bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of vacancies on the structural and relaxor properties of (Sr,Ba,Na)Nb2O6
Authors: Belous Anatolii , V’yunov Oleg , Mishchuk Dmitrii , Kamba Stanislav , Nuzhnyy Dmitry
Published: July 2007

It has been shown that the aliovalent substitution of sodium ions for strontium or barium ions in strontium barium niobate relaxor ferroelectric causes a decrease in vacancy content of the samples and a linear variation of unit cell volume with sodium concentration. The variation of c lattice parameter with sodium concentration is determined by deformations of the NbO6 oxygen octahedrons in the 001 direction. Dielectric studies carried out in a wide frequency range showed that the decrease in the vacancy content of the Sr0.6−xBa0.4Na2xNb2O6 and Sr0.6Ba0.4−yNa2yNb2O6 results in suppression of relaxor ferroelectric properties, which was manifested in both low-frequency and submillimeter region. Simultaneously, rise of the ferroelectric phase transition temperature was observed with increasing sodium concentration.
Tunable surface plasmon mediated emission from semiconductors by using metal alloys
Authors: Lei D. Y. , Li J. , Ong H. C.
Published: July 2007

The authors have explored the possibility of using binary metal alloys on surface plasmon mediated emission from semiconductor. By adjusting the alloy composition, they have found that the surface plasmon resonance energy can be tuned to match with the emission energy of semiconductor so that the energy transfer process between the semiconductor and surface plasmons can be optimized. They have calculated the plasmonic density of states and Purcell factor for ZnO and ZnTe at different alloy compositions and the results support the argument. Experimentally, they have prepared AlxAg1−x∕ZnO films at different compositions and have measured their photoluminescence. The band-edge emission from Al0.8Ag0.2∕ZnO is found to be ∼60 times stronger than that of bare ZnO, which is consistent with the theoretical prediction. As a result, metal alloys can be considered as a simple and effective means in optimizing the surface plasmon mediated emission.
A study of the phase diagram of (K,Na,Li)NbO3 determined by dielectric and piezoelectric measurements, and Raman spectroscopy
Authors: Klein Naama , Hollenstein Evelyn , Damjanovic Dragan , Trodahl H. J. , Setter Nava , Kuball Martin
Published: July 2007

A composition-temperature phase diagram of the system (1−x)(K0.5Na0.5)NbO3–xLiNbO3 is presented for 0⩽x⩽0.1. Using dielectric and piezoelectric resonance measurements, and Raman spectroscopy, ceramic samples containing 2%–10% LiNbO3 were studied over a temperature range of 7–770K showing a complex sequence of phase transitions. Analysis of the different Raman, piezoelectric, and dielectric data shows distinct transitions from cubic to tetragonal to orthorhombic to rhombohedral phase for x=0.02–0.05. The symmetries of the phases were assigned using analogy to phase diagram of (K0.5Na0.5)NbO3 single crystals and ceramics (x=0). At x>0.07 only one transition between ferroelectric phases occurs where tetragonal phase transforms to another phase, possibly of rhombohedral, orthorhombic, or monoclinic symmetry. In the region between x=0.05 and x=0.08, the phase transition sequence is more complex. Below 100K this phase with unidentified symmetry creates a vertical boundary with the rhombohedral phase present near 5% Li. A triple point between the tetragonal, orthorhombic, and the new phase is identified.
Size dependence of thermal properties of armchair carbon nanotubes: A first-principles study
Authors: Upadhyay Kahaly Mousumi , Waghmare Umesh V.
Published: July 2007

The authors use first-principles pseudopotential-based density functional theory calculations of phonon dispersions to determine the size dependence of thermal properties of armchair single wall carbon nanotubes (SWCNTs), such as their negative thermal expansion and specific heat. While the specific heat is found to depend rather weakly on the diameter of SWCNTs, their negative thermal expansion behavior determined within a quasiharmonic approximation (QHA) exhibits a relatively strong dependence on the diameter. They identify the low energy vibrational modes that are responsible for the negative thermal expansion.
Nondestructive micropatterning of living animal cells using focused femtosecond laser-induced impulsive force
Authors: Kaji Takahiro , Ito Syoji , Miyasaka Hiroshi , Hosokawa Yoichiroh , Masuhara Hiroshi , Shukunami Chisa , Hiraki Yuji
Published: July 2007

Micropatterning of mouse NIH3T3 fibroblast cells was performed using focused femtosecond laser-induced impulsive force in a culture medium. The cells were detached from an upper substrate by the force and transferred to an underlying substrate with less than spatial resolution of 80μm full width at half maximum. About 80% of the cells were confirmed to be alive at 3h after the patterning. The force exerted to the cell was investigated by high-speed imaging and estimated to be an order of micronewtons. The force origin was not only due to cavitation bubble but also due to shockwave and jet flow.
Investigation of fluid cell resonances in intermittent contact mode atomic force microscopy
Authors: Kokavecz J. , Mechler A.
Published: July 2007

In fluid, the probe resonance curve of the atomic force microscope contains several apparent resonance peaks whose origin is not well understood. In this work, the authors focus on identifying the cause of these peaks and finding the optimal imaging conditions for acoustic intermittent contact mode in fluid environment. The authors demonstrate that the peaks are also present in the spectrum of the fluid movement and in that of the shaker piezo. These peaks may or may not coincide with the natural resonance of a probe in liquid, thus it is possible to drive the probes off-resonance. Numerical calculations show the feasibility of off-resonance imaging, but predict much higher imaging force.
Controlled domain wall injection into ferromagnetic nanowires from an optimized pad geometry
Authors: McGrouther D. , McVitie S. , Chapman J. N. , Gentils A.
Published: July 2007

