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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 1,747 records · Page 97
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Dislocation-related trap levels in nitride-based light emitting diodes

Deep level transient spectroscopy was performed on InGaN/GaN multiple quantum well light emitting diodes (LEDs) in order to determine the effect of the dislocation density on the deep intragap electronic levels. The LEDs were grown by metalorganic vapor phase epitaxy on GaN templates with a high dislocation density of 8 × 10{sup 9} cm{sup −2} and a low dislocation density of 3 × 10{sup 8} cm{sup −2}. Three trapping levels for electrons were revealed, named A, A1, and B, with energies E{sub A} ≈ 0.04 eV, E{sub A1} ≈ 0.13 eV, and E{sub B} ≈ 0.54 eV, respectively. The trapping level A has a much higher concentration in the LEDs grown on the template with a high density of dislocations. The logarithmic dependence of the peak amplitude on the bias pulse width for traps A and A1 identifies the defects responsible for these traps as associated with linearly arranged defects. We conclude that traps A and A1 are dislocation-related intragap energy levels.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Characteristics of intermediate state related to anti-Stokes luminescence of ZnO single crystals

Anti-Stokes luminescence from ZnO is supposed to be a two-step two-photon absorption process with an intermediate state. The intermediate state is assumed to be a localized state with two different excited and relaxed states. One of the localized states is believed to be the well-known 2.4 eV green luminescence; the other is difficult to observe experimentally. We found an interesting 2.25 eV deep luminescence from ZnO, which has been shown to relate to anti-Stokes luminescence. The 2.25 eV yellow luminescence was observable only below the band gap excitation and through a time-resolved observation after the excitation light was turned off. The intermediate states were found to be a photo-excited donor-acceptor pair and its lattice relaxation state. The characteristics and the role of the intermediate state of ZnO related to the anti-Stokes luminescence are discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

AlGaN composition dependence of the band offsets for epitaxial Gd{sub 2}O{sub 3}/Al{sub x}Ga{sub 1−x}N (0 ≤ x ≤ 0.67) heterostructures

Gd{sub 2}O{sub 3} films were prepared on (0001)-oriented Al{sub x}Ga{sub 1−x}N (0 ≤ x ≤ 0.67) thin film substrates via reactive molecular-beam epitaxy. X-ray diffraction revealed that these films possessed the cubic bixbyite structure regardless of substrate composition and were all 111-oriented with in-plane rotations to account for the symmetry difference between the oxide film and nitride epilayer. Valence band offsets were characterized by X-ray photoelectron spectroscopy and were determined to be 0.41 ± 0.02 eV, 0.17 ± 0.02 eV, and 0.06 ± 0.03 eV at the Gd{sub 2}O{sub 3}/Al{sub x}Ga{sub 1−x}N interfaces for x = 0, 0.28, and 0.67, respectively.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Comparison of x-ray absorption spectra between water and ice: New ice data with low pre-edge absorption cross-section

The effect of crystal growth conditions on the O K-edge x-ray absorption spectra of ice is investigated through detailed analysis of the spectral features. The amount of ice defects is found to be minimized on hydrophobic surfaces, such as BaF{sub 2}(111), with low concentration of nucleation centers. This is manifested through a reduction of the absorption cross-section at 535 eV, which is associated with distorted hydrogen bonds. Furthermore, a connection is made between the observed increase in spectral intensity between 544 and 548 eV and high-symmetry points in the electronic band structure, suggesting a more extended hydrogen-bond network as compared to ices prepared differently. The spectral differences for various ice preparations are compared to the temperature dependence of spectra of liquid water upon supercooling. A double-peak feature in the absorption cross-section between 540 and 543 eV is identified as a characteristic of the crystalline phase. The connection to the interpretation of the liquid phase O K-edge x-ray absorption spectrum is extensively discussed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Exploring Mbar shock conditions and isochorically heated aluminum at the MEC end station of the LCLS

Recent experiments performed at the Matter in Extreme Conditions end station (MEC) of the Linac Coherent Light Source (LCLS) have demonstrated the first spectrally resolved measurements of plasmons from isochorically heated aluminum. The experiments have been performed using a seeded 8-keV x-ray laser beam as a pump and probe to both volumetrically heat and scatter x-rays from aluminum. Collective x-ray Thomson scattering spectra show a well-resolved plasmon feature that is down-shifted in energy by 19 eV. In addition, Mbar shock pressures from laser-compressed aluminum foils using Velocity Interferometer System for Any Reflector (VISAR) have been measured. The combination of experiments fully demonstrates the possibility to perform warm dense matter studies at the LCLS with unprecedented accuracy and precision.

