bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of rapid thermal annealing on the noise properties of InAs∕GaAs quantum dot structures
Authors: Arpatzanis N. , Tsormpatzoglou A. , Dimitriadis C. A. , Song J. D. , Choi W. J. , Lee J. I. , Charitidis C.
Published: September 2007

Self-assembled InAs quantum dots (QDs) were grown by molecular beam epitaxy (MBE) on n+-GaAs substrates, capped between 0.4μm thick n-type GaAs layers with electron concentration of 1×1016cm−3. The effect of rapid thermal annealing at 700°C for 60s on the noise properties of the structure has been investigated using Au∕n-GaAs Schottky diodes as test devices. In the reference sample without containing QDs, the noise spectra show a generation-recombination (g-r) noise behavior due to a discrete energy level located about 0.51eV below the conduction band edge. This trap is ascribed to the M4 (or EL3) trap in GaAs MBE layers, related to a chemical impurity-native defect complex. In the structure with embedded QDs, the observed g-r noise spectra are due to a midgap trap level ascribed to the EL2 trap in GaAs, which is related to the InAs QDs dissolution due to the thermal treatment.
A model for electromigration-induced degradation mechanisms in dual-inlaid copper interconnects: Effect of microstructure
Authors: Sukharev Valeriy , Zschech Ehrenfried , Nix William D.
Published: September 2007

A developed physical model and a simulation algorithm are used to predict electromigration (EM)-induced stress evolution in dual-inlaid copper interconnects. Incorporation of all important atom migration driving forces into the mass balance equation and its solution, together with the solution of the coupled electromagnetics and elasticity problems, allows the simulation of EM-induced degradation in a variety of dual-inlaid copper interconnect segments with different dominant paths for mass transport [V. Sukharev and E. Zschech, J. Appl. Phys. 96, 6337 (2004)]. The results of the numerical simulation have been proven experimentally by EM degradation studies on fully embedded dual-inlaid copper interconnect test structures and by subsequent microstructure analysis, mainly based on electron backscatter diffraction (EBSD) data. The EM-induced void formation and its virtual movement and growth in a copper interconnect were continuously monitored in an in situ scanning electron microscopy experiment. The copper microstructure, particularly the orientation of grains and grain boundaries, was determined with EBSD. It has a significant influence on grain boundary diffusivity and consequently on the mass transport along grain boundaries. For interconnects with interfaces that resist atomic transport and where grain boundaries are the important pathways for atom migration [E. Zschech et al., Z. Metallkd. 96, 996 (2005)], degradation and failure processes are completely different for microstructures with randomly oriented grain boundaries compared with “bamboolike” microstructures. The correspondence between simulation results and experimental data indicates the applicability of the developed model for optimization of the physical and electrical design rules. Simulation-based optimization of the interconnect architecture, segment geometry, material properties, and some of the process parameters can produce on-chip interconnect systems with a high immunity to EM-induced failures.
Defect-induced trap-assisted tunneling current in GaInNAs grown on GaAs substrate
Authors: Loke W. K. , Yoon S. F. , Wicaksono S. , Tan K. H. , Lew K. L.
Published: September 2007

We present the reverse-bias current-voltage and deep-level transient spectroscopy (DLTS) characteristics of a Ga0.90In0.10N0.033As0.967∕GaAs positive-intrinsic-negative photodiode (Eg=0.92 eV) and a trap-assisted tunneling model which considers generation-recombination and tunneling mechanisms. Using trap parameters obtained from the DLTS measurement, the model generates current-voltage characteristics of the photodiode, which were found to be in good agreement with experimental current-voltage curves at different temperature. The model also suggests that high dark current at low reverse-bias voltage is caused by the presence of traps which have low activation energy. Furthermore, it is predicted that approximately ten times reduction in the dark current can be achieved when the trap concentration of type H-1 (Ea=0.15 eV) is reduced by one order. On the other hand, a similar reduction in defect concentration of type H-2 (Ea=0.40 eV), which is nearer to midgap does not produce the same effect.
Improvement of laser gain by microdomain compensation effects in Nd:SrBa(Nb3O)2 lasers
Authors: Molina P. , Rodríguez E. Martín , Solé J. García , Bausa L. E. , Jaque D. , Kamisnkii A. A.
Published: September 2007

The laser performance of poled and unpoled neodymium-doped strontium barium niobate crystals has been compared. We have found that the random distribution of microferroelectric domains, characteristic of as-grown crystals, leads to a relevant improvement in the laser performance. From the analysis of the optical distortion effects induced by both poled and unpoled crystals we have concluded that the compensation effects taking place between adjacent microdomains inside the active volume are at the origin of this improvement.
Possible colloid crystal formation in a magnetized and inhomogeneous semiconductor plasma
Authors: Salimullah M. , Ehsan Z. , Zubia K. , Shah H. A. , Murtaza G.
Published: September 2007

We investigate the detailed properties of the modification of the Debye-Hückel and the consequent oscillatory wake potentials in the presence of a density inhomogeneity and external static uniform electric and magnetic fields in an ion-implanted n-type piezoelectric semiconductor. It is found that the external fields and the density nonuniformity have significant effects on the static and dynamical electrostatic potentials. The possibility of the long-ranged order formation of colloids of the implanted ions, leading to modification of the electrical, thermodynamic, and optical properties of the “soft matter,” has been pointed out. The periodic wakefields besides the usual static Coulombian Debye-Hückel potential may cause the long-range ordered structures of the charged colloidal particulates within the semiconductor to show various additional properties.
Tip-enhanced Raman spectroscopy for investigating adsorbed species on a single-crystal surface using electrochemically prepared Au tips
Authors: Wang Xi , Liu Zheng , Zhuang Mu-De , Zhang Hai-Ming , Wang Xiang , Xie Zhao-Xiong , Wu De-Yin , Ren Bin , Tian Zhong-Qun
Published: September 2007

