bluuebunny/crossref_metadata_2025 huggingface dataset:
Dual input AND gate fabricated from a single channel poly(3-hexylthiophene) thin film field effect transistor
Authors: Pinto N. J. , Pérez R. , Mueller C. H. , Theofylaktos N. , Miranda F. A.
Published: June 2006

A regio-regular poly(3-hexylthiophene) (RRP3HT) thin film transistor having a split gate architecture has been fabricated on a doped silicon/silicon nitride substrate and characterized. This device demonstrates AND logic functionality. The device functionality was controlled by applying either 0 or −10V to each of the gate electrodes. When −10V was simultaneously applied to both gates, the device was conductive (on), while any other combination of gate voltages rendered the device resistive (off). The p-type carrier charge mobility was about 5×10−4cm2∕Vs. The low mobility is attributed to the sharp contours of the RRP3HT film due to substrate nonplanarity. A significant advantage of this architecture is that AND logic devices with multiple inputs can be fabricated using a single RRP3HT channel with multiple gates.
Shape dependent thermal effects in apertured fiber probes for scanning near-field optical microscopy
Authors: Ambrosio A. , Fenwick O. , Cacialli F. , Micheletto R. , Kawakami Y. , Gucciardi P. G. , Kang D. J. , Allegrini M.
Published: June 2006

Metal-coated, “pulled,” and conically shaped fiber probes used in scanning near-field optical microscopy (SNOM) typically undergo a thermal expansion when injected with laser light, due to partial energy absorption by the metallic film. Here, we report investigations into the thermal behavior of fiber probes produced by selective chemical etching that in our experience provide high light throughputs (10−3–10−4 vs 10−6 for the pulled fibers). Unexpectedly, we find a shortening of such probes in response to “high-power” laser injection (>1mW). Thermal stress due to prolonged high-power laser injection (∼9mW at 325nm; compared to powers <1mW often used in SNOM experiments) determines permanent alterations of the probes, after which their thermomechanical behavior reverts to the commonly observed elongation in response to laser injection. Scanning electron microscopy after high-power irradiation on such probes shows partial detachment of the metallic coating near the fiber termination. This, however, does not appear to compromise the probe’s performance in terms of light confinement outside the aperture area, suggesting that the detachment only affects the coating over the fiber cladding and confirming the operational robustness of these probes. In comparison, tube-etched, conical probes display substantial damage of the coating, up to several microns from the apex, after being injected with a comparable high-power laser beam (>10mW at 633nm). Although the vertical feedback mechanism of the microscope can compensate for dilations/contractions of the probes, these findings are of general importance to the field. More specifically they are significant for the achievement of a detailed understanding of apertured-SNOM operation, for the selection and operation of near-field probes, and for preventing potential artifacts in imaging and lithography, due to uncontrolled alteration of the probe properties and/or light leakage from cracks of the opaque coating induced by thermal fatigue. In addition, our results demonstrate that it is important for probe design to also consider the probe’s thermal regime during operation, so as to prevent cracks in the functional parts of the coating and thus spurious, undesired sample illumination from regions other than the probe intended aperture.
Investigation of nanoscale composition fluctuations in InGaN using optical transmission spectroscopy and near-field scanning optical microscopy
Authors: Han B. , Wessels B. W. , Ulmer M. P.
Published: June 2006

Nanoscale composition fluctuations in InGaN alloy epitaxial layers have been investigated by optical transmission spectroscopy and near-field scanning optical microscopy. A spatial variation of the optical transmission at 488nm is observed, which is attributed to an inhomogeneous indium distribution. Both the characteristic wavelength and the normalized root mean square magnitude of the compositional fluctuations increase with increasing In concentration. The fluctuations are well described in terms of a spinodal decomposition model.
Energy transfer and frequency upconversion involving triads of Pr3+ ions in (Pr3+, Gd3+) doped fluoroindate glass
Authors: Rátiva Diego J. , de Araújo Cid B. , Messaddeq Younes
Published: June 2006

Blue and ultraviolet luminescence in (Pr3+, Gd3+) doped fluoroindate glass is studied for excitation in the red region (≈590nm). Frequency upconversion (UC) is observed due to energy transfer (ET) among three Pr3+ ions initially excited to the D21 state corresponding to the ET process D21+D21+D21→S01+H53+H53. Additionally, UC luminescence from states P7∕26 and I7∕26 of Gd3+ is observed for an excitation wavelength resonant with transitions of the Pr3+ ions. The characterization of the luminescence signals allowed to determine ET rate among the Pr3+ ions and provides evidence of interconfigurational ET between Gd3+ and Pr3+ ions.
Phase maps of microelectromechanical switches in the presence of the Casimir force and finite plasmon frequency corrections
Authors: Palasantzas G. , DeHosson J. Th. M.
Published: June 2006

In this work we explore the influence of self-affine roughness on the phase maps for microelectromechanical switches in the presence of Casimir and electrostatic forces by taking into account finite plasmon frequency corrections for plate separations smaller than the plasmon wavelength λP. The phase map depends significantly on the characteristic self-affine roughness parameters (roughness amplitude w, lateral correlation length ξ, and Hurst exponent H) leading to decreasing phase area with increasing roughening at short and/or long roughness wavelengths. The roughness influence is shown to be the prominent factor on phase maps for initial plate separations either smaller or larger than the plasmon wavelength λP.
Observation of electric field enhancement in a water streamer using Kerr effect
Authors: Sarkisov G. S. , Zameroski N. D. , Woodworth J. R.
Published: June 2006