The authors present an improved geometry for a micron-scale pad for the injection of vortex domain walls (VDWs) into ferromagnetic nanowires. The pad supports a single vortex magnetization state, the chirality of which can be controlled simply by field saturation along a specific direction. We show, using Lorentz transmission electron microscopy, that utilization of such pads allows the chirality of VDWs injected into the attached wire to be predetermined. Furthermore, the pad vortex state is highly stable and survives repeated injection and depinning of VDWs from an asymmetric notch located some distance along the wire.
Nanodomains in morphotropic lead zirconate titanate ceramics: On the origin of the strong piezoelectric effect
Authors: Theissmann Ralf , Schmitt Ljubomira A. , Kling Jens , Schierholz Roland , Schönau Kristin A. , Fuess Hartmut , Knapp Michael , Kungl Hans , Hoffmann Michael J.
Published: July 2007

The outstanding piezoelectric properties of lead zirconate titanate (PZT) ceramics with compositions close to the morphotropic phase boundary of the quasibinary phase diagram of lead zirconate and lead titanate are still under debate. A combination of ex situ and in situ transmission electron microscopy and high resolution x-ray diffraction revealed that the extrinsic piezoelectric effect in morphotropic PZT is closely connected to the existence of nanodomains. The in situ transmission electron microscopy investigations with applied electric field show that mainly the nanodomains respond to the electric field while the microdomain structure does not change noticeably in our experiments.
Asymmetric nematic liquid crystal cells containing lead zirconium titanate (PZT) films
Authors: Marino S. , Castriota M. , Strangi G. , Cazzanelli E. , Scaramuzza N.
Published: July 2007

Interest in thin films of PZT (lead zirconium titanate PbZr0.47Ti0.53O3) has increased because of their potential applications. For instance, the insertion of PZT in asymmetric nematic liquid crystal cells induces a rectified electro-optical response similar to that observed in such cells because of the insertion of metal oxide layers with ionic conductivity. In the present work several PZT films have been investigated, obtained by different thermal treatments after sol-gel synthesis and spin coating deposition. The observed rectifying effect, due to the insertion of PZT films in asymmetric liquid crystal cells, has been attributed to the presence of an internal field made from the reorientation of nanodomains of PZT. The presence of such internal fields is demonstrated and an estimation of it is given. Moreover, asymmetric nematic liquid crystal cells made with PZT films show considerable improvements in both contrast and response time.
Fluorine-vacancy complexes in Si-SiGe-Si structures
Authors: Abdulmalik D. A. , Coleman P. G. , El Mubarek H. A. W. , Ashburn P.
Published: July 2007

Fluorine-vacancy (FV) complexes have been directly observed in the Si0.94Ge0.06 layer in a Si-SiGe-Si structure, using variable-energy positron annihilation spectroscopy (VEPAS). These complexes are linked to the significant reduction of boron diffusion in the SiGe layer via interstitial trapping. Vacancies were introduced into the samples by ion implantation with 185 keV F+ at doses in the range 9×1014 to 1×1016 cm−2; the samples were subsequently subjected to rapid annealing in nitrogen ambient at 950 °C for 30 s. The VEPAS results, in combination with F profiles obtained by secondary ion mass spectrometry, are consistent with F4nVn complexes being associated with the SiGe layer and that they preferentially accumulate at the Si/SiGe interfaces. Their concentration is critically dependent on annealing temperature, decreasing significantly after annealing at 1000 °C.
Inverse tunnel magnetoresistance in magnetic tunnel junctions with an Fe4N electrode
Authors: Sunaga Kazuyuki , Tsunoda Masakiyo , Komagaki Kojiro , Uehara Yuji , Takahashi Migaku
Published: July 2007

The magnetotransport properties of Fe4N∕MgO∕CoFeB and Fe/MgO/CoFeB magnetic tunnel junctions (MTJs) were investigated at room temperature. In the Fe/MgO/CoFeB-MTJ, normal tunnel magnetoresistance (TMR) effect and roughly symmetric bias voltage (VB) dependence were observed, similar to the MTJs exhibiting coherent tunneling such as Fe/MgO/Fe. On the other hand, the inverse TMR effect, showing higher tunnel resistance for parallel magnetization configuration than for antiparallel configuration, and strong asymmetric VB dependence of TMR ratio were observed in the Fe4N∕MgO∕CoFeB-MTJ. The maximum TMR magnitude of 18.5% was obtained at VB=−200 mV, where the current flows from Fe4N to CoFeB. The enhancement of the inverse TMR ratio around VB=−200 mV is due to the broad peak of tunnel conductance in antiparallel configuration of Fe4N and CoFeB magnetizations. A large peak of the density of state at +300 meV from the Fermi level for minority spin electrons of bulk Fe4N might be an origin of this phenomenon.
Testing system for ferromagnetic shape memory microactuators
Authors: Ganor Y. , Shilo D. , Messier J. , Shield T. W. , James R. D.
Published: July 2007