INST, MATSCI, PHYS, XFEL↗
DOE OSTI2014

Surface structures of In-Pd intermetallic compounds. II. A theoretical study

The (110) surface of the InPd intermetallic compound and the In–Pd surface alloy properties are investigated in the framework of the density functional theory, within the projector augmented plane-wave method. Surface segregation is calculated to be energetically unfavorable at stoichiometric InPd(110) surfaces, while indium antisites are shown to segregate to the surface in off-stoichiometric InPd(110) systems. Concerning surface alloys obtained by burying In-doped Pd layers in Pd(111), we demonstrated that the most stable ones are those presenting atomic indium concentrations below 50 at. % (11 at. %, 25 at. %, 33 at. %). According to our calculations, the In-doped Pd layers with concentration above or equal to 50% lead to In-doped Pd multilayers, each presenting an atomic indium concentration below 50 at. %. Alloying and segregation effects in InPd intermetallic compound and In–Pd surface alloys clearly agree with the larger bonding strength of In–Pd (−0.44 eV) compared to In–In (−0.29 eV) and Pd–Pd (−0.31 eV)

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

An experimental and theoretical investigation into the excited electronic states of phenol

We present experimental electron-energy loss spectra (EELS) that were measured at impact energies of 20 and 30 eV and at angles of 90° and 10°, respectively, with energy resolution ∼70 meV. EELS for 250 eV incident electron energy over a range of angles between 3° and 50° have also been measured at a moderate energy resolution (∼0.9 eV). The latter spectra were used to derive differential cross sections and generalised oscillator strengths (GOS) for the dipole-allowed electronic transitions, through normalization to data for elastic electron scattering from benzene. Theoretical calculations were performed using time-dependent density functional theory and single-excitation configuration interaction methods. These calculations were used to assign the experimentally measured spectra. Calculated optical oscillator strengths were also compared to those derived from the GOS data. This provides the first investigation of all singlet and triplet excited electronic states of phenol up to the first ionization potential.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Langmuir probe measurements and mass spectrometry of plasma plumes generated by laser ablation of La{sub 0.4}Ca{sub 0.6}MnO{sub 3}

The plasma formed in vacuum by UV nanosecond laser ablation of La{sub 0.4}Ca{sub 0.6}MnO{sub 3} in the fluence range of 0.8 to 1.9 J cm{sup −2} using both Langmuir probe analysis and energy-resolved mass spectrometry has been studied. Mass spectrometry shows that the main positive ion species are Ca{sup +}, Mn{sup +}, La{sup +}, and LaO{sup +}. The Ca{sup +} and Mn{sup +} energy distributions are quite broad and lie in the 0–100 eV region, with the average energies increasing with laser fluence. In contrast, the La{sup +} and LaO{sup +} distributions are strongly peaked around 10 eV. The net time-of-arrival signal derived from the measured positive ion energy distributions is broadly consistent with the positive ion signal measured by the Langmuir probe. We also detected a significant number of O{sup −} ions with energies in the range of 0 to 10 eV. The Langmuir probe was also used to measure the temporal variation of the electron density and temperature at 6 cm from the ablation target. In the period when O{sup −} ions are found at this position, the plasma conditions are consistent with those required for significant negative oxygen ion formation, as revealed by studies on radio frequency excited oxygen plasma.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Ab-initio calculations of electronic, transport, and structural properties of boron phosphide

We present results from ab-initio, self-consistent density functional theory calculations of electronic and related properties of zinc blende boron phosphide (zb-BP). We employed a local density approximation potential and implemented the linear combination of atomic orbitals formalism. This technique follows the Bagayoko, Zhao, and Williams method, as enhanced by the work of Ekuma and Franklin. The results include electronic energy bands, densities of states, and effective masses. The calculated band gap of 2.02 eV, for the room temperature lattice constant of a=4.5383 Å, is in excellent agreement with the experimental value of 2.02±0.05 eV. Our result for the bulk modulus, 155.7 GPa, agrees with experiment (152–155 GPa). Our predictions for the equilibrium lattice constant and the corresponding band gap, for very low temperatures, are 4.5269 Å and 2.01 eV, respectively.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Low energy electron induced cytosine base release in 2′-deoxycytidine-3′-monophosphate via glycosidic bond cleavage: A time-dependent wavepacket study

