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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 577 records · Page 32
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Ambient pressure x-ray photoelectron spectroscopy study of water formation and adsorption under two-dimensional silica and aluminosilicate layers on Pd(111)

Ambient pressure x-ray photoelectron spectroscopy (AP-XPS) supported by density functional theory (DFT) calculations was used to characterize the interaction of water with two-dimensional (2D) silica and aluminosilicate bilayers on Pd(111). Starting with oxygen adsorbed at the SiO2/Pd interface, exposure to water caused the SiO 2 -derived XPS peaks to shift to higher binding energy and the removal of an O 1s feature associated with interfacial adsorbed oxygen. These observations were attributed to the formation of a mixed water–hydroxyl interface, which eliminates the interfacial dipolar layer, and its associated electrostatic potential, created by adsorbed oxygen. Interfacial oxygen also reacted with H 2 to produce adsorbed water which also caused an upward binding energy shift of the SiO 2 peaks. Spectra recorded under 0.5 Torr water revealed additional water adsorption and a further shift of the overlayer peaks to higher binding energy. Incorporating Al into the 2D material caused the bilayer peaks to shift to lower binding energy which could be explained by electron donation from the metal to the bilayer. Although the stronger interaction between the bilayer and Pd substrate should restrict interfacial adsorption and reaction, similar trends were observed for water and hydrogen exposure to interfacial adsorbed oxygen. Less water adsorption was observed at the aluminosilicate interface which is a consequence of Al strengthening the bond to the metal substrate. The results reveal how the sensitivity of XPS to interfacial dipoles can be exploited to distinguish reactions taking place in confined spaces under 2D layers and how tuning the composition of the 2D layer can impact such reactions.

Density functional theory↗
DOE OSTI2021

Phonon gas model for thermal conductivity of dense, strongly interacting liquids

Developing predictive thermal property models for liquids based on microscopic principles has been elusive. The difficulty is that liquids have gas-like and solid-like attributes that are at odds when considering the frameworks of microscopic models: Models for gases are simple due to randomness and low density, whereas models for crystalline solids rely on symmetry and long-range order for easier calculation. The short-range order in liquids does, however, provide structure to neighboring molecules similar to amorphous solids, and there have been recent advances indicating that collective vibrational modes store heat in liquids. Models combining Debye approximations from solid-state physics and Frenkel’s theory of liquids can accurately predict the heat capacity of liquids. Phonon-like dispersions in liquids have also been widely observed in neutron scattering experiments. These developments motivate us to propose a model where high-frequency vibrational modes, which travel at the speed of sound and have a mean free path on the order of the average intermolecular distance, conduct heat in liquids. We use this liquid phonon gas model to calculate the thermal conductivity of liquids with varying intermolecular interaction energies from strongest to weakest—Coulomb, hydrogen-bonding, Keesom, and London dispersion energy. Generally, the model is more accurate as the intermolecular interaction energy and density of liquids increase. The calculated thermal conductivity of Coulombic-bound molten sodium nitrate and hydrogen-bonded water is within 1.46% and 2.98% of the experimentally measured values, respectively, across their entire temperature ranges. Further modal analysis of the velocity and the mean free path of collective vibrations could establish the liquid phonon gas model as an accurate model for weakly interacting liquids as well.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Complex Fermi–Löwdin orbital self-interaction correction

Here, this paper introduces the use of complex Fermi orbital descriptors (FODs) in the Fermi–Löwdin self-interaction-corrected density functional theory (FLOSIC). With complex FODs, the Fermi–Löwdin orbitals (FLOs) that are used to evaluate the SIC correction to the total energy become complex. Complex FLO-SIC (cFLOSIC) calculations based on the local spin density approximation produce total energies that are generally lower than the corresponding energies found with FLOSIC restricted to real orbitals (rFLOSIC). The cFLOSIC results are qualitatively similar to earlier Perdew–Zunger SIC (PZ-SIC) calculations using complex orbitals. The energy lowering stems from the exchange–correlation part of the self-interaction correction. The Hartree part of the correction is more negative in rFLOSIC. The energy difference between real and complex solutions is greater for more strongly hybridized FLOs in atoms and for FLOs corresponding to double and triple bonds in molecules. The case of N2 is examined in detail to show the differences between the real and complex FLOs. We show that the complex triple-bond orbitals are simple, and physically appealing combinations of π and σ g orbitals that have not been discussed before. Consideration of complex FODs, and resulting unitary transformations, underscores the fact that FLO centroids are not necessarily good guesses for FOD positions in a FLOSIC calculation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Nonthermal Bonding Origin of a Novel Photoexcited Lattice Instability in SnSe

