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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 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Ultraviolet spectra of extreme nearby star-forming regions: Evidence for an overabundance of very massive stars

ABSTRACT As deep spectroscopic campaigns extend to higher redshifts and lower stellar masses, the interpretation of galaxy spectra depends increasingly upon models for very young stellar populations. Here we present new HST/COS ultraviolet spectroscopy of seven nearby (<120 Mpc) star-forming regions hosting very young stellar populations (∼4–20 Myr) with optical Wolf–Rayet stellar wind signatures, ideal laboratories in which to benchmark these stellar models. We detect nebular C iii] in all seven, but at equivalent widths uniformly <10 Å. This suggests that even for very young stellar populations, the highest equivalent width C iii] emission at ≥15 Å is reserved for inefficiently cooled gas at metallicities at or below that of the SMC. The spectra also reveal strong C iv P-Cygni profiles and broad He ii emission formed in the winds of massive stars, including some of the most prominent He ii stellar wind lines ever detected in integrated spectra. We find that the latest stellar population synthesis prescriptions with improved treatment of massive stars nearly reproduce the entire range of stellar He ii wind strengths observed here. However, we find that these models cannot simultaneously match the strongest wind features alongside the optical nebular line constraints. This discrepancy can be naturally explained by an overabundance of very massive stars produced by a high incidence of binary mass transfer and mergers occurring on short ≲10 Myr time-scales, suggesting these processes may be crucial for understanding systems dominated by young stars both nearby and in the early Universe.

Senchyna, Peter↗
DOE OSTI2024

Coherent charge hopping suppresses photoexcited small polarons in ErFeO 3 by antiadiabatic formation mechanism

Polarons are prevalent in condensed matter systems with strong electron-phonon coupling. The adiabaticity of the polaron relates to its transport properties and spatial extent. To date, only adiabatic small polaron formation has been measured following photoexcitation. The lattice reorganization energy is large enough that the first electron–optical phonon scattering event creates a small polaron without requiring substantial carrier thermalization. We measure that frustrating the iron-centered octahedra in the rare-earth orthoferrite ErFeO 3 leads to antiadiabatic polaron formation. Coherent charge hopping between neighboring Fe 3+ –Fe 2+ sites is measured with transient extreme ultraviolet spectroscopy and lasts several picoseconds before the polaron forms. The resulting small polaron formation time is an order of magnitude longer than previous measurements and indicates a shallow potential well, even in the excited state. The results emphasize the importance of considering dynamic electron-electron correlations, not just electron-phonon–induced lattice changes, for small polarons for transport, catalysis, and photoexcited applications.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Development of organic-inorganic hybrid selective layers via vapor phase infiltration to enhance the durability of perovskite solar cells

Despite the rapid increase in power conversion efficiency (PCE) of perovskite solar cells (PSCs) over the last decade, stability remains a major roadblock to commercialization. This work proposes the use of vapor phase infiltration (VPI) as a tool to create hybrid organic-inorganic layers that improve the stability of organic charge transport layers, such as hole-selective spiro-OMeTAD in these PSC and other organic electronic devices. Using X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and grazing incident wide-angle X-ray scattering (GIWAXS), we identify that infiltration of TiO X via VPI hinders the crystallization of the spiro-OMeTAD layer by likely preventing the π- π stacking of the molecules. Infiltrated PSCs retained around 80% of their original efficiency after an operando stability test of 60 h at 75 °C, double the efficiency retained by devices without infiltration. This work provides a blueprint for using VPI to stabilize organic charge transport layers via prevention of π- π stacking that leads to deleterious crystallization that shortens device lifetimes.

14 SOLAR ENERGY↗
DOE OSTI2023

Carrier and Phonon Dynamics in Multilayer WSe 2 Captured by Extreme Ultraviolet Transient Absorption Spectroscopy

Carrier and phonon dynamics in a multilayer WSe2 film are captured by extreme ultraviolet (XUV) transient absorption (TA) spectroscopy at the W N 6,7 , W O 2,3 , and Se M 4,5 edges (30–60 eV). After the broadband optical pump pulse, the XUV probe directly reports on occupations of optically excited holes and phonon-induced band renormalizations. Here, when compared with density functional theory calculations, XUV transient absorption due to holes is identified below the W O 3 edge whereas signals at the Se M 4,5 edges are dominated by phonon dynamics. Therein, 0.4 ps hole relaxation time, 1.5 ps carrier recombination time, and 1.7 ps phonon heating time are extracted. The acquisition of hole and phonon-induced signals in a single experiment can facilitate the investigation of the correlations between electron and phonon dynamics. Furthermore, the simultaneous observation of signals from different elements can be further extended to explore photochemical processes in multilayers and alloys, thereby providing key information for their applications in electronics, photocatalysts, and spintronics.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Mapping wave packet bifurcation at a conical intersection in CH 3 I by attosecond XUV transient absorption spectroscopy

