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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 433 records · Page 24
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Structure–property relations in graphitic pebbles for nuclear applications

This work presents an analytical approach for holistically characterizing graphitic matrix pebbles for nuclear applications whereby the macrostructure, microstructure, and thermophysical properties of pebbles are determined. A systematic sectioning method was applied to several pebbles to describe the regional properties of the samples. Intact matrix-only spheres and sections of spheres fabricated by Kairos Power were characterized via optical imaging, x-ray computed tomography, x-ray diffraction, and ellipsometry to determine 2D and 3D macrostructure and anisotropy. The thermophysical properties of these materials were determined via measurements of density, specific heat, thermal expansion, and thermal diffusivity. The results of this study indicate that the pebble fabrication methods and their resultant effect on microstructure have a nontrivial effect on thermophysical properties, confirming the importance of robust characterization of these components. A discussion of the characterization approach and its applicability to nuclear fuel development activities is also included.

42 ENGINEERING↗
DOE OSTI2014

Correlation Between Optical Properties And Chemical Composition Of Sputter-deposited Germanium Cxide (GeO x ) Films

Germanium oxide (GeO x ) films were grown on (1 0 0) Si substrates by reactive Direct-Current (DC) magnetron sputter-deposition using an elemental Ge target. The effects of oxygen gas fraction, Г = O 2 /(Ar + O 2 ), on the deposition rate, structure, chemical composition and optical properties of GeOx films have been investigated. The chemistry of the films exhibits an evolution from pure Ge to mixed Ge + GeO + GeO 2 and then finally to GeO 2 upon increasing Г from 0.00 to 1.00. Grazing incidence X-ray analysis indicates that the GeO x films grown were amorphous. The optical properties probed by spectroscopic ellipsometry indicate that the effect of Г is significant on the optical constants of the GeO x films. The measured index of refraction (n) at a wavelength (λ) of 550 nm is 4.67 for films grown without any oxygen, indicating behavior characteristic of semiconducting Ge. The transition from germanium to mixed Ge + GeO + GeO 2 composition is associated with a characteristic decrease in n (λ = 550 nm) to 2.62 and occurs at Г = 0.25. Finally n drops to 1.60 for Г = 0.50–1.00, where the films become GeO 2 . A detailed correlation between Г, n, k and stoichiometry in DC sputtered GeO x films is presented and discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Effects of humidity during formation of zinc oxide electron contact layers from a diethylzinc precursor solution

Here, this work focuses on the role of humidity in the formation of ZnO thin films from a reactive diethylzinc precursor solution for use as the electron contact layer (ECL) in organic photovoltaic (OPV) devices. This method is well suited for flexible devices because the films are annealed at 120 °C, making the process compatible with polymer substrates. ZnO films were prepared by spin coating and annealing at different relative humidity (RH) levels. It is found that RH during coating and annealing affects the chemical and physical properties of the ZnO films. Using x-ray photoelectron spectroscopy it is found that increasing RH during the formation steps produces a more stoichiometric oxide and a higher Zn/O ratio. Spectroscopic ellipsometry data shows a small decrease in the optical band gap with increased humidity, consistent with a more stoichiometric oxide. Kelvin probe measurements show that increased RH during formation results in a larger work function (i.e. further from vacuum). Consistent with these data, but counter to what might be expected, when these ZnO films are used as ECLs in OPV devices those with ZnO ECLs processed in low RH (less stoichiometric) had higher power conversion efficiency than those with high-RH processed ZnO due to improved open-circuit voltage. The increase in open-circuit voltage with decreasing humidity was observed with two different donor polymers and fullerene acceptors, which shows the trend is due to changes in ZnO. The observed changes in open-circuit voltage follow the same trend as the ZnO work function indicating that the increase in open-circuit voltage with decreasing humidity is the result of improved energetics at the interface between the bulk-heterojunction and the ZnO layer due to a vacuum level shift.

