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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 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Measuring calcium content in plants using NEXAFS spectroscopy

Calcium is important for the growth and development of plants. It serves crucial functions in cell wall and cell membrane structure and serves as a secondary messenger in signaling pathways relevant to nutrient and immunity responses. Thus, measuring calcium levels in plants is important for studies of plant biology and for technology development in food, agriculture, energy, and forest industries. Often, calcium in plants has been measured through techniques such as atomic absorption spectrophotometry (AAS), inductively coupled plasma–mass spectrometry (ICP-MS), and electrophysiology. These techniques, however, require large sample sizes, chemical extraction of samples or have limited spatial resolution. Here, we used near-edge X-ray absorption fine structure (NEXAFS) spectroscopy at the calcium L- and K-edges to measure the calcium to carbon mass ratio with spatial resolution in plant samples without requiring chemical extraction or large sample sizes. We demonstrate that the integrated absorbance at the calcium L-edge and the edge jump in the fluorescence yield at the calcium K-edge can be used to quantify the calcium content as the calcium mass fraction, and validate this approach with onion epidermal peels and ICP-MS. We also used NEXAFS to estimate the calcium mass ratio in hypocotyls of a model plant, Arabidopsis thaliana , which has a cell wall composition that is similar to that of onion epidermal peels. These results show that NEXAFS spectroscopy performed at the calcium edge provides an approach to quantify calcium levels within plants, which is crucial for understanding plant physiology and advancing plant-based materials.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2020

Quantification of the carbon bonding state in amorphous carbon materials: A comparison between EELS and NEXAFS measurements

In this report the quantitative determination of the carbon hybridization is critical for establishing processing-structure-properties relationships for carbon-based materials, including amorphous carbon coatings. While several techniques have been employed to characterize the amount of sp 2 and sp 3 carbon in these materials, direct comparisons between analytical results are limited. Here, we compare near edge X-ray absorption fine structure (NEXAFS) spectra of a silicon- and oxygen-containing hydrogenated amorphous carbon (a-C:H:Si:O) coating acquired in synchrotron-based scanning transmission X-ray microscopy (STXM) mode with electron energy loss spectra (EELS) obtained from the same a-C:H:Si:O lamella. While the fractions of sp 2 carbon computed from STXM and EELS spectra are in close agreement, the comparison of NEXAFS spectra acquired in STXM mode with NEXAFS spectra collected in partial electron yield mode on a flat a-C:H:Si:O surface indicated that the destructive preparation of thin lamellae for STXM analyses induces variations in the structure of a-C:H:Si:O, namely the breakage of carbon-silicon and carbon-hydrogen bonds, a change in ordering of sp 2 -bonded carbon, and an increase in the sp 2 carbon fraction. These findings can help scientists in the careful interpretation of spectroscopic results obtained from the analysis of samples made of metastable materials after the destructive preparation of specimens for analytical purposes.

36 MATERIALS SCIENCE↗
DOE OSTI2023

A library of calcium mineral reference spectra recorded by parallel imaging using NEXAFS spectromicroscopy

Calcium minerals are ubiquitous in geology and life chemistry. Understanding the phase and chemical state of calcium minerals is important for numerous processes including materials chemistry, hard tissue biogenesis and geological processes. Photoemission spectroscopies such as near edge X-ray absorption fine structure (NEXAFS) and scanning transmission X-ray microscopy have been instrumental in identifying and characterizing calcium minerals in all these areas. In this work, we have recorded reference spectra for a range of different calcium minerals including a series of calcium carbonates, calcium oxalates and calcium phosphates. While collections of reference spectra for several calcium minerals can be found in the literature, these spectra have been reported in different contexts using a variety of instruments. We, here, report a comprehensive list of references recorded in parallel in a single experiment by imaging an array of calcium minerals using a NEXAFS microscope. We present reference NEXAFS spectra at the calcium L-, carbon K- and oxygen K-edges.

