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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 343 records · Page 19
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Photoinduced phase separation in the lead halides is a polaronic effect

We present a perspective on recent observations of the photoinduced phase separation of halides in multi-component lead-halide perovskites. The spontaneous phase separation of an initial homogeneous solid solution under steady-state illumination conditions is found experimentally to be reversible, stochastic, weakly dependent on morphology, yet strongly dependent on composition and thermodynamic state. Regions enriched in a specific halide species that form upon phase separation are self-limiting in size, pinned to specific compositions, and grow in number in proportion to the steady-state carrier concentration until saturation. These empirical observations of robustness rule out explanations based on specific defect structures and point to the local modulation of an existing miscibility phase transition in the presence of excess charge carriers. A model for rationalizing existing observations based on the coupling between composition, strain, and charge density fluctuations through the formation of polarons is reviewed.

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

Effect of indium alloying on the charge carrier dynamics of thick-shell InP/ZnSe quantum dots

In this work, thick-shell InP/ZnSe III–V/II–VI quantum dots (QDs) were synthesized with two distinct interfaces between the InP core and ZnSe shell: alloy and core/shell. Despite sharing similar optical properties in the spectral domain, these two QD systems have differing amounts of indium incorporation in the shell as determined by high-resolution energy-dispersive x-ray spectroscopy scanning transmission electron microscopy. Ultrafast fluorescence upconversion spectroscopy was used to probe the charge carrier dynamics of these two systems and shows substantial charge carrier trapping in both systems that prevents radiative recombination and reduces the photoluminescence quantum yield. The alloy and core/shell QDs show slight differences in the extent of charge carrier localization with more extensive trapping observed in the alloy nanocrystals. Despite the ability to grow a thick shell, structural defects caused by III–V/II–VI charge carrier imbalances still need to be mitigated to further improve InP QDs.

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

Molecular beam homoepitaxy of N-polar AlN on bulk AlN substrates

N-polar AlN epilayers were grown on the N-face of single-crystal bulk AlN substrates by plasma-assisted molecular beam epitaxy. A combination of in situ thermal deoxidation and Al-assisted thermal desorption at high temperature aided in removing native surface oxides and impurities from the N-polar surface of the substrate enabling successful homoepitaxy. Subsequent epitaxial growth of the AlN layer on the in situ cleaned substrates, grown in a sufficiently high Al droplet regime, exhibited smooth surface morphologies with clean and wide atomic steps. KOH etch studies confirmed the N-polarity of the homoepitaxial films. Secondary ion mass spectrometry profiles show Si and H impurity concentrations below the noise levels, whereas O and C impurity concentrations of [Formula: see text] and [Formula: see text] atoms/cm 3 are observed, respectively. Although the structural defect densities are low, they interestingly appear as inversion domains of different dimensionalities.

Singhal, Jashan↗
DOE OSTI2023

Chirality reversal of magnetic solitons in chiral Cr 1/3 TaS 2

Ferromagnetism in two-dimensional (2D) materials provides an ideal platform to study emergent electromagnetic phenomena in low dimensions for future spintronics. In magnetic-element intercalated transition metal dichalcogenides, topologically nontrivial spin textures, such as chiral helimagnetic spin states and chiral soliton lattices, are realized due to the chiral lattice distortions induced by intercalated magnetic ions. Consequently, the magnetic chirality is predictably determined by the sign of antisymmetric exchange interaction (or Dzyaloshinskii–Moriya interaction, DMI) vector that is coupled to the underlying crystal chirality. Here, using cryogenic Lorentz phase microscopy, we directly observed the chirality reversal behavior of the chiral soliton lattices in Cr 1/3 TaS 2 across the structural defects. We show that a partial 1 T stacking in 2H-TaS 2 locally reduces DMI, leading to magnetic chirality reversal with direct atomic resolution imaging. Furthermore, our experimental results show that manipulation of stacking sequence provides a viable way to control the chirality of topologically nontrivial soliton lattices in 2D magnets.