We report the observation of the Kerr effect in a streamer head in experiments with electrical breakdown of water. The propagation of the streamer in water is associated with localized enhancement of the electric field surrounding the streamer head. Polarizational measurements show that the electric field around the streamer head is estimated to be 1.7–2.2MV∕cm, which is approximately six times higher than the maximum interelectrode electric field of 0.37MV∕cm. Electrostatic simulations support the experimental data.
Matrix effects on the magnetic properties of γ-Fe2O3 nanoparticles dispersed in a multiblock copolymer
Authors: Guskos N. , Glenis S. , Likodimos V. , Typek J. , Maryniak M. , Roslaniec Z. , Kwiatkowska M. , Baran M. , Szymczak R. , Petridis D.
Published: June 2006

The magnetic properties of γ-Fe2O3 ferrimagnetic nanoparticles embedded in a multiblock poly(ether-ester) copolymer have been investigated by static magnetization and ferromagnetic resonance (FMR) measurements at two different dispersion states. Significant variation of the magnetic response is identified below T≈120K, most pronounced in the marked resonance field shift of the FMR spectra, independently of the dispersion state of the nanocomposites. This behavior correlates favorably with the dynamic relaxation of the copolymer, indicating a matrix freezing effect that is attributed to the magnetoelastic coupling of the oxide nanoparticles with the surrounding polymer. At low temperatures, the dc magnetization and FMR measurements vary considerably for the two nanocomposites, indicating essential differences in their ground state, related to the different morphology of the samples and the concomitant variation of interparticle interactions.
Effect of gate-drain/source overlap on the noise in 90nm N-channel metal oxide semiconductor field effect transistors
Authors: Srinivasan R. , Bhat Navakanta
Published: June 2006

In this paper, we have studied the effect of gate-drain/source overlap (LOV) on the drain channel noise and induced gate current noise (SIg) in 90nm N-channel metal oxide semiconductor field effect transistors using process and device simulations. As the change in overlap affects the gate tunneling leakage current, its effect on shot noise component of SIg has been taken into consideration. It has been shown that “control over LOV” allows us to get better noise performance from the device, i.e., it allows us to reduce noise figure, for a given leakage current constraint. LOV in the range of 0–10nm is recommended for the 90nm gate length transistors, in order to get the best performance in radio frequency applications.
Resonant modes in continuous metallic grids over ground and related spatial-filtering applications
Authors: Siddiqui Omar F. , Eleftheriades George V.
Published: June 2006

We use the theory of periodic structures, full-wave electromagnetic, and microwave circuit simulations to explain the resonant modes that propagate in metallic grids having rectangular unit cells constructed over a ground plane. We show that these metallic grids can support two types of resonant modes that have rectangular and hyperbolic isofrequency dispersion contours. By exploiting the spatial dispersion properties of these modes, a microwave 3GHz∕6GHz harmonic splitter and a highly selective 5.8GHz∕6.2GHz diplexer are designed and simulated. Furthermore, we provide experimental results for the diplexer and for the harmonic splitter, synthesized in microstrip technology. The proposed metallic grids utilize continuous unloaded transmission-line segments thus leading to spatial-filtering devices that are easy to fabricate and are scalable to terahertz frequencies and beyond.
Electrical properties of single-wall carbon nanotube-polymer composite films
Authors: Kymakis Emmanuel , Amaratunga Gehan A. J.
Published: June 2006

The electrical properties of single-walled carbon nanotubes (SWNTs) embedded in a poly(3-octylthiophene) matrix have been investigated as a function of SWNT concentration. The electrical conductivity and its temperature dependence were measured as a function of the SWNT concentration. As the nanotube concentration increased from 0to20wt%, the conductivity of the resulting films is dramatically increased by six orders of magnitude. The enhancement in conductivity can be explained by means of a three dimension simple percolation path theory, resulting in an estimated threshold of 4wt%. The temperature dependence of the SWNT conductivity mat obeys a three-dimensional variable range hopping. In contrast, the polymer-nanotube composite conductivity follows a fluctuation induced tunneling model. The main divergence is that in the polymer-nanotube composite, the nanotubes are coated with polymer, which acts a barrier in bundle to bundle hopping.
Equivalent ion temperature in Ta plasma produced by high energy laser ablation
Authors: Torrisi L. , Gammino S. , Andó L. , Laska L. , Krasa J. , Rohlena K. , Ullschmied J. , Wolowski J. , Badziak J. , Parys P.
Published: June 2006

High energy laser, 400ps pulse duration, irradiating heavy targets in vacuum produce intense plasma and generate emission of various energetic ion groups. The ion intensity is high along the normal to the irradiated target surface and high charge state and high velocity ions are produced. The characteristics of the ion streams were investigated by using an electrostatic ion energy analyzer and different ion collectors were placed at various angles with respect to the target normal. The ion energy distribution as a function of the ion charge state was measured and the comparison of the properties of different ion groups generated by laser beams at two different energies was carried out. Measurements point out that five ion groups or more can be generated by the laser interaction with the preformed plasma, with different “equivalent ion temperatures.” Slow, thermal, fast, and very fast ions follow a Boltzmann-like distribution; the equivalent temperatures of different ion groups were estimated to reach values up to about 80keV.
Molecular beam epitaxy growth of exchange-biased PtMn∕NiFe bilayers with a spontaneously ordered PtMn layer
Authors: Choi Y. S. , Petford-Long A. K. , Ward R. C. C. , Fan R. , Goff J. P. , Hase T. P. A.
Published: June 2006

