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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 1,711 records · Page 95
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Reducing disorder in graphene nanoribbons by chemical edge modification

We present electronic transport measurements on etched graphene nanoribbons on silicon dioxide before and after a short hydrofluoric acid (HF) treatment. We report on changes in the transport properties, in particular, in terms of a decreasing transport gap and a reduced doping level after HF dipping. Interestingly, the effective energy gap is nearly unaffected by the HF treatment. Additional measurements on a graphene nanoribbon with lateral graphene gates support strong indications that the HF significantly modifies the edges of the investigated nanoribbons leading to a significantly reduced disorder potential in these graphene nanostructures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Modification of porous silicon rugate filters through thiol-yne photochemistry

Porous silicon (PSi) has a considerable potential as biosensor platform. In particular, the ability to modify the surface chemistry of porous silicon is of interest. Here we present a generic method to modify the surface of porous silicon through thiol-yne photochemistry initiated by a radical initiator. Firstly, a freshly etched porous silicon substrate is modified through thermal hydrosilylation with 1,8-nonadiyne to passivate the surface and introduce alkyne functionalities. The alkyne functional surface could then be further reacted with thiol species in the presence of a radical initiator and UV light. Functionalization of the PSi rugate filter is followed with optical reflectivity measurements as well as high resolution X-ray photoelectron spectroscopy (XPS)

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

Band gap structure modification of amorphous anodic Al oxide film by Ti-alloying

The band structure of pure and Ti-alloyed anodic aluminum oxide has been examined as a function of Ti concentration varying from 2 to 20 at. %. The band gap energy of Ti-alloyed anodic Al oxide decreases with increasing Ti concentration. X-ray absorption spectroscopy reveals that Ti atoms are not located in a TiO{sub 2} unit in the oxide layer, but rather in a mixed Ti-Al oxide layer. The optical band gap energy of the anodic oxide layers was determined by vacuum ultraviolet spectroscopy in the energy range from 4.1 to 9.2 eV (300–135 nm). The results indicate that amorphous anodic Al{sub 2}O{sub 3} has a direct band gap of 7.3 eV, which is about ∼1.4 eV lower than its crystalline counterpart (single-crystal Al{sub 2}O{sub 3}). Upon Ti-alloying, extra bands appear within the band gap of amorphous Al{sub 2}O{sub 3}, mainly caused by Ti 3d orbitals localized at the Ti site.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Modification of energy band alignment and electric properties of Pt/Ba{sub 0.6}Sr{sub 0.4}TiO{sub 3}/Pt thin-film ferroelectric varactors by Ag impurities at interfaces

We report on the effects of Ag impurities at interfaces of parallel-plate Pt/Ba{sub 0.6}Sr{sub 0.4}TiO{sub 3}/Pt thin film ferroelectric varactors. Ag impurities occur at the interfaces due to diffusion of Ag from colloidal silver paint used to attach the varactor samples with their back side to the plate heated at 600–750 °C during deposition of Ba{sub 0.6}Sr{sub 0.4}TiO{sub 3}. X-ray photoelectron spectroscopy and secondary ion mass spectrometry suggest that amount and distribution of Ag adsorbed at the interfaces depend strongly on the adsorbent surface layer. In particular, Ag preferentially accumulates on top of the Pt bottom electrode. The presence of Ag significantly reduces the barrier height between Pt and Ba{sub 0.6}Sr{sub 0.4}TiO{sub 3} leading to an increased leakage current density and, thus, to a severe degradation of the varactor performance.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Luminescence of silicon dioxide different polymorph modification: Silica glass, α-quartz, stishovite, coesite

Stishovite, coesite, oxygen deficient silica glass as well as irradiated α-quartz, exhibit two luminescence bands: a blue one and an UV one both excitable in the range within optical gap. There are similarities in spectral position and in luminescence decay kinetics among centers in these materials. The interpretation was done on the model of Oxygen Deficient Centers (ODC) [1]. The ODC(II) or twofold coordinated silicon and ODC(I) are distinguished. ODC(I) is object of controversial interpretation. The Si-Si oxygen vacancy [2] and complex defect including latent twofold coordinated silicon [3] are proposed. Remarkably, this luminescence center does not exist in as grown crystalline α-quartz. However, destructive irradiation of α-quartz crystals with fast neutrons, γ rays, or dense electron beams [4–6] creates ODC(I) like defect. In tetrahedron structured coesite the self trapped exciton (STE) luminescence observed with high energetic yield (∼30%) like in α-quartz crystals. STE in coesite coexists with oxygen deficient-like center. In octahedron structured stishovite STE was not found and only ODC exists.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Modification of near-wall coherent structures by inertial particles

