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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.

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At least 505 records · Page 28
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Role of Lithium Codoping in Enhancing the Scintillation Yield of Aluminate Garnets

The aim of this work is to clarify the scintillation-yield enhancement in Lu YAG : Pr scintillators obtained by Li codoping via integrated study of the valence state of activators, the preferential site occupancy of Li codopants, and defect structures from experimental and theoretical insights. Here, with Li codoping, the light yield and energy resolution of 10 × 10 × 10 mm 3 Lu YAG : Pr samples are improved from 156000 to 248000 photons/MeV, and 5.3 to 4.3% at 662 keV,respectively.The optical absorption spectra indicate that Li codoping does not induce conversion of stable Pr 3 + to Pr 4 + in Lu YAG : Pr single crystals. Based on the formation energies of substitutional and interstitial Li sites using density-functional-theory (DFT) calculations and the 7 Li nuclear magnetic resonance results, it is shown that the Li ions prefer to dominantly occupy the fourfold coordinated interstitial sites and fourfold coordinated Al sites. The systematic analysis of thermoluminescence glow curves, positron annihilation lifetime spectroscopies, and defect formation energies derived from DFT calculations reveals that the concentration of isolated Lu and Al vacancies as dominant acceptor defects is reduced by Li codoping, whilst the shallow Li i interstitial defects and the deep V O oxygen vacancies are introduced simultaneously. We propose that the lowering of hole trapping at defects resulting from Li codoping contributes to the scintillation-yield enhancement.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Geometrical defect detection for additive manufacturing with machine learning models

This study proposed a scheme based on Machine Learning (ML) models to detect geometric defects of additively manufactured objects. The ML models are trained with synthetic 3D point clouds with defects and then applied to detect defects in actual production. Using synthetic 3D point clouds rather than experimental data could save a huge amount of training time and costs associated with many prints for each design. Besides distance differences of individual points between source and target point clouds, this scheme uses a new concept called “patch” to capture macro-level information about nearby points for ML training and implementation. Numerical comparisons of prediction results on experimental data with different shapes showed that the proposed scheme outperformed the existing Z-difference method in the literature. Five ML methods (Bagging of Trees, Gradient Boosting, Random Forest, K-nearest Neighbors and Linear Supported Vector Machine) were compared under various conditions, such as different point cloud densities and defect sizes. Bagging and Random Forest were found the two best models regarding predictability; and the right patch size was found to be at 20. The proposed ML-based scheme is applicable to in-situ defect detection during additive manufacturing with the aid of a proper 3D data acquisition system.

Additive manufacturing↗
DOE OSTI2020

Enhancing Photocarrier Bulk Lifetime with Defect Engineering of Polycrystalline Passivated-Contact n-Cz Photovoltaic Devices

We study the photocarrier lifetime evolution of n-Cz Si material throughout the processing sequence for polycrystalline passivated contact devices. We show that a high temperature annealing pretreatment (known as Tabula Rasa) has a clear effect on enhancing bulk lifetimes of n-Cz Si. We further this development by integrating such defect engineering into the lower-temperature annealing of passivated contact. By applying oxidizing ambient gases during these anneals we report a photocarrier lifetime enhancement over an N 2 environment. This enhancement is exhibited in a 1-sun iV OC of 735 mV when annealed in O 2 rather than 708 mV in N 2.

14 SOLAR ENERGY↗
DOE OSTI2020

Radiative capture rates at deep defects from electronic structure calculations

We present a methodology to calculate radiative carrier capture coefficients at deep defects in semiconductors and insulators from first principles. Electronic structure and lattice relaxations are accurately described with hybrid density functional theory. Calculations of capture coefficients provide an additional validation of the accuracy of these functionals in dealing with localized defect states. We also discuss the validity of the Condon approximation, showing that even in the event of large lattice relaxations the approximation is accurate. We test the method on GaAs:VGa-TeAs and GaN:CN, for which reliable experiments are available, and demonstrate very good agreement with measured capture coefficients.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Exchange-correlation functional challenges in modeling quaternary chalcogenides

