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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 415 records · Page 23
All sourcesNASA NTRSDOE OSTI
NASA NTRS1997

Effective Collision Strengths for Electron Impact Excitation of Inelastic Transitions in S III

We have calculated electron collisional excitation strengths for all electric dipole forbidden, semi-forbidden, and allowed transitions among the lowest 17 LS states 3s(exp 2)3p(exp 2) P-3, D-1, S-1, 3s3p(exp 3)S-5(exp 0), D-3(exp 0), P-3(exp 0), P-1(exp 0), S-3(exp 0), D-1(exp 0), 3S(exp 2)3p3d D-1(exp 0), F-3(exp 0), P-3(exp 0), D-3(exp 0), F-3(exp 0), P-1(exp 0), and 3S(exp 2)3p4S P-3(exp 0), P-l(exp 0) of S III using the R-matrix method. These S m states are represented by fairly extensive configuration-interaction wave functions that yield excited state energies in close agreement with recent laboratory measurements. Rydberg series of resonances converging to the excited state thresholds are explicitly included in the scattering calculation. The effective collision strengths are determined assuming Maxwellian distribution of electron energies. These are listed over a wide temperature range ([0.5-10] x 10(exp 4) K) and compared, where possible, with other available calculations. Subject headings: atomic data - atomic processes

Tayal, S. S.↗
DOE OSTI2025

Investigation of Oxidation Mechanisms in HfS 2 and ZrS 2 via In Situ Electron Microscopy

Transition metal dichalcogenides (TMD) combine semiconducting properties with stability and are therefore widely studied for future microelectronic devices. However, applications require integration with a suitable dielectric at a planar, defect-free interface. For Hf- and Zr-based TMDs, the good dielectric properties of hafnia and zirconia imply that direct oxidation is a promising strategy, provided that a high-quality interface and suitable oxide morphology can be formed. Here, in this study, we investigate HfS 2 and ZrS 2 oxidation, aiming to understand pathways toward planar oxide layer formation and the mechanisms that determine the characteristics of the semiconductor/dielectric interface. We use conventional and environmental transmission electron microscopy to reveal changes in morphology and composition of the TMDs under different gaseous environments. We show that oxidation at ambient conditions causes compositional heterogeneities, with local replacement of S by O in both materials. Thermal oxidation causes desulfurization and formation of a smooth but defective oxide layer. In contrast, plasma oxidation appears the best-suited to forming a controlled TMD/dielectric interface with a smooth oxide layer and without major defects. These results provide insight into the opportunities available from HfS 2 and ZrS 2 through oxidation and suggest a materials processing strategy for electronic device fabrication.

HfS2↗
NASA NTRS1985

On 3d bonding in the transition metal trimers - The electronic structure of equilateral triangle Ca3, Sc3, Sc3(+), and Ti3(+)

It is pointed out that transition metals and transition metal (TM) compounds are currently of considerable interest because of their relevance to catalysis and to materials science problems such as hydrogen embrittlement and crack propagation in metals. The present paper is concerned with complete active space Self-Consistent Field (SCF) externally contracted configuration interaction (CASSCF/CCI) calculations for the low-lying states of Sc3 and Sc3(+). A comparison is conducted regarding the bonding in the Ca3, Sc3, and Cu3 molecules. This comparison makes it possible to predict general trends for the TM trimers. Attention is given to the qualitative features of the bonding in the TM trimers, the basis sets and other technical details of the calculations, the calculated results for Sc3 and Sc3(+), and conclusions from this work.

Walch, S. P.↗
DOE OSTI2024

Biaxial Tensile Strain Enhances Electron Mobility of Monolayer Transition Metal Dichalcogenides

Strain engineering can modulate the material properties of two-dimensional (2D) sem- iconductors for electronic and optoelectronic applications. Recent theory and experiments have found that uniaxial tensile strain can improve the electron mobility of monolayer MoS 2 , a 2D semiconductor, but the effects of biaxial strain on charge transport are not well-understood in 2D semiconductors. Here, we use biaxial tensile strain on flexible substrates to probe the electron mobility in monolayer WS 2 and MoS 2 transistors. Further, this approach experimentally achieves ~2× higher on-state current and mobility with ~0.3% applied biaxial strain in WS 2 , the highest mobility improvement at the lowest strain reported to date. We also examine the mechanisms behind this improvement through density functional theory simulations, concluding that the enhancement is primarily due to reduced intervalley electron-phonon scattering. These results underscore the role of strain engineering 2D semiconductors for flexible electronics, sensors, integrated circuits, and other opto-electronic applications.