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

VerSoX B07-B: a high-throughput XPS and ambient pressure NEXAFS beamline at Diamond Light Source

The beamline optics and endstations at branch B of the Versatile Soft X-ray (VerSoX) beamline B07 at Diamond Light Source are described. B07-B provides medium-flux X-rays in the range 45-2200 eV from a bending magnet source, giving access to local electronic structure for atoms of all elements from Li to Y. It has an endstation for high-throughput X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine-structure (NEXAFS) measurements under ultrahigh-vacuum (UHV) conditions. B07-B has a second endstation dedicated to NEXAFS at pressures from UHV to ambient pressure (1 atm). The combination of these endstations permits studies of a wide range of interfaces and materials. The beamline and endstation designs are discussed in detail, as well as their performance and the commissioning process.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2025

Approaching the Lower Temporal Limit of Laser‐Produced Plasma Sources for Table‐Top Soft X‐Ray NEXAFS Measurements

Abstract The increasing popularity of time‐resolved X‐ray absorption measurements for understanding dynamics in molecular and material systems has led to many advances in table‐top sources for pulsed X‐rays. We report on a table‐top laser‐produced plasma (LPP) source that can perform soft X‐ray (SXR), near‐edge X‐ray absorption fine structure (NEXAFS) measurements using a laser source with 23 ps pulse duration. The spectrometer's key specifications, such as brilliance, resolution, and stability, are characterized against the more commonly used longer‐pulse‐duration LPP sources. The 23 ps laser produced approximately an order of magnitude weaker SXR flux than the 8 ns laser for a higher power density due to the smaller total energy absorbed by the plasma. The increased repetition rate, as well as the use of a high line‐density X‐grating, and a self‐referencing scheme still allowed for NEXAFS measurements of Si 3 N 4 and TiO 2 thin films with 2.5 minute acquisition times, a resolving power of E/ΔE=424, and a signal‐to‐noise ratio of 100. It was observed that degradation of the gas jet nozzle led to long‐term instability of the source, which can be remediated using alternative nozzle designs. This work demonstrates the feasibility of achieving higher temporal resolution in future time‐resolved X‐ray absorption measurements using table‐top LPP sources.

Nimlos, Danika [Division of Chemistry and Chemical↗
DOE OSTI2025

Direct Observation of the ππ* to nπ* Transition in 2-Thiouracil via Time-Resolved NEXAFS Spectroscopy

The photophysics of nucleobases has been the subject of both theoretical and experimental studies over the past decades due to the challenges posed by resolving the steps of their radiationless relaxation dynamics, which cannot be described in the framework of the Born–Oppenheimer approximation (BOA). In this context, the ultrafast dynamics of 2-thiouracil has been investigated with a time-resolved NEXAFS study at the Free Electron Laser FLASH. Near Edge X-ray Absorption Fine Structure spectroscopy (NEXAFS) can be used to observe electronic transitions in ultrafast molecular relaxation. We performed time-resolved UV-pump/X-ray probe absorption measurements at the sulfur 2s (L1) and 2p (L2/3) edges. We are able to identify absorption features corresponding to the S2 (ππ*) and S1 (nπ*) electronic states. We observe a delay of 102 ± 11 fs in the population of the nπ* state with respect to the initial optical excitation and interpret the delay as the time scale for the S2 → S1 internal conversion. We furthermore identify oscillations in the absorption signal that match a similar observation in our previous X-ray photoelectron spectroscopy study on the same molecule.

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

NEXAFS Spectroscopy of P3HT and PBTTT at the Sulfur K-Edge

The sulfur K-edge near-edge X-ray absorption fine-structure (NEXAFS) spectra of the common conjugated polymers P3HT and PBTTT are studied from both experimental and theoretical perspectives. Experimental angle-resolved spectra are measured to characterize both the dominant peaks and the dichroism of the polymers. First-principles calculations using the density functional theory-based many-body X-ray absorption spectroscopy (MBXAS) method are performed for the two polymers as well as for the thiophene and thienothiophene units that make up the conjugated backbones of these polymers. Through this combined approach, we are able to confidently assign the observed peaks to specific molecular orbitals and identify the orientation of their transition dipole moments (TDMs) with respect to the coordinate frame of the polymer backbone. Here, in particular, we are able to establish the character and orthogonal nature of the three main low-energy peaks at: (i) 2473.5 eV, 1s → (S–C)­π* with TDM along the π-stacking direction; (ii) 2474.1 eV, 1s → (S–C)­σ* with TDM along the backbone; and (iii) 2475.4 eV, 1s → (S–C)­σ* with TDM perpendicular to the first two. By performing both gas-phase and solid-state simulations, and with reference to the NEXAFS spectra of thiophene and thienothiophene building blocks, the influences of polymerization and molecular packing are also explored.

