Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “point defects”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 829 records · Page 46
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Top and bottom surfaces limit carrier lifetime in lead iodide perovskite films

Carrier recombination at defects is detrimental to the performance of solar energy conversion systems, including solar cells and photoelectrochemical devices. Point defects are localized within the bulk crystal while extended defects occur at surfaces and grain boundaries. If not properly managed, surfaces can be a large source of carrier recombination. Separating surface carrier dynamics from bulk and/or grain-boundary recombination in thin films is challenging. Here, we employ transient reflection spectroscopy to measure the surface carrier dynamics in methylammonium lead iodide perovskite polycrystalline films. We find that surface recombination limits the total carrier lifetime in perovskite polycrystalline thin films, meaning that recombination inside grains and/or at grain boundaries is less important than top and bottom surface recombination. As a result, the surface recombination velocity in polycrystalline films is nearly an order of magnitude smaller than that in single crystals, possibly due to unintended surface passivation of the films during synthesis.

14 SOLAR ENERGY↗
DOE OSTI2015

Nonequilibrium kinetics of the electron–phonon sybsystem of a crystal in a strong electric field as a base of the electroplastic effect

We present the results of a kinetic analysis of nonequilibrium dynamics of the electron–phonon system of a crystal in a strong electric field based on the proposed method of numerically solving a set of Boltzmann equations for electron and phonon distribution functions without expanding the electron distribution function into a series in the phonon energy. It is shown that the electric field action excites the electron subsystem, which by transferring energy to the phonon subsystem creates a large amount of short-wave phonons that effectively influence the lattice defects (point, lines, boundaries of different phases), which results in a redistribution of and decrease in the lattice defect density, in damage healing, in a decrease in the local peak stress, and a decrease in the degradation level of the construction material properties.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Radiation defect dynamics in solids studied by pulsed ion beams

The buildup of radiation damage is often a dynamic phenomenon proceeding via migration and interaction of point defects ballistically generated in collision cascades. In this work, we describe how the dynamic aspects of damage formation via inter-cascade defect interaction can be studied by pulsed ion beam bombardment. We illustrate the pulsed beam method by an example for Si bombarded at 100°C with 1.35 MeV Xe ions and compare with previously reported data for 0.5 MeV Xe ion irradiation. Results reveal that the time constant of defect relaxation strongly depends on both ion energy and the depth from the sample surface. This observation cannot be entirely explained as an effect of the collision cascade density on defect interaction dynamics, based on depth dependencies of cascade densities calculated with a recently developed model that accounts for the fractal nature of cascades. There results call for future systematic studies to better understand ion energy and cascade density effects on defect relaxation dynamics in Si.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Modified Oxygen Defect Chemistry at Transition Metal Oxide Heterostructures Probed by Hard X-ray Photoelectron Spectroscopy and X-ray Diffraction

Transition metal oxide hetero-structures are interesting due to the distinctly different properties that can arise from their interfaces, such as superconductivity, high catalytic activity and magnetism. Oxygen point defects can play an important role at these interfaces in inducing potentially novel properties. The design of oxide hetero-structures in which the oxygen defects are manipulated to attain specific functionalities requires the ability to resolve the state and concentration of local oxygen defects across buried interfaces. In this work, we utilized a novel combination of hard x-ray photoelectron spectroscopy (HAXPES) and high resolution xray diffraction (HRXRD) to probe the local oxygen defect distribution across the buried interfaces of oxide heterolayers. This approach provides a non-destructive way to qualitatively probe locally the oxygen defects in transition metal oxide hetero-structures. We studied two trilayer structures as model systems - the La 0.8 Sr 0.2 CoO 3-δ /(La 0.5 Sr 0.5 ) 2 CoO 4 /La 0.8 Sr 0.2 CoO 3-δ (LSC 113 /LSC 214 ) and the La 0.8 Sr 0.2 CoO 3-δ /La 2 NiO 4+δ /La 0.8 Sr 0.2 CoO 3-δ (LSC 113 /LNO 214 ) on SrTiO 3 (001) single crystal substrates. We found that the oxygen defect chemistry of these transition metal oxides was strongly impacted by the presence of interfaces and the properties of the adjacent phases. Under reducing conditions, the LSC 113 in the LSC 113 /LNO 214 tri-layer had less oxygen vacancies than the LSC 113 in the LSC 113 /LSC 214 tri-layer and the LSC 113 single phase film. On the other hand, LSC 214 and LNO 214 were more reduced in the two tri-layer structures when in contact with the LSC 113 layer compared to their single phase counterparts. Furthermore, the results point out a potential way to modify the local oxygen defect states at oxide hetero-interfaces.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Enhanced Electrical Resistivity and Properties via Ion Bombardment of Ferroelectric Thin Films