Direct numerical simulations are combined with two-way coupled Lagrangian point particles to study the effect of Reynolds number on particle-turbulence interaction. Turbulent planar Couette flow is simulated at a constant dispersed phase mass loading of ϕ{sub m} = 0.25 for particle Stokes numbers of St{sub K} = [O(1), O(10), O(100)] (based on the Stokes time scale of the particle and the Kolmogorov time scale of the flow) and bulk Reynolds numbers of Re{sub b} = [8100, 24000, 72000] (based on the plate velocity difference and separation distance). Statistics of swirling strength |λ{sub ci}| are used to evaluate the impact of particles on near-wall motions which are responsible for turbulent, wall-normal momentum transport. Instantaneously, the number of high-strength swirling motions near the wall decreases significantly in the presence of particles, and this trend is enhanced with increasing Re{sub b}. Conditional averages are computed using linear stochastic estimation, providing the average structures responsible for ejection events near the wall. These conditional eddies are weakened substantially by the presence of the dispersed phase, and this effect is again enhanced with increasing Re{sub b}. We propose a mechanism where particles, by interfering with the hairpin regeneration process near the wall, can influence turbulent fluxes in a way that increases with Re{sub b} despite only having direct interaction with scales on the same order as their small physical size. At the same time, turbulent momentum flux concentrated at higher wavenumbers with increasing Re{sub b} allows small particles to be effective agents for altering turbulent transport.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Field induced modification of defect complexes in magnesium-doped lithium niobate

Dielectric constant, thermally stimulated depolarization currents (TSDC), and conductivity of undoped and 5% Mg-doped LiNbO{sub 3} single crystals between −100 °C and 200 °C have been investigated. A Debye-like dielectric relaxation with an activation energy of 135 meV is observed in the Mg-doped material, but not in undoped crystals. On heating this relaxation disappears near 140 °C and does not reappear after cooling. Anomalies observed in TSDC around this temperature are attributed to the motion of lithium vacancies, in agreement with conductivity measurements. It is proposed that in thermal equilibrium the electrons from the Mg{sub Li}{sup •} donors are trapped in (4Mg{sub Li}{sup •}+4V{sub Li}{sup ′}) defect complexes. High-temperature poling breaks these defect complexes. The transition of the liberated electrons between the Mg{sub Li}{sup •} donor centers and the Nb{sub Nb} forming the conduction band gives rise to the observed dielectric relaxation.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Modification of the structural and magnetic properties of granular FePt films by seed layer conditioning

The steadily increasing amount of digital information necessitates the availability of reliable high capacity magnetic data storage. Here, future hard disk drives with extended areal storage densities beyond 1.0 Tb/in{sup 2} are envisioned by using high anisotropy granular and chemically L1{sub 0}-ordered FePt (002) perpendicular media within a heat-assisted magnetic recording scheme. Perpendicular texturing of the [001] easy axes of the individual grains can be achieved by using MgO seed layers. It is therefore investigated, if and how an Ar{sup +} ion irradiation of the MgO seed layer prior to the deposition of the magnetic material influences the MgO surface properties and hereby the FePt [001] texture. Structural investigations reveal a flattening of the seed layer surface accompanied by a change in the morphology of the FePt grains. Moreover, the fraction of small second layer particles and the degree of coalescence of the primarily deposited FePt grains strongly increases. As for the magnetic performance, this results in a reduced coercivity along the magnetic easy axis (out of plane) and in enhanced hard axis (in-plane) remanence values. The irradiation induced changes in the magnetic properties of the granular FePt-C films are traced back to the accordingly modified atomic structure of the FePt-MgO interface region.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Ionospheric modifications in high frequency heating experiments

Featured observations in high-frequency (HF) heating experiments conducted at Arecibo, EISCAT, and high frequency active auroral research program are discussed. These phenomena appearing in the F region of the ionosphere include high-frequency heater enhanced plasma lines, airglow enhancement, energetic electron flux, artificial ionization layers, artificial spread-F, ionization enhancement, artificial cusp, wideband absorption, short-scale (meters) density irregularities, and stimulated electromagnetic emissions, which were observed when the O-mode HF heater waves with frequencies below foF2 were applied. The implication and associated physical mechanism of each observation are discussed and explained. It is shown that these phenomena caused by the HF heating are all ascribed directly or indirectly to the excitation of parametric instabilities which instigate anomalous heating. Formulation and analysis of parametric instabilities are presented. The results show that oscillating two stream instability and parametric decay instability can be excited by the O-mode HF heater waves, transmitted from all three heating facilities, in the regions near the HF reflection height and near the upper hybrid resonance layer. The excited Langmuir waves, upper hybrid waves, ion acoustic waves, lower hybrid waves, and field-aligned density irregularities set off subsequent wave-wave and wave-electron interactions, giving rise to the observed phenomena.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Electron density modification in ionospheric E layer by inserting fine dust particles

