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At least 1,801 records · Page 100
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Properties of Pt-supported iron oxide ultra-thin films: Similarity of Hubbard-corrected and hybrid density functional theory description

We report a first principles study on the properties of Pt(111)-supported FeO(111) monolayer. We confront results issued from PBE+U and HSE06 approximations, and analyze the impact of the more accurate hybrid description of the electronic structure of the metal/oxide interface on a large variety of calculated characteristics of this system. In particular, we analyze the behavior of its work function and its consequences on the spontaneous charging of adsorbed Au adatoms. We also consider the FeO{sub 2} nano-oxide phase and its peculiar oxygen storage characteristics, responsible for the unusual catalytic properties of FeO{sub x}/Pt system. We show that while the hybrid approximation does indeed substantially improve the electronic characteristics of iron oxide, of individual Au adatoms, or oxygen molecules, its overall impact on the calculated properties of the composed FeO/Pt system is very small. We assign this to the relatively small effect of the hybrid approximation on the band structure alignment. This shows that the less computationally demanding DFT+U approximation remains a fully adequate tool in theoretical studies on this kind of systems. This is particularly important for calculations on realistic systems, with large-size reconstructions induced by the lattice mismatch at the interface between the two materials.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2014

Crystal growth under external electric fields

This is a review article concerning the crystal growth under external electric fields that has been studied in our lab for the past 10 years. An external field is applied electrostatically either through an electrically insulating phase or a direct injection of an electric current to the solid-interface-liquid. The former changes the chemical potential of both solid and liquid and controls the phase relationship while the latter modifies the transport and partitioning of ionic solutes in the oxide melt during crystallization and changes the solute distribution in the crystal.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Improving the efficiency of copper indium gallium (Di-)selenide (CIGS) solar cells through integration of a moth-eye textured resist with a refractive index similar to aluminum doped zinc oxide

Textured transparent conductors are widely used in thin-film silicon solar cells. They lower the reflectivity at interfaces between different layers in the cell and/or cause an increase in the path length of photons in the Si absorber layer, which both result in an increase in the number of absorbed photons and, consequently, an increase in short-circuit current density (J{sub sc}) and cell efficiency. Through optical simulations, we recently obtained strong indications that texturing of the transparent conductor in copper indium gallium (di-)selenide (CIGS) solar cells is also optically advantageous. Here, we experimentally demonstrate that the J{sub sc} and efficiency of CIGS solar cells with an absorber layer thickness (d{sub CIGS}) of 0.85 μm, 1.00 μm and 2.00 μm increase through application of a moth-eye textured resist with a refractive index that is sufficiently similar to AZO (n{sub resist} = 1.792 vs. n{sub AZO} = 1.913 at 633 nm) to avoid large optical losses at the resist-AZO interface. On average, J{sub sc} increases by 7.2%, which matches the average reduction in reflection of 7.0%. The average relative increase in efficiency is slightly lower (6.0%). No trend towards a larger relative increase in J{sub sc} with decreasing d{sub CIGS} was observed. Ergo, the increase in J{sub sc} can be fully explained by the reduction in reflection, and we did not observe any increase in J{sub sc} based on an increased photon path length.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Determination of the oxide scale growth mechanism using 18O-tracer experiments in combination with Transmission Electron Microscopy and nanoscale Secondary Ion Mass Spectrometry

