bluuebunny/crossref_metadata_2025 huggingface dataset:
Chemisorption on nickel nanoparticles of various shapes: Influence on magnetism
Authors: Cordente N. , Amiens C. , Chaudret B. , Respaud M. , Senocq F. , Casanove M.-J.
Published: November 2003

Nanoparticles of nickel have been obtained in solution through reaction of an organometallic precursor, Ni(COD)2 (COD=cycloocta-1,5-diene), with dihydrogen, in the presence of various stabilizing agents. X-ray diffraction studies evidence a face-centered-cubic structure for all samples. Spherical isolated superparamagnetic nanoparticles (d∼4.5 nm) are produced in the presence of poly(N-vinylpyrrolid-2-one). They display a magnetization value comparable to that of bulk nickel, as determined from superconducting quantum iinterference device (SQUID) measurements. Exposure of the surface of the nanoparticles to CO, leading to CO coordination as monitored by infrared spectroscopy, or to methanol, strongly reduces their magnetization. This reduction corresponds respectively to one or two magnetically inactive layers of nickel atoms at the nanoparticles surface. The production of elongated nanoparticles was favored when either trioctylphosphineoxide or hexadecylamine (HDA) were used as stabilizing agents. In this case, SQUID measurements show that only HDA preserves the magnetization of the surface. This stabilizing agent was selected for the production of nickel nanorods. The influence of ligands on the effective anisotropy is discussed.
Hydrogen storage for carbon nanotubes synthesized by the pyrolysis method using lanthanum nickel alloy as catalyst
Authors: Zhang Haiyan , Chen Yimin , Li Shunhua , Fu Xiaojuan , Zhu Yanjuan , Yi Shuangping , Xue Xinmin , He Yanyang , Chen Yulian
Published: November 2003

We have investigated the adsorption of hydrogen of multiwall carbon nanotubes synthesized by the pyrolysis method using a lanthanum nickel hydrogen storage alloy as a catalyst. The mechanism of carbon nanotubes synthesized using a La–Ni alloy catalyst is discussed. In the hydrogen atmosphere and high-temperature process of carbon nanotube preparation, the LaNi5 alloy particle may be changed into an intermetallic compound of Ni and La by the analysis of the x-ray diffraction patterns and energy-dispersive x-ray spectroscopy of the nanotubes samples using a La–Ni alloy as a catalyst. The H2 uptake capacity of the carbon nanotubes using a La–Ni alloy catalyst is about 5 wt % through to the pressure of 10 MPa. Using a La–Ni alloy as a catalyst increases the effect of chemistry adsorption in the H2 adsorption of the La–Ni alloy catalyzed carbon nanotubes. The P–C–T curve has an approximate plateau. The La–Ni alloy catalyzed carbon nanotubes have better activation for H2 uptake and larger hydrogen uptake in comparison to the carbon nanotubes without using a La–Ni alloy catalyst, and the hydrogen uptake increases with increasing hydrogen pressure from 2 MPa to 8 MPa at room temperature. The hydrogen uptake versus time showing a hydrogen uptake rate of carbon nanotubes using the La–Ni alloy catalyst was raised by the heat treatment from 1.0 wt % (heat treatment temperature 100 °C) to 2.3 wt % (heat treatment temperature 700 °C) at the H2 pressure of 5 MPa.
Effect of magnetic field on the magnetic properties of electroplated NiFe/Cu composite wires
Authors: Li X. P. , Zhao Z. J. , Seet H. L. , Heng W. M. , Oh T. B. , Lee J. Y.
Published: November 2003

The effect of the magnetic field on the magnetic properties of NiFe/Cu composite wires electroplated under a longitudinal magnetic controlling field is presented. Composite wire samples of 20-μm-diameter Cu electroplated with a layer of Permalloy™ (Ni80Fe20) under the influence of a longitudinal magnetic field of intensities ranging from 0 to 400 Oe were produced, and the microstructure and magnetic properties were measured. The results showed that the longitudinal magnetic field in the composite wire plating makes the packing of the crystals in the plated layer more orderly, and thus increases the uniformity and magnetic softness of the plated material. It also shifts the magnetic anisotropy of the plated layer from circumferential to longitudinal, and increases the critical frequency of the plated composite wire in magnetoimpedance effect testing, at which the magnetoimpedance ratio reaches the maximum.
Capillary adhesive contact between a spherical rigid punch and a piezoelectric half space
Authors: Chen Z. R. , Yu S. W.
Published: November 2003

Adhesive contact between an axisymmetric punch and a piezoelectric half space in the presence of a liquid bridge formed by capillary condensation is studied. The coupling between liquid-bridge force and piezoelectric elastic deformation is taken into account. The solution is obtained by using the integral transform method with the aid of the dual integral equation technique. Combined with the thermodynamics equilibrium of capillary condensation, two competitive factors closely related to the relative humidity are shown to affect the capillary adhesive force. Two dimensionless parameters, which represent the material properties, and geometric relative relationships between liquid-bridge and contact systems, respectively, are defined to investigate the role of the capillary bridge on adhesive contact. Due to the electromechanical coupling, the applied electric load is shown to evidently affect adhesive contact. This may provide a possible means by which to weaken the influence of the capillary bridge in atomic force microscopy piezoelectric measurements.
Effect of coupling strength on magnetic properties of exchange spring magnets
Authors: Chakka Vamsi M. , Shan Z. S. , Liu J. P.
Published: November 2003