140 nm PbTiO 3 /20 nm SrRuO 3 thin-film heterostructures were grown on SrTiO 3 (001) single-crystal substrates via pulsed-laser deposition. In order to study the properties of these films, symmetric SrRuO 3 capacitor structures were fabricated using established techniques and dramatic differences were observed in the leakage characteristics of the heterostructures. The high-pressure O 2 and O 2 /Ar heterostructures reveal more than 2 orders of magnitude higher leakage current densities as compared to the low-pressure O 2 heterostructures. In turn, the high leakage current density of the heterostructures grown in high pressures precludes measurement of closed, saturated ferroelectric hysteresis loops, while nearly-square, fully saturated and closed hysteresis loops can be measured for the low-pressure O 2 heterostructures. The defects responsible for poor transport and ferroelectric properties in PbTiO 3 thin films are identified to be isolated point defects such as Pb 3+ centers and lead vacancies, which contribute to hole conduction. Introduction of bombardment-induced defects is shown to be an effective way to quench these trap states by turning them into deep lying defect complexes and clusters which, in turn, leads to a dramatic reduction of leakage current by up to 5 orders of magnitude and improvement of ferroelectric device performance.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Thermodynamic and Kinetic Properties of Intrinsic Defects and Mg Transmutants in 3C-SiC Determined by Density Functional Theory

Density functional theory (DFT) is used to calculate the thermodynamic and kinetic properties of transmutant Mg in 3C-SiC due to high-energy neutron irradiation associated with the fusion nuclear environment. The formation and binding energies of intrinsic defects, Mg-related defects, and clusters in 3C-SiC are systematically calculated. The minimum energy paths and activation energies during point defect migration and small cluster evolution are studied using a generalized solid-state elastic band (G-SSNEB) method with DFT energy calculations. Stable defect structures and possible defect migration mechanisms are identified. The evolution of binding energies during Mg2Si formation demonstrates that the formation of Mg2Si needs to overcome a critical nucleus size and nucleation barrier. It is also found that a compressive stress field exists around the Mg2Si nucleus. These data are important inputs in meso- and macro-scale modeling and experiments to understand and predict the impact of Mg on phase stability, microstructure evolution, and performance of SiC and SiC-based materials during long-term neutron exposures.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2021

Healing of nanocracks by collision cascades in nickel

We use molecular dynamics simulations to investigate the interaction between collision cascades and pre-existing nano-cracks in nickel (Ni). We find that collision cascades cause the cracks to heal whenever a cascade initiates a thermal spike whose core overlaps with the crack. Moreover, crack healing affects the distribution of radiation-induced defects generated during a collision cascade: defect clusters, such as dislocations and stacking fault tetrahedra, dominate in the presence of crack healing, while isolated point defects dominate in its absence. Furthermore, this work indicates that irradiation may reduce the number of pre-existing crack-like flaws in metallic components, potentially improving their mechanical resilience.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Control of polarization in bulk ferroelectrics by mechanical dislocation imprint

Defects are essential to engineering the properties of functional materials ranging from semiconductors and superconductors to ferroics. Whereas point defects have been widely exploited, dislocations are commonly viewed as problematic for functional materials and not as a microstructural tool. In this work, we developed a method for mechanically imprinting dislocation networks that favorably skew the domain structure in bulk ferroelectrics and thereby tame the large switching polarization and make it available for functional harvesting. The resulting microstructure yields a strong mechanical restoring force to revert electric field–induced domain wall displacement on the macroscopic level and high pinning force on the local level. This induces a giant increase of the dielectric and electromechanical response at intermediate electric fields in barium titanate [electric field–dependent permittivity (ε 33 ) ≈ 5800 and large-signal piezoelectric coefficient ( d 33 *) ≈ 1890 picometers/volt]. Dislocation-based anisotropy delivers a different suite of tools with which to tailor functional materials.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Effective defect diffusion lengths in Ar-ion bombarded 3C-SiC

Above room temperature, SiC exhibits pronounced processes of diffusion and interaction of radiation-generated point defects. Here, we use the recently developed pulsed ion beam method to measure effective defect diffusion lengths in 3C-SiC bombarded in the temperature range of 25–200 °C with 500 keV Ar ions. Results reveal a diffusion length of ~10 nm, which exhibits a weak temperature dependence, changing from 9 to 13 nm with increasing temperature. Lastly, these results have important implications for understanding and predicting radiation damage in SiC and for the development of radiation-resistant materials via interface-mediated defect reactions.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Dissolution of Mg-enriched defects in implanted GaN and increased p-type dopant activation