Low energy electron (LEE) induced cytosine base release in a selected pyrimidine nucleotide, viz., 2′-deoxycytidine-3′-monophosphate is investigated using ab initio electronic structure methods and time dependent quantum mechanical calculations. It has been noted that the cytosine base scission is comparatively difficult process than the 3′ C–O bond cleavage from the lowest π{sup *} shape resonance in energy region <1 eV. This is mainly due to the high activation energy barrier associated with the electron transfer from the π{sup *} orbital of the base to the σ{sup *} orbital of the glycosidic N–C bond. In addition, the metastable state formed after impinging LEE (0–1 eV) has very short lifetime (10 fs) which may decay in either of the two competing auto-detachment or dissociation process simultaneously. On the other hand, the selected N–C mode may cleave to form the cytosine base anion at higher energy regions (>2 eV) via tunneling of the glycosidic bond. Resonance states generated within this energy regime will exist for a duration of ∼35–55 fs. Comparison of salient features of the two dissociation events, i.e., 3′ C–O single strand break and glycosidic N–C bond cleavage in 3′-dCMPH molecule are also provided.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Probing bismuth ferrite nanoparticles by hard x-ray photoemission: Anomalous occurrence of metallic bismuth

We have investigated bismuth ferrite nanoparticles (∼75 nm and ∼155 nm) synthesized by a chemical method, using soft X-ray (1253.6 eV) and hard X-ray (3500, 5500, and 7500 eV) photoelectron spectroscopy. This provided an evidence for the variation of chemical state of bismuth in crystalline, phase pure nanoparticles. X-ray photoelectron spectroscopy analysis using Mg Kα (1253.6 eV) source showed that iron and bismuth were present in both Fe{sup 3+} and Bi{sup 3+} valence states as expected for bismuth ferrite. However, hard X-ray photoelectron spectroscopy analysis of the bismuth ferrite nanoparticles using variable photon energies unexpectedly showed the presence of Bi{sup 0} valence state below the surface region, indicating that bismuth ferrite nanoparticles are chemically inhomogeneous in the radial direction. Consistently, small-angle X-ray scattering reveals a core-shell structure for these radial inhomogeneous nanoparticles.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2014

Investigating the plasma parameters of an Ar/O{sub 2} discharge during the sputtering of Al targets in an inverted cylindrical magnetron

The plasma parameters and reaction kinetics in an inverted cylindrical magnetron chamber have been studied with an energy resolved mass spectrometer during the sputtering of aluminum targets in an Ar/O{sub 2} discharge. Mixtures of argon and oxygen were studied as a function of oxygen percentage (0%–90%) in the discharge. The plasma was powered at 4 kW and 40 kHz at a process pressure of 5 mTorr. Al{sup +}, Al, AlO, AlO{sup +}, O{sub 2}{sup +}, O{sup +}, Al{sub 2}O{sup +}, and Ar{sup +} were among the species detected in the discharge. The deposition rate of the deposited thin film decreased with increasing oxygen percentage in the discharge and results indicated that the pure gamma-alumina was obtained when the percentage of oxygen was approximately 70%. The linear plot of energy distributions of the positively charged film forming species changed from a single peak to a bimodal distribution as the percentage of oxygen exceeds 65%. In a log plot, however, the distributions showed multiple peaks ranging from 2 eV to 78 eV. Fluctuations of about 1 eV in peak energies were observed.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Band gap and electronic structure of MgSiN{sub 2}

Density functional theory calculations and electron energy loss spectroscopy indicate that the electronic structure of ordered orthorhombic MgSiN{sub 2} is similar to that of wurtzite AlN. A band gap of 5.7 eV was calculated for both MgSiN{sub 2} (indirect) and AlN (direct) using the Heyd-Scuseria-Ernzerhof approximation. Correction with respect to the experimental room-temperature band gap of AlN indicates that the true band gap of MgSiN{sub 2} is 6.2 eV. MgSiN{sub 2} has an additional direct gap of 6.3 eV at the Γ point.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Nano-scale NiSi and n-type silicon based Schottky barrier diode as a near infra-red detector for room temperature operation

