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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 577 records · Page 32
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Highly efficient and electrically robust carbon irradiated semi-insulating GaAs based photoconductive terahertz emitters

We demonstrate here an efficient photoconductive THz source with low electrical power consumption. We have increased the maximum THz radiation power emitted from SI-GaAs based photoconductive emitters (PCEs) by two orders of magnitude. By irradiating the SI-GaAs substrate with Carbon-ions up to 2 μm deep, we have created lot of defects and decreased the lifetime of photo-excited carriers inside the substrate. Depending on the irradiation dose, we find 1 to 2 orders of magnitude decrease in total current flowing in the substrate, resulting in subsequent decrease of heat dissipation in the device. This has resulted in increasing maximum cut-off of the applied voltage across PCE electrodes to operate the device without thermal breakdown from ∼35 V to >150 V for the 25 μm electrode gaps. At optimum operating conditions, carbon irradiated (10{sup 14} ions/cm{sup 2}) PCEs give THz pulses with power about 100 times higher in comparison to the usual PCEs on SI-GaAs and electrical to THz power conversion efficiency has improved by a factor of ∼800.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Effect of growth temperature on defects in epitaxial GaN film grown by plasma assisted molecular beam epitaxy

We report the effect of growth temperature on defect states of GaN epitaxial layers grown on 3.5 μm thick GaN epi-layer on sapphire (0001) substrates using plasma assisted molecular beam epitaxy. The GaN samples grown at three different substrate temperatures at 730, 740 and 750 °C were characterized using atomic force microscopy and photoluminescence spectroscopy. The atomic force microscopy images of these samples show the presence of small surface and large hexagonal pits on the GaN film surfaces. The surface defect density of high temperature grown sample is smaller (4.0 × 10{sup 8} cm{sup −2} at 750 °C) than that of the low temperature grown sample (1.1 × 10{sup 9} cm{sup −2} at 730 °C). A correlation between growth temperature and concentration of deep centre defect states from photoluminescence spectra is also presented. The GaN film grown at 750 °C exhibits the lowest defect concentration which confirms that the growth temperature strongly influences the surface morphology and affects the optical properties of the GaN epitaxial films.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Ground state of naphthyl cation: Singlet or triplet?

We present a benchmark theoretical investigation on the electronic structure and singlet-triplet(S-T) gap of 1- and 2-naphthyl cations using the CCSD(T) method. Our calculations reveal that the ground states of both the naphthyl cations are singlet, contrary to the results obtained by DFT/B3LYP calculations reported in previous theoretical studies. However, the triplet states obtained in the two structural isomers of naphthyl cation are completely different. The triplet state in 1-naphthyl cation is (π,σ) type, whereas in 2-naphthyl cation it is (σ,σ{sup ′}) type. The S-T gaps in naphthyl cations and the relative stability ordering of the singlet and the triplet states are highly sensitive to the basis-set quality as well as level of correlation, and demand for inclusion of perturbative triples in the coupled-cluster ansatz.

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

Thermal expansion of multiwall carbon nanotube reinforced nanocrystalline silver matrix composite

Multiwall carbon nanotube reinforced silver matrix composite was fabricated by novel molecular level mixing method, which involves nucleation of Ag ions inside carbon nanotube dispersion at the molecular level. As a result the carbon nanotubes get embedded within the powder rather than on the surfaces. Micro structural characterization by X- ray diffraction and scanning electron microscopy reveals that the nanotubes are homogeneously dispersed and anchored within the matrix. The thermal expansion of the composite with the multiwall nanotube content (0, 1.5 vol%) were investigated and it is found that coefficient of thermal expansion decreases with the addition of multiwall nanotube content and reduce to about 63% to that of pure Ag.

