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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 1,567 records · Page 87
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Enhancement of Photoemission on p-Type GaAs Using Surface Acoustic Waves

We demonstrate that photoemission properties of p-type GaAs can be altered by surface acoustic waves (SAWs) generated on the GaAs surface due to dynamical piezoelectric fields of SAWs. Multiphysics simulations indicate that charge-carrier recombination is greatly reduced, and electron effective lifetime in p-doped GaAs may increase by a factor of 10× to 20×. It implies a significant increase, by a factor of 2× to 3×, of quantum efficiency (QE) for GaAs photoemission applications, like GaAs photocathodes. Conditions of different SAW wavelengths, swept SAW intensities, and varied incident photon energies were investigated. Essential steps in SAW device fabrication on a GaAs substrate are demonstrated, including deposition of an additional layer of ZnO for piezoelectric effect enhancement, measurements of current–voltage (I–V) characteristics of the SAW device, and ability to survive high-temperature annealing. Results obtained and reported in this study provide the potential and basis for future studies on building SAW-enhanced photocathodes, as well as other GaAs photoelectric applications.

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

Effect of granularity and annealing conditions on the magneto-resistance of the electron doped superconductor Nd{sub 1.85}Ce{sub 0.15}CuO{sub 4}

A single phase polycrystalline sample of Nd{sub 1.85}Ce{sub 0.15}CuO{sub 4} has been synthesized and well characterized. Detailed studies on electrical resistivity behavior of this system as a function of temperature and magnetic field reveal interesting features attributable to the granularity effects. These features have been qualitatively understood based on the interplay of Josephson junction coupling and quasi particle tunneling which dictates the evolution of the observed temperature and field dependent resistivity behavior of Nd{sub 1.85}Ce{sub 0.15}CuO{sub 4} system. The studies also reveal significant changes with respect to annealing conditions, indicating that such compounds are very sensitive to annealing conditions. It has been observed that prolonged annealing in Argon atmosphere leads to a decrease in normal state resistivity. This is associated with a decrease in the S-I-S type Josephson tunnel junctions perhaps due to improved inter-granular coupling, nevertheless they could not be completely eliminated.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1973

GaN for LED applications

In order to improve the synthesis of GaN the effect of various growth and doping parameters has been studied. Although Be, Li, Mg, and Dy can be used to overcompensate native donors, the most interesting acceptor element is Zn. The emission spectrum and the luminescence efficiency depend on the growth temperature (below 800 C), on the partial pressure of the doping impurity, and on the duration of growth. Blue-green electroluminescence with a power efficiency of 0.1 percent and a brightness of 850 fL (at 0.6 mA and 22.5 V) was obtained. Some diodes allow the color of the emitted light to change by reversing the polarity of the bias. Continuous operation of a diode over a period of 5 months showed no evidence of degradation. The luminescence properties of ion-implanted GaN were studied. Delay effects were found in the electroluminescence of diodes, although, with a dc bias, a 70-MHz modulation was possible.

Pankove, J. I.↗
DOE OSTI2019

Effect of Ce{sup 3+} Ion on Structural and Hyperfine Interaction Studies of Co{sub 0.5}Ni{sub 0.5}Fe{sub 2−x}Ce{sub x}O{sub 4} Ferrites: Useful for Permanent Magnet Applications

