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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 919 records · Page 51
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Mechanisms for the formation of thymine under astrophysical conditions and implications for the origin of life

Nucleobases are the carriers of the genetic information in ribonucleic acid and deoxyribonucleic acid (DNA) for all life on Earth. Their presence in meteorites clearly indicates that compounds of biological importance can form via non-biological processes in extraterrestrial environments. Recent experimental studies have shown that the pyrimidine-based nucleobases uracil and cytosine can be easily formed from the ultraviolet irradiation of pyrimidine in H{sub 2}O-rich ice mixtures that simulate astrophysical processes. In contrast, thymine, which is found only in DNA, is more difficult to form under the same experimental conditions, as its formation usually requires a higher photon dose. Earlier quantum chemical studies confirmed that the reaction pathways were favorable provided that several H{sub 2}O molecules surrounded the reactants. However, the present quantum chemical study shows that the formation of thymine is limited because of the inefficiency of the methylation of pyrimidine and its oxidized derivatives in an H{sub 2}O ice, as supported by the laboratory studies. Our results constrain the formation of thymine in astrophysical environments and thus the inventory of organic molecules delivered to the early Earth and have implications for the role of thymine and DNA in the origin of life.

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

A possible new origin of long absorption tail in Nd-doped yttrium aluminum garnet induced by 15 MeV gold-ion irradiation and heat treatment

When ion irradiation introduces point-defects in semiconductors/insulators, discrete energy levels can be introduced in the bandgap, and then optical transitions whose energies are lower than the bandgap become possible. The electronic transitions between the discrete level and the continuous host band are observed as a continuous tail starting from the fundamental edge. This is the well-known mechanism of the absorption tail close to the band-edge observed in many semiconductors/insulators. In this paper, we propose another mechanism for the absorption tail, which is probably active in Nd-doped yttrium aluminum garnet (Nd:YAG) after ion irradiation and annealing. A Nd:YAG bulk crystal was irradiated with 15 MeV Au{sup 5+} ions to a fluence of 8 × 10{sup 14} ions/cm{sup 2}. The irradiation generates an amorphous layer of ∼3 μm thick with refractive index reduction of Δn = −0.03. Thermal annealing at 1000 °C induces recrystallization to randomly aligned small crystalline grains. Simultaneously, an extraordinarily long absorption tail appeared in the optical spectrum covering from 0.24 to ∼2 μm without fringes. The origin of the tail is discussed based on two models: (i) conventional electronic transitions between defect levels and YAG host band and (ii) enhanced light scattering by randomly aligned small grains.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Origin and annealing of deep-level defects in GaNAs grown by metalorganic vapor phase epitaxy

Deep-level defects were investigated by deep level transient spectroscopy on the as-grown and annealed GaNAs layers of various nitrogen (N) contents. The unintentionally doped (uid) GaNAs layers were grown by metalorganic vapor phase epitaxy with N = 1.4%, 2.0%, 2.2%, and 2.4% on GaAs substrate. The possible origin and evolution of the deep-level defects upon annealing were analyzed with the use of the GaNAs band gap diagram concept [Kudrawiec et al., Appl. Phys. Lett. 101, 082109 (2012)], which assumes that the activation energy of donor traps decreases with N-related downward shift of the conduction band. On the basis of this diagram and in comparison with previous results, the N-related traps were associated with (N−As){sub As} or (N−N){sub As} split interstitials. It was also proposed that one of the electron traps and the hole trap, lying at the same level position in the bandgap of the annealed uid-GaNAs layers, can both act as one generation-recombination center partially responsible for poor optical properties of this alloy.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Spectroscopic studies of the physical origin of environmental aging effects on doped graphene

