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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 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Crack formation in strained β-(AlxGa 1−x ) 2 O 3 films grown on (010) β-Ga 2 O 3 substrates

The cracking and local strain relaxation in (010) (Al x Ga 1−x ) 2 O 3 films grown on Ga 2 O 3 substrates are assessed in terms of film composition and thickness. We utilize x-ray diffraction and electron microscopy techniques combined with simulation and modeling to investigate that film cracking on curvature (flatness) has a directly proportional relationship with film thickness and/or aluminum content. Cross section transmission electron microscopy reveals that cracks along both the (001) and (100) cleavage planes penetrate into the substrate. The diffuse scattered intensity observed in reciprocal space maps (RSMs) is directly correlated with the tilt that is introduced due to the change in the deformation conditions near the cracks. While asymmetric RSMs show that the layers are fully strained, the diffuse scattering distribution in reciprocal space can be interpreted to show that the cracking relaxes and locally tilts the lattice ∼350 nm from the crack edges, which is consistent with the larger radius of curvature associated with the films with higher crack densities. For example, a 200 nm (Al 0.13 Ga 0.87 ) 2 O 3 thick film has an average inter-crack spacing of 3.3 µm, so most of the epitaxial layer is fully strained except near the cracks where it deforms elastically and is consistent with the gallium oxide Poisson ratio. A reciprocal space model was developed, which imports the strain and tilt distributions (based on finite element modeling) to match the features observed in the experimental maps. We also note that previous studies involving (Al x Ga 1−x ) 2 O 3 films may show evidence of cracking as observed in their symmetric and asymmetric RSMs.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Formation of tubular conduction channel in a SiGe(P)/Si core/shell nanowire heterostructure

Realizing a tubular conduction channel within a one-dimensional core–shell nanowire (NW) enables better understanding of quantum phenomena and exploration of electronic device applications. Herein, we report the growth of a SiGe(P)/Si core/shell NW heterostructure using a chemical vapor deposition coupled with vapor–liquid–solid growth mechanism. The entire NW heterostructure behaves as a p-type semiconductor, which demonstrates that the high-density carriers are confined within the 4 nm-thick Si shell and form a tubular conduction channel. These findings are confirmed by both calculations and the gate-dependent current–voltage ( I d – V g ) characteristics. Atomic resolution microscopic analyses suggest a coherent epitaxial core/shell interface where strain is released by forming dislocations along the axial direction of the NW heterostructure. Additional surface passivation achieved via growing a SiGe(P)/Si/SiGe core/multishell NW heterostructure suggests potential strategies to enhance the tubular carrier density, which could be further modified by improving multishell crystallinity and structural design.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Self-assembled Ag(111) nanostructures induced by Fermi surface nesting

Scanning tunneling microscopy measurements on Ag(111)/MoS 2 reveal atomically flat preferred, or “magic,” heights occurring at 6, 10, and 14 atomic layers. We note these results are consistent with Ag growth on a variety of semiconducting substrates and correlate with electronic energy savings in electronic structure calculations of freestanding Ag(111) films. Thus, under certain conditions, Ag will spontaneously form quantized structures independent of the substrate. To explain this, we have found Fermi surface nesting vectors in the bulk Ag band structure which account for these results and the fact Ag that is gapped along the surface normal. This model extends to a range of metallic systems which exhibit electronic confinement, epitaxial growth, and minimal strain. As with Au/MoS 2 , the Ag/MoS 2 system exhibits this behavior at unusually high temperatures so that these principles might be used for control over device features at the nanometer scale under standard fabrication conditions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Interfacial magnetic characteristics of nearly compensated gadolinium iron garnet