Nanoparticles of Co{sub 0.5}Ni{sub 0.5}Fe{sub 2−x}Ce{sub x}O{sub 4} (where x = 0.0, 0.01, 0.015 and 0.02) ferrites are prepared by the modified solution combustion method using a mixture of fuels and are characterized to understand their structural, microstructural and magnetic properties. The X-ray diffraction is used to confirm the formation of a single-phase cubic spinel structure. The average crystallite sizes are calculated using the Scherrer formula and are found to be less than 50 nm. The microstructural features are obtained by the scanning electron microscopy, and the compositional analysis is done by using the energy-dispersive spectroscopy. The transmission electron microscopy (TEM) investigations show that the synthesized ferrites are made up of very fine spherical nanoparticles. The influence of a rare-earth element (Ce{sup 3+}) on the magnetic properties of the samples was studied using the Mössbauer spectroscopy. The Mössbauer spectroscopy reveals the formation of broadened Zeeman lines and quadrupole-split lines and the presence of the Fe{sup 3+} charge state at B sites in the samples. The quadrupole splitting shows that the orientation of the magnetic hyperfine field with respect to the principle axes of the electric field gradient was random. The magnetic hyperfine field values indicate that the A sites have more A-O-B superexchange interactions than the B sites. The coexistence of magnetic sextet and a doublet component on the room-temperature spectra suggests superparamagnetic properties of the nanoparticles. The low-temperature (15 K) Mössbauer spectroscopy explores the paramagnetic relaxation in the nanoparticles. The area under the sextet refers to Fe{sup 3+} concentrations in the tetrahedral and octahedral sites of the ferrite. This study confirms that the Ce{sup 3+} substitution of Fe{sup 3+} only for octahedron sites causes the decrease in Fe-O-Fe arrangement. The effect of Ce{sup 3+} doping on the magnetic properties of Co{sub 0.5}Ni{sub 0.5}Fe{sub 2}O{sub 4} is examined from the vibrating sample magnetometry (VSM) spectra. Saturation magnetization values are decreased initially and then increased, as result of Ce{sup 3+} doping. This can be explained by Neel’s two-sub-lattice model. Further, the value of coercivity is found to be increasing with increasing Ce{sup 3+} concentration. The obtained results of M-H loop with improved coercivity (from 851 to 1039 Oe) by Ce{sup 3+} doping of Co{sub 0.5}Ni{sub 0.5}Fe{sub 2}O{sub 4} demonstrate the usefulness for permanent magnet applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Effect of cation distribution on the magnetic and hyperfine behaviour of nanocrystalline Co doped Ni–Zn ferrite (Ni{sub 0.4}Zn{sub 0.4}Co{sub 0.2}Fe{sub 2}O{sub 4})

Highlights: • Cation distribution in tetrahedral and octahedral sites of spinel Ni{sub 0.4}Zn{sub 0.4}Co{sub 0.2}Fe{sub 2}O{sub 4}. • Structural analysis of observed X-ray diffraction pattern using Rietveld method. • Study of hyperfine behaviour using Mössbauer spectroscopy. • Static and dynamic magnetic measurements. • Correlation of cation distributions obtained from Rietveld analysis with the results of magnetic and Mössbauer effect measurements. - Abstract: Nanoparticles of Ni{sub 0.4}Zn{sub 0.4}Co{sub 0.2}Fe{sub 2}O{sub 4} are prepared by a simple co-precipitation method. The as dried sample is heat treated at 400, 500, 600, 700 and 800 °C to obtain different sizes of nanoparticles. The crystallographic phase of the samples is confirmed analyzing observed X-ray diffraction (XRD) by Rietveld method. Hyperfine parameters of the samples are derived from room temperature (RT) Mössbauer spectra of the samples. Magnetic properties of the samples are investigated by static and dynamic hysteresis loops. Different magneto-crystalline parameters are calculated from the variation of magnetization with temperature (M–T curve) under zero field cooled (ZFC) and field cooled (FC) conditions of the as dried sample. The cation distribution estimated from Rietveld analysis are correlated with the results of magnetic and Mössbauer effect measurements. The observed high value of saturation magnetization (72.7 emu/g at RT) of the sample annealed at 800 °C would be interesting for applications in different electromagnetic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Controlling the orbital Hall effect in gapped bilayer graphene in the terahertz regime

We study the orbital Hall effect (OHE) in the AC regime using bilayer graphene (BLG) as a prototypical material platform. While the unbiased BLG has gapless electronic spectra, applying a perpendicular electric field creates an energy band gap that can be continuously tuned from zero to high values. By exploiting this flexibility, we demonstrate the ability to control the behavior of AC orbital Hall conductivity. Particularly, we demonstrate that the orbital Hall conductivity at the neutrality point changes its signal at a critical frequency, the value of which is proportional to the perpendicular electric field. For BLG with narrow band gaps, the active frequency region for the AC OHE may extend to a few terahertz, which is experimentally accessible with current technologies. We also consider the introduction of a perpendicular magnetic field in the weak-coupling regime using first-order perturbation theory to illustrate how the breaking of time-reversal symmetry enables the emergence of the AC charge Hall effect in the charge-doped situation and modifies the AC orbital Hall conductivity. Our calculations suggest that BLG with narrow band gaps is a practical candidate for investigating time-dependent orbital angular momentum transport. Published by the American Physical Society 2024

