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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 883 records · Page 49
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Hole traps in sodium silicate: First-principles calculations of the mobility edge

Here, the structure and properties of (Na 2 O) 0.30 (SiO 2 ) 0.70 sodium silicate glass are studied by combined ab-initio and classical molecular dynamics simulations to identify the sources of electronic traps in the band gap. Structures from classical molecular dynamics melt-quench simulations are taken as initial configurations for first-principles density functional theory structural relaxation, from which electronic properties are determined. An ensemble of thirty glass structures, each containing 660 atoms, is prepared in order to perform statistical analyses. The inverse participation ratio is used as a metric to characterize localized states in the band gap and determine the mobility edge. Structures with varying amounts of local disorder (traps) are compared. The most localized and highest energy trap states are due to Si atoms with 2–3 non-bridging oxygen atoms. Control of the electronic properties of amorphous insulators and semiconductors is essential for the advancement of many technologies, such as photovoltaics and scintillators, for which the present analysis is indispensable.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Solvation Properties of Silver and Copper Ions in a Room Temperature Ionic Liquid: A First-Principles Study

Understanding the behavior of metal ions in room temperature ionic liquids (ILs) is essential for predicting and optimizing performance for technologies like metal electrodeposition; however, many mechanistic details remain enigmatic, including the solvation properties of the ions in ILs and how they are governed by the intrinsic interaction between the ions and the liquid species. Here, we utilize first-principles molecular dynamics simulations to unravel and compare the key structural properties of Ag + and Cu + ions in a common room temperature IL, 1-ethyl-3-methylimidazolium trifluoromethanesulfonate. We find that, when compared to Cu + , the larger Ag + shows a more disordered and flexible solvation structure with a more frequent exchange of the IL species between its solvation shells. In addition, our simulations reveal an interesting analog in the solvation behavior of the ions in the IL and aqueous environments, particularly in the effect of the ion electronic structures on their solvation properties. As a result, this work provides fundamental understanding of the intrinsic properties of the metal ions in the IL, while offering mechanistic understanding and strategy for future selection of ILs for metal electrodeposition processes.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Stability of the hcp Ruthenium at high pressures from first principles

The method of calculation of the elastic constants up to third order from the energy-strain relation under pressure for the hcp crystals is given and described in details. The method is applied to the hcp phase of Ruthenium. Elastic constants, lattice dynamics, and electronic structure are investigated in the pressure interval of 0–600 GPa by means of first principles calculations. The obtained parameters are in very good agreement with available experimental and theoretical data. No preconditions for phase transformation driven by mechanical or dynamical instabilities for hcp Ru were found in the investigated pressure range. The reason of stability at such high pressures is explained in the context of electronic structure peculiarities.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Understanding the stable boron clusters: A bond model and first-principles calculations based on high-throughput screening

The unique electronic property induced diversified structure of boron (B) cluster has attracted much interest from experimentalists and theorists. B{sub 30–40} were reported to be planar fragments of triangular lattice with proper concentrations of vacancies recently. Here, we have performed high-throughput screening for possible B clusters through the first-principles calculations, including various shapes and distributions of vacancies. As a result, we have determined the structures of B{sub n} clusters with n = 30–51 and found a stable planar cluster of B{sub 49} with a double-hexagon vacancy. Considering the 8-electron rule and the electron delocalization, a concise model for the distribution of the 2c–2e and 3c–2e bonds has been proposed to explain the stability of B planar clusters, as well as the reported B cages.

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

First-principles studies of p-type nitrogen-doped α-Fe{sub 2}O{sub 3-x}S{sub x} alloys

Based on spin-polarized density functional theory, the characteristics of p-type doping are investigated in the N-doped α-Fe{sub 2}O{sub 3−x}S{sub x} alloys by means of first-principles methods. Numerical results show that when N substitutes O atom in pure α-Fe{sub 2}O{sub 3}, N impurity level is a deeper acceptor state. However, the unoccupied level is much shallower when N doped in the α-Fe{sub 2}O{sub 3−x}S{sub x} alloys, which indicates N impurity can provide good and effective p-type carriers. These predicted numerical results are interesting and useful to understand the α-Fe{sub 2}O{sub 3−x}S{sub x} alloys as a new low-cost solar cell material.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2023

