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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 343 records · Page 19
All sourcesNASA NTRSDOE OSTI
DOE OSTI2011

Faster Proton dynamics of water on SnO2 compared to TiO2.

Proton jump processes in the hydration layer on the iso-structural TiO2 rutile (110) and SnO2 cassiterite(110) surfaces were studied with density functional theory molecular dynamics. We find thatthe proton jump rate is more than three times faster on cassiterite compared with rutile. A local analysisbased on the correlation between the stretching band of the O–H vibrations and the strength ofH-bonds indicates that the faster proton jump activity on cassiterite is produced by a stronger H-bondformation between the surface and the hydration layer above the surface. The origin of the increasedH-bond strength on cassiterite is a combined effect of stronger covalent bonding and stronger electrostaticinteractions due to differences of its electronic structure. The bridging oxygens form thestrongest H-bonds between the surface and the hydration layer. This higher proton jump rate is likelyto affect reactivity and catalytic activity on the surface. A better understanding of its origins will enablemethods to control these rates. © 2011 American Institute of Physics. [doi:10.1063/1.3509386]

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Theory of the electronic structure of substitutional semiconductor alloys: Analytical approaches

Methods of predicting the electronic structure of disordered semiconductor alloys involving mainly isoelectronic substitution are reviewed. Special emphasis is placed on analytical methods of studying currently available models of alloys. An approximate equation for the localization threshold of electronic states in the Lifshitz model is considered, and the inaccuracy of this equation is estimated. The contributions of the perturbation potential of an individual impurity and of crystal-lattice distortions in the vicinity of the impurity center are analyzed on the basis of the Faddeev equations. The contributions of intrinsic impurity potentials and volume effects to the formation of the electronic structure of semiconductor alloys are esti- mated. Methods of calculating matrix elements of the perturbation potentials of isoelectronic impurities in alloys with consideration for deformation effects are considered. The procedure of calculating the compositional dependence of the band gap of multicomponent alloys is described. A comparative analysis of various methods for predicting the formation of electronic states bound at individual isoelectronic impurities in semiconductors is conducted. The theory of the energy spectrum of charged impurities in isoelectronic alloys is presented.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Shape dependent electronic properties of wurzite GaN nanowire

In the present work, energetic stability and electronic behavior of triangular and square shaped wurzite GaN NW oriented along [1100] and [11 2 0] direction has been investigated by employing ab-initio DFT calculation. Structural analysis suggests that triangular shaped NW undergoes strong surface reconstruction compared to square shaped NW. However, binding energy reveals that square shaped NW is energetically more feasible than triangular NW. Further, from electronic band structure we observe that both structures are metallic with higher metallicity for triangular shaped NW.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

First-principles study of Au–Cu alloy surface changes induced by gas adsorption of CO, NO, or O{sub 2}

The surface composition of bimetallics can be strongly altered by adsorbing molecules where the metal with the strongest interaction with the adsorbate segregates into the surface. To investigate the effect of reactive gas on the surface composition of Au–Cu alloy, we examined by means of density functional theory to study the segregation behavior of copper in gold matrices. The adsorption mechanisms of CO, NO, and O{sub 2} gas molecules on gold, copper, and gold-copper low index (111), (100), and (110) surfaces were analyzed from energetic and electronic points of view. Our results show a strong segregation of Cu toward the (110) surface in the presence of all adsorbed molecules. Interestingly, the Cu segregation toward the (111) and (100) surface could occur only in the presence of CO and at a lower extent in the presence of NO. The analysis of the electronic structure highlights the different binding characters of adsorbates inducing the Cu segregation.