The environmental aging effect of doped graphene is investigated as a function of the organic doping species, humidity, and the number of graphene layers adjacent to the dopant by studies of the Raman spectroscopy, x-ray and ultraviolet photoelectron spectroscopy, scanning electron microscopy, infrared spectroscopy, and electrical transport measurements. It is found that higher humidity and structural defects induce faster degradation in doped graphene. Detailed analysis of the spectroscopic data suggest that the physical origin of the aging effect is associated with the continuing reaction of H{sub 2}O molecules with the hygroscopic organic dopants, which leads to formation of excess chemical bonds, reduction in the doped graphene carrier density, and proliferation of damages from the graphene grain boundaries. These environmental aging effects are further shown to be significantly mitigated by added graphene layers.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Electronic origin of spatial self-phase modulation: Evidenced by comparing graphite with C{sub 60} and graphene

We report unambiguous observation of spatial self-phase modulation (SSPM) in a dispersive suspension of graphite flakes. This coherent nonlinear optical effect in bulk graphite is found to be broadband and large, with a third-order nonlinear susceptibility χ{sup (3)} of 2.2 × 10{sup −9} esu (i.e., 3.1 × 10{sup −17} m{sup 2}/V{sup 2} in SI units) at 532 nm excitation. Comparison with other carbon allotropes shows that this value is 5 × 10{sup 7} times higher than that of C{sub 60} but ∼50 times lower than that of graphene, fully exhibiting the electronic origin of SSPM.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Ferroelectric-like hysteresis loop originated from non-ferroelectric effects

Piezoresponse force microscopy (PFM) has provided advanced nanoscale understanding and analysis of ferroelectric and piezoelectric properties. In PFM-based studies, electromechanical strain induced by the converse piezoelectric effect is probed and analyzed as a PFM response. However, electromechanical strain can also arise from several non-piezoelectric origins that may lead to a misinterpretation of the observed response. Among them, electrostatic interaction can significantly affect the PFM response. Nonetheless, previous studies explored solely the influence of electrostatic interaction on the PFM response under the situation accompanied with polarization switching. Here, we show the influence of the electrostatic interaction in the absence of polarization switching by using unipolar voltage sweep. The obtained results reveal that the electromechanical neutralization between piezoresponse of polarization and electrostatic interaction plays a crucial role in the observed ferroelectric-like hysteresis loop despite the absence of polarization switching. Furthermore, our work can provide a basic guideline for the correct interpretation of the hysteresis loop in PFM-based studies.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Tests for, origins of, and corrections to non-Gaussian statistics. The dipole-flip model

Linear response approximations are central to our understanding and simulations of nonequilibrium statistical mechanics. Despite the success of these approaches in predicting nonequilibrium dynamics, open questions remain. Laird and Thompson [J. Chem. Phys. 126, 211104 (2007)] previously formalized, in the context of solvation dynamics, the connection between the static linear-response approximation and the assumption of Gaussian statistics. The Gaussian statistics perspective is useful in understanding why linear response approximations are still accurate for perturbations much larger than thermal energies. In this study, we use this approach to address three outstanding issues in the context of the “dipole-flip” model, which is known to exhibit nonlinear response. First, we demonstrate how non-Gaussian statistics can be predicted from purely equilibrium molecular dynamics (MD) simulations (i.e., without resort to a full nonequilibrium MD as is the current practice). Second, we show that the Gaussian statistics approximation may also be used to identify the physical origins of nonlinear response residing in a small number of coordinates. Third, we explore an approach for correcting the Gaussian statistics approximation for nonlinear response effects using the same equilibrium simulation. Finally, the results are discussed in the context of several other examples of nonlinear responses throughout the literature.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Origin of dielectric relaxor behavior in PVDF-based copolymer and terpolymer films

Relaxor ferroelectrics exhibit frequency-dispersion of their dielectric permittivity peak as a function of temperature, the origin of which has been widely debated. Microscopic understanding of such behavior for polymeric ferroelectrics has presented new challenges since unlike traditional ceramic ferroelectrics, dielectric relaxation in polymers is a consequence of short-range molecular dynamics that are difficult to measure directly. Here, through careful analysis of atomic-level H-atom dynamics as determined by Quasi-elastic Neutron Scattering (QENS), we show that short-range molecular dynamics within crystalline domains cannot explain the macroscopic frequency-dispersion of dielectric properties observed in prototypical polyvinylidene-fluoride (PVDF)-based relaxor ferroelectrics. Instead, from multiscale quantitative microstructural characterization, a clear correlation between the amount of crystalline-amorphous interfaces and dielectric relaxation is observed, which indicates that such interfaces play a central role. These results provide critical insights into the role of atomic and microscopic structures towards relaxor behavior in ferroelectric polymers, which will be important for their future design.

