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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 379 records · Page 21
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Digital twins and deep learning segmentation of defects in monolayer MX 2 phases

Developing methods to understand and control defect formation in nanomaterials offers a promising route for materials discovery. Monolayer MX 2 phases represent a particularly compelling case for defect engineering of nanomaterials due to the large variability in their physical properties as different defects are introduced into their structure. However, effective identification and quantification of defects remain a challenge even as high-throughput scanning transmission electron microscopy methods improve. This study highlights the benefits of employing first principles calculations to produce digital twins for training deep learning segmentation models for defect identification in monolayer MX 2 phases. Around 600 defect structures were obtained using density functional theory calculations, with each monolayer MX 2 structure being subjected to multislice simulations for the purpose of generating the digital twins. Several deep learning segmentation architectures were trained on this dataset, and their performances evaluated under a variety of conditions such as recognizing defects in the presence of unidentified impurities, beam damage, grain boundaries, and with reduced image quality from low electron doses. Further, this digital twin approach allows benchmarking different deep learning architectures on a theory dataset, which enables the study of defect classification under a broad array of finely controlled conditions. It thus opens the door to resolving the underpinning physical reasons for model shortcomings and potentially chart paths forward for automated discovery of materials defect phases in experiments.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Detector performance and defect densities in CdZnTe after two-step annealing

Defects both micro-scale and nano-scale are plays an important role in CdZnTe(CZT) device performance. Typical micro-scale defect, Te inclusions, was removed by two-step annealing and its concentration were analyzed by IR transmission microscopy. In addition, transmission electron microscopy (TEM) measurement was employed to investigate the evolution of nano-scale defects after annealing process. Micro-twins and stacking faults were commonly observed defects in CZT as-grown and annealed CZT. The line shape defects which possibly related to the stress field around dislocations, were combine and disappear in in-situ the annealing at 200-220 °C. Frisch-grid CZT detector made of two-step annealed one exhibit better energy resolution and low backscattering counts in Cs-137 gamma spectra.

36 MATERIALS SCIENCE↗
DOE OSTI2025

How non-ohmic contact-layer diodes in perovskite pinholes affect abrupt low-voltage reverse-bias breakdown and destruction of solar cells

Perovskite solar cells (PSCs) rapidly degrade under reverse bias, a condition that may occur during partial shading. Here, in this study, we use electrical measurements, electron microscopy, and optical and thermal imaging to investigate abrupt breakdown and hotspotting under low reverse potentials (<|-2| V). We show that microscopic pinholes in the perovskite layer cause rapid, destructive breakdown under reverse bias despite minimally reducing power conversion efficiencies. Measurements on miniature (200-micrometer diameter) PSCs and perovskite-free transport-layer diodes indicate that abrupt, low-voltage breakdown occurs in nanoscale to micrometer-scale defects and that metal migration and filamentation are unlikely causes. Reverse-bias stability substantially improves when pinholes in the perovskite and transport layers are eliminated. Atomic layer deposition of tin oxide prevents abrupt breakdown by ensuring physical separation between electrodes-not by blocking metal ion migration. Perovskite researchers should adopt cleaner, more uniform deposition techniques to enable robust PSCs for further research and commercial applications.

14 SOLAR ENERGY↗
DOE OSTI2025

Describing Point Defect Topology in 2D Energy Materials Through Computer Vision

Point defects such as vacancies and impurity atoms strongly impact the performance of 2D materials. Traditional efforts often rely on manual detection, a process that is time-intensive, prone to human error, and challenging to scale. Here we leverage machine learning (ML) methods to identify and quantify vacancies within 2D transition metal carbides (Ti3C2, MXenes), aiming to expedite detection while improving accuracy. MXenes exhibit valuable defect-defined electrochemical properties, but we currently lack statistical understanding of defect topology needed to fully harness these materials. We employ a convolutional neural network for semantic segmentation of experimental MXene images, opening an opportunity to conduct a rigorous statistical study on defect hierarchy while investigating local relaxation in the lattice. We show how the integration of ML can yield fundamental insight into point defects, providing a powerful tool that will play an increasingly crucial role in the future of materials science.

