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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 1,585 records · Page 88
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Kinetics of short-range order formation in GeSn alloy: MBE vs CVD

Recently, short-range order (SRO) has attracted significant attention, challenging the conventional view of the atomic positions in alloys being random. Furthermore, the presence of SRO has been predicted to have profound effects on the electronic and topological properties of group-IV alloys, offering a different direction in designing group-IV materials for photoelectronic and quantum devices. However, due to the limited understanding of the formation mechanisms, developing effective methods to manipulate SRO in epitaxy is still challenging. To address this, we propose a mechanism for the GeSn alloy, revealing that surface diffusion plays a key role in SRO formation. Building on this mechanism, we show that the distinct surface conditions in MBE and CVD lead to the formation of SRO with enhanced Sn–Sn pairing in MBE-grown samples, while CVD-grown samples remain random alloys. Furthermore, our findings provide an initial understanding of the kinetic process of SRO formation, providing guidance for the design of experiments to manipulate SRO.

Alloys↗
DOE OSTI2022

Atomic-Site-Specific Surface Valence-Band Structure from X-Ray Standing-Wave Excited Photoemission

X-ray standing-wave (XSW) excited photoelectron emission was used to measure the site-specific valence band (VB) for ½ monolayer (ML) Pt grown on a SrTiO 3 (001) surface. The XSW induced modulations in the core level (CL), and VB photoemission from the surface and substrate atoms were monitored for three ℎ⁡kl substrate Bragg reflections. The XSW CL analysis shows the Pt to have a face-centered-cubic-like cube-on-cube epitaxy with the substrate. The XSW VB information compares well to a density functional theory calculated projected density of states from the surface and substrate atoms. Altogether, this Letter represents a novel method for determining the contribution to the density of states by valence electrons from specific atomic surface sites.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2015

The U{sup 5+} compound Ba{sub 9}Ag{sub 10}U{sub 4}S{sub 24}: Synthesis, structure, and electronic properties

Black crystals of Ba{sub 9}Ag{sub 10}U{sub 4}S{sub 24} have been made by direct combination of BaS, Ag, U, and S at 1273 K. This compound crystallizes in a new structure type in the space group C{sub 4v}{sup 10}−I4cm of the tetragonal system with four formula units in a cell with lattice constants a=13.9189(6) Å and c=23.7641(11) Å (V=4604(5) Å{sup 3}). Multiphoton Luminescence Spectroscopy measurements are consistent with the noncentrosymmetric nature of the structure. In the structure each U atom is octahedrally coordinated by six S atoms, whereas three of the five crystallographically independent Ag atoms are tetrahedrally coordinated to four S atoms, another has a seesaw coordination to four S atoms, and the last has a triangular coordination to three S atoms. The overall structure consists of the three-dimensional stacking of the US{sub 6}, AgS{sub 4}, and AgS{sub 3} polyhedra to leave channels in which Ba atoms reside. Based on the values of the U–S interatomic distances, the compound Ba{sub 9}Ag{sub 10}U{sub 4}S{sub 24} contains U{sup 5+} and charge balance is achieved with the formal oxidation states of 9 Ba{sup 2+}, 10 Ag{sup 1+}, 4 U{sup 5+} and 24 S{sup 2−}. DFT calculations predict an antiferromagnetic ground state and a band gap of 2.1 eV. Resistivity measurements indicate that the compound is a semiconductor with a complex activation mechanism and activation energies ranging from 0.03(1) eV to 0.08(1) eV. - Graphical abstract: General view of the Ba{sub 9}Ag{sub 10}U{sub 4}S{sub 24} structure. - Highlights: • Black crystals of Ba{sub 9}Ag{sub 10}U{sub 4}S{sub 24} have been made by direct combination of BaS, Ag, U, and S at 1273 K. • Ba{sub 9}Ag{sub 10}U{sub 4}S{sub 24} contains U{sup 5+} and charge balance is achieved with 9 Ba{sup 2+}, 10 Ag{sup 1+}, 4 U{sup 5+}, and 24 S{sup 2−}. • DFT calculations predict an antiferromagnetic ground state and a band gap of 2.1 eV. • The compound is a semiconductor with activation energies ranging from 0.03(1) eV to 0.08(1) eV.