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

Molecular, crystal, and electronic structure of the cobalt(II) complex with 10-(2-benzothiazolylazo)-9-phenanthrol

The reaction of 10-(2-benzothiazolylazo)-9-phenanthrol (HL) with cobalt(II) acetate gives the coordination compound [CoL{sub 2}] {center_dot} CHCl{sub 3} (I). The molecular and crystal structure of I is determined by X-ray diffraction. The coordination polyhedron of the Co atom in complex I is an octahedron. The anion L acts as a tridentate chelating ligand and is coordinated to the Co atom through the phenanthrenequinone O1 atom and the benzothiazole N1 atom of the moieties L and the N3 atom of the azo group to form two five-membered metallocycles. The molecular and electronic structures of the compounds HL, L, and CoL{sub 2} are studied at the density functional theory level. The results of the quantum-chemical calculations are in good agreement with the values determined by X-ray diffraction.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1999

Computational Study of Field Initiated Surface Reactions for Synthesis of Diamond and Silicon

This project involves using quantum chemistry to simulate surface chemical reactions in the presence of an electric field for nanofabrication of diamond and silicon. A field delivered by a scanning tunneling microscope (STM) to a nanometer scale region of a surface affects chemical reaction potential energy surfaces (PES) to direct atomic scale surface modification to fabricate sub-nanometer structures. Our original hypothesis is that the applied voltage polarizes the charge distribution of the valence electrons and that these distorted molecular orbitals can be manipulated with the STM so as to change the relative stabilities of the electronic configurations over the reaction coordinates and thus the topology of the PES and reaction kinetics. Our objective is to investigate the effect of applied bias on surface reactions and the extent to which STM delivered fields can be used to direct surface chemical reactions on an atomic scale on diamond and silicon. To analyze the fundamentals of field induced chemistry and to investigate the application of this technique for the fabrication of nanostructures, we have employed methods capable of accurately describing molecular electronic structure. The methods we employ are density functional theory (DFT) quantum chemical (QC) methods. To determine the effect of applied bias on surface reactions we have calculated the QC PESs in various applied external fields for various reaction steps for depositing or etching diamond and silicon. We have chosen reactions which are thought to play a role in etching and the chemical vapor deposition growth of Si and diamond. The PESs of the elementary reaction steps involved are then calculated under the applied fields, which we vary in magnitude and configuration. We pay special attention to the change in the reaction barriers, and transition state locations, and search for low energy reaction channels which were inaccessible without the applied bias.

Musgrave, Charles Bruce↗
DOE OSTI2013

Hybrid functional calculation of electronic and phonon structure of BaSnO{sub 3}

Barium stannate, BaSnO{sub 3} (BSO), with a cubic perovskite structure, has been highlighted as a promising host material for the next generation transparent oxide electrodes. This study examined theoretically the electronic structure and phonon structure of BSO using hybrid density functional theory based on the HSE06 functional. The electronic structure results of BSO were corrected by extending the phonon calculations based on the hybrid density functional. The fundamental thermal properties were also predicted based on a hybrid functional calculation. Overall, a detailed understanding of the electronic structure, phonon modes and phonon dispersion of BSO will provide a theoretical starting-point for engineering applications of this material. - Graphical Abstract: (a) Crystal structure of BaSnO{sub 3}. The center ball is Ba and small (red) ball on edge is oxygen and SnO{sub 6} octahedrons are plotted as polyhedron. (b) Electronic band structure along the high symmetry point in the Brillouin zone using the HSE06 hybrid functional. (c) The phonon dispersion curve calculated using the HSE06 hybrid functional (d) Zone center lowest energy F{sub 1u} phonon mode. Highlights: Black-Right-Pointing-Pointer We report the full hybrid functional calculation of not only the electronic structure but also the phonon structure for BaSnO{sub 3}. Black-Right-Pointing-Pointer The band gap calculation of HSE06 revealed an indirect gap with 2.48 eV. Black-Right-Pointing-Pointer The effective mass at the conduction band minimum and valence band maximum was calculated. Black-Right-Pointing-Pointer In addition, the phonon structure of BSO was calculated using the HSE06 functional. Black-Right-Pointing-Pointer Finally, the heat capacity was calculated and compared with the recent experimental result.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Structural, electronic and thermodynamic properties of britholites Ca{sub 10−x}La{sub x}(PO{sub 4}){sub 6−x}(SiO{sub 4}){sub x}F{sub 2} (0 ≤ x ≤ 6): Experiment and theory

