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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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All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Novel penta-graphene nanotubes: strain-induced structural and semiconductor–metal transitions

Research into novel one-dimensional (1D) materials and associated structural transitions is of significant scientific interest. It is widely accepted that a 1D system with a short-range interaction cannot have 1D phase transition at finite temperature. In this paper, we propose a series of new stable carbon nanotubes by rolling up penta-graphene sheets, which exhibit fascinating well-defined 1D phase transitions triggered by axial strain. Our first-principles calculations show that such penta-graphene nanotubes (PGNTs) are dynamically stable by phonon calculations, but transform from a tri-layer structure to a highly defective single-walled nanotube at low temperature in molecular dynamics simulations. We show that moderate compressive strains can drive structural transitions of (4,4), (5,5), and (6,6) PGNTs, during which the distances of neighboring carbon dimers in the inner shell have a sudden drop, corresponding to dimer–dimer nonbonding to bonding transitions. After such transition, the tubes become much more thermally stable and undergo semiconductor–metal transitions under increasing strain. The band gaps of PGNTs are not sensitive to chirality whereas they can be tuned effectively from visible to short-wavelength infrared by appropriate strain, making them appealing materials for flexible nano-optoelectronics. In conclusion, these findings provide useful insight into unusual phase transitions in low-dimensional systems.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Analysis of defects in In 2 O 3 :H synthesized in presence of water vapor and hydrogen gas mixture

Using hybrid functional-based density functional theory calculations, we analyze the structure and kinetics of defects formed in two competing synthesis routes to prepare hydrogen-doped In 2 O 3 films, using a hydrogen and oxygen gas mixture vs synthesis in the presence of water vapor. For both of these synthesis routes, we find that H + is the dominant defect species: when the Fermi level is close to the conduction band, H + has a lower formation energy than other intrinsic or extrinsic defects. Our results also suggest that water molecules spontaneously split into H + (which occupies octahedral voids) and OH – interstitials (which occupies vacant oxygen lattice sites or oxygen vacancies). From the analysis of the binding energies between these different defects, we conclude that these defects do not cluster and are most likely to stay spatially distributed throughout the films. In addition, the sum of formation energies of an oxygen (i.e., O$^{2–}_i$) and a H + interstitial is close to the formation energy of a OH – interstitial, meaning that water molecules are completely split into 2H + and O$^{2–}_i$ at the synthesis conditions. Further, in the presence of H 2 + O 2 gas mixture, oxygen interstitials occupy oxygen vacancies while hydrogen interstitials occupy vacant oxygen lattice sites and form bonds with lattice oxygens. Our analysis of the defect equilibria suggests that the hydrogen content in films synthesized in the presence of water vapor is higher than films synthesized in the presence of a hydrogen gas mixture. At high dopant concentrations, a hydrogen bond network is formed in the system and this leads to large distortions in the lattice.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Establishing defect-property relationships for 2D-nanomaterials (Final technical report)

Studies of new families of two-dimensional nanomaterials (2DNMs) have established that they possess unique properties that diverge from those of their bulk counterparts. This project aimed to determine how tolerant 2DNMs are to extreme photon and particle fluxes and to identify the mechanisms governing their radiation response. Early work indicated that graphene is not as representative of other 2DNMs as previously assumed; however, the origin of this difference remained unclear. To address these knowledge gaps, this project investigated the physical processes occurring at multiple length scales in transition metal dichalcogenides (TMDs) and quantified their structural stability and property evolution under far-from-equilibrium conditions. In-situ and ex-situ ion and electron irradiations were performed to directly control and monitor defect formation in TMDs. Complementary density functional theory (DFT) and molecular dynamics (MD) simulations were employed to elucidate the mechanisms of defect generation and evolution. The outcomes of this work established mechanistic understanding of irradiation-induced defect formation in 2DNMs, quantified the radiation tolerance of TMDs, and built a fundamental knowledge base for correlating defect structures with material properties. Collectively, these results provide new insights into the stability of low-dimensional materials in extreme environments and enable the predictive design of radiation-tolerant 2DNMs.

