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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 253 records · Page 14
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Electrical Behavior of Combinatorial Thin-Film Zr x Ta 1−x O y

Combinatorial magnetron sputtering and electrical characterization were used to systematically study the impact of compositional changes in the resistive switching of transition metal oxides, specifically the Zr x Ta 1−x O y system. Current-voltage behavior across a range of temperatures provided insights into the mechanisms that contribute to differences in the electrical conductivity of the pristine Ta 2 O 5 and ZrO 2 , and mixed Zr x Ta 1−x O y devices. The underlying conductive mechanism was found to be a mixture of charge trapping and ionic motion, where charge trapping/emission dictated the short-term cycling behavior while ion motion contributed to changes in the conduction with increased cycling number. ToF-SIMS was used to identify the origin of the “wake-up” behavior of the devices, revealing an ionic motion contribution. This understanding of how cation concentration affects conduction in mixed valence systems helps provide a foundation for a new approach toward manipulating resistive switching in these active layer materials.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Anodic electrocatalytic conversion of carboxylic acids on thin films of RuO 2 , IrO 2 , and Pt

The electrocatalytic upgrading of carboxylic acids, abundant biomass-derived molecules, remains a challenging task. In this paper, we report an electrocatalytic decarboxylation (ECDX) approach for the conversion of carboxylic acids into paraffins, olefins, and alcohols via (non-)Kolbe electrolysis on thin films (TFs). The ECDX rate, product selectivity, and current efficiency were potential and electrode dependent. For example, the ECDX activity of RuO 2 -TF was similar to that of Pt foil, but the selectivity to Kolbe products was lower on the former. RuO 2 -TF showed about five times higher rates for the oxygen evolution reaction than Pt foil, which resulted in lower current efficiency. The ECDX and O 2 evolution activities of IrO 2 -TF were potential dependent, but this electrode was selective to non-Kolbe products with low current efficiency (<10 %). This work highlights the performance of thin films as an alternative to bulk metals as anodes for oxidative upgrading of carboxylic acids.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Sequential Infiltration Synthesis (SIS) of In 2 O 3 -Based Porous Photoelectrodes for Molecular Sensitization

High-surface-area photoelectrodes are highly desirable for solar energy conversion applications, particularly in systems where surface-bound molecular chromophores are responsible for light absorption. To this end, porous Indium-Zinc-Oxide (IZO) frameworks are fabricated via Sequential Infiltration Synthesis (SIS) into poly(methyl methacrylate) (PMMA) films followed by air annealing. A more complete understanding of the multimetal infiltration process conditions and postannealing temperature allows for precise control of zinc (Zn) incorporation to afford porous, conductive, transparent, and amorphous photoelectrodes. Uniform incorporation of both indium (In) and Zn is found to require extended diethylzinc (DEZ) exposures to compensate for remarkably slow precursor diffusion in the presence of indium oxyhydroxide nuclei. SIS-fabricated IZO photoelectrodes are structurally robust, electrochemically active, and exhibit dye adsorption that suggest practical surface area enhancements greater than 100×. In conclusion, these attributes establish SIS as a rapid, scalable, and reproducible route to photoelectrodes for solar energy conversion and hybrid interface characterization.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Hybrid MBE Route to Adsorption-Controlled Growth of BaTiO 3 Membranes with Robust Polarization Switching

Freestanding ferroelectric membranes are promising for flexible electronics, nonvolatile memory, photonics, and spintronics, but their synthesis is challenged by the need for reproducibility with precise stoichiometric control. Here, we demonstrate the adsorption-controlled growth of single-crystalline, epitaxial BaTiO 3 films by hybrid molecular beam epitaxy (MBE) on a binary oxide sacrificial layer. Using a simple water-droplet lift-off method, we obtained submillimeter- to millimeter-sized membranes that retained crystallinity, as confirmed by high-resolution X-ray diffraction, and exhibited robust tetragonal symmetry, as verified by Raman spectroscopy. Impedance spectroscopy confirmed a high dielectric constant of ∼1340, reflecting the robust dielectric response of the membranes. Ferroelectric functionality was revealed by piezoresponse force microscopy (PFM) and further verified by polarization-electric field (P-E) loop measurements with Positive-Up-Negative-Down (PUND). The P-E loops exhibited a remnant polarization of ∼5 μC cm –2 and a coercive field of ∼63 kV cm –1 . Furthermore, these results were interpreted in relation to c- and a-domain configurations.