14 SOLAR ENERGY↗
DOE OSTI2018

Thermal transitions in semi-crystalline polymer thin films studied via spectral reflectance

Herein we report the finding that spectral reflectance (SR) can be used to identify thermal transitions in semi-crystalline polymer thin films. By studying film thickness as a function of temperature, we found that semi-crystalline polymers exhibit characteristic expansion and contraction profiles during the melting (T m ) and crystallization (T c ) transitions, respectively. Prior to this discovery, studies on the crystalline T m in thin films have involved more expensive and complex techniques such as atomic force microscopy (AFM), ellipsometry, and grazing-incidence wide-angle X-ray scattering (GIWAXS). We correlate T m and T c as measured by SR with differential scanning calorimetry (DSC) in the bulk and temperature-controlled GIWAXS or AFM in thin films. We show that SR is accurate for measuring changes in films with thicknesses of 500 nm down to 21 nm and for detecting melting point depression due to thin film confinement. Furthermore, we demonstrate that SR is a powerful tool for measuring thermal transitions in semi-crystalline polymer films with single-degree resolution.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Semi-transparent p-type barium copper sulfide as a back contact interface layer for cadmium telluride solar cells

Optically transparent p-type materials play a critical role in transparent electronics including photovoltaic (PV) devices. P-type sulfide materials offer an alternative to oxides for PV application due to improved hole transport properties. Here, we report the solution-based synthesis of earth-abundant p-type transparent conducting barium copper sulfide (α-BaCu 4 S 3 , BCS) thin films. These films were characterized using scanning electron microscopy, X-ray diffraction, UV–Vis–NIR spectrophotometry, Raman spectroscopy, and spectroscopic ellipsometry. BCS films of ~100 nm thickness transmit >70% of visible light. We report on tests of the hole transport properties of these BCS films for cadmium telluride (CdTe) photovoltaics, finding that the BCS deposition process forms a beneficial tellurium (Te) rich surface on CdTe by selectively removing Cd from the surface. Based on our study, the BCS interface layer plays dual functions for CdTe PV devices as a hole transport material and as an etchant, enhancing the resulting device performance. We observed a significant increase in open-circuit voltage of CdTe solar cells with the BCS buffer layer. Furthermore, we discuss semitransparent CdTe solar cells with BCS as a hole transport layer and indium tin oxide as a finishing electrode. Semitransparent CdTe solar cells shows 13.3% conversion efficiency for the front side illumination and 1.2% efficiency for back side illumination, indicating high recombination of charge carriers generated close to the rear CdTe/BCS/ITO contact.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Measurement of Poly-Si Film Thickness on Textured Surfaces by X-Ray Diffraction in Poly-Si/SiOx Passivating Contacts for Monocrystalline Si Solar Cells

Polycrystalline Silicon on tunneling silicon oxide (poly-Si/SiOx) passivating contacts have shown great potential for the next-generation monocrystalline Si (c-Si) industrial photovoltaic technology. However, these cells typically suffer from strong parasitic absorption in the thick front poly-Si layer, which is designed to reduce metal-induced recombination. In previous work, we demonstrated an improved short-circuit current density, Jsc, by thinning the front poly-Si film in an SF6 plasma using the front metal grids as a self-aligned mask, but the sub - 100 nm thick poly-Si film is difficult to measure on an alkaline textured surface. Conventional optical techniques such as spectroscopic ellipsometry cannot be used due to the high scattering nature of the random pyramids. At poly-Si thicknesses below 50 nm, secondary electron microscopy (SEM) has difficulty distinguishing the poly-Si from the underlying c-Si substrate. Here, we demonstrate X-ray diffraction as an effective method to quantitatively measure the front poly-Si thickness. The thickness calculated from the diffraction peak height of the Si(111) crystallographic plane agrees well with the cross-section SEM analysis and simulations using SunSolve. We show that by thinning the front poly-Si from 200 to 60 nm, Jsc increased by 2.4 mA/cm2, while maintaining the same Voc. This led to an absolute efficiency gain of 1.73%. In addition, we also discuss possible reasons for the premature loss of passivation before the removal of all poly-Si, which prevented an even higher gain in Jsc.

