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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 865 records · Page 48
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Directional charge transfer mediated by mid-gap states: A transient absorption spectroscopy study of CdSe quantum dot/β-Pb 0.33 V 2 O 5 heterostructures

For solar energy conversion, not only must a semiconductor absorb incident solar radiation efficiently but also its photoexcited electron—hole pairs must further be separated and transported across interfaces. Charge transfer across interfaces requires consideration of both thermodynamic driving forces as well as the competing kinetics of multiple possible transfer, cooling, and recombination pathways. In this work, we demonstrate a novel strategy for extracting holes from photoexcited CdSe quantum dots (QDs) based on interfacing with β-Pb 0.33 V 2 O 5 nanowires that have strategically positioned midgap states derived from the intercalating Pb 2+ ions. Unlike midgap states derived from defects or dopants, the states utilized here are derived from the intrinsic crystal structure and are thus homogeneously distributed across the material. CdSe/β-Pb 0.33 V 2 O 5 heterostructures were assembled using two distinct methods: successive ionic layer adsorption and reaction (SILAR) and linker-assisted assembly (LAA). Transient absorption spectroscopy measurements indicate that, for both types of heterostructures, photoexcitation of CdSe QDs was followed by the transfer of electrons to the conduction band of β-Pb 0.33 V 2 O 5 nanowires and holes to the midgap states of β-Pb 0.33 V 2 O 5 nanowires. Holes were transferred on time scales less than 1 ps, whereas electrons were transferred more slowly on time scales of ~2 ps. In contrast, for analogous heterostructures consisting of CdSe QDs interfaced with V 2 O 5 nanowires (wherein midgap states are absent), only electron transfer was observed. Interestingly, electron transfer was readily achieved for CdSe QDs interfaced with V 2 O 5 nanowires by the SILAR method; however, for interfaces incorporating molecular linkers, electron transfer was observed only upon excitation at energies substantially greater than the bandgap absorption threshold of CdSe. Furthermore, transient absorbance decay traces reveal longer excited-state lifetimes (1–3 μs) for CdSe/β-Pb 0.33 V 2 O 5 heterostructures relative to bare β-Pb 0.33 V 2 O 5 nanowires (0.2 to 0.6 μs); the difference is attributed to surface passivation of intrinsic surface defects in β-Pb 0.33 V 2 O 5 upon interfacing with CdSe.

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

Irradiation resistance of nanostructured interfaces in Zr–Nb metallic multilayers

We report that irradiation resistance of metallic nanostructured multilayers is determined by the interactions between defects and phase boundaries. However, the dose-dependent interfacial morphology evolution can greatly change the nature of the defect–boundary interaction mechanisms over time. In the present study, we used atomistic models combined with a novel technique based on the accumulation of Frenkel pairs to simulate irradiation processes. We examined dose effects on defect evolutions near zirconium–niobium multilayer phase boundaries. Our simulations enabled us to categorize defect evolution mechanisms in bulk phases into progressing stages of dislocation accumulation, saturation, and coalescence. In the metallic multilayers, we observed a phase boundary absorption mechanism early on during irradiation, while at higher damage levels, the increased irradiation intermixing triggered a phase transformation in the Zr–Nb mixture. Lastly, this physical phenomenon resulted in the emission of a large quantity of small immobile dislocation loops from the phase boundaries.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Morphological stability of Cu-Nb nanocomposites under high-energy collision cascades

We use molecular dynamics and phase field simulations to demonstrate that Cu-Nb multilayered nanocomposites with individual layer thicknesses above 2–4 nm remain morphologically stable when subjected to 100 keV collision cascades, characteristic of neutron or heavy ion irradiation. The probability of morphological instability rapidly increases as the layer thickness decreases to 1 nm, which we attribute to overlap of zones of liquid-like interdiffusion inside radiation-induced thermal spikes at neighboring interfaces in the multilayer.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Role of morphology in defect formation and photo-induced carrier instabilities in amorphous indium oxide