Reports on spin Hall magnetoresistance, magnonic spin currents from thermal gradients, and spin transfertorque magnetic random-access memory using compensated ferrimagnets largely discuss bulk magnetization but lack consideration of depth profiles or interfacial characteristics. Here, magnetic and structural characterization of profiles and interfaces was performed for nearly compensated gadolinium iron garnet (GdIG) thin films. X-ray diffraction and reciprocal space maps show that sputter deposited GdIG on Si is polycrystalline with the desired cubic garnet phase, and GdIG on gadolinium gallium garnet (GGG) is epitaxial with <0.06% compressive strain. Temperature-dependent magnetometry confirms the compensation temperatures of GGG/GdIG and Si/GdIG to be 285 and 260 K, respectively, both near room temperature. Interestingly, these measurements suggest the presence of unsaturated rare-earth moments, which result in a characteristic hysteresis between heating and cooling sequences in the magnetization-temperature curves at zero field. Depth-profile measurements from polarized neutron reflectometry (PNR) indicate up to 91% volume fraction in GdIG on Si. At the interface, PNR reveals a region containing magnetized Fe-doped GGG, a low-density GdIG at the GGG/GdIG interface, and a thin magnetically dead layer at the Si/GdIG interface. Cross-sectional transmission electron microscopy and energy dispersive x-ray spectroscopy confirm the assessment of PNR. In conclusion, the magnetic characteristics of interfacial regions are attributed to intermixing of Fe-Ga at the GGG/GdIG interface and the presence of amorphous Fe-Si at the Si/GdIG interface.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Colossal Strain Tuning of Ferroelectric Transitions in KNbO 3 Thin Films

Strong coupling between polarization ( P ) and strain (ɛ) in ferroelectric complex oxides offers unique opportunities to dramatically tune their properties. Here colossal strain tuning of ferroelectricity in epitaxial KNbO 3 thin films grown by sub‐oxide molecular beam epitaxy is demonstrated. While bulk KNbO 3 exhibits three ferroelectric transitions and a Curie temperature ( T c ) of ≈676 K, phase‐field modeling predicts that a biaxial strain of as little as −0.6% pushes its T c > 975 K, its decomposition temperature in air, and for −1.4% strain, to T c > 1325 K, its melting point. Furthermore, a strain of −1.5% can stabilize a single phase throughout the entire temperature range of its stability. A combination of temperature‐dependent second harmonic generation measurements, synchrotron‐based X‐ray reciprocal space mapping, ferroelectric measurements, and transmission electron microscopy reveal a single tetragonal phase from 10 K to 975 K, an enhancement of ≈46% in the tetragonal phase remanent polarization ( P r ), and a ≈200% enhancement in its optical second harmonic generation coefficients over bulk values. These properties in a lead‐free system, but with properties comparable or superior to lead‐based systems, make it an attractive candidate for applications ranging from high‐temperature ferroelectric memory to cryogenic temperature quantum computing.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Strain-induced creation and switching of anion vacancy layers in perovskite oxynitrides

Perovskite oxides can host various anion-vacancy orders, which greatly change their properties, but the order pattern is still difficult to manipulate. Separately, lattice strain between thin film oxides and a substrate induces improved functions and novel states of matter, while little attention has been paid to changes in chemical composition. Here we combine these two aspects to achieve strain-induced creation and switching of anion-vacancy patterns in perovskite films. Epitaxial SrVO 3 films are topochemically converted to anion-deficient oxynitrides by ammonia treatment, where the direction or periodicity of defect planes is altered depending on the substrate employed, unlike the known change in crystal orientation. First-principles calculations verified its biaxial strain effect. Like oxide heterostructures, the oxynitride has a superlattice of insulating and metallic blocks. Given the abundance of perovskite families, this study provides new opportunities to design superlattices by chemically modifying simple perovskite oxides with tunable anion-vacancy patterns through epitaxial lattice strain.

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

Molecular beam epitaxy and crystal structure of majority a -plane-oriented and substrate-strained Mn 3 Sn thin films grown directly on sapphire (0001)

The Kagome antiferromagnet Mn 3 Sn has garnered much attention due to the presence of exciting properties such as anomalous Hall and Nernst effects. This paper discusses the synthesis of crystalline Mn 3 Sn thin films, prepared on Al 2 O 3 (0001) substrates at 453 ± 5°C using molecular beam epitaxy. The growth is monitored in situ using reflection high energy electron diffraction and measured ex situ using x-ray diffraction, Rutherford back-scattering, and cross-sectional scanning transmission electron microscopy. Our analysis shows the in-plane lattice constants of a 1,M = 4.117 ± 0.027 Å and a 2,M = 4.943 ± 0.033 Å, which is a very unexpected result when compared to the bulk a-plane Mn 3 Sn. This indicates a strain in the film and makes it challenging to provide a straightforward explanation. In an effort to explain our results, we discuss two possible orientation relationships between the Mn 3 Sn films and the sapphire substrates. Samples prepared under these conditions appear to have smooth surfaces locally, but overall the film has a 3D island morphology. First-principles calculations provide atomic models of the Mn 3 Sn (112¯0) lattice on Al 2 O 3 (0001) high symmetry sites, indicating that the L3-R90° is the most stable configuration. A detailed discussion of the experimental data and theoretical results, as well as strain effects, is provided.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Carrier properties of Bi(111) grown on mica and Si(111)