A tip-enhanced Raman instrument was set up based on a homemade optical fiber Raman head, a dispersive spectrograph, and a scanning tunneling microscope (STM) system. Electrochemical preparation of tip-enhanced Raman spectroscopy (TERS) Au tips was refined by using the etching current as ending point control, resulting in a success rate as high as 90%. The high quality Au tips allow the recording of STM images with molecular resolution and TERS spectra of nonresonant surface species on a single-crystal surface.
Multidimensional integrable systems and deformations of Lie algebra homomorphisms
Authors: Dunajski Maciej , Grant James D. E. , Strachan Ian A. B.
Published: September 2007

We use deformations of Lie algebra homomorphisms to construct deformations of dispersionless integrable systems arising as symmetry reductions of anti-self-dual Yang-Mills equations with a gauge group Diff(S1).
Electron transport through single carbon nanotubes
Authors: Chai G. , Heinrich H. , Chow L. , Schenkel T.
Published: September 2007

The authors report on the transport of energetic electrons through single, well aligned multiwall carbon nanotubes (CNTs). Embedding of CNTs in a protective carbon fiber coating enables the application of focused ion beam based sample preparation techniques for the nondestructive isolation and alignment of individual tubes. Aligned tubes with lengths of 0.7–3μm allow transport of 300keV electrons in a transmission electron microscope through their hollow cores at zero degree incident angles and for a misalignment of up to 1°.
Quantum confinement of crystalline silicon nanotubes with nonuniform wall thickness: Implication to modulation doping
Authors: Yan Binghai , Zhou Gang , Zeng Xiao Cheng , Wu Jian , Gu Bing-Lin , Duan Wenhui
Published: September 2007

First-principles calculations of crystalline silicon nanotubes (SiNTs) show that nonuniformity in wall thickness can cause sizable variation in the band gap as well as notable shift in the optical absorption spectrum. A unique quantum confinement behavior is observed: the electronic wave functions of the valence band maximum and conduction band minimum are due mainly to atoms located in the thicker side of the tube wall. This is advantageous to spatially separate the doping impurities from the conducting channel in doped SiNTs. Practically, the performance of the SiNT-based transistors may be substantially improved by selective p∕n doping in the thinner side of the tube wall in the spirit of modulation doping.
Experimental studies of strong dipolar interparticle interaction in monodisperse Fe3O4 nanoparticles
Authors: Bae Che Jin , Angappane S. , Park J.-G. , Lee Youjin , Lee Jinwoo , An Kwangjin , Hyeon Taeghwan
Published: September 2007

Interparticle interaction of monodisperse Fe3O4 nanoparticles has been experimentally investigated by dispersing the nanoparticles in solvents. With increasing the interparticle distances to larger than 100nm in a controlled manner, the authors found that the blocking temperature (TB) of the nanoparticles drops continuously and eventually gets saturated with a total drop in TB of 7–17K observed for 3, 5, and 7nm samples, compared with their respective nanopowder samples. By carefully studying the dependence of TB on the interparticle distance, the authors could demonstrate that the experimental dependence of TB follows the theoretical curve of the dipole-dipole interaction.
High quantum efficiency polymer photoconductors using interdigitated electrodes
Authors: Niemeyer A. C. , Campbell I. H. , So F. , Crone B. K.
Published: September 2007

In this letter the authors investigate photoconductivity in interdigitated lateral photoconductors with aluminum contacts, and a poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-vinylene]/ {6}-1-(3-(methoxycarbonyl)propyl)-{5}-1-phenyl-[6,6]-C61 blend as the active layer. They demonstrate quantum efficiencies over 70% in 5μm devices. The quantum efficiency spectra follow the absorbance spectra and are not corrected for incomplete absorption of the films. The quantum efficiency is a monotonically increasing function of applied field. The field dependence can be explained with a modified Onsager model for charge dissociation.
Numbers of donors and acceptors from transport measurements in graphene
Authors: Novikov D. S.
Published: September 2007

A method is suggested to separately determine the surface density of positively and negatively charged impurities that limit the mobility in a graphene monolayer. The method is based on the exact result for the transport cross section, according to which the massless carriers are scattered more strongly when they are attracted to a charged impurity than when they are repelled from it.
Vibrational and electrical properties of hexagonal La2O3 films
Authors: Scarel G. , Debernardi A. , Tsoutsou D. , Spiga S. , Capelli S. C. , Lamagna L. , Volkos S. N. , Alia M. , Fanciulli M.
Published: September 2007

La 2 O 3 films are obtained by annealing in vacuum La(OH)3 layers produced upon air exposure of La2O3 layers grown using atomic layer deposition. Sample thickness determines whether hexagonal or cubic La2O3 form. The observed infrared active phonon mode with minimum frequency for hexagonal La2O3 films is located at 198cm−1, in good agreement with the first principles calculations, and at a lower frequency than the corresponding mode for cubic La2O3 is at 235cm−1. Accordingly, electrical measurements reveal a dielectric constant around 27 for hexagonal La2O3 films, significantly higher than the one around 17 derived for cubic La2O3 layers.
Comparison of different methods to calibrate torsional spring constant and photodetector for atomic force microscopy friction measurements in air and liquid
Authors: Pettersson Torbjörn , Nordgren Niklas , Rutland Mark W. , Feiler Adam
Published: September 2007

