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At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Surface equilibration mechanism controls the molecular packing of glassy molecular semiconductors at organic interfaces

Significance Because glasses are nonequilibrium materials, even a single-component system has an enormous number of distinct glass structures. Physical vapor deposition (PVD) is an important tool for preparing a wide range of these glasses. While the substrate can influence structure for hundreds of nanometers for crystalline and liquid crystalline systems, little is known about the structure of PVD glasses near an underlying substrate. This is important for organic electronics, as layers of PVD glasses as thin as 10 nm are used as active elements in organic light-emitting diodes, and molecular packing is vital to performance. We show that the free surface controls molecular packing of PVD glasses near a buried interface rather than the substrate, in sharp contrast with crystalline systems.

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

Long-Term Stability of PVD/Sputtered Au Electrical Contacts to Thermoelectric Bi 2 Te 2 : Re-evaluation of SAND2015-7203

The fabrication of long-lived electrical contacts to thermoelectric Bi 2 Te 3 -based modules is a challenging problem due to chemical incompatibilities and rapid diffusion rates. Previously, technical guidance from SAND report 2015-7203 selected electroplated Au as the preferred method for fabrication of long-lived contacts because of concerns that the grain structure of sputtered/physical vapor deposited (PVD) Au contacts can evolve during aging. We have re- evaluated PVD Au contacts and show that they are appropriate for long-life service. We measure grain size and morphology at different aging times under accelerated temperature gradient conditions, and we show that the PVD Au contacts are stable and remain relatively unchanged. The PVD Au fabricated here is not subject to the deterioration observed in the previous report.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Long-Term Stability of PVD/Sputtered Au Electrical Contacts to Thermoelectric Bi 2 Te 2 (Re-evaluation of SAND2015-7203)

The fabrication of long-lived electrical contacts to thermoelectric Bi 2 Te 3 -based modules is a challenging problem due to chemical incompatibilities and rapid diffusion rates. Previously, technical guidance from SAND report 2015-7203 selected electroplated Au as the preferred method for fabrication of long-lived contacts because of concerns that the grain structure of sputtered/physical vapor deposited (PVD) Au contacts can evolve during aging. We have re-evaluated PVD Au contacts and show that they are appropriate for long-life service. We measure grain size and morphology at different aging times under accelerated temperature gradient conditions, and we show that the PVD Au contacts are stable and remain relatively unchanged. The PVD Au fabricated here is not subject to the deterioration observed in the previous report.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Growth of Large‐Sized 2D Ultrathin SnSe Crystals with In‐Plane Ferroelectricity

Abstract Tin (II) selenide (SnSe) is an emerging 2D material with many intriguing properties, such as record‐high thermoelectric figure of merit (ZT), purely in‐plane ferroelectricity, and excellent nonlinear optical properties. To explore these functional properties and related applications, a crucial step is to develop controllable routes to synthesize large‐area, ultrathin, and high‐quality SnSe crystals. Physical vapor deposition (PVD) constitutes a reliable method to synthesize 2D SnSe, however, effects of various growth parameters have not yet been systematically investigated, and current PVD‐synthesized flakes are often thick (>10 nm) with small lateral sizes (<10 µm). In this work, high‐quality 2D SnSe crystals are synthesized via low‐pressure PVD, which display in‐plane ferroelectric domains observed by piezoresponse force microscopy and polarization‐dependent reflection spectroscopy. Detailed studies regarding the roles of various parameters are further carried out, including substrate pre‐annealing, growth duration, temperature, and pressure, which enable to rationally optimize the growth and obtain 2D SnSe crystals with lateral sizes up to ≈23.0 µm and thicknesses down to ≈2.0 nm (3–4 layers). This work paves the way for the controlled growth of large‐area 2D SnSe, facilitating the future exploration of many interesting multiferroic properties and applications with atomic thickness.