2D materials↗
DOE OSTI2017

Effects of defects and dephasing on charge and spin currents in two-dimensional topological insulators

Using the nonequilibrium Keldysh Green's function formalism, we investigate the effect of defects on the electronic structure and transport properties of two-dimensional topological insulators (TI). We demonstrate how the spatial flow of charge changes between the topologically protected edge and bulk states and show that elastically and inelastically scattering defects that preserve the time-reversal symmetry of the TI lead to qualitatively different effects on the TI's local electronic structure and its transport properties. Moreover, we show that the recently predicted ability to create highly spin-polarized currents by breaking the time-reversal symmetry of the TI via magnetic defects [J. S. Van Dyke and D. K. Morr, Phys. Rev. B 93, 081401 (2016)] is robust against the inclusion of a Rashba spin-orbit interaction and the effects of dephasing, and remains unaffected by changes over a wide range of the TI's parameters. We discuss how the sign of the induced spin currents changes under symmetry operations, such as reversal of bias and gate voltages, or spatial reflections. Finally, we show that the insight into the interplay between topology and symmetry of the magnetic defects can be employed for the creation of intriguing quantum phenomena, such as highly localized magnetic fields inside the TI.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Dislocation-Assisted Quasi-Two-Dimensional Semiconducting Nanochannels Embedded in Perovskite Thin Films

Defect engineering in perovskite thin films has attracted extensive attention recently due to the films’ atomic-scale modification, allowing for remarkable flexibility to design novel nanostructures for next generation nanodevices. However, the defect-assisted three-dimensional nanostructures in thin film matrices usually has large misfit strain and thus causes unstable thin film structures. In contrast, defect-assisted one- or two-dimensional nanostructures embedded in thin films can sustain large misfit strains without relaxation, which make them suitable for defect engineering in perovskite thin films. Here, we reported the fabrication and characterization of edge-type misfit dislocation-assisted two-dimensional BiMnO x nanochannels embedded in SrTiO 3 /La 0.7 Sr 0.3 MnO 3 /TbScO 3 perovskite thin films. The nanochannels are epitaxially grown from the surrounding films without noticeable misfit strain. Diode-like current rectification was spatially observed at nanochannels due to the formation of Schottky junctions between BiMnO x nanochannels and conducting La 0.7 Sr 0.3 MnO 3 thin films. Such atomically scaled heterostructures constitute more flexible ultimate functional units for nanoscale electronic devices.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

Luminescence of impurity Ce{sup 3+} centers in KH{sub 2}PO{sub 4} : Ce crystals

The photoluminescence, X-ray luminescence, and cathodoluminescence spectra of KH{sub 2}PO{sub 4} : Ce single crystals contain a nonelementary band of radiation with an energy of 3.55 eV and decay time of 27–33 ns. It is formed by fast radiative interconfiguration d → f transitions between the excited and ground states of Ce{sup 3+} ions, with the ground state is split by a crystalline field. In the range of concentrations studied (0.5–3 × 10{sup –2} wt %), Ce3+ ions enter the KH{sub 2}PO{sub 4} : Ce crystal lattice as substitution ions. Local charge compensation takes place by means of defects in the crystal structure that cause luminescence with a large Stokes shift in the region of 2.4–2.2 eV. The presence of hydrogen sublattice defects decreases the efficiency of energy transport by free charge carriers to the luminescent centers. The interaction of defects and impurity centers manifests itself as a slow inertial building-up of the stationary X-ray luminescence yield of Ce{sup 3+} centers.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Point defects in Ga 2 O 3