We report the direct epitaxial growth of equiatomic ordered antiferromagnetic PtMn layers by molecular beam epitaxy. Such layers are used in giant magnetoresistance spin valve sensors as the antiferromagnetic pinning layer. Structural characterization and phase identification confirmed the spontaneous formation of the chemically ordered face-centered-tetragonal (L10) phase of PtMn with about 87.4% ordering. Based on the antiferromagnetic PtMn layer, we prepared exchange-biased PtMn∕NiFe bilayers with various PtMn thicknesses. The exchange anisotropy field of the bilayer with NiFe grown on PtMn stabilizes at about 50Oe beyond a PtMn thickness of 15nm. Although the exchange anisotropy field is small compared to that of the polycrystalline system, the antiferromagnetic domain structure is stable over repetitive external magnetic field cycling and no training effect is observed.
Influence of bias stressing and irradiation on poly-three-hexylthiophene based field effect transistors
Authors: Devine R. A. B.
Published: June 2006

Preliminary measurements of positive and negative bias stress and radiation effects in poly-three-hexylthiophene based field effect transistors are reported. Radiation up to 0.5Mrad (SiO2) is found to have little effect on channel carrier mobility though bias stressing does. A strong positive bias stress induced positive threshold voltage shift is suppressed when devices are simultaneously irradiated. There is no evidence for significant radiation effects in the organic semiconductor. Recovery effects are observed following removal of bias stress and radiation.
Origin of uniaxial magnetic anisotropy in epitaxial InMnAs thin films
Authors: Chiu P. T. , May S. J. , Wessels B. W.
Published: June 2006

The in-plane magnetic anisotropy of (001) oriented InMnAs epitaxial films grown on InAs and GaSb were measured by magnetometry and the longitudinal magneto-optic Kerr effect. InMnAs epitaxial layers when grown on InAs exhibit a significant in-plane uniaxial anisotropy field of −0.66kG at room temperature. The uniaxial magnetic anisotropy is attributed to differences in Mn atomic density along the [110] and [1¯10] directions, resulting from the uniaxial surface symmetry of the InAs (001) surface. The asymmetry of the Mn density is preserved in InMnAs films grown on InAs by a two-dimensional growth mechanism. However, disorder as evidenced by increasing surface pitting perturbs the growth from an ideal layer-by-layer mechanism. This results in a decrease in the magnitude of the uniaxial anisotropy field with increasing thickness between 320–630nm. In contrast, InMnAs films grown on GaSb are magnetically isotropic for a thickness range of 50–420nm. The isotropic behavior is attributed to three-dimensional island growth of films grown on GaSb that eliminates long range order.
Dielectric and tunable properties of K-doped Ba0.6Sr0.4TiO3 thin films fabricated by sol-gel method
Authors: Sun Xiaohua , Zhu Bailin , Liu Tao , Li Meiya , Zhao Xing-Zhong , Wang Danyang , Sun Chengliang , Chan Helen L. W.
Published: June 2006

Ba 0.6 Sr 0.4 Ti O 3 (BST) thin films doped by K (BSTK) from 1to20mol% were fabricated by sol-gel method on a Pt∕TiO2∕SiO2∕Si substrate. Thermal evolutionary process of the Ba0.6Sr0.4TiO3 and (Ba0.6Sr0.4)0.95K0.05TiO3 dry gel was carried out by thermogravimetry and differential thermal analysis system. The structure and surface morphology of BST thin films were investigated as functions of K concentration by x-ray diffraction and atomic force microscopy. The dielectric measurements were conducted on metal-insulator-metal capacitors at the frequency from 100Hzto1MHz. The K concentration in BST thin films has a strong influence on the material properties including surface morphology and dielectric and tunable properties. The grain size, surface root-mean-square roughness, dielectric constant, dissipation factor, and tunability all increased with increasing K content up to 7.5mol% and then decreased with increasing K content from 7.5to20mol% in the BSTK thin films at 1MHz. The effects of K doping on the microstructure and dielectric and tunable properties of Ba0.6Sr0.4TiO3 thin films were analyzed. The (Ba0.6Sr0.4)92.5%K7.5%TiO3 thin film exhibited the highest dielectric constant of 1040 and the largest tunability of 73.6%. The dielectric constant, dielectric loss, and tunability of K-doped BST thin films with the optimal K content of 5mol% were about 971, 0.023, and 69.96%, respectively. In addition, its figure of merit showed a maximum value of approximately 28.52.
Theory of Q-Controlled dynamic force microscopy in air
Authors: Hölscher H. , Ebeling D. , Schwarz U. D.
Published: June 2006

