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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 1,279 records · Page 71
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Large bipolaron density at organic semiconductor/electrode interfaces

Bipolaron states, in which two electrons or two holes occupy a single molecule or conjugated polymer segment, are typically considered to be negligible in organic semiconductor devices due to Coulomb repulsion between the two charges. Here we use charge modulation spectroscopy to reveal a bipolaron sheet density >10 10 cm –2 at the interface between an indium tin oxide anode and the common small molecule organic semiconductor N,N'-Bis(3-methylphenyl)-N,N'-diphenylbenzidine. We find that the magnetocurrent response of hole-only devices correlates closely with changes in the bipolaron concentration, supporting the bipolaron model of unipolar organic magnetoresistance and suggesting that it may be more of an interface than a bulk phenomenon. These results are understood on the basis of a quantitative interface energy level alignment model, which indicates that bipolarons are generally expected to be significant near contacts in the Fermi level pinning regime and thus may be more prevalent in organic electronic devices than previously thought.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Metal foils with ordered crystal structure and method for producing metal foils

A method for producing a metal foil comprising depositing metal onto an oxidizable substrate to form a metal film on the substrate; oxidizing the substrate at an interface between the metal film and the substrate; and removing the metal film from the substrate to yield a metal foil. A method for forming a thin metal film comprising pre-polarizing a single-crystal Si substrate by application of a potential which is negative of a potential at which Si oxidizes, which pre-polarization occurs in the presence of metal ions to form metal growth nucleation sites on the substrate, followed by application of a potential at which both oxidation of Si and electrodeposition of the metal occur to grow the metal film and oxidize the Si to SiOx, which potential is more positive than the potential applied in the pre-polarization step.

Switzer, Jay A.↗
DOE OSTI2019

Néel vector reorientation in ferromagnetic/antiferromagnetic complex oxide nanostructures

In this study, we report on a temperature-driven antiferromagnetic (AF) spin reorientation transition in micro- and nanostructures of AF/ferromagnetic (FM) LaFeO 3 /La 0.7 Sr 0.3 MnO 3 thin film bilayers. Using a combination of x-ray photoemission electron microscopy and x-ray absorption spectroscopy, the Néel vector is shown to reorient 90° as a result of the competition between a shape-imposed anisotropy in the AF layer and interface coupling to the adjacent FM layer. We demonstrate how a temperature dependence of the AF/FM spin configuration in line-shaped nanomagnets can be tuned by variation of their linewidth. This work provides insight into the AF/FM interface exchange coupling in complex oxide heterostructures and the possibilities of spin control by nanostructuring in thin film spintronics.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2023

Sources of error and methods to improve accuracy in interface state density analysis using quasi-static capacitance–voltage measurements in wide bandgap semiconductors

Characterizing interface trap states in commercial wide bandgap devices using frequency-based measurements requires unconventionally high probing frequencies to account for both fast and slow traps associated with wide bandgap materials. The C - ψs technique has been suggested as a viable quasi-static method for determining the interface trap state densities in wide bandgap systems, but the results are shown to be susceptible to errors in the analysis procedure. Here this work explores the primary sources of errors present in the C - ψs technique using an analytical model that describes the apparent response for wide bandgap MOS capacitor devices. Measurement noise is shown to greatly impact the linear fitting routine of the 1/C$^{*2}_s$ vs Ψ s plot to calibrate the additive constant in the surface potential/gate voltage relationship, and an inexact knowledge of the oxide capacitance is also shown to impede interface trap state analysis near the band edge. In addition, a slight nonlinearity that is typically present throughout the 1/C$^{*2}_s$ vs Ψ s plot hinders the accurate estimation of interface trap densities, which is demonstrated for a fabricated n-SiC MOS capacitor device. Methods are suggested to improve quasi-static analysis, including a novel method to determine an approximate integration constant without relying on a linear fitting routine.

42 ENGINEERING↗
DOE OSTI2016

Catalytically enhanced thermal decomposition of chemically grown silicon oxide layers on Si(001)

The thermal decomposition of Si dioxide layers formed by wet chemical treatment on Si(001) has been studied by low-energy electron microscopy. Independent nucleations of voids occur into the Si oxide layers that open by reaction at the void periphery. Depending on the voids, the reaction rates exhibit large differences via the occurrence of a nonlinear growth of the void radius. This non-steady state regime is attributed to the accumulation of defects and silicon hydroxyl species at the SiO{sub 2}/Si interface that enhances the silicon oxide decomposition at the void periphery.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2023

Asymmetric nanoparticle oxidation observed in-situ by the evolution of diffraction contrast