Ferromagnetic shape memory alloys are a class of smart materials that exhibit a unique combination of large strains and fast response when exposed to magnetic field. Accordingly, these materials have significant potential in motion generation applications such as microactuators and sensors. This article presents a novel experimental system that measures the dynamic magnetomechanical behavior of microscale ferromagnetic shape memory specimens. The system is comprised of an alternating magnetic field generator (AMFG) and a mechanical loading and sensing system. The AMFG generates a dynamic magnetic field that periodically alternates between two orthogonal directions to facilitate martensitic variant switching and to remotely achieve a full magnetic actuation cycle, without the need of mechanical resetting mechanisms. Moreover, the AMFG is designed to produce a magnetic field that inhibits 180° magnetization domain switching, which causes energy loss without strain generation. The mechanical loading and sensing system maintains a constant mechanical load on the measured specimen by means of a cantilever beam, while the displacement is optically monitored with a resolution of approximately 0.1μm. Preliminary measurements using Ni2MnGa single crystal specimens, with a cross section of 100×100μm2, verified their large actuation strains and established their potential to become a material of great importance in microactuation technology.
Metal-enhanced superoxide generation: A consequence of plasmon-enhanced triplet yields
Authors: Zhang Yongxia , Aslan Kadir , Previte Michael J. R. , Geddes Chris D.
Published: July 2007

The authors report significant enhancements in the generation of superoxide for fluorophores in close proximity to silver nanoparticles. A distance dependence study of the fluorophores from the metallic nanostructures, combined with carefully chosen control samples, confirms that the enhancements in superoxide generation are due to plasmon-enhanced triplet yields, a consequence of the distance dependent sensitizer extent of excitation. This observation strongly agrees with current models developed by the authors. Given that the generation of superoxide and other oxygen species is important for many chemical and biological applications, then we believe that our findings are likely to fuel a wealth of oxygen-based plasmon-enhanced triplet assays.
Epitaxial growth of ZrO2 on GaN templates by oxide molecular beam epitaxy
Authors: Gu Xing , Izyumskaya Natalia , Avrutin Vitaly , Xiao Bo , Morkoç Hadis
Published: July 2007

Molecular beam epitaxial growth of ZrO2 has been achieved on GaN (0001)∕c-Al2O3 substrates employing a reactive H2O2 oxygen source. A low temperature buffer followed by in situ annealing and high temperature growth has been employed to attain monoclinic, (100)-oriented ZrO2 thin films. The typical full width at half maximum of a 30-nm-thick ZrO2 (100) film rocking curves is 0.4arcdeg and the root-mean-square surface roughness is ∼4Å. ω−2θ and pole figure x-ray diffraction patterns confirm the monoclinic structure of ZrO2. Data support an in-plane epitaxial relationship of ZrO2 [010]‖GaN[112¯] and ZrO2 [001]‖GaN[11¯00]. X-ray diffraction and reflection high-energy electron diffraction analyses reveal in-plane compressive strain, which is mainly due to the lattice mismatch.
Relaxation and anharmonic couplings of the O–H stretching vibration of asymmetric strongly hydrogen-bonded complexes
Authors: Gündoğdu Kenan , Bandaria Jigar , Nydegger Michael , Rock William , Cheatum Christopher M.
Published: July 2007

We present infrared transient grating measurements of complexes of formic acid with pyridine and pyrazine at four excitation frequencies within the broad proton-stretching band. These experiments investigate the mechanism of the line broadening of the O–H stretching vibration. The transients show coherent oscillations that decay within a few hundred femtoseconds and population relaxation on two time scales. We fit the data using a simple model of three coupled oscillators that relax via sequential kinetics through an intermediate state. Based on this model, we conclude that the coherent oscillations result from superpositions of Fermi-resonance-coupled states involving formic acid overtone and combination states.
Formation of the Bi-2212 phase in a BiSrCaO thick film screen printed on Cu∕Ni∕Cu substrate
Authors: Park In Soo , Lee Kee-Sun , Han Sang Chul , Sung Tae Hyun
Published: July 2007

To adopt Cu as a substrate material for Bi superconductors, Ni and Cu were electroplated on a Cu sheet; in this structure, Ni restrained the oxidation of Cu, and Cu acted as the source element to provide Cu oxides. Superconducting phases such as Bi-2212 were obtained by sequential heat treatments in Ar gas and Ar–(20vol%)O2 gas. Particularly, the (00l) plane of the Bi-2212 phase tended to develop in the exterior region close to the free surface rather than in the interior region. The critical onset temperatures of the specimens produced by the sequential heat treatments ranged from 73.4to78.5K.
Dislocation reduction in GaN grown on stripe patterned r-plane sapphire substrates
Authors: Chen Hou-Guang , Ko Tsung-Shine , Ling Shih-Chun , Lu Tien-Chang , Kuo Hao-Chung , Wang Shing-Chung , Wu Yue-Han , Chang Li
Published: July 2007

Extended defect reduction in GaN can be achieved via direct growth on stripe patterned (11¯02) r-plane sapphire substrates by metal organic chemical vapor deposition. The striped mesa is along [112¯0] with two etched sides in {0001} and {11¯01} faces. GaN grown on both etched facets in epitaxy exhibit different crystallographic relationships with sapphire substrate which are (11¯02)sapphire‖(112¯0)GaN and [112¯0]sapphire‖[1¯100]GaN, and (0001)sapphire‖(0001)GaN and [112¯0]sapphire‖[1¯100]GaN, respectively. The dislocation densities can be significantly reduced through epitaxial growth on the inclined lateral faces of mesas. Dislocation density in the order of ∼107cm−2 can be achieved in the tilted GaN.
Decrease in switching voltage fluctuation of Pt∕NiOx∕Pt structure by process control
Authors: Jung Ranju , Lee Myoung-Jae , Seo Sunae , Kim Dong Chirl , Park Gyeong-Su , Kim Kihong , Ahn Seungeon , Park Youngsoo , Yoo In-Kyeong , Kim Jin-Soo , Park Bae Ho
Published: July 2007