36 MATERIALS SCIENCE↗
DOE OSTI2014

High temperature ferromagnetism in Ni doped ZnO nanoparticles: Milling time dependence

We report on the room temperature ferromagnetism (RT FM) in the Zn{sub 1−x}Ni{sub x}O (x = 0, 0.03, and 0.05) nanoparticles (NPs) synthesized by a ball milling technique. X-ray diffraction analysis confirms the single crystalline ZnO wurtzite structure with presence of small intensity secondary phase related peak which disappear with increasing milling time for Ni doped samples. HRTEM lattice images show that the doped NPs are single crystalline with a dspacing of 2.44 Å. Energy-dispersive X-ray spectroscopy analysis confirms the presence of Ni ions in the ZnO matrix. Magnetic measurement (RT) exhibits the hysteresis loop with saturation magnetization (M{sub s}) of 1.6–2.56 (emu/g) and coercive field (H{sub c}) of 296–322 Oe. M-T measurement shows a Curie temperature of the order of 325°C for 3% Ni doped sample. Micro -Raman studies show doping/disorder induced additional modes at ∼510, 547, 572 cm{sup −1} in addition to 437 cm{sup −1} peak of pure ZnO. UV-Vis absorption spectra illustrate band gap shift due to doping. Alteration of M{sub s} value with the variation of doping concentration and milling time has been studied and discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Electrical and optical properties of gadolinium doped bismuth ferrite nanoparticles

Multiferroic bismuth ferrite (BFO) and gadolinium (Gd) doped bismuth ferrite had been synthesized by a sol-gel method. Particle size had been estimated by Transmission electron microscopy (TEM) and found to decrease with Gd doping. We studied the temperature and frequency dependence of impedance and electric modulus and calculated the grain and grain boundary resistance and capacitance of the investigated samples. We observed that electrical activation energy increases for all the doped samples. Optical band gap also increases for the doped samples which can be used in photocatalytic application of BFO.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Structural and magnetic properties of Co{sub 2}Ti{sub 1−x}Fe{sub x}Al (0 ≤ x ≤ 0.5) alloys

In this work we studied the effect of partial Fe substitution for Ti on the structural and magnetic properties of the Co{sub 2}TiAl. X-ray diffraction analysis indicates the presence of B2 type disorder for x > 0, (111) reflections are absent for x > 0 which is the characteristic of B2 type disorder. XRD analysis also shows presence of second phase. Magnetization measurements also confirm the presence of dual phase. Curie temperature of the alloys increases with increase in Fe concentration. Saturation magnetic moments agree very well with those calculated by Slater-Pauling rule.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Facile functionalization of Fe{sub 2}O{sub 3} nanoparticles to induce inherent photoluminescence and excellent photocatalytic activity

Herein, we report the emergence of intrinsic multicolor photoluminescence in Fe{sub 2}O{sub 3} nanoparticles (NPs) ranging from blue, cyan, to green, upon facile functionalization and further surface modification with a small organic ligand, Na-tartrate. Moreover, we have found unprecedented photocatalytic property of the functionalized Fe{sub 2}O{sub 3} NPs in the degradation of a model water-contaminant. Meticulous investigation through UV-visible absorption and fluorescence study along with theoretical support from literature unfolds that ligand-to-metal charge-transfer transition from the tartrate ligand to the lowest unoccupied energy level of Fe{sup 3+} of the NPs and d−d transitions centered over Fe{sup 3+} ions in the NPs play the key role in the emergence of multiple photoluminescence from the ligand functionalized Fe{sub 2}O{sub 3} NPs. Moreover, vibrating sample magnetometry measurements demonstrate that the surface modification changes the magnetic behaviour of Fe{sub 2}O{sub 3} NPs upon functionalization. We believe that the great potential of our versatile, ferromagnetic, multicolor photoluminescent Fe{sub 2}O{sub 3} NPs would stimulate the development of numerous opportunities toward their biological and technological applications.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2014

Local control of magnetic damping in ferromagnetic/non-magnetic bilayers by interfacial intermixing induced by focused ion-beam irradiation

The influence of interfacial intermixing on the picosecond magnetization dynamics of ferromagnetic/non-magnetic thin-film bilayers was studied. Low-dose focused-ion-beam irradiation was used to induce intermixing across the interface between a 10 nm Ni{sub 81}Fe{sub 19} layer and a 2–3 nm capping layer of either Au or Cr. Time-resolved magneto-optical Kerr effect was used to study magnetization dynamics as a function of ion-beam dose. With an Au cap, the damping of the un-irradiated bilayer was comparable with native Ni{sub 81}Fe{sub 19} and increased with increasing ion dose. In contrast, for Ni{sub 81}Fe{sub 19}/Cr the damping was higher than that for native Ni{sub 81}Fe{sub 19}, but the damping decreased with increasing dose.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Optimization of excess Bi doping to enhance ferroic orders of spin casted BiFeO{sub 3} thin film