The main feature of an exchange spring magnet can be characterized by ferromagnetic exchange coupling between a magnetically soft phase and a magnetically hard phase. The coupling constant J, a measure of the coupling strength between the two phases, is a key parameter in controlling the spring magnet properties. A ferromagnetically coupled hard–soft bilayer (FCB) has been used in this article to develop analytical expressions for a spring magnet that correlate the magnetic properties of a FCB and its layer parameters. These analytical expressions have been developed by solving a modified Stoner–Wohlfarth model. A set of analytical solutions describing the magnetic properties of the FCB at the different stages of J have been derived. The evolution of the magnetization reversal in a FCB, as a function of the coupling constant J, and an applied field H, is analyzed in detail. As a result, the approach to enhance the maximum energy product (BH)max is revealed.
Modeling of fluorine-based high-density plasma etching of anisotropic silicon trenches with oxygen sidewall passivation
Authors: Blauw M. A. , van der Drift E. , Marcos G. , Rhallabi A.
Published: November 2003

The kinetics of high aspect ratio, anisotropic silicon etching in a SF6–O2 plasma is investigated with a combination of Monte Carlo simulations and inductively coupled plasma etching experiments. The spontaneous reaction of atomic fluorine is dominant at room temperature and Knudsen transport of the radicals is the only limitation in narrow structures. At low temperatures (typically between −125 and −95 °C) oxygen passivation becomes effective and anisotropic profiles are obtained because the oxygen passivation can only be removed by the directional ion bombardment. The input parameter settings for the Monte Carlo model are based on measurements with plasma diagnostics. Simulations show that anisotropy is controlled by the oxygen sidewall passivation which depends on the oxygen flux, the oxygen adsorption coefficient, and the aspect ratio. The simulated trench profiles and the aspect ratio dependent etch rate are consistent with the experimental results. Experimentally the etch rate behavior can be tuned from aspect ratio dependent to aspect ratio independent by decreasing the ion flux. This effect can be described well by the recently developed chemically enhanced ion-neutral synergy model. It turns out that aspect ratio independent etching is obtained if the downwards depletion of fluorine radicals due to Knudsen transport is compensated by an increase of the available reaction sites.
Manipulation of the structural and optical properties of InAs quantum dots by using various InGaAs structures
Authors: Kim Jin Soo , Lee Jin Hong , Hong Sung Ui , Han Won Seok , Kwack Ho-Sang , Lee Chul Wook , Oh Dae Kon
Published: November 2003

The structural and optical properties of self-assembled InAs quantum dots (QDs) with various InGaAs structures were investigated by transmission electron microscopy (TEM) and photoluminescence (PL). The emission peak position of InAs QDs covered by a 6 nm In0.15Ga0.85As layer was 1.26 μm with PL linewidth of 31 meV, which is narrower than that of QDs in a GaAs matrix. By inserting a 1 nm In0.15Ga0.85As layer below the InAs QD layer with a 6 nm In0.15Ga0.85As overgrowth layer, the emission peak position was redshifted with larger energy-level spacing between the ground states and the first excited states compared to that of QDs with an In0.15Ga0.85As overgrowth layer only. By covering the InAs QDs on a 1 nm In0.15Ga0.85As layer with an 8 nm InxGa1−xAs layer having graded In composition, the emission peak position was 1.32 μm with relatively larger energy-level spacing and narrower PL linewidth compared to QDs covered by an In0.15Ga0.85As layer. The longer emission wavelength with relatively larger energy-level spacing was largely related to the change in the QD shape and size, especially the aspect ratio (height/width), which was confirmed by cross-sectional TEM images.
Physical characteristics and electrical properties of Sr0.8Bi2+xTa2O9 films on Al2O3/Si annealed at high temperature
Authors: Lan Ban-Chiang , Huang Chih-Yuan , Chen San-Yuan
Published: November 2003

Bismuth-containing layered perovskite Sr0.8Bi2+xTa2O9 (SBT) thin films with x=0–0.8 were prepared to investigate annealing temperature effect, especially at high temperature, on physical characteristics and electrical properties of SBT films on Al2O3 (10 nm)/Si. At 800 °C, the Sr0.8Bi2Ta2+xO9/Al2O3/Si exhibits ferroelectric mode and the width of memory window decreases with the increase of Bi content that is dependent on the effective coercive field. However, at a higher annealing temperature of 900 °C, a larger ferroelectric memory window was obtained for Bi-rich Sr0.8Bi2+xTa2O9 (x=0.4 or 0.8) films compared to SBT film (x=0) that should be related to the reduced leakage current due to the formation of rod-shape grains and amorphous SBT composite layer. The leakage current of Sr0.8Bi2.4Ta2O9/Al2O3/Si annealed at 900 °C is about 1.2×10−9 A/cm2 that has two orders of magnitude lower than that of 800 °C-annealed SBT films measured at −100 kV/cm. However, the Sr0.8Bi2Ta2O9 (x=0) film on Al2O3/Si capacitor shows no obvious change with the increase of annealing temperature.
Stress induced charge trapping effects in SiO2/Al2O3 gate stacks with TiN electrodes
Authors: Kerber A. , Cartier E. , Groeseneken G. , Maes H. E. , Schwalke U.
Published: November 2003

Strong polarity dependent charge trapping effects have been observed in as-deposited SiO2/Al2O3 gate stacks with TiN gate electrodes on n- and p-type Si substrates using current–voltage (I–V) and capacitance–voltage (C–V) sensing techniques. For substrate injection, electron trapping occurs mainly in the bulk of the Al2O3, resulting in positive voltage shifts for both I–V and C–V measurements. In the case of gate injection, positive charge trapping near the SiO2/Al2O3 interface leads to negative voltage shifts for C–V and positive shifts for I–V measurements. The polarity dependent charging effects are explained in terms of the difference in barrier height for substrate and gate injection and of the inherent asymmetry of the dual layer gate dielectric.
Application of Bryan’s algorithm to the mobility spectrum analysis of semiconductor devices
Authors: Chrastina D. , Hague J. P. , Leadley D. R.
Published: November 2003