Raman spectroscopy↗
DOE OSTI2025

Low-temperature oxidation of methane and methanol on iridium oxides

Iridium oxides (IrO 2 ) are of significant interest for low-temperature oxidation of small molecules such as CH 4 and CH 3 OH, although the physical origin of their high activity remains under debate. Here, we demonstrate that the enhanced activity of IrO 2 arises from the formation of coordinatively unsaturated (CUS) oxygen species. By combining ambient-pressure X-ray spectroscopy and density functional theory calculations, we present evidence for the formation of CUS oxygen during CH 4 and CH 3 OH oxidation. Such surface speciation correlates with the conversion of methane to carbon dioxide and methanol to methyl formate on rutile IrO 2 and hydrous IrO 2 powder catalysts in a plug-flow reactor at room temperature. These findings extend the understanding of the physical origin of the higher activity of iridium oxide thin-film catalysts to powder catalysts and provide insights into the tuneability of iridium-oxide-containing catalysts for low-temperature C–H and O–H bond activation.

AP-XPS↗
DOE OSTI2021

Molecular beam epitaxy of phase-pure antiperovskite Sr 3 SnO thin films

The antiperovskite oxide Sr 3 SnO has attracted substantial interest due to its topologically non-trivial band structure. Sr-deficient Sr3-xSnO can become superconducting, making it a candidate intrinsic topological superconductor. Here, we show that epitaxial, phase-pure Sr 3-x SnO films can be synthesized by molecular beam epitaxy (MBE) using solid Sr and SnO 2 sources. Here, we show that Sn-rich growth conditions result in a large amount of a Sn-rich impurity phase, which is challenging to detect in x-ray diffraction. Carrier densities and the amount of the impurity phase change systematically with the growth conditions, indicating that MBE provides excellent control over the films' stoichiometry. We discuss the electrical properties, including quantum interference phenomena, which support the topological nature of the films.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Determining the Oxygen Stoichiometry of Cobaltite Thin Films

Transition metal oxides (TMO) are promising materials to realize low-power neuromorphic devices. Their physical properties critically depend on their oxygen vacancy concentrations, whose experimental determination remains a challenging task. Here we focus on cobaltites, in particular La 1-x Sr x CoO 3-d (LSCO), and we present a strategy to identify fingerprints of oxygen vacancies in X-ray absorption (XA) spectra. Using a combination of experiment and theory, we show that the variation of the oxygen vacancy concentration in the perovskite phase of LSCO is correlated with the change of the relative peak positions of the O K-edge XA spectra. Furthermore, we also identify an additional geometrical fingerprint that captures both the changes of the Co-O bond length and Co-O-Co bond angle in the material due to the presence of oxygen vacancies. Finally, we predict the oxygen vacancy concentration of experimental samples and show how the resistivity of the oxide material may be tuned as a function of the defect concentration present in the system.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Emergent and robust ferromagnetic-insulating state in highly strained ferroelastic LaCoO 3 thin films

Transition metal oxides are promising candidates for the next generation of spintronic devices due to their fascinating properties that can be effectively engineered by strain, defects, and microstructure. An excellent example can be found in ferroelastic LaCoO 3 with paramagnetism in bulk. In contrast, unexpected ferromagnetism is observed in tensile-strained LaCoO 3 films, however, its origin remains controversial. Here we simultaneously reveal the formation of ordered oxygen vacancies and previously unreported long-range suppression of CoO 6 octahedral rotations throughout LaCoO 3 films. Supported by density functional theory calculations, we find that the strong modification of Co 3d-O 2p hybridization associated with the increase of both Co-O-Co bond angle and Co-O bond length weakens the crystal-field splitting and facilitates an ordered high-spin state of Co ions, inducing an emergent ferromagnetic-insulating state. Our work provides unique insights into underlying mechanisms driving the ferromagnetic-insulating state in tensile-strained ferroelastic LaCoO 3 films while suggesting potential applications toward low-power spintronic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Multielectrode electrochemical cell for in situ structural characterization of amorphous thin-film catalysts using high-energy X-ray scattering

