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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 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Study of InGaAs-based modulation doped field effect transistor structures using variable-angle spectroscopic ellipsometry

Variable-angle spectroscopic ellipsometry was used to estimate the thicknesses of all layers within the optical penetration depth of InGaAs-based modulation doped field effect transistor structures. Strained and unstrained InGaAs channels were made by molecular beam epitaxy (MBE) on InP substrates and by metal-organic chemical vapor deposition on GaAs substrates. In most cases, ellipsometrically determined thicknesses were within 10% of the growth-calibration results. The MBE-made InGaAs strained layers showed large strain effects, indicating a probable shift in the critical points of their dielectric function toward the InP lattice-matched concentration.

Alterovitz, S. A.↗
DOE OSTI2016

Synthesis of graphene and graphene nanostructures by ion implantation and pulsed laser annealing

In this paper, we report a systematic study that shows how the numerous processing parameters associated with ion implantation (II) and pulsed laser annealing (PLA) can be manipulated to control the quantity and quality of graphene (G), few-layer graphene (FLG), and other carbon nanostructures selectively synthesized in crystalline SiC (c-SiC). Controlled implantations of Si{sup −} plus C{sup −} and Au{sup +} ions in c-SiC showed that both the thickness of the amorphous layer formed by ion damage and the doping effect of the implanted Au enhance the formation of G and FLG during PLA. The relative contributions of the amorphous and doping effects were studied separately, and thermal simulation calculations were used to estimate surface temperatures and to help understand the phase changes occurring during PLA. In addition to the amorphous layer thickness and catalytic doping effects, other enhancement effects were found to depend on other ion species, the annealing environment, PLA fluence and number of pulses, and even laser frequency. Optimum II and PLA conditions are identified and possible mechanisms for selective synthesis of G, FLG, and carbon nanostructures are discussed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Effect of doping, microstructure, and CO{sub 2} on La{sub 2}NiO{sub 4+{delta}}-based oxygen-transporting materials

Alkaline earth-free La{sub 2}NiO{sub 4+{delta}} based materials were synthesized by a sol-gel method and studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques as well as oxygen permeation experiments. Effects of doping the nickel position with a variety of cations (Al, Co, Cu, Fe, Mg, Ta, and Zr) were investigated with regards to oxygen flux and microstructure. Doping was always found to diminish the oxygen flux as compared to the reference composition. However, larger grains, which were achieved by longer annealing times at 1723 K have a minor negative impact on oxygen permeation flux in case of La{sub 2}NiO{sub 4+{delta}} and La{sub 2}Ni{sub 0.9}Fe{sub 0.1}O{sub 4+{delta}} system. Moessbauer spectroscopy shows that the iron-doped system exhibits a secondary phase, which was identified by high-resolution transmission electron microscopy (HRTEM) as a higher Ruddlesden-Popper phase. In-situ XRD in an atmosphere containing 50 vol% CO{sub 2} and long-term oxygen permeation experiments using pure CO{sub 2} as the sweep gas revealed a high tolerance of the materials towards CO{sub 2}. - Graphical Abstract: The vibrational polished La{sub 2}Ni{sub 0.9}Fe{sub 0.1}O{sub 4+{delta}} membrane revealed formation of secondary phases, which were confirmed by Moessbauer spectroscopy. Highlights: Black-Right-Pointing-Pointer La{sub 2}NiO{sub 4+{delta}} systematically doped with different valent cations. Black-Right-Pointing-Pointer Large grains have a negative impact on oxygen-permeation performance. Black-Right-Pointing-Pointer CO{sub 2}-stability proved by in-situ XRD in 50 vol% CO{sub 2}. Black-Right-Pointing-Pointer Constant long-term permeation in presence of CO{sub 2}. Black-Right-Pointing-Pointer Moessbauer and TEM reveal multiple phases in La{sub 2}Ni{sub 0.9}Fe{sub 0.1}O{sub 4+{delta}} system.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS2006

Effects of Doping on Thermal Conductivity of Pyrochlore Oxides for Advanced Thermal Barrier Coatings

Pyrochlore oxides of general composition, A2B2O7, where A is a 3(+) cation (La to Lu) and B is a 4(+) cation (Zr, Hf, Ti, etc.) have high melting point, relatively high coefficient of thermal expansion, and low thermal conductivity which make them suitable for applications as high-temperature thermal barrier coatings. The effect of doping at the A site on the thermal conductivity of a pyrochlore oxide La2Zr2O7, has been investigated. Oxide powders of various compositions La2Zr2O7, La(1.7)Gd(0.3)Zr2O7, La(1.7)Yb(0.3)Zr2O7 and La(1.7)Gd(0.15)Yb(0.15)Zr2O7 were synthesized by the citric acid sol-gel method. These powders were hot pressed into discs and used for thermal conductivity measurements using a steady-state laser heat flux test technique. The rare earth oxide doped pyrochlores La(1.7)Gd(0.3)Zr2O7, La(1.7)Yb(0.3)Zr2O7 and La(1.7)Gd(0.15)Yb(0.15)Zr2O7 had lower thermal conductivity than the un-doped La2Zr2O7. The Gd2O3 and Yb2O3 co-doped composition showed the lowest thermal conductivity.

