Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “VALENCE”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,279 records · Page 71
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Position-sensitive change in the transition metal L-edge fine structures

Studying the structure and composition of solid-state materials on the atomic scale has become nearly routine in transmission electron microscopy with the development of novel electron optics and electron sources. In particular, with spatial resolutions better than 0.1 nm and energy resolution smaller than 100 meV, the stoichiometry, bonding, and coordination can now be examined on similar scales. Aberration-corrected scanning transmission electron microscopy and electron energy-loss spectroscopy (EELS) have played a crucial role in identifying charge ordering, valence, and as spin state transitions in transition metal perovskite oxides. In this letter, we investigate the effects of ever-decreasing electron-probe sizes on the measured near-edge fine-structure of the transition metal core-loss edge using EELS. We find that for certain transition metal perovskites, the position of the electron probe with respect to the atomic column is crucial in determining the correct valence state. Several reasons for the observed position-sensitive EELS fine-structure are discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Ionization and photofragmentation of Ru{sub 3}(CO){sub 12} and Os{sub 3}(CO){sub 12}

In this paper, we use a combination of photoelectron spectroscopy, mass spectrometry, and density functional theory calculations to get a detailed understanding of valence single and double ionization and the subsequent dissociation processes. This is exemplified on benchmark systems, trimetallo-dodecacarbonyls M{sub 3}(CO){sub 12} with M = Ru, Os, where the energy remaining in the molecule after photoionization can be retrieved by measuring the degree of fragmentation of the molecular ion. The intensity of different mass peaks can thus be directly related to ionization cross sections obtained by photoelectron spectroscopy. We find that the M—CO dissociation energy rises as the number of CO ligands decreases due to dissociation. Moreover, ionization of the CO ligands has a higher cross section than that of the metal center for both single and double ionization. After advanced fragmentation, a CO bond can break and the carbon atom remains bonded to the metal core. In addition, we found that the valence ionization cross sections of M{sub 3}(CO){sub 12} are maximal at about 40 eV photon energy thus showing a more pronounced shape resonance than Ru and Os-complexes with a single metal atom center. Finally, an np → nd giant resonance absorption causes a significant increase of the ionization cross section above 50 eV for Ru{sub 3}(CO){sub 12}.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Many-body ab initio diffusion quantum Monte Carlo applied to the strongly correlated oxide NiO

We present a many-body diffusion quantum Monte Carlo (DMC) study of the bulk and defect properties of NiO. We find excellent agreement with experimental values, within 0.3%, 0.6%, and 3.5% for the lattice constant, cohesive energy, and bulk modulus, respectively. The quasiparticle bandgap was also computed, and the DMC result of 4.72 (0.17) eV compares well with the experimental value of 4.3 eV. Furthermore, DMC calculations of excited states at the L, Z, and the gamma point of the Brillouin zone reveal a flat upper valence band for NiO, in good agreement with Angle Resolved Photoemission Spectroscopy results. To study defect properties, we evaluated the formation energies of the neutral and charged vacancies of oxygen and nickel in NiO. A formation energy of 7.2 (0.15) eV was found for the oxygen vacancy under oxygen rich conditions. For the Ni vacancy, we obtained a formation energy of 3.2 (0.15) eV under Ni rich conditions. These results confirm that NiO occurs as a p-type material with the dominant intrinsic vacancy defect being Ni vacancy.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Band alignment and photon extraction studies of Na-doped MgZnO/Ga-doped ZnO heterojunction for light-emitter applications

Ultraviolet photoelectron spectroscopy is carried out to measure the energy discontinuity at the interface of p-type Na-doped MgZnO (NMZO)/n-type Ga-doped ZnO (GZO) heterojunction grown by dual ion beam sputtering. The offset values at valence band and conduction band of NMZO/GZO heterojunction are calculated to be 1.93 and −2.36 eV, respectively. The p-type conduction in NMZO film has been confirmed by Hall measurement and band structure. Moreover, the effect of Ar{sup +} ion sputtering on the valence band onset values of NMZO and GZO thin films has been investigated. This asymmetric waveguide structure formed by the lower refractive index of GZO than that of NMZO indicates that easy extraction of photons generated in GZO through the NMZO layer into free space. The asymmetric waveguide structure has potential applications to produce ZnO-based light emitters with high extraction efficiency.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Effects of local field and inherent strain in reflectance anisotropy spectra of A{sup III}B{sup V} semiconductors with naturally oxidized surfaces

