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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 1,261 records · Page 70
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Surface Defect Chemistry and Electronic Structure of Pr 0.1 Ce 0.9 O 2-δ Revealed in Operando

Understanding the surface defect chemistry of oxides under functional operating conditions is important for providing guidelines for improving the kinetics of electrochemical reactions. Ceria-based oxides have applications in solid oxide fuel/electrolysis cells, thermo-chemical water splitting, catalytic convertors, and red-ox active memristive devices. The surface defect chemistry of doped ceria in the regime of high oxygen pressure, pO 2 , approximating the operating conditions of fuel cell cathodes at elevated temperatures, has not yet been revealed. In this paper, we investigated the Pr 0.1 Ce 0.9 O 2-δ (PCO) surface by in operando X-ray photoelectron and absorption spectroscopic methods. We quantified the concentration of reduced Pr 3+ , at the near-surface region of PCO as a function of electrochemical potential, corresponding to a wide range of effective pO 2 . We found that the Pr 3+ concentration at the surface was significantly higher than the values predicted from bulk defect chemistry. This finding indicates a lower effective defect formation energy at the surface region compared with that in the bulk. Finally, in addition, the Pr 3+ concentration has a weaker dependence on pO 2 compared to that in the bulk.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Surface Reorganization Leads to Enhanced Photocatalytic Activity in Defective BiOCl

Introducing defects into semiconductor photocatalysts has been identified as an effective approach to extend the visible-light absorption and achieve high-efficiency solar energy conversion. However, the band gap model system of defect states may not truly describe the evolutions in real materials as the narrower band gap would limit the photocatalytic activity via suppressing the charge separation. We report that reorganizing the surface termination in a defective semiconductor plays a key role in determining the photocatalytic performance. We directly observed that the surface reorganizations are accompanied by the formation of defects in layered structured bismuth oxychloride (BiOCl). Both experimental and theoretical results demonstrate that varying terminations have strong effects on the electronic structure and electron–hole pair recombination, which is shown to be the driving force of the promotion of visible-light photocatalytic activity in BiOCl. We also reveal that the surface reorganization induces a novel transfer path and high-dielectric surface to prevent the trapping of charge carriers, highlighting an efficient way of improving the photocatalytic activity by surface reorganization.

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 OSTI2022

Point Defects Control Guest Molecule Diffusion in the 1D Pores of Zn(tbip)

Molecular diffusion plays a critical role in metal-organic frameworks (MOFs) within the application of kinetic chemical separations. We carefully study in this work the unexpected role of point defects for short-chain alkanes diffusing in Zn(tbip), an MOF with rigid one-dimensional (1D) channels. Inside a defect-free Zn(tbip), guest molecules are expected to follow single-file diffusion along 1D channels. It has been found previously by Heinke et al. that these parallel 1D channels are connected for molecular diffusion. Our density functional theory (DFT) calculations suggest that linker vacancy defects could arise under experimentally relevant conditions by removing a pair of linkers. Further climbing-image nudged elastic band (cNEB) DFT calculations indicate that hopping of short-chain alkanes between adjacent 1D channels over defect windows can occur at moderate temperatures. In addition to providing insights into connected adjacent 1D channels in Zn(tbip), Heinke et al. also inferred that most 1D pores are blocked from a microkinetic model. Additionally, we explored the influence of hydrolyzed linker created by the formation of linker removal inside 1D pores. Our DFT calculations show that the linkers can effectively block the pores and the linker diffusion in 1D channels is slow. Our results, for the first time, offer a mechanistic explanation of the unexpected molecular diffusion behavior in this MOF with 1D channels.

