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At least 289 records · Page 16

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NASA NTRS2007

Thin Film Ceramic Strain Sensor Development for Harsh Environments

The need to consider ceramic sensing elements is brought about by the temperature limits of metal thin film sensors in propulsion system applications. In order to have a more passive method of negating changes of resistance due to temperature, an effort is underway at NASA GRC to develop high temperature thin film ceramic static strain gauges for application in turbine engines, specifically in the fan and compressor modules on blades. Other applications include on aircraft hot section structures and on thermal protection systems. The near-term interim goal of this research effort was to identify candidate thin film ceramic sensor materials to test for viability and provide a list of possible thin film ceramic sensor materials and corresponding properties to test for viability. This goal was achieved by conducting a thorough literature search for ceramics that have the potential for application as high temperature thin film strain gauges chemically and physically compatible and selecting potential candidate materials for with NASA GRC's microfabrication procedures and substrates.

Fralick, Gustave C.↗

NASA NTRS1993

The effect of fluctuations on the electrical transport behavior in YBa2Cu3O(7-x)

The excess conductivity behavior of highly oriented YBa2Cu3O(7-x) thin films prepared by both coevaporation and laser ablation was studied in detail in the reduced-temperature range 9 x 10(exp -4) is less than t is less than 1. The excess conductivity in all the films studied was found to diverge sharply near T(sub c), in agreement with the conventional mean-field theory. However, the detailed temperature dependence could not be fitted to either the power-law or the logarithmic functional forms as predicted by the theory. The excess conductivity of all the films was found to be exponentially dependent on the temperature over nearly three decades for 9 x 10(exp -4) is less than t is less than 10(exp -1), in contradiction to the mean-field theory.

Vitta, Satish↗

DOE OSTI2025

Origin of Stabilization of Ligand-Centered Mixed Valence Ruthenium Azopyridine Complexes: DFT Insights for Neuromorphic Applications

Redox-driven conductance changes are critical processes in molecular- and coordination-complex-based memristive thin films and devices that are envisioned for neuromorphic technologies, but fundamental mechanisms of conductance switching are not fully understood. Here, we explore charge disproportionation (CD) processes in [Ru II L 2 ](PF 6 ) 2 molecular systems that intrinsically involve interfragment charge transfer (IFCT). Using a combination of ab initio molecular dynamics simulation (AIMD), time-dependent density functional theory (TD-DFT), and density functional theory (DFT) calculations, we investigate the electron transfer mechanisms and the roles of temperature and cell volumetric expansion in facilitating the counterion movements and electronic transitions required for low-cost IFCT and charge redistribution. A detailed analysis of the density of states and TD-DFT calculations highlights that unpaired electrons play a crucial role in low-energy transitions, with the azo (N=N) groups of the ligand serving as the primary sites for electronic transport between molecular fragments, further stabilizing the asymmetric state. Localization of added electrons on azo ligands occurs with negligible change at the Ru centers, supported by atomic volume expansions up to +4.74 bohr 3 , and goes along with a progressive reduction of the HOMO−LUMO gap across redox states, suggesting enhanced conductivity. The TD-DFT analysis reveals a dominant IFCT excitation at 2082.76 nm in the doubly reduced (22) state, while a stabilization energy of 1.20 eV of the asymmetric (13) state relative to the symmetric (22) state is predicted by constrained DFT. Periodic DFT and AIMD simulations emulating a molecular film show that the stabilization of the asymmetric state, relative to a symmetric one, translates in net charge separation values (order of ∼0.33 e) that are strongly linked to increased counterion mobility (average counterion displacements exceeding 0.7 Å per atom during CD events) and the involvement of azo groups in electron redistribution. These findings, which align with previously reported experimental and computational data, provide key insights into the IFCT mechanisms and electronic transport facilitated by azo groups, with important implications for redox-driven memristive and neuromorphic technologies.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2023

Enhanced Photostability of “Hollow” Mixed Halide Wide-Bandgap Perovskite Films

We investigated the role of hollow perovskite architectures in enhancing the photostability of mixed halide wide-bandgap perovskites. We focused on mitigating photoluminescence (PL) peak shifts caused by phase segregation when exposed to light. By analyzing the optical and structural properties of mixed bromide/iodide hollow perovskite thin films, we observed that the incorporation of hollow structures reduced the ionic conductivity in the films, leading to improved photostability compared to non-hollow perovskite samples. The mixed halide hollow perovskite thin films exhibited increased the bandgap. High-power laser irradiation was used to induce phase segregation, and changes in the PL emission spectra were measured as a function of irradiation time. The mixed halide hollow perovskite thin films exhibited reduced PL peak shifts compared to the control samples. The inclusion of enI 2 (en = ethylenediamine) resulted in a reduction in the overall ionic conductivity of the films and a lower trap density. Hollow perovskite films incorporated in solar cells indicated that while the initial efficiency of the solar cells decreased with increasing enI 2 concentration, the open-circuit voltage value increased, potentially due to the slight enhancement of the band gap. Furthermore, the findings highlight the potential of hollow perovskite architectures in enhancing the photostability of mixed halide perovskites

