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At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Prospects for atomic resolution in-line holography for a 3D determination of atomic structures from single projections

Abstract It is now established that the 3D structure of homogeneous nanocrystals can be recovered from in-line hologram of single projections. The method builds on a quantitative contrast interpretation of electron exit wave functions. Since simulated exit wave functions of single and bilayers of graphene reveal the atomic structure of carbon-based materials with sufficient resolution, we explore theoretically how the approach can be expanded beyond periodic carbon-based materials to include non-periodic molecular structures. We show here theoretically that the 3D atomic structure of randomly oriented oleic acid molecules can be recovered from a single projection.

3D molecular imaging↗
DOE OSTI2017

Atomic Structure and Dynamics of Single Platinum Atom Interactions with Monolayer MoS 2

In this work, we have studied atomic level interactions between single Pt atoms and the surface of monolayer MoS 2 using aberration-corrected annular dark field scanning transmission electron microscopy at an accelerating voltage of 60 kV. Strong contrast from single Pt atoms on the atomically resolved monolayer MoS 2 lattice enables their exact position to be determined with respect to the MoS2 lattice, revealing stable binding sites. In regions of MoS 2 free from surface contamination, the Pt atoms are localized in S vacancy sites and exhibit dynamic hopping to nearby vacancy sites driven by the energy supplied by the electron beam. However, in areas of MoS 2 contaminated with carbon surface layers, the Pt atoms appear at various positions with respect to the underlying MoS 2 lattice, including on top of Mo and in off-axis positions. These variations are due to the Pt bonding with the surrounding amorphous carbon layer, which disrupts the intrinsic Pt–MoS 2 interactions, leading to more varied positions. Density functional theory (DFT) calculations reveal that Pt atoms on the surface of MoS 2 have a small barrier for migration and are stabilized when bound to either a single or double sulfur vacancies. DFT calculations have been used to understand how the catalytic activity of the MoS 2 basal plane for hydrogen evolution reaction is influenced by Pt dopants by variation of the hydrogen adsorption free energy. Finally, this strong dependence of catalytic effect on interfacial configurations is shown to be common for a series of dopants, which may provide a means to create and optimize reaction centers.

2D materials↗
DOE OSTI2023

Interface atomic structures in a cadmium arsenide/III–V semiconductor heterostructure

The interface atomic structure between an epitaxial thin film of the prototype topological semimetal cadmium arsenide (Cd 3 As 2 ) and a III–V semiconductor layer is investigated using high-angle annular dark-field imaging in an aberration-corrected scanning transmission electron microscope. We find that the interface unit cell adopts a defined stoichiometry that is CdSb-like, which is achieved through the insertion of periodically arranged Cd vacancies in the terminating Cd-plane of Cd 3 As 2 . This interface stoichiometry is consistent with the Sb-termination of the III–V layer and the fact that CdSb is the thermodynamically stable phase in the Cd–Sb binary system. We find at least two distinct alignments of the film with respect to the buffer layer, which are characterized by a $\frac{1}{4}$ <100> Cd 3 As 2 shift parallel to the interface. We show that steps of half unit cell height in the III–V layer can produce these distinct interface structures.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2024

Spectral scheme for atomic structure calculations in density functional theory

In this study, we present a spectral scheme for atomic structure calculations in pseudopotential Kohn-Sham density functional theory. In particular, after applying an exponential transformation of the radial coordinates, we employ global polynomial interpolation on a Chebyshev grid, with derivative operators approximated using the Chebyshev differentiation matrix, and integrations using Clenshaw-Curtis quadrature. We demonstrate the accuracy and efficiency of the scheme through spin-polarized and unpolarized calculations for representative atoms, while considering local, semilocal, and hybrid exchange-correlation functionals. In particular, we find that $\mathcal{O}$(200) grid points are sufficient to achieve an accuracy of 1 microhartree in the eigenvalues for optimized norm conserving Vanderbilt pseudopotentials spanning the periodic table from atomic number Ζ = 1 to 83.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2020