A multielectrode-based electrochemical cell allows the structural characterization of an amorphous thin-film water oxidation catalyst under various electrochemical potentials using high-energy X-ray scattering and atomic pair distribution function (PDF) techniques. A multielectrode with five electrodes provides a sufficiently low background signal to enable high-energy X-ray scattering (HEXS) measurements and amplifies the extremely low HEXS signals from samples for high-resolution PDF analysis of in situ data from thin-film catalysts. Glassy carbon (GC) creates a relatively low intensity HEXS pattern and is used as a working electrode. Instead of a three-dimensional (3D) porous electrode architecture, the flat geometry of the electrode enables various deposition techniques to be used for the preparation of a highly conductive metal oxide layer. PDF analysis demonstrates high spatial resolution for a 230 nm thick amorphous iridium oxide film deposited on two roughened 60 µm thick GC electrodes. The PDF analysis resolves the domain size and distinguishes changes in fine structure which are directly correlated with the structure and function of the catalysts. In conclusion, the results bring the opportunity to analyze the structure of nanometre-scale amorphous thin-film catalysts in an electrolyte-compatible and compact 3D-printed electrochemical cell in a three-electrode configuration.

36 MATERIALS SCIENCE↗
DOE OSTI2021

In Situ X-ray Measurements to Follow the Crystallization of BaTiO 3 Thin Films during RF-Magnetron Sputter Deposition

Here, we report on adding an important dimension to the fundamental understanding of the evolution of the thin film micro structure evolution. Thin films have gained broad attention in their applications for electro-optical devices, solar-cell technology, as well storage devices. Deep insights into fundamental functionalities can be realized via studying crystallization microstructure and formation processes of polycrystalline or epitaxial thin films. Besides the fundamental aspects, it is industrially important to minimize cost which intrinsically requires lower energy consumption at increasing performance which requires new approaches to thin film growth in general. Here, we present a state of the art sputtering technique that allows for time-resolved in situ studies of such thin film growth with a special focus on the crystallization via small angle scattering and X-ray diffraction. Focusing on the crystallization of the example material of BaTiO 3 , we demonstrate how a prototypical thin film forms and how detailed all phases of the structural evolution can be identified. The technique is shaped to enable a versatile approach for understanding and ultimately controlling a broad variety of growth processes, and more over it demonstrate how to in situ investigate the influence of single high temperature sputtering parameters on the film quality. It is shown that the whole evolution from nucleation, diffusion adsorption and grain growth to the crystallization can be observed during all stages of thin film growth as well as quantitatively as qualitatively. This can be used to optimize thin-film quality, efficiency and performance.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Acceptor and compensating donor doping of single crystalline SnO (001) films grown by molecular beam epitaxy and its perspectives for optoelectronics and gas-sensing

(La and Ga)-doped tin monoxide [stannous oxide, tin (II) oxide, SnO] thin films were grown by plasma-assisted and suboxide molecular beam epitaxy with dopant concentrations ranging from ≈ 5 × 10 18 to 2 × 10 21 cm -3 . In this concentration range, the incorporation of Ga into SnO was limited by the formation of secondary phases observed at 1.2 × 10 21 cm -3 Ga, while the incorporation of La showed a lower solubility limit. Transport measurements on the doped samples reveal that Ga acts as an acceptor and La as a compensating donor. While Ga doping led to an increase in the hole concentration from 1 × 10 18 -1 × 10 19 cm -3 for unintentionally doped (UID) SnO up to 5 × 10 19 cm -3 , La-concentrations well in excess of the UID acceptor concentration resulted in semi-insulating films without detectable n-type conductivity. Ab initio calculations qualitatively agree with our dopant assignment of Ga and La and further predict In Sn to act as an acceptor as well as Al Sn and B Sn as donors. These results show the possibilities of controlling the hole concentration in p-type SnO, which can be useful for a range of optoelectronic and gas-sensing applications.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Effect of substrate and growth method on vanadium dioxide thin films by RF magnetron sputtering: Vanadium metal oxidation vs reactive sputtering