2D materials↗
DOE OSTI2024

Optical and microstructural studies of erbium-doped TiO 2 thin films on silicon, SrTiO 3 , and sapphire

Rare-earth ion doped oxide thin films integrated on silicon substrates provide a route toward scalable, chip-scale platforms for quantum coherent devices. Erbium-doped TiO 2 is an attractive candidate: the Er 3+ optical transition is compatible with C-band optical fiber communications, while TiO 2 is an insulating dielectric compatible with silicon process technology. Through structural and optical studies of Er-doped TiO 2 thin films grown via molecular beam deposition on silicon, SrTiO 3 , and sapphire substrates, we have explored the impact of polycrystallinity and microstructure on the optical properties of the Er emission. Comparing polycrystalline TiO 2 (rutile)/Si with single-crystalline TiO 2 (rutile)/r-sapphire and polycrystalline TiO 2 (anatase)/Si with single-crystalline TiO 2 (anatase)/SrTiO 3 , we observe that the inhomogeneous linewidth (Γ inh ) of the most prominent peak in the Er spectrum (the Y 1 –Z 1 transition, 1520 and 1533 nm in rutile and anatase TiO 2 ) is significantly narrower in the polycrystalline case. This implies a relative insensitivity to extended structural defects and grain boundaries in such films (as opposed to, e.g., point defects). We show that the growth of an undoped, underlying TiO 2 buffer on Si can reduce Γ inh by a factor of 4–5. Expectedly, Γ inh also reduces with decreasing Er concentrations: we observe a ∼2 order of magnitude reduction from ∼1000 ppm Er to ∼10 ppm Er. Γ inh then gets limited to a residual value of ∼5 GHz that is insensitive to further reduction in the Er concentration. Based upon the above results, we argue that the optical properties in these thin films are limited by the presence of high “grown-in” point defect concentrations.

Chemical elements↗
DOE OSTI2025

Suppression of phase separation in AlGaInAs compositionally graded buffers for 1550 nm photovoltaic converters on GaAs

We investigate strategies to suppress phase separation and reduce threading dislocation density (TDD) in AlGaInAs compositionally graded buffers (CGBs) that span the lattice constant range from GaAs to InP. Combining the results from high resolution x-ray diffraction, cathodoluminescence, transmission electron microscopy, and photovoltaic device measurements, we correlate the choices of epitaxial growth conditions with the defect structure of the CGBs and subsequent device performance. Both the use of substrates with high misorientation off (100) toward the (111)A plane and Zn doping instead of Si doping are shown to suppress phase separation and reduce TDD. We demonstrate a 0.74 eV GaInAs device grown on a GaAs substrate offcut 19.5° toward (111)A using a Zn-doped AlGaInAs CGB with TDD = 3.5 ± 0.2 × 10 6 cm -2 that has a bandgap-open circuit voltage offset of only 0.434 V measured under the AM1.5G solar spectrum. We characterized this device under high-intensity irradiance from a 1570 nm laser and measured a 31.9% peak laser power conversion efficiency at 3.6 W/cm 2 irradiance. These results provide a roadmap to the manufacture of laser- and thermal-power conversion devices with the performance and cost-effectiveness needed to drive adoption of these technologies at scale.

14 SOLAR ENERGY↗
DOE OSTI2020

Effect of hydrothermal synthesis conditions on up-conversion luminescence intensity of β-NaYF{sub 4} : Er{sup 3+}, Yb{sup 3+} submicron particles

The differences in the luminescence intensities of up-conversion β-NaYF{sub 4} : Er{sup 3+}, Yb{sup 3+} particles synthesised by the hydrothermal method under various synthesis conditions are studied. The results of the study lead to the conclusion that in order to achieve the maximum luminescence intensity in such particles, it is necessary to use ammonium fluoride and a medium with pH = 3. In this case, the length of the particles increases, up to the formation of rod-shaped particles. Based on the data on the size of the coherent scattering region and on microstresses, we can assume that the particles are polycrystals. At the same time, limiting the size of the coherent scattering region is possible due to the defective structure. When the nanoparticles are synthesised in a medium with pH = 3, hydrolysed regions containing OH groups are formed on the crystallite surface. The presence of these groups does not affect the intensity of up-conversion luminescence of submicron-size particles. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Ion irradiation of iron chalcogenide superconducting films

