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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 559 records · Page 31
All sourcesNASA NTRSDOE OSTI
DOE OSTI2010

Electronic Structure and Gas-Phase Behaviour of the Heaviest Elements

Electronic structures and gas-phase adsorption behaviour of the heaviest elements 112, 113 and 114 and of their lighter homologs Hg, Tl and Pb is studied on the basis of ab initio Dirac-Coulomb atomic and four-component Density Functional Theory molecular and cluster calculations. The heaviest elements were shown to have low adsorption enthalpies on Teflon and should, therefore, be well transported through Teflon capillaries from the target chamber to the chemistry set up. Adsorption enthalpies of these elements on the Au(111) surface are predicted as -44.5 kJ/mol, -158.6 kJ/mol and -68.5 kJ/mol, respectively, giving the following sequence in the adsorption temperatures 113>114>112.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Structures, Energetics and Spectroscopic Fingerprints of Water Clusters n = 2-24

Water's function as a universal solvent and its role in mediating several biological functions that are responsible for sustaining life has created tremendous interest in the understanding of its structure at the molecular level.1 Due to the size of the simulation cells and the sampling time needed to compute many macroscopic properties, most of the initial simulations are performed using a classical force field whereas several processes that involve chemistry are subsequently probed with electronic structure based methods. A significant effort has therefore been devoted towards the development of classical force fields for water.2 Clusters of water molecules are useful in probing the intermolecular interactions at the microscopic level as well as providing information about the subtle energy differences that are associated with different bonding arrangements within a hydrogen bonded network. They moreover render a quantitative picture of the nature and magnitude of the various components of the intermolecular interactions such as exchange, dispersion, induction etc. They can finally serve as a vehicle for the study of the convergence of properties with increasing size.

08 HYDROGEN↗
DOE OSTI2015

Optical conductivity of partially oxidized graphene from first principles

We investigate the geometry, electronic structure, and optical properties of partially oxidized graphene using density functional theory. Our calculations show that oxygen atoms are chemisorbed on graphene plane and distort carbon atoms vertically, with almost no change in the in-plane structure. The ground state configurations for different oxygen coverages ranging from 2% to 50% (O/C ratio) are calculated and show the strong tendency of oxygen adatoms to aggregate and form discrete islands on graphene plane. It is found that the opened band gap due to oxygen functionalization depends on the oxygen density and the adsorption configuration. The gap is not significant for oxygen densities lower than 8%. The optical conductivities are calculated in the infrared, visible, and ultraviolet regions and show different characteristic features depending on the degree of oxidation. These results imply that optical measurement techniques can be employed to monitor oxidation (or reduction) process as contact-free methods.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

On the nexus between atom probe microscopy and density functional theory simulations

The capability of precisely determining the atomic identity and position, coupled with an understanding of the inter-atomic interactions is crucial to enabling microstructure-property relationships in modern materials science. Two approaches to the investigation of materials at the atomic-scale are atom probe microscopy (APM) and density functional theory (DFT). On the one hand, APM offers an ultimate in 3D elemental analysis by determining the chemical identity and atomic position in a material. On the other, first principles DFT simulations allow the prediction of atomic structure and, critically, the electronic structure which is the gateway to calculating the real-world properties of materials. Both approaches are undertaken at the atomic-scale, and so are intrinsically complementary. This paper provides a review of the opportunities and challenges in combining these approaches to do materials science. We first summarise the status of these approaches and set out a framework for the current main workflows by which APM experiments inform DFT simulations and vice-versa. We point out certain gaps between the approaches and ways that these may be bridged via a series of case-studies organised into the aforementioned workflow framework.

36 MATERIALS SCIENCE↗
DOE OSTI2014

The electronic structure of the antimony chalcogenide series: Prospects for optoelectronic applications

In this study, density functional theory is used to evaluate the electronic structure of the antimony chalcogenide series. Analysis of the electronic density of states and charge density shows that asymmetric density, or ‘lone pairs’, forms on the Sb{sup III} cations in the distorted oxide, sulphide and selenide materials. The asymmetric density progressively weakens down the series, due to the increase in energy of valence p states from O to Te, and is absent for Sb{sub 2}Te{sub 3}. The fundamental and optical band gaps were calculated and Sb{sub 2}O{sub 3}, Sb{sub 2}S{sub 3} and Sb{sub 2}Se{sub 3} have indirect band gaps, while Sb{sub 2}Te{sub 3} was calculated to have a direct band gap at Γ. The band gaps are also seen to reduce from Sb{sub 2}O{sub 3} to Sb{sub 2}Te{sub 3}. The optical band gap for Sb{sub 2}O{sub 3} makes it a candidate as a transparent conducting oxide, while Sb{sub 2}S{sub 3} and Sb{sub 2}Se{sub 3} have suitable band gaps for thin film solar cell absorbers. - Graphical abstract: A schematic illustrating the interaction between the Sb{sup III} cations and the chalcogenide anions and the change in their respective energy levels down the series. - Highlights: • The electronic structure of the antimony chalcogenide series is modelled using DFT. • Asymmetric density is present on distorted systems and absent on the symmetric telluride system. • Asymmetric density is formed from the mixing of Sb 5s and anion p states, where the anti-bonding combination is stabilised by the Sb 5p states. • The asymmetric density weakens down the series due to the increase in energy of chalcogenide p states. • The increase in energy of the anion p states reduces the fundamental and optical band gaps.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