Ab initio molecular dynamics liquid-quench simulations and hybrid density functional calculations are performed to model the effects of room-temperature atomic fluctuations and photo-illumination on the structural and electronic properties of amorphous sub-stoichiometric In 2 O 2.96 . A large configurational ensemble is employed to reliably predict the distribution of localized defects as well as their response to the thermal and light activation. The results reveal that the illumination effects on the carrier concentration are greater in amorphous configurations with shorter In–O bond length and reduced polyhedral sharing as compared to the structures with a more uniform morphology. Further, the obtained correlation between the photo-induced carrier density and the reduction in the number of fully coordinated In-atoms implies that metal oxides with a significant fraction of crystalline/amorphous interfaces would show a more pronounced response to illumination. Photo-excitation also produces In–O 2 –In defects that have not been previously found in sub-stoichiometric amorphous oxides; these defects are responsible for carrier instabilities due to overdoping.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2013

Molecular beam epitaxy growth of high electron mobility InAs/AlSb deep quantum well structure

InAs/AlSb deep quantum well (QW) structures with high electron mobility were grown by molecular beam epitaxy (MBE) on semi-insulating GaAs substrates. AlSb and Al{sub 0.75}Ga{sub 0.25}Sb buffer layers were grown to accommodate the lattice mismatch (7%) between the InAs/AlSb QW active region and GaAs substrate. Transmission electron microscopy shows abrupt interface and atomic force microscopy measurements display smooth surface morphology. Growth conditions of AlSb and Al{sub 0.75}Ga{sub 0.25}Sb buffer were optimized. Al{sub 0.75}Ga{sub 0.25}Sb is better than AlSb as a buffer layer as indicated. The sample with optimal Al{sub 0.75}Ga{sub 0.25}Sb buffer layer shows a smooth surface morphology with root-mean-square roughness of 6.67 A. The electron mobility has reached as high as 27 000 cm{sup 2}/Vs with a sheet density of 4.54 Multiplication-Sign 10{sup 11}/cm{sup 2} at room temperature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

First principles density functional theory study of tritium species adsorption on Ni(111) surface and diffusion in nickel-sublayer for tritium storage

The nickel-plated zircaloy-4 is used as a tritium ( 3 H) getter in the tritium-producing burnable absorber rods (TPBARs) to capture 3 H produced in the 6 Li-riched annular γ-LiAlO 2 pellet under neutron irradiation. The experimental data and our previous theoretical results showed that the 3 H species produced from the γ-LiAlO 2 pellet were mainly 3 H 2 and 3 H 2 O. These 3 H species diffuse from the surface of the LiAlO 2 pellet across vacuum to the nickel-plated zircaloy-4 getter and then further diffuse into the getter to chemically form metal hydrides. While a number of studies show that oxygen binds strongly as compared to 3 H on the nickel (Ni) layer, the detailed mechanism of 3 H species absorption and diffusion across the Ni plate and Ni/Zr interface are still unclear. By employing density functional theory calculations, here we explored the 3 H 2 and 3 H 2 O species adsorption and dissociation on the Ni(111) surface and diffusion into the Ni sublayer. Our results indicated that the 3 H 2 and 3 H 2 O dissociate on the Ni(111) surface. The NiO x and Ni(O 3 H) x could be formed in the Ni layer due to the higher oxygen (O) diffusion energy barrier and formation of Ni vacancy defects. The oxygen was found to be retained in the Ni layer from diffusing across the Ni–Zr interface. This was revealed by comparing the diffusion barriers for 3 H with O. 3 H was found to have nearly three times smaller diffusion barrier than for O, making 3 H comparatively easier to diffuse through the Ni layer. In conclusion, the obtained results provide guidelines for experimental measurements on 3 H retention behavior in TPBARs and may open further avenues to explore the impurity effects on 3 H diffusion and storage at the Ni/zircaloy interfaces.