First-principles molten salt phase diagrams through thermodynamic integration

Precise prediction of phase diagrams in molecular dynamics simulations is challenging due to the simultaneous need for long time and large length scales and accurate interatomic potentials. Here, we show that thermodynamic integration from low-cost force fields to neural network potentials trained using density-functional theory (DFT) enables rapid first-principles prediction of the solid–liquid phase boundary in the model salt NaCl. We use this technique to compare the accuracy of several DFT exchange–correlation functionals for predicting the NaCl phase boundary and find that the inclusion of dispersion interactions is critical to obtain good agreement with experiment. Importantly, our approach introduces a method to predict solid–liquid phase boundaries for any material at an ab initio level of accuracy, with the majority of the computational cost at the level of classical potentials.

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

First-principles study of direct and narrow band gap semiconducting β -CuGaO 2

Semiconducting oxides have attracted much attention due to their great stability in air or water and the abundance of oxygen. Recent success in synthesizing a metastable phase of CuGaO 2 with direct narrow band gap opens up new applications of semiconducting oxides as absorber layer for photovoltaics. Using first-principles density functional theory calculations, we investigate the thermodynamic and mechanical stabilities as well as the structural and electronic properties of the β-CuGaO 2 phase. Our calculations show that the β-CuGaO 2 structure is dynamically and mechanically stable. The energy band gap is confirmed to be direct at the Γ point of Brillouin zone. In conclusion, the optical absorption occurs right at the band gap edge and the density of states near the valance band maximum is large, inducing an intense absorption of light as observed in experiment.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Mechanism of Na accumulation at extended defects in Si from first-principles

Sodium (Na) impurities in silicon solar cells are considered to play an important role in potential-induced degradation (PID), a significant cause of solar cell degradation and failure. Shorting due to Na accumulation at extended defects has been suggested as a culprit for PID. However, it is not clear how the extended defects are decorated by Na impurities. Using first-principles density functional theory calculations, we find that Na impurities segregate from the bulk into extended defects such as intrinsic stacking faults and Sigma 3 (111) grain boundaries. The energy barrier required for Na to escape from the extended defects is substantial and similar to the sum of the barrier energy in bulk Si (1.1-1.2 eV) and the segregation energy to the stacking fault (~0.7 eV). Surprisingly, the migration barrier for Na diffusion within the extended defects is even higher than the energy barrier for escaping. Furthermore, the results suggest that the extended defects likely accumulate Na as the impurities segregate to the defects from the bulk, rather than because of migration through the extended defects. Published by AIP Publishing.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Efficient First-Principles Methodology for the Calculation of the All-Phonon Inelastic Scattering in Solids

Inelastic scattering experiments are key methods for mapping the full dispersion of fundamental excitations of solids in the ground as well as nonequilibrium states. A quantitative analysis of inelastic scattering in terms of phonon excitations requires identifying the role of multiphonon processes. Here, we develop an efficient first-principles methodology for calculating the all-phonon quantum mechanical structure factor of solids. We demonstrate our method by obtaining excellent agreement between measurements and calculations of the diffuse scattering patterns of black phosphorus, showing that multiphonon processes play a substantial role. The present approach constitutes a step towards the interpretation of static and time-resolved electron, x-ray, and neutron inelastic scattering data.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Comprehensive View of the Ligand–Gold Interface from First Principles

A large number of ligands have been used to stabilize and functionalize the gold surfaces and nanoclusters, but there has been no systematic comparison about their binding strengths with gold. In this work, we studied the interaction between 27 ligands of six different types (thiolates, phosphines, amines, aryl radicals, alkynyls, and N-heterocyclic carbenes) with the model Au surfaces by first-principles density functional theory (DFT). On the perfect Au(111), we found the order of binding strengths to be bulky N-heterocyclic carbenes (NHCs) ≈ alkynyls > thiolates ≈ phosphines > aryls ≈ less sterically bulky NHCs > alkylamines. The much stronger interaction of bulky carbenes to Au than the less sterically bulky NHCs arises from the van der Waals (vdW) attraction of bulky side groups with gold surface via the short Au···HCH2R contact. Further, we showed that the presence of a gold adatom on Au(111) leads to enhanced binding and a similar order for most of the ligands examined. Overall, bulky NHCs and alkynyl groups form the strongest interaction to both Au(111) and Auad–Au(111). This suggests that NHCs can be employed as alternatives to the currently widely used thiolates and the emerging alkynyl ligands for the preparation of more stable self-assembled monolayer structures on metal surfaces. Further, this insight allowed us to design viable magic-number gold clusters with NHCs as the protecting ligands.