Resistance change random access memory devices using NiOx films with resistance switching properties have immense potential for high-density nonvolatile memory exceeding currently used flash memory. The only critical failure of a NiOx film is to write wrong information due to large fluctuations of switching voltages during successive resistance switching operations. The authors show that failure-free NiOx film can be grown directly on Pt electrode just by process control. Intensive analyses show that the superior resistance switching behaviors of their simple Pt∕NiOx∕Pt structure may result from a very thin Ni–Pt layer self-formed at the bottom interface during deposition of NiOx.
Microwave absorbing performances of multiwalled carbon nanotube composites with negative permeability
Authors: Deng Longjiang , Han Mangui
Published: July 2007

The permittivity and permeability behaviors of composites made from the multiwalled carbon nanotubes with magnetic impurity Ni and the wax have been studied in 3–18GHz. The unusual permittivity dispersion behaviors have been explained based on the Cole-Cole model and the conductivity contribution model. Permeability is found to have negative imaginary parts within 3–11GHz. The composites are found to show good microwave absorbing performances (reflection loss <−20dB): matching thickness is 1.5mm and absorbing frequency band is 11.6–12.4GHz, and the absorbing performance can be explained by the “geometrical effect.”
Fine structure in the tunneling characteristic of MgB2 thin films
Authors: Schneider R. , Geerk J. , Zaitsev A. G. , Löhneysen H. v.
Published: July 2007

The authors report on the observation of theoretically predicted fine structures in the negative second derivative of the current-voltage characteristic measured on sandwich-type tunnel junctions on MgB2 thin films with a Tc of 35K. The high resolution was achieved by a progress in the in situ preparation of MgB2 thin films by thermal evaporation of Mg combined with B sputtering. They were able to increase the substrate temperature up to 550°C by sputtering boron from a red-hot sintered target. The film properties, such as Tc and the residual resistivity, significantly improved compared to films deposited at lower substrate temperatures. Better crystalline order within the MgB2 grains is evidently the key to the details of the electron-phonon coupling.
Si-SiO
2
-Si
and Si-CaCO3-Si core–double-shell nanoparticles: Tuning light emission from infrared to ultraviolet
Authors: Caetano E. W. S. , Maia F. F. , Freire V. N. , Albuquerque E. L. , Medeiros S. K.
Published: July 2007

Confinement features of excitons in Si-SiO2-Si and Si-CaCO3-Si spherical nanoparticles (NPs) with a core–double-shell structure are studied in this work. In order to compute carrier and exciton ground-state energies, we estimate electron and hole effective masses in CaCO3 calcite using first-principles calculations. A comparison is made between the recombination energies and oscillator strengths of excitons in SiO2 and CaCO3 NPs as a function of diameter and barrier (middle shell) thicknesses. The recombination energy spans the energy range of the visible spectrum by changing the barrier width. The excitonic oscillator strength is orders of magnitude larger for Si-CaCO3-Si NPs in comparison to Si-SiO2-Si NPs. The results allow us to suggest that Si-CaCO3-Si nanoparticles emitting light in any wavelength between the infrared and ultraviolet can be fabricated, and are more efficient than SiO2-based ones.
Scanning thermal imaging of an electrically excited aluminum microstripe
Authors: Samson Benjamin , Aigouy Lionel , Latempa Rossella , Tessier Gilles , Aprili Marco , Mortier Michel , Lesueur Jérôme , Fournier Danièle
Published: July 2007

We study the Joule heating of a 1.25 μm wide aluminum microstripe excited by an electrical current. The temperature changes are measured with a scanning thermal microscope that uses a small fluorescent particle as a sensor. The lateral resolution observed for this sample is better than 300 nm. We have compared the temperature distribution in the stripe with a simple analytical model of heat propagation in the wire and the substrate. A good qualitative agreement is observed, although the measured temperature is much smaller than the estimated one, showing that the heat transfer between the hot wire and the fluorescent probe is not fully efficient.
Numerical evaluation of effective dielectric properties of three-dimensional composite materials with arbitrary inclusions using a finite-difference time-domain method
Authors: Wu Dagang , Chen Ji , Liu Ce
Published: July 2007

In this article, we introduce a numerical procedure to evaluate effective dielectric properties of arbitrary multicomponent three-dimensional mixtures. Recognizing that many mixtures have periodic extend in all directions, we only need to analyze a unit element for effective electrical properties extraction. The numerical technique used here is a finite-difference time-domain method with a periodic boundary condition that generalizes many boundary conditions used in previous works. Several numerical examples are provided to demonstrate the effectiveness of this method. Using this developed procedure, we study the effects of frequency, inclusion shapes, inclusion volume, and inclusion conductivity on mixture’s electrical properties. It is observed that these parameters can significantly change the electrical properties of mixtures.
Quantitative atomic force measurement with a quartz tuning fork
Authors: Lee Manhee , Jahng Junghoon , Kim Kyungho , Jhe Wonho
Published: July 2007

The authors demonstrate a simple yet robust method for quantitative measurement of dynamic atomic force using the quartz tuning fork for both electrically driven mode and mechanically driven mode. It is shown that both modes can be made fully equivalent and also allow accurate force measurement. The quartz tuning fork is now expected to be widely employed as a quantitative force measurement probe in addition to its capability to surface image in the atomic scale.
Enhanced electro-optical properties in gold nanoparticles doped ferroelectric liquid crystals
Authors: Kaur S. , Singh S. P. , Biradar A. M. , Choudhary Amit , Sreenivas K.
Published: July 2007

