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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 739 records · Page 41
All sourcesNASA NTRSDOE OSTI
DOE OSTI2010

Multi-Center Electronic Structure Calculations for Plasma Equation of State

We report on an approach for computing electronic structure utilizing solid-state multi-center scattering techniques, but generalized to finite temperatures to model plasmas. This approach has the advantage of handling mixtures at a fundamental level without the imposition of ad hoc continuum lowering models, and incorporates bonding and charge exchange, as well as multi-center effects in the calculation of the continuum density of states.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Negative ion photoelectron spectroscopy of P 2 N 3 – : Electron affinity and electronic structures of P 2 N 3 ˙

The recent successful synthesis of P 2 N 3 – , a planar all-inorganic aromatic molecule, represents a breakthrough in inorganic chemistry, because, like its isolobal counterparts C 5 H 5 – and cyclo-P 5 – , P 2 N 3 – has potential to serve as a new ligand for transition metals and a building block in solid-state molecular architectures. In light of its importance, we report here a negative ion photoelectron spectroscopy (NIPES) and ab initio study of P 2 N 3 – , to investigate the electronic structures of P 2 N 3 – and its neutral P 2 N 3 • radical. The adiabatic detachment energy of P 2 N 3 – (electron affinity of P 2 N 3 •) was determined to be 3.765 ± 0.010 eV, indicating high stability for the P 2 N 3 – anion. Ab initio electronic structure calculations reveal five low-lying electronic states in the neutral P 2 N 3 • radical. Calculation of the Franck-Condon factors (FCFs) for each anion-to-neutral electronic transition and comparison of the resulting simulated NIPE spectrum with the vibrational structure in the observed spectrum allows the first four excited states of P 2 N 3 • to be determined to lie 6.2, 6.7, 11.5, and 22.8 kcal/mol -1 above the ground state of the radical, which is found to be a 6π-electron, 2 A 1 , σ state.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Atomic and electron structure of reconstructed (111) surface in ZnSe and CdSe crystals

The atomic and electron structure of four variants of polar (111)-(2 × 2) surfaces in ZnSe and CdSe terminated by a cation, namely, the ideal, relaxed, reconstructed, and relaxed after reconstruction surfaces, are calculated for the first time from the first principles. The surface is simulated by a film with a thickness of 12 atomic layers and a vacuum gap of ~16 Å in the layered superlattice approximation. Four fictitious hydrogen atoms with a charge of 0.5 electrons each are added for closing dangling Se bonds on the opposite side of the film. Ab initio calculations are performed using the QUANTUM ESPRESSO software based on the density functional theory. It is shown that relaxation results in splitting of atomic layers. We calculate and analyze the band structures and total and layer-wise densities of electron states for four variants of the surface.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Investigating the electronic structure of high explosives with X-ray Raman spectroscopy

Abstract We investigate the sensitivity and potential of a synergistic experiment-theory X-ray Raman spectroscopy (XRS) methodology on revealing and following the static and dynamic electronic structure of high explosive molecular materials. We show that advanced ab-initio theoretical calculations accounting for the core-hole effect based on the Bethe-Salpeter Equation (BSE) approximation are critical for accurately predicting the shape and the energy position of the spectral features of C and N core-level spectra. Moreover, the incident X-ray dose typical XRS experiments require can induce, in certain unstable structures, a prominent radiation damage at room temperature. Upon developing a compatible cryostat module for enabling cryogenic temperatures ( $$\approx$$ ≈ 10 K) we suppress the radiation damage and enable the acquisition of reliable experimental spectra in excellent agreement with the theory. Overall, we demonstrate the high sensitivity of the recently available state-of-the-art X-ray Raman spectroscopy capabilities in characterizing the electronic structure of high explosives. At the same time, the high accuracy of the theoretical approach may enable reliable identification of intermediate structures upon rapid chemical decomposition during detonation. Considering the increasing availability of X-ray free-electron lasers, such a combined experiment-theory approach paves the way for time-resolved dynamic studies of high explosives under detonation conditions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1988

Scanning tunneling microscopy characterization of the geometric and electronic structure of hydrogen-terminated silicon surfaces

Scanning tunneling microscopy (STM) methods are used to characterize hydrogen-terminated Si surfaces prepared by a novel method. The surface preparation method is used to expose the Si-SiO2 interface. STM images directly reveal the topographic structure of the Si-SiO2 interface. The dependence of interface topography on oxide preparation conditions observed by STM is compared to the results of conventional surface characterization methods. Also, the electronic structure of the hydrogen-terminated surface is studied by STM spectroscopy. The near-ideal electronic structure of this surface enables direct tunnel spectroscopy measurements of Schottky barrier phenomena. In addition, this method enables probing of semiconductor subsurface properties by STM.

