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At least 865 records · Page 48
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Improved joint X-ray and neutron refinement procedure in Phenix

Neutron diffraction is one of the three crystallographic techniques (X-ray, neutron and electron diffraction) used to determine the atomic structures of molecules. Its particular strengths derive from the fact that H (and D) atoms are strong neutron scatterers, meaning that their positions, and thus protonation states, can be derived from crystallographic maps. However, because of technical limitations and experimental obstacles, the quality of neutron diffraction data is typically much poorer (completeness, resolution and signal to noise) than that of X-ray diffraction data for the same sample. Further, refinement is more complex as it usually requires additional parameters to describe the H (and D) atoms. The increase in the number of parameters may be mitigated by using the `riding hydrogen' refinement strategy, in which the positions of H atoms without a rotational degree of freedom are inferred from their neighboring heavy atoms. However, this does not address the issues related to poor data quality. Therefore, neutron structure determination often relies on the presence of an X-ray data set for joint X-ray and neutron (XN) refinement. In this approach, the X-ray data serve to compensate for the deficiencies of the neutron diffraction data by refining one model simultaneously against the X-ray and neutron data sets. To be applicable, it is assumed that both data sets are highly isomorphous, and preferably collected from the same crystals and at the same temperature. However, the approach has a number of limitations that are discussed in this work by comparing four separately re-refined neutron models. To address the limitations, a new method for joint XN refinement is introduced that optimizes two different models against the different data sets. This approach is tested using neutron models and data deposited in the Protein Data Bank. The efficacy of refining models with H atoms as riding or as individual atoms is also investigated.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Structural investigation of the temperature-stable relaxor dielectric Ba 0.8 Ca 0.2 TiO 3 -Bi(Mg 0.5 Ti 0.5 )O 3

Aberration corrected scanning transmission electron microscopy (STEM) and electron diffraction have been used to disclose local structure and nano-chemistry in a Ca modified BaTiO 3 -Bi(Mg 0.5 Ti 0.5 )O 3 relax or dielectric ceramic which exhibits high and near-invariant relative permittivity over a wide temperature range. High resolution, synchrotron X-ray diffraction indicated a globally cubic structure ($Pm$$\overline{3}$$m$), but direct atomic-scale imaging by STEM revealed local tetragonal distortions. Nanopolar clusters were identified from B-site atomic displacement vectors measured relative to oxygen ion positions along < 100 > and < 110 > zone axes of integrated differential phase contrast (iDPC) STEM images, highlighting cluster sizes of 2–5 nm. Chemical analysis by STEM-energy dispersive X-ray spectroscopy and full pattern refinements of X-ray powder diffraction data each implied high levels of Bi vacancies within the matrix. The possibility that these A-site vacancies modulate the nanopolar structure and promote flattening of the permittivity-temperature response in this class of dielectric is discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Mechanistic insights into selective oxidation and corrosion of multi-principal element alloys from high resolution and in situ microscopy

Multiple principal elements alloys (MPEAs) have drawn significant recent interest from the metallurgical research community. These novel alloys have the potential to replace traditional alloys with a range of tailored mechanical properties, corrosion resistance, and radiation tolerance enabled by complex compositions. The broad alloy design space of MPEAs presents a great opportunity to discover new forms of oxidation and corrosion resistance in extreme environments that break away from conventional corrosion resistant alloy design criteria. However, understanding the oxidation and corrosion response of MPEAs can be a challenging task due the complex composition, structural, and chemical state changes occurring in the corrosion product and at its interface with the base alloy. This perspective discusses the utility of high resolution and in situ microscopy techniques for revealing unique aspects of MPEA oxidation behavior at the near-atomic level. Challenges of studying these alloys and several open questions are presented.

Kautz, Elizabeth J.↗
DOE OSTI2020

Atomistic Positioning of Defects in Helium Ion Treated Single-Layer MoS 2

Structuring materials with atomic precision is the ultimate goal of nanotechnology and is becoming increasingly relevant as an enabling technology for quantum electronics/spintronics and quantum photonics. Here, we create atomic defects in monolayer MoS 2 by helium ion (He-ion) beam lithography with a spatial fidelity approaching the single-atom limit in all three dimensions. Using low-temperature scanning tunneling microscopy (STM), we confirm the formation of individual point defects in MoS 2 upon He-ion bombardment and show that defects are generated within 9 nm of the incident helium ions. Atom-specific sputtering yields are determined by analyzing the type and occurrence of defects observed in high-resolution STM images and compared with Monte Carlo simulations. Both theory and experiment indicate that the He-ion bombardment predominantly generates sulfur vacancies.