Bansal, Narottam P.↗
DOE OSTI2018

Effect of doping on the performance of high-crystalline SrMnO 3 perovskite nanofibers as a supercapacitor electrode

Perovskite oxides are promising multi-functional materials with enhanced physical and chemical properties. In this study, high-crystalline SrMnO 3 perovskite oxide nanofibers have been successfully synthesized by sol–gel electrospinning followed by calcination at different temperatures, using polyvinylpyrrolidone as a sacrificial polymeric binder. The change in porosity and grain size with calcination temperature imparted a substantial effect on the electrochemical properties of the obtained SrMnO 3 nanofibers. The SrMnO 3 nanofiber electrode calcined at 700 °C exhibits an electrochemical capacitance of 321.7 F g –1 at a discharge current density of 0.5 A g –1 . Here, the effect of doping Ba/Ca on Sr, and Co/Fe/Ni on Mn, respectively, on the specific capacitance of SrMnO 3 nanofibers is studied. 20 mol% Ba loading shows the best performance as a supercapacitor electrode with a specific capacitance of 446.8 F g –1 at a discharge current density of 0.5 A g –1 . The nanofibers retained 87% its initial capacitance after 5000 successive cycles. The device fabricated using the nanofibers show an energy density of 37.3 W h kg –1 at a power density of 400 W kg –1 , and it is retained as 15.7 W h kg –1 even at a high-power density of 8006 W kg –1 , indicating the potential of this electrode material for high-rate charge/discharge operations in supercapacitors.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Effect of silver doping on the surface of La{sub 5/8}Ca{sub 3/8}MnO{sub 3} epitaxial films

Thin film manganese oxides (manganites) display remarkable properties, such as colossal magnetoresistance and charge ordered phases, and became a focal point of research in the past two decades owing to potential applications ranging from oxide spintronics to resistive switching-based memories. La{sub x}Ca{sub 1−x}MnO{sub 3} (LCMO), a widely studied manganite, is known to substantially improve its transport properties when doped with Ag. However, despite the abundance of studies on LCMO, the effect of silver on the surface structure is unknown. Here, through in-situ methods, scanning tunneling microscopy (STM) is performed on La{sub 5/8}Ca{sub 3/8}MnO{sub 3} films grown by pulsed laser deposition. Films doped by silver, as confirmed by in-situ X-ray photoelectron spectroscopy, display large-scale reconstructions, interpreted as being of type (√10 × √10)R18.4°, while films lacking silver display a (√2 × √2)R45° reconstruction that may be associated with a surface charge-ordered state. It is posited that the possible cause of the varied reconstructions is due to a vacancy ordering on top of the existing (√2 × √2)R45° reconstruction. These studies highlight the influence of Ag on the surface structure, and therefore a route towards modifying the surface properties of manganites.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

The effect of doping on global lattice properties of magnetite Fe{sub 3-x}Me{sub x}O{sub 4} (Me=Zn, Ti and Al)

X-ray powder diffraction was measured in Fe{sub 3-x}Me{sub x}O{sub 4} (Me=Zn, Ti, Al; x<0.065), in T range 70-300 K to see the effect of different doping on global lattice properties. The experimental results have shown that some lattice properties (e.g., the cell volume) are dopand specific. This can be attributed to the difference in preferential sites occupation by dopants. As confirmed by EXAFS, Zn enters tetrahedral, while Ti octahedral lattice sites, differently affecting crucial octahedral iron positions in the spinel lattice. However, despite this fact, it was found that T dependence of both monoclinic angle and lattice parameters is universal for studied samples above and below the Verwey transition temperature T{sub V}. So, not the iron atoms in octahedral positions individually, but interactions between them are responsible for the Verwey transition character change with doping. - Graphical abstract: A low temperature magnetite cell volume vs. dopants content. Apparently, Zn, Ti and Al atoms have different effect on the global lattice properties at individual temperatures. However, the Verwey transition reacts to dopants in a similar manner, despite the different way the octahedral iron positions are affected. Highlights: Black-Right-Pointing-Pointer We measure powder diffraction and EXAFS on Fe{sub 3-x}Me{sub x}O{sub 4}, Me=Zn, Ti, Al (x<0.065), in T range 70-300 K. Black-Right-Pointing-Pointer XRD: atom-type independent changes of lattice parameters with T. Black-Right-Pointing-Pointer EXAFS: Zn replaces Fe on tetrahedral positions, Ti on octahedral positions. Black-Right-Pointing-Pointer Thus, some secondary interactions between ordering orbitals, not the primary one driving the Verwey transition, control the transition order.

