the exchange coupling and magnetic ordering. The emergent magnetism at oxide interfaces can be controlled by ferroelectric polarization and has a strong effect on spin-dependent transport properties of oxide heterostructures, including magnetic and ferroelectric tunnel junctions. In this work, using prototype La 2/3 Sr 1/3 MnO 3 /BaTiO 3 heterostructures, we show that ferroelectric polarization of BaTiO 3 controls the orbital hybridization and magnetism at heterointerfaces. We observe changes in the enhanced orbital occupancy and significant charge redistribution across the heterointerfaces, affecting the spin and orbital magnetic moments of the interfacial Mn and Ti atoms. Notably, we find that the exchange coupling between Mn and Ti atoms across the interface is tuned by ferroelectric polarization from ferromagnetic to antiferromagnetic. Our findings provide a viable route to electrically control complex magnetic configurations at artificial multiferroic interfaces, taking a step toward low-power spintronics.

36 MATERIALS SCIENCE↗
NASA NTRS2005

Structures Self-Assembled Through Directional Solidification

Nanotechnology has created a demand for new fabrication methods with an emphasis on simple, low-cost techniques. Directional solidification of eutectics (DSE) is an unconventional approach in comparison to low-temperature biomimetic approaches. A technical challenge for DSE is producing microstructural architectures on the nanometer scale. In both processes, the driving force is the minimization of Gibb's free energy. Selfassembly by biomimetic approaches depends on weak interaction forces between organic molecules to define the architectural structure. The architectural structure for solidification depends on strong chemical bonding between atoms. Constituents partition into atomic-level arrangements at the liquid-solid interface to form polyphase structures, and this atomic-level arrangement at the liquid-solid interface is controlled by atomic diffusion and total undercooling due to composition (diffusion), kinetics, and curvature of the boundary phases. Judicious selection of the materials system and control of the total undercooling are the keys to producing structures on the nanometer scale. The silicon-titanium silicide (Si-TiSi2) eutectic forms a rod structure under isothermal cooling conditions. At the NASA Glenn Research Center, directional solidification was employed along with a thermal gradient to promote uniform rods oriented with the thermal gradient. The preceding photomicrograph shows the typical transverse microstructure of a solidified Si-TiSi2 eutectic composition. The dark and light gray regions are Si and TiSi2, respectively. Preferred rod orientation along the thermal gradient was poor. The ordered TiSi2 rods have a narrow distribution in diameter of 2 to 3 m, as shown. The rod diameter showed a weak dependence on process conditions. Anisotropic etch behavior between different phases provides the opportunity to fabricate structures with high aspect ratios. The photomicrographs show the resulting microstructure after a wet chemical etch and a dry plasma etch. The wet chemical etches the silicon away, exposing the TiSi2 rods, whereas plasma etching preferentially etches the Si-TiSi2 interface to form a crater. The porous architectures are applicable to fabricating microdevices or creating templates for part fabrication. The porous rod structure can serve as a platform for fabricating microplasma devices for propulsion or microheat exchangers and for fabricating microfilters for miniatured chemical reactors. Although more work is required, self-assembly from DSE can have a role in microdevice fabrication.

Dynys, Frederick W.↗
DOE OSTI2025

Experimental and theoretical investigation of the crystalline surface, film, and interface properties of antiperovskite Mn 3 GaN grown by molecular beam epitaxy on MgO(001)

Here, we present a study of the epitaxial growth, characterization, and theoretical modeling of thin film antiperovskite Mn 3 GaN, an antiferromagnetic material with kagome structure which is grown on MgO (001) substrates using N-plasma-assisted molecular beam epitaxy. Reflection high energy electron diffraction is used to assess the in-plane evolution of the film structure during growth, and the surface is investigated in-situ using scanning tunneling microscopy and Auger electron spectroscopy. These results are combined with precision measurements done ex-situ determining the film lattice constants using a combination of x-ray diffraction with reciprocal space mapping and scanning transmission electron microscopy. Overall, a uniform, homogeneous film with an atomically smooth vacuum surface and atomically sharp substrate interface is found having very small in-plane tensile strain and mild out-of-plane compressive strain. First-principles theoretical calculations are applied in order to ascertain the lowest energy models for both the Mn 3 GaN surface and the Mn 3 GaN/MgO film/substrate interface. Models including MnGa versus MnN surface layers and MnGa versus MnN interfacial layers are considered as functions of both the Mn and Ga chemical potentials. The predictions are discussed in comparison to the experimental results. The overall findings suggest that Mn 3 GaN on MgO(001) is a viable epitaxial film which can be further explored in connection with antiferromagnetic spintronics.