2D materials↗
DOE OSTI2024

Nanoscale Three-Dimensional Imaging of Integrated Circuits Using a Scanning Electron Microscope and Transition-Edge Sensor Spectrometer

X-ray nanotomography is a powerful tool for the characterization of nanoscale materials and structures, but it is difficult to implement due to the competing requirements of X-ray flux and spot size. Due to this constraint, state-of-the-art nanotomography is predominantly performed at large synchrotron facilities. We present a laboratory-scale nanotomography instrument that achieves nanoscale spatial resolution while addressing the limitations of conventional tomography tools. The instrument combines the electron beam of a scanning electron microscope (SEM) with the precise, broadband X-ray detection of a superconducting transition-edge sensor (TES) microcalorimeter. The electron beam generates a highly focused X-ray spot on a metal target held micrometers away from the sample of interest, while the TES spectrometer isolates target photons with a high signal-to-noise ratio. This combination of a focused X-ray spot, energy-resolved X-ray detection, and unique system geometry enables nanoscale, element-specific X-ray imaging in a compact footprint. The proof of concept for this approach to X-ray nanotomography is demonstrated by imaging 160 nm features in three dimensions in six layers of a Cu-SiO 2 integrated circuit, and a path toward finer resolution and enhanced imaging capabilities is discussed.

47 OTHER INSTRUMENTATION↗
NASA NTRS1996

Collision Strengths for Electron Impact Excitation of Inelastic Transitions in Ar II

We have calculated collision strengths for electron impact excitation of inelastic transitions in Ar II using the R-matrix method in two independent nine- and 19-state close-coupling approximations. In the nine-state calculation the 3s(sup 2)3p(sup 5)2p(sup 0), 3S(sup 3)p(sup 6)S-2, 3p(sup 4)(P-3)4s(sup 2)P, 3p(sup 4)(P-3)3d(sup 2)P, 3p(sup 4)(D-1)4s(sup 2)D, 3p(sup 4)(P-3)3d(sup 2)D, 3p(sup 4)(S-1)4s(sup 2)S, 3p(sup 4)(D-1)3d(sup 2)S and 3p(sup 4)(D-1)4d(sup 2)S states are included, while in the 19-state calculation these states plus an additional ten states 3p(sup 4)(P-3)3d(sup 2)F, 3p(sup 4)(P-3)4p(sup 2)D(sup 0), 2p(sup 0), 2S(sup 0), 3p(sup 4)(D-1)4p(sup 2)P(sup 0), 2D(sup O), 2F(sup 0), 3p(sup 4)(D-1)3d(sup 2)D, p-2 and 3p(sup 4)(S-1)4p(sup 2)P(sup 0) are considered. These target states are represented by fairly extensive configuration-interaction wavefunctions which yield excitation energies and oscillator strengths that are generally in good agreement with the available most accurate calculations and the experimental values. Rydberg series of resonances converging to the excited state thresholds are included in the calculation. The effective collision strengths are obtained assuming a Maxwellian distribution of electron energies which are tabulated over the temperature range (0.5-20) x 10(exp 4) K.

Tayal, S. S.↗
DOE OSTI2025

Purely electronic insulator-metal transition in rutile VO 2

Volatile resistive switching in neuromorphic computing can be tuned by external stimuli such as temperature or electric-field. However, this type of switching is generally coupled to structural changes, resulting in slower reaction speed and higher energy consumption when incorporated into an electronic device. The vanadium dioxide (VO 2 ), which has near room temperature metal-insulator transition (MIT), is an archetypical volatile resistive switching system. Here, we demonstrate an isostructural MIT in an ultrathin VO 2 film capped with a photoconductive cadmium sulfide (CdS) layer. Transmission electron microscopy, resistivity experiments, and first-principles calculations show that the hole carriers induced by CdS photovoltaic effect are driving the MIT in rutile VO 2 . The insulating-rutile VO 2 phase has been proved and can remain stable for hours. Our finding provides a new approach to produce purely electronically driven MIT in VO 2 , and widens its applications in fast-response, low-energy neuromorphic devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2026