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2024

Increased thermal conductivity and decreased electron–phonon coupling factor of the aluminum scandium intermetallic phase (Al 3 Sc) compared to solid solutions

Aluminum scandium alloys and their intermetallic phases have arisen as potential candidates for the next generation of electrical interconnects. Here, in this work, we measure the in-plane thermal conductivity and electron–phonon coupling factor of aluminum scandium alloy thin films deposited at different temperatures, where the temperature is used to control the grain size and volume fraction of the Al 3 Sc intermetallic phase. As the Al 3 Sc intermetallic formation increases with higher deposition temperature, we measure increasing in-plane thermal conductivity and a decrease in the electron–phonon coupling factor, which corresponds to an increase in grain size. Our findings demonstrate the role that chemical ordering from the formation of the intermetallic phase has on thermal transport.

Chemical elements↗

DOE OSTI2025

Tailoring optical response of MXene thin films

Due to their attractive optical properties, 2D MXenes have garnered interest in nanophotonic and optoelectronic applications. However, tuning their properties typically requires the iterative synthesis of MXenes with a specific set of properties, such as the absorption band position, electronic conductivity, and dielectric constant. We demonstrate how to tailor the optical properties of MXene thin films over a broad 1500-nm wavelength range by mixing different ratios of highly conductive Ti 3 C 2 T x with poorly conductive Nb 2 CT x . By changing the MXene film composition, the epsilon-near-zero (ENZ) point, where the optical properties transit from dielectric to metallic, was varied in the spectral range from 1.1 to 2.6 µm. Additionally, we observed a reduction in absorption in some compositions compared to the absorption of the pure MXene films. Compared to other methods, this approach enables simple and continual tuning of MXene optical properties without requiring multiple time-consuming synthesis steps.

2D materials↗

DOE OSTI2025

Delineating Confinement Regimes for Nafion Thin Films via Simultaneous QCM-D and Spectroscopic Ellipsometry

Proton-conducting ionomers used in electrodes of electrochemical devices form nanometer-scale films covering metallic catalyst surfaces, wherein they experience confinement and interfacial effects absent in the bulk polymer. This confinement alters the physical properties of the ionomer film, which is postulated to increase the density and is attributed to the transport limitations observed in fuel-cell electrodes. Despite studies showing reduced swelling, no systematic measurement has validated this phenomenon by demonstrating both densification and stiffening as the film gets thinner. Here, this study aims to fill this gap by reporting the humidity-driven changes in swelling, mass uptake, and density of Nafion ionomer films cast at varying thicknesses (10–320 nm) onto a gold-plated sensor. The films were simultaneously probed during humidification using spectroscopic ellipsometry (SE) and a quartz crystal microbalance with dissipation (QCM-D), which allowed for determination of the density and shear stiffness. The effects of confinement were prominently observed below ∼30 nm, with films showing increased density along with decreased swelling and mass uptake during hydration. In addition, the confined films show a negative excess volume upon initial water sorption, implying a larger proportion of absorbed water might bound to ionic groups in accord with notion of localized densification.

Espinet, Kevin B. [University of California, Berke↗

NASA NTRS2004

Charge Storage, Conductivity and Charge Profiles of Insulators as Related to Spacecraft Charging