2D materials↗
DOE OSTI2022

Properties and microstructure evolution of silicon nitride and zirconium nitride following Ni ion irradiation

We report that silicon nitride and zirconium nitride have been proposed as potential materials for multiple nuclear applications (inert matrix fuels, accident tolerant fuels, space nuclear power, fusion reactor design), yet knowledge on their behavior under irradiation remains limited. Ion irradiations were performed using 15 MeV Ni 5+ ions on Si 3 N 4 and ZrN samples, with midrange doses (around 3 µm) from 1 to 50 dpa and temperatures from 300 to 700°C. Volumetric lattice swelling was determined by grazing incidence X-ray diffraction, defect production and evolution were tracked using Transmission Electron Microscopy, and nanoindentation was performed to quantify the ceramics’ mechanical properties evolution. The results from these irradiation studies on nitride ceramics help fill the current gap present in the literature. Behavior consistent with past work on irradiated Si 3 N 4 was observed with respect to mechanical properties and defect formation up to 15 dpa and 500°C. Failure of the grain boundary sintering aid in Si 3 N 4 was observed above these conditions. Different behavior was observed in both nitrides at 50 dpa and 700°C, where lattice swelling increased past potential saturation values. Unreported cavity formation was witnessed in both materials under all irradiation conditions, with stable number density and slight size increase above 15 dpa. The mechanism for the cavity formation remains to be determined.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Phase-operation for conduction electron by atomic-scale scattering via single point-defect

In order to propose a phase-operation technique for conduction electrons in solid, we have investigated, using scanning tunneling microscopy, an atomic-scale electron-scattering phenomenon on a 2D subband state formed in Si. Particularly, we have noticed a single surface point-defect around which a standing-wave pattern created, and a dispersion of scattering phase-shifts by the defect-potential against electron-energy has been measured. The behavior is well-explained with appropriate scattering parameters: the potential height and radius. This result experimentally proves that the atomic-scale potential scattering via the point defect enables phase-operation for conduction electrons.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

High-temperature phase transformation and topochemical nature in ferroelastic (NH{sub 4}){sub 2}SO{sub 4}

The electrical conductivity of ferroelastic ammonium sulfate (NH{sub 4}){sub 2}SO{sub 4} revealed an anomaly at around 130 °C (=403 K, T{sub P}) on heating with large and irreversible thermal hysteresis through thermal cycle. Ferroelastic domain walls and surface morphology of (NH{sub 4}){sub 2}SO{sub 4} were investigated by hot-stage polarizing microscopy. Structural phase transition from an orthorhombic ferroelastic phase to a hexagonal paraelastic phase was not identified at T{sub P} upon heating. On further heating above T{sub P}, microscopic spots appeared and grew on the crystal surface, suggesting that the high-temperature anomaly at T{sub P} was an indication of an onset of thermal decomposition controlled by topochemical factors. The increase of electrical conductivity above T{sub P} was attributed to proton migration. - Graphical abstract: Surface morphology of the (100) face of (NH{sub 4}){sub 2}SO{sub 4} on heating, showing chemical reaction at the surface. - Highlights: • We investigate the high-temperature phase transformation of ammonium sulfate. • The increasing conductivity upon heating is attributed to proton migration. • Structural phase transition from orthorhombic to hexagonal phase is not confirmed. • High-temperature anomaly is related to an onset of thermal decomposition. • The nature of the high-temperature anomaly is topochemical controlled by defects.

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

Thermal transport across nanoscale damage profile in sapphire irradiated by swift heavy ions

In this work, we studied the degradation of thermal conductivity in single crystal sapphire (α-Al 2 O 3 ) irradiated by 167 MeV Xe swift heavy ions (SHIs) over the multiple fluences in the range of 10 12 –10 14 ions/cm 2 . Thermal conductivity was measured primarily in the cross-plane direction using a noncontact ultrafast optical pump-probe technique called picosecond time domain thermoreflectance (TDTR). Multiple samples with variable ion fluences allowed us to probe distinct regions resulting from different regimes of microstructure evolution caused by electronic energy loss. By tuning the penetration depth of the thermal waves using different modulation frequencies, two regions with distinct conductivities were identified and the values of which were found to be consistent with phonon-mediated thermal transport models while the microstructure was confirmed by electron microscopy characterization. These damaged regions were determined to be a several micrometer thick ion track region and several tens of nanometer-thick amorphous layer present only above 5.0 × 10 13 ions/cm 2 . These results demonstrate the applicability of TDTR to resolve thermal transport behavior in SHI irradiated oxides having nonhomogeneous damage profile on a nanometer scale. The presented approach facilitates future studies aiming at resolving the impact of distinct damage resulting from electronic and nuclear stopping regimes under irradiation.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2018