A comparison is presented between properties of Bi(111) films grown by Stranski-Krastanov epitaxy on Si(111) 7 × 7 and by van der Waals epitaxy on mica substrates. Thin film morphologies and electronic transport properties of Bi(111) films of variable thickness are investigated for each growth method. Atomic force micrographs for Bi(111) films on mica reveal clearly defined triangular regions consisting of layered steps with height 0.4 nm, corresponding to the Bi(111) bilayer height. Variable-temperature electronic transport measurements show the existence of a quantum confinement-induced energy gap in the film interiors, resulting in a semimetal-to-semiconductor transition. Magnetotransport analysis in a three-carrier model including metallic electrons in surface states and electrons and holes in the films' interiors provides a detailed study of densities, mobilities, and mean-free paths of the three carrier types. Further, improved electronic transport properties are found in Bi(111) films of higher thickness on mica compared to on Si(111), a likely result of the largely strain-free van der Waals epitaxial growth.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Elucidating the Role of InGaAs and InAlAs Buffers on Carrier Dynamics of Tensile-Strained Ge Double Heterostructures

Extensive research efforts of strained germanium (Ge) are currently underway due to its unique properties, namely, (i) possibility of band gap and strain engineering to achieve a direct band gap, thus exhibiting superior radiative properties, and (ii) higher electron and hole mobilities than Si for upcoming technology nodes. Realizing lasing structures is vital to leveraging the benefits of tensile-strained Ge (ε-Ge). Here, we use a combination of different analytical tools to elucidate the effect of the underlying InGaAs/InAlAs and InGaAs overlaying heterostructures on the material quality and strain state of ε-Ge grown by molecular beam epitaxy. Using X-ray analysis, we show the constancy of tensile strain in sub-50 nm ε-Ge in a quantum-well (QW) heterostructure. Further, effective carrier lifetime using photoconductive decay as a function of buffer type exhibited a high (low) defect-limited carrier lifetime of ~68 ns (~13 ns) in 0.61% (0.66%) ε-Ge grown on an InGaAs (InAlAs) buffer. These results correspond well with the measured surface roughness of 1.289 nm (6.303 nm), consistent with the surface effect of the ε-Ge/III–V heterointerface. Furthermore, a reasonably high effective lifetime of ~78 ns is demonstrated in a QW of ~30 nm 1.6% ε-Ge, a moderate reduction from ~99 ns in uncapped ε-Ge, alluding to the surface effect of the overlying heterointerface. Thus, the above results highlight the prime quality of ε-Ge that can be achieved via III–V heteroepitaxy and paves a path for integrated Ge photonics.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Vertical Strain-Driven Antiferromagnetic to Ferromagnetic Phase Transition in EuTiO 3 Nanocomposite Thin Films

Three-dimensional (3D) strain induced in self-assembled vertically aligned nanocomposite (VAN) epitaxial films provides an unrivaled method to induce very large strains in thin films. Here, by growing VAN films of EuTiO 3 (ETO)–Eu 2 O 3 (EO) with different EO fractions, the vertical strain was systematically increased in ETO, up to 3.15%, and the Eu–Ti–Eu bond angle along $\langle$111$\rangle$ decreased by up to 1°, leading to a weakening of the antiferromagnetic interactions and switching from antiferromagnetic to ferromagnetic behavior. Our work has shown for the first time that Eu–Ti–Eu superexchange interactions play a key role in determining the magnetic ground state of ETO. Broadly, our work serves as an exemplar to show that multifunctionalities in strong spin–lattice coupling perovskite oxides can be uniquely tuned at the atomic scale using simple VAN structures.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Symmetry-engineered chiral magnetotransport in the correlated oxide SrNbO 3