2D materials↗
DOE OSTI2023

Surface Equilibration Mechanism Controls the Stability of a Model Codeposited Glass Mixture of Organic Semiconductors

While previous work has identified the conditions for preparing ultrastable single-component organic glasses by physical vapor deposition (PVD), little is known about the stability of codeposited mixtures. Here, we prepared binary PVD glasses of organic semiconductors, TPD (N,N'-Bis(3-methylphenyl)-N,N'-diphenylbenzidine) and m-MTDATA (4,4',4"-Tris[phenyl(m-tolyl)amino]triphenylamine), with a 50:50 mass concentration over a wide range of substrate temperatures (T sub ). The enthalpy and kinetic stability are evaluated with differential scanning calorimetry and spectroscopic ellipsometry. Binary organic semiconductor glasses with exceptional thermodynamic and kinetic stability comparable to the most stable single-component organic glasses are obtained when deposited at T sub = 0.78–0.90T g (where T g is the conventional glass transition temperature). When deposited at 0.94T g , the enthalpy of the m-MTDATA/TPD glass equals that expected for the equilibrium liquid at that temperature. Thus, the surface equilibration mechanism previously advanced for single-component PVD glasses is also applicable for these codeposited glasses. Furthermore, these results provide an avenue for designing high-performance organic electronic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Metastable multimeric G-quadruplex 2′FY-RNA aptamers that selectively bind pyoverdines

Two 2′FY-RNA aptamers with distinct sequences were selected for specific binding to pyoverdine-Pf5 (PVD-Pf5), increasing chromophore fluorescence upon binding. They also recognized the peptide portion of pyoverdines, as shown by their differential specificity for related variants. Computational analysis and experimental data (NMM binding, CD spectra) identified G-quadruplex structures that were thermally metastable but reformed in the presence of PVD-Pf5. Further structural studies mainly with one aptamer revealed imino proton peaks in 1D H-NMR and pressure stability up to 2 kbar. Electrophoretic evidence identified dimeric G-quadruplexes formed by the 2′FY-RNA aptamers and their RNA equivalents. While cations were necessary for PVD-Pf5 binding, they were not required for G-quadruplex formation. Given the established role of G-quadruplexes as protein interaction sites, multimeric G-quadruplexes offer a potential framework for structure-based regulatory mechanisms in cellular RNAs. In addition to previously characterized multimeric G-quadruplexes, these aptamers contribute novel sequences that expand the repertoire of known multimeric G-quadruplexes.

2′FY-RNA↗
DOE OSTI2024

Synthesis, microstructure and micro-mechanical characterization of metal (Nb, Ti) – MAX phase (Ti 2 AlC) nanolaminates

We utilize elevated temperature physical vapor deposition (PVD) techniques to design metal/MAX multilayered nanocomposite thin films with alternating nanoscale metallic (Nb, Ti) and MAX phase (Ti 2 AlC) layer thicknesses. These metal/MAX nanolaminate architectures attempt to exploit a unique hierarchical topology – as interfaces between the layers are expected to be in direct competition with the internal interfaces within the MAX layers, to drive their tunable macroscopic mechanical behavior. Two metal/MAX nanolaminates – Nb/Ti 2 AlC and Ti/Ti 2 AlC – were deposited. The Nb/Ti 2 AlC metal/MAX system showed highly diffused layer interfaces with distinct Ti – rich and Nb–Al – rich layers, with the presence of MAX phase alongside TiC and other Ti–Al and Nb–Al intermetallic phases. The Nb/Ti 2 AlC system possessed a layered architecture, though the MAX phases were not found to be continuously present in each alternating layer. The second Ti/Ti 2 AlC system showed a non-lamellar nanocomposite microstructure and the formation of mixed Ti n+1 AlC n phases (a mix of n = 1, 2), and no indication of layering. Diffusion occurring between the metal/MAX layers in both cases, likely due to the elevated temperatures during the deposition process, is speculated as the likely cause of these resultant microstructures. The mechanical properties of both systems were evaluated using micromechanical (nanoindentation and micro-pillar compression) techniques, which demonstrated high strengths for both systems (Nb system: yield and instability strengths of 4.88 ± 0.1 GPa and 5.57 ± 0.03 GPa, Ti system: yield and instability strength of 5.61 ± 0.28 GPa and 6.21 ± 0.25 GPa). In conclusion, this work highlights the promising mechanical properties of metal/MAX multilayered depositions and summarizes the challenges in PVD synthesis of metal/MAX multilayered nanolaminates.