Dissipation of charges built up near the surface of insulators due to space environment interaction is central to understanding spacecraft charging. Conductivity of insulating materials is key to determine how accumulated charge will distribute across the spacecraft and how rapidly charge imbalance will dissipate. To understand these processes requires knowledge of how charge is deposited within the insulator, the mechanisms for charge trapping and charge transport within the insulator, and how the profile of trapped charge affects the transport and emission of charges from insulators. One must consider generation of mobile electrons and holes, their trapping, thermal de-trapping, mobility and recombination. Conductivity is more appropriately measured for spacecraft charging applications as the "decay" of charge deposited on the surface of an insulator, rather than by flow of current across two electrodes around the sample. We have found that conductivity determined from charge storage decay methods is 102 to 104 smaller than values obtained from classical ASTM and IEC methods for a variety of thin film insulating samples. For typical spacecraft charging conditions, classical conductivity predicts decay times on the order of minutes to hours (less than typical orbit periods); however, the higher charge storage conductivities predict decay times on the order of weeks to months leading to accumulation of charge with subsequent orbits. We found experimental evidence that penetration profiles of radiation and light are exceedingly important, and that internal electric fields due to charge profiles and high-field conduction by trapped electrons must be considered for space applications. We have also studied whether the decay constants depend on incident voltage and flux or on internal charge distributions and electric fields; light-activated discharge of surface charge to distinguish among differing charge trapping centers; and radiation-induced conductivity. Our experiments also show that "Malter" electron emission occurs for hours after turning off the electron beam. This Malter emission similar to emission due to negative electron affinity in semiconductors is a result of the prior radiation or optical excitations of valence electrons and their slow drift among traps towards the surface where they are subsequently emitted. This work is supported through funding from the NASA Space Environments and Effects Program.

Dennison, J. R.↗

DOE OSTI2022

Nanoscale laser flash measurements of diffuson transport in amorphous Ge and Si

The thermal properties of amorphous materials have attracted significant attention due to their technological importance in electronic devices. In addition, the disorder-induced breakdown of the phonon gas model makes vibrational transport in amorphous materials a topic of fundamental interest. In the past few decades, theoretical concepts, such as propagons, diffusons, and locons, have emerged to describe different types of vibrational modes in disordered solids. However, experiments can struggle to accurately determine which types of vibrational states carry the majority of the heat. In this study, we use nanoscale laser flash measurements (front/back time-domain thermoreflectance) to investigate thermal transport mechanisms in amorphous Ge and amorphous Si thin-films. We observe a nearly linear relationship between the amorphous film’s thermal resistance and the film’s thickness. The slope of the film’s thermal resistance vs thickness corresponds to a thickness-independent thermal conductivity of 0.4 and 0.6 W/(m K) for a-Ge and a-Si, respectively. This result reveals that the majority of heat currents in amorphous Si and Ge thin films prepared via RF sputtering at room temperature are carried by diffusons and/or propagons with mean free paths less than a few nanometers.

36 MATERIALS SCIENCE↗

DOE OSTI2020

Reversible Interlayer Sliding and Conductivity Changes in Adaptive Tetrathiafulvalene-Based Covalent Organic Frameworks

Ordered interlayer stacking is intrinsic in two-dimensional covalent organic frameworks (2D COFs) and has strong implications on COF's optoelectronic properties. Reversible interlayer sliding, corresponding to shearing of 2D layers along their basal plane, is an appealing dynamic control of both structures and properties, yet it remains unexplored in the 2D COF field. In this paper, we demonstrate that the reversible interlayer sliding can be realized in an imine-linked tetrathiafulvalene (TTF)-based COF TTF-DMTA. The solvent treatment induces crystalline phase changes between the proposed staircase-like sql net structure and a slightly slipped eclipsed sql net structure. The solvation-induced crystallinity changes correlate well with reversible spectroscopic and electrical conductivity changes as demonstrated in oriented COF thin films. In contrast, no reversible switching is observed in a related TTF-TA COF, which differs from TTF-DMTA in terms of the absence of methoxy groups on the phenylene linkers. This work represents the first 2D COF example of which eclipsed and staircase-like aggregated states are interchangeably accessed via interlayer sliding, an uncharted structural feature that may enable applications such as chemiresistive sensors.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2024

Parasitic conduction loss of lithium niobate on insulator platform

Thin-film lithium niobate on insulator (LNOI) has recently emerged as an exciting platform for nonlinear integrated photonic devices, drawing parallels with the impact of silicon on insulator (SOI) in the electronic device realm. While the influence of interfacial states of SOI on electronic device performance, particularly their contribution to parasitic radio frequency (RF) losses, is well-documented, this aspect remains relatively unexplored in LNOI, despite their shared silicon/oxide interface structure. Here, we investigate the RF loss arising from mobile charges at the silicon/oxide interface of the LNOI platform. We show that this parasitic conduction can be exacerbated, especially during plasma etching, and leads to substantial RF losses. Through strategic incorporation of a trap-rich poly-silicon layer at this interface, we effectively eliminate this parasitic conduction, resulting in the suppression of RF losses. This work establishes crucial guidelines for harnessing LNOI in applications such as high-frequency electro-mechanical devices, high-speed electro-optic modulators, and high-efficiency microwave-to-optics converters.

