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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 469 records · Page 26
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

AENET–LAMMPS and AENET–TINKER : Interfaces for accurate and efficient molecular dynamics simulations with machine learning potentials

Machine-learning potentials (MLPs) trained on data from quantum-mechanics based first-principles methods can approach the accuracy of the reference method at a fraction of the computational cost. To facilitate efficient MLP-based molecular dynamics and Monte Carlo simulations, an integration of the MLPs with sampling software is needed. Here, we develop two interfaces that link the atomic energy network (ænet) MLP package with the popular sampling packages TINKER and LAMMPS. The three packages, ænet, TINKER, and LAMMPS, are free and open-source software that enable, in combination, accurate simulations of large and complex systems with low computational cost that scales linearly with the number of atoms. Scaling tests show that the parallel efficiency of the ænet–TINKER interface is nearly optimal but is limited to shared-memory systems. The ænet–LAMMPS interface achieves excellent parallel efficiency on highly parallel distributed memory systems and benefits from the highly optimized neighbor list implemented in LAMMPS. We demonstrate the utility of the two MLP interfaces for two relevant example applications: the investigation of diffusion phenomena in liquid water and the equilibration of nanostructured amorphous battery materials.

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

Polar liquids at charged interfaces: A dipolar shell theory

The structure of polar liquids and electrolytic solutions, such as water and aqueous electrolytes, at interfaces underlies numerous phenomena in physics, chemistry, biology, and engineering. In this work, we develop a continuum theory that captures the essential features of dielectric screening by polar liquids at charged interfaces, including decaying spatial oscillations in charge and mass, starting from the molecular properties of the solvent. The theory predicts an anisotropic dielectric tensor of interfacial polar liquids previously studied in molecular dynamics simulations. We explore the effect of the interfacial polar liquid properties on the capacitance of the electrode/electrolyte interface and on hydration forces between two plane-parallel polarized surfaces. In the linear response approximation, we obtain simple formulas for the characteristic decay lengths of molecular and ionic profiles at the interface.

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

Trap passivation of 4H-SiC/SiO 2 interfaces by nitrogen annealing

We report an N 2 based annealing treatment to passivate interface traps (D it ) in n- and p-type 4H-SiC. The process has the potential to replace the commonly used hazardous and expensive gas nitric oxide (NO). N 2 postoxidation annealing reduces D it in both the upper and lower halves of the 4H-SiC bandgap, with a greater impact at the valence band edge. N 2 annealing at 1500 °C is observed to be more effective in passivating traps and positive fixed charges than NO annealing for p-type devices, whereas for n-type devices, the opposite is true. The breakdown voltages for these devices are found to be lower than that of NO annealed devices. X-ray photoelectron spectroscopy has been performed to estimate the nitrogen areal density at the interface. D it is measured as a function of nitrogen areal densities in the near interfacial regions for the different processes. Additionally, theoretical analysis through density functional theory is consistent with the measured D it profiles by showing the generation of additional states near the valence band edge due to increased nitrogen concentration. In addition to reporting the effect of N 2 annealing on both n- and p-type 4H-SiC, this work correlates the nitrogen areal densities at the interface to the D it and explains the difference in D it characteristics with nitrogen areal density between n- and p-type interfaces.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Interface-modulated kinetic differentials in electron and hole transfer rates as a key design principle for redox photocatalysis by Sb2VO5/QD heterostructures

The efficient conversion of solar energy to chemical energy represents a critical bottleneck to the energy transition. Photocatalytic splitting of water to generate solar fuels is a promising solution. Semiconductor quantum dots (QDs) are prime candidates for light-harvesting components of photocatalytic heterostructures, given their size-dependent photophysical properties and band-edge energies. A promising series of heterostructured photocatalysts interface QDs with transition-metal oxides which embed midgap electronic states derived from the stereochemically active electron lone pairs of p-block cations. Here, we examine the thermodynamic driving forces and dynamics of charge separation in Sb2VO5/CdSe QD heterostructures, wherein a high density of Sb 5s2-derived midgap states are prospective acceptors for photogenerated holes. Hard-x-ray valence band photoemission spectroscopy measurements of Sb2VO5/CdSe QD heterostructures were used to deduce thermodynamic driving forces for charge separation. Interfacial charge transfer dynamics in the heterostructures were examined as a function of the mode of interfacial connectivity, contrasting heterostructures with direct interfaces assembled by successive ion layer adsorption and reaction (SILAR) and interfaces comprising molecular bridges assembled by linker-assisted assembly (LAA). Transient absorption spectroscopy measurements indicate ultrafast (<2 ps) electron and hole transfer in SILAR-derived heterostructures, whereas LAA-derived heterostructures show orders of magnitude differentials in the kinetics of hole (<100 ps) and electron (∼1 ns) transfer. The interface-modulated kinetic differentials in electron and hole transfer rates underpin the more effective charge separation, reduced charge recombination, and greater photocatalytic efficiency observed for the LAA-derived Sb2VO5/CdSe QD heterostructures.