Influential electro-optical and textural properties in the smectic C* phase have been observed using gold nanoparticles (Au NPs) in ferroelectric liquid crystals (FLCs). An application of mere 0.1V brings fivefold increase in optical tilt and doping creates a strong intrinsic field inside the sample generating high tilt and a reproducible observation of memory effect. The lowering of threshold voltage and the enhanced optical contrast is probably due to the interaction of electron wave oscillation in Au NPs with the incident light traversing through the FLC molecules.
Design of high-efficiency GaN-based light emitting diodes with vertical injection geometry
Authors: Kim Hyunsoo , Kim Kyoung-Kook , Choi Kwang-Ki , Kim Hyungkun , Song June-O , Cho Jaehee , Baik Kwang Hyeon , Sone Cheolsoo , Park Yongjo , Seong Tae-Yeon
Published: July 2007

The authors report on the design and fabrication of high-efficiency GaN-based light emitting diodes (LEDs) with vertical-injection geometry. Based on the analyses of LED test patterns fabricated with various n-electrode dimensions, a design rule for vertical LEDs is proposed. It is found that the suppression of the vertical current under n electrodes and the efficient injection of the spreading current across the n layers are essential to fabricate high-efficiency LEDs. Introduction of the current blocking layer along with well-designed branched n electrodes results in a large enhancement of power efficiency by a factor of 1.9, compared with that of reference LEDs.
Defect reduction of GaAs epitaxy on Si (001) using selective aspect ratio trapping
Authors: Li J. Z. , Bai J. , Park J.-S. , Adekore B. , Fox K. , Carroll M. , Lochtefeld A. , Shellenbarger Z.
Published: July 2007

Metal-organic chemical vapor deposition growth of GaAs on Si was studied using the selective aspect ratio trapping method. Vertical propagation of threading dislocations generated at the GaAs∕Si interface was suppressed within an initial thin GaAs layer inside SiO2 trenches with aspect ratio >1, leading to defect-free GaAs regions up to 300nm in width. Cross-sectional and plan-view transmission electron microscopies were used to characterize the defect reduction. Material quality was confirmed by room temperature photoluminescence measurements. This approach shows great promise for the fabrication of optoelectronic integrated circuits on Si substrates.
Fluorescence suppression within Raman spectroscopy using annular beam excitation
Authors: Cormack I. G. , Mazilu M. , Dholakia K. , Herrington C. S.
Published: July 2007

This letter demonstrates how the use of an annular beam can significantly reduce the detected fluorescence from optical components within a Raman spectroscopic arrangement. The fluorescence from the excitation beam has a reduced generation and collection efficiency compared to the standard Gaussian beam. Crucially, the Raman signal from the annular beam is collected through the dark center of the excitation light field. This robust and simple technique may be immediately applied to all Raman configurations and may find broader applicability for any spectroscopic methodology which utilizes a high numerical aperture lens and requires the detected fluorescence to be suppressed.
Oxygen plasma treatment and postaging of pentacene field-effect transistors for improved mobility
Authors: Shin Kwonwoo , Yang Sang Yoon , Yang Chanwoo , Jeon Hayoung , Park Chan Eon
Published: July 2007

The authors fabricated pentacene transistors with high mobilities by controlling the morphology of pentacene film through adjustments to the surface energy of the gate dielectrics with oxygen plasma treatment, and then by improving the interfacial properties through postaging. The increased surface energy of poly(methylmethacrylate) dielectric that results from the oxygen plasma treatment improves the interconnections between grains and enlarges the grain size. The postaging of transistors is presumed to rearrange the interface functional groups and as a result decrease the polar functionality without changing pentacene film morphology, which reduces the number of trap states and increases the mobility to 0.73cm2∕Vs.
Bias-induced insulator-metal transition in organic electronics
Authors: Wei J. H. , Xie S. J. , Mei L. M. , Yan YiJing
Published: July 2007

The authors investigate the bias-induced insulator-metal transition in organic electronics devices on the basis of the Su-Schrieffer-Heeger model [W. P. Su et al., Phys. Rev. B 22, 2099 (1980)] combined with the nonequilibrium Green’s function formalism. The insulator-metal transition is explained with the energy level crossover that eliminates the Peierls phase [R. Peierls, Quantum Theory of Solids (Oxford University Press, Oxford, 1955)] and delocalizes the electron states near the threshold voltage. This may account for the experimental observations on the devices that exhibit intrinsic bistable conductance switching with large on-off ratio.
Thin crystalline 3C-SiC layer growth through carbonization of differently oriented Si substrates
Authors: Severino A. , D’Arrigo G. , Bongiorno C. , Scalese S. , La Via F. , Foti G.
Published: July 2007

The growth of thin cubic silicon carbide (3C-SiC) buffer layers in an horizontal hot-wall chemical vapor deposition reactor, through the carbonization of differently oriented Si surfaces, is presented. A qualitative and quantitative study has been performed on statistical parameters related to voids due to the buffer layer growth on the different substrate orientations emphasizing shape, size, and density as a function of the substrate orientation. Variation in the void parameters can be attributed to the atomic packing density related to the substrate orientations, which were (100) Si, (111) Si, and (110) Si in this study. Scanning electron microscopy and transmission electron microscopy were performed to analyze the surface and the crystalline quality of the 3C-SiC films grown and, eventually, an empirical model for the carbonization of Si surfaces formulated. Large platens characterize the 3C-SiC films with shapes related to the orientations of the substrate. These platens derive from the two-dimensional growth of different SiC islands which enlarge during the process due to the continuous reaction between Si and C atoms. The interior part of platens was characterized by the presence of a pure crystalline material with the presence of small tilts affecting some grains in the 3C-SiC layer in order to relief the stress generated with the substrate.
Thermal stability of Co∕SiO2 multilayers for use in the soft x-ray region
Authors: Ishino Masahiko , Koike Masato , Kanehira Mika , Satou Futami , Terauchi Masami , Sano Kazuo
Published: July 2007