42 ENGINEERING↗
DOE OSTI2018

Origin and roles of oxygen impurities in hexagonal boron nitride epilayers

Photoluminescence emission spectroscopy and electrical transport measurements have been employed to study the origin and roles of oxygen impurities in hexagonal boron nitride (h-BN) epilayers grown on sapphire substrates. The temperature dependence of the electrical resistivity revealed the presence of a previously unnoticed impurity level of about 0.6 eV in h-BN epilayers grown at high temperatures. The findings suggested that in addition to the common nitrogen vacancy (V N ) shallow donors in h-BN, oxygen impurities diffused from sapphire substrates during high temperature growth also act as substitutional donors (O N ). The presence of O N gives rise to an additional emission peak in the photoluminescence spectrum, corresponding to a donor-acceptor pair recombination involving the O N donor and the C N (carbon occupying nitrogen site) deep level acceptor. Moreover, due to the presence of O N donors, the majority charge carrier type changed to electrons in epilayers grown at high temperatures, in contrast to typical h-BN epilayers which naturally reflect “p-type” character. The results offered a more coherent picture for common impurities/defects in h-BN as well as a better understanding of the growth mediated impurities in h-BN epilayers, which will be helpful for finding possible ways to further improve the quality and purity of this emerging material.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2018

The origin of holomorphic states in Landau levels from non-commutative geometry and a new formula for their overlaps on the torus

Holomorphic functions that characterize states in a two-dimensional Landau level have been central to key developments such as the Laughlin state. Their origin has historically been attributed to a special property of “Schrödinger wavefunctions” of states in the “lowest Landau level.” In this work, it is shown that they instead arise in any Landau level as a generic mathematical property of the Heisenberg description of the non-commutative geometry of guiding centers. When quasiperiodic boundary conditions are applied to compactify the system on a torus, a new formula for the overlap between holomorphic states, in the form of a discrete sum rather than an integral, is obtained. The new formula is unexpected from the previous “lowest-Landau level Schrödinger wavefunction” interpretation.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Origins and effects of mix on magnetized liner inertial fusion target performance

In magneto-inertial-fusion experiments, energy losses such as a radiation need to be well controlled in order to maximize the compressional work done on the fuel and achieve thermonuclear conditions. One possible cause for high radiation losses is high-Z material mixing from the target components into the fuel. In this work, we analyze the effects of mix on target performance in Magnetized Liner Inertial Fusion (MagLIF) experiments at Sandia National Laboratories. Our results show that mix is likely produced from a variety of sources, approximately half of which originates during the laser heating phase and the remainder near stagnation, likely from the liner deceleration. By changing the “cushion” component of MagLIF targets from Al to Be, we achieved a 10× increase in neutron yield, a 60% increase in ion temperature, and an ~50% increase in fuel energy at stagnation.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2019

Vibrational probe of the origin of singlet exciton fission in TIPS-pentacene solutions

We use native vibrational modes of the model singlet fission chromophore 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-Pn) to examine the origins of singlet fission in solution between molecules that are not tethered by a covalent linkage. We use the C—H stretch modes of TIPS side groups of TIPS-Pn to demonstrate that singlet fission does not occur by diffusive encounter of independent molecules in solution. Instead, TIPS-Pn molecules aggregate in solution through their TIPS side groups. This aggregation breaks the symmetry of the TIPS-Pn molecules and enables the formation of triplets to be probed through the formally symmetry forbidden symmetric alkyne stretch mode of the TIPS side groups. The alkyne stretch modes of TIPS-Pn are sensitive to the electronic excited states present during the singlet fission reaction and provide unique signatures of the formation of triplets following the initial separation of triplet pair intermediates. These findings highlight the opportunity to leverage structural information from vibrational modes to better understand intermolecular interactions that lead to singlet fission.

