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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 1,027 records · Page 57
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

An experimental feasibility study on the use of CO 2 -soluble polyfluoroacrylates for CO 2 mobility and conformance control applications

We report high molecular weight polyfluoroacrylate (PFA) is an amorphous, sticky, hydrophobic, oil-phobic, CO 2 -soluble homopolymer. 1 wt% PFA-in-CO 2 solutions are four times as viscous as pure CO 2 . Although these solutions provided modest mobility control, unexpectedly large increases in pressure drop occurred because a portion of the PFA adsorbed onto the pore surfaces, altering wettability and significantly permeability. Further, PFA exhibited diminished solubility in CO 2 in the presence of light extracted hydrocarbons. Although these effects rendered CO 2 -PFA solutions inappropriate for mobility control, excellent conformance control for dual parallel brine-saturated cores was attained when a CO 2 -PFA solution was first injected into the isolated high permeability sandstone core. This core was then placed in parallel with a low permeability sandstone core, and all of the subsequently injected CO 2 was diverted to the low permeability core. Conformance control was not as effective in limestone, possibly due the low pH-induced erosion of the limestone that mitigated PFA adsorption.

02 PETROLEUM↗
DOE OSTI2018

The effect of Cu-Zn disorder on charge carrier mobility and lifetime in Cu2ZnSnSe4

Cu-Zn disorder is one possible origin for the limited efficiencies of kesterite solar cells and its impact on the band gap and band tails have been intensively studied. However, the effect on charge transport and recombination, which are key properties for solar cells, has not been investigated so far. Therefore, we probe the impact of the Cu-Zn order on charge carrier mobility and lifetime. To this end, we change the Cu-Zn order of a co-evaporated Cu2ZnSnSe4 thin film by sequential annealing and probe the impact by time-resolved terahertz spectroscopy. Aside from of the well-known band gap shift, we find no significant change in mobility and lifetime with Cu-Zn order. This finding indicates that Cu-Zn disorder is not limiting efficiencies of kesterite solar cells at their current status by means of charge carrier recombination and transport.

14 SOLAR ENERGY↗
DOE OSTI2017

Direct Visualization and Control of Atomic Mobility at {100} Surfaces of Ceria in the Environmental Transmission Electron Microscope

We report that ceria is one of the world’s most prominent material for applications in heterogeneous catalysis, as catalyst support or catalyst itself. Despite an exhaustive literature on the structure of reactive facets of CeO 2 in line with its catalytic mechanisms, the temporal evolution of the atomic surface structure exposed to realistic redox conditions remains elusive. Here, we provide a direct visualization of the atomic mobility of cerium atoms on {100} surfaces of CeO 2 nanocubes at room temperature in high vacuum, O 2 , and CO 2 atmospheres in an environmental transmission electron microscope. Lastly, through quantification of the cationic mobility, we demonstrate the control of the surface dynamics under exposure to O 2 and CO 2 atmospheres, providing opportunities for a better understanding of the intimate catalytic mechanisms.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Defect-Limited Mobility and Defect Thermochemistry in Mixed A-Cation Tin Perovskites: (CH3NH3)1-xCsxSnBr3