Chemistry↗
DOE OSTI2024

Thermal conductance of interfaces between titanium nitride and group IV semiconductors at high temperatures

Measuring the temperature dependence of material properties is a standard method for better understanding the microscopic origins for that property. Surprisingly, only a few experimental studies of thermal boundary conductance at high temperatures exist. This lack of high temperature data makes it difficult to evaluate competing theories for how inelastic processes contribute to thermal conductance. To address this, we report time domain thermoreflectance measurements of the thermal boundary conductance for TiN on diamond, silicon-carbide, silicon, and germanium between 120 and 1000 K. In all systems, the interface conductance increases monotonically without stagnating at higher temperatures. For TiN/SiC interfaces, G ranges from 330 to 1000 MW/m2-K, with a room temperature conductance of 750 MW/m2-K. The interface conductance for TiN/diamond ranges from 140 to 950 MW/m2-K. Notably, for all four interfacial systems, the conductance continues to increase with temperature even after all phonon modes in the vibrationally soft material are thermally excited. This observation suggests that inelastic processes are significant contributors to the thermal conductance in all four interfacial systems, regardless of whether the materials forming the interface are vibrationally similar or dissimilar. Our study fills a notable gap in the literature for how interfacial conductance evolves at high temperatures and tests burgeoning theories for the role of inelastic processes in interfacial thermal transport.

Physics↗
DOE OSTI2025

Type-I and type-II interfaces in a MoSe 2 /WS 2 van der Waals heterostructure

We report experimental evidence that MoSe 2 and WS 2 allow the formation of type-I and type-II interfaces, according to the thickness of the former. Heterostructure samples are obtained by stacking a monolayer WS 2 flake on top of a MoSe 2 flake that contains regions of thickness from one to four layers. Photoluminescence spectroscopy and transient absorption measurements reveal a type-II interface in the regions of monolayer MoSe 2 in contact with monolayer WS 2 . In other regions of the heterostructure formed by multilayer MoSe 2 and monolayer WS 2 , features of type-I interface are observed, including the absence of charge transfer and dominance of intralayer excitons in MoSe 2 . Furthermore, the coexistence of type-I and type-II interfaces in a single heterostructure offers opportunities to design sophisticated two-dimensional materials with finely controlled photocarrier behaviors.

2D materials↗
DOE OSTI2025

Quantitative characterization of ZrO2 gate dielectric interface with tellurium

Tellurium (Te) has recently emerged as a promising p-type semiconductor that can be processed at low temperatures, compatible with back end of line CMOS integration. Characterization of tellurium–dielectric interfaces is essential for further device advancements. Here, the interface quality of Te with ZrO2 gate dielectric is studied in a metal-oxide semiconductor capacitor structure. The interface trap density (Dit) is measured as a function of atomic layer deposition (ALD) temperature, without the use of a seed layer. Given the low thermal budget of Te, the ALD temperature is shown to be particularly important. The lowest Dit of 5 × 1012 states/cm2 eV is obtained at a low ALD process temperature of 120 °C. To further assess the impact of Dit on device performance, field-effect transistors (FETs) were fabricated. The subthreshold swing and effective hole mobility of the FETs were analyzed in relation to Dit, emphasizing the importance of a defect-minimized interface for enhancing Te transistor performance.

Byeon, Kyeong-Jae↗
DOE OSTI2025

A fluctuating hydrodynamics model for nanoscale surfactant-laden interfaces

A multispecies diffuse interface model is formulated in a fluctuating hydrodynamics framework for the purpose of simulating surfactant interfaces at the nanoscale. The model generalizes previous work to ternary mixtures, employing a Cahn-Hilliard free energy density combined with incompressible, isothermal fluctuating hydrodynamics where dissipative fluxes include both deterministic and stochastic terms. The intermolecular parameters in the free energy are chosen such that one species acts as a partially miscible surfactant. From Laplace pressure measurements, we show that in this model the surface tension decreases linearly with surfactant concentration, leading to Marangoni convection for interfaces with concentration gradients. In the capillary wave spectrum for interfaces with and without surfactant, we find that for the former, the spectrum deviates significantly from classical capillary wave theory, presumably due to Gibbs elasticity. In non-equilibrium simulations of the Rayleigh-Plateau instability, deterministic simulations showed that the surfactant delays pinching of a fluid cylinder into droplets. However, stochastic simulations indicate that thermal fluctuations disrupt the surfactant's stabilizing effect. Similarly, the spreading of a patch of surfactant, driven by Marangoni convection, was found to be partially suppressed by thermal fluctuations.

