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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 505 records · Page 28
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Electron Scattering from 1-Methyl-5-Nitroimidazole: Cross-Sections for Modeling Electron Transport through Potential Radiosensitizers

In this study, we present a complete set of electron scattering cross-sections from 1-Methyl-5-Nitroimidazole (1M5NI) molecules for impact energies ranging from 0.1 to 1000 eV. This information is relevant to evaluate the potential role of 1M5NI as a molecular radiosensitizers. The total electron scattering cross-sections (TCS) that we previously measured with a magnetically confined electron transmission apparatus were considered as the reference values for the present analysis. Elastic scattering cross-sections were calculated by means of two different schemes: The Schwinger multichannel (SMC) method for the lower energies (below 15 eV) and the independent atom model-based screening-corrected additivity rule with interferences (IAM-SCARI) for higher energies (above 15 eV). The latter was also applied to calculate the total ionization cross-sections, which were complemented with experimental values of the induced cationic fragmentation by electron impact. Double differential ionization cross-sections were measured with a reaction microscope multi-particle coincidence spectrometer. Using a momentum imaging spectrometer, direct measurements of the anion fragment yields and kinetic energies by the dissociative electron attachment are also presented. Cross-sections for the other inelastic channels were derived with a self-consistent procedure by sampling their values at a given energy to ensure that the sum of the cross-sections of all the scattering processes available at that energy coincides with the corresponding TCS. This cross-section data set is ready to be used for modelling electron-induced radiation damage at the molecular level to biologically relevant media containing 1M5NI as a potential radiosensitizer. Nonetheless, a proper evaluation of its radiosensitizing effects would require further radiobiological experiments.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2018

Electron-driven and thermal chemistry during water-assisted purification of platinum nanomaterials generated by electron beam induced deposition

Focused electron beam induced deposition (FEBID) is a versatile tool for the direct-write fabrication of nanostructures on surfaces. However, FEBID nanostructures are usually highly contaminated by carbon originating from the precursor used in the process. Recently, it was shown that platinum nanostructures produced by FEBID can be efficiently purified by electron irradiation in the presence of water. If such processes can be transferred to FEBID deposits produced from other carbon-containing precursors, a new general approach to the generation of pure metallic nanostructures could be implemented. Therefore this study aims to understand the chemical reactions that are fundamental to the water-assisted purification of platinum FEBID deposits generated from trimethyl(methylcyclopentadienyl)platinum(IV) (MeCpPtMe 3 ). The experiments performed under ultrahigh vacuum conditions apply a combination of different desorption experiments coupled with mass spectrometry to analyse reaction products. Electron-stimulated desorption monitors species that leave the surface during electron exposure while post-irradiation thermal desorption spectrometry reveals products that evolve during subsequent thermal treatment. In addition, desorption of volatile products was also observed when a deposit produced by electron exposure was subsequently brought into contact with water. The results distinguish between contributions of thermal chemistry, direct chemistry between water and the deposit, and electron-induced reactions that all contribute to the purification process. We discuss reaction kinetics for the main volatile products CO and CH 4 to obtain mechanistic information. The results provide novel insights into the chemistry that occurs during purification of FEBID nanostructures with implications also for the stability of the carbonaceous matrix of nanogranular FEBID materials under humid conditions.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Effect of electron-electron interaction on cyclotron resonance in high-mobility InAs/AlSb quantum wells

We report observation of electron-electron (e-e) interaction effect on cyclotron resonance (CR) in InAs/AlSb quantum well heterostructures. High mobility values allow us to observe strongly pronounced triple splitting of CR line at noninteger filling factors of Landau levels ν. At magnetic fields, corresponding to ν > 4, experimental values of CR energies are in good agreement with single-electron calculations on the basis of eight-band k ⋅ p Hamiltonian. In the range of filling factors 3 < ν < 4 pronounced, splitting of CR line, exceeding significantly the difference in single-electron CR energies, is discovered. The strength of the splitting increases when occupation of the partially filled Landau level tends to a half, being in qualitative agreement with previous prediction by MacDonald and Kallin [Phys. Rev. B 40, 5795 (1989)]. We demonstrate that such behaviour of CR modes can be quantitatively described if one takes into account both electron correlations and the mixing between conduction and valence bands in the calculations of matrix elements of e-e interaction.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Temperature Dependent n-Type Self Doping in Nominally 19-Electron Half-Heusler Thermoelectric Materials

