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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 433 records · Page 24
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Hydrogen and fluorine co-decorated silicene: A first principles study of piezoelectric properties

A low-buckled silicene monolayer being centrosymmetric like graphene, in contrast to a piezoelectric hexagonal boron nitride (h-BN), is not intrinsically piezoelectric. However, based on first principles calculations, we show that chemical co-decoration of hydrogen (H) and fluorine (F) on opposite sides of silicene (i.e., one side is decorated with H, while the other one is with F) breaks the centrosymmetry. Redistributing the charge density due to the electronegativity difference between the atoms, non-centrosymmetric co-decoration induces an out-of-plane dipolar polarization and concomitant piezoelectricity into non-piezoelectric silicene monolayer. Our piezoelectric coefficients are comparable with other known two-dimensional piezoelectric materials (e.g., hydrofluorinated graphene/h-BN) and some bulk semiconductors, such as wurtzite GaN and wurtzite BN. Moreover, because of silicene's lower elastic constants compared to graphene or h-BN, piezoelectric strain constants are found significantly larger than those of hydrofluorinated graphene/h-BN. We also predict that a wide range of band gaps with an average of 2.52 eV can be opened in a low-buckled gapless semi-metallic silicene monolayer by co-decoration of H and F atoms on the surface.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Charge transition levels of oxygen, lanthanum, and fluorine related defect structures in bulk hafnium dioxide (HfO{sub 2}): An ab initio investigation

Intrinsic defect structures and impurity atoms are one of the main sources of leakage current in metal-oxide-semiconductor devices. Using state of the art density functional theory, we have investigated oxygen, lanthanum, and fluorine related defect structures and possible combinations of them. In particular, we have calculated their charge transition levels in bulk m-HfO{sub 2}. For this purpose, we have developed a new scaling scheme to account for the band gap underestimation within the density functional theory. The obtained results are able to explain the recent experimental observation of a reduction of the trap density near the silicon valence band edge after NF{sub 3} treatment and the associated reduction of the device degradation.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Highly improved photo-induced bias stability of sandwiched triple layer structure in sol-gel processed fluorine-doped indium zinc oxide thin film transistor

In order to improve the reliability of TFT, an Al{sub 2}O{sub 3} insulating layer is inserted between active fluorine doped indium zinc oxide (IZO:F) thin films to form a sandwiched triple layer. All the thin films were fabricated via low-cost sol-gel process. Due to its large energy bandgap and high bonding energy with oxygen atoms, the Al{sub 2}O{sub 3} layer acts as a photo-induced positive charge blocking layer that effectively blocks the migration of both holes and V {sub o}{sup 2+} toward the interface between the gate insulator and the semiconductor. The inserted Al{sub 2}O{sub 3} triple layer exhibits a noticeably low turn on voltage shift of −0.7 V under NBIS as well as the good TFT performance with a mobility of 10.9 cm{sup 2}/V ⋅ s. We anticipate that this approach can be used to solve the stability issues such as NBIS, which is caused by inescapable oxygen vacancies.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

The physical mechanism on the threshold voltage temperature stability improvement for GaN HEMTs with pre-fluorination argon treatment

In this paper, a normally-off AlGaN/GaN MIS-HEMT with improved threshold voltage (V{sub TH}) thermal stability is reported with investigations on its physical mechanism. The normally-off operation of the device is achieved from novel short argon plasma treatment (APT) prior to the fluorine plasma treatment (FPT) on Al{sub 2}O{sub 3} gate dielectrics. For the MIS-HEMT with FPT only, its V{sub TH} drops from 4.2 V at room temperature to 0.5 V at 200 °C. Alternatively, for the device with APT-then-FPT process, its V{sub TH} can retain at 2.5 V at 200 °C due to the increased amount of deep-level traps that do not emit electrons at 200 °C. This thermally stable V{sub TH} makes this device suitable for high power applications. The depth profile of the F atoms in Al{sub 2}O{sub 3}, measured by the secondary ion mass spectroscopy, reveals a significant increase in the F concentration when APT is conducted prior to FPT. The X-ray photoelectron spectroscopy (XPS) analysis on the plasma-treated Al{sub 2}O{sub 3} surfaces observes higher composition of Al-F bonds if APT was applied before FPT. The enhanced breaking of Al-O bonds due to Ar bombardment assisted in the increased incorporation of F radicals at the surface during the subsequent FPT process. The Schrödinger equation of Al{sub 2}O{sub x}F{sub y} cells, with the same Al-F compositions as obtained from XPS, was solved by Gaussian 09 molecular simulations to extract electron state distribution as a function of energy. The simulation results show creation of the deeper trap states in the Al{sub 2}O{sub 3} bandgap when APT is used before FPT. Finally, the trap distribution extracted from the simulations is verified by the gate-stress experimental characterization to confirm the physical mechanism described.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