Local atomic structure and Ni nanophase segregation in Zn 1-x Ni x S thin films

Here, the local atomic structure of Zn 1-x Ni x S thin films was investigated using X-ray absorption spectroscopy. The films were grown using RF-sputtering at atomic concentrations x = 0.00, x = 0.04, x = 0.08 and x = 0.14. X-ray diffraction shows that the lattice parameter contracts with increasing Ni concentration, consistent with the smaller atomic radii of Ni compared to that one of Zn. Optical absorption reveals a reduction of the bandgap as Ni concentration increases, except for sample x = 0.14 where a complex behavior is observed. Results from X-ray absorption near edge structure (XANES) spectroscopy indicate a shift in the valence state of Ni for the x = 0.14 sample. The analysis of the extended X-ray absorption fine structure (EXAFS) spectra indicate a contraction of ~0.7 Å in the Ni–S bond distance when compared to the Zn–S bond length, which generates local lattice distortions and an increment of the static disorder as the Ni concentration increases. The EXAFS results for sample x = 0.14 show the presence of a Zn 1-x Ni x S phase and a nanoscopic metallic Ni phase with domain sizes below the diffraction limit. These results reveal that the local atomic structure differs in a significant manner from the average crystalline structure, implying its importance for the determination of the electronic properties of this material.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Effects of Nonequilibrium Atomic Structure on Ionic Diffusivity in LLZO: A Classical and Machine Learning Molecular Dynamics Study

To improve the performance of electrochemical devices, it is essential to understand the effects of nonequilibrium motifs in solids, such as grain boundaries, amorphous phases, and highly strained regions, on atomic-scale transport and stability. Molecular dynamics simulations are used to explore the combined effect of far-from-equilibrium atomic structures and the choice of interatomic potential on ionic diffusivity predictions for Li 7 La 3 Zr 2 O 12 (LLZO), a promising solid electrolyte for all-solid-state batteries. Amorphization and high strain are considered using both classical Buckingham interatomic potentials and machine learning force fields. Here we find that both crystalline expansion and amorphization tend to slow diffusion, although the different physical encodings in the two potentials impact the properties in different ways. We trace these variations to a combination of structural and transport factors, the contributions of which are deconvoluted computationally. Graph-based analysis reveals that the variations for amorphous LLZO arise from the connectivity of diffusion pathways within the predicted structures, which generally correlates with diffusivity and is notably higher for structures generated by the machine learning force fields. Our study provides additional insight into the relationship between atomic structure and diffusivity in LLZO, while also highlighting the need for care in choosing and validating potentials to simulate far from equilibrium structures.

25 ENERGY STORAGE↗
DOE OSTI2015

The effect of atomic structure on interface spin-polarization of half-metallic spin valves: Co{sub 2}MnSi/Ag epitaxial interfaces

Using density functional theory calculations motivated by aberration-corrected electron microscopy, we show how the atomic structure of a fully epitaxial Co{sub 2}MnSi/Ag interfaces controls the local spin-polarization. The calculations show clear difference in spin-polarization at Fermi level between the two main types: bulk-like terminated Co/Ag and Mn-Si/Ag interfaces. Co/Ag interface spin-polarization switches sign from positive to negative, while in the case of Mn-Si/Ag, it is still positive but reduced. Cross-sectional atomic structure analysis of Co{sub 2}MnSi/Ag interface, part of a spin-valve device, shows that the interface is determined by an additional layer of either Co or Mn. The presence of an additional Mn layer induces weak inverse spin-polarisation (−7%), while additional Co layer makes the interface region strongly inversely spin-polarized (−73%). In addition, we show that Ag diffusion from the spacer into the Co{sub 2}MnSi electrode does not have a significant effect on the overall Co{sub 2}MnSi /Ag performance.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2021

Anomalous Behavior in the Atomic Structure of Nb$^{3}$Sn under High Pressure

In the present study, the local atomic structure of a Nb3Sn superconductor sample has been probed by X-ray absorption fine structure (XAFS) as a function of hydrostatic pressure (from ambient up to 26 GPa) using a diamond anvil cell set-up. The analysis of the Nb-K edge extended X-ray absorption fine structure (EXAFS) data was carried out combining standard multi shell structural refinement and reverse Monte Carlo method to provide detailed in situ characterization of the pressure-induced evolution of the Nb local structure in Nb3Sn. The results highlight a complex evolution of Nb chains at the local atomic scale, with a peculiar correlated displacement of Nb–Nb and Nb–Nb–Nb configurations. Such a local effect appears related to anomalies evidenced by X-ray diffraction in other superconductors belonging to the same A15 crystallographic structure.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Size and shape of grain boundary network components and their atomic structures in polycrystalline nanoscale materials