Tafen, De Nyago [National Energy Technology Lab. (↗
DOE OSTI2020

Coherent control and high-fidelity readout of chromium ions in commercial silicon carbide

Abstract Transition metal ions provide a rich set of optically active defect spins in wide bandgap semiconductors. Chromium (Cr 4+ ) in silicon-carbide (SiC) produces a spin-1 ground state with a narrow, spectrally isolated, spin-selective, near-telecom optical interface. However, previous studies were hindered by material quality resulting in limited coherent control. In this work, we implant Cr into commercial 4H-SiC and show optimal defect activation after annealing above 1600 °C. We measure an ensemble optical hole linewidth of 31 MHz, an order of magnitude improvement compared to as-grown samples. An in-depth exploration of optical and spin dynamics reveals efficient spin polarization, coherent control, and readout with high fidelity (79%). We report T 1 times greater than 1 s at cryogenic temperatures (15 K) with a T 2 * = 317 ns and a T 2 = 81 μs, where spin dephasing times are currently limited by spin–spin interactions within the defect ensemble. Our results demonstrate the potential of Cr 4+ in SiC as an extrinsic, optically active spin qubit.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2014

Atomic layer epitaxy of Ruddlesden-Popper SrO(SrTiO{sub 3}){sub n} films by means of metalorganic aerosol deposition

We report an atomic layer epitaxial growth of Ruddlesden-Popper (RP) thin films of SrO(SrTiO{sub 3}){sub n} (n = ∞, 2, 3, 4) by means of metalorganic aerosol deposition (MAD). The films are grown on SrTiO{sub 3}(001) substrates by means of a sequential deposition of Sr-O/Ti-O{sub 2} atomic monolayers, monitored in-situ by optical ellipsometry. X-ray diffraction and transmission electron microscopy (TEM) reveal the RP structure with n = 2–4 in accordance with the growth recipe. RP defects, observed by TEM in a good correlation with the in-situ ellipsometry, mainly result from the excess of SrO. Being maximal at the film/substrate interface, the SrO excess rapidly decreases and saturates after 5–6 repetitions of the SrO(SrTiO{sub 3}){sub 4} block at the level of 2.4%. This identifies the SrTiO{sub 3} substrate surface as a source of RP defects under oxidizing conditions within MAD. Advantages and limitations of MAD as a solution-based and vacuum-free chemical deposition route were discussed in comparison with molecular beam epitaxy.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Electrical and photovoltaic properties of CdTe/ZnTe n-i-p junctions grown by molecular beam epitaxy

Preliminary studies have been performed on photoelectrical properties of CdTe/ZnTe n-i-p junctions grown using the molecular beam epitaxy technique. Photovoltaic properties of the cells have been investigated by the measurements of current-voltage (I-V) characteristics under 1-sun illumination. I-V characteristics yield efficiencies of the cells varying from 3.4% to 4.9%. The low efficiency can be due to the presence of electrically active defects. In order to study the origin of defects in CdTe/ZnTe photovoltaic junctions, space charge techniques (C-V and deep level transient spectroscopy (DLTS)) have been applied. From the C-V measurements, a doping profile was calculated confirming charge accumulation in the i-CdTe layer. The results of the DLTS studies revealed the presence of four traps within a temperature range from 77–420 K. Three of them with activation energies equal to 0.22 eV, 0.45 eV, and 0.78 eV have been ascribed to the hole traps present in the i-CdTe material and their possible origin has been discussed. The fourth, high-temperature DLTS peak observed at ∼350 K has been attributed to extended defects as its amplitude and temperature position depends on the value of the filling pulse width. It is assumed that the defects related to the trap are either located in the i-CdTe layer or at the i-CdTe/ZnTe interface. However, it was found that the trap exhibits twofold nature: it behaves as a majority or as a minority trap, depending on the filling pulse height, which is a characteristic feature of recombination centers. This trap is presumably responsible for the low efficiency of the cells.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Substrate preparation effects on defect density in molecular beam epitaxial growth of CdTe on CdTe (100) and (211)B

