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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 1,495 records · Page 83
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Resistivity scaling and electron relaxation times in metallic nanowires

We study the resistivity scaling in nanometer-sized metallic wires due to surface roughness and grain-boundaries, currently the main cause of electron scattering in nanoscaled interconnects. The resistivity has been obtained with the Boltzmann transport equation, adopting the relaxation time approximation of the distribution function and the effective mass approximation for the conducting electrons. The relaxation times are calculated exactly, using Fermi's golden rule, resulting in a correct relaxation time for every sub-band state contributing to the transport. In general, the relaxation time strongly depends on the sub-band state, something that remained unclear with the methods of previous work. The resistivity scaling is obtained for different roughness and grain-boundary properties, showing large differences in scaling behavior and relaxation times. Our model clearly indicates that the resistivity is dominated by grain-boundary scattering, easily surpassing the surface roughness contribution by a factor of 10.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2025

Probing air-water interfaces of dibutyl phosphoric acid (HDBP) aqueous solutions using vibrational sum frequency generation (vSFG) spectroscopy

Liquid-liquid extraction is a separation technique implemented in a wide variety of areas, achieving particular success in both the nuclear and biomedical fields. In this work, vibrational sum frequency generation spectroscopy (VSFG) and surface tension measurements were used to investigate the adsorption of dibutyl phosphate (DBP) at air-aqueous interfaces to simulate liquid-liquid systems relevant to the Plutonium Uranium Redox Extraction (PUREX) Process. The objective of this work is to establish qualitative relationships between changes in the bulk aqueous phase concentrations of DBP and its concentration and structure at air-liquid interface as probed with VSFG. Nitric acid concentration and solution ionic strength were varied to examine their effect on the interfacial DBP.. Introduction of DBP into neat water resulted in reduction of the VSFG spectral intensity in the dangling O-H region (3680 – 3800 cm -1 ) but large increase in the H-bonded O-H stretch frequency region (3000 – 3500 cm-1) and the appearance of the CH 3 symmetric stretch and CH 3 Fermi resonance peaks at ~ 2880 and 2945 cm -1 , respectively, indicating DBP at the air-water interface. The intensity of the C-H strecth peaks increased as DBP concentration increased from 0.24 to 32 mM, accompanied by a decreasing surface tension values. At fixed DBP concentration, the addition of either or both of HNO 3 and NaNO 3 to an ionic strength of 1 M or 3 M led to significant reduction of the O-H VSFG peaks and enhancement of the C-H peaks. The origins of these experimental observations are attributed to both the increased HDBP molecules partitioning and adsorption to the interface and the protonation of the interfacial DBP- molecules.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Measuring the Surface Photovoltage of a Schottky Barrier under Intense Light Conditions: Zn/p-Si(100) by Laser Time-Resolved Extreme Ultraviolet Photoelectron Spectroscopy

A metal-semiconductor heterojunction is investigated by Auger and photoelectron spectroscopy to characterize the structural and electronic properties of the metallic film and to obtain the time-resolved electronic response induced by femtosecond laser excitation of the semiconductor material. The 3.5 monolayer (ML) Zn films deposited on p-type Si(100) at liquid nitrogen temperature grows in a layer-by-layer fashion. Electronic structure measurements by extreme ultraviolet (XUV) photoelectron spectroscopy indicate that the films are metallic in nature, creating a Schottky barrier at the 3.5 ML Zn/p-Si(100) interface. Utilizing a 35 fs, 800 nm pump pulse at a pump intensity of (2.5-6) × 10 9 W/cm 2 to excite the Si and a time-delayed extreme ultraviolet pulse to probe the Zn, we observed large transient surface photovoltage shifts of 0.3-2.2 eV at carrier densities of (1.5-4.5) × 10 20 cm -3 . Three shifts are determined the Zn 3d core level, the photoemission onset, and the metallic Fermi level. The photovoltages increase with laser excitation intensity, and the Zn 3d core level exhibits the largest binding energy shifts due to pronounced screening of the core level. The large observed shifts are rationalized on the basis of the energetics of band flattening and carrier accumulation in the metallic layer of the Zn/p-Si(100) heterojunction at high carrier densities. The observed carrier recombination dynamics are biexponential in character, with similar time constants for both the Zn 3d and photoemission onset binding energy shifts. The Zn 3d core level shifts are also found to be sensitive to the electron temperature. These results show that core-level photoemission can be used to monitor valence electron dynamics, allowing separation of charge dynamics in heterojunctions and solids composed of multiple elements.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Identification of Possible Pathways for C–C Bond Formation during Electrochemical Reduction of CO 2 : New Theoretical Insights from an Improved Electrochemical Model