Kaiser, W. J.↗
DOE OSTI2025

Dichotomous Temperature Response in the Electronic Structure of Epitaxially Grown Altermagnet MnTe

The altermagnet candidate MnTe has recently gained significant interest due to its unconventional magnetic ordering. One of the key features of altermagnetism is the momentum-dependent spin-split band and its temperaturedependent evolution. Yet a fully momentum-resolved experimental investigation, including out-of-plane direction, is still lacking. Here, we systematically investigate the electronic structure of epitaxially grown MnTe by using angle-resolved photoemission spectroscopy (ARPES). Our photon-energy-dependent ARPES data reveal significant out-of-plane dispersions consistent with previous theoretical calculations. More interestingly, we identify two distinct temperature-dependent electronic band structure evolutions at different out-of-plane momentum positions: momentum-dependent energy shifts at the nodal plane and substantial spectral weight suppression at the off-nodal plane. These findings may suggest the importance of considering both the itinerant and localized nature of the magnetic ordering and momentum-dependent interactions. Our work provides crucial insights into the complex correlation between momentum, temperature, and electronic structure in MnTe, contributing to a deeper understanding of altermagnetism.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2015

Effect of hydrostatic pressure and uniaxial strain on the electronic structure of Pb 1-x Sn x Te

The electronic structure of Pb 1–x Sn x Te is studied by using the relativistic Korringa-Kohn-Rostoker Green function method in the framework of density functional theory. For all concentrations x, Pb 1–x Sn x Te is a direct semiconductor with a narrow band gap. In contrast to pure lead telluride, tin telluride shows an inverted band characteristic close to the Fermi energy. It will be shown that this particular property can be tuned, first, by alloying PbTe and SnTe and, second, by applying hydrostatic pressure or uniaxial strain. Furthermore, the magnitude of strain needed to switch between the regular and inverted band gap can be tuned by the alloy composition. In conclusion, there is a range of potential usage of Pb 1–x Sn x Te for spintronic applications.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Investigation on Electronic Structures and Magnetic Properties of (Mn, Ga) Co-doped SnO{sub 2}

The electronic structures and magnetic properties of (Mn, Ga) co-doped SnO{sub 2} are studied by the first-principles calculations in full-potential linearized augmented plane wave formalism within generalized gradient approximations. We found that the doped system favors the ferromagnetic state and room-temperature ferromagnetism can be expected in it. The origin of ferromagnetism can be attributed to p-d exchange interactions as Mn-O-Mn. Ga acts as acceptor in (Mn, Ga) co-doped SnO{sub 2} host and can donate one hole, increasing the carrier concentration and thus leading to a more stable ferromagnetic state. These results suggest a recipe obtaining promising dilute magnetic semiconductor by doping nonmagnetic elements in SnO{sub 2} matrix.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

FP-LAPW study of structural, electronic, elastic, mechanical and thermal properties of AlFe intermetallic

The structural, electronic, elastic, mechanical and thermal properties of AlFe intermetallic compound in B{sub 2}-type (CsCl) structure have been investigated using first-principles calculations. The exchange-correlation term was treated within generalized gradient approximation. Ground state properties i.e. lattice constants (a{sub 0}), bulk modulus (B) and first-order pressure derivative of bulk modulus (B’) are presented. The density of states are derived which show the metallic character of present compound. Our results for C{sub 11}, C{sub 12} and C{sub 44} agree well with previous theoretical data. Using Pugh’s criteria (B/G{sub H} < 1.75), brittle character of AlFe is satisfied. In addition shear modulus (G{sub H}), Young’s modulus (E), sound wave velocities and Debye temperature (θ{sub D}) have also been estimated.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Electronic structure, surface doping, and optical response in epitaxial WSe 2 thin films