Lattice dynamics measurements are often crucial tools for understanding how materials transform between different structures. Here, we report time-resolved x-ray scattering-based measurements of the nonequilibrium lattice dynamics in SnSe, a monochalcogenide reported to host a novel photoinduced lattice instability. By fitting interatomic force models to the fluence dependent excited-state dispersion, we determine the nonthermal origin of the lattice instability to be dominated by changes of interatomic interactions along a bilayer-connecting bond, rather than of an intralayer bonding network that is of primary importance to the lattice instability in thermal equilibrium.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Optimization of the Reax force field for the lithium–oxygen system using a high fidelity charge model

Studies using molecular dynamics (MD) have long struggled to simulate the failure modes of materials, predicting unrealistically high ductility and failing to capture brittle fracture. The primary cause of this shortcoming is an inadequate description of bond breaking. While reactive force fields such as ReaxFF show improvements compared to traditional force fields, the charge models used yield unphysical partial charges, especially during dissociation of ionic bonds. This flaw may be remedied by using the atom-condensed Kohn–Sham density functional theory (DFT) approximated to a second order (ACKS2) charge model for determining partial charges. In this work, we present a new ACKS2-enabled Reax force field for fracture simulations of lithium oxide systems, which was obtained by training against an extensive set of DFT, multireference configuration interaction (MRCI), and MRCI+Q reference data using genetic optimization techniques. This new force field significantly improves the bond breaking behavior, but still cannot fully capture the brittle fracture in MD simulations, suggesting more research is needed to improve simulation of brittle fracture.

36 MATERIALS SCIENCE↗
DOE OSTI2022

ThAu 2 – , ThAu 2 O – , and ThAuOH – anions: Photoelectron spectroscopic and theoretical characterization

The thorium–gold negative ions ThAu 2 – , ThAu 2 O – , and ThAuOH – have been observed and experimentally characterized by anion photoelectron spectroscopy. These experiments are accompanied by extensive ab initio electronic structure calculations using a relativistic composite methodology based primarily on coupled cluster singles and doubles with perturbative triples calculations. The theoretical electron affinities (EAs) at 0 K agree with the experimental adiabatic EAs to within 0.02 eV for all species. Two separate isomers were located in the calculations for ThAuOH – , and detachment from both of these appears to be present in the photoelectron spectrum. Excited electronic states of the neutral molecules are reported at the equation of motion-coupled cluster singles and doubles level of theory. Atomization energies and heats of formation are also calculated for each neutral species and have expected uncertainties of 3 and 4 kcal/mol, respectively. The σ bonds between Th and Au are determined by natural bond orbital analysis to consist of predominately sd hybrids on Th bonding with the Au 6s orbital. In order to investigate the correspondence between the bonding in Th–Au and Th–F molecules, a limited number of calculations were also carried out on most of the F-analogs of this study. These results demonstrate that Au does behave like F in these cases, although the Th–F σ bonds are much more ionic compared to Th–Au. This results in an EA for ThF 2 that is 10 kcal/mol smaller than that of ThAu 2 . The EA values for the Th(IV) species, i.e., ThX 2 O and ThXOH, only differed, however, by 3–4 kcal/mol.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Real-time equation-of-motion CC cumulant and CC Green’s function simulations of photoemission spectra of water and water dimer