A powerful method for mobility spectrum analysis is presented, based on Bryan’s maximum entropy algorithm. The Bayesian analysis central to Bryan’s algorithm ensures that we avoid overfitting of data, resulting in a physically reasonable solution. The algorithm is fast, and allows the analysis of large quantities of data, removing the bias of data selection inherent in all previous techniques. Existing mobility spectrum analysis systems are reviewed, and the performance of the Bryan’s algorithm mobility spectrum (BAMS) approach is demonstrated using synthetic data sets. Analysis of experimental data is briefly discussed. We find that BAMS performs well compared to existing mobility spectrum methods.
Charging effects on the carrier mobility in silicon-on-insulator wafers covered with a high-k layer
Authors: Halley D. , Norga G. , Guiller A. , Fompeyrine J. , Locquet J. P. , Drechsler U. , Siegwart H. , Rossel C.
Published: November 2003

The carrier mobility μ in low-doped silicon-on-insulator wafers is found to be strongly modified by the deposition of a thin ZrO2 or SrZrO3 top layer grown by molecular-beam epitaxy. Pseudo-metal–oxide–semiconductor field-effect-transistor measurements performed on several samples clearly show a correlation between μ and the density of interface traps (Dit) at the Si/buried-oxide interface. The reduction of Dit by a forming gas anneal leads to a corresponding increase in mobility. Moreover, the high-k/Si interface can contribute to the total drain current via the creation of an inversion channel induced by trapped charges in the high-k layer. Using Hall-effect measurements, we took advantage of this additional current to evaluate the carrier mobility at the high-k/Si interface, without the need of a top gate electrode.
Photoelectron spectroscopic studies of bismuth-excess strontium bismuth tantalate thin films and their high-pressure-O2-annealing effects
Authors: Takahashi M. , Noda M. , Okuyama M.
Published: November 2003

The electronic structure of strontium bismuth tantalate (SBT) thin films has been studied by x-ray photoelectron spectroscopy and ultraviolet-ray photoyield spectroscopy (UV-PYS). The SBT films were prepared by metalorganic decomposition (MOD) method using a chemical solution of Sr:Bi:Ta=0.8:2.3:2. The XPS spectra imply that pyrochlore-dominant MOD-SBT films include bismuth oxides other than oxidized bismuth [(Bi2O2)2+] layers after high-pressure-O2 annealing. The UV-PYS spectra exhibited that high-pressure-O2-annealed SBT thin films with dominant pyrochlore structures have electron excitation energy of about 6.0 eV, which is excitation from the Fermi level to the vacuum level, while the as-crystallized SBT thin films with dominant perovskite structures exhibit about 5.5 eV. These results indicate that the high-pressure-O2 annealing oxidizes Bi atoms more in the pyrochlore-dominant SBT thin films than in the perovskite-dominant ones. The high-pressure-O2 annealing tends to suppress current through the pyrochlore-dominant MOD-SBT thin films, which is considered to be partly because bismuth oxides other than (Bi2O2)2+ layers have been produced at the surface by the high-pressure-O2 annealing.
Three modes in a radio frequency atmospheric pressure glow discharge
Authors: Shi J. J. , Deng X. T. , Hall R. , Punnett J. D. , Kong M. G.
Published: November 2003

Fundamentally not requiring a vacuum chamber, atmospheric pressure glow discharges (APGDs) offer an exciting prospect for a wide range of material processing applications. To characterize their operation and establish their operation range, a radio frequency (rf) APGD is studied experimentally with measurement of discharge voltage, current, dissipated plasma power and plasma impedance. Different from the current understanding that rf APGD are operative only in the abnormal glow mode, we show the presence of two additional modes namely the normal glow mode and the recovery mode. It is shown that all three modes are spatially uniform and possess key characteristics of a glow discharge. So rf APGD have a much wider operation range than previously believed. To provide further insights, we investigate the transition from the abnormal glow mode to the recovery mode. It is established that the cause responsible for the mode transition is sheath breakdown, a phenomenon that is known in low- and moderate-pressure glow discharges but has not been reported before for atmospheric-pressure glow discharges. Finally we demonstrate that plasma dynamics, hence plasma stability, in these three modes are influenced crucially by the impedance matching between the plasma rig and the power source.
Influence of electron beam injection on plasma parameters and sheath in a dc discharge plasma
Authors: Pal A. R. , Boruah D. , Adhikary N. C. , Bailung H. , Chutia Joyanti
Published: November 2003

Influence of an electron beam on plasma characteristics and boundary sheath has been investigated experimentally in a dc discharge plasma. This article reports the behavior of the ion sheath formed in front of a negatively biased plate immersed in the plasma, produced in a double plasma device. When a low energy electron beam (Eb=10–50 eV) is injected into the target section, the density as well as the positive ion flux into the sheath increase. As a result, sheath contraction takes place. It is also observed that the floating potential of the plate can be controlled with the help of an electron beam.
Study of a growth instability of γ-In2Se3
Authors: Amory C. , Bernède J. C. , Marsillac S.
Published: November 2003