light/electron beam induced current↗
DOE OSTI2016

Strain-induced optical band gap variation of SnO 2 films

In this paper, thickness dependent strain relaxation effects are utilized to study the impact of crystal anisotropy on the optical band gap of epitaxial SnO 2 films grown by pulsed laser deposition on (0001)-oriented sapphire substrates. An X-ray diffraction analysis reveals that all films are under tensile biaxial in-plane strain and that strain relaxation occurs with increasing thickness. Variable angle spectroscopic ellipsometry shows that the optical band gap of the SnO 2 films continuously increases with increasing film thickness. This increase in the band gap is linearly related to the strain state of the films, which indicates that the main origin of the band gap change is strain relaxation. The experimental observation is in excellent agreement with results from density functional theory for biaxial in-plane strain. Our research demonstrates that strain is an effective way to tune the band gap of SnO 2 films and suggests that strain engineering is an appealing route to tailor the optical properties of oxide semiconductors.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Morphology-Driven Control of Metabolite Selectivity Using Nanostructure-Initiator Mass Spectrometry

Nanostructure-initiator mass spectrometry (NIMS) is a laser desorption/ionization analysis technique based on the vaporization of a nanostructure-trapped liquid "initiator" phase. Here in this work, we report an intriguing relationship between NIMS surface morphology and analyte selectivity. Scanning electron microscopy and spectroscopic ellipsometry were used to characterize the surface morphologies of a series of NIMS substrates generated by anodic electrochemical etching. Mass spectrometry imaging was applied to compare NIMS sensitivity of these various surfaces toward the analysis of diverse analytes. The porosity of NIMS surfaces was found to increase linearly with etching time where the pore size ranged from 4 to 12 nm with corresponding porosities estimated to be 7-70%. Surface morphology was found to significantly and selectively alter NIMS sensitivity. The small molecule (<2k Da) sensitivity was found to increase with increased porosity, whereas low porosity had the highest sensitivity for the largest molecules examined. Estimation of molecular sizes showed that this transition occurs when the pore size is <3× the maximum of molecular dimensions. While the origins of selectivity are unclear, increased signal from small molecules with increased surface area is consistent with a surface area restructuring-driven desorption/ionization process where signal intensity increases with porosity. In contrast, large molecules show highest signal for the low-porosity and small-pore-size surfaces. We attribute this to strong interactions between the initiator-coated pore structures and large molecules that hinder desorption/ionization by trapping large molecules. Lastly, this finding may enable us to design NIMS surfaces with increased specificity to molecules of interest.

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

Short-Range Order Tunes Optical Properties in Long-Range Disordered ZnSnN2-ZnO Alloy

Local site ordering offers a new paradigm for property control in functional materials. However, systems that exhibit a propensity for local order and global disorder are often challenging to characterize, and demonstrations of ordering-induced property tuning are few and far between. Here, we demonstrate that short-range ordering tunes the optical absorption edge in the long-range disordered alloy system (ZnSnN2)1-x(ZnO)2x at x = 0.25. We use combinatorial cosputtering to synthesize a set of thin-film samples spanning this alloy space. X-ray diffraction demonstrates lattice contraction as a function of alloy composition, confirming that a mixed-anion and -cation alloy has been synthesized. Using N and O K-edge X-ray absorption near-edge structure in conjunction with simulations of cation-disordered supercell structures, we find that samples exhibit octet-rule-breaking motifs around both anions. Upon annealing at an alloy composition of x = 0.25, X-ray absorption analysis suggests that local motif structure shifts toward octet-rule-conserving while long-range disorder is maintained. Spectroscopic ellipsometry reveals that local ordering increases the absorption edge energy at constant composition. Additionally, alloy-induced optical absorption edge tuning is demonstrated. This work paves the way toward property tuning with short-range ordering in (ZnSnN2)1-x(ZnO)2x and beyond.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Site-Selective Atomic Layer Deposition at Thermally Generated Surface Oxygen Vacancies on Rutile TiO 2

