bluuebunny/crossref_metadata_2025 huggingface dataset:
Energy-loss straggling caused by the inhomogeneity of target material
Authors: Tosaki Mitsuo
Published: February 2006

Rutherford backscattering spectroscopy (RBS) accompanying with the sharp 4.808-MeV resonance of proton beams in carbon has been applied to examine the energy-loss straggling in detail, which causes the broadening of proton beam energy in the penetrating path. In the present measurements, RBS peak profiles of homogeneous and inhomogeneous carbon materials have been obtained with an incident energy of 5.5MeV. Careful analyses of those profiles have revealed that the energy-loss straggling can be separated into two parts; one is collision straggling and the other is density straggling. The collision straggling is caused by the statistical fluctuation in collisions of proton with target atoms, which has been intensively studied since the theoretical work by Bohr [Philos. Mag. 30, 581 (1915)]. The density straggling is caused by the statistical fluctuation in local density of target material, which has been discussed in the present work. The random inhomogeneity as a measure of the fluctuation in the local density is introduced into the existing theory of the energy-loss straggling. Following the theoretical treatment, we have successfully deduced the random inhomogeneity of various carbon materials and the spatial spread of 4.808-MeV resonance protons in the materials. Some applications of the present method for the RBS with the sharp resonance are also presented.
Thermodynamic and dynamic anomalies for a three-dimensional isotropic core-softened potential
Authors: Barros de Oliveira Alan , Netz Paulo A. , Colla Thiago , Barbosa Marcia C.
Published: February 2006

Using molecular-dynamics simulations and integral equations (Rogers-Young, Percus-Yevick, and hypernetted chain closures) we investigate the thermodynamics of particles interacting with continuous core-softened intermolecular potential. Dynamic properties are also analyzed by the simulations. We show that, for a chosen shape of the potential, the density, at constant pressure, has a maximum for a certain temperature. The line of temperatures of maximum density (TMD) was determined in the pressure-temperature phase diagram. Similarly the diffusion constant at a constant temperature, D, has a maximum at a density ρmax and a minimum at a density ρmin<ρmax. In the pressure-temperature phase diagram the line of extrema in diffusivity is outside of the TMD line. Although this interparticle potential lacks directionality, this is the same behavior observed in simple point charge/extended water.
Adhesive wafer bonding
Authors: Niklaus F. , Stemme G. , Lu J. -Q. , Gutmann R. J.
Published: February 2006

Wafer bonding with intermediate polymer adhesives is an important fabrication technique for advanced microelectronic and microelectromechanical systems, such as three-dimensional integrated circuits, advanced packaging, and microfluidics. In adhesive wafer bonding, the polymer adhesive bears the forces involved to hold the surfaces together. The main advantages of adhesive wafer bonding include the insensitivity to surface topography, the low bonding temperatures, the compatibility with standard integrated circuit wafer processing, and the ability to join different types of wafers. Compared to alternative wafer bonding techniques, adhesive wafer bonding is simple, robust, and low cost. This article reviews the state-of-the-art polymer adhesive wafer bonding technologies, materials, and applications.
A cubic phase of C3N4 synthesized in the diamond-anvil cell
Authors: Ming L. C. , Zinin P. , Meng Y. , Liu X. R. , Hong S. M. , Xie Y.
Published: February 2006

A cubic phase of C3N4 was discovered. It was recovered at ambient conditions from the graphite-like C3N4 (g-C3N4) phase subjected to pressures between 21 and 38 GPa in a diamond-anvil cell, laser heated to temperatures between 1600 and 3000 K. The x-ray-diffraction data of the phase are best explained by a cubic unit cell with the lattice parameters a=3.878±0.001Å. With an assumption of 1molecule∕unit cell (Z=1) for the cubic phase, the molar volume of the cubic phase is 35.126cm3∕mol and the density is 2.62g∕cm3. The density of the cubic phase is less than that which was predicted for the high-pressure phases but is 12% denser than the low-pressure graphitic phase (ρ=2.336g∕cm3). The cubic phase has not been predicted theoretically and represents an unknown structure in C3N4.
Nondispersive hole transport in carbazole- and anthracene-containing polyspirobifluorene copolymers studied by the charge-generation layer time-of-flight technique
Authors: Laquai Frédéric , Wegner Gerhard , Im Chan , Bässler Heinz , Heun Susanne
Published: February 2006

Nondispersive hole transport in two polyspirobifluorene copolymers containing either 10% anthracene or 10% carbazole was studied in detail by the charge-generation layer time-of-flight (TOF) technique over a wide range of electric fields and temperatures. The TOF transients of both polymers showed a clear plateau indicating nondispersive transport of charge carriers. Zero-field mobilities were found to be in the order of 10−6cm2∕Vs at room temperature. Results were analyzed within the framework of the Gaussian disorder model to extract the parameters of the charge-carrier transport. The width of the transport density of states was determined to be 83meV for the polyspirobifluorene-anthracene copolymer and 89meV for the polyspirobifluorene-carbazole copolymer. At lower temperatures a change of slope in the temperature dependence of the zero-field mobility was observed. At higher temperatures the TOF transients were modified by a cusp. Both phenomena can be explained within the framework of the Gaussian disorder model.
Effect of Li substitution on dielectric and ferroelectric properties of ZnO thin films grown by pulsed-laser ablation
Authors: Dhananjay  , Nagaraju J. , Krupanidhi S. B.
Published: February 2006