Capillary waves↗
DOE OSTI2020

Deep learning of interface structures from simulated 4D STEM data: cation intermixing vs. roughening ∗

Abstract Interface structures in complex oxides remain an active area of condensed matter physics research, largely enabled by recent advances in scanning transmission electron microscopy (STEM). Yet the nature of the STEM contrast in which the structure is projected along the given direction precludes separation of possible structural models. Here, we utilize deep convolutional neural networks (DCNN) trained on simulated 4D STEM datasets to predict structural descriptors of interfaces. We focus on the widely studied interface between LaAlO 3 and SrTiO 3 , using dynamical diffraction theory and leveraging high performance computing to simulate thousands of possible 4D STEM datasets to train the DCNN to learn properties of the underlying structures on which the simulations are based. We test the DCNN on simulated data and show that it is possible (with >95% accuracy) to identify a physically rough from a chemically diffuse interface and create a DCNN regression model to predict step positions. We quantify the applicability of the model to different thicknesses and the transferability of the approach. The method shown here is general and can be applied for any inverse imaging problem where forward models are present.

42 ENGINEERING↗
DOE OSTI2021

Seismic waves in medium with poroelastic/elastic interfaces: a two-dimensional P-SV finite-difference modelling

SUMMARY We present a new methodology of the finite-difference (FD) modelling of seismic wave propagation in a strongly heterogeneous medium composed of poroelastic (P) and (strictly) elastic (E) parts. The medium can include P/P, P/E and E/E material interfaces of arbitrary shapes. The poroelastic part can be with (i) zero resistive friction, (ii) non-zero constant resistive friction or (iii) JKD model of the frequency-dependent permeability and resistive friction. Our FD scheme is capable of subcell resolution: a material interface can have an arbitrary position in the spatial grid. The scheme keeps computational efficiency of the scheme for a smoothly and weakly heterogeneous medium (medium without material interfaces). Numerical tests against independent analytical, semi-analytical and spectral-element methods prove the efficiency and accuracy of our FD modelling. In numerical examples, we indicate effect of the P/E interfaces for the poroelastic medium with a constant resistive friction and medium with the JKD model of the frequency-dependent permeability and resistive friction. We address the 2-D P-SV problem. The approach can be readily extended to the 3-D problem.

58 GEOSCIENCES↗
DOE OSTI2024

Molecular-Resolution Electron Imaging of Defects and Dynamics at the Ice-Water Interface

Water crystallization into hexagonal ice (type I h ) is one of the most critical processes relevant to the Earth’s environment and human activities. However, despite recent breakthroughs in imaging non-equilibrium condensed ice structures, the ice-water interface has never been imaged at a molecular resolution. This is primarily due to the low stability of the hydrogen bonds in ice under high-resolution microscopy conditions and a lack of methods to prepare compatible samples. Here, this presentation describes the first molecular-resolution imaging of ice crystallized from liquid water and the ice-water interface using high-resolution transmission electron microscopy (HRTEM). By encapsulating deionized (DI) water between two amorphous carbon (a-C) TEM grids and subsequently freezing it with liquid N 2 on a cryo sample stage, we generated two types of ice: non-equilibrium, condensed ice from the atmosphere and encapsulated ice from the DI water (Fig. 1A). Condensed ice usually shows irregular, spherulitic shapes (Fig. 1B). Selected area electron diffraction (SAED) shows that they are a mixture of cubic and hexagonal crystals (Fig. 1C). On the contrary, encapsulated ice forms thin films that contain large-area single-crystalline regions of hexagonal ice oriented along the [0001] zone axis (Fig. 1D). Differential electron energy-loss spectroscopy (EELS) confirmed the high purity of the encapsulated ice free from organic contaminations that are common in other encapsulation methods for HRTEM such as graphene liquid cells. These single-crystalline areas are robust under the electron beam up to ~100 e/Å 2 s. Aberration-corrected HRTEM imaging in these areas achieved a line resolution of ~1.3 Å (Fig. 1E and F). This platform allows us to study near-equilibrium ice structures and dynamics at an unprecedented spatial resolution (Fig. 2). For example, we discovered subdomain-rich regions near the defective crystal edges despite the structure appearing single-crystalline according to diffraction criteria. These subdomains connect via low-angle grain boundaries with flat energy landscapes as a function of tilt angles (according to simulations), showing the high tolerance of ice to defect structures. When we tuned the sample temperature and electron flux rate, we observed radiolysis-controlled bubble generation and dissolution in ice single crystals near a steady state of bubble dynamics. Furthermore, rich beam-induced melting and recrystallization dynamics were observed at the ice-water interface with lattice resolution. These data represent the first observation of the ice-water phase transformation at the sub-nanometer level. In summary, the methods developed in this work enabled molecular-resolution observations of ice and the ice-water interface and shed light on the microstructures and phase transformation pathways. Finer control on the temperature, electron irradiation profile, and imaging detector could eventually lead to real-time observation of ice nucleation in water and address long-standing questions in the nucleation pathways.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2026