γ- In 2 Se 3 thin film are deposited for various substrate temperatures in the range of 523–673 K. This study shows that at 573 and 673 K the thin films are well crystallized with grains aligned along the c axis. Between these temperatures, a domain of instability appears where the γ-In2Se3 thin films have a randomly orientation and the c-lattice parameter increases. The presence of the metastable phase κ-In2Se3, during the growth, can explain the existence of this domain of instability. The insertion of Zn during the preparation process allows us to stabilize the phase κ at room temperature which confirms our hypothesis that the presence of κ-In2Se3 perturbs the crystallization of γ-In2Se3. The increase of the c-lattice parameter can be explain by the fact that κ-In2Se3 has a larger unit cell than γ-In2Se3 or by a competition during the growth between the grains of both phases which could generate constraints along the c axis. During the cooling, the κ phase disappears in favor of the γ phase.
Probing temperature during laser spot welding from vapor composition and modeling
Authors: He X. , DebRoy T. , Fuerschbach P. W.
Published: November 2003

Measurement of weld pool temperature during laser spot welding is a difficult task because of the short pulse duration, often lasting only a few milliseconds, highly transient nature of the process, and the presence of a metal vapor plume near the weld pool. This article describes recent research to estimate weld pool temperatures experimentally and theoretically. Composition of the metal vapor from the weld pool was determined by condensing a portion of the vapor on the inner surface of an open ended quartz tube which was mounted perpendicular to the sample surface and coaxial with the laser beam. It was found that iron, chromium, and manganese were the main metallic species in the vapor phase. The concentrations of Fe and Cr in the vapor increased slightly while the concentration of Mn in the vapor decreased somewhat with the increase in power density. The vapor composition was used to determine an effective temperature of the weld pool. A transient, three-dimensional numerical heat transfer and fluid flow model based on the solution of the equations of conservation of mass, momentum and energy was used to calculate the temperature and velocity fields in the weld pool as a function of time. The experimentally determined geometry of the spot welds agreed well with that determined from the computed temperature field. The effective temperature determined from the vapor composition was found to be close to the numerically computed peak temperature at the weld pool surface. Because of the short process duration and other serious problems in the direct measurement of temperature during laser spot welding, estimating approximate values of peak temperature from metal vapor composition is particularly valuable.
Thermal deformation noise in large optical systems
Authors: Tolomeo Jason , Klavins Andy , Tenerelli Domenick , Dickens John
Published: November 2003

Future large space-based telescope systems require precise optical surface quality and wave-front stability. One source of noise for very large precise optical systems is ambient thermal energy which induces statistical fluctuations in the strain energy state of the structure. We broadly model such optical systems as bending energy dominated or membrane in-plane energy dominated and derive analytical expressions for the governing parameters that determine noise magnitude. It is shown that for bending-based systems thermal noise increases as aperture is increased and as bending stiffness is decreased, while for membrane mirror systems it is the in-plane pretension level that determines the noise magnitude. The analysis is extended to numerical finite element techniques to illustrate the effects on very general large damped structures where we address the form of equivalent thermal loading density required in modeling such distributed structures. Calculations show that temporal rms deformation noise on the order of a picometer or less can be expected for apertures up to about 10 m and therefore is probably not significant. For lightweight precision aperture systems greater than 10 m, thermal noise may need to be considered in the design.
Diffusion of phosphorus in relaxed Si1−xGex films and strained Si/Si1−xGex heterostructures
Authors: Christensen J. S. , Radamson H. H. , Kuznetsov A. Yu. , Svensson B. G.
Published: November 2003

Phosphorus diffusion has been studied in relaxed Si1−xGex samples (x=0.11 and 0.19) and strained Si/Si1−xGex/Si heterostructures (x=0.08, 0.13, and 0.18). The diffusivity of P is found to increase with increasing Ge content, while the influence of compressive strain results in a decrease in diffusivity as compared to that in relaxed material. The effect of strain is found to be equivalent to an apparent activation energy of −13 eV per unit strain, where the negative sign indicates that the P diffusion is mediated by interstitials in Si1−xGex (x<0.20). This conclusion is also supported by an experiment utilizing injection of Si self-interstitials, which results in an enhanced P diffusion in strained Si1−xGex. Further, P is found to segregate into Si across Si/Si1−xGex interfaces and the segregation coefficient increases with increasing Ge concentration.
Microstructures and thermoelectric properties of Fe0.92Mn0.08Six alloys prepared by rapid solidification and hot pressing
Authors: Chen H. Y. , Zhao X. B. , Lu Y. F. , Mueller E. , Mrotzek A.
Published: November 2003

Iron disilicide based thermoelectric materials Fe0.92Mn0.08Six (1.9⩽x⩽2.5) were prepared by rapid solidification (melt spinning) and hot uniaxial pressing at 1248 K with 50 MPa for 30 min, followed by annealing at 1073 K for 20 h. X-ray diffraction and scanning electron microscopy showed excess silicon phase for samples with x⩾2.1, and both the configurations and the amounts of secondary silicon particles varied with an increase in x. Hall measurements carried out at room temperature showed that the carrier concentrations for Fe0.92Mn0.08Six (1.9⩽x⩽2.5) were between 2.6×1018 and 5.6×1018 cm−3. The Seebeck coefficient, electrical conductivity and thermal conductivity were measured from room temperature to 973 K. It was found that a little excess silicon in the sample, x=2.1, enhanced the Seebeck coefficient weakly, but was effective for decreasing the thermal conductivity. A maximum figure of merit, ZT=0.17, was obtained for Fe0.92Mn0.08Si2.0 at 973 K.
Reduced temperature dependence of refractive-index in TlInGaAs quaternary alloys grown on InP substrates
Authors: Imada A. , Lee H.-J. , Fujiwara A. , Emura S. , Hasegawa S. , Asahi H.
Published: November 2003