Abstract The use of transmission electron microscopy (TEM) to observe real-time structural and compositional changes has proven to be a valuable tool for understanding the dynamic behavior of nanomaterials. However, identifying the nanoparticles of interest typically require an obvious change in position, size, or structure, as compositional changes may not be noticeable during the experiment. Oxidation or reduction can often result in subtle volume changes only, so elucidating mechanisms in real-time requires atomic-scale resolution or in-situ electron energy loss spectroscopy, which may not be widely accessible. Here, by monitoring the evolution of diffraction contrast, we can observe both structural and compositional changes in iron oxide nanoparticles, specifically the oxidation from a wüstite-magnetite (FeO@Fe 3 O 4 ) core – shell nanoparticle to single crystalline magnetite, Fe 3 O 4 nanoparticle. The in-situ TEM images reveal a distinctive light and dark contrast known as the ‘Ashby-Brown contrast’, which is a result of coherent strain across the core – shell interface. As the nanoparticles fully oxidize to Fe 3 O 4 , the diffraction contrast evolves and then disappears completely, which is then confirmed by modeling and simulation of TEM images. This represents a new, simplified approach to tracking the oxidation or reduction mechanisms of nanoparticles using in-situ TEM experiments.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Reaction Mechanism for Direct Propylene Epoxidation by Alumina-Supported Silver Aggregates. The Role of the Particle / Support Interface

Sub-nanometer Ag aggregates on alumina supports have been found to be active toward direct propylene epoxidation to propylene oxide by molecular oxygen at low temperatures, with a negligible amount of carbon dioxide formation (Science 328, p. 224, 2010). In this work, we computationally and experimentally investigate the origin of the high reactivity of the sub-nanometer Ag aggregates. Computationally, we study O 2 dissociation and propylene epoxidation on unsupported Ag 19 and Ag 20 clusters, as well as alumina-supported Ag 19 . The O 2 dissociation and propylene epoxidation apparent barriers at the interface between the Ag aggregate and the alumina support are calculated to be 0.2 and 0.2~0.4 eV, respectively. These barriers are somewhat lower than those on sites away from the interface. The mechanism at the interface is similar to what was previously found for the silver trimer on alumina and can account for the high activity observed for the direct oxidation of propylene on the Ag aggregates. The barriers for oxygen dissociation on these model systems both at the interface and on the surfaces are small compared to crystalline surfaces, indicating that availability of oxygen will not be a rate limiting step for the aggregates, as in the case of the crystalline surfaces. Experimentally, we investigate Ultrananocrystalline Diamond (UNCD)-supported silver aggregates under reactive conditions of propylene partial oxidation. The UNCD-supported Ag clusters are found to be not measurably active toward propylene oxidation, in contrast to the alumina supported Ag clusters. This suggests that the lack of metal-oxide interfacial sites of the Ag-UNCD catalyst, limits the epoxidation catalytic activity. This combined computational and experimental study shows the importance of the metal-oxide interface as well as the non-crystalline nature of the alumina-supported sub-nanometer Ag aggregate catalysts for propylene epoxidation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1988

Producing CCD imaging sensor with flashed backside metal film

A backside illuminated CCD imaging sensor for reading out image charges from wells of the array of pixels is significantly improved for blue, UV, far UV and low energy x-ray wavelengths (1-5000.ANG.) by so overthinning the backside as to place the depletion edge at the surface and depositing a thin transparent metal film of about 10.ANG. on a native-quality oxide film of less than about 30.ANG. grown on the thinned backside. The metal is selected to have a higher work function than that of the semiconductor to so bend the energy bands (at the interface of the semiconductor material and the oxide film) as to eliminate wells that would otherwise trap minority carriers. A bias voltage may be applied to extend the frontside depletion edge to the interface of the semiconductor material with the oxide film in the event there is not sufficient thinning. This metal film (flash gate), which improves and stabilizes the quantum efficiency of a CCD imaging sensor, will also improve the QE of any p-n junction photodetector.

Janesick, James R.↗
NASA NTRS1991

CCD imaging sensor with flashed backside metal film

A backside illuminated CCD imaging sensor for reading out image charges from wells of the array of pixels is significantly improved for blue, UV, far UV and low energy x-ray wavelengths (1-5000.ANG.) by so overthinning the backside as to place the depletion edge at the surface and depositing a thin transparent metal film of about 10.ANG. on a native-quality oxide film of less than about 30.ANG. grown on the thinned backside. The metal is selected to have a higher work function than that of the semiconductor to so bend the energy bands (at the interface of the semiconductor material and the oxide film) as to eliminate wells that would otherwise trap minority carriers. A bias voltage may be applied to extend the frontside depletion edge to the interface of the semiconductor material with the oxide film in the event there is not sufficient thinning. This metal film (flash gate), which improves and stabilizes the quantum efficiency of a CCD imaging sensor, will also improve the QE of any p-n junction photodetector.