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

Insights into the Interaction of Redox Active Organic Molecules and Solvents with the Pristine and Defective Graphene Surfaces from Density Functional Theory

A systematic study is reported on the interaction of two representative redox active organic molecules and two solvent molecules with pristine and defective graphene surfaces as a model of an electrode surface of a redox flow battery (RFB). The redox active molecules include a catholyte, 2,5-di-tert-butyl-1,4-dimethoxybenzene (DDB), and an anolyte molecule, benzothiadiazole (BTZ), and the solvent molecules include acetonitrile (MeCN) and ethylene carbonate (EC). The graphene defects investigated include a single vacancy, double vacancy, zigzag step edge, and armchair step edge. Computations suggest that the interactions of all molecules with a pristine graphene surface are relatively weak (0.2 to 0.8 eV) and dominated by van der Waals effects; therefore, these molecules are chemically stable upon interacting with pristine nondefective graphene. The BTZ, DDB, MeCN, and EC molecules interact strongly (1.5 to 5.5 eV) with the single vacancy and zigzag step edges of graphene that leads to the possible decomposition of the molecules with strength of interaction in the order of MeCN > BTZ > DDB > EC and MeCN > EC > BTZ > DDB, respectively. Calculations show that the BTZ, DDB, MeCN, and EC molecules interact less strongly (0.2 to 1.4 eV) to the double vacancy and armchair step edge than to the single vacancy and zigzag step edge. The binding energies of the molecules were significantly reduced when interacting with the passivated defects, suggesting that the passivation of the defects could help prevent unwanted chemical interactions between the neutral molecules and the electrode surface. In all organic RFBs, one of the crucial bottlenecks is the stability of the constituent molecules with the electrode material, and this study provides insights into the chemical interaction of selected candidate species with a model carbon electrode.

Howard, Jason D.↗
DOE OSTI2016

Oxygen Point Defect Chemistry in Ruddlesden–Popper Oxides (La 1- x Sr x ) 2 MO 4±δ (M = Co, Ni, Cu)

Here, stability of oxygen point defects in Ruddlesden–Popper oxides (La 1- x Sr x ) 2 MO 4±δ (M = Co, Ni, Cu) is studied with density functional theory calculations to determine their stable sites, charge states, and energetics as functions of Sr content ( x ), transition metal (M), and defect concentration (δ). We demonstrate that the dominant O point defects can change between oxide interstitials, peroxide interstitials, and vacancies. In general, increasing x and atomic number of M stabilizes peroxide over oxide interstitials as well as vacancies over both peroxide and oxide interstitials; increasing δ destabilizes both oxide interstitials and vacancies but barely affects peroxide interstitials. We also demonstrate that the O 2p-band center is a powerful descriptor for these materials and correlates linearly with the formation energy of all defects. The trends of formation energy versus x , M, and δ and the correlation with O 2p-band center are explained in terms of oxidation chemistry and electronic structure.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Improving the Carrier Lifetime of Tin Sulfide via Prediction and Mitigation of Harmful Point Defects

Tin monosulfide (SnS) is an emerging thin-film absorber material for photovoltaics. An outstanding challenge is to improve carrier lifetimes to >1 ns, which should enable >10% device efficiencies. However, reported results to date have only demonstrated lifetimes at or below 100 ps. In this study, we employ defect modeling to identify the sulfur vacancy and defects from Fe, Co, and Mo as most recombination-active. We attempt to minimize these defects in crystalline samples through high-purity, sulfur-rich growth and experimentally improve lifetimes to >3 ns, thus achieving our 1 ns goal. This framework might prove effective for unlocking the lifetime potential in other emerging thin-film materials by rapidly identifying and mitigating lifetime-limiting point defects.

14 SOLAR ENERGY↗
DOE OSTI2018

Direct Evidence of Topological Defects in Electron Waves through Nanoscale Localized Magnetic Charge

Topological concepts play an important role in, and provide unique insights into, many physical phenomena. In particular topological defects have become an active area of research due to their relevance to diverse systems including condensed matter and the early universe. These defects arise in systems during phase transitions or symmetry-breaking operations that lead to a specific configuration of the order parameter that is stable against external perturbations. In this work, we experimentally show that excitations or defects carrying magnetic charge in artificial spin ices introduce a topological defect in incident coherent electron waves. This results in the formation of a localized electron vortex beam carrying orbital angular momentum that is directly correlated with the magnetic charge. Furthermore, this work provides unique insight into the interaction of electrons with magnetically charged excitations and the effect on their topology thereby opening new possibilities to explore exotic scattering and quantum effects in nanoscale condensed-matter systems.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Energetics of 2D and 3D Gold Nanoparticles on MgO(100): Influence of Particle Size and Defects on Gold Adsorption and Adhesion Energies