Multiferroic Bismuth Ferrite (BiFeO{sub 3}) thin films with varying excess bismuth (Bi) concentration were grown by chemical solution deposition technique. Room temperature multiferroic properties (ferromagnetism, ferroelectricity, and piezoelectricity) of the deposited BiFeO{sub 3} thin films have been studied. High resolution X-ray diffraction and Raman spectroscopy studies reveal that the dominant phases formed in the prepared samples change continuously from a mixture of BiFeO{sub 3} and Fe{sub 2}O{sub 3} to pure BiFeO{sub 3} phase and, subsequently, to a mixture of BiFeO{sub 3} and Bi{sub 2}O{sub 3} with increase in the concentration of excess Bi from 0% to 15%. BiFeO{sub 3} thin films having low content (0% and 2%) of excess Bi showed the traces of ferromagnetic phase (γ-Fe{sub 2}O{sub 3}). Deterioration in ferroic properties of BiFeO{sub 3} thin films is also observed when prepared with higher content (15%) of excess Bi. Single-phased BiFeO{sub 3} thin film prepared with 5% excess Bi concentration exhibited the soft ferromagnetic hysteresis loops and ferroelectric characteristics with remnant polarization 4.2 μC/cm{sup 2} and saturation magnetization 11.66 emu/g. The switching of fine spontaneous domains with applied dc bias has been observed using piezoresponse force microscopy in BiFeO{sub 3} thin films having 5% excess Bi. The results are important to identify optimum excess Bi concentration needed for the formation of single phase BiFeO{sub 3} thin films exhibiting the improved multiferroic properties.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Performance of the beam chamber vacuum system of K = 500 cyclotron at Variable Energy Cyclotron Centre Kolkata

The beam chamber of Variable Energy Cyclotron Centre, Kolkata's K = 500 superconducting cyclotron is pumped by liquid helium cooled cryopanel with liquid nitrogen cooled radiation shield. Performance of the vacuum system was evaluated by cooling the cryopanel assembly with liquid nitrogen and liquid helium. Direct measurement of beam chamber pressure is quite difficult because of space restrictions and the presence of high magnetic field. Pressure gauges were placed away from the beam chamber. The beam chamber pressure was evaluated using a Monte Carlo simulation software for vacuum system and compared with measurements. The details of the vacuum system, measurements, and estimation of pressure of the beam chamber are described in this paper.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

Optical density of states in ultradilute GaAsN alloy: Coexistence of free excitons and impurity band of localized and delocalized states

Optically active states in liquid phase epitaxy-grown ultra-dilute GaAsN are studied. The feature-rich low temperature photoluminescence spectrum has contributions from excitonic band states of the GaAsN alloy, and two types of defect states—localized and extended. The degree of delocalization for extended states both within the conduction and defect bands, characterized by the electron temperature, is found to be similar. The degree of localization in the defect band is analyzed by the strength of the phonon replicas. Stronger emission from these localized states is attributed to their giant oscillator strength.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Ultraviolet-B radiation enhancement in dielectric barrier discharge based xenon chloride exciplex source by air

A single barrier dielectric barrier discharge tube of quartz with multi-strip Titanium-Gold (Ti-Au) coatings have been developed and utilized for ultraviolet-B (UV-B) radiation production peaking at wavelength 308 nm. The observed radiation at this wavelength has been examined for the mixtures of the Xenon together with chlorine and air admixtures. The gas mixture composition, chlorine gas content, total gas pressure, and air pressure dependency of the UV intensity, has been analyzed. It is found that the larger concentration of Cl{sub 2} deteriorates the performance of the developed source and around 2% Cl{sub 2} in this source produced optimum results. Furthermore, an addition of air in the xenon and chlorine working gas environment leads to achieve same intensity of UV-B light but at lower working gas pressure where significant amount of gas is air.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Time evolution studies of laser induced chemical changes in InAs nanowire using Raman spectroscopy

We report the study of time evolution of chemical changes on the surface of an InAs nanowire (NW) on laser irradiation in different power density regime, using Raman spectroscopy for a time span of 8–16 min. Mixture of metastable oxides like InAsO{sub 4,} As{sub 2}O{sub 3} are formed upon oxidation, which are reflected as sharp Raman peaks at ∼240–254 and 180–200 cm{sup −1}. Evidence of removal of arsenic layer by layer is also observed at higher power density. Position controlled laser induced chemical modification on a nanometer scale, without changing the core of the NW, can be useful for NW based device fabrication.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2014