Janesick, James R.↗
DOE OSTI2011

Direct determination of flat-band voltage for metal/high {kappa} oxide/semiconductor heterointerfaces by electric-field-induced second-harmonic generation

We have employed electric-field-induced second-harmonic (EFISH) generation to determine the flat-band voltage (V{sub FB}) of Cr/ALD-Al{sub 2}O{sub 3}/MBE-HfO{sub 2}/n-Si (001) MOS structure. Due to the phase sensitivity of EFISH signal to the electric field in the space charge region, the V{sub FB} of -1.20{+-}0.07 V was determined by analyzing the relative phase change in the EFISH signal as a function of the applied gate voltage. The obtained value is in good agreement with that estimated by the capacitance-voltage measurement. This study demonstrated an all-optical technique to directly determine the flat-band voltage for the high {kappa} oxide/Si heterointerfaces.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Shell-mediated control of surface chemistry of highly stoichiometric magnetite nanoparticles

Magnetite (Fe 3 O 4 ) nanoparticles are one of the most studied nanomaterials for different nanotechnological and biomedical applications. However, Fe 3 O 4 nanomaterials gradually oxidize to maghemite (γ-Fe 2 O 3 ) under conventional environmental conditions leading to changes in their functional properties that determine their performance in many applications. Here we propose a novel strategy to control the surface chemistry of monodisperse 12 nm magnetite nanoparticles by means of a 3 nm-thick Zn-ferrite epitaxial coating in core/shell nanostructures. We have carried out a combined Mössbauer spectroscopy, dc magnetometry, X-ray photoelectron spectroscopy and spatially resolved electron energy loss spectroscopy study on iron oxide and Fe 3 O 4 /Zn 0.6 Fe 2.4 O 4 core/shell nanoparticles aged under ambient conditions for 6 months. Our results reveal that while the aged iron oxide nanoparticles consist of a mixture of γ-Fe 2 O 3 and Fe 3 O 4 , the Zn-ferrite-coating preserves a highly stoichiometric Fe 3 O 4 core. Therefore, the aged core/shell nanoparticles present a sharp Verwey transition, an increased saturation magnetization and the possibility of tuning the effective anisotropy through exchange-coupling at the core/shell interface. The inhibition of the oxidation of the Fe 3 O 4 cores can be accounted for in terms of the chemical nature of the shell layer and an epitaxial crystal symmetry matching between the core and the shell.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
NASA NTRS2020

Mineralogy of Vera Rubin Ridge in Gale Crater from the Mars Science Laboratory CheMin instrument

Gale crater was selected as the landing site for the Mars Science Laboratory Curiosity rover because of orbital evidence for a variety of secondary minerals in the lower slopes of Aeolis Mons (aka Mount Sharp) that indicate changes in aqueous conditions over time. Distinct units demonstrate orbital spectral signatures of hematite, phyllosilicate (smectite), and sulfate minerals, which suggest that ancient aqueous environments in Gale crater varied in oxidation potential, pH, and water activity. Vera Rubin ridge (VRR) is the first of these units identified from orbit to have been studied by Curiosity. Orbital near-infrared data from VRR show a strong band at 860 nm indicative of hematite. Before Curiosity arrived at VRR, the hypotheses to explain the formation of hematite included (1) precipitation at a redox interface where aqueous Fe2+ was oxidized to Fe3+, and (2) acidic alteration of olivine in oxic fluids. Studying the composition and sedimentology of the rocks on VRR allow us to test and refine these hypotheses and flesh out the depositional and diagenetic history of the ridge. Here, we focus on the mineralogical results of four rock powders drilled from and immediately below VRR as determined by CheMin.