Extreme ultraviolet (XUV) transient absorption spectroscopy has emerged as a sensitive tool for mapping the real-time structural and electronic evolution of molecules. Here, attosecond XUV transient absorption is used to track dynamics in the A-band of methyl iodide (CH 3 I). Gaseous CH 3 I molecules are excited to the A-band by a UV pump (277 nm, ~20 fs) and probed by attosecond XUV pulses targeting iodine I(4d) core-to-valence transitions. Owing to the excellent temporal resolution of the technique, passage through a conical intersection is mapped through spectral signatures of nonadiabatic wave packet bifurcation observed to occur at 15 ± 4 fs following UV photoexcitation. The observed XUV signatures and time dynamics are in agreement with previous simulations [H. Wang, M. Odelius, and D. Prendergast, J. Chem. Phys. 151, 124106 (2019)]. Furthermore, due to the short duration of the UV pump pulse, coherent vibrational motion in the CH 3 I ground state along the C-I stretch mode (538 ± 7 cm -1 ) launched by resonant impulsive stimulated Raman scattering and dynamics in multiphoton excited states of CH 3 I are also detected.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Elucidating the Influence of Residual Polymer and Gas Environment on the Electronic Structure of a Graphene Layer Using in Situ APXPS

We use in situ ambient pressure X-ray photoelectron spectroscopy (APXPS) and ultraviolet photoelectron spectroscopy (UPS) to develop an effective method for studying changes in the graphene (Gr) electronic structure according to certain circumstance. The amount of polymethyl methacrylate (PMMA) residual polymer (RP), inevitably generated during the Gr transfer process, is significantly reduced from Gr surface by thermal annealing. This processed Gr is then sequentially exposed to specific gas environments (Ar, N 2 , O 2 , and CO 2 ), and APXPS or UPS is carried out to investigate the variations in the Gr electronic structure including the work function. When the amount of PMMA RP on Gr is reduced, the position of the main carbon peak shifts by more than 0.4 eV to a higher binding energy (in XPS spectra), and the secondary electron cutoff moves by about 0.2 eV to a lower binding energy (in UPS spectra). These changes are generally caused by a decrease in the Gr work function. On the other hand, exposure to the gas environments at different temperatures that we investigated did not produce significant changes in the work function and chemical states of Gr. These results confirm that the material in contact with Gr should be considered to achieve the desired Gr performance in electronics.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Time-Domain Extreme-Ultraviolet Diffuse Scattering Spectroscopy of Nanoscale Surface Phonons

Here, we report the observation of dynamic fringe patterns in the diffuse scattering of extreme ultraviolet light from surfaces, following femtosecond optical excitation. At each point on the detector, the diffuse scattering intensity exhibits oscillations at well-defined frequencies that correspond to surface phonons with wave vectors determined by the scattering geometry, indicating that the optical excitation generates coherent surface phonons propagating in all directions and spanning a wavelength range from 60 to 300 nm. This phenomenon is observed on a variety of samples, including single-layer and multilayer metal films, as well as bulk semiconductors. The measured surface phonon dispersions show good agreement with theoretical calculations. By comparing signal amplitudes from samples with different surface morphologies, we find that the excitation mechanism is linked to the natural surface roughness of the samples. However, the signal is still detectable on extremely smooth surfaces with subnanometer roughness. Our findings demonstrate a simple and effective method for optically exciting coherent surface phonons with nanoscale wavelengths on a wide range of solid samples and establish a foundation for surface phonon spectroscopy in a wave vector range well beyond the limit of conventional surface Brillouin scattering.

Capotondi, F. [Elettra-Sincrotrone Trieste (Italy)↗
DOE OSTI2022

Revealing aggregation of non-fullerene acceptors in intermixed phase by ultraviolet-visible absorption spectroscopy

Non-fullerene acceptor (NFA) aggregation is crucial in determining bulk-heterojunction (BHJ) organic photovoltaic (OPV) performance. However, it is still a big challenge to characterize the nanostructure of NFAs in the disordered donor-acceptor intermixed phase. Here, we demonstrate a method to characterize NFA aggregation and composition in the intermixed phase by measuring NFA concentration-dependent ultraviolet-visible (UV-vis) absorption spectroscopy of BHJ films. In various polymer:NFA films, an absorption shift as a function of increasing molecular concentration (ASIMC) phenomenon is observed, and different NFA aggregation behaviors can be distinguished. The ASIMC method was then applied to study the influence of processing conditions on the NFA concentration in the intermixed phase of devices to establish a correlation with device efficiency. The current work provides a feasible tool to study the nanostructure of NFAs in the complex polymer matrix and to understand the variations in the NFA concentration in the intermixed phase under non-equilibrium conditions.