The theory of dynamic force microscopy in air is developed with respect to the application of the Q-Control technique, which allows to increase or decrease the effective Q factor of the cantilever via an active external feedback. Analytical as well as numerical approaches are applied to solve the equation of motion describing the cantilever dynamics with and without Q-Control in the presence of a model tip-sample interaction force. Based on this analysis, the characteristics of Q-Controlled dynamic force microscopy are compared to conventional dynamic force microscopy carried out in amplitude modulation mode without active Q-Control (“tapping mode”). In the case of negligible tip-sample interaction (i.e., with the tip “far” from the surface), the theory describes how Q-Control alters the shape of the resonance curves of the cantilever by modifying the effective Q factor and shifting the resonance peak. Explicit consideration of tip-sample forces then permits insight into the imaging properties of Q-Controlled dynamic force microscopy. In particular, it is found that an increased Q factor prevents the oscillating cantilever to jump into a repulsive imaging regime during tip-sample approach, which often occurs during conventional tapping mode imaging in air. The analysis reveals in detail which parts of the tip-sample force curve are contributing to the contrast formation for the different imaging conditions if plotted as a function of the tip-sample distance. Based on these findings, we conclude that the restriction of the maximal tip-sample force to specific parts of the attractive regime, which is triggered by the activation of the Q-Control feedback, is the main reason for the enhanced imaging quality reported in several experimental studies if compared to conventional tapping mode imaging without Q-Control.
Spin relaxation under identical Dresselhaus and Rashba coupling strengths in GaAs quantum wells
Authors: Cheng J. L. , Wu M. W.
Published: June 2006

Spin relaxation under identical Dresselhaus and Rashba coupling strengths in GaAs quantum wells is studied in both the traditional collinear statistics, where the energy spectra do not contain the spin-orbit coupling terms, and the helix statistics, where the spin-orbit couplings are included in the energy spectra. We show that there is only marginal difference between the spin relaxation times obtained from these two different statistics. We further show that with the cubic term of the Dresselhaus spin-orbit coupling included, the spin relaxation time along the (1,1,0) direction becomes finite, although it is still much longer than that along the other two perpendicular directions. The properties of the spin relaxation along this special direction under varied conditions are studied in detail.
Solid phase epitaxy of SrTiO3 on (Ba,Sr)O∕Si(100): The relationship between oxygen stoichiometry and interface stability
Authors: Norga G. J. , Marchiori C. , Rossel C. , Guiller A. , Locquet J. P. , Siegwart H. , Caimi D. , Fompeyrine J. , Seo J. W. , Dieker Ch.
Published: June 2006

Key aspects of the growth process of epitaxial SrTiO3 with crystalline interface on silicon are outlined. An important step in this process is the solid phase epitaxy in ultrahigh vacuum of amorphous SrTiO3 on top of a few monolayer thick, low-temperature grown, epitaxial (Ba,Sr)O∕Si(100) template. Insufficient oxygen supply during the SrTiO3 deposition step causes the formation of amorphous alkaline-earth silicates and TiSix at the Si∕epitaxial oxide interface during ultrahigh vacuum annealing. Performing SrTiO3 deposition in excess O2, this interfacial reaction is suppressed, and a metal-insulator-semiconductor capacitance equivalent to 0.5nm of SiO2 is obtained for a 10unit cell SrTiO3∕1unit cell (Ba,Sr)O∕p-Si(100) stack.
Injection and transport processes in organic light emitting diodes based on a silole derivative
Authors: Huby N. , Hirsch L. , Wantz G. , Vignau L. , Barrière A. S. , Parneix J. P. , Aubouy L. , Gerbier P.
Published: June 2006

This paper reports on charge injection and transport in electroluminescent devices based on a silole derivative 1,1-dimethyl-2,5-bis(p-2,2′-dipyridylaminophenyl)-3,4-diphenylsilole (DMPPS). The devices are composed of tin doped indium oxide (In2O3:Sn or I TO)/poly(3,4-ethylene dioxythiophene doped with poly(styrene sulfonate)/DMPPS/metal. Current-voltage and luminance-voltage characteristics are first performed as a function of the electron injection barrier height and of the organic layer thickness. The voltage dependence of current and luminance varies with the metal cathode, i.e., Ca, Al, Cu, and Au. The effect of the DMPPS thickness in a double carrier device shows that electrons predominate and are bulk limited. An accurate investigation is carried out as a function of temperature for hole-only and bipolar devices, i.e., with gold and calcium cathodes. Hole-only devices (with Au cathode) exhibit an Ohmic behavior for low voltages. A hopping mechanism (thermally assisted tunnel transfer between localized states) agrees with experimental data, since activation energy is found close to 50meV. The electron transfer limitation is located at the DMPPS/cathode interface and the Fowler-Nordheim mechanism is qualitatively consistent with experimental data at high voltages. With a Ca cathode, electron conduction is preponderant and is bulk limited. A power dependence J∝Vm with m>2 is consistent with the model of trap charge limited conduction. The total electron trap density was estimated to be 2×1018cm−3.
Effect of Ce substitution on magnetic and dielectric properties of BiMn2O5
Authors: Sun Z. H. , Cheng B. L. , Dai S. , Jin K. J. , Zhou Y. L. , Lu H. B. , Chen Z. H. , Yang G. Z.
Published: June 2006