Reflectance anisotropy (RA) spectra of naturally oxidized (001) surfaces of GaAs and InAs crystals are measured for photon energies from 1.5 up to 5.5 eV. The differential high-accuracy RA spectra reveal features substantially different from those caused by either a reconstruction of clean surface or a built-in near-surface electric field. Models of atomic structure with anisotropic transition layers of excess arsenic atoms specific for GaAs(001)/oxide and InAs(001)/oxide interfaces are proposed. In conformity with these models, a general theory of reflectance anisotropy is developed for semiconductor/oxide interfaces within the Green's function technique. The theory takes into account the combined effect of local field due to interface dipoles and of intrinsic near-surface strain of the crystal. Measured RA spectra are analyzed in the model of valence-bond dipoles occupying a rectangular lattice in a multilayer medium. Comparing the measured and calculated spectra, we conclude that RA spectra of oxidized GaAs(001) and InAs(001) surfaces are simultaneously influenced by interface and near-surface anisotropies. The former is responsible for the broad-band spectral features which are associated with polarizability of the valence bonds attached to As atoms at the crystal/oxide interface. The near-surface anisotropy is due to inherent uniaxial straining the near-surface region of crystal. The effect of strain on RA spectra is experimentally and theoretically substantiated for GaAs crystal wafers undergone a uniaxial applied stress. Basically, this work results in the following. It establishes the physical nature of different levels of RA spectra observed in a majority of papers, but never analyzed. It demonstrates how the studied features of RA spectra could be applied for optical characterization of strained interfaces and atomic layers.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Cobalt (II) oxide and nickel (II) oxide alloys as potential intermediate-band semiconductors: A theoretical study

Solar cells based on single pn junctions, employing single-gap semiconductors can ideally achieve efficiencies as high as 34%. Developing solar cells based on intermediate-band semiconductors (IBSCs), which can absorb light across multiple band gaps, is a possible way to defy this theoretical limit and achieve efficiencies as high as 60%. Here, we use first principles quantum mechanics methods and introduce CoO and Co{sub 0.25}Ni{sub 0.75}O as possible IBSCs. We show that the conduction band in both of these materials is divided into two distinct bands separated by a band gap. We further show that the lower conduction band (i.e., the intermediate band) is wider in Co{sub 0.25}Ni{sub 0.75}O compared with CoO. This should enhance light absorption from the valence band edge to the intermediate band, making Co{sub 0.25}Ni{sub 0.75}O more appropriate for use as an IBSC. Our findings provide the basis for future attempts to partially populate the intermediate band and to reduce the lower band gap in Co{sub 0.25}Ni{sub 0.75}O in order to enhance the potential of this material for use in IBSC solar cell technologies. Furthermore, with proper identification of heterojunctions and dopants, CoO and Co{sub 0.25}Ni{sub 0.75}O could be used in multi-color light emitting diode and laser technologies.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Importance of hydrophobic traps for proton diffusion in lyotropic liquid crystals

The diffusion of protons in self-assembled systems is potentially important for the design of efficient proton exchange membranes. In this work, we study proton dynamics in a low-water content, lamellar phase of a sodium-carboxylate gemini surfactant/water system using computer simulations. The hopping of protons via the Grotthuss mechanism is explicitly allowed through the multi-state empirical valence bond method. We find that the hydronium ion is trapped on the hydrophobic side of the surfactant-water interface, and proton diffusion then proceeds by hopping between surface sites. The importance of hydrophobic traps is surprising because one would expect the hydronium ions to be trapped at the charged headgroups. The physics illustrated in this system should be relevant to the proton dynamics in other amphiphilic membrane systems, whenever there exist exposed hydrophobic surface regions.