Density functional theory↗
DOE OSTI2024

Visualizing the SEI formation between lithium metal and solid-state electrolyte

The solid electrolyte interphase (SEI) is regarded as the most important factor affecting the durability of lithium-metal anode in all-solid-state batteries (ASSBs). Despite its significance, the nucleation and growth mechanism of SEI is not yet well understood. Here, we elucidate the thermodynamics and kinetics governing SEI formation at the Li|β-Li 3 PS 4 interface at the atomic scale via thermodynamic phase equilibrium analysis and machine-learning-potential-assisted molecular dynamics (MD) simulations. An accurate moment tensor potential using the machine-learning method is developed for a reactive model of Li|β-Li 3 PS 4 . This potential enabled us to perform large-scale MD simulations with the model size expanded to the experimental dimensions (~40 nm) while maintaining the same level of accuracy as density functional theory calculations. The results reveal a four-stage evolution process at the Li|β-Li 3 PS 4 interface, namely (i) fast ion diffusion, (ii) nucleation, (iii) Li 2 S growth, and (iv) stabilization. Notably, we demonstrate that the SEI can be categorized into crystalline and amorphous regions. The simulated SEI thickness, structure, and composition closely match experimental findings, validating the accuracy of the MD simulations. We further disclose the significant impact of ion diffusion kinetic limitations on the phase formation and crystallization of interfacial products. Furthermore, we shed light on the detailed potential energy (PE) distribution of lithium along the direction perpendicular to the interface. Finally, this information is crucial for better understanding interfacial ion mobility.

25 ENERGY STORAGE↗
DOE OSTI2021

Optically Induced Static Magnetization in Metal Halide Perovskite for Spin‐Related Optoelectronics

Abstract Understanding the feasibility to couple semiconducting and magnetic properties in metal halide perovskites through interface design opens new opportunities for creating the next generation spin‐related optoelectronics. In this work, a fundamentally new phenomenon of optically induced magnetization achieved by coupling photoexcited orbital magnetic dipoles with magnetic spins at perovskite/ferromagnetic interface is discovered. The depth‐sensitive polarized neutron reflectometry combined with in situ photoexcitation setup, constitutes key evidence of this novel effect. It is demonstrated that a circularly polarized photoexcitation induces a stable magnetization signal within the depth up to 7.5 nm into the surface of high‐quality perovskite (MAPbBr 3 ) film underneath a ferromagnetic cobalt layer at room temperature. In contrast, a linearly polarized light does not induce any detectable magnetization in the MAPbBr 3 . The observation reveals that photoexcited orbital magnetic dipoles at the surface of perovskite are coupled with the spins of the ferromagnetic atoms at the interface, leading to an optically induced magnetization within the perovskite’s surface. The finding demonstrates that perovskite semiconductor can be bridged with magnetism through optically controllable method at room temperature in this heterojunction design. This provides the new concept of utilizing spin and orbital degrees of freedom in new‐generation spin‐related optoelectronic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Interface structures of inclined ZnO thin film on (0 1 1)-MgO substrate with bulk-like optical properties

Combining different phase structure materials with unique properties to design novel devices plays a significant role in the development of modern electronics. Here, we explore the characteristics of this type of complex interface and epitaxy structures based on the coupling between hexagonal ZnO film and cubic MgO substrate. The ZnO film was prepared by the molecular beam epitaxy technique on the MgO (0 1 1) substrate. The analysis results from the in situ reflection high energy electron diffraction patterns, X-ray diffraction (XRD)-pole figures and high resolution transmission electron microscopy images demonstrate that the film exhibits two-fold symmetry domains with a growth direction deviated from c-axis at about 31º along the [0 1 0] MgO or [0 $\bar{1}$ 0] MgO azimuth. Despite the intertwined diffusion from Zn and Mg atoms in the interface, which is the possible origin of a blue shift of about 0.083 eV in the Photoluminescence (PL) spectrum, the inclined film shows a full width at half maximum value that is close to the reported value from the high quality film. Finally, this work hopefully provides useful insights to the design and exploration of the novel optoelectronic devices that involve the integration of materials with different structure and different properties.

77 NANOSCIENCE AND NANOTECHNOLOGY↗