In the field of high-power electronics, gallium oxide (Ga 2 O 3 ) is attracting attention due to its wide band gap and ability to be doped n-type. Point defects, including vacancies, impurities, and dopants, play important roles in optimizing device performance. This tutorial discusses the fundamental properties of point defects in monoclinic β-Ga 2 O 3 and the methods employed to study them. Oxygen vacancies are deep donors that do not cause n-type conductivity but may compensate acceptors. Gallium vacancies are deep acceptors that can be partially passivated by hydrogen. Substitutional magnesium is a promising acceptor that produces semi-insulating material and also forms a complex with hydrogen. Calcium and iron also have deep acceptor levels. Iridium deep donors are introduced into crystals grown from a melt in an Ir crucible. Here, other defects are introduced by irradiation with energetic particles such as neutrons or protons. In addition to altering the electronic properties, defects give rise to UV/visible emission bands in photoluminescence and cathodoluminescence spectra.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Multi-Level Structural Damage Characterization Using Sparse Acoustic Sensor Networks and Knowledge Transferred Deep Learning

Standard structural health monitoring techniques face well-known difficulties for comprehensive defect diagnosis in real-world structures that have structural, material, or geometric complexity. This motivates the exploration of machine-learning-based structural health monitoring methods in complex structures. However, creating sufficient training data sets with various defects is an ongoing challenge for data-driven machine (deep) learning algorithms. The ability to transfer the knowledge of a trained neural network from one component to another or to other sections of the same component would drastically reduce the required training data set. Also, it would facilitate computationally inexpensive machine learning based inspection systems. In this work, a machine-learning-based multi-level damage characterization is demonstrated with the ability to transfer trained knowledge within the sparse sensor network. A novel network spatial assistance and an adaptive convolution technique are proposed for efficient knowledge transfer within the deep learning algorithm. Proposed structural health monitoring method is experimentally evaluated on an aluminum plate with artificially induced defects. It was observed that the method improves the performance of knowledge transferred damage characterization by 50% during localization and 24% during severity assessment. Further, experiments using time windows with and without multiple edge reflections are studied. Results reveal that multiply scattered waves contain rich and deterministic defect signatures that can be mined using deep learning neural networks, improving the accuracy of both identification and quantification. In the case of a fixed sensor network, using multiply scattered waves shows 100% prediction accuracy at all levels of damage characterization.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Defect formation during chlorine-based dry etching and their effects on the electronic and structural properties of InP/InAsP quantum wells

The general objective is the investigation of the defects formed by dry etching tools such as those involved in the fabrication of photonic devices with III–V semiconductors. Emphasis is put on plasma exposures with chlorine-based chemistries. In addition to identifying these defects and describing their effects on the electro-optic and structural properties, the long-term target would be to predict the impact on the parameters of importance for photonic devices, and possibly include these predictions in their design. The work is first centered on explaining the experimental methodology. This methodology starts with the design and growth of a quantum well structure on indium phosphide, including ternary indium arsenide/phosphide quantum wells with graded arsenic/phosphor composition. These samples have then been characterized by luminescence methods (photo- and cathodoluminescence), high-resolution transmission electron microscopy, and secondary ion mass spectrometry. As one of the parameters of importance in this study, the authors have also included the doping level. The samples have been exposed to the etching plasmas for “short” durations that do not remove completely the quantum wells, but change their optical signature. No masking layer with lithographic features was involved as this work is purely oriented to study the interaction between the plasma and the samples. A significant difference in the luminescence spectra of the as-grown undoped and doped samples is observed. A mechanism describing the effect of the built-in electric field appearing as a consequence of the doping profile is proposed. This mechanism involves quantum confined Stark effect and electric-field induced carrier escape from the quantum wells. In the following part, the effects of exposure to various chlorine-based plasmas were explored. Differences are again observed between the undoped and doped samples, especially for chemistries containing silicon tetrachloride. Secondary ion mass spectrometry indicates penetration of chlorine in the structures. Transmission electron microscopy is used to characterize the quantum well structure before and after plasma bombardment. By examining carefully the luminescence spectral properties, the authors could demonstrate the influence of the etching plasmas on the built-in electric field (in the case of doped samples), and relate it to some ionic species penetrating the structures. Etching plasmas involving both chlorine and nitrogen have also been studied. The etching rate for these chemistries is much slower than for some of the silicon tetrachloride based chemistries. Their effects on the samples are also very different, showing much reduced effect on the built-in electric field (for the doped samples), but significant blue-shifts of the luminescence peaks that the authors attributed to the penetration of nitrogen in the structures. Nitrogen, in interstitial locations, induces mechanical compressive stress that accounts for the blue-shifts. Finally, from the comparison between secondary ion mass spectrometry and luminescence spectra, the authors suggest some elements for a general mechanism involved in the etching by chloride-chemistries, in which a competition takes place between the species at the surface, active for the etching mechanism, and the species that penetrate the structure, lost for the etching process, but relevant in terms of impact on the electro-optic and structural features of the exposed materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