Two-dimensional materials↗
DOE OSTI2023

Real-time insight into the multistage mechanism of nanoparticle exsolution from a perovskite host surface

In exsolution, nanoparticles form by emerging from oxide hosts by application of redox driving forces, leading to transformative advances in stability, activity, and efficiency over deposition techniques, and resulting in a wide range of new opportunities for catalytic, energy and net-zero-related technologies. However, the mechanism of exsolved nanoparticle nucleation and perovskite structural evolution, has, to date, remained unclear. Herein, we shed light on this elusive process by following in real time Ir nanoparticle emergence from a SrTiO 3 host oxide lattice, using in situ high-resolution electron microscopy in combination with computational simulations and machine learning analytics. We show that nucleation occurs via atom clustering, in tandem with host evolution, revealing the participation of surface defects and host lattice restructuring in trapping Ir atoms to initiate nanoparticle formation and growth. These insights provide a theoretical platform and practical recommendations to further the development of highly functional and broadly applicable exsolvable materials.

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

Selective volatile organic compound gas sensor based on carbon nanotubes functionalized with ZnO nanoparticles

Here this paper describes the theory and results for a new class of low-cost chemoresistive gas sensors designed for selective hydrocarbon gas detection. The sensors utilize a multiwalled carbon nanotube (MWCNT) backbone functionalized with metal oxide nanocrystals. Specifically, nanoparticles were grown on the surface of the MWCNTs using atomic layer deposition. The crystallinity of the ZnO-MWCNTs' heterostructure was examined by using a high-resolution transmission electron microscope. The structure of the ZnO/MWCNTs was analyzed using a scanning electron microscope and energy dispersive x ray. The Hall effect measurement shows p-type characteristics of the MWCNTs, supporting the typical PN junction formation with n-type ZnO nanocrystals. The electron-donating ability of ZnO provided a strong response to the ppm levels of toluene at room temperature (25 °C) and showed strong selectivity with other volatile organic compound gases such as benzene, methane, and formaldehyde.

47 OTHER INSTRUMENTATION↗
DOE OSTI2025

Two-photon Rydberg EIT resonances in non-collinear beam configurations

We study the modifications of Rydberg EIT resonances in non-collinear geometry in which the two required optical fields cross at a small non-zero angle. We observe a strong broadening and amplitude reduction even for small angles when compared to exact counter-propagating and co-propagating collinear geometries. We confirm that such EIT peak deterioration results from the additional Doppler broadening due to the transverse velocity distribution of atoms. The numerical simulation closely matches the experimental measurements. While a non-collinear geometry provides improved spatial resolution for Rydberg EIT electrometry, we conclude that the crossing angle must be small to maintain field sensitivity.

Su, Kevin↗
NASA NTRS1984

Star formation in the nucleus of NGC 253

The B-alpha and B-gamma lines of atomic hydrogen have been observed at eight positions near the nucleus of NGC 253 with 6 arcsec spatial resolution. Most if not all of the bolometric luminosity of the central 200 pc of this galaxy is probably produced by O and B stars which have recently formed. The spatial distribution of the line fluxes is similar to that of the 10 microns flux, supporting the hypothesis that the 10 microns radiation is powered by the newly formed stars. The extinction to the ionized gas varies over small distances in this region, explaining the discrepancies between extinction and Lyman continuum luminosity derived separately from visual and infrared line observations.

Beck, S. C.↗
NASA NTRS1986

Observational exobiology

The Earth's atmosphere absorbs partially or completely many ultraviolet, infrared and submillimeter wavelengths. Atmospheric seeing distorts small images, imposing a limit on the achievable angular resolution at optical and infrared wavelengths that is much poorer than the intrinsic capability of telescope optics. The atomic and molecular species of the atmosphere confuse or prevent the spectral studies of similar compounds outside of the terrestrial environment. Telescopes placed in orbit above the atmosphere avoid these problems and enjoy a unique view of the universe. There are many complex questions pertaining to the origin and evolution of the biogenic elements and compounds and the existence of terrestrial types of planets elsewhere that can be only tackled from orbiting facilities. The detailed nature of the spacecraft, platforms and instrumentation most likely to be launched by the United States and Europe in the near future in an attempt to determine what observational programs would be tractable and which areas of interest to exobiology required hardware capabilities beyond those currently envisioned are considered.