Here, we have carried out a periodic Kohn-Sham density functional theory investigation of the pathways by which carbon-carbon bonds could be formed during the electrochemical reduction of CO 2 on Cu(100) using a model that includes the effects of the electrochemical potential, solvent, and electrolyte. The electrochemical potential was set by relating the applied potential to the Fermi energy and then calculating the number of electrons required by the simulation cell for that specific Fermi energy. The solvent was included as a continuum dielectric, and the electrolyte was described using a linearized Poisson-Boltzmann model. The calculated potential of zero charge for a variety of surfaces agrees with experiment to within a mean average error of 0.09 V, thereby validating the assumptions of the model. Analysis of the mechanism for C–C bond formation revealed that at low-applied potential, C–C bond formation occurs through a CO dimer. However, at high applied potentials, a large activation barrier blocks this pathway; therefore, C–C bond formation occurs through reaction of adsorbed CHO and CO. Rate parameters determined from our calculations were used to simulate the kinetics of ethene formation during the electrochemical reduction of CO over a Cu(100) surface. An excellent match was observed between previously reported measurements of the partial current for ethene formation as a function of applied voltage and the variation in the partial current for C–C bond formation predicted by our microkinetic model. The electrochemical model reported here is simple, fairly easy to implement, and involves only a small increase in computational cost over calculations neglecting the effects of the electrolyte and the applied field. Therefore, it can be used to study the effects of applied potential and electrolyte composition on the energetics of surface reactions for a wide variety of electrochemical reactions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Chemical Reactivity Descriptor for the Oxide-Electrolyte Interface in Li-Ion Batteries

Understanding electrochemical and chemical reactions at the electrode–electrolyte interface is of fundamental importance for the safety and cycle life of Li-ion batteries. Positive electrode materials such as layered transition metal oxides exhibit different degrees of chemical reactivity with commonly used carbonate-based electrolytes. Here we employed density functional theory methods to compare the energetics of four different chemical reactions between ethylene carbonate (EC) and layered (LixMO2) and rocksalt (MO) oxide surfaces. EC dissociation on layered oxides was found energetically more favorable than nucleophilic attack, electrophilic attack, and EC dissociation with oxygen extraction from the oxide surface. In addition, EC dissociation became energetically more favorable on the oxide surfaces with transition metal ions from left to right on the periodic table or by increasing transition metal valence in the oxides, where higher degree of EC dissociation was found as the Fermi level was lowered into the oxide O 2p band.

25 ENERGY STORAGE↗
DOE OSTI2024

Correlated topological flat bands in rhombohedral graphite

Flat bands and nontrivial topological physics are two important topics of condensed matter physics. With a unique stacking configuration analogous to the Su–Schrieffer–Heeger model, rhombohedral graphite (RG) is a potential candidate for realizing both flat bands and nontrivial topological physics. Here, in this study, we report experimental evidence of topological flat bands (TFBs) on the surface of bulk RG, which are topologically protected by bulk helical Dirac nodal lines via the bulk-boundary correspondence. Moreover, upon in situ electron doping, the surface TFBs show a splitting with exotic doping evolution, with an order-of-magnitude increase in the bandwidth of the lower split band, and pinning of the upper band near the Fermi level. These experimental observations together with Hartree–Fock calculations suggest that correlation effects are important in this system. Our results demonstrate RG as a platform for investigating the rich interplay between nontrivial band topology, correlation effects, and interaction-driven symmetry-broken states.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Ultrafast direct electron transfer at organic semiconductor and metal interfaces