Resonance laser-plasma excitation of coherent terahertz phonons in the bulk of fluorine-bearing crystals under high-intensity femtosecond laser irradiation

The dynamics of coherent phonons in fluorine-containing crystals was investigated by pump-probe technique in the plasma production regime. Several phonon modes, whose frequencies are overtones of the 0.38-THz fundamental frequency, were simultaneously observed in a lithium fluoride crystal. Phonons with frequencies of 1 and 0.1 THz were discovered in a calcium fluoride crystal and coherent phonons with frequencies of 1 THz and 67 GHz were observed in a barium fluoride crystal. Furthermore, in the latter case the amplitudes of phonon mode oscillations were found to significantly increase 15 ps after laser irradiation. (interaction of laser radiation with matter)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

FLUORINE VARIATIONS IN THE GLOBULAR CLUSTER NGC 6656 (M22): IMPLICATIONS FOR INTERNAL ENRICHMENT TIMESCALES

Observed chemical (anti)correlations in proton-capture elements among globular cluster stars are presently recognized as the signature of self-enrichment from now extinct, previous generations of stars. This defines the multiple population scenario. Since fluorine is also affected by proton captures, determining its abundance in globular clusters provides new and complementary clues regarding the nature of these previous generations and supplies strong observational constraints to the chemical enrichment timescales. In this paper, we present our results on near-infrared CRIRES spectroscopic observations of six cool giant stars in NGC 6656 (M22): the main objective is to derive the F content and its internal variation in this peculiar cluster, which exhibits significant changes in both light- and heavy-element abundances. Across our sample, we detected F variations beyond the measurement uncertainties and found that the F abundances are positively correlated with O and anticorrelated with Na, as expected according to the multiple population framework. Furthermore, our observations reveal an increase in the F content between the two different sub-groups, s-process rich and s-process poor, hosted within M22. The comparison with theoretical models suggests that asymptotic giant stars with masses between 4 and 5 M {sub Sun} are responsible for the observed chemical pattern, confirming evidence from previous works: the difference in age between the two sub-components in M22 must be not larger than a few hundred Myr.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

FLUORINE ABUNDANCES OF GALACTIC LOW-METALLICITY GIANTS

With abundances and 2{sigma} upper limits of fluorine (F) in seven metal-poor field giants, nucleosynthesis of stellar F at low metallicity is discussed. The measurements are derived from the HF(1-0) R9 line at 23358 A using near-infrared K-band high-resolution spectra obtained with CRIRES at the Very Large Telescope. The sample reaches lower metallicities than previous studies on F of field giants, ranging from [Fe/H] = -1.56 down to -2.13. Effects of three-dimensional model atmospheres on the derived F and O abundances are quantitatively estimated and shown to be insignificant for the program stars. The observed F yield in the form of [F/O] is compared with two sets of Galactic chemical evolution models, which quantitatively demonstrate the contribution of Type II supernova (SN II) {nu}-process and asymptotic giant branch/Wolf-Rayet stars. It is found that at this low-metallicity region, models cannot well predict the observed distribution of [F/O], while the observations are better fit by models considering an SN II {nu}-process with a neutrino energy of E {sub {nu}} = 3 Multiplication-Sign 10{sup 53} erg. Our sample contains HD 110281, a retrograde orbiting low-{alpha} halo star, showing a similar F evolution as globular clusters. This supports the theory that such halo stars are possibly accreted from dwarf galaxy progenitors of globular clusters in the halo.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2010