Highlights: • {sup 18}O-tracer experiments to determine oxide scale growth mechanism on a Fe-Cr alloy. • Cr{sub 2}O{sub 3} scale growth primarily by means of metal outwards diffusion in air at 850 °C • nanoSIMS mapping on TEM lamella more accurate than SIMS sputter depth profiling • nanoSIMS mapping on TEM lamella reveal information about local oxide growth. • Strong local variation of oxide scale growth - Abstract: Two-stage {sup 18}O{sub 2}/{sup 16}O{sub 2} exposures can be used to investigate the effect that alloying elements, secondary phases, or surface treatments have on the high temperature oxidation behaviour of certain materials. During subsequent exposures to {sup 16}O{sub 2}- and {sup 18}O{sub 2}-rich atmospheres, {sup 16}O- and {sup 18}O-rich layers are formed. Analysis of the layers using Secondary Ion Mass Spectrometry (SIMS) depth profiling allows for conclusions to be drawn about the oxide scale growth mechanism. The conclusions are, however, not entirely unambiguous due to the limited lateral resolution of the technology. Rough surface topography and the thickness variation of the oxide scale over the analysed volume add to the ambiguity of the findings. In this study, an Fe-20%Cr alloy was exposed to both {sup 18}O- and {sup 16}O-rich environments at 850 °C. Two methods were used to analyse the thermally grown Cr{sub 2}O{sub 3} scale: (1) traditional SIMS depth profiling and (2) preparation of a cross-sectional lamellae for Transmission Electron Microscopy (TEM), which, subsequently, was analysed in a NanoSIMS. The NanoSIMS {sup 16}O and {sup 18}O elemental maps were then superimposed on the TEM image. In comparison with traditional SIMS depth profiling, the nanoSIMS elemental maps reveal detailed information about local oxide growth in different parts of an oxide scale. Moreover, a clear {sup 16}O/{sup 18}O interface can be seen in the nanoSIMS maps, which is not the case in the sputter depth profiles. The findings of this study show that the aforementioned issues associated with sputter depth profiling can be eliminated by mapping a cross-section of an oxide scale using high resolution nanoSIMS.

36 MATERIALS SCIENCE↗
DOE OSTI2017

A rationally designed polysulfide-trapping interface on the polymeric separator for high-energy Li–S batteries

A coating layer consisting of a layer of graphene oxide (GO) inserted in between two layers of multi-walled carbon nanotubes (MWCNT) forms a multifunctional polysulfide-trapping triple-interface on the polymeric separator for lithium–sulfur (Li–S) batteries. The top and bottom highly conductive MWCNT interfaces are able to physically inhibit polysulfide diffusion and effectively reutilize the trapped active material during cycling. The inserted ultrathin GO layer with negatively charged carboxyl groups, on the other hand, electrostatically repels the migrating polysulfide anions. The stacked GO sheets also extensively prolong the migration routes of the polysulfides. Additionally, the “buffer zones” interspersed within the triple-interface accommodate the migrating polysulfides. Thus, the sandwiched separators enable Li–S cells with a high-sulfur-loading (10 mg cm -2 ), high areal capacity (10.9 mA h cm -2 ), and stable cycling ability even though the coating layer is ultra-thin (~17 μm) and light-weight (~0.23 mg cm -2 ).

25 ENERGY STORAGE↗
DOE OSTI2015

CO oxidation on gold-supported iron oxides: New insights into strong oxide–metal interactions

Very active FeO x –Au catalysts for CO oxidation are obtained after depositing nanoparticles of FeO, Fe 3 O 4 , and Fe 2 O 3 on a Au(111) substrate. Neither FeO nor Fe 2 O 3 is stable under the reaction conditions. Under an environment of CO/O 2 , they undergo oxidation (FeO) or reduction (Fe 2 O 3 ) to yield nanoparticles of Fe 3 O 4 that are not formed in a bulk phase. Using a combined experimental and theoretical approach, we show a strong oxide–metal interaction (SOMI) between Fe 3 O 4 nanostructures and Au(111), which gives the oxide special properties, allows the formation of an active phase, and provides a unique interface to facilitate a catalytic reaction. This work highlights the important role that the SOMI can play in enhancing the catalytic performance of the oxide component in metal–oxide catalysts.