Experimental confirmation of the magnetic ordering transition induced by an electronic structure change in the metallic triangular antiferromagnet Co 1/3⁢ TaS 2

We report angle-resolved photoemission spectroscopy (ARPES) studies combined with DFT+DMFT calculations to confirm that the magnetic ordering vector transition from Q=(1/2,0,0) to Q=(1/3,0,0) in the metallic triangular antiferromagnets Co 1/3±𝜀⁢ TaS 2 (𝜀 ≈ 0.007) is induced by the electronic structure change in the system. The ARPES-measured Fermi surface (FS) maps of Co 0.325 ⁢TaS 2 show two hexagonal and one circular holelike FSs around Γ, which matches well with the triple- Q state by taking into account the contribution of nesting vectors occurring between Co 3⁢𝑑 and Ta 5⁢𝑑 orbitals. In the case of Co 0.340 ⁢TaS 2 , a new electron pocket around K appears, and the FS geometry changes as a result of the correlation effect of Co 4 ⁢S 18 tripods forming in the system. The magnetic susceptibility calculations based on the charge-self-consistent DFT+DMFT band structures and the random phase approximation indicate that the most stable magnetic ordering vector (1/2,0,0) split into (1/6,0,0) and (1/2,0,0), which is consistent with the magnetic phase transition around 𝑥=1/3 in Co 𝑥 ⁢TaS 2 .

Fermi surface↗
NASA NTRS1984

Theoretical study of electron correlation effects in transition metal dimers

Introduction of partially localized orbitals is shown to reduce the number of terms needed to describe the bonding in transition metal clusters. Using this formalism, it is possible to compute the various intra- and inter-atomic electron correlation contributions to the bond energy. Calculations demonstrate the relative importance of several kinds of electron correlation terms involving the 3p, 3d, and 4s electrons. Improved interaction potentials are obtained for the dimers V(2) and Cr(2) when additional correlation is added to the CAS SCF results of Walch, Bauschlicher, Roos, and Nelin (1983).

Das, G. P.↗
DOE OSTI2020

Compression of curium pyrrolidine-dithiocarbamate enhances covalency

Curium is unique in the actinide series because its half-filled 5f 7 shell has lower energy than other 5f n configurations, rendering it both redox-inactive and resistant to forming chemical bonds that engage the 5f shell. This is even more pronounced in gadolinium, curium’s lanthanide analogue, owing to the contraction of the 4f orbitals with respect to the 5f orbitals. However, at high pressures metallic curium undergoes a transition from localized to itinerant 5f electrons. This transition is accompanied by a crystal structure dictated by the magnetic interactions between curium atoms. Therefore, the question arises of whether the frontier metal orbitals in curium(III)–ligand interactions can also be modified by applying pressure, and thus be induced to form metal–ligand bonds with a degree of covalency. Here we report experimental and computational evidence for changes in the relative roles of the 5f/6d orbitals in curium–sulfur bonds in [Cm(pydtc) 4 ] – (pydtc, pyrrolidinedithiocarbamate) at high pressures (up to 11 gigapascals). We compare these results to the spectra of [Nd(pydtc) 4 ] – and of a Cm(III) mellitate that possesses only curium–oxygen bonds. Compared with the changes observed in the [Cm(pydtc) 4 ] – spectra, we observe smaller changes in the f–f transitions in the [Nd(pydtc) 4 ] – absorption spectrum and in the f–f emission spectrum of the Cm(III) mellitate upon pressurization, which are related to the smaller perturbation of the nature of their bonds. These results reveal that the metal orbital contributions to the curium–sulfur bonds are considerably enhanced at high pressures and that the 5f orbital involvement doubles between 0 and 11 gigapascal. Furthermore, our work implies that covalency in actinides is complex even when dealing with the same ion, but it could guide the selection of ligands to study the effect of pressure on actinide compounds.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Electronic structure in a transition metal dipnictide TaAs 2