Highlights: • The structural and electronic properties of apatites with the general formula Ca{sub 10−x}La{sub x}(PO{sub 4}){sub 6−x}(SiO{sub 4}){sub x}F{sub 2} (0 ≤ x ≤ 6) have been investigated by means of the density functional theory. • The calculated enthalpy of formation of the compounds increased with the substitution degree. • The decrease in stability is probably due to the disorder induced in the Ca/LaO{sub 6}F polyhedron, following the substitution. - Abstract: The apatite-type compounds Ca{sub 10−x}La{sub x}(PO{sub 4}){sub 6−x}(SiO{sub 4}){sub x}F{sub 2} with 0 ≤ x ≤ 6 were prepared using a high temperature solid state reaction and were characterized by X-ray diffraction. The crystal structure, chemical bonding, electronic structure and formation energy of all relaxed structures were analyzed by density functional theory (DFT). The calculated results show that the predicted geometry can well reproduce the structural parameters. The incorporation of La{sup 3+} into the fluorapatite (FA) structure induced especially at the level of the S(2) sites a certain disorder which is responsible for the weakening in the stability with x. Excellent agreement were obtained between the calculated and experimental results. Moreover, the band structure indicates that despite the reduction of the band gap with x content all materials remain insulating.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Electronic properties of III-nitride semiconductors: A first-principles investigation using the Tran-Blaha modified Becke-Johnson potential

In this work, we use density functional theory to investigate the influence of semilocal exchange and correlation effects on the electronic properties of III-nitride semiconductors considering zinc-blende and wurtzite crystal structures. We find that the inclusion of such effects through the use of the Tran-Blaha modified Becke-Johnson potential yields an excellent description of the electronic structures of these materials giving energy band gaps which are systematically larger than the ones obtained with standard functionals such as the generalized gradient approximation. The discrepancy between the experimental and theoretical band gaps is then significantly reduced with semilocal exchange and correlation effects. However, the effective masses are overestimated in the zinc-blende nitrides, but no systematic trend is found in the wurtzite compounds. New results for energy band gaps and effective masses of zinc-blende and wurtzite indium nitrides are presented.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Rietveld refinement, electronic structure and ionic conductivity of Sr{sub 4}La{sub 6}(SiO{sub 4}){sub 6}F{sub 2} and Sr{sub 4}La{sub 6}(SiO{sub 4}){sub 6}O ceramics

In this paper, we report the effect of the tunnel anions on the ionic conductivity of Strontium-Lanthanum silicate apatites. The Sr{sub 4}La{sub 6}(SiO{sub 4}){sub 6}F{sub 2} and Sr{sub 4}La{sub 6}(SiO{sub 4}){sub 6}O ceramics were prepared by the solid state reaction method. X-ray diffraction, NMR spectroscopy and Raman measurements were performed to investigate the crystal structure and vibrational active modes. Moreover, the electronic structures of the crystals were evaluated by the first-principles quantum mechanical calculation based on the density functional theory. Finally, the ionic conductivity was studied according to the complex impedance method. - Graphical abstract: The relaxed primitive unit cell for Sr{sub 4}La{sub 6}Fap. Display Omitted.