Hybrid organic-inorganic semiconductors crystallizing in the perovskite structure present a significant opportunity for realizing defect-tolerant semiconductors. In this work, we examine the solid solution, (CH3NH3)1-xCsxSnBr3, and identify the thermochemistry dictating the intrinsic carrier concentrations and how local structural distortions influences this electronic behavior. This family of compounds exhibits the expected systematic trend in decreasing optical gap with the cesium to methylammonium A-site mixing ratio, x, in the visible region. However, the carrier mobility, as determined from time-resolved microwave conductivity measurements, trends opposite to that expected from first-principles calculations combined with Boltzmann scattering theory calculations of the carrier mobility. We propose that this is a result of increasing carrier scattering with x in (CH3NH3)1-xCsxSnBr3 due to a significant increase in the carrier density with x. By examining the dependence of the carrier density as a function of x, we infer the compositional-dependence of the average enthalpy and nonconfigurational entropy per defect. While diffraction reveals a cubic aristotypic perovskite structure as a function of x at room temperature, the pair distribution functions obtained from synchrotron X-ray total scattering from these materials are better described by symmetry-adapted displacement modes of the Pm3m crystal structure, which we attribute to a large degree of anharmonic dynamics of the atom positions. This analysis shows that CH3NH3+-rich compositions retain mostly linear Sn-Br-Sn bonding environments through the displacements, while Cs+-rich compositions lead to more significantly bent Sn-Br-Sn environments. We propose that these bent bonding arrangements in Cs-rich compositions yield a higher propensity for defect formation. This also provides a rationale for carrier trapping that gives rise to anomalous microwave transients. Together, these results provide insight into the structure-dynamics-properties relationships in this highly anharmonic system with high amplitude atomic motions and low defect formation energies.

defect thermochemistry↗
DOE OSTI2024

Seasonal Patterns in Mobile Colistin Resistance Gene Variants in Wastewater Bioaerosols and Liquid Sludge

To address the growing threat of global antimicrobial resistance, a one-health approach is needed to understand the complex socioecological cycling of antibiotic resistance genes. In this study, a metagenomics approach using DNA shotgun sequencing, metagenome assembly, and antibiotic resistance gene (ARG) annotation was used to examine seasonal patterns in the abundance of mobile colistin resistance (mcr) gene variants in bioaerosols and liquid sludge in three wastewater treatment plants (WWTPs). ARGs represented 0.2–0.8 and 0.1–0.2% of the bioaerosol and liquid sludge metagenomes, respectively, while mcr genes represented 0–0.3 and 0–0.5% of the identified ARGs in bioaerosol and liquid sludge metagenomes. Seven of the ten known mcr variants were detected in wastewater bioaerosol and liquid samples, with mcr-5 and mcr-8 being the most prevalent across all seasons and sites. Furthermore, additional functional and taxonomic annotation of mcr-containing metagenomic contigs showed that mcr genes were often located on contigs with other co-occurring ARGs and mobile genetic elements and may be harbored by opportunistic human pathogens and other bacterial taxa not previously associated with mcr genes. Atmospheric dispersion modeling showed that mcr-containing bioaerosols can be transported kilometers away from the WWTPs, resulting in the possible dissemination of these ARGs into surrounding environments and communities.

Antimicrobial agents↗
DOE OSTI2022

Stabilized Synthesis of 2D Verbeekite: Monoclinic PdSe 2 Crystals with High Mobility and In-Plane Optical and Electrical Anisotropy

PdSe 2 has a layered structure with an unusual, puckered Cairo pentagonal tiling. Its atomic bond configuration features planar 4-fold-coordinated Pd atoms and intralayer Se–Se bonds that enable polymorphic phases with distinct electronic and quantum properties, especially when atomically thin. PdSe 2 is conventionally orthorhombic, and direct synthesis of its metastable polymorphic phases is still a challenge. Here, we report an ambient-pressure chemical vapor deposition approach to synthesize metastable monoclinic PdSe 2 . Monoclinic PdSe 2 is shown to be synthesized selectively under Se-deficient conditions that induce Se vacancies. These defects are shown by first-principles density functional theory calculations to reduce the free energy of the metastable monoclinic phase, thereby stabilizing it during synthesis. Further, the structure and composition of the monoclinic PdSe 2 crystals are identified and characterized by scanning transmission electron microscopy imaging, convergent beam electron diffraction, and electron energy loss spectroscopy. Polarized Raman spectroscopy of the monoclinic PdSe 2 flakes reveals their strong in-plane optical anisotropy. Electrical transport measurements show that the monoclinic PdSe 2 exhibits n-type charge carrier conduction with electron mobilities up to ~298 cm 2 V –1 s –1 and a strong in-plane electron mobility anisotropy of ~1.9. The defect-mediated growth pathway identified in this work is promising for phase-selective direct synthesis of other 2D transition metal dichalcogenides.