In this work, the adsorption of gold vapor onto MgO(100) films grown on Mo(100) was studied at 300 and 100 K using single crystal adsorption calorimetry (SCAC). The Au particle morphology was investigated using He + low-energy ion scattering spectroscopy (LEIS) and X-ray photoelectron spectroscopy (XPS). The LEIS data combined with particle shape measurements from the literature reveal that, at both 300 K and 100 K, Au grows as 2D islands with bilayer thickness (~0.41 nm) up to a diameter of ~7 nm. At higher coverage, the islands thicken with little increase in diameter. The island densities are 3.0 x10 11 and 5.4 x10 11 per cm 2 at 300 K and 100 K, respectively. The initial sticking probability of Au is 0.90 at 300 K and 0.95 at 100 K. The surface residence time of the Au atoms that do not stick is <10 ms, implying that gold monomers bind to MgO(100) weakly (<69 kJ/mol). The adsorption energies indicate that Au particles of the same size bind more strongly to MgO(100) when grown at 300 K than at 100 K, which we attribute to Au binding to step edges or defects at 300 K, but at perfect MgO(100) terraces at 100 K (because Au diffusion is too slow to find defects). The adsorption energy of Au onto ~30-atom Au clusters is 285 kJ/mol at 300 K, ~68 kJ/mol higher than at 100 K, attributed to the difference between particles on defects versus terraces. Similarly, the adhesion energy of Au nanoparticles to MgO(100) extracted from the adsorption energies at 300 K is much higher (1.8 J/m 2 for ~7 nm particles at defects) than at 100 K (0.3 J/m 2 for ~7 nm particles at terraces). This 100 K adhesion energy is close to that estimated from electron-microscopy shape measurements of Au particles at terraces on MgO(100) (0.45-0.67 J/m 2 ). The heat of Au adsorption and Au 2 chemical potential change by >100 kJ/mol as gold’s 2D island size increases from 0.7 to 7 nm diameter, implying dramatic changes in catalytic activity and sintering rates with 2D diameter. This is the first experimental measurement of any metal adsorption energy on any oxide as a function of island diameter when making 2D islands, as well as the first direct comparison of any adhesion energy found from calorimetric adsorption energies to that from particle shape analysis.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Defect Creation by Linker Fragmentation in Metal-Organic Frameworks and Its Effects on Gas Uptake Properties

We successfully demonstrate an approach based on linker fragmentation to create defects and tune the pore volumes and surface areas of two metal-organic frameworks, NU-125 and HKUST-1, both of which feature copper paddlewheel nodes. Depending on the linker fragment composition, the defect can be either a vacant site or a functional group that the original linker does not have. In the first case, we show that both surface area and pore volume increase, while in the second case they decrease. The effect of defects on the high-pressure gas uptake is also studied over a large temperature and pressure range for different gases. We found that despite an increase in pore volume and surface area in structures with vacant sites, the absolute adsorption for methane decreases for HKUST-1 and slightly increases for NU-125. However, the working capacity (deliverable amount between 65 and 5 bar) in both cases remains similar to parent frameworks due to lower uptakes at low pressures. In the case of NU-125, the effect of defects became more pronounced at lower temperatures, reflecting the greater surface areas and pore volumes of the altered forms.

Barin, G↗
DOE OSTI2025

Atomic-Scale Dynamics of Five-Fold Twin Mediated Coalescence: Pathway-Dependent and Defect-Governed Nonclassical Growth Mechanisms