Annealing Mg-implanted homoepitaxial GaN at temperatures above 1400 °C eliminates the formation of inversion domains and leads to improved dopant activation efficiency. Extended defects, in the form of inversion domains, contain electrically inactive Mg after post-implantation annealing at temperatures as high as 1300 °C (one GPa N 2 overpressure), which results in a low dopant activation efficiency. Triple-axis x-ray data reveal that implant-induced strain is fully relieved after annealing at 1300 °C for 10 min, indicating that strain-inducing point defects formed during implantation have reconfigured and inversion domains are formed. However, annealing at temperatures of 1400–1500 °C (one GPa N 2 overpressure) eliminates the presence of the inversion domains. While residual defects, such as dislocation loops, still exist after annealing at and above 1400 °C, chemical analysis at multiple dislocation loops shows no sign of Mg segregation. Meanwhile, an overall decreasing trend in the dislocation loop density is observed after annealing at the higher temperatures and longer times. Additionally, once inversion domains are formed and the samples are cooled to room temperature, they are shown to dissolve with subsequent annealing above 1400 °C. While such defects have been observed before, the important finding that such defects can be dissolved with a short, higher temperature step is key. Earlier work [Breckenridge et al., J. Appl. Phys. Lett. 118, 022101 (2021)] addressing electrical measurements of these types of samples showed that annealing at 1400 °C leads to a dopant activation efficiency that is an order of magnitude higher than that observed at 1300 °C. In conclusion, this work complements earlier work by identifying the inversion domains, which incorporate Mg, and points to the benefits, in terms of defect density and p-type dopant activation, of using higher temperature (>1400 °C) annealing cycles to activate Mg in GaN, even if the Mg-containing inversion domains had been formed during lower temperature annealing.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Intermittent Defect Fluctuations in Oxide Heterostructures

Abstract The heterogeneous nature, local presence, and dynamic evolution of defects typically govern the ionic and electronic properties of a wide variety of functional materials. While the last 50 years have seen considerable efforts into development of new methods to identify the nature of defects in complex materials, such as the perovskite oxides, very little is known about defect dynamics and their influence on the functionality of a material. Here, the discovery of the intermittent behavior of point defects (oxygen vacancies) in oxide heterostructures employing X‐ray photon correlation spectroscopy is reported. Local fluctuations between two ordered phases in strained SrCoO x with different degrees of stability of the oxygen vacancies are observed. Ab‐initio‐informed phase‐field modeling reveals that fluctuations between the competing ordered phases are modulated by the oxygen ion/vacancy interaction energy and epitaxial strain. The results demonstrate how defect dynamics, evidenced by measurement and modeling of their temporal fluctuations, give rise to stochastic properties that now can be fully characterized using coherent X‐rays, coupled for the first time to multiscale modeling in functional complex oxide heterostructures. The study and its findings open new avenues for engineering the dynamical response of functional materials used in neuromorphic and electrochemical applications.

36 MATERIALS SCIENCE↗
DOE OSTI2018

On the role of electro-migration in the evolution of radiation damage in nanostructured ionic materials

We report that radiation resistant materials are needed for a large number of applications. One route to enhancing radiation resistance is to introduce a high density of defect sinks such as grain boundaries. However, there are still important questions regarding the role of grain boundaries in enhancing radiation tolerance, particularly in ionic materials. Experiments have found large improvements in the amorphization resistance of oxides at temperatures where defect mobilities are too low to easily reach the boundaries. Standard reaction-diffusion models are inadequate in explaining this behavior. Here, we examine the role of electro-migration in the overall transport of irradiation-induced defects in ionic systems. We find that electro-migration can have a large impact on the steady state point defect concentrations as compared to models that do not include the effect of electro-migration. In particular, defect concentrations can change by nearly seven orders of magnitude for certain conditions. Finally, we conclude that radiation tolerance in nanocrystalline ionic ceramics will be driven both by enhanced sink density via small grain sizes and, critically, the effects of electro-migration induced by charge segregation to grain boundaries.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Dependence of simulated radiation damage on crystal structure and atomic misfit in metals