The discovery of a semiconducting ground state X y YZ (y = 0.8 or 0.75) in nominally 19-electron half-Heusler materials warrants a closer look at their apparently metallic properties that often make them good thermoelectric (TE) materials. Here, by systematically investigating the temperature dependence of off-stoichiometry (x) in V 0.8+x CoSb, Nb 0.8+x CoSb, and Ti 0.75+x NiSb it is found that x invariably increases with increasing temperature, leading to an n-type self-doping behavior. In addition, there is also a large phase width (range of x) associated with each phase that is temperature dependent. Thus, unlike in typical 18-electron half-Heuslers (e,g, TiNiSn), the temperature dependence of vacancy and carrier concentration (n) in nominally 19-electron half-Heuslers links its transport properties to synthesis conditions. The temperature dependence of x and n are understood using density functional theory based defect energies (E d ) and phase diagrams. E d are calculated for 21 systems which can be used in predicting solubility in this family of compounds. Using this simple strategy, suitable composition and temperature synthesis conditions are devised for obtaining an optimized n to engineer TE properties in phase-pure V 0.8+x CoSb, and the previously unexplored Ta 0.8+x CoSb.

19-electron half-Heuslers↗
DOE OSTI2017

Electron-acoustic rogue waves in a plasma with Tribeche–Tsallis–Cairns distributed electrons

The problem of electron-acoustic (EA) rogue waves in a plasma consisting of fluid cold electrons, nonthermal nonextensive electrons and stationary ions, is addressed. A standard multiple scale method has been carried out to derive a nonlinear Schrödinger-like equation. The coefficients of dispersion and nonlinearity depend on the nonextensive and nonthermal parameters. The EA wave stability is analyzed. Interestingly, it is found that the wave number threshold, above which the EA wave modulational instability (MI) sets in, increases as the nonextensive parameter increases. As the nonthermal character of the electrons increases, the MI occurs at large wavelength. Moreover, it is shown that as the nonextensive parameter increases, the EA rogue wave pulse grows while its width is narrowed. The amplitude of the EA rogue wave decreases with an increase of the number of energetic electrons. In the absence of nonthermal electrons, the nonextensive effects are more perceptible and more noticeable. In view of the crucial importance of rogue waves, our results can contribute to the understanding of localized electrostatic envelope excitations and underlying physical processes, that may occur in space as well as in laboratory plasmas.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Electron postmastectomy chest wall plus comprehensive nodal irradiation technique using Electron Monte Carlo dose algorithm

For left-sided postmastectomy patients requiring chest wall plus comprehensive nodal irradiation, sometimes traditional techniques such as partial wide tangents or electron/tangents combination are not feasible due to abnormal chest wall contour or heart position or unusually wide excision scar. We developed electron chest wall irradiation technique using Electron Monte Carlo (EMC) dose algorithm that will achieve heart sparing with acceptable ipsilateral lung dose, minimal contralateral lung, and breast dose. Ten left-sided postmastectomy patients with very challenging anatomy were selected for this dosimetry study. The en face electron fields were designed from a single isocenter and gantry angle with different energy beams using different cutouts that matched on the skin. Smaller energy was used in the central thin chest wall part and higher energy in the medial internal mammary nodes (IMN) area, superior part of the thick chest wall, and/or axillary nodal area. The electron fields were matched to the photon supraclavicular field in the superior region. Daily field junctions were used to feather the match lines between all the fields. Electron field dose calculations were done with Monte Carlo. Five patients' chest wall fields were planned with 6/9MeVcombination, 1 with 6/12 MeV, 2 with 9/12 MeV, 1 with 9/16 MeV, and 1 with 6/9/12 MeV. Institutional criteria of prescription dose of 50 Gy for target volumes and normal tissue dose were met with this technique in spite of the challenging anatomy. Mean heart dose averaged 3.0 Gy ± 0.8 Gy. For ipsilateral lung, V20Gy and V5Gy averaged 33.2% ± 4.5% and 64.6% ± 9.6%, respectively. For contralateral lung, V5Gy averaged 5.1% ± 5.0%. For contralateral breast, V5Gy averaged 3.3% ± 3.1%. The electron chest wall irradiation technique using EMC dose algorithm can provide adequate dose coverage to the chest wall, IMNs, and/or axillary nodes while achieving heart sparing with acceptable ipsilateral lung dose, minimal contralateral lung, and breast dose.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2015