Refractive-index dispersion for the TlInGaAs quaternary alloys, grown on (100) InP substrates by gas-source molecular-beam epitaxy, has been measured at temperatures from 290 to 350 K in the photon-energy range of 1.2–2.0 eV by using spectroscopic ellipsometry. The temperature coefficient of the refractive-index decreases with increasing Tl composition. The result corresponds to the already reported reduced temperature dependence of the band-gap energy for the TlInGaAs because of the alloy of semiconductor InGaAs and semimetal TlAs. The result also agrees with the recently reported small temperature-variation of the lasing-wavelength for the TlInGaAs/InP laser diodes.
Structural characterization of ultrathin cuprate artificial superconducting structures by x-ray synchrotron radiation
Authors: Aruta C. , Angeloni M. , Balestrino G. , Medaglia P. G. , Orgiani P. , Tebano A.
Published: November 2003

Grazing incidence x-ray diffraction and reflectivity measurements were carried out on ultrathin (a few unit cells thick) superconducting heterostructures, based on Ba0.9Nd0.1CuO2+x and CaCuO2 individual blocks, using undulator radiation from a third-generation synchrotron. We investigated films with different thicknesses of the intermediate CaCuO2 block, grown on (001)SrTiO3 substrates by the pulsed-laser deposition technique with no in situ diagnostic. In this communication we demonstrate that the thickness of such heterostructures can be controlled at the single unit cell level. This analysis shows that the interfaces between the different constituent blocks are very sharp with roughness less than one unit cell.
Switching behavior of plasma polymer films containing silver nanoparticles
Authors: Kiesow A. , Morris J. E. , Radehaus C. , Heilmann A.
Published: November 2003

A reversible electronic switching effect has been observed in plasma polymer films with embedded silver nanoparticles. The electrical and nanostructural properties of the films have been investigated, and three different structure types were observed: metallic, percolation, and dielectric. While for the metallic and dielectric types, respectively, metallic conduction and thermally activated tunneling can be identified as the dominant electronic conduction mechanisms, switching appears only in percolation structures. These drastic, abrupt changes of up to six orders of magnitude in the current–voltage behavior are highly reversible for these nanocomposite materials, and are defined as threshold switching.
Growth of high quality crack-free AlGaN films on GaN templates using plastic relaxation through buried cracks
Authors: Bethoux J.-M. , Vennéguès P. , Natali F. , Feltin E. , Tottereau O. , Nataf G. , De Mierry P. , Semond F.
Published: November 2003

A method is presented to achieve thick high quality crack-free AlGaN layers on GaN. This method uses jointly plastic relaxation and lateral growth. In a first step, plastic relaxation by cracking and misfit dislocation introduction is realized. Then the cracks are overgrown to obtain a smooth surface. By this reproducible technique, we grew smooth metal-organic chemical vapor deposition Al0.2Ga0.8N films with a threading dislocation density as low as 5×108 cm−2. This result is the best ever reported for crack-free AlGaN growth over a large area. The control of the complete plastic relaxation opens up perspectives for the realization of high performance devices. In order to explain the mechanisms involved in the full relaxation of the AlGaN/GaN heterostructure, we propose a relaxation scheme and discuss its different steps.
Vibrational properties of the rare earth aluminum garnets
Authors: Papagelis K. , Ves S.
Published: November 2003

In this work we present single crystal infrared (IR) reflectance spectra for the Dy3Al5O12 and Er3Al5O12 compounds, their corresponding optical constants, as well as a discussion concerning the vibrational, dielectric, and elastic properties of various members of the rare earth aluminum garnet family. In the framework of the rigid ion model, the frequencies of the experimentally unresolved infrared active modes, the IR eigenvectors and the partial phonon density of states have been calculated. The role of the different atomic species in the various regions of the IR vibrational spectrum has been elucidated. In particular, it is found that the rare earth ionic mass plays a crucial role in the low frequency IR modes (<200 cm−1), while the high frequency modes are related mainly to the molecular modes of the AlO4 tetrahedral subunit. From the volume dependence of the IR active mode frequencies, their mode Grüneisen parameters are estimated and discussed. Finally, by using a modification of the semiempirical Brout’s formula, the bulk moduli have been determined and are compared to those derived from interatomic potential calculations.
Young’s modulus mapping on SCS-6 SiCf/Ti-6Al-4V composite by electromagnetic-resonance-ultrasound microscopy
Authors: Tian Jiayong , Ogi Hirotsugu , Tada Toyokazu , Hirao Masahiko
Published: November 2003

Electromagnetic-resonance-ultrasound microscopy was applied to visualize Young’s-modulus distribution in SCS-6 SiC fiber/Ti-6Al-4V composite. Resonance of a piezoelectric langasite (La3Ga5SiO14) crystal is excited and detected by a surrounding solenoid coil without any contact between them. Point contact with the specimen causes a shift in resonance frequency of the order of 10−4, depending on the local elastic stiffness of the contacting material. Two-dimensional stepping on the specimen surface provides the image of Young’s-modulus distribution. Calibration measurements based on contact theory allow determination of Young’s moduli of the carbon core (150 GPa), SiC sheaths (180–505 GPa), and Ti-6Al-4V matrix (90 GPa). The radial variation of Young’s modulus depicts the concentric multi-phase layers in an SCS-6 fiber.
Electromigration induced resistance decrease in Sn conductors
Authors: Lloyd J. R.
Published: November 2003

The resistance of thick film (2 μm) Sn conductors deposited onto Ni/Cr underlays was found to decrease several percent during electromigration stressing. The application of dc was necessary to initiate the resistance decrease. Equivalent ac current (rms values equal) did not produce the effect and extensive aging at elevated temperature did not either, indicating that the effect is not purely thermally induced. It is suggested that the reduction in the electromigration induced chemical potential from reductions in resistivity associated with reorientation of the microstructure is responsible for the effect.
Observation of twist nematic liquid-crystal lines
Authors: Škarabot M. , Kralj S. , Rastegar A. , Rasing Th.
Published: November 2003