Tang, Qing↗
DOE OSTI2018

First-Principles Study of Spinel MgTiS 2 as a Cathode Material

Spinel intercalation hosts are well known to to facilitate high rate capability and high voltage Li-ion batteries. A recent experimental study has shown that Mg can reversibly intercalate in spinel TiS 2 , demonstrating the viability of Li intercalation host crystal structures for Mg-ion batteries. We report on a first-principles statistical mechanics study of Mg insertion into spinel TiS 2 , accounting for occupancy on both octahedrally and tetrahedrally coordinated interstitial sites. In contrast to Li-containing spinels, we predict mixed octahedral and tetrahedral site occupancy at nondilute Mg concentrations consistent with the recent experimental study of Sun et al. The onset of mixed occupancy is correlated with an increase in the spinel volume upon Mg insertion, which is more pronounced in MgxTiS 2 than in its Li counterpart. Lastly, the results in this study suggest that the degree of mixed occupancy could be controlled through the volume of the host with addition of electrochemically inactive species.

25 ENERGY STORAGE↗
DOE OSTI2020

A first principles method to determine speciation of carbonates in supercritical water

The determination of the speciation of ions and molecules in supercritical aqueous fluids under pressure is critical to understanding their mass transport in the Earth's interior. Unfortunately, there is no experimental technique yet available to directly characterize species dissolved in water at extreme conditions. Here we present a strategy, based on first-principles simulations, to determine ratios of Raman scattering cross-sections of aqueous species under extreme conditions, thus providing a key quantity that can be used, in conjunction with Raman measurements, to predict chemical speciation in aqueous fluids. Due to the importance of the Earth's carbon cycle, we focus on carbonate and bicarbonate ions. Our calculations up to 11 GPa and 1000 K indicate a higher concentration of bicarbonates in water than previously considered at conditions relevant to the Earth's upper mantle, with important implications for the transport of carbon in aqueous fluids in the Earth's interior.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

First-principles investigation of the resistive switching energetics in monolayer MoS 2 : insights into metal diffusion and adsorption

A deeper understanding of resistive switching (RS) in 2D materials is essential for advancing neuromorphic computing. The Dissociation-Diffusion-Adsorption (DDA) model offers a useful framework for probing RS mechanisms in non-volatile memory (NVM) and in-memory computing. We have employed first-principles density functional theory (DFT) to explore dissociation, diffusion, and adsorption phenomena within the DDA model, focusing on the interactions between exemplary metal atoms (Au, Ag, Cu) and monolayer MoS 2 . Nudged elastic band (NEB) calculations evaluated diffusion barriers in pristine and sulfur-vacancy MoS 2 . Charged systems were modeled to assess the impact of applied bias on migration pathways. We also examined metal dissociation from bulk electrodes and adsorption at S vacancies. Ag/MoS 2 shows the lowest dissociation barrier (~0.034 eV), while Au and Cu exhibit similar values (~0.32 eV). These insights highlight Ag as a promising candidate for low-energy RS applications and provide guidance for optimizing switching efficiency in 2D memory devices.

Atomistic models↗
DOE OSTI2014

First-principle investigations of K{sub 2}NiF{sub 4}-type double perovskite oxides La{sub 4}B′B″O{sub 8} (B′B″ = Fe, Co, Ni)

The K{sub 2}NiF{sub 4}-type structure La{sub 4}CoNiO{sub 8} (LCNO), La{sub 4}FeCoO{sub 8} (LFCO), and La{sub 4}FeNiO{sub 8} (LFNO) are studied by using the first-principle electronic structure calculations. Our results indicate that the ground state of LCNO is a ferrimagnetism (FiM) with a large energy gap about 1.9 eV, LFCO and LFNO are antiferromagnetism with energy gaps about 1.3 and 1.4 eV, respectively. Their orthorhombic distortions, out-of-plane elongation, and tilting of octahedron are discussed. It is indicated that LFCO and LFNO have stronger crystal distortion than LCNO. Our calculations indicate that the in-plane magnetic exchange interaction of LCNO is much stronger than LFCO and LFNO, thus LCNO should have much higher magnetic ordering temperature than LFCO and LFNO.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