Chantler, Paul Alexander [Monash University, Clayt↗
DOE OSTI2024

NEXAFS spectroscopy of alkylated benzothienobenzothiophene thin films at the carbon and sulfur K-edges

Alkylated benzothienobenzothiophenes are an important class of organic semiconductors that exhibit high performance in solution-processed organic field-effect transistors. In this work, we study the near-edge x-ray absorption fine-structure (NEXAFS) spectra of 2,7-didecyl[1]benzothieno[3,2-b][1]benzothiophene (C 10 -BTBT) at both the carbon and sulfur K-edges. Angle-resolved experiments of thin films are performed to characterize the dichroism associated with molecular orientation. First-principles calculations using the density functional theory-based many-body x-ray absorption spectroscopy (MBXAS) method are also performed to correlate the peaks observed and their dichroism with transitions to specific antibonding molecular orbitals. Interestingly, the dichroism of the dominant, lowest energy peak is opposite at the carbon and sulfur K-edges. While the low-energy peak at the carbon K-edge is assigned to carbon 1s → π* transitions with transition dipole moment (TDM) perpendicular to the planar BTBT core, the dominant low energy peak at the sulfur K-edge is assigned to sulfur 1s → σ* transitions with TDM oriented along the long axis of the BTBT core. These differences at the sulfur and carbon K-edges are understood through the MBAXS simulations that find a reordering of the energy of the lowest energy π* and σ* transitions at the sulfur K-edge due to the strong localization of the σ* orbital over the sulfur atom. This work highlights differences in the NEXAFS spectra of organic semiconductors at carbon and sulfur K-edges and provides new insights into peak assignment and x-ray dichroism relevant for studying the molecular orientation of organic semiconductor films.

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

Association between soil organic carbon and calcium in acidic grassland soils from Point Reyes National Seashore, CA

Organo-mineral and organo-metal associations play an important role in the retention and accumulation of soil organic carbon (SOC). Recent studies have demonstrated a positive correlation between calcium (Ca) and SOC content in a range of soil types. However, most of these studies have focused on soils that contain calcium carbonate (pH > 6). To assess the importance of Ca-SOC associations in lower pH soils, we investigated their physical and chemical interaction in the grassland soils of Point Reyes National Seashore (CA, USA) at a range of spatial scales. Multivariate analyses of our bulk soil characterisation dataset showed a strong correlation between exchangeable Ca (Ca Exch ; 5–8.3 c.mol c kg –1 ) and SOC (0.6–4%) content. Additionally, linear combination fitting (LCF) of bulk Ca K-edge X-ray absorption near-edge structure (XANES) spectra revealed that Ca was predominantly associated with organic carbon across all samples. Scanning transmission X-ray microscopy near-edge X-ray absorption fine structure spectroscopy (STXM C/Ca NEXAFS) showed that Ca had a strong spatial correlation with C at the microscale. The STXM C NEXAFS K-edge spectra indicated that SOC had a higher abundance of aromatic/olefinic and phenolic C functional groups when associated with Ca, relative to C associated with Fe. In regions of high Ca-C association, the STXM C NEXAFS spectra were similar to the spectrum from lignin, with moderate changes in peak intensities and positions that are consistent with oxidative C transformation. Through this association, Ca thus seems to be preferentially associated with plant-like organic matter that has undergone some oxidative transformation, at depth in acidic grassland soils of California. Our study highlights the importance of Ca-SOC complexation in acidic grassland soils and provides a conceptual model of its contribution to SOC preservation, a research area that has previously been unexplored.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2025