Polycrystalline BiMn2O5 and Bi0.9Ce0.1Mn2O5 have been prepared by a solid-state reaction. A crystalline structure study shows that all x-ray diffraction patterns can be indexed successfully in an orthorhombic phase. Substitution of Ce for Bi ions induced in the unit cell a slight distortion and enlargement. Magnetic and dielectric measurements all reveal that Ce substitution induces great effects: BiMn2O5 is in the antiferromagnetic (AFM) phase with a Néel transition temperature at about 42K, whereas Bi0.9Ce0.1Mn2O5 is in the ferromagnetic (FM) phase with a Curie temperature at about 46K. The permittivity of Bi0.9Ce0.1Mn2O5 is much higher than that of BiMn2O5, and two dielectric relaxation peaks have been observed in the former, instead of one dielectric loss peak as observed in BiMn2O5. The valent state of Ce ions has been analyzed mainly as tetravalent by the core-level spectrum of Ce 3d from x-ray photoemission spectroscopy. The possible mechanisms of the magnetic transition from AFM to FM and the peculiar dielectric relaxation peaks induced by Ce substitution have been discussed in detail.
Nanoscale morphology and photoemission of arsenic implanted germanium films
Authors: Petö G. , Khanh N. Q. , Horváth Z. E. , Molnár G. , Gyulai J. , Kótai E. , Guczi L. , Frey L.
Published: June 2006

Germanium films of 140nm thickness deposited onto Si substrate were implanted with 70keV arsenic ions with a dose of 2.5×1014cm−2. The morphology of the implanted films was determined by Rutherford backscattering and cross-sectional transmission electron microscopy. Concentration of oxygen and carbon impurities and their distribution in the implanted layer were detected by secondary-ion-mass spectroscopy and nuclear reaction analysis using the O16(He4,He4)O16 reaction. The depth dependence of the valence band density of states was investigated by measuring the energy distribution curve of photoelectrons using Ar ion etching for profiling. The morphology of As implanted film was dominated by nanosized (10–100nm) Ge islands separated by empty bubbles at a depth of 20–50nm under the surface. At depth ranges of 0–20 and 70 to a measured depth of 140nm, however, morphology of the as-evaporated Ge film was not modified. At a depth of 20–50nm, photoelectron spectra were similar to those obtained for Ge amorphized with heavy ion (Sb) implantation [implantation induced (I.I.) a-Ge]. The depth profile of the morphology and the photoemission data indicate correlation between the morphology and valence band density of states of the ion I.I. a-Ge. As this regime was formed deep in the evaporated film, i.e., isolated from the environment, any contamination, etc., effect can be excluded. The depth distribution of this I.I. a-Ge layer shows that the atomic displacement process cannot account for its formation.
Modeling of nanosecond laser ablation with vapor plasma formation
Authors: Wu Benxin , Shin Yung C.
Published: June 2006

A thermal model for nanosecond pulsed laser ablation is developed, where the heat conduction equation in the target and the gas dynamic equations in the vapor and ambient gas phase are coupled through the Knudsen layer (KL) relations for evaporation/recondensation at the target-vapor interface. The plasma formation and laser-plasma interactions are simulated in the model, which are found to have a significant effect on the laser-induced evaporation process. The shielding effect of the plasma reduces the laser energy reaching the target surface and therefore decreases the surface temperature, and the laser energy deposition in the plasma contributes to the increase of the vapor pressure above the KL. All of these will make the transition earlier from sonic evaporation stage to the subsonic evaporation and then to the recondensation stage, and therefore decrease the laser-induced evaporation depth. The simulation results are compared with experimental data for the plasma transmissivity, plasma front locations and velocities, laser ablation depth, and average plasma temperatures, and reasonably good agreements are obtained. This model is valid when the phase explosion does not occur, that is, when the target surface temperature does not reach or exceed the target material critical temperature.
Dependence of device performance on carrier escape sequence in multi-quantum-well p-i-n solar cells
Authors: Alemu A. , Coaquira J. A. H. , Freundlich A.
Published: June 2006

This work is a study relating device performance and carrier escape sequence in a large set of InAsP∕InP p-i-n multi-quantum-well solar cells. The devices encompass nearly identical i-region thickness and built-in electric field and present similar absorption threshold energies. The escape sequence of the first confined electron-to-conduction band continuum and heavy/light holes-to-valence band continuum is extracted from the photoluminescence versus temperature analysis and by comparing the measured activation energies to calculated hole/electron well depths and thermionic escape times. Light holes, as expected for most III-V nanostructure systems, are found to be the fastest escaping carriers in all samples. The escape of electrons prior to heavy holes is shown to be a prerequisite to prevent severe open circuit voltage degradation. A possible explanation of the origin of this effect is offered. InP∕InAsP multi-quantum-well solar cells with high built-in electric field and fast electronic escape time display better open circuit voltage and performance.
Spin disorder and magnetic anisotropy in Fe3O4 nanoparticles
Authors: Lima E. , Brandl A. L. , Arelaro A. D. , Goya G. F.
Published: June 2006

We have studied the magnetic behavior of dextran-coated magnetite (Fe3O4) nanoparticles with median particle size ⟨d⟩=8nm. Magnetization curves and in-field Mössbauer spectroscopy measurements showed that the magnetic moment MS of the particles was much smaller than the bulk material. However, we found no evidence of magnetic irreversibility or nonsaturating behavior at high fields, usually associated to spin canting. The values of magnetic anisotropy Keff from different techniques indicate that surface or shape contributions are negligible. It is proposed that these particles have bulklike ferromagnetic structure with ordered A and B sublattices, but nearly compensated the magnetic moments. The dependence of the blocking temperature with frequency and applied fields, TB(H,ω), suggests that the observed nonmonotonic behavior is governed by the strength of interparticle interactions.
Experimental investigation of temperature and volume fraction variations on the effective thermal conductivity of nanoparticle suspensions (nanofluids)
Authors: Li Calvin H. , Peterson G. P.
Published: June 2006