Li-doped ZnO thin films (Zn1−xLixO, x=0.05–0.15) were grown by pulsed-laser ablation technique. Highly c-axis-oriented films were obtained at a growth temperature of 500 °C. Ferroelectricity in Zn1−xLixO was found from the temperature-dependent dielectric constant and from the polarization hysteresis loop. The transition temperature (Tc) varied from 290 to 330 K as the Li concentration increased from 0.05 to 0.15. It was found that the maximum value of the dielectric constant at Tc is a function of Li concentration. A symmetric increase in memory window with the applied gate voltage is observed for the ferroelectric thin films on a p-type Si substrate. A ferroelectric P-E hysteresis loop was observed for all the compositions. The spontaneous polarization (Ps) and coercive field (Ec) of 0.6μC∕cm2 and 45kV∕cm were obtained for Zn0.85Li0.15O thin films. These observations reveal that partial replacement of host Zn by Li ions induces a ferroelectric phase in the wurtzite-ZnO semiconductor. The dc transport studies revealed an Ohmic behavior in the lower-voltage region and space-charge-limited conduction prevailed at higher voltages. The optical constants were evaluated from the transmission spectrum and it was found that Li substitution in ZnO enhances the dielectric constant.
Fabrication of tunable nanostructure arrays using ion-polishing-assisted nanosphere lithography
Authors: Zheng Y. B. , Wang S. J. , Huan A. C. H. , Wang Y. H.
Published: February 2006

A simple technique has been developed for fabricating tunable nanostructure arrays, combining ion polishing and nanosphere lithography. Nanohole arrays with the controllable shape and in-plane dimensions are demonstrated. The combination of ion polishing and nanosphere lithography yields tunable masks and provides an easy way for flexible nanofabrication.
One-dimensional postwetting layer in InGaAs∕GaAs(100) quantum-dot chains
Authors: Wang Zh. M. , Mazur Yu. I. , Shultz J. L. , Salamo G. J. , Mishima T. D. , Johnson M. B.
Published: February 2006

Long chains of quantum dots formed in InGaAs∕GaAs(100) multiple layers have been systematically investigated by scanning electron, transmission electron, and atomic force microscopies. In addition to the usual two-dimensional wetting layer involved in the Stranski-Krastanov growth, we have directly observed a one-dimensional postwetting layer along the [01−1] direction that strings together the quantum dots in each chain. In sharp contrast with the two-dimensional wetting layer, which exists before the quantum-dot chains form, this one-dimensional postwetting layer develops during the GaAs capping of the existing dot chains. This one-dimensional layer forms through the anisotropic surface diffusion of In atoms that accompanies the change in strain profile during capping and therefore produces the steady-state material distribution that includes a one-dimensional postwetting layer as a result.
Crystal structure and thermoelectric properties of the type-I clathrate compound Ba8Ge43 with an ordered arrangement of Ge vacancies
Authors: Okamoto Norihiko L. , Oh Min Wook , Nishii Takumi , Tanaka Katsushi , Inui Haruyuki
Published: February 2006

The crystal structure of the type-I clathrate compound Ba8Ge43 has been investigated by x-ray diffraction and transmission electron microscopy. The thermoelectric properties of Ba8Ge43 have also been investigated. The crystal structure of Ba8Ge43 is different from that reported for the usual type-I clathrate compounds with the space group of Pm3¯n but is a superlattice structure based on the usual type-I clathrate structure due to the ordering of Ge vacancies in half the 6c sites of the usual type-I clathrate structure. The crystal structure of Ba8Ge43 belongs to the space group of Ia3¯d and Ge vacancies exclusively occupy the 24c sites. The thermoelectric properties of Ba8Ge43 are not particularly good, as exemplified by the rather low ZT value of 0.057 because of the high value of electrical resistivity, which may arise from the existence of Ge vacancies.
Structural phase-transition region and electrical properties of Pb(Ni1∕3Nb2∕3)O3–Pb(Zn1∕3Nb2∕3)O3–PbTiO3 ceramics
Authors: Pan Jing-Song , Zhang Xiao-Wen
Published: February 2006

The phase structures, and dielectric and piezoelectric properties of three series of ceramic compositions in the Pb(Ni1∕3Nb2∕3)O3–Pb(Zn1∕3Nb2∕3)O3–PbTiO3 ternary system have been systematically investigated. The existence of the morphotropic phase boundary (MPB) line is proposed and a wide linear MPB region is verified. The contents of the coexisting rhombohedral and tetragonal phases vary irregularly along the MPB line, while the sole increase of PT with a fixed ratio of Ni∕Zn leads to the decrease of the rhombohedral phase in the MPB region. Compositions on the Ni-rich side of the MPB line show better electrical properties than those on the Zn-rich side. The MPB composition of 0.56Pb(Ni1∕3Nb2∕3)O3–0.14Pb(Zn1∕3Nb2∕3)O3–0.30PbTiO3 was found to be with high values of εm and d33(εm=24500 and d33=740pC∕N), which is superior to those of many reported ferroelectric ceramics.
Effects of PbF2 doping on structure and spectroscopic properties of Ga2O3–GeO2–Bi2O3–PbO glasses doped with rare earths
Authors: Zhang Q. Y. , Li T. , Shi D. M. , Yang G. F. , Yang Z. M. , Jiang Z. H. , Buddhudu S.
Published: February 2006