The thermal stability of Co∕SiO2 multilayers was evaluated. Multilayer samples were deposited on Si substrates by means of the ion beam sputtering method and annealed at temperatures from 100 °C to 600 °C in a vacuum furnace. For the structural and optical evaluations, small-angle x-ray diffraction measurements, soft x-ray reflectivity measurement in the 1 keV energy region, and transmission electron microscopy observations were carried out. As the results, the Co∕SiO2 multilayers annealed up to 400 °C maintained the initial multilayer structure, and kept almost the same soft x-ray reflectivity as the as-deposited sample. A deterioration of the multilayer structure caused by the growth of cobalt grains was found on the samples annealed over 500 °C, and the soft x-ray reflectivity dropped in accordance with the deterioration of the multilayer structure.
Dynamic force microscopy study of the Ga-rich c(8×2) and As-rich c(4×4) reconstructions of the GaAs(001) surface
Authors: Kawai Shigeki , Rose Franck , Ishii Takanori , Tsukamoto Shiro , Kawakatsu Hideki
Published: July 2007

As-rich and Ga-rich GaAs(001) surfaces were studied with frequency-modulation dynamic force microscopy. By simply changing the parameters of argon sputtering and annealing during sample preparation, surfaces reconstructed with the As-rich c(4×4) phase or the Ga-rich c(8×2) phase could be obtained. True atomic resolution of the c(8×2) reconstruction is achieved by using constant frequency shift imaging. We show that tip functionalization allows selective species imaging. The presence at the tip apex of empty Ga dangling bonds or of fully filled As dangling bonds leads to selective atomic resolution of the As or Ga sublattices of the c(8×2) reconstructed surface, respectively. Our observations support the ζ model for the c(8×2) reconstruction (but no dimers were found) and the α model for the c(8×2) reconstruction (individual As–As dimers were resolved by dynamic force microscopy).
Cross-sectional scanning tunneling microscopy of biased semiconductor lasers
Authors: Cobley R. J. , Teng K. S. , Brown M. R. , Wilks S. P.
Published: July 2007

Cross-sectional scanning tunneling microscopy is applied to semiconductor lasers which are biased and producing light. Two device structures are investigated—a double quantum well laser and a buried heterostructure device with two-dimensional growth variation. Scanning tunneling microscopy (STM) images are collected as a function of sample drive bias. Changes that occur in the STM image as a result of powering the sample are observed, which are due to changes in the energy band structure and the carrier concentration in the device as it responds to bias. The observed changes are largely reversible and a model is presented which matches and confirms this behavior. Once these effects are confirmed and decoupled, the technique can be used to study device-specific behavior resulting from physical changes in lasers as they are operated.
Comparative study of the crystal-field splitting of trivalent neodymium energy levels in polycrystalline ceramic and nanocrystalline yttrium oxide
Authors: Gruber John B. , Sardar Dhiraj K. , Nash Kelly L. , Yow Raylon M.
Published: July 2007

Absorption and fluorescence spectra of commercially available polycrystalline ceramic Nd3+:Y2O3 and nanocrystalline aggregates of Nd3+:Y2O3 grown in our laboratory are optically characterized at nominal 8K and room temperature. In comparing the spectra obtained from these two different sources, we find similarities in terms of the detailed crystal-field splitting of the energy levels of the Nd3+ ion in the Y2O3 host. The 8K spectra are analyzed for the energy (Stark) level transitions between the LJ2S+1 multiplet manifolds of Nd3+(4f3). The analysis includes the manifolds of I9∕24 (the ground state), I11∕24, I13∕24, I15∕24, F3∕24, F5∕24, H9∕22, F7∕24, S3∕24, and F9∕24. The results of this study are also compared with a crystal-field splitting analysis reported earlier for single-crystal Nd3+:Y2O3 grown by a modified flame fusion method. We find that the spectroscopic properties of our composites compare favorably with other ceramic and single-crystal forms of Nd3+:Y2O3 currently available.
Topography of metallic surfaces subjected to plastic strain: Roughness, spatial correlations, and eigenvalue spectral entropy
Authors: Hubbard J. B. , Stoudt M. R. , Levine L. E.
Published: July 2007

This paper describes an analysis technique that integrates high-resolution topographical imaging and rigorous matrix-based three-dimensional characterization methods. We employ scanning confocal laser microscopy to obtain topographic images of the surface of an aluminum alloy subjected to various levels of uniaxial plastic strain. These images are discretized into a square array of pixels, each of which is then assigned a numerical value corresponding to the deviation of surface height relative to an averaged background. The result is a set of real, non-Hermitian n×n matrices, which are diagonalized to produce a collection of spectra, each of which consists of n complex eigenvalues. These eigenvalue spectra are observed to change systematically as the degree of plastic strain is varied. Because this approach is based solely on the behavior of the eigenvalue spectra, it eliminates the need for the a priori assumptions about surface character used in conventional topographic analyses. The information contained within an eigenvalue spectrum is distilled into a scalar measure of topographic disorder, referred to as the “spectral entropy.” The spectral entropy is observed to decrease monotonically with increasing plastic strain. This behavior is consistent with the observed topographical changes induced by plastic strain. In addition, the spectral entropy can be decomposed into a constant term that is independent of all spatial correlations that occur in the surface roughness and a term that incorporates these correlations at all levels of complexity.
Impedance spectroscopy of unetched CdTe/CdS solar cells—equivalent circuit analysis
Authors: Proskuryakov Y. Y. , Durose K. , Taele B. M. , Oelting S.
Published: July 2007