High quality WSe 2 films have been grown on bilayer graphene (BLG) with layer-by-layer control of thickness using molecular beam epitaxy. The combination of angle-resolved photoemission, scanning tunneling microscopy/spectroscopy, and optical absorption measurements reveal the atomic and electronic structures evolution and optical response of WSe 2 /BLG. We observe that a bilayer of WSe 2 is a direct bandgap semiconductor, when integrated in a BLG-based heterostructure, thus shifting the direct–indirect band gap crossover to trilayer WSe 2 . In the monolayer limit, WSe 2 shows a spin-splitting of 475 meV in the valence band at the K point, the largest value observed among all the MX 2 (M = Mo, W; X = S, Se) materials. The exciton binding energy of monolayer-WSe 2 /BLG is found to be 0.21 eV, a value that is orders of magnitude larger than that of conventional three-dimensional semiconductors, yet small as compared to other two-dimensional transition metal dichalcogennides (TMDCs) semiconductors. Lastly, our finding regarding the overall modification of the electronic structure by an alkali metal surface electron doping opens a route to further control the electronic properties of TMDCs.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Temperature-modulated electronic structure of graphene on SiC: Possible roles of electron-electron interaction and strain

We have iobserved the electron band structure of graphene epitaxially grown on an SiC substrate using angle-resolved photoemission spectroscopy. The conical energy spectrum of graphene exhibits a minimum slope at ~50 K, which is accompanied by the minimum separation between its two branches. These observations provide a viable route towards the engineering of the electronic properties of graphene using temperature, while the latter suggests a possible evidence of gap engineering via strain induced by the substrate and modulated by temperature.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Effect of isovalent non-magnetic Fe-site doping on the electronic structure and spontaneous polarization of BiFeO{sub 3}

We report the results of our first-principles calculations on the effect of isovalent, non-magnetic, Al{sup 3+} ion doping on the electronic structure and spontaneous polarization of multiferroic BiFeO{sub 3}. Our calculations reveal that Al{sup 3+} doping in BiFeO{sub 3} results in the reduction of Fe–O–Fe bond angle, leading to the weakening of antiferromagnetic superexchange interaction, further substantiated by the reduction of exchange interaction constant with increasing doping level. Lowering of well-depth is suggestive of reduced switching potential and improved P-E loop with lowered coercivity. Chemical bonding analysis by electron localization function shows that cation–oxygen bonding is of mixed ionic–covalent character, with marginal increase in the covalent character with increasing doping concentration. Large spontaneous polarization of undoped BiFeO{sub 3} is retained with lower doping level (6.25%), while for higher doping content (31.25%), the spontaneous polarization is reduced, primarily due to larger c/a ratio at higher doping level.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Local Electronic Structure and Density of Edge and Facet Atoms at Rh Nanoclusters Self-Assembled on a Graphene Template

The chemical and physical properties of nanoclusters largely depend on their sizes and shapes. This is partly due to finite size effects influencing the local electronic structure of the nanocluster atoms which are located on the nanofacets and on their edges. Here we present a thorough study on graphene-supported Rh nanocluster assemblies and their geometry-dependent electronic structure obtained by combining high-energy resolution core level photoelectron spectroscopy, scanning tunneling microscopy, and density functional theory. We demonstrate the possibility to finely control the morphology and the degree of structural order of Rh clusters grown in register with the template surface of graphene/Ir(111). By comparing measured and calculated core electron binding energies, we identify edge, facet, and bulk atoms of the nanoclusters. We describe how small interatomic distance changes occur while varying the nanocluster size, substantially modifying the properties of surface atoms. The properties of under-coordinated Rh atoms are discussed in view of their importance in heterogeneous catalysis and magnetism.