We report an intense room-temperature blue upconversion in Tm3+∕Yb+3-codoped Ga2O3–GeO2–Bi2O3–PbO(PbF2) glasses upon excitation with a convenient 980nm laser diode. Effects of PbF2 doping on the thermal stability, structure, and upconversion properties of Tm3+∕Yb3+-codoped gallate-germanium-bismuth-lead glasses have been investigated. We find that the presence of PbF2 provides two potentials: shortening the ultraviolet cutoff band and decreasing the phonon energy of host glasses. The blue-upconversion intensity has a cubelike dependence on incident pump-laser power, indicating a three-photon process. Energy-transfer process and nonradiative phonon-assisted decay could be responsible for the population of the G41 emitting level of the Tm3+. The results indicate the potential possibility towards the development of an oxide-based blue-upconversion glass-fiber laser.
Universal time relaxation behavior of the exchange bias in ferromagnetic/antiferromagnetic bilayers
Authors: Dho Joonghoe , Leung C. W. , Blamire M. G.
Published: February 2006

The resilience of the exchange bias (Hex) in ferromagnet/antiferromagnet bilayers is generally studied in terms of repeated hysteresis loop cycling or by protracted annealing under reversed field (training and long-term relaxation, respectively). In this paper we report measurements of training and relaxation in NiFe films coupled with polycrystalline FeMn and epitaxial α-Fe2O3. We show that Hex suppressed both by training and relaxation was partially recovered as soon as a field cycling for consecutive hysteresis loop measurement was stopped or the magnetization of the ferromagnet was switched back to the biased direction. In both cases we can model the observed logarithmic time relaxation behavior, and its film thickness and temperature dependence, in terms of a thermally activated reversal of the antiferromagnetic domain configuration to reduce the total magnetic energy.
Nucleation-type magnetization behavior in FePt (001) particulate films
Authors: Shima T. , Takanashi K. , Takahashi Y. K. , Hono K. , Li G. Q. , Ishio S.
Published: February 2006

Nucleation-type magnetization behavior is reported in sputtered FePt (001) films with an island structure, where the particles show a multiple-domain structure. A large coercivity HC of more than 50 kOe is achieved at an initial applied field of only 6 kOe. The magnetization behavior and the magnetic domain observation indicate clearly that domain walls are wiped out completely at a low applied field, and once domain walls are wiped out, it is hard to nucleate reversed domains in the particles, resulting in high HC. The remarkable nucleation-type behavior of magnetization is also found to give rise to anomalous minor loops when the applied magnetic field is alternated around zero and increased gradually.
Photoconductivity in nanocrystalline GaN and amorphous GaON
Authors: Koo A. , Budde F. , Ruck B. J. , Trodahl H. J. , Bittar A. , Preston A. , Zeinert A.
Published: February 2006

In this work we present a study of the optoelectronic properties of nanocrystalline GaN (nc-GaN) and amorphous GaON (a-GaON) grown by ion-assisted deposition. The two classes of film show very distinct photoconductive responses; the nc-GaN has a fast small response while the a-GaON films have a much larger response which is persistent. To describe the observed intensity, wavelength, and temperature dependence of the photoconductivity in each class of film, we build a model which takes into account the role of a large density of localized states in the gap. The photoconductivity measurements are supplemented by thermally stimulated conductivity, measurement of the absorption coefficient, and determination of the Fermi level. Using the model to aid our interpretation of this data set, we are able to characterize the density of states in the gap for the two materials.
Investigation of shape, strain, and interdiffusion in InGaAs quantum rings using grazing incidence x-ray diffraction
Authors: Sztucki Michael , Metzger Till Hartmut , Chamard Virginie , Hesse Anke , Holý Václav
Published: February 2006

The formation of nanoscopic InGaAs ring structures on a GaAs(001) substrate takes place when InAs quantum dots, grown by Stranski-Krastanov self-organization, are covered by a thin layer of GaAs. The shape transformation into rings is governed by strain, diffusion, and surface tension, physical parameters which are of importance to monitor the magneto-optical and electronic properties of the rings. In this work we report on the characterization of morphology and structure of the rings in three dimensions (such as strain and chemical composition). To this end we apply grazing incidence small angle x-ray scattering (GISAXS) and grazing incidence diffraction (GID). From GISAXS the shape is found to be of circular symmetry with an average outer radius of 26nm, a height of about 1.5nm, and a hole in the middle, in good agreement with atomic force microscopy measurements. Information about strain and interdiffusion is derived from intensity mappings in reciprocal space close to the (220) and (22¯0) reflections done in the surface sensitive GID geometry. From a comparison of the intensity maps with finite-element model calculations the InGaAs interdiffusion profile in the ring is determined. It strongly depends on the crystallographic orientation. In the ring we find a maximum InAs concentration of more than 80% along [11¯0], while along [110] it is below 20%. This results from the preferred diffusion of InAs along [11¯0].
Mean free path for excitation energy migration in Nd3+-doped glasses as a function of concentration
Authors: Serqueira E. O. , Monte A. F. G. , Dantas N. O. , Morais P. C.
Published: February 2006