3D Bragg coherent diffractive imaging of five-fold multiply twinned gold nanoparticle

The formation mechanism of five-fold multiply twinned nanoparticles has been a long-term topic because of their geometrical incompatibility. So, various models have been proposed to explain how the internal structure of the multiply twinned nanoparticles accommodates the constraints of the solid-angle deficiency. Here, we investigate the internal structure, strain field and strain energy density of 600 nm sized five-fold multiply twinned gold nanoparticles quantitatively using Bragg coherent diffractive imaging, which is suitable for the study of buried defects and three-dimensional strain distribution with great precision. Our study reveals that the strain energy density in five-fold multiply twinned gold nanoparticles is an order of magnitude higher than that of the single nanocrystals such as an octahedron and triangular plate synthesized under the same conditions. This result indicates that the strain developed while accommodating an angular misfit, although partially released through the introduction of structural defects, is still large throughout the crystal.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Polytypic Zn–(In,Ga)–Se Nanocrystals with Tunable Emission

While the polymorphism of chalcopyrite semiconductors has been widely studied, the wurtzite analogues of defect-chalcopyrite II–III 2 –VI 4 compositions, such as Zn(In,Ga) 2 (S,Se) 4 , remain underexplored. Here, we report the synthesis of polytypic Zn–(In,Ga)–Se multipods via cation exchange using ZnSe as a template. With cubic cores and wurtzite arms elongated along the hexagonal c-axis, the multipods retain the structure and morphology of the ZnSe template. Optical characterization reveals composition-dependent absorption and photoluminescence, tunable from the visible to the near-infrared region, with spectral features distinct from those of previously reported defect-chalcopyrite structures. Temperature-dependent measurements demonstrate strong emission at cryogenic temperatures, which is quenched near room temperature due to thermally activated nonradiative processes. Here, we illustrate the use of ZnSe as a platform for cation exchange toward wurtzite multinary chalcogenides, unlocking access to novel structures with colorful optical properties.

cation exchange↗
NASA NTRS2011

The Role of Water in the Stability of Cratonic Keels

Cratons are typically underlain by large, deep, and old lithospheric keels (to greater than 200 km depth, greater than 2.5 Ga old) projecting into the asthenosphere (e.g., Jordan, 1978; Richardson et al., 1984). This has mystified Earth scientists as the dynamic and relatively hot asthenosphere should have eroded away these keels over time (e.g., Sleep, 2003; O'Neill et al., 2008; Karato, 2010). Three key factors have been invoked to explain cratonic root survival: 1) Low density makes the cratonic mantle buoyant (e.g., Poudjom Djomani et al., 2001). 2) Low temperatures (e.g., Pollack, 1986; Boyd, 1987), and 3) low water contents (e.g., Pollack, 1986), would make cratonic roots mechanically strong. Here we address the mechanism of the longevity of continental mantle lithosphere by focusing on the water parameter. Although nominally anhydrous , olivine, pyroxene and garnet can accommodate trace amounts of water in the form of H bonded to structural O in mineral defects (e.g., Bell and Rossman, 1992). Olivine softens by orders of magnitude if water (1-1000 ppm H2O) is added to its structure (e.g., Mackwell et al., 1985). Our recent work has placed constraints on the distribution of water measured in peridotite minerals in the cratonic root beneath the Kaapvaal in southern Africa (Peslier et al., 2010). At P greater than 5 GPa, the water contents of pyroxene remain relatively constant while those of olivine systematically decrease from 50 to less than 10 ppm H2O at 6.4 GPa. We hypothesized that at P greater than 6.4 GPa, i.e. at the bottom of the cratonic lithosphere, olivines are essentially dry (greater than 10 ppm H2O). As olivine likely controls the rheology of the mantle, we calculated that the dry olivines could be responsible for a contrast in viscosity between cratonic lithosphere and surrounding asthenosphere large enough to explain the resistance of cratonic root to asthenospheric delamination.

