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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,153 records · Page 64
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Spin-polarized surface resonances accompanying topological surface state formation

Topological insulators host spin-polarized surface states born out of the energetic inversion of bulk bands driven by the spin-orbit interaction. Here we discover previously unidentified consequences of band-inversion on the surface electronic structure of the topological insulator Bi 2 Se 3 . By performing simultaneous spin, time, and angle-resolved photoemission spectroscopy, we map the spin-polarized unoccupied electronic structure and identify a surface resonance which is distinct from the topological surface state, yet shares a similar spin-orbital texture with opposite orientation. Its momentum dependence and spin texture imply an intimate connection with the topological surface state. Calculations show these two distinct states can emerge from trivial Rashba-like states that change topology through the spin-orbit-induced band inversion. As a result, this work thus provides a compelling view of the coevolution of surface states through a topological phase transition, enabled by the unique capability of directly measuring the spin-polarized unoccupied band structure.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Distinct Turbulence Structures in Stably Stratified Boundary Layers With Weak and Strong Surface Shear

Turbulence structures and exchange of momentum and heat in the nocturnal stable boundary layer (SBL) show distinct features under different stability conditions prompting interest in their connection. Here eddy covariance data collected at four different heights on a 62-m meteorological tower over a large open flat terrain are used to characterize different SBL states and associated turbulence structures. In a SBL characterized by strong near-surface winds, turbulent eddy sizes scale with their observational heights and the SBL experiences enhanced turbulent mixing of momentum and heat throughout (a state hereafter referred to as a “coupled” state). Conversely, in a decoupled SBL, weak winds occur near the surface and turbulent eddies are depressed and detached from the boundary leading to suppressed vertical mixing and layered SBL profiles. Because the transport of momentum and heat to the surface across SBL layers is determined by turbulent eddies, cross-layer correlation and the aforementioned SBL coupling states can be delineated by distinct turbulence structures.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2019

Defect Formation under Nitrogen-Ion Implantation and Subsequent Annealing in GaAs Structures with an Uncovered Surface and a Surface Covered with an AlN Film

The concentration profiles of defects produced in structures upon the implantation of nitrogen ions into GaAs epitaxial layers with an uncovered surface and that covered with an AlN film and subsequent annealing are studied. The ion energies and the implantation doses are chosen so that the nitrogen-atom concentration profiles coincided in structures of both types. Rutherford proton backscattering spectra are measured in the random and channeling modes, and the concentration profiles of point defects formed are calculated for the samples under study. It is found that the implantation of nitrogen ions introduces nearly the same number of point defects into structures of both types, and the formation of an AlN film by ion-plasma sputtering is accompanied by the formation of an additional number of defects. However, the annealing of structures of both types leads to nearly the same concentrations of residual defects.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Optimal packing size of non-ligated CdSe nanoclusters for microstructure synthesis

Structural and electrostatic properties of nanoclusters of CdSe of diameter 1–2 nm are studied with first principle calculations to determine the optimal size for synthesizing microstructures. Based on robustness of the core structure, i.e., the retention of tetrahedral geometry, hexagonal ring structure, and overall wu{sup ¨}rtzite structure to surface relaxations, we conclude that nanoclusters of ~2 nm diameter are the best candidates to form a dense microstructure with minimal interstitial space. Se-terminated surfaces retain a zigzag structure as Se atoms are pulled out and Cd atoms are pulled in due to relaxation, therefore, are best suited for inter-nanocluster formations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Long-term deformational simulation of PC bridges based on the thermo-hygro model of micro-pores in cementitious composites

Creep deflections that greatly exceed the predicted values by the linear creep law are being found in measurements on actual PC bridge viaducts. In this study, structural creep deformations were reproduced by using the multi-scale coupled thermo-hygro and mechanical modeling which enables to deal with an interaction of chemo-physical events of differing dimensions ranging from the kinematics of moisture in micro-pores to the macroscopic structural mechanics, and the effect of various factors was analytically investigated. The numerical analysis approximately reproduced the excessive deflection measured on an actual bridge viaduct. It was confirmed that the creep bending of the viaduct having the hollow cross-section varies significantly due to the ambient temperature, humidity and the structural specific surface area. The macroscopic structural responses in association with the thermodynamic state of moisture in the micro-pores are also discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Performance demonstration of vacuum microwave components critical for the operation of the ITER low-field side reflectometer