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

Origins of hydrogen that passivates bulk defects in silicon heterojunction solar cells

Silicon heterojunction solar cell fabrication incorporates a significant amount of hydrogen into the silicon wafer bulk, and the amount of injected hydrogen is comparable to that introduced by silicon nitride films during a high-temperature firing step. In this work, the origins of the hydrogen injected during heterojunction cell processing have been identified. We demonstrate that the hydrogen plasma treatment that is routinely included to improve surface passivation considerably increases the hydrogen concentration in the wafers. We also show that the hydrogenated amorphous silicon i/p+ stack is more effective than the i/n+ stack for bulk hydrogen incorporation, and both are more effective than intrinsic films alone.

Sun, Chang (ORCID:0000000282140458)↗
DOE OSTI2020

Origin of core-to-core x-ray emission spectroscopy sensitivity to structural dynamics

Recently, coherent structural dynamics in the excited state of an iron photosensitizer was observed through oscillations in the intensity of Kα x-ray emission spectroscopy (XES). Understanding the origin of the unexpected sensitivity of core-to-core transitions to structural dynamics is important for further development of femtosecond time-resolved XES methods and, we believe, generally necessary for interpretation of XES signals from highly non-equilibrium structures that are ubiquitous in photophysics and photochemistry. Here, we use multiconfigurational wavefunction calculations combined with atomic theory to analyze the emission process in detail. The sensitivity of core-to-core transitions to structural dynamics is due to a shift of the minimum energy metal–ligand bond distance between 1s and 2p core-hole states. A key effect is the additional contraction of the non-bonding 3s and 3p orbitals in 1s core-hole states, which decreases electron– electron repulsion and increases overlap in the metal–ligand bonds. The effect is believed to be general and especially pronounced for systems with strong bonds. The important role of 3s and 3p orbitals is consistent with the analysis of radial charge and spin densities and can be connected to the negative chemical shift observed for many transition metal complexes. The XES sensitivity to structural dynamics can be optimized by tuning the emission energy spectrometer, with oscillations up to ±4% of the maximum intensity for the current system. The theoretical predictions can be used to design experiments that separate electronic and nuclear degrees of freedom in ultrafast excited state dynamics.

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

Mechanistic origins of methyl-driven Overhauser DNP

The Overhauser effect in the dynamic nuclear polarization (DNP) of non-conducting solids has drawn much attention due to the potential for efficient high-field DNP as well as a general interest in the underlying principles that enable the Overhauser effect in small molecules. We recently reported the observation of 1 H and 2 H Overhauser effects in H 3 C- or D 3 C-functionalized Blatter radical analogs, which we presumed to be caused by methyl rotation. In this work, we look at the mechanism for methyl-driven Overhauser DNP in greater detail, considering methyl librations and tunneling in addition to classical rotation. Here, we predict the temperature dependence of these mechanisms using density functional theory and spin dynamics simulations. Comparisons with results from ultralow-temperature magic angle spinning-DNP experiments revealed that cross-relaxation at temperatures above 60 K originates from both libration and rotation, while librations dominate at lower temperatures. Due to the zero-point vibrational nature of these motions, they are not quenched by very low temperatures, and methyl-driven Overhauser DNP is expected to increase in efficiency down to 0 K, predominantly due to increases in nuclear relaxation times.