A detailed study of electric and structural properties has been carried out on CdTe/CdS solar cells which deliberately were not subjected to etching by a nitric-phosphoric (NP) or bromine-methanol (Br-Me) acids, conventionally employed for the formation of Te-rich layer before back contacting. In the previous work [J. Appl. Phys. 101, 014505 (2007)] we have shown that cells that were not etched provide more extensive information on sample/material properties than the etched ones, as analyzed by admittance spectroscopy. Although seemingly being able to describe the distribution of defect energy levels, the admittance spectroscopy approach has a significant drawback because the underlying theoretical formulation does not take into account the frequency-dependent contribution from the back contact together with its influence on the trap contributions. In this work we use an alternative methodology for analysis of impedance data measured in dark conditions, which applies an equivalent circuit model to the experimental spectra. In particular, a complete model consisting of 10–12 elements is suggested, which describes all the sets of data taken at different temperatures, unambiguously separating the respective roles of p-n junction parameters, defect trap levels, back contact, as well as spatial inhomogeneities within the cell. It is essential that the values of the parameters used to describe ac response from trap levels and that from the back contact are found to be consistent with admittance and I-V measurements. In addition, the temperature dependence of the dark conductance (GJ) and capacitance (CJ) of the main p-n junction, as well as temperature dependence of back contact resistance (RB), were obtained and analyzed. It was found that GJ(T) follows exp(T/T0) behavior which is characteristic of temperature-assisted tunneling, while CJ(T) agrees well with values of the high-frequency capacitance of the cell CHF(T). The T dependence of RB is found to follow activation behavior defined by a Schottky barrier with a height of (0.545±0.015) eV, that being close to the value obtained from dark I-V measurements.
Enhancement of glass-forming ability of FeCoNiBSiNb bulk glassy alloys with superhigh strength and good soft-magnetic properties
Authors: Shen Baolong , Chang Chuntao , Zhang Zhefeng , Inoue Akihisa
Published: July 2007

The effect of Zr addition on the glass-forming ability (GFA) of FeCoNiBSiNb glassy alloys in [(Fe0.6Co0.3Ni0.1)0.75B0.2Si0.05]96−xNb4Zrx system was investigated. In addition to slight increases of glass transition temperature from 818to822K and supercooled liquid region from 60to65K, the 1at.% Zr addition was found to be effective in approaching alloy to a eutectic point as well as decreasing liquidus temperature from 1427to1400K, resulting in an increase in GFA. By copper mold casting, [(Fe0.6Co0.3Ni0.1)0.75B0.2Si0.05]95Nb4Zr1 bulk glassy alloys (BGAs) with diameters in the range up to 6mm were produced. The BGA exhibits a superhigh fracture strength of 4180MPa, and Young’s modulus of 200GPa, combined with an elastic strain of 0.02. The glassy alloy exhibits good soft-magnetic properties as well, i.e., rather high saturation magnetization of 1.1T, low coercive force of 2A∕m, and high permeability of 16 700 at 1kHz under a field of 1A∕m. The reason why only 1at.% Zr is effective in improving GFA and the fracture mechanisms during compression of this Fe-based BGA were discussed.
Observation of 300K high energy magnetodielectric contrast in the bilayer manganite (La0.4Pr0.6)1.2Sr1.8Mn2O7
Authors: Cao J. , Rai R. C. , Brown S. , Musfeldt J. L. , Tackett R. , Lawes G. , Wang Y. J. , Wei X. , Apostu M. , Suryanarayanan R. , Revcolevschi A.
Published: July 2007

Large high energy magnetodielectric effects are observed in the bilayer manganite (La0.4Pr0.6)1.2Sr1.8Mn2O7, a direct consequence of exploiting magnetoresistance changes associated with the field driven spin-glass insulator to ferromagnetic metal transition and its high temperature remnant. The low temperature magnetodielectric contrast is as large as ∼100% near 0.8eV at 10T and over 10 000% in selected phonon regions. The 300K magnetodielectric contrast is ∼20% near 1.1eV at 30T. The results are potentially useful for magnetic memory applications away from the dc limit.
Performance improvement of polymer solar cells by using a solution processible titanium chelate as cathode buffer layer
Authors: Tan Zhan’ao , Yang Chunhe , Zhou Erjun , Wang Xiang , Li Yongfang
Published: July 2007

A solution processible titanium chelate, titanium (diisopropoxide) bis (2,4-pentanedionate) (TIPD), was used as the cathode buffer layer in the polymer solar cells (PSCs) based on the blend of poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene] and [6,6]-phenyl-C61-butyric acid methyl ester. Introducing TIPD buffer layer reduced the interface resistance between the active layer and Al electrode, leading to a lower device resistance. The power conversion efficiency of the PSC with TIPD buffer layer reached 2.52% under the illumination of AM1.5, 100mW∕cm2, which is increased by 51.8% in comparison with that (1.66%) of the device without TIPD buffer layer under the same experimental conditions.
High efficiency of CdSe quantum-dot-sensitized TiO2 inverse opal solar cells
Authors: Diguna Lina J. , Shen Qing , Kobayashi Junya , Toyoda Taro
Published: July 2007