Peslier, Anne H.↗
DOE OSTI2018

GeV ion irradiation of NiFe and NiCo: Insights from MD simulations and experiments

Concentrated solid solution alloys have attracted rapidly increasing attention due to their potential for designing materials with high tolerance to radiation damage. To tackle the effects of chemical complexity in defect dynamics and radiation response, we present in this paper a computational study on swift heavy ion induced effects in Ni and equiatomic Ni -based alloys (Ni 50 Fe 50 , Ni 50 Co 50 ) using two-temperature molecular dynamics simulations (2T-MD). The electronic heat conductivity in the two-temperature equations is parameterized from the results of first principles electronic structure calculations. A bismuth ion (1.542 GeV) is selected and single impact simulations performed in each target. We study the heat flow in the electronic subsystem and show that alloying Ni with Co or Fe reduces the heat dissipation from the impact by the electronic subsystem. Simulation results suggest no melting or residual damage in pure Ni while a cylindrical region melts along the ion propagation path in the alloys. In Ni 50 Co 50 the damage consists of a dislocation loop structure (d = 2 nm) and isolated point defects, while in Ni 50 Fe 50 , a defect cluster (d = 4 nm) along the ion path is, in addition, formed. The simulation results are supported by atomic-level structural and defect characterizations in bismuth-irradiated Ni and Ni 50 Fe 50 . Finally, the significance of the 2T-MD model is demonstrated by comparing the results to those obtained with an instantaneous energy deposition model without consideration of e-ph interactions in pure Ni and by showing that it leads to a different qualitative behavior.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Research Update: Emerging chalcostibite absorbers for thin-film solar cells

Copper antimony chalcogenides CuSbCh 2 (Ch=S, Se) are an emerging family of absorbers studied for thin-film solar cells. These non-toxic and Earth-abundant materials show a layered low-dimensional chalcostibite crystal structure, leading to interesting optoelectronic properties for applications in photovoltaic (PV) devices. This research update describes the CuSbCh 2 crystallographic structures, synthesis methods, competing phases, band structures, optoelectronic properties, point defects, carrier dynamics, and interface band offsets, based on experimental and theoretical data. Correlations between these absorber properties and PV device performance are discussed, and opportunities for further increase in the efficiency of the chalcostibite PV devices are highlighted.

14 SOLAR ENERGY↗
DOE OSTI2019

Special Features of the Structure of Titanium Nitride Coatings in Substrate Surface Cuts

Elemental compositions and structure-phase states of titanium nitride coatings on the flat surface and in a cut of substrate from 10Cr18Ni10Ti steel are investigated by the methods of x-ray microspectral analysis, Auger electron spectroscopy, and dark-field electron microscopy analysis of the bending-torsion of the crystal lattice. It is established that the coating in the cut, unlike the coating on the frontal surface, has a modified elemental composition and a two-layer structure whose special features and thicknesses change with distance from the sample surface. It is shown that these changes are caused by phase transformations, decreased number of defects, dispersion of layer structures, and increased concentration of impurities with increasing distance from the substrate surface. Based on the results obtained, possible conditions for coating structure formation in the cut of the substrate surface are discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Designing functionality in perovskite thin films using ion implantation techniques: Assessment and insights from first-principles calculations