Abstract The family of transition-metal dipnictides has been of theoretical and experimental interest because this family hosts topological states and extremely large magnetoresistance (MR). Recently, T a A s 2 , a member of this family, has been predicted to support a topological crystalline insulating state. Here, by using high-resolution angle-resolved photoemission spectroscopy (ARPES), we reveal both closed and open pockets in the metallic Fermi surface (FS) and linearly dispersive bands on the ( 2 ‾ 01 ) surface, along with the presence of extreme MR observed from magneto-transport measurements. A comparison of the ARPES results with first-principles computations shows that the linearly dispersive bands on the measured surface of T a A s 2 are trivial bulk bands. The absence of symmetry-protected surface state on the ( 2 ‾ 01 ) surface indicates its topologically dark nature. The presence of open FS features suggests that the open-orbit fermiology could contribute to the extremely large MR of T a A s 2 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Magnetic imaging and statistical analysis of the metamagnetic phase transition of FeRh with electron spins in diamond

Magnetic imaging based on nitrogen-vacancy (NV) centers in diamond has emerged as a powerful tool for probing magnetic phenomena in fields ranging from biology to physics. A key strength of NV sensing is its local-probe nature, enabling high-resolution spatial images of magnetic stray fields emanating from a sample. However, this local character can also form a drawback for analyzing the global properties of a system, such as a phase transition temperature. Here, we address this challenge by using statistical analyses of magnetic-field maps to characterize the first-order temperature-driven metamagnetic phase transition from the antiferromagnetic to the ferromagnetic state in FeRh. After imaging the phase transition and identifying the regimes of nucleation, growth, and coalescence of ferromagnetic domains, we statistically characterize the spatial magnetic-field maps to extract the transition temperature and thermal hysteresis width. By analyzing the spatial correlations of the maps in relation to the magnetocrystalline anisotropy and external magnetic field, we detect a reorientation of domain walls across the phase transition. The employed statistical approach can be extended to the study of other magnetic phenomena with NV magnetometry or other sensing techniques.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Mixed excitonic nature in water-oxidized BiVO 4 surfaces with defects

BiVO 4 is a promising photocatalyst for efficient water oxidation, with surface reactivity determined by the structure of active catalytic sites. Surface oxidation in the presence of oxygen vacancies induces electron localization, suggesting an atomistic route to improve the charge transfer efficiency within the catalytic cycle. In this paper, we study the effect of oxygen vacancies on the electronic and optical properties at BiVO 4 surfaces upon water oxidation. We use density functional theory and many-body perturbation theory to explore the change in the electronic and quasiparticle energy levels and to evaluate the electron-hole coupling as a function of the underlying structure. Further we show that while the presence of defects alters the atomic structure and largely modifies the wave-function nature, leading to defect-localized states at the quasiparticle gap region, the optical excitations remain largely unchanged due to the substantial hybridization of defect and nondefect electron-hole transitions. Our findings suggest that defect-induced surface oxidation supports improved electron transport, both through bound and tunable electronic states and via a mixed nature of the optical transitions, expected to reduce electron-hole defect trapping.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Direct Demonstration of Unified Brønsted-Evans-Polanyi Relationships for Proton-Coupled Electron Transfer Reactions on Transition Metal Surfaces

Brønsted-Evans-Polanyi (BEP) relationships, which relate elementary reaction barriers to reaction thermodynamics, have long been discussed in electrochemical science, but demonstration of their existence in heterogeneous electrocatalysis, across many different catalyst surfaces and voltages, is lacking. In this contribution, the BEP model is demonstrated to describe both multiple catalyst surfaces and variable voltages in electrochemical environments. For proton-coupled electron transfer (PCET) to surface nitrogen (N*) and nitric oxide (NO*), reaction energies and activation barriers are calculated using Density Functional Theory (DFT) on a parallel plate capacitor model at three different potentials and for nine different transition metal surfaces. Linear BEP relations that describe all potentials and catalyst surfaces are obtained for these elementary reactions, and the slopes of the correlations are shown to be directly related to the fractional coordinate of the transition states (FCTS) of the reactions. The results, which are explained in terms of Marcus Theory, prove a direct equivalence between unified BEP coefficients, describing both variable catalyst surfaces and voltages, and electrochemical symmetry factors and provide a straightforward means of estimating this quantify for PCET reactions on transition metal surfaces. The resulting relationships, in turn, could lead to predictions of electrocatalytic reactivity trends of enhanced accuracy and efficiency.

Electrochemistry↗