Communication: Enhanced oxygen reduction reaction and its underlying mechanism in Pd-Ir-Co trimetallic alloys

Based on a combined density functional theory and experimental study, we present that the electrochemical activity of Pd{sub 3}Co alloy catalysts toward oxygen reduction reaction (ORR) can be enhanced by adding a small amount of Ir. While Ir tends to favorably exist in the subsurface layers, the underlying Ir atoms are found to cause a substantial modification in the surface electronic structure. As a consequence, we find that the activation barriers of O/OH hydrogenation reactions are noticeably lowered, which would be mainly responsible for the enhanced ORR activity. Furthermore, our study suggests that the presence of Ir in the near-surface region can suppress Co out-diffusion from the Pd{sub 3}Co substrate, thereby improving the durability of Pd-Ir-Co catalysts. We also discuss the relative roles played by Ir and Co in enhancing the ORR activity relative to monometallic Pd catalysts.

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

Hydrogen molecule on lithium adsorbed graphene: A DFT study

Electronic structure calculations for the adsorption of molecular hydrogen on lithium (Li) decorated and pristine graphene have been studied systematically using SIESTA code [1] within the framework of the first-principle DFT under the Perdew-Burke-Ernzerhof (PBE) form of the generalized gradient approximation (GGA)[2], including spin polarization. The energy of adsorption of hydrogen molecule on graphene is always enhanced by the presence of co-adsorbed lithium. The most efficient adsorption configuration is when H{sub 2} is lying parallel to lithium adsorbed graphene which is in contrast to its adsorption on pristine graphene (PG) where it prefers perpendicular orientation.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2015

Electronic spectra and excited state dynamics of pentafluorophenol: Effects of low-lying πσ{sup ∗} states

Multiple fluorine atom substitution effect on photophysics of an aromatic chromophore has been investigated using phenol as the reference system. It has been noticed that the discrete vibronic structure of the S{sub 1}←S{sub 0} absorption system of phenol vapor is completely washed out for pentafluorophenol (PFP), and the latter also shows very large Stokes shift in the fluorescence spectrum. For excitations beyond S{sub 1} origin, the emission yield of PFP is reduced sharply with increase in excess vibronic energy. However, in a collisional environment like liquid hydrocarbon, the underlying dynamical process that drives the non-radiative decay is hindered drastically. Electronic structure theory predicts a number of low-lying dark electronic states of πσ{sup ∗} character in the vicinity of the lowest valence ππ{sup ∗} state of this molecule. Tentatively, we have attributed the excitation energy dependent non-radiative decay of the molecule observed only in the gas phase to an interplay between the lowest ππ{sup ∗} and a nearby dissociative πσ{sup ∗} state. Measurements in different liquids reveal that some of the dark excited states light up with appreciable intensity only in protic liquids like methanol and water due to hydrogen bonding between solute and solvents. Electronic structure theory methods indeed predict that for PFP-(H{sub 2}O){sub n} clusters (n = 1-11), intensities of a number of πσ{sup ∗} states are enhanced with increase in cluster size. In contrast with emitting behavior of the molecule in the gas phase and solutions of nonpolar and polar aprotic liquids, the fluorescence is completely switched off in polar protic liquids. This behavior is a chemically significant manifestation of perfluoro effect, because a very opposite effect occurs in the case of unsubstituted phenol for which fluorescence yield undergoes a very large enhancement in protic liquids. Several dynamical mechanisms have been suggested to interpret the observed photophysical behavior.