An experimental investigation was conducted to examine the effects of variations in the temperature and volume fraction on the steady-state effective thermal conductivity of two different nanoparticle suspensions. Copper and aluminum oxide, CuO and Al2O3, nanoparticles with area weighted diameters of 29 and 36nm, respectively, were blended with distilled water at 2%, 4%, 6%, and 10% volume fractions and the resulting suspensions were evaluated at temperatures ranging from 27.5to34.7°C. The results indicate that the nanoparticle material, diameter, volume fraction, and bulk temperature, all have a significant impact on the effective thermal conductivity of these suspensions. The 6% volume fraction of CuO nanoparticle/distilled water suspension resulted in an increase in the effective thermal conductivity of 1.52 times that of pure distilled water and the 10% Al2O3 nanoparticle/distilled water suspension increased the effective thermal conductivity by a factor of 1.3, at a temperature of 34°C. A two-factor linear regression analysis based on the temperature and volume fraction was applied and indicated that the experimental results are in stark contrast to the trends predicted by the traditional theoretical models with respect to both temperature and volume fraction. The available models are reviewed and the possible reasons for the unusually high effective thermal conductivity of nanofluids are analyzed and discussed.
Capture barrier for DX centers in gallium doped Cd1−xMnxTe
Authors: Placzek-Popko Ewa , Nowak Anna , Szatkowski Jan , Sieranski Kazimierz
Published: June 2006

We report on the capture barrier for the gallium related DX center in Cd0.99Mn0.01Te. In order to determine the barrier height, two methods were applied: an analysis of the persistent photoconductivity decay and the optical deep level transient spectroscopy technique. Over a range of temperatures varying from 77to105K, the capture barrier height, deduced from the decay time constants of photoconductivity, has been found to be equal to 0.22eV. An apparent hole trap, observed with the optical deep level transient spectroscopy, was attributed to the thermally activated capture cross section of a DX center with a 0.23eV capture barrier. The obtained data are close to 0.21eV, the value of the capture barrier which we determined earlier with the help of the deep level transient spectroscopy method.
High-efficiency generation of both forward and backward optical phase conjugations in one-dimensional nonlinear photonic crystals
Authors: Xie Ping , Zhang Zhao-Qing
Published: June 2006

By using only one pump beam, we show theoretically that both forward and backward phase conjugations can be generated simultaneously and efficiently in one-dimensional χ(3)-nonlinear photonic crystals of only a few micrometers in size. The photonic crystal considered here is a CdS∕SiO2 superlattice. By using realistic optical parameters in our calculations, we find that the phase-conjugate generation efficiency can be enhanced by four to five orders of magnitude as compared with that in a homogeneous CdS medium of the same nonlinearity and length. The enhancement is caused by multiple reflections as well as strong field localization introduced by the shifted band-edge state, the gap localized state, or the shifted defect state.
Residual internal stress in partially crystallized photothermorefractive glass: Evaluation by nuclear magnetic resonance spectroscopy and first principles calculations
Authors: Zwanziger J. W. , Werner-Zwanziger U. , Zanotto E. D. , Rotari E. , Glebova L. N. , Glebov L. B. , Schneider J. F.
Published: June 2006

In some circumstances, the mechanical and optical properties of multiphase brittle materials strongly depend on the level of residual micromechanical stresses that arise upon cooling due to thermal and elastic mismatch between the constituent phases. Here we study the residual internal stress in a partially crystallized oxyfluoride glass, best known as photothermorefractive (PTR) glass. This material is composed of a glass matrix with embedded nanosize sodium fluoride (NaF) crystals. Using both the Selsing model and solid-state nuclear magnetic resonance in combination with first principles calculations we found that the crystals are under a tensile stress field of approximately 610–800MPa. For this stress level the estimated critical crystal diameter for spontaneous cracking is about 2300–1900nm, which greatly exceeds the observed diameters of 7–35nm. Hence no spontaneous cracking is expected for the PTR glasses. First principles calculations indicate that the stress induced change of the refractive index of the NaF crystals is about −0.08%, which agrees with the observed refractive index changes.
The hysteresis curves of nanoparticles obtained by Monte Carlo method based on the Stoner-Wohlfarth model
Authors: Du H. F. , Du A.
Published: June 2006

A modified Metropolis method is performed to simulate the hysteresis of three-dimensional noninteracting nanoparticles. Unlike the method of Xu et al. [J. Appl. Phys. 89, 3403 (2001)] and that of Jalil [J. Appl. Phys. 93, 8050 (2003)], in which the particles are always restricted to stay in the minimum energy state in the former, or are allowed to fluctuate among five states around the minimum energy state in the latter, we completely consider the random thermal fluctuation of the magnetic moment in three-dimensional space at low temperature. Hence our method is more accurate compared with physical reality. The simulation results show that the hysteresis loops appear over a wider range of temperature compared with those of Xu et al. The remanence and coercivity of the system decrease as temperature increases and disappear at high temperature.
Laser damage threshold studies on urea L-malic acid: A nonlinear optical crystal
Authors: Vanishri S. , Bhat H. L. , Deepthy A. , Nampoori V. P. N. , de Matos Gomes E. , Belsley M.
Published: June 2006