In this paper, we have developed the kinetics of E-region ionospheric plasma comprising of fine dust grains and shown that the electron density in E-layer can purposely be reduced/enhanced up to desired level by inserting fine dust particles of appropriate physical/material properties; this may certainly be promising for preferred rf-signal processing through these layers. The analytical formulation is based on average charge theory and includes the number and energy balance of the plasma constituents along with charge balance over dust particles. The effect of varying number density, work function, and photo-efficiency of dust particles on ionospheric plasma density at different altitude in E-layer has been critically examined and presented graphically.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Magnetic modification at sub-surface of FeRh bulk by energetic ion beam irradiation

Ferromagnetic layered structure has been made at sub-surface of the antiferromagnetic FeRh bulk samples by high energy He ion beam irradiation. In accordance with the Transport of Ions in Matter simulation, such ion beam can effectively deposit the elastic collision energy in several μm regions in the depth from the surface. Measurement with a superconducting quantum interference device reveals the irradiated samples to be ferromagnetic. Assuming that only the part the energy deposited can be modified to be ferromagnetic, the corresponding irradiation induced magnetization is consistent with the data that we previously reported. On the other hand, the X-ray magnetic circular dichroism (XMCD) spectra for the irradiated samples are totally unchanged as those for the unirradiated samples. Since XMCD signal in total emission yield method is considered to be surface sensitive with a typical probing depth of several nm, the surface magnetic state is maintained to be antiferromagnetic. By utilizing these phenomena, three-dimensional magnetic patterning of FeRh can be realized, which may potentially be used for future magnetic exchange device application such as nano-scale sensors and memories.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Measurement and image processing evaluation of surface modifications of dental implants G4 pure titanium created by different techniques

Foreign substances and organic tissue interaction placed into the jaw in order to eliminate tooth loss involves a highly complex process. Many biological reactions take place as well as the biomechanical forces that influence this formation. Osseointegration denotes to the direct structural and functional association between the living bone and the load-bearing artificial implant's surface. Taking into consideration of the requirements in the manufacturing processes of the implants, surface characterizations with high precise measurement techniques are investigated and thus long-term success of dental implant is emphasized on the importance of these processes in this study. In this research, the detailed surface characterization was performed to identify the dependence of the manufacturing techniques on the surface properties by using the image processing methods and using the scanning electron microscope (SEM) for morphological properties in 3D and Taylor Hobson stylus profilometer for roughness properties in 2D. Three implant surfaces fabricated by different manufacturing techniques were inspected, and a machined surface was included into the study as a reference specimen. The results indicated that different surface treatments were strongly influenced surface morphology. Thus 2D and 3D precise inspection techniques were highlighted on the importance for surface characterization. Different image analyses techniques such as Dark-light technique were used to verify the surface measurement results. The computational phase was performed using image processing toolbox in Matlab with precise evaluation of the roughness for the implant surfaces. The relationship between the number of black and white pixels and surface roughness is presented. FFT image processing and analyses results explicitly imply that the technique is useful in the determination of surface roughness. The results showed that the number of black pixels in the image increases with increase in surface roughness.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Modification of structure and magnetic anisotropy of epitaxial CoFe₂O₄ films by hydrogen reduction

Heteroepitaxial CoFe₂O₄ (CFO) thin films with different thicknesses were deposited on MgO (001) substrates. The as-deposited CFO films show a clear switching of magnetic anisotropy with increasing film thickness. The thinner films (<100 nm) show a perpendicular magnetic anisotropy due to the out-of-plane compressive strain. The thicker films exhibit an in-plane easy axis owing to the dominating shape anisotropy effect. The magnetostriction coefficient of CFO films is estimated to be λ[001] =-188 × 10⁻⁶. Metallic CoFe₂ films were obtained by annealing the as-deposited CFO films in forming gas (Ar 93% + H₂ 7%) at 450 °C. XRD shows that CoFe₂ films are textured out-of-plane and aligned in-plane, owing to lattice matching between CoFe₂ and MgO substrate. TEM results indicate that as-deposited films are continuous while the annealed films exhibit a nanopore mushroom structure. The magnetic anisotropy of CoFe₂ films is dominated by the shape effect. The results demonstrate that hydrogen reduction can be effectively used to modify microstructures and physical properties of complex metal oxide materials.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Fluorine-ion-beam modification of magnetic properties of thin GaMnAs films