FLUORINE ABUNDANCES IN GALACTIC ASYMPTOTIC GIANT BRANCH STARS

An analysis of the fluorine abundance in Galactic asymptotic giant branch (AGB) carbon stars (24 N-type, 5 SC-type, and 5 J-type) is presented. This study uses the state-of-the-art carbon-rich atmosphere models and improved atomic and molecular line lists in the 2.3 {mu}m region. Significantly lower F abundances are obtained in comparison to previous studies in the literature. This difference is mainly due to molecular blends. In the case of carbon stars of SC-type, differences in the model atmospheres are also relevant. The new F enhancements are now in agreement with the most recent theoretical nucleosynthesis models in low-mass AGB stars, solving the long-standing problem of F in Galactic AGB stars. Nevertheless, some SC-type carbon stars still show larger F abundances than predicted by stellar models. The possibility that these stars are of larger mass is briefly discussed.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

EVOLUTION OF FLUORINE IN THE GALAXY WITH THE {nu}-PROCESS

We calculate the evolution of fluorine in the solar neighborhood with the {nu}-process of core-collapse supernovae, the results of which are in good agreement with the observations of field stars. The {nu}-process operating in supernovae causes the [F/O] ratio to plateau at [O/H] {approx}< - 1.2, followed by a rapid increase toward [O/H] {approx} - 0.5 from the contribution of asymptotic giant branch stars. The plateau value of [F/O] depends on the neutrino luminosity released by core-collapse supernovae and may be constrained by using future observations of field stars at low metallicities. For globular clusters, the handful of [F/O] measurements suggest that the relative contribution from low-mass supernovae is smaller in these systems than in the field.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

Determination of fluorine distribution in shark teeth by laser-induced breakdown spectroscopy

Quantifying the chemical composition of fast-growing hard tissues in the environment can shed valuable information in terms of understanding ecosystems both prehistoric and current. Changes in chemical composition can be correlated with environmental conditions and can provide information about the organism's life. Sharks can lose 0.1 to 1.1 teeth/day, depending on species, which offers a unique opportunity to record environmental changes over a short duration of time. Shark teeth contain a biomineral phase that is made up of fluorapatite [Ca 5 (PO 4 ) 3 F], and the F distribution within the tooth can be correlated to tooth hardness. Typically, this is determined by bulk acid digestion, energy-dispersive X-ray spectroscopy (EDS), or wavelength-dispersive spectroscopy. Here we present laser-induced breakdown spectroscopy (LIBS) as an alternative and faster approach for determining F distribution within shark teeth. Using a two-volume laser ablation chamber (TwoVol3) with innovative embedded collection optics for LIBS, shark teeth were investigated from sand tiger (Carcharias Taurus), tiger (Galeocerdo Cuvier), and hammerhead sharks (Sphyrnidae). Fluorine distribution was mapped using the CaF 603 nm band (CaF, Β 2 Σ + → X 2 Σ + ) and quantified using apatite reference materials. In addition, F measurements were cross referenced with EDS analyses to validate the findings. Distributions of F (603 nm), Na (589 nm), and H (656 nm) within the tooth correlate well with the expected biomineral composition and expected tooth hardness. This rapid methodology could transform the current means of determining F distribution, particularly when large sample specimens (350 mm 2 , presented here) and large quantities of specimens are of interest.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Desorption and sublimation kinetics for fluorinated aluminum nitride surfaces