The development of next-generation quaternary chalcogenides, such as Cu2ZnSnS4 (CZTS) and Cu2ZnGeS4 (CZGS), for solar energy and thermoelectric applications hinges upon both careful experimentation and accurate quantum mechanical modeling. To address the latter, many have turned to density functional theory (DFT), which offers several choices for the approximate treatment of electron exchange and correlation (XC). Popular XC functionals include the Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) and the recently developed strongly constrained and appropriately normed (SCAN) meta-GGA. Extensions of DFT functionals, such as adding a Hubbard U correction and introducing a fraction of the Fock exchange (hybrid functionals), have been used widely to model systems containing 3d metal ions. However, no studies yet have compared comprehensively PBE(+U) and SCAN(+U) in the quality of their predictions of the bulk and defect thermodynamics of quaternary chalcogenides, which play a critical role in device fabrication and performance. Hence, here we calculate the (i) 0 K formation energies of bulk Ge compounds and (ii) neutral defect formation energies including charge-balanced (e.g., CuZn + ZnCu) and charge-imbalanced (e.g., CuSn) combinations of antisites and vacancies in CZTS and CZGS using the PBE, PBE +U, SCAN, SCAN +U, and the hybrid Heyd-Scuseria-Ernzerhof XC frameworks. We find that the formation energies of charge-imbalanced defects are more sensitive to the choice of the XC functional than those of charge-balanced defects, which can be explained by the differences in the extent of penalization of defect-generated delocalized electrons/holes by PBE, PBE +U, SCAN, and SCAN +U. Additionally, our results show that SCAN systematically underbinds Ge-containing compounds, thus highlighting the need for even further improvement of XC functionals. Based on our findings, we recommend the use of SCAN for modeling quaternary chalcogenides because its errors are systematic, and it has the firmest theoretical underpinning. Our work provides guidance for future modeling of quaternary chalcogenides.

14 SOLAR ENERGY↗
DOE OSTI2020

Split Ga vacancies and the unusually strong anisotropy of positron annihilation spectra in β–Ga 2 O 3

We report a systematic first-principles study on positron annihilation parameters in the β–Ga 2 O 3 lattice and Ga monovacancy defects complemented with orientation-dependent experiments of the Doppler broadening of the positron-electron annihilation. We find that both the β–Ga 2 O 3 lattice and the considered defects exhibit unusually strong anisotropy in their Doppler broadening signals. This anisotropy is associated with low symmetry of the β–Ga 2 O 3 crystal structure that leads to unusual kind of one-dimensional confinement of positrons even in the delocalized state in the lattice. In particular, the split Ga vacancies recently observed by scanning transmission electron microscopy produce unusually anisotropic positron annihilation signals. Furthermore, we show that in experiments, the positron annihilation signals in β–Ga 2 O 3 samples seem to be often dominated by split Ga vacancies.

36 MATERIALS SCIENCE↗
DOE OSTI2025

High-throughput spin-bath characterization of spin defects in semiconductors

Detailed knowledge of the local environments of spin defects in semiconductors, such as nitrogenvacancy (NV) centers in diamond or divacancies in silicon carbide, is crucial for optimizing control and entanglement protocols in quantum sensing and information applications. However, at present a direct experimental characterization of individual defect environments is not scalable, as conventional spin-bath measurements are time consuming and difficult to automate. Achieving high-throughput characterization requires short experiments to probe the spin bath. However, with fewer and noisier measurements, the inverse problem of recovering spin-bath properties from measured data becomes ill posed, with multiple spin baths having a high likelihood of yielding the same data. In this work, we present a set of computational tools to resolve the ill-posed inverse problem of recovering the atomic positions and hyperfine couplings of random nuclei surrounding spin defects from sparse, noisy experimental coherence data, which can be obtained in hours. Here, we use a trans-dimensional Bayesian approach that incorporates ab initio data to yield full posterior distributions over nuclear spin environments, enabling robust recovery from limited data. We also provide practical tools and guidelines to determine the limits of detectability for hyperfine couplings under specific dynamical decoupling sequences and sampling conditions. In addition, we demonstrate how the tools developed here, in combination with ab initio simulations of spin baths, can guide the design of efficient experimental protocols for application-specific high-throughput screening. To showcase the utility of our approach, we apply it to design fast dynamical decoupling experiments to characterize the spin baths often individual NV centers in diamond. While the primary focus is on accelerating spin-bath characterization of spin defects, this Bayesian approach also lays the foundation for digital-twin studies of spin defects, where a virtual model of the spin-defect system evolves in real time with ongoing experimental measurements. Together, the set of tools we designed and applied paves the way for scalable deployment of spin defects in semiconductors for quantum sensing and information applications.