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

Picoscale structural insight into superconductivity of monolayer FeSe/SrTiO 3

Remarkable enhancement of the superconducting transition temperature ( c ) has been observed for monolayer (ML) FeSe films grown on SrTiO 3 substrates. The atomic-scale structure of the FeSe/SrTiO 3 interface is an important determinant of both the magnetic and interfacial electron-phonon interactions and is a key ingredient to understanding its high- T c superconductivity. We resolve the atomic-scale structure of the FeSe/SrTiO 3 interface through a complementary analysis of scanning transmission electron microscopy and in situ surface x-ray diffraction. We find that the interface is more strongly bonded for a particular registration, which leads to a coherently strained ML. We also determine structural parameters, such as the distance between ML FeSe and the oxide, Se-Fe-Se bond angles, layer-resolved distances between Fe-Se, and registry of the FeSe lattice relative to the oxide. This picoscale structure determination provides an explicit structural framework and constraint for theoretical approaches addressing the high- T c mechanism in FeSe/SrTiO 3 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Photoluminescence polarization anisotropy for studying long-range structural ordering within semiconductor multi-atomic alloys and organic crystals

Long-range structural ordering within multi-component semiconductor alloys and organic crystals leads to significant optical anisotropy and, in particular, to anisotropy of the photoluminescence (PL) emission. The PL emission of ternary and quaternary semiconductor alloys is polarized if there is some amount of the atomic ordering within the crystal structure. We analyze the polarization of the PL emission from the quaternary GaInAsP semiconductor alloy grown by Liquid Phase Epitaxy (LPE) and conclude that it could be caused by low degree atomic ordering within the crystal structure together with the thermal biaxial strain due to difference between the thermal expansion coefficients of the layer and the substrate. We also study the state of polarization of the PL from organic crystals in order to identify different features of the crystal PL spectrum.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Atomic study on the ordered structure in Al melts induced by liquid/substrate interface with Ti solute

Atomic ordering in Al melts induced by liquid/substrate interface with Ti solute was investigated by ab initio molecular dynamics simulations and in-situ synchrotron X-ray diffraction. It is predicted that deformed nanoscale ordering Al layers with a rhombohedral-centered hexagonal structure (R3{sup ¯}m space group) instead of the intrinsic fcc structure (Fm3{sup ¯}m space group) form on substrate at temperature above Al liquids. With Al atoms stacking away from the interface, the ordering structure reaches a critical thickness, which inhibits the consecutive stacking of Al atoms on substrates. The locally stacking reconstruction induced by Ti atom relieves the accumulated elastic strain energy in ordered Al layers, facilitating fully heterogeneous nucleation on substrate beyond the deformed ordering Al layer around the melting point. The roles of liquid/substrate interface with Ti solute in the physical behavior of heterogeneous nucleation on substrate were discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Edge-Confined Excitons in Monolayer Black Phosphorus

Quantum confinement of two-dimensional excitons in van der Waals materials via electrostatic trapping, lithographic patterning, Moiré potentials, and chemical implantation has enabled significant advances in tailoring light emission from nanostructures. While such approaches rely on complex preparation of materials, natural edges are a ubiquitous feature in layered materials and provide a different approach for investigating quantum-confined excitons. Here, we observe that certain edge sites of monolayer black phosphorus (BP) strongly localize the intrinsic quasi-one-dimensional excitons, yielding sharp spectral lines in photoluminescence, with nearly an order of magnitude line width reduction. Through structural characterization of BP edges using transmission electron microscopy and first-principles GW plus Bethe–Salpeter equation (GW-BSE) calculations of exemplary BP nanoribbons, we find that certain atomic reconstructions can strongly quantum-confine excitons resulting in distinct emission features, mediated by local strain and screening. Here we observe linearly polarized luminescence emission from edge reconstructions that preserve the mirror symmetry of the parent BP lattice, in agreement with calculations. Furthermore, we demonstrate efficient electrical switching of localized edge excitonic luminescence, whose sites act as excitonic transistors for emission. Localized emission from BP edges motivates exploration of nanoribbons and quantum dots as hosts for tunable narrowband light generation, with future potential to create atomic-like structures for quantum information processing applications as well as exploration of exotic phases that may reside in atomic edge structures.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1981

Quantum chemical calculation of the equilibrium structures of small metal atom clusters

A decomposition of the molecular energy is presented that is motivated by the atom superposition and electron delocalization physical model of chemical binding. The energy appears in physically transparent form consisting of a classical electrostatic interaction, a zero order two electron exchange interaction, a relaxation energy, and the atomic energies. Detailed formulae are derived in zero and first order of approximation. The formulation extends beyond first order to any chosen level of approximation leading, in principle, to the exact energy. The structure of this energy decomposition lends itself to the fullest utilization of the solutions to the atomic sub problems to simplify the calculation of the molecular energy. If nonlinear relaxation effects remain minor, the molecular energy calculation requires at most the calculation of two center, two electron integrals. This scheme thus affords the prospects of substantially reducing the computational effort required for the calculation of molecular energies.