Recent studies have demonstrated that growth of CdTe on CdTe (100) and (211)B substrates via molecular beam epitaxy (MBE) results in planar defect densities 2 and 3 orders of magnitude higher than growth on InSb (100) substrates, respectively. To understand this shortcoming, MBE growth on CdTe substrates with a variety of substrate preparation methods is studied by scanning electron microscopy, secondary ion mass spectrometry, x-ray photoelectron spectroscopy, cross sectional transmission electron microscopy, and atom probe tomography (APT). Prior to growth, carbon is shown to remain on substrate surfaces even after atomic hydrogen cleaning. APT revealed that following the growth of films, trace amounts of carbon remained at the substrate/film interface. This residual carbon may lead to structural degradation, which was determined as the main cause of higher defect density.

14 SOLAR ENERGY↗
DOE OSTI2023

Infusible Thermoplastic Composites for Wind Turbine Blade Manufacturing: Fatigue Life of Thermoplastic Laminates under Ambient and Low-Temperature Conditions

Traditionally, thermoset resins such as polyesters (PE) and epoxies are used as the polymer matrix for construction of wind turbine blades. However, concern about their end-of-life treatment garners interest to use thermoplastics for increased recyclability. However, the high viscosity of molten thermoplastics inhibits their use in manufacturing wind turbine blades with injection or compression molding. A recently developed, infusible, reactive thermoplastic resin overcomes this technological barrier. Toward verifying that this recyclable resin is suitable for use in wind turbine blades, a dataset of R?=?0.1 and R?=?10 fatigue data for glass fiber-reinforced acrylic composites is provided and equal fatigue life to industry standard epoxy and unsaturated PE resin systems is demonstrated. Specifically, R?=?0.1 fatigue data for acrylic composites at room temperature and -30?degrees C for verification of low-temperature performance are tabulated. To elucidate failure mechanisms, in situ mechanical testing with X-ray computed tomography demonstrates that damage accumulation occurs by crack propagation along the fiber-matrix interface under cyclic loading. Infrared (IR) thermography predicts failure points in composites specimens with porosity defects introduced from nonideal manufacturing processes. Furthermore, these manufacturing defects are shown to compromise the fatigue life of the acrylic laminates by an order of magnitude.

effect of defects↗
DOE OSTI2016

Temperature dependence of the radiation tolerance of nanocrystalline pyrochlores A 2 Ti 2 O 7 (A = Gd, Ho and Lu)

A potentially enhanced radiation resistance of nanocrystalline materials, as a consequence of the high density of interfaces and surfaces, has attracted much attention both to understand the fundamental role of these defect sinks and to develop them for high-radiation environments. Here, irradiation response of nanocrystalline A 2 Ti 2 O 7 (A = Gd, Ho and Lu) pyrochlore powders with grain sizes of 20–30 nm was investigated by 1-MeV Kr 2+ ion bombardment. In situ transmission electron microscopy (TEM) revealed that the critical amorphization fluence for each nanocrystalline compound at room temperature was greater than that for their coarse-grained counterparts, indicating an enhanced amorphization resistance. The effect of temperature on the irradiation response of one of these compounds, nanocrystalline Lu 2 Ti 2 O 7 , was further examined by performing ion irradiation at an elevated temperature range of 480–600 K. The critical amorphization temperature (T c ) was found to be noticeably higher in nanocrystalline Lu 2 Ti 2 O 7 (610 K) than its coarse-grained counterpart (480 K), revealing that nanocrystalline Lu 2 Ti 2 O 7 is less resistant to amorphization compared to its coarse-grained phase under high temperatures. We interpret these results with the aid of atomistic simulations. Molecular statics calculations find that cation antisite defects are less energetically costly to form near surfaces than in the bulk, suggesting that the nanocrystalline form of these materials is generally less susceptible to amorphization than coarse-grained counterparts at low temperatures where defect kinetics are negligible. In contrast, at high temperatures, the annealing efficiency of antisite defects by cation interstitials is significantly reduced due to the sink properties of the surfaces in the nanocrystalline pyrochlore, which contributes to the observed higher amorphization temperature in the nano-grained phase than in coarse-grained counterpart. Altogether, these results provide new insight into the behavior of nanocrystalline materials under irradiation.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Magnetotransport in MgO-based magnetic tunnel junctions grown by molecular beam epitaxy (invited)