A number of atomic force microscopy cantilevers have been exhaustively calibrated by a number of techniques to obtain both normal and frictional force constants to evaluate the relative accuracy of the different methods. These were of either direct or indirect character—the latter relies on cantilever resonant frequencies. The so-called Sader [Rev. Sci. Instrum. 70, 3967 (1999)] and Cleveland [Rev. Sci. Instrum. 64, 403 (1993)] techniques are compared for the normal force constant calibration and while agreement was good, a systematic difference was observed. For the torsional force constants, all the techniques displayed a certain scatter but the agreement was highly encouraging. By far the simplest technique is that of Sader, and it is suggested in view of this validation that this method should be generally adopted. The issue of the photodetector calibration is also addressed since this is necessary to obtain the cantilever twist from which the torsional force is calculated. Here a technique of obtaining the torsional photodetector sensitivity by combining the direct and indirect methods is proposed. Direct calibration measurements were conducted in liquid as well as air, and a conversion factor was obtained showing that quantitative friction measurements in liquid are equally feasible provided the correct calibration is performed.
Time interval measurement device based on surface acoustic wave filter excitation, providing 1ps precision and stability
Authors: Panek Petr , Prochazka Ivan
Published: September 2007

This article deals with the time interval measurement device, which is based on a surface acoustic wave (SAW) filter as a time interpolator. The operating principle is based on the fact that a transversal SAW filter excited by a short pulse can generate a finite signal with highly suppressed spectra outside a narrow frequency band. If the responses to two excitations are sampled at clock ticks, they can be precisely reconstructed from a finite number of samples and then compared so as to determine the time interval between the two excitations. We have designed and constructed a two-channel time interval measurement device which allows independent timing of two events and evaluation of the time interval between them. The device has been constructed using commercially available components. The experimental results proved the concept. We have assessed the single-shot time interval measurement precision of 1.3ps rms that corresponds to the time of arrival precision of 0.9ps rms in each channel. The temperature drift of the measured time interval on temperature is lower than 0.5ps∕K, and the long term stability is better than ±0.2ps∕h. These are to our knowledge the best values reported for the time interval measurement device. The results are in good agreement with the error budget based on the theoretical analysis.
Optical loss and interface morphology in AlGaAs∕GaAs distributed Bragg reflectors
Authors: Zhang Z. , von Würtemberg R. Marcks , Berggren J. , Hammar M.
Published: September 2007

It is shown that n-type doping of AlGaAs∕GaAs distributed Bragg reflectors (DBRs) grown by metal-organic vapor-phase epitaxy has a profound negative impact on the performance of vertical-cavity surface-emitting lasers (VCSELs) based on such mirrors. Using an intracavity contact scheme, 1.3-μm-range InGaAs VCSELs with and without doping in the bottom DBR are directly compared. Doped mirrors lead to lower slope efficiency, lower output power, and higher threshold current. From x-ray diffraction, high-accuracy reflectance measurements, and atomic force microscopy studies, it is suggested that this performance degradation is due to the doping-enhanced Al–Ga interdiffusion, leading to interface roughening and increased scattering loss.
Tunable lead-chalcogenide on Si resonant cavity enhanced midinfrared detector
Authors: Felder F. , Arnold M. , Rahim M. , Ebneter C. , Zogg H.
Published: September 2007

Midinfrared tunable resonant cavity enhanced detectors have been realized. The linewidths of 0.07μm are determined by the finesse of the cavities, while the length of the cavity can be changed with a movable mirror. This allows tuning across the 4–5.5μm midinfrared wavelength range. The thin (0.3μm) photodiodes inside the cavity are based on lead-chalcogenide narrow gap semiconductor layers grown epitaxially onto a Si substrate. Due to the thin active layer, a higher sensitivity at the higher operation temperatures is achieved as compared to conventional thick photodiodes.
Spin polarization switched by a ferroelectric resonant tunneling diode
Authors: Li M. K. , Kim N. M. , Kang T. W.
Published: September 2007

The spin transport through a resonant tunneling diode with a ferroelectric quantum well is studied by nonequilibrium Green’s function method. The results show that the spin polarization can be switched by changing the dipole direction of the ferroelectric quantum well in a resonant tunneling diode. At a special bias, the authors can get highly spin polarized current or evenly spin mixed current depending on the dipole direction.
Electron mobility enhancement in strained-germanium n-channel metal-oxide-semiconductor field-effect transistors
Authors: Yang Y.-J. , Ho W. S. , Huang C.-F. , Chang S. T. , Liu C. W.
Published: September 2007

The dependence of electron mobility on strain, channel direction, and substrate orientation is theoretically studied for the germanium n-channel metal-oxide-semiconductor field-effect transistors. For the unstrained channel, (111) substrate can provide the highest mobility among the three orientations, mainly due to its largest quantization mass and smallest conductivity mass in L valley. The tensile strain parallel to the [1¯10] channel direction on (111) substrate gives 4.1 times mobility of Si at 1MV∕cm, and the mobility enhancement starts to saturate for the strain larger than 0.5%. The compressive strain of ∼1.5% transverse to [1¯10] on (111) substrate yields 2.9 times mobility enhancement at 1MV∕cm.
Hydrogen bonding at grain surfaces and boundaries of nanodiamond films detected by high resolution electron energy loss spectroscopy
Authors: Michaelson Sh. , Ternyak O. , Hoffman A. , Williams O. A. , Gruen D. M.
Published: September 2007