Chiral transport in topological materials, arising from the interplay between topology and chirality, holds significant potential for energy-efficient spintronics and quantum information technologies through dissipationless, coherent charge flow. However, the emergence of chiral transport by tuning the electronic states of novel chiral materials is largely unexplored. Here, we report chiral transport driven by correlated Dirac fermions in SrNbO 3 epitaxial thin films, where strain-induced nonsymmorphic symmetry of oxygen octahedra tunes the material from metallic to Dirac states. Such symmetry-driven Dirac fermions feature a remarkable enhancement of electron mobility and magnetoresistance. Signatures of chiral transport induced by the chiral anomaly, including negative longitudinal magnetoresistance and twofold planar Hall oscillation, are observed in the Dirac semimetallic state, whereas they are absent in the metallic state of SrNbO 3 thin films. This work highlights a crucial role of symmetry engineering in generating chiral charge transport in oxide Dirac semimetals, opening an avenue to novel oxide-based topological and quantum devices.

Materials science↗
DOE OSTI2021

Understanding, quantifying, and controlling the molecular ordering of semiconducting polymers: from novices to experts and amorphous to perfect crystals

Molecular packing and texture of semiconducting polymers are often critical to the performance of devices using these materials. Although frameworks exist to quantify the ordering, interpretations are often just qualitative, resulting in imprecise use of terminology. Here, we reemphasize the significance of quantifying molecular ordering in terms of degree of crystallinity (volume fractions that are ordered) and quality of ordering and their relation to the size scale of an ordered region. We are motivated in part by our own imprecise and inconsistent use of terminology in the past, as well as the need to have a primer or tutorial reference to teach new group members. We strive to develop and use consistent terminology with regards to crystallinity, semicrystallinity, paracrystallinity, and related characteristics. To account for vastly different quality of ordering along different directions, we classify paracrystals into 2D and 3D paracrystals and use paracrystallite to describe the spatial extent of molecular ordering in 1–10 nm. We show that a deeper understanding of molecular ordering can be achieved by combining grazing-incidence wide-angle X-ray scattering and differential scanning calorimetry, even though not all aspects of these measurements are consistent, and some classification appears to be method dependent. We classify a broad range of representative polymers under common processing conditions into five categories based on the quantitative analysis of the paracrystalline disorder parameter (g) and thermal transitions. A small database is presented for 13 representative conjugated and insulating polymers ranging from amorphous to semi-paracrystalline. Lastly, we outline the challenges to rationally design more perfect polymer crystals and propose a new molecular design approach that envisions conceptual molecular grafting that is akin to strained and unstrained hetero-epitaxy in classic (compound) semiconductors thin film growth.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Stress-induced phase transitions in nanoscale CuInP 2 S 6

Using the Landau-Devonshire approach and available experimental results, we constructed multiwell thermodynamic potential of the layered ferroelectric CuInP 2 S 6 (CIPS). The analysis of temperature dependences of the dielectric permittivity and lattice constants for different applied pressures unexpectedly reveals the critically important role of a nonlinear electrostriction in this material. With the nonlinear electrostriction included we calculated the temperature and pressure phase diagrams and spontaneous polarization of a bulk CIPS, within the assumed range of applicable temperatures and applied pressures. Using the developed thermodynamic potential, we revealed the strain-induced phase transitions in thin epitaxial СIPS films, as well as the stress-induced phase transitions in СIPS nanoparticles, the shape of which varies from prolate needles to oblate disks. Furthermore, w also revealed the strong influence of a mismatch strain, elastic stress, and shape anisotropy on the phase diagrams and polar properties of a nanoscale CIPS, and derived analytical expressions allowing for elastic control of the nanoscale CIPS polar properties. Hence obtained results can be of particular interest for the strain engineering of nanoscale layered ferroelectrics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Predicting metal–metal interactions. I. The influence of strain on nanoparticle and metal adlayer stabilities