Physics↗

NASA NTRS1991

Characterization of devices, circuits, and high-temperature superconductor transmission lines by electro-optic testing

The development of a capability for testing transmission lines, devices, and circuits using the optically-based technique of electro-optics sampling was the goal of this project. Electro-optic network analysis of a high-speed device was demonstrated. The project involved research on all of the facets necessary in order to realize this result, including the discovery of the optimum electronic pulse source, development of an adequate test fixture, improvement of the electro-optic probe tip, and identification of a device which responded at high frequency but did not oscillate in the test fixture. In addition, during the process of investigating patterned high-critical-temperature superconductors, several non-contacting techniques for the determination of the transport properties of high T(sub c) films were developed and implemented. These are a transient, optical pump-probe, time-resolved reflectivity experiment, an impulsive-stimulated Raman scattering experiment, and a terahertz-beam coherent-spectroscopy experiment. The latter technique has enabled us to measure both the complex refractive index of an MgO substrate used for high-T(sub c) films and the complex conductivity of a YBa2Cu3O(7-x) sample. This information was acquired across an extremely wide frequency range: from the microwave to the submillimeter-wave regime. The experiments on the YBCO were conducted without patterning of, or contact to, the thin film. Thus, the need for the more difficult transmission-line experiments was eliminated. Progress in all of these areas was made and is documented in a number of papers. These papers may be found in the section listing the abstracts of the publications that were issued during the course of the research.

Whitaker, John F.↗

DOE OSTI2021

Influence of Heterointerfaces on the Kinetics of Oxygen Surface Exchange on Epitaxial La 1.85 Sr 0.15 CuO 4 Thin Films

Considerable attention has been directed to understanding the influence of heterointerfaces between Ruddlesden–Popper (RP) phases and ABO 3 perovskites on the kinetics of oxygen electrocatalysis at elevated temperatures. Here, we report the effect of heterointerfaces on the oxygen surface exchange kinetics by employing heteroepitaxial oxide thin films formed by decorating LaNiO 3 (LNO) on La 1.85 Sr 0.15 CuO 4 (LSCO) thin films. Regardless of LNO decoration, tensile in-plane strain on LSCO films does not change. The oxygen surface exchange coefficients (kchem) of LSCO films extracted from electrical conductivity relaxation curves significantly increase with partial decorations of LNO, whereas full LNO coverage leads to the reduction in the kchem of LSCO films. The activation energy for oxygen exchange in LSCO films significantly decreases with partial LNO decorations in contrast with the full coverage of LNO. Optical spectroscopy reveals the increased oxygen vacancies in the partially covered LSCO films relative to the undecorated LSCO film. We attribute the enhanced oxygen surface exchange kinetics of LSCO to the increased oxygen vacancies by creating the heterointerface between LSCO and LNO.

25 ENERGY STORAGE↗

DOE OSTI2023

Anisotropic topological surface states in thin-film monoclinic Ag 2 Se

Silver chalcogenide systems have recently attracted significant attention due to their promising topological insulating properties. Here we conducted systematic low-temperature local scanning tunneling microscopy/spectroscopy and first-principles studies on the surface states of monoclinic Ag 2 Se thin films grown using molecular beam epitaxy. Through the use of quasi-particle interference patterns, we have observed evidence for topological surface states on the selenium-terminated surfaces with different types of defect densities. The results of ab initio calculations confirm the existence of nontrivial topological surface states in the monoclinic Ag 2 Se structure, for which such properties have not been previously reported. The energy dispersion determined using voltage-dependent standing wave patterns suggests that these topological states have an anisotropic Dirac cone structure. In conclusion, this discovery may lead to new applications for monoclinic Ag 2 Se in the rapidly growing fields of nanoelectronics and spintronics.

36 MATERIALS SCIENCE↗

DOE OSTI2022

3ω techniques for measurement of volumetric heat capacity and anisotropic thermal conductivity of a solution processable, hybrid organic/inorganic film, Te-PEDOT:PSS

We report that measuring the thermal properties of anisotropic films of hybrid materials poses a challenge to existing metrology techniques. We have developed a new approach for measuring the volumetric heat capacity and anisotropic thermal conductivity of these systems using the 3ω method. While there exist many avenues for measuring the thermal properties of thin films, most carry with them difficult requirements such as smooth surfaces or advanced lithography. Here, we present measurements of a film's in-plane and cross-plane conductance and its volumetric heat capacity using relatively simple sample configurations, each requiring a single heater. For the measurement of volumetric heat capacity, we present a new model fitting method, relying on a standard film-on-substrate configuration. For the measurement of in-plane thermal conductance by 3ω, we have developed the use of an embedded micro-wire heater in suspended drop cast films, allowing for a 12 μm wide heater without the need for advanced lithography. We also expose the surprisingly significant effect of thermal radiation in the suspended film measurement and its associated error. Our measurements reveal a large anisotropy in the thermal conductivity of our test material, Te-PEDOT:PSS, of k in-plane / k cross-plane = 19, consistent with the nanoscale morphology of the material.