Evidence of topological charge polarization at active-passive interfaces in acoustically powered active liquid crystals

Theoretical and computational studies predict topological charge separation at active-passive interfaces in active nematics, but reliable experimental validation has been lacking. We utilize a synthetic acoustically energized liquid crystal to experimentally investigate the dynamics of topological defects across an active-passive interface. The interference pattern induced by the acoustic wave inside the experimental cell creates a spatial distribution of activity, resulting in the formation of active-passive interfaces with the liquid-crystalline director field aligned parallel to those interfaces. The activity gradient drives the reorientation of positively charged topological defects along its direction, causing them to migrate into the passive zone and form a layer with net topological charge. That layer “discharges” upon cessation of activity through annihilation of positive and negative topological charges, resembling the discharge of a capacitor.

Active defects↗
DOE OSTI2020

Effective spin-mixing conductance of topological-insulator/ferromagnet and heavy-metal/ferromagnet spin-orbit-coupled interfaces: A first-principles Floquet-nonequilibrium Green function approach

The spin-mixing conductance (SMC) is a key quantity determining efficiency of spin transport across interfaces. Thus, knowledge of its precise value is required for accurate measurement of parameters quantifying numerous effects in spintronics, such as spin-orbit torque, spin Hall magnetoresistance, spin Hall effect, and spin pumping. However, the standard expression for SMC, provided by the scattering theory in terms of the reflection probability amplitudes, is inapplicable when strong spin-orbit coupling (SOC) is present directly at the interface. This is the precisely the case of topological-insulator/ferromagnet and heavy-metal/ferromagnet interfaces of great contemporary interest. We introduce an approach where first-principles Hamiltonian of these interfaces, obtained from noncollinear density functional theory (ncDFT) calculations, is combined with charge-conserving Floquet-nonequilibrium-Green-function formalism to compute directly the pumped spin current $I^{S_z}_L$ into semi-infinite left lead of two-terminal heterostructures Cu/X/Co/Cu or Y/Co/Cu—where X = Bi 2 Se 3 and Y = Pt or W—due to microwave-driven steadily precessing magnetization of the Co layer. This allows us to extract an effective SMC as a prefactor in $I^{S_z}_L$ versus precession cone angle θ dependence, as long as it remains the same, $I^{S_z}$ ∝ sin 2 θ , as in the case where SOC is absent. By comparing calculations where SOC in switched off versus switched on in ncDFT calculations, we find that SOC consistently reduces the pumped spin current and therefore the effective SMC.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Interface magnetism and anomalous Hall effect in La 0.7 ⁢Sr 0.3⁢ Mn⁢O 3 /SrIr⁢O 3 bilayers

Epitaxial interfaces combining 3d and 5d transition metal oxides are a fertile playground to examine the interplay between topology and electron correlations. In this work, we explore magnetism and transport of bilayers combining ferromagnetic La 0.7⁢ Sr 0.3⁢ Mn⁢O 3 and the strong spin-orbit coupling material SrIr⁢O 3 . We have found an interfacial magnetic proximity effect driving an intrinsic contribution to the anomalous Hall effect of topological origin. The interfacial proximity interaction is enabled by the Mn-O-Ir bonding reconstruction and depends on layer sequence. While bilayers with the La 0.7⁢ Sr 0.3 ⁢Mn⁢O 3 on top featuring a robust Mn magnetism at the interface show the intrinsic AHE contribution, it is absent in bilayers with the inverted layer sequence (SrIr⁢O 3 layer on top) showing strongly suppressed magnetism at the interface. These results illustrate the leading role of interfacial atomic reconstructions on the interplay between topology and correlations at 3d/5d oxide interfaces. This finding may be of interest in future oxide topological spintronics and spin-orbitronics.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Colloquium : Sliding and pinning in structurally lubric 2D material interfaces