The energy-transfer process and the related migration mechanism of excitation energy, important in the optical dynamics of Nd-doped glasses, were investigated. In order to study the migration mechanism and transfer process, Nd3+-doped oxide glasses were produced with doping concentration (N) ranging from 0.1to1.9wt% as Nd2O3. A microluminescence technique was used to measure the spatial distribution of the emitting light as a function of the distance from the center of the laser-excitation spot with different Nd3+-ion concentrations. Efficient long-range migration of excitation energy of Nd3+ ions was observed at 1.1wt% of Nd2O3. The critical distance between Nd3+ ions, estimated from the observed migration length, shows that dipole-dipole interaction is not the dominant mechanism for energy transfer. The mean free path for migratory excitation energy in the investigated material, assisted by absorption and scattering by defects or phonons, is the most probable mechanism for energy transfer.
Investigation of thickness dependence of the ferroelectric properties of Pb(Zr0.6Ti0.4)O3 thin-film capacitors
Authors: Bouregba R. , Le Rhun G. , Poullain G. , Leclerc G.
Published: February 2006

The ferroelectric properties of Pt∕Pb(Zr0.6Ti0.4)O3∕Pt∕TiO2∕SiO2∕Si thin-film capacitors with different thicknesses are investigated. According to the literature data, tilting of the hysteresis loops and marked increase of the coercive fields are observed when the thickness of the film is reduced. The degradation of the switching properties is fully reproduced by simulations including nonferroelectric space-charge layers at both ferroelectric/electrode interfaces. Based on the theoretical results, a converse model is constructed from which the overall interface capacitance, the total interface built-in potential, and both dielectric permittivity and polarization of the bulk ferroelectric layer are determined for each film. Remarkably the polarization loop due to the switching domains, calculated for each Pb(Zr,Ti)O3 (PZT) capacitor, exhibited a squarelike shape with coercive fields in agreement with the bulk value. Moreover, a unique set of parameters was found whatever the film thickness. From our results it is concluded that the degradation of the ferroelectric properties of metal-PZT thin-film-metal capacitors, often encountered when their thickness is reduced, probably arises from a mechanism of modulation of density and sign of the space charge at both interfaces.
Measuring the electron temperature by optical emission spectroscopy in two temperature plasmas at atmospheric pressure: A critical approach
Authors: Yanguas-Gil A. , Cotrino J. , González-Elipe A. R.
Published: February 2006

The measurement of the electron mean kinetic energy by identifying the electron temperature and the excitation temperature obtained by optical emission spectroscopy is theoretically studied for two temperature argon plasmas at atmospheric pressure. Using a 32-level collisional radiative model in which both electron impact and argon-impact inelastic collisions are taken into account, it has been found that under certain conditions the argon inelastic collisions may cause a decrease of the argon excitation temperature so that the relation Te>Texc>T0 is satisfied. This inequality also appears when electron losses due to diffusion are important and the electron density is lower than its equilibrium value.
Optical properties of highly faceted ZnO rods
Authors: Djurišić A. B. , Kwok W. M. , Chan W. K. , Phillips D. L. , Leung Y. H. , Xie M. H. , Chen H. Y. , Wu C. L. , Gwo S.
Published: February 2006

Highly faceted ZnO rods with 18 side surfaces were synthesized. Their optical properties were characterized by variable temperature photoluminescence, time-resolved photoluminescence, and time-integrated photoluminescence. Low-temperature photoluminescence is dominated by a narrow donor bound exciton peak, while at room temperature ultraviolet emission and green emission can be observed. In spite of the presence of the defect emission, the samples have excellent crystal quality based on the long lifetime of spontaneous emission, with the time constants of biexponential decay equal to 116ps and 1.2ns. With increasing pump fluence, stimulated emissions due to excitonic and electron-hole plasma effects were observed. The lasing dynamics in both emission regimes is discussed.
Determination of the interface trap density of rubrene single-crystal field-effect transistors and comparison to the bulk trap density
Authors: Goldmann C. , Krellner C. , Pernstich K. P. , Haas S. , Gundlach D. J. , Batlogg B.
Published: February 2006

In order to gain further insight into the details of charge transport in organic semiconductor devices it is necessary to characterize the density of trap states at the semiconductor∕gate dielectric interface. Here we use the technique of gate bias stress to quantitatively determine the interface trap density in rubrene single-crystal field-effect transistors with two different types of interfaces. A reversible and reproducible shift of the I-V characteristics is observed upon both negative and positive gate bias stress, whose physical origin is identified as charge trapping and detrapping at the crystal∕SiO2 insulator interface. We can thus quantify the density of interface traps that are alternately filled and emptied on a time scale of ≅1h in the energy range defined by the applied bias stress. For a typical rubrene∕SiO2 interface we extract a density of ∼2×1012cm−2 at a stress bias of ±50V, corresponding to a volume density of ≅1019∕(cm3eV). An octadecyltrichlorosilane treatment of the SiO2 dielectric surface reduced this charge density by more than a factor of 2. The bulk trap density derived from space-charge-limited current measurements is typically three orders of magnitude lower, highlighting the dominant role in charge trapping played by the crystal∕dielectric interface.
Dislocation reduction in GaN grown on porous TiN networks by metal-organic vapor-phase epitaxy
Authors: Fu Y. , Yun F. , Moon Y. T. , Özgür Ü. , Xie J. Q. , Ni X. F. , Biyikli N. , Morkoç H. , Zhou Lin , Smith David J. , Inoki C. K. , Kuan T. S.
Published: February 2006