Kahn, L. R.↗
DOE OSTI2017

Extended Gamma-Ray Emission from the G25.0+0.0 Region: A Star-forming Region Powered by the Newly Found OB Association?

We report a study of extended γ -ray emission with the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope , which is likely to be the second case of a γ -ray detection from a star-forming region (SFR) in our Galaxy. The LAT source is located in the G25 region, 1.°7 × 2.°1 around ( l , b ) = (25.°0, 0.°0). The γ -ray emission is found to be composed of two extended sources and one pointlike source. The extended sources have similar sizes of about 1.°4 × 0.°6. An ∼0.°4 diameter subregion of one has a photon index of Γ = 1.53 ± 0.15, and is spatially coincident with HESS J1837−069, likely a pulsar wind nebula. The other parts of the extended sources have a photon index of Γ = 2.1 ± 0.2 without significant spectral curvature. Given their spatial and spectral properties, they have no clear associations with sources at other wavelengths. Their γ -ray properties are similar to those of the Cygnus cocoon SFR, the only firmly established γ -ray detection of an SFR in the Galaxy. Indeed, we find bubble-like structures of atomic and molecular gas in G25, which may be created by a putative OB association/cluster. The γ -ray emitting regions appear confined in the bubble-like structure; similar properties are also found in the Cygnus cocoon. In addition, using observations with the XMM-Newton , we find a candidate young massive OB association/cluster G25.18+0.26 in the G25 region. We propose that the extended γ -ray emission in G25 is associated with an SFR driven by G25.18+0.26. Based on this scenario, we discuss possible acceleration processes in the SFR and compare them with the Cygnus cocoon.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

Structural transition and chemical reactivity of atomic carbon chains

The sp-hybridized carbon chain (carbyne) is a representative 1D atomic material, whose bonding structure and chemical reactivity have remained a mystery for a century. Here, in this work, we report the unexpected alternating bond orders of 1.4 and 2.6 for the most stable carbon chain and the in situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) detection of the temperature-dependent reversible change of the bond order alternation. Moreover, we revealed its reactivities with O 2 , H 2 , and CO 2 at temperatures up to 600 °C and created an end-group-protection strategy to stabilize it. These observations open a new door to the chemistry of atomic materials.

25 ENERGY STORAGE↗
DOE OSTI2023

The Synthescope: A Vision for Combining Synthesis with Atomic Fabrication

Here, the scanning transmission electron microscope, a workhorse instrument in materials characterization, is being transformed into an atomic-scale material-manipulation platform. With an eye on the trajectory of recent developments and the obstacles toward progress in this field, a vision for a path toward an expanded set of capabilities and applications is provided. The microscope is reconceptualized as an instrument for fabrication and synthesis with the capability to image and characterize atomic-scale structural formation as it occurs. Further development and refinement of this approach may have substantial impact on research in microelectronics, quantum information science, and catalysis, where precise control over atomic-scale structure and chemistry of a few “active sites” can have a dramatic impact on larger-scale functionality and where developing a better understanding of atomic-scale processes can help point the way to larger-scale synthesis approaches.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Selective laser pumping of magnetic sublevels in the hyperfine structure of the cesium atom

The evolution of the populations of the magnetic sublevels of the cesium atom ({sup 133}Cs isotope) in resonant laser fields with linear polarization is analyzed using the equations for the density matrix. Analytic expressions are derived for stationary populations resulting from laser-induced optical transitions on the hyperfine structure components F{sub g} = 3 ↔ F{sub e} = 2, 3 and F{sub f} = 4 ↔ F{sub e} = 3, 4 of lines D{sub 1} (6{sup 2}S{sub 1/2} → 6{sup 2}P{sub 1/2}) and D{sub 2} (6{sup 2}S{sub 1/2} → 6{sup 2}P{sub 3/2}) depending on the initial values of the populations. The numerical solution of the evolution equations gives the characteristic times of stabilization of the steady regime as functions of laser field intensities and detuning from optical resonance. We determine the sequences of optical transitions increasing (by more than an order of magnitude) the population of the lower sublevel 6{sup 2}S{sub 1/2}F{sub g} = 3 M = 0 of the “clock” microwave transition F{sub g} = 3 M = 0 ↔ F{sub f} = 4 M = 0 in the cesium frequency standard, which increases the signal intensity in the recording system by the same proportion.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Atomic-resolution visualization and doping effects of complex structures in intercalated bilayer graphene