First-Principles Calculations on the Wettability of Li Atoms on the (111) Surfaces of W and Mo Substrates

Comprehension over the interactions between lithium (Li) atoms and tungsten (W) or molybdenum (Mo) are crucial to improve the wettability of the flowing liquid Li, a candidate plasma facing material in fusion devices, on the surfaces of supported substrate metals. In this work, we utilize first-principles density- functional theory calculations to figure out the adsorption and diffusion properties of Li atoms and clusters on the (111) surfaces of W and Mo. It is found that single Li atom in the fcc-hollow site is the most favored configuration. For the multiple Li atoms adsorption on the substrates, the planar construction is more stable than the stacking one. The electronic structure analysis shows that the lateral interaction between Li atoms is very weak and the binding between Li atom and the substrates is strong; therefore, it can be inferred that the liquid Li is “wetting” intrinsically on the surfaces of the W and Mo substrates. We also investigate the effect of defects (vacancy, H, C, and O) and find that the preexisted vacancy in the substrates has little effect on the wettability; however, the impurities (especially O atom) will hinder the movement of Li atoms on the metal substrates.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Highly dispersed Pt atoms and clusters on hydroxylated indium tin oxide: a view from first-principles calculations

Supported single-atom and small cluster catalysts have become highly popular in heterogeneous catalysis. These catalysts can maximize the metal atom utilization while still showing superior catalytic performance. One of the main challenges in producing these small cluster catalysts is their low binding strength with the support, which causes these small clusters to sinter into larger nanoparticles. We have used first-principles simulations to study small Pt n ( n : 1,2,3) clusters on indium oxide, tin doped indium oxide, and hydroxylated tin doped indium oxide. We report that the Pt n cluster is stabilized in the presence of tin and that this is especially the case for Pt single atoms on the hydroxylated indium tin oxide support, which are anchored to the support via the hydroxyl group. On this support, the Pt single atoms become more stable than Pt 2 and Pt 3 clusters, hence decreasing sintering. These findings provide a promising way to design single-atom catalysts on electrically conducting supports for electrocatalytic applications and to better understand how functional groups on supports can increase the adhesion of cluster catalysts.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Effects of structure distortion on optical phonon properties of crystalline beta-BaTeMo{sub 2}O{sub 9}—A novel nonlinear optical material: Infrared and Raman spectra as well as first-principles calculations

The infrared dielectric property of monoclinic BaTeMo{sub 2}O{sub 9} single crystals is studied by polarized IR reflectance spectra from 20 to 1800 cm{sup −1}. Based on the modified Lorentz model, the frequencies, strengths, and dampings of TO modes as well as the orientations of the dipole momenta are determined, agreeing well with Raman spectra and results from First-principles calculation. The observed modes are visually assigned to the specific atoms' motions in the primitive cell based on the theory calculations. A large shift of the internal modes of the anion groups relative to free anion co-ordination polyhedra is observed, which can be used to indicate the distortions of co-ordination polyhedra related to the nonlinear optical properties. Further, the experimental results of the strengths of the oscillators support the elimination and splitting of degenerate modes in free regular polyhedrons. These results offer a way to evaluate the nonlinear optical properties by use of traditional IR reflectivity spectra.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Effects of nitrogen-related defects on visible light photocatalytic response in N{sup +} implanted TiO{sub 2}: A first-principles study

It was found that the visible-light responsiveness of a nitrogen ion-implanted TiO{sub 2} photocatalyst was attributable to the predominant chemical states of nitrogen, depending on the local nitrogen concentration near the surface. In the present study, we examined the effects of conceivable nitrogen-related defects on the visible light responsiveness, based on electronic structures calculated from first principles. Possible chemical states were proposed by comparing previously reported experiments with the present theoretical N-K X-ray absorption spectra. The theoretically predicted visible light absorption spectra and carrier trap states due to the bandgap states associated with the defects well explained the relationship between the catalytic reactivity and the proposed chemical states.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