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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 379 records · Page 21
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Multi-layered heterochromatin interaction as a switch for DIM2-mediated DNA methylation

Functional crosstalk between DNA methylation, histone H3 lysine-9 trimethylation (H3K9me3) and heterochromatin protein 1 (HP1) is essential for proper heterochromatin assembly and genome stability. However, how repressive chromatin cues guide DNA methyltransferases for region-specific DNA methylation remains largely unknown. Here, we report structure-function characterizations of DNA methyltransferase Defective-In-Methylation-2 (DIM2) in Neurospora . The DNA methylation activity of DIM2 requires the presence of both H3K9me3 and HP1. Our structural study reveals a bipartite DIM2-HP1 interaction, leading to a disorder-to-order transition of the DIM2 target-recognition domain that is essential for substrate binding. Furthermore, the structure of DIM2-HP1-H3K9me3-DNA complex reveals a substrate-binding mechanism distinct from that for its mammalian orthologue DNMT1. In addition, the dual recognition of H3K9me3 peptide by the DIM2 RFTS and BAH1 domains allosterically impacts the DIM2-substrate binding, thereby controlling DIM2-mediated DNA methylation. Together, this study uncovers how multiple heterochromatin factors coordinately orchestrate an activity-switching mechanism for region-specific DNA methylation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Zn acceptors in β-Ga 2 O 3 crystals

Electron paramagnetic resonance (EPR) is used to identify and characterize neutral zinc acceptors in Zn-doped β-Ga 2 O 3 crystals. Two EPR spectra are observed at low temperatures, one from Zn ions at tetrahedral Ga(1) sites (the $Zn$$^{0}_{Ga 1}$ acceptor) and one from Zn ions at octahedral Ga(2) sites (the $Zn$$^{0}_{Ga 2}$ acceptor). These Zn acceptors are small polarons, with the unpaired spin localized in each case on a threefold coordinated oxygen O(I) ion adjacent to the Zn ion. Resolved hyperfine interactions with neighboring 69 Ga and 71 Ga nuclei allow the EPR spectra from the two acceptors to be easily distinguished: $Zn$$^{0}_{Ga 1}$ acceptors interact equally with two Ga(2) ions and $Zn$$^{0}_{Ga 2}$ acceptors interact unequally with a Ga(1) ion and a Ga(2) ion. The as-grown crystals are compensated, with the Zn ions initially present as singly ionized acceptors ($Zn$$^{-}_{Ga 1}$ and $Zn$$^{-}_{Ga 2}$). Exposing a crystal to 325 nm laser light, while being held at 140 K, primarily produces neutral $Zn$$^{0}_{Ga 2}$ acceptors when photoinduced holes are trapped at $Zn$$^{-}_{Ga 2}$ acceptors. This suggests that there may be significantly more Zn ions at Ga(2) sites than at Ga(1) sites. Warming the crystal briefly to room temperature, after removing the light, destroys the EPR spectrum from the shallower $Zn$$^{0}_{Ga 2}$ acceptors and produces the EPR spectrum from the more stable $Zn$$^{0}_{Ga 1}$ acceptors. Furthermore, the $Zn$$^{0}_{Ga 2}$ acceptors decay in the 240–260 K region with a thermal activation energy near 0.65 eV, similar to $Mg$$^{0}_{Ga 2}$ acceptors, whereas the slightly deeper $Zn$$^{0}_{Ga 1}$ acceptors decay close to room temperature with an approximate thermal activation energy of 0.78 eV.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Role of deep levels and barrier height lowering in current-flow mechanism in 150 μ m thick epitaxial n-type 4H–SiC Schottky barrier radiation detectors

Schottky barrier detectors (SBDs) require larger surface areas than conventional electronics to increase the detection efficiency although such SBDs manifest large diode ideality factors due to inhomogeneous areal distribution of surface barrier height (SBH). Inhomogeneous SBH distributions lead to various current flow mechanisms in SBDs, which need to be identified to optimize detector performance. In this Letter, we identify the current flow mechanism in large area Schottky barrier diodes for radiation detection fabricated on 150 μm thick n-4H–SiC epitaxial layers. The analysis of temperature-dependent forward current–voltage (I–V–T) characteristics of SBDs revealed two linear regions in current–voltage curves up to 450 K, one corresponding to the current flow through a low barrier patch, while the other corresponds to that of average barrier distribution. Applying a SBH distribution model to the reverse I–V–T characteristics, an activation energy of 0.76 eV for the current flow over the Schottky barrier was calculated. Additionally, the activation energy did not directly correspond to any of the defect levels observed from the deep level transient spectroscopy (DLTS). Above 450 K, a Schottky type barrier lowering suggested a current flow through a low barrier patch of ≈ 0.8 eV. The absence of any SBH lowering below 450 K indicated that the current corresponded to a neutrally charged trap level at ≈ 0.6 eV below the conduction band edge, which was consistent with DLTS measurements revealing the presence of an electron trap level Z1/2 at 0.59 eV below the conduction band edge.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2024