36 MATERIALS SCIENCE↗
DOE OSTI2014

Electric field geometries dominate quantum transport coupling in silicon nanoring

Investigations on the relation between the geometries of silicon nanodevices and the quantum phenomenon they exhibit, such as the Aharonov–Bohm (AB) effect and the Coulomb blockade, were conducted. An arsenic doped silicon nanoring coupled with a nanowire by electron beam lithography was fabricated. At 1.47 K, Coulomb blockade oscillations were observed under modulation from the top gate voltage, and a periodic AB oscillation of ΔB = 0.178 T was estimated for a ring radius of 86 nm under a high sweeping magnetic field. Modulating the flat top gate and the pointed side gate was performed to cluster and separate the many electron quantum dots, which demonstrated that quantum confinement and interference effects coexisted in the doped silicon nanoring.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Competition of three-dimensional magnetic phases in Ca2Ru1-xFexO4: A structural perspective

The crystalline and magnetic structures of Ca 2 Ru 1 - x Fe x O 4 ( x = 0.02 , 0.05, 0.08, and 0.12) have been studied using neutron and x-ray diffraction. The Fe-doping reduces the Ru-O bond length in both apical and planar directions. The smaller Ru( Fe ) O 6 octahedron leads to its reduced distortion. The P b c a space group is maintained in all the Fe dopings, so is the octahedral flattening. Warming has a similar effect on the lattice to that of the Fe doping in releasing the distorted octahedra but precipitates an abrupt octahedral elongation near the N e ' el temperature. Two competing antiferromagnetic orders, A - and B -centered phases have been observed. The Fe-doping-relaxed crystal structure prefers the latter to the former. As the doping increases, the B -centered phase continuously grows at the cost of the A -centered one and eventually replaces it at x = 0.12 . The absence of the two-dimensional antiferromagnetic critical fluctuations above the magnetic transition temperature and the three-dimensional magnetic correlation below the transition, together with the anomalous lattice response, point to an important role of orbital degree of freedom in driving the magnetic phase competition.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1975

Diffusion length measurements using the scanning electron microscope

A measurement technique employing the scanning electron microscope is described in which values of the true bulk diffusion length are obtained. It is shown that surface recombination effects can be eliminated through the application of highly doped surface field layers. The influence of high injection level effects and low-high junction current generation on the resulting measurement was investigated. Close agreement is found between the diffusion lengths measured by this method and those obtained using a penetrating radiation technique.

Weizer, V. G.↗
NASA NTRS1979

Studies of silicon p-n junction solar cells

To provide theoretical support for investigating different ways to obtain high open-circuit voltages in p-n junction silicon solar cells, an analytical treatment of heavily doped transparent-emitter devices is presented that includes the effects of bandgap narrowing, Fermi-Dirac statistics, a doping concentration gradient, and a finite surface recombination velocity at the emitter surface. Topics covered include: (1) experimental determination of bandgap narrowing in the emitter of silicon p-n junction devices; (2) heavily doped transparent regions in junction solar cells, diodes, and transistors; (3) high-low-emitter solar cell; (4) determination of lifetimes and recombination currents in p-n junction solar cells; (5) MOS and oxide-charged-induced BSF solar cells; and (6) design of high efficiency solar cells for space and terrestrial applications.

Neugroschel, A.↗
DOE OSTI2015

Rapid synthesis of Co, Ni co-doped ZnO nanoparticles: Optical and electrochemical properties