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

Hetero-epitaxial EuO interfaces studied by analytic electron microscopy

With nearly complete spin polarization, the ferromagnetic semiconductor europium monoxide could enable next-generation spintronic devices by providing efficient ohmic spin injection into silicon. Spin injection is greatly affected by the quality of the interface between the injector and silicon. Here, we use atomic-resolution scanning transmission electron microscopy in conjunction with electron energy loss spectroscopy to directly image and chemically characterize a series of EuO|Si and EuO|YAlO{sub 3} interfaces fabricated using different growth conditions. We identify the presence of europium silicides and regions of disorder at the EuO|Si interfaces, imperfections that could significantly reduce spin injection efficiencies via spin-flip scattering.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Irreversible electrical manipulation of magnetization on BiFeO{sub 3}-based heterostructures

We prepared several heterostructures, Co/Bi{sub 0.90}La{sub 0.10}FeO{sub 3} on surface oxidized Si or (111) SrTiO{sub 3} and NiFe/Bi{sub 0.90}La{sub 0.10}FeO{sub 3} on (001) SrTiO{sub 3} substrates using LaNiO{sub 3} as bottom electrode. With different strategies of voltage application, the exchange bias field H{sub E} decreased with increasing voltage irreversibly for all the heterostructures at room temperature. The chemical state at the NiFe/Bi{sub 0.90}La{sub 0.10}FeO{sub 3} interface was studied by X-ray photoelectron spectroscopy before and after the electrical manipulation. The oxidization of the metallic ferromagnetic layer at interface after the electrical manipulation has been confirmed, which might explain the irreversibility.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Study of interface reaction in a B 4 C/Cr mirror at elevated temperature using soft X-ray reflectivity

Boron carbide is a prominent material for high-brilliance synchrotron optics as it remains stable up to very high temperatures. The present study shows a significant change taking place at 550°C in the buried interface region formed between the Cr adhesive layer and the native oxide layer present on the silicon substrate. An in situ annealing study is carried out at the Indus-1 Reflectivity beamline from room temperature to 550°C (100°C steps). The studied sample is a mirror-like boron carbide thin film of 400 Å thickness deposited with an adhesive layer of 20 Å Cr on a silicon substrate. The corresponding changes in the film structure are recorded using angle-dependent soft X-ray reflectivity measurements carried out in the region of the boron K-edge after each annealing temperature. Analyses performed using the Parratt recursive formalism reveal that the top boron carbide layer remains intact but interface reactions take place in the buried Cr–SiO2 region. After 300°C the Cr layer diffuses towards the substrate. At higher temperatures of 500°C and 550°C the Cr reacts with the native oxide layer and tends to form a low-density compound of chromium oxysilicide (CrSiOx). Depth profiling of Si and Cr distributions obtained from secondary ion mass spectroscopy measurements corroborate the layer model obtained from the soft X-ray reflectivity analyses. Details of the interface reaction taking place near the substrate region of boron carbide/Cr sample are discussed.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2016

A novel technique to measure interface trap density in a GaAs MOS capacitor using time-varying magnetic fields

Interface trap density (D{sub it}) in a GaAs metal-oxide-semiconductor (MOS) capacitor can be measured electrically by measuring its impedance, i.e. by exciting it with a small signal voltage source and measuring the resulting current through the circuit. We propose a new method of measuring D{sub it} where the MOS capacitor is subjected to a (time-varying) magnetic field instead, which produces an effect equivalent to a (time-varying) voltage drop across the sample. This happens because the electron chemical potential of GaAs changes with a change in an externally applied magnetic field (unlike that of the gate metal); this is not the voltage induced by Faraday’s law of electromagnetic induction. So, by measuring the current through the MOS, D{sub it} can be found similarly. Energy band diagrams and equivalent circuits of a MOS capacitor are drawn in the presence of a magnetic field, and analyzed. The way in which a magnetic field affects a MOS structure is shown to be fundamentally different compared to an electrical voltage source.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Irradiation-induced reactions at the CeO 2 /SiO 2 /Si interface