X-Ray Radar Imaging Technique Using a 2 Mev Linear Electron Accelerator

X-ray radar imaging, patented in 2013 by James R. Wood, combines standard radar techniques with the penetration power of x-rays to image scenes. Our project strives to demonstrate the technique using a 2 MeV linear electron accelerator (linac) to generate the S-band–modulated x-ray signals. X-ray detectors such as photodiodes and scintillators are used to detect the signals in backscatter and transmission detection schemes. The S-band microstructure is imposed on the variable-width electron pulse, and this modulation carries over to the bremsstrahlung x-rays after the electron beam is incident upon a copper-tungsten alloy target. Using phase/distance calculations and a low-jitter timing system, we expect to detect different object distances by comparing the measured phase differences. The experimental setup, which meets strict jitter requirements, and preliminary experimental results are presented.

43 PARTICLE ACCELERATORS↗
DOE OSTI2018

Methods for Conducting Electron Backscattered Diffraction (EBSD) on Polycrystalline Organic Molecular Thin Films

Electron backscattered diffraction (EBSD) is a technique regularly used to obtain crystallographic information from inorganic samples. When EBSD is acquired simultaneously with emitting diodes data, a sample can be thoroughly characterized both structurally and compositionally. For organic materials, coherent Kikuchi patterns do form when the electron beam interacts with crystalline material. However, such patterns tend to be weak due to the low average atomic number of organic materials. This is compounded by the fact that the patterns fade quickly and disappear completely once a critical electron dose is exceeded, inhibiting successful collection of EBSD maps from them. In this paper, a new approach is presented that allows successful collection of EBSD maps from organic materials, here the extreme example of a hydrocarbon organic molecular thin film, and opens new avenues of characterization for crystalline organic materials.

42 ENGINEERING↗
DOE OSTI2021

Intrinsic Li Distribution in Layered Transition-Metal Oxides Using Low-Dose Scanning Transmission Electron Microscopy and Spectroscopy

Understanding Li distribution in layered lithium transition-metal oxide (LiTMO) cathodes in Li-ion batteries has been a major challenge at the atomic scale and nanoscale. Li is extremely difficult to study by transmission electron microscopy (TEM) because the high-energy electrons impart significant energy and cause massive migration. Here, we directly map the intrinsic spatial distribution and bonding of Li in LiNiO 2 -layered cathode materials using low-dose and low-loss electron energy loss spectroscopy (EELS). EELS spectra of the Li–K edge are measured simultaneously with O–K and Ni–L, M 3,2 edges from layered, cation-mixed, and rock-salt phases and directly matched with atomic-resolution scanning TEM images to correlate the changes in peak intensities and positions to the stoichiometry changes with continual loss of Li and O. Changes in the Li content in the LiNiO 2 particles as a function of electron beam dose are studied by sequential Li spectroscopic mapping. We show that the “intrinsic” Li distribution can be observed using a total dose of less than ~1.5 × 10 8 e – nm –2 at an accelerating voltage of 80 kV. The method of nanoscale mapping of Li distribution introduced in this study is applicable to high-Ni LiTMO cathode materials (>89% of Ni) as well as LiNiO 2 . Further study on the extra peaks of the Li–K edge reveals that the peak at ~59 eV is from the Li ions intercalated in between NiO 2 layers barely interacting with each other with less Li K shell electrons pulled to the L shell electrons in NiO 2 . Furthermore, the results shown here provide improved low-loss TEM characterization approaches that can be used to understand the intrinsic fundamental behaviors in Li-ion batteries.

25 ENERGY STORAGE↗
DOE OSTI2020

Vibrational Spectroscopy Reveals Effects of Electron Push–Pull and Solvent Polarity on Electron Delocalization in Radical Anions of Donor–Acceptor Compounds

Herein, the nature of excess electrons has been studied in donor–acceptor (D–A) compounds based on substituted triarylamines and a nitrile-functionalized fluorene, by changing the substituents on the triarylamines and also the solvent polarity. We observed that both electron push-pull capability at the distant location in the amine donor unit and solvation in solvents of varying polarity significantly affect the nitrile $ν(C≡N)$ vibrations of the fluorene acceptor unit in radical anions of these D–A compounds. Quantum calculations show that the push-pull capability translates the position of an excess electron while keeping its width relatively constant. On the other hand, solvation changes both, making an electron more compact in polar media. The current study points to the idea that solvation plays a more significant role in controlling the nature of excess electrons, while synthetic modification that influences electron push-pull capability enables further tuning.