Final design studies in preparation for manufacturing have been performed for functional components of the vacuum portion of the ITER Low-Field Side Reflectometer (LFSR). These components consist of an antenna array, electron cyclotron heating (ECH) protection mirrors, phase calibration mirrors, and vacuum windows. Evaluation of these components was conducted at the LFSR test facility and DIII-D. The antenna array consists of six corrugated-waveguide antennas for simultaneous profile, fluctuation, and Doppler measurements. A diffraction grating, incorporated into the plasma-facing miter bend, provides protection of sensitive components from stray ECH at 170 GHz. For in situ phase calibration of the LFSR profile reflectometer, an embossed mirror is incorporated into the adjacent miter bend. Measurements of the radiated beam profile indicate that these components have a small, acceptable effect on mode conversion and beam quality. Baseline transmission characteristics of the dual-disk vacuum window are obtained and are used to guide ongoing developments. Preliminary simulations indicate that a surface-relief structure on the window surfaces can greatly improve transmission. The workability of real-time phase measurements was demonstrated on the DIII-D profile reflectometer. The new automated real-time analysis agrees well with the standard post-processing routine.

47 OTHER INSTRUMENTATION↗
DOE OSTI2018

Molecular structure and sour gas surface chemistry of supported K 2 O/WO 3 /Al 2 O 3 catalysts

Molecular structures of the unprompted and K 2 O-promoted supported WO 3 /Al 2 O 3 catalysts were studied with in situ Raman and UV-vis spectroscopy. In situ Raman spectra revealed that supported 20% WO 3 /Al 2 O 3 corresponds to near monolayer coverage of isolated and oligomeric surface WOx species on Al 2 O 3 . Above monolayer surface WO x coverage (21% WO 3 /Al 2 O 3 ), crystalline WO 3 nanoparticles are also present. The addition of K 2 O to the supported WO 3 /Al 2 O 3 catalyst increased the concentration of isolated surface WO x species and did not form K 2 WO 4 nanoparticles. The reducibility of the tungsten oxide structures depends on their structures (2D or 3D) and the K 2 O promoter. Their interaction with acidic CO 2 and SO 2 gases was also investigated. Adsorption of CO 2 creates several surface carbonate species of varying acidity that were detected using a combination of in situ IR and mass spectroscopy. Adsorbed bicarbonate form on weakly basic surface sites on tungsten oxide monolayer WO 3 /Al 2 O 3 catalyst as well as in the presence of low 2.5% K 2 O loading. At high 5% K 2 O loading, the presence of the strong surface basic sites yields adsorbed carbonates. In conclusion, after SO 2 pretreatment, however, new strongly adsorbed sulfate appears on the surface that inhibit CO 2 adsorption.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Atomistic modeling of energy band alignment in CdSeTe surfaces

Atomistic modeling based on Desnity Functional Theory (DFT)- 1/2 method coupled with surface Green’s function has been employed to investigate the energy band alignment results in cadmium selenium telluride (CdSeTe) surfaces. The structural and electronic properties of the bulk ternary alloy were established before cleaving the CdSeTe low index surface facets. The dependency of atomic-scale electronic features on different plane orientations was explored by studying the energy band alignment diagrams in the unreconstructed and reconstructed surfaces. While the low index CdSeTe surface geometry reconstructions replicate the CdTe surface geometries, the energy band alignment features of the unreconstructed and reconstructed CdSeTe surfaces differ from those observed in CdTe low index surfaces. The structurally relaxed reconstructed CdSeTe surface results in purely flat bands as opposed to an internal cusp feature observed in CdTe surfaces. The presence of an internal energy cusp in CdTe(111) surfaces is expected to play a key role in enhancing the hole charge transport towards the back of CdTe-only solar cell device and the absence of such feature in CdSeTe surface may explain one of the reason for the lower performance of CdSeTe-only solar cell seen experimentally.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Peptide-Directed PdAu Nanoscale Surface Segregation: Toward Controlled Bimetallic Architecture for Catalytic Materials

Bimetallic nanoparticles are of immense scientific and technological interest given the synergistic properties observed when mixing two different metallic species at the nanoscale. This is particularly prevalent in catalysis, where bimetallic nanoparticles often exhibit improved catalytic activity and durability over their monometallic counterparts. Yet despite intense research efforts, little is understood regarding how to optimize bimetallic surface composition and structure synthetically using rational design principles. Recently, it has been demonstrated that peptide-enabled routes for nanoparticle synthesis result in materials with sequence-dependent catalytic properties, providing an opportunity for rational design through sequence manipulation. In this study, bimetallic PdAu nanoparticles are synthesized with a small set of peptides containing known Pd and Au binding motifs. The resulting nanoparticles were extensively characterized using high-resolution scanning transmission electron microscopy, X-ray absorption spectroscopy and high-energy X-ray diffraction coupled to atomic pair distribution function analysis. Structural information obtained from synchrotron radiation methods were then used to generate model nanoparticle configurations using reverse Monte Carlo simulations, which illustrate sequence-dependence in both surface structure and surface composition. Replica exchange solute tempering molecular dynamic simulations were also used to predict the modes of peptide binding on monometallic surfaces, indicating that different sequences bind to the metal interfaces via different mechanisms. As a testbed reaction, electrocatalytic methanol oxidation experiments were performed, wherein differences in catalytic activity are clearly observed in materials with identical bimetallic composition. Finally, taken together, this study indicates that peptides could be used to arrive at bimetallic surfaces with enhanced catalytic properties, which could be leveraged for rational bimetallic nanoparticle design using peptide-enabled approaches.