The influence of high-energy (1.6 MeV) Ar 2+ irradiation on the interfacial interaction between cerium oxide thin films (~15 nm) with a SiO 2 /Si substrate is investigated using transmission electron microscopy, ultrahigh vacuum x-ray photoelectron spectroscopy (XPS), and a carbon monoxide (CO) oxidation catalytic reaction using ambient pressure XPS. The combination of these methods allows probing the dynamics of vacancy generation and its relation to chemical interactions at the CeO 2 /SiO 2 /Si interface. The results suggest that irradiation causes amorphization of some portion of CeO 2 at the CeO 2 /SiO 2 /Si interface and creates oxygen vacancies due to the formation of Ce 2 O 3 at room temperature. The subsequent ultra-high-vacuum annealing of irradiated films increases the concentration of Ce 2 O 3 with the simultaneous growth of the SiO 2 layer. Interactions with CO molecules result in an additional reduction of cerium and promote the transition of Ce 2 O 3 to a silicate compound. Furthermore, thermal annealing of thin films exposed to oxygen or carbon monoxide shows that the silicate phase is highly stabile even at 450 °C.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Enhancing the Carbon Monoxide Oxidation Performance through Surface Defect Enrichment of Ceria-Based Supports for Platinum Catalyst

Effective synthesis and application of single-atom catalysts on supports lacking enough defects remain a significant challenge in environmental catalysis. Herein, we present a universal defect-enrichment strategy to increase the surface defects of CeO 2 -based supports through H 2 reduction pretreatment. The Pt catalysts supported by defective CeO 2 -based supports, including CeO 2 , CeZrO x , and CeO 2 /Al 2 O 3 (CA), exhibit much higher Pt dispersion and CO oxidation activity upon reduction activation compared to their counterpart catalysts without defect enrichment. Specifically, Pt is present as embedded single atoms on the CA support with enriched surface defects (CA-HD) based on which the highly active catalyst showing embedded Pt clusters (Pt C ) with the bottom layer of Pt atoms substituting the Ce cations in the CeO 2 surface lattice can be obtained through reduction activation. Embedded PtC can better facilitate CO adsorption and promote O 2 activation at Pt C –CeO 2 interfaces, thereby contributing to the superior low-temperature CO oxidation activity of the Pt/CA-HD catalyst after activation.

36 MATERIALS SCIENCE↗
DOE OSTI2014

A general mechanism for stabilizing the small sizes of precious metal nanoparticles on oxide supports

We recently discovered that MgAl2O4 spinel {111} nano-facets optimally stabilize the small sizes of platinum nanoparticles even after severe high temperature aging treatments. Here we report the thermal stabilities of other precious metals with various physical and chemical properties on the MgAl2O4 spinel {111} facets, providing important new insights into the stabilization mechanisms. Besides Pt, Rh and Ir can also be successfully stabilized as small (1-3 nm) nanoparticles and even as single atomic species after extremely severe (800 °C, 1 week) oxidative aging. However, other metals either aggregate (Ru, Pd, Ag, and Au) or sublimate (Os) even during initial catalyst synthesis. On the basis of ab initio theoretical calculations and experimental observations, we rationalize that the exceptional stabilization originates from lattice matching, and the correspondingly strong attractive interactions at interfaces between the spinel {111} surface oxygens and epitaxial metals\metal oxides. On this basis, design principles for catalyst support oxide materials that are capable in stabilizing precious metals are proposed.

anti-sintering↗
DOE OSTI2026

Pattern-enhanced Resonant Soft X-ray Scattering for Operando monitoring of electrochemical solid-liquid interfaces

Unveiling interfaces at sub-nanometer scales is essential for advancing the understanding of complex chemical transformations. However, characterizing solid-liquid interfaces with high dimensional sensitivity and temporal resolution remains challenging, due to their dynamic nature and inaccessibility by conventional probes. Here we present an approach, Pattern-enhanced Resonant Soft X-ray Scattering, to overcome the challenges. Rooted in a “sample-as-optics” philosophy, this technique utilizes precisely engineered line-grating nanopatterns to modulate near-field X-ray illumination, coherently enhancing scattering signals from the line-gratings. We implement the method using Ni line-grating nanopatterns in electrochemical water oxidation. The periodic nanostructures serve as diffractive optical elements to reveal the Ni oxidation gradients and structural dynamics at the electrode-electrolyte interfaces. Finite-element simulations corroborate the observed trends by modeling variations in compositions and structures during electrocatalysis. Through integrating advanced sample design with coherent wave nature of soft X-rays, our approach opens accessible pathways to operando exploring chemical evolution and sub-nanometer dimensional variations simultaneously in electrochemical systems. This non-destructive method is efficient and element-specific, making it valuable for probing chemical and dimensional dynamics with appropriate modeling.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1991