2D material↗
DOE OSTI2021

Human Mobility to Parks Under the COVID-19 Pandemic and Wildfire Seasons in the Western and Central United States

In 2020, people's health suffered a great crisis under the dual effects of the COVID-19 pandemic and the extensive, severe wildfires in the western and central United States. Parks, including city, national, and cultural parks, offer a unique opportunity for people to maintain their recreation behaviors following the social distancing protocols during the pandemic. However, massive forest wildfires in western and central US, producing harmful toxic gases and smoke, pose significant threats to human health and affect their recreation behaviors and mobility to parks. In this study, we employed the geographically and temporally weighted regression (GTWR) Models to investigate how COVID-19 and wildfires jointly shaped human mobility to parks, regarding the number of visits per capita, dwell time, and travel distance to parks, during June - September 2020. We detected strong correlations between visitations and COVID-19 incidence in southern Montana, western Wyoming, Colorado, and Utah before August. However, the pattern was weakened over time, indicating the decreasing trend of the degree of concern regarding the pandemic. Moreover, more park visits and lower dwell time were found in parks further away from wildfires and less air pollution in Washington, Oregon, California, Colorado, and New Mexico, during the wildfire season, suggesting the potential avoidance of wildfires when visiting parks. This study provides important insights on people's responses in recreation and social behaviors when facing multiple severe crises that impact their health and wellbeing, which could support the preparation and mitigation of the health impacts from future pandemics and natural hazards.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2013

Improved fluid simulations of radio-frequency plasmas using energy dependent ion mobilities

Symmetric and asymmetric capacitively coupled radio-frequency plasmas in oxygen at 40 Pa, 300 V voltage amplitude and a discharge gap of 40 mm are investigated by means of one-dimensional numerical semi-kinetic fluid modeling on the basis of a simplified reaction scheme including the dominant positive and negative ions, background gas, and electrons. An improved treatment, by accounting for the dependence of ion mobilities on E/N, is compared to the standard approach, based on using zero-field mobility values only. The charged particle dynamics as a result of direct electron impact ionization of oxygen, secondary electron release from the electrodes, the spatial distribution of all involved particles as well as impact of geometry and model modification on ion energies is analyzed and compared to independent simulations and experiments.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
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 OSTI2013

Energy resolution and related charge carrier mobility in LaBr{sub 3}:Ce scintillators

The scintillation response of LaBr{sub 3}:Ce scintillation crystals was studied as function of temperature and Ce concentration with synchrotron X-rays between 9 keV and 100 keV. The results were analyzed using the theory of carrier transport in wide band gap semiconductors to gain new insights into charge carrier generation, diffusion, and capture mechanisms. Their influence on the efficiency of energy transfer and conversion from X-ray or γ-ray photon to optical photons and therefore on the energy resolution of lanthanum halide scintillators was studied. From this, we will propose that scattering of carriers by both the lattice phonons and by ionized impurities are key processes determining the temperature dependence of carrier mobility and ultimately the scintillation efficiency and energy resolution. When assuming about 100 ppm ionized impurity concentration in 0.2% Ce{sup 3+} doped LaBr{sub 3,} mobilities are such that we can reproduce the observed temperature dependence of the energy resolution, and in particular, the minimum in resolution near room temperature is reproduced.