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

Effects of chemical disorder and spin-orbit coupling on the electronic-structure and Fermi-surface topology of YbSb-based monopnictides

Here, in this work, we study the influence of disorder on the electronic structure of YbSb—a rare-earth monopnictide featuring a simple rocksalt (B1) crystal structure and a well-defined Fermi-surface topology—by employing first-principles density-functional theory. We focus on chemical disorder introduced through Te and Al doping, selected based on their thermodynamic stability in alloyed configurations, to understand how such perturbations modify the electronic states of YbSb. Our results indicate that Te doping predominantly introduces electronlike states at the 𝑋 and 𝐿 points, while Al doping leads to a suppression of holelike states at Γ, effectively driving the system from a semimetallic state to one characterized by very narrow-gap behavior at Γ. This modulation of the Fermi surface, particularly the reduction of central hole pockets at Γ, plays a central role in altering interpocket scattering—a mechanism critical for tuning quantum transport properties, including superconductivity. This disorder-driven modulation of the Fermi surface, particularly the suppression of central hole pockets at Γ, controls interpocket scattering, which is essential for optimizing quantum transport properties, including superconductivity. Our results show that disorder can be effectively used for engineering band topology, thereby tuning quantum related response through a tailored electronic structure.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Insights into the electronic origin of enhancing the catalytic activity of Co 3 O 4 for oxygen evolution by single atom ruthenium

The surface electronic structure of transition-metal oxide catalysts plays a decisive role in binding the intermediates of the oxygen evolution reaction (OER) to the oxide surface, in turn influencing the catalytic activity of these materials. However, the approaches to modulating the electronic structure of surface metal ions are rare and far behind the demands. Here, we report a surface single atom decoration for adjusting the surface electronic structure of Co 3 O 4 , leading to enhanced electrocatalytic activity for OER, in which the isolated Ru single atoms were uniformly deposited on the surface of Co 3 O 4 by an atomic layer deposition technology. As the OER catalyst, the as-made catalysts have exhibited a significantly enhanced catalytic activity (with increasing to 95.5 times) and a dramatically decreased overpotential. The density functional theory calculations reveal that the single-atom Ru acts as a promotor to adjust the 3d electronic structure of adjacent Co atoms and to tune the binding energy between intermediates and activity sites, finally leading to enhanced catalytic activity.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Water dissociative adsorption on NiO(111): Energetics and structure of the hydroxylated surface

The energetics of the reactions of water with metal oxide surfaces are of tremendous interest for catalysis, electrocatalysis, and geochemistry, yet the energy for the dissociative adsorption of water was only previously measured on one well-defined oxide surface, iron oxide. In the present paper, the enthalpy of the dissociative adsorption of water is measured on NiO(111)-2 × 2 at 300 K using single-crystal adsorption calorimetry. The differential heat of dissociative adsorption decreases with coverage from 170 to 117 kJ/mol in the first 0.25 ML of coverage. Water adsorbs molecularly on top of that, with a heat of ~92 kJ/mol. Density functional theory (DFT) calculations reproduce the measured energies well (all within 17 kJ/mol) and provide insight into the atomic-level structure of the surfaces studied experimentally. They show that the oxygen-terminated O-octo(2 × 2) structure is the most stable NiO(111)-2 × 2 termination and gives reaction energies with water that are more consistent with the calorimetry results than the metal-terminated surface. They show that water adsorbs dissociatively on this (2 × 2)-O-octo surface to produce a hydroxyl-covered surface with a heat of adsorption of 171 ± 5 kJ/mol in the low-coverage limit (very close to 170 kJ/mol experimentally) and an integral heat that decreases by 14 kJ/mol up to saturation (compared to ~30 kJ/mol experimentally). As a result, sensitivity of this reaction’s energy to choice of DFT method is tested using a variety of different exchange correlation functionals, including HSE06, and found to be quite weak.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Determination of crystallographic orientation of lead-free piezoelectric (K,Na)NbO{sub 3} epitaxial thin films grown on SrTiO{sub 3} (100) surfaces