Newly developed coupled-cluster (CC) methods enable simulations of ionization potentials and spectral functions of molecular systems in a wide range of energy scales ranging from core-binding to valence. This paper discusses the results obtained with the real-time equation-of-motion CC cumulant (RT-EOM-CC) approach and CC Green’s function (CCGF) approaches in applications to the water and water dimer molecules. Here, we compare the ionization potentials obtained with these methods for the valence region with the results obtained with the coupled-cluster with singles, doubles, and perturbative triples formulation as a difference of energies for N and N – 1 electron systems. All methods show good agreement with each other. They also agree well with the experiment with errors usually below 0.1 eV for the ionization potentials. We also analyze unique features of the spectral functions, associated with the position of satellite peaks, obtained with the RT-EOM-CC and CCGF methods employing single and double excitations, as a function of the monomer OH bond length and the proton transfer coordinate in the dimer. Finally, we analyze the impact of the basis set effects on the quality of calculated ionization potentials and find that the basis set effects are less pronounced for the augmented-type sets.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Isolation, bonding and reactivity of a monomeric stibine oxide

Abstract In contrast to phosphine oxides and arsine oxides, which are common and exist as stable monomeric species featuring the corresponding pnictoryl functional group (Pn=O/Pn + –O − ; Pn = P, As), stibine oxides are generally polymeric, and the properties of the unperturbed stiboryl group (Sb=O/Sb + –O − ) remain unexplored. We now report the isolation of the monomeric stibine oxide, Dipp 3 SbO (where Dipp = 2,6-diisopropylphenyl). Spectroscopic, crystallographic and computational studies provide insight into the nature of the Sb=O/Sb + –O − bond. Moreover, isolation of Dipp 3 SbO allows the chemistry of the stiboryl group to be explored. Here we show that Dipp 3 SbO can act as a Brønsted base, a hydrogen-bond acceptor and a transition-metal ligand, in addition engaging in 1,2-addition, O-for-F 2 exchange and O-atom transfer. In all cases, the reactivity of Dipp 3 SbO differed from that of the lighter congeners Dipp 3 AsO and Dipp 3 PO.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Interfacial charge transfer and interaction in the MXene/TiO 2 heterostructures

Hybrid materials of MXenes [two-dimensional (2D) carbides and nitrides] and transition-metal oxides have shown great promise in electrical energy storage (EES) and 2D heterostructures have been proposed as the next-generation electrode materials to expand the limits of current technology. Here we use first principles density functional theory to investigate the interfacial structure, energetics, and electronic properties of the heterostructures of MXenes (Ti n+1 C n T 2 ; T = terminal groups) and anatase TiO 2 . We find that the greatest work-function differences are between OH-terminated MXene (1.6 eV) and anatase TiO 2 (101) (6.4 eV), resulting in the largest interfacial electron transfer (~0.9e/nm 2 across the interface) from MXene to the TiO 2 layer. This interface also has the strongest adhesion and is further strengthened by hydrogen bond formation. For O–, F–, or mixed O–/F– terminated Ti n+1 C n MXenes, electron transfer is minimal and interfacial adhesion is weak for their heterostructures with TiO 2 . The strong dependence of the interfacial properties of the MXene/TiO 2 heterostructures on the surface chemistry of the MXenes will be useful to tune the heterostructures for EES applications.

2-dimensinal systems↗
DOE OSTI2022

Structural and dynamic properties of solvated hydroxide and hydronium ions in water from ab initio modeling

Predicting the asymmetric structure and dynamics of solvated hydroxide and hydronium in water from ab initio molecular dynamics (AIMD) has been a challenging task. The difficulty mainly comes from a lack of accurate and efficient exchange–correlation functional in elucidating the amphiphilic nature and the ubiquitous proton transfer behaviors of the two ions. By adopting the strongly constrained and appropriately normed (SCAN) meta-generalized gradient approximation functional in AIMD simulations, we systematically examine the amphiphilic properties, the solvation structures, the electronic structures, and the dynamic properties of the two water ions. In particular, we compare these results to those predicted by the PBE0-TS functional, which is an accurate yet computationally more expensive exchange–correlation functional. We demonstrate that the general-purpose SCAN functional provides a reliable choice for describing the two water ions. Specifically, in the SCAN picture of water ions, the appearance of the fourth and fifth hydrogen bonds near hydroxide stabilizes the pot-like shape solvation structure and suppresses the structural diffusion, while the hydronium stably donates three hydrogen bonds to its neighbors. We apply a detailed analysis of the proton transfer mechanism of the two ions and find the two ions exhibit substantially different proton transfer patterns. In conclusion, our AIMD simulations indicate that hydroxide diffuses more slowly than hydronium in water, which is consistent with the experimental results.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Elucidating the water–anatase TiO 2 (101) interface structure using infrared signatures and molecular dynamics