Interfacial defect reduction enhances universal power law response in Mo–SiNx granular metals

Granular metals (GMs), consisting of metal nanoparticles separated by an insulating matrix, frequently serve as a platform for fundamental electron transport studies. However, few technologically mature devices incorporating GMs have been realized, in large part because intrinsic defects (e.g., electron trapping sites and metal/insulator interfacial defects) frequently impede electron transport, particularly in GMs that do not contain noble metals. Here, we demonstrate that such defects can be minimized in molybdenum–silicon nitride (Mo–SiN x ) GMs via optimization of the sputter deposition atmosphere. For Mo–SiN x GMs deposited in a mixed Ar/N 2 environment, x-ray photoemission spectroscopy shows a 40%–60% reduction of interfacial Mo-silicide defects compared to Mo–SiN x GMs sputtered in a pure Ar environment. Electron transport measurements confirm the reduced defect density; the dc conductivity improved (decreased) by 10 4 –10 5 and the activation energy for variable-range hopping increased 10×. Since GMs are disordered materials, the GM nanostructure should, theoretically, support a universal power law (UPL) response; in practice, that response is generally overwhelmed by resistive (defective) transport. Here, the defect-minimized Mo–SiN x GMs display a superlinear UPL response, which we quantify as the ratio of the conductivity at 1 MHz to that at dc, Δσ ω ⁠. Remarkably, these GMs display a Δσ ω up to 10 7 , a three-orders-of-magnitude improved response than previously reported for GMs. By enabling high-performance electric transport with a non-noble metal GM, this work represents an important step toward both new fundamental UPL research and scalable, mature GM device applications.

36 MATERIALS SCIENCE↗
DOE OSTI2025

On the scale of heterogeneity in composite electrodes of batteries

An electrode in cylindrical or pouch cell batteries contains millions of active particles embedded in a conductive network. Battery performance, such as voltage, capacity, and cyclic efficiency, is a collective response of the particle network. We use optical microscopy to measure the local, heterogeneous state of charge of individual particles upon charging and discharging. The optical reflectivity is proportional to Li composition in the ternary oxide LiNi x Mn y Co z O 2 (NMC) cathode. Through clustering analysis, we determine the scale of heterogeneity where a representative volume in the composite electrode contains 100 to 1,000 particles. The heterogeneous activity in the particle network can be described by Weibull defect population at the particle interface with the conductive matrix.

batteries↗
DOE OSTI2021

Oxygen Evolution on Iron Oxide Nanoparticles: The Impact of Crystallinity and Size on the Overpotential

The oxygen evolution reaction (OER) is a critical reaction in electrolysis and photoelectrolysis of water to generate and store clean energy. Therefore, the development of low-cost and efficient electrocatalysts for the OER is of great scientific and technological importance. Although promising iron oxide-based electrocatalysts have been recently developed for the OER, an in-depth experimental and theoretical analysis of the OER mechanism on iron oxide-based electrocatalysts is still needed to provide guidelines to optimize the performance of iron oxide-based electrocatalysts further. To address this need, we synthesized a series of monodisperse iron oxide nanoparticles to analyze their intrinsic OER activities. Using nanoparticles of the same size but different crystallinity, we show that amorphous iron oxide nanoparticles have better OER activity than crystalline ones. The size effect studies further revealed that the edge/defect sites are the active sites for the OER. Density functional theory calculations demonstrated that the edge/defect sites provide bridge sites to adsorb OER intermediates, resulting in low OER overpotential. These calculations confirm that the high OER activity of amorphous nanoparticles results from a high concentration of defect sites on their surface. These results provide novel strategies to increase the performance of iron oxide-based and likely other oxide-based OER electrocatalysts.