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

Dual-comb spectroscopy in the deep ultraviolet

This Letter reports dual-frequency-comb spectroscopy in the deep ultraviolet used to characterize transient laser-produced plasmas. Dual-comb spectroscopy at these wavelengths enables access to a large number of strong electronic transitions in neutral and ionized atoms and molecules. This broadband dual-frequency-comb system at 265 nm measures multiple transitions in neutral and singly ionized iron atoms in a laser-produced plasma. Analysis of absorption spectra yields time-resolved ion and neutral column densities, excitation temperatures, and electron densities, which are measured down to 10 14 cm —3 levels. The results provide insights into low-temperature plasma properties.

47 OTHER INSTRUMENTATION↗
DOE OSTI2020

Hollow Au Nanosphere-Cu 2 O Core–Shell Nanostructures with Controllable Core Surface Morphology

Design of metal-semiconductor interfaces and heterostructures is of strong interest for various catalytic applications including photocatalysis. Here, a series of hollow Au nanosphere (HGN)-Cu 2 O core-shell nanostructures with varying core surface rugosities are synthesized and investigated for possible photocatalytic applications. HGN surface rugosity is tuned by pH modification during galvanic exchange, and carboxyl groups are utilized as coordination sites to deposit uniform Cu 2 O shells onto the gold surfaces. Final core-shell structures are verified by transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy-dispersive X-ray spectrometry (EDS), and X-ray diffraction (XRD). Information regarding chemical state and electronic band structure is acquired by X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). Ultrafast transient absorption (TA) reveals that charge separation in bumpy HGN (bHGN)-Cu 2 O may effectively provide longer-lived photoexcited carriers, offering great potential for utilization in advanced photocatalytic processes.

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

Supramolecular Assembly of Lanthanide-Binding Tag Peptides for Aqueous Separation of Rare Earth Elements

Selective and eco-friendly separation and purification methods for rare earth elements (REEs) are necessary to meet the increasing demand for these valuable metals, which are extensively used in modern electronics and clean energy technologies. Mining feedstocks consist of REE mixtures as stable trivalent cations (Ln 3+ ) that are difficult to separate due to their identical charge and similar size. Lanthanide-binding tags (LBTs), peptide chelates that coordinate Ln 3+ in binding pockets, show promise as selective, high-affinity extractants. We demonstrate that the LBT variant LBTLLA 5– , designed for high selectivity for Tb 3+ , is an effective extractant, forming complexes with REEs in solution that subsequently organize into self-assembling structures rich in Ln 3+ . These structures condense into aggregates that can be separated, enabling an efficient, all-aqueous, eco-friendly separation process. The self-assembled structures are studied using dynamic light scattering, ζ-potential measurements, transmission electron microscopy, anomalous small-angle X-ray scattering, inductively coupled plasma optical emission spectroscopy, and ultraviolet–visible absorption spectroscopy, which confirm LBTLLA 5– peptide-REE ion binding and the further assembly of micron-scale structures rich in REEs. Molecular dynamics simulations reveal the interactions promoting aggregation as well as the integrity of the binding pocket upon self-assembly. We find that LBTLLA 5– :Ln 3+ complexes recruit excess cations within the macrostructures, and we demonstrate that aggregation and selective separation can be controlled by manipulating the metal-peptide ratio in solution. Furthermore, we demonstrate separation from equimolar mixtures of REE pairs Tb 3+ -Lu 3+ and Tb 3+ -La 3+ , supporting the application of LBT peptides as a platform for the selective separation of REEs.

LBT peptides↗
DOE OSTI2023

A beamline for ultrafast extreme ultraviolet magneto-optical spectroscopy in reflection near the shot noise limit

High harmonic generation (HHG) makes it possible to measure spin and charge dynamics in materials on femtosecond to attosecond timescales. However, the extreme nonlinear nature of the high harmonic process means that intensity fluctuations can limit measurement sensitivity. Here we present a noise-canceled, tabletop high harmonic beamline for time-resolved reflection mode spectroscopy of magnetic materials. We use a reference spectrometer to independently normalize the intensity fluctuations of each harmonic order and eliminate long term drift, allowing us to make spectroscopic measurements near the shot noise limit. Here, these improvements allow us to significantly reduce the integration time required for high signal-to-noise (SNR) measurements of element-specific spin dynamics. Looking forward, improvements in the HHG flux, optical coatings, and grating design can further reduce the acquisition time for high SNR measurements by 1–2 orders of magnitude, enabling dramatically improved sensitivity to spin, charge, and phonon dynamics in magnetic materials.