Multiscale analysis of nanoindentation-induced defect structures in gum metal

Using ex-situ transmission electron microscopy and the recently developed nanoprobe diffraction (NPD) technique, we characterize a nanoindented solution treated gum metal. Lattice rotations are resolved at a 1.2 nm length-scale and shown to be continuous within the nanoindentation pit; further, it is revealed that these can be accommodated by a reasonable number of geometrically necessary dislocations at a density of ~10 15 /m 2 . We furthermore provide direct evidence that dislocations within the nanoindent, rather than secondary phase nanoparticles, can serve as potent barriers to dislocation motion. We also demonstrate that plasticity in these alloys under nanoindentation can be accommodated solely by dislocation nucleation and propagation, with no competing deformation mechanisms present. Conventional transmission electron microscopy and “ g•b ” analysis reveal the presence of dislocations on &lang;$ \bar{1}11 $&rang;{110} slip systems and highly localized plastic deformation in the form of shear bands on <111>{$ \bar{1} \bar{1}2 $} slip systems, similar to previously observed “giant faults”.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Diffusion and recrystallization of B implanted in crystalline and pre-amorphized Ge in the presence of F

Although the diffusion control and dopant activation of Ge p-type junctions are straightforward when using B{sup +} implantation, the use of the heavier BF{sub 2}{sup +} ions or even BF{sup +} is still favored in terms of shallow junction formation and throughput—because implants can be done at higher energies, which can give higher beam currents and beam stability—and thus the understanding of the effect of F co-doping becomes important. In this work, we have investigated diffusion and end-of-range (EOR) defect formation for B{sup +}, BF{sup +}, and BF{sub 2}{sup +} implants in crystalline and pre-amorphized Ge, employing rapid thermal annealing at 600 °C and 800 °C for 10 s. It is demonstrated that the diffusion of B is strongly influenced by the temperature, the presence of F, and the depth of amorphous/crystalline interface. The B and F diffusion profiles suggest the formation of B–F complexes and enhanced diffusion by interaction with point defects. In addition, the strong chemical effect of F is found only for B in Ge, while such an effect is vanishingly small for samples implanted with F alone, or co-implanted with P and F, as evidenced by the high residual F concentration in the B-doped samples after annealing. After 600 °C annealing for 10 s, interstitial-induced compressive strain was still observed in the EOR region for the sample implanted with BF{sup +}, as measured by X-ray diffraction. Further analysis by cross-sectional transmission electron microscopy showed that the {311} interstitial clusters are the majority type of EOR defects. The impact of these {311} defects on the electrical performance of Ge p{sup +}/n junctions formed by BF{sup +} implantation was evaluated.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Electrical Activity of Extended Defects in Multicrystalline Silicon

The excess carrier lifetime (τ) distribution in multicrystalline silicon grown by the Bridgman technique from high-purity metallurgical silicon (HPMG-Si) is studied. The features of the variation in τ, caused by the grain-boundary structure of ingots, are revealed. The grain boundaries, dislocations, and impurity microinclusions are studied by electron probe microanalysis (EPMA) and scanning electron microscopy (SEM) using selective acid etching. The electrical activity of extended defects is measured by the electronbeam- induced-current (EBIC) method.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Deep Learning Segmentation of Complex Features in Atomic-Resolution Phase-Contrast Transmission Electron Microscopy Images

Phase-contrast transmission electron microscopy (TEM) is a powerful tool for imaging the local atomic structure of materials. TEM has been used heavily in studies of defect structures of two-dimensional materials such as monolayer graphene due to its high dose efficiency. However, phase-contrast imaging can produce complex nonlinear contrast, even for weakly scattering samples. It is, therefore, difficult to develop fully automated analysis routines for phase-contrast TEM studies using conventional image processing tools. For automated analysis of large sample regions of graphene, one of the key problems is segmentation between the structure of interest and unwanted structures such as surface contaminant layers. In this study, we compare the performance of a conventional Bragg filtering method with a deep learning routine based on the U-Net architecture. We show that the deep learning method is more general, simpler to apply in practice, and produces more accurate and robust results than the conventional algorithm. We provide easily adaptable source code for all results in this paper and discuss potential applications for deep learning in fully automated TEM image analysis.