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

A family of rare earth molybdenum bronzes: Oxides consisting of periodic arrays of interacting magnetic units

The family of rare earth molybdenum bronzes, reduced ternary molybdates of composition LnMo{sub 16}O{sub 44,} was synthesized and a detailed structural study carried out. Bond valence sum (BVS) calculations clearly show that the molybdenum ions in tetrahedral coordination are hexavalent while the electron count in the primitive unit cell is odd. Yet, measurements show that the phases are semiconductors. The temperature dependence of the magnetic susceptibility of samples containing several different rare earth elements was measured. These measurements verified the presence of a 6.5 K magnetic phase transition not arising from the rare earth constituent, but likely associated with the unique isolated ReO{sub 3}-type Mo{sub 8}O{sub 36} structural subunits in this phase. To better understand the behavior of these materials, electronic structure calculations were performed within density functional theory. Results suggest a magnetic state in which these structural moieties have an internal ferromagnetic arrangement, with small ~1/8 μ{sub B} moments on each Mo. We suggest that the Mo{sub 8}O{sub 36} units behave like pseudoatoms with spin 1/2 derived from a single hole distributed over the eight Mo atoms that are strongly hybridized with the O atoms of the subunit. Interestingly, while the compound is antiferromagnetic, our calculations suggest that a field-stabilized ferromagnetic state, if achievable, will be a narrow band half-metal. - Graphical abstract: LnMo{sub 16}O{sub 44} phases comprise corner sharing tetrahedral and octahedral molybdenum ions. The MoO{sub 6} octahedra form Mo{sub 8}O{sub 36} units that are well separated and act like pseudo-atoms, accommodating 11 electrons each. - Highlights: • Single crystal X-ray diffraction refinements of LnMo{sub 16}O{sub 44} single crystals for Ln=Ce, Pr, Nd, Tb, Dy and Ho. • DFT calculations based on LaMo{sub 16}O{sub 44}. • [Mo{sub 8}O{sub 36}] units behaving as superatoms with a net magnetic moment of 1 µ{sub B}. • Bronze structure containing equal number of molybdenum tetrahedra and octahedral.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2012

A photoactive titanate with a stereochemically active Sn lone pair: Electronic and crystal structure of Sn{sub 2}TiO{sub 4} from computational chemistry

TiO{sub 2} remains the most widely studied metal oxide for photocatalytic reactions. The standard approach to reduce the band gap of titania, for increasing the absorption of visible light, is anion modification. For example the formation of an oxynitride compound, where the nitrogen 2p states decrease the binding energy of the valence band. We demonstrate that cation modification can produce a similar effect through the formation of a ternary oxide combining Ti and an ns{sup 2} cation, Sn(II). In Sn{sub 2}TiO{sub 4}, the underlying Ti 3d conduction states remain largely unmodified and an electronic band gap of 2.1 eV (590 nm) is predicted by hybrid density functional theory. Our analysis indicates a strong potential for Sn{sub 2}TiO{sub 4} in visible-light driven photocatalysis, which should prove superior to the alternative (SnO{sub 2}){sub 1-x}(TiO{sub 2}){sub x} solid-solution. - Graphical abstract: Sn{sub 2}TiO{sub 4} is predicted to be a semiconductor with potential for bipolar conductivity and visible-light photocatalysis. Highlights: Black-Right-Pointing-Pointer Tin titanate adopts the Pb{sub 3}O{sub 4} crystal structure with a sterically active Sn(II) lone pair. Black-Right-Pointing-Pointer Tin titanate is thermodynamically stable with respect to TiO{sub 2} and SnO. Black-Right-Pointing-Pointer Tin titanate is predicted have a band gap of 590 nm, ideal for photocatalytic applications.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Band gap bowing parameter in pseudomorphic Al{sub x}Ga{sub 1−x}N/GaN high electron mobility transistor structures