The ability to control direct electron transfer can facilitate the development of new molecular electronics, light-harvesting materials, and photocatalysis. However, control of direct electron transfer has been rarely reported, and the molecular conformation–electron dynamics relationships remain unclear. We describe direct electron transfer at buried interfaces between an organic polymer semiconductor film and a gold substrate by observing the first dynamical electric field–induced vibrational sum frequency generation (VSFG). In transient electric field– induced VSFG measurements on this system, we observe dynamical responses (<150 fs) that depend on photon energy and polarization, demonstrating that electrons are directly transferred from the Fermi level of gold to the lowest unoccupied molecular orbital of organic semiconductor. Transient spectra further reveal that, although the interfaces are prepared without deliberate alignment control, a sub-ensemble of surface molecules can adopt conformations for direct electron transfer. Density functional theory calculations support the experimental results and ascribe the observed electron transfer to a flat-lying polymer configuration in which electronic orbitals are found to be delocalized across the interface. The present observation of direct electron transfer at complex interfaces and the insights gained into the relationship between molecular conformations and electron dynamics will have implications for implementing novel direct electron transfer in energy materials.

36 MATERIALS SCIENCE↗
NASA NTRS1999

Transmission Measurement of the Third-Order Susceptibility of Gold

Gold nanoparticle composites are known to display large optical nonlinearities. In order to assess the validity of generalized effective medium theories (EMT's) for describing the linear and nonlinear optical properties of metal nanoparticle composites, knowledge of the linear and nonlinear susceptibilities of the constituent materials is a prerequisite. In this study the inherent nonlinearity of the metal is measured directly (rather than deduced from a suitable EMT) using a very thin gold film. Specifically, we have used the z-scan technique at a wavelength near the transmission window of bulk gold to measure the third-order susceptibility of a continuous thin gold film deposited on a quartz substrate surface-modified with a self-assembled monolayer to promote adhesion and uniformity without affecting the optical properties. We compare our results with predictions which ascribe the nonlinear response to a Fermi-smearing mechanism. Further, we note that the sign of the nonlinear susceptibility is reversed from that of gold nanoparticle composites.

Smith, David D.↗
DOE OSTI2018

Conformational explosion: Understanding the complexity of short chain para -dialkylbenzene potential energy surfaces

The single-conformation ultraviolet and infrared spectroscopy of three short-chain para-dialkylbenzenes (para-diethylbenzene, para-dipropylbenzene, and para-dibutylbenzene) is reported for the jet-cooled, isolated molecules. The present study builds off previous work on single-chain n-alkylbenzenes, where an anharmonic local mode Hamiltonian method was developed to account for stretch-bend Fermi resonance in the alkyl CH stretch region [D. P. Tabor et al., J. Chem. Phys. 144, 224310 (2016)]. The jet-cooled molecules are interrogated using laser-induced fluorescence (LIF) excitation, fluorescence dip infrared spectroscopy, and dispersed fluorescence. The LIF spectra in the S1 ← S0 origin region show a dramatic increase in the number of resolved transitions with increasing length of the alkyl chains, reflecting an explosion in the number of unique low-energy conformations formed when two independent alkyl chains are present. Since the barriers to isomerization of the alkyl chain are similar in size, this results in an “egg carton” shaped potential energy surface. A combination of electronic frequency shift and alkyl CH stretch infrared spectra is used to generate a consistent set of conformational assignments. Using these experimental techniques in conjunction with computational methods, subsets of origin transitions in the LIF excitation spectrum can be classified into different conformational families. Two conformations are resolved in para-diethylbenzene, seven in para-dipropylbenzene, and about nineteen in para-dibutylbenzene. These chains are largely independent of each other as there are no new single-chain conformations induced by the presence of a second chain. A cursory LIF excitation scan of para-dioctylbenzene shows a broad congested spectrum at frequencies consistent with interactions of alkyl chains with the phenyl π cloud

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

Ligand-Dependent Energetics for Dehydrogenation: Implications in Li-Ion Battery Electrolyte Stability and Selective Oxidation Catalysis of Hydrogen-Containing Molecules