The authors have demonstrated an approach to sensitized-type solar cells, based on TiO2 inverse opal and the use of CdSe quantum dots (QDs) as sensitizers. CdSe QDs were grown in situ on TiO2 inverse opal electrodes, utilizing a chemical bath deposition method. All of the photovoltaic performances, including short circuit photocurrent density, open circuit voltage, fill factor, and efficiency, were significantly improved by surface modification with ZnS and fluoride ions. A power conversion efficiency of about 2.7% has been attained, under solar illumination of 100mW∕cm2. This value is relatively high for metal oxide solar cells, sensitized with semiconductor QDs.
Giant magnetoelectric effect (under a dc magnetic bias of 2Oe) in laminate composites of FeBSiC alloy ribbons and Pb(Zn1∕3,Nb2∕3)O3–7%PbTiO3 fibers
Authors: Dong Shuxiang , Zhai Junyi , Xing Zhengping , Li Jiefang , Viehland D.
Published: July 2007

Giant magnetoelectric (ME) voltage and charge coefficients have been found in long-type composites of high-permeability magnetostrictive FeBSiC alloy ribbons laminated together with piezoelectric Pb(Zn1∕3,Nb2∕3)O3–7%PbTiO3 single crystal fibers. The maximum ME voltage and charge coefficients at low frequencies were 10.5V∕cmOe and 1nC∕Oe under a notably low dc magnetic bias of 2Oe; at resonance, these coefficients were dramatically increased to 400V∕cmOe and 42nC∕Oe, respectively. These values are much higher, and the required dc magnetic bias much lower, than those of previously reported Terfenol-D based ME laminates.
Nonlithographic fabrication of surface-enhanced Raman scattering substrates using a rastered atmospheric-pressure microplasma source
Authors: Chiang Wei-Hung , Cochey Matthew , Virnelson R. Craig , Sankaran R. Mohan
Published: July 2007

A nonlithographic patterning technique based on a numerically controlled atmospheric-pressure microplasma source has been developed to fabricate surface-enhanced Raman scattering (SERS) substrates. Microstructures in silver (Ag) films on glass are created by localized physical sputtering of Ag atoms using an argon (Ar) microplasma horizontally scanned across the glass substrate. Detection of crystal violet on patterned substrates shows an enhancement of the Raman scattering signal intensity by eight to ten orders of magnitude higher than bare Ag/glass substrates. The SERS enhancement depends on the pattern geometry showing that the mechanism is related to surface irregularities in the sputtered holes.
Effect of post-growth annealing on the optical properties of InAs/GaAs quantum dots: A tight-binding study
Authors: Santoprete R. , Kratzer P. , Scheffler M. , Capaz Rodrigo B. , Koiller Belita
Published: July 2007

We present an atomistic study of the strain field, the one-particle electronic spectrum and the oscillator strength of the fundamental optical transition in chemically disordered InxGa1−xAs pyramidal quantum dots (QDs). Interdiffusion across the interfaces of an originally “pure” InAs dot buried in a GaAs matrix is simulated through a simple model, leading to atomic configurations where the abrupt heterointerfaces are replaced by a spatially inhomogeneous composition profile x. Structural relaxation and the strain field calculations are performed through the Keating valence force field model, while the electronic and optical properties are determined within the empirical tight-binding approach. We analyze the relative impact of two different aspects of the chemical disorder, namely: (i) the effect of the strain relief inside the QD, and (ii) the purely chemical effect due to the group-III atomic species interdiffusion. We find that these effects may be quantitatively comparable, significantly affecting the electronic and optical properties of the dot. Our results are discussed in comparison with recent luminescence studies of intermixed QDs.
Yield strength of molybdenum at high pressures
Authors: Jing Qiumin , Bi Yan , Wu Qiang , Jing Fuqian , Wang Zhigang , Xu Jian , Jiang Sheng
Published: July 2007

In the diamond anvil cell technology, the pressure gradient approach is one of the three major methods in determining the yield strength for various materials at high pressures. In the present work, by in situ measuring the thickness of the sample foil, we have improved the traditional technique in this method. Based on this modification, the yield strength of molybdenum at pressures has been measured. Our main experimental conclusions are as follows: (1) The measured yield strength data for three samples with different initial thickness (100, 250, and 500μm) are in good agreement above a peak pressure of 10GPa. (2) The measured yield strength can be fitted into a linear formula Y=0.48(±0.19)+0.14(±0.01)P (Y and P denote the yield strength and local pressure, respectively, both of them are in gigapascals) in the local pressure range of 8–21GPa. This result is in good agreement with both Y=0.46+0.13P determined in the pressure range of 5–24GPa measured by the radial x-ray diffraction technique and the previous shock wave data below 10GPa. (3) The zero-pressure yield strength of Mo is 0.5GPa when we extrapolate our experimental data into the ambient pressure. It is close to the tensile strength of 0.7GPa determined by Bridgman [Phys. Rev. 48, 825 (1934)] previously. The modified method described in this article therefore provides the confidence in determination of the yield strength at high pressures
dc electric field tunable bulk acoustic wave solidly mounted resonator using SrTiO3
Authors: Saddik G. N. , Boesch D. S. , Stemmer S. , York R. A.
Published: July 2007

A tunable SrTiO3 solidly mounted bulk acoustic wave resonator has been designed, fabricated, and tested. The solidly mounted resonator was implemented using an acoustical Bragg reflector of alternating high and low acoustic impedance layers. The resonator demonstrated a frequency tunability of 1%, from 5.87to5.81GHz with an applied bias of 0–7.5V, respectively. A return loss of −6.5dB was observed at an applied bias of 7.5V. The quality factor at the resonant frequency was 78 and relatively constant with applied bias. The effective electromechanical coupling coefficient was 2% and near linear with applied bias.