A method for evaluation of aluminium composition in pseudomorphic Al{sub x}Ga{sub 1−x}N layer from the measured photoluminescence (PL) peak energy is presented here. The layers were grown by metalorganic chemical vapor deposition and characterized by high resolution X-ray diffraction (HRXRD), PL, cathodoluminescence, and atomic force microscopy. We estimated the value of biaxial stress in pseudomorphic Al{sub x}Ga{sub 1−x}N layers grown on sapphire and silicon carbide substrates using HRXRD scans. The effect of biaxial stress on the room temperature band edge luminescence in pseudomorphic Al{sub x}Ga{sub 1−x}N/GaN layers for various aluminium compositions in the range of 0.2 < x < 0.3 was determined. The value of pressure coefficient of band gap was also estimated. The stress corrected bowing parameter in Al{sub x}Ga{sub 1−x}N was determined as 0.50 ± 0.06 eV. Our values match well with the theoretically obtained value of bowing parameter from the density functional theory.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Benchmarking third-order cluster perturbation theory for electronically excited states

In this study, we investigate the reliability of cluster perturbation (CP) theory applied to the calculation of electronically excited states through a comprehensive benchmark. In CP theory, perturbative corrections are added to the properties of a parent excitation space, which converge toward the properties of a target excitation space. For the CPS(D-n) model, perturbative corrections through order n are added to the coupled cluster singles (CCS) excitation energies to target the coupled cluster singles and doubles (CCSD) excitation energies. Through a comparative analysis of excitation energy calculations across a diverse set of molecules and wavefunction methods, we present a comprehensive evaluation of the accuracy of the third-order CPS(D) model, CPS(D-3), in calculating excitation energies. Further, our findings demonstrate that CPS(D-3) is a reliable alternative to established methods, particularly CCSD, while systematically overestimating the excitation energies compared to high-level coupled cluster methods such as CC3. These results highlight the strengths and limitations of CPS(D-3), as well as the promising directions for its future development.

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

Projected Commutator DIIS Method for Accelerating Hybrid Functional Electronic Structure Calculations

The commutator direct inversion of the iterative subspace (commutator DIIS or C-DIIS) method developed by Pulay is an efficient and the most widely used scheme in quantum chemistry to accelerate the convergence of self-consistent field (SCF) iterations in Hartree–Fock theory and Kohn–Sham density functional theory. The C-DIIS method requires the explicit storage of the density matrix, the Fock matrix, and the commutator matrix. Hence, the method can only be used for systems with a relatively small basis set, such as the Gaussian basis set. We develop a new method that enables the C-DIIS method to be efficiently employed in electronic structure calculations with a large basis set such as planewaves for the first time. The key ingredient is the projection of both the density matrix and the commutator matrix to an auxiliary matrix called the gauge-fixing matrix. The resulting projected commutator-DIIS method (PC-DIIS) only operates on matrices of the same dimension as that consists of Kohn–Sham orbitals. The cost of the method is comparable to that of standard charge mixing schemes used in large basis set calculations. The PC-DIIS method is gauge-invariant, which guarantees that its performance is invariant with respect to any unitary transformation of the Kohn–Sham orbitals. We demonstrate that the PC-DIIS method can be viewed as an extension of an iterative eigensolver for nonlinear problems. We use the PC-DIIS method for accelerating Kohn–Sham density functional theory calculations with hybrid exchange–correlation functionals, and demonstrate its superior performance compared to the commonly used nested two-level SCF iteration procedure. Furthermore, we demonstrate that in the context of ab initio molecular dynamics (MD) simulation with hybrid functionals one can extrapolate the gauge-fixing matrix to achieve the goal of extrapolating the entire density matrix implicitly along the MD trajectory. Here, numerical results indicate that the new method significantly reduces the number of SCF iterations per MD step, compared to the commonly used strategy of extrapolating the electron density.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Investigation of the Substituting Effect of Chromium on the Electronic Structures and the Half-Metallic Ferromagnetic Properties of BaO