Rampe, E. B.↗
DOE OSTI2021

Tocopherol controls D1 amino acid oxidation by oxygen radicals in Photosystem II

Photosystem II (PSII) is an intrinsic membrane protein complex that functions as a light-driven water:plastoquinone oxidoreductase in oxygenic photosynthesis. Electron transport in PSII is associated with formation of reactive oxygen species (ROS) responsible for oxidative modifications of PSII proteins. In this study, oxidative modifications of the D1 and D2 proteins by the superoxide anion (O 2 •- ) and the hydroxyl (HO•) radicals were studied in WT and a tocopherol cyclase (vte1) mutant, which is deficient in the lipid-soluble antioxidant α-tocopherol. In the absence of this antioxidant, high-resolution tandem mass spectrometry was used to identify oxidation of D1: 130 E to hydroxyglutamic acid by O 2 •- at the Pheo D1 site. Additionally, D1: 246 Y was modified to either tyrosine hydroperoxide or dihydroxyphenylalanine by O 2 •- and HO • , respectively, in the vicinity of the nonheme iron. We propose that α-tocopherol is localized near PheoD1 and the nonheme iron, with its chromanol head exposed to the lipid–water interface. This helps to prevent oxidative modification of the amino acid’s hydrogen that is bonded to PheoD1 and the nonheme iron (via bicarbonate), and thus protects electron transport in PSII from ROS damage.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2015

Comparison of gate dielectric plasma damage from plasma-enhanced atomic layer deposited and magnetron sputtered TiN metal gates

Fully depleted silicon-on-insulator transistors were fabricated using two different metal gate deposition mechanisms to compare plasma damage effects on gate oxide quality. Devices fabricated with both plasma-enhanced atomic-layer-deposited (PE-ALD) TiN gates and magnetron plasma sputtered TiN gates showed very good electrostatics and short-channel characteristics. However, the gate oxide quality was markedly better for PE-ALD TiN. A significant reduction in interface state density was inferred from capacitance-voltage measurements as well as a 1200× reduction in gate leakage current. A high-power magnetron plasma source produces a much higher energetic ion and vacuum ultra-violet (VUV) photon flux to the wafer compared to a low-power inductively coupled PE-ALD source. The ion and VUV photons produce defect states in the bulk of the gate oxide as well as at the oxide-silicon interface, causing higher leakage and potential reliability degradation.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Two-dimensional electron gas in GaAs/SrHfO{sub 3} heterostructure

The III-V/perovskite-oxide system can potentially create new material properties and new device applications by combining the rich properties of perovskite-oxides together with the superior optical and electronic properties of III-Vs. The structural and electronic properties of the surface and interface are studied using first-principles calculations for the GaAs/SrHfO{sub 3} heterostructure. We investigate the specific adsorption sites and the atomic structure at the initial growth stage of GaAs on the SrHfO{sub 3} (001) substrate. Ga and As adsorption atoms preferentially adsorb at the top sites of oxygen atoms under different coverage. The energetically favorable interfaces are presented among the atomic arrangements of the GaAs/SrHfO{sub 3} interfaces. Our calculations predict the existing of the two-dimensional electron gas in the GaAs/SrHfO{sub 3} heterostructure.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2012

Identification of a paramagnetic recombination center in silicon/silicon-dioxide interface

A paramagnetic recombination center having an orthorhombic symmetry with g[110] = 2.0095(2), g[001] = 2.0038(2), and g[110] = 2.0029(2) is found at the interface between silicon and native oxide. The center is referred to P{sub m} center and observed by a spin dependent recombination based electron paramagnetic resonance detection that has the sensitivity of {approx}10{sup 11} spins/cm{sup 2}. The employment of an isotopically enriched {sup 28}Si sample with the concentration of {sup 29}Si nuclear spins reduced to 0.017% leads to narrowing of the resonance line. This narrowing is the key for the accurate determination of the angular dependence of the g-factor.

36 MATERIALS SCIENCE↗
DOE OSTI2012

The presence of a (1 x 1) oxygen overlayer on ZnO(0001) surfaces and at Schottky interfaces

The atomic surface and interface structures of uncoated and metal-coated epi-polished ZnO(0001) Zn-polar wafers were investigated via surface x-ray diffraction. All uncoated samples showed the presence of a fully occupied (1 x 1) overlayer of oxygen atoms located at the on-top position above the terminating Zn atom, a structure predicted to be unstable by several density functional theory calculations. The same oxygen overlayer was clearly seen at the interface of ZnO with both elemental and oxidized metal Schottky contact layers. No significant atomic relaxations were observed at surfaces and interfaces processed under typical device fabrication conditions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1974

Radiation hardening of MOS devices by boron

A technique is described for radiation hardening of MOS devices and specifically for stabilizing the gate threshold potential at room temperature of a radiation subjected MOS field-effect device with a semiconductor substrate, an insulating layer of oxide on the substrate, and a gate electrode disposed on the insulating layer. The boron is introduced within a layer of the oxide of about 100 A-300 A thickness immediately adjacent the semiconductor-insulator interface. The concentration of boron in the oxide layer is preferably maintained on the order of 10 to the 18th power atoms/cu cm. The technique serves to reduce and substantially annihilate radiation induced positive gate charge accumulations.

Danchenko, V.↗