The strong impact of molecular beam epitaxy growth and Synchrotron Radiation characterization tools in the understanding of fundamental issues in nanomagnetism and spintronics is illustrated through the example of fully epitaxial MgO-based Magnetic Tunnel Junctions (MTJs). If ab initio calculations predict very high tunnel magnetoresistance (TMR) in such devices, some discrepancy between theory and experiments still exists. The influence of imperfections in real systems has thus to be considered like surface contaminations, structural defects, unexpected electronic states, etc. The influence of possible oxygen contamination at the Fe/MgO(001) interface is thus studied, and is shown to be not so detrimental to TMR as predicted by ab initio calculations. On the contrary, the decrease of dislocations density in the MgO barrier of MTJs using Fe{sub 1−x}V{sub x} electrodes is shown to significantly increase TMR. Finally, unexpected transport properties in Fe{sub 1−X}Co{sub x}/MgO/Fe{sub 1−X}Co{sub x} (001) are presented. With the help of spin and symmetry resolved photoemission and ab initio calculation, the TMR decrease for Co content higher than 25% is shown to come from the existence of an interface state and the shift of the empty Δ1 minority spin state towards the Fermi level.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

In Situ Visualization of the Pinning Effect of Planar Defects on Li Ion Insertion

Longevity of Li ion batteries strongly depends on the interaction of transporting Li ions in electrode crystals with defects. However, detailed interactions between the Li ion flux and structural defects in the host crystal remain obscure due to the transient nature of such interactions. Here, by in situ transmission electron microscopy and density function theory calculations, we reveal how the diffusion pathways and transport kinetics of a Li ion can be affected by planar defects in a tungsten trioxide lattice. We uncover that changes in charge distribution and lattice spacing along the planar defects disrupt the continuity of ion conduction channels and dramatically increase the energy barrier of Li diffusion, thus, arresting Li ions at the defect sites and twisting the lithiation front. In conclusion, the atomic-scale understanding holds critical implications for rational interface design in solid-state batteries and solid oxide fuel cells.

25 ENERGY STORAGE↗
DOE OSTI2016

Defective Pollen Wall 2 ( DPW2 ) Encodes an Acyl Transferase Required for Rice Pollen Development

Aliphatic and aromatic lipids are both essential structural components of the plant cuticle, an important interface between the plant and environment. Although cross links between aromatic and aliphatic or other moieties are known to be associated with the formation of leaf cutin and root and seed suberin, the contribution of aromatic lipids to the biosynthesis of anther cuticles and pollen walls remains elusive. In this study, we characterized the rice (Oryza sativa) male sterile mutant, defective pollen wall 2 (dpw2), which showed an abnormal anther cuticle, a defective pollen wall, and complete male sterility. Compared with the wild type, dpw2 anthers have increased amounts of cutin and waxes and decreased levels of lipidic and phenolic compounds. DPW2 encodes a cytoplasmically localized BAHD acyltransferase. In vitro assays demonstrated that recombinant DPW2 specifically transfers hydroxycinnamic acid moieties, using v-hydroxy fatty acids as acyl acceptors and hydroxycinnamoyl-CoAs as acyl donors. Thus, The cytoplasmic hydroxycinnamoyl-CoA:v-hydroxy fatty acid transferase DPW2 plays a fundamental role in male reproduction via the biosynthesis of key components of the anther cuticle and pollen wall.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2021