Hydrogenated nanodiamond films consisting of 300 and 10–30nm grain sizes were examined by high resolution electron energy loss spectroscopy. C–H stretching modes were identified at 350, 360, and 375meV. The mode at 375meV was enhanced in the case of 10–30nm grain size and it is stable up to in situ annealing to >800°C. Complete hydrogen desorption occurs upon annealing to 1000°C. Exposure of the nanodiamond film to atomic hydrogen results in a strong quenching of the 375meV C–H mode, most likely due to preferential etching of (sp2)-carbon-hydrogen at the surface and grain boundaries of the films.
Maxwell-Wagner space charge effects on the Pb(Zr,Ti)O3–CoFe2O4 multilayers
Authors: Ortega N. , Kumar Ashok , Katiyar R. S. , Scott J. F.
Published: September 2007

Electrical properties of Pb(Zr0.53Ti0.47)O3∕CoFe3O4 (PZT/CFO) multilayers (MLs) were investigated in the light of Maxwell-Wagner space charge effects and the data differ from those of BiFeO3∕CoFe3O4 nanocomposites. These MLs are of special interest as weak field sensors of magnetism. These data fit the Maxwell-Wagner space charge model of Catalan et al. [Appl. Phys. Lett. 77, 3078 (2000)]. Very large temperature shifts in dielectric loss peak (>200K) are indicative of high space charge density at the PZT/CFO interfaces. These conclusions are confirmed by spurious hysteresis loop that are similar to those of highly conducting specimens.
GaN Schottky barrier photodetectors with SiN∕GaN nucleation layer
Authors: Jhou Y. D. , Chang S. J. , Su Y. K. , Lee Y. Y. , Liu C. H. , Lee H. C.
Published: September 2007

GaN Schottky barrier photodetectors with SiN∕GaN nucleation layer were fabricated. It was found that leakage current was much smaller and much less bias dependent for the photodetector with SiN∕GaN nucleation layer, as compared to the photodetector with conventional low-temperature GaN nucleation layer. It was also found that effective Schottky barrier height increased from 1.27to1.53eV with the insertion of the SiN layer. Furthermore, it was found that the authors can effectively suppress internal gain of the detector and enhance ultraviolet to visible rejection ratio by using the SiN∕GaN nucleation layer.
Polarity inversion of type II InAs∕GaSb superlattice photodiodes
Authors: Nguyen Binh-Minh , Hoffman Darin , Delaunay Pierre-Yves , Razeghi Manijeh , Nathan Vaidya
Published: September 2007

The authors demonstrated the realization of p-on-n type II InAs∕GaSb superlattice photodiodes. Growth condition for high quality InAsSb layer lattice matched to GaSb was established for the use of an effective n-contact layer. By studying the effect of various GaSb capping layer thicknesses on the optical and electrical performances, an optimized thickness of 160nm was determined. In comparison to as grown n-on-p superlattice photodiodes, this inverted design of p on n has shown similar quality. Finally, by analyzing Fabry-Perot interference fringes in the front side illuminated spectral measurement, the refractive index of the superlattice was determined to be approximately 3.8.
Variation in the properties of the interface in a CoFeB∕MgO∕CoFeB tunnel junction during thermal annealing
Authors: Jang Youngman , Nam Chunghee , Lee Ki-Su , Cho B. K. , Cho Y. J. , Kim Kwang-Seok , Kim K. W.
Published: September 2007

Variation in the quality of the interface in a CoFeB∕MgO∕CoFeB tunnel junction during thermal annealing was investigated using x-ray photoemission spectroscopy. The formation of B oxide and the reduction of Fe oxide at the bottom interface after thermal annealing near Ta=300°C were found to enhance the tunneling magnetoresistance ratio significantly. At the same time, an asymmetry of the conductance (dV∕dI) in the bias polarity and a local minimum of conductance in a positive bias state were measured which were attributed to the presence of a minority state at the bottom interface. The authors believe that the existence of the Bloch state was also responsible for the failure of the application of the Brinkman-Dynes-Rowell or Simmons models to the CoFeB∕MgO∕CoFeB junction.
First-principles study of pressure-induced metal-insulator transition in BiNiO3
Authors: Cai M. Q. , Yang G. W. , Tan X. , Cao Y. L. , Wang L. L. , Hu W. Y. , Wang Y. G.
Published: September 2007

First-principles calculation is used to study the pressure-induced metal-insulator transition in BiNiO3. It is found that the G-antiferromagnetic insulator triclinic phase in BiNiO3 transforms to the ferromagnetic half-metallic orthorhombic phase with a volume collapse of 6.2% when the applied pressure is 3.46GPa. The interaction between neighbor Ni atoms creates the energy band gap of 1.96eV in the majority density of states. The pressure suppresses the charge disproportion of Bi. The strong hybridizations of Ni–O and Bi–O lead to the decrease of the spin magnetic moment with 1.74μB compared with the Ni3+ with d7 configuration 3μB.
Polymer constraint effect for electrothermal bimorph microactuators
Authors: Chu Duc T. , Lau G. K. , Sarro P. M.
Published: September 2007