Molecules intercalating two-dimensional materials form complex structures that have been characterized primarily by spatially averaged techniques. In this work, we use aberration-corrected scanning transmission electron microscopy and density-functional-theory (DFT) calculations to study the atomic structure of bilayer graphene (BLG) and few-layer graphene (FLG) intercalated with FeC l 3 . In BLG, we discover two distinct intercalated structures that we identify as monolayer FeC l 3 and monolayer FeC l 2 . The two structures are separated by atomically sharp boundaries and induce large free-carrier densities on the order of 10 13 c m - 2 in the graphene layers. In FLG, we observe multiple FeC l 3 layers stacked in a variety of possible configurations with respect to one another. Lastly, we find that the microscope's electron beam can convert the FeC l 3 monolayer into FeOCl monolayers in a rectangular lattice. These results reveal the need for a combination of atomically resolved microscopy, spectroscopy, and DFT calculations to identify intercalated structures and study their properties.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Structure of the toxic core of α-synuclein from invisible crystals

We report that the protein α-synuclein is the main component of Lewy bodies, the neuron-associated aggregates seen in Parkinson disease and other neurodegenerative pathologies. An 11-residue segment, which we term NACore, appears to be responsible for amyloid formation and cytotoxicity of human α-synuclein. Here we describe crystals of NACore that have dimensions smaller than the wavelength of visible light and thus are invisible by optical microscopy. As the crystals are thousands of times too small for structure determination by synchrotron X-ray diffraction, we use micro-electron diffraction to determine the structure at atomic resolution. The 1.4 Å resolution structure demonstrates that this method can determine previously unknown protein structures and here yields, to our knowledge, the highest resolution achieved by any cryo-electron microscopy method to date. The structure exhibits protofibrils built of pairs of face-to-face β-sheets. X-ray fibre diffraction patterns show the similarity of NACore to toxic fibrils of full-length α-synuclein. Finally, the NACore structure, together with that of a second segment, inspires a model for most of the ordered portion of the toxic, full-length α-synuclein fibril, presenting opportunities for the design of inhibitors of α-synuclein fibrils.

Lewy bodies↗
DOE OSTI2019

Engineering medium-range order and polyamorphism in a nanostructured amorphous alloy

Like crystalline materials, the properties of amorphous materials can be tailored by tuning the local atomic-to-nanoscale structural configurations. Polyamorphism is evident by the coexistence of kinetically stabilized amorphous structures with tailorable short-to-medium-range orders, providing a viable means to engineer the degree of local order and heterogeneity. Here, we report experimental evidence of the coexistence of liquid-like and solid-like amorphous phases in a Ni82P18 amorphous alloy with enhanced thermal stability and plasticity prepared by pulsed electrodeposition. The two amorphous phases, of comparable volume fraction of ~50% each, have similar short-range order but are distinguished by packing at the medium-range length scale (>6 Å). Upon heating, a structure crossover at ~450 K was observed, where the liquid-like structure transforms to the solid-like structure, as evidenced by the enthalpy release and an anomalous contraction of atomic structure over the medium-range length scale, due to the metastable nature of the liquid-like structure.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Estimations of Mo X-pinch plasma parameters on QiangGuang-1 facility by L-shell spectral analyses

Plasma parameters of molybdenum (Mo) X-pinches on the 1-MA QiangGuang-1 facility were estimated by L-shell spectral analysis. X-ray radiation from X-pinches had a pulsed width of 1 ns, and its spectra in 2–3 keV were measured with a time-integrated X-ray spectrometer. Relative intensities of spectral features were derived by correcting for the spectral sensitivity of the spectrometer. With an open source, atomic code FAC (flexible atomic code), ion structures, and various atomic radiative-collisional rates for O-, F-, Ne-, Na-, Mg-, and Al-like ionization stages were calculated, and synthetic spectra were constructed at given plasma parameters. By fitting the measured spectra with the modeled, Mo X-pinch plasmas on the QiangGuang-1 facility had an electron density of about 10{sup 21} cm{sup −3} and the electron temperature of about 1.2 keV.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