47 OTHER INSTRUMENTATION↗
DOE OSTI2024

Quantifying magnetic field driven lattice distortions in kagome metals at the femto-scale using scanning tunneling microscopy

A wide array of unusual phenomena has recently been uncovered in kagome solids. The charge density wave (CDW) state in the kagome superconductor AV 3 Sb 5 , in particular, intrigued the community; the CDW phase appears to break the time-reversal symmetry despite the absence of spin magnetism, which has been tied to exotic orbital loop currents possibly intertwined with magnetic field tunable crystal distortions. To test this connection, precise determination of the lattice response to an applied magnetic field is crucial but can be challenging at the atomic scale. We establish a scanning tunneling microscopy (STM) based method to study the evolution of the AV 3 Sb 5 atomic structure as a function of magnetic field. The method substantially reduces the errors of typical STM measurements, which are at the order of 1% when measuring an in-plane lattice constant change. Here, we find that the out-of-plane lattice constant of AV 3 Sb 5 remains unchanged (within 10 —6 ) by the application of both in-plane and out-of-plane magnetic fields. We also reveal that the in-plane lattice response to magnetic field is at most at the order of 0.05%. Our experiments provide further constraints on time-reversal symmetry breaking in kagome metals and establish a tool for higher-resolution extraction of the field-lattice coupling at the nanoscale applicable to other quantum materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

How Substitutional Point Defects in Two-Dimensional WS 2 Induce Charge Localization, Spin–Orbit Splitting, and Strain

Control of impurity concentrations in semiconducting materials is essential to device technology. Because of their intrinsic confinement, the properties of two-dimensional semiconductors such as transition metal dichalcogenides (TMDs) are more sensitive to defects than traditional bulk materials. The technological adoption of TMDs is dependent on the mitigation of deleterious defects and guided incorporation of functional foreign atoms. The first step toward impurity control is the identification of defects and assessment of their electronic properties. Here, we present a comprehensive study of point defects in monolayer tungsten disulfide (WS 2 ) grown by chemical vapor deposition using scanning tunneling microscopy/spectroscopy, CO-tip noncontact atomic force microscopy, Kelvin probe force spectroscopy, density functional theory, and tight-binding calculations. We observe four different substitutional defects: chromium (Cr W ) and molybdenum (Mo W ) at a tungsten site, oxygen at sulfur sites in both top and bottom layers (O S top/bottom), and two negatively charged defects (CD type I and CD type II). Their electronic fingerprints unambiguously corroborate the defect assignment and reveal the presence or absence of in-gap defect states. Cr W forms three deep unoccupied defect states, two of which arise from spin-orbit splitting. The formation of such localized trap states for Cr W differs from the Mo W case and can be explained by their different d shell energetics and local strain, which we directly measured. Utilizing a tight-binding model the electronic spectra of the isolectronic substitutions O S and Cr W are mimicked in the limit of a zero hopping term and infinite on-site energy at a S and W site, respectively. The abundant CDs are negatively charged, which leads to a significant band bending around the defect and a local increase of the contact potential difference. In addition, CD-rich domains larger than 100 nm are observed, causing a work function increase of 1.1 V. While most defects are electronically isolated, we also observed hybrid states formed between CrW dimers. Finally, the important role of charge localization, spin-orbit coupling, and strain for the formation of deep defect states observed at substitutional defects in WS 2 as reported here will guide future efforts of targeted defect engineering and doping of TMDs.

2D materials↗
DOE OSTI2025

Self-Learning Kinetic Monte Carlo Simulations of Radiation Damage in Nuclear Fuels

Understanding how irradiation affects the thermo-physical and mechanical properties of nuclear materials, such as thermal conductivity degradation in fuels and embrittlement of structural components, is critical to the safety and efficiency of nuclear reactors. These effects are largely governed by the formation and evolution of atomic-scale point defects and defect clusters. Due to their small sizes, however, these defects are invisible under high-resolution scanning transmission electron microscopy. This project aims to fill this experimental knowledge gap by integrating density functional theory (DFT), machine learning interatomic potential (MLIP), and kinetic Monte Carlo (KMC) techniques to predict longtime evolution of irradiation-induced defects in nuclear fuels.