Electrochemistry↗
NASA NTRS1990

The utilization of an infrared imaging system as a cooling slot blockage detector in the inspection of a transpiration cooled nozzle

A comprehensive examination of the 8 foot temperature tunnel's transpiration cooled nozzle was completed using an infrared imaging radiometer to locate regions of cooling flow irregularities caused by obstruction of three or more adjacent cooling slots. Restrictions in the cooling flow were found and cataloged. Blockages found were due primarily to the presence of residual phosphoric acid being discharged from some of the cooling slots. This acid was used during construction of the nozzle components and was to have been purged prior to its delivery to the NASA Langley Research Center (LaRC). In addition, a radial displacement of one selection of discs located in the spool piece was inspected and cataloged for future reference. There did not seem to be a serious restriction of flow in this defect, but evidence from the infrared images indicated reduced slot activity within the gouge. The radiometer survey uncovered regions where closer inspection is recommended but did not cover the entire surface area of the three nozzle subsections due to equipment limitations. A list of areas with suspected problems is included in Appendix A.

Borg, Stephen E.↗
DOE OSTI2023

Selective Uptake of Ethane/Ethylene Mixtures by UTSA-280 is Driven by Reversibly Coordinated Water Defects

The metal–organic framework (MOF) UTSA-280 has been shown previously to separate ethane/ethylene mixtures. We show that this separation is kinetic; if full equilibrium is reached, the MOF is not selective. The time scales associated with this kinetic separation are sufficient to make the material potentially interesting for practical separations. The observation of kinetic separation is surprising because the MOF’s one-dimensional (1D) channels are small enough that molecular motion along the channels would be expected to take place by single-file diffusion, which is not observed. We show using a combination of experimental and modeling techniques that diffusion of molecules between adjacent 1D channels through local defects associated with coordinated water molecules allows net mixing of molecules and, moreover, that this process is potentially responsible for the selective nature of the observed kinetic separation. We show how activation and regeneration conditions play an essential role in controlling the density of these local defects.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Quantifying the Influence of Defects on Selectivity of Electrodes Encapsulated by Nanoscopic Silicon Oxide Overlayers

Encapsulation of electrocatalysts and photocatalysts with semipermeable nanoscopic oxide overlayers that exhibit selective transport properties is an attractive approach to achieve high redox selectivity. However, defects within the overlayers-such as pinholes, cracks, or particle inclusions-may facilitate local high rates of parasitic reactions by creating pathways for facile transport of undesired reactants to exposed active sites. Scanning electrochemical microscopy (SECM) is an attractive method to determine the influence of defects on macroscopic performance metrics thanks to its ability to measure the relative rates of competing electrochemical reactions with high spatial resolution over the electrode. Here, we report the use of SECM to determine the influence of overlayer defects on the selectivity of silicon oxide (SiOx) encapsulated platinum thin-film electrocatalysts operated under conditions where two competing reactions-the hydrogen evolution and Fe(III) reduction reactions-can occur. After an SECM methodology is described to determine spatially resolved selectivity, representative selectivity maps are correlated with the location of defects that are characterized by optical, electron, and atomic force microscopies. This analysis reveals that certain types of defects in the oxide overlayer are responsible for ∼60-90% of the partial current density toward the undesired Fe(III) reduction reaction. By correcting for defect contributions to Fe(III) reduction rates, true Fe(III) permeability values for the SiOxoverlayers were determined to be over an order of magnitude lower than permeabilities determined from analyses that ignore the presence of defects. Lastly, different types of defects were studied revealing that defect morphology can have varying influence on both redox selectivity and calculated permeability. This work highlights the need for spatially resolved measurements to evaluate the performance of oxide-encapsulated catalysts and understand their performance limits.

25 ENERGY STORAGE↗
DOE OSTI2022

Thermal Hydroquinone Oxidation on Co/N-doped Carbon Proceeds by a Band-Mediated Electrochemical Mechanism