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

The role of high-level calculations in the assignment of the Q-band spectra of chlorophyll

We recently established a novel assignment of the visible absorption spectrum of chlorophyll-a that sees the two components Q{sub x} and Q{sub y} of the low-energy Q band as being intrinsically mixed by non-adiabatic coupling. This ended 50 years debate as to the nature of the Q bands, with prior discussion poised only in the language of the Born-Oppenheimer and Condon approximations. The new assignment presents significant ramifications for exciton transport and quantum coherence effects in photosystems. Results from state of the art electronic structure calculations have always been used to justify assignments, but quantitative inaccuracies and systematic failures have historically limited usefulness. We examine the role of CAM-B3LYP time-dependent density-functional theory (TD-DFT) and Symmetry Adapted Cluster-Configuration Interaction (SAC-CI) calculations in first showing that all previous assignments were untenable, in justifying the new assignment, in making some extraordinary predictions that were vindicated by the new assignment, and in then identifying small but significant anomalies in the extensive experimental data record.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Confirmation of intrinsic electron gap states at nonpolar GaN(1-100) surfaces combining photoelectron and surface optical spectroscopy

The electronic structure of GaN(1–100) surfaces is investigated in-situ by photoelectron spectroscopy (PES) and reflection anisotropy spectroscopy (RAS). Occupied surface states 3.1 eV below the Fermi energy are observed by PES, accompanied by surface optical transitions found in RAS around 3.3 eV, i.e., below the bulk band gap. These results indicate that the GaN(1–100) surface band gap is smaller than the bulk one due to the existence of intra-gap states, in agreement with density functional theory calculations. Furthermore, the experiments demonstrate that RAS can be applied for optical surface studies of anisotropic crystals.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Lowest-energy cage structures of medium-sized (ZnO){sub n} clusters with n = 15 − 24

Fullerene-like cage structures of medium-sized (ZnO){sub n} clusters with n = 15 − 24 were generated by spiral algorithm and optimized using density functional theory calculations. Most of these lowest-energy cage structures contain only four-membered and six-membered rings, whereas eight-membered rings were found in the lowest-energy cages of (ZnO){sub n} (n = 19, 20, 23, 24). Our best cage configurations either reproduce or prevail the previously reported ones. The size-dependent electronic properties were also discussed.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2015

Atomic-scale observation of dynamical fluctuation and three-dimensional structure of gold clusters

Unravelling three-dimensional structures and dynamical fluctuation of metal nanoclusters is critical to understanding reaction process and the origin of catalytic activity in many heterogeneous catalytic systems. We obtain three-dimensional structures of ultra-small Au clusters by combining aberration-corrected scanning transmission electron microscopy, density functional theory calculations, and imaging simulations. The configurations of unique Au clusters are revealed at the atomic scale and the corresponding electronic states are given. The sequential observations reveal a transition of ultra-small Au clusters with about 25 atoms from a near-square to an elongated structure. We also find a transition from two dimensions to three dimensions for the Au clusters. The obtained three-dimensional geometry and associated electronic states help to clarify atomistic mechanism of shape- and number-dependent catalytic activities of Au clusters.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Magnetic, Mechanical, and Electronic Properties of RhH{sub x} (x = 0, 0.25, 0.75, 1) from First-Principles Calculations

The magnetic, mechanical, and electronic properties of RhH{sub x} (x = 0, 0.25, 0.75, 1) have been investigated by first-principles calculations based on density functional theory. It is shown that RhH{sub x} (x = 0.25, 0.75, 1) in the cubic structure are magnetic except for Rh. The total magnetic moments of RhH{sub x} (x = 0.25, 0.75, 1) decrease with increasing content of H, which mainly results from the change of the Rh moments. The calculated elastic constants indicate that RhH{sub x} (x = 0, 0.25, 0.75, 1) in the cubic structure are mechanically stable. With increasing H content, the bulk modulus, shear modulus, and Young modulus gradually decrease except the bulk modulus of RhH. By means of the values of the B/G ratio, it is concluded that H doping improves the ductile properties of Rh. The strong hybridization between H-s and Rh-d is the origin of magnetism for RhH{sub x} (x = 0.25, 0.75, 1) and decreases with increasing H content.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Understanding the conductance switching of permethyloligosilanes: A theoretical approach

On the basis of ab initio density functional theory coupled with non-equilibrium Green’s function technique, we have presented a molecular level understanding on the stereoelectronic switching of conducting properties of oligosilane molecules. Su et al. [Nat. Chem. 7, 215–220 (2015)] demonstrated that these types of oligosilane molecules exhibit three stereoconformers which show two distinct conducting profiles. On the basis of break-junction technique, the authors show that manipulating a specific dihedral angle and thereby controlling the length of the molecular contact, it is possible to switch the conducting states minutely. However, their discussions scarce the proper energy level alignment upon which the molecule-lead tunneling amplitude depends. On the basis of electronic structure and non-equilibrium electron transport calculations, we interpret such switching behavior and thus quantify the switching parameter demonstrating how the metal-molecule contact geometry along with the electronic energy level alignment is responsible for such kind of junction process. We also provide the variation of switching parameter and the type of majority carrier with the conjugation length of the oligosilanes.