36 MATERIALS SCIENCE↗

DOE OSTI2025

Strain-driven oxygen vacancy ordering in LaNiO 3 thin films revealed by integrated differential phase contrast imaging in scanning transmission electron microscopy

Rare-earth nickelates, such as LaNiO 3 (LNO), exhibit complex electronic properties, with ordered oxygen vacancies (OOV) influencing conductivity and magnetic behavior. We investigate the structural stability of strain-induced OOV phases in LNO thin films grown on SrTiO 3 substrates and the impact of Ruddlesden–Popper (RP) faults. Using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and integrated differential phase contrast (iDPC) STEM imaging, we conducted atomic-scale structural and compositional analyses of OOV. Geometric phase analysis (GPA) was employed to measure the strain in fault-free and RP fault regions, while density functional theory (DFT) calculations explored different OOV arrangements in the LNO phase. Simulated iDPC-STEM imaging of energy-stabilized structures was performed to correlate with experimental results. Here, our findings reveal superstructure modulation in the chemical composition and atomic-scale lattice structure in LNO, primarily due to the formation of the OOV in Ni–O layers of the LaNiO 2.5 phase. The out-of-plane compressive strain of about 2% stabilizes this phase, reducing the strain, diminishing OOV, and transforming them into LNO.

36 MATERIALS SCIENCE↗

DOE OSTI2022

Diffraction and thermal effect of a Bessel-Gaussian laser for Ag nanoparticle deposition

Nanoparticles are known to sinter at much lower temperatures than the corresponding bulk or micro size particles. A laser-assisted sintering process is considered in this study to sinter Ag nanoparticles by dispensing Ag paste onto an indium tin oxide-coated Si substrate. The Gaussian beam of a CO 2 laser source is propagated through axicon and biconvex lenses, and the resulting hollow beam is focused on the Ag paste with a hollow parabolic mirror. A Bessel-Gaussian irradiance distribution is obtained at the focal plane of the parabolic mirror due to the interference of the hollow laser cone. The Fresnel diffraction approximation is considered to determine the phasor of the laser and an analytical approach is implemented to calculate the irradiance distribution of the Bessel-Gaussian beam. This irradiance distribution is utilized as a heat source in a heat conduction model and the temperature distribution is analyzed for thin Ag films formed during the laser sintering of Ag nanoparticles. An analytical expression is obtained for the temperature distribution by solving the heat conduction equation using Fourier transform for finite media. The widths of the deposited Ag lines are predicted from the temperature profiles and the model predictions compare well with the experimental results. The isotherms are found to be geometrically noncongruent with convex and concave tips depending on the locally maximum and minimum irradiances of the Bessel-Gaussian beam, respectively. The convex and concave tips, however, appear in the same isotherm for sufficiently high substrate speed relative to the laser beam.

77 NANOSCIENCE AND NANOTECHNOLOGY↗

DOE OSTI2026

Powder‐to‐Film Conversion of Nickel Single‐Atom Catalysts into Binder‐Free and Resistant Electrodes

Although a few binder-free and self-supported single-atom electrodes have been reported, achieving mechanically robust, defect-engineered, and reproducible films that preserve atomic dispersion under electrochemical operation remains challenging. This work addresses this limitation by presenting a versatile and generalizable strategy to transform powders into standalone, defect-engineered thin films hosting atomically dispersed Ni centers within conductive 2D frameworks. The physicochemical and electronic properties of these materials are thoroughly characterized using a comprehensive set of spectroscopic and microscopic techniques and confirmed the homogeneous dispersion and monoatomic nature of the Ni centers (0.94 wt.%) on the electrode films. Electrochemical testing via cyclic voltammetry and electrochemical impedance spectroscopy under a range of experimental conditions revealed that integration of Ni single atoms markedly enhanced performance and stability compared to carbon nanotube-only electrodes, maintaining integrity after 15 h of continuous operation. This improvement is accompanied by a notable reduction in charge transfer resistance (30.50 Ω) and an increase in double-layer capacitance (295.45 µF). Post-electrochemical analyses corroborated the structural integrity and robustness of the electrodes. Overall, this work bridges atomically precise catalysis and device-level electrochemistry, opening a route toward reproducible and scalable single-atom electrodes for sensing and energy conversion.

36 MATERIALS SCIENCE↗