The fabrication of nano-scale NiSi/n-Si Schottky barrier diode by rapid thermal annealing process is reported. The characterization of the nano-scale NiSi film was performed using Micro-Raman Spectroscopy and X-ray Photoelectron Spectroscopy (XPS). The thickness of the film (27 nm) has been measured by cross-sectional Secondary Electron Microscopy and XPS based depth profile method. Current–voltage (I–V) characteristics show an excellent rectification ratio (I{sub ON}/I{sub OFF} = 10⁵) at a bias voltage of ±1 V. The diode ideality factor is 1.28. The barrier height was also determined independently based on I–V (0.62 eV) and high frequency capacitance–voltage technique (0.76 eV), and the correlation between them has explained. The diode photo-response was measured in the range of 1.35–2.5 μm under different reverse bias conditions (0.0–1.0 V). The response is observed to increase with increasing reverse bias. From the photo-responsivity study, the zero bias barrier height was determined to be 0.54 eV.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Band offsets of TiZnSnO/Si heterojunction determined by x-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy (XPS) was utilized to measure the valence band offset (ΔE{sub V}) of the TiZnSnO (TZTO)/Si heterojunction. TZTO films were deposited on Si (100) substrates using magnetron sputtering at room temperature. By using the Zn 2p{sub 3/2} and Sn 3d{sub 5/2} energy levels as references, the value of ΔE{sub V} was calculated to be 2.69 ± 0.1 eV. Combining with the experimental optical energy band gap of 3.98 eV for TZTO extracted from the UV-vis transmittance spectrum, the conduction band offset (ΔE{sub C}) was deduced to be 0.17 ± 0.1 eV at the interface. Hence, the energy band alignment of the heterojunction was determined accurately, showing a type-I form. This will be beneficial for the design and application of TZTO/Si hybrid devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Defect density and dielectric constant in perovskite solar cells

We report on measurement of dielectric constant, mid-gap defect density, Urbach energy of tail states in CH{sub 3}NH{sub 3}PbI{sub x}Cl{sub 1−x} perovskite solar cells. Midgap defect densities were estimated by measuring capacitance vs. frequency at different temperatures and show two peaks, one at 0.66 eV below the conduction band and one at 0.24 eV below the conduction band. The attempt to escape frequency is in the range of 2 × 10{sup 11}/s. Quantum efficiency data indicate a bandgap of 1.58 eV. Urbach energies of valence and conduction band are estimated to be ∼16 and ∼18 meV. Measurement of saturation capacitance indicates that the relative dielectric constant is ∼18.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

First principles study of the structural, elastic, electronic and phonon properties of CdX{sub 2}O{sub 4} (X=Al, Ga, In) spinel-type oxides

We have performed ab-initio calculations of the structural, electronic, elastic and dynamical properties for the spinel compounds CdX{sub 2}O{sub 4} (X=Al, Ga, In) using the plane wave pseudo-potential method within the generalized gradient approximation (GGA). The calculated lattice parameters, elastic constants for these compounds are in good agreement with the previous calculated values. The computed direct band gaps of CdAl{sub 2}O{sub 4}, CdGa{sub 2}O{sub 4} and CdIn{sub 2}O{sub 4} are 2.90 eV, 1.92 eV and 1.16 eV, respectively. The lattice vibrations were calculated by direct method. The calculated phonon dispersion curves show that all compounds are dynamically stable in the spinel structure.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

A study on dependence of the structural, optical and electrical properties of cadmium lead sulphide thin films on Cd/Pb ratio

Cadmium Lead Sulphide thin films with systematic variation in Cd/Pb ratio are prepared at 333K by CBD, adjusting the reagent-molarity, deposition time and pH. XRD exhibits crystalline-amorphous transition as Cd% exceeds Pb%. AFM shows agglomeration of crystallites of size ∼50±5 nm. EDAX assess the composition whereas XPS ascertains the ternary formation, with binding energies of Pb4f{sub 7/2} and 4f{sub 5/2}, Cd3d{sub 5/2} and 3d{sub 3/2} and S2p at 137.03, 141.606, 404.667, 412.133 and 160.218 eV respectively. The optical absorption spectra reveal the variance in the direct allowed band gaps, from 1.57eV to 2.42 eV as Cd/Pb ratio increases from 0.2 to 2.7, suggesting possibility of band gap engineering in the n-type films.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