The authors report on the analysis of the polymer constraint effect and its use for a micromachined electrothermal bimorph actuator. The actuated displacement is enhanced due to the polymer constraint effect. Both the thermal expansion and apparent Young’s modulus of the constrained polymer blocks are significantly improved, compared with the no constraint case. The calculation that agrees well with experimental results provides the means to optimize the design of the constrained polymer stack electrothermal microactuator.
Screening model for nanowire surface-charge sensors in liquid
Authors: Sørensen Martin Hedegård , Mortensen Niels Asger , Brandbyge Mads
Published: September 2007

The conductance change of nanowire field-effect transistors is considered a highly sensitive probe for surface charge. However, Debye screening of relevant physiological liquid environments challenge device performance due to competing screening from the ionic liquid and nanowire charge carriers. The authors discuss this effect within Thomas-Fermi and Debye-Hückel theory and derive analytical results for cylindrical wires which can be used to estimate the sensitivity of nanowire surface-charge sensors. They study the interplay between the nanowire radius, the Thomas-Fermi and Debye screening lengths, and the length of the functionalization molecules. The analytical results are compared to finite-element calculations on a realistic geometry.
Formation of Ge nanocrystals using Si1.33Ge0.67O2 and Si2.67Ge1.33N2 film for nonvolatile memory application
Authors: Chen Wei-Ren , Chang Ting-Chang , Hsieh Yen-Ting , Sze Simon M. , Chang Chun-Yen
Published: September 2007

The authors proposed a formation mechanism of Ge nanocrystals embedded in the dielectric by using Si1.33Ge0.67O2 and Si2.67Ge1.33N2 films for nonvolatile memory application in this study. Because of internal competition reaction, this formation process reduced the thermal budget and eliminated the use of high pressure H2 treatment or steam process. In this research, the preannealing capping oxide step is a critical process for nonvolatile memory effect. Transmission electron microscope shows the shape and density of nanocrystals in the dielectric. Moreover, the memory structure with Ge nanocrystal embedded in SiNx has better charge storage ability and data retention than Ge nanocrystal embedded in SiOx.
Ultrasensitive, label-free, and real-time immunodetection using silicon field-effect transistors
Authors: Kim Ansoon , Ah Chil Seong , Yu Han Young , Yang Jong-Heon , Baek In-Bok , Ahn Chang-Guen , Park Chan Woo , Jun Myung Sim , Lee Sungjae
Published: September 2007

Ultrasensitive, label-free, and real-time prostate-specific antigen (PSA) sensor was developed using n-type silicon nanowire-based structures configured as field-effect transistors using the conventional “top-down” semiconductor processes. Specific binding of PSA with antibody of PSA (anti-PSA) immobilized on the Si surface through covalent linkage leads to a conductivity change in response to variations of electric field at the surface. The conductance changes depending on PSA concentrations and pH values in solution according to isoelectric point of PSA provide the evidence of the real-time detection of 1fg∕ml PSA. The authors also explored the sensitivity of PSA immunodetection depending on both Si–field-effect transistors (FET) dimensions and doping concentrations to provide strategy for fabrication of an ultrasensitive Si-FET biosensor.
Fabrication and characterization of “on-edge” molecular junctions for molecular electronics
Authors: Shamai T. , Ophir A. , Selzer Y.
Published: September 2007

A high throughput fabrication method of molecular junctions with a typical area of 0.005–0.01μm2 is presented. The small size is determined by one optical lithography step. The structure of junctions is metal-SAM-metal, where SAM is a self-assembled molecular layer with <105 molecules. The effect of attributes such as temperature, type of metal films, and molecular structure of the SAM on the I-V characteristics of the junctions is found to be in agreement with previous results and theoretical predictions. The prospect advantages of the junctions for future research are also mentioned.
Alignment dependent mechanical responses of carbon nanotubes to light
Authors: Lu Shaoxin , Ahir Samit V. , Terentjev Eugene M. , Panchapakesan Balaji
Published: September 2007

The authors report the orientation dependent elastic responses of carbon nanotubes to infrared photons. Unaligned and partially aligned samples of single wall carbon nanotubes (SWCNTs) and multiwall carbon nanotubes (MWCNTs) were studied for orientation dependent mechanical responses. While partial alignment in MWCNT ensembles changed the mechanical response to photons from expansion to contraction, the speeds of photomechanical responses were increased at least an order of magnitude by nanotube partial alignment in both SWCNT and MCWNT samples. The unique alignment dependent reversible photomechanical responses of carbon nanotubes are critical for actuator applications.
Elastic and electronic properties of TcB2 and superhard ReB2: First-principles calculations
Authors: Wang Yuan Xu
Published: September 2007

The structural, elastic, and electronic properties of TcB2 and recent synthesized superhard ReB2 were studied by the first-principles calculations. Both ReB2 and TcB2 are found to be elastically stable with the hexagonal and orthorhombic structures. For the two materials, the hexagonal structure is more stable than the orthorhombic one. Moreover, hexagonal ReB2 and TcB2 have stronger directional bonding between ions than OsB2, IrN2, OsN2, and PtN2. The author’s calculations show that both ReB2 and TcB2 are potentially superhard materials on the basis of their large bulk moduli and also their large shear to bulk modulus ratios. The band structure shows that experimentally synthesized hexagonal ReB2 is metallic. A pseudogap appears around the Fermi level of the total density of states of hexagonal ReB2, which may contribute to its stability.
Electric field induced conversion in the nature of the phase transition from the first order to the second order for Langmuir-Boldgett polymer films
Authors: Meng X. J. , Kliem H. , Lin T. , Chu J. H.
Published: September 2007