Molecules catalyze aerobic oxidation reactions via redox cycling at the metal center to effect sequential activation of O 2 and the substrate. Metal surfaces can catalyze the same transformations by coupling independent half-reactions for oxygen reduction and substrate oxidation mediated via the exchange of band-electrons. Metal/nitrogen-doped carbons (MNCs) are promising catalysts for aerobic oxidation that consist of molecule-like active sites embedded in a conductive carbon host. Owing to their combining molecular and metallic features, it remains unclear whether they proceed via the sequential redox cycling pathways of molecules or the band-mediated pathways of metals. Herein, we simultaneously track the potential of the catalyst and the rate of the turnover of aerobic hydroquinone oxidation on a cobalt-based MNC catalyst contacted to a catalytically inactive carbon electrode. By comparing operando measurements of rate and potential with the current-voltage behavior of each constituent half-reaction under identical conditions, we show that these molecular materials can display the band-mediated reaction mechanisms of extended metallic solids. Further, we show that the action of these band-mediated mechanisms explains fractional reaction orders in both oxygen and hydroquinone, the time evolution of catalyst potential and rate, and the dependence of rate on overall reaction free energy. Selective poisoning experiments suggest that oxygen reduction proceeds at cobalt sites, whereas quinone oxidation proceeds at native oxidic defects on the carbon. These findings highlight that molecule-like active sites can take advantage of band-mediated mechanisms when coupled to conductive hosts.

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

Operando Tailoring of Defects and Strains in Corrugated β-Ni(OH) 2 Nanosheets for Stable and High-Rate Energy Storage

Nickel hydroxide represents a technologically important material for energy storage, such as hybrid supercapacitors. It has two different crystallographic polymorphs, α- and β-Ni(OH) 2 , showing advantages in either theoretical capacity or cycling/rate performance, manifesting a trade-off trend that needs to be optimized for practical applications. In this work, the synergistic superiorities in both activity and stability of corrugated β-Ni(OH) 2 nanosheets are demonstrated through an electrochemical abuse approach. With ≈91% capacity retention after 10 000 cycles, the corrugated β-Ni(OH) 2 nanosheets can deliver a gravimetric capacity of 457 C g -1 at a high current density of 30 A g -1 , which is nearly two and four times that of the regular α- and β-Ni(OH) 2 , respectively. Operando spectroscopy and finite element analysis reveal that greatly enhanced chemical activity and structural robustness can be attributed to the in situ tailored lattice defects and the strain-induced highly curved micromorphology. This work demonstrates a multi-scale defect-and-strain co-design strategy, which is helpful for rational design and tuned fabrication of next-generation electrode materials for stable and high-rate energy storage.

36 MATERIALS SCIENCE↗
DOE OSTI2021

DMRG on Top of Plane-Wave Kohn–Sham Orbitals: A Case Study of Defected Boron Nitride

In this paper, we analyze the numerical aspects of the inherent multi reference density matrix renormalization group (DMRG) calculations on top of the periodic Kohn-Sham density functional theory using the complete active space approach. The potential of the framework is illustrated by studying hexagonal boron nitride nanoflakes embedding a charged single boron vacancy point defect by revealing a vertical energy spectrum with a prominent multireference character. We investigate the consistency of the DMRG energy spectrum from the perspective of sample size, basis size, and active space selection protocol. Results obtained from standard quantum chemical atom-centered basis calculations and plane-wave based counterparts show excellent agreement. Furthermore, we also discuss the spectrum of the periodic sheet which is in good agreement with extrapolated data of finite clusters. These results pave the way toward applying the DMRG method in extended correlated solid-state systems, such as point defect qubit in wide band gap semiconductors.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Nanoscale electrostatic control in ultraclean van der Waals heterostructures by local anodic oxidation of graphite gates

In an all-van der Waals heterostructure, the active layer, gate dielectrics and gate electrodes are assembled from two-dimensional crystals that have a low density of atomic defects. This design allows two-dimensional electron systems with very low disorder to be created, particularly in heterostructures where the active layer also has intrinsically low disorder, such as crystalline graphene layers or metal dichalcogenide heterobilayers. A key missing ingredient has been nanoscale electrostatic control, with existing methods for fabricated local gates typically introducing unwanted contamination. Here we describe a resist-free local anodic oxidation process for patterning sub-100 nm features in graphite gates, and their subsequent integration into an all-van der Waals heterostructure. We define a quantum point contact in the fractional quantum Hall regime as a benchmark device and observe signatures of chiral Luttinger liquid behaviour, indicating an absence of extrinsic scattering centres in the vicinity of the point contact. In the integer quantum Hall regime, we demonstrate in situ control of the edge confinement potential, a key requirement for the precision control of chiral edge states. In conclusion, this technique may enable the fabrication of devices capable of single anyon control and coherent edge-state interferometry in the fractional quantum Hall regime.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Oxygen Vacancies Enriched Bi Based Catalysts for Enhancing Electrocatalytic CO2 Reduction to Formate