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

Theoretical predictions of properties and volatility of chlorides and oxychlorides of group-4 elements. I. Electronic structures and properties of MCl{sub 4} and MOCl{sub 2} (M = Ti, Zr, Hf, and Rf)

Relativistic, infinite order exact two-component, density functional theory electronic structure calculations were performed for MCl{sub 4} and MOCl{sub 2} of group-4 elements Ti, Zr, Hf, and element 104, Rf, with the aim to predict their behaviour in gas-phase chromatography experiments. RfCl{sub 4} and RfOCl{sub 2} were shown to be less stable than their lighter homologs in the group, tetrachlorides and oxychlorides of Zr and Hf, respectively. The oxychlorides turned out to be stable as a bent structure, though the stabilization energy with respect to the flat one (C{sub 2v}) is very small. The trend in the formation of the tetrachlorides from the oxychlorides in group 4 is shown to be Zr < Hf < Rf, while the one in the formation of the oxychlorides from the chlorides is opposite. All the calculated properties are used to estimate adsorption energy of these species on various surfaces in order to interpret results of gas-phase chromatography experiments, as is shown in Paper II.

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

G-centers in irradiated silicon revisited: A screened hybrid density functional theory approach

Electronic structure calculations employing screened hybrid density functional theory are used to gain fundamental insight into the interaction of carbon interstitial (C{sub i}) and substitutional (C{sub s}) atoms forming the C{sub i}C{sub s} defect known as G-center in silicon (Si). The G-center is one of the most important radiation related defects in Czochralski grown Si. We systematically investigate the density of states and formation energy for different types of C{sub i}C{sub s} defects with respect to the Fermi energy for all possible charge states. Prevalence of the neutral state for the C-type defect is established.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Vacancies in ordered and disordered titanium monoxide: Mechanism of B1 structure stabilization

The electronic structure and stability of three phases of titanium monoxide TiO{sub y} with B1 type of the basic structure have been studied. Cubic phase without structural vacancies, TiO, and two phases with structural vacancies, monoclinic Ti{sub 5}O{sub 5} and cubic disordered TiO{sub 1.0}, was treated by means of first-principles calculations within the density functional theory with pseudo-potential approach based on the plane wave's basis. The ordered monoclinic phase Ti{sub 5}O{sub 5} was found to be the most stable and the cubic TiO without vacancies the less stable one. The role of structural vacancies in the titanium sublattice is to decrease the Fermi energy, the role of vacancies in the oxygen sublattice is to contribute to the appearance of Ti–Ti bonding interactions through these vacancies and to reinforce the Ti–Ti interactions close to them. Listed effects are significantly pronounced if the vacancies in the titanium and oxygen sublattices are associated in the so called “vacancy channels” which determine the formation of vacancy ordered structure of monoclinic Ti{sub 5}O{sub 5}-type. - Graphical abstract: Changes in total DOS of titanium monoxide when going from vacancy-free TiO to TiO with disordered structural vacancies and to TiO with ordered structural vacancies. Highlights: • Ordered monoclinic Ti{sub 5}O{sub 5} is the most stable phase of titanium monoxide. • Vacancy-free TiO is the less stable phase of the titanium monoxide. • Ordering of oxygen vacancies leads to the appearance of Ti–Ti bonding interactions. • Titanium vacancies contribute significantly to the decreasing of the Fermi energy.

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

Effect of disorder on electronic and magnetic properties of Co{sub 2}VGa Heusler alloy

This paper presents the effect of disorder on electronic, magnetic and half-metallic properties of Co{sub 2}VGa Heusler alloy using density functional theory. Binary mixing is the most common form of atomic disorder in these compounds. We have considered three types of disorders: DO{sub 3}, A2 and B2 disorder which corresponds to X-Y, X-Z and Y-Z mixing respectively. After structural optimization, we found that A2 disorder has high formation energy and is most unlikely to occur. The half-metallic nature of the alloy is destroyed in presence of DO{sub 3} and A2 disorder. The destruction of half-metallicity is due to reconstruction of energy states. Also the loss of half-metallicity is accompanied by reversal of spin-polarization at the Fermi level. B2 disorder retains the half-metallic nature of the alloy but spin-polarization value is reduced as compared to the ordered alloy.

36 MATERIALS SCIENCE↗