Ferroelectric and dielectric properties of the copolymer of vinylidene fluoride and trifluoroethylene films derived from the Langmuir-Blodgett technology have been investigated at different temperatures. The influence of the dc electric field on the phase transition is emphasized and the results show that the phase transition temperature increases with increasing applied electric field. Simultaneously, the nature of the phase transition converts from the first order to the second order when the applied electric field is high enough.
Bilayer gate dielectric study by scanning tunneling microscopy
Authors: Ong Y. C. , Ang D. S. , Pey K. L. , O’Shea S. J. , Goh K. E. J. , Troadec C. , Tung C. H. , Kawanago T. , Kakushima K. , Iwai H.
Published: September 2007

An advanced bilayer gate dielectric stack consisting of Sc2O3∕La2O3∕SiOx annealed in nitrogen at 300°C was studied by scanning tunneling microscopy using bias dependent imaging. By changing the sample bias, electrical properties of different layers of the dielectric stack can be studied. At a sample bias of +3.5V, the conduction band of the La2O3 layer is probed revealing a polycrystalline film with an average grain size of about 27nm, in good agreement with that determined from planar transmission electron microscopy. High conductivity at grain boundaries, due possibly to dangling bonds, can be observed in this layer, as also observed in grain boundary assisted current conduction in metal-oxide-silicon structures. Imaging at a sample bias of −4V probes the interfacial SiOx layer and an amorphouslike image of the interfacial layer is obtained.
Transparent conducting C60:LiF nanocomposite thin films for organic light-emitting diodes
Authors: Zhao Y. Q. , Huang C. J. , Ogundimu T. , Lu Z. H.
Published: September 2007

C 60 : Li F nanocomposite thin films were synthesized by physical vapor coevaporation. It is found that the nano-composite films are extremely conductive even at high LiF concentrations of up to 75wt% and that the films form Ohmic contacts with Al electrodes. IR measurements showed evidence of charge transfer from LiF to C60. Scanning electron microscope studies showed that C60:LiF (75wt%) nanocomposite forms uniform films. Compared with an archetypical tris(8-hydroxyquinolinato)alumina based device, OLEDs with the C60:LiF composite electron transport materials have lower driving voltages and higher power efficiencies.
Using nonlinear ac electrokinetics vortex flow to enhance catalytic activities of sol-gel encapsulated trypsin in microfluidic devices
Authors: Wang Shau-Chun , Chen Hsiao-Ping , Lai Yi-Wen , Chau Lai-Kwan , Chuang Yu-Chun , Chen Yi-Jie
Published: September 2007

A novel microstirring strategy is applied to accelerate the digestion rate of the substrate Nα-benzoyl-L-arginine-4-nitroanilide (L-BAPA) catalyzed by sol-gel encapsulated trypsin. We use an ac nonlinear electrokinetic vortex flow to stir the solution in a microfluidic reaction chamber to reduce the diffusion length between the immobilized enzyme and substrate in the solution. High-intensity nonlinear electroosmotic microvortices, with angular speeds in excess of 1 cm/s, are generated around a small (∼1.2 mm) conductive ion exchange granule when ac electric fields (133 V/cm) are applied across a miniature chamber smaller than 10 μl. Coupling between these microvortices and the on-and-off electrophoretic motion of the granule in low frequency (0.1 Hz) ac fields produces chaotic stream lines to stir substrate molecules sufficiently. We demonstrate that, within a 5-min digestion period, the catalytic reaction rate of immobilized trypsin increases almost 30-fold with adequate reproducibility (15%) due to sufficient stirring action through the introduction of the nonlinear electrokinetic vortices. In contrast, low-frequency ac electroosmotic flow without the granule, provides limited stirring action and increases the reaction rate approximately ninefold with barely acceptable reproducibility (30%). Dye molecules are used to characterize the increases in solute diffusivity in the reaction reservoir in which sol-gel particles are placed, with and without the presence of granule, and compared with the static case. The solute diffusivity enhancement data show respective increases of ∼30 and ∼8 times, with and without the presence of granule. These numbers are consistent with the ratios of the enhanced reaction rate.
Relationship between follicle size and oocyte developmental competence in prepubertal and adult pigs
Authors: Bagg Melanie A. , Nottle Mark B. , Armstrong David T. , Grupen Christopher G.
Published: September 2007