Unique atomic arrangements (i.e., defects) on material surfaces may exhibit distinctive reactivity that affords opportunities for selective surface chemistry exclusively at those sites. While theoretically appealing, site-selective atomic layer deposition (ALD) processes remain largely unrealized experimentally. Here we provide evidence for site-selective ALD that exclusively targets oxygen vacancies thermally generated at the surface of TiO 2 single crystals. High-resolution in situ ellipsometry reveals the nucleation behavior of dimethylaluminum isopropoxide (DMAI) and water at process conditions precisely tuned for selective hydroxylation of surface defects. An island growth model for nucleation and atomic force microscopy (AFM) imaging are consistent with a site-selective growth mechanism that depends on the surface oxygen vacancy density. This approach to direct ALD reactions at defective surface sites may provide future opportunities for targeted remediation of electronically imperfect interfaces.

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

Generic Behavior of Ultrastability and Anisotropic Molecular Packing in Codeposited Organic Semiconductor Glass Mixtures

Vapor-deposited glass mixtures of organic semiconductors commonly serve as active layers in organic electronic devices, whose lifetime and performance are strongly influenced by the stability and structure of these mixed glasses. Here, we study the stability and anisotropic molecular packing of six co-deposited organic semiconductor glass mixtures with 50:50 weight ratio, by differential scanning calorimetry and spectroscopic ellipsometry. We also find that all six binary systems exhibit high kinetic stability and significantly reduced enthalpy relative to the corresponding liquid-cooled glassy mixtures (ultrastable behavior), even for systems where the glass transition temperatures of the components differ by more than 90 K. Furthermore, we demonstrate that the birefringence of a co-deposited glass mixture, a measure of its anisotropic packing, can be predicted from the birefringence of glasses of the two pure components. These results for stability and structure are expected to be applicable to other co-deposited organic semiconductor glass mixtures, so long as the two components mix well in the glass and individually can form ultrastable glasses. Therefore, our findings are significant for designing novel electronic devices with enhanced device lifetime and increased operational efficiency.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Tailoring the Interfacial Interactions of Porous Polymer Membranes to Accelerate Atomic Layer Deposition: The Latent Path to Antifouling Membranes

Atomic layer deposition (ALD) is a powerful strategy to engineer hybrid organic-inorganic membranes with emergent functionalities. The combination of atomic-level thickness control, a wide materials palette, and unprecedented conformality allows the physiochemical properties (e.g., hydrophilicity) of mesoporous polymer membranes to be precisely tuned. The nucleation of ALD material growth on polymer surfaces relies on Lewis acid-base interactions and remains an overlooked motif with tremendous potential to accelerate ALD nucleation and growth. Strategies to enhance these interactions could enable desirable properties such as antifouling behavior to be imparted on inert polymer surfaces that lack the necessary functional groups for ALD nucleation. Here, in this study, we demonstrate that the reactivity of polyacrylonitrile (PAN) membranes toward ALD metal oxide (MO) precursors with Lewis acid characteristics is enhanced by introducing strong Lewis base functional groups (amidoxime: Am) on the PAN backbone (Am-PAN). The resulting Lewis acid-base interactions accelerate the MO nucleation in Am-PAN and reduce the number of deposition cycles required to achieve hydrophilicity compared with the untreated PAN membrane. Unveiling the reaction mechanism, in situ Fourier transform infrared (FTIR) spectroscopy measurements established enhanced interaction dynamics between the ALD MO precursors and the Am-PAN membrane, unlike the PAN membrane. Spectroscopic ellipsometry and thermogravimetric analysis measurements revealed higher MO loadings in Am-PAN membranes compared to PAN membranes for the same number of ALD cycles. We found that strong Lewis acid-base interactions accelerated the ALD for a range of materials including Al 2 O 3 , TiO 2 , SnO 2 , and ZnO. More broadly, our work demonstrates that tailoring metal-precursor-polymer interactions is a powerful strategy to accelerate and modulate the ALD. We used this design strategy to fabricate Al 2 O 3 -Am-PAN hybrid membranes that showed 2-fold higher antifouling capability compared to pristine PAN membranes prepared with an equivalent number of Al 2 O 3 ALD cycles. Our approach expands the scope of design options for fouling-resistant porous hybrid inorganic-organic membranes and may ultimately reduce the operational costs of water treatment.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Static and dynamic optical properties of La 1-x Sr x FeO 3-δ : The effects of A-site and oxygen stoichiometry