We have observed that a single groove made by an atomic force microscope (AFM) tip in a polyimide layer strongly aligns nematic liquid crystals locally and have used this phenomenon for studying twist nematic lines. We have measured the intensity profile of light transmitted across a single line and the azimuthal surface energy as a function of the spatial separation of grooves. From these measurements, we have determined the azimuthal surface anchoring energy of the AFM structured and of the untreated polyimide. We find that the twist coherence length, which determines the width of TN lines, is approximately proportional to the cell thickness, while the surface anchoring energy can change it for a factor of 2 at maximum.
Electromagnetic wave absorption properties of α-Fe/Fe3B/Y2O3 nanocomposites in gigahertz range
Authors: Liu Jiu Rong , Itoh Masahiro , Machida Ken-ichi
Published: November 2003

Nanocomposites α-Fe/Fe3B/Y2O3 were prepared by a melt-spun technique, and the electromagnetic wave absorption properties were measured in the 0.05–20.05 GHz range. Compared with α-Fe/Y2O3 composites, the resonance frequency (fr) of α-Fe/Fe3B/Y2O3 shifted to a higher frequency range due to the large anisotropy field (HA) of tetragonal Fe3B (∼0.4 MA/m). The relative permittivity (εr=εr′−jεr″) was constantly low over the 0.5–10 GHz region, which indicates that the composite powders have a high resistivity (ρ=∼100 Ω m). The effective electromagnetic wave absorption (reflection loss <−20 dB) was obtained in a frequency range of 2.7–6.5 GHz on resin composites of 80 wt % α-Fe/Fe3B/Y2O3 powders, with thickness of 6–3 mm, respectively. A minimum reflection loss of −33 dB was observed at 4.5 GHz with an absorber thickness of 4 mm.
Room-temperature soldering with nanostructured foils
Authors: Wang J. , Besnoin E. , Duckham A. , Spey S. J. , Reiss M. E. , Knio O. M. , Powers M. , Whitener M. , Weihs T. P.
Published: November 2003

Self-propagating formation reactions in nanostructured multilayer foils provide rapid bursts of heat and can act as local heat sources to melt solder layers and join materials. This letter describes the room-temperature soldering of stainless steel specimens using freestanding, nanostructured Al/Ni foils. The products, heats, and velocities of the reactions are described, and the microstructure and the mechanical properties of the resulting joints are characterized. A tensile shear strength of 48 MPa was measured for the reactive foil joints, compared to 38 MPa for conventional joints. Both numerical predictions and infrared measurements show limited heat exposure to the components during reactive joining.
Characterization of isomers in aluminum tris(quinoline-8-olate) by one-dimensional 27Al nuclear magnetic resonance under magic-angle spinning
Authors: Utz Marcel , Nandagopal Magesh , Mathai Mathew , Papadimitrakopoulos Fotios
Published: November 2003

Solid-state 27Al NMR spectra under magic-angle spinning of different forms of aluminum tris(quinoline-8-olate) (Alq3) are presented. Alq3 is an organometallic complex of great importance in the context of organic light-emitting diodes. Our results demonstrate a strong difference in the asymmetry of the electric field gradient (EFG) tensor at the aluminum site between the α and the recently discovered δ polymorph of Alq3. While the EFG is nearly planar (η≈1) in the α phase, it is nearly axially symmetric (η≈0) for the δ phase. This result provides strong support to the hypothesis that the δ phase contains the facial isomer of Alq3. While the spectra of both the α and the δ polymorphs exhibit sharp features, highly disordered forms of Alq3 obtained from rapid vapor deposition onto a cold substrate, yield broadened spectra, indicating substantial structural disorder in the local geometry of different Alq3 molecules.
Dispersive properties of cylindrical Rayleigh waves
Authors: Wu Xianmei , Qian Menglu , Cantrell John H.
Published: November 2003

An analytical model of cylindrical Rayleigh waves generated by a laser source is developed that predicts a previously unknown maximum in the Rayleigh wave velocity. The maximum value CR max of the cylindrical Rayleigh wave velocity CR occurs at a critical small value of ωR/CR where ω is the angular frequency and R is the radius of the cylinder. CR max is smaller in magnitude than the longitudinal velocity but larger than the shear velocity in the material. Experimental data, generated by a cylindrically focused laser source, confirm the theoretical predictions for aluminum cylinders.
Far-infrared absorption in GaAs:Te liquid phase epitaxial films
Authors: Cardozo B. L. , Haller E. E. , Reichertz L. A. , Beeman J. W.
Published: November 2003

The far-infrared absorption spectrum of n-type GaAs is of interest for applications such as GaAs photoconductors and blocked impurity band detectors. The linear optical absorption coefficients α for three n-type GaAs films of varying doping concentrations have been measured in the range of 10 to 100 cm−1 using Fourier transform infrared spectrometry. These results show α having maximum values of between 46 cm−1 at 1×1015 cm−3 and approximately 800 cm−1 at 2.1×1016 cm−3. The formation and widening of a donor impurity band with increasing impurity concentration is clearly demonstrated.
Defect generation by preferred nucleation in epitaxial Sr2RuO4/LaAlO3
Authors: Zurbuchen Mark A. , Jia Yunfa , Knapp Stacy , Carim Altaf H. , Schlom Darrell G. , Pan X. Q.
Published: November 2003