The first-principle methods of density functional theory were used to investigate the doping effect of 3d chromium (Cr) impurities on the electronic properties and the ferromagnetic arrangement of (BaO) barium oxide such as Ba{sub 1−x}Cr{sub x}O compounds at concentrations x = 0.25, 0.5, and 0.75. The p–d exchange coupling between 2p states of oxygen (O) and 3d (Cr) partially filled levels induces magnetism in the Ba{sub 1−x}Cr{sub x}O compounds. The majority- and minority-spin states exhibit respectively metallic and semiconducting features, leading to a half-metallic (HM) character with 100% spin polarization for all concentrations. The Ba{sub 0.75}Cr{sub 0.25}O, Ba{sub 0.5}Cr{sub 0.5}O, and Ba{sub 0.25}Cr{sub 0.75}O compounds have integral total magnetic moments of 4 μ{sub B} per Cr atom with HM gaps of 0.055, 0.218, and 0.308 eV, respectively. These behaviors form accurate half-metallic ferromagnets and make Ba{sub 1−x}Cr{sub x}O as potential materials for possible spintronics applications.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Energetics of intrinsic defects and their complexes in ZnO investigated by density functional calculations

Formation energies of various intrinsic defects and defect complexes in ZnO have been calculated using a density-functional-theory-based pseudopotential all-electron method. The various defects considered are oxygen vacancy (V{sub O}), zinc vacancy (V{sub Zn}), oxygen at an interstitial site (O{sub i}), Zn at an interstitial site (Zn{sub i}), Zn at V{sub O} (Zn{sub O}), O at V{sub Zn}(O{sub Zn}), and an antisite pair (combination of the preceding two defects). In addition, defect complexes like (V{sub O}+Zn{sub i}) and Zn-vacancy clusters are studied. The Schokkty pair (V{sub O}+V{sub Zn}) and Frenkel pairs [(V{sub O}+O{sub i}) and (V{sub Zn}+Zn{sub i})] are considered theoretically for the first time. Upon comparing the formation energies of these defects, we find that V{sub O} would be the dominant intrinsic defect under both Zn-rich and O-rich conditions and it is a deep double donor. Both Zn{sub O} and Zn{sub i} are found to be shallow donors. The low formation energy of donor-type intrinsic defects could lead to difficulty in achieving p-type conductivity in ZnO. Defect complexes have charge transitions deep inside the band gap. The red, yellow, and green photoluminescence peaks of undoped samples can be assigned to some of the defect complexes considered. It is believed that the red luminescence originates from an electronic transition in V{sub O}, but we find that it can originate from the antisite Zn{sub O} defect. Charge density and electron-localization function analyses have been used to understand the effect of these defects on the ZnO lattice. The electronic structure of ZnO with intrinsic defects has been studied using density-of-states and electronic band structure plots. The acceptor levels introduced by V{sub Zn} are relatively localized, making it difficult to achieve p-type conductivity with sufficient hole mobility.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Runtime performance of a GAMESS quantum chemistry application offloaded to GPUs

Summary Computational chemistry is at the forefront of solving urgent societal problems, such as polymer upcycling and carbon capture. The complexity of modeling these processes at appropriate length and time scales is mainly manifested in the number and types of chemical species involved in the reactions and may require models of several thousand atoms and large basis sets to accurately capture the chemical complexity and heterogeneity in the physical and chemical processes. The quantum chemistry package General Atomic and Molecular Electronic Structure System (GAMESS) has a wide array of methods that can efficiently and accurately treat complex chemical systems. In this work, we have used the GAMESS Effective Fragment Molecule Orbital (EFMO) method for electronic structure calculation of a challenging mesoporous silica nanoparticle (MSN) model surrounded by about 4700 water molecules to investigate the strong scaling and GPU offloading on hybrid CPU‐GPU nodes. Experiments were performed on the Perlmutter platform at the National Energy Research Scientific Computing Center. Good strong scaling and load balancing have been observed on up to 88 hybrid nodes for different settings of the execution parameters for the calculation considered here. When GPUs are oversubscribed by offloading work from multiple CPU processes, using the NVIDIA multi‐process service (MPS) has consistently reduced time to solution and energy consumed. Additionally, for some configuration parameter settings, oversubscription with MPS improved performance by up to 5.8% over the case without oversubscription.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