The adsorption and desorption of halogen and other gaseous species from surfaces is a key fundamental process for both wet chemical and dry plasma etch and clean processes utilized in nanoelectronic fabrication processes. Therefore, to increase the fundamental understanding of these processes with regard to aluminum nitride (AlN) surfaces, temperature programmed desorption (TPD) and x-ray photoelectron spectroscopy (XPS) have been utilized to investigate the desorption kinetics of water (H{sub 2}O), fluorine (F{sub 2}), hydrogen (H{sub 2}), hydrogen fluoride (HF), and other related species from aluminum nitride thin film surfaces treated with an aqueous solution of buffered hydrogen fluoride (BHF) diluted in methanol (CH{sub 3}OH). Pre-TPD XPS measurements of the CH{sub 3}OH:BHF treated AlN surfaces showed the presence of a variety of Al-F, N-F, Al-O, Al-OH, C-H, and C-O surfaces species in addition to Al-N bonding from the AlN thin film. The primary species observed desorbing from these same surfaces during TPD measurements included H{sub 2}, H{sub 2}O, HF, F{sub 2}, and CH{sub 3}OH with some evidence for nitrogen (N{sub 2}) and ammonia (NH{sub 3}) desorption as well. For H{sub 2}O, two desorption peaks with second order kinetics were observed at 195 and 460 °C with activation energies (E{sub d}) of 51 ± 3 and 87 ± 5 kJ/mol, respectively. Desorption of HF similarly exhibited second order kinetics with a peak temperature of 475 °C and E{sub d} of 110 ± 5 kJ/mol. The TPD spectra for F{sub 2} exhibited two peaks at 485 and 585 °C with second order kinetics and E{sub d} of 62 ± 3 and 270 ± 10 kJ/mol, respectively. These values are in excellent agreement with previous E{sub d} measurements for desorption of H{sub 2}O from SiO{sub 2} and AlF{sub x} from AlN surfaces, respectively. The F{sub 2} desorption is therefore attributed to fragmentation of AlF{sub x} species in the mass spectrometer ionizer. H{sub 2} desorption exhibited an additional high temperature peak at 910 °C with E{sub d} = 370 ± 10 kJ/mol that is consistent with both the dehydrogenation of surface AlOH species and H{sub 2} assisted sublimation of AlN. Similarly, N{sub 2} exhibited a similar higher temperature desorption peak with E{sub d} = 535 ± 40 kJ/mol that is consistent with the activation energy for direct sublimation of AlN.

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

Growth mechanism and optical properties of Ti thin films deposited onto fluorine-doped tin oxide glass substrate

In this work, a detailed study of the influence of the thickness on the morphological and optical properties of titanium (Ti) thin films deposited onto rough fluorine-doped tin oxide glass by d.c. magnetron sputtering is carried out. The films were characterized by several methods for composition, crystallinity, morphology, and optical properties. Regardless of the deposition time, all the studied Ti films of 400, 1500, 2000, and 2500 nm in thickness were single crystalline in the α-Ti phase and also very similar to each other with respect to composition. Using the atomic force microscopy (AFM) technique, the authors analyzed the roughness evolution of the Ti films characteristics as a function of the film thickness. By applying the dynamic scaling theory to the AFM images, a steady growth roughness exponent α = 0.72 ± 0.02 and a dynamic growth roughness exponent β = 0.22 ± 0.02 were determined. The value of α and β are consistent with nonlinear growth model incorporating random deposition with surface diffusion. Finally, measuring the reflection spectra of the samples by a spectrophotometer in the spectral range of 300–1100 nm allowed us to investigate the optical properties. The authors observed the increments of the reflection of Ti films with thickness, which by employing the effective medium approximation theory showed an increase in thickness followed by an increase in the volume fraction of metal.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Pilot-scale electron cyclotron resonance-metal organic chemical vapor deposition system for the preparation of large-area fluorine-doped SnO{sub 2} thin films

The authors report the surface morphology, optical, electrical, thermal and humidity impacts, and electromagnetic interference properties of fluorine-doped tin oxide (SnO{sub 2}:F or “FTO”) thin films on a flexible polyethylene terephthalate (PET) substrate fabricated by a pilot-scale electron cyclotron resonance–metal organic chemical vapor deposition (PS ECR-MOCVD). The characteristics of large area FTO thin films were compared with a commercially available transparent conductive electrode made of tin-doped indium oxide (ITO), prepared with an identical film and PET thickness of 125 nm and 188 μm, respectively. The results revealed that the as-prepared FTO thin films exhibited comparable performances with the incumbent ITO films, including a high optical transmittance of 97% (substrate-subtracted), low electrical resistivity of about 5 × 10{sup −3} Ω cm, improved electrical and optical performances due to the external thermal and humidity impact, and an excellent shielding effectiveness of electromagnetic interference of nearly 2.3 dB. These excellent performances of the FTO thin films were strongly attributed to the design of the PS ECR-MOCVD, which enabled a uniform plasma environment resulting from a proper mixture of electromagnetic profiles and microwave power.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Nanostructured Crystals of Fluorite Phases Sr{sub 1–x}R{sub x}F{sub 2+x} and Their Ordering: 12. Influence of Structural Ordering on the Fluorine-Ion Conductivity of Sr{sub 0.667}R{sub 0.333}F{sub 2.333} Alloys (R = Tb or Tm) at Their Annealing