Vanadium dioxide (VO 2 ) undergoes a metal–insulator phase transition at ~70 °C and has attracted substantial interest for potential applications in electronics, including those in neuromorphic computing. The vanadium–oxygen system has a rather complicated phase diagram, and controlling the stoichiometry and the phase of thin films of vanadium oxides is a well-known challenge. Here, we explore the novel combination of two methods of VO 2 thin film deposition using off-axis RF magnetron sputtering on (100)- and (111)-oriented yttria-stabilized zirconia (YSZ) substrates: reactive sputtering of vanadium in an oxygen environment and sputtering of vanadium metal followed by oxidation to VO 2 . Interestingly, the reactive sputtering process on both substrate orientations yields the metastable semiconducting VO 2 (B) phase, which is structurally stabilized by the YSZ surface. The metal sputtering and oxidation process on YSZ produces mainly the equilibrium monoclinic (or M1) phase of VO 2 that exhibits a metal–insulator transition. Using this method, we obtained thin films of (010)-textured polycrystalline VO 2 (M1) that show a metal–insulator transition with an on/off ratio larger than 1000.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Codesigning Alloy Compositions of CdSe y Te 1− y Absorbers and Mg x Zn 1− x O Contacts to Increase Solar Cell Efficiency

Thin‐film solar cells such as CdTe are a major commercial photovoltaic technology, with more than 25 GW installed worldwide and levelized costs of electricity competitive with fossil fuels. Further progress may result from integrating CdSe y Te 1− y absorbers with Mg x Zn 1− x O contacts, but the device efficiency is difficult to maximize due to coupled dependence on chemical composition of both alloys. Herein, a high‐throughput approach is demonstrated to codesign chemical compositions in alloyed Mg x Zn 1− x O/CdSe y Te 1− y thin‐film solar cells, using combinatorial libraries of PV devices with orthogonal composition gradients in CdSe y Te 1− y absorbers and Mg x Zn 1− x O contacts. It is found that the solar cell performance is a strong and coupled function of both elemental compositions, with efficiency up to 17.7% ( V OC = 836 mV, fill factor = 69%, J SC = 30.6 mA cm −2 ) at atomic compositions of Mg/(Mg + Zn) ≈18% and average Se/(Se + Te) ≈4%. These performance trends among >100 devices are explained by >100 ns lifetime of photoexcited charge carriers at the Mg x Zn 1− x O/CdSe y Te 1− y interface where strong Se accumulation is also observed. This study reports the optimal compositions of the commercially relevant Mg x Zn 1− x O/CdSe y Te 1− y solar cells and demonstrates a general approach to codesigning performance of alloyed thin‐film solar cells and other optoelectronic devices.

14 SOLAR ENERGY↗
DOE OSTI2025

Local structure of zinc–indium–tin oxide films via grazing-incidence x-ray pair-distribution functions and theoretical methods