We report on the effectiveness of porous TiN nanonetworks on the reduction of threading dislocations (TDs) in GaN grown by metal-organic vapor-phase epitaxy (MOVPE). The porous TiN networks were formed by in situ annealing of thin-deposited Ti films deposited ex situ on GaN templates within the MOVPE growth chamber. Different annealing parameters in relation to surface porosity of TiN networks were investigated. Transmission electron micrographs indicated dislocation reduction by factors of up to 10 in GaN layers grown on the TiN nanonetwork, compared with a control sample. TiN prevented many dislocations present in the GaN templates from penetrating into the upper layer. Microscale epitaxial lateral overgrowth of GaN above TiN also contributed to TD reduction. The surface porosity of the TiN network had a strong impact on the efficiency of TD reduction. X-ray-diffraction and time-resolved photoluminescence measurements further confirmed the improved GaN quality.
Structure and magnetic properties of N-doped L10-ordered FePt–C nanocomposite films
Authors: Mi W. B. , Jiang E. Y. , Bai H. L. , Liu Hui
Published: February 2006

As-deposited and annealed N additive FePt–C nanocomposite films fabricated using facing-target sputtering were investigated systemically. Structure analyses reveal that the as-deposited films are composed of ∼3nm amorphous metal granules buried in a-CN matrix, and transform into L10-ordered FePt particles followed by an increase of particle size after being annealed. The N atomic fraction increases, and the size of FePt particles in the 650°C annealed films reduces, with increasing N2 partial pressure (PN). The diffusion of Fe and Pt atoms into the positions of the ordered L10 lattice was improved by the decompounding of Fe–N bonds and the escape of N atoms from the film during the annealing. Magnetic measurements indicate that high-temperature annealing turns the superparamagnetism (at PN<10%) and paramagnetism (at PN⩾10%) of the as-deposited films into ferromagnetism. The coercivity of the 650°C annealed N-doped films decreases with increasing PN due to the particle size reduction.
Influence of in situ ultrasound treatment during ion implantation on formation of silver nanoparticles in silica
Authors: Romanyuk Andriy , Spassov Vladislav , Melnik Viktor
Published: February 2006

We report on the effect of in situ ultrasound treatment on the clustering process of silver atoms in ion-implanted SiO2. Cross-sectional transmission electron microscopy shows single-crystal Ag spheres with an increased cluster size when prepared using ultrasound vibrations. Time-of-flight secondary-ion-mass spectrometry demonstrates an enhanced yield of [Ag2]216 complexes in structures treated with acoustic waves. An analysis of the influence of ultrasound on defect reaction kinetics as well as on different stages of the clustering process is performed.
Characterization of radio frequency plasma using Langmuir probe and optical emission spectroscopy
Authors: Nisha M. , Saji K. J. , Ajimsha R. S. , Joshy N. V. , Jayaraj M. K.
Published: February 2006

The radio frequency plasma generated during the sputtering of Indium Tin Oxide target using Argon was analyzed by Langmuir probe and optical-emission spectroscopy. The basic plasma parameters such as electron temperature and ion density were evaluated. These studies were carried out by varying the RF power from 20to50W. A linear increase in ion density and an exponential decrease in electron temperature with rf power were observed. The measured plasma parameters were then correlated with the properties of ITO thin films deposited under similar plasma conditions.
Electrical shorting of organic photovoltaic films resulting from metal migration
Authors: Suemori Kouji , Yokoyama Masaaki , Hiramoto Masahiro
Published: February 2006

Migration of Au nanoparticles to a depth of more than 100nm from vacuum-deposited Au electrodes into perylene pigment (Me-PTC) films, used for organic photovoltaic cells, was clearly observed by transmission electron microscopy. The depth profile of the number of migrated Au particles was shown to coincide well with cell thickness profile of the magnitude of leakage current for the cells with a Au∕Me-PTC interface. This clearly indicated that electrical shorting of very thin organic photovoltaic cells is mainly caused by the metal migration. Influence of metal migration into organic semiconductor films must be eliminated to realize high efficiency and large area solar cells.
Tunneling magnetoresistance in devices based on epitaxial NiMnSb with uniaxial anisotropy
Authors: Liu J. , Girgis E. , Bach P. , Rüster C. , Gould C. , Schmidt G. , Molenkamp L. W.
Published: February 2006

We demonstrate tunnel magnetoresistance junctions based on a trilayer system consisting of an epitaxial NiMnSb, an aluminum oxide, and a CoFe trilayer. The junctions show a tunneling magnetoresistance of ΔR∕R of 8.7% at room temperature which increases to 14.7% at 4.2K. The layers show a clear separate switching and a small ferromagnetic coupling. A uniaxial in-plane anisotropy in the NiMnSb layer leads to different switching characteristics depending on the direction in which the magnetic field is applied, an effect which can be used for sensor applications.
The influence of void space on antireflection coatings of silica nanoparticle self-assembled films
Authors: Yancey S. E. , Zhong W. , Heflin J. R. , Ritter A. L.
Published: February 2006