Iron-chalcogenide FeSe x Te 1-x (Fe(Se,Te)) superconductors have attracted a great deal of interests in both fundamental physics and potential applications. While Majorana zero-modes hosted by the Fe(Se,Te) superconductors hold great promise for topological quantum computing, low-anisotropy and very high upper critical fields make Fe(Se,Te) an excellent candidate for high magnetic field and energy applications. Recently, we demonstrated a route for creating structural defects by ion irradiation in the FST films, leading to strong flux pinning. Ion irradiation into Fe(Se,Te) films significantly improved the critical current density J c over a wide field range and for all orientations with respect to magnetic field. In particular, a robust enhancement of critical temperature T c and J c was realized simultaneously in the Fe(Se,Te) film irradiated with proton. Here, we provide a review of the ion irradiated Fe(Se,Te) films, including the film growth and characteristics before and after irradiations, design of irradiation process for different ion energy and species with irradiation simulations and experimental setups that are scalable for long length Fe(Se,Te) coated conductors.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

A High-Speed Rotational Diamond Anvil Cell for In Situ Analysis of Hierarchical Microstructural Evolution of Metallic Alloys during Extreme Shear Deformation

High speed shear deformation is ubiquitous in engineering applications, ranging from material processing methods such as friction stir processing/extrusion and in tribological contacts. However, analyzing the microstructural evolution of materials while they are undergoing high speed shear deformation have been a long-standing challenge. This led to predominant reliance on ex situ microscopy before and after shear deformation. But ex situ microscopy lacks the ability to analyze dynamic and transient hierarchical microstructural evolution mechanisms that could occur during shear deformation of materials. Therefore, to better understand the dynamic mechanisms of mass and energy transfer in materials under shear deformation, we developed a first of its kind high-speed rotational diamond anvil cell (HS-RDAC) for synchrotron-based in situ high-energy x-ray diffraction (XRD). We studied the time resolved lattice strain evolution, XRD peak broadening and changes in spatial variation of shear deformation induced alloying in pure metal and metal alloy sheets and powder mixture using the HS-RDAC. These in situ results were combined with detailed ex situ microstructural characterization before and after the shear deformation using transmission electron microscopy and atom probe tomography, which revealed the different stages of evolution of a shear deformation induced hierarchical nanostructure. Multiscale computational simulations including computational fluid dynamics, crystal plasticity, molecular dynamic simulation and density functional theory uncovered the mechanisms behind morphological changes, evolution of defect structures and changes in driving force for shear deformation induced intermixing. In conclusion, this in situ HS-RDAC capability, in combination with ex situ microstructural characterization and computational simulations, can provide new insights into the hierarchical microstructural evolution pathway during shear deformation.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Chirality reversal at finite magnetic impurity strength and local signatures of a topological phase transition

Here, we study the honeycomb lattice with a single magnetic impurity modeled by adding imaginary next-nearest-neighbor hopping 𝑖⁢ℎ on a single hexagon. This Haldane defect gives a topological mass term to the gapless Dirac cones and generates chirality. For a small density of defects, Neehus et al. [Phys. Rev. Lett. 135, 126604 (2025)] found that the system's chirality reverses at a critical ℎ 𝑐 ≈ 0.95 associated with an unexpected tricritical point of Dirac fermions at zero defect density. We investigate this zero-density limit by analyzing a single defect and computing two experimentally relevant measures of chirality: (1) orbital magnetization via local Chern marker, a bulk probe of all occupied states; and (2) electronic currents of low-energy states. Both probes show a chirality reversal at a critical ℎ 𝑐 ≈ 0.9–1.0. Motivated by this consistency, we propose a defect-scale toy model whose low-energy states reverse their chirality at ℎ$^{'}_{c}$ ≈ 0.87. Remarkably, the same pair of zero-energy bound states also generates the critical point ℎ 𝑐 in the full impurity projected T-matrix. Our results show how the chirality reversal produced by an impurity can be observed either in local probes or in the global topology, and suggest a possible role of the microscopic defect structure at the critical point.