Highly focused and efficient terahertz radiation generation by photo-mixing of lasers in plasma in the presence of magnetic field

A mechanism of efficient and highly focused terahertz (THz) radiation generation by photo-mixing of top-hat like lasers with frequencies ω{sub 1}, ω{sub 2} and wave numbers k{sub 1}, k{sub 2} in pre-formed rippled density (corrugated) plasma is proposed. In this mechanism, intensity variation of lasers offers nonlinear ponderomotive force at frequency ω{sup ′}=ω{sub 1}−ω{sub 2} and wave number k{sup ′}=k{sub 1}−k{sub 2} which couples with density ripples in the plasma and leads to a strong nonlinear oscillatory current that resonantly excites highly focused and intense THz radiation at frequency ω{sub UH}=√((ω{sub p}{sup 2}+ω{sub c}{sup 2})) (where ω{sub c} is electron cyclotron frequency). The efficiency of emitted THz radiation of the order of 15% is obtained under optimum conditions. It is observed that focus and intensity of emitted radiation can be controlled by selecting a proper profile index of the lasers, ripple parameters, and tuning of external magnetic field.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Chemical potential shift and gap-state formation in SrTiO{sub 3−δ} revealed by photoemission spectroscopy

In this study, we report on investigations of the electronic structure of SrTiO{sub 3} annealed at temperature ranging between 550 and 840 °C in an ultrahigh vacuum. Annealing induced oxygen vacancies (O{sub vac}) impart considerable changes in the electronic structure of SrTiO{sub 3}. Using core-level photoemission spectroscopy, we have studied the chemical potential shift (Δμ) as a function of annealing temperature. The result shows that the chemical potential monotonously increases with electron doping in SrTiO{sub 3−δ}. The monotonous increase of the chemical potential rules out the existence of electronic phase separation in the sample. Using valence band photoemission, we have demonstrated the formation of a low density of states at the near Fermi level electronic spectrum of SrTiO{sub 3−δ}. The gap-states were observed by spectral weight transfer over a large energy scale of the stoichiometric band gap of SrTiO{sub 3} system leading finally to an insulator-metal transition. We have interpreted our results from the point of structural distortions induced by oxygen vacancies.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Strain induced Z{sub 2} topological insulating state of β-As{sub 2}Te{sub 3}

Topological insulators are non-trivial quantum states of matter which exhibit a gap in the electronic structure of their bulk form, but a gapless metallic electronic spectrum at the surface. Here, we predict a uniaxial strain induced electronic topological transition (ETT) from a band to topological insulating state in the rhombohedral phase (space group: R3{sup ¯}m) of As{sub 2}Te{sub 3} (β-As{sub 2}Te{sub 3}) through first-principles calculations including spin-orbit coupling within density functional theory. The ETT in β-As{sub 2}Te{sub 3} is shown to occur at the uniaxial strain ϵ{sub zz} = −0.05 (σ{sub zz} = 1.77 GPa), passing through a Weyl metallic state with a single Dirac cone in its electronic structure at the Γ point. We demonstrate the ETT through band inversion and reversal of parity of the top of the valence and bottom of the conduction bands leading to change in the ℤ{sub 2} topological invariant ν{sub 0} from 0 to 1 across the transition. Based on its electronic structure and phonon dispersion, we propose ultra-thin films of As{sub 2}Te{sub 3} to be promising for use in ultra-thin stress sensors, charge pumps, and thermoelectrics.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Quantum beats from the coherent interaction of hole states with surface state in near-surface quantum well

We report tunneling assisted beating of carriers in a near-surface single GaAsP/AlGaAs quantum well using transient reflectivity measurement. The observed damped oscillating signal has a period of 120 ± 6 fs which corresponds to the energy difference between lh1 and hh2 hole states in the quantum well. Comparing the transient reflectivity signal at different photon energies and with a buried quantum well sample, we show that the beating is caused by the coherent coupling between surface state and the hole states (lh1 and hh2) in the near-surface quantum well. The dependence of decay of coherence of these tunneling carriers on the excitation fluence is also reported. This observation on the coherent tunneling of carrier is important for future quantum device applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