A plethora of two-dimensional (2D) materials have been introduced in physics and engineering in the past two decades. Their robust, membranelike sheets permit (mostly require) deposition, giving rise to solid-solid dry interfaces whose mobility, pinning, and general tribological properties under shear stress are currently being understood and controlled, both experimentally and theoretically. Here, in this Colloquium simulated case studies of twisted graphene systems are used as a prototype workhorse tool to demonstrate and discuss the general picture of 2D material interface sliding. First highlighted is the crucial mechanical difference, often overlooked, between small and large incommensurabilities, which corresponds to, for example, small and large twist angles in graphene interfaces. In both cases, focusing on flat, structurally lubric or “superlubric” geometries, the generally separate scalings with the area of static friction in pinned states and of kinetic friction during sliding are elucidated and reviewed, tangled as they are with the effects of velocity, temperature, load, and defects. The roles of island boundaries and elasticity are also discussed, and compared when possible to results in the literature for systems other than graphene. It is proposed that the resulting picture of pinning and sliding should be applicable to interfaces in generic 2D materials that are of importance for the physics and technology of existing and future bilayer and multilayer systems.

2-dimensional systems↗
DOE OSTI2020

Impact of interface materials on side permeation in indirect encapsulation of organic electronics

This work demonstrates the impact of the contact interface between barrier films and adhesives on the side permeation of moisture into packaged devices. When barrier films are brought into contact with the adhesive layer during indirect encapsulation, permeation along defects at this interface can occur due to the imperfect nature of contact, resulting in the formation of pores. The connected network of pores can act as capillaries and be an alternative pathway for water permeation as opposed to the bulk of the adhesive or edge seal materials used for barrier attachment to the package. The rate of water permeation through the capillaries is governed by surface energies of the materials at the interfaces. Experimental results demonstrate that the rate of water permeation is significantly lowered by using materials with higher contact angles at the interface.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Investigating the magnetic and atomic interface configuration for a model Fe/CrN bilayer system

A bilayer of iron on chromium nitride (Fe/CrN) is an interesting system for exchange biasing and sensing applications as the Néel temperature of CrN is 280 K and the Curie temperature of Fe is 1043 K. In this paper, we study the crystal and magnetic structures of the Fe/CrN interface at the atomic level. High quality epitaxial Fe/CrN bilayers prepared by molecular beam epitaxy grow in 001 orientation on MgO(001) substrates with uniform layer thicknesses and sharp interfaces. Our data reveal the epitaxial correlation between Fe and CrN crystals and their magnetic structures at the interface. The magnetic anisotropy directions of Fe and CrN are found parallel to [110] MgO . We studied the electronic and magnetic properties of the interface by performing the first-principles total-energy calculations. Here, we present a model that combines the crystal and magnetic structures of the Fe/CrN bilayer and fully explains all results.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

In situ x-ray photoelectron spectroscopy analysis of electrochemical interfaces in battery: Recent advances and remaining challenges

An in-depth understanding of charge transfer processes at the electrochemical interfaces is a critical knowledge gap impeding the design of energy storage materials. X-ray photoelectron spectroscopy plays an important role in analyzing electronic structures of heterogeneous interfaces, such as electrode-electrolyte interphases. Correspondingly, ex situ studies based on postmortem analysis of electrode materials using x-ray techniques are widely reported in the literature but often fail to capture intermediate and transient species, which are critical for a predictive understanding of the charge transfer process. The lack of extensive in situ/operando x-ray analysis of buried interfaces in energy storage systems can be mainly attributed to technical limitations, such as the requirement of high vacuum conditions. However, in the past decade, considerable efforts have been devoted to overcoming these technical barriers and enable investigation of the solid/solid and solid/liquid interfaces. This review catalogs some of the recent progresses and new experimental designs in the application of in situ and operando x-ray photoelectron spectroscopy toward characterizing interfacial processes and emergent properties, which can help build the design strategy for advanced batteries. The remaining challenges and future research directions are also discussed, as potential paths forward in this field.

Wi, Sungun↗