This study investigates the evolution of radiation damage in three metals in the low temperature and high radiant flux regime using molecular statics and a Frenkel pair accumulation method to simulate up to 2.0 displacements per atom. The metals considered include Fe, equiatomic CrCoNi, and a fictitious metal with similar bulk properties to the CrCoNi composed of a single atom type referred to as an A-atom. CrCoNi is found to sustain higher concentrations of dislocations than either the Fe or A-atom systems and more stacking faults than the A-atom system. The results suggest that the difference between the concentrations of vacancies and interstitials is substantially smaller for CrCoNi than the A-atom system, perhaps reflecting that the sink capture radius is smaller in CrCoNi due to the roughened potential energy landscape. A model that partitions the major contributions from defects to the stored energy is described, and serves to highlight a general need for higher fidelity approaches to point defect identification.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Advances in Quantum Defect Embedding Theory

Quantum defect embedding theory (QDET) is a many-body embedding method designed to describe condensed systems with strongly correlated electrons localized within a given region of space, for example spin defects in semiconductors and insulators. Although the QDET approach has been successful in predicting the electronic properties of several point defects, several limitations of the method remain. Here, in this work, we propose multiple advances to the QDET formalism. We derive a doublecounting correction that consistently treats the frequency dependence of the screened Coulomb interaction, and we illustrate the effect of including unoccupied orbitals in the active space. In addition, we propose a method to describe hybridization effects between the active space and the environment, and we compare the results of several impurity solvers, providing further insights into improving the reliability and applicability of the method. We present results for defects in diamond and for molecular qubits, including a detailed comparison with experiments.

Chen, Siyuan [University of Chicago, IL (United St↗
DOE OSTI2017

First-principles calculation of intrinsic defect chemistry and self-doping in PbTe

Semiconductor dopability is inherently limited by intrinsic defect chemistry. In many thermoelectric materials, narrow band gaps due to strong spin-orbit interactions make accurate atomic level predictions of intrinsic defect chemistry and self-doping computationally challenging. For this study, we use different levels of theory to model point defects in PbTe, and compare and contrast the results against each other and a large body of experimental data. We find that to accurately reproduce the intrinsic defect chemistry and known self-doping behavior of PbTe, it is essential to (a) go beyond the semi-local GGA approximation to density functional theory, (b) include spin-orbit coupling, and (c) utilize many-body GW theory to describe the positions of individual band edges. The hybrid HSE functional with spin-orbit coupling included, in combination with the band edge shifts from G0W0 is the only approach that accurately captures both the intrinsic conductivity type of PbTe as function of synthesis conditions as well as the measured charge carrier concentrations, without the need for experimental inputs. Our results reaffirm the critical role of the position of individual band edges in defect calculations, and demonstrate that dopability can be accurately predicted in such challenging narrow band gap materials.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Tuning of the electronic and vibrational properties of epitaxial MoS 2 through He-ion beam modification

Abstract Atomically thin transition metal dichalcogenides (TMDs), like MoS 2 with high carrier mobilities and tunable electron dispersions, are unique active material candidates for next generation opto-electronic devices. Previous studies on ion irradiation show great potential applications when applied to two-dimensional (2D) materials, yet have been limited to micron size exfoliated flakes or smaller. To demonstrate the scalability of this method for industrial applications, we report the application of relatively low power (50 keV) 4 He + ion irradiation towards tuning the optoelectronic properties of an epitaxially grown continuous film of MoS 2 at the wafer scale, and demonstrate that precise manipulation of atomistic defects can be achieved in TMD films using ion implanters. The effect of 4 He + ion fluence on the PL and Raman signatures of the irradiated film provides new insights into the type and concentration of defects formed in the MoS 2 lattice, which are quantified through ion beam analysis. PL and Raman spectroscopy indicate that point defects are generated without causing disruption to the underlying lattice structure of the 2D films and hence, this technique can prove to be an effective way to achieve defect-mediated control over the opto-electronic properties of MoS 2 and other 2D materials.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2023

Wide-field probing of silica laser-induced damage precursors by photoluminescence photochemical quenching

Here, we describe a wide-field approach to probe transient changes in photoluminescence (PL) of defects on silica surfaces. This technique allows simultaneous capture of spatially resolved PL with spontaneous quenching behavior. We attribute the quenching of PL intensity to photochemical reactions of surface defects and/or subsurface fractures with ambient molecules. Such quenching curves can be accurately reproduced by our theoretical model using two quenchable defect populations with different reaction rates. The fitting parameters of our model are spatially correlated to fractures in silica where point defects and mechanical stresses are known to be present, potentially indicating regions prone to laser-induced damage growth. We believe that our approach allows rapid spatial resolved identification of damage prone morphology, providing a new pathway to fast, non-destructive predictions of laser-induced damage growth.

47 OTHER INSTRUMENTATION↗