The ionic conductivity of Sr{sub 0.667}R{sub 0.333}F{sub 2.333} alloys (rational Sr{sub 2}RF{sub 7} compositions) in SrF{sub 2}–RF{sub 3} systems (R = Tb or Tm), prepared by spontaneous crystallization, has been investigated for the “as-grown” state and after annealing in CF{sub 4} at 900 ± 20°C for 96 h. As-grown samples of both compositions, prepared by fast (200°C/min) melt crystallization, exhibit partial (nonequilibrium) ordering, which increases from Tb to Tm. Annealing of Sr{sub 0.667}R{sub 0.333}F{sub 2.333} alloys yields strong ordering (equilibrium for the annealing temperatures) of the fluorite structure (CaF{sub 2} type, sp. gr. Fm3̅m, Z = 4) at the formation of t-Sr{sub 2}RF{sub 7} tetragonal compound (sp. gr. I4/m, Z = 30). It is established that ordering of the alloy fluorite structure reduces the fluorine-ion conductivity. After the annealing, the conductivity of Sr{sub 0.667}R{sub 0.333}F{sub 2.333} alloys with the initial (nonequilibrium) ordering stage of t-Sr{sub 2}RF{sub 7} phases with almost complete (equilibrium) ordering decreases by a factor of 3–4.5.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Optimization of the deposition and annealing conditions of fluorine-doped indium oxide films for silicon solar cells

Fluorine-doped indium oxide (IFO) films are deposited onto (pp{sup +})Si and (n{sup +}nn{sup +})Si structures made of single-crystal silicon by ultrasonic spray pyrolysis. The effect of the IFO deposition time and annealing time in an argon atmosphere with methanol vapor on the IFO chemical composition, the photovoltage and fill factor of the Illumination-U{sub oc} curves of IFO/(pp{sup +})Si structures, and the sheet resistance of IFO/(n{sup +}nn{sup +})Si structures, correlating with the IFO/(n{sup +})Si contact resistance, is studied. The obtained features are explained by modification of the properties of the SiO{sub x} transition layer at the IFO/Si interface during deposition and annealing. Analysis of the results made it possible to optimize the fabrication conditions of solar cells based on IFO/(pp{sup +})Si heterostructures and to increase their efficiency from 17% to a record 17.8%.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Effect of fluorine, nitrogen, and carbon impurities on the electronic and magnetic properties of WO{sub 3}

Within electron density functional theory with the use of the Vienna ab-initio simulation package (VASP), the effect of the sp substitutional impurities of fluorine (n-type dopant), nitrogen, and carbon (p-type dopants) on the electronic and magnetic properties of tungsten trioxide WO{sub 3} is studied. It is established that these impurities induce the transformation of tungsten trioxide (nonmagnetic semiconductor) into nonmagnetic metal (WO{sub 3}:F), magnetic semimetal (WO{sub 3}:N), or magnetic metal (WO{sub 3}:C) states.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Spectroscopy of PbS and PbSe Quantum Dots in Fluorine Phosphate Glasses

Transmission spectra of the narrow-bandgap semiconductor quantum dots PbS and PbSe in the fluorine phosphate glass are experimentally studied at different temperatures. Energies of 1s exciton transitions observed in the absorption spectra are used to determine characteristic sizes of the quantum dots under study. A nontrivial temperature behavior of the ground and excited quantum confined states in the quantum dots of different sizes are observed.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Do Chemical Effects Affect the Accumulation of Structural Damage during the Implantation of Fluorine Ions into GaN?

The accumulation of structural damage in GaN under irradiation with accelerated F and Ne ions with energies of 1.3 and 3.2 keV/amu is investigated. It is shown that chemical effects during implantation of fluorine ions within the doses under consideration do not noticeably affect the formation of stable structural damage both in the bulk and on the surface of GaN.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