A detailed study of surface laser damage performed on a nonlinear optical crystal, urea L-malic acid, using 7ns laser pulses at 10Hz repetition rate from a Q-switched Nd:YAG laser at wavelengths of 532 and 1064nm is reported. The single shot and multiple shot surface laser damage threshold values are determined to be 26.64±0.19 and 20.60±0.36GWcm−2 at 1064nm and 18.44±0.31 and 7.52±0.22GWcm−2 at 532nm laser radiation, respectively. The laser damage anisotropy is consistent with the Vickers mechanical hardness measurement performed along three crystallographic directions. The Knoop polar plot also reflects the damage morphology. Our investigation reveals a direct correlation between the laser damage profile and hardness anisotropy. Thermal breakdown of the crystal is identified as the possible mechanism of laser induced surface damage.
Field-dependent behavior of a magnetic force microscopy tip probed by means of high coercive nanomagnets
Authors: Rastei M. V. , Abes M. , Bucher J. P. , Dinia A. , Pierron-Bohnes V.
Published: June 2006

Single domain uniformly magnetized CoPt dots are used to probe magnetic force microscopy (MFM) tip characteristics. Magnetic contrasts obtained in MFM experiments are exploited to get the tip hysteresis loop in the influence of the dot. Coercivity, remnant magnetization, as well as details about the magnetization reversal of the tips are obtained. Well defined dot geometries allow a quantitative determination of the stray field of the dots acting on the tip. The tip’s hysteresis loop refers to an effective tip height, involved in the imaging, which is much larger than the extension of a magnetic domain of the CoCr tip coating. When MFM tips and sample dots have widely different characteristics, it is possible to get separate magnetic information, such as hysteresis loops, on both tip and sample, depending on the magnitude of the out-of-plane external field.
Effect of solvents on the chemical and physical properties of ionic polymer-metal composites
Authors: Nemat-Nasser Sia , Zamani Shahram , Tor Yitzhak
Published: June 2006

Ionic polymer-metal composites (IPMCs) consist of a perfluorinated ionomer membrane (usually Nafion® or Flemion®) plated on both faces with a noble metal such as gold or platinum and neutralized with a certain amount of counterions that balance the electrical charge of anions covalently fixed to the backbone ionomer. IPMCs are electroactive materials with potential applications as actuators and sensors. Their electrical-chemical-mechanical response is dependent on the cations used, the nature and the amount of solvent uptake, the morphology of the electrodes, the composition of the backbone ionomer, the geometry and boundary conditions of the composite element, and the magnitude and spatial and time variation of the applied potential. With water as the solvent, the applied electric potential must be limited to less than 1.3V at room temperature, to avoid electrolysis. Moreover, water evaporation in open air presents additional problems. These and related factors limit the application of IPMCs with water as the solvent. We present the results of a series of tests on both Nafion- and Flemion-based IPMCs with ethylene glycol, glycerol, and crown ethers as solvents. IPMCs with these solvents have greater solvent uptake and can be subjected to relatively high voltages without electrolysis. They can be actuated in open air for rather long time periods, and at low temperatures. They may be good actuators when high-speed actuation is not necessary. In addition, their slow response in open air allows direct observation of the physical characteristics of the cathode and anode surfaces of a cantilever during actuations. This can provide additional clues for unraveling the underpinning micromechanisms of their actuation. Remarkably, solvents are found to have profound effects on the nature of the IPMCs’ actuation. For example, Nafion-based IPMCs in Li+ form show very small back relaxation when hydrated, but extensive back relaxation with all other solvents that we have considered. On the other hand, the same membrane in the K+ form has extensive back relaxations when solvated with water, or ethylene glycol, or glycerol, but none with 18-Crown-6.
Improving fatigue resistance of Pb(Zr,Ti)O3 thin films by using PbZrO3 buffer layers
Authors: Mensur Alkoy Ebru , Uchiyama Kiyoshi , Shiosaki Tadashi , Alkoy Sedat
Published: June 2006

Ferroelectric Pb(Zr0.52Ti0.48)O3 (PZT) thin films with PbZrO3 (PZ) buffer layers were prepared on Pt(111)∕Ti∕SiO2∕Si(100) substrates using a hybrid rf magnetron sputtering and sol-gel process. Texture of PZT films was found to depend on Pb content of PZ buffer layers. Buffered PZT films displayed comparable ferroelectric properties (2Pr=38–53μC∕cm2,2Ec=136–170kV∕cm) with unbuffered PZT. Asymmetric leakage current and fatigue behavior with superior fatigue resistance was observed in PZ buffered PZT compared to unbuffered films. PZ buffer layers were found to affect crystallization and texture of PZT, and act as a capacitive interface layer possibly blocking charge injection from electrodes.
Reevaluation of the mechanism for ultrananocrystalline diamond deposition from Ar∕CH4∕H2 gas mixtures
Authors: May P. W. , Harvey J. N. , Smith J. A. , Mankelevich Yu. A.
Published: June 2006