The present study compared the distribution and steroid composition of 3-, 4- and 5–8-mm follicles on the surface of prepubertal and adult ovaries, and determined the relationship between follicle size and developmental competence of oocytes following parthenogenetic activation. The effect of 1 mm dibutyryl cAMP (dbcAMP) for the first 22 h of in vitro maturation (IVM) on the embryo development of prepubertal oocytes from the three follicle size cohorts was also determined. Compared with adult, prepubertal ovaries contained a higher proportion of 3-mm follicles (46 v. 72%, respectively), but a lower proportion of 4-mm (33 v. 22%, respectively) and 5–8-mm follicles (21 v. 6%, respectively). Adult follicular fluid (FF) contained 11-fold higher levels of progesterone (P4) than prepubertal FF, with similar levels observed between all adult follicle sizes. In prepubertal FF, the P4 concentration increased with follicle size from 3 to 4 to 5–8 mm. Rates of blastocyst development following parthenogenetic activation of adult oocytes from all three follicles sizes were similar (approximately 55%), whereas rates from prepubertal oocytes increased with increasing follicle size from 3 (17%) to 4 (36%) to 5–8 mm (55%). Treatment with dbcAMP for the first 22 h of IVM led to a 1.5-fold increase in the rate of blastocyst development for prepubertal oocytes from 3-mm follicles, but had no effect on prepubertal oocytes from the 4 and 5–8 mm classes. Mean blastocyst cell number increased with follicle size in prepubertal ovaries and was similar for all follicle sizes in adult ovaries. The present study demonstrates that the low efficiency of in vitro embryo production observed using prepubertal compared with adult pig oocytes is due to a greater proportion of 3-mm follicles on prepubertal ovaries, which contain oocytes of inferior developmental competence.
Translocation reverses birth sex ratio bias depending on its timing during gestation: evidence for the action of two sex-allocation mechanisms
Authors: Linklater W. L.
Published: September 2007

Many sex allocation mechanisms are proposed but rarely have researchers considered and tested more than one at a time. Four facultative birth sex ratio (BSR) adjustment mechanisms are considered: (1) hormone-induced conception bias; (2) sex-differential embryo death from excess glucose metabolism; (3) sex-differential embryo death from embryo–uterine developmental asynchrony; and (4) pregnancy hormone suppression and resource deprivation. All mechanisms could be switched on by the corticoadrenal stress response. A total of 104 female rhinoceroses (Rhinocerotidae), translocated from 1961 to 2004 at different stages of gestation or conceived soon after arrival in captivity, were used to test for a reversal in BSR bias as evidence for the action of multiple sex-allocation mechanisms. Translocation induced a statistically significant BSR reversal between early gestation (86% male births from 0 to 0.19 gestation) and mid-gestation (38% male from 0.2 to 0.79 gestation). Captivity also induced a strongly male-biased (67% male) BSR for conceptions after arrival in captivity. The results indicate the action of at least two sex-allocation mechanisms operating in sequence, confirm the important role of sex-differential embryo death around implantation and of stress in sex allocation, and lend support to suggestions that sex-differential glucose metabolism by the preimplantation embryo likely plays a role in facultative BSR adjustment.
Ovarian activity in Arabian leopards (Panthera pardus nimr): sexual behaviour and faecal steroid monitoring during the follicular cycle, mating and pregnancy
Authors: de Haas van Dorsser Florine J. , Green Daphne I. , Holt William V. , Pickard Amanda R.
Published: September 2007

The Arabian leopard is a critically endangered subspecies endemic to the Arabian Peninsula. A fundamental understanding of the ovarian activity of the leopard is important to enhance the success with which it breeds in captivity. The objective of the present study was to characterise the endocrinology of the follicular cycle, ovulation and pregnancy in captive females using faecal steroid hormone analyses and observations of sexual behaviour. The follicular cycle of the leopard was shown to last 18–23 days based on the interval between consecutive peaks of faecal oestrogen conjugates, and the occurrence of silent heats was high. Puberty had commenced at 2 years of age, but faecal steroid profiles did not match those of the adult female until 3 years of age. No seasonal change in ovarian steroid excretion was observed, although behavioural oestrus was suppressed in summer. Significant rises in faecal progestagen concentrations were only recorded in mated leopards, indicating that these females were strictly induced ovulators. However, only 60% of these mating periods were ovulatory. Progestagen concentrations during pregnancy were significantly higher than those of the non-pregnant luteal phase. The average duration of the non-pregnant and pregnant luteal phases was 39 and 97 days, respectively. The basic features of the reproductive cycle of the Arabian leopard described here form an important foundation for further study into its reproduction.
Effect of donor age on success of spermatogenesis in feline testis xenografts
Authors: Kim Yeunhee , Selvaraj Vimal , Pukazhenthi Budhan , Travis Alexander J.
Published: September 2007

Ectopic xenografting of ‘donor’ feline testicular tissue into a ‘recipient’ immunodeficient mouse is a promising tool to preserve the male genome from genetically valuable felids. To define parameters under which the technique can succeed, we compared the effect of donor age on xenograft spermatogenesis among four age groups of domestic cats (Felis catus; age range 8 weeks to 15 months). In all cases, fresh tissue was grafted into castrated mice and collected 10, 30 and 50 weeks later. The percentage of xenografts recovered decreased as donor age increased. Mature testicular spermatozoa were observed in xenografts from the 8 and 9–16 week age groups; only a single 7-month-old donor produced elongating spermatids and xenografts from donors ≥ 8 months of age degenerated. Seminal vesicle weight, an indicator of bioactive testosterone, was not significantly different between donors aged 8 weeks to 7 months and controls, suggesting that xenograft Leydig cells were ultimately functional even in the 5–7 month age group. Regardless of donor age, production of mature spermatozoa from xenografts was markedly delayed compared with controls. Comparison of xenografts that produced sperm with normal controls revealed a decrease in tubule cross-sections having post-meiotic germ cells. Together, these results indicate that the maximum practical donor age was just before the onset of puberty and that even successful xenografts had abnormalities in spermatogenesis.
Effects of cryopreservation on mitochondrial function and heterogeneity, lipid raft stability and phosphatidylserine translocation in koala (Phascolarctos cinereus) spermatozoa
Authors: Zee Yeng Peng , Holt William V. , Allen Camryn D. , Nicolson Vere , Burridge Michelle , Lisle Allan , Carrick Frank N. , Johnston Steve D.
Published: September 2007