Strain-engineering of bimetallic nanomaterials is an important design strategy for developing new catalysts. In this paper, we introduce an approach for including strain effects into a recently introduced, density functional theory (DFT)-based alloy stability model. The model predicts adsorption site stabilities in nanoparticles and connects these site stabilities with catalytic reactivity and selectivity. Strain-based dependencies will increase the model’s accuracy for nanoparticles affected by finite-size effects. In addition to the stability of small nanoparticles, strain also influences the heat of adsorption of epitaxially grown metal-on-metal adlayers. In this respect, we successfully benchmark the strain-including alloy stability model with previous experimentally determined trends in the heats of adsorption of Au and Cu adlayers on Pt (111). For these systems, our model predicts stronger bimetallic interactions in the first monolayer than monometallic interactions in the second monolayer. We explicitly quantify the interplay between destabilizing strain effects and the energy gained by forming new metal–metal bonds. While tensile strain in the first Cu monolayer significantly destabilizes the adsorption strength, compressive strain in the first Au monolayer has a minimal impact on the heat of adsorption. Hence, this study introduces and, by comparison with previous experiments, validates an efficient DFT-based approach for strain-engineering the stability, and, in turn, the catalytic performance, of active sites in bimetallic alloys with atomic level resolution.

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

Atomic Structure of the Initial Nucleation Layer in Hexagonal Perovskite BaRuO 3 Thin Films

Hexagonal perovskites are an attractive family of materials due to their various polymorphs and rich structure–property relationships. BaRuO 3 (BRO) is a prototypical hexagonal perovskite, in which the electromagnetic properties are significantly modified depending on its lattice structure. Whereas thin-film synthesis would allow the study of the diverse properties of hexagonal perovskites by epitaxially stabilizing various metastable polymorphs, the epitaxial growth of hexagonal perovskites, especially at the initial growth stage, is yet to be addressed. In this study, it is shown that an intriguing nucleation behavior takes place during the initial stabilization of a hexagonal 9R BRO thin film deposited on a (111) SrTiO 3 (STO) substrate. It is found that a nanoscale strained layer composed of different RuO 6 octahedral stacking is initially formed at the interface followed by a relaxed single crystal 9R BRO thin film. Within the interface layer, hexagonal BRO is nucleated on the (111) STO substrate by both face- and corner-sharing octahedra. More interestingly, it is found that boundaries between differently stacked nucleation layers (heterostructural boundaries) facilitate strain relaxation, in addition to the formation of conventional misfit dislocations evolved from homostructural boundaries. The observations reveal an important underlying mechanism to understand the thin-film epitaxy and strain accommodation in hexagonal perovskites.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Giant linear magnetoelectric effect at the morphotropic phase boundary of epitaxial Sr 0.5 Ba 0.5 MnO 3 films

First-principles calculations are conducted to compute linear magnetoelectric coupling coefficients in epitaxial (001) Sr 0.5 Ba 0.5 MnO 3 films. A large enhancement of different linear magnetoelectric elements is found in a strained-induced morphotropic phase boundary region. Such enhancement is demonstrated to originate from the behavior of the dielectric susceptibility, thanks to a simple phenomenological model that is presently shown to be relevant and accurate. Finally, this work can thus provide a promising approach towards designing highly desired single-phase multiferroic with a colossal magnetoelectric conversion.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Breaking $\mathrm{OER}$ and $\mathrm{CER}$ scaling relations via strain and its relaxation in $\mathrm{RuO}$ 2 (101)

Green hydrogen production from abundant water sources is an important component of renewable energy storage. Water oxidation catalysts are typically considered bound by adsorbate scaling relations, limiting their activity for the oxygen evolution reaction (OER) as well as selectivity between OER and the chlorine evolution reaction (CER) that compete in saline water streams. RuO 2 is highly active for both reactions, and recent measurements have shown the OER activity is greater on undercoordinated, high index facets compared to the lowest-energy (110) facet often studied. The growth of such orientations as epitaxial films, however, can result in appreciable strain and potential surface faceting via its relaxation. Here, we find the activity and selectivity towards OER and CER vary with thickness in epitaxial (101) RuO 2 thin films: OER activity decreases 4x as film thickness increases from 8 nm to 48 nm, while CER activity is comparable. Thus, strain and its relaxation can be used to break scaling relationships between OER and CER, highlighting the important role that defects play in selective oxidation processes on RuO 2 in chloride-containing media.

36 MATERIALS SCIENCE↗