Defective crystals with distinct properties have been discovered in many systems. However, the growth mechanism of defective crystals is still poorly understood. Here, in this work, using a 5-fold twinned gold (Au) nanocrystal (NC) as a model system, three new coalescence pathways involving detwinning or twinning have been uncovered through atomic-scale dynamic observations in an aberration-corrected transmission electron microscope coupled with atomistic simulations. This demonstrates that beyond crystal size, coalescence growth dynamics involving 5-fold twins (5-FTs) are highly dependent on crystal defect density and the approach pathways of the crystals. When a 5-FT encounters a smaller 5-FT or a smaller NC in a face-to-face way, a new, larger 5-FT is produced at a relatively fast coalescence growth rate; while in a corner-to-corner way, the coalescence dynamics are more retarded and sluggish, which is conducive to the formation of complex multitwined structures rather than 5-FTs. This highlights that the planar defect density and crystal approach pathway influence the coalescence dynamics of crystals containing 5-FT. Moreover, a column-by-column grain boundary (GB) migration mechanism, which results in bent GBs, was also unveiled during the crystal coalescence process. These results enrich the general understanding of the crystal growth theory and provide new insights into the controllable fabrication of 5-FTs by crystal coalescence mechanisms.

crystallization↗
DOE OSTI2021

Comparison of Biotite Elastic Properties Recovered by Spherical Nanoindentations and Atomistic Simulations — Influence of Nano-Scale Defects in Phyllosilicates

Phyllosilicate minerals, due to their sheets structure and morphology, are known to cause anisotropy in bulk rock properties and make the bulk rock more compliant. Accurately characterizing the micromechanical behavior of phyllosilicate minerals from laboratory observations, which eventually translates to the bulk rock behavior, is still challenging due to their fine-grained nature. Recent advances in atomistic simulations open the possibility of theoretically investigating such mineral mechanical behavior. We compare the elastic properties of biotites recovered by spherical nanoindentation with those predicted from density functional theory (DFT) simulations to investigate to what extent theoretical predictions reproduce actual phyllosilicate properties. Spherical nanoindentation was conducted using schist rocks from Poorman Formation, South Dakota, USA, to recover continuous indentation stress-strain curves. Loading in the layer-normal orientation shows an average indentation modulus (M) of about 35 GPa, while loading in the layer-parallel orientation gives a higher average of about 95 GPa. To facilitate comparison, the elastic stiffness constants (cij) determined from DFT were converted to indentation modulus (M) using solutions proposed in this study. The majority of the nanoindentation modulus results are below the values inferred from the simulation results representing ideal defect-free minerals. We suggest that crystal defects present at the nano-scale, potentially ripplocations, are the dominant cause of the lower indentation modulus recovered from nanoindentation compared to those inferred from DFT simulations. Results highlight the importance of acknowledging the defects that exist down to the nano-scale as it modifies the mechanical properties of phyllosilicates compared to its pure defect-free form.

58 GEOSCIENCES↗
DOE OSTI2016

Enhancing radiation tolerance by controlling defect mobility and migration pathways in multicomponent single-phase alloys

A grand challenge in material science is to understand the correlation between intrinsic properties and defect dynamics. Radiation tolerant materials are in great demand for safe operation and advancement of nuclear and aerospace systems. Unlike traditional approaches that rely on microstructural and nanoscale features to mitigate radiation damage, this study demonstrates enhancement of radiation tolerance with the suppression of void formation by two orders magnitude at elevated temperatures in equiatomic single-phase concentrated solid solution alloys, and more importantly, reveals its controlling mechanism through a detailed analysis of the depth distribution of defect clusters and an atomistic computer simulation. The enhanced swelling resistance is attributed to the tailored interstitial defect cluster motion in the alloys from a long-range one-dimensional mode to a short-range three-dimensional mode, which leads to enhanced point defect recombination. Finally, the results suggest design criteria for next generation radiation tolerant structural alloys.

36 MATERIALS SCIENCE↗
DOE OSTI2017

The self-healing of defects induced by the hydriding phase transformation in palladium nanoparticles

Nanosizing can dramatically alter material properties by enhancing surface thermodynamic contributions, shortening diffusion lengths, and increasing the number of catalytically active sites per unit volume. These mechanisms have been used to explain the improved properties of catalysts, battery materials, plasmonic materials, etc. Here we show that Pd nanoparticles also have the ability to self-heal defects in their crystal structures. Using Bragg coherent diffractive imaging, we image dislocations nucleated deep in a Pd nanoparticle during the forward hydriding phase transformation that heal during the reverse transformation, despite the region surrounding the dislocations remaining in the hydrogen-poor phase. We show that defective Pd nanoparticles exhibit sloped isotherms, indicating that defects act as additional barriers to the phase transformation. Our results resolve the formation and healing of structural defects during phase transformations at the single nanoparticle level and offer an additional perspective as to how and why nanoparticles differ from their bulk counterparts.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Optical charge state control of spin defects in 4H-SiC