Spectroscopic Investigation into P(NDI2OD-T2) Charge Localization

P(NDI2OD-T2), commonly referred to as N2200, stands out as a promising electron-transporting (n-type) polymer for low-cost, flexible (photo)electrochemical applications due to its reversible two-electron reduction and high electron mobility. UV–vis spectroelectrochemistry in the tetrabutyl ammonium hexafluorophosphate/acetonitrile electrolyte shows two sets of chemically reversible redox signals in the cyclic voltammetry corresponding to the reduction of the neutral polymer film to polaronic and bipolaronic species. These electrochemical signatures suggest a distinct electronic reorganization upon reduction (polaron/bipolaron formation), highlighting the need for molecular-level insights into how charges are accommodated within the polymer backbone. While it has been previously hypothesized that charge predominantly localizes on the naphthalene diimide (NDI) unit during reductive charging, specific changes in atomic environments that confirm this localization have not been characterized in n-type polymers. Herein, we use near-edge X-ray absorption fine structure (NEXAFS) spectroscopy to probe electronic transitions in an electrochemically charged polymer to deduce charge localization. The O K-edge (1s) spectra exhibit two distinct π* peaks; the intensity of the lower-energy π* a peak that corresponds to an excitation to a largely localized carbonyl state decreases with reductive potentials relative to the higher-energy π* b peak. We corroborate this with Raman spectroscopy at different potentials, which shows a decrease in intensity on the C–C/C═C and C═O stretching bands of NDI as well as a red shift of the carbonyl band due to the formation of a polaron on the NDI. Additionally, new Raman active NDI signals associated with elongated C═O and C═C bonds are observed at lower energy during the formation of charged states. Together with theoretical calculations, these findings show that the injected charge spatially localizes on the NDI units and is dominantly distributed on the carbonyl groups. In conclusion, the combination of NEXAFS, optical and vibrational spectroscopies, and theoretical calculations is generalizable to other pi-conjugated polymers and can identify charge localization for the further development of organic semiconductors.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Structure of Layers Formed by [2-(3,6-Disubstituted-9 H -carbazol-9-yl)ethyl]phosphonic Acids on Metal Oxides

[2-(9 H -Carbazol-9-yl)ethyl]phosphonic acid (2PACz) and its derivatives are being used extensively as hole-transport layers in organic and perovskite solar cells due to their ability to modify electrode work function, surface wettability, and in some cases, to improve active-layer adhesion, while minimizing interfacial energy losses. The orientation and coverage of surface modifiers significantly impact these functional properties; however, the detailed structure and packing in these overlayers are challenging to investigate, leading to a lack of understanding of how this structure and packing influence performance and limiting the ability to rationally design molecular modifiers. Here, we investigate monolayers of 2-(9 H -carbazol-9-yl)ethyl phosphonic acid derivatives (from here on referred to as X-2PACz) on indium tin oxide (ITO) and alpha phase aluminum oxide (α-Al 2 O 3 ) using a combination of X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and X-ray reflectivity (XRR). By correlating elemental ratios, molecular orientation, and electron density profiles, we directly quantify surface coverage, layer thickness, and molecular tilt across a series of chemically related monolayers. We find that 2PACz and the X-2PACz derivatives form dense monolayers on α-Al 2 O 3 and ITO, with similar surface coverages on either substrate that are inversely proportional to molecular steric bulk. These surface coverage results indicate that X-2PACz is sterically limited in its monolayer surface packing density, as opposed to site limited. Despite surface packing density differences between molecules, the NEXAFS data show a constant average molecular orientation of 61° to 65° between the plane of the carbazole and the substrate for all the molecules on both substrates. These results increase our general understanding of 2PACz derivatives as they become increasingly useful in high performance solar cells.

NEXAFS↗
DOE OSTI2021

Oxidation of Cu 2 O(111) by NO 2 : An Ambient Pressure X-ray Photoelectron Spectroscopy Study

With NO x persisting as a problematic pollutant, understanding the interactions and reactivity of NO x species at atmospherically relevant pressures with surfaces remains essential. Synchrotron-based ambient pressure x-ray photoelectron spectroscopy (APXPS) and near-edge x-ray absorption spectroscopy fine structure (NEXAFS) were used to investigate the interactions of NO2 with a Cu 2 O(111) surface. The NEXAFS data reveals a pressure dependence of the oxidation of the Cu 2 O surface to CuO by NO 2 . APXPS results show NO 2 chemi- and physisorption on the Cu 2 O surface and decomposition to NO and O, or to N and O, with the amount of the species also being pressure dependent. Finally, the combination of the two spectroscopic techniques shows a comprehensive picture of NO 2 adsorption with implications for catalysis, gas sensing, and gas filtration materials.