The atomic structure of the film–substrate interface of a (001) Sr2RuO4/(100)c LaAlO3 film, determined by high-resolution transmission electron microscopy and simulation, is reported. The structure of superconductivity-quenching Δc≈0.25 nm out-of-phase boundaries (OPBs) in the film is also reported. Growth in one region on the La-terminated surface is observed to nucleate with a SrO layer. Because two structurally equivalent SrO layers exist within the unit cell, two neighboring nuclei with differing growth order (SrO-RuO2-SrO or RuO2-SrO-SrO) will nucleate an OPB where their misaligned growth fronts meet. Strategies to avoid OPB generation by this mechanism are suggested, which it is hoped may ultimately lead to superconducting Sr2RuO4 films.
Effects of the morphology and nanostructure on the optical nonlinearities of Au:BaTiO3 nanocomposite films
Authors: Wang Weitian , Yang Guang , Wu Weidong , Chen Zhenghao
Published: November 2003

Metal nanocomposite thin films formed by nanometer-sized gold particles embedded in BaTiO3 matrices (Au:BaTiO3) were fabricated on MgO (100) and glass substrates using a pulsed-laser deposition technique. The high-resolution transmission electron microscopy analysis indicated that the morphology of gold particles and the structure of BaTiO3 matrices changed remarkably with the change of the substrate temperatures. Crystal lattice fringes from the Au nanocrystals and BaTiO3 matrices were clearly observed. The nonlinear optical properties of the Au:BaTiO3 nanocomposite films were measured using the Z-scan method at the wavelength of 532 nm. A sign reversal was demonstrated in the nonlinear absorption coefficient (β) and the nonlinear refractive index (n2) of Au:BaTiO3 films with the same Au concentration but a different nanostructure.
Through-thickness superconducting and normal-state transport properties revealed by thinning of thick film ex situ YBa2Cu3O7−x coated conductors
Authors: Feldmann D. M. , Larbalestier D. C. , Feenstra R. , Gapud A. A. , Budai J. D. , Holesinger T. G. , Arendt P. N.
Published: November 2003

A rapid decrease in the critical current density (Jc) of YBa2Cu3O7−x (YBCO) films with increasing film thickness has been observed for multiple YBCO growth processes. While such behavior is predicted from two-dimensional collective pinning models under certain assumptions, empirical observations of the thickness dependence of Jc are believed to be largely processing dependent at present. To investigate this behavior in ex situ YBCO films, 2.0- and 2.9-μm-thick YBCO films on ion beam assisted deposition-yttria stabilized zirconia substrates were thinned and repeatedly measured for ρ(T) and Jc(H). The 2.9 μm film exhibited a constant Jc[77 K, self-field (SF)] through thickness of ∼1 MA/cm2 while the 2.0 μm film exhibited an increase in Jc(77 K, SF) as it was thinned. Neither film offered evidence of significant dead layers, suggesting that further increases in critical current can be obtained by growing thicker YBCO layers.
Dipolar induced, spatially localized resonance in magnetic antidot arrays
Authors: Yu Chengtao , Pechan Michael J. , Mankey Gary J.
Published: November 2003

Dipole induced, spatially localized ferromagnetic resonances (at 35 GHz) are observed in micron-sized antidot arrays in permalloy films fabricated with photolithography. All square (3 μm×3 μm) and rectangular (3 μm×4, 5, and 7 μm) array samples exhibit double resonances, with each resonance possessing uniaxial in-plane anisotropy. Interestingly, the easy axes of the two resonances are orthogonal in all cases. The magnitude of the induced dipolar anisotropy decreases with increasing rectangular aspect ratio for one of the resonances, but remains essentially constant for the other. Micromagnetic simulations reveal that the two resonance peaks are the consequence of a dipole field distribution producing two areas with distinctly different demagnetizing field patterns.
Chemical gating of In2O3 nanowires by organic and biomolecules
Authors: Li Chao , Lei Bo , Zhang Daihua , Liu Xiaolei , Han Song , Tang Tao , Rouhanizadeh Mahsa , Hsiai Tzung , Zhou Chongwu
Published: November 2003

In 2 O 3 nanowire transistors were used to investigate the chemical gating effect of organic molecules and biomolecules with amino or nitro groups. The nanowire conductance changed dramatically after adsorption of these molecules. Specifically, amino groups in organic molecules such as butylamine, donated electrons to In2O3 nanowires and thus led to enhanced carrier concentrations and conductance, whereas molecules with nitro groups such as butyl nitrite made In2O3 nanowires less conductive by withdrawing electrons. In addition, intrananowire junctions created by partial exposure of the nanowire device to butyl nitrite were investigated, and pronounced rectifying current–voltage characteristics were obtained. Furthermore, chemical gating by low-density lipoprotein cholesterol, the offending agent in coronary heart diseases, was also observed and attributed to the amino groups carried by the bio species.
Tunable high-frequency band-stop magnetic filters
Authors: Kuanr Bijoy , Celinski Z. , Camley R. E.
Published: November 2003

We present results on Fe- and Permalloy™-based microstrip microwave band-stop filters. These structures, prepared on GaAs substrates, are compatible in size and growth process with on-chip high-frequency electronics. We observed power attenuation of 100 dB/cm for Permalloy and 180 dB/cm for Fe. The insertion loss is low: 2–3 dB for Permalloy and 3–5 dB for the Fe-based structures. Our geometry includes a significant boost to the zero-field operational frequency due to the shape anisotropy of the magnetic element in the microstrip. Using the shape anisotropy, we create a Fe-based filter that operates at 11 GHz with zero applied field.
Epitaxial growth of Fe3Si/GaAs(001) hybrid structures
Authors: Herfort Jens , Schönherr Hans-Peter , Ploog Klaus H.
Published: November 2003