This study investigates the deposition by ionic self-assembly of alternating silica nanoparticle and poly(allyamine hydrochloride) layers with the goal to create a single-material antireflection coating. The condition that the optical thickness of the film be equal to λ∕4 can be satisfied by depositing the requisite number of bilayers to obtain minimum reflectivity at the chosen wavelength. The second condition for antireflection, that the index of refraction of the film be equal to nc=n1n2, where n1 and n2 are the refractive indices of the media on each side of the film, requires that nc=1.22 for a film with air on one surface and glass (assuming n=1.50) on the other. Such a low index of refraction can be created in films consisting of nanoparticles if the proper volume fraction of void space exists in the film. In the wavelength range λ=350–700nm, minimum reflectivities of ⩾2.0%, ⩽0.2%, and ⩽0.2% were obtained with films created on both sides of a glass slide using 15, 45, and 85nm average diameter silica nanoparticles, respectively. The maximum transmittances for the corresponding films were 97%, ⩾98%, and ⩾97%. The minimum reflectance of films prepared with 15nm average diameter silica nanoparticles was limited by insufficient void volume in the films. The maximum transmittances of films prepared with 45 and 85nm average diameter silica nanoparticles were limited by diffuse scattering arising from the inhomogeneous morphology of the films. The extinction of normal incident light (=1−R−T, where R and T are the reflectance and transmittance, respectively) provides a measure of diffuse scattering for light with wavelength longer than the absorption edge of the film. It was found that the extinction is proportional to 1∕λ4 for λ>450nm suggesting that the mechanism for extinction at long wavelengths is Rayleigh scattering. The Rayleigh slope (diffuse scattering intensity versus 1∕λ4) increased with increasing diameter silica nanoparticles. For a given average diameter silica nanoparticle, the Rayleigh slope increased with increasing film thickness for films less than approximately 150nm thick, but did not depend on film thickness, within experimental scatter, for films that were thicker than 150nm. The results suggest that the source of Rayleigh scattering was not in the bulk of the film (such as, fluctuations in the index of refraction), but rather was primarily associated with surface roughness.
Electronic properties of H-terminated diamond in electrolyte solutions
Authors: Nebel C. E. , Rezek B. , Shin D. , Watanabe H. , Yamamoto T.
Published: February 2006

The electronic properties of hydrogen-terminated single-crystalline chemical-vapor deposited diamond in electrolyte solutions between pH 2 and 12 have been characterized by cyclic voltammetry experiments and pH-sensitive measurements using ion-sensitive field-effect transistor structures. The data show the formation of surface conductivity in diamond if immersed into electrolytes. The drain-source conductivity is pH dependent, with about 66mV∕pH. Due to strong Coulomb repulsion between positive ions in the electrolyte (hydronium ions) and the H+-surface termination of diamond, an enlarged tunneling gap is established which prevents electronic interactions between the electrolyte and diamond. This gap is the “virtual gate insulator” of diamond ion-sensitive field-effect transistor structures, with an interface resistance of about 108Ω. The application of potentials larger than the oxidation threshold of +0.7V (pH 13) to +1.6V (pH 1) gives rise to strong leakage currents and to partial surface oxidation.
Enhanced magnetoresistance in a magnetic-particle chain
Authors: Xu C. , Tong F. Q. , Ma Y. Q.
Published: February 2006

We study the magnetotransport in a magnetic-particle chain by Monte Carlo simulation. The influence of the correlations between magnetic particles on the system’s conductance is studied and an unusual tunneling magnetoresistance (TMR) effect is shown in the magnetic-particle chain. A large negative TMR at room temperature is obtained due to the strong magnetic interaction between particles. A separated hysteresis loop of TMR is observed at low temperature, and a large positive TMR peak appears at the coercive field. Both the theoretical calculation and the Monte Carlo simulation show an enhancement of TMR as the magnetic energy increases.
Theory of current-induced dynamics in molecular-scale devices
Authors: Jorn Ryan , Seideman Tamar
Published: February 2006

We develop a theoretical framework for the study of inelastic resonant transport and current-driven dynamics in molecular nanodevices. Our approach combines a Born-Oppenheimer solution of the coordinate-, energy-, and voltage-dependent self-energy with a time-dependent scattering solution of the vibrational dynamics. The formalism is applied to two classic problems in current-triggered dynamics. As a simple example of bound-bound events in the nuclear subspace we study the problem of current-induced oscillations in Au–C60–Au heterojunctions. As a well-studied example of bound-free events in the nuclear subspace we revisit the problem of scanning-tunneling-microscopy-triggered H-atom desorption from a Si(100) surface. Our numerical results are supported by a simple analytically soluble model.
All-metamaterial-based subwavelength cavities (λ∕60) for ultrathin directive antennas
Authors: Ourir Abdelwaheb , de Lustrac André , Lourtioz Jean-Michel
Published: February 2006

In this letter, we present the characterization and modeling of a metamaterial-based resonant cavity for ultrathin directive printed antennas. A planar artificial magnetic conductor is used for the two reflectors of the Fabry–Pérot-type resonant cavity. One reflector behaves as a high impedance surface, and serves as a substrate for the printed antenna. The other reflector is a partially reflective surface used as a transmitting window. The cavity is operated on subwavelength modes, the smallest cavity thickness being of the order of λ∕60. A drastic enhancement of the antenna directivity and gain is obtained over a relatively wide band from 7.5to10.1GHz, corresponding to a range of cavity thicknesses from ∼λ∕3 to ∼λ∕60. The cavity resonance is seen to be correctly predicted from the standard ray theory approach.
Photoinduced electron transfer and geminate recombination in liquids on short time scales: Experiments and theory
Authors: Goun Alexei , Glusac Ksenija , Fayer M. D.
Published: February 2006