Bayesian methods↗
DOE OSTI2021

Site Mixing for Engineering Magnetic Topological Insulators

The van der Waals compound, MnBi 2 Te 4 , is the first intrinsic magnetic topological insulator, providing a materials platform for exploring exotic quantum phenomena such as the axion insulator state and the quantum anomalous Hall effect. However, intrinsic structural imperfections lead to bulk conductivity, and the roles of magnetic defects are still unknown. With higher concentrations of the same types of magnetic defects, the isostructural compound MnSb 2 Te 4 is a better model system for a systematic investigation of the connections among magnetism, topology, and lattice defects. In this work, the impact of antisite defects on the magnetism and electronic structure is studied in MnSb 2 Te 4 . Mn-Sb site mixing leads to complex magnetic structures and tunes the interlayer magnetic coupling between antiferromagnetic and ferromagnetic. The detailed nonstoichiometry and site mixing of MnSb2Te4 crystals depend on the growth parameters, which can lead to ≈40% of Mn sites occupied by Sb and ≈15% of Sb sites by Mn in as-grown crystals. Single-crystal neutron diffraction and electron microscopy studies show nearly random distribution of the antisite defects. Band structure calculations suggest that the Mn-Sb site mixing favors a ferromagnetic interlayer coupling, consistent with experimental observation, but is detrimental to the band inversion required for a nontrivial topology. Overall, our results suggest a long-range magnetic order of Mn ions sitting on Bi sites in MnBi 2 Te 4 . The effects of site mixing should be considered in all layered heterostructures that consist of alternating magnetic and topological layers, including the entire family of MnTe(Bi 2 Te 3 )n, its Sb analogs, and their solid solution.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Nanoscale electronic inhomogeneities in 1⁢ T ⁢-TaS 2

Here, we report a set of scanning tunneling microscopy (STM) and spectroscopy (STS) experiments studying native defects in CVT grown 1⁢T–TaS 2 . Six different sample surfaces from four bulk crystals were investigated. Wide area imaging reveals a prevalence of nanometer-scale electronic inhomogeneities due to native defects, with pristine regions interspersed. These inhomogeneities appear in typical as-grown crystals and coexist with a well-formed commensurate charge density wave of 1T–TaS 2 at low temperatures. Electronic inhomogeneities show up both as variations in the apparent height in STM and in the local density of states in STS; the bands can shift by 60 meV and the gap varies by more than 100 meV across inhomogeneities. These inhomogeneities are present in similar concentration across large-scale areas of all samples studied, but do not influence the charge density wave formation on local or global scales. The commensurate charge density wave exhibits long-range order and remains locally intact in the presence of these inhomogeneities.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Intentional carbon doping reveals CH as an abundant charged impurity in nominally undoped synthetic WS 2 and WSe 2