Various mechanisms for the growth and renucleation of ultrananocrystalline diamond (UNCD) films are discussed and evaluated in the light of experimental and theoretical evidences in recent publications. We propose that the most likely model for UNCD growth is that where most of the diamond is formed via a similar mechanism to that of microcrystalline diamond films, i.e., gas phase H atoms abstracting surface hydrogens, followed by a CHx, x=0–3, addition. Calculations of the gas composition close to the substrate surface in the microwave plasma reactor for both the microcrystalline diamond and the UNCD growth, at substrate temperatures of 1073 and 673K, suggest that CH3 and C atoms are the most likely precursors for the growth of UNCD. However, the deposition is interrupted by an event which prevents the smooth growth of a continuous layer, and instead creates a surface defect which changes the growth direction and acts as a renucleation site. The possible nature of this event is discussed in detail. Using estimates for reaction rates of various species (including H atoms, Ar* metastables, Ar+ and ArH+ ions) on the diamond surface, a number of mechanisms are discussed and discounted. We propose that the most likely causes for the renucleation required for the UNCD growth are (i) the attachment of C1 species (especially C atoms) followed by local surface restructuring, (ii) the reduction of the efficiency of the β-scission reaction resulting in an increase in the number of long-chained hydrocarbons on the surface, or (iii) a combination of these two processes.
Principles and mechanisms of gas sensing by GaN nanowires functionalized with gold nanoparticles
Authors: Dobrokhotov Vladimir , McIlroy D. N. , Norton M. Grant , Abuzir A. , Yeh W. J. , Stevenson Ian , Pouy R. , Bochenek J. , Cartwright M. , Wang Lidong , Dawson J. , Beaux Miles , Berven Chris
Published: June 2006

Electrical properties of a chemical sensor constructed from mats of GaN nanowires decorated with gold nanoparticles as a function of exposure to Ar, N2, and methane are presented. The Au nanoparticle decorated nanowires exhibited chemically selective electrical responses. The sensor exhibits a nominal response to Ar and slightly greater response for N2. Upon exposure to methane the conductivity is suppressed by 50% relative to vacuum. The effect is fully reversible and is independent of exposure history. We offer a model by which the change in the current is caused by a change in the depletion depth of the nanowires, the change in the depletion depth being due to an adsorbate induced change in the potential on the gold nanoparticles on the surface of the nanowires.
Elastic properties of the bcc structure of bismuth at high pressure
Authors: Gutiérrez Gonzalo , Menéndez-Proupin Eduardo , Singh Anil K.
Published: June 2006

First-principles calculations of the single-crystal elastic constants of the body-centered cubic phase of bismuth are carried out in the pressure range of 31–191GPa. The calculations are performed in the framework of density functional theory with generalized gradient approximation using a plane wave basis set and pseudopotential scheme. These results confirm the conclusions drawn in a recent study based on the x-ray diffraction data under nonhydrostatic compression. The calculated unit cell volumes and the bulk moduli as a function of pressure agree well with the experimentally measured values.
Magnetic force microscopy sensors using iron-filled carbon nanotubes
Authors: Winkler Andreas , Mühl Thomas , Menzel Siegfried , Kozhuharova-Koseva Radinka , Hampel Silke , Leonhardt Albrecht , Büchner Bernd
Published: June 2006

Probes for magnetic force microscopy (MFM) were prepared by pinning iron-filled multiwall carbon nanotubes to conventional scanning force microscopy probes. These nanotube MFM probes reveal a great potential for high spatial resolution of both topography and magnetic stray field. The ends of the high aspect ratio iron nanowires within the nanotubes can be considered as stationary effective magnetic monopole moments which opens the possibility of quantitative stray field measurements in a straightforward manner. The carbon shells around the iron nanowires provide wear resistance and oxidation protection.
Cyclotron effective mass and Landé g factor in GaAs–Ga1−xAlxAs quantum wells under growth-direction applied magnetic fields
Authors: de Dios-Leyva M. , Porras-Montenegro N. , Brandi H. S. , Oliveira L. E.
Published: June 2006

We have performed a theoretical study of the cyclotron effective mass and electron effective Landé g‖ factor in semiconductor GaAs–Ga1−xAlxAs quantum wells under an applied magnetic field parallel to the growth direction of the quantum well. The theoretical approach is within the nonparabolic and effective-mass approximation and via an Ogg-McCombe effective Hamiltonian [Proc. Phys. Soc. London 89, 431 (1969); Phys. Rev. 181, 1206 (1969)] for the electron in the conduction band of the GaAs–Ga1−xAlxAs heterostructure, which allows a unified treatment of both the cyclotron mass and g‖ factor. Calculations are performed for different widths of the GaAs–Ga1−xAlxAs quantum wells and as functions of the applied magnetic field, with results in very good agreement with reported experimental measurements of the electron cyclotron effective mass and g‖ factor.
Work function of (8,0) single-walled boron nitride nanotube at the open tube end
Authors: Zhang Jia , Loh Kian Ping , Deng Mu , Sullivian Michael B. , Zheng Jianwei , Wu Ping
Published: June 2006

The effects of hydrogen (H) and fluorine (F) chemisorption on the work function (Φ) and polarization field of (8,0) boron nitride nanotubes (BNNT) have been investigated using periodic density functional theory. The binding energies of coadsorbed H and F atoms on B and N sites in BNNT are much higher than that of single adsorption case. The work function at the open tube end can be affected by adsorption on the tube wall due to changes in the macroscopic polarization field along the tube, as well as the shifting of the Fermi level. Among the various adsorption patterns on the wall, we found that adsorption of H,F pairs, or H,H pairs, on the tube wall to form a continuous, armchair chain along the tube axis, produces strong enhancement of the intrinsic polarization field. With wall adsorption of (H,H) pairs, the Φ at the B terminal becomes 3.25eV, compared with 5.40eV of the pristine tube, and with (H,F) pairs wall adsorption, the Φ at the B terminal is reduced to 3.56eV.