A detailed experimental and theoretical study on the local (r ≤ 4.5 Å) atomic structure of amorphous and crystalline zinc–indium–tin oxide (ZITO) thin films using grazing-incidence x-ray Pair-Distribution Functions (PDFs), ab initio Molecular Dynamics (MD), and Empirical Potential Structure Refinement (EPSR) Monte Carlo simulations is presented. High-energy synchrotron x rays, a two-dimensional detector, and different incident angles were used to probe the depth uniformity of five (ZnO) 0.15 (In 2 O 3 ) 0.70 (SnO 2 ) 0.15 films that were deposited via pulsed-laser deposition at growth temperatures (T G ) ranging from 25 to 300 °C. Films deposited at T G ≤ 150 °C were amorphous. The partially crystalline (T G = 200 °C) and fully crystalline (T G = 300 °C) films were highly textured. Both crystalline and amorphous structures were investigated using ab initio MD and EPSR Monte Carlo simulations. The density of the amorphous films determined from the experimental data agreed with MD calculations. Coordination numbers, bond lengths, and distortion for metal–oxygen and for both the edge- and corner-shared In–metal shells up to 4.5 Å obtained from PDF analysis closely agreed with MD and EPSR simulations. There is a pronounced decrease in the edge- and corner-shared In–Zn distances arising from the shorter Zn–O bond length, Zn–O tetrahedral coordination, and In–O–Zn angle in amorphous ZITO compared to its crystalline counterpart. A maximum in electrical mobility was observed for the amorphous film just before crystallization occurred. While the peak is broad, consistent with nearly unchanged overall cation–oxygen coordination in the amorphous films, ESPR results indicate that the tetrahedral coordination follows the conductivity trend.

Grazing Incidence X-ray↗
DOE OSTI2025

Understanding how defects and dopant atoms in copper surface oxides affect reactivity

Copper and its oxides are key catalytic materials, on which reactions often occur at the metal/oxide interface. Here, in this work, we directly connect the induction period observed during methanol-driven reduction of thin-film copper oxides to their atomic-scale structural order. Using temperature-programmed desorption (TPD) methanol titrations combined with scanning tunneling microscopy, we show that highly ordered oxide phases – particularly the “29” structure with its low defect density – exhibit long induction periods and initially low reactivity. The induction period, defined as the number of methanol TPD cycles required to reach half of the maximum formaldehyde yield, scales with oxide order and oxygen coverage. Enhanced reactivity of well-ordered oxides emerges only after repeated methanol adsorption/desorption cycles generate oxygen vacancies and new Cu(111)/Cu x O interfacial sites. In contrast, disordered or sub-stoichiometric oxides, which contain more intrinsic defects and interfaces, are active from the first TPD cycle. We further examine how dilute Pt and Rh dopants influence oxide order and reactivity: 1% Pt increases defect density and catalytic activity, while 1% Rh promotes oxide ordering and longer induction periods. These findings demonstrate that dilute alloying provides a potential method for tuning the structure and reactivity of Cu(111)/Cu x O interfaces.

Cu(111)Methanol oxidation↗
DOE OSTI2022

Surface stability of SrNbO 3+δ grown by hybrid molecular beam epitaxy

4d transition metal oxides have emerged as promising materials for numerous applications including high mobility electronics. SrNbO 3 is one such candidate material, serving as a good donor material in interfacial oxide systems and exhibiting high electron mobility in ultrathin films. However, its synthesis is challenging due to the metastable nature of the d 1 Nb 4+ cation and the limitations in the delivery of refractory Nb. To date, films have been grown primarily by pulsed laser deposition (PLD), but development of a means to grow and stabilize the material via molecular beam epitaxy (MBE) would enable studies of interfacial phenomena and multilayer structures that may be challenging by PLD. To that end, SrNbO 3 thin films were grown using hybrid MBE for the first time using a tris(diethylamido)(tert-butylimido) niobium precursor for Nb and an elemental Sr source on GdScO 3 substrates. Varying thicknesses of insulating SrHfO 3 capping layers were deposited using a hafnium tert-butoxide precursor for Hf on top of SrNbO 3 films to preserve the metastable surface. Grown films were transferred in vacuo for x-ray photoelectron spectroscopy to quantify elemental composition, density of states at the Fermi energy, and Nb oxidation state. Ex situ studies by x-ray absorption near edge spectroscopy and scanning transmission electron microscopy illustrate that the SrHfO 3 capping plays an important role in preserving the crystalline quality of the material and the Nb 4d 1 metastable charge state under atmospheric conditions.

36 MATERIALS SCIENCE↗