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

Origins of the high-energy electroluminescence peaks in long-wavelength (~495–685 nm) InGaN light-emitting diodes

We investigate the unexpected high-energy electroluminescence (EL) peaks observed in long-wavelength InGaN light-emitting diodes (LEDs) with ground state emission peaks between ~495 and 685 nm by studying the EL spectra of LEDs with varying quantum well (QW) thicknesses and indium compositions. In addition to the ground state emission, two high-energy emission peaks were observed in the LEDs with thick QWs and high indium compositions. The less energetic high-energy emission peak (2.4–2.6 eV) is attributed to the optical transitions involving excited states. Factors influencing the excited state transitions, such as the QW thickness and indium compositions, were also examined by simulations to better understand the occurrence of these transitions. The more energetic high-energy emission peak (2.8–3.1 eV) originates from V-defect sidewalls and was verified through micro-photoluminescence measurements. Identification of the high-energy emission peaks is essential as it enables targeted epitaxial or growth optimizations to minimize or eliminate these undesirable emission peaks. In conclusion, this work demonstrates the importance of using thin QWs to suppress the unwanted high-energy emissions due to excited state transitions and V-defect sidewalls for long-wavelength InGaN LEDs.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Origin of proton irradiation-induced deep acceptors in Al 0.70 Ga 0.30 N

Deep level defect introduction and carrier removal were characterized using steady-state photocapacitance (SSPC), deep level transient spectroscopy (DLTS) and lighted capacitance–voltage for proton irradiated n-type Al 0.70 Ga 0.30 N Schottky diodes grown by metal-organic vapor phase epitaxy on AlN-on-sapphire templates. SSPC observed deep levels in the as-grown diode with zero-phonon transition energies of 2.20, 2.65, 3.10, 3.40, and 4.65 eV relative to the conduction band minimum (E c ), and an additional deep level emerged at 1.20 eV with irradiation. Lighted capacitance–voltage measurements quantified the deep level concentration (N t ) of states detected by SSPC, and it was observed that N t increased with proton fluence only for the 1.2 and 4.65 eV levels. Carrier removal was much larger than the increase in N t of the 1.2 and 4.65 eV deep levels, suggesting that radiation-induced deep level compensators existed beyond what was detected with SSPC. DLTS detected additional, proton-induced deep acceptors at E c —0.55, 0.82, and 1.16 eV, the latter of which is likely the same 1.20 eV deep state observed by SSPC. The concentration of the E c —0.82 eV defect state was large enough to reconcile carrier removal with total deep level introduction. Comparing the E c —0.82 and 1.16 eV deep acceptor levels to previous experimental and theoretical reports suggests that their atomistic origins could be the nitrogen vacancy (V N ) and oxygen substituting on the nitrogen sub-lattice (O N ), respectively. This defect behavior contrasts starkly with GaN, where V N and O N are shallow donors, and demonstrates that the electronic properties of defects can evolve drastically within the AlGaN alloy system ranging from wide bandgap GaN to ultra-wide bandgap AlN.

Armstrong, Andrew Michael [Sandia National Laborat↗
DOE OSTI2016

Origins of ultralow velocity zones through slab-derived metallic melt

Understanding the ultralow velocity zones (ULVZs) places constraints on the chemical composition and thermal structure of deep Earth and provides critical information on the dynamics of large-scale mantle convection, but their origin has remained enigmatic for decades. Recent studies suggest that metallic iron and carbon are produced in subducted slabs when they sink beyond a depth of 250 km. Here we show that the eutectic melting curve of the iron-carbon system crosses the current geotherm near Earth’s core-mantle boundary, suggesting that dense metallic melt may form in the lowermost mantle. If concentrated into isolated patches, such melt could produce the seismically observed density and velocity features of ULVZs. Depending on the wetting behavior of the metallic melt, the resultant ULVZs may be short-lived domains that are replenished or regenerated through subduction, or long-lasting regions containing both metallic and silicate melts. Slab-derived metallic melt may produce another type of ULVZ that escapes core sequestration by reacting with the mantle to form iron-rich post-bridgmanite or ferropericlase. The hypotheses connect peculiar features near Earth’s core-mantle boundary to subduction of the oceanic lithosphere through the deep carbon cycle.

58 GEOSCIENCES↗