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

The role of Co valence in charge transport in the entropy‐stabilized oxide (Mg 0.2 Co 0.2 Ni 0.2 Cu 0.2 Zn 0.2 )O

Abstract Many of the studies on the entropy‐stabilized oxide (Mg 0.2 Co 0.2 Ni 0.2 Cu 0.2 Zn 0.2 )O have been heavily application‐based. Previous works have studied effects of cation stoichiometry on the entropy‐driven reaction to form a single phase, but a fundamental exploration of the effects of anion stoichiometry and/or redox chemistry on electrical properties is lacking. Using near‐edge X‐ray absorption fine structure (NEXAFS) and electrical measurements, we show that oxidizing thin film samples of (Mg 0.2 Co 0.2 Ni 0.2 Cu 0.2 Zn 0.2 )O affects primarily the valence of Co, leaving the other cations in this high‐entropy system unchanged. This oxidation increases electrical conduction in these thin films, which occurs via small polaron hopping mediated by the Co valence shift from 2+ to a mixed 2+/3+ state. In parallel, we show that bulk samples sintered in an oxygen‐rich atmosphere have a lower activation energy for electrical conduction than those equilibrated in a nitrogen (reducing) atmosphere. Combining feasible defect compensation scenarios with electrical impedance measurements and NEXAFS data, we propose a self‐consistent interpretation of Co redox‐mediated small polaron conduction as the dominant method of charge transfer in this system.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Extracting structural insights from soft X-ray scattering of biological assemblies

Resonant soft X-ray scattering (RSoXS), a technique that combines X-ray absorption spectroscopy and X-ray scattering, can probe the nano- and meso-scale structure of biological assemblies with chemical specificity. RSoXS experiments yield scattering data collected at several photon energies, for example across an elemental absorption edge of interest. Collecting a near-edge X-ray absorption fine structure (NEXAFS) spectrum complements RSoXS experiments and determines X-ray energies that are best suited for RSoXS measurements. The analysis of RSoXS data is similar in many ways to analysis of small angle X-ray scattering using hard X-rays, with an added dimension that includes an X-ray energy dependence. This chapter discusses procedures for predicting scattering contrast and thereby identifying energies suitable for RSoXS measurements using NEXAFS spectra, analyses of 2D RSoXS images through integration into 1D profiles, and strategies for elucidating the origin of RSoXS scattering features. As a result, it also discusses existing and potential methods for interpretation of RSoXS data to gain detailed structural insights into biological systems.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2021

Surfactant inhibition mechanisms of carbonate mineral dissolution in shale

Surfactants are common additives to hydraulic fracturing and enhanced oil recovery (EOR) fluids, and are under consideration for amendment to supercritical carbon dioxide for geological carbon sequestration (GCS). The effect of a common anionic surfactant, internal olefin sulfonate (IOS), on mineral dissolution from shale into brine was evaluated. When added to brine at concentrations exceeding the critical micelle concentration (94 mg/L), IOS inhibited carbonate mineral dissolution in an Eagle Ford shale, as well as dissolution of optical quality calcite (the dominant carbonate in the shale). Laser profilometry images provide spatial resolution across > 3 orders of magnitude, and indicate that IOS addition to brine both enhances the formation of new etch pits in calcite, and impedes their further growth. Time-of-flight secondary ion mass spectrometry surface profiles show for the first time that IOS preferentially adsorbs at calcite pit edges versus flat calcite surfaces (i.e., terraces). Surface pressure calculations, sulfur K-edge near edge X-ray absorption fine structure (NEXAFS) spectroscopy results, and density functional theory (DFT) calculations support this observation; the DFT results indicate that the sulfonate head group of the IOS molecule binds strongly to the calcite step site as compared to the terrace site. The S K-edge NEXAFS results indicate that IOS adsorbed more to etched calcite surfaces compared to smooth calcite surfaces. Overall, the results indicate that weak adsorption on flat calcite surfaces (i.e., terraces) disrupts water structure and enhances mass transfer of dissolution, while strong adsorption on calcite pit edges displaces adsorbed water and inhibits further etch pit growth. This work provides the first direct evidence of preferential adsorption of IOS to etched calcite surfaces and links it to macroscopic dissolution kinetics. Finally, this work has implications for surfactant-containing fluids used in hydraulic fracturing, EOR and potentially GCS for subsurface injection into carbonate rich reservoirs.

36 MATERIALS SCIENCE↗