Understanding the physical properties and controlling the generation of intrinsic and extrinsic defects is central to the technological adoption of 2D materials in devices. Here we identify a charged carbon-hydrogen complex at a chalcogen site (CH X ) as a common, charged impurity in synthetically grown transition metal dichalcogenides (TMDs). This conclusion is drawn by comparing high resolution scanning probe microscopy measurements of nominally undoped and intentionally carbon doped TMD samples. While CH impurity densities in undoped CVD-grown WS 2 and MOCVD-grown WSe 2 can range anywhere from parts per million to parts per thousand, CH densities in the percentage levels were selectively generated by a post-synthetic methane plasma treatment. Our study indicates that methane plasma treatment is a selective and clean method for the controlled introduction of a charged carbon-hydrogen complex at a surface chalcogen site, adefect that is commonly present in synthetic TMDs.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Accelerating Discovery of Atomistic Defects via Machine Learning

The quantification of defects such as vacancies in crystalline structures is a cornerstone of materials science research. 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 a crystalline lattice, aiming to expedite detection while improving accuracy. Additionally, we explore the transferability of these ML techniques, identifying characteristics of atomistic imaging data that complicate this task. We show how the integration of ML can drive innovation, providing a powerful tool that will play an increasingly crucial role in the future of materials science.

2D materials↗
DOE OSTI2026

Oxygen Vacancy Evolution at Li x V 2 O 5 /LiPON Solid State Electrochemical Interfaces Using Depth Resolved Cathodoluminescence Spectroscopy

The formation of oxygen vacancies at buried LiPON/ Li x V 2 O 5 interfaces has been observed on a near-nanometer scale and nondestructively using depth-resolved cathodoluminescence spectroscopy (DRCLS) and interfacial markers. Before electrochemical cycling, as-deposited LiPON/Li x V 2 O 5 exhibits a 1.6 eV defect optical emission, which density functional theory calculations identify as originating from oxygen vacancies. This defect appears first within a few nanometers of the buried LiPON/Li x V 2 O 5 interface without cycling, indicating that spontaneous O diffusion from the Li x V 2 O 5 lattice into LiPON may have caused these interface-localized oxygen vacancy defects. DRCLS measured the intensity and spatial distribution of this oxygen vacancy signal as a function of electrochemical cycling in a LiPON/Li x V 2 O 5 half-cell, showing oxygen vacancy signal increasing and moving deeper into the electrode with increased cycle number. Significant electrochemical irreversibility was also observed, with poor Coulombic efficiency and a 15% drop in capacity over 50 cycles. Theoretical simulations predict that the presence of oxygen vacancies increases the energy barrier for lithium diffusion significantly, indicating that this aggregation of oxygen vacancies could be another battery degradation mechanism accompanying lithiation induced phase changes.

36 MATERIALS SCIENCE↗
DOE OSTI2025

First-Principles Framework for the Prediction of Intersystem Crossing Rates in Spin Defects: The Role of Electron Correlation

Optically active spin defects in solids are promising platforms for quantum technologies. In this work, we present a first-principles framework to investigate intersystem crossing processes, which represent crucial steps in the optical spin-polarization cycle used to address spin defects. Considering the nitrogen-vacancy center in diamond as a case study, we demonstrate that our framework effectively captures electron correlation effects in the calculation of many-body electronic states and their spin-orbit coupling and electron-phonon interactions, while systematically addressing finite-size effects. We validate our predictions by carrying out measurements of fluorescence lifetimes, finding excellent agreement between theory and experiments. The framework presented here provides a versatile and robust tool for exploring the optical cycle of varied spin defects entirely from first principles.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2026

Unraveling Defect-Dependent Conductivity-Type Switching in CuFe 2 O 4 for Enhanced Photoelectrocatalytic Reduction of Benzaldehyde