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

Synthesis of a mixed-valent tin nitride and considerations of its possible crystal structures

Recent advances in theoretical structure prediction methods and high-throughput computational techniques are revolutionizing experimental discovery of the thermodynamically stable inorganic materials. Metastable materials represent a new frontier for these studies, since even simple binary non-ground state compounds of common elements may be awaiting discovery. However, there are significant research challenges related to non-equilibrium thin film synthesis and crystal structure predictions, such as small strained crystals in the experimental samples and energy minimization based theoretical algorithms. Here, we report on experimental synthesis and characterization, as well as theoretical first-principles calculations of a previously unreported mixed-valent binary tin nitride. Thin film experiments indicate that this novel material is N-deficient SnN with tin in the mixed II/IV valence state and a small low-symmetry unit cell. Theoretical calculations suggest that the most likely crystal structure has the space group 2 (SG2) related to the distorted delafossite (SG166), which is nearly 0.1 eV/atom above the ground state SnN polymorph. This observation is rationalized by the structural similarity of the SnN distorted delafossite to the chemically related Sn{sub 3}N{sub 4} spinel compound, which provides a fresh scientific insight into the reasons for growth of polymorphs of metastable materials. In addition to reporting on the discovery of the simple binary SnN compound, this paper illustrates a possible way of combining a wide range of advanced characterization techniques with the first-principle property calculation methods, to elucidate the most likely crystal structure of the previously unreported metastable materials.

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

Ultrafast and band-selective Auger recombination in InGaN quantum wells

In InGaN quantum well based light-emitting diodes, Auger recombination is believed to limit the quantum efficiency at high injection currents. Here in this paper, we report the direct observation of carrier loss from Auger recombination on a sub-picosecond timescale in a single InGaN quantum well using time-resolved photoemission. Selective excitations of different valence sub-bands reveal that the Auger rate constant decreases by two orders of magnitude as the effective hole mass decreases, confirming the critical role of momentum conservation.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

High temperature and low pressure chemical vapor deposition of silicon nitride on AlGaN: Band offsets and passivation studies

In this work, we employed X-ray photoelectron spectroscopy to determine the band offsets and interface Fermi level at the heterojunction formed by stoichiometric silicon nitride deposited on Al{sub x}Ga{sub 1-x}N (of varying Al composition “x”) via low pressure chemical vapor deposition. Silicon nitride is found to form a type II staggered band alignment with AlGaN for all Al compositions (0 ≤ x ≤ 1) and present an electron barrier into AlGaN even at higher Al compositions, where E{sub g}(AlGaN) > E{sub g}(Si{sub 3}N{sub 4}). Further, no band bending is observed in AlGaN for x ≤ 0.6 and a reduced band bending (by ∼1 eV in comparison to that at free surface) is observed for x > 0.6. The Fermi level in silicon nitride is found to be at 3 eV with respect to its valence band, which is likely due to silicon (≡Si{sup 0/−1}) dangling bonds. The presence of band bending for x > 0.6 is seen as a likely consequence of Fermi level alignment at Si{sub 3}N{sub 4}/AlGaN hetero-interface and not due to interface states. Photoelectron spectroscopy results are corroborated by current-voltage-temperature and capacitance-voltage measurements. A shift in the interface Fermi level (before band bending at equilibrium) from the conduction band in Si{sub 3}N{sub 4}/n-GaN to the valence band in Si{sub 3}N{sub 4}/p-GaN is observed, which strongly indicates a reduction in mid-gap interface states. Hence, stoichiometric silicon nitride is found to be a feasible passivation and dielectric insulation material for AlGaN at any composition.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Hubbard physics in the PAW GW approximation

It is demonstrated that the signatures of the Hubbard Model in the strongly interacting regime can be simulated by modifying the screening in the limit of zero wavevector in Projector-Augmented Wave GW calculations for systems without significant nesting. This modification, when applied to the Mott insulator CuO, results in the opening of the Mott gap by the splitting of states at the Fermi level into upper and lower Hubbard bands, and exhibits a giant transfer of spectral weight upon electron doping. The method is also employed to clearly illustrate that the M{sub 1} and M{sub 2} forms of vanadium dioxide are fundamentally different types of insulator. Standard GW calculations are sufficient to open a gap in M{sub 1} VO{sub 2}, which arise from the Peierls pairing filling the valence band, creating homopolar bonds. The valence band wavefunctions are stabilized with respect to the conduction band, reducing polarizability and pushing the conduction band eigenvalues to higher energy. The M{sub 2} structure, however, opens a gap from strong on-site interactions; it is a Mott insulator.