The hydrogen adsorption energetics on the surface of inorganic compounds can be used to predict electrolyte stability in Li-ion batteries and catalytic activity for selective oxidation of small molecules such as H 2 and CH 4 . Using first-principles density functional theory (DFT), the hydrogen adsorption was found to be unfavorable on high-band-gap insulators, which could be attributed to a lower energy level associated with adsorbed hydrogen relative to the bottom of the conduction band. In contrast, the hydrogen adsorption was shown to be the most favorable on metallic and semiconducting compounds, which results from an electron transfer from adsorbed hydrogen to the Fermi level or the bottom of the conduction band. Of significance, computed hydrogen adsorption energetics on insulating, semiconducting, and metallic oxides; phosphates; fluorides; and sulfides were decreased by lowering the ligand p band center, while the energy penalty for ligand vacancy formation was increased, indicative of decreased surface reducibility. A statistical regression analysis, where 16 structural and electronic parameters such as metal–ligand distance, electronegativity difference, Bader charges, bulk and surface metal and ligand band centers, band gap, ligand band width, and work function were examined, further showed that the surface ligand p band center is the most accurate single descriptor that governs the hydrogen adsorption tendency, and additional considerations of the band gap and average metal–ligand distance further reconcile the differences among compounds with different ligands/structures, whose ligand bands are different in shape and width. We discuss the implications of these findings for passivating coatings and design of catalysts and the need for novel theoretical methods to accurately estimate these quantities from first principles. Furthermore, these results establish a universal design principle for future high-throughput studies aiming to design electrode surfaces to minimize electrolyte oxidation by dehydrogenation in Li-ion batteries and enhance the H–H and C–H activation for selective oxidation catalysis.

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

Large surface conductance and superconductivity in topological insulator microstructures

Controllable geometric manipulation via micromachining techniques provides a promising tool for enhancing useful topological electrical responses relevant to future applications such as quantum information science [P. J. W. Moll, “Focused ion beam microstructuring of quantum matter,” Annu. Rev. Condens. Matter Phys. 9, 147 (2018); Jang et al., “Observation of half-height magnetization steps in Sr 2 RuO 4 ,” Science 331, 186 (2011); Moll et al., “Transport evidence for Fermi-arc-mediated chirality transfer in the Dirac semimetal Cd 3 As 2 ,” Nature 535, 266 (2016); Moll et al., “Evidence for hydrodynamic electron flow in PdCoO 2 ,” Science 351, 1061 (2016)]. Here, we present microdevices fabricated with a focused ion beam from an indium-doped topological insulator Pb 1–x Sn x Te. Furthermore, with the device thickness on the order of 1 μm and an extremely large bulk resistivity, we achieve an unprecedented enhancement of the surface contribution to about 30% of the total conductance near room temperature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

NuSTAR discovery of a young, energetic pulsar associated with the luminous gamma-ray source HESS J1640–465

We report the discovery of a 206 ms pulsar associated with the TeV γ-ray source HESS J1640–465 using the Nuclear Spectroscopic Telescope Array (NuSTAR) X-ray observatory. PSR J1640–4631 lies within the shell-type supernova remnant (SNR) G338.3–0.0, and coincides with an X-ray point source and putative pulsar wind nebula (PWN) previously identified in XMM-Newton and Chandra images. It is spinning down rapidly with period derivative P-dot = 9.758(44) × 10{sup –13}, yielding a spin-down luminosity E-dot = 4.4 × 10{sup 36} erg s{sup –1}, characteristic age τ{sub c}≡P/2 P-dot = 3350 yr, and surface dipole magnetic field strength B{sub s} = 1.4 × 10{sup 13} G. For the measured distance of 12 kpc to G338.3–0.0, the 0.2-10 TeV luminosity of HESS J1640–465 is 6% of the pulsar's present E-dot . The Fermi source 1FHL J1640.5–4634 is marginally coincident with PSR J1640–4631, but we find no γ-ray pulsations in a search using five years of Fermi Large Area Telescope (LAT) data. The pulsar energetics support an evolutionary PWN model for the broadband spectrum of HESS J1640–465, provided that the pulsar's braking index is n ≈ 2, and that its initial spin period was P {sub 0} ∼ 15 ms.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Electronic coupling between a FeSe monolayer film and SrTiO 3 substrate

Several experimental groups have reported superconductivity in single unit cell layers of FeSe on SrTiO 3 and a few other substrates, with critical temperature $T_c$ reports ranging up to 100 K, and a variety of theoretical work has been done. In this work we examine more closely the interaction of a single FeSe layer with a TiO 2 terminated SrTiO 3 (001) (STO) substrate. Several situations are analyzed: the underlying ideal interface, the effect of zpolarized longitudinal optic [LO($\hat{z}$)] phonons in STO, electron doping of the STO substrate, substitution of Se by the bordering chalcogenides S and Te, and doping by adsorption of K on the FeSe surface. These results complement earlier studies of O and Se vacancies. The O p y , p y surface band of STO persists at the interface, and by sharing holes with the hole pocket of FeSe it plays a part in the behavior around the interface, initially by determining the Fe Fermi level lineup with the STO band gap. The LO($\hat{z}$) mode causes strong band shifts around the interface but the strength of coupling to the Fe bands cannot be obtained with our methods. Adsorption of 25% K (one K per four Fe) fills the small O interface hole pocket and donates the rest of the electrons to the Fe hole pockets, filling them.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Sulfur passivation of semi-insulating GaAs: Transition from Coulomb blockade to weak localization regime