Microstructure in polycrystalline materials is composed of grain boundary plane, triple junction line, and vertex point. They are the integral parts of the grain boundary network structure and the foundation for the structure-property relations. In polycrystalline, especially nanocrystalline, materials, it becomes increasingly difficult to probe the atomistic structure of the microstructure components directly in experiment due to the size limitation. Here, we present a numerical approach using pair correlation function from atomistic simulation to obtain the detailed information for atomic order and disorder in the grain boundary network in nanocrystalline materials. We show that the atomic structures in the different microstructural components are related closely to their geometric size and shape, leading to unique signatures for atomic structure in microstructural characterization at nanoscales. The dependence varies systematically with the characteristic dimension of the microstructural component: liquid-like disorder is found in vertex points, but a certain order persists in triple junctions and grain boundaries along the extended dimensions of these microstructure components.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Atomic structure considerations for the low-temperature opacity of Xenon

Here, we have begun a preliminary investigation into the opacity of Xe at low temperatures (<50 eV). The emissivity and opacity of Xe is a crucial factor in determining the utility of Xe in EUV lithography, with numerous industrial applications. To this end, we have been exploring the accuracy of some approximations used in opacity models for the relevant ion stages of Xe (6 times ionized through 20 times ionized). Due to the complexity of Xe atomic structure, one needs to use full configuration-interaction to properly describe the strong mixing in the various n=4 sub-shells that give rise to the Δn=0 and Δn=1 transitions that dominate the opacity spectrum at low temperatures. Since calculations that include full configuration-interaction for large numbers of configurations quickly become computationally prohibitive, we have explored hybrid calculations, in which full configuration-interaction is retained for the most important transitions, while intermediate coupling is employed for all other transitions. After verifying atomic structure properties, local-thermodynamic-equilibrium (LTE) opacities are generated using the ATOMIC code at selected temperatures and densities and compared to various experimental results.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2020

Characterizing electronic and atomic structures for amorphous and molecular metal oxide catalysts at functional interfaces by combining soft X-ray spectroscopy and high-energy X-ray scattering

Amorphous thin film materials and heterogenized molecular catalysts supported on electrode and other functional interfaces are widely investigated as promising catalyst formats for applications in solar and electrochemical fuels catalysis. However the amorphous character of these catalysts and the complexity of the interfacial architectures that merge charge transport properties of electrode and semiconductor supports with discrete sites for multi-step catalysis poses challenges for probing mechanisms that activate and tune sites for catalysis. This minireview discusses advances in soft X-ray spectroscopy and high-energy X-ray scattering that provide opportunities to resolve interfacial electronic and atomic structures, respectively, that are linked to catalysis. This review discusses how these techniques can be partnered with advances in nanostructured interface synthesis for combined soft X-ray spectroscopy and high-energy X-ray scattering analyses of thin film and heterogenized molecular catalysts. Further, these combined approaches enable opportunities for the characterization of both electronic and atomic structures underlying fundamental catalytic function, and that can be applied under conditions relevant to device applications.

14 SOLAR ENERGY↗
DOE OSTI2020

Improving the picture of atomic structure in nonoriented polymer domains using the pair distribution function: A study of polyamide 6

Abstract Investigations of the atomic structures within polyamides started over 80 years ago and continue today. These weakly ordered materials diffract X‐rays poorly and typically require postprocessing to obtain idealized samples for structural studies. An important goal remains to develop techniques to study the local structure in its natural state, with atomic resolution, and with sensitivity to subtle changes due to synthesis conditions or other technologically relevant processing procedures. Here, we compare the structures of as‐produced, nonoriented polyamide 6 ([C 6 H 11 NO] n ) from both hydrolytic and anionic processes. A total scattering pair distribution function approach is used to elucidate information about the atomic bonding, molecular conformation, chain packing, crystallite size, and ratio of ordered to disordered domain content. The results are compared with those from standard analytical methods.

Terban, Maxwell W.↗
DOE OSTI2015

Voronoi analysis of the short–range atomic structure in iron and iron–carbon melts

In this work, we simulated the atomic structure of liquid iron and iron–carbon alloys by means of ab initio molecular dynamics. Voronoi analysis was used to highlight changes in the close environments of Fe atoms as carbon concentration in the melt increases. We have found, that even high concentrations of carbon do not affect short–range atomic order of iron atoms — it remains effectively the same as in pure iron melts.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2026

Atomic structures for studying elemental segregation at garnet grain boundaries

This is a dataset containing atomic structures of bulk and grain boundaries of doped/undoped Li7La3Zr2O12 solid electrolytes, which were obtained from atomic simulations in a journal paper "Elemental Segregation at Garnet Grain Boundaries: Eliminating Its Detrimental Role in Conductivity and Lithium Nucleation" by Kai Yao et al.