Perovskite oxides are a promising material class for photovoltaic and photocatalytic applications due to their visible band gaps, nanosecond recombination lifetimes, and great chemical diversity. However, there is limited understanding of the link between composition and static and dynamic optical properties, despite the critical role these properties play in the design of light-harvesting devices. To clarify these relationships, we systemically studied the optoelectronic properties in La 1-x Sr x FeO 3-δ epitaxial films, uncovering the effects of A-site cation substitution and oxygen stoichiometry. Variable-angle spectroscopic ellipsometry was used to measure static optical properties, revealing a linear increase in absorption coefficient at 1.25 eV and a red-shifting of the optical absorption edge with increasing Sr fraction. The absorption spectra can be similarly tuned through the introduction of oxygen vacancies, indicating the critical role that nominal Fe valence plays in optical absorption. Dynamic optoelectronic properties were studied with ultrafast transient reflectance spectroscopy, revealing similar nanosecond photoexcited carrier lifetimes for oxygen deficient and stoichiometric films with the same nominal Fe valence. Furthermore, these results demonstrate that while the static optical absorption is strongly dependent on nominal Fe valence tuned through cation or anion stoichiometry, oxygen vacancies do not appear to play a significantly detrimental role in the recombination kinetics.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Sequential Infiltration Synthesis of Multinary Metal Oxides: The Role of Precursor Diffusion and Reactivity

Understanding volatile metal–organic precursor transport in polymer matrices during sequential infiltration synthesis (SIS) is critical to the design of hybrid materials of tailored composition and uniform structure. Here, we investigate the diffusion and reaction behavior of diethylzinc (DEZ) in poly(methyl methacrylate) (PMMA) matrices with variable indium and zinc incorporation. Unlike conventional SIS where precursors adduct or react directly with the polymer backbone, DEZ shows minimal interaction with PMMA. However, DEZ may interact strongly with In–OH or Zn–OH moieties that are previously incorporated into a PMMA thick film. Using spectroscopic ellipsometry, X-ray photoelectron spectroscopy depth profiling, and EDX line scans, we quantify Zn incorporation as a function of exposure time, polymer thickness, and previous infiltration steps. The Zn distribution evolves from surface-localized to deep within the ∼1 μm thick film with increasing DEZ exposures up to 30 min. A reaction-diffusion model incorporating a hindering factor to capture diffusivity decay due to product accumulation reproduces the Zn depth profile and yields physically meaningful parameters to guide future growth. Furthermore, this study provides a framework for interpreting metal–organic precursor infiltration behavior and highlights the utility of model-guided SIS process design.

diethylzinc↗
DOE OSTI2018

Area-Selective Atomic Layer Deposition of Metal Oxides on Noble Metals through Catalytic Oxygen Activation

Area-selective atomic layer deposition (ALD) is envisioned to play a key role in next-generation semiconductor processing and can also provide new opportunities in the field of catalysis. In this work, we developed an approach for the area-selective deposition of metal oxides on noble metals. Using O 2 gas as co-reactant, area-selective ALD has been achieved by relying on the catalytic dissociation of the oxygen molecules on the noble metal surface, while no deposition takes place on inert surfaces that do not dissociate oxygen (i.e., SiO 2 , Al2O 3 , Au). The process is demonstrated for selective deposition of iron oxide and nickel oxide on platinum and iridium substrates. Characterization by in situ spectroscopic ellipsometry, transmission electron microscopy, scanning Auger electron spectroscopy, and X-ray photoelectron spectroscopy confirms a very high degree of selectivity, with a constant ALD growth rate on the catalytic metal substrates and no deposition on inert substrates, even after 300 ALD cycles. We demonstrate the area-selective ALD approach on planar and patterned substrates and use it to prepare Pt/Fe 2 O 3 core/shell nanoparticles. Finally, the approach is proposed to be extendable beyond the materials presented here, specifically to other metal oxide ALD processes for which the precursor requires a strong oxidizing agent for growth.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Sequential Infiltration Synthesis of Electronic Materials: Group 13 Oxides via Metal Alkyl Precursors