Koala sperm mitochondria were examined by cryomicroscopy using the fluorescent probe JC-1, which distinguishes high (red) and low (green) mitochondrial membrane potential (MMP). At normal body temperature, ~70% of live and untreated spermatozoa exhibited high MMP whereas <3% of live untreated spermatozoa exhibited low potential. A third class, in which single midpieces contained mixed mitochondrial populations, was also detected. Heterogeneity was noted in the level of MMP between individual koalas, individual spermatozoa and even between mitochondrial gyres within single midpieces. MMP of the live sperm population was not significantly affected by glycerol but was suppressed by freezing and thawing treatments. After thawing, MMP declined significantly during rewarming, especially as the temperature increased from 5 to 35°C. The distribution of the ganglioside GM1 was examined using fluorescent-labelled cholera toxin B. In fresh, untreated koala spermatozoa GM1 was detected on the head and midpiece, but not on the principal piece. No significant redistribution of GM1 was observed after chilling and cryotreatment. Phosphatidylserine translocation across the plasma membrane was examined using fluorescent-labelled annexin V. Few fresh spermatozoa exhibited phosphatidylserine translocation (~1%); this was not increased by chilling or cryopreservation, thus implying that cryotreatment had little effect on plasma membrane lipid asymmetry.
Society for Reproductive Biology Founders' Lecture 2007. Insights into germ cell biology: from the bench to the clinic
Authors: Roy Angshumoy , Matzuk Martin M.
Published: September 2007

The germline is unique among tissues in being the only lineage that is transmitted through generations. The gonadal somatic cells that interact with male and female germ cells are equally important for their juxtacrine and paracrine signalling pathways that lead to the formation of functionally mature gametes and healthy progeny. The present review summarises exciting new studies that our group and others have achieved at the frontier of male and female germ cell biology and in studying transforming growth factor-β signalling pathways in oocyte–somatic cell interactions and gonadal growth and differentiation. In the process, we have produced over 70 transgenic and knockout models to study reproduction in vivo. These models have helped us identify novel and unexplored areas of germ cell biology and translate this work into the fertility clinic.
Osteopontin regulates hindlimb-unloading-induced lymphoid organ atrophy and weight loss by modulating corticosteroid production
Authors: Wang Kathryn X. , Shi Yufang , Denhardt David T.
Published: September 2007

Osteopontin (OPN), a multifunctional secreted phosphoglycoprotein, plays diverse roles in bone biology, immune regulation, cell survival, inflammation, and cancer metastasis. Here we show its role in determining lymphocyte homeostasis and body mass in response to hindlimb unloading (HU), a model for evaluating effects of weightlessness on the musculoskeletal and other physiological systems. Using this stress model, we compared OPN−/−mice with OPN+/+mice subjected to HU for 3 days. Whereas OPN+/+mice suffered a marked reduction of body weight and significant spleen and thymus atrophy, OPN−/−mice exhibited minor weight loss and much less spleen and thymus atrophy. The HU-induced lymphoid organ atrophy was the result of dramatically diminished numbers, respectively, of T and B cells in the spleen and CD4+CD8+double-positive cells in the thymus of OPN+/+mice. Increased levels of corticosterone, which modulates lymphocyte activation responses and apoptosis during stress, were found only in OPN+/+mice. Apoptotic cell death was evident in the spleen and thymus of OPN+/+mice subjected to HU but not in OPN−/−mice. Importantly, lymphocytes from both OPN+/+and OPN−/−mice were equally sensitive to corticosteroid-induced apoptosis. These results reveal that OPN is required for enhanced corticosterone production, immune organ atrophy, and weight loss in mice subjected to HU.
Carbon dioxide enrichment alters plant community structure and accelerates shrub growth in the shortgrass steppe
Authors: Morgan Jack A. , Milchunas Daniel G. , LeCain Daniel R. , West Mark , Mosier Arvin R.
Published: September 2007

A hypothesis has been advanced that the incursion of woody plants into world grasslands over the past two centuries has been driven in part by increasing carbon dioxide concentration, [CO
2
], in Earth's atmosphere. Unlike the warm season forage grasses they are displacing, woody plants have a photosynthetic metabolism and carbon allocation patterns that are responsive to CO
2
, and many have tap roots that are more effective than grasses for reaching deep soil water stores that can be enhanced under elevated CO
2
. However, this commonly cited hypothesis has little direct support from manipulative experimentation and competes with more traditional theories of shrub encroachment involving climate change, management, and fire. Here, we show that, although doubling [CO
2
] over the Colorado shortgrass steppe had little impact on plant species diversity, it resulted in an increasingly dissimilar plant community over the 5-year experiment compared with plots maintained at present-day [CO
2
]. Growth at the doubled [CO
2
] resulted in an ≈40-fold increase in aboveground biomass and a 20-fold increase in plant cover of
Artemisia frigida
Willd, a common subshrub of some North American and Asian grasslands. This CO
2
-induced enhancement of plant growth, among the highest yet reported, provides evidence from a native grassland suggesting that rising atmospheric [CO
2
] may be contributing to the shrubland expansions of the past 200 years. Encroachment of shrubs into grasslands is an important problem facing rangeland managers and ranchers; this process replaces grasses, the preferred forage of domestic livestock, with species that are unsuitable for domestic livestock grazing.