Tarter, J.↗
NASA NTRS1985

High spectral resolution Fabry-Perot interferometer measurements of Comet Halley at H-alpha and 6300 A

A 40.6-cm Newtonian telescope was interfaced to the Fabry-Perot interferometer at the Arecibo Observatory to make high-spectral-resolution measurements of Comet Halley emissions at 6562.72 A (H-alpha) and 6300.3 A (O I). The atomic hydrogen outflow velocity was found to be approximately 7.9 + or - 1.0 km/s. In general, the H-alpha spectra are highly structured, and indicative of a multiple component atomic hydrogen velocity distribution. For O (1D), implied production rates of 6.44 + or - 3.0 x 10 to the 28th/s and 5.66 + or - 2.7 x 10 to the 28th/s were found on March 15 and 17, respectively.

Kerr, R. B.↗
NASA NTRS1995

Model atmospheres for M (sub)dwarf stars. 1: The base model grid

We have calculated a grid of more than 700 model atmospheres valid for a wide range of parameters encompassing the coolest known M dwarfs, M subdwarfs, and brown dwarf candidates: 1500 less than or equal to T(sub eff) less than or equal to 4000 K, 3.5 less than or equal to log g less than or equal to 5.5, and -4.0 less than or equal to (M/H) less than or equal to +0.5. Our equation of state includes 105 molecules and up to 27 ionization stages of 39 elements. In the calculations of the base grid of model atmospheres presented here, we include over 300 molecular bands of four molecules (TiO, VO, CaH, FeH) in the JOLA approximation, the water opacity of Ludwig (1971), collision-induced opacities, b-f and f-f atomic processes, as well as about 2 million spectral lines selected from a list with more than 42 million atomic and 24 million molecular (H2, CH, NH, OH, MgH, SiH, C2, CN, CO, SiO) lines. High-resolution synthetic spectra are obtained using an opacity sampling method. The model atmospheres and spectra are calculated with the generalized stellar atmosphere code PHOENIX, assuming LTE, plane-parallel geometry, energy (radiative plus convective) conservation, and hydrostatic equilibrium. The model spectra give close agreement with observations of M dwarfs across a wide spectral range from the blue to the near-IR, with one notable exception: the fit to the water bands. We discuss several practical applications of our model grid, e.g., broadband colors derived from the synthetic spectra. In light of current efforts to identify genuine brown dwarfs, we also show how low-resolution spectra of cool dwarfs vary with surface gravity, and how the high-regulation line profile of the Li I resonance doublet depends on the Li abundance.

Allard, France↗
NASA NTRS1992

Definition of the Spatial Resolution of X-Ray Microanalysis in Thin Foils

The spatial resolution of X-ray microanalysis in thin foils is defined in terms of the incident electron beam diameter and the average beam broadening. The beam diameter is defined as the full width tenth maximum of a Gaussian intensity distribution. The spatial resolution is calculated by a convolution of the beam diameter and the average beam broadening. This definition of the spatial resolution can be related simply to experimental measurements of composition profiles across interphase interfaces. Monte Carlo calculations using a high-speed parallel supercomputer show good agreement with this definition of the spatial resolution and calculations based on this definition. The agreement is good over a range of specimen thicknesses and atomic number, but is poor when excessive beam tailing distorts the assumed Gaussian electron intensity distributions. Beam tailing occurs in low-Z materials because of fast secondary electrons and in high-Z materials because of plural scattering.