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

Communication: XUV transient absorption spectroscopy of iodomethane and iodobenzene photodissociation

Time-resolved extreme ultraviolet (XUV) transient absorption spectroscopy of iodomethane and iodobenzene photodissociation at the iodine pre-N{sub 4,5} edge is presented, using femtosecond UV pump pulses and XUV probe pulses from high harmonic generation. For both molecules the molecular core-to-valence absorption lines fade immediately, within the pump-probe time-resolution. Absorption lines converging to the atomic iodine product emerge promptly in CH{sub 3}I but are time-delayed in C{sub 6}H{sub 5}I. We attribute this delay to the initial π → σ{sup *} excitation in iodobenzene, which is distant from the iodine reporter atom. We measure a continuous shift in energy of the emerging atomic absorption lines in CH{sub 3}I, attributed to relaxation of the excited valence shell. An independent particle model is used to rationalize the observed experimental findings.

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

Isovalent substitutes play in different ways: Effects of isovalent substitution on the thermoelectric properties of CoSi{sub 0.98}B{sub 0.02}

Boron-added CoSi, CoSi{sub 0.98}B{sub 0.02}, possesses a very high thermoelectric power factor of 60 μW cm{sup −1} K{sup −2} at room temperature, which is among the highest power factors that have ever been reported for near-room-temperature thermoelectric applications. Since the electrical properties of this material have been tuned properly, isovalent substitution for its host atoms is intentionally employed to reduce the lattice thermal conductivity while maintaining the electronic properties unchanged. In our previous work, the effect of Rh substitution for Co atoms on the thermoelectric properties of CoSi{sub 0.98}B{sub 0.02} has been studied. Here, we present a study of the substitution of Ge for Si atoms in this compound. Even though Ge and Rh are isovalent with their corresponding host atoms, they play different roles in determining the electrical and thermal transport properties. Through the evaluation of the lattice thermal conductivity by the Debye approximation and the comparison between the high-temperature Seebeck coefficients, we propose that Rh substitution leads to a further overlapping of the conduction and the valence bands, while Ge substitution only shifts the Fermi level upward into the conduction band. Our results show that the influence of isovalent substitution on the electronic structure cannot be ignored when the alloying method is used to improve thermoelectric properties.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Correlation consistent basis sets for lanthanides: The atoms La–Lu

Using the 3rd-order Douglas-Kroll-Hess (DKH3) Hamiltonian, all-electron correlation consistent basis sets of double-, triple-, and quadruple-zeta quality have been developed for the lanthanide elements La through Lu. Basis sets designed for the recovery of valence correlation (defined here as 4f5s5p5d6s), cc-pVnZ-DK3, and outer-core correlation (valence + 4s4p4d), cc-pwCVnZ-DK3, are reported (n = D, T, and Q). Systematic convergence of both Hartree-Fock and correlation energies towards their respective complete basis set (CBS) limits are observed. Benchmark calculations of the first three ionization potentials (IPs) of La through Lu are reported at the DKH3 coupled cluster singles and doubles with perturbative triples, CCSD(T), level of theory, including effects of correlation down through the 4s electrons. Spin-orbit coupling is treated at the 2-component HF level. After extrapolation to the CBS limit, the average errors with respect to experiment were just 0.52, 1.14, and 4.24 kcal/mol for the 1st, 2nd, and 3rd IPs, respectively, compared to the average experimental uncertainties of 0.03, 1.78, and 2.65 kcal/mol, respectively. The new basis sets are also used in CCSD(T) benchmark calculations of the equilibrium geometries, atomization energies, and heats of formation for Gd{sub 2}, GdF, and GdF{sub 3}. Except for the equilibrium geometry and harmonic frequency of GdF, which are accurately known from experiment, all other calculated quantities represent significant improvements compared to the existing experimental quantities. With estimated uncertainties of about ±3 kcal/mol, the 0 K atomization energies (298 K heats of formation) are calculated to be (all in kcal/mol): 33.2 (160.1) for Gd{sub 2}, 151.7 (−36.6) for GdF, and 447.1 (−295.2) for GdF{sub 3}.