Defects in silicon carbide (SiC) have emerged as a favorable platform for optically active spin-based quantum technologies. Spin qubits exist in specific charge states of these defects, where the ability to control these states can provide enhanced spin-dependent readout and long-term charge stability. We investigate this charge state control for two major spin qubits in 4H-SiC, the divacancy and silicon vacancy, obtaining bidirectional optical charge conversion between the bright and dark states of these defects. We measure increased photoluminescence from divacancy ensembles by up to three orders of magnitude using near-ultraviolet excitation, depending on the substrate, and without degrading the electron spin coherence time. This charge conversion remains stable for hours at cryogenic temperatures, allowing spatial and persistent patterning of the charge state populations. As a result, we develop a comprehensive model of the defects and optical processes involved, offering a strong basis to improve material design and to develop quantum applications in SiC.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Identifying substitutional oxygen as a prolific point defect in monolayer transition metal dichalcogenides

Chalcogen vacancies are generally considered to be the most common point defects in transition metal dichalcogenide (TMD) semiconductors because of their low formation energy in vacuum and their frequent observation in transmission electron microscopy studies. Consequently, unexpected optical, transport, and catalytic properties in 2D-TMDs have been attributed to in-gap states associated with chalcogen vacancies, even in the absence of direct experimental evidence. Here, we combine low-temperature non-contact atomic force microscopy, scanning tunneling microscopy and spectroscopy, and state-of-the-art ab initio density functional theory and GW calculations to determine both the atomic structure and electronic properties of an abundant chalcogen-site point defect common to MoSe 2 and WS 2 monolayers grown by molecular beam epitaxy and chemical vapor deposition, respectively. Surprisingly, we observe no in-gap states. Our results strongly suggest that the common chalcogen defects in the described 2D-TMD semiconductors, measured in vacuum environment after gentle annealing, are oxygen substitutional defects, rather than vacancies.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Spatial defects nanoengineering for bipolar conductivity in MoS 2

Understanding the atomistic origin of defects in two-dimensional transition metal dichalcogenides, their impact on the electronic properties, and how to control them is critical for future electronics and optoelectronics. Here, we demonstrate the integration of thermochemical scanning probe lithography (tc-SPL) with a flow-through reactive gas cell to achieve nanoscale control of defects in monolayer MoS 2 . The tc-SPL produced defects can present either p- or n-type doping on demand, depending on the used gasses, allowing the realization of field effect transistors, and p-n junctions with precise sub-μm spatial control, and a rectification ratio of over 10 4 . Doping and defects formation are elucidated by means of X-Ray photoelectron spectroscopy, scanning transmission electron microscopy, and density functional theory. We find that p-type doping in HCl/H 2 O atmosphere is related to the rearrangement of sulfur atoms, and the formation of protruding covalent S-S bonds on the surface. Alternatively, local heating MoS 2 in N 2 produces n-character.

36 MATERIALS SCIENCE↗
DOE OSTI2021

The role of chalcogen vacancies for atomic defect emission in MoS 2

For two-dimensional (2D) layered semiconductors, control over atomic defects and understanding of their electronic and optical functionality represent major challenges towards developing a mature semiconductor technology using such materials. Here, we correlate generation, optical spectroscopy, atomic resolution imaging, and ab initio theory of chalcogen vacancies in monolayer MoS 2 . Chalcogen vacancies are selectively generated by in-vacuo annealing, but also focused ion beam exposure. The defect generation rate, atomic imaging and the optical signatures support this claim. We discriminate the narrow linewidth photoluminescence signatures of vacancies, resulting predominantly from localized defect orbitals, from broad luminescence features in the same spectral range, resulting from adsorbates. Vacancies can be patterned with a precision below 10 nm by ion beams, show single photon emission, and open the possibility for advanced defect engineering of 2D semiconductors at the ultimate scale.

36 MATERIALS SCIENCE↗