47 OTHER INSTRUMENTATION↗
DOE OSTI2025

Chloride-Based Volatility for Waste Reduction and/or Reuse of Metallic-, Oxide- and Salt-Based Reactor Fuels

The objective of the chloride based volatility project (CBV) was to demonstrate the ability to separate uranium from used fuel to enable process improvements resulting in 10x reduction of waste volume, while maintaining safeguards standards and global backend costs at $\$$1/MW-hr. Current industrial practices perform separations of used fuel using solvent media in the form of aqueous and molten salt processes, resulting in contaminated process waste. The CBV approach utilized solid state chemistry with no solvent media and was successful in chlorinating uranium and fission product oxides that sublimed into the gas phase and were collected in targeted condensation zones based on temperature gradients of chlorinated products. Recovery of better than 95% of initial uranium in the form of UCl4 was demonstrated when simulated used nuclear fuel was used. Laser induced breakdown spectroscopy, LIBS, and ultraviolet-visible spectroscopy, UV-Vis, were combined into a high temperature flow cell design and utilized as process monitoring techniques to observed chlorinated products leave the reaction vessel in real time.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2022

Silver Nanoparticles Functionalized Nanosilica Grown over Graphene Oxide for Enhancing Antibacterial Effect

The continuous growth of multidrug-resistant bacteria due to the overuse of antibiotics and antibacterial agents poses a threat to human health. Silver nanoparticles, silica-based materials, and graphene-based materials have become potential antibacterial candidates. In this study, we developed an effective method of enhancing the antibacterial property of graphene oxide (GO) by growing nanosilica (NS) of approximately 50 nm on the graphene oxide (GO) surface. The structures and compositions of the materials were characterized through powdered X-ray diffraction (P-XRD), transmission electron microscopy (TEM), scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDS), ultraviolet–visible spectroscopy (UV–VIS), dynamic light scattering (DLS), Raman spectroscopy (RM), Fourier-transform infrared spectroscopy (FTIR), Brunauer–Emmet–Teller (BET) surface area, and pore size determination. The silver nanoparticles (AgNPs) with an average diameter of 26 nm were functionalized on the nanosilica (NS) surface. The composite contained approximately 3% of silver nanoparticles. The silver nanoparticles on nanosilica supported over graphene oxide (GO/NS/AgNPs) exhibited a 7-log reduction of Escherichia coli and a 5.2-log reduction of Bacillus subtilis within one hour of exposure. Both GO/NS and GO/NS/AgNPs exhibited substantial antimicrobial effects against E. coli and B. subtilis.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Characterization of Carrier Cooling Bottleneck in Silicon Nanoparticles by Extreme Ultraviolet (XUV) Transient Absorption Spectroscopy

Silicon nanoparticles have the promise to surpass the theoretical efficiency limit of single-junction silicon photovoltaics by the creation of a “phonon bottleneck,” a theorized slowing of the cooling rate of hot optical phonons that in turn reduces the cooling rate of hot carriers in the material. Verifying the presence of a phonon bottleneck in silicon nanoparticles requires simultaneous resolution of electronic and structural changes at short timescales. Here, extreme ultraviolet transient absorption spectroscopy is used to observe the excited-state electronic and lattice dynamics in polycrystalline silicon nanoparticles following 800 nm photoexcitation, which excites carriers with 0.35 ± 0.03 eV excess energy above the Δ 1 conduction band minimum. The nanoparticles have nominal 100 nm diameters with crystalline grain sizes of about ~16 nm. The extracted carrier–phonon and phonon–phonon relaxation times of the nanoparticles are compared to those for a silicon (100) single-crystal thin film at similar carrier densities (2 × 10 19 cm –3 for the nanoparticles and 6 × 10 19 cm –3 for the film). The measured carrier–phonon and phonon–phonon scattering lifetimes for the polycrystalline nanoparticles are 870 ± 40 fs and 17.5 ± 0.3 ps, respectively, versus 195 ± 20 fs and 8.1 ± 0.2 ps, respectively, for the silicon thin film. Furthermore, the reduced scattering rates observed in the nanoparticles are consistent with the phonon bottleneck hypothesis.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