MAX phase↗
DOE OSTI2025

Investigating the impact of preparation routes on the properties of copper-decorated silicon particles as anode materials for lithium-ion batteries

In recent years, the calendar life of Si has been recognized as a significant issue that must be addressed prior to technology deployment: The carbon conductive additive is a potential source of parasitic side reactions. However, carbon remains essential due to the low electronic conductivity of Si. In this study, we investigate the use of Cu as a conductive additive and potential alternative to carbon. Some Cu-decorated silicon particles (Si Cu ) were prepared using physical vapor deposition (PVD) via sputtering and high-energy milling. Other Si Cu particles were prepared by using a solution method and examined briefly. The milling method caused Cu to appear as island-like features on the Si surface, whereas the PVD method initially produced similar island-like features that gradually developed into a continuous coating around the Si as sputtering time increased. Electrodes fabricated from Si Cu exhibited lower overall resistivity, demonstrating the beneficial effect of Cu in improving electronic percolation through the electrode. Electrochemical tests showed that the milled SiCu exhibited higher capacity retention, improved rate capability, and lower overpotential. Furthermore, Si Cu coupled with an NMC811 cathode exhibited lower leakage currents compared with the baseline silicon, indicating that incorporating Cu provided an additional advantage of minimizing parasitic currents in the cells.

25 ENERGY STORAGE↗
DOE OSTI2024

Characterization of MoS 2 films via simultaneous grazing incidence X-ray diffraction and grazing incidence X-ray fluorescence (GIXRD/GIXRF)

Physical vapor deposited (PVD) molybdenum disulfide (nominal composition MoS 2 ) is employed as a thin film solid lubricant for extreme environments where liquid lubricants are not viable. The tribological properties of MoS 2 are highly dependent on morphological attributes such as film thickness, orientation, crystallinity, film density, and stoichiometry. These structural characteristics are controlled by tuning the PVD process parameters, yet undesirable alterations in the structure often occur due to process variations between deposition runs. Nondestructive film diagnostics can enable improved yield and serve as a means of tuning a deposition process, thus enabling quality control and materials exploration. Grazing incidence X-ray diffraction (GIXRD) for MoS 2 film characterization provides valuable information about film density and grain orientation (texture). However, the determination of film stoichiometry can only be indirectly inferred via GIXRD. The combination of density and microstructure via GIXRD with chemical composition via grazing incidence X-ray fluorescence (GIXRF) enables the isolation and decoupling of film density, composition, and microstructure and their ultimate impact on film layer thickness, thereby improving coating thickness predictions via X-ray fluorescence. We have augmented an existing GIXRD instrument with an additional X-ray detector for the simultaneous measurement of energy-dispersive X-ray fluorescence spectra during the GIXRD analysis. This combined GIXRD/GIXRF analysis has proven synergetic for correlating chemical composition to the structural aspects of MoS 2 films provided by GIXRD. We present the usefulness of the combined diagnostic technique via exemplar MoS 2 film samples and provide a discussion regarding data extraction techniques of grazing angle series measurements.

MoS2↗
DOE OSTI2024

High Density Two-Component Glasses of Organic Semiconductors Prepared by Physical Vapor Deposition

Physical Vapor Deposition (PVD) is widely utilized for the production of organic semiconductor devices due to its ability to form thin layers with exceptional properties. Although the layers in the device usually consist of two or more components, there is limited understanding about the fundamental characteristics of such multi-component vapor-deposited glasses. Here, spectroscopic ellipsometry was employed to characterize the densities, thermal stabilities and optical properties of co-vapor deposited NPD and TPD glasses across the entire range of composition. We find that co-deposited NPD and TPD form high density glasses with enhanced thermal stability. The dependences of density and stability upon substrate temperature are correlated, and the birefringence of the co-deposited glasses is determined by the reduced substrate temperature of mixtures. Additionally, we observe that the transformation of a highly stable and dense two-component glass into its supercooled liquid initiates from the free surface and propagates into the bulk at constant velocity, like single component PVD glasses. As a result, all these features are consistent with the surface equilibration mechanism.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2026