We report for the first time a rapid preparation of Zn{sub 1−2x}Co{sub x}Ni{sub x}O nanoparticles via a versatile and environmentally friendly route, microwave-assisted hydrothermal (MAH) method. The Co, Ni co-doped ZnO nanoparticles present an effect on photoluminescence and electrochemical properties, exhibiting excellent electrocatalytic performance compared to undoped ZnO sample. Photoluminescence spectroscopy measurements indicated the reduction of the green–orange–red visible emission region after adding Co and Ni ions, revealing the formation of alternative pathways for the generated recombination. The presence of these metallic ions into ZnO creates different defects, contributing to a local structural disorder, as revealed by Raman spectra. Electrochemical experiments revealed that the electrocatalytic oxidation of dopamine on ZnO attached to multi-walled carbon nanotubes improved significantly in the Co, Ni co-doped ZnO samples when compared to pure ZnO. - Graphical abstract: Rapid synthesis of Co, Ni co-doped ZnO nanoparticles: optical and electrochemical properties. Co, Ni co-doped ZnO hexagonal nanoparticles with optical and electrocatalytic properties were successfully prepared for the first time using a microwave hydrothermal method at mild conditions. - Highlights: • Co{sup 2+} and Ni{sup 2+} into ZnO lattice obtained a mild and environmentally friendly process. • The heating method strongly influences in the growth and shape of the particles. • Short-range defects generated by the ions insertion affects the photoluminescence. • Doped ZnO nanoparticles improve the electrocatalytic properties of pure oxide.

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

Electrical, luminescent, and deep trap properties of Si doped n-GaN grown by pendeo epitaxy

Electrical and luminescent properties and deep trap spectra of Si doped GaN films grown by maskless epitaxial lateral overgrowth (MELO) are reported. The dislocation density in the wing region of the structure was 10{sup 6 }cm{sup −2}, while in the seed region it was 10{sup 8 }cm{sup −2}. The major electron traps present had activation energy of 0.56 eV and concentrations in the high 10{sup 15 }cm{sup −3} range. A comparison of diffusion length values and 0.56 eV trap concentration in MELO GaN and epitaxial lateral overgrowth (ELOG) GaN showed a good correlation, suggesting these traps could be effective in carrier recombination. The doped MELO films were more uniform in their electrical properties than either ELOG films or undoped MELO films. We also discuss the differences in deep trap spectra and luminescence spectra of low-dislocation-density MELO, ELOG, and bulk n-GaN samples grown by hydride vapor phase epitaxy. It is suggested that the observed differences could be caused by the differences in oxygen and carbon contamination levels.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Synergistic enhancement of nitrogen and sulfur co-doped graphene with carbon nanospheres insertion for electrocatalytic oxygen reduction reaction

A nitrogen and sulfur co-doped graphene/carbon black (NSGCB) nanocomposite for the oxygen reduction reaction (ORR) was synthesized through a one-pot annealing of a precursor mixture containing graphene oxide, thiourea, and acidized carbon black (CB). The NSGCB showed excellent performance for the ORR with the onset and half-way potentials at 0.96 V and 0.81 V (vs. RHE), respectively. It is significantly improved over that of the catalysts derived from only graphene (0.90 V and 0.76 V) or carbon nanosphere (0.82 V and 0.74 V). The enhanced catalytic activity on the NSGCB electrode could be attributed to the synergistic effect of N/S co-doping and the enlarged interlayer space resulted from the insertion of carbon nanosphere into the graphene sheets. The four-electron selectivity and the limiting current density of the NSGCB nanocomposite are comparable to that of the commercially Pt/C catalyst. Furthermore, the NSGCB nanocomposite was superior to Pt/C in terms of long-term durability and tolerance to methanol poisoning.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Effect of silver ions and clusters on the luminescence properties of Eu-doped borate glasses

Highlights: • Ag{sup +} and Ag clusters are investigated in the borate glasses via ion exchange method. • The aggregation of silver ions to the clusters was controlled by the ion exchange concentration. • Eu{sup 3+}/Eu{sup 2+} ions emission was enhanced with the sensitization of the silver species. • Energy transfer process from Ag ions and Ag clusters to Eu ions is identified by the lifetime measurements. - Abstract: Silver ions and clusters were applied to Eu{sup 3+}-doped borate glasses via the Ag{sup +}–Na{sup +} ion exchange method. Eu{sup 3+}/Eu{sup 2+} ion luminescence enhancement was achieved after silver ion exchange. Absorption spectra showed no band at 420 nm, which indicates that silver nanoparticles can be excluded as a silver state in the glass. Silver ion aggregation into clusters during the ion exchange process may be inferred. The effect of silver ions and clusters on rare earth emissions was investigated using spectral information and lifetime measurements. Significant luminescence enhancements were observed from the energy transfer of Ag{sup +} ions and clusters to Eu{sup 3+}/Eu{sup 2+} ions, companied with the silver ions aggregated into the clusters state. The results of this research may extend the current understanding of interactions between rare-earth ions and Ag species.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Mitigation of Pitting on Nitrogen-Doped Niobium Surfaces through Two-Step Electropolishing