Magnetic and electrical transport properties of fluorine-ion-beam irradiated GaMnAs films were studied as a function of ion fluence and energy of impinging ions. The different nature of defects created by ions of low- and high-energies is explored in this work by means of transport and magnetization measurements. Our results show that the saturation magnetization of the irradiated samples is suppressed as the ion fluence is increased. Interestingly, however, the same effect is not observed in the case of critical temperature, which remains nearly the same for irradiated and non-irradiated samples measured by superconducting quantum interference device. Magnetotransport measurements appear to provide more reliable results regarding the critical temperature, since they are consistent with the ion-irradiation-induced disorder in the GaMnAs film, quantified here as the increase of the resistivity. We discuss this behavior based on the inhomogeneity of damages caused by the irradiation process.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Modification of opto-electronic properties of ZnO by incorporating metallic tin for buffer layer in thin film solar cells

In this report, the effect of incorporation of metallic tin (Sn) on opto-electronic properties of ZnO thin films is presented. ZnO thin films were deposited through ‘automated chemical spray pyrolysis’ (CSP) technique; later different quantities of ‘Sn’ were evaporated on it and subsequently annealed. Vacuum annealing showed a positive effect on crystallinity of films. Creation of sub band gap levels due to ‘Sn’ diffusion was evident from the absorption and PL spectra. The tin incorporated films showed good photo response in visible region. Tin incorporated ZnO thin films seem to satisfy the desirable criteria for buffer layer in thin film solar cells.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Structural modification of nanocrystalline diamond films via positive/negative bias enhanced nucleation and growth processes for improving their electron field emission properties

Electron field emission (EFE) properties of nanocrystalline diamond (NCD) films synthesized by the bias-enhanced growth (beg) process under different bias voltages were investigated. The induction of the nanographitic phases is presumed to be the prime factor in enhancing the EFE properties of negative biased NCD films. Transmission electron microscopic investigations reveal that a negative bias voltage of −300 V increases the rate of growth for NCD films with the size of the grains changing from nano to ultranano size. This effect also is accompanied by the induction of nanographitic filaments in the grain boundaries of the films. The turn-on field (E{sub 0}) for the EFE process then effectively gets reduced. The EFE process of the beg-NCD{sub −300V} films can be turned on at E{sub 0} = 3.86 V/μm, and the EFE current density achieved is 1.49 mA/cm{sup 2} at an applied field of 7.85 V/μm. On the other hand, though a positive-bias beg process (+200 V) results in the reduction of grain size, it does not induce sufficient nanographitic phases to lower the E{sub 0} value of the EFE process. Moreover, the optical emission spectroscopic investigation indicates that one of the primary causes that changes the granular structure of the NCD films is the increase in the proportion of C{sub 2} and CH species induced in the growing plasma. The polarity of the bias voltage is of less importance in the microstructural evolution of the films.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Enhancement of the anti-damping spin torque efficacy of platinum by interface modification

We report a strong enhancement of the efficacy of the spin Hall effect (SHE) of Pt for exerting anti-damping spin torque on an adjacent ferromagnetic layer by the insertion of ≈0.5 nm layer of Hf between a Pt film and a thin, ≤2 nm, Fe{sub 60}Co{sub 20}B{sub 20} ferromagnetic layer. This enhancement is quantified by measurement of the switching current density when the ferromagnetic layer is the free electrode in a magnetic tunnel junction. The results are explained as the suppression of spin pumping through a substantial decrease in the effective spin-mixing conductance of the interface, but without a concomitant reduction of the ferromagnet's absorption of the SHE generated spin current.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Ultra-violet absorption induced modifications in bulk and nanoscale electrical transport properties of Al-doped ZnO thin films

Using conductive atomic force microscopy and Kelvin probe force microscopy, we study local electrical transport properties in aluminum-doped zinc oxide (ZnO:Al or AZO) thin films. Current mapping shows a spatial variation in conductivity which corroborates well with the local mapping of donor concentration (∼10{sup 20 }cm{sup −3}). In addition, a strong enhancement in the local current at grains is observed after exposing the film to ultra-violet (UV) light which is attributed to persistent photocurrent. Further, it is shown that UV absorption gives a smooth conduction in AZO film which in turn gives rise to an improvement in the bulk photoresponsivity of an n-AZO/p-Si heterojunction diode. This finding is in contrast to the belief that UV absorption in an AZO layer leads to an optical loss for the underneath absorbing layer of a heterojunction solar cell.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