Growth of Low-Defect WSe 2 Film via High-Purity van der Waals Crystal Precursor

Two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) exhibit exceptional electrical and optical properties, empowering their promising prospects for future nanoelectronics. Despite major advances in n-type 2D semiconductors, the field has yet to synthesize high-mobility p-type 2D TMDs, in particular WSe 2 , and systematically query the influence of defects. In this study, we unveil the pivotal role of substitutional impurity defects vis-à-vis the precursor used and growth method employed in defining the quality of 2D p-type WSe 2 . Density functional theory calculations suggest the adverse effect of Fe-, Co-, Ni- and Si-substituted W impurity defects on the mobility of WSe 2 , whereas defects such as O-, S-substituted Se and Mo-substituted W pose negligible impact. Guided by the theory, we pinpoint van der Waals (vdW) crystals, commonly used in mechanical exfoliation, as the optimal precursor, and develop a facile vdW crystal physical vapor deposition (PVD) method to grow high-purity monolayer 2D WSe 2 film (VPVD-WSe 2 ) that is continuous across a centimeter scale. A suite of spectroscopies confirms the markedly reduced defect density of the as-synthesized WSe 2 compared to those by typical chemical vapor deposition methods, and by PVD with commercial or hydrothermal precursors. Scanning tunneling microscopy further evidence the ultralow substitutional impurity defect density of VPVD-WSe 2 , greatly outperforming the control samples and approaching the mechanically exfoliated counterparts. The VPVD-WSe 2 based field-effect transistors exhibit notable electrical performance with record-high field-effect hole mobility up to 112 cm 2 V –1 s –1 at room temperature, exceeding the best-reported monolayer WSe 2 synthesized by chemical vapor deposition and rivaling the mechanically exfoliated 2D WSe 2 flakes.

WSe2↗
DOE OSTI2026

The effect of high-power transient events on tungsten and tungsten coatings used for radio frequency launcher applications

High-temperature plasma-facing material coatings used for radio frequency (RF) launchers need to be robust enough to survive RF breakdown arcing or other transient events from the plasma (e.g., an edge localized mode) without causing a catastrophic failure of the coating. High-power transient effects are being explored by using an RF-induced vacuum arc to determine the robustness of tungsten coatings made by a variety of manufacturing methods. A 1/4-wavelength resonant section of vacuum transmission line terminated with an open circuit electrode structure with a well-defined electric field (30-60 kV/mm) produces repeatable arcing conditions. The initial focus is on tungsten as a plasma-facing material, including sintered tungsten, tungsten coatings on steel produced via physical vapor deposition (PVD), and functionally graded tungsten/steel coatings deposited by low-pressure plasma-spraying (LPPS). Thin PVD coatings (1-2 microns) fail catastrophically from an arc and result in severe delamination of the coating. The arc-induced damage of thicker coatings, such as those made via LPPS, tend to be restricted to the top few microns of the surface. Arcing often initiates on sharp surface microstructures and causes localized melting of tungsten at the surface of all the materials studied and results in resolidified melt pools with surface cracks. The resolidified surface results in a reduction in overall deuterium retention when exposed to typical RF plasma sheath conditions.

Caughman, John [ORNL] (ORCID:0000000206091164)↗
DOE OSTI2023

Vapor-to-glass preparation of biaxially aligned organic semiconductors

Physical vapor deposition (PVD) provides a route to prepare highly stable and anisotropic organic glasses that are utilized in multi-layer structures such as organic light-emitting devices. While previous work has demonstrated that anisotropic glasses with uniaxial symmetry can be prepared by PVD, here, we prepare biaxially aligned glasses in which molecular orientation has a preferred in-plane direction. With the collective effect of the surface equilibration mechanism and template growth on an aligned substrate, macroscopic biaxial alignment is achieved in depositions as much as 180 K below the clearing point TLC−iso (and 50 K below the glass transition temperature Tg) with single-component disk-like (phenanthroperylene ester) and rod-like (itraconazole) mesogens. The preparation of biaxially aligned organic semiconductors adds a new dimension of structural control for vapor-deposited glasses and may enable polarized emission and in-plane control of charge mobility.