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

Electronic Coherence and Coherent Dephasing in the Optical Control of Electrons in Graphene

Electronic coherence is of utmost importance for the access and control of quantum-mechanical solid-state properties. Using a purely electronic observable, the photocurrent, we measure a lower bound of the electronic coherence time of 22 ± 4 fs in graphene. The photocurrent is ideally suited to measure electronic coherence, as it is a direct result of coherent quantum-path interference, controlled by the delay between two ultrashort two-color laser pulses. The maximum delay for which interference between the population amplitude injected by the first pulse interferes with that generated by the second pulse determines the electronic coherence time. In particular, numerical simulations reveal that the experimental data yields a lower bound on the electronic coherence time, masked by coherent dephasing due to the broadband absorption in graphene. Overall, we expect that our results will significantly advance the understanding of coherent quantum control in solid-state systems ranging from excitation with weak fields to strongly driven systems.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2020

Electron-phonon coupling induced intrinsic Floquet electronic structure

Floquet states are a topic of intense contemporary interest, which is often induced by coherent external oscillating perturbation (e.g., laser, or microwave) which breaks the continuous time translational symmetry of the systems. Usually, electron–phonon coupling modifies the electronic structure of a crystal as a non-coherent perturbation and seems difficult to form Floquet states. Surprisingly, we found that the thermal equilibrium electron–phonon coupling in M(MoS) 3 and M(MoSe) 3 (where M is a metallic element) exhibits a coherent behavior, and the electronic structure can be described by the Floquet theorem. Such a coherent Floquet state is caused by a selective giant electron–phonon coupling, with thermodynamic phonon oscillation serving as a driving force on the electronic part of the system. The quasi-1D Dirac cone at the Fermi energy has its band gap open and close regularly. Similarly, the electric current will oscillate even under a constant voltage.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Electronic structure of cobalt doped CdSe quantum dots using soft X-ray spectroscopy

Here, the electronic structure and magnetic properties of cobalt doped CdSe quantum dots (QDs) are studied using electron microscopy, soft X-ray spectroscopy, and magnetometry. Magnetometry measurements suggest these QDs are superparamagnetic, contrary to a spin-glass state observed in the bulk analogue. Electron microscopy shows well formed QDs, but with cobalt existing as doped into the QD and as unreacted species not contained in the QD. X-ray absorption measurements at the Co L3-edge suggest that changes in spectra features as a function of particle size can be described considering combination of a cobalt ion in a tetrahedral crystal field and an octahedrally coordinated (impurity) phase. With decreasing particle sizes, the impurity phase increases, suggesting that small QDs can be difficult to dope.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Intermittent laser-plasma interactions and hot electron generation in shock ignition

We study laser-plasma interactions and hot electron generation in the ignition phase of shock ignition through 1D and 2D particle-in-cell simulations in the regime of long density scale length and moderately high laser intensity. These long-term simulations show an intermittent bursting pattern of laser-plasma instabilities, resulting from a coupling of the modes near the quarter-critical-surface and those in the lower density region via plasma waves and laser pump depletion. The majority of the hot electrons are found to be from stimulated Raman scattering and of moderate energies. However, high energy electrons of preheating threat can still be generated from the two-plasmon-decay instability.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Two-dimensional electron gas in monolayer InN quantum wells

We report in this letter experimental results that confirm the two-dimensional nature of the electron systems in a superlattice structure of 40 InN quantum wells consisting of one monolayer of InN embedded between 10 nm GaN barriers. The electron density and mobility of the two-dimensional electron system (2DES) in these InN quantum wells are 5 × 10{sup 15 }cm{sup −2} (or 1.25 × 10{sup 14 }cm{sup −2} per InN quantum well, assuming all the quantum wells are connected by diffused indium contacts) and 420 cm{sup 2}/Vs, respectively. Moreover, the diagonal resistance of the 2DES shows virtually no temperature dependence in a wide temperature range, indicating the topological nature of the 2DES.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