Ion implantation has been widely used in the semiconductor industry for decades to selectively control electron/hole doping for device applications. Recently, experimental studies on ion implantation into more structurally and electronically complex materials have been undertaken in which defect generation has been used to control a variety of functional phenomena. Of particular interest, are recent findings demonstrating that low doses of low energy helium ions into single crystal films can be used to tailor the structural properties. These initial experimental studies have shown that crystal symmetry can be continuously controlled by applying increasingly large doses of He ions into a crystal. The observed changes in lattice structure were then observed to correlate with functional changes, such as metal-insulator transition temperature2 and optical bandgap3. In these preliminary experimental studies, changes to lattice expansion was proposed to be the direct result of chemical pressure originating predominantly from the implanted He applying chemical pressure at interstitial sites. However, the influence of possible secondary knock-on damage arising from the He atoms transferring energy to the lattice through nuclear-nuclear collision with the crystal lattice remains largely unaddressed. In this work, we focus on a SrRuO3 model system to provide a comprehensive examination of the impact of common defects on structural and electronic properties, obtain calculated defect formation energies, and define defect migration barriers. Our model indicates that, while interstitial He can modify the crystal properties, a dose significantly larger than those reported in experimental studies would be required. The true origin of the observed structural changes is likely the result of a combination of secondary defects created during He implantation. Of particular importance, we observe that different defect types can generate greatly varied local electronic structures and that the formation energies and migration energy barriers vary by defect type. Thus, we may have identified a new method of selectively inducing controlled defect complexes into single crystal materials. Development of this approach would have a broad impact on both our ability to probe specific defect contributions in fundamental studies and allow a new level of control over functional properties driven by specific defect complexes.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Investigation of the tunnel magnetoresistance in junctions with a strontium stannate barrier

In this paper, we experimentally investigate the structural, magnetic, and electrical transport properties of La 0.67 Sr 0.33 MnO 3 based magnetic tunnel junctions with a SrSnO 3 barrier. Our results show that despite the high density of defects in the strontium stannate barrier, due to the large lattice mismatch, the observed tunnel magnetoresistance (TMR) is comparable to tunnel junctions with a better lattice matched SrTiO 3 barrier, reaching values of up to 350% at T = 5K. Further analysis of the current-voltage characteristics of the junction and the bias voltage dependence of the observed tunnel magnetoresistance show a decrease of the TMR with increasing bias voltage. In addition, the observed TMR vanishes for T > 200K. Finally, our results suggest that by employing a better lattice matched ferromagnetic electrode, and thus reducing the structural defects in the strontium stannate barrier, even larger TMR ratios might be possible in the future.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2024

Midgap state requirements for optically active quantum defects

Abstract Optically active quantum defects play an important role in quantum sensing, computing and communication. The electronic structure and the single-particle energy levels of these quantum defects in the semiconducting host have been used to understand their optoelectronic properties. Optical excitations that are central for their initialization and readout are linked to transitions between occupied and unoccupied single-particle states. It is commonly assumed that only quantum defects introducing levels well within the band gap and far from the band edges are of interest for quantum technologies as they mimic an isolated atom embedded in the host. In this perspective, we contradict this common assumption and show that optically active defects with energy levels close to the band edges can display similar properties. We highlight quantum defects that are excited through transitions to or from a band-like level (bound exciton) such as the T center and Se S i + in silicon. We also present how defects such as the silicon split-vacancy in diamond can involve transitions between localized levels that are above the conduction band or below the valence band. Loosening the commonly assumed requirement on the electronic structure of quantum defects offers opportunities in quantum defects design and discovery especially in smaller band gap hosts such as silicon. We discuss the challenges in terms of operating temperature for photoluminescence or radiative lifetime in this regime. We also highlight how these alternative type of defects bring their own needs in terms of theoretical developments and fundamental understanding. This perspective clarifies the electronic structure requirement for quantum defects and will facilitate the identification and design of new color centers for quantum applications especially driven by first principles computations.

Xiong, Yihuang (ORCID:0000000178096047)↗