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

Dielectric Characterization of Mylar and The Effects of Doping Processes.

Mylar® polymer is a bi-axially oriented polyethylene terephthalate (PET) polymer film used widely as a dielectric, specifically in capacitors. The dielectric characteristics of Mylar have been well studied and documented over the years; however, many of the mechanisms responsible for dielectric breakdown and failure are not understood for modified versions of the material. Previous studies on Mylar confirm that factors such as temperature, humidity, and voltage ramp rates can also have a significant effect on the dielectric properties and measurement of the dielectric properties. This study seeks to determine how dielectric properties, including permittivity, dielectric loss, and breakdown strength, are affected by doping of the polymer. To do this, two types of Mylar films, virgin film and film doped with a small-molecule electron-acceptor, are tested. Both types of materials are tested under a variety of environmental and experimental conditions, including testing at elevated temperatures, varying relative humidity, and varying ramp rates in dielectric breakdown testing. Analysis of permittivity, dielectric loss, and breakdown strength will be presented comparing virgin and doped Mylar to gain insight into the effects of doping with electron-acceptor molecules on dielectric properties under these varying environmental and test conditions.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Nondegenerate n-type doping phenomenon on molybdenum disulfide (MoS{sub 2}) by zinc oxide (ZnO)

Highlights: • We have demonstrated nondegenerate n-type doping phenomenon of MoS{sub 2} by ZnO. • ZnO doping improved the electrical parameters of MoS{sub 2} transistor (I{sub on}↑, μ{sub FE}↑, n↑). • The reduction of ZnO doping effect (ΔV{sub TH}: ∼75% ↓) was observed in air. • The highest photoresponsivity of ZnO-doped MoS{sub 2} photodetector was 3.18 × 10{sup 3} A/W. • The highest detectivity of ZnO-doped MoS{sub 2} photodetector was 5.94 × 10{sup 12} Jones. - Abstract: In this paper, we have demonstrated nondegenerate n-type doping phenomenon of MoS{sub 2} by ZnO. The ZnO doping effects were systematically investigated by Raman spectroscopy and electrical/optical measurements (I{sub D}–V{sub G} with/without exposure to 520, 655, 785, and 850 nm laser sources). The ZnO doping improved the performance parameters of MoS{sub 2}-based electronics (I{sub on}↑, μ{sub FE}↑, n↑) owing to reduction of the effective barrier height between the source and the MoS{sub 2} channel. We also monitored the effects of ZnO doping during exposure to air; reduction in ΔV{sub TH} of about 75% was observed after 156 h. In addition, the optoelectronic performance of the MoS{sub 2} photodetector was enhanced due to the reduction of the recombination rate of photogenerated carriers caused by ZnO doping. In our results, the highest photoresponsivity (about 3.18 × 10{sup 3} A/W) and detectivity (5.94 × 10{sup 12} Jones) of the ZnO-doped photodetector were observed for 520 nm laser exposure.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Effect of doping on the electron transport in polyfluorene

In this paper, electron transport of pure and DMC doped polyfluorne (PF) films have been studied at various doping concentrations. Pure films show space charge limited conduction with field and temperature dependent mobility. The J–V characteristics of doped PF were ohmic at low voltages due to thermally released carriers from dopant states. At higher voltages the current density increases nonlinearly due to field dependent mobility and carrier concentration thereby filling of tail states of HOMO of the host. The conductivity of doped films were analyzed using the Unified Gaussian Disorder Model (UGDM). The carrier concentration obtained from the fitting show a non-linear dependence on doping concentration which may be due to a combined effect of thermally activated carrier generation and increased carrier mobility.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Theory of nitrogen doping of carbon nanoribbons: Edge effects

Nitrogen doping of a carbon nanoribbon is profoundly affected by its one-dimensional character, symmetry, and interaction with edge states. Using state-of-the-art ab initio calculations, including hybrid exact-exchange density functional theory, we find that, for N-doped zigzag ribbons, the electronic properties are strongly dependent upon sublattice effects due to the non-equivalence of the two sublattices. For armchair ribbons, N-doping effects are different depending upon the ribbon family: for families 2 and 0, the N-induced levels are in the conduction band, while for family 1 the N levels are in the gap. In zigzag nanoribbons, nitrogen close to the edge is a deep center, while in armchair nanoribbons its behavior is close to an effective-mass-like donor with the ionization energy dependent on the value of the band gap. In chiral nanoribbons, we find strong dependence of the impurity level and formation energy upon the edge position of the dopant, while such site-specificity is not manifested in the magnitude of the magnetization.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2022