36 MATERIALS SCIENCE↗
DOE OSTI2013

High carrier mobility of CoPc wires based field-effect transistors using bi-layer gate dielectric

Polyvinyl alcohol (PVA) and anodized Al{sub 2}O{sub 3} layers were used as bi-layer gate for the fabrication of cobalt phthalocyanine (CoPc) wire base field-effect transistors (OFETs). CoPc wires were grown on SiO{sub 2} surfaces by organic vapor phase deposition method. These devices exhibit a field-effect carrier mobility (μ{sub EF}) value of 1.11 cm{sup 2}/Vs. The high carrier mobility for CoPc molecules is attributed to the better capacitive coupling between the channel of CoPc wires and the gate through organic-inorganic dielectric layer. Our measurements also demonstrated the way to determine the thicknesses of the dielectric layers for a better process condition of OFETs.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Room-temperature observations of the weak localization in low-mobility graphene films

We report room-temperature observations of the quantum conductance corrections caused by the weak localization in graphene films synthesized using solid-state-source chemical vapor deposition. Both Raman spectroscopy and Hall measurements showed strong disorder in the samples with a low mobility of ∼430 cm{sup 2}/V s. The emergence of weak localization at room temperature arises from the competition between the valley-dependent scattering and the thermal dephasing in such low-quality samples, although quantum effects normally appear in the samples that have an ideal structure at cryogenic temperatures. The large disorder in our low-mobility samples unexpectedly preserved the quantum mechanical weak localization.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

Scattering due to Schottky barrier height spatial fluctuation on two dimensional electron gas in AlGaN/GaN high electron mobility transistors

A scattering mechanism related to the Schottky barrier height (SBH) spatial fluctuation of the two dimensional electron gas (2DEG) in AlGaN/GaN heterostructures is presented. We find that the low field mobility is on the order of 10{sup 4}–10{sup 6} cm{sup 2}/Vs. The 2DEG transport properties are found to be influenced by both the mobility and 2DEG density variations caused by the SBH fluctuation. Our results indicate that a uniform Schottky contact is highly desired to minimize the influence of SBH inhomogeneity on the device performance.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Stable organic field-effect-transistors with high mobilities unaffected by supporting dielectric based on phenylene-bridged thienobenzothiophene

We report on the electrical properties of organic field-effect transistors (OFET) based on a new class of organic semiconductors. The molecules consist of the same thieno[2,3-b][1]benzothiophene building blocks, connected by different π-bridge spacers (ethylene, phenylene, and fluorophenylene). Molecular orbitals and highest occupied molecular orbital/lowest unoccupied molecular orbital energies were calculated and compared with results from cyclic voltammetric and UV-vis absorption measurements. In order to study the influence of the bridge groups on the molecular arrangement and surface interaction, the transistor performance on a wide range of dielectrics has been investigated in detail. These include as grown SiO{sub 2} and Al{sub 2}O{sub 3} and also treated with octadecyltrichrolosilane and octadecylphosphonic acid, as well as Cytop and Parylene C. An extended study of the multitude of combinations of these materials revealed mobilities up to ∼1 cm{sup 2}/Vs, measured for devices made of the phenylene-bridged compound. Surprisingly, the mobility was quite independent of the supporting gate dielectric. Stability over time has been observed with no degradation after 5 months. By eliminating the hysteresis using Cytop, we were able to show that some of the molecules form films without long-term charge carrier trapping. These are interesting features for practical industrial processing of organic electronics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Engineering the (In, Al, Ga)N back-barrier to achieve high channel-conductivity for extremely scaled channel-thicknesses in N-polar GaN high-electron-mobility-transistors

Scaling down the channel-thickness (t{sub ch}) in GaN/(In, Al, Ga)N high-electron-mobility-transistors (HEMTs) is essential to eliminating short-channel effects in sub 100 nm gate length HEMTs. However, this scaling can degrade both charge density (n{sub s}) and mobility (μ), thereby reducing channel-conductivity. In this study, the back-barrier design in N-polar GaN/(In, Al, Ga)N was engineered to achieve highly conductive-channels with t{sub ch} < 5-nm using metal organic chemical vapor deposition. Compositional-grading was found to be the most effective approach in reducing channel-conductivity for structures with t{sub ch} ∼ 3-nm. For a HEMT with 3-nm-thick-channel, a sheet-resistance of 329 Ω/◻ and a peak-transconductance of 718 mS/mm were demonstrated.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