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

Asymmetric band offsets in silicon heterojunction solar cells: Impact on device performance

Amorphous/crystalline silicon interfaces feature considerably larger valence than conduction band offsets. In this article, we analyze the impact of such band offset asymmetry on the performance of silicon heterojunction solar cells. To this end, we use silicon suboxides as passivation layers—inserted between substrate and (front or rear) contacts—since such layers enable intentionally exacerbated band-offset asymmetry. Investigating all topologically possible passivation layer permutations and focussing on light and dark current-voltage characteristics, we confirm that to avoid fill factor losses, wider-bandgap silicon oxide films (of at least several nanometer thin) should be avoided in hole-collecting contacts. As a consequence, device implementation of such films as window layers—without degraded carrier collection—demands electron collection at the front and hole collection at the rear. Furthermore, at elevated operating temperatures, once possible carrier transport barriers are overcome by thermionic (field) emission, the device performance is mainly dictated by the passivation of its surfaces. In this context, compared to the standard amorphous silicon layers, the wide-bandgap oxide layers applied here passivate remarkably better at these temperatures, which may represent an additional benefit under practical operation conditions.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

Infrared absorption and visible transparency in heavily doped p-type BaSnO 3

The recent experimental work shows that perovskite BaSnO 3 can be heavily doped by K to become a stable p-type semiconductor. Here, we find that p-type perovskite BaSnO 3 retains transparency for visible light while absorbing strongly in the infrared below 1.5 eV. The origin of the remarkable optical transparency even with heavy doping is that the interband transitions that are enabled by empty states at the top of the valence band are concentrated mainly in the energy range from 0.5 to 1.5 eV, i.e., not extending past the near IR. In contrast to n-type, the Burstein-Moss shift is slightly negative, but very small reflecting the heavier valence bands relative to the conduction bands.

36 MATERIALS SCIENCE↗
DOE OSTI2017

The effects of core-level broadening in determining band alignment at the epitaxial SrTiO 3 (001)/ p -Ge(001) heterojunction

Chemical effects at the surface and interface can broaden core-level spectra in X-ray photoemission for thin-film heterojunctions, as can electronic charge redistributions. We explore these effects and their influence on the measurement of valence and conduction band offsets at the epitaxial SrTiO 3 (001)/p-Ge(001) heterojunction. Here, we observe a clear broadening in Ge 3d and Sr 3d core-level X-ray photoelectron spectra relative to those of clean, bulk Ge(001), and homoepitaxial SrTiO 3 (001), respectively. Angle-resolved measurements indicate that this broadening is driven primarily by chemical shifts associated with surface hydroxylation, with built-in potentials playing only a minor role. The impact of these two interpretations on the valence band offset is significant on the scale of transport energetics, amounting to a difference of 0.2 eV.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Gaussian basis sets for use in correlated molecular calculations. XI. Pseudopotential-based and all-electron relativistic basis sets for alkali metal (K–Fr) and alkaline earth (Ca–Ra) elements

New correlation consistent basis sets based on pseudopotential (PP) Hamiltonians have been developed from double- to quintuple-zeta quality for the late alkali (K–Fr) and alkaline earth (Ca–Ra) metals. These are accompanied by new all-electron basis sets of double- to quadruple-zeta quality that have been contracted for use with both Douglas-Kroll-Hess (DKH) and eXact 2-Component (X2C) scalar relativistic Hamiltonians. Sets for valence correlation (ms), cc-pVnZ-PP and cc-pVnZ-(DK,DK3/X2C), in addition to outer-core correlation [valence + (m-1)sp], cc-p(w)CVnZ-PP and cc-pwCVnZ-(DK,DK3/X2C), are reported. The –PP sets have been developed for use with small-core PPs [I. S. Lim et al., J. Chem. Phys. 122, 104103 (2005) and I. S. Lim et al., J. Chem. Phys. 124, 034107 (2006)], while the all-electron sets utilized second-order DKH Hamiltonians for 4s and 5s elements and third-order DKH for 6s and 7s. The accuracy of the basis sets is assessed through benchmark calculations at the coupled-cluster level of theory for both atomic and molecular properties. Not surprisingly, it is found that outer-core correlation is vital for accurate calculation of the thermodynamic and spectroscopic properties of diatomic molecules containing these elements.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