The coupled processes of intermolecular photoinduced forward electron transfer and geminate recombination between the (hole) donor (Rhodamine 3B) and (hole) acceptors (N,N-dimethylaniline) are studied in three molecular liquids: acetonitrile, butyronitrile, and benzonitrile. Two color pump-probe experiments on time scales from ∼100fs to hundreds of picoseconds give information about the depletion of the donor excited state due to forward electron transfer and the survival kinetics of the radicals produced by forward electron transfer. The data are analyzed with a model presented previously that includes distance dependent forward and back electron transfer rates, donor and acceptor diffusion, solvent structure, and the hydrodynamic effect in a mean-field theory of through solvent electron transfer. The forward electron transfer is in the normal regime, and the Marcus equation for the distance dependence of the transfer rate is used. The forward electron transfer data for several concentrations in the three solvents are fitted to the theory with a single adjustable parameter, the electronic coupling matrix element Jf at contact. Within experimental error all concentrations in all three solvents are fitted with the same value of Jf. The geminate recombination (back transfer) is in the inverted region, and semiclassical treatment developed by Jortner [J. Chem. Phys. 64, 4860 (1976)] is used to describe the distance dependence of the back electron transfer. The data are fitted with the single adjustable parameter Jb. It is found that the value of Jb decreases as the solvent viscosity increases. Possible explanations are discussed.
Efficient polymer:polymer bulk heterojunction solar cells
Authors: Koetse Marc M. , Sweelssen Jörgen , Hoekerd Kornel T. , Schoo Herman F. M. , Veenstra Sjoerd C. , Kroon Jan M. , Yang Xiaoniu , Loos Joachim
Published: February 2006

An organic bulk heterojunction photovoltaic device based on a blend of two conjugated polymers, a polyphenylenevinylene as the electron donor and a red emitting polyfluorene as the acceptor, is presented with a maximum external quantum efficiency of 52% at 530nm and a power conversion efficiency, measured under AM1.5G, 100mW∕cm2 conditions, of 1.5% on an active area of 0.36cm2.
Nanosecond discharge in air at atmospheric pressure as an x-ray source with high pulse repetition rates
Authors: Tarasenko Victor F.
Published: February 2006

The properties of x-ray radiation and runaway electrons produced using a nanosecond volume discharge are examined. X-ray radiation at a pulse repetition rate of 3kHz was obtained time in a gas diode filled with air at atmospheric pressure. The current pulse width (FWHM) for runaway electrons generated in the gas diode was ∼100ps. A prepulse was observed on an oscilloscope trace of the main runaway electron beam current.
Extension of the dielectric tunability range in ferroelectric materials by electric bias field antiparallel to polarization
Authors: Budimir Marko , Damjanovic Dragan , Setter Nava
Published: February 2006

While in the paralectric phase of ferroelectrics, and in centrosymmetric materials in general, the dielectric tunability is symmetrical for positive and negative electric bias fields, in noncentrosymmetric materials this is not the case. By using the thermodynamic theory, and tetragonal phase of monodomain single-crystal perovskites BaTiO3 and Pb(Zr,Ti)O3, as examples, we show that the theoretical tunability region of ferroelectrics may, in principle, be considerably broadened and tunability greatly enhanced by applying electric-bias field antiparallel to polarization. Available experimental data are in qualitative agreement with the theoretical predictions.
Observation of saturable and reverse-saturable absorption at longitudinal surface plasmon resonance in gold nanorods
Authors: Elim Hendry I. , Yang Jian , Lee Jim-Yang , Mi Jun , Ji Wei
Published: February 2006

Saturable and reverse saturable absorption at longitudinal surface plasmon resonance (SPR) in gold nanorods have been observed using Z scan and transient absorption techniques with femtosecond laser pulses. At lower excitation irradiances, the wavelength dispersion of saturable absorption has been determined near the longitudinal mode of SPR with a recovery time determined to be a few ten picoseconds on the SPR resonance. With higher excitation irradiances, reverse saturable absorption occurs and becomes dominant. The underlying mechanisms are discussed. Such reversible saturable absorption makes Au NRs an ideal candidate for optical limiting applications.
Calibrating laser beam deflection systems for use in atomic force microscopes and cantilever sensors
Authors: Beaulieu L. Y. , Godin Michel , Laroche Olivier , Tabard-Cossa Vincent , Grütter Peter
Published: February 2006

Most atomic force microscopes and cantilever-based sensors use an optical laser beam detection system to monitor cantilever deflections. We have developed a working model that accurately describes the way in which a position sensitive photodetector interprets the deflection of a cantilever in these instruments. This model exactly predicts the numerical relationship between the measured photodetector signal and the actual cantilever deflection. In addition, the model is used to optimize the geometry of such laser deflection systems, which greatly simplifies the use of any cantilever-based instrument that uses a laser beam detection system.
Inducing graphite tube transformation with liquid gallium and flash discharge
Authors: Fujita Jun-ichi , Ichihashi Toshinari , Nakazawa Shotaro , Okada Satoshi , Ishida Masahiko , Ochiai Yukinori , Kaito Takashi , Matsui Shinji
Published: February 2006