The sequential infiltration synthesis (SIS) of group 13 indium and gallium oxides (In 2 O 3 and Ga 2 O 3 ) into poly(methyl methacrylate) (PMMA) thin films is demonstrated using trimethylindium (TMIn) and trimethylgallium (TMGa), respectively, with water. In situ Fourier transform infrared (FTIR) spectroscopy reveals that these metal alkyl precursors reversibly associate with the carbonyl groups of PMMA in analogy to trimethylaluminum (TMAl), however, with significantly lower affinity. This is demonstrated to have important kinetic consequences that dramatically alter the synthetic parameters required to achieve material growth. Ab initio density functional theory simulations of the methyl methacrylate monomer with group 13 metal alkyls corroborate association energy that is 3X greater for TMAl than for either TMIn or TMGa. As a consequence, the kinetics of activated diffusion within the film is observed to be far more rapid for TMIn and TMGa than for TMAI. Spectroscopic ellipsometry and scanning electron microscopy, in combination with Hall effect measurements of SIS-derived In 2 O 3 films, demonstrate that SIS enables rapid growth of thin films with continuous electrically conductive pathways after postannealing. Notably, SIS with TMIn and water enables the growth of In 2 O 3 at 80°C, well below the onset temperature of atomic layer deposition (ALD) using these precursors.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Molecular Orientation for Vapor-Deposited Organic Glasses Follows Rate-Temperature Superposition: The Case of Posaconazole

The anisotropic properties of organic glasses produced by physical vapor deposition (PVD) depend upon substrate temperature and deposition rate. In recent work, it was shown for a liquid crystalline system that the anisotropic structure of the glass was controlled by a single combined variable as indicated by the observation of deposition rate-substrate temperature superposition (RTS). Here we test the utility of RTS for posaconazole, a molecule that does not form liquid crystals. We prepare glasses of posaconazole utilizing a range of deposition rates covering 2 orders of magnitude and an 18 K range in substrate temperature. We characterize the glasses using ellipsometry and X-ray scattering. Consistent with RTS, we find that decreasing the deposition rate has the same effect upon molecular orientation as increasing the substrate temperature during deposition. Thus, RTS can be used to predict and control the structure of glasses prepared at a wide range of deposition conditions. We use RTS to infer a characteristic time for molecular reorientation at the surface of posaconazole.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Quantifying Charge Carrier Localization in PBTTT Using Thermoelectric and Spectroscopic Techniques

Chemically doped poly[2,5-bis(3-alkylthiophen-2-yl)thieno[3,2- b ]thiophene] (PBTTT) shows promise for many organic electronic applications, but rationalizing its charge transport properties is challenging because conjugated polymers are inhomogeneous, with convoluted optical and solid-state transport properties. Herein, we use the semilocalized transport (SLoT) model to quantify how the charge transport properties of PBTTT change as a function of iron(III) chloride (FeCl 3 ) doping level. We use the SLoT model to calculate fundamental transport parameters, including the carrier density needed for metal-like electrical conductivities and the position of the Fermi energy level with respect to the transport edge. We then contextualize these parameters with other polymer-dopant systems and previous PBTTT reports. Additionally, we use grazing incidence wide-angle X-ray scattering and spectroscopic ellipsometry techniques to better characterize inhomogeneity in PBTTT. Our analyses indicate that PBTTT obtains high electrical conductivities due to its quickly rising reduced Fermi energy level, and this rise is afforded by its locally high carrier densities in highly ordered microdomains. Ultimately, this report sets a benchmark for comparing transport properties across polymer-dopant-processing systems.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