Chern insulators↗
DOE OSTI2026

Vortex motion induced losses in tantalum resonators

Tantalum (Ta)-based superconducting circuits have been demonstrated to enable ultrahigh qubit quality factors (𝑄), motivating a careful study of the microscopic origin of the remaining losses that limit their performance. We have recently shown that the losses in Ta-based resonators are dominated by two-level systems at low microwave powers and millikelvin temperatures. We also observe that some devices exhibit loss that is exponentially activated at a lower temperature inconsistent with the superconducting critical temperature (T 𝑐 ) of the constituent film. Specifically, dc resistivity measurements show a T 𝑐 of over 4 K, while microwave measurements of resonators fabricated from these films show losses that increase exponentially with temperature with an activation energy as low as 0.3 K. Here, in this study, we present a comparative study of the structural and thermodynamic properties of Ta-based resonators and identify vortex motion-induced loss as the source of thermally activated microwave loss. Through careful magnetoresistance and x-ray diffraction measurements, we observe that the increased loss occurs for films that are in the clean limit, where the superconducting coherence length (𝜉) is shorter than the mean free path (𝑙). Vortex motion-induced losses are suppressed for films in the dirty limit, which show evidence of structural defects that can pin vortices. We verify this hypothesis by explicitly pinning vortices via patterning, and we find that we can suppress the loss by microfabrication.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Diffuse scattering from dynamically compressed single-crystal zirconium following the pressure-induced α → ω phase transition

The prototypical α → ω phase transition in zirconium is an ideal test bed for our understanding of polymorphism under extreme loading conditions. After half a century of study, a consensus had emerged that the transition is realized via one of two distinct displacive mechanisms, depending on the nature of the compression path. However, recent dynamic-compression experiments equipped with diffraction diagnostics performed in the past few years have revealed new transition mechanisms, demonstrating that our understanding of the underlying atomistic dynamics and transition kinetics is in fact far from complete. We present classical molecular dynamics simulations of the α → ω phase transition in single-crystal zirconium shock compressed along the [0001] axis using a machine-learning-class potential. The transition is predicted to proceed primarily via a modified version of the two-stage Usikov-Zilberstein mechanism, whereby the high-pressure ω phase heterogeneously nucleates at boundaries between grains of an intermediate β phase. We further observe the fomentation of atomistic disorder at the junctions between β grains, leading to the formation of highly defective interstitial material between the ω grains. We directly compare synthetic x-ray diffraction patterns generated from our simulations with those obtained using femtosecond diffraction in recent dynamic-compression experiments, and show that the simulations produce the same unique, anisotropic diffuse scattering signal unlike any previously seen from an elemental metal. Our simulations suggest that the diffuse signal arises from a combination of thermal diffuse scattering, nanoparticlelike scattering from residual kinetically stabilized α and β grains, and scattering from interstitial defective structures. Published by the American Physical Society 2024

36 MATERIALS SCIENCE↗
DOE OSTI2021

Twin-boundary-mediated flexoelectricity in LaAlO 3

Flexoelectricity has garnered much attention owing to its ability to bring electromechanical functionality to nonpiezoelectric materials and its nanoscale significance. In order to move towards a more complete understanding of this phenomenon and improve the efficacy of flexoelectric-based devices, it is necessary to quantify microstructural contributions to flexoelectricity. Here we directly measure the flexoelectric response of bulk centrosymmetric LaAlO 3 crystals with different twin-boundary microstructures. We show that twin-boundary flexoelectric contributions are comparable to intrinsic contributions at room temperature and enhance the flexoelectric response by ~4× at elevated temperatures. Additionally, we observe time-dependent and nonlinear flexoelectric responses associated with strain-gradient-induced twin-boundary polarization. Finally, these results are explained by considering the interplay between twin-boundary orientation, beam-bending strain fields, and pinning site interactions, and directly demonstrate that macroscopic flexoelectric responses are very sensitive to structural defects.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Controlling magnetism and transport at perovskite cobaltite interfaces via strain-tuned oxygen vacancy ordering

Complex oxides such as perovskite cobaltites exhibit rich phenomena at interfaces due to the complex interplay between their structural, defect, electronic, and magnetic degrees of freedom. Here, we study this interplay here in the ferromagnetic metallic cobaltite La 1−𝑥 ⁢Sr 𝑥⁢ CoO 3−𝛿 , using specific substrates to vary both the heteroepitaxial strain (compressive vs tensile) and growth orientation ((001) vs (110)). Transmission electron microscopy, electron energy-loss spectroscopy, high-resolution X-ray diffraction, magnetometry, polarized neutron reflectometry, and electronic magnetotransport measurements are applied. Lattice mismatch and growth orientation are found to enable the precise control of interfacial oxygen vacancy ordering in La 1−𝑥⁢ Sr 𝑥 ⁢CoO 3−𝛿 , thus dictating strain relaxation and oxygen vacancy depth profiles, in turn controlling thickness-dependent magnetic and electronic properties. In particular, compressive strain and (110) orientations minimize deleterious magnetic/electronic dead layer effects, leading to the optimization of interfacial magnetism and transport. Strain and orientation tuning of oxygen vacancy ordering are thus established as powerful means to control physical properties at cobaltite-based interfaces, relevant to several fields.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Electron Paramagentic Resonance Investigation of Mechanism of Light- and Elevated-Temperature-Induced Degradation in Ga-doped Cz Si