Chemistry↗
DOE OSTI2022

Characterization of Nb films with varying RRR values at low temperatures [Poster]

Superconducting properties of Nb in microwave fields vary significantly with lattice defects and impurity content, where sub-at.% impurity level can reduce or increase microwave surface resistance by an order of magnitude. In this study, we studied the microwave properties of Nb films, deposited by different PVD techniques, at low microwave fields correlating microwave properties at dilution fridge temperatures with material properties characterized with surface science techniques. We are studying the microwave properties of Nb films, deposited by different PVD techniques, at low microwave fields and low temperatures. The first Nb film, deposited onto an elliptical niobium cavity by HiPIMS technique at CERN, was tested at low temperatures. The first results show that film performance was similar to that of the bulk niobium cavities.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Vapor Deposition Process for Engineering of Dispersed PEMFC ORR Pt/NbO x /C Catalysts

With support from the Department of Energy, and in partnership with Oak Ridge National Laboratory, Exothermics, Northeastern University, the University of Michigan, and IRD Fuel Cells, Ford has led an effort to use physical vapor deposition (PVD) to produce oxygen reduction reaction (ORR) catalysts for proton exchange membrane fuel cells (PEMFCs). Conventional ORR catalysts are made by wet chemical processes, and experience loss of electrochemical surface area and activity over voltage cycling due to Pt dissolution and Pt particle coalescence. Not only was PVD processing researched as a means to reduce solvent waste and enhance reproducibility in catalyst manufacturing, but it was also perceived as a means to deposit preferential phases of niobium oxide onto the catalyst, which would then serve to prevent Pt particle movement and extend PEMFC lifetime.

08 HYDROGEN↗
DOE OSTI2026

PIE-Enabled Study of Aqueous Corrosion & Zr Hydriding in Cr-Coated Cladding: M3GV-23PN0101132

Chromium-coated zirconium alloy cladding is under investigation as an accident-tolerant fuel (ATF) concept to extend performance to higher burnups via improved oxidation resistance and reduced hydrogen pickup. However, hydrothermal corrosion behavior and hydrogen transport mechanisms governing in-service hydriding of these coatings remain poorly understood. In a collaborative study between Pacific Northwest National Laboratory (PNNL), Idaho National Laboratory (INL), and the University of Huddersfield, cold spray (CS) and physical vapor deposition (PVD) Cr-coated Optimized ZIRLO™ cladding samples were characterized after exposure to PWR-simulated water chemistry under both in-core (neutron irradiation) and out-of-core (aqueous-only) conditions at the MIT Research Reactor. Multi-scale characterization was performed independently at PNNL and INL to evaluate coating integrity, microstructural evolution, Cr/Zr interface chemistry, and hydride formation. Both laboratories observed a consistent divergence in hydriding behavior: out-of-core CS Cr-coated cladding exhibited elevated hydride concentrations exceeding those of uncoated cladding, whereas in-core CS samples showed substantially suppressed hydriding. In-core specimens also displayed irradiation-specific features, including nanoscale voids within the Cr coating and radiation-induced segregation clusters in the Zr substrate. CS coatings retained an interdiffusion-free Cr/Zr bond with no intermetallic layer, whereas PVD coatings exhibited inferior quality with visible cracks and pores. An automated image-based hydride quantification method systematically overestimated bulk hydrogen content relative to inert gas fusion measurements, underscoring the need for standardized sample preparation and reference standards. Interpretation of hydride nucleation and growth is complicated by the open inner-diameter of the specimens, which provides additional hydrogen pathways. The results motivate further foundational studies to support predictive models of hydrothermal corrosion and hydrogen transport in unirradiated and irradiated Cr-coated cladding.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2025

Harnessing Machine Learning to Predict MoS 2 Solid Lubricant Performance

Physical vapor deposited (PVD) molybdenum disulfide (MoS 2 ) solid lubricant coatings are an exemplar material system for machine learning methods due to small changes in process variables often causing large variations in microstructure and mechanical/tribological properties. Here, in this work, a gradient boosted regression tree machine learning method is applied to an existing experimental data set containing process, microstructure, and property information to create deeper insights into the process-structure–property relationships for molybdenum disulfide (MoS 2 ) solid lubricant coatings. The optimized and cross-validated models show good predictive capabilities for density, reduced modulus, hardness, wear rate, and initial coefficients of friction. The contribution of individual deposition variables (i.e., argon pressure, deposition power, target conditioning) on coating properties is highlighted through feature importance. The process-property relationships established herein show linear and non-linear relationships and highlight the influence of uncontrolled deposition variables (i.e., target conditioning) on the tribological performance.

MoS2↗