A methodology to identify and quantify mobility-reducing defects in 4H-silicon carbide power metal-oxide-semiconductor field-effect transistors

In this paper, we present a methodology for the identification and quantification of defects responsible for low channel mobility in 4H-Silicon Carbide (SiC) power metal-oxide-semiconductor field-effect transistors (MOSFETs). To achieve this, we use an algorithm based on 2D-device simulations of a power MOSFET, density functional simulations, and measurement data. Using physical modeling of carrier mobility and interface traps, we reproduce the experimental I-V characteristics of a 4H-SiC doubly implanted MOSFET through drift-diffusion simulation. We extract the position of Fermi level and the occupied trap density as a function of applied bias and temperature. Using these inputs, our algorithm estimates the number of possible trap types, their energy levels, and concentrations at 4H-SiC/SiO{sub 2} interface. Subsequently, we use density functional theory (DFT)-based ab initio simulations to identify the atomic make-up of defects causing these trap levels. We study silicon vacancy and carbon di-interstitial defects in the SiC side of the interface. Our algorithm indicates that the D{sub it} spectrum near the conduction band edge (3.25 eV) is composed of three trap types located at 2.8–2.85 eV, 3.05 eV, and 3.1–3.2 eV, and also calculates their densities. Based on DFT simulations, this work attributes the trap levels very close to the conduction band edge to the C di-interstitial defect.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Physical understanding of electron mobility in asymmetrically strained InGaAs-on-insulator metal-oxide-semiconductor field-effect transistors fabricated by lateral strain relaxation

In this paper, we fabricated asymmetrically tensile-strained In{sub 0.53}Ga{sub 0.47}As-on-insulator (-OI) metal-oxide-semiconductor field-effect transistors (MOSFETs) using a lateral strain relaxation technique. A stripe-like line structure, fabricated in biaxially strained In{sub 0.53}Ga{sub 0.47}As-OI can lead to the lateral strain relaxation and asymmetric strain configuration in In{sub 0.53}Ga{sub 0.47}As-OI with the channel width of 100 nm. We have found that the effective mobility (μ{sub eff}) enhancement in In{sub 0.53}Ga{sub 0.47}As-OI MOSFETs with uniaxial-like asymmetric strain becomes smaller than that in In{sub 0.53}Ga{sub 0.47}As-OI MOSFETs with biaxial strain. We have clarified from a systematic analysis between the strain values and the μ{sub eff} characteristics that this mobility behavior can be understood by the change of the energy level of the conduction band minimum due to the lateral strain relaxation.

36 MATERIALS SCIENCE↗
DOE OSTI2014

High thermoelectric performance in the multi-valley electronic system Zr{sub 3}Ni{sub 3−x}Co{sub x}Sb{sub 4} and the high-mobility Zr{sub 3}Ni{sub 3−x}Cu{sub x}Sb{sub 4}

We report synthesis and thermoelectric performance of the p-type Zr{sub 3}Ni{sub 3−x}Co{sub x}Sb{sub 4} and the n-type Zr{sub 3}Ni{sub 3−x}Cu{sub x}Sb{sub 4}, which are derived from the same parent semiconductor Zr{sub 3}Ni{sub 3}Sb{sub 4}. We found a high thermoelectric performance for both the p-type compound (the figure-of-merit ZT is 0.52 at 760 K) and the n-type compound (ZT = 0.41 at 670 K). ZT of the p-type compound exceeded the value of the p-type half-Heusler compounds consisting of similar elements. The Hall-coefficient measurements indicate that the high ZT in the n-type compounds is a consequence of the high electron mobility of 52.4 cm{sup 2}/Vs. In contrast, the p-type compounds showed higher ZT in spite of much lower mobility. We discuss the mechanism for the high ZT in the p-type compound from the viewpoint of the multi-valley band structure.

36 MATERIALS SCIENCE↗