The practical application of CO2 conversion through electrochemical reduction is still hindered by the lack of high performance and cost-effective catalysts. It is thus imperative to explore feasible strategies for scalable fabrication of catalysts for CO2 reduction to targeting products with high efficiency. Herein, for the first time, a hybrid of BiOx encapsulated in carbon thin layer is developed as high active electrocatalyst for CO2 conversion, which is synthesized by pyrolysis using Bi MOF as precursor and sacrificial template. The carbon layer overcoating BiOx catalyst exhibits superior performance for CO2 reduction to formate, achieving a maximum Faradaic efficiency of 89.3 % and a maximum current density of 37.8 mA cm-2. It is found that the as-prepared BiOx catalyst is enriched with oxygen vacancies stabilized by the encapsulating carbon layer. Both the carbon layer and oxygen vacancies improve the charge transfer of the catalyst, in favor of the high formate current density. The defects of oxygen vacancies play a vital role in CO2 adsorption and activation and thereby further enhancing the electrocatalytic ability for formate production.

oxygen vacancy, bismuth, carbon coating, CO2 elect↗
DOE OSTI2020

Atomic defects, functional groups and properties in MXenes

MXenes, a new family of functional two-dimensional (2D) materials, have shown great potential for an extensive variety of applications within the last decade. Atomic defects and functional groups in MXenes are known to have a tremendous influence on the functional properties. In this review, we focus on recent progress in the characterization of atomic defects and functional group chemistry in MXenes, and how to control them to directly influence various properties ( e.g ., electron transport, Li + adsorption, hydrogen evolution reaction (HER) activity, and magnetism) of 2D MXenes materials. Dynamic structural transformations such as oxidation and growth induced by atomic defects in MXenes are also discussed. The review thus provides perspectives on property optimization through atomic defect engineering, and bottom-up synthesis methods based on defect-assisted homoepitaxial growth of MXenes.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Defect Termination in the UiO-66 Family of Metal–Organic Frameworks: The Role of Water and Modulator

The defect concentration in the prototypical metal–organic framework UiO-66 can be well controlled during synthesis, leading to precisely tunable physicochemical properties for this structure. However, there has been a long-standing debate regarding the nature of the compensating species present at the defective sites. Here, we present unambiguous spectroscopic evidence that the missing-linker defect sites in an ambient environment are compensated with both carboxylate and water (bound through intermolecular hydrogen bonding), which is further supported by ab initio calculations. In contrast to the prevailing assumption that the monocarboxylate groups (COO – ) of the modulators form bidentate bonding with two Zr 4+ sites, COO – is found to coordinate to an open Zr 4+ site in an unidentate mode. The neighboring Zr 4+ site is terminated by a coordinating H 2 O molecule, which helps to stabilize the COO – group. Lastly, this finding not only provides a new understanding of defect termination in UiO-66, but also sheds light on the origin of its catalytic activity.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Interfacial Unit-Dependent Catalytic Activity for CO Oxidation over Cerium Oxysulfate Cluster Assemblies

Atomically precise cerium oxo clusters offer a platform to investigate structure–property relationships that are much more complex in the ill-defined bulk material cerium dioxide. We investigated the activity of the MCe 70 torus family (M = Cd, Ce, Co, Cu, Fe, Ni, and Zn), a family of discrete oxysulfate-based Ce 70 rings linked by monomeric cation units, for CO oxidation. CuCe 70 emerged as the best performing MCe 70 catalyst among those tested, prompting our exploration of the role of the interfacial unit on catalytic activity. Temperature-programmed reduction (TPR) studies of the catalysts indicated a lower temperature reduction in CuCe 70 as compared to CeCe 70 . In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) indicated that CuCe 70 exhibited a faster formation of Ce 3+ and contained CO bridging sites absent in CeCe 70 . Isothermal CO adsorption measurements demonstrated a greater uptake of CO by CuCe 70 as compared to CeCe 70 . The calculated energies for the formation of a single oxygen defect in the structure significantly decreased with the presence of Cu at the linkage site as opposed to Ce. Furthermore, this study revealed that atomic-level changes in the interfacial unit can change the reducibility, CO binding/uptake, and oxygen vacancy defect formation energetics in the MCe 70 family to thus tune their catalytic activity.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