Kim, Kwangnam [Lawrence Livermore National Laborat↗
DOE OSTI2014

On the local electronic and atomic structure of Ce 1-x Pr x O 2-δ epitaxial films on Si

The local electronic and atomic structure of (111)-oriented, single crystalline mixed Ce 1-x Pr x O 2-δ (x = 0, 0.1 and 0.6) epitaxial thin films on silicon substrates have been investigated in view of engineering redox properties of complex oxide films. Non-destructive X-ray absorption near edge structure reveals that Pr shows only +3 valence and Ce shows only nominal +4 valence in mixed oxides. Extended x-ray absorption fine structure (EXAFS) studies were performed at K edges of Ce and Pr using a specially designed monochromator system for high energy measurements. They demonstrate that the fluorite lattice of ceria (CeO₂) is almost not perturbed for x = 0.1 sample, while higher Pr concentration (x = 0.6) not only generates a higher disorder level (thus more disordered oxygen) but also causes a significant reduction of Ce–O interatomic distances. The valence states of the cations were also examined by techniques operating in highly reducing environments: scanning transmission electron microscopy-electron energy loss spectroscopy and X-ray photoemission spectroscopy; in these reducing environments, evidence for the presence of Ce³⁺ was clearly found for the higher Pr concentration. Thus, the introduction of Pr³⁺ into CeO₂ strongly enhances the oxygen exchange properties of CeO₂. This improved oxygen mobility properties of CeO₂ are attributed to the lattice disorder induced by Pr mixing in the CeO₂ fluorite lattice, as demonstrated by EXAFS measurements. Thus, a comprehensive picture of the modifications of the atomic and electronic structure of Ce 1-x Pr x O 2-δ epitaxial films and their relation is obtained.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Curved-line search algorithm for ab initio atomic structure relaxation

Ab initio atomic relaxations often take large numbers of steps and long times to converge, especially when the initial atomic configurations are far from the local minimum or there are curved and narrow valleys in the multidimensional potentials. An atomic relaxation method based on on-the-flight force learning and a corresponding curved-line search algorithm is presented to accelerate this process. In this work, results demonstrate the superior performance of this method for metal and magnetic clusters when compared with the conventional conjugate-gradient method.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Atomic structure of Pt nanoclusters supported by graphene/Ir(111) and reversible transformation under CO exposure

We investigate the atomic structure of graphene/Ir(111) supported platinum clusters with on average fewer than 40 atoms by means of surface x-ray diffraction (SXRD), grazing incidence small angle x-ray scattering (GISAXS), and normal incidence x-ray standing waves (NIXSW) measurements, in comparison with density functional theory calculations (DFT). GISAXS revealed that the clusters with 1.3 nm diameter form a regular array with domain sizes of 90 nm. SXRD shows that the 1–2 monolayer high, (111) oriented Pt nanoparticles grow epitaxially on the graphene support. From the combined analysis of the SXRD and NIXSW data, a three-dimensional (3D) structural model of the clusters and the graphene support can be deduced which is in line with the DFT results. For the clusters grown in ultrahigh vacuum the lattice parameter is reduced by (4.6±0.1)% compared to bulk platinum. The graphene layer undergoes a strong Pt adsorption induced buckling, caused by a rehybridization of the carbon atoms below the cluster. Lastly, in situ observation of the Pt clusters in CO and O 2 environments revealed a reversible change of the clusters' strain state while successively dosing CO at room temperature and O 2 at 575 K, pointing to a CO oxidation activity of the Pt clusters.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2021

Atomic structures of interfacial solute gateways to θ' precipitates in Al-Cu alloys

Many materials employed in critical structural applications depend upon metastable strengthening precipitates that transform or dissolve at elevated temperatures. Herein, aberration-corrected scanning transmission electron microscopy and first-principles calculations are used to accurately determine the atomic structure of the highly mobile, semi-coherent precipitate interfaces that control this process in the classic θ' (Al 2 Cu) precipitate in the Al-Cu system. Semi-coherent {110} interfaces are found to be composed of an array of unexpected misfit dislocations that are arranged in two different structural units. Dislocations accommodate nearly all of the misfit between the Al matrix and strengthening phase. Cu is observed to segregate to the compressed edge of the dislocation cores at specific sites in this interface. First-principles calculations revealed the energetic landscape that facilitates these sites to become entry and exit gateways of Cu atoms in this semi-coherent interface. In conclusion, this investigation reveals critical features within semi-coherent interfaces that determine the thermal stability of precipitation-hardened alloys.

36 MATERIALS SCIENCE↗