Strong Fermi-level pinning at metal contacts to halide perovskites

The performance of halide perovskite-based electronic and optoelectronic devices is often related to interfacial charge transport. To shed light on the underlying physical and chemical properties of CH 3 NH 3 PbI 3 (MAPbI 3 ) in direct contact with common electrodes Al, Ti, Cr, Ag, and Au, the evolution of interfacial properties and Fermi level pinning is systematically studied. Given a unique experimental facility, pristine interfaces without any exposure to ambient air were prepared. We observe aggregation of substantial amounts of metallic lead (Pb 0 ) at the metal/MAPbI 3 interface, resulting from the interfacial reaction between the deposited metal and iodine ions from MAPbI 3 . It is found that the Schottky barrier height at the metal/MAPbI 3 interface is independent of the metal work function due to strong Fermi level pinning, possibly due to the metallic Pb 0 aggregates, which act as interfacial trap sites. The charge neutrality level of MAPbI 3 is consistent with the energy level of Pb 0 -related defects, indicating that Pb 0 interfacial trap states can be nonradiative recombination sites. Here this work underlines that control of chemical bonding at interfaces is a key factor for designing future halide perovskite-based devices.

36 MATERIALS SCIENCE↗
DOE OSTI2022

The Diffusion Mechanism of Ge During Oxidation of Si/SiGe Nanofins

A recently discovered, enhanced Ge diffusion mechanism along the oxidizing interface of Si/SiGe nanostructures has enabled the formation of single-crystal Si nanowires and quantum dots embedded in a defect-free, single-crystal SiGe matrix. Here, we report oxidation studies of Si/SiGe nanofins aimed at gaining a better understanding of this novel diffusion mechanism. Here, a superlattice of alternating Si/Si 0.7 Ge 0.3 layers was grown and patterned into fins. After oxidation of the fins, the rate of Ge diffusion down the Si/SiO 2 interface was measured through the analysis of HAADF-STEM images. The activation energy for the diffusion of Ge down the sidewall was found to be 1.1 eV, which is less than one-quarter of the activation energy previously reported for Ge diffusion in bulk Si. Through a combination of experiments and DFT calculations, we propose that the redistribution of Ge occurs by diffusion along the Si/SiO 2 interface followed by a reintroduction into substitutional positions in the crystalline Si.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Diffusion and recrystallization of B implanted in crystalline and pre-amorphized Ge in the presence of F

Although the diffusion control and dopant activation of Ge p-type junctions are straightforward when using B{sup +} implantation, the use of the heavier BF{sub 2}{sup +} ions or even BF{sup +} is still favored in terms of shallow junction formation and throughput—because implants can be done at higher energies, which can give higher beam currents and beam stability—and thus the understanding of the effect of F co-doping becomes important. In this work, we have investigated diffusion and end-of-range (EOR) defect formation for B{sup +}, BF{sup +}, and BF{sub 2}{sup +} implants in crystalline and pre-amorphized Ge, employing rapid thermal annealing at 600 °C and 800 °C for 10 s. It is demonstrated that the diffusion of B is strongly influenced by the temperature, the presence of F, and the depth of amorphous/crystalline interface. The B and F diffusion profiles suggest the formation of B–F complexes and enhanced diffusion by interaction with point defects. In addition, the strong chemical effect of F is found only for B in Ge, while such an effect is vanishingly small for samples implanted with F alone, or co-implanted with P and F, as evidenced by the high residual F concentration in the B-doped samples after annealing. After 600 °C annealing for 10 s, interstitial-induced compressive strain was still observed in the EOR region for the sample implanted with BF{sup +}, as measured by X-ray diffraction. Further analysis by cross-sectional transmission electron microscopy showed that the {311} interstitial clusters are the majority type of EOR defects. The impact of these {311} defects on the electrical performance of Ge p{sup +}/n junctions formed by BF{sup +} implantation was evaluated.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