The sulfur passivation of the semi-insulating GaAs bulk (SI GaAs) grown in an excess phase of arsenic is used to observe the transition from the Coulomb blockade to the weak localization regime at room temperature. The I–V characteristics of the SI GaAs device reveal nonlinear behavior that appears to be evidence of the Coulomb blockade process as well as the Coulomb oscillations. The sulfur passivation of the SI GaAs device surface results in enormous transformation of the I–V characteristics that demonstrate the strong increase of the resistance and Coulomb blockade regime is replaced by the electron tunneling processes. The results obtained are analyzed within frameworks of disordering SI GaAs surface that is caused by inhomogeneous distribution of the donor and acceptor anti-site defects which affects the conditions of quantum- mechanical tunneling. Weak localization processes caused by the preservation of the Fermi level pinning are demonstrated by measuring the negative magnetoresistance in weak magnetic fields at room temperature. Finally, the studies of the magnetoresistance at higher magnetic fields reveal the h/2e Aharonov–Altshuler–Spivak oscillations with the complicated behavior due to possible statistical mismatch of the interference paths in the presence of different microdefects.

36 MATERIALS SCIENCE↗
NASA NTRS1975

Efficiency of silicon solar cells as a function of base layer resistivity

This paper reports on a theoretical study of the limitations on silicon solar-cell efficiency for both n(+)-p and n(+)-p-p(+) type cells. Detailed calculations have been made of solar-cell operation using a general computer analysis program for semiconductor devices. The computer program, which simultaneously solves Poisson's equation and the electron and hole quasi-Fermi level equations, provides an accurate numerical solution of solar-cell operation without limiting assumptions or approximations. It is found that minority-carrier lifetime and heavy doping effects in the n(+) surface region present serious limitations to efficiency in low-resistivity silicon solar cells.

Dunbar, P. M.↗
NASA NTRS1996

Dust and Sputtered Particle Streams in Cometary Atmosphere

Dynamics of dust particles in a comet atmosphere was investigated using Monte-Carlo simulation method taking into account radiation and gravitational forces applied to the particles. The model distribution function found out in VEGA experiments was chosen for calculations. Dust particles with masses more then 10(exp -10) g were considered. The delay effect for the big mass particles is shown. In this paper the boundary distribution function for sputtered particles drifting off the surface was determined using Nochilla's model. General dependences for the normal component of the relative mass velocity of sputtered particles are given as the function of particle energy normalized by Tomas-Fermi energy.

Svirschevsky, Stanislav B.↗
DOE OSTI2015

Sign change in the tunnel magnetoresistance of Fe{sub 3}O{sub 4}/MgO/Co-Fe-B magnetic tunnel junctions depending on the annealing temperature and the interface treatment

Magnetite (Fe{sub 3}O{sub 4}) is an eligible candidate for magnetic tunnel junctions (MTJs) since it shows a high spin polarization at the Fermi level as well as a high Curie temperature of 585°C. In this study, Fe{sub 3}O{sub 4}/MgO/Co-Fe-B MTJs were manufactured. A sign change in the TMR is observed after annealing the MTJs at temperatures between 200°C and 280°C. Our findings suggest an Mg interdiffusion from the MgO barrier into the Fe{sub 3}O{sub 4} as the reason for the change of the TMR. Additionally, different treatments of the magnetite interface (argon bombardment, annealing at 200°C in oxygen atmosphere) during the preparation of the MTJs have been studied regarding their effect on the performance of the MTJs. A maximum TMR of up to -12% could be observed using both argon bombardment and annealing in oxygen atmosphere, despite exposing the magnetite surface to atmospheric conditions before the deposition of the MgO barrier.

36 MATERIALS SCIENCE↗