Chemistry↗
DOE OSTI2018

The Electronic Structure and Spectra of Triphenylamines Functionalized by Phenylethynyl Groups

We study the features of the electronic structure and the IR, UV, and visible spectra of a series of triphenylamines substituted with phenylethynyl groups. The analysis is performed at the level of the density functional theory (DFT) and its nonstationary version in comparison with the experimental data of IR and electron spectroscopy. It is shown that, in the excited state, there is a change in the alternation of single, double, and triple bonds in accordance with the character of bonding and antibonding in the lowest vacant molecular orbital. The gradual introduction of additional phenylethynyl groups does not cause frequency shifts in the IR spectra of the molecules under study, but significantly affects the intensity of the corresponding IR bands. A similar effect is also observed in the electronic-absorption spectra of these compounds. This can be used for optical tuning of triphenylamines as promising materials for organic light-emitting diodes and solar cells.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Phase stability, electronic structures, and superconductivity properties of the BaPb 1 -x Bi x O 3 and Ba 1 -x K x BiO 3 perovskites

With extensive first-principles calculations, we investigate the phase stability,electronic structures, and superconductivity properties of the BaPb 1 -x Bi x O 3 (BPBO) and Ba 1 -x K x BiO 3 (BKBO) perovskites with the cubic (C), tetragonal (T), and orthorhombic (O) phases. Our calculations show that the tetragonal superconducting phases of both perovskites are metastable. However, the orthorhombicphase of the BPBO perovskite in the superconductivity region is only slightly more stable than the tetragonal phase. The small energy difference between the T and O phases and the discontinuous T-to-O phase transition account for the experimentally observed coexistence of the T and O phases. On the other hand, the BKBO perovskite involves a large energy difference between the T and O phases,which induces a low equilibrium temperature of the discontinuous T-to-O phase transition, in agreement with the experimental observation that the tetragonal BKBO is maintained down to low temperatures. Moreover, the electronic structures of both BPBO and BKBO superconductors show a flat band near the Fermi level, which is favorable for superconductivity. Furthermore, we find that the longer the total length of the flat band segment is, the higher the critical temperature of the BPBO or BKBO perovskite is. This key finding could be generalized straightforwardly to other unconventional superconductors and can be used to design and find optimal composition with maximum Tc for new unconventional superconductors.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Density functional theory investigation of the electronic structure and thermoelectric properties of layered MoS{sub 2}, MoSe{sub 2} and their mixed-layer compound

First principles density functional theory calculations were carried out for the 2H-MoQ{sub 2} (Q=S and Se) and their hypothetical mixed-layer compound. Due to the different electronegativities of S and Se atoms on MoQ{sub 2}, the band gap size could be adjusted in mixed-layer compound MoS{sub 2}/MoSe{sub 2}. Also, the indirect band gap in pure MoQ{sub 2} compounds is changed to the pseudo direct band gap in mixed-layer MoS{sub 2}/MoSe{sub 2} which is similar to the monolayer compound. The layer mixing enhances the thermoelectric properties because of the increased density of states around the Fermi level and the decreased band gap size. Therefore, we suggest that this layer mixing approach should be regarded as a useful way to modulate their electronic structures and to improve their thermoelectric properties. -- Graphical abstract: On the basis of density functional calculations we predict that the mixed-layer compounds 2H-MoS{sub 2}/2H-MoSe{sub 2}, in which two different layers 2H-MoS{sub 2} and 2H-MoSe{sub 2}, have enhanced thermoelectric properties because of the increased density of states around the Fermi level and the decreased band gap size. Highlights: • We explored a way of improving TE properties of 2H-MoQ{sub 2} on DFT methods. • The mixed-layer compounds MoS{sub 2}/MoSe{sub 2} have enhanced thermoelectric properties. • This is caused by modulated electronic structure of mixed layer compound. • Layer mixing approach should be regarded as a useful way to improve TE properties.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Electronic structure of nitinol surfaces oxidized by low-energy ion bombardment

We have studied the electronic structure of nitinol exposed to low-energy oxygen-ion bombardment, using x-ray photoemission spectroscopy (XPS) and near-edge x-ray absorption fine structure (NEXAFS) spectroscopy. XPS spectra reveal a gradual transformation of nitinol surfaces into TiO{sub 2} with increased dose of implanted oxygen. No oxidation of Ni atoms has been detected. NEXAFS spectra around O K-edge and Ti L{sub 2,3}-edge, reflecting the element-specific partial density of empty electronic states, exhibit features, which can be attributed to the creation of molecular orbitals, crystal field splitting, and the absence of long-range order, characteristic of the amorphous TiO{sub 2}. Based on these results, we discuss the oxidation kinetics of nitinol under low-energy oxygen-ion bombardment.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