Evidence from transmission electron microscopy for an oxynitride layer in oxidized Si3N4

Microstructural and energy dispersive spectrometry evidence is produced, from transmission electron microscopy, to show that a silicon oxynitride inner layer is produced by the oxidation of silicon nitride in dry oxygen at 1350 C as proposed by Tressler et al. (1989). However, details of the microstructures at the oxide/nitride interface do not agree entirely with the rest of the Tressler model for the oxidation of Si3N4.

Ogbuji, Linus U. J. T.↗
DOE OSTI2016

The BiCu{sub 1−x}OS oxysulfide: Copper deficiency and electronic properties

An oxysulfide series of nominal compositions BiCu{sub 1−x}OS with x<0.20 has been prepared and its structural properties characterized by combining powder X-ray diffraction and transmission electron microscopy techniques. It is found that this oxysulfide, crystallizing in the P4/nmm space group, tends to adopt a constant amount of copper vacancy corresponding to x=0.05 in the BiCu{sub 1−x}OS formula. The presence of Cu vacancies is confirmed by HAADF-STEM analysis showing, in the Cu atomic columns, alternating peaks of different intensities in some very localized regions. For larger Cu deficiencies (x>0.05 in the nominal composition), other types of structural nanodefects are evidenced such as bismuth oxysulfides of the “BiOS” ternary system which might explain the report of superconductivity for the BiCu{sub 1−x}OS oxysulfide. Local epitaxial growth of the BiCuOS oxysulfide on top of CuO is also observed. In marked contrast to the BiCu{sub 1−x}OSe oxyselenide, these results give an explanation to the limited impact of Cu deficiency on the Seebeck coefficient in BiCu{sub 1−x}OS compounds. - Graphical abstract: High resolution TEM image showing a Bi(Cu)OS/Bi{sub 2}O{sub 2}S interface and corresponding dislocation region. The Bi(Cu)OS structure adopts a rather constant Cu content (near 0.95); starting from BiCuOS leads to the formation of defects such as the Bi{sub 2}O{sub 2}S oxysulfide.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Electronic structure investigation of atomic layer deposition ruthenium(oxide) thin films using photoemission spectroscopy

Analyzing and manipulating the electronic band line-up of interfaces in novel micro- and nanoelectronic devices is important to achieve further advancement in this field. Such band alignment modifications can be achieved by introducing thin conformal interfacial dipole layers. Atomic layer deposition (ALD), enabling angstrom-precise control over thin film thickness, is an ideal technique for this challenge. Ruthenium (Ru{sup 0}) and its oxide (RuO{sub 2}) have gained interest in the past decade as interfacial dipole layers because of their favorable properties like metal-equivalent work functions, conductivity, etc. In this study, initial results of the electronic structure investigation of ALD Ru{sup 0} and RuO{sub 2} films via photoemission spectroscopy are presented. These experiments give insight into the band alignment, growth behavior, surface structure termination, and dipole formation. The experiments were performed in an integrated vacuum system attached to a home-built, stop-flow type ALD reactor without exposing the samples to the ambient in between deposition and analysis. Bis(ethylcyclopentadienyl)ruthenium(II) was used as precursor and oxygen as reactant. The analysis chamber was outfitted with X-ray photoemission spectroscopy (LIXPS, XPS). The determined growth modes are consistent with a strong growth inhibition situation with a maximum average growth rate of 0.21 Å/cycle for RuO{sub 2} and 0.04 Å/cycle for Ru.{sup 0} An interface dipole of up to −0.93 eV was observed, supporting the assumption of a strongly physisorbed interface. A separate experiment where the surface of a RuO film was sputtered suggests that the surface is terminated by an intermediate, stable, non-stoichiometric RuO{sub 2}/OH compound whose surface is saturated with hydroxyl groups.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

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