We have established an optimized growth temperature range, namely, 150 °C<TG<250 °C, where ferromagnetic Fe3Si/GaAs(001) hybrid structures with high crystalline and interfacial quality can be fabricated by molecular-beam epitaxy. The composition of the Fe3Si layers, which can be regarded as a Heusler alloy, was tuned within the stable Fe3Si phase. The layers show high magnetic moments with a value of 1050 emu/cm3, which is close to that of bulk Fe3Si.
Development of epitaxial NbN/MgO/NbN–superconductor-insulator-superconductor mixers for operations over the Nb gap frequency
Authors: Kawakami Akira , Uzawa Yoshinori , Wang Zhen
Published: November 2003

We have developed fabrication processes for epitaxial NbN/MgO/NbN trilayers and epitaxial NbN/MgO/NbN tunnel junctions. We estimated the junction specific capacitances (CS) for designing fully epitaxial NbN–superconductor-insulator-superconductor (SIS) mixers by using Fiske steps. Tunnel junctions measuring 40×3 and 20×3 μm2 were fabricated, and Fiske steps caused by external magnetic fields were clearly observed in the I–V characteristics. The CS was estimated as a function of the product of the normal-state resistance and the junction area (RNA), and they were 73–120 fF/μm2 at 8800–90 Ω μm2. We fabricated and tested a fully epitaxial NbN/MgO/NbN–SIS mixer. The mixer showed low-noise properties over the Nb gap frequency, and the lowest DSB receiver noise was 260 K at 795 GHz.
Structural and elastic anisotropy of carbon phases prepared from fullerite C60
Authors: Lyapin A. G. , Mukhamadiarov V. V. , Brazhkin V. V. , Popova S. V. , Kondrin M. V. , Sadykov R. A. , Tat’yanin E. V. , Bayliss S. C. , Sapelkin A. V.
Published: November 2003

We show that application of nonhydrostatic pressure to cluster-based molecular material, like fullerite C60, provides an opportunity to create elastically and structurally anisotropic carbon materials, including two-dimensional polymerized rhombohedral C60 and superhard graphite-type (sp2) disordered atomic-based phases. There is direct correlation between textured polymerized and/or textured covalent structure and anisotropic elasticity. Whereas this anisotropy is induced by the uniaxial pressure component, in the case of disordered atomic-based phases, it may be governed by the uniform pressure magnitude.
High temperature structural studies of HgS and HgSe quantum dots
Authors: Qadri Syed B. , Kuno Masaru , Feng C. R. , Rath B. B. , Yousuf M.
Published: November 2003

We report the structural investigations of β-HgS and HgSe quantum dots as a function of temperature between 300 and 600 K using x-ray diffraction. For both the chalcogenides, the zinc-blende structure remains stable up to 600 K without undergoing any phase transformation. The crystallite size increases as a function of temperature. However, for nanocrystallite ∼5.0 nm, lattice parameters show reduction in comparison to their bulk values. With increase in temperature, the lattice parameter increases and approaches the equilibrium value as the crystallite sizes grow to more than 10.0 nm. We attribute the temperature induced increase in crystallite size primarily to normal grain growth, a phenomenon observed in crystalline solids when the crystallite size undergoes gradual increase as function of time at elevated temperatures with accompanying recrystallization of new crystallite nuclei, and we rule out the possibility of size-dependent melting.
Effect of the Si/SiO2 interface on self-diffusion of Si in semiconductor-grade SiO2
Authors: Fukatsu Shigeto , Takahashi Tomonori , Itoh Kohei M. , Uematsu Masashi , Fujiwara Akira , Kageshima Hiroyuki , Takahashi Yasuo , Shiraishi Kenji , Gösele Ulrich
Published: November 2003

Self-diffusion of ion-implanted Si30 in SiO2 formed directly on Si substrates by thermal oxidation was studied as a function of the temperature and SiO2 thickness (200, 300, and 650 nm). The diffusion coefficient increases by about an order of magnitude with decreasing SiO2 thickness from 650 to 200 nm when silicon–nitride capping layers are placed on top of the SiO2, i.e., the distance between the Si30 diffusers and Si/SiO2 interface has a strong influence. Because the stress on SiO2 by nitride estimated for such a change in diffusivity is unrealistically large, Si species, most likely SiO, generated at the Si/SiO2 interface and diffusing into SiO2 must be affecting the self-diffusion of Si in SiO2.
Niobium nanoclusters studied with in situ transmission electron microscopy
Authors: Vystavel T. , Palasantzas G. , Koch S. A. , De Hosson J. Th. M.
Published: November 2003

Structural aspects of deposited niobium nanoclusters approximately 10 nm in size have been explored by means of high-resolution transmission electron microscopy. The niobium clusters have a bcc structure and a crystal habit of rhombic dodecahedron. In situ heating up to ∼800 °C revealed a resistance to high temperatures, in the sense that the cluster habit is preserved. However, the internal structural order of the clusters is altered due to formation of niobium oxide domains within the clusters. Coalescence does not occur even at the highest temperatures, which is attributed to the presence of facets and the occurrence of oxidation during heat treatment.
High-temperature biquadratic coupling in Fe/Cr trilayers
Authors: Aernout D. , Van Eek S. M. , Croonenborghs B. , L’abbé C. , Rots M. , Meersschaut J.
Published: November 2003

We report on the study of epitaxial Fe/Cr trilayers with a Cr thickness of 8 nm grown at different temperatures. We show that it is possible to produce systems that exhibit interlayer exchange coupling above room temperature, while being uncoupled at lower temperatures. Through perturbed angular correlation measurements, this macroscopic effect is identified as a direct result of the growth induced characteristics of the antiferromagnetic spin density wave ordering in the Cr spacer.