Spin-triplet superconductivity from excitonic effect in doped insulators

Despite being of fundamental importance and potential interest for topological quantum computing, spin-triplet superconductors remain rare in solid state materials after decades of research. In this work, we present a three-particle mechanism for spin-triplet superconductivity in multiband systems, where an effective attraction between doped electrons is produced from the Coulomb repulsion via a virtual interband transition involving a third electron [V. Crépel, L. Fu, Sci. Adv. 7, eabh2233 (2021)]. Our theory is analytically controlled by an interband hybridization parameter and explicitly demonstrated in doped band insulators with the example of an extended Hubbard model. Our theory of exciton-mediated pairing reveals how, as a matter of principle, a two-particle bound state can arise from the strong electron repulsion upon doping, opening a viable path to Bose–Einstein condensate (BEC)–Bardeen–Cooper–Schrieffer (BCS) physics in solid state systems. In light of this theory, we propose that recently discovered dilute superconductors such as ZrNCl, WTe 2 , and moiré materials can be spin-triplet and compare the expected consequences of our theory with experimental data.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Effect of Doping on the Properties of Hydrogenated Amorphous Silicon Irradiated with Femtosecond Laser Pulses

A comparative analysis of the effect of femtosecond laser irradiation on the structure and conductivity of undoped and boron-doped hydrogenated amorphous silicon (a-Si: H) is performed. It is demonstrated that the process of nanocrystal formation in the amorphous matrix under femtosecond laser irradiation is initiated at lower laser energy densities in undoped a-Si: H samples. The differences in conductivity between undoped and doped a-Si: H samples vanish almost completely after irradiation with an energy density of 150–160 mJ/cm{sup 2}.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Defect properties of Sb- and Bi-doped CuInSe{sub 2}: The effect of the deep lone-pair s states

Bi or Sb doping has been used to make better material properties of polycrystalline Cu{sub 2}(In,Ga)Se{sub 2} as solar cell absorbers, including the experimentally observed improved electrical properties. However, the mechanism is still not clear. Using first-principles method, we investigate the stability and electronic structure of Bi- and Sb-related defects in CuInSe{sub 2} and study their effects on the doping efficiency. Contrary to previous thinking that Bi or Sb substituted on the anion site, we find that under anion-rich conditions, the impurities can substitute on cation sites and are isovalent to In because of the formation of the impurity lone pair s states. When the impurities substitute for Cu, the defects act as shallow double donors and help remove the deep In{sub Cu} level, thus resulting in the improved carrier life time. On the other hand, under anion-poor conditions, impurities at the Se site create amphoteric deep levels that are detrimental to the device performance.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

High magnetic field induced spin flip/flop behavior and magnetic phase diagram of CuFe{sub 1−x}Ga{sub x}O{sub 2}

The structure and magnetic properties of non-magnetic Ga{sup 3+} ion doped CuFe{sub 1−x}Ga{sub x}O{sub 2} (x=0, 0.03, and 0.05) single crystal samples were investigated. X-ray diffraction patterns analysis confirms that the samples are single-phase crystallizing. Doping effect on the magnetic behavior of the ground state and the field-induced spin flip/flop transitions were detected. The transition temperatures and critical magnetic fields of the spin flip/flop, as well as the magnetic hysteresis directly depend on the Ga{sup 3+} doping level. Such doping effects may associate with the competition between dilution effect (partial release of spin frustration) and the induced local magnetic moment, which is the result of the changed magnetic coupling both inter- and intra-planes of Fe ions. Based on the experimental results, the effects of Ga{sup 3+} doping on the spin flip/flop behavior and a detailed high field magnetic diagram were assumed. - Graphical abstract: Temperature dependence of the critical fields for CuFe{sub 1−x}Ga{sub x}O{sub 2} (x=0, 0.03, 0.05). (a) and (b) The magnetic field's parallel and perpendicular cases respectively. In which, the square, circle and triangular represent x=0, 0.03 and 0.05 respectively. - Highlights: • Single crystal samples of CuFe{sub 1−x}Ga{sub x}O{sub 2} (x=0, 0.03 and 0.05) were grown. • Magnetic field dependence of magnetization was measured in pulsed high magnetic field up to 54 T with different temperatures. • The magnetic field induced multi-step-like transitions were observed in all the single crystal samples. • A detailed magnetic diagram was assumed based on the experimental results.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