We found that a flash motion of Ga droplets with Joule heating transformed an amorphous carbon pillar into multiwalled carbon nanotube. Flush discharge into the pillar quickly heated up Ga droplets, which originally incorporated in the pillar due to the fabrication technique with focused-ion-beam-induced chemical vapor deposition, inducing a flash motion of Ga toward negative electrode. Thus, the Ga droplets dug out a tunnel inside the pillar, simultaneously inducing a catalytic transformation into a multiwalled graphitic tube.
Improving work function tuning by preimplanting multiple dopants in Ni fully silicided gate
Authors: Liu J. , Kwong D. L.
Published: February 2006

A suitable work function (4.29eV) ideal for n-type transistor is realized by preimplanting multiple n-type dopants in Ni fully silicided gate. Flexible adjustments of work function in Si band gap are realized by coimplanting B with n-type dopants. Leakage current and long-term reliability are not degraded by preimplantation of multiple dopants in Ni fully silicided gate.
In situ organic coating of metal nanoparticles
Authors: Lei Haile , Tang Yongjian , Li Jun , Luo Jiangshan , Li Xibo
Published: February 2006

A physical approach has been developed to in situ coat metal nanoparticles with a nanolayer of organic hydrocarbon film. Copper nanoparticles produced by the flow-levitation method were in situ coated with hydrocarbon films via a dielectric barrier discharge. The hydrocarbon-coated Cu nanoparticles have been analyzed by transmission electron microscopy and infrared absorption to determine the size distribution and dispersion of nanoparticles as well as the chemical components and thickness of the coatings.
Seed biology implications for the maintenance and establishment of Tetratheca juncea (Tremandraceae), a vulnerable Australian species
Authors: Bellairs S. M. , Bartier F. V. , Gravina A. J. , Baker K.
Published: February 2006

Tetratheca juncea Smith is an endemic and vulnerable shrub species with apparently poor recruitment from New South Wales, Australia. Lack of understanding of seed biology limits management options for promoting survival of existing populations and recruitment of new populations. This study investigated the soil seed bank, seed viability, germination and seed dormancy. The plants release seeds in January but no seeds germinated from soil collected near the plants in September 1999, irrespective of the soil being treated by smoke, heat or fire. When sampled again in February 2001, seeds were present in the soil seed bank but were not viable. Viability testing of seeds collected from the plants determined that at least a third of the seeds being produced were viable, even after storage for 6 months. Seed germination and dormancy investigations found that the seeds germinated following exposure to smoke or scarification of the seed coat. Untreated seeds did not germinate during the preliminary study, although a proportion of untreated seeds germinated from a later seed lot. Fire management is important for promoting the survival of this species and the soil seed bank cannot be relied on for re-establishment of populations as the longevity of the soil seed bank seems to be short. Many other species of Tetratheca are also rare or threatened and this study suggests that hand-collected seeds treated with fire-related stimuli may be important for re-establishing those species and that the soil seed bank requires assessment before being relied on as a source of propagules or for maintaining a population.
Significance of post-germination buoyancy in Helmholtzia glaberrima and Philydrum lanuginosum (Philydraceae)
Authors: Prentis Peter J. , Meyers Noel M. , Mather Peter B.
Published: February 2006

Post-germination buoyancy has been proposed to reduce seedling establishment in amphibious plants if seeds germinate under flooded conditions. We tested this hypothesis in two amphibious species, Helmholtzia glaberrima (Hook.) and Philydrum lanuginosum (Banks & Sol.). We tested whether seed germination was affected by different levels of inundation, whether seedlings germinating underwater floated and whether seedlings established after prolonged floatation at rates comparable to seedling establishment in waterlogged soil. Germination underwater and in waterlogged soil was similar for both species. Seeds germinating underwater exhibited post-germination buoyancy and established at similar rates to seedlings in waterlogged soil. The results demonstrated that if seeds germinated underwater, post-germination buoyancy conferred the potential to avoid inundation and promote establishment, when waters recede and/or in areas of high soil moisture that are not submerged. More generally, this finding showed that post-germination buoyancy can promote seedling dispersal and the deposition of seedlings in conditions more favourable for establishment if seeds germinate under flooded conditions or in permanent wetlands.
Delayed autonomous selfing and inbreeding depression in the Australian annual Hibiscus trionum var. vesicarius (Malvaceae)
Authors: Seed Leahwyn , Vaughton Glenda , Ramsey Mike
Published: February 2006

The Australian annual, Hibiscus trionum var. vesicarius, produces large, showy flowers typical of an outcrossing species, yet flowers autonomously self-pollinate. We used experimental pollinations to examine self-compatibility, inbreeding depression and the efficiency and mechanism of autonomous selfing. Seed set of self- or cross-pollinated flowers did not differ, indicating that plants were fully self-compatible. Seed set following autonomous selfing varied among plants, and was 11–103% of that following hand-selfing. Autonomous selfing was delayed, and styles curved and stigmas contacted the anthers before flowers closed. Delayed selfing was facultative and curvature depended on the number of pollen grains on stigmas, with 50 or more grains preventing curvature. Both self- and cross-pollen prevented curvature. Similarly to unpollinated styles, styles that were pollinated with dead pollen curved fully, indicating that either pollen germination or pollen-tube growth prevents curvature. Within flowers, the five styles acted independently, depending on the amount of pollination that each received. Although plants exhibit a high potential for selfing, crossed progeny outperformed selfed progeny and cumulative inbreeding depression was 0.64, which is high for a self-compatible annual. Despite this high inbreeding depression, delayed selfing would be advantageous under variable pollinator conditions, providing reproductive assurance.