We report on the electron paramagnetic resonance (EPR) spectra recorded at 6 K for Ga-doped Czochralski (Cz) Si in the initial stages of light- and elevated-temperature-induced degradation (LeTID). We show that the narrow-range EPR spectrum depends on the duration of simultaneous heat and light exposure. After prolonged LeTID, the EPR intensity increases at ~334 mT. The EPR signature at ~334 mT is characteristic of Si dangling bonds, and the increase in the dangling bond density can be attributed to the bulk and not the surface. Additionally, we show that the inverse of the minority carrier lifetime in Ga-doped Cz Si samples, measured by quasi-steady-state photoconductance (QSSPC) decay, correlates with the increase in the intensity of the Si dangling bond signal measured by EPR. Based on these observations, we put forth a hypothesis connecting H to LeTID and the corresponding defect structure.

Cz Si↗
DOE OSTI2022

Fracture mechanisms of sodium silicate glasses

Reactive classical molecular dynamics simulations of sodium silicate glasses, xNa 2 O–(100 – x)SiO 2 (x = 10–30), under quasi-static loading, were performed for the analysis of molecular scale fracture mechanisms. Mechanical properties of the sodium silicate glasses were consistent with experimentally reported values, and the amount of crack propagation varied with reported fracture toughness values. The most crack propagation occurred in NS20 systems (20-mol% Na 2 O) compared with the other simulated compositions. Dissipation via two mechanisms, the first through sodium migration as a lower activation energy process and the second through structural rearrangement as a higher activation energy process, was calculated and accounted for the energy that was not stored elastically or associated with the formation of new fracture surfaces. Here, a correlation between crack propagation and energy dissipation was identified, with systems with higher crack propagation exhibiting less energy dissipation. Sodium silicate glass compositions with lower energy dissipation also exhibited the most sodium movement and structural rearrangement within 10 Å of the crack tip during loading. Therefore, high sodium mobility near the crack tip may enable energy dissipation without requiring formation of structural defects. Therefore, the varying mobilities of the network modifiers near crack tips influence the brittleness and the crack growth rate of modified amorphous oxide systems.

36 MATERIALS SCIENCE↗
DOE OSTI2025

The effects of antimony incorporation on the optoelectronic properties of Cd(Se,Te) photovoltaics

One historical limitation of CdTe PV devices has been the open circuit voltage (VOC) deficit (difference between theoretical maximum VOC and measured values). While non-radiative recombination at extended structural defects and interfaces negatively impacts VOC, intentionally adding impurities (dopants) to increase the concentration of free carriers in the photovoltaic (PV) absorber material can increase the VOC. In this contribution we present a detailed analysis of the Sb doped CdTe and the Cd(Se,Te) alloy system via electron beam induced current (EBIC) measurements and the presence as well as spatial distribution of defect levels through cathodoluminescence spectrum imaging (CLSI).

14 SOLAR ENERGY↗
DOE OSTI2021

NMR spectroscopy of coin cell batteries with metal casings

Battery cells with metal casings are commonly considered incompatible with nuclear magnetic resonance (NMR) spectroscopy because the oscillating radio-frequency magnetic fields (“rf fields”) responsible for excitation and detection of NMR active nuclei do not penetrate metals. Here, we show that rf fields can still efficiently penetrate nonmetallic layers of coin cells with metal casings provided “B 1 damming” configurations are avoided. With this understanding, we demonstrate noninvasive high-field in situ 7 Li and 19 F NMR of coin cells with metal casings using a traditional external NMR coil. This includes the first NMR measurements of an unmodified commercial off-the-shelf rechargeable battery in operando, from which we detect, resolve, and separate 7 Li NMR signals from elemental Li, anodic β-LiAl, and cathodic Li x MnO 2 compounds. Real-time changes of β-LiAl lithium diffusion rates and variable β-LiAl 7 Li NMR Knight shifts are observed and tied to electrochemically driven changes of the β-LiAl defect structure.

25 ENERGY STORAGE↗