Crystallographic structure of sol-gel-processed lead-free (K,Na)NbO{sub 3} (KNN) epitaxial films on [100]-cut SrTiO{sub 3} single-crystalline substrates was investigated for a deeper understanding of its piezoelectric response. Lattice parameter measurement by high-resolution X-ray diffraction and transmission electron microscopy revealed that the orthorhombic KNN films on SrTiO{sub 3} (100) surfaces are [010] oriented (b-axis-oriented) rather than commonly identified c-axis orientation. Based on the crystallographic orientation and corresponding ferroelectric domain structure investigated by piezoresponse force microscopy, the superior piezoelectric property along b-axis of epitaxial KNN films than other orientations can be explained.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Effect of hole doping on the electronic structure and the Fermi surface in the Hubbard model within norm-conserving cluster pertubation theory

The concentration dependences of the band structure, spectral weight, density of states, and Fermi surface in the paramagnetic state are studied in the Hubbard model within cluster pertubation theory with 2 Multiplication-Sign 2 clusters. Representation of the Hubbard X operators makes it possible to control conservation of the spectral weight in constructing cluster perturbation theory. The calculated value of the ground-state energy is in good agreement with the results obtained using nonperturbative methods such as the quantum Monte Carlo method, exact diagonalization of a 4 Multiplication-Sign 4 cluster, and the variational Monte Carlo method. It is shown that in the case of hole doping, the states in the band gap (in-gap states) lie near the top of the lower Hubbard band for large values of U and near the bottom of the upper band for small U. The concentration dependence of the Fermi surface strongly depends on hopping to second (t Prime ) and third (t Double-Prime ) neighbors. For parameter values typical of HTSC cuprates, the existence of three concentration regions with different Fermi surfaces is demonstrated. It is shown that broadening of the spectral electron density with an energy resolution typical of contemporary ARPES leads to a pattern of arcs with a length depending on the concentration. Only an order-of-magnitude decrease in the linewidth makes it possible to obtain the true Fermi surface from the spectral density. The kinks associated with strong electron correlations are detected in the dispersion relation below the Fermi level.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Thermodynamic stability and structures of iron chloride surfaces: A first-principles investigation

In this study, we report a comprehensive density functional theory investigation of the structure and thermodynamic stability of FeCl{sub 2} and FeCl{sub 3} surfaces. Calculated lattice constants and heats of formation for bulk FeCl{sub 2} and FeCl{sub 3} were found to be in relatively good agreement with experimental measurements. We provide structural parameters for 15 distinct FeCl{sub 2} and FeCl{sub 3} surfaces along the three low-index orientations. The optimized geometries for all surfaces are compared with analogous bulk values. Ab initio atomistic thermodynamic calculations have been carried out to assess the relative thermodynamic stability of FeCl{sub 2} and FeCl{sub 3} surfaces under practical operating conditions of temperatures and pressures. The FeCl{sub 2} (100-Cl) surface is found to afford the most stable configuration at all experimentally accessible gas phase conditions.

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

In Situ/Operando Probing of Dynamic Phase Structures of Alumina‐Supported Ultrasmall Copper‐Gold Alloy Nanoparticles Under Reaction Conditions

Abstract The ability to control phase structures and surface sites of ultrasmall alloy nanoparticles under reaction conditions is essential for preparing catalysts by design. This is, however, challenging due to limited understanding of the atomic‐scale phases and their correlation with the ensemble‐averaged structures and activities of catalysts during catalytic reactions. We reveal here a dynamic structural stability of alumina‐supported ultrasmall and equiatomic copper‐gold alloy nanoparticles under reaction conditions as a model system in the in situ/operando study. In situ atomic‐scale morphological tracking under oxygen reveals temperature‐dependent dynamic crystalline‐amorphous dual‐phase structures, showing dynamic stability over an elevated temperature range. This atomic‐scale dynamic phase stability coincides with a “conversion plateau” observed for carbon monoxide oxidation on the catalyst. It is substantiated by the stable lattice ordering/disordering structures and surface sites with oscillatory characteristics shown by operando ensemble‐average structural tracking of the catalyst during the oxidation reaction. The understanding of the atomic‐scale dynamic phase structures in correlation with the ensemble‐average dynamic ordering/disordering phase structures and surface sites provides fresh insights into the unique synergy of the supported alloy nanoparticles. This understanding has implications for the design and structural tuning of active and stable ultrasmall alloy catalysts under elevated temperatures.

Chemistry↗
DOE OSTI2016

Vectorial mapping of noncollinear antiferromagnetic structure of semiconducting FeSe surface with spin-polarized scanning tunneling microscopy

Antiferromagnetic semiconductors gain increasing interest due to their possible application in spintronics. Using spin polarized scanning tunneling microscopy operating in a vector field, we mapped the noncollinear antiferromagnetic spin structure of a semiconducting hexagonal FeSe surface on the atomic scale. The surface possesses an in-plane compensated Néel structure which is further confirmed by first-principles calculations.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