Williams, D. B.↗
NASA NTRS2014

A Global Upper Atmosphere Observatory Using of Lidar on the International Space Station

A concept for hosting a lidar facility for the upper atmosphere on the International Space Station (ISS) is presented and discussed. The concept is based on utilizing an existing Large Space Optics mirror having a 2.37-m aperture as the primary mirror in its receiver. This large aperture provides for hosting several transmitter systems to retrieve density, temperature, and wind measurements for several upper atmospheric species. Thus the concept provides for measurements over a wide altitude range (80-600 km), at various time and spatial resolutions, and hosting on the ISS provides nearly global coverage. The baseline concept includes transmitters and receivers for atomic oxygen (80-500 km), metastable helium (400-600 km), and sodium (80-110 km). The facility is conceived as being flexible such that other transmitter/receiver systems could be added to allow the possibility of other species to be studied, such as iron. The presentation discusses the transformative science that would be gained by such an observatory by combining the nearly global coverage afforded by the ISS orbit with the extension of powerful lidar techniques to high altitudes. The challenges in realizing such an observatory are discussed, as are current plans and partnerships to meet those challenges. The presentation also reports on the development status of several components, primarily various independent transmitter/receiver systems, that are under consideration for the baseline observatory. Several institutions are performing these developments.

Clemmons, J. H.↗
NASA NTRS2006

CFD Growth of 3C-SiC on 4H/6H Mesas

This article describes growth and characterization of the highest quality reproducible 3C-SiC heteroepitaxial films ever reported. By properly nucleating 3C-SiC growth on top of perfectly on-axis (0001) 4H-SiC mesa surfaces completely free of atomic scale steps and extended defects, growth of 3C-SiC mesa heterofilms completely free of extended crystal defects can be achieved. In contrast, nucleation and growth of 3C-SiC mesa heterofilms on top of 4H-SiC mesas with atomic-scale steps always results in numerous observable dislocations threading through the 3C-SiC epilayer. High-resolution X-ray diffraction and transmission electron microscopy measurements indicate non-trivial in-plane lattice mismatch between the 3C and 4H layers. This mismatch is somewhat relieved in the step-free mesa case via misfit dislocations confined to the 3C/4H interfacial region without dislocations threading into the overlying 3C-SiC layer. These results indicate that the presence or absence of steps at the 3C/4H heteroepitaxial interface critically impacts the quality, defect structure, and relaxation mechanisms of single-crystal heteroepitaxial 3C-SiC films.

Neudeck, Philip G.↗
DOE OSTI2022

Precision measurement of Compton scattering in silicon with a skipper CCD for dark matter detection

Experiments aiming to directly detect dark matter through particle recoils can achieve energy thresholds of $\mathcal{O}(1\,\mathrm{eV})$. In this regime, ionization signals from small-angle Compton scatters of environmental $\gamma$-rays constitute a significant background. Monte Carlo simulations used to build background models have not been experimentally validated at these low energies. We report a precision measurement of Compton scattering on silicon atomic shell electrons down to 23$\,$eV. A skipper charge-coupled device (CCD) with single-electron resolution, developed for the DAMIC-M experiment, was exposed to a $^{241}$Am $\gamma$-ray source over several months. Features associated with the silicon K, L$_{1}$, and L$_{2,3}$-shells are clearly identified, and scattering on valence electrons is detected for the first time below 100$\,$eV. We find that the relativistic impulse approximation for Compton scattering, which is implemented in Monte Carlo simulations commonly used by direct detection experiments, does not reproduce the measured spectrum below 0.5$\,$keV. The data are in better agreement with $ab$$initio$ calculations originally developed for X-ray absorption spectroscopy.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
NASA NTRS1993

158 micron forbidden C II mapping of NGC 6946 - Probing the atomic medium

A well-sampled map (23 x 17 kpc) of the strong 158 micron forbidden C II cooling line in the Scd galaxy NGC 6946 at 55 arcsec resolution is presented which was taken with the MPE/UCB Far-infrared Imaging Fabry-Perot Interferometer (FIFI) in the Kuiper Airborne Observatory. It is concluded that the line emission in NGC 6946 is present in three spatially distinct components including nucleus, spiral arms, and extended region. An extended emission region is a source of most of the forbidden C II luminosity in NGC 6946. The 1 arcmin nuclear component has a line luminosity of 1.5 x 10 exp 7 solar luminosity and contributes 0.15 percent of the galaxy's total FIR luminosity. An extended component of forbidden C II emission is found to exist past the molecular extent of the galaxy and to be present to at least the full dimensions of the map. This component is attributed to a mixture of neutral and atomic clouds.

Madden, S. C.↗