Surface processing of niobium superconducting RF cavities used in particle accelerators is crucial for determining their performance. The nitrogen (N)-doping method, developed at Fermilab, enhances the cavities’ quality factor by doping the cavity’s interior surface with N atoms at 800 °C. After N-doping, electropolishing (EP) is employed to remove the niobium nitride phases and attain the desired concentration of interstitial N atoms. This study explores the effects of EP conditions on N-doped niobium samples. The samples were electropolished at 5 to 40 °C and voltages between 3 and 22 V. Different EP voltage ranges produced two types of pits, classified based on their sizes. Larger pits, linked to higher voltages, are also affected by the reaction rate. Pitting could be reduced at a lower temperature and adequate EP voltage. However, to effectively minimize gas evolution forming large-sized pitting, peeling off the top nitride layer before EP might be beneficial. This was achieved through precise low-voltage etching, forming a two-step EP process. The process involves removing the top 100–200 nm layer at a low voltage, followed by standard EP to remove the required material. This two-step EP process effectively mitigated the pitting risk associated with gas evolution on N-doped surfaces.

43 PARTICLE ACCELERATORS↗
DOE OSTI2016

Fabrication and characterization of spark plasma sintered Ce:LuAG ceramic for scintillation application

Rare earth Cerium doped Lutetium Aluminum Garnet (Ce:LuAG) ceramics are widely used as phosphor material in medical imaging and high-energy physics. Due to its technological importance, an attempt has been made to fabricate Ce:LuAG ceramics by using spark plasma sintering (SPS) technique. XRD patterns of SPS sintered Ce:LuAG ceramics reveals a mixed LuAG and CeO{sub 2} (antisite defect) phases. The microstructures of SPS sintered Ce:LuAG ceramics shows limited densification, inappropriate compaction of particles and existence of residual pores, voids between the grain boundaries affects the transparency of Ce:LuAG ceramics. Relative density and hardness of post sintered Ce:LuAG ceramic is also determined. The effect of Ce{sup 3+} doping concentration and sintering temperature on optical luminescence behavior of Ce:LuAG ceramic is presented.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Coherent vertical electron transport and interface roughness effects in AlGaN/GaN intersubband devices

We investigate electron transport in epitaxially grown nitride-based resonant tunneling diodes (RTDs) and superlattice sequential tunneling devices. A density-matrix model is developed, and shown to reproduce the experimentally measured features of the current–voltage curves, with its dephasing terms calculated from semi-classical scattering rates. Lifetime broadening effects are shown to have a significant influence in the experimental data. Additionally, it is shown that the interface roughness geometry has a large effect on current magnitude, peak-to-valley ratios and misalignment features; in some cases eliminating negative differential resistance entirely in RTDs. Sequential tunneling device characteristics are dominated by a parasitic current that is most likely to be caused by dislocations; however, excellent agreement between the simulated and experimentally measured tunneling current magnitude and alignment bias is demonstrated. This analysis of the effects of scattering lifetimes, contact doping and growth quality on electron transport highlights critical optimization parameters for the development of III–nitride unipolar electronic and optoelectronic devices.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Effect of Intrinsic Defects on Electronic and Magnetic Properties in Tm-Doped GaN: First-Principles Calculations

Based on the density functional theory, we investigate the electronic and magnetic properties of various types of defect complexes formed by dopant Tm and Ga vacancies, N vacancies, or O interstitial in Tm-doped GaN. Formation energies are first calculated for all defect complexes to assess their stability. The single Tm dopant is found to introduce the local magnetic moment of about 2 μ{sub B}/Tm in GaN. However, in the case of defect complexes, the magnetic moments of Tm can be suppressed by the existence of Ga vacancies around it, while the presence of N vacancies or O interstitial does not influence the magnetic moment of Tm. In addition, each Ga vacancy in the neutral charge state induces the local magnetic moment of about 2.1 μ{sub B} and one octahedral O interstitial can lead to the local moment of about 1.6 μ{sub B}.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