The structure and dynamics of water on solid surfaces critically affect the chemistry of materials in ambient and aqueous environments. Here, we investigate the hydrogen bonding network of water adsorbed on the majority (101) surface of anatase TiO 2 , a widely used photocatalyst, using polarization- and azimuth-resolved infrared spectroscopy combined with neural network potential molecular dynamics simulations. Our results show that one monolayer of water saturates the undercoordinated titanium (Ti 5c ) sites, forming one-dimensional chains of molecule hydrogen bonded to surface undercoordinated bridging oxygen (O 2c ) atoms. As the coverage increases, water adsorption on O 2c sites leads to significant restructuring of the water monolayer and the formation of a two-dimensional hydrogen bond network characterized by tightly bound pairs of water molecules on adjacent Ti 5c and O 2c sites. This structural motif likely persists at ambient conditions, influencing the reactions occurring there. In conclusion, the results reported here provide critical details of the structure of the water–anatase (101) interface that were previously hypothesized but unconfirmed experimentally.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Structural disorder-driven topological phase transition in noncentrosymmetric BiTeI

In this work, we investigate using local structural disorder to induce a topologically nontrivial phase in a solid state system. Using first-principles calculations, we introduce structural disorder in the trivial insulator BiTeI and observe the emergence of a topological insulating phase. By modifying the bonding environments, the crystal-field splitting is enhanced, with spin-orbit interactions producing a band inversion in the bulk electronic structure. Analysis of the Wannier charge centers and the surface electronic structure reveals a strong topological insulator with Dirac surface states. Finally, we propose a prescription for inducing topological states from disorder in crystalline materials. Understanding how local environments produce topological phases is a key step for predicting disordered and amorphous topological materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Atomic-scale defects generated in the early/intermediate stages of dielectric breakdown in Si/SiO 2 transistors

In this study, electrically detected magnetic resonance and near-zero-field magnetoresistance measurements were used to study atomic-scale traps generated during high-field gate stressing in Si/SiO 2 MOSFETs. The defects observed are almost certainly important to time-dependent dielectric breakdown. The measurements were made with spin-dependent recombination current involving defects at and near the Si/SiO 2 boundary. The interface traps observed are P b0 and P b1 centers, which are silicon dangling bond defects. The ratio of P b0 /P b1 is dependent on the gate stressing polarity. Electrically detected magnetic resonance measurements also reveal generation of E' oxide defects near the Si/SiO 2 interface. Near-zero-field magnetoresistance measurements made throughout stressing reveal that the local hyperfine environment of the interface traps changes with stressing time; these changes are almost certainly due to the redistribution of hydrogen near the interface.

42 ENGINEERING↗
DOE OSTI2024

Dynamics of hydride anion and acetyloxyl radical production by electron attachment to acetic acid

Here, we investigate the dynamics and site-selectivity in the dissociation of transient anions formed upon attachment of low energy electrons to acetic acid by anion fragment momentum imaging experiments. The resonances at 6.7 and 7.7 eV are confirmed to dissociate exclusively by the O–H bond, while a third resonance at 9.1 eV dissociates primarily by both C–H break and O–H break. A fourth resonance near 10 eV is found to dissociate by O–H break. For each resonance, the measured kinetic energy release indicates two-body dissociation produces a neutral radical in the ground electronic state, for all four resonances. The measured angular distributions are consistent with all four resonances having A' symmetry.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Ultrafast formation of a transient two-dimensional diamondlike structure in twisted bilayer graphene

Due to the absence of matching carbon atoms at honeycomb centers with carbon atoms in adjacent graphene sheets, theorists predicted that a sliding process is needed to form AA, AB', or ABC stacking when directly converting graphite into sp 3 bonded diamond. Here, using twisted bilayer graphene, which naturally provides AA and AB' stacking configurations, we report the ultrafast formation of a transient two-dimensional diamondlike structure (which is not observed in aligned graphene) under femtosecond laser irradiation. This photoinduced phase transition is evidenced by the appearance of bond lengths of 1.94 and 3.14 Å in the time-dependent differential pair distribution function using MeV ultrafast electron diffraction. Molecular dynamics and first-principles calculation indicate that sp 3 bonds nucleate at AA and AB' stacked areas in a moiré pattern. This work sheds light on the direct graphite-to-diamond transformation mechanism, which has not been fully understood for more than 60 years.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Understanding Oxide–Metal Interactions During Hot Isostatic Pressing to Diffusion Bond Aluminum Alloy 6061 Plates