36 - MATERIALS SCIENCE↗
DOE OSTI2018

Structural Features of PLZT Films

Lead zirconate titanate (PZT) films doped with lanthanum, Pb({sub 1–х)}La{sub х}(Zr{sub 0.48}Ti{sub 0.52}) (х = 0, 0.02, 0.05, 0.08, or 0.01), have been investigated by electron microscopy and X-ray diffraction. Films were formed on Si–SiO{sub 2}–TiO{sub 2}–Pt substrates by chemical vapor deposition from a solution and annealed at temperatures T = 650 and 750°C. The main structural features of the films, differing them from undoped PZT films fabricated by the same method, have been established. It is found that doping with lanthanum delays the pyrochlore–perovskite transformation in the film bulk, i.e., in the regions distant from the film–substrate interface. The fraction of metastable pyrochlore phase increases with an increase in the La molar content in the films. The main reason for the delay is the deficit of lead in the intergranular perovskite space, especially in the upper part of the film. Annealing at T = 750°C reduces the content of pyrochlore phase but does not completely remove it, which was never observed for undoped PZT films. Doping with lanthanum leads to a change in the lattice period c and a tetragonal distortion of the perovskite lattice (c/a ratio). Hence, the [100] texture of the films obtained, in contrast to the typical [111] texture of PZT films, is due to the increase in the lattice mismatch between the film and platinum layer when lead atoms are replaced with lanthanum. Lattice distortions of “transrotational” character, whose value exceeds 160 deg/μm, are found to arise in growing crystals.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Two Outer Membrane Proteins Contribute to Caulobacter crescentus Cellular Fitness by Preventing Intracellular S-Layer Protein Accumulation

Surface layers, or S-layers, are two-dimensional protein arrays that form the outermost layer of many bacteria and archaea. They serve several functions, including physical protection of the cell from environmental threats. The high abundance of S-layer proteins necessitates a highly efficient export mechanism to transport the S-layer protein from the cytoplasm to the cell exterior. Caulobacter crescentus is unique in that it has two homologous, seemingly redundant outer membrane proteins, RsaF a and RsaF b , which together with other components form a type I protein translocation pathway for S-layer export. These proteins have homology to Escherichia coli TolC, the outer membrane channel of multidrug efflux pumps. Here we provide evidence that, unlike TolC, RsaF a and RsaF b are not involved in either the maintenance of membrane stability or the active export of antimicrobial compounds. Rather, RsaF a and RsaF b are required to prevent intracellular accumulation and aggregation of the S-layer protein RsaA; deletion of RsaF a and RsaF b led to a general growth defect and lowered cellular fitness. Using Western blotting, transmission electron microscopy, and transcriptome sequencing (RNA-seq), we show that loss of both RsaF a and RsaF b led to accumulation of insoluble RsaA in the cytoplasm, which in turn caused upregulation of a number of genes involved in protein misfolding and degradation pathways. These findings provide new insight into the requirement for RsaF a and RsaF b in cellular fitness and tolerance to antimicrobial agents and further our understanding of the S-layer export mechanism on both the transcriptional and translational levels in C. crescentus . IMPORTANCE Decreased growth rate and reduced cell fitness are common side effects of protein production in overexpression systems. Inclusion bodies typically form inside the cell, largely due to a lack of sufficient export machinery to transport the overexpressed proteins to the extracellular environment. This phenomenon can conceivably also occur in natural systems. As one example of a system evolved to prevent intracellular protein accumulation, our study demonstrates that Caulobacter crescentus has two homologous outer membrane transporter proteins that are involved in S-layer export. This is an interesting case study that demonstrates how bacteria can evolve redundancy to ensure adequate protein export functionality and maintain high cellular fitness. Moreover, we provide evidence that these two outer membrane proteins, although being the closest C. crescentus homologs to TolC in E. coli , do not process TolC functionality in C. crescentus .

59 BASIC BIOLOGICAL SCIENCES↗