Photoelectrocatalytic (PEC) reduction provides a sustainable route for upgrading biomass-derived feedstocks with reduced energy requirements, yet remains largely unexplored beyond hydrogen evolution and CO 2 reduction due to the scarcity of stable photocathodes. Here, we report a defect-engineered CuFe 2 O 4 photocathode that enables directly quantified PEC reduction of benzaldehyde to benzyl alcohol using a singlecomponent, earth-abundant oxide. By controlling annealing temperature and oxygen partial pressure, CuFe 2 O 4 is systematically tuned from n-type to p-type conductivity. Electrochemical measurements, X-ray and ultraviolet photoelectron spectroscopy, and firstprinciples defect calculations collectively show that oxygen-rich annealing conditions suppress deep donor-type oxygen vacancies while stabilizing shallow acceptor-type copper vacancies, resulting in enhanced hole concentration and improved charge transport. In a mixed acetonitrile/water electrolyte employing 1,4-benzoquinone as a redox mediator, the optimized CuFe 2 O 4 photocathode achieves stable photoelectrochemical operation over 18 h under continuous illumination with a benzyl alcohol production rate of 2.57 μmol/h at −0.50 V vs Ag/AgNO 3 , corresponding to a Faradaic efficiency of 51.3%. This PEC approach lowers the required applied potential by ∼1 V compared to traditional electrocatalytic methods, offering a more energy-efficient route for carbonyl reduction. These findings establish CuFe 2 O 4 as a viable photocathode platform for sustainable photoelectrocatalytic organic transformations under mild reaction conditions.

benzaldehyde reduction↗
DOE OSTI2023

Migration of Ga vacancies and interstitials in β – Ga 2 O 3

Pathways and energy barriers for the migration of Ga vacancies (V Ga ) and Ga interstitials (Ga i ) in β–Ga 2 O 3 are explored using hybrid functional calculations and the nudged elastic band method. Considering β–Ga 2 O 3 as primarily being an n-type semiconductor, we focus on defect charge states relevant under such conditions: $V^{3 –}_{Ga}$, Ga$^{3+}_{i}$, and Ga$^{+}_{i}$. Notably, we describe a mechanism by which V Ga can transform between its different split configurations. In all cases, the intermediate state consists of a vacancy split between three Ga sites—a three-split vacancy—which enables passage over a significantly lower energy barrier. This is because it avoids the unfavorable simple vacancy at the tetrahedral Ga site. The proposed mechanism lowers the overall barrier for $V^{3–}_{Ga}$ diffusion along the [001] crystal direction from 1.73 to 0.97 eV, whereas the 2.08 eV barrier for the [100] and [010] directions is unaffected. For Ga$^{3+}_{i}$, we obtain similar overall migration barriers of 0.72, 0.80, and 1.02 eV for the [010], [001], and [100] directions, respectively. Ga$^{+}_{i}$ exhibits a strong preference for diffusion within the large eight-sided channel; the overall migration barrier is 0.92 eV for the [010] direction, and 2.16 eV for the [001] and [100] directions. The limiting step for the two latter directions involves ionization of Ga$^{+}_{i}$ followed by a jump to an adjacent large eight-sided channel as Ga$^{3+}_{i}$. Finally, our results are discussed in light of experimental observations of thermally activated recovery processes in irradiated material.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Effects of transition metals on the evolution of polymer-derived SiOC ceramics

This work focuses on catalytic graphitization from different metals, iron (Fe), cobalt (Co), and nickel (Ni), for inducing carbon growth in silicon oxycarbides (SiOCs) during the pyrolysis. Fe, Co, and Ni-modified SiOCs were synthesized by pyrolysis to 900, 1100, and 1300 °C respectively in Ar. The transition metals induced the formation of the corresponding metal silicides, β-SiC, and graphitic carbon with the catalytic activity in the order of Fe > Co > Ni, in agreement with the activation energy calculation based on the carbon types and amounts. Lateral growth of turbostratic carbon followed a 2D grain growth process and its point-like defect density decreased based on the catalytic order of the transition metals, with SiOC/Fe having the lowest defect density. Quantitative analysis of the XPS results with Rietveld refinement illustrated that the phase separation of SiOC is more dominant than local carbothermal reduction between SiO 2 and C in the SiOC/M (M = Ni, Co, or Fe) systems. Understanding the catalytic graphitization effect of Ni, Co, and Fe on polymer derived ceramics offers new strategies in increasing high-temperature phase amounts and thus creating novel materials for heterogeneous catalysis, magnetic, and other applications.

36 MATERIALS SCIENCE↗