The interaction between Mg, Si, and Al 2 O 3 during hot isostatic pressing diffusion bonding of aluminum alloy 6061 (AA6061) plates was investigated through thermodynamic calculations and experimental microstructural characterization. Thermodynamic calculations as functions of temperature, pressure, and composition revealed that the interaction among Mg, Si, and Al 2 O 3 yields Mg 2 Si and either MgO + Al or MgAl 2 O 4 + Al, facilitating the reduction of Al 2 O 3 and allowing Al/Al metallic bonds to form. Total pressure variation had a negligible influence on the oxygen partial pressure, and consequently, the reaction product formation. Oxygen partial pressure variation as a function of temperature and initial amount of Al 2 O 3 determined the formation of either MgO or MgAl 2 O 4 . Experimental Hot Isostatic Pressure (HIP) bonding at 723 K and 833 K under a constant pressure of 1017 atm documented the cooling rate-dependent formation of β–Mg 2 Si precipitates. High-resolution transmission electron microscopy imaging and selected area electron diffraction patterns verified the formation of β–Mg 2 Si and MgO at the interface but did not detect MgAl 2 O 4 . In conclusion, findings from this study clarify the role of thermochemical interactions in oxide disruption and bonding mechanisms during HIP diffusion bonding of AA6061 and provide guidance for optimizing joining processes for monolithic nuclear fuel assemblies.

11 - NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2025

Microscopic insights into the solvation of polyethylene glycol chains in water: A machine learning potential approach

Polyethylene glycol (PEG) is a structurally simple, nontoxic, and water-soluble polymer widely utilized in medical and pharmaceutical applications. Notably, when a PEG chain is immersed in water, the surrounding water molecules play a key role in driving conformational changes of this macromolecule. In this study, we explore the solvation behavior of PEG under mechanical strain using molecular dynamics simulations, with an interatomic potential obtained from machine learning. Our focus is on the transition from the favored coil-like conformation to an extended one under external force. Through analyses of radial distribution functions, hydrogen bonding, and solvation dynamics, we uncover how mechanical stretching influences the local hydration environment. Furthermore, we disentangle the enthalpic and entropic contributions to the conformational stability of PEG in water. Surprisingly, our neural network potential model identifies dewetting of PEG C-atoms, and not water H-bonding with PEG O-atoms, as the main enthalpic driving force for the coiling of PEG in water.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Atomistic simulations for investigation of substrate effects on lipid in-source fragmentation in secondary ion mass spectrometry

In beam-based ionization methods, the substrate plays an important role on the desorption mechanism of molecules from surfaces. Both the specific orientation that a molecule adopts at a surface and the strength of the molecule-surface interaction can greatly influence desorption processes, which in turn will affect the ion yield and the degree of in-source fragmentation of a molecule. In the beam-based method of secondary ion mass spectrometry (SIMS), in-source fragmentation can be significant and molecule specific due to the hard ionization method of using a primary ion beam for molecule desorption. To investigate the role of the substrate on orientation and in-source fragmentation, we have used atomistic simulations—molecular dynamics in combination with density functional theory calculations—to explore the desorption of a sphingolipid (palmitoylsphingomyelin) from a model surface (gold). We then compare SIMS data from this model system to our modeling findings. Using this approach, we found that the combined adsorption and binding energy of certain bonds associated with the headgroup fragments (C3H8N+, C5H12N+, C5H14NO+, and C5H15PNO4+) was a good predictor for fragment intensities (as indicated by relative ion yields). This is the first example where atomistic simulations have been applied in beam-based ionization of lipids, and it presents a new approach to study biointerfacial lipid ordering effects on SIMS imaging.

59 BASIC BIOLOGICAL SCIENCES↗